Decoupled Lithospheric Folding, Lower Crustal Flow Channels, and the Growth of Tibetan Plateau (original) (raw)
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Large-scale crustal deformation of the Tibetan Plateau
Journal of Geophysical Research, 2001
The topography, velocity, and strain fields calculated from a three-dimensional Newtonian viscous model for large-scale crustal deformation are generally in good agreement with results from geological, geodetic and earthquake studies in and around the Tibetan Plateau, provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau (equivalent to a viscosity of 10 12 Pa s within a 250-mthick channel or 10 18 Pa s within a 15-km-thick channel). Model studies and observations show a plateau at ϳ5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins. Rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins (at rates ϳ20 mm/yr). Model results show north-south shortening (ϳ10 mm/yr) in reasonable agreement with GPS data (5-8 mm/yr of north-south shortening across the northern two thirds of the plateau) and east-west stretching (10 -15 mm/yr) across the eastern half of the high plateau, in reasonable agreement with seismic, geologic, and GPS data. Upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east. Clockwise rotation of material around the east Himalayan syntaxis (at rates up to ϳ10 mm/yr) occurs partly as a result of dextral shear between Indian and Asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east. There is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust, except in southern and northern Tibet where margin-perpendicular extension occurs only for the case of an extremely weak lower crust. Our results suggest that the Tibetan Plateau is likely to have gone through a twostage development. The first stage produced a long, narrow, high orogen whose height may have been comparable to the modern plateau. The second stage produced a plateau that grew progressively to the north and east. East-west stretching, eastward plateau growth and dextral rotation around the east Himalayan syntaxis probably did not begin until well into the second stage of plateau growth, perhaps becoming significant after ϳ20 m.y. of convergence.
Earth, Planets and Space, 2014
A number of tectonic models have been proposed for the Tibetan Plateau, which origin, however, remains poorly understood. In this study, investigations of the shear wave velocity (Vs) and density (ρ) structures of the crust and upper mantle evidenced three remarkable features: (1) There are variations in Vs and ρ of the metasomatic mantle wedge in the hanging wall of the subduction beneath different tectonic blocks of Tibet, which may be inferred as related to the dehydration of the downgoing slab. (2) Sections depicting gravitational potential energy suggest that the subducted lithosphere is less dense than the ambient rocks, and thus, being buoyant, it cannot be driven by gravitational slab pull. The subduction process can be inferred by the faster SW-ward motion of Eurasia relative to India as indicated by the plate motions relative to the mantle. An opposite NE-ward mantle flow can be inferred beneath the Himalaya system, deviating E and SE-ward toward China along the tectonic equator. (3) The variation in the thickness of the metasomatic mantle wedge suggests that the leading edge of the subducting Indian slab reaches the Bangoin-Nujiang suture (BNS), and the metasomatic mantle wedge overlaps with a region with poor Sn-wave propagation in north Tibet. The metasomatic layer, north of the BNS, deforms in the E-W direction to accommodate lithosphere shortening in south Tibet.
Southward extrusion of Tibetan crust and its effect on Himalayan tectonics
2001
The Tibetan Plateau is a storehouse of excess gravitational potential energy accumulated through crustal thickening during India-Asia collision, and the contrast in potential energy between the Plateau and its surroundings strongly influences the modern tectonics of south As•a. The distribution of potential energy anomalies across the region, derived from geopotential models, indicates that the H•malayan front is the optimal location for focused dissipation of excess energy stored in the Plateau. The modem pattern of deformation and erosion in the Himalaya provides an efficient mechanism for such dissipation, and a review of the Neogene geological evolution of southern Tibet and the Himalaya shows that this mechanism has been operational for at least the past 20 million years. This persistence of deformational and erosional style suggests to us that orogens, like other complex systems, can evolve toward "steady state" configurations maintained by the continuous flow of energy. The capacity of orogenic systems to self-organize into temporally persistent structural and erosional patterns suggests that the tectonic history of a mountain range may depend on local energetics as much as it does on far-field plate interactions.
2008
The Tibetan Plateau is the product of crustal thickening caused by collision between India and Asia. Plate tectonic reconstructions suggest continuous northward movement of the Indian plate relative to stable Eurasia at nearly 50 mm/yr for the last 50 My. The plateau is now at~5 km elevation with steep topographic gradients across the southern and northern margins. These steep topographic gradients are also related to large lateral variations in the geoid and gravity anomalies. In a SSW to NNE cross section, the Bouguer gravity anomaly decreases over a distance of 500 km from about 0 mGal in the India plate to~−500 mGal on the Tibetan Plateau. The geoid anomaly also presents steep gradients on both the Himalayan front and the northern margin, reaching values between 20-30 m on the plateau, suggesting a pronounced thinning of the lithospheric mantle. Uplift late in the tectonic evolution of the plateau, the widespread extension, and the associated magmatism have been attributed to convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere. Here we present a twodimensional lithospheric thermal and density model along a transect from the Indian plate to Asia, crossing the Himalaya front and the Tibetan Plateau. The model is based on the assumption of local isostatic equilibrium, and is constrained by the topography, gravity and geoid anomalies and by thermal data within the crust. Our results suggest that the height of the Tibetan Plateau is compensated by thick crust in the south and by hot upper mantle to the north. The Tibetan Plateau as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the northern plateau is required to explain the high topography, gravity, geoid and crustal temperatures. The lithosphere reaches a maximum depth of~260 km beneath the southern Plateau, and thins abruptly northward to~100 km under the central and northern Plateau. The lithosphere depth increases again beneath the Qaidam basin and the Qilian Shan to~160 km.
Propagation of the deformation and growth of the Tibetan–Himalayan orogen: A review
Earth-Science Reviews, 2015
Long-standing problems in the geological evolution of the Tibetan-Himalayan orogen include where the India-Asia convergence was accommodated and how the plateau grew. To clarify these problems, we review the deformations and their role in the plateau's growth. Our results show that ~1630 km of shortening occurred across the Tibetan-Himalayan orogen since ~55 Ma, with more than ~1400 km accommodated by large-scale thrust belts. These thrust belts display an outward expansion from central Tibet and couple with the surficial uplift. The development of the Tibetan plateau involved three significant steps: Primitive plateau (~90-55 Ma), Proto-plateau (~55-40 Ma), and Neoteric plateau (~40-0 Ma). Several processes have collaborated to produce the Proto-plateau, including the pre-existing Primitive plateau, the India-Asia collision, and subductions of Greater India and Songpan-Ganzi beneath the Lhasa-Qiangtang terrane. Since ~40 Ma, the Proto-plateau, which was dominated by a topographic gradient, lower crustal flow and continuous India-Asia convergence, experienced three periods of rapid outward growth (~40-23, ~23-10, and ~10-0 Ma) in general. The N-S trending rifts were caused by the eastward growth of
Crustal-lithospheric structure and continental extrusion of Tibet
Journal of the …, 2011
Crustal shortening and thickening to c. 70–85 km in the Tibetan Plateau occurred both before and mainly after the c. 50 Ma India–Asia collision. Potassic–ultrapotassic shoshonitic and adakitic lavas erupted across the Qiangtang (c. 50–29 Ma) and Lhasa blocks (c. 30–10 Ma) indicate a hot mantle, thick crust and eclogitic root during that period. The progressive northward underthrusting of cold, Indian mantle lithosphere since collision shut off the source in the Lhasa block at c. 10 Ma. Late Miocene–Pleistocene shoshonitic volcanic rocks in northern Tibet require hot mantle. We review the major tectonic processes proposed for Tibet including ‘rigid-block', continuum and crustal flow as well as the geological history of the major strike-slip faults. We examine controversies concerning the cumulative geological offsets and the discrepancies between geological, Quaternary and geodetic slip rates. Low present-day slip rates measured from global positioning system and InSAR along the Karakoram and Altyn Tagh Faults in addition to slow long-term geological rates can only account for limited eastward extrusion of Tibet since Mid-Miocene time. We conclude that despite being prominent geomorphological features sometimes with wide mylonite zones, the faults cut earlier formed metamorphic and igneous rocks and show limited offsets. Concentrated strain at the surface is dissipated deeper into wide ductile shear zones.
Geological Society, London, Special Publications, 2006
Many seismic and magnetotelluric experiments within Tibet provide proxies for lithospheric temperature and lithology, and hence rheology. Most data have been collected between c. 888E and 958E in a corridor around the Lhasa-Golmud highway, but newer experiments in western Tibet, and inversions of seismic data utilizing wave-paths transiting the Tibetan Plateau, support a substantial uniformity of properties broadly parallel to the principal Cenozoic and Mesozoic sutures, and perpendicular to the modern NNE convergence direction. These data require unusually weak zones in the crust at different depths throughout Tibet at the present day. In southern Tibet these weak zones are in the upper crust of the Tethyan Himalaya, the middle crust in the southern Lhasa terrane, and the middle and lower crust in the northern Lhasa terrane. In northern Tibet, north of the Banggong-Nujiang suture, the middle and probably the lower crust of both the Qiangtang and Songpan-Ganzi terranes are unusually weak. The Indian uppermost mantle is cold and seismogenic beneath the Tethyan Himalaya and the southernmost Lhasa terrane, but is probably overlain by a northward thickening zone of Asian mantle beneath the northern Lhasa terrane. Beneath northern Tibet the upper mantle has not been replaced by subducting Indian and Asian lithospheres, and is warmer than to the south. These inferred vertical strength profiles all have minima in the crust, thereby permitting, though not actually requiring, some form of channelized flow at the present day. Using the simplest parameterization of channel-flow models, I infer that a Poiseuille-type flow (flow between stationary boundaries) parallel to India -Asia convergence is occurring throughout much of southern Tibet, and a combination of Couette (top-driven, between moving boundaries) and Poiseuille lithospheric flow, perpendicular to lithospheric shortening, is active in northern Tibet. Explicit channel-flow models that successfully replicate much of the large-scale geophysical behaviour of Tibet need refinement and additional model complexity to capture the full details of the temporal and spatial variation of the India-Asia collision.
Is the Underthrust Indian Lithosphere Split beneath the Tibetan Plateau?
International Geology Review, 2007
Detailed examination of geologic, and geophysical data from the Himalayan orogen and Tibetan Plateau reveals noticeable differences in lithospheric structure, magmatism, and deformation styles between the western and eastern Tibetan plateaus, challenging the widely accepted single-slab model of underthrusting of the Indian plate beneath Eurasia. We propose a slab tear model in which the Indian lithosphere has split into two slabs: a northward-moving slab subducting steeply beneath the western sector of the Plateau, and a northeastward-moving slab subducting at a low angle beneath the eastern sector of the Plateau and the Three Rivers region (Lanchangjiang, Jinshajiang, and Nujiang rivers). The tectonic boundary between these realms is identified as the Yadong-Anduo-Golmud (YAG) tectonic corridor.
Earth-Science Reviews, 2005
Cenozoic magmatism on the Tibetan plateau shows systematic variations in space and time that must be considered in models concerning Tibetan tectonic evolution. After the India -Asia collision, which started in the early Tertiary and terminated the Gangdese arc magmatism in the Lhasa terrane made of the southern Tibetan plateau, widespread potassium-rich lavas and subordinate sodium-rich basalts were generated from f 50 to 30 Ma in the Qiangtang terrane of northern Tibet. Subsequent post-collisional magmatism migrated southwards, producing ultrapotassic and adakitic lavas coevally between f 26 and 10 Ma in the Lhasa terrane. Then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f 13 Ma in the western Qiangtang and Songpan -Ganze terranes. Such spatial -temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the Indian continental lithospheric mantle started thrusting under Asia by involving rollback and breakoff of the subducted Neo-Tethyan slab followed by removal of the thickened Lhasa lithospheric root. We propose that only after the lithospheric removal, which occurred at f 26 Ma, could the Indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the Himalayan -Tibetan orogenesis. Consequently, the Tibetan plateau is suggested to have risen diachronously from south to north. Whereas the southern part of the plateau may have been created and maintained since the late Oligocene, the northern plateau would have not attained its present-day elevation and size until the mid-Miocene when the lower part of the western Qiangtang and Songpan -Ganze lithospheres began to founder owing to the push of the underthrust Indian mantle lithosphere. D