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Joint and Fracture Patterns in Basement and Sedimentary Rocks in the Sudetes Mountains
An attempt was made to characterize faults and joints occurring in the Sudetes in three rock complexes displaying different geological histories and chosen especially for this purpose. They are as follows: (1) the region of highly folded metamorphic rocks occurring in the Klodzko-Orlica dome and in the Kaczawa Mountains, (2) the region of the great plutonic massif of the Karkonosze Mountains, (3) the platform rocks of Cretaceous age occurring in the Sudetes and in the Sudetic fore-land. All the areas are cut by faults and joints arranged in sets or systems which are frequently orthogonal, or nearly orthogonal. The development of joint patterns in each of the areas was, however, completely different and highly complicated.
In the region of folded metamorphic rocks (the Klodzko-Orlica dome), the development of joint and fracture systems at first was due to a mechanism of regional kinking and was intimately associated with the development of late brittle fold structures – kink bands and joint drags. Then a conjugate system was developed, and the planes belonging to the two complementary sets cut each other along approximately the lines (vector σ2) steadily steepened and finally assumed a nearly vertical position enabling a system of steep, approximately orthogonal fractures to develop. Such structural foundations in the course of subsequent differential uplifting during the Saxonian orogeny influenced the development of steep joints and faults affecting both the metamorphic basement and Cretaceous cover.
The region of the plutonic massif of the Karkonosze granite is cut by a joint system composed of two differently oriented joint sets. The sets cut each other at fairly variable angles, sometimes perpendicularly. According to Mierzejewski (1973), this angular variability is mainly due to the differential competence of the deformed material as well as to the deviation of essentially horizontal principal compressive stresses. This deviation in stress vectors resulted from the arcuate trend of fold structures existing in the country rocks of the massif.
The system of steep joints recorded in rocks of the Cretaceous platform cover is always roughly orthogonal, and appears to be the most constant one of all joint and fracture systems described in other Sudetic regions. Jerzykiewicz (1968) claims that those systematic joints are due to relaxation of stresses which came to exist in the Cretaceous rocks during their lithification. The stress relaxation is initiated and controlled by planetary strain which may result from continental drift.
When joints in Sudetes are examined, starting from the folded basement through the late-kinematic Karkonosze granite up to the platform cover, one can notice that joint and fracture patterns get steadily more and more simple. It may be expected that, when in the Cretaceous period, the basement of the Sudetes was cooled and became highly brittle, it displayed a uniform reaction and resistance to orogenic stresses. Under such conditions regular joint systems were developed. The most stable systems are encountered in the rocks of the sedimentary cover.
Evidence of large vertical displacements of the Cretaceous rocks is commonly found in the Sudetes. Proof of roughly horizontal displacements is rarely recorded. Post-Cretaceous movements took place at least twice. They are referred to the so-called Saxonian orogeny recognized in rocks of the Mesozoic cover in central and northwestern Europe.
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