Exhumation history of the deepest central Himalayan rocks, Ama Drime range: Key pressure-temperature-deformation-time constraints on orogenic models

Abstract

[1] The Ama Drime range located at the transition between the high Himalayan range and south Tibet is a N–S active horst that offsets the South Tibetan Detachment System (STDS). Within the horst, a paragneissic unit, possibly attributed to the upper Himalayan crystalline series, overly the lower Himalayan crystalline series Ama Drime orthogneissic unit containing large metabasite layers and pods that have experienced pressure ≥1.4 GPa. Combining structural analysis with new and published pressure-temperature (P-T) estimates as well as U-Th/Pb, 39Ar/40Ar and (U-Th)/He ages, the P-T-deformation-time (P-T-D-t) paths of the main units within and on both sides of the horst are reconstructed. They imply that N–S normal faults initiated prior to 11 Ma and have accounted for a total exhumation ≤0.6 GPa (22 km) that probably occurred in two phases: the first one until ∼9 Ma and the second one since 6 to 4 Ma at a rate of ∼1 mm/yr. In the Ama Drime unit, 1 to 1.3 GPa (37 to 48 km) of exhumation occurred after partial melting since ∼30 Ma until ∼13 Ma, above the Main Central Trust (MCT) and below the STDS when these two fault systems were active together. The switch from E–W (STDS) to N-S (Ama Drime horst) normal faulting between 13 and 12 Ma occurs at the time of propagation of thrusting from the MCT to the Main Boundary Thrust. These data are in favor of a wedge extrusion or thrust system rather than a crustal flow model for the building of the Himalaya. We propose that the kinematics of south Tibet Cenozoic extension phases is fundamentally driven by the direction and rate of India underthrusting.