Contemporaneous plutonism and strike‐slip faulting: A case study from the Tonale fault zone north of the Adamello pluton (Italian Alps)

Abstract

A field, microstructural, and geochronological study of contemporaneous plutonism and strike‐slip faulting along the eastern Tonale fault zone provides new insights into the interrelationships between magmatic emplacement, contact metamorphism and shearing, and it places new time constraints for the Periadriatic Fault System. Although pluton emplacement and shearing were not caused by each other, they mutually interacted during contact metamorphism. The character of the composite Tonale fault zone varies considerably with its proximity to the northern margin of the Adamello pluton, from a paired greenschist facies mylonite belt (Northern mylonite zone) and cataclastic fault zone in the west, to a fault zone consisting of Northern mylonite zone, Cataclastic fault zone and a second mylonite belt (Southern mylonite zone) formed by contemporaneous shearing and contact metamorphism in the east. Ongoing strike‐slip motion led to telescoping and advective cooling of the deforming contact aureole. New U/Pb zircon ages for the northernmost Adamello intrusions (the Avio at 34.6‐1.0 Ma and the Presanella at 32.0‐2.3 Ma) date the onset of contact metamorphism. Rb‐Sr and K‐Ar cooling ages and new zircon fission track ages provide evidence that postintrusive cooling to below 300°C was achieved rapidly, i.e., approximately 30 Myr ago being consistent with a shallow crustal emplacement at ambient temperatures of approximately 250°C. Cataclastic dextral strike‐slip faulting continued until about 20 Ma, when the Tonale fault zone became offset by a number of minor sinistral shears, and by the Giudicarie fault. Hence dextral strike‐slip motion along the Periadriatic Fault System east of the Bergell pluton lasted from approximately 35–20 Myr ago.

The Tauern Window (Eastern Alps, Austria): a new tectonic map, with cross-sections and a tectonometamorphic synthesis

“…Figure 5 clearly shows that the western part of the Tauern Window, and the area around the northern part of the Giudicarie Fault, where it offsets the Periadriatic Fault (Stipp et al 2004;Pomella et al 2011), are located in an intermediate location where present-day slab polarity is undetermined, whereas the eastern part of the Tauern Window is located in the area characterized by an Adriatic slab presently descending NE-ward below the Alps. Given the unchanged architecture of the nappes that characterize the crustal geometry of the Alpine edifice, this change in polarity primarily affected a decoupled mantle configuration and was induced at a late stage during Alpine orogeny, i.e.…”

“…The North Giudicarie Fault is a Miocene sinistral strike-slip zone that overprints Oligocene dextral strike-slip associated with a formerly straight Periadriatic Fault (Stipp et al 2004;Pomella et al 2011;see Viola et al 2001, for a differing view). It forms the westernmost branch of a set of sinisitrally transpressive overthrusts defining a broader Giudicarie Belt located outside Fig.…”

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

“…These magmas rose along the Periadriatic line, which is the main Alpine crustal break (Figure 1 (a)), and subsequently emplaced in the form of plutons, dykes, and volcanites (Berger, Thomsen, Ovtcharova, Kapferer, & Mercolli, 2012;Bernardelli, Castelli, & Rossetti, 2000;Bigioggero, Colombo, Del Moro, Gregnanin, Macera, & Tunesi, 1994;Callegari, Cigolini, Medeot, & D'Antonio, 2004;Carraro & Ferrara, 1968;Dolenec, 1994;Fodor et al, 2008;Kapferer, Mercolli, Berger, Ovtcharova, & Fügenschuh, 2012;Mayer, Cortiana, Dal Piaz, Deloule, De Pieri, & Jobstraibizer, 2003;Müller, Mancktelow, & Meier, 2000;Oberli, Meier, Berger, Rosenberg, & Gieré, 2004;Pamić & Palincaš, 2000;Romer, Schärer, & Steck, 1996;Romer & Siegesmund, 2003;Rosenberg, 2004;Rossetti, Agangi, Castelli, Padoan, & Ruffini, 2007;Stipp, Fügenschuh, Gromet, Stünitz, & Schmid, 2004;Van Merke de Lummen & Vander Auwera, 1990;Von Blanckenburg, Früh-Green, Diethelm, & Stille, 1992;Von Blanckenburg et al, 1998;Zanoni, 2010;Zanoni et al, 2008). The Periadriatic line nucleated due to the contrasting rheology between rocks of the Southalpine domain and rocks of the Austroalpine and Penninic domains, during Alpine nappe structural development (Figure 1(a)).…”

Late Oligocene to early Miocene lateral extrusion at the eastern border of the Lepontine dome of the central Alps (Bergell and Insubric areas, eastern central Alps)

“…Regarding the end of vertical brittle offset on the Tonale fault, a discrepancy exist between the central and eastern segments of the fault, where upper and lower Miocene ages have been proposed, respectively, on the basis of thermochronologial studies [Viola, 2000;Ciancaleoni, 2005;Rahn, 2005]. Because the Tonale fault is offset by the Giudicarie Fault sinistral reactivation and related faults at $19-16 Ma due to Giudicarie indentation (Figure 1), the last dextral strike-slip movements along the Tonale fault took place during the Burdigalian [Schönborn, 1992;Schmid et al, 1996a;Viola et al, 2001;Stipp et al, 2004]. Depending on authors, right-lateral movements along the Insubric mylonites and Tonale Fault since the Oligocene, are assumed to be on the order of 30 to 60 km [Heitzmann, 1987b;Giger, 1991;Viola et al, 2001] or up to some 100 km [Schmid and Kissling, 2000].…”

“…This late Oligocene exhumation event was associated with local sets of brittle-ductile south dipping steep pro-thrusts, considered as antithetical conjugate sets to the vertical movements on the Insubric Mylonites (D3, Figure 2b) [Huber and Marquer, 1996]. South and east of the Bergell pluton, vertical offset is progressively replaced by pure dextral strike-slip along the greenschist facies Insubric mylonites, claiming for an eastward change from transpressional to pure strike-slip kinematics [Wiedenbeck, 1986;Schmid et al, 1989;Werling, 1992;Meier, 2003;Stipp et al, 2004].…”