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


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