Depth‐variant azimuthal anisotropy in Tibet revealed by surface wave tomography

Pandey, Shantanu; Yuan, Xiaohui; Debayle, E.; Tilmann, Frederik; Priestley, Keith; Li, Xueqing

doi:10.1002/2015gl063921

Cited by 51 publications

(55 citation statements)

References 53 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In the upper mantle beneath eastern Tibet, the NNE-SSW fast-propagation directions that we determine here are consistent with those in large-scale waveform tomography of Pandey et al (2015) and Schaeffer et al (2016). The amplitude of radial anisotropy in the upper mantle shows lateral variations different from those in the crust (Agius & Lebedev 2014): upper-mantle radial anisotropy is the strongest beneath northeastern Tibet (3.4-8.6 per cent), weaker just beyond high-plateau boundaries, and weak in central Tibet (e.g., Agius & Lebedev 2013;Zhang et al 2016a).…”

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

confidence: 82%

“…In these locations, this depth range is occupied by the asthenosphere (Agius & Lebedev 2013). Multi-mode surface wave tomography studies map anisotropy in the eastern Tibet with similar fast-propagation azimuths (Priestley et al 2006;Pandey et al 2015;Schaeffer et al 2016). …”

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

confidence: 99%

“…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%

“…A number of such tomographic models were focussed on Tibet or presented with a spotlight on the India-Asia collision zone (Priestley et al 2006;Yang et al 2010a;Pandey et al 2015;Schaeffer et al 2016). In this study, in contrast, we aim to constrain azimuthal anisotropy beneath Tibet more accurately than previously by using only a few tens of interstation dispersion curves (even though each interstation curve is determined from tens to hundreds of single-event dispersion curves).…”

mentioning

confidence: 99%

See 3 more Smart Citations

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

Agius

Lebedev²

2017

Geophysical Journal International

View full text Add to dashboard Cite

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.

show abstract

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

confidence: 82%

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

confidence: 99%

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

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

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

Agius

Lebedev²

2017

Geophysical Journal International

View full text Add to dashboard Cite

show abstract

“…In the Taiwan orogen, γ peaks only at the transition between the two anisotropy domains, unlike in Japan where γ peaks twice. The orogen‐parallel paired with convergence‐parallel anisotropy at different depths is also observed in Tibet on a much larger vertical scale (Pandey et al, ). The mechanisms that caused the layered deformation in these regions may be entirely different and are a subject beyond the scope of this study.…”

Section: Discussionmentioning

confidence: 84%

Crustal Magmatism and Deformation Fabrics in Northeast Japan Revealed by Ambient Noise Tomography

et al. 2018

View full text Add to dashboard Cite

Mapping both isotropic and anisotropic velocity structures of the crust provides insight into the dominant mechanisms that produce the deformation. We performed ambient noise tomography for shear velocity Vs in the Tohoku region, Japan, where plate kinematics remains simple as the Pacific plate is subducting under the Okhotsk plate. Cross‐correlation functions were retrieved from Hi‐net recordings and phase velocities of 3–16 s were measured, followed by a one‐step, wavelet‐based multiscale inversion for both Vs and its azimuthal anisotropy. The isotropic model shows that high Vs anomalies underlie the Cretaceous aged Kitakami and Abukuma Mountains from shallow crust to lower crust. Low Vs is found to track the active volcanoes with the strongest anomalies concentrated at roughly 10 km depth, which we interpret as the shallow magma reservoirs of the Quaternary magmatic system. The resolved azimuthal anisotropy exhibits a switch of anisotropy in the upper crust from island‐parallel to a random or suppressed state in 5–15 km depth interval and back to island‐parallel. The island‐parallel fast direction may map the structural fabrics of the crustal deformation, and the spatial correlation between velocity and anisotropy patterns suggests that continuous volcanic activities and magma ponding disrupt the otherwise consistent fabrics in the upper crust. Below 20 km depth, the anisotropy turns to convergence‐parallel, which may result from shearing either imposed by the return flow in the mantle wedge or frozen‐in from the last stage of extension of the continental margin that opened the Sea of Japan.

show abstract

Shear wave splitting analyses in Tian Shan: Geodynamic implications of complex seismic anisotropy

Cherie

Gao

Liu

et al. 2016

Geochem Geophys Geosyst

View full text Add to dashboard Cite

The Tian Shan is a tectonically complex intracontinental orogenic belt situated between the Tarim Basin and the Kazakh Shield. The vast majority of the previous shear wave splitting (SWS) measurements were presented as station averages, which are only valid when the anisotropy structure can be approximated by a single layer of anisotropy with a horizontal axis of symmetry, i.e., a model of simple anisotropy. A variety of anisotropy‐forming hypotheses have been proposed based on the station‐averaged measurements. In this study, we measure the splitting parameters at 25 stations that recorded high‐quality data from a wide back azimuthal range for the purpose of identifying and characterizing complex anisotropy. Among the 25 stations, 15 of them show systematic azimuthal variations in the observed splitting parameters with a 90° periodicity that is consistent with a model of two‐layered anisotropy. The fast orientations of the upper layer range from 50° to 90° measured clockwise from the north, which are subparallel to the strike of the orogenic belt, and the splitting times are between 0.9 and 1.9 s. The corresponding values for the lower layer are −45° to −85° and 1.2–2.2 s, respectively. The remaining 10 stations demonstrate azimuthally invariant splitting parameters with strike‐parallel fast orientations, and can be represented by a single layer of anisotropy with a horizontal axis of symmetry. We propose that the strike‐parallel anisotropy is caused by lithospheric shortening, and anisotropy in the lower layer is associated with WNW‐ward flow of asthenospheric material sandwiched between the subducting Tarim lithosphere and the thick Kazakh lithospheric root.

show abstract

Depth‐variant azimuthal anisotropy in Tibet revealed by surface wave tomography

Cited by 51 publications

References 53 publications

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

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

Crustal Magmatism and Deformation Fabrics in Northeast Japan Revealed by Ambient Noise Tomography

Shear wave splitting analyses in Tian Shan: Geodynamic implications of complex seismic anisotropy

Contact Info

Product

Resources

About