Thinning and Flow of Tibetan Crust Constrained by Seismic Anisotropy

Shapiro, N. M.; Ritzwoller, M. H.; Molnár, Péter; Levin, Vadim

doi:10.1126/science.1098276

Cited by 291 publications

(300 citation statements)

References 26 publications

(7 reference statements)

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“…Several recent Himalayan-Tibetan tectonic investigations have proposed a weak middle-lower crust, which allows crustal flow, to explain the building of the southeastern margin of the Tibetan Plateau [Clark and Royden, 2000;Beaumont et al, 2001;Schoenbohm et al, 2006]. This idea is supported by results from geophysical studies that show evidence for localized partial melt within the middle crust in Tibet [Kind et al, 1996;Wei et al, 2001] and a radial anisotropy that can be caused by channel flow within the mid-to-lower Tibetan crust [Shapiro et al, 2004]. However, contrary to the notion that the north -south rift zones in the Tibetan Plateau are shallow features, formed by the eastward motion of the shallow crust that are decoupled from the mantle lithosphere by a low-viscosity lower crust, our results suggest an upper mantle origin of the rift zones, at least in southeastern Tibet, where mantle lithosphere delamination plays a key role in the process of the rise of the plateau.…”

Section: Resultssupporting

confidence: 71%

Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts

Ren

Shen

2008

J. Geophys. Res.

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[1] While several mechanisms have been suggested to explain the evolution of the Tibetan Plateau, observational constraints on the deep lithospheric processes have been sparse, and previous seismic studies were mostly along profiles perpendicular to the collision front of the Indian and Eurasian plates. In this study, we show tomographic evidence for the delamination of the mantle lithosphere beneath southeastern Tibet, a process in which the entire mantle lithosphere peels away from the crust along the Moho and thus is a mechanism for rapid thinning of the lithosphere. Our P and S wave velocity models show the presence of a low-velocity anomaly in the crust and upper mantle down to $300 km depth beneath a north-south trending rift zone in southeastern Tibet. This low-velocity anomaly is situated above a tabular, high-dipping-angle, high-velocity anomaly that extends into the upper mantle transition zone. The V P /V S ratio of this high-velocity anomaly suggests that temperature variations are not the only cause of the anomaly and a highly melt-depleted mantle is required. These observations suggest a causal relationship between the delamination of mantle lithosphere and the formation of the north-south trending rift in southeastern Tibet.

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Section: Resultssupporting

confidence: 71%

Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts

Ren

Shen

2008

J. Geophys. Res.

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“…Radial anisotropy (the difference between the vertically and horizontally polarized waves: V SV and V SH , respectively, in the case of S waves) is also well documented (e.g., Shapiro et al 2004;Huang et al 2010;Duret et al 2010;Guo et al 2012;Xie et al 2013;Li et al 2016). …”

Section: Azimuthal and Radial Anisotropy Beneath Tibet: A Brief Synthmentioning

confidence: 97%

“…Radial anisotropy in the middle and lower crust is thought to be primarily due to near-horizontal alignment of the sheet-like crystals of mica (e.g., biotite and muscovite) (e.g., Shapiro et al 2004;Meissner et al 2006). It can be large: V SH exceeds V SV by up to ∼10 per cent in central Tibet (e.g., Shapiro et al 2004;Duret et al 2010;Agius & Lebedev 2014).…”

Section: Origin Of Anisotropy and Its Relationship To Deformationmentioning

confidence: 99%

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Complex, multilayered azimuthal anisotropy beneath Tibet: evidence for co-existing channel flow and pure-shear crustal thickening

Agius

Lebedev²

2017

Geophysical Journal International

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SUMMARYOf the two debated, end-member models for the late-Cenozoic thickening of Tibetan crust, one invokes "channel flow" (rapid viscous flow of the mid-lower crust, driven by topography-induced pressure gradients and transporting crustal rocks eastward) and the other-"pure shear" (faulting and folding in the upper crust, with viscous shortening in the mid-lower crust). Deep-crustal deformation implied by each model is different and would produce different anisotropic rock fabric. Observations of seismic anisotropy can thus offer a discriminant. We use broadband phase-velocity curves-each a robust average of tens to hundreds of measurements-to determine azimuthal anisotropy in the entire lithosphere-asthenosphere depth range and constrain its amplitude. Inversions of the differential dispersion from path pairs, region-average inversions and phase-velocity tomography yield mutually consistent results, defining two highly anisotropic layers with different fast-propagation directions within each: the middle crust and the asthenosphere.In the asthenosphere beneath central and eastern Tibet, anisotropy is 2-4 per cent and has a NNE-SSW fast-propagation azimuth, indicating flow probably driven by the NNEward, shallow-angle subduction of India. The distribution and complexity of published shear-wave splitting measurements can be accounted for by the different anisotropy in the mid-lower crust and asthenosphere. The estimated splitting times that would be ac-2 M.R. Agius and S. Lebedev cumulated in the crust alone are 0.25-0.8 s; in the upper mantle-0.5-1.2 s, depending on location. In the middle crust (20-45 km depth) beneath southern and central Tibet, azimuthal anisotropy is 3-5 and 4-6 per cent, respectively, and its E-W fast-propagation directions are parallel to the current extension at the surface. The rate of the extension is relatively low, however, whereas the large radial anisotropy observed in the middle crust requires strong alignment of mica crystals, implying large finite strain and consistent with high-rate horizontal flow. Together, radial and azimuthal anisotropy suggest eastward mid-crustal channel flow in central Tibet, along the regional topography gradient.In NE high Tibet, mid-crustal azimuthal anisotropy is 4-8 per cent and has WNW-ESE and NW-SE fast-propagation directions, parallel to the net extension at the surface. These fast directions are inconsistent with channel flow following the SW-NE regional topography gradient. Instead, they suggest similar net deformation in the (decoupled) shallow and deep crust. In the brittle upper crust, it is accommodated by strike-slip faulting; in the ductile mid-lower crust-by shear oriented at ∼45 • to the faults. Although mid-crustal flow beneath NE Tibet may transport some material towards the plateau periphery at a low region-average rate, the dominant mid-crust deformation pattern is shear parallel to the plateau boundary. This implies that channel flow from central Tibet is not the main cause of the on-going crustal thickening farther northeast.

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“…The channel flow model is based on both geological constraints from the Greater Himalaya and geophysical constraints from the INDEPTH seismic experiments in southern Tibet (Zhao et al 1993;Nelson et al 1996). Intermediate-period Rayleigh and Love waves propagating across Tibet indicate a radial anisotropy in the middle-lower crust consistent with a 20-40% thinning of the middle crust and consistent with channel flow (Shapiro et al 2004). The geologically constrained parameters include a 10-20 km thick middle crust composed of sillimanite-K-feldspar grade gneisses, migmatites and leucogranites bounded by an inverted metamorphic sequence above the Main Central Thrust along the base and a right-way-up metamorphic sequence beneath a major low-angle normal fault, the South Tibetan Detachment, along the top (Searle et al , 2010b.…”

Section: Himalayan Channel Flow Modelsmentioning

confidence: 99%

Crustal–lithospheric structure and continental extrusion of Tibet

Searle

Elliott

Phillips

et al. 2011

JGS

237

154

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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 strikeslip 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.

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Thinning and Flow of Tibetan Crust Constrained by Seismic Anisotropy

Cited by 291 publications

References 26 publications

Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts

Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts

Complex, multilayered azimuthal anisotropy beneath Tibet: evidence for co-existing channel flow and pure-shear crustal thickening

Crustal–lithospheric structure and continental extrusion of Tibet

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