The Vienna Basin

Abstract The Vienna Basin was formed by middle Miocene (Karpatian-Badenian, 17.5-13.0 Ma) extension and contains up to 6 km of Miocene to Quaternary sedimentary rocks. This basin is partly superimposed on the north-vergent nappes of the outer West Carpathian flysch belt and partly on the nappes of the inner Carpathian belt. The obvious rhombohedral shape of the Vienna Basin, the left-stepping pattern of en echelon faults within the basin, and the southward migration of basin extension through time strongly suggest that this basin is a pull-apart feature formed during middle Miocene left slip along a northeast-trending fault system. This interpretation is supported by geologic mapping in the Carpathians. These left-slip faults appear to have functioned mainly as tear faults within the Carpathian nappes and separated the areas of active north-vergent thrusting east of the Vienna Basin from areas west of the basin where thrusting had already been completed. Reflection seismic lines show that the autochthonous European-plate basement continues beneath the allochthonous Carpathian nappes and beneath the Vienna Basin, and that in general the European plate is not significantly disrupted by the normal faults that bound the basin. Thus both the normal faults and the associated strike-slip faults appear to merge into a gently southeast-dipping detachment at depth. In this way, extension of the Vienna Basin appears to have been restricted mainly to shallow crustal levels above that detachment (that is, restricted mainly to the allochthonous nappes of the Carpathians). Detailed analyses of subsidence and heat-flow data indicate that little or no heating of the lithosphere occurred during extension of the Vienna Basin, and support the interpretation that extension was confined to shallow crustal levels. This interpretation explains why hydrocarbons mature at much greater depths in the Vienna Basin (a5 km) than in the neighboring Pannonian Basin.