Davide Zanoni

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.

Structural and petrographic map of the Sassa gabbro complex (Dent Blanche nappe, Austroalpine tectonic system, Western Alps, Italy)

The Sassa gabbro complex outcrops in the upper Valpelline and is part of the Permian gabbros of the Collon – Matterhorn group, set in the Dent Blanche nappe of the Western Austroalpine tectonic system. This 1:2500-scale map was constructed through the synergic use of meso- and microscopic analysis of fabrics to identify the relationships of the superposed foliations with the growth of equilibrium mineral assemblages. Throughout the foliation trajectories, a mosaic of rock volumes, in which strain is differently partitioned, is highlighted on the map for all the successive deformation stages. Therefore, this mosaic shows deformation gradients spanning from unstrained or weakly strained domains, where the primary magmatic features are still well preserved, to domains in which the Alpine fabrics are pervasive and obliterate the previous structures. The pre-Alpine history is characterized by the polyphasic emplacement of the Sassa gabbro and by a coronitic growth of successive mineral assemblages during crustal thinning-related exhumation. The Alpine history is characterized by multistage heterogeneous deformation consisting of a first D1 stage developed in blueschist facies conditions followed by D2 and D3 stages developed in greenschist facies conditions. In summary, by applying this analytical method, the geologic traces of the pre-Alpine lithospheric thinning and Alpine subduction histories have been separated.

Taking advantage of petrostructural heterogeneities in subduction-collisional orogens, and effect on the scale of analysis

Since the beginning of the last century, tectonic history of polyphase metamorphic tectonites of orogenic basement complexes is often related to primary links with metasediments, of presumably known origin, and location of their original basins. However such history is worth to be compared with results of an alternative, independent investigation that pursues: i) an objective reconstruction of the evolutionary steps modifying the lithostratigraphic setting and of its deformation-metamorphism interactions during plate-scale events, and ii) a privileged reconstruction of the rock memory for the structural and metamorphic correlation of crystalline basement units. Interpretative merging of data gathered from these affine rock properties made interpretations of orogenic zones more actualistic and based on recognition of tectonic trajectories of units through evolving geodynamic contexts. In this account a refinement of the analytical approach to inferring deformation and metamorphic paths and constructing geological histories of basements in axial zones of orogenic belts is presented and examples are synthesized from the Western Alps and the Canadian Cordillera, based on detailed structural and lithostratigraphic mapping in harmony with macro- and micro- structural techniques of analysis, are reported from the two belts.

Pre- to post-Cordilleran transposition history of Joss Mountain: Insights into the exhumation of the Shuswap complex, southeastern Canadian Cordillera

We present new multiscale structural, mineral chemical, and U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data in order to unravel part of the tectono-metamorphic evolution of the Shuswap complex in the southern Canadian Cordillera. We reconstructed the pressure-temperature-deformation-time ( P - T - d - t ) history of the Joss Mountain domain within the Shuswap complex. The west-dipping Greenbush shear zone separates the Joss Mountain domain from the structurally lower Thor-Odin culmination to the east, the southern culmination of the Monashee complex, and one of the structurally deepest parts of the Shuswap complex. At Joss Mountain, the protolith of an orthogneiss crystallized at ca. 360 Ma which is consistent with Late Devonian arc magmatism along the western paleomargin of North America. Joss Mountain metasedimentary rocks and orthogneiss were transposed at ∼21–29 km depth over a period of at least 20 m.y., and possibly more than 38 m.y., during Late Cretaceous to Paleocene mature stages of Cordilleran continental collision. This mature collision took place while slow detachment of the subducted oceanic lithosphere occurred and thermal conditions were approaching those of a crust undergoing postorogenic thermal relaxation. Transposition at Joss Mountain ended earlier and exhumation started earlier than in the Monashee complex. Exhumation occurred under conditions of near-isothermal decompression and geothermal gradients consistent with lithospheric thinning. Earlier and slower exhumation of the Joss Mountain domain than of the adjacent northwestern Thor-Odin culmination may have resulted from normal movement along the Greenbush shear zone contributing to the exhumation of the Shuswap complex.

The Variscan evolution in the basement cobbles of the Permian Ponteranica Formation by microstructural and petrologic analysis

This work reconstructs the metamorphic evolution recorded in the cobbles of the Ponteranica Formation to be compared with the evolutions of the basements of the central Southern Alps and of the clasts of Aga and Vedello and Dosso dei Galli conglomerates. The Ponteranica Formation is part of the lower volcano-sedimentary cycle of the Permian sequences of the western Orobic basin. These sequences cap the Variscan basement in which different tectonometamorphic units were recognized and contoured. We sampled all the crystalline rock types preserved as clasts such as different types of metapelites, meta-intrusives, granitoids, and tourmalinites and reconstructed the relative chronology of the superposed equilibrium assemblages. Only for metapelitic rocks the PT paths were reconstructed on the ground of mineral chemical analyses and geothermobarometry. The cobbles of the Pontereanica Formation record two types of metamorphic evolution. In Type 1 cobbles, the M1 metamorphic assemblage is characterised by white mica, biotite, garnet, staurolite, ± kyanite and is interpreted as the result of the record of the Variscan collision. This assemblage is followed by M2 assemblage characterised by white mica, chlorite, ± biotite that represents the Variscan exhumation. In type 2 cobbles, relict andalusite and possible cordierite characterise the pre-M2 metamorphic re-equilibration that developed under a high thermal regime. The PT paths recorded in cobbles are consistent with the tectono-metamorphic evolutions recorded in the basement units suggesting that during the deposition of conglomerates, the type of tectono-metamorphic units exposed to erosion were equivalent to that presently exposed in the Orobic Alps.The evolution of type 1 cobbles is similar to the evolution recorded in the clasts of Aga and Vedello conglomerates, whereas the clasts of Dosso dei Galli Conglomerate recorded only the effects of the Variscan tectonic burial. The occurrence of tourmalinite cobbles may imply an age up to Upper Permian / Lower Triassic for the Ponteranica Formation. In this view, the age of the Permian conglomerates of central Southalpine decreases westward with the increase of the thermal maturity of the metamorphic imprints recorded in clasts. These observations are consistent with a westward migration of the extensional tectonics responsible for the basin opening.

UHP Ti-chondrodite in the Zermatt-Saas serpentinite: Constraints on a new tectonic scenario

Abstract We focus on the key role of different Ti-humite minerals in subducted serpentinites as possible indicators of extreme pressure conditions. The occurrence of Ti-chondrodite and/or Ti-clinohumite assemblages in the eclogitized serpentinites of the Zermatt-Saas Zone (ZSZ) of the Western Alps allows the recrystallization of such rocks at UHP conditions (P = 2.8–3.5 GPa, T = 600–670 °C) to be determined. Such conditions are similar to those registered by the nearby Cignana unit, a main Alpine area for UHP metamorphism, where coesite and microdiamond have been found. In ZSZ serpentinites, the new UHP assemblage predates the previously recognized HP-UHP paragenesis, which was recently dated at 65 Ma. This finding opens up a new interpretation for the petrologically and structurally well-constrained HP/ UHP records, especially because all other ages for HP-UHP metamorphism in the ZSZ are much younger, and for the size of UHP units. Our findings suggest that ophiolites in the axial zone of collisional belts are a mosaic of oceanic lithosphere slices that recorded contrasted thermal and mechanical evolutions during their physical trajectories in the subduction wedge.

Structure and petrography of the southwestern margin of the Biella pluton, Western Alps

This work presents a new form surface map of the southwestern margin of the Biella pluton at the scale 1:10,000. The Biella pluton is part of the Periadriatic intrusives of the Alps and is emplaced in the continental Sesia-Lanzo Zone of the western Austroalpine domain. The country rocks consist of metapelites and different metagranitoids. Pre-intrusive HP (high pressure) mineral assemblages are dominant in country rocks with the exception of metagranitoids dominated by HT (high temperature) assemblages. The plutonic rocks consist of monzonite with minor syenite. The ductile polyphasic deformation of the country rocks predates the pluton emplacement, with the exception of syn-intrusive folding and shearing, which were recorded in the country rocks of the deeper part of the pluton. Syn-intrusive deformation may be represented by brittle structures that bear mineralisation; however, the majority of the brittle deformation postdates the final emplacement of the pluton. The orientation of the pervasive foliation in the country rocks controls the space available for magma intrusion and possible magmatic flow during emplacement. The inferred diffusion of the thermal aureole in the country rocks is based on the variation in contact metamorphic minerals, which is described by microscopic analysis. The extent of the aureole appears to be controlled by the type of dominant mineral assemblages, rock permeability, and the orientation of the regional foliation in country rocks with respect to the pluton margin. The multiscale structural analysis reveals that the Biella pluton emplaced at a depth as shallow as the greenschist facies conditions or shallower.

Structural analysis of a subduction-related contact in southern Sesia-Lanzo Zone (Austroalpine Domain, Italian Western Alps)

ABSTRACT A new foliation trajectory map at 1:10000 scale, represented here with an interpretative structural map, is derived from an original field analysis at 1:5000 scale in the southern Sesia-Lanzo Zone (SLZ). It shows the relative chronology of overprinting foliations, characterised by the mineral assemblages that mark superposed fabrics in each rock type. This map and the associated cross-sections, which synthesise the 3D structural outline of the tectonic contact between the Eclogitic Micaschists Complex (EMC), the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex, allow the correlation of the structural and metamorphic imprints that developed in these crustal and mantle complexes during Alpine subduction. Furthermore, the map and cross-sections allow the immediate perception of the metamorphic environments in which the structural imprints developed in each complex successively under eclogite, blueschist and greenschist facies conditions. The represented structural and metamorphic evolution of the southern end of the SLZ (internal Western Alps) has been inferred based on multiscale structural analysis. The dominant fabrics at the regional scale are two superposed mylonitic foliations that developed under blueschist and greenschist facies conditions, respectively. Metamorphic assemblages underlying the successive fabrics in the different metamorphic complexes allow us to identify contrasting metamorphic evolutions indicating that the tectonic contacts between the EMC, the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex developed under blueschist facies conditions and were successively reactivated during the greenschist facies retrogression.

Dating of ultramafic rocks from the Western Alps ophiolites discloses Late Cretaceous subduction ages in the Zermatt-Saas Zone

The Zermatt-Saas Zone was part of the Middle to Late Jurassic Tethyan lithosphere that underwent oceanic metamorphism during Mesozoic time and subduction during Eocene time (HP to UHP metamorphism). In upper Valtournanche, serpentinite, metarodingite and eclogite record a dominant S2 foliation that developed under 2.5±0.3 GPa and 600±20°C during Alpine subduction. Serpentinites contain clinopyroxene and rare zircon porphyroclasts. Clinopyroxene porphyroclasts show fringes within S2 with similar compositions to that of grains defining S2. Zircon cores show zoning typical of magmatic growth and thin fringes parallel to the S2 foliation. These features indicate crystallization of clinopyroxene and zircon fringes during HP syn-D2 metamorphism, related to the Alpine subduction. The U–Pb zircon dates for cores and fringes reveal crystallization at 165±3.2 Ma and 65.5±5.6 Ma, respectively. The Middle Jurassic dates are in agreement with the known ages for the oceanic accretion of the Tethyan lithosphere. The Late Cretaceaous - Paleocene dates suggest that the Zermatt-Saas Zone experienced high-pressure to ultra-high-pressure (HP–UHP) metamorphism at c. 16 Ma earlier than previously reported. This result is in agreement with the evidence that in the Western Alps the continental Sesia-Lanzo Zone reached the subduction climax at least from 70 Ma and was exhumed during ongoing oceanic subduction. Our results are further evidence that the Zermatt-Saas ophiolites diachronically recorded heterogeneous HP–UHP metamorphism.

Tectonometamorphic evolution of the Lago della Vecchia metaintrusive and its country rocks, Sesia-Lanzo Zone, Western Alps

In the Sesia-Lanzo Zone different complexes are separated on the basis of rock types and dominant metamorphic imprints: one of these is the Eclogitic Micaschists Complex (EMC) that is distinguished by a dominant Alpine eclogite facies fabric. In this contribution, we present the results of a multiscale structural analysis devoted to reconstruct the tectonometamorphic evolution of the Lago della Vecchia metaintrusive and its country rocks (upper Cervo valley, Biella), which are part of the EMC. Seven groups of superposed structures have been detected. Igneous pre-Alpine M0 relicts are preserved in metaintrusive. Alpine structures consist of: S1 foliation (D1) developed under eclogite facies conditions; S2 foliation (D2) is the most pervasive fabric and developed under low-pressure blueschist facies conditions; localised shear zones and open folds (D3 and D4) are associated with the development of greenschist facies assemblages; a kilometre-long shear zones (D5) developed under greenschist facies conditions. Finally, andesitic dikes intrusion postdates D1-D5 structures. On the basis of deformation-metamorphism relationships geothermobarometric estimates were performed to constrain the PT conditions characterising subsequent deformation stages. The resulting Pressure-Temperature-relative time of deformation (P-Td-t) path shows a pre-Alpine magmatic emplacement at around 15 km depth at T=710±19°C that is followed by re-equilibration indicating the conditions of the Alpine eclogitic peak (D1: 2.23±0.18 GPa; 537±43°C), compatible with a cold-subduction environment. A near-isothermal exhumation postdates D1, down to 1.12±0.11 GPa at 477±39°C during D2 deformation stage. Afterwards, temperature and pressure decreased from D2 low-pressure blueschist to D3-D4 greenschist re-equilibrations (P=0.6±0.10 GPa and T=302±24°C). In contrast with the most part of the EMC, where the dominant fabric developed under eclogitic facies conditions, at Lago della Vecchia the dominant fabric developed under low-pressure blueschist facies conditions. This difference suggests that the entire EMC should be characterised by kilometre-scale structural heterogeneities in the dominant tectonometamorphic record.