|Abstract or Summary
- Active tectonics of a deformation front constrains the kinematic evolution and structural interaction between the fold-thrust belt and the most-recently accreted foreland basin. At the Himalayan deformation front, the thrust front is blind, characterized by a broad fold (the Suruin-Mastgarh anticline (SMA)), and displays no emergent faults cutting the southern limb. Dated deformed terraces on the Ujh River constrain the structural style of deformation and shortening rates. Six terraces are recognized, and three yield OSL ages of 53 ka, 33 ka, and 0.4 ka. Terrace restorations through long profiles reveal a deformation pattern characterized by uniform uplift across the anticlinal axis and northern limb, and variable uplift due to rotation of the southern limb. Offset terraces occur between the fold trace and the northern limb. Bedrock dips, stratigraphic thicknesses, and cross sections suggest that a SW-directed duplex at depth drives uniform uplift in the north, and a NE directed wedge thrust drives variable tilt in the south. Localized faulting at the fold axis introduces asymmetrical fold geometry. Folding of the oldest dated terrace suggests rock uplift rates across the SMA range between 1.8 and 2.0 mm/yr. Assuming a 25°-dip for the duplex ramp on the basis of dip data constraints, the shortening rate across the SMA ranges between 3.8-5.4 mm/yr or ~4.6 mm/yr since ~53 ka. Of that rate, 2.7-1.1 mm/yr is likely absorbed by faulting at the fold axis. In comparison, long-term bedrock shortening rates are consistent with our data of Pleistocene shortening rates. Cross sections at the Ujh River transects and Chenab reentrant indicate 6.5 mm/yr and ~4 mm/yr, respectively, using an onset age of thrusting of ~ 3 Ma.
Within the Sub-Himalaya deformation belt, new geomorphic mapping demonstrates that active emergent thrust faulting occurs north of the deformation thrust front in the Kashmir Himalaya. The Riasi thrust (RT) comprises the southeastern strand of a ~250 km-long seismically active fault system in Pakistan and Indian Kashmir. Multiple fault strands with Quaternary activity characterize the fault zone near the Chenab River. Vertical separation of ~272 m of Pleistocene fluvial deposits marks the main strand of the RT, or the Main Riasi thrust (MRT). A shortening rate of 10.8-2.8 mm/yr characterizes a ~91-39 ka time interval for the MRT at this location. A fault scarp and offset Holocene terrace mark the southernmost and frontal splay of the RT fault zone, called here the Frontal Riasi thrust (FRT). Differential uplift (21.6±1m) of a Holocene terrace yields a preferred shortening rate of 8.8-4.4 mm/yr. Contact relationships in a trench across the FRT date the last surface rupture of the RT fault zone at ~4,500 yrs ago. Average shortening rates since ~100 ka across the MRT and the FRT range from 10.8 mm/yr to 4.4 mm/yr (7.6±3.3 mm/yr), consistent with long-term bedrock shortening of ~10-14 mm/yr since 5 Ma.
Given that Himalayan-Indian convergence is ~11-18 mm/yr, the sum of the intermediate-term shortening rates for the RT (10.8-4.4 mm/yr) and the deformation front (5.4-3.8 mm/yr), accounts for most of the total geological shortening in Kashmir. Using average rates, internal faults (RT) absorb 50% of the Himalayan shortening, comparatively more than the deformation front (~30%) in Kashmir. Similarities in the structural setting between the Riasi thrust and the Balakot-Bagh fault, including millennia recurrence, suggest the potential of Mw 7 to 8 earthquakes on the RT fault zone, similar to the source of the Mw 7.6 2005 Kashmir earthquake on the Balakot-Bagh fault segment. Lower rate at the deformation front may suggest an even longer recurrence interval, but nonetheless potentially devastating with predicted Mw of 7.5-7.6 along the 180-250 km-long structure.
Apatite and zircon (U-Th)/He cooling ages are used to quantify the pattern of distributed deformation and overall thrust-belt kinematics at longer timescales across the Kashmir Himalayas. Apatite (U-Th)/He (AHe) cooling ages for the foreland molasse sediments are consistently younger than the sediment age, indicating that ages of Sub-Himalayan belt samples are reset. We interpret regional pattern of young AHe cooling ages (<3 Ma) in the foreland with peak cooling at 1.9-3.2 Ma to represent rapid coeval fault-related exhumation on multiple structures across the Sub-Himalaya. In the hinterland front ranges of the Pir Panjal, the MBT and MCT thrusts are characterized by older cooling (>4.7 Ma). For zircon (U-Th)/He (ZHe), probability density plots of samples from both detrital cooling ages in the foreland and reset cooling ages in the hinterland show a pronounced spike in cooling between ~16 and 20 Ma, a period where MCT motion is well documented throughout the Himalaya. Estimates of exhumation rates for the sum of Sub-Himalayan structures are higher (2.8-2.2 mm/yr) than across the MCT/MBT (<1 mm/yr) in the Pir Panjal Range. Cooling patterns across the Kashmir Himalayas do not correlate with monsoon precipitation gradients, suggesting climate forcing is decoupled from erosion and exhumation. Distributed rather than localized forward-propagating deformation characterizes fault-related exhumation for the orogenic wedge development in the Sub-Himalayan belt since at least ~2-3 Ma. In the hinterland, coeval young cooling ages (< 3 Ma) and high exhumation rates (2.8-1.3 mm/yr) across the Kishtwar Window, >100 km north of the deformation front, suggest tectonic-driven rapid exhumation across the orogenic wedge coincides with localities of predicted changes in ramp geometry and/or active orogenic growth. We attribute a pattern of distributed deformation and coeval faulting across the Sub-Himalaya to the effects of pre-existing basin architecture in Kashmir.