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
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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.
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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
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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.
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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
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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.
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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.
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Figure 3.2. Main Eurasian tectonic and magmatic features with age color-coded ophiolites, left.
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The Qiangtang terrane may be divided into (south)west and (north)east sections according to the outcropping metamorphic belt, glaucophane schists (Hennig, 1915), volcanics and Mesozoic granite. This could suggest a diachronous collision between
Lhasa and Qiangtang terranes (Zhang and Tang, 2009), if both segments were not part of the same block, which rifted off Gondwana (e.g. Metcalfe, 1988; Sengor, 1987).
Cathasian (Kidd et al., 1988) and Gondwanan facies (Jin, 2002; Norin, 1946; Sun, 1993), located in east and west Qiangtang, respectively, support the distinction between east and west Qiangtang. The Bangong-Nujiang suture marks its southern boundary.
Bangong-Nujiang suture (Tibet)
The Bangong-Nujiang suture (BNS, Fig. 3.1) formed when the Lhasa block collided with the Qiangtang terrane along a northward-dipping subduction zone (e.g. Metcalfe,
2006; Taylor et al., 2003). Triassic-Jurassic bivalves in the Yeba volcanic sediments, northeastern Lhasa terrane, show that marine conditions were prevalent ~180 Ma (Yin and Grant-Mackie, 2005). Field mapping and geochronological studies near Shiquanhe, far-west Lhasa terrane, uncovered the remnants of a subduction-accretion complex and forearc basin, which was attributed to the closure of the Late Jurassic-Early Cretaceous
Bangong-Nujiang Ocean (Kapp et al., 2003). Further east, metamorphism and magmatism in exposed Cambrian rocks in the Amdo basement, northeast Lhasa terrane
(Fig. 3.2) indicate a continental arc formed ~185-170 Ma (Guynn et al., 2006).
Several ophiolites were obducted onto the Lhasa block’s northern margin in the Late
Jurassic-Early Cretaceous (e.g. Dewey et al., 1988; Girardeau et al., 1984; Pearce and
Deng, 1988). The ophiolitic belt is anomalously wide in Donqiao, southwest of Amdo
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(Fig. 3.2), where Jurassic ophiolite fragments (Tang and Wang, 1984) produced a metamorphic aureole ~180-175 Ma due to obduction (Zhou et al., 1997). The ophiolitic belt is also anomalously wide further west where it is considered to represent the collision of at least one intra-oceanic arc (e.g. Girardeau et al., 1984; Matte et al., 1996;
Pearce and Deng, 1988). The Xainxa ultramafic rocks (Fig. 3.2), considered a klippe of the Donqiao ophiolite ~200 km further northeast, were emplaced during the Early
Cretaceous (Girardeau et al., 1985). The latter matches the Amdo basement magmatism and metamorphism and could possibly represent an intra-oceanic arc that collided with the Lhasa block (Segnôr and Celal, 1981). Radiolarians from west-central BNS, reveal that deep marine conditions must have prevailed here until the Early Aptian/121 Ma
(Baxter et al., 2009). Only the western BNS was reactivated in the Late Cretaceous-
Paleocene (Matte et al., 1996), or Miocene (Ratschbacher et al., 1994), following dextral strike-slip from the Karakoram fault.
Lhasa terrane (Tibet)
The Lhasa terrane (Fig. 3.1) is situated in southern Tibet, south of the Qiangtang terrane and the BNS. The oldest Lhasa terrane rocks are Precambrian gneisses, they are overlain by Carboniferous glacial tilliods, Permo-Jurassic shallow shelf marine carbonates, and
Cretaceous terrestrial redbeds and andesites of the Takena Formation (Chang et al.,
1986a).
Detrital zircons identify the Lhasa block as a Gondwana-derived fragment (Zhu et al.,
2011a). Stratigraphic studies show that the Lhasa and Qiangtang blocks were a continuous platform until the Late Triassic before transtensional rifting and Mid-Late
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Cretaceous transpression (Schneider et al., 2003). A study of Late Devonian to Eocene stratigraphic succession in the Tethyan Himalayas, India’s northern margin, shows that
Carboniferous rifting culminated in seafloor spreading by the Early Permian, verifying that the Lhasa terrain rifted from Gondwana by then (Sciunnach and Garzanti, 2012).
Metcalfe (2006) originally suggested that the Lhasa terrane rifted from Gondwana along with the West Myanmar (Burma) block (Argoland). Initial contact between the Lhasa terrane and Eurasian margin has been dated to the Late Jurassic (Dewey et al., 1988;
Metcalfe, 2006). Early Cretaceous clastic strata in north Lhasa (Zhang, 2004), and coeval deformation forming the Qiangtang anticline (Kapp et al., 2003), support the closure of a foreland basin via northward underthrusting of Lhasa by the Qiangtang terrane, as does ~120 Ma plutonism in the central-north Lhasa terrane (Xu et al., 1985).
Yang et al. (2009) divide the Lhasa terrane into north and south segments based on eclogites located in the eastern part of the terrane, which they SHRIMP U-Pb-dated
~242-292 Ma. They explained the eclogites as a Permo-Triassic southward relocation of southward-directed oceanic subduction zone. The crustal thickness of the Lhasa block, increasing from ~65 to 80 km going east to west across the dextral Jiali fault (Fig. 3.2), might also suggest two blocks but that effect has been linked to the ongoing convergence between India and Eurasia (Zhang and Klemperer, 2005).
The Lhasa block has also been divided into three northwest-trending ribbons, according to magmatic belts and different sedimentary cover rocks (Zhu et al., 2011b). The North
Lhasa terrane Jurassic-Cretaceous cover rocks contain exposed Mesozoic volcanic rocks and plutonic rocks, emplaced ~124-107 Ma (Zhu et al., 2009a) and ~140-80 Ma (Zhao
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et al., 2008), respectively. The cover rocks in the Central Lhasa terrane include Permo-
Carboniferous metasediments, Jurassic-Cretaceous volcaniclastics and minor pre-
Jurassic limestones (Zhu et al., 2011b). The volcanic rocks mainly include the Zenong
Group, dated ~143-102 Ma (Zhu et al., 2009a), and plutons dated ~215-95 Ma (He et al.,
2006). In the South Lhasa terrane, Late Triassic-Cretaceous sedimentary cover is interspersed with volcanic rocks, including the Lower Jurassic Yeba (Zhu et al., 2008) and Lower Cretaceous Sangri Formations (Zhu et al., 2009b).
Ophiolites obducted onto the Lhasa block’s northern margin have been interpreted to mark the end of north-dipping subduction and Lhasa-Eurasia collision (e.g. Zhou et al.,
1997). Ophiolites running northwest to southeast through the terrane were interpreted as another suture (Girardeau et al., 1985; Matte et al., 1996), though they could also represent remnants from one giant ophiolite nappe rooted at the BNS (Chang et al.,
1986b; Girardeau et al., 1984). These ophiolites include the Albian-Aptian emplaced
Xainxa ophiolites (Girardeau et al., 1985), located ~100 km south of Amdo (Fig. 3.2).
The southern margin of the Lhasa terrane experienced Andean-type, calc-alkaline magmatism from the Jurassic to the Late Eocene (Scharer et al., 1984; Zhu et al., 2008;
Zhu et al., 2006). Extensive calc-alkaline granitoids of the Gangdese batholith and the associated subaerial andesitic extrusive Linzizong Formation, dominate the southern margin of the Lhasa block (e.g. Chan et al., 2009). A major regional unconformity exists between the folded and eroded Late Mesozoic Takena Formation (e.g. Liu, 1988) and the gently-dipping Palaeocene-Eocene Linzizong volcanics (e.g. Allegre et al., 1984;
Coulon et al., 1986; Maluski et al., 1982). This contact suggests that the majority of crustal thickening and shortening within the Lhasa terrane occurred before the final
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India-Eurasia collision (e.g. England and Searle, 1986; Leier et al., 2007; Yin and
Harrison, 2000).
Lower Cretaceous crustal thickening is also supported by fission track ages from
Cretaceous granitoids and Jurassic metasediments from a peneplain in Tibet (Hetzel et al., 2011). Their study suggests that cooling and exhumation of the granitoids occurred between 70 and ~55 Ma, followed by a rapid decline in the exhumation rate and stabilizing ~48 Ma. A recent thermochronological study also suggests that the Tibetan plateau grew locally from the Late Cretaceous, spanning the region by 45 Ma
(Rohrmann et al., 2012). Mesozoic plutonic rocks emplaced from the Late Triassic to
~72 Ma, occur as smaller relics within the Tertiary Gangdese batholith (Ji et al., 2009).
Aitchison et al. (2002a) identified an unconformable contact between the Oligocene–
Miocene Loubusa conglomerates and foliated metamorphic rocks of the southern Lhasa terrane, instead of the north-dipping, southward-directed Gangdese backthrust (Harrison et al., 2000; Yin et al., 1999; Yin et al., 1994), attributed to Late Eocene Tibetan Plateau uplift.
Karakoram terrane (NW India and Pakistan)
Further west, the Karakoram terrane (Fig. 3.1) extends east from the Afghanistan-
Pakistan border and is truncated from the Tarim Basin, further north, along the
Karakoram fault. The Shyok suture separates Karakoram terrane from Kohistan-Ladakh arc, further south. Heuberger (2004) describes the Karakoram terrane geology as
Precambrian crystalline basement, unconformably overlain by Paleozoic marine sediments, unconformably overlain by Permian–Jurassic carbonates, which document
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its migration as part of the Cimmeride continent. Late Devonian-Early Carboniferous rifting is recorded along its northern margin.
The Karakoram terrane can be linked to Gondwana based on its fossil assemblages
(Sharma et al., 1980; Srivastava and Agnihotri, 2010; Xingxue and Xiuyuan, 1994) and zircon ages (White et al., 2011), though the latter authors also identify sediment source links to the Pamir region, further north, and the Qiangtang terrane, further east.
Karakorum has also been proposed a dextrally-offset counterpart to the Lhasa block, in
Tibet, as both blocks have similar sequences of Cambro-Ordovician sediments overlying Cambrian gneisses, (Rolland, 2002). The Karakoram fault has disrupted continuity between the east and west terranes. Late Tertiary transpression along the
Karakoram fault exhumed granites and migmatites, which appear to be dextrally offset by up to 150 km (Searle et al., 1998). Much larger offsets (~1000 km) were initially proposed for the Karakoram fault (Peltzer and Tapponnier, 1988) but field mapping shows this not to be the case (Searle, 1996).
The Karakoram terrane can be divided into three geological zones from north to south.
These are the northern sediments, the batholith and the metamorphic complex. The sediments consist of Carboniferous shales and Permian-Mesozoic carbonates, conglomerates and tuffs, intruded by gneisses ~115-94 Ma and cross-cut by leucograntie pegmatites ~70-58 Ma (Rex et al., 1988). The Karakoram batholith is a long but relatively thin subduction-related igneous crustal lineament of mainly massive or weakly foliated biotite-granodiorite dated ~77 Ma (Pudsey, 1986). It has undergone episodic magmatism for at least 80 My, suggesting it is a major crustal lineament (Rex et al., 1988). The Karakorum metamorphic complex, to the south, is composed of
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metasedimentary sequences, felsic gneisses and migmatites, and a prominent melange that includes ultramafic rock originating from between the batholith and Shyok suture, further south (Rex et al., 1988). Crustal thickening caused the metamorphic episodes from the Latest Cretaceous to Tertiary (Searle et al., 1999).
Collision between Karakoram and Eurasia formed the Tirich Boundary Zone (TBZ) and early Jurassic red sandstones, which are cross-cut by the 115 Ma Tirich Mir pluton
(Heuberger, 2004). The study also describes ferriferous-alkaline intrusions in the
Karakoram batholith and Campanian carbonates, suggesting a marine basin formed after a collision caused regional metamorphism in eastern Karakoram 80-60 Ma.
Shyok suture (NW India and Pakistan)
The Shyok suture (also known as the Northern Suture, Fig. 3.3) divides the Karakoram and Kohistan-Ladakh terranes but has been shifted southeast by a thrust system in the upper 15 km of crust (Searle et al., 1987). The Shyok suture mélange zone reaches up to
4 km wide and contains volcanic greenstone, limestone, red shale, conglomerate, quartzite and serpentinite blocks in a slate matrix (Pudsey, 1986). Based on 111-62 Ma intrusions within Albian-Aptian limestons, the latter study dates the suture to the Late
Cretaceous but incorporates the closure of a backarc basin, which extended along the
Yarlung-Tsangpo suture (Fig. 3.1). Based on two post-collisional granites in northern
Kohistan with isotopic links to India, Khan et al. (2009), propose a far younger age of
~47-41 Ma for the Shyok suture. They also link the ~50 Ma Shyok suture to the
Yarlung–Tsangpo suture, which they date ~51 Ma.
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Kohistan-Ladakh terrane (NW India and Pakistan)
Kohistan-Ladakh (Fig. 3.1) lies south of the Karakorum terrane and is interpreted as the remains of an intraoceanic-arc. In Ladakh, the Dras Group of arc and intrusive units
(Fig. 3.3) consist of Jurassic to Upper Cretaceous tholeiitic to andesitic rocks and volcaniclastic sediments (Coward et al., 1987; Rolland, 2002). The arc was initiated in the Late Jurassic-Early Cretaceous, thickened by 95 Ma, underwent granulite-facies metamorphism ~90 Ma before post-collisional intrusions were emplaced (Petterson,
2010). Ladakh is typically correlated with the Kohistan but tectonic thinning around the
Nanga Parbat syntaxis divides them (Khan et al., 1993). The Kohistan arc contains andesitic lavas, tuffs, volcaniclastics, slates and limestones, metamorphosed to greenschist facies. Kohistan-Ladakh has been divided into six mainly Cretaceous units.
From north to south, they include the Yasin sediments, Chalt volcanics, Kohistan batholith, Chilas ultramafic complex, Kamila amphibolte belt, and Jilal ultramafic complex (Petterson and Windley, 1985, 1992; Pudsey, 1986).
The Albian-Aptian Yasin Group of slates, turbidites and limestones formed in intra-arc basins and are overlain by the Chalt volcanic group of pillow-bearing island arc tholeiitic lavas, succeeded by calc-alkaline andesites to rhyolites. The Yasin sediments and Chalt volcanics (Fig. 3.3) were intruded by the Kohistan batholith, which can be divided into two stages: the early deformed group and younger undeformed group
(Petterson and Windley, 1985). The older group contains the Matum Das trondhjemite pluton dated ~102 Ma. The pluton was intruded by a 75 Ma suite of dykes between Jutal and Chalt in Gilgit, northeast Kohistan (Petterson and Windley, 1992). Since only the older batholith is folded, deformation must have occurred between ~100 and 75 Ma.
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The younger group are cross-cut by granodiorites dated ~54 Ma, and granites dated ~40
Ma, e.g. Shirot and Gilgit granitoids (Petterson, 1985, unpublished Ph.D. Thesis). The
75 Ma intrusions are considered to post-date the collision, as recorded in the fold structures and penetrative fabrics of the older group (Petterson and Windley, 1992).
The Kamila complex (Fig. 3.3) thrusts southward onto the Tethyan Himalaya along the
Kamila shear zone. It is composed of intensely deformed amphibolites and metasediments Ar/Ar dated ~80 Ma (Treloar et al., 1989). The complex has been suggested to represent a more southerly metamorphosed equivalent of the Kohistan arc, or an earlier accreted arc (Searle et al., 1987).
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Figure 3.3. Geological units of the Kohistan-Ladakh terrane (K-L).
The Jijal complex (Fig. 3.3) is the deepest and most southern outcropping of the arc. It is a 150 km2 wedge of garnet-bearing granulite-facies meta-gabbro, intruded by a 14 km2, genetically unrelated, garnet-free ultramafic body, composed of mainly diopsidites, dunites, peridotites and harzburgites (Jan and Howie, 1981). The high-grade metamorphic rocks occur in the hanging wall of the Indus suture, between the arc and the Tethyan Himalaya. The ultramafic body was initially explained as a faulted slab of upper mantle/sub-oceanic crust, an orogenic diapir, or ultramafic rocks isolated from basaltic magma, which intruded the garnet granulites after being independently metamorphosed before reaching present surroundings (Jan and Howie, 1981). The contact between them was also suggested to represent the sub-arc petrological moho
(Burg et al., 1998). Garrido et al. (2006) propose that the assemblage formed in situ during early arc magmatism before the Kohistan arc underwent crustal thickening from
~110-90 Ma, creating the Jijal complex before the arc sutured to Eurasia. The granulites formed between ~118-83 Ma (Yamamoto and Nakamura, 2000) but were probably emplaced before peak metamorphic conditions were reached during crustal thickening
(Jan and Weaver, 2004).
The Chilas complex (Fig. 3.3) intrudes the southern part of the Kohistan batholith and is composed of layered basic cumulates, including noritic gabbro and dunites, metamorphosed to granulite facies (Coward et al., 1986). The complex was thought to represent the magma chamber of the arc (Rai and Pande, 1978). It has also been suggested that intra-arc rifting formed the Chilas complex (Burg et al., 1998; Khan et al.,
1996) before renewed compression forced the granitic plutons to intrude the Kohistan
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batholith. The undeformed gabbronorites contain xenoliths of strongly deformed biotite- schists and gneisses, suggesting their emplacement followed collision with Eurasia
(Treloar et al., 1996). The complex was U-Pb zircon dated ~85-84 Ma (Schaltegger et al., 2002; Zeitler et al., 1981).
Treloar et al. (1996) re-evaluated the stratigraphy of the arc using published geochemical data plus new field mapping and structural, and provided a revised geological and tectonic summary. The Kamila belt is the oldest unit and represents a subduction basement, intruded by arc-type gabbroic sheets and plutons, overlain by the
Jaglot Group composed of turbiditic metasediments (Yasin) and basaltic lavas (Chalt) erupted in an extensional basin. Eurasian collison ~100 Ma formed major intra-arc shear zones and a regionally-penetrative steep cleavage. The Chilas Complex intruded the
Kamila amphibolites and Jaglot Group ~85 Ma, after Eurasian suturing. This caused regional amphibolite facies metamorphism, melting of the lower arc, and plutonism.
The early plutons were unroofed and eroded during compression between 80-55 Ma.
Younger plutons were emplaced until 40 Ma, extruding basaltic to rhyolitic rocks.
These events took place over three phases of extension, rifting and compression, associated with a retreating subduction zone. Following extension until 104 Ma, collision with Eurasia caused granitiod plutomism at Matum Das, shearing within the
Kamila shear zone and development of the main compressional fabrics. Extension resumed from 85-75 Ma with emplacement of the Chilas gabbronorite, metamorphism, lower arc melting and stage 2 plutonism. Compression resumed from 75-55 Ma, causing uplift of the stage 2 Swat and Dir batholiths, before extension caused the collapse of the arc’s southern margin, acidic volcanism and emplacment of younger stage 2 plutons
(48-40 Ma). The Indus Confluence granite sheets were emplaced ~34 ±14 Ma.
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Indus suture (NW India and Pakistan)
The Indus suture separates Khoistan-Ladakh arc from the Tethyan Himalayas (India).
Ar/Ar dating reveals that blueschists, identified in the Indus suture’s hanging wall, underwent a metamorphic peak ~75-80 Ma (Anczkiewicz et al., 2000; Maluski and
Matte, 1984). This was identified as the timing of the Kohistan arc’s obduction onto
India (Bard, 1983; Maluski and Matte, 1984). Geochemical investigations suggest an island arc or transitional MORB setting for the Late Cretaceous blueschists (Honegger et al., 1989). Isotopic analysis of zircons in Kohistan-Ladakh suggests arc magmatism stopped ~61 Ma, leading to ~60 Ma age for the suture (Khan et al., 2009). These authors suggest this was a result of India colliding with the arc before the arc collided with
Eurasia. Age data from rocks in the Pakistan Himalaya show that peak metamorphism occurred ~47 Ma, ~20 Ma earlier than along central and eastern Himalaya, also suggesting obduction occurred during the Late Cretaceous-Early Palaeocene (Searle and
Treloar, 2010).
Younger ages for the Indus suture have also been proposed. The development of 60-40
Ma calc-alkaline plutons in two thirds of the Kohistan batholith (Petterson and Windley,
1985) suggests the arc did not collide with India until after 40 Ma. A deformed granodiorite identified east of the Kohistan arc indicates an Upper Eocene/Lower
Oligocene collision for the arc (Bard, 1983). Due to the calc-alkaline lavas, sediments and Eocene fossils, covering the Kohistan batholith, Petterson and Windley (1985) also date the Indus Suture to the Eocene. They also identify the ~34 and 29 Ma Rb-Sr-dated layered aplite-pegmatite sheets intruding the Kohistan batholith as post-collisional.
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Yarlung-Tsangpo suture (YTS, NE India and Himalaya)
The Yarlung-Tsangpo suture zone (YTS, Fig. 3.2) is the tectonic boundary between
Eurasia and India. There are three main components to the suture zone: the Xigaze forearc basin, the Yarlung-Tsangpo ophiolitic belt and associated mélange, and the accretionary prism.
The Xigaze terrane (Fig. 3.2) lies along the southern margin of the Lhasa terrane and is considered a prime example of an exposed fore-arc basin (Durr, 1996). It is partly folded but mainly dips steeply to the north, having undergone dextral strike-slip and oceanic thrusting prior to obduction (Searle et al., 1987). It contains a succession
Middle to Upper Cretaceous volcaniclastic turbidites and sediments that record basin inversion into the Eocene, which do not continue into the Lhasa terrane due to lateral translation (Aitchison et al., 2011). An abrupt change from Upper Cretaceous Xigaze marine flysch to Eocene Qiuwa conglomerates, the latter originating from the Gangdese batholith, was interpreted as a product of the India-Eurasia collision (Searle et al., 1987).
Abundant ophiolites (Fig. 3.2) outcrop along the YTS but appear to coalesce into two age groups. The older ophiolites include those around the Eastern syntaxis, dated ~200
Ma (Geng et al., 2006), and Naga ophiolite, located in Nagaland near West Myanmar, recently dated as Upper Jurassic via radiolarian assemblages (Baxter et al., 2011). The
Chin Hills ophiolite, between India and West Myanmar, is also dated at ~160 Ma
(Mitchell, 1981). Southeast of the Linzizong volcanics, the Zedong and Loubusa ophiolites were Ar/Ar date at ~161 Ma (McDermid et al., 2002) and 177 Ma (Robinson et al., 2004), respectively. Near the terminus of the Karakoram fault, Kiogar and
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Jungbwa ophiolites were U-Pb dated to ~160 Ma and 123 Ma (Chan et al., 2007b), respectively, though Jungbwa has also been Ar/Ar dated to 152 Ma (Miller et al., 2003).
Further west, the Najiu ophiolite was recently U-Pb dated at ~364 Ma (Dai et al., 2011a).
This implies that some PalaeoTethys oceanic relic is ingrained at this suture, which the authors explain as a Paleo-Tethys Ocean branch operating in the Meso-Tethys. This is hard to envisage as their reconstruction shows East and West Qiangtang offset by over
3,000 km but with East Qiangtang over 2,000 km west of West Qiangtang while the
Lhasa block is ~1000 km to its northeast. One cannot imagine how the blocks migrated to their present position.
The younger ophiolites include the Dazhuqu terrane, ~150 km east of Xigaze, dated to the Late Barremian to Late Aptian according to radiolarians (Ziabrev et al., 2003).
Southeast of Xigaze, radiolarian biostratigraphy from the Bainang terrane, an intra- oceanic island arc subduction complex, dates its deep marine sedimentation from the
Late Triassic to the Late Aptian (Ziabrev et al., 2004). The Xigaze ophiolites near
Bainang were Ar/Ar dated ~123.6-127.7 Ma (Guilmette et al., 2009). The Buma ophioliic mélange, outcropping south of the central Linzizong volcanics, was Ar/Ar dated at ~128 Ma (Guilmette et al., 2009). Saga and Sangsang ophiolites do not yet have age data but the Saga mélange has yielded a metamorphic sole that reached peak conditions between 132 and 127 Ma (Guilmette et al., 2011). West of Saga and
Sangsang, the Zhongba ophiolite dates at ~127 Ma (pers. comm. from Dai and Wang, quoted in Hebert et al. 2011). Clustering southwest of the Linzizong volcanics,
Danqiong (Chan et al., 2007a), and Xiugugabu (Wei et al., 2006) ophiolites both date at
~126 Ma.
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An intra-oceanic island arc sequence was recently identified in the central YTS (Fig.
3.2), though it may not all be related to the same arc (Aitchison et al., 2002a; Aitchison et al., 2000). These include the overturned Late Jurassic Zedong arc (McDermid et al.,
2002), the Dazhuqu ophioite (Abrajevitch et al., 2005; Ziabrev et al., 2003), dated U-Pb dated ~126 Ma, though two metamorphic soles are Ar/Ar dated ~90-80 Ma (Malpas et al., 2003), and the Mid Cretaceous Bainang accretionary wedge (Ziabrev et al., 2004).
Though they are truncated against the overlying Xigaze volcaniclastic turbidites, the
Dazhuqu ophiolitic rocks also underwent a distinct structural evolution (Aitchison et al.,
2002a) and paleomagnetic results show it formed at sub-equatorial latitudes with some fragments having undergone counter-clockwise rotation (Abrajevitch et al., 2005).
Tethyan Himalaya
The Tethyan Himalaya (High Himalaya) blend into the southern YTS and contain the exposed remains of the NeoTethys Ocean. It is separated from Tibetan sediments along a shallow north-dipping normal fault, south of which southward thrusting dominates
(Burg and Chen, 1984; Burg et al., 1984). The predominantly metamorphic basement is composed of sillimanite, kyanite, staurolite, garnet and muscovite-bearing gneisses, schists and leucogranites (Searle et al., 1987). Similar rocks outcrop south of the Indus suture, where small-scale structures indicate south-eastward thrusting (Coward et al.,
1987). The main central outcrop of Himalayan granites originate from Palaeozoic and older rocks, which partially melted off the subducting Indian shield (Searle and Fryer,
1986). Radiometric ages are quite young e.g. monazites from Makalu granite at Mount
Everest date ~22-24 Ma (Scharer, 1984). Searle (1986) argues that large-scale thrusting
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caused the inversion of metamorphic isograds along several major thrust zones so that radiometric cooling ages show only the youngest metamorphic event.
In Northern Pakistan, the Sapat mafic-ultramafic complex lies between the Kamila amphibolites and Indus suture in southern Kohistan, and was initially identified as the base of the Kohistan arc due to its mineral chemistry (Jan et al., 1993; Searle et al.,
1999). Khan et al. (2004) investigated the mineral chemistry of the complex, including peridot-bearing dunite and serpentinite, overlain by ultramafic cumulates and layered to isotropic gabbros and basaltic metavolcanics. They suggest the dunite may have formed in a supra-subduction zone setting of fore-arc affinity and attribute the complex to other ophiolite sequences observed along the Indus suture.
The Spongtang ophiolite and mélange in the northwest Tethyan Himalaya, roughly 30 km south of Ladakh (Fig. 3.2), occurs as a ~5 km section within the Zanskar carbonate platform (Clift et al., 2000). The ophiolite overlies allochthonous sediments and mélanges similar to those exposed in the nearby Indus suture, including exotic limestones, Dras volcanics, radiolarian cherts, amphibolites and greenshists. The north- dipping Spongtang ophiolite, thrust over Indian Palaeocene–Eocene limestones, was considered to have obducted during the India-Eurasia collision (Fuchs, 1982). Searle
(1986) suggested a Cretaceous-Tertiary obduction where the ophiolite was only thrust onto India after collision with Eurasia but Garzanti et al. (2005) noted there was no stratigraphic evidence of its earlier obduction in the Zanskar range.
Early Cretaceous radiolaria were recently collected near the Spongtang massif (Baxter et al., 2010). Those authors suggest that the first evidence of its emplacement is the
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appearance of ophiolitic detritus in sedimentary units, including the Palaeocene Chogdo
Formation in the Zanskar valley (Searle et al., 1990) and Dibling (Garzanti et al., 1987).
This formation has been classified as mostly Asian-derived and therefore not related to
Indian collision (Henderson et al., 2010a; Wu et al., 2007). Pedersen et al. (2001) dated the Spontang ophiolite at ~177 Ma and an andesitic sample from the overlying Spong arc was dated at 88 Ma (a long-lived oceanic arc).
Further east along the Indus suture, the Nidar ophoilite (Fig. 3.2) was recently been dated ~126 Ma according to radiolarians (Zyabrev et al., 2008), a reasonable match to the ~124 Ma radiometric age of the ophiolite (Maheo et al., 2004), and the YTS
Cretaceous ophioites.
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SE ASIAN GEOLOGY
Myanmar and Thailand
Fan and Ko (1994) provide a geological overview of Myanmar (Burma, Fig. 3.4), which can be divided into three geological terranes, including Arakan Yoma, Central Myanmar and Shan-West Malaysia-Sumatra. The eastern terrane can also be subdivided into West
Kachin (north), East Kachin Shan and Karen Tenasserim (south), along two NE and SE trending faults (Fig. 5). Our model suggests that Arakan Yoma, Central Myanmar and even Karen Tenasserim, extending south to the Thai Peninsula, could contain fragments of Central Argoland and north Greater India (Gascoyne block).
The region commonly referred to as West Myanmar incorporates Arakan Yoma and
Central Myanmar, which are located west of the Sagaing fault (Fig. 3.4). Some models refer to West Burma as Argoland (Metcalfe, 2006), which may have accreted there in the Late Cretaceous (Heine and Müller, 2005). Conversely, Fan and Ko (1994) describe two microcontinents that rifted from Gondwana and accreted to Eurasia far earlier:
Indosinia collided with the Yanzi-Huanan terrane along the Red River suture (Fig. 3.4) in the Devonian or Early Carboniferous, while SinoMyanmarlaya collided with East
Asia during the Indosinian orogeny between the Triassic and Jurassic (Hutchison, 1989).
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Figure 3.4. Regional tectonic map of SE Asia.
Karen Tenasserim, south of the Three Pagodas Fault (Fig. 3.5), consists of
Carboniferous-Permian limestone, Triassic-Jurassic limestone and Jurassic-Cretaceous red sandstone and conglomerate, some of which can be traced to Mergui, in SW
Thailand (Fan and Ko, 1994). The Phuket-Tenasserim plutonic arc formed after the
Cretaceous-Paleocene allochthonous collision between Arakan Yoma and Central
Myanmar. Arakan Yoma then moved ~450 km north relative to Karen Tanasserim because of the dextral Sagaing strik-slip fault (Mitchell, 1981). Lower Cretaceous I- and
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S-type granites intruded into marine clastic rocks due to westward migration of the eastward-dipping subduction zone (Fan and Ko, 1994).
CHINA
INDIA
WK
EK CMB
LAOS
(Three Pagodas) KT
THAILAND
Figure 3.5. Regional tectonic setting of Myanmar adapted from Fan and Ko (1994).
CMB is the Central Burmese Basin subterrane, EK is East Kachin subterrane, KT is
Karen Tenasserim and WK is West Kachin subterrane.
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Central Myanmar (Fig. 3.5) consists of Upper Triassic flysch basement uncomformably overlain by Cretaceous limestone and over 10 km of Late Oligocene to Quaternary sediments eroded from the Tibetan Plateau (Rodolfo, 1969). Central Myanmar is divided by a north-south volcanic arc with oil-bearing fields in the fore-arc to the west
(Fan and Ko, 1994). Volcanic materials in the Lower Eocene deposits come from the
Peg Yoma (Central Myanmar) volcanic arc, a granodiorite pluton, which intruded basaltic andesites in the Mid Cretaceous (Mitchell, 1981). The arc formed as a result of eastward subduction beneath Arakan Yoma in the Late Mesozoic (Fan and Ko, 1994).
Arakan Yoma (Fig. 3.5) is regarded by Fan and Ko (1994) as an Oliogocene uplifted subduction system consisting of Mesozoic flysch from an Early Cretaceous, eastward- dipping subduction zone. The subduction zone led to the formation of the Arakan Yoma
Ranges, as well as the volcanic arc striking through Central Myanmar and a plutonic arc extending north from Mergui, in SW Thailand. The Indo-Burmese Ranges of Arakan
Yoma (Mitchell, 1993) and the NW Australian shelf (Gradstein, 1992; von Rad et al.,
1992), share a Late Jurassic uplift erosional unconformity, which links West Myanmar to East Gondwana. Halobia bivalve affinities between Timor and Myanmar also support a Gondwana origin for West Myanmar (Charlton et al., 2009; McRoberts, 2010).
133 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
Sumatra and Java
Argoland was considered part of the Sikuleh (Fig. 3.4), West Myanmar and West
Sulawesi continental fragment, which supplied sediments to northwest Timor during the
Triassic and Jurassic until it separated from NW Australia (Metcalfe, 1996).
Geochronological studies show that the Malay Peninsula, Thailand and Indochina have remained uplifted throughout the Cenozoic following a ~90 Ma thermo-tectonic event
(Hall, 2002).
The Woyla Group (Fig. 3.4), along Sumatra’s southwestern margin, includes fragments of volcanic arc and imbricated oceanic crust, intruded by the Sikuleh Batholith of northwest Sumatra. The Sikuleh batholith has been attributed to a marginal basin formed by oblique subduction (Cameron et al., 1980), and an allochthonous terrane whose collision overrode West Sumatra forming the Woyla Nappe in the Albian-Aptian
(Barber, 2000; Barber and Crow, 2009). Its younger complex, a homogenous, unfoliated, biotite-hornblende granodiorite containing mafic xenoliths and flow foliation, was K/Ar dated ~97.7 Ma (Bennett et al., 1981). The Manunggal Batholith, a composite of leucogranodiorite, granodiorite, granite and pyroxene-quartz diorite, intruded the Woyla
Group in Natal (west-central Sumatra) ~87 Ma (Kanao, 1971, unpublished but quoted in
Barber 2000).
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METHODOLOGY AND DATA
We use the same methodology as described for the Indian Ocean studies incorporating constraints from seafloor offshore West Australia, East Antarctica and East India
(Gibbons et al., submitted; Gibbons et al., in prep). We use the combined timescales of
Cande and Kent (1995) and Gradstein et al. (1994) for Cenozoic and Mesozoic times, respectively.
The extents of the continental fragments are based on available potential field data and tectonic constraints (see discussion). Argoland is defined as a ribbon terrane reaching east to west from Papua New Guinea to East Africa. Greater India is defined as a narrow indenter, for its northern half. The southern margin of East Argoland is constructed from published NW Australian COB (Müller et al., 2005).
The geometry of the Eurasian margin is taken from van Hinsbergen et al. (2011b), which incorporates Cenozoic deformation. The Mesozoic tectonic evolution of the
Eurasian terranes is based on the geological evidence from the Eurasian margin (see discussion). The Eurasian intra-oceanic arcs, which collide with our modeled continental fragments, are developed as narrow slivers, migrating from the Eurasian margin. Their time of their formation and rate of migration is based on geological evidence from the Eurasian margin (see discussion).
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PLATE KINEMATIC CONSTRAINTS
Robust plate tectonic models are built on regional constraints, and can recreate observable tectonic features and avoid misfits and unlikely plate motions. This has been undertaken for the Indian Ocean (Gibbons et al., submitted; Gibbons et al., in prep), and is expanded in this study to incorporate constraints from the Eurasian margin.
Constraints for the extent of Argoland and Greater India
The main continental blocks in our study include Argoland and Greater India. Argoland formed East Gondwana’s northern margin during the Jurassic, stretching from East
Africa to Papua New Guinea. This extended outline negates the initiation of a ~8000 km-long tranform fault across the Central MesoTethys. A Jurassic sliver, dredged ~1000 km off West Australia in 2008 (Gibbons et al., submitted; Gibbons et al., in prep), necessitates a drastically reduced size for Greater India, so that its northern extent consists of a ~100 km-wide indenter, which was originally conjugate to the Exmouth
Plateau. The revised outline of these blocks, the outline of the Eurasian margin, and their respective relative motion through time, dictates the onset of their collisions.
Argoland’s migration is based on the magnetic anomaly picks in the Argo abyssal plain until ~136 Ma, when Greater India began to migrate from Gondwana and Argoland became fixed to Greater India. The motion of Greater India and its northern indenter, the Gascoyne block, are governed by constraints dictated by the seafloor offshore West
Australia and East Antarctica. These include seafloor spreading magnetic anomalies, initial fit reconstructions that avoid gaps and overlap, submerged plateaus, and fracture
136 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
zones. Several reconstructed fits between India, Madagascar, Sri Lanka and the
Seychelles, were tight (e.g. Lawver et al., 1998; Marks and Tikku, 2001). Our model adopts a revised 83 Ma-position fit (Williams et al., 2011), which shifts Australia several hundred kilometres further east relative to Antarctica than previously modeled, leaving additional accommodation space for Sri Lanka, Madagascar and the Seychelles.
Motion for Greater India
The initial motion for Greater India is tightly constrained in a giant vice, consisting of
Madagascar and the Wallaby-Zenith Fracture Zone (WZFZ), a prominent linear-oblique feature offshore West Australia. Greater India’s motion formed the WZFZ and our model reconstructs the wide fracture zone by ‘unzipping’ India from Gondwana. This also alleviates compression between India and Madagascar, as the latter migrated from
Antarctica. Our reassignment of younger ages to magnetic anomaly picks in the
Enderby Basin (Gibbons et al., in prep), also minimises overlap between India and
Madagascar and solves the persistent problem of two-way strike-slip between them, as seen in previous models (e.g. Gaina et al., 2007).
During the Cretaceous Normal Superchron (CNS), a period of no magnetic reversals from 120-83.5 Ma, Greater India’s motion relative to Australia is constrained by the necessity of generating no overlap between the two continents along the WZFZ (as there is no evidence for major compression along this transform zone), and by modelling a full spreading rate of ~70 mm/yr, a continuation of the spreading rate documented in the pre-CNS seafloor. A further constraint is given by the observed prominent bend of the
Kerguelen and Wharton Basin fracture zones in the CNS. This bend signifies the onset
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of relative motion between India and Madagascar, culminating in their separation via seafloor spreading, progressing from south to north, from 94-84 Ma in the model presented here, independently supported by other dates proposed for their separation by seafloor spreading, e.g. ~86.5 Ma (Yatheesh et al., 2006).
For other Indian Ocean abyssal plains, where we have not reinterpreted potential field data, our model incorporates published magnetic anomaly picks and ensures that there is agreement between age picks in coeval oceanic basins. Of the several models available that describe the initial motion for Africa-Antarctica (Mozambique and Lazarev/Weddel
Basins) and Africa-Madagascar (Somali Basin), we adopt Konig and Jokat, (2010) and
Müller et al. (2008), respectively. We adopt these models because, in both studies, seafloor spreading between Africa and Madagascar, and Africa and Antarctica, started
~M26/155 Ma and ceased ~M0/120.4 Ma. This ensures continuity between the seafloor spreading corridors and avoids overlap in the initial fit reconstruction. Fig. 2.8 (Chapter
2) shows that an alternative model where seafloor spreading ceased ~M10N, causes unacceptable overlap between Madagascar, India and Antarctica, pre-rifting. After the
CNS, we incorporate Cretaceous magnetic anomaly picks from the Southwest Indian
Ridge (Bernard et al., 2005; Marks and Tikku, 2001; Nankivell, 1998), and Rodrigues
Triple Junction (Cande et al., 2010; Müller et al., 1997; Royer and Chang, 1991), the latter includes picks from the Southwest Indian and Carlsberg Ridges. A more detailed account of the tectonic model can be found in our associated papers (Gibbons et al., submitted; Gibbons et al., in prep).
The Mesozoic Eurasian margin and oceanic arcs
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The Mesozoic Eurasian margin we adopted for this study incorporates the Karakoram,
West Qiangtang and Lhasa terranes, as the Cimmerian continent. We distinguish the
Western Qiangtang terrane, which contains Gondwanan facies (Jin, 2002; Norin, 1946;
Sun, 1993), from the East Qiantang terrane, which contains Cathasian facies (Kidd et al.,
1988). We adopt a blueschist-facies metamorphic belt, running southeast to northwest, proposed to be 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), as their boundary.
We incorporate the Lhasa terrane as the Eurasian margin as stratigraphic studies show that the Lhasa and Qiangtang blocks were a continuous platform until the Late Triassic
(Schneider et al., 2003). They may have rifted from Gondwana even earlier, since the
Late Devonian to Eocene stratigraphic succession in the Tethyan Himalayas shows that
Carboniferous rifting culminated in seafloor spreading by the Early Permian (Sciunnach and Garzanti, 2012). Metcalfe (2006) originally suggested that the Lhasa terrane rifted from Gondwana with the Argoland (West Burma/Myanmar block) but this could not have accreted to the Eurasian margin before ~80 Ma, given our modeled motions for
Argoland, so we discard this model. Initial contact between the Lhasa terrane and
Eurasian margin, forming the Bangong-Nujiang suture (BNS), has been dated to the
Late Jurassic (Dewey et al., 1988; Metcalfe, 2006) and Early Cretaceous. The younger collision is based on clastic strata in north Lhasa (Zhang, 2004), coeval deformation forming the Qiangtang anticline (Kapp et al., 2003) and ~120 Ma plutonism in the central-north Lhasa terrane (Xu et al., 1985). Radiolarians from west-central BNS, also reveal that deep marine conditions must have prevailed here until the Early Aptian/121
Ma (Baxter et al., 2009). To satisfy the array of ages reported for the BNS, we model
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the Lhasa terrane a rifting from from West Qiangtang in the Late Jurassic and re- collidig by the Mid Cretaceous. This is also required to form the Xigaze fore-arc basin
(Fig. 3.2), which has a succession Middle to Upper Cretaceous volcaniclastic turbidites and sediments (Aitchison et al., 2011). The latter study reported that sediment source/Hinterland mismatches suggested >500 km lateral translation, which is also featured in our model.
Further west, the Karakoram terrane was linked to Gondwana based on its fossil assemblages (Sharma et al., 1980; Srivastava and Agnihotri, 2010; Xingxue and
Xiuyuan, 1994) and zircon ages (White et al., 2011). The latter authors identify
Karakoram sediment source links to the Qiangtang terrane, while another study proposed it a counterpart to the dextrally-offset Lhasa block, due to similar sequences of
Cambro-Ordovician sediments overlying Cambrian gneisses (Rolland, 2002). This could all be explained if the Qiangtang, Lhasa and Karakoram terranes were once a
Gondwana-derived continental fragment, so we include Karakoram in our initial
Eurasian margin.
Further south, the Kohistan batholith can be divided into two stages: the early deformed group dated ~104 Ma, which were intruded by a younger undeformed group ~75 Ma
(Petterson and Windley, 1985). The authors concluded that the Kohistan-Ladakh terrane must have accreted to Eurasia between ~100 and 75 Ma. The Matum Das tonalite was dated ~154 Ma (Schaltegger et al., 2004). We model the Kohistan-Ladakh terrane to have formed as a back-arc of the allochthonous Lhasa terrane from as early as ~150 Ma but suggest that its obduction occurred ~60 Ma.
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There is evidence of an intra-oceanic arc in the YTS, though it may not all be related to the same arc (Aitchison et al., 2002a; Aitchison et al., 2000). The components include the overturned Late Jurassic Zedong arc (McDermid et al., 2002), the Dazhuqu ophioite
(Abrajevitch et al., 2005; Ziabrev et al., 2003), dated U-Pb dated ~126 Ma, though two metamorphic soles are Ar/Ar dated ~90-80 Ma (Malpas et al., 2003), and the Mid
Cretaceous Bainang accretionary wedge (Ziabrev et al., 2004). Paleomagnetic results show the oceanic arc reached sub-equatorial latitudes (Abrajevitch et al., 2005).
There are abundant Barremian-Aptian ophiolites along the YTS (Fig. 3.2), include the
Dazhuqu terrane (Ziabrev et al., 2003) Bainang terrane (Ziabrev et al., 2004) Xigaze ophiolites (Guilmette et al., 2009), Buma (Guilmette et al., 2009), Saga and Sangsang
(Guilmette et al., 2011), Zhongba (pers. comm. from Dai and Wang, quoted in Hebert et al. 2011), Danqiong (Chan et al., 2007a), and Xiugugabu (Wei et al., 2006). Their common age suggests that an obduction event occurred ~126 Ma.
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REVISED TECTONIC MODEL
By the Middle Mesozoic, the Karakoram, West Qiangtang and Lhasa terranes, formed
Eurasia’s southern margin, while Argoland formed East Gondwana’s northern margin.
A subduction zone operated at the Eurasian margin to subduct the MesoTethys Ocean.
At ~155 Ma, Argoland began migrating north from Gondwana and the Lhasa terrane (L,
Fig. 3.6) began migrating west from the West Qiangtang terrane (Q, Fig. 3.6). An intra- oceanic arc (IA, Fig. 3.6) and subduction zone (thick dashed blue line, Fig. 3.6), advanced from the Lhasa terrane towards the equator. The central MesoTethys spreading ridge became extinct (light blue line, Fig. 3.6a) due to the initiation of the new spreading centre south of Argoland (thick grey line, Fig. 3.6a). The subduction zone was still located south of the Eurasian margin. This new spreading regime was isolated from the western MesoTethys (north of Africa) by a major transform fault, stretching north from East Africa to Iran.
At ~135 Ma (Fig. 3.6b), starting from a triple junction located off NW Australia,
Greater India and its northern indenter, the Gascoyne block, began to unzip from
Gondwana. The Lhasa terrane was still migrating southwest, creating a back-arc basin
(Kohistan-Ladakh) between the Lhasa and Karakoram-Qiangtang terranes. Argoland and the oceanic arc approached the equator and the Eurasian subduction zone was located south of the oceanic arc.
At ~125 Ma (fig. 3.6c), West Argoland collided with the western portion of the oceanic arc, causing the arc and Lhasa terrane, which became fixed to the arc, to retreat back to the Eurasian margin. Two subduction zones now operated simultaneously; one was
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located south of the Qiangtang terrane, subducting the back-arc basin to the Lhasa terrane. The other subduction zone, located south of the oceanic arc, subducted the
MesoTethys between Argoland and the arc.
By ~95 Ma (Fig. 3.3d), the Lhasa terrane had accreted to Eurasia. Both subduction zones were still operating, the trench south of the arc was obliquely subducting the
NeoTethys and MesoTethys, either side of Argoland. The other trench, now relocated to south of the Lhasa terrane, was obliquely subducting the back-arc ocean between the oceanic arc and Eurasia. Greater India began advancing towards the arc, which was retreating towards Eurasia. This followed relative motion between India and
Madagascar (MAD, Fig. 3.6) ~98 Ma, causing the curved Kerguelen and Wharton Basin fracture zones to form off East Antarctica and West Australia, respectively. Seafloor spreading, progressing from south to north, separated India from Madagascar between
94-84 Ma.
At ~55 Ma (Fig. 3.6e), Greater India collided with the oceanic arc, obliterating
Argoland and ending the subduction zone south of the arc. The other subduction zone was still operating south of the Lhasa terrane. The Gascoyne block (GB, Fig. 3.6e) either collided with the arc or with West Myanmar, depending on the geometry of the arc.
At ~35 Ma (Fig. 3.6f), Greater India collided with the Eurasian margin, starting from the west. NE Greater India obliterated the Gascoyne Block as it collided with Myanmar, which it began to shift northwards to its current location, southeast of the Lhasa terrane.
143 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
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Figure 3.6. Mercator-projected reconstructions of the Tethys and Indian Oceans at (a)
155 Ma, (b) 135 Ma, (c) 125 Ma and (d) 95 Ma (e) 55 Ma, (f) 35 Ma, constructed using
144 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
GPlates exported geometries with Australia fixed in present-day coordinates. Showing pseudofaults (light green lines), extinct ridges (light blue lines), COB and micro- continental fragments (thin black lin, filled in yellow), isochrons (red lines), spreading centres (thick dark brown lines). Countries are outlined in grey (some countries are not shown so that the Eurasia terranes are visible) and large igneous provinces are shown in red. Showing the 85°E Ridge (85°ER), 90°E Ridge (90°ER), Bruce Rise (BR), Conrad
Rise 4000 m isobath (CR), Crozet Hotspot (Cr), Elan Bank (EB), Intra-oceanic arc (IA),
Karakoram terrane (KK), Kerguelen Plateau (KP), Kerguelen Fracture Zone (KFZ),
Kohistan-Ladakh arc (KL), Laxmi Ridge (L), Lhasa block (LB), Madagascar (MAD),
Madagascar Ridge 3500 m isobath (MR), Naturaliste Plateau (NP), Seychelles (S) and
Sri Lanka (SL), Qiangtang terrane (Q). Black stars show the locations of hotspots featured in this study fixed in their present day locations, including Conrad (Co), Crozet
(Cr), Marion (M), and Kerguelen (K). For continental micro-fragments affiliated with the West Australian margin, please refer to Gibbons et. al., (submitted).
145 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
DISCUSSION
Our revised model of the Indian Ocean assimilates a variety of key geological observations from the India-Eurasia collision zone into a regional plate kinematic model
(Gibbons et al., submitted; Gibbons et al., in prep). Our Mesozoic Eurasian margin includes the Karakoram, West Qiangtang and Lhasa terranes. Metcalfe (2006) originally suggested that the Lhasa terrane rifted from Gondwana with the Argoland (West
Burma/Myanmar block) but this could not have accreted to the Eurasian margin before
~80 Ma, given this study’s motions for Argoland.
Yang et al. (2009) divide the Lhasa terrane into north and south segments based on eclogites located in the east, which they explained as a Permo-Triassic southward relocation of southward-directed subduction zone but this even predates the Cimmerian collision in the Mid-Late Triassic (Metcalfe, 2006). The crustal thickness of the Lhasa block, increasing from ~65 to 80 km going east to west across the dextral Jiali fault (Fig.
3.2), might also suggest two blocks but the effect has been linked to the ongoing convergence between India and Eurasia (Zhang and Klemperer, 2005). Due to magmatic belts and different sedimentary cover rocks, the Lhasa block was attributed to three ribbons (Zhu et al., 2011b). We suggest the magmatic belts may have arisen from subduction-related magmatic episodes and the oblique motion of the Lhasa terrane along Eurasia. Ophiolites striking northwest to southeast through the Lhasa terrane were interpreted as another suture (Girardeau et al., 1985; Matte et al., 1996), but our model supports the suggestion that they represent remnants of one giant ophiolite nappe rooted at the BNS (Chang et al., 1986b; Girardeau et al., 1984).
146 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
The Jurassic ophiolites (Fig. 3.2) appear to outcrop mainly at the eastern or western edges of the Eurasian terranes. These include the Eastern syntaxis ophiolites (Geng et al.,
2006), Nagaland (Baxter et al., 2011), Chin Hills, between India and West Myanmar,
(Mitchell, 1981), Zedong (McDermid et al., 2002) and Loubusa (Robinson et al., 2004), southeast of the Linzizong volcanics, Kiogar (Chan et al., 2007b) and Jungbwa (Miller et al., 2003), near the terminus of the Karakoram fault. Though this is outside of the scope of our model, we suggest their obduction could be related to the Late Triassic-
Jurassic accretion of the Cimmerian continent (Sengor, 1987), where the edges of the
Karakoram, West Qiangtang and Lhasa terranes converged against the SE Asian margin, in the east, and a transform fault separating the Western and Central MesoTethys
Oceans, in the west.
Studies report that the southern margin of the Lhasa terrane experienced Andean-type, calc-alkaline magmatism from the Jurassic to the Late Eocene (Scharer et al., 1984; Zhu et al., 2008; Zhu et al., 2006). This supports our subduction zone modeled at Lhasa’s southern margin from the time it rifted from Eurasia until it collided with India. A major regional unconformity between the folded and eroded Late Mesozoic Takena Formation
(e.g. Liu, 1988) and the gently-dipping Palaeocene-Eocene Linzizong volcanics (e.g.
Allegre et al., 1984; Coulon et al., 1986; Maluski et al., 1982) suggests that the majority of crustal thickening and shortening within the Lhasa terrane occurred before the final
India-Eurasia collision (e.g. England and Searle, 1986; Leier et al., 2007; Yin and
Harrison, 2000). This is supported by fission track ages from Cretaceous granitoids and
Jurassic metasediments from a peneplain in Tibet, which finds that cooling and exhumation of the granitoids occurred between 70 and ~55 Ma (Hetzel et al., 2011). A recent thermochronological study also suggests that the Tibetan plateau grew locally
147 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
from the Late Cretaceous, and spanned the region by 45 Ma (Rohrmann et al., 2012).
Our model correlates this to oceanic subduction beneath the Lhasa terrane.
The Shyok suture, between Karakoram and Kohistan-Ladakh, has been dated to the Late
Cretaceous, based on 111-62 Ma intrusions within Albian-Aptian limestones (Pudsey,
1986). Khan et al. (2009), date the suture to ~47-41 Ma and link it to the Yarlung–
Tsangpo suture, which they date ~51 Ma, but this is incompatible with the Kohistan-
Ladakh terrane’s Cretaceous magmatism. A collision would also explain why there is a
~104 Ma deformed and ~75 Ma undeformed group in the Kohistan batholith (Petterson and Windley, 1985). Treloar et al. (1996) suggest a ~100 Ma collision caused the granitiod plutomism at Matum Das, major intra-arc shear zones and a regionally- penetrative steep cleavage. Some studies suggest that the first evidence of the arc’s emplacement is the appearance of ophiolitic detritus in sedimentary units, including the
Palaeocene Chogdo Formation in the Zanskar valley (Searle et al., 1990) and Dibling
(Garzanti et al., 1987). This formation has been classified as mostly Asian-derived and therefore cannot related to Indian collision (Henderson et al., 2010a; Wu et al., 2007). In our model, the extinct spreading centre between the oceanic arc and Lhasa terrane was obliquely subducting along the Karakoram terrane margin 60 Ma, which could have caused obduction of the arc.
We adopt the Cretaceous age for the Shyok suture and model the Kohistan-Ladakh terrane to have formed as a result of the allochthonous Lhasa terrane rifting by ~155 Ma.
This is a good match to the Early Cretaceous radiolaria, recently collected near the
Spongtang massif, ~30 km south of Ladakh (Baxter et al., 2010). The Lhasa terrane may have rifted as early as the Lower Jurassic since Triassic-Jurassic bivalves in the Yeba
148 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
volcanic sediments, northeastern Lhasa terrane, show that marine conditions were prevalent ~180 Ma (Yin and Grant-Mackie, 2005). Field mapping and geochronological studies near Shiquanhe, far-west Lhasa terrane, uncovered the remnants of a subduction- accretion complex and forearc basin, which was attributed to the closure of the Late
Jurassic-Early Cretaceous Bangong-Nujiang Ocean (Kapp et al., 2003). Further east, metamorphism and magmatism in exposed Cambrian rocks in the Amdo basement, northeast Lhasa terrane (Fig. 3.2) indicate a continental arc formed ~185-170 Ma
(Guynn et al., 2006).
West Argoland collision
If no oceanic arc existed, the collision between Argoland and Eurasia would have occurred ~70 Ma, which may be supported by indirect evidence, such as the eruption of the calc-alkaline Lingzizong volcanics ~64 Ma (e.g. Chen et al., 2010) and the abrupt change in sedimentation from Cretaceous Xigase marine flysch to Eocene Qiuwa conglomerates in the Yarlung-Tsangpo suture (Searle et al. 1987). Leucograntie pegmatites in the North Karakoram, dated ~70-58 Ma (Rex et al. 1988), and the
Karakorum batholith intrusions, dated ~111-62 Ma (Pudsey, 1986), could also be contemporaneous, but are more likely related to oceanic subduction. However, since there is evidence of an equatorial intra-oceanic arc in the YTS (Aitchison et al., 2002a;
Aitchison et al., 2000), this feature is included in our model. An island arc complex can explain why there is no stratigraphic record of ophiolite obduction onto India’s northern margin during Cretaceous times (Sciunnach and Garzanti, 2012). It also helps explain why we find no direct evidence of Argoland at the Eurasian margin. This may be expected from a narrow sliver of stretched, underplated and sunken continental crust,
149 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
which was essentially wedged in the subduction trench before Greater India collided with the arc ~55 Ma, likely completely destroying any remnant of Argoland.
East Argoland collision
Argoland was considered part of the Sikuleh (Fig. 3.4), West Myanmar and West
Sulawesi continental fragment, which supplied sediments to northwest Timor during the
Triassic and Jurassic until it separated from NW Australia (Metcalfe, 1996). Our model suggest that East Argoland reached Sumatra ~80 Ma. This may be supported by geochronological studies, which show that the Malay Peninsula, Thailand and Indochina have remained uplifted throughout the Cenozoic following a ~90 Ma thermo-tectonic event (Hall, 2002). The Woyla Group (Fig. 3.4), along Sumatra’s southwestern margin, includes fragments of volcanic arc and imbricated oceanic crust, intruded by the Sikuleh
Batholith of northwest Sumatra. The Sikuleh batholith has been attributed to a marginal basin formed by oblique subduction (Cameron et al., 1980), and an allochthonous terrane whose collision overrode West Sumatra forming the Woyla Nappe in the
Albian-Aptian (Barber, 2000; Barber and Crow, 2009). The Manunggal Batholith, a composite of leucogranodiorite, granodiorite, granite and pyroxene-quartz diorite, intruded the Woyla Group in Natal (west-central Sumatra) ~87 Ma (Kanao, 1971, unpublished but quoted in Barber 2000). These may correspond to the East Argoland accretion though, as a thinned, underplated, sunken continental fragment, it may also have been located at the subduction trench.
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The Gascoyne block collision
The region commonly referred to as West Myanmar incorporates Arakan Yoma and
Central Myanmar, which are located west of the Sagaing fault (Fig. 3.4). Some models refer to West Myanmar (Burma) as Argoland (Metcalfe, 2006), which may have accreted there in the Late Cretaceous (Heine and Müller, 2005). Conversely, Arakan
Yoma (Fig. 3.5) is regarded by Fan and Ko (1994) as an Oliogocene uplifted subduction system consisting of Mesozoic flysch from an Early Cretaceous, eastward-dipping subduction zone. The subduction zone led to the formation of the Arakan Yoma Ranges, as well as the volcanic arc striking through Central Myanmar and a plutonic arc extending north from SW Thailand. The Indo-Burmese Ranges of Arakan Yoma
(Mitchell, 1993) and the NW Australian shelf (Gradstein, 1992; von Rad et al., 1992), share a Late Jurassic uplift erosional unconformity, which could have affected the
Gascoyne block. Halobia bivalve affinities between Timor and Myanmar also support a
Gondwana origin for West Myanmar (Charlton et al., 2009; McRoberts, 2010). The
Gascoyne block accretion to West Myanmar matches the emplacement of a ~50 Ma granitic batholith (Fan and Ko, 1994) but, since we cannot accurately constrain the far eastern extent of the NeoTethys island arc, the Gascoyne block may have accreted to the arc instead. Our modeled collision narrowly precedes a decrease in the convergence rate between India and Eurasia ~43 Ma (e.g. Cande et al. 2010). The Gascoyne block would likely have been destroyed by Greater India’s collision with Myanmar ~38 Ma.
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Greater India collisions
Many studies date the onset India-Eurasia collision to ~55-50 Ma (Garzanti et al., 1987;
Leech et al., 2005; Sciunnach and Garzanti, 2012; e.g. Searle et al., 1987). An abrupt change from Upper Cretaceous Xigaze marine flysch to Qiuwa conglomerates, the latter originating from the Gangdese batholith, was interpreted as a product of the India-
Eurasia collision (Searle et al., 1987). The latter study identified the Qiuwa conglomerates as Eocene, which only matches our India-arc collision, but stratigraphic relationships now indicate an Upper Oligocene-Lower Miocene age (Aitchison et al.,
2002b), which supports our model.
Younger ages for the Indus suture, between India and Kohistan-Ladakh, have also been proposed. The development of 60-40 Ma calc-alkaline plutons in two thirds of the
Kohistan batholith (Petterson and Windley, 1985) suggests that oceanic subduction continued until 40 Ma. A deformed granodiorite identified east of the Kohistan arc indicates an Upper Eocene/Lower Oligocene collision (Bard, 1983), which corresponds well to Greater India’s arrival. Due to the calc-alkaline lavas, sediments and Eocene fossils, covering the Kohistan batholith, Petterson and Windley (1985) dated the Indus
Suture to Eocene. They also Rb-Sr-dated layered aplite-pegmatite sheets intruding the
Kohistan batholith at ~34 and 29 Ma, identifying them as post-collisional but we suggest they might better be explained by a collision. Treloar et al. (1996) identify that younger plutons emplaced in the Khostan-Ladakh terrane until 40 Ma, before the Indus
Confluence granite sheets were emplaced ~34 ±14 Ma, again a good match to our collision at NW Greater India.
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Docking between India and Eurasia began simultaneously at Myanmar and Iran ~40 Ma but the main portion of Greater India did not reach the Eurasian margin, specifically at the Kohistan-Ladakh terrane, as modeled by van Hinsbergen (2011a), until ~35 Ma.
Suturing then progressed towards the east, reaching Nagaland at the East Indian syntaxis
~10 Ma. A ~35 Ma collision is supported by the apparent lack of Indian-plate input in the Indus Basin sedimentary rocks deposited in the Indus suture, between India and
Kohistan-Ladakh dated ~50 Ma (Henderson et al., 2010b).
A collision at the Eocene-Oligocene boundary also coincides with several significant climatic events, including abrupt cooling and glaciation in Antarctica (DeConto and
Pollard, 2003; Zachos and Kump, 2005), the disappearance of playa lake deposits in northeastern Tibetan plateau (Dupont-Nivet et al., 2007), and cooling and aridification in Asia (Ivany et al., 2000). Tibetan uplift was previously invoked to explain these events but this had already concluded by 38 Ma (Dupont-Nivet et al., 2008). Aitchison et al. (2007) also suggest a 35 Ma India-Eurasia collision with the identification of
Lower Eocene marine sediments in the Pengqu Formation near the Zephure mountains near the central YTS (Wang et al., 2002).
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The final docking for India coincides well with a small Indian plate deceleration at 25-
20 Ma (Molnar and Stock, 2009) and also matches the onset of Alio Shan shear zone/Red River fault separating West Myanmar from Indochina, which activated between 21 Ma (Searle, 2006) and 35 Ma (Gilley et al., 2003; Leloup et al., 2007;
Scharer et al., 1994; Zhang and Scharer, 1999).
154 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
CONCLUSION
A new tectonic model, incorporating a revision of potential field data in all the abyssal plains off West Australia and Antarctica, in a global framework, is the first to accurately include many anomalous tectonic features in the Indian Ocean in a regional tectonic model in a self-consistent way (Gibbons et al., submitted; Gibbons et al., in prep). New geological data offshore West Australia has also led to the identification of a northeast indenter to Greater India (Gibbons et al., submitted). This model also suggests that
Argoland was a ~5000 km-long continental sliver, which reached from Africa to Papua
New Guinea. These new constraints have implications for the timing of collision between Argoland, Greater India and the Eurasian margin. Evidence for these accretions should be prevalent along the Eurasian and SE Asian margins. Our investigation of literature of the geology of Eurasia and SE Asia has limited the possible scenarios for the collision events.
Argoland migrated north from Greater India and Australia ~156 Ma, while the Lhasa block rifted from Eurasia, forming Xigaze fore-arc and Kohistan-Ladakh back-arc, initiating the Zedong oceanic arc. Greater India and the Gascoyne block, a northeast indenter originally conjugate to the Exmouth Plateau, began rifting from East
Gondwana ~136 Ma. West Argoland collided equatorially with the (Zedong-Bainang-
Dazhuqu) oceanic arc ~126 Ma. Greater India migrated west until a ~98 Ma spreading reorganization initiated its separation from Madagascar, progressing northward from 94-
84 Ma. East Argoland reached Sumatra ~80 Ma. Greater India collided with the Zedong arc ~55 Ma, destroying evidence of Argoland. The Gascoyne block reached the eastern end of the oceanic arc, near Myanmar, ~50 Ma. Greater India finally accreted to the
155 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 3 - Eurasian margin
Eurasian margin from ~35, suturing from west to east, likely destroying evidence of the
Gascoyne block.
Our model of East Gondwana breakup satisfies rigorous constraints from the West
Australian and East Antarctic margins, comprising a regional plate-tectonic framework.
Our model can also account for direct geological evidence of the Early-Mid Cretaceous obuduction of a Neotethyan oceanic arc, as well as the youngest marine sediments to be found between India and Eurasia, and explains the high-impact climatic events, dated to the Oligocene-Eocene boundary.
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