1. Introduction

The Ligurian Alps are a peculiar segment of the Alpine chain characterized by a long-lasting tectonic evolution [i.e. Capponi et al. 2009; Molli et al. 2010; Seno et al. 2003; Vanossi 1986]. Starting from the middle of the last century, geologists mainly focused their attention on the Alps–Apennine relationships [Elter et al. 1966; Elter and Pertusati 1973; Grandjacquet and Haccard 1977; Laubscher 1991; Marroni et al. 2017; Mosca et al. 2009] and/or the study of the high pressure–low temperature (HP–LT) metamorphic units with particular attention to their exhumation-related retrograde paths [i.e. Voltri Group, Spagnolo et al. 2007; Malatesta et al. 2012]. In the last twenty years, various scientific contributions [Carminati 2001; d’Atri et al. 2016; Mueller et al. 2020; Piana et al. 2021; Sanità et al. 2020; Seno et al. 2005], were focused on the western Ligurian Alps where a SW-verging tectonic pile is composed of both continental- and oceanic-derived units [i.e. Di Giulio 1992; Haccard 1961; Lanteaume 1962, 1968; Vanossi 1986, 1991; Vanossi et al. 1994]. Unraveling the deformation history of such a crustal-scale and composite, tectonic pile is of paramount importance for a better understanding of Alpine collision tectonics in this sector of the Alpine chain [see discussions in Carminati 2001; d’Atri et al. 2016; Di Giulio 1988; Piana et al. 2021; Sanità et al. 2020]. In order to achieve this objective, it is necessary to identify key areas where the relationships between the different units are well exposed with a clear structural framework and relevant kinematic indicators. Such requirements are found in the High Tanaro Valley, a remarkably exposed area at the border between Piemonte and Liguria Regions, and located at the junction between Maritime and Ligurian Alps (Figure 1a), south of the Marguareis Massif, whose structural architecture was recently updated and reappraised [Sanità et al. 2020, 2021]. This work is aimed to provide new high-resolution structural data to depict the structural architecture and the deformation history of the tectonic units outcropping in this part of the Ligurian Alps and to discuss their tectonic evolution in the framework of the Western Alps geodynamics.

2. Geological context of the Western Ligurian Alps

In the Western Alps the orogenic history started in Late Cretaceous times [i.e. Dewey et al. 1989; Rebay et al. 2018; Rosenbaum et al. 2002], with the subduction of the Ligure–Piemontese Ocean due to the convergence between Europe and Adria Plates. The prolonged convergence has resulted, in the first stage, in the involvement of the European continental margin into the Alpine wedge through continental subduction, and subsequently in the collision starting from middle Eocene [Coward and Dietrich 1989; Handy et al. 2010; Rosenbaum et al. 2002; Stampfli et al. 2001; Simon-Labric et al. 2009].

In the framework of the tectonic scenario proposed for the western Ligurian Alps [Seno et al. 2003, 2005; Vanossi et al. 1984] the lowermost portion of the SW-verging tectonic pile (Figure 1a) is represented by the Briançonnais Units characterized by a typical Meso-Cenozoic succession diagnostic of an origin from the thinned portion of the European continental margin [Decarlis et al. 2013; Vanossi et al. 1984]. The latter underwent a HP–LT metamorphic imprint that decreases progressively westward [Bonazzi et al. 1987; Messiga et al. 1981]. These units underthrust the oceanic-derived unit (Ligure-Piemontese Ocean), called Moglio–Testico Unit [Boni and Vanossi 1972; Lanteaume 1968], represented by Late Cretaceous–Paleocene(?) oceanic sediments showing very low-grade metamorphic imprint [Bonazzi et al. 1987; Messiga et al. 1981; Seno et al. 2005]. The Helminthoid Flysch Unit (FH) [cf. San Remo-Monte Saccarello Unit, Vanossi et al. 1984], including its basal complex (Lanteaume, 1968), forms the top of the tectonic pile (Figure 1a). It consists of a Late Cretaceous non-metamorphic sedimentary succession detached from its original basement, whose paleogeographic provenance is still debated [Di Giulio 1992; Mueller et al. 2018; Sanità et al. 2020; Vanossi 1991]. Interestingly, inverted superposition relationships between these units, i.e. the Briançonnais units thrust onto the FH, can be locally observed [Di Giulio 1988; Merizzi and Seno 1991]. In the more external sectors of the Ligurian Alps (toward west) the FH is thrust onto the Dauphinois/Provençal Units (Figure 1a) that represent the internal portions of the European continental margin [i.e. Decarlis et al. 2013].

3. High Tanaro Valley: geological overview

In High Tanaro Valley (northwestern Ligurian Alps), the tectonic pile is composed of SW-vergent continental- and/or oceanic-derived units (Figure 1b). According to the available paleogeographic reconstructions [Decarlis et al. 2013; Lemoine et al. 1986; Seno et al. 2005; Stampfli 1993; Vanossi et al. 1984 and quoted references; Vanossi 1991] it can be assumed that: (i) the continental units (Marguareis Unit—MU, Chambeuil Slices—CS) exhibit a Meso-Cenozoic succession typical of the Briançonnais Domain; (ii) the FH deposits are regarded as the sedimentary cover of the ocean–continent transition of the Adria Plate [Sanità et al. 2020] or conversely of an oceanic domain [see Mueller et al. 2018, and references therein]. At the regional scale, the relationships between the different tectonic units and their deformation history are still debated. According to Mueller et al. [2020], the structural setting is characterized by a SW-vergent tectonic pile in which the Late Cretaceous non-metamorphic FH [cf. Sanremo Unit of Mueller et al. 2020] is thrust onto the Dauphinois Units. These two units are separated by highly sheared Cenozoic siliciclastic turbidite deposits [i.e. Flysch Noir, or Boaria Formation of Sanità et al. 2020]. Moving from SW toward NE, the FH shows an overturned tectonic contact with the Ormea Briançonnais units [cf. Marguareis Unit of Sanità et al. 2020]. As already outlined by Cabella et al. [1987], Di Giulio [1988] or Vanossi et al. [1984], all these tectonic contacts are deformed in turn by a SW-verging later folding event that is responsible for the overturned relationships between these units. Moreover, north of the study area, in the Marguareis Massif, Sanità et al. [2020] recognized a structural architecture in which the FH is located in between Briançonnais units. All the previously cited authors recognized a finite strain pattern resulting from superposed folding events, and according to Mueller et al. [2020] or Sanità et al. [2020], the FH and the Briançonnais Units exhibit contrasting internal deformation patterns. Conversely, d’Atri et al. [2016] and Piana et al. [2021] suggested that the Helminthoid Flysch and Briançonnais units are bounded by a NW–SE-oriented high-angle fault network developed during brittle and progressive transpressional tectonics.

4. The tectonic units in High Tanaro Valley: lithostratigraphy and structural mapping

In the valley of Tanaro river (boundary between Piemonte and Liguria Regions about 50 km southwest of the town of Cuneo), we detected a well-exposed tectonic pile allowing litho-structural mapping at 1:10.000 scale to be performed (Figure 1b,c). The mapped area covers about 32 km² from Upega village (easternmost boundary) to Monte Bertrand area (westernmost boundary) along the Italy–French border until the Colle del Vescovo (northernmost boundary) area. The tectonic pile, from top to bottom, is composed of: (i) the MU (Briançonnais Domain), which extends to the northwest along the southwestern sector of the Marguareis Massif [Sanità et al. 2020]; (ii) the FH (Ligurian Domain) with its basal complex [basal complexes of Lanteaume 1957] extends to the north-northwest until Limone Piemonte village (Figure 1a); (iii) and the CS characterized by Meso-Cenozoic sequences. Tectonic slices composed of fragments of the FH are also present along the tectonic contact separating it from CS (Figures 1b,c, 2a).

The MU is mostly represented by the Upega Formation and the Boaria Formations [Sanità et al. 2020]. The first one is composed of Late Cretaceous–Paleocene(?) hemipelagic marly limestones [i.e. Calcshistes Planctoniques of Fallot and Faure-Muret 1954], whereas the second one is represented by fine-grained turbidites interlayered with chaotic deposits of middle Eocene age [Gidon 1972; Sanità et al. 2020—cf. Flysch Noir of Lanteaume 1968]. However, older formations [i.e. Val Tanarello Limestone and Rio di Nava Limestone, Boni et al. 1971] are exposed along the easternmost border of the study area (Figure 1b).

The FH consists of a non-metamorphic sedimentary succession. It is composed of its basal complex characterized by basinal plain deposits [San Bartolomeo Formation or basal complexes of Lanteaume 1957] represented by red to green manganese-rich shales of Late Cretaceous age [Hauterivian–Campanian, Cobianchi et al. 1991; Manivit and Prud’Homme 1990]. In the topmost portion, fine-grained siliciclastic turbidites represented by dm-thick lenticular beds of arenites are also present. The basinal plain deposits passing upward to deep-sea fan coarse- to medium-grained turbidites are represented by m-thick beds of siliciclastic arenites capped by marls and cm- to dm-thick beds of shales [Bordighera Sandstone, Sagri 1980]. The calcareous nannofossil content recognized within samples collected at the bottom of the Bordighera Sandstone yielded a Campanian age in accordance with Sanità et al. [2020] and Manivit and Prud’Homme [1990].

The CS (Figures 1b,c, 2a,b) are represented by Jurassic platform deposits (i.e. Val Tanarello Limestone), Late Cretaceous–Paleocene(?) Upega Formation, middle Eocene nummulite-rich limestones (Madonna dei Cancelli Limestone) and middle Eocene Boaria Formation of Sanità et al. [2020]. Despite the intense deformation, in each slice the primary stratigraphic relationships between different formations are preserved (Figure 2c). Tectonic slices with comparable structural position, i.e. located at the base of the Helminthoid Flysch–Briançonnais couple, were documented also in the Embrunais-Ubaye sector and they are regarded as witnesses of the Sub-Briançonnais Domain [see Thum et al. 2015, and quoted references]. However, based on widely accepted stratigraphic observations, we reaffirm that the CS outcropping in the study area can be regarded as fragments of Briançonnais units (see also Section 6).

5. Deformation history recorded by the tectonic units

Detailed mapping and micro- to map-scale structural analysis, were performed in order to unravel the finite strain pattern of the different units outcropping in the investigated area. We preferred to retain the labelling and the deformational event classifications used in Sanità et al. [2020] to avoid any misunderstanding among the readers.

Structural analysis allowed us to reconstruct specific internal strain patterns for each unit and to distinguish in the whole area of interest the superposition of pre- (Section 5.1) syn- (Section 5.2) and post-stacking (PS) (Section 5.3) tectonic structures.

5.1. The pre-stacking structures

The MU recorded two well-distinct deformation events called D1_MU and D2_MU. The D1_MU event is mostly represented by a S1_MU pervasive foliation preserved as relict foliation between D2_MU microlithons. At the microscale (Figure 3b), the S1_MU is a slaty cleavage highlighted by syn-metamorphic white micas, quartz, plagioclase, calcite and chlorite with strong preferred shape orientations, as well as stylolitic surfaces underlined by concentrations of insoluble minerals, particularly oxides. S1_MU shows a NW–SE trend and dips toward NE and SW with variable angles (Figure 4). F1 folds are rarely observed and they show (Figure 3a) similar geometry and scattered fold axes with axial planes of NW–SE strike and dips toward SW and NE (Figure 4). The D2_MU is the most prominent deformation event that can be observed in this area of the MU, thus S2_MU foliation is the dominant structural feature and is represented by a pervasive spaced crenulation cleavage (Figure 3c,d). At the microscale (Figure 3d) it is marked by stylolitic surfaces, rare syn-metamorphic white micas, re-oriented grains of calcite, quartz, chlorite and oxides. S2_MU foliation is associated with NE-verging F2_MU fold system developed from micro- to map-scale (Figures 1b,c, 3c). F2 folds (Figure 3c) show parallel to similar geometry with NW–SE trending fold axes plunging toward NW and SE and axial planes with NW–SE direction and dips toward SW and NE with variable angles (Figure 4).

In the FH the oldest deformation event (D1_FH) is testified by S1_FH foliation, mostly preserved as relict in hinge zone of D2_FH-related microlithons. In thin section (Figure 3f), S1_FH is a slaty cleavage outlined by stylolitic surfaces, phyllosilicates with strong preferred shape orientation, elongated grains of calcite, quartz and feldspars. At mesoscale S1_FH can be well observed in the hinge zone of the F2_FH-related folds. It is also present in the slices at the bottom of the FH where it is associated with F1 folds (Figure 3e) characterized by a NE–SW trending fold axes plunging NE and SW and axial planes with NE–SW direction and dips toward the SE (Figure 4). The D2_FH phase is the most prominent deformation event recorded in the FH. It is testified by S2_FH foliation well-evident only in the fine-grained rocks (marls and shales). At the microscale (Figure 3i) S2_FH is a crenulation cleavage marked by stylolitic surfaces and re-oriented grains of calcite, quartz. The S2_FH is associated with SW-verging F2 fold system (Figures 1b,c, 3h) showing parallel geometry, NW–SE trending fold axes plunging toward NW and SE and axial planes with NW–SE direction and dips toward SW and NE (Figure 4).

The CS located at the bottom of the FH recorded the same polyphase pre-stacking deformation history (Figure 2d,f) to the ones we documented in the MU. Therefore, we reaffirm that the deformations observed in the tectonic slices can be correlated with those of the MU.

5.2. The syn-stacking structures

The syn-stacking deformation events, which were largely observed in the whole study area, are responsible for the coupling of the units and for their folding. The thrust surfaces are testified by two main unit-bounding shear zone systems marked by decametric-thick high-strain cataclastic zones (Figure 5a,b). The lowermost is located at the bottom of the FH (Figure 5) and regarded as in-sequence thrusting events. It shows NW–SE strike and dips toward NE and SW with medium- to high-angles (Figure 5e). The stretching lineations on the fault planes show NE–SW trend and plunge NE and SW (Figure 5e). Top-to-SW kinematic indicators are present and they are represented by sigma-shaped dm-sized siliciclastic and carbonate clasts (Figure 5c) observed in various lithologies (i.e. Bordighera Sandstone, Boaria Formation and Upega Formation). The uppermost shear zone system separates the overlying MU from the underlying FH (Figure 5) and is regarded as an out-of-sequence thrusting event. It shows NW–SE strike and dips at various angles towards NE and SW (Figure 5f). The stretching lineations show NE–SW trend and plunge NE and SW (Figure 5f). Kinematic indicators pointing toward SW are represented by cm-sized sigma-shaped carbonate and siliciclastic clasts (Figure 5d).

A major knee-shaped folding event affecting only the MU is observed. It is characterized by a NW–SE trending fold axes plunging NW and SE and axial planes showing a NW–SE direction and a generally northeastward dipping axial plane (Figure 5). This event was already documented in the MU exposed in the southwestern sectors of the Marguareis Massif [D3_MU of Sanità et al. 2020] and is geometrically consistent with the out-of-sequence thrusting event. The tectonic foliation related with this folding event is rarely observed and it is not associated with metamorphic re-crystallization. This suggests that this folding event took place at very shallow structural levels [as suggested by Sanità et al. 2020, 2021].

5.3. The post-stacking deformation events

All the previously described pre- and syn-stacking structures are deformed by the so-called PS fold system and faults. The PS fold system is mostly located in zones where vertical or sub-vertical layering is present (Figures 1b,c, 5a). PS fold system is testified by recumbent open folds showing parallel geometry and rounded hinge-zone (Figure 6a–c). The PS fold system shows (Figure 6e) a NW–SE trending fold axes (A_PS) and sub-horizontal to southwestward-dipping axial planes (AP_PS). In the field the associated tectonic foliation is only observed in the less competent rocks (shales or marls). In thin section (Figure 6d) it is marked by stylolitic surfaces (S_PS) without any metamorphic re-crystallization, suggesting its development at shallow structural levels. The faults show a NNE–SSW main direction (Figure 1b) and cut at high angles all the previously described structures including the AP_PS. A minor NW–SE-oriented fault system is also present.

6. The role of thrust tectonics in Helminthoid Flysch–Briançonnais relationships and comparison with other key areas of the Western Alps

The geological survey carried out in the junction area between Maritime and Ligurian Alps first clearly confirms that the relationships between the major Alpine tectonics units of interest can only be interpreted in the framework of a polyphase tectonic evolution and not as a result of a single transpressive brittle tectonic event as proposed by Piana et al. [2021]. Secondly, it demonstrates that the very low-grade FH is located in between tectonic units (i.e. Briançonnais Units, Figures 1b,c, 5) showing higher metamorphic conditions. In Figure 7 we report two geological cross-sections showing the structural architecture of two areas located in the southwestern sector of the Marguareis Massif (Figure 7a,b) and in the area investigated in this paper (Figure 7c). The two cross-sections clearly highlight amazing structural analogies. In both the sections the MU forms the topmost portion of the tectonic pile and is thrust, with an out-of-sequence thrust, onto the FH and its basal complex. The FH is thrust onto Briançonnais units both represented by tectonic slices (i.e. Cima del Becco Slice, and CS in this paper) and units [i.e. the Cabanaira Unit, Sanità et al. 2020]. Each unit recorded different pre-stacking deformation histories responsible for development of fold systems at different structural levels. The related axial planes are cut by the unit-bounding shear zone systems, which show a top-to-SW sense of shear, and are responsible for the coupling of the units. Thus, this indicates that the thrusting events superimposed onto the pre-stacking structures. So, the thrusting events are suggested to have occurred during the syn-stacking tectonics.

Recognizable in the geological cross-sections, the out-of-sequence thrusting and the associated knee-shaped folding affecting the MU can be observed from Upega village to Col di Perla area (Figure 7), thus on more than 70 km. In contrast with the previous authors [Di Giulio 1988; Merizzi and Seno 1991; Mueller et al. 2020; Piana et al. 2021], we hold that the reconstructed thrust surfaces and the associated knee-shaped folding must be considered as first-order structural features in this area and, consequently, that syn-stacking tectonics played a significant role in the finite structural architecture of the units exposed in the junction area between Maritime and Ligurian Alps (Figure 8). This hypothesis is corroborated by the fact that the inverted structural relationships between the FH and Briançonnais Units (Figure 7) can be appreciated at the regional scale and not just at the local scale. This type of scenario was detected not only by Sanità et al. [2020] in the Marguareis Massif (north of the study area, but also in the Embrunais-Ubaye sector [French western Alps, see Figure 7 of Merle and Brun 1984]. In this emblematic key target for studying the lithology and the structure of the Helminthoid Units [see Kerckhove 1963, 1969, with references therein], a tectonic evolution entirely consistent with the one proposed in this paper was reconstructed [cf. Figures 8 and 7 of Merle and Brun 1984].

7. Kinematic interpretation in the framework of Alpine collision tectonics

The pre-stacking structures documented in the Briançonnais Units exposed in the study area (i.e. the MU and the CS) demonstrate that the D1_MU phase developed under ductile conditions during a southwest-ward motion of the unit, as proposed by Carminati [2001] and Sanità et al. [2020]. The same authors also considered that the metamorphic peak conditions were reached during this deformation phase in accordance with the mineral recrystallizations along the S1_MU tectonic foliation. Some authors [Dumont et al. 2012] proposed an early N–S to NW-directed compressional phases in the Emrunais-Ubaye sector close to the Pelvoux Massif and mostly recorded into the Dauphinois units. To the contrary, in the study area this roughly N-directed compressional phase is not recorded by the Briançonnais units. However, at the scale of the western Alps, the D1_MU phase is the witness to the involvement (underthrusting) of the European continental units (here Briançonnais units) in the Alpine continental collision as proposed by Lanari et al. [2014]. The D2_MU phase can be related to a NE-directed-backthrusting event, developed at lower P–T metamorphic conditions, during which the Briançonnais units start to be exhumed [as suggested by Carminati 2001]. Based on these considerations, we confirm the reconstruction proposed by Sanità et al. [2020] in which the pre-stacking deformation history recorded by the Briançonnais Units are the result of the involvement in the suture zone of the thinned Europe continental margin that underwent underthrusting, accretion and later exhumation into the Alpine orogenic wedge.

The FH, instead, recorded a different deformation history developed at shallow structural levels as suggested by the recrystallization along the related S1_FH and S2_FH foliations. They are associated with two different fold systems. The F1_FH folds and kinematic indicators indicated a NW-directed tectonic transport, whereas the structural features related to D2_FH phase indicate a tectonic transport direction toward southwest. These kinematic interpretations for the FH emphasize an origin from the innermost sectors of the Alpine chain. These structural evidences are consistent with the stratigraphic analogies between the FH and the External Ligurian units successions [Northern Apennine, Elter et al. 1966; Marroni et al. 2010], which are regarded as the sedimentary cover of the ocean–continent transition along the Adria Plate [Marroni et al. 2017, and quoted references].

Geochronological data on the age of the deformations recorded by each unit are not yet available. Nevertheless, based on the age of deposits at the top of the sedimentary sequences and, the cross-cutting relationships between different tectonic structures a possible timing for the tectonic evolution considered can be proposed at the regional scale. In the Marguareis Massif (north of the study area), structural and stratigraphic constraints support a Middle to Late Eocene time span for the development of the pre-stacking structures [Sanità et al. 2020]. This is in agreement with the 45 Ma ⁴⁰Ar–³⁹Ar age on mica obtained by Lanari et al. [2014].

The syn-stacking events, responsible for the coupling of the units, is represented here by the unit-bounding shear zone system and by the knee-shaped folding event, both developed at shallow structural levels. Consequently, this structural setting was acquired after the main exhumation of the Briançonnais Units and in uppermost Late Eocene time [Maino et al. 2015; Vanossi et al. 1984]. In such a case both the out-of-sequence thrusting and knee-shaped folding can be constrained to the Late Eocene–Early Oligocene time span, in accordance with the onset of compressional deformation in the former European continental margin [i.e. External Crystalline Massifs, Simon-Labric et al. 2009; Sanchez et al. 2011].

The post-stacking deformation history is represented by fold and fault system development. We propose to interpret the post-stacking fold system as the result of vertical shortening and folding of pre-existing inclined layers starting from Early Oligocene. This kind of deformation is typical of extensional tectonics as outlined in other sectors of the Alpine chain [Froitzheim 1992; Ratschbacher et al. 1989; Wheeler and Butler 1994]. The last deformation event depicted in the study area is represented by a sub-vertical fault system with NE–SW direction that locally juxtaposes different tectonic units. However, the faulting event did not play a significant role in the finite strain pattern recorded by the units exposed in this area of the western Ligurian Alps as proposed instead by some authors [d’Atri et al. 2016; Piana et al. 2021].

8. Conclusion

In this paper we document a tectonic pile of units exposed in High Tanaro Valley (Ligurian Alps). These intricate finite strain patterns recorded are characterized by the superimposition of pre-, syn- and post-stacking structures, developed in different time spans and at different structural levels. Our investigations demonstrate that:

• The tectonic coupling is the result of a polyphase ductile tectonic evolution and not the result of a single transpressive brittle tectonic event.
• The syn-stacking tectonic events correspond to top-to-SW thrusting and folding developed during Late Eocene–Early Oligocene times. They brought the units to a specific and original structural configuration in which the non-metamorphic FH is tectonically located in between the Briançonnais units, the latter showing higher metamorphic conditions.
• The pre-stacking ductile structures documented in the Briançonnais units are the witness to the underthrusting of the European thinned continental margin during the Alpine continental collision.
• The kinematic indicators observed in the FH unit, document first a NW-directed tectonic transport followed by a motion toward the SW (i.e. tectonic coupling with Briançonnais units). The early tectonic and kinematic indicators are compatible with an origin of the FH in innermost sectors of the Alpine belt, probably at the boundary with the Adria Plate.

Conflicts of interest

Authors have no conflict of interest to declare.

Acknowledgements

We thank the reviewers for their constructive and challenging tips. We thank also the editor, Carole Petit, for the editorial work. The authors are thankful to University of Pisa (PRA project) for financial support of this project.