Vitale Brovarone

RSCM THERMOMETRY IN THE ALPI APUANE (NW TUSCANY, ITALY): NEW CONTRAINTS FOR THE METAMORPHIC AND TECTONIC HISTORY OF THE INNER NORTHERN APENNINES

Keywords: RSCM Thermometry, Thermal structure, Tectonics, Alpi Apuane, Northern Apennines

Highlight:

-first RSCM data for the northern Apennines -peak temperature in the Alpi Apuane metamorphic core and surroundings region -new definition of the thermal structure and nappe architecture of the inner northern Apennines

INTRODUCTION

The thermal structure and the field metamorphic gradient in mountain belts may be first order variables directly related with the tectonic setting and the crustal-scale architecture of the orogens (between others Chopin et al., 1991;Goscombe et al., 2005[START_REF] Brown | Metamorphic patterns in orogenic systems and the geological record[END_REF], Vitale Brovarone et al 2013, Agard and Vitale Brovarone, 2013 and references). Data on the thermal structure of metamorphic units combined with those constraining kinematic history delineates the extent to which heat and mass are transferred from mid to upper crust [START_REF] Selverstone | Evidence for east-west crustal extension in the Eastern Alps implications for the unroofing history of the Tauern Window[END_REF][START_REF] Jolivet | Midcrustal shear zones in post-orogenic extension: the northern Tyrrhenian Sea case[END_REF][START_REF] Ring | Exhumation processes[END_REF][START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF][START_REF] Cottle | Metamorphic history of the South Tibetan Detachment System, Mt. Everest region, revealed by RSCM thermometry and phase equilibria modelling[END_REF]. Therefore, the understanding of the thermal structure of the internal parts of the mountain belt and its evolution represent a key element to unravel the tectonic frame and kynematics of the development of the orogenic system, allowing a better definition of past geological boundary conditions as inputs for large-scale thermokinematic and geodynamics simulations [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF][START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF]Avouac et al., 2012).

In the inner northern Apennines thermal characters and evolution of the main units of the nappe stack were defined through the classical methods of clay mineralogy, vitrinite reflectance, fluid inclusions and illite cristallinity for the shallowest units of the nappe stack (Cerrina et al., 1980;Carter et al., 1998;Carosi et al., 2005;[START_REF] Reutter | Coalification studies in the Northern Apeninnes and paleogeothermal implications[END_REF]Reutter et al., 1980;Ellero et al., 2001;[START_REF] Montomoli | Pressure fluctuation during uplift of the Northern Appenines (Italy): a fluid inclusions study[END_REF]Montomoli et al., 2002;[START_REF] Botti | Sedimentary and tectonic burial evolution of the Northern Apennines in the Modena-Bologna area: constraints from combined stratigraphic, structural, organic matter and clay mineral data of Neogene thrust-top basins[END_REF][START_REF] Dellisanti | Use of Tmax as a thermal maturity indicator in orogenic successions and comparison with clay mineral evolution[END_REF][START_REF] Caricchi | Reconstruction of maximum burial along the Northern Apennines thrust wedge (Italy) by indicators of thermal exposure and modelling[END_REF][START_REF] Ventura | Thermal history and exhumation of the Northern Apennines Relationship between Tmax and clay minerals 129 (Italy): evidence from combined apatite fission track and vitrinite reflectance data from foreland basin sediments[END_REF], while in the lowermost metamorphic units Al-silicates, chloritoidchlorite thermometry, calcite-dolomite, stable isotope geothermometers, and conodonts color were used (Franceschelli et al., 1990;[START_REF] Franceschelli | Prograde and retrograde chloritoid zoning in low temperature metamorphism[END_REF][START_REF] Franceschelli | Zoning of chloritoid from kyanite-facies metapsammites, Alpi Apuane, Italy[END_REF]Molli et al., 2000a,b;[START_REF] Molli | Tectono-metamorphic evolution of the Alpi Apuane Metamorphic Complex: new data and constraints for geodynamic models[END_REF][START_REF] Cortecci | Oxygen-and carbon-isotopic composition of gypsum-calcitedolomite crystals and metamorphic marble assemblages[END_REF][START_REF] Cortecci | Stable isotope composition and geothermometry of metamorphic rocks from the Apuane Alps, northern Tuscany, Italy[END_REF]Costagliola et al., 2002;Consani, 2002). More recently, thermodinamical modeling was also applied in the Paleozoic basement terms of the Tuscan metamorphic units (Lo Po' and Braga, 2014;Lo Po' et al., 2017).

Due to the patchy distribution of suitable rock types in the different units and within the nappe stack, however, we still lack a precise definition of the regional thermal structure and of the paleothermal field gradients.

As a consequence the overall tectonic setting shaping the present day nappe-architecture is still a matter of debate, with crustal contractional and crustal extensional models supported by different groups of authors [START_REF] Carmignani | Crustal extension in the Northern Apennines: the transition from compression to extension in the Alpi Apuane Core Complex[END_REF]Boccaletti et al., 1993;Carosi et al., 2003;Decandia et al., 1995;[START_REF] Storti | Tectonics of the Punta Bianca promontory: Insights for the evolution of the Northern Apennines-Northern Tyrrhenian Sea basin[END_REF][START_REF] Jolivet | Midcrustal shear zones in post-orogenic extension: the northern Tyrrhenian Sea case[END_REF][START_REF] Thomson | Thermochronologic evidence for orogen-parallel variability in wedge kinematics during extending convergent orogenesis of the northern Apennines, Italy[END_REF]Sani et al., 2014;Musumeci et al., 2016). This in turn leaves open some relevant questions such as the relationships between exhumation processes and crustal/litospheric thinning, which shaped the present day Moho Versione 12 12 2017 4 depth and thermal anomaly, characteristic features of the inner northern Apennine in Tuscany (Di Stefano et al., 2011;[START_REF] Vedova | Deep temperatures and surface heat flow distributions[END_REF]Spada et al., 2013 and references).

In this study, we applied the same geothermometer, i.e. Raman spectroscopy on carbonaceous material (RSCM), to a large range of lithologies from the main units of the inner northern Apennines, with special focus on the Alpi Apuane metamorphic core and nearby areas. Thanks to the irreversible process of graphitization of carbonaceaos material (CM), this geothermometer can provide peak temperature (T) estimates for CM-bearing metasedimentary rocks independently from their mineralogical assemblage, and is therefore aplicable to a wider range of rock types in the considered units compared to previous studies.

Our data allow us to constraining the field thermal structure and the exhumation-related paleogradients and therefore to improving our understanding of the orogenic processes recorded in the inner northern Apennines. Moreover, our data document the thermal architecture and metamorphic signature of a mid-shallow orogenic wedge and its relationships with the regional structures, the nappe and thrust stack styles and their internal deformation.

REGIONAL GEOLOGY

The northern Apennines (Fig. 1) are characterized by a pile of thrust-sheets and fold nappes derived from the distal part of the Adria continental margin (the Tuscan Domain) presently lying below the remnants of a former intraoceanic accretionary wedge represented by the Ligurian and sub-Ligurian units relicts of the former Mesozoic western alpine Tethys ocean [START_REF] Elter | Introduction à la géologie de l'Apennin septentrional[END_REF][START_REF] Bortolotti | Ophiolites, Ligurides and the tectonic evolution from spreading to convergence of a Mesozoic Western Tethys segment[END_REF][START_REF] Bernoulli | Mesozoic-Tertiary Carbonate Platforms, Slopes and Basins of the external Apeninnes and Sicily[END_REF][START_REF] Liotta | D2 asymmetric folds and their vergence meaning in the Montagnola Senese Versione 12 12 2017 25 metamorphic rocks (inner northern Apennines, central Italy)[END_REF]Molli, 2008 and ref.). The recent to active tectonic framework of the northern Apennines is characterized by crustal-scale extension in the inner-western (Tyrrhenian) side of the orogen and shortening in its external (Po Plain and Adriatic) eastern side (e.g. [START_REF] Elter | Introduction à la géologie de l'Apennin septentrional[END_REF][START_REF] Barchi | The CROP03 profile: a synthesis of results on deep structures of the Northern Apennines[END_REF]Doglioni et al., 1998;Fellin et al., 2002;Bennett et al., 2012;Boccaletti et al., 2010;Eva et al., 2014;Faccenna et al., 2014;[START_REF] Clemenzi | Fluid pressure cycles, variations in permeability, and weakening mechanisms along low-angle normal faults: The Tellaro detachment, Italy[END_REF]Le Breton et al., 2017).

In the NW of Tuscany, the Alpi Apuane form the largest tectonic window in the inner northern Apennines and expose the deepest crustal units of the belt (Tuscan Metamorphic Units) (Fig. 1).

Three major stratigraphic and tectono-metamorphic Adria-derived continental units are traditionally distinguished in the region, the Tuscan Nappe, the Massa unit and the Apuane unit (Fig. 2). The Tuscan Nappe consists of Mesozoic carbonates and Tertiary deep water and turbiditic sequence mainly detached from their original basement along the décollement level of the Norian anidrites and dolostones to be compared with the Anidriti di Burano (Ciarapica and Passeri, 2002 and references therein). These anydrites and dolostones are transformed almost everywhere (with some relevant exceptions) into cataclastic breccias called Calcare Cavernoso or "cellular" limestone (Baldacci et al., 1967;Gandin et al., 2000). The post-Norian sequence continues with Rhaetian to Hettangian shallow water limestones (Rhaetavicula Contorta and Calcare Massiccio), Lower Liassic to Cretaceous pelagic limestones, radiolarites and shales (Calcare selcifero, Marne a Posidonomya, Diaspri, Maiolica), grading to hemipelagic deposits of the Scaglia (Cretaceous-Oligocene), to end with the siliciclastic foredeep turbidites of the Macigno (Late Oligocene-Early Miocene). The entire sequence, has a thickness between 2000 and 4000 m (Fazzuoli et al., 1986;Ciarapica and Passeri, 2002;[START_REF] Molli | Structural inheritance and style of reactivation at mid-crustal levels: A case study from the Alpi Apuane (Tuscany, Italy)[END_REF];

The Apuane Unit forming most of the Alpi Apuane tectonic window is made up of a Paleozoic basement unconformably overlain by an Upper Triassic-Oligocene metasedimentary sequence. The Paleozoic basement is represented by metasedimentary and metavolcanic rocks including Upper Cambrian-Lower Ordovician phyllites and quarzites, Middle Ordovician metavolcanics and metavolcanoclastics, Upper Ordovician quartzitic metasandstones and phyllites, Silurian black phyllites and Orthoceras-bearing metadolostones [START_REF] Conti | Prealpine basement in the Alpi Apuane (Northern Apennines, Italy)[END_REF][START_REF] Gattiglio | Stratigraphic correlation forms of the Tuscan Palaeozoic basement[END_REF][START_REF] Pandeli | The Paleozoic basement of the Northern Apennines: stratigraphy, tectono-metamorphic evolution and alpine hydrothermal processes[END_REF]. The Paleozoic lithostratigraphic units were deformed and metamorphosed under lowgrade conditions, not well precisely defined, during the Variscan orogeny [START_REF] Conti | The overprint of the Alpine tectonometamorphic evolution on the Hercynian orogen: an example from the Apuane Alps (Northern Apennines, Italy)[END_REF][START_REF] Conti | Prealpine basement in the Alpi Apuane (Northern Apennines, Italy)[END_REF].

The Mesozoic cover-rocks, where complete, include thin Triassic continental to shallow-water Verrucano-like deposits followed by Upper Triassic-Liassic carbonate platform metasediments comprising dolostone (Grezzoni Fm.), dolomitic marble, and marble (the Carrara Marble), in turn covered by Middle Liassic-Lower Cretaceous cherty metalimestone, cherts, and calcschists, and Lower Cretaceous to Lower Oligocene sericitic phyllites and calcschists with marble interlayers. Oligocene-early Miocene (?) metasediments related to turbiditic systems (Pseudomacigno Fm.) complete the sedimentary succession (Patacca et al., 2011 and reference therein).

The Massa unit is exposed in the westernmost part of the Alpi Apuane tectonic window and includes a litho-stratigraphic sequence formed by a Paleozoic basement similar to that of the Apuane unit and a characteristic and distinctive Upper Permian-Upper Triassic metasedimentary succession, including a Mid Triassic continental (conglomerates and pelites) to marine (carbonate platform derived deposits) succession associated with intraplate alkaline basalts (Martini et al, 1985). Lithostratigraphic terms younger than late Mid Triassic are not described in the literature, although (see below) they may be locally found as tectonic lenses and small scale remnants within a cataclastic fault zone below the contact with the overlaying Tuscan Nappe [START_REF] Molli | Tectono-metamorphic evolution of the Alpi Apuane Metamorphic Complex: new data and constraints for geodynamic models[END_REF]Conti et al., 2004). Traditional literature (Baldacci et al., 1967;[START_REF] Elter | Introduction à la géologie de l'Apennin septentrional[END_REF] based on the similarities in Versione 12 12 2017 6 stratigraphic contents of the pre-Late Triassic sequences considered the metamorphic exposures of Punta Bianca and M.Pisano the same of the Massa unit although different peak metamophic conditions were recorded in the recent papers [START_REF] Storti | Tectonics of the Punta Bianca promontory: Insights for the evolution of the Northern Apennines-Northern Tyrrhenian Sea basin[END_REF]Carosi et al., 2014;Leoni and Pertusati, 2003;Molli, 2008 and references therein).

The deformation structures of the Tuscan metamophic units of the Alpi Apuane and surroundings metamorphic cores may be referred to two main tectono-metamorphic regional events (D1 and D2 phases of [START_REF] Carmignani | Crustal extension in the Northern Apennines: the transition from compression to extension in the Alpi Apuane Core Complex[END_REF], which are regarded (Molli et al., 2000[START_REF] Molli | Tectono-metamorphic evolution of the Alpi Apuane Metamorphic Complex: new data and constraints for geodynamic models[END_REF] as recording progressive deformation of the distal Adriatic continental margin during continental subduction and the syn-to post-contractional exhumation [START_REF] Carmignani | Crustal extension in the Northern Apennines: the transition from compression to extension in the Alpi Apuane Core Complex[END_REF][START_REF] Jolivet | Midcrustal shear zones in post-orogenic extension: the northern Tyrrhenian Sea case[END_REF]Carmignani et al., 2001;[START_REF] Molli | Northern Apennine-Corsica orogenic system: an updated review[END_REF]. The two main regional events D1 and D2 were related to different fold generations or folding phases as illustrated by [START_REF] Molli | Structural inheritance and style of reactivation at mid-crustal levels: A case study from the Alpi Apuane (Tuscany, Italy)[END_REF] in the Alpi Apuane. The same deformation patterns may be also recognized in the Punta Bianca and M.Pisano metamorphic cores (Storti, 1996;Montomoli, 2002;Carosi et al., 2007;[START_REF] Molli | Northern Apennine-Corsica orogenic system: an updated review[END_REF][START_REF] Balestrieri | Age and temperature constraints on metamorphism and exhumation of the syn-orogenic metamorphic complexes of Northern Apennines, Italy[END_REF]. D1, which can be associated with underplating and antiformal stacking of the metamorphic units, is defined by a main axial-plane foliation of isoclinal folds observable on a micro-to kilometer-scale (Fig. 2) and is associated with a regionally NE-oriented stretching lineation interpreted as recording the main transport direction of the inner northern Apennines [START_REF] Carmignani | Structural evolution of the Apuane Alps: an example of continental margin deformation in the Northern Apennine[END_REF][START_REF] Molli | Northern Apennine-Corsica orogenic system: an updated review[END_REF][START_REF] Molli | Structural inheritance and style of reactivation at mid-crustal levels: A case study from the Alpi Apuane (Tuscany, Italy)[END_REF].

During D2, the previously formed structures were reworked by different generations of folds and high-strain zones, related to exhumation of the metamorphic units within the inner portion of the northern Apenninic wedge [START_REF] Molli | Northern Apennine-Corsica orogenic system: an updated review[END_REF]. During the early stage of D2, folds with subhorizontal axial plane crenulation formed, whereas later deformation was associated with semibrittle and brittle structures, represented by kink and open folds, and low-angle normal faults (Molli and Meccheri, 2000;[START_REF] Molli | Fault zone structure and fluid-rock interaction of a high angle normal fault in Carrara marble (NW Tuscany, Italy)[END_REF]. The final stages of deformation are recorded by systemes of high-angle normal to oblique-normal and trascurrent faults related with the recent to active deformation regime affecting the area [START_REF] Corti | Interaction between normal faults and pre-existing thrust systems in analogue models[END_REF]Bennett et al., 2012;Molli et al., 2015 a,b). Available pressure (P), T and geochronological data for the Alpi Apuane metamorphism are summarized in Molli et al. (2000[START_REF] Molli | Tectono-metamorphic evolution of the Alpi Apuane Metamorphic Complex: new data and constraints for geodynamic models[END_REF]; Fellin et al. (2007 and references therein); Carosi et al. (2007Carosi et al. ( , 2013)).

In the metamorphic units of the Alpi Apuane, peak conditions are roughly related to T between 450 and 350 °C and P of 0,8-0.4 GPa (Molli et al., 2002 and references therein). In particular, kyanite + chloritoid + phengitic muscovite chracterize the assemblages in metapelites of the Massa unit where peak conditions in the range of 0,6-0,8 GPa and 420-500°C [START_REF] Franceschelli | Regional distribution of Al-silicates and metamorphic zonation in the low-grade Verrucano metasediments from the Northern Apennines, Italy[END_REF][START_REF] Jolivet | Midcrustal shear zones in post-orogenic extension: the northern Tyrrhenian Sea case[END_REF][START_REF] Franceschelli | Zoning of chloritoid from kyanite-facies metapsammites, Alpi Apuane, Italy[END_REF]Molli et al., 2000b) were defined. Instead, pyrophyllite + chloritoid + chlorite + phengitic muscovite in metapelites may be found in the Apuane unit, with peak metamorphic conditions estimated in the range of 0.4-0.6 GPa and 350-450 °C [START_REF] Franceschelli | Regional distribution of Al-silicates and metamorphic zonation in the low-grade Verrucano metasediments from the Northern Apennines, Italy[END_REF]Di Pisa et al., 1987;[START_REF] Jolivet | Midcrustal shear zones in post-orogenic extension: the northern Tyrrhenian Sea case[END_REF]Molli et al., 2000b).

The early deformation D1 in the metamophic units of the Alpi Apuane occurred during early Miocene at 27-20 Ma [START_REF] Kligfield | Dating of deformational phases using K-Ar and 40Ar/39Ar techniques: results from the Northern Apennines[END_REF], whereas the syn-metamorphic D2 structures developed at T higher than 250 °C, predated 11 Ma according to zircon fission-track ages of [START_REF] Fellin | Thermochronologic evidence for exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy[END_REF].

In contrast to the Tuscan metamorphic units, the Tuscan nappe was accreted at a shallow crustal level within the northern Apennines wedge beginning in early Miocene (Cerrina Feroni et al., 2004;[START_REF] Molli | Northern Apennine-Corsica orogenic system: an updated review[END_REF]. Burial occurred under a sequence of thrust sheets now preserved in the overlying sub-Ligurian and Ligurian units (Figs. 1,2). Early thrusting is documented by top-to-the east small scale shear zones and an early generation of east vergent tight to isoclinal folds within incompetent stratigraphic layers (Gianmarino & Giglia, 1990;Carter, 1992;Molli et al., 2011). The early formed structures in the Tuscan nappe were subsequently overprinted by small to large (kilometer-scale) refolding associated with sub-horizontal crenulation cleavage observable in pelitic rock units, lowangle normal faults and later upright folds (Carmignani et al., 1995;[START_REF] Storti | Tectonics of the Punta Bianca promontory: Insights for the evolution of the Northern Apennines-Northern Tyrrhenian Sea basin[END_REF]Carosi et al., 2003). Metamorphism to anchizone grade [START_REF] Cerrina Feroni | Contributo alla conoscenza dei processi metamorfici di grado molto basso (anchi-metamorfismo) a carico della falda toscana nell'area del ricoprimento apuano[END_REF]Carosi et al., 2003;Molli et al., 2011) during burial and deformation has been tectono-stratigraphically constrained to a maximum depth of 7 km for the Macigno sandstones [START_REF] Reutter | Coalification studies in the Northern Apeninnes and paleogeothermal implications[END_REF][START_REF] Montomoli | Pressure fluctuation during uplift of the Northern Appenines (Italy): a fluid inclusions study[END_REF]Montomoli, 2002;[START_REF] Fellin | Thermochronologic evidence for exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy[END_REF].

RAMAN SPECTROSCOPY OF CARBONACEOUS MATERIAL

Raman spectra were obtained using a Renishaw InVIA Reflex microspectrometer (IMPMC Paris).

We used a 514 nm Laser Physics argon laser in circular polarization. The laser was focused on the sample by a DMLM Leica microscope with a 100× objective (NA = 0.85), and the laser power at the sample surface was set around 1 mW. The Rayleigh diffusion was eliminated by edge filters, and to achieve nearly confocal configuration the entrance slit was closed down to 15 µm. The signal was finally dispersed using a 1800 gr/mm grating and analyzed by a Peltier cooled RENCAM CCD detector. Before each session, the spectrometer was calibrated with a silicon standard. Because Raman spectroscopy of CM can be affected by several analytical mismatches, we closely followed the analytical and fitting procedures described by Beyssac et al. (2002[START_REF] Beyssac | On the characterization of disordered and heterogeneous carbonaceous materials using Raman spectroscopy[END_REF]. Measurements were done on polished thin sections cut perpendicularly to the main fabrics and CM was systematically analyzed below a transparent adjacent mineral, generally quartz. Between 10-20 spectra were recorded for each sample in the extended scanning mode (1000-2000 cm-1) with acquisition times from 30 to 60 s. Spectra were then processed using the software Peakfit (following Beyssac et al., 2002). Based on the obtained spectra, T from samples characterized by lower-T metamorphism (i.e. ~200 b T b 350 °C) were estimated using the correlation proposed by Lahfid et al. (2010). At higher-T (i.e. ~350 b T b 650 °C), the T was calculated using the calibration of Beyssac et al. (2002), with a calibration-attached accuracy of ±50 °C due to uncertainties on petrologic data used for the calibration. Relative uncertainties on T are, however, much smaller, around 10-15 °C [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF].

For each sample, several spectra were acquired to assess the potential within-sample structural heterogeneity, with the exception of a few samples containing very little CM amount (Table 1). The latter samples have therefore higher uncertainty compared to the others, and were not considered for tectonic interpretations even though the obtained T is in the range of the neighbouring ones. In a few samples, isolated spectra of crystalline graphite (no defect bands) were interpreted as detrital CM incorporated in the sedimentary rocks prior to metamorphism, and not considered for thermometric estimates. Some structural heterogeneity was found in the samples, with a maximum standard error (stadard deviation devided by √n) < 8 for most samples (Table 1).

The Raman spectra obtained from the selected samples set show a large between-sample variation in structural organization ranging from poorly organized CM (lower-T) to well cristallized graphite (higher-T) (Table 1). The collected data allow distinguishing samples characterized by T gaps in the order of about 20 °C (Fig. 3), as already observed for this method along continuous metamorphic gradients in Alpine settings [START_REF] Vitale Brovarone | The blueschisteclogite transition in the Alpine chain: PT paths and the role of slow-spreading extensional structures in the evolution of HP-LT mountain belts[END_REF]

RSCM TEMPERATURE RECORD IN THE ALPI APUANE AND SOURROUNDINGS

Our dataset consists of 74 analyzed samples (Table 1) selected among 120 collected in various stratigraphic and structural positions within the different tectonic units. The samples were collected along a regional SW-NE oriented section across the inner (western) northern Apennines (Fig. 1).

The uppermost continental unit of the nappe pile was sampled west of the Alpi Apuane in the La Spezia area (Fig. 1,2), where the stratigraphic sequence of the Tuscan Nappe is exposed as part of a regional-scale fold called La Spezia fold (e.g. Gianmarino and Giglia, 1991;Carter, 1992;Carosi et al., 2002;Molli et al., 2011 and references therein), in the eastern promontory of La Spezia (between Lerici and P.Bianca) (Storti, 1996;Clemenzi et al., 2015 and references), north and east of the Alpi Apuane in the Lunigiana and Lima Valley [START_REF] Clemenzi | Fluid pressure cycles, variations in permeability, and weakening mechanisms along low-angle normal faults: The Tellaro detachment, Italy[END_REF]Clemenzi et al., 2014 and references).

A more systematic sampling was performed across the Alpi Apuane in order to enlighten the thermal structure of the metamorphic core that was poorly documented so far (Figs. 2,4 andTable 1). Some samples of the metamorphic units were also sampled in the eastern promontory of La Spezia as well as southeastward of the Alpi Apuane in the M.Pisano (Fig. 2).

The stratigraphic units sampled in our study are mainly from the Mesozoic to Tertiary cover sequence and in particular from siliciclastic sandstones of Macigno fm. (Late Oligocene-Early Miocene in age) and impure Triassic limestones for the Tuscan Nappe. The metamorphic equivalents (metasandstones, slates, schists, calcschists and impure marbles) were sampled in the Apuane and Massa units altogether with some samples from the Paleozoic basement units (mainly phyllites and schists) (Fig. 2). The samples collected in the eastern promontory of La Spezia come from the Triassic impure limestone at the base of the Tuscan Nappe and from the underlying pre-Mesozoic terms (black phyllites supposed to be Permian age) of the metamorphic Punta Bianca unit i.e. respectively in the hanging-wall and footwall of the former major thrust reworked as a low angle normal fault (Storti, 1996;Carosi et al., 1998[START_REF] Clemenzi | Fluid pressure cycles, variations in permeability, and weakening mechanisms along low-angle normal faults: The Tellaro detachment, Italy[END_REF]. Moreover, the two analyzed samples in the S.Maria del Giudice unit are part of the Monte Pisano metamorphic core (Rau and Tongiorgi, 1974;Carosi et al., 1993;Montomoli, 2002;[START_REF] Leoni | Il metamorfismo delle unità Tettoniche dei M.Pisani (App. Sett.)[END_REF] and are derived from its Tertiary cover sequence (Pseudomacigno Fm.).

Results are presented in maps and projected along the SW-NE structure-orthogonal regional and local cross sections (Figs. 4,5,6). For a sake of comparison, results of previous studies including illite-crystallinity [START_REF] Cerrina Feroni | Contributo alla conoscenza dei processi metamorfici di grado molto basso (anchi-metamorfismo) a carico della falda toscana nell'area del ricoprimento apuano[END_REF]Carosi et al., 2003;Leoni et al. 2003) and Calcite/Dolomite thermometer (quoted references) are also shown where available (Fig. 2,4).

The sampling strategy allowed constraining the RSCM T within the classically defined continentderived units of the inner northern Apennines [START_REF] Elter | Introduction à la géologie de l'Apennin septentrional[END_REF][START_REF] Carmignani | Structural evolution of the Apuane Alps: an example of continental margin deformation in the Northern Apennine[END_REF][START_REF] Carmignani | Crustal extension in the Northern Apennines: the transition from compression to extension in the Alpi Apuane Core Complex[END_REF]Carmignani et al., 2001 between the others) which are in the current view (see above) subdivided into three major slices referred, from top to bottom, to as Tuscan nappe, Massa and Apuane (also called "Authochtonous") units (Fig. 1). In the La Spezia area, the Punta Bianca metamorphic occurences are associated with a pristine lithostratigraphic assemblage similar to that of the Massa unit but affected by a lower grade peak metamorphic imprint (Ciarapica and Passeri, 1985;Storti, 1996;Leoni and Pertusati, 2003;Lo Po' et al., 2017), whereas the Monte Pisano Versione 12 12 2017 10 metamorphic core is similarly considered part of the Massa unit for the presence of widespread continental "Verrucano" deposits (Rau and Tongiorgi, 1974;[START_REF] Balestrieri | Age and temperature constraints on metamorphism and exhumation of the syn-orogenic metamorphic complexes of Northern Apennines, Italy[END_REF].

The overall data sets, summarized in Fig. 7 and Table 1, show significant difference in mean peak T among the major tectonic units so far defined.

The RSCM peak T are in the range of 246-284 °C (mean 260 ± 26 °C ) for the Tuscan Nappe; 306-537 °C (mean 397 ± 64 °C, n=63) for for the Apuane and 454-509 °C (485 ± 28 °C, n=3) for the Massa unit.

In the Tuscan Nappe exposed in the La Spezia area (Figs. 1,2), a simple relationship between stratigraphic positions of the samples and peak T appears (Fig. 6). The youngest terms of the succession represented by the Macigno fm. show a mean T of 253 °C, whereas in the lowermost measured Triassic term of the Calcari a Rhaetavicula Contorta fm. (Rhaetian in age) a mean T of 289 °C was obtained.

In the eastern Promontory of La Spezia in the same stratigraphic unit i.e. the Calcari a Rhaetavicula Contorta (Rhaetian in age), a mean T of ∼ 284 °C well confirmed the T obtained in the same stratigraphic unit in the western promontory.

Samples from the Tuscan Nappe in the Lima Valley (Baldacci et al., 1967;Fazzuoli et al. 1992;Botti et al., 2009;[START_REF] Clemenzi | Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines, Italy)[END_REF] east of the Alpi Apuane show instead lower mean temperature of 227 °C obtained for Jurassic and Cretaceous pelites and marls (Table 1, Fig. 6).

In the Alpi Apuane, although the highest T were measured in samples of the Paleozoic basement (364-537 °C, mean 445 ± 66°C) and the lowest (306-511 °C, mean 368 ± 50°C) in the youngest stratigraphic terms of the cover sequence (PseudoMacigno fm.), no simple relationships between stratigraphy and T appear at the scale of the whole metamorphic core (Figs. 4,5,8).

The mean T in the north-west part of the metamorphic complex (north of the Seravezza-Monte Corchia, Turrite valley alignment) is basically the same as those defined in the south-eastern segment (south of the Seravezza-Monte Corchia, Turrite valley alignment), around ∼380 °C. The T data plotted on composite vertical cross-sections (Figs. 5,9) show, however, a clearly well-defined, yet different thermal structure and overall thermal architecture between the northern and southern Alpi Apuane.

The north-west part of the Alpi Apuane, as previously defined, (cross-sections A,B,C,D,E,F,G)

shows an inverted field metamorphic gradient. Within the Apuane unit higher T (483-507 °C, mean 465 °C) are found in the west, within a structural distance of ∼500 m from the basal contact of the Massa unit (Figs. 5,9). The T decrease eastward and structurally downward to ∼ 345 °C in the Orto di Donna sincline and in the Arni-Boana structural culmination, i.e. in the geometrically deepest parts of the Alpi Apuane metamorphic core (Kligfield et al., 1981;[START_REF] Carmignani | Crustal extension in the Northern Apennines: the transition from compression to extension in the Alpi Apuane Core Complex[END_REF]Carmignani et al., 2001;Molli and Vaselli, 2007).

The south-eastern parts of the Alpi Apuane (cross-sections H,I,L,M) are instead characterized by a normal-type metamorphic field gradient with overall upward decreasing T from 510 °C to 320 °C in a vertical structural distance of 3000 m.

Samples with the highest RSCM T are founded in the south-eastern part of the Alpi Apuane, an area known in the local geological literature as "Zona dello Stazzemese" or "Stazzemese Shear Zone" (Stazzema is village included in it). The area is characterized and recognized for a long time for its distinctive structural style (see Massa 2007 for an historical ovierview) with kilometer scale recumbent D2 isoclinal folds with highly sheared and mylonitized limbs (see Carmignani et al. 1996, Conti et al., 2009;Cinquini, 2014). The area is also peculiar with respect to the whole Alpi Apuane for the presence of characteristic mineralizations of Pb-Zn±Au Hg, Fe-Cu and Barite-Iron Oxide-Pyrite deposits studied and employed since the mid of last century (Carmignani et al., 1972;Carmignani et al., 1976;Orberger, 1985;Costagliola et al., 1990;Dini et al., 1995;[START_REF] Costagliola | Metamorphic barite-pyrite (Pb-Zn-Ag) veins at Pollone, Apuane Alps, Tuscany: vein geometry, geothermobarometry[END_REF]Biagioni et al., 2016).

COMPARISON WITH PREVIOUS DATA

Our new RSCM data find good agreement with previous available data, in particular those derived from calcite/dolomite (Ca/Do) geothermometer/analysis (Fig. 4) [START_REF] Di Pisa | Regional variation of the metamorphic temperatures across the Tuscanid 1 Unit and its implications on the alpine metamorphism (Apuan Alps, N-Tuscany)[END_REF]Molli et al., 2000;Oesterling et al., 2007). Although different in absolute value, the RSCM and Ca/Do data well document the presence of inverted metamorphism in the North/West part of the Alpi Apuane, with higher T in the westernmost and uppermost structural positions which decrease eastward and downward within the structural building.

In the West (where the T are highest) Ca/Do shows lower values with respect to RSCM whereas this is not always the case or is the opposite in the central and easternmost part of the Apuane (Fig. 4).

The comparison between Ca/Do and RSCM cannot be drawn for the lack of data in the south-east part of the Alpi Apuane ("Zona dello Stazzemese") where suitable rock-types for Ca/Do analyses are less common and therefore not deeply investigated in the literature. In this area, however, different studies (Cortecci et al., 1989;[START_REF] Cortecci | Stable isotope composition and geothermometry of metamorphic rocks from the Apuane Alps, northern Tuscany, Italy[END_REF]Costagliola et al., 1997;[START_REF] Costagliola | Metamorphic barite-pyrite (Pb-Zn-Ag) veins at Pollone, Apuane Alps, Tuscany: vein geometry, geothermobarometry[END_REF]Biagioni et al., 2013Biagioni et al., , 2016) ) which include stable isotope compositions and geothermometry, fluid inclusions and phase relationships between sulfosalts in ore bodies give host rock T around 350-400 °C and T for mineralizing fluids and ore bodies higher than 450 °C [START_REF] Costagliola | Metamorphic barite-pyrite (Pb-Zn-Ag) veins at Pollone, Apuane Alps, Tuscany: vein geometry, geothermobarometry[END_REF], which are similar to our RSCM results.

For the Massa unit we were able to find only 3 suitable samples among more than 25 analyzed since they were almost totally oxided coming mainly from Verrucano continental metasediments. Our mean RSCM T is, however, in agreement with the estimates based on Fe-Mg exchanges in coexisting chloritoid and chlorite (450-500 °C; [START_REF] Franceschelli | Zoning of chloritoid from kyanite-facies metapsammites, Alpi Apuane, Italy[END_REF]. Similar T ranges were also constrained by the stability field of the kyanite+quartz pair and chlorite+chloritoid+quartz assemblages in the FeO-Al 2 O3-SiO 2 -H 2 O (Franceschelli et al., 1998).

Using similar methods (chloritoid±chlorite Mg±Fe-exchange thermometer a peak temperature of 455 °C was found by Molli et al. (2000).

Ca-Do data in the Mid-Triassic marble of the Massa unit also give a similar T in the range of 430-520 °C (Cardaci, 1987).

SUMMARY AND DISCUSSION

The RSCM data presented in this study allowed us to better constrain the relationships between the nappe architecture and the thermal structure in the Alpi Apuane and their surroundings, while highlighting new subjects for a better and more complete understanding of the thermo-mechanical evolution and exhumation history of the inner northern Apennines in the general frame of the evolution of the Apennines orogen. Hereafter the results and the first order points of our contribution are summarized and discussed.

Peak temperature and tectonic units

A first point to be remarked is the difference in the mean peak T among the structurally/stratigraphically defined tectonic units (Fig. 7).

The classically accepted subdivision (Elter, 1975, Carmignani and[START_REF] Carmignani | Crustal extension in the Northern Apennines: the transition from compression to extension in the Alpi Apuane Core Complex[END_REF]Decandia et al., 1996, Vai andMartini, 2001) into three major continental units forming the backbone of the inner northern Apennines -from top to bottom: the Tuscan nappe, the Massa and the Apuane units -Versione 12 12 2017 13 appears to be fully supported by the new measured RSCM T (Fig. 7) which show a mean peak T of ∼ 260 °C for the Tuscan Nappe, ∼ 485 °C for the Massa unit and ∼ 385 °C for the Apuane unit. Moreover, our data suggest that the nappe stack was originally formed by units overlooked in current literature. In such group of units may be included the Punta Bianca Unit in the West, and the Monte Pisano Unit in the South. Moreover, in the southern-eastern Alpi Apuane the Panie may be inserted within this group, since they are here characterized by a RSCM T of 320 °C, an intermediate value between those of the Apuane Unit and those of the Tuscan Nappe. Other evidence of missing units, inside the Alpi Apuane, may be searched for between the Massa unit and the overlying Tuscan Nappe (Fig. 5 cross section E), where remnants of a post mid-Triassic cover (cherty metalimestones, marbles, phyllites) with a peak RSCM T of ∼350 °C were sampled (sample 16). Those remnants are also observable as structural relicts exposed as hectometer to pluridecameter thick slices and lenses (see Molli et al., 2000) and widely recognizable as clasts (meter, decimeter to centimeter in scale) within the carbonatic tectonic breccia at the base of the Tuscan Nappe in the southern Alpi Apuane (Conti et al., 2009;CARG F.260;Cinquini and Molli, 2015). These data and occurences may be interpreted as related to the former existence of a metasedimentary unit ("X unit", "missing section" in [START_REF] Fellin | Thermochronologic evidence for exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy[END_REF]) originally including the post Mid-Triassic cover sequence now only scatterly observable on top of the Massa Unit.

The rank of individual tectonic unit for the Panie (see discussion in Nardi, 1967[START_REF] Giglia | Geologia dell'Alta Versilia settentrionale (Tav. M.Altissimo)[END_REF] as well as the importance of the "X unit" to fill the gap between the base of the Tuscan Nappe and the top of the Massa unit during the syn-peak metamorphism nappe stacking are in line with what anticipated and discussed in the frame of the tectonic history of the metamorphic units by Molli et al., (2000); [START_REF] Molli | Tectono-metamorphic evolution of the Alpi Apuane Metamorphic Complex: new data and constraints for geodynamic models[END_REF]; [START_REF] Fellin | Thermochronologic evidence for exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy[END_REF]. Thanks to the newly defined RSCM data and considering the mean T as an estimate of the "whole unit" thermal peak, we may emphasize that the classical subdivision into three major Tuscan units across the Alpi Apuane and surrounding introduced in literature since [START_REF] Elter | Introduction à la géologie de l'Apennin septentrional[END_REF] as related to the original thrust stacking is due instead to tectonic excision during exhumation, i.e present day tectonic units have to be considered as remnants of a former thicker, more complete and now only partially preserved contractional nappe stack (Coli, 1989;Van den Berg, 1989;[START_REF] Carmignani | Crustal extension in the Northern Apennines: the transition from compression to extension in the Alpi Apuane Core Complex[END_REF]Carmignani et al. 1995;[START_REF] Jolivet | Midcrustal shear zones in post-orogenic extension: the northern Tyrrhenian Sea case[END_REF][START_REF] Molli | Tectono-metamorphic evolution of the Alpi Apuane Metamorphic Complex: new data and constraints for geodynamic models[END_REF].

Metamorphic field gradients and thermal architecture

For the Tuscan Nappe, the dataset in the La Spezia area (Fig. 2,6) indicates a base (Triassic Rhaetian limestone) to top (Oligocene-early Miocene Macigno sandstone) difference of ∼40 °C which occurs within a structural distance of 2000 m. Thus, assuming that the peak T are: (i) coheval in age and Versione 12 12 2017 14 (ii) related with the pre-folding thermal architecture of the unit within the orogenic prism, a peak T normal-type field gradient of 20 °C/Km may be defined (Fig. 11).

At regional scale, the collected data from the Tuscan Nappe suggest for ist westernmost exposure (La Spezia area) a mean RSCM T of 271° C and a lower mean T of 227°C in the easternmost position in the Lima Valley, east of the Alpi Apuane (Fig. 1,2). This West to East decrease of peak T, in line with previously recorded illite crystallinite and organic matter data [START_REF] Reutter | Coalification studies in the Northern Apeninnes and paleogeothermal implications[END_REF][START_REF] Cerrina Feroni | Contributo alla conoscenza dei processi metamorfici di grado molto basso (anchi-metamorfismo) a carico della falda toscana nell'area del ricoprimento apuano[END_REF], may be referred to an original different thickness of the orogenic wedge stack above the Tuscan Nappe. Moreover, assuming that the field gradient defined in the La Spezia area was constant at regional scale, this results in a variation of the wedge thickness above the Tuscan Nappe of ∼2 Km from west (La Spezia) to east (Val di Lima) (Fig. 11).

For the Alpi Apuane, taking into account the whole data set in the Apuane unit, a difference in temperatures between the pre-Mesozoic samples and the Mesozoic to Tertiary ones may be underlined (Figs. 4,5,8,9).

The pre-Mesozoic basement rocks show a mean peak RSCM T of 445 °C whereas a mean of 386°C is found in the metasedimentary Mesozoic to Tertiary cover. These data may be explained considering the higher peak T in the basement rock as a relict metamorphism of the Variscan age [START_REF] Conti | Prealpine basement in the Alpi Apuane (Northern Apennines, Italy)[END_REF], Pandeli et al., 2003) or as a relict of a Late Variscan (Permian) thermal event in line with recently acquired data (Vezzoni et al., 2017;Pieruccioni et al., 2017).

Nevertheless, since similar higher T (Fig. 8) were also found in samples with Mesozoic to Tertiary protoliths, an alternative hypothesis may also be taken into account.

In this alternative frame, an Apenninic-age heating possibly related to a fluid flow infiltration in the basement rocks (and locally in the Mesozoic cover) during its detachment from the underlying subducted middle to lower crust may be envisaged (Fig. 11). The higher peak T found in association with mineralized horizons in the southern Alpi Apuane may thus be interpreted as an evidence of Apenninic-age infiltration and hot fluid flow along high-strain localized zones of deformation, which were reworked during exhumation ("Zona dello Stazzemese" or Stazzemese Shear Zone. Figs. 9, 11).

Finally, the distribution of peak T at the scale of the whole Alpi Apuane (Fig. 9) indicates a different and systematic arrangement with respect to the overall architecture of the structures in the northwest as opposed to the south-easternmost part.

The North-West part of the Alpi Apuane (Figs. 5,9) is characterized by a SW to NE decrease of peak temperature from 495 °C in the west to 357 °C in the central and 336 °C in the eastermost position. A difference of 125° C is observed with a structural distance of ∼8 Km, thus an inverted field metamorphism with a T gradient of ∼20 °/Km could be defined.

Furthermore, the distribution of the peak temperatures and the resulting thermal architecture are different in the South-East part of the Alpi Apuane (Fig. 9) where an apparent normal-type gradient is observed with transition from temperatures higher than 500 ° C to less than 300 °C (projected base of the Tuscan Nappe) in less than 1,5 Km (e.g. cross-section L) with an apparent field gradient in excess of 90 °C / Km. This clearly points out the complexity of the thermal vs. structural architecture in the Alpi Apuane which well fits an overall "contractional" model of antiformal stacking (i.e. an orogenic wedge dominated by material advenction) in the northern part and an opposite "extensional" core complex model with condensed isograds (i.e. an orogenic wedge dominated by temperature advenction) for its southernmost part. This variability in the thermal structure between the two sectors of the Alpi Apuane, however, may be only apparent since the exposed geometrically deepest and cooler levels of the eastern Alpi Apuane are not exposed in the central-southeasternmost segment of the dome in relationship with its 3D structure, see below (Fig. 9).

Regional structures of the Alpi Apuane, kinematics and paleothermal evolution -Internal deformation of the Alpi Apuane metamorphic core

The regional structure of the Alpi Apuane metamorphic core is that of an asymmetric antiform dome-like structure defined by the attitude of the main foliation D1 dipping west along the western side of the dome and to the east along the eastern side (Carmignani and Giglia, 1979;[START_REF] Kligfield | The Northern Apennine as a collisional orogen[END_REF]Kligfield et al., 1981;Carmignani and Kligfiled, 1990;Carmignani et al., 1995;Molli and Vaselli, 2007;[START_REF] Molli | Structural inheritance and style of reactivation at mid-crustal levels: A case study from the Alpi Apuane (Tuscany, Italy)[END_REF]). This regional scale antiform (Figs. 4,5,9) shows internal complexities due to the presence of two minor culminations respectively centered in the south west and north east part of the dome (Figs. 4,9). The paleoisotherms defined by our RSCM data are deformed by the later (i.e. post-D1) dome-shaped regional structures of the core, resulting in an apparently inverted type field gradient in the North-West, where the main phase (D1) isoclinal regional fold structures (Bergiola Anticline in the Massa unit, Carrara Syncline, Vinca-Forno anticline and Orto di Donna syncline, Figs. 4,5) and related axial planar folition D1 are westdipping, and in a normal type field gradient in the east and southeast part of the Alpi Apuane, where the main phase (D1) isoclinal regional fold structures, e.g. the M.Corchia syncline (southern prolongation of the Orto di Donna-Altissimo syncline), and related axial planar foliation D1 are East-or South/East-dipping.

These relationships betwen the paleothermal architecture and regional deformation features within the Alpi Apuane and surroundings area may be inserted in the conceptual model suggested in Figure 10, which takes into account all available structural and geo-thermochronological data together with presented RSCM results.

The proposed evolutionary model envisages: i) crustal underplating of the Tuscan continental units (Fig. 10a,b) with an early stage of underthrusting and stacking below the Ligurian/subligurian lid (former Ligurian Tethys-derived accretionary wedge). Assuming a constant thermal gradient of 20 °C/Km as defined in the upper continental unit (Tuscan Nappe), a T > 400 °C was reached at crustal depth major than 20 Km, thus fitting available petrological data [START_REF] Di Pisa | Regional variation of the metamorphic temperatures across the Tuscanid 1 Unit and its implications on the alpine metamorphism (Apuan Alps, N-Tuscany)[END_REF]Molli et al, 2000) and our RSCM data. The Massa unit, originally derived from the westernmost position, records higher Temperature (and Pressure) peaks, with respect to the Apuane unit (see also [START_REF] Jolivet | Midcrustal shear zones in post-orogenic extension: the northern Tyrrhenian Sea case[END_REF]Molli et al., 2000a,b); ii) a successive duplexing stage (Fig. 10c,d) may be responsible for the formation of the internal stacking and regional fold development including the overthrusting of the Massa (higher grade) unit above the Apuane unit (lower grade). During this deformation stage (late D1 in Molli et al., 2000), and following the thermo-kinematic model proposed by Dunlop et al. (1998) and[START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF] a folding of previous paleoisotherms is suggested to produce the paleothermal and structural features in the western side of the Alpi Apuane across the contact between the Massa and the Apuane units and downward (Figs. 4,5).

During the crustal duplexing and antiformal stacking (Fig. 11d,e,f) a possible pathway for hot fluids coming from middle to lower crust may be suggested along some major tectonic contact and along the decollement level at the base of detached Paleozoic terms. This hot fluid channelling associated with ore mineralization was localized with the ongoing deformation and after the refolding of previous D1 structures in the eastern limb of the regional antiformal stack within "Stazzemese Shear Zone" (Figs. 9,10e,f).

-The Alpi Apuane metamorphic core and its boundary faults

As previously illustrated, the Alpi Apuane metamorphic core forms a regional scale northwestsoutheast trending elliptical dome, which is separated by the overlying Tuscan Nappe by meter to hundreds of meters thick levels of carbonatic breccias and cataclasites which have been considered

since Trevisan (1965) as derived from the former basal layer of the Tuscan Nappe, i.e. by an original alternation of dolomites and evaporites of Triassic age ("Calcare Cavernoso" Auct.). [START_REF] Carmignani | Crustal extension in the Northern Apennines: the transition from compression to extension in the Alpi Apuane Core Complex[END_REF] first interpreted this breccia-layer ("window fault" in [START_REF] Hodgkins | The use of fluid inclusions to constrain fault zone pressure, temperature and kinematic history: an example from the Alpi Apuane, Italy[END_REF]Casale, 2012) as the detachment horizon that separates low-grade to unmetamorphic upper-plate units (our RSCM T < 300 °C) from the underlying metamorphic lower plate units (401±65 °C mean of all our data set of RSCM T).

From the kinematic point of view the "window fault" cuts down-section the footwall regional structures along a South-West to North-East transport direction [START_REF] Molli | Structural inheritance and style of reactivation at mid-crustal levels: A case study from the Alpi Apuane (Tuscany, Italy)[END_REF] as shown in map and cross section in Figs. 4,5,9. The low angle detachment fault is therefore in contact with the uppermost structures of the western Alpi Apuane (Massa unit, Carrara Syncline, Vinca-Forno Anticline) and with progressively deeper and lower regional structures (Orto di Donna Syncline, Tambura Anticline and easternmost Apuane).

This structural cut-down section corresponds to a paleothermal architecture which shows across the "window fault" and along the transport direction an eastward decrease in the T gap between hangingwall and footwall from ∆T > 200 °C in the west to ∆T < 50 °C in the east (Fig. 9). This is mainly due to the cross-cut relationship and kinematic interaction between the window fault and the footwall antiformal stack of the metamorphic core as figured in the conceptual model of Figure 11e,f. Furthermore, using the thermochronological data in [START_REF] Fellin | Thermochronologic evidence for exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy[END_REF], we may date some steps of the proposed kinematic history with the end of distributed deformation in the footwall metamorphic core and its antiformal stack development occured before 11 Ma (by Zr FT age constrains Temperature of 240 °C), whereas the end of the activity of the "window fault" occurred at a crustal depth as low as ∼ 180 °C in a time interval between 7 to 4-5 Ma. After that time crustal extension ending in the present day tectonic setting, superficial and deep crustal morphostructure was accomodated at shallow crustal levels by high angle normal faults (Fig.,4,5,9) cross-cutting the former synmetamophic structures as well as the "window fault" (Molli et al., 2016 and references therein).

CONCLUSIONS

RSCM T estimates have proven an efficient mean to put extensive T constraints on the thermal structure of coherent units and on tectonic contacts even within very narrow T ranges (Beyssac et al., 2002;Angiboust et al., 2011[START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF][START_REF] Vitale Brovarone | Stacking and metamorphism of continuous segments of subducted lithosphere in a high-pressure wedge: The example of Alpine Corsica (France)[END_REF], 2014;Bellanger et al., 2014).

Our data in the Alpi Apuane metamorphic core well illustrate how RSCM T distribution are related/controlled in different ways by regional and local deformation structures and in turn that the observed style of deformation and type of structures are controlled/related with local paleotemperatures.

Since the seminal paper of [START_REF] Carmignani | Crustal extension in the Northern Apennines: the transition from compression to extension in the Alpi Apuane Core Complex[END_REF], the Alpi Apuane have been worldwide recognized as a metamorphic core complex (Withney et al., 2013), recording mid-crustal distributed post-orogenic "ductile" extensional deformation. This view, however, has been disputed by different authors who have suggested that extensional denudation of the Alpi Apuane was instead related with underplating and thickening of the internal northern Apennines (among others Cello and Mazzoli, 1996;[START_REF] Jolivet | Midcrustal shear zones in post-orogenic extension: the northern Tyrrhenian Sea case[END_REF][START_REF] Fellin | Thermochronologic evidence for exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy[END_REF].

Our RSCM data support this view and remark how the paleothermal architecture of the Alpi Apuane metamorphic core better fits a tectonic scenario of syn-orogenic contractional exhumation associated with a cool paleothermal gradient, which allowed the preservation of inverted metamorphism across the Alpi Apuane metamorphic core and its overall paleothermal architecture.

Our major conclusions put new contraints for the thermo-mechanical evolution and exhumation history of the northern Apennine in its inner (western) side; moreover, they may be of general and widespread interest for other orogens, showing anchimetamorphic to metamorphic mid to shallow crustal terrains and metamorphic cores. Our data clear illustrate how the thermal and metamorphic signature may have different characters and different field gradients at tens of kilometer scale calling for a careful analysis of thermal data in correlation with the associated structural architecture for a firm interpretation of the tectonic frame at the orogen-scale.

FIGURE CAPTION

Fig01. Geological setting of the Northern Apennine and regional cross sections (mod. by [START_REF] Molli | Northern Apennine-Corsica orogenic system: an updated review[END_REF] with indicated the studied areas.

Fig02. Tectonic map of the inner northern Apennines with locations of analyzed samples. In the figure are also reported previous Temperatures estimates and references. Table 1. Selected samples for RSCM thermometry. GPS coordinates in WGS84 system, number of spectra (n), mean R2ratio (Beyssac et al., 2002a) or RA1 ratio (Lahfid et al., 2010) for n spectra with corresponding standard deviation (sdv), and calculated temperature with standard error (SE).

Standard error is the standard deviation divided by √n. The absolute error on temperature is ±50 C (Beyssac et al., 2002a). *low-T samples processed with RA1 ratio. ** Samples for which the degree of graphitization of CM might reflect hydrothermal processes rather than regional metamorphism. Km, fitting the petrological data available [START_REF] Di Pisa | Regional variation of the metamorphic temperatures across the Tuscanid 1 Unit and its implications on the alpine metamorphism (Apuan Alps, N-Tuscany)[END_REF]Molli et al, 2000) and our RSCM data. Note that the Massa unit originally derived from the westernmost position recorded higher Temperature (and Pressure) peaks, with respect of the Apuane unit (see also Molli et al., 2000); c,d) successive duplexing stage with formation of internal stacking and overthrusting of Massa (high grade) above the Apuane unit (lower grade). During this deformation path, which basically follow the thermo-kinematic model proposed by [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF], a folding of previous paleoisotherms is envisiged to produce the thermal features described at page 10 (see cross sections E,F,I of Figs. 4,5). In d) are also indicated the possible path way of hot fluids coming from subducted middle to lower crust which may be associated with ore bodies, reworked within the Stazzemese Shear zone during following exhumation stage; e) antiformal stacking phase with development of: 1) finite geometry of regional deformation structures (dome shape of main foliation and related regional D1 structures); 2) "Stazzemese Shear Zone" possible deeper expression of extensional detachment system witness by the brittle structure of the "window fault" (see Molli and Vaselli, 2007); d) finite thermal structure result of the regional folding of previous isotherms by the antiformal stack-related dome.

  Significantly, the highest RSCM T are found in different protoliths (Palaeozic, Mesozoic-Cenozoic in ages) in close relationships with mineralized. The RSCM T for these samples exceed of about 180-200 °C the values obtained for the enclosing host rocks. As an example, in the Fornovolasco area (the easternmost extension of the "Zona dello Stazzemese"), Tertiary metasandstones (Pseudomacigno Fm.) show RSCM T >500°C (sample 70) if close to mineralized levels (Trimpello ore bodies), whereas the same rock type far from mineralized bodies shows T <360 °C (samples 68,69,71) (Fig. 5 and Table1).

Fig03

  Fig03 Examples of spectra RSCM Fig04 Geological map of the Alpi Apuane with measured RSCM temperatures and other data available from literatures: (1) Ca/Do from in Molli et al. 2000; (2) Di Pisa et al. 1985 and Franceschelli in Carmignani et al., 1987. Fig05 Cross-sections across the Alpi Apuane with RSCM temperatures Fig06. a) Cross section of La Spezia area and measured RSCM temperatures; b) Stratigraphy and RSCM temperatures in the Tuscan Nappe.

Fig07

  Fig07 RSCM temperatures in the different tectonic units of the Alpi Apuane and nearby metamorphic core of Punta Bianca and Monte Pisano.Fig08 Stratigraphy and RSCM temperatures in the Apuane unit.

Fig. 09 .

 09 Fig.09. Paleothermal and structural architecture of the Alpi Apuane metamorphic core.

Fig. 10 .

 10 Fig.10. Microscopic view of Tuscan carbonates in the different tectonic units with related RSCM Tmax and Ca/Do from Molli et al. (2000). All samples have the same protolith of the "Calcare Massiccio" Jurassic in age. a) Tuscan Nappe limestone; b) metalimestone from Panie Unit; c,d) Carrara marbles from Apuane unit (c,d respectively from western and eastern side of the Alpi Apuane).

Fig. 11 .

 11 Fig.11. Evolutionary model of Apenninic wedge during crustal underplating of the Tuscan continental units: a) early stage of nappe stacking below the Ligurian/subligurian lid (former accretionary wedge); b) the same as in a) with in red dashed lines the 400°, 350° and 300 °C paleoisotherms. Assuming a constant thermal gradient of 20 °C/Km as defined in the upper continental unit (Tuscan Nappe) a temperature higher of 400 °C is reached at depht major than 20

  Fig.1 Molli et al JSG 2017

  Fig.2