Determination of the crustal structure in southern Tibet by dispersion and amplitude analysis of Rayleigh waves

Cotte, Nathalie; Pedersen, Helle A.; Campillo, Míchel; Mars, Jérôme; Ni, James; Kind, R.; Sandvol, Eric; Zhao, Wenjin

doi:10.1046/j.1365-246x.1999.00927.x

Cited by 61 publications

(56 citation statements)

References 30 publications

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“…7) throughout the mid-crust in most of Tibet has also been observed in a number of previous studies (e.g. Cotte et al 1999;Rapine et al 2003;Caldwell et al 2009;Guo et al 2009;Li et al 2009;Yang et al 2012;Agius & Lebedev 2014;Jiang et al 2014;Li et al 2014;Sun et al 2014;Bao et al 2015;Deng et al 2015;Gilligan et al 2015). Our results have high resolution across the entire Tibetan Plateau and the Pamirs, whereas many previous studies have mainly focused on more restricted geographical regions.…”

Section: P O S S I B L E C Au S E O F T H E L O W V E L O C I T Y L Asupporting

confidence: 85%

Lateral variations in the crustal structure of the Indo–Eurasian collision zone

Gilligan

2018

Geophysical Journal International

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S U M M A R YThe processes involved in continental collisions remain contested, yet knowledge of these processes is crucial to improving our understanding of how some of the most dramatic features on the Earth have formed. As the largest and highest orogenic plateau on the Earth today, Tibet is an excellent natural laboratory for investigating collisional processes. To understand the development of the Tibetan Plateau, we need to understand the crustal structure beneath both Tibet and the Indian Plate. Building on previous work, we measure new group velocity dispersion curves using data from regional earthquakes (4424 paths) and ambient noise data (5696 paths), and use these to obtain new fundamental mode Rayleigh wave group velocity maps for periods from 5 to 70 s for a region including Tibet, Pakistan and India. The dense path coverage at the shortest periods, due to the inclusion of ambient noise measurements, allows features of up to 100 km scale to be resolved in some areas of the collision zone, providing one of the highest resolution models of the crust and uppermost mantle across this region. We invert the Rayleigh wave group velocity maps for shear wave velocity structure to 120 km depth and construct a 3-D velocity model for the crust and uppermost mantle of the Indo-Eurasian collision zone. We use this 3-D model to map the lateral variations in the crust and in the nature of the crust-mantle transition (Moho) across the Indo-Eurasian collision zone. The Moho occurs at lower shear velocities below northeastern Tibet than it does beneath western and southern Tibet and below India. The east-west difference across Tibet is particularly apparent in the elevated velocities observed west of 84• E at depths exceeding 90 km. This suggests that Indian lithosphere underlies the whole of the Plateau in the west, but possibly not in the east. At depths of 20-40 km our crustal model shows the existence of a pervasive mid-crustal low velocity layer (∼10% decrease in velocity, V s < 3.4 km s −1 ) throughout all of Tibet, as well as beneath the Pamirs, but not below India. The thickness of this layer, the lowest velocity in the layer and the degree of velocity reduction vary across the region. Combining our Rayleigh wave observations with previously published Love wave dispersion measurements, we find that the low velocity layer has a radial anisotropic signature with V sh > V sv . The characteristics of the low velocity layer are supportive of deformation occurring through ductile flow in the mid-crust.

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Section: P O S S I B L E C Au S E O F T H E L O W V E L O C I T Y L Asupporting

confidence: 85%

Lateral variations in the crustal structure of the Indo–Eurasian collision zone

Gilligan

2018

Geophysical Journal International

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“…Although previous geophysical observations (e.g., Nelson et al, 1996) and the results of geodynamic modeling (e.g., Beaumont et al, 2001Beaumont et al, , 2006 suggest that channel flow may occur at depths of 20-30 km beneath southern Tibet (i.e., mid-crustal levels), other recent studies suggest that channel flow may also occur in the lower crust, as indicated by widespread high-conductivity layers in the mid-lower crust (e.g., Jin et al, 2010;Wei et al, 2010), a pronounced low-velocity channel that extends from 30 to 70 km depth in the mid-lower crust (e.g., Cotte et al, 1999), and the fact that earthquakes are absent at depths of 30-65 km beneath the Lhasa terrane (e.g., Chen and Yang, 2004;Jackson 2002aJackson , 2002bJackson et al, 2004). These findings imply that mid-lower crustal material beneath Southern Tibet is ductile and therefore may take place ductile flow.…”

Section: Southeastward Ductile Flow Of the Mid-lower Crust Beneath Wementioning

confidence: 98%

Geochemical variations in Miocene adakitic rocks from the western and eastern Lhasa terrane: Implications for lower crustal flow beneath the Southern Tibetan Plateau

Chen

Zhao

et al. 2011

Lithos

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“…Since the 1980s, many Chinese and foreign seismologists have studied the crust and upper mantle structure of the Tibetan Plateau by using seismic body wave and surface wave data [5][6][7][8][9][10][11][12][13][14][15][16][17][18] , and have studied the upper mantle anisotropy beneath the Tibetan Plateau using teleseismic SKS-wave splitting [19][20][21][22][23][24][25][26][27][28] . Hirn et al [20] considered seismic anisotropy as an indicator of mantle flow beneath Himalayas and Tibet, and they thought that the anisotropy of the Plateau is caused by the re-orientation of crystal lattice or the oriented arrangement of liquid-filled cracks due to differential shear deformation in the horizontal plane.…”

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confidence: 99%

Azimuthal anisotropy of Rayleigh waves beneath the Tibetan Plateau and adjacent areas

Wang

Huang

2008

Sci. China Ser. D-Earth Sci.

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The crustal and upper mantle azimuthal anisotropy of the Tibetan Plateau and adjacent areas was studied by Rayleigh wave tomography. We collected sufficient broadband digital seismograms traversing the Tibetan Plateau and adjacent areas from available stations, including especially some data from the temporary stations newly deployed in Yunnan, eastern Tibet, and western Sichuan. They made an adequate path coverage in most regions to achieve a reasonable resolution for the inversion. The model resolution tests show that the anisotropic features of scope greater than 400 km and strength greater than 2% are reliable. The azimuthal anisotropy pattern inside the Tibetan Plateau was similar to the characteristic of tectonic partition. The crustal anisotropy strength is greater than 2% in most regions of East Tibet, and the anisotropy shows clockwise rotation surrounding the eastern Himalayan syntaxis. Vertically, the anisotropy direction indicates a coherent pattern within the upper crust, lower crust, and lithosphere mantle of the Tibetan Plateau, which also is consistent with GPS velocity field and SKS fast polarization directions. The result supports that the crust-mantle deformation beneath the Tibetan Plateau is vertically coherent. The anisotropy strength of crust and lithospheric upper mantle in Yunnan outside the Tibetan Plateau is lower than 2%, so SKS splitting from core-mantle boundary to station should largely be attributed to the anisotropy of asthenosphere.Tibetan Plateau, azimuthal anisotropy, Rayleigh wave tomography, vertically coherent deformationThe formation of Tibetan Plateau is a result of collision between the Indian plate and Eurasian plate. Since the 1970s, Earth scientists have put forward several plate collision models of the Indian and Eurasian plate in accordance with different uplift and deformation mechanisms of Tibetan Plateau, for example, the kinematic model of extrusion or escape [1] and the kinetic model of continuous deformation [2] . Deep geophysical observation provides important information to evaluate the correctness of these models. To utilize broadband seismic data from Tibetan Plateau and surrounding areas to verify the collision model will definitely deepen the understanding of plate collision course.

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Determination of the crustal structure in southern Tibet by dispersion and amplitude analysis of Rayleigh waves

Cited by 61 publications

References 30 publications

Lateral variations in the crustal structure of the Indo–Eurasian collision zone

Lateral variations in the crustal structure of the Indo–Eurasian collision zone

Geochemical variations in Miocene adakitic rocks from the western and eastern Lhasa terrane: Implications for lower crustal flow beneath the Southern Tibetan Plateau

Azimuthal anisotropy of Rayleigh waves beneath the Tibetan Plateau and adjacent areas

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