THE EVOLUTION OF THE CENTRAL HIMALAYAN MOUNTAINS: RECOVERING THE EROSIONAL HISTORY FROM THE REMAINING SEDIMENT

Cooling temperatures in the Himalayan thrust belt of western Nepal rock indicate when rocks in the hanging walls of the Main Central thrust, Ramgarh thrust, and in the Lesser Himalayan duplex were exhumed through the closing temperatures. These rocks were then exposed at the surface, eroded, transported, and deposited into the foreland basin and formed the Bhainskati and Dumri Formations and the Siwalik Group units. Petrographic, clast, zircon fission track and ⁴⁰Ar/³⁹Ar data from these sediments indicate whether Tibetan/Greater Himalayan or Lesser Himalayan rock contributed the detritus. Thus, these synorogenic sediments can be used as a proxy to indicate which unit was at the surface in the thrust belt, and the erosional surface in each reconstruction was modified accordingly. This kinematic model has 18 time frames with distinct structural events occurring in each reconstruction. Both the flexural wave and the accommodation space were fixed manually in each reconstruction frame to match the data in the foreland basin which suggests whether a unit is at the surface or buried, and the depth to which the unit may be buried. To validate the model, the amount of space produced by flexing was reproduced using a modeling program.

These data indicate that Greater Himalayan rock was exposed at the surface of the thrust belt until ~11 Ma at which time Lesser Himalayan rock began contributing a significant amount of detritus. At 11.1 Ma, the lower Siwalik unit shows a sediment accumulation rate increase. I attribute this increase to Greater and Lesser Himalayan rocks in the Lesser Himalayan duplex being thrust over a ramp in the buried upper Lesser Himalayan stratigraphy that first caused lower Lesser Himalayan rocks to be exposed at the surface and then caused rocks from the upper Lesser Himalaya to be exposed. Subsequently, the Lesser Himalayan ramp propagated southward as more Lesser Himalayan thrust sheets were clipped off and incorporated into the Lesser Himalayan duplex. The final Lesser Himalayan ramp was created when the Main Boundary thrust emplaced Lesser Himalayan rocks over the Siwalik Group sediments. The locus of thrusting shifted southward to create the Subhimalayan thrust system and Main Frontal thrust.