India–East Antarctica conjugate margins: rift-shear tectonic setting inferred from gravity and bathymetry data

The Eastern Continental Margin of India (ECMI) has evolved as a consequence of breakup of India from East Antarctica during the Early Cretaceous (ca. 130 Ma). The conjugate margin of ECMI in East Antarctica is represented by the margin extending from Gunneris Ridge in the west to about 95°E in the east. To understand the isostatic compensation mechanism operating beneath these conjugate margins, we have examined the cross spectral correlation between gravity and bathymetry along 21 profiles across the ECMI and 16 profiles across the conjugate East Antarctica Margin using both ship and satellite-derived gravity data. The ECMI is considered as a composite of two segments, one north of 16°N extending beyond 20°N, which is based on its rifted margin character, and the other, south of 16°N extending up to Sri Lanka, which has a transform-rift character. Similarly, the conjugate margin of East Antarctica is also considered to be a composite of two segments, west and east of the central bulge at 50–55°E. Admittance analysis and comparison with various isostatic models suggest a flexural plate model with an elastic thickness of 10–25 km for the northern segment of ECMI and its conjugate segment which is the east Enderby land Margin, comparable to results obtained from the eastern North American Margin. For the southern segment of ECMI, low elastic plate thickness of less than 5 km or a local compensation is obtained with matching results for the west Enderby land Margin. These, in turn, appear comparable to the low Te values inferred for the Ghana transform margin of North Africa and Grand Banks Margin of eastern Canada, thereby indicating that the southern segment of ECMI and its conjugate in East Antarctica have developed as a consequence of shearing rather than rifting in the early stages of continental separation.