Deformation and fluid-rock interaction along then reactivated Outer Hebrides fault zone, Scotland

Abstract

The Outer Hebrides Fault Zone (OHFZ) is a major, moderately E- to ESE-dipping long-lived reactivated fault zone which is developed in, and cross-cuts, crystalline amphibolite to granulite grade Lewisian basement gneisses, NW Scotland. A complex assemblage of different fault rocks and structures is presently exposed along the OHFZ, which reflects deformation at a range of crustal depths and metamorphic (temperature, pressure, fluid activity) conditions. Detailed field and microstructural observations have demonstrated that segments of the fault zone which display evidence of repeated reactivation over long periods of geological time (movements range from late Laxfordian / Grenvillian to Oligocene in age) are characterised by intense, localised greenschist facies retrogression and the development of sericite- and chlorite-bearing phyllonitic shear zones. In contrast, phyllonite is absent from segments of the fault zone which have not suffered extensive reactivation. These observations are consistent with phyllonitisation at mid-crustal depths having caused profound long-term mechanical weakening of the OHFZ,Two phases of retrogression and phyllonitisation have been recognised along the OHFZ:Upper greenschist facies, Late Laxfordian / Grenvillian phyllonitisation, which occurred at between 15 and 17km depth (Lewis and Harris only), and Lower greenschist facies, Caledonian phyllonitisation, which occurred at between 8 and 9km depth (Lewis, Harris and the Uists).Microstructural and geochemical studies of selected phyllonites from reactivated segments of the OHFZ demonstrate that greenschist facies retrogression and phyllonitisation were promoted by the influx of warm (c.250 to 450 C), hydrous iron^ and magnesium-bearing, oxidising fluids into the fault zone. Fluid flow during upper greenschist facies phyllonitisation was focused into pre-existing bands of highly strained quartzo-feldspathic mylonite, whilst fluid flow during lower greenschist facies phyllonitisation was focused predominantly into pre-existing brittle fractures and cataclastic crush zones. Thus, the distribution and intensity of fluid flow, and hence the distribution and intensity of retrogression and phyllonitisation were ultimately governed by the nature of pre-existing permeability pathways through the fault zone. It is therefore concluded that the long-term rheological evolution of reactivated basement fault zones is inexorably linked to the mid-crustal permeability evolution of such structures