Structural and paleostress analysis within a fossil slow-spreading ridge: Tectonic processes involved during ocean expansion

Abstract

This study aims to decipher the spatio-temporal chronology of tectonic processes leading to ocean widening at a slow-spreading oceanic ridge axis, involving mantle and gabbro exhumation and volcanism. Structural and petrographic studies of deformed lithologies were performed on peridotites, gabbros and basalts from the Chenaillet ophiolite (Alps, France). Inversion of fault-slip data reveals N030° and N060° σ3 paleostress trends. The dominant N030° extension, is consistent with the NNW-SSE direction of volcano feeder-dykes and ENE dipping low-angle normal faults crosscutting the mantle. The N060° extension accords with the mantle dome structure formation to the east. Low-angle faults intersect roots of volcanoes that thus belong to their hanging walls. Hydrothermalism is contemporaneous with or postdates low-angle faulting. A feeder-dyke major virgation northwards, synchronous with eruptions, suggests a dextral transform fault consistent with a N120° σ2. Oceanic expansion mechanisms are clarified. At the surface, magma eruption occurs along on-axis active high-angle normal faults, their footwalls enabling mantle exhumation. At depth, off-axis high-angle faults become low-angle faults as they spread at shallow level. With westward drift of the lithosphere, the uppermost levels of the ridge shift westward faster, such that volcanoes move to an off-axis position while their roots are cut by low-angle faults.