Abstract
AbstractThe rift phase of the Campos Basin developed during the Early Cretaceous on a heterogeneous crust comprising structures inherited from the Brasiliano‐Pan African tectonic events, mostly generated during the Neoproterozoic‐Cambrian amalgamation of western Gondwana blocks. The main rifting episode took place from the Hauterivian to the Barremian, then was succeeded by the transition and post‐rift (SAG) phases during the Aptian. Rift faults developed as a result of a progressive rotation of extension from E‐W to NW‐SE. The role of pre‐existing intra‐basement structures on the style and evolution of the rift faults was investigated using 3D high‐resolution seismic data, borehole logs and sidewall samples. Three seismic facies (SF1, SF2 and SF3) and three types of intra‐basement structures (Surfaces, Geobodies and Internal Reflections) were identified and mapped. They represent, respectively, contrasting levels of seismic anisotropy, interpreted as metamorphic foliation, and ductile shear zones that bound rock units with particular seismic facies signatures. Sidewall cores show that banded biotite‐gneiss is the predominant rock type in the eastern half of the study area, while more homogeneous granitoid is the dominant lithology on the west. Such a binary division of lithotypes is consistent with the distribution of mapped intra‐basement seismic facies and features. The contrasting basement heterogeneity across the study area is the major control in the strain distribution during rifting. Where the basement is highly heterogeneous, the pre‐existing fabric was selectively reactivated whenever its orientation was favourable, resulting in faults forming progressively as the extension direction rotates, whilst shallower low to very low angle basement fabric were cross‐cut by rift faults. Where the basement is homogeneous, only early formed faults remain active throughout the rifting.
Highlights
The influence of basement fabric on the structural styles of rift basins based on surface geology and remote sensing has been widely reported in the literature (e.g. Daly et al, 1989; Milani & Davison, 1988)
We explored the relationships between basement heterogeneity and the nucleation, growth, and structural style of the rift faults
We suggest that rift faults in this position have shorter lengths and smaller displacements because of a combination of three factors: (a) the Main Fault Zone (MFZ) focused the strain in this region, in accordance with Walsh et al (1991); (b) basement heterogeneities are less continuous and do not exhibit a persistent orientation when compared to the basement fabric near the MFZ; (c) heterogeneous basement (SF2/Seismic Facies 3 (SF3)) is progressively shallower eastwards and its internal anisotropies (Basal Geobody) are usually oriented at a high angle to rift faults
Summary
The influence of basement fabric on the structural styles of rift basins based on surface geology and remote sensing has been widely reported in the literature (e.g. Daly et al, 1989; Milani & Davison, 1988). The Oriental Terrane (eastern Ribeira Belt) and the Cabo Frio Tectonic Domain (CFTD) can be extrapolated offshore to the study area, following the trends of their tectonic boundaries (Figure 1a; Egydio-Silva et al, 2018; Schmitt et al, 2016). The former is constituted by low P-high T Neoproterozoic para-and ortho-d erived gneisses intruded by several magmatic bodies, with NE-S W fabrics that include foliation, folds and other ductile structures generated during a protracted evolution from ca. Expansion index, throw-depth and throw-distance plots (e.g. Cartwright et al, 1998; Reeve et al, 2015) could not be applied since the mapped horizons consist of unconformities
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