Extensional structures in anisotropic rocks

Abstract

Ab~'aet-A distinct class of structures can form as a result of extension along a plane of anisotropy (foliation). The effect of the foliation is to decrease the ductility of the material in this orientation so that brittle fractures or shearbands develop. Foliation boudinage is caused by brittle failure; extensional fractures cause symmetric boudinage, and shear fractures cause asymmetric boudinage. Extensional crenulation cleavage is defined by sets of small-scale ductile shear-bands along the limbs of very open microfolds in the foliation. The sense of movement on the shear-bands is such as to cause a component of extension along the older foliation. Conjugate cleavage sets indicate coaxial shortening normal to the foliation; the shortening axis bisects the obtuse angle between the sets. A single set indicates oblique or non-coaxial deformation. Extensional crenulation cleavage is microstructurally and genetically distinct from other types ofcleavage. It does not occur as an axial plane structure in folds, and has no fixed relationship to the finite strain axes. It is common in mylonite zones, and may be favoured by crystal-plastic and cataclastic deformational mechanisms. These cause grain-size reduction, and hence softening, which favour the development of shear-bands.

Mechanisms for folding of high-grade rocks in extensional tectonic settings

“…9c). Foliations may be dragged in either normal or reverse senses at different positions along the same structure during asymmetrical foliation boudinage (Platt and Vissers, 1980), as illustrated in Fig. 9d.…”

“…Where the ductile layers are less dense than the overlying semibrittle layers, the antiform in ductile layers is tighter, and no rollover of semi-brittle layers onto the fault has Figures (a) and (e) modified after Lliboutry (1982, Fig. 12.10), (b) and (f) modified after Dennis and Secor (1990) and (c -d) modified after Platt and Vissers (1980). occurred (Fig.…”

“…9c). In extensional crenulation cleavages described by Platt and Vissers (1980), deformed layers tend to undergo an average of about 17j back-rotation and remain planar except for their deflection (''drag'') into extensional shear zones. Complex folds may, however, develop with larger amounts of back-rotation.…”

Ductile and brittle extension in the southern Lofoten Archipelago, North Norway; implications for differences in tectonic style along an ancient collisional margin

“…Rare tops-west shear zones occur higher in the nappe stack, where they were observed to truncate F 3 folds; hence the late-phase shear zones are interpreted as D 4 structures. The east-northeasttrending, northwest-dipping shear zones are defined by an S 4 extensional crenulation cleavage (Platt and Vissers, 1980) that is subparallel to S 2 but generally has a steeper dip ( fig. 6A).…”

Crystal plasticity and superplasticity in quartzite; A natural example

“…), in the Sierra Alhamilla, southern Spain (Platt and Vissers, 1980;Platt et al, 1984;Platt and Behrmann, in press). This is a major intracrustal shear zone within the Betic Zone, the internal metamorphic segment of the Betic Orogen of Alpine age (Egeler and Stmon, 1969;Torres-Roldan, 1979).…”