 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; August 2005; v. 33; no. 8; p. 641–644; doi: 10.1130/G21530.1; 4 figures; Data Repository item 2005122. 641 Termination of a fossil continent-ocean fracture zone imaged with three-dimensional seismic data: The Chain Fracture Zone, eastern equatorial Atlantic Richard J. Davies* Christopher J. MacLeod    3DLab, School of Earth, Ocean and Planetary Sciences, Main Building, Park Place, Cardiff University, Cardiff CF10 3YE, UK Richard Morgan Veritas DGC Ltd., Crompton Way, Crawley, West Sussex RH10 9QN, UK Sepribo E. Briggs 3DLab, School of Earth, Ocean and Planetary Sciences, Main Building, Park Place, Cardiff University, Cardiff CF10 3YE, UK ABSTRACT We describe the first three-dimensional imaging of the termination of a continent-ocean fracture zone (COFZ), the Chain Fracture Zone, located offshore of the Niger Delta. The COFZ marks the abrupt transition between extended continental crust, comprising mul- tiple half-graben, and oceanic crust that has a pervasive seafloor-spreading fabric. It pre- serves a history of continent-continent shearing followed by oceanic crust accretion and continent-ocean shearing during the inception of Atlantic rifting. The termination is marked by steeply dipping faults with sigmoidal planform and thrusts that probably formed as a result of continent-continent or continent-ocean shearing. These are crosscut by the seafloor-spreading fabric that formed during the subsequent phase of oceanic crust accretion. The accreted oceanic crust is cut by listric and planar faults that curve in the direction of the COFZ, where they terminate. The transition from continental to oceanic crust across the COFZ is sharp and resolvable to 100–200 m. Complexes of lava flows emanate from volcanoes along the COFZ, bifurcating and trifurcating down the volcano flanks. The volcanoes are 2–5.5 km wide and 1.4 km in height relative to adjacent oceanic crust and were injected at the COFZ, probably as the spreading center migrated along it. Keywords: seismic reflection, fracture zone, transitional crust, oceanic crust. Figure 1. Free-air gravity data from Sandwell and Smith (1997) over Romanche, Chain, and Charcot Fracture Zones, and Niger Del- ta. Heavy dashed line indicates continent- ocean boundary (COB). Black box shows outline of three-dimensional seismic data set. INTRODUCTION Continent-ocean fracture zones (COFZs) are a fundamental type of tectonic plate boundary that develop during continental rift- ing and the initiation of ocean spreading. Ki- nematic models have proposed that they form by three main processes: continent-continent shearing, followed by the accretion of oceanic crust, and concomitant continent-ocean shear- ing along the COFZ (Le Pichon and Hayes, 1971; Scrutton, 1979; Mascle and Blarez, 1987). However, there has been limited sci- entific drilling and two-dimensional seismic data acquisition at fossil COFZs. Although some outcrop and side-scan sonar data are available over their ocean-ocean fracture zone counterparts, the three-dimensional structure of fracture zones in general is poorly constrained. The subsurface structure of deep-water pas- sive continental margins is increasingly being imaged by large three-dimensional seismic surveys acquired by the petroleum industry. These surveys image rifted continental and oceanic basement rocks and offer the oppor- tunity to understand the three-dimensional structure of oceanic and stretched continental crust as well as the fracture zones that can separate them. This paper describes one such *E-mail: richard.davies@earth.cf.ac.uk. three-dimensional seismic reflection data set from the west African continental margin that covers an area of 1650 km 2 and images a 45- km-long section of the Chain Fracture Zone. These data reveal a COFZ and the transition between stretched continental crust and oce- anic crust. What is exceptional is that there is evidence for multiple developmental episodes; the data set probably images the termination of the COFZ. This paper provides the first three-dimensional imaging of a fossil COFZ and descriptions of the main structural and magmatic features as well as the hitherto un- known structural character of oceanic crust that is accreted during COFZ development. DATA AND GEOLOGIC SETTING A free-air gravity map resolves the Ro- manche, Chain, and Charcot Fracture Zones (Fig. 1). The three-dimensional seismic data are located 180 km off the coast of Nigeria, at the termination of the Chain Fracture Zone. These data have a line spacing of 25 m and a 12 s record length. The present-day water depth in the study area is 2.4–3.3 km, and the basement is buried to 8.5–8.75 km below sea level by the Cenozoic Niger Delta. To the north and northeast of the Chain COFZ, high- ly extended continental crust, commonly termed transitional crust (Rosendahl et al., 1992; Wilson et al., 2003), was mapped using 9500 line km of two-dimensional seismic data (Morgan, 2003; Fig. 1). This region consists of northwest-southeast–trending, oceanward- dipping extensional faults that are offset by northeast-southwest–oriented transfer faults, which formed during rifting of the Atlantic during the Aptian (de Matos, 2000). To the south of this region, across the Chain COFZ, the crust has a different velocity structure and deformational characteristics, interpreted as oceanic crust. SEISMIC INTERPRETATION Differentiating Oceanic and Transitional Crust Representative seismic lines show three re- flections mapped throughout the survey (la- beled 1, 2, and 3; Figs. 2B–2E; uninterpreted seismic lines are in Data Repository Fig. DR1). 1 They divide the crust to the south of the COFZ into two layers. The upper layer (between reflections 1 and 2) is 0.4–2 km thick and consists of moderate to high ampli- tude continuous reflections, with a very high amplitude uppermost reflection (reflection 1). The lower layer (between reflections 2 and 3) 1 GSA Data Repository item 2005122, Figure DR1, uninterpreted seismic lines from Figures 2 and 3, is available online at www.geosociety.org/pubs/ ft2005.htm, or on request from editing@ geosociety.org or Documents Secretary, USA, P.O. Box 9140, Boulder, CO 80301-9140, USA. Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/33/8/641/3529684/i0091-7613-33-8-641.pdf by Equinor user on 16 March 2023