THE HIMALAYAN MOUNTAINS AND ANALOGOUS SYSTEMS WERE BUILT BY UNDERPLATING

Extrusion and underplating models are invoked to address Himalayan tectonic questions, including how ongoing mountain-building proceeds, and how the crystalline core developed and was emplaced. Here, extrusion involves exhumation from the orogenic middle crust to the surface between surface-breaching faults; underplating involves accretion of material from underthrusting India to the over-riding orogen. The two model types are not mutually exclusive, but the extrusion models require >>10 km normal-sense slip along the South Tibet detachment (STD) which separates the Himalayan crystalline core from the lower grade carapace. The concepts that such normal faults occur and denote extrusion have been widely exported to ancient orogenic belts which feature antithetic shear zones.

Our integrated field, analytical, and balancing research reveals that Himalayan development was dominated by underplating in concert with erosive processes. The smoking gun demonstrating that development and emplacement of the crystalline core occurred via underplating is the preserved merger of the bounding faults (the STD and the Main Central thrust) at the leading edge of the crystalline core. Here we report structural mapping, microstructural, quartz c-axis fabric, and geochronological data that demonstrate preservation of the merger in two regions of Nepal. Because the Himalaya is the source of orogen-parallel syn-convergent normal faulting criteria and yet satisfies such criteria via thrust faulting, this normal faulting concept, associated extrusion models, and understanding of STD-analogue structures worldwide requires re-evaluation. We explore Himalayan deformation subsequent to crystalline core emplacement via field mapping and balanced palinspastic reconstruction across northwest India. Resulting structural geometries require that discrete underplating processes dominated here and resolve a stratigraphy question across the India – west Nepal border (the Ramgarh thrust hanging wall problem). Balanced reconstruction indicates that the underplating was accomplished via simultaneous development of multiple duplexes along different ramps of the Himalayan sole thrust. In summary, Himalayan mountain building has been continuously dominated by underplating since the Oligocene.