Ian Alsop

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Papers by Ian Alsop

Fold and Thrust Systems in Mass‐Transport Deposits Around the Dead Sea Basin

Geophysical monograph, Nov 4, 2019

Distinguishing coeval patterns of contraction and collapse around flow lobes in mass transport deposits

Journal of Structural Geology, May 1, 2020

Abstract Gravity-driven mass transport deposits (MTDs) form by the downslope-directed movement of... more Abstract Gravity-driven mass transport deposits (MTDs) form by the downslope-directed movement of sediment associated with slope failure. Simple models suggest that extension forms at the upslope (head) area, contraction is focussed in the downslope toe of the slump, while differential shear associated with strike-slip is restricted to the lateral margins of the slump. Although the head and toe are considered to be dominated by layer-parallel shear (LPS), differential layer-normal shear (LNS) may be generated around the lateral margins of slumps and potentially also within MTDs where flow has been separated into different ‘lobes’. Despite this realisation that LNS must form, there has been little work into the geometries and spatial relationships of resulting structures. Using the late Pleistocene Lisan Formation exposed around the Dead Sea Basin as our case study, we examine detailed (

Distinguishing thrust sequences in gravity-driven fold and thrust belts

Journal of Structural Geology, Apr 1, 2018

Piggyback or foreland-propagating thrust sequences, where younger thrusts develop in the footwall... more Piggyback or foreland-propagating thrust sequences, where younger thrusts develop in the footwalls of existing thrusts, are generally assumed to be the typical order of thrust development in most orogenic settings. However, overstep or ‘break-back’ sequences, where later thrusts develop above and in the hangingwalls of earlier thrusts, may potentially form during cessation of movement in gravity-driven mass transport deposits (MTDs). In this study, we provide a detailed outcrop-based analysis of such an overstep thrust sequence developed in an MTD in the southern Dead Sea Basin. Evidence that may be used to discriminate overstep thrusting from piggyback thrust sequences within the gravity-driven fold and thrust belt includes upright folds and forethrusts that are cut by younger overlying thrusts. Backthrusts form ideal markers that are also clearly offset and cut by overlying younger forethrusts. Portions of the basal detachment to the thrust system are folded and locally imbricated in footwall synclines below forethrust ramps, and these geometries also support an overstep sequence. However, new ‘short-cut’ basal detachments develop below these synclines, indicating that movement continued on the basal detachment rather than it being abandoned as in classic overstep sequences. Further evidence for ‘synchronous thrusting’, where movement on more than one thrust occurs at the same time, is provided by displacement patterns on sequences of thrust ramp imbricates that systematically increases downslope towards the toe of the MTD. Older thrusts that initiate downslope in the broadly overstep sequence continue to move and therefore accrue greater displacements during synchronous thrusting. Our study provides a template to help distinguish different thrust sequences in both orogenic settings and gravity-driven surficial systems, with displacement patterns potentially being imaged in seismic sections across offshore MTDs.

The Caledonian strike-swing and associated lineaments in NW Ireland and adjacent areas: sedimentation, deformation and igneous intrusion patterns

Journal of the Geological Society, May 1, 1996

Part of the major swing in strike associated with the Greenland-Labrador Promontory of the North ... more Part of the major swing in strike associated with the Greenland-Labrador Promontory of the North Atlantic Caledonides is exposed in considerable detail in the metasedimentary and meta-igneous rocks of the Neo-Proterozoic Dalradian Supergroup in northwest Ireland. We show that the strike swing controlled Dalradian stratigraphy and sedimentation patterns and is therefore at least c. 600 Ma in age. Our interpretation of the stratigraphy allows a reconstruction of this promontory in the Laurentian continental margin and shows that it separated a NE-trending. gently inclined and broad continental shelf in the north from an E-trending. more steeply inclined and narrower shelf to the south. Once established in the geometry of the Dalradian sequence, the strike swing and basement promontory had a major influence on Caledonian deformation patterns. It controlled: the nucleation of major folds: the geometry of the regionally developed primary cleavages; the facing direction of the early folds: and it also acted as a major buttress in perturbing and deflecting. on a regional scale, the orogenic transport vector. Accurate restoration of the sinistral displacement on a late Caledonian fault system shows that the apices of the strike swing at different stratigraphic levels, and their associated sedimentary facies changes, define a line trending approximately N12ºE. This line is coincident with (again, after fault restoration): six out of the eight Devonian Donegal granites: the vast majority of the mantle-related, ultrabasic ‘Appinite’ suite (with most of these bodies concentrated at the intersection of this line and the NE-trending Main Donegal Granite shear zone): and the highest density of the Neo-Proterozoic basic tholeiite suite. Thes e and other data are used to suggest that the Donegal Lineament is the expression of a major steeply inclined fault in the basement beneath the Dalradian which is intimately related to the morphology of the promontory. The fault, which might be as old as 1800 Ma, probably reached down into the lithospheric mantle. controlling igneous activity, including the ascent, emplacement site and perhaps sources of the magmas. Another complementary strike swing and parallel lineament is described from the Dalradian rocks of the adjacent part of Scotland. It is suggested that these and other major lineaments (i.e old pre-Caledonian faults) which have either NNE. NE, or ESE trends reflect not so much the location of the northern British Isles at a 120º triple junction during Iapetus rifting. but rather the exploitation of these pre-existing trends in the pre-Caledonian Atlantic borderlands during Iapetus opening.

Discussion on major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: implications for Caledonian tectonics and mid-crustal shear zones

Geological Magazine, May 1, 1994

Deformation bands and their impact on fluid flow: Insights from geometrical modelling and multi-scale flow simulations in sandstones

Journal of Structural Geology, Dec 1, 2020

Abstract Deformation bands may show permeabilities that are a few orders of magnitude lower than ... more Abstract Deformation bands may show permeabilities that are a few orders of magnitude lower than the surrounding host rocks and therefore have a negative impact on fluid flow in sandstone hydrocarbon reservoirs. Understanding the geometrical attributes of individual bands and their patterns is a critical step in quantifying their connectivity and interpreting how they may compartmentalise reservoirs in the subsurface. In order to gain insights into the shapes, cross-cutting relationships and connectivity of these structures, we developed 3D geometrical models (scaled to centimetres) of deformation bands and joining ladder structures, using a sliced small-scale rock sample from outcrops at Hopeman, Moray Firth (Scotland) and near Goblin Valley, Utah (USA). This 3D geometrical model is used for two-phase flow simulations (water-oil and gas-oil) in order to investigate the effect of deformation bands on time to water breakthrough, waterfront propagation and sweep efficiency. We also investigated capillary effects on flow. Flow simulation results show that the presence of deformation bands led to various time to water and gas breakthroughs, irregular shape of water and gas front propagation and various sweep efficiency for different flow-system. Furthermore, understanding the dynamic behaviour of flow through these structures can lead to better predictions of the influence of deformation bands on fluid flow in subsurface reservoirs.

Ben Hee. Scotland Sheet 108W. Bedrock. 1:50,000 Geology Series

The Dalradian of Central Donegal : an example of polyphase mid-crustal thrusting

The geometry and kinematic development of a major syn-metamorphic slide zone, the Central Donegal... more The geometry and kinematic development of a major syn-metamorphic slide zone, the Central Donegal slide, is described from the Dalradian Succession of Central Donegal, Ireland. The earliest manifestation of this structure is a large scale reversal in the direction of younging and stratigraphy across the slide plane, a feature that probably developed at the lower greenschist facies of metamorphism. Kinematic indicators associated with this early deformation have been destroyed by later higher metamorphic grade overprinting, but by correlation with other early structures in Donegal, and with reference to sub-basin development in South and Central Donegal it was probably a Grampian thrust with North West directed transport. This tectonic package was then redeformed at lower amphibolite facies by a kilometric scale shear zone associated with slides and large scale sheath folds. Kinematic indicators show that the sheath folds 'intrude' downwards to the south east although originally they are regarded as having faced upwards to the south east. Following this phase of deformation the Dalradian stratigraphy of Central and South Donegal was largely uninverted and probably lay on the upper normal limb (and represents a deeper level) of the upward facing Glenelly Anticline, a major fold to the east in the Sperrin Mountains which is equated with the Tay Nappe. The zone was then reworked during another (mid-amphibolite facies) event during which large scale south facing and verging folds were formed: the Ballybofey Antiformal complex. Earlier formed sheath folds were refolded, slides reactivated and new slides formed. This phase of deformation is associated with major oblique dextral overthrusting, which with increasing metamorphic grade eventually carries the entire Dalradian cover sequence over Moine - like Proterozoic basement 30 km to the south along the Lough Derg slide. This second amphibolite facies metamorphism can be directly associated with the large scale South East directed overthrusting of the entire Dalradian stratigraphy, and related to crustal thickening and loading via the development of a nappe pile towards the south.

The effect of rock type on natural water flooding and residual oil saturation below free water level and oil water contact: A case study from the Middle East

Journal of Petroleum Science and Engineering, Oct 1, 2020

The presence of residual oil below the present day free water level (FWL) and oil water contact (... more The presence of residual oil below the present day free water level (FWL) and oil water contact (OWC) is common in many fields of the Middle East. This residual oil is seen in reservoirs prior to the start of production. A rock typing study was carried out to investigate the effect of rock types on the presence and distribution of residual oil below the free water level (FWL) and oil water contact (OWC) and potential recovery from ROZ's following water flooding. The rock typing study was carried out by integration of different data sources i.e. core description, thin section study, conventional core analysis (CCAL), special core analysis (SCAL) and petrophysical logs. Six major rock types with unique reservoir quality and behaviour were classified in cored wells and subsequently predicted in un-cored wells. Reservoir quality increases from rock type 1 to 6. The residual oil saturation (Sor) of each rock type was defined and it was concluded that all rock types except for rock type 1 contain residual oil below the present day FWL and OWC. The absence of residual oil in rock type 1 was related to the fact that it is unlikely that oil migration into it ever occurred and consequently no residual oil is seen below the present day FWL and OWC. It was also revealed that rock type 6, with better porosity and permeability, includes the highest residual oil saturation. This was attributed to its bimodal pore throat size distribution and the effect of this on water flooding and hence potential recovery.

format_quoteCapillary pressure curves reveal properties across rock types, indicating significant differences in reservoir behaviors across the study area.format_quote

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Coseismic horizontal slip revealed by sheared clastic dikes in the Dead Sea Basin

Geological Society of America Bulletin, Mar 24, 2016

Despite the hazard caused by near-surface destructive horizontal displacements during earthquakes... more Despite the hazard caused by near-surface destructive horizontal displacements during earthquakes, field evidence for coseismic slip along horizontal discontinuities is exceptionally rare, mainly due to the lack of adequate exposure and markers. However, within the seismically active Dead Sea basin, the Late Pleistocene Lisan Formation contains vertical clastic dikes at maximum depth of 15 m that are sheared laterally and thereby provide unique profiles of such horizontal displacement. In order to investigate how coseismic horizontal shearing is distributed near the surface, we document a ~1 m thick brittle shear zone, comprising up to eleven slip surfaces that can be traced for tens of meters in the Lisan Formation. Displacements along Manuscript Click here to download Manuscript Sheared dikes 22_12_2015.docx individual slip surfaces are up to 0.6 m and the total displacement across the shear zone is up to 2.0 m. Displacement profiles and gradients indicate that the brittle shear zone formed by simple shear, and deformation was associated with slip partitioning and transfer between primary and secondary slip surfaces. Evidence for concurrent displacement along slip surfaces during a single event indicates that the brittle shear zone was formed during a coseismic event post 30 ka. We consider the mechanical effect of seismic-wave related transient stress, which when added to the initial static effective stress, may result in concurrent horizontal shear failure along detrital-rich layers in the Lisan Formation. The exceptional quality of exposures and markers enables us to document for the first time, the details of near-surface horizontal shearing, and indicates that displacement along horizontal bedding planes is a viable mechanism to absorb coseismic deformation in well-bedded near surface strata.

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Tectonic controls on residual oil saturation below the present-day fluid contact level in reservoirs of the Persian Gulf

Journal of Asian Earth Sciences, Apr 1, 2020

The presence of residual oil below the present-day free water level (FWL) and oil water contact (... more The presence of residual oil below the present-day free water level (FWL) and oil water contact (OWC) is common in many oil fields in the Middle East, particularly those in the Persian Gulf. This residual oil is seen in both clastic and carbonate reservoirs prior to the start of production. The characterisation and modelling of these fields is difficult in practice. Also, these residual oils below the FWL and OWC could become classified as reserves if ways to produce them could be found. However, the first step is to better understand their origin. Therefore, the goal of this study was to investigate the role of geological events on the presence of the residual oil zone (ROZ) below the FWL and OWC. It has been suggested that the presence of residual oil below the present day FWL and OWC is related to the geotectonic history of the region. From the middle Miocene, reverse faulting and overfolding propagated over the Zagros, leading to the amplification of folds and the migration of the Zagros orogeny towards its foreland basin (Persian Gulf). In response to this additional massive loading on the continental margin, the forebulge amplitude was increased, its location migrated towards the uplifted Zagros Mountains, and consequently the Persian Gulf became narrower. This exerted a north to north-east downward tilting of the entire basin, including all the structures and reservoirs previously filled by hydrocarbons. This basin tilting changed the equilibrium of the structures and their fluid contents, and resulted in the hydrocarbons and water attempting to find a new equilibrium. Under these conditions, the early migrated and accumulated oil was flushed out by water (imbibition), and a ROZ was left below the present day FWL and OWCs. The angle of regional basin tilt has been calculated to be 0.836° based on seismic sections.

format_quoteA comprehensive literature review integrated geological, petrophysical data to model residual oil presence below fluid contacts.format_quote

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Magnetic fabrics as strain markers in folded soft-sediment layers

Journal of Structural Geology, Feb 1, 2022

We exploit the potential of magnetic fabrics acting as strain markers in folded layers, by analys... more We exploit the potential of magnetic fabrics acting as strain markers in folded layers, by analysing an exceptionally well-exposed, recent (<1 kyr) slump horizon in unlithified lake deposits within the Dead Sea basin. The ~3-m-long folded soft-sediment layer, together with an underlying basal detachment, and an 'undeformed' reference layer are extensively sampled (n=97) for an anisotropy of magnetic susceptibility (AMS) analysis. This analysis reveals deformation fabrics within the folded layer which are significantly different from fabrics detected in the 'undeformed' layer. The maximum magnetic susceptibility axes (K1) show a hinge-parallel orientation, and the minimum magnetic susceptibility axes (K3) show a trail of orientations directed eastward parallel to the direction of downslope slumping toward the depocenter of the basin. In terms of shape of the AMS, samples from the 'undeformed' layer are oblate, while the majority of samples from the fold backlimb are oblate to neutral, and those from the forelimb and hinge zones are more prolate. We postulate that the deformation shown by the AMS analysis approximates well to sections through the strain ellipsoid in the folded layer, suggesting that magnetic fabrics serve as strain markers that are invisible to the naked eye. The deformation fabrics are created by particles moving relative to one another and reorganising during hydroplastic deformation. Particles physically rotate in the hinge zone, resulting in shortening of the intermediate axes and creation of more prolate shapes. The combination of two types of fabrics (deposition and deformation) in the hinge zones increases the intensity of the lineation due to the intersection of the primary and secondary fabrics (foliations). Based on the dense sampling scheme, we produce GIS-based interpolation maps that show the spatial distribution of the AMS parameters in the folded layer. These maps are compared to data from classical strain analyses, providing a benchmark for combining traditional structural methods and AMS analyses in studying folding and softsediment deformation.

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Bed-parallel slip: Identifying missing displacement in mass transport deposits

Journal of Structural Geology, Feb 1, 2020

Bed-parallel slip (BPS), where neighbouring beds slide past one another along bedding planes, is ... more Bed-parallel slip (BPS), where neighbouring beds slide past one another along bedding planes, is notoriously difficult to identify without reference to pre-existing features such as steep faults or dykes that act as markers to record BPS offset. While BPS is intuitively thought to operate during downslope sliding of mass transport deposits (MTDs) in sedimentary basins, there is a conspicuous lack of supporting outcrop or seismic data to corroborate this and BPS displacement may therefore have remained unaccounted for. This study addresses this gap in knowledge by investigating late-Pleistocene MTDs developed around the Dead Sea Basin and provides the first detailed analysis of BPS that pervades a gravity-driven setting. In particular, we examine the role of BPS that crosscuts earlier normal and reverse faults that act as markers to allow metre-scale patterns of horizontal displacement to be identified in MTDs. The studied BPS always forms with a consistent top-to-the east sense of offset that corresponds with gravity-driven downslope movement towards the depo-centre of the basin. BPS frequently develops adjacent to competent detrital-rich beds and forms discrete glide planes with little or no visible deformation in sediments on either margin, although detachment folds are occasionally developed above the slip plane and confirm directions of easterly movement. Early downslope-dipping normal faults that are cut by later BPS planes results in older over younger stratigraphic relationships across the BPS surface, together with 'sawtooth' patterns where multiple BPS planes have developed. Conversely, early upslope-dipping normal faults that are cut by BPS create younger over older stratigraphic relationships combined with missing section and 'staircase' patterns where multiple BPS planes exist. As BPS in subhorizontal sequences does not have a vertical component of displacement, it may be examined in terms of horizontal 'heave'. BPS increases heave in upslope-dipping normal faults, whereas it reduces heave in downslope-dipping normal faults and may even become negative where sections are repeated across sawtooth profiles. In addition, BPS increases heave in upslope-dipping reverse/thrust faults and reduces heave across downslope dipping 'backthrusts'. Although individual BPS planes may have limited displacement, the net consequence of multiple planes of BPS that form in the shallow-subsurface is to distort patterns and estimates of extension and contraction across fault zones in MTDs.

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$Research paper thumbnail of Fault and fracture patterns around a strike-slip influenced salt wall$

Fault and fracture patterns around a strike-slip influenced salt wall

Journal of Structural Geology, 2018

The trends of faults and fractures in overburden next to a salt diapir are generally considered t... more The trends of faults and fractures in overburden next to a salt diapir are generally considered to be either parallel to the salt margin to form concentric patterns, or at right angles to the salt contact to create an overall radial distribution around the diapir. However, these simple diapir-related patterns may become more complex if regional tectonics influences the siting and growth of a diapir. Using the Sedom salt wall in the Dead Sea Fault system as our case study, we examine the influence of regional strike-slip faulting on fracture patterns around a salt diapir. This type of influence is important in general as the distribution and orientation of fractures on all scales may influence permeability and hence control fluid and hydrocarbon flow. Fractures adjacent to the N-S trending salt wall contain fibrous gypsum veins and injected clastic dykes, attesting to high fluid pressures adjacent to the diapir. Next to the western flank of the salt wall, broad (~1000 m) zones of upturn or 'drape folds' are associated with NW-SE striking conjugate extensional fractures within the overburden. Within 300 m of the salt contact, fracture patterns in map view display a progressive ~30°-35° clockwise rotation with more NNW-SSE strikes immediately adjacent to the salt wall. While some extensional faults display growth geometries, indicating that they were syndepositional and initiated prior to tilting of beds associated with drape folding, other fractures display increasing dips towards the salt, suggesting that they have formed during upturn of bedding near the diapir. These observations collectively suggest that many fractures developed to accommodate rotation of beds during drape folding. Extensional fractures in the overburden define a mean strike that is ~45° anticlockwise (counter-clockwise) of the N-S trending salt wall, and are therefore consistent with sinistral transtension along the N-S trending Sedom Fault that underlies the salt wall. Our outcrop analysis reveals fracture geometries that are related to both tilting of beds during drape folding, and regional strike-slip tectonics. The presence of faults and fractures that interact with drape folds suggests that deformation in overburden next to salt cannot be simply pigeon-holed into 'end-member' scenarios of purely brittle faulting or viscous flow.

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

Geological Magazine, May 1, 2001

The Outer Hebrides Fault Zone is a major ESE-dipping reactivated structure within Lewisian baseme... more 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.

The geometry and structural evolution of a crustal-scale Caledonian fold complex: the Ballybofey Nappe, northwest Ireland

Geological Magazine, Jul 1, 1994

The gross geometries exhibited by crustal-scale fold nappes are considered a consequence of both ... more The gross geometries exhibited by crustal-scale fold nappes are considered a consequence of both original stratigraphic relationships associated with sub-basin configuration, coupled with the nature of the structural regime and tectonic processes involved in the generation of the nappe pile. The Neo-Proterozoic Dalradian metasediments of northwestern Ireland provide a well-constrained and correlatable stratigraphy which defines a sequence of sub-reclined, tight-isoclinal Caledonian (c. 460 Ma) fold nappes. Within this fold complex, the dominant structure is the crustal-scale Ballybofey Nappe, which may be traced for 40 km along strike and is responsible for a regional (500 km2) stratigraphie inversion. The gentle, NE-plunging attitude of this fold results in a complete spectrum of tectonic levels and deformation gradients being exposed. Relatively low strains in the upper fold limb gradually increase down through the nappe, resulting in the generation of composite foliations and lineations and the development of a 10 km thick shear zone which culminates in a high strain basal detachment with underlying pre-Caledonian basement. The Ballybofey Nappe nucleated and propagated along a major zone of lateral sedimentary facies variation, coincident with the margin of a major Dalradian sub-basin. The large amplitude of the nappe is strongly influenced by the lateral heterogeneity within the metasedimentary sequence, and is associated with a minimum of 25–30 km ESE-directed translation concentrated within the overturned limb. Additional significant displacement is also focused along the basal décollement. Generation of the nappe complex resulted in significant crustal thickening and amphibolite facies metamorphism consistent with 15–18 km of burial, induced by a sequence of nappes propagating in the direction of overshear. The ESE-directed translation of the major fold nappes is away from the Caledonian foreland and a gravity-driven mechanism of nappe emplacement is suggested. Rigorous structural analysis within the cohesive stratigraphie framework enables relationships between the tectonic evolution and stratigraphic patterns to be distinguished, thus allowing models of fold nappe generation and mid-crustal deformation to be evaluated.

Caledonian extension in the north Irish Dalradian: implications for the timing and activation of gravity collapse

Journal of the Geological Society, 1993

... An Ordovician ophiolite in County Tyrone, Ireland. Nature, 315, 210-212. McCLAY, KR, NORTON, ... more

Major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: implications for Caledonian tectonics and mid-crustal shear zones

Geological Magazine, Mar 1, 1993

The dominant structure controlling the disposition of Dalradian stratigraphy in mid-Ulster has hi... more The dominant structure controlling the disposition of Dalradian stratigraphy in mid-Ulster has hitherto been regarded as a southeast-facing gently inclined F1 anticline, a gross geometry modelled on, and thought to be a possible correlative of, the Tay Nappe in Scotland. Remapping of the supposedly inverted southern limb of this major fold reveals that much of it is in fact the correct way up. However, a stratigraphie repetition coupled with a reversal in younging does occur in the Sperrin Mountains, much further south than previously realized. This hitherto unrecognized upward southeast-facing isoclinal Sperrin Nappe is, however, a D2 structure, traceable for at least 40 km along strike and responsible for a regional stratigraphie inversion over an area of 300 km2. Following D2, a major 10 km thick D3 ductile shear zone resulted in translation towards the east-southeast. In the south, this deformation carried the Dalradian over Ordovician volcanics of the Tyrone Igneous Complex along the Omagh Thrust. Penecontemporaneity of magmatism with deformation clearly demonstrates that D3 is Caledonian (Arenig-Llanvirn). This deformation correlates with similar southeast-directed Caledonian thrusting in southern Donegal and Connemara. The apparent absence of Dalradian deformation of this age in southwestern Scotland may imply that Caledonian collision of outboard terranes with the miogeoclinal margin was initiated in Ireland and/or subsequent strike-slip has removed the evidence for deformation of this age from southwestern Scotland. The D3 shear zone in the Sperrin Mountains affects a very large volume of psammitic rocks. Within this shear zone the strain is not markedly higher than surrounding areas; however, its existence is demonstrated by the reorientation of mineral lineations over a large area. Such broad zones of only moderate strain may, we believe, be typical of translatory tectonics in areas of the mid-crust where there is little lithological diversity.

Precambrian crustal evolution and Cretaceous–Palaeogene faulting in West Greenland: Structural analysis of the northern Nagssugtoqidian orogen, West Greenland: an example of complex tectonic patterns in reworked high-grade metamorphic terrains

Geological Survey of Denmark and Greenland Bulletin, Dec 1, 2006

Abstract: The area north-east of Kangaatsiaq features polyphase grey orthogneisses, supracrustal ... more Abstract: The area north-east of Kangaatsiaq features polyphase grey orthogneisses, supracrustal rocks and Kangaatsiaq granite exposed within a WSW–ENE-trending synform. The supracrustal rocks are comprised of garnet-bearing metapelites, layered amphibolites and layered, likewise grey biotite paragneisses. Their association and geochemical compositions are consistent with a metamorphosed volcano-sedimentary basin (containing both tholeiitic and calc-alkali lavas) and is similar to other Archaean greenstone belts. The ...

Discussion on evidence for a major Neoproterozoic orogenic unconformity within the Dalradian Supergroup of NW Ireland

Journal of the Geological Society, 2005

Geoff Tanner writes: Firstly, I would like to congratulate the authors for presenting a very care... more Geoff Tanner writes: Firstly, I would like to congratulate the authors for presenting a very carefully documented, and splendidly illustrated, account of the origin of the Stralinchy conglomerates. The aim of this contribution is to focus on one important question: is their controversial model for the Donegal Dalradian supported by studies of the stratigraphically equivalent Dalradian rocks of the SW Highlands of Scotland? If an orogenic unconformity is present in the Dalradian Supergroup, evidence for this should be found at the base of, or within, the Easdale Subgroup in this region. The field relationships seen at this stratigraphical level in Scotland are reviewed briefly, and an attempt made to reconcile the results with the Donegal model. Jura Quartzite–Easdale Subgroup relationship in SW Highlands of Scotland. The contact between the Jura Quartzite and the Scarba Conglomerate Formation (which represents the base of the Easdale Subgroup) is best seen in excellent coastal exposures between Jura and Loch Creran. It is exposed on the SE side of the island of Jura; is excised by faulting on the islands of Scarba and Lunga; and re-appears on the SE side of the Rubha Aird Peninsula, 7 km north of Oban. At Lussagiven on Jura [NR 6387 8666], the top of the Jura Quartzite is marked by cross-bedded quartzite followed by 23 m of thinly bedded, commonly gritty, sandstones with conglomerate lenses. These beds, which display cross-lamination, graded bedding and channel structures, pass upwards with perfect conformity into a 36 m thick unit of laminated slate with sandstone ribs, the Jura Slate member of the Scarba Conglomerate Formation. Gritty sandstone and conglomerate beds up to 1 m thick, that characterize the Scarba Conglomerate, appear 280m farther up sequence They contain pebbles to 3 cm; rip-up clasts of mudrock; and less common rafts of sandy sediment up …

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