Evidence for Pliocene-Quaternary normal faulting in the hinterland of the Bhutan Himalaya

In the central Himalaya, past researchers have identified a distinctive transition from the physiographic Lower Himalayan ranges in the south to the Higher Himalayan ranges in the north. Local relief and hillslope gradient, as well as erosion and surface uplift rates, increase abruptly across this transition to the north. In the eastern Himalaya, the same physiographic transition exists, but it is less dramatic. We describe here a previously undocumented steep, north-dipping, brittle structure that is roughly coincident with this physiographic transition in eastern Bhutan-the Lhuentse fault. Low-temperature (U-Th)/He apatite data suggest that the Lhuentse fault has been active since the Pliocene, and (U-Th)/He dates on offset hydrothermal hematite deposits from within the fault zone demonstrate a component of Quaternary slip. Although we identified no definitive evidence of fault kinematics based on field or petrographic analysis of the fault rocks, the disrupted pattern of (U-Th)/He apatite dates suggests normal-sense displacement, contrary to what was expected given previous studies of an analogous transition in the central Himalaya. We regard the existence and activity of the Lhuentse fault as evidence of (1) recent evolution in the tectonic regime of the eastern Himalaya from one of near-exclusive north-south shortening to one in which both transcurrent and normal faulting are increasingly important in the region north of the Himalayan deformation front, or (2) an active duplex south of the physiographic transition in the middle latitudes of Bhutan.