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UNIVERSITY OF PISA
DEPARTMENT OF EARTH SCIENCES
POSTGRADUATE COURSE IN ‘SCIENZE E TECNOLOGIE GEOLOGICHE’GEOLOGICAL MAPPING AND STRUCTURAL
ANALYSIS OF THE ALPINE UNITS IN THE
PEDANI AREA
(ALPINE CORSICA)
Candidate Supervisor
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ABSTRACT
The objective of this thesis is the geological and structural study of a sector of Alpine Corsica situated between the villages of Ponte Leccia and Morosaglia. The main re-sult is the geological map at the scale of 1:10.000 that covers an area of 35 Km2, this geological map is the basis for the produc-tion of a DEM.
The study of this area followed five main phases:
Study of the regional Corsican geol-ogy using the available literature.
A geo-structural survey of the study area was carried out, during this sur-vey measurements of linear and planar structural elements were collected.
Structural stations were selected for a mesoscale structural analysis and sam-ples were collected for a microstruc-tural analysis.
Microstructural analysis was carried out on thin sections made from the collected samples.
The structural evolution of the study area was reconstructed integrating all collected data.
The study area is made up of a complex stack of tectonic units that have a varying
metamorphic grade. During the survey of the study area seven tectonic units were rec-ognized. The Canavaggia unit is the deepest one within the stack of tectonic units. In the study area this unit lies beneath the Pedani unit. Both these units have a continental af-finity and register a polyphase deformation of Alpine age that is associated to a low grade metamorphic imprint. Above these two units are the Scoltola and Venato units. These two units also have a continental af-finity and also register Alpine polyphase deformation structures. The units that oc-cupy the highest position within the stack are the oceanic units derived from the Lig-ure-Piemontese oceanic basin. These units are the Lento unit (that belongs to the Schistes Lustrès complex; Levi et al., 2007) and the Pineto and Serra Debbione units (that belong to the Nappe Superièure; e.g. Nardi, 1975; Levi et al., 2007).
From a lithostratigraphic point of view the Canavaggia unit is made up of Permo-Carboniferous meta-granite that has in-truded in the Rocce Brune complex. The Rocce Brune complex is a polymetamorphic and polydeformed complex that acquired its deformation and metamorphic features dur-ing the Pan-African, Varisican and Alpine orogenesis. An unconformity separates these two lithotypes from the Permian Volcano-Sedimentary complex. The Meta-Volcano-Sedimentary complex is in turn covered by the Padule Meta-Breccia (Eocene) via another unconformity. The Pedani unit is made up of metamorphic
Tri-assic-Jurassic carbonate deposits that sur-mount the Permian Meta-Volcano-Sedimentary complex. The Venato unit is made up of the Eocene Venato Meta-Arenite formation, whereas the Scoltola unit is made up of the Scoltola Meta-Breccia for-mation that also has an Eocene age. As far as the units derived from the Ligure-Piemontese oceanic domain are concerned the Serra Debbione unit is mainly made up of serpentinites intruded by rare gabbroic bodies, the Pineto unit is mainly composed of gabbros. Both units have middle to upper Jurassic age. Lastly the Lento unit is made up of meta-ophiolites (meta-serpentinites, meta-gabbros and meta-basalts) that have a middle to upper Jurassic age. These rocks are covered by deep sea deposits, also meta-morphic, that include meta-radiolarites and the Erbajolo formation (alternating marble layers with calc-schist and mica-schist lay-ers).
As far as the general structure is concerned the Canavaggia and Pedani units are at the center of an antiform that, on the northern side, is separated from the Serra Debbione unit by a normal fault. On the southern side the Lento, Scoltola and Venato units are thrust over the Pedani and Canavaggia units. The western sector of the study area is cut by a tectonic lineation that has been de-scribed in literature as the Corsican Central Fault Zone (CCFZ) (Waters, 1990; Molli & Tribuzzio, 2004; Lacombe & Jolivet, 2005). The CCFZ is a fault zone with sinistral strike-slip kinematics and a thickness of a
few hundred meters, it cuts the westward side of the Pedani-Canavaggia antiform.
The stratigraphic successions as well as a structural analysis at all scales have been described for each unit. The structural analysis is complex due to the different en-tity of deformation that has been registered by the various lithotypes that make up the tectonic units. For this reason the structural analysis of the various units has been carried out separately for the Canavaggia, Pedani, Venato and Scoltola units. Owing to the low metamorphic grade and absence of deforma-tion structures in the Pineto and Serra Deb-bione units the structural analysis of these units has not been carried out. The deforma-tion phases can be summarized in the fol-lowing way:
The Canavaggia unit has been deformed by three main phases: The first two phases make up a composite foliation that is visible in the Meta-Granite where highly deformed domains are alternated with relatively unde-formed domains. Within the Meta-Volcano-Sedimentary complex the first two phases develop as a penetrative composite foliation except in the F2 hinges. In the Meta-Volcano-Sedimentary complex the D2 phase consists of non cylindrical F2 folds with high angle axes. At the microscale the D1 phase crystallizes chlorite whereas the D2 phase crystallizes muscovite, the S1 fo-liation can be classified as a slaty cleavage and the S2 foliation as an a4/a5 crenulation cleavage (of Passchier and Trouw, 2005).
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These structures can be recognized in all the lithotypes. The D3 phase can be seen in the Meta-Volcano-Sedimentary complex and in the Padule Meta-Breccia as a sub horizontal AP3 axial plane foliation and on the geo-logical sections. Three deformation phases have been recognized in the Pedani unit. The deformation structures vary according to the lithology: in the Meta-Volcano-Sedimentary complex the D1 and D2 phase generate a composite foliation where the S1 foliation is visible as a continuous cleavage within the microlithons defined by the S2 foliation; most of the carbonate lithotypes register the D1 phase as a disjunctive cleav-age and the D2 phase develops F2 folds. However the Laminated Meta-Limestone formation registers the S1 foliation as a con-tinuous foliation and the D2 and D3 phases a s a crenulation clevage. F2 folds develop during the D2 phase in the carbonate litho-types, the folds are open and have sub-horizontal axes. At the microscale in the Meta-Volcano-Sedimentary complex chlo-rite crystallizes during the D1 phase and muscovite during the D2 phase. The D3 phase deforms all lithotypes in a similar way, long wave F3 folds can be seen on the geological section. The Venato unit registers three phases (D1, D2 and D3). The first phase is represented by a continuous schis-tosity that is visible within the microlithons in the D2 phase. F2 non cylindrical isoclinal folds develop during the D2 phase, these folds are accompanied by the formation of a crenulation cleavage that can be classified as an a3/a4 type crenulation cleavage of
Passchier and Trouw (2005). F3 open folds with a sub-horizontal axial plane develop during the third phase (D3), they are accom-panied by an a2 type crenulation cleavage (of Passichier and Trouw, 2005) that can be observed at the microscale. Three deforma-tion phases also develop in the Scoltola unit. A continuous schistosity marks up the D1 phase, this foliation is present within the microlithons that belong to the D2 phase. During the D1 phase both muscovite and chlorite crystallize. F2 non cylindrical iso-clinal folds develop during the D2 phase, a composite S1/S2 foliation can be recognized on the F2 fold limbs whereas a type a3/a4 S2 crenulation cleavage (of Passchier and Trouw, 2005) can be recognized in the F2 hinges. F3 open folds with a sub-horizontal axis develop during the D3 phase, the devel-opment of these folds is accompanied by an a2 type S3 crenulation cleavage (of Passchier and Trouw, 2005). The Lento unit registers four deformation phases. The D1 phase is characterized by a relic S1 foliation that can be seen in the microlithons that de-velop during the D2 phase. At the mesoscale F2 isoclinal folds develop during the D2 phase, at the microscale within the Erbajolo Formation an S2 slaty cleavage develops on the F2 fold limbs and a type a3/a4 S2 crenu-lation cleavage (of Passchier and Trouw, 2005) develops in the F2 hinges. F3 open to close folds develop during the D3 phase and are accompanied by the development of an a2/a3 type S3 crenulation cleavage (of Passchier and Trouw, 2005). The D4 phase can be recognized at the kilometer scale on
the geological sections where long wave F4 folds deform the contacts with the other units and the larger D1, D2 and D3 struc-tures within the Lento unit.
The main tectonic lineations in the study area have been analyzed. The main lineation is the CCFZ, good outcrops of this lineation are visible along the D71 at about one kilo-meter from Ponte Leccia. The study of this fault zone is based on the collection of fault plane and slikenline measurements. These measurements were then processed using Wintensor and confirmed the sinistral strike-slip kinematics that most authors agree on (Waters, 1990; Molli & Tribuzzio, 2004; Lacombe & Jolivet, 2005). Measurements were also collected on the contact between the Pedani and Canavaggia units with the Serra Debbione unit. These measurements were also processed using Wintensor and confirmed the normal dip-slip kinematics of this fault. The tectonic contact between the Lento and Scoltola unit has been analyzed: measurements of the S-C planes that de-velop in these contacts confirm a top to the west kinematics.
In conclusion the study area is formed by a complex stack of tectonic units. Both conti-nental and oceanic units have been involved in underplating processes, they reached their metamorphic peak during subduction and the continental units crystallized chlorite and muscovite. The D1 structures formed during this phase. In literature similar conti-nental units that have been involved in sub-duction processes register a low-grade
epi-dote-blueschist metamorphic peak (Malasoma et al., 2006). Later the exuma-tion of continental units caused the coupling of continental and oceanic units with the formation of the D2 phase in all units and the D3 phase in the Canavaggia and Lento units. The most important structure that de-veloped during the D2 phase is the antiform that is responsible for the overturning of the Pedani carbonate succession and the crystal-lization of muscovite in the Permian forma-tions. The D2 phase developed during the late Eocene-early Oligocene and caused the stacking the Lento, Serra Debbione and Pineto oceanic units on the Canavaggia, Pedani, Venato and Scoltola continental units. This stacking caused the coupling of units with a different tectonic and metamor-phic history. Subsequently an extensional phase developed and deformed the stack of tectonic units, forming long wave folds with a sub-horizontal axial plane and fault zones with both high and low dip angle. This phase is responsible for the D3 phase in the Pedani, Scoltola and Venato units and the D4 phase in the Canavaggia and Lento units. The last main deformation structure in the study area is the CCFZ: a sinistral strike slip fault with a N-S strike that separates Alpine Corsica from Hercynian Corsica. The CCFZ is caused by the strike slip move-ment of the Adria tectonic plate relative to the Sardo-Corsican tectonic plate, during the rotation of the Sardo-Corsican tectonic plate in the early Miocene (Speranza et al., 2002).
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1. INTRODUCTION
This thesis analyses a sector of central Cor-sica, not far from the town of Ponte Leccia. The study area has a size of about 35 Km2 and its border is defined by the N193 to the north and to the west, the Casaluna River to the south and the village of Morosaglia to the east (Fig 1.1a and b). The highest point in the study area is Cima Pedani (910 m.a.s.l) and the lowest is situated at the base of the Golo valley (160 m.a.s.l.) The aim of this thesis is the structural study of the tectonic units that have been deformed during the Alpine orogenesis. During this study a geological map and a DEM of the area have been produced at a scale of 1:10.000 along with two geological sec-tions. The mapping was carried out using the Vescovato 43409 OT topographic map produced by the IGN (Institut Géographi-que National) at a scale of 1:25.000. A tec-tonic analysis of the area has been made at
all scales and a tectonic reconstruction of the area has been proposed. The tectonic analisis has been carried out using strati-graphic and tectonic geometric elements such as bedding, axial plane foliation, fault plane, mineral lineation and fold hinge measurements. The measurements related to the faults have been elaborated using Wintensor, while all other measurements were plotted on stereonet diagrams to facili-tate their interpretation. Measurements of the P/T metamorphic conditions of the Ped-ani unit were attempted using samples col-lected in carbonate formations and analyzed with a SEM-EDS but due to small grain size and low percentage of calcite grains no conclusions could be extracted from the data.
a
a b
2 GEOLOGICAL
FRAME-WORK
From a geological point of view, the Cor-sica Island can be divided in to two main domains: the ―Hercynian‖ and ―Alpine‖ Corsica (Fig 2.2). These domains are sepa-rated by a tectonic line that runs from Ile-Rousse (to the NNW of the island) to So-lenzara (to the SSE of the island), reported in the literature as Central Corsica Fault Zone (Lacombe and Jolivet, 2005).
The Hercynian Corsica corresponds to the south-western part of the island. It mainly consists of Carboniferous to Permian grani-toids intruded into a Paleozoic basement (Paquette et al., 2003). Remnants of a Late Carboniferous?-Jurassic to Eocene sedi-mentary cover have been preserved in the easternmost areas of the Hercynian Corsica (Durand-Delga, 1984), close to the Central Corsica Fault Zone.
The Alpine Corsica corresponds to the east-ern part of the island, it is made up of an imbricate stack of tectonic units that is
re-Fig 2.1 - Simplified tectonic map of Italy, showing the major tectonic and geodynamic settings in the region. The faults in this map as in the following figures are from the Structural map of Italy (Consiglio Nazionale delle Ricerche, 1992). BT, Bradanic trough; CA, Carnic Alps GL, Giudicarie Line; PM, Peloritani Mountains (From Carminati and Doglioni, 2012).
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garded as the southern continuation of the alpine collisional belt (Fig 2.1). These units are derived from oceanic and continental domains of the Liguro-Piemontese oceanic basin as well as from the Europe and Adria continental plate margins (Dallanand Nardi, 1984; Durand-Delga, 1984; Malavieille et al.,1998).
2.1 HERCYNIAN CORSICA
The Hercynian Corsica belongs to the Euro-pean continental plate margin and it is made up of Carboniferous to Permian granites intruded into a Paleozoic basement. The granites were intruded during the
post-collisional phase of the Varisican orogeny, they have been divided in to three main plu-tonic suites (Bonin, 1880; Orsini, 1980; Paquette et al., 2003; Renna et al., 2006). The oldest plutonic suite of Carboniferous age (340-320 Ma) consists of Mg- and K- rich granitoids with shoshonitic geochemi-cal signature; this plutonic suite is followed by Late Carboniferous intrusions (305 Ma) that are dominated by low-K horneblende granitoids with calc-alkaline geochemical signature (Orsini, 1980; Paquette et al., 2003; Renna et al., 2006). Lastly, gabbroic sequences and A-type granites intruded the
Fig 2.2 - Tectonic map of Corsica (from Zarki-Jakni et al., 2004 )
Fig 2.3 - General stratigraphic log of Hercynian Corsi-can autochthonous deposits. Key: 1 - Metamorphic basement and batholith; 2 - Detritic part of the first sequence; 3 - Carbonate part of the first mega-sequence (Malm); 4 - Unconformity - hiatus; 5-6 - Second detritic mega-sequence (upper Cretaceous); 7 - Eocene breccia and carbonate breccia; 8 - Eocene sedi-ments. (Da Amaudric Du Chaffaut, 1980).
batholith during the Permian time (290-280 Ma; Renna et al., 2006).
The sedimentary cover has been divided in two successions (Fig 2.3) (Amaduric du Chaffaut, 1980):
The first succession shows at its base a detritic level that contains fragments of the crystalline basement. This level is followed by Triassic-Early Jurassic shallow-water carbonate rocks. At the top of the succession a detritic level that contains both carbonate and meta-morphic clasts has been found.
The later succession is made up of Eocene sediments that uncomfortably cover the first succession and the un-derlying basement rock; it consists of breccias and siliciclastic arenites with fragments of micaschists, carbonates, granites etc.
2.2 ALPINE CORSICA
The Alpine Corsica consists of a complex stack of variably metamorphosed units de-rived from both oceanic and continental domains belonging to the Tethyan realm. These units have been divided into three main groups (Nardi, 1968; Alvarez, 1972; Jolivet et al., 1990; Malavieille et al., 1998; Durand-Delga, 1997; Marroni et al., 2000; Molli, 2008; Brovarone et al., 2012):
- C o n t i n e n t a l - d e r i v e d u n i t s
(parautochthonous) -Schistes Lustrés complex -Nappes Superieurs
2.2.1 CONTINENTAL -DERIVED
UNITS
Most of the continent derived units crop out in the western part of the belt (Fig 2.4) and can be divided into two main groups: the Tenda Unit and the External Continental Units also known as Parautochthonous Units (Nardi, 1986; Molli et al, 2006; Malasoma & Marroni, 2007; Brovarone et al., 2012).
The Tenda Unit
Located in the westernmost part of Alpine Corsica, to the W this unit is separated from the Balagne Nappe by a strike slip fault known as the CCFZ (Cfr. Ostriconi Fault by Lacombe and Jolivet 2005; Waters, 1990; Molli, 2008). To the E the Tenda Unit is separated by the East Tenda Shear Zone (ETSZ) from the SL complex (Jolivet et al., 1990; Molli et al., 2006; Maggi et al., 2012). The ETSZ has been interpreted as a thick HP polyphase shear zone, reactivated at lower grade metamorphic conditions dur-ing an extensional phase (Jolivet et al., 1990; Fournier et al., 1991; Molli et al., 2006; Maggi et al., 2012; Brovarone et al., 2012). The Tenda massif consists of amphi-bole-biotite granodiorites associated to co-genetic dacitic volcano-sedimentary forma-tions, these include gabbroic intrusions and
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are crosscut by dykes whose composition ranges from basaltic to rhyolitic (Ohnensetter and Rossi, 1985; Rossi et al., 1992; Molli et al., 2006).
External continental units
The S. Lucia nappe, Caporalino-S. Angelo nappe, Corte units and Palasca unit make up the External continental units (Marroni et al., 2001). The Corte units are regarded
Fig 2.4 - Tectonic sketch map of Alpine Corsica (AC). Key: 1. Hercynian basement; 2. Sedimentary cover of Her-cynian basement; 3. Parautochthonous units (C: aporalino-S.Angelo Unit, A: Annunciata Unit); 4. Santa Lucia Unit; 5. External Continental Units undifferentiated; CS: Corte Slices); 6. Tenda Massif; 7. Innermost continental units (C: Centuri Unit, FU: Farinole Unit, OS: Serra di Pigno-Oletta Unit); 8. Schistes Lustrés Complex (CAS: Castagniccia Unit, INZ: Inzecca Unit); 9. Upper Units (Nappes supérieures) (B: Balagne Nappe, N: Nebbio Unit, P: Pineto-Tribbio Unit, BO: Bas-Ostriconi Unit, M: Macinaggio Unit); 10. Miocene deposits (SF: Saint Florent Basin, F: Francardo-Ponte Leccia Basin); 11. Plio-Quaternary deposits (from Malasoma, 2006).
as fragments of the European continental margin consisting mainly of Hercynian granitoids with minor volumes of host-rock basement, both are covered by a sedimentary succession of Permian to Mesozoic age (Dallanand Nardi, 1984; Durand-Delga, 1984; Molli et al., 2006; Malasoma, 2006). In the Corte slices a pervasive deformation associated to HP/LT is well documented in the literature (Amaduric du Caffaut et al., 1976; Amaduric du Cahffaut & Saliot 1979; Bézert and Caby, 1988; Malasoma and Marroni, 2007). The Caporalino-S. Angelo unit is a non-metamorphic unit (Malasoma, 2006). It is made up of a granitic basement with a thick sedimentary cover that includes: Permian volcano-sedimentary deposits, Triassic carbonate deposits, Middle Jurassic (Durand delga, 1984) or Eocene (Puccinelli et al., 2012) polymict conglomerate/sandstone, Late Jurassic-Berrasian limestone and Eocene Flysch de Tonda. The Palasca unit is made up of a thick succession of siliciclastic turbidites reported as the Annunciata Formation (Middle Eocene) (Malasona, 2006). The Santa Lucia unit is the most external of the Nappes superieurs. It is made up of
fragments of the Permian lower crust covered by cretaceous sediments (Durand Delga, 1984; Rossi et al., 1992; Caby and Jacob, 2000; Zibra et al., 2009). It has a variable metamorphic facies, the eastern portion has undergone a low grade polyphase Alpine orogenic evolution (Egal, 1992) and the western part is almost undeformed (Zibra et al., 2009).
2.2.2 SCHISTES LUSTRES COMPLEX The SL Complex makes up most of Alpine
Corsica, and consists of continental and oceanic sequences deformed and metamor-phosed under HP/LT conditions. These units have been folded together in a N-S trending antiform (Fig 2.5) (Caron, 1977; Jolivet et al., 1990; Molli and Malavielle, 2010), this structure has two axial culmina-tions that define the SAC (Southern Alpine Corsica) and the NAC (Northern Alpine Corsica) separated by a central axial de-pression known as the CAD (Brovarone et al., 2012). These structures crop out respec-tively in the Castagniccia region, the Cap Corse and an area between Bastia and Saint Florant. Due to the complexity and large number of tectonic slices implicated in the
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S L
Complex many classifications have been suggested. In this study we propose the lithostratigraphical classification by Delcey (1974), where the SL Complex is made up of different units each consisting of slices of meta-ophiolites and/or metasedimentary rocks (Ohnenstetter, 1981; Lagabrielle et al., 1997). The meta-ophiolites, showing an N-MORB affinity (Beccaluva et al., 1977; Saccani et al., 2000), are made up of ser-pentinites, metagabbros and metabasalts that reflect a Tethyan-type magma-poor Ju-rassic basin. The metasediments can be di-vided into four main groups belonging to different domains (Delcey, 1974; Caron and Bonin, 1980; Lahondère et al., 1992; Guieu et al., 1994):
Castagniccia domain
San Pietro di Tenda domain
Bagliacone-Riventosa domain
Inzecca domain
2.2.3 NAPPES SUPERIERUS
The uppermost structural ensamble of the present day nappe pile is made up of the Balagne nappe, Nebbio, Rio Magno and Pineto units (Fig 2.6) (Nardi, 1968; Durand -Delga 1984). These units consist of a com-plexly deformed association of ophiolitic and continent-derived rocks affected by a greenschist metamorphic facies (Fournier et al., 1991).
The Balagne nappe
Out of the Nappes Superieurs the Balagne nappe is the westernmost one; it is sepa-rated from the Tenda massif by a high angle normal fault, it is thrust over the Varisican crystalline basement and its Middle Eocene sedimentary cover (Gibbons and Horak, 1984; Marroni et al., 2001). To the west the Balagne nappe is limited at the base by a group of thrust sheets belonging to the Ex-ternal Continental Units. From a composi-tional point of view the Balagne nape is made up of a Jurassic ophiolite sequence and its related sedimentary cover, it has
Fig 2.6 - Geological section of the Balagne nappe. Key: 1. Hercynian basement; 2. Numulite limestone and con-glomerates (middle Eocene); 3. Flysch (middle Eocene); 4. Meta-conglomerate (Eocene); 5. Annunciata Formation belonging to the Palasca unit (middle Eocene); 6. Ophilolites (Jurassic); 7. Ophiolite sedimentary cover (upper Ju-rassic - upper Cenomanian; 8. Alturaja arkose (age?); 9. MCT Hercynian metamorphic basement; 10. Thrust; 11. Strike-slip fault; 12. Normal fault (From Marroni and Pandolfi, 2003).
been divided into two main units referred to as the Navaccia and Toccone units affected by a polyphse deformation that developed under very low grade metamorphic condi-tions (Egal, 1992). The geochemical com-position of these basalts has an E-MORB affinity, this suggests that the Balagne nappe originates from the early stages of oceanic spreading (Dal Piaz et al., 1977; Durand-Delga et al., 1997; Saccani et al., 2000).
The Nebbio unit
The Nebbio unit is a small non metamorphic thrust sheet that overlies the SL complex (Rossi et al., 2001). According to Saccani et al. (2005) this unit is a Balagne-type ophio-litic unit. Miocene (Burdigalian – Serraval-lian) deposits cover this unit via an uncon-formity; the ophiolite sequence includes massive flow and pillow basalts, cherts, Calpionella limestone and a Cretaceous siliciclastic deposits (Dallan and Puccinelli, 1995).
The Pineto unit
The Pineto unit (Cfr. The Pineto gabbroic Massif, Saccani et al., 2000) crops out on the east side of the GoloRiver, between Francardo and Ponte Leccia. To the north a slice of Inzecca type SL is thrust over the Pineto unit (Durand-Delga et al., 2005). Ac-cording to Durand-Delga et al. (2005) the Pineto unit ophiolites have an N-MORB signature that distinguishes them, paleo-geographicaly, from the Balagne nappe. The sedimentary sequence that overlies the
ophiolites has been described by Durand-Delga et al. (2005) as made up of radio-larites, pelites with ‗Palombini‘ type beds that have been dated at the base with Cal-pionellopsis as late Berrasian, hardened olive green marl with thin beds of limestone or quartzite and finally the pelitic or micro-breccia ‗Flysch de Balliccione‘. This unit is not affected by high pressure/low tempera-ture metamorphic conditions (Sanfilippo and Tribuzio, 2013).
The Rio Magno unit
This unit crops out discontinuously along the south-eastern border of Alpine Corsica, it overlies the SL complex and is formed by an ophiolitic basement and sedimentary cover, both non metamorphic (Padoa et al., 2001). The Rio Magno unit contains N-MORB basalts and Durand-Delga et al., (2005) assign it (along with the Pineto unit) to the Apennine Ligurides.
2.3 GEODYNAMIC EVOLUTION
The geodynamic evolution of Alpine Cor-sica is correlated to that of the Western Alps (Dallan and Nardi, 1984; Malavielle et al., 1998; Marroni and Pandolfi, 2003; Sac-cani et al., 2000; Marroni and Pandolfi, 2007; Brovarone et al., 2012). Alpine Cor-sica has in fact been considered as the southernmost extremity of the Western Alps, that was isolated from the main Al-pine Belt by the opening of the Liguro-Provençal basin (during the
Oligo-15
Miocene) due to the anticlockwise rotation of the Corsica-Sardinia block (Jolivet et al., 1991; Brunet et al., 2000; Speranza et al., 2002). However many geodynamic models whose interpretation of the collisional and orogenic evolution differ in many points have been proposed. The geodynamic evo-lution is generally accepted to have begun during the Middle to Late Jurassic time span, as an extensional phase caused the opening of the Liguro-Piemontese oceanic basin between the European and Adria plates. Marroni and Pandolfi (2007) suggest
that the initial opening was caused by asymmetric spreading where the role of lower plate was played by the Adria plate; subsequently the oceanic spreading lead to the formation of an oceanic basin with a very slow to slow-spreading ridge. The tec-tonic slices preserved in the Apennine and Alpine Corsican belts belonging to the Liguro-Piemontese basin contain a mantle basement covered by thin volcano-sedimentary complexes (Marroni and Pan-dolfi, 2007). Paleogeographic reconstruc-tions of the Liguro-Piemontese oceanic
ba-Fig 2.7 - Palaeotectonic sketch maps of the western Mediterranean for: (a) Late Cretaceous; (b) Late Eocene; (c) Late Oligocene; (d) Middle Miocene (mainly based on Gueguen et al. 1997; Stampfli et al. 1998; Se´ranne 1999; Neugebauer et al. 2001; Michard et al. 2002; Dezes et al. 2004 and references) (from Molli, 2008).
sin suggest that it reached a maximum width of 600 km (Bortolotti et al., 1990). This oceanic basin underwent a compres-sive phase during the Late Cretaceous (Fig 2.7a), as the Europe-Corsica plate plays the role of lower plate in an east dipping alpine subduction zone that was probably within the Ligurio-Piemonte oceanic basin (Fig 2.8a) (Boccaletti et al., 1971; Abbate et al., 1980; Doglioni, 1991; Marroni et al., 2010). This change from an extensional to a com-pressive tectonic regime is generally be-lieved to be due to the E-SE movement of the Iberic microplate towards the Adria mi-croplate (Lagabrielle & Polino, 1988; Schmidt et al., 1996; Stampfli et al., 1998; Michard et al., 2002). A thick accretionary complex developed during subduction, tes-tified by the SL Complex that was de-formed under HP/LT conditions. According to Brunet et al. (2000) the highest metamor-phic conditions were reached between 90 and 60 Ma, with an eclogite metamorphic peak in the Farinole Morteda unit and Castagnaccia schists; Lahondere and Guerra (1997) have dated this metamorphic peak (in eclogite facies) at 84 ± 5 Ma. Be-tween 45-38 Ma Cap Corse eclogites were exhumed to blueschist facies, meanwhile continental and oceanic units that up to this point had not been involved in the orogenic wedge built up, began to be subducted and were buried in a blueschist metamorphic facies (Brunet et al. and references therein, 2000). During the last subduction stages (45 -32 Ma) the Tenda massif and Corte slices were subject to underthrusting below the
Schistes Lustrés (Brunet et al., 2000; Molli et al., 2006; Molli, 2008). Due to the buoy-ancy of continental crust the subduction ended with the inception of the collisional phase during the Middle Eocene (Fig 2.8b & 2.8c) (Marroni et al., 2010). According to Brunet et al. (2000) the collapse of the ac-cretionary wedge (33 – 22 Ma) corresponds to the onset of an extensional phase that caused the opening of the Liguro-Provençal basin (Fig 2.7c; Fig 2.8e). Marroni et al., (2010) propose a model in which the break off of the alpine slab and the beginning of the west dipping Apennine subduction also happened during this time span. The west dipping subduction was favored by the presence of an oceanic crust bordered by a sub-continental mantle covered by a thinned continental crust to the east of Cor-sica (Fig 2.8d). Meanwhile a N-S trending zone of sub-vertical sinistral strike-slip faults known as the Central Corsica Fault Zone (CCFZ) developed along most of the boundary between Alpine and Varisican Corsica (Fig 2.7b) due to a 24 degree anti-clockwise rotation of the Sardo-Corsican micrpolate (Muttoni et al., 2000). This structure is formed by a sinistral strike-slip fault system made up of anastomosing and overlapping – overstepping faults, whose tectonic activity can roughly be constrained between the Late Eocene and early Mio-cene (Maluski et al. 1973; Jourdan, 1988; Waters, 1990; Molli & Tribuzio, 2004; La-combe & Jolivet, 2005). According to Sper-anza et al. (2002) this rotation must have begun after 19 Ma, this assumption is based
17
Fig 2.8 - Simplified geological sections. a. During the upper Cretaceous; b. During the Paleocene - lower Eocene; c. During the middle Eocene; d. During the upper Eocene; e. During the Oligocene. (from Molli and Malavielle, 2010).
on Ar/Ar ages; they do however state the uncertainty of total rotation and its timing. The whole of Alpine Corsica has been af-fected by broad regional-scale folding caused by a late Miocene tectonics. These folds have N-S trending axes and sub-vertical axial planes and are responsible for the Tenda massif, Cap Corse, Castagniccia antiforms and the Balagne, Nebbio and Francardo-Ponte Leccia synforms. A final deformation phase was responsible for the tectonic thinning of continental lithosphere to the east of Corsica during the late Mio-cene to PlioMio-cene, resulting in the opening of the Tyrrhenian Sea (Fig 2.7c) (Speranza et al., 2002).
19
3 GEOLOGICAL SETTING
OF THE STUDY AREA
3.1 Tectonic units
Seven tectonic units have been distin-guished in the study (Fig 3.1) area that dif-fer in lithology, metamorphic facies and structural position:
Serra unit is made up of oceanic crust, mainly by serpentinites with gabbroic dykes that registers a low metamor-phic facies.
Pineto unit is also made up of oceanic crust; in this case it is composed mostly of gabbros that also register a low metamorphic facies.
Lento unit is made up of oceanic crust and associated sediments deformed under HP/LT metamorphic facies, in literature this unit has been classified as part of the Schistes Lustrés Com-plex (Levi et al., 2007).
Scoltola unit probably has continental
affinity and includes meta-breccias and meta-sandstone all of which are strongly deformed
Venato unit is similar to the meta-sandstone of the Scoltola unit but with a lower deformation imprint.
Pedani unit is made up of a Mesozoic sedimentary succession that has an affinity with that of a carbonate shelf and overlies a poorly exposed volcano -sedimentary sequence.
Canavaggio unit is made up of country rock intruded by Permian granites and a volcano-sedimentary sequence, cou-pled by an unconformity with an Eo-cene breccia.
These units have been ordered according to their structural position, where the highest units (structurally speaking) are at the beginning of the list. In this thesis the last four units have been named according to their location, therefore they cannot be found in other older publications
Fig 3.1 - Sketch of the tectonic units in the study area. A. Pineto unit; B. Serra Debbione unit; C. Lento unit; D. Scoltola unit; E. Venato unit; F. Pedani unit; G. Canavaggia unit.
3.2 LITHOSTRATIGRAPHY
The successions that make up the tectonic units found in the study area can be assigned to two main domains: the Ligure-Piemonetse oceanic domain and the Euro-pean continental margin. The oceanic units are thrust over the continental ones.
By integration of all the data collected in the successions of the units derived from the Ligure-Piemontese oceanic basin (Lento, Pineto and Serra Debbione units) a complete stratigraphic log can be reconstructed. The stratigraphic log includes a middle to late Jurassic ophiolite sequence, made up of per-idotites, gabbros, ophiolitic breccias and ba-salts, covered by a Late Jurassic to Late Cre-taceous sedimentary cover. Each unit in-cludes all or only a part of this succession. In the same way, the stratigraphic log repsentative of the European margin can be re-constructed by integration of all data col-lected in the successions of the continental units (Pedani, Canavaggia, Scoltola and Ve-nato units). This log includes a Paleozoic Basement, made up of Carboniferous gran-ites and their country rock, both covered by a volcano-sedimentary complex, showing a transition to a Triassic-Jurassic, mainly car-bonate, sequence unconformably covered by breccias and siliciclastic turbidites of Eo-cene age. Each unit includes only a part of this succession.
The successions of the units from the study area will be described separately from the uppermost to the lowermost one.
Units derived from the Ligure-Piemontese oceanic domain
These units are represented from top to bot-tom by the Serra Debbione, Pineto and Lento units. The Lento unit has been classi-fied as part of the Upper Ophiolitic units belonging to the Schistes Lustrés (Rossi et al., 1994; Levi et al., 2007). However, Rossi et al. (1994) have regarded the Serra Deb-bione peridotites as belonging to the Lento unit, whereas Levi et al. (2007) have inter-preted the absence of HP/LT metamorphism in the gabbros intruded in the Serra Deb-bione peridotites as proof that these rocks represent an independent tectonic unit be-longing to the Nappes Superieurs. The Pineto unit is also regarded as belonging to the Nappes Superieurs. Durand-Delga et al. (2005) and Sanfilippo and Tribuzio (2013) report a sedimentary cover for the Pineto unit, however this sedimentary cover crops out on the western side of the Golo River in the vicinity of Ponte Leccia, these outcrops do not fall within the boundaries of the study area.
Serra Debbione unit
The Serra Debbione unit, recognized by Levi et al. (2007), is represented by peri-dotites cut by gabbro bodies and basaltic dykes (Fig 3.2).
Λ – SERPENTINITES (Cfr. “Péridotites
serpentinisées; serpentinites” by Rossi et al., 1994)
21
Outcrop– The main body of serpentinites crops out over a large area in the NW sec-tion of the study area below Cima Terseto and Serra Debbione.
Lithology –In some areas (such as some of the outcrops along the D71 between Ponte Leccia and Padule) weathering processes have completely obliterated the original and metamorphic textures, but in several out-crops the serpentinites preserve remnants of
original clino- and orthopyroxene crystals. These rocks are intruded by a trondhjemite dykes (Fig 3.3). The serpentinites have been described by Levi et al. (2007) as intruded by gabbroic bodies and crosscut by several basaltic dykes. Rossi et al. (1994) assign the serpentinites to the ―Ensamble ultrama-fique‖ of the Scistes Lustrés.
Γb – GABBROS (Cfr. “Métagabbros
indif-férenciés” by Rossi et al., 1994)
Outcrop – One small isolated outcrop of gabbros has been identified in the area of Bocca di Riscamone. Other outcrops have been found along the contact with the Lento unit along the Golo valley.
Lithology – These gabbros display a me-dium- to coarse-grained holocrystalline tex-ture consisting of plagioclase, olivine and pyroxene.
Pineto unit
Fig 3.2 - Serra Beddione stratigraphic log. Key: Λ – SERPENTINITES; Γb – GABBROS.
Fig 3.3 - Trondhjemite dikes within the serpenitinite of the Serra debbione unit.
Fig 3.4 - Serra Beddione stratigraphic log. Key: Γa–
This unit is mainly represented by gabbros cut by basaltic dykes (Saccani et al., 2000; Durand Delga et al., 2005; Sanfilippo and Tribuzio, 2013) (Fig 3.4). Sanfilippo and Tribuzio (2013) provide the evidence of striking micro-structural and geochemical similarities of the gabbros from Pineto unit with the Ligurian ophiolites.
Γa– GABBROS (Cfr. “Ensamble mafique”
by Rossi et al., 1994)
Outcrop – The gabbros belonging to this unit crop out extensively throughout the western section of the study area and, in par-ticular on the Cima D‘Alpa in the Forete Territorial de Pineto.
Lithology – According to Sanfilippo and Tribuzio (2013) the thickness of this unit is roughly 1.5 Km. The Pineto unit is charac-terized by troctolites and minor olivine
gab-bros in the SE sector and by clinopyroxene-rich gabbros to gabbronorites. These gab-bros show a well developed layering repre-sented by compositional and grain size ani-sotropies (Fig 3.5). The crystal size varies from a few mm to 5-6 cm.
Age – The gabbros of the Pineto unit are generally thought to have a Jurassic age (Sanfilippo and Tribuzio, 2012).
Lento Unit
Fig 3.5 - Grain size layering within the gabbros belong-ing to the Pineto unit.
Fig 3.6 - Stratigraphic log of the Lento unit. Key: πΛ – META-SERPENTINITES; πΓ – META-GABBROS; πΣ –META-BASALTS; MJR – META-RADIOLARITES; ERF – ERBAJOLO FORMATION
23
The Lento unit is made up of a meta-ophiolite sequence and its metasedimentary cover (Fig 3.6) that has been strongly de-formed under blueschist facies metamor-phism (Levi et al., 2007). The meta-ophiolites are Middle to Late Jurassic in age, their protoliths are represented by a peridotite-gabbro basement that is covered by a thin volcano-sedimentary complex fol-lowed by a thick sedimentary pelagic suc-cession (Levi et al., 2007).
πΛ – META-SERPENTINITES (Cfr.
“Péridotites serpentinisées; serpentinites” by Rossi et al., 1994)
Outcrops– The main body of Meta-Serpentinites is made up of a large outcrop across the whole study area with a N-S di-rection and is cut by the D17 road near Bocca a Serna, the N193 road to the north
and the D139 road to the south.
Lithology –The Meta-Serpentinites consist of foliated rocks that are mainly made up of serpentine minerals with rare relics of ortho- and clinopyroxene. The Meta-Serpentinites are crosscut by a network of fibrous serpen-tinite veins with a thickness that can reach 5 -6 cm (Fig 3.7). The Meta-Serpentinites are intruded by gabbroic bodies and basaltic dykes as described by Levi et al. (2007). Age – The Meta-Serpentinites can be attrib-uted to the Middle Jurassic (Levi et al., 2007).
Fig 3.7 - Aspect of the serpentinites belonging to the
πΓ – META-GABBROS (Cfr. “Métagabbros indifférenciés” byRossi et al., 1994)
Outcrops – The main body of Meta-Gabbros crops out in an area to the South of Cima Pedani, close to the tectonic contact with the Pedani unit to the west, and in the southern edge of the study area.
Lithology –These rocks are generally foli-ated with rare relics of magmatic clinopy-roxene. However sometimes it is possible to recognize boudins where the magmatic tex-ture is preserved. These rocks display layers with different crystal size where the plagio-clase and pyroxene crystals are easily recog-nizable and reach a maximum size of 3-4 cm. Levi et al. (2007) described the proto-liths of the Meta-Gabbros as ranging from Mg – to Fe – gabbros.
Age – The ophiolitic sequence of the Lento unit can be attributed to the Middle to Late Jurassic (Levi et al., 2007).
πΣ –META-BASALTS (Cfr.“Métabasaltes
en coussins, massifs et/ou agglomérats”by Rossi et al., 1994)
Outcrops – The main outcrops are found on the southern edge of the study area near Ponte de Scoltola along the D139 road. Out-crops occur also along the N193 and D615 roads along the Golo valley.
Lithology – The protoliths of the meta-basalts are represented by pillow lavas, hya-loclastites and ophiolite breccias. The meta-basalts, although strongly deformed, still
show relics of pillow structures (Fig 3.8). Epidotite-bearing mineralizations are wide-spread.
Age – The ophiolitic sequence of the Lento unit can be attributed to the Middle to Late Jurassic (Levi et al., 2007).
MJR – META-RADIOLARITES (Cfr.
“Jaspes à radiolaires” by Rossi et al., 1994)
Outcrops – The Meta-Radiolarites are found in relatively small bodies identified along the D71 road near Bocca a Serna and to the north of Santa Maria di Riscamone.
Lithology – The Meta-Radiolarites are made up of meta-quartzite layers with a thickness of 1-10 cm that in some cases are separated by thin layers of schists.
Age –The Meta-Radiolarites do not yield fossils that can be used to determine their age, however the age of radiolarites found in the Balagne nappe with a similar strati-graphic position has been reported as Call-ovian-Oxfordian to Tithonian (Rossi et al., 1994).
ERF – ERBAJOLO FORMATION (Cfr.
“Formation d’Erbajolo: alternances de schistes et calcaires” by Rossi et al., 1994) Outcrop – The Erbajolo formation crops out extensively in the eastern section of the study area and in the Golo valley near the contact with the Serra Debbione unit. Lithology – This Formation is made up of thin to medium layers of metalimestone and
25
thick layers of schists (Fig 3.9). The occur-rence of metalimestone layers increases to-wards the top of the formation. This forma-tion can be compared to the ―Argille a Palombini formation‖ that crops out in the northern Apennines and the ―Formation de la Replatte‖ that has been found in the Alps (Rossi et al., 1994).
Age – Rossi et al. (1994) suggest an early Cretaceous age for this formation.
Units derived from the European conti-nental margin
In the study area four units derived from the European continental margin have been rec-ognized, from top to bottom they are: the Scoltola unit, the Venato unit, the Pedani unit and the Canavaggia unit. In the litera-ture these four units were grouped in to two
units. According to Rossi et al. (1994), the Venato and Scoltola units belong to the Nappe de Santa Lucia. The Pedani and Ca-navaggia units were in turn reported as the single Caporalino Pedani unit by Rossi et al. (1994).
Scoltola unit
The Scoltola unit has been recognized for the first time in this thesis and is made up solely of the Scoltola Meta-Sandstone (Fig 3.11). Previously it was classified as partly as the Conglomeràt du Tomboni belonging to the Nappe de Santa Lucia and partly as the Breches de Francardo belonging to the Unité Prépiémontaise de Caporalino-Pedani by Rossi et al. (1994).
SMB – SCOLTOLA META-BRECCIA (Cfr. “Conglomeràt du Tomboni” & “Brèches de Francardo” by Rossi et al., 1994)
Outcrop – This formation crops out exten-sively along the D39 road in the area of
Fig 3.9 - An carbonate arenite layer belonging to the Erbajolo formation of the Lento unit.
Fig 3.10 - Stratigraphic log of the Scoltola unit. Key: SMB –SCOLTOLA META-BRECCIA
Ponte de Scoltola, below Cima Pedani and along the Casaluna River between Canavag-gia and Ponte de Scoltola.
Lithology – This formation comprises both meta-breccia and minor meta-sandstone. The meta-breccia is characterized by well developed foliation with cm-sized clasts that are strongly deformed in most areas, al-though some outcrops show a lesser degree of deformation (Fig 3.11). The meta-breccia appears to include clasts of meta-limestones, meta-basic rocks and meta-quarztites in a matrix consisting of quartz, muscovite and calcite.
Age –Although Rossi et al. (1994) consider it to belong to the Jurassic, this formation can be regarded as an Eocene deposit on the basis of lithological similarities with the other Eocene deposits in the study area and with the Eocene deposits described by Puc-cinelli et al. (2012).
Venato unit
The Venato unit has been recognized for the first time in this thesis and is made up solely by the Venato Meta-Sandstone formation (Fig 3.13). Rossi et al. (1994) regarded this unit as belonging to the Nappe de Santa Lu-cia.
VMS – VENATO META-SANDSTONE
Fig 3.11 - Relatively undeformed aspect of the Scoltola Meta-Breccia.
Fig 3.12 - Stratigraphic log of the Venato unit. Key: VMS – VENATO META-SANDSTONE.
27
(Cfr. “Conglomeràt du Tomboni” by Rossi et al., 1994)
Outcrop – Thisformation crops out along the D39 road near Ponte de Scoltola.
Lithology – This formation is manly made up of meta-arenites and meta-pelites (Fig 3.13). The meta-arenites show varying grain size from fine to coarse. In thin section the grains have been recognized as lithic frag-ments, feldspars and quartz grains.
Age –Although Rossi et al. (1994) consider this formation to belong to the Jurassic, this formation can be regarded as an Eocene de-posit on the basis of lithological similarities with the other Eocene deposits in the study area and with the Eocene deposits described by Puccinelli et al. (2012).
Pedani unit
This unit is made up of a metamorphic suc-cession whose protoliths are represented by Triassic to Jurassic meta-carbonate se-quence that overlies the Permian Meta-Volcano-Sedimentary formation that repre-sents the base of this succession (Fig 3.14). On the whole the succession of this unit is considered by Rossi et al. (1994) and Amaduric du Chaffaut (1980) as derived from a carbonate shelf located on the Euro-pean margin of the Ligure-Piemontese oce-anic basin.
VSC – META-VOLCANO-SEDIM-
ENTARY COMPLEX (Cfr. “Permien volcanogène” by Rossi et al., 1994)
Outcrop – This formation crops out in a small area along the D71 road, not far from Bocca a Serna.
Lithology – The Meta-Volcano-Sedimentary complex consists of orthogneisses whose protoliths are represented by hyperalkaline volcanic products like rhyolites and dacites.
Fig 3.14 - Striatigraphic log of the Pedani unit. Key: VSC – MET A-VOLCANO-SEDIMENT ARY COMPLEX; CML – ―CAVERNOSO‖ META-LIMESTONE; LMD – LOWER META-DOLOSTONE; LML – LUMACHELLE BEARINGMETA-LIMESTONE; UMD – UPPER META-DOLOSTONE; LAM – LAMINATED META-LIMESTONE; LAMa - MEMBER OF THE META-POLYMICTIC BRECCIA; MCB – META-CARBONATE BRECCIA
These rocks include mm-sized quartz and feldspar grains in a recrystallized matrix made up of phyllosilicates and quartz. Age – According to Rossi et al. (1994) these rocks can be assigned to the second Permian cycle
CML – ―CAVERNOSO‖
META-LIMESTONE (Cfr. ―Cargneules” by Rodriguez, 1981)
Outcrop – This formation crops out in one very small area along the D71 road near Bocca a Serna.
Lithology – The ―Cavernoso‖ Meta-Limestone is made up of a meta-breccia with an abundant recrystallized calcite ma-trix where mm- to cm-sized fragments of basement rocks and meta-carbonate rocks occur. The main features of these rocks are
cavities with a diameter that can reach 2-3 centimeters.
Age – Rodriguez (1981) assigns this forma-tion to the middle Triassic on the basis of its stratigraphic position.
LMD – LOWER META-DOLOSTONE (Cfr. ―Trias superieur (Norien?) Dolomies” byRossi et al., 1994)
Outcrop – This formation crops out exten-sively on the Punta di Quercia Tonda and Cima Pedani. Large outcrops have been rec-ognized along the D71 road on the northern side of the study area as well as along the D39 road on the southern side of study area. Lithology – This formation consists of well bedded dolostone. In some cases meta-dolostone layers are separated by thick lay-ers of meta-conglomerates (Fig. 3.15) and
Fig 3.15 - Conglomerate layer within the Lower Meta-Dolostone Formation.
Fig 3.16 - Fragments of the Permian basement within the Lower Meta-Dolostone Formation (indicated by the white arrow)
29
by thinner levels of meta-pelites. The proto-lith is represented by thick layers (up to 50 centimeters) of dolostone with structures attributed to algal laminae (Rodriguez, 1981). According to Rodriguez (1981) the lower section of the dolostone was depos-ited in a lagoon environment where the transformation into dolomite was diagenetic, this process was helped by periods of expo-sure above sea level that are testified by the conglomerates. The middle part of the de-posit belongs to a tidal sedimentary faces and the upper part was deposited in a su-pratidal environment (Rodriguez, 1981). Near Bocca a Serna the meta-dolostone con-tains fragments of the Permian basement (Fig 3.16). Rodriguez (1981) reports a level of volcano-sedimentary deposits towards the top of the formation. Rodriguez (1981) com-pares this deposit to a similar volcano-sedimentary one recognized by Durand Delga (1978) that crops out along the RN 193 road south of Bistuglio and dated be-tween the Norien and Rhaetien. Amaduric Du Chaffaut (1980) reports levels of vol-cano sedimentary origin trhougout the for-mation.
Age – According to Rodriguez (1981) this formation belongs to the Norian.
LML – LUMACHELLE BEARINGMETA-LIMESTONE (Cfr. ―Trias supérieur Rhétien and. Calcaires et lumachelles” by Rossi et al 1994)
Outcrop – This formation crops out in the area of the top of Punta di Quercia Tonda,
on the southern flank of Cima Pedani and along the D71 road, not far from Bocca a Serna.
Lithology – The protolith of this formation, whose thickness doesn‘t exceed fifteen me-ters, is represented by alternating beds of fossil-bearing limestone with meta-pelites and meta-arenites. The meta-limestone (Fig 3.17) contains fragments of bivalves and crinoids, lamellibranchs, gasteropodes, echi-noderms, foramnifera as well fish teeth (Rodriguez, 1981). Hollande (1877) reports Avicula contorta PORTL., Terebratula gre-garia SUESS.. Rodriguez (1981) and Amaduric du Caffaut (1980) describe one layer that containes oolites with a bioclastic nucleus. According to Rodriguez (1981) this formation was deposited in a facies that varyes form supratidal and infratidal to shal-low sea bed. A layer made up of
meta-Fig 3.17 - Fragments of bivalves and other fossils in the Lumachelle Bearing Meta-Limestone.
arenites was recognized on the eastern side for Cima Pedani. At the micro-scale sample (CH40) shows the clasts to have a volcano-sedimentary origin.
Age –The age of this formation has been determined as Rhaetian by Hollande (1877) and by Rodriguez (1981) thanks to the pres-ence of A. Contorta.
UMD – UPPER META-DOLOSTONE (Cfr. “Hettangien” dolomites litées” by Rossi et al., 1994)
Outcrop – The formation crops out exten-sively on the south-eastern side of Punta di Quercia Tonda and below Cima Pedani, but also along the D71 road in the Quercioli val-ley. There is also an isolated outcrop along the D39 road roughly half way between Ca-navaggia and Ponte de Scoltola.
Lithology – This formation is made up of massive meta-dolostones. The sedimentary features of this formation can be fully recog-nized. The sedimentary facies varies from intratidal for the lower part of the formation to supratidal for the upper part (Rodriguez, 1981). According to Rodriguez (1981) this formation can be divided into three ―microfacies‖ on the basis of the primary structures seen in thin section: 1) cies ―oodolomicrosparitique‖, 2) microfa-cies made up of bioturbated dolomitic lime-stone that contains layers of sandlime-stone and micrtitic intraclasts and 3) a dolo-microsparitic microfacies that contains sandstone whose grains are made up of dolomicrite with inclusions of clay and iron
oxides. According to Rodriguez (1981) the dolomitisation happened in the lower part of the formation after deposition and during diagenesis.
Age – The age of this formation has been dated according to stratigraphic continuity as Hettangian, and has been compared by Rodriguez (1981) to Liassic carbonates be-longing to the Saint-Florent region that con-tain Arietitide (Gindrat, 1942) and Gry-phaea cf. arcuate (Ricour, 1949). Gelmini and Mantovani (1982) suggested that this formation belongs to the Anisian on the ba-sis of Glomospira and Glomospirella.
LAM – LAMINATED
META-LIMESTONE (Cfr. “Sinemurien - Calcaire slités” byRossi et al., 1994)
Outcrop – This formation crops out in a large area on the south-western flank of
Fig 3.18 - General aspect of the Laminated Meta-Limestone.
31
Cima Pedani. Outcrops of this formation are also found along the D39 road between Ca-navaggia and Ponte de Scoltola and along the D71 road in the Quercioli valley.
Lithology – This formation is made up of thin bedded meta-limestone. The beds have a relatively constant thickness ranging be-tween 1-2 centimeters (Fig 3.18). At the mi-croscale Rodriguez (1981) describes them as homogenous micritic rocks that also contain phyllosilicates and iron oxides. Bioclasts are rare, however Rodriguez (1981) reports the presence of ostracodes, radiolarites, echino-derm fragments and sponge spicola. At the base of this formation a meta-breccia (Fig 3.16) has been classified as the Member of the Meta-Polymictic Breccia (LAMa) (Cfr. “Brèches”by Rossi et al., 1994) has a thick-ness that is very variable, from 20 up to 0 meters. The blocks of the meta-breccia are
composed mainly of meta-limestone and meta-dolostone as well as fragments of the basement rock (Fig 3.19), that can reach 1 meter in diameter. The matrix is mainly made up of a pelitic-micritic cement that containes iron oxides, spunge spicola and radiolarites are present (Rodriguez, 1981). Levels of fine-grained breccia and sandstone are also found in the central part of the for-mation. The sandstone has a carbonate ce-ment that also contains phyllosilicates, the grain composition varies: 1) at the base of the formation the clasts are made up of car-bonate fragments belonging to the underly-ing carbonate formations; 2) in the central part of the formation the sandstone contains a small portion of quartz, fragments of the volcano-sedimentary complex and
mi-caschists; 3) the quartz,
volcano-sedimentary fragments and micaschists are the main components of the sandstone (Rodriguez, 1982).
Age – According to Rossi et al. (1994) this formation belongs to the Sinemurian. MCB – META-CARBONATE BRECCIA (Cfr. “Conglomérats a blocs calcaires et dolomitiques” by Rossi et al., 1994)
Outcrop – This formation crops out at the base of the Casaluna valey along the D39 road about 1.5 Km from Pont du Scoltola. Other outcrops have been found to the NE of Canavaggia and Prumezzano.
Lithology – This formation is made up of a clast supported meta-breccia, that is almost monogenic. The block size varies between 2
Fig 3.19- General aspect of the Member of the Meta
-Polymictic Breccia, a large polimetric carbon-are block is visible in the lower half of the photo.
-0.5 m, and are composed mainly of meta-limestone that containes fragments of mol-lusks and rarer meta-dolostone blocks. This formation presents lithological similarities with the Eocene breccia at the top of the Caporalino Sant‘ Angelo unit as described by Puccinelli et al., (2012).
Age – The age of this formation can be at-tributed to the Eocene on the basis of simi-larities with the Ecoene deposits described by Puccinelli et al. (2012).
Canavaggia unit
This unit consists of Meta-Granites and their host rocks covered by a Meta-Volcano-Sedimentary complex (Fig 3.20). These lithotypes are uncomfortably covered by Eocene deposits.
K – ROCHES BRUNES (Cfr. “Roches
brunes: cornéennes et terrains ante-batholitiques plus ou moins métamorfiques indifférenciés” byRossi et al., 1994)
Outcrop – The Roches Brunes crop out in the western area of the map, along the D71 road in the area of Padule and Bocca di Ris-camone and above the D39 road near Casaluna.
Lithology – The Roches Brunes consists of a deformed and metamorphosed complex, de-formation happened during the Caledonian and Variscan ororgeny. This complex in-cludes foliated amphibolites, paragneisses, orthogneisses and schists.
Age – According to Rossi et al. (1994) the Roches Brunes in the area of Santo-Pietro di Tenda are pre-Carboniferous. Rodriguez (1981) suggests that they deposited either during a Silurian or a Devonian-Carboniferous time span.
γ – META-GRANITE (Cfr.
“Monzogranites leucocrates à biotite” by Rossi et al., 1994)
Outcrop – These rocks crop out in the area
Fig 3.20 - Stratigraphic log of the Canavaggia unit. Key: K – ROCHES BRUNES; γ – METAGRANITE; VSC – META-VOLCANO-SEDIM- ENTARY COMPLEX; γa – META-GRANITIC DYKES; PMB –
PADULE BRECCIA; PMB – PADULE META-BRECCIA.
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of Padule along the D71 road. The outcrops are found both on the southern and northern side of the D71 road and to the west of Pa-dule. A small outcrop has also been found in the area of Bocca di Riscamone near the D71 road.
Lithology – These rocks are fine to medium grained biotite bearing granites. With a holocrystalline texture made up of quartz and feldspar phenocrysts. Biotite content is low and biotite crystals are often altered into chlorite. Quartzite veins are often present (Fig 3.21).
Age – According to Rossi et al. (1994) these rocks have a Permo-Carboniferous age.
VSC – META-VOLCANO-SEDIM-
ENTARY COMPLEX (Cfr. “Permien volcanogène” by Rossi et al., 1994)
Outcrop – This formation crops out along the D71 road at Padule and Bocca di Risca-mone areas and in the northwestern side of Punta di Quercia Tonda. It also crops out in the area of Prumezzano.
Lithology – The Meta-Volcano-Sedimentary complex is made up of orthogneisses whose protoliths are represented by hyperalkaline volcanic products such as rhyolites and dacites. These rocks include mm-sized quartz and feldspar grains (Fig 3.22) in a recrystallized matrix made up of phyllosili-cates and quartz. In the Meta-Volcano-Sedimentary complex paragneisses and schists also occur whose protoliths can be classified as arkosic wakes with abundant matrix that reaches up 70% of the rock. Dykes of quartz are also common.
Age – According to Rossi et al. (1994) these rocks belong to the second hyperalkaline
Fig 3.21 - General aspect of the Meta-Granite. Fig 3.22 - Quartz and feldspar garins (indicated with the arrow) within the Meta-Volcano-Sedimentary complex.
volcanic Permian cycle
γa – META-GRANITIC DYKES (Cfr.
“Microgranites alcalins” by Rossi et al., 1994)
Outcrop – This formation crops out along the D71 road at the boundary with the Serra unit, Padule and Bocca di Riscamone on the northwestern side of Punta di Quercia Tonda. It also crops out in the area of Pru-mezzano.
Lithology – These rocks display aporphyiric texture with quartz and mono feldspar crys-tals in a fine-grained matrix.
PMB – PADULE META-BRECCIA (Cfr.
“Brèches de Padule” by Rossi et al., 1994) Outcrop – This formation crops out on the western to southern side of Cima Pedani and Punta di Quercia Tonda. Outcrops have been identified along the D39 road in the Salgi and Canavaggia areas and along the D71 road in the Padule area.
Lithology – This formation is represented by a matrix supported polygenic meta-breccia (Fig 3.23) where clasts of meta-limestone, meta-dolostone, quartzite, meta-basalt and meta-granite in a fine-grained recrystallized matrix. The clasts have a variable size, from a few centimeters up to several meters. One granite block was found to have a diameter of roughly 10 m. In thin section the matrix is made up of phyllosilicates and recrystal-lized quartz, and mainly contains quartz and feldspar crystals as well as a few lithic grains.
Age – Although Rossi et al. (1994) consider this formation to belong to the Jurassic, it can be regarded as an Eocene deposit on the basis of similarities with the Ecoene depos-its described by Puccinelli et al. (2012).
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4. STRUCTURAL ANALYSIS
The structural analysis have been per-formed on the Canavaggia, Pedani, Scotola, Venato and Lento units. These units are characterized by metamorphic rocks that display a polyphased structural history. In contrast, the Pineto and Serra Debbione units are characterized by peridotites and/or gabbros that underwent a very low-grade orogenic metamorphism without the devel-opment of relevant deformation structures. These features hamper the development of folds and/or foliations in the Pineto and Serra Debbione units, where the only evi-dent deformations are represented by brittle faults.
Concerning the Canavaggia, Pedani, Scotola, Venato and Lento units the struc-tural analisis will be described separately in each unit. In each unit the structural analy-sis has been carried out at all the scales. The methodology used followed three main steps:
Collection of data concerning the atti-tude for the sedimentary layers as well as linear and planar structural elements like foliation, fold axes and lineations. This data was then plotted on stereonet diagrams to illustrate the setting of the different structural ele-ments.
Detailed analysis at the mesoscale of significant outcrops where the fea-tures of the deformation phases are
clear and/or where the interference among the different phases is well exposed. In these outcrops, samples for structural analysis have been col-lected.
Micro-structural analysis of the col-lected samples to estimate the micro-scale deformation features and the associated recrystallization developed during the different phases recog-nized at the mesoscale.
4.1 THE CANAVAGGIA UNIT
The deformation in the Canavaggia unit consists of four main phases. These phases are evident in all the formations of this unit except in the ―Rocce Brune‖. This forma-tion records deformaforma-tion phases that belong to Alpine, Varisican and Pan-African oro-genesis. This feature hampers a clear dis-tinction between the phases belonging to the different orogenesis.
D1 phase
The D1 phase can be seen at the micro-scale in all the lithotypes (but has been al-most completely transposed in the Meta-Granites) within the microlithons bounded by the F2 foliation and is defined by lepido-blastic layers of white mica and chlorite that define a continuous schistosity (S1) (Fig 4.1).
D2 phase
