Anisotropic low‐velocity lower crust beneath the northeastern margin of<scp>T</scp>ibetan<scp>P</scp>lateau: Evidence for crustal channel flow

Shen, Xuzhang; Yuan, Xiaohui; Ren, Junsheng

doi:10.1002/2015gc005952

Cited by 38 publications

(26 citation statements)

References 44 publications

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“…Amphibole anisotropy can produce fast-axis symmetry and is also invoked [Sun et al, 2012], and fast-axis symmetry in the crust may be assumed in receiver function inversions [Bianchi et al, 2015]. Most receiver function studies that target splitting of the Moho Ps converted phase [McNamara and Owens, 1993;Peng and Humphreys, 1997;Nagaya et al, 2008;Nagaya et al, 2011;Liu and Niu, 2012;Sun et al, 2012;Shen et al, 2015;Sun et al, 2015;Kong et al, 2016;Niu et al, 2016;Wang et al, 2016] solve for a fast horizontal axis orientation, as is standard in shear wave splitting studies. Surface wave studies targeting crustal radial anisotropy (vertical symmetry axis) find dominantly V SH > V SV [Moschetti et al, 2010;Xie et al, 2013] indicating slow-axis symmetry.…”

Section: Hexagonal Symmetry: Fast Versus Slow Axis Off-axis Behaviormentioning

confidence: 99%

“…The first is splitting of the P-to-S converted phase [McNamara and Owens, 1993;Peng and Humphreys, 1997], which accumulates in anisotropic layers above the converting contrast. Anisotropy of significant magnitude and thickness is required to accumulate a measurable signal, and while the technique has experienced a recent resurgence in popularity for estimating bulk crustal anisotropy from splitting of the Moho Ps phase [Nagaya et al, 2008[Nagaya et al, , 2011Liu and Niu, 2012;Sun et al, 2012;Ruempker et al, 2014;Shen et al, 2015;Sun et al, 2015;Kong et al, 2016;Niu et al, 2016;Wang et al, 2016], it will not detect shear zones of a few kilometers thickness and a few percent contrast in anisotropy, and splitting may be conflated with conversions from thinner anisotropic layers [Liu and Park, 2017]. A second receiver function method that is capable of resolving such structures robustly is based on analyzing azimuthal variations in the amplitude and polarity of converted phases.…”

Section: Effects On Seismic Signatures and Interpretationmentioning

confidence: 99%

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Characteristics of deep crustal seismic anisotropy from a compilation of rock elasticity tensors and their expression in receiver functions

et al. 2017

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Rocks in the continental crust are long lived and have the potential to record a wide span of tectonic history in rock fabric. Mapping rock fabric in situ at depth requires the application of seismic methods. Below depths of microcrack closure seismic anisotropy presumably reflects the shape and crystallographic preferred orientations influenced by deformation processes. Interpretation of seismic observables relevant for anisotropy requires assumptions on the symmetry and orientation of the bulk elastic tensor. We compare commonly made assumptions against a compilation of 95 bulk elastic tensors from laboratory measurements, including electron backscatter diffraction and ultrasound, on crustal rocks. The majority of samples developed fabric at pressures corresponding to middle to lower crustal condition. Tensor symmetry is a function of mineral modal composition, with mica‐rich samples trending toward hexagonal symmetry, amphibole‐rich samples trending toward an increased orthorhombic symmetry component, and quartz‐feldspar‐rich samples showing a larger component of lower symmetries. Seventy‐seven percent of samples have a best fit hexagonal tensor with slow‐axis symmetry, as opposed to mantle deformation fabric that usually has fast‐axis symmetry. The best fit hexagonal approximation for crustal tensors is not elliptical but deviates systematically from elliptical symmetry with increasing anisotropy, an observation that affects the magnitude and orientation of anisotropy inferred from receiver function and surface wave observations. We present empirical linear relationships between anisotropy and ellipticity for crustal rocks. The maximum out‐of‐plane conversion amplitudes in receiver functions scale linearly with degree of anisotropy for nonelliptical symmetry. The elliptical assumption results in a bias of up to 1.4 times true anisotropy.

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Section: Hexagonal Symmetry: Fast Versus Slow Axis Off-axis Behaviormentioning

confidence: 99%

Section: Effects On Seismic Signatures and Interpretationmentioning

confidence: 99%

Characteristics of deep crustal seismic anisotropy from a compilation of rock elasticity tensors and their expression in receiver functions

et al. 2017

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“…The presence of substantial azimuthal anisotropy beneath Tibet is well established in studies using different techniques and data types, including surface-wave imaging (e.g., Griot et al 1998;Huang et al 2004;Su et al 2008;Yao et al 2010;Yi et al 2010;Yang et al 2010b;Ceylan et al 2012;Legendre et al 2015;Pandey et al 2015;Schaeffer et al 2016;Xie et al 2016;Chen et al 2016), shear-wave splitting analysis (e.g., McNamara et al 1994;Hirn et al 1995;Sandvol et al 1997;Sol et al 2007;Zhao et al 2010;Leon Soto et al 2012;Eken et al 2013;Chang et al 2015;Wu et al 2015a;Chen et al 2015;Liu et al 2016;Singh et al 2016;Ye et al 2016), receiver functions (e.g., Vergne et al 2003;Levin et al 2008;Shen et al 2015;Liu et al 2015;Kong et al 2016), attenuation studies (Bao et al 2012) and P-wave arrival times (e.g., Wei et al 2013;Huang et al 2014;Zhang et al 2016b;Wei et al 2016). 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: 99%

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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“…Here the slow V SV (Li et al, ) and V SH from our inversion as well as positive radial anisotropy ( V SH > V SV ; Li et al, ) in the midlower crust beneath the QOB and northern SGT (Figures b, c, and ) may also imply that a horizontal melt‐rich layer likely exists in the NE Tibetan plateau. This horizontal melt‐rich layer could result in strong crustal azimuthal anisotropy inferred from receiver function analysis (Shen, Yuan, & Ren, ) and shear wave splitting measurement (Li et al, ) and correlates well with relatively low‐resistivity (Xiao et al, ) and high Lg‐wave attenuation belt (Zhao et al, ) in the lower crust and relatively high surface heat flow (Zhang et al, ).…”

Section: Discussionmentioning

confidence: 91%

Lithospheric SH Wave Velocity Structure Beneath the Northeastern Tibetan Plateau From Love Wave Tomography

Xiao

2019

JGR Solid Earth

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The northeastern (NE) Tibetan plateau has been a prime site to understand the dynamic processes responsible for the rise and lateral growth of the Tibetan plateau. Here we construct a high‐resolution lithospheric scale isotropic SH wave velocity model (down to the depth of 130 km) for the NE Tibetan plateau and its adjacent regions based on the measurements of fundamental‐mode Love wave dispersions (20–100 s) with seismic data recorded by ChinArray II and China Digital Seismic Array using two‐plane wave method. Prominent slow SH wave velocity (VSH) is observed in the midlower crust of the NE Tibetan plateau, specifically in the Qilian Orogenic Belt and the Songpan‐Ganzi Terrane regions, coincident with previously indicated significantly slow SV wave velocity (VSV), probably implying the existence of partial melts in the midlower crust. This low SH wave velocity anomaly could be traced down to the uppermost mantle, indicating a weak and warm lithosphere, which we interpret as a consequence of asthenosphere upwelling after lithospheric mantle removal in the NE Tibetan plateau. The asthenosphere upwelling and associated deep‐seated thermal buoyancy could explain the occurrence of partial melting in the mid‐lower crust and account for parts of high elevations in the NE Tibetan plateau. The western Qinling orogenic belt is characterized with a high velocity anomaly in most of lithosphere, which is inconsistent with the existence of channel flow within the lithosphere along the Qinling orogenic belt from the Tibetan plateau.

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Anisotropic low‐velocity lower crust beneath the northeastern margin ofTibetanPlateau: Evidence for crustal channel flow

Cited by 38 publications

References 44 publications

Characteristics of deep crustal seismic anisotropy from a compilation of rock elasticity tensors and their expression in receiver functions

Characteristics of deep crustal seismic anisotropy from a compilation of rock elasticity tensors and their expression in receiver functions

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

Lithospheric SH Wave Velocity Structure Beneath the Northeastern Tibetan Plateau From Love Wave Tomography

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