CONSTRAINTS ON HIMALAYAN DEFORMATION INFERRED FROM VERTICAL VELOCITY-FIELDS IN NEPAL AND TIBET

Spirit leveling data from the Nepal Himalaya between 1977 and 1990 indicate localized uplift at 2-3 mm/yr in the Lesser Himalaya with spatial wavelengths of 25-35 km and at 4-6 mm/yr in the Greater Himalaya with a wavelength of almost-equal-to 40 km. Leveling data with significantly sparser spatial sampling in southern Tibet between 1959 and 1981 suggest that the Himalayan divide may be rising at a rate of 7.5+/-5.6 mm/yr relative to central Tibet. We use two-dimensional dislocation modeling methods to examine a number of structural models that yield vertical velocity fields similar to those observed. Although these models are structurally nonunique, dislocation models that satisfy the data require aseismic slip rates of 2-7 mm/yr on shallow dipping faults beneath the Lesser Himalaya and rates of 9-18 mm/yr on deep thrust faults dipping at almost-equal-to 25-degrees-N near the Greater Himalaya. Unfortunately, the leveling data cannot constrain long-wavelength uplift (>100 km) across the Himalaya, and unequivocal estimates of aseismic slip in central Nepal are therefore not possible. In turn, the poor spatial density of leveling data in southern Tibet may inadequately sample the processes responsible for the uplift of the Greater Himalaya. Despite these shortcomings in the leveling data, the pattern of uplift is consistent with a crustal scale ramp near the Greater Himalaya linking shallow northward dipping thrust planes (3-6-degrees) beneath the Lesser Himalaya and southern Tibet. Aseismic slip on the potential rupture surface of future great earthquakes beneath the Nepal Himalaya south of this ramp appears not to exceed 30% of the total convergence rate between India and southern Tibet resulting in an accumulating slip deficit of 13+/-8 mm/yr.