Investigating the Nature of the Seismic Layer 2A/2B Boundary at Mid-ocean Ridges


DOEI Project Funded: 2007


Understanding the nature and properties of the upper oceanic crust is fundamental to quantify magmatic processes occurring along mid-ocean ridges, as well as the interrelated tectonic, biological, and hydrothermal processes that are known as “ridge crest processes”. Submarine volcanic eruptions along mid-ocean ridges are the mechanism generating the uppermost part of the oceanic crust, a layer of highly fractured basaltic lavas that is typically several hundreds of meters thick. This highly porous layer, in which water-rock interactions and mineralization processes are common, hosts deep-sea hydrothermal springs and provides a unique geological environment for sub-seafloor microbial communities. Geophysicists commonly used remote-sensing methods such as seismic profiling to infer the thickness and properties of the lava layer (seismic layer 2A) and the underlying dike complex (seismic layer 2B) in order to investigate the magmatic and tectonic processes that create them and their evolution with time.Recent observations in two sections of oceanic crust formed about 1 million year ago challenge such geophysical-lithological correlation, suggesting that lithology plays a secondary role in defining the time-evolving nature of the seismic layer 2A/2B boundary, which is primarily controlled by alteration.

The aim of this proposal is to constrain the nature of seismic layer 2A/2B boundary in recently accreted crust, and to evaluate the possible dependence of such nature on the magmatic state of the ridge.Achieving these goals will allow us to better quantify magmatic process at the ridge crest, and make reliable comparisons of ridge crest processes between sections of the global mid-ocean ridge system in different magmatic state and/or spreading apart at different rates. We will perform high-resolution, two dimensional seismic tomography inversions to resolve the structure of the upper ocean crust along a section of the Galapagos Spreading Center along which clear changes in magmatic state are well documented. The available data and method we intend to apply will resolve lateral variations in seismic properties at scales of 1 km or less within the upper ~700-1000 m of the crust.