CENOZOIC SLIP HISTORIES OF THE TIDDING AND LOHIT THRUSTS IN THE NORTHERN INDO-BURMA RANGES, AND IMPLICATIONS FOR CRUSTAL THICKENING AND EXHUMATION OF GANGDESE LOWER ARC CRUST ALONG THE INDIA-ASIA SUTURE ZONE

Crustal thickening has been the dominant form of Cenozoic deformation along the India-Asia suture zone since the onset of continental collision. However, the timings, geometries, displacement magnitudes, and along-strike continuity of major thrusts along this boundary, such as the Great Counter thrust (~25-10 Ma) and Gangdese thrust (~27-18 Ma), remain inadequately understood. In this study, we present the findings of new geological mapping and thermochronologic, geochronologic, and geothermobarometric analyses across two key contractional structures along the easternmost India-Asia suture zone in the northern Indo-Burma Ranges—the north-dipping Lohit and Tidding thrusts—and their respective hanging wall rocks including the latest Jurassic-Cretaceous Gangdese batholith (i.e., Lohit Plutonic Complex) and the Indus-Yarlung suture zone (i.e., Tidding mélange complex). Our field observations confirm that ductile deformation of the Tidding mélange complex is concentrated along the basal, top-south Tidding thrust, which contrasts the geometry and top-north kinematics of the Great Counter thrust at the same structural position to the west. Mélange rocks experienced peak upper amphibolite-facies metamorphism at depths of ~33-38 km prior to exhumation during slip along the Tidding thrust at ~36-30 Ma. To the north, the ~5-km-wide Lohit thrust shear zone and discrete Lohit thrust fault near its base have subvertical geometries and north-side-up kinematics. Cretaceous granitoids of the Lohit Plutonic Complex in the hanging wall were emplaced at depths up to ~39 km in crust that was ~38-52 km thick at that time and subsequently exhumed to mid-crustal depths during ~25-23-Ma slip along the Lohit thrust. Given that the Lohit and Gangdese thrusts are at the same structural position and have comparable geometries, kinematics, and lifespans, we interpret that these thrusts formed segments of a single, blind thrust system that served as the preeminent crustal thickening structure along the Neotethys-southern Lhasa terrane margin during the Oligocene-Miocene. Whereas much of the Gangdese thrust remains concealed along this margin, the local exposure of the Lohit and Gangdese thrusts in the northern Indo-Burma Ranges and eastern Himalayan syntaxis is attributed to the Late Miocene initiation of focused duplexing of structurally lower Indian continental rocks and resultant exhumation of the overlying blind thrust system and its hanging wall rocks, possibly related to the onset of clockwise crustal flow around northeastern India.