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

Research output: Contribution to journalArticlepeer-review

Authors

  • Christine Peirce
  • G Reveley
  • AH Robinson
  • Matthew Funnell
  • Roger Searle
  • Nuno Simao
  • Christopher MacLeod

Colleges, School and Institutes

External organisations

  • Durham University
  • Cardiff University

Abstract

The 13 degrees 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 similar to 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 similar to 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 degrees 20'N OCC. The 13 degrees 20'N OCC is distinct in that it does not show higher relative velocity at shallower crustal depth like its neighbours, while the 13 degrees 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 degrees N segment are not interconnected at depth. To the north of the 13 degrees 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 degrees N segment and the southern ridge-transform intersection outside corner. Along profile, Marathon fracture zone offsets younger (similar to 1 Myr versus similar to 8 Myr) oceanic crust than Mercurius fracture zone (similar to 8 Myr versus similar to 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 degrees N segment OCCs rather than normally accreted oceanic crust, particularly in the upper-to-middle crust.

Details

Original languageEnglish
Pages (from-to)988-1010
JournalGeophysical Journal International
Volume217
Issue number2
Early online date6 Feb 2019
Publication statusPublished - 1 May 2019

Keywords

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