Evolution of the Pannonian Basin System: 1. Tectonics

Abstract

The Carpathian arc formed during Cretaceous to Miocene time by continental collision between Europe and smaller continental fragments following southward and westward subduction of ocean floor. During the last stages of thrusting in the outer Carpathians, a set of discrete basins formed behind the Carpathian loop. These basins are regions of middle to late Miocene extension and are connected to each other and to areas of coeval shortening in the outer Carpathian thrust belt by a system of strike‐slip faults. Sinistral northeast trending and dextral northwest trending sets of conjugate shears reflect overall east‐west extension of the intra‐Carpathian region during middle to late Miocene time. Palinspastic reconstruction of the basins indicates about 100 (±50) km of east‐west extension across the intra‐Carpathian region during this time. We interpret the Vienna basin, which is superimposed partly on the flysch nappes of the outer West Carpathians, as the result of thin‐skinned extensional tectonics above a shallow detachment surface within the crust. Extension of upper crustal rocks above this detachment was probably accomodated by thrusting of the outermost nappes of the West Carpathians over the European platform. Extension of basins farther inside the Carpathian loop probably involved rocks at deeper crustal levels and within the upper mantle. The most internally situated basins formed by extensional processes that involved the entire lithosphere. Thus the depth to which extension occurred seems to be closely related to the proximity of each basin to the active thrust front in the Carpathians. This can be related to the geometry of the subduction zone. Extension within the intra‐Carpathian basins began first in the northernmost basins and migrated southward and eastward over a ten million year period. Palinspastic reconstruction of the basins at three different times during the Miocene (17.5, 16.5 and 13.0 Ma) predicts eastward migration of the zone of thrusting and shortening within the Carpathian mountains to accomodate the synchronous eastward and southward migration of areas of extension. From these reconstructions, about 120 (±60) km of east‐west shortening is predicted across the outer East Carpathians from about 17.5 Ma until present. Both predictions are in good agreement with the timing and magnitude of shortening in the Carpathian thrust belt as determined directly from rocks within the belt. From this we infer a close relationship for middle to late Miocene extension of the intra‐Carpathian region and middle to late Miocene crustal shortening within the outer Carpathians. During middle and late Miocene time, deformation of the Pannonian fragment was dominated by extensional and wrench tectonics, except along a narrow zone in the outer Carpathian thrust belt. Middle and late Miocene thrusting in the Carpathians is interpreted as the result of simple shear caused by relative motion between Europe and the overriding Pannonian fragment at the subduction boundary, rather than of regional compression of lithospheric rocks at depth. The initiation of extensional tectonics within the intra‐Carpathian region in middle Miocene time can be related to reorganization of major fragment boundaries of the Eastern European Alpine system.