Finite strain estimates from the dalradian dolomitic formation, Islay, Argyll, Scotland

Sedimentary dykelets in the Dolomitic Formation of Islay have been rotated relative to bedding during the primary deformation, which imposed a penetrative, slaty cleavage on the dolomitic lithologies. The dykelets have been used to determine the principal strain ratios in two dimensions and, by a new method these results have been combined to define the three-dimensional finite strain state. The results show that the shortening normal to the cleavage is in the range 33–66%, and the maximum extension within the cleavage is in the range 25–157%. The deformation is predominantly of the flattening type (1 > k 0).     

Repeated parallel deformation in part of the eastern Sierra Nevada,California and its implications for dating structural events

Study of a thick section of late Paleozoic to mid-Cretaceous sedimentary and volcanogenic rocks in eastcentral Sierra Nevada has revealed an involved structural succession not readily apparent when analysed under the traditional assumptions of structural analysis (e.g. parallel structures are of the same age).Earliest structures in the area occur as sparse folds in late Paleozoic rocks, whereas in Triassic to mid-Cretaceous rocks earliest structures occur as penecontemporaneous slumps. Upon these earliest structures are superimposed slaty cleavage with associated lineations and subsequent crenulations. The slaty cleavage across the area is statistically parallel, as are the axial planes of crenulations which fold the slaty cleavage. Such a succession would traditionally be interpreted as representing two periods of deformation, the first forming the slaty cleavage and the second the crenulation of the slaty cleavage. There is evidence, however, to indicate that the slaty cleavage itself was formed during more than one period of deformation and the same may be true for the crenulations. Dykes emplaced in Jurassic rocks have been dated (U/Pb) as mid-Cretaceous and lie parallel to what is probably an early slaty cleavage direction. The dykes, however, also bear a slaty cleavage, albeit weaker than in the host rock. In addition, quantitative strain determinations of rocks in the area show that the older units are more strongly deformed than the younger units. These and other data suggest that the statistically parallel slaty cleavage and related structures (folds, lineations, etc.) found in the Jurassic and older rocks have formed during at least two, and possibly three, increments of strain, each increment separated by a lengthy period of geologic time, possibly as much as 45 Ma or more. Crenulations of the slaty cleavage at any point (subsequently formed after each period of slaty cleavage formation) may even predate slaty cleavage formed later at another nearby point.While it is possible to set up a chronology between earlier (tectonic and/or penecontemporaneous slumps) and later structures (slaty cleavage, folds, lineations, etc.), it is not valid to designate for the entire area a relative time sequence of formation of slaty cleavage and crenulations in the Jurassic or older rocks by the usual methods (e.g. S₂, S₃, F₂, F₃, etc.). These later structures can only be designated as Only in the youngest stratigraphic unit in the area, which has been subjected to one deformation (mid-Cretaceous), can a valid structural succession be applied areally.We suggest that multiphase, parallel structures, comparable to those we have described, may be a relatively common phenomenon in orogenic belts. Until one arrives at a thorough understanding of the detailed stratigraphy and the absolute ages of units in key relationships to the structures, it may only be possible to delineate the broadest of time sequences for the structures concerned.     

Tectonically dewatered slates in the Ludlovian of the Lake District,England

Multiple siltstone dykes intruded parallel to cleavage in the Ludlovian argillites and graywackes of the Lake District throw new light on the origin of slaty cleavage. Pore pressures equalling lithostatic pressure developed during the Caledonian orogeny and caused tectonic dewatering of the partially lithified sediments. Slaty cleavage was initiated as an essentially planar structure by the intrusion of thin pelitic folia during the escape of the pore water. Cleavage folia and fold axial planes may have a common geometric relationship to the deforming stresses, but are independent of each other in origin. Continued deformation after tectonic dewatering refracted the cleavage by rotation of competent layers. Critical examination of evidence supporting traditional theories of slaty-cleavage formation shows that all observed relationships are compatible with the tectonic-dewatering theory.     

Soft‐sediment deformation structures in a Pleistocene glaciolacustrine delta and their implications for the recognition of subenvironments in delta deposits

The development of soft‐sediment deformation structures in clastic sediments is now reasonably well‐understood but their development in various deltaic subenvironments is not. A sedimentological analysis of a Pleistocene (ca 13·1 to 15 ¹⁰Be ka) Gilbert‐type glaciolacustine delta with gravity‐induced slides and slumps in the Mosty‐Danowo tunnel valley (north‐western Poland) provides more insight, because the various soft‐sediment deformation structures in these deposits were considered in the context of their specific deltaic subenvironment. The sediments show three main groups of soft‐sediment deformation structures in layers between undeformed sediments. The first group consists of deformed cross‐bedding (inclined, overturned, recumbent, complex and sheath folds), large‐scale folds (recumbent and sheath folds) and pillows forming plastic deformations. The second group comprises pillar structures (isolated and stress), clastic dykes with sand volcanoes and clastic megadykes as examples of water‐escape structures. The third group consists of faults (normal and reverse) and extensional fissures (small fissures and neptunian dykes). Some of the deformations developed shortly after deposition of the deformed sediment, other structures developed later. This development must be ascribed to hydroplastic movement in a quasi‐solid state, and due to fluidization and liquefaction of the rapidly deposited, water‐saturated deltaic sediments. The various types of deformations were triggered by: (i) a high sedimentation rate; (ii) erosion (by wave action or meltwater currents); and (iii) ice‐sheet loading and seasonal changes in the ablation rate. Analysis of these triggers, in combination with the deformational mechanisms, have resulted – on the basis of the spatial distribution of the various types of soft‐sediment deformation structures in the delta under study – in a model for the development of soft‐sediment deformation structures in the topsets, foresets and bottomsets of deltas. This analysis not only increases the understanding of the deformation processes in both modern and ancient deltaic settings but also helps to distinguish between the various subenvironments in ancient deltaic deposits.     

Les plis hercyniens kilométriques couchés vers l'ouest-sud-ouest dans la région du pic du Midi d'Ossau–col du Somport (zone axiale des Pyrénées occidentales)Kilometric westward Hercynian recumbent folds in the western Pyrenees (Pic du Midi d'Ossau area)

The Palaeozoic of the western Pyrenees shows two superposed tectonics easily defined by their different geometry and the major unconformity of the Permian sediments and volcanics on the Devono-Carboniferous series: an Hercynian tectonic found only in the pre-Permian series, characterised by kilometric westward recumbent folds with a weak cleavage; a Pyrenean tectonic, characterised by tight east–west folds, upright to overturned to the south with slaty cleavage, which is the only deformation found in the Permian and Mesozoic series and the second deformation in the pre-Permian Palaeozoic. The Hercynian folding, roughly perpendicular to the trend of the Pyrenees characterises the northern branch of the Ibero-Armorican virgation. To cite this article: P. Matte, C. R. Geoscience 334 (2002) 773–779.     

Evaluation of soft sediment deformation structures along the Fethiye–Burdur Fault Zone,SW Turkey

Burdur city is located on lacustrine sedimentary deposits at the northeastern end of the Fethiye–Burdur Fault Zone (FBFZ) in SW Turkey. Fault steps were formed in response to vertical displacement along normal fault zones in these deposits. Soft sediment deformation structures were identified at five sites in lacustrine sediments located on both sides of the FBFZ. The deformed sediments are composed of unconsolidated alternations of sands, silts and clay layers and show different morphological types. The soft sediment deformation structures include load structures, flame structures, slumps, dykes, neptunian dykes, drops and pseudonodules, intercalated layers, ball and pillow structures, minor faults and water escape structures of varying geometry and dimension. These structures are a direct response to fluid escape during liquefaction and fluidization mechanism. The driving forces inferred include gravitational instabilities and hydraulic processes. Geological, tectonic, mineralogical investigations and age analysis were carried out to identify the cause for these soft sediment deformations. OSL dating indicated an age ranging from 15161±744 to 17434±896 years for the soft sediment deformation structures. Geological investigations of the soft sediment deformation structures and tectonic history of the basin indicate that the main factor for deformation is past seismic activity.     

Post‐tectonic clastic dykes in the Dalradian of Scotland and Ireland: implications for delayed lithification and deformation of sediments

Structures associated with soft‐sediment deformation are preserved in the Neoproterozoic Dalradian Supergroup of SW Scotland and NW Ireland. Clastic dykes display a range of age relationships to regional Caledonian D₁ folds and fabrics from pre‐tectonic to hitherto unrecognized post‐tectonic. Evidence for the post‐D₁ timing of some dykes includes the emplacement of centimetre‐scale injections along regional S₁ cleavage, the disorientation and transport of cleaved wall‐rock clasts within larger dykes, and clastic dykes which markedly cross‐cut and transect F₁ fold hinges and axial planes. Collectively, these observations are compatible with the earliest regional (D₁) structures deforming a sequence which contained locally overpressured and unlithified pockets of sediment. These critical relationships indicate that overpressured pockets of unlithified sediment were possibly retained within the Dalradian for significant periods of time spanning at least 120 Ma given existing isotopic constraints. Copyright © 2000 John Wiley & Sons, Ltd.     

Neptunian dykes along a drowned carbonate platform margin: an indication for recurrent extensional tectonic activity?

Three successive Mesozoic neptunian dyke generations and related unconformities suggest recurrent extensional fracturing and periods of relative sea-level rise along the NW Trento Plateau margin in the Southern Alps, Italy. The first neptunian dyke generation was induced by NNW–SSE directed extension of Early Jurassic skeletal oolitic periplatform deposits generating micritic early Middle Liassic neptunian dykes with orthogonal orientation. The second generation of neptunian dykes was possibly caused by marginal extension at the drowned platform edge penetrating Late Jurassic, red pelagic limestones with a pelagic matrix of Albian/Cenomanian age and nearly orthogonal fracture orientation. The third generation of neptunian dykes occurred after a prolonged period of submarine exposure and erosion (Aptian/Albian to Late Maastrichtian) during the rapid burial of the submarine Trento Plateau margin relief. The Late Maastrichtian neptunian dykes were caused by extension of Early to Middle Jurassic oolitic periplatform limestones along steep (inclination > 10°) submarine slopes. Generally successive neptunian dyke generations along drowned carbonate platform margins could be caused by repeated extensional brittle fracturing of lithified periplatform deposits and the filling of micritic matrix derived from overlying pelagic sediment sequences under substantial hydrostatic pressure. This would suggest that recurrent extensional fracturing is continuously recorded by neptunian dyke formation which could be used to indicate extensional tectonic activity at a foundering deep-marine carbonate platform edge.     

Neptunian dykes and associated breccias (Southern Alps, Italy and Switzerland): role of gravity sliding in open and closed systems

Neptunian dykes and sills of Middle Jurassic pelagic limestone within Lower Jurassic shallow-water carbonate host rocks occur at many localities in the Southern Alps of Italy and Switzerland, especially on what were the upper slopes of tilted half-grabens created during the Early Jurassic rifting stage of a passive margin that faced the Middle and Late Jurassic Tethyan Ocean. The host rocks were dilated by cracking, folding, and brecciation during movements of shallow-based gravity-driven slides and slumps of semibrittle platform strata, commonly along décollement contacts between layers of different competence. In most places, the network of cavities in the dilated strata connected to the sea floor, and pelagic sediments trickled from above into the open spaces. In other places, the brittle strata were overlain by somewhat impermeable sediments that formed a partial seal. Sudden dilation of the brittle beds resulted in forceful injection of the overlying weakly consolidated or plastic sediments into open spaces. The filling in both open and closed systems was commonly episodic, resulting in complex internal-sediment stratigraphy and cross-cutting dykes. Stable isotopic data on internal sediments and early-formed cement lie within the field of normal sea water, and none of the sedimentological or stable isotopic data supports a subaerial, dissolution (karst) origin for the Jurassic neptunian dykes of this region.     

Solution transfer syn-S2: an inferred means of deriving fault fill in the lake Moondarra area, Mt. Isa, Queensland, Australia, based on oxygen isotope results

Many of the faults within the Lake Moondarra area cropout as wide zones which are filled with massive or fibrous milky quartz. Additionally, a variety of rock fragments from the surrounding metasediments are present in the fault zones. The second regional slaty cleavage, S₂, is locally observed through the quartz and country rock fragments within some of the zones, indicating that these fault fills originated pre- or early syn-S₂. The absence of the first regional slaty cleavage, S₁, suggests that the fault fills developed post or late syn-S₁. Evidence supporting an early syn-S₂ timing for the development of these fills is provided by the results of oxygen isotope analysis carried out on quartz specimens collected from the faults and also by the nature of the quartz. Specimens of quartz taken from the various fault zones have δ¹⁸O values between 12.4 and 14.5‰. This suggests that metamorphic water isotopically equilibrated with connate formation water was the aqueous fluid which transported the silica. δ¹⁸O values for the silica in the quartz-rich, sometimes dolomitic, metasediments of the Mount Isa and Haslingden Groups within the Lake Moondarra area are similar to the quartz within the fault fills. The lack of any metamorphic event between the first and second deformation, together with the sometimes fibrous nature of the fault-filling material with fibres parallel to the mineral elongation in S₂ (i.e. L₂²), suggests that the quartz in the faults was derived syntectonically at grain to grain contacts in the metasediments within the Lake Moondarra area, early in the development of S₂. The quartz is believed to have moved by the process of solution transfer to the faults as they underwent dilation during D₂. Subsequently the quartz was precipitated as a result of a chemical potential gradient.     

The establishment of recumbent folds in the Lower Palaeozoic near Queanbeyan,New South Wales

Lower Palaeozoic sedimentary and volcanic rocks east of Queanbeyan, N.S.W., have undergone multiple deformation resulting in four systems of folds. The first of these consists of large isoclinal, recumbent folds (F1). The second generation folds (F2) are the most pronounced; they consist of flattened flexural‐slip folds with well developed axial‐plane slaty cleavage. Minor variants of this system are associated with meridionally‐trending faults. Third and fourth generation folds are minor kink systems.

The existence of first generation folds was established on the basis of F2 fold‐facing determinations, and their likely form was deduced from the geometrical variations of F2 folds. It is thought that all fold phases developed during the Late Silurian Bowning Orogeny.     

The development of slaty cleavage, fleurieu peninsula, south australia

In low grade, biotite-rich metasiltstones and slates from the western side of Fleurieu Peninsula, the slaty cleavage is defined by elongate deformed old biotites and by the coincident elongate dimensions and (001) planes of thin, well aligned new biotites. Histograms of frequency versus (001)-S₁ angle (S₁ defined by aligned thin muscovite used as the reference plane) were determined in thin section for both populations. The old biotites show a symmetrical bimodal distribution of (001) about S₁, with maxima at around 20° either side ofS₁. This distribution, together with the preserved intracrystalline strain, indicates that these old grains deformed largely by slip on (001) with some modification of grain boundaries by diffusive transfer. The new biotite grains are very well aligned (standard deviation 3.8°) and show no sign of mechanical deformation. They have not been mechanically rotated into alignment but must have nucleated and grown in a specific orientation. The proportion of new to old grains increases with metamorphic grade, causing a rapid strengthening of the crystallographic alignment. In slates and phyllites with similar microstructure, the mica fabric determined by X-ray texture goniometry cannot be used as a quantitative measure of the geometry and magnitude of the bulk strain, as the intensity of the crystallographic fabric will be strongly influenced by the proportion of new mica, which is itself greatly affected by the metamorphic grade during slaty cleavage formation.     

Deformation history of the Hampden Synform in the Eastern Fold Belt of the Mt Isa terrane

The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590–1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first‐generation tight to isoclinal folds and an associated axial‐planar slaty cleavage. Preservation of recumbent first‐generation folds in the hinge zones of second‐generation folds, and the approximately northeast‐southwest orientation of restored L¹ ₀ intersection lineation suggest recumbent folding occurred during east‐west to northwest‐southeast shortening. First‐generation folds are refolded by north‐south‐oriented upright non‐cylindrical tight to isoclinal second‐generation folds. A differentiated axial‐planar cleavage to the second‐generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second‐generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late‐orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin‐skinned crustal shortening during the development of first‐generation structures to thick‐skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.     

Character and kinematics of faults within the turbidite-dominated Lachlan Orogen: implications for tectonic evolution of eastern Australia

Fault zones within turbidite-dominated orogenic systems, typified by the Lachlan Orogen of eastern Australia, are characterised by higher than average strain and intense mica fabrics, transposition foliation and isoclinal folds, poly-deformation with overprinting crenulation cleavages, and steeply to moderately plunging meso- and micro-folds. They have a different character compared to the brittle–ductile fault zones of classic foreland fold-and-thrust belts such as the Appalachians and the Canadian Rocky Mountains. Multiple cleavages and transposition layering record a progressive shear-related deformation history. An intense mica fabric evolves initially during shortening of the overlying sedimentary wedge, but is progressively modified during rotation and emplacement to higher structural levels along the steep parts of inferred listric faults. The deformed wedge outside the fault zones generally undergoes one phase of deformation, shown by a weak to moderately developed slaty cleavage which is parallel to the axial surface of upright, subhorizontally plunging chevron-folds. Other faults within the turbidites of the Lachlan Orogen include the steep zones of ‘ductile’ strike-slip deformation that bound a centrally located, high T/low P metamorphic complex. Characterised by S–C mylonites, these ductile shear zones indicate a southward passage of the metamorphic complex as a crustal wedge, with emplacement to higher structural levels along a leading-edge, ductile thrust-fault. Ar–Ar dating constrains the timing of regional deformation to be mostly Late Ordovician through Silurian across the Lachlan Orogen. Faults in the low grade turbidite sequences record the kinematic evolution of the developing Lachlan Orogen and indicate progressive deformation associated with simultaneous, eastward propagating and migrating deformation fronts in both the western and eastern parts of the fold belt. These deformation fronts are related to ‘accretionary style’ deformation at the leading edges of overriding plates, in an inferred southwest Pacific-type subduction setting from the Late Ordovician to the mid-Devonian, along the former Gondwana margin. The fault zones effectively accommodate and preserve movements within the structurally thickening, migrating and prograding accretionary wedge.     

Regional slaty cleavage formation and fold axis rotation by re-use and reactivation of pre-existing foliations: the Fiery Creek Slate Belt, North Queensland

A deformation history, comprised of six separate deformation events of differing intensity, has affected the rocks of the South Palmer River region of the the Hodgkinson Province, north Queensland. Within this region, a zone of pervasive slaty cleavage, herein termed the Fiery Creek Slate Belt, has developed as a result of the superposition of fabrics formed during several of these events. The most important processes in the formation of this composite cleavage were the re-use and reactivation of the favourably oriented, steep, N-S-trending S₂ foliation by the intense fourth deformation event, D₄. This produced micro-, meso- and macroscopic folds in an originally shallow S₃ foliation, produced during the intervening D₃ deformation, with an axial planar S₂–S₄ foliation. The D₄ stretching lineation, L⁴₄, plunges subvertically to steeply north and indicates that shear during D₄ was oriented steeply north-south. In the Fiery Creek Slate Belt, D₂ fold axes are interpreted to have formed in much shallower orientations than their present moderately N-S-plunging to subvertical orientations. We consider this to be a result of D₄ shear, which caused variable degrees of rotation of D₂ fold axes toward the D₄ stretching lineation due to subparallelism of the bulk shortening directions of the D₂ and D₄ events. Near-total destruction of the pre-D₄ foliations during slaty cleavage formation has produced a misleading impression of a simple deformation history. There is no relationship between metamorphic grade and intensity of slaty cleavage development.     

Bundled slaty cleavage in laminated argillite, north-central minnesota

Exceptional bundled slaty cleavage (defined herein) has been found in drill cores of laminated, folded, weakly metamorphosed argillite at several localities in the early Proterozoic Animikie basin of north-central Minnesota. The cleavage domains are more closely spaced within the cleavage bundles than outside them, the mean tectosilicate grain size of siltstone layers, measured normal to cleavage, is less in the cleavage bundles than outside them, and the cleavage bundles are enriched in opaque phases and phyllosilicates relative to extra-bundle segments. These facts suggest that pressure solution was a major factor in bundle development. If it is assumed that opaque phases have been conserved during pressure solution, the modal differences in composition between intra-bundle and extra-bundle segments of beds provide a means for estimating bulk material shortening normal to cleavage. Argillite samples from the central part of the Animikie basin have been shortened a minimum of about 22%, as estimated by this method. These estimates are similar to the shortening values derived from other strain markers in other rock types interbedded with the argillite, and are also consistent with the regional pattern of deformation.     

Impact-related clastic injections in the marine Ordovician Lockne impact structure, Central Sweden

Clastic injections generated in connection with the formation of impact craters show many similarities to injections created by other geological processes. However, circumstances such as their position relative to the impact structure and the evidence of forceful processes indicate an impact origin. The Ordovician Lockne impact structure was formed in a marine environment with both sedimentary (Cambrian and Ordovician) and underlying crystalline (Proterozoic) target rocks. Sea water played a substantial part in the cratering process, especially in the modification of the newly formed crater as the water surged back into the structure. In the Lockne area clastic dykes and sills have long been known and have earlier been interpreted as neptunian dykes and conglomerates. So far seven cases of dykes and sills are known in the area. In this work these are interpreted as clastic injections formed in connection with the Lockne impact. The clastic injections occur in the crystalline basement and the sedimentary sequence. The material in the injections comes from all local lithologies (both sedimentary and crystalline) but the sedimentary sequence dominates as a source. The dykes and sills were injected simultaneously with the fracturing and dilation of the host rock in the cratering process, and occur at different stratigraphic levels. In some dykes, clasts from the host rock wall can be fitted back to their original position; the clasts are slightly rotated and surrounded by exotic material. Quartz grains with planar deformation features were observed in the injected material. Most of the sills within the bedded Ordovician limestone are restricted to marly beds, except for the feeder dykes which cut through the overlying beds. This circumstance demonstrates how the decompression has opened the strata along weaker layers and that the underpressure created subsequently sucked the material down. Laminar flow is a conspicuous internal structure in the dykes and sills and indicates viscous flow of injected material. The lamination in the injected material is parallel to the walls in each case. The material was lithified prior to the event and was crushed, mobilized in a water/sediment slurry and injected as dykes and sills.     

Structural pattern in the Precambrian rocks of Sonua-Lotapahar region,North Singhbhum,eastern India

In the western part of the North Singhbhum fold belt near Lotapahar and Sonua the remobilized basement block of Chakradharpur Gneiss is overlain by a metasedimentary assemblage consisting of quartz arenite, conglomerate, slate-phyllite, greywacke with volcanogenic material, volcaniclastic rocks and chert. The rock assemblage suggests an association of volcanism, turbidite deposition and debris flow in the basin. The grade of metamorphism is very low, the common metamorphic minerals being muscovite, chlorite, biotite and stilpnomelane. Three phases of deformation have affected the rocks. The principal D1 structure is a penetrative planar fabric, parallel to or at low angle to bedding. No D1 major fold is observed and the regional importance of this deformation is uncertain. The D2 deformation has given rise to a number of northerly plunging major folds on E-W axial planes. These have nearly reclined geometry and theL ₂lineation is mostly downdip on theS ₂surface, though some variation in pitch is observed. The morphology of D2 planar fabric varies from slaty cleavage/schistosity to crenulation cleavage and solution cleavage. D3 deformation is weak and has given rise to puckers and broad warps on schistosity and bedding. The D2 major folds south of Lotapahar are second order folds in the core of the Ongarbira syncline whose easterly closure is exposed east of the mapped area. Photogeological study suggests that the easterly and westerly closing folds together form a large synclinal sheath fold. There is a continuity of structures from north to south and no mylonite belt is present, though there is attenuation and disruption along the fold limbs. Therefore, the Singhbhum shear zone cannot be extended westwards in the present area. There is no evidence that in this area a discontinuity surface separates two orogenic belts of Archaean and Proterozoic age.     

Late Caledonian dyke-swarms in Southern Scotland: New field,petrological and geochemical data for the Wigtown Peninsula,Galloway

Several hundred, dominantly NE-trending dykes invade Silurian flysch sediments of the Wigtown Peninsula. The most basic dykes are K-rich biotite-lamprophyres, hornblende-lamprophyres and appinites ('diorites' of previous maps). More acidic (qz and cm normative) dykes include restricted andesitic compositions ('porphyritcs', c. 57 per cent SiO₂) and a continuum from dacitic to rhyolitic compositions ('acid porphyrites' and 'porphyries', c. 63–71 per cent SiO₂). The relative abundance of basic to intermediate + acidic dykes, based on examination of 180 thin sections and 70 new whole-rock analyses, is approximately 7:1. The biotite- and hornblende-lamprophyres are distinct in chemistry and distribution, biotite-lamprophyres being confined to the south of the Peninsula. Neither type has any compositional equivalent among the plutonic rocks of the area. Porphyrite and porphyry dykes do not represent simple lamprophyre fractionates, but may be derived from at least two (intermediate and acidic) parent magmas, which may represent crustally-contaminated lamprophyre or entirely separate crustal melts. Emplacement of all dyke compositions was apparently episodic through Wenlock to early Devonian times (c. 425-395 Ma). Early dykes post-date the main deformation (D₁) but predate minor (D₂) folding, both D₁ and D₂ being related to thrusting. On the current accretionary model for the Southern Uplands, this implies that dyke emplacement commenced before the end of accretion in a compressional tectonic regime—scarcely consistent with the moderate (up to 6 per cent) crustal extension represented by the dykes. The geochemistry of the lamprophyres requires a deep mantle origin several hundred kilometres behind any subduction zone. This suggests that the 'lapetus suture', at the time of dyke emplacement, was much farther south than its currently inferred position and places the Southern Uplands in a back-arc situation.     

A structural cross-section from the Aureole of the Main Donegal Granite

Seven phases of deformation are recognised in Dalradian metasediments within the NW aureole of the Main Donegal Granite. Major NW facing F2 folds (the Aghla Anticline and Errigal Syncline) refold an originally NW facing slaty cleavage, which is usually parallel to bedding. D3 structures cross-cut the major F2 folds and verge and face to the SE on their normal limbs. A fourth phase of deformation intensifies towards the granite contact and is shown to be broadly coeval with intrusion and responsible for a major structure in the aureole. Three later phases are variably developed throughout the aureole. The kinematics of F4 folds in relation to the granite intrusion are briefly discussed.