Seismic structure of the continental margin of French Guiana : implications for the rifting and early spreading of the equatorial Atlantic
Recent studies of passive continental margins suggest that the rifting process produces a variety of structural styles. The along-strike continuity of these rifted margins is punctuated by significant lateral offsets, known as transform margins. Whilst many general features of both rift- and transform-style margins have been identified, the extent to which they are inter-related is not well understood. The equatorial Atlantic exhibits a high number of large offset mid-ocean ridge transform faults and associated fracture zones, which indicate the highly segmented nature of its margins. As such, this location provides an ideal setting for a study of the deep structure of both rift- and transform-style margin structures. This investigation forms part of the Amazon Cone Experiment, a large-scale geophysical study of the French Guiana and northeast Brazil margin in the west equatorial Atlantic. This study will provide evidence of crustal structure from recently acquired seismic, gravity and magnetic data, along two transects of the margin. The acquisition comprised 962 km of coincident multi-channel reflection and wide-angle refraction seismic data which were recorded by 20 ocean-bottom seismographs per transect, deployed at 10 km spacing. Ray-trace forward modelling of traveltime data from these instruments has resulted in two P-wave velocity-depth models of the subsurface structure which have been tested and further constrained by independent gravity free-air anomaly data. Interpretation of the resulting models suggests that the pre rift continental crust is 35- 37 km thick. While at the oceanward end or each transect oceanic crust is identified which, at 3.5-5.0 km thickness, is considered to be unusually thin. The manner in which this transition is accomplished is dramatically different between the two models. In the south of the survey area the crust thins abruptly by a factor of 6.4 over a distance of ~70 km, adjacent to a ~ 45 km ocean-continent transition zone. To the north, however, more gradual thinning over ~ 320 km associated with an abrupt transition to oceanic crust is observed. Neither profile shows evidence of the tilted fault blocks characteristic of rifted margins. There is no evidence for rift-related magmatism, commonly manifest as high P- wave velocity underplating or packages of seaward-dipping reflectors, along either profile. Hence, the margin is interpreted as non-volcanic, which suggests that rifting was not very rapid. On the basis of these results, a model of transtensional rifting is proposed, in which a component of motion oblique to the margin results in the production of relatively wide, leaky' transform margins. This model suggests that the French Guiana margin is segmented into rift- and transform-style structures. However, the transform margins exhibit unusually wide zones of continental crustal thinning as a result of the transtensional extension. For the Amazon Cone Experiment as a whole, anomalously thin oceanic crust is observed over a wide areal extent. This crust indicates that magma flow from the mantle is low and is interpreted to be a result of relatively cool asthenospheric mantle, slow spreading and the effect of large-scale fracture zones. The results of this study have implications for our understanding of the effects of transtensional stresses during rifting and mode of opening of the equatorial Atlantic.