Tertiary brittle faulting and stress field evolution in the Gerecse Mountains, northern Hungary

Abstract

The Gerecse Mountains of the Transdanubian Range in northern Hungary have suffered polyphase deformation during the Tertiary. The evolution of the stress fields was determined by the detailed structural survey of the area: the statistical analysis of microtectonic observations, the mapping and reinterpretation of fault pattern and the study of basin stratigraphy and geometry. The stress fields show an apparent clockwise rotation from Eocene until Quaternary times. Three main phases were reconstructed. During the Eocene-Early Oligocene E-W- to ENE-WSW-oriented compression and perpendicular tension affected the area. The formation and evolution of several sedimentary basins were controlled by fault systems activated by this phase. After a period of intensive erosion in the Early Oligocene the stress field rotated approx, 30–40°: NW-SE-directed compression and NE-SW-trending tension existed during the Late Oligocene and Early Miocene. After correction with palaeomagnetic data, an original Palaeogene σ 1 orientation is deduced NNE-SSW, reflecting the collisional processes of Europe and the Apulian microplate. The Middle Miocene to Pliocene period was characterised mainly by extensional deformation: the direction of the minimal stress axis ( σ 3) varied between E-W and NW-SE. The characteristic clockwise rotation pattern of the Tertiary palaeostress fields in the Gerecse Mts. is explained by the combined effect of the regional stress field changes in the Carpatho-Pannonian region and the rotation of the North Pannonian Unit.