Constraints on crustal structure of adjacent OCCs and segment boundaries at 13°N on the Mid-Atlantic Ridge

Christine Peirce*, G Reveley, AH Robinson, Matthew Funnell, Roger Searle, Nuno Simao, Christopher MacLeod, Tim Reston

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

17 Citations (Scopus)
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Abstract

The 13°N segment of the Mid-Atlantic Ridge is an example of a morphologically well-studied slow spreading ridge segment populated with oceanic core complexes (OCCs). In this paper we present the results of an ∼200-km-long 2-D seismic and gravity transect through this segment, the bounding fracture zones to the south and the ridge discontinuity to the north. We use this transect to consider the two end-member models of OCC evolution in which one, referred to as the Segment-scale model, implies they are interconnected with their detachments being part of a single segment-long feature, and the other, the Localized model, that each OCC is structurally isolated. We show, using the 7.5 km s-1 velocity contour as the base of crust marker, that the crust is consistently relatively thin ridge-parallel, at ∼5 km thick on average, and that, beneath the OCCs, the Moho marks the top of a velocity gradient transition into the mantle, rather than a distinct velocity discontinuity. Although each OCC is not traversed in an identical structural location, they show a different crustal velocity-density structure with depth, with along axis variations in this structure mirrored by the bathymetric deeps between them. Older OCCs have a contrasting velocity-depth signature to the currently active 13°20′N OCC. The 13°20′N OCC is distinct in that it does not show higher relative velocity at shallower crustal depth like its neighbours, while the 13°30′N OCC has an apparently thinner crust. Our combined P-wave seismic traveltime tomography and gravity forward modelling suggests that the OCCs of the 13°N segment are not interconnected at depth. To the north of the 13°30′N OCC, our modelling also suggests that the crust is being magmatically refreshed, or that the ridge axis is currently undergoing magmatic accretion with an associated ridge tip propagation occurring across the ridge discontinuity that marks its northern edge. The profile also crosses the Marathon and Mercurius fracture zones that mark the southern limit of the 13°N segment and the southern ridge-transform intersection outside corner. Along profile, Marathon fracture zone offsets younger (∼1 Myr versus ∼8 Myr) oceanic crust than Mercurius fracture zone (∼8 Myr versus ∼11 Myr). When considered in combination, both seismic and gravity modelling suggest crustal thinning in the direct vicinity of the bathymetric valley of Marathon fracture zone, coupled with a region of low density that, most likely, reflects serpentinization of the uppermost mantle. In addition, the crust captured between fracture zones appears relatively rotated about an E-W axis and uplifted to the north, with the upwards motion accommodated on the northern lateral edge of the bathymetric depression rather than in its centre. Both the outside corner and the crust bounded by fracture zones have velocity-depth characteristics similar to that of the 13°N segment OCCs rather than normally accreted oceanic crust, particularly in the upper-to-middle crust. Overall, our results support the Localized model of OCC evolution and suggest that fracture zones do not become locked immediately on transform-to-fracture transition as current models dictate.

Original languageEnglish
Pages (from-to)988-1010
Number of pages23
JournalGeophysical Journal International
Volume217
Issue number2
Early online date6 Feb 2019
DOIs
Publication statusPublished - May 2019

Bibliographical note

Funding 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 (Re-ston & 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 MCS data were processed using ClaritasTM and 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 On-line (dro.dur.ac.uk). We thank the three reviewers for their comments.

Publisher Copyright:
© Crown copyright 2019.

Keywords

  • controlled-source seismology
  • crustal imaging
  • crustal structure
  • mid-ocean ridge processes

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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