Thrusting and faulting in metamorphic and sedimentary units of Ligurian Alps: an example of integrated field work and geochemical analyses

Abstract

In a sector placed in the SE part of the AlpsApennine junction, a kilometre-scale shear zone has been identified as the Grognardo thrust zone (GTZ), which caused the NE-directed thrusting of metaophiolites (Voltri Group) and polymetamorphic continental crust slices (Valosio Unit) of Ligurian Alps onto Oligocene sediments of an episutural basin known as ''Tertiary Piemonte Basin''. The structural setting of the GTZ is due to syn-to late-metamorphic deformation, followed by a brittle thrusting that occurred in the Late Aquitanian times and can thus be related to one of the main contractional tectonic events suffered by northern Apennines. The GTZ was then sealed by Lower Burdigalian carbonate platform sediments (Visone Formation). Transtensive faulting followed in post-Burdigalian times along NW-SE regional faults and displaced the previously coupled sedimentary and metamorphic units. The GTZ thus underwent a plastic-to-brittle evolution, during which carbonate-rich fluids largely sustained the deformation. In these stages, a complex vein network originated within both the metamorphic and sedimentary rocks. Field data and stable isotopic analyses ( 13 C and 18 O) of bulk rocks and veins show that fluid-rock interaction caused the carbonatisation of the rocks in the late-metamorphic stages and the cataclasis and recementation, by the action of isochemical cold carbonate groundwater during the thrusting events. Carbonate veins largely developed also during the transtensive faulting stages, with composition clearly different from that of the veins associated to thrust faults, as indicated by the strong depletion in 13 C of carbonate fillings, suggesting the presence of exotic fluids, characterised by a high content of organic matter.

Geological setting of the southern termination of Western Alps

“…Since about 26 Ma, a transpressional regime coherent with regional N-S shortening directions induced right-lateral movements of the NW-SE-and E-W-oriented fault systems displacing the Argentera Massif (Sanchez et al 2011b), while in the adjacent internal syn-orogenic basins (Tertiary Piemonte Basin) significant uplifting occurred at about 20 Ma, controlled by N-S to NE-SW compressional tectonic regime (Festa et al 2005;Piana et al 2006;Maino et al 2013) and recorded by regional unconformities (Piana et al 1997;d'Atri et al 2002;Dela Pierre et al 2003) and by the onset of new depocenters (Rossi et al 2009). …”

“…This age-span could correspond, in the Maritime Alps and Western Ligurian Alps, to the switch from sinistral tectonics to a compressional one (Maino et al 2013 (Piana et al 1997(Piana et al , 2006Dela Pierre et al 2003). This event induced the dextral reactivation of the main NW-SE striking shear zones at regional scale (Sanchez et al 2011a;Rolland et al 2012;Bauve et al 2014), although in the study area sinistral movements are still locally recorded along the D2 event (see Fig.…”

Retrogressive fabric development during exhumation of the Voltri Massif (Ligurian Alps, Italy): arguments for an extensional origin and implications for the Alps–Apennines linkage

“…Although this structural setting may be also explained by means out-of-sequence thrusting (e.g. Hoogerduijn Strating 1994;Capponi et al 1998;Capponi et al 1999;Vissers et al 2001;Piana et al 2006), the consistence of our kinematic data (from ductile-to-brittle conditions) and the preservation of local tectonic elision along the LTC/UTC contact argue for the extensional reworking of an early structured compressional edifice, whose HP remnants were dismembered by top-to-the-W shearing. Further radiometric investigations in the HP units of the UTC are needed to support this tectonic reconstruction.…”

New data for the kinematic interpretation of the Alps–Apennines junction (Northwestern Italy)

“…1; cfr. geological maps by Lorenz 1969;Cazzola et al 1981;Mutti et al 1995), NNW-SSE fault systems, such as the Grognardo thrust zone (Piana et al 2006), and a northern segment of the Celle-Sanda fault system (as intended in this study, see below). The WSW-ENE fault systems accommodated regional left-lateral displacements, while the NNW-SSE faults are east-verging reverse faults.…”