Building a silicic effusive volcano: Geology, structure, and tectonics of Monte Amiata (Middle Pleistocene, Italy)

Monte Amiata (305-231 ka; Italy) is an uncommon example of completely effusive silicic (trachydacite) composite volcano. We applied the Unconformity Bounded Stratigraphic Unit criteria, volcanic facies analysis, and structural analysis to decipher its geological evolution and structure, and to infer the relations between regional tectonics and the shallow plumbing structure. Based on the occurrence of a first-order geological unconformity representing a paleoweathering surface of paleoclimatic significance, we recognize two major volcanic stratigraphic units and seven phases of activity. Monte Amiata emplaced in the releasing step-over of a large NE-SW striking oblique shear zone, which (a) favored the emplacement at 5-6 km depth of a silicic intrusion during Pliocene, (b) promoted the rising of deeper mafic magmas and the formation of an apical dike-rift zone during Pleistocene, and (c) accounts for the effusive eruptions of degassed magmas and the growth of the volcano.