The Ötztal Nappe in the Eastern Alps is a thrust sheet of Variscan metamorphic basement rocks and their Mesozoic sediment cover. It has been argued that the main part of the Ötztal Nappe and its southeastern part, the Texel Complex, belong to two different Austroalpine nappe systems and are separated by a major tectonic contact. Different locations have been proposed for this boundary. We use microprobe mapping of garnet and structural field geology to test the hypothesis of such a tectonic separation. The Pre-Mesozoic rocks in the area include several lithotectonic units: Ötztal Complex s.str., Texel Complex, Laas Complex, Schneeberg Complex, and Schneeberg Frame Zone. With the exception of the Schneeberg Complex which contains only single-phased (Eoalpine, i.e. Late Cretaceous) garnet, all these units have two-phased garnet with Variscan cores and Eoalpine rims. The Schneeberg Complex represents Paleozoic sediments with only low-grade (sub-garnet-grade) Variscan metamorphism which was thrust over the other units and their Mesozoic cover (Brenner Mesozoic) during an early stage of the Eoalpine orogeny, before the peak of Eoalpine metamorphism and garnet growth. Folding of the thrust later modified the structural setting so that the Schneeberg Thrust was locally inverted and the Schneeberg Complex came to lie under the Ötztal Complex s.str. The hypothesized Ötztal/Texel boundaries of earlier authors either cut across undisturbed lithological layering or are unsupported by any structural evidence. Our results support the existence of one coherent Ötztal Nappe, including the Texel Complex, and showing a southeastward increase of Eoalpine metamorphism which resulted from southeastward subduction.

<p>Following a tectonic scheme proposed by Janák et al. (2011; Journal of Metamorphic Geology 29, 317-332) and Pleuger et al. (2011; Zeitschrift der Deutschen Gesellschaft für Geowissenschaften 162, 171-192), the Rhodopes are composed of four nappe complexes, from bottom to top the Lower, Middle, and Upper Allochthon and the Circum-Rhodope Belt. Rocks derived from Adria and/or Pelagonia (Lower Allochthon) are separated from rocks of European origin (Upper Allochthon) by lithologically variegated thrust sheets containing sporadic occurrences of ophiolites (Middle Allochthon). These ophiolites typically yield magmatic protolith ages of c. 160 Ma and were metamorphosed under amphibolite- to eclogite-facies conditions. They represent Neotethyan lithosphere subducted below Europe in the Late Cretaceous to Palaeogene whereas the Circum-Rhodope Belt contains ophiolites of the same protolith age but with lower metamorphic grade (greenschist facies at most) and was obducted onto the former European margin in the Jurassic. We present LA-ICP-MS U-Pb zircon and additional geochemical data from the Luda Reka Unit in the Bulgarian Eastern Rhodopes. This unit consists mostly of amphibolite, metagabbro, and metadiorite that yielded two protolith ages of 163.5±2.6 Ma and 154.2±1.0 Ma. The trace element patterns resemble those of typical back-arc basalts and lower oceanic crustal cumulates. Initial epsilon Nd values of six samples calculated to 154 Ma were +10.8 ±0.8 (2σ; n = 6), in agreement with average basalts derived from depleted ambient mantle. A pegmatite crosscutting the Luda Reka Unit yielded a magmatic age of 52.04±1.1 Ma. Such pegmatites are widespread in the Luda Reka Unit (Middle Allochthon) suggesting that emplacement of this unit over the Bjala Reka Orthogneiss Unit (Lower Allochthon) where such pegmatites are lacking happened only after c. 52 Ma. The Bjala Reka Orthogneiss Unit forms the footwall of the top-to-the-SSW Bjala Reka Detachment that became active in the Late Eocene. Where the Luda Reka Unit is lacking, the Bjala Reka Orthogneiss Unit is overlain by rocks that were collectively described as “Low-grade Mesozoic Unit” (e.g. Bonev & Stampfli 2008; Lithos 100, 210-233). Based on peak temperatures determined by Raman spectroscopy of organic matter, two tectonic units can be distinguished in the “Low-grade Mesozoic Unit”. The temperature peak was at c. 530 °C in the Mandrica Unit below and at c. 285 °C in the Maglenica Unit above. For the Mandrica Unit, minimum peak pressures of c. 1.4 GPa were obtained by Raman spectroscopy of quartz inclusions in garnet, indicating that this unit underwent subduction-related metamorphism. Because of this marked difference in peak metamorphic grade, we attribute only the anchimetamorphic Maglenica Unit to the Circum-Rhodope Belt while the high-pressure Mandrica Unit probably represents the Upper Allochthon. Both units are presently separated by the top-to-the-NW Mandrica Detachment that was active before the Bjala Reka Detachment. Our new findings show that the easternmost Rhodopes expose a condensed section through all four nappe complexes, notably including the Neotethys suture.</p>

<p>The Permian was a time of strong crustal extension in the area of the later-formed Alpine orogen. This involved extensional detachment faulting and the formation of metamorphic core complexes. We describe (1) an area in the Southern Alps (Valsassina, Orobic chain) where a metamorphic core complex and detachment fault have been preserved and only moderately overprinted by Alpine collisional shortening, and (2) an area in the Austroalpine (Schneeberg) where Alpine deformation and metamorphism are intense but a Permian low-angle normal fault is reconstructed from the present-day tectonometamorphic setting. In the Southern Alps case, the Grassi Detachment Fault represents a low-angle detachment capping a metamorphic core complex in the footwall which was affected by upward‐increasing, top‐to‐the‐southeast mylonitization. Two granitoid intrusions occur in the core complex, c. 289 Ma and c. 287 Ma, the older of which was syn-tectonic with respect to the extensional mylonites (Pohl, Froitzheim, et al., 2018, Tectonics). Consequently, detachment‐related mylonitic shearing took place during the Early Permian and ended at ~288 Ma, but kinematically coherent brittle faulting continued. Considering 30° anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally ~N‐S and the sense of transport top-South. In this area, there is no evidence of Permian strike-slip faulting but only of extension. In the Schneeberg area of the Austroalpine, a unit of Early Paleozoic metasediments with only Eoalpine (Cretaceous) garnet, the Schneeberg Complex, overlies units with two-phased (Variscan plus Eoalpine) garnet both to the North (Ötztal Complex) and to the South (Texel Complex). The basal contact of the Schneeberg Complex was active as a north-directed thrust during the Eoalpine orogeny. It reactivated a pre-existing, post-Variscan but pre-Mesozoic, i.e. Permian low-angle normal fault. This normal fault had emplaced the Schneeberg Complex with only low Variscan metamorphism (no Variscan garnet) on an amphibolite-facies metamorphic Variscan basement. The original normal fault dipped south or southeast, like the Grassi detachment in the Southern Alps. As the most deeply subducted units of the Eoalpine orogen (e.g. Koralpe, Saualpe, Pohorje) are also the ones showing the strongest Permian rift-related magmatism, we hypothesize that the Eoalpine subduction was localized in a deep Permian rift system within continental crust.</p>

<p>The Austroalpine Ötztal Nappe shows pervasive Eoalpine and local Variscan high-pressure metamorphism and deformation in its southeastern end, which obscure pr<span>i</span>or structures. We used magmatic and detrital zircon U-Pb dating by laser ablation ICP-MS to identify the precursor units of the Ötztal Nappe and the relationships among them.</p><p>Magmatic protolith dating of granitoid othogneisses in the Ötztal basement yielded Ordovician ages (450 – 470 Ma). The zircons of the Ordovician magmatism are important markers in the detrital zircon record. The paragneisses of the Ötztal basement, in which the Ordovician granitoids intruded, show no Ordovician zircons. The partly calcareous metasediments of the Schneeberg Complex and the Laas Series record some Ordovician detrital zircons. While the Schneeberg Complex is in tectonic contact to the Ötztal Nappe (Klug & Froitzheim, subm.), the Laas Series is the post-Ordovician sedimentary cover of the Ötztal basement. A Permo-Triassic basal metasandstone of the Brenner Mesozoic shows next to a strong Ordovician zircon age population some Variscan and Permo-Triassic zircons.</p><p>Zircon dating allowed to identify pre-Ordovician basement with Ordovician intrusions covered by post-Ordovician-pre-Variscan and Permo-Mesozoic sediments. This supports the concept of a non-tectonic unity in the southeastern Ötztal Nappe outside of the Schneeberg Complex.</p><p> </p>