Pre-collisional architecture of the European distal margin: Inferences from the high-pressure continental units of central Corsica (France)

5.1 Tectonic setting

The study area (Figures 1, 2 and 4) is located between the E-W trending Asco valley, in the north, and Tavignano valley, in the south. To the west, it is bounded by the eastern border of the Hercynian Corsica and to the east by the CCFZ that extend from Ponte Leccia, in the north, to Corte, in the south (Figure 4). This area has been mapped first at 1:50,000 scale by Rossi et al. [36] and later in detail at 1:10,000 scale by Di Rosa et al. [12] and Malasoma et al. [13] and studied from a tectono-metamorphic point of view by Bezert and Caby [37], Malasoma et al. [38], and Di Rosa et al. [8,9,10,11]. The detailed mapping revealed that in the study area, three continental affinity tectono-metamorphic units belonging to the Lower Units (i.e., the CPU, CAU, and PPU from west to east, respectively) (Figures 2 and 4) are stacked towards the west. These units are thrusted onto the Hercynian Corsica by an E-dipping shear zone with top-to-W kinematics. Within this shear zone, slices of granitoids, pre-Variscan basement and tertiary deposits (i.e., the Parautochthonous Units) are stacked [12,13] and separated from each other by minor ductile shear zones trending parallel to eastern Hercynian Corsica boundary [8,12,13] (Figures 2 and 4).

The three Lower Units are affected by complex folding pattern and are cut by N-S trending minor shear zones and E-W trending high-angle faults [8,12,13]. In map view (Figures 2 and 4), the thickness of the CAU progressively decreases toward S and the CPU disappears below the quaternary deposits northward of Popolasca village. Klippen of Schistes Lustrés Complex (Inzecca and Lento Units), located south of the Francardo basin, and Upper Units (PIU and SDU), located west of Ponte Leccia are preserved above CAU and PPU. Thin slices of Schistes Lustrés Complex are also mapped along the shear zones bounding the contact between each Lower Units [12,13]. The Lower Units and their tectonic boundaries are lately deformed by E-verging folds, producing the verticalization, or the overturning of the previous structures [8,9,12,13] as in Castirla and Castiglione areas.

To the east, the CCFZ separates the Lower Units from the Santa Lucia Unit (SLU in Figure 2), the not metamorphic Eocene classic deposits of the Caporalino-Sant’Angelo Unit (i.e., Parautoctonous units; SA in Figure 2) [58] and the weakly metamorphic Middle to Late Jurassic ophiolite sequence of the PIU (i.e., Upper Unit; Figure 2) [24,44,58,59]. The later contact is locally sealed by the Late Burdigalian to Langhian marine and continental deposits of the Francardo Basin [21] (Figure 2).

5.2 Stratigraphic setting

The stratigraphic section of the European margin reconstructed from the three Lower Units (CPU, CAU, and PPU) exposed in the study area [12,13] is shown in Figure 5. The oldest rocks of the sequence are represented by an assemblage of mica schists, quartzites, paragneisses, and amphibolites deformed and metamorphosed during the Pan-African and Variscan orogenies [16,60]. This sequence, commonly referred to as “Roches Brunes,” is covered by a Late Carboniferous–Early Permian thick sequence of metarhyodacites and metarhyolites interbedded with metavolcaniclastics (mainly consisting of metarkoses [36]).

Figure 5

Stratigraphic log of the European margin sequence reconstructed by integrating all the data available in the study area for these units (Section 5.2). The colors of the different formations are the same as used in Figure 4.

Early Permian (290–280 Ma) metagranitoids, mainly consisting of peralkaline to slightly peraluminous A-type granites, are predominantly dominated by amphibole-biotite granites and pink biotite-bearing granites (U3 Rossi et al. [15]). The latter are intruded into the “Roches Brunes” and the metavolcanites and metavolcaniclastites, locally producing hornfels. They are lately cut by Early Permian mafic and silicic dyke swarms [61]. The metagranitoids and their host rocks are locally covered, as observed near the Castirla village, by a thin level of Late Permian Fonde-Fuata Conglomerates characterized by clasts of quartzites, granitoids, rhyolites, and mica schists [9,36] (Figure 6a). The Mesozoic sedimentary sequence starts with a thin level of Early Triassic Verrucano Fm. consisting of coarse-grained metasandstones and quartz-rich conglomerates associated with green to violet metapelites. It then continues with the Middle Triassic Detritic Metadolomites Fm. made up of metadolostones with intercalations of metabreccias containing metadolostones clasts (Figure 6b). The Triassic sequence continues with the Carnian Carniole Fm. (i.e., metabreccias with clasts of dolostones and limestones) and the Norian-Rhaetian Metadolomites Fm. (Figure 6c). This formation is characterized by thick layers of metadolostones containing purple metapelite layers, interpreted as paleosoil horizons (Figure 6c) and intercalations of metavolcanites and breccia consisting of metadolostones fragments in a micrite matrix [36]. The Metadolomites Fm. gradually passes to the Metaconglomerates Fm. (Figure 6d) probably Rhaetian in age [9]. This formation occurs as discontinuous lenses containing metadolostones and metavolcanites clasts embedded in a carbonate matrix. The Metadolomites and Metaconglomerates Fms. are both topped by the Metalimestones and Metadolomites Fm. (Figure 6e) made up of medium thick beds of metalimestones and metadolostones of probable Hettangian–Sinemurian age [36].

Figure 6

Field photos of the stratigraphic features in selected outcrops of pre- and syn-rift lithotypes recognized in the Lower Units. (a) Fonde-Fuata Conglomerates Fm., Late Permian (pre-rift deposits). (b) Detritic Metadolomites Fm., Middle Triassic (syn-rift deposits). (c) Metadolomites Fm., Norian-Rhaetian (syn-rift deposits). (d) Metaconglomerates Fm., supposed of Rhetian (syn-rift deposits). (e) Metalimestones and Metadolomites Fm., Hettangian-Sinemurian (syn-rift deposits). (f) Lumachella Metalimestones Fm., Sinemurian-Toarcian (syn-rift deposits). (g) Thin-bedded Metalimestones Fm. (syn-rift deposits). (h) Detritic Metalimestones Fm., Early Jurassic (syn-rift deposits).

The latter formation is topped by the Sinemurian-Toarcian Lumachella Metalimestones Fm. (Figure 6f) showing a well-preserved association of lamellibranchs, bryozoa, and echinoids. The Lumachella Metalimestones Fm. passes to Early Jurassic thin-bedded Metalimestones Fm. [36] (Figure 6g) represented by thin beds of metalimestones alternating with very thin beds of metapelites. The above formation is the Cherty Metalimestones Fm. The last formation of Mesozoic age is represented by the Early Jurassic Detritic Metalimestones Fm. (Figure 6h) consisting of a matrix-supported polymict metabreccias, containing Triassic dolostones, mica schists, and rhyolites clasts frequently organized in well graded beds. Middle Jurassic to Paleocene deposits in the study area were not documented.

The Mesozoic succession is unconformably topped by the Eocene foredeep deposits sequence of the Metabreccias Fm. (Figure 7a) and the Metaturbidites Fm. [9,12] (Figure 7b). The Metabreccias Fm. consists of subrounded to subangular clasts of orthogneiss, paragneiss, mica schist, metagranite, quartzite, and marble embedded in a fine-grained matrix ranging in size from metapelites to metarenites (Figure 7a). The Metabreccias Fm. also includes lenses of nummulite-bearing metalimestones. It passes upward with a stratigraphic, gradual transition to the Metaturbidites Fm. (Figure 7b). It crops out as medium thick beds of metarenites and metapelites still preserving the Bouma sequence. The occurrence of Nummulites sp. indicates a Middle to Late Eocene age for both the Metabreccias and the Metaturbidites Fms. [36]. The Metabreccias Fm. are unconformably lying over all the previous formations from the “Roches Brunes” to the Detritic Metalimestones Fm. [9,12,13] (Figure 5).

Figure 7

Field photos of the stratigraphic features in selected outcrops of the Eocene deposits in the Lower Units. (a) Metabreccias Fm. and (b) Metaturbidites Fm.

5.3 Stratigraphic differences among the three units

The presence of a Pre-Eocene sequence in the three studied units (Figure 8) allows to depict the stratigraphic setting of the European distal margin before its involvement in the Late Eocene–Early Oligocene continental subduction. The three tectonic units preserve different Paleozoic to Mesozoic successions: more complete in the PPU and more reduced in CAU and CPU.

Figure 8

Reconstruction of the stratigraphic logs of the CPU, CAU, and PPU units. Pre- and syn-rift deposits are grouped following the subdivisions adopted in Figure 4.

The PPU preserve the more complete Mesozoic succession of the Lower Units ranging from the Verrucano Fm. (Early Triassic) to the Detritic Metalimestones Fm. (Early Jurassic). It is worth to note that this sequence is characterized by episodes of coarse-grained deposits within the Middle Triassic Detritic Metadolomites Fm., the Late Triassic Metaconglomerates Fm., and the uppermost Early Jurassic Detritic Metalimestones Fm. Other important points for stress are the sharp change in the thickness of the Mesozoic formation and the presence of coarser grain size deposits within the thicker formations. These features are not a consequence of ductile tectonics but seems to be inherited from the pristine stratigraphic setting indicating that the source areas of these deposits can be identified in structural highs of the European distal continental margin. In the PPU, the Eocene deposits are very thick and generally occur at the top of the Detritic Metalimestones Fm. (i.e., the uppermost level of the outcropping sedimentary sequence). Locally, they show stratigraphic relationships either with the Metalimestones and Metadolomites Fm., as well as with the thin-bedded Metalimestone Fm. This indicates that the Eocene deposits unconformably overlie previously tilted sequence from lowermost to uppermost Early Jurassic age.

Completely different is the picture reconstructed from the CAU, where the Middle to Late Eocene deposits crops out over the metagranitoids or their host rocks (i.e., the “Roches Brunes”). The only exception is represented by a small outcrop of Eocene deposits at the top of the thin-bedded Metalimestones Fm., but the structural position of this outcrop is unclear so that a correlation with the PPU can also be hypothesized. The occurrence of very thin Eocene deposits at the top of metagranitoids and their host rocks suggests that the CAU was derived from a structural high where the Mesozoic sequence was totally or partially not deposited and/or eroded. However, no evidence to discriminate why the Mesozoic sequence is lacking have been detected during the field mapping.

More complex is the stratigraphic setting of the CPU where the Eocene deposits are found at the top of the Late Carboniferous–Early Permian metavolcanoclastics in the Castirla area and over the Early Jurassic Detritic Metalimestone Fm. in the Tavignano valley. In the Popolasca area, the pristine relationships between the Eocene and Mesozoic deposits are instead obliterated by the pervasive ductile deformation that affects the boundaries between the different lithotypes. Overall, the thickness of Eocene deposits of the CPU and PPU is comparable.

7.1 Tentative reconstruction of the European distal continental margin before the Eocene

The stratigraphic setting of the Lower Units indicates that their successions belong to a continental margin that was characterized by a rugged morphology before the sedimentation of the Metabreccias and Metaturbidites Fms., i.e., the Eocene foredeep deposits. This evidence is supported by the fact that these deposits lie unconformably at the top of formations characterized by different thicknesses. In the PPU, the Eocene deposits are found at the top of the Mesozoic carbonate sequence (i.e., from Detritic Metadolomites Fm. to Detritic Metalimestones Fm.) whereas in the CAU, the same deposits lie directly over the Paleozoic basement. In contrast, the Eocene deposits from the CPU are at the top of the Paleozoic basement as well as at the top of the Early Jurassic carbonate rocks.

The geodynamic picture proposed for the Middle Triassic to Early Jurassic interval (Figure 10a) indicates the occurrence of a wide area of the European continental crust affected by a rifting process predating the opening of the Ligurian-Piedmont oceanic basin [41,66].

Figure 10

Schematic section showing the hypothetic pre-Eocene stratigraphic setting for the Lower Units. (a) Location of the paleogeographic domain of origin of the Lower Units in the frame of the European continental margin and the neighboring Ligurian-Piedmont oceanic basin. (b) Detail of the European distal continental margin with possible location of the three studied units. See text for further discussion.

This process was characterized by the development of normal faults to accommodate the thinning of the continental crust. Our hypothetic reconstruction (Figure 10b) suggests that the differences in the stratigraphic logs of the Lower Units can be mainly related to the rough topography developed through normal faulting with horsts bounded by basins, as observed in the present-day continental margins [67,68] and detected in several fossil examples [66,69]. In this reconstruction, the CAU would represent a structural high, whereas the PPU could record deposition in a fault-bounded basin. In this picture, the CPU is a more articulated domain affected by a rugged morphology with small, fault-bounded basins. This picture is supported by the presence of clastic deposits at different levels of the Middle Triassic to Early Jurassic succession that suggests the occurrence of source areas able to provide frequent coarse-grained debris. In our reconstruction, illustrated in Figure 10, these source areas are represented by fault scarps at the border of the horsts.

The same picture arises from the Pedani area where the Lower Units are exposed (Figure 9). In this area, the Lower Units are in fact characterized by two different successions, one of these, i.e., the PE, includes a complete succession up to thin-bedded Metalimestones Fm. of Early Jurassic age whereas in the other unit, i.e., in the CN, the Eocene deposits have been found at the top of both the “Roches Brunes” and the metavolcanics and metavolcaniclastics. The pristine relationships between the Lower Units from Pedani area and those from the area between the Asco and Tavignano Valleys are unfortunately masked by the fault system of CCFZ, but the picture of their Pre-Eocene depositional setting is similar and indicate an architecture of the European distal continental margins defined by structural highs and lows bounded by normal faults.

7.2 Comparison with the successions of European distal margin from Western Alps

The correlation between the Lower Units and the units of Western Alps representative of the distal European continental margin, i.e., the Pre-Piedmont Units (Figure 11), has been previously proposed by Amaudric du Chaffaut [35], Durand-Delga et al. [70], and Durand-Delga [7]. These reconstructions were based on paleontological and geological data that have been subsequently deeply modified by Puccinelli et al. [58]. Ligurian Alps in Italy, which represents the Northern prosecution of Alpine Corsica, comprise several units all derived from the Pre-Piedmont domain: Case Tuberto, Calizzano-Pallare-Savona, Monte Sotta, and Arnasco-Castelbianco Units [69,71,72]. These units are thrusted over the Brianconnais Units, i.e., the remnants of the more internal segment of the European continental margin. The Pre-Piedmont Units include a Pre-Late Carboniferous basement unconformably covered by a Late Carboniferous to Tertiary succession, both strongly deformed and metamorphosed during the Late Eocene–Early Oligocene continental subduction [72]. Among them, the Monte Sotta Unit shows a complete Early Triassic to Early Jurassic succession [73] that strictly resembles the log reconstructed for the Lower Units in this contribution. The Monte Sotta succession includes Early Triassic metasandstones, and thick Middle to Late Triassic sequence of metadolostones interfingered with carbonate metabreccias, that can be correlated with the Lower Units shallow-water sequence comprising Detritic Metadolomites, Metadolomites, and Metaconglomerates Fms. In both Monte Sotta and Lower Units, the Early Jurassic deposits consists of well-bedded metalimestones, associated with thick levels of metabreccias. In addition, the other Pre-Piedmont Units of the Ligurian Alps also show the same stratigraphic features when the Mesozoic succession is preserved, as in the Case Tuberto, and Arnasco-Castelbianco Units [72]. In the Ligurian Alps, the Eocene foredeep deposits consisting of metasandstones are preserved only in two independent slices, known as Leverone and Colle Domenica Elements [72].

Figure 11

Tectonic sketch map of the Ligurian Alps showing the location of the Pre-Piedmont Units discussed in the text (redrawn from [86]).

A comparison can also be made with representative units of the distal continental margin cropping out in the southern area of the Western Alps (Figure 1) along the Maira, Varaita, and Grana Valleys, few kilometers NW of the Ligurian Alps [74,75]. In this area, several Pre-Piedmont Units, known as Dronero, Sampeyre, Maira, and Grana units, are outcropping in an imbricate stack thrust over the Brianconnais Units. Among these Pre-Piedmont Units, the Grana Unit preserved the most complete Mesozoic succession that consists of Middle to Late Triassic metadolostones showing a transition to Early Jurassic metalimestones with intercalations of carbonate metabreccias. Here as well, the succession corresponds to the stratigraphic log reconstructed in the Lower Units of Alpine Corsica ranging from Detritic Metadolomites to Metabreccias Fms. The Early Triassic deposits are instead preserved in the Sampeyre Unit where Late Permian–Early Triassic sequence of quartz-rich metapelites, metasandstones, and metaconglomerates has been reconstructed [74,75]. The Paleozoic basement of these units is preserved in the Dronero Unit where a pre-Alpine metamorphic complex is associated with metagranitoids and metasediments, the latter interfingering with metavolcanics of probably Permian age [74]. Thus, the basement also shows a strong similarity with that of the Lower Units of Alpine Corsica.

Overall, strong similarities between these units and the Lower Units of Alpine Corsica can be described for both the Paleozoic basement and the Triassic-Jurassic succession, indicating a common stratigraphic setting of the remnants of the European distal continental margin.

7.3 Lower units of Alpine Corsica and the geodynamic history from opening to closure of the Ligurian-Piedmont oceanic basin

The stratigraphic reconstruction of the HP Lower Units studied in the area between the Asco and Tavignano Valleys, integrated with the already published data from the Pedani area, allows reconstructing the architecture of the European continental margin before the collision-related deformation and the geodynamic history of the European continental margin during the opening and closure of the Ligurian-Piedmont oceanic basin.

The post-Variscan history of the Lower Units was dominated by a widespread magmatic activity, represented by the Late Carboniferous–Early Permian metavolcanics and metavolcaniclastics and by the Early Permian metagranitoids. This igneous activity is described by many authors in several tectonic units derived from passive European continental margin and has been interpreted by them (e.g., [76]) as a post-collisional magmatism directly linked to the collapse of the collisional belt and its lithospheric roots at the end of the Variscan orogeny.

The Mesozoic evolution was a two-stage process: the Late Permian–Early Triassic deposition of continental deposits, followed by a Middle Triassic–Early Jurassic transition from continental to shallow-water and pelagic deposits.

Several authors have suggested that the Late Permian–Early Triassic stage was characterized by coexistence of a high geothermic regime associated with the development of several extensional continental basins, both related to mantle upwelling below the European continental crust [77,78]. The only record of this stage in the Lower Units is represented by the very thin continental deposits, i.e., the Fonde-Fuata Conglomerates and Verrucano Fm., that indicate the erosion of the previously exhumed Variscan belt. The characteristics of these deposits, consisting of quartz-rich conglomerates and quartz-rich arenites, probably representing the Late Permian–Early Triassic deposition of a fluvio-deltaic succession, in different sectors of the Alpine-Mediterranean region is documented (e.g., [79]).

The second Middle Triassic–Early Jurassic stage is well represented by the succession ranging from Detritic Metadolomites Fm. to Detritic Metalimestones Fm. The collected data indicate that this succession was deposited in a sedimentary setting characterized by topographic highs and lows bounded by normal faults, as suggested by the differences between the stratigraphic logs of the studied units. In the Early Jurassic, these basins recorded a progressive sinking as suggested by the transition from the thin-bedded Metalimestones Fm. to Cherty Metalimestones and Detritic Metalimestones Fms., i.e., by the transition from shallow-water to pelagic deposits. This evolution is associated with the Late Triassic volcanic activity as detected in the Norian Metadolostone Fm. [63]. This picture fits very well with the history proposed for the European continental margin as reconstructed in the Western Alps, where the Middle Triassic to Early Jurassic time is characterized by the rifting onset and the subsequent widespread stretching of European continental crust by pure shear-dominated extension [41,66]. The crustal stretching originated a network of high-angle normal and/or transtensional faults bounding large rift basins where shallow water deposits were sedimented [42,69,74,75,80]. These faults were probably characterized by important offset producing scarps where the basement rocks were exhumed thus producing clastic deposits, as those detected in the Early Jurassic deposits of the study area. Unfortunately, the lack of Middle to Late Jurassic deposits hampers to collect information about the subsequent rifting-related events.

There is no record of Cretaceous to Early Tertiary sedimentation in the continental units from Alpine Corsica, but some inferences can come from the ophiolite sequence of the Balagne nappe of the Upper Units. These ophiolites are considered as representative of the OCT to the European continental margin [7,24], and are characterized by coarse-grained, polymictic deposits interlayered throughout the sequence from the Jurassic basalts up to the Late Cretaceous sedimentary deposits [7,23,24,25]. All these deposits contain clasts derived from the European continental crust such as granitoids, acidic volcanics, metamorphic rocks, Triassic dolostones, and Triassic to Jurassic limestones. This feature suggests that during the Cretaceous, the European continental margin was still characterized by persistent structural highs that represent the source area of the clastic deposits found in the ophiolite sedimentary cover.

The unconformity at the base of the Eocene clastic deposits and the sharp change in their thickness indicate that structural highs were still existing until the Tertiary. Therefore, we can hypothesize that the rifting-related normal faults were probably reactivated when the continental crust underwent forebulging as approaching subduction was subjected to forebulge, as postulated not only for the European continental margin in the Western Alps [81,82] but also for the Lower Units of Alpine Corsica (e.g., [9]).

Collisional belts are typically characterized by evidence of inversion tectonics [82,83,85] (i.e., the reworking of older normal faults into pure thrusts). We can therefore speculate that the Lower Units-bounding ductile shear zones re-worked the normal faults affecting several stages at the European continental margin. Thus, the Lower Units can be regarded as fragments of the European continental margin detached from their substratum and transferred at depth to the orogenic wedge by ductile thrusts originated in correspondence to the pre-existing weakness zones represented by the Mesozoic normal faults.