Outer Hebrides Fault Zone Research Articles

U−Pb geochronology and microstructural analysis of apatite within a Proterozoic crustal-scale shear zone, Outer Hebrides, NW Scotland

U–Pb apatite geochronology is increasingly recognized as a valuable tool for constraining the age of mid-crustal ductile shear zones. The crustal-scale Outer Hebrides Fault Zone (OHFZ) within the Laurentian foreland of the Scottish Caledonides has long been of uncertain age and tectonic significance. Earliest deformation within the OHFZ was associated with top-to-the-NW ductile thrusting that formed a belt of greenschist facies mylonites within host Archean−Paleoproterozoic basement gneisses. Previous estimates for the timing of thrusting vary between c . 1600 Ma and c . 430 Ma. The mylonitic fabrics are defined by a recrystallized assemblage of quartz + albite/oligoclase + sericite + actinolite + epidote + apatite ± calcite, consistent with deformation temperatures of 400–500°C and within the range of reported closure temperatures for Pb diffusion in apatite. U–Pb (LA−ICP−MS) dating of two texturally distinct apatite grain types within the mylonites has yielded ages mostly in the range c . 1100–900 Ma. The OHFZ is therefore interpreted as a Grenville–Sveconorwegian structure that formed during the tripartite collision of Laurentia, Baltica, and Amazonia and the assembly of Rodinia. Supplementary material : U–Pb isotopic data (Table S1), trace element data (Table S2) and laser ablation spot images are available at https://doi.org/10.6084/m9.figshare.c.7084925

Stress fields of ancient seismicity recorded in the dynamic geometry of pseudotachylyte in the Outer Hebrides Fault Zone, UK

Heterogeneous sequences of exhumed fault rocks preserve a record of the long-term evolution of fault strength and deformation behaviour during prolonged tectonic activity. Along the Outer Hebrides Fault Zone (OHFZ) in NW Scotland, numerous pseudotachylytes record palaeoseismic slip events within sequences of mylonites, cataclasites and phyllonites. To date, the kinematics and controls on seismicity within the long active history of the OHFZ have been poorly constrained. Additional uncertainties over the relative location of a meteorite impact and possible pre-OHFZ brittle faulting also complicate interpretation of the diffuse seismic record. We present kinematic analyses of seismicity in the OHFZ, combining observations of offset markers, en echelon injection veins and injection vein geometry to reconstruct slip directions and stress fields. This new dataset indicates that a range of fault orientations, slip directions and slip senses hosted seismicity in the OHFZ. Such complexity requires several stress field orientations, in contrast with the NW–SE Caledonian compression traditionally attributed to frictional melting along the OHFZ, indicating that seismicity had a long-term presence across the fault zone. Persistence of strong frictional failure alongside the simultaneous development of weak fault rocks and phyllonitic shear zones in parts of the OHFZ has significant implications for understanding seismic hazard along mature continental faults. Supplementary material: Tables listing analysed orientation measurements plus further information and sensitivity testing of palaeostress analysis parameters are available at https://doi.org/10.6084/m9.figshare.c.5134797

Geochemical and petrographic variations in pseudotachylyte along the Outer Hebrides Fault Zone, Scotland

The islands of the Outer Hebrides, Scotland host a c. 200 km long fault zone that was active between 1100 and 250 Ma, during which time a belt of fault rock several kilometres wide was created. The fault zone hosts crush breccias, phyllonite, mylonite and pseudotachylyte. The latter unit is often found in major fault zones and within meteorite impact structures. Although pseudotachylyte is widely believed to be a product of frictional melting within a fault zone, there is still controversy over the specific mechanisms of formation. Pseudotachylyte was collected from the southern block of the Outer Hebrides Fault Zone. A geochemical mixing model was applied to reconstruct the geochemistry of the pseudotachylyte matrix using the locally available lithologies. The results show that the pseudotachylyte could have been derived from the local country rock, although some mixing was necessary to provide accurate reconstructions, suggesting a degree of mixing between melts derived from different protoliths. Petrographic observations identified additional characteristics of melt transportation, including transported clasts, aligned fragments, melt segregation and injection of the melt into tensile fractures. The latter probably served as the transport mechanism, drawing melt into the low-pressure tensile structures that developed adjacent to the melt-forming zones within the fault plane. Supplementary Material : Sample site co-ordinates and electron microprobe settings are available at: https://doi.org/10.6084/m9.figshare.c.4567931

Basement-influenced rifting and basin development: a reappraisal of post-Caledonian faulting patterns from the North Coast Transfer Zone, Scotland

Abstract The post-Caledonian development of the West Orkney Basin is regularly cited as a classic example of basement-influenced rifting. This paper presents the first detailed multidisciplinary analysis of the three-dimensional (3D) geometries and distribution of post-Caledonian faults in onshore northernmost Scotland, examining their relationships to basement fabrics and comparing them to rift-related structures developed offshore in the West Orkney Basin. Two phases of rift-related faulting are distinguished: 1) Devonian ENE–WSW extension localized in the east of the basin and related to regional sinistral transtension along the Great Glen Fault; and 2) Permo-Triassic NW–SE extension focused to the west of the basin and probably contemporaneous with movements along the Minch Fault. A complex North Coast Transfer Zone is developed along the northern Scottish coast linking Mesozoic rifts that reactivated Caledonian structures in the West Orkney Basin (Naver Thrust) to those bounding the North Minch Basin (Outer Hebrides Fault Zone). Polymodal faulting patterns are widespread in onshore exposures. Fault patterns vary due to changes in the obliquity between regional rifting vectors and variably orientated pre-existing structures in each basement terrane. The geometric complexity and spatial variations in fault patterns onshore can be correlated with changes in basement structures, despite limited direct reactivation of pre-existing fabrics.

Kinematic and thermal constraints on the reactivation of the Outer Hebrides Fault Zone, NW Scotland

AbstractThe Outer Hebrides Fault Zone is a major easterly dipping reactivated shear zone which displaces Lewisian gneiss of the Laurentian craton, NW Scotland. Despite a number of detailed field studies, the fault zone remains poorly understood with regard to both its age of inception and precise conditions of reactivation. The island of Scalpay in the northern portion of the Outer Hebrides Fault Zone provides exceptional exposures through a variety of reactivated fault rock types and therefore represents an ideal location to investigate fault zone evolution via fluid inclusion studies of syn-tectonic quartz veins. This fluid inclusion study constrains reactivation temperatures more precisely than hitherto possible with top-to-the-NW ductile thrusting occurring at 500 ± 30°C. Subsequent phyllonitization is associated with oblique sinistral top-to-the-NE strike-slip at 230 ± 20°C, followed by a discrete system of top-to-the-NE/SE extensional detachments at 150 ± 20°C. Other recent fluid inclusion studies in the southern portion of the Outer Hebrides Fault Zone constrain phyllonitization associated with top-to-the-E displacement to 370 ± 20°C, with subsequent top-to-the-NE extensional detachments operating at 150–210°C. Thus, late-stage extensional detachment systems record consistent conditions of reactivation along the strike length of the Outer Hebrides Fault Zone. However, our results also clearly emphasize that conditions of earlier fault zone reactivation and phyllonitization were highly heterogeneous between the northern and southern portions, thus suggesting a spatial and temporal variation in the deformation and/or fluid flux system.

The initiation and early tectonic significance of the Outer Hebrides Fault Zone, Scotland

The Outer Hebrides Fault Zone is a major ESE-dipping detachment exposed within basement gneisses of the Archaean–Palaeoproterozoic Lewisian Complex, northwest Scotland. The fault zone exhibits a long-lived displacement history and was active during Proterozoic, end-Silurian, Carboniferous and Mesozoic times. The earliest deformation event preserved onshore was associated with top-to-the-NW ductile thrusting. A previous study proposed that thrust-related protomylonitic and mylonitic fabrics are cross-cut by amphibolites (‘Younger Basics’) and Laxfordian granite and pegmatite sheets. This evidence was used to suggest that ductile thrusting occurred during the Palaeoproterozoic Inverian event at c. 2500 Ma. Our observations demonstrate, however, that mylonitic fabrics within the ductile thrust zone are superimposed on all components of the gneiss complex including amphibolites and Laxfordian intrusions. It therefore follows that the Outer Hebrides Fault Zone cannot be older than c. 1685 Ma, the age of recently dated Laxfordian granites in the Outer Hebrides. Geochronological studies have shown that the basement blocks of the northern Outer Hebrides and Scottish mainland have different geological histories and were amalgamated during Proterozoic times at or after c. 1700 Ma. We propose that early ductile thrusting along the Outer Hebrides Fault Zone formed part of this amalgamation process leading to burial and reheating of the footwall gneisses in Lewis and north Harris. This would account for the c. 1100 Ma thermal event recorded by previous workers and implies that ductile thrusting along the Outer Hebrides Fault Zone is of Grenvillian age.

A reappraisal of the Sibson‐Scholz fault zone model: The nature of the frictional to viscous (“brittle‐ductile”) transition along a long‐lived, crustal‐scale fault, Outer Hebrides, Scotland

The widely cited Sibson‐Scholz conceptual fault zone model suggests that seismically active, upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10–15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep. The crustal‐scale Outer Hebrides Fault Zone (OHFZ) in NW Scotland has been described as the type example of such a continental fault zone. It cuts Precambrian basement gneisses and is deeply exhumed, allowing direct study of the deformation products and processes that occur across a wide range of crustal depths. A number of fault rock assemblages are recognized to have formed during a long‐lived displacement history lasting in excess of 1000 Myr. During Caledonian movements that are recognized along much of the 190 km onshore fault trace, brittle, cataclasite‐bearing faults in the west of the OHFZ are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones. Field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust‐related crustal thickening or shear heating. Microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids. This led to retrogression, with the widespread development of new finegrained phyllosilicate‐bearing fault rocks (“phyllonites”), and the onset of fluid‐assisted, grain size‐sensitive diffusional creep in the most highly deformed and altered parts of the fault zone. Phyllonitic fault rocks also occur in older, more deeply exhumed parts of the fault zone, implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature (depth) range within crustal‐scale faults. Our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal‐scale faults containing interconnected networks of phyllosilicate‐bearing fault rocks will be characterized by long‐term relative weakness and shallow (∼5 km) frictional‐viscous transition zones. Similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the San Andreas Fault.

Extensional tectonics of the Outer Hebrides Fault Zone, South Uist, northwest Scotland

The Outer Hebrides Fault Zone is a major ESE-dipping reactivated structure within Lewisian basement gneisses of the Laurentian craton, northwest Scotland. Detailed mapping in South Uist reveals important new evidence that contributes to a better understanding of the kinematic evolution of the fault zone. Large quantities of pseudotachylite which characterize the fault zone on South Uist may in part be lithologically controlled, and therefore of little value in determining areas of greatest deformation and displacement. Only limited evidence is preserved for ductile and brittle thrust-sense movements along this portion of the fault zone. The tectonics of the fault zone on South Uist are dominated by structures associated with several episodes of pervasive top-down-to-the-SE to -ENE brittle extensional deformation, which are progressively overprinted by protophyllonitic and phyllonitic fabrics associated with top-down-to-the-E to -ENE extension. A series of late-stage high-angle normal faults record top-down-to-the-ESE to -ENE extension and cut the phyllonites. Fluid inclusion studies from syntectonic quartz veins constrain the conditions of phyllonite formation at 370 ± 20 °C. Field evidence suggests that this section of the Outer Hebrides Fault Zone may have been largely unaffected by sinistral strike-slip reactivation as reported along-strike to the north, suggesting both a varied and compartmentalized tectonic and evolutionary history along the length of the Outer Hebrides Fault Zone.

Contrasting modes of reactivation in the Outer Hebrides Fault Zone, northern Barra, Scotland

The gently ESE-dipping Outer Hebrides Fault Zone may be traced for c . 170 km through the Lewisian gneisses of northwestern Scotland and defines broad, arcuate swings in strike from NE–SW to almost north–south. This study evaluates the kinematic evolution of the fault zone within the hinge region of such a strike swing exposed in the small isles north of Barra. New structural mapping of this area has concentrated on the geometry, kinematic significance and the relative age of different structural elements of the fault zone, and provides a comparison with the better known tectonics of the northern portions of the Outer Hebrides Fault Zone. Hitherto unrecognized top-to-the-NW ductile thrusting in this southern portion of the fault zone, coupled with abundant evidence of extensional reactivation and lack of sinistral strike-slip in phyllonites as reported further north, suggests that the present segmented and arcuate geometry is a primary feature of the fault system. The original thrust template governed the subsequent reactivation and relative importance of both late Caledonian strike-slip and extensional collapse components. Inherited structural architectures may thus provide geometric constraints on the nature and patterns of subsequent reactivation and tectonism.

Fault-zone weakening processes along the reactivated Outer Hebrides Fault Zone, Scotland

The Outer Hebrides Fault Zone is a major reactivated structure cutting amphibolite-grade Lewisian basement gneisses in NW Scotland. During a regionally important phase of sinistral strike-slip movements, the influx of chemically active hydrous fluids along the fault zone was associated with the formation of a network of greenschist-facies phyllonitic shear zones. Later ESE-directed extensional strain was preferentially focused into these pre-existing zones of weakness. The syn-tectonic alteration of a relatively strong, feldspar/hornblende-dominated load-bearing framework microstructure to an interconnected weak layer microstructure of fine-grained, strongly aligned phyllosilicate aggregates leads to the long-term weakening in the fault zone. Comparison with experimental data suggests that this produces a shallowing of the frictional-viscous creep ('brittle-ductile') transition and a substantial reduction in total crustal strength. Similar processes may account for the apparent weakness of many long-lived fault zones.

Evidence for Caledonian sinistral strike-slip motion and associated fault zone weakening, Outer Hebrides Fault Zone, NW Scotland

The Outer Hebrides Fault Zone is a major ESE-dipping reactivated fault within the Lewisian gneisses of the Laurentian craton in NW Scotland. Early thrust structures are overprinted by a network of retrograde ductile shear zones in which fluid channelling has hydrated the pre-existing basement rocks at low temperatures, forming chlorite-white mica phyllonites in regions of highest strain. Strike-parallel mineral lineations and shear-sense indicators suggest sinistral displacements that are thought to be late Caledonian based on isotopic data and regional considerations. The phyllonites have focused extensional movements that overprint strike-slip fabrics and may control the location of the Mesozoic Sea of Hebrides and Minch basins.

The Outer Hebrides fault: a major Proterozoic structure in NW Britain

Mylonites in the Outer Hebrides fault zone on Scalpay, Hams, are cut by metabasite sheets which are probably of Scourie dyke age (2400–2200 Ma) and by pegmatites of late Laxfordian age (c. 1750 Ma). The mylonites contain relicts of a gneissose banding which probably formed in the Archaean c . 2700 Ma ago. These age constraints suggest that the fault formed in the Inverian event c. 2500 Ma ago, not in the Caledonian (Silurian-Devonian) as widely reported. The fault is a long-lived structure which was reactivated as an extensional fualt in the Laxfordian, late Proterozoic (Torr-idonian) and the Carboniferous/Early Mesozoic, and as a thrust in the Caledonian. In a regional context this interpretation has important implications for the structural evolution of the Lewisian complex, for the nature of the Caledonian event in the Laurentian craton (Caledonian foreland) and for the age and geometry of the sedimentary basins in the hanging wall of the Outer Hebrides fault.

The Outer Hebrides Fault Zone: evidence for normal movements

Summary The Outer Hebrides Fault Zone is a major structural feature within the Lewisian of the Outer Hebrides. It is well exposed in the Uists where it consists of high-strain strands of ultracataclasite, cataclasite and phyllonite in a crush melange. Microstructural studies of the fault rocks indicate that the sequence crush melange-cataclasite-phyllonite is typical of that for c -type mylonite development and reflects increasing retrogression. This, plus the anastomosing pattern of the high-strain zones, suggests that the fault rocks in the Uists mark the edge of a major fault zone. Fold and shear band asymmetries indicate a major phase of normal movement. It is not possible to assign definite ages to either, but it is clear that the formation of the crush melange preceded that of the high-strain strands, which appear to be immediately post-Caledonian.