Tuscan Units Research Articles

Deformation and pressure-temperature-time history of the External Tuscan Units in the Northern Apennines (Italy): The case of the Punta Bianca Unit

In this study we investigated through a multidisciplinary approach the still poorly known tectono-metamorphic evolution of the Punta Bianca Unit in the Northern Apennines. The Punta Bianca Unit is part of the Tuscan Metamorphic Units, a group of units derived from the Adria passive margin, metamorphosed at different conditions, and forming the backbone of the Northern Apennine belt. We combined meso- and microstructural analyses, 40Ar/39Ar white-mica geochronology and multi-equilibrium geothermobarometry from high-resolution X-ray chemical maps, to unravel the deformation and metamorphic history of this part of the belt. Meso- and microstructural data indicate that the Punta Bianca Unit recorded two main phases of ductile deformation (here referred to Dp-1 and Dp) associated with syn-kinematic growth of K-white mica, chlorite, calcite, quartz on the related tectonic foliations (Sp-1 and Sp), followed by a later ductile deformation phase (Dp+1) lacking of metamorphic blastesis. P-T estimates complemented by microstructural data suggest that peak metamorphic conditions reached ∼0.8 GPa and ∼350°C and occurred synchronously with the first deformation phase (Dp-1). Temperature values were also confirmed by Raman spectroscopy of carbonaceous material on selected samples. This stage was followed by the exhumation of the Punta Bianca Unit, as testified by decreasing pressure and temperature down to ∼0.4 GPa and ∼300°C respectively, together with the development of the main foliation (Sp). At the regional scale, the Tuscan Metamorphic Units have been mostly affected by HP-LT metamorphic gradients equilibrated under blueschist-facies conditions (up to ∼1.4 GPa). Results from the present work on the contrary, suggest that the Punta Bianca Unit never reached such HP-LT conditions, testifying that it was deformed at relatively upper structural levels, thus highlighting an important variation in the tectono-metamorphic evolution of the Tuscan Metamorphic Units along strike in the Northern Apennines. 40Ar/39Ar laserprobe data (using both the in-situ and step-heating techniques) indicate a minimum age for the onset of continental subduction of ∼20 Ma (Dp-1), which was followed in close succession by exhumation at ∼16 Ma. This approach, if applied to different tectonic units building up the nappe pile of the Northern Apennines, could be successful in better unravelling the tectonic history.

Coexistence of contractional and extensional tectonics during the northern Apennines orogeny: the late Miocene out‐of‐sequence thrust in the Elba Island nappe stack

AbstractIn the northern Tyrrhenian Sea, the Elba Island is one of the westernmost portions of the northern Apennine inner belt. One of its noteworthy features is the anomalous tectonic repetition of continental‐derived (Tuscan Unit) and oceanic‐derived (Ligurian units) thrust sheets, lately intruded by late Miocene granitoids. Moreover, in detail, a slice of strongly deformed Ligurian peridotites results tectonically sandwiched between two thrust sheets of Tuscan units. This tectonic setting results from a middle Miocene folding and thrusting of the Apenninic nappe stack with development of large‐scale antiform and out‐of‐sequence thrust. In central‐eastern Elba Island, the folding of an imbricate stack is bracketed between Langhian (middle Miocene) and Messinian (late Miocene). Consequently, the anomalous repetition of Tuscan and Ligurian units thrust sheets gives evidence of middle‐late Miocene shortening deformation post‐dating nappe stack and pre‐dating late Miocene–Pliocene granite emplacement. We suggest that the architecture of the Elba Island nappe stack documents the coexistence of early‐middle Miocene contractional and extensional tectonics in an overall convergent tectonic setting in the westernmost zone of northern Apennines. Extensional tectonics in the upper portion of the wedge, balancing transient gravitational instabilities due to over‐thickened conditions, were followed by a renewal of contractional deformation leading to development of large‐scale out‐of‐sequence thrust responsible for inversion of the stack order. Copyright © 2016 John Wiley & Sons, Ltd.

Reply to: Barazzuoli P., Bertini G., Brogi A., Capezzuoli E., Conticelli S., Doveri M., Ellero A., Gianelli G., La Felice S., Liotta D., Marroni M., Manzella A., Meccheri M., Montanari D., Pandeli E., Principe C., Ruggieri R., Sbrana A., Vaselli V., Vezzoli L., 2015. COMMENT ON: “Borgia, A., Mazzoldi, A., Brunori, C.A., Allocca, C., Delcroix, C., Micheli, L., Vercellino, A.,

The volcanic spreading model by Borgia et al. (2014) is accurate in describing the extensional structures found on the edifice and the radial compressional structures existing all around the base of Amiata Volcano. Volcanic conduits, extensional structures, and direct contact between the volcanic rocks and the Tuscan Units, constitute the hydraulic connection between the potable fresh-water aquifer contained in the volcanites and the underlying hydrothermal system. Therefore, gaseous phases tend to flow upward (particularly through faults) carrying pollutants into the freshwater aquifer, while the freshwater recharges (also through primary permeability) the exploited geothermal fields.

Detrital fission-track-compositional signature of an orogenic chain-hinterland basin system: The case of the late Neogene Quaternary Valdelsa basin (Northern Apennines, Italy)

Detrital thermochronological data collected in syn-tectonic basin deposits are a promising tool for deciphering time and processes of the evolution of orogenic belts. Our study deals with the Valdelsa basin, one of the wider basins of central Tuscany, Italy. The Valdelsa basin is located at the rear of the Northern Apennines, a collisional orogen whose late Neogene Quaternary development is alternatively attributed to extensional and compressional regimes. These contrasting interpretations mostly rely on different reconstructions of the tectono-sedimentary evolution of several basins formed at the rear of the chain since the late Tortonian. Here, we explore the detrital thermochronological–compositional signature of tectonic and surface processes during the Valdelsa basin development. For this aim, detrital apatite fission-track analysis of 21 sand samples from the latest Messinian Gelasian fluvial to shallow marine basin deposits, has been accompanied by a clast composition analysis of 7 representative outcrops of the conglomerate facies. The grain-age distributions of the sediment samples are generally characterized by two distinct components, one younger peak (P1) varying between 5.5±2.8 and 9.5±1.0Ma and one older peak (P2) varying from 15.0±8.0 to 41.0±10Ma. By comparison with some bedrock ages obtained from the E-NE basin shoulder, we attributed the P2 peak to the Ligurian Units and the P1 peak to the Macigno Formation (Tuscan Units). These units are arranged one upon the other in the complex nappe pile forming the Northern Apennines orogen. While the gravel composition indicates a predominant feeding from the Ligurian units all along the sedimentary succession with a subordinate occurrence of Macigno pebbles slightly increasing upsection, the P1 peak is present even in the oldest collected sandy sediments. The early P1 occurrence reveals that the Macigno was exposed in the E-NE basin shoulder since at least the latest Messinian–early Zanclean. This means that deeper part of the structural edifice was exposed earlier than previously reported. This was likely made possible by exposures of Macigno at the cores of anticlines or on the footwall of backthrusts. Thus, the Macigno early supply claims a role for shortening in the Northern Apennines hinterland basin evolution. Our study revealed the suitability of the detrital thermochronological/compositional analysis on syn-tectonic infill of hinterland basins in discriminating between different tectonic processes affecting the late evolution of an orogenic chain.

Markers of MIS 5 sea level fluctuations in the Bergeggi Marine Cave (NW Mediterranean Sea)

REGIONAL SETTING The onshore territory of the Liguria region (Fig.1) is represented by an arch-shaped mountain ridge, presenting a steep seaward slope and with its watershed very close to the Mediterranean sea. From a geological point of view, the region can be divided into two parts. The western part of the arch (west of Genova), NE–SW oriented, is considered to be the southern continuation of the Western Alps (Ligurian Alps, Vanossi et al., 1994. In this part, Provencal, Flysch, SubBrianconneese, Brianconneese, Piedmonteese, and metaophiolitic units and related metamorphic covers outcrop. West of Genova (Fig.2) an ophiolitic complex (named the Voltri Group) marks the transition between the Alpine and the Apenninic domains (Cortesogno & Haccard, 1984) (the latter stretching NW–SE), where Ligurian, SubLigurian and Tuscan units outcrop (Gianmarino et al., 2002). Fig.1 Geographical position of the Liguria Region in the Italian peninsula

A sagging along the eastern Chianti Mts., Italy

A deep-seated gravitational slope deformation (DGSD) affects the eastern side of the Chianti Mts. Ridge. It develops in an N−S to NW−SE direction and is > 10 km wide and 3-4 km long. This area corresponds to the eastern side of a main antiform, characterised by east-verging folds and thrusts involving bedrock of the Mesozoic−Paleogene Tuscan Units, particularly sandstones containing interlayered highly fractured and deformed Ligurian rocks (shales and limestones with olistostromes). The foot of the slope is characterised by tilted Plio-Pleistocene deposits unconformably sealing the bedrock structures as folds, thrusts and faults. The most significant morphological features are a main escarpment, trenches, several secondary and counter-slope escarpments that together indicate large-scale gravitational phenomena. The main escarpment is responsible for the headward retreat of the slope, and is deeply segmented by numerous arcuate niches that reveal differential movements of single blocks. The DGSD is also dissected by SW−NE trending streams that often deepen inside the N−S trenches. Minor landslides due to local instability are also present. At the foot of the slope, the older continental Pliocene deposits of the Upper Valdarno Basin crop out. Although tilted by tectonic movements, the deposits have not been severely affected by gravitational deformations. This indicates that the movement is a typical sagging, a large landslide at an embryonic stage, affecting the upper part of the slope but not reaching the valley bottom. The deformations are absorbed in the rock mass which is also partially drained by stream incision that prevents high pore pressure. The occurrence of down-slope and down-movement facing escarpments and up-slope and up-movement facing counter-slope escarpments indicate a sagging characterised by a listric spoon-shaped geometry. The DGSD has a style similar to crustal extensional tectonics such as Morton and Black's crustal attenuation model. Although few chronological indications of movements are present, the fact that Late Pleistocene debris deposits, widespread in the northern and central Apennines, are not found at the contact between the escarpment and the trenches suggests a post-glacial activity for at least part of the movements. Recognizing embryonic-stage collapse is of primary importance in assessing geological hazard and risk because rapid evolution and collapse could follow.

The Barga Basin (Tuscany): A record of Plio-Pleistocene mountain building of the Northern Apennines, Italy

The Barga Basin, located between the Apuane Alps and the Northern Apennines, was the site of a fines-dominated meandering alluvial plain during the Early-Middle Pliocene. Rivers came from the Tyrrhenian side, across the Apuane Alps, and carried gravels mostly composed of Tuscan units that capped the metamorphic complex. Apatite fission tracks indicate that the metamorphic core of the Apuane Alps was deeper than 4 km. To the east, the Apennine divide did not exist and the area was part of the Po plain. A major unconformity marks the transition to the Late Pliocene–Early Pleistocene sequence, mostly made of metamorphic gravels deposited by braided rivers still coming from WNW. At ca. 2 Ma, the Apuane Mountains, following uplift, underwent severe denudation. Wind gaps along the Apuane ridge suggest that the drainage still partially extended westward in the area that is now drowned under the Ligurian-Tyrrhenian sea. Since the Early Pleistocene, rapid uplift of the Apennine Ridge led to the present-day drainage and the deposition of two alluvial terraces. The older terrace at Monte Perpoli is scarcely represented while the Late Pleistocene one is made of large west-flowing telescopic alluvial fans. Since late Early Pleistocene, uplift of the Apennine divide, associated with the activity of high-angle normal faults, generated reversed rivers. During this period the activity of the Versilia fault isolated the Apuane ridge and further drowned the eastern Ligurian basin. A new interpretation and timing of the tectonic evolution of this sector of the Apennine and the adjacent Tyrrhenian sea and peri-Adricatic Basin emerges.

Late tectonic evolution of the Northern Apennines: the role of contractional tectonics in the exhumation of the tuscan units

Structural analysis performed in the continental tuscan units of the Northern Apennines (Italy) highlight the presence of transversal and parallel structures, with respect to the main trend of the belt (NW-SE), affecting the tectonic nappe pile. These structures, following the main synmetamorphic phases associated to isoclinal folds with axial plane foliations, are upright folds, brittle and brittle-ductile shear zones. Nearly upright folds are related to NW-SE and NE-SW directions of shortening. The two perpendicular directions of shortening, associated with a contraction setting, induced the vertical growth of the metamorphic domes enhancing the process of exhumation of the metamorphites. The presence of later NE-verging brittle-ductile shear zones crosscutting both systems of folds testifies that contraction long lasted during the orogenesis. Fluid inclusion analyses, performed in syntectonic veins coeval with NW-SE trending upright folds, point out that, after the first tectonic phase, pressure values decrease supporting that this tectonic unit was exhuming during the development of upright folds. Post-collisional extensional tectonics gave rise to collapse folds and low- and high-angle normal faults in the latest stages of deformation.

Late Miocene remagnetization within the internal sector of the Northern Apennines, Italy

Paleomagnetic and geologic evidence indicates that Upper Jurassic radiolarian cherts of both the Tuscan Cherts Formation (continental margin, Tuscan Units) and the Monte Alpe Cherts Formation (oceanic crust, Ligurian Units) were remagnetized during Miocene orogenesis of the Northern Apennines of Italy. Characteristic overprint magnetizations with reversed polarities have been found over a large area within the internal sector of the Northern Apennines, including eastern Liguria, Elba Island and the Thyrrenian margin, and west of the Middle Tuscan Ridge. The reversed-polarity overprint (average direction: D=177°, I=−52°, α 95=15°) was most likely acquired during Late Miocene uplift and denudation of the orogenic chain, and thermochemical remagnetization was a probable consequence of increased circulation of orogenic fluids. Similarly, mostly reversed-polarity directions of magnetization have been found by other workers in overlying post-orogenic Messinian sediments ( D=177°, I=−57°, α 95=3°), which show little counterclockwise (CCW) vertical-axis rotation with respect to stable Europe (−8±5°). The Monte Alpe Cherts sampled at sites in the external sector of the Northern Apennines, close to major tectonic features, have normal-polarity overprint directions with in situ W–SW declinations. Since the overlying post-orogenic Messinian sediments have not been substantially rotated about vertical axes, the evidence points to an earlier, pre-Late Miocene remagnetization in the external parts of the orogenic chain.

THE EVOLUTION OF THE LIGURIAN UNITS IN SOUTHERN TUSCANY: A NEW PROPOSAL

To attempt the reconstruction of the stratigraphic – tectonic framework of the oceanic units pertaining to the Ligure – Piemontese Ocean (Ligurian Units Auctt.: Helminthoid Flysch Units, Vara Units, Lanciaia Fm.) cropping out in Southern Tuscany, sedimentological, biostratigraphical, petrographical and structural studies were performed. These analyses were carried out in some key outcrops (Gambassi-Montaione; S. Donato in Poggio; Northern Monti della Gherardesca; Micciano-Libbiano and Serrazzano; Lanciaia-Montecastelli Pisano,). Moreover, field data were integrated with those coming from the geothermal wells. The biostratigraphic and petrographic analyses and the sedimentological observation permit to group the Cretaceous helminthoid flysches of these areas in a single formation, the Monteverdi Marittimo Fm., subdivided in three members: i) Larderello Member deposited in the middle part of the basin,, ii) Montaione Member, deposited in the inner (western?) part and close to an ophiolitic ridge because of the occurrence of ophiolitic olistoliths and breccias iii) San Donato Member, that represents the distal eastern part of the turbiditic system. Despite a complex polyphasic evolution, the piling up of four main Ligurian tectonic units can be recognized (from bottom to top of the pile): Castelnuovo Val di Cecina Unit (CVU), Vara Unit (VU) and Lanciaia Fm., Castelluccio Unit (CU), Montignoso Unit (MU). The Lupicaia Creek Unit (LU) is also present locally at the top of the pile (Montecatini Val di Cecina and south of the Montaione-Gambassi). In the Early and to Late Paleocene, the thrust of the VU onto the Monteverdi M.mo Fm. (CVU) happened and was sealed by the sedimentation of the Lanciaia Fm. which can be considered an earlier Epiligurian unit. Since Late Eocene also the Lanciaia Fm. was deformed by the out-ofsequence thrust of the CU and, finally, of the MU. Preliminary structural data point out to two main tectonic events, at least perpendicular, that affect the Ligurian Units. The first one (axes direction approximately E-W) strongly affects all Ligurian Units while is absent in the Tuscan Units and in the neogenic sediments. On the other hand the second tectonic event (axes direction NW-SE) affect only the Tuscan Unit and the neogenic sediments and refold the first event. In particular, the structural data point out, at least for the Ligurian Units of this sector of the Northern Apennines, to a structural evolution likely linked, until the Middle – Upper Eocene, to a transpressive tectonic context probably focused along the transform fault of the Ligure-Piemontese Ocean.

THE BARGONASCO - UPPER VAL GRAVEGLIA OPHIOLITIC SUCCESSION, NORTHERN APENNINES, ITALY

The Northern Apennines ophiolitic successions pertain to the Ligurian orogenic units, which were thrust onto the Adria continental margin (Tuscan and Umbrian Units) and show two different types of stratigraphic settings (Fig. 1) (Abbate et al., 1980): a- as “oceanic basement” of the Vara Supergroup, one of the higher units of the tectonic pile (“Internal Ligurides”); b- as olistoliths of variable sizes, accompanied by ophiolitic breccias, included in the Upper Cretaceous - Eocene terrigenous successions of the “External Ligurides” tectonic units. The Val Graveglia-Bargonasco area represented in the map, is located in eastern Liguria, on the northwesternmost side of the Northern Apennine, where most and more complete ophiolitic sequences of type a- crop out (the “Mt. Porcile Unit, Fig. 1). They consist of mantle peridotites (from clinopyroxene-poor lherzolites to harzburgites) and gabbros, which form the “oceanic basement”, metamorphosed and tectonically denudated before the volcano-sedimentary cover (ophiolitic breccias, basalts, cherts, carbonate and carbonate- shaly pelagites, and finally siliciclastic turbidites) was deposited. The tectonic scheme (Fig. 1, partially modified from the “schema tettonico” of the geologic map) shows that the Porcile Unit, object of our study, includes seven subunits separated by minor thrusts, i.e., from west to east, Mt. Domenico, M. Bianco, Rio Gromolo, Mt. Ciazze, M. Bocco, Rivo Orti, Mt. Tregin Subunits. The stratigraphic successions of all the Subunits pertain to the Vara Supergroup (see the stratigraphic Scheme on the geologic map). This short note summarises a more extensive one, about the geologic evolution of this same area, which will be published in the Atti Ticinensi di Scienze della Terra, Pavia, Italy, 2004. We decided to write this short summary to advertise the map of this key area for understanding the geology of the Northern Apennine ophiolites.

THE HP-LP META-OPHIOLITIC UNIT AND VERRUCANO OF THE CALA GRANDE AREA IN THE ARGENTARIO PROMONTORY (SOUTHERN TUSCANY, ITALY): STRUCTURAL-METAMORPHIC EVOLUTION AND REGIONAL CONSIDERATIONS

Ophiolite-bearing HP-LT metamorphic rocks (Schistes Lustres or Calcschists with ophiolithes Auctt.) are present in southern Tuscany and Tuscan Archipelago (Mt. Argentario Promontory, Giglio Island, Elba Island and Gorgona Island). They are interpreted as fragments of oceanic crust which experienced an initial HP-LT metamorphic evolution during the closure of the Ligurian ocean and successively, mooving eastwards, were enclosed together the Ligurian and Tuscan Units in the Apennine chain. The HP-LT ophiolitic succession of the Mt. Argentario is that of the Cala Grande Unit which is constituted by grey-green to red-violet metapelites, marble, calc- schists, metacherts and associated metamorphic ophiolitic rocks (metaserpentinites, metagabbros and metabasalts). The petrographical-structural studies performed by the authors allow to reconstruct the complex metamorphic and tectonic evolution of such successions. In the metabasites, the primary femic magmatic minerals and those of the oceanic metamorphism are overprinted by: 1) a polystadial HP-LT event in the blueschist facies (Dp, peak conditions P = 7-8kb for T = 340-350°C); 2) a greenschist facies event(s) characterized by the chloritization of the Na-amphiboles, development of a main penetrative schistosity and of top-to-NE and top-to-SW shear bands. In the associated metasediments, the HP-LT event is testified by the schistosity of the NE-vergent Dp event which is overprinted by two greenschists tectono-metamorphic events (NE-vergent Dgs1 and SW-vergent Dgs2). The tectonic slices of Verrucano (Triassic quartz-rich metasediments of the Tuscan Monticiano-Roccastrada Unit) intercalated in the Cala Grande Unit show a similar structural-metamorphic evolution, but with different orientation of the mineralogic lineations within the D1 foliations. This data point to a post-D1 emplacement of the Verrucano slices onto the Cala Grande Unit, probably during the D2 event of the Northern Apennines tectogenesis. The structural-metamorphic evolution of the studied area is also compared to that of other metamorphic successions of the Tuscan Archipelago and of the Southern Tuscany.

STRUCTURAL EVOLUTION OF ANCHI-/EPIMETAMORPHIC UNITS OF CENTRAL AND EASTERN ELBA (ORTANO, ACQUADOLCE, MONTICIANO-ROCCASTRADA AND GRASSERA UNITS)

The complex framework of the Elba tectonic pile has always been a matter of debate within the geology of the Northern Apennines and of the Tyrrhenian area. The structural data collected on the epi- and anchimetamorphic units of central and eastern Elba (Ortano Unit, Acquadolce Unit, Monticiano-Roccastrada Unit and Grassera Unit) allow more detailed reconstructions of the complicate tectonic evolution of the Elba stack. Because of their similar structural framework, the Ortano Unit and the Monticiano-Roccastrada Unit acquired their main tectono-metamorphic imprints together as part of a single Paleozoic to Tertiary Tuscan-type succession. The probable Ligurian Piedmontese Acquadolce Unit, which occurs tectonically intercalated between the Tuscan epimetamorphic units, is characterized by a different tectono-metamorphic framework (e.g. shears show opposite sense relative to the Tuscan units). Moreover, the anchimetamorphic Grassera Unit, that is another possible Ligurian Piedmontese unit, is located in between the non-metamorphic Ophiolitic Unit and the Tuscan Nappe. The present, complex piling up of Tuscan and Piedmontese units occurred within the time interval of 19 Ma (according to the radiometric age of the main foliation of the Acquadolce Unit) and 5 Ma (according to the radiometric age of the La Serra-Porto Azzurro granitoid which produced a well preserved thermometamorphic aureole throughout the stacked units). In more recent times (5-2 Ma), the uprising of the cooled La Serra-Porto Azzurro magmatic body remobilized the eastern Elba tectonic pile onto the Porto Azzurro Unit by low-angle detachment faults (Zuccale Fault). The whole Eastern Elba tectonic pile has been affected by a final, weak folding phase which occurred either during or immediately after the Zuccale Fault, and then by high angle normal faulting which is characterized be hematite-rich mineralizations.

3 - SOUTHERN TUSCANY - A - ARGENTARIO PROMONTORY

The Argentario Promontory consists of a complex stack of Tuscan, Ligurian and Ligurian-Piedmontese Units (Lazzarotto et al., 1964; Decandia and Lazzarotto, 1980). From bottom to top they are (Fig. 1): 1. Monticiano-Roccastrada Unit. It comprises, from bottom to top: a- Mt. Argentario Formation. Graphitic phyllites with metasiltstone interbeds grading upwards into grey-whitish quartzites and metaconglomerates, with local graphite-rich phyllitic intercalations. This formation has been correlated to the Upper Carboniferous-Lower Permian successions cropping out along the Mid Tuscan Ridge (e.g. the St. Lorenzo Schists of the Pisani Mts. and Iano Schists and Sandstones) and in eastern Elba (Rio Marina Fm.). b- “Verrucano”. Alternating violet and greenish phyllites with local carbonate nodules and levels (caliche), quartzites and quartzitic white-pink metaconglomerates of Late Ladinian?- Carnian age. This succession, which shows an overall fining- and thinning-upward trend (including many metric/decametric positive cycles), is related to continental braided rivers passing upward to a coastal plain environment (Uncini, 1985). In the syntectonic quartz-rich segregations of the “Verrucano” rocks, Theye et al. (1997) found Mg-carpholite which testifies a HP-LT event (P> 8kb T = 300-400°C). c- Tocchi Formation. It consists of vacuolar, polymictic breccia made up of yellowish carbonatic and grey to green phyllitic clasts. These diagenetic and/or tectonic breccias derive from Carnian siliciclastic-carbonatic lithotypes which constitute the transition from the continental “Verrucano” and the Norian carbonatic shelf deposits. 2. Tuscan Nappe. This non-metamorphic unit is constituted by the “Calcare Cavernoso”: grey and dark grey, vacuolar calcareous and calcareous-dolomitic breccias with local karst structures. At places, these breccias include boulders and slices of stratified calcareous-dolomitic successions. Cataclastic textures are frequently recognisable. 3. Cala Piatti Unit. It is made up of Triassic recrystallised carbonate succession: a. Lower?-Middle Triassic stratified grey-pinkish limestones, b. Middle-Upper Triassic stratified black limestones, c. Upper Triassic? massive or poorly bedded grey dolostones. This succession represents one of the rare evidence of Early?-Middle Triassic marine sedimentation in the Northern Apennines (see also the Punta Bianca-Massa succession: Martini et al., 1986). 4. Cala Grande Unit. It is made up of HP-LT metamorphic rocks. In particular, two lithologic associations are recognisable: a. Varicoloured (reddish and grey-green) phyllites with grey and dark grey calcschists and recrystallised limestones; b. Metaophiolites (gabbros, basalts, serpentinites and ophiolitic breccias). Particularly, in b. HP-LP minerals are present: glaucophanitic amphibole (Mgriebeckite to crossite)+lawsonite+phengitic mica±Na-pyroxene (Gottardi, 1957; Ricci, 1972; Azzaro et al., 1977; Theye et al., 1997). The presence of Fe-carpholite relics within syn-tectonic quartz veins of the metasedimentary lithotypes (Theye et al., 1997) suggests T= 300-350°C and P>7 kb peak metamorphic conditions in agreement with the Blueschists facies minerals of the Metaophiolites. This unit was correlated to the “Schistes Lustres” of the Alpine Corsica (Ricci, 1972; Ricci and Serri, 1975). Polymictic tectonic breccias, mainly made up of elements and slices of “Calcare Cavernoso” and/or “Verrucano” (but locally also of the two uppermost units), separate the Tuscan Nappe from the overlying Cala Piatti Unit and this latter from the Cala Grande Unit. The complex tectonic stack of Mt. Argentario is related to the Oligo-Miocene compressional stage (which produced the piling up of the “Schistes Lustres” onto the Tuscan Units and the metamorphism of the Monticiano-Roccastrada Unit), but also to the post-compressional events (Burdigalian- early Tortonian) linked to the extensional tectonics of the Apennines and the Tyrrhenian Sea area (Bartole, 1995; Carmignani et al., 1995, cum bibl.). During the extensional events, the tectonic lamination of the Tuscan Nappe through low-angle normal faults, and the formation of the Mt. Argentario polymictic breccias occurred.

3 - SOUTHERN TUSCANY - B - COLLINE METALLIFERE

In the last decades the geology of Southern Tuscany (Giannini et al., 1971; Boccaletti et al., 1982, cum bibl.) has been greatly improved by lithostratigraphic, biostratigraphic and geophysic studies, and by deep geothermal drillings (with a depth down to 3000 m b.g.l.) (Bertini et al., 1991; 1994; Elter and Pandeli, 1996, cum bibl.). The geological setting of this area is composed, from top to bottom, of the following tectono-stratigraphic units (Figs. 1 and 2): a. Neo-autochthon (post-orogenic). It is formed by the sediments filling the extensional basins, generally oriented along to the Apenninic chain. They unconformably lie on the stack of Ligurian and Tuscan Units, and are represented by lacustrine-marine (late Tortonian-Messinian), marine (Pliocene) and lacustrine-fluvial (early Villafranchian-Quaternary) sediments. b. Ligurian, sub-Ligurian and epi-Ligurian Units. They are composed of five tectonic units: 1- Upper Ophiolitic Unit (Late Jurassic-Early Cretaceous: Vara Supergroup), largely represented by Palombini Shales with some ophiolitic outcrops. 2- Lower Ophiolitic Unit, (Jurassic-Early Cretaceous: Vara Supergroup), including thick ophiolitic successions. This unit is stratigraphically capped by: 3- The syn-orogenic Upper Paleocene- Middle Eocene epi-Ligurian Lanciaia Fm. (ophiolitic breccias and sandstones, grading upward to a mainly carbonate-siliciclastic flysch). 4- Monteverdi Marittimo Unit (Southern Tuscany Elminthoid Flysch Group): consisting of a calcareousmarly- arenaceous flysch of Late Cretaceous- Early Paleocene age. 5- Canetolo Unit (Paleocene- Middle/Late Eocene), made up of shales with limestone, calcarenite and graywacke intercalations. This unit belongs to the sub-Ligurian paleogeographic Domain (close to the Adria margin). c. Tuscan Nappe. It is composed, from top to bottom, of the following non-metamorphic formations: Macigno (Late Oligocene-Early Miocene), “Scisti Policromi” (Early Cretaceous- Oligocene), Tuscan Cherts (Malm), Posidonia Marlstones (Dogger), “Rosso Ammonitico” and Limano Cherty Limestones (Middle-Late Liassic), “Calcare Massiccio” (Late Triassic?-Early Liassic), Spezia Fm. (Rhaetian), Burano Anhydrites (Norian-Rhaetian). In this succession lateralvertical facies changes and sedimentary unconformities are present, mainly in Middle-Late Jurassic times. Furthermore, the Tuscan Nappe is often tectonically laminated or locally lacks (“Serie Toscana Ridotta” = Reduced Tuscan Series Auctt.) by low-angle extensional faulting with flat-ramp-flat geometry (Fig. 3). The Tuscan Nappe lies on the Tuscan Metamorphic succession (Monticiano-Roccastrada Unit) which is quoted below. d. Monticiano-Roccastrada Unit. The upper part of this unit is a complex pile of tectonic slices made up of epimetamorphic Lower to Upper Paleozoic formations, Triassic “Verrucano”, Tocchi Fm. and non-metamorphic Upper Triassic anhydrite-carbonate successions. The lower part is made up of ?Upper Cambrian-?Lower Ordovician siliciclastics and metabasites (Phyllitic-quartzitic Complex), which include relics of Hercynian schistosity. In the subsurface of Larderello geothermal area, the Phyllitic-quartzitic Complex overlies ?Lower Paleozoic/?pre-Paleozoic Micaschist Complex (see Stop 5). The Colline Metallifere area is characterised by high heat flow (>650 mWm-2 in the Larderello geothermal field), by lithosphere thinning (Moho at a depth of about 22 km in the Grosseto area), by the uplift of the metasomatised and partially melted mantle, and by anatectic crustal magmatism (Boccaletti et al., 1985; Mongelli et al., 1991).

A TRANSECT IN SOUTHERN TUSCANY, FROM THE BACCINELLO BASIN TO THE CETONA RIDGE

The Northern Apennines (NA) is a NE-verging thrust and fold belt (Fig. 1) produced by the Cenozoic collision between the Corso-Sardinian block and the Adria Plate (Boccaletti and Guazzone, 1972; 1974; Principi and Treves, 1984; Malinverno and Ryan, 1986, among others). The NA thrust-nappe pile consists of sequences deposited on the Adria Plate continental margin (Tuscan and Umbria-Marche Units) which were overthrust by the Ligurian Units during continental collision. The Ligurian Units, which consist of ophiolites and their sedimentary covers, represent the remnants of the Ligurian-Piedmont paleo-ocean (Abbate et al., 1970). The NA evolution, after the Oligocene continental collisional phase, has been referred to two different models. In the first one, the development of an external thrust belt shifting eastwards, coupled with an internal extensional area is envisaged. In this process both the crust and the lithosphere would be involved (Merla, 1951; Boccaletti and Guazzone, 1974; Elter et al., 1975; Carmignani et al., 1980; Boccaletti et al., 1990; Patacca et al., 1990). In front of the migrating chain a foredeep basin developed during Late Oligocene-Miocene, whose deposits were progressively annexed to the chain (Ricci Lucchi, 1986; Boccaletti et al., 1990). In the second model, the accretionary prism, thickened by crustal collision, would collapse to recover its equilibrium conditions (Carmignani and Kligfield, 1990). As a consequence of the gravitational collapse, the development of core complex structures and of extensional tectonics has taken place since Burdigalian-Langhian (Carmignani et al., 1994). In this model, the Northern Apennines thrust belt is explained as due to gravitational tectonics active since the Early Miocene (Decandia et al., 1993; Carmignani et al., 1995). Starting in late Tortonian, the internal area (hinterland) of NA was characterized by the development of continental and marine basins, mostly striking from NW-SE to N-S, and subparallel to the main thrust fronts of the chain. Their formation has been commonly referred to the extensional processes related to the opening of the Tyrrhenian Basin, either in a back-arc regime (Boccaletti and Guazzone, 1972; 1974; Malinverno and Ryan, 1986; Royden et al., 1987; Boccaletti et al., 1990; Patacca et al., 1990), or as due to late orogenic gravity collapse (Carmignani and Kligfield, 1990; Cameli et al., 1993; Carmignani et al., 1994; Keller et al., 1994). The eastward younging trend of the hinterland basins has been related to the eastward shifting of the extensional front, which followed the migration of the thrust front (Merla 1951; Elter et al., 1975). However, in the last two decades structural studies have shown that the deposits of the hinterland basins have been affected by widespread compressional deformations and by major angular unconformities (Boccaletti et al., 1995, and references therein) that allow to subdivide the entire succession into five Unconformity Bounded Stratigraphic Units (UBSUs, Salvador, 1987).) since the unconformities that limited each UBSU have been correlated on a regional scale, (Boccaletti et al., 1994; 1995). The deformations affecting the basin fill, their architecture, the presence of regional unconformities strongly suggest that they have mostly developed under a compressional tectonic regime, tied to reactivation of thrust faults affecting the pre-Neogene substrate (Boccaletti et al., 1995; 1997; Bonini and Moratti, 1995; Landi et al., 1995). The objective of this excursion, whose road log is reported in Fig. 2, is to illustrate the relationships between tectonics and sedimentation in the hinterland basins along a transect across Southern Tuscany including the Baccinello-Cinigiano, Velona and Siena-Radicofani Basins. In this area, basin development has been mainly related to the activity of two crustal thrusts: the Mid-Tuscany Metamorphic Ridge and the Cetona Mountain thrusts (Fig. 3).

Applicability of the dipole-dipole method for structural studies: examples from the Northern Apennines

We present some results of a geoelectrical investigation program conducted in the Northern Apennines, namely in the Val d'Aveto and Bobbio window and surrounding areas. Field activity included the execution of more than 50 vertical electrical soundings with continuous polar dipole-dipole spread. We image the geometries of some deep geological structures; in particular we found a resistive background, whose resistivity is different along the geoelectrical profiles. In our interpretation the resistive background consists of subligurid and tuscan units underlying the alloctone Ligurid units in the area surrounding the Val d'Aveto and Bobbio window. The resistive background was not found, at least at the same depths, toward north-east. Therefore, the geoelectrical survey revealed the position of the front of the subligurid and Tuscan nappes toward the plain for a depth of about one kilometer.