2‐D V_p and V_s Models of the Indian Oceanic Crust Adjacent to the NinetyEast Ridge

Abstract 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 V p and V s models 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 ( V p ∼ 4.2 km/s and V s ∼ 2.3 km/s). At the flanks of the outcrop basement, the sediment cover is 200–300 m thick and ν values are similar (0.28–0.3), but V p and V s values are higher (4.5–4.8 and 2.4–2.6 km/s, respectively). We interpret the relatively lower V p and V s around 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 V p – V s trend is linear and follows a ν value of 0.28–0.29, however, at the seismic layer 2/3 transition, the V p – V s trend abruptly changes following a ν value of 0.25–0.26. These reduced observed ν values 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.