Shear wave velocity structure and crustal lithology beneath the ultraslow spreading Southwest Indian Ridge at 50° E

SUMMARY Shear wave velocities provide an important constraint on crustal lithology. Limited shear data are available from the ultraslow spreading mid-ocean ridges. We combine observations of both compressional (P) and (S) waves in ocean–bottom seismometer Southwest Indian Ridge to determine P-wave velocity (Vp), S-wave (Vs), Vp/Vs Poisson's ratio variations along ridge at 49°17′E–50°49′E. Similar layered structures were revealed beneath magmatically robust segment centres (Vp/Vs 1.76–1.94, 0.26–0.32) non-transform discontinuity (NTD) between them 1.76–2.03 0.26–0.34). Because laboratory measurements show overlap mafic igneous ultramafic rocks, particularly Vp values typical oceanic Layer 3, it can be difficult distinguish composition using this parameter only. However, our observed gradients 0.1 ± s−1 suggest that area, 3 consists primarily rocks NTD. Oceanic Layers 2A 2B above likely also consist with some evidence for increased fracturing