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

Enrico Capezzuoli,Domenico Liotta,Martina Zucchi,Amalia Spina,Andrea Brogi,Giancarlo Molli,Renzo Regoli,Gabriella Bagnoli

doi:10.3390/geosciences11020084

Abstract

The Pre-Mesozoic units exposed in the inner Northern Apennines mostly consist of Pennsylvanian-Permian successions unconformably deposited on a continental crust consolidated at the end of the Variscan orogenic cycle (Silurian-Carboniferous). In the inner Northern Apennines, exposures of this continental crust, Cambrian?-Devonian in age, have been described in Northern Tuscany, Elba Island (Tuscan Archipelago) and, partly, in scattered and isolated outcrops of southern Tuscany. This paper reappraises the most significant succession (i.e., Risanguigno Formation) exposed in southern Tuscany and considered by most authors as part of the Variscan Basement. New stratigraphic and structural studies, coupled with analyses of the organic matter content, allow us to refine the age of the Risanguigno Fm and its geological setting and evolution. Based on the low diversification of palynoflora, the content of sporomorphs, the structural setting and the new field study, this formation is dated as late Tournaisian to Visean (Middle Mississippian) and is not affected by pre-Alpine deformation. This conclusion, together with the already existing data, clearly indicate that no exposures of rocks involved in the Variscan orogenesis occur in southern Tuscany.

Highlights

Stratigraphic reconstructions of the deep successions involved in orogens later affected by post-collisional extensional tectonics are always tempting, since these are normally metamorphosed and involved in polyphase deformation, are laterally segmented and, are exposed in scattered outcrops
The Risanguigno Fm is mostly dominated by black to grey phyllite, locally intercalated by cm-thick level and lenses of metasandstone and metasiltstone (Figure 6a,b)
The new palynological-fossiliferous data for the Risanguigno Fm, coupled with its sedimentary and deformational setting, make it possible to assign it to the Middle Mississippian and to exclude its encompassment in the Variscan basement

Summary

Introduction

Stratigraphic reconstructions of the deep successions involved in orogens later affected by post-collisional extensional tectonics are always tempting, since these are normally metamorphosed and involved in polyphase deformation, are laterally segmented and, are exposed in scattered outcrops. 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]. To strengthen this approach, several studies of the pre-Alpine metamorphic rocks of the Tuscan Archipelago and Apuan Alps have incorporated palaeontological, stratigraphic. Acycccloersd(iMngislysi,swsiepprei-acno-nesairdlyerPtehremRiiasnanagnudigmnoidFdolrem-laatteioPne, rwmhiiacnh)i[s5r,e2g6]a.rdAecdcoasrdpianrgtloy,f wthee Treu-csocannsidCerrysthtaellRiniseanBgauseigmneonFt oarnmdactoionns,tiwtuhtiecshtihsereogldaredstedouatscproaprtsoifntshoeuTtuhsecrannTCursycsatnayll.inIne tBhaissevmieewnt, athnids cisoonlsattietudtaesndthsecoalrdceeslyt osututcdrioepdsfionrmsoauttihoenrnstrTounscgalynyi.nIfnlutehniscevdietwh,etrheiscoisnosltartuecdtainodn socfatrhceeleynstitruedPierde-AfolrpminaetiAonpesntrnoinngiclysuincflceusesniocne.dTthheererefocoren,stthruecatiimonooffththise peanptierre iPsrteoAdolpciunme eAnpteannndiniilclusustcrcaetsesiaonn.eTwhlyeredfiosrceo,vtehreeadimpaolyf nthoifslopraaplecroinstteondt oacnudm, ecnotnasenqduiellnutsltyr,atteo parnoevwidlye dthisecporveecriesde pagaelyonfotflhoeraRlicsaonngteunigt nanodF,mco, ntosegqeutheenrtlwy,ittoh pitrsosvtirduecttuhreapl rseecttisinega.gTehoisf wthiellRleisaadntgoudigetneormFmin,attoiognetohfeVrawriistchanitsfosrtmruacttiuornasl isnettthiinsgk.eTyhsiescwtoirllolfeNadorttohderenteArmpeinnantiinoens forfoVmaraissctarantifgorrampahtiiconans dinptahliasekoegyeosgecrtaoprhoicf NpeorrstpheecrtnivAe.pennines from a stratigraphic and palaeogeographic perspective

Methods

Results

Discussion

Conclusion