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A region of oceanic core complexes (OCCs) exists at 13°N on the Mid-Atlantic Ridge that is regarded as a type site. This site includes two OCCs at 13°20′N and 13°30′N, thought to be in the active and dying stages of evolution, and two together called the Ashadze Complex (centred at 13°05′N) that are considered to be relict. Here we describe the results of S-wave seismic modelling along an ∼200-km-long 2-D transect traversing, south-to-north, through both the Mercurius and Marathon fracture zones, the southern outside corner of the 13°N segment, the OCCs, the ridge axis deviation in trend centred at 13°35′N, and the youngest oceanic crust of the eastern ridge flank to the north. Our inversion model, and the corresponding Vp/Vs ratio, show that the majority of the crust beneath the 13°30′N OCC comprises metamorphosed lithologies that have been exhumed to the shallowest subseabed level, while basaltic lithologies underlie the 13°20′N OCC. The transition between these contrasting crustal structures occurs over a distance of <5 km, and extends to at least ∼2 km depth below seafloor. The northern and southern OCCs of the Ashadze Complex have contrasting structures at shallow depth, with the northern OCC having a faster S-wave velocity in the upper crust. A Vp/Vs ratio of >1.9 (and equivalent Poisson's ratio of >0.3) indicates exhumed and/or metamorphosed lithologies beneath the bathymetric depression between them and within the crust beneath the southern OCC. Between the northern and southern flanks of the Marathon fracture zone and northern flank of Mercurius fracture zone, the lower crust has a relatively low Vp/Vs ratio suggesting that the deformation associated with Marathon fracture zone, which facilitates fluid ingress, extends laterally within the lower crust. Marathon fracture zone itself is underlain by a broad zone of low S-wave velocity (∼2.0 km s−1) up to ∼20 km wide from the seabed to at least the mid-crust, that is mirrored in a high Vp/Vs ratio and lower density, particularly deeper than ∼1 km below seabed within its bathymetric footprint. Volcanic domains are highlighted by a low Vp/Vs ratio of <1.6 (and equivalent Poisson's ratio of <0.15). Our combined seismic and density models favour the localized model of OCC evolution. They also show a considerable ridge-parallel variability in the amount and distribution of magmatic versus metamorphosed crust. Our results suggest that the current focus of magmatism lies to the north of the 13°20′N OCC, where the magmatic accretion-type seabed morphology observed is mirrored in the pattern of microseismicity, suggesting that its inward-facing median-valley-wall fault may link to the 13°20′N OCC detachment surface. Magmatism and active faulting behind (to the west) the footwall breakaway of the 13°30′N OCC, and the microseismicity concentrated in a band along its southern flank, suggest a readjustment of ridge geometry along axis is underway. As part of this, a transform offset is forming that will ultimately accommodate the 13°30′N OCC in its inside corner on the eastern flank of the ridge axis to the north.
Bibliographical noteFunding Information:
This research project was funded by the Natural Environment 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. The WA data were manipulated for plotting using Seismic Unix. All figures were prepared using the Generic Mapping Tools (GMT) package (Wessel & Smith 1998). 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 three reviewers for their very positive and constructive 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 process
ASJC Scopus subject areas
- Geochemistry and Petrology
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- 1 Finished
1/07/15 → 31/12/19
Project: Research Councils