Dyke emplacement and its interaction with fracture systems and regional stress fields: Combination of a field study and geochronology in Cserhát Hills, Hungary

Widespread andesitic volcanism with several eruption centres occurred during the Middle Miocene in the Cserhát Hills, central – northern Hungary. In this time, an extensive dyke system developed in the area, where some dykes have exposed maximum length of 23 km, and maximum width of 25 m. This dyke system shows a change in its strike from E–W to NNW–SSE. Here we integrate new and previous field observations to derive structural maps and study dykes and fractures in the Cserhát Hills. K/Ar geochronology is used to understand the temporal evolution of regional fault patterns before, during and after the formation of the dykes and also to gain insights into the interaction between the dyke emplacement and the regional stress field. Fault-slip data were collected at 27 different sites along the dykes and were combined with reinterpreted datasets from 16 sites located at a distance from dykes. The field observations, integrated with the geochronological data sets suggests that dykes with different orientations were emplaced in two different eruptive cycles around 15.4 and 14.7 Ma. The deformation history of the Pannonian Basin involved a clockwise change in regional minimal stress axis, probably as a result of regional vertical–axis counter-clockwise block rotation. Our field observations suggest strike-slip stress regime may occur near propagating dyke tips, and the direction of minimal stress axis may have locally rotated counter-clockwise where dykes changed their strikes and emplaced along pre-existing fractures, mostly normal faults.