Geological Consequences of Dike Intrusion at Mid-Ocean Ridge Spreading Centers

Abstract

Dike intrusion is a fundamental process of crustal accretion at mid-ocean ridge spreading centers. Although many studies of ophiolites and spreading centers treat dikes as passive infillings of tensile fractures, observations and mechanical models from subaerial rift zones demonstrate that widespread, temporally and spatially complex mechanical effects accompany dike intrusion. These effects can potentially have a profound influence on the processes attending seafloor spreading. Specifically, dike intrusion should change the magnitude and orientation of the local stress field as a dike tip propagates along a spreading center. Consequently, dike emplacement could cause significant short-term (days to months), long-term (years to decades), and permanent changes in the structure of the axial crust. The mechanical effects of intrusion of even a single dike from a subaxial magma chamber should influence the entire narrow ( 10 km) median valley. At fast-spreading ridges the effects of intrusion are associated with recent microearthquake swarms, graben subsidence, fissures, eruptions, and hydrothermal megaplume events. In contrast, relatively infrequent dike intrusions with only local effects on ridge-axis morphology and geology may be reflected by scattered volcanic edifices and large-scale faulting at slow-spreading ridges. We suggest that repeated dike intrusion will create systematically contrasting crustal assemblages that should reflect the varying relationship between faulting and magmatism at different mid-ocean ridge spreading centers.