Abstract

Investigations of the Clipperton transform fault, a fast-slipping (11 cm/yr) transform plate boundary offsetting the East Pacific Rise, indicate that the most common types of rocks occurring in this setting comprise a distinctive suite of sedimentary breccias and mudstones. These rocks occur within a narrow (<5 km wide) domain of rugged terrain inferred to be the surface expression of strike-slip faulting.

The sedimentary breccias are composed of angular basaltic clasts set in a fine-grained matrix of very angular basaltic mineral and glass fragments and a subordinate amount of their altered equivalents. The mudstones consist of material similar to the breccia matrix but also include a small amount of pelagic carbonate. These sediments represent the deposits of small, active debris-fan systems located at the base of slopes flanking the elongate, transform-parallel basins of the region.

The active mass wasting and breccia formation occur in young crust as it passes through the transform fault domain. The sediments are the products of physical and chemical disintegration and tectonic abrasion of basaltic crust, initially formed at the ridge-transform intersections. Most of the fine-grained fraction consists of rock flour, formed by frictional movement along fault surfaces and shear fractures. Its abundance reflects the high strain rates of this fast-slipping transform fault.

As a result of differential vertical movements of crustal blocks, associated with wrench tectonics, these sediments undergo multiple cycles of erosion and redeposition. As sections of the floor of the transform valley escape into the aseismic limbs of the fracture zone, the breccias and mudstones an preserved in the basal sedimentary stratigraphy of the oceanic crust. These distinctive assemblages may serve as indicators of oceanic crust from transform fault zones expose in some ophiolites.

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