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  • 1
    Description / Table of Contents: The world's mountain ranges are the clearest manifestations of long-term deformation of the continental crust. As such they have attracted geological investigations for centuries. Throughout this long history of research a few keynote publications stand out. One of the most important is the Geological Survey's 1907 Memoir on The Geological Structure of the North-West Highlands of Scotland. The Memoir summarized some of the Geological Survey's finest work, and outlined many of the principles of field-based structural and tectonic analysis that have subsequently guided generations of geologists working in other mountain belts, both ancient and modern. The thematic set of 32 papers in this Special Publication celebrate the 100th anniversary of the 1907 Memoir by placing the original findings in both historical and modern contexts, and juxtaposing them against present-day studies of deformation processes operating not only in the NW Highlands, but also in other mountain belts.
    Pages: Online-Ressource (VII, 872 Seiten)
    ISBN: 9781862393004
    Language: English
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  • 2
    Publication Date: 2010-06-21
    Description: Fold and fabric patterns developed within a major Caledonian thrust nappe in NW Scotland reflect a progressive increase in regional D2 strain towards the basal ductile detachment. Within the upper greenschist to lower amphibolite facies thrust sheet, the main gently east-dipping foliations and SE-plunging transport-parallel lineations maintain a broadly similar orientation over c. 600 km2. Associated main phase, thrust-related folds (F2) are widely developed, and towards the base of the thrust sheet display progressive tightening and increasing curvilinearity of fold hinges ultimately resulting in sheath folds. Secondary folds (F3) are largely restricted to high-strain zones and are interpreted as flow perturbation folds formed during non-coaxial, top-to-the-NW ductile thrusting. These features are consistent with a structural model that incorporates plane strain pure-shear flattening with a superimposed and highly variable simple shear component focused into high-strain zones. The increase in strain over a distance of 30 km across strike is similar to the increasing deformation observed when structures are traced along strike to the north, and which are apparently related to proximity to basement-cover contacts. A U-Pb zircon age of 415{+/-}6 Ma obtained from a syn-D2 meta-granite confirms that regional deformation occurred during the Scandian phase of the Caledonian orogeny.
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  • 3
    Publication Date: 2010-06-21
    Description: The Moine Supergroup of NW Scotland is a thick sequence of early Neoproterozoic sedimentary rocks, with minor igneous intrusions, that display evidence for multiple phases of regional deformation and metamorphism. The descriptions and interpretations of the Moine Schists' provided by the 1907 memoir (Peach et al. 1907) have been proved to be essentially correct and have laid the groundwork for a century of distinguished and influential research that has reached far beyond the confines of NW Scotland. The Survey workers recognized the sedimentary protoliths of these rocks, realized that they had been deposited unconformably on inliers of reworked basement gneisses that now occupy the cores of major folds, and understood the likely complexity of folding and the kinematic significance of mineral lineations. Further advances in understanding of the Moine rocks were mainly achieved through two techniques that were not available to the Survey workers of 100 years ago - geochronology and palaeomagnetism. Isotopic studies have confirmed the view of the Survey workers that the Moine rocks are of Precambrian age, and furthermore have demonstrated a complex, polyorogenic history.
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  • 4
    Publication Date: 2010-06-21
    Description: The world's mountain ranges are the clearest manifestations of long-term deformation of the continental crust. As such they have attracted geological investigations for centuries. Throughout this long history of research a few keynote publications stand out. One of the most important is the Geological Survey's 1907 memoir, The Geological Structure of the North-West Highlands of Scotland. The papers in this Special Publication celebrate the 100th anniversary of this remarkable book, placing the original findings in a present-day context by juxtaposing them against modern studies, not only from the NW Highlands, but also from elsewhere around the world. On first sight, the 1907 memoir by B. N. Peach and colleagues is simply a description of regional geology, albeit an exceptionally detailed one. However, the memoir outlines the Geological Survey's chief discoveries, and an abundance of minor ones, resulting from a decade of fieldwork in NW Scotland. At its height this activity occupied many of the most talented field geologists of Peach and Horne's generation and a significant number of the Scottish branch of the Survey. This team performed wondrous deeds, recognizing and interpreting many deformation structures on a variety of scales. The memoir describes these structures in a matter-of-fact manner, with occasional musings on their significance for broader tectonic processes. By focusing on the detail and establishing the supreme importance of getting the basic field relationships correct, the memoir's authors established the NW Highlands of Scotland as a prime location for training. Subsequent generations of geologists, from young students to...
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  • 5
    Publication Date: 2010-06-21
    Description: The Himalaya-Tibet and Caledonide orogens are comparable in scale and are similar in various aspects. Regional suture zones are recognizable in both, although their identification is more problematic in the deeply eroded Caledonide orogen. Crustal-scale thrust belts, regional Barrovian metamorphism characterized by clockwise P-T paths, and migmatitic cores with crustally-derived leucogranite complexes are the dominant structural feature of both orogens. Both orogens also record calc-alkaline magmatism attributed to subduction activity prior to collision. Syn-orogenic extension accompanied crustal thickening in both orogens, however, the Caledonides also have a protracted record of late- to post-orogenic extension that is attributed to lithospheric delamination in combination with oblique plate divergence. The oblique nature of the Caledonian collision is also reflected in the development of regionally significant sinistral strike-slip faults and shear zones, whereas such structures are apparently not as significant within the Himalayan orogen. The major difference between the two orogens relates to their contrasting gross structure: the Caledonides has bivergent geometry with thrust belts developed in the pro- and retro-wedges, whereas the Himalaya has a thrust belt located only in the pro-wedge segment. These differing geometries are probably explicable with reference to pre-collision contrasts in rheology and/or inherited structures. As such, there is no reason to suggest that either example should be viewed as being a typical' product of collisional orogenesis - they likely represent end-members of a range of possible orogenic profiles.
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  • 6
    Publication Date: 2010-06-21
    Description: Sharp lateral changes in structural geometry of ductile thrust stacks are not widely reported. A regional-scale lateral culmination wall forms the southern boundary of the Cassley Culmination in Moine rocks in the Caledonides of Sutherland, Northern Scotland. This culmination wall is part of the Oykel Transverse Zone (OTZ), a kilometre-scale shear zone characterized by constrictional finite strain fabrics aligned sub-parallel to the regional WNW-directed thrust transport direction. Main phase folds and fabrics in the transverse zone hanging wall are folded by main phase folds and fabrics in the footwall, thus recording foreland-propagating ductile deformation. South of the Cassley Culmination, shortening occurred uniformly, without development of discrete subsidiary thrusts; distributed deformation (fold development) alternated with localized thrusting within the culmination. The classic ESE-plunging mullions at Oykel Bridge are an integral part of the OTZ and were generated by constriction aligned sub-parallel to the transport direction. Constriction is attributed to differential, transtensional movement across the OTZ during culmination development. Subsequent formation of the underlying Assynt Culmination further accentuated upward-bulging of the Cassley Culmination, amplifying the lateral change across the transverse zone. The OTZ aligns with a pronounced gravity gradient on the south-western side of the Lairg gravity low. Interpretive modelling relates this gradient to a buried basement ramp that possibly controlled the location of the transverse zone.
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  • 7
    Publication Date: 2010-06-21
    Description: 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.
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  • 8
    Publication Date: 2012-09-01
    Description: Contrasting tectonic models for the Highland Border Complex in the Scottish Caledonides view it either as part of the rifted Laurentian margin of the Iapetus Ocean or as an oceanic terrane. Detrital zircon data from sandstones of the complex yield age peaks at 2.8–2.6, 1.3 and 1.1–1.0 Ga and minor peaks at c . 1.7–1.9 Ga. These characteristics compare closely with those of the upper Dalradian Supergroup of the adjacent Grampian terrane, and with the record of eastern Laurentia. The data are also consistent with the Laurentian provenance indicated by palaeontological evidence from the complex, and field evidence for continuity with the Dalradian Supergroup. Detrital ages for the Cambrian Salterella Grit of the Caledonian foreland compare with those for approximately age-equivalent sandstones from the Highland Border Complex. Both were contemporaneous with the regressive Hawke Bay event, accounting for similarities in provenance, and further linking the Highland Border Complex to Laurentia. The Grampian terrane was being uplifted and shedding detritus throughout the Ordovician and Silurian. The absence of this event from the detrital zircon records of either the Midland Valley or Southern Upland terranes suggests that these blocks cannot have been in their current location relative to the Grampian terrane before the end of the Silurian. Supplementary material: The complete analytical dataset and cathodoluminescence images are available at http://www.geolsoc.org.uk/SUP18531 .
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 9
    Publication Date: 2011-10-26
    Description: : An integrated in situ monazite laser-ablation inductively coupled plasma mass spectrometry and metamorphic equilibria study is used to establish the P – T conditions and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides. The results have implications for the structural setting of the Unst ophiolite, which was obducted onto metasedimentary rocks of the Dalradian Supergroup. Metapelites in the footwall of the ophiolite yield U–Pb ages between 462 and 451 Ma with P – T conditions varying from c . 7.5 kbar and 550 °C directly below the ophiolite to c . 10 kbar and 775 °C at structurally deeper levels. The timing of peak metamorphism corresponds closely to that of Grampian ( c . 450–470 Ma) metamorphism in mainland Scotland and Ireland, and Taconic ( c . 450–460 Ma) metamorphism in the Appalachians, thus confirming the near-synchroneity of this important arc accretion event along the Laurentian margin. There is a significant metamorphic contrast between the low-grade rocks associated with the Unst ophiolite and the P – T conditions recorded in its footwall. If published K–Ar ages of c. 470 Ma broadly record obduction of the ophiolite, its present basal contact is probably a younger tectonic break that was associated with the excision of at least c . 10 km of crustal section. Supplementary material: Electron microporobe chemical analyses of garnet grains are available at www.geolsoc.org.uk/SUP18494
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 10
    Publication Date: 2011-01-07
    Description: : The Moine Thrust Zone in the Scottish Highlands developed during the Scandian Event of the Caledonian Orogeny, and now forms the boundary between the Caledonian orogenic belt and the undeformed foreland. The Scandian Event, and the formation of the Moine Thrust Zone, have previously been dated by a range of isotopic methods, and relatively imprecise ages on a suite of alkaline intrusions localized along the thrust zone have provided the best age constraints for deformation. Recent British Geological Survey mapping has improved our understanding of the structural relationships of some of these intrusions, and this work is combined with new U–Pb dates in this paper to provide significantly improved ages for the Moine Thrust Zone. Our work shows that a single early intrusion (the Glen Dessarry Pluton) was emplaced within the orogenic belt to the east of the Moine Thrust Zone at 447.9 ± 2.9 Ma. A more significant pulse of magmatism centred in the Assynt area, which temporally overlapped movement in the thrust zone, occurred at 430.7 ± 0.5 Ma. Movement in the thrust zone had largely ceased by the time of emplacement of the youngest intrusions, the late suite of the Loch Borralan Pluton, at 429.2 ± 0.5 Ma, and the Loch Loyal Syenite Complex.
    Print ISSN: 0016-7649
    Topics: Geosciences
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