Geomorphic and paleoseismic evidence for late Quaternary deformation in the southwest Kashmir Valley, India: Out-of-sequence thrusting, or deformation above a structural ramp?

In the northwest Himalaya, partitioning of Indian-Eurasian convergence across multiple active structures, including a fold at the deformation front, and the Riasi thrust 60 km to the north, suggests that strain is partially accommodated by out-of-sequence thrusting. Deformation of the Plio-Pleistocene Karawa deposits (KD) and latest Pleistocene fluvial terraces on the southwest side of the Kashmir Valley (KV) indicate that deformation also occurs 100 km north of the deformation front. A historical record of 13 earthquakes in the valley over the last millennium, including damaging earthquakes in 1555 and 1885, further suggests that the KV is a locus of active deformation. We use geomorphic mapping, terrace profiling, paleoseismic trenching, and radiometric dating to constrain the extent, timing, rate and style of deformation in the KV. Tectonic geomorphic mapping on high-resolution satellite imagery reveals a series of discontinuous scarps, which we call the Balapora fault (BF), cutting the KD and younger terraces over 45-60 km south of the Jehlum River. Near the north end of the BF, only the highest three of six strath terraces that cross the fault along the Shaliganaga River are deformed, and optically stimulated luminescence (OSL) ages on the highest undeformed terrace show that the fault has not moved there in 50 +/-3 ka. To the south, a flight of five strath terraces along the Sasara River have been uplifted by the BF. Correlating soil and loess stratigraphy from the youngest deformed terrace dated terraces in nearby drainages suggests that deformation has occurred since ~50 ka. Further south, along the Rembiara River (RR), the BF deforms two regionally extensive terraces. Using an OSL age of 51 +/-11 ka collected from fluvial deposits a few meters above the lower strath, and a measured strath elevation above the river of 19 +/- 1 m at the fault, we calculate an average incision rate of 0.3-0.5 mm/yr. An exposure on the left bank of the RR reveals that the BF is sub-vertical, suggesting incision rate is a proxy for minimum slip rate for the BF. Paleoseismic trenches where the BF crosses the RR reveal evidence for at least one event between 1.5-18.7 ka, and another between 33.4-38.4 ka, with as many as four events in ~40 ka. Thus the BF did not rupture during any documented historical events. Furthermore, the high angle, sub-mm/yr slip rate, and long average earthquake recurrence all suggest that the BF is not a major out-of-sequence thrust that accommodates significant shortening related to the Indian-Eurasian collision. Long terrace profile surveys along the RR show that terraces diverge upstream of the BF, reflecting ramping of an underlying décollement toward the southwest side of the KV. We speculate that the BF is a minor accommodational structure produced by a change in dip of the décollement, and that strain accumulation at this inflection zone resulting from down-dip loading could be the source of frequent earthquakes documented in the KV.