Abstract

Although intraplate deformation is commonly observed on the continents, there are only a few places in the oceans where deformation is distributed over a broad area rather than being localized at a single plate boundary. The central Indian Ocean has long been recognized as a site of active widespread compressive deformation of the oceanic lithosphere, reflecting an exceptionally high stress level. However, the strain pattern, derived from indirect and incomplete observations, remains poorly known. A more complete data set was obtained during the Phèdre cruise (1991) by means of deep seismic reflection profiling. One 2100-km-long profile runs along the 81°E meridian from lat 14°S to the coast of Sri Lanka, and provides us with the first long cross section through the region of observed crustal-level faults. Fault analysis offers new estimates of the amount of shortening, which was obtained across individual faults by measuring the maximum vertical uplift of sedimentary reflectors at the base of the sedimentary cover and converting it to horizontal throw under various assumptions of the fault geometry. The contribution of the seismically resolvable faults amounts to 22-37 km of shortening or 2.5%-4.3%. This is two to three times greater than previous estimates extrapolated from local measurements on shorter seismic reflection profiles.

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