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4 Chapter 3 Eurasian Margin The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 – Eurasian margin Chapter 3 - A revised history of terrane, arc and continental collisions between India and Eurasia The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin ABSTRACT Models for the Mesozoic and Cenozoic accretions along the southern margin of Eurasia suffer from disparity arising from the variety and differing interpretations of geological observations. We revisit the dispersal of East Gondwana by simultaneously considering geophysical data from all the abyssal plains offshore West Australia and East Antarctica, as well as new Jurassic age date from the Wharton Basin, which limits the original size of northern Greater India to a narrow indenter, delaying its collision with Eurasia until ~35 Ma. This coincides with the youngest marine deposits located between India and Eurasia, and drastic changes in global climate conditions. Our model implies that Argoland, an extended ribbon terrane reaching over 6000 km from East Africa to Papua New Guinea, rifted from Gondwana in the Late Jurassic, forming the NeoTethys Ocean and the northern margins of Australia and Greater India. Greater India began migrating from West Australia and East Antarctica ~136 Ma, unzipping from NW of Australia, to part Southern India and Sri Lanka from Antarctica ~126 Ma. Several micro-continental fragments were transferred from India to Australia during this process. While there is no direct evidence remaining for the portion of Argoland that existed west of the Argo abyssal plain, the fragment was likely a narrow, thinned continental sliver, which may have been underplated in the India-Eurasia collision zone. Several ophiolites along the Yarlung-Tsangpo suture zone, between India and Eurasia, are attributed to an equatorial intra-oceanic arc that records a widespread obduction event ~126 Ma – yet Greater India was ~3000 km south of the equator at this time. We propose that West Argoland accreted to the equatorial island arc ~126 Ma while Central Argoland reached the eastern portion of the arc ~80 Ma, around the same time that East 104 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin Argoland reached Sumatra. Greater India and its northeastern indenter, the Gascoyne block, then collided with the arc ~55 Ma, likely destroying any remnants of Argoland along the subduction zone. The Gascoyne block then collided with Myanmar ~50 Ma while Greater India collided with Eurasia from ~35 Ma, suturing from west to east. These accretions and their provenances are supported by geological evidence, including the discovery of Upper Eocene shallow marine strata in southern Tibet, a ~52 Ma global tectonic event, Late Jurassic uplift and an erosional unconformity shared between the Indo-Burmese Ranges and the NW Australian shelf, Halobia bivalve affinities between Timor and Myanmar, Sumatra’s ~87 Ma Woyla Group Manunggal Batholith, and a thermo-tectonic uplift event affecting the Malay Peninsula, Thailand and Indochina from ~90 Ma. 105 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin INTRODUCTION The timing of continental accretions along the Eurasian margin (Fig. 3.1) has been debated for decades mainly due to conflicting geological evidence from a long-lived, highly complex and actively deforming subduction/collision zone, stretching ~8000 km from the Mediterranean to SE Asia. The continental blocks that accreted to Eurasia rifted from the northern margin of East Gondwana, creating and closing a succession of Tethyan Oceans (e.g. Metcalfe, 2006). Conflicts between competing tectonic models that describe the breakup of East Gondwana reflect the uncertainty in deciphering Eurasian margin accretions in the absence of a regional plate tectonic model that is built upon a synthesis of both offshore and onshore key geophysical and geological data. For instance, there are variations of up to several thousand kilometers proposed for the extents of Greater India (e.g. Ali and Aitchison, 2005; van Hinsbergen et al., 2011a), and Argoland (e.g. Fullerton et al., 1989; Heine and Müller, 2005). We develop a revised, regional plate-tectonic model, built on a re-examination of marine potential field data off West Australia and East Antarctica, as well as on recently collected data in the Wharton Basin (Gibbons et al., submitted; Gibbons et al., in prep). These margins fringe relatively undeformed oceanic crust, punctuated by fracture zones, submerged plateaus and volcanic edifices (Fig. 3.1). This model reveals that several continental fragments apart from Greater India rifted away from Gondwana in the Late Jurassic and Cretaceous. The main fragments include Argoland, a ribbon terrane located north of Greater India and Australia, and the Gascoyne block, Greater India’s northern indenter, which was once conjugate to the Exmouth Plateau off NW Australia. The plate kinematic model based on evidence preserved on the Indian Ocean floor constrains the 106 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin relative motion of all the major plates involved, as well as some ribbon terranes. However, such a model cannot be used to infer the timing and geometry of collisions along the Eurasian margin, because unravelling the history of these events involves a knowledge of whether or when back-arc basins opened, or when they were destroyed, as well as a knowledge of regional geological events that help constrain the timing and spatial extent of individual collision, as well as the nature of the objects that collided, for instance distinguishing between continent-continent and arc-continent collisions. Geological data used here to help link a regional plate kinematic model to a detailed history of collisions along the Eurasian margin include correlative suture zone ages and the identification of continental crust with Gondwana affinities, both along the Eurasian and SE Asian marginal terranes, as well as obducted ophiolites and island arcs. We review the Eurasian and SE Asian geology to constrain the continental collisions. 107 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin EURASIAN GEOLOGY Many authors (e.g. Allegre et al., 1984; Searle et al., 1987; Yin and Harrison, 2000) describe the sub-parallel eastwest-trending terranes and suture zones of Eurasia Eurasian margin (Fig. 3.1). The terranes have been divided into east and west partitions by two main faults running southeast and southwest of the Tarim Basin, which acts as a giant anvil about which these terranes slide (Fig. 3.1). The dextral Karakoram fault isolates the western terranes, which from north to south include Karakoram, Kohistan- Ladakh, Himalaya and India, respectively divided by the Shyok, Indus and Main Boundary Thrust sutures. The sinistral Altyn Tagh Fault isolates the eastern terranes, which from north to south include Eastern Kunlun-Quaidam (Kunlun), Songpan-Ganzi- Hoh Xil (Songpan), Qiangtang, Lhasa, Himalaya and India, respectively divided by the Anyimaqen-Kunlun-Muztagh (AKMS), Jinsha, Bangong-Nujiang (BNS), Yarlung- Tsangpo (YTS) sutures and the Main Boundary Thrust (MBT). Ophiolites, now known to derive from back-arc basins/suprasubduction-zone settings (e.g. Bloomer et al., 1995), are scattered throughout the terranes and suture zones, and can help unravel their collision history. Geochronological studies of the Eurasian ophiolites (Fig. 3.2) along the southern sutures mainly fall into two distinct groups, dating to the Mid Jurassic and Lower-Mid Cretaceous (e.g. Dai et al., 2011a; 2011b; Hebert et al., 2011). The Trans-Himalayan batholith is also integral to Eurasian margin studies. It is known by several names depending on location, including the Gangdese plutonic complex, Kailas tonalite, and Ladakh and Kohistan batholith (Fig. 3.3). The 2,500 km-long feature has a mainly uniform composition of biotite-hornblende granodiorite and is considered a product of Andean-type subduction, having both mantle and lower 108 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin continental crustal melts (Searle et al., 1987). Only the western portion intrudes a folded intra-oceanic arc, which may have stretched across the Eurasian margin but was either eroded or did not outcrop further east. Two main stages of plutonism formed the Gangdese batholith from either ~188 Ma (Chu et al., 2006) or 103 to 80 Ma, then 65 to 46 Ma (Wen et al., 2008). Qiangtang terrane (Tibet) The Qiangtang terrane (Fig. 3.1) is separated from the Songpan terrane, further north, by the Jinsha suture, a linear ophiolitic mélange zone with peridotites, radiolarites, gabbro and basalts, volcaniclastic sediments and granitoid intrusions (Roger et al., 2003). Late Carboniferous glacio-marine sequences in this terrane point to a Gondwanan origin (e.g. Chang et al., 1986b; Metcalfe, 1988). The western sediments include Late Permian- Jurassic limestones and shales, interbedded with lava flows (Matte et al., 1996), unconformably overlain by Cretaceous-Paleocene red conglomerates and sandstones, unconformably overlain by Eocene rhyolite (Norin, 1946). A northwest-trending, 600- km long, 300-km wide anticlinorium has been identified with metamorphic rocks and Upper Paleozoic strata at its core, and Jurassic to Upper Cretaceous strata on its northern and southern limbs (Yin et al., 1998). A blueschist-facies metamorphic belt running southeast to northwest was variously alternately as pre-Devonian basement (Chang et al., 1986a), an extensional basin (Deng et al., 1996), Triassic-Jurassic core complex (Kapp et al., 2000), or a Triassic suture zone (Li et al., 1995). The latter two studies Ar/Ar dated the eclogite samples at ~223 Ma. 109 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin Figure 3.1. Main Eurasian faults, tectonic blocks (bold) and boundaries featuring the Indus Suture, InS, Karakoram, KK, Karakoram Fault, KkF, Kohistan-Ladakh, K-L, Longmu-Goza Fault, Rushan Pshart, RP, the Shyok Suture, ShS, and West Burma (Myanmar), WB. 110 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin Figure 3.2.
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