How steep are the Himalaya? Characteristics and implications of along-strike topographic variations

Differences in the topography of central Nepal and Bhutan are characterized by map and profile patterns of elevation, slope, relief, and stream gradients. Nepal exhibits a narrow hinterland zone of extreme relief, steep slopes and channels, and deep fluvial dissection and a wide foreland zone of lower relief; active shortening on the Main Frontal thrust occurs far outboard of the steep topography. In contrast, Bhutan has two high-relief zones separated by a narrow, low-relief step in the topographic and river profiles; the southern zone has the higher relief, rising abruptly from the Main Frontal thrust to a series of 4000 m peaks, whereas the northern zone consists of low-relief, high-elevation, plateau-like terrain cut by widely spaced, steep-walled valleys. Spatial relationships between these geomorphic features, and geologic contrasts, are used to examine scenarios of tectonic and surface-process interactions. The observations favor a model of along-strike segmentation in which timing differences in the varying balance of uplift and erosion produce spatial differences of topographic and geologic patterns, although other interpretations are possible.