modelling the mid late holocene evolution of the huelva estuary and its human colonization south western spain abstract the major changes that occurred in the southwestern estuaries of the spanish atlantic coast during the last 6500 yr bp were simultaneous to human settlement and therefore the understanding of their coastal evolution will help interpreting human patterns in these areas the study of the morpho sedimentary features of new outcrops appearing in the middle sector of saltes island huelva estuary spain has been used to develop a model to understand the complex evolution of sand barriers than can be applied to similar inlets along the atlantic iberian coast the first human settlements 6000 4000 yr bp in the early huelva estuary tinto and odiel rivers were located in the ancient coastal banks or in the nearby hills from 4000 yr bp onwards the estuarine sediments started to emerge as sand barriers and chenier plains prograding towards the mouth as the littoral strands stabilized morphologically they were colonized by human settlements in successive periods the oldest inland almendral and more recent outward cascajera the study of the upper sedimentary layers of la cascajera barrier display a tempestitic sequence of landward progradational washover fans the calibrated and modelled ams dates in marine shells provide a storminess time range between the second half of first century bce and the entire first century ce sedimentary records are useful to evaluate environmental changes either from natural or anthropogenic causes such as global and climate change the interrelationship between the archaeological findings mainly salting fish factories and old ports and the morpho sedimentary evolution at the mouth of the tinto and odiel rivers allows us to highlight not only the huelva estuary s dynamics evolution but also the possible regional patterns of human habitation from the beginning of the present sea level highstand middle holocene

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

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

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

lamprophyric volcanism of cenozoic age on the sierra leone rise implications for regional tectonics and the stratigraphic time scale abstract igneous and sedimentary rocks have been recovered by dredging the upper slopes of krause seamount a feature lying on the sierra leone rise near 9 n 20 w in the eastern equatorial atlantic the igneous samples consist of lamprophyres and an ankaramite which are rarely encountered in an oceanic setting associated with these rocks are breccias made up of lamprophyric fragments set in a lithified calcareous matrix containing a rich planktonic foraminiferal assemblage of early eocene ypresian subzone p8a age whole rock k ar dating of the igneous samples proved unsuccessful owing to advanced alteration and consequent argon loss however within the breccias are numerous fresh crystal clasts of ti rich amphibole pyroxene and biotite which have compositions closely similar to phenocrysts observed in the lamprophyric rocks suggesting that they are derived directly from them k ar dating of separates of the three minerals yielded ages in the range 53 3 55 4 ma the concordant results provide a maximum age for the ypresian subzone p8a and are consistent with its position within recently established cenozoic time scales they indicate that krause seamount is appreciably younger than the adjacent gambia and sierra leone basins in common with other occurrences of lamprophyric rocks in the oceans those exposed on krause seamount are located in an area of old oceanic basement close to a large fracture zone which has been reactivated by late stage volcanic activity these relatively rare alkaline rocks may be generated by remelting of a volatile rich metasomatized layer which is present in the upper mantle beneath mesozoic ocean floor

en echelon vein array development in extension and shear a subdivision of en echelon vein arrays is proposed based on the geometry of sets of transcurrent arrays from the lower carboniferous rocks in the se midlands of ireland the parameter vein zone angle is proportional to the array orientation for arrays developed through secondary failure in shear zones shear arrays for arrays developed during propagation of an extension vein extension arrays is proportional to the overlap of veins in the array p o criteria for field distinction together with the different methods of kinematic interpretation that are required for the two types of arrays are described for shear arrays the average trends of the two members of the conjugate set are calculated and the axis of principal shortening strain z bisects these the average trend of extension arrays is parallel to that of extension veins and is therefore normal to the maximum extensional strain axis x

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

a tectonomagmatic model for the genesis and emplacement of caledonian calc alkaline lamprophyres petrogenetically related caledonian calc alkaline lamprophyric and shoshonitic dykes in eastern ireland appear coeval with sinistral transtension these dykes are exposed in a 10 km wide zone trending sw from clogher head that strikes anticlockwise of the surface trace of the iapetus suture lamprophyre dykes intruded sinistral riedel shears with a mean strike trend at 040 riedel shears that host lamprophyre dykes strike anticlockwise of primary sinistral shears by 35 consistent with sinistral transtension primary shears strike 070 parallel to the mean strike trend of shoshonitic dykes sinistral primary shears may have controlled the emplacement of caledonian shoshonitic intrusive rocks using lamprophyres from clogher head as examples a new model is proposed for lamprophyre genesis and emplacement related to simple shear in this model transient decompression of metasomatized mantle during simple shear triggers release of volatile rich potassic magma which ascends to crustal levels along steep strike slip faults an association between lamprophyre magmatism and deep faulting suggests that relatively deep 60km seismicity may precede magma ascent this model can account for several paradoxes of caledonian magmatism

short paper movement on proposed terrane boundaries in northern britain constraints from ordovician devonian igneous rocks regional chemical and isotopic differences seen in ordovician devonian igneous rocks of northern britain are thought to be due to subduction related and collision magmatism tapping a variety of mantle and crustal sources matching of spatial chemical variation across major fault zones restricts post magmatic transcurrent movement north of the southern upland fault suf to a few tens of kilometres on each fault a unique mantle isotopic signature present in the same area may restrict post 1 5 ga movement to less than a few hundred kilometres no such restrictions exist on displacement across the suf to the south of which all magmas show a pb isotopic component derived from southern sources

model for the intrusion of batholiths associated with the eruption of large volume ash flow tuffs pyroclastic eruption and the intrusion of batholiths associated with large volume ash flow tuffs may be driven by a decrease in reservoir pressure caused by the low density of the magma column due to vesiculation batholithic intrusion would then be accomplished by the subsidence and settling of kilometer sized crustal blocks through the magma chamber resulting in eventual collapse to form large caldera structures at the surface such a model does not require the formation of a large laterally extensive shallow magma chamber before the onset of large volume ash flow eruptions eruption could commence directly from a deeper reservoir with only a small channelway being opened to the surface before the onset of catastrophic ash flow eruptions of the scale of yellowstone or long valley such a model has wide ranging implications and explains many of the problems inherent in the simple collapse model involving shallow magna chambers as well as the process and timing of batholith intrusion in such cases

a late ordovician to middle wenlockian graptolite sequence from a borehole within the rathkenny tract eastern ireland and its relation to the paleogeography of the iapetus ocean a borehole drilled within the rathkenny tract southwest of navan eastern ireland yielded a rich graptolite fauna including three excellent basal silurian graptolite assemblages from a deep water pelagic and hemipelagic sequence ranging in age from uppermost caradocian to middle wenlockian from immediately below carboniferous strata biostratigraphic analysis of the graptolites shows that there are eight discrete faunal biostratigraphic units and these together with the lithology indicate the graptolite bearing sequence is mostly inverted and faulted at several levels biogeographic affinities of several of the llandoverian and of the wenlockian graptolite species as well as some lithological data suggest that the rathkenny tract lay on the southeast side of the iapetus ocean the presence of the youngest graptolites further suggests that some element at least of the iapetus ocean though probably narrow by this time was clearly still in existence until at least middle wenlockian

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

the great tonalite sill emplacement into a contractional shear zone and implications for late cretaceous to early eocene tectonics in southeastern alaska and british columbia the late cretaceous to early tertiary great tonalite sill of southeast alaska and british columbia is a very long 1 000 km and thin 25 km orogen parallel composite batholith which lies between the insular superter rane including the alexander and wrangellia terranes and the intermontane superterrane including the stikine and cache creek terranes the batholith is delineated by many steep sheet like plutons which are dominated by northwest southeast striking concordant fabrics with steep lineations that formed during deformation in a country rock shear zone prior to the complete crystallization of the magmas deformation in this shear zone is dominated by northeast southwest directed contraction orthogonal to the orogenic strike associated with a component of northeast over southwest high angle shear the steep multiple dike like nature of the body and its emplacement during orogenic contraction imply that ascent and emplacement have been achieved by dike wedging mechanisms along the deep reaching shear zone the remarkable narrowness of the great tonalite sill is probably the result of melting at the base of a very localized zone of thickened crust produced by the associated narrow contractional shear zone extending along the orogen length such a shear zone of late cretaceous to early tertiary age lying along 800 km of the possible boundary between the insular and intermontane superterranes implies that it may represent the actual boundary between them if this hypothesis is correct it implies that the large scale tectonic regime during emplacement of the great tonalite sill was predominantly orthogonal and not obliquely dextral as has been indicated from paleomagnetic data

late caledonian extensional reactivation of a ductile thrust in nw scotland evidence is given for ductile extension across a fault zone located on the western seaboard of the northern highlands of scotland the fault zone lies within the metamorphic caledonides and extensional movement occurred during the waning stages of caledonian orogenic activity an earlier phase of westerly directed overthrusting took place on the same fault zone at higher metamorphic temperatures but after the peak of caledonian metamorphism the compression may be broadly coeval with early displacements on the moine thrust zone which probably lies offshore some 10 km to the northwest the extensional reactivation could relate to local crustal instability resulting directly from overthrusting it may have been due to local gravitational collapse following overthickening of the crust alternatively the extensional movement may witness the onset of regional gravitational collapse of the orogen during the late silurian to early devonian reactivation accords with interpretations of the movement histories of similarly eastward dipping offshore faults recognized on seismic reflection profiles

evidence for silurian sinistral accretion of avalon composite terrane in canada the kingston dyke complex forms a regionally developed belt of sheeted bimodal dykes within the avalon composite terrane of southern new brunswick canada dyke orientation relative to the belt suggests their injection accompanied sinistral movement and the belt has previously been attributed to transform activity within the late precambrian magmatic arc regime typical of the avalon composite terrane in the northern appalachians however rhyolite dykes of the sheeted complex yield a u pb age of 435 5 1 5 ma that is interpreted to date their emplacement the complex therefore provides evidence of early silurian sinistral displacement in the northern appalachians such movement is suggested by palaeomagnetic data and may record the accretion of the avalon composite terrane to cratonic north america

the role of fluorine and oxygen fugacity in the genesis of the ultrapotassic rocks the effects of h2o co2 ch4 and hf on partial melting of a model phlogopite harzburgite mantle are considered with regard to the production of ultrapotassic magmas fluorine has a polymerising effect in h2o poor conditions but in the presence of abundant h2o where hf rather than f is dominant the overall effect is depolymerisation methane also dissolves by forming oh groups and so has a depolymerising effect group i ultrapotassic rocks lamproites probably originate from primary magmas with sio2 contents ranging from around 40 wt to at least 52 wt this range can be explained by differing depths of origin from a similar source with a similar reduced h2o ch4 hf volatile mixture the formation of silica rich initial melts from a model phlogopite harzburgite is assisted by the presence of ch4 and hf dissociation of less than 0 1 wt h2o driven by h2 loss is sufficient to cause oxidation during emplacement to observed oxidation states silica poor ultrapotassic rocks could be produced at higher pressures in a reduced environment or in an oxidised environment with high co2 co2 h2o ratios

igneous emplacement in a shear zone termination the biotite granite at strontian scotland deformed xenolith strain data together with other structural information indicate that the biotite granite body 90 sq km of the strontian complex scotland was emplaced in the extensional termination of a dextral transcurrent shear zone this shear zone is a splay of the major great glen fault which lies along the southern boundary of the granite siting of the shear zone splay was probably controlled by a a slight releasing bend in the great glen fault in this area and b a large pre existing asymetric synform in proterozoic metasedimentary country rocks which intersects the great glen fault trace at a high angle a model is proposed in which moine thrust caledonian compression at ca 435 ma activated the great glen fault dextrally dextral movements around the releasing bend detached a flat segment from the inside fault wall and the biotite granite was emplaced side ways at depths of about 15 km as a sheet into this extensional listric fault bounded cavity

production of jurassic rhyolite by anatexis of the lower crust of patagonia abstract the mid jurassic marifil and chon aike volcanic rocks of eastern patagonia are part of one of the largest silicic igneous provinces known rb sr geochronology indicates eruptive ages of 175 190 ma for the marifil complex mostly toarcian aalenian the majority of the rocks are isotopically uniform with initial 87sr 86sr 0 7067 0 0003 and endt 4 2 primary magmas of andesitic composition were generated by partial melting of mafic grenvillian lower crust indentified by depleted mantle model ages of 1150 1600 ma lower crustal pyroxene granulite xenoliths can be modelled as residual jurassic source although they may alternatively be co genetic cumulates the dacite rhyolite suite formed by crystal liquid fractionation processes from the primary andesites involving multistage crystallization and re melting during magma ascent with the later stages of evolution being explicable by fractional crystallization of plagioclase amphibole and accessory minerals the province represents large scale lower crustal reworking associated with the unique tectonic and igneous environment of gondwana break up

microstructural tectonometamorphic processes and the development of gneissic layering a mechanism for metamorphic segregation the mary granite in the east athabasca mylonite triangle northern saskatchewan provides an example and a model for the development of non migmatitic gneissic texture gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase orthopyroxene and matrix in the igneous rocks plagioclase phenocrysts were progressively deformed and recrystallized first into core and mantle structures and ultimately into plagioclase rich layers or ribbons garnet preferentially developed in the outer portions of recrystallized mantles and with further deformation produced garnet rich sub layers within the plagioclase rich gneissic domains orthopyroxene was replaced by clinopyroxene and garnet and hornblende if sufficient water was present which were in turn drawn into layers with new garnet growth along the boundaries the igneous matrix evolved through a number of transient fabric stages involving s c fabrics s c c fabrics and ultramylonitic domains in addition quartz veins were emplaced and subsequently deformed into quartz rich gneissic layers moderate to highly strained samples display extreme mineralogical compositional segregation yet most domains can be directly related to the original igneous precursors the mary granite was emplaced at approximately 900 c and 1 0 gpa and was metamorphosed at approximately 750 c and 1 0 gpa the igneous rocks crystallized in the medium pressure granulite field opx pl but were metamorphosed on cooling into the high pressure grt cpx pl granulite field the compositional segregation resulted from a dynamic mutually reinforcing interaction between deformation metamorphic and igneous processes in the deep crust the production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust

structural trends in central britain from images of gravity and aeromagnetic fields digital image processing techniques have been used to analyse the regional gravity and aeromagnetic datasets in central britain colour and shaded relief images have been generated which convey information on both anomaly amplitude as colour and anomaly gradient as relief and highlight structural trends lineaments and textural contrasts not easily discernible on standard contour maps we have identified some of the more important features and have tried to relate these to the evolution of the crust in each region on a broad scale the images emphasize caledonoid ene trends to the north of the solway line and both tornquist se and caledonoid ne to ene trends to the south the pattern of magnetic anomalies over central england seems to define the extent of the shallow late precambrian early palaeozoic basement of the midlands microcraton and delineates crustal elements and characteristic trends within it magnetic lineaments and the grain of gravity anomalies trending in northeasterly and northwesterly directions on either side of the microcraton seem to relate to structures which originated during the evolution of the welsh and eastern english caledonides respectively where lower palaeozoic and precambrian rocks form the concealed basement as in most of england the gravity lineaments tend to reflect the influence of basement fractures on the subsequent pattern of sedimentation whereas many of the features on the magnetic images are more directly related to structures within the basement itself