a history of cleavage and folding an example from the goldenville formation nova scotia temporal relationships between cleavage formation and folding in sedimentary rock sequences can vary from either phenomena preceding the other to both occurring simultaneously knowledge of this relationship however is required to understand fully the relationship of cleavage orientation to finite strain and the origin of cleavage refraction across competent incompetent layers evidence from the goldenville formation provides an example of cleavage formed before folding by 50 pressure solution removal of material resulting in cleavage planes normal to bedding during active folding this layer parallel shortening was accompanied by excess pore pressure that in some cases exceeded lithostatic pressure causing formation of bedding parallel quartz veins with crack seal textures folds developed by a mechanism of differential flexural flow with no loss of coherence across bedding surfaces en echelon veins formed sequentially across some beds during active folding and subsequently were deformed by various increments of bedding parallel shear after folds reached 80 limb dips continued horizontal shortening was accommodated by additional pressure solution but no recognizable new cleavage was formed because cleavage in the goldenville formation formed perpendicular to bedding before folding and was passively rotated and extended by bedding parallel simple shear during fold amplification the resulting cleavage planes cannot lie in the xy plane of the finite strain ellipsoid differential shear between metagray wacke and slate beds accounts for the observed cleavage refraction and the bedding cleavage angle provides a good estimate of the shearing across these beds during active folding if cleavage can be shown to postdate folding in other examples it is possible that cleavage planes will lie in the xy plane of the finite strain ellipsoid

practical application of the sequential balancing technique an example from the irish variscides the process of sequential restoration presented by cooper trayner 1986 is applied to a n s cross section through the irish variscides to demonstrate that this approach improves the accuracy of thin skinned structural modelling and allows the progressive deformation of a marginal belt to be examined the principles and methods of sequential restoration are illustrated the cross section is restored in four steps and gives a bulk shortening of 52 in this model the fold belt evolved as the basal decollement migrated northwards and considerable ductile shortening by folding and cleavage formation preceded faulting although faulting gradually became more important towards the north the section is divided into four structural domains based upon different deformation histories each is restored independently by removing the ductile strain before restoring the faulting from this example it is clear that many models derived from single step balancing especially those through ductile deformed areas should be re examined

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

the major ion composition of silurian seawater one hundred fluid inclusions in silurian marine halite were analyzed in order to determine the major ion composition of silurian seawater the samples analyzed were from three formations in the late silurian michigan basin the a 1 a 2 and b evaporites of the salina group and one formation in the early silurian canning basin australia the mallowa salt of the carribuddy group the results indicate that the major ion composition of silurian seawater was not the same as present day seawater the silurian ocean had lower concentrations of mg2 na and so42 and much higher concentrations of ca2 relative to the ocean s present day composition furthermore silurian seawater had ca2 in excess of so42 evaporation of silurian seawater of the composition determined in this study produces kcl type potash minerals that lack the mgso4 type late stage salts formed during the evaporation of present day seawater the relatively low na concentrations in silurian seawater support the hypothesis that oscillations in the major ion composition of the oceans are primarily controlled by changes in the flux of mid ocean ridge brine and riverine inputs and not global or basin scale seawater driven dolomitization the mg2 ca2 ratio of silurian seawater was 1 4 and the k ca2 ratio was 0 3 both of which differ from the present day counterparts of 5 and 1 respectively seawaters with mg2 ca2 2 e g modern seawater facilitate the precipitation of aragonite and high magnesian calcite therefore the early paleozoic calcite seas were likely due to the low mg2 ca2 ratio of seawater not the pco2 of the silurian atmosphere

global positioning system measurements of present day crustal movements in the arabia africa eurasia plate collision zone we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1994 at 54 sites extending east west from the caucasus mountains of southern russia georgia and armenia to the aegean coast of turkey and north south from the southern edge of the eurasian plate pontus block to the northern edge of the arabian platform viewed from a eurasia fixed reference frame sites on the northern arabian platform move n38 13 w at 20 3 mm yr roughly consistent with the velocity implied by nuvel 1a circuit closure n23 7 w at 24 2 mm yr the motion of arabia appears to be transferred directly to the region of turkey north of the suture however eastern turkey is characterized by distributed deformation while central western turkey is characterized by coherent plate motion involving westward displacement and counterclockwise rotation of the anatolian plate internal deformation within the central part of the anatolian plate is less than 2 mm yr the anatolian plate is decoupled from eurasia along the right lateral strike slip north anatolian fault naf this different response in eastern and western turkey to the collision of arabia may result from the different boundary conditions the hellenic arc forming a free boundary to the west and the asian continent and oceanic lithosphere of the black and caspian seas forming a resistant boundary to the north and east we derive a best fitting euler vector for anatolia eurasia motion of 29 2 0 8 n 32 9 0 4 1 3 0 1 m y the mapped surface trace of the naf corresponds well to a small circle about this pole the new euler vector implies an upper bound for naf slip rate of 30 2 mm yr i e assuming all relative motion is accommodated along the naf using the nuvel 1a euler vector for arabia eurasia and the gps euler vector for anatolia eurasia we determine an arabia anatolia euler vector of 31 2 n 45 2 e 0 9 0 1 m y and an upper bound on the east anatolian fault slip rate of 15 3 mm yr the aegean trough region of western turkey deviates significantly from coherent plate rotation in addition to rotating with anatolia this region shows roughly n s extension at a rate of 14 5 mm yr taken together with satellite laser ranging results along the hellenic arc the contemporary pattern of deformation indicates increasing motions toward the arc suggesting that the westward displacement and counterclockwise rotation of anatolia is driven both by pushing from the arabian plate and by pulling or basal drag associated with the foundering african plate along the hellenic subduction zone

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

cleavage strain in the variscan fold belt county cork ireland estimated from stretched arsenopyrite rosettes abstract in south west ireland hydrothermally formed arsenopyrite crystals in a devonian mudstone have responded to variscan deformation by brittle extension fracture and fragment separation the interfragment gaps and terminal extension zones of each crystal are infilled with fibrous quartz stretches within the cleavage plane have been calculated by the various methods available most of which can be modified to incorporate terminal extension zones the strain reversal method is the most accurate currently available but still gives a minimum estimate of the overall strain the more direct hossain method which gives only slightly lower estimates with this data is more practical for field use a strain ellipse can be estimated from each crystal rosette composed of three laths assuming the original interlimb angles were all 60 and because actual rather than relative stretches are estimated this provides a lower bound to the area increase in the plane of cleavage based on the average of our calculated strain ellipses this area increase is at least 114 and implies an average shortening across the cleavage of at least 53 however several lines of evidence suggest that the cleavage deformation was more intense and more oblate than that calculated and we argue that a 300 area increase in the cleavage plane and 75 shortening across the cleavage are more realistic estimates of the true strain furthermore the along strike elongation indicated is at least 80 which may be regionally significant estimates of orogenic contraction derived from balanced section construction should therefore take into account the possibility of a substantial strike elongation and tectonic models that can accommodate such elongations need to be developed

the major ion composition of permian seawater abstract the major ion mg2 ca2 na k so42 and cl composition of permian seawater was determined from chemical analyses of fluid inclusions in marine halites new data from the upper permian san andres formation of texas 274 272 ma and salado formation of new mexico 251 ma analyzed by the environmental scanning electron microscopy esem x ray energy dispersive spectrometry eds method along with published chemical compositions of fluid inclusions in permian marine halites from north america two formations of different ages and the central and eastern european basins eight formations of four different ages show that permian seawater shares chemical characteristics with modern seawater including so42 ca2 at the point of gypsum precipitation evolution into mg2 na k so42 cl brines and mg2 k ratios 5 permian seawater however is slightly depleted in so42 and enriched in ca2 although modeling results do not rule out ca2 concentrations close to those in present day seawater na and mg2 in permian seawater are close to slightly below their concentrations in modern seawater permian and modern seawater are both classified as aragonite seas with mg2 ca2 ratios 2 conditions favorable for precipitation of aragonite and magnesian calcite as ooids and cements the chemistry of permian seawater was modeled using the chemical composition of brine inclusions for three periods lower permian asselian sakmarian 296 283 ma lower permian artinskian kungurian 283 274 ma and upper permian tatarian 258 251 ma parallel changes in the chemistry of brine inclusions from equivalent age evaporites in north america central europe and eastern europe show that seawater underwent secular variations in chemistry over the 50 million years of the permian modeled so42 concentrations are 20 mmol per kg h2o mmolal and 19 mmolal in the asselian sakmarian and artinskian kungurian with higher concentrations in the upper permian tatarian 23 mmolal modeled ca2 is at or above its concentration in modern seawater throughout the permian mg2 is close to slightly below its concentration in modern seawater 55 mmolal in the asselian sakmarian 52 mmolal and tatarian 52 mmolal but slightly higher than modern seawater in the artinskian kungurian 60 mmolal mg2 ca2 ratios are 3 5 total range 2 7 to 5 5 in the lower permian and rose slightly to 3 7 total range 3 1 to 5 8 in the upper permian primarily due to decreases in ca2 these results are consistent with models that predict oscillations in the major ion composition of phanerozoic seawater on the basis of changes in the midocean ridge river water flux ratio driven by changes in the rate of midocean ridge crust production the permian was characterized by low sea levels icehouse conditions and southern hemisphere glaciation such conditions analogous to the present ice age and the similarities between permian seawater and modern seawater all suggest that general phanerozoic supercycles driven by mantle convection and global volcanicity also control the major ion chemistry of seawater

a note on photography in structural geology abstract when photographs of outcrops are required for detailed and or accurate structural work the photographic image should be parallel to the outerop surface a method has been developed which allows oblique photographs to be corrected when processed the method imposes certain restrictions on the combinations of camera and enlarger lenses which will yield a correctly proportioned print and graphs are presented which predict suitable combinations for a wide range of conditions the technique has been successfully applied to several problems one of which is presented as an illustrative example

morphotectonics and strain partitioning at the iberia africa plate boundary from multibeam and seismic reflection data abstract the gulf of cadiz off sw iberia and the nw moroccan margin straddles the cryptic plate boundary between africa and eurasia a region where the orogenic alpine compressive deformation in the continental collision zone passes laterally to the west to strike slip deformation a set of new multibeam bathymetry multi channel and single channel seismic data presented here image the main morphological features of tectonic origin of a significant part of the gulf of cadiz from the continental shelf to the abyssal plain these morphotectonic features are shown to result from the reactivation of deeply rooted faults that changed their kinematics from the early mesozoic rifting through the late cretaceous paleogene collision to the pliocene quaternary thrusting and wrenching the old faults control deep incised more than 100 km long canyons and valleys several effects of neotectonics on deep water seabed are shown these include i the complex morphology caused by wrenching on the 230 km long wnw ese faults that produced en echelon folds on the sediments ii the formation of up to 5 km wide crescent shaped scours at roughly 4 km water depth by reactivation of thrusts iii 10 km long creep folds on the continental slope and iv the formation of landslides on active fault escarpments the present day deformation is partitioned on ne sw thrusts and wnw ese to w e strike slip faults and is propagating northwards on n s trending thrusts along the west iberia margin from 35 5 n to 38 n which should be considered for seismic hazard

stratigraphic controls on eformation patterns in fault related folds a detachment fold example from the sierra madre oriental northeast mexico abstract deformation patterns are described by the style intensity and distribution of structures throughout a volume of rock in fault related folds presently observed deformation patterns are controlled by structural factors like kinematic history environmental factors like burial history and stratigraphic factors like layer thickness and competence to demonstrate the role of stratigraphy in controlling fold related deformation patterns we document the stratigraphic variability of mesostructures at one structural position on a map scale detachment fold unique assemblages of faults fractures and cleavages in certain stratigraphic intervals define a mechanical stratigraphy comprised of several units each exhibiting a characteristic deformation response to the folding process the varying response of each mechanical unit suggests that folding mechanisms such as orthogonal flexure flexural slip or flexural flow were sequentially or coevally operative in each of the units using the simple theoretical strain distributions associated with each folding mechanism and the fact that each mechanical unit accommodates fold related strain by developing a characteristic set of mesostructures we are able to predict general deformation patterns elsewhere in the fold the significance of this approach is that it recognizes that even in a single structure fold kinematics and deformation response may be dramatically different in various parts of a stratigraphic section because of this the quality and character of reservoir rocks in anticlinal traps may change markedly over short stratigraphic distances predictions of fold related deformation based solely on fold kinematics are unlikely to resolve these variations

the evolution of a thick shallow marine succession the south munster basin ireland the relatively fine grained late devonian to early carboniferous cork beds succession of the south munster basin includes continuous sections of paralic facies that are over 1000 m thick and individual sandstone units over 300 m thick however the succession does not reflect prolonged phases when facies belts were stationary but rather multiple stacking of small scale high frequency sequences each associated with pronounced migration of shorelines what seems to have been unusual about the south munster basin succession was that the geographical positioning of these high frequency sequences was fixed this resulted from an unusual combination of tectonics shelf hydrodynamics sedimentation rates and the textural maturity of the sediment within the basin of these tectonics was probably most critical particularly the juxtaposition of rapid subsidence in basinal areas and a basin margin zone to the north that was sufficiently up standing to pin the maximum extent of transgression during repeated highstands of sea level yet not so upstanding as to have diverted the major regional drainage system the embayed palaeogeography of the area may also have been influential

crustal seismic structure beneath portugal and southern galicia western iberia and the role of variscan inheritance abstract the crustal structure in western iberia is the result of a complex geodynamic history most of the surface is covered by rocks dating to the variscan orogeny the coastal ranges dominated by mesozoic structures and mesocenozoic basins covering partially the mainland in this study we present the results of a local earthquake tomographic study performed to image in depth this complex crustal structure down to 24 km depth the obtained tomograms present a good correlation with the surface geology in general with higher vp velocities in the north and lower vp velocities in the south the heterogeneity observed on the surface geology of the galicia tras os montes zone is well marked being a relatively thin layer over the smoother structure of the centro iberia zone ciz the ciz crustal block confined between the porto tomar ferreira do alentejo and the manteigas braganca faults having higher vp values enhancing the contrast to the lusitanian basin to the west the ossa morena zone corresponds to the unit presenting the greater heterogeneity in both vp and vp vs models showing also with a complex transition to the south portuguese zone and a relatively smooth transition to the centro iberian zone unexpectedly the south portuguese zone present an east west velocity variation with no apparent correspondence with the surface geology the transition to the coastal mesocenozoic basins is clearly marked the lower tagus valley corresponding to a 4 km thick low velocity region the relocation inland recorded seismicity in the period 2000 2014 allows cleansing some of the alignments and establish their correlation with some of the active structures in portugal the model features and seismicity pattern reveal the strong role played by the variscan heritage

a chronology of foreland deformation ultra violet laser 40ar 39ar dating of syn late orogenic intrusions from the variscides of southwest ireland abstract the upper palaeozoic sedimentary rocks of the munster and south munster basins southern ireland lie within the rhenohercynian zone of the european variscan orogeny this foreland region accommodated shortening during the asturian phase of variscan deformation at the end of the carboniferous by the development of kilometre scale and lower order folding high angle reverse faulting and regional fabric development at the southwestern extremity of the belt lies the black ball white ball heads area of the beara peninsula where high level igneous intrusions locally exhibit a close relationship with both the ductile and late brittle phases of variscan deformation 40 ar 39 ar ultra violet laser analysis of phlogopite crystals from these intrusions principally trachytic dykes has yielded ages that when combined with structural field relationships help to constrain the timing of variscan deformation in southern ireland these ages include 314 44 1 00 ma for a penetratively deformed lamprophric pipe on black ball head 301 98 1 47 298 08 0 61 ma for dyke material associated with later stage brittle deformation and a date of 296 88 0 60 ma for an undeformed post orogenic dyke on white ball head

conical fold terminations in the bannisdale slates of the english lake district abstract the structure of an area of silurian rocks in the non metamorphic caledonides near kendal cumbria u k is briefly described the main folds are periclinal and die away in the southeast of the area the uniform thickness of sandstone beds around folds shows that they deformed dominantly by buckling and only locally is flattening important a model is developed to explain the conical attitude of bedding within the periclinal terminations the model is erected on the basis of buckling alone and is developed to examine the effects of flattening inhomogeneity in buckling and flattening over the area is demonstrated using the model and discussed the pattern of folding is compared to that predicted by earlier published experimental work

crust mantle coupling in the gulf of cadiz sw iberia 1 in the gulf of cadiz and cape st vincent region at the sw iberian margin seismic catalogues report seismicity down to 100 km depth according to structural models for the region this includes earthquakes that occur in the uppermost continental mantle we invert regional waveforms to estimate moment tensors and centroid depths for moderate earthquakes obtaining eight solutions mw 3 8 to 5 3 between 6 and 60 km depth moment tensor solutions and previous source estimates show predominately reverse and strike slip faulting style with nnw sse oriented p axes close to plate convergence between africa and eurasia we obtain similar stress tensor estimates from crustal and upper mantle earthquakes both with sub horizontal nnw 1 axes this indicates analogous deformation in both domains and that earthquakes are unrelated to any subduction process

quantitative assessment of chemical and mineralogical changes due to progressive low temperature alteration of east pacific rise basalts from 0 to 9 ma many studies have focused on low temperature alteration of the oceanic crust nevertheless little is known about the initial processes of low temperature alteration and their influence on the mineralogical and chemical composition of the oceanic crust once it leaves the spreading axis the lack of such studies is primarily related to the difficulty of recovering samples representative of this initial alteration style such information is nevertheless important not only for crust ocean mass balances but also to characterize an important input to the subduction zones we have studied dredged basalts from the eastern flank of the east pacific rise at 14 15 s concentrating on the products of a single spreading segment in a corridor perpendicular to the spreading axis and covering a range of crustal ages from 0 to 9 ma electron microprobe x ray fluorescence x ray diffraction and icp ms analyses have been carried out to examine the mineralogical and chemical changes in the basalts which make up the surface of the upper crust caused by low temperature alteration fresh rocks were sampled at the ridge axis off axis basalts show features of progressive alteration celadonite is the main alteration component in 0 12 4 6 ma old rocks whereas phillipsite is more abundant in rocks older than 4 6 ma changing compositions of secondary minerals progressive sealing of fractures and the occurrence of more alteration rinds on older rocks show evidence for a slight change in redox conditions from an oxidizing water dominated to a more reducing rock dominated environment with time iron oxyhydroxide and celadonite are the first alteration products partly replaced or covered by saponite under more reducing conditions the fe necessary for the formation of these minerals is furnished by the dissolution of glass and the breakdown of olivine phillipsite is present in fractures and veins in rocks older than 1 ma analyses also indicate an illite smectite mixed layered mineral which is believed to be an intermediate between saponite and celadonite and small amounts of a chlorite smectite mixed layered mineral all samples are characterized by the lack of minerals formed by hydrothermal processes we conclude that the alteration took place under seawater dominated conditions at low temperature a comparison of trace element analyses from altered whole rock samples and their appendant fresh glass chips provides a record of element mobility during alteration off axis basalts show significant uptakes of rb cs and ba which are supplied by seawater and incorporated in or on secondary minerals an enrichment of u is also apparent and appears to be especially strong when alteration conditions are oxidative k2o is also gained in all altered off axis basalts believed to be linked to the formation of celadonite however the volume of rock being altered is so small relative to the volume of the oceans that this scavenging has no noticeable effect on the composition of the oceans the composition of the oceanic crust on the other hand is affected significantly

seismotectonics of portugal and its adjacent atlantic area abstract a study of focal mechanisms of earthquakes and stress indicators are presented for portugal and its adjacent atlantic margins recently riberio et al 1996 published a work on this subject in this study we introduce new focal mechanisms of nine earthquakes we use these results together with 12 other focal mechanisms and the main tectonic features in the region to define more accurately the type of mechanism and the stress pattern of the region all the focal mechanisms used in this study differ partially but are compatible with the regional stress field the stress indicators resulting from focal mechanisms and other geological and geophysical data show that the portugal continental and its atlantic margins are under horizontal pressure in nnw sse direction with a greater proportion of strike slip and reverse oblique mechanisms for the whole area

stress pattern in portugal mainland and the adjacent atlantic region west iberia the portuguese mainland territory is located close to the azores gibraltar plate boundary in a tectonic setting responsible for significant neotectonic and seismic activities however few data concerning the present regional lithospheric stress field were available as testified by recently published maps of stress indicators for the europe and mediterranean regions one of the authors already presented a synthesis on this subject cabral 1993 where geological and geophysical stress indicators were considered in this paper we introduce new information mainly a considerable amount of borehole breakout data the updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145 standard deviation 21 for the maximum horizontal stress direction shmax on the average the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth while the borehole elongation results are consistent with the mean shmax trend these differences in stress trend suggest a regional progressive rotation of the shmax direction from nnw sse to wnw ese since the upper pliocene to estimate stress trajectories new and published stress indicators in the adjacent atlantic area and northern africa were also investigated showing a very uniform nw se shmax trend in west iberia a high level of horizontal compressive stress acting oblique to the western portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model of activation of this passive margin with the nucleation of a subduction zone in the atlantic sw of iberia at the gorringe submarine bank which is propagating northward along the base of the continental slope at the transition between thinned and normal continental crust

morphotectonics in a low tectonic rate area analysis of the southern portuguese atlantic coastal region south portugal is characterized by low tectonic rates 0 3 mm a with infrequent large seismicity recent studies indicate a coastal region in southwest portugal uplifting at higher rates 0 11 0 01 mm a than the remaining southern portion of portugal 0 04 mm a however the mechanisms that drive this uplift are poorly understood with the purpose of investigating the regional quaternary deformation and its patterns as well as the difference in the uplift rate 77 exorheic drainage basins along 460 km of the southern portuguese coastline were analysed through the application of geomorphic indices in this study we applied stream channel sinuosity s basin relief ratio rh elongation re basin shape ratio bs valley height width ratio vf basin asymmetry factor af hypsometry hi and the stream length gradient index sl and we propose the terminal basin shape index tbs this study aims to 1 identify quaternary deformation along presumed tectonic structures 2 recognize uplift or subsidence along the coastline and 3 test the application of geomorphic indices in low deformation rate environments the cross correlation of results led to the recognition of the sao teotonio aljezur sinceira fault system and the sao marcos quarteira fault as major regional quaternary faults as well as to the interpretation of quaternary activity for other structures spatial differences in uplift rates are identified through basin shape indices and valley height width ratios even for low vertical motion rates whereas other indices were found to be not as sensitive to variations in uplift rate

the discovery of a new devonian tetrapod trackway in sw ireland a tetrapod trackway has been discovered in devonian sedimentary rocks of sw ireland it is the first discovery of its kind in europe details of the footprints are not preserved probably because of strong variscan pressure solution cleavage however after removing the effect of variscan deformation the large number of footprints more than 150 makes it possible to calculate the original dimensions of the footprint pattern stride approximately 34 cm and pace approximately 35 cm these values are higher than those for other traces of comparable age the total length of the tetrapod is estimated to be 1 m lithostratigraphical and biostratigraphical arguments indicate an age of mid to late devonian

strain and stress fields along the gibraltar orogenic arc constraints on active geodynamics abstract the gibraltar orogenic arc in the western mediterranean represents a complex region of active deformation related to the oblique nubia eurasia convergence process to increase the knowledge on the ongoing active processes in this region we have used the most up to date and comprehensive geodetic crustal motion and stress fields to this end we analyzed both continuous and campaign mode gps data collected between 1999 00 and 2011 00 across the area and compiled a multidisciplinary dataset of well constrained stress indicators to be compared with the geodetic results the main results highlight the oblique nature of the nubia eurasia convergence which provides the largest component of the observed stress pattern and is responsible for a significant strain rate field along the gibraltar orogenic arc we discuss our findings with respect to available geological seismological and geophysical data in order to verify their coherency compared to more relevant geodynamical models proposed in literature according to previous studies we confirmed how much of the secondary stress pattern can be related to the gravitational potential energy field which may also be responsible for some 2d stress strain rate angular discrepancies observed in large areas of the betics in addition taking into account the sub orthogonal azimuthal relationship between the shmax and ehmin directions and the fast polarization directions we conjectured a deep dynamic process controlling both the crustal stress field and the surface deformation on large areas of the orogenic arc finally although the models proposed to explain the geodynamic pattern of the gibraltar orogenic arc are supported by a discrete number of geological and geophysical observations it is only the back arc extension and westward rollback model that is able to adequately account for the vast majority of the observations based on our findings and other evidences we retain that this process could still be active beneath the gibraltar orogenic arc

tectono sedimentary ripples in devonian shallow marine siliciclastic sediments southern ireland abstract tectonically altered sedimentary ripples occur commonly in the marine devonian old head sandstone formation of southern ireland these tectono sedimentary ripples are dominantly wave produced and have sharp v shaped troughs where spaced pressure solution cleavage surfaces are located at a high angle to bedding the ripples control the spacing of cleavage and influence cleavage orientation the profile shape of the ripple is a function of stress refraction and concentration within the stiff sandstone layers of a multilayer sequence ripple symmetry is affected by tectonic deformation which plays an important role in the development of tectono sedimentary ripples tectonically flattened wave produced ripples occur where cleavage is oriented parallel to bedding due to an anomalous deformation in the old head sandstone formation at galley head selected ripple profiles have been undeformed by computer simulation to reconstruct ancient wave conditions this use of tectono sedimentary ripples for this purpose has its limitations and a number of guidelines are given for application of this technique elsewhere

structural evolution of the irish variscides the effects of the variscan orogeny on the upper palaeozoic succession are recognizable throughout central and southern ireland the deformation can be zoned using structural styles and trends which illustrate a northward decrease in deformation intensity coupled with increasing control by pre variscan structures in the basement the most intense deformation occurs to the south of the dingle dungarvan line where shortening approaches 50 as a result of layer parallel shortening followed by buckling and finally thrusting the deformation is considered to have developed above a shallow dipping sole thrust which has been located offshore by the swat seismic reflection profiles cross faults which are common throughout the orogen resulted from n s shear couples this is consistent with their origin as accommodation structures due to differential shortening along strike the original shape of the munster basin has exerted a control on the distribution and overall facing of the major structures the sole thrust extends to the north of the dingle dungarvan line and the structural zonation of the orogen is due to variations in the stratigraphic level of this thrust the leinster massif is considered to be essentially autochthonous slip on the sole thrust decays towards the massif which acted as a rigid passive block and was partially responsible for the arcuate trend of the orogen

on the secular variations in the composition of phanerozoic marine potash evaporites in a recent paper l a hardie proposed that the secular changes in the mineralogy of marine nonskeletal limestones and in the mineralogy of marine potash evaporites during the phanerozoic are the result of changes in the composition of seawater caused primarily by fluctuations in the flux of seawater through mid ocean ridges he suggested that even quite small variations 25 in this flux have a profound effect on the composition of seawater we show that the effects of changes in the hydrothermal flux are much smaller than he proposed nevertheless a doubling of the hydrothermal flux could probably alter the composition of seawater sufficiently to affect the mineralogy of marine potash evaporites an alternative explanation for the variations in the mineralogy of these deposits invokes differences in the degree of dolomitization during the evaporation of seawater the apparent near constancy of the k sup content of seawater during the phanerozoic supports this hypothesis it seems likely that during periods of rapid seafloor spreading the rate of seawater cycling is greater than today that sea level is higher that very large carbonate platforms are more abundant and that changes in seawater chemistry together with increases in dolomitization during the evaporation of seawatermore lead to mgso sub 4 free potash deposits the relative importance of these several effects can probably be determined with certainty only when we have analyses of relatively unaltered seawater from the several phanerozoic periods 27 refs 1 fig 1 tab less

imaging active faulting in a region of distributed deformation from the joint clustering of focal mechanisms and hypocentres application to the azores western mediterranean region abstract the matching between linear trends of hypocentres and fault planes indicated by focal mechanisms fms is frequently used to infer the location and geometry of active faults this practice works well in regions of fast lithospheric deformation where earthquake patterns are clear and major structures accommodate the bulk of deformation but typically fails in regions of slow and distributed deformation we present a new joint fm and hypocentre cluster algorithm that is able to detect systematically the consistency between hypocentre lineations and fms even in regions of distributed deformation we apply the method to the azores western mediterranean region with particular emphasis on western iberia the analysis relies on a compilation of hypocentres and fms taken from regional and global earthquake catalogues academic theses and technical reports complemented by new fms for western iberia the joint clustering algorithm images both well known and new seismo tectonic features the azores triple junction is characterised by fms with vertical pressure p axes in good agreement with the divergent setting and the iberian domain is characterised by nw se oriented p axes indicating a response of the lithosphere to the ongoing oblique convergence between nubia and eurasia several earthquakes remain unclustered in the western mediterranean domain which may indicate a response to local stresses the major regions of consistent faulting that we identify are the mid atlantic ridge the terceira rift the trans alboran shear zone and the north coast of algeria in addition other smaller earthquake clusters present a good match between epicentre lineations and fm fault planes these clusters may signal single active faults or wide zones of distributed but consistent faulting mainland portugal is dominated by strike slip earthquakes with fault planes coincident with the predominant nne ssw and wnw ese oriented earthquake lineations clusters offshore sw iberia are predominantly strike slip or reverse confirming previous suggestions of slip partitioning

orogen parallel extension and oblique tectonics the relation between stretching lineations and relative plate motions many collisional orogens exhibit two distinct deformation phases the first and relatively early phase forms within an obliquely subducting continent and is characterized by regionally consistent stretching lineations subparallel to the orogen and to associated isoclinal and sheath folds second phase folds and stretching lineations develop during and after imbrication of the subducting plate into the overriding plate and they become progressively disharmonic with stretching lineations oriented perpendicular to the orogen structural morphology and stretching lineations indicate that a fundamental kinematic transition occurs as material is imbricated from footwall to hanging wall kinematic modeling based on modern oblique convergent boundaries and considerations of buoyancy forces suggest that only the relatively early structures reflect the relative plate motion but that the correspondence is not simple in particular the orientation of stretching lineations is a function of finite strain kinematics of deformation and angle of subduction and convergence and it will consistently underestimate the angle of oblique convergence second phase structures reflect isostatic emergence of the hanging wall and are independent of relative plate convergence examples are drawn from the south central canadian rockies the western alps the variscides of ireland and france and the himalaya

the palaeoenvironment and deformation of beaconites like burrows in the old red sandstone at gortnabinna sw ireland the gortnabinna area contains a number of well preserved deformed burrows within the fluviatile old red sandstone of s ireland detailed logging of the local succession enables subdivision into four facies facies 1 represents sheet flood deposits facies 2 ephemeral stream channels facies 3 marginal sheet flood deposits and facies 4 low energy settling of fines the burrows occur in facies 1 and 4 the sediments are now folded into a large upright westward plunging anticline the fold has a well developed axial planar cleavage and is extended along strike by listric normal faults the burrows in facies 1 were originally circular and orthogonal to bedding and reveal a n s layer parallel shortening strain of 2 1 followed by a shear strain of 1 48 during folding these burrows have a bimodal distribution of diameters with peaks in the ranges 50 80 and 110 170 mm the burrows of facies 4 are sub parallel to the bedding plane and are generally curved with a maximum width of 180 mm and lengths of up to 2 3 m these burrows display passive infill following abandonment the burrows are morphologically similar though distinct from beaconites barratti which has been described from elsewhere in the old red sandstone and are tentatively interpreted as lungfish burrows

a geophysical study of the carcavai fault zone portugal the algarve province is located a few hundred kilometres north of the crossing of the e w eurasia africa plate boundary in an area of diffuse seismicity and broad deformation it is characterised by a moderate seismicity with some important historical and instrumental earthquakes causing loss of lives and significant material damages the area is affected not only by plate boundary earthquakes but also by moderate to large events generated by local sources the assessment of onshore local sources is therefore of vital importance for an evaluation of the regional seismic hazard this paper discusses the application of geophysical data to the study of the carcavai fault zone an outcropping structure more than 20 km long which is seen to deform sediments of the plio quaternary age the location of some sectors of the fault zone as well as the vertical offsets of the structure are still to be confirmed in order to estimate these and to study the geometry of the fault zone at depth geophysical data were acquired together with new geological data where the location of the fault was less certain em and seismic reflection profiles with coarse spatial sampling were carried out after the detailed location of the fault zone seismic reflection profiles with a more dense spatial resolution were acquired the integrated interpretation of the geological and geophysical data confirmed the presence of a large fault zone the total fault length is still unknown as its extension offshore is still being studied together with estimated values of the throw obtained this data set has improved understanding the seismic hazard in the area by providing more refined estimates of co seismic rupture maximum expected earthquake and return periods

flexural shear in a periclinal fold from the irish variscides abstract periclinal folds occur in a structural culmination in the centre of the irish variscan fold belt the culmination marks a zone of enhanced shortening which is associated with a locally attenuated stratigraphy and an anomalous deformation sequence with respect to the remainder of the orogen originally vertical burrows have been used along with cleavage fold relations to study both the three dimensional evolution of the periclinal folding and the temporal relationship between folding and cleavage as well as cleavage forming flattening strains the burrows record along strike shear related to the propagation of the periclinal folds a comparison with strain measurements made outside the culmination zone shows that the galley head area suffered higher strains integration of field data with published analyses of plasticine modelling of periclinal folds yields models for the propagation of a pericline and for the geometry of its lateral terminations

simulations of strong ground motion in sw iberia for the 1969 february 28 ms 8 0 and the 1755 november 1 m 8 5 earthquakes ii strong ground motion simulations summary this is the second paper of a series of two concerning strong ground motion in sw iberia due to earthquakes originating from the adjacent atlantic area the aim of this paper is to use the velocity model that was proposed and validated in the companion paper for seismic intensity modelling of the 1969 m s 8 0 and 1755 m 8 5 8 7 earthquakes first we propose a regression to convert simulated values of peak ground velocity pgv into modified mercalli intensity mmi in sw iberia and using this regression we build synthetic isoseismal maps for a large m s 8 0 earthquake that occurred in 1969 based on information on the seismic source provided by various authors we show that the velocity model effectively reproduces macroseismic observations in the whole region we also confirm that seismic intensity distribution is very sensitive to a small number of source parameters rupture directivity fault strike and fault dimensions then we extrapolate the method to the case of the great m 8 5 8 7 1755 earthquake for a series of hypotheses recently proposed by three authors about the location of the epicentral region the model involving a subduction

connecting onshore structures in the algarve with the southern portuguese continental margin the carcavai fault zone abstract the algarve is located a few hundred kilometres north of the crossing of the e w eurasia africa plate boundary and is characterised by a moderate seismicity with some important historical and instrumental earthquakes causing loss of lives and significant material damages the area is affected not only by plate boundary generated earthquakes but also by local events capable of generating moderate to large earthquakes the assessment of onshore local sources and its connections with the plate border is therefore of vital importance for an evaluation of the regional seismic hazard this paper discusses the application of geophysical data to study a large fault zone which is the offshore prolonging of the carcavai fault zone cf an onshore outcropping structure more than 20 km long which is seen to deform sediments of plio quaternary age offshore and onshore aeromagnetic data offshore gravimetric and seismic reflection data shows the existence of a long over 200 km wsw ene trending fault zone affecting the palaeozoic basement with a normal geometry which is probably segmented by nnw sse to n s faults seismic data shows that this fault zone has been reactivated as a left lateral strike slip fault and inverted in the cenozoic with the upthrust of the northwestern block in agreement with the onshore cf characteristics recent work carried out onshore and offshore near the coastline that shows deformation of plio quaternary sediments suggests that this is an active fault some of the faults segments have instrumental seismicity associated though faults very rarely rupture along its entire length several fault segments have a length of about 30 km and may produce an earthquake of magnitude about7 the proximity of the onshore segment to the city of faro and of the offshore segments to the main population centres of the algarve makes it a serious threat to the algarve

model of seawater composition for the phanerozoic we present an inverse model of phanerozoic seawater com position calibrated against updated paleoseawater compositions from fluid inclusions in marine halites the model considers step wise alteration of seawater composition via 1 variable input of river water 2 variable rates of alteration of seawater through reactions at mid ocean ridges and 3 variable rates of alteration of seawater through reactions on ridge flanks and across the ocean floor in general the model achieves agreement with paleoseawater fluid inclusion data for na 1 c a 2 1 s o 4 2 2 and k 1 particularly when variable runoff is considered variable rates of basalt seawater interactions at both ridges and ridge flanks are required to understand the evolution of seawater particularly the observed near constant concentration of k 1 through time

a comparison of sanderson s and panozzo s strain measurement methods using calcite grain boundaries from the variscan fold and thrust belt in ireland abstract two methods of determining strain from the two dimensional preferred orientation of lines by sanderson and panozzo are compared using grain boundaries in limestones from s ireland the panozzo method utilizes the orientations and lengths of lines whereas the sanderson method uses only the orientations the panozzo method is shown to be more sensitive to inhomogeneous deformation or dubious data than the simpler method of sanderson

cambrian sea water preserved as inclusions in marine low magnesium calcite cement the existence of temporal changes in the chemical composition of the oceans which could provide constraints on the potential variability of the ocean atmosphere system remains an open question assessments of the chemistry of ancient oceans have relied largely on analysis of marine precipitates generally carbonate and evaporite minerals1 these studies suggest that whereas marine salinity has remained relatively stable over phanerozoic time1 magnesium calcium and sulphate concentrations of ancient oceans and co2 partial pressure of ancient atmospheres may have changed2 4 the ratios of isotopes of carbon oxygen sulphur and strontium also appear to have varied3 5 6 here we present analyses of primary one phase fluid inclusions in cambrian and ordovician marine cements which appear to represent aliquots of early palaeozoic oceans the cements have trace element stable isotope and strontium isotope contents that are consistent with their having been precipitated in a cambrian ordovician marine environment and the fluids have marine salinities as these apparently primary cements are low magnesium calcite unlike the predominantly high magnesium calcite and aragonite of today s carbonate precipitates the chemistry of the cambrian ocean atmosphere system seems to have been different from that of today

a model for atmospheric co 2 over phanerozoic time a new model has been constructed for calculating the level of atmospheric co sub 2 over phanerozoic time which is much simpler mathematically than the blag model but more complex geologically and biologically results suggest that there has been a notable pattern of varying atmospheric co sub 2 level over the past 570 my with high levels during the mesozoic and early paleozoic and low levels during the permo carboniferous and late cenozoic sensitivity analysis shows that within reasonable limits of the principal governing parameters this qualitative trend of varying co sub 2 is relatively insensitive to the actual values chosen for these parameters causes of the co sub 2 variation are multiple and no single geological or biological process can be called upon to explain all co sub 2 variation for all time the calculated trend of co sub 2 over time agrees well with independent estimations of paleoclimates thus the greenhouse theory of paleoclimate on a long geological time scale is supported by the results of the present study

secular oscillations in the carbonate mineralogy of reef building and sediment producing organisms driven by tectonically forced shifts in seawater chemistry the primary mineralogy of oolites and early marine carbonate cements led sandberg nature 305 1983 19 22 to divide the phanerozoic eon into three intervals of aragonite seas and two intervals of calcite seas hardie geology 24 1996 279 283 has shown that these oscillations together with synchronous oscillations in the mineralogy of marine potash evaporites can be explained by secular shifts in the mg ca ratio of seawater driven by changes in spreading rates along mid ocean ridges the hardie model also predicts that high mg calcite should precipitate along with aragonite as it does in today s aragonite sea we have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms ones that have produced massive skeletons large reefs or voluminous bodies of sediment that correspond to sandberg s aragonite seas and calcite seas and that are predicted by the hardie model particular groups of corals sponges and algae appear to have been dominant reef builders only when favored by an appropriate mg ca ratio in seawater in early and middle paleozoic calcite seas calcite i reefs were dominated by calcitic tabulate heliolitid and rugose corals and calcitic stromatoporoids in contrast during the period of late paleozoic early mesozoic aragonite seas aragonite ii aragonitic groups of sponges scleractinian corals and phylloid algae as well as high mg calcitic red algae were principal reef builders during late cretaceous time at the acme of calcite ii massive rudists displaced aragonitic hermatypic corals in today s aragonite sea aragonite iii scleractinian corals are again dominant reef builders along with high mg calcitic coralline algae major sediment producing algae exhibit temporal patterns similar to those of reef builders calcitic receptaculitids flourished during calcite i whereas aragonitic dasycladaceans did not become dominant rock formers until aragonite ii during calcite ii calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the late cretaceous after the mg ca ratio of seawater had reached a very low value and calcium concentration a very high value as the mg ca ratio of seawater rose and calcium concentration fell during the cenozoic era individual coccoliths on average became less massive and encrusted cells less thickly by pliocene time during aragonite iii the prominent genus discoaster secreted only narrow rayed coccoliths that covered less than 25 of the cell surface also during aragonite iii the aragonitic green alga halimeda emerged as the dominant skeletal sediment producer in reef tracts the influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification such groups include algae sponges corals and bryozoans morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs this linkage in addition to the basic chemical demands of hypercalcification has given the mg ca ratio of seawater strong control over the success of individual reef building taxa more generally this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history we conclude that throughout phanerozoic time a chain of causation has extended from mid ocean ridge processes via seawater chemistry to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits

evaluating seawater chemistry from fluid inclusions in halite examples from modern marine and nonmarine environments fluid inclusions from marine halites have long been studied to determine the chemical composition of ancient seawater chemical analyses of the major ions in fluid inclusions in halites from the solar saltwork of great inagua island bahamas and from the supratidal sabkha baja california mexico show that modern marine halites faithfully record the chemical signature of seawater the major ions in great inagua and baja california fluid inclusions display distinctive linear trends when plotted against one another ie na k and so42 vs mg2 and cl which track the evaporation path of seawater as it evolved during the crystallization of halite these evaporation paths defined for the major ions by fluid inclusions in halite overlap findings of computer simulations of the evaporation of modern seawater by the harvie moller and weare hmw computer program the close match between the hmw seawater evaporation paths and the great inagua fluid inclusion data is not surprising considering the carefully controlled inflow evaporation and discharge of seawater at the great inagua saltwork the major ion chemistry of fluid inclusions from the baja california halites matches the hmw seawater evaporation paths in most respects but one baja fluid inclusion has lower concentrations of mg2 than evaporated seawater nonmarine inflows and syndepositional recycling of preexisting salts in the baja california supratidal setting were not large enough to override the chemical signature of evaporating seawater as the primary control on the baja fluid inclusion compositions fluid inclusions in halites from the nonmarine qaidam basin qinghai province western china have a distinctly different major ion chemical signature than does global seawater the fluid inclusion chemistries from the qaidam basin halites do not lie on the evaporation pathways defined by modern seawater and can clearly be differentiated from fluid inclusions containing evaporated seawater if fluid inclusions in halites from modern natural settings contain unmistakable samples of evaporated seawater then evaluation of the chemistry of ancient seawater by chemical analysis of fluid inclusions in ancient marine halites by means of the same approach should be valid

the global water cycle and continental erosion during phanerozoic time 570 my asensitivity study is conducted to determine what effect two mechanisms namely continental size and latitudinal location might have on the global wtaer cycle the global water cycle over phanerozoic time 570 my is established first by analyzing the distribution of rainfall p evaporation e and runoff d by bands of 10 of latitude on the present day earth and then applying these values to the ancient phanerozoic world in this backward projection the global cosmic climatic and orographic conditions prevailing today are supposed constant through time while the only variable parameter controlling the system are the latitudinal positions of the continents and the relative area of continents and oceans the calculated phanerozoic global runoff clearly shows that the humid periods are the cretaceous 100 my the devonian silurian 400 my and the cambrian 500 my while the dry periods are close to the present day 0 my and the permo triassic 200 my the continal global runoff together with the survival rate of the volcanics are used to predict the fluctuations of the isotopic ratio 87sr 86sr in marine sediments predicted and measured values appear to be in good agreement except for the present which somewhat validate the calculations the continental runoff is also related to the sedimentation rate and used to estimate the specific erosion rate of the continents through the phanerozoic thus the sedimemtation rate variations are understood as a consequence of the continental runoff fluctuations through time

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

micro seismicity in the gulf of cadiz is there a link between micro seismicity high magnitude earthquakes and active faults abstract the gulf of cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes occasionally punctuated by high magnitude events such as the m 8 7 1755 great lisbon earthquake and the m 7 9 event of february 28th 1969 micro seismicity was recorded during 11 months by a temporary network of 25 ocean bottom seismometers obss in an area of high seismic activity encompassing the potential source areas of the mentioned large magnitude earthquakes we combined micro seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the gulf of cadiz area three controlling mechanisms are explored i active tectonic structures ii transitions between different lithospheric domains and inherited mesozoic structures and iii fault weakening mechanisms our results show that micro seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric rheological domains even though the crustal structure is well imaged in the seismic profiles and in the bathymetry crustal faults show low to negligible seismic activity a possible explanation for this is that the crustal thrusts are thin skinned structures rooting in relatively shallow sub horizontal decollements associated with aseismic serpentinization levels at the top of the lithospheric mantle therefore co seismic slip along crustal thrusts may only occur during large magnitude events while for most of the inter seismic cycle these thrusts remain locked or slip aseismically we further speculate that high magnitude earthquake s ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers

geoalgar project first results on seismicity and fault plane solutions the project geoalgar initiated in may 2000 is devoted to the geodynamic monitoring and seismic characterization of the algarve region a brief description of the project goals as well as the first results concerning the analysis of the recent seismic digital data from 1999 and 2000 are presented after simultaneous inversion of the seismic data and the velocity model parameters the relocation of the hypocenters was performed for two selected areas twenty five earthquakes were used for area i and 125 earthquakes for area 2 selected from the period 1999 2000 and with magnitude ml 2 0 the results show that there are two main regions where there are more events the monchique region inland and another one in the area of guadalquivir bank ranging from 36 4 n 08 w to 36 8 n 7 2 w for area 1 the hypocentral corrections are relatively small with the focus slightly deeper than that in the old solution for the more regional events the hypocenters corrections are bigger with the focus becoming more shallow

a theory of finite strain variation through contrasting layers and its bearing on cleavage refraction abstract a theory of finite strain variation in contrasting viscous layers is presented the theory is applicable to layers which are oblique to two principal strains but parallel to the third and is not restricted to plane strain results may be obtained algebraically or by use of the mohr diagram for strain solutions are given for various examples of layering attitude and viscosity ratio measured with respect to a reference layer it is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers and in some cases the principal axes may be exchanged low strain is indicated in relatively more viscous layers and high strain with extension at a smaller angle to the layering in less viscous layers the latter may approximate simple shear parallel to layering if the strain is sufficiently high the geological implications of the theory are strong strain variations in layered rock sequences it is suggested that in general strain will be inhomogeneous from layer to layer both in orientation and amount the relationship of strain and cleavage are reviewed in the introduction to establish the validity of predicting cleavage patterns from strain data qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the xy planes of strain with this assumption some three dimensional features of planar and linear fabric refraction in contrasting rocks are predicted

analytical method for estimating palaeowave climate and water depth from wave ripple marks analytical expressions are developed for evaluating palaeodepth and palaeowave conditions from wave ripple marks palaeodepth and palaeowave climate estimates can be extremely powerful predictors in reconstructions of vertical movements related to basin subsidence local and regional ancient wave climate trends and the basin shape the analytical expressions developed herein are applied to wave ripple marks preserved in the lower marine molasse of switzerland western austria and southern germany the results suggest that in contrast to traditional concepts of an eustatic sea level change the lower marine molasse regression was a result of mainly depositional processes the subsidence rate greatly exceeded the rate of sea level fall over a wide geographical area furthermore the widening of the lower marine molasse basin to the ene is well documented by the increase of the ancient wave period and wave power calculated from wave ripple marks found in storm deposits

increased production of calcite and slower growth for the major sediment producing alga halimeda as the mg ca ratio of seawater is lowered to a calcite sea level abstract laboratory experiments were conducted to subject the calcareous alga halimeda a prolific producer of aragonite in the modern ocean to a variety of ambient mg ca ratios these experiments were based on three predictions 1 that halimeda would calcify at a lower rate when the ambient mg ca ratio was reduced from its high level in modern seawater 5 2 2 that this alga would grow more slowly under these conditions because its calcification yields as a byproduct co2 which is available for photosynthesis and 3 that the calcite content of halimeda s skeleton would increase as a percentage of total caco3 as predicted reduction of the mg ca ratio of ambient artificial seawater from its modern level to estimated early oligocene and early eocene levels 2 5 and 1 5 respectively resulted in progressively lower rates of calcification production of organic matter and linear growth for halimeda incrassata colonies the ambient mg ca ratio exerted a stronger influence over the rate of calcification than ca2 which in the geologic past has risen in seawater whenever the ambient mg ca ratio has declined thus although elevation of ca2 at such times would have promoted the calcification and growth of halimeda the concomitant decline of the mg ca ratio would have had a stronger negative effect in addition the percentage of skeletal material consisting of calcite rose from 8 in modern day seawater to 16 in artificial early oligocene seawater and 46 in artificial early eocene seawater magnesium was incorporated into this calcite as a function of the ambient mg ca ratio with a fractionation pattern differing only slightly from that of nonskeletal calcite halimeda exerts less biological control over its biomineralization than the related genera penicillus and udotea which in previous experiments produced only 22 25 of skeletal caco3 as calcite in seawater having a very low mg ca ratio 1 0 nonetheless it is likely that many taxa of green algae that produce little or no calcite today would have produced a substantial proportion of skeletal low mg caco3 as calcite in ancient seas such as those of the cretaceous whose mg ca ratios were close to unity

geological evolution of seawater boron isotopic composition recorded in evaporites the abundance of boron isotopes in ancient marine carbonates can be used to estimate oceanic ph that reflects atmospheric co 2 levels this proxy requires that the boron isotopic composition of seawater at the time the carbonate has formed is known and thus the past changes in seawater chemistry here we report the boron isotopic composition of selected ancient marine halites and modern sea salts the signal interpreted as marine reveals a clear evolution of the boron isotopic composition of seawater to 8 variations over the cenozoic comparison between this reconstructed curve and the mg ca ratio reveals a high level of consistency that will help to better define oceanic geochemical cycles

hydrothermal circulation through mid ocean ridge flanks fluxes of heat and magnesium thermally driven convection of seawater occurs through oceanic crust of all ages at the seafloor spreading axis on mid ocean ridge flanks and in the ocean basins at the ridge axis and on the flanks circulating seawater produces a major chemical exchange between the oceans and the crust based on heat budget constraints and the composition of hot springs between 10 and 40 of the river flux of mg can be taken up during high temperature alteration of the basaltic crust along the ridge axis most of the hydrothermal heat loss however occurs on the mid ocean ridge flanks where the temperatures are lower and the seawater flux correspondingly larger the estimated heat loss on the flanks is so large that upwelling must occur over a large fraction 5 30 of the seafloor less than 65 ma in age if temperatures are 20 c and seepage velocities are on the order of 10 to 100 cm y the circulating seawater needs to lose on average less than 1 2 of its mg content in order to solve the mg mass balance for the oceans chemical fluxes through mid ocean ridge flanks are poorly known because of the wide range of crustal conditions that prevail there and the paucity of study to date the most critical parameter for characterizing crustal conditions is temperature in basement which is a function of crustal age basement topography and sediment thickness and permeability basement temperature which can usually be inferred from heat flow and seismic reflection surveys largely determines the change in the composition of seawater circulating through basement this change can be inferred in turn from profiles of sediment porewater chemistry all sites studied to date with temperatures at the sediment basement interface 25 c have a large component of advective heat loss and show only a small 80 mg loss both types of sites may be important for chemical fluxes whether the cooler sites are important depends on how much the seawater changes in composition as it circulates through the crust only small changes are needed whether the warmer sites are important depends on how much heat is lost by advection in this type of setting the warmer sites could produce significant chemical fluxes even if they are scarce they could account for the entire river input of mg to the oceans even if they represent only 8 20 of the total advective heat loss on ridge flanks

ambient seismic noise tomography of sw iberia integrating seafloor and land based data we used ambient seismic noise recorded by 24 broadband ocean bottom seismometers obs deployed in the gulf of cadiz during the ec funded nearest project and seven broadband land stations located in the south of portugal to image the sedimentary and crustal structure beneath the eastern atlantic and sw iberia we computed ambient noise cross correlations to obtain empirical green s functions egfs between all station pairs using land seismometers and both obs sensors seismometers and hydrophones despite the great difference in the recording conditions and local crustal structure between the obss and land stations we could compute egfs by applying a linear cross correlation with running absolute mean average time normalization followed by a time frequency phase weighted stack dispersion analysis was then applied to the egfs between 4 and 20s period the obtained dispersion curves allowed mapping the lateral variation of rayleigh wave group velocities as a function of period finally dispersion curves extracted from each cell of the 2d group velocity maps were inverted as a function of depth to obtain the 3d distribution of the shear wave velocities the 3 d shear wave velocity model computed from joint inversion of obss and land stations data allowed to estimate the thickness of sediments and crust and the moho depth despite the gap that exists between the obss and land station locations our model displays a good correlation with the known geological structure the derived sedimentary layer and crustal thicknesses and the obtained moho depth are locally in agreement with the models proposed by other studies using near vertical refraction and wide angle seismic profiling we conclude that ambient noise tomography could be a valuable tool to image oceanic domains and also that it is possible to integrate seafloor and land based stations to derive a structure model in the transition domain between continent and ocean

major cation gradients in dsdp interstitial waters the role of diffusive exchange between seawater and upper oceanic crust abstract interstitial water analyses from dsdp legs 1 53 were surveyed to identify sites at which the dominant process supporting gradients of major seawater cations is alteration of the underlying oceanic crust sites with thin sediment cover are generally without gradients sites with appreciable sediment accumulation are generally with gradients the transition is probably the result of cessation of convective circulation within the sediments a model is developed for the diffusive flux through an accumulating unreactive sedimentary layer supported by continued reaction within the crust the model is consistent with the observations provided that convection within the crust extends to depths 1 km and that convective flow within the sediments ends once 100 150 m of sediment accumulates the model suggests a steady state gradient inversely proportional to the sedimentation rate this dependence is apparent in the data however some variation in the reaction rate is also required fluxes of mg and na are systematically related to the flux of ca which because of its mobility serves as a tracer of hydrolysis of the basalt when the ca flux is low the mg flux serves as the principal sink for alkalinity produced during basalt hydrolysis when the ca flux is high the mg flux becomes diffusion limited and nabearing alteration phases must form the fluxes resulting from these alteration reactions exert little influence on either the chemistry of seawater or that of the upper crust

zebra and boxwork fabrics in hydrothermal dolomites of northern canada indicators for dilational fracturing dissolution or in situ replacement zebra and boxwork fabrics in manetoe and presqu ile hydrothermal dolomites provide critical data for discrimination between hypotheses for their origin in a unique occurrence sets of weakly imbricated decimetre sized curvilinear lenticles of white dolospar are developed within sub metre sized tongues of greyish white dolomitized crinoidal packstone and grainstone this nascent zebra fabric has developed along wavy to nodular intra bed sedimentary partings primary bedding can be traced through dolomitized masses containing these fabrics into the surrounding undolomitized limestone all zebra and boxwork fabrics are confined within single beds consistent with an early burial time of origin no change in bed thickness occurs across zebra or boxwork fabrics within beds this indicates a dissolutional rather than a dilational origin for the creation of pore space partly cemented with white dolospar the dolomitized groundmass displays an abruptly gradational or sharp transition to centripetal saddle dolomite cement which partially to nearly completely occludes vugs circulation of geothermally heated hypersaline devonian elk point basinal sea water brines led to dissolution and replacive dolomitization of limestone adjacent to vugs and near contemporaneous precipitation of white dolospar within dissolutional vugs consistent with geochemical simulations the presence of downward extending galleries of white dolospar cemented solution collapse breccia provides further support for an interpretation of regional thermally driven convection of hydrothermal evaporative sea water brines across a broad area of northern canada the absence of gases under pressure in vacuoles within fluid inclusions from manetoe and presqu ile dolospars is also more consistent with a dissolutional and contemporaneous dolomitization origin for these fabrics rather than an origin involving dilational fracturing for space creation and dolospar precipitation the ubiquitous presence of zebra and boxwork fabrics in hydrothermal dolomite reservoirs indicates that they are not confined fault zones and instead occur wherever precursor limestones had relatively greater porosity and permeability prior to dolomitization

neogene to recent contraction and basin inversion along the nubia iberia boundary in sw iberia the sw of iberia is currently undergoing compression related to the convergence between nubia and iberia multiple compressive structures and their related seismic activity have been documented along the diffuse nubia iberia plate boundary including the gorringe bank west of the gulf of cadiz and the betic rif orogen to the east despite seismic activity indicating a dominant compressive stress along the algarve margin in the gulf of cadiz the structures at the origin of this seismicity remain elusive this paper documents the contractional structures that provide linkage across the gulf of cadiz and play a major role in defining the present day seismicity and bathymetry of this area the structures described in this paper caused the neogene inversion of the jurassic oblique passive margin that formed between the central atlantic and the ligurian tethys this example of a partially inverted margin provides insights into the factors that condition the inversion of passive margins

cleavage geometry and the development of the church bay anticline co cork ireland abstract the church bay anticline is one of a series of e w trending hercyman folds in southern ireland the fold has a crudely axial planar cleavage which was initiated early in the buckling history the cleavage was deformed subsequently by a number of discrete bedding parallel shear zones as the fold continued to amplify this has resulted in a characteristically sigmoidal cleavage geometry in the fold profile plane the cleavage also displays kink bands and sharp angular changes across bedding planes in the fold hinge as a result of shearing within these sheared zones the cleavage is intensified indicating that conditions for cleavage formation were maintained throughout the development of the fold no new cleavage has formed despite the rotation of the cleavage during the deformation history