the metagabbros orthogneisses and paragneisses of the connemara complex western ireland although now largely disappeared within the 400 ma old galway granite the syntectonic connemara metagabbro gneiss complex which crystallized at 490 ma arenig is the largest intrusion into the dalradian rocks it is part of widespread cambro ordovician volcanism and plutonic injection into continental crust in the appalachian caledonide orogenic belt being synchronous in crystallization with the widely obducted 490 ma ophiolites within the orogeny the peridotites and hornblende gabbros were injected broken up and shouldered aside by quartz diorite granodiorite granite and trondhjemite gneisses and it is shown that these orthogneisses are not the in situ differentiation products of the gabbro magma the hornblende in the metagabbros is mainly igneous but is partly metamorphic and replaces olivine fo 75 92 orthopyroxene en 65 92 clinopyroxene wo 48 5 en 50 39 fs 5 14 and plagioclase typically an 80 92 range an 50 96 an aureole at least 10 km wide in the sillimanite zone was imposed and cordierite garnet sillimanite dalradian hornfelses were partially melted at temperatures up to 1000 o c near to the gabbros at 4 5 1 kbar corresponding to c 18 km depth this suggests that the complex forms part of the root of a now eroded batholith of calc alkaline affinities the gabbros corresponding to the transition zone to a basic bottom which is often supposed to underlie many batholiths within the orthogneisses are sheets of paragneiss formed from dalradian metasediments which have been folded up from the floor and down into sides of the magma chamber which process could be illustrative of the means by which basic magma can intimately interact with crustal material the 18 o 16 o d h nd and sr isotope evidence and the existence of inherited zircons in the gneisses indicate that the gabbro and gneiss magmas were significantly contaminated with crustal possibly dalradian material both before and after intrusion with the lowest initial 87 sr 86 sr at 490 ma being 0 708 even in samples far removed from metasediment the metagabbros are tholeiitic to high alumina basalt in character but the gneisses have distinct calc alkaline geochemistry the chemistry igneous hornblende and highly calcic plagioclase match many arc plutons in the western usa the syntectonic nature and the ubiquitous gneissic fabric in the quartz diorites trondhjemites and granites is a consequence of early movements which were probably in part connected with the separation of the connemara massif and which culminated in slicing off the largely inverted complex and carrying it southwards on the mannin thrust which underlies the whole of connemara the complex is thus a fragment of a now disrupted cambro ordovician continental magmatic arc or continent island arc environment which can be traced along the se edge of the laurentian continent

the provenance of granite boulders in conglomerates of the northern and central belts of the southern uplands of scotland rb sr whole rock isochrons from several suites of granite boulders which occur in ordovician and silurian conglomerates in the southern uplands of scotland yield ages of around 1200 ma 600 to 700 ma and 470 to 490 ma the plutonic and tectonic history recorded by these clasts corresponds more closely with that of nw newfoundland than with that of scotland the petrography of the clasts can also be matched with exposed granite plutons in newfoundland since the conglomerates were derived from the nw it is concluded that the southern uplands lay to the se of newfoundland a total of 1500 km strike slip displacement occurred between the caradoc and the early devonian consistent with oblique subduction of the iapetus ocean below the north american continental margin

resurgent strike slip duplex development along the hitra snasa and verran faults more trondelag fault zone central norway abstract lineament analysis of the ene wsw verran and hitra snasa faults of the long lived more trondelag fault zone mtfz has revealed the presence of a complex array of anastomosing faults and fractures this has the geometric configuration of a major dextral strike slip fault zone and compares well with characteristic patterns of braided wrench faulting e g the san andreas fault system of california the fault array recognized in conjunction with the verran fault is clearly post caledonian and is considered to define a dextral strike slip duplex system associated with the parallel hitra snasa fault are riedel like structures which tend to point to an earlier component of sinistral movement rock products present along the hitra snasa fault and its secondary faults comprise mylonites hydrothermally altered rocks and small scale recrystallized breccias along the main verran fault evidence of late polyphasal deformation is seen in several discrete episodes of brecciation hydrothermal alteration and locally pervasive prehnite and stilbite veining the fault structures occurring along the hitra snasa and verran faults are thought to have originated as a sinistral fault system of late devonian age especially for the hitra snasa lineament subsequently strike slip movement reversed in sense and shifted locus towards the verran fault system during the late jurassic or early cretaceous during this strike slip reversal some earlier fractures related to the sinistral system were rejuvenated within the stress field of the evolving dextral duplex system strike slip displacements of a similar age are known from fault complexes on the norwegian continental shelf and in the northern north sea the regional picture indicates that the mtfz was almost certainly linked to the fault systems of northern scotland prior to the opening of the viking graben

strike slip tectonics and granite petrogenesis transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts individual transcurrent shear zones may not only control the ascent paths siting and emplacement mechanisms of plutons but they may also cause the genesis of the granitoids themselves two contrasting situations are examined in the hercynian shear zones of iberia thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile gondwanan sediments to produce syntectonic granites in the northern part of the british caledonides the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the moho

contrasting structural features in the granulite gneiss charnockite granite complex lake baikal u s s r evidence for diverse geotectonic regimes in early proterozoic times abstract evidence for two major geotectonic regimes in a deep level early proterozoic crystalline complex is provided by its structural metamorphic and igneous features the earlier one is characterized by 1 isoclinal to tight folds in gneisses and granulites formed by dynamothermal metamorphism 2 asymmetrical folds and tectonically emplaced bodies of ultramafic material and 3 tight to open upright folds and minor granite bodies these structural metamorphic and igneous features are considered to be the results of collisional tectonism they are more than 100 ma older than other products interpreted as resulting from transcurrent faulting extensional tectonism and which show many similarities with the basin and range province of the u s a here patchy distribution of charnockite is a distinctive feature associated with knuckle shaped asymmetrical folds with moderately dipping axial planes which exert a strong structural control on the process of charnockitization this is related to the passage of co 2 whose source is considered to be crystallizing mantle derived basic magma later upright folds steeply inclined shears and successively emplaced pyroxene bearing granitoids biotite bearing leucogranites and pegmatites are interpreted as features associated with caldera formation the igneous products are considered to be the result of progressive fractionation of basic magma possibly with incorporated melted crustal material some movement on gently dipping faults that cut all other structures is likely to be associated with himalayan rifting rather than with precambrian tectonism and to represent a third geotectonic regime

caledonian plutonism and major lineaments in the sw scottish highlands within the sw grampian highlands several ne sw trending shear zones and faults related to the caledonian transpressional collision are recognized as being distinct from an intersect ing set of nw se trending pre caledonian crustal lineaments which were reactivated during caledonian orogenesis the geochemical and isotopic characteristics of the 425 400 ma granitoids of this region show that they were derived by anatexis of the lower crust associated with a mantle component which provided an influx of mantle heat and or metasomatic fluids their spatial distribution suggests a close genetic relationship with caledonian shear zones having made the fundamental distinction between ascent transport and emplacement arrival configurations possible for granitoid magmas new data are presented for these plutons which show that i a common modal sequence of intrusive phases can be recognized ii these phases are all sited at shear zone or lineament intersections where transtensional zones allowed and facilitated ascent iii emplacement was often by a process of localized in situ ballooning the existence of such lineaments in these orientations in the sw highlands may indicate that the structure of the lower crust can be regarded as a series of blocks bounded by intersecting ductile zones of high strain the establishment of such a pattern of structural interactions within the lower crust could provide a mechanistic framework within which the location of anatectic zones siting and ascent pathways and any subsequent emplacement phenomena can be explained in orogenic belts

sequence stratigraphy of the palaeozoic welsh basin the timing and extent of unconformities in the welsh basin are investigated using rock preservation curves derived from outcrop stratigraphic logs four basin wide unconformities occur focussed in late precambrian late tremadoc pusgillian early ashgill and mid devonian times these bound three megasequences equating with newly defined lithostratigraphic units the dyfed gwynedd and powys supergroups less extensive unconformities bound 18 component sequences the majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea level change the megasequence boundaries are attributed to late precambrian to early cambrian onset of rifting to form a passive margin tremadoc onset of subduction with intra arc then back arc extension late caradoc end to subduction and late early devonian collisional deformation the megasequences and controlling events can be tentatively matched with other basins on the avalonian margin more generally this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins

late caledonian dolerite sills from sw connacht ireland bimodal silurian igneous activity in sw connacht occurred during basin subsidence and sedimentation and before basin deformation mafic sills averaging 3 m thickness were preferentially intruded into and supported by a thin horizon of fine grained siliceous sediment tonalee fm multiple injection produced the thicker sills a primary igneous lithology of augite plagioclase olivine phyric dolerite has in some sills been obliterated by pervasive hydration and carbonation expressed in a biotite calcite quartz albite mineralogy this represents volatile influx into magma at a sub intrusive level and contrasts with syn intrusive assimilation and post intrusive alteration that introduced silica and potassium into and removed magnesium from marginal dolerite the sw connacht magmatism occurred in an extensional tectonic setting probably in a nne ssw structurally controlled basin and was coeval with bimodal magmatism in other parts of ireland and britain on both sides of the iapetus sutures

polyphase magnetization in lower carboniferous rocks of s scotland palaeomagnetic and tectonic considerations abstract lower carboniferous lavas and intrusives from the southern margin of the midland valley scotland have yielded a principal magnetization termed the b component with d i 192 12 95 5 9 this magnetization is thought to have formed essentially during the original cooling but evidence of low temperature mineralogical changes as well as the presence of dual polarity remanence structures suggest that the magnetization processes continued beyond the deuteric stage covering at least the time span of geomagnetic field inversions an overprinted magnetization the a1 component of inferred lower tertiary age has a mean direction of d i 188 49 95 3 5 this secondary magnetization is found principally in three sites two from silica veins and one from a basaltic neck located within the southern uplands fault zone at dunbar microscopic evidence suggests that the a1 overprint was acquired thermochemically through alpine tectonic re activation of the fault

the british caledonides interpretations from cenozoic analogues recent interpretations of cenozoic arc systems and collision belts facilitate reinterpretation of some aspects of british caledonide evolution end cambrian grampian collision of the passive dalradian foreland following southeastwards subduction beneath an island arc was accompanied by initiation of the highland boundary fault as a high angle south directed oblique slip thrust mid ordovician to early devonian northwestward oblique subduction of the iapetus ocean beneath the grampian orogen resulted in a continental margin magmatic arc back arc thrusting and development of an accretionary prism while southeastward subduction led to arc magmatism and back arc extension followed by initiation of the rheic ocean as a back arc marginal basin this syn subduction n s asymmetry of the iapetus ocean margins was analogous to the e w asymmetry of the modern pacific closure of iapetus was diachronous earlier in the northeast during end silurian collision the southern caledonides behaved as a passive foreland post collision foreland thrusting resulted in deposition and deformation of lower old red sandstone foreland basin deposits in wales and probably in northwest directed back thrusting in the region of the longford down accretionary prism subsequent dextral movement in the suture zone juxtaposed the southern caledonides with scotland and northern ireland beneath which northwestward subduction had continued into the early devonian

ophiolitic melange separates ortho and para tectonic caledonides in western ireland the nature of the contact between the orthotectonic late pre cambrian to cambrian and the paratectonic ordovician and silurian caledonides has long been a source of discussion in western ireland this junction occurs in a complex zone which crops out along the southern shores of clew bay county mayo where dalradian middle cambrian and ordovician arenig rocks are separated by a pre silurian rock unit termed the deer park complex1 fig 1 the complex contains both meta sedimentary and meta igneous rocks and has been variously interpreted as representing a pre caledonian basement ridge1 an imbricate thrust zone2 related to the development of a major fault system the fair head clew bay line3 or an arenig ophiolite4 we report here geological geochemical and geophy sical studies on the deer park complex which indicate that it is a northerly dipping melange unit containing the components of a dismembered ophiolite these findings imply a source of ophiolitic rocks to the north possibly within the orthotectonic caledonides

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

the ordovician evolution of the south mayo trough western ireland the south mayo trough a broad synclinorium containing a 10 km sequence of early ordovician turbidites passing up into shallow water sandstones is interpreted as a forearc basin with an ophiolitic basement that formed a backstop to an accretionary prism on the northern edge of a north facing arc the arc collided with the laurentian margin in early llanvirn times to deform the dalradian supergroup rifted margin clastic sequence prior to the late llandovery sinistral transcurrent faulting transposed the connemara dalradian terrane to its present position on the outboard ocean ward side of the south mayo trough the south mayo trough was fed principally by arc and ophiolite derived elastics during the arenig with an increasing metamorphic component during the llanvirn the late ordovician transpressional docking of connemara provided the southerly source for high grade metamorphic clasts in the deny veeny formation

the trace element and isotope geochemistry of the sabaloka igneous complex sudan the sabaloka igneous complex comprises 2 igneous centres of 9within plate9 magmatism intruded into precambrian gneisses in northern sudan the cauldron complex is the larger centre and was emplaced at 383 14 ma the earliest felsic magmas are highly fractionated peraluminous rhyolites with isotope and trace element characteristics indicative of either a biotite tonalite granodiorite source or of amphibole fractionation from a metalumi nous melt the rhyolites are overlain by ignimbrites which have similar trace element characteristics to the rhyolites but are less aluminous a mica granite boss intrudes both rhyolites and ignimbrites and has an unuaual trace element geochemistry which is ascribed to volatile loss during emplacement the cauldron complex is circumscribed by a microgranite ring dyke which has isotopic and trace element characteristics which allow the microgranite magma to be the source from which the rhyolites fractionated the microgranite itself requires a monzonite or syenite source the subsidiary tuleih complex was emplaced at 465 14 ma and is largely composed of a silica saturated marginally undersaturated syenite trace elements and isotopes from the syenite indicate a garnet eclogite source of mantle origin the sabaloka intrusions provide an example of several periods of magmatism contributing to the same igneous complex although not necessarily to the same igneous centre

the sedimentology of the south connemara group western ireland a possible ordovician trench fill sequence the stratigraphy of the ordovician south connemara group is described the group is approximately 3 km thick and includes sedimentary and volcanic rocks the sedimentary elements of the succession include conglomerates sandstones shales cherts and minor limestone breccias the lettermullen formation exhibits the thickest continuous sedimentary sequence in the group it is divided into five facies a conglomerate facies thick sandstone facies medium sandstone facies sandstone siltstone facies and chert facies the sedimentary features of these rocks include channelling and grading and are described in terms of bouma units where applicable the coarse elements of the formation are thought to represent deposition in or proximal to submarine channels feeding a more distal depositional site represented by the finer elements igneous rocks of the group include pillowed and non pillowed lavas massive amphibolites and minor tuffs often in conformable contact with the sediments petrography of conglomerate clasts and sandstones indicates derivation from a continental source area palaeocurrent analysis reveals a primary easterly mode with a subsidiary northerly mode it is suggested that the group exhibits some characteristics of trench fill sequences and the succession may be related to similar environments within the southern uplands accretionary wedge

syn orogenic alkaline magmatism and its relationship to the moine thrust zone and the thermal state of the lithosphere in nw scotland the alkaline complexes of nw scotland lie in a zone sub parallel to the moine thrust and represent the nw edge of the otherwise overwhelmingly calc alkaline late orogenic magmatism of the caledonian fold belt the emplacement of intrusives in assynt overlapped the thrust movements and new u pb zircon ages for the loch ailsh intrusion 439 4 ma provide are liable bracket when combined with existing age data for the loch borralan intrusion 430 4 ma for the main movements on the ben more thrust plane a detailed history of the main tectonic and igneous events in the moine thrust zone is tabulated zircons from the undeformed cnoc nan cuilean syenite emplaced in moine rocks near loch loyal give an age of 426 9 ma in contrast to the deformed glen dessary syenite for which earlier work gave 456 5 ma the linear distribution of the alkaline complexes to the west of the calc alkaline plutons and the ultra potassic character of the loch borralan complex the most westerly are consistent with but do not prove generation related to a w dipping subduction zone the persistence of the magmatism over 30 ma in a belt being shortened by thrusting suggests a source in the mantle lithosphere the absence of magmatism further to the nw and the production of calc alkaline magmas to the se of this zone is compatible with a model in which the thermal state of the lithosphere exercises the most important control on the nature of late caledonian magmatism

displaced terranes in britain and ireland a meeting entitled displaced terranes in britain and ireland was held at burlington house on 2 november 1988 this was the third terranes meeting held in britain since the inception of international geological correlation project no 233 terranes in the circum atlantic palaeozoic orogens the others having been held at cardiff in 1986 and glasgow in 1987 igcp 233 grew out of igcp 27 the caledonide orogen many of the developments promoted by which are recorded within two special publications of the geological society harris et al 1979 harris fettes1988 over the last few years research on displaced terranes in the british caledonides has accelerated with new data now challenging what have become traditional applications of plate tectonic theory to the british caledonides the reports of work in progress on caledonian terranes and it was thought appropriate to take advantage of the rapid publication facilities afforded by the short papers scheme because of the four page limit on short papers much introductory material has had to be omitted so that we provide here an introduction to the concept of displaced terranes and its applications to british palaeozoic geology displaced terranes the essential value of the terrane concept lies in its recognition of the likelihood of large scale lateral fault movements occurring along active plate margins the concept of terrane tectonics is therefore no more than a fuller appreciation of the kinematic implications of plate tectonic theory the impact of this recognition upon our perception of geotectonic processes in general and the

comparison of truly undrained and constant volume direct simple shear tests the paper describes a chamber for the norwegian geotechnical institute s direct simple shear device that enables undrained tests with pore pressure measurements to be run the results of undrained

does the golden spike still glitter a modus operandi for precambrian stratigraphy is suggested which combines the geochronometrical scale and late proterozoic chronostratigraphy the development of phanerozoic stratigraphical classification is followed from the pioneering days of murchison through the growing understanding of fossils in relation to time and the later distinction of rock and time rock to the advent of the golden spike criticisms of chronostratigraphy global standard stratigraphy are assessed and its value upheld stratigraphical procedures such as those of event stratigraphy are useful in particular situations but correlative methods in the phanerozoic are dominated by biostratigraphy examples are taken of its use and potential in north american cordilleran palaeozoic stratigraphy in the triassic system as a whole and in the caledonides of britain and ireland

a magnetic deep structural geology interpretation of ireland a new 10 nt compilation of magnetic data for ireland and the immediately adjacent sea area shows distinct geophysical provinces of characteristic magnetic signature separated by major magnetic linears which can be related to important geological lines in the far northwest there is a structurally coherent grampian tectonic thermal zone bounded to the southeast by the fair head clew bay line between the fair head clew bay line and the western extension of the southern uplands line which forms the northern margin to the southern uplands of scotland is a segmented zone of relatively small magnetic anomalies and geological bodies of mixed crustal affinity grampian massifs lower palaeozoic sedimentary basins bounded by major shear faults relics of lower palaeozoic oceanic and related crust and possible older microcontinent fragments these separate areas have quite individual structural and metamorphic histories there is good evidence for the grampian massifs having undergone significant lateral movement during lower palaeozoic times and it can be inferred that many of the now adjacent bodies are only docked the original position of each block is now not known in the far southeast of ireland are relics of a cadomian craton which are now tectonically imbricated with lower palaeozoic rocks this is probably the segmented margin of the stable cadomian craton which is inferred beneath wales and south britain between the southern uplands line and the northernmost extent of the relics of the southern craton is a zone of deep seated low amplitude magnetic anomalies and major linears these are about 30 oblique to the southern margin of this vestiges of the iapetus ocean zone but are nearly parallel with the northern margin

crystallization and emplacement chronology of the fournier oceanic fragment canadian appalachians collision zones between the plates that form the earth s surface are commonly delineated by displaced fragments of oceanic crust and mantle these fragments are important because they can be used to constrain the timing of the generation and destruction of former oceans by means of radiometrie and fossil studies in the northern appalachian caledonian orogen of canada the united kingdom and scandinavia the majority of the oceanic fragments crystallized between 480 and 500 myr ago in basins marginal to the lapetus ocean and were emplaced onto continental margins at least 10 myr later here we report a significant exception u pb geochronology of zircons extracted from a plagiogranite in the fournier oceanic fragment of the canadian appalachians reveals a much younger crystallization age of 461 3 2 myr whereas fossil evidence indicates continental incorporation in the silurian some lapetan marginal basins therefore continued to develop or were created after the early ordovician

seismological evidence for shallow crystalline basement in southern uplands scotland new evidence is presented and old data re appraised to indicate that the southern uplands of scotland contain crystalline rocks of continental affinity at shallow depth 1 5 km in at least two zones roughly parallel to the caledonoid strike this is in contrast with earlier interpretations especially of the lispb seismic profile1 2 which suggested that the lack of such rocks implied a major discontinuity across the caledonides near the southern uplands fault the new interpretations suggest that basement indistinguishable seismologically from that below the midland valley continues 15 20 km southwards below the southern uplands and that another such crustal block underlies the eskdalemuir eka seismological array further to the south the existence of such blocks radically affects plate tectonic reconstructions of the caledonides especially the margins of the lapetus ocean3 6 only drilling is likely to show whether the contact between basement and the overlying lower palaeozoic greywackes and shales is structural igneous or least likely simple overstep

strain partitioning in transpression zones abstract transprcssional strain acting upon structurally anisotropic rocks can be partitioned into separate deformational domains of pure shear and simple shear this contrasts with homogeneous transpression in which both the pure shear and the simple shear strain components are uniformly distributed across the zone of deformation structural weaknesses capable of partially or fully accommodating one component of deformation include lithological contacts rheological heterogeneities and faults or shear zones situated within the deformation zone or lying along its boundaries partitioning of transprcssional strain can occur when stress is applied oblique to pre existing structural weaknesses or can occur during later stages of progressive strain when the early deformation of isotropic rocks imparts sufficient anisotropy to allow subsequent strain to be partitioned partitioning of transpressional strain into domains lying parallel to the deformation zone boundaries can be distinguished from fault stepped transpression in which strain is partitioned along the length of a segmented fault zone mesofracture analysis of rocks affected by mid devonian deformation on both sides of the highland boundary fault zone hbfz in central scotland shows that strain was not homogeneous the mesofracture data suggest that regional north south compression orientated oblique to the pre existing ne sw trending hbfz was partitioned into separate deformational domains the hbfz accommodated most of the simple shear component whilst the rocks flanking the zone were deformed predominantly by pure shear a contemporary example is the san andreas fault in central california where analyses of neo tectonic stresses show that the direction of principal compression is perpendicular to the fault zone many examples of the partitioning of transpressional strain have also been recognised at destructive plate margins where the direction of plate motion is oblique to the edge of the over riding plate

the northern belt of the longford down inlier ireland and southern uplands scotland an ordovician back arc basin the northern belt of the southern uplands longford down zone contains two compositionally distinct clastic petrofacies suites each ranging from mid to late ordovician age they extend throughout the length of the belt and are generally separated by a strike fault the northwestern metaclast petrofacies suite is characterized by felsic igneous and metamorphic detritus derived from a continental source to the nw whereas the southeastern basic clast petrofacies suite contains abundant calc alkaline detritus derived from an island arc and plutonic metamorphic root zone complex to the s or se a similar source but located to the north is indicated for younger clastic sequences of similar composition in the northern part of the central belt immediately south of a major sinistral shear zone which defines the boundary between the northern and central belts it is concluded in light of other facets of regional geology and possible palaeogeographic configurations that this pattern reflects a back arc basin in the northern belt margined by a volcanic arc massif which once occupied the space now represented by the shear zone and a fore arc basin to the se central and southern belts the northern belt back arc basin is postulated to have been closed and deformed at the end ordovician and subsequently overthrust northwestward by the allochthonous thrust stack imbricated central and southern belts fore arc sequence at the end silurian this in the northern belt resulted in the development of a major recumbent structure downward facing on regional end silurian cleavage

structure of a continental strike slip fault from deep seismic reflection walls boundary fault northern british caledonides reprocessing of four offshore deep reflection profiles across the walls boundary strike slip fault postulated as the northward continuation of the great glen fault over the shetland platform provides useful constraints on the lithospheric structure of a major continental transform and some insight into the manner in which strike slip faults deform the reprocessing was aimed at delineating complex structure and reducing noise contamination and consisted of analyses of diffractions and amplitude variations and experimentation with seismic migration the results demonstrate a highly variable crustal and uppermost mantle structure for the walls boundary fault along more than 150 km of its length six key observations can be made from this study 1 as middle and lower crustal reflections approach the fault they are truncated by and do not continue across the downward projection of the fault 2 trends in reflection structure of the crust and or uppermost mantle differ markedly from one side of the fault to the other 3 diffractions typically are concentrated at the level of the moho discontinuity directly beneath the fault 4 after seismic migration steeply dipping reflectors are imaged in places directly beneath the fault in the lower crust 5 in one location the moho reflection is deflected across the fault into a narrow synform or keel structure which is exactly bisected by the downward projection of the fault and 6 in a separate location a prominent upper mantle reflector is truncated across the fault a vertical or steeply dipping structure is deduced for the fault in much of the crust and perhaps parts of the uppermost mantle however more diverse reflecting structures are interpreted for the lower crust such as the moho deflection and steeply dipping reflectors directly beneath the fault that may have been related to a localized component of transpression and resultant crustal thickening the presence of diffractions at the level of the moho discontinuity where the downward projection of the fault intersects it suggests points of structural roughness that may be related to deformation of the moho by late motion along the fault integrating the interpretation of the reflection data with previous geological studies implies that the age of the moho deflection is post early cretaceous but that the dipping structure adjacent to and cut off by the fault is probably caledonian silurian early devonian although the disparate structure of the crust on opposite sides of the fault supports geologic interpretations of large scale displacement a major step in the moho discontinuity directly beneath the walls boundary fault cannot be substantiated from the seismic data

the application of microchemical analysis of alluvial gold grains to the understanding of complex local and regional gold mineralization a case study in the irish and scottish caledonides terranes of the appalachian caledonian orogen north of the iapetus suture host gold mineralization in north america scotland and ireland to date commercial exploitation has been confined to the appalachian terrane of canada in scotland and ireland regional analysis of the types of gold mineralization has been handicapped by the relatively small number of bedrock gold occurrences available for study however there are a large number of alluvial gold localities a technique of microchemical characterization of gold grains has been applied to facilitate the classification of alluvial gold localities in terms of the style or styles of source mineralization thereby permitting a more complete interpretation of regional gold mineralization the technique of microchemical characterization used in this study classifies populations of gold grains from each locality in terms of the assemblage of opaque mineral inclusions enclosed within the gold grains and the concentration of minor alloying elements in the gold a total of 2 400 gold grains collected from 44 sites in ireland and scotland have been studied the microchemical signatures of populations of gold grains extracted from bedrock samples correlate closely with those from adjacent alluvial populations and furthermore our data are compatible with the descriptions of the vein mineralogy of bedrock mineralization reported by other workers this result has facilitated informed speculation on the type of source mineralization of other alluvial occurrences where the bedrock source remains undiscovered populations of alluvial gold from individual localities have been grouped into larger regional data sets on the basis of similarities in microchemical signature the signatures of these data sets were analyzed using the same criteria and 10 classes of gold were identified regional analysis of the distribution of the gold classes showed that the occurrence of the four major classes correlates strongly with host terrane irrespective of geographical location gold from the southern uplands terrane is characterized by inclusions of base metal sulfides and sulfarsenides with the silver content of the gold being less than 15 percent in contrast populations of alluvial gold from the grampian and highland terranes commonly exhibit median silver contents of over 20 percent and frequently contain silver telluride and bismuth telluride inclusions which are extremely rare in gold from the southern uplands terrane however in other cases the style of mineralization is controlled by specific host or source rocks and is independent of the terrane two examples are gold mineralization within porphyry systems and gold associated with red bed sediments the study has confirmed the value of the technique of gold grain characterization using information from both concentrations of alloying elements and inclusion assemblages as a powerful tool in interpreting complex local and regional gold mineralization

an oxygen and carbon isotopic study of multiple episodes of fluid flow in the dalradian and highland border complex stonehaven scotland the carbon and oxygen systematics of rocks north of stonehaven scotland were studied to provide new constraints on the nature and timing of metamorphic and post metamorphic fluid infiltration carbon and oxygen isotopic data were collected from carbonated spilites and carbonate bearing veins in the highland boundary fault from dalradian metacarbonate layers and pervasively carbonated schists and from three generations of carbonate bearing veins and a quartz porphyry dyke within the dalradian isotopic evidence for syn metamorphic fluid infiltration in the dalradian is preserved in quartz calcite veins in the upper chlorite zone and in staurolite zone metacarbonates 18 o values for coexisting quartz 18 o quartz 13 6 and calcite 18 o calcite 11 6 in a vein from the upper chlorite zone are consistent with precipitation of both minerals from a single fluid at c 375 c oxygen isotopic data from staurolite zone metacarbonate layers 18 o calcite 10 4 to 12 9 suggest major isotopic exchange with an external fluid equilibrated with a quartzo feldspathic reservoir such as the dalradian metasediments at metamorphic temperatures low negative carbon isotope data from these metacarbonate layers 13 c calcite 12 4 to 17 7 may indicate a component of oxidized organic material in the infiltrating fluid post metamorphic fluids derived from or equilibrated with a high temperature silicate reservoir such as felsic igneous rocks were responsible for precipitation of post metamorphic vein carbonate carbonation of pelitic schists and alteration of metamorphic feldspar porphyroblasts from barrow s famous spotted chloritoid schists in an area geographically associated with several carbonated quartz porphyry dykes some veins in the biotite and garnet zones have been conduits for multiple generations of fluid flow quartz from this generation of veins preserves a metamorphic isotopic signature 18 o quartz 11 2 to 13 2 whereas dolomite and calcite are considered to be post metamorphic based on textural evidence and isotopic data indicating extreme oxygen isotopic disequilibrium between vein quartz and carbonate minerals 18 o dolomite 23 7 to 24 2 18 o calcite 25 3 18 o quartz 11 2 to 13 2 variably depleted carbon isotopes from carbonate minerals in these veins 13 c dolomite 8 1 to 13 4 13 c calcite 12 2 may reflect input of oxidized organic material in the mineralizing fluid isotopic data from carbonate veins and variably carbonated spilites from the highland boundary fault are consistent with hydrothermal alteration of basalts by seawater which had its carbon isotope systematics modified by addition of magmatic co 2 and or oxidation of organic carbon such alteration may have occurred on the seafloor or during emplacement in the highland boundary fault

the deep structure of northern england and the iapetus suture zone from birps deep seismic reflection profiles the nec deep seismic reflection profile images structures in the lower crust which represent the iapetus suture zone beneath southern scotland and northern england on the basis of reflectivity characteristics of the lower crust and reflection moho we recognize four different crustal zones which correspond to four crustal terrane types midland valley zone a a sub continental subduction complex zone b lake district zone c and midland platform zone d the junctions between each of these zones are interpreted as being tectonic a and b originate from the northern continental margin of the iapetus ocean and are separated from c and d which are derived from a southern continent by strong northerly dipping reflections between approximately 15 and 30 km deep in the crust the structure responsible for these reflections does not displace the reflection moho north of the ab cd junction the base of zone b and base of the reflective crust is marked by a pair of parallel reflectors persistent for a horizontal length of about 55 km these reflectors are interpreted as the top and bottom of a slice of remnant oceanic crust at their northerly limit the deepest of the two parallel reflectors transects the moho to a depth of 6 7 km into the uppermost mantle we suggest that their termination delimits the northerly extent of a decollement or shear zone at the present continental moho the shear zone was active during the late stages of caledonian collision when mantle originally from beneath the southern continent underthrust old and newly created crust of the collision zone

palaeomagnetism of the loch doon granite complex southern uplands of scotland the late caledonian palaeomagnetic record and an early devonian episode of true polar wander abstract the southern uplands terrane is an ordovician silurian back arc foreland basin emplaced at the northern margin of the iapetus ocean and intruded by granite complexes including loch doon 408 3 1 5 ma during early devonian times protracted cooling of this 130 km 3 intrusion recorded magnetic remanence comprising a predominant a magnetisation linked to initial cooling with dual polarity and mean direction d i 237 64 95 4 palaeopole at 316 e 21 n subsidiary magnetisations include mesozoic remanence correlating with extensional tectonism in the adjoining irish sea basin b d i 234 59 and minority populations c d i 106 2 and d d i 199 1 recording emplacement of younger 395 ma granites in adjoining terranes and the variscan orogenic event the a directions have an arcuate distribution identifying anticlockwise rotation during cooling a comparable rotation is identified in the orthotectonic caledonides to the north and the paratectonic caledonides to the south following closure of iapetus continental motion from midsoutherly latitudes 40 s at 408 ma to equatorial palaeolatitudes by 395 ma is identified and implies minimum rates of continental movement between 430 and 390 ma of 30 70 cm year more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander silurian devonian palaeomagnetic data from the british scandinavian caledonides define a 430 385 ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from gondwana they reconcile pre 430 ma and post 380 ma apw from this supercontinent and show that laurentia baltica avalonia lay to the west of south america with a relict rheic ocean opening to the north which closed to produce variscan orogeny by a combination of pivotal closure and right lateral transpression

accretion and collision related deformation in the southern uplands accretionary wedge southwestern scotland recent models for the tectonic development of the southern scottish caledonides are reviewed in terms of the deformation sequence observed within the southern uplands of scotland the early stages of this history are consistent with deformation within an accretionary prism formed above a northwest directed subduction zone later stages show a reversal in transport direction and also evidence for sinistral shear a deformational history for the area is proposed in terms of recent data on the deep structure of the southern uplands and adjacent regions the southern uplands is considered to be a large scale popup structure between a forethrust the iapetus suture and a backthrust which partially obducted it toward the northwest

the geology of the western part of the fintona block northern ireland evolution of carboniferous basins the western part of the fintona block is divided into four fault bounded segments that contain red bed sediments formerly assigned to the lower old red sandstone dating by miospores indicates the presence of deposits of early devonian age in the irvinestown segment late visean early silesian age in the tempo lisbellaw segment and late visean early silesian and late silesian ages in the milltown segment northward migration of the early carboniferous marine transgression in the northern part of ireland coincided with the sequential propagation of back stepping faults and resulted in the development of diachronous facies belts between late courceyan and arundian times tectonic uplift of a possible southwesterly extension of the tyrone igneous complex gave rise to the deposition of asbian to pendleian red beds to the south of a massif an interface between these red beds and contemporaneous marine sediments farther to the south is recognized and dated a new non marine basin containing brigantian and pendleian red beds also developed to the north of the massif a waterlogged floodplain that developed during westphalian a times may be coeval with more widespread coal bearing sequences elsewhere in ireland alluvial fans prograded southwards over this plain during westphalian b times when faults bordering a northern landmass were reactivated

tomographic evidence for the mantle upwelling beneath southwestern japan and its implications for arc magmatism abstract the seismotectonics and volcanotectonics of southwestern sw japan have been affected by the subduction of the old pacific and the young philippine sea slabs we have attempted to determine the three dimensional p and s wave velocity structures of the upper mantle in sw japan by using a large amount of high quality arrival time data recorded by the dense seismograph network on the japanese islands to understand the tectonic framework of sw japan our results show the existence of a large low velocity anomaly below the philippine sea slab in the chugoku and shikoku districts at least down to depths of 300 km we infer that the low velocity zone is an upwelling flow from depth two shallower low velocity zones derived from this low velocity zone are detected one is toward the quaternary volcanoes in the chugoku district and the other is toward the uppermost mantle beneath the kii peninsula the former suggests that the magma for the quaternary volcanoes in the chugoku district is attributable to the upwelling flow in the upper mantle this idea supports the geochemical and petrological observations that the erupted alkalic basalt in the chugoku district should have originated from a more primitive or enriched mantle the latter is responsible for the absence of a high velocity anomaly corresponding to the subducted philippine sea slab beneath the kii peninsula the philippine sea slab could have been more hydrated locally beneath the kii peninsula than that in the surrounding regions by the interaction with the mantle upwelling consequently dehydration reactions could be involved during the progressive metamorphism accompanied by intensive intraslab seismicity lowering the seismic velocity in the slab aqueous fluids thus released to the overriding plate might be a possible source of mantle type helium and trigger a long term swarm activity in the crust of the overriding plate our results indicate the importance of the heterogeneous structure below the philippine sea slab with regard to the tectonics of sw japan

is the southern uplands of scotland really an accretionary prism we argue that there is little which demands the accretionary prism model for the widespread silurian rocks of the southern uplands of scotland and ireland with new data from ireland we reinterpret these rocks as the turbidite fill of a successor basin that formed after the subduction of iapetus oceanic crust finally stopped at the end of the ordovician the structure of the basin is a sinistral transcurrent imbricate stack the result of oblique terminal collision of cadomia with laurasia at the end of the silurian

slab breakoff a model for caledonian late granite syn collisional magmatism in the orthotectonic metamorphic zone of scotland and donegal ireland abstract none of the existing models for calc alkaline late granite siluro devonian genesis in the metamorphic caledonian orogenic belt of ireland and scotland fully explains their spatial age or chemical character a consistent model must involve the closure of iapetus ocean where slab breakoff is a natural consequence of attempted subduction of continental crust expected outcome is a long linear belt of high k calc alkaline magmas some with characteristic trace element signatures specifically high ba sr and zr other features include the critical magmatic association of coeval appinite and granite rapid uplift erosion and the low grade regional metamorphism in the southern uplands the linear heat pulse on breakoff is spatially intensity and time limited producing small volume melts emplaced as separated plutons over a short time span magmatism in the caledonian metamorphic belt is accurately accounted for by slab breakoff on collision of baltica with the scoto greenland margin during the scandian orogeny following iapetus ocean closure the two chemically isotopically and areally distinctive suites in the metamorphic belt in scotland viz the argyll and cairngorm suites can be modelled by reference to the donegal granites where sequential partial melting of new lamprophyric underplated crust then shallower old crust as heat conduction moved up through the crust on slab breakoff produced magmas characteristic of the two suites

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

geochemical evolution of an ordovician island arc south mayo ireland early ordovician volcanic rocks exposed in the south mayo region of western ireland document the history of a volcanic arc complex produced following the initiation of south dipping subduction within the iapetus ocean in the late cambrian or early ordovician lavas of the lough nafooey group tremadoc arenig show an eruption history marked by an initial period of tholeiitic basaltic volcanism followed by a shift to increasing silica compositions of calc alkaline affinity subsequent volcanism of the tourmakeady group arenig shows a mixture of rhyolitic and andesitic compositions which are in turn succeeded by tuffs of the rosroe and mweelrea formations of the murrisk group llanvirn these are also rhyolitic and andesitic but are generally more siliceous the overall increase in silica contents may reflect increasing fractional crystallization due to crustal thickening trace element studies of the basaltic members of the lough nafooey group show a strong subduction zone influence while absolute levels of immobile trace and rare earth elements decrease upsection relative depletion of the most incompatible elements decreases over the same time period these trends suggest decreasing degrees of partial melting of a progressively more depleted source mantle upsection the basaltic bohaun volcanic formation tremadoc arenig shows greater depletion in its incompatible trace elements than the lough nafooey group so indicating a more depleted mantle source comparison with modern arc systems suggests that the bohaun volcanic formation was probably generated shortly after the initiation of subduction and was erupted in a more trenchward position than the lough nafooey group i e in the forearc this confirms a south dipping polarity to the subduction zone and suggests an age of initiation during the late cambrian or earliest ordovician

geological constraints structural evolution and deep geology of the northwest scottish caledonides the front of the caledonian orogenic belt in nw scotland is marked by a zone of foreland thrusting which contains imbricates of lewisian basement proterozoic cover and cambrian shelf sediments faults within this moine thrust belt in most localities propagated in a foreland directed sequence therefore the earliest of these thrusts was the moine thrust which carried a basement assemblage of moine and lewisian rocks over the cambrian shelf the development of large duplex and imbricate structures in the footwall postadted movement on this thrust hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the cambrian shelf no synsedimentary or prethrust extensional features are recognised within the cambrian succession which displays a remarkable stratigraphic and thickness consistency this implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to caledonian thrusting a balanced cross section constructed across the northern part of the moine thrust belt has given a restored width for the cambrian shelf of 54 km geophysical data suggest that the present foreland crustal thickness is 28 km if this is taken to be the crustal thickness beneath the cambrian shelf during caledonian thrusting a minimum cross sectional area of 1512 km of lewisian rocks must remain beneath the present outcrop of the moine thrust sheet this foreland basement wedge may have been imbricated subsequent to the development of the cambrian imbricates caledonian structures within the moine thrust sheet are interpreted as forming an imbricate stack with the moine thrust acting as a floor thrust the ductile imbricate thrusts possibly roofed into the naver slide late open folds which warp foliation slides and metamorphic isograds within the moine sheet may represent culminations in the foreland lewisian imbricates beneath the moine thrust these postulated culminations may have been sufficient to initiate gravity driven structures this may explain late movements on high level faults within the moine thrust belt

the glen kyllachy granite and its bearing on the nature of the caledonian orogeny in scotland the tomatin findhorn granite in the nw grampian highlands has been separated into two distinct complexes a late tectonic glen kyllachy granite tectonically foliated and a post tectonic findhorn granite flow foliated for the glen kyllachy complex rb sr analyses of muscovites from the granite and from an associated suite of cross foliated pegmatites yield an emplacement age of 443 5 15 ma whole rock rb sr data support field and textural evidence that the pegmatite and granite emplacement was late tectonic to the last f3 phase of caledonian folding initial granite 87 sr 86 sr of 0 7176 supports field and geochemical evidence of derivation from upper crustal metasediments first metamorphosed during the grenville event for the findhorn granite concordant rb sr and k ar mineral data establish an age of 413 5 ma and an initial 87 sr 86 sr of c 0 706 indicates a lower crustal and or mantle source this age and isotopic contrast between these granites is characteristic of the whole grampian region in which a plutonic hiatus between c 440 and 415 ma coincides with the peak of sedimentary accretion in the southern uplands and may be explained in terms of the lack of hydrous materials passing down the associated subduction zone structural metamorphic and radiometric evidence suggests a that the late f3 glen kyllachy pegmatites are comparable with the 442 7 ma old syn f3 pegmatites in the n highlands both pegmatite suites are situated in the axial zone of the metamorphic caledonides displaced by the great glen fault and b that the caledonian c 454 ma old peak tectonic metamorphic event d2 previously documented in the n highlands coincides with a caradocian basin closure along the highland boundary fault for silurian events in the n highlands we propose a separate subduction history in this region the peak of calc alkaline plutonism was accompanied by continuous folding and westward thrusting at the same time as a plutonic hiatus in the grampian highlands current evidence suggests that the c 160 km or greater sinistral movements on the great glen fault were not initiated until late silurian early devonian time

palaeozoic terrane accretion in the western irish caledonides the southern boundary of the early caledonides in the british isles the dalradian supergroup is represented by the major highland boundary fair head clew bay line in the west of ireland accretion along this laurasian margin occurred by sinistral strike slip during two phases the earliest was responsible for the docking of the connemara terrane a displaced part of the dalradian outboard of the south mayo trough in the llanvirnian llandeilian this event is recorded independently by radiometrically dated sinistral strike slip deformation in the ox mountains of western ireland following deposition of a silurian overlap sequence sinistral movements were renewed between the upper llandoverian and the siegennian this event is part of a major phase of sinistral strike slip throughout the british isles involving accretion of the cadomian terrane and the devonian accretion of the avalon terrane in the northern appalachians

shear wave splitting around the northern atlantic frozen pangaean lithospheric anisotropy abstract although the number of teleseismic shear wave splitting measurements has considerably increased world wide during the last decade there is no consensus on a unique and universal interpretation of these data splitting might primarily originate in the lithospheric and or asthenospheric mantle but the technique has no vertical resolution and therefore leaves this question of anisotropy location unresolved generally the consequences of the two possibilities are used to infer which is more likely if splitting occurs in the lithosphere the splitting parameters the orientation of the fast split shear wave o and the delay time t should be related to frozen or active deformation on the other hand if the splitting occurs in the asthenosphere the observed splitting parameters should be related to the present day flow induced by the differential motion between the tectonic plates and the underlying mantle the aim of this paper is to test these hypotheses against shear wave splitting observations from western europe and the eastern u s indeed except in a few places splitting on either side of the atlantic on hercynian caledonian or grenvillian structures do not appear to have been deeply and pervasively affected by the opening of this ocean this provides us the opportunity to place these results in the pangaean reference frame of the hercynian caledonian belt before the opening of the atlantic in many places the close parallelism between lithospheric structure and the absolute plate motion apm impedes a reliable identification of the origin of the anisotropy especially an asthenospheric flow channelled around the deeply rooted north american craton may be compatible with the large scale anisotropy pattern in the eastern u s despite this ambiguity fast polarization directions o observed on or near some major lithospheric structures correlate better with the trend of the local tectonic structures than with the expected asthenospheric flow therefore we suggest that these lithospheric structures developed before the opening of the atlantic remain frozen since hercynian caledonian or grenvillian times and are responsible for the observed splitting at these places

major southeast directed caledonian thrusting and folding in the dalradian rocks of mid ulster implications for caledonian tectonics and mid crustal shear zones the dominant structure controlling the disposition of dalradian stratigraphy in mid ulster has hitherto been regarded as a southeast facing gently inclined f 1 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 d 2 structure traceable for at least 40 km along strike and responsible for a regional stratigraphie inversion over an area of 300 km 2 following d 2 a major 10 km thick d 3 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 d 3 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 d 3 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

does the great glen fault really disrupt moho and upper mantle structure many previous interpretations of deep seismic reflection profiles across the great glen strike slip fault system have postulated that the fault penetrates the crust and upper mantle as a vertical plane that sharply offsets the moho discontinuity such an interpretation has become a general feature of more recent regional syntheses of the deep geology of the british isles reprocessing of portions of four profiles across the fault improves the resolution of lower crustal and upper mantle structure and calls into question the moho step interpretation diffraction analysis and seismic migration applied to reflection data north of shetland across the northward continuation of the great glen fault walls boundary fault indicate a narrow synform or keel developed on the moho directly beneath the fault this keel is itself underlain by a possible mantle reflection that is cut off by the downward projection of the fault north and west of ireland analyses of amplitudes frequencies and the geometrical behavior of reflections upon migration show that structures previously interpreted as moho steps may be better explained as distinct packages of reflectivity that are uppermost mantle or possibly out of the plane in origin in one location north of ireland where some three dimensional control exists the vertically downward projection of the great glen fault intersects without disrupting dipping structure in the upper mantle this observation leads to a model for displacement on the fault system in which motion is laterally transferred along a dipping ramp or blind thrust in the uppermost mantle somewhat analogous to models developed for the san andreas fault that indicate displacement along the fault to be laterally offset within the middle crust one of the principal conclusions of this study that major vertical steps on the moho beneath the great glen fault are difficult to support from the available seismic data is consistent with theologically based studies which predict that moho topography such as vertical steps is unlikely to be preserved over long periods of geologic time

british paleomagnetism iapetus ocean and the great glen fault new paleomagnetic data for britain establish the width of the iapetus ocean as 1000 km in mid ordovician time but do not yet allow precise estimates of the rate of its subsequent closure the new lower paleozoic data from northern scotland are similar to data from the rest of britain but are not compatible with north america data on the bullard et al reassembly if this discrepancy is correct lines of major transcurrent dislocation have yet to be identified either in the north atlantic continental shelves and margins or in eastern north america and the great glen fault was not the main locus of offset between britain and north america the tectonic complexities associated with this configuration require further evaluation

response of continental magmatic arcs to regional tectonic changes recorded by synorogenic plutons in the middle crust an example from the coast mountains of british columbia abstract the coast mountains of british columbia record an increase in magmatic activity acceleration in exhumation rate and a change from transpression to extension between 60 and 52 ma structural analysis of fabrics in three mid crustal plutons and country rocks leads to conclusions about pluton emplacement mechanisms and strain partitioning during changing tectonic conditions the quottoon and kitlope plutons 60 ma have steep foliations and lineations consistent with partitioned transpressional deformation the chief matthew s pluton 57 55 ma intruded during the formation of a sub horizontal transposition foliation and has radially distributed lineations consistent with sub vertical flattening during extension the change in orientation of the foliation represents an almost orthogonal rotation of the shortening direction from sub horizontal to sub vertical the chief mathew s pluton is interpreted to intrude initially into gently dipping fractures perpendicular to the steeply dipping foliation these melt filled fractures acted as conduits for melt triggering horizontal flow and eventually formed kilometer scale sills the steeply dipping fabrics of the coast shear zone provided pathways for melt to ascended from lower to middle crustal depths partitioning deformation into three dimensional domains of flattening simple shear and constrictional strain created the space for plutons this pattern of deformation reflects the interaction of regional deformation with magma emplacement

tectonic evolution of the southeastern margin of the yangtze block constraints from shrimp u pb and la icp ms hf isotopic studies of zircon from the eastern jiangnan orogenic belt and implications for the tectonic interpretation of south china abstract the jiangnan orogenic belt is a ne sw trending fold belt in the southeastern yangtze block of south china the eastern segment of the belt located in northeastern jiangxi and southern anhui provinces is predominated by greenschist facies metamorphosed volcanic sedimentary strata with minor ophiolite and their protoliths are considered to have formed in a back arc basin zircon shrimp u pb dating reveals that these rocks were deposited emplaced in the period 842 825 ma zircon la icp ms hf analyses show that most igneous zircons from the volcanic sedimentary rocks possess positive hf t values of 0 09 12 74 corresponding to depleted source model ages of 0 9 1 4 ga this suggests a significant neoproterozoic juvenile crustal growth event in the southeastern part of the yangtze block which is also documented by gabbro 833 ma from ophiolite with positive hf t values of 5 24 13 25 and hafnium model ages of 0 87 1 20 ga these new data combined with previous results indicate the presence of an arc and back arc basin system along the southeastern margin of the yangtze block at 842 825 ma collision of the arc with the proposed shuangxiwu composite terrane which had previously formed away from the southeastern margin of the yangtze block through possible subduction and collision led to the closure of the back arc basin and the formation of the jiangnan belt in the period 825 815 ma we suggest that south china is an accretionary orogen and that orogen parallel motion along terrane bounding shear zones may have played a significant role in its amalgamation

the pontesford lineament welsh borderland the ne trending pontesford lineament centres on the pontesford linley and adjacent faults in the shelve longmynd inlier to the ne it is usually drawn along the se edge of the cheshire basin and to the sw along a variety of lines parallel to or converging with the church stretton fault new mapping and reassessment of published data suggest that sw of the pontesford linley fault the lineament is logically drawn along the clun forest disturbance a post pridoli late silurian fold fault belt then across the nw side of the llandrindod inlier to flank the tywi lineament the lineament was intermittently active at least from mid ordovician through to triassic time with both strike slip and dip slip displacements that involved the upper part of the precambrian basement late ordovician and early silurian strike slip was particularly important the lineament controlled the sites of several silurian sedimentary basins ordovician contrasts across the lineament can be explained either by contemporary fault control or by subsequent large displacements juxtaposing unlike terranes the long history of reactivation and basement control on the pontesford lineament is typical of lineaments throughout wales and the borderland it suggests that both caledonian and variscan structural trends n of the variscan front may be inherited from older pre silurian or even precambrian basement structures

snowdon basalts and the cessation of caledonian subduction by the longvillian basalts of the bedded pyroclastic formation snowdon volcanic group north wales of longvillian caradoc age are heterogeneous in trace element composition forming a mixing trend between ocean island basalt oib type and magmatic arc basalt the two end member compositions were partial melts of two distinct upper mantle sources the oib end member is similar to icelandic oib and is the only known example of basalt of this type in the ordovician silurian welsh basin since ocean island basalt magma type is thought never to erupt above well established subduction zones the eruption of the longvillian oib provides a minimum age for the end of subduction of iapetus oceanic lithosphere below the welsh basin a model for the evolution of the mantle below the welsh basin is proposed which accounts for the geochemistry of magmas erupted from the tremadoc to the early silurian

palaeomagnetic study of the cairnsmoor of fleet granite and criffel dalbeattie granodiorite contact aureoles caledonian tectonics of the southern uplands of scotland and devonian palaeogeography the plutons of cairnsmoor of fleet 392 2 ma and criffel dalbeattie 397 2 ma both mineral isochron ages comprise two of four major post tectonic granitic complexes emplaced into the southern uplands an ordovician silurian back arc and foreland basin complex formed at the northern margin of the iapetus suture to expand the palaeomagnetic record of the southern uplands we have studied palaeomagnetism and magnetic fabrics in traverses spanning contacts of these intrusions with host mudrocks a uniform anisotropy of magnetic susceptibility ams fabric across the cairnsmoor of fleet contact has been enhanced by recrystallization into hornfels near the contact and records a late acadian regional stress operative during or soon after emplacement of the pluton in middle devonian times magnetization during slow cooling recorded a dual polarity a remanence in granite and hornfels with mean direction d i 92 2 95 6 5 yielding a palaeopole q 6 at 2 n 265 e linked to cooling at c 392 ma subsidiary magnetizations are overprints imparted during variscan tectonism b d i 194 6 and jurassic rifting within the adjoining irish sea basin c c 160 140 ma d i 172 52 the criffel dalbeattie pluton has more complex ams fabrics recording both deformation and emplacement effects hematite of secondary hydrothermal origin is a significant feature of the rock magnetic record in the aureole which is otherwise dominated by paramagnetism the granodiorite is more strongly magnetized than the country rocks accounting for a positive aeromagnetic anomaly a fairly dispersed dual polarity remanence mean d i 115 55 95 18 in granodiorite and late tectonic porphyrite dykes is probably the oldest magnetization preserved in this pluton because it correlates with an excursion of britain into southerly palaeolatitudes at c 410 ma and indicates an early devonian emplacement age the palaeofield at c 397 ma the currently accepted isotopic age is recorded by a minority overprinted remanence mean d i 272 2 95 12 similar to the record in the cairnsmoor of fleet pluton and granites from the adjoining lake district terrane granite complexes of the southern uplands block collectively record regional rotation and excursion of britain into southerly latitudes between c 410 and 390 ma comparable silurian devonian palaeomagnetic poles identify common apparent polar wander apw in paratectonic and orthotectonic terranes from the variscan front in the south to the laurentian foreland in the north following climactic acadian deformation apw between 430 and 390 ma embracing the post closure history of the caledonian orogen is a loop executed at rates much higher than typical rates of plate motion and appears to record a component of true polar wander the 110 arc length is identical to polar shift identified between mid silurian and lower middle devonian poles from gondwana the two paths superimpose to show that the western margin of gondwana was in proximity to the se margin of laurentia during acadian deformation in early middle devonian times and remote from the caledonides the residual rheic ocean subsequently closed by a combination of pivotal and left lateral strike slip motions

the loch quoich line a limit of early palaeozoic crustal reworking in the moine of the northern highlands of scotland the loch quoich line a major structural line at least 60 km in length within the moine of the northern highlands of scotland has hitherto been variously interpreted as a major tectonic dislocation the root zone of a nappe and an unconformity the work reported here was carried out at the type locality of the line and was directed at resolving these ambiguities the lithostratigraphic succession established by sedimentary structures shows that the rocks of the glenfinnan division are older than but are stratigraphically continuous into those of the loch eil division rocks of the loch eil division crop out extensively to the w of the loch quoich line and are not restricted in outcrop to the area e of the line as was previously thought the early d 1 d 2 structures were initially sub horizontal steeply inclined nne trending d 3 structures are weakly developed e of the axial trace of a major fold the beinn beag synform in this area the metasediments remain essentially in their post d 2 gently inclined attitude w of the axial trace of the synform the earlier folds and fabrics were severely reworked into steep attitudes during d 3 producing nne trending zones of 9platy9 rocks which mark regions of severe strain on d 3 fold limbs earlier d 1 d 2 structures are clearly preserved in d 3 hinge zones which mark zones of low d 3 strain it is concluded that the loch quoich line coincides with the axial trace of the beinn beag synform and that it is the eastern limit of severe tectonic reworking probably caledonian over printing early probably precambrian structures recognition of structural and stratigraphic continuity between the glenfinnan and loch eil divisions implies that the loch quoich line cannot be correlated across the great glen fault with the putative unconformity separating the grampian cover and central highland basement divisions in the moine rocks of the central highlands

mavis an upper crustal seismic refraction experiment in the midland valley of scotland summary the results of the midland valley investigation by seismology mavis are described the experiment consists of three refraction profiles of between 80 and 100 km length across the upper palaeozoic basins of the scottish midland valley three refractors are recognized thus defining four upper crustal layers the top two layers are interpreted as carboniferous plus upper old red sandstone and lower old red sandstone plus lower palaeozoic the structure of the refractor an unconformity separating the two layers mirrors surface structure deeper refractors associated with midland valley crystalline basement show no correlation with surface structure the mavis survey allows the refinement of earlier gross interpretation of upper crustal structure in the midland valley but confirms the existence of anomalously shallow 6 4 km s basement and shows in conjunction with results of previous work that a large area of the central and southern midland valley is characterized by one velocity signature probably indicative of a single terrane

a sequential back arc and foreland basin thrust duplex model for the southern uplands of scotland thrust imbrication of ordovician and silurian submarine fan sequences overlapping pelagic deposits in the southern uplands has been interpreted in terms of an accretionary prism formed above a nw directed subduction zone structural features invoked to support accretion are not definitive and could be explained in terms of a thin skinned thrusting model new palaeocurrent and compositional evidence from llandeilo to mid llandovery age turbidites in the northern part of the southern uplands proves interdigitation of sediments with strongly contrasting petrography turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz rich formations this implies a back arc situation the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south oblique collision of the opposing continental margins of the iapetus ocean during the llandovery caused the cessation of subduction underthrusting of the southern margin initiated a se propagating thrust stack which deformed the back arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc a southward migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late llandovery hawick group and wenlock sequences mid to end silurian sinistral strike slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep seated structures allowed the intrusion of lamprophyre dykes and granites

the application of spatial derivatives to non potential field data interpretation source body edge detection is a common feature in the processing and interpretation of potential field data sets a wide range of spatial derivatives is available to enhance the information contained in the basic data here the ability of these procedures to assist with the mapping interpretation of non potential field data is considered the study uses airborne electromagnetic conductivity data but also provides a general context for other conductivity resistivity data provided the non potential field nature of active and thus spatially focused measurements is acknowledged the study discusses and demonstrates the application of a range of common spatial derivative procedures including the analytic signal and upward continuation to both magnetic and conductivity data the ability of the tilt derivative to provide enhanced mapping of conductivity data is considered in detail tilt and its associated functions are formed by taking combinations of vertical and horizontal derivatives of the data set theoretical forward modelling studies are first carried out to assess the performance of the tilt derivative in relation to the detection and definition of concealed conductivity structure the tilt derivative embodies automatic gain control that normalizes the detection and definition of both weak and strong conductivity gradients across an appropriate subsurface depth range the use of high order spatial derivatives inevitably results in a degree of noise cultural perturbation amplification that is survey and technique specific both of these aspects are considered using practical case studies of jointly obtained magnetic and conductivity data at a variety of spatial scales

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

regional scale fluid flow and element mobility in barrow s metamorphic zones stonehaven scotland the geochemistry of metamorphic quartz vein formation in barrow s index mineral zones north of stonehaven scotland was investigated in order to assess regional fluid flow and mass transfer metamorphic grade in the dalradian metasediments increases to the north northwest away from the highland boundary fault hbf and associated ophiolitic rocks of the highland border complex hbc passing through the chlorite chl biotite bt garnet grt chloritoid cld and staurolite st zones syn metamorphic fluid infiltration at 462 8 8 ma breeding et al in am mineral 89 1067 2004 produced considerable quartz veining vein abundance varies from about 5 to 15 volume percent of the outcrops veins tend to be more abundant in metapelitic layers than in metapsammitic ones metamorphic veins are surrounded by centimeter to decimeter wide zones of chemical and mineralogical alteration selvages porphyroblasts particularly bt grt cld and st are typically larger in selvages than in wallrocks distal to veins the altered selvages underwent fluid driven addition of na ca and sr and loss of k rb and ba alteration is most intense within 750 m of the hbf but is still very significant at the northern end of the field area some 2 km away mg fet fet total iron was either unchanged or increased due to alteration silica was added at some chl and bt zone localities near the hbf pb mass transfer was variable although pb was added at a number of locations rare earth elements ree were generally immobile but light ree and possibly heavy ree were lost at one field site the gain of na and ca and loss of k promoted the growth of plagioclase at the expense of micas particularly muscovite in selvages and wallrock inclusions throughout the field area and probably some calcite and or dolomite growth directly adjacent to the hbf the ca gains were also critical for epidote production in the bt zone gains of ca and increases in mg fet helped to stabilize grt at the expense of cld and st in some selvages hornblende and cummingtonite were discovered in strongly altered metapelitic rocks at one cld zone locality the metasomatism puts important constraints on the processes of mass transfer and suggests two models for regional fluid flow in the first model fluid flow in a direction of increasing temperature downward along the hbf added na and ca and removed k from the dalradian in the second model fluid flow upward from the hbc transported na and ca into the overlying dalradian and at the same time stripped out k the latter model is favored because it can most readily account for silica addition near the hbf but neither model can be ruled out at present in either case the veins represent fractures that transmitted very large time integrated fluid fluxes of at least 104 m3 fluid m2 rock consequently the veins were conduits for regional fluid flow that caused considerable open system chemical and mineralogical alteration during metamorphism

structural evolution in the moine of northwest scotland a caledonian linked thrust system a model is proposed whereby the caledonian metamorphic basement cover complex of northwest scotland the moine is considered as a linked thrust system this system lies between the moine thrust at its base and the naver sgurr beag slide at its top ductile fold and thrust zones which developed at mid crustal levels at metamorphic grades from greenschist to amphibolite facies are interpreted as decoupling from a detachment presently situated at relatively shallow depths this model is illustrated by two preliminary balanced cross sections these imply shortening across the northwest scottish caledonides in excess of 130 km and probably over 200 km when these structures are restored onto a crustal template a considerable quantity of lower crust is found to be required at depth the most likely location for the lower crustal wedge is beneath the grampian highlands

igneous emplacement in a transpressive shear zone ox mountains igneous complex the central and southwestern ox mountains inlier in western ireland comprises amphibolite facies metasediments and metavolcanic rocks which have been deformed by a major sinistral shear zone granitoid emplacement occurred in two distinct styles in the ox mountains shear zone the older pluton the ox mountains granodiorite was emplaced syn kinematically with respect to the main sinistral transpressional events in its wall rocks it was emplaced as a series of compositionally distinct sheets in the central part of the shear zone system thus suggesting the formation of transient dilational sites occurred in response to the transpressive deformation the younger lough talt and easky lough adamellites were emplaced as individual intrusions in dilational cavities produced by reactivated movements on a discrete zone within the ox mountains inlier the different intrusive styles are considered to be a product of the tectonic regimes operating during emplacement intrusion into transient dilational cavities may be one method by which granitoid magmas are emplaced into fault zones undergoing active contractional deformation

major southward thrusting of the dalradian rocks of connemara western ireland the connemara schists are of dalradian late precambrian cambrian age and since the work of kilburn et al 1 it has been apparentthat they occupy an anomalous geographical position south southeast of the strike continuation of the dalradian rocks of scotland and ireland and south of the inferred position of the highland boundary fault fig 1 also the connemara schists are unusual in having reached a higher metamorphic grade and having had a different metamorphic history from most other parts of the dalradian the model which we propose here for the connemara dalradian that of an allochthonous thrust sheet offers an explanation for these anomalies and highlights hitherto unsuspected similarities between the tectonic histories of the south east margins of the metamorphic caledonides in the british isles and scandinavia

u pb zircon age constraints on the timing and duration of wenlock silurian paleocommunity collapse and recovery during the big crisis high precision isotope dilution u pb zircon dating was conducted on three volcanic ash fall bentonite samples from the swedish island of gotland and on a fourth bentonite from the west midlands england zircons from the ireviken grotlingbo djupvik gotland and wren s nest hill 15 west midlands bentonites yielded weighted mean pb 206 u 238 ages of 431 83 0 23 0 67 ma 428 45 035 0 73 ma 428 06 0 2110 66 ma and 427 86 032 0 71 ma respectively analytical total uncertainties these biostratigraphically well controlled age dates effectively bracket the wenlock epoch of the silurian period and provide control for the duration of one of the major paleozoic biotic events and associated perturbations to the global carbon cycle the big crisis or lundgreni event graptolites the niulde event conodonts the mulde excursion carbon isotopes these new data suggest an older and shorter duration for the recalibration of the wenlock series and demonstrate that the cascade of biological and chemical events that took place during the big crisis happened on time scales of tens to hundreds of thousands of years less

the iapetus suture in the british isles the position of the iapetus ocean suture can be traced between ordovician fossil localities in the southern uplands and the lake district which contain respectively distinct north american and european faunas the southern uplands contains north american caradoc faunas in the northern belt and is considered to have been accreted onto the laurentian north american margin between the llandeilo and the end of the wenlock deep seismic reflection profiling shows that a surface probably parallel to the suture zone dips down to the northwest from the solway line in eastern ireland the surface trace of the suture coincides with the navan fault which separates the longford down massif from ordovician sediments containing european faunas the navan silvermines fault may represent the surface expression of the suture but it is masked by wenlock turbidites which appear to have crossed over the plate boundary northward subduction of continental crust below the margin of laurentia during the late silurian may explain the influx of turbidite fans from the north or north west onto depressed crust to the south of the suture termination of subduction coincides with the widespread acadian orogeny in the emsian at 395 5 ma

an ordovician ophiolite in county tyrone ireland ophiolites are regarded widely by geologists as remnants of old oceanic crust that has been obducted onto the continental margins during ocean or marginal basin closure and orogeny therefore they provide evidence for the existence of old oceans and oceanic basins and are a key to the extension of plate tectonic modelling into pre mesozoic rock assemblages in the caledonide appalachian orogen ophiolites are known although very few of these are dated well enough to provide accurate constraints on plate tectonic kinematics we report here the discovery of an ophiolitic sequence within the rocks of the tyrone igneous complex in northern ireland u pb dating combined with field evidence indicates that the tyrone ophiolite was formed and then obducted onto the northern margins of the iapetus ocean close to 470 myr bp arenig

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

the newer granite problem a geotectonic view the siluro devonian suite of granitic plutons in the british caledonides known as the newer granites together with their associated extrusive rocks represent one of the most extensively researched examples of calc alkaline magmatism apparently related to orogeny although recent chemical studies have credibly interpreted some of the scottish intrusions and volcanic rocks as part of a continental margin magmatic arc generated by the subduction of lapetus oceanic lithosphere beneath laurentia insurmountable problems of distribution and timing arise when attempts are made to relate the magmatic activity as a whole to a traditional two plate collision model for the orogeny newer granite magmatism is here discussed in the context of more mobilistic models for the post grampian evolution of the british caledonides which involve e w closure between laurentia and baltica terminated by collision in the silurian followed by the northward accretion of gondwana derived terranes in the early devonian the former produced the main caledonian tectonometamorphism of the northern highlands of scotland the latter the late caledonian deformation of the slate belts in the paratectonic caledonides these models imply much more complex convergence geometries which can in principle account for the whole newer granite suite as a series of subduction generated magmatic arcs overlapping in space and time the model proposed involves three late caledonian magmatic arcs in addition to the ordovician borrowdale arc which is not considered in this paper one is related to laurentia baltica convergence with westward subduction beneath the scottish sector of the laurentian margin in the ordovician and early silurian which generated the early members of the newer granite suite in the highlands a second is related to northward silurian early devonian subduction at the solway line which generated the younger newer granites and volcanic rocks north of the highland border and a third related to northward accretion of the armorican terrane in early devonian time produced intrusive and extrusive magmatism as far south as southeast ireland and the english midlands

the origin and significance of the delaney dome formation connemara ireland dalradian meta sediments of the laurentian margin and mafic intrusions thereof in sw connemara ireland tectonically overlie meta rhyolites of the delaney dome formation the two units are separated by the mannin thrust a new u pb age of 474 6 5 5 ma shows that the delaney dome formation is a temporal equivalent of arc volcanic rocks preserved in the adjacent south mayo trough the tourmakeady volcanic group erupted during the collision of an oceanic island arc with the laurentian margin in the grampian orogeny new rare earth and high field strength element data show that the delaney dome formation and tourmakeady volcanic group are chemically similar and arc like in character this suggests that the delaney dome formation is an along strike equivalent of the tourmakeady group strike slip faulted south of the south mayo trough during or after the grampian orogeny further correlation of these units with northern appalachian rhyolites is also possible the delaney dome formation is an extrusive temporal equivalent of intrusions that penetrate the connemara dalradian thus movement along the mannin thrust brought mid crustal plutons and dalradian country rocks tectonically above the extrusive volcanic sequence the mannin thrust is identified as a major imbricating structure within a continental arc but not a terrane boundary

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

the significance of a reliable ordovician age for some highland border rocks in central scotland a lower arenig fauna with north american affinities has been rediscovered at dounans aberfoyle central scotland this discovery makes part of the highland border complex younger than the adjacent dalradian rocks and suggests a provenance from the s

short paper stratigraphical correlations adjacent to the highland boundary fault in the west of ireland an ordovician age for at least part of the clew bay supercomplex a dismembered ophiolite together with contiguous deep water sediments and volcanics occurring adjacent to the highland boundary fault in western ireland supplement comparisons with the highland border complex of scotland based on lithological similarities an early phase of terrane development associated with ophiolite emplacement is represented in western ireland whilst the recognition of similar intra terrane magnafacies patterns permit a more complete analysis of the evolution of the highland border terrane

inversion of electromagnetic induction data for the iapetus suture zone in the uk abstract the iapetus suture zone isz corresponding to the join between the european crust of northern england and the north american derived crust of scotland has been examined by numerous seismic and induction experiments over a 20 year period the university of edinburgh electromagnetic em induction group is in the process of building a database comprising magnetotelluric and magnetic variation stations from this region in this paper we present 2d inversions of selected induction sites along roughly north south traverses across the isz in northern england and southern scotland the first inversion involves both the transverse electric te and transverse magnetic tm modes for 10 audio magnetotelluric approximately 100 0 1 hz sites care has been taken to remove the effects of static shifts this inversion resolves a conductive feature beginning under the southern northumberland trough at depths of 10 km dipping to the south below the alston block in the second inversion 12 longer period magnetic variation sites were added to the original data set improving the resolution of this feature which correlates well with a dipping bright reflector detected by the north east coast nec seismic reflection experiment in addition the southern uplands anomaly is imaged no north dipping mid to deep crustal conductor is observed which would correlate to north dipping palaeozoic subduction under laurentia

the ochil fault and the kincardine basin key structures in the tectonic evolution of the midland valley of scotland the ochil fault one of the most prominent tectonic features in the midland valley of scotland juxtaposes lower devonian volcanic rocks against late westphalian strata implying a possible vertical displacement of up to 4 km the kincardine basin in its hanging wall although actively subsiding during the greater part of the silesian trends generally n s perpendicular to the ochil fault and its silesian sedimentary record shows little sign of tectonic control by that fault it is proposed that the ochil fault was initiated possibly as a sinistral strike slip feature in the devonian but acted as a sidewall fault during the early evolution of the kincardine basin attributed to extension on a hypothetical pre brigantian fault along the bo9ness line on the east side of the basin until late namurian times when active extension ceased both faults were probably buried during late namurian and westphalian times reactivation of the ochil fault during end carboniferous n s extension dated by quartz dolerite emplacement was responsible for probably at least 2 km displacement including the presently visible footwall uplift it is suggested that the hanging wall may have contained a permian basin now removed