Fluid expulsion in accretionary wedges: what can we learn from Miocene seep carbonates (Emilia and Tuscan Apennines, Italy)

Seepage of hydrocarbon-rich fluids is a common process in accre onary wedges where tectonic thickening and underpla ng generate pore-fluid overpressures and induce fluid migra on (Bohrmann et al., 2002; Gill et al., 2005; Ding et al., 2010; Crutchley et al., 2015; Klaucke et al., 2015). The migra on and expulsion of methane-rich fluids promote the precipita on of a variety of authigenic minerals on the seafloor or within the sediments, i.e. carbonates (aragonite, calcite, dolomite), sulfides and sulphates, forming crusts, concre ons and build-ups. Reduced compounds in the fluids also sustain peculiar microbial consor a and macrofaunal communi es (Teichert et al., 2005; Campbell, 2006) (Fig. 1). It is widely recognized that the most reliable indicators of fossil cold seeps are the anomalously nega ve carbon 13 isotope composi on of the carbonates, with d C values as low as -60‰ VPDB (Judd and Hovland, 2009), the peculiar chemosynthe c bivalves (Lucinidae and Vesicomyidae) and specific biomarkers (e.g. Taviani, 2014; Peckmann et al., 2002; Grillenzoni et al., 2017). In the northern Apennine orogenic belt, several outcrops of authigenic carbonates tes fy for Miocene seepage ac vity (Terzi et al., 1994; Dela Pierre et al., 2010; Con  et al., 2017). These carbonates show highly depleted d13C signatures and host large chemosynthe c bivalve assemblages (mainly giant lucinids)(Grillenzoni et al., 2017), thus tes fying a long and complex history of methane-rich fluid ven ng on the sea floor. Apenninic carbonate deposits bearing fossil chemosynthe c fauna have been found in various domains and basin types, from wedge-top basins to the slope of the accre onary wedge, and in the inner foredeep (Con  and Fontana, 1999; Argen no et al., 2019a) (Figs 2, 3). In the following, we summarize and describe the main features of the Apennine seep carbonates, from field observa ons to geochemical features.