Preliminary Geophysical Results from the Ninetyeast Ridge Expedition

Geologic and geophysical data were collected along the Ninetyeast Ridge (NER) between 5.5°N and 26.1°S during the NSF-funded KNOX06RR cruise of R/V Roger Revelle during June to August 2007. Scientific objectives were to obtain site survey data for proposed drilling and to test the hotspot formation model of NER. During 48.6 days underway, 3861 km of high-resolution multichannel seismic reflection data were collected at seven sites, along with multibeam echo-sounder bathymetry, 3.5 kHz echo-sounder profiles, magnetic, and gravity data while underway. In addition, 33 dredges were collected along the ridge, recovering basalt for geochemical and geochronologic analysis at 24 sites. Bathymetric data show changes in morphology along NER from large, individual seamounts in the north, to smaller seamounts and ridges in the central NER, and to a high, narrow, continuous ridge in the south. Seismic and bathymetric data indicate that NER is pervasively faulted, indicating a greater role for tectonic deformation than is generally appreciated. The east side of the ridge exhibits a steep, linear escarpment, 1-2 km high, especially along the south NER. In contrast, western NER slopes are gentle, apparently unfaulted volcanic flanks. Morphology implies that the scarps resulted from strike-slip faulting nearly parallel to the ridge. Seismic data also show faulting of volcanic basement with trends highly oblique to the NER trend. This set of faults displays horst and graben structures, creates sediment-filled troughs, and sometimes nearly dissects the ridge. Many of these faults form half- grabens and may be related to seafloor spreading. Thick sediment covers northern NER volcanoes (typical thickness of 400-500 m) resulting from proximity to the Bengal fan. Sediment cover decreases from north to south, with typical thicknesses less than approximately 100 m at the south end of NER, where sediment thickness seems to be controlled mainly by topography.