Regional-scale paleofluid system across the Tuscan Nappe – Umbria Marche Arcuate Ridge (northern Apennines) as revealed by mesostructural and isotopic analyses of stylolite-vein networks

Abstract

Abstract. We report the results of a multi-proxy study that combines structural analysis of fracture-stylolite network and isotopic characterization of calcite vein cements/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 pathways network during the main stages of contraction in the Tuscan Nappe and Umbria Marche arcuate ridge (Northern Apennines).We reconstruct four continuous steps of deformation at the scale of the belt: burial that developed sedimentary stylolites, Apenninic-related layer parallel shortening with a contraction striking NE-SW, local extension related to folding, then a late stage of fold tightening under a contraction still striking NE-SW. In order to assess the timing and burial depth of strata at all stages, we combine a paleopiezometric tool based on 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 the western part of the ridge, layer-parallel shortening started in Serravalian time (~ 12 Ma), then folding started at Tortonian time (~ 8 Ma), late stage fold tightening started in early Zanclean (~ 5 Ma) and likely lasted until recent/modern extension occurred (~ 3 Ma onward). This timing provides important constraints on the temperature that expectedly prevailed in the studied strata through its history. 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 are local, or flowed laterally from the same reservoir. However, the western edge of the ridge recorded pulses of eastward squeegee-type migration of hydrothermal fluids (> 140 °C), that can be related to the difference in structural style of the subsurface between the eastern Tuscan Nappe and the Umbria Marche Ridge.