The Geomorphic Response of An Active Metamorphic Core-Complex: An Example from the Lunggar Rift, Southern Tibet

The N-trending Lunggar Range parallels a rift valley in west-central Tibet. The Lunggar Rift valley is ~70km long and 5-10km wide. The central part of the Lunggar Rift is bounded on its western flank by a <40°E-dipping normal fault that juxtaposes variably deformed biotite granite, leucogranite, and mylonitic gneiss in the footwall against Paleozoic strata and Neogene alluvial fan, fluvial and lacustrine rocks in the hanging wall. We investigated the along-strike geomorphic response of the footwall and hanging wall by analyzing stream channel profiles. Because of the effects of lithologic and structural controls on channel profiles, we limited our analysis to the central Lunggar Rift because of the uniform lithology and structure. We subdivide individual channel profiles into three regions based on the following; an upper glacial dominated reach, an intermediate reach characterized by debris flow dominated incision with low channel concavity, and a downstream fluvial dominated reach with higher channel concavity. Concavity increases southward along the Lunggar Range, with channel profiles to the north dominated by debris flow and glacial dominated reaches. These preliminary observations suggest that the Lunggar Range is likely a transient landscape responding to along-strike variations in magnitude of fault slip. An additional observation is that a drainage divide occurs in the central Lunggar Rift in the region of maximum inferred extension. To understand the relationship between the hanging wall drainage divide and the footwall, rift parallel topographic swath profiles were generated for basin elevations and a 4km swath spanning the range crest. When comparing elevation versus distance relative to the region of inferred maximum extension, maximum basin elevations coincide with the region of inferred maximum slip. This is in contrast to Tibet rifts with smaller magnitudes of displacement (< 5km's) which have minimum basin elevations in the central rift, consistent with a half graben geometry.