Abstract
Abstract. 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.
Highlights
The upper crust is the locus of omnipresent fluid migrations that occur at all scales, leading to strain localization, earthquake triggering, and georesource generation, distribution and storage (e.g. Cartwright, 2007; Andresen, 2012; Bjørlykke, 1994, 1993; Lacombe and Rolland, 2016; Lacombe et al, 2014; Roure et al, 2005; Agosta et al, 2016)
Skinned Umbria–Marche Apennine Ridge reveals the occurrence of several fracture/stylolite sets that support a threestagestructural evolution of the Apenninic contraction: (1) layer-parallel shortening is reconstructed by a set of joint/veins striking NE–SW to E–W, perpendicular to the local trend of the fold, alongside with stylolite peaks striking NE–SW and early folding bedding-parallel reverse faults; (2) the folding stage is recorded by fold-parallel mode I joints and veins; (3) the late-stage fold tightening is recorded by post-tilting, late folding stylolite peaks and joints and veins, and mesoscale reverse and strike-slip faults
Thanks to burial models coupled to bedding-parallel stylolite paleopiezometry, along with U– Pb absolute dating of strike-slip faults related to late-stage fold tightening, we were able to reconstruct the timing of the onset and the duration of the Apennine contraction
Summary
The upper crust is the locus of omnipresent fluid migrations that occur at all scales, leading to strain localization, earthquake triggering, and georesource generation, distribution and storage (e.g. Cartwright, 2007; Andresen, 2012; Bjørlykke, 1994, 1993; Lacombe and Rolland, 2016; Lacombe et al, 2014; Roure et al, 2005; Agosta et al, 2016). It is a fundamental topic to depict the history of fluid migration in deformed carbonates. Such knowledge no only impacts both the prediction and monitoring of energy prospects and potential storage areas and may help refine our understanding of what mechanisms facilitate fluid migrations during diagenesis of the sedimentary rocks, along with temporal and spatial scales of fluid flow. Fluid migration and accumulation events are strongly controlled by tectonics, especially by the related development of large-scale faults and fracture networks. In fold-andthrust belts and orogenic forelands, the complex deformation history can be unravelled by studying the development of mesoscale structures such as faults, veins, and stylolites (Tavani et al, 2015). Once contraction is accommodated by strata tilting syn-folding, strata curvature-related structures develop under local extension. Mesoscale structures developing during the so-called postfolding events are kinematically unrelated with folding
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