Joachim Kuhlemann

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.

Regional synthesis of Mediterranean atmospheric circulation during the Last Glacial Maximum.

Atmospheric circulation leaves few direct traces in the geological record, making reconstructions of this crucial element of the climate system inherently difficult. We produced a regional Mediterranean synthesis of paleo-proxy data from the sea surface to alpine altitudes. This provides a detailed observational context for change in the three-dimensional structure of atmospheric circulation between the Last Glacial Maximum (LGM, ∼23,000 to 19,000 years ago) and the present. The synthesis reveals evidence for frequent cold polar air incursions, topographically channeled into the northwestern Mediterranean. Anomalously steep vertical temperature gradients in the central Mediterranean imply local convective precipitation. We find the LGM patterns to be analogous, though amplified, to previously reconstructed phases of enhanced meridional winter circulation during the Maunder Minimum (the Little Ice Age).

Post-Eocene evolution of the North Alpine Foreland Basin and its response to Alpine tectonics

Abstract The post-flysch (Oligocene–Miocene) palaeogeographic evolution of the entire North Alpine Foreland Basin (NAFB) between Savoy (France) and Lower Austria is presented in eight sketch maps. The compilation considers the palinspastic evolution of the Alps. It includes intramontane deposits, which represent a continuous marginal facies of the NAFB during Rupelian to Early Burdigalian times. The facies distribution in the NAFB was driven by two major types of processes, which are related to the tectonic evolution of the Alpine orogen. The first type, representing tectonic processes at the thrust front of the Alps, directly influenced the facies distribution of the narrowing NAFB. The second type represents an indirect impact of Alpine uplift and tectonics to the NAFB, transformed by varying sediment discharge. A strong increase in sediment discharge due to uplift of the Alps is the major reason for the generally regressive coarsening- and shallowing-upward cycle from the Lower Marine to the Lower Freshwater Molasse (UMM, USM) between 33 and 21 Ma. The development of the “Burdigalian Seaway” at ∼20 Ma was coeval with a reduction of thrust advance rates in the western and central part of the NAFB. Shallow marine conditions (Upper Marine Molasse, OMM) prevailed for ∼3 million years. In the Eastern Alps, reduction of relief due to lateral (east–west-oriented) extension caused a strong reduction in sediment discharge. Closure of the “Burdigalian Seaway” around 17 Ma occurred during a phase of tectonic reorganisation in the Alpine orogen and is coeval with a short-term increase in sediment discharge. Between 17 and 12 Ma, the NAFB was constantly overfilled (Upper Freshwater Molasse, OSM), despite strongly decreasing sediment discharge. Termination of sedimentation in the (unfolded) NAFB occurred diachronously in an undramatic process. It started in the western NAFB in the course of uplift of the Swiss Jura Mountains after 11 Ma and reached Lower Austria around 6–5 Ma. Strong uplift of the Alps and the NAFB started at around 6 Ma in the Swiss and Western Alps and at 4–3 Ma in the Eastern Alps. The uplift was followed by reworking and erosion of more than 2 km of Molasse sediments in the western NAFB.

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.

Toward a comprehensive provenance analysis: A multi-method approach and its implications for the evolution of the Central Alps

In this study, we discuss potential problems connected with using geochronological data from foreland basins to unravel exhumation histories of the hinterland. In particular, we compare the results of a provenance analysis solely based on zircon fission-track ages from the foreland basin with a multi-method approach based on (i) the aforementioned zircon fission-track data, (ii) Nd isotope ratios of detrital epidote, and (iii) sediment accumulation rates in the foreland basins. For the example of the Central European Alps, we demonstrate that the multi-method approach can lead to highly different interpretations in terms of hinterland exhumation and geodynamic evolution. This is due to the fact that fission-track dating on detrital zircons alone only monitors the exhumation and erosion of zircon-containing lithologies and therefore only of restricted areas of the hinterland while the combination with Nd isotope ratios on detrital epidote also includes the erosion of zircon-free or -poor units such as basic magmatic rocks. A comparison of zircon fission-track and epidote Nd data with the sediment accumulation curve shows whether hinterland exhumation was predominantly caused by tectonic or by erosional denudation. Furthermore, we discuss some problems that may arise from using geochronological data from foreland basins to assess the maturity of a mountain belt in the hinterland. Applied to the Central Alps, our combined approach shows that the metamorphic core became exposed simultaneously over large areas by one sudden pulse of exhumation between 21 and 20 Ma. The main trigger for that exhumation event was tectonic denudation which is consistent with a geodynamic setting of large-scale extension. The Central Alps did not achieve exhumational steady-state conditions before 14 Ma.

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.

Miocene siliciclastic deposits of Naxos Island: Geodynamic and environmental implications for the evolution of the southern Aegean Sea (Greece)

An interdisciplinary study has been carried out on Naxos Island, located in the southern Aegean Sea (Greece), which shows Miocene geodynamic and environmental changes in a classic example of a collapsing orogen. Early to Mid-Miocene siliciclastic deposits on Naxos have been shed from an uplifting mountainous realm in the south, which included a patchwork of at least four source terrains of different thermal histories.Petrography of pebbles suggests that the source units formed part of a passivecontinental margin succession (external Pelagonian unit), and an ophiolite succession mainly of deep-water cherts and limestones deposited on basalt substratum (Pindos unit). The continental margin source contributed rounded zircon crystals of Late Jurassic to Early Cretaceous age and broadly scattering Paleozoic zircon fission-track cooling ages. A distal pebble assemblage of Paleogene shallow-water carbonates passing into flysch-like, mixed calcarenitic and siliciclastic components with volcanic arc components is subordinately present. High-grade metamorphic components from the nearby metamorphic core complex are not present. The depositional evolution reflects increasing relief and, in some parts, a fluvial succession with rhythmic channel deposition, possibly due to runoff variability forced by orbital cyclicity. Upsection, the depositional trend indicates increasing seasonality and decreasing humidity in the source region. The Miocene sedimentary succession has been deposited on an ophiolite nappe. Juxtaposition of this ophiolite nappe occurred as an extensional allochthon during large-scale extension in the Aegean region at the margins of an exhuming metamorphic core complex.

Survival of ancient landforms in a collisional setting as revealed by combined fission track and (U-Th)/He thermochronometry: A case study from Corsica (France)

The age of high-elevation planation surfaces in Corsica is constrained using new apatite (U-Th)/He data, field observations, and published work (zircon fission track, apatite fission track [AFT] data and landform/stratigraphical analysis). Thermal modeling results based on AFT and (U-Th)/He data, and the Eocene sediments uncomformably overlapping the Variscan crystalline basement indicate that present-day elevated planation surfaces in Corsica are the remnants of an erosion surface formed on the basement between ∼120 and ∼60 Ma. During the Alpine collision in the Paleocene-Eocene, the Variscan crystalline basement was buried beneath a westward-thinning wedge of flysch, and the eastern portion was overridden by the Alpine nappes. Resetting of the apatite fission track thermochronometer suggests an overburden thickness of >4 km covering Variscan Corsica. Protected by soft sediment, the planation surface was preserved. In the latest Oligocene to Miocene times, the surface was re-exposed and offset by reactivated faults, with individual basement blocks differentially uplifted in several phases to elevations of, in some cases, >2 km. Currently the planation surface remnants occur at different altitudes and with variable tilt. This Corsican example demonstrates that under favorable conditions, paleolandforms typical of tectonically inactive areas can survive in tectonically active settings such as at collisional plate margins. The results of some samples also reveal some discrepancies in thermal histories modeled from combined AFT and (U-Th)/He data. In some cases, models could not find a cooling path that fit both data sets, while in other instances, the modeled cooling paths suggest isothermal holding at temperature levels just below the apatite partial annealing zone followed by final late Neogene cooling. This result appears to be an artifact of the modeling algorithm as it is in conflict with independent geological constraints. Caution should be used when cross-validating the AFT and (U-Th)/He systems both in the case extremely old terrains and in the case of rocks with a relatively simple, young cooling history.