characterization of seismicity at volcan baru panama may 2013 through april 2014 abstract volcan baru in the western province of chiriqui is panama s youngest and most active volcano although baru has experienced no historic eruptions there have been four eruptive episodes in the last 1600 years the most recent occurring 400 500 years ago sherrod et al 2007 in addition there have been four reported earthquake swarms in the last 100 years the most recent swarm occurred in may of 2006 prompting a usgs hazard assessment sherrod et al 2007 in order to characterize local seismicity and provide a reference for future monitoring efforts we established a seismic network that operated from may 2013 through april 2014 the network consisted of eight temporary single component short period sensors loaned by osop panama and three permanent stations distributed over a 35 by 15 km area during operation of the network 91 local events were detected and located these events were used to calculate a new 1 d velocity model for the region surrounding baru of particular interest was a cluster of events located less than 5 km east of baru at a depth of roughly 5 km we used a template matching detection technique to identify another 47 smaller magnitude events in the area of this cluster the largest events in the cluster exhibit a broad range of frequencies extending up to 20 hz and focal mechanisms calculated for eight of the events suggest strike slip and reverse faulting are the predominant source processes the directions of maximum compression shown by these focal mechanisms are approximately radial with respect to the volcano which may reflect the influence of a magmatic source beneath the edifice

lithospheric uplift in eastern gondwana evidence for a long lived mantle plume system recent work has demonstrated that modern drainage systems on rifted continental margins may be the product of ancient lithospheric doming in areas of flood basalt magmatism this uplift is thought to reflect the existence of widespread mafic underplating and excess melt generation at rift zones here i show that uplift due to the initiation of a thermal anomaly plume head in the asthenosphere may be evidenced by the formation of discrete rift structures and persistent paleodrainage sediment dispersal patterns for up to 150 m y prior to the commencement of igneous activity and final continental breakup volcanism thus occurs as a comparatively late stage geochemical response to the long term residence of hot plume material beneath continental mantle lithosphere

constraining the components of the kerguelen mantle plume a hf pb sr nd isotopic study of picrites and high mgo basalts from the kerguelen archipelago 1 we report geochemical and hf pb sr nd isotopic analyses of a suite of picrites and associated high mgo volcanic rocks 6 17 wt mgo from the kerguelen archipelago which are rare compared to other oceanic islands to better constrain the nature and the origin of components present in the kerguelen mantle plume source the sr and nd isotopic compositions of the transitional to mildly alkalic picrites and high mgo basalts closely match those of the 24 ma mildly alkalic basalts from the courbet peninsula whose compositions are considered to reflect the present geochemical expression of the enriched component of the kerguelen mantle plume however linear trends in pb isotopic compositions in the studied samples reflect involvement of a component with lower 206pb 204pb and 208pb 204pb than that inferred for the enriched kerguelen plume contamination of the mgo rich magmas by the kerguelen plateau cannot account for the observed hf pb sr nd isotopic variations isotopic systematics in the picrites and the high mgo basalts are inconsistent with simple binary mixing between two distinct end members and indicate the presence of small scale heterogeneities within the kerguelen plume itself as has been observed in other hot spot environments such as hawaii and iceland the 34 to 26 ma kerguelen plume related basalts that formed when the archipelago was close to the ridge axis 50 to 250 km show geochemical evidence for significant involvement of a southeast indian ridge seir like source in contrast the 24 25 ma mildly alkalic basalts from the eastern and southeastern parts of the archipelago which erupted about 400 km away from the seir and the picrites and high mgo basalts from this study show little or no contribution from a seir like component thus chemical interaction between the seir axis and the kerguelen plume effectively ceased prior to 25 ma

the dykes and structural setting of the volcanic front in the lesser antilles island arc the orientations of dykes from many of the islands of the lesser antilles island arc have been mapped most of these dykes can be interpreted in terms of local or regional swarms derived from specific volcanoes of known age with distinct preferred orientations dykes are known from all cenozoic epochs except the palaeocene but are most common in pliocene miocene and oligocene rocks a majority of the sampled dykes are basaltic intrude volcaniclastic host rocks and show a preference for widths of 1 1 25 m locally dyke swarms dilate their hosts by up to 9 over hundreds of metres and up to 2 over distances of kilometres the azimuths of dykes of all ages show a general ne sw preferred orientation with a second nw se mode particularly in the miocene rocks of martinique the regional setting for these minor intrusions is a volcanic front above a subduction zone composed of three segments saba montserrat guadeloupe martinique st lucia grenada the spacing of volcanic centres along this front is interpreted in terms of rising plumes of basaltic magma spaced about 30 km apart this magma is normally intercepted at crustal depths by dioritic plutons and andesitic dacitic magma generated there plumes which intersect transverse fracture systems or which migrate along the front can avoid these crustal traps throughout its history the volcanic front as a whole has migrated episodically towards the backarc at an average velocity of about 1 km ma the local direction of plate convergence is negatively correlated with the local preferred orientation of dykes the dominant ne sw azimuth mode corresponds closely to the direction of faulting in the sedimentary cover of the backarc and the inferred tectonic fabric of the oceanic crust on which the arc is founded a generalised model of the regional stress field that controls dyke intrusion outside of the immediate vicinity of central volcanic vents is proposed in which the maximum horizontal stress parallels the volcanic front except in the northern segment where subduction of the barracuda rise perturbs the stress field there is also evidence of specific temporal changes in the stress field that are probably due to large scale plate kinematics

the 1995 1998 eruption of the soufriere hills volcano montserrat wi eruption of the soufriere hills volcano began on 18 july 1995 after three years of elevated seismic activity four months of increasingly vigorous phreatic activity culminated in mid november 1995 with the initiation of dome growth growth rates increased unevenly through early march 1996 with fluctuations on time scales from hours to months since march 1996 gravitational collapse of the unstable dome flank has affected an ever increasing area with pyroclastic flows surges and ashfalls major collapse of the eastern flank on 17 september 1996 resulted in a sub plinian explosive eruption later that day by february 1997 the dome had outgrown the confines of the crater and begun to spill into the surrounding valleys a large collapse on 25 june 1997 caused pyroclastic flows and surges on the northern flanks and resulted in the only deaths of the eruption in august september and october 1997 vulcanian explosions followed further collapses on the western and northern flanks the largest event of the eruption occurred on 26 december 1997 with failure of the southwestern flank of the volcano producing a debris avalanche and large dome collapse pyroclastic flows dome growth ceased in early march 1998 but residual volcanic activity has continued and consists of ash venting mild explosions and dome collapse pyroclastic flows

seismic expression of magma induced crustal strains and localized fluid pressures during initial eruptive stages soufriere hills volcano montserrat 1 we explore volcanotectonic earthquake activity during the early stages of volcanic activity 1995 96 at soufriere hills volcano we focus on several zones of temporally confined seismic activity that comprise assemblages of small scale structural elements 2 4 km distant from the volcano at depths 2 4 km below sea level and consider seismicity in relation to regional tectonics and heterogeneity of stiffness and strength as revealed by the sea calipso tomography experiment the clustered seismicity and relatively aseismic zones are interpreted to reflect a broad weakened tectonic zone of ese trend that crosses montserrat and the ascent of a magmatic dike of nne trend which altered the stress distribution to promote localized fault movements and caused localized dilatation with changes in pore fluid pressures to either weaken or strengthen the rock mass depending on the local polarity of strain

nature of the continent ocean transition on the non volcanic rifted margin of the central great australian bight a region of 50 120km width defines the continent ocean transition cot in the central great australian bight it is characterized by a thin apron of post break up sediments overlying complexly deformed sediments and intruded crust bounded landward by a basement ridge complex and oceanward by rough oceanic basement recently acquired deep reflection and refraction seismic data have significantly enhanced understanding of the cot and basement ridge modelled gravity and magnetic data and features interpreted from seismic data are consistent with aspects of extensional and break up models proposed for the west iberia margin many of the features and relationships observed beneath the outer margin of the central great australian bight can be explained by extension within a lithosphere scale pure shear environment involving four layers brittle upper crust and upper mantle and ductile lower crust and lower lithospheric mantle the cot is interpreted to be underlain by extended continental lithosphere thus the continentocean boundary is unequivocally defined between oceanic crust and the cot and appears to be associated with sea floor spreading magnetic anomaly 33 indicating that break up and sea floor spreading did not commence until c 83 ma early carnpanian time later than the currently accepted 95ma age the major part of the basement ridge complex is probably a combination of serpentinized peridotites and mafic intrusions or extrusions derived by mantle upwelling and limited partial melting the magmatic products of this process probably cooled during chron 34 producing a distinctive magnetic anomaly but one that does not relate to break up and sea floor spreading

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

interaction between regional and magma induced stresses and their impact on volcano tectonic seismicity abstract recent seismological observations have reported volcano tectonic vt earthquakes with fault plane solutions exhibiting a change of 90 in their pressure axes relative to the regional stress field interestingly they are recorded mainly during periods preceding eruptive activity and coexisting with those vts showing a regional trend this study explains the occurrence of such trends in vt seismicity and discusses the possible patterns of earthquake locations related to the interaction of regional and magma induced stresses caused by pressurization or depressurization of magmatic sources our analysis shows that in the presence of a dominant regional stress field faulting will occur on faults whose associated slip direction is close to or in agreement with the background regional stress failure on faults with an opposite slip direction is unlikely to occur as magma pressure starts counter acting the regional stresses the likelihood of faults to slip in either a regional or opposite sense of slip relative to regional maximum compression increases allowing the co existence of possible failure with both slip tendencies however the spatial distribution of possible faulting differs as the pressure is progressively increased the stress patterns gradually approach those corresponding to the absence of a regional stress field the presented modeling results have implications for volcanic monitoring routines aiming to detect changes in stress patterns they will ultimately help to improve the correct interpretation of volcano tectonic seismicity

tectonic significance of the lambert graben east antarctica reconstructing the gondwanan rift antarctica s lambert graben australia s north west shelf and the eastern indian peninsula all host thick fault bounded permian triassic successions these terranes were adjacent to each other in gondwana the lambert graben intersects the modern coastline strikes oblique to shelf architecture and has a geophysical signature that can be traced 1000 km inland vitrinite reflectance data from the graben margins record permian triassic infill australia s north west shelf is the relict of an intracontinental carboniferous permian rift that was infilled during the permian triassic then driven to oceanic completion during jurassic cretaceous gondwana breakup this rift was compartmentalized over length scales of 650 km corresponding to accommodation zones margin normal geophysical lineaments and long lived crustal weaknesses in eastern india similar compartmentalization is marked by extensive coal bearing graben systems gondwana reconstructions indicate that the lambert graben corresponds to the orientation and length scale of carboniferous permian rift compartmentalization the lambert graben represents an accommodation zone of a wide intracontinental rift that extended from australia s north west shelf between india and antarctica to southern africa this rift collected gondwana s thick permian triassic sedimentary blanket and rich alluvial coal deposits

submarine basalts of the northern kerguelen plateau interaction between the kerguelen plume and the southeast indian ridge revealed at odp site 1140 during ocean drilling program leg 183 basaltic cores were retrieved from the northern kerguelen plateau nkp at site 1140 on the extreme north of the plateau 270 km north of the kerguelen archipelago amongst the six basement units recovered five were pillow basalts with fresh glassy rims together with flow lobes this is the first evidence for submarine eruption anywhere on the kerguelen plateau each flow unit has distinct geochemical and isotopic characteristics that span the range from southeast indian ridge seir mid ocean ridge basalt to tholeiitic transitional basalts derived from the kerguelen mantle plume relationships between element abundance ratios involving incompatible elements and isotopic ratios reflect mixing of near primary melts from an seir source and the kerguelen plume with an increasing role for an incompatible element rich end member in the order unit 1

breakup between australia and antarctica a brief review in the light of new data abstract the arguments justifying the revised timing of breakup between australia and antarctica cande and mutter 1982 and the reconstruction of broken ridge and kerguelen plateau mutter and cande 1983 are reviewed and considered with respect to new subsidence data the age of breakup was revised from anomaly 22 time 55 my b p to anomaly 34 time 85 my b p the rough topography of the diamantina zone can be attributed to very slow spreading 5 mm yr beginning between the times of anomaly 34 and anomaly 19 the reconstruction of broken ridge and kerguelen plateau at anomaly 34 time shows overlap of these two features but the overlap problem is nearly resolved by anomaly 18 time 42 my b p normal seafloor spreading rates 22 mm yr commenced at anomaly 19 time 43 my b p subsidence patterns calculated from biostratigraphic data from wells drilled along australia s southern margin are interpreted as more consistent with the revised age of australia antarctic breakup subsidence curves systematically show rapid subsidence associated with the rift phase of margin development followed by much slower thermally controlled subsidence during the drift phase the timing of the rift to drift transition is believed to coincide with the age of breakup 60 to 110 my b p in addition the subsidence curves indicate a west to east propagation of breakup along the southern margin magnetic anomaly patterns and stratigraphie observations are consistent with this hypothesis

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

seismicity associated with dome growth and collapse at the soufriere hills volcano montserrat varied seismicity has accompanied growth and collapse of the lava dome of the soufriere hills volcano montserrat earthquakes have been classified as either volcano tectonic long period or hybrid and daily variations in the numbers of events have mapped changes in the style of eruption repetitive hybrid earthquakes were common during the first months of dome growth in july 1996 the style of seismicity changed and regular short lived hybrid earthquake swarms became common this change was probably caused by an increase in the magma flux earthquake swarms have preceded almost all major dome collapses and have accompanied cyclical deformation thought to be due to a build up of pressure in the upper conduit which is later released by magma moving into the dome

astrochronology of the valanginian stage from reference sections vocontian basin france and palaeoenvironmental implications for the weissert event abstract high resolution gamma ray measurements performed on five biostratigraphically well dated reference sections from the vocontian basin south eastern france are used to develop a new astrochronology of the valanginian stage and its subdivisions i e ammonite and calcareous nannofossil zones and subzones spectral analyses show a pervasive dominance of 405 kyr eccentricity cycles with the expression of 100 kyr eccentricity obliquity and precession previous rough estimates of valanginian stage duration ranged from 3 9 to 6 5 myr but were generally based on less reliable or indirect methods this study provides a precise duration of 5 08 myr tuning the series to the 405 kyr eccentricity cycle thus further enhancing the accuracy of the geologic time scale new durations of magnetochrons are proposed based on intercalibration between ammonite and calcareous nannofossil biozones cyclostratigraphy and carbon isotope stratigraphy after integrating orbitally calibrated durations and available radio isotopic ages for the berriasian hauterivian interval a link between the weissert event and the emplacement of the parana etendeka traps now appears unlikely strontium carbon and oxygen isotope stratigraphy sedimentological and palaeontological features clay mineralogy and the orbital record during the upper valanginian all suggest that a transient cool arid climate occurred after the carbon isotope event possibly associated with limited polar ice

volcano tectonic seismicity at unzen volcano japan 1985 1999 abstract the eruption of unzen volcano in southwestern japan began on november 17 1990 and an extrusion of a dacite lava dome lasted from may 20 1991 until early february 1995 the seismic network operated by the shimabara earthquake and volcano observatory of kyushu university sevo located about 4000 volcano tectonic earthquakes during the period from 1985 to 1999 the seismicity was greatest in the period between one year before the eruption and the start of the lava dome extrusion the hypocenters were mostly distributed within a 12 20 km area on the western side of the volcano an inclined seismic aseismic boundary shallows eastward to the summit probably reflecting the thermal structure beneath the volcano within 4 km west from the summit however the seismicity is very low it was suggested that the high temperature ductile and low q bodies exist there we obtained focal mechanism solutions for about 90 earthquakes among them orientations of p axes for the earthquakes occurring on the upper side of the inclined seismic aseismic boundary suggest a pressure source located below this boundary although most of the focal mechanisms have t axes nearly horizontal in the n s direction consistent with the n s extensional tectonic stress several earthquakes whose focal mechanisms are inconsistent with the tectonic stress were observed within a horizontal distance of 5 km from the summit this is because the pressurization of volcanic gas or magma beneath the summit produced a strong local stress in the summit surroundings

a test of alternative caribbean plate relative motion models two contrasting models have been proposed to describe the present day motions between the caribbean plate and neighboring plates one model based on both north america caribbean and cocos caribbean data assumes that north america caribbean motion is reflected by the spreading rate approximately 2 cm yr inferred from magnetic anomalies at the cayman spreading center and the azimuths of nearby transforms jordan 1975 this geometry was used by minster and jordan 1978 in deriving global plate motion model rm2 the other model based only on north america caribbean data uses rates and azimuths inferred from the geometry of the lesser antilles wadati benioff zone sykes et al 1982 the cayman spreading center data were discounted assuming that they underestimate the full north america caribbean motion owing to the complex tectonics of the greater antilles the latter model assumes plate convergence at the lesser antilles at a rate about twice as fast as and an azimuth differing by 25 from the jordan model the two models also differ in their predictions of south america caribbean motion the jordan model predicts oblique convergence whereas sykes et al predict oblique divergence we use the nuvel 1 global relative motion data set which incorporates recent data not used in earlier studies to discriminate between the alternative models we find that the jordan geometry provides a better fit to the direction of north america caribbean motion moreover we find that models based on this geometry better fit the direction of cocos caribbean motion observed in earthquake slip vectors along the middle america trench since the prediction of this direction depends strongly on the rate of north america caribbean motion we conclude that the jordan model better describes caribbean plate motions the discrepancy between the two models results from the different methods of estimating plate motions sykes et al 1982 report that they determined a caribbean north america convergence rate of approximately 4 cm yr from the length of the lesser antilles wadati benioff zone we employ a similar procedure and obtain a rate closer to 2 cm yr we also find that the method they report using to determine the convergence direction is unable to distinguish between the alternative azimuths we thus conclude that estimates of the rate and direction of plate motion from wadati benioff zone configurations are insufficiently robust and that such estimates are better derived using rates from magnetic anomalies and azimuths from transform faults and slip vectors we further conclude that successful estimates of relative motion require internal consistency between the euler vectors of all relevant plates as demonstrated by the powerful cocos caribbean constraint not incorporated in the sykes et al model

relations between fracture patterns seismicity and plate motions in the lesser antilles abstract measurements of orientations of joints that have been made on the leeward and windward islands lesser antilles from st martin to st vincent are reported the preferred orientations of the joints have been determined by a statistical analysis assuming the joints to be shearing phenomena the principal directions of the stress field that caused them can be calculated the latter are in very good agreement with the principal stress directions obtained by a statistical analysis of fault plane solutions of earthquakes in the area the inferred maximum compression direction being nw se ca n 130 e this corresponds to a model which assumes that the american plate is subducted in that direction beneath the caribbean plate

dominant symmetry of a conjugate southern australian and east antarctic magma poor rifted margin segment synthesis and modeling of published deep seismic and potential field data from the conjugate magma poor rifted margins of the great australian bight southern australia and central wilkes land east antarctica show that there is pronounced symmetry of structures in a 300 km wide zone straddling the axis of final breakup this symmetry is observed consistently for a distance of some hundreds of kilometers along strike from inboard to outboard both margins comprise a narrow zone of attenuation of the crystalline continental crust an approximately 4 km high basement ridge interpreted as unroofed peridotites at the location of maximum thinning of the continental crust and a 60 70 km wide continent ocean transition zone that contains a sedimentary basin that may be underlain by altered mantle and fragments of crystalline continental crust the marked breakup symmetry described here is in contrast to the asymmetry of the iberia newfoundland margin and is consistent with the operation of a symmetrical extensional detachment system deforming the whole crust in the center of the rift as envisaged by some numerical models for the continental rifting process

seismic hydraulic fracture migration originated by successive deep magma pulses the 2011 2013 seismic series associated to the volcanic activity of el hierro island in this manuscript we present a new interpretation of the seismic series that accompanied eruptive activity off the coast of el hierro canary islands during 2011 2013 we estimated temporal variations of the gutenberg richter b value throughout the period of analysis and performed high precision relocations of the preeruptive and syneruptive seismicity using a realistic 3 d velocity model our results suggest that eruptive activity and the accompanying seismicity were caused by repeated injections of magma from the mantle into the lower crust these magma pulses occurred within a small and well defined volume resulting in the emplacement of fresh magma along the crust mantle boundary underneath el hierro we analyzed the distribution of earthquake hypocenters in time and space in order to assess seismic diffusivity in the lower crust our results suggest that very high earthquake rates underneath el hierro represent the response of a stable lower crust to stress perturbations with pulsatory character linked to the injection of magma from the mantle magma input from depth caused large stress perturbations to propagate into the lower crust generating energetic seismic swarms the absence of any preferential alignment in the spatial pattern of seismicity reinforces our hypothesis that stress perturbation and related seismicity had diffusive character we conclude that the temporal and spatial evolution of seismicity was neither tracking the path of magma migration nor it defines the boundaries of magma storage volumes such as a midcrustal sill our conceptual model considers pulsatory magma injection from the upper mantle and its propagation along the moho we suggest within this framework that the spatial and temporal distributions of earthquake hypocenters reflect hydraulic fracturing processes associated with stress propagation due to magma movement

updated gondwana permian cretaceous earth history of australia abstract permo carboniferous glaciation confined to icecaps and mountain glaciers was followed by permian coal measures and early triassic barren measures and redbeds in the east terminally deformed in the mid triassic coal deposition resumed during the late triassic and tholeiite was erupted in the southeast after rifting the western margin was formed by the opening of the indian ocean at 156 and 132 ma at 140 ma a brief glaciation affected central australia by the 99 ma mid cretaceous the southern margin was finally shaped by the opening of the southeastern indian ocean the shoreline retreated to the present coast from the maximum aptian shoreline of an epeiric sea and the eastern highlands were uplifted to produce the present morphology of australia new data relate to the permo carboniferous and early cretaceous glaciations the kiaman reversed paleomagnetic interval events about the permian triassic boundary including possible impact craters advances in palynology invertebrate paleontology macrofloral paleontology and paleobiogeography the provenance of sediments by u pb ages and host rock affinity of zircons stable isotopes and biomarkers in petroleum systems coal environments calibrating the time scale with u pb ages of zircons fission track thermotectonic imaging geothermal energy and terranes split off the western margin

the breakup of east gondwana assimilating constraints from cretaceous ocean basins around india into a best fit tectonic model published models for the cretaceous sea oor spreading history of east gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and or violate particular constraints from at least one of the associated ocean basins we link east gondwana spreading corridors by integrating magnetic and gravity anomaly data from the enderby basin off east antarctica within a regional plate kinematic framework to identify a conjugate series of east west trending magnetic anomalies m4 to m0 126 7 120 4 ma the mid ocean ridge that separated greater india from australia antarctica propagated from north to south starting at 136ma northwest of australia and reached the southern tip of india at 126 ma sea oor spreading in the enderby basin was abandoned at 115 ma when a ridge jump transferred the elan bank and south kerguelen plateau to the antarctic plate our revised plate kinematic model helps resolve the problem of successive two way strike slip motion between madagascar and india seen in many previously published reconstructions and also suggests that sea oor spreading between them progressed from south to north from 94 to 84 ma this timing is essential for tectonic ow lines to match the curved fracture zones of the wharton and enderby basins as greater india gradually began to unzip from madagascar from 100 ma in our model the 85 east ridge and kerguelen fracture zone formed as conjugate anks of a leaky transform fault following the 100ma spreading reorganization our model also identi es the afanasy nikitin seamounts as products of the conrad rise hotspot

pre breakup continental extension in east gondwanaland and the early opening of the eastern indian ocean abstract extension of continental crust by up to 360 km on a north northeast azimuth in the great australian bight occurred prior to the middle cretaceous 96 ma onset of seafloor spreading between australia and antarctica this large amount of continental extension is constrained to the late jurassic 160 ma middle cretaceous interval about a pole estimated to lie about 40 southeastward of southeastern australia using bathymetric data combined with seismic and magnetic determinations of the continent ocean boundaries off australia india and antarctica we determine a revised fit of east gondwanaland prior to continental extension and at various stages thereafter in the first stage from 160 to 132 5 ma m11 india australia rotated from antarctica with continental crustal extension between australia and antarctica and largely transform motion between the coromandel coast margin of india and the kron prins olav kyst margin of antarctica in the second stage from 132 5 to 96 ma india rotated northwestward away from australia antarctica producing m10 and younger magnetic anomalies along the western margin of australia continental extension was greatest between australia and antarctica during this stage the newly determined 132 5 96 ma rotation between india and australia provides an improved fit between the orientation of observed m10 m0 magnetic anomalies and the southward decrease in spreading rate off the western australian margin together with bathymetry the new rotation parameters lead to identification of the abandoned 96 ma spreading ridge along the lost dutchmen and dirck hartog ridges the naturaliste fracture zone is found to lie nearly parallel to the early cretaceous transform direction between india and antarctica suggesting that slow seafloor spreading required between australia and antarctica to accommodate the continental extension further east occurred between naturaliste fracture zone and the m4 m0 anomalies adjacent to the naturaliste plateau the revised fit of india in east gondwanaland reduces the eastward extent of greater india to 95 e but does not change its extent as far north as the cape range fracture zone the new pole determined for the early cretaceous rotation between india and australia antarctica lies within 15 of the southern tip of india significantly closer than in previous determinations thereby reducing the amount of seafloor spreading generated between africa and antarctica by the india antarctica rotation and thus offering a possible solution to the perceived spreading rate problems between m11 and m0 in the somali mozambique and antarctic basins

temporal and spatial variation of local stress fields before and after the 1992 eruptions of crater peak vent mount spurr volcano alaska we searched for changes in local stress field orientation at mount spurr volcano alaska between august 1991 and december 2001 this study focuses on the stress field orientation beneath crater peak vent the site of three eruptions in 1992 and beneath the summit of mount spurr local stress tensors were calculated by inverting subsets of 140 fault plane solutions for earthquakes beneath crater peak and 96 fault plane solutions for earthquakes beneath mount spurr we also calculated an upper crustal regional stress tensor by inverting fault plane solutions for 66 intraplate earthquakes located near mount spurr during 1991 2001 prior to the 1992 eruptions and for 11 months beginning with a posteruption seismic swarm the axis of maximum compressive stress beneath crater peak was subhorizontal and oriented n67 76 e approximately perpendicular to the regional axis of maximum compressive stress n43 w the strong temporal correlation between this horizontal stress field rotation change in position of the 1 3 axes relative to regional stress and magmatic activity indicates that the rotation was related to magmatic activity and we suggest that the crater peak stress field rotation resulted from pressurization of a network of dikes during the entire study period the stress field beneath the summit of mount spurr also differed from the regional stress tensor and was characterized by a vertical axis of maximum compressive stress we suggest that slip beneath mount spurr s summit occurs primarily on a major normal fault in response to a combination of gravitational loading hydrothermal circulation and magmatic processes beneath crater peak online material regional and local fault plane solutions

volatiles in submarine basaltic glasses from the northern kerguelen plateau odp site 1140 implications for source region compositions magmatic processes and plateau subsidence submarine pillow basalts 34 ma recovered from the northern introduction kerguelen plateau at odp site 1140 contain abundant unaltered the kerguelen plateau and broken ridge formed mostly glass providing the first opportunity to measure the volatile contents during the cretaceous as a single giant oceanic plateau of tholeiitic basaltic magmas related to the kerguelen mantle plume that is interpreted to represent voluminous magmatism the glasses have la sm and nb zr ratios that vary from values associated with emplacement of a mantle plume head similar to southeast indian ridge seir morb unit 1 to below young indian ocean lithosphere duncan slightly more enriched unit 6 to values transitional between storey 1992 weis et al 1992 during ocean drilling seir morb and basaltic magmas formed by melting of the program odp leg 183 igneous basement rock and kerguelen plume units 2 and 3 volatile contents for glasses in sediment cores were obtained from five sites on the units 1 and 6 are similar to depleted mid ocean ridge basalt kerguelen plateau and two on broken ridge fig 1 the morb values 0 25 0 27 wt h2o 1240 1450 ppm s results combined with subsequent radiometric dating 42 54 ppm cl in contrast h2o contents are higher for the demonstrate that from south to north the age of the enriched glasses unit 2 0 44 wt h2o unit 3 0 69 wt uppermost crust decreases from 120 110 ma in the as are s 1500 ppm and cl 146 206 ppm cl k ratios southern kerguelen plateau to 110 to 95 ma in the for all glasses are relatively low 0 03 0 04 indicating that central kerguelen plateau broken ridge and elan bank assimilation of hydrothermally altered material did not occur during to 35 ma in the northern kerguelen plateau coffin shallow level crystallization h2o ce for the enriched glasses et al 2000 pringle duncan 2000 duncan 2002 at units 2 and 3 is significantly lower than pacific and south six of seven basement sites as well as at four sites drilled atlantic morb values suggesting that low h2o ce may be an on earlier odp cruises subaerially erupted basalt flows inherent characteristic of the kerguelen plume source vapor saturation were recovered indicating that the growth rate of the pressures calculated using the h2o and co2 contents of the glasses kerguelen large igneous province lip was sufficient to indicate that 1700 m of subsidence has occurred on this part of form a subaerial landmass however a sequence of the plateau since eruption of the basalts at 34 ma subsidence submarine erupted pillow basalt flows of 90 m thickness estimates for site 1140 and other odp drill sites indicate that the was drilled at site 1140 on the northern kerguelen various parts of the kerguelen plateau subsided at a rate comparable plateau coffin et al 2000 with that for normal indian ocean lithosphere the site 1140 submarine glasses provide the first

dome growth behavior at soufriere hills volcano montserrat revealed by relocation of volcanic event swarms 1995 1996 abstract we have relocated a subset of events from the digital waveform catalogue of 17 000 volcanic microearthquakes recorded between july 1995 and february 1996 at soufriere hills volcano shv montserrat using a cross correlation based phase repicking technique with a joint location method hypocenters were estimated for 3914 earthquakes having five or more corrected p wave picks the seismic source region collapsed to a volume of 1 km 3 from an initial 100 km 3 relocated events represent 36 swarms each containing nearly identical waveforms having source dimensions of 10 to 100 m in diameter and spatial separations on the order of 500 m or less each swarm occurred over a span of several hours to a few days triggered data appear to miss between 65 and 98 of the events that occur within these swarms based on review of helicorder records visual estimates of summit dome growth show a rough correspondence between episodes of intense swarming and increases in extruded magma although dome observations are too sparse to make a direct comparison for this time period the limited depth range over which dome growth related events occur is consistent with a dynamic model of cyclic plug extrusion behavior in the shallow conduit governed by magma supply rate overpressure buildup and physical properties of the magma and conduit geometry seismic sources may occur in locally overpressured regions that result from microlite formation in a zone of rapid decompression we propose that this zone exists in the vicinity of a detachment plane associated with the cyclic plug extrusion

a process based model of pre eruption seismicity patterns and its use for eruption forecasting at dormant stratovolcanoes abstract using long term data sets the eruptive size timing and style can typically be successfully forecast for dormant volcanoes using current monitoring data and knowledge of precursory eruptive patterns the usaid usgs volcano disaster assistance program vdap utilizes seismic data over multiple eruption cycles and across volcano types to successfully forecast eruption size style and onset times as well as changes within ongoing eruptions from millions of seismic event observations we show that seismic data a proxy for rate dependent strain changes in the magmatic system combined with a process based conceptual geologic model enables eruption forecasting especially for stratovolcanoes dormant 20 years we show that this model often works for some open systems ones that sufficiently reseal to plug the conduit and also where a deep intrusion occurs after phreatic or phreatomagmatic explosions have begun this model is divided into four seismicity and geologic stages we describe each of the four stages of seismicity in detail and how they relate to the ongoing geologic process of that stage stage 1 deep intrusion and seismicity occurring at 10 to 40 km depth below the volcanic edifice stage 2 distal vt seismicity induced at distances from 2 to 30 km laterally from the summit by magma intruding into and inflating the upper crustal magma storage region stage 3 vent clearing seismicity associated with the initial vent creation and clearing phase of eruptive activity stage 4 repetitive event seismicity associated with the final ascent of magma from 2 km to the surface we present 36 examples of this seismic progression from 26 volcanoes we believe that this progression generally correlates with the magma flux and the eventual explosivity and is invaluable for forecasting eruptions

identifying sources of respirable quartz and silica dust in underground coal mines in southern west virginia western virginia and eastern kentucky prior research has suggested that the source of respirable silica dust in underground coal mines is typically the immediate top or bottom lithology adjacent to the mined seam not mineral matter bound within the mined coal bed geochemical analyses were applied in an effort to identify the specific source rock of respirable quartz dust in coal mines the analyses also demonstrate the compositional changes that take place in the generation of the respirable dust fraction from parent rock material all six mine sites were mining coal with relatively low mineral matter content although two mines were operating in the fire clay coal bed which contains a persistent tonstein interpretations of ca mg mn na and k concentrations strongly suggest that the top strata above the mined seam is the primary source of mineral dust produced during mining one site indicates a mixed or bottom source possibly due to site specific conditions respirable dust compositional analyses suggest a direct relationship between the quantity of mineral si and the quantity of quartz si a similar relationship was not found in either the top or bottom rocks adjacent to the mined seam an apparent loss of elemental al was noted in the respirable dust fraction when compared to potential parent rock sources elemental al is present in top and bottom rock strata within illite kaolinite feldspar and chlorite a possible explanation for loss of al in the respirable dust samples is the removal of clays and possibly chlorite minerals it is expected that removal of this portion of the al bearing mineral matter occurs during rock abrasion and dust transport prior to dust capture on the samplers

nature of the continent ocean transition zone along the southern australian continental margin a comparison of the naturaliste plateau sw australia and the central great australian bight sectors we document the interpretation of three crustal sections from coincident deep seismic reflection gravity and magnetic data acquired on australia s southern margin one section from the naturaliste plateau and the diamantina zone and two in the great australian bight gab interpretations are based on an integrated study of deep multichannel seismic gravity and magnetic data together with sparse sonobuoy and dredging information all interpreted sections of the margin show a transition from thinned continental crust through a wide continent ocean transition zone cotz in the gab the transition is to slow sea floor spreading oceanic crust that dates from breakup in the campanian c 83 ma in the naturaliste diamantina margin the earliest oceanic crust is undated the cotz on these margins is geologically and geophysically complex but interpretation of all data including dredge hauls is consistent with the presence of a mixture of modified continental lower crust breakup related volcanics and exhumed continental mantle serpentinized detachment faults are not well imaged but have been inferred from high amplitude magnetic signatures interpreted to arise from magnetite associated with the hydration of peridotites alternative models for the structure of the cotz involving either mafic underplating or aborted sea floor spreading have been explored but are considered unlikely on this margin similarity in the final architecture of these margins has major implications for the nature of rifting in the southern rift system and may point to the entire 4000 km long system being non volcanic in character second order differences in geometry and morphology of the two areas studied are unlikely to be a function of strain rate instead they probably reflect complexities owing to the multiple tectonic events that occurred during final gondwanide fragmentation the most dramatic of these is the impact of hotspot activity in the kerguelen plateau which commenced some 50 ma prior to final breakup in that sector

a global scale plate reorganization event at 105 100 ma abstract a major plate reorganization is postulated to have occurred at approximately 100 ma however this reorganization has received limited attention despite being associated with the most prominent suite of fracture zone bends on the planet and many other geological events we investigate tectonic events from the period 110 to 90 ma and show that the reorganization occurred between 105 and 100 ma was global in scale and affected all major plates seafloor evidence for plate motion changes is abundant during this period with either fracture zone bends or terminations preserved in all ocean basins long lived eastern gondwanaland subduction ended along a 7000 km long section of the margin while elsewhere around the proto pacific rim subduction continued and there is evidence that compressional stresses increased in the overriding plates thrusting in western north america transpression and basin inversion in eastern asia and development of the present day andean style margin along western south america occurred contemporaneous with the development of an extensional regime in eastern gondwanaland basin instability in africa and western europe further demonstrates that lithospheric stress regime changes were widespread at this time considering the timing of the reorganization and the nature of associated plate boundary changes we suggest that eastern gondwanaland subduction cessation is the most likely driving mechanism for the reorganization subduction is the dominant driver of plate motion and therefore this event had the potential to strongly modify the balance of driving forces acting on the plates in the southwestern proto pacific and neighboring plates whereby producing widespread changes in plate motion and continental lithospheric stress patterns we propose that major changes in ridge trench interaction triggered the cessation of subduction the progressive subduction of two closely spaced perpendicular mid ocean ridges at the eastern gondwanaland subduction zone to the east of australia and new zealand respectively resulted in very young crust entering the trench and we suggest that by 105 100 ma there was insufficient negative buoyancy to drive subduction finally we propose that the plume push force of the bouvet plume that erupted near the african antarctic south american triple junction contributed to plate motion changes in the southern atlantic region

temporal and spatial variation of the stress state of popocatepetl volcano mexico abstract the fault plane solutions fps of volcano tectonic events vts can help in the determination of the stress state of a volcano before during and after an eruptive period hence for popocatepetl we have calculated the fps from 1996 to 2003 and we have compared the results with its volcanic activity for popocatepetl there are two main areas of vts one below the crater and another in the southeastern zone for the below crater we have 398 solutions and for the southeastern zone 64 the fps below the crater have pressure axes p with a preferential orientation of s70 w n70 e which is consistent with the regional stress state for the tension axes t a radial tension is observed although a great amount of axes exists with a direction of nw se most of the fps are normal solutions and these are present at depths mainly between 0 and 3 km asl although we have periods where an increase is observed in the number of reverse events approximately between 3 and 0 km asl the variability of the solutions with depth is probably associated with the intrusion of magma inside the volcano the group of events located in the southeastern part of the volcano have pressure axes p with directions of s n and w e and the same orientations are observed for the tension axes t most of the mechanisms in this area are strike slip solutions which follow two possible faults which were activated during the early magma intrusions inside of the volcano

spatial variations in the frequency magnitude distribution of earthquakes at soufriere hills volcano montserrat west indies the frequency magnitude distribution of earthquakes measured by the b value is determined as a function of space beneath soufriere hills volcano montserrat from data recorded between august 1 1995 and march 31 1996 a volume of anomalously high b values b 3 0 with a 1 5 km radius is imaged at depths of 0 and 1 5 km beneath english s crater and chance s peak this high b value anomaly extends southwest to gage s soufriere at depths greater than 2 5 km volumes of comparatively low b values b 1 are found beneath st george s hill windy hill and below 2 5 km depth and to the south of english s crater we speculate the depth of high b value anomalies under volcanoes may be a function of silica content modified by some additional factors with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths

joint analysis of geodetic and earthquake fault plane solution data to constrain magmatic sources a case study from kīlauea volcano abstract a joint analysis of geodetic and seismic datasets from k lauea volcano during a period of magmatic unrest in 2006 demonstrates the effectiveness of this combination for testing and constraining models of magma dynamics for a complex multi source system at the end of 2003 k lauea s summit began a four year long period of inflation due to a surge in magma supply to the volcano in 2006 for the first time since 1982 k lauea s southwest rift zone swrz also experienced inflation to investigate the characteristics of active magma sources and the nature of their interactions with faults in the swrz during 2006 we integrate through coulomb stress modeling contemporary geodetic data from insar and gps with a new catalogue of double couple fault plane solutions for volcano tectonic earthquakes we define two periods of inflation during 2006 based on the rate of deformation measured in daily gps data spanning february to 15 march 2006 period 1 and 16 march to 30 september 2006 period 2 insar data for these two periods are inverted to determine the position change in size and shape of inflation sources in each period our new models are consistent with microseismic activity from each period they suggest that during period 1 deformation in the swrz can be explained by pressurization of magma in a spherical reservoir beneath the south caldera and that during period 2 magma was also aseismically intruded farther to the southwest into the swrz along a sub horizontal plane our coulomb stress analysis shows that the microseismicity recorded in the swrz is induced by overpressurization of the south caldera reservoir and not by magma intrusion into the swrz this study highlights the importance of a joint analysis of independent geophysical datasets to fully constrain the nature of magma accumulation

evidence from the ad 2000 izu islands earthquake swarm that stressing rate governs seismicity magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits that cannot be associated with the diffusion of pore fluids or heat such swarm seismicity also migrates with time and often exhibits a dog bone shaped distribution the largest earthquakes in swarms produce aftershocks that obey an omori type exponential temporal decay but the duration of the aftershock sequences is drastically reduced relative to normal earthquake activity here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion a 1 000 fold increase in seismicity rate and a 1 000 fold decrease in aftershock duration occurred during the two month long dyke intrusion we find that the seismicity rate is proportional to the calculated stressing rate and that the duration of aftershock sequences is inversely proportional to the stressing rate this behaviour is in accord with a laboratory based rate state constitutive law suggesting an explanation for the occurrence of earthquake swarms any sustained increase in stressing rate whether due to an intrusion extrusion or creep event should produce such seismological behaviour

m sequence geomagnetic polarity time scale mhtc12 that steadies global spreading rates and incorporates astrochronology constraints 1 geomagnetic polarity time scales gptss have been constructed by interpolating between dated marine magnetic anomalies assuming uniformly varying spreading rates a strategy to obtain an optimal gpts is to minimize spreading rate fluctuations in many ridge systems however this has been possible only for a few spreading centers we describe here a monte carlo sampling method that overcomes this limitation and improves gpts accuracy by incorporating information on polarity chron durations estimated from astrochronology the sampling generates a large ensemble of gptss that simultaneously agree with radiometric age constraints minimize the global variation in spreading rates and fit polarity chron durations estimated by astrochronology a key feature is the inclusion and propagation of data uncertainties which weigh how each piece of information affects the resulting time scale the average of the sampled ensemble gives a reference gpts and the variance of the ensemble measures the time scale uncertainty we apply the method to construct mhtc12 an improved version of the m sequence gpts late jurassic early cretaceous 160 120 ma this gpts minimizes the variation in spreading rates in a global data set of magnetic lineations from the western pacific north atlantic and indian ocean nw of australia and it also accounts for the duration of five polarity chrons established from astrochronology cm0r through cm3r this gpts can be updated by repeating the monte carlo sampling with additional data that may become available in the future

volcano tectonic earthquakes a new tool for estimating intrusive volumes and forecasting eruptions abstract we present data on 136 high frequency earthquakes and swarms termed volcano tectonic vt seismicity which preceded 111 eruptions at 83 volcanoes plus data on vt swarms that preceded intrusions at 21 other volcanoes we find that vt seismicity is usually the earliest reported seismic precursor for eruptions at volcanoes that have been dormant for decades or more and precedes eruptions of all magma types from basaltic to rhyolitic and all explosivities from vei 0 to ultraplinian vei 6 at such previously long dormant volcanoes because large eruptions occur most commonly during resumption of activity at long dormant volcanoes vt seismicity is an important precursor for the earth s most dangerous eruptions vt seismicity precedes all explosive eruptions of vei 5 and most if not all vei 4 eruptions in our data set surprisingly we find that the vt seismicity originates at distal locations on tectonic fault structures at distances of one or two to tens of kilometers laterally from the site of the eventual eruption and rarely if ever starts beneath the eruption site itself the distal vt swarms generally occur at depths almost equal to the horizontal distance of the swarm from the summit out to about 15 km distance beyond which hypocenter depths level out we summarize several important characteristics of this distal vt seismicity including swarm like nature onset days to years prior to the beginning of magmatic eruptions peaking of activity at the time of the initial eruption whether phreatic or magmatic and large non double couple component to focal mechanisms most importantly we show that the intruded magma volume can be simply estimated from the cumulative seismic moment of the vt seismicity from log10 v 0 77 log moment 5 32 with volume v in cubic meters and seismic moment in newton meters because the cumulative seismic moment can be approximated from the size of just the few largest events and is quite insensitive to precise locations the intruded magma volume can be quickly and easily estimated with few short period seismic stations notable cases in which distal vt events preceded eruptions at long dormant volcanoes include nevado del ruiz 1984 1985 pinatubo 1991 unzen 1989 1995 soufriere hills 1995 shishaldin 1989 1999 tacana 1985 1986 pacaya 1980 1984 rabaul 1994 and cotopaxi 2001 additional cases are recognized at frequently active volcanoes including popocateptl 2001 2003 and mauna loa 1984 we present four case studies pinatubo soufriere hills unzen and tacana in which we demonstrate the above mentioned vt characteristics prior to eruptions using regional data recorded by neic we recognized in near real time that a huge distal vt swarm was occurring deduced that a proportionately huge magmatic intrusion was taking place beneath the long dormant sulu range new britain island papua new guinea that it was likely to lead to eruptive activity and warned rabaul volcano observatory days before a phreatic eruption occurred this confirms the value of this technique for eruption forecasting we also present a counter example where we deduced that a vt swarm at volcan cosiguina nicaragua indicated a small intrusion insufficient to reach the surface and erupt finally we discuss limitations of the method and propose a mechanism by which this distal vt seismicity is triggered by magmatic intrusion

characterization of seismicity at mt etna volcano italy by inter event time distribution abstract the space time inter event time iet distributions of earthquakes occurring from 1988 to 2011 at mt etna are analysed in order to identify the periodicity or stationary behaviour of seismicity and to correlate it with the volcano tectonic features of the region the comparison between iet distributions at etna with those obtained both for sicily and italy shows that iets at a larger scale are well modelled by a gamma distribution whereas at etna local scale they are characterised by a bimodal curve in which the two peaks are related to i the contribution of local seismic swarms with very short inter event times and ii the background regional stationary seismicity iet analysis is an important tool to investigate the behaviour of seismicity at different crustal levels in the etna region distinguishing sectors that are influenced by volcano dynamics or regional tectonics indeed the spatial variation of iet distributions obtained by analysing different etna crustal sectors shows that seismicity shallower than 5 km is almost entirely characterised by short inter event times and is mainly confined to the summit area for earthquakes deeper than 5 km occurring in the eastern flank of the volcano as well as in eastern sicily iet distributions are characterised by independent events which suggest that both areas are influenced by the same extensional regional regime by contrast iet distributions obtained for the western flank and north western sicily are marked by two peaks indicating that the compressional stress is acting in both areas

phenocryst hosted melt inclusions record stalling of magma during ascent in the conduit and upper magma reservoir prior to vulcanian explosions soufriere hills volcano montserrat west indies the mechanics of explosive eruptions influence magma ascent pathways vulcanian explosions involve a stop start mechanism that recurs on various timescales evacuating the uppermost portions of the conduit during the repose time between explosions magma rises from depth and refills the conduit and stalls until the overpressure is sufficient to generate another explosion we have analyzed major elements cl s h2o and co2 in plagioclase hosted melt inclusions sampled from pumice erupted during four vulcanian events at soufriere hills volcano montserrat to determine melt compositions prior to eruption using fourier transform infrared spectroscopy we measured values up to 6 7 wt h2o and 80 ppm co2 of 42 melt inclusions 81 cluster between 2 8 and 5 4 wt h2o 57 to 173 mpa or 2 7 km suggesting lower conduit to upper magma reservoir conditions we propose two models to explain the magmatic conditions prior to eruption in model 1 melt inclusions were trapped during crystal growth in magma that was stalled in the lower conduit to upper magma reservoir and during trapping the magma was undergoing closed system degassing with up to 1 wt free vapor this model can explain the melt inclusions with higher h2o contents since these have sampled the upper parts of the magma reservoir however the model cannot explain the melt inclusions with lower h2o because the timescale for plagioclase crystallization and melt inclusion entrapment is longer than the magma residence time in the conduit in model 2 melt inclusions were originally trapped at deeper levels of the magma chamber but then lost hydrogen by diffusion through the plagioclase host during periodic stalling of the magma in the lower conduit system in this second scenario which we favor the melt inclusions record re equilibration depths within the lower conduit to upper magma reservoir

full fit palinspastic reconstruction of the conjugate australian antarctic margins 1 despite decades of study the prerift configuration and early rifting history between australia and antarctica is not well established the plate boundary system during the cretaceous includes the evolving kerguelen broken ridge large igneous province in the west as well as the conjugate passive and transform margin segments of the australian and antarctic continents previous rigid plate reconstruction models have highlighted the difficulty in satisfying all the available observations within a single coherent reconstruction history we investigate a range of scenarios for the early rifting history of these plates by developing a deforming plate model for this conjugate margin pair potential field data are used to define the boundaries of stretched continental crust on a regional scale integrating crustal thickness along tectonic flow lines provides an estimate of the prerift location of the continental plate boundary we then use the prerift plate boundary positions along with additional constraints from geological structures and large igneous provinces within the same australian and antarctic plate system to compute full fit poles of rotation for australia relative to antarctica our preferred model implies that the leeuwin and vincennes fracture zones are conjugate features within gondwana but that the direction of initial opening between australia and antarctica does not follow the orientation of these features rather the geometry of these features is likely related to the earlier rifting of india away from australia antarctica previous full fit reconstructions based on qualitative estimates of continental margin overlaps generally yield a tighter fit than our preferred reconstruction based on palinspastic margin restoration

overview of the eruption of soufriere hills volcano montserrat 18 july 1995 to december 1997 the onset of phreatic volcanic activity at the soufriere hills volcano montserrat on 18 july 1995 followed a three year period of heightened volcano seismic activity beneath the island phreatic explosions gave way to continuous eruption of juvenile andesitic magma in the form of a lava dome on or around 15 november 1995 magma production rate has varied leading to changes in eruptive style an explosive eruption on 17 september 1996 followed a period of enhanced dome growth and large scale gravitational collapses from its eastern flank increasing dome volume led to stressing and overtopping of the confining crater walls to the southwest north and west during early 1997 sustained high magma production rate since june 1997 has led to three periods of major gravitational dome collapses followed by vulcanian explosive eruptions dome growth re started immediately after the cessation of the latest of these explosive phases in october 1997 and continues as of december 1997

origin and evolution of the kerguelen plateau broken ridge and kerguelen archipelago editorial large igneous provinces lips are constructed when two objectives of odp leg 183 were to sample the copious amounts of mantle derived magma enter the submarine lip in as many locations as possible to 1 earth s crust in localized regions although the kerguelen determine the temporal variation in magma production plateau and broken ridge are now submarine they 2 constrain the origin and evolution of the volcanic formed as a contiguous largely subaerial lip during rocks forming the lip studies of igneous basement cretaceous time in the eastern indian ocean subrecovered during leg 183 complement continuing studies sequently they were separated by sea floor spreading of the cenozoic subaerial lavas that form the kerguelen beginning in middle eocene time along the southeast archipelago this volume includes a collection of papers indian ridge seir fig 1 the kerguelen archipelago that address these objectives by using data for lavas is a cenozoic feature constructed on the northern kerforming the kerguelen plateau broken ridge and kerguelen plateau nkp fig 1 together with the guelen archipelago 82 38 ma hotspot track formed by the ninetyeast high quality radiometric age determinations are critridge the kerguelen plateau broken ridge and kerical for understanding rates of lip formation their causguelen archipelago represent 119 myr of volcanism ative dynamic mantle processes and their temporal that has been attributed to the kerguelen mantle plume relationships to potentially related environmental although the igneous basement of the kerguelen plateau changes duncan employs the ar ar technique to had been sampled at sites 738 747 749 and 750 fig determine ages at all of the odp leg 183 igneous 1 by ocean drilling program odp legs 119 and 120 basement drill sites fig 1 in general ages young leg 183 was the first leg dedicated to understanding the northwards on the kerguelen plateau from 119 ma origin and evolution of an oceanic lip five sites 1136 at site 1136 in the south to 34 ma at site 1140 in the 1137 1138 1139 1140 penetrated igneous basement north emplacement of the conjugate central kerguelen of the kerguelen plateau and at two sites 1141 1142 plateau ckp and broken ridge falls within an 5 myr the first basement samples were obtained from broken ridge fig 1 window from 100 to 95 ma an unanticipated result

late cenozoic increase in accumulation rates of terrestrial sediment how might climate change have affected erosion rates abstract accumulation rates of terrestrial sediment have increased in the past few million years both on and adjacent to continents although not everywhere apparently erosion has increased in elevated terrain regardless of when last tectonically active or what the present day climate in many regions sediment coarsened abruptly in late pliocene time sparser data suggest increased sedimentation rates at 15 ma approximately when oxygen isotopes in benthic foraminifera imply high latitude cooling if climate change effected accelerated erosion understanding how it did so remains the challenge some obvious candidates such as lowered sea level leading to erosion of continental shelves or increased glaciation account for increased sedimentation in some but not all areas perhaps stable climates that varied slowly allowed geomorphic processes to maintain a state of equilibrium with little erosion until 3 4 ma when large oscillations in climate with periods of 20 000 40 000 years developed and de

magmatic system geometry at mount st helens modeled from the stress field associated with posteruptive earthquakes earthquakes following the may 18 1980 and june 12 1980 explosive eruptions of mount st helens were concentrated at depths below 6 km the hypocenters of these posteruption earthquakes define two seismic lobes separated by an aseismic volume of 2 km width that is located beneath the position of the crater the timing and location of the posteruption seismicity suggests that the earthquakes occurred in the country rock surrounding a magma body located within the aseismic volume the withdrawal of magma caused a pressure decrease within the reservoir and the earthquakes occurred as a brittle response to the resulting stress change focal mechanism solutions for posteruption earthquakes distributed within the seismic lobes are calculated using polarity and amplitude data results show that a perturbation from the regional stress field occurs between depths of 7 and 11 km this anomalous stress field is successfully modeled by a decrease in pressure within a cylindrical magma body located between the seismic lobes and subject to conditions representing the regional shear stress regime in the best fitting model the magma body is centered at 46 204 n 122 187 w 600 m to the north of the present dome and is under a regional shear stress of 20 bars the radius of the chamber is calculated to be between 0 65 and 0 75 km when using pressure drops within the cylinder of 220 and 150 bars respectively the magma body extends from 7 to 11 km and has a volume of between 5 and 7 km3 posteruption earthquakes located below 11 km fall on a northeast striking fault that is preferentially aligned with the regional tectonic stress regime this fault may be a conduit to transport magma to the shallow reservoir from greater depths within the crust

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

comment on major australian antarctic plate reorganization at hawaiian emperor bend time whittaker et al reports 5 october 2007 p 83 presented reconstructions for australia and antarctica showing a change in relative plate motion 53 million years ago coincident with an inferred major global plate reorganization this comment addresses problematic areas in their assumptions and the geological consequences of their reconstructions

analysis and forward modeling of seismic anisotropy during the ongoing eruption of the soufriere hills volcano montserrat 1996 2007 1 volcanic stress field analysis has been lauded as a potentially powerful tool for midterm to long term eruption forecasting however because tectonic processes can also produce localized stress field reorientations evidence for a direct causal link between observed stress field reorientations and magmatic activity is of critical importance in this study we show that local stress field reorientations preceding changes in volcanic activity at the soufriere hills volcano montserrat are observable using two independent measures of crustal stress local volcano tectonic earthquake fault plane solutions and measurements of shear wave splitting in regional earthquakes we further demonstrate that the local stress field orientation during a 6 month period preceding the onset of eruptive activity at soufriere hills in 1999 is highly localized and spatiotemporally variable and that the spatial pattern of precursory local stress orientations is consistent with numerically modeled patterns of stress resulting from pressurization of a vertical dike these observations provide compelling evidence for a direct causal link between pressurization of midlevel volcanic conduit systems by ascending magma and precursory local stress field reorientations and demonstrate that seismological analysis can be used to detect subtle local changes in stress that herald eruptive activity

transtensional deformation of montserrat revealed by shear wave splitting here we investigate seismic anisotropy of the upper crust in the vicinity of soufriere hills volcano using shear wave splitting sws analysis from volcano tectonic vt events soufriere hills which is located on the island of montserrat in the lesser antilles became active in 1995 and has been erupting ever since with five major phases of extrusive activity we use data recorded on a network of seismometers between 1996 and 2007 partially spanning three extrusive phases shear wave splitting in the crust is often assumed to be controlled either by structural features or by stress aligned cracks in such a case the polarization of the fast shear wave would align parallel to the strike of the structure or to the maximum compressive stress direction previous studies analyzing sws in the region using regional earthquakes observed temporal variations in which were interpreted as being caused by stress perturbations associated with pressurization of a dyke our analysis which uses much shallower sources and thus only samples the anisotropy of the upper few kilometres of the crust shows no clear temporal variation however temporal effects cannot be ruled out as large fluctuations in the rate of vt events over the course of the study period as well as changes in the seismic network configuration make it difficult to assess average delay times of approximately 0 2 s similar in magnitude to those reported for much deeper slab events suggest that the bulk of the anisotropy is in the shallow crust we observe clear spatial variations in anisotropy which we believe are consistent with structurally controlled anisotropy resulting from a left lateral transtensional array of faults which crosses the volcanic complex

patterns of volcanotectonic seismicity and stress during the ongoing eruption of the soufriere hills volcano montserrat 1995 2007 abstract the ongoing eruption of the soufriere hills volcano montserrat has been accompanied throughout by varying levels of high frequency volcanotectonic vt seismicity these earthquakes reflect the brittle response of the host rock to stresses generated within the magmatic system and thus reveal interesting and useful information about the structure of the volcanic conduit system and processes occurring within it in general systematic changes in the rate location and fault plane solutions of vt earthquakes correspond to changes in the volcano s behavior and indicate that the main conduit for the eruption is a dike or system of dikes trending ne sw and centered beneath the eruptive vent to date the eruption has comprised three extrusive phases separated by two 1 2 year long periods of residual activity prior to the start of each extrusive phase vt earthquakes with fault plane solution p axes oriented perpendicular to inferred regional maximum compression dominate the data set consistent with stresses induced by the inflation of the mid level conduit system 90 rotated vt fault plane solutions are also observed preceding a change in eruption style from effusive to explosive in 1997 while increases in the rate of vt earthquakes precede eruption phase onsets high rates of vt seismicity are also observed during the first period of residual activity and in this case appear to reflect the relaxation of host rock following withdrawal of magma from the mid crustal system most vt earthquakes are located directly beneath the eruptive vent although two distal vt clusters were observed during the first six months of the eruption late 1995 early 1996 both of these distal clusters likely resulted from stresses generated during the establishment of the main conduit system

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

results from the broadband seismic network on montserrat a digital broadband seismic network has been installed around soufriere hills volcano on montserrat while several distinctive types of seismic events with frequencies ranging from 0 5hz to 30hz could be identified the emphasis is on two types of low frequency events which indicate the involvement of a fluid phase in the source mechanism the so called long period events and the hybrid events the latter occur in swarms with distinct periodicities of 4 to 12 hours and preceed major dome collapses and explosions the swarms correlate very well with the tilt observed at the flanks of the volcanic edifice and hence can be linked to the pressurization of the magmatic system occasionally separate hybrid events merge and form harmonic tremor which sometimes has a shifting spectral content this reveals temporary changes in the source parameters low frequency seismic signals on montserrat are considered to be key parameters for the monitoring of the internal dynamics of the volcano

deep crustal structure in the southwestern yilgarn craton western australia abstract the archaean rocks of the yilgarn craton comprise the largest geological entity in western australia and one of the largest areas of archaean crust anywhere in the world however poor outcrop has prevented a thorough understanding of the structure of the crust in the region a subject of particular interest being the relationship between high grade metamorphic rocks in the west and lower grade granitoid greenstone terrains further east interpretation of previously unpublished seismic refraction data combined with the re interpretation of published data allows crustal velocity structure in the southwest of the craton to be resolved overall the structure is similar to shield areas elsewhere in the world the data defining a two layered crust with an average thickness of about 35 km however significant lateral variations in velocity structure are also observed and these coincide with a terrane boundary previously postulated on the basis of geological observations and seismic reflection data the terrane boundary dips to the east extends through the entire crust and is notable for an area of anomalously high seismic velocity in the lower crust two possible interpretations of the high velocity zone are proposed firstly it is due to the presence of mafic to ultramafic intrusions secondly and the preferred interpretation the high velocity zone is a fault bounded mega sliver which may be a suspect terrane in its own right perhaps with oceanic affinities the results of the seismic experiment strongly support tectonic models for yilgarn craton based on terrane tectonics the possibility that the terrane boundary is being reactivated by contemporary stresses also provides a plausible explanation for the local intra plate seismicity

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

mechanisms of unrest and eruption at persistently restless volcanoes insights from the 2015 eruption of telica volcano nicaragua many of earth s volcanoes experience well defined states of quiescence and unrest with unrest occasionally culminating in eruption some volcanoes however experience an unusually protracted i e decades long period of noneruptive unrest and are thus categorized as persistently restless volcanoes prvs the processes that drive persistently restless volcanism are poorly understood as our knowledge of prvs is currently based on a small number of case studies here we examine multidisciplinary observations of the 2015 eruptive episode at telica volcano nicaragua in the context of its long term behavior we suggest that the latter phases of the 2015 eruption were ultimately driven by destabilization of its shallow magma reservoir based on previous geodetic seismic studies of telica geirsson et al 2014 https doi org 10 1016 j jvolgeores 2013 11 009 rodgers et al 2013 https doi org 10 1016 j jvolgeores 2013 08 010 and 2015 https doi org 10 1016 j jvolgeores 2014 11 012 and on multiparameter observations at telica over a 7 year period we propose that three distinct states of unrest occur at telica over decadal timescales a stable open state involving steady conduit convection and two distinct unstable states that may lead to eruptions in the weak sealing state phreatic explosions result from steady conduit convection underlying a weak seal in the destabilized state destabilization of the top of the convecting magma in the conduit leads to rapid accumulation of high pressures leading to strong impulsive phreatomagmatic explosions our observations and interpretations suggest that continuous seismic ground based deformation gas emission and thermal monitoring and interpretation of these data within a paradigm of sustained conduit convection modulated by episodes of sealing and destabilization of shallow magma reservoirs may allow robust forecasting of eruption potential energy and duration at telica and similar prvs worldwide

numerical models of volcanotectonic earthquake triggering on non ideally oriented faults 1 calculation of inflation induced coulomb stress changes on faults of varying orientation indicates that local faults of almost any orientation are susceptible to slip in response to dike inflation additionally as dikes typically inflate in the direction of regional minimum compressive stress the sense of inflation induced strike slip faulting predicted by coulomb stress change models is opposite to that expected from regional tectonic forces observations of volcanotectonic vt earthquake fault plane solutions with pressure p axes oriented 90 to regional maximum compression confirm this prediction in detail imperfect agreement between coulomb stress change models and the ranges of fault plane orientations observed in vt fault plane solution studies and observed locations of vt seismicity indicates that the orientation and location of pre existing faults around the conduit ultimately determine the seismic response of host rock to an inflating dike

earthquake classification location and error analysis in a volcanic environment implications for the magmatic system of the 1989 1990 eruptions at redoubt volcano alaska abstract determination of the precise locations of seismic events associated with the 1989 1990 eruptions of redoubt volcano posed a number of problems including poorly known crustal velocities a sparse station distribution and an abundance of events with emergent phase onsets in addition the high relief of the volcano could not be incorporated into the hypoellipse earthquake location algorithm this algorithm was modified to allow hypocenters to be located above the elevation of the seismic stations the velocity model was calibrated on the basis of a posteruptive seismic survey in which four chemical explosions were recorded by eight stations of the permanent network supplemented with 20 temporary seismographs deployed on and around the volcanic edifice the model consists of a stack of homogeneous horizontal layers setting the top of the model at the summit allows events to be located anywhere within the volcanic edifice detailed analysis of hypocentral errors shows that the long period lp events constituting the vigorous 23 hour swarm that preceded the initial eruption on december 14 could have originated from a point 1 4 km below the crater floor a similar analysis of lp events in the swarm preceding the major eruption on january 2 shows they also could have originated from a point the location of which is shifted 0 8 km northwest and 0 7 km deeper than the source of the initial swarm we suggest this shift in lp activity reflects a northward jump in the pathway for magmatic gases caused by the sealing of the initial pathway by magma extrusion during the last half of december volcano tectonic vt earthquakes did not occur until after the initial 23 hour long swarm they began slowly just below the lp source and their rate of occurrence increased after the eruption of 01 52 ast on december 15 when they shifted to depths of 6 to 10 km after january 2 the vt activity migrated gradually northward this migration suggests northward propagating withdrawal of magma from a plexus of dikes and or sills located in the 6 to 10 km depth range precise relocations of selected events prior to january 2 clearly resolve a narrow steeply dipping pencil shaped concentration of activity in the depth range of 1 7 km which illuminates the conduit along which magma was transported to the surface a third event type named hybrid which blends the characteristics of both vt and lp events originates just below the lp source and may reflect brittle failure along a zone intersecting a fluid filled crack the distribution of hybrid events is elongated 0 2 0 4 km in an east west direction this distribution may offer constraints on the orientation and size of the fluid filled crack inferred to be the source of the lp events

tephra fall deposits from the 1992 eruption of crater peak alaska implications of clast textures for eruptive processes the 1992 eruption of crater peak mount spurr alaska involved three subplinian tephra producing events of similar volume and duration the tephra consists of two dense juvenile clast types that are identified by color one tan and one gray of similar chemistry mineral assemblage and glass composition in two of the eruptive events the clast types are strongly stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third tan clasts have average densities between 1 5 and 1 7 g cc and vesicularities phenocryst free of approximately 42 gray clasts have average densities between 2 1 and 2 3 g cc and vesicularities of approximately 20 both contain abundant microlites average maximum plagioclase microlite lengths 13 15 m in gray clasts in the upper layer are similar regardless of eruptive event and therefore the repose time between them and are larger than average maximum plagioclase microlite lengths 9 11 m in the tan clasts in the lower layer this suggests that microlite growth is a response to eruptive processes and not to magma reservoir heterogeneity or dynamics furthermore we suggest that the low vesicularities of the clasts are due to syneruptive magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater

structural styles in the perth basin associated with the mesozoic break up of greater india and australia abstract the perth basin constitutes one of the principal tectonic features along the western continental margin of australia and formed through permo cretaceous rifting of greater india and australia it is composed of northerly striking sub basins troughs and ridges bounded by faults which display evidence for normal dip slip and possibly dextral strike slip movement transfer faults divide the basin into compartmentalised regions of similar structural style extension and transtension occurred during the jurassic early cretaceous associated with continental break up stretching was accommodated mainly by the reactivation of pre existing northerly oriented faults which display both planar and listric geometries the darling fault system along the eastern margin of the basin has a planar profile the hanging wall succession in the adjoining perth basin has undergone block rotation and tilting with syn sedimentary depositional thickening of rock units towards the fault listric normal faults occur in both the onshore and offshore portions of the basin and influence geometry of the sub basins dip on these faults decreases through the basin sequence and probably soles out in the upper level of basement rollover anticlines develop within the hangingwall of these structures extension oblique to the main structural trends of the basin in the late jurassic the earliest cretaceous resulted in dextral strike slip movement along major north striking faults and the development of en echelon folds in the basin compressive deformation developed during this event at constraining bends of the major fault systems two orientations of transfer zones are recognised and significantly influence basin structures the first are east west trending transfer faults which formed in the permian and were reactivated during jurassic extension but can only be recognised in the northernmost perth basin these transfer faults link basin segments of different characteristics and transfer dip displacement among the northerly striking faults the transfer zones striking nw only influence late jurassic early cretaceous age deformation features they are characterised by either a swing in a left lateral sense in the orientation of northerly striking fractures as they cross the transfer zones or a termination of the fracture at the transfer zone the transfer zones are parallel to and may be related to transform belts in the indian oceanic crust basin wide uplift occurred during the break up and resulted in basin inversion and erosion of up to thousands of metres of the sedimentary succession folding by dextral strike slip motion along the darling urella fault system also inverted the pre break up sequences in major onshore depocentres

from two way time to depth and pressure for interpretation of seismic velocities offshore methodology and examples from the wallaby plateau on the west australian margin abstract the effect of water loading can account for sub seafloor seismic p wave velocity increases exceeding 1 km s from shallow to deep water but it is commonly ignored in offshore studies direct comparative analysis of interval velocity patterns between areas of significantly different water depths requires various water pressure related changes in velocity to be accounted for this apparently simple task is not easy to implement in practical terms one approach is to try to predict velocity increase at any given depth due to the pressure effect of water layer above and then to reduce the observed interval velocity at that depth by subtraction of the predicted velocity increase interval velocities at different locations thus reduced can be compared and conclusions about rock lithology can subsequently be made the application of water depth adjustments to seismic velocities and the methods used to apply them remain controversial the presentation of velocity models as a function of pressure rather than two way time or depth emerges as a possible solution analysis of appropriately adjusted seismic interval velocities from geoscience australia s 2008 09 seismic survey ga 310 in conjunction with seismic reflection interpretation provides new insights into the geology of the wallaby plateau offshore of western australia seismically distinctive divergent dipping reflector sequences ddrs identified in the area are similar in seismic character to seaward dipping reflector sequences sdrs of inferred volcanic composition the water depth adjustment of seismic velocities analysed in our study reduces the distinction between sdrs ddrs and sedimentary strata such that discrimination between volcanic and sedimentary strata in ddrs or sdrs is equivocal a major uncertainty of this interpretation is caused by a lack of global reference velocity models of sdrs and ddrs

early india australia spreading history revealed by newly detected mesozoic magnetic anomalies in the perth abyssal plain the seafloor within the perth abyssal plain pap offshorewestern australia is the only section of crust that directly records the early spreading history between india and australia during the mesozoic breakup of gondwana however this early spreading has been poorly constrained due to an absence of data including marine magnetic anomalies and data constraining the crustal nature of key tectonic features here we present new magnetic anomaly data from the pap that shows that the crust in the western part of the basin was part of the indian plate the conjugate flank to the oceanic crust immediately offshore the perth margin australia we identify a sequence of m2 and older anomalies in the west pap within crust that initially moved with the indian plate formed at intermediate half spreading rates 35 mm yr consistent with the conjugate sequence on the australian plate more speculatively we reinterpret the youngest anomalies in the east pap finding that the m0 age crust initially formed on the indian plate was transferred to the australian plate by a westward jump or propagation of the spreading ridge shortly afterm0 time samples dredged from the gulden draak and batavia knolls at the western edge of the pap reveal that these bathymetric features are continental fragments rather than igneous plateaus related to broken ridge these microcontinents rifted away from australia with greater india during initial breakup at 130ma then rifted from india following the cessation of spreading in the pap 101 103ma

re examination of the magnetic lineations of the gascoyne and cuvier abyssal plains off nw australia this thesis addresses causes of two forms of secondary hawaiian volcanism rejuvenated onshore eruptions and offshore hawaiian arch flows it is proposed that secondary volcanism is generated as a direct consequence of lithospheric flexural uplift that surrounds new volcanic shields as they grow this uplift causes decompression of the underlying asthenosphere which is assumed to be chemically and isotopically heterogeneous near its solidus and derived from the hawaiian mantle plume uplift is modeled as the axisymmetric response of an elastic plate to a volcanic point load that grows linearly in time to model flow in the asthenosphere the rate of flexure of the lithosphere is taken as the upper boundary condition on an isoviscous incompressible fluid half space the first feature of secondary volcanism this model explains is the observed spatial gap between secondary volcanism and active shields best agreement is found with the majority of the observed spatial gaps with a lithosphere of effective elastic thickness te 25 35 km secondly this work demonstrates that the flexural model can produce observed crustal production if some magma focusing toward individual eruption sites occurs from the mantle over an area two to ten times the eruption area the third feature this model addresses is that secondary lavas are isotopically distinct from shield lavas in this model melting the same two component mantle forms the secondary and shield lavas but the components are sampled by melting at rates that differ between the locations as predictable functions of depth flexural decompression produces melts that preferentially sample the mantle component that begins melting shallowest and which is associated with sr and nd isotope ratios most like those of secondary lavas melting in the center of a mantle plume is assumed to generate shield volcanism and is predicted to preferentially sample the component that begins melting deepest which is associated with sr and nd isotope ratios more similar to shield lavas models therefore successfully predict the observed mean difference in 87sr 86sr and 143nd 144nd compositions between the secondary and shield lavas the fourth feature addressed is that secondary lavas are alkalic and shield lavas are dominantly tholeiitic to explain this difference the mean extent of partial melting is computed and it is found that a model plume composed mostly of depleted peridotite 90 and some pyroxenite 10 will yield a lower extent of melting for secondary lavas than shield lavas this particular model assumes lithospheric thicknesses 90 100 km and plume potential temperatures mean of 1550 c that are consistent with independent studies of the hawaiian hotspot thus asthenospheric melting by flexural decompression is a viable mechanism of intraplate volcanism which can explain many general characteristics of secondary hawaiian volcanism

seismicity and stress in the vicinity of mount spurr volcano south central alaska focal mechanism solutions and hypocenters for earthquakes near mount spurr volcano in south central alaska reveal spatial perturbations in the regional stress field and in the maximum depth of seismicity at the volcano practically all the shocks during the dormant decade 1981 1991 have depths shallower than 5 km and are characterized by nearly pure normal slip in contrast earthquakes located outboard of the volcano mostly concentrate in the depth range 3 18 km and exhibit predominantly strike slip oblique reverse slip or reverse slip the regional stress field is characterized by subhorizontal maximum principal stress directed n30 w concordant with the direction of convergence between the north american and pacific plates whereas the maximum principal stress is probably more nearly vertical beneath the volcano we suggest that the shoaling of seismicity beneath the volcano reflects localized elevation of the depth to the midcrustal transition from brittle failure to plastic flow we attribute this localized elevation to relict magmatic heat associated with past volcanic eruptions the available data are not sufficient to determine the cause of the suggested rotation of the maximum principal stress axis beneath the volcano possible mechanisms include an increase in the vertical stress from the weight of the mount spurr massif and a decrease in the maximum horizontal stress associated with doming of the shallow crust from magmatic processes

temporal change of characteristics of shallow volcano tectonic earthquakes associated with increase in volcanic activity at kuchinoerabujima volcano japan abstract shallow volcano tectonic vt earthquakes recorded at the kuchinoerabujima island volcano in southwest japan are analyzed in order to clarify the role of hydrothermal activity in the development of volcanic seismicity from analysis of shallow vt earthquakes in 2006 two specific episodes of elevated seismicity are observed in april and november 2006 the vt earthquakes have hypocenters at depths of 0 0 4 km beneath the summit crater and normal fault focal mechanisms with wnw ese extension consistent with the tensional stress field indicated by the alignment of craters and fissures although the hypocenters and focal mechanisms are found to be largely invariant during these episodes the corner frequencies of the vt earthquakes underwent a pronounced increase and decrease accompanying the changes in seismicity rates the corner frequencies increased to 20 25 hz approximately one month prior to the onset of elevated seismicity and then decreased to 10 15 hz in the period of peak seismicity the rupture length also decreased at the onset of seismicity thereafter increasing as the seismicity continued the peak seismicity in terms of the daily number of vt events was accompanied by inflation around the crater suggestive of a pressure increase in the volcanic system it is inferred that the increase in shallow vt seismicity and rupture length is related to the development of a fractured zone the pressure increase in the volcanic system is attributed to the intrusion of hydrothermal fluids which is supported by an observed increase in fumarolic temperature and activity the preceding monochromatic events are thus considered to be generated by the effect of fluid filled cracks the shortening of rupture length is then inferred to be related to the closing of non fluid filled cracks in the fracture zone under the increasing pressure field leading to a transition from monochromatic events to low frequency and shallow vt seismicity