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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 2219
Nature of the South Indian precambrian crust
Dr. Shyam Rai
Seismic Tomography National Geophysical Research Institute IASPEI
Sandeep Gupta, S. Jagdeesh, Vinod Gaur
South India is a collage of several crustal blocks formed by geodynamic processes operating from mid-
Archean to Neo-Proterozoic. The main geological province constitutes Dharwar craton, Granulite terrain
and Cuddapah basin. Dharwar craton is an Archean continental fragment with a continuously exposed
crustal section from low grade gneisses and greenstone belts in the north to granulites in south.
Western part of craton hosts the 3.4 Ga greenstone belt while its northern part is covered with 65Ma
Deccan volcanics. What forms the basement of the basalt is an unresolved issue. Eastern part of the
Dharwar craton is wrapped by the Proterozoic Cuddapah basin. To the south of the craton is the high
grade metamorphic terrain with imprint from Archean to late Proterozoic. These diverse crustal
fragments of south India make it ideally suited to model the nature of the Archean crust and its possible
deformation with time. We model the crustal structure of south India at 35 broadband seismograph
locations through joint inversion of the receiver functions with the surface wave group velocity
measurements. Moho depth varies significantly from 35 km beneath the late Archean eastern Dharwar
craton (EDC), and Deccan volcanic province (DVP) to over 45 km beneath the mid Archean granitegreenstone
belt in the western Dharwar craton (WDC), and ~40 km beneath the garnulite terrain. The
average shear velocity of south Indian crust varies from 3.66 km/s in upper part to ~ 3.9 km/s in the
lower half and is higher than the global average for Precambrian terrains. We present the details of
intra-terrain velocity variation in the crust, correlate with the surface geology and speculate the possible
rock composition with depth in different terrains. These S- velocity derived composition sections along
with recently acquired heat production in different rock types of south India have been used to infer the
present day crustal contribution to the surface heat flow and therefore the mantle heat contribution.
Together with these shear velocity and heat flow, we have used the reliable estimates of P-velocity with
depth, nature of seismic reflector, conductivity variation with depth derived from MT measurements and
the geochemical measurements to understand the crust composition and possible models of evolution.
Keywords: south india, precambrian, crust