Tectonometamorphic evolution of the Himalayan metamorphic core in the Makalu-Arun region, eastern Nepal

Abstract

The metamorphic core of the Himalayan orogen, once viewed as a structurally contiguous unit, is now known to be divided into multiple packages by a series of structural discontinuities. While these structures have been identified based on distinct metamorphic and kinematic histories of the bounding packages, the number of structures identified across the exhumed former midcrustal core varies throughout the Himalaya. The scarcity of field evidence for these structures not only impedes their identification, but also hinders our understanding of their role in the development of the Himalaya. This study characterizes the metamorphic and geochronological history of garnet, sillimanite, and kyanite bearing gneisses in Makalu-Arun region, Nepal. Phase equilibria modelling coupled with monazite U/Th-Pb petrochronology delineates four rock packages with distinct metamorphic histories separated by at least three thrust-sense structures. The earliest thrust activity and subsequent in-sequence thrusting lasted from ca. 23-14 Ma, with initiation of late out-of-sequence thrust after ca.14 Ma. These results are consistent with foreland thrust migration and juxtaposition consistent with models that incorporate underplating, metamorphism and exhumation of midcrustal rocks during orogenesis. Moreover, the demonstrated complex evolution of the metamorphic core in Makalu region is inconsistent with models that incorporate singular, orogenic-wide structures in the Himalaya. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6420244