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

the atlantic water boundary current in the nansen basin transport and mechanisms of lateral exchange data from a shipboard hydrographic survey near 30 e in the nansen basin of the arctic ocean are used to investigate the structure and transport of the atlantic water boundary current two high resolution synoptic crossings of the current indicate that it is roughly 30 km wide and weakly mid depth intensified using a previously determined definition of atlantic water the transport of this water mass is calculated to be 1 6 0 3 sv which is similar to the transport of atlantic water in the inner branch of the west spitsbergen current at the time of the survey a small anti cyclonic eddy of atlantic water was situated just offshore of the boundary current the data suggest that the feature was recently detached from the boundary current and due to compensating effects of temperature and salinity on the thermal wind shear the maximum swirl speed was situated below the hydrographic property core two other similar features were detected within our study domain suggesting that these eddies are common and represent an effective means of fluxing warm and salty water from the boundary current into the interior an atmospheric low pressure system transiting south of our study area resulted in southeasterly winds prior to and during the field measurements a comparison to hydrographic data from the pacific water boundary current in the canada basin under similar atmospheric forcing suggests that upwelling was taking place during the survey this provides a second mechanism related to cross stream exchange of heat and salt in this region of the nansen basin this article is protected by copyright all rights reserved

relationships between marginal thrusting and movement on major internal shear zones in the northern highland caledonides scotland abstract the development of the syn metamorphic sgurr beag slide zone a major ductile shear zone of initially low dip caused at least 50 km north western thrust displacement of part of the internal metamorphic complex of the northern highland caledonides of scotland initiation of the zone and movements upon it were earlier than formation of the marginal moine thrust zone movement on the zone followed but overlapped the peak caledonian metamorphism and the mid to high amphibolite facies mineral assemblages fabrics and structures produced during the development of the slide zone and those surviving from earlier events were reworked under greenschist facies conditions during mylonitization associated with initiation of the moine thrust zone displacements on the slide zone and thrust movements were separated by emplacement of a regional suite of pegmatites and a considerable change of metamorphic grade thus they may not constitute members of a progressive sequence of caledonian thrusts formed over a short time interval rather preliminary isotopic data may imply an interval of c 25 ma between movement on the slide zone and final ductile translation along the moine thrust zone

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

thermal effects and timing of thrusting in the moine thrust zone illite crystallinity studies in the foreland to the moine thrust belt show a widespread heating event which is interpreted as a thermal response to thrust sheet emplacement temperatures reached 275 c approximately 24 ma after the main thrusting events based on the relationships between syn thrusting intrusions and the thrust planes combined with radiometric dates from the moine sheet it is argued that movement on the moine thrust started between 440 and 430 ma ago contractional slip in the moine thrust belt ceased at about 425 ma ago and it is proposed that illite metamorphism attained temperatures of 275 c at about 408 ma ago

implications for caledonian plate tectonic models of chemical data from volcanic rocks of the british old red sandstone volcanic rocks associated with the continental sediments known as the old red sandstone outcrop widely in northern britain and form a dominantly calc alkaline suite comparable to those developed on modern continental margins rocks rich in ni and cr are unusually abundant and are thought to have incompatible element concentrations little different from those of primary magmas such rocks show pronounced spatial chemical variation with concentrations of sr ba k light rare earth elements and p 2 o 5 and the ratio la y showing up to a six fold increase in a northwestward traverse across scotland in more evolved rocks a similar increase is shown by sr ba and la y but regional comparisons are made difficult by the varying effects of fractional crystallization the chemistry of all old red sandstone volcanic rocks n of the southern uplands fault is consistent with the former existence of a subduction zone that changed in strike from ene in ireland and southern scotland to n in the north sea this orientation in conjunction with almost e w plate motion may explain the great contrast in end silurian deformation styles between britain and scandinavia old red sandstone volcanic and plutonic rocks s of the southern uplands fault are thought to be younger and are perhaps unrelated to subduction activity

paleozoic evolution of the armorica plate on the basis of paleomagnetic data new paleomagnetic data support the contention that armorica and gondwana formed a coherent block until ordovician time however by early devonian time armorica collided with the north american northern european assembly to form the old red continent and the collision itself was responsible for the acadian orogeny not for the earlier taconic orogeny as postulated previously the carboniferous collision of gondwana with the old red continent subsequently formed pangaea and produced the appalachian hercynian orogeny

suspect terranes and accretionary history of the appalachian orogen the north american connection or the grenvillian continental crust that separated from eastern north america during the initiation of the appalachian orogenic cycle is poorly defined within the present orogen a variety of terranes occur outboard of the north american miogeocline all structurally uncoupled and therefore suspect stratigraphic analysis indicates that appalachian accretion progressed from the miogeocline outward the boundaries of the earliest accreted western terranes are marked by melange and ophiolite complexes later boundaries between eastern terranes are steep mylonitic zones and brittle faults accretionary events defined by stratigraphic analysis correspond to times of major deformation plutonism and metamorphism in the history of the orogen

the thrust and shear zones of the moine thrust zone and the nw scottish caledonides the caledonian thrust belts of nw scotland exhibit many features typical of foreland fold and thrust belts the major structures propagated westwards towards the foreland the thin carapace of cambro ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets on a broad scale the rules of thrust geometry are obeyed but many local anomalies are found particularly in the assynt region in assynt several large fault bounded gravity spreading structures surge zones have been identified extensional faults have been traced into strike slip and then contractional faults there are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow similar structures have been found along the length of the moine thrust belt suggesting that gravity spreading was an important mechanism during the later stages of movement within the main caledonian fold belt there are several major tectonic slides which are recognized by a local increase in intensity of deformation the majority are nw verging and were originally gently dipping in the highlands se of the great glen there are early major se verging recumbent folds associated with considerable crustal thickening reflected in high pressure 12kbar metamorphism many of the major slides may have formed by spreading away from this thickened crust the caledonian deformation and metamorphism occurred later in the moines and moine thrust zone than in the dalradian and grampian moines to the se of the great glen this may reflect different levels of deformation alternatively it may argue for a large strike slip displacement on the great glen fault juxtaposing crustal blocks which had experienced different ages of caledonian deformation and metamorphism the late movement on the moine thrust involved spreading from an area of thickened caledonian crust formed either by the stair step trajectory of the basal thrust to the moines and or by thickening due to intrusion of large masses of late caledonian granites

the structural setting and tectonic significance of the glen dessary syenite inverness shire the glen dessary syenite crops out in the steep belt of the northern highland moine it is enclosed within a psammite envelope which was previously assigned to the glenfinnan division but which is here correlated with the psammites of the loch eil division exposed to the e of the loch quoich line three episodes of deformation have been identified in the moine metasediments of the glen dessary area but only the latest d 3 is thought to have deformed the syenite imposing on it a strong steeply plunging mineral lineation the syenite is thought to occupy the core of a large intensely curvilinear d 3 synform the d 3 structures which deform the syenite can be correlated with upright structures which reworked previously flat lying already crystalline rocks recognized at loch quoich upright reworking must be younger than the glen dessary syenite dated by u pb methods at 456 5 ma two hitherto unrecorded small bodies of granitic gneiss similar to the c 1020 ma ardgour gneiss have been identified in psammites of the loch eil division near the w end of loch arkaig their presence suggests that the loch eil division has a grenville history and is part of the 9old moine9

the metamorphic environment of the sgurr beag slide a major crustal displacement zone in proterozoic moine rocks of scotland new evidence from the sw northern highlands of scotland establishes the southerly extension of the sgurr beag slide interpretation of changes in the mineralogy of calcsilicate rocks particularly the use of the anorthite content of plagioclase feldspar as an index of metamorphic grade together with consideration of the texture and mineralogy of pelitic rocks suggests that the slide juxtaposes crustal segments of different metamorphic grade and history movement on the slide post dates a major phase of regional metamorphism and two phases of regional deformation of precambrian age was synchronous with a regional fold forming event and regional metamorphism at or before c 467 ma caledonian and was followed by regional folding models of the thermal effects of displacement across syn metamorphic shear zones are investigated with particular reference to the roles of thermal relaxation and levels of shear strain a model based on the development of a ductile syn metamorphic asymmetric shear zone best explains the metamorphic patterns associated with the sgurr beag slide an original low to moderate easterly dip is implied with north westerly upthrusting of the eastern crustal block

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

folds and the strain ellipsoid a general model abstract a new general model is proposed to account for non orthogonal relationships of fold axes and axial surfaces to the axes of the finite strain ellipsoid descriptions in the literature of such relationships e g plunge variations and transecting cleavage are reviewed and the development of the model is outlined the model is described in which plane strain k 1 or slight flattening strain k 0 71 or slight constrictional strain k 1 85 are imposed on a suite of layers with oblique orientations to the bulk strain axes x y z folds will initiate perpendicular to the direction of shortening in the layer and develop as non material lines moving through the layer by angular migration at geologically realistic finite strains the folds will be transected by xy cleavage in slight flattening strain folds are likely to be cylindroidal with axes not measurably oblique to xy in slight constrictional strain en echelon periclines are predicted with marked transection by cleavage in some examples the interference of two directions of folding will give rise to severe plunge variations and even downward facing although strain markers frequently indicate finite strains in the flattening field this may result from the superimposition of slightly constrictional tectonic strain on compactional flattening

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

a model for the crustal evolution of southern scotland in the late silurian southern scotland was a mature accretionary active margin of sumatra type on the southeastern edge of north america while england was on a passive margin on the opposite side of the iapetus ocean southward subduction below england during the middle ordovician produced a magmatic arc which now lies close to the iapetus suture and thus argues for a missing forearc terrane tectonic erosion or strike slip removal may be responsible for some of this missing terrane to the north of the lake district but we now suggest that underthrusting below southern scotland may also be an important cause if the ordovician forearc terrane of northern england is now below the southern uplands we would have an explanation for the evidence for similar lower crustal foundations on either side of the surface trace of the iapetus suture velocity structure from the lispb study sr u pb isotopic signatures and geophysical attributes of the c 400 ma granites the under thrusting model still leaves room for considerable modification of both margins by large strike slip faults seismic refraction profiles of modern accretionary active margins sumatra sw japan allow us to propose a likely velocity profile for late silurian southern scotland we choose a mesozoic forearc truncated by strike slip faulting the queen charlotte islands margin of the ne pacific as an analogue for the late silurian southern iapetus margin following previous suggestions of highly oblique and slow convergence during the final stages of closure of iapetus we combine the two profiles by under thrusting the southern margin to produce a model which involves minimal shortening in the collision zone the crustal profile resulting from such a collision would be very similar to the modern southern scotland profile both in terms of the scale of the tectono stratigraphic terranes involved and of the resulting crustal velocity structure the cessation of accretion after the mid wenlock may be related to impingement of the english continental crust against the scottish trench subsequent uplift of the southern uplands may be connected with the appearance of coarse detritus in the lake district during the silurian

the caledonian metamorphic core an alpine model the neogene evolution of the central swiss alps is outlined and used as an analogue for the grampian orogeny of scotland a direct comparison is made between the three tectonothermal stages which characterize the alps and similar events in the scottish highlands geometric similarities are emphasised using geological cross sections kinematic and mechanistic similarities are inferred a net convergence model which links the metamorphic and uplift denudation history of the alps to its tectonic development in a crustal scale duplex is argued to describe appropriately the history of the dalradian and northern scottish highlands

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

long term fault slip rates distributed deformation rates and forecast of seismicity in the western united states from joint fitting of community geologic geodetic and stress direction data sets 1 the long term average velocity field of the western united states is computed with a kinematic finite element code community data sets include fault traces geologic offset rates geodetic velocities principal stress directions and euler poles there is an irreducible minimum amount of distributed permanent deformation which accommodates one third of pacific north america relative motion in california much of this may be due to slip on faults not included in the model all data sets are fit at a common rms level of 1 8 datum standard deviations experiments with alternate weights fault sets and euler poles define a suite of acceptable community models in pseudoprospective tests fault offset rates are compared to 126 additional published rates not used in the computation 44 are consistent another 48 have discrepancies under 1 mm a 1 and 8 have larger discrepancies updated models are then computed novel predictions include dextral slip at 2 3 mm a 1 in the brothers fault zone two alternative solutions for the mendocino triple junction slower slip on some trains of the san andreas fault than in recent hazard models and clockwise rotation of some domains in the eastern california shear zone long term seismicity is computed by assigning each fault and finite element the seismicity parameters coupled thickness corner magnitude and spectral slope of the most comparable type of plate boundary this long term seismicity forecast is retrospectively compared to instrumental seismicity the western united states has been 37 below its long term average seismicity during 1977 2008 primarily because of temporary reduced activity in the cascadia subduction zone and san andreas fault system