on the trace of the iapetus suture in ireland and britain the iapetus suture in ireland and britain is that line which separates caledonian rocks of the laurentian and avalonian continents the suture is cryptic nowhere is there an exposed fault zone containing ophiolite remnants blue schist melanges or trench deposits instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the iapetus suture zone which contains two or more tectonostratigraphic terranes several data sets are utilized to constrain this trace the distribution of faunal provinces caused by oceanic separation can be used most reliably to define early ordovician terranes fauna1 intermingling in the mid ordovician reduces provinciality and the confidence of terrane identification using faunal data the late ordovician and particularly silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes elimination of the iapetus ocean and development of the suture zone had begun by the ashgill therefore the resolution of the suture trace becomes unreliable where ordovician rocks do not crop out because silurian turbidite fans of laurentian provenance may have dispersed across a significant width of avalonian crust similarly the precision of structural identification of the suture zone by correlating the suture with a major fold facing confrontation may be weakened by late overthrusting tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in the lower crust do not always coincide with their inferred projections in outcrop tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by nd and pb isotopic patterns the rb sr patterns of granites record a stronger distinction between north and south of the suture despite these difficulties we depict a possible suture trace in ireland that departs significantly from the traditionally placed line by following a ne sw aligned trace through the slane fault and the navan tipperary lineament and thence through a dog leg to pass north of the dingle peninsula

short paper detrital mineral ages from the southern uplands using 40ar 39ar laser probe detrital mica ages in the southern uplands range from 458 to 502 ma the evidence favours a source in a basement which was part of an arc complex to the north rather than from the dalradian block which also has mica ages of this range no grenville ages have yet been recorded detrital volcanic clasts dispersed from the south have cooling ages of 560 and 530 ma indicating that they did not come from a contemporary arc but an older arc terrane of uncertain affinities

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

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

ordovician faunas in mass flow deposits southern scotland in the ordovician northern belt of the southern uplands basal volcanics arenig are followed by cherts llanvirn llandeilo then by graptolite shales llandelio lower caradoc and finally by caradoc greywackes within the greywackes kirkcolm formation are a number of occurrences of fossiliferous conglomerate and overlying mudstone these can be traced along strike for some 30 km the conglomerates and especially the mudstones yield rich mid caradoc shelly assemblages brachiopods 20 spp trilobites 14 spp gastropods bivalves bryozoans and the first known palaeozoic scleractiniamorph coral these occurrences are interpreted as mass flow deposits derived by downslope movement from a now vanished shelf to the north and may belong either to a single gigantic debris flow event or to a series of smaller but roughly contemporaneous flows strong faunal similarities to faunas at girvan western scotland and tyrone northern ireland lying north of the southern upland fault may suggest sinistral strike slip movement of no more than a few hundred kilometres

models for the deposition and simple shear deformation of a turbidite sequence in the south georgia portion of the southern andes back arc basin the cumberland bay formation on south georgia south atlantic is an upper jurassic lower cretaceous andesitic turbidite sequence 8 km thick derived from a contemporary volcanic are and deposited in part of the southern andes back arc basin 4 main lithofacies are recognized which show coarsening and thickening upward cycles a lower cretaceous palinspastic reconstruction shows that lobes of sand rich material were banked against the fault bounded sw margin of the basin adjacent to the volcanic arc the basin had a northerly directed palaeoslope and evidence for both axial and lateral palaeocurrents is preserved sedimentation of the formation cannot be explained by the classical fan model but resembles that described from linear tectonically controlled troughs cf grenada basin lesser antilles folding resulted from mid cretaceous closure of the basin and increases in intensity to the ne open upright folds pass into trains of inclined chevron folds with progressive decrease in interlimb angle axial plane dip and wavelength the turbidites were affected by up to 55 shortening and had a minimum original cross strike width of about 60 km the geometry of the structures is consistent with deformation above a gently inclined simple shear zone resulting from limited underthrusting or subduction of the basin floor beneath the arc terrane extension of both sedimentary and tectonic models to the southern andes is discussed

mechanical twinning in quartz shock experiments impact pseudotachylites and fault breccias abstract increasing use of diffraction methods to study preferred orientation of minerals has established that quartz in deformed rocks not only displays characteristic c axis orientation patterns but that there is also generally a distinct difference in the orientation of positive and negative rhombs in the trigonal quartz crystal structure positive and negative rhombs are structurally different and particularly negative rhombs e g 01 1 1 are much stiffer than positive rhombs e g 10 1 1 here we focus on the role of mechanical dauphine twinning under stress as a cause of this difference and illustrate with ebsd measurements ubiquitous twinning in quartz bearing rocks subjected to high stresses characteristic twinning is observed in experimentally shocked sandstones and stishovite bearing quartzites from the vredefort meteorite impact site in south africa similar twinning is documented for quartz associated with pseudotachylites from the santa rosa mylonite zone in southern california whereas quartz in underlying ductile mylonites are more or less twin free it suggests that twinning was produced by local seismic stresses that caused fracture and frictional melting on fault surfaces quartz bearing breccias from the safod san andreas fault observatory at depth drilling project also show evidence of twinning and suggest high seismic stresses in the currently creeping segment of the san andreas fault at parkfield from these observations it appears that dauphine twin microstructures can be diagnostic of high local and transient stresses

microscale chemical heterogeneity in groundwater ronen d magaritz m gvirtzman h and garner w 1987 microscale chemical hetereogeneity in groundwater j hydrol 92 173 178 large variations in the concentration of cl no a and so42 in the upper water layers of a polluted aquifer were found between samples that were vertically separated by as little as 3 cm this was disclosed when a multilayer dialysis cell device was used to obtain undisturbed real time samples from test wells this first time presentation of chemical fine structure provides new insights into mechanisms of contamination of an aquifer by surface inputs and raises the question of the validity of groundwater quality data based on single samples however obtained

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

structural histories in the sw southern uplands scotland the detailed structural histories in the sw part of the southern uplands are reviewed the evolutionary sequences described from the southern part of the central belt are characteristic of a fold thrust fault system which has been affected by strike slip deformation the present structure is characterized by tectonic domains usually less than 2 km across strike which show different internal deformation patterns and histories the generalized deformation history of the area involves downslope movement thrusting and folding followed by strike slip deformation the domain character of the area is considered to have developed by the juxtaposition of blocks throughout this deformation history the concentration of anastomosing stratal disruption zones developed by mixing of partially lithified sediment during deformation and the geometry of folds and faults indicate the presence of an important shear zone towards the base of the central belt a complex relationship between faulting folding and cleavage development exists within this shear zone although cleavage development appears to post date movement in some disruption zones there is evidence that the disruption zone displacement was synchronous with or continued after cleavage development these features indicate that the shear zone had a complex movement history spanning the lithification of the sediments the structural histories described are compatible with an accretionary wedge model for the area although the nature of the basement which underlay the southern uplands during its evolution cannot be assessed from the outcrop scale structures

early paleozoic paleogeography and accretionary history of the newfoundland appalachians new paleomagnetic results were obtained from several units in the central mobile belt of the north central newfoundland appalachians combined with recently published data the results place important limits on interpretations of early paleozoic paleogeography and evolution of the appalachian caledonian orogen the ordovician appalachian margin of north america located at low paleolatitudes and facing roughly south was separated from the southern margin of iapetus avalon armorica by 3500 km primary directions in pillowed volcanic rocks of the moreton9s harbour group that are part of an early ordovician island arc show that these rocks were formed near the north american margin at a paleolatitude of 12 s early to middle ordovician volcanic rocks of the roberts arm chanceport and summerford groups on the other hand were extruded at paleolatitudes of 30 40 s unconformably overlying volcanics of the silurian botwood group were extruded subaerially at a paleolatitude of 24 s combined with tectonostratigraphic data these results suggest the following tectonic history in early ordovician time northerly subduction of the main iapetus basin was accompanied by the formation of a volcanic arc and a back arc basin near the north american margin middle ordovician emplacement of ophiolites on the north american margin reflects the closure of this back arc basin iapetus i and continued northerly subduction in silurian time closed the main basin iapetus ii

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

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

tectonic development of the southern belt of the southern uplands accretionary complex detailed structural studies in the wenlock age southern belt of the southern uplands have identified major controls on tectonic development original thickness and composition of sediment control the style of faulting thin poorly consolidated interbedded sandstone mudstone sequences form broad sheared zones while thicker well consolidated sandstone dominated sequences fail along thin bedding parallel brittle fractures with vein infill the structural evolution of the sheared zones provides an insight into melange development depth of burial within the accretionary pile governs the development of cleavage and intensity of post fl deformational phases the intensity of imbrication within accreted material probably relates to subduction rate with early wenlock sequences being highly imbricated and mid wenlock sediments preserved in internally coherent fault bounded packets sinistral strike slip movement controls the development of steeply plunging folds sub horizontal stretching directions and sideways facing bedding cleavage relationships and strongly influences fabric development in sheared zones determination of the relative importance of these factors permits a fuller understanding of the deformational history

late caledonian dyke swarms in southern scotland a regional zone of primitive k rich lamprophyres and associated vents siluro devonian magmatism in the southern uplands scotland and northern ireland includes a hitherto neglected regional sw ne zone c 10 km wide and 300 km long of calc alkaline lamprophyre dykes lamprophyres were locally emplaced contemporaneously with small 100 km depths b a contribution from subducted sedimentary material and c volatile components added metasomatically in the crust or mantle during ascent rb sr isochron and new k ar biotite hornble

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

early veins as evidence of detachment in the lower palaeozoic rocks of the welsh basin abstract a suite of small scale structures mostly represented by veins is widely developed in the lower palaeozoic elastic sedimentary rocks of the welsh basin the structures were produced after considerable dewatering and diagenesis had taken place but before the host rocks were folded and cleaved by the regional deformation bedding parallel veins the most common of the structures are attributed to hydraulic jacking and mineralization by overpressured pore fluid probably at burial depths of several hundred metres or more the various burial processes including the production of the veins are regarded as being diachronous in effect propagating up through the sediment pile as sedimentation and burial progressed the bedding parallel veins are considered to represent inclined detachment surfaces sole planes down which rock masses slid due to gravity these surfaces were initiated in rocks originally inclined from the horizontal for sedimentological reasons or in horizontal host rocks which were later tilted by tectonic processes other structures in the suite were produced as a consequence of the downslope movement for example by compression in the toe regions of the glide sheet and by extension at the trailing edges slip directions recorded by striations on the bedding parallel veins were mainly wnw ese in w wales and n s in ne wales it is anticipated that these structures will be found to be widespread in other sedimentary basins that they have escaped attention elsewhere is probably because individual structures are small and seemingly insignificant whilst in deformed rocks they may have been attributed to folding or faulting

style scale and significance of sand bodies in the northern and central belts southwest southern uplands sedimentological and biostratigraphical data from the rhinns of galloway and adjacent areas in sw scotland confirm that deep water depositional systems consistently operated along and were sourced from the northwestern margin of an asymmetrical basin during the late ordovician and early silurian while pelagic facies accumulated simultaneously to the se providing a scenario analogous to many modern trench systems most of the observed sedimentological anomalies with regard to this general model can be explained within the context of the varied styles of trench filling depositional systems briefly reviewed here and the major stratigraphic and sedimentologic features can be best explained in terms of a geotectonically evolving fore arc region two main phases of development are recognized a llandeilo late ashgill during this time interval the fore arc trench region was tectonically juxtaposed against an active continental margin arc small to medium scale se prograding sand rich fans were formed within a relatively narrow trench leading to axial diversion of the fans initially to ne but later mainly to sw simultaneously a coarse volcanilithic sediment apron flanking the arc migrated gradually northeastwards probably in response to relative fault displacement of the arc and trench b llandovery during this time interval the fore arc trench region was dominated by a variety of mainly fan type depositional systems which were exclusively sourced at least until the uppermost llandovery from the northwestern margin the juxtaposition of coeval depositional systems differing significantly in sedimentological style and composition strongly suggests that the llandovery sedimentation in this region occurred in several structurally separated sub basins within the main trench system

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

structural constraints on closure geometry across the iapetus suture in eastern ireland the east coast of ireland is one of the few places where lower palaeozoic rocks are exposed adjacent to the supposed surface trace of the iapetus suture north of the tinure fault the local surface expression of the suture llandovery and wenlock strata correlated with the central and southern belts of the southern uplands were deformed by post wenlock deformation d1 characterized by southward verging northerly inclined upright f1 folding and nne trending faulting with a component of sinistral transpression south of the tinure fault d1 structures are northward verging southerly inclined upright folds also associated with sinistral transpression structures on both sides of the tinure fault were modified by northwest directed thrusting and north verging folding with a component of dextral transpression this second event affected lamprophyres intruding d1 deformed rocks of wenlock age but pre dates the emplacement of uncleaved probably lower devonian lamprophyre dykes and the unconformable deposition of lower carboniferous clastics and limestones other southerly dipping thrusts with a dextral component of motion which occur in lower palaeozoic rocks in ireland and britain may be of caledonian rather than variscan age as is commonly assumed the northward thrusting accounts for the mismatch between the inferred surface trace of the iapetus suture and that projected from seismic reflectors at 10 km depth

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

paleogeography of some vestiges of iapetus paleomagnetism of the ordovician robert s arm summerford and chanceport groups central newfoundland in the central mobile belt of newfoundland three volcanic groups of early middle ordovician age robert9s arm summerford and chanceport are thought to have formed within the iapetus ocean demagnetization analyses of 328 samples from these three groups has yielded similar magnetic directions calculated by combining stable end point directions and great circle poles the three groups yield similar poles located at 6 n 140 e 8 n 140 e and 10 n 131 e corresponding to paleolatitudes of about 30 s the magnetization of the robert9s arm group passes the tilt test and one site shows normal polarity antipodal to the directions of the other sites the paleolatitude of 31 s places the volcanic rocks at the time of their formation in the middle of iapetus between the laurentian margin at about 15 s and the location of the avalon terrane at 50 s subsequent accretion to laurentia and the presently adjacent island arc rocks represented by the moreton9s harbour group occurred after northward drift in the later ordovician earliest silurian and before the middle silurian

the ordovician biogeography of the grangegeeth terrane and the iapetus suture zone in eastern ireland the caradoc trilobite and brachiopod faunas of the grangegeeth terrane show marked scoto appalachian laurentian affinities contrary to some plate tectonic models this indicates that the terrane must lie to the north of the iapetus suture more limited graptolite evidence from the llanvirn of grangegeeth suggests that the terrane may have been closer to the gondwanan margin or at least at higher latitudes earlier in the ordovician and migrated to low latitudes prior to its eventual accretion onto the laurentian margin

stratal extension in thrust footwalls makran accretionary prism implications for thrust tectonics small scale stratal extension is commonly associated with thrust faults in the makran accretionary prism of southwest pakistan low angle normal faults dipping in the direction of transport relative to the thrust are common in gouge zones and duplexes and may also cut the footwall and hanging wall where they cause stratal extension these faults are probably riedel shears we have studied a broad zone of more complex extensional deformation beneath one bedding parallel thrust the zone disrupts earlier compressional structures and affects a significant volume of the footwall normal faults initiating in a rearward sequence branch off the thrust and cut down into the footwall merging at a lower stratigraphic level part of the thrust displacement was thereby transfe red to that level and fragments of the footwall were detached and transferred southward beneath the hanging wall the latter remained undeformed and accommodation structures are confined to the footwall these effects may have been caused by variations in sliding resistance along the fault if a thrust transfers part or all of its displacement down section along normal faults in this way before eventually cutting back up toward the surface it will detach a rootless fragment of footwall rocks from a position forward of the initial ramp through that horizon this process can be referred to as footwall plucking it can explain rootless basement slices along thrusts for example and it has important implications for section restoration techniques and the interpretation of thrust belts

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 structure of the western part of the southern uplands of scotland the structures of two areas one in the northern belt and one in the central belt of the southern uplands are described and compared the main structural histories are the same both show a main deformation phase related to se directed thrusting with associated fold and cleavage formation both areas exhibit late minor nw directed thrusting with related refolding and crenulation cleavage development in the case of the central belt both areas also show evidence of sinistral strike slip or transpressive deformation which reactivates steepened thrust faults when these deformation histories are reviewed in the context of geophysical radiometric and biostratigraphical data from the region a forearc accretionary setting for the whole of the southern uplands appears the most likely the strike slip deformation has internally disrupted an accretionary prism but has not juxtaposed originally unrelated terranes the effects of the late silurian closure of iapetus and subsequent events are also represented in the southern uplands