1. 3-D Finite Element Modeling of the Rise and Fall of the Himalayan-Tibetan Plateau Mian Liu and Youqing Yang Dept. of Geological Sciences, University of Missouri-Columbia

2. Some of the fundamental questions of the Tibetan tectonics What causes the E-W extension in Tibet? When did the “collapse” start? When did the Tibetan plateau uplift? What are the temporal and spatial evolution of the mountain building in Tibet? How was the >2000 km crustal shortening accommodated? What controlled the strain partitioning between crustal thickening and lateral extrusion?

3. GPS velocity relative to stable Siberia Data from Larson et al., 1999; Chen et al., 2000; Wang et al., 2001 Earthquake focal mechanism showing E-W extension Data from Harvard Catalog

4. Active crustal deformation in Tibet: 3-D finite element model & rheology

5. Crustal Stress Indicated by Earthquake Data Predicted Stress State in the upper crust

6. Elevation reduced to 50% of present values No E-W extension predicted Basal shear = 30 MPa under Himalayas and south Tibet A narrow zone of nearly N-S Extension – South Tibetan Detachment Fault system?

7. Long-term history of the Himalayan-Tibetan orogen Viscous thin-sheet model (England & Houseman, 1986) Plasticine analog model (Tapponnier et al., 1986)

8. 3-D finite strain model x10 vertical exaggeration Indian plate Tarim Sichuan Basin Asia

10. 40 Ma 20 Ma Present 10 Ma

11. Near surface velocity Because of the depth- variable lithospheric rheology, the surface velocity can be significantly different from that in the lower crust.

12. The predicted plateau formation is sensitive to the lower crustal rheology Lower crustal viscosity 2.5 times higher than the base model Lower crustal viscosity 0.5 times the base model value

13. Predicted uplift history of the different parts of the Tibetan Plateau Accelerated uplift but different history at various parts of Tibet; Most parts reached >3 km ~10-20 Myr ago.

14. Predicted Crustal Mass Distribution

15. Conclusions The E-W extension in Tibet can be explained by gravitational collapse of the plateau. Major E-W extension started only when the plateau reached ~75% of its present values. Assuming a flat Asian continent before the Indo-Asian collision, the plateau would have grown from S to N, and from W to E; most part of the plateau probably reached >3 km 10-20 million years ago. The lower crust flow largely controls the topographic evolution; the motion of the upper crust may be significantly different from that in the lower crust. Partitioning of the shortened crustal mass between thickening and lateral extrusion/erosion changed with time. Mass accommodated by mountain building may have peaked ~10 Myr ago; extrusion and erosion become increasingly important.

16. If lithostatic stress condition was applied at the south China block, strike-slip would be promoted and the elevation in the eastern Tibetan plateau should be 1 km lower, velocity higher, slop steeper.