Current Seismicity and Tectonics Near the Gulf of Cambay: Evidences for the Khambat Plume Induced Activity

The Western Indian shield is seismically active as shown by both historical seismicity records and recently acquired instrumental data. Moderate to large earthquakes have occurred in the region but they do not clearly correlate with known tectonic features or faults. Current seismicity for this part of the shield for the period 1977-97. as evidenced from Gauribidanur Seismic Array (GBA) data, is found to be largely restricted to an area of about 80 km radius which accounts for almost 85 percent of the activity. The area is referred to as the Surat-Daman Seismic Zone (SDSZ) which has a general correspondence to the regional pattern of seismicity over the Western Indian shield established from both instrumental and non-instrumental data for a longer time span. The Indian shield crust is known to exhibit a sharp reduction in crustal thickness from 36 km below the Cambay Rift to about 20 km below the SDSZ within a distance of about 70 km. The thin shield crust is dissected into several faulted blocks that arc deformed to various degrees; some of these crustal blocks are seismically active. These features suggest that seismic activity in the region possibly relates to the fossil trace of the Khambat Plume along the Western continental margin near the Gulf of Cambay. The fossil plume appears to be located at least 100 km eastward onland. Bouguer anomalies over the fossil plume are largely positive with amplitudes upto 130 mgal, rendering further support to the presence of anomalous crust in the region. Available stress data from focal mechanism solutions indicate compressive stress in east-west to NNE-WSW direction. This suggests that stresses responsible for rift tectonics are not significant at the present stage; rather, the plume tectonics seems to be regionally predominant.