Crystallographic orientation of microcracks in quartz and inferred deformation processes: a study on gneisses from the German Continental Deep Drilling Project (KTB)

This study was carried out on four gneiss samples from the German Continent al Deep Drilling Project (KTB) taken at depths between 556 and 8633 m. The crystallographic orientation of microcracks in quartz was determined by a c ombination of the electron channelling pattern method (ECP) and U-stage mic roscopy. The distinct preferred crystallographic orientations of various cr ack generations point to different processes of crack initiation and propag ation which partly seem to be depth-dependent. For older healed cracks, a c rystalplastic initiation due to dislocation pile-up and related lattice dis tortion is indicated by cracking normal to prominent slip directions, which are (a) at lower or [c] at comparatively higher temperatures. In contrast, younger open cracks preferentially formed on crystallographic planes with low surface energy (rhombs and prisms), which is interpreted in terms of pu re elastic crack mechanisms. Because of the thermoelastic anisotropy of qua rtz, internal stresses resulting from thermal contraction during cooling ma y be the main driving force for the initiation or further propagation of cr acks parallel to the c-axis. Crack propagation preferentially affects grain s with crystal lattices and operating crack mechanisms being in a direction appropriate to the applied external (tectonic) stresses. This selective cr acking explains why cracks also show constant orientations with respect to geographic directions up to regional dimensions. (C) 1999 Elsevier Science B.V. All rights reserved.