Tectonics and sedimentation in the Fohnsdorf-Seckau Basin (Miocene, Austria): from a pull-apart basin to a half-graben

The Miocene intramontane Fohnsdorf-Seckau Basin is situated at the junction of the sinistral Mur-Murz-fault system and the dextral Pols-Lavanttal faul t system. The basin comprises a 2,400-m-thick coal-bearing fluviodeltaic-la custrine succession (Lower to Middle Miocene, Upper Karpatian?/Lower Badeni an) which is overlain by a 1,000-m-thick alluvio-deltaic conglomeratic succ ession (Apfelberg Formation, ?Middle/Upper Badenian) in the south. A three- stage model for the basin evolution has been reconstructed from structural analysis and basin fill geometries. During a first pull-apart phase, subsid ence occurred along ENE-trending, sinistral strike-slip faults of the Mur-M urz fault system and NE-SW to N-S-trending normal faults, forming a composi te pull-apart basin between overstepping en-echelon strike-slip faults. The Seckau and Fohnsdorf sub-basins are considered as two adjacent pull-aparts which merged into one basin. During the second phase, N-S to NNW-SSE exten sion and normal faulting along the southern basin margin fault formed a hal f-graben, filled by wedge-shaped alluvial strata (Apfelberg Formation). Dur ing the third phase, after the end of basin sedimentation, the dextral Pols -Lavanttal fault system reshaped the western basin margin into a positive f lower structure.