S U M M A R YGeophysical data from the Amazon Cone Experiment are used to determine the structure and evolution of the French Guiana and Northeast Brazil continental margin, and to better understand the origin and development of along-margin segmentation. A 427-km-long combined multichannel reflection and wide-angle refraction seismic profile acquired across the southern French Guiana margin is interpreted, where plate reconstructions suggest a rift-type setting.The resulting model shows a crustal structure in which 35-37-km-thick pre-rift continental crust is thinned by a factor of 6.4 over a distance of ∼70 km associated with continental breakup and the initiation and establishment of seafloor spreading. The ocean-continent boundary is a transition zone up to 45 km in width, in which the two-layered oceanic-type crustal structure develops. Although relatively thin at 3.5-5.0 km, such thin oceanic crust appears characteristic of the margin as a whole.There is no evidence of rift-related magmatism, either as seaward-dipping sequences in the reflection data or as a high velocity region in the lower crust in the P-wave velocity model, and as a such the margin is identified as non-volcanic in type. However, there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins. Consequently, the thin oceanic crust, the rapidity of continental crustal thinning and the absence of characteristic riftrelated structures leads to the conclusion that the southern French Guiana margin has instead developed in an oblique rift setting, in which transform motion also played a significant role in the evolution of the resulting crustal structure and along-margin segmentation in structural style.

International audienceWe present results from multibeam bathymetric data acquired during 2005 and 2006, in the region of maximum slip of the 26 Dec. 2004 earthquake (Mw 9.2). These data provide high-resolution images of seafloor morphology of the entire NW Sumatra forearc from the Sunda trench to the submarine volcanic arc just north of Sumatra. A slope gradient analysis of the combined dataset accurately highlights those portions of the seafloor shaped by active tectonic, depositional and/or erosional processes. The greatest slope gradients are located in the frontal 30 km of the forearc, at the toe of the accretionary wedge. This suggests that long-term deformation rates are highest here and that probably only minor amounts of slip are accommodated by other thrust faults further landward. Obvious N–S oriented lineaments observed on the incoming oceanic plate are aligned sub-parallel to the fracture zones associated with the Wharton fossil spreading center. Active strike-slip motion is suggested by recent deformation with up to 20–30 m of vertical offset. The intersection of these N–S elongated bathymetric scarps with the accretionary wedge partly controls the geometry of thrust anticlines and the location of erosional features (e.g. slide scars, canyons) at the wedge toe. Our interpretation suggests that these N–S lineaments have a significant impact on the oceanic plate, the toe of the wedge and further landward in the wedge. Finally, the bathymetric data indicate that folding at the front of the accretionary wedge occurs primarily along landward-vergent (seaward-dipping) thrusts, an unusual style in accretionary wedges worldwide. The N–S elongated lineaments locally act as boundaries between zones with predominant seaward versus landward vergence

S U M M A R YThe Amazon Cone Experiment acquired two transects (Profiles D & A) across the Demerara Plateau, part of the French Guiana-Northeast Brazil continental margin, to better understand rift and transform-style margin evolution. Profile A images an intermediate-type margin formed as a result of trans-tensional extension. In this paper we describe the modelling of wide-angle and multichannel seismic and gravity data from Profile D, to reveal whole crustal structure and features exhibiting transform characteristics. Combining these results with other studies in the region and comparing our results with 'young' rift analogues, we develop a model of along-margin segmentation that explains the evolution of the west equatorial Atlantic.Interpretation of the velocity-depth model for Profile D shows a 35-37 km thick continental crust which thins to 10-11 km over a distance of 320 km. This thinning is accommodated in two regions. The narrowest region, associated with the ocean-continent transition, is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break-up geometry of the Central Atlantic. Given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning, located 162 km landward of the ocean-continent transition, is a result of rifting suborthogonal to the profile orientation. There is no evidence of rotated faulted blocks, half graben structures or riftrelated magmatism, manifest as either seaward-dipping reflectors or as a high-velocity region within the lower crust. The Demerara Plateau is, therefore, interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform-type zones in which trans-tensional extension is accommodated. In contrast to Profile A, modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift-type setting.Offshore, unusually thin oceanic crust of 3.3-5.7 km thickness is identified which is consistent with similar observations further south. In the absence of identifiable magnetic anomalies, best estimates of the initial half-spreading rate of ∼20 mm yr −1 suggest that the thin crust throughout the region is unlikely to be a result of ultra-slow spreading but, instead, it is most likely due to a reduced magma supply at numerous, long-lived transform faults reflected by those presently offsetting the Mid-Atlantic Ridge in this equatorial setting.

[1] Seismic and gravity data have been used to determine the structure of the sediments, crust, and upper mantle that underlie the Amazon continental margin, offshore NE Brazil. Seismic reflection profile data reveal a major unconformity at $7 s two-way travel time (TWTT) which we interpret as marking the onset of the transcontinental Amazon River and the formation of the Amazon deep-sea fan system in the late Miocene. Seismic refraction data show mean sediment velocities that decrease by >1.5 km s À1 in a seaward direction. We attribute this decrease to facies changes associated with sediment progradation and the development of topset, foreset, and bottomset beds. Seismic refraction data show that the sediments are underlain by oceanic crust that has a similar velocity structure compared to elsewhere in the Atlantic Ocean but is unusually thin ($4.2 km). We attribute the thin crust to either slow seafloor spreading or a limited magma supply during the initial rifting of South America and Africa in the Early Cretaceous. The seismic data have been used to construct a new sediment thickness grid that together with gravity anomaly data, suggests the Amazon fan loaded lithosphere with an unusually high flexural strength. While a high-strength lithosphere explains the overall depth of the seismic Moho, there are discrepancies (of up to 2 km) beneath the upper fan, where the modeled flexed Moho is shallower than the seismic Moho, and beneath the middle fan, where it is deeper. Gravity and seismic modeling suggest these discrepancies are caused by lateral changes in subcrustal density such that the mantle underlying the upper fan is denser than it is beneath the middle fan. We attribute these lateral density differences to proximity to the Ceara Rise, which is believed to have formed during the Late Cretaceous in a mid-ocean ridge setting. Fan loading of a relatively strong, dense, and, hence, cold lithosphere predicts stress orientations that are consistent with borehole breakout data and the location and height of the Gurupé Arch onshore. Despite its proximity to ''leaky'' transform faults, the margin that underlies the Amazon fan appears to be of nonvolcanic origin. The main differences with other nonvolcanic margins, such as West Iberia and Newfoundland, are a greater sediment accumulation, a narrower zone of transitional crust, and a lack of any evidence for extreme extension and mantle serpentinization.

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