The formation of triangle zones in already deformed belts: the case study of an external area of the northern Apennines (Italy)

Abstract

This study reconstructs the sequence of deformation that affected a sector of the northern Apennines accretionary wedge (external Ligurian Units located in the Parma and Piacenza provinces). This external sector of the belt underwent several superposed events due to oceanic accretion and subsequent continental collision, at shallow structural levels. To this purpose, microstructural analysis was applied to the calcite veins. Twinned calcite crystals represent stress-strain markers and make it possible to estimate (1) pressure-temperature conditions, (2) amount and pressure of circulating fluids and (3) paleostress determinations. This methodology permitted to reconstruct the peculiar kinematics and tentatively interpreting this frontal zone of the belt as a tectonic wedge (triangle zone). The Ligurian wedge in the studied area differs from classical triangle zones, which typically form at foreland margins of thrust belts, in that its para-autocthon was already deformed when passively uplifted by the wedging event.