PRELIMINARY CONSTRAINTS ON THE LITHO-TECTONIC FRAMEWORK OF THE EASTERNMOST HIMALAYA, SIANG VALLEY REGION, BASED ON GEOLOGIC MAPPING AND U-PB ZIRCON GEOCHRONOLOGY

The geometric and age relationships of rock units and structures of the easternmost Himalaya located near the eastern syntaxis remain inadequately understood, despite being important to understanding the evolution of the entire orogen. In this study, we present the preliminary results of detailed field mapping and detrital zircon U-Pb geochronology across the easternmost Himalaya of the Siang Valley region, NE India. Our work shows that rock units from SE to NW consist of (1) sandstone and conglomerate of the Siwalik Group, (2) quartz arenite and greywacke of the Yinkiong Formation(?), (3) shale, sandstone, and carbonate of the Gondwana Group, (4) quartzite, slate, muscovite schist, calc-schist, paragneiss, and phyllite of the Bichom Formation and Miri-Buxa Group (Lesser Himalayan Sequence; LHS) and basalt and volcaniclastic rocks of the Abor volcanics, (5) paragneiss, marble, and garnet biotite schist of the Bomdila Group (Lesser Himalayan Crystalline Complex; LHC), and (6) augen gneiss, schist, migmatite, and paragneiss with relict cross-beds of the Sela Group (Greater Himalayan Sequence; GHS). The Siwalik Group is bounded to the south by the Main Frontal thrust and internally shortened by the NW-dipping, brittle Mingo thrust. The NW-dipping Main Boundary thrust (MBT) is expressed as a ~5-m-wide brittle fault zone with top-SE kinematics and juxtaposes the Miri-Buxa Group atop the Yinkiong Formation(?). In the hanging-wall of the MBT, the structurally-lower exposure of the Main Central thrust (MCT-I) is expressed as a ~8-km-wide, NW-dipping mylonite zone with top-SE kinematics and places the Bomdila Group atop the Miri-Buxa Group. The structurally-higher MCT-II is expressed as a ~1-km-wide, NW-dipping mylonite zone with top-SE kinematics and juxtaposes the Sela Group atop the Bomdila Group. Internal deformation (e.g., isoclinal folding and slaty cleavage) is pervasive within LHS, LHC, and GHS rocks. Our mapping confirms previous workers’ observations of an inverted greenschist-upper amphibolite facies sequence that spans >30 km leading up to the MCT-II. U-Pb geochronology of the LHC, LHS, Yinkiong Formation(?), and Siwalik Group yields ranges of maximum depositional ages of ~1023-883 Ma, ~529-431 Ma, ~133-97 Ma, and ~26-13 Ma, respectively. Our revision of the litho-tectonic architecture of the Siang Valley region provides a framework to further understand orogen-parallel variations in the geometric configurations and ages of rock units and structures that influenced and/or developed during Himalayan orogenesis.