[Show abstract][Hide abstract] ABSTRACT: Seismic anisotropy in the upper mantle has been inferred from shear wave splitting measurements at the temporary seismic array (ANTILOPE-2) in the central-southern Tibet from September 2005 to October 2006. Teleseismic shear waves (mainly SKS and SKKS) are used to determine the splitting parameters (fast polarization direction and delay time). Weak splitting with average delay time of 0.3 s is observed at stations in the southern Lhasa terrane and northernmost Tethyan Himalaya. Significant seismic polarization anisotropy with delay times from 0.8 to 1.5 s and NE-oriented fast direction is obtained for stations located farther north in the northern Lhasa terrane. The transition in shear wave splitting characteristics occurs at 30.5°N and suggests distinct upper mantle deformation from south to north. The negligible anisotropy south of this transition indicates that there is no appreciable large-scale mantle seismic polarization anisotropy. The substantial anisotropy in the north may be caused by the eastward flow in a lithospheric crush zone which is squeezed and sheared between the advancing Indian plate to the south and Eurasian plate to the north.
[Show abstract][Hide abstract] ABSTRACT:  We employ the P and S receiver function technique to data from the 44 seismic stations deployed in the eastern Himalayan syntaxis to investigate the crustal thickness, the average Poisson's ratio, and the depth of the lithosphere-asthenosphere boundary (LAB). The observed crustal thickness exhibits an overall NE-deepening trend, varying from 55 to 75 km. Two anomalous areas lie in the west and east of the Namche Barwa syntaxis characterized by thinner and thicker crust, respectively. The average Poisson's ratios within the study area are low in the north and moderate elsewhere with some high values in the south, consistent with felsic and intermediate rocks forming the crust. Our migrated images reveal that (1) the LAB of the Tibetan plate exists at relatively shallow depths (~110 km) and exhibits a gap beneath the Namche Barwa syntaxis, which may have formed by the delamination of mantle lithosphere due to local mantle upwelling, and (2) the LAB of the Asian plate is observed at a depth of ~180 km, which implies that the Asian plate has advanced southward to about 30°N under the Lhasa terrane. Our results provide new insights into the understanding of continental subduction and lithospheric deformation of the eastern Himalayan syntaxis.
[Show abstract][Hide abstract] ABSTRACT: Receiver function image along a temporary seismic array (ANTILOPE-2)
reveals detailed information of the underthrusting of the Indian crust
beneath southern Tibet. The Moho dips northward from ~50 km to 80 km
depth beneath Himalaya terrane and reaches locally a depth of 90 km
beneath the Indus-Yalung suture (IYS). It remains at ~80 km in Lhasa
terrane and shallows to ~70 km in Qiangtang terrane. A lower crustal
interface at ~60 km depth beneath Lhasa terrane can be clearly followed
southward through the Main-Himalaya-Thrust (MHT) and connects the
Main-Boundary-Thrust (MBT) at the surface, which represents the border
of the Indian crust that is underthrusting the Tibetan crust until south
of Bangong-Nujiang Suture (BNS) at ~32°N. We also observed a
wide-spread mid-crustal low velocity zone with increasing depth from ~15
km in Lhasa terrane southward to ~35 km beneath high Himalaya that is
terminated at the MHT. The low-velocity zone is thought to be formed by
partial melt and/or aqueous fluids.
[Show abstract][Hide abstract] ABSTRACT: The fate of the colliding Indian and Asian tectonic plates below the
Tibetan high plateau may be visualized by, in addition to seismic
tomography, mapping the deep seismic discontinuities, like the
crust-mantle boundary (Moho), the lithosphere-asthenosphere boundary
(LAB), or the discontinuities at 410 and 660 km depth. We herein present
observations of seismic discontinuities with the P and S receiver
function techniques beneath central and western Tibet along two new
profiles. The LAB of the Indian and Asian plates is well-imaged by
several profiles and suggests a changing mode of India-Asia collision in
the east-west direction. From eastern Himalayan syntaxis to the western
edge of the Tarim Basin, the Indian lithosphere is underthrusting Tibet
at an increasingly shallower angle and reaching progressively further to
the north. A particular lithospheric region called Tibetan Plate was
found in northern and eastern Tibet between the two colliding plates,
the existence of which is marked by high temperature, low mantle seismic
velocity (correlating with late arriving signals from the 410
discontinuity), poor Sn propagation, east and southeast oriented global
positioning system displacements, and strikingly larger seismic (SKS)
anisotropy. The crustal shortening in the southern Tibet is accommodated
by underthrusting of the Indian crust below the Asian crust that may
reach further north than the YZS. In northern Tibet, crustal shortening
is accommodated by homogeneous crustal thickening. The more rugged and
higher topography in west Tibet can be supported by the rigid mantle
lithosphere there, whereas to the east the lithosphere is weaker due to
the existence of the crush zone. Under pressure by Indian and Asian
plates, the subducted Indian lithospheric materials moved eastward and
divided into four directions when meeting the Sichuan basin, two
horizontal (southeastern ward forming Yun-Gui-Chuan plateau,
northeastern ward to Erdos) and two vertical(upward forming Longmen Shan
and down ward entering deep mantle).
[Show abstract][Hide abstract] ABSTRACT: We investigate the crustal thickness, Poisson’s ratio, and the thickness variation of the mantle transition zone using the P receiver function technique on data recorded by the 68 seismic station of the ASCENT experiment beneath the northeast Tibetan plateau. Our results reveal a distinct crustal and upper mantle structure contrast between the western and eastern part of the northeast Tibetan plateau along two migrated profiles. In the eastern region the Moho is relatively smooth and continuous and varies in depth between ∼80 and 51 km, whereas the depth of the Moho beneath the western region varies between ∼77 and 52 km and is offset ∼20 km beneath the boundary between the Kunlun fault and Qaidam basin. The delay times for the Ps phases from both the 410-km and 660-km discontinuities in the western region are ∼1.2 s later than that in the eastern region, indicating that the average velocities of the upper mantle in the western region are lower than that in the eastern region. Crustal rocks with low- to moderate Poisson’s ratios are interpreted to be dominated by felsic to intermediate compositions, implying that large scale middle-lower crustal flow does not occur easily. The nearly constant thickness of the mantle transition zone, with an average value of ∼255 km (24.4 s) from south to north, implies that no lithospheric fragments correlating to the India–Asia collision have entered into the mantle transition zone.
Physics of The Earth and Planetary Interiors 04/2013; 217:1–9. · 2.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the results of a seismic wide-angle reflection/refraction profile across the central Qaidam basin, the largest basin within the Qinghai-Tibetan plateau. The 350-km‐long profile extends from the northern margin of the East-Kunlun Shan to the southern margin of the Qilian Shan. The P- and S-wave velocity structure and Poisson's ratio data provide constraints on composition. The crust here consists of a near-surface sedimentary layer and a four-layered crystalline crust having several significant features. (1) The sedimentary fill of the Qaidam basin reaches a maximum thickness of 8 km, and the basin shape mirrors the uplifted Moho. (2) Within the four layers of the crystalline crust, P- (S-) wave velocities increase with depth and fall within the following velocity ranges: 5.9–6.3 km/s (3.45–3.65 km/s), 6.45–6.55 km/s (3.7 km/s), 6.65 km/s (3.8 km/s), and 6.7–6.9 km/s (3.8–3.9 km/s), respectively; (3) low-velocity zones with a 3–5% reduction in seismic velocity are detected in the lower half of the crust beneath the Qaidam basin and its transition to the Qilian Shan. (4) The crystalline crust is thickest beneath the northern margin of the basin towards the Qilian Shan (58–62 km) and thinnest beneath the center of the basin (52 km). Variations in crustal thickness are caused most pronouncedly by thickness variations in the lowermost layer of the crust. (5) Poisson's ratio and P-wave velocity values suggest that the Qaidam crust has an essentially felsic composition with an intermediate layer at its base. Based on the crustal structure reported here, we suggest that late Cenozoic convergence is accommodated by thick-skinned tectonic deformation with thickening involving the entire crust across the Kunlun–Qaidam–Qilian system.
[Show abstract][Hide abstract] ABSTRACT: A detailed 3-D P-wave velocity model beneath central Tibet was obtained
using 26 741 arrival times from 1025 teleseismic events recorded by the
portable stations of the Hi-CLIMB project. In the crustal correction, we
consider both vertical and lateral velocity variations. Our teleseismic
P-wave tomography result reveals that the Indian lithospheric mantle
underthrusts no further than 32.5°N. In addition, the presence of
low velocity anomalies under the Indus-Tsangpo suture suggests that
subduction is not a simple and continuous process. We suggest that the
delamination of Indian mantle lithosphere induces mantle upwelling
beneath the rifts, which in turn created cracks or a break in the
subducted plate. Moreover, the formation of active rifts near the
profile is related to the mantle upwelling.
Geophysical Journal International 09/2012; 190(3):1325-1334. · 2.85 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A three-dimensional P wave velocity model of the crust and upper mantle
down to 400-km depth beneath eastern Tibet is obtained using many
temporary seismic stations of the ASCENT project and the Namche Barwa
Broadband Seismic Network. We collected 16,508 arrival times of P, Pn
and Pg phases from 573 local and regional earthquakes and 7,450 P wave
arrivals from seismograms of 435 teleseismic events. Our high-quality
data set enables us to reconstruct the 3-D velocity structure under
eastern Tibet in more detail than the previous studies. In the shallow
depth, our results show that the low velocity zones are not
interconnected well (no wide-spread low velocity zones), which may
reflect the complex pattern of material flowing in the study region.
Below the Moho, we find that the Indian lithospheric mantle underthrusts
sub-horizontally under eastern Tibet, and the extent of the northward
advancing Indian lithosphere decreases from west to east. In the north,
the Asian lithospheric mantle is detected under the vicinity of the
Qaidam Basin. Between the Indian and Asian lithospheric mantles, there
is an obvious low-velocity anomaly which may reflect an upwelling mantle
[Show abstract][Hide abstract] ABSTRACT: For better studying the relationship between the rifts and deep structure, a detailed P-wave velocity structure under eastern
Tibet has been modeled using 4767 arrival times from 169 teleseismic events recorded by 51 portable stations. In horizontal
slices through the model, a prominent low-velocity anomaly was detected under the rifts from the surface to a depth of ∼250
km; this extends to a depth of ∼400 km in the vertical slice. This low-velocity anomaly is interpreted as an upper mantle
upwelling. The observations made provide seismic evidence for the formation of north-south trending rifts. East of the low-velocity
anomaly, a clear high-velocity anomaly is found between depths of 40 and 200 km. Due to its shallow depth, we suggest that
it consists of materials from an ancient continental closure rather than the Indian Plate. From depths of 250 to 400 km, a
high-velocity anomaly appears to the south of the Jiali Fault. This anomaly may correspond to the northern edge of the Indian
Plate that detached from the surface under the Himalayan block. We suggest that the Indian Plate underthrusts no further than
the Jiali Fault in eastern Tibet.
Keywordstomography–upper mantle upwelling–Indian plate subduction–north-south trending rifts–Tibetan Plateau
Chinese Science Bulletin 01/2011; 56(23):2450-2455. · 1.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The fate of the colliding Indian and Asian tectonic plates below the Tibetan high plateau may be visualized by, in addition to seismic tomography, mapping the deep seismic discontinuities, like the crust-mantle boundary (Moho), the lithosphere-asthenosphere boundary (LAB), or the discontinuities at 410 and 660 km depth. We herein present observations of seismic discontinuities with the P and S receiver function techniques beneath central and western Tibet along two new profiles and discuss the results in connection with results from earlier profiles, which did observe the LAB. The LAB of the Indian and Asian plates is well-imaged by several profiles and suggests a changing mode of India-Asia collision in the east-west direction. From eastern Himalayan syntaxis to the western edge of the Tarim Basin, the Indian lithosphere is underthrusting Tibet at an increasingly shallower angle and reaching progressively further to the north. A particular lithospheric region was formed in northern and eastern Tibet as a crush zone between the two colliding plates, the existence of which is marked by high temperature, low mantle seismic wavespeed (correlating with late arriving signals from the 410 discontinuity), poor Sn propagation, east and southeast oriented global positioning system displacements, and strikingly larger seismic (SKS) anisotropy.
Proceedings of the National Academy of Sciences 06/2010; 107(25):11229-33. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: By analyzing teleseismic waveforms recorded by 53 stations of Hi-Climb profile passing through the central Bangong-Nujiang
suture (BNS), a total of 4764 high- quality receiver functions are obtained. The average crustal thickness and Poisson’s ratio
beneath each station are estimated using the travel time of Ps and PpPs of the Moho. The discontinuities such as the Moho,
the 410- and 660-km interfaces are also studied using the common converted points (CCP) time to depth migration of receiver
functions. The main results are as follows: (1) The Moho of Lhasa terrane and that of Qiangtang terrane nearby BNS are overridden
and offset by ∼10 km. The structural geometry shows a northward uplifting and the southward deepening for the Moho of Lhasa
terrane and Qiangtang terrane, respectively, which is related to the reactivated structure beneath BNS since Cenozoic era.
(2) The variation range of Poisson’s ratios along the profile is between 0.237 and 0.280, indicating that the crust is mainly
composed of felsic and intermediate rocks. The anti-correlation between the crustal thickness and Poisson’s ratio suggests
that thicker crust beneath the southern Qiangtang terrane may be related to the successive thrust of felsic and intermediate
rock of Lhasa terrane. (3) The thickness of the mantle transition zone along the profile remains about 255 km, implying that
the tectonic activities caused by the India-Asia collision are confined to the depths above 410 km.
Chinese Science Bulletin 01/2010; 55(7):607-613. · 1.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The method of 2-D travel time inversion, which can be applied to determining 2-D velocity structure and interfaces simultaneously,
is used in this paper to reprocess the data of Paiku Co-Pumoyingcuo seismic profile across the Nyima-Tingri rift and Shenzha-Dinggye
rift. P-wave velocity structure and interfaces beneath the profile are obtained. The interfaces in the crust near Tingri and
Dinggye which are located on rifts have a tendency to uplift, and velocities of middle and lower crusts are high. Low velocity
layer in upper crust has an offset. Compared with the distribution of the earthquakes in this region, it is speculated that
normal faults near Tingri and Dinggye extend to the upper mantle. Apparently it is affected by deep material: the uplift of
mantle causes partial melting in the crust, thus the thickness of crust in this area becomes thin, and tension failures occur
in this region easily. On the basis of the characteristics of the earthquakes’ distribution and the structures of the crustal
velocity and interfaces, materials from the mantle still uplifts and the failures are still active.
[Show abstract][Hide abstract] ABSTRACT: In this paper, a 2D velocity structure of the crust and the upper mantle of the northern margin of the Tarim Basin (TB) has
been obtained by ray tracing and theoretical seismogram calculation under the condition of 2D lateral inhomogeneous medium
using the data of seismic wide angle reflection/refraction profile from Baicheng to Da Qaidam crossing the Kuqa Depression
(KD) and Tabei Uplift (TU). And along the Baicheng to Da Qaidam profile, 4 of the 10 shot points are located in the northern
margin of the TB. The results show that the character of the crust is uniform on the whole between the KD and TU, but the
depth of the layers, thickness of the crust and the velocity obviously vary along the profile. Thereinto, the variation of
the crust thickness mainly occurs in the middle and lower crust. The Moho has an uplifting trend near the Baicheng shot point
in KD and Luntai shot point in TU, and the thickness of the crust reduces to 42 km and 47 km in these two areas, respectively.
The transition zone between the KD and TU has a thickest crust, up to 52 km. In this transition zone, there are high velocity
anomalies in the upper crust, and low velocity anomalies in the lower crust, these velocity anomalies zone is near vertical,
and the sediment above them is thicker than the other areas. According to the velocity distributions, the profile can be divided
into three sections: KD, TU and transition zone between them. Each section has a special velocity structural feature, the
form of the crystalline basement and the relationship between the deep structure and the shallow one. The differences of velocity
and tectonic between eastern and western profile in the northern margin of the Tarim Basin (NMTB) may suggest different speed
and intensity of the subduction from the Tarim basin to the Tianshan orogenic belt (TOB).
Chinese Science Bulletin 01/2008; 53(10):1544-1554. · 1.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have obtained velocity images of the uppermost mantle beneath China by performing tomographic inversion of both Pn and Sn traveltimes. From the Annual Bulletin of Chinese Earthquakes, 99,139 Pn arrivals and 43,646 Sn arrivals were selected. Pn anisotropy was also obtained simultaneously with Pn velocity. Average Pn and Sn velocities are 8.05 and 4.55 km/s, respectively, and maximum velocity perturbations are about 3–4%. The Pn and Sn velocities are low in eastern China and high in western China. Particularly high velocities are associated with old basins (for example, Tarim, Junggar, Turpan-Hami, Qaidam, and Sichuan) and stable craton (for example, Ordos). Low Sn velocities are found mainly throughout North China. In addition, velocities are relatively low beneath the central Tibetan Plateau and the North-South Seismic Zone (along 103°E). In Tarim and central China where we observe strong anisotropy, the fast Pn velocity directions are consistent with the directions of maximum principal compressive stress as well as directions of crustal movement determined from Global Positioning System. Beneath the India-Eurasia collision zone, the Pn anisotropy direction is parallel to the collision arc and nearly perpendicular to both the direction of maximum compression and crustal movement resulting from pure shear deformation. Both the velocity variations and anisotropy indicate that the Tibetan Plateau was extruded, and the mantle material beneath the plateau has flowed around the East Himalaya Syntax, while the remaining material has diverted northwestward beneath the Tarim Basin.
Journal of Geophysical Research 01/2007; 112. · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have selected 10,899 ML amplitude readings from 1732 events re- corded by 91 stations, as reported in the Annual Bulletin of Chinese Earthquakes (ABCE), and have used tomographic imaging to estimate the lateral variations of the quality factor Q0 (Q at 1 Hz) within the crust of Northern China. Estimated Q0 values vary from 115 to 715 with an average of 415. Q0 values are consistent with tectonic and topographic structure in Eastern China. Q0 is low in the active tectonic regions having many faults, such as the Shanxi and Yinchuan Grabens, Bohai Bay, and Tanlu Fault Zone, and is high in the stable Ordos Craton. Q0 values are low in several topographically low-lying areas, such as the North China, Taikang-Hefei, Jianghan, Subei-Yellow Sea, and Songliao basins, whereas it is high in mountainous and uplift regions exhibiting surface expressions of crystalline basement rocks: the Yinshan, Yanshan, Taihang, Qinlin, Dabie and Wuyi Mountains, and Luxi and Jiaoliao Uplifts. Quality-factor estimates are also consistent with Pn- and Sn-velocity patterns. High- velocity values, in general, correspond with high Q0 and low-velocity values with low Q0. This is consistent with a common temperature influence in the crust and uppermost mantle.
Bulletin of the Seismological Society of America 01/2006; 96:1560-1566. · 1.94 Impact Factor