Figure Caption. Schematic block diagram of the Tethyan, Greater, and Lesser Himalaya system. Schematic shows the mantle fabric orientation, striking N63E and dipping 44° NNW, compared with the major crustal scale structures (i.e., the Main Himalaya Thrust, MHT) which generally strike east-west and dip ~10° or less [Zhao et al.; 1993]. Abbreviations are: MFT=Main Frontal Thrust, MBT=Main Boundary Thrust, MCT=Main Central Thrust, STD=South Tibetan Detachment (normal fault), LH=Lesser Himalaya.

Possible Upper Mantle Reflection Fabric on Seismic Profiles from the Tethyan Himalaya: Identification and Tectonic Interpretation*

Douglas Alsdorf1, Larry Brown1, and Douglas Nelson2
1Cornell University, Institute for the Study of the Continents, Dept. of Geological Sci., Ithaca, NY
2Syracuse University, Dept. of Geological Sci., Syracuse, NY

*Published in Journal of Geophysical Research, v. 101, B11, pp. 25305-25320, 1996

Abstract. 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 re-processed 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 suggests that a dipping reflection "fabric" is present in the upper mantle. Based on 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 ~N63E, significantly oblique to the ~EW strike of crustal deformation; however, the fabric dip direction (~44°, N27W) 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.

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