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Dating syntectonic sedimentary sequences is often seen as the unique way to constrain the initiation, duration, and rate of folding as well as the sequence of deformation in the shallow crust. Beyond fold growth, however, deformation mesostructures accommodate the internal strain of pre-folding strata before, during, and after strata tilting. Absolute dating of syn-folding mesostructures may help constrain the duration of fold growth in the absence of preserved growth strata. Absolute dating of mesostructures related to early-folding layer-parallel shortening and late fold tightening provides an access to the timing and duration of the entire folding event. We compile available ages from the literature and provide new U–Pb ages of calcite cements from veins and faults from four folds (Apennines, Pyrenees, Rocky Mountains). Our results not only better constrain the timing of fold growth but also reveal a contraction preceding and following folding, the duration of which might be a function of the tectonic style and regional sequence of deformation. This study paves the way for a better appraisal of folding lifetime and processes and stress evolution in folded domains. [ABSTRACT FROM AUTHOR]

We report the results of a multiproxy study that combines structural analysis of a fracture–stylolite network and isotopic characterization of calcite vein cements and/or fault coating. Together with new paleopiezometric and radiometric constraints on burial evolution and deformation timing, these results provide a first-order picture of the regional fluid systems and pathways that were present during the main stages of contraction in the Tuscan Nappe and Umbria–Marche Apennine Ridge (northern Apennines). We reconstruct four steps of deformation at the scale of the belt: burial-related stylolitization, Apenninic-related layer-parallel shortening with a contraction trending NE–SW, local extension related to folding, and late-stage fold tightening under a contraction still striking NE–SW. We combine the paleopiezometric inversion of the roughness of sedimentary stylolites – that constrains the range of burial depth of strata prior to layer-parallel shortening – with burial models and U–Pb absolute dating of fault coatings in order to determine the timing of development of mesostructures. In the western part of the ridge, layer-parallel shortening started in Langhian time (∼15 Ma), and then folding started at Tortonian time (∼8 Ma); late-stage fold tightening started by the early Pliocene (∼5 Ma) and likely lasted until recent/modern extension occurred (∼3 Ma onward). The textural and geochemical (δ18O , δ13C , Δ47CO2 and 87Sr/86Sr) study of calcite vein cements and fault coatings reveals that most of the fluids involved in the belt during deformation either are local or flowed laterally from the same reservoir. However, the western edge of the ridge recorded pulses of eastward migration of hydrothermal fluids (>140 ∘ C), driven by the tectonic contraction and by the difference in structural style of the subsurface between the eastern Tuscan Nappe and the Umbria–Marche Apennine Ridge. [ABSTRACT FROM AUTHOR]

E-twinning is a common plastic deformation mechanism in calcite deformed at low temperature. Strain rate, temperature and confining pressure have negligible effects on twinning activation that is mainly dependent on differential stress. The critical resolved shear stress (CRSS) required for twinning activation is dependent on grain size and strain hardening. This CRSS value may obey the Hall-Petch relation, but due to sparse experimental data its actual evolution with grain size and strain still remains a matter of debate. In order to provide additional constraints on twinning activation and development, new mechanical tests were carried out at room temperature on unconfined single crystals of calcite, with different sizes and crystallographic orientations. Uniaxial deformation was performed at controlled displacement rate, meanwhile the sample surface was monitored using optical microscopy and high resolution CCD camera. The retrieved macroscopic stress-strain behavior of the crystals was correlated with the surface observations of the deformation process. Results show (1) the onset of crystal plasticity with the activation of the first isolated mechanical twins during the strain hardening stage, and (2) the densification and thickening of twin lamellae during the steady state flow stress stage. Such thickening of twin lamellae at room temperature emphasizes that calcite twin morphology is not only controlled by temperature. The different values for the CRSS obtained for the activation of isolated twins and for the onset of twin densification and thickening questions the appropriate value to be considered when using calcite twin data for stress inversion purposes. [ABSTRACT FROM AUTHOR]

Stylolites are rough dissolution surfaces commonly found in carbonates, characterized by peaks oriented parallel to the maximum principal stress active at the time the dissolution occurred. The development of the Stylolite Roughness Inversion Technique (SRIT) applied to sedimentary bedding-parallel stylolites (BPS) provides an access to the magnitude of the vertical maximum principal stress experienced by the host carbonates, and thus to their maximum paleo-burial depth. A recent study of a BPS population hosted in poorly tectonized Middle Jurassic carbonates of the Paris basin sub-surface (France) unravelled that stylolite morphology controls the maximum vertical stress at which the stylolite was still active. That led to possibly reconstruct the burial history of strata from a BPS population regardless of the fluid pressure and temperature. However, the reliable use of BPS to better constrain the burial depth in tectonized contexts where unroofing by erosion likely occurred such as fold-and-thrust belt was not tested.This study reports a critical comparison of the results of SRIT applied to BPS stylolite populations in various fold-and-thrust belts: a first population hosted in deformed Paleozoic carbonates of the Bighorn basin in the Laramide fold-and-thrust belt (Wyoming, USA); a second population hosted in deformed Jurassic carbonates of the Umbria-Marche fold-and-thrust belt (Apennines, Italy); and a third population hosted in Jurassic carbonates of the Bearnaise Chains (Pyrenees, France). Results highlight how a population of BPS can record intermediate to maximum burial depths, as long as the maximum principal stress remains vertical, i.e., until it switches to horizontal in response to tectonic stress build-up. Especially suture and sharp peak BPS are better suited to estimate the maximum depth at which the principal maximum stress switch from vertical to horizontal, whereas seismogram pinning BPS record preferentially intermediate depths, providing new insights into possibly complex burial history, including tectonic burial by thrust sheets. [ABSTRACT FROM AUTHOR]

Stylolites that develop commonly in carbonates are stress-related pressure solution features of which roughness signal can be used as a paleopiezometer. While sedimentary stylolites records the depth of burial reached by strata as long as the maximum principal stress remains vertical, tectonic stylolites records the absolute horizontal stress magnitudes during the contraction phase they formed. This contribution reports the paleopiezometric inversion of the roughness of bedding-parallel and systematic tectonic stylolites to reconstruct the long-term stress magnitude history that prevailed from strata burial to early Laramide layer-parallel shortening in the Paleozoic carbonates of the Bighorn basin, Wyoming, USA. Our quantitative stress estimates are combined with the results of earlier microstructural, paleostress and thermochronological studies to refine the scenario of stress evolution in both the western part (Rattlesnake Mountain) and the eastern part (Sheep Mountain, Little Sheep Mountain Anticlines, Bighorn Mountains) of the basin. Tectonic stylolite paleopiezometry yields differential stress magnitudes in the range 15-50 MPa for the ~E-W directed Sevier layer-parallel shortening and in the range 3-27 MPa for the NE-directed early Laramide layer-parallel shortening. This results therefore unravels a systematic difference in the level of differential stress sustained by sedimentary cover rocks, with unexpected higher magnitude during thin-skinned Sevier shortening than during thick-skinned Laramide shortening. When integrated with U-Pb absolute ages of calcite veins kinematically related to these two stages of layer-parallel shortening, our results illustrate how distinctively deformation propagates and stress build-up occurs in thin-skinned and thick-skinned tectonics, and how the tectonic style affects the magnitude of the orogenic stress transmitted towards the continental interior. [ABSTRACT FROM AUTHOR]