Sub-Continental Lithospheric Mantle Discontinuities beneath the Eastern Himalayan Plate Boundary System, NE India

We use P-wave receiver function (P-RF) analysis and joint inversion with Rayleigh wave group velocity dispersion data to model the shear-wave velocity (Vs) structure of sub-continental lithospheric mantle (SCLM) discontinuities beneath northeast (NE) India. The most prominent SCLM discontinuity is the Hales Discontinuity (H-D) observed beneath the Eastern Himalayan Foreland Basin (Brahmaputra Valley) and Shillong Plateau. The P-to-SV converted phase from the H-D (P_hs) is a positive amplitude arrival at ∼10–12 s and has positive move out with increasing ray-parameter. From joint inversion, the H-D is modeled at a depth range of 90–106 km, with 9–12% Vs increase beneath the Brahmaputra Valley. Beneath the Shillong Plateau the H-D is at a depth range of 86–102 km, with 6–9% Vs increase. An intra-lithospheric discontinuity (ILD) has been identified in the Shillong Plateau station P-RFs, as a positive amplitude P_ILDs phase, arriving at 8–8.5 s. This is modeled at a depth range of 65–75 km with Vs increase of ∼7±4%. We construct 2D profiles of depth-migrated common conversion-point stack of P-RFs to distinguish the SCLM discontinuity arrivals from crustal phases. 3D spline-interpolated surface of the H-D has been constructed to visualize its lateral variations. We use xenolith data from the Dharwar Craton, which has similar geological age, petrology and seismic structure as the Shillong Plateau, to petrologically model the SCLM H-D and ILD Vs structure in NE-India. From the calculated Vs structure we conjecture that the H-D is a petrological boundary between mantle peridotite and kyanite-eclogite, with its origin as metamorphosed paleo-subducted oceanic-slab, similar to other global observations. We further speculate that the shallower ILD could be formed as a contact between frozen asthenosphere-derived metasomatic melts within the SCLM.