Laura Federico

Ophiolite mélange zone records exhumation in a fossil subduction channel

Recent models propose that the exhumation of high-pressure rocks occurs by means of return flow inside a low-viscosity channel of serpentinite situated between the plates. To test this hypothesis, we investigated a serpentinite melange in the Western Alps, which contains exotic mafic and metasedimentary tectonic blocks, recording heterogeneous metamorphic evolutions and variable high-pressure ages. The peak metamorphic conditions range from eclogite- to garnet-blueschist-facies. The structural evidence and the pressure-temperature paths of the different blocks suggest coupling between blocks and matrix, at least in the blue-schist facies. 39 Ar- 40 Ar dating indicates eclogite-facies peak at ca. 43 Ma and blueschist-facies peak at ca. 43 and 40 Ma in different blocks, respectively. These data point to diachronous metamorphic paths resulting from independent tectonic evolutions of the different slices. We therefore propose that this melange formed during exhumation of subducted rocks equilibrated at different depths inside a subduction channel. This mechanism can be extended to other serpentinite melanges in the Alps and other orogens (e.g., the Cyclades, the Coast Ranges of California) for which a growing heterogeneity in the timing of metamorphic equilibration and of pressure-temperature paths can be expected with further investigations.

Different PT paths recorded in a tectonic mélange (Voltri Massif, NW Italy): implications for the exhumation of HP rocks

The Cascine Parasi Mélange (CPM) of the high-pressure, meta-ophiolitic Voltri Massif (Ligurian Western Alps), consists of a foliated chlorite-actinolite greenschist matrix enclosing lenses of metabasites and metasediments. The surrounding units consist of serpentinites not enclosing these metamorphic rocks. The matrix records three sets of folds: (i) Dm1/Dm2 (blueschist to greenschist-facies conditions), which can be correlated to folds in the metasedimentary blocks; (ii) Dm3, which are the most obvious in the field and which partially re-orient the previous structures. The metabasite lenses preserve internal High-Pressure (HP) schistosities unrelated to the matrix foliation. The lenses equilibrated at different peak metamorphic conditions (ranging from eclogite- to blueschist-facies) and some recorded the prograde transition from lawsonite-bearing assemblages to garnet blueschists. Individual lenses display different segments of typical subduction PT paths which apparently converge in the blueschist facies. A late stage greenschist-facies re-equilibration is particularly widespread at the rims of the HP lenses. These structural and metamorphic features suggest that the mélange was active during early phases of the structural evolution of the area, at least through the exhumation and emplacement of the HP blocks into shallower crustal levels at conditions transitional from blueschist- to greenschist-facies; the older history is only preserved inside the blocks.

Structure of the Millen Schist Belt (Antarctica): Clues for the tectonics of northern Victoria Land along the paleo‐Pacific margin of Gondwana

Northern Victoria Land (Antarctica) belonged to the active proto-Pacific margin of Gondwana, which was the site of convergence during the Paleozoic. This study provides new insights into the structural architecture of northern Victoria Land, focusing on the boundary area between the Bowers and Robertson Bay terranes, i.e., in the Millen Schist Belt. It is a high-strain equivalent of the adjoining terranes, presently delimited by the Leap Year and the Handler faults. Our study reveals that these two faults overprint a preexisting transitional deformational boundary and are associated with a significant syntectonic circulation of fluids and mineralization. The Millen Schist Belt consists of two lithotectonic packages, juxtaposed along the Crosscut-Aorangi duplex thrust system, related to late Ross deformation. As there is increasing evidence of a post-Ross contractional event in northern Victoria Land, we suggest that the structural architecture of the Bowers-Robertson Bay terrane boundary results from a long-lasting SW-NE contractional regime, during the Ross-Delamerian Orogeny and still active afterward. This points to an extension of the Australian Lachlan Orogeny in Antarctica. The similarity of the structural architecture, the gold mineralization, the rock type, and the age supports the correlation of the Bowers and the Robertson Bay terranes with the Stawell Zone of the Lachlan Fold Belt. In our new tectonic scenario the Lanterman Fault (northern Victoria Land) plays the same role as the Moyston Fault (southeastern Australia), and the Leap Year and Handler faults correlate with the “intra-zone faults” of the Stawell Zone (e.g., the Ararat-Stawell Fault Zone).

Exhumation in a fossil subduction channel: an example from the Ligurian Alps

Recent models propose that the exhumation of high-pressure rocks occurs by means of a return flow inside a subduction channel of low viscosity serpentinite between the confronting plates. Model predictions include different pressure-temperature paths and ages in the exhumed subduction complexes. We studied a serpentinitic melange in the Ligurian Alps to test this hypothesis. It contains exotic tectonic blocks with respect to the surrounding metaophiolites, which equilibrated at different peak metamorphic conditions and record different segments of a typical subduction P-T path. Both structural evidence and P-T paths of the different blocks suggest coupling between blocks and matrix in the blueschist facies. We obtained 39 Ar/ 40 Ar ages for the eclogite facies peak (43.2±0.5 Ma) and for the blueschist facies peak (39.95±0.37 Ma and at 43.4±0.5 Ma) in different blocks; these data point to diachronous metamorphic trajectories and independent tectonic evolution of the different slices inside the channel. Inasmuch as the geochronological, petrographic and structural data fit the predictions of numerical models in terms of different pressure-temperature paths and variable metamorphic ages, we suggest that the studied melange has been originated in the subduction channel. This mechanism can be extended to other serpentinitic melanges in the Alps and other orogens, for which we expect further investigations will show a growing heterogeneity in the timing of metamorphic equilibration and of P-T paths.

Fluid-controlled deformation in blueschist-facies conditions: plastic vs brittle behaviour in a brecciated mylonite (Voltri Massif, Western Alps, Italy)C. MALATESTA AND OTHERSDeformation and fluid flow in subduction zones

A blueschist-facies mylonite crops out between two high-pressure tectono-metamorphic oceanic units of the Ligurian Western Alps (NW Italy). This mylonitic metabasite is made up of alternating layers with different grain size and proportions of blueschist-facies minerals. The mylonitic foliation formed at metamorphic conditions of T = 220–310 °C and P = 6.5–10 kbar. The mylonite shows various superposed structures: (i) intrafoliar and similar folds; (ii) chocolatetablet foliation boudinage; (iii) veins; (iv) breccia. The occurrence of comparable mineral assemblages along the foliation, in boudin necks, in veins and in breccia cement suggests that the transition from ductile deformation (folds) to brittle deformation (veining and breccia), passing through a brittle–ductile regime (foliation boudinage), occurred gradually, without a substantial change in mineral assemblage and therefore in the overall P–T metamorphic conditions (blueschist-facies). A strong fluid–rock interaction was associated with all the deformative events affecting the rock: the mylonite shows an enrichment in incompatible elements (i.e. As and Sb), suggesting an input of fluids, released by adjacent high-pressure metasedimentary rocks, during ductile deformation. The following fracturing was probably enhanced by brittle instabilities arising from strain and pore-fluid pressure partitioning between adjacent domains, without further external fluid input. Fluids were therefore fixed inside the rock during mylonitization and later released into a dense fracture mesh that allowed them to migrate through the mylonitic horizon close to the plate interface. We finally propose that the fracture mesh might represent the field evidence of past episodic tremors or ‘slow earthquakes’ triggered by high pore-fluid pressure.

Geology of the Pontinvrea area (Ligurian Alps, Italy): structural setting of the contact between Montenotte and Voltri units

This geological map at the 1:10,000 scale shows the structural setting of two poly-deformed metaophiolite units, with different metamorphic peak conditions, i.e. the blueschist facies Montenotte Unit and the eclogite facies Voltri Unit, in a selected area of 8.2 km2 within the Ligurian Alps (northern Italy). This study focuses on the tectonic contact between the two tectono-metamorphic units and on their relationships with the Oligocene sediments of the Tertiary Piedmont Basin. The map is a composite report of our field and laboratory study of structures and metamorphism, that explains our interpretation of the tectonic history of the study area. It shows that the two units were coupled during their exhumation path, along a blueschist facies mylonitic contact. This contact has been later involved in thrust faults that caused the superposition of the metamorphic basement on top of the Oligocene sediments.

Geology of the Eastern Ligurian Alps: a review of the tectonic units

The Alpine and Apennine belts come into contact in Central Ligu ria, in an extremely complex structural arrangement; this junction is usually referred to as the Ligurian Knot. The geological mapping for the 213-230 Genova and 212 Spigno Monferrato 1:50,000 quadrangles necessitated the updating of the stratigraphic, structural and metamorphic descriptions of this area, and a redefinition of some of the units and geological ensembles already established in Central Liguria, i.e. the Voltri Group, the Sestri-Voltaggio Zone and the Val Polcevera Unit. We propose a new definition of tectonic and tectono-metamorphic units, grouped in three main associations, as follows: a) units derived from oceanic crust and mantle; b) units derived from a continental margin; c) Flysch units derived from the sedimentary cover of an oceanic basement. In group a), we propose to keep the already established Palmaro-Caffarella, Cravasco-Voltaggio, Varazze and Figogna units and to establish a new Voltri tectonometamorphic Unit, including all the rocks showing a metamorphic climax in eclogite facies conditions, with a variably developed greenschist facies overprint. In group b) we propose to keep the already established Gazzo-Isoverde Unit and to define a new Angassino- Terma tectono-metamorphic Unit, encompassing limited outcrops of quartzite and dolomitic limestone associated with the Voltri rocks. In group c) we propose to replace the former Val Polcevera Unit with new Ronco, Montanesi and Mignanego tectonic units. We propose to abandon the term Voltri Group, for the term Voltri Massif that can be used to define a set of units, including the Voltri, Varazze, Palmaro-Caffarella and Angassino-Terma units, irrespective of their different lithology and/or paleogeographic derivation and/or metamorphism. For the Sestri-Voltaggio Zone, we propose to use this term only for the Cravasco-Voltaggio and Gazzo-Isoverde units (and not the Figogna Unit), to emphasize their common tectonometamorphic evolution.

Stratigraphic vs structural contacts in a late orogenic basin: the case of the Tertiary Piedmont Basin in the Sassello area (Ligurian Alps, Italy)

This geological map (1:10.000 scale) of the ‘Sassello Basin’ remnant covers an area of about 33.4 km2 of Liguria (NW Italy); it highlights the occurrence of two main types of contacts between the sediments of the Tertiary Piedmont Basin and the metamorphic substratum (Voltri Unit): (i) stratigraphic and (ii) structural (thrust or steeply dipping faults). (i) Stratigraphic contacts are represented by the main transgressive surface and the nonconformity between the metamorphic rocks of the substratum and the subaerial deposits. They are locally folded and occur along the steeply dipping short limbs of asymmetric folds related to the late-alpine/apennine tectonics. (ii) Structural contacts are related to the late-alpine/apennine tectonics (thrust faults) or (mostly) to Plio-Quaternary extensional/transtensional faulting.