A new model for the evolution of oceanic transform faults based on 3D Broadband Seismic observations from São Tomé and Príncipe in the eastern Gulf of Guinea.

Abstract

<p>Oceanic Transform faults are one of manifestations of the three major plate boundaries and a key tectonic feature of oceanic crust. They are broadly considered to accommodate strike-slip displacement along simple vertical faults and to be largely without magmatic addition. We present the first observations from broadband 3D seismic of buried, Cretaceous-aged transform faults in the Gulf of Guinea with complex internal architectures including crustal scale detachments and rotated packages of volcanics within oceanic crust. In the study area, several Oceanic Fracture Zones (OFZ) are described from Top Crust to Moho. OFZ scarps are observed to connect at depth with zones of low angle reflectivity which dip into the OFZ and perpendicular to the spreading orientation. At depth they detach onto the Moho, necking the adjacent crust in the manner of extensional shear zones. Thickly stacked and tilted reflectors, interpreted as extrusive lava flows, are common above the shear zones and infill up to 75% of the crustal thickness within the OFZ. The entire OFZ stratigraphy is overlain and sealed by late-stage lavas that are continuous from the abyssal hills of the trailing spreading ridge. These insights demonstrate complexity previously only predicted in numerical simulations. We propose a model with inside corner extension at a high angle to the spreading orientation along a low angle shear zone that acts as a conduit for decompression related melt and volcanism. Late-stage lavas indicate a second stage of magmatic accretion as dykes propagate through the transform to adjacent crust, as inferred from bathymetric studies. These observations come from a unique 3D seismic dataset and are placed within a kinematic model which combines insights from numerical models. We conclude that these oceanic transforms were non-conservative and not simple strike slip fault zones, contradicting the conventional view.</p>