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

Abstract

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.

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

“…In this ductile setting, ramps and flats on the Main Central Thrust resulted in imbrication or interleaving of the Lesser Himalayan Sequence and Greater Himalayan Sequence in the immediate vicinity of the thrust. Deformation was subsequently transferred to the Ramgarh thrust (Pearson & DeCelles 2005;Robinson & Pearson 2013;Webb 2013), which was responsible for finally exhuming the deformed Daling rocks in its hanging wall and thrusting them upon the Buxa rocks, inverting the original Daling-Buxa sedimentary relationship in the Lesser Himalayan Sequence (Fig. 2c).…”

Deformation mechanisms in the frontal Lesser Himalayan Duplex in Sikkim Himalaya, India

“…The Lesser Himalayan belt bounded between the MCT and MBT has experienced multiple phases of thrusting and duplex formation (e.g., Schelling and Arita 1991;DeCelles et al 2002). In this region, the Ramgarh thrust (Valdiya 1980;Srivastava and Mitra 1994;DeCelles et al 2002;Pearson and DeCelles 2005) is considered as a regional fault in the western and the Nepal Himalayas that has been locally recognized as the north Kalijhora thrust in Darjiling-Sikkim Himalaya (Mukul 2000) and Shumar thrust in Bhutan Himalaya (McQuarrie et al 2008). It was mapped earlier as an unnamed fault (Sinha Roy 1967, 1974Acharyya 1976Acharyya , 1980Schwan 1980).…”

“…This fault was previously mapped as the Tendong thrust in the northern part of the Rangit window and as the Rammam-Rangit fault on the southern part of the window (Raina 1982). Given that the Ramgarh thrust elsewhere in the Himalaya (Valdiya 1980;Srivastava and Mitra 1994;DeCelles et al 2002;Pearson and DeCelles 2005) have similar structural position (footwall of the MCT thrusts) and transports a similar dominantly pelitic sequence in its hanging wall, we correlate the Tendong and Rammam-Rangit faults with the regionally recognized Ramgarh thrust. Similarly, the Ramgarh thrust can also be correlated with the north Kalijhora thrust (NKT) (Mukul 2000) in the frontal part of the Darjiling Himalaya as both faults carry the Daling sequence in their hanging wall and have Gondwana rocks of the MBT sheet in its footwall.…”

Overburden-Induced Flattening Structure in the Himalaya: Mechanism and Implication

“…Recent studies from different parts of the Himalaya have recognized a regional thrust, named as Ramgarh Thrust (RT), which is now considered as a major regional Himalayan thrust, like other major thrusts (e.g. MCT, MBT, MFT) of the Himalayan fold-thrust belt, that transports upper Lesser Himalayan rocks (Pre-Cambrian sequence) over lower Lesser Himalayan rocks (Permian-EoceneMiocene) 8,15,[17][18][19][20][21][22][23] . The area of study encompasses a small part of the RT sheet in the Lesser Himalaya in the DSH (Figure 1).…”