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Low-Temperature Thermochronology: Methodological Studies and Application in Collisional Orogens

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Low-Temperature Thermochronology: Methodological Studies and Application in Collisional Orogens Low-temperature thermochronology: methodological studies and application in collisional orogens Der Fakultät für Geowissenschaften, Geotechnik und Bergbau der Technischen Universität Bergakademie Freiberg eingereichte DISSERTATION zur Erlangung des akademischen Grades doctor rerum naturalium (Dr.rer.nat.), vorgelegt von Dipl.-Geol. Konstanze Stübner geboren am 03.03.1979 in Leipzig Freiberg, den Acknowledgements I am grateful for the help of many people who made this dissertation possible. Most of all I thank Lothar Ratschbacher for the splendid opportunity to work in the Pamirs, and for patience and guidance in the field and during data analysis and interpretation. Raymond Jonckheere is acknowledged for his advice on the methodological part of my thesis and on fission-track analy- sis for the applied projects. The most interesting ideas were born in vigorous discussions of the fission-track group at Freiberg. Thank you to Uwe Martens who involved me into the Guatemala project by inviting me to participate in his field work. I would also like to thank Mike McWilliams and Jim Metcalf who supported me during my visit at Stanford University; together with many more colleagues there they made it a very memorable and successful stay for me. Pieter Vermeesch facilitated apatite (U-Th)/He measurements at ETH Zürich. Field work in Tajikistan would not have been the same without Klaus Stanek, Richard Gloaguen and our Tajik collaborators Vladisav Minaev, Negmat Rajabov, Mustafo and Ilhom- jon, and the most outstanding driver, Ibrahim. István Dunkl conducted the majority of (U- Th)/He analyses at Univ. Göttingen and provided a large batch of last-minute results; the study of Cenozoic exhumation in the Southwest Pamirs greatly benefited from his commitment. Thanks also to all the colleagues and friends who supported me throughout my PhD, and my family who encouraged me in all my endeavours. This thesis was funded by a fellowship of the Studienstiftung des Deutschen Volkes and a one-year travel grant by the German Academic Exchange Council. Abstract Apatite (U-Th)/He and fission-track dating are low-temperature geothermochronometers of increasing importance in geological research, in particular for resolving the timing, rate and magnitude of vertical displacements of the upper crust resulting from climatic or tectonic fac- tors. The calculation of radiometric ages and thermal histories from the fission-track record in a mineral relies on counts and measurements of etched fission tracks under an optical microscope. In recent work, it has been recognized that the sources of error are due to observational factors related to the manner in which the tracks are etched. The first part of this thesis therefore aims at a better understanding of the etching of nuclear tracks in minerals. A model is proposed based on detailed observations of track etch pits in minerals and numerical simulations that explains the characteristic features of track etch pits and their evolution with etching time. In Chapter 2 the etch-pit geometries are interpreted in terms of periodic bond chains follow- ing the atomistic theory of crystal growth and dissolution. Periodic bond chains are identified, which relate triangular etch pits to tri-octahedral micas and diamond-shaped etch pits to di- octahedral lattices. A first-order estimate of the bond strengths between neighboring atoms along the periodic bond chains is in agreement with the relative track etch rates in the different investigated minerals. Hexagonal and star-shaped etch pits in tri-octahedral mica violate the lat- tice symmetries; they are explained by simultaneous etching of the front and back of mica sub- units as a result of the fact that these have become detached during track formation by high- energy particles (swift-ion tracks, fission tracks). This work emphasized the influence of the lat- tice structure on the track formation process that is at odds with existing theories. Periodic bond chains account for the outlines of etched nuclear tracks in different minerals but fall short of explaining a number of other observations, most prominently the curvature of etch pits, their complex diameter distribution, and variable growth rates according to the defect (alpha-recoil track, fission track, dislocation, etc.) and as a function of etching time. In Chapter 3, an atomistic etching model is proposed and implemented in a Monte Carlo computer simula- tion. Apparent horizontal and vertical growth rates during four stages of etch pit evolution are related to bond strength between neighbouring lattice units; the onset and duration of these stag- es are determined by the bond strength and the extent of the defect. Results show that high bond-strength minerals are better able to sustain angular features than low bond-strength miner- als. Implications for particle track dating include: (1) the increase of the number of etched alpa- recoil tracks with etching time due to bulk etching is non-linear because the bulk etch rate is not constant; (2) the evolution of the etch-pit shape with continued etching can cause loss of tracks due to observation effects related to loss of contrast. The second part focuses on two case studies integrating thermochronologic, structural, pe- trologic, and geochronologic data. Chapter 4 is a comprehensive study of the tectonic evolution of Central America. Previously unpublished petrologic, structural, and geochronologic data con- strain Paleozoic amalgamation of Pangea, development of Mesozoic proto-Pacific arcs, and Ju- rassic intra-arc rifting. In the Early Cretaceous intra-arc rifting reverted to subduction and colli- sion. A subduction zone developed along the south margin of southern Mexico; oblique sub- duction led to ophiolite emplacement and sinistral collision of the Caribbean arc (future Cuba, Hispaniola, Jamaica) along the Motagua suture zone in the Late Cretaceous. At ~40 Ma the Chortís block separated from southern Mexico under sinistral transtension; subsequently it was displaced ≤1100 km eastward to its present location in central Guatemala, south of the Motagua suture zone. Apatite fission track ages and structural data along the fault zones constrain indi- vidual faults strands through the Neogene and indicate that the Polochic fault zone, north of the Motagua suture zone, constitutes the active plate boundary. Chapter 4 demonstrates how geoch- ronology, thermochronology, structural geology and petrology work together to unravel >400 Ma of a complex interplay of arc formation, rifting, and collision. The second case study focuses on the Pamir Mountains in Tajikistan, which have received little attention to date due to their remote location in a politically unstable area. The Shaxdara gneiss dome of southwestern Pamirs has previously been interpreted as Archean−Proterozoic basement accreted to the Asia continental margin during amalgamation of Eurasia. It is re- interpreted in Chapter 5 as a Paleozoic−Mesozoic marine sediment sequence, metamorphosed in a Cretaceous Andean-type magmatic arc and overprinted subsequently by the India-Asia colli- sion. The major achievement of this study is the recognition of large-scale extension in the Pa- mirs. Continued convergence between India and Asia in the Tertiary caused sinistral transpres- sion along the western Pamirs and the formation of en echelon antiforms. Thickening is com- pensated by upper crustal extension confined to the thermally weakened gneiss domes. Apatite fission-track ages reveal late Miocene exhumation of the Shaxdara dome by tectonic denudation along the South Pamir normal fault. Apatite (U-Th)/He ages from high elevation samples reflect this cooling event; low-elevation samples indicate later exhumation by rapid incision of the Pjansch River, likely commencing in the Pleistocene. The Shaxdara dome study highlights the possibilities that low-temperature thermochronology opens up in the fields of geomorphology and neotectonics. Zusammenfassung Apatit (U-Th)/He- und Spaltspurendatierung sind Niedrigtemperatur-Geothermochrono- meter, die in den Geowissenschaften zunehmend an Bedeutung gewinnen, insbesondere bei der zeitlichen Einordnung von vertikalen Bewegungen in der oberen Kruste, von Hebungsraten und Hebungsbeträgen. Solche Hebungen resultieren aus tektonischen oder klimatischen Verände- rungen. Die Berechnung von radiometrischen Altern und Temperaturgeschichten aus Spaltspu- ren im Mineral beruht auf Zählungen und Messungen geätzter Spuren unter dem Mikroskop. Neuere Studien zeigen, daß die Hauptfehlerquelle dabei Beobachtungsfaktoren im Zusammen- hang mit der Ätzung der Spuren sind. Darum befaßt sich der erste Teil der Arbeit mit der Ät- zung von Partikelspuren in Mineralien. Basierend auf Beobachtungen von Ätzgruben und einer numerischen Simulation wird ein Modell vorgeschlagen, das charakteristische Eigenschaften von Ätzgruben und deren Entwicklung mit zunehmender Ätzzeit erklärt. In Kapitel 2 werden geometrische Eigenschaften von Ätzgruben mit Hilfe der atomistischen Theorie von Kristallwachstum und –lösung interpretiert. „Periodic bond chains“ (Bindungs- ketten) werden identifiziert, welche die dreieckigen Ätzgruben in tri-oktaedrischen und rauten- förmigen Ätzgruben in di-oktaedrischen Glimmern erklären. Eine grobe Abschätzung der Bin- dungsstärken zwischen benachbarten Atomen entlang der Bindungsketten stimmt mit den relati- ven Ätzgeschwindigkeiten in verschiedenen Mineralien überein. Hexagonale und sternförmige Ätzgruben in tri-oktaedrishen Glimmern stehen im Widerspruch zur Kristallsymmetrie;
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