[1] The plate motion of India changed dramatically between 50 and 35 Ma, with the rate of convergence between India and Asia dropping from $15 to $4 cm/yr. This change is coincident with the onset of the India-Asia collision, and with a rearrangement of plate boundaries in the Indian Ocean. On the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate, we perform force balance calculations for the precollision and postcollision configurations. We show that the observed Euler poles for the Indian plate are well explained in terms of their locations and magnitudes if (1) the resistive force induced by mountain building in the Himalaya-Tibet area is $5-6 Â 10 12 N/m, (2) the net force exerted upon the Indian plate by subduction zones is similar in magnitude to the ridge-push force ($2.5 Â 10 12 N/m), and (3) basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame. The third point implies an asthenospheric viscosity of $2-5 Â 10 19 Pa s, assuming a thickness of 100-150 km. Synthetic Euler poles show that crustal thickening in the Tibetan Plateau was the dominant cause of the Cenozoic slowdown of the Indian plate.

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S U M M A R YSurface velocities in parts of the India-Asia collision zone are compared to velocities calculated from equations describing fluid flow driven by topographically produced pressure gradients. A good agreement is found if the viscosity of the crust is ∼10 20 Pa s in southern Tibet and ∼10 22 Pa s in the area between the Eastern Syntaxis and the Szechwan Basin. The lower boundary condition of the flow changes between these two areas, with a stress-free lower boundary in the area between the Szechwan basin and the Eastern Syntaxis, and a horizontally rigid but vertically deformable boundary where strong Indian lithospheric material underlies southern Tibet. Deformation maps for olivine, diopside and anorthite show our findings to be consistent with laboratory measurements of the rheology of minerals. Gravitationally driven flow is also suggested to be taking place in the Indo-Burman Ranges, with a viscosity of ∼10 19 -10 20 Pa s. Flow in both southern Tibet and the Indo-Burman Ranges provides an explanation for the formation of the geometry of the Eastern Himalayan Syntaxis. The majority of the normal faulting earthquakes in the Tibetan Plateau occur in the area of southern Tibet which we model as gravitationally spreading over the Indian shield.

[1] This paper examines how active faulting in the Turkey-Iran-Caucasus region accommodates the Arabia-Eurasia collision and the velocity field observed by GPS. The overall shortening across the zone is, in general, spatially separated (''partitioned'') into right-lateral strike slip in the Turkish-Iranian Plateau and thrusting in the Greater Caucasus. A band of counterclockwise rotating NW-SE right-lateral strike-slip faults accommodates a NW-SE gradient in NE directed velocity (relative to Eurasia) between the Black and Caspian seas. A NNW-SSE band of previously unrecognized oblique normal faults is present on the Turkey-Iran border. We estimate the offsets on faults from geomorphological features and show that these offsets can be achieved in 5 ± 2 Ma at present rates. This implies a reorganization of deformation in the collision zone at that time, after the initial collision at $12 Ma, probably in response to mantle-induced dynamic uplift.