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

作者：Greenroyd C J^*; Peirce C; Rodger M; Watts A B; Hobbs R W

来源：Geophysical Journal International, 2008, 172(2): 549-U19.

DOI：10.1111/j.1365-246X.2007.03662.x

• composition of the oceanic crust
• controlled source seismology
• continental margins : transform
• crustal structure

The 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 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 similar to 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.

• 出版日期2008-2