The Mid-Atlantic Ridge at 13°N is regarded as a type locality for oceanic core complexes (OCCs), as it contains, within ∼70 km along the spreading axis, four that are at different stages of their life cycle. The wealth of existing seabed observations and sampling makes this an ideal target to resolve contradictions between the existing models of OCC development. Here we describe the results of P-wave seismic tomographic modelling within a 60 × 60 km footprint, containing several OCCs, the ridge axis and both flanks, which determines OCC crustal structure, detachment geometry and OCC interconnectivity along axis. A grid of wide-angle seismic refraction data was acquired along a series of 17 transects within which a network of 46 ocean-bottom seismographs was deployed. Approximately 130 000 first arrival traveltimes, together with sparse Moho reflections, have been modelled, constraining the crust and uppermost mantle to a depth of ∼10 km below sea level. Depth slices through this 3-D model reveal several independent structures each with a higher P-wave velocity (Vp) than its surrounds. At the seafloor, these features correspond to the OCCs adjacent to the axial valley walls at 13°20′N and 13°30′N, and off axis at 13°25′N. These high-Vp features display dipping trends into the deeper crust, consistent with the surface expression of each OCC's detachment, implying that rocks of the mid-to-lower crust and uppermost mantle within the footwall are juxtaposed against lower Vp material in the hangingwall. The neovolcanic zone of the ridge axis has systematically lower Vp than the surrounding crust at all depths, and is wider between OCCs. On average, throughout the 13°N region, the crust is ∼6 km-thick. However, beneath a deep lava-floored basin between axial OCCs the crust is thinner and is more characteristically oceanic in layering and velocity–depth structure. Thicker crust at the ridge axis suggests a more magmatic phase of current crustal formation, while modelling of the sparse Moho reflections suggests the crust–mantle boundary is a transition zone throughout most of the 13°N segment. Our results support a model in which OCCs are bounded by independent detachment faults whose dip increases with depth and is variable with azimuth around each OCC, suggesting a geometry and mechanism of faulting that is more complicated than previously thought. The steepness of the northern flank of the 13°20′N detachment suggests that it represents a transfer zone between different faulting regimes to the south and north. We propose that individual detachments may not be linked along-axis, and that OCCs act as transfer zones linking areas of normal spreading and detachment faulting. Along ridge variation in magma supply influences the nature of this detachment faulting. Consequently, not only does magma supply control how detachments rotate and migrate off axis before finally becoming inactive, but also how, when and where new OCCs are created.
Bibliographical noteFunding Information:
This research project was funded by the Natural Environmental Research Council (NERC) grants NE/J02029X/1, NE/J022551/1 and NE/J021741/1. We would like to thank all involved in the planning and acquisition of data during research cruise JC132 (Reston & Peirce 2016), including the officers, engineers and crew of the RRS James Cook, the scientific party, and all seagoing technicians. The NERC Ocean-Bottom Instrumentation Facility (Minshull et al. 2005) provided the OBSs used in this project, together with their technical support at sea. All figures were prepared using the Generic Mapping Tools (GMT) package (Wessel et al. 2013). All data from cruises JC102, JC109 and JC132 are archived at the NERC’s British Oceanographic Data Centre (www.bodc.ac.uk), and the final accepted version of this manuscript is available through Durham Research Online (dro.dur.ac.uk). We thank our two reviewers, Satish Singh and Michele Paulatto, for their positive and thoughtful comments.
© The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society.
- Controlled source seismology
- Crustal imaging
- Crustal structure
- Mid-ocean ridge processes
ASJC Scopus subject areas
- Geochemistry and Petrology