WESTERN HIMALAYA GIANBUL GNEISS DOME GEOCHRONOLOGY REVEALS INITIATION OF DOMING IN THE MIDDLE CRUST, BUOYANCY-DRIVEN ASCENT, AND EXHUMATION BY NORMAL FAULTING

A general lack of consensus about origin of Himalaya gneiss domes—seemingly paradoxical extensional structures in the predominantly contractional India-Asia collision zone—hinders accurate thermomechanical modeling of the orogen. To test whether doming resulted from passive-roof faulting (e.g. thrust duplex formation, antiformal bending above thrust ramp, etc.), channel flow, or via the buoyant rise of anatectic melts, this study investigates the depth and timing of doming processes for Gianbul dome in the western Himalaya. Structural observations, monazite U/Th-Pb geochronology, and ⁴⁰Ar/³⁹Ar mica thermochronology along a northeast-southwest transect provide critical new constraints for the tectonometamorphic evolution of Gianbul dome.

The dome is composed of Greater Himalayan sequence migmatite, Paleozoic orthogneiss, and metasedimentary rock cut by multiple generations of leucogranite dikes. Contractional deformation and Barrovian metamorphism related to crustal thickening occurred from ~37 to 33 Ma. Gianbul underwent subsequent near-isothermal decompression as extension occurred in the top-to-the SW Khanjar shear zone on the southwestern flank, and in the top-to-the NE Zanskar shear zone (western equivalent to the South Tibetan detachment system) on the northeastern flank. The emplacement of post-tectonic dikes at ~22.6 Ma in the southwest and ~21 Ma in the northeast mark the end of extensional ductile shearing. ⁴⁰Ar/³⁹Ar muscovite ages from gneisses decrease northeastward across the dome from 22.1 to 20.2 Ma, suggesting that Gianbul dome was exhumed as part of a rigid block—tilted 5–10° top-to-the SW—in the footwall of the Zanskar normal fault. Post-tectonic dike muscovite ages indicate that the entire dome cooled below muscovite Ar closure (<400° C) by ~19.8 Ma.

We conclude that doming i) initiated in the middle crust during the early stages of extension, ii) was driven by a positive feedback among dehydration melting, buoyancy, ductile exhumation, and decompression, and iii) culminated with the injection of anatectic melts into the upper levels of the dome at ~22–21 Ma. As normal-sense displacement transferred from the ductile Zanskar shear zone to the brittle Zanskar normal fault, Gianbul dome continued to be exhumed as part of a rigid footwall block.