Possible upper mantle reflection fabric on seismic profiles from the Tethyan Himalaya: Identification and tectonic interpretation

Upper mantle structures of the Himalaya collision system are not well-defined, yet are integral to our understanding of the lithospheric scale processes which have built the Tibetan plateau. A 100-km-long deep seismic reflection profile (Tib-1) and a short cross line (Tib-2) were recorded within the Tethyan Himalaya to help constrain the geometry of the crust and mantle. At long travel times, the returned seismic energy is especially weak compared to the noise; therefore the data were reprocessed with signal-to-noise enhancing techniques to identify coherent events at mantle travel times. Visual comparisons of shot gathers, recordings of the ambient noise, common midpoint stacks, and synthetic sections suggest that a dipping reflection ''fabric'' is present in the upper mantle. On the basis of geometrical and velocity constraints, the fabric is not likely related to sideswipe, P-to-S phase conversions, multiples, or surface waves. The fabric is most likely within the underthrust Indian lithosphere beneath the peaks of the High Himalaya. The fabric strikes similar to N63 degrees E, significantly oblique to the similar to E-W strike of crustal deformation; however, the fabric dip direction (similar to 44 degrees, N27 degrees W) is essentially the same as the fast direction of polarization inferred from a previous SKS shear wave splitting study. The crust versus mantle strike discordance suggests that the fabric may predate the collision of India with Tibet and mark previous deformation of the Indian lithosphere (e.g., rifting episodes that formed the Tethyan ocean). Conversely, if the fabric is the result of collision, the discordance would suggest that mantle deformation is decoupled from crustal deformation within the Himalayas.