Looking at the Seafloor Without Water
Over the past few weeks, we have surveyed the Enriquillo-Plantain Garden fault along the Jamaica Passage from west (near Jamaica) to east (near Haiti). The trace of the fault along the seafloor is manifested by linear ridges as high as 1,000 meters. South of the fault there are three large basins: the Moran Basin (60 km long), the Navassa Basin (30 km) and Motely Basin (20 km). The basins are bounded by the fault to the north and by a series of ridges to the south. Huge underwater landslides extend from the southern ridges into the basins. The deposits are tens of kilometers in length and include boulders the size of trucks. This suggests that the southern border of this fault zone is tectonically active and earthquakes may be a mechanism that initiate downslope failures and the huge slumps.
So far we have recovered 31 gravity cores. In the cores we discovered a turbidite —sediment deposited by a current of rapidly moving, turbulent water moving down a slope — that contains iron-rich minerals and organic material, possibly wood or algae. This is unusual since most sediments that we obtained are “calcareous oozes” composed of tiny shelled creatures. The turbidite’s iron-rich minerals are derived from the fault ridges. We will learn more when we open the cores at the Lamont-Doherty Core Repository, but we anticipate finding more turbidites that will shed light into the earthquake history of the region and help to better assess the hazard for Jamaica and Haiti.