UC Berkeley

The Cenozoic accretionary wedge of Calabria, Southern Italy, consists of several units of continental and oceanic affinity accreted beneath the former continental margin of the Sardinia-Corsica block. Each of these units bears the imprint of blueschist-facies metamorphism, indicating that it has been subducted to high-pressure/low-temperature conditions during the Alpine Orogeny. Structurally higher units, having been accreted first, record the early metamorphic history of the orogen; lower units, which are accreted later, record correspondingly later sedimentary and metamorphic events.

In this dissertation, I use metamorphic petrology, structural geology, and geochronology, in the context of field relationships, to study several of the nappe units. The result is a new age and tectonic framework for the Calabrian orogen.

The Zangarona Schist, often the structurally highest unit in the accretionary wedge, records the early history of subduction in Calabria. I show that the high-pressure/high-temperature event recorded in these rocks did not occur at the beginning of Alpine-age subduction; instead, it was formed during an earlier event which took places in the Hercynian Orogen. The lack of a high-temperature metamorphic event, along with the considerable age of units at the time of their subduction, suggests that subduction started off in a cold thermal environment.

The Diamante-Terranova ophiolite unit is a metabasalt and associated sedimentary cover which was metamorphosed in the lawsonite-blueschist facies. It is generally considered to have a greenschist-facies overprint--one which is common in circum-Mediterranean orogens. I show that this greenschist-facies overprint does not exist and that the rocks followed a counterclockwise pressure-temperature-time history. The lack of a warm-overprint allows for Ar/Ar geochronology to be used on high-pressure phengites, indicating a crystallization age of about 48 Myr. This age links the rocks to a west-dipping subduction zone that led to volcanism in Sardinia.

The Frido Unit is a dominantly phyllite and quartzite unit which records changes in sedimentation during the approach of a unit of thinned continental crust to the Calabrian subduction zone. Although much of it appears to be a coherent unit, I show that the upper levels of the unit include chaotic blocks of metabasalt and serpentinite, deposited as a sedimentary mélange. Many of the Calabrian ophiolite units, previously considered to be distinct nappe units, have similar block-in-matrix relationships and also may be sedimentary mélanges consisting of previously-subducted material which has been re-deposited into the trench by underwater slides.

Together, this re-evaluation of the Calabrian nappe stack indicates that subduction likely began in the Eocene in a cold thermal environment. Early-formed blueschist units were exhumed and re-deposited in the trench, eventually undergoing a second high-pressure metamorphic event. This same subduction zone exists today, to the southeast, as Ionian oceanic crust continues to subduct beneath Calabria.