Early (pre-8 Ma) fault activity and temporal strain accumulation in the central Indian Ocean

The diffuse deformation zone in the central Indian Ocean is the classical example of distributed deformation of the oceanic lithosphere with shortening between the Indian and Capricorn plates manifest as reverse faulting (5–10 km spaced faults) and long-wavelength (100–300 km) folding. The onset of this deformation is commonly regarded as a key far-field indicator for the start of major uplift of the Himalayas and Tibet, some 4000 km further to the north, due to increased deviatoric stresses within the wider India-Asia area. There has been disagreement concerning the likely timing for the onset of deformation between plate motion inversions and seismic reflection-based studies. In the present study, fault displacement data from seismic reflection profiles within the central Indian Ocean demonstrate that compressional activity started much earlier, at around 15.4–13.9 Ma. We reconstruct that 12% of the total reverse fault population had been activated, and 14% of the total strain accumulated, prior to a sharp increase in the deformation rate at 8.0–7.5 Ma. There is no evidence for any regional unconformity before 8.0–7.5 Ma, early shortening was accommodated by activity on single isolated fault blocks. Total strain estimates derived are more variable and complex than those predicted from plate inversion and do not show simple west to east increase

Reconciling plate kinematic and seismic estimates of lithospheric convergence in the central Indian Ocean

The far-field signature of the India-Asia collision and history of uplift in Tibet are recorded by sediment input into the Indian Ocean and the strain accumulation history across the diffuse plate boundary between the Indian and Capricorn plates. We describe the history of India-Capricorn convergence from updated estimates of India-Somalia-Capricorn plate rotations and observations derived from seismic reflection data. New India-Capricorn plate rotations for the past 20 m.y. are consistent with slow north-south convergence from 18 Ma about a stationary or nearly stationary pole near the eastern edge of the Chagos-Laccadive ridge, simpler than predicted by previous models based on many fewer data. The new rotations suggest that convergence began between 18 and 14 Ma, consistent with marine seismic evidence for an onset of deformation at 15.4–13.9 Ma. They further show that convergence rates doubled at 8 Ma, in agreement with a sharp increase in fault activity at 8–7.5 Ma seen on seismic reflection profiles. A discrepancy between the total strain estimated from kinematic and seismic reflection data can be reconciled if pervasive reverse faulting within the diffuse plate boundary is accompanied by block rotations of 1°–3°. <br/