QUATERNARY SHORTENING AND PALEOSEISMICITY IN THE FOLD-THRUST BELT OF BHUTAN HIMALAYA

Little work has been done on the paleoseismicity of the major faults in Bhutan, particularly the Main Frontal Thrust (MFT). Previous scientific research reports that most of the shortening along the Himalayan mountain front is concentrated along the MFT and that shortening rates along the MFT increase from west to east. Our preliminary observations in southern Bhutan suggest that the system is more complex, including out of sequence faulting north of the MFT, within the Main Boundary Thrust (MBT) zone. A series of alluvial cut-and-fill and bedrock (strath) river terraces near the town of Gelephu records a prolonged history of aggradation and incision in response to climatic changes as well as surface uplift and discrete fault offsets associated with the MBT. In 2013 we investigated these geologic markers revealing a flight of Quaternary aged fluvial terraces (Q3-Q1, oldest to youngest). Methods used to investigate the geomorphic evolution of these river terraces include: geologic mapping, surveying, and OSL geochronology. Geologic mapping places the flight of terraces north of the MFT within the MBT zone. Surveying of the oldest terrace tread (Q3) reveals that this surface is back tilted. Field investigations have also identified back tilted alluvial deposits below the Q3 surface. North dipping fault scarps, representing normal faulting have been identified that cut and offset Q3 and Q2 surfaces, creating a graben. Back tilting of the Q3 surface and alluvial deposits is interpreted to represent a kilometer-scale, east-west striking anticline with the graben forming along the axis of the anticline. In the northern limb of this anticline, a south-dipping, 3-4 m thrust fault scarp has been identified that offsets the Q2 surface and may represent the last surface rupture of the MBT. Degradation modeling of surveyed scarp profiles will be used to determine scarp ages helping to constrain the timing of fault surface ruptures. A suite of samples was collected from the various deposits to provide age control of the Q3-Q1 surfaces via ¹⁴C and OSL (optically-stimulated luminescence) geochronology. Age control of the Quaternary surfaces will be used to constrain the timing and rates of deformation and faulting of these respective surfaces, thus providing insight to the complex nature of the activity north of the MFT within the MBT zone.