Interplay between faults and lava flows in construction of the upper oceanic crust:: The East Pacific Rise crest 9°25′-9°58′N

[1] The distribution of faults and fault characteristics along the East Pacific Rise (EPR) crest between 9 degrees 25'N and 9 degrees 58'N were studied using high-resolution side-scan sonar data and near-bottom bathymetric profiles. The resulting analysis shows important variations in the density of deformational features and tectonic strain estimates at young seafloor relative to older, sediment-covered seafloor of the same spreading age. We estimate that the expression of tectonic deformation and associated strain on old'' seafloor is similar to 5 times greater than that on young'' seafloor, owing to the frequent fault burial by recent lava flows. Thus the unseen, volcanically overprinted tectonic deformation may contribute from 30% to 100% of the similar to 300 m of subsidence required to fully build up the extrusive pile ( Layer 2A). Many longer lava flows ( greater than similar to 1 km) dam against inward facing fault scarps. This limits their length at distances of 1 - 2 km, which are coincident with where the extrusive layer acquires its full thickness. More than 2% of plate separation at the EPR is accommodated by brittle deformation, which consists mainly of inward facing faults ( similar to 70%). Faulting at the EPR crest occurs within the narrow, similar to 4 km wide upper crust that behaves as a brittle lid overlying the axial magma chamber. Deformation at greater distances off axis ( up to 40 km) is accommodated by flexure of the lithosphere due to thermal subsidence, resulting in similar to 50% inward facing faults accommodating similar to 50% of the strain. On the basis of observed burial of faults by lava flows and damming of flows by fault scarps, we find that the development of Layer 2A is strongly controlled by low-relief growth faults that form at the ridge crest and its upper flanks. In turn, those faults have a profound impact on how lava flows are distributed along and across the ridge crest.