Sekwencja deformacji skał metamorficznych z otworu wiertniczego Niedźwiedź IG 2

Abstract

 Wyróżniono struktury tektoniczne powstałe w czasie faz deformacji D1-D4. Pozwoliły one ustalić sekwencję deformacji w przewierconych amfibolitach i blastomylonitach. Charakterystyczny dla tych serii liletamorficznych był silny rozwój stref mylonitycznych i kataklastycznych, szczególnie w fazie D2. Faza deformacji D1 należy prawdopodobnie do fałdowań starobretońskich, natomiast młodsze fazy (D2-4 związane były z procesami kopułotwórczymi i/lub deformacjami synintruzyjnymi granitoidówżulowskich. SEQUENCES OF DEFORMATIONS OF METAMORPHIC ROCKS IN THE BOREHOLENIEDŹWIEDŹ IG 2 (LOWER SILESIA) The paper presents results of detail tectonic studies on core material from the borehole NiedźwiedźIG 2 (Fig. 1), situated between Ziębice and Paczków (Lower Silesia). The borehole penetrated amphibolites at depth of 1638.4 m to enter underlaying series of mylonitic-blastomylonitic rocks, in which it was ended at depth of 1694.0 m. At least four stages of deformations were identified (D1, D2, D3, and D4).The oldest identified tectonic structures originated in the phase D1. They include regional foliation Sl and occasionally preserved folds Fl. Two morphological varieties of foliation Sl of the schistosity type were differentiated in the amphibolite series (Fig. 2): homogeneous and inhomogeneous foliations, and three varieties in the series of mylonitic and blastomylonitic rocks (Fig. 3): foliation of the laminated schistosity type, foliation of the dispersed schistosity type, and foliation of the lenticular schistosity type.The bulk (90%) of values of angles of dip for foliation Sl fall within the range from 32° to 58°.Deviations from that range are innumerous and explainable as due to origin of younger fold structures (F3 ?), up to about a dozen meters in size. The deformation phase D1 was synmetamorphic, developing under conditions of amphibolite facies.The second phase of deformations (D2) resulted in folding of surfaces of foliation S1 in result of origin of numerous folds F2. The majority of these folds belong to the group of non-cylindrical, conical or sheath folds. With reference to 4 characteristic features (i.e. thickness of zone, mean size of diminution of grain in zone, development of new fabric, and relation of zone to folds F2), two groups of planar structures S2 were differentiated: ductile shear (S2r) and mylonitic (S2m) zones. The shear zones S2r + S2m pass into a planar structure, more and more penetrative at the expense of foliation S1 from the depth of 1416 m (but especially 1570 m). The deformation phase D2 took place under conditions of shearing stresses. The most intense deformations related to shearing are found in rocks underlaying the amphibolite series and at the base of that series. Fracturing and ductile shears of individual parts of the tectogene were accompanied by a process of recrystallization under conditions of epidote-amphibolite facies M2.The phase D3 resulted in origin of planar structures S3 of the type of delamination zones (S3r) and mylonitic-cataclastic zones (S3m), folds F3, and intersectional lineations L3i. Structures of phase D4 originated in rock domains markedly more rigid than in times of the proceeding phases. This is indicated by nature of cataclastic zones S4k, tectonic breccias S4b, boudinage structures L4b, and kink folds F4.Assuming that the above reconstructions of sequences of deformations and spatial orientation of tectonic structures are correct, 3 models of non-coaxial (rotational) history of deformations were outlined. The models were constructed making some simplifications and assumptions, e.g. neglecting changes in volume of the deformed rocks and major stress values, and inferring planar deformations in the course of individual deformation phases (X> Y = 1 > Z).The model 1, characterized by the smallest and constant (sinistral) rotations of deformation ellipsoid in the phases D2-4, is based on the hypothesis of tangential to subtangential nature of all the deformations. The model 2 assumes contribution of dome-forming phases in the phases D1-4, and the model 3 assumes the greatest and varying rotations of deformation ellipsoid and contribution of both tangential and dome-forming deformations, regarded as related to the Variscan orogenic cycle.