What geological data can be used to define the timing of plate collision?

The difficulty in defining the timing of plate collision is in part because it can be diachronous, but the main issue is what geological data are used to determine when collision occurred. Many consider India-Asia collision is marked by (1) a change from marine to continental sedimentation in the Indus-Tsangpo Suture Zone (ITSZ); (2) the end of I-type granite production in the Trans-Himalayan Batholith and (3) the first development of south-facing folds and thrusts in the ITSZ. Yet there are two possible periods when sedimentation changed from marine to continental in the ITSZ, multiple switches of I- and S-type granite genesis occurred at different times along the orogen and it is difficult to precisely date episodes of shortening. Metamorphic ages are also interpreted to mark collision, yet such rocks often form due to extension.

Others propose that collision can be identified by deceleration of the Indian plate at ~65-50 Ma. However, as India was situated thousands of kilometres south of Eurasia at this time, this scenario is only possible if one extends the northern margin of India or the southern margin of Eurasia to arbitrarily “fill the gap”. It is more likely that India’s rapid deceleration was caused by collision with an intraoceanic island arc and death of one of the subduction zones that existed between India and Eurasia during the late Cretaceous.

Studies of much younger collision zones in SE Asia show that uplift is rapid and commonly associated with phases of extension rather than crustal shortening, meaning that a change from marine to continental sedimentation does not require plate collision. Pulses of rapid uplift also mean rapid erosion will occur, so key evidence is often removed. It is also important to consider the geometry of colliding continental margins. SE Asia provides some insight into the complexity of collision, extension and margin geometries. For example, continued northward motion of Australia has already resulted in collision between different parts of the former Australian continent. Such complexities are rarely considered in tectonic models of the India-Eurasia system. Arguments concerned with the timing of India and Asia collision will continue unless we reconsider the criteria used to define collision.