The geometry of the Main Himalayan Thrust in Central Nepal (85°E) derived from thermo-kinematic modeling of thermochronological data in the Gyirong region (southern China)

The subsurface structure of the Himalaya is dominated by the seismically active Main Himalayan Thrust fault, which accommodates about half of the plate convergence between India and Asia. In Central Nepal, geological, geophysical, and geodetic studies indicate that the Main Himalayan Thrust has a flat-ramp-flat geometry, which causes cooling, erosion, and exhumation of rocks to be faster above the mid-crustal ramp than above the adjacent upper and lower flats (Brewer & Burbank 2006; Robert et al. 2011; Hubbard et al. 2016). However, in the northern High Himalaya the ramp geometry, in particular its width and depth are not well resolved. To place further constraints on the northward extent and depth of the ramp, we present low-temperature thermochronological data and U-Pb zircon ages from the Gyirong region, China. Our zircon U-Pb data show that the protolith of the High Himalayan orthogneisses is 478±4 Ma old, and was intruded by a large leucogranite at 19.5±0.7 Ma. The Pliocene-Quaternary cooling of the study area is constrained by apatite fission track ages and apatite and zircon (U-Th)/He ages between ~5 Ma and ~0.5 Ma, which show a marked trend of southward-younging ages (Wolff et al. 2022). Together with published cooling ages from Nepal farther south, the ages define a U-shaped pattern across the Main Himalayan Thrust, with ages increasing to the north and south from a minimum in the High Himalaya. A thermo-kinematic model, in which the geometry of the mid-crustal ramp was varied, explains the age data and suggests that the ramp has a dip of ~22°N. Compared to previous models, our new ages require that the ramp has a greater width (~55 km) and reaches a greater depth (~34 km) below the northern High Himalaya. Still farther north, the fault is presumably developed as a gently-dipping ductile shear zone, which forms the prominent reflector visible in seismic reflection data. References Brewer, I.D., Burbank, D.W. (2006). J. Geophys. Res. 111, B09409. Hubbard, J., Almeida, R., Foster, A., et al. (2016). Geology 44, 639-642. Robert, X., Van der Beek, P., Braun, J., et al. (2011). J. Geophys. Res. 116, B05202. Wolff, R., Hölzer, K., Hetzel, R., et al. (2022). Tectonophysics 834, 229378.