On the tectonics of the Ligurian Apennines (northern Italy)

Abstract

Four nappes have been recognized in the Ligurian Apennines. In the Lavagna Nappe very low-grade metamorphism is combined with very large, originally W-facing isoclinal folds. In the other nappes, no evidence for metamorphism is found and all eutectonic folding was originally E- to NE-facing. Tectonic transport along the major nappe contacts was in an E- to NE-direction. A tectonic model is presented, which explains the generation of the large, originally W-facing folds as a result of originally E-inclined subduction within a young oceanic basin. Young oceanic lithosphere (maximum age approximately 25 Ma) subducted beneath oceanic lithosphere with a maximum age of approximately 40 Ma, under the influence of horizontally oriented compressional forces. Within the tectonic wedge, associated with the subduction, originally W-facing isoclinal folding and metamorphism occurred. Decrease and/or termination of compression resulted in the cessation of the subduction movements, followed by uplift of the region above the subducted plate by means of buoyancy. This uplift formed a slope from which sequences slid in an E- to NE-direction, causing E- to NE-facing folds. Ultimately, detachment and thrusting of gravitational nappes took place, by which process rock sequences of oceanic origin have been externally transported to attain ensialic (continental) domains. The Triassic-Early Oligocene tectonic events recognized in the Ligurian Apennines correlate quite well with the events that preceded the collision phase of the Alps.