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

Searle, R. C., Cowie, P. A., Mitchell, Neil Charles, Allerton, S., MacLeod, Christopher John, Escartin, J., Russell, S. M., Slootweg, P. A. and Tanaka, T. 1998. Fault structure and detailed evolution of a slow spreading ridge segment: the Mid-Atlantic Ridge at 29 degrees N. Earth and Planetary Science Letters 154 (1-4) , pp. 167-183. 10.1016/S0012-821X(97)00160-X

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Abstract

We present preliminary results of a detailed near-bottom study of the morphology and tectonics of the 29°N “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 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 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 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−1 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.

Item Type: Article
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: Mid-Atlantic Ridge; Median valley; Aea-floor spreading; Acoustical surveys; Magnetic anomalies; Faults
Publisher: Elsevier
ISSN: 0012-821X
Last Modified: 04 Jun 2017 02:51
URI: http://orca.cf.ac.uk/id/eprint/12747

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