The Romanche fracture zone influences the segmentation of the equatorial margin of Brazil

Abstract

This study addresses the crustal segmentation of the Brazilian equatorial margin (BEM) controlled by the Romanche fracture zone (RFZ). It uses a combination of satellite and marine gravity anomalies and seismic reflection data to constrain the continent-ocean crust transition. We propose a simplified evolution model of the BEM that involves dextral strike-slip plate motion along the RFZ in an oblique margin transform setting, which caused differential extension of a 31 km thick continental crust and the subsequent generation of a 5.0–10 km thick oceanic crust in the early Cretaceous. As a result, the trend of the coastline is controlled by two main directions: NW-SE and E-W. Two NW-SE-oriented segments, which are related to the oblique spreading centers, formed during the initial generation of oceanic crust adjacent to the margin. The western segment exhibits an ~160-km-wide hyperextended crust orthogonal to the shoreline, with a constant thickness of 10 km. In contrast, the eastern segment exhibits a 90-km-wide continental crust and a 13 km-thick oceanic crust. The central E-W-oriented segment is related to the RFZ and presents a sharp and steep boundary between the continental and oceanic crusts. The breakup geometry and shoreline orientations and the subsequent oceanic spreading centers are oblique (~40°) to the transform. In the Santonian (~83.5 ± 8 Ma), when the transform margin reached its passive stage, isochron 34 became orthogonal to the transform. At present, the RFZ forms well-defined ridges in the equatorial Atlantic that rise to ~2.5 km above the surrounding basins. These ridges and a series of NW-SE-oriented seamounts acted as a topographic barrier and prevented the deposition of a significant volume of sediment sourced from the continent. In addition, neotectonic reactivation of the fracture zone with a major normal component along the continental-oceanic lithospheric boundary (COB) indicates that the RFZ is a long-lived fault.