LITHOSPHERIC-SCALE TECTONICS OF THE INDIA-ASIA COLLISIONAL SYSTEM

Abstract

The India-Asia orogenic system is the largest and among the most well studied orogens on Earth. Despite its status as an archetype for large hot orogens, there remain many unknowns regarding the first-order evolution of this system. Part of this uncertainty concerns the behavior of the continental lithospheric mantle (CLM).

This thesis uses 2D mantle-scale geodynamical models to investigate how behavior of the CLM has influenced the evolution of the India-Asia collisional system. By incorporating diverse sets of geological observations, the focus of this research is to show that these diverse, often enigmatic observations of the system, can be explained self-consistently with geodynamical models.

For the Himalayan-Tibetan orogen, model results show that early CLM delamination of a weak central Qiangtang terrane, followed by delamination of the southern Lhasa CLM, can produce far-field crustal deformation and magmatism which later migrates south, contrary to expectations of continent-continent collision. Qiangtang delamination in the models drives opening of the subduction channel resulting in upwelling hot asthenosphere under the Himalayan crust, explaining early Eohimalayan metamorphism and predicting tectonic quiescence in Himalayan growth. Tectonic quiescence predicts significantly more shortened Asian crust than Indian crust, which may explain the relative narrow width of the Himalaya as compared to the Tibetan plateau.

For the Hindu Kush and Pamir, seismic evidence shows opposing slabs, with a steeply north-dipping slab inferred to be Indian CLM under the Hindu Kush, and a shallowly south-dipping slab, inferred to be Asian CLM under the Pamir. Models show that if the western-most part of the indenting India plate comprised thin, marginal lithosphere, this can lead to steep subduction of Indian lithosphere as observed for the Hindu Kush. In contrast, full thickness Indian lithosphere can indent Asia, leading to forced delamination of Asian CLM producing the observed south-dipping slab under the Pamir and the Pamir orocline.

Given the usefulness of geochemical data for understanding lithospheric processes, a secondary goal of this research was to develop novel machine learning techniques for studying geochemical data. Results of this research show that unrecognized geochemical features reflecting tectonic processes can be discovered using these methods.