Seismic constraints on the crustal structure within the Vema Fracture Zone

Abstract

Results from recent studies of the Kane and Oceanographer fracture zones suggest that significant variations in crustal thickness are associated with parts of these fracture zones, with total crustal thicknesses in places less than half the thickness of normal oceanic crust. The extent of this anomalous crust and its origin are still very poorly constrained by available geophysical data. Here we report results from a 75 km long refraction line shot in late 1977 over three Hawaii Institute of Geophysics ocean bottom seismometers deployed within the Vema transform valley near its western intersection with the Mid‐Atlantic Ridge (10°53′N, 43°39′W). We show that the apparent crustal thickness beneath this portion of the Vema transform valley is about 5 km, not substantially different from crustal thicknesses typically associated with oceanic crust. However, the Vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the Kane and Oceanographer fracture zones. It is characterized by low compressional wave velocities, a relatively uniform velocity gradient throughout most of the crust, and the absence of a typical layer 3 refractor. This anomalous crust is interpreted as intensely fractured, hydrothermally altered and serpentinized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls. The large variation in Moho depths found in these large‐offset, slowly slipping transforms is attributed to differences in the depth and extent of serpentinization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer. One important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor.