Figure 14. Block diagram interpretation of the INDEPTH survey area north of the Zangbo suture. Numbers with circles refer to reflections in Figures 2, 5, 6, 9, 10, and 11. Solid lines denote reflections imaged on the seismic data or taken from the geologic map whereas dotted lines are inferred. See Figures 7 and 8 for close-ups. The Gangdese batholith formed from arc magmatism of the subducting Tethyan oceanic plate from ~100 to ~40 Ma. Deformation within the backarc may be marked by the southwest dipping reflections, #4. Reactivated deformation along these events may have occurred after collision. Well after collision, i.e., 25-15 Ma, the extensive Gangdese thrust (GTS, #5) may have formed a duplex (e.g., Figure 12), hence significantly thickening the crust. After the GTS, deformation occurred along backthrusts (e.g., the Renbu-Zedong thrust, RZT) within the Tethyan Himalaya and crustal thickening occurred along the MHT (Figure 2). Thus, the original suture zone at collision time may be marked by the steeply dipping event, #8, with significant post-collision modification by the GTS and RZT. After the crust was over-thickened, it may have heated by a combination of radiogenic and viscous heating [e.g., England and Thompson, 1986; Zhao and Yuen, 1987]. This melt zone may have facilitated the distribution of post-collisional shortening throughout the crust. From 8 Ma to present, ~EW extension has occurred along high and low-angle faults (e.g., the Nyainqentanglha Shear Zone, NSZ, #3).

Crustal Deformation of the Lhasa Terrane, Tibet Plateau from INDEPTH Seismic Reflection Profiles*

Douglas Alsdorf1, Larry Brown1, Douglas Nelson2,Yizhaq Makovsky3, and Simon Klemperer3
1Cornell University, Institute for the Study of the Continents, Dept. of Geological Sci., Ithaca, NY
2Syracuse University, Dept. of Geological Sci., Syracuse, NY
3Stanford University, Dept. of Geophysics, Stanford, CA

*Submitted to Tectonics, 1997

Abstract. Deep seismic reflection data collected by Project INDEPTH across the southern Lhasa terrane of the Tibetan plateau provide new details of crustal structure related to plateau evolution. Four main lines (averaging 23 km in length) and two cross lines (average of 7 km length) were recorded within the Yangbajain-Damxung graben north of the Zangbo suture. Shallow reflections (typically less than 1 km) and relatively slow refracted arrivals (~2500 m/s) are interpreted as thin, Quaternary graben fill. Underlying stratified reflections extending locally to ~11.5 km depth are interpreted to result from the Paleozoic-Cenozoic supracrustal sequence of the Lhasa block. Geologic controls combined with these depths suggest that Quaternary, high-angle extension is minor compared to late-Miocene, low-angle extension that exposes the basement (e.g., extension along the Nyainqentanglha Shear Zone). Local "bright spots" within a band of prominent reflections at 12 km to 18 km depth are characterized by negative reflection coefficients, strong P-to-S conversions, low velocities and high conductivities that suggest the presence of magma. The geometry of this band of reflections is suggestive of periodic deformation of the basement that began prior to the collision of India with Asia. Three gently north-dipping reflections at 15 km, 21 km, and 28 km depth beneath the Gangdese batholith may be indicative of a crustal-scale duplex of the late-Oligocene, early-Miocene Gangdese thrust system. There is no evidence north of the Zangbo suture for the continuation of the Main Himalayan Thrust, the crustal-scale decollement south of the suture along which India is currently underthrusting southern Tibet. Few reflections are observed below ~20 km with the exception of a deep event that lies directly beneath the Gangdese batholith, dipping steeply (40 degrees) to the north and extending from 40 km to 60 km depth. Correlating this steeply dipping event with wide-angle reflections previously reported from Sino-French work suggests that it may mark an imbrication of the lower crust. Alternatively, since the event projects updip toward the surface exposure of the Yarlung-Zangbo suture, it may instead delineate the suture's position in the lower crust. Either case, combined with the evidence for compressional deformation at ~12 km to ~18 km depth and large-scale (Gangdese) thrusting, supports distributed shortening at depth as a major factor in crustal thickening and plateau uplift.

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