Teleseismic tomographic evidence for contrasting crust and upper mantles in south Indian Archaean terrains

Abstract

We compare the velocity structure of the crust and upper mantle of the Archaen granulite terrain of southernmost India with that of the Archaean granite-greenstone (Dharwar craton) terrain just to the north, through P-wave teleseismic travel time measurements. The teleseismic rays recorded by a coarse network of 11 portable seismic stations show an anomalous pattern of late arrivals (delays) over the granulite terrain in contrast to fast arrivals on the Dharwar craton. Such a pattern of time residuals amongst these Archaean terrains, which have remained inert in the last 2 Ga, may indicate the presence of compositional heterogeneity within the crust and upper mantle beneath them. Three-dimensional P-wave velocity tomography using teleseismic rays from a variety of azimuths indicates the existence of contrasting P-wave crust and upper-mantle velocity patterns: (1) in the crust (0–40 km) the western Dharwar craton and the granulite terrain have lower velocity (up to −3%) compared with the higher velocity (1–3%) observed over the eastern Dharwar craton, and (2) in the upper mantle (40–177 km), there is 2–3% lower velocity beneath the granulite terrain compared with the western Dharwar craton. The existence of such lateral velocity variation in the crust and upper mantle, and its preservation since late Archaean times, points to the presence of possibly thick and chemically distinct lithospheres that did not participate in mantle convection.