Thermoelastic stress: How important as a cause of earthquakes in young oceanic lithosphere?

Abstract

We present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere (less than 35 m.y. in age). In support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m.y. where cooling rates are relatively high. Most near‐ridge earthquakes have focal depths between the Moho and the depth of the 800°C isotherm. Thrust and strike‐slip events dominate the shallowest seismicity, while the focal mechanisms of the deeper events are generally characterized by normal faulting. To assess the importance of thermal stress in the generation of near‐ridge earthquakes, we model the lithosphere as an elastic half‐space in which the response to cooling is computed using the method of thermoelastic displacement potentials. A key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere. A further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature. Normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half‐space cooling. Models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust‐ and normal‐faulting earthquakes near mid‐ocean ridges. The addition of a “ridge push” stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m.y. but probably contributes little to the stress field at younger ages. Though these simple models are limited by several simplifying assumptions, particularly in the immediate vicinity of the ridge axis, they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere.