The Greater Himalayan thrust belt: Insight into the assembly of the exhumed Himalayan metamorphic core, Modi Khola valley, central Nepal

Abstract

"Strike-parallel discontinuities within the Himalayan metamorphic core are interpreted to reflect thrust-sense movement. There is, however, disagreement on the nature and sense of movement across one such structure in central Nepal, which has hampered efforts to understand its kinematics and potentially correlate it with structures at similar structural levels. Using an integrated approach, this study characterizes multiple structural breaks in the Modi Khola region. Thermobarometric calculations combined with petrochronological investigation show that the rocks across the Sinuwa thrust record similar histories with prograde garnet growth ca. 35 Ma and peak pressures of ~ 11.0 kbar, anatexis at ca. 28 Ma followed by cooling and exhumation between ca. 24 and 15 Ma. Whereas, rocks below the Bhanuwa fault record similar garnet growth at ca. 35 Ma and pressures of ~11.5 kbar, but experienced melting and retrogression after ca. 21 Ma. Finally, the rocks in the footwall of Main Central thrust record prograde metamorphism ca. 17 - 13 Ma with peak pressures of ~ 7.0 kbar. This down-structural migration of prograde metamorphism, anatexis, and subsequent cooling and exhumation of the footwall is consistent with models of progressive underplating and in-sequence thrusting. Paired with published cooling ages across the Bhanuwa fault, results from this study indicate reactivation of normal-sense motion across the structure later during mid-late Miocene time. This new dataset shows that the final assembly of the Himalayan metamorphic core is a result of progressive deformation and juxtaposition of multiple thrust sheets. We refer to this as the Greater Himalayan thrust belt."