Fault structure and detailed evolution of a slow spreading ridge segment: the Mid-Atlantic Ridge at 29°N

R.C. Searle (1) , P.A. A Cowie (2) , N.C. C Mitchell , S. Allerton , C.J. J Macleod (3) , J. Escartin (4) , S.M. M Russell , P.A. A Slootweg , T. Tanaka

1 Department of Earth Sciences [Durham]
2 UiB - Department of Earth Science [Bergen]
3 School of Earth and Ocean Sciences [Cardiff]
4 IPGP - Institut de Physique du Globe de Paris

We present preliminary results of a detailed near-bottom study of the morphology and tectonics of the 298N ''Broken Spur'' segment on the slow spreading Mid-Atlantic Ridge, using principally the TOBI deep-towed instrument. The survey covered two-thirds of the segment length, including all of its southern non-transform boundary, and extended off-axis of 40 Ž. km 3.3 Ma on either side. We obtained nearly complete near-bottom sidescan sonar coverage and deep-towed three-component magnetic observations along 2-km-spaced E-W tracks. Sidescan data reveal new details of fault structure and evolution. Faults grow by along-axis linkage. In the inside corner, they also link in the axis-normal direction by curving to Ž. meet the next outer older fault; this leads to wider-spaced faults compared to segment centre or outside corner. Outward facing faults exist but are rare. The non-transform offset is characterised by faults that are highly oblique, not parallel, to the spreading direction, and show cross-cutting relations with ridge-parallel faults to the north, suggesting along-axis migration of the offset. Almost all volcanic activity occurs within 5 km of the axis. Most fault growth is complete within 15 km of the Ž. axis 1.2 Ma , though large scarps continue to be degraded by mass-wasting beyond there. Crustal magnetisation is strongly three-dimensional. The current neovolcanic zone is slightly oblique to earlier reversal boundaries, and its magnetisation rises to a maximum of 30 A m y1 near its southern tip. The central magnetisation high tapers southwards and is asymmetric, with a sharp western but gradual eastern boundary. We infer a highly asymmetric accretion of layer 2 near the segment end. Older magnetic anomalies are kinked and sometimes missing. We interpret these observations as evidence of a rapid, 18 km southward migration of the segment boundary during the past 1.8 Ma, and present a series of reconstructions illustrating this tectonic history.

Mots clés: Mid-Atlantic Ridge; median valley; sea-floor spreading; acoustical surveys; magnetic anomalies; faults

Domaines: Sciences de la Terre

R.C. Searle, P.A. A Cowie, N.C. C Mitchell, S. Allerton, C.J. J Macleod, et al.. Fault structure and detailed evolution of a slow spreading ridge segment: the Mid-Atlantic Ridge at 29°N. Earth and Planetary Science Letters, 1998, 154 (1-4), pp.167-183. ⟨10.1016/S0012-821X(97)00160-X⟩. ⟨hal-02325892⟩