Himalayan sub-Moho earthquakes suggest crustal faults trigger eclogitized-drip tectonics

Along the 1400-km Himalayan arc, 100 + sub-Moho earthquakes have been detected from their Sn/Lg amplitude ratios or S-P delay times, concentrated densely beneath a ~ 300-km segment in south Tibet where they reach ~ 110-km depth. Explanations for these earthquakes include Moho-penetrating faults and the dripping of eclogitized lower crust. We estimate the geological strain-rates, temperatures, and timescales for these two processes from seismological, thermal, geological, and geodetic datasets. We constrain the eclogite viscosity to \(\:\lesssim\:\)1–5×10 ²¹ Pa⋅s through numerical modeling of viscous Rayleigh-Taylor dripping within the available geological timescale (~ 20 Ma). Thermal analysis shows it is unlikely that brittle failure in olivine-dominated lithospheric mantle occurs below the 70-km Moho, hence a deeply penetrating fault cannot by itself explain the 70–110-km seismicity. Stronger eclogitized lower crust at upper-mantle depths would enable brittle failure, but an eclogite drip by itself cannot explain the dominating dextral-slip focal mechanisms. We propose that eclogitization of mafic granulites in the Indian lower crust occurs along lower-crustal shear zones associated with active faults and fluid intrusion, creating the density anomaly that drives Rayleigh-Taylor instability. As the eclogite drip grows, high strain within the drip creates brittle faulting to upper-mantle depths, albeit in what are crustal lithologies.