The Main Central Thrust, a Possible Early Paleozoic Structure Reactivated During the Cenozoic: Insights from the NW Himalaya

The northern Indian passive margin has witnessed at least two Wilson-cycle-related collisional events since the Proterozoic: the Early Paleozoic Bhimphedian Orogeny and the Cenozoic Himalayan Orogeny. The latter, triggered by the India–Asia collision, produced a series of orogen-scale structures including the Main Frontal Thrust, Main Boundary Thrust, Main Central Thrust (MCT), and South Tibetan Detachment System (STDS), which cut the Himalaya into distinct lithotectonic belts. However, the extent to which vestiges of the Early Paleozoic tectonism persist remains uncertain due to extensive overprinting by Cenozoic deformation. This ambiguity has revived debates on whether the major Himalayan fault systems are exclusively Cenozoic tectonic boundaries or the reactivation of long-lived, inherited Early Paleozoic structures.

This study investigates the tectonic evolution of the MCT, a several-kilometre-thick, foreland-propagating, high-strain shear zone that activated ca. 27–11 Ma. It structurally juxtaposes the Neoproterozoic Greater Himalayan Sequence (GHS) over the Paleoproterozoic Lesser Himalayan Sequence. Integrated structural mapping and U-Pb zircon geochronology were conducted on the GHS rocks from the proximal hanging wall of the MCT in the Dhauliganga (Garhwal) and Pabbar (Himachal) valleys of the NW Himalaya. Ductilely deformed psammitic, ortho- and aplite gneisses, leucogranite, and migmatite display NE-dipping mylonite foliation, NNE-plunging stretching lineation, and persistent top-to-the-SW ductile shearing, consistent with regional MCT kinematics.

Detrital zircon spectra constrain the maximum depositional age of the GHS metasedimentary protoliths to 849 ± 6.7 Ma. These country rocks were syn- to post-tectonically intruded by orthogneiss and leucogranite along a major crustal conduit, the proto-MCT, during the Early Paleozoic Bhimphedian Orogeny. In the Dhauliganga Valley, three distinct tectonothermal pluses are recorded at 554 ± 6.8 Ma, 489 ± 2.8 Ma, and 471 ± 3.2 Ma. In the Pabbar Valley, coeval crustal anatexis along the proto-MCT produced stromatic migmatite at 508 ± 6.7 Ma and 473 ± 3.1 Ma. These ages collectively reflect magmatism, regional metamorphism, and pervasive deformation along the proto-MCT during the Bhimphedian Orogeny. During the Cenozoic Himalayan Orogeny, this dormant proto-MCT was reactivated and subsequently evolved to present-day MCT. This is evident by tectonothermal pulses at 20 ± 3.0 Ma and 16 ± 1.2 Ma, recorded in the leucogranite and aplite gneiss in the Dhauliganga Valley. Notably, a comparable tectonic evolution of the STDS during the Early Paleozoic (a~499–467 Ma) and Cenozoic (~34–25 Ma and 23–13 Ma) has been documented in the upper reaches of the Dhauliganga Valley.

Together, these findings demonstrate that both the MCT and STDS originated as coeval Early Paleozoic proto-tectonic structures and were subsequently reactivated during Late Eocene to Early Miocene Himalayan deformation phases. This dual-stage tectonic evolution underscores that the Himalayan crustal architecture is fundamentally inherited from Early Paleozoic orogenesis, with Cenozoic deformation preferentially exploiting these pre-existing anisotropies. Therefore, comprehensive Himalayan tectonic models must integrate the contributions of Early Paleozoic tectonism, rather than attributing these major shear zones solely to the Cenozoic displacement.