Indus-Yarlung Suture Research Articles

Reply to comment by Eduardo Garzanti on “When and where did India and Asia collide?”

] We thank Eduardo Garzanti for his comment[Garzanti, 2008] on our recent paper [Aitchison et al.,2007]. A traditional approach is to apply Occam’s razorand view our planet’s most significant orogenic zone as aresponse to closure of a one-ocean, two-continent conver-gence system. However, a vast body of new data has beenobtained in the decades following widespread acceptance ofthis conservative model by the scientific community. Wecontend that significant details are lost with this conven-tional view. Critically, it is now clear that the northernmargin of India experienced not one but two quite discretecollisions during the Paleogene.[

Geological records of the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of central Tibet

A geological and geochronologic investigation of the Nima area along the Jurassic–Early Cretaceous Bangong suture of central Tibet (∼32°N, ∼87°E) provides well-dated records of contractional deformation and sedimentation during mid-Cretaceous and mid-Tertiary time. Jurassic to Lower Cretaceous (≤125 Ma) marine sedimentary rocks were transposed, intruded by granitoids, and uplifted above sea level by ca. 118 Ma, the age of the oldest nonmarine strata documented. Younger nonmarine Cretaceous rocks include ca. 110–106 Ma volcanic-bearing strata and Cenomanian red beds and conglomerates. The Jurassic–Cretaceous rocks are unconformably overlain by up to 4000 m of Upper Oligocene to Lower Miocene lacustrine, nearshore lacustrine, and fluvial red-bed deposits. Paleocurrent directions, growth stratal relationships, and a structural restoration of the basin show that Cretaceous–Tertiary nonmarine deposition was coeval with mainly S-directed thrusting in the northern part of the Nima area and N-directed thrusting along the southern margin of the basin. The structural restoration suggests >58 km (>47%) of N–S shortening following Early Cretaceous ocean closure and ∼25 km shortening (∼28%) of Nima basin strata since 26 Ma. Cretaceous magmatism and syncontractional basin development are attributed to northward low-angle subduction of the Neotethyan oceanic lithosphere and Lhasa-Qiangtang continental collision, respectively. Tertiary syncontractional basin development in the Nima area was coeval with that along the Bangong suture in westernmost Tibet and the Indus-Yarlung suture in southern Tibet, suggesting simultaneous, renewed contraction along these sutures during the Oligocene-Miocene. This suture-zone reactivation immediately predated major displacement within the Himalayan Main Central thrust system shear zone, raising the possibility that Tertiary shortening in Tibet and the Himalayas may be interpretable in the context of a mechanically linked, composite orogenic system.

Continuous Metamorphic Zircon Growth and Interpretation of U-Pb SHRIMP Dating: An Example from the Western Himalaya

Ultrahigh-pressure (UHP) rocks in the northwest Himalaya are some of the youngest on earth, and allow testing of critical questions of UHP metamorphism and exhumation and the India-Asia collision. The Tso Morari Complex (TMC) is a UHP subduction-zone complex in eastern Ladakh in the western Himalaya, south of the Indus-Yarlung suture zone. U-Pb SHRIMP dating of zircon shows the TMC has a Proterozoic protolith, preserves a Pan-African magmatic history, and shows continuous metamorphic zircon growth during the Early to Middle Eocene, hence constraining the timing of collision, subduction, and exhumation in the western Himalaya. Zircon dating indicates that UHP metamorphism occurred at 53.3 ± 0.7 Ma, followed by 8 m.y. of continual zircon crystallization to amphibolite-facies metamorphic conditions at 45.2 ± 0.7 Ma. Similar continuous zircon growth during UHP metamorphism and through early exhumation to amphibolite-facies conditions occurs in other UHP subduction complexes, including the Sulu terrane, where coesite-bearing inclusions within dated zircon prove conclusively that zircon dates record UHP metamorphism. U-Pb SHRIMP dating of zircon for both the TMC and the Sulu belt demonstrate that zircon continues to crystallize at temperatures 400° ± 50°C based on 40Ar/39Ar dating yielding the same ages.

Discovery and significance of Cretaceous fossils from the Xigaze Forearc Basin, Tibet

The Xigaze Cretaceous forearc basin located between the Lhasa Block and Indus-Yarlung Suture was filled with flysch deposits during the mid-Cretaceous, as the Tethys oceanic crust was being subducted underneath Eurasia. The flysch comprises the Xigaze Group, dominated by the Angren Formation. Well preserved, newly discovered foraminifera within the Angren Formation constrain its age to late Albian to late Coniacian. Radiometric dating of the ophiolite belt impinging on the southern edge of the forearc basin yielded an age of 120+10 Ma, which implies that the deposition in the forearc basin occurred shortly after initiation of the subduction of the Neotethys.

Structural elements of the southern Tethyan Himalaya crust from wide‐angle seismic data

A deep seismic common midpoint (CMP) profile, shot across the southern margin of the Tethyan Himalaya as the first stage of Project International Deep Profiling of Tibet and the Himalaya (Indepth), imaged the top (Main Himalayan Thrust or MHT) and bottom (Moho) of the Indian continental crust underthrusting the Himalaya. We used portable seismographs to record the CMP‐profile shots at a wide range of offsets, up to 155 km. Our short‐offset data corroborate the CMP‐profile data, while our large‐offset data are dominated by a band of reflectivity. We interpret the bright onset of this reflective band as the basal detachment of the South Tibetan Detachment system and a phase at the end of the reflective band as the MHT. We used the CMP‐profile first‐break data to model the uppermost 2 km in detail. The depth of young extensional basins of the Yadong rift system along the CMP profile was constrained to a maximum of 2 km, yielding a throw of 4.6 km across the eastern flank of the rift and a rough estimate of approximately 1.5% east‐west extension by normal faulting in the Tethyan Himalaya. The wide‐angle data were used to construct a crustal seismic‐velocity model down to the MHT. The South Tibetan Detachment System (STDS) basal detachment reflector is observed dipping 12.5°NNE from a depth of about 6 km beneath the surface under the south end of the CMP profile to a depth of 22 km, then flattening to a dip of only 2.5°NNE. Thus our observations suggest that the STDS basal detachment is a deep‐rooted basement fault. For the MHT we observe a dip of 7.5°NNE from 20 km depth below sea level at the crest of the High Himalaya crystalline sheet to 36 km below sea level (40 km beneath the surface) at a distance of about 70 km south of the Indus‐Yarlung Suture (IYS). From geometrical arguments we suggest that Indian crust underthrusts the surface expression of the IYS within the crust but does not form the lower crust of central and northern Tibet.

The Xigaze forearc basin: evolution and facies architecture (Cretaceous, Tibet)

The mid-Cretaceous to Eocene flysch deposits of the Xigaze forearc basin in southern Tibet were investigated in a 120 km segment along the Indus-Yarlung suture zone. The basin evolved south of the magmatic arc (Gangdise belt) of the Lhasa block on top of trapped oceanic or transitional crust. Remnants of shelf carbonates are preserved along the northern edge of the basin. The mapped segment was shortened by about 65%; metamorphism reached low-grade conditions along the northern margin of the basin. The flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic (andesitic to dacitic) material shed from the magmatic-arc Gangdise belt. Particularly in the western part of the study area, plutonic and sedimentary rocks from deep erosion levels and/or more distal sources contributed to the basin fill. Rivers from the Lhasa block acted as point sources and fed five major deep-sea channel systems. Turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone, thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing. The forearc flysch is subdivided into at least three megasequences, which begin with wide (up to several km) incised (10 to 50 m), coarse-grained channel fills and their associated fan deposits. The upper parts of the megasequences contain hemipelagic dark shales and marls (deposited above the calcite compensation depth). Lateral channel migration, channel-lobe switching, but also volcanic pulses generated a predominantly fining-upward, high-frequency cyclicity. After continental collision, the marine sedimentation in the forearc basin was replaced by fluvial deposits of the Eocene-Oligocene Qiuwu formation, which is time-equivalent to the Kailas and Indus molasses farther west and rich in coarse gravel derived from the Gangdise belt. Both forearc flysch and Qiuwu formation were deformed simultaneously during the Miocene. We assume that the molasse-type Qiuwu formation represents the final continental facies of the forearc basin filling.

Mesozoic neo‐tethyan pre‐orogenic deep marine sediments along the Indus–Yarlung Suture, Himalaya

Shelf, forereef and basin margin (slope) olistoliths (Exotic blocks of limestone) of Permian–Jurassic age are tectonically juxtaposed within the Triassic to Eocene age pre‐orogenic, deep abyssal plain turbidites of the Lamayuru. The pre‐collision tectonic setting and depositional environment of the limestone olistoliths can be reconstructed from within the neighbouring Zanskar range. The disorganized Ophiolitic Melange Zone, an association of different tectonic rock slivers of Jurassic–Eocene age, is tectonically underlain by the overthrusted Lamayuru Formation and tectonically overlain by the Nindam Formation. Tectonic slivers of Late Jurassic–Early Cretaceous age red radiolarian cherts represent a characteristic lithotectonic unit of the Ophiolitic Melange Zone, those occurring near the contact zone with the Lamayuru Formation, were deposited within the neo‐Tethyan deep‐ocean floor of the Indian passive margin below the carbonate compensation depth. These tectonic slivers accumulated along the northern margin of the Indus–Yarlung Suture Zone of the Ladakh Indian Himalaya during subduction accretion associated with the initial convergence of the Indian plate beneath the Eurasian plate.

On the Permian biogeographic boundary between Gondwana and Eurasia in Tibet, China as the eastern section of the Tethys

As is well known, Glossopteris as a cool climate flora and the bivalve Eurydesma, the rugose coral Lytvolasma, the brachiopod Globiella and the fusulinid Monodiexodina as a cold-cool water fauna often occur with tillites and tilloids of glacial-glaciomarine origin or else closely following such rocks in the northern margin of the Godwanaland in Early Permina times (Asselian-Sakmarian). Therefore, they are usually considered as the indicators of the Gondwana Tethys facies. The case of the Gondwana Tethys is quite different from that of the Cathayan Tethys as the southern margin of Eurasia. The biota of south China and the Kunlun Mountains belonging to the Cathayan Tethys is characterized by tropical flora and warm water fauna. Evidently south China and the Kunlun Mountains area were covered by a warm water sea during the whole Permian period and perhaps from the late Middle Permian stage onwards the seawater became tropical. The Indus-Yarlung suture is the biogeographic boundary between Gondwana and Eurasia based on the discoveries of Glossopteris flora and Lytvolasma fauna in Qubu and Kangmar, southern Tibet. Recently, with the rapid advance of geological and palaeontological data the cool climate flora and the cold-cool water fauna with glaciomarine deposits have been found in Early Permian rocks in many localities of the Gangdise-Nyainqentanglha and Qiantang blocks. Therefore, they definitely belong to the epeiric sea of the marginal Gondwanaland. The biogeographic boundary between Gondwana and Eurasia should be moved from the Yarlung suture and the Bangongco-Nujiang suture northwards to the Longmuco-Yushu suture (i.e., the boundary passes between the Songban-Ganze block and the Qiangtang block.) It is very interesting that the Permian biota of Tibet such as that of the Salt Range shows the three developmental stages: the cold-cool water type, the mixture type and the tropical-warm water type. The change of flora and fauna might be caused by the Gondwanaland glacial action in Early Permian times (Asselian-Sakmarian), the worldwide warming of climate and the rifting and rapidly northwards drifting of Gondwanaland during the early Middle Permian (Artinskian). In the late Middle Permian the Gondwana Tethys became still warmer and even tropical and the typical warm tropical fauna of Neoschwagerina and Verbeekina quickly replaced the cool water one.

Oxygen isotopes in mantle related and geothermally altered magmatites of the Transhimalayan (Gangdese) ranges

In closed magma systems SiO2 approximately measures differentiation progress and oxygen isotopes can seem to obey Rayleigh fractionation only as a consequence of the behaviour of SiO2. The main role of δ18O is as a sensitive indicator of contamination, either at the start of differentiation (δ18Oinit) or as a proportion of fractionation in AFC. Plots of δ18O vs SiO2-allow to determine initial δ18O values for different sequences for source comparison. For NBS-28=9.60, the δ18O at 48% SiO2-varies between a high 6.4‰ for Kiglapait (Kalamarides 1984), 5.9‰ for Transhimalaya, 5.8‰ for Hachijo-Jima (Matsuhisa 1979), 5.6‰ for Koloula (Chivas et al. 1982) and a low 5.3‰ for the Darran Complex, New Zealand. The Transhimalayan batholiths (Gangdese belt) were emplaced in the ‘Ladakh-Lhasa terrane’, between the present-day Banggong-Nujiang, and Indus-Yarlung Tsangbo suture zones, after its accretion to Eurasia. The gradient of the least contaminated continuous (δ18O vs SiO2-igneous trend line is similar to that of Koloula, and AFC calculations suggest a low secondary assimilation rate of less than 0.05 times the rate of crystallisation. Outliers enriched in 18O are frequent in the Lhasa, and apparently rare in the Ladakh transsect. Low-δ18O (5.0‰−0‰) granitoids and andesites on the Lhasa-Yangbajain axis are the result of present day or recent near-surface geothermal activity; their quartzes still trace the granitoids to the Transhimalaya δ18O trend line, but the distribution of low total rock or feldspar δ18O values could be a guide to more recent heat flow and thermally marked tectonic lineaments. Two ignimbrites from Maqiang show hardly any 18O-contamination by crustal material.

Indian Gondwana Plate margin and its evolutionary history

The outline of Indian Plate in the Gondwanaland Plate mosaic has been reconstructed. The basic premise for the reconstruction lies in the identification of the suture zone along Indus-Yarlung tectonic zone and Indo-Burman range, both of which are wreathed with ophiolite complexes. The north eastern margin of the Indian part of the Gondwana Plate, which was ill-defined in many earlier reconstructions, is now more precisely delineated with the find of slide-generated olistostrom bodies representing plate marginal trench setting around Ukhrul-Paoyi-Kiphire area of the ophiolite belt of Manipur-Nagaland. The recent report of continental Gondwanas close to this suture zone lends credence to this palaeogeographic reconstruction. On the north, the continental sediments having distinct Gondwana entity rarely extend to the Tethyan basin and as such the Indus-Yarlung Suture truly delimits the Gondwana Plate domain. The Himalayan front is regarded as Tethys-facing margin of the Gondwana continent. Along the eastern margin of Indian Plate, rifting as a sequel to ocean floor spreading led to the evolution of coastal troughs of Cauvery, Palar, Godavari-Krishna and Athgarh which bears records of marine transgressions during Aptian-Albian time from a juvenile Indian Ocean. These oceanward tilted troughs may represents the rifted arm of a triple junction formed during the continental fragmentation. The discovery of such troughs in the Upper Assam and Bengal Basin suggests that the separation of India from Eastern Gondwanaland occurred in a NE-SW direction. The Cambay and the Kutch basins document similar evolutionary history along the western margin of the Indian Plate. As a consequence of crustal tension accompanying the fragmentation, the outpour of tholeiitic basalt took place in Rajmahal, Khasi-Garo-Mikir Hills and Upper Assam at 100-105 million years along the west coast. The earliest manifestation of volcanism has been recorded in Saurashtra which is considered to be contemporaneous with Rajmahal volcanicity. It is suggested that both the eastern and western margins of the Indian Gondwana Plate bear closely related records of fragmentation in the Early Cretaceous time.