the bristol mendip foreland thrust belt the thrusts and associated fold structures of the bristol mendip area are interpreted in terms of a thin skinned piggy back foreland thrust belt of variscan age with dominant northward transport strike normal balanced sections and compatible strike parallel sections have been constructed to illustrate the 3 d geometry of the linked thrust system north south shortening of c 20 km 40 is calculated from deformed and restored strike normal sections a thick and extensive thrust wedge of pre old red sandstone basement in the west of the area is responsible for culminations expressed at the surface as the mendip periclines uplifted culminations permit erosion to lower stratigraphic levels in the west and south of the area strike parallel sections and a hanging wall cut off map indicate that the majority of thrusts in the area are mainly oblique ramps rather than simple frontal and lateral ramps or flats the en echelon pattern of the mendip periclines may be related to the oblique ne climb of two of these thrusts the angle of frontal ramp climb through the dinantian stratigraphy is generally low and this explains the presence of numerous klippen of lower on upper carboniferous rocks whilst suggesting that certain thrust sheets were much more extensive than present outcrops indicate early pre thrusting movement on basement lineaments in the variscan foreland created uplifts angular unconformities and a predominance of n s structures in the north of the area

reactivation of relay ramps and structural evolution of the mere fault and wardour monocline northern wessex basin new geological field mapping along a 24 km long portion of the mere fault in the northern part of the wessex basin together with seismic reflection and other subsurface data allow an analysis of displacement both along the length and down the dip of the reactivated fault the principal segments of the mere fault dip south at about 70 and display components of both syndepositional normal displacement and later reversal of movement during basin contraction minimum estimates of the largest down to the south displacements range from less than 100 m at the surface to 350 m at the top of the pre permian basement and these values decrease to zero toward the fault segment tips estimates that allow for reverse movement along the fault suggest that there must have been at least 500 m of normal displacement along the central portion of the segment stratigraphical separation at the surface indicates that the largest down to the north displacements associated with later fault reversal are at least 200 m and occur in the east where reversal of movement has taken place on an early high angle fault segment in the west the principal fault strands are eroded to deeper stratigraphical levels where largely normal slip is preserved and segments are linked by normal and oblique transfer faults the wardour monocline was developed during basin contraction in part by movement along a concealed fault segment overstepping from the mere fault at the surface and in part over a relay ramp between the two fault segments

the modification of chevron folds by simple shear examples from north cornwall and devon this contribution considers the effect of a horizontal simple shear deformation on an initially upright chevron fold pair using geometrical models the models utilize an initial fold geometry based on structures observed in the coastal sections of north cornwall and devon and consider various mechanisms by which the simple shear could be accommodated due to their different initial orientations the limbs of the chevron folds rotate at different rates during simple shear and this is accommodated by the modification of the fold structure possible modification structures include normal faulting in the opposite direction to the sense of shear on either the axial planes or the limbs of the folds and folding on the limb which dips in the same general direction as the sense of shear the results of the modelling can be used to interpret the structures observed in the studied sections it is shown that a southerly directed simple shear deformation acted in discrete zones resulting in the local modification of certain chevron folds this deformation is in the opposite direction to the main variscan movements in sw england nevertheless it is argued that the simple shear is an integral component of the main deformation and represents an analogous process to the mechanism of backthrusting present in most thrust belts

deep structure of the east mendip hills from gravity aeromagnetic and seismic reflection data beacon hill pericline is the most easterly pericline of the mendip hills whose folds and thrusts were formed during the variscan orogeny by earth movements directed from the south the silurian volcanic rocks making up the core of the pericline are shown to be responsible for the large linear magnetic anomalies present over eastern mendip magnetic anomalies within the radstock coalfield to the north however are related to volcanic horizons in the carboniferous limestone a three dimensional model shows the presence of silurian rocks beneath the spine of the eastern mendip hills seismic reflection sections are used to control two dimensional interpretations of potential field data from beacon hill pericline although the magnetic anomalies are adequately explained by the seismic model the gravity anomalies require the presence of an additional low density body beneath the pericline9s southern flank this body is interpreted as a thrust wedge of old red sandstone emplaced in its present position by a previously unrecognized post variscan normal fault the considerable lateral extent of the structure is demonstrated by the trend of the maxima of the horizontal gradient of the gravity anomalies

variscan inversion within the cheshire basin england carboniferous evolution north of the variscan front abstract a comprehensive seismic reflection and borehole database over the whole of the cheshire basin has allowed examination of the carboniferous succession beneath a thick permo triassic cover construction of a seismic sequence stratigraphy involving the recognition of unconformity bounded packages has allowed the examination of the control by tectonics of sedimentation within the pre permo triassic of the cheshire basin the carboniferous has been divided into two unconformity bound megasequences i e dinantian ms10 0 and namurian westphalian ms20 0 these are deformed by a hitherto unknown series of variscan structures interpretation of these structures has allowed assessment of the variscan orogeny in this area of the variscan foreland considerable variscan inversion in the area exists 100 150 km north of the variscan fold thrust front and is suggested to illustrate the effects of predominantly e w directed compression especially to the north of the northern apex of the adjacent midlands microcraton compression occurred during two late carboniferous pulses of thrusting

mathematical modeling of basal coastal cliff erosion in uniform strata a theoretical approach abstract this article utilizes the general assumption that cliff erosion is dependent on the strength of wave assailing forces and the resistance of the cliff material it considers the possibility of understanding and describing coastal cliff erosion processes particularly basal erosion through a mathematical analysis of one of the clearest manifestations of its action i e the development of the basal cliff notch through mechanical wave action it analyzes profile change through time assuming a cliff composed of uniform strata deterioration of the cliff is expressed by means of the erosion function which in this study is related to the cliff erosion rate an equation termed the belov davies williams bdw equation has been formulated for a moving surface and this is proposed as a mathematical model of cliff profile change explicit solutions of the bdw equation are found for both steady state and time dependent erosion rates in both cases it is assumed that wave erosion intensity decreases

intracanyon basalt lavas of the debed river northern armenia part of a pliocene pleistocene continental flood basalt province in the south caucasus abstract late pliocene to early pleistocene 3 25 2 05 ma 200 400 m thick basalt lavas outcrop in the south caucasus region including the kars erzurum plateau northeastern turkey the javakheti plateau georgia armenia and the lori plateau northern armenia these fissure fed rapidly erupted fluid lavas filled pre existing river valleys over many tens of kilometres the basalts exposed in the debed river canyon northern armenia are 200 m thick and of three morphological types 1 basal pillow basalts and hyaloclastites overlain by 2 columnar jointed pahoehoe sheet flows in turn overlain by 3 slabby pahoehoe and rubbly pahoehoe flows the lower and middle lavas show evidence for damming of river drainage like many lavas of the columbia river flood basalt province scotland ireland and iceland there is also evidence for syn volcanic faulting of the early lavas related basalts also outcrop in the gegham uplands and the hrazdan river basin in armenia this 3 25 2 05 ma south caucasus basalt province covering parts of turkey georgia and armenia has an estimated areal extent of 15 000 km2 and volume of 2250 km3 because its main geological features are remarkably like those of many continental flood basalt cfb provinces we consider it a true albeit small cfb province it is the smallest and youngest cfb in the world an analogue closely similar in major features is the late miocene altos de jalisco cfb province in the western trans mexican volcanic belt both provinces formed during lithospheric pull apart and transtensional faulting their broader significance is in showing flood basalt size distribution to be a continuum without natural breaks with implications for geodynamic models

cooling fractures in impact melt deposits on the moon and mercury implications for cooling solely by thermal radiation we study the distribution morphology and geometrical properties of fractures in several young impact melt deposits on the moon and mercury and the ways that these fractures may form from cooling by thermal radiation in each impact melt complex the topography of the underlying terrain determines the orientation of cooling fractures such that interior fractures that formed in the relatively thick interior areas of the melt unit are wider and have a larger spacing than marginal fractures that formed in the relatively thin areas near the unit s margins solid debris entrained in molten deposits provides prefracture flaws that can seed cooling fractures but too much solid debris prevents cooling fractures from growing to macroscopic sizes the appearance of subparallel fractures is mainly caused by subsidence of the deposits during the process of cooling and solidification tensile stresses caused by thermal radiation are large enough to initiate cooling fractures on both the moon and mercury which may represent the initial stage of columnar joints formation but the cooling rate caused solely by thermal radiation is not large enough to form well organized columnar joints that feature polygonal colonnades we therefore propose that thermal conduction and convection are the major contributors in the formation of columnar joints on planetary bodies

the eruption environment of multi tiered columnar basalt lava flows field studies on the eruption environment and prevailing cooling conditions of multi tiered lava flows of the type exemplified by the giant s causeway were carried out at localities on the columbia river plateau southern iceland on the islands of mull and staffa and around the giant s causeway ne ireland common features can be recognized in the volcanology of such flows in all the localities examined these include palaeotopographic evidence of river valleys giving conditions conducive to the ponding of flows and the damming of existing drainage evidence of high rainfall regimes or abundant surface water association of interlava lacustrine and fluviatile sediments association of hyaloclastite flow foot breccias within multi tiered basalt lava successions and evidence of large volume lava flows and high eruption rates examination of the petrography of the upper curvi columnar parts of the lava the entablature shows textural signs of quenching compared with the lower more regular columnar zone the colonnade these common field and petrographic features confirm the model of multi tiered flow formation by modification of the cooling regime in the upper part of the flow following inundation of the hot lava surface by displaced river drainage

how do you grade peperites a spectacular graded peperite of volcanic origin from the palaeogene mull lava field of nw scotland the carraig mhor bed cmb consists of a basal facies of pale sub horizontally aligned fluidal shaped clasts of hydrothermally altered basalt set in a brown complexly laminated silt to clay grade host siliciclastic sediment the basal facies of the cmb grades upwards over an interval of 10 m into an upper facies dominated by smaller and more angular clasts of identical composition set in a subordinate proportion of the same sediment matrix the distinctive textural characteristics of the cmb together with its remarkably uniform grading motif may be explained in terms of the mingling of basaltic magma and fluidised siliciclastic sediment resulting in the ductile fragmentation of magma and production of the fluidal clasts which dominate the basal facies cooling of the system led to limited brittle fragmentation yielding the finer grained sediment poor upper facies of the cmb during cooling the larger fluidal shaped clasts settled to the base of the unit through the saturated sediment producing the vertical stratigraphic grading now preserved grading occurred essentially in situ during peperite formation and cannot be attributed to remobilisation mass flow or pyroclastic phenomena

from feeder dykes to scoria cones the tectonically controlled plumbing system of the rauðholar volcanic chain northern volcanic zone iceland the rau holar volcanic chain located in the northern volcanic zone of iceland has been variably eroded such that in the northern part the original scoria cones are preserved while the central and southern parts expose their shallow feeders the chain thus offers insight into the inner workings of the near surface feeder system of scoria cones the volcanic chain was mapped in 3d using gps the en echelon arranged volcanic chain can be divided into three parts the southernmost part contains only plugs and necks with a thin pyroclastic cover as well as multi tiered lava flows the central part combines partially eroded scoria cones feeder dyke intersections and welded scoria interbedded within rootless and clastogenic lava flows the welded scoria is composed of different kinds of lithics and bombs the northern part preserves almost intact overlapping scoria cones with voluminous lapilli sized scoriaceous deposits the overall dyke trend is orthogonal but shows radial patterns in individual cone complexes feeder dykes observed to depths of about 200 m below the volcanic chain are up to 8 m thick and flare in to conduits in the uppermost 20 50 m the exposed shallow plumbing system shows that magma pathways through the volcanic edifice are very complex with incremental repeated intrusions we interpret the arcuate shape to be the result of a local change in the orientation of the stress field because the rau holar volcanic chain is located within a major relay structure between volcanoes on the eastern fremrinamur rift arm and a rift extension with grabens on the western periphery

palaeogeographical evolution of the rattray volcanic province central north sea the rattray volcanics member at the triple junction of the north sea rift is here subdivided into two informal sub members based on analysis of core wireline and seismic data the lower and upper rattray volcanics were emplaced during two distinct phases of volcanism separated by a sustained volcanic hiatus the presence of hyaloclastite and abundant freshwater algae at the base of the lower rattray indicates that large lakes were present in the area prior to the volcanism possibly indicating that collapse of the regional jurassic central north sea dome began prior to volcanism pulsed subsidence probably occurred through the duration of the volcanism with lacustrine conditions becoming re established during the mid volcanic hiatus sediments were deposited across the rattray volcanic province in fluvial systems and floodplain coal swamps after the final cessation of volcanism with later marine transgression leading to drowning of the area in the callovian to oxfordian in terms of hydrocarbon prospectivity no evidence is currently found to confirm the presence of an intra basaltic play analogous to the rosebank field in the faroe shetland basin although post volcanic pentland formation sedimentary sequences in the fisher bank basin area indicate the possibility of supra basaltic prospectivity in the triple junction supplementary material a table containing the complete palynological record is available at https doi org 10 6084 m9 figshare c 4857195

strains developed in the hangingwalls of thrusts due to their slip propagation rate a dislocation model reply abstract fault planes propagate radially from a point source and this failure propagation process is very similar to the movement of a dislocation through a crystal lattice an elastic strain represented by an extra half plane is necessarily imparted to the lattice at an edge dislocation and this is equivalent to the ductile bead which accompanies the propagating tip of a thrust fault this ductile bead migrates with the fault tip and imparts a characteristic internal strain to the thrust sheet a two dimensional model is presented to illustrate the inter relationship between fault plane slip fault tip propagation and internal strain in multilayered sequences internal strain is usually represented by asymmetric folds verging in the thrust transport direction a simple technique the displacement distance plot utilizes the fact that displacement dies out towards the fault tip this plot can be used to quantify the relative stretch measured parallel to the fault movement which is dependent on the slip propagation rate and it may be used to define exactly the position of the fault tip on a cross section examples of fold thrust structures from devon england and pembrokeshire wales are used to illustrate the technique

pleistocene volcanism and the geomorphological record of the hrazdan valley central armenia linking landscape dynamics and the palaeolithic record abstract the southern caucasus lies at the intersection of africa the levant and eurasia and is thus a region of considerable interest in the study of pleistocene hominin population dynamics and behaviour while palaeolithic archaeological sites in the region such as dmanisi and nor geghi 1 attest to such palaeogeographic significance a greater understanding of the chronology and nature of climatic and geomorphic changes in the region is needed to fully understand hominin settlement dynamics the hrazdan river valley central armenia has the potential to offer such insights given its rich palaeolithic record and complex history of pleistocene infill as a result of alluvial lacustrine aeolian and volcanic processes we therefore present a stratigraphic framework for basin infill and hominin activity during the pleistocene based on extensive geomorphological and geological mapping published chronometric results 40ar 39ar and k ar and archaeological survey we demonstrate that the onset of pleistocene volcanism in the gegham range to the immediate east of the hrazdan valley occurred around 700 ka bp after which there were several phases of effusive eruption lasting until 200 ka interbedded with lava emplaced by these eruptions are alluvial and lacustrine sequences some with evidence of pedogenesis and several of which have yielded palaeolithic artefacts taken together these sequences suggest a cyclical model of infill whereby lava flow along the valley resulted in the blockage of the palaeo hrazdan river and lake formation in the lea of the lava dams breaching of these dams resulted in a shift to predominately fluvial deposition and the consequent development of floodplain soils hominin populations exploited the floodplains at times when the last of these phases coincided with interglacial and interstadial climates but they also occupied the surrounding valley sides during the same warm humid phases

structure of blackdown pericline mendip hills from gravity and seismic data blackdown pericline is the western pericline of the mendip hills which were formed during the variscan orogeny at the end of the carboniferous unlike the more easterly periclines blackdown pericline has no associated magnetic anomaly indicating the absence of the silurian volcanic rocks responsible for the anomaly in eastern mendip the gravity field of blackdown pericline cannot be adequately modelled by the known geology of the mendip hills and requires the presence of an anomalously dense body at shallow depth the geometry of the body is inferred from a seismic reflection profile and is interpreted as a wedge of precambrian basement rocks its high density and negligible susceptibility may indicate an amphibolitic composition three models involving variscan thrusting and post variscan listric normal faulting are presented in explanation of the presence of the wedge the anomalous body is shown to be laterally continuous to west and east a linear negative gravity anomaly to the north of blackdown pericline is interpreted to be caused by a small occurrence of coal measures which have not before been described from this area

geometry and kinematics of fault propagation folds with variable interlimb angle abstract several conceptual approaches have been proposed to account for the development of fault propagation folds whose geometry and kinematics depend on the amount of displacement along a basal decollement level the ramp angle and the slip to propagation ratio among these the variable interlimb angle model of mitra 1990 is able to explain open and close natural folds but its application is limited because the fold geometry and bed thickness evolution rely on imposed parameters that cannot be measured directly here we use the ramp and the interlimb angles as input data to develop a forward fold model that accounts for thickness variations in the forelimb the relationship between the fold amplitude and fold wavelength is subsequently applied to construct balanced geological cross sections from surface parameters only and to propose a kinematic restoration of the folding through time the model can catter for a wide variety of folds reconstruct the deep architecture of anticlines and deduce the kinematic evolution of the folding with time we consider three natural examples to validate the variable interlimb angle model along strike thickness variation in the forelimb of the turner valley anticline in the alberta foothills of canada precisely corresponds to the theoretical values proposed by our model reconstruction at depth of the alima anticline in the southern tunisian atlas implies that the decollement level is localised in the triassic liassic series as highlighted by seismic imaging the kinematic reconstruction of the ucero anticline in the spanish castilian mountains is also in agreement with the fold geometry derived from two cross sections the variable interlimb angle model predicts that the fault propagation fold can be symmetric normal asymmetric with a greater dip value in the forelimb than in the backlimb or reverse asymmetric with greater dip in the backlimb depending on the shortening amount

crustal structure beneath southern england from deep seismic reflection profiles seismic reflection events from recently acquired profiles in southern england are interpreted in terms of stratigraphic and structural interfaces between rock units in particular evidence is brought forward that a series of reflection events dipping between 22 and 27 with azimuth averaging 160 represents 2 major thrust zones occurring in the variscan floor beneath mesozoic cover a geological model based upon borehole information and regional geological considerations is presented the tentative recognition of crystalline basement overlain and overstepped by late proterozoic rocks of variable thickness suggests that basement in this area was unstable from late precambrian times the mesozoic structural evolution can be related to tectonic re activation of major concealed thrust zones in the palaeozoic 9basement9

entablature fracture types and mechanisms entablature is the term used to describe zones or tiers of irregular jointing in basaltic lava flows it is thought to form when water from rivers dammed by the lava inundates the lava flow surface and during lava meltwater interaction in subglacial settings a number of different fracture types are described in entablature outcrops from the burfell lava and older lava flows in jorsardalur southwest iceland these are striae bearing column bounding fractures and pseudopillow fracture systems that themselves consist of two different fracture types master fractures with dimpled surface textures and subsidiary fractures with curved striae the interaction of pseudopillow fracture systems and columnar jointing in the entablature produces the chevron fracture patterns that are commonly observed in entablature cube jointing is a more densely fractured version of entablature which likely forms when more coolant enters the hot lava the entablature tiers display closely spaced striae and dendritic crystal shapes which indicate rapid cooling master fracture surfaces show a thin band with an evolved composition at the fracture surface mineral textures in this band also show evidence of quenching of this material this is interpreted as gas driven filter pressing of late stage residual melt that is drawn into an area of low pressure immediately preceding or during master fracture formation by ductile extensional fracture of hot partially crystallised lava this melt is then quenched by an influx of water and or steam when the master fracture fully opens our findings suggest that master fractures are the main conduit for coolant entering the lava flow during entablature formation

millimeter scale modal layering and the nature of the upper solidification zone in thick flood basalt flows and other sheets of magma abstract water flooding the surface of the holyoke flood basalt flow of connecticut and massachusetts was probably responsible for its entablature developing quench textures to depths of 80 m beneath the flow s surface despite the rapid cooling from above the entablature colonnade boundary is displaced above the center of the flow which can only mean that crystal mush sank from the roof zone to the floor of the flow during solidification the entablature of the holyoke flow exhibits a subtle quasi horizontal millimeter scale modal layering with sheets of ophitic pyroxene plagioclase clusters alternating with discontinuous sheets of residual liquid the layering is believed to have formed by repeated nucleation of pyroxene plagioclase clusters in a rapidly advancing thermal boundary layer the alternating layers of crystals and residual liquid produced a mush with planes of weakness along which the dense crystal mush could separate from the roof the layering parallels the cuspate entablature colonnade boundary which must therefore be a primary magmatic feature rather than simply where downward and upward propagating fractures met the modal layering has been found in other flood basalt flows but also in the roof zone of the palisades sill this texture may therefore be common in the roof zones of many sheet like bodies of magma

crustal evolutionary model for the variscides of ireland and wales from swat seismic data the swat seismic reflection profiles recently acquired across the celtic sea demonstrate that the crust which underlies the munster and pembroke variscan foldbelts contains extensive south dipping reflections interpreted as hercynian foreland directed ramp thrusts largely involving the pre devonian basement these large scale crustal ramps developed without any intervening flat lying decollement levels and are thought to have originated in a deep infra crustal decoupling zone not imaged on the swat profiles because it lies within the present day layered lower crust at approximately 25 km depth the pre existing extensional basinal pattern is assumed to have highly influenced the geometry of these deeply rooted thrusts and in particular that of the frontal ramp which emerges along a discontinuous n100 trending thrust zone located above the faulted extensional margin of the munster pembroke and mendips devonian basins shortening of the irish and welsh external zones is hence assigned to dominantly crustal stacking achieved along steep ramps dying out upwards and giving way to mainly undisrupted folds in the thick overlying devonian infill in addition a shallow thin skinned fold thrust system developed above an intra namurian master decollement still preserved in the footwall of the frontal ramp where it either stays blind ireland or emerges pembroke above a dominantly wrench faulted caledonian basement compiling swat seismic data and structural geology leads to a crustal evolutionary model for the irish and south welsh external variscides from a devonian basin template to a final deformation pattern

geochemistry and volcanology of the tertiary basalts of the giant s causeway area northern ireland the columnar jointed basalts of the giant9s causeway in northern ireland are a localized group of flows the causeway tholeiite member within the tertiary antrim lava group major and trace element analyses including rare earth element ree data are presented which show a range of compositions from hy normative tholeiites with mid ocean ridge basalt morb characteristics to qz normative examples with sio 2 54 and k 2 o 1 0 within the causeway tholeiites are three well defined chemical sub groups which can be correlated stratigraphically across the area zr nb data suggest a single mantle source for the lavas with the range of composition the result of variable degrees of partial melting of a progressively depleted mantle followed by fractional crystallization of the magmas at higher crustal levels mass balance calculations are used to model the fractionation processes involved the lavas of the causeway result from tectonically controlled decompression melting with magma equilibration in ephemeral chambers at high crustal levels followed by eruption as sheet flows disruption of the contemporaneous drainage system and the subsequent flooding of hot flow surfaces modified the cooling regime and formed the characteristic multi tiered flows with colonnade and entablature development

column on column structures as indicators of lava ice interaction ruapehu andesite volcano new zealand abstract lava flows of the mangawhero formation ca 15 60 ka on ruapehu volcano erupted during the last glaciation in a distal flow lobe at tukino on the east side of the mountain small secondary columns 10 20 cm thick have formed on the sides of large rectangular primary 0 5 3 m thick cooling columns thick 10 m zones of such small columns form a lateral and basal outer rind of the lobe as they do not mark glassy zones of quenching these secondary columns are interpreted as being formed by a second cooling event at temperatures below the boundary between the low creep and elastic regimes 600 c by rapid influx of copious amounts of water temperature drops deduced from extensional strains of the two sets of columns were used to gauge the viability of such a two stage process absence of reliable data on andesite contraction coefficients was overcome by using a sliding scale to assess a large range of values the estimates indicate that two stage chilling is feasible after flowing across relatively ice poor terrain the lava flow must have interacted with a valley glacier that provided water for further chilling the already formed primary columns and formation of the outer rind small columns given this evidence for lava ice interaction it is likely that prominent thick flows elsewhere in the mangawhero formation may have been constrained to their ridge top locations by ice conditions similar to those described by lescinsky and sisson lescinsky d t sisson t w 1998 ridge forming ice bounded lava flows at mount rainier washington geology 26 351 354

a reappraisal of the facing confrontation in north cornwall fold or thrust dominated tectonics the problem of conflicting fold or thrust solutions to the interpretation of the structure of north cornwall is discussed via new detailed mapping of the coastal sections in addition to facing evidence shear criteria have been extensively applied in order to sustain new interpretations the facing confrontation at polzeath is confirmed but the high strain zone centred upon tintagel is ascribed to later superimposed northwards directed ductile shearing peak metamorphism was synchronous with this event subsequent northwards displacement became confined to discrete thrust planes dismembering the south facing structures which developed during an intermediate phase of backthrusting the major structures are thrust dominated though large recumbent folds were generated during the early ductile phases the facing confrontation is probably allochthonous having been carried northwards on a blind thrust it is suggested that the important rusey fault zone is the roof thrust to a triangle zone in which the lower grade rocks of the culm basin are uplifted over an underlying antiformal stack

penetration of water into hot rock boundaries of magma at grimsvotn the grimsvotn geothermal area is located within one of the most active caldera volcanoes in iceland in the interior of the ice cap vatnajokull at an altitude of 1 400 m fig 1 its setting oilers unique conditions for calorimetric measurement of the heat release from the subglacial geothermal system the melting of ice due to the geothermal activity creates a depression in the surface of the ice cap ice and water are diverted towards the depression from a 300 km2 drainage basin the meltwater accumulates in a 30 km2 subglacial lake periodic outbursts of water jokulhlaups drain the lake subglacially down to the skeidararsandur plain we propose here that penetration of water into hot rock is the primary reason for the intense heat release 5 000 mw thermal of the subglacial grimsvotn geothermal area injection of water into boundary rocks of magma should be considered as a method of heat exploitation

geological and 40ar 39ar age constraints on late stage deccan rhyolitic volcanism inter volcanic sedimentation and the panvel flexure from the dongri area mumbai abstract post k pg boundary deccan magmatism is well known from the mumbai area in the panvel flexure zone represented by the salsette subgroup it shows characters atypical of much of the deccan traps including rhyolite lavas and tuffs mafic tuffs and breccias spilitic pillow basalts and intertrappean sedimentary or volcanosedimentary deposits with mafic intrusions as well as trachyte intrusions containing basaltic enclaves the intertrappean deposits have been interpreted as formed in shallow marine or lagoonal environments in small fault bounded basins due to syn volcanic subsidence we report a previously unknown sedimentary deposit underlying the dongri rhyolite flow from the upper part of the salsette subgroup with a westerly tectonic dip due to the panvel flexure we have obtained concordant 40ar 39ar ages of 62 6 0 6 ma 2 and 62 9 0 2 ma 2 for samples taken from two separate outcrops of this rhyolite the results are significant in showing that i danian inter volcanic sedimentary deposits formed throughout mumbai ii the rock units are consistent with the stratigraphy postulated earlier for mumbai iii shale fragments known in some dongri tuffs were likely derived from the sedimentary deposit under the dongri rhyolite iv the total duration of extrusive and intrusive deccan magmatism was at least 8 9 million years and v panvel flexure formed or continued to form after 63 ma possibly even 62 ma and could not have formed by 65 64 ma as concluded in a recent study

stratigraphy of the fort selkirk volcanogenic complex in central yukon and its paleoclimatic significance ar ar and paleomagnetic data brunhes matuyama kaena and mammoth age basaltic lava flows tertiary quaternary selkirk volcanics were sampled in west central yukon the mean characteristic remanent magnetization chrm direction of the flows sampled in this and previous studies has a declination of 348 7 and an inclination of 70 8 n 42 k 99 6 95 2 2 all on lower hemisphere the time range represented in this study ca 3 25 to ca 0 004 ma is great enough to have confidently averaged secular variation sediment associated with the basalt has a mean declination of 7 6 and inclination of 78 8 n 5 k 5 6 95 35 7 a new 40ar 39ar date on the reversely magnetized basal basalts at ne ch e ddhawa places the eruption in the mammoth subchron of the gauss normal chron the newly dated basal basalt at ne ch e ddhawa precedes the initial continental glaciation in yukon and is older than the fort selkirk vent lower mushroom which was previously thought to be the oldest eruption at fort selkirk volcanic complex

the 3d facies architecture of flood basalt provinces and their internal heterogeneity examples from the palaeogene skye lava field quantifying the facies architecture of flood basalt provinces is important as it can be used to understand the physical volcanology and rock property variations throughout the lava sequence the 3d facies architecture and internal heterogeneity of the skye lava field for example provides important insights into the evolution of the british tertiary igneous province and valuable information to aid in the exploration of potential offshore reservoirs underlying significant flood lavas along the north atlantic margin the volcanic stratigraphy of the talisker bay area of skye comprises 1 lower compound braided lavas flow lobes 3 the transitional sequence 7 4 km 3 and the upper sequence 17 0 km 3 the lower sequence lavas formed on the flanks of a shield volcano and were sourced from the ne volcanological features such as lava feeder tubes pahoehoe textures and lobes indicate a scale of volcanism similar to that of present day hawaii the within flow heterogeneity of the basalts is characterized using an intrafacies scheme allowing comparison of variations in lithofacies with characteristic geophysical rock properties of compressional wave velocity and density

a reinterpretation of the concealed variscan structure beneath southern england by section balancing recently chadwick et al 1983 presented an interpretation of the concealed variscan structure beneath southern england based on deep seismic reflection profiles we re evaluate the status of reflection events identified along some of the profiles and draw attention to major uncertainties in the interpretation presented the geological section of chadwick et al is admissible in that it portrays similar structures to those seen to the west in the mendips however the section is not a valid balanced section as it cannot be restored to a pre thrusting stratigraphy using the published seismic reflection data and borehole information we present a new balanced cross section of the variscan structures for which a total tectonic shortening of about 20 is calculated

the terminology of structures in thrust belts abstract a review of structures and geometric relationships recognized in thrust belts is presented a thrust is defined as any contractional fault a corollary being that thrusts must cut up section in their transport direction flats are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ramps are those portions of thrusts which cut across datum surfaces ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike slip movement an array of thrusts which diverge in their transport direction may form by either of two propagation models these are termed piggy back propagation which is foreland directed and overstep propagation which is opposed to the thrust transport direction an array of thrust surfaces is termed an imbricate stack and should these surfaces anastamose upwards a duplex will result the fault bounded blocks are termed horses a duplex is bounded by a higher roof thrust and a lower floor thrust the intersection of any two thrust planes is termed a branch line thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut a further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one the movement of thrust sheets over corrugated surfaces or the local development of thrust structures beneath will fold higher thrust sheets these folds are termed culminations and their limbs are termed culmination walls accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust these accommodation surfaces are termed hangingwall detachments and they need not root down into the active thrust this category of detachment includes dip slip hangingwall drop faults which are developed by differential uplift of duplex roofs and out of the syncline thrusts which develop from overtightened fold hinges back thrusts as well as forming as hangingwall detachments may also form due to layer parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp

an interpretation of the variscan structures in sw england the variscan structures in sw england apart from the high level structures in the culm synclinorium of n devon are recumbent with an overall gentle southward dip of cleavages thrusts and recumbent folds translation was towards the nnw the structures are interpreted on a thin skinned model with a basal decollement dipping gently s related to a s dipping subduction zone s of the lizard the granites are thought to have been generated to the s of their present position and injected northwards above the decollement zone

quantitative modeling of compaction in the holyoke flood basalt flow hartford basin connecticut compaction in the holyoke flood basalt flow is modeled using a quantitative phase equilibrium routine based on the melts algorithm that is coupled with compaction driven heat and mass transport equations for a crystal liquid mush that is cooling from both the top and the bottom the compaction is qualitatively different in the upper and lower parts of the flow the solid fraction in the lower part increases as a result of concurrent compaction and solidification in contrast cooling from the upper boundary results in an increase in viscosity and a decrease in permeability toward the top this causes the velocity of the solid matrix to increase downward and the upper part to undergo dilation of the solid matrix the result is an s shaped profile for compatible elements and an inverse profile for incompatible elements the region of modeled maximum dilation matches the location of segregation sheets in the holyoke flow discrepancy between observed deformation as calculated from textural deformation of plagioclase chain networks versus compositional profiles suggest some crystal accumulation and or compaction prior to the formation of the plagioclase chains

structural controls on upper palaeozoic sedimentation in south east wales marked thickness changes in early dinantian courceyan chadian sediments occur across the cardiff cowbridge anticline a major east west variscan fold in the vale of glamorgan they resulted from differential subsidence over an active concealed basement fault zone the vale of glamorgan axis which is coincident with the hinge of the anticline the latter formed in response to inversion on this basement fault a caledonian ancestry for the vale of glamorgan axis is indicated by pronounced upper old red sandstone overstep across it across the severn estuary between south wales and the bristol mendips region there is a significant offset of isopachs and facies belts in both the courceyan chadian and arundian sequences it is suggested that these result from intra carboniferous dextral strike slip along a major fault zone underlying the severn estuary the severn estuary fault zone and that the vale of glamorgan axis was a synthetic structure developed in conjunction with this strike slip movement by inference the severn estuary fault zone shares a common history with the vale of glamorgan axis extending back to the devonian and possibly earlier late dinantian holkerian facies distribution reflects uplift and emergence on the north south usk anticline and malvern lineament continued uplift is recorded in silesian sequences in south east wales and the bristol mendips area the orientation of structures bordering the severn estuary is consistent with their development in a right lateral strike slip zone

in situ exhumation from bedrock of large rounded boulders at the giant s causeway northern ireland an alternative genesis for large shore boulders mega clasts abstract very large boulders mega clasts are found on some coasts the size and position of the boulders has been used to suggest that contemporary marine processes acting within their normal spatial and energy range are unlikely to have moved them explanations for the presence of such boulders include transport by infrequent very high energy marine processes storms or tsunamis mass movement from backing cliffs transport by ice or exhumation from glacial deposits this paper advances an alternative explanation which does not involve transport by any of the marine or glacial processes or gravity it is proposed that in a very specific geological and topographic setting large boulders are exhumed in situ by storm waves acting on heavily weathered jointed basalts eventually wave action liberates residual blocks from the deeply weathered matrix these liberated boulders will be mobile only if they lie within wave competence and the larger ones will remain as stationary residuals the same in situ weathering processes followed by removal of the friable matrix material debris by wave action also progressively round the boulders consequently despite their appearance of being transport rounded the larger boulders have not transported at all in specific locations the assumption that the presence and rounding of such large clasts in the shore zone can be attributed to marine transport can lead to erroneous interpretations of very high energy storm wave or tsunami activity

end carboniferous fold thrust structures oxfordshire uk implications for the structural evolution of the late variscan foreland of south central england interpretation of commercial seismic reflection data from the west of banbury south central england has provided information on the late carboniferous structural evolution of the oxfordshire syncline end carboniferous structures are preserved in the footwall of the main basin bounding faults of the permo triassic worcester graben restoration of major pre triassic normal faults has allowed reconstruction of earlier compressional structures in contrast to the wnw ese trend of structures to the south in the variscan foreland fold thrust belt major faults and folds in oxfordshire are approximately n s trending the structures seen to deform westphalian d stephanian aged sequences on the seismic profiles are a combination of steep 30 40 dip fault bounded uplifts and less dominant thin skinned tectonics apparently showing transport to the east in the south some thin skinned structures are present and in the north cover sequences are uplifted in a narrow zone of deformation on reactivated basement faults the amount of shortening also changes along strike from c 1600 m in the south to c 950 m in the north reactivation of major basement faults with differential basement shortening accounts for the main structures localized detachment in the young and weak sediments close to the palaeo land surface explains the thin skinned structures seen on some of the seismic lines these end carboniferous structures in oxfordshire and structures in the malvern abberley hills represent the structural limits of a large n s striking intra stephanian uplift in the late variscan foreland it is suggested that the structures formed during a discrete phase of intra stephanian e w directed regional shortening prior to the final phase of movement in the variscan belt

particle size and shape in pebble beach sedimentation an in depth investigation was made of beach gravel fabrics at two beach sites located on the glamorgan heritage coast wales u k granulometric data from 31 080 pebbles were obtained and analysis confirmed the existence of a basic shape differentiation structure when the b axis was used at gileston beach a low energy location and nash beach a high energy location the use of the b axis in size and shape analysis seems to have led some workers to incorrect rejection of size as a factor in determining the transportational and depositional potential of a particle an inferred energy model can be constructed where the influence of particle size on sorting is predominant when energy conditions are high and particle shape factors predominate when energy conditions are low the c axis standard deviation value was the most effective discriminator of variation in shore parallel sedimentation structures at both beaches for shore normal structural discrimination the c axis mean and standard deviation values were the most effective

displacement distance methods in the analysis of fold thrust structures and linked fault systems a new method for analysing the internal strain in thrust sheets is described the displacement distance diagram can be used to quantify the relative shortening or relative stretch between the hangingwall and footwall beds of a thrust and is a useful way of analysing commonly occurring fold thrust structures the stretch is a function of the relative rates of thrust propagation and thrust slip the cumulative displacement distance diagram described here can be used to calculate the absolute shortening or extension when this is accommodated above a single decollement in a linked fault system this method offers a useful way of analysing duplex structures and large scale foreland thrust zones as well as extensional fault systems and can be used as an alternative to or in conjunction with balanced cross sections both methods can be used to quantify the variation in shortening above a single detachment zone this variation may be mechanically significant reflecting lithological control on thrust or normal fault growth

the formation of columnar joints produced by cooling in basalt at staffa scotland columnar jointing in basaltic lava flows on the island of staffa nw scotland was studied using a combination of field mapping and measurement of column dimensions sample petrology and measurements of plagioclase crystal size distributions csds interpreted using theoretical models of cooling four different lava flow units were measured and column ordering was assessed using the hexagonality index and relative standard deviations of column side length top area and internal angle upper and lower colonnades consist of dominantly 5 6 and 7 sided columns with a hexagonality index value very similar to that of giant s causeway and other basaltic columnar jointed localities csds from samples at different heights within one colonnade were used to infer the propagation of the solidus isotherm which was consistent with a convective cooling mechanism within the colonnade interior sample petrology and csd measurements suggest that entablature can form both by the interaction of propagating joint sets and flooding of the flow surface by water and the most widely exposed unit on staffa shows evidence of both mechanisms operating on the same flow crystal size distribution measurements can provide a useful tool for field interpretation of lava flow cooling mechanisms

palaeo stress fields in the variscan foreland during the carboniferous from microstructural analysis in the british isles abstract north of the variscan front the carboniferous series of the british isles is affected by mainly n s trending folds which are almost perpendicular to this e w oriented front flat and unconformable overlying permian strata demonstrate a variscan age for the deformation a microtectonic survey made in dinantian and to a lesser extent namurian calcareous layers has revealed two successive events according to chronological criteria the first event maximum stress e w trending led to the formation of n s folds with local reorientation in the vicinity of major pre existing faults this compression may have been active since the namurian it pre dates a second compressional event orientated n s which is related to the final emplacement of the variscan nappes

large phreatomagmatic vent complex at coombs hills antarctica wet explosive initiation of flood basalt volcanism in the ferrar karoo lip the mawson formation and correlatives in the transantarctic mountains and south africa record an early eruption episode related to the onset of ferrar karoo flood basalt volcanism mawson formation rocks at coombs hills comprise mainly 80 vol structureless tuff breccia and coarse lapilli tuff cut by irregular dikes and sills within a large vent complex 30 km2 quenched juvenile fragments of generally low but variable vesicularity accretionary lapilli and country rock clasts within vent fill and pyroclastic density current deposits point to explosive interaction of basalt with groundwater in porous country rock and wet vent filling debris metre scale dikes and pods of coherent basalt in places merge imperceptibly into peperite and then into surrounding breccia steeply dipping to sub vertical depositional contacts juxtapose volcaniclastic rocks of contrasting componentry and grainsize these sub vertical tuff breccia zones are inferred to have formed when jets of debris steam water passed through unconsolidated vent filling deposits these jets of debris may have sometimes breached the surface to form subaerial tephra jets which fed subaerial pyroclastic density currents and fall deposits others however probably died out within vent fill before reaching the surface allowing mixing and recycling of clasts which never reached the atmosphere most of the ejecta that did escape the debris filled vents was rapidly recycled as vents broadened via lateral quarrying of country rock and bedded pyroclastic vent rim deposits which collapsed along the margins into individual vents the unstratified poorly sorted deposits comprising most of the complex are capped by tuff lapilli tuff and tuff breccia beds inferred to have been deposited on the floor of the vent complex by pyroclastic density currents development of the extensive coombs hills vent complex involved interaction of large volumes of magma and water we infer that recycling of water as well as recycling of pyroclasts was important in maintaining water supply for phreatomagmatic interactions even when aquifer rock in the vent walls lay far from eruption sites as a consequence of vent complex widening the proportion of recycled water increased with vent complex size in the same way that the proportion of recycled tephra did though water recycling leaves no direct rock record the volcaniclastic deposits within the vent complex show through their lithofacies structural architecture lithofacies characteristics and particle properties clear evidence for extensive and varied recycling of material as the complex evolved

pliocene and pleistocene volcanic interaction with cordilleran ice sheets damming of the yukon river and vertebrate palaeontology fort selkirk volcanic group west central yukon canada abstract neogene volcanism in the fort selkirk area began with eruptions in the wolverine creek basin ca 4 3 ma and persisted to ca 3 0 ma filling the ancestral yukon river valley with at least 40 m of lava flows activity at the ne ch e ddhawa eruptive center overlapped with the last stages of the wolverine creek eruptive centers hyaloclastic tuff was erupted between ca 3 21 and 3 05 ma this eruption caused or was coincident with damming of yukon river the first demonstrable incursion of a cordilleran ice sheet into the fort selkirk area was coincident with a second eruption of the ne ch e ddhawa eruptive center ca 2 1 ma the ne ch e ddhawa subglacial mound was erupted beneath at least 300 m of glacial ice ne ch e ddhawa glaciation the eruption of the fort selkirk center occurred between the last eruption of ne ch e ddhawa and fort selkirk glaciation ca 2 1 1 5 ma till and outwash from fort selkirk glaciation are conformably overlain by nonglacial sediments that contain the fort selkirk tephra fission track dated at ca 1 5 ma these nonglacial sediments also preserve a short magnetic reversal reversed to normal identified as the gilsa polarity excursion temporal control and sedimentology constrain fort selkirk glaciation and the fort selkirk local fauna to marine isotope stage 54 rapid and extensive eruption of the pelly eruptive center filled the yukon river valley with 70 m of lava which buried these glacial and nonglacial sediments and dammed yukon river local striations and erratic pebbles occur on the last of these lava flows they document a subsequent incursion of glacial ice during the last 500 ka of the matuyama chron forks glaciation the last major eruption of mafic lava occurred in the middle pleistocene west of early holocene volcano mountain in basin of black creek lava flowed down the valley presently occupied by black creek and dammed yukon river in the area of the black creek confluence this eruption predated the middle pleistocene reid glaciation minor volcanism has continued in this area since the middle pleistocene at volcano mountain

discovery of columnar jointing on mars we report on the discovery of columnar jointing in marte valles mars these columnar lavas were discovered in the wall of a pristine 16 km diameter impact crater and exhibit the features of terrestrial columnar basalts there are discontinuous outcrops along the entire crater wall suggesting that the columnar rocks covered a surface area of at least 200 km 2 assuming that the rocks obliterated by the impact event were similarly jointed we also see columns in the walls of other fresh craters in the nearby volcanic plains of elysium planitia amazonis planitia which include marte vallis and in a well preserved crater in northeast hellas

a mountain front model for the variscan deformation of the south wales coalfield access to the considerable geological database of british coal together with seismic evidence of subsurface relationships has enabled the development of a structural model for the south wales coalfield detailed site investigations are used as key examples the simple broad synclinal nature of the coalfield masks a more complex structural system thrusting of variscan age is largely confined to the lower and middle coal measures and throughout the central and northern coalfield is dominantly a forethrust system however along the south crop a major backthrust system is developed which in places penetrates the upper coal measures the south crop shows features analogous with a mountain front and is interpreted as a triangle zone in which pre existing basement faults have facilitated uplift the lack of thrusting within the upper coal measures in contrast to the underlying lower and middle coal measures suggests they have acted as a passive roof to the thrust system geographical variation in the intensity of the deformation can be related to the proximity of and buttressing against the caledonian massif although broadly analogous with current models of foreland basin development and deformation the south wales coalfield demonstrates the importance of pre existing massifs and lineaments in determining the precise evolution of an area

the importance of pahoehoe abstract pahoehoe lava flows are common in every basaltic province and their submarine variants pillow lavas and sheet flows cover the bulk of the earth pahoehoe flows are emplaced by inflation the injection of molten lava underneath a solidified crust only in the past few years has an understanding of the inflation process and the ability to recognize ancient inflated lava flows been achieved all large terrestrial basaltic flow fields studied to date including flood basalts were emplaced as thermally efficient inflated compound pahoehoe sheet flows this leads us to propose that this is the standard way of emplacing large lavas the swell hypothesis the atmospheric impact of such flood basalt eruptions could have been protracted and severe providing a plausible link between flood basalt eruptions and mass extinctions

late cenozoic columnar jointed basaltic lavas in eastern and southeastern china morphologies structures and formation mechanisms late cenozoic basaltic lavas in eastern and southeastern china commonly display spectacular columnar joints in four volcanic fields in this region we have observed and classified five types of colonnade vertical inclined horizontal fanning upwards and fanning downwards that provide information on the cooling regime and emplacement mechanism of the lava flows in total we analyzed the geometry and morphology of joints across ten different lava flow areas in the changle volcanic field cvf luhe volcanic field lvf eastern zhejiang volcanic field ezvf and niutoushan linjin island nanding island volcanic field nlnvf field observations show that columnar jointed alkaline basalts commonly exhibit narrow and irregular shapes indicating fast cooling tholeiites with columnar joints in contrast show wide and regular polygons indicating slower cooling vertical columnar joints and other types horizontal and fanning shaped have different mature patterns with the implication that lavas flowed on relatively flat land and in paleo valleys respectively this study suggests a simple qualitative description of different types of colonnade can be made based on their alignment patterns which represents a direct and easy way to derive solidification processes after lava eruption from colonnaded lavas in outcrop

the columbia river basalts between 17 million and 6 million years ago 200 000 square kilometers of the american northwest were flooded by basaltic lava that erupted through fissures in the crust up to 150 kilometers long larger individual eruptions covered over a third of the columbia plateau in a few days the lavas represent partial melts of the earth s mantle that were only slightly modified by near surface upper crustal processes the abundant chemical and mineralogical data now available offer an opportunity to study mantle composition and the processes involved in the evolution of the earth s crust

structures textures and cooling histories of columbia river basalt flows flood basalt flows of the columbia river basalt group commonly exhibit well developed colonnade and entablature structures formed during cooling of individual flows the colonnade refers to the well defined columnar structure of many flows which appears in the lowermost 10 to 30 and in places the uppermost 10 to 20 entablature refers to those parts of a flow that have smaller column diameters and more irregular pattern fractures than those of colonnade entablature commonly occupies the central 60 to 70 of a flow in some cases however the entablature is interrupted by colonnade in such cases the entablature and colonnade may be repeated once or twice within a single flow in other cases flows may lack an entablature entirely petrographic examination reveals a pronounced correlation between structural and textural features thin sections of colonnade basalt have relatively coarse cruciform or octahedral fe ti oxide crystals and 20 glassy mesostasis the central entablature basalt in contrast has much more mesostasis as much as 60 and has textural features such as feathery fe ti oxide crystals indicative of more rapid cooling these features are the reverse of what might be expected for normal conductive cooling because cooling rates at the flow margins should be greater than those in the flow interior we reconciled the observed basalt textures with thermal models by taking into account rainfall and deranged drainage during crystallization of the flow downward migration of water through cracks would cause convective cooling of underlying unsolidified lava this effect and its consequences have been modeled numerically in order to clarify the physical constraints on the mechanism by which the central part of the lava can be quenched we have found that a water infiltration front can explain the presence of a thick quenched flow interior provided that the infiltration front moves downward just behind the lava solidification front despite heavy rainfall on hawaii for example 250 cm yr at kilauea iki exposed prehistoric lava flows on that island exhibit only colonnades we deduce from this that heavy rainfall alone is inadequate to quench flow interiors extensive flooding by deranged drainage or perhaps extremely heavy rainfall 250 cm yr is apparently required to produce entablatures the model also can be used to explain the occurrence of repeated entablature and colonnade by considering the effects of intermittent inundation the imposition of boundary conditions that mimic the removal and reoccurrence of flooding yields cooling histories that are consistent with the development of multiple colonnades and entablatures

the nature and significance of rift related near surface fissure fill networks in fractured carbonates below regional unconformities fissure fill networks are a widely recognized but relatively little described near surface phenomenon supplementary material appendix a is available at https doi org 10 6084 m9 figshare c 5023103

the measurement of pebble impacts and wave action on shore platforms and beaches the swash force transducer swashometer abstract an instrument has been designed that can measure pebble impacts and wave forces in a harsh high energy turbulent near shore environment the instrument weighs some 50 kg and stands 60 cm in height and has a pebble sensor head that is interchangeable in that the size and shape can be representative of beach pebbles e g discoid spherical blade or rod like impacts on the sensor are recorded on a computer and the instrument can be programmed to run for varying time periods two characteristic force versus time traces representing a different wave activity are presented the instrument performed well throughout several tidal cycles and under a variety of energy conditions

is the water still hot sustainability and the thermal springs at bath england the hot springs at bath are the largest natural thermal source in britain sustainable use of the waters for a spa requires maintenance of their temperature and flow rate together with smaller springs at hotwells bristol they form the outflow from a regional thermal aquifer that occurs where the carboniferous limestone is buried at depths 2 7 km in the bristol bath structural basin the aquifer is recharged via limestone outcrops forming the south and west portions of the basin rim current knowledge of the basin s structure is reviewed and important uncertainties identified concerning the hydrogeological role of thrust faults which may cut the limestone at depth a simple numerical model is used to determine the possible influence of thrusts upon groundwater flow within the thermal aquifer comparison of the modelled flow patterns with geochemical data and structure contours eliminates the hypothesis that thrusts completely disrupt the continuity of the aquifer the most successful model is used to simulate the possible impact of dewatering by large quarries at the limestone outcrops north and south of bath substantial reductions in modelled flow at bath result from proposed dewatering in the eastern mendips although the steady state approach adopted has severe limitations in that it does not take account of the incremental staging of actual dewatering nor allow for partial restitution of groundwater levels the geological uncertainties highlighted by the modelling could be addressed by future research into the effect of thrusts on the continuity of the carboniferous limestone more refined modelling to predict the timing of possible impacts of quarry dewatering will require measurements of the storativity of the thermal aquifer

pseudopillow fracture systems in lavas insights into cooling mechanisms and environments from lava flow fractures abstract detailed field observations of structures within the flow front of a holocene trachyandesite lava from snaefellsnes iceland are presented the lava provides exceptional three dimensional exposure of complex brittle and ductile deformation textures that record processes of lava fracture and quenching driven by external water the flow front interior is characterised by structures consisting of a large metre scale curviplanar master fracture with many smaller centimetre scale subsidiary fractures perpendicular to the master fracture such structures have previously been recognised in a range of lava compositions from basalt to dacite and called pseudopillows or pseudopillow fractures we propose the term pseudopillow fracture systems to emphasise the consistent package of different fracture types occurring together all documented occurrences of pseudopillow fracture systems are in lavas that have been inferred to interact with an aqueous coolant i e liquid water ice or snow we use fracture surface textures and their orientation in relation to flow banding to identify three distinct types of master fracture and two types of subsidiary fractures master fracture surface textures used to identify fracture mechanisms include chisel marks striae cavitation dimples river lines and rough smooth fracture surface textures these indicate both brittle and ductile fracture happening on different types of master fracture chisel marks on subsidiary fractures indicate comparative cooling rates cooling directions and isotherm orientations at the time of fracture we propose a model for pseudopillow fracture system formation taking into account all the various fracture types textures and fracture propagation mechanisms and discuss their implications for interaction mechanisms between lava flows and external coolants

surface morphology of columnar joints and its significance to mechanics and direction of joint growth columnar joints in basaltic lava flows display conspicuous bands oriented normal to column axes new observations show that each band contains a single plumose structure and thus represents an individual crack or joint segment formed during a discrete growth event analysis of plumose structure and intersections of cracks leads to a new kinematic model of columnar jointing and provides the first direct proof that columnar joints grow incrementally from exterior to interior regions of solidifying magma bodies columnar joints form by nucleation and growth of new cracks on the edges of older cracks each new crack begins at a point and propagates mostly normal to column axes and along the leading edge of a developing column face where thermal stress is concentrated inward propagation of cracks toward hotter regions is limited by a decrease of thermal stress and by the brittle ductile transition of lava outward and lateral propagation is limited by mechanical interaction with previous cracks and by low thermal stress in already fractured lava cracks often diverge slightly from the planes of previous cracks probably because of spatial and temporal changes in directions of local principal stresses mechanical interaction causes a diverging crack to overlap curve toward and usually intersect the previous crack behind its edge leaving a blind tip that points in the overall growth direction of the columnar joints this and other directional criteria are applied to determine joint growth patterns in several lava flows of the western united states in two tiered and multi tiered flows downward growing columnar joints usually meet upward growing joints well below the middle of the flows which indicates very rapid cooling of upper portions relative to lower portions this supports the idea that convection of water in columnar joints connected to the surface may be an important mechanism for cooling the upper portions of these flows whereas conduction is probably the dominant cooling mechanism at the bases

the onset of flood volcanism in the north western part of the siberian traps explosive volcanism versus effusive lava flows abstract the siberian traps large igneous province was formed during the end permian about 252 ma ago basaltic melt was injected into the organic and salt rich tunguska sedimentary basin forming interconnected sill complexes and associated hydrothermal vent complexes thick deposits of basaltic tuff and tephra covered the paleosurface before the onset of flood volcanism commonly taken as direct evidence for the explosive nature of the initial phase of volcanism the field work in this study revealed that tuffs are virtually absent along a 150 km long transect along the dyupkun lake and kureika river even though tuff is shown on available geological maps towards the south and west the transition between the end permian sediments and the flood basalts is either characterized by thin 2 5 m to no tephra deposits khantaika area hyaloclastites and associated lake deposited tephra kureika area or massive tephra deposits from local eruptive centers severnaya area the new results can be put into the context of other studies about volcanic tuff horizons in siberia and questions the notion of province scale explosive volcanism in siberia during the onset of flood volcanism moreover the main thicknesses of explosive tuff deposits up to 700 m are located in the central and southern parts of the province where the lip erupted through thick cambrian salt and carbonate sequences since numerous phreatomagmatic pipes are present in these areas we suggest a causal relationship between deep magma sediment interactions explosive eruptions and the resulting environmental stress that initiated the end permian mass extinction

vertical salt marsh accretion since the roman period in the severn estuary southwest britain abstract using three methods of measurement and averaging over the whole estuary the vertical sediment accretion on mature clastic salt marshes amounts to 1 22 m since the roman period ca a d 150 1 05 m since the medieval period ca a d 1250 0 54 m since ca a d 1845 and 0 21 m since ca a d 1945 these data are cautiously interpreted to mean that the trend of relative sea level in the severn estuary is continuing upward shorter term negative tendencies are not excluded and that the current rate of rise could be a few millimetres per year the measured sediment thicknesses cannot be equated directly with relative movements of sea level however because of effects related mainly to 1 changes in the tidal range with general deepening 2 the possible transgression up the severn vale of the estuary as a whole and 3 the possible raising of tidal levels due to wetland reclamation after estimating for all these factors the true relative rise since the roman period could have amounted to 1 6 1 7 m

pillow lavas and fluvio lacustrine deposits in the northeast of parana continental magmatic province brazil abstract the northeast edge of the parana continental magmatic province was marked by a topographical barrier that gave rise to a predominantly peridesertic environment this high allowed the recurrent of above average rainfall in this area of the province in contrast with other areas where many evidences indicate that lava flows have settled in an arid climate in a desert environment as a result alluvial fan debris flows and fluvio lacustrine systems developed occasionally this paper presents one of these sequences which is remarkable for the presence of sedimentary rocks and two sets of pillow lavas both sets of pillow lavas show interaction with the fluvio lacustrine sediments with distinctive grain size from pebbly sand to clay that locally preserve the original sedimentary structures such as bedding and lamination this interaction gives rise to peperites which show a diversity of textures for different sedimentary granulometries hyaloclastites and fragments of lava also occur in the interpillow material and may be intricately mixed with sediment these features based mainly on detailed field observations in the northeastern region demonstrate that the palaeorelief sedimentary environment stratigraphy and morphology of sedimentary units and basaltic flows are contrasting from the central and southern regions of the parana continental magmatic province

structural synthesis of s staffordshire uk implications for the variscan evolution of the pennine basin the presence of strata of westphalian a to possibly stephanian age in south staffordshire located at the southern margin of the pennine basin provides a rare control on the timing of late carboniferous deformation in central england during westphalian a c times sedimentation in south staffordshire was controlled by thermal subsidence with a superimposed influence of synsedimentary normal and reverse faulting alkaline basic igneous activity occurred during westphalian c times synchronous with uplift of the southern margin of the pennine basin uplift culminated during westphalian c d times in a phase of variscan inversion along basement lineaments and the development of a regional unconformity renewed subsidence and deposition of a relatively thick sedimentary sequence of westphalian d to possibly stephanian age was followed by a final phase of variscan deformation comprising possibly two pulses of uplift tilting and erosion observations presented for south staffordshire in conjunction with studies from other parts of the pennine basin indicate a complex interrelationship between rifting thermal subsidence and variscan basin inversion pulses of n s extension and e w to nw se compression are interpreted as the product of regional strike slip deformation superimposed upon a regime of thermal subsidence alkaline igneous activity is considered to be the product of mantle partial melting in response to limited decompression due to transtensional displacements on crustal thickness faults

deep structure of the vale of glamorgan south wales uk the deep structure of the vale of glamorgan is investigated using recently acquired seismic data reflection sections from a commercial seismic survey in the vale enable the carboniferous limestone old red sandstone and silurian sequences to be mapped in the subsurface the old red sandstone sequence thins westwards under the southern part of the vale and in the southwest corner of the vale the base of the old red sandstone is interpreted to overstep across the underlying silurian sequence in an analogous manner to its regional overstep in west wales the precambrian basement surface is not clearly represented on the sections but may coincide with the base of a reflective sequence observed in places along one of the reflection lines basement depth estimates from the reflection sections are compared with estimates derived from a new time term analysis of data from long seismic refraction lines basement depths remain uncertain to 1 2 km but appear to be greater under the central part of the vale than under the western part and along the south coastal zone a linked system of variscan forethrusts is traced to outcrop in the core of the cowbridge anticline but none involves major displacement the ty9n y nant moel gilau fault system of the south wales coalfield is shown to be a component of the linked fault system and its current net extension is attributed to mesozoic reactivation of a variscan thrust involving only limited displacement the individual thrusts appear to connect to a basal thrust that may extend northwards into or under the south wales coalfield the amount of displacement along the basal thrust cannot be determined reliably but it may exceed 10 km and involve large scale repetition of palaeozoic sequences under the vale

diachronous variscan late orogenic collapse as a response to multiple detachments a view from the internides in france to the foreland in the irish sea