Alpine structural evolution of the Internal Piedmont Zone in the Upper Viù Valley (Lanzo Valleys Ophiolite, Western Alps) (original) (raw)
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Minerals
Ophiolites of the Alpine belt derive from the closure of the Mesozoic Tethys Ocean that was interposed between the palaeo-Europe and palaeo-Adria continental plates. The Alpine orogeny has intensely reworked the oceanic rocks into metaophiolites with various metamorphic imprints. In the Western Alps, metaophiolites and continental-derived units are distributed within two paired bands: An inner band where Alpine subduction-related high-pressure (HP) metamorphism is preserved, and an outer band where blueschist to greenschist facies recrystallisation due to the decompression path prevails. The metaophiolites of the inner band are hugely important not just because they provide records of the prograde tectonic and metamorphic evolution of the Western Alps, but also because they retain the signature of the intra-oceanic tectono-sedimentary evolution. Lithostratigraphic and petrographic criteria applied to metasediments associated with HP metaophiolites reveal the occurrence of distinct t...
Geological Magazine
The eclogite-facies Monviso meta-ophiolite Complex in the Western Alps represents a well-preserved fragment of oceanic lithosphere and related Upper Jurassic – Lower Cretaceous sedimentary covers. This meta-ophiolite sequence records the evolution of an oceanic core complex formed by mantle exhumation along an intra-oceanic detachment fault (the Baracun Shear Zone), related to the opening of the Ligurian–Piedmont oceanic basin (Alpine Tethys). On the basis of detailed geological mapping, and structural, stratigraphic and petrological observations, we propose a new interpretation for the tectonostratigraphic architecture of the Monviso meta-ophiolite Complex, and discuss the role played by structural inheritance in its formation. We document that subduction- and exhumation-related Alpine tectonics were strongly influenced by the inherited Jurassic intra-oceanic tectonosedimentary physiography. The latter, although strongly deformed during a major Alpine stage of non-cylindrical W-ver...
Episodes, 2015
The Late Jurassic Monviso ophiolite in the Western Alps is a multiply deformed, eclogite-facies metaophiolite that represents a remnant of the Alpine Tethyan oceanic lithosphere. The recent recognition of a pre-Alpine detachment fault in the Lower Tectonic Unit of this ophiolite has led to the discovery of an oceanic core complex, which developed during the initial stages of the tectonic evolution of the Alpine Tethys. The NNW-striking, 20-25-km-long shear zone (Baracun Shear Zone) contains ductilely to cataclastically deformed blocks and clasts of Fe-Ti and Mg-Al metagabbros in a matrix made of mylonitic serpentinite and talc-chlorite schist with high Ni-Cr concentrations and high Cl contents. Intensely sheared ophicarbonate rocks and brecciated serpentinite within this shear zone are deformed by the Alpine-phase S1 foliation and D2 folds, providing a critical age constraint for the timing of its formation. Metabasaltic-metasedimentary rocks in the hanging wall increase in thickness away from the shear zone, characteristic of syn-extensional rock sequences in supradetachment basins. A Lower Cretaceous post-extensional sedimentary sequence unconformably cover the synextensional strata, the detachment shear zone, and the ophiolitic footwall, establishing a strong structural evidence for the intraoceanic, seafloor spreading origin of the tectonic fabric of the Monviso ophiolite, prior to the onset of subduction zone tectonics in the Alpine Tethys. The Monviso ophiolite and the Baracun Shear Zone represent a peridotite-localized oceanic core complex, which survived both the subduction and continental collision tectonic stages of the Alpine orogeny. Intraoceanic detachment faults and oceanic core complexes may play a significant role in subduction initiation, and hence their recognition in orogenic belts is an important step in reconstructing the record of ocean basin collapse and closure. However, detailed, field-based structural, petrological and geochemical studies of the seafloor spreading and extensional tectonic history of these ophiolites have been scarce. This has been in part due to the strong overprint of the Alpine-stage subduction-collision related deformation-metamorphic events that obscures the previously developed rift-drift and seafloor spreading generated structures and mineral assemblages in these units. In this paper, we document through detailed geological mapping, systematic structural and stratigraphic observations, petrographic and geochemical analyses the internal structure, tectonic fabric and evolution history of the Monviso ophiolite, one of the best preserved ophiolites in the Western Alps. We show that this ophiolite is an on-land exposure of an oceanic core complex, which formed through simple-shear seafloor spreading kinematics during the opening of the Ligurian-Piedmont ocean basin within the Alpine Tethys. This inferred oceanic core complex origin of the Monviso ophiolite is significant in that: (1) it better explains the dismembered and highly attenuated crustal architecture of the ophiolites in the Western Alps; (2) it presents a first coherent documentation of the intraoceanic extensional tectonic history of the Jurassic oceanic lithosphere preserved in the Western Alps, demonstrating that it is possible "to see through" the subduction-collision induced metamorphic overprint in multiply deformed orogenic belts; and (3) it provides a regionally consistent tectonic framework for the rift-drift, seafloor spreading, and contractional episodes of the Ligurian-Piedmont ocean basin evolution. Our data and observations from the Monviso ophiolite complement the diverse datasets available from the modern oceanic core complexes, and provide further insights into the geometry, internal structure, and stratigraphy of supradetachment basin sequences, which are missing from in-situ oceanic core complexes. 2. Regional Geology of the Western Alps and its Tethyan connection The Western Alps (Fig. 1) developed due to the collision between Adria (upper plate) and Europe (lower plate) as the intervening oceanic lithosphere of the Jurassic Alpine Tethys Ocean was consumed (see e.g. Coward and Dietrich 1989; Laubscher 1991; Dilek, 2006). The collision zone (i.e. the axial section of the Alpine belt) involves an exhumed fossil subduction complex bounded by the Penninic front and the Insubric and Canavese lines (Fig. 1A). Tectonic units of this subduction complex are overthrust WNW onto the European foreland (Ricou and Siddans, 1986; Platt et al. 1989; Schmid and Kissling 2000; Butler et al. 2013). Different meta-ophiolite units (i.e. the Piedmont Zone; see e.g. Dal Piaz et al., 2003) are tectonically sandwiched between the European and Adriatic continental margin units (see Bigi et al., 1990), and display varying metamorphic facies conditions ranging from high-pressure (HP) to ultra high-pressure (UHP) (Frey et al., 1999 and reference therein). The Piedmont Zone is distinguished by eclogite-facies units (i.e. the Zermatt-Saas Zone auct.; Bearth, 1967) and blueschist-facies ones (i.e. the Combin Zone auct., Fig. 1A). The orogenic structural architecture of the Western Alps as seen in the field today (Fig. 1) was built up during three main phases of deformation-metamorphism events (Balestro et al., 2015): (1) Edipping subduction zone tectonics and eclogite-facies metamorphism in the Paleocene to middle Eocene, during which contractional deformation (D1) structures, mainly S1 foliation, were developed; (2) Continental collision tectonics in the late Eocene-early Oligocene that caused W-vergent folding and thrusting (D2). Blueschist-to greenschist-facies metamorphic re-equilibration took place during this event, producing S2 foliation; (3) Crustal exhumation (D3) and deep crust/mantle indentation in
Swiss Journal of Geosciences
In the Western Alps, different shear zones acting at different depths have been investigated for explaining multistage exhumation of (U)HP units, and several exhumation models have been proposed for explaining present-day stacking of different tectonometamorphic units. This study aims to reconstruct the tectonic evolution of the Susa Shear Zone (SSZ), a polyphasic first-order shear zone, outcropping in the Susa Valley. The SSZ consists of a thick mylonitic zone, along which units characterized by different Alpine metamorphic P–T peaks are coupled. In the study area, the footwall of the SSZ mostly consists of oceanic units (i.e., Internal Piedmont Zone), which record eclogitic conditions, whereas the hanging wall consists of oceanic units (i.e., External Piedmont Zone), which record blueschist-facies conditions. These tectonic units were deformed during subduction- and exhumation-related Alpine history, throughout four main regional deformation phases (from D1 to D4), and were couple...
Minerals
We present a detailed description of the tectono-stratigraphic architecture of the eclogite-facies Internal Piedmont Zone (IPZ) metaophiolite, exposed in the Lanzo Valleys (Western Alps), which represents the remnant of the Jurassic Alpine Tethys. Seafloor spreading and mantle exhumation processes related to the Alpine Tethys evolution strongly conditioned the intra-oceanic depositional setting, which resulted in an articulated physiography and a heterogeneous stratigraphic succession above the exhumed serpentinized mantle. “Complete” and “reduced” successions were recognized, reflecting deposition in morphological or structural lows and highs, respectively. The “complete” succession consists of quartzite, followed by marble and calcschist. The “reduced” succession differs for the unconformable contact of the calcschist directly above mantle rocks, lacking quartzite and gray marble. The serpentinite at the base of this succession is intruded by metagabbro and characterized at its to...
Lithos, 2021
A multidisciplinary approach to the study of collisional orogenic belts can improve our knowledge of their geodynamic evolution and may suggest new tectonic models, especially for (U)HP rocks inside the accretionary wedge. In the Western Alps, wherein nappes of different origin are stacked, having recorded different metamorphic peaks at different stages of the orogenic evolution. This study focuses on the External (EPZ) and Internal (IPZ) ophiolitic units of the Piedmont Zone (Susa Valley, Western Alps), which were deformed throughout four tectonometamorphic phases (D1 to D4), developing different foliations and cleavages (S1 to S4) at different metamorphic conditions. The IPZ and EPZ are separated by a shear zone (i.e. the Susa Shear Zone) during which a related mylonitic foliation (SM) developed. S1 developed at high pressure conditions (Epidote-eclogite vs. Lawsonite-blueschist facies conditions for IPZ and EPZ, respectively), as suggested by the composition of white mica (i.e. phengite), whereas S2 developed at low pressure conditions (Epidote-greenschist facies conditions in both IPZ and EPZ) and is defined by muscovite. White mica defining the SM mylonitic foliation (T1) is mostly defined by phengite, while the T2-related disjunctive cleavage is defined by fine-grained muscovite. The relative chronology inferred from meso-and micro-structural observations suggests that T1 was nearcoeval respect to the D2, while T2 developed during D4. A new set of radiometric ages of the main metamorphic foliations were obtained by in situ Ar/Ar dating on white mica. Different generations of white mica defining S1 and S2 foliations in both the IPZ and EPZ and SM in the SSZ, were dated and two main groups of ages were obtained. In both IPZ and EPZ, S1 foliation developed at ~46-41 Ma, while S2 foliation developed at ~40-36 Ma and was nearly coeval with the SM mylonitic foliation (~39-36 Ma). Comparison between structural, petrological and geochronological data allows to define time of coupling of the different units and consequently to infer new tectonic implications for the exhumation of meta-ophiolites of the Piedmont Zone within axial sector of the Western Alps.
Eclogitization of the Monviso ophiolite (W. Alps) and implications on subduction dynamics
Journal of Metamorphic Geology, 2012
To constrain deep (40-100 km) subduction dynamics, extensive P-T data are provided on the eclogitic Monviso ophiolite derived from the subducted Liguro-Piemontese oceanic lithosphere (which was exhumed, together with associated continental units, before the Alpine collision). The Monviso ophiolite has so far been interpreted either as a fossilized subduction channel, with tectonic blocks detached from the slab at different depths and gathered in a weak serpentinized matrix, or as a more or less continuous portion of oceanic lithosphere. To evaluate potential heterogeneities within and between the various subunits, extensive sampling was undertaken on metasedimentary rocks and Fe-Ti metagabbros. The results indicate that the Monviso ophiolite comprises two main coherent tectonic subunits (the Monviso and Lago Superiore Units) detached during subduction at different depths and later juxtaposed at epidote-blueschist facies during exhumation along the subduction interface. Raman spectroscopy of carbonaceous material suggests (i) a difference in peak temperature of 50°C between these two subunits and (ii) a good temperature homogeneity within each subunit. Pseudosections and average P-T estimates using THERMOCALC THERMOCALC in the Lago Superiore Unit suggest for the first time homogeneous HP to UHP conditions (550°C, 26-27 kbar). Parageneses, peak conditions and tectonic setting are very similar to those of the Zermatt-Saas ophiolite, 200 km northwards, thus suggesting a common detachment mechanism for the whole Western Alpine belt.
Tectonostratigraphy of the northern Monviso Meta-ophiolite Complex (Western Alps)
Italian Journal of Geosciences, 2014
The Monviso Meta-Ophiolite Complex is a remnant of the Piedmont-Ligurian oceanic lithosphere stacked in the Western Alps, and consisting of dismembered HP meta-ophiolite sequences. In this work, focused on the northern sector of the Complex, we differentiate six tectonic units which structural, petrographic and stratigraphic characteristics are described in detail and discussed in the light of a comparison with the overall geology of the Monviso Meta-Ophiolite Complex. The structural evolution has been referred to i) an early syn-eclogitic deformation phase (D 1 ), ii) a main deformation phase (D 2 ) occurred in the blueschist-to greenschist-facies transition and characterized by the development of a regional foliation (S 2 ) that is parallel to the tectonic contacts and to the axial plane of map-scale W-verging folds, and iii) a late-metamorphic deformation phase (D 3 ) characterized by westward extensional tectonic. The northern Monviso Meta-Ophiolite Complex is characterized by a poor preservation of HP paragenesis and a widespread overprint of the blueschist-to greenschist-facies metamorphism, but the occurrence of garnet-, omphacite-, talc-and lawsoniteassemblage in a Fe-Ti metagabbro indicates P-T eclogitic conditions (2.5-2.7 GPa for 550-570 °C) very similar to those calculated in the southern sector of the Complex. The stratigraphic characteristics of the meta-ophiolite sequences point out that, differently from the southern sector of the Complex where basalt-poor and basalt-rich oceanic units have been distinguished, in the northern Monviso Meta-Ophiolite Complex the different types of metasediments may be the key to restore the oceanic tectonostratigraphy, marked by gabbro and mantle peridotite exposition on a puzzle-like ocean floor where basalt effusion and different sedimentation processes took place.
Journal of Maps
In the Valmala sector of the southern Dora Maira Massif (Western Alps), two different eclogiteand blueschist-facies units (i.e. the Rocca Solei and Dronero units, respectively), are separated by a shear zone (i.e. the Valmala Tectonic Unit), which peculiarly consists of mixed slices of ophiolitic and continental rocks. A detailed geological map at 1:10,000 scale allowed to point out that the tectonic slices within the Valmala Tectonic Unit consist of 'native' rock slices wrenched from the overlying Dronero Unit, and 'exotic' rocks likely sourced from other units of the Dora Maira and from a continental margin and an oceanic basin. On the contrary, rock slices sourced from the underlying Rocca Solei Unit are lacking. The overall tectonic stack results after an early subduction-related deformation phase (i.e. the D1), and the pervasive overprinting of two subsequent exhumation-related deformation phases (i.e. the D2 and D3). The Valmala Tectonic Unit is inferred to have played a role in decoupling the southern Dora Maira Massif during subduction, and/or in driving exhumation of the ultra-high pressure rocks occurring in the adjoining Brossasco-Isasca Unit.
Journal of Structural Geology, 2011
Prograde lawsonite during the flow of continental crust in the Alpine subduction: Strain vs. metamorphism partitioning, a field-analysis approach to infer tectonometamorphic evolutions (Sesia-Lanzo Zone, Western Italian Alps) a b s t r a c t Detailed mapping of superposed fabrics and their mineral support allows for reconstruction of the tectonometamorphic evolution of the Ivozio Complex, within the inner portion of the Sesia-Lanzo Zone (Western Italian Alps). The resulting evolution is characterized by a multi-stage structural and metamorphic reequilibration during Alpine subduction, starting from the pre-Alpine igneous association (Amp 0 þ Cpx 0 ). The prograde associations begin with S 1a marked by Amp I þ Zo I which pre-date the growth of Grt I (S 1b ); successive increase in pressure stabilizes a second generation of Amp þ Grt (S 1c Amp II þ ZoI þ Grt II ). The growth of prograde lawsonite and omphacite occur during S 1d (Omp I þ Lws þ Grt II þ Amp II ) within lawsonite-bearing eclogites, while S 1e is associated with the break-down of lawsonite, producing the association Omp I þ Ky þ Zo II þ Grt II þ Amp II (lws-bearing eclogites); S 1d-e stages are associated with Amp II þ ZoI þ Grt II þ Omp I in eclogites. The second generation of penetrative foliation (S 2 ), describing the retrograde evolution, is divided into S 2a (Amp II þ GrtII þ Pg þ Zo II ) and S 2b (Chl þ AmpIII þ Pg þ Ab). The comparison between the reconstructed evolution of the Ivozio Complex and PeT paths inferred in the Southern Sesia-Lanzo Zone suggests a non-uniqueness of the Sesia-Lanzo Zone continental crust, during the Alpine subduction.