Towards a mechanical model for the formation of the Pannonian basin

Abstract

New maps showing crustal and lithospheric thickness variations in the Pannonian basin and the surrounding East Alpine, Carpathian and Dinaric mountains have been prepared on the basis of recent seismic, seismologic and electromagnetic data. A map presenting Miocene faults of regional significance has been also constructed for the same region by using a wealth of recently available national and/or more local studies. It is suggested that observed crustal and lithospheric structural contrasts have been controlled primarily by Neogene kinematic history of the region. Kinematic history is characterized by the following main patterns: 1. (1) indentation by Adria and large-scale backthrusting in the Southern Alps; 2. (2) eastward escape and extension of the Eastern Alps; 3. (3) extensional collapse in the Pannonian basin area; 4. (4) formation of broad wrench fault systems along strike in the Dinarides and Southern Carpathians, and the Western Carpathians with dextral and sinistral shear, respectively; 5. (5) compression and accretion of the external thrust and fold belt in the Eastern Carpathians. These kinematic patterns are thought to be interrelated and all are manifestations of the late-stage evolution of an overthickened orogenic wedge. In order to arrive at a better understanding of the mechanism of extension, which formed the Pannonian basin, deep crustal seismic profiles, hydrocarbon exploration reflection lines and borehole data have been analyzed in the Little Hungarian Plain. This plain represents the transition zone between the Alps and the Pannonian lowlands, and the results are illustrated by eight interpreted cross-sections. One result of regional importance is the clear recognition that the Transdanubian Central Range at the southeastern flank of the Little Hungarian Plain is composed of Alpine (pre-Senonian) thrust sheets. This finding marks the end of a century of debate: the allochthony of the substrata of the Pannonian basin can be now considered proven. Another result of more general interest is that these cross-sections document the mode of lithospheric extension. Preexisting compressional detachment planes reactivate as low-angle normal faults and lead to tectonic unroofing of deeply buried metamorphic terranes characterized by ductile flow along subhorizontal lineation.