ALONG-STRIKE STRAIN VARIATIONS IN THE WESTERN AND CENTRAL HIMALAYAN OROGEN

Conventional studies on the tectonics of Himalayan orogen are often conducted along transects cross-cutting the orogen. This approach often overlooks or is incapable of recognizing variations in strain along the strike of the orogen. Here, we integrate published geological maps and cross-sections and invoking the 3D kriging algorithm in Midland Valley MOVE, we constructed 3D surfaces of the Main Himalayan Thrust (MHT), Main Central Thrust (MCT), and South Tibet Detachment (STD), which are 3 major shear zones that bound the high-grade metamorphosed core of the orogen and play important roles in models explaining accommodating strain within the Himalayan orogen. The thickness of the metamorphosed core, bounded by the STD at the top and the MCT and MHT at the frontal and hinterland parts of the bottom respectively, varies significantly in the hinterland along the strike of the orogen, from ~25-26 km in the western Himalaya to ~34-42 km in the central Himalaya. Correspondingly, the STD is structurally low in western Himalaya and structurally high in central Himalaya with significant structural relief, as much as ~37 km, due to the change of thickness of the metamorphosed core below it. Both the thickness of the metamorphic orogenic core and the elevation of the STD abruptly change across the Gurla-Mandhata metamorphic core complex along the strike of the orogen, suggesting a dramatic change in style of deformation. We propose that the western and central Himalaya have different strain-accommodating mechanisms along the same convergent boundary. The strain in western Himalaya, where oblique convergence has been ongoing at least since the Pliocene due to the curvature of the orogen, was partitioned into orogen-parallel stretching and orogen-normal shortening components. The dominating stretching strain caused vertical crustal thinning and extensional features, such as the Zhada basin developed in the hinterland. Along with the Gurla Mandhata detachment system these regional structures are envisioned to be kinematically linked with structures in the western syntaxis to form a forearc sliver. The central Himalaya has little obliquity in convergence and therefore the strain was accumulated dominantly by orogen-normal shortening and vertical thickening, causing relatively thick metamorphosed core and high STD.