Indian Plate motion and shape: constraints on the geometry of the Himalayan orogen

Sea-floor palaeomagnetic data that reflect variations in rate and vector of Indian Plate movement and rotation suggest that initial collision between India and Asia occurred at about 50–55 Ma ago. As the pre-collisional Indian Plate was diamond shaped, with the northern margin comprised of two oblique boundaries, collision was earliest where these boundaries meet, or in what is now the northwest Himalaya. Oblique convergence along each of these two boundaries would generate rotation of thrust sheets as they climb on to the Indian Plate. The oroclinal shape of the main Himalayan chain to the east of the northwest Himalayan syntaxes reflects a combination of the effects of oblique convergence, post-collisional anticlockwise rotation of the Indian Plate, and the pinning of the main thrusts at their northwestern terminations by crust thickened during the earliest collisional stage. The Indian Plate rotation enhances a strike-slip component of movement along the western oblique margin, with the transpressively sinistral Chaman fault zone now acting as a continental escape structure.