Applications of morphochronology to the active tectonics of Tibet
Résumé
The Himalayas and the Tibetan Plateau were formed as a result of the collision of India and Asia, and provide an excellent opportunity to study the mechanical response of the continental lithosphere to tectonic stress. Geophysicists are divided in their views on the nature of this response, advocating either (1) homogeneously distributed deformation with the lithosphere deforming as a fluid continuum or (2) highly localized deformation with the lithosphere deforming as a system of blocks. The resolution of this issue has broad implications for understanding the tectonic response of continental lithosphere in general. Homogeneous deformation is supported by relatively low decadal, geodetic slip-rate estimates for the Altyn Tagh and Karakorum faults. Localized deformation is supported by high millennial, geomorphic slip rates constrained by both cosmogenic and radiocarbon dating on these faults. Based upon the agreement of rates determined by radiocarbon and cosmogenic dating, the overall linearity of offset versus age correlations, and the plateau-wide correlation of landscape evolution and climate history, the disparity between geomorphic and geodetic slip-rate determinations is unlikely to be due to the effects of surface erosion on the cosmogenic age determinations. Similarly, based upon the consistency of slip rates over various observation intervals, secular variations in slip rate appear to persist no longer than 2000 yr and are unlikely to provide reconciliation. Conversely, geodetic and geomorphic slip-rate estimates on the Kunlun fault, which does not have significant splays or associated thrust faults, are in good agreement, indicating that there is no fundamental reason why these complementary geodetic and geomorphic methods should disagree. Similarly, the geodetic and geomorphic estimates of shortening rates across the northeastern edge of the plateau are in reasonable agreement, and the geomorphic rates on individual thrust faults demonstrate a significant eastward decrease in the shortening rate. This rate decrease is consistent with the transfer of slip from the Altyn Tagh fault to genetically related thrust mountain building at its terminus. Rates on the Altyn Tagh fault suggest a similar decrease in rate, but the current data set is too small to be definitive. Overall, the high, late Pleistocene-Holocene geomorphic slip velocities on the major strike-slip faults of Tibet suggest that these faults absorb as much of India's convergence relative to Siberia as the Himalayan Main Frontal Thrust on the southern edge of the plateau.