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Tectonic Evolution of the Qiangtang Terrane, Central Tibetan Plateau

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Tectonic Evolution of the Qiangtang Terrane, Central Tibetan Plateau Tectonic evolution of the Qiangtang terrane, Central Tibetan Plateau Zhao Zhongbao Eberhard Karls Universität Tübingen Structural Geology Group 2015 Tectonic evolution of the Qiangtang terrane, central Tibetan Plateau Dissertation der Mathematisch-Naturwissenschaftlichen Fakultät der Eberhard Karls Universität Tübingen zur Erlangung des Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) vorgelegt von Zhongbao Zhao aus Shanxi, China Tübingen, 2015 Erklärung Ich erkläre hiermit, dass ich die zur Promotion eingereichte Arbeit selbständig verfasst, nur die angegebenen Quellen und Hilfsmittel benutzt und wörtlich oder inhaltlich übernommene Stellen als solche gekennzeichnet habe. Ich erkläre, dass die Richtlinien zur Sicherung guter wissenschaftlicher Praxis der Universität Tübingen (Beschluss des Senats vom 25.5.2000) beachtet wurden. Ich versichere an Eides statt, dass diese Angaben wahr sind und dass ich nichts verschwiegen habe. Mir ist bekannt, dass die falsche Abgabe einer Versicherung an Eides statt mit Freiheitsstrafe bis zu drei Jahren oder mit Geldstrafe bestraft wird. Tübingen, June 2015 Datum der mündlichen Prüfung: 28th. August 2015 Dekan: Prof. Dr. Wolfgang Rosenstiel 1. Berichterstatter: Prof. Dr. Paul D Bons 2. Berichterstatter: Prof. Dr. Genhou Wang II Thesis organization and list of publications This PhD-thesis is organized in a cumulative manner. It is separated into 6 chapters. Here, the general organization of the thesis and a list of included publications are given. Chapter 1 presents an introduction covering the context and the main ideas of the thesis. In Chapter 2-4 three different research papers which were published in the framework of the present study are represented. Chapter 5 represents results from (U-Th)/He thermochronology that are going to be a paper written for concerning on Cretaceous uplifting and shortening of Qiangtang terrane. Chapter 6 use numerical modeling methods prove our new exhumation mechanism which is also going to publish soon. Chapter 7 recapitulates the conclusions and open research questions are given. Publications Zhao, Z., Bons, P., Wang, G., Soesoo, A., and Liu, Y.: Tectonic evolution and high-pressure rock exhumation in the Qiangtang Terrane, Central Tibet, Solid Earth, 6, 457-473, 2015. Zhao, Z., Bons, P. D., Wang, G., Liu, Y., and Zheng, Y.: Origin and pre-Cenozoic evolution of the south Qiangtang basement, Central Tibet, Tectonophysics, 623, 52-66, http://dx.doi.org/10.1016/j. tecto.2014.03. 016, 2014. Yang. Y., Zhao, Z. B., Yuan, T. Y., Liu, Y., and Li, C. Y.: Ordovician parallel unconformity in Qiangtang terrane, northern Tibet: Implications to Early Paleozoic evolution of northern Tibetan region. Acta Petrologica Sinica, 30, 2381-2392, 2014. Z. Zhao., P. D. Bons., K. Stübner., T. A. Ehlers and G. Wang.: Early Cretaceous exhumation of the Qiangtang Culmination and collision of the Qiangtang and Lhasa terranes, central Tibet. (In preparation) Paul D. Bons., Evgene Burov., Enrique Gomez-Rivas., Alavar Soesoo., Douwe D.J. van Hinsbergen., Kun Wang., Zhongbao Zhao (alphabetical order).: Subduction reversal: an efficient mechanism for the exhumation of Ultra-high-pressure rocks. (In preparation) III Abstract The highest plateau in the world has attracted geologist’s attention for centuries. Significant crustal shortening, which led to the eventual construction of the Cenozoic Tibetan Plateau, began in the Himalaya in the south and the Qilian Shan in the north (over than 1000km) at approximately the Middle Cenozoic. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogeny exerted a strong control on the Cenozoic strain history and strain distribution. Moreover, the Tibetan plateau formed by several terrane slices each of which is about several hundred kilometer wide. Assuming that the lithosphere thickness is also on the 100 km scale, the small width-thickness ratio must have induced a strong influence of mantle dynamics on crustal deformation because of the strong crustal deformation that is located mainly at the continent edges. The Qiangtang terrane in Central Tibet, with its high-pressure rocks, is a key area to unravel the evolution of the Paleo-Tethys. The presence of widespread Mesozoic subduction mélange and high-pressure rocks in the Qiangtang terranes can have importance consequences for the formation of the Cenozoic Tibetan plateau. Long-lasting and detailed mapping work offer us the possibly to unravel the geological evolution of the Qiangtang terrane and further test existing geodynamic models in order to better understand the evolution of small terranes. 1:50,000 mapping results from the Rongma area in the central Qiangtang terrane show that the Qiangtang metamorphic belt can be separated into a Paleozoic autochthonous basement and an overlying allochthonous thrust stack of subduction mélange that contains high-pressure rocks and Permian sediments. Detrital zircon ages (youngest peak at 591 Ma) and an ~ 470 Ma granite intrusion age constrain the age of the Qiangtang Basement to be between the Late Precambrian to Middle Ordovician. This is in agreement with the observed unconformity between basement and overlying Mid–Late Ordovician strata. Based on detrital age spectra and an Early Ordovician unconformity we prefer that the Qiangtang terrane has Gondwana affinity during the Early Paleozoic. The occurrence of Late Triassic eclogite and glaucophane-bearing schists in the Central Qiangtang terrane indicates the existence of a suture zone between the North and South Qiangtang terranes before the Late Triassic. Detailed 3D mapping analyses indicate that the high-pressure rocks were exhumed from underneath the south Qiangtang terrane in an extensional setting caused by the pull of the northward subducting slab of the Shuanghu–Tethys. High-pressure rocks, sedimentary mélange and margin sediments were thrusted on top of the ophiolitic mélange that was scraped off the subducting plate. Both IV units were subsequently thrusted on top of the south Qiantang terrane continental basement. These conclusions are also supported by sediments and magma ages. Onset of Late Triassic sedimentation marked the end of the amalgamation of both Qiangtang terranes and the beginning of spreading between Qiantang and north Lhasa to the south, leading to the deposition of thick flysch deposits in the Jurassic. Strongly folded Jurassic flysch is unconformably overlain by weakly deformed mid-Cretaceous conglomerate and volcanics. This relationship demonstrates closure of Banggong Hu-Nujiang Ocean during Early Cretaceous. Collision between the Lhasa and Qiangtang terranes led to fast exhumation in the Qiangtang terrane during ~140-90 Ma, with exhumation rates about 0.15-0.3 mm/y. The Qiangtang terrane was above sea level up to the mid-Cretaceous and experienced significant denudation prior to mid-Cretaceous times. This model implies substantial crustal thickening and perhaps plateau formation in central Tibet prior to the Indo-Asian collision. A new model for the exhumation of (ultra-) high-pressure ((U)HP) rocks was developed, based on the observation of exhumed and obducted (U)HP rocks in the study area. It is suggested that exhumation was caused here by pulling up of the subducted slab in a double divergent subduction setting. The long northward subducting slab is envisaged to have extracted the short southward subducting slab. This model was further investigated with numerical modelling. It is sugested that the proposed mechanism may explain the very fast and recent exhumation of eclogites of the d'Entrecasteaux Island off the coast of Papua New Guinea, as well as the fast opening of the Woodlark Basin. V Zusammenfassung Das höchste Plateau der Welt hat seit Jahrhunderten die Aufmerksamkeit von Geologen auf sich gezogen. Eine wesentliche Verkürzung der Erdkruste von mehr als 1000 km begann im Süden des Himalaya und dem Norden des Qilian Shan etwa im mittleren Känozoikum. Die tektonische Geschichte des Himalya- Tibet-Orogens im Paläozoikum und Mesozoikum nahm einen starken Einfluss auf Geschichte und Verteilung der Deformation zu dieser Zeit. Außerdem entstand das Tibetplateau aus mehreren Terranstücken, welche jeweils über eine Breite von mehreren hundert Kilometer verfügen. Unter der Annahme, dass die Stärke des lithosphärischen Mantels ebenfalls etwa bei 100 km liegt, müssen kleinere Dicke-Breite Verhältnisse auf einen starken Einfluss der Manteldynamik auf die Krustendeformation hinweisen, da die starke Krustendeformation hauptsächlich auf den Kontinentalrand bezogen ist. Mit seinen Hochdruckgesteinen ist das Qiangtang Terran in Zentraltibet eine Schlüsselregion, um die Evolution der Paleo-Thetys zu enträtseln. Das Vorliegen einer ausgedehnten Subduktionsmélange zusammen mit Hochdruckgesteinen im Qiangtang Gebiet kann wichtige Konsequenzen für die Entstehung des känozoischen Tibetplateaus haben. Lange und detaillierte Kartierungen bieten die Möglichkeit, die geologische Geschichte des Qiangtang Gebietes zu erklären und ferner existierende geodynamische Modelle zu testen, um die Entstehung kleiner Terrane zu verstehen. Die Ergebnisse der Kartierungen im Maßstab 1:50,000 aus der Rongma Region im zentralen Qiangtang Terran zeigen die mögliche Einteilung des metamorphen Qiangtanggürtels in ein paläozoisches autochtones Grundgebirge und einen darüberliegenden allochtonen Stapel von Überschiebungen der Subduktionsmélange, die Hochdruckgesteine und permische Sedimente beinhalten. Detritische Zirkonalter (jüngstes Maximum bei etwa 591 Ma) und eine ca. 470
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