Controls of Stratigraphic Architecture on Along Strike Cooling Age Patterns

Cooling related to moving rocks over an similar to 8.5 km vertically thick ramp in the Main Himalayan thrust (MHT) produced 0.8-2 Ma zircon (U-Th)/He (ZHe) and apatite fission track (AFT) ages from the Main Central thrust to 40 km south in central Nepal. The ramp size is controlled by thicker Proterozoic stratigraphy. ZHe ages in NW India and eastern Bhutan over a similar 40 km wide swath are 1.4-12 Ma. In addition, the ZHe ages are consistently 2-5 million years older than the AFT ages. We argue that the MHT ramps are smaller because of thinner Proterozoic stratigraphy, with two vertically thick ramps of 2.5 km each in eastern Bhutan and one vertically thick ramp of similar to 5 km in NW India. Thus, the original stratigraphy and associated weaker decollement horizons control location and size of active MHT ramps, resulting vertical uplift, and along strike pattern of cooling ages.