Decoding fault kinematics from the roof thrust of the Lesser Himalayan duplex: Insights from paleostress reconstruction, Ramgarh thrust, Darjeeling-Himalaya

Paleostress reconstruction in fractured rocks is generally conducted through fault-slip inversion. Estimating spatio-temporal variation of paleostress directions from deformed rocks with prolonged deformation history is extremely challenging due to heterogeneity, non-coaxiality of deformation, ascertaining relative timing of formation of different fracture sets, genetic association with the larger structure, and probable reactivation among other factors.  Therefore, fault-slip-based kinematic studies from roof thrusts of duplexes are generally less common. In this context, we attempt to deduce the fault kinematics from the leading-edge exposure of the Ramgarh thrust (RT) sheet in Darjeeling-Himalaya, which hosts a thrust-related antiform. The RT also acts as the roof thrust of the Lesser Himalayan duplex (Bhattacharyya et al., 2015). We studied the slickenline data from near the footwall contact of the RT zone (part of the overturned forelimb of the antiform) up to ~3.4km into the RT sheet (backlimb). As part of an ongoing study (Ammu and Bhattacharyya, in revision), we have deciphered a first-order relative timing among the fracture sets along with fold-fracture relative timing using various factors, for example, spatial variation of shear fracture-bedding angles, offset, relative abundance at different locations, fold test, correlation between the dihedral angles of conjugate faults and depth. We used the PBT method (Huang and Charlesworth, 1989; Sperner et al., 1993) to invert the fault-slips and reconstruct the stress regimes. This multi-proxy workflow can be used to systematically reconstruct the fault kinematics from structurally complex settings.

The major fault, RT, is oriented ~72°, 304°, along a cross-section with a bearing of ~130-310° and has a top-to-the-south vergence. The shear fractures (n=208) record normal (~61%), inverse (~39%), sinistral (~54%), dextral (~46%), oblique- (~74%), dip- (~18%) and strike- (~8%) slip movement. Although the RT sheet records a heterogeneous fault-slip population, proximal to the RT zone, the shear fractures show dominantly inverse (~58%) and sinistral (~67%) sense, i.e., a similar sense of slip as that of the major fault. We divided the heterogeneous fault-slip data into fourteen homogeneous subsets, which were categorized into pre-, syn-, and post-folding stages. The RT sheet records eight post-folding fault-slip subsets with ~NNE-SSW compression, ~NW-SE, and ~NE-SW extension, and strike-slip regimes with ~NE-SW, ~E-W, and ~NW-SE compression. The post-folding ~NNE-SSW compressional regime conforms to the present-day orientation of the regional tectonic stress field. Most stress regimes exhibit an anticlockwise rotation of stress fields proximal to the fault as compared to the interior of the RT sheet. The rotation of stress fields is observed across the pre-, syn-, and post-folding stages. Thus, the RT caused stress perturbations and influenced fracture kinematics within the RT sheet across space and time.