István Dunkl

Palinspastic reconstruction and topographic evolution of the Eastern Alps during late Tertiary tectonic extrusion

This paper presents a new palinspastic restoration of the Eastern Alps for Neogene time and an attempt to reconstruct the Neogene palaeogeology, palaeotopography and palaeohydrography in connection with the structural evolution. The Eastern Alps underwent radical horizontal displacement during the Neogene due to large strike-slip systems and formation of structural windows. Our palinspastic reconstruction considers: (a) the rearrangement of tectonic units dismembered during tectonic extrusion, (b) the tectonic denudation driven by displacement of the crystalline blocks, (c) geochronological arguments, and (d) the sedimentary record of the syn-extrusion basins. The rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre-Miocene unstretched pattern of the Eastern Alps, reduced to 65% of its present-day E–W elongation. Using this structural frame and considering the sedimentary record, a set of palaeogeologic and palaeotopographic sketch maps with the palaeo-river systems is presented for three time slices (pre-, syn- and post-extension situation). In Late Oligocene and Early Miocene times, the western Eastern Alps were already mountainous, whereas the eastern part of the orogen formed lowlands or hilly areas. Enhanced block movement in the course of the extrusion process around the Early/Middle Miocene boundary led to the formation of intramontane sedimentary basins and a fault-induced reorientation of the drainage pattern, which forms the basis of the modern river system in the area east of the Tauern window. This region, where pre-Miocene land surfaces are preserved, probably became a mountainous area not before Late Miocene time and never reached the elevations of the areas further west.

Lithospheric structure of the Pannonian basin derived from seismic, gravity and geothermal data

Abstract The structure of the Pannonian basin is the result of distinct modes of Mid-Late Miocene extension exerting a profound effect on the lithospheric configuration, which continues even today. As the first manifestation of extensional collapse, large magnitude, metamorphic core complex style extension took place at the beginning of the Mid-Miocene in certain parts of the basin. Extrapolation of the present-day high heat flow in the basin, corrected for the blanketing effect of the basin fill, indicates a hot and thin lithosphere at the onset of extension. This initial condition, combined with the relatively thick crust inherited from earlier Alpine compressional episodes, appears to be responsible for the core complex type extension at the beginning of the syn-rift period. This type of extension is well documented in the northwestern Pannonian basin. Newly obtained deep reflection seismic and fission-track data integrated with well data from the southeastern part of the basin suggests that it developed in a similar fashion. Shortly after the initial period, the style of syn-rift extension changed to a wide-rift style, covering an area of much larger geographic extent. The associated normal faults revealed by industry reflection seismic data tend to dominate within the upper crust, obscuring pre-existing structures. However, several deep seismic profiles, constrained by gravity and geothermal modeling, image the entire lithosphere beneath the basin. It is the Mid-Miocene synrift extension which is still reflected in the structure of the Pannonian lithosphere, on the scale of the whole basin system. The gradually diminishing extension during the Late Miocene/Pliocene could not advance to the localization of extension into narrow rift zones in the Pannonian region, except some deep subbasins such as the Makó/Békés and Danube basins. These basins are underlain coincidently by anomalously thin crust (22–25 km) and lithosphere (45–60 km). Significant departures (up to 130 mW m−2) from the average present-day surface heat flow (c. 90 mW m−2) and intensive Pliocene alkaline magmatism are also regarded as evidence for the initiation of two newly defined narrow rift zones (Tisza and Duna) in the Pannonian basin system. However, both of these narrow rifts failed since the final docking of the Eastern Carpathians onto the European foreland excluded any further extension of the back-arc region.

Late Cretaceous exhumation of the metamorphic Gleinalm dome, Eastern Alps: kinematics, cooling history and sedimentary response in a sinistral wrench corridor

Abstract The metamorphic Gleinalm dome, Eastern Alps, was uplifted and exhumed within a releasing structure in a sinistral wrench corridor during the Late Cretaceous. The dome is confined by a system of ductile shear zones including low-angle normal faults and steep sinistral tear faults which define a large releasing structure with the metamorphic dome in its center. The fabrics developed within all ductile shear zones record processes which were operating during decreasing temperatures from initial epidote-amphibolite/upper greenschist facies conditions (with crystal plastic fabrics in quartz) to temperatures below ca. 300°C (with predominantly cataclastic fabrics). A cooling path based on 40Ar/39Ar amphibole (95.4 ± 1.2 Ma) and muscovite ages (87.6 ± 0.6; 84.3 ± 0.7 Ma) together with sphene, zircon and apatite fission track data indicate cooling through ca. 500°C at ca. 94 Ma to below ca. 250-200°C at 65 Ma. Subsidence of the adjacent Late Cretaceous Kainach Gosau basin occurred synchronously with cooling and uplift of the Gleinalm dome. Internal depositional patterns record rapid subsidence at the time of cooling with internal synsedimentary block rotation above an intra-crustal ductile normal fault. The sinistral wrench corridor of the Eastern Alps developed by sinistral displacement of the Austroalpine units against a relatively stable Europe during the Late Cretaceous.

Timing of low-temperature metamorphism and cooling of the Paleozoic and Mesozoic formations of the Bükkium, innermost Western Carpathians, Hungary

K-Ar ages of illite-muscovite and fission track ages of zircon and apatite were determined from various lithotypes of the Bükkium, which forms the innermost segment of the Western Carpathians. The stratigraphic ages of these Dinaric type formations cover a wide range from the Late Ordovician up to the Late Jurassic. The grade of the orogenic dynamo-thermal metamorphism varies from the late diagenetic zone through the ‘anchizone’ up to the ‘epizone’ (chlorite, maximally biotite isograd of the greenschist facies). The K-Ar system of the illite-muscovite in the < 2 μm grain-size fraction approached equilibrium only in ‘epizonal’ and high-temperature ‘anchizonal’ conditions. The orogenic metamorphism culminated between the eo-Hellenic (160-120 Ma) phase connected to the beginning of the subduction in the Dinarides, and the Austrian (100-95 Ma) phase characterized by compressional crustal thickening. No isotope geochronological evidence was found for proving any Hercynian recrystallization. The stability field of fission tracks in zircon was approached using the thermal histories of the different tectonic units. A temperature less than 250°C and effective heating time of 20–30 Ma had only negligible effects on the tracks, whereas total annealing was reached between 250 and 300°C. Apatite fission track ages from the Paleozoic and Mesozoic formations show that the uplift of the Bükk Mountains occurred only in the Tertiary (not earlier than ca. 40 Ma ago). Thermal modeling based on apatite fission track length spectra and preserved Paleogene sediment thickness data proved that the Late Neogene burial of the recently exhumed plateau of the Bükk Mountains exceeded 1 km.

The Palaeogene forearc basin of the Eastern Alps and Western Carpathians: subduction erosion and basin evolution

Scarce Palaeogene sediment remnants in the Eastern Alps and Western Carpathians are interpreted as remains of a continuous forearc basin. New apatite fission-track geochronological data corroborate mild Paleocene–Eocene exhumation and relief formation in the Eastern Alps. Palinspastic restoration and nine palaeogeographical maps of the Eastern Alps and Western Carpathians ranging from the Paleocene to the Late Oligocene epoch illustrate west to east migration of subsidence in the forearc basin. Subsidence isochrons indicate that oblique subduction of the European plate below the Adriatic plate was responsible for forearc basin migration at a rate of 8 mm a−1. The Periadriatic Lineament was formed as a result of shearing by oblique subduction. The Neogene to recent Sumatra forearc basin is an analogue for the evolution of the East Alpine–West Carpathian forearc basin.

Miocene rotations in the Eastern Alps - palaeomagnetic results from intramontane basin sediments

A palaeomagnetic study of late Early Miocene to late Middle Miocene sediments from the Eastern Alpine intramontane basins revealed counterclockwise rotations in the Ennstal, in the western part of the Noric and in the Lavanttal depressions. For the basal strata of the basins, declinations are between 273° and 315°, while for the younger strata they are between 321° and 333°, in perfect agreement with observations from the Klagenfurt basin. The results suggest synsedimentary rotation during the lateral tectonic extrusion of the Eastern Alps (ca. 17‐13 Ma), which was also responsible for the formation of the mostly transtensional basins. We propose a model in which domino-shaped blocks, separated by NNW‐SSE trending dextral faults, rotated counterclockwise due to faster eastward motion in the south relative to areas further north. The protrusion of the Bohemian spur inhibited eastward motion in the northern part of the study area, thus creating a sinistral wrench corridor. After lateral extrusion had ceased the counterclockwise rotation continued probably as ‘en bloc’ rotation of ca. 30°. Clockwise rotation was observed in the eastern part of the Noric depression, which appears to belong to a limited area with complex rotation pattern near the eastern margin of the Alps. This area is wedged between the counterclockwise rotated Eastern Alps and the similarly rotated North Pannonian area in Hungary.

Exhumation of the Rechnitz Window at the border of the Eastern Alps and Pannonian Basin during Neogene extension

Abstract The Rechnitz Window is the easternmost Penninic window of the Alps and the only one that is partly covered by Neogene sediments. Zircon and apatite fission-track ages have been measured on the Penninic metasediments of the Rechnitz series to better understand the exhumation of the window at the Alpine-Pannonian border. The zircon FT ages range from 21.9 to 13.4 Ma, similar to the white mica K/Ar ages (19 and 23 Ma). The exhumation rate during the Early Miocene extension was high (∼40°C/Ma). Apatite samples display FT ages of 7.3 and 9.7 Ma, thus the cooling rate was significantly less (7–11°C/Ma) during the post-rift uplift uplift in Late Miocene-Pliocene than in the Early Miocene escape period. Zircon FT ages decrease eastward due to the gradual southeastward slide of the Austroalpine cover along low-angle normal faults. The exhumation of the Rechnitz metamorphic core complex is younger than the unroofing of the eastern Tauern Window. Morphology of detrital zircon crystals of the Penninic metasediments was used for the differentiation of tectonic subunits and to trace the internal structure of the window. One population was probably derived from undifferentiated calc-alkaline granitoids and tholeiitic granitoid sources, while the other derived from evolved calc-alkaline granitoids and alkaline granitoids of anorogenic complexes. The areal distribution of zircon populations is in harmony with the eastward shift of zircon cooling ages. These data indicate the exhumation of deep levels of Penninic metasediments in the centre of the window.

Thermal effects of exhumation of a metamorphic core complex on hanging wall syn-rift sediments: an example from the Rechnitz Window, Eastern Alps

The removal of overburden by tectonic processes displaces the rock masses and their temperature with respect to the surface. Consequently high cooling rates and increased heat flow have been reported from rapidly exhuming terrains. Previous studies were mainly focused on the greenschist- and amphibolite-facies tectono-thermal evolution of the footwall. A two-dimensional numerical thermal model presented in this study concentrates on the thermal history of the hanging wall and the footwall blocks and evaluates the variation of the near-surface heat flow above an evolving metamorphic core complex. Based on structural, thermobarometric and geochronological constraints from the Rechnitz core complex of the Eastern Alps the modelling suggests that rapid movement along the extensional fault took place between 22 and 17 Ma. During this period the heat flow reached values of 140‐150 mW=m 2 in the hanging wall in a 10‐15-km-wide zone parallel to the fault trace. The increased heat flow caused the high coal rank in the Miocene syn-rift sediments deposited on the hanging wall. Apatite fission track geochronology of the thermally overprinted sedimentary deposits gives the time at which the bottom of the syn-rift sediment pile passed through 110oC.13:6 0:9 Ma). The vitrinite reflectance data (Rr: 0.67‐1.04%) suggest significant burial of the margin of the hanging wall at the time of core complex formation. Considering the coal rank data and the thermal conductivity of the sedimentary deposits by modelling of the organic maturation we calculate syn-rift (Early‐Middle Miocene) burial of the hanging wall margin of the Rechnitz core complex to be 1100‐1600 m. This value shows the importance of the post-Middle Miocene erosion of the region, which was related to the inversion of the Pannonian basin.

Paleogeography and catchment evolution in a mobile orogenic belt: the Central Alps in Oligo-Miocene times

Abstract In this study, we reconstruct the surface evolution of the Oligo–Miocene Central Alps using geochronological, geochemical and petrographical methods on the foreland basin sediments of both flanks of the mountain range. Our model is illustrated in four sketch maps of different time slices between mid-Oligocene to Middle Miocene times. For each time slice, we try to (1) give a palinspastic reconstruction of the Central Alps, based on the post-collisional lateral extrusion model, (2) show which tectonic units had become exposed to the surface due to exhumation processes in the Central Alps, (3) describe the thermochronologic evolution of lithological units formerly exposed but completely eroded today, (4) differentiate the catchment areas of the paleo-river systems which delivered debris to the foreland basins, and (5) describe the position of the main drainage divide relative to the exposed tectonic units.

Combination of Single-Grain Fission-Track Chronology and Morphological Analysis of Detrital Zircon Crystals in Provenance Studies: Sources of the Macigno Formation (Apennines, Italy)

Fission track (FT) analyses on unannealed detrital min- erals provide a powerful tool both for refining provenance models de- rived from traditional methods and for collecting information about erosion rates of the source area. Their power is increased if they are coupled with the study of zircon morphology. This combination of methods is applied to the Chattian-Aquitanian (25-23 Ma) Macigno turbidite complex. Basin-fill patterns and petrographical studies con- sistently identify the uplifting western Central Alps as the main source region for the Macigno Formation. Most zircon grains fall into a young age cluster ( ; 40-30 Ma), de- rived from a rapidly exhuming crystalline source region with a high cooling rate. Within this cluster, two age subgroups can be distin- guished at 30 and 40 Ma. In the younger subgroup, the zircon mor- phology supports the presence of two main populations: (1) from ig- neous rocks (S-type euhedral zircons), which appear to be partly de- rived from airborne tuffs; and (2) from metasedimentary units. In huge volumes of these metamorphic rocks, mica Ar-Ar and zircon fission- track thermochronometers have been reset, because of high geothermal gradients in the vicinity of the Periadriatic intrusives in mid-Oligocene times. At the present surface of the Alps, zircon FT ages around and slightly less than 30 Ma are reported in the Sesia-Lanzo zone, the Gran Paradiso Massif, the Upper Pennine nappes, the Monte Rosa Massif, and the Dent Blanche complex. The older subgroup of the Tertiary zircons (40 Ma) may have been supplied by metamorphic and mig- matitic rocks affected by an Eocene high-temperature phase. A Late Cretaceous age cluster ( ; 70-60 Ma) is related to cooling after the main Austroalpine metamorphic event at 110-100 Ma. Most of the recently exposed Austroalpine nappe complex displays mica cooling ages and zircon FT ages between 95-70 Ma and 99-55 Ma, respectively. Finally, an ill-defined Jurassic age cluster, with a mean in Late Ju- rassic times, is related to rift-shoulder heating of the Austroalpine/ South-Alpine crystalline basement due to rifting of the Pennine oceanic domain. Presently, the Silvretta nappe complex, situated at the western termination of the Austroalpine realm, and the South-Alpine basement west of the Canavese Line, display similar zircon FT ages. Therefore, a westward continuation of the Silvretta complex prior to deep Neo- gene erosion is suggested.