Neotectonics of the Kumaun-Garhwal region of the Indian Himalaya

Field observations, geomorphologic evidence, and precise U-Th ages of deformed speleothems lend new insight into the nature of active deformation in the Kumaun-Garhwal region of the Indian Himalaya, ~75 km north of the Main Frontal Thrust. Growth strata and minor displacements evident in fluvial-lacustrine fan sequences indicate regional tilting ~10-20 km south of the Main Central Thrust near the town of Uttarkashi, Uttarakhand, yet these features do not appear to correlate with prominent surface-breaking fault traces or previously mapped structures. Approximately 300 km along-strike near the border with western Nepal, new U-Th ages from deformed speleothems (cave deposits) indicate ongoing Quaternary (~2 ka or less) northeast-directed rotations and deformation that, similar to the deformation observed at Uttarkashi, do not appear to be related to slip along previously mapped faults. This region coincides with a sharp, ~200 km long, NW-SE striking boundary that defines significant northward increases in topography, mean hillslopes, normalized river steepness (ksn), and mean annual rainfall, and is located more than ~600 km along-strike from the well-studied physiographic transition recognized in Central Nepal (i.e. Wobus et al., 2003). The landscape transition of northern India also coincides roughly with an order of magnitude decrease in published Ar-Ar cooling ages, with the youngest (late Miocene) ages occurring nearest the landscape transition (Celerier et al., 2009). These observations collectively suggest that enhanced erosion, and most likely heightened differential rock uplift, has occurred in the region north of the landscape transition since the late Miocene. While these data do not preclude the existence of an active surface-breaking thrust fault high in the range, the existing data preliminarily suggest that, similar to Central Nepal, the neotectonics of the Kumaun-Garhwal region of India could be best explained by continual growth of an active duplex and/or underplating at depth along a structural ramp that flattens southward and links up-dip with the Main Frontal Thrust.