Geophysical Studies of Oceanic Core Complexes: The Mid-Atlantic Ridge, 13 - 14°N

Abstract

Marine geophysical studies have revealed that crustal accretion processes at mid-ocean ridges are largely dependent on the relative contribution of magmatism and tectonism to plate separation. At high levels of magmatic accretion, the ridge is associated with large amounts of surficial volcanism and the formation of closely spaced, continuous abyssal hills that parallel the ridge axis. In contrast, when magmatic accretion is low, tectonic extension prevails and crustal structure is markedly different. In this latter scenario, long-lived detachment faulting forms oceanic core complexes (OCCs) that expose lower crustal rocks and mantle material at the seafloor. Whilst many of the general features of this ‘magma-poor’ terrain have been identified, there is still widespread debate as to the exact conditions that necessitate and regulate OCC formation.
This study presents the results of a deep-towed and shipboard geophysical survey of the Mid-Atlantic Ridge between 13 – 14°N. This area is an ideal place to test and develop models for OCC structure and formation as the ridge axis exhibits multiple core complexes at various evolutionary stages. The data presented here include: deep-towed TOBI sidescan sonar and tri-axial magnetometer data, shipboard gravity and bathymetric data, and detailed seabed sampling results.
High resolution sidescan sonar data reveal that active core complexes in this region are associated with a neovolcanic hiatus within the axial valley. In contrast, a recently terminated near-axis OCC is associated with widespread neovolcanism, thus confirming that OCC formation is regulated by variations in melt supply to the ridge axis. Forward modelling of gravity data shows that beneath the domal sections of OCC footwalls (from which serpentinised peridotites were ubiquitously sampled), a low density zone (LDZ) exists. This area is interpreted as comprising predominantly gabbroic material that has been captured by each detachment fault at depth beneath the ridge axis (thus explaining the gap in neovolcanism at the surface). Furthermore, magnetic data suggest that OCC footwalls are highly heterogeneous, reflecting significant compositional and thickness variations of the LDZ.
Analyses of sidescan sonar imagery, combined with models of gravity and magnetic anomalies, reveal significant across- and along-axis asymmetry in the region. In general, the quadrants of the survey area within the inside-corners of the non-transform offset at 13°38’N are associated with widespread OCC formation, elevated tectonic strain (Tε = 25 – 30%), thin crust and faster spreading (20 – 40%) compared with conjugate ridge flanks; these areas are characterised by crust that is typically ~0.5 – 1.0 km thicker than OCC-forming areas, and tectonic strain (Tε = 10 – 15%) is partitioned across numerous, small faults.
On the basis of these results, a model is presented for the structure of the 13 – 14°N region, and for the life cycle of oceanic core complexes. These models have implications for our current understanding of the magmato-tectonic conditions within which OCCs are expected to operate, and, in a broader sense, for the long wavelength processes that govern crustal accretion at low levels of magmatism.