STRUCTURAL AND TECTONIC IMPLICATIONS OF THE RAMGARH THRUST, HIMALAYAN FOLD-THRUST BELT OF NEPAL

In terms of volume, rocks of the Lesser Himalayan (LH) zone form the bulk of the Himalayan fold-thrust belt in Nepal. Recent research shows that considerable structural complexity exists within these rocks, yet most syntheses and models of the fold-thrust belt’s evolution focus only on the importance of the South Tibetan Detachment system and on the Main Central (MCT), Main Boundary (MBT), and Main Frontal thrusts. Structural mapping across Nepal has revealed the presence of the Ramgarh thrust (RT), which carries a far-traveled thrust sheet of LH zone rocks. The RT and a large duplex in Lesser Himalayan zone rocks (called the LH duplex by DeCelles et al., 2000) are the two main structures that define the structural geometry of the LH zone. The RT is a regional structure that can be traced for more than 1000 km across Nepal and in the adjacent Kumaon region of northern India. Minor details notwithstanding, the structural characteristics of the Ramgarh thrust sheet remain relatively consistent along the length of the fold-thrust belt in Nepal.

The RT always carries rocks of the lowermost LH sequence, the Kushma and Ranimata Formations or their lateral equivalents. Additionally, the RT sheet is always located in the proximal footwall of the MCT, within a zone of strained rocks that many workers call the MCT zone. Regional balanced cross-sections from far-western Nepal (Robinson, 2001) and central Nepal suggest that the RT accommodated a magnitude of tectonic shortening similar to that accommodated by the MCT.

Constraints from structural and thermochronologic datasets as well as from foreland basin provenance studies suggest that the RT became active during the middle Miocene. Following emplacement of the RT sheet, displacement was fed into the LH duplex system during the late Miocene. This deformation within LH zone rocks during the middle to late Miocene occurred between emplacement of the MCT and MBT sheets. The MCT became active during the latest Oligocene to early Miocene (Guillot, 1999; Hodges, 2000), and the MBT became active during the latest Miocene to early Pliocene (Robinson et al., in press). Accommodation of tectonic shortening through the southward translation of thrust sheets within the Himalayan fold-thrust belt suggests that deformation has achieved a steady state since at least the latest Oligocene.