Neil S. Mancktelow ¹, Andreas Meier^* 1, Giulio A.M. Viola ¹, Wolfgang Müller ¹, Bernhard Fügenschuh ², Diane Seward ¹, Igor M. Villa ³

The Periadriatic fault is the most obvious map-scale feature in the Alps. It separates the Penninic and Austroalpine nappes, intensively reworked in the Alpine orogenesis, from the Southern Alps which, on the contrary, do not present a strong Alpine metamorphic or structural overprint. The tectonic history of the Eastern Alps north of the Periadriatic fault is complex and heterogeneous in its development. Multiphase pre-Alpine deformation, metamorphism and magmatic intrusion occurred in the Ordovician, Carboniferous and Permian followed by important periods of deformation related to the Alpine cycle in the Liassic, Cretaceous, and Tertiary. This intermittent tectonic activity over a long time, together with the lack of Mesozoic sediments as temporal markers throughout most of the area, often makes it difficult to assign a particular feature (shear zone, fold, metamorphic assemblage etc.) to a specific event without direct geochronological control. This common lack of constraint determined the aim of the current long-term cooperative project – to date, as far as possible, specific structures or fabrics with well-characterized geometry and kinematics in order to build up a consistent tectonic history on a more quantitative basis. We have concentrated on the Alpine history and particularly on major shear zones that are (or have been) related to the Periadriatic fault system. Understanding these structures, however, requires their integration into a consistent regional scheme. Two important intrusive time markers are very useful in establishing a relative chronology: (1) the Permian gabbroic to granodioritic plutons and particularly the widespread light-coloured pegmatites with coarse muscovite and common tourmaline, intruded around 260 Ma; and (2) the Oligocene "Periadriatic" tonalitic plutons and surprisingly widespread calc-alkaline dykes of age 30-32 Ma. The intrusives and late pegmatites of (1) have in the past been used to distinguish between "Alpine" and "pre-Alpine" deformation, but it is now clear that there is also an important Late Permian deformation under high-T low-to-moderate P conditions that locally overprints some of these bodies. This phase is apparently related to underplating of the basal crust by extensive gabbroic intrusions and associated (failed) rifting in the Permian. Effects of the Liassic rifting leading to formation of the Alpine passive margin have not yet been recognized in our study area. In the Cretaceous, there is clear evidence for local high-pressure metamorphism around 100 Ma (e.g. Thöni and Jagoutz, 1993), followed by more moderate-pressure metamorphism reaching upper amphibolite facies north of Meran (e.g. Thoeni, 1983). This metamorphism is apparently related to initially ca. west-directed thrusting and subsequent ESE-directed low-angle normal faulting terminating around the Cretaceous-Tertiary boundary (e.g. Froitzheim et al., 1997). The most important manifestations of the latest Cretaceous extension are the widespread mylonite zone in the uppermost Oetztal basement and overlying Brenner Mesozoic metasediments, the Pejo Fault and the probably equivalent Mortirolo Fault. The Mortirolo and Pejo faults, marking the same tectonic boundary between the Campo Crystalline and the Tonale unit, have previously been considered to represent two different deformation events. The change in metamorphic conditions from Alpine greenschist facies in the footwall (N) to well-preserved high-T pre-Alpine rocks in the hanging wall (S) observed across both faults is consistent with the present day geometry and kinematics and is best explained by top-to-the-E directed extensional shearing. Deformation ages related to the Pejo fault, obtained from direct ⁴⁰Ar/³⁹Ar dating of pseudotachylytes and Rb/Sr dating of mylonites (structurally controlled microsampling), are around 75 Ma. The Mortirolo fault has previously been interpreted as a pre-Alpine structure, supposedly contact metamorphosed by the Permian Serottini intrusion. However, detailed structural mapping has unequivocally established that distinct shear zones with similar kinematics to the Mortirolo fault deform the intrusions on a wide range of scales. Rb/Sr biotite ages from Permian dykes within the mylonitized footwall of the Mortirolo fault are around 78 Ma (Del Moro and Notarpietro, 1987). It is therefore most likely that the Mortirolo fault belongs to the system of E- to SE-directed normal faults (e.g. Corvatsch, Schlinig and Pejo) responsible for Late Cretaceous extension in the Eastern Alps (Froitzheim et al., 1997). The Mortirolo fault can be followed further to the west as a belt of mylonites marking the northern border of the Tonale unit east of the Bergell intrusion and is probably responsible for the westward tectonic thinning of the underlying Austroalpine units, as observable on the map scale. In a second stage, Mortirolo and Pejo faults were reactivated along N/NNE-directed, steeply S-dipping thrusts, best documented in the hanging wall of the Cretaceous structures. The low-grade mylonites show metamorphic conditions at the transition from brittle to ductile defomation. Stepwise-heating ⁴⁰Ar/³⁹Ar dating of pseudotachylytes and Rb/Sr dating by microsampling synkinematically recrystallized white mica from mylonites demonstrate a Late Eocene to Early Oligocene age for reactivation. Both faults are folded by open, regional-scale, WSW-ENE-striking structures. The NE- to N-dipping northern limb, as exposed in Valtellina, shows a dextral top-to-the-NE transport direction, consistent with the expected sense after folding. A superposed second set of NW-SE striking open folds results in a broad dome and basin interference pattern at outcrop and regional scale. Both these sets of folds apparently post-date the mid-Oligocene (~30-32 Ma) intrusion of calc-alkaline dykes. Locally, a high-T fabric petrographically similar to the pre-Alpine fabric of the Tonale unit is also preserved in the footwall of the Mortirolo fault. The distinction between an Alpine pervasively overprinted Campo unit (footwall) and a Tonale unit preserving mainly pre-Alpine fabrics (hanging wall), as previously assumed, cannot always be applied. It is more appropriate to consider differing degrees of Alpine overprint, both in the hanging wall and footwall, variably affecting a similar pre-Alpine protolith. This complex distribution of pre-Alpine and Alpine fabrics in the investigated area is also the subject of a poster presentation at this meeting (The Mortirolo and Pejo fault system: new structural and geochronological constraints, A. Meier and G. Viola). South of the Tauern window, initial Eo-Oligocene deformation is largely related to the regionally important Defereggen-Antholz-Vals (DAV) fault that separates Tertiary Rb-Sr biotite ages in the north from pre-Alpine ages in the south (e.g. Borsi et al., 1978). In the northern block, it is not always straightforward to distinguish between potentially Permian, Cretaceous or Tertiary deformation, since all overprint the ~ 260 Ma old pegmatites, the kinematics in current coordinates are similar (largely sinistral strike-slip) and the metamorphic conditions of deformation cannot always be unequivocally determined. Movement on the DAV is younger than the juxtaposition of Penninic and Eastern-Alpine nappes, whose contact is now exposed at the southern boundary of the Tauern window and lasted until the early stages of intrusion related to the group (2) Periadriatic intrusives. Early dykes not present in the main plutons are weakly to mylonitically overprinted by the DAV fabric as are andalusite porphyroblasts due to contact metamorphism immediately south the major Rieserferner body. However, the plutons themselves regionally crosscut the DAV fabric, DAV mylonites are contact metamorphosed (e.g. south of Zinnsnock) and deformation within the plutons shows combined dextral plus top-to-SE reverse shearing directly related to dextral-transpressive movements on the Periadriatic fault. It appears, therefore, that the Periadriatic intrusions mark an important transition in regional kinematics, from probably sinistral-transtensive to dextral transpressive. Since then, regional tectonics has been dominated by movements on the Periadriatic fault and concommitant updoming and exhumation of the Tauern window, involving folding and top-to-SE directed reverse faulting together with orogen-parallel low-angle normal faulting (the Brenner fault major movement between 20-18 Ma). In the central portion of the Periadriatic Fault, where a major change in its otherwise almost constantly E-W striking direction occurs, the Periadriatic fault is not a single straight structure, but is rather a fault system consisting of a number of distinct segments. These are the Giudicarie, the Mauls, the Passeier and the Jaufen faults. The understanding of the temporal and structural relationships between them is a key issue in order to shed light on the tectonic setting of this part of the Alpine edifice. A detailed structural study of these fault segments, together with a dense sampling for fission-track analysis, has been performed in order to reconstruct the deformation history of this system and to try to evaluate the real sinistral displacement accommodated by the Giudicarie fault, one of the still most debated topics. Based on the preliminary results of this study, it seems possible to characterize two main tectonic phases: a) A thrusting event around 32 Ma with transport direction towards 100-110°, through which the Austroalpine system overrode the Southern Alps. This early event is largely recorded by basement and limestones mylonites along the Giudicarie and Mauls faults. b) A later sinistral strike slip displacement, mainly characterized by structures at the ductile-brittle transition. It overprinted the top to SE thrusting related mylonites but its strain was also partitioned into a large system of transcurrent faults in the southalpine domain.

It was probably during this late event (b) that the trend of the Periadriatic fault was modified, however not in proportions exceeding a sinistral displacement that the authors of this contribution evaluate in 10-15 km. In fact, the recently discovered ca. 10km of sinistral displacement of the Jaufen mylonites through the Passeier brittle fault provide an efficient and unique tool to constrain the real amount of this throw. Fission-track analysis would also confirm this working hypothesis. The distribution of the tonalitic lamellae along the Giudicarie fault is also in agreement with this estimate: in fact they are continuously outcropped along the fault with a single exception consisting in a gap of slightly more than 10 km. This amount corresponds exactly to the sinistral displacement due to the fault, which cut off these thin plutonic bodies, previously emplaced during the thrusting event. Furthermore, a direct structural connection has been established between the Brenner and the Jaufen fault. This structural link also provides a good constraint on the timing of the phase b), as it necessarily has to postdate the main exhumation phase for the Tauern Window at 19 Ma.

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Del Moro A., Notarpietro A. (1987), Schweiz. Min. Petr. Mitt. 67, 295-306.

Froitzheim N., Conti P., Van Daalen M. (1997), Tectonophysics 280, 267-293.

Thöni, M. (1983), Mem. Sci. Geol. 36, 211-238.

Thöni, M. and Jagoutz, E. (1993), Schweiz. Min. Petr. Mitt. 73, 177-189.