Constraints on 3‐D stress in the crust from support of mid‐ocean ridge topography

Abstract

The direction of crustal stresses acting at mid‐ocean ridges is well characterized, but the magnitude of these stresses is poorly constrained. We present a method by which the absolute magnitude of these stresses may be constrained using seafloor topography and gravity. The topography is divided into a short‐wavelength portion, created by rifting, magmatism, and transform faulting, and a long‐wavelength portion associated with the cooling and subsidence of the oceanic lithosphere. The short‐wavelength surface and Moho topography are used to calculate the spatially varying 3‐D stress tensor in the crust by assuming that in creating this topography, the deviatoric stress reached the elastic‐plastic limiting stress; the Moho topography is constrained by short‐wavelength gravity variations. Under these assumptions, an incompressible elastic material gives the smallest plastic failure stress associated with this topography. This short‐wavelength topographic stress generally predicts the wrong style of earthquake focal mechanisms at ridges and transform faults. However, the addition of an in‐plane regional stress field is able to reconcile the combined crustal stress with both the ridge and transform focal mechanisms. By adjusting the magnitude of the regional stress, we determine a lower bound for in situ ridge‐perpendicular extension of 25–40 MPa along the slow spreading mid‐Atlantic ridge, 40–50 MPa along the ultra–slow spreading ridges in the western Indian Ocean, and 10–30 MPa along the fast spreading ridges of the southeastern Indian and Pacific Oceans. Furthermore, we constrain the magnitude of ridge‐parallel extension to be between 4 and 8 MPa in the Atlantic Ocean, between −1 and 7 MPa in the western Indian Ocean, and between −1 and 3 MPa in the southeastern Indian and Pacific Oceans. These observations suggest that a deep transform valley is an essential feature of the ridge‐transform spreading center.