NW Italian Alps Research Articles

Dating deformation in the Gran Paradiso Massif (NW Italian Alps): Implications for the exhumation of high-pressure rocks in a collisional belt

The Gran Paradiso massif, situated in the internal part of the Western Italian Alps, records a complex tectono-metamorphic history involving high-pressure metamorphism and subsequent exhumation during retrograde metamorphism. The exact timing of deformation and, consequently, the geodynamic evolution of this part of the Western Alps is still debated and is addressed here by the application of Rb/Sr geochronology, 40Ar/39Ar step heating and 40Ar/39Ar total fusion dating techniques. Geochronological results are presented from shear zone samples in the core of the Gran Paradiso massif (Piantonetto Valley), and in the area closer to the contact with the overlying Piedmont ophiolitic domain (south and southwest of Pont Valsavarenche). The shear zones operated during crustal thinning and exhumation of the Gran Paradiso massif. 40Ar/39Ar step heating results from shear zones in the Piantonetto Valley show acceptable plateau ages that are interpreted to represent two events of mica growth. Similar ages, and an additional younger age cluster, are recognised in the 40Ar/39Ar total fusion analyses, indicating that specific cleavage domains operated at 39.2±0.2, 36.5±0.6 and 33.3±0.4Ma. P–T pseudosections show a progressive decrease in metamorphic conditions during deformation, suggesting that the age of incipient exhumation and the related deformation in the Piantonetto Valley is equal to or older than 39.2±0.2Ma. In the Pont area, the last increments of deformation in a top-to-W shear zone postdate 36.6±0.6Ma (Rb/Sr mineral data), whereas the present-day top-to-W contact of the Gran Paradiso massif with the overlying Piedmont domain is dated at 41.2±1.1Ma (Rb/Sr multi-mineral isochron age). We propose a model that considers exhumation of the Gran Paradiso nappe at 41–34Ma. During this period, the nappe was coupled with the Zermatt-Saas zone, forming an extruding wedge. The kinematics associated with this wedge involved top-to-W shearing within the Gran Paradiso nappe (e.g. Pont area shear zones) and top-to-E shearing at the top of the extruding wedge (e.g. Orco shear zone). Subsequent deformation (after ~34Ma) was characterised by coaxial strain involving orogenic-scale backfolding and backthrusting.

Polyphase seismic faulting in the Ivrea zone (Italian Alps) revealed by 40Ar/39Ar dating of pseudotachylytes

Terra Nova, 23, 162–170, 2011AbstractMost fault‐related pseudotachylytes in the Val Sesia area, Ivrea zone, NW Italian Alps, occur in two areas: near the Canavese Line in the Kinzigite Formation paragneisses and in an elongate belt in the Mafic Complex gabbros away from the Canavese Line. 40Ar/39Ar dating indicates that pseudotachylytes from the Kinzigite Formation were formed in the late Eocene, likely during the early stages of the collision between the NW corner of the Adriatic indenter and the European margin. Pseudotachylytes from the Mafic Complex were formed under a transpressive strain regime, as suggested by kinematic indicators, and yield Early Cretaceous (Albian) 40Ar/39Ar ages. Their formation is possibly related to shortening during the inception of the tectonic inversion of the Adriatic passive margin.

Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps)

The Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones 1) to steep inclination (shear zones 2). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern.

Red fox (Vulpes vulpes) cannibalistic behaviour and the prevalence of Trichinella britovi in NW Italian Alps

Food habits of the red fox (Vulpes vulpes) were studied in the Aosta Valley region (NW Italian Alps) and were related to the prevalence of Trichinella infection in the red fox itself and in two Mustelid species (the stone marten (Martes foina) and the badger (Meles meles)). The search of Trichinella by the automatic digestion of muscles samples led us to determine a prevalence of 3.5+/-1.2% in red foxes, 7.9+/-4.3% in stone martens and 1.9+/-1.8% in badgers, with no significant differences among the species. All larvae were identified as Trichinella britovi. The fox diet was assessed through the analysis of both faeces (n=180) and the stomach contents of road-killed animals (n=109). Our results confirmed the opportunistic feeding behaviour of the red fox, which is able to use various trophic resources, both of animal and vegetal origin: e.g. wild and cultivated fruits (F%=47.1; V%=67.3), rodents (F%=22.8; V%=64.8) and carrion (F%=15.6; V%=78.6) formed the bulk of the fox's diet. The frequency of occurrence of potential events of cannibalism was 1.0%, even if the complete absence of undigested remains, other than hairs, suggested the possibility of confusing cannibalism with coat-cleaning. We suggest that intra-specific necrophagy could not represent the unique way of transmission of the nematode in natural conditions.

Brittle post-metamorphic tectonics in the Gran Paradiso Massif (north-western Italian Alps)

New geological mapping (1:10.000 scale) and structural analysis in the Orco valley (NW Italian Alps), give insights on the post-metamorphic brittle tectonic evolution of the Gran Paradiso Unit and adjoining Piemonte Ophiolitic Unit and Sesia-Lanzo Unit. According to existing geochronological data, the metamorphic evolution of the studied domain ended in Early Oligocene and was followed by a progressive exhumation during Neogene. This work describes the brittle fault network developed during the post-metamorphic tectonic stages coeval to exhumation. The geometry and kinematics of this fault network allow to interpret the study area as a part of a larger lowstrain, fault-bounded block, whose main bordering discontinuities are located outside the mapped region. The post-metamorphic kinematic evolution was characterised by: (a) ESE-WNW extension, attested by NNE-SSW normal faults and extensional joints; (b) dextral shearing along a major E-W shear zone (Orco shear zone). The individual faults of the studied area are considered as P- or R-shears of the Orco shear zone, which could be in turn a part of a larger NE-SW regional shear zone bounded to the N by the Chamonix-Rhône- Simplon Line.

Syn-convergent exhumation implied by progressive deformation and metamorphism in the Valle dell’Orco transect, NW Italian Alps

Structural, metamorphic and geochronological data from the Valle del'Orco are best explained by an orogenic wedge model in which crustal slices from continent and ocean acted as passive blocks at different times in the Alpine orogeny. Deformation generally migrated westwards in this section, and metamorphic recrystallization at any given point was largely controlled by the pervasiveness of strain. The data suggest a dynamic crustal stacking system lasting from 70 to 35 Ma, rather than continental collision following long-lived B-type subduction, and offer a relevant and simple model for eclogite exhumation that requires exhumation of ductile shear-bounded slices within the orogenic wedge. The easternmost, internal unit of the Sesia-Lanzo Zone, the Eclogitic Micaschist Complex, was subducted and pervasively recrystallized under eclogite facies conditions during the early stages of convergence in the late Cretaceous. During the development of the major preserved structures, the eastern Eclogitic Micaschist Complex was undergoing general non-coaxial shear with an overall top-down to the east vorticity, suggesting a potential mechanism of exhuming an eclogitic 'pip'. Slices of pre-Alpine basement, the Vasario-Sparone Subunit, were incorporated into the lower (western) parts of this unit during the latest part of this phase, but do not show penetrative effects of the early Alpine deformation. They were passively folded during the early decompressional part of the P—T history, prior to the unit being juxtaposed against the underlying Gneiss Minuti Complex under blueschist-greenschist facies conditions. The Gneiss Minuti Complex itself was then recrystallized at high strain in the greenschist-facies in the latest Eocene. Fabrics developed at this stage indicate top-down-to-east extension which may have accommodated the exhumation of eclogitized Piemont oceanic material and the European continental material of the Gran Paradiso massif. Evidence of northwest-directed thrusting within the Gran Paradiso unit at the same time suggests again that the crustal block was transported on a deeper reverse-sense shear zone that presumably links to the more external thrusts now exposed to the west. High-pressure mineral lineations within the Piemont zone cannot be unequivocally assigned to either the burial or exhumation paths, but are consistent with the generally east-west transport directions observed in the structurally higher units.

Home range and habitat selection by the Nutcracker Nucifraga caryocatactes during autumn in the Alps

In the Mont Avic Natural Park (NW Italian Alps), the Nutcracker Nucifraga caryocatactes occurred in coniferous forests where Arolla Pines Pinus cembra, its preferred habitat, were absent. A radiotelemetric study carried out during autumn 1992 and 1993 showed that, from September, Nutcrackers concentrated in the few Hazel Corylus avellana hedges of the rural zones of the park to collect nuts, which were eaten or stored. Individual home ranges overlapped to a high degree until October, when birds started to abandon the Hazel areas. From October onward, individual home ranges significantly expanded and their degree of overlap diminished compared to September. The home range analysis and the observations in the field suggested that the population of Nutcrackers in the park during autumn was made up of at least three different categories of birds, which displayed different ranging behaviour patterns. A few untagged birds probably stored nuts in their breeding territories, whereas all the radio‐tagged birds (n = 15) gave no indication of territorial use, since from October they either stored nuts in the coniferous woods near the Hazel hedges or abandoned the park to fly to other Hazel areas. Hence, during autumn the ranging behaviour of the Nutcrackers seems to be largely independent of their reproductive territorial system. Habitat selection analysis and transect counts revealed that birds positively selected Hazels, especially in the coniferous woods around Hazel hedges, and avoided the other coniferous forests. Nevertheless, from October some individuals reversed their habitat choices and positively selected coniferous forests (especially Mountain Pine Pinus uncinata woods) while avoiding Hazel.

Can crustal extension be distinguished from thrusting in the internal parts of mountain belts? A case history of the Entrelor shear zone, Western Alps

Criteria for the distinguishing between structures that accommodate crustal-scale extension or crustal-scale shortening may be applied to late orogenic shear zones developed in the internal parts of mountain belts. This is done for a large shear zone that has been mapped in the NW Italian Alps, around the internal basement massif of Gran Paradiso. The structure involved is termed the Entrelor shear zone. It carries greenschist facies European continental rocks (the Briançonnais—Grand St. Bernard unit) onto an old subduction complex of oceanic (Piemonte) and continental (Gran Paradiso) material that preserve eclogitic assemblages. Evidence for the map-scale continuity of the Entrelor shear zone is presented together with kinematic data to show how folds, stretching lineations and shear criteria relate to a regional episode of ESE-directed shearing. However, folds initiated systematically oblique to the shear direction, on a NE-SW axis. With continued shearing, the fold population remains skewed E-W of the population of stretching lineations. The shear zone may be traced for 70 km around the dome of the Gran Paradiso massif and displays a minimum displacement of 20 km. This Entrelor structure shows variable offset of metamorphic grade. However, for much of its length, greenschist facies rocks are carried in its hanging-wall onto eclogites in the footwall. Tracing these units regionally shows that this apparent extensional offset of metamorphic facies is the result of restacking of an originally ‘inverted’ metamorphic sequence (i.e. HP Piemonte on LP Briançonnais). This contractional behavior is consistent with the profile of the shear zone relative to the modern surface. The Entrelor shear climbs up from generally buried in the west to generally eroded in the east. The conclusion that the Entrelor shear zone is probably a thrust rather than an extensional structure relies on linking outcrop data into a semi-regional context. False interpretations may result from examining only small parts of the structure. However, although these map and structural criteria are consistent with contractional kinematics associated with crustal shortening, confirmation is required through linked petrological and isotopic studies. The Entrelor shear zone is thus a good illustration of the need for caution in inferring the contractional or extensional nature of structures developed late in the history of the internal parts of mountain belts.

Isotope systematics in vein gold from Brusson, Val d'Ayas (NW Italy), 1. Pb/Pb evidence for a Piemonte metaophiolite Au source

The Monte Rosa Gold District (NW Italian Alps) comprises hundreds of late Alpine, epigenetic, gold-bearing quartz veins. We report a lead isotope study on an important one of these veins, Fenilia (Brusson, Val d'Ayas), which reveals a complex isotope geochemistry of petrographically cogenetic vein minerals. Petrographic observations allow to distinguish a quartz-dominated main-stage and a quartz-poor final-stage mineral assemblage. Most of the free gold crystallized in the beginning of the final stage, whereas sulphides were deposited during its waning. Three distinct lead isotope fields result for the paragenetic sequence of vein minerals. The variable lead isotope signature (15.517 < 207Pb/ 204Pb < 15.623) trapped in final-stage free gold, and minor quartz can be interpreted as a variable mixture between a primitive MORB-type and a radiogenic Pb component. It differs distinctly from the radiogenic lead signature of the main-stage mineral assemblage ( 207Pb/ 204Pb > 15.706), as well as from the uniform radiogenic lead (15.649 < 207Pb/ 204Pb < 15.691) characteristic of the waning final stage. We favour a model where the deeply circulating final-stage fluid, the ultimate source of which remains obscure, acquired lead and thus possibly its gold in Piemonte metaophiolitic rocks beneath the Monte Rosa nappe. During ascent, the primitive gold-bearing fluid was variably contaminated by an evolved crustal lead (15.706 < 207Pb/ 204Pb < 15.743) compatible with derivation from Monte Rosa nappe rocks. During deposition of final-stage minerals the Pb isotope composition of the fluid changed to a constant signature becoming again dominated by a radiogenic lead source. Thus within an assemblage defined as cogenetic on petrographic grounds the Pb isotopic signature is not necessarily uniform and source modelling based on lead isotopes of associated minerals may be inappropriate. It remains open at present, whether the inferred Piemonte metaophiolite gold source is confined to the Fenilia vein in Val d'Ayas only, or whether it might be more widespread in the Monte Rosa Gold District.

Bulk composition and mineral parageneses of sapphirine‐bearing rocks along a gabbro‐lherzolite contact at Finero, Ivrea Zone, N Italy

Sapphirine occurs in a 3‐5 m wide zone between amphibole‐lherzolite and garnetiferous metagabbro at Finero in the Ivrea Zone, NW Italian Alps. Layers consisting of plag + hb + sa + cpx + opx + sp + gt are interbanded with spinel pyroxenites, which may contain sapphirine replacing spinel. All minerals are very magnesian, with XMg between 0.78 and 0.92. Bulk rock analyses suggest that precursors to the sapphirine‐bearing rocks were igneous cumulates of plagioclase + olivine + hornblende + spinel. Up to 16wt% CaO does not inhibit sapphirine formation and it is the unusually Mg‐rich nature of the host rocks which allows sapphirine development. The early igneous assemblage was replaced by one of cpx + sa + hb +± plag at a pressure of 9 ± 1 kbar and temperatures of 900 ± 50°C. Subsequent rapid uplift caused the instability of gt, gt + hb, hb and sa + cpx to form opx + plag ± sp ± sa symplectites.