The India-Asia pile up

Abstract

Many ideas about the tectonic history of the Himalayan orogen hinge on the arguments about the timing of collision (~70 Ma, ~50 Ma or ~34 Ma). One of these is whether the tectono-thermal evolution of the Himalaya involved dominantly episodic processes or continuous protracted deformation/heating. This thesis reviews the definitions proposed for the India-Asia collision and investigates whether the evolution of the Himalayan orogen involved episodic or continuous tectonic processes.

This study used SHRIMP U/Pb geochronology to determine if the granitoids of the Ladakh and Karakorum batholiths of NW India were emplaced episodically or continuously during the Himalayan orogeny. The SHRIMP results indicate that magmatism was episodic. However, we also found that zircons with high-uranium concentrations produce older apparent ages when analysed with SHRIMP. This presented a problem when interpreting the age of particular granitoid samples. We therefore investigated the cause of this problem and found that older apparent ages were related to matrix effects associated with radiation damage in the zircon.

One of the most widely used definitions to determine the timing of India-Asia collision suggests that this event is marked by a switch from 1- to S-type magmatism in the Trans-Himalayan batholith. Many consider this to occur at ~50 Ma. However, the work that is presented here suggests there was no switch from 1- to S-type magmatism where it is proposed to occur in NW India. However, U/ Pb SHRIMP and petrological data suggest there is potentially a switch from 1- to S-type magmatism in the Karakorum Batholith between 32 Ma and 18 Ma. This indicates that India-Asia collision occurred much later than many consider, or the I-S classification scheme should not be used to define the timing of collision.

This thesis also reviews the published rotation data for the India-Eurasia plate circuit and presents a new methodology for presenting the data and decisions that are adopted in plate reconstructions. The review shows the problems associated with existing plate reconstructions of the Indian plate, and highlights the need to ensure the highest resolution data is used in a given plate reconstruction. The most appropriate rotation data was compiled to build a new plate reconstruction of India's motion relative to Eurasia. This reconstruction implies the velocity of the Indian plate was episodic over the past 100 Ma. The episodes of plate acceleration and deceleration correspond with the timing of many tectonic events recognised along the Alpine-Tethyan orogen. However, the changes in plate velocity are not necessarily related to the India-Asia collision as other geodynamic scenarios explain these changes in velocity.

This thesis also questions the use of inherited/detrital age spectra of zircon to define the location of plate boundaries. New U/Pb data from the Karakorum Terrane suggests that it has a similar age spectra to the Himalayan Terrane to the south and the Pamir to the north. These results indicate that the Karakorum Terrane was once a component of Gondwana (or derived from Gondwanan material) and highlights the problems associated with using age spectra to "fingerprint" terranes.

A synthesis of all of the results indicates that the Himalayan orogen involved multiple accretion events, not just one collision between India and Asia. Its evolution is therefore analogous to a multi-vehicle freeway pile-up, rather than a head-on collision between two vehicles.