ANDREA BROGI (*), (**) & LORENZO FABBRINI (*) KEY WORDS: Tuscan Nappe, collisional structure, Northern Apennines, southern Tuscany, Monte Amiata area.

Transcript

1 Ital.J.Geosci. (Boll.Soc.Geol.It.), Vol. 129, No. 1 (2010), pp , 13 figs. (DOI: /IJG ) The Monte Penna thrust (southern Tuscany, Italy): geometry and kinematic of a collisional structure affecting the Tuscan Nappe during the Northern Apennines orogenic building ANDREA BROGI (*), (**) & LORENZO FABBRINI (*) ABSTRACT In this paper we describe the geometry and kinematics of a contractional structure, consisting of a thrust and related minor structures, which doubled the Tuscan Nappe cropping out in the southern side of the Monte Amiata geothermal region (Selvena-Castell Azzara area), during the building of the Northern Apennines. Such a thrust (hereafter the Monte Penna thrust, MPT) is documented, in this area, for the first time. The integration of fieldwork with mine data from two different cinnabar/stibnite mines helped us to better define the thrust geometrical setting both at the surface and underground. Thrusting took place during the Early Miocene and produced the tectonic repetition of the Tuscan Nappe succession. The thrust gave rise to two tectonic subunits (SU1 and SU2) separated by a contractional shear zone up to 3m thick. Such a shear zone consists of three structural domains characterised by different structures such as folds, a tectonic foliation, reverse faults and Riedel shear fractures. They indicate a top-to-the ESE sense of shear, coherently with the general staking pattern of the Northern Apennines belt at least for the study area and surroundings. The thrust-sheet was deformed by cartographic- to mesoscopic folds, N135 to N220 striking on average, NE- and SE-verging, respectively. The tectonic setting recognised in the study area is coherent with that of the Tuscan Nappe described for the Monte Amiata region and the whole southern Tuscany, resulting characterised by duplex system. Such a duplex system was affected by extensional structures since the Middle Miocene. Presently, the thrusts consist of relic structures preserved within extensional horses. The whole evidence possibly make the Tuscan Nappe duplex system comparable with the structure of the Tuscan Metamorphic «basement», in terms of geometry, stacking pattern and timing, suggesting a probable common structural evolution even if a different structural levels. KEY WORDS: Tuscan Nappe, collisional structure, Northern Apennines, southern Tuscany, Monte Amiata area. RIASSUNTO Il sovrascorrimento del Monte Penna (Toscana meridionale): geometria e cinematica di una struttura collisionale sviluppata nella Falda Toscana durante l orogenesi dell Appennino Settentrionale. In questo lavoro vengono descritte la geometria e cinematica di una struttura contrazionale rappresentata da un sovrascorrimento e dalle sue relative strutture minori, che hanno interessato la Falda Toscana esposta nella parte meridionale dell area geotermica del Monte Amiata durante l edificazione della Catena appenninica settentrionale. Il sovrascorrimento del Monte Penna (di seguito MPT) viene documentato per la prima volta in questo lavoro. L integrazione di dati geologici di superficie con quelli derivati (*) Dipartimento di Scienze della Terra, Università degli Studi di Siena, Via Laterina, Siena, Italy. (**) Corresponding Author: brogiandrea@unisi.it dall attività di ricerca ed estrazione mineraria in due miniere di cinabro ed antimonite, ha permesso di ricostruire la geometria del sovrascorrimento sia in superficie che in profondità. La sua età è probabilmente da riferire al Miocene inferiore poiché la deformazione ha interessato la formazione del Macigno, attribuita all Oligocene superiore-miocene inferiore. Il sovrascorrimento ha causato lo sviluppo di due sottounità di Falda Toscana (SU1 e SU2) appilatesi da ovest verso est. Il senso di movimento del sovrascorrimento è stato determinato attraverso l analisi cinematica di strutture minori riconosciute entro la zona di taglio corrispondente alla superficie di sovrascorrimento. Tale zona di taglio, spessa fino a 3m, è caratterizzata da tre diversi domini strutturali che mostrano diverse caratteristiche e strutture, quali pieghe, una foliazione tettonica, faglie inverse e fratture di taglio di Riedel. Queste strutture minori indicano un senso di taglio top-to-the ESE, coerentemente con quello che è il generale senso di accavallamento della Catena appenninica settentrionale, come documentato nelle aree circostanti alla zona di studio. Il sovrascorrimento, così come le strutture minori, sono state successivamente deformate da pieghe riconoscibili alla scala mesoscopica e cartografica, mediamente orientate in direzione da N135 a N220 e vergenti verso NE e SE. L assetto tettonico ricostruito per la Falda Toscana nell area di studio è coerente con l assetto della Falda Toscana descritta nella regione amiatina che risulta essere caratterizzata da una strutturazione in duplex. Il sistema di duplex è stato successivamente interessato da faglie dirette. Ne consegue che parte dell originaria strutturazione a sovrascorrimenti è oggi preservata entro horse estensionali nei quali possono essere riconosciute strutture contrazionali relitte discontinuamente esposte. L insieme delle evidenze strutturali rende l assetto tettonico della Falda Toscana molto simile a quello del «basamento» metamorfico toscano, affiorante nelle aree circostanti o perforato in sondaggio, in termini di geometria dei sovrascorrimenti, cinematica delle strutture minori associate ed età del loro sviluppo, suggerendo che sia la Falda Toscana che il «basamento» metamorfico abbiano avuto una simile evoluzione strutturale durante la fase di convergenza e di edificazione dell Appennino Settentrionale, sia pure entro livelli strutturali diversi. TERMINI CHIAVE: Falda Toscana, strutture collisionali, Appennino Settentrionale, Toscana meridionale, area del Monte Amiata. INTRODUCTION Contractional structures usually can develop at different scales and in a variety of tectonic styles within the different crustal levels during horizontal shortening (RAMSAY, 1967; RAMSAY & HUBER, 1987 cum. bibl). Brittle-ductile contractional structures develop within the upper crustal levels during the formation of fold-and-thrust belts, defining the main structure and architecture of the chains (BOYER & ELLIOT, 1982; BUTLER, 1982; SANDERSON, 1982; SUPPE, 1983; SUPPE & MEDWEDEFF, 1990; MITRA, 1993; MITRA & SUSSMAN, 1997; THORBJORNSEN & DUNNE, 1997; WIBBERLEY, 1997).

2 THE MONTE PENNA THRUST, SOUTHERN TUSCANY 75 The Northern Apennines is a fold-and-thrust belt formed by accretionary units stacked since the Cretaceous, during the convergence and collision of the African (Adria microplate) and the European plate, represented by the Sardinia-Corsica Massif (BOCCALETTI et alii, 1971; CASTELLARIN et alii, 1992; MANTOVANI et alii, 1995; FAC- CENNA et alii, 2001). The inner zone of the Northern Apennines, represented by the Tuscany, the Northern Latium and the Northern Tyrrhenian Sea, was affected by extension since the Early-Middle Miocene (CARMIGNANI et alii, 1994; JOLIVET et alii, 1998; BRUNET et alii, 2000; BROGI et alii, 2005a, 2005b), acting coevally with compression in the outer Northern Apennines (ELTER et alii, 1975; LAVECCHIA et alii, 2004; BARCHI et alii, 2006). Consequently, contractional structures are broadly exposed and preserved in the outer part of the chain. In the inner zone, contractional structures were dismembered by extensional ones, such as low- and high-angle normal faults (LAVECCHIA, 1988; BERTINI et alii, 1991; LIOTTA et alii, 1998; BROGI et alii, 2005a, 2005b; COLLETTINI et alii, 2006; BROGI, 2008). Thus, contractional structures can be only recognised as relicts within extensional horses (BROGI, 2004a, 2004b; BROGI & CERBONESCHI, 2007). Thrusts were broadly documented in southern Tuscany, mainly within the Tuscan metamorphic «basement», where duplex structures have been described both at surface (COSTANTINI et alii, 1988; LAZZAROTTO et alii, 2003) and at depth, mainly in the Larderello Geothermal field (PANDELI et alii, 1991; CAMELI, 1994). Concerning the overlying non-metamorphic tectonic units, thrusting gave rise to the Ligurian tectonic pile which overthrusted the Tuscan Nappe, the deepest non-metamorphic tectonic unit of the Northern Apennines (CARMIGNANI et alii, 2001). In the recent years, it has been demonstrated, in several places of southern Tuscany, that the Tuscan Nappe was affected by widespread internal thrusting, which produced multiple tectonic repetitions of the stratigraphic succession (BROGI, 2005; BROGI et alii, 2005b). Such a thrusting indicates that the Tuscan Nappe recorded a severe shortening during the building of the Northern Apennines belt. In this paper, we document for the first time the occurrence of a thrust (hereafter Monte Penna thrust, MPT) doubling the Tuscan Nappe succession cropping out in the southern Monte Amiata geothermal region (fig. 1), in the Selvena-Castell Azzara area (fig. 2). New detailed fieldwork and structural analyses integrated with mine data from the Morone-Dainelli and Cornacchino cinnabar/stibnite mines, allowed us to better constrain the geometry and kinematics of the MPT. The MPT shows analogous features with respect to other thrusts doubling the Tuscan Nappe succession in other places of southern Tuscany (BROGI & LAZZAROTTO, 2002; BROGI, 2004a, 2004b; BROGI et alii, 2005a, 2005c, 2005d; BROGI & CER- BONESCHI, 2007). These evidences suggest that thrusting regionally affected the Tuscan Nappe during the collisional event, at least in the inner zone of the Northern Apennines. GEOLOGICAL SETTING The Mt. Amiata geothermal area (fig. 1) was deeply investigated for geothermal and mining purposes mainly in the last decades (CALAMAI et alii, 1970; ARISI ROTA et alii, 1971; ELTER & PANDELI, 1991; BATINI et alii, 2003). Its geological setting has been reconstructed integrating fieldwork, mining, geothermal boreholes and geophysical investigations (BATINI et alii, 2003 cum. bibl.). Several regional tectono-stratigraphic units have been documented, from top to bottom (fig. 3A): 1) The Magmatic Complex, mainly represented by the Mt. Amiata volcano (FERRARI et alii, 1996 cum. bibl.). This complex consists of dacitic, rhyodacitic and olivinelatitic rocks containing mafic enclaves, erupted in a period ranging from 300 to 190 ka (FERRARI et alii, 1996; CADOUX & PINTI, 2008). 2) Neogene and Quaternary deposits. They consist of Middle Miocene-Quaternary continental and marine sediments filling the tectonic depressions, and Quaternary travertines. These deposits unconformably overlie the pre-neogene substratum; 3) The Ligurian Complex. This includes the Ligurian Units (S. Fiora Unit: Early Cretaceous-Eocene; and Ophiolitic Unit: Middle Jurassic-Early Cretaceous) and the Subligurian Unit (Eocene-Oligocene). The Ligurian Units are composed of remnants of the Jurassic oceanic basement and its pelagic sedimentary cover. The Subligurian Unit is composed of a pelagic sequence belonging to a paleogeographical domain interposed between the Ligurian Domain and the Tuscan Domain. The Ligurian and Subligurian Units were thrusted eastwards over the Tuscan Domain during the Latest Oligocene-Early Miocene. 4) The Tuscan Nappe. This is related to part of the Late Triassic-Early Miocene sedimentary cover of the Adria continental paleomargin (CIARAPICA & PASSERI, 1994; FAZZUOLI et alii, 1994). Its stratigraphic succession is made up of (from bottom to top): evaporite (Late Triassic), carbonate (Late Triassic-Early Cretaceous) and clayey-turbiditic (Cretaceous-Early Miocene) successions. The Tuscan Nappe was detached from its substratum along the Triassic evaporite horizon and was thrusted over the outer palaeogeographical domains (Umbria- Marchigian Domain) during the Late Oligocene-Early Miocene contraction. 5) Below the Tuscan Nappe, the Tuscan metamorphic complex has been encountered by deep boreholes (ELTER & PANDELI, 1991). This complex is composed only of the Monticiano-Roccastrada Unit, which has been drilled down to 4900 m. The complex is composed of very low-grade metamorphic sequence belonging to two groups: (a) the Triassic Verrucano Group, composed of continental metapelites, metasandstones and metaconglomerates; (b) the Palaeozoic Group, composed of graphitic phyllites and meta-sandstones with dolostone layers, and Late Permian fusulinid-bearing limestones and dolostones. The occurrence of the Micaschist Group and the Gneiss Complex at depth, as documented by borehole data in the Larderello-Travale geothermal area, has been documented by xenoliths in the Quaternary lavas of the Monte Amiata volcano (VAN BERGEN, 1983). The Selvena-Castell Azzara area (fig. 2) belongs to a broad structural culmination about 50Km 2 wide, where the Tuscan Nappe emerges from the surrounding overthrusted Ligurian Units (fig. 1). Fieldwork and subsurface geological data acquired during mining allowed us to reconstruct the tectono-stratigraphic setting of the Tuscan Nappe.

3 76 A. BROGI & L. FABBRINI Fig. 1 - Location of the study area: a) The study area in the Northern Apennines; b) location of the two main geothermal areas of southern Tuscany; c) geological sketch map of the Monte Amiata Geothermal region and location of the study area enlarged in fig. 2 (after BROGI, 2006a, modified). Ubicazione dell area di studio: a) ubicazione dell area nell Appennino Settentrionale; b) ubicazione delle due più importanti aree geotermiche della Toscana meridionale; c) carta geologica schematica dell area geotermica del Monte Amiata ed ubicazione dell area di studio indicata in fig. 2 (da BROGI, 2006a, modificata). The Ligurian Units are only represented by the S. Fiora Unit (COSTANTINI et alii, 1977; CARMIGNANI et alii, 2001), corresponding to the External Ligurides Units (MARRONI et alii, 2001) for the Northern Apennines, mainly characterised by Cretaceous marly-calcareous turbidites (Helmintoid Flysh units, CERRINA FERONI et alii, 2004 cum. bibl.). The S. Fiora Unit mainly consists of its basal succession (S. Fiora Fm), Cretaceous in age, composed of shales, calcilutites and marls (Sillano Fm belonging to the Calvana Supergroup described north to the Arno River, BORTO- LOTTI et alii, 2001 cum. bibl.), with interbedded arenaceous turbidites (Pietraforte Fm) (fig. 3B). The stratigraphic succession of the Tuscan Nappe has been described by ELTER (1955), LOSACCO (1960), CANUTI & MARCUCCI (1971), DECANDIA et alii (1994), BONCIANI et alii (2005). It consists of the complete stratigraphic succession ranging in age from the Late Triassic and the Early Miocene (fig. 3). The relationships between the Tuscan Nappe and Ligurian Units were driven by the contractional structures developed during the collisional event, and overall by the Middle Miocene-Pliocene extensional events, abundantly described for the surroundings areas (see CALAMAI et alii, 1970; CARMIGNANI et alii, 1994; BROGI, 2004a, 2004b, 2006a; BONCIANI et alii, 2005; BROGI et alii, 2005a). Concerning the extensional structures, they produced widespread tectonic omissions within the Tuscan Nappe and S. Fiora units, implying the direct superimposition of the S. Fiora Fm on all the Tuscan Nappe formations (ELTER, 1955; DECANDIA et alii, 1994; BONCIANI et alii, 2005). Sub-

4 THE MONTE PENNA THRUST, SOUTHERN TUSCANY 77 Fig. 2 - Tectonic sketch showing the structure of the Tuscan Nappe in the Selvena-Castell Azzara area. Its location is given in fig. 1c. Letters A, B and C indicate the station of structural analyses (see the text for more details). Schema tettonico che illustra la struttura della Falda Toscana nell area di Selvena-Castell Azzara. L ubicazione è indicata nella fig. 1c. Le lettere A, B e C indicano le stazioni di analisi strutturale (si veda il testo per maggiori dettagli).

5 78 A. BROGI & L. FABBRINI Fig. 3 - Tectono-stratigraphic units recognised in the Monte Amiata area (A, after BATINI et alii, 2003) and in the Selvena-Castell Azzara area (B, reconstructed by this study). Key: BUR - Burano Fm: dark dolostones with interbedded gypsum and/or anhydrite levels, only documented by mining (Late Trias); RET - Calcari a Rhaetavicula contorta Fm: dark limestones interbedded with minor levels of marls (Late Trias); MAS - Calcare Massiccio Fm: massive grey and pink limestones (Early Lias); SEL - Calcare Selcifero Fm: cherty limestones (Middle-Late Lias); POD - Marne a Posidonomya Fm: marls and marly-clays (Middle Jurassic); DSD - Diaspri Fm: radiolarites (Late Jurassic); MAI - Maiolica Fm: cherty limestones and marls (Early Cretaceous); Scaglia Toscana Fm (BRO - Argilliti di Brolio: shales and rare limestones and manganesiferous radiolarites mainly occurring at the top of the succession (Late Cretaceous); SUG - Marne del Sugame: marls and limestones (Late Cretaceous- Eocene); DUD1 - «Sottonummulitico» Auctt. (LOSACCO, 1960): calcarenites, marls and clays (Middle-Late Eocene); NUM - «Nummulitico» Auctt. (Calcarenti di Montegrossi in CANUTI et alii, 1965): nummulite bearing biocalcarenites and biocalcirudites (Late Eocene); DUD2 - «Sopranummulitico» Auctt. (LOSACCO, 1960): yellow and red marls interbedded with calcarenites and limestones (Late Eocene-Oligocene). MAC - Macigno Fm: turbiditic quartz-feldspar micaceous sandstones and silts (Late Oligocene-Early Miocene); FIA - S. Fiora Fm; PTF - Pietraforte. Relazioni tettono-stratigrafiche riconosciute nell area del Monte Amiata (A, da BATINI et alii, 2003) ed in quella di Selvena-Castell Azzara (B, ricostruita in questo lavoro). Spiegazione delle sigle: BUR - Formazione di Burano: dolomie grigie con intercalati livelli di anidriti e gessi documentati solo in miniera (Triassico superiore); RET - Formazione dei Calcari a Rhaetavicula contorta: dolomie grigie e nere con intercalazioni di livelli marnosi (Triassico superiore); MAS - Formazione del Calcare Massiccio: calcari massicci grigi e rosati (Liassico inferiore); SEL - Formazione del Calcare Selcifero: calcari selciferi grigi stratificati (Liassico medio-superiore); POD - Formazione delle Marne a Posidonomya: marne ed argille marnose (Dogger) DSD - Formazione dei Diaspri: radiolariti (Malm); MAI - Formazione della Maiolica: calcari grigio chiari selciferi (Cretecico inferiore); Formazione della Scaglia Toscana: BRO - Argilliti di Brolio: argilliti, rari calcari e radiolariti manganesifere presenti nella parte superiore della successione (Cretacico superiore); SUG - Marne del Sugame: marne e calcari (Cretaceo superiore-eocene); DUD1 - «Sottonummulitico» Auctt. (LOSACCO, 1960): calcareniti, marne e argilliti (Eocene medio-superiore); NUM - «Nummulitico» Auctt. (Calcareniti di Montegrossi in CANUTI et alii, 1965): biocalcareniti e biocalciruditi a Nummuliti (Eocene superiore); DUD2 - «Sopranummulitico» Auctt. (LOSACCO, 1960): jmarne gialle e rosse, calcari e calcareniti (Eocene superiore-oligocene); MAC - Formazione del Macigno: arenarie quarzoso feldspatiche micacee con intercalazioni di livelli di siltiti (Oligocene superiore - Miocene inferiore); FIA - Formazione di S. Fiora; PTF - Pietraforte.

6 THE MONTE PENNA THRUST, SOUTHERN TUSCANY 79 surface geological information from the Morone-Dainelli mine (fig. 2) documented that the S. Fiora Unit overlies the Late Triassic Tuscan Nappe succession (Calcari a Rhaetavicula contorta and Burano Fms). This peculiar tectonic setting, known as the «serie ridotta» (TREVISAN, 1955; DECANDIA et alii, 1993) has been kinematically and dynamically discussed in BROGI (2006a) for the Monte Amiata area and surroundings, and has been documented for the first time in the study area by ELTER (1955) and DECANDIA et alii (1994). DEFORMATION The MPT doubled the Tuscan Nappe succession and does not propagate within the Ligurian Units. Taking into account the aim of this paper, we describe only the contractional structures within the Tuscan Nappe and address to the abundant literature for the extensional structures (CALAMAI et alii, 1970; DECANDIA et alii, 1993; CARMIGNANI et alii, 1994; BONCIANI et alii, 2005; BROGI, 2004a; BROGI et alii, 2005a). Contractional structures can be referred to two superimposed deformational events, analogously to the tectonic framework described for the Tuscan Nappe in the whole Tuscany (PERTUSATI et alii, 1977; CARMIGNANI et alii, 1991; ELTER & SANDRELLI, 1995; CAMPETTI et alii, 1995, 1999; BONINI, 1999; BROGI, 2004a, 2004b; CAROSI et alii, 2004a, 2004b). The MPT developed during the oldest contractional event recognised in the study area. It is exposed in the western, southern and eastern sides of the Monte Penna (figs. 2 and 4a), close to the Selvena village (fig. 2), and in the neighbouring of the Castell Azzara village (fig 4). Thrusting gave rise to two tectonic subunits (hereafter SU1 and SU2) (fig. 3B). The uppermost subunit (SU1) consists of an exposed stratigraphic succession ranging from the Late Cretaceous clayey succession (Scaglia Toscana Fm, Argilliti di Brolio, BRO in fig. 3B) and the Early Miocene turbidites (Macigno Fm, MAC in fig. 3B). The lowermost subunit (SU2) is characterised by the stratigraphic succession ranging from the Late Triassic evaporites (Burano Fm, BUR in fig. 3B) and the Early Miocene turbidites (Macigno Fm). The Burano Fm has been only documented at depth by boreholes during mining exploration; the deepest exposed formation belonging to the SU2 consists of the Late Triassic carbonate succession (Calcari a Rhaetavicula contorta Fm, RET in fig. 3B). The damage zone related to the MPT was also documented and geometrically constrained by mining. Particularly, it was encountered by the Rigo gallery (fig. 5), N30 striking, which crossed, at 780 m above sea level, the Monte Penna-Monte Civitella ridge connecting the Cornacchino mine with the northern side of the Monte Civitella (fig. 2). The gallery (fig. 5) encountered the Sottonummulitico belonging to the SU1 subunit and overlying the Nummulitico representing the topmost succession of the SU2 subunit (figs. 3 and 5). Such a subunit was also drilled down to the Maiolica Fm (fig. 5). Miners have described the shear zone separating the SU1 and SU2 as strongly damaged rock mass, several metres thick. Unfortunately, no information on the kinematics were reported; the gallery is unusable. Nevertheless, the thrust plane is exposed at the surface. It consists of a metre thick contractional shear zone only exposed in few areas where structural analyses have been carried out (structural stations A, B and C indicated in figs. 2 and 4). On the whole, the tectonic event which gave rise to the thrusting (D 1 deformational event) produced minor structures (F 1 folds, S 1 tectonic foliation, reverse faults and shear fractures sets) within the thrust damage zone. Furthermore, the thrust and related minor structures were deformed by NE- and SE-verging F 2 folds system (fig. 4), described in the following paragraph. THRUST DAMAGE ZONE Thrusting produced a brittle-ductile contractional shear zone up to 3m thick, mainly affecting the basal succession of the overthrusted subunit SU1, composed by a shaly succession with scarce carbonate beds (Argilliti di Brolio, BRO in fig. 3B). The turbiditic succession (Macigno Fm, MAC in fig. 3B) belonging to the footwall block results intensely deformed only in its upper part, very close to the thrust plane, where the maximum shear deformation accommodated. Overall, the damage zone may be subdivided in three main structural domains (A, B and C structural domain) typified by different fabrics and deformation features (fig. 6). The deepest structural domain (A structural domain) is discontinuously exposed, but well represented in the structural station named C and shown in the figs. 2 and 4b. The deformation strongly affected the uppermost part (up to 1m) of the footwall succession composed of quartzfeldspar micaceous turbitides (Macigno Fm). The A structural domain (fig. 6) is characterised by pervasively fracturated rocks, resulting affected by shear fractures forming angles ranging from and with respect to the bedding surface (fig. 7A), sub-horizontal and about parallel with respect to the thrust plane. Shear fractures are closely spaced, up to fractures per metre, on average (fig. 7A). The B structural domain (fig. 6) is located at the top of the thrust plane. It is characterised by about 1-1,5 m thick of clayey gouge, occasionally containing disrupted millimetre to centimetre clasts of carbonate rocks originally interbedded within the shaly basal succession of the overthrusted sub-unit (Scaglia Toscana Fm, Argilliti di Brolio), and very altered centimetre arenaceous clasts belonging to the footwall turbiditic succession (Macigno Fm) (fig. 7B). The C structural domain ranges from 0,5 to 1 m in thickness (fig. 6). It affected the clayey succession of the Argilliti di Brolio, here characterised by shales with interbedded carbonate rocks (beds up to 5 cm of siliceous calcilutites and marly-limestones), and marly succession of the Marne del Sugame, characterised by marls and marly limestones several metres thick. The main structures characterising such a structural domain consist of decimetre to metre sized ESE-verging asymmetric folds (fig. 7C), often detached (fig. 7D). WNW-dipping reverse faults whit metre displacements have been also observed mainly affecting the marly lithotypes. The fault planes consist of clean surfaces, striated, locally sealed by millimetre thick fibrous calcite veins. Nor tectonic breccias neither fault gouges have been observed. KINEMATICS A, B and C structural domains (fig. 6) are not completely represented in the whole stations of structural

7 80 A. BROGI & L. FABBRINI Fig. 4 - Geological maps and cross-sections of two key areas illustrating the exposure of the Monte Penna thrust. Their location is given in fig. 2. Letters A, B and C indicate the stations of the structural analyses (see the text for more details). Carte geologiche e relative sezioni geologiche di due aree chiave che mostrano il sovrascorrimento di Monte Penna. La loro ubicazione è indicata in fig. 2.

8 THE MONTE PENNA THRUST, SOUTHERN TUSCANY 81 Fig. 5 - Geological section across a gallery mine traced in the fig. 2: a) Simplified geological section highlighting the relationships between the Tuscan Nappe Subunit 1 and Subunit 2, as reconstructed by field and mine data; b) Detail of the geological section as reconstructed through mine data which have been reinterpreted. The original data, mainly consisting of lithological information, are also given in the bars A and B located below the section; they indicate: A - lithological information from mining woks dated at 1929; B - lithological information from mining works dated at Key: 1) «Scisti policromi»; 2) «Sottonummulitico e strati di galestro»; 3) «Bancone»; 4) «Scisti del Creteceo»; 5) «Calcare e scisti a fucoidi». Sezione geologica attraverso una galleria mineraria la cui traccia è riportata in fig. 2: a) sezione geologica semplificata che mette in evidenza le relazioni tra la Sottounità 1 e la Sottounità 2 di Falda Toscana come ricostruito secondo i dati minerari; b) Dettaglio della sezione geologica come ricostruita attraverso i dati della galleria mineraria che sono stati reinterpretati. I dati originali, principalmente indicanti le informazioni litologiche, sono riportate nelle barre A e B sotto alla sezione geologica; essi indicano: A - informazioni litologiche dai lavori minerari del 1929; B - informazioni litologiche dai lavori minerari del I muneri indicano: 1) Scisti policromi; 2) Sottonummulitico e strati di galestro; 3) Bancone; 4) Scisti del Creteceo; 5) Calcare e scisti a fucoidi. Fig. 6 - Unscaled cartoon showing the three domains of the Monte Penna damage zone. See the text for explanation. Schema non in scala che mostra i tre domini riconosciuti entro la zona di taglio del sovrascorrimento di Monte Penna. Vedere il testo per maggiori dettagli. analyses given in the figs. 2 and 4. Particularly, in the Selvena area (structural stations named A and B, shown in figs. 2 and 4A) the C and B structural domains mainly occur. Differently, in the Castell Azzara area (structural station named C, shown in figs. 2 and 4B) the A structural domain is mainly represented. Different structures and fabrics characterise the A and C structural domains, but all structures confirm the top-to-the-ese sense of shear

9 82 A. BROGI & L. FABBRINI Fig. 7 - Examples of mesoscopic structures developed during the thrusting and recognised within the shear zone related to the Monte Penna thrust: A - R-R 1 Riedel shear fracture affecting the Macigno Fm (A structural domain); B - disrupted carbonate, turbiditic and clayey beds involved in the shearing (B structural domain); C - asymmetrical F 1 folds and related S 1 tectonic foliation (slaty-cleavage) affecting the Argilliti di Brolio (C structural domain); D - detached and isoclinal F 1 folds with related S 1 tectonic foliation (stylolitic-cleavage) affecting marly lithotype of the Marne del Sugame (C structural domain). Alcuni esempi di strutture alla scala mesoscopica sviluppate durante il sovrascorrimento e riconosciute entro la zona di taglio del sovrascorrimento del Monte Penna. Vedere il testo per maggiori dettagli: A - Fratture di taglio di Riedel R-R 1 sviluppate entro le arenarie del Macigno appartenenti al dominio strutturale C; B - porzioni di strati calcarei, arenacei e argillosi deformati entro la zona di taglio collegata allo sviluppo del sovrascorrimento e presenti entro il dominio strutturale B; C - pieghe asimmetriche F 1 e relativa foliazione tettonica S 1 (slaty-cleavage) che interessano i litotipi argillosi delle Argilliti di Brolio entro il dominio strutturale C; D - pieghe F 1 scollate ed isoclinali e relativa foliazione tettonica S 1 (clivaggio stilitico) che interessa i litotipi marnosi delle Marne del Sugame appartenenti al dominio strutturale B. as indicated by: i) the vergence of minor folds developed within the shear zone; ii) the attitude of the S 1 tectonic foliation related to the shearing; iii) the orientation of reverse faults and, iv) the angular relationships between Riedel shear fractures. Shearing was partitioned within the three different shear domains, mainly in response to their rheological properties inherited by the lithological composition. The R and R 1 Riedel shear fractures affecting the turbiditic beds of the Macigno Fm of the A structural domain recognised in the Castell Azzara area (structural station C, figs. 2 and 4B) are eastward dipping and N10 - N20 striking on average (fig. 8A). The angular relationships between the two fractures sets define a lineation N 15 striking (fig. 8A), perpendicular to the direction of the tectonic transport. This latter is top-to-the ESE as indicated by plunging of the R- and R 1 fractures sets. The clayey and marly rocks characterising the C structural domain were mainly folded and affected by a widespread and locally pervasive tectonic foliation (S 1 ) (sensu TWISS & MOORE, 1992) (fig. 7D). Folds (F 1 ), centimetre to decimetre in size, are often detached and asymmetric (fig. 7C), and show interlimb angles ranging from 10 to 30, resulting in subisoclinal to tight folds (RAM- SAY, 1967; RAMSAY & HUBER, 1987). Their hinge lines mainly strike N15, mostly plunge to the north, both vergence and facing (sensu BELL, 1981) being ESE-directed (fig. 8C). The F 1 axial planes are subhorizontal or very gently inclined to the west. The S 1 foliation, normally subhorizontal or WNW-dipping, frequently exhibits a

10 THE MONTE PENNA THRUST, SOUTHERN TUSCANY 83 Fig. 8 - Kinematic data (Schmidt diagram, lower hemisphere) from the A, B and C structural stations indicated in figs. 2 and 4: A) kinematic data (Schmidt diagram, lower hemisphere) from the C structural station (Macigno Fm); 1) great circles and poles of R shear surfaces; 2) great circles and poles of the R 1 shear surfaces; 3) great circles of the R and R 1 shear surface and the arrow indicating the sense of shear; 4) rose diagram showing the lineation of intersection between R and R 1 surfaces; B) kinematic data (Schmidt diagram, lower hemisphere) from the B structural station and photograph showing a reverse fault dissected by a normal fault with relative stereographic diagrams (Marne del Sugame); 1) great circles of reverse fault and arrow indicating the sense of movement; 2) hinge lines of drag folds occurring in the hangingwall of the reverse fault; C) kinematic data (Schmidt diagram, lower hemisphere) from the A structural station and photograph showing F 1 minor folds with relative stereographic diagrams (Argilliti di Brolio); 1) F 1 folds hinge lines; 2) contouring of the F 1 folds hinge lines. Analisi cinematica (diagramma di Schmidt, emisfero inferiore) relativa alle stazioni di analisi strutturali A, B e C riportate nelle figg. 2 e 4: A) analisi cinematica (diagramma di Schmidt, emisfera inferiore) realizzata nella stazione strutturale C e foto che illustra una faglia inversa dislocata da una faglia diretta con i relativi diagrammi stereografici (Macigno); 1) ciclografica e poli delle superfici R; 2) ciclografica e poli delle superfici R 1 ; 3) ciclografiche delle superfici R e R 1 ; la freccia indica il senso di taglio; 4) rose diagram delle lineazioni di intersezione tra le superfici R ed R 1 ; B) analisi cinematica (diagramma di Schmidt, emisfera inferiore) realizzata nella stazione strutturale B; 1) ciclografiche delle faglie inverse; la freccia indica il senso di trasporto tettonico (Marne del Sugame); 2) linee assiali delle pieghe di trascinamento presenti nel blocco di tetto; C) analisi cinematica (diagramma di Schmidt, emisfera inferiore) realizzata nella stazione strutturale A e foto che illustra pieghe minori F 1 ed i relativi diagrammi stereografici (Argilliti di Brolio); 1) linee assiali delle pieghe F 1 ; 2) contouring. subparallel attitude with respect to the bedding (hereafter S 0 ) in the pelitic dominant lithotypes, whereas S 1 intersects S 0 at higher angles in the marly rocks (fig. 7D). The S 1 morphology consists of a locally penetrative foliation within the pelitic lithotypes (slaty-cleavage). Contrarily, it consists of a stylolytic cleavage within marly beds. The S 1 slaty cleavage results very penetrative both at the fold hinges and limbs. Contrarily, the S 1 stylolytic cleavage

11 84 A. BROGI & L. FABBRINI Fig. 9 - Mesoscopic F 2 NE-verging and NE-facing folds affecting the Nummulite bearing calcarenites («Nummulitico» Auctt.) and relative stereographic diagram (Schmidt diagram, lower hemisphere) of: 1) hinge lines and axes of the F 2 folds; 2) contouring of F 2 hinge lines and axes. Pieghe F 2 alla scala mesoscopica, caratterizzate da una vergenza nord-orientale, che deformano calcareniti a Nummuliti (Nummulitico Auctt.) e relativo diagramma stereografico (diagramma di Schmidt, emisfera inferiore) che rappresenta: 1) linee assiali ed assi delle pieghe F 2 ; 2) relativo contouring. only occurs at the fold hinges. No dynamic recrystallisation has been observed on the S 1 foliation. Reverse faults characterised by decimetre to metre displacements have been also recognised (fig. 8B). Such faults mainly affected the most competent lithotypes such as the marly rocks, and developed at high angle (about ) with respect to bedding (fig. 8B). Such structures show WNW-dipping fault planes and are characterised by top-to-the ESE tectonic transport (fig. 8B). Any kind of structure, such as folds, tectonic foliation, reverse fault, shear fracture, useful to reconstruct the shearing, have been recognised within the B structural domain. This latter consists of a very disrupted tectonic level where the primary sedimentary features and rock fabrics have been completely obliterated. STRUCTURES DEFORMING THE MONTE PENNA THRUST The MPT as well as the related minor structures (D 1 deformational event) were deformed by a later folding event (D 2 deformational event) that produced cartographic-scale (figs. 4 and 5) and mesoscopic F 2 folds (fig. 9). They are characterised by NE to SE structural facing, and show SW- and NW-dipping axial planes ranging from 5 to 50 (fig. 9A). Their hinge lines, as well as their axial trending range from N135 to 220 on average (fig. 11C). F 2 folds are characterised by interlimb angles varying from 40 to 110 and predominantly belong to the 1B and 1C Ramsay s classes (RAMSAY, 1967). S 2 axial planar tectonic foliation rarely developed. The S 2 foliation only developed at the fold hinge zones mainly within the pelitic lithotypes. Here, the S 2 foliation mainly consists of a disjunctive, spaced discrete cleavage, with centimetre spacing. Within the carbonate rocks (limestones and marls), the S 2 scarcely developed, and becomes a spaced discrete stylolytic cleavage only developed at the F 2 hinge zones (fig. 9). No dynamic recrystallisation has been observed on the S 2 tectonic foliation. Low-angle detachments and later high angle normal to transtensional faults dissected (i) the F 2 folds, (ii) the previously developed MPT and (iii) the associated minor F 1 folds (fig. 8). The high angle faults dissected the lowangle detachments (fig. 10). DISCUSSION The structural setting documented in the Selvena- Castell Azzara area strongly supports for a polyphase folding evolution of the Tuscan Nappe.

12 THE MONTE PENNA THRUST, SOUTHERN TUSCANY 85 Fig Geological section across the western side of the Monte Penna. Its trace is given in fig. 2. Note the extensional structures (low-angle detachment and high angle normal fault) superimposed on the contractional structures. Sezione geologica attraverso il versante occidentale del Monte Penna. La traccia della sezione è riportata in fig. 2. Si notino le strutture estensionali, costituite da una faglia a basso angolo ed una ad alto angolo, che si sono sovraimposte alle strutture contrazionali. The internal doubling of the Tuscan Nappe succession in the study area is here documented for the first time. Nevertheless, Late Oligocene-Early Miocene quartzfeldspar sandstones (Macigno Fm) underlying the Eocene calcarenites and calcirudites were also pointed out by ELTER (1955). He explained such a «anomaly» as the result of cartographic-scale open fractures affecting the Eocene calcarinetes, filled by the overlying Macigno Fm. We have documented that the doubling of the stratigraphic succession here described was the result of a contractional structure: the Monte Penna thrust. The deformation related to the development of such a contractional structure (D 1 deformational event) gave rise to mesoscopic scale detached folds, shear fractures and a tectonic foliation consisting of slaty cleavage and/or stylolytic cleavage, mainly developed in the pelitic and marly lithotypes, respectively. These structures, only concentrated within the thrust damage zone, are the oldest contractional structures recognised in the study area. Such structures were folded by cartographic-scale F 2 folds (D 2 deformational event), as recognised in the Mt. Penna (fig. 4). F 2 folds are characterised by orientation and vergence (toward ENE or ESE) comparable with the F 2 fold system described in other places of southern Tuscany (ELTER & SANDRELLI, 1994; BONINI, 1999; BROGI & LAZ- ZAROTTO, 2002). The D 2 deformational event took place in a shallower structural level with respect to the D 1 structures, such as the MTP, as highlighted by the different deformational features. This tectonic frame agrees with the structural setting documented for the neighbouring areas (Monte Aquilaia- Monte Buceto, Monte Labbro, Bagnore, fig. 1) where the Fig Cartoon showing the hypothetical duplex system which affected the Tuscan Nappe in the Monte Amiata area as reconstructed by the geological evidence described in CRESCENTI & GIUSSANI (1969), CALAMAI et alii (1970), BROGI & LAZZAROTTO (2002), BROGI (2004a, 2004b), BROGI et alii (2005a, 2005b). Note that the stratigraphic horizon corresponding to the Scaglia Toscana Fm played as a decoupling horizon during the thrusting as documented in the study area. Schema che illustra l ipotetico sistema a duplex che ha interessato la Falda Toscana nell area del Monte Amiata come ricostruito da evidenze geologiche documentate in CRESCENTI & GIUSSANI (1969), CALAMAI et alii (1970), BROGI & LAZZAROTTO (2002), BROGI (2004a, 2004b), BROGI et alii (2005a, 2005b). Si noti come il livello argillitico appartenente alla Scaglia Toscana possa aver funzionato come orizzonte di disaccoppiamento durante lo sviluppo dei sovrascorrimenti come documentato nell area di studio.

13 86 A. BROGI & L. FABBRINI Fig Representative tectono-stratigraphic logs reconstructed for the indicated localities as documented in literature. The Monte Penna thrust shows a stacking pattern very similar with respect to other ones documented in the Mt. Amiata area. Rapporti tettono-stratigrafici come documentati in letteratura per alcune località dell area del Monte Amiata. Il sovrascorrimento di Monte Penna mostra alcune caratteristiche confrontabili con quelle documentate in altre aree della regione amiatina. F 2 folds deforming thrust planes have been described. In other places of the Northern Apennines (the Alpi Apuane region), a fold system (F 2 folds) deforming a regionally distributed tectonic foliation (S 1 ), possibly related to isoclinals F 1 folds affecting the Tuscan Nappe, were described since the 70s (PERTUSATI et alii, 1977). This picture, abundantly documented by several authors (CARMIGNANI et alii, 1991; AQUE et alii, 2002; CAROSI et alii, 2003, 2004a, 2004b) in the last decade, agrees with the tectonic setting documented in the study area and surroundings. In other words, we hypothesise that the contractional structures of the D 1 deformational event described in this paper can be compared, in terms of tectonic framework and age, with those described for the Tuscan Nappe in the Alpi Apuane region (see PERTUSATI et alii, 1977; CARMIGNANI et alii, 1991; CAROSI et alii, 2004a, 2004b). In this view, we hypothesise that the regional S 1 tectonic foliation described for the Alpi Apuane could be coeval, and developed in a similar tectonic framework, with the MPT and other thrusts documented in the Monte Amiata region. Nevertheless, the D 1 structures documented for the study area are local structural features, only concentrated within the damage zone of the MPT. This evidence possibly suggests that the Tuscan Nappe exposed in the Mt. Amiata region was affected by deformation during the D 1 deformational event in a shallower structural level with respect to the Tuscan Nappe exposed in the Alpi Apuane region. THRUST STACKING PATTERN The stacking pattern documented for the study area (fig. 3) indicates that the MPT produced the tectonic repetition of the uppermost part of the Tuscan Nappe succession (Scaglia Toscana and Macigno Fms forming the SU1, fig. 3). The overthrusted subunit (SU1) detached at the base of the Cretaceous red shales (Argilliti di Brolio, BRO in fig. 3) suggesting that this stratigaphic horizon played a fundamental role during the Tuscan Nappe deformation, acting as a regional decoupling level besides the Late Triassic evaporites (fig. 11). Both the red shales and evaporites levels consist of weak stratigraphic horizons within the Tuscan Nappe succession, giving rise to a multilayered setting characterised by the alternance of stratigraphic levels with different mechanical behaviour. During the stacking process the deformation mainly accommodated within the weak stratigraphic horizons, acting as suited levels for enucleating internal thrusting (fig. 11). This is confirmed not only by the stacking pattern of the MPT, but also by those described for the Tuscan Nappe exposed in the Monte Amiata area and surroundings, such as the Poggio Zoccolino (BROGI, 2004b; PANDELI et alii, 2005); Monte Aquilaia-Monte Buceto (BROGI & LAZZAROTTO, 2002; BROGI, 2004a, 2006b); Bagno Vignoni (BROGI et alii, 2005d; BROGI et alii, 2007); Monte Labbro (CRESCENTI & GIUSSANI, 1969); Bagnore (CALAMAI et alii, 1970) (fig. 12). All the thrusts doubling the Tuscan Nappe succession in the Monte Amiata region were characterised by eastward movement according to the general tectonic transport of the Northern Apennines belt (CARMIGNANI et alii, 2001 cum. bibl.). The whole data, coupled with those described for the Larderello Geothermal area (fig. 1B) (BROGI et alii, 2005c; BROGI & CERBONESCHI, 2007), indicate that the Tuscan Nappe was affected by a regional stacking process followed by a folding event (D 2 deformational event), during the Northern Apennines structural evolution. Although the occurrence of thrusts within the Tuscan Nappe has been recently described for the southern Tuscany, the following D 2 deformational event, mainly giving rise to F 2 folds, were described in the whole Tuscany (CARMIGNANI et alii, 1991; ELTER & SANDRELLI, 1994; STORTI, 1995; CAMPETTI et alii, 1995, 1999; FAZZUOLI et alii, 1998; BONINI, 1999; BROGI & LAZZAROTTO, 2002; MONTOMOLI et alii, 2001; BROGI, 2004a; CAROSI et alii, 2004a, 2004b). The relationships between F 2 folds and the low-angle detachments (CARMIGNANI et alii, 1991; ELTER & SAN- DRELLI, 1994; MONTOMOLI, 2002; BROGI & LAZZAROTTO, 2002; CAROSI et alii, 2004b; BROGI & CERBONESCHI, 2007) are still under discussion; we have no data contributing to the scientific debate. THRUSTING AGE Several considerations allow us to constrain the age of the MPT development: a) the Macigno Fm (Chattian-Aquitanian) was doubled during thrusting, implying that the tectonic activity was younger than the Aquitanian; b) the Ligurian Units were not involved in the duplex structure of the Tuscan Nappe; they overthrusted the Tuscan Nappe during the Late Oligocene-Early Miocene (CARMIGNANI et alii, 2001 cum. bibl.); c) the MPT and related minor structures were deformed by the F 2 folds and low-angle normal faults, developed, in southern Tuscany, during the Middle-

14 THE MONTE PENNA THRUST, SOUTHERN TUSCANY 87 Fig Cartoon showing the hypothetical duplex system which affected the Tuscan Metamorphic «basement» and the Tuscan Nappe resulting influenced by the occurrence of a multilayered crust characterised by weak stratigraphic horizons which strongly controlled the geometry of the trusts. Schema che illustra l ipotetico sviluppo del sistema a duplex che ha interessato il «basamento» metamorfico toscano e la Falda Toscana. Secondo questa ipotesi il sistema a duplex è stato fortemente condizionato dalla presenza di una crosta stratificata con presenza di orizzonti a comportamento relativamente duttile che hanno fortemente influenzato la geometria dei sovrascorrimenti. and Middle-Late Miocene (Late Langhian-Messinian), respectively (BROGI & LIOTTA, 2006; BROGI & CERBO- NESCHI, 2007). Following these consideration we can ascribe the development of the MPT to the Early Miocene (Aquitanian-Burdigalian). TUSCAN NAPPE VS TUSCAN METAMORPHIC «BASEMENT» DUPLEX STRUCTURE One of the timely theme dealing with the tectonic evolution of the Northern Apennines deals with the relationships between the tectonic evolution of the cover tectonic units (for instance the Tuscan Nappe) and the Tuscan metamorphic «basement». This is crucial for the knowledge of the structural evolution of the collisional edifice, and to understand the thin-skinned vs. thick-skinned tectonics of the Northern Apennines. The structural setting of the Tuscan Nappe in southern Tuscany shows important analogies with the structural setting described for the Tuscan metamorphic «basement» in the Middle Tuscan Range (COSTANTINI et alii, 1988; CONTI et alii, 1991; LIOTTA, 2002; LAZZAROTTO et alii, 2003; ALDINUCCI et alii, 2005; BROGI, 2008; CASINI et alii, 2008). Nevertheless, difference in the structural styles are related to the different structural levels where the deformation took place. The Tuscan Nappe shows a stacking pattern like to that of the Tuscan metamorphic «basement» exposed at the surface in the Middle Tuscan Range (COSTANTINI et alii, 1988; LAZZAROTTO et alii, 2003), or drilled at depth in the Larderello geothermal area (the so called «complesso a scaglie» in PANDELI et alii, 1991). The «complesso a scaglie» has been referred to a contractional duplexing developed during the Late Oligocene-Early Miocene (CARMIGNANI et alii, 2001 cum. bibl.), which produced thrusts with main eastward movement, affecting the Palaeozoic and Triassic siliciclastic successions of the Tuscan Domain. Such a thrusting gave rise to a thick tectonic pile drilled for at least 3500m in the neighbouring of the Larderello geothermal area (CAMELI, 1994). Nevertheless, although the thrusting within the Tuscan metamorphic «basement» and the Tuscan Nappe can be considered coeval, it developed at different structural levels as indicated by the different metamorphic imprinting. It has been estimated that the metamorphic units of the southern Tuscany suffered about 8-10Kbar and C during the collisional event. As a consequence, they were affected by a prograde green-schist to HP-LT metamorphism (FRANCESCHELLI et alii, 1986; GIORGETTI et alii, 1998; THEYE et alii, 1997; JOLIVET et alii, 1998; BRUNET et alii, 2000). Contrarily, the Tuscan Nappe was deformed at about 2,5Kbar and C during the collisional event as extimated for the La Spezia area, in the western side of the Northern Apennines (MONTOMOLI et alii, 2001; CAROSI et alii, 2003), and was affected by a regional (CER- RINA FERONI et alii, 1983; AQUE et alii, 2002) or localised (BROGI, 2006b) very low-grade metamorphism. The tectonic piles characterising the Tuscan metamorphic «basement» and the Tuscan Nappe were separated by a decoupling regional stratigraphic horizon corresponding to the Late Triassic evaporites (fig. 13). Such a stratigraphic horizon was involved in the duplexing of both the Tuscan metamorphic «basement» (BURGASSI et alii, 1979; BERTINI et alii, 1991, 1994) and the Tuscan Nappe (BROGI, 2004a; BROGI et alii, 2005c), suggesting the fundamental role of the Late Triassic evaporites during the Tertiary shortening of the Northern Apennines crust, decoupling