Interpretation of the results of microtectonic measurements performed with respect to mesozoic formations of the Villány Hills, Hungary

Abstract

In the framework of OTKA project, a complex scientific investigation is being carried out with the aim of preparing
a comprehensive palaeogeographical reconstruction of the Tisza structural unit. I have joined this investigation with
microtectonical measurements performed in the area of the Villány Hills. The task I have set myself is to establish the
deliminitation the phases of structural transformation, and determine their relative and numeric ages. It has been an
important issue to clarify, how far the events of structural transformation postulated by me, can be integrated into the
regional geological processes. I have analysed the tectonical conditions of the area at issue relying upon some
stereographical projections, figures and models constructed by means of various structure-geological softwares. The
study demonstrates — in addition to describing the structural elements of the 7 quarries investigated — the related stress
fields. I have recorded, in seven quarries in the Villány Hills, the data of over 600 striations, 500 lithoclases, various
plastic deformation elements (folds, flexures), as well as stylolites. Based on the data recorded, I have tried to determine
the structural, palaeotectonic conditions of the individual quarries. To summarize the first part of the investigation it can
be stated that most of the quarries investigated in the Villány Hills show a relatively uniform, compression-transpression
picture from the point of view of tectonic styles. Tensor fields of tension character have been encountered in the
Szobrosbánya, at Beremend, and these stress fields can be releated to the opening of the pull-apart basins in the early
Miocene. The structural elements, folds, lythoclases investigated, adjust themselves well to the stress fields of the region.
Based on the measurements 6 structural phases were delimitated in this region. The Villanyian Imbrications and the high
amplitude folds were formed by the Late Cretaceous compressional stress field. It was followed by two tensional fields
until the Late Miocene, which was resulted in the opening of pull-apart basins. The inversion in the Villány Hills started
in the Late Miocene. First stress field was N–S transpressional, that was followed by a NE–SW transpressional one and
finally an E–W compressional one. The reactivation of the imbrications and main faults, the opening of the
transpressional basins were generated by these shortening deformations. The E–W compressional field could be
observed only in the Harsány-hegy quarry. This stress field was evolved by the result of a local block rotation or the
rotation of the Villanyian imbrications. This latter statement can be emphasized by the stress fields rotation from the Late
Miocene, which began N–S shortening in the Late Miocene, NE–SW shortening in the Pliocene and finally E–W
shortening in also Pliocene.