Site Surveys of the Central and Southern Ninetyeast Ridge for the Ocean Drilling Program, Leg 121

The Ninetyeast Ridge is one of the most striking tectonic features of the world oceans. This ridge is situated in the Indian Ocean and runs almost parallel to the 90° E meridian. The origin of this feature has been explained by either a hot spot or leaky transform fault hypothesis. The primary objective of drilling on the ridge is to ascertain its age at certain areas. The knowledge of the age in combination with paleolatitude data should help the determination of the exact rate of motion of the Indian plate before and after the collision of India with Asia as well as establishing the time of the rate change. We should be able to relate this to the events of Himalayan orogeny, and to consider the implications for models of plate-driving forces. The northward motion curve should also help to understand the dynamic, sedimentary and tectonic development of the ridge, as well as clarify some questions regarding its origin. A secondary objective is to determine the chemical composition of the underlying basalts, especially their radiogenic isotopic ratios. This should help to define the extent of mantle compositional heterogeneity, and from that one can infer mantle dynamic processes. The chemical composition information will also constrain the origin and behavior of the ridge and the Indian Ocean hot spots, Kerguelen, Amsterdam, and St. Paul islands. Two site surveys for ODP were conducted at the central and southern areas of the Ninetyeast Ridge between DSDP sites 214 and 253 and between sites 253 and 254, respectively. The surveys were performed by the R/V CONRAD in the summer of 1986 during cruises RC 2707 and RC 2708. The survey was conducted using a single channel streamer and two 80 cubic inch water guns as sources. The single channel reflection data and the data obtained by a sonobuoy refraction survey were subsequently processed and interpreted at the University of Texas, Institute for Geophysics (UTIG), Austin, Texas. Bathymetry and basement maps were created, as well as isopach maps of the important seismostratigraphic horizons and maps of residual gravity and magnetics. The lithologic composition of five different horizons were interpreted at the central survey site, four at the southern site. Lithologies were based on the assumed mode of evolution of the ridge, lithologic composition of the neighboring DSDP sites 214, 253 and 254, and velocities obtained by the refraction surveys. At both survey sites the seismostratigraphic horizons were divided into two main groups, upper and lower. The sediments of the upper group were interpreted to be mainly deep water pelagic oozes, while the sediments of the lower group were mainly volcaniclastic sediments interlayered with basaltic lava flows deposited in shallow water or possibly subaerially. Two possible drilling sites are suggested at the central area. Their positions are based on the thickness of sedimentary cover, the depositional setting and the depth of water (regarding time of core recovery). The suggested drilling sites in the central survey area are located at seismic line intersections for 3-dimensional control. Five seismostratigraphic units are predicted at the two suggested drilling sites. The first two units are deep water pelagic oozes; the third unit is estimated to be either chalk or a green volcanic ash; unit number four is inferred to be black volcanic ash, possibly interbedded with volcanically derived siltstone or sandstone and the last sedimentary 2 unit consists of lava flows interbedded with volcaniclastic sediments. Total thicknesses of the sediments at the suggested drilling sites are 493 and 401 meters, respectively. The first three horizons show little or no faulting, while the last two horizons are faulted extensively. An important tectonic feature trending in the eastwest direction has been interpreted. This feature changes from an overthrust on the west side of the ridge to the normal fault on the east side, and according to interpretation of magnetic survey data may be accompanied by a basaltic dike. The age of the overthrust appears to be approximately 42 My, which is time of anomaly 18 (Middle to Late Eocene). This may suggest that this compressional feature is causally connected with a change of rate of northward motion of the Indian plate. The normal fault on the eastern side of the ridge seem to be active until present time. Four possible drilling sites are suggested in the southern survey area. Criteria for their placement are the same as above. Unfortunately, it is not possible to place the drilling sites onto seismic line intersections because of the excessive ruggedness of sea floor and underlying layers. Four stratigraphic units are interpreted at this survey site. The upper two are again deep water pelagic oozes and belong to the upper group, the lower two horizons are composed of volcaniclastic sediments and belong to the lower sedimentary group. Thicknesses of the sediments at the suggested drilling sites in the southern survey area are 338, 186, 177 and 255 m, respectively. Tectonic setting of this survey site is different than the setting of the central site. There is no evidence for compressional tectonic activity similar to that of the central survey site.