Mesozoic to Tertiary Evolution of the Southwestern Proto-Pacific Gondwana Margin

University of Sydney, PhD Thesis, Kayla T. Maloney, 2012.

The University of Sydney School of Geosciences

Mesozoic to Tertiary evolution of the southwestern proto-Pacific Gondwana margin

Kayla T. Maloney 2012

Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy

The University of Sydney School of Geosciences Division of Geology and Geophysics Madsen Building (F09) Sydney, NSW, 2006 Australia University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. i

DECLARATION I declare that this thesis is less than 100,000 words in length, and that the work contained in this thesis has not been submitted for a higher degree at any other university or institution.

Kayla T. Maloney August, 2012

PREFACE

This PhD thesis consists of a collection of papers that are published or prepared for submission with international peer-reviewed journals appropriate to the discipline of geology. The publications form part of an integrated project and are presented in an order that represents the related elements of a connected thesis. The thesis contains an introductory section that provides an outline of the thesis, a summary of the contribution of the work to the field of geology, and a critical evaluation of the role of the thesis in informing further research in the field. Common themes in the papers are tied together and a discussion and conclusion of the whole thesis is presented at the end. No animal or ethical approvals were needed during the completion of this study. Data and interpretations in the thesis are the work of the author except where stated in the text.

ACKNOWLEDGEMENTS

Looking back at the past three and a half years of work I am reminded of the contributions and support of a great many people without which this thesis would never have come to be. Thank you so much to G. Clarke, I could not have asked for a better supervisor; to K. Klepeis; to the Earthbyte Group at the University of Sydney; to all my friends and colleagues who have made my time here so enjoyable; and to my family and L. Mondy for your encouragement and support. University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. ii

ABSTRACT The Andean margin of South America formed part of the proto-Pacific margin of Gondwana, and has been the site of long-lived subduction. The tectonic evolution of the margin has been influenced by the breakup of that supercontinent and interactions with the subduction zone and the subducting proto-Pacific plates. Since Late Jurassic times the margin has experienced two distinct tectonic regimes. The first was principally controlled by extension associated with Gondwana dispersal, resulting in the development of extensional basins along the length of the margin. The second regime, principally controlled by convergence with minor episodes of extension, initiated at different times along the margin and continues today. During this second period the margin experienced major uplift, leading particularly to the development of the Altiplano-Puna Plateau system, as well as numerous fold and thrust belts. The role of subduction along the proto-Pacific margin in contributing to the development of these events is quantitatively examined in the framework of a recently developed kinematic plate model. The absolute velocity of the overriding South American plate, the convergence velocity between the downgoing oceanic slab and the South American plate, and the age of the subducting slab are resolved along the margin back to 170 Ma to determine correlations between these parameters and deformation along the Andean margin. Any single parameter examined in isolation did not produce a correlation with deformation in the overriding plate, indicating that more complex interactions among several factors are required to produce the observed pattern of deformation. The development of extensional basins floored by mafic/oceanic crust was associated with periods when the motion of the South American plate was directed away from the trench in conjunction with the subduction of oceanic crust older than c.50 Myr. Contraction events, such as the development of fold and thrust belts and of the Altiplano-Puna Plateau, were associated with convergence velocities greater than 4 cm/yr. The absolute motion of South America was directed trenchward during these events, although the magnitude of motion was very small. The development of extensional basins that did not progress to the formation of mafic crust was accompanied by diverse conditions along the subduction zone, as were large magmatic fluxes leading to batholith emplacement, indicating that these events are not primarily controlled by any of the parameters investigated. In addition, no correlations could be established related to the structural style of fold and thrust belts.

Whereas plate scale processes control the tectonic regime along the margin, other mechanisms that act within the overall context of the imposed plate setting can affect deformation at more regional or local scales. The Rocas Verdes Basin in southern Patagonia is an unusual setting as it is one of only two extensional basins floored by ocean crust in a string of Mesozoic extensional ensialic basins developed along the Andean margin. The Rocas Verdes Basin is interpreted to have undergone different evolutional histories along the length of the basin. Negative (i.e. extensional) convergence rates were present in the southern portion of the basin during opening, while positive convergence rates affected the north, are implicated as contributing factors resulting in different magnitudes of extension.

The Cordillera Darwin metamorphic complex in southernmost Patagonia exposes amphibolite facies kyanite-staurolite metapelitic schist, the highest grade metamorphic rocks in the Andean chain south of Ecuador. Closure of the Rocas Verdes Basin in this area resulted in continental underthrusting beneath the arc, resulting in the amphibolite facies metamorphism. Thrust-controlled exhumation terminated metamorphism. Garnet in metapelitic schist has patchy textures where regions of clear grossular-rich garnet with fine-grained S1 inclusion trails are cut by turbid regions of spessartine-pyrope-rich garnet containing inclusions of biotite, muscovite, plagioclase, and quartz that align with S2 in the matrix. Aqueous micro-inclusions in the turbid regions suggest that garnet recrystallization to form the patchy textures was facilitated by fluid ingress; recrystallization appears contemporaneous with the growth of matrix kyanite and staurolite. Pseudosection modelling in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–

H2O–TiO2–Fe2O3 (NCKFMASHTO) describes a P-T path for rocks of the Cordillera Darwin of nearly isothermal decompression from 12 to 9 kbar at T≈620°C. Along this path, garnet mode is predicted to decrease from a maximum of c.5% to <1%, driving garnet dissolution. In situ ion-probe U-Th-Pb dating University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. iii of S2 matrix monazite, texturally associated with the growth of S2 kyanite and staurolite, shows that exhumation of the Cordillera Darwin was underway by 72.6 ± 1.1 Ma. Later granitic intrusions produced contact aureoles including sillimanite bearing migmatites in the adjacent rocks at P≈6 kbar.

The Seno Otway region lies midway between the comparatively well studied regions of Ultima Esperanza in the north, and Cordillera Darwin in the south. The area includes rocks that reflect the two distinct tectonic regimes of the South American margin: there is a complete record from the development of the Jurassic – Late Cretaceous Rocas Verdes marginal basin to the Cretaceous Magallanes foreland basin succession. Sedimentation into the Rocas Verdes Basin consisted of the hemipelagic mudstones and fine- grained sandstones of the Zapata and Canal Bertrand formations. The formations are mudstone dominated with an increasing abundance of sand in the upper portions recording the onset of contraction and basin closure. Thrusting that obducted portions of the mafic marginal basin floor onto the South American continent was contemporary with the development of the Magallanes foreland basin. Sedimentation into this basin included repeated fining upward sequences of interbedded mudstone and sandstone turbidites called the Latorre formation, and the overlying Escarpada Formation, composed of interbedded sandstone, mudstone, and conglomerate. Emergent portions of the Rocas Verdes Basin and Late Jurassic volcanic rock, exposed through thrusting, were eroded to form parts of the Late Cretaceous sedimentary units. The Magallanes fold and thrust belt involved the metamorphic Paleozoic basement in this region. Folding styles are tight to isoclinal near the main obduction thrusts and broaden to become close to open toward the foreland. An accompanying axial planar cleavage is fine-scaled and pervasive in regions of tight folding and is only weakly developed toward the foreland. Metamorphic grade in the region does not exceed greenschist facies. Deformation in portions of the Latorre and Escarpada formations is low enough to allow for the preservation of original sedimentary features, including various paleocurrent indicators. These indicators record the presence of multiple submarine fans in the Magallanes foreland basin in the Late Cretaceous.

The Andean margin of South America is commonly considered the type example of an ocean – continent convergent margin. Geological variability along its length provides a unique opportunity to validate macroscopic models for lithospheric weakening leading to the formation and destruction of marginal basins. During the Early Mesozoic most of the Andean margin lay below sea level as a series of extensional basins formed above a subduction zone. Cretaceous compression related to intraplate shortening resulted in basin inversion, with highly variable thickening and uplift. Crustal thickening in the area now known as Cordillera Darwin resulted in high-grade metamorphism from tectonic processes commonly associated with regions of continent – continent convergence. Such diversity in the Rocas Verdes Basin thus provides valuable insights into the evolution of a margin perhaps too commonly regarded as uniform and coherent. Basin development and inversion reflect related but distinct extensional and contractional regimes along the length of the Andean margin, and events in the history of the basin are demonstrably linked with plate-scale processes. University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. iv

This thesis is based on the following three papers, referred to by their roman numerals:

I Maloney, K. T., G. L. Clarke, K. A. Klepeis, L. Quevedo, reviewed and ready for resubmission, The Late Jurassic to present evolution of the Andean margin: drivers and the geological record, Tectonics.

II Maloney, K. T., G. L. Clarke, K. A. Klepeis, C. M. Fanning, and W. Wang (2011), Crustal growth during back-arc closure: Cretaceous exhumation history of Cordillera Darwin, southern Patagonia, Journal of Metamorphic Geology, 29(6), 649-672, doi:10.1111/j.1525-1314.2011.00934.x.

III Maloney, K. T., G. L. Clarke, K. A. Klepeis, ready for submission, Tectonic inversion of Rocas Verdes marginal basin; the record in Seno Otway, Patagonia, Journal of South American Earth Sciences. University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. v

TABLE OF CONTENTS

Abstract ii

Introduction 1

Scope of Thesis 2

Thesis Outline 2

Paper I: The Late Jurassic to present evolution of the Andean margin: drivers and the geological record 5

Paper II: Crustal growth during back-arc closure: Cretaceous exhumation history of Cordillera Darwin, Southern Patagonia 28

Paper III: Tectonic inversion of the Rocas Verdes marginal basin: the record in Seno Otway, Patagonia 53

Discussion 72

Conclusions 79

References 80 University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 1

INTRODUCTION involving growth, inversion, and the development of a foreland basin (McAtamney et al., 2011; The Andean margin of South America is commonly Paper III). Relationships at Seno Otway enable the considered the type example of an ocean – dependencies of this evolution on subduction to be continent convergent margin. Initially situated evaluated. Growth of the Rocas Verdes Basin has along the south-western margin of Gondwana prior been associated with absolute plate velocities of to its dispersal, this margin has been the site of the South American continent being directed away subduction since at least the Jurassic (James, 1971; from the trench, whereas trench-ward absolute Coira et al., 1982; Pankhurst et al., 2000; Oliveros velocities have been implicated in basin closure et al., 2006). During this long-lived subduction the (Ramos, 2010; Paper I). Distinctions between the Andean margin developed a varied and commonly geology of Seno Otway and of Cordillera Darwin, complex sequence of superimposed deformation particularly with respect to metamorphic grade are and magmatic events. Post-Jurassic deformation discussed and evaluated to examine variable effects along the western margin of South America can of deformation related to convergence. be placed into either of two broad categories: A recently developed plate kinematic model extension during the Late Jurassic and Cretaceous, (Seton et al., 2012) allows the reconstruction of or contraction from the Late Cretaceous to the tectonic plate configuration back to 200 Ma. It present. Distinctions in the location, timing of was used to investigate relationships between initiation and duration of deformation events give subduction and geologic events recorded by the insights into the influence of a subduction zone on overriding South American plate. The model and overriding plate deformation. associated oceanic crust age grid (M. Seton & R.D. Numerous conditions have been implicated as Müller, unpublished data) were used to investigate drivers of deformation and magmatism, including convergence rates, the absolute motion of South the rate of convergence between subducting and America and the age of subducting slabs in relation overriding plates (Pilger, 1984; Yañez & Cembrano, to the timing of the development of extensional 2004; Capitanio et al., 2011; Ramos, 2010; Daly, basins, fold and thrust belts, major uplift of the 1989), the absolute velocity of South America in a Altiplano-Puna plateau, and major magmatic mantle or hotspot reference frame (Ramos, 2010; events. Yañez & Cembrano, 2004; Martinod et al., 2010), The northernmost and southernmost regions the age of the subducting slab (Capitanio et al., of the margin are unique in preserving mafic- 2011; Yañez & Cembrano, 2004; Pilger, 1984), floored basins with mid ocean ridge affinities: the flat slab subduction (Pilger, 1981, 1984; Martinod Colombian Marginal Seaway in the northern Andes, et al., 2010), and trench rollback velocity (Pindell and the Rocas Verdes Basin in Patagonia (Dalziel & Tabbutt, 1995; Ramos, 2010). The relative et al., 1974; Stern, 1980; Alabaster & Storey, 1990; influence of each of these various conditions on Mpodozis & Allmendinger, 1993; Calderón et al., deformation is unclear, as are the effects of any 2007; Paper I). The Rocas Verdes Basin formation, potential interaction between the conditions. As closure and inversion, and development of the yet no single parameter has been satisfactorily foreland Magallanes Basin is examined here as an in identified as a primary controller of overriding plate depth case study on the progression of deformation deformation, implying that interaction between and crustal evolution at a regional scale. Although the conditions likely plays an important role in deformation in this region was considered determining tectonic regime and deformation style. homogeneous for the purposes of the plate scale Geological relationships at two locations, investigation, when a basin-scale view is taken Seno Otway and Cordillera Darwin, located along significant differences in the geology of northern the length of the Rocas Verdes Basin in Patagonia and southern regions become apparent. This is are investigated in detail to establish plausible consistent with distinct processes and events having relations between subduction and deformation occurred along the length of the basin. For example, in the overriding plate at a regional scale. Seno there is a profound change in metamorphic grade in Otway records what is commonly regarded as a the region of the Cordillera Darwin, which includes complete sequence of marginal basin evolution, the highest grade Cretaceous metamorphic rocks University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 2 south of Ecuador (Klepeis & Austin, 1997). This microscopic scales, and the processes that drive distinction reflects significantly greater shortening deformation. This requires the integration of various of the southern Rocas Verdes Basin, compared with investigation techniques including the analysis that experienced by rocks in northern regions. The of kinematic models, collection and analysis of strike of the orogen also changes from being north- structural data, observation of mineral textural and trending in the north to east-trending in the south. compositional relationships, and quantification There are also distinctions in the depositional facies of timing of metamorphism and metamorphic of characteristic sedimentary formations (Katz, conditions experienced by rocks. Work done 1963; Wilson, 1991; Olivero & Martinioni, 2001; advances the understanding of processes taking Fildani & Hessler, 2005). place during and after Gondwana dispersal, as well Cordillera Darwin forms a topographic high as crustal evolution along the Andean margin, the that lies, on average, more than 1 km above the type example of crustal growth associated with surrounding mountains in the Fuegian Andes ocean – continent convergence. Specifically this (Kranck, 1932; Nelson et al., 1980; Klepeis, thesis addresses: 1994; Kohn et al., 1995; Cunningham, 1995). The (1) The influence of the subduction zone on Cordillera Darwin amphibolite schists include a deformation in the overriding plate (Paper I). A narrow belt of moderate-high pressure (7-11 kbar) recently developed kinematic plate model allowed metamorphic rocks and contain the only known for the evaluation of potential correlations between Cretaceous kyanite in the Andes (Darwin, 1846; modelled plate velocities and oceanic slab ages, Nelson et al., 1980; Kohn et al., 1993). These rocks and deformation in the South American plate over include complex garnet textures, where grains with the past 170 Ma. early inclusions trails and prograde compositional (2) Partitioning of crustal evolution at a local zoning were partially recrystallised to honeycomb scale, namely in the case of the Rocas Verdes and atoll-style textures at upper amphibolite Basin in southern Patagonia (Papers II & III). The facies conditions. The Cretaceous history of these development of models of the tectonic evolution kyanite-bearing rocks north of the Beagle Channel of distinct areas within the basin helps to qualify is interpreted by integrating field relationships with differences in deformational history. the results of mineral equilibria modelling and in- (3) The connection between metamorphic situ U-Pb dating of metamorphic monazite. textures in Cordillera Darwin and distinct phases in The Seno Otway region lies between the development of the Rocas Verdes Basin (Paper comparatively well studied regions to the north II). Garnet growth, patchy garnet recrystallization, and south, making it an ideal location to study the and the development of other mineral assemblages changes in geology that occur along the strike of are all associated with events in the evolution of the Rocas Verdes Basin. Metamorphism in this the Cordillera Darwin region, aided by constraints region does not exceed greeschist facies. Original from geochronology. sedimentary features including paleocurrent (4) The integration of structural and indicators are preserved, allowing for an sedimentalogical data from a previously little interpretation of sediment dispersal patterns to studied area along the Rocas Verdes Basin with the be developed. To facilitate comparison between evolution of the region (Paper III). locations along strike in the Rocas Verdes Basin, models of the Late Jurassic to Paleogene evolution of both the Cordillera Darwin and Seno Otway THESIS OUTLINE regions are developed. Field mapping and sampling were conducted in southern Patagonia during the austral autumn of SCOPE OF THESIS 2009. Petrographic and structural analyses were conducted at the University of Sydney. Raman The main aim of the thesis is to investigate the spectroscopy was conducted at the Australian development of deformation in an ocean – continent Centre for Microscopy and Microanalysis at the convergent margin setting from plate to local and University of Sydney. Microprobe analyses were University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 3 conducted at the Electron Microscope Unit at and the third author is a close collaborator, both of the University of New South Wales. Sensitive whose research grants funded the work and who high-resolution ion microprobe (SHRIMP) helped in the preparation of the manuscript. The geochronology was conducted at the Research fourth author is a collaborator who helped with School of Earth Sciences at the Australian National the SHRIMP geochronology. The fifth author is a University. PhD student who helped with the mineral equilibria Paper I The Late Jurassic to present evolution modelling. of the Andean margin: drivers and the geological Paper III Tectonic inversion of Rocas Verdes record, compiles published Andean literature to marginal basin; the record in Seno Otway, Patagonia, produce a database of deformation and magmatic focuses on a region north of Cordillera Darwin events from 170 Ma to present along the Andean and combines structural and sedimentalogical margin. Convergence velocities, absolute data to produce a model for the evolution of the overriding plate velocities and subducting slab age Rocas Verdes Basin in Seno Otway, and to examine are determined from a kinematic plate model from along-strike differences in the evolution of the 170 Ma to present, and compared to the database basin. A representative cross-section is produced of events to draw out correlations in timing and structural style is evaluated, demonstrating between processes occurring along the subduction decreasing intensity away from obduction thrusts. margin and deformation and magmatism in the Basement involvement indicates greater shortening overriding plate. Potential causal relationships than in regions to the north. Sedimentary facies include: the development of extensional back arc are used to interpret depositional environments, basins involving mafic/oceanic crust associated and paleocurrent indicators imply the presence of with overriding plate velocity directed away from multiple submarine fans in the developing foreland the trench and with modelled subducting slab age basin. I am first author on this paper and 100% of it older than 50 Myr; and the development of major can be credited to this thesis. The second author is fold and thrust belts and plateau uplift associated my PhD supervisor and the third author is a close with convergence velocity higher than 4 cm/yr. I collaborator, both of whose research grants funded am first author on this paper and 100% of itcan the work and who helped in the preparation of the be credited to this thesis. The second author is manuscript. my PhD supervisor and the third author is a close The Discussion provides a concise summary collaborator; both helped in the preparation of the of all the important findings as well as suggestions manuscript. The fourth author is a post-doctoral for future research. The major findings of this fellow and helped with computer scripting. thesis are integrated with the current theories for Paper II Crustal growth during back- the tectonic evolution of the Andean margin and arc closure: Cretaceous exhumation history the Rocas Verdes marginal basin. of Cordillera Darwin, southern Patagonia, examines the mineral assemblages present in the Cordillera Darwin amphibolite facies terrane and integrates the metamorphic data with structural data and geochronology to produce a model for the tectonic evolution of the region. Calculated P-T pseudosections indicate that continental underthrusting to depths of c.35 km produced the metamorphism, and subsequent thrust driven exhumation coupled with an influx of fluid resulted in the development of patch textures within garnet grains. SHRIMP geochronology on monazite allowed the age of thrust-related fabric development to be constrained. I am first author on this paper and 100% of it can be credited to this thesis. The second author is my PhD supervisor I University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 5

The Late Jurassic to present evolution of the Andean margin: drivers and the geological record

K. T. MALONEY1, G. L. CLARKE1, K. A. KLEPEIS2, AND L. QUEVEDO1 1 School of Geosciences, University of Sydney, NSW 2006, Australia 2 Department of Geology, University of Vermont, Burlington, VT, 05405, USA

ABSTRACT Uncommonly long-lived subduction and variable plate geometry along the South American Andean plate margin resulted in diverse relationships between magmatic flux, extensional and contractional deformation, as recorded by the overriding continental plate. Convergence velocities, absolute overriding plate velocities and subducting slab ages were resolved along the trench from 170 Ma to the present using a recently developed kinematic global plate model to identify any relationship between subduction conditions, deformation style and magmatic features in the overriding plate. Key correlations reflect the dependence of macroscopic crustal strain style on subduction mechanism and relative plate vectors. Extensional back arc basins involving mafic/ oceanic crust developed only when the overriding plate velocity of South America was directed away from the trench and the modelled age of the subducting slab was older than 50 Myr. The development of fold and thrust belts, and uplift of major plateaus was accompanied by trench normal convergence velocities in excess of 4 cm/yr. Parameters investigated in this study revealed no correlation with the timing of major magmatic events, nor was any correlation observed with the structural style of fold and thrust belts.

Key words: Andes, subduction, kinematics, plate velocities, deformation

INTRODUCTION and duration. The influence of any given subduction zone(s) The Andean margin of South America is commonly on deformation in the overriding plate has been considered the type example of an ocean – investigated along the South American margin continent convergent margin. Initially situated by various authors. Numerous conditions have along the southwestern margin of Gondwana prior been implicated as drivers of deformation and to its dispersal, this margin has been the site of magmatism, including the rate of convergence subduction since at least the Jurassic (James, 1971; between subducting and overriding plates (Pilger, Coira et al., 1982; Pankhurst et al., 2000; Oliveros 1984; Yañez & Cembrano, 2004; Capitanio et al., et al., 2006). During this long-lived subduction the 2011; Ramos, 2010; Daly, 1989), the absolute Andean margin developed a varied and commonly velocity of South America in a mantle or hotspot complex sequence of superimposed deformation and reference frame (Ramos, 2010; Yañez & Cembrano, magmatic events. This paper examines relationships 2004; Martinod et al., 2010), the model age of the between conditions imposed on the margin by the subducting slab (Capitanio et al., 2011; Yañez subduction zone(s) and the development of specific & Cembrano, 2004; Pilger, 1984), the dip of the significant geological events in the arc and back subducting slab (Pilger, 1981, 1984; Jarrard, 1986; arc regions of the overriding South American Lallemand et al., 2005; Martinod et al., 2010), plate over the past 170 Ma. Geological events trench velocity (Pindell & Tabbutt, 1995; Oncken identified in this analysis needed to be of sufficient et al., 2006; Ramos, 2010) and the lateral width of magnitude to have been influenced by plate-scale the subducting slab (Schellart, 2008; Schellart et processes, with reasonable control on their timing al., 2007). The relative influence of each of these University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 6

80°W 70°W 60°W 80°W 70°W 60°W A B 10°N 10°N

EC CMS Ma c oored back arc basin

0° Continental rift basin 0° Fold and thrust belt Foreland basement uplifts NPSA Plateau Batholith CBB

10°S 10°S CHB MAR PCB HCB PCB SA

APP 20°S 20°S SB

AP SBS

30°S CCB 30°S LR SP COMB AC CUMB MAL CM G AG AL 40°S 40°S NCCB

P PFB PFB

50°S (Tierra del 50°S Fuego) MAG RVB

Fig. 1. A) Late Jurassic to Cretaceous deformation and magmatic events along the Andean margin. Note that as much of the crust flooring the mafic back arc basins is no longer extant the outlines shown are interpreted potential extents. In the case of the Rocas Verdes Basin the northernmost ophiolite fragments are reported at 52°S, so this has been used as the northernmost point in the analysis, but the basin may have stretched farther north. CMS: Colombian Marginal Seaway, PCB: Peruvian Coastal Batholith, HCB: Huarmey and Cañete Basins, SB: Salta Basin, CCB: Central Chilean Basin, PFB: Patagonian and Fuegian Batholiths, RVB: Rocas Verdes Basin. B) Cenozoic deformation and magmatic events along the Andean margin. EC: Eastern Cordillera of Colombia, NPSA: North Peruvian Sub-Andean fold and thrust belt (FTB), MAR: Marañon FTB, CBB: Cordillera Blanca Batholith, CHB: Callejon de Huaylas Basin; PCB: Peruvian Coastal Batholith, APP: Altiplano-Puna Plateau, SA: Sub- Andean FTB, SBS: Santa Bárbara System, AP: Argentine Precordillera, SP: Sierras Pampeanas, AC: Aconcagua FTB, LR: La Ramada FTB, MAL: Malargüe FTB, CM: Chos Malal FTB, G: Guañacos FTB, AG: Agrio FTB, COMB: Coya-Machalí Basin, CUMB: Cura Mallín Basin, AL: Aluminé FTB, NCCB: Ñirihuau-Collón Curá Basin, PFB: Patagonian and Fuegian Batholiths, P: Patagonian FTB, MAG: Magallanes FTB. Data sources include: Ramos et al., 1996; Hervé et al., 2007; Atherton & Petford, 1996; Kley & Monaldi, 2002; Fosdick et al., 2011; Ramos et al., 2002; Klepeis & Austin 1997; Gubbels et al., 1993; Allmendinger et al., 1997; Atherton & Aguirre, 1992; Marquillas et al., 2005; Zapata & Folguera, 2005; Vietor & Echtler, 2006; Mégard, 1984; Spalletti & Dalla Salda 1996; García Morabito & Ramos, 2012; Paredes et al., 2009; Ramos & Kay, 2006; Mora et al., 2009; Giovanni et al., 2010. University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 7 various conditions on deformation is unclear, as major magmatic events. This method implicitly are the effects of any potential interactions between accepts the kinematic model; uncertainties with the conditions. As yet no single parameter has been respect to plate position and motion increase with satisfactorily identified as a primary controller age, particularly in the Pacific region. The model of overriding plate deformation, implying that was not itself explicitly tested. However, a first interaction between the conditions likely plays an order evaluation of the modelling outcomes is important role in determining tectonic regime and provided by the geological record, as discussed deformation style. below. Though slab dip and mantle flow are A well constrained dataset is essential in commonly considered significant parameters elucidating relationships between subduction influencing geometry, plate coupling and tectonic zone dynamics and the geological evolution setting (Jarrard, 1986; Ramos, 2010) they were of the overriding South American plate. The omitted from this study as there are no accurate identification of plausible correlations between models of dip or mantle flow patterns back through subduction parameters and deformation requires time developed independently of the geology of the that the initiation and duration of any deformation margin. event be precisely constrained. Recent advances in dating techniques have provided refined constraints on event ages. In addition, data from recent field OVERVIEW OF DEFORMATION IN THE studies (eg. Villagómez et al., 2011; McAtamney et ANDES al., 2011) has provided an improved understanding of key parts of the Andean margin, and more The effects of post-Jurassic deformation along the detail in the nature and timing of deformation and western margin of South America can be placed into magmatic events. either of two broad categories: extension during A recently developed plate kinematic model the Late Jurassic and Cretaceous, and contraction (Seton et al., 2012) allows the reconstruction of from the Late Cretaceous to the present (Fig. 1). tectonic plate configuration back to 200 Ma. It The margin is divided into five regions: northern, was used to investigate relationships between Peruvian, central, south-central, and southern subduction and geologic events recorded by Andes. This arbitrary division simply facilitates the the overriding South American plate. A hybrid discussion and evaluation of complex detail in the absolute reference frame comprising a moving history of the Andean margin, and highlights any Indian/Atlantic hotspots reference frame (O’Neill dependence between particular geological events et al., 2005) from 100 Ma to the present and a and the edge of a subduction zone in the geological paleomagnetically derived true polar wander evolution the region. corrected reference frame (Steinberger & Torsvik, 2008) back to 200 Ma is used for the model, with Northern Andes (8°N to 5.5°S) the Pacific reference frame modified from Wessel The region defined here as the northern Andes and Kroenke’s (2008) fixed Pacific hotspots to extends from Colombia to northern Peru. Tectonic remove certain unrealistically fast plate motions. style in the region was influenced by its interaction This model incorporates the breakup of the Ontong- with the Caribbean plate system (Pindell & Java, Hikurangi and Manihiki plateaus (Taylor, Kennan, 2009). During the Mesozoic, the Northern 2006) in the south Pacific, which has repercussions Andean margin comprised a series of accreted and in the calculated locations of ridge subduction and para-autochthonous terranes juxtaposed against oceanic plate velocities experienced by the Andean autochthonous continental crust (McCourt et al., margin. The model and associated oceanic crust 1984; Nivia et al., 2006; Villagómez et al., 2011). age grid (M. Seton & R.D. Müller, unpublished Extension and lithospheric thinning of the margin data) were used to investigate convergence rates, occurred from the Late Jurassic to Early Cretaceous. the absolute motion of South America and the ages The igneous Quebradagrande Complex consists of of subducting slabs in relation to the timing of the both MORB and arc-related rocks, erupted through development of extensional basins, fold and thrust attenuated crust (Villagómez et al., 2011). This belts, uplift of the Altiplano-Puna plateau, and event resulted in the Colombian Marginal Seaway University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 8

(8°N–5.5°S), a c.140 – 130 Ma north-trending Ma resulted in the still active extensional Callejon basin floored by oceanic crust. The basin has been de Huaylas Basin (8.5–10°S; Giovanni et al., 2010; extrapolated to the proposed Proto-Caribbean McNulty & Farber, 2002). The Callejon de Huaylas Seaway, which closed before c.100 Ma (Kennan Basin is one of only a handful of locations along & Pindell, 2009; Aspden & McCourt, 1986; Nivia the margin to record Cenozoic extension (Fig. 1). et al., 2006). Shortening in the early Cenozoic led to out of sequence reactivation of Mesozoic Central Andes (14°S to 34°S) extensional rifts in and uplift the Eastern Cordillera The region defined here as the central Andes of Colombia (6– 2°N) at c.60 – 20 Ma (Parra et al., comprises the Altiplano-Puna plateau and the 2009a; 2009b; 2012). adjacent foreland of southernmost Peru, extending through northern and central Chile to western parts Peruvian Andes (5.5°S to 14°S) of Argentina. The southern portion of the Cañete The region defined here as the Peruvian Andes basin and the southernmost units of the Peruvian extends from northern to southern Peru, terminating Coastal Batholith both extend southwards into this at the the start of the Altiplano-Puna plateau. Late part of the Andes. Cretaceous extension resulted in Jurassic and Cretaceous extension continued along the opening of the c.130 – 70 Ma continental Salta this portion of the margin and led to the Tithonian basin (22–27.5°S) comprising a series of seven sub- (c.147 Ma; Gradstein et al., 1994) aged opening basins radiating from a central high (Marquillas of the Huarmey and Cañete basins (Cobbing, et al., 2005; Carrera et al., 2006; Reyes, 1972) 1978; Atherton & Aguirre, 1992). Sheeted mafic and, immediately to the south, the opening of the dyke swarms and pillow lavas in the Huarmey Central Chilean Basin (27–34°S) during Aptian basin (8–12°S) are interpreted to reflect extensive and Cenomanian (c.120 – 94 Ma) times (Mpodozis crustal extension (Atherton et al., 1983), on a scale & Allmendinger, 1993; Levi & Aguirre, 1981). inadequate to form new oceanic crust (Ramos, The Altiplano-Puna plateau of the central 2010). Crustal extension without the development Andes is second only to the Tibetan Plateau in height of new oceanic crust also formed the Cañete basin and extent, and is the highest plateau associated (12–16°S) (Atherton & Aguirre, 1992). The main with abundant and on-going arc magmatism phase of subsidence in both basins occurred in (Allmendinger et al., 1997). It is divided into two the Albian (c.99 Ma) and sediment accumulation regions based on distinctions in uplift history and ceased soon after (Cobbing, 1978). Components morphology: the Altiplano portion to the north of the c.105 – 37 Ma Peruvian Coastal Batholith (14–22°S) and the higher Puna portion to the south include several super-units (Mukasa, 1986) distinct (22–28°S; Whitman et al., 1996; Isacks, 1988). in timing and geochemistry. The most voluminous South of the Puna plateau Andean altitudes remain flux is recorded by parts of the c.86 – 70 Ma Santa higher than 3 km to c.34°S (Whitman et al., 1996; Rosa and Tiabaya super-units (Mukasa, 1986). Allmendinger et al., 1997). Crustal uplift to form Cenozoic contraction first developed in this region the Altiplano may have begun as early as the with the mid to late Eocene thin-skinned Marañon Eocene, but most uplift is thought to have occurred fold and thrust belt (7–12.5°S). Reactivation of between c.25 and 12 Ma. Uplift of the Puna plateau these thrusts occurred during a second phase of commenced between c.20 and 15 Ma and continued contraction from c.20 – 13 Ma (Mégard, 1984). The until c.2 – 1 Ma (Allmendinger et al., 1997). This locus of deformation then shifted eastward at c.11 region has the thickest crust of the Andean margin, Ma to the presently active thin-skinned northern being c.60 – 65 km thick beneath the Altiplano, Peruvian Sub-Andean fold and thrust belt (3–11°S; and approximately 10 km thinner beneath the Puna Audebaud et al., 1973; Rousse et al., 2003; Mégard, portion (Beck et al., 1996; Beck & Zandt, 2002; 1984). To the west of these fold and thrust belts Gerbault et al., 2005). the c.13 – 3 Ma Cordillera Blanca Batholith was On the eastern flank of the Altiplano, in the emplaced approximately 100 km inboard of the Eastern Cordillera, the thick-skinned reactivation Peruvian Coastal Batholith (Atherton & Petford, of earlier Paleozoic compressive and Mesozoic 1996). Detachment faulting along the western flank extensional structures is interpreted to have of the Cordillera Blanca Batholith beginning at c.5 occurred between c.40 and 20 Ma; the locus of University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 9 deformation is inferred to have migrated eastward South-Central Andes (34°S to 42.5°S) after 20 Ma to develop a thin-skinned Sub-Andean The region defined here as the south-central fold and thrust belt that is still active (McQuarrie Andes extends south from the termination of the et al., 2005; Gubbels et al., 1993; Burchfiel et al., Puna plateau through to central Chile. Widespread 1981; Isacks, 1988). The continuous nature of the extension and rifting took place in this region prior Sub-Andean fold and thrust belt, which developed to the period investigated in the study with the along the length of the Altiplano, contrasts with opening of the Triassic Neuquén Basin (Vergani et the varied deformation fabrics in foreland crustal al., 1995) and reactivation of these structures plays material adjacent the Puna portion of the plateau. an important role in the later tectonic evolution The Puna foreland comprises several fold and of the region. An early phase of contractional thrust belts and basement terranes of distinct style deformation in the late Mesozoic resulted in and age. Flat slab subduction of the Nazca plate the formation of a series of fold and thrust belts. between 27 and 33°30’S (Barazangi & Isacks, The northernmost of these, the Chos Malal fold 1976) introduced further tectonic complexity in and thrust belt (36–37.5°S), records thin-skinned deformed rocks of the southernmost Puna foreland. deformation with a detachment surface along Late A structural inversion of the Cretaceous Salta Basin Jurassic evaporites (Folguera et al., 2007). Timing began after c.9 Ma (Reynolds et al., 2000) to form of deformation in the belt is poorly constrained to the thick-skinned thrust belt of the Santa Bárbara sometime between the mid to late Cretaceous, with System (23–27°S) at the northern limit of flat slab a pulse of uplift recorded at c.70 Ma (Burns, 2002; subduction (Grier et al., 1991; Kley & Monaldi, Burns et al., 2006; Kay et al., 2006). Following 2002). The influence of flat-slab subduction of the south from the Chos Malal belt, the Agrio fold and Nazca plate is also seen in foreland basement uplifts thrust belt (37.5–38.5°S) records a combination of of the Sierras Pampeanas (27–33.5°S) where large deformation styles from c.102 – 70 Ma with thick- basement blocks were uplifted in the late Cenozoic, skinned reactivation of Neuquén Basin normal starting from c.7 Ma (Ramos et al., 2002). Directly faults in the inner sector, and thin-skinned thrusting east of the Sierras Pampeanas thin-skinned thrusting in the outer, forelandward sector (Zamora Valcarce in the Argentine Precordillera (28–33°S) developed et al., 2006; Zapata & Folguera, 2005). Reactivation along a Cambo-Ordivician limestone detachment of Neuquén Basin faults is also recorded from at surface starting at c.21 Ma (Jordan et al., 1993; least c.75 – 65 Ma in the thick-skinned Aluminé Vietor & Echtler, 2006). To the east and south of the fold and thrust belt (38.5–40.5°S; García Morabito Argentine Precordillera a string of fold and thrust & Ramos, 2012). belts developed in close proximity, and at about This region records Cenozoic (Oligocene – the same time, but display distinct structural styles. Miocene) extension in a string of basins, with rare The La Ramada fold and thrust belt (31.5–32.5°S) equivalent structures elsewhere along the margin. records alternating episodes of thin- and thick- Active extensional faulting in the northernmost skinned thrusting (Cristallini & Ramos, 2000). basin, the Coya-Machalí Basin (33–36°S), took Thin-skinned thrusting, involving a detachment place from c.34 – 23 Ma (Charrier et al., 2002; floored by evaporite, occurred betweenc .20 and 14 Godoy et al., 1999). Farther south, extension in the Ma. The reactivation of Triassic normal faults then Cura Mallín Basin (36–38°S) occurred from c.26 – led to thick-skinned style deformation between c.14 20 Ma (Burns et al., 2006; Jordan et al., 2001). The and 12.7 Ma. This produced a sticking point in the southernmost of these basins is the c.34 – 18 Ma foreland propagation of orogeny and resulted in out Ñirihuau-Collón Curá Basin (40–42.5°S; García of sequence thin-skinned thrusting on the west side Morabito & Ramos, 2012; Franzese et al., 2011; of the belt between c.12 and 9.2 Ma (Cristallini & Rapela et al., 1983; Giacosa et al., 2005). Ramos, 2000; Ramos et al., 1996). Thin-skinned The Oligocene – Miocene extensional phase thrusting in the Aconcagua fold and thrust belt was followed by a period of renewed contraction (32.5–34°S) began at c.22 Ma, coeval with, or just that involved the development of new fold and prior to, uplift in the Puna plateau, and continued thrust belts and the reactivation of pre-existing until c.8 Ma (Ramos et al., 1996; Ramos, 1985). ones. The Malargüe fold and thrust belt (34–36°S), developed immediately south of the Aconcagua belt University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 10

(central Andes), and is characterized primarily by the North Patagonian Batholith (40–47°S), the thick-skinned deformation (Kozlowski et al., 1993; South Patagonian Batholith (47–53°S), and Ramos et al., 1996) that involved the reactivation the Fuegian Batholith (53–56°S). Each of the of extensional structures of the Triassic Neuquén batholiths formed mostly during two episodes of Basin (Giambiagi et al., 2008). The main phase magma flux, of distinct ages. A first episode of the of deformation in the Malargüe fold and thrust North Patagonian Batholith lasted from c.140–95 belt occurred between c.15 and 8 Ma, although Ma, and a second from c.20–8 Ma (Pankhurst et deformation continued until c.1 Ma (Giambiagi al., 1999). The South Patagonian Batholith mostly et al., 2008; Turienzo, 2010; Ramos et al., 1996). formed between c.157 and 137 Ma (Hervé et al., Dominantly thick-skinned reactivation in the Chos 2007), with a subsidiary episode between c.25 Malal fold and thrust belt occurred from c.15 – 12 and 15 Ma (Hervé et al., 2007). The c.157 Ma Ma and involved the inversion of Early Jurassic episode was partially contemporaneous with silicic extensional structures as well as the reactivation volcanism of the Tobifera Formation (Hervé et of Late Cretaceous thrust faults (Kay et al., 2006 al., 2007). Episodic development of this batholith Folguera et al., 2007). West of the Chos Malal belt continued throughout much of the Cretaceous, the closure of the Cura Mallín Basin accompanied magma flux migrating westward with time until the development of the Guañacos fold and thrust c.75 Ma. The Fuegian Batholith in Tierra del Fuego belt (36.5–37.5°S) starting at c.9 Ma (Folguera et records a c.141–81 Ma episode, followed by a 20 al., 2006; Burns et al., 2006). Deformation in this Myr hiatus before magmatism resumed in a second belt is predominantly thin-skinned north of 37°S, episode that extended from c.60–34 Ma (Hervé et with a mix of thick and thin-skinned styles south al., 1984; Halpern, 1973). of 37°S (Folguera et al., 2006; 2007). South of The opening of the Rocas Verdes back the Chos Malal belt, the Agrio fold and thrust belt arc basin (52–56°S) at c.152 Ma (Calderón et al., experienced thick-skinned reactivation from c.7 – 2007) was accompanied by the development of 5 Ma associated with the closure of the southern oceanic crust (Dalziel et al., 1974). Basin inversion portion of the Cura Mallín Basin (Zapata & has been inferred to be related to a change in the Folguera, 2005). Reactivation of the Aluminé fold absolute plate motion of South America at c.100 and thrust belt from c.11 – 3 Ma accompanied the Ma (Somoza & Zaffarana, 2008; Ramos, 2010; this closure of the Ñirihuau-Collón Curá Basin (García study). By c.92 Ma convergence had resulted in the Morabito et al., 2011; Rosenau et al., 2006), formation of the thin-skinned Magallanes fold and the southernmost of the Oligocene – Miocene thrust belt (52–56°S) and associated foreland basin extensional basins. (Fildani et al., 2003; Fildani & Hessler, 2005; Nelson et al., 1980). The Magallanes fold and thrust Southern Andes (42.5°S to 56.5°S) belt is the southernmost segment of the Patagonian The region defined here as the southern Andes fold and thrust belt (47–56°S). Constraints on the consists of central to southernmost Chile, and timing of deformation in northern segments are central to southernmost western Argentina. It is poor; deformation is variously recorded to have south of the former Ñirihuau-Collón Curá Basin occurred sometime between the Maastrichtian and and extends to the southernmost point of South Middle Miocene (c.71 – 13 Ma) in some locations America. Widespread grabens and half grabens (Suárez et al., 2000; Kraemer, 1998). Thrusting that include silicic volcanic rocks of the Tobifera continued in the Magallanes fold and thrust belt Formation developed along this portion of the until the Late Oligocene/Early Neogene (Klepeis, Andean margin between c.178 and 153 Ma; they 1994). Sinistral strike-slip faults observed in the have been interpreted as marking the onset of foreland (Cunningham, 1993) are inferred to have Gondwana dispersal (Bruhn et al., 1978; Pankhurst dominated deformation in the region after the main et al., 2000). thrusting events (Klepeis, 1994). Arc-derived plutonic rocks extend continuously along the plate margin in this region to Tierra del Fuego (Stern & Stroup, 1982; Hervé et al., 1984). They form a distinct belt that includes University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 11

−80˚ Andean margin that experienced oroclinal bending, such as the Bolivian or Patagonian Oroclines, the 0˚ 170-153 Ma 152-121 Ma 120-86 Ma 85-56 Ma effect of block rotations are likely to be appreciable. (160 Ma) (135 Ma) (100 Ma) (70 Ma) FAR For instance, parts of the Patagonian Orocline record FAR FAR rotations of c.90° relative to the South American −20˚ FAR craton (Cunningham et al., 1991). In these regions care must be taken when interpreting the results, CHA and the study can be viewed as an investigation of PHX −40˚ only first-order effects. PHX The shape and position of the defined

CAT subduction trench relative to the South American

ANT craton were also kept constant. Therefore, changes

ANT ANT in shape or position of the trench resulted in some −60˚ of the fifty five points being slightly offset from the actual trench position. In extreme cases, several COC COC of the northernmost points were no longer within 55-24 Ma 23-21 Ma 20-0 Ma (40 Ma) (22 Ma) (5 Ma) reasonable distance of the active subduction trench; for example, some were located in the central regions of the Caribbean plate. Therefore, data NAZ NAZ FAR for these points after 70 Ma were ignored. As the development of the Eastern Cordillera of Colombia occurred during this interval that event has been neglected from the analysis. Several other geologic events have necessarily been excluded from ANT analysis due to inadequate constraints on timing of initiation and duration of the event. These excluded events include the Cretaceous aged first episode of Fig. 2. The various plate configurations along the Andean thrusting in the Chos Malal fold and thrust belt in margin from 170 Ma to the present in a fixed South America the south-central Andes, and the development of reference frame. Time ranges for when the given configuration existed are given, while the age in brackets is the actual portions of the Patagonian fold and thrust belt of reconstruction age show. FAR: Farallon Plate, PHX: Phoenix the southern Andes north of 52°S. Plate, ANT: Antarctic Plate, CHA: Chasca Plate, CAT: Catequil Though South America was the only Plate, COC: Cocos Plate, NAZ: Nazca Plate continental plate of interest, the Pacific margin SUBDUCTION PARAMETERS interacted with several different oceanic plates. These include the Farallon, Phoenix, Chasca, Parameters characterizing plate motion at the Catequil, Antarctic, Nazca and Cocos plates; the subduction margin were resolved at fifty five points evolving plate configuration is illustrated in Fig. 2. along a defined Andean trench from 170 Ma to the The Chasca and Catequil plates are new plates that present. Point spacing was chosen to give good are introduced in the kinematic model developed coverage of the margin. The plate model used in by Seton et al. (2012). this study is composed of rigid, undeformable There is a possibility that that Antarctic plates, with South America treated as a single rigid Peninsula may have lain immediately west of the block such that the overriding plate velocity used southernmost extent of the South American Pacific in the analysis is that of the South American craton. margin during parts of the Mesozoic (Lawver et Any smaller block rotations, such as that between al., 1998; Ghidella et al., 2002; Jokat et al., 2003; Patagonia and the rest of South America, and the Hervé et al., 2006). Therefore, subduction might not introduction of the Scotia Plate in southernmost have occurred along the posited margin for some South America at 40 Ma have not been considered. of the earliest times investigated, as the earliest This oversimplification is likely valid for only some evidence of subduction along this portion of the portions of the Andean margin. In regions of the margin corresponds to c.157 Ma components of the University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 12

−100˚ −80˚ −60˚

120 Age of Subducting Slab at Point 1

80 40 1 0 Age of Slab (Myr) Uplift of Altiplano and Puna plateaus 120 Age of Subducting Slab at Point 7 Emplacement of major plutons 80 0˚ Extension and rifting without 0˚ 40 7 formation of oceanic crust 0 Rifting resulting in the formation of Age of Slab (Myr) oceanic crust Development of thin-skinned fold 120 Age of Subducting Slab at Point 13 and thrust belt 80 13 Development of basement thrusts in 40 the foreland

0 Development of thick-skinned fold

Age of Slab (Myr) and thrust belt Farallon Plate

120 Age of Subducting Slab at Point 19 Phoenix Plate Chazca Plate 80 19 Catequil Plate 40 Antarctic Plate 0 Cocos Plate Age of Slab (Myr) 25 Nazca Plate −20˚ −20˚ Location of 120 Age of Subducting Slab at Point 25 subducting 80 spreading ridges

40

0

Age of Slab (Myr) 31

120 Age of Subducting Slab at Point 31

80

40

0

Age of Slab (Myr) 37

120 Age of Subducting Slab at Point 37

80

40

0 −40˚ Age of Slab (Myr) 43 −40˚

120 Age of Subducting Slab at Point 43

80

40

0 Age of Slab (Myr)

120 Age of Subducting Slab at Point 49 80 49 40

0 Age of Slab (Myr)

120 Age of Subducting Slab at Point 55

80

40

0 Age of Slab (Myr) 160 140 120 100 80 60 40 20 0 55 Time (Ma)

−100˚ −80˚ −60˚ Fig. 3. Age of subducting slab as a function of time for 10 of the 55 points with the duration of the deformation and magmatic events overlain on the plots. Map indicates the present day location of the 10 points. The colour of the x-axis line indicates which slab was subducting at that location for a given time. University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 13

Trench Normal Absolute Velocity of South America at Point 1 Trench Parallel Absolute Velocity of South America at Point 1 4 4

2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Absolute Velocity of South America at Point 7 Trench Parallel Absolute Velocity of South America at Point 7 4 4 2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Absolute Velocity of South America at Point 13 Trench Parallel Absolute Velocity of South America at Point 13 4 4

2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Absolute Velocity of South America at Point 19 Trench Parallel Absolute Velocity of South America at Point 19 4 4

2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Absolute Velocity of South America at Point 25 Trench Parallel Absolute Velocity of South America at Point 25 4 4 2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Absolute Velocity of South America at Point 31 Trench Parallel Absolute Velocity of South America at Point 31 4 4 2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Absolute Velocity of South America at Point 37 Trench Parallel Absolute Velocity of South America at Point 37 4 4 2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Absolute Velocity of South America at Point 43 Trench Parallel Absolute Velocity of South America at Point 43 4 4 2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Absolute Velocity of South America at Point 49 Trench Parallel Absolute Velocity of South America at Point 49 4 4 2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Absolute Velocity of South America at Point 55 Trench Parallel Absolute Velocity of South America at Point 55 4 4

2 2 0 0 −2 −2 −4 Velocity (cm/yr) Velocity (cm/yr) 160 140 120 100 80 60 40 20 0 160 140 120 100 80 60 40 20 0 Time (Ma) Time (Ma) Fig. 4. Trench normal absolute plate velocity of South America and trench parallel absolute plate velocity of South America as functions of time with the duration of the deformation and magmatic events overlain on the plots. Points and colours are the same as in Fig. 3. University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 14

Trench Normal Convergence Velocity at Point 1 Trench Parallel Convergence Velocity at Point 1 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Convergence Velocity at Point 7 Trench Parallel Convergence Velocity at Point 7 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Convergence Velocity at Point 13 Trench Parallel Convergence Velocity at Point 13 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Convergence Velocity at Point 19 Trench Parallel Convergence Velocity at Point 19 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Convergence Velocity at Point 25 Trench Parallel Convergence Velocity at Point 25 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Convergence Velocity at Point 31 Trench Parallel Convergence Velocity at Point 31 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Convergence Velocity at Point 37 Trench Parallel Convergence Velocity at Point 37 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Convergence Velocity at Point 43 Trench Parallel Convergence Velocity at Point 43 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Convergence Velocity at Point 49 Trench Parallel Convergence Velocity at Point 49 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr)

Trench Normal Convergence Velocity at Point 55 Trench Parallel Convergence Velocity at Point 55 12 12 8 8 4 4 0 0 −4 −4 Velocity (cm/yr) Velocity (cm/yr) 160 140 120 100 80 60 40 20 0 160 140 120 100 80 60 40 20 0 Time (Ma) Time (Ma) Fig. 5. Trench normal convergence velocity between South America and the relevant downgoing slab, and trench parallel convergence velocity between South America and the relevant downgoing slab as functions of time with the duration of the deformation and magmatic events overlain on the plots. Points and colours are the same as in Fig. 3. University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 15

Table 1. Timing, location and conditions imposed by the subduction zone under which various geological events along the margin developeda,b Normal Normal Normal Conv Conv Conv Abs Abs Abs Slab Velocity Velocity Velocity Approximate Timing Velocity Velocity Velocity Age (norm) (norm) (norm) Event Location (Ma) <0cm/yr >0cm/yr >2cm/yr >50 Ma <0cm/yr >0cm/yr >4cm/yr

Continental Extension Sub-Andean rifting (north) 8°N to 7°S 190 - 140 X X X Sub-Andean rifting (south) 7°S to 14°S 190 - 140 X X X Huarmey and Cañete Basins 8°S to 16°S 147 - 99 X X X X X Callejon de Huaylas Basin 8.5°S to 10°S <5 X X X Salta Basin 22°S to 27.5°S 130 - 70 X X X X Central Chilean Basin 27°S to 34°S 120 - 94 X X X X Coya-Machalí Basin 33°S to 36°S 34 - 23 X X X Cura Mallín Basin 36°S to 38°S 26 - 20 X X X Ñirihuau-Collón Curá Basin 40°S to 42.5°S 34 - 18 X X Gondwana Dispersal Rifting 50°S to 56°S 178 - 153 X X X

Extension With New Oceanic Crust Colombian Marginal Seaway 8°N to 5.5°S 140 - 130 X X X Rocas Verdes Basin (north) 52°S to 54.5°S 152 - 100 X X Rocas Verdes Basin (south) 54.5°S to 56°S 152 - 100 X X X

Fold and Thrust Belts (FTB) North Peruvian Sub-Andean FTB 3°S to 11°S <11 X X X Marañon FTB 7°S to 12.5°S 20 - 13 X X X X Sub-Andean FTB 14°S to 22°S <20 X X X X Santa Bárbara System 23° to 27°S <9 X X X X Sierras Pampeanas Uplift 27°S to 33.5°S <8 X X X Argentine Precordillera 28°S to 33°S <21 X X X X La Ramada FTB 31.5°S to 32.5°S 20 - 9 X X X X Aconcagua FTB 32.5°S to 34°S 22 - 8 X X X X Malargüe FTB 34°S to 36°S 15 - 8(- 1)c X X X X Guañacos FTB 36.5°S to 37.5°S <9 X X X Chos Malal FTB 36°S to 37.5°S 15 - 12 X X X X Agrio FTB (1) 37.5°S to 38.5°S 102 - 70 X X Agrio FTB (2) 37.5°S to 38.5°S 7 - 5 X X X Aluminé FTB (1) 38.5°S to 40.5°S 75 - 65 X X X Aluminé FTB (2) 38.5°S to 40.5°S 11 - 3 X X X Magallanes FTB 52°S to 56°S 92- 20 X X X

Plateau Uplift Altiplano Plateau Uplift 14° to 22°S 25 - 12 X X X X Puna Plateau and Southern Uplifts 22°S to 34°S 20 - 1 X X X X

Magmatic Events Peruvian Coastal Batholith 8°S to 16°S (105 -)86 - 70(- 37)c X X X North Patagonian Batholith (1) 40°S to 47°S 140 - 95 X X X X X North Patagonian Batholith (2) 40°S to 47°S 20 - 8 X X X South Patagonian Batholith (1) 47°S to 53°S 157 - 75 X X X X X Fuegian Batholith (1) 53°S to 56°S 141 - 81 X X X Fuegian Batholith (2) 53°S to 56°S 60 - 34 X X X X South Patagonian Batholith (2) 47°S to 53°S 25 - 15 X X X Cordillera Blanca Batholith 8°S to 10°S 13 - 3 X X X aSee text for references bTimescale from Gradstein et al . (1994) used to convert stratigraphic ages to absolute ages cParentheses in Timing indicate minor phases of deformation or magmatic events. The ages not in parentheses indicate the major phase of deformation, or period of most voluminous magmatism, while ages within parentheses indicate periods of minor deformation or less voluminous magmatic emplacement.

South Patagonian Batholith (Hervé et al., 2007). the defined trench; the position of a representative However, as both the position of the peninsula and ten of the fifty five points are shown in Fig. 3. the timing of exactly when overlap between the The absolute plate velocity of South America was peninsula and southernmost South America ceased determined normal and parallel to the trench (Fig. are poorly constrained, subduction parameters 4). In the case of the trench normal absolute velocity, were extracted as if subduction did occur along this positive values indicate that the overriding South portion of the South American margin for the entire American plate was moving toward the trench at period investigated. that time, whereas negative values reflect motion The first subduction parameter is the age of away from the trench. Trench parallel northward the subducting slab, established by sampling the motion of the overriding South American plate reconstructed age grid at fifty five points along is indicated by positive values, negative values University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 16 indicate trench parallel motion to the south. the model age of the subducted slabs individually Similarly, relative convergence velocities between correlate well with the progression from crustal South America and the various subducting oceanic extension to back arc spreading (Figs 3 & 4). Though slabs were determined both normal and parallel east–directed motion of the South American plate to the trench (Fig. 5). Positive trench normal occurred along much of the margin between c.160 convergence velocities indicate convergence and 120 Ma, oceanic crust developed only at those between South America and the subducting plate, segments of the margin where comparatively old whereas negative values indicate divergence. In the ocean crust was subducted. Examples include the case of trench parallel convergence rates, positive formation of the Colombian Marginal Seaway in values indicate right-lateral motion along the the northern Andes, and the Rocas Verdes Basin of margin and negative velocities indicate motion was the southern Andes (Fig. 1). A strong correlation left-lateral. (coefficient 0.33) can also be established between these events and normal convergence velocities less than 0 cm/yr, though this association is weakened RESULTS by being absent from northern parts (north of 54.5°S) of the Rocas Verdes Basin. A comparison of model data pertinent to The development of the Altiplano-Puna convergence rate, absolute plate motion and the plateau of the central Andes and its flanking fold model ages of subducting slabs along the South and thrust belts was accompanied by high relative America Pacific margin establishes some distinctive trench normal convergence rates (magnitude >4 correlations with its geologic record, including the cm/yr). This relation can be extrapolated for almost development of extensional basins, fold and thrust all compressional events recorded by the margin. belts, uplift of plateaus, and major magmatic events An exception is the Magallanes fold and thrust belt (Table 1). All correlation coefficients found during in the Southern Andes (Fig. 1); its development the analysis are low (less than 0.39), but all have was accompanied comparatively low positive or p-values of much less than 1%, indicating that negative convergence rates (magnitude <2 cm/ they are statistically significant. Low coefficients yr). The correlation coefficient for contraction are expected. There will be errors in the attributed events that developed contemporaneous with high age and duration of recorded geological events. as convergence rates is 0.21, but increases to 0.32 well as in the oceanic crust ages and plate motions if the Magallanes fold and thrust belt is excluded of the model used. Slab age and plate motion data from the analysis. become more uncertain with age. All of these factors can be expected to contribute to decreasing any correlation between the variables considered. DISCUSSION In this context, correlations are considered strong if they have correlation coefficients with an absolute Temporal and spatial associations between value over 0.30 (for the parameters stipulated), geological events and the plate parameters outlined are considered weak for coefficients between 0.30 above beg explanatory geodynamic mechanisms. and 0.10, and are uncorrelated for parameters with In a global context, subduction zone polarity coefficients less than 0.10. appears to be a strong determinant in the likelihood Periods of recorded crustal extension of developing back-arc basins, with most marginal accompanied a range of tectonic scenarios. basins being developed over west-dipping However, there is a strong correlation (coefficient subduction zones (Dickinson, 1978; Doglioni, 0.32) between the development of marginal basins 1995; Doglioni et al., 2007). The marginal basins floored by oceanic crust, and the absolute motion of the Andean margin are thus part of a rarer group of South America having been directed away from of back-arc basins developed along east-dipping the trench, accompanied the subduction of oceanic subduction zones, and likely required the presence crust older than 50 Myr (Table 1), where Myr of unique conditions along the subduction zone to denotes million years since the crust formed at a enable formation. The opening of mafic-floored ridge. Neither the motion of South America, nor back arc basins along the South American margin University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 17 was clearly associated with the trench normal first order explanation for the correlation could be absolute velocity vector being directed away from suction force, being derivative from the effect of the trench. This conclusion confirms the findings subducted lithosphere displacing mantle material, of other studies, which have indicated that absolute in turn causing the overriding continental plate to continental plate velocity controls the tectonic be pulled toward the trench (Elsasser, 1971; Forsyth regime of ocean-continent convergent margins & Uyeda, 1975). As older oceanic lithosphere (Heuret & Lallemand, 2005; Oncken et al., 2006; tends to be thicker (Parsons & McKenzie, 1978), Ramos, 2010). However this condition alone the subduction of relatively old oceanic lithosphere appears inadequate to facilitate crustal separation should displace proportionately more mantle and, for example, micro-continent dispersal. The material, inducing a stronger suction force than in development of back arc basins in the northern the case of the subduction of younger lithosphere. and southern Andes regions was associated with In situations where the velocity of the overriding the subduction of oceanic slabs older than 50 Myr. plate is negative, the tensile stress in the back arc The effect of slab age on subduction dynamics is region can be envisaged as sufficient to induce ambiguous. The buoyancy (age, thermal regime), crustal splitting, provided a pre-existing weakness rigidity, and the extent of coupling with the is present (Shemenda, 1993). overriding plate, together with other parameters, An alternative explanation for the development have been shown to have a dependency on slab age, of mafic floored back arc basins at only the northern yet are also considered to have distinct, commonly and southernmost extents of the subduction margin competing effects (Forsyth & Uyeda, 1975; Di involves a model whereby lateral slab width controls Giuseppe et al., 2009; Yáñez & Cembrano, 2004). deformation in the overriding plate (Schellart et Trench rollback, whereby the subduction zone al., 2007; Schellart, 2008). In this model, mantle hinge moves away from the overriding plate, is flow around the edges of a subducting slab enables likely an important condition in the development rollback, facilitating lithospheric extension more of mafic floored back arc basins (Oncken et al., easily at the edges of long subduction zones. 2006; Ramos, 2010). However, as the model used Conversely, the lack of mantle escape flow in in this analysis does not incorporate deformation of central portions of a subduction zone retards trench the trench it cannot be used to provide a record of rollback. This effect may also explain why the rollback along the margin. Slab age is sometimes development of mafic floored back arc basins does employed as a proxy for trench rollback as rollback not occur at some other locations along the margin is commonly considered a consequence of the where the conditions of a negative overriding plate subduction of old, dense oceanic crust; younger velocity and subducting slab age greater than 50 slabs are commonly considered comparatively Myr are met (e.g. Point 49, Figs 3 & 4). However, buoyant and thus able to resist subduction leading location along the subduction zone cannot control to trench advance (Molnar & Atwater, 1978; the timing of the development of extension, and Dewey, 1980; Garfunkel et al., 1986). This could under this alternative model the direction of the present a tidy explanation for the opening of mafic velocity of the overriding plate still seems to be a floored back arc basins facilitated by slab retreat primary control on the initiation of mafic floored from the subduction of old oceanic crust. However, back arc basin extension. in recent years the accepted relationship between Modelled trench normal convergence rates slab age and trench movement has been challenged. less than 0 cm/yr (i.e. extensional) were associated A global compilation of absolute motions with the opening of the Colombian Marginal established the inverse relationship: older slabs Seaway (Northern Andes) and the southern portion advance toward the overriding plate and younger of the Rocas Verdes Basin (southern Andes). ones retreat (Heuret & Lallemand, 2005). Older, However, this condition does not appear essential stronger oceanic lithosphere supposedly resists to the development mafic-floored basins, as it was bending at the subduction zone, resulting in trench not associated with the opening of the northern advance, whereas younger, weaker crust subducts portion of the Rocas Verdes Basin. Geological more easily resulting in trench retreat (Di Guiseppe complexity of the Rocas Verdes Basin confounds et al., 2009). Instead of trench rollback, a potential easy interpretations. Geochemical distinctions University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 18 between mafic rocks in southern and northern slabs older than 50 Myr is largely absent from the portions of the basin indicate that the basin was development of fold and thrust belts in the Peruvian wider in the south than in the north, originally and south-central Andes, especially during the early leading to an interpretation that crustal separation episodes of thrusting in the Agrio and Aluminé fold initiated in the south and propagated northward and thrust belts (Fig. 3). Clearly if subducting slab (de Wit & Stern, 1981; Stern & de Wit, 2003). age does play a role in contractional deformation However, recent detrital zircon spectra from Rocas in the overriding plate it is not straightforward. Verdes sedimentary rocks are more consistent with It should also be noted that although the trench synchronous rifting along the length of the basin normal absolute velocity of South America is low (McAtamney et al., 2011) requiring an alternative in magnitude during these compression events, explanation for distinctions between the southern it is still positive, potentially contributing to the and northern mafic rocks. Convergence rates less compressive regime. than 0 cm/yr in the south but not in the north could The Magallanes fold and thrust belt of the have facilitated greater extension in the south, and southern Andes presents an exception to the could explain the differences in basin width. correlations established from other fold and The development of fold and thrust belts and thrust belts along the South American margin. plateau uplift along the South American margin, Its development was accompanied by low trench features commonly associated with crustal normal convergence velocity: always less than compression, seems to require contributing effects 4 cm/yr and occasionally negative (tensional). from parameters additional to those investigated The most probable causes of this inconsistency here. Trench normal convergence velocity in excess involve simplifications made in investigating the of 4 cm/yr was correlated with the development subduction parameters, namely that both internal of most contractional features, excluding the deformation of South America and any change in Magallanes fold and thrust belt. A high convergence trench geometry were disregarded. The portion of rate could reasonably be expected to facilitate the South American plate now in the southernmost a compressive strain regime in the overriding Andes experienced shortening sufficient to bury continental plate, assuming regular plate coupling. components to c.10-12 kbar (Klepeis et al., 2010; However, convergence rates along the margin have Paper II), but the global plate model used currently been as high or higher during periods of extensional has no mechanism to incorporate the effects of deformation or no contractional deformation such shortening on the geometry of a plate. The (Fig. 5). Therefore, while high convergence rates Patagonian Orocline initiated during or has become may facilitate the development of fold and thrust more pronounced since the mid Cretaceous (e.g. belts and plateau uplift, in isolation from other Kraemer, 2003) resulting in changes in the shape of conditions they are insufficient to drive overriding the margin that should influence plate velocities at plate contraction. The age of the subducting slab relevant parts of the trench. has been suggested to have influenced the rise An essential point to recall when interpreting of the Andes, as the age of the Nazca plate, with these results is that correlations highlighted by oldest crust subducting in the centre of the margin, this analysis do not necessarily imply causal correlates well with height and crustal thickening relationships. Geodynamic mechanisms outlined along the Andean margin (Capitanio et al., 2011; above indicate that potential causal relationships Yañez & Cembrano, 2004). However, when the may exist, but are not required by the results. In effect of slab age was examined in this analysis it cases where the proposed mechanism agrees with was found that correlation coefficients decreased results of previous studies, such as the tectonic slightly from 0.21 for convergence rate alone to control provided by the direction of overriding 0.20 for combined convergence rate and slab age. plate motion the likelihood of the existence of a The effect was slightly larger when the Magallanes causal relationship is high. fold and thrust belt was excluded from the analysis; Several non-correlations are additionally the coefficients decreased from 0.32 to 0.30 for notable, as they allow parameters modelled in convergence rate alone and combined convergence this study to be discounted as the primary drivers rate and slab age, respectively. The subduction of of a given geological event. For instance, periods University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 19 inferred to have been accompanied by a high potential energy of a region which results in ductile magmatic flux do not seem to correlate with any flow of the lithosphere to reduce those contrasts of the investigated parameters, either singly or in (Rey et al., 2001). Therefore gravitational forces combination. Thus it is inferred that high recorded can lead to deformation independent of the rates of magmatic flux were not controlled by any subduction zone parameters, and may be a factor in of subducting slab age, absolute overriding plate some of the observed discrepancies. velocity, or convergence velocity. In addition, the This investigation has assumed that the subduction of spreading ridges, commonly invoked kinematic model used to determine the subduction as a cause of orogenesis initiation (Palmer, 1968; related parameters accurately represents conditions Ramos, 2005), has no correlation with any type along the margin back to 170 Ma. However, there of deformation and thus appears to have had a can be significant variations between models, negligible effect on foreland deformation along particularly at older times. The method outlined in the Andean margin (Figs 3-5). This study also this paper could easily be adapted to test several separated structural styles of fold and thrust belts different models to determine which best reproduces but failed to determine any correlation between the major geological events in the overriding plate. any of the modelled subduction parameters and the Such a test could determine whether the observed development of thin- or thick-skinned thrusting. The correlations are robust across the models, or reactivation of Mesozoic extensional structures to potential artefacts of specific models. form thick-skinned fold and thrust belts is common along the margin, recorded in the Eastern Cordillera of Colombia, the Santa Bárbara System and various CONCLUSIONS thrust belts of the Neuquén Basin (Parra et al., 2009; Reynolds et al., 2000; Grier et al., 1991; Giambiagi The examination of the Andean margin over the et al., 2008; Ramos et al., 1996; Zapata & Folguera, past 170 Ma has allowed for the determination of 2005; Folguera et al., 2007; García Morabito et al., the conditions imposed by the subduction zone 2011). However, earlier Mesozoic deformation necessary for the development of geological events does not appear to be uniquely deterministic of in the overriding plate. Key findings include: contractional deformation style. For instance, the mid – late Cretaceous episode of thrusting the 1. Continental extension may reflect a series of Chos Malal fold and thrust belt developed a thin- distinct conditions for which no correlation skinned style due to the availability of detachment could be established. Nonetheless, extension surfaces despite the presence of earlier extensional that progresses to forming new oceanic crust features which were reactivated in the second requires that the trench normal absolute deformation episode of the belt (Folguera et al., velocity of the overriding plate be directed 2007). In contrast with Kley et al. (1999), rather away from the trench and that the age of the than being dominantly controlled by inherited subducting slab be older than 50 Myr. Mesozoic extensional structures, thrusting style appears to be the result of the interplay of various 2. Plateau uplift and the development of fold and factors including Mesozoic deformation, contrasts thrust belts generally developed in the presence between differing lithologies or sediment thickness, of trench normal convergence velocities and the orientation of pre-existing structures. greater than 4 cm/yr. This investigation focused on the relationships between geology and the forces acting at the 3. Trench normal convergence velocities greater subduction zone at the boundary of the interacting than 4 cm/yr well prior to the development plates, but forces acting internally on the overriding of contractional features indicate that while plate have been omitted as a quantitative assessment high convergence velocities may facilitate of these forces would be beyond the scope of this shortening of the overriding crust other study. Thickening or thinning of the lithosphere conditions are necessary for the initiation of in response to subduction along the margin can contraction. introduce lateral contrasts in the gravitational University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 20

4. The direction of the motion of the overriding 1-9, doi: 10.1016/0895-9811(96)00023-5. plate appears to control the overall tectonic Atherton, M. P., W. S. Pitcher, and V. Warden (1983), regime. The Mesozoic marginal basin of central Peru, Nature, 305(5932), 303-306, doi:10.1038/305303a0. 5. No correlations could be established Audebaud, E., R. Capdevilla, B. Dalmayrac, J. Debelmas, G. Laubacher, C. Lefèvre, R. Marocco, C. Martinez, between the modelled subduction parameters M. Mattauer, and F. Mégard (1973), Les traits investigated and major magmatic events, or géologiques essentiels des Andes centrales (Pérou- with the structural style developed in fold and Bolivie), Revue de Géographie Physique et de thrust belts. Géologie Dynamique, 15(1-2), 73-114. Barazangi, M., and B. L. Isacks (1976), Spatial distribution 6. Spreading ridge impingement on the margin of earthquakes and subduction of the Nazca plate was uncorrelated with the initiation of beneath South America, Geology, 4(11), 686-692, doi: orogenesis. 10.1130/0091-7613(1976)4<686:sdoeas>2.0.co;2. Beck, S. L., and G. Zandt (2002), The nature of orogenic crust in the central Andes, Journal of Geophysical Research, 107(B10), 2230, ACKNOWLEDGEMENTS doi:10.1029/2000jb000124. Beck, S. L., G. Zandt, S. C. Myers, T. C. Wallace, P. KTM is supported by an Australian Government G. Silver, and L. Drake (1996), Crustal-thickness Endeavour International Postgraduate Research variations in the central Andes, Geology, 24(5), 407- Scholarship with travel support from Statoil. LQ is 410, doi:10.1130/0091-7613(1996)024<0407:ctvitc> supported by funding from the Australian Research 2.3.co;2. Council (DP0986377). Funding provided by the Bruhn, R. L., C. R. Stern, and M. J. De Wit (1978), Field National Science Foundation (EAR-0635940 to and geochemical data bearing on the development of a KAK), the Australian Academy of Sciences (Travel Mesozoic volcano-tectonic rift zone and back-arc basin in southernmost South America, Earth and Planetary Grant to GLC) and the School of Geosciences at Science Letters, 41(1), 32-46, doi:10.1016/0012- the University of Sydney is also acknowledged. 821X(78)90039-0. We thank M. Seton for help with GPlates and the Burchfiel, B. C., P. Molnar, and G. Suaréz (1981), Possible kinematic model. GPlates is supported by AusScope think-skin tectonics in the eastern Andes of Bolivia in Sydney. Thorough, supportive reviews by two and Peru (abstract), Eos Transactions AGU, 62(45), anonymous reviewers substantially improved this 1047, doi:10.1029/EO062i045p00794. work. Burns, W. (2002), Tectonics of the Southern Andes from stratigraphic, thermochronologic, and geochemical perspectives [Ph. D. thesis]: Ithaca, NY, Cornell University, p. 204. REFERENCES Burns, W. M., T. E. Jordan, P. Copeland, and S. A. Kelley (2006), The case for extensional tectonics in Allmendinger, R. W., T. E. Jordan, S. M. Kay, and B. L. the Oligocene-Miocene Southern Andes as recorded Isacks (1997), The evolution of the Altiplano-Puna in the Cura Mallín basin (36°–38°S), Geological plateau of the Central Andes, Annual Review of Earth Society of America Special Papers, 407, 163-184, and Planetary Sciences, 25(1), 139-174, doi:10.1146/ doi:10.1130/2006.2407(08). annurev.earth.25.1.139. Calderón, M., A. Fildani, F. Hervé, C. M. Fanning, A. Aspden, J. A., and W. J. McCourt (1986), Mesozoic Weislogel, and U. Cordani (2007), Late Jurassic oceanic terrane in the central Andes of Colombia, bimodal magmatism in the northern sea-floor remnant Geology, 14(5), 415-418, doi:10.1130/0091- of the Rocas Verdes basin, southern Patagonian 7613(1986)14<415:MOTITC>2.0.CO;2. Andes, Journal of the Geological Society, London, Atherton, M. P., and L. Aguirre (1992), Thermal and 164(5), 1011-1022, doi:10.1144/0016-76492006-102. geotectonic setting of Cretaceous volcanic rocks Capitanio, F. A., C. Faccenna, S. Zlotnik, and D. R. near Ica, Peru, in relation to Andean crustal thinning, Stegman (2011), Subduction dynamics and the origin Journal of South American Earth Sciences, 5(1), 47- of Andean orogeny and the Bolivian orocline, Nature, 69, doi:10.1016/0895-9811(92)90059-8. 480(7375), 83-86, doi:10.1038/nature10596. Atherton, M. P., and N. Petford (1996), Plutonism and Carrera, N., J. A. Muñoz, F. Sàbat, R. Mon, and E. Roca the growth of Andean Crust at 9°S from 100 to 3 Ma, (2006), The role of inversion tectonics in the structure Journal of South American Earth Sciences, 9(1-2), University of Sydney, PhD Thesis, Paper I, Evolution of the Andean margin, Kayla T. Maloney, 2012. 21

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CrustalCrustal growth during back-arcback-arc closure: closure: Cretaceous Cretaceous exhumation exhumationhistory of Cordillera history of Darwin, Cordillera southern Darwin, Patagonia Southern Patagonia

K. T. MALONEY,1 G. L. CLARKE,1 K. A. KLEPEIS,2 C. M. FANNING3 AND W. WANG1 1School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia ([email protected]) 2Department of Geology, University of Vermont, Burlington, VT, USA 3Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia

ABSTRACT The Cordillera Darwin metamorphic complex is unique in the Andes in exposing kyanite–staurolite schist north of the Beagle Channel in southern Patagonia. Garnet in amphibolite facies pelitic schists from Bahı´ a Pia has patchy textures whereby some grains consist of clear, grossular-rich garnet with fine-grained S1 inclusion trails truncated by regions of turbid spessartine–pyrope-rich garnet with biotite, muscovite, plagioclase and quartz inclusions. Micron-scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress; S2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix. Pseudosection modelling in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) is used to infer a P–T path dominated by decompression from 12 to 9 kbar at T 620 °C, coupled with garnet mode decreasing from 5% to <1%. U–Th–Pb in situ dating of S2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72.6 ± 1.1 Ma. Contact aureoles developed adjacent to late granite intrusions include sillimanite-bearing migmatites formed at P 6 kbar after 72 Ma. Metamorphism of southern Cordillera Darwin induced by continental underthrusting beneath the arc, related to closure of the Rocas Verdes back-arc basin, was terminated by thrusting-controlled exhumation, with the rocks at P 9 kbar by c. 73 Ma and 6 kbar by c. 70 Ma. 

Key words: Cordillera Darwin; in situ monazite; pseudosection; THERMOCALC.

Nelson et al., 1980; Cunningham, 1994). This rift basin INTRODUCTION is unusual in the Andes because it is the only one of Cordillera Darwin, in southernmost South America several mid to Late Jurassic basins south of Ecuador (Fig. 1), forms a topographic high that lies, on aver- that was floored by basaltic crust with mid-ocean ridge age, more than 1 km above the surrounding moun- affinities (Stern, 1980; Allen, 1982; Alabaster & Storey, tains in the Fuegian Andes (Kranck, 1932; Nelson 1990; Mpodozis & Allmendinger, 1993; Caldero´ n et al., 1980; Klepeis, 1994a; Cunningham, 1995; Kohn et al., 2007). Part of the oceanic floor and sedimentary et al., 1995). It includes a basement of Palaeozoic fill of the Rocas Verdes basin occurs tectonically schists (Herve´ et al., 2010) surrounded by mid- interleaved with basement rocks in Cordillera Darwin Jurassic and younger volcano-sedimentary cover, and (Klepeis et al., 2010). However, heterogeneous effects intruded by granite suites and mafic dykes (Fig. 2). of Cretaceous deformation and metamorphism make North of the Beagle Channel, upper amphibolite distinguishing the protoliths of units in high-grade facies regional metamorphic assemblages (Darwin, components of Cordillera Darwin difficult (Nelson 1846; Nelson et al., 1980; Halpern, 1973; Kohn et al., et al., 1980; Herve´ et al., 2010). 1993) occur in basement and cover rocks exposed in a This paper addresses two major problems: crustal partially ice-covered mountain range between Bahı´ as growth mechanisms in a back-arc setting and the Pia and Parry (Fig. 2). This belt of high-grade rocks is metamorphic history of mineral assemblages unique restricted to a south-east-trending lens (50 · 30 km) along the Andean chain. We use new data to evaluate that includes exposures at Bahı´ a Pia, Ventisquero conflicting models proposed to explain the occurrence Roncagli and Bahı´ a Parry (Dalziel & Corte´ s, 1972; of the Cretaceous regional metamorphic rocks, and Nelson et al., 1980; Herve´ et al., 1981; Klepeis et al., propose a tectonic model that accounts for the suc- 2010). cession of mineral assemblages in the context of field Low-grade remnants of the mid-Jurassic to Early relationships. These data include the first monazite Cretaceous Rocas Verdes back-arc basin occur south ages that precisely establish that regional metamor- of the Beagle Channel (Katz, 1973; Dalziel et al., 1974; phism occurred during the Late Cretaceous, field

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Patagonian Batholith Sth. Cordillera Darwin Ultima Esperanza 48°S Hervé etDeseado al. (2007) Nelson Massif et al. (1980) Wilson (1991) and others Patagonian Batholith Magallanes

foreland v v v

v v v

v

v v v Cerro Toro Fm Patagonian Icecap Limit of v Undefined basin thrust belt

Upper K (sedimentary)Compression v K3 Upper 126-75 Punta Barrosa v LowerFm K (sedimentary) Lago Viedma Cretaceous

K2 v

v

v v v v v 136-127

v Rocas Verdes

v v v

v v v

v v Upper J Tobifera &equiv.

v v back-arc basin

v v v v

v

v K1 v

v Zapata Fm

v Yaghan Fm

v v v

v v 144-137

v v v v v v

v Lower v v

v v v

v v

v v v

v Volcanoclastic rocks v v v

v Lago Argentino

v v

v v v v v

v v v 145 Tortuga Sarmiento

v v

v v v v v mafic rocks

v v v v v v

v v v v

v

v v

Extension

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v v

v

v Ophiolite

v

v v v

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v v v Tobifera Fm v

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v v v v Darwin v

Upper v Jurassic Patagonian Batholith

51°S Granite

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v Pre-Jurassic ‘‘basement’’

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v Palaeozoic

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v v v

v Basement v v

v 6000 v v v

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v v

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1000 Magallanes Basin

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v Malvinas Basin

v v v SarmientoSarmiento v v ophiolite ((UltimaUltima 4000 Esperanza) 2000 Tierra del 3000 Fuego South

AmAmericaerica

v v

v v

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55°S v v

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Cordillera v v v v v

v Fuegian

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v

v

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Darwin v v v

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v v Andes

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v v v v 0 50 100 km Fig. 2 v v Tortuga v 75°W 70°W ophiolite

Fig. 1. Simplified geology and stratigraphy of the southern Andes, after Dalziel (1981) and Wilson (1991). Magallanes Basin structure contours demarcate depth to top of Jurassic volcanic rocks, in metres (from Biddle et al., 1986). In the stratigraphic column, K1, K2 and K3 indicate Cretaceous stages of pluton emplacement within the Patagonian Batholith identified by Herve´ et al. (2007). Inset locates this figure in southernmost South America. Box indicates the location of Fig. 2a. structural relationships, detailed petrology and mineral recently published U–Pb zircon data from rocks in equilibria modelling that build on previous work nearby areas (Barbeau et al., 2009; Herve´ et al., 2010; (Nelson et al., 1980; Kohn et al., 1993, 1995) and Klepeis et al., 2010).

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(a) (b) (c) 69°40'0"W A’ 69°20'0"W 54°40'S BPr 54°40'S C Icecap O R D I PT L L E ? R A pe: Grt-Bt D 0502 A ±St±Ky R 65 W 89 ma: I Grt-Hbl-Czo N 66

49 0504

0506 Chl-Bt 10 40 54°50'S BP 54°50'S 0501 SC 44 41 80 50 0510 33 A 0708 anal Beagle 12 60 C 60 74 21 67 TB R 0513

BR 0703 Isla Greenschist Facies Gordon

69°40'0"W 69°20'0"W Amphibolite Facies - kyanite 40 34 Amphibolite Facies U. Jur.-L. Cret. marine sedimentary sequences, incl. mudstone, 26 24 - sillimanite sandstone, volcaniclastic rock, tuff, & minor limestone of the 0 5 km Yahgan Fm. U. Jur.-L. Cret. pillow basalt, dolerite & gabbroic dykes of the Tortuga ophiolite complex intruding Early Cretaceous. 05 21 Foliation (S0/S1), lineation (L1) a b U. Jur. (a) silicic volcanic & volcaniclastic rock of the Tobifera Fm. (b) felsic-interm. granite & granitic orthogneiss & dykes of the 15 Darwin Suite, intruded by mafic dykes. 38 Thrust fault with foliation (S2), lineation (L2)

U. Cret. granite, granodiorite, tonalite of the 21 Beagle Suite. 55 Crenulation cleavage (S3), lineation (L3) Undifferentiated Mesozoic and Cenozoic granite, Strike-slip, oblique-slip or normal fault, granodiorite, tonalite & gabbro of the Patagonian batholith. arrows show relative horizontal motion, ball and tick show normal motion on down thrown side. L. Palaeozoic (mostly Ord.-Dev.) interlayered meta-arenite and pelitic 23 schist, incl. staurolite-kyanite schist (upper amphibolite facies). 15 Shear zone foliation, lineation associated with strike-slip, oblique-slip and normal faults L. Palaeozoic (mostly Ord.-Dev.) interlayered meta-arenite, phyllite and pelitic schist (greenschist facies).

Regions covered by ice

Staurolite

Kyanite

(d)

Fig. 2. (a) Structural map of a region within the Cordillera Darwin. A and A¢ locate the cross-section in (d). Sample 0703 located on map. SC: Seno Cerrado, TB: Bahı´ a Tres Brazos, BP: Bahı´ a Pia, BR: Bahı´ a Romanche, R: Ventisquero Roncagli; BPr: Bahı´ a Parry; PT: Parry Thrust. After Klepeis et al. (2010). (b) Structural map of Bahı´ a Pia with fault orientations and site locations discussed in the text. (c) Location of diagnostic metamorphic mineral assemblages found in Bahı´ a Pia, m: mafic protolith, pe: pelitic protolith. (d) Cross-section with metamorphic isograds from Bahı´ a Pia to Bahı´ a Parry, with rock type patterns as given in legend of (a). After Klepeis et al. (2010).

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silt-dominated sequences conformably overlie silicic REGIONAL SETTING pyroclastic deposits and mafic pillow basalts (Allen, Cordillera Darwin forms part of the Fuegian Andes, 1982; Fuenzalida & Covacevich, 1988; Fildani & which comprise a comparatively topographically sub- Hessler, 2005). This sedimentary fill includes rocks of dued portion of the Andean mountain chain. The the Zapata and Yahgan Formation, and equivalents geology of this belt has been divided (after Kranck, (Dalziel & Elliot, 1971; Halpern & Rex, 1972; Dott 1932; Nelson et al., 1980; Dalziel, 1981; Winslow, et al., 1982; Wilson, 1991; Olivero & Martinioni, 2001) 1982; Sua´ rez et al., 1985) into four main elements: (i) that might represent turbidite deposits (Fuenzalida & pre-Late Jurassic Gondwanide elements; (ii) the Late Covacevich, 1988). These units thicken to the south- Jurassic to Early Cretaceous Rocas Verdes marginal east from the Ultima Esperanza region (51.5°S; Fig. 1), basin (Dalziel et al., 1974; Bruhn et al., 1978; Allen, reflecting a widening of the back-arc basin with 1982; Hanson & Wilson, 1993); (iii) Cretaceous to distance south. These units conformably overlie the Cenozoic plutonic rocks of the Patagonian batholith, Tobifera Formation. interpreted as the root of a Pacific magmatic arc; and Compression and crustal shortening beginning in the (iv) a fold and thrust belt comprising uppermost Late Cretaceous resulted in a series of events that mark Cretaceous to Palaeogene sedimentary rocks of the the onset of Andean orogenesis in Patagonia (Dott Magallanes and Malvinas foreland basins (Fig. 1). et al., 1977; Dalziel, 1981; Wilson, 1991). These include Palaeozoic to Jurassic metasedimentary rocks and the formation of the flexural Magallanes foreland metavolcanics deposited along the proto-Pacific basin (Natland et al., 1974; Dott et al., 1982; Winslow, Gondwana margin and interpreted to form much of 1982; Biddle et al., 1986; Wilson, 1991; Fildani & Cordillera Darwin (Fig. 1) have widespread equiva- Hessler, 2005), and the Magallanes fold-thrust belt lents in, for example, Antarctica (Dalziel & Elliot, intercalating components of the Palaeozoic basement 1971; Stump, 1995), South Africa (de Wit, 1977) and and mafic floor and sedimentary fill of the Rocas the Lachlan Fold belt of eastern Australia (Foster & Verdes basin (Dalziel et al., 1974; Nelson et al., 1980; Gray, 2000), although metasedimentary rocks forming Caldero´ n et al., 2007; Klepeis et al., 2010). Fine to a basement to Cordillera Darwin differ in provenance medium-grained sandstones that overlie rift basin to those underlying the Magallanes foreland basin sedimentary rocks of the Rocas Verdes basin mark the (Herve´ et al., 2010). In the Fuegian Andes, care is beginning of foreland basin sedimentation. These required to separate pre-Mesozoic orogenic features include sequences of the Punta Barrosa Formation from those related to their tectonic reworking during that conformably overlie the Zapata Formation closure of the Rocas Verdes marginal basin (Nelson (Wilson, 1991; Fildani & Hessler, 2005). Detrital zir- et al., 1980; Kohn et al., 1993). Detrital zircon age con spectra suggest that the Punta Barrosa Formation patterns for samples taken from areas focussed on for is no older than c. 92 Ma, in the Ultima Esperanza work in this paper (Bahı´ a Pia) have a range of peaks region (Fildani et al., 2003). In Tierra de Fuego, the most prominent of which is Cambro-Ordovician in the equivalent units may be somewhat younger age, indicating their protoliths are Palaeozoic (Herve´ (McAtamney et al., 2009). Crustal shortening in the et al., 2010). Magallanes foreland propagated northwards, termi- During the Early and Middle Jurassic, voluminous nating in the Eocene (Alvarez-Marro´ n et al., 1993; basaltic and less abundant rhyolitic volcanism erupted Ghiglione & Ramos, 2005; Barbeau et al., 2009; onto Gondwana in its incipient stages of dispersal Gombosi et al., 2009). (Encarnacio´ n et al., 1996; Duncan et al., 1997; Minor Rocks now exposed in Cordillera Darwin experi- & Mukasa, 1997). In Patagonia and the Antarctic enced convergence-related burial followed by exhu- Peninsula, Jurassic silicic volcanic rocks, basal breccia, mation (Kohn et al., 1995), contemporary with the conglomerate and fossiliferous shale and sandstone, of development of the Magallanes foreland basin and the Tobifera Formation (Natland et al., 1974; Gust thrust belt (Nelson et al., 1980; Halpern, 1973; et al., 1985; Hanson & Wilson, 1991; Pankhurst et al., Cunningham, 1995; Klepeis et al., 2010). Kyanite– 2003; Caldero´ n et al., 2007) record this phase of crustal staurolite-bearing assemblages reflect metamorphic extension. These felsic volcanics and related granitoids conditions of 580–630 °C and 7–12 kbar (Kohn et al., in Patagonia are interpreted to reflect the later stages 1993; below). Fission-track thermochronology (175–160 Ma) of a plate margin migration of mag- (Nelson, 1982) and 40Ar ⁄ 39Ar cooling ages (Kohn matism related to continental break-up (Pankhurst et al., 1995) reveal an initial stage of cooling (from 550 et al., 2000). Widespread basaltic and gabbroic dyke to 325 °C) and exhumation from c. 90 to 70 Ma. A swarms in pre-Jurassic ÔbasementÕ rocks record further second stage of cooling (<250 °C) and further exhu- crustal extension, correlated with Early Cretaceous mation from the Palaeocene to the Middle Eocene pillow basalts and hyaloclastite interlayered with chert (Kohn et al., 1995; Barbeau et al., 2009; Gombosi and siltstone that record the opening of the Rocas et al., 2009). Verdes marginal basin (Dalziel et al., 1974; Stern et al., Granite, dolerite and gabbro intruded Palaeozoic 1992; Stern & de Wit, 2003) between c. 152 and basement, and rift-related sequences including the 142 Ma (Caldero´ n et al., 2007). Interbedded shale and Tobifera Formation and sedimentary fill of the Rocas

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Verdes basin (Nelson et al., 1980; Sua´ rez et al., 1985; rocks (Fig. 2; Table 1). Recent U–Pb dating of detrital Cunningham, 1994; Klepeis et al., 2010). Two granitic zircon grains in staurolite–kyanite-bearing schists suites have been observed in southern Cordillera (Herve´ et al., 2010) has confirmed a long-standing Darwin: (i) the Late Jurassic Darwin suite (Nelson interpretation based on Rb–Sr whole-rock data (Herve´ et al., 1980; Herve´ et al., 1981, 2010); and (ii) the Late et al., 1981) of Palaeozoic protoliths. These schists are Cretaceous Beagle suite (Nelson et al., 1980; Herve´ commonly referred to as ÔbasementÕ to the Rocas et al., 1984; Klepeis et al., 2010). The granite suites are Verdes series (Nelson et al., 1980; Herve´ et al., 1981). a local manifestation of the south Patagonian batho- The oldest foliation that can be identified in the schists lith, which records c. 150 Myr of subduction related is a layer-parallel foliation (S1) cut by c. 160 Ma felsic magmatism (Herve´ et al., 2007). In Cordillera Darwin, orthogneiss (Fig. 3a) of the Darwin Suite (Klepeis gabbroic sills and dolerite dykes represent magmatism et al., 2010). S1 is commonly interpreted as preserving that accompanied the opening of the Rocas Verdes evidence for pre-Jurassic orogeny, interpreted to pre- back-arc basin (Nelson et al., 1980; Cunningham, cede the emergence of the Andean mountain chain 1994). Low-grade remnants of this magmatism now (Herve´ et al., 1981). In the Bahı´ a Pia amphibolite form the Sarmiento and Tortuga Ophiolite complexes facies schists, S1 contains intrafolially folded quartz (Fig. 1; Sua´ rez & Pettigrew, 1976; Stern, 1980; Allen, veins and laminae and it seems likely that S1 is a 1982; Caldero´ n et al., 2007). Pre-Jurassic basement composite fabric that includes pre-Jurassic and rocks retain evidence for a foliation that predates Cretaceous components. This interpretation is com- emplacement of the Darwin Granite Suite (Nelson patible with that of Nelson et al. (1980). et al., 1980) and may be Palaeozoic in age, but pre- Felsic orthogneiss of the Darwin Granite Suite is Jurassic metamorphic conditions evidently did not cut by numerous mafic dykes (Fig. 3a,b), which are exceed lower greenschist facies (Nelson et al., 1980; interpreted as being equivalent to the Tortuga Kohn et al., 1993, 1995). High-grade metamorphic and Sarmiento Ophiolite (Caldero´ n et al., 2007). fabrics in eastern Cordillera Darwin are cut by the Amphibolite facies assemblages in the felsic orthog- Beagle Suite (Nelson et al., 1980; Moore, 1990; Kohn neiss and recrystallized mafic dykes define a moder- et al., 1995), which appears to have been emplaced ately ( 60°) south-west-dipping S2 foliation over a period between c. 86 and 70 Ma (Klepeis et al., (Fig. 3c,d), which includes a down-dip mineral line- 2010; S. A. Thomson & K. A. Klepeis, unpublished ation and is commonly a spaced crenulation cleavage U–Pb zircon data). in graphitic schist (Fig. 3e). Silicic schists mainly Greenschist facies strike-slip and normal faults contain S1 folia, tightly folded (Fig. 3c) or trans- occur along the Beagle Channel (Dalziel & Brown, posed into S2 (Fig. 3d,e). Garnet in both metapelitic 1989; Cunningham, 1993, 1995). These cut all upper schists and metamorphosed dykes may preserve fine- amphibolite facies fabrics and plutons in Cordillera grained S1 inclusion trails oblique to S2 (below), Darwin (Klepeis et al., 2010), and record the effects supporting the interpretation that S1 is a composite of strike-slip faulting that has occurred from at least age fabric. the late Tertiary to the present (Gombosi et al., In fiords further west (Seno Garibaldi, Seno 2009). Ventisquero), shallowly dipping S2–L2 fabrics and thrust faults that juxtapose ÔbasementÕ schists and ÔcoverÕ ´ units are interpreted to record the north-east-directed BAHIA PIA AND VENTISQUERO RONCAGLI thrusting of the ÔbasementÕ nappe onto the larger South Multiply deformed amphibolite facies metapelitic American craton (e.g. Nelson et al., 1980). In Seno schist occurs interlayered with mafic and felsic Parry, to the north of Bahı´ a Pia, north-east directed orthogneiss in Bahı´ a Pia (Nelson et al., 1980), and in thrusting places amphibolite facies orthogneiss over the vicinity of Ventisquero Roncagli (Cunningham, greenschist facies metapelites along the Parry Thrust 1995). Kohn et al. (1993, 1995) characterized varia- (Fig. 2a,d), which forms the northern limit of high- tions in metamorphic grade through central Cordillera grade metamorphic rocks (Klepeis et al., 2010). Lower Darwin based on the occurrence of a series of index grade Tobifera rocks in southern Bahı´ a Pia contain a minerals, specifically chlorite–biotite, garnet, kyanite, fabric interpreted as equivalent to S2 in amphibolite staurolite and ⁄ or sillimanite. Following early work by facies schists of northern Bahı´ a Pia and Ventisquero Nelson et al. (1980), Kohn et al. (1993) identified a Roncagli area (Fig. 2a,b). pronounced metamorphic discontinuity across the S2 is deformed by macroscopic, reclined F3 folds north-west arm of the Beagle Channel – whereas with a north-east dipping axial plane that deform all kyanite-grade schists occur north of the channel, only metamorphic fabrics but which lack pervasive fabric chlorite–biotite-grade schists were identified in similar development (Fig. 2d). These folds are interpreted to lithologies to the south (Fig. 2a,c). reflect the post-metamorphic back-folding of a north- Bahı´ a Pia exposes north-west-trending, interlayered east vergent nappe pile (Nelson et al., 1980). S1 and S2 amphibolite facies schists, felsic and mafic orthogneiss, fabrics, and F3 folds are all cut by c. 86–70 Ma felsic all in fault contact with chlorite–biotite grade (after plutons of the Beagle Suite (Kohn et al., 1995; Klepeis Kohn et al., 1993), south-dipping Tobifera Formation et al., 2010) which have contact aureoles with weakly

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Table 1. Timing of major events and formation of fabrics in Cordillera Darwin.

Time (Ma) Event Foliation Nature Lineation Mineral Reference assemblage

Palaeozoic to Jurassic Deposition of ÔbasementÕ rocks Herve´ et al. (1979, 1981, 2010) Pre-Late Jurassic Pre-Darwin Granite orogeny S1 Intrafolial Inferred Nelson et al. (1980) greenschist 164 ± 1 Darwin Granite; U–Pb zircon Mukasa & Dalziel (1996) 157 ± 7 Rb–Sr Herve´ et al. (1981) 159 ± 1 U–Pb on zircon; rhyolite dyke Herve´ et al. (2010) 172 ± 1 U–Pb on zircon from Tobifera Pankhurst et al. (2000) volcanic 163 ± 2 U–Pb on zircon from Tobifera Herve´ et al. (2010) sandstone Verdes basin) 152–142 Mafic dyke swarm (related to Herve´ et al. (1979, 1981), formation of Rocas Verdes basin) Nelson et al. (1980), Caldero´ n et al. (2007) Penetrative fabric development S1 Amphibolite Klepeis et al. (2010); this Underthrusting (Pia); obduction facies paper (Seno Ventisquero, western Beagle Pelite: Grt–Bt– Channel) Ms ± Ep* 86 ± 1 Early Beagle Suite granite that cuts Mafic: Grt–Hbl– Klepeis et al. (2010) obduction structures Czo–Pl 73 ± 1 Staurolite–kyanite growth Patchy S2 SE-trending, Down dip Grt–Bt–Ky–St Monazite; this paper recrystallization of garnet folded (exhumation of Cordillera Darwin) Dips SW 65° 90 ± 2, 69 ± 1 Beagle Granite; U–Pb zircon Bt–Sil–Ms Mukasa (in Kohn et al., c. 70 U–Pb zircon migmatites 1995) in contact S. Thomson & K. Klepeis aureoles (unpublished data) 73–65 Ma Hbl, Kfs Ar–Ar cooling ages Kohn et al. (1995) Continued uplift of Cordillera Darwin Macroscopic overturned folds F3 NW-trending Klepeis et al. (2010) Palaeogene or younger Strike slip, oblique slip, normal faults Gombosi et al. (2009)

*Mineral symbols used here and elsewhere are after Kretz (1983), extended by Bucher & Frey (1994). deformed sillimanite–garnet–biotite migmatites in the northernmost and easternmost portions of Bahı´ a Pia PETROGRAPHY (stations 0502, 0510, Fig. 2c). Similar aureoles with The focus of this paper involves amphibolite facies metapelitic assemblages that include sillimanite, stau- basement schists and orthogneiss to the north of rolite, garnet and biotite were identified in Seno Tobifera exposures in Bahı´ a Pia, and equivalent schists Romanche, south of the Beagle Channel. All rocks are forming a prominent ridge to east of Ventisquero cut by strike-slip and oblique-slip normal faults and Roncagli (Fig. 2a). This paper concentrates on com- shear zones (Fig. 2a) that are possibly as old as the plex garnet textures in metapelitic schists, building on Miocene (Klepeis, 1994b; Klepeis & Austin, 1997; the work of Nelson et al. (1980) and Kohn et al. (1993, Lodolo et al., 2003; Ghiglione & Ramos, 2005; Bar- 1995). Tobifera exposures in Bahı´ a Pia are much finer beau et al., 2009) and control much of the current grained (minerals 0.1–0.2 mm) than the basement topography. schists, being formed from felsic schists comprising The outlet valley and ridges adjacent to Ventisquero chlorite, biotite, titanite, plagioclase and quartz inter- Roncagli include staurolite–kyanite schists and layered with mafic schists comprising actinolite, chlo- orthogneiss (Cunningham, 1995) that represent along- rite, biotite, epidote, plagioclase and quartz. Garnet strike equivalents of the schists exposed in Bahı´ a Pia. was not observed in these rocks, and grain size is an Metamorphic grade and structural relationships record order of magnitude smaller than in the amphibolite a history similar to that of the rocks in Bahı´ a facies schists. Pia (Fig. 2). Cunningham (1995) interpreted a Most basement schists in Bahı´ a Pia are felsic, com- macroscopic antiform defined successively by garnet, prising plagioclase–quartz-rich layers alternating with kyanite–staurolite, garnet and biotite zones on a ridge muscovite–biotite layers on a cm-scale (Fig. 3e). parallel to the valley. This succession corresponded, Grains are mostly 1–2 mm across. Ilmenite and less respectively, to orthogneiss, schist and orthogneiss abundant rutile occur in either laminae, with or with- units in our field traverses, with mineral zones mostly out garnet. Micaceous laminae may range in size up to reflecting variations in lithology, not grade. The schists decimetre or metre-scale, where they additionally include kyanite–garnet–staurolite-bearing assemblages contain garnet, staurolite and kyanite. A distinctive that assist with the interpretation of the Bahı´ a Pia layer of graphite-rich aluminous schist is continuous assemblages. across the two arms of Bahı´ a Pia; most aluminous

Ó 2011 Blackwell Publishing Ltd University of Sydney, PhD Thesis, Paper II, Cretaceous exhumation of Cordillera CRETACEOUSDarwin, Kayla T. EXHUMATIONMaloney, 2012. OF CORDILLERA DARWIN 347

(a) (b)

(c) (d)

(e) (f)

Fig. 3. (a) Composite S1 in Palaeozoic basement, cut by c. 160 Ma Darwin granite, which contains S2. Bahı´ a Pia site 0502. (b) Deformed c. 145 Ma mafic dyke swarm cutting Darwin granite. Bahı´ a Pia site 0504. (c) Dykes of Beagle Granite cutting S3 in Palaeozoic basement schists. Roncagli area. (d) Metamorphosed and deformed mafic dykes that preserve S2 in Palaeozoic schists that preserve S1 and S2 foliations. Bahı´ a Pia site 0506. (e) Transposition of S1 foliation into S2 in carbonaceous schist in Palaeozoic basement. Bahı´ a Pia site 0501. (f) Cruciform twinning in post-S1 staurolite in Grt–St–Ky–Bt schist. Bahı´ a Pia site 0513. minerals in this layer have abundant fine-grained inclusions of S1 plagioclase, epidote, quartz, rutile, graphite inclusions. The penetrative foliation in most ilmenite, muscovite and ⁄ or biotite, with or without samples is S2, which largely comprises a spaced cren- graphite (Figs 4a–d & 5a,b). Individual S1 grains are ulation of S1 in the graphite-rich schists (Fig. 3e). commonly <0.1 mm across. Curved and folded S1 Centimetre-scale garnet grains at both Bahı´ a Pia and inclusion trails are consistent with garnet having grown Ventisquero Roncagli may contain curved or folded syn-tectonically with respect to D2 (Fig. 4d).

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In comparison with other textures described below, lite and kyanite in the metapelitic schists are partially to garnet with distinct S1 inclusion trails has a clear habit, completely pseudomorphed by randomly orientated locally clouded by graphite inclusions. Cruciform intergrowths of chlorite and muscovite (Fig. 4c). Gar- twins of staurolite (Fig. 3f) have planar S2 quartz, net in metabasite is commonly partially retrogressed to rutile and ilmenite inclusions, at a high angle to curved randomly oriented chlorite. Felsic orthogneiss inter- S1 inclusions in garnet (Figs 4a & 5c), consistent with layered with metapelitic schist at Bahı´ a Pia comprises staurolite growth late in, or after, the development of S2 plagioclase, microcline, quartz, biotite and ilmenite S2 (also Cunningham, 1994). Kyanite also has inclu- with or without garnet and ⁄ or muscovite. Recrystal- sions of S2 rutile and ilmenite, consistent with it having lized mafic dykes comprise S2 hornblende, plagioclase, formed at the same time as staurolite. S2 is the domi- biotite and ilmenite, with or without garnet, rutile, nant foliation observed within the matrix, and is pri- epidote and titanite (grains 1–2 mm across). Garnet marily defined by biotite, muscovite and quartz may have oriented inclusions of epidote, titanite and (Fig. 4a). Minerals involved in the S2 paragenesis are plagioclase, indicating complexity in the development garnet rims, staurolite, kyanite, biotite, muscovite, of S2 and ⁄ or age of protolith(s) (Fig. 4f). rutile, ilmenite, plagioclase and quartz. At Bahı´ a Pia locations 0502 and 0510, migmatite Garnet in amphibolite facies schists of Bahı´ a Pia containing sillimanite, plagioclase, biotite and ilmenite, may preserve complex textures, which include patches with or without garnet, as well as garnet-bearing and core or rim domains of turbid garnet intergrown metasedimentary schist similar to that described with comparatively coarse-grained (0.5–1 mm diame- above, have coarse-grained intergrowths of muscovite, ter) randomly oriented biotite, muscovite, plagioclase, biotite and fibrolitic sillimanite (Fig. 4g) that cut S2 ilmenite, rutile, quartz, monazite and zircon. Such and are weakly aligned with a late shear fabric. grains with ÔpatchÕ textures of turbid garnet may occur Randomly oriented chlorite partially pseudomorphs adjacent to clear garnet grains that preserve folded S1 biotite, garnet and sillimanite. Similar, weakly foliated inclusions trails (Fig. 5d). Inclusions 1–2 lm across textures were found in Seno Romanche (0709) south of cause the turbid appearance to the garnet patches; the Beagle Channel, where fine-grained (0.5 mm) their small size hampers identification. Turbid garnet metasedimentary rocks include calc-schist layers of may occur as comparatively small parts of otherwise plagioclase, hornblende, biotite, ilmenite, epidote and clear garnet grains (Fig. 5c), form most of a grain quartz with or without diopside. These occur inter- (Fig. 5e), or anything in between (Fig. 5f). At an layered with metapelitic layers comprising plagioclase, incipient stage of development of this texture (e.g. biotite, garnet, quartz, ilmenite and fibrolitic sillima- Fig. 5c) isolated coarse-grained mineral inclusions, one nite. Another finely layered rock at the same location to two orders of magnitude larger than those forming comprises: (i) comparatively coarse-grained (1 mm) S1 inclusion trails, occur apparently entirely sur- metapelitic layers of garnet, biotite, muscovite, stau- rounded by garnet, but with turbid garnet adjacent the rolite, sillimanite, ilmenite and quartz interlayered large mineral inclusions (Fig. 5c). Garnet that is with; (ii) metapsammite layers comprising plagioclase, dominantly turbid commonly has fracture traces that quartz, biotite, muscovite and ilmenite. These are the connect isolated large inclusions (Figs 4e & 5h), or first reported occurrences of garnet and sillimanite isolated patches of incipient turbid garnet, to the host south of the Beagle Channel, and are interpreted as grain boundaries (Fig. 5c). In cases, atoll-like struc- contact effects of Beagle Suite plutons on metasedi- tures are developed (Fig. 6), comprising a core or rim mentary hosts at both Bahı´ a Pia and Seno Romanche. domain of turbid garnet intergrown with large ran- Garnet in the samples from Bahı´ a Pia aureoles lacks domly oriented minerals, enclosed by a mantle of the patch textures evident outside the aureoles. ÔclearÕ garnet that may preserve fine-grained S1 inclu- sion trails. Large syn- to post-S2 staurolite and kyanite porphyroblasts commonly occur adjacent to garnet GARNET MINERAL CHEMISTRY AND RAMAN grains with the patchy turbid textures (Figs 4e & 5f). ANALYSIS Garnet with turbid patches truncates S1 inclusions The main features of the mineral chemistry are trails (e.g. Fig. 5d), but many recrystallized grain cores presented in Kohn et al. (1993, 1995), with detail rele- have ambiguous timing relationships with respect to vant to the samples used for this work summarized in the matrix S2 foliation. On the basis of S1 truncations Table 2. Analyses were performed on the Cameca at turbid-clear boundaries, and our inferred relation- SX-50 Camebax microprobe housed at the University of ship between the development of the turbid garnet and New South Wales, operating with an accelerating volt- staurolite and kyanite growth, we infer that the turbid age of 15 kV, a beam width of 1–5 lm and PAP data garnet formed during and after the development of S2. reduction techniques supplied by the manufacturer. For Curved inclusion trails similar to those in clear garnet the purpose of analysing the complex garnet textures from Bahı´ a Pia occur in the Ventisquero Roncagli and applying the mineral equilibria modelling described samples, but garnet from this area appears to lack the below, this paper concentrates on garnet relationships development of turbid textures (Fig. 5a). At both Bahı´ a in key metapelitic samples from Bahı´ a Pia (0708) and Pia and Ventisquero Roncagli, garnet, biotite, stauro- the ridge east of Ventisquero Roncagli (0703).

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A detailed compositional profile was collected grossular-rich clear regions and pyrope–spessartine- through a garnet grain in sample 0703 at a high angle to rich turbid regions (Fig. 6). S1 (Fig. 5g). The profile extends from a spessartine– grossular-rich core to a comparatively pyrope–alman- dine-rich rim (Fig. 7a). An inner bell-shape defined by GEOCHRONOLOGY both grossular and spessartine is similar to the inter- Three sensitive high-resolution ion microprobe preted growth zoning used by Kohn et al. (1993) to (SHRIMP) mounts were prepared from segments of two recover a prograde path for Cordillera Darwin involv- thin sections of garnet-bearing metapelitic schist sam- ing 500–620 °C at 6–9 kbar. A broad outer mantle of ples 0501 (mounts Z5490 & Z5491) and 0708 (mount comparatively spessartine–grossular-poor garnet (left Z5495) from Bahı´ a Pia, for the purpose of U–Th–Pb side of Fig. 7a) reflects areas that have inclusions monazite dating. Monazite was analysed in situ within aligned with S2 (Fig. 5g), matched by a compositionally selected areas of the polished thin sections, recast into similar, narrow zone on the other grain boundary. epoxy discs together with reference monazite grains A traverse made through a garnet grain from Bahı´ a (44069). Reflected and transmitted light photomicro- Pia sample 0708 with turbid patches (Fig. 5h) has graphs were prepared for navigation, as were Scanning complex chemical zoning (Fig. 7b). The pattern is Electron Microscope (SEM) back-scattered and sec- similar for an orthogonal traverse that is oblique to the ondary electron (BSE) images of the monazite internal foliation (not shown). Spessartine content varies structure (Fig. 8). The images were used to ensure that smoothly from core to rim in a fashion similar to that in the 20 lm SHRIMP analysis spot was wholly within a sample 0703. However, grossular, pyrope and alman- single age component within the sectioned grains. dine contents in the grain vary complexly, in patterns The U–Th–Pb analyses were made using SHRIMP coupled with analyses collected from either clear or II at the Research School of Earth Sciences, The turbid sections of the garnet grain. Turbid portions of Australian National University, Canberra, Australia the garnet grain have lower grossular and higher following procedures given in Williams (1998, and pyrope content than clear adjacent portions of the references therein). As the monazite grains were grain. Turbid areas within the garnet grain have similar expected to be, and are, Cretaceous in age, no energy pyrope content to that near grain boundaries (Fig. 7b). filtering was applied to the extracted secondary ion Raman spectroscopy was performed on the garnet beam. As such, common Pb correction was made using grains using an excitation line of 514 nm. Spectra are the measured 207Pb ⁄ 206Pb ratios as outlined by broadly similar between the turbid and clear areas and Williams (1998). Each analysis consisted of six scans are dominated by garnet bond peaks (after Bersani through the mass range, with the 44069 reference et al., 2009), but significant differences are observed monazite grains analysed after each group of three (Fig. 7c). Turbid garnet has a broad peak between unknown analyses. Data were reduced using the 3300 and 3700 cm)1 lacking in spectra collected from SQUID Excel Macro of Ludwig (2001). The Pb ⁄ U clear garnet, associated with the stretching mode of the ratios have been normalized relative to a value of O–H bond of water, with small, superimposed peaks 0.0668 for the 44069 reference monazite, equivalent to indicating impurities of aqueous solution (Dubessy an age of 424.9 Ma (see Aleinikoff et al., 2006). et al., 2001). The strongest band in both spectra is the Uncertainty in the U–Pb calibration was 1.19%, garnet Si–O stretching mode at 910 cm)1 (Bersani 0.62% and 0.45% for the three polished thin sections et al., 2009). This band increases in wave number from analysed, Z5495, Z5490 and Z5491, respectively. 906 to 917 cm)1 with the change from grossular to Uncertainties given for individual analyses (ratios and spessartine–pyrope rich garnet. Both spectra show a ages) are at the one sigma level (Table 3). Tera & series of peaks from 2840 to 3100 cm)1 consistent with Wasserburg (1972) concordia plots, probability density the presence of hydrocarbons (Wo¨ rner et al., 1993; plots with stacked histograms, and weighted mean Dubessy et al., 2001; Fan et al., 2004). 206Pb ⁄ 238U age calculations (Fig. 9) were carried out Maps of Mn, Mg and Ca X-ray intensity were also using ISOPLOT ⁄ EX (Ludwig, 2003). Weighted mean collected over part of patchily recrystallized garnet 206Pb ⁄ 238U ages (207Pb corrected) were calculated, texture in sample 0506 (Fig. 6). Semi-quantitative including addition of the U ⁄ Pb ratio calibration uncer- cation proportions were then calculated using matrix tainty (added in quadrature) and the uncertainties are corrections and analyses of garnet isolated from other reported as 95% confidence limits. minerals by using a mixture of element thresholds Thirteen analyses were performed on monazite (Clarke et al., 2001). Clear garnet has high, variable grains in mount Z5490, 13 on mount Z5491 and 11 on grossular content, whereas turbid garnet is rich in mount Z5495. Monazite grains in the samples were all pyrope, similar to relationships in sample 0708. A located within the quartz–mica-rich domains outside narrow rim of the garnet grain in sample 0506 has large garnet porphyroblasts (Fig. 8a,c); no monazite spessartine, pyrope and grossular contents that match was found in any garnet examined under the SEM. All its turbid core. Turbid parts of the garnet grain have observed monazite is aligned with S2 in the matrix, and higher, homogeneous spessartine contents in contrast occurs in contact with biotite, muscovite, plagioclase with that in the clear domains, resulting in garnet with and quartz. Monazite grains range in size from 20 to 

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(a)

(b) (c)

(d) (e)

(f) (g)

Ó 2011 Blackwell Publishing Ltd University of Sydney, PhD Thesis, Paper II, Cretaceous exhumation of CordilleraCRETACEOUS Darwin, Kayla T. EXHUMATION Maloney, 2012. OF CORDILLERA DARWIN 1138

150 lm, and pleochroic halos are common in adjacent internally consistent thermodynamic data set of Hol- minerals. Xenotime also occurs in the quartz–mica-rich land & Powell (1990; data set created on 22 November domains of the rock, and inside garnet. Monazite 2003). Calculations involved the phases chlorite, grains are all anhedral to subhedral, and, on the basis quartz, ilmenite, rutile, titanite, epidote, garnet, of SEM imaging, lack chemical zoning (Fig. 8b). All plagioclase, K-feldspar, biotite, muscovite, kyanite, monazite is inferred to be metamorphic in origin, with sillimanite, silicate melt and water. Activity–composi- no age or zoning patterns that might reflect inherited tion relationships of phases follow White et al. (2003). grains; the metamorphic event that produced the The bulk rock composition used in the modelling could monazite is inferred to be the same as the one that not be determined directly from X-ray fluorescence formed the dominant S2 foliation. (XRF) whole-rock analyses, due to the pronounced Ages from the grains in each sample were generally zoning in garnet and therefore the sequestering of consistent and gave an overall age of 72.6 ± 1.1 Ma. elements (e.g. Marmo et al., 2002). Instead, represen- Some slightly older ages between c. 78 and 74 Ma from tative microprobe data of the main rock-forming S2 mount Z5495 were interpreted to be a consequence of minerals in 0708b were weighted according to mineral machine U ⁄ Pb ratio calibration anomalies arising from mode determined by point counting and converted to the relative large expanse of this particular SHRIMP mole per cent to calculate a model rock composition mount, the older analyses having being made near the (see Wei et al., 2009). Ferric content was fixed at a low edge of this probe mount. Monazite grains from the value (FeO:FeO3 ⁄ 2 = 20:1) reflecting the occurrence other two mounts agree to within analytical uncertainty. of graphite. A further simplification employed in the discussion below is to assume that, on the basis of equivalent mineral assemblage and similar modal data, MINERAL EQUILIBRIA MODELLING AND P–T the bulk rock composition of sample 0708b from Bahı´ a PATH Pia is broadly equivalent to that of sample 0703 from Pseudosections show the stable multivariant equilibria Ventisquero Roncagli. Difficulties related to site access in a given chemical system for a specific rock compo- meant it was impossible to return large samples for sition or rock compositional vector (Powell et al., XRF analysis from most locations. 1998). P–T pseudosections allow observed assemblages Figure 10a shows the equilibria modelled for to be quantitatively constrained for a specified rock 550–700 °C and 5–12.5 kbar, prepared with quartz in composition (e.g. Powell et al., 1998; White et al., excess. As it is difficult to estimate the amount of H2O 2002). The chosen model chemical system should present for temperature conditions above the water- describe the major chemical components in the rock saturated solidus, the method of White et al. (2001) system as fully as possible. The model system of Na2O– was used, and these equilibria were calculated with CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 fixed water content at a value that reflects the highest (NCKFMASHTO) was chosen to calculate phase pressure sub-solidus assemblage. Most equilibria are relationships and reaction processes in the Cordillera tri or quadrivariant, with two divariant equilibria: one Darwin schists. This system represents the combina- narrow, curved field involving the main S2 paragenesis tion of the NCKFMASH model of White et al. (2001) of biotite, garnet, kyanite, staurolite, muscovite, pla- and KFMASHTO model of White et al. (2002) as gioclase, quartz, rutile and ilmenite, and another at outlined in White et al. (2003). Though important, higher temperature involving biotite, garnet, kyanite, MnO was not included in the model system because muscovite, quartz, plagioclase, rutile, ilmenite and refined a-X models are yet to be constructed for min- liquid. The diagram has epidote-bearing equilibria at erals other than garnet. The main effect of including high-P and low-T, and staurolite-bearing equilibria MnO in the model system is to expand the garnet- restricted to 550–630 °C and <9.3 kbar, outlined by a bearing field, with small modes of spessartine-rich green line. Garnet-bearing assemblages occur at high- garnet appearing at lower P–T conditions than shown P–T conditions with a complex lower limit marked by in NCKFMASHTO pseudosections. a red line (Fig. 10a). Isopleths of garnet mode for Calculations were undertaken using THERMOCALC relevant fields are shown (as percentages) in Fig. 10a, (version 3.32; Powell & Holland, 1988) with the and x(grt) = Fe ⁄ (Fe + Mg) and z(grt) = Ca ⁄

Fig. 4. (a) Garnet with S1 inclusion trails and complex internal structures, including patchy turbid areas. Staurolite contains S2 inclusion trails that parallel the penetrative matrix foliation. Sample 0708, base of image 25 mm. (b) Simply twinned staurolite over- growing folded S1 defined by biotite, muscovite, plagioclase, quartz and ilmenite. Sample 0501a, base of image 3.5 mm. (c) S2 staurolite, kyanite and garnet in mica schist from Ventisquero Roncagli area, partially retrogressed to randomly oriented muscovite and biotite. Sample 0703, base of image 3.5 mm. (d) Snowball inclusion trail texture in garnet core defined mostly by S1 quartz inclusion trails, enclosed in an inclusion-poor rim. Sample 0513, base of image 1.5 mm. (e) Complex structure in patchily recrystallized garnet. Turbid areas are inferred to be later than the clear area at the top of the grain, and, on the basis of microstructures inferred to have formed contemporary with the development of post-S1 growth of kyanite and staurolite. Sample 0708, base of image 1.5 mm. (f) Garnet with inclusions trails from a metamorphosed mafic dyke. Matrix hornblende defines S2 Sample 0501b, base of image 3.5 mm. (g) Sillimanite, biotite, garnet and plagioclase in a deformed migmatite from the contact aureole of a Beagle Suite granite intrusion. Garnet lacks the patchy recrystallization textures evident in samples from outside the aureoles. Sample 0510, base of image 3.5 mm.

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(a) (b)

(c) (d)

(e) (f)

(g) (h)

Ó 2011 Blackwell Publishing Ltd University of Sydney, PhD Thesis, Paper II, Cretaceous exhumation of CordilleraCRETACEOUS Darwin, Kayla T. EXHUMATION Maloney, 2012. OF CORDILLERA DARWIN 1340

Fig. 6. Maps of Mn, Mg and Ca cations per 24 oxygen over part of a patchily recrystallized garnet grain involving a mantle of clear grossular-rich garnet and a core of turbid pyrope–spessartine-rich garnet intergrown with randomly oriented mica, plagioclase and quartz. Maps of X-ray intensity were collected using the University of NSW Cameca SX-50 microprobe, with matrix correction and image enhancement procedures following Clarke et al. (2001). (Ca + Fe + Mg) contents in Fig. 10b,c, respectively. and staurolite-bearing equilibria overlap (in the The main S2 assemblages reflect a restricted P–T kyanite field). The narrowness of this divariant field domain near P 9 kbar and T 640 °C where garnet reflects the near co-linearity of garnet and staurolite  

Fig. 5. (a,b) Garnet core S1 epidote inclusion trails, enclosed in a comparatively inclusion-poor rim. Sample 0706B, Roncagli area, base of image 3 mm. (c) Garnet with a large biotite inclusion surrounded by turbid garnet (dark areas), and an isolated patch of turbid garnet apparently developed unrelated to the grain boundary. Adjacent staurolite has an intense S2 foliation defined mostly by ilmenite, which is oblique to weak S1 inclusion trails in garnet. Sample 0708, base of image 3.5 mm. (d) Contrasting textures in two adjacent garnet porphyroblasts. The left-hand porphyroblast preserves curved S1 inclusions trails defined by quartz, plagioclase, biotite and muscovite, whereas the right-hand porphyroblast has straight S1 inclusion trails truncated by turbid garnet patches that include large randomly oriented grains of biotite and muscovite. Sample 0501a, base of image 5 mm. (e) Clear garnet with fine-grained S1 inclusion trails (running across the top of the grain) mostly recrystallized to turbid garnet, biotite and plagioclase. Staurolite is adjacent to the upper right-hand side of garnet. Sample 0708, base of image 3.5 mm. (f) Two clear garnet grains with turbid areas and kyanite developed at their mutual boundary. S2 staurolite, biotite, plagioclase and quartz occur in the matrix. Sample 0708, base of image 3.5 mm. (g) Garnet with curved S1 inclusion trails, mostly defined by quartz and ilmenite. Grain has pronounced chemical zoning as shown the compositional profile collected oblique to the S1 inclusion trail along the dashed yellow line in Fig. 6a. Sample 0703, base of image 1 mm. (h) Garnet with turbid patches developed both at the grain edge and internally. Large random muscovite, biotite and plagioclase intergrown with turbid garnet truncate S1 inclusion trails in clear areas. Sample 0708, base of image 5 mm.

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Table 2. Summary of EMP mineral chemistry data for metapelitic samples from Cordillera Darwin.

Sample Garnet Garnet Biotite Muscovite Pl St Ep Chl Hbl

Core Rim In Grt S2 In Grt S2 In Grt S2 S2 In Grt Post-S2 S2

Alm Sps Prp Grs Alm Sps Prp Grs XMg XMg Si pfu Si pfu An An XMg Czo XMg XMg

Pia1a Grt–St–Bt 0.77 – 0.01 0.13 0.81 – 0.12 0.07 0.46 6.1 0.55 0.73 0.16 Pia1b Grt–Hbl 0.64 – 0.06 0.30 0.13 0.38 Pia2c Grt–Sil–Bt 0.78 0.06 0.10 0.05 0.45 6.0 0.63 Pia5b Grt–St–Bt 0.71 0.08 0.12 0.11 0.69 0.14 0.14 0.06 0.29 5.3 0.25 0.50 0.20 0.59 Pia 7c Grt–St–Bt 0.75 0.06 0.12 0.04 0.45 0.48 6.0 6.0 0.72 0.81 0.27 0.41 Pia 6a Grt–Ky–St–Bt 0.68 0.03 0.04 0.18 0.79 0.05 0.10 0.04 0.46 6.0 0.25 0.26 0.10 0.25 Pia 7c Grt–St–Bt 0.71 0.02 0.10 0.06 0.75 0.06 0.12 0.03 0.41 0.45 6.0 6.0 0.38 0.18 0703 Grt–Ky–St–Bt 0.69 0.04 0.14 0.12 0.54 0.12 0.05 0.25 0.43 0.26 0.16 0.56 0.54 0708 Grt–Ky–St–Bt 0.61 0.14 0.05 0.25 0.74 0.07 0.12 0.08 0.52 0.52 6.2 6.3 0.35 0.74 0.24

with respect to kyanite in terms of XFe = Fe ⁄ (Fe + mantle compositions (red dot 2, Fig. 10a). Lower Mg) composition. temperature conditions for garnet cores would be Conditions of P 12 kbar and T 620 °C can be recovered in a model system involving MnO (Wei recovered for S1 inclusion assemblages involving bio- et al., 2009). tite, epidote, muscovite, albite-rich plagioclase and The complex garnet textures in sample 0708 from rutile, together with the composition of the garnet core Bahı´ a Pia can be modelled using a similar, more [x(grt) = Fe ⁄ (Fe + Mg) = 92 and z(grt) = Ca ⁄ restricted P–T path than that inferred for the Ventis- (Ca + Fe + Mg) = 30] in sample 0703 from Ventis- quero Roncagli sample. This restriction might reflect a quero Roncagli (red dot labelled 1 in Fig. 10a). This sampling bias in the core analyses used, possibly core composition is inferred to reflect the earliest stage related to partial grain recrystallization, along-strike of garnet growth, as patch textures associated with the variation in pressure conditions in Cordillera Darwin development of S2 were not observed in garnet from (Fig. 2), or errors associated with the bulk rock cal- Ventisquero Roncagli; were MnO to be considered in culations. Taking the most calcic part of the clear the model system subtly lower temperature conditions garnet domains [x(grt) = 9, z(grt) = 24] as reflecting would probably be inferred. The pronounced chemical the earliest garnet growth, conditions for the quad- zoning of garnet from this sample from a grossular- rivariant S1 assemblage biotite, epidote, garnet, rich core to an almandine–pyrope-rich rim (Fig. 7a) muscovite, plagioclase, and rutile involved P that is inferred to be contemporary with S2 staurolite 10.5 kbar and T 610 °C (marked by white dot and kyanite can be modelled as reflecting decompres- labelled 1, Fig. 10a). There are no epidote inclusions in sion from 12 to 9 kbar through the quadrivariant garnet from sample 0708, but epidote does occur as field involving biotite, epidote, garnet, muscovite, inclusions in metapelitic garnet elsewhere in Bahı´ a Pia. plagioclase and rutile, and the trivariant field involving Garnet mode is again modelled as coupled with biotite, epidote, garnet, muscovite, plagioclase, rutile increasing temperature conditions, possibly up to 5% and ilmenite. Garnet mode is modelled as initially in the trivariant field involving biotite, epidote, garnet, increasing on a prograde path; the composition of the muscovite, plagioclase, quartz, rutile and ilmenite. The intermediate mantle of garnet in this sample can be composition of the garnet rim, and turbid parts of modelled as reflecting P 10.7 kbar and T 630 °C garnet from this rock [x(grt) = 84, z(grt) = 13] is (red dot labelled as 2, Fig. 10a). The composition of modelled as falling at P 8.8 kbar and T 620 °C in the garnet rim [x(grt) = 84, z(grt) = 13] overlaps with the low-P part of the trivariant field involving biotite, the narrow divariant field involving the S2 paragenesis garnet, staurolite, muscovite, plagioclase, quartz, rutile of biotite, garnet, kyanite, staurolite, muscovite, pla- and ilmenite, shown by the white dot labelled 2 gioclase, quartz, rutile and ilmenite, at P 9 kbar and (Fig. 10a). This lies just outside the divariant field T 625 °C. Garnet growth in this sample can thus be reflecting the main S2 paragenesis, but probably within modelled via a P–T path dominated by decompression error when issues related to the method of whole-rock from 12 to 9 kbar, over 610–630 °C. Temperature calculation are considered. A decompressional path conditions inferred for the garnet rim composition are extending below the divariant field involving biotite, slightly (5–10 °C) lower, but within error of the peak, garnet, kyanite, staurolite, muscovite, plagioclase, inferred to have accompanied the formation of garnet quartz, rutile and ilmenite, is inferred to have resulted

Ó 2011 Blackwell Publishing Ltd University of Sydney, PhD Thesis, Paper II, Cretaceous exhumation of CordilleraCRETACEOUS Darwin, Kayla T. EXHUMATION Maloney, 2012. OF CORDILLERA DARWIN 1542

absolute pressure conditions inferred above, related to analytical errors and the means by which a whole-rock composition was established. Nonetheless, the relative pressure change between conditions inferred for the inclusion and matrix assemblage could be expected to be more robust (e.g. Worley & Powell, 2000). The presence of sillimanite in contact aureoles around Beagle Suite plutons at various locations in Bahı´ a Pia and Seno Romanche reflects exhumation and localized heating from the kyanite field to silli- manite field. Migmatitic assemblages at sample site 0510 involving sillimanite, plagioclase, biotite and ilmenite, with or without garnet (Fig. 4g), reflect T 690 °C at P 6 kbar (Fig. 10). These sillimanite assemblages are not widespread (cf. Kohn et al., 1993). Post-S2 chlorite reflects retrogression at low-grade conditions, as reflected by the small chlorite-bearing fields at the low-P–T extent of Fig. 10a.

DISCUSSION

Patchy recrystallization of garnet The honeycomb and patch textures (Figs 4a & 5h) of turbid, spessartine–pyrope-rich garnet in the Bahı´ a Pia samples might be interpreted as early formed garnet, overgrown by later grossular-rich garnet. However, several features counter this interpretation. The ÔatollÕ- like textures are always incomplete, the minerals intergrown with turbid garnet are commonly randomly oriented, and one to two orders of magnitude larger than S1 inclusions. The minerals intergrown with tur- bid garnet do not define S1, but may cut S1 and be continuous with minerals defining S2 in the sur- rounding quartz–mica-rich matrix (Fig. 5e). Mineral intergrowths with turbid garnet are comparable in grain size to matrix S2 minerals. They are also generally the same minerals, being dominated by quartz and mica. In detail, turbid, spessartine–pyrope- Fig. 7. (a) Traverse through garnet grain from sample 0703 from rich garnet also encloses grossular-rich garnet (Fig. 6) Ventisquero Roncagli, showing variation in Xalm = Fe ⁄ and its development seems enhanced near sites of (Fe + Mn + Mg + Ca), Xspess = Mn ⁄ (Fe + Mn + Mg + staurolite and kyanite growth (Fig. 5e,f). Accepting Ca), Xpy = Mg ⁄ (Fe + Mn + Mg + Ca) and Xgross = Ca ⁄ that turbid, spessartine–pyrope-rich garnet formed (Fe + Mn + Mg + Ca). XFe = Fe ⁄ (Fe + Mg) and zCa = Ca ⁄ (Fe + Mg + Ca) values corresponding to isopleth values later than the grossular-rich garnet begs an interpre- modelled in THERMOCALC are shown for parts of the grain tation for a mechanism responsible for garnet recrys- core, mantle and rim. All analyses of mineral inclusions were tallization decoupled from the grain margin, that is, removed from the profile. (b) Traverse through a garnet grain in one apparently less affected by kinetic aspects influ- sample 0708, with irregularly developed turbid patches. All encing diffusion. variables as given for this figure. Though a bell-shaped pattern is still defined by spessartine content, equivalent trends in Turbid garnet from Bahı´ a Pia (Fig. 5c,e) texturally almandine, pyrope and grossular contents are highly disrupted in resembles chemically homogeneous ÔhoneycombÕ gar- areas corresponding to the turbid garnet. (c) Raman spectra net in high-pressure rocks from the Tauern Window of from a garnet from sample Pia7C ⁄ 1 from clear and turbid areas the Eastern Alps, interpreted to result from precipita- within the garnet. tion from a wetting fluid along quartz-quartz grain boundaries (Hawkins et al., 2007). Minerals inter- in a substantial decrease in garnet mode from a max- grown with the turbid garnet from Bahı´ a Pia are imum of 5% to <1% garnet. This predicted change dominated by muscovite and biotite, abundant quartz provides a plausible mechanism for the development of and, apparently, reflect a process coupled with reduced the patchy turbid textures, as discussed below. Sub- garnet mode. Turbid garnet regions also have a similar stantial errors (1–3 kbar) could be expected for all the appearance to cloudy garnet observed in the Acadian

Ó 2011 Blackwell Publishing Ltd 16UniversityK. T. of MALONEY Sydney, PhDET Thesis AL. , Paper II, Cretaceous exhumation of Cordillera Darwin, Kayla T. Maloney, 2012. 43

(a) (b)

(c) (d)

Fig. 8. (a) Monazite grain with visible pleochroic halo from grain mount Z5490 prepared from sample 0708a. Square indicates monazite. Monazite occurs in S2 biotite. Base of image is 1.5 mm. (b) Backscattered electron (BSE) image of monazite in (a); there is minimal zoning. (c) Monazite grain with visible pleochroic halo from grain mount Z5495 prepared from sample 0501. Monazite occurs in S2 defined by intergrown muscovite and staurolite (left). Turbid garnet is visible on the right of the image. Base of image is 1.5 mm. (d) BSE image of monazite in (c); there is minimal zoning. zone of western New England, north-eastern United resorbed all Mn from earlier-formed garnet. Large States of America, interpreted to be the result of mica and plagioclase grains intergrown with the turbid 1–2 lm fluid and mineral inclusions developed during garnet, and large staurolite and kyanite adjacent to fluid assisted metasomatism along grain boundaries garnet, would probably require that intergranular dif- and cracks (Hames & Menard, 1993; Whitney et al., fusion occurred on a scale at least as large as the garnet 1996a,b). Fluid ingress late in the development of S2, porphyroblasts (e.g. Carmichael, 1969). The lack of the possibly coupled with the fracturing and site-specific development of turbid garnet in the Ventisquero partial recrystallization of garnet (Whitney, 1991), can Roncagli examples most probably reflects lower fluid- also account, for example, from Bahı´ a Pia. rock ratios in those rocks, possibly related to them Evidence for the presence of fluid within turbid being more distant to areas of Rocas Verdes rocks garnet was found during Raman spectroscopy, indi- undergoing prograde metamorphism. cating the presence of an aqueous saline solution (Fig. 7c). Fluid could have been sourced from under- thrust sheets of Rocas Verdes metasedimentary rocks A model for metamorphism driven by back-arc basin experiencing contemporary prograde metamorphism. closure Hydro-fracturing could have then exposed parts of The southern sector of the Rocas Verdes basin gener- earlier-formed grain cores of compositions formed at ally is interpreted to have been wider and undergone higher pressure and lower temperature (e.g. Carlson & greater subsidence than northern parts of the basin Gordon, 2004) and enhanced reaction coupled with (Dalziel & Elliot, 1971; Dott et al., 1982; Wilson, 1991; staurolite and kyanite growth. Reduced garnet mode, Olivero & Martinioni, 2001; Fig. 11a). This interpre- interpreted as related to terrane-wide uplift, can tation focuses attention on the need for a mechanism explain the higher spessartine content in the turbid that accounts for the fate of a larger area of mafic garnet whereby reduced modes of secondary garnet ocean floor to the southern part of the basin (Fig. 11a).

Ó 2011 Blackwell Publishing Ltd University of Sydney, PhD Thesis, Paper II, Cretaceous exhumation of CordilleraCRETACEOUS Darwin, Kayla T. EXHUMATION Maloney, 2012. OF CORDILLERA DARWIN 1744

Table 3. Summary of SHRIMP U–Pb data for monazite grains from Cordillera Darwin.

206 204 206 Grain. U Th Th ⁄ U Pb* Pb ⁄ Pb f206% Total Radiogenic Age (Ma) spot (ppm) (ppm) (ppm) 238U ⁄ 206Pb ± 207Pb ⁄ 206Pb ± 206Pb ⁄ 238U±206Pb ⁄ 238U±

Z5495 4 3045 17 975 5.90 29.1 0.000888 0.79 89.93 1.03 0.0537 0.0006 0.0110 0.0001 70.7 0.8 5.1 3500 19 570 5.59 33.8 0.000729 0.87 88.93 1.04 0.0544 0.0007 0.0111 0.0001 71.5 0.8 5.2 4038 23 226 5.75 45.3 0.004155 7.34 76.57 1.30 0.1058 0.0035 0.0121 0.0002 6 3540 22 147 6.26 34.4 0.000754 0.47 88.44 1.18 0.0512 0.0006 0.0113 0.0002 72.1 1.0 7 4518 25 456 5.63 45.2 0.000357 0.46 85.94 1.02 0.0511 0.0007 0.0116 0.0001 74.2 0.9 9 3072 8876 2.89 30.9 0.000464 0.36 85.44 0.93 0.0504 0.0006 0.0117 0.0001 74.7 0.8 10 3811 17 501 4.59 39.5 0.003030 5.20 82.81 0.89 0.0887 0.0017 0.0114 0.0001 73.4 0.8 8 3044 10 700 3.52 30.6 0.000377 0.33 85.40 0.93 0.0501 0.0007 0.0117 0.0001 74.8 0.8 2 3098 12 341 3.98 31.1 0.000281 0.23 85.46 0.94 0.0493 0.0006 0.0117 0.0001 74.8 0.8 1 3194 13 865 4.34 32.2 0.000629 0.41 85.19 0.93 0.0508 0.0010 0.0117 0.0001 74.9 0.8 3 3061 18 879 6.17 29.8 0.000376 0.38 88.20 0.97 0.0504 0.0007 0.0113 0.0001 72.4 0.8 Z5490 7 4521 19 885 4.40 45.5 0.000734 0.92 85.34 0.94 0.0548 0.0006 0.0116 0.0001 74.4 0.8 8 4915 44 461 9.05 47.6 0.000846 0.64 88.64 0.95 0.0525 0.0005 0.0112 0.0001 71.9 0.8 9 3358 18 278 5.44 32.5 0.000520 0.36 88.72 0.97 0.0503 0.0006 0.0112 0.0001 72.0 0.8 10 3530 17 448 4.94 34.1 0.000472 0.47 88.92 0.97 0.0512 0.0006 0.0112 0.0001 71.8 0.8 12 4214 27 185 6.45 42.6 0.001763 3.07 84.96 0.91 0.0718 0.0006 0.0114 0.0001 73.1 0.8 11 4167 18 095 4.34 41.9 0.001934 3.84 85.36 0.92 0.0779 0.0018 0.0113 0.0001 72.2 0.8 14 4074 28 099 6.90 39.5 0.000268 0.44 88.69 0.95 0.0509 0.0007 0.0112 0.0001 72.0 0.8 15 3657 23 703 6.48 36.3 0.000267 0.52 86.48 0.94 0.0516 0.0006 0.0115 0.0001 73.7 0.8 13 3695 22 416 6.07 36.2 0.000954 1.53 87.69 0.95 0.0596 0.0008 0.0112 0.0001 72.0 0.8 4 3838 20 962 5.46 36.3 0.000266 0.33 90.72 0.98 0.0500 0.0006 0.0110 0.0001 70.4 0.8 3 3860 42 263 10.95 36.7 0.000565 0.73 90.41 0.98 0.0532 0.0011 0.0110 0.0001 70.4 0.8 2 4284 23 300 5.44 41.4 0.000393 0.57 88.96 0.96 0.0519 0.0005 0.0112 0.0001 71.6 0.8 5 4549 20 918 4.60 45.2 0.001084 1.86 86.38 0.93 0.0622 0.0009 0.0114 0.0001 72.8 0.8 Z5491 11.2 3719 21 484 5.78 37.0 0.000878 2.04 86.33 0.93 0.0637 0.0011 0.0113 0.0001 72.7 0.8 3 4080 19 263 4.72 41.3 0.000124 0.53 84.93 0.93 0.0517 0.0006 0.0117 0.0001 75.1 0.8 4 3825 21 630 5.66 37.6 – 0.38 87.45 0.96 0.0505 0.0006 0.0114 0.0001 73.0 0.8 2 3640 18 820 5.17 36.3 0.000174 0.44 86.23 0.94 0.0510 0.0009 0.0115 0.0001 74.0 0.8 5 3618 20 838 5.76 35.7 0.000460 1.27 87.16 0.95 0.0576 0.0006 0.0113 0.0001 72.6 0.8 1 4435 28 392 6.40 42.7 0.000267 0.80 89.21 0.97 0.0538 0.0006 0.0111 0.0001 71.3 0.8 6 4465 43 131 9.66 43.5 0.000000 0.27 88.20 0.95 0.0496 0.0007 0.0113 0.0001 72.5 0.8 7 3752 21 150 5.64 36.8 0.000152 0.41 87.64 0.96 0.0507 0.0006 0.0114 0.0001 72.8 0.8 8 4087 23 707 5.80 40.0 0.000121 0.34 87.71 0.96 0.0502 0.0007 0.0114 0.0001 72.8 0.8 9 3980 34 858 8.76 38.3 0.000107 0.69 89.16 0.97 0.0529 0.0006 0.0111 0.0001 71.4 0.8 12 5778 42 830 7.41 56.5 0.000012 0.61 87.91 0.96 0.0523 0.0006 0.0113 0.0001 72.5 0.8 11.1 7558 77 235 10.22 71.5 0.000061 0.99 90.80 0.98 0.0552 0.0006 0.0109 0.0001 69.9 0.8 10 14247 166 607 11.69 143 0.000092 2.19 85.73 0.93 0.0649 0.0007 0.0114 0.0001 73.1 0.8

Age ±no std ±include std wtd ave most 72.61 0.46 1.55 1.13 36 of 37 analyses, MSWD = 2.9 wtd ave Z5490 71.97 0.61 1.05 0.76 12 of 13 analyses, MSWD = 1.5 wtd ave Z5491 72.60 0.47 0.79 0.57 11 of 13 analyses, MSWD = 0.92

Uncertainties given at the one sigma level; the error in the 44069 reference monazite calibration was 1.19%, 0.62% and 0.45% for the analytical sessions; f206% denotes the percentage of 206Pb that is common Pb; and the correction for common Pb for the U ⁄ Pb data has been made using the measured 238U ⁄ 206Pb and 207Pb ⁄ 206Pb ratios following Tera & Wasserburg (1972) as outlined in Williams (1998).

This issue, and the burial of continental crust to (Fig. 11c) accounts for the prograde S1 assemblages 40 km as a consequence of tectonic under-thrusting now preserved as S1 inclusion trails in garnet from during back-arc basin closure can be explained by a Cordillera Darwin. This tectonic loading on the lead- thickening of continental crust following incipient ing edge of the continent was accompanied by the subduction of the back arc basinÕs mafic floor development of the flexural Magallanes foreland basin (Fig. 11b). The following discussion draws on regional (Natland et al., 1974; Biddle et al., 1986; Wilson, relationships and geochronology (Table 1) to evaluate 1991). Foreland basin sedimentation had begun by c. tectonic models for the history of Cordillera Darwin 92 Ma in northern Patagonia (Fildani et al., 2003) and schists. The combined effects of tectonic imbrication possibly slightly later in Tierra del Fuego (McAtamney and further subduction appear to have juxtaposed et al., 2009). Cross-cutting relationships between plu- continental crust of the Gondwana margin and the tons of the Beagle Suite at Seno Ventisquero indicate volcanic arc, in relative positions close to where they that continental underthrusting and then initial are currently. obduction occurred prior to c. 86 Ma, while subduc- Attempted subduction and thickening of the leading tion continued along the proto-Pacific margin (Klepeis edge of the continental margin under the volcanic arc et al., 2010). Relative mantle and continental crustal

Ó 2011 Blackwell Publishing Ltd University18 K. T. of MALONEY Sydney, PhDET Thesis AL. , Paper II, Cretaceous exhumation of Cordillera Darwin, Kayla T. Maloney, 2012. 45

(a) Data-point error ellipses are 68.3% conf . (b) 7 Mean = 71.97 ± 0.61 (0.85%) 95% conf. Wtd by data-pt errs only, 1 of 13 rej. 0.10 6 MSWD = 1.5, probability = 0.11 Relative probability

5

0.08 4 Pb Pb

207 206 3 Number

0.06 2

1

88 84 80 76 72 68 0.04 0 70 74 78 82 86 90 94 98 68 70 72 74 76 78 80 238U/ 206Pb 206Pb/238 U Age (Ma) (c) (d) 7 6 Mean = 72.80 ± 0.64 (0.88%) 95% conf. Wtd by data-pt errs only, 1 of 13 rej. 6 MSWD = 1.6, probability = 0.091 Relative probability

5 Relative probability

5 4

4

3 umber Number 3 N

2 2

1 1

0 0 68 70 72 74 76 78 80 68 70 72 74 76 78 80 82 206Pb/238 U Age (Ma) 206Pb/238 U Age (Ma)

Fig. 9. (a) Tera-Wasserburg diagram showing 207Pb ⁄ 206Pb v. 238U ⁄ 206Pb ratios for monazite analyses collected from all three grains mounts. Error ellipses are shown at 68.3% confidence. (b) Histogram of ages from grain mount Z5490, sample 0708. (c) Histogram of ages from grain mount Z5491, sample 0708. (d) Histogram of ages from grain mount Z5495, sample 0501. Due to limited data and scatter a meaningful age for this mount was not found, however, results are consistent with the other two mounts. buoyancy could have limited the descent of the that uplift and cooling of the high-grade rocks in Gondwana margin (e.g. Aitchison et al., 1995; Harris Cordillera Darwin was initially comparatively rapid. et al., 2000). Fission-track ages (Nelson, 1982; Gombosi et al., 2009) Monazite in the Bahı´ a Pia schists most probably record continued, somewhat slower, uplift of Cordillera formed from the partial breakdown of xenotime. As it Darwin into the Neogene. Termination of the Creta- only occurs in micaceous S2 assemblages that include ceous metamorphism could have been ultimately con- staurolite and kyanite, the 72.6 ±1.1 Ma monazite age trolled by oceanic slab break-off, and foundering of the is inferred to reflect a stage of the terrane’s exhumation leading Rocas Basin mafic floor (Fig. 11e). Continued (Figs 10 & 11d). High-grade rocks in Bahı´ a Pia thus crustal shortening during and after uplift of the high- underwent nearly isothermal decompression to form grade rocks in Cordillera Darwin is evident in the staurolite-kyanite-bearing assemblages at P 9 kbar development of basement-cored macroscopic folds and by c. 73 Ma, and migmatitic sillimanite assemblages at back thrusts, with the eastward propagation of con- P 6 kbar by c. 70 Ma (Fig. 11d; S. Thomson & K. A. temporary thrust faults into the Magallanes foreland Klepeis, unpublished U–Pb ages of Beagle suite plu- basin (Klepeis et al., 2010; Fig. 11e). tons). These data are consistent with c. 70–69 Ma Although there is a general consensus that high- 40Ar ⁄ 39Ar muscovite and biotite cooling ages, and c. grade metamorphism in Cordillera Darwin rocks was 76–73 Ma hornblende ages obtained from high-grade induced by tectonic burial of the Gondawana margin rocks in Bahı´ a Pia (Grunow et al., 1992; Kohn et al., and quasi-oceanic crust associated with closure of the 1995). Model closure temperatures for Ar retention in Rocas Verdes back-arc basin, alternative models for hornblende, muscovite and biotite, together with the their exhumation also have been proposed. Dalziel & 40Ar ⁄ 39Ar ages were used by Kohn et al. (1995) to infer Brown (1989) inferred that the high-grade rocks

Ó 2011 Blackwell Publishing Ltd University of Sydney, PhD Thesis, Paper II, Cretaceous exhumation of CordilleraCRETACEOUS Darwin, Kayla T. EXHUMATION Maloney, 2012. OF CORDILLERA DARWIN 1946

(a) NCKFMASHTO 3 4 5 6 (b) 1 2 Grt Bt Ms 89 88 84 78 Grt Bt Ms Pl Ep Rt 12 Roncagli 1 Pl Ep Rt Ilm Liq Bt Ms Ttn G03 core Ilm H2O x92 z30 Pl Ep Rt 4 10 H2O Bt Ms Grt Bt Ms Grt Bt Ms 78 Pl Ep Rt Pl Ep Rt Pl Ky Rt Grt Bt Ms 11 Pl Ky H2O H2O Ilm H2O 2 Rt Ilm Liq 80 Pia G08b ‘core’ 1 x90 z24 8 82 10 Grt Bt Ms Pl Ky Ilm 4 Liq H2O 84 83 Grt Bt Ms 2 x(grt) Pl Ky St 1 3 Bt Ms Rt Ilm H2O 9 2 Pl Ky Ilm G03 rim (c) 1 H2O rim x84 z13 Grt Bt

P kbar x85 z12 Pl Ky Bt Ms Bt Ilm Liq 8 Bt Ms Pl Ky Pl Ky Pl Ep Bt Ms Rt Ilm H2O Ilm Liq 16 Rt Ilm Pl St H2O Rt Ilm 26 H2O Bt Ms 14 H2O Pl St Ky 24 Rt Ilm 7 12 H2O 22 contact aureole 20 10 Bt Ms Pl Chl Bt Ms 6 Rt Ilm Pl Sil Bt Pl Sil 18 Rt Ilm H2O Ilm Liq H2O H O 12 Bt Pl Sil 2 z(grt) SiO2 Al2O3 CaO MgO FeO K2O Na2 O TiO2 O Rt Ilm Bt Pl Sil 66.74 13.40 2.68 4.73 6.25 2.16 3.20 0.76 0.30 Ilm H2O H2O with H2O=8.5 5 St Liq Grt 550 600 650 700 T °C

Fig. 10. (a) Calculated Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) pseudosection indicating the position of mineral inclusion assemblages and inferred P–T path followed by samples 0708 (White dots; Bahı´ a Pia) and 0703 (Red dots; Roncagli). A representative whole-rock composition was calculated from modal data and representative microprobe analyses of S2 minerals and garnet rims. Garnet cores were excluded from the calculations in a manner following Marmo et al. (2002). Isopleths of garnet mode are shown for relevant fields. Green line indicates the boundaries of staurolite-bearing equilibria, red line indicates the lower pressure limit of garnet-bearing equilibria and the yellow line indicates the solidus. Contact aureole refers to sillimanite-bearing assemblages that surround plutons of the Beagle Suite grantoids. (b,c) Isopleths of x(grt) = Fe ⁄ (Fe + Mg) and z(grt) = Ca ⁄ (Ca + Fe + Mg) for garnet in relevant fields, used to locate the P–T position of core and mantle inclusion assemblages for the two samples. defined a metamorphic core complex, on the basis of exhumation of high-grade rocks in Cordillera Darwin similarities between Cordillera Darwin and the was controlled by transpression related to oblique-slip Shuswap metamorphic core complex in the North faulting observed along the Beagle Channel. This American Cordillera, where uplift and exhumation is model requires an increase in metamorphic grade inferred to have been driven by extension (Armstrong, towards the channel, inferred as being present at 1982). However, the southern boundary of amphibo- Ventisquero Roncagli by Cunningham (1995), but lite facies rocks in Bahı´ a Pia is a normal fault, which lacking at Bahı´ a Pia and elsewhere along the western acts as a step-over between sinistral strike-slip faults reaches of the Beagle Channel. In addition, our (Figs 2a, d: Klepeis et al., 2010), and the northern observations at Ventisquero Roncagli suggest that boundary is the Parry Thrust in Seno Parry (Klepeis variations in mineral assemblage in this region are et al., 2010). mostly controlled by lithology, not grade. Greenschist In an alternative model, Cunningham (1995) sug- facies rocks immediately north of the Beagle Channel gested that basement-involved shortening and the (Fig. 2) in Bahı´ a Pia indicate that metamorphic grade

Ó 2011 Blackwell Publishing Ltd 20UniversityK. T. of MALONEY Sydney, PhDET Thesis AL. , Paper II, Cretaceous exhumation of Cordillera Darwin, Kayla T. Maloney, 2012. 47

(a) Jurassic-Early Cretaceous back arc basin 148 Ma Yahgan Fm. Zapata Fm. Basaltic crust of the Rocas ? Verdes basin Mafic dykes Mafic dykes Darwin granite Silicic volcanic rock of the (b) Mid-Cretaceous (before c. 86 Ma) Tobifera Fm.

Beagle Suite 50 km granitoids y Increased z Sedimentary fill arc magmatism ? of the Magallanes foreland basin Incipient subduction of basaltic crust Gondwana margin

(c) Late Cretaceous (before c. 73 Ma) Late S1 garnet Obduction Magallanes foreland basin Syn-post S2 St + Ky

86 Ma Underthrusting, imbrication, crustal thickening late stage S1, Ventisquero Roncagli: P ≈ 12 kbar T ≈ 600 °C

(d) Late Cretaceous (before c. 70 Ma)

Contact metamorphism, post S2 sillimanite, Bahía Pia: P ≈ 6 kbar T > 670 °C Beginning of exhumation during S2, Ky + St growth syn-post S2, Ventisquero Roncagli and Bahía Pia: P ≈ 9 kbar T ≈ 620–630 °C

Internal thickening, uplift, exhumation, cooling (e) Palaeogene Thrust belt propagation until Eocene-Oligocene

Cordillera Darwin Magallanes fold-thrust belt

Patagonian batholith

Paleosubduction zone

Fig. 11. Cartoon summarizing the Late Jurassic to Palaeogene evolution of Cordillera Darwin, the Rocas Verdes basin and the Magallanes foreland fold-thrust belt at the latitude of Tierra del Fuego. (a) Rifting, dyking and bimodal volcanism forms the quasi- oceanic Rocas Verdes rift basin by the Late Jurassic. Basin fill comprising the Zapata and Yahgan Formations thickens to the south, its width is uncertain, and arc magmatism was absent. (b) Compression initiated by c. 100 Ma led to subduction of the basaltic floor beneath the batholith to form a narrow thrust wedge comprised of mafic floor fragments and deformed volcanic and sedimentary basin fill. (c) Collision between the Patagonian batholith and South American continental crust resulted in continental crust and sequences of silicic volcanic rock (i.e. the Tobifera Formation) being underthrust beneath the batholith, contemporary with prograde S1 mineral inclusion trails in garnet that reflect P 12 kbar and T 600 °C in rocks at Bahı´ a Pia and Ventisquero Roncagli. High-grade shear zones exposed at Bahı´ as Pia and Parry cut all penetrative foliations and reflect tectonic uplift prior to c. 86 Ma, the age of the oldest Beagle suite granitoid in western Cordillera Darwin. Crustal loading and flexure created the Magallanes foreland basin. (d) Syn- to post-S2 kyanite–staurolite-bearing assemblages in parts of Cordillera Darwin now exposed between Bahı´ a Pia and Ventisquero Roncagli reflect initial uplift to P 9 kbar by c. 73 Ma. Sillimanite-bearing migmatitic assemblages in contact aureoles to Beagle Suite plutons in Bahı´ a Pia reflect further uplift to P 6 kbar by c. 70 Ma. (e) On-going collision between the Patagonian batholith and South American continental crust resulted in internal thickening, uplift and exhumation of hinterland thrusts in Cordillera Darwin. In response to this thickening of the internal part of the wedge, the Magallanes fold-thrust belt propagated into the foreland, deformation being over by the Eocene–Oligocene. Late Tertiary strike-slip faults omitted from the profiles for simplicity.

Ó 2011 Blackwell Publishing Ltd University of Sydney, PhD Thesis, Paper II, Cretaceous exhumation of CordilleraCRETACEOUS Darwin, Kayla T. EXHUMATION Maloney, 2012. OF CORDILLERA DARWIN 2148 increases away from the channel, inconsistent with a Armstrong, R.L., 1982. Cordilleran metamorphic core com- model of exhumation having been driven by trans- plexes – from Arizona to southern Canada. Annual Review of Earth and Planetary Science, 10, 129–154. pression along the Beagle Channel. In addition, all Barbeau, D.L., Jr., Olivero, E.B., Swanson-Hysell, N.L., Zahid, observed strike-slip faults cut F3 structures and all K.M., Murray, K.E & Gehrels, G.E., 2009. Detrital-zircon exhumation-related fabrics, indicating that they post- geochronology of the eastern Magallanes foreland basin: date the main phase of exhumation (Klepeis et al., implications for Eocene kinematics of the northern Scotia Arc 2010). U–Pb zircon data indicate that most strike-slip and Drake Passage. Earth and Planetary Science Letters, 284, 489–503. and oblique-slip features formed after the Eocene Bersani, D., Ando` , S., Vignola, P. et al., 2009. Micro-Raman (Gombosi et al., 2009). The inverted northern meta- spectroscopy as a routine tool for garnet analysis. Spectro- morphic profile (Fig. 2d) and all other field relation- chimica Acta Part A, 73, 484–491. ships discussed in this paper are most consistent with Biddle, K.T., Uliana, M.A., Mitchum, R.M., Fitzgerald, M.G. & Wright, R.C., 1986. The stratigraphic and structural evolution exhumation having been synchronous with, and of the central and eastern Magallanes basin, southern South controlled by, thrusting. America. Special Publication of the International Association of Sedimentologists, 8, 41–61. Bruhn, R., Stern, C. & de Wit, M., 1978. Field and geochemical ACKNOWLEDGEMENTS data bearing on the development of a Mesozoic volcano-tec- tonic rift zone and back-arc basin in southernmost South KTM and WW are supported by Australian America. Earth and Planetary Science Letters, 41, 32–46. Government Endeavour International Postgraduate Bucher, K. & Frey, M., 1994. Petrogenesis of Metamorphic Research Scholarships. We thank DIFROL (Direc- Rocks, 6th edn. Springer-Verlag Telos, Berlin, pp. 309–310. cio´ n Nacional de Fronteras y Lı´ mites del Estado) and Caldero´ n, M., Fildani, A., Herve´ , F., Fanning, C.M., Weislogel, A. & Cordani, U., 2007. Late Jurassic bimodal magmatism in the Chilean Navy for permission to visit and sample the northern sea-floor remnant of the Rocas Verdes basin, localities along the Beagle Channel. Logistic and southern Patagonian Andes. Journal of the Geological Society, analytical expenses were met with funding provided 164, 1011–1022. by the National Science Foundation (EAR-0635940 to Carlson, W.D. & Gordon, C.L., 2004. Effects of matrix grain KAK), an Australian Academy of Sciences Travel size on the kinetics of intergranular diffusion. Journal of Metamorphic Geology, 22, 733–742. Grant (GLC) and the School of Geosciences at the Carmichael, D.M., 1969. On the mechanism of prograde meta- University of Sydney. We thank M. De Paoli for help morphic reactions in quartz-bearing pelitic rocks. Contribu- with the microprobe work, and L. Carter for help tions to Mineralogy and Petrology, 20, 244–267. with Raman analysis. We are greatly indebted to Cheng, H., Nakamura, E., Kobayashi, K. & Zhou, Z., 2007. ´ Origin of atoll garnets in eclogites and implications for the J. Alvarez and F. Poblete, (Universidad de Chile), redistribution of trace elements during slab exhumation in a P. Betka (University of Texas), and S. 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Ó 2011 Blackwell Publishing Ltd III University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 53

Tectonic inversion of the Rocas Verdes marginal basin: the record in Seno Otway, Patagonia K. T. MALONEY1, G. L. CLARKE1, AND K. A. KLEPEIS2 1 School of Geosciences, University of Sydney, NSW 2006, Australia 2 Department of Geology, University of Vermont, Burlington, VT, 05405, USA

ABSTRACT Geological elements of the north-trending Jurassic–Lower Cretaceous Rocas Verdes marginal basin and Upper Cretaceous Magallanes foreland basin, southernmost South America, record along strike distinctions related to both basin development and closure. Seno Otway, located approximately midway between the better studied regions of Ultima Esperanza and Cordillera Darwin, displays a complete sequence of basin evolution from: (i) back-arc basin extension; (ii) tectonic inversion; and (iii) the development of an overlying foreland basin, the Magallanes Basin. Clastic sedimentation into the mafic floored Rocas Verdes Basin is recorded by mudstones and fine sandstones of the Zapata and Canal Bertrand formations. The foreland basin records the arrival of coarse-grained sediments comprising the Latorre and overlying Escarpada formations. Paleocurrent indicators in these formations record multiple Late Cretaceous foreland submarine fans. Thrusting during Late Cretaceous contraction obducted the floor of the Rocas Verdes Basin. The development of the Magallanes fold and thrust belt then exhumed components of the Rocas Verdes basin and underlying Late Jurassic felsic volcanic rocks to Earth’s surface, to act as a source of Late Cretaceous sediments. The intensity of deformation in the Seno Otway region decreases toward the foreland, away from the major obduction thrusts. Metamorphic grade in the region does not exceed greenschist facies.

Key words: Rocas Verdes Basin, Magallanes Basin, Seno Otway, marginal basin inversion, foreland sedimentation

INTRODUCTION little is known of the basin form and dimension prior to its closure. It has been widely considered Late Paleozoic to present rocks forming the (Dalziel, 1981; Stern & de Wit, 2003) that the basin southernmost part of South America (Dalziel et al., first opened in the south due to subduction-related 1974; Olivero & Martinioni, 2001) record a history back-arc extension and that extension propagated of subduction (Hervé et al., 2007), widespread northward. However recent detrital zircon volcanism (Gust et al., 1985; Biddle et al., 1986; spectra from rocks collected along the basin are Bruhn et al., 1978), back-arc extension (Dalziel more consistent with an interpretation involving et al., 1974; Bruhn, 1979), batholith intrusion simultaneous opening along the length of the basin (Hervé et al., 2007), foreland folding and thrusting (McAtamney et al., 2011). (Wilson, 1991; Fosdick et al., 2011), and foreland Significant differences in the geology of basin sedimentation (Wilson, 1991; Fildani & northern and southern regions of the basin are Hessler, 2005). South of 49°S, discontinuous consistent with distinct processes and events having orogen-parallel lenses of mafic rocks have been occurred in the two areas. For example, there is a interpreted as proto-oceanic crustal remnants of the profound change in metamorphic grade in the region Jurassic to Cretaceous Rocas Verdes Basin, formed of the Cordillera Darwin (Fig. 1), which includes along the southwest margin of Gondwana (Fig. 1; the highest grade Cretaceous metamorphic rocks Katz, 1972; Dalziel et al., 1974). Comparatively south of Ecuador (Klepeis & Austin, 1997). This University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 54

48oS (this paper) and amphibolite facies regional Patagonian batholith

Patagoni metamorphism in Cordillera Darwin (Klepeis Upper Cretaceous sediments (Latorre and Limit of Escarpada formations and equivalents) thrust belt et al., 2010; Paper II). Along strike variations in

an icecap Lower Cretaceous sediments (Zapata Formation and equivalents) depositional and contractional features of the Rocas Upper Jurrassic and Lower Cretaceous volcaniclastic rocks (Yahgan Formation and Verdes Basin are explored in this paper, as they equivalents) Upper Jurassic silicic volcanics and present an uncommonly well preserved history for volcaniclastics (Tobifera and equivalents) the complete evolution of a marginal basin cycle Ophiolite o 51 S Patagonian batholith including: initial extension, development of mafic Metamorphic basement crust, basin closure and ophiolite obduction, and

Seno foreland propagation of thrusting accompanied by Skyring Magallanes Seno the development of a foreland basin. Paleocurrent Otway Basin indicators, preserved in some of the foreland basin

Sarmiento sequence sedimentary rocks, provide a record of ophiolite the changes in depositional patterns through the (Ultima 0901 Esperanza) evolution of the foreland basin portion of the cycle. Tierra del Undertaking fieldwork in the region is complicated South Fuego America 55oS by its remoteness, and many areas are best accessed Cordillera N by water. Field observations to complete this Darwin Beagle Channel Tortuga 0 50 100 km project were undertaken using a boat and zodiac, ophiolite 75oW 70oW over April and May 2009. Fig. 1. Overview of the geology of southern Patagonia, modified from Fildani & Hessler (2005) and Wilson (1991). Box in Seno Otway region indicates location of Fig. 2. Site GEOLOGIC AND TECTONIC SETTING 0901, outside the main focus area of fieldwork, is indicated on this map. Rocks forming present day South America once distinction reflects significantly greater shortening bound the proto-Pacific margin of Gondwana, a site of the southern Rocas Verdes Basin, compared with of long-lived subduction. Late Paleozoic to early that experienced by rocks in northern regions. The Triassic metasedimentary and metavolcanic rocks strike of the orogen also changes from being north- (Dalziel et al., 1974; Nelson et al., 1980; Hervé et trending in the north to east-trending in the south. al., 1981) represent Gondwana relics and form a There are also distinctions in the depositional facies basement to other rocks in the southernmost region of characteristic sedimentary formations, notably of South America. The Late Paleozoic accretionary the Zapata and Yahgan formations and equivalents Madre de Dios complex (Hervé et al., 2007) records (Katz, 1963; Wilson, 1991; Olivero & Martinioni, the oldest evidence of subduction along the margin 2001; Fildani & Hessler, 2005). The Seno Otway in this region. Paleomagnetic and structural data region lies between the comparatively well studied from the Madre de Dios indicate that Late Paleozoic regions of Ultima Esperanza to the north, and subduction occurred obliquely from the northwest Cordillera Darwin and the Tortuga ophiolite to the (Rapalini et al., 2001). Late Jurassic volcanic and south (Fig. 1), making it an ideal location to study plutonic components of the Patagonian Batholith the changes in geology that occur along the strike are the earliest recognized parts of the magmatic of the Rocas Verdes Basin. arc (Katz & Watters, 1966; Hervé et al., 2007). A combination of thick and thin-skinned The lack of arc-related intrusive rocks prior to the thrusting styles has been suggested for parts of Late Jurassic can be interpreted to be a result of: (i) the Cretaceous contraction of the Rocas Verdes oblique Paleozoic subduction (Hervé et al., 2007); Basin (Fildani & Hessler, 2005). Though limited or (ii) the Antarctic Peninsula having lain west of basement involvement has been interpreted in Patagonia prior to 155 Ma, after which it began to the Ultima Esperanza region (Harambour, 2002; move south, facilitating development of the current Fildani & Hessler, 2005), significantly more arc (Calderón et al., 2007; Hervé et al., 2006; Jokat shortening is indicated in the south by basement- et al., 2003). cover intercalation in the Seno Otway region The earliest stage of Gondwana dispersal University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 55 is commonly attributed to a period of global of closed-system crystal – liquid fractionation, volcanism accompanied by crustal extension others interpreted as wall rock xenoliths (de Wit & (Dalziel et al., 1987). This volcanism was primarily Stern, 1981; Stern & de Wit, 2003). In contrast, the basaltic, perhaps best represented by the Karoo and Tortuga ophiolite in the south (Fig. 1) has uniform Ferrar provinces of South Africa and Antarctica, basic rocks of MORB affinity, lacking silicic and respectively (Storey 1995). In Patagonia and the intermediate rocks (Stern & de Wit, 2003). Further Antarctic Peninsula widespread contemporary differences between the two ophiolite complexes ignimbrite and rhyolite volcanism reflected include the contact between the gabbroic and the extensive crustal anatexis (Bruhn et al., 1978; sheeted dyke units being abrupt in the Sarmiento Gust et al., 1985; Dalziel et al., 1987; Kay et al., ophiolite, but gradual in the Tortuga ophiolite 1989; Pankhurst et al., 2000). These silicic rocks (Stern & de Wit, 2003). Such differences have been unconformably overlie the Paleozoic metamorphic interpreted as representing distinct stages in the basement (Bruhn et al., 1978; Wilson, 1991), and evolution of the Rocas Verdes Basin. Geological will be referred to in this paper as the Tobifera relationships in the Tortuga ophiolite are interpreted Formation (after Thomas, 1949), also called the El to represent a younger stage of evolution involving Quemado Formation, or the Ibañez Formation in localized spreading broadly similar to modern western Patagonia (Pankhurst et al., 2000), or the ocean ridges, whereas the Sarmiento ophiolite has Lemaire Formation in Argentina (Kranck, 1932). It been interpreted as representing an initial extension includes volcanic and volcaniclastic rock, deposited related to broadly distributed magmatism (de Wit in both subaerial and submarine environments & Stern, 1981). (Olivero & Martinioni, 2001; Pankhurst et al., 2000). Plutonic rocks of the Patagonian Batholith Its thickness varies throughout the region, though were emplaced in a linear belt along the paleo- it is usually inferred to have been between 1000 Pacific Gondwanan margin from the Late Jurassic and 2000 m thick (Wilson, 1991). The variability through to the Neogene (Hervé et al., 2007). These in thickness is interpreted to reflect its deposition in rocks indicate that subduction overlapped with the graben and half graben structures, eruption having formation and inversion of the Rocas Verdes Basin, been coeval with regional extension in a volcano- and then continued. The batholith is primarily tectonic rift setting (Bruhn et al., 1978; Gust et al., granitic to dioritic plutonic material emplaced in 1985). Pankhurst et al. (2000) reported U-Pb zircon episodic pulses (Hervé et al., 2007). A first episode crystallization ages of 178.4 ± 1.4 to 153.0 ± 1.0 of magmatism occurred between 157 ± 3 to 144 ± Ma for the unit. Calderón et al. (2007) dated two 1 Ma, partially coeval with the Tobifera Formation samples at c.148 Ma and c.142 Ma. (Hervé et al., 2007). This arc and a sliver of host Extension in the Late Jurassic to Mid- continental basement were rifted westward away Cretaceous resulted in a series of extensional from the continental mainland during formation of basins along the Andean margin. The northernmost the Rocas Verdes Basin (Dalziel et al., 1974). and southernmost regions of the margin are unique Clastic rocks that initially filled the Rocas in preserving mafic-floored basins with mid ocean Verdes Basin include Lower Cretaceous mudstone ridge affinities: the Colombian Marginal Seaway that overlies both the Tobifera Formation and the in the northern Andes, and the Rocas Verdes mafic floor of the basin. The characteristics of Basin in Patagonia (Dalziel et al., 1974; Stern, cognate sedimentary units vary significantly along 1980; Alabaster & Storey, 1990; Mpodozis & strike of the basin. As a consequence, there has Allmendinger, 1993; Calderón et al., 2007; Paper been a proliferation of unit names. In the Ultima I). Initial rifting of the Rocas Verdes Basin at c.152 Esperanza region the initial fill of the Rocas Verdes Ma (Calderón et al., 2007) temporally overlaps with Basin consists of the Zapata Formation (Katz, 1963), the latest stage of Tobifera Formation volcaniclastic primarily composed of black marine mudstones, sedimentation. Along-strike differences are with subordinate thin siltstone and fine-grained observed in ophiolitic relics of the volcanic floor to sandstone (Wilson, 1991; Fildani & Hessler, 2005). the Rocas Verdes Basin. The Sarmiento Complex The unit conformably overlies ophiolite of the (Fig. 1) includes both mafic and felsic igneous Rocas Verdes Basin, sitting, for example, directly rocks, some felsic rocks interpreted as products on the pillow lava of the Sarmiento complex University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 56

(Fildani & Hessler, 2005; Stern & de Wit, 2003), Basin (Olivero & Martinioni, 2001). Volcaniclastic and volcanic rock of the Tobifera Formation (Katz, Yahgan sedimentary rocks are intercalated with 1963; Fildani & Hessler, 2005). The name ‘Zapata’ ophiolitic pillow lava along the margins of the mafic is generally used, though it has historically also complex (Stern & de Wit, 2003). Interpretations been known as the ‘Erezcano Formation’ (Cecioni, of age relationships within the formation rely on 1956). It is commonly c.1000 – 1200 m thick in sparse fossils. Basal parts are considered Tithonian western parts of the basin, thinning eastward to – Neocomian (Olivero & Martinioni, 2001), and less than 630 m (Katz, 1963; Wilson, 1991; Fildani the upper part of the formation thought to be Albian & Hessler, 2005). Deposition commenced in the (Dott et al., 1977; Olivero & Martinioni, 1996). Tithonian (Wilson, 1991; Fildani & Hessler, 2005) The Beauvoir Formation, found north of the and continued through the Early Cretaceous for Yahgan Formation in Tierra del Fuego, is considered c.40 – 50 myr. A gradational conformable contact a lateral equivalent of the Yahgan Formation between the Zapata and overlying Punta Barrosa (Olivero & Martinioni, 2001). It is primarily formations is approximately 150 m thick (Fildani black marine mudstone with some tuffaceous & Hessler, 2005). rock (Olivero & Martinioni, 2001), making it Farther south, in the region between Seno lithologically similar to the Zapata Formation. Skyring and Seno Otway (Fig. 1) a succession Though the age of the unit is poorly constrained, of volcaniclastic breccia and lava interbedded some fossil and isotopic data indicate deposition with turbidite has been called the Canal Bertrand occurred during the Early Cretaceous (Olivero & Formation. This unit is thought to overlie the Zapata Martinioni, 2001). Formation, but precede development of the foreland Contraction and closure of the Rocas Verdes Magallanes Basin (Castelli et al., 1993; Mpodozis Basin, associated with the change to westward et al., 2007). Following McAtamney et al. (2011), motion of the South American plate that accompanied the Zapata and Canal Bertrand formation names are opening in the Atlantic (Paper I; Somoza & applied to the sedimentary fill of the Rocas Verdes Zaffarana, 2008; Ramos, 2010), was underway by Basin in the Seno Otway region. the start of the Late Cretaceous (Fildani & Hessler, The Yahgan Formation (Kranck, 1932) was 2005; Fosdick et al., 2011). Crustal loading related deposited in the southern portion of the Rocas to this orogeny is interpreted as having resulted in Verdes Basin, now called Tierra del Fuego (Fig. 1). the formation of the adjacent Magallanes foreland The unit is commonly considered equivalent to the basin (Fig. 1), which also displays regional Zapata Formation to the north (Fildani & Hessler, variation in the nature of its sedimentary fill. 2005). Facies interpretations for rocks forming the Lowermost parts of Magallanes Basin sequence in Yahgan Formation are more varied than those of the the Ultima Esperanza region involve interbedded Zapata, as the unit includes mudstone, tuffaceous sandstone and turbidite mudstone of the Punta rock, sandstone, and conglomerate (Olivero & Barrosa Formation (Cecioni, 1956; Katz, 1963). Martinioni, 2001). Conglomerate is only observed The appearance of decimetre scale medium-grained on the archipelago south of Tierra del Fuego; north sandstone is used to distinguish the Punta Barrosa of the Beagle Channel mudstone, sandy turbidite, from underlying Zaparta Formation strata (Wilson, tuffaceous rock, and sandstone are common 1991; Fildani & Hessler, 2005). The lower portion (Olivero & Martinioni, 2001). The thickness of of the formation comprises at least 50% shale, and the formation varies. In the southernmost region there is an overall upward grading in grain size and adjacent to parts of the Patagonian Batholith, bed thickness (Fildani & Hessler, 2005; Wilson, the formation reaches its maximum thickness of 1991). The base of the Punta Barrosa Formation c.6000 m (Olivero & Martinioni, 2001). It thins has been dated at 95 ± 2 and 92 ± 1 Ma, and northward, accompanied by a decrease in grain uppermost units at c.85 Ma (Fildani et al., 2003; size, to being c.1500 m thick north of the Beagle Fildani & Hessler, 2005; Romans et al., 2010). The Channel (Olivero & Martinioni, 2001; Fig. 1). Such formation is conformably overlain by the Santonian features have led to an interpretation of the Yahgan – Campanian Cerro Toro Formation (Cecioni, Formation being a volcaniclastic wedge of arc 1956; Katz, 1963), a shale-dominated unit more detritus deposited into a deepening Rocas Verdes than 2000 m thick that includes sandstone turbidite University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 57 72.5 W 72 W

16

6 0996 F* A’ 40 0998 31 FF 09112 0999 24 75 09118

45

EB 44

27 FS 40 22 53.25 S 9 09120 53.25 S 25 47 35 20 09125 CJ 2619 62 12 53 FW BN 09132 17 39 20 44 30 28 A EC 49 41

CJ 25

09145 09148

53.5 S 53.5 S Escarpada Formation (Upper Cretaceous) 17 Bedding Orientation

Latorre Formation (Upper Cretaceous) Syncline

Zapata/Canal Bertrand Formations Anticline (Upper Jurassic - Lower Cretaceous) Overturned Syncline Rocas Verdes gabbros and basalts (Upper Jurassic - Lower Cretaceous) Overturned Anticline Patagonian Batholith (Upper Jurassic - Upper Cretaceous) Lithologic Contact

Tobifera Formation (Upper Jurassic) Thrust Fault 0 5 10 15 km Paleozoic metamorphic basement

72.5 W 72 W Fig. 2. Geological map of field area in Seno Otway with locations of several of the sites visited.The location of the cross-section drawn in Fig. 7 is indicated by A to A’. Site labels indicate locations referred to in text, or locations of the paleocurrent plots in Fig. 4. CJ – Canál Jerónimo; FF – Fiordo Fanny; EB – Estero Bending; EC – Estero Condor; BN – Brazo Nuñez; FS – Fiordo Sullivan; FW – Fiordo Wickham. and extensive, thickly-bedded conglomerate deep-water deposit in the basin that preceded the (Wilson, 1991; Hubbard & Shultz, 2008). The shallow marine Maastrichtian – Danian Dorotea Cerro Toro Formation has a conformable contact Formation (Katz, 1963; Shultz & Hubbard, 2005). with the overlying Campanian–early Maastrichtian In the Seno Skyring region Late Cretaceous Tres Pasos Formation (Katz, 1963; Wilson, 1991). equivalents of the Punta Barrosa and Cerro Toro The Tres Pasos Formation is c.1500 m thick, and formations have been named the Latorre Formation consists mainly of sandy turbidite. It is the last and the Escarpada Formation, respectively University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 58

(Mpodozis et al., 2007). Following the convention Paleocurrent indicators were observed at of McAtamney et al. (2011), the Latorre Formation several locations in Seno Otway, mostly in the and Escarpada Formation names are informally Latorre and Escarpada formations. Ripple foresets applied to the Late Cretaceous strata in the Seno are the most common indicator, but several sites Otway region. also display flute casts and imbricated clasts. In all In Tierra del Fuego, Late Cretaceous and cases, the bedding orientation in which the indicator younger sedimentary strata are variable in was found has been returned to horizontal in order composition and commonly poorly defined. An to determine the restored paleocurrent direction. exhaustive description of this variation is outside the aims of this paper; however it should be noted Pre- Rocas Verdes Units that the units generally coarsen upward, reflecting In the Seno Otway region Paleozoic to early a trend similar to the shallowing depositional Jurassic basement rocks are represented by environment as observed in the Ultima Esperanza greenschist facies quartz-mica phyllite and schist region (Olivero & Malumián, 2008; Olivero & with prominent chlorite. Fine scale phyllitic and Martinioni, 2001). schistose fabrics define penetrative S1, commonly Late Cretaceous thrusting associated with the deformed in a D2 crenulation cleavage (Fig. 3a). development of the Magallanes fold and thrust belt Quartz rodding is common in this unit. was underway by c.101 Ma (Fosdick et al., 2011). Outcrop of the volcaniclastic Tobifera Early thrusting obducted ophioliote segments of Formation in the region occurs primarily as the Rocas Verdes Basin floor onto the Gondwanan massive, metamorphosed silicic ignimbrite. margin (Dalziel et al., 1974; Nelson et al., Individual outcrops generally display a strong 1980; Calerón et al., 2007; Klepeis et al., 2010). planar fine-scale cleavage, which has completely Convergence related burial of basement rocks to obliterated any depositional features (Fig. 3b). depths of c.35 km resulted in amphibolite facies Flattened quartz and plagioclase grains define a schists of the Cordillera Darwin, subsequently penetrative fabric that envelops larger quartz clasts exhumed through on-going thrusting (Nelson, (3 – 15 cm in diameter; Fig. 3b). Sigmoidal clasts 1982; Kohn et al., 1995; Klepeis et al., 2010; Paper indicate a top to the northeast sense of shear (Fig. II). Crustal shortening through the early Paleogene 3c). Areas of lower grade rocks (e.g. site 0901, propagated toward the foreland and terminated Fig. 1) comprise volcaniclastic sandstone beds in the Eocene (Alvarez-Marrón et al., 1993; 1 – 3 m thick interbedded with 0.5 – 5 m thick Ghiglione & Ramos, 2005; Barbeau et al., 2009; conglomerate beds (Fig. 3d). Cross-laminations in Gombosi et al., 2009). As shortening in the region sandstone indicate a paleocurrent oriented toward diminished, strike-slip and trans-tensional motion the southwest (Fig. 4a). Clasts in the conglomerate became dominant, evident in the development of layer range from 1 – 15 cm in diameter and are the South American – Scotia plate transform during composed of andesite, rhyolite, and greenschist, the late Oligocene or early Neogene (Cunningham, with rare granite and granodiorote. The greenschist 1993; Klepeis & Austin, 1997; Lodolo et al., 2003; clasts contain crenulated foliations and closely Gombosi et al., 2009). resemble basement rocks observed elsewhere in the field area (Fig. 3e). Outcrop of the Patagonian Batholith is GEOLOGY OF SENO OTWAY limited to the westernmost region of the field area. Intermediate and felsic intrusions are relatively The Seno Otway region is extensively faulted, with fine grained and are composed primarily of quartz, a series of northeast-verging thrust faults disrupting plagioclase, and amphibole, with some pyrite. and imbricating the sedimentary strata (Fig. 2). Outcrops are mostly undeformed and display very Conformable contacts are rare; most units are in little foliation. tectonic contact across the thrust faults. Faulting, folding, and the development of extensive cleavage Rocas Verdes Associated Units in some units make estimating unit thickness Metamorphosed basalt and gabbro are considered difficult. relics of the mafic floor of the Rocas Verdes Basin University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 59

A B

S1

S2

C SW D

0 10 20 cm

E F

G H

Fig. 3. (A) Crenullated basement schist displaying S1 and S2 foliations, (B) Silicic ignimbrite in the Tobifera Formation with planar foliation and large quartz clasts, (C) Quartz clast with sigmoidal deformation tails in the Tobifera Formation displaying top to the northeast sense of shear, (D) Basal conglomerate and underlying silicic volcaniclastic sandstone of the Tobifera Formation, (E) Basement clast in the Tobifera Formation basal conglomerate, (F) Interbedded mudstones and fine grained sandstones in the Zapata Formation with bedding parallel cleavage, (G) Tight folds in the Zapata Formation, (H) Clastic sandstone dyke in the Zapata Formation immediately above the Rocas Verdes mafic rocks. University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 60

A 0901 B 0996 No pillow lavas were observed. However, evidence of a submarine environment is chert locally occurs with basalt. Outcrops are generally unfoliated, n=1 n=13 although some sites display a planar cleavage defined by chlorite. The sedimentary fill of the Rocas Verdes Basin comprises the mudstone dominated Zapata Tb Fm Lt Fm and Canal Bertrand formations. Mudstones of the C 09112 D 09118 Zapata Formation are interbedded with thin beds of fine-grained sandstone that tend to increase in abundance up section (Fig. 3f). A pervasive slaty n=5 n=10 cleavage, axial planar to tight to isoclinal folds (Fig. 3g), occurs throughout the Zapata/Canal Bertrand formations. The extensive folding has thickened the unit, making it difficult to estimate its true Lt Fm Lt Fm thickness. The Zapata Formation is apparently in E 09125 F 0998 conformable contact with the underlying Tobifera Formation and with the mafic rocks that floored the Rocas Verdes Basin. Later thrust faulting has n=5 n=21 dissected and imbricated the sequence such that most contacts occur across thrust faults, but the original depositional contacts can be observed in several locations. Mudstones and siltstones Lt Fm Es Fm immediately above the depositional contact with the Rocas Verdes mafic rocks contain several G 0998 H 0999 clastic sandstone dykes (Fig. 3h).

n=8 n=1 Foreland Basin Units The base of the Latorre Formation in Seno Otway is marked by the appearance of a 0.5 – 0.8 m thick sandstone unit at the base of an interbedded Es Fm Es Fm sandstone and mudstone sequence that fines Fig. 4. Rose diagrams showing paleocurrent data from sites (A) upward (Fig. 5a). Sandstone beds tend to be coarser 0901 Tobifera Formation (Tb Fm), (B) 0996 Latorre Formation and more abundant than found in the Zapata and (Lt Fm), (C) 09112 Latorre Formation, (D) 09118 Latorre Canal Bertrand formations, and are generally 4 – Formation, (E) 09125 Latorre Formation, (F) 0998 Escarpada 20 cm thick. Sandstone beds thicker than this are Formation (Es Fm), (G) 0998 Escarpada Formation (flute marks and imbricated clasts), (H) 0999 Escarpada Formation rare, with the notable exception of the thick basal (imbricated clasts). Paleocurrent indicators are from ripple unit. The repeated fining upward sequences result foresets unless indicated otherwise. Some of the data displayed in mudstone dominated sections of the Latorre in these plots has also been published by McAtamney et al. Formation that have a similar appearance to (2011). upper portions of the Zapata and Canal Bertrand in the region. No ultramafic rocks were observed. formations. Mudstone layers frequently display Plagioclase, pyroxene and hornblende phenocrysts a slaty cleavage, whereas sandstone layers lack persist in weakly deformed gabbro and leucogabbro. cleavage and original sedimentary features are Outcrops are generally highly altered, commonly commonly preserved. Ripple cross-laminations containing chlorite and epidote, giving the rocks can be observed at several locations and were the greenish colour that led to their name. Basaltic used to determine paleocurrent directions for the dykes intrude a variety of host rocks, including Latorre Formation (Figs 4, 5b). Multiple paleoflow gabbro, felsic plutons of the Patagonian Batholith, indicators were recorded at each site. Although and volcaniclastic rock of the Tobifera Formation. the overall current trend is to the northeast, some University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 61

A B

C D

E Fig. 5. (A) Basal sandstone of the Latorre Formation, (B) Ripple cross-laminations preserved in the sandstones of the Latorre Formation, (C) Basal conglomerate of the Escarpada Formation, (D) Large clast in the Escarpada Formation displaying interbedded sandstone and mudstone with internal deformation, (E) Open style fold in the Escarpada Formation.

indicators record east or southeast directed flow. layers record less strain. A larger variety of These data indicate flow generally perpendicular to paleocurrent indicators are preserved in the sandy the northwest orogen trend in the region, with only layers of the Escarpada Formation than seen in the minor components parallel to the orogen. Latorre Formation, including ripple foresets, flute The Escarpada Formation conformably overlies casts, and clast imbrications. Paleocurrent directions the Latorre Formation. The base of the formation determined from these indicators record multiple is marked by the first appearance of a cobble flow directions in the unit, even at single sites (Fig. conglomerate horizon with a sand dominated matrix 4). Paleoflow directions range from northeast to (Fig. 5c). Clasts tend to be rounded and spherical southeast, but unlike the Latorre Formation there and have a variety of compositions, including does not appear to be a dominant flow direction sandstone, mudstone, basalt, andesite, and felsic within that range. Fold styles tend to be more open volcanic material. Some large clasts (c.2 m) of than those seen in underlying units (Fig. 5e). One sandstone display internal deformation (Fig. 5d), section of the Escarpada Formation immediately indicating that slumping occurred before the clasts northeast of fault F* (Figs 6, 7) records higher strain were ripped off of the channel wall by turbidity than the rest of the unit. Here tight, upright, gently currents. The Escarpada Formation consists of plunging synform-antiform pairs result in bedding interbedded sandstone and conglomerate layers that is vertical to sub-vertical, and the development with some mudstone that indicates fining upward of a pervasive fine scale slaty cleavage. The section sequences. Slaty cleavage is present in some of the is mud dominated, though it still contains pebble to mudstone layers, but sandstone and conglomerate meter sized clasts throughout. University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 62

A 72.5 W 72 W

22 26 20 34 37 Undeformed intrusive rocks

F* 0998 Tight to isoclinal northeast verging folding; 09102C pervasive S2 axial planar slaty to schistose cleavage 09102D 09102A 09102E Upright isoclinal folding; pervasive S2 axial 09116 planar slaty cleavage 70 24 67 Tight to close nearing upright folding; S2 axial 68 planar spaced cleavage developed in 31 27 muddy units 43 09120 53.25 S 53.25 S 27 52 50 45 58 37 Close to open folding; little development of 43 42 cleavage 30 33 40 26 48 49 32 48 16 Orientation of S2 axial planar cleavage 25 34 26 55 09132 0 5 10 15 km

32 69 70 50 58 35 64 22 40 48 38 54 32 45 09145 40 09148 37

B Thrust Faults Normal Faults Sinistral Faults Dextral Faults

09102C 09102E 0998 09102A 0915 09102A

09116 09145 09102A 09102D 09102C 09116

09148 09102E 09102E 09120 09132 Fig. 6. (A) Intensity of deformation in the Seno Otway region based folding style and cleavage development. Strike/dip symbols indicate the orientation of the axial planar cleavage. Site locations indicate sites where fault measurements were taken. (B) Fault plane solutions shown on lower hemisphere equal area stereonets, divided into observed fault populations. Fault plane (thick black line), slickenline striae orientation (dot), and sense of motion of hanging wall (arrow) are all indicated. Faulting involved normal faulting. Structures strike between Large northeast verging thrust faults dominate north and northeast, with slickenline striae plunging structure in the Seno Otway region. They formed to the north or northwest (Fig. 6). The second during Late Cretaceous compression and have population involved thrust faulting. Fault strike is dissected and imbricated the stratigraphy of the between northwest and northeast and striae plunge area (Figs 2, 7). These faults outcrop poorly, and in a wide range of directions, though plunges at they are generally defined by strain gradients in the any single site are generally consistent (Fig. 6). adjacent rock units, as well as notable stratigraphic The third population involves sinistral strike-slip inversions. Smaller faults throughout the region faults that are east-striking, and include striae that could be directly measured, and several populations generally plunge shallowly to the west (Fig. 6).The of faults are interpreted. The first population fourth population involves dextral strike-slip faults University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 63 of northeast strike that have shallow northeast- A’ plunging striae (Fig. 6). NE Solutions were determined by plotting all of the fault planes, striae, and sense of motions F* determined for each population at a particular site, and using this information to calculate P and T axes. An unweighted moment tensor summation was calculated using linked Bingham distribution Thrust Fault Thrust statistics to produce an average kinematic fault S2 cleavage plane solution for each population at each site, scale Vertical = Horizontal following the methods of Marrett & Allmendinger (1990). Calculations were performed using the program FaultKin 5 (Marrett & Allmendinger, 1990; Allmendinger et al., 2012). Large scale thrust faulting at site 09145 (Fig. 6) placed older basement rocks over the Tobifera Formation, and a fault plane solution was determined for this site. Fault plane solutions for most of the other thrust faults observed were similar, and are interpreted as being formed during the same deformation event. The strike-slip faults generally cut younger features, such as diorite dykes that intrude at site 09120 (Fig. 6), and are interpreted as the youngest stage of faulting. Zapata/Canal BertrandZapata/Canal Formations Formation Latorre Escarpada Formation

CROSS-SECTION OF SENO OTWAY

Due to a lack of subsurface constraints, no attempt was made to produce a balanced cross-section across the Seno Otway region. Instead, all of the surface data has been integrated to produce a cross- section that reflects the geology at the surface as well as giving an interpretation of the changing structure on a transect perpendicular to the regional strike. Most of the data gathered during field work were collected along the shores of Canál Jerónimo and the connected fjords (Fig. 2). The transect runs along Canál Jerónimo, and integrates data from both shores. Since the entirety of the transect is at, or only slightly above, sea level, the effects of topographic variation are ignored. Although the geology does not match up exactly across the canal, data from both the northern and southern shores have been integrated to produce a representative section. The cross-section shows an overall decrease in Tobifera Formation Tobifera the intensity of deformation towards the northeast, metasedimentary basement and metavolcanic Paleozoic Batholith Patagonian Basin oor Verdes and ma c dykes of the RocasGabbros that is, farther into the foreland and away from the

main obduction structures (Figs 6, 7). The northeast- shown on Fig. 2. Formlines have been drawn to correspond with surface bedding and cleavage measurements. A’ to A . Representative geological cross-section from A verging thrust faults are the dominant structural 0 5km Fig. 7 University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 64 feature of the cross-section, with several large- lower deformation have much less well developed scale thrusts causing major displacement across cleavage. In these areas it is slaty and more spaced, units. Splays off of the main thrusts cause second and is only developed in the mudstone dominated order imbrication of the sequence. The formlines sections, with sandstone dominated sections being of the cross-section have been drawn to correspond relatively unaffected. In the areas of least intense to the orientation of bedding at the surface. The deformation it is rarely seen even in the mudstone resulting folds are tighter with smaller amplitudes dominated units. in the southwest, and become broader with longer wavelengths in the northeast, which matches with the structural style observed in the field. Folding DISCUSSION style in the region of most intense deformation tends to be tight to isoclinal with inclined axial The initial phase of evolution in the Rocas Verdes planes (Figs 6, 7). Folds are asymmetric and verge marginal basin formed part of a widespread to the northeast. Northeast along the transect the Mid to Late Jurassic volcano tectonic rift zone intensity of deformation decreases, and folding (Bruhn et al., 1978; Fig. 8a). Crustal extension style goes from tight to close and finally to open, and accompanying silicic volcanism associated and the fold axial planes become more steeply with this phase resulted in graben and half graben inclined, approaching upright. Folds maintain the structures along the length of the incipient marginal northeast sense of vergence, but are significantly basin. Further crustal extension was accompanied less asymmetric. The exception to the trend of by the emplacement of gabbro and basalt as decreasing deformation to the northeast is seen basal units of the Rocas Verdes Basin. These in the section of upright, tight to isoclinal, gently incipient stages of ocean floor development rifted plunging synform-antiform pairs immediately a continental sliver attached to the magmatic arc northeast of fault F* (Figs 6, 7). This narrow band away from the South American continent (Fig. of more intense deformation is associated with 8b). The nature of the Rocas Verdes Basin seems fault F*, likely developing as the Magallanes fold to have varied significantly along strike: the and thrust belt propagated into the foreland. southern portion of the basin was both wider and The Seno Otway region can be divided experienced more subsidence, leading to greater into thrust-dominant domains and fold- sediment accumulation, than in the north (Stern dominant domains. Thrust-dominant domains & de Wit, 2003; McAtamney et al., 2011; Wilson, are characterized by closely spaced thrust faults 1991; Olivero & Martinioni, 2001). In contrast with and overturned folds. These domains are present the more extensive Sarmiento ophiolite, gabbro between Brazo Nuñez and Fiordo Sullivan, and and basalt in the Seno Otway region lacks crustal between Fiordo Fanny and fault F* (Figs 2, 7). xenoliths. This has been interpreted as indicating Fold-dominant domains tend to have more widely that Seno Otway records more advanced crustal spaced thrust faults, and a somewhat more open extension related to the evolution of the Rocas style of folding with little overturning. A fold- Verdes Basin. dominant domain is present between the two thrust- In the Ultima Esperanza region the Zapata dominant domains (Figs 2, 7). Formation is interpreted as representing hemipelagic The dominant cleavage in the region is the sedimentation, involving periodic fine-grained S2 axial planar cleavage associated with the turbidite deposition on the northeastern flank of the development of the fold and thrust belt. The S1 Rocas Verdes Basin (Fildani & Hessler, 2005). In fabric present in some basement rocks appears contrast, the Yahgan and Beauvoir formations on to record an earlier deformation event. The S2 Tierra del Fuego represent arc-related deposition axial planar cleavage dips consistently southwest, on the southwestern flank of the back-arc basin and is oriented parallel to the strike of the major (Olivero & Martinioni, 2001). The Seno Otway thrust faults (Fig. 6). In the regions of highest Zapata and Canal Bertrand formations contain deformation it is slaty to schistose, very finely more abundant sandy material than the Zapata spaced, and pervasive in all lithological units. Formation in Ultima Esperanza, but are otherwise Accompanying the change in fold styles, regions of similar and are interpreted to have formed in a University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 65

Middle to Late Jurassic Volcano-Tectonic Rift Zone A NE

B Early Cretaceous Rocas Verdes Basin NE

C Mid Cretaceous Initiation of Magallanes Foreland Basin NE

D Late Cretaceous to Paleogene Propagation of Magallanes Fold and Thrust Belt

NE

Metamorphic basement Arc-related sediment

Calc-alkaline pluton of volcanic arc Lower Cretatceous mudstones (Zapata/Canal Bertrand Formations)

Tobifera Formation Upper Cretaceous turbidites (Latorre Formation)

Rocas Verdes gabbros and basalts Upper Cretaceous turbidites and conglomerates (Escarpada Formation)

Fig. 8. (A) Middle to Late Jurassic: Initial extensional phase associated with Gondwanan dispersal, the formation of a widespread volcano-tectonic rift zone, accompanied by the deposition of the volcanic and volcaniclastic Tobifera Formation. (B) Early Cretaceous: Localization of extension results in the separation of the arc and a continental sliver from the South American continent and the development of the mafic floored Rocas Verdes marginal basin. Rocas Verdes gabbros and basalts are emplaced, and the Zapata/Canal Bertrand mudstone dominated sediments are deposited into the partitioned basin. (C) Mid Cretaceous: Initiation of contraction with associated marginal basin closure and obduction of Rocas Verdes ophiolitic remnants onto the South American continent. Crustal loading from thrust-induced shortening and thickening drives the formation of the Magallanes foreland basin, into which the Latorre Formation is deposited. (D) Late Cretaceous to Paleogene: Continued contraction continues to develop the Magallanes fold and thrust belt, which propagates into the foreland. The Escarpada Formation is deposited into the foreland basin. University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 66 similar depositional environment. The greater in the Magallanes fold and thrust belt (Fig. 8d). abundance of sand in the Seno Otway region could Paleocurrent directions vary by approximately 90°, reflect the partitioning of the Rocas Verdes Basin from being orogen parallel to being perpendicular into subsidiary horst and graben structures (Fildani to the orogen. The orogen parallel component of & Hessler, 2005), and consequent complexity paleocurrent flow is stronger in the Escarpada in depositional patterns. Taken in the context of Formation than in the Latorre Formation, and distinctions between ophiolitic rocks in the two could indicate that the orogen parallel deposition regions, along strike distinctions between the controlling the distribution of the Punta Barrosa sedimentary fill of the marginal basin are consistent Formation in the Ultima Esperanza region extended with the Rocas Verdes Basin having been wider in as far south as the Seno Otway region during this Seno Otway than Ultima Espernza, but was likely time. significantly narrower than in the Cordillera Darwin Convergence controlling the development of region. An overall coarsening of units in the upper the Magallanes fold and thrust belt had initiated Zapata/Canal Bertrand formations would seem by the earliest Late Cretaceous. Thrust sheets to best reflect proximity to source, and is likely a responsible for obducting the floor of the Rocas result of initial contraction in the region that would Verdes Basin onto the continent had formed prior to lead to closure of the Rocas Verdes Basin and the c.86 Ma (Klepeis et al., 2010; Fosdick et al., 2011; development of the Magallanes fold and thrust belt Figs 8c, d). Thrusting in the Seno Otway region and foreland basin (Fig. 8c). also exhumed greenschist facies metamorphic Sedimentary sequences of the Latorre Paleozoic basement (Figs 2, 7). A combination Formation are similar to those described for the of thick and thin-skinned thrusting styles has Punta Barrosa Formation in the Ultima Esperanza been inferred for the Ultima Esperanza region, as region, and are interpreted similarly: being the basement involvement is reported from proprietary first deposits related to the developing Magallanes seismic sections (Harambour, 2002; Fildani & foreland basin. Paleocurrent indicators in the Punta Hessler, 2005). However, at Ultima Esperanza the Barrosa Formation are directed parallel to the thrusts are not emergent, and significantly more orogen, indicating the Punta Barrosa was deposited basement involved shortening could be expected as part of a submarine fan that ran along the length to have affected the Seno Otway region. A trend of the northern part of the orogen (Fildani & Hessler, of greater crustal shortening with distance to the 2005). In contrast, the Latorre Formation displays south continues in the Cordillera Darwin region, much greater variation in paleocurrent directions, where continental underthrusting buried Paleozoic with a predominant flow direction perpendicular to basement rocks to depths of c.35 km and resulted the trend of the orogen. The nature of the submarine in amphibolite facies metamorphism (Klepeis et fans must have either been distinct in the northern al., 2010; Paper II). Burial to these depths required and central parts of the Magallanes Basin, or the much greater shortening than occurred at either Punta Barrosa and Latorre formations were both Ultima Esperanza or Seno Otway. deposited in a single large submarine fan with a Deformation in the Seno Otway region is most large diversity of flow directions. Differences in intense near the obduction structures, where tight sediment provenance and composition between the folds and pervasive fine scale cleavage developed two formations (McAtamney et al., 2011), are most throughout the Zapata/Canal Bertrand formations. consistent with the presence of multiple submarine The interpreted cross-section (Fig. 7) reflects more fans in the Late Cretaceous. open folding in structurally-overlying foreland Initial conglomerate deposition in the Escarpada basin units away from the obduction structures. Formation at c. 81-80 Ma (McAtamney et al., 2011) Cleavage is less developed in these units, enabling marks further complexity in sediment provenance. the preservation of delicate sedimentary features Andesitic clasts reflect the sedimentation of such as ripples and flute casts. This indicates that volcanic arc detritus. In addition, basaltic and as deformation propagated into the foreland the rhyolitic clasts indicate that the Rocas Verdes intensity of deformation decreased, and motion mafic rocks and the felsic Tobifera Formation were along later thrust faults was likely of lower both emergent at this time, consequent to thrusting magnitude than that along the early obduction University of Sydney, PhD Thesis, Paper III, Rocas Verdes marginal basin inversion in Seno Otway, Kayla T. Maloney, 2012. 67 thrusts. In the Cordillera Darwin region a later REFERENCES phase of thrusting resulted in a series of out-of- sequence thrusts and back thrusts that exhumed Alabaster, T., and B. C. 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(1929), Zur mechanic geologisher Geological Society of America Bulletin, 105(11), 1462- brucherscheinungen, Zentralblatt für Mineralogie, 1477, doi:10.1130/0016-7606(1993)105<1462:etcanc Geologie und Paleontologie B, p. 354-368 >2.3.co;2. (Abhandlung). Mpodozis, C., P. Alvarez, S. Elgueta, P. Mella, F. Romans, B. W., A. Fildani, S. A. Graham, S. M. Hervé, and M. Fanning (2007), Revised Cretaceous Hubbard, and J. A. Covault (2010), Importance of stratigraphy of the Magallanes Foreland Basin at Seno predecessor basin history on sedimentary fill of a Skyring: Regional implications of new SHRIMP age retroarc foreland basin: provenance analysis of the data on detrital zircon populations, paper presented Cretaceous Magallanes basin, Chile (50–52°S), at GEOSUR 2007 International Congress on the Basin Research, 22(5), 640-658, doi:10.1111/j.1365- Geology and Geophysics of the Southern Hemisphere, 2117.2009.00443.x. Univ. Católica de Chile, Santiago, 20–21 Nov. Shultz, M. R., and S. M. 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(Cretaceous), southern Chile, Journal of Sedimentary Research, 75(3), 440-453, doi:10.2110/jsr.2005.034. Somoza, R., and C. B. Zaffarana (2008), Mid-Cretaceous polar standstill of South America, motion of the Atlantic hotspots and the birth of the Andean cordillera, Earth and Planetary Science Letters, 271(1-4), 267- 277, doi:10.1016/j.epsl.2008.04.004. Stern, C. (1980), Geochemistry of Chilean ophiolites: evidence for the compositional evolution of the mantle source of back-arc basins, Journal of Geophysical Research, 85(B2), 955-966, doi:10.1029/ JB085iB02p00955. Stern, C., and M. J. de Wit (2003), Rocas Verdes ophiolites, southernmost South america: remnants of progressive stages of development of oceanic- type crust in a continental margin back-arc basin, Geological Society, London, Special Publications, 218, 665-683, doi:10.1144/GSL.SP.2003.218.01.32. Storey, B. C. (1995), The role of mantle plumes in continental breakup: case histories from Gondwanaland, Nature, 377(6547), 301-308, doi:10.1038/377301a0. Tchalenko, J. S. (1970), Similarities between Shear Zones of Different Magnitudes, Geological Society of America Bulletin, 81(6), 1625-1640, doi:10.1130/0016-7606(1970)81[1625:sbszod]2.0. co;2. Thomas, C. R. (1949), Geology and petroleum exploration in Magallanes Province, Chile, Bulletin of the American Association of Petroleum Geologists, 33, 1553-1578, doi:10.1306/3D933DEE-16B1-11D7- 8645000102C1865D. Wilson, T. J. (1991), Transition from back-arc to foreland basin development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile, Geological Society of America Bulletin, 103(1), 98-111, doi:10.1130/0016- 7606(1991)103<0098:TFBATF>2.3.CO;2. DISCUSSION University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 72

DISCUSSION initial rifting phase of Gondwana dispersal (Storey, 1995). A presently active continental marginal The South American Andean margin is a unique basin analogous to those formed in the Mesozoic natural laboratory for investigating crustal along the Andean margin is New Zealand’s Taupo evolution at ocean – continent convergent margins. volcanic zone, where voluminous arc magmatism The long (c.200 Myr) history of subduction, is accompanying extremely high rates of crustal coupled with the absence of any major episodes extension (Stern, 1987; Audoine et al., 2004). of continental collision, together with the physical The early stages of Gondwana break-up were length of the margin both make it ideal for this accompanied by voluminous plume related basaltic purpose. Semi-continuous crustal growth along magmatism in the Karoo province of Africa (Cox, the margin during the 200 Myr period is evident in 1992; Duncan et al., 1997) and the Ferrar province the form of numerous marginal basins, voluminous of Antarctica (Elliot, 1992). The lateral migration of arc magmatism, fold and thrust belts, and the parts of the Karoo plume towards the proto-Pacific products of high-grade burial metamorphism. Gondwanan margin in what is now Patagonia The margin also has the second highest of Earth’s resulted in voluminous silicic volcanism along the continental plateaus, in the Altiplano-Puna region. convergent margin (Pankhurst et al., 2000; Riley In isolation, many of these macroscopic features et al., 2001). Accompanying widespread extension might be ascribed to tectonic scenarios distinct helped set the stage for mafic-floored marginal from the causal ocean – continent convergent basin development in southernmost South America. margin. Geological detail of the Andean margin A contractional regime has dominated the thus provides a basis for a critical evaluation of Andean margin from the Late Cretaceous to the our understanding of components of plate tectonic present. Extensional basin inversion, ophiolite theory. This Discussion integrates results from the obduction, fold and thrust belt formation, uplift three papers to review the key tectonic processes of the extensive Altiplano-Puna plateau, and a that shaped the Andean margin. Two distinct stages few instances of high-P metamorphism have can be identified: a Late Jurassic and Cretaceous characterized this regime (Paper I, Fig. 1). During extensional stage; and a contractional stage from this period the absolute velocity of the overriding the Late Cretaceous to the present, with only minor continental plate was directed almost exclusively limited episodes of extensional deformation. Along trenchward as South America rifted away from the Patagonian portion of the margin a third stage of Africa and moved westward with the opening of the strike-slip tectonics dominates from the Oligocene South Atlantic. Several of the features developed to the present. during this regime, in particular amphibolite facies metamorphism and plateau development, The Andean Margin: an overview would commonly be associated with continent Late Jurassic and Cretaceous crustal extension – continent collisional settings. The Altiplano- resulted in a string of extensional marginal basins Puna plateau is second in height and extent only stretching from present day Colombia down to to the Tibetan plateau formed during the India – Chile (Atherton et al., 1983; Atherton & Aguirre, Eurasia collision, the type example of continental 1992; Marquillas et al., 2005; Carrera et al., collision (e.g. Harrison et al., 1992). Crustal 2006; Mpodozis & Allmendinger, 1993; Paper thickening in the absence of continental collision I, Fig. 1). Development of the northernmost and in Cordillera Darwin resulted in amphibolite facies southernmost basins progressed to an extent that metamorphism, in contrast with most models new oceanic crust was produced (Bourgois et al., of Barrovian-style metamorphism occurring in 1987; Aspden & McCourt; 1986; Nivia et al., continental collision settings (e.g. Thompson & 2006; Dalziel et al., 1974). Modern analogues England, 1984). of these mafic-floored basins in present day Geological elements of the Andean margin Colombia and Patagonia include the Japan Sea and formed since the Mesozoic include: extensional Bransfield Strait (Dalziel et al., 1974; Saunders basins, both of mafic and continental crustal & Tarney, 1984). Basin development along the affinity; thick and thin-skinned fold and thrust belts; Andean margin was contemporaneous with the extensive plateaus; and high-grade metamorphic University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 73 terranes. These all developed in the same setting floored back-arc basins along the South American involving continuous subduction, in the absence margin, including the Rocas Verdes Basin, was of continental collision. The variety of features associated with trench normal absolute velocity highlights two important concerns. The first is vectors of the continent being directed away the difficulty in using simplistic assumptions to from the trench (Paper I, Fig. 4). This conclusion identify tectonic settings based purely on geological confirms the findings of other studies, which have features, if the setting itself is no longer extant. The indicated that absolute continental plate velocity Cordillera Darwin metamorphic terrane provides provides a primary control on the tectonic regime a particularly striking example of this problem, as of ocean – continent convergent margins (Heuret without the context of the known subduction history, & Lallemand, 2005; Ramos, 2010). However the simplistic interpretation for these rocks would this condition alone was insufficient for crustal be a continental collision orogenesis. The second separation and, for example, microcontinent issue underscored by the wide range of features is dispersal. Back-arc basins were developed in the the importance of changing conditions, potentially Northern Andes and Patagonia in continental crust imposed by plate motions or slab characteristics overlying a subducting oceanic slab older than 50 such as age or topography, in determining the Myr (Paper I, Fig. 3), yet they did not lead to the ultimate evolutional history of a given setting. dispersal of marginal continental fragments. The The record provides an insight into the effect of slab age on subduction dynamics seems dominant factors controlling crustal extension ambiguous. The buoyancy (age, thermal regime), or contraction along the margin, and enables rigidity, and the extent of coupling with the specific structures and units to be linked to plate overriding plate, together with other parameters, scale processes. Temporal and spatial associations have been shown to have a dependency on slab between geological events and the plate parameters age, yet also have distinct, commonly competing, imposed by the subduction zone require explanatory effects (Forsyth & Uyeda, 1975; Di Giuseppe et al., geodynamic mechanisms. 2009; Yáñez & Cembrano, 2004). The velocity of the trench (i.e. trench advance or Extension Regime retreat) is commonly considered to be an important Widespread opening of marginal basins with parameter affecting deformation in the overriding continental and oceanic affinities in the back-arc plate, and the interaction between the absolute region of the Andean margin characterized the Late velocity of South America and the trench velocity Jurassic to Cretaceous extensional regime. Normal likely played an important role in the evolution faulting, resulting in the development of graben of the Andean margin, both during the extension and half graben structures was common. Back-arc and contraction regimes (Oncken et al., 2006; extension is most commonly associated with Pacific Ramos, 2010). However, the methods outlined in subduction zones where it was first identified this thesis are unable to quantify the relationship (Karig, 1970). In a global context, subduction zone between trench velocity and overriding plate polarity appears to be a strong determinant in the deformation as trench velocity is itself a function likelihood of developing back-arc basins, with most of shortening or stretching in the overriding plate, marginal basins being developed over west-dipping i.e. of overriding plate deformation (Ramos, 2010). subduction zones (Dickinson, 1978; Doglioni, Trench retreat, or rollback, has also commonly 1995; Doglioni et al., 2007). The marginal basins been considered a consequence of the subduction of the Andean margin are thus part of a rarer group of old, dense oceanic crust, whereas younger slabs of back-arc basins developed along east-dipping are commonly considered comparatively buoyant subduction zones, and likely required the presence and thus able to resist subduction leading to trench of unique conditions along the subduction zone to advance (Molnar & Atwater, 1978; Dewey, 1980; enable formation. Garfunkel et al., 1986). This could present a Marginal basins with continental affinity that convenient explanation for the opening of mafic- opened during the Late Jurassic and Cretaceous floored back-arc basins facilitated by slab retreat developed under variable conditions imposed by the from the subduction of old oceanic crust. However, subduction zone. However, the opening of mafic- in recent years this accepted relationship between University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 74 slab age and trench movement has been challenged. less than 0 cm/yr (i.e. extensional) were associated A global compilation of absolute motions with the opening of both the Colombian Marginal established the inverse relationship: older slabs Seaway (northern Andes) and the southern portion advance toward the overriding plate and younger of the Rocas Verdes Basin (Paper I, Fig. 5). ones retreat (Heuret & Lallemand, 2005). Older, However, this condition does not appear essential to stronger oceanic lithosphere supposedly resists the development mafic-floored basins, as it was not bending at the subduction zone, resulting in trench associated with the opening of the northern portion advance, whereas younger, weaker crust subducts of the Rocas Verdes Basin. Along-strike geological more easily resulting in trench retreat (Di Guiseppe complexity of the Rocas Verdes Basin confounds et al., 2009). Instead of trench rollback, a potential simplistic interpretations. In contrast with the first order explanation of the correlation could be more extensive Sarmiento ophiolite in the Ultima suction force, being derivative from the effect of Esperanza area (Paper III, Fig. 1), gabbro and basalt subducted lithosphere displacing mantle material in in the Seno Otway region lack crustal xenoliths turn causing the overriding continental plate to be (Paper III). This has been interpreted as indicating pulled toward the trench (Elsasser, 1971; Forsyth that Seno Otway records more advanced crustal & Uyeda, 1975). As older oceanic lithosphere extension related to the evolution of the Rocas tends to be thicker (Parsons & McKenzie, 1978), Verdes Basin, though likely less advanced than the the subduction of relatively old oceanic lithosphere Tortuga ophiolite near Cordillera Darwin. In the should displace proportionately more mantle Ultima Esperanza region the Zapata Formation is material, inducing a stronger suction force than interpreted to represent hemipelagic sedimentation, in the case of the otherwise similar subduction of which involved periodic fine-grained turbidite younger lithosphere. In situations where the velocity deposition on the northeastern flank of the Rocas of the overriding plate is negative, the tensile stress Verdes Basin (Fildani & Hessler, 2005). In contrast, in the back-arc region can be envisaged as sufficient the Yahgan and Beauvoir formations in Tierra to induce crustal splitting, provided a pre-existing del Fuego represent arc-related deposition on the weakness is present (Shemenda, 1993). southwestern flank of the back-arc basin (Olivero An alternative explanation for the development & Martinioni, 2001). The Seno Otway Zapata and of mafic floored back arc basins at only the northern Canal Bertrand formations contain more abundant and southernmost extents of the subduction margin sandy material than the Zapata Formation at involves a model whereby lateral slab width controls Ultima Esperanza, but otherwise are similar deformation in the overriding plate (Schellart et and are interpreted to have formed in a similar al., 2007; Schellart, 2008). In this model, mantle depositional environment. A higher proportion of flow around the edges of a subducting slab enables sandstone in the Seno Otway region could reflect rollback, facilitating lithospheric extension more the partitioning of the Rocas Verdes Basin into easily at the edges of long subduction zones. subsidiary horst and graben structures (Fildani Conversely, the lack of mantle escape flow in & Hessler, 2005), and consequent complexity central portions of a subduction zone retards trench in depositional patterns. Taken in the context of rollback. This effect may also explain why the distinctions between ophiolitic rocks in the two development of mafic floored back arc basins does regions, along strike distinctions between the not occur at some other locations along the margin sedimentary fill of the marginal basin are consistent where the conditions of a negative overriding plate with the Rocas Verdes Basin having been wider velocity and subducting slab age greater than 50 in Seno Otway than Ultima Espernza, but was Myr are met (e.g. Paper I, Figs 3 & 4, Point 49). likely significantly narrower than in the Cordillera However, location along the subduction zone Darwin region. These distinctions originally lead cannot control the timing of the development of to an interpretation that crustal separation initiated extension, and under this alternative model the in the south and propagated northward (de Wit direction of the velocity of the overriding plate still & Stern, 1981; Stern & de Wit, 2003). However, seems to be a primary control on the initiation of recent detrital zircon spectra from Rocas Verdes mafic floored back arc basin extension. sedimentary rocks are more consistent with Modelled trench normal convergence rates synchronous crustal separation along the length of University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 75 the basin (McAtamney et al., 2011) requiring an into the overriding plate (Dewey & Bird, 1970). alternative explanation for distinctions between The evolution in conditions along the subduction the southern and northern mafic rocks. Negative zone likely contributed to the fulfilment of those convergence rates in the south but not in the north conditions, and enabled the development of some of could have facilitated greater extension in the the unique contractional features in the overriding south, and could explain the differences in basin South American plate. width (Paper I, Fig. 5). Contributing effects from multiple parameters Extension first ceased in the mafic-floored were required to produce the deformation pattern Colombian Marginal Seaway and Rocas Verdes recorded during the contraction regime. The Basin at the northernmost and southernmost development of fold and thrust belts and plateau extremes of the Andean margin, respectively. The uplift along the South American margin, features end of active extension approximately coincided commonly associated with crustal compression, with the change in absolute velocity of the overriding was associated with trench normal convergence South American plate from being eastward directed velocities in excess of 4 cm/yr (Paper I, Fig. 5). to westward directed (Paper I, Fig. 4). Extension A high convergence rate could reasonably be in the marginal basins of continental affinity in the expected to facilitate a compressive strain regime central portions of the Andean margin continued in any continental plate overriding subduction, into the Late Cretaceous. In contrast with the mafic- assuming regular plate coupling. However, floored basins, no association was seen between convergence rates along the margin have been the cessation of extension in these regions with as high or higher during periods of extensional conditions along the subduction zone. deformation or no contractional deformation (Paper I, Fig. 5). Therefore, while high convergence rates Contraction Regime may facilitate the development of fold and thrust Contraction along the Andean margin is reflected belts and plateau uplift, in isolation from other in the inversion of Mesozoic marginal basins, the conditions they are insufficient to drive overriding development of fold and thrust belts, the uplift of plate contraction. It should also be noted that the Altiplano-Puna plateau, and the development although the trench normal absolute velocity of of high-grade amphibolite facies metamorphism. South America is low in magnitude during these Late Cretaceous inversion of the Rocas Verdes compression events, it is still positive, potentially Basin and the subsequent development of the contributing to the compressive regime. Magallanes fold and thrust belt and amphibolite Age of the subducting slab has been suggested facies metamorphism marked the first Andean to have influenced the rise of the Andes, as the age contractional episodes. Elsewhere along the of the Nazca plate, with oldest crust subducting in margin contraction only initiated after c.25 Ma the centre of the margin, correlates well with height with the uplift of the Altiplano-Puna plateau and and crustal thickening along the Andean margin the formation of various thin- and thick-skinned (Capitanio et al., 2011; Yañez & Cembrano, 2004). foreland fold and thrust belts. These contraction However, when the effect of slab age was examined features are commonly associated with convergent in this analysis it was found that correlation margins. Foreland fold and thrust belts tend to coefficients decreased slightly from 0.21 for form in any of ocean – continent, arc – continent, convergence rate alone to 0.20 for combined or continent – continent tectonic settings. In convergence rate and slab age. The effect was contrast, plateau uplift and Barrovian style slightly larger when the Magallanes fold and thrust metamorphism are more commonly associated belt was excluded from the analysis; the coefficients with continental collision. Orogenesis along ocean decreased from 0.32 to 0.30 for convergence rate – continent convergent margins requires that two alone and combined convergence rate and slab age, conditions be met: (i) the overriding continental respectively. The subduction of slabs older than 50 plate must be in compression; and (ii) coupling Myr is largely absent from the development of fold between the downgoing oceanic slab and the and thrust belts in the Peruvian and south-central overriding continental plate must be sufficient to Andes, especially during the early episodes of enable the transmission of compressive stresses thrusting in the Agrio and Aluminé fold and thrust University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 76 belts (Paper I, Fig. 3). Clearly if subducting slab have affected the Seno Otway region. Continuing age does play a role in contractional deformation in the trend of greater crustal shortening with distance the overriding plate it is not straightforward. to the south, the burial depth recorded in kyanite The Magallanes fold and thrust belt of the and staurolite bearing schists of Cordillera Darwin southern Andes presents an exception to the required much greater shortening than occurred correlations established from other fold and at either Ultima Esperanza or Seno Otway. In the thrust belts along the South American margin. Its Cordillera Darwin region a later phase of thrusting development was accompanied by low trench normal resulted in a series of out-of-sequence thrusts and convergence velocity: always less than 4 cm/yr and back thrusts that exhumed the high-grade basement occasionally negative (i.e. tensional; Paper I, Fig. schists of the Cordillera Darwin (Klepeis et al., 5). The most probable causes of this inconsistency 2010; Paper II). Evidence of out-of-sequence thrusts involve the simplifications made in investigating is recorded at Seno Otway, although thrusting the subduction parameters, namely that both potentially accommodated less shortening than in internal deformation of South America and any Cordillera Darwin (P. Betka, unpublished data). change in trench geometry were disregarded. The This phase of thrusting is absent from the Ultima portion of the South American plate now in southern Esperanza region. Patagonia experienced shortening sufficient to bury Metamorphism associated with thrust-driven components to c. 10-12 kbar (Klepeis et al., 2010; exhumation in Cordillera Darwin was dated using Paper II), but the global plate model used currently U-Th-Pb monazite geochronology at 72.61 ± 1.13 has no mechanism to incorporate the effects of such Ma (Paper II). From the location of monazite shortening on the geometry of a plate, as plates in exclusively within S2 assemblages, it can be the model are treated as rigid, undeforming blocks. inferred that monazite growth occurred during the The Patagonian Orocline initiated during or has prograde exhumation, most probably at P≈9 kbar become more pronounced since the mid Cretaceous and T≈625°C (Paper II, Fig.10). Monazite growth, (e.g. Cunningham et al., 1991; Kraemer, 2003) probably from partial breakdown of xenotime, is resulting in changes in the shape of the margin that interpreted as having been contemporary with would likely influence plate velocities at relevant staurolite and kyanite growth. The 72.61 ± 1.13 parts of the trench. Ma monazite age thus post-dates the initiation of Differential shortening that developed the exhumation; maximum pressure conditions of c.12 Patagonian Orocline is clearly recorded within the kbar in Cordillera Darwin were attained prior to c.73 Rocas Verdes Basin. Along the basin length thrust Ma. Therefore the change from burial to exhumation sheets responsible for obducting the floor of the is constrained to having occurred between c.86 Ma Rocas Verdes Basin onto the continent had formed and c.73 Ma. It is interesting to note that an abrupt prior to c.86 Ma (Klepeis et al., 2010; Fosdick et increase of c.2 cm/yr in trench normal convergence al., 2011). The early foliation preserved in garnet rate falls within this age bracket, occurring at c.75 grains from the metapelitic schists of Bahía Pia Ma (Paper I, Fig. 5). However, as no such increase in Cordillera Darwin possibly formed during in convergence rate is observed during the earlier this event, as continental underthrusting buried burial event any relationship that may exist between Paleozoic basement rocks to depths of c.35 km convergence rate and metamorphic events is not and resulted in amphibolite facies metamorphism straightforward, and likely involves the interaction (Klepeis et al., 2010; Paper II). Thrusting in the of various specific conditions. Seno Otway region also exhumed greenschist The decrease in pressure resulting from facies metamorphic Paleozoic basement (Paper III). exhumation coupled with fluid ingress and the A combination of thick and thin-skinned thrusting partial fracturing of garnet grains likely drove the styles has been inferred for the Ultima Esperanza development of the patchy garnet recrystallization region, as basement involvement is inferred from textures associated with matrix staurolite and proprietary seismic data (Harambour, 2002; Fildani kyanite observed at Bahía Pia. Evidence for the & Hessler, 2005). However, at Ultima Esperanza presence of fluid within turbid garnet was found such thrusts are not emergent, and significantly more during Raman spectroscopy, reflecting the presence basement involved shortening could be expected to of an aqueous saline solution (Paper II, Fig. 7). University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 77

Fluid could have been sourced from Rocas Verdes Deformation structures in the Seno Otway metasediments, under-thrust and undergoing region are most intense near obduction structures, contemporary prograde metamorphism. Hydro- where tight folds and pervasive fine scale cleavage fracturing could have then exposed parts of earlier- developed throughout the Zapata/Canal Bertrand formed grain cores of compositions formed at formations. A more open folding style developed higher pressure and lower temperature (e.g. Carlson in structurally-overlying foreland basin units away & Gordon, 2004) and enhanced reaction coupled from the obduction structures (Paper III, Fig. 7). with staurolite and kyanite growth. Reduced garnet Cleavage is less developed in these units, enabling mode, interpreted as being related to terrane-wide the preservation of delicate sedimentary features uplift, can explain the higher spessartine content such as ripples and flute casts. The decreasing in the turbid garnet whereby reduced modes of deformation gradient toward the foreland (Paper secondary garnet resorbed all manganese from III, Fig.6) indicates that as deformation propagated earlier garnet (Paper II, Figs 6 & 10). Large mica into the foreland the intensity of deformation and plagioclase grains intergrown with the turbid decreased, and motion along later thrust faults was garnet, and large staurolite and kyanite adjacent to likely of lower magnitude than that along the early garnet, would probably require that intergranular obduction thrusts. diffusion occurred on a scale at least as large as Crustal loading from thickening and uplift the garnet porphyroblasts (e.g. Carmichael, 1969). along convergent margins commonly results in The lack of the development of turbid garnet in continental lithospheric flexure, leading to the the Ventisquero Roncagli examples most probably development of foreland basins. These basins reflects lower fluid-rock ratios in those rocks, are bounded by a foreland propagating thrust possibly related to them being more distant to front on the side closest to the crustal loading, areas of Rocas Verdes rocks undergoing prograde and a flexural uplift called a foreland bulge on metamorphism. the other margin. Lithospheric flexure results in The age bracket for the start of exhumation asymmetric accommodation space that is greatest is consistent with previous geochronology work. adjacent to the mountains and decreases toward the Grunow et al. (1992) and Kohn et al. (1995) foreland bulge; sediment thickness varies likewise performed 40Ar/39Ar dating on the amphibolite (Flemings & Jordan, 1990; Gómez et al., 2005). schists in Bahía Pia and found consistent results A major sediment source to these basins is the of c.70 – 69 Ma from muscovite and biotite and uplifting mountains or arc along the margin. Basin c.76 – 73 Ma from hornblende. These results paired stratigraphy provides information on the evolution with the results of this study indicate that uplift of orogenic events. and cooling in Cordillera Darwin was very rapid, Contraction, initial Andean uplift, and as they trace a cooling history from about 620°C thrusting in the region of the inverted Rocas Verdes for the formation of monazite, through (inferred) Basin resulted in the formation of the Magallanes closure temperatures of c.505°C, c.370°C, and foreland basin. The composition of the lowermost c.320°C for hornblende, muscovite, and biotite, sedimentary units in the basin vary along its respectively (Kohn et al., 1995). Fission track length, and include the Punta Barrosa Formation ages (Nelson, 1982; Gombosi et al., 2009) record at Ultima Esperanza, and its lateral equivalent the the continued uplift of Cordillera Darwin into the Latorre Formation at Seno Otway. Sedimentary Neogene. Localized sillimanite growth reflects sequences of the Latorre Formation are similar to spatially restricted at P≈6 kbar heating due to the those described for the Punta Barrosa Formation at intrusion of the c. 86 Ma to 68 Ma tonalitic Beagle the Ultima Esperanza region, and are interpreted Suite (Klepeis et al., 2010; Halpern & Rex, 1972; similarly: being the first deposits related to the Mukasa & Dalziel, 1996). No evidence is recorded developing Magallanes foreland basin. Paleocurrent of heating due to magmatism prior to the intrusion indicators in the Punta Barrosa Formation are of the Beagle Suite, supporting the idea that no directed parallel to the orogen (Paper III, Fig. 4), active magmatic arc was present south of Tierra del indicating the Punta Barrosa was deposited as part Fuego prior to the Late Cretaceous (Klepeis et al., of a submarine fan that ran along the length of the 2010; McAtamney et al., 2011). northern part of the orogen (Fildani & Hessler, University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 78

2005). In contrast, the Latorre Formation displays the period examined in this thesis. Continental much greater variation in paleocurrent directions, transforms exhibit complex structures, and with a predominant flow direction perpendicular deformation can be distributed across zones to the trend of the orogen. The nature of the that vary from tens of kilometres to a thousand submarine fans must have either been distinct in kilometres depending on lithospheric strength (e.g. the northern and central parts of the Magallanes Eckstein & Simmonsi, 1978; Atwater, 1970). The Basin, or the Punta Barrosa and Latorre formations transform boundary in Patagonia is expressed as were both deposited in a single large submarine fan the Magallanes-Fagnano fault system and imposed with a diversity of flow directions. Distinctions in a sinistral transtensional regime on the region sediment provenance and composition between the (Lodolo et al., 2003; Menichetti et al., 2008). Left- two formations (McAtamney et al., 2011), are most lateral convergence rates are modelled along the consistent with the presence of multiple submarine southernmost portion of the margin during this fans in the Late Cretaceous. period (Paper I, Fig. 5). Initial conglomerate deposition in the c.81 – 80 In Seno Otway strike-slip and normal-slip Ma Escarpada Formation (McAtamney et al., 2011) populations of faults generally cut folds, as well as marks further complexity in sediment provenance. dykes younger than the folding and thrusting. These Andesitic clasts reflect the sedimentation of features indicate that they are a later stage of faulting volcanic arc detritus. In addition, basaltic and that the initial thrust faults, and are interpreted as rhyolitic clasts indicate that the Rocas Verdes mafic being associated with the transtensional regime. In rocks and the felsic Tobifera Formation were both Cordillera Darwin strike-slip and oblique slip thrust emergent at this time, as a consequence of thrusting faults cut fabrics associated with the exhumation in the Magallanes fold and thrust belt. Paleocurrent event, indicating that they post-dated the main phase directions vary by approximately 90°, from being of exhumation (Klepeis et al., 2010). U-Pb zircon orogen parallel to being perpendicular to the orogen. dates indicate that most strike-slip and oblique slip The orogen parallel component of paleocurrent features occurred after the Eocene (Barbeau et al., flow is stronger in the Escarpada Formation than 2009). These faults, similar to those recorded in in the Latorre Formation, and could indicate that Seno Otway, were interpreted using a Riedel shear the orogen parallel deposition controlling the model (Riedel, 1929; Tchalenko, 1970; Naylor et distribution of the Punta Barrosa Formation in the al., 1986) with the sinistral faults synthetic the Ultima Esperanza region extended as far south as dextral faults antithetic to the orientation of the the Seno Otway region during this time. plate boundary in the region (Klepeis & Austin, Contraction in Patagonia declined as a sinistral 1997). A similar situation likely occurred in Seno strike-slip regime took over during the Oligocene. Otway, with both sets of faults forming during the However, along the remainder of the Andean margin same, most recent, tectonic regime. contraction only initiated at c.25 Ma with the uplift of the Altiplano-Puna plateau. The majority of the Subduction Correlations – Null Results margin has remained in contraction to the present In the analysis of associations between overriding (e.g. Reynolds et al., 2000; Ramos et al., 2002). continental plate deformation and conditions The inversion of Mesozoic extensional basins and imposed by the subduction zone, several non- development of fold and thrust belts propagating correlations are notable as they allow parameters into foreland basins along the eastern flank of modelled in this study to be discounted as the the uplifted Andes Mountains has characterized primary drivers of a given geological feature. this regime. Several of these fold and thrust belts For instance, periods inferred to have been remain active today. accompanied by a high magmatic flux do not seem to correlate with any of the investigated parameters, Strike-Slip Regime – Patagonia either singly or in combination. Thus it is inferred The Oligocene development of the South America that high recorded rates of magmatic flux were – Scotia transform plate boundary marks the first not controlled by any of subducting slab age, shift in tectonic setting from ocean – continent absolute overriding plate velocity, or convergence convergence along the Andean margin over of velocity. In addition, the subduction of spreading University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 79 ridges, commonly invoked as a cause of orogenesis older than c.50 Myr; and development of fold and initiation (Palmer, 1968; Ramos, 2005), has no thrust belts and major plateau uplift associated with correlation with any type of deformation and thus trench normal convergence rates greater than c.4 appears to have had a negligible effect on foreland cm/yr. Overriding plate velocity, though commonly deformation along the Andean margin (Paper I, small in magnitude, was generally directed toward Figs 3-5). the trench during contraction events. No correlations This study also separated structural styles of were observed with any of the plate parameters fold and thrust belts but failed to determine any and either the style (i.e. thick- or thin-skinned) of correlation between any of the modelled subduction thrusting developed, or with high magmatic fluxes parameters and the development of thin- or thick- resulting in batholith emplacement. skinned thrusting. The reactivation of Mesozoic The Rocas Verdes Basin is one of two mafic- extensional structures to form thick-skinned fold floored back-arc basins that opened along the and thrust belts is common along the margin, margin during the extensional tectonic regime. recorded in the Eastern Cordillera of Colombia, the Distinctions in geochemistry and sedimentary Santa Bárbara System and various thrust belts of deposition between northern and southern portions the Neuquén Basin (Parra et al., 2009; Reynolds of the basin indicate the basin was likely narrower et al., 2000; Grier et al., 1991; Giambiagi et al., in the north and wider in the south. Positive 2008; Ramos et al., 1996; Zapata & Folguera, (convergent) convergence rates in the north and 2005; Folguera et al., 2007; García Morabito et negative (tensional) convergence rates in the south al., 2011). However, earlier Mesozoic deformation could be a contributing factor in the different does not appear to be uniquely deterministic of growth histories. contractional deformation style. For instance, the In the Rocas Verdes Basin the initiation of mid – late Cretaceous episode of thrusting the Chos the contraction regime resulted in the inversion Malal fold and thrust belt developed a thin-skinned of the marginal basin and the development of the style due to the availability of detachment surfaces foreland Magallanes Basin. Distinctions between despite the presence of earlier extensional features the evolution of the northern and southern portions which were reactivated in the second deformation continued under this regime. Thrusting obducted episode of the belt (Folguera et al., 2007). In ophiolitic relics of the Rocas Verdes Basin floor contrast with the suggestions of Kley et al. (1999), onto the continent by c.86 Ma. In Cordillera rather than being dominantly controlled by inherited Darwin, Paleozoic continental crust was buried Mesozoic extensional structures, thrusting style to c.10-12 kbar under the magmatic arc during appears to be the result of the interplay of various crustal shortening, resulting in amphibolite facies factors including Mesozoic deformation, contrasts metamorphism. Thrust related exhumation was between differing lithologies or sediment thickness, underway by 72.61 ± 1.13 Ma. A 2 cm/yr increase and the orientation of pre-existing structures. in trench normal convergence rates at c.75 Ma occurred around the time of the change from terrane burial to exhumation. Exhumation coupled with CONCLUSIONS fluid ingress drove garnet recrystallization observed in Bahía Pia. Sillimanite formed in contact aureoles The Andean margin is the site of long-lived adjacent to Beagle Suite intrusions and is causally subduction. Deformation along the margin is unrelated with the regional metamorphic gradient. divided into two dominant regimes: Late Jurassic Metamorphism in Seno Otway does not exceed and Cretaceous extension, and Late Cretaceous greenschist facies as significantly less shortening to present contraction. Margin-wide correlations occurred there than in Cordillera Darwin. between conditions at the subduction zone and Sediment deposition in the region records deformation on the overriding plate reflecting these hemipelagic sedimentation in the northern portion regimes include: opening of mafic-floored back-arc of the Rocas Verdes Basin, and arc detritus basins associated with an overriding plate velocity dominated sedimentation in the southern portion. directed away from the trench in conjunction with The inversion of the marginal basin is recorded in the subduction of oceanic slabs with modelled ages coarsening of the upper portions of the Zapata and University of Sydney, PhD Thesis, Kayla T. Maloney, 2012. 80

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