Uplift and convergence along the Himalayan Frontal Thrust of India

Abstract

Along the Himalayan thrust front in northwestern India, terrace deposits exposed 20 to 30 m above modern stream level are interpreted to have been uplifted by displacement on the underlying Himalayan Frontal Thrust. A radiocarbon age limits the age of the terrace to ≤1665±215 calendar BC (≤3663±215 radiocarbon years before present), yielding a vertical uplift rate of ≥6.9±1.8 mm/yr. In combination with published studies constraining the dip of the Himalayan Frontal Thrust fault to about 30° in the study area, the observed uplift rate equates to horizontal shortening across the Himalayan Frontal Thrust of ≥11.9±3.1 mm/yr and the slip rate of the Himalayan Frontal Thrust of ≥13.8±3.6 mm/yr. This is similar to previously reported rate estimates along the Himalayan arc based on displacement of older Plio‐Miocene age rocks, or the much shorter records of geodesy and historical seismicity. The similarity is consistent with the idea that convergence across the Himalayan front has occurred at a relatively steady rate through time. The seismic expression of this deformation includes several great (M∼8) historical earthquakes which, due to lack of surface rupture during those events, have been attributed to their occurrence on blind thrusts. Yet, the occurrence of a possible fault scarp in the field area indicates that past earthquakes have been sufficiently large to rupture to the surface and produce coseismic scarps. These observations suggest a potential for earthquakes along the Himalayan Frontal Thrust larger than those observed historically.