E.A. Pickett2,3 and S. Allerton2


Ocean Drilling Program Leg 159 to the Côte d’Ivoire-Ghana Transform Margin provided an invaluable opportunity to document the structures of a continental transform margin from the Albian to the present. Observations of core and thin sections, together with analyses of structural and bedding dip data, have led to a better understanding of the styles of deformation involved. The cores from all sites (959-962) display a wide range of deformation styles with the most intense concentration of faults, veins, and microfolds occurring toward the base of each site; possibly related to deformation during the syntransform stage. At Sites 959 and 960, the downhole increase in deformation coincides with the unconformity between Lower Cretaceous clastic sediments and overlying undeformed late Albian–Turonian carbonates. Normal faults are the most common fault type, whereas evidence for strike-slip is surprisingly rare. Veins are common throughout the successions and commonly display several generations of vein fill. At Site 959 a second deformation event has been recorded, affecting sediments below 750 meters below seafloor (early Eocene and older). Changes in physical properties of the sediments, vein compositions, and a decrease in bedding dip above this level indicate a decrease in tectonic activity at this time. A possible mechanism, which may have caused this change in tectonic regime, may be related to a switch from uncoupled to coupled linkage between the continental and oceanic crust.

1Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998. Proc. ODP, Sci. Results, 159: College Station, TX (Ocean Drilling Program).
2Department of Geology and Geophysics, Grant Institute, West Mains Road, Edinburgh EH9 3JW, United Kingdom.
3Present address: British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, United Kingdom. eapi@wpo.nerc.ac.uk