THE EFFECTS OF THE BRITTLE-DUCTILE TRANSITION ON THE INDIA-ASIA INTRACONTINENTAL SUBDUCTION ZONE

Abstract:

The intracontinental subduction zone between India and Asia is termed the Main Himalayan thrust. Beneath the range front and Lesser Himalaya the subduction interface is locked and only ruptures during great earthquakes. However beneath the High Himalaya and southern Tibet a rheologic transition from brittle to ductile deformation along the subduction zone has been defined based on microseismicity, geodesy, and thermal modelling. Geodetic data suggests that the crust above the transition zone strains to accommodate much of India-Asia interseismic convergence as southern Tibet creeps toward the locked portion of the Main Himalayan thrust. We combine regional geology with stream profile analysis, thermal modelling, geodesy, and microseismicity to understand spatial and temporal characteristics of the intracontinental subduction in this region of the Himalaya. This continuous flux of material against the locked portion of the subduction zone stores elastic strain which drives great earthquakes that rupture the plate boundary from southern Tibet to the range front. While the brittle-ductile transition zone stores a significant amount of interseismic strain it has also been a locus of permanent shortening and crustal thickening as shown by the presence of the Dolpo and Manaslu folds in Nepal and Kakhtang thrust in Bhutan, which were all active in the Middle Miocene. The forelimb of the Dolpo fold, a 400 km long 50 km half wavelength gentle fold, coincides with a 200 km x 50 km swath of oversteepened river channels indicative of rapid rock uplift suggesting the fold is currently being uplifted and shortened. The Dolpo fold experienced persistent shortening and crustal thickening despite being located adjacent to, and active concurrently with the Gurla Mandhata core complex indicating simultaneous N-S shortening and E-W extension implying a constrictional train field since the Middle Miocene. We interpret the Dolpo fold to reflect an active antiformal duplex along the Main Himalayan thrust in which shear zones stack ductile lower-middle crust and propagate into the seismogenic upper crust becoming nucleation sites for microseismicity. We predict the position of the duplex to be controlled not by a footwall ramp but rather by the transition from ductile to brittle rheology along the Main Himalayan thrust.