Volcanic Episodicity and a Non‐Steady State Rift Valley Along Northeast Pacific Spreading Centers: Evidence From Sea MARC I

Abstract

Sea MARC I side‐looking sonar images and Sea Beam bathymetry along a 400‐km stretch of the Juan de Fuca Ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading. An elongate summit depression (ESD), or rift valley, is commonly observed in the spine of this crestal ridge. The crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis. The landform has a plan‐form shape that resembles a side view of an archer's bow with the long dimension of the bow form parallel to the strike of the ridge. In cross section, the bow form is flat on top and has steep flanks. These bow‐form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge‐parallel dikes, similar to Icelandic fissure eruptions. Both the variable dimensions of the ESD along axis of the Juan de Fuca Ridge and the relationship among volcanic flow morphologies within and beyond the ESD suggest the four different segments of the Juan de Fuca Ridge presented in detail here display different stages in a cycle of oceanic crust accretion. This cycle includes episodes in which there is (1) extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression, (2) collapse within the summit region of the crestal ridge to form an ESD during a phase of volcanic inactivity, and (3) renewed magmatism in the ESD as its floor widens by extension and brittle fracture of the upper crust. This episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time.