The South Tibetan Detachment System: A Cambro–Ordovician terrane boundary reactivated during Cenozoic Himalayan collision in the NW Himalaya

The South Tibetan Detachment System (STDS) represents a major extensional shear zone separating the Greater Himalayan Sequence (GHS) from the Tethyan Himalayan Sequence (THS), yet its long-term tectonomagmatic evolution remains poorly understood. The Dhauliganga Valley (Uttarakhand, NW Himalaya) is one of the few transects where the STDS is excellently exposed from Malari to Goting for ~10 km along its strike, remarkably showing cross-cutting relationship of the Paleozoic orthogneisses and Neoproterozoic mylonites and migmatite, which are further intruded by at least two generations of Mio–Oligocene leucogranites. Zircon U–Pb geochronology integrated with major, trace and rare-earth element geochemistry of representative granites and leucogranites constrain the timing and source characteristics of the magmatic events associated with the STDS and are crucial to develop a tectonic model that explains the role of the STDS as a fundamental Cambro–Ordovician terrane boundary that was subsequently reactivated during Cenozoic orogeny.

Field observations and microstructural analyses in the STDS zone document a polyphase deformational history characterized by multi-phase shearing, syn-tectonic melt emplacement and overprinting brittle deformation. Structural and microstructural fabrics capture an evolution from D2 top-to-SW thrusting to D3 top-to-NE brittle–ductile extension with oblique-slip and transtensional components, producing a high-angle shear geometry unique to this transect.

Detrital zircon populations from folded psammitic gneiss in the footwall preserve Neoproterozoic inheritance (~1075–860 Ma), overprinted by the Cambro–Ordovician granitoids with crystallization ages of 498.92 ± 5.5 Ma and 486.54 ± 2.3 Ma. It is hypothesized that STDS facilitated the emplacement of an extensive ~200km Cambro–Ordovician granite belt from Sutlej to Dhauliganga Valleys in NW Himalaya during the Kurgiakh/Bhimphedian Orogeny. In the post-orogenic phase, the STDS acted as a proto-tectonic marginal extensional boundary that facilitated the denudation of these granites and gneisses into the Tethyan basin resulting in a >10 km-thick THS.

Leucogranites within the uppermost GHS yield ages from Late Eocene to Early Oligocene (35–23 Ma) indicating magmatism associated with SW-directed contraction and crustal thickening (35.3 ± 1.8 Ma, 33.99 ± 1.07 Ma). These leucogranites exhibit tight isoclinal folds with NE-dipping axial surfaces. Syn-tectonic emplacement occurred in the Late Oligocene–Early Miocene (25.03 ± 0.54 Ma 23.68 ± 0.94 Ma), followed by extension-induced exhumation. Magmatic activity abruptly ceased along the STDS in this transect by 13.30 ± 0.30 Ma, after a protracted melt generation and emplacement for nearly 10.0 Ma.

Whole-rock geochemistry indicates that both Paleozoic granitoids and Miocene leucogranites represent syn-collisional, peraluminous crustal melts generated via fluid-absent muscovite-dehydration melting under high-pressure conditions.

This study proposes a tectonomagmatic evolutionary framework for the STDS in the Dhauliganga Valley, a Paleozoic terrane boundary and a reactivated Cenozoic extensional structure mediating melt emplacement, strain localization and exhumation. The persistence of a complex thermal history suggests a foundation for future isotopic and thermochronological investigations into the architecture and rheology of the Himalayan orogen.