SC structure
The SC structure is a tectonic structural element that is created by the deformation of crystalline rocks, especially granitoids. It is found mainly in transpressive, ductile shear zones .
etymology
The abbreviation SC is from the French words schistosité ( foliation ) for the letter S and cisaillement ( shear ) for the letter C have been derived.
description
The SC structure, also C / S structure, English S-C structure or SC fabrics , is defined by the two main structural elements of a shear zone: the S surfaces, which represent the XY surfaces of the finite deformation ellipsoid and thus inclined foliation planes , and the C- Areas that run parallel to the edge of the shear zone and absorb very high deformations due to concentrated shear. The S-surfaces are originally at an angle of 45 ° to the edge of the shear zone. As the deformation progresses, however, they rotate progressively into the shear zone (in the direction of the structural attraction), so that the included angle of the two structural elements is now usually well below 45 °. At the contact with the shear zone edge, the S-surfaces show drag, so that they finally run parallel to the edge and form a sigmoidal structure .
Occurrence
SC structures arise in only moderately foiled, semi-ductile mylonites with a low mica content . They occur most frequently in moderate shear zones, but above all in tectonically stressed granitoids in which C-surfaces are surrounded by feldspar porphyroclasts . These C-type shear bands may originate right next to the feldspars - at points of high differential tension, from where they pull through the rock like a network in order to unite with other shear bands.
Emergence
SC structures should represent an expression of inhomogeneous simple shear (English inhomogeneous simple shear ). In contrast to shear bands of the C'-type (English shear band cleavage or sbc for short ), they probably arise at the very beginning of the mylonitization. The oblique foliation in the microlithon areas increases even further as the shear bands continue to grow. If SC structures are stretched , they can be overprinted by shear bands of the C 'type. In the process, inclined C 'shear surfaces are formed which intersect the S and C surfaces at a flat angle. The 'C' areas usually provide micro deportations ( Abschiebungskrenulation or so-called ECC structure , English extensional crenulation cleavage or abbreviated ecc ), and are good shear sense indicators.
Individual evidence
- ↑ D. Berthé, P. Choukroune and P. Jegouzo: Orthogneiss, mylonite and non-coaxial deformation of granites: the example of the South Armorican shear zone . In: Journal of Structural Geology . tape 1 , 1979, p. 31-42 .
- ↑ S. Hanmer and CW Passchier: Shear sense indicators: a review . In: Geol. Surv. Can. Pap. Tape 90 , 1991, p. 1-71 .