UPLIFT IN THE NEPAL HIMALAYA REVEALED BY SPIRIT LEVELING

A 250-km-long first-order leveling line linking the geodetic surveys of Tibet and India through the Kingdom of Nepal has been measured twice in the past 15 years. Changes in relative bench mark elevation projected along a 200 km line orthogonal to the mean structural trend in the Himalaya indicate a pattern of uplift with wavelengths of 30-50 km not related to earthquake occurrence. Relative vertical velocities of bench marks of up to 4 mm/year show no evident correlation with level line slope. An absence of microearthquake data prevents localized uplift from being attributed to slip on mapped or buried faults, although circumstantial evidence suggests this is likely. In the North, in line with the high Himalayan peaks, relative uplift rates of 4 +/- 1 mm/yr and a wavelength of 50 km are coincident with a steepening in the Main Boundary Thrust and the Moho proposed by Lyon-Caen and Molnar [1985]. In the South, uplift rates of 2 +/- 0.3 mm/yr with a wavelength of 30 km correspond to active folding of an anticline on the southern flank of the Lesser Himalaya. Observed tilt rates of 0.06-mu-rad/year are similar to the long term tilt rates needed to produce the 40-70-degrees flanks of the anticline. A range of detachment geometries and slip velocities can generate the observed deformation field, but geologically realistic models fall into a group of horizontal to shallow dipping, subhorizontal faults between 12 and 6 km in depth. No unique solution exists, although the wavelength of the observed deformation field permits models with significant slip closer than 4 km to the surface to be excluded.