Giovanni Capponi

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.

Structural history and tectonic evolution of the boundary between the Wilson and Bowers terranes, Lanterman Range, northern Victoria Land, Antarctica

Abstract The boundary between the Wilson and Bowers terranes is one of the major tectonic discontinuities of northern Victoria Land (Antarctica) and is referred to as the Lanterman Fault Zone by most authors. Fieldwork carried out in this area indicates that the structural evolution of this boundary is polyphase and four main tectonic phases can be recognized: (1) west over east thrusting of Ross age (i.e. around 500 Ma), coeval with amphibolite and amphibolite–greenschist facies transition metamorphism; (2) sinistral strike-slip shearing, coeval with greenschist facies metamorphism; this event may belong to the Ross Orogeny or may represent evidence of the Borchgrevink Orogeny (i.e. around 360 Ma); (3) large-wavelength folding of late-Ross or Borchgrevink age; (4) Cenozoic brittle tectonics, expressed by small to km-scale structures, with dextral strike-slip displacement. Evidence for this structural evolution is not confined to a single tectonic lineament, but occurs throughout the area close to the boundary between the Wilson and Bowers terranes. It follows that the docking of the Wilson and Bowers terranes was complex and polyphase and it is not possible to define a single tectonic setting for the Lanterman Fault Zone, since it was the site of contrasting tectonic regimes at different times and at different structural levels.

Dynamics and seismotectonics of the West-Alpine arc

Abstract In this paper we present a comparative review of structural, seismic and focal mechanism data from the West-Alpine arc. In the Western Alps, seismic activity is concentrated along an external belt, corresponding to the Penninic front, and an internal belt, corresponding to the Austro-Alpine front and its southern extension. These seismic belts are connected in the NE by a seismic lineament, corresponding to the Simplon and Centovalli Line and in the south by the E-W-trending seismic Stura “couloir”, located between the Argentera and Dora Maira massifs. The SimplondashCentovalli-Tonale system and the Stura “couloir” are dextral and sinistral strike-slip systems respectively; this implies a westward translation of the West-Alpine arc and the Po Plain. Based on these observations, a seismotectonic model is proposed in which the frontal Penninic thrust and the basal surface of the accretionary wedge corresponding to the Penninic sole thrust are reactivated. Activity along the frontal thrust increases the arcuate shape of the Western Alps and disengages them from the Central and the Ligurian Alps along the tear faults of the SimplondashCentovalli-Tonale system and the Stura “couloir” respectively. In the wider framework of northern Italy, the sector of the eastern Alps north of the Gailtal Line, is moving in an orogendashparallel direction towards the east. Areas with lateral escape in opposite directions, towards the west for the Po Plain and towards the east for the Eastern Alps, north of the Gailtal Line, are separated by the South Alpine Atesine indenter. A model postulating a double lateral escape and a central indenter matches most of the features of the models of “poinconnement”. The seismic features of northern Italy agree with the seismicity of the stable sector of northern Europe up to the North Sea. This area is fragmented into blocks bounded by seismic bands, some of which probably reactivate pre-existing structures. The overall stress field of Cratonic Europe is dominated by ndashNW-directed trajectories of maximum horizontal compression. The seismicity of the European Alpine foreland is interpreted as the response of a stable block to the same stress field of northern Italy, which is induced by the ondashgoing NW-SE Africa-Europe convergence. We suggest that the westward translation of the West-Alpine arc continues at present and is the driving mechanism of the observed seismicity.

Quartz fabric and strain partitioning in sheath folds: an example from the Voltri Group (Western Alps, Italy)

Abstract A mesoscopic sheath fold, defined by an almost pure quartzitic level in quartz-mica-schists of the Voltri Group (southeastern border of the Western Alps), was sampled and studied in detail. Microstructural analysis was carried out on thin sections from different structural domains in the quartzitic layer, with the emphasis to analyse quartz-shape fabrics and the crystallographic preferred orientation of the c -axes of this mineral. Quartz was dynamically recrystallized but its degree of preferred orientation is not as high as it would be expected in a high shear-strain regime required for the development of sheath folds. This may suggest strain partitioning between the quartzitic level and the micaceous matrix. Crystallographic preferred orientation of c -axes and grain-shape fabric asymmetry indicate the same sense of shear on both fold limbs in the XZ -plane; this is in agreement with a model of sheath-fold development by the passive rotation of material lines. Clustering of the c -axis poles shows a configuration indicative of a different strain state in different domains of the sheath fold and suggests an evolution of the fold characterized by combined simple shear and coaxial progressive deformation.

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.

Late structural evolution in an accretionary wedge: insights from the Voltri Massif (Ligurian Alps, Italy)

The Voltri Massif underwent a polyphasic tectono-metamorphic evolution that records both the Alpine and part of the Apennine deformation events. So this is a key-area to investigate the relationships between Alpine and Apennine deformation events. This paper focus on the upper crustal deformations (UCD) that characterize the last stages of the tectonics of the Voltri Massif. In the Voltri Massif UCD are characterized by the superpositions of ductile, brittle-ductile and brittle structures that can be attributed to three main tectonic events (from D3 to D5). The oldest UCD event (D3) developed folds and reverse shear zones under ductile to brittle-ductile conditions. Main compressive NW-SE oriented regime characterized D3 event. Brittle-ductile to brittle reverse shear zones and important strike-slip/transpressive systems overprinted D3 structures. This D4 event was significant at the regional scale and occurred under main transpressive, NE-SW oriented, regime. The latest normal and transtensional brittle structures, that formed during UCD D5 event, locally reactivated the older structures.

The suture between the Alps and Apennines in the Ligurian sector based on geological and geomagnetic data

Abstract A ground magnetic survey was carried out over a wide area of northwestern Italy, including western and central Liguria, the southeastern Piedmont and a part of the Ligurian Sea. Several cross-sections, cutting the main structural elements (the Ligurian Sea, the Ligurian Alps, the Monferrato Hills) have been performed and magnetic models have been developed. The interpretation of the most characteristic cross-section (A1-NNW), is presented in this paper. A model involving the presence of a major suture between the Ligurian Alps and the Monferrato Hills fits the positive magnetic anomaly running from the surroundings of Turin towards Asti. This model implies that slabs of upper mantle or lower continental crust were trapped in the suture, as in the internal zone of the Western Alps, where the Ivrea body comes in contact at depth with the Pennidic nappes. To the south, this suture line does not appear throughout the Ligurian Sea, probably owing to the recent crustal thinning, which is revealed by both the magnetic and the gravimetric features.

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.