Microstructure, c-axis pattern, microstrain and kinematics of some S-C mylonites in grenville gneiss

Abstract

In a major tectonic zone late extension related SC mylonites locally overprint the predominant coarser quartz microstructures, which are related to earlier thrusting. Some of the S C mylonites display a microstructural evolution which began with the formation of deformation bands in the coarser pre-existing microstructure and continued with the formation of asymmetric quartz microfoliations, either by continued formation of deformation bands or fine new grains oblique to the deformation band boundaries. The orientation of boudinaged and passively reoriented rutile needles show that (i) the formation of deformation bands was preceded and accompanied by the accumulation of strain; (ii) that the deformation bands and oblique microfoliation which formed directly from them lie close to the finite stretching direction; whereas (iii) other microfoliations form oblique to deformation bands and extended rutile needles near the probable instantaneous stretching direction. The latter are therefore interpreted to be strain insensitive, steady-state foliations. The crystallographic preferred orientation of the original deformation bands appears to determine that of the microfoliations, the two types of microfoliations showing distinct but related patterns. The element common to both types is the presence of two maxima near Y in a YZ girdle—a feature inherited from the deformation bands, which were formed in the initial stages of shortening of the aggregate, favourably disposing it for rhomb slip and providing nucleation sites for subsequent recrystallization. The data confirm that, despite the fact that a variety of microstructures and crystallographic microfabrics result from recrystallization processes, kinematic information is usually recoverable from the crystallographic microfabrics owing to the primacy of intracrystalline slip processes.