Structural geology and regional tectonic significance of the Ramgarh thrust, Himalayan fold-thrust belt of Nepal

摘要

The Ramgarh thrust is one of the major fault systems of the Himalayan thrust belt in Nepal and northern India. The Ramgarh thrust sheet is ～0.2-2.0 km thick and can be traced along strike the entire length of the Himalaya in Nepal. The fault generally places the oldest Paleoproterozoic rocks in the Lesser Himalayan series upon younger Lesser Himalayan rocks or lower Miocene foreland basin deposits. Regional balanced cross sections suggest that the Ramgarh thrust had at least ～120 km of initial south vergent displacement. Subsequently, the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying Lesser Himalayan rocks. Ramgarh hanging wall strata are greenschist-grade phyllite, quartzite, and augen gneiss, all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top-to-the-south sense of shear. Structural fabrics in the Ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet. Regional and local mapping of the Ramgarh thrust in Nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat, except where local lateral ramps complicate its geometry. Similarly, the structurally overlying Main Central thrust always places a hanging wall flat in Greater Himalayan series rocks upon the regionally flat Ramgarh thrust sheet. These geometric relationships preclude kinematic and thermal models that elevate Greater Himalayan and lower Lesser Himalayan rocks along high-angle thrust ramps in the vicinity of the present traces of the Ramgarh and Main Central thrust faults. Instead, the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the Main Central thrust. The present steep dips of the Ramgarh and Main Central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the Lesser Himalayan zone. The Ramgarh thrust sheet overlaps a bed length of at least 100 km in lower Miocene foreland basin deposits, indicating that a significant amount of displacement on the thrust must have occurred after ～15 Ma. Growth of the Lesser Himalayan duplex and additional slip on the frontal part of the Ramgarh thrust occurred from ～12 to 5 Ma. The presence of a major greenschist-grade metasedimentary thrust sheet composed of Lesser Himalayan rocks directly below the Main Central thrust suggests that the famous "inverted metamorphism" in this region is a result of structural inversion. Similarly, the concept of a broad zone of intense shear strain related exclusively to emplacement of the Main Central thrust sheet is probably invalid in Nepal.