Structure and evolution of an obliquely sheared continental margin:

Rio Muni, West Africa

Jonathan Turner1, Bruce Rosendahl2 & Paul Wilson1

1 University of Birmingham, School of Geography, Earth & Environmental Sciences, Birmingham B15 2TT, UK

2 Rosenstiel School of Marine & Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA

j.p.turner@bham.ac.uk

Shear margins are divergent continental terraines whose breakup and early drift evolution are characterized by significant obliquity in the plate divergence vector relative to the strike of the margin. We focus on the Rio Muni shear margin, equatorial West Africa, where the c.70km-wide Ascension Fracture Zone (AFZ) exhibits oblique-slip faulting and synrift half-graben formation that accommodated oblique extension (10º-15º to the strike of the margin) during the period leading up to and immediately following whole-lithosphere failure and continental breakup (c.117Ma.). Oblique extension is recorded also by strike-slip fault geometries within the AFZ and buckling of Aptian synrift rocks in response to block rotation and local transpression. Rio Muni shares basic characteristics of both rifted and transform margins, the end members of a spectrum of shear margin types. Thus, Rio Muni displays marginal ridges, rapid synrift subsidence in pull-apart basins, a steep continental slope, and a relatively short distance between cratonic rocks and the ocean-continent boundary. It is characterized by modest extensional strain across much of the shelf, increasing rapidly toward the foot of the continental slope. As at transform margins, continental breakup and the onset of oceanic spreading (drifting) are separate episodes recorded by discrete breakup and drift unconformities. On some shear margins they may form a single composite unconformity, depending on i) time elapsing between breakup and drift and ii) rate of sedimentation during the breakup-drift transition. Along highly segmented margins such as the equatorial South Atlantic, where the type of margin (i.e. rifted, transform or shear) may change according to the variation in strike between adjacent segments, the ocean-continent boundary cannot be assumed to be coeval. A shear or transform segment may have undergone breakup whilst its adjacent rifted segment is still stretching and thinning immediately prior to breakup. Oceanic opening will proceed immediately following breakup on a rifted margin, whereas transform and shear margins may experience several tens of millennia between breakup and drift. Non-coeval breakup and drift have important consequences in regard to the 'fit' of the equatorial South American and African margins.