Journal olStrucmral Geology. Vol. 6 No. 3, pp. 273 to 281, 1984 0191-8141184 $03.01)+ (I.111) Printed in Great Britain © 1984 Pergamon Press Lid. The generation of ductile and brittle shear bands in a low-angle myionite zone CEES W. PASSCHIER Department of Geology, University College of Swansea, Swansea, SA2 8PP, U.K (Received 27 January 1983; accepted in revised form 15 September 1983) Abstract--In the Saint-Barth61emy Massif, French Pyrenees, a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies. The last major structures formed in the zone are isolated shear bands, divided into three types. Anastomosing, inhomogenous ultramylonitic shear bands (Type I) are subparallel to the mylonitic foliation in the gneiss (Sg). Most of these bands developed by ductile deformation processes only. Planar, homogeneous ultramylonite bands (Type II) are usually oblique to Sg. They generated as pseudo- tachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation. Type III shear bands are planar and oblique to Sg. They consist of pseudotachylyte, weakly affected by ductile deformation. Type 1, I1 and I11 bands seem to represent progressively younger structures on a local scale, linked to falling P-T conditions. The systematic variation in orientation of the different shear bands with respect to Sg is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy, INTRODUCTION THE OCCURRENCE of late zones of concentrated shear strain in domains of non-coaxial ductile deformation has been a subject of interest for a number of years (White 1979, Platt & Vissers 1980, White et al. 1980, Passchier 1982a). Microstructurally these zones, called shear bands, show evidence of both brittle and ductile defor- mation processes (Platt & Vissers 1980). The bands usually form a crenulation and have a consistent orienta- tion with respect to major foliation planes of the rock in which they were formed; they seem to be reliable mar- kers for sense of shear (Platt & Vissers 1980, White et al. 1980). Shear bands are thought to be the result of hardening in a mylonite zone (White et al. 1980). This study treats the spatial relationships of several types of major shear bands from a mylonite zone in the Pyrenees. The bands are characterized by wide spacing, strong localization of shear strain and a change of domin- ant deformation mechanism with age. They represent the final stages of ductile deformation in a major mylo- nite zone, before it enters the field of dominant brittle fracturing. The microstructural characteristics of these shear bands were treated in an earlier paper (Passchier 1982b). REGIONAL GEOLOGY The Saint-Barth61emy Massif is one of the Northern Pyrenean Massifs in France (Fig. 1). It consists of a gneiss core covered by sediments of Palaeozoic age. The gneiss core can be subdivided into a basal unit, probably of Precambrian age, overlain by intrusive and migmatitic gneisses of Variscan age (Zwart 1954, Guchereau 1975, Zwart 1979, Passchier 1982a). A main structural event of ductile thrusting caused intense deformation in the gneiss (Passchier 1982a). The l [ mesozoic • • non metamorl~iC ~*., "~ I | migmahhc gneiss "~% - • I d,f0rmatlon l:'>/~,drfrh ki,,'Tl I I I I I I~4 m main myLonite band r--/~,r,.." _.~c_.'_.:~._~llq.~ i i I~ Ill - ~ ~ ~"\\~'~11 I t 1 ?"- li ~ ] II ~ ~ "~"".,LII I) ,. , • * * * * ~ llllol~illOu Fig. I. Simplified map and profile of the Saint Barth61emy Massif based on the maps of Passchier (1982a) and Zwart (1954). Dots--shear band localities. 273