Satellite-based Observation of the Tectonics of Southern Tibet
The Himalayas and the Tibetan Plateau were formed as a result of the collision of India and Asia, and provide an excellent natural laboratory for the investigation of the mechanical response of the outer 100 km of the Earth (the lithosphere) to tectonic stress. Geophysicists are divided in their views on the nature of this response with one group advocating homogeneously distributed deformation in which the lithosphere deforms as a fluid continuum while others contend that deformation is highly localized with the lithosphere deforming as a system of rigid blocks. These rigid blocks or plate undergo little internal deformation. The latter group draws support from the high slip-rates recently observed on strike-slip faults along the northern edge of the Plateau (the Altyn Tagh Fault, ATF), coupled with seismic observations suggesting that these faults penetrate the entire lithosphere. These ''lithospheric faults'' define continental lithospheric plates and facilitate the eastward extrusion of the ''central Tibet plate''. If extrusion of a rigid Tibet occurs then there must be equivalent features at its southern boundary with slip-rates similar to those in the north. The southern boundary of Tibet, defined by the Main Himalayan Thrust (MHT), has no lateral component of motion and is therefore kinematically incompatible with motion in the north. However, a series of features, the Karakorum Fault, the Karakorum-Jiali Fracture Zone (KJFZ), the Jiali Fault and the Red River Fault which lie to the north of the MHT may define the actual, kinematic, southern boundary of this ''central Tibet plate''. We have investigated the rate of slip along the Karakorum Fault (KKF), the major strike-slip fault in southwestern Tibet. If the KKF represents the actual, kinematic, southern boundary of this Tibet, and is the only feature accommodating eastward extrusion of Tibet, then its slip-rate should be similar to that of the ATF in the north. Offsets along the Karakorum Fault ranging from tens of meters to kilometers have been mapped using satellite imagery and field mapping, and samples ages determined by cosmic-ray exposure dating. Near Bulong Kol (39{sup o}N, 75{sup o}E) cosmogenic dating of a 40 m fluvial offset yields a slip rate of {approx}6.5 mm/yr. Near Mt. Kailas (31.5{sup o}N, 80.7{sup o}E), a glacial moraine offset by {approx}350 m has been dated at 32.3 {+-} 9.5 thousand years, yielding a slip rate of 10.8 {+-} 3.6 mm/yr. In the Gar Valley (32{sup o}N, 80{sup o}E) a river channel incised in glacial sediments yields an offset of 1750 m and an age of 283,000 years equivalent to a slip-rate of 6 mm/yr. Relative to the ATF, the slip rates on the KKF are lower than expected, and since these measurements cover almost the entire length of the KKF, the disparity cannot be attributed to along strike variation in the rate. Based upon the analysis of satellite images along the Karakorum Fault, we believe that this apparent slip deficit may be to the en echelon arrangement of multiple strike slip fault segments that characterize what should more appropriately be called the Karakorum Fault Zone. The geometric arrangement of parallel fault segments produces the ''pull apart'' basins that form the valleys along the KKF. Hence, at any given latitude, slip along the KKF may be distributed among numerous fault segments. This investigation supports efforts to understand the structure and mechanical response of the Earth's crust and supports the application of remote sensing methods.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15004043
- Report Number(s):
- UCRL-ID-151776; TRN: US201015%%283
- Country of Publication:
- United States
- Language:
- English
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