Crustal structure of the French Guiana margin, West Equatorial Atlantic

Geophysical 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 break-up 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 rift-related 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

Seismic structure, gravity anomalies, and flexure of the Amazon continental margin, NE Brazil

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-1 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 lithosphère 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. Copyright 2009 by the American Geophysical Union

Do fracture zones define continental margin segmentation? - Evidence from the French Guiana margin

Plate reconstructions suggest that the French Guiana margin in the west equatorial Atlantic is a highly segmented margin with both rift- and transform-style features. We describe here the results of modelling coincident multi-channel and wide-angle seismic, gravity and magnetic data acquired along two transects of this margin. The resulting models not only highlight the degree of structural segmentation but also demonstrate the effect of trans-tension on margin evolution. As a whole, the margin is characterised by 35-37 km thick pre-rift continental crust which is separated from unusually thin oceanic crust (3-4 km) by thinned continental and/or transitional regions. To the north, the margin exhibits a 320 km wide zone of thinned continental crust adjacent to a narrow ocean-continent transition, and is interpreted as a transform margin where the wide zone of thinned crust is a result of profile orientation being highly oblique to the direction of rifting. Approximately 240 km to the south, the margin is characterised by a 70 km wide zone of thinned continental crust which is wider than typical for transform, and narrower than typical for rifted margins. This crustal structure is interpreted to reflect a "leaky" transform formed by trans-tensional extension. These observations suggest that fracture zone influenced geometry of equatorial Atlantic rifting, did not produce a well-defined margin crustal structure, but instead resulted in margin segments which display characteristics of both rift and transform tectonic processes. The associated abundance of fracture zones has likely also affected the post-rift evolution of the margin, and provided topographic basement highs which acted as sediment dams to the northwards flux of sediment from the Amazon. © 2008 Elsevier B.V. All rights reserved

Demerara Plateau - the structure and evolution of a transform passive margin

The Amazon Cone Experiment acquired two transects (Profiles D and 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 35-37km thick continental crust which thins to 10-11km over a distance of 320km. 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 162km 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 rift-related 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. © 2007 The Authors Journal compilation © 2007 RAS

Evidence for unusually thin oceanic crust and strong mantle beneath the Amazon fan

We used seismic and gravity data to determine the structure of the crust and mantle beneath the Amazon Fan. Seismic data suggest that the crust is of oceanic-type and is unusually thin (<4 km) compared to elsewhere in the Atlantic. We attribute the thin crust to ultraslow seafloor spreading following the breakup of South America and Africa during the Early Cretaceous. Gravity data suggest that the fan was emplaced on lithosphere that increased its elastic thickness, Te, and hence strength, following rifting. The increase, from 10 km to 40 km, is greater, however, than would be expected if Te were determined by a single controlling isotherm, based on a cooling plate model. Hence, we conclude that the Amazon Fan has been emplaced on, and is supported by, unusually thin oceanic crust and strong mantle