1. Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA
2. Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, USA
3. Royal Bank of Canada, 71 Queen Victoria Street, London EC4V 4DE, UK
4. Present address: Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, 4412 Spicewood Springs Road, #600, Austin, Texas 78759-8500, USA

Correspondence to: W. Roger Buck¹ Correspondence and requests for materials should be addressed to W.R.B. (Email: buck@ldeo.columbia.edu).

Abstract

Abyssal-hill-bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the Earth. The recognition that these faults form at plate spreading centres came with the plate tectonic revolution. Recent observations reveal a large range of fault sizes and orientations; numerical models of plate separation, dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations. Plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast-spreading centres. Conversely, plate stretching, with differing amounts of constant-rate magmatic dyke intrusion, can explain the great variety of fault offset seen at slow-spreading ridges. Very-large-offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion.

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