Geology of the Piemonte-Ligurian units of the Urtier area (Northwestern Alps – Italy)

ABSTRACT Detailed geological mapping of the Urtier Valley in the Western Alps allowed to realize a new geological map 1:10.000 in scale. In the mapped area, a complete nappe stack of the axial part of the Alpine belt crops out. The study focused on the two units derived from the Piemonte-Ligurian oceanic domain, Broillot and Bardonney units, for which, in addition to the classic petrographic and structural considerations, a lithostratigraphic approach was applied. The lithostratigraphic approach, combined with structural analysis, led to reconstruct a nappe stacking significantly different from that generally proposed for the Western Alps. In the Urtier Valley in fact, the eclogite facies units are at the top of the nappe pile, overlapped on the blueschist facies units. This anomalous stacking order could be explained by interpreting the study area as an inverted limb of a regional fold with the eclogite facies units at the core.


Introduction
In the Italian side of the Western Alps, the Urtier Valley is one of the three great valleys open up to the head of the Cogne valley, a dextral tributary of the Aosta Valley ( Figure 1). The Urtier Valley trends EW and extends for about 10 km from the watershed that divides it from the Champorcher Valley, to its confluence with Cogne Valley near Lillaz village.
In the Urtier Valley the complete nappe stack of the axial part of the Alpine belt is exposed, from the lowermost Gran Paradiso Penninic basement, the superposed oceanic units derived from the Piemonte-Ligurian oceanic domain, up to the uppermost Sesia continental basement slivers. This nappe stack is the result of the convergence between the European lower plate and Adria upper plate that, starting in the Cretaceous, led to the closure of the interposed Piemonte-Ligurian oceanic domain within a subduction zone, resulted in the continental collision during the Cenozoic (Argand, 1916;Butler et al., 2013;Dewey et al., 1989;Rosenbaum & Lister, 2005). Geodynamic models and paleogeographic reconstructions provide a foreland-directed nappe stack, related to a subduction process that started in the innermost units of Adria domain and then progressively involving the more external units of oceanic domain and finally the European passive margin (Dal Piaz et al., 2003;Schmid et al., 2004).
This geodynamic evolution seems to be confirmed by the distribution of radiometric age data corresponding to peak-pressure mineral assemblages, which progressively become younger from the continental basement units of the Adria plate at the top, to the lowermost continental units derived from the European margin (Amato et al., 1999;Dal Piaz et al., 2001;Inger et al., 1996;Lapen et al., 2003Lapen et al., , 2006Liati et al., 2002Liati et al., , 2005Ramsbotham et al., 1994;Rubatto et al., 1999;Thöni, 1999;Villa et al., 2014).
The interposed tectonic units derived from the Piemonte-Ligurian oceanic domain are characterized by intermediate radiometric ages, and are classically subdivide into two different nappe systems (Figure 2), a structurally higher blueschist facies unit (i.e. the Combin Unit) superposed on an eclogite facies unit (i.e. the Zermatt-Saas Unit) (Bearth, 1967). This structural setting, described for the first time on the Swiss Alps and in the northern Aosta Valley, was considered reliable for the whole belt and therefore extended to the southern Aosta Valley and to the French-Italian Alps (e.g. Balestro et al., 2019;Ballèvre & Merle, 1993;Dal Piaz, 1999;Michard et al., 1996).
An example of this complexity is represented in the Urtier Valley, where a structural stacking characterized by the eclogite facies oceanic units superposed on the blueschist facies oceanic units was proposed (Ellero & Loprieno, 2014, 2018Loprieno et al., 2009).
In this contribution, a new geological map of the Urtier Valley area is presented, accompanied by geological cross-sections. The geological mapping was carried out paying particular attention to the lithostratigraphic characters of the Piemonte-Ligurian Units and to the reconstruction of the geometric relationships between the tectonic units, with the aim of contributing to a better understanding of the distribution, deformation history and structural setting of the Piemonte-Ligurian Units in the southern side of the Aosta Valley.
Among all possible methodological approaches, accurate geological mapping has been proven to be one of the most important tools for a reliable geometric reconstruction of the nappe pile that represents the essential basis for any geodynamic model.

Geological setting of the Urtier Valley
In the Urtier Valley, five different tectono-metamorphic units were distinguished (Figure 3). From bottom to top, they are: (i) the Gran Paradiso Unit; (ii) the Pene Blanche Unit; (iii) the Broillot Unit; (iv) the Bardonney Unit; (v) the Tour Ponton and Acque Rosse units (Ellero & Loprieno, 2014, 2018. The lowermost Gran Paradiso Unit belongs to Briançonnais-type Penninic basement resulting from the originary European continental margin. This unit represents the southern limit of the mapped area and consists of augen gneisses derived from late-Variscan granitoids intruded into metasedimentary rocks (Gneiss Minuti) (Bertrand, 1968;Compagnoni et al., 1974). The Gran Paradiso Unit is characterized by an eclogite facies metamorphism dated at c. 33.7 Ma (Radulescu et al., 2009).
The overlying Pene Blanche Unit consists of a Briançonnais-like facies metasedimentary succession, which start with Lower Triassic quartzite and Liassic marble unevenly preserved along the stratigraphic contact with the Gran Paradiso basement (Ellero & Loprieno, 2018). The metasedimentary succession continues with Lower Triassic to Liassic limestones (Elter, 1972), followed upward by Liassic calcschists and marbles not associated with ophiolites, outcropping in the northern side of the Urtier Valley as dismembered tectonic slices (cfr. 'Faisceau de Cogne' of Elter, 1972). For the Pene Blanche Unit an eclogite facies metamorphism was suggested (Beltrando et al., 2008).
lowermost Broillot Unit and the uppermost Bardonney Unit (Ellero & Loprieno, 2018). These two Piemonte-Ligurian units represent the main topic of this study and therefore they will be described in detail in the following section.

Methods
The geological map results from original fieldwork at 1:10.000 scale realized during Master's Degree theses by A. Loprieno and A. Ellero (Ellero, 1994;Loprieno, 1994), subsequently integrated and corrected during several additional mapping performed by the authors at 1:10.000 scale. Geological mapping was performed using topographic maps (Carta Tecnica Regionale) produced by the Regione Autonoma Valle d'Aosta, represented on a vector topographic map derived from the CTRN (Carta Tecnica Regionale Numerica 1:10.000; Coordinate System represented in meters ED50-European Datum 1950, UTM Zone 32 Nord). These topographic maps have been scaled at 1:12.500 and the raster format were digitized to be used for the Main Map.
Structural elements documented in the field (e.g. foliations, fold axes, stretching lineations) are represented in the Main Map using a synoptic representation that allows differentiating their tectonic superposition order. The Quaternary geology resulted from original mapping. The Main Map is accompanied by (i) a tectonic sketch map of the study area where the main fold axial planes are shown, (ii) five geological cross-sections and (iii) two panoramic photos where the nappe pile of the area is highlighted.
Unlike the methodology usually applied for the study of the Piemonte-Ligurian Units units, in which the different tectonic units are distinguished mainly by their different metamorphic grade, we used a lithostratigraphic approach combined with structural and petrologic analyses. The evidence on the outcrops of lithological boundaries that retain the original meaning of primary stratigraphic contacts, associated to the knowledge of the stratigraphic succession that characterizes the homologous oceanic units of the Northern Apennines (Decandia & Elter, 1972;Elter, 1975), allowed us to use a stratigraphic approach, based on concepts such as stratigraphic polarity or younging direction, to perform an accurate reconstruction of the geometries that characterize the nappe pile. In this work, we therefore felt authorized to use the term 'stratigraphic', despite being aware of the intense re-elaboration due to the tectonic-metamorphic evolution.

The Piemonte -Ligurian units of the Urtier Valley
In the Urtier Valley have been recognized two tectonics units belonging to the Piemonte -Ligurian oceanic domain, the Broillot Unit and the Bardonney Unit (Ellero & Loprieno, 2018). These units have a different metamorphic evolution. The Broillot Unit is characterized by a general metamorphic retrogression under greenschist facies conditions. Quantitative petrological analyses are lacking for the Broillot Unit, with the only indications on the peak metamorphism deriving from qualitative petrological studies, which interpreted the garnet-chloritoid paragenesis of micaschists as indicative of blueschist facies conditions (Bousquet, 2008;Ellero & Loprieno, 2018), or, alternatively, of eclogite facies conditions (Beltrando et al., 2008).
Instead, in the Bardonney Unit, the peak metamorphic conditions correspond to the eclogite facies, as evidenced by garnet + omphacite + rutile paragenesis preserved in meta-Fe-Ti gabbros included in the Mg-Fe gabbro blocks. A complex metamorphic retrogression history, characterized by several stages of recrystallization, can be observed starting from the replacement of garnet + omphacite by glaucophane + clinozoisite, continuing with barroisite + actinolite replacing glaucophane, and finally with the formation of chlorite, epidote, albite and titanite. The tectonometamorphic history of the Bardonney Unit is therefore characterized by a metamorphic peak under eclogite-facies conditions, followed by a retrogression with progressive decrease of pressure, from blueschist-to greenschist-facies conditions, respectively (Ellero & Loprieno, 2018;Tartarotti et al., 2019a).
Another important difference between the two tectonic units consists in a different lithostratigraphic succession that, in the case of Broillot Unit, maintains its original stratigraphic order despite the complex deformation history. It is precisely these different lithostratigraphic characters that will be described and interpreted in this study.

The Broillot Unit
The Broillot Unit mostly crops out in central and northern parts of the mapped area, whereas some small and discontinuous slices cropping out directly above the Gran Paradiso Unit in Bardonney and Acque Rosse valleys (see the Main Map; Figure 3). In the Chatillon sheet of the new Geological Map of Italy (Dal Piaz et al., 2008) this unit is generally included within the Zermatt-Saas Unit.
However, the only study that deals specifically with the Piemonte-Ligurian units of this area, describes a 'Blueschist facies Piemonte unit' cropping out in the Urtier Valley, correlating this unit, which partly corresponds to the Broillot Unit, with the Combin Unit (Beltrando et al., 2008).
The most important feature of this unit is the wellpreserved lithostratigraphic succession that is diagnostic of a typical oceanic crust associated with its sedimentary cover (Figure 4(a)). The ophiolitic basement is mainly represented by prasinites and minor serpentinites (Colonna iron-mine area, NE of Cogne). Prasinites are interpreted as metamorphic equivalent of originary basalts. They are characterized by greenschists paragenesis, consisting of albite, chlorite, green amphibole, epidote, garnet and titanite. In some cases, prasinites show ovardite textures, characterized by porphyroblast albite with inclusions of green-brown amphibole, titanite, epidote and chlorite. The ultramafic rocks consist of antigorite serpentinites, with subordinate magnetite and relics of pyroxenes.

The Bardonney Unit
The Bardonney Unit crops out on the southern flank of Urtier Valley, and in the Tour Ponton area, below the continental basement sliver (see the Main Map; Figure 3). This unit partly corresponds to the 'Eclogite facies Piemonte unit' described by Beltrando et al. (2008), correlated to the Zermatt-Saas unit by these authors. The Bardonney Unit can be partly correlated also to the 'Grivola-Urtier unit' (Dal Piaz et al., 2008;Tartarotti et al., 2019b), which is instead separate by the Zermatt-Saas unit.
The distinctive character of the Bardonney Unit is the total absence of stratigraphic order, with a chaotic tectono-stratigraphic complex, characterized by ophiolitic blocks of variable size, origin and lithology, interpreted as originary olistoliths, embedded in a sedimentary matrix (Ellero & Loprieno, 2018). The largest ophiolitic blocks reach cartographic scale and consist of serpentinites and metagabbros ( Figure 5). The serpentinites are foliated and show a paragenesis of serpentine, chlorite, amphibole, pyroxene (diopside), epidote, metamorphic olivine, titanian clinohumite and magnetite. The serpentinite blocks are associated with a monogenic metabreccia consisting of clasts of serpentinite embedded in a heterogeneous carbonate matrix ( Figure 5(b)), characterized by calcite mixed with serpentine, chlorite, epidote, garnet, amphibole, clinopyroxene, white mica, albite, titanian clinohumite, tourmaline and titanite, suggesting a detrital origin for these deposits that can be interpreted as an originary reworked meta-ophicalcite. A similar interpretation has been proposed for many other locations within the Piemonte-Ligurian domain relicts in the Western Alps (e.g. Lafay et al., 2017;Lemoine, 1980;Tartarotti et al., 2017bTartarotti et al., , 2019a.
The metagabbro blocks consist of Mg-Fe gabbro containing pods of eclogitic Fe-Ti metagabbros characterized by omphacite, garnet and rutile assemblage.
Ophiolitic blocks of smaller size, ranging from centimeter to metric scale, generally consist of amphibolites (Figure 6(a,b)), with a paragenesis characterized by amphibole, chlorite, epidote, albite, garnet, rutile and/or titanite. In well-preserved ophiolitic blocks, the amphibole consists of glaucophane usually associated with garnet and rutile. Blocks in matrix can be derived not only by the ophiolitic basement, but also from Piemonte-Ligurian sedimentary sequence. In this case, the slide-blocks consist of marbles and calcschists associated to amphibolites embedded in a carbonate matrix (Figure 6(c)). The matrix of the chaotic complex consists of alternate layers of carbonate, quartzite and metabasite, likely derived from the erosion of ophiolitic basement and sedimentary cover (Figure 6(d)). Carbonate layers are calcschists and impure marble, composed by calcite, white mica, quartz, chlorite, epidote and titanite. Garnet porphyroblasts are common, containing inclusions of glaucophane, rutile, clinopyroxene and quartz. The quartzite matrix is represented by metasandstones containing quartz, white mica, chlorite, garnet, amphibole, albite, rutile and/or titanite. Metabasic matrix consists of prasinites and amphibolite layers, generally alternated with metasandstones or calcschists. Metabasites are composed by amphibole, garnet, albite, zoisite, chlorite, biotite, rutile and/or titanite, quartz and calcite, with diffused relics of high-pressure assemblages, represented by glaucophane associated to garnet and rutile.
In the field, the most evident deformation phase is represented by the D2, which developed F2 isoclinal folds associated with S2 greenschist facies axial plane foliation (Figure 7(a,b)). L2 stretching lineations are parallel to the E-W oriented A2 fold axes, suggesting pervasive shearing during the D2 phase. Shear bands and asymmetric porphyroclasts indicate a top-to-the-W transport direction (Figure 7(c)). The evidence of the tectonic boundaries deformed by F2 folds suggested that the different tectonic units were juxtaposed during the D1 or alternatively in the early stages of the D2. In the eclogitic units, the S2 is a composite schistosity progressively developed during the exhumation processes (Ellero & Loprieno, 2018), initially characterized by the oriented growth of glaucophane and epidote typical of blueschist facies, then replaced by greenschist assemblages represented by Ca-Amphibole, chlorite, albite and titanite growth along the same schistosity. Instead, in the Broillot Unit, the S2 has developed entirely within greenschist metamorphic facies conditions (Ellero & Loprieno, 2018).
The last D3 deformation phase is testified by open to closed F3 folds (Figure 7(b)) with A3 fold axes parallel to A2 axes and L2 stretching lineations, associated with gently NE or SW S3 dipping axial plane crenulation cleavage (Figure 7(d)). The superposition between F3 and F2 folds produced type 3 interference pattern of Ramsay (1967) (Figure 7(b)).
The interference pattern between the three deformation phases D1, D2, and D3 is particularly evident in the Miserin lakes area, where the marker is represented by the tectonic contact between Acque Rosse and Bardonney units (Figure 7(e)).

Discussion
During the geological mapping of the Piemonte-Ligurian units outcropping in the Urtier Valley, a lithostratigraphic approach was applied to reconstruct the different units in the field and the large-scale structures. The two Piemonte-Ligurian units recognized in the Urtier Valley can be distinguished not only for their different peak metamorphic conditions, but also for their different stratigraphic features. In fact, while the Broillot Unit retains an almost complete cover sequence associated with its ophiolitic basement, the Bardonney Unit consists of a chaotic complex, without any lithostratigraphic order.
The preserved stratigraphic boundaries in the Broillot Unit succession represent the key for the reconstruction of the regional structures of the nappe pile in Urtier Valley. These boundaries, easily recognizable at the outcrop, can be followed in the field for kilometers allowing the recognition normal or reverse flanks of the fold structures taking into account of the normal or reverse polarity of stratigraphic succession. This approach led to the reconstruction of a nappe pile proposed in Ellero and Loprieno (2018), characterized by a geometric order of superposition between Piemonte-Ligurian units different from previously proposed models for the study area and in general for the south of Aosta Valley (e.g. Ballèvre et al., 1986;Battiston et al., 1984;Beltrando et al., 2008). The Tour Ponton structure at the map scale summarizes the effective geometrical order of the nappe pile (Figure 3, Main Map). The opposite stratigraphic polarity of Broillot Unit succession, north and south of the Gneiss Minuti Complex of Tour Ponton Unit, testify reverse and normal limbs of a F2 synform. Retrodeforming this F2 fold, a geometrical stack characterized from top to bottom by Tour Ponton Unit, Bardonney Unit and Broillot Unit is shown. The structural stack thus obtained differs from what is usually proposed for the Western Alps, because it is characterized by the eclogite facies units (Tour Ponton and Bardonney units) onto the blueschists facies unit (Broillot Unit). This apparently anomalous stack order, was explained by a complex multistage D2 exhumation phase, in which, the tectonic contacts, developed in the early stages, were folded during the latest D2 stages (Ellero & Loprieno, 2018). In the proposed model, the Tour Ponton structure corresponds to an overturned limb of a F2 regional isoclinal fold involving eclogitic units in the core of blueschist units, whose normal limb crops out in the northern Aosta Valley (Figure 16 in Ellero & Loprieno, 2018).
Considering the preserved stratigraphic succession of the Broillot Unit as an effective marker for the reconstruction of the structural stack geometries, it is possible to observe how, the anomalous stack order described for the Tour Ponton structure, characterizes the entire study area. In this regard, particularly significant is the finding of slivers of typical Broillot metasedimentary succession, with the association of prasinite, quartzite and calcschists, tectonically above the Gran Paradiso Unit (Figure 3, Main Map). The direct superposition of the blueschists facies Broillot Unit above the eclogite facies Gran Paradiso Unit was interpreted as a normal fault-type contact, originated during the early stages of the D2 exhumation phase (Ellero & Loprieno, 2018).
The structural stack created during the exhumation phases, was subsequently refolded by the D3 deformation phase that was correlated with the D3 nappe refolding phase of Bucher et al. (2003Bucher et al. ( , 2004 (Ellero & Loprieno, 2018). Finally, this regional structure was dislocated by the Late Oligocene-Neogene Aosta-Ranzola normal fault system (ARL in Figure 2(c)) (Bistacchi et al., 2001), with the inverted limb of the regional fold outcropping south of Aosta Valley.

Conclusions
Detailed field mapping in the Urtier Valley was performed, leading to a new 1:12.500 scaled geological map, accompanied by geological cross-sections.  (Ramsay, 1967) in the Gneiss Minuti of Acque Rosse Unit. The interference pattern is drawn in the sketch at the bottom right. (b) Isoclinal hinge F2 refolded by F3 fold producing type 3 interference pattern (Ramsay, 1967) in the serpentinites of Bardonney Unit. (c) West-dipping shear bands developed during D2 deformation phase in the calcschists of Broillot Unit. (d) S3 crenulation cleavage in the serpentinites of Bardonney Unit. (e) Interference pattern between D1, D2 and D3 structures at the Miserin Lakes.
Geological mapping was accomplished by applying an unusual approach for a highly deformed and metamorphic environment such as the Western Alps. The two tectonics units belonging to the Piemonte -Ligurian oceanic domain were in fact distinguished on the basis of their respective lithostratigraphic characters. The two oceanic units recognized in the study area, named Broillot and Bardonney units, can be correlated respectively to the blueschist facies Combin Unit and to the eclogite facies Zermatt-Saas Unit, traditionally distinguished inside the Piemonte -Ligurian oceanic domain of the Western Alps. The Broillot Unit is characterized by a well-preserved stratigraphic succession which can be interpreted as the metamorphic equivalent of the pelagic deposits described for the Internal Liguride units in the Northern Apennines. Conversely, the Bardonney Unit shows the characteristics of a chaotic complex, without any stratigraphic order. The preserved stratigraphic contacts observed in the Broillot Unit have been used as markers for the reconstruction of the structures at the map scale, thanks to their evidence and continuity in the field. In addition, the stratigraphic polarity allowed to clearly recognize normal or reverse limbs of map scale folds. This study highlighted a structural stack characterized by eclogite facies units at the top of the nappe pile. This structural setting can be explained assuming a regional scale fold structure with eclogite facies unit at the core, with the inverted and normal limbs outcropping respectively south and north of Aosta Valley.
Geological and structural mapping even of little areas, proved to be a necessary tool for a robust geodynamic reconstruction at regional scale.
Moreover, this study showed that, also in a severely metamorphosed and deformed environment, the lithostratigraphic approach can be the most valuable tool to reconstruct the nappe stack.

Software
The map database has been implemented in QGIS environment (QGIS 3.10). The final map layout was assemblated using Adobe Illustrator. Finally, we have used Adobe Illustrator and Adobe Photoshop for figures and photo assemblages.