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

short paper terrane rotations in the irish caledonides palaeozoic palaeomagnetic data from scotland and ireland have been compared on a terrane model and early caledonide clockwise rotation of a terrane through 65o 30 connemara massif and later anticlockwise rotation of up to 50 in the vicinity of clew bay have been deduced these are the firfirst estimates of terrane rotations in the european part of the caledonian appalachian orogen terrane analysis of orogenic belts is currently a major field of study to a large extent this has been stimulated by studies in the north american cordillera where large rotations and translations have been determined palaeomagnetically see beck et al 1986 and references therein terrane analysis may be applied to older orogenic belts such as the caledonides which have been particularly intensively studied previously published palaeomagnetic data from connemara morris tanner 1977 and nw galway morris et al 1973 had indicated local block rotations but furnished few constraints on either their age and magnitude or on the identities of the blocks this paper relates the currently available palaeomagnetic dataseto a terrane model of w ireland hutton dewey 1986 see fig 1 new data new palaeomagnetic data from the irish caledonides comprise the following connemara gabbro robertson 1988 the dalradian rocks of the connemara massif are unique in the british caledonides in lying to the south of a caledonian ophiolite and also south of a thick sequence of ordovician shales and volcanics of low metamorphic grade in clew bay and the south mayo trough respectively the structural

connemara its position and role in the grampian orogeny in the irish and british caledonides the early ordovician grampian orogeny was the result of collision between the laurentian rifted margin and an oceanic island arc the connemara terrain in western ireland differs in position and character from all other parts of the exposed dalradian rocks of the grampian orogen in lying south of the collided arc and fore arc and in having north verging fold nappes that developed synchronously with the intrusion of huge volumes of calc alkaline magmas that provided the heat for regional barrovian metamorphism we have tested this hypothesis with a numerical model which demonstrates its admissablity connemara is not a terrane displaced with respect to the remainder of the grampian orogen but was overridden northwards by the arc and its fore arc basin south mayo trough frontal ophiolite complex deer park and accretionary complex killadangan deposition in the south mayo trough occurred below sea level and above the evolving grampian orogen which developed

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

the numerical simulation of non supercell tornadogenesis part i initiation and evolution of pretornadic misocyclone circulations along a dry outflow boundary abstract high resolution three dimensional simulations are used to study misocyclone initiation and development along the leading edge of an outflow boundary model conditions were designed such that this development could be simulated independent of moist processes the outflow boundary is allowed to propagate into a region of southerly low level flow which results in a vertical vortex sheet along the outflow s leading edge lobe and cleft instabilities follow and provide perturbations for the subsequent development of horizontal shearing instabilities these growing instabilities are the inaugural circulations of leading edge misocyclones with wavelengths ranging from 1 6 to 3 2 km the structure of these modeled misocyclones compares favorably to observed pretornadic misocyclones along outflow boundaries in northeast colorado vortex sheet dynamics are observed to exert substantial control over the structure of the evolving outflow leading edge the vertical vortex sheet passes through discrete evoluti

a cryptic caledonian flysch terrane in scotland a substantial part of the lower old red sandstone crawton group in the north east midland valley of scotland has a first cycle provenance from the south and east i e from a source block within the midland valley from the clast assemblages in proximal conglomerates it is clear that this provenance comprised a flysch terrane cut by granodioritic and tonalitic plutons angular clasts of lithic arenite and metagreywacke derived from this cryptic flysch sequence show it to be distinct from a metasedimentary sequence which flanked the lower old red sandstone basin to the north and north east the basin must overlie or lie adjacent to a boundary between two quite different sedimentary assemblages petrographic examination and chemical analysis of the clasts show that the southerly cryptic succession includes an important mafic ultramanc provenance implying that ophiolitic source rocks may have originally contributed to it sr and nd isotopic compositions for the clasts establish that the midland valley flysch is isotopically unrelated to either southern uplands shales and greywackes or dalradian metasedimentary rocks the clasts have lower sr contents than either suite and sr isotopic compositions 87 sr 86 sr in the range 0 7099 0 7176 at 450 ma which are more radiogenic than the southern uplands but less radiogenic than the dalradian crustal residence ages t dm have a narrow range of 1 76 1 94 ga the mean of 1 87 0 06 ga compares with a mean residence age of 1 49 0 21 ga for southern uplands greywackes and shales mixing calculations demonstrate that the incorporation of young ophiolite derived basaltic detritus into the caledonian sedimentary mass can explain properties of the nd data previously attributed to a bimodal proterozoic provenance the distinct compositions of the southern uplands and midland valley flysch sediments suggest they originally tapped different parts of the laurentian margin and have subsequently been brought together structurally as displaced fore arc slivers the diverse isotopic compositions of outboard flysch terranes has implications for the nature of the sedimentary component incorporated in inboard magma suites

finite strains in transpression zones with no boundary slip abstract a mathematical model describing finite strains has been developed for transpression zones where no slip is allowed in any direction at the zone boundaries this has been adapted to model transpression zones where the boundaries are inclined the model can also be used to describe finite strain patterns where material is incorporated into the deforming zone from one or both sides after the initiation of deformation so that the deforming zone remains at a constant width thus allowing large transpressional strains to be accommodated without the deforming zone closing to zero thickness this model of steady state transpression can be used to explain some of the features of the south mayo trough a late silurian transpression zone in the west of ireland

a theory for strong long lived squall lines abstract we study herein the mechanics of long lived line oriented precipitating cumulus convection squall lines using two and three dimensional numerical models of moist convection these models used in juxtaposition enable us to address the important theoretical issue of whether a squall line is a system of special long lived cells or whether it is a long lived system of ordinary short lived cells our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but on occasion a squall line may be composed of essentially steady supercell thunderstorms the numerical experiments presented herein show that either type of squall line may develop from an initial line like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line with shallow shear oriented perpendicular to the line a long lived line evolves containing individually short lived cells our analysis of this type o

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

a simulation of the development of successive cells along a cold outflow boundary abstract a three dimensional numerical simulation is presented in which five new cells clouds develop in succession over a 4 h period the cells that develop have common characteristics including a lifetime of 40 min they form at 30 min intervals in a convergent region along the cold outflow boundary that is established by previous cells the propagation of the cloud system is several meters per second faster than individual cell movement due to the discrete development of new cells on the right flank of previous ones analysis of the simulation indicates that the most pronounced changes near the surface are due to downdraft development in contrast there is only a weak signature of new updraft development near the surface in the convergence field each new updraft develops along the cold outflow boundary which moves slowly away from the previous cell the air participating in the generation of a new cell appears to have originated 1 1 5 km below cloud base 2 6 km as the cell grows and rain begins

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

slab flattening and the rise of the eastern cordillera colombia abstract the topographic growth of a mountain belt is commonly attributed to isostatic balance in response to crustal and lithospheric thickening however deeper mantle processes may also influence the topography of the earth here we discuss the role of these processes in the eastern cordillera ec of colombia the ec is an active double vergent fold and thrust belt that formed during the cenozoic by the inversion of a mesozoic rift and topography there has risen up to 5 000 m cocuy sierra the belt is located 500 km away from the trench where two separate portions of the nazca plate subduct below the south american plate north of 5 n the ec rises above a flat slab subduction region volcanic arc migration implies slab shallowing by 10 ma and flattening up to the present day configuration at 6 ma the occurrence of a high vp vs anomaly and clustered seismicity below the belt at 160 km depth delineates the slab geometry and has been related to dehydration of the slab suggesting the presence of a hydrated mantle wedge we compiled thermochronologic data and inverted for the exhumation history of the chain over the last 20 ma using the age elevation relationship and the different closure temperatures of multiple thermochronologic systems results indicate that exhumation rates increased during the plio pleistocene at different wavelengths and amplitudes the small wavelength and large amplitude signals could be related to shallow crustal deformation whereas the source of the long wavelength and moderate amplitude signal has yet to be identified pulses of fast exhumation are found to be concomitant with the uplift that occurred from 7 ma to the present day previous studies suggested that the high topography of the chain cannot be achieved solely through isostatic adjustment the highest residual topography is centered on the highest elevations of the ec whereas the lowest residual topography corresponds to the magdalena valley following the regional slab geometry we propose that the recent uplift and exhumation events were triggered by the transition from regular to flat slab subduction along with the hydration of the mantle wedge above the slab we test the dynamic feasibility of our hypothesis with a series of numerical models for the present day state predicting the correct trends in elevation requires a flat slab geometry and a weak and buoyant mantle wedge

the initiation and organization of convective cells atop a cold air outflow boundary abstract on 13 july 1986 a cold air outflow from thunderstorms over illinois and missouri propagated through the mist microburst and severe thunderstorm network over northern alabama the study of this outflow is important since the gust front was solely responsible for the initiation of numerous convective cells previous studies have documented the initiation of convection due to colliding gust fronts in addition there was a pronounced mesoscale organization of the cells atop the outflow boundary this was most likely due to a combination of kelvin helmholtz k h and internal gravity ig wave activity in contrast to previous cases the k h wave crests were oriented nearly perpendicular to the gust front within the analysis area the resulting intersections between the circulations associated with the k h waves and the gust front produced favorable locations for the initiation of convection subsequently the convective cells remained along the updraft side of the k h wave circulations as they pro

a geological and tectonic cross section of the caledonides of western ireland geological and geophysical criteria allow division of the caledonides of western ireland into four principal zones zone 1 in the north contains grenvillian basement with late proterozoic supra crustals zone 2 contains the westport complex the deer park complex and the south mayo trough connemara and the delany dome formation occupy zone 3 zone 4 contains the south connemara group and mid to late ordovician and silurian sediments and volcanic rocks the various tectonic models proposed for these zones are discussed and a minimum tectonic model is presented which involves early ordovician collision of the northern part of zone 2 with the laurentian margin zone 1 followed by subduction flip northern part of zone 4 the site of early collision between zones 1 and 2 subsequently underwent orogenic collapse and strike slip dismemberment leading to the sinistral transpressive emplacement of zone 3 assemblages in zone 4 were accreted to the now south facing arc northwards stress transfer consequent on suturing deformed the thinned crust and overlying silurian sequences in zones 2 and 3

observations of the sea breeze front during cape part i single doppler satellite and cloud photogrammetry analysis abstract an analysis of two sea breeze events on 6 august an onshore flow event and 12 august an offshore flow event 1991 is presented using single doppler observations satellite images and cloud pictures collected during the convection and precipitation electrification cape experiment documentation of the alongfrontal variability at the leading edge of the sea breeze circulation is presented for the first time the horizontal structure of the front was strongly modulated by the near perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 august these intersection points also appeared to be preferred locations for cloud development along the front horizontal convective rolls were also documented on 6 august however their orientation was nearly parallel to the sea breeze front as a result extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection po

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

numerical simulation of the interaction between the sea breeze front and horizontal convective rolls part i offshore ambient flow abstract a three dimensional cloud resolving model is used to investigate the interaction between the sea breeze circulation and boundary layer roll convection horizontal convective rolls hcrs develop over land in response to strong daytime surface heating and tend to become aligned parallel to the vertical wind shear vector whereas the land sea heating contrast causes the formation of the sea breeze front sbf along the coastline the ability of hcrs to modulate the along frontal structure of the sbf is examined complementing and extending previous observational and numerical studies three simulations are discussed the first two demonstrating that the model can produce both phenomena independently the third is initialized with offshore mean flow and vertical shear perpendicular to the coastline and results in a sharply defined inland propagating sbf that encounters hcrs aligned perpendicular to it before the interaction takes place the sbf is nearly two dimensional and devoid of along front

nonhydrostatic simulation of frontogenesis in a moist atmosphere part iii thermal wind imbalance and rainbands abstract the dynamical mechanisms contributing to the cross front ageostrophic circulation are identified in high resolution 40 to 5 km nonhydrostatic simulations of moist frontogenesis in a first step the importance of the alongfront ageostrophic circulation is assessed the structure of the intense thermal wind imbalance twi occurring in the vicinity of the surface cold front is diagnosed and explained using a budget of the alongfront vorticity it allows one to propose a new balance in terms of the steadiness of the field in the system moving framework the twi is thus found nearly equal to the total cross front transport by resolved and subgrid scales it is shown that first the deviation of the prefrontal air toward the front enhanced by the surface friction and cloud diabatic processes allows generation of a layer of positive near at the top of the pbl second the frontal lifting of this structure is responsible for the basic structure of the twi in a second step a general for

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

siluro devonian palaeomagnetism terrane emplacement and rotation in the caledonides of western ireland summary the palaeomagnetism of a silurian succession lough mask formation lavas and sediments and intrusive rocks kilbride dolerites and andesites and microgranodiorites killary harbour area in the connemara mayo segment of the irish caledonides is described magnetizations are related to the regional tectonic history with the aid of fold conglomerate and contact tests they describe sequences of shallow to intermediate inclinations in both the eastern and western sectors of this caledonian inlier the shallow components appear to pre date siluro devonian folding while the intermediate ones post date this episode the change in palaeofield inclination is identical to the transition described by contemporaneous deep level silurian to shallow and surface level lower devonian bodies in the british and scandinavian caledonides but declinations are rotated consistently to the west the lower middle silurian vector is rotated clockwise by c 100 this difference is progressively reduced to c 50 by the time of the siluro devonian d2 folding showing that this fold generation now oriented e w developed in parallelism with contemporaneous folds on the ne strike continuation of the caledonides post folding remanences continue to deviate in a clockwise sense from the remainder of the caledonides showing that rotation continued into middle upper devonian times and finally ceased during carboniferous times the integrated effect of this rotation is recognized in ordovician magnetizations from the dalradian metamorphic terrane of south connemara it can be explained by block rotations within a zone of distributed deformation according to the mckenzie jackson model and illustrated by many neovolcanic zones palaeomagnetism defines a tectonic regime commencing with rotations of c 3 myr and lasting from mid silurian to carboniferous times western ireland is sited within a closure gap along the iapetus suture extending from ireland to newfoundland and these terranes were apparently emplaced by progressive elimination of this gap during sinistral transpression across the caledonian orogen

geomorphology morphometry spatial distribution and ages of mafic monogenetic volcanoes of the peinado and incahuasi fields southernmost central volcanic zone of the andes abstract the back arc southern puna region south of 24 s in the central volcanic zone of the andes contains hundreds of miocene to quaternary mafic monogenetic volcanoes roughly distributed in several volcanic fields analysis of the geomorphology morphometry spatial distribution and age of 36 mafic monogenetic centers in the peinado and incahuasi fields at the southwestern margin of the puna plateau gives insights on the links that mafic monogenetic volcanism can have with main and secondary structures and the effects of degradation on scoria cone shape degree of preservation morphometry and six new unspiked k ar ages indicate two distinct age groups old centers 2 ma and young centers 1 ma the older mafic centers consist of degraded scoria cones and lava flows with an estimated cumulative bulk volume of 2 2 km3 the younger mafic centers display a variety of well preserved landforms scoria cones lava flows maars tuff rings domes and have an estimated cumulative volume of 6 2 km3 their distribution in a main nne ssw and secondary ene wsw trends suggests a control by the regional nne ssw striking peinado and san francisco fault systems in a dextral strike slip configuration scoria cone and lava flow morphometries are similar in both fields but a marked difference indicating hydrovolcanic activity is the presence of tuff rings maars and tephra fallout deposits only in the peinado field morphometric analysis of dated pristine and degraded scoria cones of the southern puna region shows that parameters related to crater depth and inner crater steepness decrease consistently with age suggesting that crater infilling is the degradation related process that is most sensitive to age at scoria cones in the southern puna high values of crater depth crater width crater depth cone height and mean inner crater slope are indicative of a young 1 ma age as young cones can have initially low values

ages of regional metamorphism and ductile deformation in the central and southern appalachians abstract a summary of available geochronology of deformational fabrics formed at greenschist and higher grades in the central and southern appalachians is presented as a map showing the ages of regional metamorphism and ductile deformation the ca 480 435 ma taconic event affected most of the piedmont and blue ridge provinces the ca 380 340 ma acadian event was milder and was centered in the west central piedmont and eastern blue ridge the 330 230 ma alleghanian event exhibits an abrupt ductile deformation front in the eastern piedmont and extends an unknown distance eastward below the atlantic coastal plain

numerical modeling of precipitation cores on cold fronts abstract a nonhydrostatic three dimensional mesoscale model including cloud physics is used to simulate the structure of a narrow cold frontal rainband ncfr the model simulations reproduce the observed core gap structure of the ncfr trapped gravity waves triggered by regions of stronger convection on the cold front induce subsidence and regions of warming aloft in these regions precipitation is suppressed thereby creating precipitation gaps along the front separated by precipitation cores the advection of hydrometeors is responsible for the parallel orientation and the elliptical shapes of the precipitation cores gravity waves produce pressure perturbations just behind the cold front which modify the wind and thermal structure parts of the front behave locally like a gravity current traveling at the theoretical gravity current speed in a direction perpendicular to the local orientation of the front but the motion of the front as a whole is not well described by the gravity current spe

moist convection at a surface cold front abstract the motion of a surface cold front in an environment that is unstable to moist convection is studied with the aid ofboth hydrostatic and nonhydrostatic two dimensional models simulations with the hydrostatic model essentially extend the work reported by ross and orlanski it is shown that when deep convection occurs coriolis turning of the flow into the convective line creates a poleward low level jet ahead of the front it is also shown that after the generation and decay of the first convective element another line develops on the order of a day later it is found that the intensity of this line increases significantly ira north south gradient of moisture is specified the periodicity in convective activity at the front is explained in terms of an inertial gravity oscillation in the low level convergence the first convective system which decays when the subeloud layer is dried out by the convection forces a geostrophic imbalance in the surface front and the surrounding environment in ret

did the dalradian slides originate as extensional faults rocks of the dalradian supergroup1 2 of scotland and ireland were deposited in late proterozoic early cambrian times on what became the southern margin of the laurentian plate and underwent metamorphism and deformation when this margin was affected by the grampian orogeny in the late cambrian3 4 the resulting complex structure is still not fully understood and a continuing problem is presented by the origin and significance of the slides a series of ductile faults which lie sub parallel to bedding5 although numerous slides have been recognized within the dalradian terrain6 the term has seldom been used outside the british caledonides it is generally assumed that these slides are synmetamorphic and were formed during grampian deformation however a consideration of their geometry stratigraphic relationships and context now suggests that they may have been initiated as syndepositional extensional faults which were only modified and not formed during the grampian orogeny

oroclinal bending in the caledonides of western ireland new palaeomagnetic data from the caledonides of western ireland indicate that the silurian rocks of south mayo underwent oroclinal bending following folding in siluro devonian time bending was accommodated on faults cutting the silurian sequence and was driven by strike slip motion across the antrim galway line a recently recognized major curvilinear lineament we propose that the silurian rocks in the east of the region suffered up to 30 clockwise rotation increasing towards the west to approximately 70 this newly inferred differential rotation of the silurian rocks about a sub vertical axis implies that the underlying dalradian rocks of the connemara massif to the south must also have undergone clockwise rotation in late silurian early devonian time this is consistent with published palaeomagnetic data from the connemara gabbro and accounts for the swing in strike of the caledonian orogen in this part of western ireland from this we deduce that the swing in strike of the caledonian rocks elsewhere on the west coast of ireland might have a similar explanation

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones xi comparisons between observational and theoretical aspects of rainbands abstract comparisons are made between the characteristics of several types of rainbands observed in an extratropical cyclone and dynamical mechanisms relevant on the mesoscale the warm sector flow ahead of the cold front and above the cold frontal zone aloft was unstable to conditional symmetric instability and theoretical predictions for this mechanism am consistent with several aspects of the warm sector and wide cold frontal rainbands in the case of the warm sector rainbands other mechanisms e g wave cisk and mixed dynamic convective instabilities may have also played a role the core structure of the narrow cold frontal rainbands appeared to be affected by an instability that derived its energy from the horizontal shear across the surface front also many aspects of the narrow cold frontal rainband were similar to a density current shear induced gravity waves appeared to be responsible for the wavelike rainbands observed in the vicinity of the cold frontal zone aloft the orientation of the

the role of boundary layer convergence zones and horizontal rolls in the initiation of thunderstorms a case study abstract the initiation of thunderstorms is examined through a combined observational and modeling case study the study is based on doppler radar aircraft mesonet balloon sounding and profiler and photographic data from the convection initiation and downburst experiment cinde conducted near denver colorado the study examines the initiation of a line of thunderstorms that developed along a preexisting quasi stationary boundary layer convergence line on 17 july 1987 the storms were triggered at the intersection of the convergence line with horizontal rolls where enhanced updrafts were present the primary effect of the convergence line was to deepen the moist layer locally and provide a region potentially favorable to deep convection the critical factor governing the time of storm development was apparently related to the attainment of a balance between horizontal vorticity in the opposing flows on either side of the convergence line the effect was to cause the updrafts in the convergence line

massive middle miocene gypsic paleosols in the atacama desert and the formation of the central andean rain shadow abstract the central andes of south america are one of the highest and broadest mountain ranges on earth and have a profound influence on climate in the southern hemisphere yet the timing of andean uplift and its subsequent impact on atmospheric circulation is not well known here we use fossil soils from along an 800 km transect 19 25 s in the atacama desert of northern chile to identify the initiation of extreme aridity along the west coast of south america and infer a minimum age for the development of the central andean rain shadow paleosols record the onset of extreme aridity at the boundary between the middle and late miocene 12 10 ma lower miocene 24 20 ma soils are chemically weathered and contain soil carbonate indicative of a vegetated landscape and semi arid climate in contrast gypsum dominates massive 2 5 m thick upper middle miocene soils that formed on a hyperarid landscape devoid of vegetation the development of extreme hyperaridity is linked to the formation of the central andean rain shadow through the divergence of oxygen isotope values oxygen isotope values of lower miocene soil carbonates in the atacama desert 5 0 1 1 vpdb and central andes 7 8 1 are similar indicating no or a limited rain shadow by 15 mya however 18 o values of soil carbonate differ by 8 with values of soil carbonate in altiplano paleosols getting lower due to uplift while soil carbonate values in the atacama desert get higher due to aridity therefore by 15 mya there is clear evidence for a rain shadow from the central andes by the latest middle miocene ca 12 10 ma paleosols in the altiplano decreased to 11 8 1 0 while soils in the atacama desert increase to 1 5 4 5 this almost 14 divergence in the 18 o values of soil carbonate for soils only 250 km apart is evidence of a strong central andes rain shadow effect in place during the late middle miocene 12 10 ma

the antrim galway line a resolution of the highland border fault enigma of the caledonides of britain and ireland the westward continuation of the highland border fault of scotland hbfz into ireland is problematic it is widely thought to follow a pronounced magnetic and gravity lineament the fair head clew bay line fcl the advantage of this interpretation is that it places all the ordovician ophiolitic complexes and associated sedimentary basins to the south of the fcl which would represent the contact between laurentia and the outboard terranes its main shortcomings are that both the deep structure and timing of strike slip are different on the hbfz and fcl in ireland the fcl is a north dipping feature that can be traced to the moho on birps profiles while the hbfz has no such signature terrane amalgamation in western ireland was completed by the late ordovician while in scotland the midland valley terrane did not finally dock until the early devonian these considerations suggest that in western ireland a branch of the hbfz exists which was active in post ordovician time and must lie south of connemara an examination of irish geological geophysical and image processed magnetic data shows that a profound lineament can be traced from antrim to galway bay the antrim galway line stitching plutons date movement on it as pre 405 ma we propose that the antrim galway line represents the continuation of the scottish hbfz while the fcl is a preserved ordovician splay of the hbfz system whose northdipping geometry is a product of ordovician collapse of the orogen in western ireland

end silurian modifications of ordovician terranes in western ireland the ordovician elements in the caledonides of western ireland may be divided into the clew bay south mayo north connemara and south connemara sub zones evidence suggests the presence of a number of proximal suspect terranes and the remnants of one or possibly two arc complexes palaeontological and sedimentary evidence suggests an allochthonous relationship between the silurian rocks of croagh patrick and north galway these successions may therefore not represent a single sedimentary overlap to the ordovician suspect terranes end silurian deformation was the result of transpression which was responsible in part for the present configuration of the area s geology deep seated shear zones with fuchsite and gold mineralization were foci for lateral translation concomitant with southward directed thrusting northward directed back thrusts and variable degrees of metamorphism being of higher grade in the north the end silurian event is intermediate between the high grade metamorphism in parts of the appalachians and the non metamorphic silurian sequences of scotland

a basin model for the silurian of the midland valley of scotland and ireland identification of terranes is frequently based on the apparent disparity of source area for sediment supply palaeontological evidence and the presence of faults whose sense and amount of displacement is sometimes conjectural as an example of such superficially unrelated stratigraphies the silurian successions of the midland valley of scotland and the northern part of ireland are compared a diachronous late llandovery early middle telychian transgressive episode which is earlier in the midland valley inliers is common to many of the successions the transgression resulted in the development of comparable sedimentary facies and fossil assemblages the clast compositions of llandovery and wenlock conglomerates suggest erosion of a common provenance of volcanic rocks founded on a metaquartzite basement areas of active volcanism appear to have migrated in a north westerly direction with time a similar sequence is evident in canada it is suggested that the common features of the scottish irish successions may be accommodated in a unified basin model for silurian sedimentation and volcanism such a basin may have formed in an intra arc environment and been controlled by oblique slip fault mechanisms although evidence for the presence of a silurian arc is equivocal

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 relationships between tectonics climate and exhumation in the central andes 18 36 s evidence from low temperature thermochronology abstract the central andes between 18 s and 36 s latitude strike north south for 2000 km along the chilean subduction margin cross several climate zones from hyperarid to humid and exhibit mean elevations in excess of 4000 m a s l here we investigate the relationships between tectonics climate and exhumation by inverting low temperature thermochronological data compiled from the literature 824 ages from 549 samples and new data 238 ages from 146 samples to quantify the exhumation rate history of the central andes since 80 ma our inferred exhumation rates west of the drainage divide and between 18 and 32 s did not exceed 0 25 km ma such low exhumation rates are consistent with low shortening rates and arid conditions in this region local pulses of exhumation occurred only during the eocene as response to active deformation and during the miocene probably as response to uplift of the western andean slope east of the drainage divide between 18 and 28 s the observed exhumation pattern reflects the onset and eastward propagation of deformation here exhumation occurred locally since the middle to late eocene in the eastern cordillera and the altiplano puna and subsequently affected larger parts of these regions and the north western sierra pampeanas during the oligocene in the early miocene 20 ma the interandean zone started exhuming and at 12 10 ma exhumation propagated into the subandean zone enhanced shortening rates and intensified precipitation along the eastern deformation front associated with the onset of the south american monsoon led to increased exhumation rates in the eastern interandean and the subandean zones in the plio pleistocene 0 6 km ma higher pleistocene exhumation rates are also observed in the northern sierra pampeanas 1 5 km ma that can be related to rock uplift along steep reverse faults coupled with high precipitation south of 32 s on the western side exhumation rates in the principal cordillera increased from ca 0 25 km ma in the miocene to rates locally exceeding 2 km ma in the pleistocene whereas the tectonic regime in the southern principal cordillera remained unchanged since the late miocene these higher rates are likely associated with enhanced erosion resulting from intensified pleistocene precipitation and glacial growth in this region reinforced by isostatic rock uplift and active tectonics our study shows that the onset of exhumation correlates mainly with the initiation of horizontal shortening and crustal thickening whereas the magnitude of exhumation is largely set by the amount of precipitation and glacial erosion and by the style of deformation which is controlled by inherited structures and the amount of sediments in the foreland

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

numerical study of precipitation core gap structure along cold fronts abstract the mechanism responsible for the core gap structure of precipitation along narrow cold frontal rainbands ncfrs is investigated through analyses of idealized cloud resolving simulations of cold fronts the control simulation in which the prefrontal thermal stratification is characterized by a weak convective instability at low levels with convective available potential energy cape of 60 j kg 1 reproduces the typical alongfront variability of observed ncfrs the simulated ncfr is broken up into regularly spaced ellipsoidal cores oriented at a clockwise angle to the cold front while horizontal shear instability hsi has frequently been proposed as a mechanism leading to the alongfront variability of ncfrs no characteristic features of hsi are recognized in the simulated vertical vorticity field at the leading edge of the cold front the alongfront variability in precipitation is attributed to the formation of a wavelike disturbance just above the leading edge of the cold front the wave

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

rapid geodetic shortening across the eastern cordillera of nw argentina observed by the puna andes gps array exxonmobil through the convergent orogenic systems analysis achievement rewards for college scientists foundation peter j coney fellowship

source or sink an assessment of the role of the old red sandstone in the genesis of the irish zn pb deposits the zn pb ba deposits of central ireland are hosted by lower carboniferous waulsortian limestone and navan group carbonate sediments the deposits are located near the margins of subbasins in close association with extensional faults the source of metals and the flow pathways utilized by the hydrothermal fluids responsible for ore genesis have long been a matter of conjecture the devonian to lower carboniferous old red sandstone has been suggested as a possible regional aquifer and metals source therefore a petrographic and lead isotope study of this unit was conducted to assess its putative role in ore formation hydrothermally unaltered old red sandstone has 206pb 204pb 207pb 204pb and 208pb 204pb ratios ranging from 18 71 to 22 45 15 52 to 15 86 and 38 57 to 42 71 respectively the sedimentological characteristics of the red beds coupled with results of isotopic modeling support the conclusion that the lead isotope signature of the unit is predominantly controlled by radiogenic caledonian granite detritus that has an avalonian like character the modeled lead isotope composition of the old red sandstone at the time of the lower carboniferous ore forming event is too radiogenic for it to have acted as the main source of lead for the irish ore field examination of drill core outcrop and underground exposures reveals that hydrothermal alteration within the old red sandstone forms a halo around faults indicating that fluid flow in the unit was predominantly structurally focused these hydrothermally altered sediments have 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 18 21 to 21 77 15 54 to 15 91 and 38 02 to 41 57 respectively unlike the unaltered sediments several altered samples have lead isotope compositions similar to galena from the lower carboniferous deposits petrographic examination reveals that these samples contain trace hydrothermal sulfides that are probably responsible for the orelike signature therefore it appears that at least locally the old red sandstone functioned as a sink for lead during the lower carboniferous mineralizing event the conclusions from this study eliminate the old red sandstone as the main source of lead in the ore field and suggest that deposit lead is derived primarily from lower paleozoic rocks precambrian crystalline basement in view of these results regional gravity driven fluid flow models calling upon an old red sandstone focused flow regime and metals source can be discounted rather deep circulation of fluids through the fractured basement underlying central ireland plays an integral role in the evolution of the ore forming system

structural controls and origin of gold silver mineralization in the grampian terrane of scotland and ireland gold bearing mineral deposits occur over a strike distance of 300 km within the grampian terrane of scotland and ireland this terrane consists of neoproterozoic lower ordovician rocks of the dalradian supergroup that were polyphase deformed and metamorphosed during the c 470 ma grampian orogeny sulphide rich au ag deposits occur in scotland at calliachar urlar burn tombuie tyndrum and cononish and in ireland at curraghinalt omagh cavanacaw croagh patrick cregganbaun and bohaun they are hosted by 0 1 6 m thick quartz veins and have a similar overall mineralogy including native gold as cu fe pb and sn sulphides with hessite tetrahedrite and electrum present in the first six localities above the mineralized quartz veins which are characterized by open space textures crystallized at c 3 5 km depth in the crust all of the deposits were structurally controlled and apart from curraghinalt occur within second order riedel r r and t fractures resulting from a regional n s trending maximum principal stress these deposits are of upper silurian to lower devonian post scandian age and are inferred to have crystallized from hot silica rich metamorphic fluids derived from dehydration reactions at the greenschist amphibolite facies boundary curraghinalt is an older grampian thrust related deposit plutonic igneous rocks mainly granitoid contributed in part to the fluids which were channelled into major orogen parallel strike slip faults to be injected by fault valve pumping into the damage zones and fault breccias of newly formed riedel fractures any residual fluid probably percolated to the ground surface to form rhynie chert type hot springs

shear instability wave along a snowband instability structure evolution and energetics derived from dual doppler radar data abstract this article presents a detailed analysis of a meso scale 17 km wavelength shear instability wave along a snowband using a series of dual doppler radar data the wave developed along a low level shear line that formed under the strain wind field caused by an adjacent mesoscale vortex the horizontal wind shear across the line was largest at lower levels and the eddy component horizontal winds and the retrieved pressure anomaly showed a bottom intensified structure as well the resultant vertical pressure gradient force was found to be responsible for the enhancement of alternating updrafts and downdrafts that were subsequently related to the formation of the reflectivity core gap structure of the wave eddy kinetic energy eke budgets of the evolving disturbance were investigated using time series of retrieved kinematic and thermodynamic data the wave grew at an approximately constant growth rate for about 40 min from its onset the eke in this quasi linear growth period was primarily gen

gravity gradients and upper crustal tectonic fabrics ireland gravity gradients derived from an updated bouguer anomaly map of ireland reveal large scale lineaments many of which can be related to geological structures and tectonic development first horizontal derivatives of the anomaly calculated at various azimuths accentuate linear trends in different directions while the second derivative helps resolve near surface structure the dominant ne sw caledonian trending lineaments are interpreted as due to fault boundaries and folds in pre carboniferous basement and caledonian fault reactivation during the variscan orogeny subsidiary nne ssw trending lineaments are present as well as weaker nw se trending lineaments the largest negative anomalies are produced by late caledonian granites and widespread buried granitic rock in the middle to upper crust is proposed both exposed and inferred granites appear to be spatially related to intersections of ne sw with nne ssw and nw se trending lineaments the orientation of the dominant gravity trend changes from ne sw towards ene wsw going westwards across ireland and forms part of a more regional change in the caledonian gravity fabric recognised throughout the northeast atlantic region variscan e w gravity trends in the south of ireland overlay the caledonian trends relationships between gravity lineaments and anomalies support a thin skinned model for the variscan cover

widening of the andes an interplay between subduction dynamics and crustal wedge tectonics shortening of the continental lithosphere is generally accommodated by the growth of crustal wedges building above megathrusts in the mantle lithosphere we show that the locus of shortening in the western margin of south america has largely been controlled by the geometry of the slab numerical models confirm that horizontal subduction favors compression far from the trench above the asthenospheric wedge and steeply dipping segment of the subducting slab as a result a second crustal wedge grows in the hinterland of the continent and widens the andes in the bolivian orocline this wedge corresponds to the eastern cordillera whose growth was triggered by a major episode of horizontal subduction when the slab returned to a steeper dip angle shortening and uplift pursued facilitated by the structural and thermo chemical alteration of the continental lithosphere we review the successive episodes of horizontal subduction that have occurred beneath south america at different latitudes and show that they explain the diachronic widening of the andes we infer that the present day segmented physiography of the andes results from the latitudinally variable transient interplay between slab dynamics and upper plate tectonics over the cenozoic we emphasize that slab flattening or absence thereof is a major driving mechanism that sets the width of the andes at any latitude

structural constraints on the evolution of the central asian orogenic belt in sw mongolia we provide a detailed description of the structures along a 300 km long and 50 km wide transect across the central asian orogenic belt caob in southwestern mongolia covering the precambrian dzabkhan continental domain with overthrust neoproterozoic ophiolites in the north lake zone a silurian devonian passive margin association gobi altai zone and oceanic domain trans altai zone in the center and a continental area south gobi zone in the south structural analysis suggests late cambrian collapse of the thickened lake zone continental crust leading to stretching of the lithosphere and followed by silurian devonian formation of oceanic crust in the trans altai domain subsequent emplacement of devonian carboniferous and late carboniferous magmatic arcs occurred on the gobi altai and south gobi zone crusts respectively during e w shortening finally the entire system was affected by n s convergence from the permian to jurassic leading to heterogeneous shortening of the orogenic domain the model best fitting these observations is one of generalized westward drift of the tuva mongol dzabkhan baydrag ribbon continents during the silurian devonian associated with westward subduction of the mongol okhotsk ocean and sequential growth of syn convergent magmatic arcs back arc basins opened during this period in the area of the western paleoasian ocean the present day shape of the caob in southern mongolia was probably formed during permian to mesozoic anticlockwise rotation and folding of the tuva mongol dzabkhan baydrag continental ribbons combined with a strike slip transpressional reactivation of ancient transform boundaries in the paleoasian oceanic domain all continental and oceanic crustal domains were reactivated and intensely deformed during this convergence in a style controlled by crustal rheology and a heterogeneous permian magmatic thermal input the sequence of tectonic events is tested against published paleomagnetic data paleogeographic reconstructions and tectonic models leading to a revised model for the accretion of juvenile crust to a continental margin in the caob of southern mongolia

short paper ophiolitic detritus in the ordovician sediments of south mayo ireland sandstones from ordovician sediments of the south mayo trough western irish caledonides show marked stratigraphic variation in their major and trace element geochemistry high cr ni and mg concentrations of ultramafic derivation succeed high ti and fe concentrations of mafic provenance this sequence is interpreted as the result of the progressive unroofing of an ophiolite a possible sedimentary linkage with ophiolite lithologies in the clew bay domain to the north would indicate minimal post arenig transcurrent displacement the south mayo flysch with its ophiolitic detritus is typical of caledonian flysch sequences which pre date the mid ordovician taconic deformation but differs in character from those of late ordovician age

retroarc basin reorganization and aridification during paleogene uplift of the southern central andes u s national science foundation ear 1049605 robert r shrock foundation at indiana university nsf ear award 1338583 project pdts unsj e985

palaeomagnetism of silurian sediments in w ireland evidence for block rotation in the caledonides summary a palaeomagnetic study of mid silurian wenlock sediments of the salrock formation in nw galway yields a reliable silurian palaeomagnetic pole position for w ireland at latitude 1 6 s longitude 287 7 e dp 2 5 dm 4 6 n 27 sites a further 12 sampling sites were located in silurian redbeds on clare island s mayo where a palaeomagnetic remanence of silurian age was also identified both remanences pre date local late caledonian folding however the clare island result has an anomalous azimuth when compared with the salrock result and with published silurian results for britain the two irish results can be reconciled by anticlockwise block rotation of the clare island region through 40 50 the rotation brings both palaeomagnetic and structural trends within the two sampling regions into alignment strike slip displacement across the clew bay fault zone producing a sinistral shear couple might have enabled such a rotation the timing and geographic extent of the proposed block rotation is as yet poorly defined however a younger age limit for rotation is provided by a late devonian or carboniferous partial magnetic overprint observed throughout both study areas and from geological evidence it seems likely that rotation is confined to the clew bay fault zone the dalradian rocks of the connemara massif located to the south of the salrock district have an e w structural grain some workers have proposed clockwise rotation of the connemara massif of ca 30 at between 500 and 390 ma which would bring the structural trends in w ireland into parallelism with caledonian trends in scotland because the silurian salrock succession which rests unconformably on the dalradian basement of connemara shows no such anomaly it provides a younger age limit for the possible rotation of connemara

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

contrasted metamorphic and structural evolutions across a major ductile brittle displacement zone in nw connemara western ireland nw connemara is transected by a steeply dipping e w to ese wnw trending structure the renvyle bofin slide rbs which shows evidence of both ductile and brittle displacement structural observations indicate a polyphase d1 d4 deformation history in the adjacent rock units dominated by the development during d2 of a regional l s fabric comprising an e w to ese wnw striking foliation and a strike parallel stretching lineation on the north side of the rbs the stretching lineation is locally oriented n s and associated with a system of linked ductile duplex structures in the contact aureole of the dawros currywongaun doughruagh complex dcdc folding of the n s stretching lineation by f3 folds suggests it too formed during d2 synchronous with the more strike parallel stretching lineation peak metamorphic conditions and qualitative ptt paths derived from gibbs method thermodynamic modelling of garnet zoning indicate both sides of the rbs underwent burial during d2 but that the south side of the rbs was buried more c 1 5 kbar 5 km it is suggested that d2 involved predominantly sinistral displacement on the rbs but east of tully mountain around the dcdc there was substantial dipslip displacement resulting from transpression where the slide markedly bends around tully mountain modelling of metamorphic mineral assemblages developed across the rbs and analysis of published mineral ages provide evidence that the rbs moved repeatedly during and after regional metamorphism that the d2 dip slip component around the dcdc may have been substantial during garnet growth assisting burial of rocks on the southern side of the rbs and that dip slip movements were probably dominant and variable during exhumation

geometry and late mesozoic cenozoic evolution of the salar de atacama basin 22 30 24 30 s in the northern central andes new constraints from geophysical geochronological and field data abstract the late mesozoic cenozoic evolution of the central andes of northern chile consists of a series of events related to changing conditions along the plate margin leading to different episodes of compression and extension a long lasting basin which records these changes corresponds to the salar de atacama basin its tectonic history has been blurred by several interpretations of seismic reflection lines and multiple chronostratigraphic correlations between subsurface surface units as such many aspects of its tectonic evolution are still unclear in this study we performed an e w gravimetric profile across the barros arana syncline and the basin we analyzed interval velocities obtained from seismic reflection studies and reviewed the profiles closest to the gravity survey four u pb samples for detrital zircon analysis were obtained from late cretaceous paleogene units geophysical data show a segmentation of physical properties within the basin with denser units and faster interval velocities around the late mesozoic early cenozoic units and lighter slower units in the actual salt pan the main contrast in physical properties is observed in the evaporitic and fine grained continental deposits of the san pedro formation geophysical and geological constraints show that its thickness varies between 800 and 1200 m and that it was deformed during later events the u pb data show that the rocks within the barros arana syncline reach well into the paleogene and that they were mainly deformed during the late eocene incaic event where an east verging thrust system also involving permo triassic units was developed establishing the actual structural configuration the late oligocene early miocene units were then deposited in a post orogenic setting with minor compression a stronger phase around the late miocene reactivated previous structures and folded these deposits this continuous compressive history of the basin which is mostly a result of the incaic event is in agreement with the regional evolution of the central andes

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

non hydrostatic simulations of a cold front observed during the fronts 87 experiment a two dimensional non hydrostatic cloud model is used to simulate a frontal system observed on 11 november 1987 during the fronts 87 experiment the cloud resolving model is able to reproduce a large variety of frontal bands starting from only the meso scale fields produced by a hydrostatic forecast model a narrow cold frontal rainband is identified at the surface cold front two classes of wide bands are also simulated a wide cold frontal rainband and warm sector wide rainbands the vertical velocity fields exhibit similar banded features in both the simulations and the doppler radar observations with bands having the same position intensity and width several sensitivity experiments are presented one of them shows that the initial knowledge of the ageostrophic circulation is not necessary to give a successful simulation other experiments show that the along front baroclinicity drives the wide band formation whereas the along front deformation is not important the heat moisture and horizontal momentum budgets are also investigated in order to determine the rainband effects on larger scales

terrane amalgamation in the clew bay region west of ireland the caledonides of the west of ireland provide a well exposed and well mapped example of an oblique collision zone the east northeast trending deer park and achill beg fault system is a crustal scale ductile sinistral strike slip duplex of late ordovician age imbricating late precambrian granulite facies lower crustal rocks near eclogite facies supracrustal rocks up to amphibolite facies dalradian metasedimentary rocks and greenschist facies cambro ordovician rocks this fault system is correlated with a pre devonian component of the highland boundary fault system in southern scotland in the clew bay area the high pressure low temperature facies metamorphic rocks in tectonic contact with greenschist facies cambro ordovician rocks are together interpreted as an accretionary prism complex related to northwestward directed subduction both of these are allocthonous terrains with respect to the dalradian terrane to the north north west mayo to the south the cambro ordovician rocks docked with a probable dalradian block containing ultramafic intrusives deer park complex during the late ordovician the deer park complex and south mayo trough linked earlier during the arenig silurian and lower middle devonian redbed successions sit unconformably on the metamorphic rocks deposition and deformation of these cover rocks was controlled by oblique strike slip movements on the leek fault whose strike swings from west northwest to north northeast following earlier basement trends as it is traced eastwards from clew bay the leek fault system may be correlated with the leannan fault of northwest donegal a splay of the great glen fault system of central scotland east of clew bay this sinistral shear generated local dilation on the more northerly trending bend of the leek fault lower and middle old red sandstone redbeds were developed here the west northwest trend of the leek fault in clew bay acted as a compressional bend during these sinistral movements and transpressional southwest directed thrusting developed in silurian rocks post middle old red sandstone pre late tournaisian dextral displacement on the leek fault reversed this pattern with transtension in clew bay allowing intrusion of small carbonated peridotite bodies into silurian rocks and easterly directed thrusting of middle old red sandstone rocks east of the bay on the transpressional north south bend a tectonic model for the region is presented here this model involves a northwestward directed subduction system 150 to 750 km of arenig sinistral strike slip movement and eastwards insertion of the connemara block with formation of the ordovician south mayo trough as a pull apart basin subsequently a further 130 to 650 km eastward displacement of rocks took place south of the deer park fault in later ordovician times the magnitudes of these estimates are directly proportional to an assumed maximum wavelength of 1500 km for promontories on the original laurentian margin and using the current juxtaposition of terranes a minimum wavelength of 300 km is inferred

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

the organization of convection in narrow cold frontal rainbands abstract narrow cold frontal rainbands are lines of intense precipitation that straddle surface cold fronts recent observational work has revealed that the rainfall within the band is organized into regularly spaced ellipsoidal cells called precipitation cores the rainband is coincident with a line of intense cyclonic shear associated with a low level jet that lies ahead of and parallel to the surface cold front numerous authors have suggested that the organization of cells in the rainband is the result of shear instability of the horizontal wind field to investigate this hypothesis a linear stability analysis was performed on an idealized frontal zone consisting of a line of convection coincident with a line of cyclonic shear to model the convective processes the air inside the rainband was assumed to be unstably stratified the presence of horizontal wind shear in an unstably stratified environment resulted in the existence of a mode with a short wave cutoff the coupling between the convective p

gravity waves generated during frontogenesis abstract gravity waves forced by nonhydrostatic and nongeostrophic processes within a frontal zone are discussed in particular stationary waves immediately above and below the surface front are considered the waves that appear above the front are horizontally stationary with respect to the front but are vertically propagating the vertical wavelength here is given by 2 n since the waves are nearly hydrostatic the horizontal wavelength of the waves above the front is determined by standing waves that set up below the front these waves corrugate the frontal surface and these corrugations in turn determine the horizontal scale of the waves above the front the waves under the front are standing and are trapped between the earth s surface and the frontal zone which due to its conditions of flow reversal and small ri is assumed to be a reflector of gravity waves the horizontal scale of the standing waves is determined by their vertical wavelength and the slope of the frontal surface these waves

short paper the silurian of clew bay ireland part of the midland valley of scotland a new agnathan dominated biota found near louisburgh co mayo in the west of ireland is correlated with post llandovery biotas of the midland valley of scotland mvs it is argued that this biotic similarity implies a 500 km elongate aquatic connection between the two areas an alternative hypothesis that the fault bounded irish outcrop is a displaced terrane originally in closer contiguity with the mvs inliers is rejected this results from a detailed structural analysis of the bounding fault emlagh thrust which constrains the post llandovery strike slip motion to less than 25 km

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

airborne dual doppler observations of an intense frontal system approaching the pacific northwest coast airborne doppler radar data collected off the pacific northwest coast by a noaa wp 3d orion aircraft over an 8 h period on 8 december 1993 during the coastal observations and simulations with topography experiment reveal the mesoscale structure of an intense frontal system while it was well offshore and as it approached within 20 km of the oregon coastline during the offshore stage a portion of the narrow coldfrontal rainband was characterized by deep convective cores pseudo dual doppler analyses characterize the kinematic and precipitation structure of the deep convection pseudo dual doppler analyses describe the subsequent evolution of the narrow cold frontal rainband as it approached to within 20 km of the oregon coast deformation of the frontal zone appeared to cause the dissipation of one of three precipitation cores contained within the dual doppler area the precipitation cores and the strong convergence zone associated with the front conformed to some degree to the shape of the coastline near cape blanco oregon as the front neared the coast changes in the prefrontal flow that occurred as the front approached the coast were qualitatively consistent with theoretical and numerical studies of upstream orographic influence comparison of pseudo dual doppler derived velocity profiles with idealized numerical model calculations suggests that the nearshore evolution of the frontal rainband was significantly affected by an upstream influence of the coastal orography

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

large along strike variations in the onset of subandean exhumation implications for central andean orogenic growth abstract plate tectonics drives mountain building in general but the space time pattern and style of deformation is influenced by how climate geodynamics and basement structure modify the orogenic wedge growth of the subandean thrust belt which lies at the boundary between the arid high elevation central andean plateau and its humid low elevation eastern foreland figures prominently into debates of orogenic wedge evolution we integrate new apatite and zircon u th he thermochronometer data with previously published apatite fission track data from samples collected along four subandean structural cross sections in bolivia between 15 and 20 s we interpret cooling ages vs structural depth to indicate the onset of subandean exhumation and signify the forward propagation of deformation we find that subandean growth is diachronous south 11 3 ma vs north 6 2 ma of the bolivian orocline and that subandean exhumation magnitudes vary by more than a factor of two similar north south contrasts are present in foreland deposition hinterland erosion and paleoclimate these observations both corroborate diachronous orogenic growth and illuminate potential propagation mechanisms of particular interest is an abrupt shift to cooler more arid conditions in the altiplano hinterland that is diachronous in southern bolivia 16 13 ma vs northern bolivia 10 7 ma and precedes the timing of subandean propagation in each region others have interpreted the paleoclimate shift to reflect either rapid surface uplift due to lithosphere removal or an abrupt change in climate dynamics once orographic threshold elevations were exceeded these mechanisms are not mutually exclusive and both would drive forward propagation of the orogenic wedge by augmenting the hinterland backstop either through surface uplift or spatially variable erosion in summary we suggest that diachronous subandean exhumation was driven by piecemeal hinterland uplift orography and the outward propagation of deformation

megathrust shear force controls mountain height at convergent plate margins the shear force along convergent plate boundary faults megathrusts determines the height of mountain ranges that can be mechanically sustained1 4 however whether the true height of mountain ranges corresponds to this tectonically supported elevation is debated4 7 in particular climate dependent erosional processes are often assumed to exert a first order control on mountain height5 12 although this assumption has remained difficult to validate12 here we constrain the shear force along active megathrusts using their rheological properties and then determine the tectonically supported elevation using a force balance model we show that the height of mountain ranges around the globe matches this elevation irrespective of climatic conditions and the rate of erosion this finding indicates that mountain ranges are close to force equilibrium and that their height is primarily controlled by the megathrust shear force we conclude that temporal variations in mountain height reflect long term changes in the force balance but are not indicative of a direct climate control on mountain elevation

oligocene to present shallow subduction beneath the southern puna plateau abstract the southern puna plateau is a conspicuous example of a high elevation orogenic plateau in a non collisional setting this orogenic sector is currently located above an anomalously shallow subduction segment in which timing and relation to upper plate tectonics have been widely overlooked this subduction segment here referred to as the southern puna shallow subduction spss is characterized by a 200 km wide shallow area located at 300 km from the trench at a depth of 100 120 km and dipping 10 12 to the east to determine the onset of the spss and its link to the tectonic and magmatic activity in this region we analyzed the tectonomagmatic record of the southern puna plateau from preexisting datasets also we present a new approach based on global subduction data that provides a straightforward methodology to extract potential paleo slab angles from the bedrock arc record this analysis reveals that a pronounced eastward arc front migration and magmatic broadening took place at 26 ma and was preceded by 4 ma of reduced magmatic activity which we link to the inception of the spss as expected in shallow subduction settings a change to basement cored distributed deformation south of 25 s in the southern puna plateau coincides with the beginning of shallow subduction also the spss is coincident with the enigmatic post eocene intraplate deformation of the otumpa hills located at 950 km from the trench we suggest that this succession of events is not fortuitous and that the development of the spss impacted directly the overriding plate since the oligocene contributing to the building of one of the largest topographies 3 km and thickest orogenic crusts 70 60 km on earth the shallow subduction would have acted jointly with cenozoic changes in plate kinematics and climate enhancing andean orogenesis at studied latitudes

deformation history of the puna plateau central andes of northwestern argentina abstract two tectonic shortening events created the first order structural characteristics of the puna plateau one in the paleozoic and a second during the andean orogeny in the cenozoic to constrain the structural characteristics and timing of andean deformation in the puna plateau we focus on differentiating these two shortening events and provide cooling ages apatite fission track and apatite u th he to determine the amount of andean exhumation on the hanging wall of major cenozoic faults two contrasting expressions of shortening are documented in ordovician strata strain requires four stages folding flattening through cleavage formation low angle faulting and local distributed shear in the mesozoic and cenozoic strata shortening is expressed as reverse faults fault propagation folds and dipping panels dissimilarities reflect different p t conditions and amount of strain as structures in the ordovician rocks require higher temperatures and strain than andean structures in the mesozoic and cenozoic strata furthermore local cretaceous aft cooling ages in pervasively cleaved ordovician rocks reveal these rocks did not reach temperatures compatible with cleavage formation during the andean event lastly our data show a middle eocene to oligocene exhumation history that supports an overall continuous development of the andean thrust belt in the puna plateau since the early cenozoic

oroclinal bending in the caledonides of western ireland a mid palaeozoic feature controlled by a pre existing structural grain abstract the caledonides of the west of ireland mark a segment of the caledonian appalachian orogen where the structural grain swings from the ne sw trend that typifies the caledonides of northern britain to an e w orientation the origin of this arcuate structure has previously been proposed to be either primary reflecting the original geometry of promontories and embayments in the laurentian margin in this sector of the orogen or secondary as evidenced by palaeomagnetically determined clockwise rotations in silurian rocks in the region we report the results of a palaeomagnetic investigation of the early to middle ordovician mweelrea ignimbrites llanvirn from an accreted taconic arc the south mayo trough in that part of the western irish caledonides that has an e w trend after removal of a low temperature component of magnetisation l interpreted to represent a viscous component of the earth s present field a high temperature component of magnetisation h was isolated at fifteen sites in four ignimbrite flows h passes a palaeomagnetic fold test yielding a tilt corrected mean remanence direction d 132 6 i 29 1 95 17 9 which corresponds to a palaeopole at 36 4 e 9 9 s the pole is in reasonable agreement with established early to middle ordovician reference poles for laurentia and indicates that no significant rotation occurred between the e w south mayo trough and the laurentian margin after accretion and therefore that the curvature of this segment of the orogen is likely to be primary this contrasts with other results from silurian sequences to the south which record significant post silurian clockwise rotations examination of all reliable palaeomagnetic data from the region indicates that the clockwise rotations are restricted to the rocks south of the south mayo trough we propose that the rotations in the south are linked with the superposition of the pre existing structural grain on the younger rock units during late silurian and early devonian sinistral transpression

episodic out of sequence deformation promoted by cenozoic fault reactivation in nw argentina abstract the central andes are the product of contractional deformation related to subduction of the nazca plate beneath the south american continent in this geodynamic setting thrust belt propagation in the central andean retroarc in nw argentina and southern bolivia has been linked with the critical wedge model implying progressive eastward migration of the deformation front during the cenozoic however eocene unconformities and growth strata within the orogenic hinterland of the eastern cordillera and puna plateau suggest selective reactivation of pre andean and andean faults revealing a pattern of diachronous and spatially disparate range uplifts the calchaqui valley in nw argentina is characterized by basin and range morphology which is intimately linked with the heterogeneous neoproterozoic basement cretaceous rift structures and the cenozoic foreland in this study we analyze this complex deformation by reconstructing cenozoic fault reactivation and sedimentation patterns in the eastern cordillera detailed structural mapping along the eastern calchaqui border tonco area reveals five cenozoic unconformities in the former foreland deposits related to multiple episodes of fault reactivation during the eocene to pleistocene a refined chronostratigraphy u pb zircon new ahe thermochronological data apatite u th sm he and existing evidence for deformation indicate episodic out of sequence deformation and increased eo oligocene puna plateau to miocene eastern cordillera fault displacement apparently controlled by pre cenozoic heterogeneities the coeval eastward propagation of deformation and continuing deformation in the west generally challenge the applicability of the critical wedge model