Letter

Nature 455, 790-794 (9 October 2008) | doi:10.1038/nature07333; Received 24 March 2008; Accepted 6 August 2008

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

J. Escartín1, D. K. Smith2, J. Cann3, H. Schouten2, C. H. Langmuir4 & S. Escrig4

  1. Marine Geosciences Group, CNRS Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France
  2. Geology and Geophysics Department, Woods Hole Oceanographical Institute, Woods Hole, Massachusetts 02543, USA
  3. School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
  4. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

Correspondence to: J. Escartín1 Correspondence and requests for materials should be addressed to J.E. (Email: escartin@ipgp.jussieu.fr).

The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on sea floor formed along ridges spreading at less than 80 km Myr–1 (refs 1–4). These faults can accommodate extension for up to 1–3 Myr (ref. 5), and are associated with one of the two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge. The first mode is asymmetrical accretion involving an active detachment fault6 along one ridge flank. The second mode is the well-known symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and the formation of abyssal hills on both flanks. Here we present an examination of approx2,500 km of the Mid-Atlantic Ridge between 12.5 and 35° N, which reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified so far in the study region is closely associated with asymmetrical accretion, which also shows high levels of near-continuous hydroacoustically and teleseismically recorded seismicity. Increased seismicity is probably generated along detachment faults that accommodate a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which occurs primarily at segment ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic evidence and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths or because the rising mantle is cooler. We suggest that much of the variability in sea-floor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachment faults in oceanic lithospheric accretion.

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