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

The evolution of the early/middle Miocene Fohnsdorf Basin has been studied using borehole data, reflection seismic lines, and vitrinite reflectance. T he basin is located along the sinistral Mur-Murz fault system and probably formed as an asymmetric pull-apart basin, which was subsequently modified b y halfgraben tectonics, as a consequence of eastward lateral extrusion. Sed imentation started with the deposition of fluvio-deltaic sediments. Thick c oal accumulated in the northwestern basin. Thereafter subsidence rates incr eased 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 sedime nts showing a coarsening-upward trend due to southward prograding deltaic l obes. 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 c oarse-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 basi n formation. Miocene heat flow was approximately 65-70 mW/m(2), which is si gnificantly lower than in other basins along the Mur-Murz fault system. The present-day southwestern basin margin is a recent feature, which is relate d to transpression along the dextral Pols-Lavanttal fault system. It is for med by reverse faults constituting the northeastern part of a flower struct ure. Miocene sediments in the Feeberg valley are preserved along its southw estern part. Uplift of the central part of the flower structure was at leas t 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 an d Seckau basins, and in the erosion of 1750 m of sediments along the northe rn basin margin.