The Greater Himalayan Thrust Belt: Insight Into the Assembly of the Exhumed Himalayan Metamorphic Core, Modi Khola Valley, Central Nepal. Issue 9 (2nd September 2020)

• Record Type: Journal Article
• Authors: Shrestha, Sudip; Larson, Kyle P.; Martin, Aaron J.; Guilmette, Carl; Smit, Matthijs A.; Cottle, John M.
• Is Part Of: Tectonics. Volume 39:Issue 9(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-09-02
• Subjects: Himalayan metamorphic core -- tectonometamorphic discontinuities -- P‐T‐t paths -- Greater Himalayan thrust belt -- petrochronology -- phase equlibria modeling; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2020TC006252
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: Strike‐parallel tectonometamorphic discontinuities within the Himalayan metamorphic core are typically interpreted to reflect thrust‐sense movement. However, there is disagreement on the nature and sense of movement across one such structure in central Nepal. Using an integrated approach, this study characterizes multiple structural breaks in the Modi Khola region. Thermobarometric calculations combined with petrochronological investigation show that 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, and followed by cooling and exhumation between ca. 24 and 15 Ma. Rocks below the structurally lower 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, rocks in the footwall of the Main Central thrust, the structurally lowest break investigated, 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. When paired with published cooling ages across the Bhanuwa fault, results from this study are consistent with normal‐sense reactivation of the structure during middle‐late Miocene time. This new data set shows that the final assembly of the Himalayan metamorphic core is a result of progressive deformation and juxtaposition of multiple thrust sheets through a combination of in‐sequence thrusting, out‐of‐sequence thrusting, and normal faulting. We refer to this as the Greater Himalayan thrust belt. Key Points: Structural breaks within exhumed metamorphic core in the Modi Khola valley include Sinuwa thrust, Bhanuwa fault, and Main Central thrust Rocks record similar prograde but varying history of anatexis, cooling, and exhumation indicating down‐structural migration of metamorphism P‐T‐t paths outline in‐sequence thrusting, development of the Greater Himalayan thrust belt followed by reactivation of the Bhanuwa fault