Characterising brittle reactivation in basement: an example from the Lewisian Gneiss Complex, NW Scotland
It is widely accepted that pre-existing ductile and brittle fabrics in continental lithosphere influence the orientation and spatial characteristics of subsequent deformation structures. By using quantitative techniques to describe geometric and spatial attributes it should be possible to determine whether these pre-existing weaknesses affect fracture network development.
To test this hypothesis, the mainland Lewisian Gneiss Complex (LGC) is being used as a case study. The Late Archaean - Early Proterozoic LGC comprises several terranes of TTG gneisses; mafic and ultramafic dykes together with subordinate supracrustal sequences, with numerous ductile shear zones of variable width and metamorphic grade. Three main regional fracture sets are recognised (from oldest to youngest): (1) steeply-dipping NW-SE Paleoproterozoic faults that are preferentially developed as foliation-parallel structures in pre-existing ductile shear zones; (2) N-S to ENE-WSW trending hematite stained normal fault ladder fractures associated with the deposition of the overlying Neoproterozoic (1.2 Ga.) Stoer Group sediments; (3) NE-SW-trending younger (Mesozoic?) faults and fractures??. Some of the older faults are locally reactivated during other regionally significant deformation episodes that affect NW Scotland, notably during the Silurian development of the Caledonian Moine Thrust Zone. Each fault set is associated with characteristic fault rock and mineral assemblages.
The present project focuses on characterising these fracture sets from regional to outcrop scale using a range of remote and fieldwork analysis techniques. Regional data comprises 2D lineament maps created from high resolution NEXTMap digital elevation models. Outcrop data consists of 1D sample lines and 2D photo-mosaics which have allowed fracture attribute characterisations to be made. This study centres on the Assynt and Rhiconich terranes to assess the heterogeneity in fracture networks due to variations in lithology and metamorphic grade.
Results from statistical analysis of outcrop and regional orientation data show that outcrops in the granodioritic Assynt terrane exhibit a correlation between intense foliation and preferential development of fracture sets in greenschist-facies shear zones. This does not necessarily require direct reactivation of pre-existing fabrics, e.g. the Stoer Group ladder fractures preferentially occur within shear zones even though they cut across the foliation. These correlations do not apply in the granitic Rhiconich terrane, where it is apparent that fractures do not reactivate pre-existing fabrics and instead cut across the foliation, even within pre-existing (amphibolite facies) shear zones.
In conclusion, the heterogeneity of the fracture sets within the mainland LGC can be attributed to variations in lithology, metamorphic grade and pre-existing ductile and brittle basement structure, particulary in relation to phyllosilicates (greenschist facies) in shear zones which give a strong anisotropy, localising or impeding deformation depending on the orientation of the principal stress/strain axes.