2-D Vp and Vs Models 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 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 (nu) of 0.28-0.29 (V-p similar to 4.2 km/s and V-s similar to 2.3 km/s). At the flanks of the outcrop basement, the sediment cover is 200-300 m thick and nu 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 nu value of 0.28-0.29, however, at the seismic layer 2/3 transition, the V-p-V-s trend abruptly changes following a nu value of 0.25-0.26. These reduced observed nu values at the layer 2/3 transition are lower than those reported by laboratory measurements for gabbro (nu similar to 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.

Automated summary

This study uses simultaneous P- and S-wave data to investigate the seismic structure of the oceanic crust. The results show different seismic properties at different locations, indicating hydrothermal alteration and compaction processes. The abrupt change in seismic properties at the layer 2/3 transition is attributed to epidotization and crack-change properties.