Detailed Configuration of the Underthrusting Indian Lithosphere Beneath Western Tibet Revealed by Receiver Function Images

Abstract

We analyze the teleseismic waveform data recorded by 42 temporary stations from the Y2 and ANTILOPE‐1 arrays using the P and S receiver function techniques to investigate the lithospheric structure beneath western Tibet. The Moho is reliably identified as a prominent feature at depths of 55–82 km in the stacked traces and in depth migrated images. It has a concave shape and reaches the deepest location at about 80 km north of the Indus‐Yarlung suture (IYS). An intracrustal discontinuity is observed at ~55 km depth below the southern Lhasa terrane, which could represent the upper border of the eclogitized underthrusting Indian lower crust. Underthrusting of the Indian crust has been widely observed beneath the Lhasa terrane and correlates well with the Bouguer gravity low, suggesting that the gravity anomalies in the Lhasa terrane are induced by topography of the Moho. At ~20 km depth, a midcrustal low‐velocity zone (LVZ) is observed beneath the Tethyan Himalaya and southern Lhasa terrane, suggesting a layer of partial melts that decouples the thrust/fold deformation of the upper crust from the shortening and underthrusting in the lower crust. The Sp conversions at the lithosphere‐asthenosphere boundary (LAB) can be recognized at depths of 130–200 km, showing that the Indian lithospheric mantle is underthrusting with a ramp‐flat shape beneath southern Tibet and probably is detached from the lower crust immediately under the IYS. Our observations reconstruct the configuration of the underthrusting Indian lithosphere and indicate significant along strike variations.

Imaging the Moho and the Main Himalayan Thrust in Western Nepal With Receiver Functions

“…Our interpretation is based on the most robust results revealed by RF imaging (Figures and ). The Moho geometry connects well with the one derived from P wave RFs in southern Tibet (Xu et al, ), with discrepancies of less than 5 km at their southernmost stations, reasonable for lateral variation over the ~75‐km distance separating the two profiles.…”

“…Our observations of the Moho and MHT are represented in red solid lines, and the extrapolations of the MHT in red dashed lines. Dashed black line on the right side indicates the intracrustal discontinuity as drawn by Xu et al (). White arrows represent motion of the India plate relative to Tibet.…”

“…Extended profile and interpretation. (a) Composite receiver function image from Figure (left) and from Xu et al (; right, reduced color intensity). Green crosses: Moho depths derived by H‐K stacking in this study.…”

“…Black dots: locally detected seismicity by Hi‐KNet (Hoste‐Colomer et al, ). Note that the Xu et al () profile is offset by ~75 km to the west and has been processed differently than our profile, but we juxtaposed it properly to our image using their station‐wise Moho depth estimates (green crosses based on differential delay times of Ps and PpPs Moho phases from station‐stacked P receiver functions). Their thick black dashed lines: interpretation of the Moho and an intracrustal discontinuity.…”

“…Cross section AA′ is shown on Figures and . The thick black line locates the southern end of Xu et al () profile (Figure ). Black dashed line locates the Chainpur geological cross section (Robinson & McQuarrie, ).…”