Parnell-Turner, Ross, Escartin, Javier, Olive, Jean-Arthur, Smith, Deborah K. and Petersen, Sven (2018) Genesis of corrugated fault surfaces by strain localization recorded at oceanic detachments. Earth and Planetary Science Letters, 498 . pp. 116-128. DOI 10.1016/j.epsl.2018.06.034.

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Abstract

Seafloor spreading at slow and ultraslow rates is often taken up by extension on large-offset faults
called detachments, which exhume lower crustal and mantle rocks, and in some cases make up domed
oceanic core complexes. The exposed footwall may reveal a characteristic pattern of spreading-parallel
corrugations, the largest of which are clearly visible in multibeam bathymetric data, and whose nature
and origin have been the subject of controversy. In order to tackle this debate, we use available nearbottom
bathymetric surveys recently acquired with autonomous deep-sea vehicles over five corrugated
detachments along the Mid-Atlantic Ridge. With a spatial resolution of 2 m, these data allow us to
compare the geometry of corrugations on oceanic detachments that are characterized by differing fault
zone lithologies, and accommodate varying amounts of slip. The fault surfaces host corrugations with
wavelengths of 10-250 m, while individual corrugations are finite in length, typically 100-500 m. Power
spectra of profiles calculated across the corrugated fault surfaces reveal a common level of roughness, and
indicate that the fault surfaces are not fractal. Since systematic variation in roughness with fault offset
is not evident, we propose that portions of the exposed footwalls analyzed here record constant brittle
strain. We assess three competing hypotheses for corrugation formation and find that the continuous
casting and varying depth to brittle-ductile transition models cannot explain the observed corrugation
geometry nor available geological observations. We suggest a model involving brittle strain localization on
a network of linked fractures within a zone of finite thickness is a better explanation for the observations.
This model explains corrugations on oceanic detachment faults exposed at the seafloor and on normal
faults in the continents, and is consistent with recently imaged corrugations on a subduction zone
megathrust. Hence fracture linkage and coalescence may give rise to corrugated fault zones, regardless of
earlier deformation history and tectonic setting.

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