From Upper Palaeozoic extensional basin fill to late Alpine low grade metamorphic core complex: preliminary note on the sedimentary and tectonic development of the Monticiano-Roccastrada-Zone (MRZ; Southern Tuscany, Italy)
published: Jan 1, 1997
The Lower Devonian to Permian, predominantly siliciclastic sedimentary formations of the MRZ are interpreted as deposits of a successor basin formed by extensional collaps of a thickened accretionary wedge near the palaeo-adriatic suture. Up to Lower Carboniferous, slow and relatively uniform subsidence is reflected by a thin and shallow marine sequence made up of pelites, dolomites, cherts and Culm-type lydites. Not later than Middle Carboniferous, a rift pulse triggered the bathymetric and facial differentiation of coeval formations following upward with erosional unconformity. Till ?Upper Carboniferous, continued and quick input of terrigeneous siliciclasts smoothed basin topography and shallowed the sedimentary environment. Permian regressive deltaic and litoral sediments correspond to the onset of late hercynian rise of geothermal gradients, retarding the depositional area's rate of subsidence. In ?Upper Permian to ?Lower Triassic, basin uplift and inversion caused ?Middle Triassic subaerial exposition of the upper part of the basin fill: substantial thickness-fluctuations (0--350 m) of the Permian and clast analysis of the lower part of the Verrucano-Group ensure that the erosional base of the ?Anisian redbeds reworked Upper Carboniferous. Subsequent to ?Middle Triassic-?Lower Carnian peneplanation, the Upper Palaeozoic failed rift was sealed by transgressive passive margin sediments of the mesozoic Tethys-rift. Cretaceous to Eocene convergence and Lower Oligocene Apennine thrusting buried the failed rift below a ~ 12 km thick stack of tectonic nappes of Ligurian and Tuscan sediments. The ?Middle Carboniferous zone of crustal weakness and Upper Triassic evaporites favoured the development of the well-known Appennine remote thrusts. During Middle Oligocene to Tortonian, compensatory crustal thinning produced nearly complete unroofing of the failed-rift-hangingwall, the rise of the geotherms and low grade metamorphism. Continued extension and thermal uplift of the failed rift led to Upper Pliocene acid fissure eruption and finally to exhumation of the metasediments of the Monticiano-Roccastrada -- metamorphic -- core-complex.