Oligocene?Miocene Tectonic Evolution of the Central Part of the Mid-Hungarian Shear Zone

A detailed tectonic evaluation of Oligocene and Miocene structures in the central part of the Mid-Hungarian Shear Zone in the Pannonian Basin, between Jászberény and Adony, was undertaken using industrial seismic data. Four partly overlapping 3D seismic surveys provided the base of the project, and were supplemented by 2D seismic sections. Seismic interpretation results were corroborated by section restoration.

The Mid-Hungarian Shear Zone, lying between the Balaton Line in the northwest and the Mid-Hungarian Line in the southeast, was a narrow northeast-southwest striking convergent fold and thrust belt in the Late Oligocene to Early Miocene. It was characterised by thin-skinned thrust ridges and synsedimentary synclines or ramp basins. The intensity of shortening increased southwestwards. We relate this to vertical axis rotational events within the shear zone. The thrust belt started to built up around Jászberény in the northeast. The Tóalmás and Monor areas (in the central part of the study area) were only mildly deformed, while the Bugyi and Adony areas (in the southwest) underwent intense shortening, uplift and erosion in several phases.

A first clockwise rotation occurred during the Egerian, and resulted in increased shortening of the southwestern parts, most likely even a thrust nappe in the Adony area. This was followed by a second clockwise rotation (most likely) in the Ottnangian.

Being in the convergence and translation zone between the oppositely rotating ALCAPA and Tisza terranes, the Mid-Hungarian Shear Zone is regarded a pure shear dominated transpressional orogen in the Late Oligocene to Early Miocene, with strain partitioning and internal rotations being important factors in the deformation history of the zone.

The Bugyi High is a Sarmatian?earliest Pannonian age sinistral pop-up structure superposed on an earlier thrust, related to a counterclockwise rotation event within the shear zone. During the Pannonian (or slightly earlier), sinistral strike-slip movements initiated in a wide zone, with localised deformation being mainly concentrated along the Tóalmás Zone. This resulted in the spindle-shaped Mende-Sülysáp pull-apart basin and related structures. The strike-slip zone, being essentially superposed on the Balaton Line, crosscut earlier thrusts and basins at acute angles, thus resulting in complex geometries.

The Adony Basin is an early Pannonian pull-apart basin of the Tóalmás Zone superposed on a Late Oligocene to Middle Miocene ramp basin. Strike-slip faulting was partly detached from the basement on local salt of Badenian age. The margins of the basin coincided with the fronts of earlier thrusts. In the basin, gravity sliding on the steep margins resulted in salt welds and detachment folds.