The eo-Alpine high-pressure belt: Kinematics of the emplacement of Cretaceous eclogites in the Austoalpine units of the Eastern Alps

Geochronological data have shown that some of the most important basic eclogites from the Austroalpine units of the Eastern Alps are definitely the product of an eo-Alpine subduction-exhumation event, a result, which is in clear contrast to the traditional "pre-Alpine basement view". Due to a linear present-day array of the eo-Alpine eclogites in the Eastern Alps, Thöni & Jagoutz (1993) suggested the existence of an "eo-Alpine high-pressure belt (EHB)", located within the southern Austroalpine basement N of the Periadriatic lineament and striking at least some 350km in an E-W direction. While important aspects like the pressure-temperature conditions or the timing of emplacement are still not well constrained, present information lacks mainly from the following basic questions: (i) Did the host rocks, which are typically of crustal origin, experience a common Alpine PT evolution with the eclogites? (ii) Are the different eclogite occurrences genetically related in space and time, and thus confirming the existence of EHB? If remnants of an EHB exist, models are required, which can be tested in the field by structural, geochronological and petrological methods. A model for the EHB has to be consistent with all available geological data and should account for a mechanism to subduct and exhume the eclogites on the scale of the Alpine belt as a whole. Basically, two extreme scenarios may be sketched: (i) One coherent subduction zone or (ii) distinct subduction zones separated by transfer faults and or propagating with time from E to W. These models could clearly be tested by means of combined structural, geochronological and petrological methods within the actual geologic frame. In order to test these models and to prove the existence of an EHB the proposed project is designed to focus on the following points: (i) Structural mapping of the eclogite occurrences in the well-exposed western part of the EEP (i.e. Schober and Texel Group). Correlation of the structures with the immediate eclogite host rocks. (ii) Well- selected samples from the two areas will be analysed, putting emphasis on an interdisciplinary approach using structural, petrological and geochronological methods. (iii) In order to test the models of either one coherent or distinct subduction zones a detailed comparison of the geological data from the Schober and Texel Group as well as the investigation of a probable linkage between these two (now separated) eclogite occurrences are necessary. These results of the western parts will be compared with data from the less well exposed eclogites from the eastern parts of the EHB (i.e. Saualpe, Koralpe). (iv) The main aim of the proposed project, however, is to search the exact location of the EHB. In fact the proposed investigations will focus on the eo-Alpine zones of high-strain in the vicinity and, especially, to the S of the investigated eclogites, which probably represent the base of the hanging wall of the eo-Alpine subduction zone. The present-day arrangement of areas, which suffered eo-Alpine high- pressure metamorphism, can be only understood if the tectonic processes during the Tertiary events are considered. Although the proposed project will investigate the complete exhumation history of the high-pressure rocks, the main interest is clearly on the eo-Alpine events. The younger tectonic history will be discussed in collaboration with other working groups. The vision is, to test a model that operates by lateral and vertical displacement, which is, however, at present merely hypothetical A new geological data base combined with geodynamic modelling could probably explain some of the mechanisms that played a key role during the early steps of Alpine mountain building.

Project P13227-N06 was focused on the investigation of rock series that crystallized at high pressure (P) conditions (eclogite facies) and which have subsequently been exhumed rapidly and brought to the earth`s surface by fast tectonic processes. The investigated high-pressure rock series are exposed within the southern Austroalpine basement, immediately to the north of the Periadriatic Lineament, and extending over some 400 km along strike, from the Texelgruppe in the west (South Tyrol) to the Pohorje Mountains (Slovenija) at the eastern end of the Alpine chain (Eo-Alpine high-pressure belt; Thöni & Jagoutz, 1993). Within the project, field and analytical work were concentrated on rocks from the Texel Complex and the Schneeberg-Monteneve Complex in the southern Ötztal basement. The major goal was to constrain - by a tight interdisciplinary approach - conditions, timing and processes operative during the exhumation and emplacement of the high-pressure rocks. Detailed outcrop mapping and (micro-)structural investigations were the basis for deciphering the complex deformation history in this area, allowing an effective sampling strategy for further analytical studies. Three ductile and two brittle deformation phases have been recognized, all of which, however, are interpreted to pertain to one single, coherent orogenic cycle. Thermobarometric methods and thermodynamic modeling allowed to constrain the pressure and temperature (PT) conditions close to the metamorphic peak (maximum burial) and during initial exhumation as 1.30.1 GPa / 58040 C for the eclogites from the Texel Complex, and 1.0-0.8 GPa at 600-550 C for the Schneeberg Complex. Metamorphic andalusite was found for the first time in this area which gave further valuable information concerning the decompression path of the rocks. Age determinations using the Sm-Nd (garnet) and Rb-Sr (white mica) methods clearly show that, with very few exceptions, mineral content and deformation structures are younger than Permian, i.e., they were formed during the Alpine orogenesis. The metamorphic peak is constrained by these data at c. 90 (4) Ma. In the Late Cretaceous, the Schneeberg Complex operated as a major, some 3-5 km thick, shear zone which allowed to extrude the high-P metamorphic Texel Complex as its footwall towards SE, while the low-grade metamorphic (hangingwall) Ötztal basement was displaced towards NW, along this "Schneeberg Normal Fault". At c. 70 Ma B.P., th rocks had cooled down to temperatures of =300 C. The more easterly parts of the Eo-alpine HP-belt (Schobergruppe; Saualpe-Koralpe-Pohorje) were subducted during the Cretaceous to depths of = 50-60 km (P = 2.0 GPa). New geochronological data (Sm-Nd, Lu-Hf, U-Pb methods), however, yielded consistently young peak-pressure ages of c. 87-92 Ma for these areas as well, indicating near-contemporaneous exhumation along the whole Eo-Alpine high-pressure belt, with exhumation rates of 2-5 mm/y for the time c. 90-80 Ma B.P..