A quantitative study of the influence of pre-existing compositional and fabric heterogeneities upon fracture-zone development during basement reactivation

Abstract

Abstract In common with many other regions of exposed continental basement, the Late Archaean to Palaeoproterozoic Lewisian Complex, NW Scotland, preserves numerous examples of faults that appear to reactivate pre-existing compositional and structural heterogeneities in the host gneisses. A regionally recognized set of late Laxfordian sinistral strike-slip faults and fractures are spatially associated with pre-existing NW-SE-trending ductile shear zones of Inverian and Laxfordian age. Field observations suggest that most of the sinistral displacements have been accommodated along laterally persistent faults (here termed principal displacement zones (PDZ)) that lie sub-parallel to the pre-existing foliation in the shear zones. Geometric and orientation data collected during structural logging of the PDZ faults have been used to quantitatively test the influence of lithology and pre-existing structural geometry on the spatial patterns of fault development. Stereographic analysis shows a strong geometrical correspondence between the intensity and form of the pre-existing anisotropy and the alignment of the PDZ brittle faults. Spatial clustering of PDZ faults varies depending on lithology (amphibolite v. acid gneiss v. quartz-mica schist). A close correlation exists between the geometry and intensity of the pre-existing foliation and fault spatial clustering. The results demonstrate that reactivation of pre-existing anisotropies in typical continental basement gneisses exert a significant control on brittle fault development and growth in the upper crust.