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 Continental Deep Drilling Project (KTB) taken at depths between 556 and 8633 m. The crystallographic orientation of microcracks in quartz was determined by a combination of the electron channelling pattern method (ECP) and U-stage microscopy. The distinct preferred crystallographic orientations of various crack generations point to different processes of crack initiation and propagation which partly seem to be depth-dependent. For older healed cracks, a crystalplastic initiation due to dislocation pile-up and related lattice distortion 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 pure elastic crack mechanisms. Because of the thermoelastic anisotropy of quartz, internal stresses resulting from thermal contraction during cooling may be the main driving force for the initiation or further propagation of cracks parallel to the c-axis. Crack propagation preferentially affects grains with crystal lattices and operating crack mechanisms being in a direction appropriate to the applied external (tectonic) stresses. This selective cracking 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.