Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Testa, Gibbons

Late orogenic structural control on the regional geometry of basement massifs: An example from the Jeseník Mountains, Czech Republic

Journal of the Czech Geological Society, volume 41 (1996), issue 3-4, 147 - 155

During a visit to the Jeseník Mountains in the summer of 1993, Radek Melka and Wes Gibbons became interested in anomalous steep zones along the eastern margin of the Desná Complex. At this time, the origin and significance of these steep zones remained unclear. It was their hope and intention to continue and develop this work during 1994. Sadly, due to Radek's untimely death, this was not to be so. The present authors returned to the area in the summer of 1994 to further the study of this problem. This paper presents their findings and discusses possible broader implications drawn from their conclusions. Variscan metamorphic rocks in the Jeseník Mountains, (NE Czech Republic) include a prominent asymmetric dome-like structure known as the Desná antiform. This regional fold exposes a core of pre-Devonian basement (Desná Complex) flanked by metasediments of probable Devonian age. Both basement and cover within the area are pervaded by a "top-to-southwest" main mylonitic fabric of probable Variscan age, which in the basement rocks transposes an earlier migmatitic foliation thought to be of Cadomian age. The main mylonitic fabric on the northwestern side of the antiform dips gently or moderately to the northwest and is parallel to the regional sheet dip. In contrast, within both the hinge region and southeastern limb of the antiform the main mylonitic fabric is folded about southeasterly verging folds with associated steep to moderate northwest dipping axial planar crenulation fabric. Local overturning of the steepened mylonitic fabric around these late folds reverses the apparent sense of mylonitic shear from sinistral (top-to-southwest) to dextral (top-to-northeast). The presence of near-vertical fold limbs along this southeast side of the Desná Complex has produced local steep zones which, like the domal geometry of the Desná Complex itself, are a product of the late southeasterly-directed compression. The outcrop pattern of the Desná Complex and its metasedimentary cover is therefore attributed neither to early antiformal nappe stacking nor core complex collapse, but to late orogenic cross folding.