Imaging Hales Discontinuity beneath India: Seismological and Petrological Model

We model the depth and Vs structure of the Hales discontinuity (H-D) beneath Eastern Dharwar Craton (EDC) and Southern Granulite Terrain (SGT) using P-wave receiver function (P-RF) analysis and joint inversion with Rayleigh wave phase velocity dispersion. We calculate P-RFs at higher frequency (fmax 0.46 Hz), compared to previous studies, to show that the P-to-S converted phase from the H-D (Phs) is distinct from the crustal reverberations. The Phs at stations in the EDC arrive at ~10 s beneath GBA, and ~11 s beneath HYB. From joint inversion the H-D is modeled at 97 ± 5 km and 108 ± 5 km depth, with 5% and 3% Vs increase, beneath GBA and HYB, respectively. For KOD, in SGT, the Phs coincides with the mid-crustal PpSs+PsPs reverberation at most ray-parameters, causing destructive intereference. This explains the apparent absence of Phs in previous studies. We isolated P-RFs where Phs is distinct at ~10.5 s and model it at depth of 101 ± 5 km with Vs increase of 3%. We demonstrate through forward calculation that the spinel-garnet mineral transformation cannot explain the H-D Vs increase. From data of mantle xenoliths in the Wajrakarur kimberlite field, Southern India, we calculate Vs of mantle peridotite and eclogite, using published bulk rock compositions through Perple-X. At the H-D depth and temperature derived from Indian shield geotherm, we observed a perfect match to the Vs. We hypothesize that H-D marks the surface of a paleo-subducted eclogitic oceanic slab embeded within the upper mantle peridotite. Observations of mantle faults within the Canadian lithosphere, at similar depth, has been related to relict-subduction zones and therefore independently supports our model.