Burial-deformation history of an arcuate fold unraveled by fracture analysis, stylolite paleopiezometry and vein cement geochemistry: a case study in the Cingoli Anticline (Umbria-Marches, northern Apennines)

<p>The timing and duration of fold-related deformation - including layer-parallel shortening (LPS) – related to fold growth, are difficult to estimate because of a lack of data in most natural cases. We propose an original combination of direct and indirect dating methods to reconstruct the burial-deformation history of the Mesozoic carbonates that crop out in the Cingoli Anticline in the Umbria-Marche Apennine Ridge.</p><p>The Cingoli anticline displays an arcuate shape in map view, trending N140 in its northern part and N160 in its southern part). We first study the fracture-stylolite network to characterize the successive stages of deformation. Several sets of mesostructures were discriminated according to their orientation and relative chronology:</p><ul><li>(i) N-S trending vertical joints (after unfolding), likely related to foreland flexure/forebulge development;</li> <li>(ii) N045 trending vertical, bedding and fold-axis perpendicular joints/veins, associated with early folding stylolites with N045-oriented peaks and reverse faulting associated with a N045 σ<sub>1</sub> (after unfolding), reflecting LPS;</li> <li>(iii) bedding-perpendicular and fold axis -parallel joints/veins, e., trending N140 in the northern part and N160 in the southern part of the anticline, reflecting outer-arc extension associated to strata curvature at fold hinge, the variation about 20° in orientation between the northern and southern parts of the fold being consistent with the arcuate shape of the anticline;</li> <li>(iv) tectonic stylolites with horizontal peaks striking N045, along with conjugate vertical strike-slip faults, associated with a horizontal N045 contraction affecting the strata after the fold was locked, corresponding to the late stage of fold tightening (LSFT).</li> </ul><p>These results suggest that the Cingoli anticline developed under a continuous N045 contraction and that its arcuate shape is likely primary and was achieved in relation to the reactivation of an N-S normal fault inherited from the Tethyan rifting, without any vertical-axis rotation of the fold axis.</p><p>We further reconstructed burial curves considering sedimentary formation thicknesses, corrected from physical and chemical compaction. We also quantified the vertical stress experienced by sedimentary stylolites from a roughness inversion technique, allowing derivation of the maximum depth experienced by the strata prior to contraction (using bedding-parallel sedimentary stylolites) and during exhumation (using horizontal sedimentary stylolites related to a post-folding compaction). Oxygen and carbon isotope ratios measured in tectonic vein cements point towards a locally-sourced fluid system with limited vertical migration at the scale of the carbonate core, enabling the use of the absolute temperatures obtained from CO<sub>2</sub> clumped isotope (D<sub>47</sub>) to reconstruct the depth during layer-parallel shortening and folding. The comparison of reconstructed depth at which each deformation phase occurred with the burial curve provides absolute timing for the development and exhumation of the Cingoli Anticline. Together with U-Pb ages of calcite vein cements and fault coatings from the nearby San Vicino Anticline, located west of the Cingoli Anticline, our data suggest that contraction started by ~8 Ma (LPS) and ended by ~3 Ma (LSFT), and that the growth of the Cingoli anticline lasted from ~5.5 to 4 Ma.  </p>