Formation of diapiric structure in the deformation zone, central Indian Ocean: a model from gravity and seismic reflection data

Analyses of bathymetry, gravity and seismic reflection data of the diffusive plate boundary in the central Indian Ocean reveal a new kind of deformed structure besides the well-reported structures of long-wavelength anticlinal basement rises and high-angle reverse faults. The structure (basement trough) has a length of about 150 km and deepens by up to 1 km from its regional trend (northward dipping). The basement trough includes a rise at its center with a height of about 1.5km. The rise is about 10 km wide with rounded upper surface and bounded by vertical faults. A broad freeair gravity low of about 20 mGal and a local high of 8 mGal in its center are associated with the identified basement trough and rise structure respectively. Seismic results reveal that the horizontal crustal compression prevailing in the diffusive plate boundary might have formed the basement trough possibly in early Pliocene time. Differential loading stresses have been generated from unequal crust/sediment thickness on lower crustal and upper mantle rocks. A thin semi-ductile serpentinite layer existing near the base of the crust that is interpreted to have been formed at mid-ocean ridge and become part of the lithosphere, may have responded to the downward loading stresses generated by the sediments and crustal rocks to inject the serpentinites into the overlying strata to form a classic diapiric structure

Fault reactivation in the central Indian Ocean and the rheology of oceanic lithosphere

The intraplate deformation in the central Indian Ocean basin is a well-known example of a deviation from an axiom of plate tectonics: that of rigid plates with deformation concentrated at plate boundaries. Here we present multichannel seismic reflection profiles which show that high-angle reverse faults in the sediments of the central Indian Ocean extend through the crust and possibly into the uppermost mantle. The dip of these faults, which we believe result from the reactivation of pre-existing faults formed at the spreading centre, is ˜40° in the basement, which is consistent with the distribution and focal mechanisms of earthquakes on faults now forming at spreading centres. This style of deformation, coupled with the observation of large earthquakes in the mantle lithosphere, indicates that brittle failure of the oceanic lithosphere may nucleate in the vicinity of the brittle/ductile transition and propagate through the crust

Lithospheric wide-angle seismic profiles using stacked airgun shots

The signal-to-noise ratio of marine wide-angle seismic profiles can be significantly enhanced by stacking multiple shots. Signals detected from airgun shots from a seismic ship repeated many times within a small source area (called “stack shots”) can be stacked in a manner somewhat similar to common-mid-point processing of reflection seismic data. We collected two such “stack shot” profiles across the eastern margin of Newfoundland. At each shooting site, 36 closely spaced airgun shots were fired consecutively, and recorded along a profile made of about 400 land receivers at offsets of 100 to 455 km. While the data can be stacked in several different ways we show that a two-step technique or “two-pass stack” is the most effective. The traces of each receiver gather are first stacked at an aperture of 0.5 km along a slope of 8 km/s (stacking with linear moveout). The stacked traces are then reordered by increasing offset and stacked a second time along the same slope and with an aperture of 0.5 km or less. This technique is superior to a direct stack in which all the traces would be stacked in one pass because it allows improvement in the data quality by the various methods designed for each phase of the two-pass stack. Our results show that the “stack shot” technique coupled with the “two-pass stack” is a viable alternative, with less environmental impact, to using large, single explosions at sea

Annotated record of the detailed examination of Mn deposits from DSDP Leg 42 (Holes 372 and 379A)

Leg 42, Part 1 of the Deep Sea Drilling Project was scheduled to continue the geological exploration of the Mediterranean. The strategy was to search for areas where the Mediterranean Evaporite formation had been largely or completely removed by erosion, such is the case for Site 372. For Leg 42, Part 2, the project entered the Black Sea. In particular, Site 379 was located the central portion of the Black Sea