Pseudotachylytes in the Ulten zone, Tonale nappe: melting processes, 40Ar-39Ar geochronology and tectonic implications

Pseudotachylytes in the Ulten zone, Tonale nappe: melting processes, 40Ar-39Ar geochronology and tectonic implications\n\nSilvana Martin1, Alfio Viganò2, Giorgio Ranalli3, Gaston Godard4 and Marinella Ada Laurenzi5\n\n1) Dipartimento di Geoscienze, Università di Padova, Italy, silvana.martin@unipd.it\n2) Dipartimento di Ingegneria civile e ambientale, Università di Trento, Italy\n3) Department of Earth Sciences and Ottawa-Carleton Geoscience Centre, Carleton University, Ottawa, Canada\n4) IPGP-CNRS-Université Denis Diderot, Paris, France\n5) Istituto di Geoscienze e Georisorse, CNR, Pisa, Italy\n\n\nPseudotachylytes of the Tonale nappe\n\nPseudotachylytes (PSTs) are interpreted as the product of high-velocity frictional slip along faults. In the Alps, PSTs in the crust are mostly recognized along major faults, i.e., at the base of allochtonous units and along the Periadriatic Lineament fault system. Here we describe fault- and injection-PSTs associated to exhumed faults and shear zones in the Ulten zone, in the NE sector of the Austroalpine Tonale nappe (NE Italy). In this area, at the intersection between three main fault systems, i.e., the Peio, Tonale and Giudicarie, PSTs are extensively present (Figure 1).\n\n\n\n \n\nFigure 1: Simplified structural map of the Tonale nappe, with distribution of pseudotachylytes in basement rocks. Faults and minor shear zones are inferred from ductile and brittle data from field observations and previous studies. The area where pseudotachylytes are more abundant (shaded area) is part of the Ulten zone.\n\n\nIn the field, PSTs are commonly associated to (ultra)mylonites and cataclasites, within felsic migmatites and gneiss of the Ulten unit. New collected structural data confirm the presence of a complex system of faults (i.e., Rumo, Val dell’Acqua, Passo Palù and Val Clapa; Figure 1) with the presence of both ductile and brittle indicators of deformation (cf. Martin et al., 2004). Fault-veins show typical thickness between few millimeters to 1–2 cm, with variable lengths up to 3–5 m. Injection-veins can attain 7–10 cm in thickness and few tens of centimeters in length.\nVeins are banded, with sharp boundaries between different bands and sheat-fold-like structures, or with an homogeneous matrix. Bands exhibit chemical component variations in some elements, for example K20. Clasts within the glassy matrix are mostly quartz with minor plagioclase and rare sillimanite, kyanite, apatite and K-feldspar. Plagioclase clasts show rims with evident cathodoluminescence, inversely correlated to the size of clasts. Microlite layers are generally parallel to the vein walls, sometimes forming coronas around clasts.\n\nData and results\n\nPetrology\nA mass-balance study, performed to quantify the partial melting process, shows that clasts are mainly composed of quartz, plagioclase, sillimanite (or kyanite), suggesting that biotite, muscovite and garnet have almost completely melted. The proportion of the melted rock is on the order of 0.6.\nA thermodynamic modelling of the melting process shows that the energy necessary to melt 1 cm3 of rock is in the order of 4–5 kJ. Temperature reached inhomogeneous values up to ~1700 °C for quartz clasts, with an average temperature in the 1200–1400 °C range for the PST melt before quenching.\n\nGeochronology\n40Ar-39Ar in-situ ages vary across the veins, due to relevant variations of K/Cl ratios and, in some extent, to the 40Ar yield. Older ages are justified by the presence of clasts. The ages of four veins, supported by in-situ and step-heating analyses, are comprised between about 35 and 21 Ma, with a sample with an older age of ~50 Ma.\n\nTectonic implications\nThe dated PSTs, also compared with other results from the literature, confirm a peak of deformation of the NE sector of Tonale nappe (Ulten unit) during the Late Eocene-Early Oligocene. This tectonic event was contemporaneous with the major magmatic pulse along the Periadriatic Lineament. Most of the pseudotachylytes developed under metamorphic conditions at temperatures compatible with low-T mylonite generation and with closure temperature of zircons (minimum P-T conditions from Godard et al., 1996; 0.4–0.5 GPa, 450–500 °C). A late activity of the North Giudicarie fault is confirmed by zircon fission tracks of Early Miocene age and coeval pseudotachylytes (~21 Ma).\n\nReferences\n\nGodard, G., Martin, S., Prosser, G., Kienast, J.R. and Morten, L., 1996: Variscan migmatites, eclogites and garnet-peridotites of the Ulten zone, Eastern Austroalpine system, Tectonophysics 259, 313-341.\nMartin, S., Fellin, M.G., Massironi, M., Surian, N., Tommasi, L. and Cocco, S., 2004: Recent surface tectonics in a Paleozoic deeply-exhumed basement: The Bresimo Valley (Western Trentino), in Mapping Geology in Italy, SELCA, 273-278.

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