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.