Distribution of Shortening between the Indian and Australian Plates in the Central Indian-Ocean

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Earth and Planetary Science Letters
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We analyze a single-channel seismic (SCS) reflection profile that completely crosses the zone of deformed oceanic lithosphere in the central Indian Ocean at 78.8 degrees E. By summing the apparent shortening on all seismically resolvable faults (throws > similar to 10 m), we find that 11.2 +/- 2 km of shortening has occurred at this longitude during the past 7.5 m.y. This estimate, together with the 27 +/- 5 km of shortening previously estimated from a multichannel seismic (MCS) profile farther east at 81.5 degrees E, are consistent with the west-to-east increase in shortening predicted by Euler poles which treat the Indian and Australian plates as separate tectonic units. Our result therefore provides direct evidence from the deformation itself that the compression of oceanic lithosphere in the central Indian Ocean, originally regarded as 'intraplate', is better described as constituting part of a broad boundary zone between distinct Indian and Australian plates.We also examine the size statistics of faults revealed in SCS and MCS profiles running nearly normal to the deformation trends in the longitude band 78.8 degrees E-81.5 degrees E. The N-S extent of the deformation does not change appreciably over these longitudes. We find that the average fault spacing remains constant at about 7 km, whereas the mean throw increases systematically from west to east (similar to 74 m to similar to 177 m). Basically the contribution of 'small' faults (those with throws of 10-50 m) decreases systematically across the deforming region (i.e., with increasing amount of shortening). This suggests that the deformation occurs by reactivation of a select fault population, that these faults continue to add displacement with time and that relatively few new faults are initiated. We also infer from the fault size statistics that the contribution to the deformation of faults below the resolution of the seismic methods (similar to 10 m for SCS and similar to 50 m for MCS) is likely to be quite limited.


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