Fault scarps and tectonic strain in young seafloor

Fault scarps at Mid-Ocean Ridges (MOR) are well recognized on the seafloor and often measured to estimate the tectonic component of plate spreading. However, tectonic strain estimates based on the dimensions of fault scarps that can be traced on seafloor topographic maps (which we refer to as apparent tectonic strain) differ from the actual whole tectonic strain. This is clearly the case at relatively melt-poor slow-ultraslow ridge segment ends, where strain is accommodated by detachment faults that do not produce linear fault scarps at the seafloor. This contribution explores the relation between actual and apparent tectonic strain in magma robust MOR regions (at fast, intermediate spreading ridges, and in the magmatically robust segment centers of slow-ultraslow ridges). We use high-resolution (1-2 m) bathymetry data at 8 MOR sites, which span a broad range of spreading rates (14-110 km/Ma) and melt fluxes. To the first degree, apparent tectonic strain is highest at slow spreading ridges, which have the lowest melt fluxes, and decreases as melt flux increases (fast spreading ridges). We examine how faults nucleate and evolve on the young axial seafloor, while establishing the relationships to volcanism. Apparent tectonic strain derived from the dimensions of fault scarps on the young seafloor is reduced due to lava flows that cover pre-existing faults. Apparent tectonic strain also includes a component of strain that is not related to far-field tectonic stresses but to stalled dike intrusions that induce extensional faults in the shallow crust. This mechanism is probably responsible for the high apparent tectonic strain estimated at domal volcanos found at the center of intermediate-slow-ultraslow ridge segments. Apparent tectonic strain at and near MOR thus poorly reflects the real tectonic component of plate divergence, instead relating to the interplay between tectonic and magmatic processes over different time scales.