Structural restoration of the ultrathin, Camamu-Almada rifted-passive margin, northeastern Brazil: Relations between crustal stretching, sedimentation, and uplift

Abstract:

The Camamu and Almada non-volcanic, passive margin off the northeastern coast of Brazil covers an area of 22,000 km² and includes: 1) thin (15 km-thick) to ultra-thin (5 km-thick) Precambrian continental trust; 2) a necked zone of undetermined transitional crust (4 km-thick); and 3) Aptian and younger oceanic crust (6 – 8 km-thick). Fission track studies onshore have shown three periods of post-rift (post-Aptian) uplift during the late Cretaceous, Eocene, and Miocene, but little work has been done in the offshore. We investigate the overall tectonic evolution of both the Camamu and Almada basins by structurally restoring two Kirkchhoff PSDM seismic lines recorded to a depth of 40 km.

In both the Camamu and the Almada, a marginal rift developed throughout the Berrasian to Aptian continental breakup between South America and Africa that localized up to 7 km of pre- and syn-rift sedimentation. Crustal extension on both continental and transitional crust occurred through large-offset, basinward-stepping normal faults. Large salt-withdrawal minibasins up to 2.5 km-thick formed above these thick sediments immediately following evaporitic deposition, pushing the salt basinward into salt pillows and diapirs. A large, Early Eocene unconformity eroded down to late-rift Aptian sedimentation and matches the largest uplift event quantified onshore, corroborating the previously-interpreted spreading-rate-controlled uplift mechanism. The basin underwent a complicated rifting history of basinward-stepping large-offset normal faults with previously little-discussed salt tectonism immediately following evaporitic deposition before transferring into a classic passive margin setting post-Eocene. These results are significant in that they quantify the tectonic history of a triple junction with an atypical rift history of both transtension and transpression throughout the rifting process.