Late Palaeozoic tectonics in Central Mediterranean: a reappraisal

Abstract

A revision of late Palaeozoic tectonics recorded in Tuscany, Calabria and Corsica is here presented. We propose that, in Tuscany, upper Carboniferous-Permian shallow-marine to continental sedimentary basins, characterized by unconformities and abrupt changes in sedimentary facies, coal-measures, red fanglomerate deposits and felsic magmatism, may be related with a transtensional setting where upper-crustal splay faults are linked with a mid-crustal shear zone. The remnants of the latter can be found in the deep-well logs of Pontremoli and Larderello-Travale in northern and southern Tuscany respectively. In Calabria (Sila, Serre and Aspromonte), a continuous pre-Mesozoic crustal section is exposed, where the lower-crustal portion mainly includes granulites and migmatitic paragneisses, together with subordinate marbles and metabasites. The mid-crustal section, up to 13 km-thick, includes granitoids, tonalitic to granitic in composition, emplaced between 306 and 295 Ma. They were progressively deformed during retrograde extensional shearing, with a final magmatic activity, between 295 ± 1 and 277 ± 1 Ma, when shallower dykes were emplaced in a transtensional regime. The section is completed by an upper crustal portion, mainly formed by a Palaeozoic sedimentary succession deformed as a low-grade fold and thrust belt, and locally overlaying medium-grade paragneiss units. As a whole, these features are reminiscent of the nappe zone domains of the Sardinia Variscan Orogen. In Corsica, besides the well-known effusive and intrusive Permian magmatism of the “Autochthonous” domain, the Alpine Santa Lucia Nappe exposes a kilometer-scale portion of the Permian lower to mid-crust, exhibiting many similarities to the Ivrea Zone. The distinct Mafic and Granitic complexes characterizing this crustal domain are juxtaposed through an oblique-slip shear zone named Santa Lucia Shear Zone. Structural and petrological data witness the interaction between magmatism, metamorphism and retrograde shearing during Permian, in a temperature range of c. 800–400 °C. We frame the outlined paleotectonic domains within a regional-scale, strain–partitioned, tectonic setting controlled by a first-order transcurrent/transtensional fault network that includes a westernmost fault (Santa Lucia Fault) and an easternmost one (East Tuscan Fault), with intervening crustal domains affected by extensional to transtensional deformation. As a whole, our revision allows new suggestions for a better understanding of the tectonic framework and evolution of the Central Mediterranean during the late Palaeozoic.

Reconsidering the Variscan Basement of Southern Tuscany (Inner Northern Apennines)

“…This succession is formed by phyllite, metasandstone, metaconglomerate with local carbonate levels attributed to Mississippian-late Permian on the basis of the fossil [16,18,44,[47][48][49], palynoflora content [24,25] and radiometric dating [23]. Its evolution is related to rifting [1], transcurrent/transtensive pull-apart basins [5,50] or to late Variscan compressional events [9,19].…”

“…A similar age (up to lower Permian) is also testified for the continental succession (Scisti di San Lorenzo Fm; [132]) exposed in the northernmost area of Tuscany (Figure 11). This Carboniferous-lower Permian deposition was succeeded by a second middlelate Permian sedimentary cycle (see, e.g., [5] for a review) where the older deposits were partially dismantled and accumulated in the new one. This is also testified in the Monticiano-Roccastrada area by the occurrence of numerous clastic fragments relatable to the Risanguigno Fm [4], or by the presence of middle Carboniferous (late Visean-early Namurian: [16,[47][48][49]133,134]) clasts, bioclasts and olistoliths embedded within the middlelate Permian Farma and Carpineta formations [70].…”

“…In this view, the presence of a Variscan tectonic phase in explaining the angular unconformity separating the Risanguigno Fm with the overlying Poggio al Carpino Fm and attributed to the Sudetic [15,43] or Bretonian phase [9,18,38] is denied. Therefore, such an angular unconformity is to be considered as having developed during the Carboniferous-Permian post-collisional tectonic regime [32,45,53], giving rise to short-lived, possibly pull-apart basins, dominated by continental to shallowmarine conditions [5].…”

“…This is even crucial for the metamorphosed, deep successions of the Northern Apennines [1], which experienced the Variscan sedimentary and tectonic evolution (Devonian-Carboniferous), then the Alpine cycle (Triassic-Oligocene), and ultimately the extensional process leading to the opening of the Tyrrhenian Basin (Miocene-Quaternary). Nowadays, the so-called Tuscan Crystalline Basement (Cambrian?-Devonian [2]) is discontinuously exposed, and the scarcity of fossils remains inhibits precise age determination [3][4][5]. Thus, in absence of fossil records, the Tuscan Basement is traditionally related to the well-known and better exposed Palaeozoic succession of southeastern Sardinia, where the Alpine deformation is relatively minor [6][7][8][9].…”

“…More recently, radiometric (Ar/Ar, U/Th [21][22][23]) and palynological studies [24,25] have served to motivate a review of the entire Tuscan Palaeozoic successions based on bio/chronological markers. These studies contribute more precise age datings and provide new evolutionary scenarios in the context of two distinct Palaeozoic cycles (Mississippian-early Permian and middle-late Permian) [5,26]. Accordingly, we re-consider the Risanguigno Formation, which is regarded as part of the Tuscan Crystalline Basement and constitutes the oldest outcrops in southern Tuscany.…”