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We investigate the seismic attenuation along a 160 km profile from the Lower to the High Himalaya in the Garhwal region, India, through the analysis of Lg waveforms from regional earthquakes recorded on 18 broadband seismographs with interstation spacing of 7–10 km. Lateral variability in attenuation is derived through the inversion of 36 two-station measurements using a global optimization scheme. We observe a contrasting attenuation property in the two Himalayan belts: the Lower Himalaya has of 742 ± 235 similar to those observed in the Indian shield, whereas the High Himalaya is characterized by an unusually low value of 30–60. The seismic attenuation is also well correlated with the P-wave teleseismic traveltime residual pattern: faster arrival (approximately 0.2 s) at stations in the Lower Himalaya as compared to azimuthally independent time delay of ∼0.75 s for the High Himalayan stations. The high attenuation and low velocity in the High Himalaya suggests that low viscosity and partial melt in the crust can be correlated to the presence of Miocene leucogranite plutons in this Himalayan belt, a magmatic product of the Indo-Asian collision and presumably evidence of a partial melting event. This shallow low viscosity channel possibly connects the mid-crustal channel beneath Tibet. [ABSTRACT FROM AUTHOR]

In south Karakorum, the western prolongation of southern Tibet, three distinct types of magmatic rocks were emplaced during the Neogene: (1) 22–24 Myr old lamprophyres, characterized by strong enrichment in large ion lithophile (LILE) and light rare earth elements (LREE), 87Sr/86Sr(i) = 0·7096, εNd(i) = –7, and εHf = –9, interpreted to reflect partial melting of a previously metasomatized spinel-lherzolite mantle source; (2) the 21–26 Myr old Baltoro high Ba–Sr granitoids, likewise strongly enriched in LILE and LREE, with 87Sr/86Sr(i) = 0·7034–0·7183, εNd(i) = –6·5 to –11·0, and εHf = –1·8 to –8·0, produced by partial melting of amphibole-bearing rocks in the lower crust, possibly the root of south Karakorum Cretaceous magmatic arc; (3) the 8–9 Myr old Hemasil syenite and its associated lamprophyre, also both enriched in incompatible elements but with isotopic compositions closer to those of depleted mantle (87Sr/86Sr(i) = 0·7043–0·7055, εNd(i) = ·5 – 4·3, and εHf = 10·4 – 11·2). The Hemasil syenite is interpreted as the product of partial melting of a time-integrated depleted spinel-lherzolite source that was enriched in K and LREE during a recent metasomatic event. We propose that the lamprophyres were formed during partial melting of the South Asian mantle previously metasomatized by fluids derived from the subducted Indian continental crust. This melting episode is interpreted to be related to a break-off event that occurred within the subducting Indian continental lithosphere. Intrusion of the resulting lamprophyric melts into the previously thickened south Karakorum crust caused partial melting of calc-alkaline igneous protoliths and generation of the Baltoro granitoids. Late-stage syenitic magmas were produced by low-degree partial melting during upwelling and adiabatic decompression of depleted mantle along the Shigar strike-slip fault. [ABSTRACT FROM AUTHOR]