STRUCTURAL EVOLUTION OF THE HIMALAYAN FOLD-THRUST BELT IN CENTRAL NEPAL

Geological mapping along a north south transect through the Himalayan Fold-Thrust Belt (HFTB) has generated a new structural model for central Nepal. We document the existence of two new major thrusts: the Ramgarh Thrust (RT) and the Trishuli Thrust (TT). A new regional balanced cross-section shows at least 300 km of shortening.

Both the RT and TT occur in rocks of the Lesser Himalayan Zone (LHZ). The RT was first recognized in the Kumaon Himalaya (Valdiya, 1980) and in western Nepal (DeCelles et. al., 1998), where it serves as the roof thrust for a duplex of LHZ rocks. In central Nepal, the RT carries the Dunga and Robang Formations, which were previously mapped as the uppermost portion of the LHZ. However, structural relationships combined with U-Pb zircon and Sm-Nd whole rock isotopic data indicate that these rocks correlate with the lower part of the LHZ. Therefore, the Dunga and Robang Formations are probably the distal facies of the lower part of the Kuncha Formation.

We have mapped the RT in Langtang National Park, near Kodari at the Nepal/China border, in the Trishuli River drainage along the north flank of the Kathmandu Complex (KC), and near Hetauda on the south flank of the KC. In the Langtang and Kodari areas, the RT occurs directly below the MCT. Fabric attitudes are identical in the hanging-wall and foot-wall of the MCT, implying a flat-on-flat thrust relationship. On both the north and south flanks of the KC, the RT sheet forms the foot-wall for the MT. Fabric attitudes for rocks in the hanging-wall of the MT and in the RT sheet are concordant, suggesting a flat-on-flat thrust relationship for the KC. This structural evidence, combined with isotopic data, supports the idea that the MT is the southward continuation of the MCT.

The TT sheet is structurally below the RT, and contains a complete and apparently continuous section of LHZ rocks including the Ulleri Augen Gneiss. The TT sheet's leading edge is buried under the KC, and extends north to Langtang National Park. The TT serves as the roof thrust for a duplex in LHZ rocks north of the KC. Duplexing caused passive folding of the RT and MT/MCT into the broad Gorkha-Pokhara anticlinorium and the Kathmandu synclinorium.

Integrating this new structural model with thermobarometric and geochronologic data sets will lead to new insights regarding the tectonic evolution of the Himalayan orogeny.