HR: 1330h
AN: S32A-0836    [PDF]
TI: Anomalous Deep Moho and Low S-velocities in the Lower Crust of an Early Archaean Greenstone Belt: Western Dharwar Craton, South Indian Shield
AU: Rai, S
EM: SSRAI_NGRI@rediffmail.com
AF: NGRI, Uppal Road, Hyderabad, 500 007 India
AU: Gupta, S
EM: sandeep_ngri@yahoo.co.in
AF: NGRI, Uppal Road, Hyderabad, 500 007 India
AU: * Priestley, K
EM: keith@esc.cam.ac.uk
AF: Bullard Laboratories, Madingley Rise, Madingley Road, Cambridge, CB30EZ United Kingdom
AU: Gaur, V K
EM: gaur@ccmacs.ernet.in
AF: CMMACS, NAL Belur Campus, Bangalore, 560 037 India
AB: We present a shear velocity image across the early Archaean ($>$3.4-3.0 Ga) granite-greenstone crust of the western Dharwar craton of the south Indian shield. We determine this image through a receiver function analysis of teleseismic waveforms recorded at six broadband seismographs deployed for a year along a 200 km long profile across the granite-greenstone belt. We analyze more than 60 high signal-to-noise ratio seismograms from each station. All stations are sited on amphibolite facies rock with $\sim$6 kb paleo-pressure indicating about 10 km of erosion. Results from a grid search over the H-Vp/Vs domain and from a formal inversion of the receiver function waveforms reveals that the crustal thickness increases from $\sim$35$\pm$2 km in the late Archaean terrain ($\sim$2.5 Ga) of the east Dharwar craton (SE India) to $\sim$53$\pm$2 km in the early Archaean greenstone belt of the west Dharwar craton (SW India). We find significantly reduced S-velocity (3.2-3.4 km/s) and high Poisson's ratio ($\sim$0.3) in the lower crust of the deepest part of the greenstone belt. The anomalous deep Moho and very low-velocity, lower crustal layer in the early Archaean nucleus suggest hydrated mantle condition and subduction of ocean lithosphere at $\sim$3.4 Ga. This is the oldest evidence known to us for plate tectonics in the Archaean.
DE: 1242 Seismic deformations (7205)
DE: 7205 Continental crust (1242)
DE: 8150 Plate boundary--general (3040)
SC: Seismology [S]
MN: 2003 Fall Meeting