Subtle structural testimony of Active faults: examples from Peninsular India

Studies of damaging earthquakes that occurred in Peninsular India over 50 years suggest that they occur along favourably oriented pre-existing weaker planes/faults in the ongoing compressional tectonic regime. Many of these pre-existing structural weaknesses developed until the collision between India and Eurasia reversed the style of movement post-Miocene, from a general normal sense of movement to either reverse or strike-slip. However, identifying neotectonic signatures from the plate interior, especially in tropical climatic regions, is very challenging since erosional agents can nullify the signature of tectonic movement. The present article focused on identifying active faults from the zones of two major NW-SE trending structures, separated by about 1600 km, that are being widely deliberated for the reconstruction of the Gondwana assembly, viz., the Mahanadi Shear Zone and the Achankovil Shear Zone.

The NW-SE trending Neoproterozoic Mahanadi Shear Zone opened up as rift basins around 300 to 100 million years ago for the deposition of Gondwana sediments. Our studies at two locations, ~140 km apart along the strike direction, indicate that the litho-contact between crystalline and sedimentary can be easily made out through geomorphic expressions, drainage patterns, and nature of vegetation. The study identified badland topography and structurally controlled meandering of drainages in the area, which are associated with neotectonic adjustments. The brittle faulting, with a reverse sense of movement, identified in crystalline rocks shows wide damage zones with gouge injecting into fractures and also onto the surface, where the soil cover is negligible. The extrusion of gouge is preserved as a conical heap above the surface level. The study also identified the gouge injection into Quaternary sediments at several locations. The reverse faulting is also reflected in the laterite cap that developed over younger sediments deposited over the Gondwana formation.

The NW-SE trending Achankovil shear zone is a major Pan-African structure located close to the southern end of peninsular India, cutting through the Western Ghats. Earlier studies identified two major faults at the southern end of this shear system, viz., the Thenmala and Thenmala South faults, for which there exists a sharp geomorphic expression in the Western Ghats. However, its expression in the plain area east of the mountain terrain is very weak. The present study identified badland topography, abandoned river paths, and anomalous natural depressions associated with these faults as results of neotectonic adjustments in this area. Perturbation of land into the sea along the strike continuity of both faults in the southern side and the drainage divide between them are the other significant effects of neotectonism associated with these faults. Field investigations identified surface ruptures along the faults, preserved in hard laterite that was observed above crystalline rocks. Studies based on the trapped aeolian deposits within hard laterite suggest at least two faulting events within the last 4400 years.   

The present series of studies identified a host of geomorphic and structural evidences that can be used to identify active faults. These clues can be touchstones for future studies in the field of active fault evaluation in such terrains.