Long-Term Slow Exhumation in the Middle of the Greater Himalayan Series in the Modi Khola Transect, Central Nepal

Recent thermobarometric constraints on rocks exposed along the Modi Khola suggest the presence of a large normal fault, the Bhanuwa fault, ~700 m structurally above the Main Central thrust, entirely within Greater Himalayan rocks. Rocks in the footwall and hanging wall of the Bhanuwa fault record pressures of ~14.5 and 10 kbars, respectively. This pressure difference equates to a depth difference of ~17 km and suggests ~35 kilometers of slip on the Bhanuwa fault. We retrieve cooling rates for proximal hanging wall and footwall rocks near their peak temperature by modeling retrograde Fe-Mg diffusion profiles in garnets in contact with biotite. We infer cooling rates at lower temperatures based on muscovite ⁴⁰Ar/³⁹Ar ages and current conditions. For both types of data we consistently find much slower cooling rates in the hanging wall compared to the footwall of the Bhanuwa fault. This difference in cooling rates suggests long-term slower exhumation of the hanging wall, an interpretation supported by the difference in estimated pressures across the fault. Cooling and exhumation rates for Greater Himalayan rocks ~1.5 km structurally above the proximal hanging wall rocks, and possibly across a thrust, are much faster than for the proximal hanging wall rocks and are similar to the rates for proximal footwall rocks of the Bhanuwa fault. Thus we find a minimum in long-term exhumation rates near the middle of the Greater Himalayan series sandwiched between regions of long-term fast exhumation near its structural top and base. This spatial pattern of exhumation rates is not well explained by existing dynamic models but follows simply from the kinematics of motion on the Bhanuwa fault and nearby thrust faults. Diagrams that hold Earth's surface fixed and show the motion of hanging wall and footwall rocks relative to it help elucidate exhumation patterns expected for thrust and normal faults.