Long-Term Morphotectonic Evolution and Denudation Chronology of the Antioqueño Plateau, Central Cordillera, Colombia

LONG-TERM MORPHOTECTONIC EVOLUTION AND DENUDATION CHRONOLOGY OF THE ANTIOQUEÑO PLATEAU, CORDILLERA CENTRAL, COLOMBIA

SERGIO ANDRÉS RESTREPO-MORENO

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2009

To my family: my preferred spiritual niche and my most precious source of love and awareness.

ACKNOWLEDGMENTS

This degree is linked to what seems now an incredible amount of work. Although this research venture represents yet another step in my apparently endless academic career, it is by no means an individual conquest and many people and organizations participated in this endeavor so I could bring my doctoral project to fruition.

I owe the deepest gratitude to my family for their continuing support in the various projects

I have embarked, not only in academe but in every aspect of life. And I also owe them an apology as they generously allowed me to steal some of our precious time together. But the family has grown in this long 7 years or more so I want to specifically thank members of the “old family”: my parents Mariana and Miguel Angel for the gift of life and for their guidance and their infinite capacity to love and give; to my siblings Clara, Camilo and Joche because with them I was introduced, at an early age, to the delight of laughter and the privileges of extended childhood, I am also grateful to them for their long-term solidarity and the great moments we

have shared together. And, of course, I want to thank the “new family”: another magical group of

people that has resulted from sharing my life with Isa which in turn brought Luna and Inti

Miguel into the scene. These three new members of my “whole family” have made my life even

better than it already was while increasing my degree of compromise with society. To Isa’s

family also my gratitude for their decided support to Isa, the kids and, hence, to me.

The enormous task of administration and paperwork is at the basis of academic success in

our Department, although the work of these “invisible” people usually remains unnoticed. I want

thank all of those who have dedicated long hours of work so others can engulf in book-laboratory

life, those who have passed through room Williamson 241 and the various generations of

committed workers, in particular the generation with which I got to share more time: Ron Ozbun,

Ileana McCray, Mary Ploch, and Jody Gordon.

I owe gratitude to David Foster, my supervisor. David valued my intentions of conducting an independent research project. His support came in many forms, though most appreciated were his critiques of my ideas, and his invitation to share his devotion for kid’s foot-ball, something they call soccer here in the US. Each of the other members of my committee had something to teach me, Mark Brenner, Willie Harris, and Ellen Martin are thanked for their guidance through the intricacies of both academic matters and life in general. Through their company and permanent support this doctoral work truly acquired its “Ph” part as they were always ready to

“waste” some time philosophizing around. Helpful academic discussions with them enhanced

this work. They also represent an important aspect of my now deep interest in lakes, soils,

climate change, and the interactions between humans and the natural environment, which are all

essential components of my academic and social quandaries. I also want to thank Dave Hodell

for his opportune comments about my research, particularly during committee meeting time and

Mike Binford for his interesting approach to the use of the distant eyes (remote sensing) and GIS

to study the complexities of Earth. I am thankful to Mike Perfit. Although not strictly a

committee member, he has provided much appreciated long-term guidance and support and he

played a fundamental role as a graduate advisor during my first years at UF. He was also the

person in Geological Sciences that believed in my project and made everything possible so I

could tap the initial financial support that allowed me to successfully pursue this track.

The comradeship of Richard Barclay (geology), Phill D’Amo (geology), Lara Maxwell

(veterinary medicine), Monica Lipinsky (geology), Victoria Mejia (geology), Carlos Jaramillo

(geology), Mary Lindenberg (geology), Pedro Santos (zoology), Greg Babitt (zoology), Anne

Lindenberg (engineering), the soccer crew in zoology and botany, and a few other geology and

non-geology students was fundamental in acclimatizing to the rapid changes in life that came

with leaving family and land behind. Their company also deepened my thinking in very interesting realms inside and outside of academe. Thanking George Kamenov is a must as his support in the technical aspects of this dissertation, our shared passion for foot-ball (something they call soccer here in the US), and our common nature as foreigners (and then parents!) gradually brought us close together and made us share the path of laughter quite often.

Various organizations and colleagues in Colombia and The US have contributed intellectually to this project. Listing is always a bad thing to do because you constantly forget to include someone/something, but here I go: I want to thank Miguel Angel Restrepo and the rest of the crew at CIER, Colombian NGO for the financial and logistic support; Norberto Parra,

Alberto Arias, and Kenneth Cabrera, great geomorphologists and hilarious friends at ICNE at

Universidad Nacional de Colombia for the permanent academic advise; Daniel Stockli and other researchers at the Isotope Geology Lab of the University of Kansas for a very productive time for me as their apprentice in the meticulous work of apatite helium dating. I am most grateful to the generosity of Paul O’Sullivan and Ray Donelick at Apatite to Zircon Inc. Their scientific work in perfecting a new way to date apatite crystals by fission tracks using LA-ICP-MS is a breakthrough. Not only are they great scientists, but also possess a very humane approach to the interaction with others in academe. They came to UF, on their budget, to teach George, David and I the fundamentals of LA-ICP-MS fission track dating, sharing their knowledge, opening up a new avenue of research for one of our facilities at UF, and tremendously facilitating my process of data acquisition with this tool. Big thanks also go to my colleagues and friends Carlos

Suescún and Camilo Polanco for their assistance in the field, to Misty Stroud for her valuable help with whole-rock chemical analysis at UF, and to my cousin Juan David Moreno and “Paty”

Monsalve for their dedicated work with some of the graphics that accompany this text. Many

thanks also go to Andre Stephenson at Application Support Center at UF for his invaluable help in finishing up this manuscript.

Financial support was of paramount importance in undertaking this scientific project. The independent character of my research made it a bit more difficult, but I always managed to find very receptive individuals out there ready to contribute to my scholarly efforts. Most thanked are

Anne Donnelly and Emila Hodges for the SEAGEP Fellowship and the various research grants provided, Susan Jacobson for awarding me the Compton Foundation-Tropical Conservation and

Development Scholarship and Research Grant, Cindy Martinez and others at the American

Geological Institute for a long history of financial support and mentoring; administrators in the

Geology Department, The Center for Latin American Studies-Tinker Foundation-UF, The

College of Liberal Arts and Sciences, The Office of Graduate Studies-UF, Women’s Club-UF,

CIER, the Office of Minorities at the Graduate School-UF, GSA, (unfortunately I have to close with and “etc.” right here), etc. for the opportune financial support.

Once again, I present my sincere gratitude to all those individuals and organizations that directly and indirectly contributed to this work, whether listed here or not, this dissertation is as much the product of their hard labor and dedication as it is mine.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS...... 4

LIST OF TABLES...... 11

LIST OF FIGURES ...... 12

ABSTRACT ...... 14

CHAPTER

1 INTRODUCTION...... 16

Intro to an Intro...... 16 Study Site ...... 17 Objectives and Significance of this Investigation ...... 19 Scientific and Societal Importance of Erosion ...... 20 Natural Erosion ...... 21 Anthropogenic Erosion...... 24 Assessing Erosion Rates in the Colombian Andes ...... 26 Modern Erosion ...... 26 Long-term Erosion ...... 27 This Investigation in the Context of the Geological Discipline ...... 31 Methods ...... 33 Erosional Exhumation and Low Temperature Thermochronology ...... 33 Apatite low temperature thermochronology ...... 33 Post Crystallization Cooling and Petrology of the Antioqueño Plateau ...... 39 Spot U-Pb and Hf isotopic analysis in magmatic zircons...... 39 Zircon cathodoluminescence imaging (CL) ...... 40 Whole-rock major-trace element, and radiogenic isotopic analysis ...... 41 Thesis Outline ...... 42

2 FORMATION AGE AND MAGMA SOURCES OF THE ANTIOQUEÑO AND OVEJAS BATHOLITHS, CENTRAL CORDILLERA, COLOMBIA ...... 50

Introduction ...... 50 Study Site ...... 53 Geologic Setting ...... 53 The Antioqueño and Ovejas Batholiths ...... 55 Previous Thermochronology and Isotopic Analysis ...... 56 Tectonic and Geodynamic Setting ...... 57 Methods ...... 59 Sample Collection and Preparation ...... 60 Zircon separation, grain description and cathodoluminescence ...... 61 Whole rock sample preparation ...... 63

U/Pb and Hf Spot Analysis in Zircons ...... 64 Whole Rock Geochemical and Isotopic Analysis ...... 67 Major and trace element analysis ...... 67 Nd isotopic analysis ...... 67 Pb isotopic analysis ...... 68 Results ...... 68 Major Elements ...... 68 Zircon U-Pb and Hf ...... 69 Pb and Nd Isotopic Compositions ...... 70 Rare Earth Elements ...... 70 Discussion and Interpretation ...... 71 Major Elements ...... 71 Zircon CL, U-Pb and Hf ...... 72 Trace Element and Radiogenic Isotopes ...... 74 Radiogenic Pb and Nd ...... 79 A Feasible Geodynamic Setting ...... 79 Conclusions ...... 82

3 LONG-TERM EROSION AND EXHUMATION OF THE “ALTIPLANO ANTIOQUEÑO”, NORTHERN ANDES (COLOMBIA) FROM APATITE (U-TH)/HE THERMOCHRONOLOGY ...... 102

Introduction ...... 102 Study Site ...... 104 Geological and Physiographic Overview ...... 104 The Antioqueño Plateau ...... 106 Tectonic Setting ...... 108 Apatite (U-Th)/He Thermochronology ...... 110 Methods ...... 111 Results ...... 114 Discussion and Interpretation ...... 116 Conclusions ...... 123

4 FURTHER CONSTRAINTS ON LONG-TERM EROSIONAL EXHUMATION OF THE “ALTIPLANO ANTIOQUEÑO”, NORTHERN ANDES, COLOMBIA, BY LA- ICP-MS APATITE FISSION TRACK ANALYSIS ...... 138

Introduction ...... 138 Study Site ...... 140 Geomorphologic Considerations ...... 141 Geological Considerations ...... 143 Tectonic Considerations ...... 146 Uplift and Denudation in the Colombian Andes ...... 147 Low Temperature Thermochronology and Erosional Exhumation ...... 150 Apatite Fission Track Analysis ...... 150 Apatite Fission-Track Analysis in the Antioqueño Plateau ...... 152 Methods ...... 153

Sample Collection and Preparation ...... 153 Age Measurements by LA-ICP-MS ...... 154 Track Length Measurements ...... 156 Annealing Kinetics and Dpar Parameterization...... 156 Fission-track Modeling...... 157 Results ...... 158 Interpretation and Discussion ...... 160 Conclusions ...... 171

5 GENERAL CONCLUSIONS AND FURTHER RESEARCH...... 186

Antioqueño and Ovejas Batholith Age and Petrogenensis ...... 186 Low Temperature Thermochronology ...... 187

LIST OF REFERENCES ...... 190

BIOGRAPHICAL SKETCH ...... 221

LIST OF TABLES

Table page

2-1 Summary of major element data for the Antioqueño and Ovejas batholiths...... 85

2-2 U-Pb ages and present εHf values for the Antioqueño and Ovejas batholiths...... 86

2-3 Whole rock trace element concentrations for the Antioqueño and Ovejas batholiths...... 87

2-4 Lead and Neodymium isotopic compositions for the Antioqueño and Ovejas batholiths...... 88

3-1 Summary of (U-Th)/He results for Matasanos and La García vertical transects...... 127

4-1 Instrumentation, operating conditions, and data acquisition parameters used for LA- ICP-MS apatite fission-track dating...... 173

4-2 Summary of apatite fission-track data...... 174

LIST OF FIGURES

Figure page

1-1 Physiographic Units of Colombia...... 46

1-2 The Antioqueño Plateau a geomorphic feature carved on the Antioqueño Batholith...... 47

1-3 The concept of the partial annealing zone (PAZ) and partial retention zone (PRZ) in apatite fission track and (U-Th)/He thermochronology...... 48

1-4 Vertical profile approach to low-temperature thermochronology in the Antioqueño Plateau...... 49

2-1 Geologic and structural map of the study site...... 88

2-2 Color photographs of samples in their hand specimen...... 89

2-3 Photomicrographs of grey-scale CL imaging of typical zircons from the Antioqueño and Ovejas batholiths...... 90

2-4 Classification of the Antioqueño and Ovejas batholith based on their SiO2 vs. Na2O + K2O concentrations...... 91

2-5 Sub-alkalinity degree for the Antioqueño and Ovejas batholiths...... 92

2-6 Major element Harker variation diagrams for the Antioqueño and Ovejas batholiths...... 93

2-7 U-Pb Age-Elevation relationship and range of previously reported ages for the Antioqueño and Ovejas batholiths ...... 94

2-8 Tera-Wasserburg diagrams...... 95

2-9 U-Pb Age-Present εHf relationship...... 96

2-10 Pb/Pb correlation diagrams for the Antioqueño and Ovejas batholiths...... 97

2-11 εNd vs. εHf relationship for the Antioqueño and Ovejas batholiths...... 98

2-12 Primitive mantle normalized Rare Earth Elements patterns (REE) for the Antioqueño and the Ovejas batholiths...... 99

2-13 Multi-element plot for the Antioqueño and the Ovejas batholiths...... 100

2-14 Post-crystallization cooling path for the Antioqueño Batholith...... 101

3-1 Shaded relief map of the Northern Andes, Colombia. T ...... 128

3-2 Geology of the study site...... 129

3-3 Three dimensional representation and simplified topographic/geologic cross section across the Antioqueño Plateau...... 130

3-4 Modern tectonic setting...... 131

3-5 Apatite (U-Th)/He Age-elevation Relationship...... 132

3-6 Relation between apparent (U-Th)/He and fission-track ages in apatite...... 133

3-7 Simplified sketch illustrating the extent of the crustal section removed since ca. 25 Ma ...... 134

3-8 Simplified cartoon of cross-sections depicting the paleogeographic and geologic evolution of the Northern Andes at the latitude of the study site...... 135

4-1 Regional geomorphology...... 175

4-2 Geology of the study site...... 176

4-3 Tectonic Setting...... 177

4-4 Age-elevation relationship...... 178

4-5 AFT Age vs. Elevation Relationship for The La García profile...... 179

4-6 Age vs. mean track length relationship for the Matasanos-Porce profile...... 180

4-7 Representative Time-temperature paths modeled with AFTSolve®...... 181

4-8 Relevant HeFTy Time-temperature paths...... 182

4-9 AFT and AHe time-temperature paths...... 183

4-10 Long-term spatially averaged erosion...... 184

4-11 Timing and magnitude of Cenozoic changes in convergence between the Farallon- Nazca and South American plates...... 185

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

LONG-TERM MORPHOTECTONIC EVOLUTION AND DENUDATION CHRONOLOGY OF THE ANTIOQUEÑO PLATEAU, CORDILLERA CENTRAL, COLOMBIA

Sergio Andrés Restrepo-Moreno

May 2009

Chair: David A. Foster Cochair: Ellen Martin Major: Geology

Erosion is now recognized as a fundamental component of the functioning of Earth systems. Erosion by water is one of the most efficient mechanisms of exhumation of rocks in the tropical Andes and exerts control on myriad of processes such as landscape evolution, orogenic belt development and style, tectonic process, sediment production and transfer to water bodies, weathering and soil development, chemistry of oceans and continental water bodies, climate, etc.

In this investigation zircon U-Pb geochronology and Hf isotopic analysis, whole rock geochemistry, and apatite (U-Th)/He and fission track low-temperature thermochronology are used to reconstruct the long-term denudation chronology of the Antioqueño Plateau in the northern portion of the Colombian Cordillera Central . This approach contributes to an understanding of the Cenozoic morphotectonic evolution of this major elevated plateau in the context of tectonic perturbations in the region. In addition, this investigation allows generating the first quantitative, landscape-specific figures of long-term rates of erosion as a backdrop that permits a preliminary comparison with modern, anthropogenically enhanced erosion rates.

Morphotectonic evolution of this province seems to be strongly influenced by the

Antioqueño Batholith (AB). U-Pb dating and Hf isotopic analysis in zircon grains,

complemented with whole-rock major and trace element analysis plus Pb and Nd isotopic data,

constrain formation age, post-crystallization cooling history, tectonic setting and magma sources

associated with this large granodiorite mass. Results suggest rapid assemblage from ca. 71 to 77

Ma involving discrete magmatic injections from different sources and with varying degrees of

crustal contamination and/or fractional crystallization. Geochemical data point to a tectonic

setting quite similar to the modern Northern Andean Volcanic Zone. Apatite low temperature

thermochronology records two discrete pulses of exhumation at ca. 45 and 23 Ma separated by a

period of tectonic quiescence. Both pulses are synchronous with orogenic phases throughout the

Andes indicating continental-scale tectonic controls. Exhumation appears to be controlled by well documented changes in the rate of convergence between the Farallon (Nazca) and South

American plates. Erosion rates at the climax of exhumation are ~0.1 mm/yr while the long-term averages are ~0.02-0.008 mm/yr. The latter are considered low rates for an active orogen and are characteristic of cratonic areas. Morphologic and structural features of the Antioqueño Plateau define a decoupled morphotectonic system (non-equilibrium relict surface), that exhibits lagged

response to tectonic input so that the most recent and strongest phase of orogenic activity in the

Andes (Miocene-Pliocene) is not recorded by either apatite fission track or (U-Th)/He.

CHAPTER 1 INTRODUCTION

Intro to an Intro

Overall, I devised this academic exercise as a philosophical journey that would allow me to reflect, simultaneously, on scientific and societal problems, if such categories of human tribulations admit any separation at all. The initiative was to instigate a personal immersion in geological work in light of J. Lubchenco's proposal for a new and urgent social contract for the sciences [1]. I chose to work in the tropical Andes, specifically the Cordillera Central of

Colombia, for various reasons including but not restricted to: 1) the indisputable importance of that portion of the Andean chain geologically, geographically (human and physical), biologically, and ecologically, 2) the social debt I have acquired via education with the peoples of that region, and 3) the spiritual affinity I have with that territory.

It was the concern about the interactions of humans with the landscape, i.e., the technosphere[2], particularly in the context of the capacity of humans to transform geomorphic dynamics [3-5], and the intention to understand erosional forces past and present, that pushed me into pursuing this investigation. The literature review that marked the development of this dissertation and my personal experience as a traveler aware of soil degradation, indicate that enhanced erosion due to human perturbation of natural ecosystems has the characteristics of an environmental catastrophe with an associated major threat to the sustainability of the human enterprise [6-9].

The trigger of this research adventure was thus related to an independently developed exploration of anthropogenic and natural erosion rates. However, and mainly due to the lack of reliable information on modem erosion rates and the difficulty in generating them over a reasonable time frame, the investigation migrated to topics that are at the same level of interest.

Such themes include the possibility to quantitatively determine the long-term landscape evolution and the exhumation history of an interesting morphotectonic unit in the Colombian

Cordillera Central , known as the Antioqueño Plateau (Figure 1-1), in light of proposed theories for the morphotectonic, geologic and paleogeographic evolution of the region [10-17], and continental-scale tectonic perturbations [18-20]. In fact, it was this latter migration of the study in new directions what turned out to be the core of the research project presented in this dissertation.

Study Site

The Colombian territory can be roughly divided into four physiographic provinces: a) five discrete mountainous areas including the Andean Region, the Sierra Nevada de Santa Marta, the

Serranía de Baudó, the Serranía de la Macarena, and the Serranía de Chiribiquete; b) two major plains represented by the Caribbean Plains in the north, and the vast Eastern Plains encompassing the savannas of the Orinoquía and the tropical forest of the Amazonía; c) two interandean valleys associated to the Cauca and Magdalena rivers; and d) the relatively flat areas west of the Andean Region (Pacific Coast Plains) occupied by the valleys of the rivers Atrato,

San Juan and Patía (Figure 1-1).

The mountainous domain of the Andean Region has a trident shape that encompasses the sub-parallel ranges of the Cordillera Occidental, Cordillera Central and Cordillera Orietal

(Western, Central and Eastern cordilleras), roughly oriented in a N-S direction and separated by major intramontane depressions. The Cauca trough separates the Cordillera Occidental and

Cordillera Central while the wider Magdalena Valley defines the boundary between the

Cordillera Central and Cordillera Oriental. The study site falls on northern tip of the Cordillera

Central. Lithologically, the area is dominated by Paleozoic polymetamorphic basement, with small remnants of shallow marine sedimentary sequences, Lower Cretaceous in age, and late

Cretaceous massive granitoids emplaced in the axis of the range such as the Antioqueño

batholith [21]. Structurally speaking the region has functioned as a coherent semi rigid crustal

bloc, with low internal disruption by faults [22] and bound by two major inherited structures, the

Cauca-Romeral and Palestina fault systems to the west and east respectively [23]. The tectonic

history of this portion of the Andean orogen has been determined by subduction dynamics

between the Nazca (Farallon), Caribbean and South American plates and interactions of major

lithospheric blocks (Andean and Panama-Choco blocks) and an intense accretionary process from the Paleozoic to the late Cenozoic [23-26].

The plateau morphology of the study site is a striking feature of the Andean range in

Colombia [10, 12, 27, 28]. This portion of the Cordillera Central known as the Antioqueño

Plateau (AP), bears resemblance with a truncated and asymmetric pyramid with a steep face coinciding with the Cauca-Romeral depression (occupied by the narrow valley of the Cauca

River) and a more gentle decline towards the area structurally controlled by the Palestina fault system (towards the much wider Magdalena valley). This cordilleran segment is in clear contrast with the typical sierra-like ranges of the Western and Eastern Cordilleras and the southern portion of the Cordillera Central. In addition, the AP is deeply incised by the Medellin-Porce fluvial system (Figure 1-2) creating regional scarps with differences in elevation in excess of 1.5 km. The combination of lithologic, structural, and geomorphic features make the AP an ideal milieu to conduct low temperature thermochronology studies along vertical profiles, which permits assessing the type of questions proposed in this investigation.

In the following sections of this Introduction the objectives and significance of the project, the importance of erosion, and the status of erosional studies in Colombia will be addressed.

Additionally, the advantages of the multi-tool approach implemented in this research will be

succinctly presented with emphasis on the utility of low-temperature thermochronology.

Objectives and Significance of this Investigation

To date, the morphotectonic history of the Northern Andes of Colombia remains largely

unexplored. However, this portion of the Andean chain represents an important segment of the

orogen with a great potential in providing valuable information about the interactions between

tectonics and surface processes, particularly through the application of low temperature

thermochronologic techniques. In this investigation, the lithologic, structural and morphologic

characteristics of the AP are exploited to reconstruct the Cenozoic erosional and morphotectonic

history of the northern segment of the Cordillera Central. A better understanding of the long-

term landscape development and erosional exhumation history of the AP is needed for defining

the morphotectonic and uplift history of the Northern Andes, the relationships between erosion

and tectonics, the paleogeographic evolution of the region (Duque-Caro, 1990; Gómez, 2005;

Gregory-Wodzicki, 2000; Hooghiemstra, et al., 2006; Hoorn, et al., 1995; Van der Hammen,

1960; Van der Hammen, et al., 1973), and regional geodynamics trends from the Late Mesozoic

to the Cenozoic which have controlled large scale phenomena, from voluminous magma

generation and emplacement of the Antioqueño batholith to the development of the AP as a

geomorphic anomaly. The findings of this study have implications for refining morphotectonic

response of the overriding plate to variations in convergence rates between the Nazca (Farallon)

and the South American plates during the Cenozoic [18] and patterns of regional uplift. Low

temperature thermochronology also permits to define a base-line for geologic (natural) erosion

rates that will be valuable in future geomorphologic investigations as the backdrop against which

modern (anthropogenic) erosion rates can be compared.

The specific objectives of the investigation include: 1) establishing the timing and magnitude of erosional denudation events in this orogenic segment, 2) testing whether the AP functions as an elevated, relict erosional surface, i.e., a portion of the orogen that has not

achieved equilibrium between erosion and tectonics; 3) reconstructing long-term, background

erosion rates for the Paleogene and Neogene as a way to understand landscape development

patterns for the AP and as the first attempt to quantify natural (pre-anthropogenic) erosion rates;

and 4) defining more precisely the geodynamic environment and the time of emplacement of the

Antioqueño Batholith, the initial high-temperature portion of its cooling path, and the first phases of morphotectonic development possibly associated to this major granitic intrusion that appears

to have imposed lithologic control on the morphotectonic evolution of the AP.

Results are analyzed and interpreted in light of proposed theories for erosional phases in the Colombian Andes [16], regional basin development [14, 29], other uplift-exhumation- deformation episodes reported for the Andes and the Caribbean [30-40], well characterized episodes of increased convergence between Nazca (Farallon) and South America [18-20, 41, 42], and paleogeographic evolution in Northern South America [15, 17, 43]. More precisely defining the crystallization age of the Antioqueño Batholith (through U-Pb dating) and a feasible geodynamic environment of formation (via geochemical and isotopic data) is also crucial in unraveling possible causality between increased convergence, enhanced magmatism [44-47], crustal thickening, and the initial phases of uplift during the late Mesozoic-Early Cenozoic reported for the region [16].

Scientific and Societal Importance of Erosion

Earth’s surface constitutes a complex interface between the geosphere, the hydrosphere, the atmosphere, and the biosphere. At this interface, myriad of interactions between the spheres take place at a great range of spatiotemporal scales and through multiple feed-back mechanisms,

in a rather sophisticated and intricate fashion that Phillips (1999) called “the music of the spheres”. In turn, such interactions control numerous geologic, climatic and biological processes, also at various scales of space and time [48-56]. In the Holocene, and particularly after the last

glaciation, another sphere came into play as humans became increasingly capable of modifying

the natural environment, i.e., the technosphere [2]. This relatively new sphere has increasingly affected the natural systems of Earth leading to a conflict between the technosphere and the ecosphere leading to a global environmental crisis that is manifested in several realms such as

such as global warming, loss of fertile soil, water and air pollution, pauperization of fisheries,

etc.[6, 57, 58]. Although neglected compared to other global environmental problems such as global warming and nitrogen pollution, accelerated erosion due to anthropogenic causes has the potential to become a major ecological, and hence social, catastrophe that certainly warrants further consideration [8, 59-61]. Defining background erosion rates is an important endeavor in understanding erosion past and present not only as a natural phenomenon and in the context of the interactions between tectonics, climate and surficial process, but also as a necessary step in unraveling the effect of humans on natural geomorphic dynamics. The techniques utilized in this investigation, particularly low-temperature thermochronology, are of great importance to these goals.

Natural Erosion

Erosion is an intricate and fundamental natural process taking place right at the interface referred to above. Erosion is affected by, and exerts direct influences on, apparently unrelated domains such as climate [49, 51, 52, 62-65], weathering and pedogenesis [66, 67], chemistry of the ocean and the atmosphere [48, 49, 68], sediment production and transport at a variety of temporal scales [69-74], basin evolution [14], geometry and kinematics of orogens [75-79], and subduction dynamics [80]. Therefore, constraining variations in erosion rates across space and

time in active mountain chains helps to shed light on long-term landscape evolution and on the

dynamic coupling between geomorphic processes, tectonics and climate [54, 81-83].

The study of surface processes e.g., erosion, has thus become of first-order importance to

improving our understanding of the functioning of orogenic belts and of Earth’s dynamic

systems in general. It is usually accepted that erosional activity is sensitive to both tectonic and

climatic input [51, 84-87] but a definite answer to the question of which mechanism dominates

over the other [52] still remains unresolved [50]. A more profound comprehension of how

erosional, climatic, and tectonic forces interact to shape mountains has the potential to foster

clearer insights into Earth’s history [54, 81Pinter, 1997 #419]. This desirable situation requires

not only the development of more robust physically-based theory and models about the processes

that control fluvial erosion over time [50, 88], but also improved data on erosion rates with better

spatial and temporal resolution both in active and inactive tectonic settings [54].

Removal of crustal material via fluvial erosion is one of the most efficient means of

exhumation of deeply buried rocks in mountain chains [89-92]. Geomorphic dynamics, i.e.,

erosion and deposition, are central to the development of topography [81, 93-95] and to the

processes of mountain genesis [96-98] and basin evolution [14]. After almost a century of

visualizing erosion as the feeble sister of tectonics, many Earth scientists have begun to put

forward the idea that erosion may be equally important in the overall development of mountain

belts [97] not only at the surface but deep within the orogen [77, 79, 84]. Orogenic systems are

now approached as the result of multiple and complex interactions and feedback mechanism

among tectonic, erosional and climatic processes [81, 99]; and recent compilations reflect that

fact [100]. All three processes exert control on the shape and maximum height of mountain

chains, the geometry and distribution of deformation belts, the distribution of metamorphic

zones, the thermal and mechanic characteristics of the lithosphere, and the amount of time necessary to build or destroy mountain ranges [75, 76, 78, 92, 101]. So intricate are the

interactions between climate, erosion, tectonics and topography that simple inputs can lead to

complex outputs that cause surface processes, such as climate and erosion, to profoundly influence tectonic processes throughout the crust and even the mantle [92]. Unraveling the spatial variability and distribution of erosion throughout mountainous regions is therefore fundamental to understanding long-term evolution of orogenic belts (e.g., morphotectonic, metamorphic, thermo mechanic, etc.) as well as patterns of long-term landscape development.

Erosion by water is one of the most important mechanisms of exhumation of rocks in the

tropical Andes of Colombia. Substrate stripping via erosion is usually accompanied by some

form of soil removal [8, 66, 102-104]. The velocity at which erosion takes place is related to various factors including climate, topography, substrate characteristics, and vegetation cover.

Although it has been argued that modern sediment yield is not an appropriate way to reconstruct natural erosion rates, due to the pervasive effects of human activities on sediment production and transport [105], there seems to be consensus regarding the fact that fluvial networks show that erosion takes place at a higher pace in areas with steep topography. Sediment delivered by fluvial systems indicate that mountainous regions in tectonically active tropical settings tend to exhibit high erosion rates [98, 106-110]. This results from a combination of steep topography, high precipitation rates and substrate instability induced by earth quakes. In a recent compilation of erosion rates worldwide, alpine-type topography is usually associated with erosion rates in the order of 1 mm/yr whereas cratonic regions erode at velocities of about 0.001 mm/yr [8]. Some of the highest long-term (geologic) erosion rates have been reported at the Himalayan syntaxes were erosion takes place at rates close to 10 mm/yr [92] whereas for the Andes, various studies

have concluded that erosion rates at the climax of erosive pulses are in the order of 1-0.5 mm/yr and close to ~0.02 mm/yr during intervals of tectonic quiescence [31, 32, 35, 111, 112].

However, as further information on spatiotemporal patterns of erosion and exhumation is produced in the Andes, the spatial complexities become more apparent, mainly as a result of structural and climatic heterogeneities. It is important to continue to document these important processes along the entire Andean range, particularly in its northern portion where information is notoriously scant [113].

Anthropogenic Erosion

When natural rates of soil production are overcome by human-induced erosion, an imbalance takes place in which soil degradation in various forms is the ultimate consequence. To some authors, changes in erosion rates (e.g., accelerated soil stripping) are leading global society towards a severe environmental crisis [8, 58, 61, 114-117] of a magnitude comparable to global warming. In Colombia loss of fertile soil and other associated environmental problems go still unnoticed and/or underestimated despite its evident severity. Nonetheless, water erosion occurring as sheet erosion and rill-gully erosion is considered the major form of soil degradation in the Colombian Andes, particularly since post-Colonial times [61, 118-122] when a new

“ethnoecological” paradigm was introduced whose main purpose was the rapid transformation of natural goods an services into cash-for-the-crown, i.e., a lucrative approach to the interaction of humans with the landscape [123]. Introduction of new cultigens, new animal species [124] and a completely different perception of the natural environment lead to environmental deterioration in the whole Andean territory (Restrepo, unpublished; Popenoe, personal communication).

Natural geomorphic dynamics, and hence natural erosional process, can be drastically changed by human activities [5, 7, 105, 125-133]. Recent studies have demonstrated order of magnitude differences between modern (anthropogenic) and natural (geologic) erosion rates in

tropical mountainous settings [134, 135]. In areas under conventional agriculture, erosion rates fluctuate between ~1 and 100 mm/yr, far exceeding natural soil replenishment values [8, 115].

Further, humans have been categorized as the most important geomorphic agent on Earth [4, 5,

7] and some authors have advanced the idea that we may have entered the “Anthropocene”

[136].

The processes that determine the velocity and extent of water erosion are directly and indirectly affected by land cover and land use. From the factors that are believed to control erosion rates (climate, topography, soil properties, vegetation, and land use), vegetation is one of the most important variables and one of the most susceptible to be changed by human action

[105, 116, 118, 137, 138]. Plants moderate hydrological activity and act as a barrier between climatic agents and the soil surface. Vegetation directly protects the soil surface by reducing raindrop impact and by slowing down runoff and increasing infiltration [66]. It also improves soil structure via the physical binding of soil particles by roots, the generation of organic matter, and the increased biological activity [61, 66, 138, 139]. Transformation of natural vegetation to agricultural systems leads to a deterioration of soil properties increasing the potential for soil losses due to erosion [59, 104, 118, 140]. However, assessing substrate stripping and predicting erosion trends has proven problematic and the topic remains hotly debated in the current scientific literature [55, 141-144]. The application of different predictors yields results which are sometimes several orders of magnitude different [58, 108, 145]. This is possibly related to the

inherent complexity of sediment production, transport and deposition [146, 147] and to

alterations of geomorphic dynamics due to human actions [74, 110]. To better understand the impact of humans on modern geomorphic dynamics, it is important to increase our knowledge on both modern (anthropogenic) and long-term (natural) erosion rates.

Assessing Erosion Rates in the Colombian Andes

Modern Erosion

The study of modern erosion rates in Colombia is marked by the lack of a systematic and unified approach. Most research has focused on the local scale, trying to reconstruct modern erosion under different vegetation covers [122, 148, 149]. A transect approach across the

Cordillera Central [149], south of the study region of this dissertation, showed that under undisturbed forest, normal Andean erosion rates are on the order of 0.1 mm/yr while removal of vegetation to introduce agriculture and grazing can increase erosion rates by a tenfold. Sediment accumulation in dams within the Antioqueño Plateau conducted by repeated bathymetric studies in five dams [150] have been interpreted as basin wide erosion rates on the order of 1 mm/yr.

The study by Suárez de Castro and Rodriguez (1962) at the plot scale (10-120 m2) and for

periods close to a decade gave average soil losses as low as ~0.02 mm/yr on plots with a 53%

slope (established shade coffee) and as high as ~6 mm/yr on plots with a 21% slope (yearly rotation of pasture and corn). Losses from bare plots with 21% slope ranged from 51-87 mm/yr.

Similar results were reported in the Sierra Nevada de Santa Marta through a study of soil properties [118] At a larger spatial scale, coarse predictions using the universal soil loss equation

(USLE) [151] have provided figures for erosion potential values derived at the regional similar to the ones just discussed.

In relation to sediment load as an approach to infer erosion rates, it has been estimated that the Magdalena has an average sediment yield in excess of 6 t/ha*yr [72, 152]. Latter work shows that some sub-basins within the Magdalena, such as the Carare river, have sediment yields in excess of 2000 t/km2*y [153]. This value makes Magdalena the major contributor of sediment to

the Caribbean Sea and Atlantic Ocean in South America. Both records suggest that erosion rates

for the entire Magdalena basin are close to ~1 mm/yr, a relatively high figure even for

mountainous watersheds. If the fact that the Magdalena possesses extensive areas for temporary sediment storage and that the sediment delivery rations rarely approaches a value of 1, erosion rates for this populous watershed could be even higher. Some authors concur that the possible reasons for such high rates are mainly related to human activities [152-154].

The Anthropocene is “playing hard” on society as environmental degradation and obliteration of our base of natural gods and services is on the rise [58, 155]. Looked from a different perspective, these new period of Earth’s history also offers great opportunities as it exposes the scientific community to new and challenging intellectual problems. Put it in the words of Haff (2003): “Today we stand on the brink of a change to the Earth’s surface that may be as profound and long lasting as the impact of land vegetation. Because of the short time-scales involved, and the fact that what will happen will affect us directly in many ways, it is in our interest to engage the study of these changes in a head-on way”.

However, a better understanding of the role played by our species in enhancing erosion

rates implies accurate figures on both modern (anthropogenic) and long-term (natural) erosion

rates. Low-temperature thermochronology in the AP reveals relatively low long-term erosion

rates (0.02-0.008 mm/yr), despite its location in a tectonically active mountainous setting. This

low pace of erosion is several orders of magnitude less than estimated modern substrate striping

for the Magdalena basin and the AP [121, 150](1 mm/yr).

Long-term Erosion

Studies of long-term erosional dynamics and associated exhumational response in the

Northern Andes of Colombia are rather scarce. Morphotectonic evolution of the Colombian

Andes is poorly constrained [113, 156]. The majority of studies that have tackled uplift and erosional exhumation have concentrated on the Cordillera Oriental and have applied tools such as stratigraphy and palynology [13, 157, 158] with a minor contribution from thermochronology

[34, 159, 160]. Pioneering sedimentstratigraphic studies aimed at unraveling orogeny in the

Colombian Andes took place mainly in the Cordillera Oriental and between 1940-1960 [161,

162] leading to the recognition of discrete pulses of erosion that affected the entire Colombian

Andes from the Paleocene to Mio-Pliocene times [16]. These periods of enhanced denudation were attributed to uplift and were named the “orogenetic phases” Laramic (~60-57 Ma), Pre-

Andina I-II (~54-40 Ma), Proto-Andina (~25-22 Ma), and Eu-Andina I-IV (~14-2 Ma) [16].

However, the magnitude of these events, their varying efficiencies in removing crustal material, and the spatial distribution of erosion and exhumation throughout the orogen are themes that remain largely unconstrained.

Diachronous uplift and exhumation are illustrated by sediment-stratigraphic and pollen data that yield different timing of uplift-exhumation for the Eastern and Cordillera Central with the former experiencing major phases of uplift approximately in the last 12 Ma [15, 158, 163] while the latter was the locus of denudation since the late Cretaceous [16]. Subsequent research in the sedimentary formations of the Magdalena Valley has confirmed this chronology [14, 17,

29]. But space-specific information on uplift driven exhumation for the Colombian Andes continues to be scarce. Perhaps the only examples are a series of investigations in the Cordillera

Oriental between Colombia and Venezuela, with rather limited thermochronology datasets, that have suggested uplift in excess of 3 km [164] and major cooling episodes by fission-track data between 22 and 27 Ma [34, 160]. More recent studies incorporating apatite fission-track age and length data, paleobotany and geological data, also in the Eastern Cordillera, suggest increased exhumation rates in the northeastern Andes at ~3 Ma and asymmetrical denudation of this range due to orographic effects [159].

Major consideration has been given to the recent phase of uplift-exhumation in the

Colombian Andes, i.e., the Eu-Andina panes of Van der Hammen (1960), as the present configuration of the Andes and most of the uplift has been attributed to uplift pulses during this interval [113, 156]. Apatite fission-track ages between Venezuela and Colombia cluster in two sets point to important exhumation events associated with the Eu-Andina phase [160]. Similarly, results for rapid cooling with apatite fission-track ages of 9 and 12 Ma have been reported for a small area of the Cordillera Oriental [163]. Pliocene uplift has been extensively documented in the Sabana de Bogotá by detailed stratigraphy, palynology, K/Ar and fission track dating [158,

165]. It is now widely accepted that the final major tectonic uplift of the Cordillera Oriental in the Bogotá area occurred between approximately 5 and 3 Ma [15, 113]. However, the specific of other potentially important phases of uplift and exhumation in the Colombian Andes associated with major tectonic plate reorganization during the Eocene and Oligocene [18, 19] await to be documented. Data sets aimed at elucidating uplift and denudation patterns for the Cordillera

Central are even scarcer. Kroonenberg et al. (1990) suggested that onset of uplift for the Nevado del Ruiz area occurred between 10 and 4 Ma, supporting their contingence on local stratigraphic and geochronology data. For the northern part of the Cordillera Central, a set of restricted apatite fission-track ages are interpreted as reveling an Eocene cooling event driven by exhumation

[166]. In the Cordillera Occidental recent uplift phases are suggested by the presence of 11 Ma dioritic stocks exposed at elevations of ~4000 m [156].

Despite the great lithologic and topographic potential of the Colombian Orogen

(abundance of apatite rich igneous and sedimentary units, deep intramontane valleys, presence of major elevated erosional surfaces, etc.), high-resolution low temperature thermochronology studies implemented along vertical profiles have never been undertaken. Further, the highly

complementary systems fission-track and (U-Th)/He analysis have not been applied

simultaneously, in fact, (U-Th)/Ha dating has never been applied in the Colombian Andes. In

this investigation the Cenozoic morphotectonic history of the northern tip of the Cordillera

Central is constrained by simultaneously applying apatite fission-track and (U-Th)/He analysis along two separate vertical profiles covering ~2 km of paleocrustal depths in the AP, the largest high elevation erosional surface in the Northern Andes.

The complementary nature of low temperature thermochronology data generated in this research permit to establish a compelling temporal framework for the episodes of exhumation in this portion of the orogen allowing to identify two pulses that are synchronous with well documented phases of increased plate convergence in the Eocene, and Oligocene [18, 19]. These

major exhumation episodes are separated by intervals of tectonic-geomorphic quiescence in

excess of ~15 Ma that coincide with periods of slow convergence rates between the Nazca and

South American plates [18] implying that multiple phase of planation and relief generation have

been the norm in the Northern Andes and possibly in the rest of the orogen [167-169]. Pulses of

denudation here reported are in agreement with the proposed model of orgenetic phases advances

by Van der Hammen (1961) based on sedimentstratigraphic analysis and with more recent

models of development of continental basins in the Magdalena [14] and Cauca [168].

In addition to defining an accurate temporal framework, the magnitude of denudation is

also well resolved by both systems indicating order of magnitude difference of denudation

intensity between climax (0.2 mm/yr) and quiescence (0.04-0.008 mm/yr). Further, long-term

figures for erosion define the first quantitative framework for background erosion in the

Northern Andes which may serve as the basis to establish a valid comparison between modern

(anthropogenic) and long-tern (natural) erosion rates in the region; an important contribution to the understanding of man as a geomorphic agent.

This Investigation in the Context of the Geological Discipline

During the last two decades, for the scientific and societal reasons outlined above, studies of erosion have been gaining importance in geological and environmental sciences and have shifted from simply focusing on local, short-scale surface processes to broadening their scope to include a wide range of spatial (local to regional to global) and temporal (geologic to modern) scales [8, 9, 61, 62, 66, 69, 84, 86, 92, 109, 127, 133, 170-180]. Progress in assessing erosion at contrasting spatiotemporal scales is not only related to a change in philosophical scope [180-

182], but also to the emergence of several low-temperature thermochronometric tools [54, 171,

183-185] as well as other isotopic techniques, such as terrestrial cosmogenic nuclides [69].

Assessing erosion at multiple scales permits increasing our comprehension of mountain belt evolution and allows comparisons between natural (i.e., long-term, geologic) and anthropogenically enhanced (i.e., short-term, modern) erosion rates.

Combining data on erosion rates for such wide ranges of space and time allows me to assessing important scientific problems on two main fronts: 1) reconstructing long-term exhumation, landscape evolution, and morphotectonic response of the AP, and 2) establishing the framework for a quantitative comparison between long-term (i.e., pre-anthropogenic, natural, geologic) and modern (i.e., anthropogenic) erosion rates in the same geographic milieu. These two aspects of my investigation fall under two main scientific disciplines: geology (particularly geomorphology and tectonics) and geography.

Over the last decade, geomorphology has emerged as a fundamental line of research in

Earth sciences. This is well supported by the resurgence of geologic literature on geomorphology-related topics accompanied by revitalized efforts to tackle landscape evolution

in a more quantitative way [180, 186]. In the words of Summerfield (2005b): “These are certainly interesting times for geomorphology: from a discipline regarded by the great majority of Earth scientists as being, at best, of peripheral interest and having as its subject matter the

understanding of a trivial property of the Earth (the morphology of its land surface), geomorphology has now become a prominent focus of research for those concerned with how the

Earth works”. Despite the fact that geomorphology is only a thin slice of the body of science, it has both scholarly and practical value [181], and the field can contribute significantly to the debate over the likely future of the Earth’s surface [7]. Further, the last two decades have brought about the consolidation of a relatively new scientific field know as tectonic geomorphology. As suggested by its name, this discipline resulted from the blending of tectonics and geomorphology, two previously competing academic domains, into one of the most interdisciplinary fields found today in the physical sciences. According to Burbank and

Anderson (2001), very few fields easily merge traditionally unrelated and disparate topics such as seismology, climate change, geochronology, structure, geodesy, paleobotany, and geomorphology.

Finally, the general consensus in Earth sciences is that surface processes are of first-order importance to improve our understanding of the functioning of Earth’s dynamic systems [54].

Erosion can be a significant agent in active tectonic systems, particularly at larger spatial scales, therefore, interpretation of mountain belts past and present requires serious consideration of erosion as an important element of analysis [92]. As stated by Pinter and Brandon (1997) a more profound comprehension of how erosional, climatic, and tectonic forces interact to shape mountains permits clearer insights into the Earth’s history.

Methods

Erosional Exhumation and Low Temperature Thermochronology

Crustal rocks cool either because a thermal pulse wanes or a given parcel of rock approaches the surface via exhumation. Exhumation occurs by three processes: 1) erosion, 2) normal faulting, and 3) ductile thinning [90]. In areas without recent magmatism, near-surface cooling occurs by either tectonic or erosional denudation. AHe and AFT thermochronology can effectively constrain the timing and intensity of erosional episodes at the regional scale and on timescales of ~105-107 years [54, 86, 91, 171, 172, 174, 177, 184, 187, 188]. This spatiotemporal

domain is germane for understanding orogenic evolution (e.g., mountain ranges growth and

decay cycles, erosional surface development and uplift, paleorelief, etc.) and tectonics-

climate/tectonics-exhumation feedback responses.

This study encompasses one of the first routine applications of a newly advanced

methodology for apatite fission-track dating by laser ablation, inductively coupled plasma mass spectrometry (LA-ICP-MS) [189] and conventional (U-Th)/He analysis [183, 190]. The simultaneous application of conventional (U-Th)/He dating and LA-ICP-MS AFT analysis provides an opportunity to cross-check the consistency of both datasets with regards to ongoing arguments about increased alpha retention due to radiation damage and discrepancies between

AFT and He ages [191-194].

Apatite low temperature thermochronology

Apatite ([Ca5(PO4)3(F,Cl,OH)]) possesses various characteristics that make it suitable for

(U-Th)/He and fission track dating [171, 183, 187, 190, 192, 195-197] including: euhedral character; appropriate U concentrations; relatively minor U zonation compared to the more complex zoning patterns of zircon [198, 199]; adequate grain sizes (>60 μm); transparency that allows optical examination for inclusions; common occurrence in all major rock groups (igneous,

sedimentary and metamorphic); resistance to chemical weathering; ease of mineral separation, mounting, polishing and etching procedures; reproducible etching characteristics; and well- characterized and reproducible response to elevated temperatures. However, it is important to note that various recent articles have raised concerns about complications in apatite He and fission track dating related to increased He retention potentially resulting from accumulated radiation alpha recoil damage within the crystal lattice [192] and/or U zoning [198, 199]. But the

“problems” of He kinetics under alpha particle trapping seem to be better understood now [194] and have in fact lead to important new applications of AHe as apatite’s effective closure temperature to He evolves through time allowing its use in reconstructing complex thermal histories [200].

Apatite fission track analysis (AFTA) and (U-Th)/He dating (AHe) are well established low temperature thermochronometers and offer highly complementary information in the lowest temperature range available, i.e., ~40-110ºC [184, 190, 195, 196, 201]. AFTA and AHe apparent ages provide a measure of long-term erosion rates and landscape evolution [86, 89, 171, 172,

183, 184, 188, 190, 200, 202, 203] because both systems record the time at which a rock cools through the upper 3 km of the crust, the domain strongly influenced by surficial processes such as erosion [54, 90, 186]. Simultaneous application of both methods and comparison of results are being increasingly used to validate and cross-check data sets.

One of the most common approaches to LTTC of both AHe and AFT involves sampling near vertical profiles such as steep valley, cliffs, regional scarps, etc., or paleocrustal depths along faulted structures [171, 172, 174, 201, 204-206]. In addition to using surface samples collected from vertical profiles, it is possible to use samples from different depths in a borehole

[207-209]. These approaches allow exploiting the spatial link between the samples in the vertical

dimension of the crust, i.e., paleocrustal depths, so that lower elevations yield samples that were hotter and younger than the higher elevation samples. Application of the vertical profile

approach led to the recognition of zones of partial annealing of fission tracks [210] and partial

retention of He [211, 212] in the case of AFT and AHe respectively (Figures 1-2 and 1-3). These two zones are now commonly recognized as the PAZ (for AFT; ~60-110ºC) and PRZ (for

AHe;~40-70 ºC). When exhumed, such “thermal bands” represent paleocrustal depths and permit

inferring the timing of cooling and the magnitude of unroofing directly from the shape of the age-elevation relationship [171, 172, 174, 184, 187, 201]. In addition, the distribution of confined track lengths provides further constraints on the reconstruction of denudation histories

[86, 171, 187, 196, 197]. The timing of a cooling event is usually inferred from the age-elevation relationship as a sharp inflection point, i.e., “break in slope” (Figures 1-2 and 1-3). The elevation of the inflection point is used to constrain the amount of denudation. Further, the gradient of the various segments in the age-elevation profile is used to constrain the rate of denudation, and potentially the amount of rock uplift and surface uplift [91, 171, 190, 213].

Apatite (U-Th)/He dating. Over the last decade apatite (U-Th)/He thermochronometry

(AHe) has emerged as a fundamental tool for quantifying the cooling history of rocks as they pass through the upper 1-3 km of the crust [54, 183, 190]. The low closure temperature of this thermochronometer (40-70ºC) has attracted the interest of geomorphologists and tectonicists because it is applicable to interdisciplinary studies in landform evolution, erosion dynamics, structural geology, and geodynamics [54, 83, 89, 91, 214-217].

Current revival of interest in dating minerals via 4He accumulation has been stimulated by

an improved understanding of the behavior of He in various minerals, predominantly in apatite

[194, 212, 218-220]. This fact, in combination with the increasing development of robust

analytical techniques [54, 190, 199] has made it possible to address important geologic questions, particularly in the domain of morphotectonics [183, 190, 201, 214, 217].

Helium analysis is based on measurements of 4He (emitted and retained α particles) resulting from the radioactive decay of 235U, 238U and 232Th. Thermo kinetics of helium diffusion and α-particle stopping distances in apatite (~20μm) are well constrained so that measurements of U, Th and He by mass spectrometry allows calculation of the closure time of the system [190,

194]. Radiogenic helium found in various mineral phases is largely produced by radioactive

decay of U and Th (and their intermediate daughter products) to Pb. Minor contribution of

radiogenic He results from the decay of Sm to Nd. AHe ages are calculated from the following

equation [190, 218]:

4He = 8238U(eλ238t -1) + 7235U(eλ235t -1) + 6323Th(eλ232t -1) + 147Sm(eλ147t -1) (1)

where 4He, 238U, 232Th, and 147Sm represent the amounts of such isotopes as measured in the

sample by mass spectrometry, t is the accumulation time (retention) or radiometric age, and λs are the radioactive decay constants for each isotope [190]. Detailed reviews of AHe dating fundamentals, techniques and applications are available in the literature [183, 190, 191, 221].

Apatite fission track analysis. Unlike other geochronology methods, AFT ages are

calculated from the number of linear tracks of structural damage in the crystal found over a given

area, i.e., fission tracks, and the concentration of parent atoms of 238U in the same area. The use of fission-track analysis as a geological dating tool was initially proposed in the early 1960s.

However, over the past two decades, AFTA has experienced a major increase in its application to a variety of geological problems in settings that range from cratonic regions to active orogenic systems [31, 32, 35, 111, 160, 171, 177, 185, 187, 191, 195-197, 207, 210, 222]. This is partly due to advances in understanding the temperature dependence of fission track annealing and of

the information contained in fission track length distributions, accompanied by the development of robust analytical and modeling techniques [184, 187, 196, 203, 223-226]. AFTA provides detailed information on the low-temperature thermal histories of rocks, below ca. 120±ºC. Such temperature sensitivity applies to the study of multiple geologic phenomena including long-term chronology and magnitude of continental exhumation, structural evolution of orogenic belts, sedimentary provenance, thermal history modeling of sedimentary basins and hydrocarbon maturation, etc. [171, 177, 184, 185, 195].

AFT dating relies on the measurable amounts of the density and lengths of lattice defects

(tracks) induced by spontaneous nuclear fission of 238U from which apatite ages and cooling histories can be extracted. When an atom of 238U experiences spontaneous fission (a process that

occurs with a very low probability) two positively charged fission fragments are created and

repel each other traveling at high speed through the mineral lattice and knocking other atoms out

of their sites. Thus, the trajectories of the two fission fragments create a single, long and thin

region of damage (~15 x 1 um) known as fission track. Such trails of damage do not occur at any

preferred direction so that the orientations are random. Further, the fission process itself is

intrinsic to the atom and is not dependent on the external environment, e.g., heat and pressure.

An additional characteristic of fission tracks is particularly useful in constraining cooling-

exhumation histories. After track formation, radiation damage in the mineral lattice remains

unaffected over geologic time unless changed by environmental effects such as heat. Laboratory

and field observations have shown that heating of an apatite grain triggers a progressive

annealing of the radiation damage, which is proportional to time elapsed and amount of heat. The

work of a number of researchers [192, 226-233] has triggered a growing understanding of the qualitative and quantitative properties of fission track thermal annealing in apatite which is at the

basis of the use of this system in thermal history modeling and hence exhumation chronologies

[192, 193]. Track length distributions are now routinely used in several modeling strategies to unravel thermal histories in multiple geological settings [203, 224, 234]. In a rather summarized version, unimodal, symmetrical track length distributions with high mean values (e.g., ~14-15 um) and small standard deviations (e.g., 1-2 um) are characteristic of rapid cooling [171, 187,

235] On the contrary, slow monotonic cooling produce length distributions that, although unimodal, have lower means, higher standard deviations and are negatively skewed. Samples that have experienced more complex thermal histories, e.g., heating to some degree and subsequent abrupt cooling followed by lesser amounts of heating, tend to yield track length distributions equally complex, usually bimodal, with intermediate means.

Traditionally, the external detector method (EDM) has been applied to dating apatite grains by fission tracks. (For a description of the method see [171, 187, 236]. A new approach to AFT

dating by LA-ICP-MS is employed in this investigation [189]. In brief, samples are prepared in a

similar fashion as for the EDM so as to expose internal faces of the apatite grains in mounted specimens and mounts are etched to reveal spontaneous tracks which are then counted over specific areas to derive track densities. However, 238U concentrations are not obtained through

the long, tedious and radioactive-involving process of inducing 235U fission by neutron irradiation, re-counting tracks, etc. characteristic of the EDM. Instead, 238U and 43Ca are directly measured by LA-ICP-MS on the surface where tracks for each individual grain analyzed are counted [189]. It is assumed that 43Ca in apatite is present in fixed stoichiometric amount and obtaining the ratio 238U/43Ca by LA-ICP-MS analysis, here referred as P (uppercase Greek letter

238 43 rho), is analogous to obtaining the ratio ρi/ρd from the EDM. Thus, Ρi = ( U/ Ca) for apatite

grain i; 238U = background-corrected 238U cps (counts per second); 43Ca = background-corrected

43Ca cps. The basic age equation is:

1  ρ  =  + λ ζ s,i  ti ln1 d MS g  (2) λd  Ρi 

where subscript i refers to grain i; ti = fission-track age of grain i; λd = total decay constant of 238U, ζMS = ζ-calibration factor based on LA-ICP-MS of fission-track age standards; g = 0.5 for internal grain surfaces upon which spontaneous fission tracks are counted; ρsi = spontaneous

fission-track density for grain i which equals (Ns,i/Ωi) where Ns,i is the number of spontaneous

fission tracks counted over an area Ωi. derived by the sum of the spontaneous counts divided by

the sum of the induced counts. Pooled ages are used in this investigation and they are obtained as

the sum of the spontaneous counts divided by the sum of the induced counts (Galbraith and

Laslett, 1993). Other ways to calculate ages include mean ages, determined by the arithmetic

mean of the individual ratios of spontaneous to induced tracks counted in each grain, and central

ages obtained as the weighted mean of the log normal distribution of single grain ages (Galbraith

and Laslett, 1993; Galbraith, 2005).

Post Crystallization Cooling and Petrology of the Antioqueño Plateau

Spot U-Pb and Hf isotopic analysis in magmatic zircons

Various physical and chemical characteristics of the mineral zircon ([Zr(SiO4)]), e.g., its refractory nature, resistance to physical and chemical weathering, relatively high U-Th concentrations, and high abundance in a wide range of lithologies, make it highly appropriate for geochronology/geochemistry and one of the most versatile tools for examining a wide range of

Earth processes, e.g., crystallization age and magma sources of batholithic masses. Earliest proof of the potential use of LA-ICP-MS to perform in situ determination of 207Pb/206Pb on zircon with

sufficient precision to be a practical tool for dating Proterozoic zircons took place in the early

1990s [237]. During the second half of the 1990s the technique started to be widely used to

dating zircons by the Pb/U decay schemes [238-241]. This progress in the method was related to

the development of ultraviolet laser ablation and high-sensitivity ICP-MS instrumentation.

Today, LA-ICP-MS U–Pb dating is one of the cheapest, fastest, most reliable and most widely

available techniques for in situ (spot analysis) U-Pb Dating [242].

Zircon grains constitute a geochemical data repository of unparalleled quality [243, 244].

Zircon preserves a high-quality record of near-initial Hf-isotope ratios, which can be used both in provenance studies and as a petrogenetic indicator [245]. In the recent years routine use of LA-

ICP-MS for in situ U-Pb dating in zircons has incorporated in situ determinations of within-grain isotopic variations of Hf [246, 247]. Zircon contains relatively high concentrations of Hf

(frequently at the percent to several percent level) which substitutes for Zr. Due to its high content, Hf isotopic composition is usually not significantly affected by in situ decay of the much less abundant 176Lu. Direct measurement of 176Hf/177Hf ratios by LA- ICPMS in magmatic

zircons is now being applied in investigations of the evolution of plutonic belts and provenance

studies. In addition, spot LA-ICP-MS determinations of zircon age and Hf compositions allow

studying the crustal evolutionary processes related to silicic magmatism such as magma mixing,

crustal contamination and/or assimilation, etc. [248-252].

Zircon cathodoluminescence imaging (CL)

Since the late 19th Century, it was recognized that some mineral species, including zircon,

exhibit luminescence when bombarded with electrons [253]. Cathodoluminescence imaging (CL)

is a powerful tool to resolve micro-scale chemical heterogeneities even in compositionally

complex zircons providing some of the best resolution to reveal internal structures [254, 255].

Zircon crystals ubiquitously display fine-scale compositional zoning [243, 256, 257] that

demonstrates zircon’s capacity to retain geochemically important trace and rare earth elements

(REE) over a range of geological conditions [257]. This attribute has resulted in the pervasive application of zircon geochemistry to a variety of Earth Science disciplines (e.g., Schärer and

Allègre, 1982; Froude et al., 1983 ; Fedo [258]. In this study, some crystals on which LA-ICP-

MS was undertaken were studied by CL to reveal the spatial distribution of trace elements that were incorporated into the growing crystal at the melt/crystal interface at various stages of crystallization.

Whole-rock major-trace element, and radiogenic isotopic analysis

Whole-rock trace (e.g., REE) and major element analysis combined with radiogenic isotope analysis (Nd, Pb) has a long tradition in petrogenetic studies and provides valuable data for deciphering magmatic process (fractionation, contamination, crustal mixing, depth of magmatic chamber, etc.), tectonic setting and genesis of intrusives and associated volcanics in

diverse geologic environments [45, 251, 259, 260]. In this investigation, whole-rock major and

trace element analyses in conjunction with Nd, Pb isotopic analysis were conducted on the six

samples from which zircon U-Pb ages and Hf isotopic data had already been derived (LA-ICP-

MS) with the aim of further constraining the petrogenetic characteristics of the Antioqueño and

Ovejas batholiths. Several studies have shown that lead isotopic compositions of igneous rocks

reflect the composition of the underlying basement so that lead isotopes can be used to map

crustal domains [261, 262], constrain tectonic settings [263-265]}, and study magma sources and

magma mixing process between mantle and crustal sources reflecting distinct geological

provinces [266]. Similar information can be derived from Nd isotopic data [267, 268].

Although the sample strategy, sample collection, preparation and analysis were mostly

undertaken by the author at UF, the variety of analytical techniques deployed for this

investigation was possible through the close collaboration with researchers from various

universities and non-academic institutions. George Kamenov and Misty Stroud from UF assisted

me with geochemical work presented in Chapter 2 (U-Pb and Hf spot analysis, and whole-rock

Nd, Pb, major and trace element analysis). At the University of Kansas, Danilel Stockli helped by providing permanent guidance in the intricacies of (U-Th)/He dating. The assistance of Paul

O’Sullivan and Ray Donelick in applying a novel methodology to apatite-fission track analysis was of paramount importance. They provided invaluable help both at UF during my training period in fission track dating and at their own facilities in Apatite to Zircon, Inc conducting analysis for a large number of my samples. All of these individuals actively participated in the discussion and analysis of the data and are co-authors in the three articles derived from this dissertation which are in the process of being published.

Thesis Outline

The main objectives of this study are organized chronologically from older to more recent geologic events and will appear in this manuscript in the following order: 1) assessment of the temporal framework for the emplacement of the Antioqueño and Ovejas batholiths, potential consanguinity of both intrusives, post-crystallization cooling history, and magmatic sources/generation/mixing and geodynamic setting associated with this important petrogenetic province; 2) constrains on the exhumation history, long-term erosion trends, patterns of landscape evolution, and morphotectonic development of the Antioqueño Plateau; and 4) generation of quantitative data on long-term erosion rates for the region to establish background erosional process which permit preliminary comparisons against modern erosion rates.

This dissertation is divided into five chapters, three of which correspond to separate research articles which comprise the core of this study. The first chapter (Chapter 1, this introduction), deals with the various subjects covered throughout the investigation particularly the matters that motivated the author to pursue this line of research, the scientific and societal

importance of erosion at multiple spatiotemporal scales, and the objectives and significance of the investigation. Very concisely, the most relevant aspects of the study site (geology, geomorphology and tectonics) and the methods and various analytical techniques utilized in addressing morphotectonic evolution and process at various scales of space and time are presented.

The next three chapters correspond to the core of the investigation and are derived from the application of three different techniques that permit tracking important geologic events in space and time. Although the emphasis of this investigation is placed in low temperature thermochronology, the findings in these three chapters are presented in chronological order from oldest to youngest. Thus, Chapter 3 constitutes the first publishable thesis-related article (to be submitted, Journal of South American Geology) and includes formation age, post crystallization cooling and petrogenetic constrains on the Antioqueño and Ovejas batholiths. These intrusive bodies represent an important volume of granitic rock, constitute the largest calc-alkaline suite in the Northern Andes, and may have exerted important controls on the geomorphic evolution of the AP, including the initial phases of uplift driven buy crustal thickening. However, formation age, mechanisms of magma genesis and magma sources, potential process of magma mixing during emplacement, and post crystallization cooling trends are poorly constrained. This information is relevant in reconstructing the evolution of the petrogenetic suite under scrutiny and also provides important clues about the initial phases of cooling and uplift related to crustal thickening and sustained rates of rapid convergence in the region during the Late Cretaceous.

The work in Chapter 2 is based on U-Pb dating and Hf isotopic spot analysis in zircon grains by

La-ICP-MS, complemented by whole rock isotopic, trace, and major element analysis.

In Chapters 3 and 4 two separate applications of low temperature thermo-chronology

(LTTC) along vertical profiles are presented. In both Chapters, apatite grains are used to reconstruct the long-term erosional history of the Antioqueño Plateau by producing, analyzing and modeling thermochronology data in the systems (U-Th)/He (Restrepo-Moreno et al., 2009;

Chapter 3) and fission track (submitted, The Journal of Geology, Chapter 4). This part of the dissertation provides important data on the long-term (Cenozoic) erosional exhumation and morphotectonic evolution of the Antioqueño Plateau, which appears to be characterized by craton-like denudation rates. Findings reported here permit defining two discrete pulses of exhumation, one in the Eocene and another in the Late Oligocene, separated by a long interval of

tectonic quiescence. These exhumation events are synchronous with similar events throughout the Andes and the Caribbean implying continental-scale controls on erosional exhumation associated with documented changes in convergence between Farallon (Nazca) and South

America, an outcome that has implications for regional geodynamic interpretations of Andean evolution. The dataset also allows a better understanding of the paleogeographic and morphotectonic evolution of the area while permitting a reconstruction of spatially averaged,

long-term (pre-anthropogenic) erosion rates that serve as a quantitative benchmark against which

modern erosion rates can be compared.

Finally, Chapter 5 encompasses a set of general conclusions in regards to the three main

subjects discussed throughout the dissertation, i.e., formation age and magma sources for the

Antioqueño and Ovejas batholiths; long-term morphotectonic development, landscape evolution

and exhumation history of the AP; and an initial comparison of modern (anthropogenically

accelerated) and geologic (natural) erosion rates. As the closing section of Chapter 5, a set of

potential lines of inquiry and further research topics that have emerged over the course of this investigation are proposed.

Figure 1-1. Physiographic Units of Colombia. Andean Region (dark green): W=Western Cordillera, C=Cordillera Central , and E=Eastern Cordillera. Isolated ranges (light green): 1=Sierra Nevada de Santa Marta, 2=Serranía de Baudó, 3=Serranía de La Macarena, and 4=Serranía de Chiribiquete. Plains (brown): CP=Caribbean Plains, EP = Eastern Plains (Orinoco Savanna and Amazon Forest), PP=Pacific Plain (Atrato, San Juan, and Patia valleys). Interandean Valleys (blue): M=Magdalena Valley; C=Cauca Valley. (Physiographic units derived from a GTOPO-30 digital elevation model).

Figure 1-2. The Antioqueño Plateau a geomorphic feature carved on the Antioqueño Batholith. Red-black circles show the location of the sampled sites for thermochronology (apatite (U-Th)/He and fission-track dating), geochronology (U-Pb spot analysis in zircons), and geochemical-isotopic analysis (major and trace elements, Hf, Nd, and Pb). The Medellín-Porce valley has deeply incised (1200 m) into the plateau (narrow green trough in the center of image) dividing the AP into a northern and southern portions. The area covered by the DEM falls approximately between geographic coordinates 7.5-4.5˚ N and 74- 76˚ W.

Figure 1-3. The concept of the partial annealing zone (PAZ) and partial retention zone (PRZ) in apatite fission track and (U-Th)/He thermochronology. The * symbols and the light grey shading delimit the crustal region over which tracks are partially annealed and/or He is partially retained. Dark gray shading represents the zone of complete FT- annealing and/or He-diffusion. White band indicates the zone of track stability and He retention over geologic time. At a time to in the pre-exhumation condition rocks above the PAZ and PRZ apatite grains register the age of an exhumation event. Subsequently, low-relief topography characterizes the region as the Antioqueño Plateau develops. Fission tracks are being annealed within the PAZ and He partially retained in the PRZ over geologic time. Apatite grains in rocks below the bottom of the PAZ and PRZ are recording an apparent age of “0”. In the Modern Topography situation the PAZ/PRZ formerly developed appears as an exhumed or fossil PAZ/PRZ that can be sampled along the valley walls of the Medellín-Porce fluvial system. In the bottom of the figure, the Age-Elevation relationships for AFT (bottom left) and AHe (bottom right) clearly display the patterns of apparent ages young toward the bottom of the profile and old toward the top expected from a vertical profile with negligible perturbation from thermal events or faulting (Adapted from Burbank and Anderson, 2001).

Figure 1-4. Vertical profile approach to low-temperature thermochronology in the Antioqueño Plateau. Sketch illustrating the regional scarp carved by the Medellín-Porce fluvial system incising into the Antioqueño Plateau that was used to collect samples from the widest range possible of paleocrustal depths.

CHAPTER 2 FORMATION AGE AND MAGMA SOURCES OF THE ANTIOQUEÑO AND OVEJAS BATHOLITHS, CENTRAL CORDILLERA, COLOMBIA

Introduction

Granitic plutonism is considered the principal agent of crustal differentiation and is evidenced by the vast granite batholiths and erupted equivalents present in continental masses

[249]. Granitoid suites are an essential constituent of continental and orogenic belt evolution in subduction settings. Throughout the Andes, the Mesozoic and Cenozoic are recognized as periods of significant magmatic activity [44, 47, 269, 270]. Calc-alkaline batholiths constitute a substantial component of the lithosphere of the Andean margin [45, 271]. A long, orogenic- parallel, Meso-Cenozoic batholithic belt is thus a conspicuous feature of the western South

American margin from Colombia to southern Chile and Argentina. The evolution of Andean magmatism is a result of a wide variety of tectonic and geochemical processes [42, 269, 270,

272]. Important changes in subduction geometry and kinematics have taken place over time and

have induced significant variations in processes such as volcanism, mountain building,

metamorphism, tectonics, and seismicity [18, 19, 44, 46, 47, 273]. The Middle to Late

Cretaceous interval is recognized as a period of important plutonism in the Andes, which is

possibly related to enhanced subduction triggered by a regional circum-Pacific superplume,

which also produced episodes of deformation and uplift in the lower Cretaceous rocks

throughout the Pacific basin [274]. In addition, a major peak of Andean magmatism that

occurred between 120 and 70 Ma has been identified [272]. Variations in magmatic activity

along strike are also related to changes in the characteristics of the subduction process, which are

partly related to variations in the rate of convergence and age of the subducting slab through time

particularly during the Cenozoic [42, 44, 47]. Soler (1990) argues that despite the continuous

subduction of Nazca (Farallon) beneath South America During the Late Cretaceous and the

Cenozoic, calc-alkaline magmatism has been episodic in nature whereby periods of voluminous

magmatism can be associated with intervals of high convergence rates (>10 cm/yr). Collision of

ribbon continents constitutes another possibility [275, 276]. Compositional diversity in Andean

plutonism appears to be related to 1) differences in the composition of the source, 2) variability in melting conditions, 3) complex chemical and physical interactions between mafic and felsic magmas and 4) crustal contamination [44, 45, 249, 260, 270, 277-280].

Mesozoic plutonic and volcanic events in the Colombian Andes are well expressed as large batholiths, stocks, lava flows, and pyroclastic formations [21, 281]. Enhanced magmatism in the

Northern Cordillera Central during the Late Mesozoic led to the development of large intrusive bodies such as the Antioqueño and Ovejas batholiths, hereafter AB and OB, respectively (Figure

2-1). The AB (7500 km2) is the largest intrusion and one of the broadest, most continuous lithologic units in the Colombian Andes. Smaller plutons similar in composition and spatially related to the AB, such as the OB, and the Unión Cupola, are considered genetically related to the AB. A precise chronological and geochemical framework that would permit a better understanding of the timing, the sources, the geodynamic setting, and potential mechanisms for the development of such large plutons has not been completed.

This investigation was carried out with the objectives of 1) precisely defining the crystallization age of the AB and OB plutons, 2) constraining magma sources and magma mixing process in the AB and OB, and 3) assessing the high-temperature portion of the post- crystallization cooling path for these intrusives. Accurate constraints in age formation and magma sources for the AB and OB are important to support our understanding of Meso-

Cenozoic crustal evolution processes and the spatiotemporal distribution of magmatic activity in the Northern Andes, a region that has traditionally lacked studies of that nature.

Because magma addition has been invoked as a potential mechanism for crustal thickening

and subsequent uplift-denudation, the information generated in this investigation is also

important for elucidating the timing of potential crustal thickening through magmatic activity

that may help explain the initial stages of uplift of the northern Cordillera Central during the

Laramic Orogenetic phase at ca. 64 Ma [16]. Magma addition has been suggested as one of the

plausible mechanisms of crustal thickening and subsequent uplift for the Andes of Bolivia and

Peru [99] and may be an important aspect of the morphotectonic evolution of the northern

Cordillera Central given the magnitude of Late Cretaceous intrusions.

To constrain magmatic evolution of both intrusive bodies their plutonic archive was unraveled by simultaneously conducting in situ hafnium (Hf) isotope analysis on precisely dated magmatic zircons (spot U-Pb) and performing whole-rock major, trace element a and radiogenic isotopic analyses (e.g., REE, Pb, Nd) . Results of this investigation support 1) a massive, Late

Cretaceous, subduction-related magmatic event, 2) crystallization and emplacement of the AB and OB over a relatively short interval of ca. 5 million years, 3) assemblage of both batholiths through rapid incremental steps by multiple injections, 4) significant magma mixing involving mantle and crustal sources as well as variations in the nature and depth of magmatic sources, and

5) rapid post-crystallization cooling (~800 to ~300°C) from 77 to 60 Ma,. Although the Late

Mesozoic seems to have been a period of enhanced subduction throughout the circum-Pacific region [282],and this has been taken to imply increased magmatic activity in these subduction zones, in this study other potential mechanisms are considered (e.g., short lived extension through slab roll-back, flare ups, etc.) and evaluated in light of the geochemical and isotopic data here presented.

Study Site

Large granitic Mesozoic batholiths of a nearly circular outcrop are intruded in the cordilleran axis of the northern portion of the Cordillera Central in the Colombian Andes. The area focus of this investigation is situated between longitude-latitude coordinates 5.7°-7.7°N and

74.6°-76°W (average elevation ~ 2500 m) and encompasses the Antioqueño and Ovejas batholiths, two massive granodiorite intrusives. In Colombia, the Andean chain is arranged in the form of three distinct and sub-parallel ranges separated by major intramontane depressions. From west to east these ranges are known as the Cordillera Occidental (Western Cordillera), the

Cordillera Central (Central Cordillera) and Cordillera Oriental (Eastern Cordillera). The Cauca and Patía rivers occupy the depression between the Occidental and Central cordilleras, while the

Magdalena River valley runs between the Central and Eastern ranges (see Chapter 1, Figure 1-1).

These three cordillera possess substantial geological differences that reflect the complex geologic evolution of this segment of the Andean range. Despite the geologic and geographic importance of the Colombian Andes, most studies have concentrated on the Central Andes of

Bolivia and Peru.

Geologic Setting

The Cordillera Occidental and Pacific coastal plains of Colombia have oceanic crustal affinity and are part of an island arc, i.e., allochthonous terrane, accreted to the continental margin in the

Cretaceous and early Cenozoic [21, 283]. The Cordillera Occidental is composed of oceanic sequences of mafic volcanic rocks and marine sedimentary sequences (turbiditic deposits and ophiolites) of upper Cretaceous and Cenozoic age, intruded and covered by Cenozoic plutons and volcanic sequences [44, 284, 285]. This oceanic domain prevails through western Colombia and is separated from an eastern continental domain by the Cauca-Romeral fault system, which is a paleosuture zone that extends all the way to Ecuador [25].

In contrast, the Central and Oriental cordillera possess crust of continental affinity and an

old crystalline basement. The Cordillera Oriental consists of continental Precambrian and

Paleozoic metamorphic and igneous rocks overlain by Paleozoic to Mesozoic sedimentary sequences [21]. The Cordillera Central encompasses a pre-Mesozoic polymetamorphic core complex [286] intruded by Meso-Cenozoic batholiths [21, 27]. Cenozoic to recent volcanics associated with subduction of the Nazca plate are restricted to the Cordillera Occidental and a segment of the Cordillera Central between 2°S and 5°30’N.

The study area includes a crystalline basement of low- to medium-grade metamorphic rocks grouped as the Cajamarca Complex [287] and high-grade rocks of the El Retiro Group and

Las Palmas gneiss [21, 288]. Collectively, these rocks comprise the so called pre-Mesozoic to

Mesozoic polymetamorphic complex of the Cordillera Central [286]. In the early Cretaceous, shallow marine sedimentary sequences were deposited over the metamorphic basement [289-

291]. Intense magmatic activity during the Mesozoic led to the emplacement of major cordilleran plutons that intruded the metamorphic complex as well as early Cretaceous sedimentary cover producing clear contact aureoles (Figure 2-1). Plutons emplaced during this era include Triassic ademelitic stocks (e.g., Amagá, Honda, El Buey),

Triassic-Jurassic intermediate batholiths (e.g., Segovia and Sonsόn) and much larger Cretaceous batholiths such as the AB and OB and other associated stocks [21]. Late Eocene continental arc volcanic rocks cover portions of this province [284, 286]. Finally, Late Neogene volcaniclastic sequences associated with modern volcanic activity further south in the Cordillera Central mantle the region [12, 292] (Figure 2-1). Mesozoic volcanic edifices associated with the emplacement of the AB and OB have been removed by a series of discrete exhumation pulses (Paleocene, Eocene and Late Oligocene) that have been revealed by recent low-temperature thermochronology

studies [293, 294] and sedimentstratigraphic investigations [16, 295]. At present, only small patches of the metamorphic basement, marine sequences, and Eocene volcanic rocks remain on top of the AB as roof pendant structures. The geologic province that encompasses the study site is considered part of the allochthonous Tahamí terrane, affected by several tectono-metamorphic episodes related to the Hercynian orogeny in Permo-Triassic times, a Devonian-Carboniferous event, and the beginning of the Andean cycle in the Cretaceous [279, 283, 286].

The Antioqueño and Ovejas Batholiths

The AB is the only major pluton of any age in western Colombia that is not aligned with the predominant NS structural grain of the orogen. Along with other major granitic bodies of lesser extent (e.g., Yarumal Stock, Amalfi Stock, Pueblito diorite, Sabanalarga batholith, and various gabbroic units), the AB and OB are part of the final and most important episode of plutonism in the northern Cordillera Central. Relatively early, the great regional extent and compositional homogeneity of the AB was noted [296-298]. More in-depth petrographic, geochemical, and cartographic studies were subsequently conducted and the denomination

“Batolito Antioqueño” (Antioqueño batholith) was assigned to this lithologic unit [299]. Further petrographic and geochemical studies were undertaken in the early 1980s [300, 301] and were complemented by the work of González (1993). Such studies have confirmed the compositional

(chemical and mineralogical) and textural homogeneity of the AB. About 95% of the samples analyzed to date fall in the range granodiorite to quartz-diorite. Recent geochemical data on major elements [166] has confirmed previous ideas in relation to the composition and homogeneity of the AB and OB while providing valuable information about the sub-alkalic character of the entire mass. Contemporaneous and much smaller mafic units (< 2% of the total outcrop) are spatially and temporally associated with the AB and OB [21], which is a common feature of most cordilleran batholiths [271]. 55

Previous Thermochronology and Isotopic Analysis

Radiometric studies of the AB and OB have not been carried out in a systematic way and several isotopic systems have been applied to this vast geologic unit. Such studies have yi el ded a rather wide age range from ca. 22 to ca. 98 Ma that have made it difficult to precisely constrain the ti me of crystalli zati on, whi ch, in turn, precludes i nterpreti ng the ori gi n of such large i gneous masses, t he sources of magma involved, the mechanisms of magma generation and emplacement, etc. i n the context of Cretaceous geodynami cs. Biotite K/Ar ages (~300ºC) range from ~68 Ma to ~90 Ma [301-303]. Three biotite Rb/Sr ages (~350 ºC) from ~56 to 66 Ma are also reported [304]. More recently, two whole-rock isochrone Rb/Sr yielded ages of ~82 and 98

Ma [305]. Zircon fission-track dates from ~49 to 67 Ma and apatite fission-track ages varying between ~49.1 and 28 Ma [166] in conjunction with apatite (U-Th)/He and FT ages of ~21-49

Ma [294] and ~30-59 Ma [293] (Chapters 3 and 4) have been used to study the low temperature history and Cenozoic exhumation of these units.

This ample variation in apparent ages is explained by the wide range of closure temperatures of the thermochronometric systems employed (ca. 50-400 °C) and can be interpreted as cooling rather than formation ages. The Rb-Sr isochrone age of ~98 Ma was proposed as the best estimate of crystallization age for the Antioqueño batholith [166, 305].

Whole rock Rb-Sr ages are highly suspect and often erroneous due to mobility of the Rb-Sr system and incorrect assumption of a single Sr initial common source. Thus, the Rb-Sr whole- rock ages obtained by rocks collected from a single outcrop to specimen-size are not always coincident with those of emplacement of granitic magma [306]. Until the investigation here presented, no U-Pb zircon ages or other robust high-temperature geochronometer has been used to determine the formation age of the AB or OB.

Constraints on magma sources and magma mixing for the AB are limited to the study of

Ordonez and Pimentel (2001) who proposed that the generation of such unusually large vol umes of

magma should be related to enhanced slab dehydration resulting from accelerated subduction rates

from ca. 110-96 Ma. Mi xi ng of mantl e-depleted fl uids and crustal components are suggested by

εNd(T) values between -2.40 and +2.66 [305].

Tectonic and Geodynamic Setting

The Late Mesozoic-early Cenozoic tectonic and geodynamic setting of northwestern South

America has been controlled by macro-, meso- and micro-scale tectonic process strongly associated with the subduction of the oceanic Farallon plate beneath the continental South

American plate and with the evolution of the Caribbean plate [44, 47, 307-311]. The Andean region of Colombia has been traditionally considered as a mosaic of allochthonous terrains accreted to the autochthonous continental crust of the northwestern South American margin since the Paleozoic and until the Late Neogene [278, 283]. Eastward displacement of the Farallon Plate relative to the Americas during the Mesozoic was characterized by subduction of oceanic crust beneath South America and associated volcanism/plutonism, deformation, and obduction/accretion of oceanic terrains in western Colombia during the Cretaceous [278]. This predominantly oblique mode of subduction induced large-scale transpression features throughout the northern Andes such as the well-defined, right-lateral shear zones along the northern border of the North Andean Block [44, 278, 312, 313]. Patterns of subduction (e.g., convergence rates and direction, dipping of the subducting slab, etc.) have changed substantially both in space and time [18, 273, 282, 314]. Variations in the subduction parameters have exerted important controls on the timing, magnitude and type of associated volcanism/plutonism since the

Mesozoic [44, 47, 278]. The Farallon Plate, which was subducted along the south-western portion of the northern Andes and Central America during the Mesozoic and part of the

Cenozoic, split as a result of thermal effects by the underlying Galapagos hot-spot at ~25 Ma

B.P. This event led to the modern configuration of regional tectonics as the Cocos and Nazca plates were created [20, 315, 316] and the vector fields were redistributed to establish a

predominantly E-W interaction between Nazca and South America [25, 47]. Such reorientation is

responsible for the current style of deformation and seismicity in the region [23, 25, 317].

At present, the Northern Andean region is located in the junction of four major tectonic

plates, including Nazca, Pacific, Caribbean and South America, and two major, fault-bounded

lithospheric blocks know as the North Andean and the Panama-Chocó blocks [23-25].

Intracontinental deformation is associated with inherited faults of large scale and both reverse

and strike slip character [25]. Orientation of topographic features and cordilleran grain/structure

in the Colombian Andes is predominantly NS to NE-SW compatible with W-E convergence

directions between major plates, i.e., Nazca and South America. Crustal deformation is primarily

associated with the collision of allochthonous terrains and interactions of several tectonic plates

and micro plates.

The Cordillera Central is bordered by reverse fault systems located along the foot hills

(east and west) which are thought to be rooted beneath the range [23, 25, 26]. The Cauca-

Romeral faults system to the west is associated with the Cauca river depression and has been

activated since the Oligocene combining strike-slip and reverse movement [25, 26]. To the east,

the Palestina fault system marks the boundary of the Cordillera Central. This range is

asymmetric with a much steeper western flank along the Cauca Interandean valley. The western

margin has been uplifted by transpressive movement along faults dipping eastward that belong to

the Cauca-Romeral fault system. The less abrupt eastern flank along the Magdalena river valley

is characterized by a west dipping reverse faults located along the foothill. Neogene

transpression is right lateral in southern Colombia and left lateral north of 4°N [23, 25].

Methods

Multiple techniques are utilized in this investigation to understand the timing of crystallization, the magma sources, the magma mixing processes, and the geodynamic environment that led to the development of the AB and OB. Those techniques include in situ U-

Pb dating, Hf isotopic analysis, and cathodoluminescence imaging (CL) in magmatic zircon grains, whole-rock major and trace element analysis, and whole-rock radiogenic Nd and Pb isotopic analysis.

Zircon grains form a highly robust phase in most geological environments and represent a geochemical data repository of unparalleled quality. Integrated application of U–Pb dating and

Hf-isotope analysis to magmatic zircon grain populations offers a rapid means for constraining the age and magma sources/processes associated with the intrusion of batholithic masses [249-

252, 318]. Zircon is readily amenable to U-Pb radiometric dating methods [243, 244].

Furthermore, zircon preserves a high-quality record of near-initial Hf-isotope ratios, which can be used both in provenance studies and as a petrogenetic indicator [245]. Hf analysis is being increasingly applied to evaluate terrestrial crust–mantle evolution [248, 249, 252, 319]. Zircon imaging by CL is a powerful complementary tool in radiometric dating and isotopic analysis because it resolves micro-scale chemical heterogeneities even in compositionally complex zircons providing some of the best resolution to reveal internal structures [245, 254]. CL is now commonly used in most studies that incorporate single grain spot measurements in zircon [243,

319, 320].

Finally, Whole-rock major and trace element analysis combined with whole-rock radiogenic isotope analysis (Nd and Pb) has a long tradition in petrogenetic studies and provides 59

valuable data for deciphering magmatic process (e.g., fractionation, contamination, crustal mixing, depth of magmatic chamber, etc.), tectonic setting, and genesis of intrusives and associated volcanics in diverse geologic environments [45, 251, 259, 260].

Sample Collection and Preparation

Samples were collected along three vertical profiles (two in the AB and one in the OB) to derive geochronology, geochemical, and isotopic information over the widest range possible of crustal depths into both batholiths. Profiles were also selected to incorporate broadly separated areas in latitude and longitude from the center to the periphery of the batholiths. A total of six samples (2-3 kg) were obtained from the highest and lowest points of each profile. Although the majority of the samples were fresh (SR-001, SR-003, SR-9, and SR-41), relatively altered samples (SR-24 and SR-42) were also included as they are useful for spot U-Pb dating and Hf analysis in zircon; provided the grains are non-metamictic.

Hand specimens correspond to mottled (black-in-white), phaneritic, coarse- to medium- grain, hypidiomorphic, equigranular, granodiorite with varying amounts of quartz and feldspar as the felsic phases and mostly biotite and some hornblende as mafic minerals (Figure 2-2).

Petrographic analysis indicate the approximate mineralogical composition consists of plagioclase

40-51% (Albite-Andesine), quartz 18-35%, hornblende 0-12%, biotite 6-12 %, K-feldspar 0-7%, clinopyroxene 0-0.5%. This mineralogy is consistent with previous petrographic results [21,

301]. Plagioclase crystals are subhedral to euhedral and maximum size of 4 mm. They are usually twinned and contain poikilitic inclusions of accessories and opaques. Quartz occurs as inequigranular anhedral grains smaller than 2.5 mm. Hornblende is green, subhedral and exhibits poikilitic texture, enclosing accessories and opaques, it is partially altered to chlorite and has a maximum size of 3 mm. Biotites are subhedral to anhedral, grain size up to , , is dark brown, and also encloses opaque minerals. Alkali feldspar is minor, smaller than 2 mm, and is altered to 60

sericite and clay minerals. Alteration of hornblende and biotite to chlorite is relatively common.

Sericitic alteration of plagioclase is also noticeable. Accessory minerals are mainly apatites,

zircons and sphene. Small amounts of opaque minerals are also present.

Zircon separation, grain description and cathodoluminescence

For zircon recovery, the samples were washed and weathered rims removed from the fresh specimens to eliminate potential surface contaminants. Samples were crushed, pulverized in a disk mill, sieved at 125-400 μm, rinsed, and air-dried. Zircon concentrates were obtained from the sand fraction (~2 kg) following a conventional protocol involving: a) Gemini water table for initial separation of heavy fractions, b) paramagnetic removal with hand magnet, c) density liquid separation using tetrabromoethane (TBE) and methylene iodide (MeI), d) separation based on magnetic susceptibility in a Frantz isodynamic®, e) further cleaning of the non-magnetic fraction in MeI, and f) final depuration by magnetic susceptibility, sieving and hand picking.

Zircon grains for the non magnetic fraction (0°, 1.0 A) were optically screened using a

petrographic microscope under transmitted (normal and polarized) and reflected light to monitor

for inclusions. Abundant, completely euhedral, clear, unfractured zircons were obtained. Most

grains are transparent with a yellowish tone. Magnetic separation provided non-magnetic zircon

populations that are virtually inclusion free, as confirmed by binocular microscope scanning.

External crystal morphology varies relatively little within sample zircon populations and even

between samples. Euhedral, complete grains dominate most concentrates with a prevalence of

the prismatic forms (101) terminated in pyramids. Most crystals are medium sized (60–150 μm),

equidimensional (i.e., low elongation ratios). Some acicular crystals present in samples SR-41

and SR-42.

Representative sets of non-magnetic fractions for the six samples plus a fraction of zircon standard FC-1 were placed along seven separate bands utilizing adhesive paper. Epoxy resin was 61

used to enclose the grains into one single mount which was ground and polished to expose

internal surfaces of zircon grains. The zircon mount was washed in 4X distilled water and further cleaned in an ultrasonic bath with diluted HNO3 for 5 minutes to remove impurities at the surface

prior to laser ablation and mass spectrometry analysis.

Conventional Cathodoluminescence (CL) procedures were followed to obtain images of

zircon populations, reveal internal structures associated with chemical zonation, and select

analyzed grains to monitor the type of zoning, if any, and identify potential inherited cores,

alterations, recrystallizations, and/or metamorphic absorptions [243, 255]. Growth zoning is

easily captured by CL [243, 321]. CL imaging was conducted at, USGS Ion Probe Lab, Stanford,

CA. The zircon mount was gold coated and placed into the CL system. Grey-scale CL imaging was performed using a Hitachi S-2250N scanning electron microscope. The photomultiplier detector operates in the broad green region of the electromagnetic spectrum (~490–525 nm) producing grey-scale images over that spectral range. CL spectra were collected at 15 kV with an

Acton Research ARC 150 Spectrograph (146 mm focal length) equipped with an 1800 groove/mm holographic grating connected to an Olympus BH-2 microscope. CL emission was detected using a Princeton Instruments TEA/CCD-576EMUV 576 X 384 charge-coupled device

(CCD) detector 2.

CL shows that all of the grains analyzed posess well developed growth zoning (Figure 2-3) reflecting compositional variations. Zoning in the majority of crystals analyzed (>98%) consists of repeated, fine- to medium-scale concentric bands (~2-10 μm) of high and low luminosity

around a relatively dark euhedral core. No alteration patterns (e.g., feathery textures, botroids,

etc.) were found in any of the mounted grains imaged by CL indicating that all of the crystals are

suitable for isotopic analysis (i.e., no Pb leaching). None of the zircons exhibited inherited cores,

a feature confirmed by the homogenous U-Pb ages obtained from all of the grains analyzed (see

U-Pb results below). Three grains in samples SR-43 and two in SR-003 display sector zoning. A combination of oscillatory and sector zoning has been reported in previous studies of intermediate to felsic suites [257, 322], and are thought to be modifications of oscillatory zoning by recrystalization (i.e., patchwork replacement of zoned by unzoned zircon). Patches of unzoned zircon appear to develop after completion of primary crystallization, and are accompanied by loss of U, Th and Pb and the removal of oscillatory zones [323]. Such grains were avoided for U-Pb dating.

Whole rock sample preparation

Sample rock chips (~1 cm in diameter) were washed to remove surface impurities and dried with an air compressor. Chips were further crushed using a small jaw-crusher with steel plates, a riffle to split the samples, and pulverization in a 99.8% pure Al2O3 planetary ball mill.

The powders were treated differently depending on the analysis to be performed. For major

element analysis a minor amount of Al was added to the sample. Trace element and radiogenic

isotopic analyses were carried following the procedure employed by Kamenov et al (2008).

Specifics for each sample preparation protocol are included in separate sections below.

Radiogenic isotopic analyses were conducted at the Department of Geological Sciences of

the University of Florida. Approximately 0.05 gr of sample powder was dissolved for 4–6 h in hot (80-100˚C) concentrated hydrofluoric acid (HF) plus concentrated nitric acid (HNO3).

Neodymium and Lead isotopes were separated using standard chromatographic methods in a clean laboratory. A small volume of the rock solution is loaded into the column, washed into the resin bed carefully with eluent, and then washed with more eluent until a fraction is collected when the desired element is released from the resin. The preliminary separation of Sr and REE was performed in a cation exchange quartz glass column packed with MitsubishiTM cation 63

exchange resin elluted with dilute 3.5 M HCl and 6N HCl. The rare earth elements (REE) are separated as a group on the cation exhange column eluted with 6N HCl acid. Separation of Sm and Nd from the REE group was accomplished by using a Hexyil di-ethyl hydrogen phosphate

(HDEHP)-coated Teflon powder (stationary phase) with 0.18N HCl acid (Richard et al., 1976).

The column was washed with 1.7N HCl to remove co-existing elements such as Ca and Mg. Ba, which negatively impacts the analysis of Nd isotopes, was removed by washing the column with

2N HNO3.The resultant solution was preserved in sealed Teflon vials for subsequent chemical

and isotopic analyses.

U/Pb and Hf Spot Analysis in Zircons

Fifteen zircon grains per samples were analyzed to produce a total of 90 U-Pb data for the six individual samples from the AB and the OB. In addition, five U-Pb-dated dated crystals per sample were examined for Hf isotopes for a total of 30 analyses. Spot U-Pb dating and Hf

isotopic analysis by laser ablation mass spectrometry were performed at the Department of

Geological Sciences of the University of Florida using a “Nu-Plasma” (Nu Instruments, UK)

laser ablation, multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS)

equipped with three ion counters and 12 Faraday detectors. The LA-MC-ICP-MS used possesses

a specifically designed collector block for synchronized acquisition of 204Pb (204Hg), 206Pb, and

207Pb signals on the ion-counting detectors and 235U and 238U signals on the Faraday detectors.

This arrange has been previously described in detail [324]. Analysis protocol followed in this

investigation has been previously described [267, 320, 325, 326].

For U-Pb dating, the mounted zircon grains were ablated using an attached New Wave 213

nm ultraviolet laser. A mix of Ar and He carrier gas (1 L/min Ar, 0.5 L/min He) was used for

sample transport into the mass spectrometer. The laser was set at 4 Hz pulse frequency, 40%

power and a laser spot diameter of 40μm for stationary spots. Prior to ablation, on-peak 64

background “zero” measurements were performed for 20 seconds on the blank He and Ar gases

with closed laser shutter. This zero was used for on-line correction of isobaric interferences,

particularly from 204Hg which is largely derived from the argon gas. Following blank acquisitions, sample ablation proceeded for 25-30 seconds as a way to minimize the depth of the

ablation pit and hence reduce large elemental fractionation. Sample analyses were bracketed by 2

analyses of FC-1 Duluth Gabbro zircon standard for every 10 unknown zircons. The U-Pb

isotopic data were acquired using the Nu Instruments Time Resolved Analysis (TRA) software.

Grains were ablated combining centers and rims. The TRA software allows for isotopic ratios to

be calculated from the desired time segment of data, which avoids variations due to grain defects

or surface contamination. Raw isotopic data obtained from the LA-MC-ICP-MS were imported

into a Microsoft Excel© spreadsheet where they were corrected for instrumental drift and mass

bias. Data reduction is based on the FC-1 zircon standard, dated at 1099.3 ± 0.3 Ma (207Pb / 206Pb

= 0.0762, 207Pb / 235U = 1.9428 and 206Pb / 238U = 0.1850) [327], as well as being dated more recently at 1099.0 ± 0.7 and 1099.1 ± 0.5 Ma [328]. Common Pb correction was applied in the above Excel© spreadsheet using the 207Pb/206Pb corrections previously outlined [329].

Representative age errors based on the long-term reproducibility of FC-1 were 2% for 206Pb=238U

(2σ) and 1% for 207Pb=206Pb (2σ). In addition, two other approaches to common Pb reduction

correction have been employed following the approach by Jackson et al (2004), namely,

selective integration of time-resolved signals (as described above) and Tera-Wasserburg

diagrams [330]. Previous studies have concluded that, for young zircons containing a significant amount of common Pb, plotting of data on Tera-Wasserburg diagrams constitutes the most effective method of evaluating and correcting for common Pb [242]. Tera-Wasserburg concordia diagrams were generated using Isoplot/Ex Version 2.4 [331]. Errors on individual analyses are

based on counting statistics and are at the 1σ level. Errors on pooled analyses are quoted at 2σ or

95% confidence. Tera-Wasserburg diagrams are very useful in identifying common Pb. This type of diagram plots 238U/206Pb on the x-axis versus 207Pb/206Pb on y-axis. The ratio 207Pb/206Pb is

especially susceptible to the presence of any common lead, particularly in young zircons.

Samples containing common Pb will plot above the concordia line, and can either be excluded or

used to construct a regression line to determine a concordia intercept age [242].

The same Nu-Plasma MC-ICP-MS and New Wave 213 nm laser devices described above were used for the laser Hf isotopic analysis of accurately U-Pb dated grains. Laser beam was set for ablation on stationary spots with 60 μm beam size, 8 to 10Hz, and 70% power. Typical ablation times were on the order of 30 seconds. Measurements were made in static mode on

Faraday detectors acquiring 180Hf, 178Hf, 177Hf, 176Hf, 175Lu, 174Hf, and 172Yb simultaneously.

Isobaric interference corrections during analyses were performed with on-line Lu and Yb, using

176Lu/175Lu=0.02653 and 176Yb/172Yb=0.5870, both ratios within the range of published values.

All isotopic ratios were corrected for mass-bias using 178Hf/177Hf=1.46718. Multiple analyses of

FC-1 zircon standard performed during the sample analyzes yielded 176Hf/177Hf = 0.282168

(±0.000016, 2 σ standard error; n=24), indistinguishable from liquid analyses of this standard

(176Hf/177Hf=0.282174; ±0.000013, 2 σ standard error). The potential of FC-1 as an Hf isotopic standard has been previously assessed [332] finding it suitable for LA-ICP-MS applications.

Measured and mass-bias corrected 176Lu/177Hf ratios were used to calculate initial 176Hf/177Hf ratios. Overall, due to the very low Lu/Hf ratios, the difference between the present-day measured and calculated initial 176Hf/177Hf ratios in most cases is less than 1 epsilon unit. εHf figures were calculated using the 176Hf/177Hf = 0.282843 as the most reasonable value for Bulk

Silicate Earth Reference [333].

Whole Rock Geochemical and Isotopic Analysis

Six whole-rock samples for major elements and trace elements (including REE) were

analyzed. Major elements were measured by XRF spectrometry. An Element 2 ICP-MS was

used for trace elements. A "Nu-Plasma" MC-ICP-MS for Nd and Pb radiogenic isotopic analyses

following a method for high-precision isotopic analyses of small samples [325].

Major and trace element analysis

Major element analyses were conducted at Geo-Labs, Ontario, by X-ray fluorescence in a wavelength-dispersive Philips PW 2400 XRF spectrometer. Samples were first run for loss on ignition (LOI) and then 700 mg of powdered sample were thoroughly mixed with 4200 mg

Spectroflux 100 (Dilithiumtetraborate [Li2B4O7]) and melted to a glass disc on which XRF analysis were performed. Analytical errors for major elements are ~1% (excluding Fe and Na

~2%) and for trace elements ~5%. For the calibration of major and trace element determination approximately 40 reference materials were used including a selection of internationally accepted geochemical reference samples from the US Geological Survey, the National Research Council of Canada, the International Working Group ‘‘Analytical standards of minerals, ores and rocks’’, and the Geological Survey of Japan (For more detail about specific analytical techniques the reader is directed to www.mndm.gov.on.ca). Whole rock samples were analyzed for trace element concentrations using and Element2 HR-ICP-MS at the Department of Geological

Sciences of the University of Florida. The analyses were performed in medium resolution, with

Re and Rh used as internal standards. Quantification of the results was done by external calibration using a combination of USGS rock standards.

Nd isotopic analysis

A small volume of the final sample solution containing the analyte ions was aspirated in the plasma through a DSN-100 desolvation nebulizer. Nd isotope measurements were performed

with the Nu-Plasma Time-Resolved Analysis (TRA) software operating with 0.2 s integration time, resulting in acquisition of 300 ratios per minute. TRA was used in static mode, simultaneously acquiring 142Nd on low-2, 143Nd on low-1, 144Nd on Axial, 145Nd on high-1, 146Nd on high-2, 147Sm on high-3, 148Nd on high-4, and 150Nd on high-5 Faraday detectors. Measured

144Nd, 148Nd, and 150Nd beams were corrected for isobaric interference from Sm using

147Sm/144Sm = 4.88, 147Sm/148Sm = 1.33, and 147Sm/150Sm = 2.03. All measured ratios were

normalized to 146Nd/144Nd = 0.7219 using an exponential law for mass-bias correction. Baseline

was measured by electrostatic analyzer (ESA) deflection of the beam. Replicate analyses of

JNdi-1 and La Jolla Nd standards were undertaken for every 4-5 samples yielding a good measure of the long-term external reproducibilities with mean values of 0.512107 (±0.000021,

2σ) and 0.511856 (±0.000020, 2σ), respectively. To further evaluate the analytical protocol, three separate dissolutions of USGS SRM BCR-1 were prepared and analyzed for Nd isotopes together with the samples. The mean value of 143Nd/144Nd for the analyses of BCR-1 was

0.512645 (±0.000011, 2σ), identical to the recently published value of 0.512645 (Weis et al.

2006).

Pb isotopic analysis

The same Nu Plasma MC-ICP-MS procedures descried above were utilized for Pb isotopic analyses applying the Tl normalization technique (Kamenov et al. 2004). Lead isotope data are relative to the following values of NBS 981: 206Pb/204Pb = 16.937 (±0.004, 2σ), 207Pb/204Pb =

15.490 (±0.003, 2σ), and 208Pb/204Pb = 36.695 (±0.009, 2σ).

Results

Major Elements

Relevant major element data for the Antioqueño and Ovejas batholiths is presented in

Table 2-2 (data includes geochemistry from this study and from Saenz (2003)). Rock

classification was determined by plotting total alkalis-silica diagram (TAS) adapted for plutonic

rocks [334]. All samples fall in the sub-alkali field, with a prevalence of intermediate to acid

compositions between the fields of diorite and granite exhibiting relatively minor compositional

ranges of SiO2 between ~69-60% (Figure 2-4). Geochemical results are in agreement with the estimated petrographic composition and are similar for other plutonic regions in the Andes [44,

45]. The SiO2 vs. K2O relationship defines the suite as a typical medium-K calc-alkaline series

(Figure 2-5). This granitoid suite display fairly small scatter of oxide variations relative to SiO2.

Negative linear correlations indicate normal differentiation trends of decreasing Al2O3,

FeO(total), CaO, MgO, and TiO2, with increasing SiO2 (Figure 2.6). P2O5 also defines a negative, linear correlation but much more scattered. Slightly positive correlations with SiO2 are found for K2O and Na2O (Figure 2.6). Magnesium numbers, usually fluctuating around 47, indicate relatively high degree of differentiation.

Zircon U-Pb and Hf

Zircon ages for the AB vary between 71±1.2 Ma and 77±1.7 Ma. In the case of the OB, U-

Pb ages yielded two values at 72±1.4 Ma and 77±1.7 Ma. Differences in ages are significantly greater than associated errors at 1 sigma and are interpreted as “real” formation ages. A summary of U-Pb ages is presented in Table 2.1. When plotted against elevation, U-Pb ages show no correlation (Figure 2-7).

Tera-Wasserburg diagrams indicate minimal perturbations of U-Pb ages by common lead.

Only one of the 90 zircon grains analyzed exhibits an ellipse that is clearly above the cluster of ellipses and the concordia line probably pointing toward a high common 207Pb/206Pb ratio

(sample SR-43). The rest of the analysis plot in relatively compact clusters providing precise ages (Figure 2-8). All of the U-Pb ages display excellent reproducibility and errors are usually below 1%. 69

Hf analysis yielded thirty values of εHf ranging between -2.61 and +5.30 (average of 1.77).

Summarized Hf data is presented in Table 2-1. Sample SR-001 is the only one to record slightly

negative εHf values (-0.53 to -2.72) while the rest display values between approximately 1 and 5

εHf units. Variations of εHf values within samples are≤ 2.2 units. Sample SR -42 exhibits the

largest range in variation (2.2 units) while sample SR-9 shows the narrowest range (0.88 units).

Pb and Nd Isotopic Compositions

Lead isotopic compositions expressed as rations of 206Pb/204Pb display little variability between 18.861 and 19.079, while 207Pb/204Pb values range from 15.612 to 15.682. Such figures define a relatively narrow field and plot within the characteristic Pb/Pb domain of the Northern

Volcanic Zone (Figure 2-10). Neodymium isotopes range from 0.51254 to 0.51281 (εNd from -

1.83 to 3.04), i.e., right around previously reported values for the Northern Volcanic Zone [269,

271]. Hafnium and Neodymium isotopes share a similar systematic and display a high positive

correlation (r = 0.72) when plotted together (Figure 2-11). Pb and Nd values indicate that these

samples do not contain old Precambrian crustal components. Further, Pb i sotope rati os pl ot

between the orogen growth and the upper-crust cur v es [265] thereby i ndicati ng a si gni fi cant

crustal component ( Fi gur e 2-10). Nd and Hf isotope data, in agreement with the Pb isotope results, also indicate incorporation of crustal component (Figure 2-11).

Rare Earth Elements

Six samples of whole rock granodiorite were analyzed for rare earth elements (REE),

whose abundances in ppm are summarized in Table 3-1. REE values (Figures 2-12 and 2-13) are

normalized to primitive mantle according to specific values previously published [337]. Light

REE (LREE) are enriched in all samples and most converge at a value of about 60-70. This trend

is typical of calk-alkaline suites. Heavy REE (HREE) patterns are generally flat and fluctuate

between 2 and 17. The portions of the patterns from Hf to Yb is also relatively flat. Eu

anomalies, although not pronounced, clearly show two populations of data, one with negative Eu

anomalies (samples SR-003 and SR-41) and another one with positive Eu anomalies (SR-9, SR-

42, SR-001). Fractionation is also quite variable (e.g., low for Sr-24 1.4 and large for Sr-42 18).

HREE patterns, although flat, show variation between samples.

Discussion and Interpretation

Major Elements

The AB-OB is a plutonic suite dominated by granodioritic composition with hypidiomorph

granular phases. Major element analyses show variations in composition from diorite to granite

suggesting differentiation. Most samples possess medium-K calc-alkaline composition, which is

a primary feature of the NVZ in the Andes [45, 269-271]. Samples with the highest degree of

differentiation fall in the subalkalic, calc-alkaline high-K which is a consequence of significant

crustal contamination. This explanation is further supported by the characteristic trend in the

SiO2 vs. major oxide Harker diagrams for Al2O3, FeO(total), MgO, CaO, and TiO2 (Figure 2-6).

This characteristics are similar to those described in fractionating systems [45]. Decrease in

MgO, FeO(total), and CaO as SiO2 increases is consistent with the removal of early-forming

plagioclase, plus olivine and/or pyroxene from the cooling liquid. MgO and FeO(total) are

incorporated into the typically early-forming mafic minerals. CaO may have been removed by a

calcic plagioclase, a clinopyroxene, or both. Rock suites that are related by fractional

crystallization and unmodified by significant crustal contamination define coherent linear trends

on Harker diagrams [271, 334]. Therefore, the relatively scatter observed in the Harker diagrams

may indicate contributions from the crust (e.g., magma mixing, crustal contamination) in addition to

fractional crystallization during pluton formation. This is also supported by the variations in the isotope data, as presented below. Presence of hydrous magmatic minerals points to high H2O contents in the magmas. Textures in which pyroxene is surrounded by hornblende, which is in

turn intergrown with biotite, also reflect the gradual increase in the concentration of H2O in the

melt throughout crystallization.

Zircon CL, U-Pb and Hf

Predominant CL patterns found in all samples correspond to the typical oscillatory zoning, which is considered as evidence for a primary magmatic origin [256] and characterizes zircons developed in intermediate to acidic plutonic rocks [243, 322]. Relative to U-Pb dating, the predominant compositional zonation is interpreted to reflect igneous growth zoning such that concordant or near-concordant analyses of multigrain zircon fractions yield true crystallization ages. Compositional variation of U and Pb evidenced in CL does not undermine the utility of zircon in U/Pb dating because, despite differences in absolute amounts of both elements, all zones possess identical ratios of U and Pb and thus yield the same U-Pb age [256].

Variations in U-Pb ages from 71±1.2 Ma to 77±1.7 Ma for the AB and OB are thus interpreted as crystallization ages of the respective plutons implying relatively rapid incremental assemblage for both batholiths near the Campanian-Maastrichtian transition. U-Pb formation ages are more consistent than previous Rb/Sr results [305] which were generated through an isochrone from samples collected over very large distances that violate the assumption of a common initial Sr. This is supported by the Hf isotopic data including multiple samples from the plutons indicating that there is a possibility that samples in this large batholitic mass are not necessarily from a single source further complicating Rb/Sr isochrone results.

These results are consistent with a recognized Mid Cretaceous pulse of magmatism in the

Andean range [44, 272].The Campanian-Maastrichtian transition is considered as a period of relatively rapid high angle convergence, a situation that may have lead to enhanced magmatism

[44, 47]. Gradual assemblage through discrete pulses of large batholithic masses has been reported elsewhere [249, 338]. 72

In addition to constraining formation ages for the AB, U-Pb zircon data derived in this

study provide relevant information regarding the early post-crystallization history. The AB cooled relatively rapidly following intrusion/crystallization at ca. 75 Ma from ~800°C (U-Pb zircon closure) to ~300°C (K/Ar biotite closure) at ca. 60 Ma with an average rate of ~53 °C/Ma

(Figure 2-14). This rate can be interpreted as in situ cooling after AB shallow emplacement into old and cool metamorphic rocks of the Cajamarca Complex in the core of the Cordillera Central.

Between 60 and 50 Ma, cooling rates decreased to about 9 °C/Ma where cooling was probably controlled by exhumation related to the Laramic Orogenic Phase [339]. Low-temperature thermochronology by zircon and apatite fission track data [166, 293] as well as apatite (U-

Th)/He [294], indicate that cooling rates slowed since the Early Eocene to less than 4°C/Ma and are related to gradual erosion of the Antioqueño Plateau punctuated by short duration-rapid cooling episodes due to distinct erosional exhumation events that climaxed during the early

Eocene and Late Oligocene/Early Miocene [16, 293] and characterized by cooling rates between

10-15°C/Ma.

Consistent with its subduction settings, εHf shows a range from -3 to +5, which reveals that the magma sources for AB and OB were a blend of upper crustal and mantle derived materials. Hf isotope data imply 1) mixing of mantle and crustal sources and 2) juxtaposition of zircons that were crystallizing in slightly different environments as to be able to record a progression of crustal assimilation. The range of variation in εHf, although considerable, is substantially less than that reported for other petrogenetic provinces (e.g., Australia, +10 to -10

[249]. Major element data and petrographic analysis for the AB and OB indicate significant compositional variations but do not point to any clear relationship between variations in composition and Hf values where more felsic phases, such as granodiorites and granites, are

marked by more negative (crustal) values while more mafic phases are associated with more positive (mantle) values. Hf results are in agreement with REE data, further indicating a combination of mantle and crustal sources. Sample compositional diversity probably resulted from variations in the composition of the sources, uneven melting conditions, complex chemical and physical interactions between mafic and felsic magmas, and crustal contamination. Similar processes have been reported in other petrologic studies of batholithic belts in the Andes and elsewhere [45, 260, 269, 340, 341].

Hf variability conflicts with previous Rb/Sr ages for the AB because changes in Hf in time and space imply heterogeneities of the different magmatic sources involved in the genesis of this petrogenetic province, calling into question the validity of assumptions made in Rb/Sr dating

[342]. This fact, combined with the consistency of U-Pb ages found throughout zircons from all of the samples imply that the range 72-75 Ma is a closer to the formation age than the previously reported 98 Ma.

Subtle changes in composition reported for the AB and OB (ranging from diorite to granite) and structures such as the lack of foliation, roof pendants, low degree of internal faulting, and clear contact aureole developed in the metamorphic and sedimentary hosting rocks suggest emplacement in the epizone, i.e., 2-4 km [343]. Acicular crystal populations in samples

SR-41 and SR-42 also point to fast cooling at epicrustal levels. According to Corfu et al., (2003), this ratio points to crystallization velocity with elongate crystals being common in rapidly crystallizing magmas and high level granitoids and gabbros.

Trace Element and Radiogenic Isotopes

Multi-elemental variation diagrams (Figures 2-12 and 2-13) show the typical signatures of continental margin plutonism (i.e., subduction-related) including: Typical enrichment of Large

Ion Lithophile Elements (LILE) and other incompatibles up to 10 time the chondrite value, and a 74

negative Nb anomaly, which indicate magma generation in a subduction setting with an

important input of continental lithosphere. Patterns for REE also show the characteristic Y and

HREE depletions of slab melts.

REE patterns range from LREE enriched (SR-42, SR-001) to relatively flat (SR-24 and

SR-41). Chondrite-normalized (cn) REE patterns are relatively flat [(La/Yb)cn=5.1–7.8;

(Tb/Yb)cn=1.2–1.8]. Despite the relatively flat trends for HREE, some variation is apparent

between samples even for specimens within the same topographic profile demonstrating

differences in the source. Lower values of HREE indicate that the residue is enriched in minerals

that retain Er, Yb, etc. so that the magma generated was depleted in such elements. At greater

depth, where mineral phases such as garnet are stable, selective incorporation of the HREE over

the LREE during fractional crystallization is a viable mechanism affecting REE slope

significantly. However, the HREE portion of most curves is usually flat, implying that garnet,

which strongly partitions among the HREE, was not in equilibrium with the melt at the time of

segregation. In other words, most of the REE patterns are characterized by relatively low Tb/YbN ratios (1.0–1.5) ruling out substantial amounts of garnet as a fractionating phase. Instead, the

residues were probably dominated by amphibole and plagioclase, and, possibly pyroxene. A

negative Eu anomaly indicates that plagioclase was a residual phase involved in the formation of

the rocks. These residual phases could reflect crustal source for the magmas, further supported

by the high CaO/Na2O content. Enrichment of incompatible and LREE elements (Figure 2-13),

and slight depletion of HFSE, suggest a crustal source for the parental magmas. Inflection between Sm and Eu plus the negative correlations between Sr and both SiO2 and K2O versus a

positive correlation CaO/Sr is also consistent with differentiation and plagioclase fractionation.

HREE slopes are intermediate between the medium- and high-k series, displaying wide spectra of HREE relative to the convergent values of LREE. This is more common in high-K series where differences in SiO2 led to spread of HREE and yet maintain convergence of the

LREE with marked enrichment of LREE relative to HREE (overall positive slopes). These trends are similar to those reported for other granitoids in the Cordillera Central (Vinasco et al., 2006) and felsic units in isolated massifs in northern Colombia (Cardona et al., 2006).

LILEs such as Sr, K, Rb, Ba are enriched (~80-100 rock/Mantle ratio, Figure 2-13) and behave differently from the High Field Strength Elements (HFSE) particularly Yb which show nearly primitive mantle-like concentrations (rock/mantle ratio of 1-7). This high LILE/HFSE pattern is now recognized as a hallmark of subduction zone magmas. Because the LILE and

HFSE elements are all incompatible and behave similarly in solid-melt exchange, the

“decoupling” of these two groups, and enrichment of the LILEs, is frequently explained by the participation of H2O-rich fluids in the genesis of subduction zone magmas. Aqueous fluids are an

important component of subduction zone petrogenesis. Since LREE are more incompatible and

more mobile than the MREE and HREE in hydrous fluids, the LREE enrichment in the samples can be explained by metasomatic enrichment from a hydrous fluid phase (Gregoire et al. 2001;

Franz et al. 2002). The relatively flat HFSE element pattern, usually between 1.0 and 12, means the concentration of these elements is higher than primitive mantle, and may suggests that the source of continental margin magmas is indeed affected by the subducted oceanic crust, the lithospheric wedge, the sub-continental crustal root, and the upper crust. High contents of K, Rb,

Ba, and Pb indicate the involvement of crustal material in magma generation.

Negative Nb anomaly in spider diagrams (Figure 2-13) are now considered a characteristic of subduction related magmas (magmatic arcs) and have been interpreted in various ways. The

low Nb concentrations can be attributed to the presence of a residual Nb-bearing mineral. Nb

behaves similarly to Ti, so rutile, ilmenite, sphene, and hornblende are potential scavengers of

Nb and Ta (Morris and Hart, 1983; Saunders et al., 1991). Immobile HFSE element

concentrations are similar to those of primitive mantle, and probably reflect overall mantle

source characteristics, while the LILE element concentrations reflect the more water soluble components from the slab. Normalized spider diagrams, in which continental arc magmas also show the decoupled LILE/HFSE pattern (high LILEs K, Rb, Sr, Ba, low HFSE Yb) and the Nb trough, constitute a set of features that is now accepted as a characteristic of subduction zone magmas. AB and OB data clearly display these trends supporting the idea that continental arc magmas are probably created by similar processes as island arc magmas, including LILE enrichment of the mantle wedge via aqueous fluids derived from dehydration of the altered oceanic crust of the subducting slab and subducted sediments. Enrichment in the NVZ and SVZ lavas is similar to that of island arcs while enrichment in the CVZ is considerably greater. Pearce

(1983) attributed the enriched central “hump” shape on normalized spider diagrams to intraplate enrichment (including both LILE elements and more mobile HFSE elements, such as Ce and P), by analogy with intraplate basalts. He suggested that the enriched sub-continental lithospheric mantle (SCLM) was responsible for the enriched pattern in the source of the CVZ magmas, which overlie thicker and older crust. He noted that such a component, containing elevated Ta,

Nb, Zr, and Hf, is present in many continental arc magmas in amounts higher than found in island arcs, and proposed that it may be a good indicator of the involvement of SCLM in magma

genesis. Davidson et al. (1990, 1991) and Wörner et al. (1994) argue that the trace element (and

isotopic) trends correlate with crustal thickness, and conclude that, instead of the lithospheric

mantle, the thickened crust is responsible for the enrichment patterns.

The crustal sources and their geochemical signatures can be quite variable, however, and difficult to interpret. In this context, variations of REE patterns trough time in the samples presented in this study may reflect slight changes in crustal thickness with time allowing for consideration of crustal thinning, i.e., extension through slab roll back, as a potential mechanism of magma generation via decompressive melting. Further, differences in Hf values within the AB and OB can be attributable to variable input of juvenile magma controlled by modifications of the compressional regime such as it has been suggested fro the Peruvian Andes [248].

Negative Eu anomalies developed in these relatively high-SiO2 batholiths are similar to those in other continental margins and can be related to the removal of plagioclase [45, 271].

Medium-K and high-K calc alkaline series, which represent the majority of samples analyzed for the AB suite , are progressively more LREE enriched. LREE enrichment is similar to that of other highly incompatible elements, such as K, and can be accomplished by low extent partial melting of a primitive mantle. An alternative explanation (e.g., Thompson et al., 1984), suggests that the mantle source for continental arc magmas is a heterogeneous mixture of depleted primitive mantle and enriched OIB mantle types further contaminated by crustal materials.

Whatever the case may be, it seems that more than one source with different incompatible element concentrations is involved reflecting variable depletion, and perhaps enrichment.

Negative Eu anomalies may also suggest an origin of the granodiorite suite from enriched lithospheric mantle sources.

Trace element and isotopic data support the hypothesis that AB and OB, as most of the products of Andean volcanism, have distinct characteristics. Spread values of HREE that characterizes NVZ suggest difference in the sources with some deep and garnet-scavenged

HREE and some more surficial and hence, richer in HREE, i.e., closer to, or above, a value of 10

(Figure 2-12).The AB and OB as part of a NVZ setting show greater degrees of differentiation

(expressed as the overall slope of the trend or La/YbN ratio) than SVZ, a condition that is possible to attribute to variance in both differentiation and contamination-assimilation by a

thicker crust. (For a defnintino of modern volcanic zones in the Andes , i.e., NVZ, CVZ, and

SVZ, the reader is reffered to Winter (2001), Wörner et al (1992), Davidson et al (1991), and

Harmon et al (1984)).

Radiogenic Pb and Nd

Lead isotope signatures are consistent with mixing of Pb from the mantle and crustal

reservoirs. Such signatures are consistent with plutonic masses in the CVZ and NVZ, and very

distinct compared to the SVZ, which further indicates the involvement of a thick crustal “filter”

in the genesis of the AB and OB. Values for εNd are consistent with Hf signatures and also

suggest mixing of mantel and crustal components (Figure 2-11).

A Feasible Geodynamic Setting

Our data support the hypothesis of a Mesozoic subduction-related magmatic arc. The

presence of calc-alkaline magmatism along continental margins is most probably related to the

subduction of ocenic lithosphere [44, 47, 271]. Changes in factors such as rate and angle of

convergence, age of the subducted plate, subduction angle, and interaction with hotspot traces

exert a major control magmatism and tectonism along active margins (Pilger 1984; Badham and

Halls, 1975). For Pilger et al. (1984) the predominant control to magmatism in the Andes of

northwestern South America is related to the changes in the approach angle of the oceanic plate

(Farallon-Nazca). Reorientations in plate motions have been well documented in South America

[18, 19, 311]. Such reorientations have produced significant changes in the approach angle of the

interacting plates strongly affecting the tectono-magmatic evolution of the common plate

boundary. From ~120 to 70 Ma a tectonic regime with relatively rapid NW-SE subduction (~10-

15 cm/yr) at a large angle (~ 80 ˚) is though to have lead to Cretaceous plutonism in the northern

Colombian Andes [44, 311]. Is this latter scenario of slab melting and induced melting of the asthenospheric wedge with varying degrees of differentiation and crustal contamination- assimilation the most feasible one to explain the early evolution of the AB and OB.

Alternatively, granodioritic to granitic calc-alkaline magmas can be generated by dehydration melting of fertile portions of the continental crust at temperatures above 780°C (e.g.,

Wolf and Wyllie, 1994; Rapp and Watson, 1995; Kemp et al., 2007; Patino-Douce and

McCarthy, 1998 and references therein). Temperatures in excess of 780°C within the continental crust require unusual tectonic circumstances, such as slow exhumation allowing for long-time heating [340]. Alternative sources of heat that would promote lower crust melting include advective input of heat from the mantle (Bergantz, 1989; von Blanckenburg et al., 1998).

Occurrence of mafic enclaves, i.e., dioritic to gabbroic rocks, in more felsic hosts or as separate intrusive bodies suggests a direct chemical input from the mantle. This is proposed here as a potential mechanism given the fact that extremely high temperatures, above ~1100°C, must be reached to generate large amounts of magma by H2O-under-saturated partial melting of mafic crustal source rocks (e.g., Rapp and Watson, 1995; Patino-Douce and McCarthy, 1998).

Age constrains and geochemical signatures in this study support the contention that granitoids are chemically complex and seldom reflect “characteristic” or “single uniform sources”. Hybrid sources and changes through time of particular sources and mechanisms of magma generation are probably a common feature of most batholithic belts. This variability in composition is often exacerbated by assimilation during ascent. In the case of mantle-derived magmas, an imprint of crustal assimilation is often isotopically detectable. In the case of crustal melts it is difficult, if not impossible, to assess the extent of assimilation processes, because

numerous crustal inputs could occur at any point from the source to the level of emplacement.

Some of these processes are reflected in the Hf and REE datasets here presented.

The crust possesses important characteristics that may play pivotal roles in the generation and evolution of continental arc magmas in various ways, including but not limited to: 1) magmas generated in the mantle wedge or subducting slab must pass through the broad layer of sialic and incompatible element-enriched crust, an efective “filter”, before reaching the surface with the potential for considerable crustal contamination of rising liquids; 2) the low density gradients between crust and mafic to intermediate magmas may significantly retard the upward movement of the latter thereby promoting enhanced assimilation, and/or differentiation in stagnated plutonic bodies; 3) assimilation can be further enhanced by a combination of the relatively low melting point of the continental crust and the heat released from magma generation in the subduction zone, ultimately leading to partial melting of continental crust and concomitant addition of large silicic components to the system; and 4) the sub-continental lithospheric mantle (SCLM) may be quite different than the lithosphere beneath the ocean basins could have been anchored to the overlying continent since its formation that the SCLM may have locally enriched during the long-term stagnation beneath the continent. If any or all of the considerations listed above are true, continental arc volcanics, on average, are expected to be more evolved and enriched than corresponding island arc volcanics and this is the case for all the samples presented in this investigation.

Partial melting of the mantle lithosphere, posibly along with melting of the subducting slab are considered as the mechanisms responsible for triggering the initial magma surge. This is followed by complex process of fractional crystallization, assimilation, magma mixing, etc. that reproduce the spectrum of igneous rock types usually found in convergent margins. This is not

new as the generation of basaltic magma from the mantle is considered a critical first step in

magma genesis in orogenic belts [44, 45, 47]. The thick silica and aluminum rich crust, and

possibly an enriched and heterogeneous mantle, make the continental arc the most complex

igneous/tectonic environment on Earth. Moreover, all of the enriched elements in the continental

arc patterns are soluble, and likely to be concentrated in fluids. The most feasible sources for

H2O in subduction zones would be water contained in the sediments and hydrated oceanic crust of the subducted slab. These fluids, enriched in LILE elements scavenged from the crust, can both

lower the melting temperature of the mantle wedge, and concentrate LILEs in the resulting hydrated magma thus playing a paramount role in magma genesis.t. Such a model is also consistent with the common occurrence of hydrous minerals found in suites like the AB and OB, and the explosive nature of arc volcanism, which are all common features of dormant (Western and northern Cordillera Central) and active (southern Cordillera Central) magmatic arcs in the

Colombian Andes. Mixed sources are characteristic of subduction zones and geochemical trends are very distinct compared to island arcs. Along continental arcs where volcanism has ceased to provide a renewed surface cover of accumulated lavas and pyroclastics, e.g., AB and OB, erosion, has gradually exposed this remarkable granitic suite.

Conclusions

The AB and OB batholiths in the Cordillera Central of Colombia are a typical medium-K calc-alkaline, granodioritic suite of with relatively high degree of differentiation. The medium-K calc-alkaline composition is a primary feature of the Northern Volcanic Zone of the Andes. Both intrusives possess geochemical and isotopic characteristics typical of continental margin, subduction-related plutonism. Radiogenic isotopic composition and Hf analysis in zircon indicate varying but significant contribution form the crust. This is expected as the relatively

thick crust acts as a filter increasing upper crust signals. REE patterns also support similarities

with the NVZ (important crustal components) as well as differentiation and fractionation.

U-Pb ages range from ca. 71 to 77 Ma and are the best estimates of formation age for this granodiorite suite. Plutons were emplaced near the transition Campanian-Maastrichtian and not in the Cenomanian as previously proposed. Variations in ages between different zones of both plutons represent specific times of emplacement of discrete magmatic injections corroborating the idea that large plutonic masses are formed incrementally [344, 345]. The relatively short span however, implies rapid assemblage though multiple consecutive pulses. The geodynamic milieu was one in which convergence between Farallon and South America was accelerated and the angle of convergence was high promoting enhanced magmatism. Changes in the compressional regime may exert an important control in the relative amounts of juvenile material and degree of interaction with the crust.

Geochronological and geochemical-isotopic data in this study suggest that the AB and OB possess significant chemical and isotopic variations that imply complex histories of multiple intrusions from different sources. Intrusions have taken place over a relatively short period, ca. 7

Ma, each carrying a slightly different geochemical signature indicating variations in the source and different magma mixing/contamination processes and leading to a rather quick process of assemblage through incremental additions. This large batholith was assembled in the epizone through incremental pulses. Emplacement in a semi-rigid lithospheric block can be a feasible explanation for the circular, discordant outcrop and the negligible structural perturbation evidenced by this rather continuous unit.

Post-crystallization cooling was rapid (800-300˚C) over the period ca. 75 to 60 Ma and mainly controlled by waning of the thermal perturbation introduced by emplacement in the epizone in

cold metamorphic rocks from the Paleozoic. Cooling from ~ 60 to present has been constrained

by previous LTTC studies indicating slow rates controlled by erosional exhumation triggered by tectonic input in various discreet pulses on the Eocene (ca.40-50 Ma), the Late Oligocene- early

Miocene (ca. 25-20 Ma), and Pliocene-Present (Restrepo-Moreno et al, 2009; Restrepo-Moreno et al., submitted).

Table 2-1. Summary of major element data for the Antioqueño and Ovejas batholiths. Major element amounts are given in % wt. SR = samples in this study, S = samples referred in Saenz (2003). Rock types are Qd-Gd = Quartz diorite – granodiorite, D = Diorite, G = Granite, GD = Granodiorite. Sample condition varies from fresh (F) to weathered (W). Values for Fe2O3 and FeO are derived from total FeO. Mg# = [(MgO)]/ [100*(MgO/40.3114)/(MgO/40.3114+Fe2O3/175.7+FeO/71.8464)]. Samples SR-001, SR-003, SR-9 and SR-41 are belong to the Antioqueño Batholith. Samples SR-24 and SR-42 belong toe the Ovejas Batholith. All tha smaples fmor Saenz (2003) belong to the Antioqueño Batholith. SR SR SR SR SR SR S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9 S-10 S-11 S-12 Sample 001 003 9 24 41 42 Al2O3 15.95 15.5 16.55 19.2 16.01 15.71 16.07 17.67 16.09 14.91 17.29 16.68 17.75 17.23 16.9 15.54 13.52 12.8 CaO 3.82 5.09 4.25 0.04 5.78 3.6 5.63 4.39 2.57 3.97 6.39 5.65 5.93 4.31 5.8 5.37 3.92 0.69 Fe2O3t 3.01 4.98 4.07 5.34 6.77 3.48 6.65 7.19 1.96 4.17 6.78 5.79 6.29 4.99 6.32 5.79 4.31 1.34 K2O 2.34 2.27 1.89 1.63 2.11 1.28 2.45 1.66 1.8 3.36 1.6 1.81 1.9 1.59 2.11 2.39 3.76 5.37 MgO 1.11 2.14 1.39 1.13 2.56 0.95 2.84 3.27 1.04 1.54 2.82 1.95 2.53 1.43 2.22 2.38 2.74 0.37 MnO 0.06 0.09 0.1 0.24 0.13 0.07 0.11 0.13 0.04 0.07 0.12 0.12 0.11 0.13 0.12 0.1 0.07 0.02 Na2O 3.46 3.39 3.79 0.04 3.01 3.59 3.13 2.2 5.47 2.99 3.76 3.27 3.26 3.64 3.27 3.05 2.55 2.15 P2O5 0.09 0.1 0.16 0.06 0.15 0.15 0.13 0.15 0.09 0.09 0.2 0.15 0.15 0.16 0.17 0.12 0.09 0.02 SiO2 64.51 64.8 67.12 61.03 62.44 68.84 61.39 58.76 70.51 67.63 59.55 63.38 60.73 65 61.76 63.95 67.02 75.8 85 TiO2 0.3 0.52 0.38 0.66 0.69 0.45 0.62 0.68 0.27 0.45 0.72 0.56 0.63 0.41 0.69 0.57 0.47 0.12 LOI 1.93 0.33 1.02 10.41 0.65 2.23 0.73 3.71 0.46 0.53 0.56 0.62 0.87 1.28 0.57 0.62 1.02 0.86 Total 96.5 99.2 100.7 99.8 100.3 100.3 99.8 99.8 100.3 99.7 99.8 99.9 100.2 100.1 99.9 99.9 99.5 99.5 Fe2O3 1.51 2.49 2.04 2.67 3.39 1.74 3.33 3.60 0.98 2.09 3.39 2.90 3.15 2.50 3.16 2.90 2.16 0.67 FeO 1.35 2.24 1.83 2.40 3.05 1.57 2.99 3.24 0.88 1.88 3.05 2.61 2.83 2.25 2.84 2.61 1.94 0.60 Mg# 50.11 53.92 48.19 36.56 50.73 42.64 53.77 55.33 59.10 50.14 53.11 47.84 52.28 43.83 48.89 52.82 63.39 42.92 Rock Qd- Qd- Qd- Qd- Qd- Qd- Qd- Qd- Qd- Type Gd Gd Gd D D Gd D D G Gd D Gd D Gd D Gd Gd GD S. Cond F F F W F W F F F F F F F F F F F F

Table 2-2. U-Pb ages and present εHf values for the Antioqueño and Ovejas batholiths. εHf calculated from the expression εHf = [(176Hf/177Hf/ 0.282843)-1]*10000 using Patchett et al. (2004) value of 176Hf/177Hf = 0.282843 equivalent to the mean of carbonaceous chondrites. Ages for each set of samples represents the average calculated age from the analysis of 15 zircons per each sample. Numbers to the right of the sample name (1 though 5) indicate the number of the analysis undertaken for Hf isotopic analysis. Samples SR-001, SR-003, SR-9 and SR-41 are belong to the Antioqueño Batholith. Samples SR-24 and SR-42 belong toe the Ovejas Batholith. All tha smaples fmor Saenz (2003) belong to the Antioqueño Batholith. Geographic Sample U-Pb Age εHf Coordinates Profile Elevation (Ma±2σ) (present) (decimal degrees) (m)

SR-9_1 1.87 SR-9_2 1.56 SR-9_3 75 ± 1.2 1.41 6.46 N/ 75.37W Matasanos 2370 SR-9_4 1.06 SR-9_5 0.99 SR-41_1 1.30 SR-41_2 0.92 SR-41_3 71 ± 1.2 2.93 6.36N / 75.59W Matasanos 1380 SR-41_4 1.94 SR-41_5 1.27 SR-24_1 3.85 SR-24_2 3.39 SR-24_3 72 ± 1.4 4.13 6.39N / 75.60W La García 2500 SR-24_4 4.49 SR-24_5 4.80 SR-42_1 3.01 SR-42_2 5.30 SR-42_3 77 ± 1.7 3.61 6.34N / 75.58W La García 1520 SR-42_4 3.15 SR-42_5 3.75 SR-003_1 0.81 SR-003_2 1.34 SR-003_3 77 ± 1.7 2.30 6.76N / 75.12W Porce 1070 SR-003_4 1.41 SR-003_5 2.12 SR-001_1 -0.53 SR-001_2 -1.70 SR-001_3 75 ± 1.5 -2.72 6.86N / 75.18W Porce 0760 SR-001_4 -2.01 SR-001_5 -2.61

Table 2-3. Whole rock trace element concentrations for the Antioqueño and Ovejas batholiths. Trace element shown in ppm (Rare Earth Elements included). Isotopic compositions displayed as ratios. Sample SR-001 SR-003 SR-9 SR-24 SR-41 SR-42 Element Li 17.96 29.74 47.26 13.24 31.40 13.42 Sc 5.94 13.80 9.46 12.67 21.71 3.70 Ti 3553.84 5895.10 5381.24 6729.04 7947.29 4767.78 V 54.57 115.18 62.01 69.12 148.94 32.46 Cr 8.28 10.87 6.84 17.82 11.21 13.31 Co 242.94 74.36 116.66 22.94 67.79 24.72 Ni 7.87 9.53 7.54 6.95 7.21 5.72 Cu 15.78 49.17 18.87 6.56 19.93 5.65 Zn 35.84 50.81 72.12 92.91 85.25 65.99 Ga 15.45 16.56 16.37 23.44 17.82 17.02 Rb 58.54 93.19 47.50 23.90 88.68 45.84 Sr 233.45 259.26 312.66 -4.52 224.29 250.66 Y 7.27 19.88 11.01 7.56 30.02 5.44 Zr 42.60 24.21 31.99 16.71 21.46 7.66 Nb 4.48 9.60 11.56 6.55 10.78 5.88 Cs 0.10 2.88 1.16 1.32 4.17 1.79 Ba 755.52 597.88 741.95 361.15 484.36 353.83 La 10.69 9.12 8.47 3.37 7.35 10.71 Ce 19.58 22.14 17.23 7.91 20.38 19.73 Pr 2.24 3.09 2.12 1.23 3.32 2.21 Nd 8.42 13.53 8.76 5.52 16.01 8.04 Sm 1.58 3.25 1.91 1.46 4.25 1.34 Eu 0.80 0.83 0.75 0.57 0.99 0.74 Gd 1.51 3.22 1.90 1.66 4.46 1.23 Tb 0.22 0.52 0.30 0.31 0.77 0.17 Dy 1.18 3.13 1.76 1.96 4.66 0.89 Ho 0.25 0.66 0.37 0.44 0.99 0.19 Er 0.69 1.97 1.07 1.35 2.90 0.53 Tm 0.10 0.30 0.17 0.21 0.45 0.08 Yb 0.67 1.97 1.15 1.49 2.98 0.51 Lu 0.10 0.31 0.18 0.23 0.46 0.06 Hf 1.53 1.12 1.04 0.91 1.09 0.27 Ta 1.04 5.00 6.66 0.87 5.30 1.45 Pb 6.07 6.79 8.68 11.25 6.86 7.46 Th 5.15 5.70 2.66 1.78 1.59 1.27 U 1.44 1.22 1.39 0.97 2.93 0.76

Table 2-4. Lead and Neodymium isotopic compositions for the Antioqueño and Ovejas batholiths. Isotopic compositions displayed as ratios, values for epsilon Neodymium (εNd) also included .

Sample SR-001 SR-003 SR-9 SR-24 SR-41 SR-42 Isotopic Ratio 206Pb /204Pb 19.079 19.001 18.913 18.861 19.074 18.735 207Pb /204Pb 15.682 15.640 15.638 15.618 15.644 15.612 208Pb /204Pb 38.952 38.820 38.666 38.578 38.625 38.545 208Pb /206Pb 2.042 2.043 2.044 2.045 2.025 2.057 207Pb /206Pb 0.822 0.823 0.827 0.828 0.820 0.833 143Nd /144Nd 0.512544 0.512747 0.512708 0.512814 0.512722 0.512794 εNd -1.83 2.13 1.37 3.43 1.64 3.04

Figure 2-1. Geologic and structural map of the study site. Red circles indicate sampling locations. AB = Antioqueño Batholith, OB = Ovejas Batholith. Adapted from González (2001) and Tapias et al. (2006).

Figure 2-2. Color photographs of samples in their hand specimen. White color is a combination of plagioclase feldspar and quartz, black color is predominantly biotite and hornblende.

Figure 2-3. Photomicrographs of grey-scale CL imaging of typical zircons from the Antioqueño and Ovejas batholiths. Internal structures are predominantly oscillatory zoning. LA-ICP-MS analytical spots appear as perfectly circular craters. Deep and narrow spots (40um) correspond to laser ablation sessions for U-Pb dating whereas shallow wide spots (60 μm) result from laser ablation sessions for Hf analysis.

Figure 2-4. Classification of the Antioqueño and Ovejas batholith based on their SiO2 vs. Na2O + K2O concentrations. Diagram includes samples for the Antioqueño batholith analyzed in this study and by Saenz (2003) (red circles and gray diamonds respectively) and the Ovejas batholith (orange circles).

Figure 2-5. Sub-alkalinity degree for the Antioqueño and Ovejas batholiths. Dotted lines define the fields for the different series (after [335, 336]). Diagram includes samples for the Antioqueño batholith analyzed in this study and by Saenz (2003).

Figure 2-6. Major element Harker variation diagrams for the Antioqueño and Ovejas batholiths. FeO(t) = Total Fe.

Figure 2-7. U-Pb Age-Elevation relationship and range of previously reported ages for the Antioqueño and Ovejas batholiths. Black triangles, squares and diamonds show the U-Pb ages found in this study for the Matasanos, Garcia and Porce profiles respectively. Error bars are ±2σ Vertical red line represents the mean value of the U-Pb ages obtained for all the samples. Horizontal dashed line at the bottom indicates the range of ages reported in the literature [299, 301, 302, 304, 305].

Figure 2-8. Tera-Wasserburg diagrams. Total of ellipses per sample = 15. Ellipses represent 2σ.

Figure 2-9. U-Pb Age-Present εHf relationship.

Figure 2-10. Pb/Pb correlation diagrams for the Antioqueño and Ovejas batholiths. Fields for the different modern volcanic zones of the Andes are displayed for comparison according to various sources [261, 262, 269-271].

Figure 2-11. εNd vs. εHf relationship for the Antioqueño and Ovejas batholiths. Hf isotope composition of magmatic zircons plotted as a function of whole-rock Nd isotope composition at the time of crystallization. Error bars represent 2 SEM. CC = Continental crust; DM = Depleted mantle.

Figure 2-12. Primitive mantle normalized Rare Earth Elements patterns (REE) for the Antioqueño and the Ovejas batholiths. Samples SR-001, SR-003, SR-9 and SR-41 belong to the Antioqueño Batholith, samples SR-24 and SR-42 correspond to the Ovejas Batholith. Yellow shade shows the field for REE patterns found in the Northern Volcanic Zone of the Andes [271].

Figure 2-13. Multi-element plot for the Antioqueño and the Ovejas batholiths. This REE Spider Diagram is normalized to the primitive mantle [337]. Samples SR-001, SR-003, SR-9 and SR-41 belong to the Antioqueño Batholith, samples SR-24 and SR-42 correspond to the Ovejas Batholith.

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Figure 2-14. Post-crystallization cooling path for the Antioqueño Batholith. Age errors are true numerical errors (± Ma). a. U-Pb zircon (this study), b. Rb/Sr [305], c. Rb/Sr biotite [304], d. K/Ar biotite [303], e. K/Ar biotite [302], f. K/Ar biotite [27], g. K/Ar biotite [301], h. FT zircon and apatite [166], i. FT and (U- Th)/He apatite [293, 294].

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CHAPTER 3 LONG-TERM EROSION AND EXHUMATION OF THE “ALTIPLANO ANTIOQUEÑO”, NORTHERN ANDES (COLOMBIA) FROM APATITE (U-TH)/HE THERMOCHRONOLOGY

Introduction

Defining the morphotectonic evolution of the Andean Cordillera is critical for understanding the tectonic processes driving uplift, climatic influences of the topography, the source of sediment to oceans and continental sedimentary basins, and changes in paleogeography [14, 43, 113, 346, 347]. Despite its importance in the geodynamic evolution of a geologically complex region, the Northern Andes in Colombia remain poorly studied compared to many segments of the Andes to the South. Little is known about the morphotectonic evolution of some major provinces, including the Antioqueño

Eastern Massif [27], which is a polymetamorphic complex [286], intruded by large

Mesozoic calc-alkaline plutons of the Antioqueño and Ovejas Batholiths [21].

Geomorphologically, the Antioqueño Eastern Massif encompasses the Altiplano

Antioqueño [12, 28], hereafter the Antioqueño Plateau (AP), which is the largest high elevation erosional surface in the Northern Andes. The AP is preserved as a wide, slightly dissected surface extending for tens of kilometers, locally and deeply incised by the

Medellín-Porce fluvial system (Figure 3-1 and 3-3). A better understanding of the long- term landscape development and erosional exhumation history of the AP is needed for defining the Cenozoic morphotectonic and uplift history of the Northern Andes, the paleogeographic evolution of the region, and the relationships between erosion and tectonics [14-16, 24, 43, 113, 158]. The findings of this study also have implications for refining morphotectonic response to variations in convergence rates between Nazca

(Farallon) and South American plates during the Cenozoic [18].

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Elevated erosional surfaces affected by deep and localized fluvial incision, such as the AP (Figure 3-1 and 3-3), constitute an important source of morphotectonic information, retrievable from low-temperature thermochronologic data. Such surfaces, usually referred to as relict surfaces, have played an important role in the reconstruction of the morphotectonic evolution of active [97, 348, 349] and passive continental margins

[350]. Elevated, relict erosional surfaces, indicate that portions of an orogen have not achieved equilibrium between erosion and tectonics [83, 96, 97]. It is generally thought that these surfaces develop by planation close to base level [351] and are subsequently uplifted via tectonic processes to their present position [12, 96, 167]. The non- equilibrium character of these denudational features implies lagged response to tectonic forcing, which allows their use for reconstructing regional, long-term erosional exhumation as well as patterns of surface/rock uplift and topographic evolution [83, 216].

Cenozoic uplift and erosion in the northern Cordillera Central is recorded by sedimentary units (e.g., associated litho facies and fluvial paleoflows) in the Middle

Magdalena Valley to the east [14, 16], the Lower Magdalena Valley to the north [16, 29], and the Cauca valley to the west [168]. The traditional interpretation suggests that since the latest Cretaceous the eastern flank of the Central Cordillera has operated as the source area for synorogenic detritus to the Middle and Lower Magdalena Valleys [14, 29]. The region was also considered as a source of terruginous material for the Terciario

Carbonífero de Antioquia during the Oligo-Miocene (Grosse, 1926), a coal-bearing sedimentary sequence later renamed Amagá Formation (González, 2001), situated in the

Cauca valley.Cenozoic uplift in the Northern Andes of Colombia has been also invoked as the cause of substantial paleogeographic changes that took place in the region,

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particularly, the reorientation-reorganization of the Orinoco and Amazon fluvial systems

[43].

Thermochronological evidence from apatite (U-Th)/He analysis from two vertical profiles (Figure 3-3) in the AP that reveal the erosional exhumation history of the plateau with implications for the timing of tectonic pulses of the Andes and the topographic evolution of the range during and after uplift.

Study Site

Geological and Physiographic Overview

From west to east, the Colombian Andes are composed of three main mountain belts known as Cordillera Occidental, Cordillera Central, and Cordillera Oriental. These

Cordillera are separated by two inter-Andean depressions occupied by the Cauca and

Magdalena rivers. The Magdalena River in the east and the Cauca Rivers in the west enclose the AP (Figure 3-1). The Magdalena River yields ~1000 t/km2yr of sediment to the delta which is the highest sediment yield of any large river in South America along the Caribbean and Atlantic coasts [121]. Such sediment yield is roughly equivalent to denudation rates on the order of ~1 mm/y. Similar figures have been reported for the AP based on a temporal series of bathymetric surveys in five reservoirs in the AP [150].

Augmented erosion in the Colombian Andes has been attributed to anthropogenic activities, e.g., deforestation, agriculture, and development of infrastructure (Restrepo and Syvitski, 2006). However, quantitative data regarding long-term, background erosion rates for the area are inexistent.

The three major cordilleras belong to two distinct domains that are separated by the

Romeral Fault system (Figs. 1 and 2). The western province consists of accreted oceanic crust and is exposed in the Cordillera Occidental and the lower western flank of the

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Cordillera Central. The eastern province is underlain by continental basement and is exposed throughout most of Cordillera Central and Cordillera Oriental [25, 281]. The

Cordillera Oriental is a fold-and-thrust belt that consists of polydeformed, continental

Precambrian and Paleozoic metamorphic and igneous rocks

The Cordillera Central is characterized by modern volcanic activity related to the subduction of the Nazca plate and a basement consisting of a Mesozoic-Cenozoic plutonic arc. This study focuses on a cordilleran segment located from 5°45’ to 7° latitude North without modern volcanism. Within the study area (Figure 3-2), the

Cordillera Central encompasses a pre-Mesozoic to Mesozoic polymetamorphic complex

[286] composed of low- to medium-grade metamorphic rocks of the Cajamarca Complex

[287], and high-grade metamorphic rocks of the El Retiro Group and Las Palmas Gneiss

[21]. Lower Paleozoic or Precambrian age has also been proposed for this metamorphic core [290]. The metamorphic basement is covered by the unmetamorphosed shallow marine to epicontinental sedimentary sequences San Luis [289], San Pablo, La Soledad

[290], and Abejorral [291], that are Early Cretaceous in age (132-127 Ma). Metamorphic and sedimentary units were intruded at a shallow crustal level (~4 km) by Meso-

Cenozoic batholiths, including the Batolito Antioqueño and Batolito de Ovejas, and overlain by Late Eocene continental-arc volcanic rocks [284, 286]. Finally, Late Neogene volcaniclastic sequences associated with modern volcanic activity in the Cordillera

Central mantle the region [12]. The northern portion of the Cordillera Central is considered to be part of an exotic terrane, the Terreno Tahamí , which was affected by

Devonian-Carboniferous, Permo-Triassic (Hercynian), and Cretaceous tectono- metamorphic events [283, 286, 352].

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The Antioqueño and Ovejas batholiths are texturally and compositionally homogenous. Approximately 95% of the samples collected from this units have been classified as quartzdiorite-granodiorite [166, 301, 353]. Previously published thermochronologic data from the Antioqueño Batholith and other associated plutons include biotite K/Ar ages, which range from 68±3 Ma [301] to 90±6 Ma [354], three biotite Rb/Sr ages from 56±7.4 to 66±9.2 Ma [304]; two whole-rock Rb/Sr ages of 82±8 and 98±27 Ma [305]; zircon fission-track dates from 49.1±2.5 to 67.1±2.1 Ma and fourteen apatite fission-track ages varying between 49.1±1.2 and 28.7±1.5 Ma [166]. The large spectrum of ages can be explained by the wide range of closure temperatures of the thermochronometric systems employed and should be interpreted as cooling rather than formation ages. Although the Rb-Sr age of 98±27 Ma has been proposed as the “best estimate of crystallization age for the Antioqueño batholith” [305], recent zircon U-Pb spot analyses (LA-ICP-MS) give ages between 71±1.2 and 77±1.7 Ma suggesting that formation occurred close to the transition Campanian-Maastrichtian [355].

The Antioqueño Plateau

The AP is a low-relief geomorphic domain, which appears to be a >5000 km2 relict surface located in the northernmost portion of the Cordillera Central [12]. It is flanked in the east and west by the Magdalena and Cauca river valleys, respectively. Mean elevation in the AP is ~2500 m and it possess subdued internal topography characterized by rolling hills with local relief <40 m and slopes <10°. Its overall geometry resembles an asymmetric, truncated pyramid with its steepest margin corresponding to the Cauca River canyon (Figure 3-1 and 3-3).

The AP is dominated by a dendritic drainage network and developed over gently rolling terrain where most hill slopes are mantled by saprolitic horizons up to 100 m

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thick. Inselbergs (e.g., bornhardts and tors) are conspicuous, sporadic geomorphic features throughout the AP and in some cases create local relief >300 m. To the north and northeast the AP descends in a series of step-like, less extensive erosional surfaces into the lowlands of the Lower Cauca and Magdalena Rivers. The internal low-relief for the

AP is interrupted by steep regional escarpments and deeply incised gorges (Figures 3-1 and 3-3). In some localities, steep topographic margins expose up to 3200 m of crustal section with slopes in excess of 50°. Most of the planar topography of the AP coincides with the Antioqueño Batholith, one of the largest (7600 km2) and most continuous lithological units in the Cordillera Central. The prominent topographic features within the

AP, like the Las Baldías Range and Páramo de Belmira in the western margin of the plateau, are held up by the more resistant metamorphic units (Figures 3-1 through 3-3).

About 150 km south of the study area, the plateau’s low-relief rapidly transitions into a more rugged portion of the Cordillera Central dominated by active volcanoes. This part of the orogen, in clear contrast to the AP, has geometry that bears resemblance to a symmetric pyramid terminated in higher summits reaching up to ~5000 m.

The AP has been interpreted to have developed as an erosion surface near sea level in Paleocene time and that was uplifted in discrete episodes during the Cenozoic, and modified by fluvial activity combined with the unique weathering properties of granitic rocks in a tropical setting [12, 297]. While this is the prevailing view, the AP’s morphotectonic history is still poorly constrained, and the age of the surface is strongly debated. Despite the fact that exhumation and paleoelevational history of the AP remain virtually undocumented, remnants of shallow, marine sedimentary sequences formed

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during an early Cretaceous marine transgression point to the origin of the region as a low- elevation, low-relief domain.

No other elevated low-relief erosional surfaces as extensive as the AP have been reported in other parts of the Northern Andes. Nevertheless, similar geomorphic features of lesser extent referred to in the literature as “elevated planation surfaces” (Coltorti and

Ollier, 1999), “elevated plateaus” (Wdowinski and Bock, 1994), “high altitude paleosurfaces” (Kennan, et al., 1997), and “erosion plateaus” (Ollier and Pain, 2000) are found in Ecuador, Peru, and Bolivia. Quantitative data that constrain erosion rates, landscape evolution, and uplift history of these important geomorphic features is still scarce.

Tectonic Setting

Orogeny in the Northern Andes is primarily driven by the collision of allochthonous terrains and interactions between several tectonic plates and micro plates

[23, 25]. This has resulted in heterogeneous deformation compared to the Central Andes

[23, 25, 26, 356]. Most authors agree that the regional tectonic history has been

dominated by the subduction of two oceanic plates, Nazca and Caribbean, beneath

continental South America and by interactions of two lithospheric provinces, namely, the

North Andean and the Panama-Chocó blocks [23-26, 356] (Figure 3-4). The Nazca plate is undergoing rapid steep subduction along the length of the Colombia-Ecuador trench at a rate of ~54 mm/a, and is responsible for the volcanism in the Cordillera Central [25,

26]. The Caribbean plate is being subducted slowly from the northwest at ~20 mm/a, at a shallow angle, and without an associated volcanic arc [23, 26]. During Paleocene until

Miocene, the collision of the Caribbean plate with South America resulted in uplift in the

Cordillera Central [357]. Starting in the Neogene time, subduction was blocked in the

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collision area between the Panama-Chocó block and the Western Cordillera of Colombia and this ongoing collision is producing deformation in the three cordilleras [23, 25].

Modern stress regimes are accommodated by major structures within the North

Andean Block, which is part of the South American plate [25], and within the Panama-

Chocó block [23, 24, 356]. The Boconό and Guaicáramo faults separate the North

Andean Block proper from the South American Plate (Figure 3-1 and3- 4).The North

Andean Block is escaping rigidly at 6±2 mm/a to the northeast with the South American craton acting as a rigid buttress while the Panama-Chocó block is in active collision at a

rate of ~25 mm/yr [26] . Finally, the area that encompasses the AP is characterized as a

lithospheric unit of high rigidity [22].

Two major variations in the rate of convergence between the Nazca (Farallon) and

South American plates have been reported for the Eocene and Oligocene [18, 19, 42,

315]. These changes may have determined morphotectonic, deformational and magmatic

trends in the whole Andes [36, 47]. Collision of the Panama-Chocó block is considered as

the trigger of the modern topographic configuration of the Northern Andes and has been

invoked as the cause of the most recent phase of uplift and exhumation in the region (Eu-

Andina Phase of Van der Hammen, 1960), as well as de driver of the Late Miocene-

Pliocene uplift of the Western and Eastern cordilleras that has produced most of the

present day structural relief [14, 15, 24, 25, 156].

The Central Cordillera may represent a crustal-scale, positive flower structure [14,

25]. The age of major deformation of the Central Cordillera at the latitude of this study is

pre-middle Eocene, as evidenced by the widespread Middle Magdalena Valley

unconformity [14]. Morphotectonic evolution of the Central Cordillera has exerted a

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major control on the tectonostratigraphic development of the Magdalena and proto-

Eastern Cordillera basins [14]. Elevated areas in the Central Cordillera were a source of detrital material for basins on both sides of this range. Good examples are the Late

Cretaceous to present sedimentary sequences in the Magdalena Basin to the East [14, 29] and the continental, coal-rich Amagá Formation with >1,500 m of sediments deposited on the west side of the Cordillera Central [21, 168].

Apatite (U-Th)/He Thermochronology

AHe dating is a well-established thermochronometer with temperature sensitivity between 40°C and 80°C [183, 190, 358, 359], a range that is lower than that of any other routinely used thermochronometer. Assuming geothermal gradients of 20-30°C and mean annual surface temperature of ~15 °C, this temperature range is equivalent to depths of

~1.2–3 km. AHe analysis has been used in diverse tectonic settings to investigate several geologic processes in the upper crust, including landscape and topographic development

(Clark et al., 2005;House et al.,1998), orogenic exhumation in collisional orogens

(Reiners et al., 2003),uplift/erosion intranspressive environments (Spotila et al., 2001), development of rifted margins and escarpments (Persano et al., 2002), and footwall exhumation in extensional settings (Stockli, 2005; Stockli et al., 2000).

AHe thermochronometry of samples collected from near-vertical elevation profiles enables study of the timing, magnitude, and rate of cooling of rocks as they traverse the upper 1-4 km of the earth’s crust, the realm strongly influenced by upper-crustal tectonic and erosional processes [54, 90, 91, 183, 359]. Improved reconstructions of erosional

exhumation by AHe are achieved where (1) geothermal gradients have remained

relatively stable through time [360]; (2) denudation rates are not excessively high (> 5

mm/y) as to promote significant heat advection in the upper crust [361, 362]; (3) pre-

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existing topography was not rough i.e., topographic amplitude >3 km, topographic wavelength < 20 km at denudation rates > 0.5 mm/y [363]; and 4) surface relief amplitude has not changed significantly since the rocks cooled through the closure temperature, as this has a strong effect on the slope of the age-elevation relationship and can lead to overestimation/underestimation of exhumation rates [364]. Although AHe can be used to reconstruct exhumation histories even when these criteria are not met, uncertainties associated with these four factors are minimized if a vertical profile for sample collection is available and if samples are collected over a large range of elevations. Extended vertical profiles allow a more thorough analysis of features displayed in age-elevation profiles such as exhumation events, helium partial retention zone (He-PRZ), and paleogeothermal gradients, which permit better constraints on denudation [190, 201, 364]. The AP possesses geologic, morphologic and tectonic characteristics that make it a perfect scenario to undertake AHe low temperature thermochronology studies along vertical profiles.

Methods

The AHe technique is based on the accumulation of 4He (α particles) resulting from

the radioactive decay of the parent isotopes 235U, 238U, 232Th, and, 147Sm. Thermokinetics

of helium diffusion and α-particle stopping distances in apatite (~20μm) are well

constrained so that measurements of 4He and its parent isotopes by mass spectrometry

allows calculation of the system’s closing time [190, 208, 220]. Measurements were

made by degassing multigrain aliquots through laser heating and evaluating 4He by

isotope-dilution gas source mass spectrometry, followed by determination of U, Th, and

Sm on the same crystals by isotope-dilution ICP-MS. Mean (U-Th)/He ages were

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calculated on the basis of 3-5 apatite replicate analyses. Propagated errors for He ages based on the analytical uncertainty associated with U, Th, and He measurements are 4%

(2 σ) for laser samples [190]. Nevertheless, a 6% (2 σ) uncertainty for all samples is reported based on the reproducibility of replicate analysis of laboratory standard samples

[201].

AHe analyses were conducted on samples from two separate elevation profiles located ~40 km apart in the central portion of the AP. The La García and Matasanos-

Porce profiles are situated on a regional scarp developed on the northern flank of the

Medellín/Porce River canyon. Samples were collected at elevation intervals of ~70 m, spanning the largest possible range of paleocrustal depths (~2 km) in the exposed structural relief along this margin of the AP (Figure 3-3). Apatite concentrates were obtained through the conventional method of heavy liquid and magnetic susceptibility separation. Nineteen samples yielded good quality inclusion-free, euhedral, non-fractured apatite with >65 µm in the prism width, suitable for AHe analysis [183, 190].

Apatite (U-Th)/He age determinations were carried out at the University of Kansas using laboratory procedures described in Farley (2002), House et al. (2001), Reiners et al.

(2003), Stockli et al. (2000). Radiogenic He was analyzed using a fully-automated mass spectrometry system consists of a Nd:YAG laser for total He laser extraction, an all-metal ultra-high-vacuum extraction line, a precise volume aliquot systems for 4He standard and

3He tracer for isotope, a cryogenic gas purification system, and a Blazers Prisma QMS-

200 quadrupole mass spectrometer for measuring 3He/4He ratios

Three to five multiple-grain aliquots per sample were prepared following the standard protocol available at the University of Kansas (U-Th)/He Laboratory. Euhedral

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crystals to be dated were hand-picked from apatite separates using a high-power (180x) stereo-zoom microscope under reflected and transmitted light and screened for inclusions. To enhance discrimination of inclusions, all of the grains selected for He mass spectrometry were further examined under the microscope by immersion in alcohol. Only transparent, inclusion-free, euhedral, unfractured apatite grains with similar shape and size (average diameter ~80 ± 10 µm) were chosen. Selected crystals were digitally measured and geometric parameters were used to characterize the morphometry of each grain and calculate α-ejection corrections. Uniform grain size minimizes differences in

He diffusion behavior (Farley, 2000). Sizes >60-70 µm require low correction factors for

He ages and increase accuracy (Farley, 2002).

After careful selection and screening, apatite crystals were placed into 1-mm Pt tubes, which were then loaded into copper sample holders with 36 sample slots. Samples were degassed at ~ 1050 °C for 5 minutes and analyzed for 4He, followed by a second

extraction (He re-extraction) to ensure complete degassing and to monitor He release

from more retentive U- and Th-bearing inclusions in analyzed apatite.

4 Once He measurements were completed, samples were dissolved in HNO3 and

spiked with mixed 230Th-235U-149Sm tracer for isotope dilution ICP-MS analysis of U, Th,

and Sm. Procedures were performed at Isotope Geology Laboratory of the University of

Kansas (IGL-KU). Each aliquot was analyzed for U, Th, Sm and selected REE using a

Fisons/VG PlasmaQuad II+XS ICP-MS. Precision and sensitivity of the instrument allow

isotopic analyses with RSD <1%. Concentrations of 147Sm were close to zero for all samples. Th/U rations can be used to monitor for the presence of Th rich phases such as monazite [215]. Normal Th/U ratios for monazite are between 5-20[365]. The mean Th/U

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for all of the replicates combined is 0.9 and most values are bellow 1 so none of the analysis had to be excluded.

Alpha ejection was corrected using the method of Farley and others (1996),

modified by Farley (2002). Standard 97MR-22 apatite, a well-characterized plutonic

sample from British Columbia (4.5 Ma) (Farley et al., 2001) was analyzed with each

batch of unknown sample in order to monitor system performance and check analytical

accuracy. Replicate analyses of 97MR-22 yielded a mean age of 4.6±0.2 Ma (2σ; n=41).

Analytical uncertainties for the University of Kansas (U-Th)/He facility are assessed ~6%

(2σ), which incorporates noble gas analysis and ICP-MS uncertainties.

Results

U, Th and He concentrations, Ft correction for grain dimensions and alpha ejection

corrected He ages are reported in Table 1-3 for the La García and Matasanos-Porce

profiles. All individual AHe sample aliquot analyses show excellent reproducibility.

Similarity in the age-elevation relationships for both profiles indicates consistency of the

data sets and allowed us to combine AHe data into one composite vertical transect

(Figure 3-5). Most aliquots yielded ages with standard deviations < 2.0 Ma. Only one sample, SR-11, yielded an aliquot with irreproducible 4He concentrations (see data supplement) and AHe age of 1088.8 Ma. This was likely caused by parentless 4He hosted

by mineral or fluid U- and/or Th-bearing micro-inclusions, which is the most common

reason for irreproducible ages [183, 190, 201]. Small, acicular mineral inclusions were

found in some resin-mounted transparent crystals from sample SR-11 when analyzed

under high magnification. Spontaneous fission-track distributions in other apatite grains

from the same separates show negligible zoning, therefore, α-emission corrections, which

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assume homogenous U and Th distributions [190], can be considered adequate for the population of grains analyzed.

AHe ages in the La García vertical transect increase systematically with elevation from 26.6±1.3 Ma at the bottom of the scarp to 46.7±2.4 Ma at the level of the plateau.

Almost identical results were obtained in the Matasanos transect, with samples ranging in age from 22.8±1.1 Ma to 48.9±2.4 Ma (Figure 3-5 and Table 3-1). Variations in AHe ages with elevation in this study are positively correlated and resemble theoretical He retention vs. depth curves [211] and age-elevation relationships obtained for other He vertical-profile studies [89, 201]. Positive correlation in our age-elevation profiles, similarity in slope between previous apatite fission-track data [166], and modern topography of the AP suggest that relief changes have been negligible since Eocene time, therefore, topographic corrections by the admittance ratio [89, 364] to our data are not required. Further, the combination of U and Th concentrations in our apatite grains

(average 19 ppm) and previous apatite fission-track ages reported for the same geologic province (26-45 Ma, Saenz, 2003) fall within the figures recently proposed as the range over which excessive He retention due to radiation damage, and concomitant increases in

AHe ages should be negligible [192]. Even though these are general guidelines, they enhance our confidence that the He kinetics we used in interpreting our data are a reasonable assumption, so that none of our AHe apparent ages are potential overestimates of the time of cooling through the He partial retention zone.

The age-elevation relationship for all data is characterized by three segments

(Figure 3-5). Segment 1 between 760 and 1600 m displays AHe ages from ~22 to ~26

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Ma, exhibits very low data dispersion (R2=0.98), and has a slope of 0.24 km/Ma.

Segment 2 at sample elevations between 1500 and 2000 m shows a broader range of AHe ages from 25-42 Ma (R2<0.2) and a lower slope than the other two segments (0.02 km/Ma). Segment 3 at elevations >2000 m is characterized by apparent ages from ~41 to

49 Ma, and data scatter higher than in segment 1 but lower than in segment 2 (R2<0.5)

and a slope of 0.07 mm/yr.

Discussion and Interpretation

Our AHe results complement apatite and zircon fission track thermochronology from the Antioqueño Batholith and associated plutons providing additional detail to constrain the exhumation history of the region during Cenozoic time. The AHe data reveal a marked cooling event at ca. 23 Ma (Figure 3-5). We interpret the almost invariant He ages of segment 1 as representing a rapid exhumation pulse starting at ~25

Ma. Segment 2 of the age-elevation profile is interpreted to be part of the lower segment of the Oligo-Miocene He-PRZ and may represent a period of tectonic quiescence lasting

~17 million years. The change in slope at ~41 Ma (~2000 m) is not as well defined as the one at 25 Ma (1500 m). It is probably indicative of an exhumation event that was less significant than the Oligocene episode.

Apatite fission-track data [166], although not derived from vertical profiles, provide additional insight to evaluate the AHe change in slope between segments 2 and 3 at ca. 41 Ma. All of the apatite fission-track apparent ages are equal or older than AHe ages with a mean age of ca. 46 Ma (Figure 3-6). The samples show unimodal fission- track length populations with a mean value of 14.1 μm indicating relatively rapid cooling in Eocene time [166]. This finding corroborates our interpretation that the segment 3 of the age-elevation plot records an Eocene exhumation event. The average AHe age for

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segment 3 (~41 Ma) is a minimum age for exhumation as these samples resided in the lower temperature part of the He PRZ when exhumation slowed. Nearly concordant AHe and apatite fission-track ages over crustal section of ~ 1500 m (Figure 3-6) suggest that rocks were exhumed from temperatures above the apatite fission-track partial annealing zone (>110°C) to below most of the AHe PRZ. This suggests that rocks within this zone cooled ca. 50°C in 5 million years, which, at a gradient of 25 °C, implies unroofing on the order of 2 km, i.e., ~0.4 mm/yr. Basal conglomerates in the Magdalena and Cauca basins deposited at this time also suggest significant erosion of the AP during the Eocene

(e.g., Gómez, 2005; Grosse, 1926).

The AHe results are consistent with interpretations that kilometer-scale uplift and exhumation in the Northern Andes, as well as in other portions of the Andes, occurred in discrete pulses in the Cenozoic [14, 16, 40, 366]. These exhumation events occurred during or just after rapid rates of convergence between the Farallon and South America

(~150 mm/yr) documented for the Middle Eocene and the Late Oligocene [18], suggesting a relationship between orogeny and subduction dynamics.

The break in slope between segments 1 and 2 of the age-elevation plot (Figure 3-5) defines the bottom of He-PRZ established towards the end of the quiescence period between ca. 45 and 25 Ma when erosional processes lead to the development of the proto-

AP. This pre-early Miocene He-PRZ was exhumed at ca. 25-23 Ma. The base of this fossil He-PRZ (~80°C paleo-isotherm) is at a modern depth relative to the present AP surface of ~1000 m. Before exhumation rates increased at ca. 25 Ma, the break in slope was at a depth of about 2.6 Km, assuming a geothermal gradient of 25 °C/km and a mean surface temperature of 15 °C. This implies rock uplift of approximately 1 km (Figure 3-

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7). Based on the assumed paleogeothermal gradient and the temperature difference across the He-PRZ (~ 40 °C), the top of the pre-Miocene He-PRZ is probably at ~3100 m,

which is the elevation average in the Las Baldías Range on top of the AP (Figure 3-3).

If our assumptions for the geothermal gradient and pre-existing topography are correct, only about 9000 m of plateau have been removed by erosion in the last 25 Ma, yielding a low erosion rate for an active orogen (~0.04 km/Ma). Erosion in the summits of the Las Baldías Range (e.g., Páramo de Belmira) must have been very limited, i.e., less than 200 m in the last 25 million years (~0.008 km/Ma) and such surfaces probably represent a position closer to a “true” relict of the original plateau. Our interpretation implies very contrasting efficiencies in erosion on the granitic units of the Antioqueño

Batholith (~0.04 km/Ma) relative to the metamorphic units in Las Baldías Range (0.008 km/Ma) where less than 200 m of crustal material (and in some cases <100 m) have been removed since Late Oligocene. The spatially restricted remnants of Early Cretaceous marine sedimentary strata present in the area are portions of the overburden removed by erosion during the development of the present AP since 25 Ma as well as during previous

exhumation pulses in the Eocene and Paleocene. The low values for background erosion

rates derived from our study are in agreement with denudation rates for subdued relief

and soil mantled regions suggested by other authors (Ahnert, 1970; Summerfield, 1991)

supporting the idea that the AP is a relict surface that has not adjusted to modern

erosional conditions of the orogen. Such rates are in clear contrast relative to proposed

figures of modern erosion in the region (Giraldo, 2005; Restrepo and Syvitski, 2006). The

discrepancy may be explained by the intensification of erosional process due to

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anthropogenic influence in the Magdalena basin, a problem not addressed in detail in this investigation, but that certainly warrants further consideration.

Chronology and intensity of exhumation pulses of the AP documented in this study coincide remarkably well with the timing and relative magnitude of the orogenic phases

Proto- Andina (24 Ma) and Pre-Andina II (43 Ma) proposed for the Northern Andes of

Colombia by Van Der Hammen (1960) based on a stratigraphic and paleobotanical study.

Further, these phases of erosion occurred at the same time as the discrete orogenic events in Peru known as the Incaic in the Eocene, and The Quechua 1 in the Early Miocene [36].

The Late Oligocene-early Mioene exhumation event recorded in our study is consistent with documented erosional phases for the Peruvian Andes and the Bolivian Eastern

Cordillera, where Gregory-Wodzicki (2000) reported an erosive phase at ca. 22 Ma that removed ~2 km of overburden. For the Eastern Cordillera of the Andes of central Peru apatite fission track data record two exhumation pulses during the Neogene at ca. 21 Ma and between 12 Ma and the Present, which removed about ~ 4 to 6 km of overburden in the past 30 million years [35]. Similarly, an exhumation pulse at ~40 Ma was reported by

Benjamin (1987) in his study of fission-track thermochronology for the Bolivian Andes,

whereas analogous timing for Eocene cooling events associated with erosion have been

indicated for the Sierras Pampeanas in Argentina [32] and northern Bolivia [30]. Rapid

exhumation events in mountain ranges in the Caribbean region [37] have also been

recognized for the Late Eocene and Mid to Late Oligocene. This evidence suggests that

Cenozoic exhumation pulses in the Northern Andes and the Caribbean are controlled by

continental-scale tectonics. The trigger of these accelerated pulses of erosion may be

related to well documented changes in convergence rates between the Nazca (Farallon)

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and the South American plates [18, 19, 315], which are in turn related to major reorganization of the plates such as the one caused by the break up of the Farallon Plate into Nazca and Cocos plates.

The period of relative tectonic quiescence between the two exhumation pulses provides enough time for the development of an erosional surface on which the Terciario

Carbonifero de Antioquia of Oligocene-Miocene age was deposited [168]. Our results are also in agreement with the tectonostratigraphic development of sedimentary sequences in the Magdalena Inter Andean depressions [14, 29] corroborating that the Cordillera

Central was an elevated massif that functioned as a source of sediment to major sedimentary basins during the Paleogene and Neogene. Late Paleogene-Neogene sedimentary sequences found in the AP (Sanchez and Parra, personal communication) imply that fluvial activity and concomitant obliteration of the original erosional surface in the AP have been in operation throughout the Neogene.

AHe data in this study do not record post-Middle Miocene exhumation events, i.e., the Eu-Andina orogenic phase of Van der Hammen (1960), which has been invoked as the phase responsible for the modern configuration of the Andean range and which has also been recorded in many massifs throughout the Andean range. The fact that the Eu-

Andina phase is not verified by AHe (this study) and previous fission-track data [166] constitutes compelling evidence that the AP is in geomorphic disequilibrium.

The modern expression of the AP developed through a long history (Figure 3-8) of multiple phases of erosional surface formation at low elevation (peneplanation) beginning with its emergence in the Santonian-Early Masstrichtian [16]. The Eocene and

Oligocene-Miocene exhumation events here reported were initiated after kilometer-scale

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orogenic uplift accompanied by fluvial incision and erosion that removed part of the cover of the Antioqueño Batholith including the majority of the Cretaceous marine formations.

Although there seems to be an agreement in regards to the timing of the major phases of surface uplift-denudation for the Andes, paleoelevation of the region remains for the most part undocumented. Gregory-Widowsky (2000) suggested that the

Colombian Andes was at ~40% of its modern elevation by 4 Ma. Similar results, e.g., uplift of the Colombian Andes concentrated in the last 10 Ma and intensified in the

Pliocene, were obtained by Van der Hammen et al., (1973) and by Hooghiemstra (2006) from studies of pollen and macrofossil vegetation assemblages from the Cordillera

Oriental. We suggest that the age of the most recent peneplain in the AP is Miocene.

During the Pre-Andina (Eocene) and Proto-Andina (Oligo-Miocene) phases the area experienced kilometer-scale uplift followed by erosion surface development (relief obliteration) during intervals of tectonic quiescence. The actual elevation of the AP was attained with the onset of the Eu-Andina phase, particularly the sub-phases II through IV

(12 Ma to present) when the erosion surface raised from close to sea level (e.g., 500 m) to ca. 3600 m. However, reliable paleoelevation data for the Central Cordillera is scant so the question about the progression of elevation within the context of orogenic pulses and uplift phases in the region have only been addressed indirectly, particularly in relation to the evolution of depocenters in the Magdalena Basin and its evolution from a foreland basin to an Interandean basin during the Cenozoic [14, 29].

Deep incision in the AP along the Medellín-Porce and Río Grande rivers is probably a recent feature developed during surface uplift in the Pliocene. Modern

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geomorphic expression of the AP is the result of recent fluvial processes controlled by nick point propagation. The closest point to a relict Mesozoic surface may be found at the summits of Páramo de Belmira, although glacial erosion during Paleocene-Holocene glacial periods may have removed some of it (~100 m). The saprolitic cover of the AP is absent above 3300 m elevation where the landscape is dominated by remnants of glacial landforms and truncated summits.

The similarity in the age versus elevation plots for both profiles (Figure 3-5) is interpreted as indicating exhumation of the entire AP as a discrete unit. Our results support the hypothesis that the Northern Cordillera Central is part of a structurally coherent crustal block within the orogen [22]. The rigidity of the Northern Cordillera

Central block, where the AP is located, may have controlled the nearly circular outcrop pattern of the Antioqueño batholith. Cross-sections presented by Cortes and Angelier

(2005) show a major structural zone at ~7°N/75°-77°W with high-angle, reverse faults flanking the AP (Palestina and Romeral fault systems in the east and west respectively).

These structures are inherited from the basement and bound the AP block. Rigidity of the block and bounding faults may have controlled the structural continuity of the area as well as the mode of emplacement and overall shape of the Antioqueño batholith, thus imposing litho-structural control in the geomorphic evolution of the AP. Reactivation of these major crustal structures during periods of enhanced convergence may have facilitated uplift of the AP as a coherent block.

The Cenozoic uplift and exhumation history of the northern Cordillera Central of

Colombia resulted from plate-tectonic reorganization and directly affected the morphotectonic evolution of the AP as well as the development of the sub-Andean fold-

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and-thrust belt and the fill and tectonostratigraphic evolution of the intramontane and foreland basins to the east (Magdalena, Llanos Basin) and west (Cauca). This is particularly clear at the end of the Oligocene when the Farallon plate broke up into the

Nazca and Cocos plates [18, 42]. Augmented convergence rates induced synchronous tectonic and geomorphic activity on the overriding plate (i.e., uplift, exhumation, deformation, shortening, etc.) from Argentina to Colombia. A similar effect was also observed during an earlier phase of rapid convergence in the middle Eocene. Additional control to the more recent morphotectonic history in the area is related to the west-to-east motion of the Caribbean plate and the collision of the Panama-Chocó block at ~10 Ma, which closed the Panama isthmus between 3.7 and 3.4 Ma [24] triggering the most recent orogenic phase in the region and giving the Colombian Andes its present topographic configuration [156]. The main phases of uplift of the Northern Andes led to important paleogeographic events [43] and to erosion of a large volume of sediments resulting in thick basal sandstones and molasse successions in the sub-Andean basins [14, 16]. Age control for two of these major periods is accurately provided by our AHe data fitting well the spatiotemporal framework for the evolution of the Colombian Andes in the context of continental tectonics.

Conclusions

AHe thermochronometry of samples collected from two elevation profiles spanning

~2 km of exhumed crustal sections reveal the long-term erosional exhumation of the AP.

Sampled profiles exhibit similar behavior in the age elevation relationship. AHe ages increase with elevation from ca. 22 Ma at the bottom of regional scarps to ca. 49 Ma on top of the AP. A marked inflection point in age versus elevation data at ca. 25 Ma defines the bottom of the exhumed post-Oligocene He-PRZ.

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Elevation-invariant ages below ca. 25 Ma record the onset of rapid exhumation and surface uplift of the AP that led to river incision. A subtle change in slope within the He-

PRZ, ca. 41 Ma, corresponds to a less intense, exhumation-related cooling episode.

Previous AFT data in the AP, although limited and not derived for samples over the same profiles, allows to further define this Eocene exhumation pulse. Exhumation pulses identified coincide remarkably well with climax of the Proto-Andina (~23 Ma) and Pre-

Andina (`43Ma) orogenic phases previously proposed for the Colombian Andes, and are

synchronous with tectonically driven exhumation events reported for the Peruvian,

Bolivian and Argentinean Andes, and for orogenic systems in the Caribbean. These

pulses are correlated with variations in the rates of convergence between Nazca

(Farallon) and South America previously documented for the Middle Eocene and the Late

Oligocene suggesting continental-scale controls on uplift and denudation throughout the

Andean range. During the Pre-Andina and Proto-Andina phases the area experienced

kilometer-scale uplift followed by erosion surface development (relief obliteration)

during intervals of tectonic quiescence. AHe results are in agreement with

tectonostratigraphic data in the Magdalena and Cauca basins and with proposed scenarios

for paleogeographic evolution in the Northern Andes during the Eocene and Oligo-

Miocene.

Similarity between AHe profiles indicates the whole AP was uplifted and exhumed

as a coherent structural block, corroborating previous evidence for the rigidity and

coherence of this crustal domain in the Northern Andes. Strength of this block in the

Northern Central Cordillera and bounding faults may have controlled the structural

continuity of the area as well as the geometry of emplacement of the Antioqueño

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batholith imposing litho-structural control in the geomorphic evolution of the AP.

Reactivation of these major crustal structures during periods of enhanced convergence probably facilitated uplift of the AP as a coherent block.

AHe data indicate that the age of the most recent peneplain in the AP is Miocene.

The actual elevation of the AP was attained with the onset of the Eu-Andina phase, particularly the sub-phases II through IV (12 Ma to present) when the erosion surface raised from close to sea level (e.g., 500 m) to ca. 3600 m. Nevertheless, paleoelevation data for the Central Cordillera is virtually inexistent so that the issues of the evolution of elevation within the context of orogenic pulses and uplift phases in the region are still unresolved. Pulses of exhumation are separated by interludes of tectonic quiescence of durations in excess of 15 million years, spans during which low-standing erosional surfaces can develop.

Modern configuration of the exhumed PRZ implies that only ~ 9000 m of plateau

have been eroded in the last 25 Ma, i.e., ~0.04-0.02 km/Ma, which is a low erosion rate

for an active orogen. Erosion in the summits of the Las Baldías Range is even lower, i.e.,

~0.008 km/Ma. This implies very contrasting efficiencies in erosion on the granitic units

of the Antioqueño Batholith relative to the metamorphic units in Las Baldías Range. Such

low values for background erosion rates are in agreement with denudation rates for

subdued relief and soil mantled regions , a finding that supporting the idea that the AP is

a relict surface that has not adjusted to modern erosional conditions of the orogen. Such

rates are in clear contrast relative to proposed figures of modern erosion in the region,

i.e., 1 mm/yr. The discrepancy may be explained by the intensification of erosional

process due to anthropogenic influence in the Magdalena basin. A more thorough

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comparison between modern (anthropogenic) and long-term (natural erosion rates constitutes an important line of inquiry not addressed in detail in this investigation, but that certainly deserves more consideration

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Table 3-1. Summary of (U-Th)/He results for Matasanos and La García vertical transects. The row marked as “A/G” indicates the number of aliquots (A) and the number of grains (G) per aliquot analyzed for each sample, for instance, in sample SR-9 a total of 3 aliquots were analyzed and each aliquot was prepared using two to three grains. Average mass of aliquots run for each sample is indicated in column “Mass (mg)”. 147Sm concentrations (ppm) in all samples analyzed were virtually 0 or below the detection limits. Sample Age ± Std A Mass U Th U/Th He Ft RawAge Radius Elev. LatN/LonW - number (Ma) (Ma) Dev G (ppm) (ppm) (ncc/mg) (Ma) (um) (m) Transect SR-9 43.6 2.2 1.6 3/3-2 2.8 17.5 9.5 1.8 76.5 0.70 30.5 52.1 2370 6.46/75.37 - M SR-11 43.4 2.2 2.5 3/2-1 2.8 13.5 7.7 1.8 54.0 0.70 30.4 53.1 2230 6.46/75.37 - M SR-15 48.9 2.4 3.9 5/2 4.6 24.8 3.4 7.3 112.3 0.73 35.7 57.6 2100 6.45/75.37 - M SR-19 40.7 2.0 2.8 5/3-2 4.0 17.7 8.0 2.2 54.0 0.73 29.7 57.5 1990 6.45/75.36 - M SR-6 33.7 1.7 2.5 3/3 9.4 20.3 29.5 0.7 82.3 0.72 24.3 56.0 1710 6.43/75.37 - M SR-2 36.6 1.8 0.8 3/2 4.0 8.4 10.8 0.8 34.5 0.70 25.6 51.7 1520 6.47/75.38 - M SR-40 31.4 1.6 2.5 4/2 3.6 44.4 55.4 0.8 156.7 0.71 22.3 53.0 1500 6.41/74.44 - M

127 SR-41 25.1 1.3 3.5 3/2 3.7 42.5 56.7 0.7 115.6 0.70 17.6 52.0 1380 6.41/75.41 - M

SR-CC3 24.2 1.2 2.6 3/2 8.0 18.7 39.7 0.5 63.4 0.80 19.4 79.0 1070 6.76/75.12 - M SR-CC2 23.9 1.2 2.8 3/2 3.1 29.0 18.2 1.6 67.0 0.70 16.7 51.5 1000 6.80/75.14 - M SR-CC1 22.8 1.1 1.5 3/2 3.6 19.0 15.0 1.3 39.7 0.70 16.0 52.4 760 6.86/75.18 - M SR-26 46.7 2.3 3.4 3/3-2 5.7 14.1 15.3 0.9 76.1 0.75 35.0 62.5 2350 6.38/75.59 - G SR-31 42.9 2.1 1.1 3/3 5.6 14.9 15.1 1.0 68.2 0.71 30.5 54.0 2170 6.37/75.59 - G SR-32 41.3 2.1 0.5 3/2 4.3 13.1 13.9 0.9 57.6 0.70 28.9 51.8 2110 6.38/75.60 - G SR-45 45.7 2.3 1.5 4/2 4.1 8.3 9.7 0.9 42.0 0.71 32.4 53.5 2015 6.36/75.59 - G SR-46 40.8 2.0 2.7 3/3 9.2 10.7 11.3 0.9 49.2 0.74 30.2 60.0 1900 6.36/75.59 - G SR-48 32.2 1.6 1.8 4/3-1 4.6 14.7 15.3 1.0 49.1 0.70 22.5 51.8 1710 6.35/75.58 – G SR-44 26.6 1.3 3.8 3/2 3.5 16.7 15.9 1.1 45.3 0.69 18.4 50.6 1640 6.34/75.58 - G

Figure 3-1. Shaded relief map of the Northern Andes, Colombia. The Andean mountain chain in Colombia is divided into three separate ranges: Cordillera Occidental (Western Range), Cordillera Central (Central Range), and Cordillera Oriental (Eastern Range). The Antioqueño Plateau in the northern Central Cordillera is part of the Antioqueño Eastern Massif, which is bounded by major fault zones R=Cauca-Romeral fault system to the west and P=Palestina fault system to the east. The Guaicáramo (G) and Boconó (B) faults define the limit between the Andean Block and the autochthonous South American Plate (craton).

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Figure 3-2. Geology of the study site. Area dominated by the polymetamorphic Cajamarca Complex (Pz), small patches of marine sedimentary sequences early Cretaceous in age (Kisp, Ksls, Kissl) and Late Cretaceous calk-alkaline intrusion of the Antioqueño Batholith and associated stocks (Ksta, dotted grey). Other units include: Q=Quaternary fluvial and alluvial deposits; Ngc+gas=Neogene sediments from Combia Formation (volcaniclastic continental) and Amagá Formation (fluvial); Ngro=Mesa Formation (detrital, continental). Ksvb=Volcanic Barroso Formation (volcanic, marine) and Ksu=Urrao Formation (clastic-chemical, marine) belonging to the western oceanic province. Jsde=Segovia Batholith, Jts=Sonsόn Batholith. P =metamorphic rocks of Neoproterozoic age. The Cauca Almaguer Fault (paleosuture) represents the boundary between the Eastern Continental Province (Central Cordillera) and the Western Oceanic Province (Western Cordillera). (Geologic information was adapted from González, 2001; Tapias et al., 2006).

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Figure 3-3. Three dimensional representation and simplified topographic/geologic cross section across the Antioqueño Plateau. The flat portion of the plateau has an average elevation of ~2500 m. MP=Medellín–Porce fluvial valley, CG=Cauca Gorge (CG). The CG coincides with the Cauca-Romeral (Cauca-Almaguer) fault system. Sampled profiles indicated with black and open small circles. Páramo de Belmira within Las Baldías Range, ~3600mof elevation. (Shaded relief DEM was generated from GTOPO-30 elevation data. Geologic information was obtained from González, 2001 and Tapias et al., 2006.).

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Figure. 3-4. Modern tectonic setting. Generalized map of tectonic plates and crustal blocks around the Northern Andes of Colombia. Arrows and numbers indicate, respectively, the direction and velocity of plate’s motion. (Adapted from Cortes and Angelier, 2005; Coates et al., 2004; Taboada 2002; Duque-Caro, 1990; and Trenkamp et al., 2002).

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Figure 3-5. Apatite (U-Th)/He Age-elevation Relationship. Relation between apparent apatite (U-Th)/He ages and elevation integrated for La García (black squares) and Matasanos (gray triangles) vertical profiles (AER). Error bars are ± 2σ. Bottom of the exhumed He-PRZ marked by inflection point at ~1500 m. Upper boundary of the He-PRZ appears to be beyond the maximum elevation sampled. Slopes and R2 values of the regression lines by segments appear in front of the segment’s symbol (S1, S2, and S3) for the two exhumation pulses discussed in the text. Block arrows in gray indicate the duration of the Pre-Andina II and Proto-Andina orogenic phases of Van der Hammen (1960).

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Figure3-6. Relation between apparent (U-Th)/He and fission-track ages in apatite. Error bars are ± 2σ. Gray shadow and block arrows indicate the extent of the Pre-Andina Phases I and II (Van der Hammen, 1960). (AFT ages from Saenz, 2003).

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Figure 3-7. Simplified sketch illustrating the extent of the crustal section removed since ca. 25 Ma (dotted area). Left panel represents crustal conditions at the onset of major exhumation 25 million years ago. Right panel is the actual condition with remnants of a Miocene surface that has begun to be obliterated by uplift related denudation during the most recent Andean phase (Eu-Andina II-IV, 12 MA to present). Deep and localized incision of the Medellín and Porce rivers is shown. Rock uplift discussed in the text is represented by the displacement of point A to A’. Bottom of He-PRZ represents the paleodepth at which A-He ages were zero before onset of rapid exhumation at 25 Ma. PB=Páramo de Belmira, AP=Antioqueño Plateau.

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Figure 3-8. Simplified cartoon of cross-sections depicting the paleogeographic and geologic evolution of the Northern Andes at the latitude of the study site. (a) Hauterivian- Coniacian: the ancestral Central Cordillera was covered by a shallow sea and marine

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sedimentary units were being deposited. The Proto-Western Cordillera was part of the subducting plate separated from the Central Cordillera by the ancestral Romeral Fault System. (b) Santonian-Maastrichtian: Increased convergence rates lead to the magmatism of the Antioqueño Batholith, a shallow (epizonal) intrusion emplaced into Paleozoic-Precambrian crystalline basement and lower Cretaceous marine sequences (contact metamorphism). Initial phase of uplift in the region. Central Cordillera emerges leading to a regional unconformity and becomes source of sediment to eastern and western marginal basins (Proto Magdalena and Proto Cauca basins respectively). Progradation to the east begins in the eastern margin of the Central Cordillera. Subsidence and sedimentation are prevalent over the region of the Proto Eastern Cordillera. (c) Paleocene, Laramic Phase, 60 Ma: Generalized coastal deposition took place, inversion of the eastern margin of the Eastern Cordillera basin is now pronounced. Central Cordillera continued to be gradually uplifted. Initial phases of uplift of the Western Cordillera begin, the Tahami terrain is now accreted. Reactivation of vertical movement along the Cauca-Romeral and Palestina fault systems. (d) Lower to Middle Eocene, Pre-Andina Phases I-II, 54-45 Ma: climax of pre-Andean Orogeny in Colombia. No deposition taking place in most of Colombia. Exposure of the future Middle Magdalena Valley as depocenter. (e) Oligo-Miocene transition, Proto-Andina Phase, 24-21 Ma: Major phase of uplift across the Colombian Andes, uplift on the three cordilleras. Amagá formation begins to develop in the Cauca depression. Western and Eastern cordilleras still low standing. Localized subsidence in the Cauca and Magdalena depocenters. Nazca plate subduction begins after splitting of the Farallon plate. (f) Miocene to Present, Eu Andina Phases I-IV, 18-0 Ma: Major uplift and initial inversion of the Eastern Cordillera by reactivation of inherited Jurassic–Cretaceous steep and deeply rooted normal faults. Amagá formation is folded and the Cauca valley is narrowed by indentation of the Panama- Chocó block, Colombian Andes attain the actual topographic configuration. All of overburden over the Antioqueño Batholith on the AP has been removed.

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Figure 3-8 Continued.

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CHAPTER 4 FURTHER CONSTRAINTS ON LONG-TERM EROSIONAL EXHUMATION OF THE “ALTIPLANO ANTIOQUEÑO”, NORTHERN ANDES, COLOMBIA, BY LA-ICP-MS APATITE FISSION TRACK ANALYSIS

Introduction

Meso-Cenozoic geologic evolution of the Andean chain appears to be strongly determined

by subduction dynamics between the South American and Nazca (Farallon) plates [18-20, 36, 40,

47, 315]. In the northern Andes (Colombia, Ecuador and Venezuela), the additional influence of

the Caribbean plate, exotic terrains, and major lithospheric blocks caused differences in

deformation, uplift and exhumation relative to areas of the Andes to the south [25, 26, 113, 164,

356, 367]. Effort has been devoted to understanding the timing, magnitude, spatial distribution,

and nature of uplift and erosional denudation in the context of orogenic development in the

Andean Range [16, 30-32, 35, 36, 38, 39, 113, 349, 368, 369]. The majority of studies, however,

were concentrated in the Central Andes of Peru-Bolivia, with little attention given to the

Northern Andes. Geochronology, low temperature thermochronology and isotopic studies have

been completed in the Central Andes, particularly around the Central Andean Plateau [31, 38,

216, 370, 371], but few investigations have been carried out in the Northern Andes of Colombia.

Tectonic forcing is considered to have been the driver of orogenesis and concomitant

erosion and exhumation pulses in the Andes throughout the Cenozoic [16, 36, 39, 346]. It has

also been proposed that climate- dominated over tectonic uplift as the controlling mechanism for

continental erosion in tropical South America since the Middle Miocene [347] and worldwide, particularly during the past 5-10 million years [51, 52]. Nevertheless, the chronology of

Cenozoic denudation for sectors of the Northern Andes is poor. This precludes an understanding of the relative roles of tectonic and climate in modulating erosion dynamics and exhumation in this important region. Recent apatite (U-Th)/He data from the Antioqueño Plateau (AP)

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demonstrate the utility of high-elevation erosional surfaces for studying interactions between

continental-scale tectonics and erosion [294], particularly the synchronous character of well

documented increased convergence between Nazca (Farallon) and South American plates in the

Oligocene and a marked pulse of exhumation in the AP ~25-23 Ma.

Erosion is a primary mechanism of exhumation, and hence cooling, of deeply buried

crustal material [90]. Spatial patterns of bedrock cooling ages in mountain ranges can be used to map variations in erosion rates and potential relationships with geomorphic or structural features, tectonic input, climatic gradients, and/or kinematic models for material flow through orogenic wedges [54]. Low temperature thermochronology is an effective tool to quantify rock uplift and exhumation [54, 171, 183-185]. With temperature sensitivity between ~110 and 60˚C, apatite fission track analysis (AFTA) enables constraint of the movement of a rock parcel relative to a shallow thermal reference frame (upper 4 km of crust). AFTA, which involves dating and examination of fission-track length distributions, can constrain the timing and intensity of

erosional episodes at the regional scale and on timescales of ~105-107 years [86, 91, 171, 172,

174, 177, 184, 187, 188]. This spatio-temporal domain is germane to understanding key aspects

of orogenic evolution (e.g., mountain range growth and decay cycles, erosional surface

development and uplift, paleorelief, etc.) and tectonics-climate/tectonics-exhumation feedback

response times [54, 81].

In this investigation, apatite fission track data from vertical profiles in the AP are used to

further constrain patterns of long-term erosional exhumation. The objective is to improve the

chronology and understand the magnitude of exhumation in the AP during the Cenozoic, with

emphasis on a potential exhumation pulse incompletely pinpointed by previous AHe data [294].

Integration of high-resolution AFT and AHe data is applied to constraining landscape evolution

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of the AP by documenting periods of significant crustal thermal change due to denudational cooling, and intervals during which the landscape remained relatively stable both geomorphically and tectonically, thus allowing planation surfaces to develop. Analysis of two vertical profiles permits identification of important relationships between changes in convergence rates and patterns of erosional exhumation in the Colombian Cordillera Central. It also provides quantitative data on long-term erosion rates for the region during periods of both enhanced tectonic activity and relative quiescence.

Study Site

The study was undertaken in the northern segment of the Cordillera Central, Colombia, a portion of the orogen that encompasses an anomalously large, high-elevation erosional surface, i.e., the AP [12]. The distribution and extent of relict landscapes can shed light on the environments in which they formed, making them an integral component of stratigraphic and morphotectonic reconstructions [83, 169, 348, 350, 372-374]. Quantifying erosional and kinematic evolution of orogenic plateaus, however, has been limited by insufficient age constraints on their erosional and deformational histories. Furthermore, most investigations have focused on large plateaus such as the Himalayas and the Bolivian Altiplano [216, 349, 370, 375,

376].

Nearly all mountain ranges in active tectonic settings, e.g., the Andes, are characterized by significant relief, rugged topography, and high erosion rates [62, 98, 108-110], making them a focus of scientific study [82]. Throughout the Colombian Andes, large amounts of detrital material in modern fluvial systems such as the Magdalena River [121], along with the thick, continental Cenozoic sedimentary formations [14, 16, 168: Reyes, 2002 #60], indicate that this mountain belt has been subject to intense erosional exhumation by fluvial activity. Orogenic response to tectonic and/or climatic perturbations via erosional exhumation often displays spatial

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and temporal differences within a single orogen due to geomorphic, lithologic, structural, and/or

climatic controls [38, 62, 99, 106]. The steep morphology of a tectonically active mountain range

can be abruptly interrupted by extensive, high-elevation erosional surfaces, i.e., elevated

plateaus. Such features are usually interpreted as paleosurfaces (relict landscapes) and are

believed to represent portions of an orogenic system that has not fully adjusted to present

erosional conditions [377], thus producing marked gradients in the spatio-temporal distribution of erosion/exhumation. Lagged geomorphic response may allow relict landscapes to become decoupled from modern tectonic conditions and therefore become passive markers of vertical displacement of the earth’s surface. Such surfaces also preserve information about long-term erosion dynamics and adjustments of the geomorphic system to past tectonic forcing [83, 96].

Morphotectonic and lithologic characteristics of the AP are exploited in this study to conduct the first high-resolution AFTA along two separate vertical profiles in the Northern Andes and to infer the geomorphic history of this portion of the Andes in response to tectonic input.

Geomorphologic Considerations

The AP is the largest high-elevation, erosional surface in the Northern Andes [12, 28]. This plateau has a broad elevated surface (>5000 km2) of subdued topography (slopes <10°; local

relief <60 m) that is dominated by transport-limited fluvial erosion. The AP is in obvious

contrast to the mountainous rouged topography typical of the tropical Andes of the Occidental

and Oriental cordilleras and southern segments of the Cordillera Central whose geomorphic

evolution is landslide-dominated [28]. Higher elevations and relatively steep relief within the AP domain are restricted to a small range in the western edge of the plateau roughly coinciding with a belt of Paleozoic metamorphic rocks (Figures 4-1 to 4-2). Most of the surface is mantled by thick horizons of deeply weathered granodiorite (gibbsite and kaolinite as final weathering products) with occasional, outstanding outcrops of intact bedrock in the form of tors, rock-

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castles, and inselbergs characteristic of etch topography [12]. The plateau topography is abruptly interrupted in the west and northwest by the deep, mountainous relief of the Cauca River canyon, approximately coinciding with the Cauca-Romeral fault system, with drops in elevation of

~2500-3000 m over a few km. To the east, the AP terminates as a series of surfaces that step down into the wider Magdalena valley, towards the Palestina fault system. Southwards, the AP transitions into the typical sierra-like topography of the southern Cordillera Central marked by volcanic summits [12, 378] (Figures 4-1 and 4-3). The Medellín-Porce River has incised the central portion of the AP creating a relatively deep canyon with differences in elevation of >1300 m from the plateau top to the valley bottom. Perched fluvial systems are common throughout the regional scarps that define the boundaries of the AP relative to the Medellin-Porce River.

Structurally, the plateau belongs to a discrete crustal block, slightly tilted to the southeast [12,

28], with minor internal perturbation [22], and bounded by major, sub-vertical crustal structures associated with the Cauca-Romeral and Palestina fault systems (Figures 4-2 and 4-3) to the west

and east, respectively [22, 23, 27].

As a disequilibrium domain, the AP is clearly discernible from typical equilibrated regions

of the Colombian Andes around it and displays geomorphic features that resemble a relict

landscape, indicating that the region has not adjusted to new morphotectonic boundary

conditions. Such features include subdued hill slope and channel gradients [12], dominance of

transport-limited fluvial erosion [28], and relatively low long-term erosion rates, i.e., 0.02-0.008

mm/yr [294]. Morphotectonic evolution of the AP and potential lithologic control by the

Antioqueño Batholith have attracted the attention of geologists since the 19th century [12, 27,

296-298, 378, 379]. Following a classic Davisan approach [380], it was proposed that this relict surface developed near sea level and was subsequently uplifted to its present position [12, 28].

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The definition of “relict surface” in this study is restricted to low-relief portions of the landscape

within the fluvial network. This differs from earlier works because it is assumed that relief on

upland surfaces is set by fluvial erosional processes responding to a common base level set by

major trunk streams.

No remnants of the primordial (possibly Paleocene) surface sensu stricto remain in the AP because the landscape was denuded during multiple Cenozoic exhumation events [294].

Relatively little is known about the chronology of uplift and exhumation pulses, the monotonic or punctuated nature of such events, the non-equilibrium character of the surface, and the morphotectonic evolution of this area in the context of regional geodynamics. Previous results of

AHe analysis in the region highlight the episodic nature of denudation and a strong response to tectonic input [294]. AFT data presented here refine our knowledge of this decoupled geomorphic domain and constrain the Eocene exhumation pulse, incompletely resolved by AHe thermochronology [294].

Geological Considerations

The Colombian Andes represent the northern termination of the Andean belt. At ~ 1.5°N latitude, the mountain chain flares out into three ranges: the Cordillera Occidental, Cordillera

Central and Cordillera Oriental (i.e., western, central and eastern cordilleras). The Cauca River depression separates the Occidental and Central cordilleras, while the Magdalena River valley marks the boundary between the Central and Oriental cordilleras (Figure 4-1). The present trident pattern of the Colombian Andes and its diverse lithologic and tectonic configuration have resulted from a complex geologic evolution since the Proterozoic, marked by: 1) interaction of major tectonic plates, microplates, crustal blocks, allochthonous terrains, island arcs and ridges

[278, 283, 316, 367, 381]; 2) marine transgressions and regressions [289, 290, 382]; 3) magmatic/volcanic pulses and tectometamorphic events [44, 47, 286], and 4)

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removal of crustal material via erosion and subsequent sedimentation in marine and continental

environments [14, 17, 168].

Two distinct geologic domains are separated by the Cauca-Romeral Fault system (Figure

4-2), which is a Lower Cretaceous suture zone [278, 284] between oceanic blocks and the autochthonous margin of South America [278, 281]. The western province, comprising the

Cordillera Occidental and lower western flank of the Cordillera Central, consists of accreted oceanic crust. The eastern province, which encompasses most of the Cordillera Central and the entire Cordillera Oriental, is characterized by continental basement [25, 281, 284]. The

Cordillera Occidental is composed of allochthonous oceanic sequences of basic volcanic rocks and marine sediments of Upper Cretaceous and Cenozoic age, intruded and covered by Cenozoic igneous rocks and volcanic sequences [284]. The Cordillera Oriental is a fold-and-thrust belt of polydeformed Precambrian and Paleozoic metamorphic and igneous rocks overlain by thick

Paleozoic to Mesozoic sedimentary sequences [25, 284]. The Cordillera Central is dominated by

Paleozoic-Proterozoic metamorphic rocks intruded by major granitic plutons of Permian to Late

Cretaceous age and overlain by Late Neogene to Quaternary volcanic rocks. Segments of the

Cordillera Central south of the study site are characterized by modern volcanism related to the subduction of the Nazca plate representing a Mesozoic-Cenozoic plutonic arc [21] (Figure 4-2).

The northern segment of the Cordillera Central (Figure 4-1) is considered part of an exotic terrane [283, 383] affected by several tectono-metamorphic episodes related to the Hercynian orogeny in Permo-Triassic times, a Devonian–Carboniferous tectono-metamorphic event, and the beginning of the Andean cycle in the Cretaceous [279, 286, 352]. Predominant lithologic units within the study site include a Paleozoic-Mesozoic polymetamorphic complex, elongated

Jurassic batholiths, small outcrops of shallow marine sedimentary sequences of early Cretaceous

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age, and extensive Late Cretaceous plutons (Figure 4-2). The polymetamorphic complex [286] is composed of low- to medium-grade metamorphic rocks of the Cajamarca Complex, and high- grade metamorphic rocks of the El Retiro Group and Las Palmas Gneiss [21, 287]. Isolated patches of possible Precambrian rocks within the Paleozoic basement have also been mapped

[290]. The lower Cretaceous marine transgression that covered the crystalline basement is represented by fossiliferous sedimentary sequences such as the San Luis and San Pablo [289-

291]. Metamorphic and sedimentary units are intruded by large, non-elongated, epizonal, calk- alkaline intrusions, such as the Antioqueño and Ovejas batholiths [27] that are the target lithologies of this investigation (Figure 4-2). Cenozoic sedimentation in the Cauca and

Magdalena basins is continental and strongly coupled to uplift-exhumation of the Cordillera

Central since the Paleocene [14, 16]. All sedimentary sequences underwent Miocene faulting and ample folding. The Cauca and Magdalena basins exhibit Quaternary deposits with variable thicknesses and degree of consolidation and with evidence of neotectonic activity [384]. Late

Neogene volcaniclastic sequences associated with modern volcanic activity in the Cordillera

Central mantle the region [12, 292].

Available geochronology for the Antioqueño Batholith displays a wide range of ages, from about 20 to 100 Ma. Such variation is attributable to the variety of closure temperatures of the geochronometers employed. Data include biotite K/Ar ages which range from 68±3 Ma to 90±6

Ma [301, 354], biotite Rb/Sr ages from 56 to 66 Ma [304]; two whole-rock Rb/Sr ages of 82±8 and 98±27 Ma [305]; zircon fission-track dates from 49.1±2.5 to 67.1±2.1 Ma, fourteen apatite fission-track ages varying between 49.1±1.2 and 28.7±1.5 Ma [166], and apatite Helium ages from 48.9±2.4 to 22.8±1.1 [294]. The most widely accepted crystallization age is ~98 Ma [305].

However, recent zircon U-Pb thermochronometry better constrains the age of Late Cretaceous

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magmatism (LA-ICP-MS on 90 zircons from the Antioqueño and Ovejas batholiths), yielding

crystallization ages of 74±3 Ma [355].

Tectonic Considerations

The Colombian Andes are situated at the junction of four major lithospheric plates: South

America, Nazca, Caribbean and Cocos [25, 26]. Cenozoic regional tectonics have been

dominated by the subduction of two oceanic plates, Nazca (Farallon) and Caribbean, beneath

continental South America, as well as by interactions of two lithospheric blocks known as the

North Andean and the Panama-Chocó (Figure 4-3) [23, 356]. Significant changes in convergence

between Nazca (Frallon) and South America have been reported, notably two increases in the

Cenozoic: ~50-44 Ma when rates reached ~165 mm/yr at the equator, and ~24-19 Ma with

maximum convergence of ~110 mm/yr [18, 19, 42]. Variations in the patterns of subduction, e.g., convergence rate and direction, dipping of the subducting slab, etc. have exerted important controls in the timing, magnitude and type of associated uplift, exhumation, magmatism and deformation on the overriding plate throughout the Andes [18, 19, 36, 44, 47, 314].

The Nazca plate is currently being subducted beneath South America along the Colombia-

Ecuador trench at a rate of ~54 mm/yr, while Caribbean subduction takes place from the northwest at ~20 mm/yr, at a shallow angle, and without an associated volcanic arc [23, 25, 26].

The North Andean Block is escaping rigidly in a NE direction at ~6 mm/yr and the Panama-

Chocó block is moving to the east relative to South America as a rigid indenter in active collision at a rate of ~25 mm/yr [26]. The Panama-Chocó block has played a crucial role in the morphotectonic evolution of the region for the last 12 Ma, producing most of the present structural relief [24, 25], narrowing the northern portion of the Cauca Valley that led to the development of the deep Cauca canyon [384], and driving the Late Miocene-Pliocene orogenic phase in the Colombian Andes [16, 24, 156].

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These relative motions of plates and blocks generated a complex stress field and

heterogeneous deformation in which crustal shortening takes place in a complex mosaic of

tectonic blocks, each with its own uplift history [156, 160, 385]. Transpressional deformation has

caused a mixture of strike-slip and vertical displacements along major crustal structures throughout Cenozoic time. The former is responsible for differential uplift of discrete portions of the orogen, while the latter led to the development of pull-apart basins, particularly along the

Cauca-Romeral fault system [384, 386]. These processes are central to the interpretation of thermochronology datasets in the AP because the cordilleran segment of this study behaves as a discrete coherent block bounded by two major, inherited crustal structures: the Cauca-Romeral fault system to the west and the Palestina Fault system to the east (Figures 4-2 and 4-3). These sub-vertical reverse fault systems are located along the foothills of the Cordillera Central and are thought to be rooted beneath the range [23, 25]. The Cauca-Romeral fault system coincides with the Cauca River depression and combines strike-slip and reverse movement. The Palestina fault system marks the east boundary of the Cordillera Central. This portion of the Cordillera Central is asymmetric, with a much steeper western flank along the Cauca Inter-Andean canyon.

Neogene transpression is left lateral north of 4°N [23, 25]. The western margin has been uplifted by transpressive movement along faults dipping eastward that belong to the Cauca-Romeral fault system [25]. Lithospheric strength of this block is a potential controlling factor on the low internal distortion and the circular (i.e., not elongated) shape of the Antioqueño Batholith [22], a situation that may have imposed litho-structural control in the development of the AP [355].

Uplift and Denudation in the Colombian Andes

It is generally accepted that the Cenozoic and Quaternary are associated with major geomorphic and tectonic events that have had a marked influence on the present topographic and paleogeographic configuration of the Northern Andes [16, 43, 113, 156, 158]. Uplift and

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concomitant denudation throughout the Andean chain [31, 32, 36, 40, 111] seem to be spatiotemporally related to periods of accelerated convergence [18, 19, 294]. However, morphotectonic evolution of the Colombian Andes is poorly constrained [113, 156]. Most studies of uplift and erosional exhumation concentrated on the Cordillera Oriental and applied chiefly stratigraphy and palynology, with a minor contribution from thermochronology [13, 34, 157-

160]. Pioneering sediment-stratigraphic studies aimed at unraveling Andean orogeny were done

in the Cordillera Oriental between 1940-1960 [16, 161, 162] and led to recognition of discrete pulses of erosion that affected the entire Colombian Andes from Paleocene to Mio-Pliocene times [16]. Periods of enhanced denudation were attributed to uplift and were named “orogenetic phases” Laramic (~60-57 Ma), Pre-Andina I-II (~54-40 Ma), Proto-Andina (~25-22 Ma), and

Eu-Andina I-IV (~14-2 Ma) [16]. The magnitude of these denudational events, their varying efficiencies in removing crustal material, and the spatial distribution of erosion and exhumation throughout the orogen remain largely unconstrained.

Geological variations and structural complexity in the Colombian Andes should determine a heterogeneous uplift-exhumation history of discrete crustal “blocks” that can be uplifted and eroded at different rates and at different times. Diachronous uplift and exhumation are illustrated by sediment-stratigraphic and pollen data that yield different timing of uplift-exhumation for the

Eastern and Central Cordillera, with the former experiencing major phases of uplift in the last

~12 Ma [15, 158, 163], while the latter experienced denudation since the Late Cretaceous [16].

Subsequent research in sedimentary formations of the Magdalena Valley confirmed this chronology [14, 17, 29]. But spatially-specific information on the timing and magnitude of uplift driven exhumation remains scarce.

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Investigations in the Cordillera Oriental between Colombia and Venezuela, despite limited

thermochronology datasets, suggest uplift in excess of 3 km [164] and major cooling episodes by fission-track data between 22 and 27 Ma [34, 160]. More recent studies in the Cordillera Oriental incorporated apatite fission-track, paleobotany, sediment-stratigraphy and other geological data, and also suggest increased exhumation rates in the northeastern Andes at ~5-3 Ma [295] and asymmetrical denudation of some segment of the range due to orographic effects [159]. Major consideration has been given to the recent phase of uplift-exhumation in the Colombian Andes, i.e., the Eu-Andina panes of Van der Hammen (1960). The present topographic configuration of the Andes has been attributed to uplift-exhumation pulses during this interval [113, 156]. Apatite fission-track ages between Venezuela and Colombia cluster in two sets, ~19-14 Ma and 7-4 Ma, pointing to important exhumation events of the Eu-Andina phase [160]. Similarly, results for rapid cooling with apatite fission-track ages of 9 and 12 Ma and unimodal track-length distributions (mean track lengths of ~14.5 µm) were reported for a small area of the Cordillera

Oriental [163], while apatite fission-track and vitrinite reflection data in the Middle Magdalena valley indicate km-scale uplift and ~3 km of erosion-related cooling at ca. 5 Ma [295]. This

Pliocene uplift has been documented in a core from the Sabana de Bogotá, Cordillera Oriental at

~2500 masl by detailed stratigraphy, palynology, K/Ar and fission track dating [158, 165]. It is now widely accepted that the final and major tectonic uplift of the Cordillera Oriental in the

Bogotá area occurred between ~5 and 3 Ma [15, 113].

Data elucidating uplift and denudation patterns for the Cordillera Central are scarcer.

Kroonenberg et al. (1990) suggested that onset of uplift for the Nevado del Ruiz area occurred

10-4 Ma, supporting their contention using local stratigraphic and geochronology data. For the northern part of the Cordillera Central, a set of restricted apatite fission-track ages were

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interpreted as revealing an Eocene cooling event driven by exhumation [166], but the magnitude

of unroofing and duration of the event are undocumented. This AFT database is insufficient for a

reliable denudation chronology because of the few samples dated, the low amount of confined

natural fission tracks examined, the low number of samples modeled, and the poor spatial

distribution of samples, which were not collected over vertical profiles. In the Cordillera

Occidental, recent uplift phases are suggested by 11-Ma dioritic stocks exposed at ~4000 m.

Low Temperature Thermochronology and Erosional Exhumation

During the last two decades, studies of erosion have shifted from a simple focus on local,

short-term surface processes to looking at a broader range of spatiotemporal scales [8, 9, 61, 62,

66, 69, 84, 86, 92, 109, 127, 133, 170-180]. Progress in assessing erosion at multiple scales is not

only related to a change in philosophy [180-182], but also to the development of several low-

temperature thermochronometric methods, particularly apatite fission-track (AFT) and U-Th/He

(Ahe) dating [54, 171, 183-185]. Due to its low closure temperature range (~60-110˚C), AFT

thermochronology enables determination of rates of rock uplift and denudational exhumation

from shallow crustal depths (upper 4 km) [54, 81, 86, 182, 196] and provides measures of long- term erosion rates and landscape evolution directly from rock samples [86, 89, 171, 172, 183,

184, 188, 190, 195, 196, 200, 202, 203]. AFT low temperature thermochronology can constrain the timing and intensity of erosional episodes at the regional scale and on timescales of ~105-107 years [86, 91, 171, 172, 174, 177, 184, 187, 188]. The technique can be used to understand

tectonics-climate/tectonics-exhumation feedback response times [54, 81] and thus orogenic

evolution.

Apatite Fission Track Analysis

Apatite fission track is a well established low temperature thermochronometer [197].

Interest in this thermochronologic system stems from the fact that AFT is more than a dating

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technique [86, 171, 187, 195, 387, 388]. Apatite fission-track analysis (AFTA) can be applied

not only to rock dating, but to the examination and modeling of track length distributions, thus

defining the thermal history of the samples greatly complementing the chronological constraints

provided by AFT dating [86, 171, 187, 196, 197, 225].

AFT thermochronology is based on the accumulation of ionization damage produced along the repulsive paths left by atomic fragments resulting from decay of 238U. Fission of 238U causes disruption of the mineral lattice, expressed as microscopic linear features (16 µm long and 0.1

µm wide) known as fission tracks [389]. The nearly invisible tracks are revealed by polishing an internal crystal surface and acid-etching so that the tracks can be counted and measured by optical microscopy. The number of tracks in an apatite grain is mainly a function of 238U

concentration in the grain (ppm) and the time over which tracks have been retained, so that the

number of tracks per unit area provides a measure of geologic age [171, 187, 196, 197, 389-391].

When exposed to temperatures >110°C and for geological times of ~107 yr, the tracks in apatite grains are progressively repaired through a process of thermally activated annealing to the point that tracks disappear completely, reducing AFT ages to zero [196, 229-233, 388, 390, 392,

393]. Annealing below 60 °C is insignificant [230]. Incomplete annealing of fission tracks occurs

in the interval between 60 and 110°C, a thermal horizon known as the partial annealing zone

(PAZ). The PAZ was recognized in deep boreholes and along vertical profiles [210, 230]. Ages across the PAZ vary with depth and increasing temperature. Since new tracks form continuously throughout geologic time, the AFT age and the distribution of track lengths in an apatite sample reflect the integrated thermal history of the host rock [171, 232]. Thus, while fission-track density provides a measure of the time elapsed since the crystal last cooled sufficiently to retain

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structural damage, fission-track lengths record the temperature history that the crystal experienced over that time interval.

Apatite Fission-Track Analysis in the Antioqueño Plateau

A common approach to AFT analysis involves sampling vertical profiles such as steep valleys, cliffs, and regional scarps or exploratory boreholes [171, 172, 174, 188, 204-206, 230].

This allows exploitation of the spatial relation among samples in the vertical dimension of the crust, in which lower elevations yield samples that were hotter and younger than their higher elevation counterparts. Further, features such as the PAZ are often recognized in the age- elevation relationships of vertical profiles. When exhumed, the PAZ represents paleocrustal depths and permits to infer the timing of cooling and the magnitude of unroofing from the shape of the age-elevation relationship [171, 172, 174, 184, 187, 201]. The time of a cooling event is usually represented by an inflection point (“break in slope”) in the age-elevation plot. The gradient of segments in the age-elevation profile constrains the rate of denudation, and potentially the amount of rock and surface uplift [91, 171, 172, 174, 190, 213]. Further, the elevation of the inflection point is used to constrain the amount of denudation after cooling.

Reliable vertical profiles for AFT are achievable where heat flow conditions and the geothermal gradient have been stable, where topography has not experienced major changes after samples cooled through the AFT closure temperature, and where erosion has not been high enough to promote major heat advection into the upper crust [93, 360, 362, 364].

In this investigation, geomorphic, lithologic and structural characteristics of the AP were exploited to do AFTA along two vertical profiles. Data were utilized to complement the

Cenozoic cooling history of the AP and to evaluate the response of this elevated plateau to orogenic pulses in the Northern Andes and Caribbean [16, 37], particularly during the Eocene

[294]. Characteristics of the AFT system, such as higher closure temperature and track-length

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distributions can be used in conjunction with AHe data to develop robust reconstructions of time- temperature paths experienced by a rock parcel during erosional exhumation. The double AFTA-

profile strategy also permits confirmation of the coherent response to uplift-denudation of the AP

as a structural lithospheric block. Finally, AFT data establish a long-term erosion rates into the

Paleogene, against which modern erosion [150, 153] can be compared. Although this later issue is not covered in detail in this investigation, estimation of long-term erosion rates is a first step in assessing the role of humans as geomorphic agents, a topic that has great significance scientifically and socially [6, 7].

Methods

Sample Collection and Preparation

A total of 22 granodioritic samples, 2-3 kg each, were collected for AFT analysis. These included granodiorite and diorite specimens from the Antioqueño and Ovejas batholiths retrieved at elevation intervals of ~100 m along the La García and Matasanos-Porce vertical profiles.

These vertical transects are ~40 km apart (Figure 4-1). Sample locations (x, y, z coordinates) were determined with 1:25,000-scale topographic maps, a hand-held GPS unit, and a barometric altimeter. Accuracy was ~5 m horizontally and ~10 m verticaly. Weathering rims were removed with a rock saw to a depth of ~2 cm. Samples were reduced to sand-sized particles using a jaw- crusher and pulverizer, and sieved to select particles between 300 and 60 µm. Apatite concentrates were obtained using standard gravimetric (water table and heavy liquids) and magnetic (Frantz Isodynamic®) mineral separation techniques. Apatite grains were enclosed in flat, square epoxy mounts (~3 x 15 x 15 mm) and cured at 45˚C for 2 hours. Mounts were ground to reveal internal apatite grain surfaces and polished. Etching to reveal tracks was accomplished by immersion in 5.5 N HNO3 at 21 °C (±1 °C) for 20.0 s (±0.5 s). Sample preparation was done at the University of Florida.

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Age Measurements by LA-ICP-MS

Although the external detector method (EDM) has been routinely applied to date apatite

grains by fission tracks [171, 187, 236], a new approach, using LA-ICP-MS is employed in this

investigation [189]. In brief, samples were prepared in a similar fashion as for the EDM.

However, 238U concentration was obtained by direct measurement of 238U and 43Ca by LA-ICP-

MS on the surface where tracks for each individual grain were counted. It has been showed that

43Ca in apatite is stoichiometrically fixed, and the 238U/43Ca ratio, here referred to as P (Greek

capital letter rho), can be considered as analogous to the ratio ρi/ρd (i.e., induced track densities/track density in a dosimeter) from the EDM [189].

The goal of LA-ICP-MS in AFT dating is to determine the 238U concentration from the ratio of 238U/43Ca in apatite grains and calculate cooling ages. The method of spot laser analysis for 238U and 43Ca employed here, analogous to the one described in Donelick et al. (in review), is based on an ablation scheme with the laser beam centered at a fixed point on the area where the tracks were counted. Details of data acquisition parameters used for LA-ICP-MS apatite fission- track dating are summarized in Table 4-1. Spot analysis has advantages over EDM, including rapid set up, grain-to-grain uniformity from target grain to target grain, reliable estimates of the surface uranium concentration, derivation of potentially useful depth-dependent information regarding uranium concentration zoning, minimal damage to the polished and etched surface of target grains, and ability to work with small target grains [189].

To obtain the best estimate of relative U concentration at the analyzed surface, a 3rd-order

polynomial was fitted to the 238U/43Ca ratio versus analysis point at each spot using routine LFIT

Press [394].The 238U/43Ca value is the y-intercept and the 1σ error is calculated from the

covariance matrix. This approach requires determination of an initial, primary zeta calibration

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factor of the desired precision during a single, extensive LA-ICP-MS session (ξMS). For each

subsequent LA-ICP-MS session, a secondary zeta calibration factor, related directly to the

primary factor, is determined.

A total of 50 scans for 238U and 43Ca were used to acquire high-precision data, while

minimizing time spent at each target grain. A scan represents a single cycle of signal counting in

the mass spectrometer to obtain counts for each studied isotope. Of these 50 scans, ~10 were run

while the laser warmed and was blocked from hitting the target grain. During this interval,

background counts were collected. Once the laser began to ablate a target grain, ~5-10 scans

were required to attain full signal status. These scans represent the transition between

background and full signal. For most grains, scans 21 -50 were at full signal. Full-signal analyses

were corrected by subtracting the mean background signal. The initial full-signal scans are most

important for use in AFT analysis because the spontaneous tracks counted form within ~10 µm

of the analyzed mineral surface. Plots of 43Ca cps were used for rapid assessment of instrument instability and data quality over the entire session. Comparing the background to uncorrected- for-background full-signal ratios helped identify sources of instability in the 43Ca cps. For details

on the methodology of apatite fission track dating by LA-ICP-MS see Donelick et al (in print).

Fission-track age of apatite grains were calculated using the ratio of the number of fission

tracks in the grain to the amount of 238U in the same area of the grain [181]. The radiation decay

equation used incorporates the LA-ICP-MS zeta calibration factor [189]. Age calculations for all samples involved LA-ICP-MS dating of the Cerro de Mercado apatite standard (30.6±0.3 Ma),

Durango, Mexico with a MS zeta calibration factor of ξMS= 14.5149 and 1σ ξMS=0.2625. This zeta calibration factor is determined for each sample during each LA-ICPMS session by analyzing the U:Ca ratio in grains from a standard at the beginning and end of each session

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[189]. Durango pooled ages yielded a value of 31.4±0.8 Ma. Fission-track ages of individual

apatite grains were measured at the University of Washington using the LA-ICP-MS. Tracks

were counted at 2000X magnification under un-polarized light at Apatite to Zircon, Inc by P.

O’Sullivan.

Track Length Measurements

Irradiating apatite grains with 252Cf-derived fission fragments can yield a 20-fold increase in the number of available fission tracks for length measurement, enhancing confined fission- track detection and analysis [395]. The second of two polished grain mounts for each dated sample was irradiated with ~107 tracks/cm2 fission fragments from a 50 µci 252Cf source in a vacuum chamber (activity of 252Cf in July, 1996). Irradiated grain mounts were immersed in

5.5N HNO3 for 20.0± 0.5 seconds at 21±1°C to reveal horizontal, confined fission tracks and track lengths were measured. Only natural, horizontal, and confined tracks, with clearly visible ends, were measured for length and angle to the c-axis. Fission tracks were viewed under

unpolarized light at 1562.5x magnification (100x dry objective, 1.25x projection tube, 12.5x

oculars). Length and crystallographic orientation of each fission track was determined using a

projection tube and a digitizing tablet calibrated with a stage micrometer and interfaced with a

personal computer. Precision of each track length was about ±0.20µm, while the precision of

each track angle to the crystallographic c-axis was about ±2 degrees. Approximately 130 fission

tracks per sample were measured.

Annealing Kinetics and Dpar Parameterization

Variations in chemical composition affect the kinetics of fission-track annealing so that

tracks are preferentially retained in chlorine-rich, relative to fluorine-rich apatites [230-232,

396]. Thus, apatite fission track ages and total etched fission track lengths are strongly correlated

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with the composition of their host apatite grain in samples that have experienced significant

residence time at temperatures >70°C [397]. It is therefore critical to include kinetic information

to generate valid interpretations of apatite fission-track data. Separating apatite grains into

kinetic populations enhances the reliability of the analysis and increases the thermal information

derived as multiple kinetic populations simultaneously constrain the thermal history of the

sample [226-228]. In this investigation, the parameter used to quantify solubility is termed Dpar.

Commonly specified in microns, Dpar refers to the mean fission-track etch pit diameter parallel

to the crystallographic c-axis for each apatite grain [196, 227]. Fission tracks in apatite grains exhibiting the smallest Dpar values usually anneal more quickly than fission tracks in apatite grains having larger Dpar values. In an apatite grain having a Dpar value near 1.50 µm (a typical fluorine-rich apatite), fission tracks generally do not survive geological heating above ~100°C. In an apatite grain having a Dpar value near 3.00 µm (a typical chlorine-rich apatite), fission tracks may survive geological heating >150°C [227]. Age and track-length measured apatite grains were classified for annealing kinetics using Dpar. Between one and four Dpar values were recorded and a mean Dpar value was determined for each grain measured for age or track length.

Dpar values were used for modeling the time temperature paths with HeFTy® [224] and

AFTSolve® [225, 234].

Fission-track Modeling

The grain-age distribution and component ages, track length distribution and grain composition proxy data (Dpar) were combined to constrain a sample’s thermal history with inverse modeling. AFT data was modeled using AFTSolve ®[225, 234] and HeFTy® [224].

AFTSolve uses search methods to quantify the range of statistically acceptable and better thermal histories for a sample that 1) adheres to user-defined constraints and 2) matches the

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measured AFT data. For samples in this AFT study that possess previous AHe data [294],

HeFTy® was utilized to derive the maximum amount of information possible from

thermochronometric data through forward and inverse modeling. An open-ended model with

minimal constraints was used initially and restrictions directed by the successive modeling

results and relevant geologic data, such as stratigraphic age, the samples pooled or component

AFT age(s), and AHe age(s) were subsequently imposed. A multi-kinetic annealing model was

used, allowing for modeling of multiple kinetic apatite populations with Dpar [226]. For the

initial general model, 15,000 simulations were run with a controlled random search (CRS)

technique for each sample. All age and track-length data were modeled as one kinetic population

projected to the c-axis to effectively remove the problems of anisotropic track-length reduction

[228]. Default initial track lengths were implemented as derived from Dpar. Calibration parameters for AHe ages were those of Durango apatite [220].

Results

LA-ICP-MS apatite fission track data and analytical results are presented in Table 4-1.

AFT data were derived from a sufficient numbers of grains (n>20 for age, n>100 for track length) and uranium concentrations to produce statistically sound age populations [192, 196].

Pooled fission track ages are. All uncertainties for AFT data presented herein are ±1σ, unless otherwise stated. Age spread was determined using the Chi-square test, which indicates the probability that all grains counted belong to a single population of ages [387]. All apatite fission track data passed the Chi-square test at 5%, implying that variations in fission-track age are due to inherent variability of the fission decay process and analytical conditions. AFT ages are much younger than formation ages of the sampled plutons [355] so that differences in grain ages are a result of thermal annealing due to denudational cooling that occurred long after batholith emplacement into relatively cool country rock and at shallow crustal levels near the Campanian-

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Maastrichtian. The majority of the samples AFT ages are also older (or concordant) with previous AHe ages [294] with the exception of three samples (SR-24, SR-26, and SR-29), which are discussed below. Dpar measurements show rather homogenous values and hence similar annealing kinetic characteristics for all of the samples along both profiles so that age differences are not related to compositional differences [226]. Most of the Dpar values for both profiles

(Mean = 1.42 μm) cluster around those of Durango (Mean = 1.5 μm), while exhibiting low dispersion (STDV= 0.096 μm). Thus, track annealing parameters of Durango are reasonably representative of the samples in this study, and less retentive than apatites having a Dpar value near 3.00 μm.

Samples between ~1640 and 2500 m were analyzed for the La García profile. Along this altitudinal range, apatite fission track ages vary between 35.5± 2.4 and 48.9± 3.0 Ma. Considered at 2σ, ages are virtually concordant, with a mean age of ~40 Ma (Figure 4-4). Average mean track length is ~14 µm with standard deviation of ~1.5 µm. Mean track lengths display a weak correlation with age (r=0.32), with older samples tending to exhibit longer track lengths than younger samples. This pattern, as discussed latter, is more clearly displayed when samples SR-

24, SR-26, and SR-29 are excluded from the analysis, when the Matasanos-Porce profile is considered alone, or when the two profiles are analyzed simultaneously.

Samples from elevations of ~700 to 2500 m in the Matasanos-Porce profile were analyzed.

Fission track ages range between 30.4± 3.6 and 47.7± 3.1 Ma. Ages are essentially concordant at

2σ, with a mean pooled age of 41 Ma (Figure 4-2). Average mean track length is ~14 μm with a standard deviation of ~1.4 μm. Mean track lengths along the profile display good positive correlation with age (r=0.91) so that older samples showing longer track lengths than their younger counterparts (Figure 4-5).

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Fission track data for both profiles display similarities with respect to age and confined

track-length distribution (Figures 4-4 to 4-7; Table 4-2). Such similarities are reflected by the

nearly identical range in single grain ages (~49-30 Ma), the narrow and unimodal track-length

distributions with standard deviations < 1.5 μm, the mean confined track length values of ~14

μm, the restricted number of confined tracks with lengths <10 μm, and the correlation between

tarck length and age (i.e. longer tracks with older grains and shorter tracks with younger grains).

The age-elevation relationships are consistent with previous AHe in both transects with the

exception of sample SR-26 (38.2 Ma, 2350 m), which yielded and AFT age younger than its

AHe counterpart. The profile dates exhibit a positive correlation between age and elevation

(r=0.73 for Matasanos Porce and r=0.52 for La Garcia). Because the region experienced

negligible changes in topographic parameters, except for the localized incision of the Medellín-

Porce River, it is not necessary to correct AFT ages using the admittance ratio (Reiners et al.,

2003). In addition, U and Th concentrations, the crystallization age of the plutons sampled, and

the range of apatite ages fall within the figures over which negligible discrepancies between AHe

and AFT systems have been reported [192].

Interpretation and Discussion

AFT data from the upper portion of both profiles (i.e., 1700-2400 m) are fundamentally

concordant at 2σ, implying rapid exhumation (Figure 4-4). Age elevation relationships of the

Matasanos-Porce profile record rapid cooling between ~50-40 Ma followed by a period of stability until at least 30 Ma. The two intervals of exhumation and quiescence are defined by regression lines through two sub sets of the data (samples above and below ~1600 m, excluding anomalously young AFT data, see Figure 4-4) whose intersection yields an inflection point at

~41 Ma. This slope change may mark the upper boundary of the A-PAZ (60˚C) exhumed in

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Eocene time. The La García profile shows largely concordant ages and an interval of rapid

cooling between ~45-40 Ma (Figure 4-5).

The Eocene cooling event occurred long after intrusion of the Late Cretaceous Antioqueño and Ovejas batholiths [355]. AFT data indicate that all analyzed samples resided at paleotemperatures in excess of the AFT closure temperature (>110˚C) before the onset of rapid cooling. High values for mean track lengths (~14 μm) as well as unimodal and narrow track length distributions (SDTV ~1.5 μm) further support the interpretation of rapid Eocene cooling through the apatite PAZ. In addition, Apatite fission-track modeling (AFTSolve®) for the majority of the samples shows a marked cooling event between ~50-43 Ma (Figure 4-7).

Minimum dates for initiation of cooling for the 22 samples modeled converge at ~43 Ma.

Furthermore, HeFTy® models indicate that samples were cooled in the Eocene through the apatite fission track PAZ and the apatite Helium PRZ, i.e., from >120˚C to ~40˚C (Figure 4 -8).

Concordant AFT and AHe ages for both profiles over a crustal section of ~1500-2000 m imply that rocks were exhumed from temperatures >110-120°C to below the AHe closure temperature

60-70°C (Fugures 4-4 and 4-5). That is to say, rocks cooled ca. 50°C in 5 million years, which, at a gradient of 25 °C/km, implies unroofing of ~2 km, or ~0.4 mm/yr. This erosion rate is comparable in intensity to the Oligocene-Miocene pulse previously identified in the region by

AHe analysis and about an order of magnitude higher than average, long-term denudation rates

[294].

Three AFT samples from higher elevations in The La García profile SR-24 (37.7 Ma, 2500 m), SR-26 (38.2 Ma, 2350 m), and SR-29 (35.5 Ma, 2250 m) are offset in the age elevation relationship towards younger ages relative previous AHe profiles [294] and AFT ages for their elevation range (Figure 4-4). Sample SR-26, dated by both methods, shows an AFT age

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significantly younger than a previously reported AHe date (46.7±2.3 Ma). This discrepancy

cannot be solely explained by rapid cooling of this crustal section during erosional exhumation.

Even though SR-24 is weathered, grain quality is similar for the three samples. A possible way to

explain the presence of these outliers might be through a

vertical displacement along a structure that would have uplifted these samples relative to the rest

of the profile. There is, however, no evidence for faulting within the profile and maps of the area

report no structures in this portion of the AP. In addition, previous AHe profiles yielded a

consistent trend in the age-elevation relationship for this portion of the profiles. Excessive

retention of He is quite improbable given the age of the plutons and the concentrations of U and

Th in apatite crystals from this suite [294]. Thus, transect samples likely belong to the same

structural block. Furthermore, crustal rigidity and structural coherence of this lithospehric block

was indicated in previous studies [22].

An important aspect of the interpretation of AFT data revolves around the time of rapid

erosional exhumation. Within ±2σ errors, all ages cluster ~41 Ma. Detailed observation of the

data set (Figure 4-6) shows a positive correlation (r=0.91 for The La García profile) between

samples with the longest tracks (between 14.0 – 14.3 μm) and the oldest AFT ages (between 45-

50 Ma). Conversely, samples with the shorter mean track lengths (13.7-13.3 μm) possess younger ages (30-35 Ma). This relationship is indicative of a group of samples that cooled

rapidly at ~50-45 Ma from temperatures > ~110˚C to below 60˚C, followed by an interval during which some of the lower samples in the profile remained at annealing temperatures of ~65–85˚C, resulting in reduced ages and track lengths. This is further supported by the AFT Solve® and

HeFTy® modeling results (Figures 4-7 and 4-8). Modeling of AFT data from samples with the highest mean track-lengths yielded earlier cooling times between ~50-45 Ma, whereas younger

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samples remained at higher temperatures, experiencing partial annealing and recording later

cooling.

Slightly shortened tracks at lower elevations suggest that after rapid cooling in the Middle

Eocene there was subsequent cooling. However, none of the samples record this younger event

in detail. A major phase of Oligocene uplift and denudation of the AP is clearly displayed by

AHe data [294], but is not resolved by AFT (FIgura 4-4). Previous AFT and ZFT low

temperature data led to the conclusion that the region did not experience uplift and denudation

during the Oligocene [166]. However, AHe ages [294], which are sensitive to lower

temperatures, resolve two Andean Orogenic Pulses for the Colombian Andes (Pre-Andina II-42

Ma and the Proto-Andina-23 Ma [16]) and allow refuting the previous conclusion. In other

words, the higher closer temperatures of both AFT and ZFT do not enable resolution of the

younger part of the cooling history. In addition, the vertical profile approach in this study shows

a flattening of the age-elevation relationship (break in slope) from ~45 to 30 Ma (Figure 4-4), that is consistent with previous AHe profiles [294]. This interval coincides with a period of tectonic quiescence related to a decrease in the rate of convergence between Farallon and South

America [18, 19]. Such periods of relative tectonic calm provide enough time for full development of erosional surfaces on which Oligocene sediments of the Terciario Carbonífero de

Antioquia (Amagá Formation) were deposited [168]. The Eocene pulse, poorly delineated by the

AHe dataset [294], is clearly recorded by AFT both in magnitude and timing, coinciding well with the proposed timing of the Pre-Andina phase [16, 162]. Combined AFT and AHe data indicate that the extent and rates of this major exhumation pulse were much greater than what is evident from AHe data alone.

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In interpreting the AFT dataset, an Eocene geothermal gradient of ~25°C/km and a mean

surface temperature of 15°C were assumed. This figure is reasonable for subduction-related

continental margins with a thick continental crust and absence of modern volcanism [86], such as

this non-volcanic segment of the Andes. If such assumptions are correct, the depth to the top of

the pre-Eocene PAZ (60˚C isotherm) would have been ~2.2 km (Figure 4 -11). The break in slope of the age vs. elevation relationship is at ~1600 m or approximately 1000 m below the mean elevation of the AP. Thus, the amount of section removed since the Eocene exhumation pulse is

~1.2 km for the AP as a whole, but only ~100-300 m for the highest areas such as the summits of the Belmira range. This implies spatially averaged, long-term erosion rates of <0.03 mm/yr.

AFT results and previous sediment stratigraphic studies [14, 16], as well as a lack of regional-scale Cenozoic extensional tectonics in the northern Cordillera Central [25, Kellogg,

1984 #724], imply that this regional cooling phase of Eocene age was mainly due to erosional exhumation by fluvial systems that predate the modern Magdalena and Cauca rivers. Cenozoic cooling events associated with erosional exhumation, constrained by AFT in this study and by

AHe [294], are synchronous with the Pre-Andina and Proto-Andina orogenic phases [16].

Voluminous amounts of detrital material produced in the Eocene Late Oligoceneare reflected in sedimentary sequences with basal conglomerates deposited over major regional unconformities in the Magdalena [14, 16] and Cauca [168] basins to the east and west respectively. The combined exhumation of the Eocene and Oligocene events removed most of the ~4-5 km of estimated overburden on the Antioqueño Batholith.

Diachronous uplift-exhumation in the Cordillera Central was a major determinant in the evolution of the proximal Middle Magdalena and distal Llanos basins to the east of the AP. The northern Cordillera Central experienced episodic pulses of uplift and exhumation since the

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Paleocene (Laramic, Pre-Andina and Proto-Andina phases [16, 294]) relative to the Cordillera

Oriental that was uplifted during the Neogene, (Eu-Andina phases [16]). AFT data presented

here and previous AHe ages [294] agree with recently proposed mechanical models for the

Middle Magdalena basin [14] that indicate an Early Eocene foreland basin coupled with

Cordillera Central uplift, a Middle Eocene basin characterized by erosion-exhumation of the

Cordillera Central, and marked basin tilting to the east triggered by enhanced Cordillera Central

uplift during the early Neogene. Cycles of uplift and tectonic quiescence, previously suggested

for this segment of the Colombian Andes [14, 16, 168, 294], and the possibility of km-scale

uplift episodes were quantitatively confirmed the preset AFT study. That is, the timing and

magnitude of uplift and denudation were explicitly derived from our low-temperature

thermochronology dataset from the Cordillera Central. AFT results correlate well with previous

paleogeographic and sediment stratigraphic studies [16, 43, 295].

Increased cooling by fluvial erosion in the AP may have been caused by enhanced uplift

driven by tectonic process associated with accelerated convergence between the Farallon and

South American plates (Figure 4-10), documented for the Eocene [18]. The Eocene pulse here

identified is synchronous with exhumation events of a similar magnitude that have been reported

in low-temperature thermochronology studies elsewhere in the Andes [30-32, 35, 36, 39, 111,

112, 167, 366, 368, 370, 398] and in the Caribbean region [37].

Eocene to Miocene pulses of uplift and denudation in the Colombian Andes can be placed

in a broader regional framework. Tectonic upheavals during the Paleogene and Neogene are also

referenced for other Andean locations. Paleocene to Eocene phases of exhumation have been

reported in some sectors of the Ecuadorian Andes [39]. Eocene (Incaic) and Miocene (Quecha 1-

3) orogenic phases are well characterized in central and northern Peru [36]. In the Peruvian

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Andes, apatite fission track analysis of the Huachon granite points to two periods of erosional

cooling during the Neogene, at ~22 Ma and between 12 Ma and the present, with 4-6 km of

unroofing [35]. Similarly, Benjamin (1987) used low temperature thermochronology (LTTC) to constrain the Tertiary exhumation and uplift history of the eastern Cordillera and Altiplano of

Bolivia, revealing discrete pulses of uplift in the intervals 5-15 Ma and 25-45 Ma. Onset of

Andean deformation and exhumation in the Sierras Pampeanas (Argentina) is represented by a

cooling event during the Late Paleocene–middle Eocene, followed by westward migration of the

exhumation “front” during the Late Miocene–Pliocene [32]. Eastward migration of the locus of

maximum denudation identified for several portions of the Andes [32, 35, 399] may also apply

for the Colombian Andes, where the eastern cordillera seems to be the focus of intense erosion

from the Miocene to the present. In the Colombian case, however, indentation of the Panama-

Chocó arc may imply concentration of denudation towards the active margin (Cordillera

occidental) in the Neogene and exposure of Miocene plutons supports this contention [156].

Pulses of Paleogene uplift and exhumation throughout the Andes display a clear correlation with increased rates of convergence between Farallon (Nazca) and South America

[18]. Although fairly well documented by paleontology, geomorphology, thermochronology, stable isotope paleo-altimetry, and sedimentary stratigraphy [15, 30, 113, 158, 216, 349, 371,

375], the most recent Late Miocene-Pliocene orogenic phase in the Andes has been harder to explain, as it does not coincide with a period of enhanced convergence between Nazca and South

America [18, 19, 80]. Local mechanisms, such as crustal delamination [375], lateral flow of lower crust [216], and sequential stacking of basement thrusts [30, 111], as in the Bolivian case, or structural relief due to accretion-controlled crustal deformation by the Panama-Chocó Block in the Colombian case [25], have been invoked to explain recent phases of uplift. However,

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synchronicity of this major episode of Neogene uplift-denudation, documented by low

temperature thermochronology studies from Chile and Argentina to Colombian and Venezuela,

and correlation with periods of enhanced plate convergence also suggest a major tectonic control

of continental scale.

Important climatic controls (enhanced erosion via aridization) have also been invoked to

explain periods of rapid uplift and exhumation in the Andes [347, 369] and elsewhere [51, 52].

The relative roles of climate and tectonics in sculpting mountainous landscapes remains strongly debated [50, 52, 97, 106, 400, 401]. However, as the database on exhumation chronology of the

Andes grows, important continental-scale controls are expressed differently for specific portions of the orogen. Perfect matches between exhumation pulses and increased convergence rates between Farallon (Nazca) and South America during the Eocene and the Late Oligocene, in

Colombia, the rest of South America, and the Caribbean, point to increased erosion brought about by tectonically-driven uplift. Nevertheless, a climatic contribution to increased erosion in the Eocene (Molnar and England, 1990) cannot be ruled out, and further evidence of Eocene climate trends in the Northern Andes of Colombia should be sought.

Several studies proposed a prevalent role of Caribbean subduction in the Eocene geologic evolution of the Northern Andes [33, 164, 313, 381]. Caribbean subduction [164] has been invoked to explain stratigraphic unconformities and tight faulting and folding, tectonostratigraphic features related to rapid regional uplift (Early Eocene tectonic phase ~53 Ma, and

Middle Eocene Caribbean Orogeny ~45 Ma) . Eocene magmatism in the Sierra Nevada de Santa

Marta [402], shown by plutons dated ~50 Ma by the K/Ar method, has also been ascribed to subduction-related processes in the Caribbean domain. Calcium carbonate compensation depths in marine sedimentary sequences now exposed in the San Jacinto Belt (northern Colombia) and

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the proto Cordillera Occidental suggest that this area also experienced several km of uplift [367]

in the Eocene. However, evidence of Eocene phases of erosional exhumation in the rest of the

Andean range, in addition to coincidence of enhanced convergence between the Farallon and

South American plates, points to larger-scale, continental control that needs to be considered for the Neogene.

Topography and low temperature thermo-chronometry can be used to define two parts of

the landscape of dissimilar age and geodynamic significance. They are: 1) an old, slowly eroding

highland of subdued relief that is associated with periods of sustained lower relief and elevation

from ~60-50 Ma and from ~40-25 Ma, and 2) younger, rapidly-incised river gorges and regional

scarps that cut across and around the AP during the Cenozoic, particularly during the latest Eu-

Andina phase. Combined AFT and AHe data support low elevation of the range in the

Paleocene, followed by two well recorded events of uplift and denudation across the plateau, as

well as local fluvial incision at ~50-45 Ma and ~24-20 Ma. The AP has had a cyclical

geomorphic evolution since emergence of the region after the end of the Cretaceous marine

transgression in the Paleocene. Multiple erosional surfaces have been obliterated throughout the

Cenozoic. The landscape probably evolved by “tracing” a rapidly advancing weathering front

(etch surface) that had time to re-develop during periods of tectonic quiescence, given the fast

rates of chemical weathering proposed for granodiorite under humid conditions [403].

Modern geomorphic configuration of the AP reflects enhanced uplift during the most

recent Mio-Pliocene Eu-Andina tectonic phase. In these later stages, erosion has concentrated

along a few alluvial valleys (e.g., Medellin Porce River) creating localized incision of >1500 m.

Differential erosion created numerous perched valleys and varying local base levels that

permitted development of smaller planar surfaces within the AP. Hanging valleys impose

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fundamental control on how the landscape evolves [404]. Nick point propagation ultimately determines the extent and velocity with which the fluvial system encroaches into the plateau to degrade the planar morphology. This process is potentially enhanced as convergence of the

Panama-Chocó block continues to maintain uplift in this portion of the Andes. Perpetuation of uplift, fluvial incision, and nick-point propagation will ultimately obliterate the planar topography of this morphotectonically odd segment of the Colombian Andes, leading to the development of typical mountainous topography (e.g., steep, v-shaped valleys), and leaving only small, discontinuous remnants of the erosional surface.

Two important phases of tectonic upheaval and exhumation in the Colombian Andes [16,

162] are not captured by this or previous thermochronology studies [166, 294]. The Eu-Andina phase from Miocene to present [16], considered as the strongest and most recent pulse of uplift for the Northern Andes [34, 113, 156, 158, 162, 349], is not resolved by AFT or AHe. This may be a consequence of the disequilibrium character of the AP and its lagged geomorphic response to tectonic input relative to other portions of the orogen. In addition, AFT and AHe do not document the initial stages of uplift and exhumation of the Northern Cordillera Central, i.e., the

Laramic phase [16]. However, remnants of Hauterivian-Albian shallow marine sequences in the

Cordillera Central, followed by a transition to continental deposition in the Maastrichtian, with clastic material of Cordillera Central affinity in several Late Cretaceous-Early Tertiary sedimentary formations, imply the establishment of this portion of the Andes as a relatively elevated massif about 65 Ma ago [14, 17]. Zircon fission-track data with ages varying between

~50-67 Ma [166] probably point to this initial phase cooling via erosional exhumation that led to the development of the first erosional surface of Paleocene age. Subsequent phases of increased tectonic convergence interrupted by long periods of tectonic quiescence led to a succession of

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planation surfaces, the youngest of which is Miocene, according to AHe ages [294], following

the denudational pulse in the Oligo-Miocene transition. The Neogene geomorphic evolution of the AP was strongly determined by the Eu-Andina orogenetic phase (Mio-Pliocene). Removal of the saprolitic cover is documented by major inselbergs (tors, rock castles) that indicate a stage of etch surface development [405]. Erosion, however, has been focused on a few fluvial systems that cut deeply into the plateau morphology. Large differences in erosion resistance created numerous perched valleys and local base levels that permitted development of smaller planar surfaces within the AP.

Shifts between rapid and slow convergence determined orogenic exhumation in the Andes and the morpho-tectonic evolution of the AP. However, relatively lagged response of the AP and low erosion rates relative to other portions of the orogen confirm that the AP is in morphotectonic disequilibrium. This is the result of many factors, for instance, lithospheric strength and structural integrity of the area, which may have controlled the mode of emplacement and geometry of the Antioqueño Batholith (circular pattern) at the center of the AP, surrounded by more resistant metamorphic units, thus providing litho-structural control to the development of the AP. AFT and AHe double profiling also confirm the behavior of the AP morpho-tectonic domain as a rigid and coherent portion of lithosphere.

Erosion rates in the humid tropics differ widely between tectonically active and inactive areas [406]. Long-term erosion rates for mountainous environments are between 0.1 and 2 mm/yr [8, 186]. Long-term erosion rates in the AP are ~0.02 mm/yr, while denudation at the climax of orogenic phases increased to ~0.2 mm/yr. Long-term denudation is low compared to rates in other sectors of the Colombian Andes and the rest of the Andean range to the south.

Exhumation rates during the climax of exhumation are comparable to rates reported for the

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Bolivian and Peruvian Andes [30-32, 38, 111] but are about an order of magnitude slower than rates in Taiwan, Nanga Parbat, the Greater Himalaya of Nepal, and the Southern Alps of New

Zealand [186, 407]. A comparison of results from this AFT study with recent thermochronology data in the Cordillera Oriental [159] confirms that neighboring regions in the same orogenic system subjected to similar large-scale tectonic input experienced long-term erosion rates that differ >100-fold. Within the same tectonic setting, crustal blocks may undergo different exhumation histories [174]. Low erosion rates in the AP result from its morphotectonic disequilibrium.

Conclusions

Apatite fission-track along two vertical profiles within the AP complements previous AHe data and constitutes the first high-resolution record of thermochronologic information in the

Colombian Andes. AFT analysis identifies a major cooling event from ~50-40 Ma, associated with erosional unroofing >2 km and exhumation rates of ~0.4 mm/yr. This erosional pulse is much stronger than previously indicated by AHe thermochronology alone. A period of tectonic quiescence ~41 to 30 Ma was also recorded. Long-term erosion rates derived from the vertical profiles are ~0.01 mm/yr.

Pulses of exhumation in the AP defined by low temperature thermochronology are synchronous with similar phases in the rest of the Andean orogen and the Caribbean. The match between pulses of exhumation and increased convergence rates between Farallon (Nazca) and

South America during the Eocene and the Late Oligocene (in Colombia, the rest of the Andes and the Caribbean) points to continental-scale tectonic controls on denudation.

Shifts between rapid and slow plate convergence determine orogenic exhumation and the morpho-tectonic evolution of the AP and the Andes in general. Multiple episodes of planation and relief development occurred during the Cenozoic in the AP region.

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The lagged response of the AP and low erosion rates relative to other portions of the

orogen confirm that the AP is in morphotectonic disequilibrium. The AP functions as an independent morphotectonic domain embedded in a rigid and coherent lithosphere block.

A preliminary comparison between long-term erosion (i.e., natural, 0.04 to 0.008 mm/yr;

Restrepo-Moreno et al, 2009 and Restrepo-Moreno, submitted) against modern erosion rates

(i.e., anthropogenic, 1 mm/yr, Giraldo, 2005) reveals a 2-3 order of magnitude increase. Such disparity can be interpreted as being related to anthropogenic perturbations of natural geomorphic dynamics.

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Table 4-1. Instrumentation, operating conditions, and data acquisition parameters used for LA- ICP-MS apatite fission-track dating. ICP-MS operating conditions Instrument Finnigan Element II Magnetic Sector ICP-MS Forward power 1.25 kW Reflected power <5 W Plasma gas Argon Coolant flow 15 l/min Carrier flow 1.0 l/min (Argon) 0.8 l/min (Helium)

ICP-MS data acquisition parameters Dwell time 10 msec per peak point Points per peak 4 Mass window 5% Scans 50 Data acquisition time 30 sec Data acquisition mode Electronic scanning Isotopes measured 43Ca and 238U

Laser operating conditions Laser type New Wave UP213 (Nd: YAG) Wavelength 213 nm Laser mode Q switched Laser output power 8 J/cm Laser warm up time 6 sec Shot repetition rate 5 Hz Sampling scheme Single spot, 16 µm

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Table 4-2. Summary of apatite fission-track data. Apatite fission-track age, length and Dpar data for the Matasanos (M) and the La García (G) profiles. Reported dates are pooled FT ages ±2σ. Ages calculated using the ξMS (secondary zeta calibration parameter for LA-ICP-MS) as proposed by Donelick et al (in review). Numbers in [ ] indicate number of grains analyzed. (MS = Mass spectrometry, FT = Fission track, MTL = Mean track length, STDV = Standard deviation, SE = Standard error, bkg:sig = Background to signal ratio).

Sample Dpar Ns Area Σ(ΡΩ) 1σ Σ(ΡΩ) ξMS 1σ ξMS 43Ca 238U Q Chi Lat/Lon Elevation Pooled MTL STDV [no. grains] (µm) (tracks) analyzed (cm2) (cm2) (bkg:sig) (bkg:sig) Square (Dec Deg) (m) FT Age (μm±1SE) MTL (cm2) (%) (Ma) (μm) -2 -4 -6 -2 -3 Durango [492] 1.52 3091 2.3x10 7.1x10 1.3x10 14.5149 0.263 2.565x10 2.166x10 0.97 31.4± 0.8 14.59±0.08 0.98 Unknowns SR-001-M [25] 1.54 232 9.2x10-4 3.9x10-5 1.5x10-6 15.4613 0.423 2.12x10-2 2.87x10-3 0.83 17.4 6.857/75.181 760 46.2±3.7 14.18±0.11 1.23 SR-003-M [25] 1.34 531 8.5x10-4 1.2x10-4 2.1x10-6 15.3485 0.418 2.42x10-2 1.20x10-2 0.30 27.0 6.758/75.115 1000 34.8±1.9 13.67±0.13 1.5 SR-2-M [22] 1.28 72 7.6x10-4 1.6x10-5 3.4x10-7 15.2356 0.414 2.46x10-2 6.61x10-2 0.48 20.7 6.424/75.379 1520 35.3±4.3 13.35±0.15 1.75 SR-3-M [25] 1.22 247 1.1x10-3 5.3x10-5 1.6x10-6 15.1228 0.410 1.20x10-1 2.73x10-1 0.13 31.8 6.426-75.376 1704 35.0±2.6 13.70±0.12 1.39 SR-5-M [20] 1.19 83 6.9x10-4 2.0x10-5 5.9x10-7 15.0099 0.405 2.78x10-2 1.12x10-2 0.30 21.7 6.427-75.371 1630 30.4±3.6 13.49±0.18 1.47 -3 -4 -6 -2 -2

174 SR-6-M [24] 1.36 622 1.1x10 1.1x10 2.3x10 14.8971 0.401 6.66x10 8.46x10 0.01 40.2 6.433/75.374 1710 41.1±2.1 13.85±0.12 1.32 SR-9-M [23] 1.41 485 9.6x10-4 7.9x10-5 1.6x10-6 14.7842 0.397 2.68x10-2 3.83x10-3 0.02 37.6 6.463/75.374 2270 45.1±2.5 13.96±0.13 1.52

SR-11-M [24] 1.39 333 8.6x10-4 5.1x10-5 1.0x10-6 14.6714 0.392 2.81x10-2 6.85x10-2 0 75.7 6.462/75.370 2230 47.7±3.1 13.99±0.12 1.41 SR-15-M [24] 1.38 925 1.1x10-3 1.5x10-4 2.9x10-6 14.5585 0.388 3.87x10-2 8.48x10-2 0 67.6 6.452/75.369 2100 45.5±2.1 14.03±0.11 1.32 SR-19-M [25] 1.47 303 8.7x10-4 4.6x10-5 9.0x10-7 14.4456 0.384 7.35x10-2 1.98x10-1 0.91 15.3 6.449/75.361 1990 47.2±3.1 14.23±0.13 1.45 SR-24-G [10] 1.46 134 2.6x10-4 2.6x10-5 1.1x10-6 14.3666 0.381 3.22x10-2 2.53x10-3 0 33.4 6.386/75.598 2400 37.7±3.8 13.96±0.19 1.7 SR-26-G [22] 1.48 264 6.9x10-4 4.9x10-5 1.2x10-6 14.2877 0.378 1.53x10-1 2.88x10-1 0 91.0 6.382/75.595 2350 38.2±2.7 14.25±0.12 1.41 SR-29-G [23] 1.49 293 7.4x10-4 5.8x10-5 1.4x10-6 14.1748 0.373 3.25x10-2 8.43x10-3 0.06 33.4 6.381/75.591 2250 35.5±2.4 13.81±0.14 1.59 SR-31-G [25] 1.49 280 7.7x10-4 4.8x10-5 1.0x10-6 14.0619 0.369 4.21x10-2 6.69x10-2 0.09 33.6 6.380/75.593 2170 41.2±2.8 13.97±0.13 1.47 SR-32-G [24] 1.39 405 8.8x10-4 5.8x10-5 1.3x10-6 13.9491 0.365 3.18x10-2 2.89x10-2 0 114.0 6.379/75.589 2110 48.9±3.0 14.02±0.12 1.41 SR-35-G [22] 1.42 215 7.5x10-4 3.8x10-5 6.9x10-7 13.8362 0.360 1.09x10-1 1.69x10-1 0.10 29.7 6.374/75.579 1900 39.3±3.0 13.52±0.14 1.62 SR-40-M [24] 1.47 835 9.1x10-4 1.4x10-4 2.3x10-6 13.7234 0.356 3.84x10-2 2.82x10-2 0.14 30.2 6.406/75.435 1500 41.4±1.9 13.85±0.13 1.44 SR-41-M [25] 1.48 1101 1.1x10-3 1.8x10-4 3.8x10-6 13.6105 0.352 1.73x10-1 3.85x10-1 0 99.6 6.413/75.411 1380 40.5±1.8 13.92±0.13 1.45 SR-44-G [25] 1.44 347 9.8x10-4 5.6x10-5 1.0x10-6 13.4977 0.347 3.27x10-2 1.26x10-2 0 59.0 6.345/75.579 1640 42.0±2.6 13.93±0.10 1.18 SR-45-G [22] 1.42 269 9.4x10-4 4.2x10-5 1.0x10-6 13.3848 0.343 6.55x10-2 7.59x10-2 0.03 34.9 6.359/75.590 2015 42.4±3.0 13.85±0.13 1.49 SR-46-G [24] 1.56 242 9.6x10-4 4.0x10-5 7.5x10-7 13.272 0.339 3.44x10-2 4.72x10-2 0.02 38.2 6.357/75.588 1900 39.8±2.9 13.82±0.12 1.42 SR-48-G [24] 1.51 255 8.6x10-4 4.2x10-5 1.0x10-6 13.1591 0.334 6.24x10-2 1.19x10-1 0.08 33.1 6.353/75.581 1710 39.6±2.8 13.70±0.15 1.75

Figure 4-1. Regional geomorphology. CO= Cordillera Occidental (Western Cordillera), CC=Cordillera Central (Central Cordillera), COr=Cordillera Oriental (Eastern Cordillera). Red circles indicate the location of the elevation profiles sampled. Mean elevation of the AP is ~2500 m. Elevation of the small range on the west margin of the AP is ~ 3600 m. Bottom of the Cauca River is ~600 m. Bottom of the Medellin River ~1200 m. (DEM generated with ERMapper® from GTOPO-30 elevation data.)

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Figure 4-2. Geology of the study site. Target lithology is granodiorite plutons of the Antioqueño and Ovejas batholiths (AB and OB). Location of sampled profiles the La Garcia (west) and Matasanos-Porce (east) marked with red circles. CA-F = Cauca Romeral (Cauca-Almaguer) fault system (paleosuture).

176

Figure 4-3. Tectonic Setting. Study region delimited by the red square. Major structural features in Colombia: RF= Romeral fault system, PF=Palestina fault system, UF=Uramita fault, GF=Guaicáramo fault, SBF=Santa Marta-Bucaramanga fault, BF=Boconó fault, OF=Oca fault. Major tectonic plates and two lithospheric blocks (PC=Panama-Chocó and Andean) are accompanied by arrows and numbers that indicate direction and velocity of motion relative to the South American Plate. Black triangles indicate zones of the Cordillera Central with active volcanism. (Faults are compiled form Taboada et al. (2000) and Tapias et al. (2006); plate velocity vectors from Trenkamp, et al. (2002); digital elevation model (DEM) derived from GTOPO-30 database).

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Figure 4-4. Age-elevation relationship. Low temperature thermochronology age data and elevation from the Antioqueño Plateau. Black triangles are AFT data for the Matasano-Porce profile. Black diamonds are AFT data for The La García profile. Segments S1 and S2 are regression lines (least square) through subsets of the data explained in the text. Red line encloses AFT data anomalously young relative to previous AHe ages. AHe data (gray squares) from Restrepo-Moreno et al. (2009) for the same profiles are displayed for comparison. Error bars are 2σ.

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Figure 4-5. AFT Age vs. Elevation Relationship for The La García profile. Atσ, 2 AFT ages are virtually concordant between ca. 1200 and 2600 m and are centered at ~41 Ma, indicating rapid exhumation at this time.

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Figure 4-6. Age vs. mean track length relationship for the Matasanos-Porce profile. All samples in the profile included (r=0.91), MTL= mean track length.

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Figure 4-7. Representative Time-temperature paths modeled with AFTSolve®. Models by (Ketcham et al., 2000; Ketcham and Donelick, 2001) were run by Restrepo-Moreno and O’Sulivan. Results show permissible thermal histories in the left panel (a) and measured (bars) versus best-fit model (solid line) track length distributions in the right panel (b). Thermal histories shown are acceptable- (dark gray), good- (light gray) and best-fit (black line). AFT pooled ages (Age ±1σ) mean track lengths (MTL±1σ), number of grain ages or track lengths measured (N), and goodness of fit (GOF) between the model results and the data for both the age and MTL data are reported at the base of each thermal history.

181

Figure 4-8. Relevant HeFTy Time-temperature paths. Paths modeled with HeFTy [224] by Restrepo-Moreno. Results show permissible thermal histories in the left panel and measured (bars) versus best-fit model (solid line) track length distributions in the right panel. Thermal histories shown are acceptable- (dark gray), good- (light gray) and best-fit (black line). Goodness of fit (GOF) between the model results and the data for both the age and MTL data are reported for each thermal history.

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Figure 4-9. AFT and AHe time-temperature paths. Black dotted lines show periods of rapid cooling-exhumation. Gray dotted arrows indicate spans that are unconstrained by AFT or AHe. The hachured “balloon” on the bottom left shows the temperature and age of emplacement of the Antioqueño and Ovejas batholith. AFT data from this study, AHe from Restrepo-Moreno et al. (2009). Age of intrusion of the Antioquenio batholith and associated Ovejas Batholith from Restrepo et al. (2007).

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Figure 4-10. Long-term spatially averaged erosion. Simplified sketch illustrating the extent of the crustal section removed by erosion since ca. 45 Ma (light gray, dotted area). Upper boundary of the APAZ (upper dotted line defining the band in dark gray). PB=Páramo de Belmira, AP=Antioqueño Plateau. Present day topography defined by curved black solid line.

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Figure 4-11. Timing and magnitude of Cenozoic changes in convergence between the Farallon- Nazca and South American Plate. Onset and termination of orogenic phases found in this study and in previous investigations (Restrepo-Moreno et al., 2009) are represented by gray block arrows labeled Pro (Proto-Andina) and Pre (Pre-Andina) between shaded bars delimited by dotted lines (Van der Hammen, 1958, 1961). Figure modified from Pardo-Casas and Molnar (1987).

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CHAPTER 5 GENERAL CONCLUSIONS AND FURTHER RESEARCH

Antioqueño and Ovejas Batholith Age and Petrogenensis

An extensive geochemical and isotopic study was carried out in the Antioqueño and

Ovejas batholiths (AB and OB respectively) with the aim of constraining the age, magma sources-mixing process and geodynamic environment. Geochemical and isotopic analysis allows to define the AB and OB as a typical medium-K calc-alkaline plutonic suite, mainly granodioritic in composition and developed though continental margin magmatism (i.e., subduction-related).The AB and OB were emplaced in the Maastrichtian and not in the Albian as previously proposed. Structures such as roof pendants, septum, copulas, well defined contact aureoles, lack of foliation and faulting, etc. suggest that these plutons are post-tectonic and were emplaced in the epizone. Epizonal emplacement and rapid cooling are supported by zircon grain morphology. This mass was assembled incrementally through a succession of magmatic injections over a relatively short span from ~72 to 77 Ma. Geochemical signatures (e.g., REE,

Pb, and Nd) are similar to magmatic products for the North Andean Volcanic Zone, indicating that at the time of the AB and OB genesis, the geodynamic setting was very simlaar to the one foud today in the Northern Andes. However, geochemical and isotopic differences between samples are notable (e.g., different slopes for the REE patterns, variable values for eHf, etc.).

This, along with clear magmatic zoning indicates different sources, different depths of equilibration and disparate degrees of fractional crystallization during the various pulses.

Overall, and this is also the case for most igneous products in continental arcs, there is a marked influence of crustal contamination expressed in the high values of LILE, high LREE and negative values of εHf. Similarity in age and geochemical-isotopic signatures for both plutons is indicative of consanguinity.

186

Potential mechanisms for magma production and emplacement are difficult to precise.

Opting for one particular model to categorically claim that these batholiths were formed through

crustal thinning and decompression due to extension, through a flare up a-la-Ducea [338], by enhanced dehydration of the oceanic slab triggered by accelerated subduction or by enhanced thermal activity and exchange between the mantle and the crust is may not be appropriate.

Perhaps a combination of mechanisms acting in concert can be invoked to explain the development of this large petrogenetic province. Whatever the mechanism, the AB and OB constitute a large mass of igneous rock and its emplacement at the end of the Cretaceous may have contribute to crustal thickening and hence to the initial pulses of uplift and denudation such as the Weak Laramic Phase (Van der Hammen, 1958).

Further sampling of the AB and OB can assist us in better constraining the variation information ages of these plutons. However, the actual results obtained in this investigation suggest that this is a robust estimate of formation age and provides an excellent age signature of magmatic zircons for the petrogenetic province under scrutiny. In conjunction with Hf data and

CL imaging here generated, this database can be used to undertake provenance studies in sedimentary formations within the Cauca and Magdalena depressions to the east, west and north.

In addition, this strategy can be extended to other smaller, plutonic units within the area of interest (more acidic and mafic) to better understand Late Cretaceous magmatism in the northern

Andes

Low Temperature Thermochronology

A detailed, high-resolution low temperature thermochronology study has been undertaken in the Antioqueño Plateau, northern segment of the Colombian Cordillera Central. The combination of AHe and AFTA provides a very coherent dataset that reveals a two-phased erosion history for the AB and OB. Pulses of uplift and exhumation here recorded are in 187

remarkable agreement with major exhumation events defined for the northern Andes. However, the most recent phase of uplift and exhumation in the region, known as the Eu-Andina Phase (12

Ma to present, and recognized as the driver for the most intense period of uplift and structural relief generation in the Colombian Andes is not recorded by either AFT or AHe. This confirms the hypothesis that the AP is a relict surface in morphotectonic disequilibrium with the rest of the cordilleran system and with a lagged response to uplift and exhumation. Long-term, spatially averaged erosion rates in this unusual geomorphic domain are similar to those of cratonic regions

(0.02-0.008 mm/yr) suggesting that the AP resembles a portion of a shield pending at high elevation in the middle of a cordilleran domain bordered by step topographic terrain to the west

(Cauca gorge and sierra like Western Cordillera), the east (Magdalena Depression regional scarp), north (end of the Central Cordillera, Cauca river regional scarp), and south (sierra-like portions of the Central Cordillera). AFT and AHe data along two vertical profiles suggest that the AP has responded as a coherent discrete crustal block to Cenozoic tectonic input. The region has been elevated from near sea level in the Paleocene to its actual average 2500 m as a consequence of intermittent pulses of tectonic-driven uplift end exhumation. Several phases of planation, i.e., erosional surface development, have taken place during intervals of relatively slow convergence between Farallon (Nazca) and South American plates. AFT and AHe data confirm that uplift-exhumation pulses in the Eocene and Oligocene in the Antioqueño Plateau have been important in the paleogeographic evolution of Northern South America and the tectonostratigraphic development of sedimentary basins to the east (Magdalena and Llannos basins) and to the west (Cauca basin).

Spatiotemporal coincidence between pulses of exhumation in the AP (and elsewhere in the

Andes and the Caribbean region) with periods of enhanced convergence between the Farallon

188

(Nazca) and the South American plates point to important, continental-scale tectonic controls on

erosion rates.

Additional information can be extracted from the AP region by implementing a horizontal

transect approach that would allow to: 1) test whether apparent “tilting” of the plateau to the

southeast is a tectonic feature of if it is related to fluvial activity; 2) evaluate the degree of

internal relief through time with an approach similar to the one employed by House et al.

{House, 1998 #39; House, 2001 #297}. Well characterized AHe ages achieved in this investigation can be further used, in combination with U-Pb/Hf in zircon, to study provenance of detrital material to the Amagá formation and to constrain variations in the elevation of modern

sources of sediment.

Finally, all line of evidence suggest that modern erosion rates (i.e., anthropogenic; 1-10

mm/yr) are 2-4 orders of magnitude greater than long-term (i.e., natural or pre-anthropogenic;

0.02-0.008 mm/yr) correlatives. This implies loss of soil (end regolith in already degraded areas) at a magnitude far greater than the tolerance thresholds for such process accepted worldwide.

Soil erosion is in itself a major environmental problem that is further amplified by the deleterious effect of excessive sediment transport and deposition along waterways and into strategic water

ecosystems such as rivers, lakes, and oceans. Today, coral reefs, estuaries, lakes, riverine

ecosystems, etc. are facing major problems due to accelerated sedimentation. In addition, the

potential to store water and generate electricity through the establishment of artificial reservoirs

is critically hampered by high rates of substrate removal by water.

189

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BIOGRAPHICAL SKETCH

Sergio Andres Restrepo Moreno (alias Yiyo) was born in Fort Collins (Colorado, USA), by the mountains in North America, in spring 1969. This happened while his father Miguel Angel was conducting his MSc in plant physiology at Colorado State University and his mother was totally engaged in the not-less-demanding and rather inspiring chores associated with rising kids

(Camilo and Clara). Sergio was prematurely taken to Medellín (Antioquia, Colombia) in the winter of 1970, right to the middle of the Northern Andes. From a mountain range to the next sort of thing! A year after, his younger brother, Jose Miguel, was born and with that a playing crew that has lasted for almost four decades was complete. Sergio’s wonderful great parents contributed to the warmth of the family niche.

In 2000 Sergio married his great friend, Isabel, and a year-and-a-half after their wedding,

December 29th 2001 to be precise, Sergio and Isabel (and everyone else in the now larger family) were already enjoying the company of the beautiful Luna. A bit afterwards Iti Miguel came along to further the level of happiness of the three families involved: Isa’s, mine, Isa’s, and mine.

Inti Miguel was born on July the 10th 2004. Night-winter girl and day-summer boy. What else can one ask? In 2003, Sergio’s alias was changed for Papa-Yiyo as Luna, his daughter, learned to identify “dad” and “Yiyo” as the same thing. Inti Miguel quickly followed Luna in their interesting conversations and, of course, into the new nickname given to Sergio. In other words,

Luna and Inti have awarded Sergio one of the greatest titles so far, that of a loving close father.

It is in the context of that spiritual wealth that Sergio’s academic career has developed.

Sergio has been in the schooling system (some have opted to call it the educational system) since age 4. Among his most fascinating experiences in that realm are his first days of kindergarten at a small preschool facility by the Sinú river at the Universidad de Cordoba were his father, rejected from the educational system in Medellín for having a long beard during the times of

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communism erradication, ended up teaching for a couple years, and the company of one of the first teachers “nun Bernardita” in the Los Sagrados Corazones Elementary School in Medellín suburbs.

Sergio never new why after having conditional matriculation for 5 straight years he graduated with honors from his high school days in 1986 and then joined as an undergraduate student the program in Geology at Universidad Nacional de Colombia, the University system that had unjustly persecuted his father several decades into the past and re-appointed him again.

Politics some say. Anyway, Sergio graduated from Universidad Nacional without any honors in

December 1994. Soon after Sergio had to flee as he and his younger brother were facing life threats in a time in Medellín town were a life threat, by any of the several hundred gangs in town, was worst than a death penalty in the more civilized United States. Maybe they are just the same stuff. Two years on the road were very meaningful educationally speaking although he was willingly deprived of any formal schooling work whatsoever. Sergio remembers reading two of his first books in English: Like Water for Chocolate which inspired him to get finally immersed in a new language, and Deschooling Society (Ivan Illich) which he had already read in Spanish at age 16 or so and who taught him, prematurely, to look at the compulsory educational system, from which he is still part, with profound criticism.

Back in Colombia Sergio has worked as a volunteer academic advisor CIER, a local NGO, a local dream, in a project to revive non compulsory educational systems in impoverished rural villages in the mountains of Colombia. Trying to spread the news that wisdom is already within them and not in fancy educational institutions is hard work against the media, against the government, against those who have made the maintains of Colombia (and the valleys, and the rivers, and the beaches, and the air) a scenery of war.

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He somehow regained his interest in academic work (it may have to do with his early encounter with fossil in Colombia or with the devotion placed in using rocks to get fruit, scare detractors, etc.) without changing his impression of the academic system as a whole, and joined the Department of Geological Sciences on a fellowship in 1999. In 2000 he quit academic work to go back to Colombia and be with Isabel in the west coast of Colombia during her semester as a rural doctor in a nation of war and bloodshed. Few days before her arrival, Isabel had to conduct autopsies on more than 80 bodies of young soldiers that the government assured were fighting a “good war” for a “good cause” mainly to defend “the interest of my nation”. Although late to support her through those painful hours by the Pacific Ocean Sergio managed to engage

her into a marriage project that took only 2 weeks to be organized. In spite of being called

molasses by one of his best friend in Gainesville, Richard Barclay, this quick and smooth

marriage is proof that Sergio can actually do some things very fast. Not a dissertation though!

Finally, Sergio was admitted to candidacy at UF in the Spring of 2004 and graduated,

without honors, in the Spring of 2009, 40 years after his wondrous birthday… this time away

from the mountains of the US and of Colombia and away (only geographically) from his first

family.

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