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Accurate characterization of hydrocarbon reservoirs requires a good understanding of the paleostress history of their host sedimentary basin. This study aims at reconstructing the paleoburial and paleostress history of the syn-rift Barremian TOCA Fm. in the Lower Congo basin (West African margin). Two oriented borehole cores drilled offshore Congo were investigated using stress inversion techniques applied to calcite mechanical twins to constrain paleostress orientations and magnitudes. The inversion of calcite twins was performed on a widespread early diagenetic cement (~129 to ~113 Ma) and revealed that the TOCA Fm. has undergone a complex polyphase stress history, including (1) extensional stress regimes with vertical σ 1 and horizontal σ 3 trending NE-SW and WNW-ESE (N100) associated with the rifting episode preceding the opening of the South Atlantic (130 to 112 Ma), and with mean differential stresses of ~47 MPa for (σ 1 -σ 3) and ~ 20 MPa for (σ 2 -σ 3). The early N100 extension is associated with the development of normal faults striking N-S that likely reactivated inherited basement structures. The direction of extension evolved during the Barremian into the main NE-SW extension marked by the dominant normal faults striking NW-SE. (2) Compressional and strike-slip stress regimes with horizontal σ 1 oriented ~N-S possibly related to the far-field intraplate transmission of orogenic stresses generated at the distant Africa-Eurasia plate boundary at ~67–60 Ma, and with mean differential stresses of 40 MPa for (σ 1 -σ 3) and 28 MPa for (σ 2 -σ 3). (3) Compressional and strike-slip stress regimes with horizontal σ 1 oriented ENE-WSW to ~E-W that we tentatively relate to the mid-Atlantic ridge push and prevailing since ~15 Ma onwards, associated with mean differential stresses of ~45 MPa for (σ 1 -σ 3) and ~ 18 MPa for (σ 2 -σ 3). Our results reveal that the TOCA Fm. recorded mainly far-field stress effects that could be related to interactions between the African plate and surrounding plates, and that divergent passive margins may experience a complex tectonic history including both extensional and compressional events. • Calcite twin inversion is used to reconstruct paleostress history on the West Africa passive margin. • The orientations and magnitudes of paleostresses affecting the syn -rift TOCA Fm. are characterized from two offshore cores in the Lower Congo basin. • The timing of the successive stress regimes in the TOCA Fm. is constrained. • The influence of far-field tectonic and gravitational lithospheric stresses transferred from distant plate boundaries on the post-rift history on the West Africa passive margin is demonstrated. [ABSTRACT FROM AUTHOR]

Mechanical twins are common microstructures in deformed calcite. Calcite twins have been used for a long time as indicators of stress/strain orientations and magnitudes. Developments during the last decade point toward significant improvements of existing techniques as well as new applications of calcite twin analysis in tectonic studies. This review summarises the recent progress in the understanding of twin formation, including nucleation and growth of twins, and discusses the concept of CRSS and its dependence on several factors such as strain, temperature and grain size. Classical and recent calcite twin measurement techniques are also presented and their pros and cons are discussed. The newly proposed inversion techniques allowing for the use of calcite twins as indicators of orientations and/or magnitudes of stress and strain are summarized. Benefits for tectonic studies are illustrated through the presentation of several applications, from the scale of the individual tectonic structure to the continental scale. The classical use of calcite twin morphology (e.g., thickness) as a straightforward geothermometer is critically discussed in the light of recent observations that thick twins do not always reflect deformation temperature above 170–200 °C. This review also presents how the age of twinning events in natural rocks can be constrained while individual twins cannot be dated yet. Finally, the review addresses the recent technical and conceptual progress in calcite twinning paleopiezometry, together with the promising combination of this paleopiezometer with mechanical analysis of fractures or stylolite roughness. [ABSTRACT FROM AUTHOR]

The paleostress history and associated deformation mechanisms affecting passive margins are seldom studied, as access to offshore parts is rather limited. We analyze an offshore wellbore core of the Albian, post-rift carbonates of the Sendji Fm which directly overlies the salt of the Aptian Loeme Fm in the Lower Congo Basin. Paleopiezometry based on stylolite roughness inversion (SRIT) and calcite twin inversion (CSIT) was combined with fracture analysis, U–Pb geochronology of calcite cement and burial modeling to unravel the orientations and magnitudes of horizontal and vertical stresses affecting the Sendji Fm over time, with a focus on the impact of salt tectonics on stress records. The results of SRIT on bedding-parallel stylolites constrain the range of depths over which the Sendji Fm strata deformed under a vertical principal stress σ1 to 650–2800 m (median ∼1300 m). Once projected onto the burial model derived from TemisFlow™ software, the range of depths converts to a period of pressure solution activity, from 101 to 15 Ma. Calcite twin measurements within the early diagenetic cement (U–Pb age = 101 ± 1 Ma) reveal three main types of stress regimes: (1) extensional stress regimes with σ3 trending ∼ N–S and ∼E-W associated with local, thin-skinned salt tectonics (101–80 Ma), (2) strike-slip stress regimes with σ1 trending ∼ NW-SE to NNE-SSW and a compressional stress regime with σ1 trending NNW-SSE, reflecting possible intraplate stress transfer from the distant Africa-Eurasia plate boundary (67–60 Ma); (3) a strike-slip and a compressional stress regime with σ1 trending E-W likely related to the mid-Atlantic ridge push possibly combined with effects of variations in elevation and density of the lithosphere and sedimentary flexural loading (15 Ma to present). The paleostress sequence suggests that the sedimentary cover was decoupled from the crust during salt tectonics and then recoupled from the late Cretaceous onwards. • The paleostresses affecting deep carbonates are characterized by studying an offshore core in the west African passive margin. • The timing of the successive stress regimes in the Sendji Fm is constrained. • Local gravitational stress from salt tectonics and far-field tectonic stress from plate boundaries influenced the margin. • The potential of a multi-proxy approach is established. [ABSTRACT FROM AUTHOR]