Shortening-related deformation in the Gerecse Mts, Transdanubian Range, Hungary

Abstract

The aim of the present work is to study and analyse the folds in the Gerecse Mountains, Transdanubian Range,
Hungary. The first task involved clarification of details about the kinematics of the folding — i.e. bedding-parallel striaes,
duplex structures, inverse faulted beds and inverse striaes suggest a compressional mechanism. Three sets of folds and
also fold axes can be distinguished: these are in E–W, NE–SW and SE–NW shortening directions. The presence of the
first set of folds is frequent; the other ones are less significant.
The main objective of the general analysis was to explain the dominance of the E–W compressional mechanism
which has resulted in a set of folds with N–S trending fold axes. This explanation provided the key to understanding the
age of the folding and, after discussion, it was dated to the Cretaceous age.
The most abundant E–W compressional direction shows a discrepancy from the general structural trend of the
Transdanubian Range. The first interpretation indicated a classic “structural bending” model of the Transdanubian
Range after BALLA and DUDKO (1989). The published palaeomagnetic data suggest the structural unity of the whole
Transdanubian Range from the Triassic to the present time. The aberration in structural directions can also be explained
by the inhomogeneity of the stress field; this can be modelled with the linear combination of the well-described NW–SE
compression and a slip stress field (although the latter is only hypothetical).
However, the most important explanation is the well-known flexural deformation model which also makes it possible
to estimate the timing of the deformation. Taking into consideration other observations and the model of MINDSZENTY et
al. (1994). and TARI (1994), the first Aptian structural event took place with W(SW)–E(NE) compressional directions;
this is well-documented. The last (i.e.third) structural event was characterised by N(NW)–S(SE) shortening; its age can
be given as Albian. The rotation of the compressional direction — which is in good agreement with other observations
— can be supposed.
The age of the three sets of folds (with NW–SE, N–S and NE–SW trending fold axes) can be estimated as Early
Aptian, Aptian–Albian and Albian, respectively. Taking into consideration the results of clay mineral, complex
biostratigraphical and rheological observations in the region, the depth of the deformation can be estimated at 2-3
kilometres. The precise interpretation of the burial process requires further exploration.