Style and mechanisms of the early phase deformation in the eastern part of the Bükk Mts. (NE Hungary)
In the Paleozoic and Mezozoic rocks of the Bükk Mountains the imprints of a polyphase deformation (eg. BALOGH 1964; CSONTOS 1999) and an anchi-epizonal metamorphism (eg. ÁRKAI 1973; DUNKL et al. 1994) were observed. According to former opinions confirmed by our investigations, the physical conditions of this metamorphism were the same as that of the earliest, ductile deformation phase. The imprints of later deformation phases can be distinguished in general by their tectonic style that includes brittle elements, too. However, neither ductile deformation nor metamorphism affects all rock bodies of the Bükk Mts. in the same degree.
On the eastern part of the mountains the most widely distributed, therefore the mostly comparable rock types are limestones. In these rocks, the texture elements that developed during the early phase deformation are preserved and dominant in most cases. Sedimentary texture elements are recognizable only in certain units. Our investigation was focused on the style of folding and on the different textural elements of limestones which developed during the early deformation phase. Observations were made on some hundreds of outcrops and on samples taken from a part of these.
According to the lithologic conditions and the geographical position, the folds show a certain style variety but most recognizable folds are class 2 multilayer folds (fold classification after RAMSAY 1967) and have a divergent fan cleavage. In many areas, in successions comprising of limestone, shale and/or chert layers, the more competent strata (chert vs. limestone and limestone vs. shale) show small-scale folding in hinge zones and boudinage on limbs of the large-scale folds. The fold geometry corresponds to the flexural shear model with pure shear in the hinges and simple shear on the limbs. The multilayer fold geometry depends on the thickness ratio and the competence contrast between the layers as shown in fig. 1. In other cases, the small-scale folding is absent and the deformation patterns correspond to the flexural slip model; this style can also characterize folds of later deformation events. There are such outcrops too, where the layers were folded passively in similar folds with sharp hinges, according to the passive shear model.
In a considerable part of the limestones no folds are observable on the outcrop scale due to the lack of bedding or other foliations predating the early phase deformation. However, other deformation structures, such as cleavage and lineations developed in them like in other bedded successions. So it seems very likely that they have been sheared strongly on limbs of large-scale folds, but these folds are not mappable because the later (mainly brittle) deformations have divided them up.
The occurrence of different textural patterns is influenced partly by the same factors as the macroscopic style. Additionally, the predeformational grain size plays an important role. The main deformation mechanism in the fine-grained matrix was the pressure solution which resulted in a shape preferred orientation. Conversely, the crystal aggregates of more than 20-30 mm grain size show signs of dynamical recrystallization which leads in some cases as far as to the development of milonitic texture and strong lattice preferred orientation. The intensity of the deformation depends also on the position inside a fold: it is relatively weak in the hinge zones and strong on the limbs.
The spatial style differences of the same rock types provide a base to define tectofacial units in the eastern part of the Bükk Mts. The borderlines of these units are zones of large-scale movements (strike-slip faulting) developed during later deformation phases.
Figure 1. Typical multilayer fold geometries with given thickness ratios and competence contrast. Dashed lines indicate cleavage
ÁRKAI P. 1973: Pumpellyte-prehnite-quartz facies Alpine metamorphism in the Middle Triassic volcanogenic-sedimentary sequences of the Bükk Mountains, NE Hungary. Acta Geologica 17/1-3, pp. 67-83.
BALOGH K. 1964: A Bükkhegység földtani képződményei, MÁFI Yearbook Vol. XLVIII. Issue 2.
CSONTOS L. 1999: A Bükk hegység szerkezetének főbb vonásai. Földtani Közlöny 129/4, pp. 611-651.
DUNKL I. - ÁRKAI P. - BALOGH KAD. - CSONTOS L. - NAGY G. 1994: A hőtörténet modellezése fission track adatok felhasználásával - a Bükk hegység kiemelkedéstörténete. Földtani Közlöny 124/1, pp. 1-24.
RAMSAY, J. G. 1967: Folding and Fracturing of Rocks. McGraw-Hill, New York