The Neogene Fohnsdorf Basin: basin formation and basin inversion during lateral extrusion in the Eastern Alps (Austria)

Abstract

The evolution of the early/middle Miocene Fohnsdorf Basin has been studied using borehole data, reflection seismic lines, and vitrinite reflectance. The basin is located along the sinistral Mur-Mürz fault system and probably formed as an asymmetric pull-apart basin, which was subsequently modified by halfgraben tectonics, as a consequence of eastward lateral extrusion. Sedimentation started with the deposition of fluvio-deltaic sediments. Thick coal accumulated in the northwestern basin. Thereafter subsidence rates increased dramatically with the formation of a lake several hundred meters deep. The lake was filled mainly from the north with more than 1500 m of sediments showing a coarsening-upward trend due to southward prograding deltaic lobes. A sequence of more than 1000 m of boulder gravels (Blockschotter) in the southeastern part of the basin are interpreted as the upper part of a coarse-grained fan delta succession, which accumulated along a normal fault along the southern basin margin. Fan deltas reached the central basin only during the early stages of sedimentation and during the late stages of basin formation. Miocene heat flow was approximately 65±70 mW/m 2 , which is significantly lower than in other basins along the Mur-Mürz fault system. The present-day southwestern basin margin is a recent feature, which is related to transpression along the dextral Pöls-Lavanttal fault system. It is formed by reverse faults constituting the northeastern part of a flower structure. Miocene sediments in the Feeberg valley are preserved along its southwestern part. Uplift of the central part of the flower structure was at least 2.4 km. North± south compression resulted in the deformation of the basin fill, uplift of the E/W-trending basement ridge separating the Fohnsdorf and Seckau basins, and in the erosion of 1750 m of sediments along the northern basin margin.

Polyphase movement on the Lavanttal Fault Zone (Eastern Alps): reconciling the evidence from different geochronological indicators

“…Constraints are given by pull-apart basins filled with intramontane molasse deposits. Sedimentation within these intramontane basins started at *18 Ma (e.g., Sachsenhofer et al 2000).…”

CENOZOIC TECTONIC EVOLUTION OF THE EASTERN ALPS – A RECONSTRUCTION BASED ON 40AR/39AR WHITE MICA, ZIRCON AND APATITE FISSION TRACK, AND APATITE (U/Th)-He THERMOCHRONOLOGY

“…1, 2) (approx. 18-16 Ma) (e.g., Sachsenhofer et al, 2000;Pischinger et al, 2008). Vertical movement of Austroalpine units adjacent to these basins can not be resolved by thermochronological methods during this phase.…”

“…Time constraints on fault activity are mainly defined by the sedimentary deposits along these faults. Sedimentation within the intramontane molasse basins started around 18 Ma (e.g., Sachsenhofer et al, 2000). Another time constraint is given by the exhumation and cooling ages of Penninic and Subpenninic units in the Tauern Window, indicating that lateral extrusion started around 23.5 Ma (Frisch et al, 2000).…”

Numerical modelling of clast rotation during soft-sediment deformation: a case study in Miocene delta deposits

“…In the upper part (>600 m) of the Ingering Formation, a coarsening upward trend, resulting in the predominance of quartz-rich coarse sands and pebbles/gravel, is (Sachsenhofer et al 2000b(Sachsenhofer et al , 2003. This sandy sequence has been interpreted as part of a prograding delta frontfluvial delta plain depositional system with large-scale cross bedding (2-3 m sets) and coarse channel fills (Ho¨lzel and Wagreich 2004).…”