Loch Eriboll Research Articles

Predictive GAM seabed maps can account for defined and fuzzy boundaries to improve accuracy in a Scottish sea loch seascape

Marine seabed mapping is an important element in marine spatial and conservation planning. Recent large scale mapping programmes have greatly increased our knowledge of the seafloor, yet at finer resolutions, large gaps remain. Loch Eriboll, Scotland, is an area of conservation interest with a diverse marine environment supporting habitats and species of conservation importance. Here we test and present strategies for a predictive seabed substrata map for Loch Eriboll using drop down Stereo Baited Remote Underwater Video (SBRUV) imagery collected as part of systematic underwater survey of the Loch. A total of 216 SBRUV deployments were made across the study site in depths of 3 m–117 m, with six seabed classes identified using an adaptation of the EUNIS (European Nature Information System) hierarchical habitat classification scheme. Four statistical learning approaches were tested, we found, Generalised Additive Models (GAMs) provided the optimal balance between over- and underfitted predictions. We demonstrate the creation of a predictive substratum habitat map covering 63 km2 of seabed which predicts the probability of presence and relative proportion of substratum types. Our method enables naturally occurring edges between habitat patches to be described well, increasing the accuracy of mapping habitat boundaries when compared to categorical approaches. The predictions allow for both defined boundaries such as those between sand and rock and fuzzy boundaries seen among fine mixed sediments to exist in the same model structure. We demonstrate that SBRUV imagery can be used to generate cost effective, fine scale predictive substrata maps that can inform marine planning. The modelling procedure presented has the potential for a wide adoption by marine stakeholders and could be used to establish baselines for long term monitoring of benthic habitats and further research such as animal distribution and movement work which require detailed benthic maps.

Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland

Glacial ripping is a newly recognized process sequence in which subglacial erosion is triggered by groundwater overpressure. Investigations in gneiss terrain in lowland Sweden indicate that ripping involves three stages of (i) hydraulic jacking, (ii) rock disruption under subglacial traction, and (iii) glacial transport of rock blocks. Evidence for each stage includes, respectively, dilated fractures with sediment fills, disintegrated roches moutonnées, and boulder spreads. Here, we ask: can glacial ripping also occur in sedimentary rocks, and, if so, what are its effects? The case study area is in hard, thinly bedded, gently dipping Cambrian quartz-arenites at Loch Eriboll, NW Scotland. Field surveys reveal dilated, sediment filled, bedding-parallel fractures, open joints, and brecciated zones, interpreted as markers for pervasive, shallow penetration of the quartz-arenite by water at overpressure. Other features, including disintegrated rock surfaces, boulder spreads, and monomict rubble tills, indicate glacial disruption and short distance subglacial transport. The field results together with cosmogenic isotope ages indicate that glacial ripping operated with high impact close to the former ice margin at Loch Eriboll at 17.6–16.5 ka. Glacial ripping thus can operate effectively in bedded, hard sedimentary rocks, and the accompanying brecciation is significant—if not dominant—in till formation. Candidate markers for glacial ripping are identified in other sedimentary terrains in former glaciated areas of the Northern Hemisphere.

Structure of the Northern Moine thrust zone, Loch Eriboll, Scottish Caledonides

This paper reviews the geology of the Late Ordovician - Silurian Caledonian Moine Thrust zone in the Loch Eriboll region, NW Scotland. We present new detailed mapping and balanced/restored cross-sections to examine its structural evolution. This thrust zone comprises four major thrust slices, carried by the Moine, Lochan Riabhach, Arnaboll and Sole thrusts. Ductile mylonites were derived from lithologies in both the hangingwall (Moine schists) and the footwall (Lewisian basement gneisses, Cambrian quartzites, Ordovician dolomites) of the Moine Thrust. The mylonites are bounded by the Lochan Riabhach thrust below and the Moine thrust above. The Arnaboll thrust sheet comprises Lewisian basement gneisses and Cambrian-Ordovician sedimentary rocks over the Sole Thrust sheet, which exposes a spectacular imbricate sequence of Cambrian-Ordovician sedimentary rocks. Although thrusts sequentially propagated in-sequence from higher (Moine) to lower (Sole) thrusts with time, there is evidence for minor out-of-sequence thrusting and breakback thrusting. Thrusts evolve from deep ductile shears where strain is concentrated along mylonite zones up into the brittle field where they become discrete planar thrust faults. The Early Silurian Scandian metamorphism (~435–415 Ma) is related to a Himalayan style structure with SSE-directed subduction and WNW directed extrusion of a migmatitic core, which we liken to channel flow. We suggest that two major crustal-scale thrusts that extend down into the upper mantle imaged on seismic profiles across the foreland, the Outer Isles and Flannan thrusts, are unrelated spatially or temporally to the Moine thrust sequence.

Sabkha Facies and the Preservation of A Falling-Stage Systems Tract At the Sauk II–III Supersequence Boundary In the Late Cambrian Eilean Dubh Formation, NW Scotland

The Durness Group of northwest Scotland represents a nearly one-kilometer-thick succession of mid-Cambrian to lower Middle Ordovician carbonates. These rocks crop out in a narrow belt along the foreland of the Caledonian orogen and in the Moine Thrust zone, stretching some 170 km from Loch Eriboll southwestwards to the Isle of Skye. The Cambrian parts of the succession represent deposition on a paleo-southeast-facing, low-latitude, passively subsiding continental margin, situated at around 20° S. Carbonate deposition in Scotland formed part of the extensive Great American Carbonate Bank. The Sauk II–III supersequence boundary occurs above an interval of thin, meter-scale shallowing-upward peritidal cycles containing intraclast dolorudstones, stromatolites, tepees, siliciclastic sand, and surface karst. Evidence of former evaporites in the succession comprises pseudomorphs of nodular and bedded anhydrite, crystal-shaped calcite-filled vugs representing growth of sulfate, dissolution breccias, and local halite casts. These features represent sabkha facies arranged into cycles that constitute a preserved falling-stage systems tract (FSST). The top of the FSST is marked by a pronounced karst surface of sandstone-filled fissures. The section preserves a rare snapshot of a seldom-preserved environment on the Cambrian Laurentian margin. Its preservation during a time of falling relative sea level under slow tectonic subsidence is discussed.

Lateglacial and Holocene relative sea‐level changes and first evidence for the Storegga tsunami in Sutherland, Scotland

ABSTRACTWe reconstruct one of the longest relative sea‐level (RSL) records in north‐west Europe from the north coast of mainland Scotland, using data collected from three sites in Loch Eriboll (Sutherland) that we combine with other studies from the region. Following deglaciation, RSL fell from a Lateglacial highstand of +6−8 m OD (Ordnance Datum = ca. mean sea level) at ca. 15 k cal a BP to below present, then rose to an early Holocene highstand and remained at ca. +1 m OD between ca. 7 and 3 k cal a BP, before falling to present. We find no evidence for significant differential Holocene glacio‐isostatic adjustment between sites on the north‐west (Lochinver, Loch Laxford), north (Loch Eriboll) and north‐east (Wick) coast of mainland Scotland. This suggests that the region was rapidly deglaciated and there was little difference in ice loads across the region. From one site at the head of Loch Eriboll we report the most westerly sedimentary evidence for the early Holocene Storegga tsunami on the Scottish mainland. The presence of the Storegga tsunami in Loch Eriboll is predicted by a tsunami wave model, which suggests that the tsunami impacted the entire north coast of Scotland and probably also the Atlantic coastline of north‐west Scotland.

Deformation temperatures, vorticity of flow and strain symmetry in the Loch Eriboll mylonites, NW Scotland: implications for the kinematic and structural evolution of the northernmost Moine Thrust zone

Abstract The Moine Thrust zone (MTZ) marks the Caledonian foreland-to-hinterland transition zone at the base of the Scandian ( c . 430 Ma) orogenic wedge. In the Loch Eriboll region, the upper ductile part of the MTZ is composed in ascending order of two regionally extensive thrust sheets (Upper Arnaboll-Creag na Faoilin and Creagan) and is overlain by the Moine Nappe. Quartz crystal fabrics, kinematic vorticity (W m ), and strain estimates from the ductile thrust sheets in this region are used to determine how pure and simple shear components of deformation are partitioned, and indicate that these processes may be thermally, structurally, and lithologically dependent. At the lowest structural levels, quartzite and gneiss in the Upper Arnaboll-Creag na Faoilin (UA-CNF) thrust sheet yield rigid grain-based arithmetic mean minimum (W m min) and mean maximum (W m max) vorticity estimates of 0.57 and 0.67, respectively (60–53% pure shear). Creagan thrust sheet mylonites yield W m min and W m max estimates of 0.59 and 0.72 (59–48% pure shear). At the highest structural levels, Moine Nappe mylonites yield W m min and W m max estimates of 0.59 and 0.71 (59–49% pure shear). Quartz c - and a -axis fabrics qualitatively indicate an increase in non-coaxial deformation (top-to-the-west) traced towards structurally higher levels, which is accompanied by increases in deformation temperature ( c . 370 °C to c . 550 °C). Integrated strain and vorticity estimates indicate that significant sub-vertical foliation normal shortening has occurred as nappe stacking progressed.

Variation in growth rate and reproductive output in British populations of the dorid nudibranch Adalaria proxima : consequences of restricted larval dispersal?

Growth and reproduction were compared among six geographically and genetically distinct intertidal populations of the annual, semelparous, dorid nudibranch Adalaria proxima (Alder & Hancock) to evaluate variation in fitness-related traits. The six populations spanned the geographic range in the northern British Isles: NE England (Cowling Scar), E Scotland (Kinkell Braes), NW Scotland (Loch Eriboll), W Scotland (Cuan Ferry), Northern Ireland (Portaferry), and N Wales (Menai Bridge). Nudibranchs from five sites were collected in July to August 1992 as post-metamorphic juveniles and were laboratory-reared under the same conditions of ambient temperature and photoperiod for up to 10 months and the completion of spawning. Individuals from the sixth site were added to the experiment in December 1992. Growth was monitored every 2 weeks, and reproductive performance was expressed as a weight-adjusted dimensionless index (ΣRI) of each individual's spawnings summed over the reproductive period. In general, larger nudibranchs produced larger first spawn masses and more total spawn than did smaller nudibranchs, but these size-related trends were observed only in some populations. The patterns of energy partitioning to spawnings varied significantly among populations, from allocations of a large number of eggs to few spawn masses (Loch Eriboll) to production of many small spawnings over a long spawning period (Portaferry). There was no relationship between maximum body size and the amount of spawn produced after the first spawning, nor to the length of the spawning period or the number of spawn produced. Both Menai Bridge and Kinkell Braes had low mean population ΣRI, reflecting a very poor reproductive performance, given their large maximum (pre-spawning) body sizes. By contrast, the Loch Eriboll, Cuan Ferry, and Portaferry populations all displayed high mean population ΣRI, albeit as a result of differing combinations of numbers and sizes of spawn masses and duration of the spawning period. This high variance of reproductive allocation among populations, and previous evidence of relatively stable among-population differences in allozyme frequencies, adult color, and embryo characteristics suggest very restricted larval transport of lecithotrophic larvae of A. proxima.

Micromechanical control of rheological anisotropy in quartz mylonite

The Moine nappe mylonites of the area of Loch Eriboll are inferred to have deformed predominantly by simple shear. Samples were taken from each limb of a late recumbent fold and the quartz crystallographic preferred orientation and optical microstructure were investigated. The pattern of preferred orientation was found to be related to the local folded orientation of the foliation and stretching lineation for the overturned limb and to the inferred imposed deformation for the normal limb. The mechanical development of the fold was modelled as a similar fold forming in a homogeneous mechanically anisotropic material with linear viscous rheology. A very high mechanical anisotropy was required for active amplification of small initial perturbations under simple shear applied at 10 ° to the initial enveloping surface. The high anisotropy also constrained the local kinematics to approximate simple shear parallel to the plane of easy shear, which contributes to the maintenance of the preferred orientation patterns relative to the local macroscopic fabric elements. The degree of mechanical anisotropy calculated from the preferred orientation assuming the operation of the common quartz slip systems is lower than that required for mechanical amplification of the folds. Consequently, it is suggested that the mechanical anisotropy derives from sliding on preferentially aligned grain boundaries.

Intraspecific variation in embryonic and larval traits of the dorid nudibranch mollusc Adalaria proxima (Alder and Hancock) around the northern coasts of the British Isles

Intraspecific variation in reproductive traits (egg size, clutch size, embryonic development rate, larval hatching success) of the dorid nudibranch mollusc Adalaria proxima (Alder and Hancock) was investigated for six intertidal populations embracing this species' geographic range throughout the northern British Isles. Populations were sampled from northeast England (Robin Hood's Bay), east Scotland (Kinkell Braes), north Scotland (Loch Eriboll), west Scotland (Cuan Ferry), north Wales (Menai Bridge) and Northern Ireland (Portaferry). The objective was to quantify within and between population variation throughout individuals' reproductive periods and to assess the putative adaptive significance of egg size variation. Significant variation both in egg size and clutch size was found to occur between populations. Mean population egg diameters varied from 160 μm (Portaferry) to 181 μm (Menai Bridge). Clutch size did not correlate with egg size between populations, neither did the observed geographic differences in egg size and clutch size exhibit a discernible pattern or clinal structure. The mean embryonic developmental period (range 53–69 d) also differed significantly between populations, but there was no correlation between egg size and developmental period or larval hatching success. None the less, a significant relationship between egg size and larval shell size was established, although whether a larger larval shell reflects a larger larval soma remains unknown. Significant variation in egg size and clutch size also were observed within populations over the spawning period. Individual adult Adalaria proxima may lay up to 11 spawn masses in the laboratory over the reproductive period. Significant decreases in both the number and size of eggs contained within successive spawns was found, with a consistent pattern of the first spawn mass comprising the largest proportion (26–46%) of individuals' total egg production. These egg and compensation for the gradual diminution of energetic reserves available for reproduction. Hatching success of spawn masses also decreased significantly with increasing position in the laying sequence. Maximum fitness may well be conferred on progeny from the first-laid spawn masses, which contain more and larger eggs, which display higher levels of hatching success and result in larger shelled larvae. Previous genetic studies have shown British populations of this species to be highly differentiated on scales of only a few kilometres: differentiation is believed to be due to random genetic drift attributable to restricted larval dispersal (and hence gene flow) and local habitat selection need not be invoked. The lack of correlation between egg size and rate of embryonic development to hatching of the veliger larva begs the question as to whether this variation is truly adaptive, or non-adaptive and merely also a reflection of genetic drift.

Sir Archibald Geikie (1835-1924) and the "Highlands Controversy": New Archival Sources for the History of British Geology in the Nineteenth Century

With the help of new archival material relating to Archibald Geikie, discovered fairly recently at the Haslemere Educational Museum, some revisions are offered to my account of the "Highlands Controversy" (relating to the structure of the Northwest Highlands of Scotland), published in 1990.1 Also further light is thrown on the so-called "Archaean Controversy," especially with regard to the debates about the rocks at St. David's, Pembrokeshire. It is shown that Geikie did visit the north coast of Scotland before 1884, but made no observations there that had a bearing on the Highlands Controversy. He was, however, well prepared for a change of mind before he actually examined the rocks at Loch Eriboll in 1884. It appears that Geikie's chief "bête noire" was the petrologist Thomas Bonney, and to a lesser extent Charles Callaway. Some new information about Edward Greenly is revealed, and also the procedures of the Wharton Committee which looked into the affairs of the Survey in 1900, and led to Geikie's retirement.

A Seasonal Analysis of the Armoured Dinoflagellates of Loch Eriboll, North Scotland

The armoured planktonic dinoflagellates present at a site adjacent to a floating fish-farm in Loch Eriboll, North Scotland, were studied by surface net-sampling, mainly at two-weekly intervals, over a period of four years 1990–1993. A total of 62 species was recorded. Some dinoflagellates were present at all times of the year but they reached their lowest numbers in May, during the spring diatom bloom, and their highest numbers in mid-summer. There was usually a seasonal pattern in which the most abundant dinoflagellate during the early summer was the heterotrophic species Protoperidinium ovatum, and this was followed in the later summer by the autotroph Ceratium fusus. In 1993 the pattern was rather different, with Gonyaulax species abundant in June and, later in the year, both Protoperidinium cerasus and P. excentricum having periods of abundance. Potential toxin-producing dinoflagellates, Alexandrium tamarense (PSP) and Dinophysis acuminata (DSP), were found in small numbers.

The spawning behaviour of escaped farmed and wild adult Atlantic salmon (Salmo salar L.) in a northern Scottish river

As a result of an accident in February 1989, 184 000 growing salmon escaped from a seawater farm located in Loch Eriboll in northern Scotland. Later the same year, the behaviour of wild adults and fish of farmed origin was compared in the nearby River Polla. Wild salmon and farmed escapees were distinguished by their morphology, supported by tissue carotenoid analysis. Radio-tagged salmon of both groups and of both sexes were monitored before and during spawning. Bankside observations were made on tagged and untagged wild and farmed fish over the same period. Carotenoid pigments were determined in ova or alevins sampled later from spawning sites along the river's length. Wild and farmed fish of both sexes spawned throughout the river's length. Overall however, wild fish tended to spawn in the river's higher reaches while farmed escapees tended to spawn lower down. Farmed males were more widely distributed at spawning than farmed females. Farmed fish, particularly females, spawned later than wild females. The extent to which these findings may be generalised to other rivers is discussed in relation to the unique character of the River Polla and particularly the presence of a smolt unit on its lower reaches.

Temporal and spatial patterns in the recruitment of Gibbula umbilicalis

Size/frequency samples have been taken from populations of the trochid gastropod Gibbula umbilicalis at a number of sites around the British Isles each year since 1978. In the North of Scotland, close to the limit of the species distribution, recruitment was generally poor and populations were sparse and dominated by large old individuals. While such poor recruitment may reflect the low density of adults and the shortage of nursery areas, a distinct temporal pattern of recruitment was also evident. At the start of our study, the majority of sites had population structures biased towards young animals implying that conditions for the settlement of larvae or their subsequent survival had been favourable over a wide area. In the years that followed only the enclosed Loch Eriboll regularly received substantial recruitment. Further to the south, in Wales and S.W. England, recruitment was usually more regular, populations were more dense and individuals smaller.

Agalmatolite and the maturity of sandstones of the Appin and Argyll groups and Eriboll Sandstone

Synopsis Agalmatolite is a term used to describe a soft waxy rock amenable to carving composed of massive, felted muscovite with very minor quartz and feldspar. The term is derived from αγαλμα meaning sculpture or statue. The occurrence of agalmatolite was recognised, during the initial geological survey of the NW Highlands, as resulting from a “peculiar Precambrian weathering of the Lewisian” which occurs beneath the Cambrian unconformity. We have examined the effects of this weathering on the Lewisian gneisses in a zone of alteration extending two to three metres below the Cambrian unconformity on the west side of Loch Eriboll on the north coast of Scotland. The mafic minerals, a large proportion of feldspar and even some quartz have been altered to a green muscovite in what we take to be a palaeosaprolite resulting from alkaline weathering. We imagine a loose Precambrian quartz-rich soil being river-borne a short distance to the Dalradian basin or reworked to form the Eriboll Sandstones, a supposition strongly supported by the intermittent development of discontinuous layers of green micaceous shale in the Eriboll Sandstone reminiscent in colour and composition of the agalmatolite. The alumina dissolved from the Lewisian is transported as a colloid or in solution in groundwater to the sea where it reacts with dissolved silica and K + , Na + or Ba ++ to form huge bodies of authigenic or hydrothermal-sedimentary feldspar now preserved in the Fucoid Beds and in the Middle Dalradian.

Strain path partitioning within thrust sheets: microstructural and petrofabric evidence from the Moine Thrust zone at Loch Eriboll, northwest Scotland

Quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the Loch Eriboll area of the Moine Thrust zone and are used to interpret the detailed processes involved in fabric evolution. The intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude. A correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry. Two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the Moine Thrust. Within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate, mutually interfering shear bands, globular low strain detrital quartz grains whose c axes are aligned sub-parallel to the principal finite shortening direction ( Z) and quartz c axis fabrics which are symmetric (both in terms of skeletal outline and intensity distribution) with respect to mylonitic foliation and lineation. Non-coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation. Tectonic models offering possible explanations for the presence of kinematic (strain path) domains within thrust sheets are considered.

Growth of a zoned plagioclase porphyroblast in a mylonite

Zoning patterns in a plagioclase porphyroblast in a blastomylonite derived from Lewisian gneiss near Loch Eriboll, northwest Scotland, suggest that the crystal grew during deformation. Discontinuities in the concentrations of CaO, K 2 O, and FeO mark the boundaries of overgrowths and define a core-overgrowth structure. The simultaneous changes in concentration of CaO and K 2 O are used to construct a qualitative chemical potential diagram that reflects changes in a Ca ++/ a 2 H + and a K + / a H + in the coexisting fluid during growth. Increases in these activity ratios are believed to have caused supersaturation of the grain boundary fluids with respect to plagioclase. The repeated periods of growth indicate repeated overstepping of equilibrium, which could have been caused by ion exchange on surfaces newly exposed during deformation. These data fit a model that relates cyclic variations of strain rate in fault zones with cyclic periods of crystal growth, which in turn could have a major influence on fault rock rheology.

A structural analysis of the Moine Thrust Zone between Loch Eriboll and Foinaven, NW Scotland

The northern part of the Moine Thrust Zone as exposed around the valley of Srath Beag, Sutherland was developed by thrusts propagating in the tectonic transport direction. Deformation on any particular thrust surface evolved from dominantly ductile to dominantly brittle with time. The foreland has been progressively accreted onto the overriding Moine thrust sheet by duplex formation, a process which has continuously folded the roof thrust and the rocks above its hanging-wall. Fold culminations and depression can be related to lateral ramps which may give the rocks above the hanging-wall a complex history of extensional and compressional strains normal to the transport direction. Folds within the thrust zone are laterally independent because they are controlled by short lived variations in deformation style on an evolving thrust footwall topography. Therefore there may be no correlation between structures across or along the thrust zone. This variation limits the construction of balanced cross sections as structure cannot be projected onto particular section lines.

The Moine Thrust Zone: an overview

Summary The geometry of structures within the Moine Thrust Zone are similar to those found in classic thin-skinned tectonic regimes such as the southern Canadian Rocky Mountains. The deformation style and strain patterns along the strike of the Moine Thrust Zone are controlled by the position of the floor (or Sole) thrust. The stacking sequence of thrust nappes is from E to W in the direction of transport. Reactivation of some thrusts has caused reversals in the stacking sequence. The mylonites associated with the Moine Thrust have undergone a long and complex history of progressive deformation. Detailed descriptions of the Moine Thrust Zone at Loch Eriboll are given and the relationships of the Moine Thrust Zone to other major Caledonian tectonic features of the NW Highlands are discussed.

Differential stress determined from deformation‐induced microstructures of the Moine Thrust Zone

The dislocation structure, recrystallization, and elongation of quartz grains in tectonites from three localities along the Moine thrust fault have been analyzed by transmission electron and optical microscopy. The dislocation density, 5×108 cm−2, and the grain size after recrystallization, 15 μ, in the basal quartzite unit at the Stack of Glencoul are independent of distance from the fault, to the maximum sampling distance of ∼100 m. The differential stress level determined from the deformation‐induced microstructures is on the order of 100 MPa. The magnitude of the differential stress at the fault decreases by a factor of 2 from Knockan Crag to Loch Eriboll, a distance of 50 km. At the Stack of Glencoul, grains in the basal quartzite are progressively elongated and recrystallized approaching the fault, corresponding to a progressive increase in strain. At 100 m from the fault, the aspect ratio of relict quartz grains is ∼1:1; at 0.01 m it is 85:1. A quantitative estimate of the strain, based on the aspect ratios of the relict quartz grains, as a function of distance from the fault has been used to calculate strain rate and temperature profiles. For a creep activation energy of 0.19 MJ mol−1 the temperature decreases away from the fault with a gradient of 7.5×10−3 °C cm−1. For a differential stress of 100 MPa, a steady state thrust sheet velocity of 11 cm yr−1 would be required to produce this gradient. At 100 m from the fault, 5% of the quartzite is recrystallized from 1‐mm to 15‐μm grains; at 0.01 m it is 100% recrystallized to 15‐μm grains. This small grain size probably favored grain boundary deformation mechanisms over grain matrix mechanisms and thus helped to concentrate the strain at the fault. Systematic changes in the elongation and recrystallization of quartz grains in Lewisian Gneiss and Moine Schist are obscured by the constraints imposed by other mineral phases. The small size of the grains after recrystallization, highly serrated grain boundaries, and large densities of free dislocations suggest that very little static recovery followed the plastic deformation event.

Variations of strain and microstructure in folded Pipe Rock in the Moine Thrust Zone at Loch Eriboll and their bearing on the deformational history

Synopsis Using deformed Skolithos pipes as strain markers, a folded sequence of Pipe Rock quartzites in the Heilam Nappe at Loch Eriboll is shown to have deformed dominantly by flexural flow. However, elliptical pipe sections on bedding evince a pre-folding strain. Microstructures observed in the hinge zone imply deformation dominated by the intercrystalline diffusion mechanism, pressure solution. On the limbs microstructural evidence for dislocation creep has almost obliterated features associated with pressure solution. It is concluded that the quartzite sequence deformed by pressure solution during an initial phase of layer parallel shortening and subsequently, when buckling was initiated and fold amplification occurred, the mechanism switched to dislocation creep. It is argued that this change in mechanism is related to changes in strain rate.