INTRAPLATE SHORTENING IN THE CENTRAL INDIAN-OCEAN DETERMINED FROM A 2100-KM-LONG NORTH-SOUTH DEEP SEISMIC-REFLECTION PROFILE

Although intraplate deformation is commonly observed on the continents , there are only a few places in the oceans where deformation is distr ibuted 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 lithosphe re, reflecting an exceptionally high stress level. However, the strain pattern, derived from indirect and incomplete observations, remains p oorly known. A more complete data set was obtained during the Phedre c ruise (1991) by means of deep seismic reflection profiling. One 2100-k m-long profile runs along the 81-degrees-E meridian from lat 14-degree s-S to the coast of Sri Lanka, and provides us with the first long cro ss section through the region of observed crustal-level faults. Fault analysis offers new estimates of the amount of shortening, which was o btained across individual faults by measuring the maximum vertical upl ift of sedimentary reflectors at the base of the sedimentary cover and converting it to horizontal throw under various assumptions of the fa ult geometry. The contribution of the seismically resolvable faults am ounts to 22-37 km of shortening or 2.5%-4.3%. This is two to three tim es greater than previous estimates extrapolated from local measurement s on shorter seismic reflection profiles.