Maria Iole Spalla

Tectonic significance of Alpine eclogites

Abstract A review of P-T peaks and paths of eo- and meso-Alpine eclogite fades rocks occurring along the axial part of the Alpine chain shows that rocks re-equilibrated under high- and low- T (group-B and -C eclogites), are, respectively, hosted within a lower and an upper tectonic level of the Penninic nappe system. If P-T estimates for eclogites are considered peak conditions the two crustal portions, otherwise undistinguishable, were sutured during the collision of the European and Adriatic continental plates, which corresponds to the latest tectonic mechanism of eclogitization. Before collision, formation and preservation of eclogitic rocks up to shallow levels was assisted by subduction of the cold oceanic crust. The two lithospheric processes of oceanic subduction and continental collision, though separated in time, contribute to continuous generation of eclogites under thermal conditions that evolve from higher to lower P-T ratios from the end of ocean consumption. Exhumation trajectories are characterized by low- or high thermal regimes in the same structural domain in different parts of the chain (Western and Eastern Austroalpine), in the same part of the chain (Penninic and ophiolites in Western, Central and Eastern Alps), or even within the same nappe (Dora-Maira, Gran Paradiso and Adula). Late orogenic collapse or slab breakoff processes may have caused late heating at very low pressure (0.3 GPa) during exhumation in some units of the Pennine nappes and ophiolites Mechanisms of nappe emplacement are demonstrably multiphase and inferences on palaeogeographic derivation of eclogitic units can be drawn from interpretation of P-T trajectories.

Numerical simulations of an ocean‐continent convergent system: Influence of subduction geometry and mantle wedge hydration on crustal recycling

The effects of the hydration mechanism on continental crust recycling are analyzed through a 2D finite element thermo-mechanical model. Oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method. Hydration is accomplished by lawsonite and serpentine breakdown; topography is treated as a free surface. Subduction rates of 1, 3, 5, 7.5 and 10 cm/y, slab angles of 30o, 45o and 60o and a mantle rheology represented by dry dunite and dry olivine flow laws, have been taken into account during successive numerical experiments. Model predictions pointed out that a direct relationship exists between mantle rheology and the amount of recycled crustal material: the larger the viscosity contrast between hydrated and dry mantle, the larger the percentage of recycled material into the mantle wedge. Slab dip variation has a moderate impact on the recycling. Metamorphic evolution of recycled material is influenced by subduction style. TPmax, generally representative of eclogite facies conditions, is sensitive to changes in slab dip. A direct relationship between subduction rate and exhumation rate results for different slab dips that does not depend on the used mantle flow law. Thermal regimes predicted by different numerical models are compared to PT paths followed by continental crustal slices involved in ancient and recent subduction zones, making ablative subduction a suitable pre-collisional mechanism for burial and exhumation of continental crust.

Emplacement at granulite facies conditions of the Sesia–Lanzo metagabbros: an early record of Permian rifting?

The evolution of the pre-Alpine Corio and Monastero metagabbros points to strong chemical and mineralogical similarities with that of other Permian gabbro bodies of the Alps, which are concentrated in the Southalpine and Austroalpine domains. The structural and metamorphic pre-Alpine evolution of these gabbros records a re-equilibration following the emplacement in the deep crust (P=0.6–0.9 GPa and T=850±70 °C), exhumation through amphibolite facies conditions (P=0.5–0.35 GPa and T=570–670 °C), followed by a greenschist facies imprint (0.25≤P≤0.35 GPa and T<550 °C). This retrograde P–T evolution suggests that the exhumation occurred in a high thermal gradient regime, such as that induced by upwelling of an asthenospheric plume during continental rifting. This would be consistent with the crustal thinning known to have occurred in both the Southalpine and Austroalpine domains during Permian times. The gabbros and their country acid granulites are spatially associated with the serpentinised subcontinental mantle of the Lanzo Massif. This lithologic association and the metamorphic evolution is similar to that of the Fedoz gabbro (Austroalpine Domain of the Central Alps) and completely different from that observed in passive margins, where no remnants of the lower crust occur and the upper granitic crust directly overlies the serpentinized lherzolites. The location of Permian gabbro bodies in the Austroalpine and Southalpine domains and their absence in the Helvetic domain is evidence for asymmetric rifting.

Deformation and metamorphism associated with crustal rifting: The Permian to Liassic evolution of the Lake Lugano-Lake Como area (Southern Alps)

Abstract In connection with movements between Europe and Adria, the Southern Alps underwent extension leading to the formation of the Middle Jurassic South-Alpine passive continental margin. As a result of mainly S-vergent Alpine thrusting and folding, a substantially preserved Mesozoic crustal section, ranging from the surface to a paleo-depth of ca. 15 km is exposed in the Lugano-Lake Como area. Tectonic processes associated with Permo-Mesozoic extension can thus be investigated at different crustal levels. Rifting began with a thermal anomaly during which rocks at middle-crustal levels were sheared at ca. 650–750°C. Deformation was distributed on a several km wide zone and late-kinematic pegmatites were emplaced at this stage. Temperatures then decreased and beginning in the Norian, extension was accommodated by a major, E-dipping normal fault, the Lugano-Val Grande normal fault. The fault which can be followed down to a paleo-depth of ca. 12 km, was listric and flattened (to 20°) at ca. 7–9 km depth. In the upper 8–10 km, deformation was only brittle, whereas at deeper crustal levels greenschist mylonites were formed. With continued normal faulting the colder hanging wall cooled the fault zone and greenschist mylonites were progressively replaced by lower-temperature ultramylonites and cataclasites. The thickness of the fault zone varies from some tens of meters in the upper 5–6 km to several hundred meters at deeper levels: deformation was thus discrete at a crustal scale.

The interaction of deformation and metamorphic reactions

Abstract Feedback relations between deformation and metamorphic mineral reactions, derived using the principles of non-equilibrium thermodynamics, indicate that mineral reactions progress to completion in high-strain areas, driven by energy dissipated from inelastic deformation. These processes, in common with other time-dependent geological processes, lead to both strain, and strain-rate, hardening/softening in rate-dependent materials. In particular, strain-rate softening leads to the formation of shear zones, folds and boudins by non-Biot mechanisms. Strain-softening alone does not produce folding or boudinage and results in low-strain shear zones; strain-rate softening is necessary to produce realistic strains and structures. Reaction–mechanical feedback relations operating at the scale of 10–100 m produce structures similar to those that arise from thermal–mechanical feedback relations at coarser (kilometre) scales and reaction–diffusion–mechanical feedback relations at finer (millimetre) scales. The dominance of specific processes at various length scales but the development of similar structures by all coupled processes leads to scale invariance. The concept of non-equilibrium mineral stability diagrams is introduced. In principle, deformation influences the position of mineral stability fields relative to equilibrium stability fields; the effect is negligible for the quartz→coesite reaction but may be important for others. Application of these results to the development of structures and mineral reactions in the Italian Alps is discussed.

The role of structural and metamorphic memory in the distinction of tectono-metamorphic units: the basement of the Como lake in the Southern Alps

The concept of ‘metamorphic field gradient’ applied to the lithologically homogeneous deep crust of the Southern Alps suggests the existence of two metamorphic units. The comparison of P-T-d-t paths, derived in adjacent portions of this basement and supported by a continuous foliation trajectory map helps to distinguish three tectono-metamorphic units, corresponding to the Domaso–Cortafo, Dervio–Olgiasca and Monte Muggio zones. The degree of granular scale reorganisation of the prevailing planar fabric is considered in the three zones together with the relative chronology of the structural imprints and the metamorphic environments in which they developed. This approach emphasizes that the dominant metamorphic imprint of each unit coincides with that of the most pervasive fabric at the regional scale when the degree of fabric evolution is sufficiently high, and not with the Tmax–PTmax recorded in each tectono-metamorphic unit. In terrains that underwent polyphase deformation and metamorphism the ‘metamorphic field gradient’ concept cannot therefore be utilised to discriminate tectono-metamorphic units.

Pre-Alpine Structural and Metamorphic Histories in the Orobic Southern Alps, Italy

Petrological and structural data on the Orobic South-Alpine basement are reviewed in order to reconstruct its thermomechanical history. P-T-t paths are discussed for Val Vedello — Passo S. Marco and Lario areas, i.e. for sectors traditionally attributed to different metamorphic grades (greenschist and amphibolite facies respectively). Structural investigations evidenced that, in these zones, the first recognizable phase of deformation (D1) developed in all the Oro-bic basement under amphibolite facies conditions during the Variscan period (ca. 330 Ma). Although some metagranitoids, emplaced in the Orobic basement, are considered to be Ordovician, no relics testify to the real pre-Variscan basement during the intrusion of the granites. After the D1 phase in the central-eastern areas (Val Vedello — Passo S. Marco) during D2 phase, a greenschist retrogression is observed. In the westernmost area (Lario basement) the D2 phase is characterized by an increase in temperature under low pressure regime. During the D2 phase of deformation, considering the available geochronological data, a late Variscan post-thickening uplift is suggested for the Val Vedello-Passo S. Marco areas, and a Permo-Mesozoic uplift related to an extensional tectonic regime for the Lario area. These contrasted evolutions result from the tectonic piling up of various crustal segments of the South-Alpine basement, each representative of different tectonic histories. The polyphased character of the two tectono-metamorphic histories appears to be in contradiction with the metamorphic zoneography traditionally accepted for the South-Alpine domain.

Integration of natural data within a numerical model of ablative subduction: a possible interpretation for the Alpine dynamics of the Austroalpine crust

A numerical modelling approach is used to validate the physical and geological reliability of the ablative subduction mechanism during Alpine convergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the Alpine belt: the Austroalpine Domain. The model predictions and the natural data for the Austroalpine of the Western Alps agree very well in terms of P-T peak conditions, relative chronology of peak and exhumation events, P-T-t paths, thermal gradients and the tectonic evolution of the continental rocks. These findings suggest that a pre-collisional evolution of this domain, with the burial of the continental rocks (induced by ablative subduction of the overriding Adria plate) and their exhumation (driven by an upwelling flow generated in a hydrated mantle wedge) could be a valid mechanism that reproduces the actual tectono-metamorphic configuration of this part of the Alps. There is less agreement between the model predictions and the natural data for the Austroalpine of the Central-Eastern Alps. Based on the natural data available in the literature, a critical discussion of the other proposed mechanisms is presented, and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results; these factors include variations in the continental and/or oceanic thickness, variation of the subduction rate and/or slab dip, the initial thermal state of the passive margin, the occurrence of continental collision and an oblique convergence.

The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions

Abstract Records of Variscan structural and metamorphic imprints in the Alps indicate that before Pangaea fragmentation, the continental lithosphere was thermally and mechanically perturbed during Variscan subduction and collision. A diffuse igneous activity associated with high-temperature (HT) metamorphism, accounting for a Permian–Triassic high thermal regime, is peculiar to the Alpine area and has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting. Intra-continental basins hosting Permian volcanic products have been interpreted as developed either in a late-collisional strike-slip or in a continental rifting setting. Two-dimensional finite element models have been used to shed light on the transition between the late Variscan orogenic evolution and lithospheric thinning that, since Permian–Triassic time, announced the opening of Tethys. Comparison of model predictions with a broad set of natural metamorphic, structural, sedimentary and igneous data suggests that the late collisional gravitational evolution does not provide a thermo-mechanical outline able to justify mantle partial melting, evidenced by emplacement of huge gabbro bodies and regional-scale high-temperature metamorphism during Permian–Triassic time. An active extension is required to obtain model predictions comparable with natural data inferred from the volumes of the Alpine basement that were poorly reactivated during Mesozoic–Tertiary convergence.

Interplay between deformation and metamorphism during eclogitization of amphibolites in the Sesia–Lanzo Zone of the Western Alps

Interactions of fabric evolution and chemical parameters driving reaction progress during the amphibolite-to-eclogite transition were investigated in eclogitized amphibolites of the Western Alps. In the Sesia–Lanzo Zone (SLZ), mafic rocks ranging from eclogitized hornblendites to true eclogites occur as layers and boudins within micaschists and are characterized by different modal amounts of amphibole, omphacite, zoisite, garnet, and phengite, constituting the Alpine HP assemblage. Across narrow zones, this array of lithologies displays gradients in planar fabrics characterized by coronitic, S-tectonitic, and mylonitic textures with different extents of eclogitization. Eclogitic parageneses are controlled not only by the bulk rock composition of the protoliths but also by the degree of fabric evolution. This is the case for omphacite occurrence, which is constrained by plagioclase composition and modal amount (NK parameter) in the protoliths, whereas the increase in modal omphacite and the concomitant decrease in modal amphibole in rocks with high NK are controlled by the strain rate. Protoliths with a low NK content develop the amphibole + garnet + epidote assemblage in eclogitized hornblendites. In protoliths with higher NK values, the co-existence of amphibole with garnet, omphacite, and epidote occurs only for low-to-medium strain textures (e.g. coronites and S-tectonites), whereas an amphibole-free assemblage defines a mylonitic foliation; in this case, amphibole relics are present exclusively as armoured inclusions in garnets and omphacite porphyroclasts. Thus, amphibole persists in the eclogitic assemblage at pressures exceeding the experimentally determined amphibole stability field. Values of confining pressure under which Sesia–Lanzo mafic rocks re-equilibrated during Alpine subduction were estimated, through equilibrium assemblage modelling, at 2.2–2.7 GPa. The amphibole-bearing eclogites of the SLZ show that large volumes of amphibole-bearing rocks can be exhumed from a depth exceeding 75 km without dehydration reactions running to completion. Petrological estimates in orogenic zones may help constrain geological and tectonic conclusions when selection of laboratory samples is assisted even by simple microstructural evaluation of planar fabric development.