Chapter 1: Introduction of subduction channels and of coseismic 1. Introduction and interseismic deformation are reviewed. The temporal framework of deformation 1.1. Overview within the study area is mentioned on the base of published data. This chapter This introductory chapter aims to present provides information about the distribution the motivations and objectives for the here of different key features along the ancient presented thesis, and to give a guide post plate interface, which was restored into its through the different chapters. In addition, former subduction geometry on the base of both natural laboratories are shortly published and own geothermobarometric introduced. data. Finally, we discussed our field data in terms of post-accretion changes, long-term The here presented project is a PhD thesis kinematics, short-term kinematics, and supported by the German National Merit fluid flow. Foundation, together with the GeoResearchCentre Potsdam (GFZ). Chapter 5 (“Temporal constraints for unstable slip – 40Ar/39Ar geochronology Chapter 2 (“Methods”) gives an overview applied to pseudotachylytes”) over the methods used in this study, which concentrates on Ar/Ar age dating of encompasses the analyses of structural pseudotachylytes sampled at the data, estimation of PT conditions, image northwestern rim of the Engadine window analyses for clast scaling, microscopic immediately above the base of the upper scale investigations using petrographical plate. Pseudotachylytes are unambiguous and binocular microscopes and SEM, evidence for fossil seismicity. Despite of structural restoration of a cross section, analytical limitations and constraints by provenance analyses, analyses of Sr recrystallization of the pseudotachylyte isotope signatures, and Rb/Sr as well as groundmass since the time of melt Ar/Ar geochronology. generation, 40Ar/39Ar geochronology provides an approximation about the time Chapter 3 (“Anatomy of recently active frame of occurrence of unstable slip. plate interfaces”) gives an overview about the “state-of-the-art” knowledge of Chapter 6 (“Abandonment of the South structures and processes obtained from Penninic-Austroalpine paleosubduction recently active convergent plate margins. interface zone, Central Alps: constraints This includes hypotheses about the from Rb/Sr geochronology”) deals with subduction channel, a database of Rb/Sr isotopic data in order to constrain geophysical investigations, and the the termination of subduction related distribution of interface earthquakes in the deformation in the study area. In addition, depth range of the seismogenic coupling Sr isotope signatures of marine (meta-) zone. carbonates are used to define the influence and nature of fluids circulating through the Chapter 4 (“Anatomy of a fossil subduction mélange. We used our isotopic subduction channel – a quantitative view data as hints for the mass transfer mode on changing structures along the plate along the fossil plate interface zone. interface”) concentrates on the analysis of Tectonic erosion as the prevailing mass the fossil subduction mélange and its upper transfer mode is referred to the geological plate in the European Alps. Therein, a evolution of the Gosau group depocenters, coarse overview about the Alpine which represents slope basins formed onto evolution and the geology of the working the upper plate during the time of area are given. In addition, basic concepts subduction. Finally, isotopic dating yielded 1 Chapter 1: Introduction information about the pre-Alpine history of exposure exhibiting the complete the study area as well. seismogenic part of a subduction channel has been analyzed as yet. However, Chapter 7 (“Final discussion and outcrops of rocks, which suffered conclusions”) summarizes the results of subduction and subsequent exhumation, the different subchapters and aims to exist e.g. in the European Alps or Southern integrate them into a model of structures Chile. and processes along the seismogenic zone of convergent plate interfaces with special The here presented study contributes to the emphasize on the spatiotemporal understanding of convergent plate subduction history of the European Alps. boundaries in the depth range of their former seismogenic zone aiming at testing Additionally, microprobe analyses, inferences and hypotheses of the various 40Ar/39Ar analytical data, and sample kinematic and mechanical concepts locations are given in the “Appendix A, B presented for the seismogenic zone. and C”. Therefore, we use the complete exposure of this part of a former plate interface in the European Alps, one of the best-studied 1.2 Objectives and motivation mountain belts that has resulted from successive subduction, accretion and Convergent plate margins generate most of collision, where we analyzed a mélange the world’s seismicity, and nearly all of the zone tracing the plate interface zone of the earthquakes with magnitudes >8. They fossil convergent plate margin. initiate within the seismogenic coupling Additionally, we included information zone, which marks the transient, from Southern Chile, where material, seismically active part of the subduction which formerly underwent deformation channel developed between the two along the plate interface, was exhumed to converging plates. Despite the enormous the surface by large scale basal accretion at social, economic and scientific importance a certain depth to the base of the upper associated with seismogenic zones of plate. This part of the study provided convergent plate interfaces (e.g. additive hints for structures and processes destructive earthquakes, tsunamis), occurring along the plate interface zone of processes occurring in the intervening convergent plate margins (i.e. within the subduction channels are poorly understood. subduction channel), at least for a To date, the plate interface of convergent restricted PT domain. plate boundaries (i.e. subduction channels) cannot be directly accessed by drilling nor through surface observations, but has been 1.3. Working areas intensely studied with geophysical methods, numerical modeling, and 1.3.1. Alps sandbox simulations. These, however, either have only poor resolution, or are A detailed discussion about the working strongly dependent on a number of poorly area in the European Alps, comprising constrained assumptions. In consequence, their geological, structural, metamorphical direct investigations of exhumed ancient and geochronological framework, as well convergent plate boundaries are requested as the general Alpine evolution as far as to achieve insights into deformation interesting for our purpose, can be found in processes, which occurred along the plate Chapters 4, 5 and 6. Here, we only present interface despite multiple overprinting a brief overview about the location of the during exhumation. No continuous 2 Chapter 1: Introduction Figure 1.1: Satellite image of the European Alps. The shown section is comparable to the geological maps presented in Chapters 4 and 6. Red rectangle outlines the working area, yellow line represents the trace of the cross section of Figure 1.2. Source: www.worldwind.arc.nasa.gov. study area, the main geological units, and by an east to southeast dipping subduction their evolution in time and space. zone related with the closure of the Meliata ocean leaving signatures of subduction- The working area for the main part of this related deformation within the study is located in the central part of the Austroalpine nappes (Adriatic plate, e.g. European Alps along the transition from Schmid et al. 2004). Stacking within the the Western to the Eastern Alps (Fig. 1.1). Austroalpine is associated with top-W, It extends from the Swiss-Austrian border locally top-SW and top-NW thrusting in the north to the Swiss-Italian border in (Froitzheim et al. 1994, Handy 1996). The the south. direction of convergence changed to north – south during the Tertiary orogenic cycle The European Alps are the result of (referred to as Mesoalpine to Neoalpine, continent-continent collision between the e.g. Wagreich 1995) with top-N thrusting European plate and the Adriatic plate and closure of the Alpine Tethys in- following the subduction and accretion of between the European and Adriatic plate intervening oceanic domains (Penninic (Froitzheim et al. 1994, Handy 1996, units) during the Mesozoic and Tertiary Schmid et al. 2004). According to times. Most models differentiate two Froitzheim et al. (1994) the transition orogenic cycles during Alpine evolution: A between top-W thrusting and top-N Cretaceous orogenic cycle (referred to as thrusting is marked by a Late Cretaceous Eoalpine, e.g. Wagreich 1995) is defined extensional phase with top-SE directed 3 Chapter 1: Introduction Figure 1.2: a) Schematic profile from NW towards SE illustrates the relationship between the different tectonic units. Note the subdivision of the Penninic domain into the North Penninic ocean, the Middle Penninic micro- continent, and the South Penninic ocean. The South Penninic ocean was subducted beneath parts of the African plate during Mesozoic and Tertiary times. b) N-S section of the present day situation of the tilted and exhumed South Penninic-Austroalpine plate interface zone, based on Schmid et al. (1996). The positions of the analyzed profiles are also indicated (in addition, see Chapter 4). Trace of cross section is shown in Figure 1.1. normal faulting, which partly reactivated to the Alpine Tethys, and a micro- deformation features of the former continent separating the oceanic