2‐D Vpand VsModels of the Indian Oceanic Crust Adjacent to the NinetyEast Ridge

Until now, few offshore seismic studies have acquired simultaneously P‐ and S‐ wave data to derive in detail the seismic structure of the oceanic crust. We present 2‐D Vpand Vsmodels using wide‐angle seismic data at the Indian basin adjacent to the NinetyEast Ridge. Here, an outcrop basement located at the middle of the seismic line presents uppermost crustal Poisson's ratios (ν) of 0.28–0.29 (Vp∼ 4.2 km/s and Vs∼ 2.3 km/s). At the flanks of the outcrop basement, the sediment cover is 200–300 m thick and νvalues are similar (0.28–0.3), but Vpand Vsvalues are higher (4.5–4.8 and 2.4–2.6 km/s, respectively). We interpret the relatively lower Vpand Vsaround the basement outcrop in terms of hydrothermal alteration, while at the flanks of the basement outcrop, hydrothermal alteration has most likely ceased by sedimentation and compaction processes. Across the seismic layer 2, the Vp–Vstrend is linear and follows a νvalue of 0.28–0.29, however, at the seismic layer 2/3 transition, the Vp–Vstrend abruptly changes following a νvalue of 0.25–0.26. These reduced observed νvalues at the layer 2/3 transition are lower than those reported by laboratory measurements for gabbro (ν∼ 0.293) and are interpreted in terms of epidotization at the dike‐gabbro contact and/or crack‐change properties around the lower part of the intrusive sheeted dike section. The Ninety(90°)East(E) Ridge (NER) is a hotspot track formed by the Kerguelen hotspot mantle plume onto the Indo‐Australian plate more than 60 Myr ago. The length and width of the NER is about 5,000 and 200 km, respectively, and it extends from the Bengal bay to the southeast Indian mid ocean ridge. West and east of the NER are sited the Indian and Wharton basin, respectively. Here, we study the structure of the Indian basin at the western flank of the NER where the seafloor is highlighted by the presence of prominent oceanic basement outcrops. Our seismic results suggest that mineral alteration processes occur both in the upper and lower crust, respectively, under a basement outcrop due to the contact of seawater with crustal rocks. Probably, crustal accretion in a setting characterized by the interaction of a hotspot mantle plume with an active spreading center affect the porosity and permeability of the ocean crust around the NER region at large‐scale. We obtain 2‐D Vpand Vsmodels from active seismic data for the Indian oceanic crustThe seismic models suggest hydrothermal alteration near a basement outcropPoisson's ratios change at the layer 2/3 transition from 0.28–0.29 to 0.25–0.26 We obtain 2‐D Vpand Vsmodels from active seismic data for the Indian oceanic crust The seismic models suggest hydrothermal alteration near a basement outcrop Poisson's ratios change at the layer 2/3 transition from 0.28–0.29 to 0.25–0.26