Apuan Alps Geopark
interpretation of the Apuan Alps
The current interpretation model of the Apuan Alps was defined in 1990 when Carmignani and Kligfield (and, separately, Coli) noted closes analogies between the evolution of the Apuan-Apennine area and northern American Core Complex, where more recent structures of extensional tectonics superimposed upon older compressive deformations.
Nowadays, almost the entire scientific community agrees on the interpretation of the Apuan Alps and the Northern Apennines as the complex result of two different and subsequent deformation phases: the first compressional-collisional and the second extensional. The Apuan Alps and the Northern Apennines belong to the same thrust and fold mountain belt originated during the Cenozoic by the overthrusting of the Ligurian Units, derived from the ocean (Alpine Tethys) internal domain, onto the external Tuscan and Umbria-Marche continental margin domains. The complex tectonic evolution of the area showed a first phase of deformation and metamorphism within a collisional geodynamic context. It was then followed by a second phase within an extensional tectonic regime, characterized by the development of low-angle shear zones and normal faults responsible for the uplift and exhumation of the deepest structural units.
In the light of crustal
plate movements in the Mediterranean area, the tectonic events began in the
Late Cretaceous when the European and Apulian (-African) continental margins
sutured the Tethys Ocean. In particular, during the Late Oligocene, the
Briançonnais microcontinent (before belonging to the European plate) moved
towards East and collided with the Adria microplate with the subduction of
the Apulian lithosphere underneath the Corsica-Sardinia block.
Thus, the Apuan Alps and the Northern Apennines underwent a first phase of structuration during a compressional tectonic phase characterized by crustal shortening and nappe emplacement towards the Apulian foreland. Since the Tortonian onwards, the subduction process contributed to the development of back-arc oceanic basins and therefore the opening of the Thyrrenian Sea, within crustal extensional processes which are clearly recognizable in the extensional features of macro and microstructures.
A large tectonic window formed in the Apuan Alps, thus exposing the ‘Metamorphic Complex of the Apuan Alps’. It is the deepest structural level outcropping in the internal parts of the Northern Apennines and it is therefore a key area to understand mechanisms and geodynamic processes which led to the formation of the Apenninic range.
Tectonic map of the Northern Apennines (Carmignani et al., 2006)
In particular, two superposed units, both belonging to the metamorphic succession of the Tuscan Domain, are identifiable in the Apuan tectonic window. The lower unit is the Autochthonous Auct. with extensive outcrops across the main ridge, both on the coastal and inland mountainsides, whereas the Massa Unit, on top of it, is only found in the westernmost part of the tectonic window.
The Massa Unit is composed of a Paleozoic basement on which lays in unconformity a thick Triassic sedimentary sequence characterized by Ladinian basic metavolcanic rocks. The Mesozoic cover begins with quartz metaconglomerates associated with metasandstones, metasiltstones and black phyllites which can be interpreted as continental and/or coastal siliciclastic deposits. Upward, they are followed by mainly calcareous rocks (marbles, metabreccias, calcareous schists and carbonate phyllites), deriving from carbonate shelf and neritic-pelagic deposits, intercalated with alkaline metabasites. The sequence ended with Carnian phyllites of continental-coastal origin and levels of anagenites: metaconglomerates with mainly quartz and quartzite clasts.
Auct. too is formed by a Paleozoic basement, deriving from
previous orogenic structures, with an initial age presumably dating
back to the Cambrian. A metasedimentary sequence, showing ages from
the Late Triassic up to the Oligocene, unconformably lies upon the
crystalline basement, like in the Massa Unit.
The Meso-Cenozoic cover begins with a classic “Verrucano” characterized by polygenic conglomerates, calcareous metasandstones and siliciclastic dolomites intercalated with metabreccias (VIN) presumably deriving from a transitional (from continental to littoral) depositional environment. They are followed by the “Grezzoni” dolomites (GRE) of the Late Triassic carbonate shelf, which are followed by micritic fossiliferous metalimestones of the “Colonnata” limestones and marlstones. They are then followed by the Megalodont-bearing Marbles (MMG) intercalated with polygenic metabreccias (BSE) and chloritoid schists (BSEa) which testify episodes of emersion of the carbonate platform with the formation of lateritic-bauxitic layers and the deposition of debris flows at the bottom of the tectonically active scarps.
Marbles with different dolomitization levels, crystalline dolomites, marbles and monogenic metabreccias are typical of the overlying formations of Dolomitic marbles (MDD) and Marbles s.s. (MAA), witnessing to the development of a new carbonate platform formed after that of the “Grezzoni” dolomites in the Late Triassic. Upward, pinkish marbles (MRZ), metalimestones with cherts (CLF), calcschists (CCI) and jaspers (DSD) are a sign of the drowning of the marble carbonate platform and the beginning of an hemipelagic sedimentation.
Possible time displacement among the different subsiding blocks brought about the formation of very different depositional environments with “reduced” and/or “condensed” sequences, the first evidence of which is polygenic metabreccias characterized by a phyllitic matrix. The sequence ends with metalimestones with cherty bands and nodules (ENT), calcschists, Nummulite limestones, sericitic phyllites (SSR) and quartz-feldspatic metasandstones (PSM) belonging respectively to pelagic and foredeep deposits reaching the Late Oligocene.
In conclusion, there is a strong correlation between the rocks of the Paleozoic basement of the Massa Unit and those of the Autochthonous Auct. which show analogous deformation and a similar greenschist facies metamorphism owing to the Hercynian orogeny. The same situation repeats when comparing the two metasedimentary covers because, despite the first being less thick than the second, their rocks are characterized by the same Alpine metamorphism, whose greenschist facies paragenesis shows differences in temperature and pressure conditions, higher in the Massa Unit.
In the proposed Geopark territory, rocks of the non-Metamorphic Succession of Tuscan Domain outcrop beyond the borders of the tectonic window. They are related to a single unit, commonly known as Tuscan Nappe geometrically overlying the Metamorphic Complex of the Apuan Alps (made up by the Massa Unit and the Autochthonous Auct.). The Tuscan Nappe rocks belong to the Meso-Cenozoic cover, the Paleozoic basement not being present at all. There are closed and frequent correlations as regards lithology, depositional environment and age between the geological formations of Autochthonous Auct. and Tuscan Nappe. The main difference lays in the degree of metamorphism which is absent or very weak in the Tuscan Nappe.
The Tuscan Nappe is in turn overthrusted by the Ligurian Units s.l., which crop out mainly in the northern part of the Geopark area, being represented by Helminthoid Flysch (Late Cretaceous) with basal complex characterized by ophiolitic detritism and olistolites and by shaly-calcereous deposits and calcareous turdidites ranging in age from Late Cretaceous to Eocene.
The proposed territory is also characterized by post-orogenic deposits which cover the whole time frame from the Late Miocene to today. This is particularly the case of polygenic Breccias of Metato (which derived from formations of the Tuscan Domaine cover), glacial and fluvio-glacial deposits (Middle-Late Pleistocene) as well as alluvial, slope and landslide deposits (Holocene).
Stratigraphic column of Autochthonous Auct.
The detailed description of the formations above mentioned is here omitted, referring to the legend of the enclosed geological map.
The complex geological history of the Apuan Alps is responsible for the great geodiversity of rocks and minerals of the territory. The composition of lithotypes, the Hercynian and Alpine deformations, metamorphism and hydrothermalism brought about a remarkable variety of minerals. Witness the large number of minerals found in the area, achieving the total of about 200. For the first time 19 mineral species have been discovered and described in the Apuan Alps [allanite-(La), apuanite, bottinoite, carraraite, dessauite, garavellite, grumiplucite, marrucciite, meneghinite, moëloite, pellouxite, pillaite, rouxelite, scainiite, stibivanite-2O, versiliaite, volaschioite, zaccagnaite, zincalstibite] most of which are exclusive to this area.