3D STRAIN PARTITIONING AND ACCUMULATION IN AN ARCUATE OROGENIC WEDGE: AN EXAMPLE FROM THE HIMALAYA

Studies on orogenic wedge growth often invoke critical taper theory which usually fails to consider orogen-parallel strain partitioning and to assess the role of rheology changes along the megathrust on structural architecture. We use the Himalayan orogen to investigate these two problems by integrating published geologic maps and cross-sections and constructing 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. Our 3D model of the core of the western and central Himalaya shows significant along-strike variation in the thickness, from ~25-26 km in the western Himalaya to ~34-42 km in the central Himalaya, indicating significant along-strike strain variation. Both the thickness of the orogenic core and the elevation of the STD abruptly change along the strike of the orogen across the western bounding fault of the Gurla Mandhata metamorphic core complex, demonstrating change in the style of strain there. Assuming the thickness of the orogenic core at 37 Ma is 17 km and constant along the strike, the western and central Himalaya have been thickened by 0.5 and 1-1.5 times respectively. We propose an oblique convergence model, in which orogen-parallel strain partitioning in the western oblique convergence sector can explain the dramatic along-strike strain variation and many associated features, such as metamorphic core complex, orogen-parallel stretching lineation and shear zones within the orogenic core and pull-apart basins of low subsidence rate. The mid-upper crustal strain partitioning could be triggered by increased convergence obliquity in western Himalaya via rotational shortening and/or oroclinal bending and/or the formation of a weak rear back via the development of the South Kailas thrust and south propagation of the KKF in Middle-Late Miocene. The comparison between our model and other datasets supports the hypothesis that the brittle-ductile transition zone along the megathrust is where strain locally accumulates, and mid-lower crustal duplexes develop, to facilitate outward growth of orogenic plateau. The hinterland-ward migration of the brittle-ductile transition zone of the MHT in far-western Nepal may have caused continuously focused thickening in a narrow hinterland, generating the thickest part of the orogenic core.