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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS013 Oral Presentation 2223 Deep density structure and EET of lithosphere in the NW Himalaya and Laddakh from analyses of gravity anomalies Dr. Virendra Mani Tiwari GRAVITY National Geophysical Research Institute IASPEI V.M. Tiwari, P. Banerjee, B. Singh We use recently recorded gravity data along a profile in the NW Himalaya along with data over Indian shield,Ganga basin and Laddakh to construct a deep density model across Himalayan orogeny belt. Density model constrained from all available geophysical observations such as teleseismic receiver functions and magnetotelluric results are utilised in defining subsurface load to estimate the effective elastic thickness (EET) of the lithosphere along a profile perpendicular to the Himalaya. Furthermore, we use a wavelet transform to compute coherence between Bouguer gravity and topography of existing data to map the variations of EET in the NW Himalaya. Our results confirm the earlier results that EET decreases to the north as Indian plate descent to the Himalaya. Keywords: himalaya, gravity, lithosphere

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS013 Oral Presentation 2224 Magmatic underplating of crust beneath the Laccadive Island, NW Indian Ocean Dr. Sandeep Gupta Seismic Tomography National Geophysical Research Institute, Hyderabad Shyam Rai, Santosh Mishra Laccadive Island is the northern segment of Chagos- Laccadive Ridge (CLR), a prominent aseismic topographic/tectonic feature in the NW Indian Ocean. Despite its significance in providing critical linkage between the hotspot and genesis of islands, it is poorly investigated using modern seismological tools. We investigate the crustal velocity structure beneath Laccadive Island through receiver function modelling of teleseismic waveform recorded on the island broadband station during January April 2005. The crustal velocity structure suggests ~16 km thick oceanic crust with layers of 0.5, 1.0, 6.0,8.0 km thickness and shear velocities of 0.65, 2.2, 2.95, 3.98 km/s, respectively at depth. This is followed by ~10 km thick layer of shear velocity ~4.3 km/s layer. Further deep, at depth of 26 km, we observer uppermost mantle shear velocity 4.8 km/s. The presence of intermediate lower crust-mantle velocity layer could be interpreted as the underplating of mantle-derived melt at the base of oceanic crust which progressive deepen the crust-mantle boundary at 26 km. The melt was produced during the interaction of Indian plate with the Reunion hotspot. Continuity of crustal underplated material beneath Laccadive could be traced to Laxmi Ridge in the north and to Reunion Island in the south, suggesting this to be trace of the hotspot. This result along with measurements over other hotspot linked oceanic islands reinforces that crustal underplating is their common heritage. Keywords: laccadive island, magmatic underplating, crustal structure

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