Field trip 2: first day
The internal zones of Alpine Corsica: between Bastia and Saint Florent
Intinerary: Saint Florent, Serra di Pigno, Cima Malaspina, Patrimonio, Saint Florent
The Internal zones of Alpine Corsica are well exposed between Bastia and S. Florent where most of the geological history of the orogen can be investigated. This multi-stage evolution ranges from the early stages of subduction to the late stages of post-orogenic extension and opening of oceanic domains, which are recorded in the structures found within the different tectonic units and in the sedimentary basins exposed in the area.
The nappe structure characterizing the area crops out in a regional scale open antiform-synform pair: the NS trending Serra di Pigno antiform and the Nebbio synform, both with an amplitude of nearly 5km. The Serra di Pigno antiform is developed in a doubly plunging axial depression of the c. 50km long Cap Corse-Castagniccia antiform.
The eastern and western limbs of the Serra di Pigno antiform are reworked by high angle normal faults (of dominant extensional to transtensional type), Miocene to present in age, which are responsible for the regional morphology (Fig. 2.1).
East of Serra di Pigno the alluvial plain of Marana (south of Bastia) occupies the northern part of the Aleria coastal plain. This domain represents the on-land part of the Corsica-channel basin, which separates Corsica from the Tuscan Archipelago (e.g. Mauffret et al., 1999). West of Aleria, the oldest onland marine sediments of this basin are Mid-Burdigalian in age (St. Antoine fm., 18.7-18.3 Ma), as determined with nannoplacton and planktonic foraminifera (Loye-Pilot et al., 2004).
West of Serra di Pigno the Neogene depositional system (Fig. 2.2) can be observed within the c.500 m thick carbonate rich sequence of the Saint Florent basin (Durand Delga, 1976; Dallan and Puccinelli, 1986; Rossi et al., 1994; Dallan and Puccinelli, 1996; Ferrandini et al., 1998; Fellin et al., 2005; Cavazza et al., 2007). The oldest sedimentary unit (F.Albino fm.) is formed by continental alluvial/fluvial deposits (conglomerates and pebbly sandstones) inferred to be early Burdigalian in age (Dallan and Puccinelli, 1996; Ferrandini et al., 1998). Conglomerate clasts in the F.Albino formation were mostly derived from the different metamorphic and non-metamorphic alpine units. The basal deposits are covered by ~250 m thick marine carbonates starting with sandy marls, grey limestones and calcarenites (Mt. Torra fm.) followed by calcarenites characterized by spectacular cross-laminations (Fig. 2.3) and a fauna rich in bryozoans of the Mt. S.Angelo fm. (Orszag-Sperber and Pilot, 1976; Ferrandini et al., 1998). The age of this unit is Mid-Burdigalian-early Langhian based on foraminifera and nannoplacton biostratigraphy (Dallan and Puccinelli, 1996; Ferrandini et al., 1998, Cavazza et al., 2007) and on magnetostratigraphy (Vigliotti and Kent, 1990; Ferrandini et al., 2003). The Mt. S.Angelo fm. is followed by marls, muddy arenites, limestones (Farinole fm) of Langhian-Serravalian age and by the Tortonian S. Florent conglomerates (Ferrandini et al., 1998; Ferrandini et al., 2003; Fellin et al., 2005; Cavazza et al., 2007).
Figure 2.3. Cross-bedded medium grained carbonatic sandstones.
Cross-bedded medium grained carbonatic sandstones (interpreted as shoreface deposits) of the Miocene of Saint Florent.
The Neogene deposits of the S. Florent basin unconformably overlie the Alpine nappe stack, including the non-metamorphic Nappe Supérieure (Nebbio) (Fig. 2.4, 2.5) and the HP/LT Schistes Lustrés nappe (Dallan and Puccinelli, 1996; Rossi et al., 2001). The basal unconformity is deformed by a kilometer-scale open synform which records, altogether with the internal geometry of sediments and their thickness variation (Ferrandini et al., 1998; Fellin et al., 2005), the syn- and post-sedimentary activity of high angle normal to transtensonal fault-systems. The faults bounding the basin were active at ~18-16 Ma, during the final stages of the Corsica-Sardinia block rotation (Vigliotti and Kent, 1990; Ferrandini et al., 2003; Speranza et al., 2002; Gattacceca et al., 2007).
Figure 2.5. Panoramic view from Serra di Pigno
Panoramic view from Serra di Pigno toward west and schematic line drawings showing the different units of the Nebbio Nappe System (modified after Dallan and Puccinelli, 1996).
Nappe Architecture
The nappe architecture of the internal zones cropping out between Bastia and S. Florent, includes from the top to the bottom:
The Nappe Supérieure System
The “Nappe Supérieure” System (“Allochtone du Nebbio”), which consists of three major units that escaped significant deformation/metamorphism in the accretionary-collisional orogenic evolution. These units, in superstructure position, show a very low grade alpine overprint and structures developed at shallow crustal depth. Their complete description will be fully analyzed during the third day of the field trip 2. Following Dallan and Puccinelli (1996) and Rossi et al. (2001) from top to bottom the following units (Fig. 2.3) can be recognized (see panoramic view of Fig. 2.6 and Fig. 2.4):
Figure 2.6. Panoramic views
Panoramic view from Serra di Pigno toward Tenda Massif, with the main stack of units from the Serra di Pigno to the Tenda Massif.
The Ligurian unit (also called Mortola-Canta Furmigola and Tramonti units) characterized by Jurassic basalts and a Late Jurassic-Late Cretaceous sedimentary cover including radiolarites, Calpionella limestone, and deep water pelagic Cretaceous sediments (Lydienne). The sequence is capped by Late Cretaceous ophiolite-rich breccias (Toccone breccia) and mixed ophiolite and continental crust-derived conglomerates (Alturaja fm); 1.2 The Nebbio Unit made up of a Late Cretaceous calcareous flysch (cfr. Flysch d’Ostriconi and Macinaggio) and Eocene siliciclastic deposits (coarse grained sandstones and conglomerates) including slices and/or olistoliths of Triassic-Jurassic limestone (Monte Tuda and Tramonti limestone);1.3 The Aiastrella unit (Unité inférieure du Nebbio) including pre-Hercynian phyllites and schists (“roches brunes”), Permo-Triassic conglomerates and quarzites and Triassic dolomites.
Bad outcropping conditions in the Nebbio region prevent a clear definition of the relationship between the different terms of the Nappe Supérieure system, which have been interpreted in the literature as sliced remnants of coherent units and/or detritic components within the Eocene deposits (see Dallan and Puccinelli, 1986; Rossi et al., 2001 and references therein).
The Schistes Lustrés nappe
The “Schistes Lustrés” nappe, is composed by several tectonic units, some of which include metaophiolites (mantle-derived rocks, metagabbros and metabasalts) and related metasedimentary sequences (quarzites, marbles and calcschists). Slices of polycyclic continental basement (para- and orthogneiss with gabbro bodies and basic dykes) and Mesozoic cover series can be observed as well as extensive metasedimentary units whose original attribution to continental or oceanic sequence has been disputed (Durand Delga, 1984; Lahondere 1996; Rossi et al. 1998; Rossi et al., 2001 and references therein). Recent studies (Serié, 2002; Meresse, 2006; Vitale Brovarone, 2011) showed that the association of continental slices and ophioliteswas established along a Mesozoic OCT (ocean-continent transition domain; see Field trip 1).
Following this interpretation and the results of new petrological studies (Ravna et al., 2010; Vitale Brovarone et al., 2010) including Raman Spectroscopy of Carbonaceous Material (RSCM) thermometry (Vitale Brovarone et al., 2010; 2011) the Schistes Lustrés nappe system can be subdivided from top to bottom into four different tectonic units:
1 - an epidote-blueschist (Ep-BS) oceanic unit mainly exposed in the two limbs of the Nebbio synform, resting to the west on top of the Serra di Pigno lawsonite-blueschist (Law-BS) unit and to the east on top of the Tenda unit;
2 - a (Law-BS) OCT-type unit that, similarly to the previous unit, consists of metasediments associated with ophiolites and continental crust slices outcropping on top of Serra di Pigno;
3 - a lawsonite-eclogite (Law-Ecl) OCT-type unit formed by rare metasediments associated with ophiolites and continental crust slices, which crops out at the bottom of the Lancone valley and extensively to the west of Bastia (between Bastia and Serra di Pigno). P-T conditions in these units have been recently redefined by Vitale Brovarone et al. (2011) at 520 ± 20°C and 2.3 GPa;
4 - the lower Castagniccia unit characterized by a metasedimentary cover (calcschists and impure marbles) inferred to be Cretaceous in age (Caron, 1990). Lawsonite and chloritoid characterize these metasediments that locally preserve carpholite. In this unit the absence of mafic rocks precludes the definition of precise pressure estimates whereas RSCM thermometry yields an average value at 470°C (Vitale Brovarone, 2011);
All units show HP/LT peak metamorphism testifying their involvement in subduction processes at different depth, from 1 GPa and 300-400°C up to 2.3 GPa and 470-520°C (Vitale Brovarone et al., 2011); later exhumation-related retrogression is highly variable (depending on rock-types and structural positions). A pervasive composite mylonitic foliation with relicts of earlier fabrics can be found in the Law-eclogite unit. Stretching lineation and shear sense indicators (Fig. 2.4 Map Serra di Pigno) associated with eclogite facies fabrics provide evidence of scarce top north/northwest shearing (Lahondère 1996), whereas the blueschist retrograde fabric formed the mappable main foliation associated with an east–west stretching lineation and dominant top-to-west kinematics (Faure & Malavieille 1981; Mattauer et al. 1981; Malavieille 1983; Harris 1985a, b). Greenschist retrogression is generally static, except for the metasediments, where it is locally associated with planar fabrics. In continental rocks and metabasalts the greenschist-facies retrogression is shown by the pseudomorphic growth of albite and chlorite porphyroblasts after blue-amphibole or Na pyroxene and garnet, respectively.
3) The Tenda unit. In the westernmost limb of the Nebbio synform the continent-derived gneisses and granitoids of the Tenda unit are exposed below the composite Schistes Lustrés nappe represented by the epidote-blueschist oceanic unit. The Tenda unit and the features of its eastern boundary will be the main topic of the second day of the Field trip (see below).
The conditions of coupling of the different units in the Cap Corse-Castagniccia antiform and the related kinematics are still to be worked out in details, being resolved only locally for the contact between the lower Castagniccia unit (2.4) and the Law-eclogite unit (2.3). The juxtaposition between these two units is well constrained to the exhumation history in the Mont San Petrone area (Vitale Brovarone et al, 2011), where the contact is characterized by epidote-blueschist fabrics parallel to the eclogitic foliations of the Law-eclogite unit.
The relationships between the Tenda and the overlying ophiolitic unit will be discussed during the second day of this Field trip.
Our general interpretation follows the early propositions of Mattauer et al. (1981) in considering the different contacts between internal units as related with sheared folds, refolding nappe contacts interleaving continental-derived and OCT-derived units during continental subduction and syn-contractional exhumation (Molli and Malavieille, 2010).
Day One Itinerary
Stop 1.1
Locality: Serra di Pigno summit (UTM 32T 532786 E 4727064)
Themes: Panoramic view and overall architecture of Alpine Corsica from Tenda Massif to Serra di Pigno and Cap Corse. The Nebbio synform and S.Florent sedimentary basin. Deformation and metamorphism of continental slices and Schistes Lustrés units.
After the description of the panoramic view from the Serra di Pigno summit (Fig. 2.6) we will analyze different aspects of the rocks by walking along the ridge crest towards the north.
The Serra di Pigno is the largest continental sliver inherited from the passive margin and OCT of Corsica. The Pigno and associated alpine tectonic units (Fig. 2.7; Fig. 2.8) are characterized by a lawsonite blueschist-facies metamorphism (Law-Bs). They consist of metaophiolites, and continental gneissic rocks with associated metasediments (e.g. Faure & Malavieille 1981). P-T estimates indicate P-T conditions of about 0.6/0.8 GPa and 300±50°C in the Serra di Pigno unit and 1.0 GPa and 350°C in the overlying Campitello unit (Lahondère, 1996). Omphacite and garnet are not observed in metamafics, which are characterized by Na-amphibole + lawsonite + phengite mineral assemblages, instead.
Figure 2.7. Geological-structural map of the Serra di Pigno area (modified after Malavieille, 1981).
Figure 2.8. Panoramic view toward north of the Serra di Pigno with distribution of the main rock-types.
We will cross the different tectonic units along the crest going structurally from the bottom to the top. The Serra di Pigno continental unit rest structurally above a thick series of metabasalts. A major tectonic contact outlined by a thin layer of strongly deformed metasediments bounds the two units,. Walking to the North, we cross the contact between the Pigno gneissic rocks and ophiolitic nappes resting on top of it. Above the thin sequence of metasediments that represents the Mesozoic cover of the continental slice, a thick unit of strongly deformed serpentinites can be observed. On top of it, ophicalcite and metasediments including marbles and metacherts outline a major contact that could represent a Mesozoic intraoceanic detachment, which accommodated the exhumation of mantle rocks. Walking around, we can observe the relationships between gabbro dikes and serpentinites close to the contact and the effects of the strong alpine shearing deformation. The section will end in the upper unit, which consists ofa large flaser gabbro body. The Cima di Gratera gabbro is a cumulate-layered to varied-textured gabbro that is only partly re-equilibrated at blueschist facies conditions, except in shear zones (Fournier et al., 1991) where the transformation has run to completion. Intense alpine strain affects these rocks, but parts of the gabbros that were incorporated into the subduction complex escaped deformation and recrystallization during both subduction and the subsequent exhumation.
If time allows it, we will try to observe the pseudotachylyte veins exposed around the peak of Cima di Gratera. The pseudotachylytes are located in the best preserved parts of the gabbro. These interesting rocks have been studied here by Austrheim & Andersen (2004) in gabbros and mantle peridotites and interpreted as pseudotachylytes formed by frictional melting on faults at seismic strain rates (related to subduction zone earthquakes).
Further to the North, in the Farinole area, the Serra di Pigno blueschist unit overlies a lawsonite eclogite-facies, metaophiolite-rich domain, which is the lowermost tectonic unit of this part of Cap Corse. This unit also includes slivers of continental basement rocks. Ophiolitic and continental metamafic rocks are characterized by variably preserved omphacite + Na-amphibole + Ca-amphibole + lawsonite + garnet + phengite mineral assemblages. This High Pressure nappe unit is relatively continuous from north to south of Alpine Corsica. P-T estimates for this unit are locally well defined, whereas modern studies of the microstructural and petrological evolution are lacking in other places. Estimates range from 1.5 GPa and 500 ± 50°C in the Farinole area (Lahondère, 1996), to 0.8 GPa and 300°C in the Sant’Andrea di Cotone area (Caron et al., 1981) and 2.2 – 2.6 GPa and 520 ± 20°C in the San Petrone area (see Field trip 1, Vitale Brovarone et al., 2011).
- Back to the road, walking down from the pass, we will observe the structural features of the Pigno continental slice, including the pre-Alpine magmatic rocks and structures reworked by alpine deformation. Afterwards, the cover rocks preserved on top of the continental sliver will be observed along the road. They reflect the pre-compressional sedimentary environment that was associated with such extensional allochthons. All these rocks are strongly deformed during continental subduction as shown by the sheath folds obseved in marbles of the metasedimentary sequence, which are associated with spectacular stretching lineations parallel to the fold axes.
Stop 1.2
Locality: Poubelle de Bastia UTM 32T 533823 E 4727217.
Themes: subduction-related deformation and metamorphism in mylonitic orthogneiss and panoramic view toward the east
This short stop will allow to observe the typical orthogneisses of the Pigno unit. Ductile deformation and shear sense indicators developed under HP/LT metamorphic conditions during continental subduction can be observed.
About one hundred meters above, after a short walk to the hill summit, metasedimentary rocks show spectacular structures. Beautiful sheath folds with eye-like structures are present in the impure marbles in the metasediments that cover the orthogneisses. A huge pebble (olistolith) of Triassic dolomitic limestone is included in the marble and wrapped by the foliation. Such early detrital input of exotic rocks in the sediments deposited close to the stretched continental margin is controlled by extensional tectonic processes and suggests the presence of a rough submarine topography, characterized by fault scarps.
A panoramic view towards the East will allow to discuss the role of Corsica in the tectonic framework of the northern Tyrrhenian sea.
Stop 1.3 (this stop will be cancelled for the large number of partecipants)
Locality: Cima Malaspina (UTM 32T 530789 E 4727332)
Themes: Metasediments/serpentinites contact
Slivers of a metasedimentary sequence ranging from marble to carbonate quartzite in direct contact and refolded (Fig. 2.9) with a large serpentinite body is exposed above the village of Patrimonio, along the Malaspina ridge (Fig. 2.7), which leads to the Serra di Pigno mountain. Ultramafics and metasediments overlie the HP/LT Farinole/Serra di Pigno gneissic units, which themselves lie structurally above HP/LT meta-ophiolitic units. Although being strongly deformed during alpine tectonics, the marble/serpentinite contact is concordant with the sedimentary bedding, as judged from lithological heterogeneities, and is marked by a continuous, centimetre-thick, weathering-resistant reaction rim of calcsilicates. The tectono-stratigraphic meaning of this contact will be discussed as it may represent an original and inherited ocean-continent transition or alternatively an Alpine tectonic contact.
Figure 2.9. Outcrop photograph of the contact between serpentinites and sediments at Malaspina.
Insert: alteration nicely reveals the zonation in the metasediments with a hard calc-silicates rim at the interface with serpentinites, a grey depressed zone with pure wollastonite, a dark zone enriched in CM and the bulk marble with calcite + quartz above.
The interface between serpentinites and sediments is nicely underlined by a jade-like cm-thick layer of diopside with minor garnet (andradite/garnet). In the sediments, the following mineralogical zoning is observed starting from the serpentinites:
A whitish 1 to 5 cm thick zone composed almost exclusively of wollastonite with minor grossular.
A 5 to 20 cm thick dark zone consisting of wollastonite + quartz + graphitic carbon (± grossular ± diopside), with no carbonate.
The bulk calcite+quartz bearing sediment, which is wollastonite free.
The transition between the two latter zones (wollastonite and carbon bearing vs. wollastonite free and low carbon content) is sharp.
Altogether, the association of wollastonite with local enrichment in carbonaceous material (CM) is interpreted as due to the local reduction of the Ca-carbonate to form elemental carbon and wollastonite according to the reaction
SiO2 + CaCO3 + H2=CaSiO3 + C + H2O.
The occurrence of this reaction is supported by a detailed geochemical and structural study of the carbonaceous material (CM) along the reaction front. The total organic carbon (TOC) significantly and sharply increases through the reaction front from about 0.5% wt in the original sediment to over 5% wt. in the reaction zone. Raman spectroscopy shows that CM is much more graphitic in the reaction zone than in the original rock. In addition, marked isotopic differences are observed on both sides of the reaction front with δ13C (CM) and δ13C (calcite) around -19‰ and 1‰ respectively in the original rock far from the reaction zone, whereas δ13C (CM) is around -2 ‰ in the reaction zone. These data are compatible with the formation of graphitic CM from the reduction of calcite, which may be a consequence of the diffusion of reducing fluids (H2 and CH4) from the underlying serpentinites.
This reaction front is locally associated with some spectacular mineralogical features that will be described in the field, such as the presence of aragonite-garnet intergrowths in the sediments close to the contact with serpentinites.