Journal of the Virtual Explorer

Locality: Bastia

Themes: Introduction to the geology of the Schistes Lustrés in southern Alpine Corsica.

We travel From Bastia along the road 193 in the direction of Ponte Leccia, crossing the units of the Schistes Lustrés complex. From East to West, we will cross most of the major tectonic units of the Schistes Lustrés (SL). These units are folded in a late antiformal structure with N-S striking axis, which is especially well exposed along the Golo valley, which provides a natural cross-section through the SL complex (Fig. 1.7a). Most of the rocks units are pervasively deformed, showing composite foliations locally related to large- to small-scale isoclinal folds. Lineations are scattered and range between 30N and 120N. These structural elements are then affected by the large scale antiformal structure.

Metamorphic conditions decrease progressively from the lowest to the uppermost structural level, with T>500°C in the lawsonite-eclogite unit, T~430°C in the lawsonite-bluschist unit and T~350°C in the low-grade blueschist unit (Vitale Brovarone et al., 2011a; Vitale Brovarone et al., in prep.) The eastern part of the valley, in the so-called Castagniccia region, is characterized by the east-dipping fabrics of the eastern limb of the SL antiform, and entirely consists of metasediments of the lawsonite-eclogite unit. Moving westward, in the western limb, the lithological composition becomes more varied and metaophiolites and continental basement rocks of the other tectono-metamorphic units are also exposed.

Figure 1.7. Schistes Lusrés complex

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Geological cross-sections trough the Schistes Lusrés complex. See Fig. 2.1a, b for locations.

Locality: Road D71, crossroad to Castello di Rostino (UTM 32T 522623 E4699190)

Themes: Geological overview of the Tethys-derived rocks of the Schistes Lustrés complex

In the village of Ponte Leccia, we will turn in the direction of Morosaglia, along the road D71. First we drive across the poorly metamorphosed Uppermost Units, consisting here of serpentinites and gabbros of N-MORB affinity (Saccani et al., 2000). Moving towards the east, we cross the subgreenschist continental basement unit of Caporalino (RSCM temperature = 300°C, Vitale Brovarone et al., in prep). Further to the east, other slices of very low-grade ophiolitic rocks crop out along the road D71, in the proximity of the crossroad to Castello di Rostino (Fig. 2.7b). There, the primary relationships between metabasalts and the early post-rift cover rocks (e.g. radiolarian cherts) can be observed. Well-preserved primary variolitic structures also characteristically found in the outer portion of pillow basalts.

Moving towards the village of Morosaglia we enter in the Schistes Lustrés complex (Fig. 1.7b, 1.8). First we drive across the upper and low-grade units of the Schistes Lustrés (Fig. 1.7b), consisting of lithological associations attributed to the distal Tethyan realm, with serpentinites, basalts and metasedimentary cover rocks. This unit equilibrated at a metamorphic temperature of about 350°C in the low-temperature blueschist facies conditions.

Figure 1.8. Panoramic view from the village of Morosaglia.

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Note the hercynian basement on the background and the very low-grade units (in green) overlying the Schistes Lustrés complex (in pale blue).

Locality: Santa Reparata

Themes: The HP units of the Schistes Lustrés and the Morosaglia conglomerates

Moving further towards the village of Morosaglia and down in the tectonic stack, we drive across the lawsonite-blueschist unit of the Schistes Lustrés (peak-metamorphism temperature ~ 430°C), consisting of meta-ophiolite, continent-derived rocks and metasediments, which originated from an Ocean-Continent Transition (OCT) zone. The Morosaglia metaconglomerate, cropping out below the Santa Reparata church, preserves spectacular evidence of continent-derived detrital material. This conglomerate mainly consists of clasts of granitic rocks and minor mafic and carbonate rocks. Metasediments locally contain porphyroclasts of fresh lawsonite. Moving from Morosaglia to the pass of Col De Prato, we cross the boundary separating the lawsonite-blueschist unit and the lawsonite-eclogite unit (T at around 520°C), also interpreted to derive from an OCT zone. The contact separating these units is not spectacular in this particular location, since it is located within monotonous metasedimentary sequences. In the lawsonite-eclogite unit, thin slices of continental basement rocks interpreted as rift-related extensional allochthons are scattered along the ridge running N-S and will be the subject of the second day of excursion.

Past Col the Prato pass, towards the hotel in Piedicroce, we drive across the eclogitic metasediments of the Castagniccia region.