Anisotropy of Magnetic Susceptibility (AMS) fabrics within many individual granite plutons have previously been interpreted as recording the regional syn-magmatic tectonic strain field. To test this hypothesis, we compiled a regional database of AMS data from multiple granite complexes across two orogens, the French Massif Central and the British and Irish Caledonides, and critically evaluated the degree to which the magnetic fabric of the granite plutons recorded the known, regional tectonic strain. AMS fabrics from nine plutons from the French Massif Central show that all intrusions recorded the syn-magmatic late Variscan extensional collapse, with the maximum susceptibility axes (i.e., magnetic lineation) aligned with the NW-SE regional stretching direction. AMS fabrics from ten late Caledonian ‘Newer Granite’ plutons appear to reliably record the changing tectonic regimes between 430 and 390 Ma, including the switch from transpression to transtension following Iapetus closure, and then the return to transpression following the onset of the Acadian Orogeny at 400 Ma. This study indicates that comparisons between AMS fabrics and regional tectonics is best achieved qualitatively by comparing the orientation of the susceptibility axes to the known strain field, and more quantitatively through Woodcock analysis. Overall, our results indicate that pluton-scale AMS fabrics from multiple complexes spaced across an orogen can record a complex and changing regional tectonic strain field. This indicates there is significant potential to utilise pluton-scale AMS studies, alongside precise geochronological ages, to refine the timings of an orogen’s tectonic evolution.

Nepheline syenites from the X 1.2 Ga Il & iacute;maussaq Complex of southern Greenland are examined to assess the utility of anisotropy of magnetic susceptibility (AMS) fabrics as proxies for silicate petrofabrics. Mineral lamination is a relatively common structural feature in cumulate rocks, including in the Il & iacute;maussaq intrusion, but there is little consensus on the process (or processes) responsible for its formation. The Il & iacute;maussaq AMS data are combined with rock magnetic experiments and electron backscatter diffraction (EBSD) measurements to characterize the magnetic mineralogy and compare the magnetic fabrics obtained to the silicate petrofabric. The data show that Na-amphibole (arfvedsonite) is most likely the dominant control on the AMS fabrics in the coarse-grained nepheline syenites (referred to as kakortokites), and that the AMS fabric is inverse relative to the observed silicate fabric. The EBSD data for a kakortokite sample suggests that the petrofabric is defned by arfvedsonite and is wholly planar, with evidence of only weak cross-lineation of c axes. The f ne-grained nepheline syenites (lujavrites), two of which have a well-developed lamination carried by Na-pyroxene (aegirine), appear to have composite AMS fabrics that are considered to be a consequence of a mixed aegirine (normal) and arfvedsonite (inverse) response. The combined datasets shed light on the mechanisms of fabric acquisition in both lithologies. In the kakortokites, the AMS fabrics and silicate crystallographic preferred orientations, as well as the lack of observed microstructural evidence for subsolidus intra-crystal deformation, support models invoking gravitationally controlled crystal mats in the development of the macro-rhythmic layering of these rocks. In the lujavrites, the strong planar fabrics revealed by both the AMS and EBSD datasets, with some evidence of subsolidus deformation, point to fabric formation and perhaps even aegirine crystallization at the postcumulus stage. The combination of EBSD and AMS fabric datasets is a powerful means of deciphering the processes responsible for mineral alignment in igneous cumulates.

Abstract The Antrim Lava Group of NE Ireland comprises a volcanic sequence dominated by basaltic lava flows. Including subsidiary sedimentary interlayers and some evolved lavas and intrusions, the overall sequence reaches a cumulative thickness of ∼800 m. The tempo of eruption of the Antrim Lava Group is poorly constrained but can be evaluated via weathering patterns and environmental reconstructions derived from lava-flow interbeds. In this contribution, we present palynology from a newly identified and well-developed 2.0–2.5 m thick sedimentary sequence (interbed) at Ross's Quarry, Ballycastle, Co. Antrim, that helps elucidate the contemporary development of environments in a setting subject to periodic basaltic volcanism. The interbed is subdivided into geologically distinct subunits of cross-bedded and parallel-bedded sandstones and sandy siltstones, all rich in visible organic remains such as rootlets and fragments of wood and bark. A total of 19 samples was collected from the sequence and subsequently analysed for palynological content. The palynomorph data point toward a diversity of inputs ranging from estuaries, chalky soils, dry soils, swamps, lakes, floodplains, sand bars, wet soils, established bogs and fenlands. In contrast to current understanding, the palynological data and their inferred environments collectively reveal the presence of flora that favour a temperate climate rather than the subtropical climate that has previously been inferred from the lateritic interbeds of the Antrim Lava Group. By combining the Ross's Quarry observations with palynological data from other quarry sites and boreholes in Antrim, we provide new insights into the climate, weathering systems and eruptive history of the Antrim Lava Group.

The Ratagain Complex is an enigmatic Late Caledonian granitic intrusion and a member of the high Ba-Sr Northern Highlands granite (NHG) suite that has been related to slab failure. Slab failure magmatism explains varying contributions of mafic and felsic magmas in post-collision orogenic settings. It is therefore of major importance in understanding crustal accretion. However, the source and nature of any mantle derived contri-bution is poorly understood. This study reveals that Ratagain is not only transitional in nature between the high Ba-Sr calc-alkaline granites and syenite intrusions of the Northern Highlands Terrane, but overlaps with the entire compositional range of the NHG suite. New lithogeochemical data from Ratagain confirm remarkably high Sr (>1600 ppm) and Ba (>2200 ppm) contents, high LREEs, notable depletions in Nb, U, P and Ti, low HREEs and negligible Eu anomalies, associated with high initial 87Sr/86Sr (0.7055 to 0.7062) and low epsilon Nd (-11.8 to-13.3). Although mafic parts of the complex have strong elemental and isotopic similarities with broadly coeval lamprophyres, signalling derivation from enriched mantle sources, details of the isotope array with respect to local crustal reservoirs indicate a significant Lewisian component. Such geochemical characteristics, combined with tectonic and petrological evidence, may be attributable to long-lived, incremental emplacement of suc-cessive magma batches originating from the same enriched mantle but differing in age and extent of assimilation -fractionation crystallisation. We therefore propose that some of the age dates for the Late Caledonian intrusions, particularly those obtained from older geochronology studies, are in need of review as they may record early crystallisation in the deep crust and not be a valid proxy for granite emplacement.

The anisotropy of magnetic susceptibility (AMS) is used to reveal subtle mineral alignment fabrics in apparently isotropic crystalline lithologies, including granites. Such petrofabrics can be produced by emplacement-related magma flow or post-emplacement tectonic strain. However, discriminating between flow-related and tectonic fabrics using field observations alone may be challenging and is usually a broad and arbitrary interpretation. In this contribution, we employ a range of magnetic analyses to characterize the origin of the petrofabric in the c. 425 Ma Ratagain Complex, NW Scotland, a composite Late Caledonian granitic intrusion. Our detailed magnetic analyses reveal that whilst all intrusive units carry an ambient tectonic overprint, critically, this has not developed into an obvious tectonic fabric and contains a horizontal shortening component indicative of transpression. This appears at odds with the well-defined Silurian (Scandian phase) regional transtensional tectonic regime from c. 420–415 Ma onwards. Accordingly, we suggest that either the complex is younger than previously thought or that it existed as a crystal-mush close to the magmatic solidus for a protracted period after its initial emplacement. This study lays the foundations for much-needed further investigations into the detailed emplacement mechanisms, timescales and petrogenesis of individual granitic intrusions, to aid understanding of Late Caledonian tectonics. Supplementary material : Supplementary data to this article are available at https://doi.org/10.6084/m9.figshare.c.5941375 Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/cc/SJG-early-career-research

Igneous intrusions record the movement and storage of magma within the Earth's crust and understanding how intrusions are emplaced is key to our understanding of how magma is transported from its source of origin, to in many cases, it's eventual eruption. Igneous intrusions come in many different shapes, sizes and compositions, with the terminology used to describe them being equally diverse. However, intrusions can be fundamentally split into two main categories; tabular intrusions, in the form of sills and dykes; and plutons, which are often volumetrically large, and are generally less tabular in morphology when compared to sills and dykes. Whilst classification of intrusions as sills, dykes or plutons (or other types of intrusion) is effective on an outcrop scale, the applicability of such classifications on regional-to-crustal scales can be more problematic. In the case of plutons, it is now generally understood that large, long-lived magma chambers do not necessarily form major components of the magmatic system. In many cases, the sills, dykes and plutons form complex interconnected networks within the crust, with sills intrusions for example inflating and coalescing with other intrusions leading to the formation of large plutonic bodies. Tabular intrusions are emplaced within the upper crust via two end-member modes. The first is the emplacement of magma in a brittle fashion, whereby fracture(s) extends ahead of the tip of the magma in a sill or dyke. In the second mode, the host rock does not behave in a brittle manner during magma intrusion, and the magma and host rock interact in a fashion akin to the interaction of two viscous fluids. Both modes of emplacement create distinctive morphologies and structures within the resulting intrusions, which can be observed in field outcrops to assist the identification and understanding of intrusion emplacement mechanisms.

As geological modelling and analysis move into 3D digital space, it becomes increasingly important to be able to rapidly integrate new data with existing databases, without the potential degradation caused by repeated manual transcription of numeric, graphical and meta-data. Digital field mapping offers significant benefits when compared with traditional paper mapping techniques, in that it can directly and interactively feed and be guided by downstream geological modelling and analysis. For surface structural data, model-building often begins with collection during field mapping. When field mapping is done digitally, with the right software tools, the user benefits from all the advantages of working with geospatial data yet loses none of the benefits of traditional paper mapping. Collection of field data in a geo-referenced digital environment allows the user to integrate multiple data types, which can then be quickly referred to in the field. The Petroleum Experts (Petex) FieldMoveTM and FieldMOVE ClinoTM apps replicate and enhance the traditional paper mapping experience. They provides the flexibility to work at multiple scales in one project. Digital mapping removes the potential for errors when generating a ‘fair copy’ map as there is no need to prepare it separately, allowing the user to directly start data analysis during the field campaign. Core geological skills can be supplemented by integrated digital workflows which lead from mapping to rule-based model construction in 2D and 3D. The aim of structural modelling in a digital environment realises the potential of information technology to de-risk geological models used in surface exploration and evaluation. Learning basic mapping through field work gives no better introduction and exposure to some of the main concepts and challenges that geologists will encounter when entering exploration companies and modelling more fully their current or potential assets. Here we present a digital field-mapping and 3D model building case study, using FieldMOVE, FieldMOVE Clino and MOVETM, from the Lower Palaeozoic Wren's Nest anticline in the West Midlands of the UK. Petex (and previously Midland Valley) continues to pioneer phone- and tablet-based mapping, which is widely used by industrial and academic partners, and continues to integrate feedback in further developments of this technology.

The developing asymmetry of rifting and continental breakup to form rifted margins has been much debated, as has the formation, mechanics and role of extensional detachments. Bespoke 3D seismic reflection data across the Galicia margin, west of Spain, image in unprecedented detail an asymmetric detachment (the S reflector). Mapping S in 3D reveals its surface is corrugated, proving that the overlying crustal blocks slipped on S surface during the rifting. Crucially, the 3D data show that the corrugations on S perfectly match the corrugations observed on the present-day block-bounding faults, demonstrating that S is a composite surface, comprising the juxtaposed rotated roots of block-bounding faults as in a rolling hinge system with each new fault propagation moving rifting oceanward; changes in the orientation of the corrugations record the same oceanward migration. However, in contrast to previous rolling hinge models, the slip of the crustal blocks on S occurred at angles as low as 20, requiring that S was unusually weak, consistent with the hydration of the underlying mantle by seawater ingress following the embrittlement of the entire crust. As the crust only becomes entirely brittle once thinned to 10 km, the asymmetric S detachment and the hyper-extension of the continental crust only developed late in the rifting process, which is consistent with the observed development of asymmetry between conjugate magma poor margin pairs. The 3D volume allows analysis of the heaves and along strike architecture of the normal faults, whose planes laterally die or spatially link together, implying overlaps in faults activity during hyper-extension. Our results thus reveal for the first time the 3D mechanics and timing of detachment faulting growth, the relationship between the detachment and the network of block-bounding faults above it and the key processes controlling the asymmetrical development of conjugate rifted margins. (C) 2019 Elsevier B.V. All rights reserved.

12 The developing asymmetry of rifting and continental breakup to form rifted margins has been 13 much debated, as has the formation, mechanics and role of extensional detachments. Bespoke 14 3D seismic reflection data across the Galicia margin, west of Spain, image in unprecedented 15 detail an asymmetric detachment (the S reflector). Mapping S in 3D reveals its surface is 16 corrugated, proving that the overlying crustal blocks slipped on S surface during the rifting. 17 Crucially, the 3D data show that the corrugations on S perfectly match the corrugations 18 observed on the present-day block-bounding faults, demonstrating that S is a composite 19 surface, comprising the juxtaposed rotated roots of block-bounding faults as in a rolling hinge 20 system with each new fault propagation moving rifting oceanward; changes in the orientation 21 of the corrugations record the same oceanward migration. However, in contrast to previous 22 rolling hinge models, the slip of the crustal blocks on S occurred at angles as low as ~20°, 23 requiring that S was unusually weak, consistent with the hydration of the underlying mantle 24

Although many intrusions are now known to have been incrementally emplaced, the mechanisms through which this takes place are generally poorly understood. The Newry igneous complex was incrementally emplaced within the Southern Uplands-DownLongford terrane of Northern Ireland during late Caledonian sinistral transtension. This study uses a variety of new and existing data and techniques to provide a fuller and firmer understanding of incremental emplacement than has previously been available, addressing both deep-crustal processes and those operating within the emplacement site. Host-rock orientations suggest that some of the accommodation space for the Newry igneous complex was generated due to pull-apart tectonics operating within the Southern Uplands-Down-Longford terrane. Local host-rock deflections, concentric igneous foliations, and concentric linear anisotropy of magnetic susceptibility (AMS) fabrics show that inflation due to magma overpressure also generated significant space. Strong AMS fabrics close to the boundaries of some magma pulses in turn suggest that inflation was accomplished by injection of individual magma pulses and was thus incremental. The dome-like orientations of mineral foliations within plutons and the truncation of steep local host-rock tracts by the Newry igneous complex imply that the complex consists of four laccolithic bodies. On a larger scale, it is suggested that the deep-seated Argyll and Newry lineaments represent faults that allowed magma generated at depth to ascend to the crustal level of the Southern Uplands-Down-Longford tract boundaries. It is also inferred that sinistral movement along the Argyll and Newry lineaments may have produced the releasing bend within the Southern Uplands-Down-Longford terrane. Higher in the crust, reduced confining pressure resulted in tectonic opening along this releasing bend. This local stress field induced horizontal magma flow and emplacement of the Newry igneous complex as laccolithic bodies. This study suggests that simplistic emplacement models should largely be abandoned in favor of holistic models incorporating the multiple interdependent processes operating during magma ascent and emplacement.

A systematic view of the vast nomenclature used to describe the structuresStructures of shallow-level intrusionsShallow-level intrusions is presented here. Structures are organised in four main groups, according to logical breaks in the timing of magma emplacement, independent of the scales of features: (1) Intrusion-related structures, formed as the magma is making space and then develops into its intrusion shape; (2) Magmatic flow-related structures, developed as magma moves with suspended crystals that are free to rotate; (3) Solid-state, flow-related structures that formed in portions of the intrusions affected by continuing flow of nearby magma, therefore considered to have a syn-magmatic, non-tectonic origin; (4) Thermal and fragmental structures, related to creation of space and impact on host materials. This scheme appears as a rational organisation, helpful in describing and interpreting the large variety of structures observed in shallow-level intrusions.

Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry and electromagnetic data can identify contemporary melt zones, magma reservoirs and/or crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs) and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community.

The Northern Arran Granite is regarded as an example of an upper-crustal granite diapir due to its sub circular outcrop and deformed aureole. However diapiric emplacement to shallow levels in the crust is physically difficult and unambiguous evidence for shouldering aside of country rocks by a diapir as opposed to shouldering due to in situ expansion of a laccolith or ballooning pluton is difficult to find. The key is in finding evidence for vertical ascent of a diapiric body where a diapir should preserve vertical stretching either around the periphery or in central portions. A laccolith on the other hand is unlikely to have consistently vertical or steep lineations but contain mainly gently plunging lineations or evidence of multiple horizontal pulses or lobes. Therefore evidence for diapiric ascent and emplacement should be dominated by vertical kinematic indicators. These structures may however be too subtle if preserved within the granite, and those in the aureole may become overprinted by late stage insitu expansion. To test whether the internal structure of the Northern Arran Granite is consistent with diapiric or laccolithic emplacement we have measured the anisotropy of magnetic susceptibility from oriented block samples from the Northern Arran Granite to determine if there are subtle or weak fabrics that will support either diapirism with vertically oriented stretching, or laccolithic emplacement where lateral emplacement is dominant and vertical motion only restricted to vertical thickening of an initially thin sheet. Our results reveal concordant dome shaped planar fabrics with mainly gently plunging or horizontal lineation, i.e. an absence of vertical stretching or flow, and possible evidence of partial lobes in the north western margins and Inner Granite. We interpret these fabrics and the deformation of the aureole in terms of dome-shaped expansion consistent with the latter model. In more detail the lineation pattern indicates convergence toward a south or south eastern point, consistent with the deepest part of the pluton (from previously published gravity modelling). Our model suggests that there may be some link to a major crustal structure (the Highland Boundary Fault) providing insight into the ascent route of the magma and possible influence of the Highland Boundary fault zone. This model also suggests that the magmatism on Arran including the central complex and sill complexes in the south of the island may have been supplied by a long lived, deeply penetrating feeder zone controlled by this major tectonic structure.

Our knowledge of igneous emplacement in sedimentary basins has been revolutionised by studies of offshore 3D seismic reflection3D seismic reflection data, where large scale structures and relationships are realised. These offshore data sets require detailed information from onshore analogues to fully understand subsurface structure of such intrusions and their potential effect on petroleum systemsPetroleum systems . The Inner HebridesInner Hebrides of Western Scotland, which contains an onshore record of the extensive Palaeogene magmatic activity that affected much of the North Atlantic, allows us to examine some of the classic sill geometries on a seismic to sub-seismic (outcrop) scale. As hydrocarbon exploration moves to more challenging basins, it is clear the need exists for us to fully understand the role in which intrusive volcanism plays in active hydrocarbon systems. Intrusions in general can have major effects on prospective sedimentary basins by forming interconnected low-permeability zones which can compartmentalise significant volumes of source and reservoir rock. We present a series of outcrop case studies which allow the potential influences of sills on what would represent potential source and reservoir rock intervals to be addressed and discuss the wider implications for sill emplacement in such basins.

The movement of magma through the shallow crust and the impact of subsurface sill complexes on the hydrocarbon systems of prospective sedimentary basins has long been an area of interest and debate. Based on 3D seismic reflection and well data, we present a regional analysis of the emplacement and magmatic plumbing system of the Palaeogene Faroe‐Shetland Sill Complex (FSSC), which is intruded into the Mesozoic and Cenozoic sequences of the Faroe‐Shetland Basin (FSB). Identification of magma flow directions through detailed seismic interpretation of approximately 100 sills indicates that the main magma input zones into the FSB were controlled primarily by the NE–SW basin structure that compartmentalise the FSB into its constituent sub‐basins. An analysis of well data shows that potentially up to 88% of sills in the FSSC are <40 m in thickness, and thus below the vertical resolution limit of seismic data at depths at which most sills occur. This resolution limitation suggests that caution needs to be exercised when interpreting magmatic systems from seismic data alone, as a large amount of intrusive material could potentially be missed. The interaction of the FSSC with the petroleum systems of the FSB is not well understood. Given the close association between the FSSC and potential petroleum migration routes into some of the oil/gas fields (e.g. Tormore), the role the intrusions may have played in compartmentalisation of basin fill needs to be taken fully into account to further unlock the future petroleum potential of the FSB.

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