--> The Equatorial Atlantic: South American Evidence for Early Opening Transpression

AAPG/SEG International Conference & Exhibition

Datapages, Inc.Print this page

The Equatorial Atlantic: South American Evidence for Early Opening Transpression

Abstract

Abstract

Early Cretaceous opening of Equatorial Atlantic Ocean was initiated as a series of isolated continental pull-apart basins between 5 major transform zones. The dextral opening, accompanied by a transpressional component, resulted in alternating sheared and divergent margin segments. Using long-record, regional 2D seismic data (27,000 km), we investigate the effects of the syn-rift to early drift transpression on the South American equatorial margin. The areas of focus for this study are: 1) the Demerara Rise, 2) the Foz do Amazonas Basin and 3) the Cearà Basin of Northern Brazil. Seismic data at the sheared margin segments reveal evidence of varying magnitudes of deformation and structures associated with shortening related to a change in plate vector. At the northeastern boundary of the Demerara Rise, folds and high angle thrusts (WNW-ESE) are observed within the pre-Albian sequences. This shortening resulted in the peneplanation of the Demerara Rise section, the Albian erosional unconformity, and the removal of approximately ∼2 km of pre-Albian sediment. On the Rise a second unconformity (Cenomanian?) is observed within the data and is likely related to the eastward passing of the mid-ocean ridge to the north. The second area of transpression lies beneath the thick post-rift sediments of the Amazon Cone where large folds are revealed in seismic. These folds are present at the southern boundary of the transform margin that extends to the offshore St Paul Fracture Zone. In the Cearà Basin a series of nappes along the southern flank of the Romanche transform is observed. These well-preserved, highly-deformed units reach a maximum thickness of 6 km and show a lateral deformation that extends 7 km to the south. Integration of near-shore well and seismic data indicates that the age of the deformation is pre-Albian. The preservation of folded pre- and syn-rift section at Cearà Basin is in sharp contrast to the extensive erosion that occurred on the Demerara Rise. As evidenced from this study, the structural style of deformation is vastly different across the region. In the north, massive folding and sub-aerial erosion are observed at the Rise, while the imbricate thrusts in the Cearà Basin occurred in an early-drift sub-marine setting. These observations have led to a better understanding of the pre-rift fit and transpression in the initial stages of opening between northern South America and equatorial West Africa.