Cataclastic deformation of quartzite in the moine thrust zone

Abstract

Deformed Cambrian quartzites from the Moine thrust zone on Skye have been examined under cathodoluminescence in the Scanning Electron Microscope (SEM). A sequence of progressive deformation from intact quartzite through protobreccia to breccia and ultrabreccia can be established for samples from a large fold with a wavelength of 2 km, and a comparable cataclasite sequence exists for samples approaching the Ord thrust plane. The microstructural evolution involves the development of extension microcracks by impingement at grain contacts, followed by the formation of small shear faults by linking of the extension microcracks. Further strain is localized on large breccia zones; the proportion of fine-grained matrix increases and fluid flow is concentrated into the dilatant zones, depositing cement and iron oxides. The dominant deformation mechanism for both folding and faulting in this part of the Moine thrust zone is cataclasis. Extension microcracks and shear faults have close analogues in experimental deformation, but the formation of the protobreccia in the hinge of the fold is an example of cataclastic flow on a much larger scale than a laboratory specimen. The conditions for the initiation of the shear faults and breccia zones are suggested by the Rudnicki and Rice model of a dilatant material. Work-hardening must occur in both cases; possible mechanisms include changes in the dilatancy factor and bulk modulus, and syntectonic cementation.