STRUCTURAL STYLE VARIATIONS IN THE HIMALAYAN FRONT: IMPLICATIONS FOR WEDGE-TOP DEPOSITION, NEPAL

Deformation at the front of the Himalayan thrust wedge is focused along the Main Frontal Thrust (MFT) resulting in shortening of the Neogene foreland basin deposits since the Pleistocene. This research investigates variations in structural style along the MFT in the Dang Valley region, west-central Nepal, and how these variations affect the distribution of modern wedge-top sediment depocenters. Four kinematic models and cross sections were constructed to evaluate plausible geometric and kinematic histories. Three structurally distinct regions are recognized: 1) the western section distinguished by north-dipping imbricate thrusts; 2) the central section characterized by south-dipping back thrusts that structurally confine the southern margin of the Dang Valley; and 3) the eastern section distinguished by north-dipping imbricate thrusts and south-dipping back thrusts that form along a shallow detachment resulting in a composite pop-up structure.
Neogene shortening estimates across the study area range from 47% to 40%. Derived shortening rates that range from 20 mm/yr to 14 mm/yr are consistent with modern geodetically derived India-Eurasia convergence rate across the Himalaya. This implies that in this region most of the shortening since initiation of tectonic activity ca. 2.3 Ma, has been accommodated between the Main Boundary thrust (MBT) and the MFT along a single basal detachment known as the Main Himalayan Thrust (MHT).
Two transfer zones are the most plausible features to explain the along-strike variation in structural style within the study area. From west to east these are: 1) the West Dang Transfer Zone (WDTZ), interpreted to be a complex zone of strike-slip faults and lateral ramps which may transfer strain to a deeper detachment level to the east; and 2) the Masot Khola Transfer Zone (MKTZ), interpreted to transfer strain along a lateral ramp to a deeper basal detachment and a secondary shallow detachment surface to the east.
The results of this study suggest there is a relationship between changes in structural style and wedge-top development. Abrupt changes in distribution of wedge-top sediments are coincident with changes in structural style.