<p>Folded sedimentary layers usually exhibit brittle mesostructures such as faults, joints, veins, and stylolites which accommodate the internal strain of strata during folding but also before strata started to be tilted, i.e. during Layer Parallel Shortening (LPS), and after tilting, i.e. during late stage fold tightening (LSFT) when shortening can no longer be accommodated by fold growth. We have established a fracture sequence in the Mirabeau anticline (SE France) using orientation data and relative chronology with respect to bedding attitude, which comprises the early-, syn- and late-folding fractures related to the folding event (1). This field-based approach is compared to 12 ages out of 32 samples analyzed by LA-ICP-MS U-Pb geochronology of selected syn-kinematic calcite, in order to provide an absolute time frame for the fracture development, along with a potential validation of the sequence of deformation. Along a section across the fold strike axis, the deformation related to the contraction lasted at least 12 Ma (or 26 Ma with uncertainties), bracketed between 52 Ma (&#177; 8 Ma) and 40 Ma (&#177; 6 Ma), indicating that the fold developed in response to the so-called Pyrenean-Proven&#231;al phase. In depth however, the sequence of deformation as inferred from the structural study of the fracture network is challenged by the absolute age of the syn-kinematic calcite: syn-folding flexural-slip surfaces are dated from 52 Ma (&#177; 8 Ma), i.e. older than early-folding reverse faults associated to LPS (46 &#177; 4 Ma). &#160;</p> <p>&#160;</p> <p>In order to better constrain the onset of Layer-Parallel Shortening (LPS), we further analyzed the bedding parallel sedimentary stylolites through the inversion of their roughness to better define the burial depth range during which the Jurassic and Upper Cretaceous formations of interest were submitted to a vertical stress &#963;1. Once projected on the burial-time model of the investigated strata, the depth range, going up to 3 km, constrains the time at which &#963;1 presumably switched from vertical to horizontal in response to the onset of tectonic loading. Altogether, this work questions the limits of absolute dating in understanding fold-fracture relationships, and forewarn of potentially misleading interpretation of absolute ages when associated with deformation features. The example of the Mirabeau Anticline enables a discussion about the compatibility and complementarity of different approaches to date mesoscale and macroscale deformation features in fold-and-thrust belts where the sedimentary record is not well preserved.</p> <p>&#160;</p> <p>Keywords : Absolute dating, fracturation, fold, carbonates</p> <p>&#160;</p> <p>(1)<em> Lacombe O.</em><em>, Beaudoin N., Hoareau G.,&#160;Labeur A., Pecheyran C. & Callot J.P., 2021. Dating folding beyond folding, from layer-parallel shortening to fold tightening, using mesostructures : Lessons from the Apennines, Pyrenees and Rocky Mountains.&#160; Solid Earth, 12, 10, 2145-2157</em></p>
<p>Our understanding of the temporal variation of past stress in the crust is usually pictured in relation to tectonic contexts, where it helps decipher the evolution of deformation of rocks at different scales. The paucity of paleostress reconstructions in passive margins makes the knowledge of the origin of stress and of its evolution very incomplete, especially in poorly accessible offshore parts. Moreover, in salt-rich passive margins like the offshore Congo margin, one may question whether the state of stress in supra-salt formations is mainly controlled by salt tectonics, since the salt usually acts as a decoupling level that prevents the transmission and record of far-field crustal stresses. This study focuses on the analysis of an offshore wellbore core of the Albian, post-rift carbonates of the Sendji Fm that directly overlies the salt of the Aptian Loeme Fm in the Lower Congo Basin. Paleopiezometry based on stylolite roughness and mechanical twins in calcite was combined with fracture analysis, laser U-Pb dating of calcite cement, and burial modeling to unravel the tectonic and burial evolution of the Sendji Fm over time. The results of bedding-parallel stylolite roughness inversion constrain the range of depth over which the Sendji Fm strata deformed under a vertical principal stress s1 to 650-2800 m (median ~1100m). Projection of this depth range onto the Sendji burial model derived from TemisFlow&#8482; basin modelling indicates that pressure solution was active from 105 to 12 Ma. Inversion of calcite mechanical twins measured within the early diagenetic cement (U-Pb age = 100 +/- 1Ma) yields two main states of stress: (1) an extensional stress regime with a horizontal &#963;3 trending ~E-W associated with sub-perpendicular N-S compression, and (2) a strike-slip stress regime with a horizontal &#963;1 trending ~E-W (changing from pure E-W compression to N-S extension through stress permutations). We interpret the former state of stress as local and related to the complex geometric interactions between moving halokinetic normal faults, while the latter presumably reflects the push effect of the Atlantic ridge, which prevailed from 12 Ma until present-day. Our results highlight that the stress history of the studied part of the offshore Lower Congo Basin passive margin has first been mainly dominated by burial and local normal faulting related to late Cretaceous to Miocene post-rift salt tectonics, then by a regional stress presumably originated from the far-field ridge push from ~12Ma onwards, which would indicate some mechanical re-coupling between the crust and the sedimentary cover during the Miocene.</p><p>Keywords: stress, paleopiezometry, calcite twins, stylolites, passive margin, salt.</p>
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