Thermomechanics of mid-ocean ridge segmentation

Abstract

The mechanics responsible for the initiation of the orthogonal pattern characterizing mid-ocean ridges and transform faults are studied using numerical models. The driving forces are thermal stresses arising from the cooling of young oceanic crust and extensional kinematic boundary conditions. Thermal stress can exert ridge-parallel tension comparable in magnitude to spreading-induced tension when selectively released by ridges and ridge-parallel structure. Two modes of ridge segment growth have been identified in plan view: an overlapping mode where ridge segments overlap and bend toward each other and a connecting mode where two ridge segments are connected by a transform-like fault. As the ratio of thermal stress to spreading-induced stress (γ) increases, the patterns of localized plastic strain change from the overlapping to connecting mode. The orthogonal pattern marks the transition from one mode to the other. Besides the amount of stress from each driving force, the rate of stress accumulation is crucial in determining the emergent pattern. This rate-dependence is characterized by the spreading rate normalized by a reference-cooling rate (Pe′). When Pe′ is paired with the ratio of thermal stress to the reference spreading-induced stress (γ′), they unambiguously define stability fields of the two modes. The obliquely connecting, the orthogonally connecting, and the overlapping mode are similar to ridge-transform fault intersections observed in ultra-slow, slow to intermediate, and fast spreading centers, respectively. The patterns are also sensitive to the strain weakening rate. Fracture zones were created in part as a response to thermal stress.