Interim report about geological survey in the Monticiano-Roccastrada metamorphic core complex (MRMCC, S-Tuscany, Italy) with considerations about its stratigraphic and tectonic development

The study area is part of the Mid Tuscan Ridge: a linear belt of metamorphic core complexes, exhumed during M-L-Tertiary times. They separate the N-Apenninic main belt from the Tuscan metamorphic zone, which originated during the opening of the N-Tyrrhenian back arc basin. This resulted from postcollisional rollback of the steeply W-dipping Adriatic subduction zone: effecting the migration of eastbound, paired compressional-extensional deformation fronts and of magmatism in the overlying crust (literature data).

The central part of the MRMCC (Farma Valley) was chosen for revision of geological field work. The already accomplished map part including resulting stratigraphic and tectonic facts are displayed:

Results: Above a fragmented and extended uniform base - Devonian pelagites - different Carboniferous, coeval, predominantly siliciclastic depositional areas developed as highstand systems tract above a downlap surface. Their recent proximality trend is westbound and their sedimentary environments, separated by normal faults, represent the shelf, lower slope, base of slope and basin margin. Sediment input was triggered by tropical storms and/or seaquakes. The deposits of these environments are covered by regressive, littoral-deltaic siliciclastics (Permian).

During E-M-Tertiary subduction related tectonic burial, the Carboniferous normal faults were reactivated, so that mentioned slope and basin deposits were thrusted onto inner shelf deposits. M-L-Tertiary unroofing caused exhumation from ca. 25 km depth. A megabreccia got exposed. It is arranged in subzones, characterised by distinctive metamorphism and deformation.

Farma Valley is probably geomorphological expression of an exhumed tear fault, part of the fracture system controlling the eastward migration of the Tuscan Magmatic Province.