Rapid bedrock incision implies late Quaternary out-of-sequence thrusting along Medlicott Wadia Thrust in the western Himalaya

Abstract:

The accommodation of Indo-Eurasian convergence within the Himalayan orogen has varied over spatial and temporal scales. To better understand the spatiotemporal evolution of the Himalaya, we need to quantify the activity of tectonic structures. In this regard, we investigate the activity of the famed Medlicott Wadia Thrust (MWT) in the western Sub-Himalaya. The Tawi River, a tributary of the Chenab River traverses through the entire Sub-Himalayan sector. In the Sub-Himalaya, near the city of Udhampur (in the union territory of Jammu), we observe at least six different fluvial terraces above the present-day course of the Tawi River on the hanging wall of the MWT. Two of the aforementioned terrace levels, residing at heights of ~120 m and ~55-60 m are identified as strath terraces. Optically Stimulated Luminescence (OSL) dating of the thin veneer of alluvium above the two strath surfaces yield depositional ages of ~22 ± 1.5 kyr and ~9 ± 1 kyr, respectively. Considering the negligible thickness of the alluvium, we recalculate the Middle Siwalik bedrock incision rates to be 5-6.25 mm/yr since the last ~22-24 kyr. Assuming a steady-state, we argue that this result reflects the uplift rate on the hanging wall of the MWT. By using field-collected data on structural orientation of the hanging wall units, we predict a late Quaternary shortening rate of ~7.1-8.9 mm/yr accommodated by the MWT. Our findings correlate well with the shortening rate estimates from the Riasi Fault system in the Chenab valley and the Jwalamukhi Thrust in the Kangra recess (contemporary to MWT). With accommodation of more than 50% of the total Himalayan shortening, the Medlicott Wadia Thrust stands as the most active structure in the western Himalaya at least over the Holocene.