a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

eruptive history of earth s largest quaternary caldera toba indonesia clarified single grain laser fusion sup 40 ar sup 39 ar analyses of individual sanidine phenocrysts from the two youngest toba indonesia tuffs yield mean ages of 73 plus minus 4 and 501 plus minus 5 ka in addition glass shards from toba ash deposited in malaysia were dated at 68 plus minus 7 ka by the isothermal plateau fission track technique these new determinations in conjunction with previous ages for the two oldest tuffs at toba establish the chronology of four eruptive events from the toba caldera complex over the past 1 2 m y ash flow tuffs were erupted from the complex every 0 34 to 0 43 m y culminating with the enormous 2500 3000 km sup 3 youngest toba tuff eruption caldera formation and subsequent resurgence of samosir island timing of this last eruption at toba is coincident with the early wisconsin glacial advance the high precision sup 40 ar sup 39 ar age eruption of such magnitude may provide an important marker horizon useful as a baseline for research and modeling of the worldwide climatic impact of exceptionally large explosive eruptions

measurement and estimation of pore shrinkage and pore distribution in a clayey till during soil water characteristic curve tests the soil water characteristic curve swcc of fine grained soils is usually determined experimentally in many applications such as design of mine waste covers and landfill liners the unsaturated permeability function k h is often derived theoretically from the measured swcc implicit in these derivations is the transformation of the swcc to a pore size distribution psd typically assumed to be constant and mono modal psds of a clayey till compacted at various water contents were measured after compaction after flexible wall permeability testing and during and after swcc tests the measurements show that the psd changes significantly during permeability and swcc testing a method is advanced for predicting the observed changes in psd during swcc testing psds are determined for soil samples subjected to the highest and lowest suctions applied during the swcc test the measured psds are transformed to account for pore trapping the transform assumes that flow occurs through two sets of randomly d

amatitlan an actively resurging cauldron 10 km south of guatemala city a 14 16 km diameter collapse caldera has been recognized 10 km south of guatemala city guatemala the caldera is north of the presently active volcano pacaya and west of agua a large stratovolcano the caldera was not previously recognized because its eastern and western margins coincide with faults that outline the guatemala city graben and because the northern margin of the caldera is buried by pyroclastic rocks the existence of the northern caldera margin is now established by gravity data and a variety of geological observations including circumferential faults hot springs well log data and lithological changes in sedimentary rocks a sequence of nine silicic pyroclastic deposits totaling a volume of more than 70 km 3 dense rock were erupted from the caldera the ages of these eruptions are mainly between about 300 000 years b p to less than 23 000 years b p the rocks erupted at the caldera and its associated vents consist of domes and nonwelded pyroclastic flow surge and fall deposits mainly of rhyolitic to dacitic composition successive pyroclastic eruptions have generally become smaller in volume and more silicic with time major and minor element chemistry distinguish amatitlan pyroclastics from those of other nearby calderas the caldera lies at the intersection of an offset of the volcanic chain the palin shear and the faults along the volcanic front jalpatagua fault zone the caldera has a heavily faulted resurgent dome crosscut by an impressive longitudinal graben the graben s alignment with the jalpatagua fault zone suggests a genetic relationship the longitudinal graben and resurgent dome are morphologically youthful and are the sites of many young silicic vents available seismic data show a heavy concentration of epicenters over the northern part of the resurgent dome near a young silicic intrusion the caldera is active and will probably erupt again over 1 million people live within 20 km and would be threatened in the event of a moderate eruption suggestions for future research focus on hazard assessment and forecasting

a gravity and volcanostratigraphic interpretation of the lake toba region north sumatra indonesia abstract in northern and central sumatra a wide distribution of ignimbrite and volcanic sediments is observed some geologists consider that these volcanic products were erupted from lake toba the subduction of the indian ocean plate along the trench of central sumatra started at least as long ago as the late miocene fitch s model 1972 obtained the plate convergence vector 67 mm yr decomposed into its components perpendicular 58 mm yr and parallel 34 mm yr to the trench axis tears seaward of the mountain range created the sumatran fault about 2 ma ago at that time the edge of the plate reached a depth of almost 100 km and a large amount of dacite magma was generated the tuk tuk dacite a kind of ignimbrite was erupted along the sumatran fault and covered a broad area the age of this ignimbrite was obtained as 1 9 0 4 ma k arage and 2 0 0 3 ma fission track age since this eruption the hinterland of this fault has been changed to a tensional field forming a graben structure at the northern end of this graben west of harranggaoul the ignimbrite haranggaoul formation was erupted 1 3 0 3 ma ago forming a caldera the lacustrine sediments of samosir island have a normal magnetization and overlie the tuk tuk dacite unconformably from this we can conclude that the graben structure has been covered with water lake toba at 0 7 0 6 ma ago this tectonic movement has continued so that it is difficult to distinguish the caldera structure on the gravity anomaly map of the northern part of lake toba from the results of the gravity survey it can be concluded that at 0 1 ma ignimbrite flowed down along the asahan river and filled the old river along the lowland between parapat and porsea forming a caldera 30 000 years ago dacite and and esite eruptions have occurred around lake toba forming the volcanic cones

rotation of the chortis block causes dextral slip on the guayape fault the chortis block is undergoing deformation as a direct result of large magnitude sinistral strike slip faulting on the north america caribbean plate boundary we have studied the chortis block using geologic mapping remote imagery and earthquake seismology to evaluate the effect that large magnitude strike slip faulting has on an adjacent microcontinental block in the eastern part of the chortis block recent geologic mapping has shown that the northeast striking guayape fault is dextral instead of sinistral as previously assumed because this is a major fault that cuts most of the central america isthmus the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the chortis block the presence of several north trending rifts in the western part of the chortis block has led previous workers to propose that the chortis block is divided into smaller independently rotating blocks we use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the chortis block however we have modified their model to account for dextral slip on the guayape fault we believe that rotation of the chortis block causes dextral slip on the guayape fault even though central america is undergoing large sinistral displacement with respect to north america

the masaya triple layer a 2100 year old basaltic multi episodic plinian eruption from the masaya caldera complex nicaragua the masaya caldera complex has been the site of three highly explosive basaltic eruptions within the last six thousand years a plinian eruption ca 2 ka ago formed the widespread deposits of the masaya triple layer we distinguish two facies within the masaya triple layer from each other la concepcion facies to the south and managua facies to the northwest these two facies were previously treated as two separated deposits la concepcion tephra and the masaya triple layer of perez and freundt 2006 because of their distinct regional distribution and internal architectures however chemical compositions of bulk rock matrix and inclusion glasses and mineral phases demonstrate that they are the product of a single basaltic magma batch additionally a marker bed containing fluidal shaped vesicular lapilli allowed us to make a plausible correlation between the two facies also supported by consistent lateral changes in lithologic structure and composition thickness and grain size we distinguish 10 main subunits of the masaya triple layer i to x with bulk volumes ranging between 0 02 and 0 22 km3 adding up to 0 86 km3 0 4 km3 dre for the entire deposit distal deposits identified in two cores drilled offshore nicaragua at a distance of 170 km from the masaya caldera complex increase the total tephra volume to 3 4 km3 or 1 8 km3 dre of erupted basaltic magma isopleth data of five major fallout subunits indicate mass discharges of 106 to 108 kg s and eruption columns of 21 to 32 km height affected by wind speeds of 2 m s to 20 m s which increased during the course of the multi episodic eruption magmatic plinian events alternated with phreatoplinian eruptions and phreatomagmatic explosions generating surges that typically preceded breaks in activity while single eruptive episodes lasted for few hours the entire eruption probable lasted weeks to months this is indicated by changes in atmospheric conditions and ash layer surfaces that had become modified during the breaks in activity the masaya triple layer has allowed to reconstruct in detail how a basaltic plinian eruption develops in terms of duration episodicity and variable access of external water to the conduit with implications for volcanic hazard assessment

impact of desiccation on monotonic and cyclic shear strength of thickened gold tailings abstractvarious forms of thickened or high density tailings characterized by exhibition of a yield stress that facilitates deposition in gently sloped stacks are increasingly used for a variety of mining operations although this technology reduces reliance on containment by dams and the associated risk of catastrophic failure regulators are concerned with remobilization of such unbounded deposits by earthquakes or extreme rainfalls therefore the monotonic and cyclic shear strength of thickened tailings layers are of significant interest thickened tailings experience a particular stress history during deposition which may include desiccation rewetting and subsequent consolidation by burial under new tailings the effect of this stress history on their shear response is investigated using a norwegian geotechnical institute ngi type simple shear device samples are alternately mechanically overconsolidated or subjected to desiccation and subsequent rewetting because of burial by fresh tailings be

unsaturated flow in hydrating porous media with application to cemented mine backfill unsaturated flow in hydrating porous media is pertinent to several engineering applications including underground and surface use of cemented tailings proper description and modelling of flow is complicated by changes in material properties due to hydration as well as by the generation of suction by the net consumption of water volume by hydration variously termed chemical shrinkage or self desiccation it is necessary to define changes in hydraulic properties with time for instance the soil water characteristic curve swcc is not unique in time in addition to being path drying versus wetting dependent to address this problem a theory for modelling unsaturated flow in hydrating materials is introduced the theory deconstructs the swcc into a pore size distribution which changes with time as hydration occurs and the water content which is a variable that must be tracked with space and time matric suction depends on the time dependent swcc and the water content variable the theory is applied

the 1245 yr bp asososca maar eruption the youngest event along the nejapa miraflores volcanic fault western managua nicaragua abstract the asososca tephra comprises the asymmetrically distributed well stratified phreatomagmatic products of the 1245 125 120 yr bp asososca maar eruption west of managua nicaragua it is the youngest eruptive unit related to the monogenetic volcanism concentrated along the n s trending nejapa miraflores active fault and intersecting e w to ne sw and nw se minor faults the close relationship between monogenetic volcanism and the structural local tectonic framework is evidenced by the lineament domains and vent alignment patterns identified by remotely sensed images and dems of the research area the asososca maar lake bathymetry suggests that the crater was formed by coalescing vents migrating during the eruptive event the asososca tephra dominantly consists of accidental lithics disrupted from the underlying stratigraphic units observed inside and around the maar crater dry base surge bedsets are dominant throughout the eruptive unit showing facies variation from proximal cross stratified beds to mid distal plane parallel and wavy parallel beds grain size analyses indicate that base surges transported dominantly 1 0 and 1 sized particles at the bedload in form of traction carpets while fine ash material in continuous suspension was minimal juvenile fragments were only identified in the finer fractions varying from vesiculated scoriae to dense sideromelane fragments sem analyses suggest that the asososca eruption resulted from a highly efficient fuel coolant interaction between a tholeiitic basaltic melt and an aquifer hosted in a shallow level between 50 and 200 m of olivine bearing scoria ash and lapilli beds the explosion triggered the country rock comminution and the production of moss like fused shaped and blocky ash shards with stepped surfaces quenching cracks pitting alteration skins and adhered particles all indicative of phreatomagmatic fragmentation the very recent age of the asososca maar eruption confirms that the densely inhabited managua area is volcanically active and that hazard studies should focus on the potential for phreatomagmatic eruptions

relationship between tectonism and volcanism along the great sumatran fault zone deduced by spot image analyses abstract satellite images provide evidence for numerous stepovers pull apart grabens and volcanic structures along the nw trending right lateral great sumatran fault zone high resolution spot images permit us to analyze the relationship between volcanic and tectonic structures this analysis reveals that releasing stepovers and pull aparts are ephemeral structures along the great sumatran fault zone and that the geometry of the strike slip fault evolves permanently through time the interpretative structural maps obtained from spot image analysis reveal that the formation of huge peculiarly shaped volcanic calderas has occurred in large releasing stepover fault zones and that the bounding faults of rectangular pull apart basins are analogous to the circular ring faults of calderas in particular detailed study of the segments at the southernmost 150 km long termination of the great sumatran fault zone from northwest of ranau lake to semangka bay allows us to describe the development of the ranau collapse caldera this caldera is located within a pull apart basin bounded by a stepover its peculiar rectangular shape and its relatively large size about 200 km 2 are controlled by the evolution of the extinct ranau releasing stepover similarly the toba elliptical caldera one of the largest volcanic caldera in the world is elongated parallel to the present trace of the great sumatran fault and appears to be related to a releasing stepover associated with a wide pull apart basin that is not active at present

fluid pressure and effective stress in sandbox models abstract we have done a series of simple tests on sandpacks involving upward flow of compressed air through the pores and its effect on the yield strength the ultimate objective is to model deformation coupled with fluid flow in sedimentary basins for all tests we used a single batch of fontainebleau sand sieved to a grain size between 0 200 and 0 315 mm poured into a cylindrical container and then fluidized the density of this sand was 1 585 g cm3 irrespective of sand thickness the lithostatic pressure was proportional to the thickness of the sandpack a yield envelope was obtained by shearing the sandpack horizontally compressed air entered the base of the sandpack and flowed upwards through the pore spaces for 69 measurements on air flow without shearing a plot of discharge velocity versus gradient of fluid pressure is close to a straight line verifying darcy s law and yielding an intrinsic permeability of about 1 7 darcy for the sand for 72 tests on simultaneous shearing and fluid flow the estimated effective stress lithostatic stress minus estimated pore fluid pressure ranged from 0 to 1600 pa and the pore fluid ratio between air pressure and lithostatic pressure from 0 0 to 1 0 a plot of shear stress versus effective stress at failure is almost linear verifying terzaghi s principle of effective stress the best fit slope coefficient of internal friction is about 0 55 and the intercept on the shear stress axis cohesion is less than 85 pa the tests show that it is feasible to use compressed air within sandpacks as a means of modelling deformation coupled with fluid flow the next step will be to build sandbox models of layered sequences and to investigate detachments caused by abnormal fluid pressures

the interaction between volcanoes and strike slip transtensional and transpressional fault zones analogue models and natural examples regional strike slip faulting can control magma movements deform volcanoes and may destabilise their flanks the aim of this study is to address this problem by comparing two natural examples basse terre island volcanoes lesser antilles and maderas volcano nicaragua with analogue experiments the field and remote sensing analyses of their structures reveal that guadeloupe volcanoes which developed in a1 45 striking sinistral transtensional fault zone are dominantly fractured in a 090 e120 direction which is parallel to the maximum principal horizontal stress and to the elongation direction of the summit graben of analogue models this graben is bordered by the sigmoid i fault or y shear structure and has facilitated the formation of the beaugendre and vieux habitants valleys by faulting erosion or collapse this structure has also influenced the injection of dykes and the transport of hydrothermal fluids the comparison of maderas volcano with the analogue models confirms that the volcano has developed parallel to a 135

extension and rotation of crustal blocks in northern central america and effect on the volcanic arc sinistral displacements across the north american caribbean plate boundary in northern central america are distributed among major arcuate faults that have been active in neogene time south of the jocotan and motagua faults in guatemala extensional tectonics has accompanied rotation of the trailing edge of the caribbean plate around these faults segmentation of the volcanic arc in northern central america hitherto attributed to transverse breaks in the subducting cocos plate may instead be a result of this block rotation accompanying changes along the arc are observed in seismicity gravity anomaly patterns volume and composition of volcanic products and topography therefore the complex volcano tectonic geology south of the main boundary faults may be explained by interaction and rotation of crustal blocks in the overriding caribbean plate above the magma production zone along the downgoing cocos slab

cenozoic tectonics of the nicaraguan depression nicaragua and median trough el salvador based on seismic reflection profiling and remote sensing data lakes nicaragua and managua are the two largest lakes in central america and they cover a combined area of 9000 km 2 of the presently active nicaraguan depression and central america volcanic front as part of the subduction factory focus area of the u s national science margins program 1925 km of shallow geophysical data were acquired over lakes nicaragua and managua in may 2006 to establish their late quaternary structural and stratigraphic history and to better constrain regional models for active tectonics in western nicaragua the gulf of fonseca and the median trough in el salvador in order to investigate regional upper crustal deformation resulting from forearc sliver transport and or slab rollback of the cocos plate these new data were integrated with relocated earthquake epicenters earthquake focal mechanisms high resolution digital topography from the national aeronautics and space administration nasa shuttle radar topography mission srtm published global positioning system gps vectors onland geologic maps previous maps of lake bathymetry and bottom sediment types a previously unpublished regional aeromagnetic data set and multichannel seismic reflection profiles from the gulf of fonseca and pacific ocean these new data sets have improved bathymetric bottom sediment and recent fault maps for both nicaraguan lakes and can be used as new constraints on the regional geology and tectonics three regional structural cross sections across the nicaraguan depression indicate that the basin is a highly asymmetrical half graben bounded to the southwest by northeast dipping oblique slip normal faults late oligocene to holocene extension resulted in footwall uplift along the elevated and folded area of the nicaraguan isthmus and the area of greatest subsidence occurs on the footwall block underlying lake nicaragua in the southeast a similar but younger pattern of footwall uplift adjacent to the down thrown footwall block is present to the northeast beneath lake managua and the gulf of fonseca we interpret this structural pattern as a time transgressive rift opening where the oldest extension late oligocene early miocene began in the southeast and migrated to the northwest gps data indicate that this earlier phase of intra arc normal rifting is presently being superimposed by arc parallel right lateral shear related to the northwestward transport of the central america forearc sliver

piecewise linear formulation of coupled large strain consolidation and unsaturated flow i model development and implementation abstracta mass conservative formulation of one dimensional 1d coupled large strain consolidation and unsaturated flow using a piecewise linear formulation is developed the volume and water conte

la peligrosa caldera 47 15 s 71 40 w a key event during the jurassic ignimbrite flare up in southern patagonia argentina pyroclastic and lava vent facies from the late jurassic el quemado complex are described at the southern lake ghio in the cordillera patagonica austral based on the comprehensive study of lithology and structures the reconstruction of the volcanic architecture has been carried out four ignimbrites and one rhyolitic lava unit affected by oblique slip normal faults have been recognized the evolution of la peligrosa caldera has been modeled in three different stages 1 initial collapse consisting of a precursory downsag subsidence related to a dilatational zone which controlled the location of the caldera 2 main collapse with the emplacement of large volume crystal rich ignimbrites and megabreccias under a progressive subsidence controlled by a pull apart structure related to a transtensional regime and 3 post collapse in which lava flows and associated domes were emplaced under an oblique extensional regime the caldera records a remarkable change from transtension to oblique extension which may represent an important variation in regional deformation conditions during jurassic times la peligrosa caldera may be considered a key event to understand the eruptive mechanisms of the flare up volcanism in the chon aike silicic province 2013 elsevier b v

the regional geology and evolution of the toba volcano tectonic depression indonesia the late quaternary toba volcano tectonic depression in the sunda arc is the largest resurgent cauldron in the world and lies in one of the largest ignimbrite fields the toba tuffs although composed of at least 3000 km 3 of acid tuffs spread over 20000 km 2 of mainland sumatra the greater part of the toba tuffs is believed to be a single ignimbrite cooling unit formed about 100 000 years ago the toba tuffs also include a 30000 year old airfall tuff which was erupted from a centre just n of the toba depression the toba depression developed through the same stages as other resurgent cauldrons it formed after lithification of the toba tuffs by collapse along regional faults crossing an eastwards bulge in the sumatran geanticline resurgent uplift raised lake sediments within the depression by about 500 m the uplift which was not confined to the depression was probably caused by continuing influx of magma and it is possible that the toba depression is still in the stage of post resurgence volcanism the eruption of the toba tuffs and of all post ignimbrite volcanism took place on the line of the western marginal fault of the depression this marginal fault once extended northwards offshore into a zone of miocene back arc rifting

a pore network model for hydromechanical coupling in unsaturated compacted clayey soils the behaviour of deformable unsaturated soils is difficult to characterize with simple relationships unsaturated hydraulic properties namely the soil water characteristic curve and the hydraulic

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

an early cretaceous spinosaurid theropod from southern china teeth from the early cretaceous napai formation of fusui county guangxi zhuang autonomous region south china initially described as the sauropterygian sinopliosaurus fusuiensis are redescribed as belonging to a spinosaurid theropod closely allied to siamosaurus suteethorni from the early cretaceous of thailand this identification extends to china the geographical range of asian spinosaurs previously reported from thailand and japan

deep volcanic tremor and magma ascent mechanism under kilauea hawaii deep harmonic tremor originating at depths around 40 km under kilauea was studied using records accumulated since 1962 at the hawaii volcano observatory of the u s geological survey the deep source of the tremor was determined by onset times and confirmed by the relative amplitude across the island wide network of seismometers the period of tremor was conclusively shown to be determined by the source effect and not by the path or station site effect because the period would change considerably in time but maintained uniformity across the seismic net during the tremor episode the tremor appeared to be primarily composed of p waves we interpret the observed period and amplitude in terms of the stationary crack model of aki et al 1977 and find that the seismic moment rates for deep tremors are considerably larger than those for shallow tremors suggesting more vigorous transport for the former we propose a kinematic source model which may be more appropriate for deep tremor according to this model a measurable quantity called reduced displacement is directly proportional to the rate of magma flow a systematic search for deep tremor episodes was made for the period from 1962 through 1979 and the amplitude period and duration of the tremor were tabulated we then constructed a cumulative reduced displacement plot over the 18 year period the result shows a generally steady process which does not seem to be significantly affected by major eruptions and large earthquakes near the surface the total magma flow estimated from the reduced displacement is however one order of magnitude smaller than that estimated by swanson 1972 it may be that most channels transport magma aseismically and only those with strong barriers generate tremor

experimental modelling of thin skinned shortening around magmatic intrusions magmatic intrusions can trigger thin skinned compression of the adjacent sedimentary cover by three processes a gravity gliding away from the topographic dome resulting from the ascending magma b fluid push from the rear resulting from forceful intrusion then lateral spreading of the magma and c increased loading by volcanic accumulation the applicability of the first two mechanisms gravity gliding and fluid push was tested using dynamically scaled experiments model results help to elucidate the kinematics and structural evolution of thrusts and folds formed by such processes and determine which geological parameters control the deformation style the results show that the presence of a weak layer within the sedimentary overburden is essential to form thrust and fold belts around the intrusion experiments suggest that although gravity gliding can dominate the early stages of deformation most of the deformation is caused by lateral spreading of the fluid magma pushing the adjacent sedimentary rocks models also suggest that true laccoliths can form only if the sedimentary section comprises a basal low viscosity stratum comparison with natural examples allows the evaluation of the applicability of experimental results

volcanic spreading forcing and feedback in geothermal reservoir development amiata volcano italia abstract we made a stratigraphic structural and morphologic study of the amiata volcano in italy we find that the edifice is dissected by intersecting grabens that accommodate the collapse of the higher sectors of the volcano in turn a number of compressive structures and diapirs exist around the margin of the volcano these structures create an angular drainage pattern with stream damming and captures and a set of lakes within and around the volcano we interpret these structures as the result of volcanic spreading of amiata on its weak substratum formed by the late triassic evaporites burano anhydrites and the middle jurassic to early cretaceous clayey chaotic complexes ligurian complex regional doming created a slope in the basement facilitating the outward flow and spreading of the ductile layers forced by the volcanic load we model the dynamics of spreading with a scaled lubrication approximation of the navier stokes equations and numerically study a set of solutions in the model we include simple functions for volcanic deposition and surface erosion that change the topography over time scaling indicates that spreading at amiata could still be active the numerical solution shows that as the central part of the edifice sinks into the weak basement diapiric structures of the underlying formations form around the base of the volcano deposition of volcanic rocks within the volcano and surface erosion away from it both enhance spreading in addition a sloping basement may constitute a trigger for spreading and formation of trains of adjacent diapirs as a feedback the hot hydrothermal fluids decrease the shear strength of the anhydrites facilitating the spreading process finally we observe that volcanic spreading has created ideal heat traps that constitute todays exploited geothermal fields at amiata normal faults generated by volcanic spreading volcanic conduits and direct contact between volcanic rocks which host an extensive fresh water aquifer and the rocks of the geothermal field constitute ideal pathways for water recharge during vapour extraction for geothermal energy production we think that volcanic spreading could maintain faults in a critically stressed state facilitating the occurrence of induced and triggered seismicity

late stage sinking of plutons many granodiorite to diorite plutons in the great basin of western north america are surrounded by rim monoclines or anticlines that suggest relative downward movement of the plutons while wall rocks were hot and ductile we propose that such plutons rise to a level of approximately neutral buoyancy and then founder as their densities increase 10 during crystallization late stage sinking of intermediate to mafic plutons should be common when wall rocks are rich in weak low density minerals such as quartz and calcite structures related to sinking will overprint those related to initial pluton emplacement and may be mistaken for regional tectonic structures

late pleistocene to holocene temporal succession and magnitudes of highly explosive volcanic eruptions in west central nicaragua the stratigraphic succession of widespread tephra layers in west central nicaragua was emplaced by highly explosive eruptions from mainly three volcanoes the chiltepe volcanic complex and the masaya and apoyo calderas stratigraphic correlations are based on distinct compositions of tephras the total tephras combine to a total on shore volume of about 37 km3 produced during the last 60 ka the total erupted magma mass including also distal volumes of 184 gt dre distributes to 84 into 9 dacitic to rhyolitic eruptions and to 16 into 4 basaltic to basaltic andesitic eruptions the widely dispersed tephra sheets have up to five times the mass of their parental volcanic edifices and thus represent a significant albeit less obvious component of the arc volcanism eruption magnitudes m log10 m 7 with m the mass in kg range from m 4 1 to m 6 3 most of the eruptions were dominantly plinian with eruption columns reaching variably high into the stratosphere but minor phreatomagmatic phases were also involved two phreatomagmatic eruptions one dacitic and one basaltic andesitic produced mostly pyroclastic surges but also fallout from high eruption columns comparison of fallout tephra dispersal patterns with present day seasonally changing height dependant wind directions suggests that 8 eruptions occurred during the rainy season while 5 took place during the dry season the tephra succession documents two major phases of erosion the first phase 17 ka ago appears to be related to tectonic activity whereas the second phase may have been caused by wet climatic conditions between 2 to 6 ka ago the apoyo caldera had two large plinian caldera forming eruptions in rapid succession about 24 ka ago and should be considered a silicic volcano with long repose times three highly explosive basaltic eruptions were generated at the masaya caldera within the last 6 ka since then frequent but small eruptions and lava effusion were largely limited to the caldera interior the dacitic chiltepe volcanic complex experienced six plinian eruptions during the last 17 ka and seems to be an accelerating system in which eruption magnitude increased while the degree of differentiation of erupted magma decreased at the same time we speculate that the chiltepe system might produce the next large magnitude silicic eruption in west central nicaragua

stratigraphy geomorphology geochemistry and hazard implications of the nejapa volcanic field western managua nicaragua abstract the nejapa volcanic field nvf is located on the western outskirts of managua nicaragua it consists of at least 30 volcanic structures emplaced along the n s nejapa fault which represents the western active edge of the managua graben the study area covers the central and southern parts of the volcanic field we document the basic geomorphology stratigraphy chemistry and evolution of 17 monogenetic volcanic structures ticomo a b c d and e altos de ticomo nejapa san patricio nejapa norte motastepe el hormigon la embajada asososca satelite refineria and cuesta el plomo a and b stratigraphy aided by radiocarbon dating suggests that 23 eruptions have occurred in the area during the past 34 000 years fifteen of these eruptions originated in the volcanic field between 28 500 and 2 130 yr bp with recurrence intervals varying from 400 to 7 000 yr most of these eruptions were phreatomagmatic with minor strombolian and fissural lava flow events a future eruption along the fault might be of a phreatomagmatic type posing a serious threat to the more than 500 000 inhabitants in western managua

atmospheric dispersion environmental effects and potential health hazard associated with the low altitude gas plume of masaya volcano nicaragua masaya volcano 560 m a s l nicaragua re sumed its degassing activity in mid 1993 with the con tinuous emission of so 2 at rates increasing from 600 metric tons t day 1 7 0 kg s 1 in 1995 to 1800 t day 1 21 0 kg s 1 in 1999 the low altitude gas plume is typically blown westward by the prevailing wind across the masaya caldera and las sierras high lands which are at a higher elevation than the gas vent in this study the areal distribution of atmospheric so 2 concentrations was monitored within 44 km of the vent with a network of passive samplers measured so 2 air concentrations ranged from 2 to 90 ppbv in 1998 and from 2 to 230 ppbv in 1999 the data suggest that the volcanic emissions influenced air quality across a 1 250 km2 area downwind local topography exerts a strong control on plume dispersal and hilltops are particularly prone to fumigation and thus to high ambient so 2 lev els in a zone 22 km2 in size located within 15 km of the source the response of vegetation to sustained exposure to high atmospheric dose of volcanic so 2 and hf result ed in a strong reduction in the number of plant communi ties a transition zone of somewhat indefinite boundary surrounds the devastated zones and exhibits vegetation damage in the form of leaf injury in addition to the envi ronmental impacts of the volcanic emissions both short and long term public health hazards may exist in areas most exposed to the plume the harmful effects of the volcanic emissions on cultivated vegetation could be di minished by using windbreaks made of gas tolerant trees and shrubs such as eugenia jambos brosimum utile and clusia rosea the current gas crisis at masaya volcano provides an unique opportunity for investigating the at mospheric environmental and medical impacts of volca nic gases and aerosols

new physical characterization of the fontana lapilli basaltic plinian eruption nicaragua the fontana lapilli deposit was erupted in the late pleistocene from a vent or multiple vents located near masaya volcano nicaragua and is the product of one of the largest basaltic plinian eruptions studied so far this eruption evolved from an initial sequence of fluctuating fountain like events and moderately explosive pulses to a sustained plinian episode depositing fall beds of highly vesicular basaltic andesite scoria sio2 53 wt samples show unimodal grain size distribution and a moderate sorting that are uniform in time the juvenile component predominates 96 wt and consists of vesicular clasts with both sub angular and fluidal elongated shapes we obtain a maximum plume height of 32 km and an associated mass eruption rate of 1 4 108 kg s 1 for the plinian phase estimates of erupted volume are strongly sensitive to the technique used for the calculation and to the distribution of field data our best estimate for the erupted volume of the majority of the climactic plinian phase is between 2 9 and 3 8 km3 and was obtained by applying a power law fitting technique with different integration limits the estimated eruption duration varies between 4 and 6 h marine core data confirm that the tephra thinning is better fitted by a power law than by an exponential trend

emplacement mechanisms of contrasting debris avalanches at volcan mombacho nicaragua provided by structural and facies analysis we study the lithology structure and emplacement of two debris avalanche deposits dads with contrasting origins and materials from the quaternary holocene mombacho volcano nicaragua a clear comparison is possible because both dads were emplaced onto similar nearly flat 3 slope topography with no apparent barrier to transport this lack of confinement allows us to study in nature the perfect case scenario of a freely spreading avalanche in addition there is good evidence that no substratum was incorporated in the events during flow so facies changes are related only to internal dynamics mombacho shows evidence of at least three large flank collapses producing the two well preserved debris avalanches of this study one on its northern flank las isletas directed northeast and the other on its southern flank el crater directed south other south eastern features indicate that the debris avalanche corresponding to the third collapse la danta occurred before las isletas and el crater events the materials involved in each event were similar except in their alteration state and in the amount of substrata initially included in the collapse while el crater avalanche shows no signs of substratum involvement and has characteristics of a hydrothermal weakening related collapse the las isletas avalanche involves significant substratum and was generated by gravity spreading related failure the latter avalanche may have interacted with lake nicaragua during transport in which case its run out could have been modified through a detailed morphological and structural description of the mombacho avalanches we provide two contrasting examples of non eruptive volcanic flank collapse we show that remarkably even with two distinct collapse mechanisms the debris avalanches developed the same gross stratigraphy of a coarse layer above a fine layer this fine layer provided a low friction basal slide layer whereas dad layering and the run outs are roughly similar the distribution of structures is different and related to lithology las isletas has clear proximal faults replaced distally by inter hummock depressions where basal unit zones are exhumed whereas el crater has faults throughout but the basal layer is hidden in the distal zone hummocky forms depend on material type with steep hummocks being formed of coherent lava units and low hummocks by matrix rich units in both avalanches extensional structures predominate the upper layers exclusively underwent longitudinal and lateral extension this is consistent with evidence of only small amounts of block to block interactions during bulk horizontal spreading the base of the moving mass accommodated transport by large amounts of simple shear we suggest that contractional structures and inter block collisions seen in many other avalanches are artifacts related to topographic confinement

stratigraphy paleomagnetism and magnetic fabric of the toba tuffs constraints on the sources and eruptive styles the toba depression in north central sumatra is a complex of several overlapping calderas resulting from three major ignimbrite forming eruptions within the depression the upland masses of samosir and northern uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment hitherto interpreted to be two parts of a single resurgent dome this study has demonstrated that the welded tuffs of samosir and uluan have different magnetic polarities and therefore at least two different ignimbrites are present the samosir uluan massif may consist of parts of two resurgent domes the first ignimbrite eruption occurred at 0 84 ma and produced a very thick 400 m densely welded unit having a reversed polarity anisotropy of magnetic susceptibility ams flow direction and lithic size data indicate that the source lies in the southern part of the toba depression and the thick deposit of uluan is thought to have ponded in a 40 km wide caldera the second ignimbrite is normally magnetized ams flow direction data indicate two separate source vents one to the north in the haranggaol caldera and another to the south the thick deposit at samosir is thought to have ponded in the southern caldera coarse sediments then accumulated over samosir and northern uluan and were capped by lacustrine deposits a renewed episode of resurgence then uplifted samosir island and possibly the northern part of uluan at approximately 0 075 ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the toba depression this ignimbrite is mostly nonwelded and normally magnetized part of the uluan dome was destroyed by collapse of the sibandung caldera and latung graben and concomitant with renewed subsidence of the haranggaol and porsea calderas

neotectonic development of western nicaragua this study presents a reconstruction of the neotectonic evolution of western nicaragua by means of structural analysis of outcrop scale joints faults and bedding planes integrated with macroscale structures the results indicate three clearly separable deformation phases which are closely related to specific volcanic events and account for the major tectonic features of nicaragua the first deformation phase was a late miocene to early pliocene shortening that produced the pacific plain anticlines and controlled the contemporaneous extrusion of the el coyol group volcanic rocks the second phase active between the pliocene and the pleistocene resulted in extensional structures such as the 500 km long and 50 km wide nw se trending nicaragua depression this phase was accompanied by a gradual trenchward migration of the volcanic activity responsible for the formation of the las sierras group the third deformation phase is still active and is responsible for the present shallow focus seismic activity associated with n s trending volcanic vents and the n s managua pull apart graben the neotectonic development of western nicaragua as interpreted here is closely related to the contemporaneous evolution of the neighboring areas of guatemala south of the jocotan fault honduras el salvador and costa rica north of the valle central the valle central of costa rica represents the southern boundary of this structurally homogeneous domain and may indicate the southern boundary of the neogene chortis block changes in deformation style and the trenchward migration of volcanism which occurred during the pliocene may indicate an increase in the subduction angle of the cocos plate and may also be correlated to pliocene shortening of jamaica and hispaniola

correlating geochemistry tectonics and volcanic volume along the central american volcanic front the central american volcanic front consists of several distinct volcanic lineaments or segments separated by right steps and or changes in strike each volcanic line is rotated slightly counterclockwise from the strike of the inclined seismic zone right stepping volcanic lines oblique to the strike of the slab create a sawtooth pattern in the depth to the slab zr nb is the first geochemical signature with consistent large offsets at the right steps in the volcanic front moreover zr nb mirrors the sawtooth variation in depth to the slab within a segment it increases from se to nw and at the right steps separating segments it abruptly decreases unfortunately there is no simple negative correlation between zr nb and depth to the slab because zr nb also has a regional variation similar to previously documented regional variations in slab tracers in central america e g ba la u th and 87sr 86sr within a segment zr nb decreases with increasing depth to slab this can be explained in two ways a nb retaining mineral e g amphibole in the subducting slab is breaking down gradually with increasing depth causing more nb to be released and consequently a smaller nb depletion in deeper melts alternatively all melts have the same initial nb depletion which is then diluted by acquiring nb from the surrounding mantle wedge as melts rise and react deeper melts have longer paths and therefore more reaction with the mantle wedge diluting the initial nb depletion within each volcanic segment there is variation in eruptive volume the largest volcanoes generally occur in the middle of the segments and the smaller volcanoes tend to be located at the ends connecting the largest volcanoes in each segment suggests an axis of maximum productivity this is likely the surface projection of the center of the melt aggregation zone the largest volcanoes tap the entire melt zone those with shallow depths to the slab tap just the front part of the melt zone and have very large nb depletions those at greater depths tap the back part of the melt zone and have much smaller nb depletions

coronae on venus and mars implications for similar structures on earth complex volcano tectonic structures referred to as coronae had not been described until the exploration of the surface of venus these large generally circular structures are characterized by an elevated surface concentric and radial fracture systems and extensive volcanism thought to be unique to venus rare circular features on mars bear a close resemblance to coronae the most prominent corona like feature on mars is alba patera a broad low relief plateau shaped volcano tectonic center surrounded by an annulus of concentric fractures 600 km in diameter a geophysical model for the formation of venusian coronae involving uplift due to an ascending mantle diapir followed by gravitationally driven relaxation is applied to mars the results indicate that alba patera could have formed by such a mechanism the formation of coronae and corona like features on venus and mars from mantle diapirs suggests that similar structures may have formed in earth9s lithosphere

analogue models of pull apart basins sandbox analogue models of pull apart basins that developed in sedimentary strata above releasing steps in underlying basement faults are characterized by rhombic basins that are flat bottomed box grabens with a subhorizontal synkinematic basin infill steep to nearly vertical sigmoidal oblique slip and segmented oblique extensional faults are the dominant bounding structures of the pull apart basins cross basin short cut faults link the offset principal displacement zones that are characterized by flower structure development the structural architectures of the physical models compare directly in form and dimensions to natural examples of strike slip pull apart basins

scaled experiments of volcanic spreading experiments were conducted to study the spreading of volcanic constructs volcanoes are simulated by a sand cone and the volcanic substratum is simulated by a sand layer brittle substratum overlying a silicone layer ductile substratum similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless numbers experiments determine values which predict whether or not spreading takes place of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano 2 and the brittle ductile ratio of the substratum 3 2 indicates that the volcano must be large enough to break the substratum before spreading occurs whereas 3 controls the style of deformation during spreading these dimensionless numbers change with time reaching values that tend toward those observed for stable configurations experimental values are compared with those from well constrained natural examples it is found that an essential requirement for volcanic spreading is the presence of a low viscosity layer within the substratum flow of the weak layer away from the excess load is responsible for the spreading the overlying edifice displays radial intersecting grabens due to concentric stretching dissected summit areas concentric zones of thrusts and folds form in the substratum around the edifice and diapirs of the ductile substratum rise within the fault zones

the managua graben and las sierras masaya volcanic complex nicaragua pull apart localization by an intrusive complex results from analogue modeling abstract there is a well documented association between pull apart basins in strike slip zones and large volcanic caldera complexes las sierras masaya volcanic complex nicaragua is a large basaltic lava and ignimbrite shield with nested calderas the widest caldera is partly underlain by a large and dense intrusive complex evidenced by a positive gravity anomaly the las sierras caldera relating to this cumulate complex is less than 30 000 years old and is probably still hot and ductile the inner masaya caldera hosts an active magmatic system the volcanic complex is in a dextral transtensional tectonic context of the nicaraguan depression the highly active managua graben is on the northern part of the volcano we speculate that the graben and volcano are linked tectonically with the graben initiating in response of a regional stress field modified around the dense ductile intrusive complex previous field work and reappraisal of structures with digital elevation model morphological analysis show that the volcano is surrounded by a rhombic fault pattern that may form a nascent pull apart basin we have done scaled analogue models to test the effect of intrusion density and rheology anomalies on strike slip fault geometries the models show that intrusion density variations alone do not significantly change fault patterns in contrast ductile rocks silicone in the models as analogue for hot mafic intrusive rocks markedly alter strike slip fault patterns in transtension the presence of a ductile intrusion causes the formation of a pull apart while in pure strike slip or transpression uplift and thrusting is generated pull apart and uplift structures are rhomb shaped even when the ductile inclusion is circular we conclude that a pull apart is developing at las sierras masaya volcanic complex in response to the transtensive regional deformation regime and the dense ductile intrusive complex we suggest that the volcano and graben are one dynamic system and should be monitored as one entity

tectonic interpretation of the morphology of honduras the morphotectonics of honduras have been studied using existing geologic and topographic maps and imaging done from satellites and the following interpretations have been made during the late oligocene to early miocene a wide strike slip boundary existed in western honduras and eastern guatemala and produced east west trending basins containing the coarse clastic rocks of the subinal formation marine geophysical studies of the continental shelf of honduras show that during the rest of the miocene and the early pliocene the boundary was inactive but motion resumed in the late pliocene to pleistocene and produced horsts on the north coast of honduras in northwestern honduras the east northeast trending alignment of the jocotan copan and charnelecon river valleys may result from crustal flexuring rather than from faulting as previously proposed because the postulated fault the jocotan chamelecon fault cannot be reconciled with available geologic maps the northwest and northeast flowing rivers of central and eastern honduras define domains of strike slip faulting that have probably rotated 30 from an original set of conjugate fractures widespread deposition of conglomerates suggests that strike slip movements began in the mid cretaceous and reoccurred at intervals culminating in the development of pull apart basins in western honduras during the late pliocene at the present time no further strike slip movement is possible as the faults defining each domain lie parallel and perpendicular to the middle america trench new conjugate fractures however cut honduras along north south and east northeast trends and have also separated into domains in the domain of north south fractures easterly directed ductile extension of the lithosphere beneath western honduras and guatemala is opening up rifts but the domain of east northeast fractures is characterized by strike slip motion that has produced left lateral pull aparts in central honduras early workers identified no rift valley extending from the gulf of fonseca to the caribbean and there is no fresh evidence upon which to base the existence of one today the notion of the honduras depression which has persisted over the last half century stems from misinterpretation of the maps and writings of karl sapper

p and s velocity structure of the yellowstone volcanic field from local earthquake and controlled source tomography the three dimensional p and s velocity from vp vs ratios distribution and improved hypocenters for the yellowstone volcanic field have been determined from inversion of first arrival times from 7942 local earthquakes and 16 controlled source explosions the p velocity model has an rms residual of 0 09 s whereas the vp vs ratio model calculated from 511 earthquakes has an rms residual of 0 29 s high p and s velocities outside the yellowstone caldera represent thermally undisturbed basement and sedimentary rocks a caldera wide 15 decrease from regional p velocities at depths of 6 to 12 km is coincident with a 60 mgal gravity anomaly and is interpreted as a hot subsolidus granitic batholith with a quasi plastic rheology localized 30 reductions from regional seismic velocities and higher vp vs ratios 8 km beneath yellowstone s resurgent domes are interpreted as partial melts and vestigial magma systems associated with youthful less than 2 ma silicic volcanism additional low seismic velocities and vp vs ratios and a 20 mgal gravity low less than 4 km beneath the northeast caldera rim are interpreted as a hydrothermal fracture zone thermally driven by underlying partial melt hypocenters relocated with the three dimensional p velocity model show subparallel alignment with nw se trending postcaldera volcanic vents and normal faults northwest of the caldera focal depths of relocated earthquakes decrease from more than 11 km outside the caldera to less than 6 km within the caldera reflecting thinning and heating of the seismogenic crust

three dimensional elastic wave velocity structure of the western and central tien shan arrival times of compressional p and shear s waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central tien shan are used to determine one and three dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt the best fit one dimensional structures suggest that the average depth of the mohorovicic discontinuity in this area is 50 km the three dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region a 7 km thick wedge of sediment beneath the chu depression is outlined at depth by a south dipping plane of seismic activity suggesting the presence of an active decollemont these low velocities extend continuously to the southeast toward issyk kul suggesting a structural relationship between the two however rather than being consumed it appears that issyk kul is overthrusting the surrounding ranges the low velocity sediments in the fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity velocities at midcrustal depths generally are lower beneath the central tien shan than beneath the western tien shan this pattern becomes more evident in the uppermost mantle with p velocity contrasts of as much as 10 across a boundary that corresponds roughly to the geographical position of the talasso fergana fault the low velocities beneath the central tien shan exceed 150 km depth but do not appear to be deeper than 300 km depth there is no evidence for a lithospheric root beneath this part of the range rather the low velocities imply the presence of a positive buoyancy force uplifting the mountains evidence that this low velocity region existed before the collision suggests that the tien shan may not owe its rejuvenation simply to its location at the northern edge of a strong tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere

the effect of volcanic constructs on rift fault patterns volcanoes in rift situations often show distinctive hour glass fault patterns with increased fault throw as the volcano is approached we present analogue models that show that this is caused by an interaction of the regional stress field with that set up by the volcano mass for faults to be reorientated there must be a ductile layer below the volcano this can be hot crust at mid ocean ridges continental rifts or weak sedimentary strata increased volcano size or mass and lower brittle ductile ratios lead to increased fault curvature whereas an increased regional extension rate decreases the effect volcanoes on one side of a rift may capture it forming the axis of a new rift with curved or en echelon zones on either side by concentrating extension magma is more easily erupted so volcanoes may erupt more magma of less evolved composition a positive feedback between increased extension and magma eruption rate will lead to rift narrowing which can favor the formation of oceanic crust

coupling hydraulic with mechanical models for unsaturated soils this paper presents an alternative method to couple the hydraulic component with the mechanical component in a constitutive model for unsaturated soils some pioneering work on hydromechanical coup

shear test results for cohesion and friction coefficients for different granular materials scaling implications for their usage in analogue modelling abstract laboratory tests have been carried out on dry granular materials such as quartz sand glass microspheres and sugar with different grain size rounding and sphericity the measurements have been made with a simple shear test machine for different values of normal stress 50 900 pa shear stress has been plotted against normal stress in order to determine the cohesion and coefficient of internal friction for the investigated materials resulting values of cohesion and coefficient of internal friction are mainly dependent on rounding and sphericity while grain size has a less significant influence further the behaviour of the materials for very small normal stresses 0 400 pa is more complex than previously assumed the fracture envelopes for all materials investigated are convex outward for this small range and converge towards a straight failure envelope with increasing normal stress finally in extensional faulting experiments there is no significant change in fault dip with increasing depth therefore the non linear behaviour for small normal stresses is best described as a dependence of the cohesion on the normal stress and not as a dependence of the coefficient of internal friction on the normal stress values for cohesion increase from 0 pa 15 pa at zero normal stress to 137 247 pa 15 pa for normal stresses greater than 250 400 pa the results show that well rounded spherical material is better suited to model brittle behaviour of rocks in crustal and lithospheric scale analogue models than less well rounded material since it has a smaller cohesion and a coefficient of internal friction which is closer to values of natural rocks

crustal structure beneath long valley caldera from modeling of teleseismic p wave polarizations and ps converted waves in this study the authors present new constraints on the nature of the low velocity zone beneath long valley caldera based on the measured propagation directions of teleseismic p waves and on modeling of p to s converted waves the low velocity body is a large asymmetrical volume which deepens to the east extending from depths of 7 to 30 km it contains lower velocities than originally proposed by earlier teleseismic studies in particular there is a tabular feature between 7 and 11 km depth that has a reduction in velocity of about 30 these low velocities imply a much greater percentage of melt in the crust beneath long valley caldera than previously estimated array analysis of large delayed arrivals identifies them to be ps converted waves from the shoulders and roof of this tabular zone these conversions bound the depth to the magma chamber roof to be within about 10 km of the surface these results are consistent with elements from several other studies and the authors present an integrated and improved model of crustal structure at long valley the concordance of the deeper low velocity zones with regional structural trends implies that the shallow low velocity feature is a cupola on topmore of an asymmetric diapiric ridge rising up from the migmatized lower crust of the basin and range the authors present two contrasting interpretations of the geometry of low velocity zones in the crust one implies a time invariant magma chamber and conduit system for long valley caldera the other implies an evolution of that system from a simple vertical regime to its current asymmetrical geometry 37 refs 19 figs 1 tab less

the geochemical variations of the upper cenozoic volcanism along the calama olacapato el toro transversal fault system in central andes 24 s petrogenetic and geodynamic implications in this paper we present new geochemical and sr nd isotopic data for several upper miocene volcanic centres aligned along one of the most extensive transcurrent lineament in the central andes the calama olacapato el toro cot the transversal volcanic belt along cot is constituted by large composite volcanoes and a caldera structure they are from nw to se puntas negras rincon tul tul del medio and pocitos tumepo quevar aguas calientes and tastil in order to compare chemical data from the different centres along the cot transect differentiation effects were minimised by using data extrapolated at 60 sio2 with least square regression method in the western sector of the cot the volcanic products of puntas negras and rincon show relatively high k2o and 87sr 86sr and low rb cs ta th la yb 143nd 144nd to the east the tumepo products have high sr and 143nd 144nd la yb and ba rb and low y 87sr 86sr in the easternmost cot sector quevar aguas calientes and tastil volcanic complexes exhibit low la yb high87sr sr86 and low 143nd 144nd on the basis of these data we propose a petrogenetic and geodynamical model for central andes at 24 s in correspondence of miocene quaternary volcanic arc puntas negras and rincon the magmas inherited a calcalkaline signature partly modified by upper crustal and or sediment assimilation in the central eastern sector melting assimilation storage and homogenisation mash processes occurred at the base of a thickened crust in this cot sector tumepo products show an evident lower crust signature and could be considered representative for mash derived magmas in the easternmost sector quevar aguas calientes and tastil products could represent magmas generated by partial melting of underthrusted brasilian shield and mixed with magmas derived by mash processes

three dimensional p and s wave velocity structures beneath taiwan deep structure beneath an arc continent collision the island of taiwan is a byproduct of one of the few active collisions between a continent represented by the eurasian continental shelf and an island arc represented by the northern extension of the luzon arc on the philippine sea plate to understand better the evolution and current tectonics of this collision we selected 1260 well recorded earthquakes from an initial set of 50 000 located by the taiwan telemetered seismograph network for the determination of one and three dimensional p and s wave velocity structures beneath the island the results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones in the east the philippine oceanic plate is underthrusting the eurasian plate east of a north south boundary that is well defined by both seismic activity and a region of high velocity in the south the eurasian continental plate is underthrusting the philippine sea plate south of an east west boundary at about 23 n that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities the dip of the subducted continent is shallow until it reaches the luzon island arc 50 km east of the main island structure under the main part of the island north of 230n reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range the dip of this low velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km this seismic zone lies in a velocity saddle and marks an apparent offset in the central low velocity region at 24 n evidence for the subduction of the eurasian continent beneath taiwan therefore exists everywhere beneath the island and the low velocity regions in the mantle support but do not require the subduction of some 6 16 km of lower continental crust to depths of at least 50 km the distinct change in both the velocity structure and the seismic activity at 23 n and the offset of the low velocity regions at 24 n argue for abrupt changes in the nature of subduction across these latitudes these changes may be caused by an interaction of the luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface finally the velocity structure of the subducting philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest velocity regions of the plate but are probably located near the upper edge of the plate there is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented and that subduction is presently occurring as far south as 22 n

early cretaceous vertebrates from the xinlong formation of guangxi southern china a review the vertebrate assemblage from the early cretaceous non marine xinlong formation of the napai basin in the south western part of guangxi zhuang autonomous region southern china is reviewed the assemblage includes chondrichthyans at least six species of hybodont sharks including hybodus thaiodus heteroptychodus and acrorhizodus actinopterygians halecomorphi and ginglymodi turtles the adocid shachemys and the carettochelyid kizylkumemys crocodilians cf theriosuchus and dinosaurs the sauropods fusuisaurus and liubangosaurus carcharodontosaurid and spinosaurid theropods iguanodontians and a possible psittacosaurid this assemblage shows many similarities to those from non marine formations of the khorat group of north eastern thailand it seems to be particularly close to that from the khok kruat formation considered as aptian in age as shown especially by sharks and turtles and by the presence of iguanodontians an aptian age is therefore proposed for the xinlong formation a study of the stable oxygen and carbon isotope compositions of reptile apatite suggests that this part of south china experienced subtropical arid conditions during the deposition of the xinlong formation in its composition the vertebrate fauna from the xinlong formation seems to be more similar to coeval faunas from se asia than to assemblages from northern china including the jehol biota although this may partly reflect different depositional and taphonomic environments fluvial for the xinlong formation versus lacustrine for the jehol biota it seems likely that during early cretaceous time southern china and se asia were part of a distinct zoogeographical province different from that corresponding to northern china this may be the result of both climatic differences with relatively cool climates in northern china versus a subtropical climate in the south and geographical barriers such as mountain chains

apparent downwind depletion of volcanic so2 flux lessons from masaya volcano nicaragua a series of 707 measurements at masaya in 2005 2006 and 2007 reveals that so2 emissions 15km downwind of the active vent appear to be 33 to 50 less than those measured only 5km from the vent measurements from this and previous studies indicate that dry deposition of sulfur from the plume and conversion of so2 to sulfate aerosols within the plume each may amount to a maximum of 10 loss and are not sufficient to account for the larger apparent loss measured however the so2 measurement site 15km downwind is located on a ridge over which local trade winds and the entrained plume accelerate greater wind speeds cause localized dilution of the plume along the axis of propagation the lower concentrations of so2 measured on the ridge therefore lead to calculations of lower fluxes when calculated at the same plume speed as measurements from only 5km downwind and is responsible for the apparent loss of so2 due to the importance of so2 emission rates with respect to hazard mitigation petrologic studies and sulfur budget calculations measured fluxes of so2 must be as accurate as possible future campaigns to measure so2 flux at masaya and similar volcanoes will require individual plume speed measurements to be taken at each flux measurement site to compensate for dilution and subsequent calculation of lower fluxes this study highlights the importance of a comprehensive understanding of a volcano s interaction with its surroundings especially for low boundary layer volcanoes

three hawaiian calderas an origin through loading by shallow intrusions the calderas of kilauea and mauna loa are highly dynamic structures and in the 200 year historic period have varied in volume by a factor of 2 and gained or lost 1 km3 per century the deeply eroded caldera of the extinct koolau volcano in oahu is wider than active hawaiian calderas and its lavas have a strong centripetal dip and funnel structure not evident at kilauea or mauna loa the differences can be attributed to the different erosion depths and the time integrated subsidence profile of kilauea is also a stepped funnel having its apex at halemaumau koolau caldera is the focus of an extraordinarily intense dike complex and an intriguing feature is the great diminution in dike concentration into the caldera it is thought that dike injection in any part of the complex generally continued until it reached 50 to 65 in outer parts of the caldera the complex was maintained at or rebuilt to this value despite subsidence in the center of the caldera where the positive bouguer anomaly is centered subsidence evidently greatly outpaced the capacity of dike injections to rebuild the complex assuming the same dike injection rate as kilauea and mauna loa yields a volumetric subsidence rate in koolau caldera exceeding 1 km3 per century hawaiian calderas are much more dynamic than calderas of silicic volcanoes shaped by frequent small events instead of a few great ones the temporal and volumetric correspondence of historical subsidence events with eruptions is poor and this and the high subsidence rates argue for a caldera forming mechanism that consumes the subsided rocks it is suggested that subsidence is caused by the great localized excess load of intrusive rocks carrying the center of the volcano into the thermally weakened lithosphere above the hawaiian hot spot it is envisaged that under steady state conditions the magma chamber rises as the injection of intrusions causes the level of neutral buoyancy at which the chamber is located to ascend and keeps pace with subsidence

late cenozoic tephrostratigraphy offshore the southern central american volcanic arc 1 tephra ages and provenance we studied the tephra inventory of 18 deep sea drill sites from six dsdp odp legs legs 84 138 170 202 205 206 and two iodp legs legs 334 and 344 offshore the southern central american volcanic arc cava eight drill sites are located on the incoming cocos plate and ten drill sites on the continental slope of the caribbean plate in total we examined 840 ash bearing horizons and identified 650 of these as primary ash beds of which 430 originated from the cava correlations of ash beds were established between marine cores and with terrestrial tephra deposits using major and trace element glass compositions with respect to relative stratigraphic order as a prerequisite for marine terrestrial correlations we present a new geochemical data set for significant neogene and quaternary costa rican tephras moreover new ar ar ages for marine tephras have been determined and marine ash beds are also dated using the pelagic sedimentation rates the resulting correlations and provenance analyses build a tephrochronostratigraphic framework for costa rica and nicaragua that covers the last 8 myr we define 39 correlations of marine ash beds to specific tephra formations in costa rica and nicaragua from the 4 15 ma lower sandillal ignimbrite to the 3 5 ka rincon de la vieja tephra from costa rica as well as another 32 widely distributed tephra layers for which their specific region of origin along costa rica and nicaragua can be constrained this article is protected by copyright all rights reserved

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

extension induced by volcanic loading in regional strike slip zones volcanoes and strike slip pull apart structures are commonly found together we use theoretical predictions and analogue modeling to evaluate the effect of volcanic loading in strike slip zones and the converse effect of regional strike slip faults on volcano structure theoretical predictions indicate that regional stress is reoriented by volcano loading to cause extension analogue models indicate that volcanoes in strike slip zones develop extensional pull apart structures a graben typically forms normal to the minimum regional stress axis 3 at a cone cones emplaced onto preexisting faults increase that fault9s tensional component if the substratum is weak enough to permit volcano spreading thrusts form normal to the maximum stress axis 1 a feedback situation can arise i e loading related extension allows more magma influx more eruption and thus increased loading this process can create large volcano tectonic depressions volcano structures may also be used as indicators of regional strike slip movements

caldera subsidence in areas of variable topographic relief results from analogue modeling calderas form in volcanic areas commonly associated with topographic relief pre existing topography plays an important role in the style of the caldera subsidence topography increases the load and affects the principle stress trajectories located between the roof of the magma chamber and the surface the morphology internal structure and temporal evolution of calderas are therefore sensitive to the local topography we carried out scaled analogue experiments to investigate the effect of pre existing topographic relief on caldera subsidence by modeling the presence of stratocones and plateaus of variable mass diameter and positions prior to collapse we induced collapse in sandbox experiments by withdrawing water from a rubber bladder to simulate caldera collapse into a large shallow reservoir deformation was first manifested by sagging of the sand at the surface followed by the upward propagation of a set of subsidence controlling faults from the bladder in experiments with no topography these faults usually reached the surface near the centre of the cylinder as evacuation and incremental growth progressed a second outer set of subsidence controlling faults developed this outer set dominated the subsidence by increasing the topographic load by 6 the subsidence efficiency increased producing calderas up to approximately 20 deeper the inner set of subsidence controlling faults steepened and controlled most of the collapse by contrast to the experiments without topography adding load also reduced outward growth of peripheral sagging and development of extensional faults leading to a diameter of sagging 20 smaller than with no topography a comparative analysis of our results with mount mazama at crater lake oregon and glass mountain at long valley california supports the interpretation that the addition of topographic load modifies the evolution of the major caldera faults

a discussion of the application of mercury intrusion porosimetry for the investigation of soils including an evaluation of its use to estimate volume change in compacted clayey soils

analogue models of caldera collapse in strike slip tectonic regimes regional scale faulting particularly in strike slip tectonic regimes is a relatively poorly constrained factor in the formation of caldera volcanoes to examine interactions between structures associated with regional tectonic strike slip deformation and volcano tectonic caldera subsidence we made scaled analogue models tabular sill like inclusions of creamed honey in a sand gypsum mix replicated shallow level granitic magma chambers in the brittle upper crust lateral motion of a base plate sited below half the sand gypsum pack allowed simulation of regional strike slip deformation our experiments modelled 1 strike slip deformation of a homogeneous brittle medium 2 strike slip deformation of a brittle medium containing a passive magma reservoir 3 caldera collapse into sill like magma reservoirs without regional strike slip deformation and 4 caldera collapse into sill like magma reservoirs after regional strike slip deformation our results show that whilst the magma chamber shape principally influences the development and geometry of volcano tectonic collapse structures regional tectonic strike slip faults riedel shears and y shears may affect a caldera s structural evolution in two main ways firstly regional strike slip faults above the magma chamber may form a pre collapse structural grain that is exploited and reactivated during subsidence our experiments show that such faults may preferentially reactivate where tangential to the collapse area and coincident with the chamber margins in this case volcano tectonic extension in the caldera periphery tends to localise on regional tectonic faults that lie just outside the chamber margins in addition volcano tectonic reverse faults may link with and reactivate pre collapse regional tectonic faults that lie just inside the chamber margins secondly where regional tectonic strike slip faults define corners in the magma chamber margin they may halt the propagation of volcano tectonic reverse faults the experiments also highlight the potential difficulties in assessing the relative contributions of volcano tectonic and regional tectonic subsidence processes to the final caldera structure seen in the field disruption of the pre collapse surface by regional tectonic faulting was preserved during coherent volcano tectonic subsidence to produce a caldera floor of differentially subsided fault blocks without definitive evidence for syn eruptive growth faulting thickness changes in caldera fill across such regional tectonic fault blocks in nature could be mistaken as evidence for piecemeal volcano tectonic collapse

the degassing character of a young volcanic center cerro negro nicaragua cerro negro volcano is a young basaltic cinder cone which is part of the nicaraguan volcanic arc eruptive activity at cerro negro is characterized by explosive strombolian to subplinian eruptions driven by volatile rich basaltic magma ascending rapidly from various crustal depths 15 to 6 km resulting in the onset of precursory activity only 30 min before an eruption in this paper we present a comprehensive degassing characterization of the volcano over a 4 year period aimed at improving our understanding of the magmatic plumbing network and its relationship with regional tectonics a total of 124 individual soil gas samples were collected between 2010 and 2013 and analyzed for stable carbon isotopes 13c from co2 high temperature fumaroles were sampled for 18o d and 3he 4he isotope analysis and major degassing zones were mapped using soil co2 flux measurements gases at cerro negro are characterized by a strong 3he 4he mantle signature 6 3 to 7 3 ra magmatic 13c ratios 2 3 to 3 0 meteoric 18o and d ratios and stable co2 fluxes 31 t d 1 the lack of 13c fractionation and an increase in the relative mantle component from 2002 to 2012 suggest that the volatile flux at cerro negro originates from the mantle and ascends to the surface via a series of crustal fractures that act as permeable conduits despite the lack of new eruptions the hydrothermal system of cerro negro continues to evolve due to seasonal inputs of meteoric water slope failures that expose and bury sites of active degassing and bursts of regional seismicity that have the potential to open up new conduits for gas release as well as magma continuing geophysical and geochemical monitoring of the main edifice and the recently formed south zone is essential as the volcano remains overdue to erupt

an insight into the water retention properties of compacted clayey soils experimental data from different testing methodologies on different compacted clayey soils with dominant bimodal pore size distribution are presented and analysed to provide a comprehensive picture of the evolution of the aggregated fabric along hydraulic and mechanical paths fabric changes are analysed both from the porous network viewpoint by means of careful mercury intrusion porosimetry investigation and from the soil skeleton viewpoint by quantifying swelling and shrinkage of the aggregates in an environmental scanning electron microscopy study the consequences of the aggregated fabric evolution on the water retention properties of compacted soils are analysed and discussed a new model for water retention domain is proposed which introduces a dependence of the intra aggregate pore volume on water content the model succeeds in tracking correctly the evolution of the hydraulic state of the different soils investigated along generalised hydromechanical paths the proposed approach brings to l

distribution of magma beneath the toba caldera complex north sumatra indonesia constrained by three dimensional p wave velocities seismicity and gravity data 1 we estimate the one and three dimensional p wave velocity structure beneath the toba caldera complex a 30 100 km topographic depression in north sumatra using arrival time data of local earthquakes recorded by a 40 station seismic network that operated for 4 months inversions reveal the presence of p velocities up to 37 below normal that likely map the distribution of magma within this subduction related volcanic system considered the world s largest in the upper 10 km of crust the largest low velocity region underlies the southern two thirds of the depression and coincides with a gravity low centered over the resurgent dome a smaller volume of low velocities is observed in the upper crust under the north end of the depression separating the two regions is a zone of locally high velocities indicating that the shallow subcaldera magma system is composed of two separate reservoirs not a single one that extends the entire length of the caldera complex above each low velocity region is a postcollapse volcano that erupted mostly mafic lavas after the last major caldera collapse 74 kyr ago a low velocity column below one of these volcanoes can be traced into the uppermost mantle and corresponds with a planar distribution of low frequency earthquakes in the 20 to 40 km depth range the low frequency earthquakes apparently record the migration of melt in the mafic roots of this large volume crustal magma system

the 1245 yr bp asososca maar new advances on recent volcanic stratigraphy of managua nicaragua and hazard implications abstract asososca maar is located at the western outskirts of managua nicaragua in the central part of the active n s trending and right lateral nejapa miraflores fault that marks an offset of the middle america volcanic arc it constitutes one of the 21 vents aligned along the fault between the chiltepe volcanic complex to the north and ticomo vents to the south asososca consists of an east west elongated crater filled by a lake which is currently used for supplying part of managua with drinking water 10 of the capital city demand the crater excavated the previous topography allowing the observation of a detailed holocene stratigraphic record that should be taken into account for future hazard analyses we present a geological map together with the detailed stratigraphy exposed inside and around asososca crater aided by radiocarbon dating of paleosols the pre existing volcanic sequence excavated by asososca is younger than 12 730 255 250 yr bp and is capped by the phreato plinian masaya tuff 2 are dominated by accidental clasts including basaltic to basaltic andesitic olivine bearing scoriae lapilli porphyritic and hypocrystalline andesite blocks and lapilli altered pumice lapilli and ash and ignimbrite fragments juvenile fragments were only identified in size fractions smaller than 1 in proportions lower than 25 and consist of black moss like fused shape and poorly vesiculated fresh glass fragments of basaltic composition sio2 48 the asososca tephra is interpreted as due to the emplacement of several pyroclastic surges originated by phreatomagmatic eruptions from asososca crater as suggested by impact sags geometry and dune crest migration structures according to radiocarbon dating these deposits were emplaced at 1245 125 120 yr bp the stratigraphic position of the asososca tephra and the well preserved morphology of the crater indicate that asososca is the youngest vent along the nejapa miraflores fault explosive eruptions might therefore occur again along this fault at the western outskirts of managua such kind of activity together with the strong seismicity associated to the active fault represents a serious hazard to urban infrastructure and a population of ca 1 3 million inhabitants

teleseismic p wave image of crust and upper mantle structure beneath the valles caldera new mexico initial results from the 1993 jtex passive array teleseismic p wave relative arrival time data collected from a temporary array during the 1993 jemez tomography experiment jtex have been inverted to image velocity anomalies beneath the valles caldera in northern new mexico instruments were deployed in two 30 km long profiles one of 8 and one of 9 stations these profiles crossed the caldera trending at azimuths of n46 w and n60 e respectively two dimensional teleseismic relative arrival time inversion of the 1993 data set supplemented with data from an overlapping 1987 profile confirms the existence of a mid crustal low velocity region 30 beneath the valles caldera in the depth range of 8 to 13 km below sea level with about a 6 km horizontal extent this feature is interpreted to be the seismic expression of the remnant magma chamber a shallow low velocity anomaly beneath san antonio mountain coincides with the region of highest thermal gradient values in the caldera a lower crust upper mantle low velocity anomaly is imaged but is not as well constrained due to the limited length of the profile we tentatively correlate this anomaly with the thermal effects of basaltic magmas ponded at the crust mantle boundary

stratigraphy of the toba tuffs and the evolution of the toba caldera complex sumatra indonesia during the past 1 2 m y a magma chamber of batholithic proportions has developed under the 100 by 30 km toba caldera complex four separate eruptions have occurred from vents within the present collapse structure which formed from eruption of the 2800 km3 youngest toba tuff ytt at 74 ka eruption of the three older toba tuffs alternated from calderas situated in northern and southern portions of the present caldera the northern caldera apparently developed upon a large andesitic stratovolcano the calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the ytt samosir island and the uluan block are two sides of a single resurgent dome that has resurged since eruption of the ytt samosir island is composed of thick ytt caldera fill whereas the uluan block consists mainly of the oldest toba tuff ott in the past 74000 years lava domes have been extruded on samosir island and along the caldera s western ring fracture this part of the ring fracture is the site of the only current activity at toba updoming and fumarolic activity the toba eruptions document the growth of the laterally continuous magma body which eventually erupted the ytt repose periods between the four toba tuffs range between 0 34 and 0 43 m y and give insights into pluton emplacement and magmatic evolution at toba

tectonic patterns and regional stresses near venusian coronae abstract many venusian coronae exhibit distinctive patterns of faults in particular we find the tectonic patterns of one such corona similar to the striking patterns of the grabens surrounding alba patera on mars pursuing this similarity we perform a stress analysis analogous to one that successfully explained the alba patera pattern and in so doing determined a value for the local regional stress local stress fields of venusian coronae are assumed to be created by either a gaussian axially symmetric upwarp or a cylindrical load due to accumulation of volcanic materials on an elastic lithosphere the local coronal stresses are combined with varying amounts of a uniform regional stress field for different elastic lithospheric thicknesses from the magnitudes and orientations of the resultant principal stresses the predicted patterns of surface tectonic features i e graben and thrust fault chains are plotted these patterns are compared to those of coronae on magellan images matching the patterns thus determines the regional stress and elastic lithospheric thickness about the coronae comparison of predicted to observed tectonic patterns for three specific coronae leads to estimated regional stresses of approximately 0 1 0 6 kbar and effective elastic lithospheric thicknesses consistent with 10 5 km although one area may have had a lithosphere as thin as 2 km during corona formation

the spinosaurid dinosaur baryonyx saurischia theropoda in the early cretaceous of portugal jaw fragments bearing teeth from the barremian of boca do chapim lisboa e setubal province portugal originally considered as crocodilian and identified as suchosaurus girardi by sauvage are redescribed and referred to the spinosaurid dinosaur baryonyx on the basis of comparison with baryonyx walkeri from the barremian of england this extends the geographical distribution of this unusual theropod genus to portugal baryonyx appears to have been a frequent component of early cretaceous dinosaur assemblages in the iberian region which may have formed a biogeographical stepping stone for baryonychine dispersal between europe and africa

bookshelf faulting in nicaragua oblique subduction at a high rate of convergence along much of the middle america trench results in northwest directed trench parallel block motion accommodation of this motion along northwest striking dextral strike slip faults has been postulated however in nicaragua such faults are not well developed we suggest instead that this motion is accommodated by bookshelf faulting that includes northeast striking left lateral faults we present earthquake epicenter and focal mechanism data and mapped fracture and fault data consistent with this model trenchward migration of the volcanic arc since the miocene and reactivation of northeast striking miocene structures may have led to the development of this arc and trench normal fault system