the red beds series in the erta ale segment north afar evidence for a 6 ma old post rift basin prior to continental rupturing abstract the erta ale rift segment north afar is regarded as the most mature rift part within the entire afar rift system very little is known about its deformation history because of limited exposures of geological records in its inner floor except volcanics of the erta ale chain and the poorly known red beds series along the flanks of the depression an integrated study combining sedimentological geochemical 40ar 39ar radiometric and tectonic approaches has been devoted to red beds series flanking the depression to the sw our new results allow to argue that 1 the 300 m thick red beds exposed section comprises alluvial deposits that enclose 2 basaltic lavas and related sill intrusions that both yield 6 ma 40ar 39ar ages and display similar geochemical affinities 3 the red beds series locally overlap unconformably basement bounding terrains and 4 are involved in a limited number of low displacement normal faults that recorded a modest amount of extension

deformation enhanced fluid distribution in feldspar aggregates and implications for ductile shear zones experiments at 900 c and 1 4 gpa show that the distribution of aqueous fluid in fine grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation the isolated pore distribution is rapidly regained during annealing following deformation the deformation enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength it also increases the bulk transport rate through the aggregate by more than an order of magnitude a change in fluid distribution is not observed in fine grained quartz aggregates deformed at similar conditions if deformation enhanced fluid distribution occurs in naturally deformed feldspathic rocks it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones

eruption processes and deposit characteristics at the monogenetic mt gambier volcanic complex se australia implications for alternating magmatic and phreatomagmatic activity the 5 ka mt gambier volcanic complex in the newer volcanics province australia is an extremely complex monogenetic volcanic system that preserves at least 14 eruption points aligned along a fissure system the complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner the facies are 1 coherent to vesicular fragmental alkali basalt effusive hawaiian spatter and lava flows 2 massive scoriaceous fine lapilli with coarse ash strombolian fallout 3 bedded scoriaceous fine lapilli tuff violent strombolian fallout 4 thin medium bedded undulating very fine lapilli in coarse ash dry phreatomagmatic surge modified fallout 5 palagonite altered cross bedded medium lapilli to fine ash wet phreatomagmatic base surges and 6 massive palagonite altered very poorly sorted tuff breccia and lapilli tuff phreato vulcanian pyroclastic flows since most deposits are lithified to quantify the grain size distributions gsds image analysis was performed the facies are distinct based on their gsds and the fine ash to coarse fine ash ratios these provide insights into the fragmentation intensities and water magma interaction efficiencies for each facies the eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles except for a magmatic horizon of effusive activity occurring at both ends of the volcanic complex simultaneously the eruption foci are located along nw se trending lineaments indicating that the complex was fed by multiple dykes following the subsurface structures related to the tartwaup fault system possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates viscosity crystallinity degassing and magma discharge rate as well as hydrological parameters

aspects of the crustal structure of the western mojave desert california from seismic reflection and gravity data seismic and gravity data taken along line 1 of the 1982 consortium for continental reflection profiling cocorp mojave desert survey n s profile 30 km long have been used to characterize the upper crust north of the san andreas fault in the western mojave block of southern california consortium for continental reflection profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds two way travel time the resultant common depth point cdp sections provided starting models for generating a refined geologic cross section using a combination of ray tracing forward modeling and gravity interpretation the forward modeling was used to validate the existence of faults and constrain their dips the gravity data were used to refine the overall model particularly in poor data areas on the cdp sections gravity data taken along three nearby profiles parallel to primary line of section were also used to determine the structural trend results from the first two seconds indicate the presence of a series of ene striking reverse faults beneath the late tertiary and quaternary sedimentary cover of the western mojave the faults dip northward and offset the sediment basement interface the largest such feature has an apparent throw of 1 8 km and exhibits a subtle scarp at the earth s surface suggesting holocene displacement the orientation of these faults although not an instantaneous representation of the present day stress field is consistent with nnw compression across the western mojave block and wnw striking san andreas fault as determined from nearby focal mechanisms and in situ stress measurements the faults also appear to be closing small sedimentary basins in the mojave block which may have formed during an earlier extensional phase similar to what is happening on a much larger scale in the los angeles basin to the south of the san andreas fault reflections between 2 and 5 s coupled with the local geology and gravity modeling are consistent with the presence of the pelona rand schist in the subsurface beneath the western mojave the upper surface of the schist i e vincent rand thrust equivalent rises southward toward the san andreas fault where it is displaced vertically up to the south at least 5 km along the e w trending hitchbrook fault such that the schist crops out between the hitchbrook and subparallel san andreas to the south the same structure may exist beneath the tehachapi mountains with the roles of the hitchbrook and san andreas faults played by the north and south branches of the garlock fault respectively the rising or arching of the basement toward the san andreas fault and toward the garlock is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western mojave furthermore the arching is also consistent with a strong component of fault normal compression

shear driven upwelling induced by lateral viscosity variations and asthenospheric shear a mechanism for intraplate volcanism abstract volcanism occurring away from ridges and subduction zones does not have an obvious plate tectonic explanation but instead must arise from sub lithospheric processes that generate upwelling flow and decompression melting several convective processes such as mantle plumes convective instability edge driven convection and richter rolls produce upwelling via the action of gravity on density heterogeneities in the mantle here we investigate an alternative mechanism the shear driven upwelling sdu which instead generates upwelling solely through the action of asthenospheric shear flow on viscosity heterogeneity using a numerical flow model we examine the effect of viscosity heterogeneity on viscous shear flow induced within an asthenospheric layer we demonstrate that for certain geometries and viscosity ratios circulatory flow develops within a cavity or step embedded into the lithospheric base or within a low viscosity pocket embedded within the asthenospheric layer for asthenosphere shearing at 5 cm yr we estimate that sdu can produce upwelling rates of up to 0 2 cm yr within a continental rift 0 5 cm yr along the vertical edge of a craton or 1 0 cm yr within a pocket of low viscosity asthenosphere in the last case the pocket must feature an aspect ratio of more than 5 occupy 20 60 of the asthenosphere s thickness and be at least 100 times less viscous than the surrounding asthenosphere such viscosity heterogeneity may be associated with thermal chemical melting volatile or grain size anomalies and is consistent with tomographic constraints on asthenospheric variability we estimate that sdu may generate up to 2 5 km myr of melt that is potentially eruptible as surface volcanism this is faster than eruption rates observed at some locations of continental basaltic volcanism we conclude that sdu could provide an explanation for intraplate volcanism occurring above rapidly shearing asthenosphere for example in the basin and range region of north america

volcanic vent self similar clustering and crustal thickness in the northern main ethiopian rift 1 the spatial distribution of volcanic vents in the northern ethiopian rift has been analysed in terms of their self similar clustering the fractal distribution of vents has been computed for the monogenetic vents cones domes and maar occurring along the rift valley and in the rift margin the self similar clustering of vents has been computed through the analysis of vent centres coordinates by means of the two point correlation method the vents of the northern main ethiopian rift display a self similar clustering characterised by a correlation coefficient of 1 41 0 02 with a size range defined by a lower cut off of 1 1 km and an upper cut off of 26 5 km this analysis indicates a probable crustal thickness of 27 km as indicated by the value of the computed upper cut off this value compares well with other estimates for the crustal and lithosphere thickness in the ethiopian rift

trapping of aqueous fluids in the deep crust free aqueous fluids in the deep ductile crust have residence times there which are very short in geological terms if not consumed by retrograde metamorphic reactions they empty very rapidly upwards this can be shown by applying the theory of magma percolation in a ductile host to the problem of aqueous fluid percolation below the brittle ductile transition thermal activation of ductility is strong enough to facilitate the rapid evolution of porosity and consequently allow rapid fluid transport in the cooler crust above the brittle ductile transition however this mechanism vanishes as a consequence fluid overpressures resulting from the accumulation of fluid in the transition zone must induce hydraulic fracturing rather than pore inflation in regions of horizontally compressive regional stress the commonest situation in stable continental crust these fractures will be horizontal aqueous fluids rising through or liberated from the lower crust will therefore probably accumulate at least initially at the brittle ductile transition in horizontally extensive reservoirs with high horizontal permeability but low vertical permeability

robust smooth magnetotelluric transfer functions jimmy c larsen randall l mackie 2 adele man e l l a adolfo fiordelisi4 and shirley rieven2 pacific marine environmental laboratory national oceanic and atmospheric administration 7600 sand point way ne seattle wa 981 15 0070 usa wassachusetts institute of technology 42 carleton street cambridge m a 02142 usa cnr international institute for geothermal research piazza soljerino 2 pisa 56126 italy enel esplorazione mineraria via andrea pisano 120 pisa 56122 italy

a systematic approach to the interpretation of magnetotelluric data in volcanic environments with applications to the quest for magma in long valley california magnetotelluric mt data in long valley california are principally sensitive to the complex structure in the rocks filling the caldera while the overall shapes of the sounding curves are set by the deep crustal structure and mantle the lateral variations of that response can be entirely explained using structure of the upper 1600 m a conductive body at depths from 1350 to 1600 m is present in the northern southern and western moats and beneath the resurgent dome this conductor effectively masks structures at intermediate depths we believe that this conductor represents fractured bishop tuff within the caldera and possibly graphitic metasediments beneath it electrical logs from wells penetrating these formations are used to corroborate our interpretation a large conductive 5 ohm m body of magma is precluded by the mt data the response to a spherical chamber of 4 km radius at a depth of 10 km is several times larger than the mismatch between our modeled and observed sounding curves small 2 km radius isolated pockets of magma may be present however we have developed a systematic approach to interpreting mt data in a complex volcanic environment we clustered sites on the basis of similar responses to perceive patterns in spite of severe distortions by local heterogeneity our model was built from the surface downward incorporating other published geophysical surveys and a time domain electromagnetic survey released by unocal three dimensional modeling was employed from the start and our interpretation was based primarily upon the phases this systematic approach of stripping off the known shallow structure in order to study the intermediate and deeper structure was quite successful an important finding of our study is that the phase is sensitive to shallow 1350 m three dimensional structure even at frequencies of 0 01 hz three dimensional modeling simply cannot be avoided in complex geological environments

volcanism in aboriginal australian oral traditions ethnographic evidence from the newer volcanics province abstract this article collects and presents nineteenth century ethnographic evidence from the newer volcanics province of australia and explores how volcanism was recorded and understood in aboriginal oral traditions it investigates whether aboriginal australian oral traditions can be understood as persistent eyewitness accounts of volcanic eruptions in the newer volcanics province how and what kind of geological and volcanological knowledge was embedded within aboriginal australian oral traditions and considers what value the ethnographic evidence has for understanding both the socio cultural and geological histories of the newer volcanics province

new criteria for systematic mapping and reliability assessment of monogenetic volcanic vent alignments and elongate volcanic vents for crustal stress analyses abstract linear arrays of monogenetic volcanic vent alignments represent an important source of stress strain data from volcanic regions of the earth and other planets currently however there is no standard methodology for mapping vent alignments or for assessing alignment reliability which are essential to defining a robust stress datum from vent alignments here we define a systematic method for defining monogenetic vent alignments by mapping the locations and shapes of vents elongation of vents into elliptical shapes is directly related to the trend of subsurface fissures that control alignments elongated vents therefore are critical for defining reliable regional vent alignments and also provide an independent means for assessing stress directions our reliability assessment system a b c d for vent alignments is based on the number of vents the standard deviation of vent center points from a best fit line the numbers and types of elongate vents the standard deviation of long axes of elliptical vents from a best fit line and in cases where alignments lack elongate vents by vent spacing distances we quantified morphometric parameters of well defined fissure fed vent alignments from polygenetic volcanoes and platform volcanic fields to establish appropriate threshold values for reliability assessments in this new method the combined analysis of vent locations and shapes optimizes the process of alignment mapping and the assessment of the reliability of alignments to be incorporated in stress analyses we provide quality ranking schemes for stress data derived from reliability assessed vent alignments and from elongate vents we demonstrate the utility of this new mapping and assessment methodology by analyzing monogenetic cinder cone fields within a platform volcanic field the san francisco volcanic field in arizona and within a volcanic field on the flank of a polygenetic volcano mount morning in antarctica

regional teleseismic tomography of the western lachlan orogen and the newer volcanic province southeast australia summary from 1998 may to september a portable array of 40 short period digital seismograph stations was operated in western victoria southeast australia across the western end of the midpaleozoic lachlan foldbelt and the newer volcanic province consisting of four parallel almost w e oriented receiver lines the array covered an area of about 270 150 km 2 the major aim of the lf98 lachlan foldbelt survey 1998 project is to map lateral variations in p wave speeds vp in the crust and upper mantle using teleseismic arrival time tomography primarily in order to investigate whether the major surface structural zones are associated with seismic velocity signatures at depth little a priori information from seismic profiling is available we invert 4067 relative arrival time residuals for a minimum structure vp model in the upper few hundred km using non linear iteration and 3 d ray tracing the most prominent negative anomaly 3 8 per cent in vp is found at a depth of about 45 km underneath the eastern part of the newer volcanic province it correlates spatially with the highest density of pliocene and pleistocene eruption centres northwest of melbourne and is therefore interpreted as a hotspot related high temperature anomaly causing reduced mantle velocities the related coherent volume of significantly lower than average velocities extends down to depths greater than 100 km in the east and extends west underneath the newer volcanic province a strong velocity contrast with average velocities 2 per cent greater in the west is found down to about 100 km across the moyston fault zone which forms the major structural boundary between the early paleozoic delamerian orogen in the west and the lachlan orogen in the east this result suggests that the moyston fault zone should be seen as a major lithospheric boundary in the south this boundary is also expressed by a distinct discontinuity in sr isotopic ratios of xenoliths the so called mortlake discontinuity and a change in the geochemistry of plutons of similar age however if the east to west velocity contrast originally existed in this southern zone it is now overprinted by the thermally reduced mantle velocities beneath the newer volcanic province

stable isotope geochemistry of cold co2 bearing mineral spring waters daylesford victoria australia sources of gas and water and links with waning volcanism abstract mineral springs in the victorian central highlands australia have high co 2 contents and naturally effervesce 13 c values of co 2 gas and dissolved inorganic carbon are 10 6 to 7 0 and 5 9 to 0 1 respectively with a net 13 c of 8 to 3 the carbon in these waters was derived from a mantle source associated with local pliocene to recent basaltic newer volcanic province rocks previously reported 3 he 4 he 1 2 3 1 relative to air and high he ne ratios are also consistent with the presence of magmatic volatiles silica contents imply that the waters were never heated above 130 c and that the system is not hydrothermal the occurrence of carbonated mineral springs in a relatively small region of the newer volcanics province where volcanic activity ceased several thousands of years ago may be due to the presence of late intrusions combined with deep circulation of water through deeply weathered and fractured ordovician basement a region of low seismic velocity under the daylesford area potentially images those intrusions most spring waters have 18 o and 2 h values of 8 to 6 and 45 to 35 respectively and lie to the left of the local and global meteoric water lines the anomalously low 18 o values results from co 2 exsolution at low temperatures which strongly partitions 18 o into the gas the lack of waters lying to the right of the local meteoric water line also implies that water rock interaction at elevated temperatures did not occur the 2 h values are lower than contemporary meteoric water suggesting that the waters may have recharged under colder climate conditions several thousand years ago the local ordovician rocks are gold bearing the present spring system is cold and would not efficiently transport au however volcanic is waning and the spring systems at the time of volcanism may have been hotter and able to redistribute au

tectonic history of northern new caledonia basin from deep offshore seismic reflection relation to late eocene obduction in new caledonia southwest pacific new high quality multichannel seismic reflection data from the western new caledonia offshore domain allow for the first time the direct continuous connection of seismic reflectors between the deep sea drilling project 208 drill hole on the lord howe rise and the new caledonia basin a novel seismic interpretation is hence proposed for the northern new caledonia basin stratigraphy which places the eocene oligocene unconformity deeper than previously thought and revisits the actual thickness of the pre oligocene sequences a causal link is proposed between the obduction of the south loyalty basin over new caledonia nc and the tectonic history of the northern new caledonia basin here it is suggested that as the south loyalty basin was being obducted during early oligocene times the nc basin subsided under the effect of the overloading and underthrusted to accommodate the compressional deformation which resulted in 1 the uplift of the northern fairway ridge and 2 the sinking of the western flank of new caledonia this event also had repercussions farther west with the incipient subsidence of the lord howe rise

magnetotelluric evidence for crustal suture zones bounding the southern great valley california a geoelectric section inferred from a regional magnetotelluric study across the coast ranges the great valley and the sierra nevada reveals significant variations in electrical resistivity zones of lower resistivity interpreted at depths from 10 km to at least 30 km lie near mapped geologic boundaries between the coast ranges and the great valley and beneath the eastern side of the great valley the former boundary is inferred by others to separate the subduction complex of the coast ranges from the mafic basement of the great valley the lower resistivities are most likely associated with metasediments trapped between the coast ranges ophiolite and the former oceanic crust beneath the great valley the latter boundary is problematic but may be evidence for a deep metasedimentary section trapped between the ophiolites beneath the great valley and the granitic rocks of the sierra nevada the lack of change in the magnetotelluric phase across the great valley indicates that a suture zone marked by lower resistivities is unlikely to be present beneath the valley however this does not preclude the existence of a resistive suture zone

moduli stress dependency in anisotropic rock masses abstract joint effects on rock mass anisotropy are examined by considering granodiorite and metasediment rock masses that were encountered in two different tunnelling projects composite moduli are determined using two simple models representing the constraints of equilibrium and compatibility the equilibrium constraint gives a lower value modulus where there is no stress redistribution as the rock mass deforms the compatibility constrained modulus characterises the stress distribution and redistribution as deformation occurs rock mass moduli and their dependance on joint orientation and imposed stress ratio environments are examined by considering rock masses encountered in two tunnelling projects it is shown that rock mass anisotropy is not only very sensitive to joint characteristics and orientation but also to in situ stresses in a very poor class 5 metasedimentary rock mass it is shown that in situ stresses induce significant anisotropies whereas initially pseudo isotropic elasticity was assumed because of the multiple jointing

late quaternary vegetation and climate change in southeastern australia palynological evidence from marine core e55 6 abstract palynological analysis of core e55 6 obtained from off shore western victoria provides a record of vegetation and climate change in the southeastern south australian and western victorian region spanning much of the last 125 000 yr an accompanying oxygen isotope record provides a chronology for the sequence the pollen record suggests that maximum effective precipitation occurred during the height of the last interglacial oxygen isotope substage 5e with wet sclerophyll forest and rainforest widespread in the region effective precipitation appears to have decreased during substage 5d promoting the expansion of dry sclerophyll communities then increased slightly during substage 5c with the limited re occurrence of wetter forest types climatic conditions gradually became drier from substage 5b with the replacement of eucalypt dominated forest by more open heath and mallee type communities in the arid glacial conditions of isotopic stage 4 a brief phase of increased effective precipitation occurred during the interstadial stage 3 with limited expansion of eucalyptus followed by a steady decline in effective precipitation and the expansion of open heath and grass communities pollen is very sparse in the top section of the core making environmental interpretation difficult nevertheless it appears that grass and heath communities dominated the region during stage 2 there is no clear record of the holocene due to the presence of mudflow sediments in the top of the core the pollen evidence also suggests that continental shelf exposed as sea levels fell was colonised by a mosaic of coastal vegetation including myrtaceae dominated scrub eucalypt and casuarina woodland asteraceae and chenopodiaceae dominated heath and grassland the charcoal record expressed as a ratio to pollen concentration gives evidence of increased burning during the height of the last interglacial period most likely in response to increased fuel loads with the expansion of eucalypt dominated forest this record also indicates a second phase of increased burning commencing in stage 2 which gives possible evidence for increased anthropogenic activity in the region

decompression melting driving intraplate volcanism in australia evidence from magnetotelluric sounding free to read a long period magnetotelluric mt survey with 39 sites covering an area of 270 by 150 km has identified melt within the thinned lithosphere of pleistocene holocene newer volcanics province nvp in southeast australia which has been variously attributed to mantle plume activity or edge driven mantle convection two dimensional inversions from the mt array imaged a low resistivity anomaly 10 30 m beneath the nvp at 40 80 km depth which is consistent with the presence of 1 5 4 partial melt in the lithosphere but inconsistent with elevated iron content metasomatism products or a hot spot the conductive zone is located within thin juvenile oceanic mantle lithosphere which was accreted onto thicker proterozoic continental mantle lithosphere we propose that the nvp owes its origin to decompression melting within the asthenosphere promoted by lithospheric thickness variations in conjunction with rapid shear where asthenospheric material is drawn by shear flow at a step at the base of the lithosphere

40ar 39ar geochronology reveals rapid change from plume assisted to stress dependent volcanism in the newer volcanic province se australia here we present 40ar 39ar ages of volcanic features in the cenozoic intraplate newer volcanic province in southeast australia the 5 ma volcanic products in the newer volcanic province can be subdivided into tholeiitic valley filling newer plains basalts and alkaline scoria cones lava shields and maars of the newer cones series plateau ages range from 3 76 0 01 to 4 32 0 03 ma 2 all sources of uncertainties included for the newer plains series with production rates of volcanism decreasing post 4 ma we suggest that magmatism is related to the complex interplay of magma upwelling due to edge driven convection and the cosgrove track mantle plume located in the northeast of the province at 6 5 5 ma plateau ages range from 1290 20 to 41 1 2 2 ka 2 for the newer cones series with a diffuse age progression in the onset of volcanism for these features from east to west analyses of the distribution and geomorphology of these volcanic features indicates a strong control of basement faults on volcanism reflected in alignment of volcanic features along paleozoic north south oriented basement faults in the east and cretaceous northwest southeast oriented extensional features in the west this age progression can be explained by a westerly migration of stress derived from the left lateral strike slip tasman fracture zone this suggests that the general mechanism of volcanism changed from upwelling due to plume assisted edge driven convection prior to 4 ma to stress dependent upwelling at around 1 3 ma

the impact of confining stress on the rock mass deformation modulus the paper presents the results of a literature review combined with additional laboratory testing to determine the significance of confining stress on the true stiffness properties of rocks classical elasticity theory is introduced to set the scene for what is precisely meant by stiffness increase with confining stress previous laboratory based stiffness testing on both intact and jointed rock specimens is reviewed and the experimental results and leading equations and theories are summarised the authors then introduce their own laboratory testing results concerning the same phenomena and present a new expression which is directly compared with both the previous and the newly conducted test data in conclusion it is seen that the rock types tested display significant increase in stiffness under confinement commonly up to 3 times for typical intact rocks and up to 20 times for weak specimens and that the implications for property definitions for numerical modelling are significant and relevant

anisotropy in jointed rock mass breakage abstract cracks in a rock material are generally closed and have no volume effect on the rock material s deformational characteristics these closed cracks however markedly effect the rock materials strength joints on the other hand do have a volume effect on the rock mass s deformational behaviour as well as its strength or stability in order to define the rock mass s deformational characteristics the deformational moduli are expressed in terms of equilibrium and or compatibility models for complete definition of the rock mass s deformational moduli both models are required these models describe the deforming rock mass when there is no stress redistribution equilibrium model or when there is stress redistribution compatibility model for joints it is shown that both the equilibrium and compatibility models are required to define the rock mass s deformational characteristics while for cracks only the equilibrium model is required it is shown that shear moduli have major affects on the rock mass s deformational behaviour by incorporating joint shear strength and griffith crack energy initiation criteria griffith strength loci for hard and soft rock masses are obtained these strength loci are produced for both shear and pseudo young s rock mass moduli for the hard rock the strength loci are significantly different indicating that the rock mass s failure mode possesses different energy balances for the soft rock mass the two strength loci are similar indicating that in the rock mass s failure from either shear or direct deformation the energy balances are nearly equal this suggests that the soft rock mass has minimal size effects

magnetotelluric evidence of lithospheric mantle thinning beneath the southern sierra nevada a wideband 0 01 1000 s magnetotelluric survey across the southern sierra nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10 20 km the eastern zone underlies the highest topography of the range and extends eastward this eastern conductive zone extends to depths in excess of 100 km based on model sensitivities and lies well below the moho depth of 32 38 km from seismological studies therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt such an interpretation is consistent with gravitational seismological and geologic evidence estimates of the partial melt fraction range from 2 to 5 at depths of 40 70 km which are consistent with fractions of melt inclusions observed in xenoliths from the eastern sierra nevada this partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations the other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes the western conductive zone also straddles the moho and extends into the mantle beneath western foothills of the sierra nevada and the great valley sediments to the west however xenoliths from this region indicate high velocity crustal rocks to depths in excess of 60 km and we therefore attribute the enhanced conductivity to graphitic metasediments and or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the sierran plutons

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

a chronology for the long pollen record from lake wangoom western victoria australia as derived from uranium thorium disequilibrium dating the chronology of a long pollen record from lake wangoom one of the few long palaeoenvironmental records from southeastern australia is discussed in light of the acquisition of new uranium thorium disequilibrium utd dates of crucial importance has been the estimation of ages for two major pre holocene phases of forest expansion recorded in the sequence with implications for the timing of periods of high effective precipitation in the region the new utd ages indicate that the oldest forest phase falls within the penultimate interglacial whereas the later phase corresponds with the last interglacial this is older than indicated by previously reported utd and radiocarbon dates but confirms a chronology based on direct correlation with the pollen record from marine core e55 6 the new chronology of the lake wangoom sequence provides evidence that phases of sustained forest development in southeast australia relate to interglacial periods in comparison interstadial periods appear to be characterised by only minor arboreal development marked differences in the pattern of vegetation development evident in each of the interglacials are thought to reflect varying climatic and anthropogenic influences copyright 2002 john wiley sons ltd

structural response and rock bolting of a rock mass abstract stability of a discontinuous rock mass represented as a blocky rock mass is examined and defined in terms of stress redistribution and formation of hinges the anisotropic compliance of the rock mass controls the initial stress distribution and as small deformational rotations occur finite deformations with the anisotropic compliances control the stress redistributions these stress redistributions induce and intensify stress gradients which in turn encourage further rotations and stress redistributions mechanistic deformations such as slip and rotation cause the blocky mass to act as a structure with the formation of hinges these hinges localise the resultant force at a hinge the force can change its magnitude and line of action but it cannot change its point of application stability of the blocky rock mass is considered in terms of redundancy and as each hinge is formed the redundancy is reduced beaming and arching as a means of load and stress redistribution are examined control of beaming and arching by using rock bolts is considered in relation to stress redistribution and inhibiting the formation of hinges

40ar 39ar ages for deep 3 3 km samples from the hawaii scientific drilling project mauna kea volcano hawaii 1 the hawaii scientific drilling project recovered core from a 3 5 km deep hole from the flank of mauna kea volcano providing a long essentially continuous record of the volcano s physical and petrologic development that has been used to infer the chemical and physical characteristics of the hawaiian mantle plume determining a precise accumulation rate via 40ar 39ar dating of the shield stage tholeiites which constitute 95 98 of the volcano s volume is challenging we applied40ar 39ar dating using laser and furnace heating in two laboratories berkeley and curtin to samples of two lava flows from deep in the core 3 3 km all determinations yield concordant isochron ages ranging from 612 159 to 871 302 ka 2 with p 0 90 the combined data yield an age of 681 120 ka p 0 77 for pillow lavas near the bottom of the core this new age when regressed with 40ar 39ar isochron ages previously obtained for tholeiites higher in the core defines a constant accumulation rate of 8 4 2 6 m ka that can be used to interpolate the ages of the tholeiites in the hsdp core with a mean uncertainty of about 83 ka for example at 3300 mbsl the age of 664 83 ka estimated from the regression diverges at the 95 confidence level from the age of 550 ka obtained from the numerical model of depaolo and stolper 1996 the new data have implications for the timescale of the growth of hawaiian volcanoes the paleomagnetic record in the core and the dynamics of the hawaiian mantle plume

fluid absent melting and the roles of fluids in the lithosphere a slanted summary abstract the presence or absence nature and compositions of lithospheric fluids are crucial factors in the evolution of igneous and metamorphic rocks despite this confusion still surrounds perceptions of the degrees of fluid involvement in many geological settings however experimental and field investigations constrain these roles and we are able to make some valid generalizations studies of prograde metamorphism have highlighted the spatial heterogeneity of fluid concentrations in many crustal environments this commonly indicates internal lithological control of volatile activities under fluid present conditions crustal fluids do not appear to mix readily fluid flow is generally structurally or lithologically controlled the transition from amphibolite to granulite facies is characterized by a decrease in ah2o as well as a temperature increase in many terranes these phenomena correlate with the appearance of migmatites some granulites are not migmatitic but have chemical and mineralogical characteristics of melt depleted restites this indicates efficient extraction of melts incongruent melting reactions involving the breakdown of crystalline hydrates in crustal rocks produce granitoid melts plus denser restites that are nearly or actually anhydrous experimental and theoretical treatments of fluid absent melting show that micas and amphiboles decompose at 700 to 900 c for mid to lower crustal pressures the melts resemble the compositions of s and i type granitoids pervasive aqueous fluids could not generally have been present during this melting since exposed granulite terranes are not characterized by high melt proportions and the magmas themselves are water undersaturated likewise the general presence of very h2o poor pervasive fluids is unlikely since this would allow only subsolidus dehydration at normally attainable crustal temperatures in this case the amphibolite granulite transition is possible at relatively low temperatures and the resultant rocks would not contain evidence of in situ melting the granulite granite connexion and effective crustal differentiation seem viable only during high temperature fluid absent metamorphism data on continental geotherms show that the lower crust is not entirely granulitic granulites are produced in regions of anomalously high heat flow the mantle connexion thus inferences on the nature of the lower crust should not be entirely based on studies of exposed granulite terranes mineral assemblages in eclogites also indicate widespread internal buffering of volatile activities and probable fluid absent conditions mantle derived basaltic magmas too are volatile bearing and it is plausible that volatile fixing phases also occur in the mantle volatiles may largely be introduced into the mantle through subduction processes and only released during the melting attending the breakdown of hydrous minerals as in the crust

magnetotelluric transect of long valley caldera resistivity cross section structural implications and the limits of a 2 d analysis twenty four magnetotelluric mt soundings have been collected in an east west profile across the center of long valley caldera the average station spacing is approximately 1 km and appears adequate to sample the important features of the upper crustal and deeper resistivity structures additional control on the shallowest resistivity is provided by a continuous profile of time domain electromagnetic soundings coincident with the western portion of the mt line our mt data set reveals numerous resistivity structures which illuminate the evolution and present state of the long valley system many of these have been quantified through two dimensional 2 d finite element modeling emphasizing the transverse magnetic tm mode important structural components include low resistivity layers 0 5 1 5 km deep under the eastern half of the caldera beneath the axial graben of the resurgent dome and under the west caldera moat most of this layering appears to lie in post caldera early rhyolite tuffs and the upp

volcano tectonic setting of the intraplate pliocene holocene newer volcanic province southeast australia role of crustal fracture zones 1 the pliocene holocene newer volcanic province in victoria southeast australia is a wide volcanic field site of the most recent volcanic activity of australia the intraplate signature and the localization on the northern side of the southern ocean have been used in the past to ascribe the volcanic activity either to the presence of a mantle hot spot or to thermal anomalies inherited from the seafloor spreading that separated australia from antarctica during the gondwana breakup the spacing and distribution of groups of eruption points suggests the presence of a 32 km deep mantle thermal anomaly alignments of eruption points indicate a strong tectonic control on magma emplacement mainly along nw se mesozoic cenozoic structures and along n s palaeozoic and e w late cretaceous structures this volcano tectonic setting relates to the interference of the left lateral kinematics of the major n s trending faults associated with the tasman fracture zone and the extensional structures of the otway basin this interpretation is also coherent with stress in situ data and focal mechanisms of earthquakes shmax oriented n150 this interpretation enhances the role of the tasman fracture zone a major sinistral transform on the reactivation of transtensional structures and on triggering the magmatism of the newer volcanic province

a late pleistocene and holocene vegetation and environmental record from lake wangoom western plains of victoria australia abstract pollen microfaunal and sedimentological evidence from the top 20 m of sediment in a closed volcanic crater lake is used to construct a detailed record of vegetation and environmental conditions through the holocene and a substantial part of the late pleistocene radiocarbon dating suggests that the sequence covers tha last 51 000 yr or so high lake levels and the presence of forest or woodland vegetation indicate that the holocene and the basal few thousand years of the record experienced wet and warm conditions the earliest period was succeeded by a long phase of lower but variable moisture levels before more arid conditions resulted in the replacement of forest and woodland by herbaceous vegetation and frequent lake drying periods of slight amelioration occurred between about 27 000 and 19 000 and before 15 000 radiocarbon yr ago maximum aridity occurred between about 19 000 and 10 000 yr b p a period incorporating the height of the last glacial during the pleistocene the lake became gradually more saline but has been fresh during the holocene even under low lake levels an increase in charcoal values is recorded probably around 20 000 years ago and this could have resulted from increased burning as a result of the activities of aboriginal people this may have also caused a change in understorey vegetation from one composed largely of asteraceae to grassland the record is compared with others from southeastern australia and there is good correspondence through the last 20 000 or 30 000 yr but before this problems of dating and the condensed nature of most sequences prevent detailed correlation

patterns of intraplate volcanism controlled by asthenospheric shear volcanism observed far from plate boundaries in the interior of oceanic and continental plates may result from flow in the underlying mantle comparison between a numerical model of mantle flow and the spatial distribution of intraplate volcanism indicates that rapid shear motion in the mantle may drive melting that causes intraplate eruptions most of earth s volcanism occurs at plate boundaries in association with subduction or rifting a few high volume volcanic fields are observed both at plate boundaries and within plates fed by plumes upwelling from the deep mantle1 the remaining volcanism is observed away from plate boundaries it is typically basaltic effusive and low volume occurring within continental interiors2 3 4 5 6 7 or creating seamounts on the ocean floor8 9 10 11 this intraplate volcanism has been attributed to various localized processes12 such as cracking of the lithosphere8 13 14 small scale convection in the mantle beneath the lithosphere15 16 17 or shear induced melting of low viscosity pockets of asthenospheric mantle that have become embedded along the base of the lithosphere18 here we compare the locations of observed intraplate volcanism with global patterns of mantle flow from a numerical model we find a correlation between recent continental and oceanic intraplate volcanism and areas of the asthenosphere that are experiencing rapid shear due to mantle convection we detect particularly high correlations in the interior of the continents of western north america eastern australia southern europe and antarctica as well as west of the east pacific rise in the pacific ocean we conclude that intraplate volcanism associated with mantle convection is best explained by melting caused by shear flow within the asthenosphere whereas other localized processes are less important

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

a fine resolution palaeoclimatic reconstruction of the last 2000 years from lake keilambete southeastern australia a fine resolution analysis of pollen charcoal magnetic susceptibility sediment fractions and total carotenoids was used to investigate the last 2000 years of environmental history at lake keilambete in western victoria australia the chronology of sedimentation was based on three radiocarbon dates supplemented by inferences from 22 previous radiocarbon ages from the site spanning approximately the last 7500 years changes in the analysed parameters infer a relatively higher effective moisture from approximately 2000 to 1800 cal bp increased instability in the lake keilambete catchment from 1750 to 1425 cal bp may reflect a decreased moisture availability and cooler temperatures at about 1450 cal bp a period of considerable environmental change is suggested at lake keilambete warmer temperatures or an increased moisture supply seem likely between 1415 and 1320 cal bp from around c 1150 until 800 cal bp overlapping the medieval warm period a series of changes in the composition of the la

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

late quaternary environmental change and human impact around lake bolac western victoria australia the late quaternary history of the lake bolac region is reconstructed from sedimentary and pollen analysis of lake and swamp sequences and is supported by archaeological remains from lunette sediments freshwater deposits recovered from lake turangmoroke appear to date from the last glacial period when the vegetation was composed of herbfield and grassland subsequent dry conditions at the end of the pleistocene caused the deflation of some accumulated sediments on refilling the lake had become saline as moisture availability increased casuarina woodland dominated by allocasuarina verticillata became the regional vegetation cover domination by casuarina ended ca 8000 7000 yr bp probably as a result of rising saline ground water tables casuarinas were replaced to some degree by eucalypts augmented by the development of a riverine eucalyptus camaldulensis woodland swamp deposition commenced in the channel of neighbouring fiery creek at ca 4000 yr bp most likely as the result of a reduction in stream flow with decreased precipitation despite suggestions of significant impact on the vegetation by aboriginal people from other sites in southeastern australia there is no evidence of any alteration here from periods of occupation dated to around 13 000 yr bp and after 3500 yr bp the period of european occupation though is marked by significant changes in fire patterns reduced tree cover loss of floristic diversity increased erosion rates and salinity levels and decreased water levels journal ot quaternary science

advancements in cosmogenic 38 ar exposure dating of terrestrial rocks abstract cosmogenic exposure dating of ca rich minerals using 38 ar on terrestrial rocks could be a valuable new dating tool to determine timescales of geological surface processes on earth here we show that advancement in analytical precision using the new generation multi collector argusvi mass spectrometer on irradiated pyroxene and apatite samples allows determination of cosmogenic 38 ar abundances above background values as well as discrimination of 38 ar 36 ar ratios 1 absolute precision of 0 3 from the non cosmogenic background value four statistically significant cosmochron 38 ar 36 ar vs 37 ar 36 ar diagrams could be constructed for southeast australian pyroxene samples from the mt elephant scoria cone for which a combined apparent exposure age of 313 179 ka 2 was obtained when using a 38 ar production rate ca of 250 atoms g ca yr this exposure age overlaps within error with the known 40 ar 39 ar eruption age of 184 15 ka 2 although apatite shows much larger 38 ar abundances than pyroxene our modelling and analyses of unirradiated apatite suggest that apatite suffers from both natural and reactor derived chlorogenic as well as natural nucleogenic contributions of 38 ar hence we suggest that cosmogenic 38 ar exposure dating on irradiated ca rich and eventually k rich but cl free terrestrial minerals is a potential valuable and accessible tool to determine geological surface processes on timescales of a few ma calculations show that with the new generation multi collector mass spectrometers an analytical uncertainty better than 5 2 can be achieved on samples with expected exposure ages of 4 ma

holocene lake level fluctuations in lakes keilambete and gnotuk southwestern victoria australia reconstructed holocene lake level curves from two saline hydrologically closed maar crater lakes in southwestern victoria australia show near synchronous lake level changes throughout the holocene we show that lake levels reconstructed from sediment particle size and ostracod valve chemistry 18o and sr ca have undergone rapid 10 m fluctuations throughout the holocene finer sampling resolution shows a more sensitive response to holocene climate than was previously presented for lake keilambete both maar crater lakes show a short lived maximum in holocene lake levels around 7 2 ka the period of lake filling leading to peak lake levels matches the phase of most effective precipitation 7 4 7 0 ka reconstructed from a high resolution speleothem record from northern tasmania water levels declined in both lakes during the mid holocene with a more substantive decline after 5 ka which coincides with the end of the southern hemisphere hypsithermal water levels continued to oscil

ultra high precision 40ar 39ar ages for fish canyon tuff and alder creek rhyolite sanidine new dating standards required abstract the 40 ar 39 ar dating technique is a high precision 40 ar 39 ar standards include sanidine from the ca 28 ma fish canyon tuff fct and the ca 1 2 ma alder creek rhyolite acr despite common usage the ages of fct and acr sanidine have proven contentious with reported values varying by 2 well outside the 0 1 aspiration of earthtime www earth time org the current study presents ultra high precision multi collector 40 ar 39 ar results for fct and acr sanidine using a new generation argusvi mass spectrometer these analyses demonstrate the significantly higher by a factor of 10 level of precision achievable using the argusvi system compared to most previous studies significantly 40 ar 39 ar step heating analyses resolve distinct age gradients for both fct and acr sanidine possible explanations for these gradients include combinations of recoil loss of 39 ar instrumental bias isotopic fractionation during step heating thermally induced argon loss and or extraneous argon contamination minor recoil loss of 39 ar and partial retention of inherited argon during extended magma residence are deemed the most plausible causes of the age discordance given this assumption we calculate revised maximum eruption ages of 28 01 0 04 ma 2 for fct and 1 178 0 002 ma 2 for acr relative to the astronomically calibrated age of a1 tephra sanidine crete rivera et al 2011 earth planet sci lett 311 420 426 nonetheless the argusvi data demonstrate that fct and acr sanidine are non ideal as high precision 40 ar 39 ar standards with different heating protocols likely to induce inter laboratory bias the current findings account for at least some of the scatter in inter calibration ages reported previously for various 40 ar 39 ar standards in broader terms the results mandate a re evaluation of the astronomically calibrated ages for these and other sanidine samples while the new generation of multi collector mass spectrometers provides the means to further evaluate 40 ar 39 ar ages used to define new decay constants and standard ages

crust and upper mantle structure beneath southeast australia from ambient noise and teleseismic tomography abstract in the last decade the lithospheric structure beneath southeast australia has been intensively studied using passive seismic data from wombat the largest transportable seismic array in the southern hemisphere the two primary imaging methods that have been applied are ambient noise tomography for the crust and teleseismic tomography for the upper mantle despite these recent studies no attempt has yet been made to provide an integrated view of the crust mantle system here we perform teleseismic tomography using wombat data that includes a detailed crustal model from ambient noise tomography in the starting model a moho surface from the australian seismological reference earth model ausrem is also included this has the dual benefit of accounting for the unresolved crustal component of the teleseismic arrival time residuals and producing a model that reveals a high level of detail in both the crust and upper mantle our new integrated p wave model contains a number of noteworthy features including i low velocity anomalies in the lower crust and high velocity anomalies in the lithospheric mantle beneath the gawler craton and curnamona province which are of paleoproterozoic archean origin ii a marked velocity transition in the crust and lithospheric mantle near the moyston fault which we interpret as the boundary between the lachlan and delamerian orogens iii a rapid eastward decrease in upper mantle velocity 200 km inboard of the east coast of australia which is consistent with a marked thinning of the lithosphere iv an increase in upper mantle velocity north of the gawler craton and curnamona province which points to the presence of thicker lithosphere associated with the precambrian shield region of the australian continent v cenozoic intraplate basaltic volcanic centres distributed exclusively above the zone of thinner lithosphere inboard of the east coast with the exception of low volume leucitite volcanics

long period magnetotelluric measurements near the central california coast a land locked view of the conductivity structure under the pacific ocean summary telluric data from hollister california were combined with magnetic data from the fresno california magnetic observatory to determine the mt response at long periods the shifted eigenstate analysis of latorraca et al 1986 was used to determine the maximum apparent resistivities and corresponding eih phases for periods from 1 to 30 h the apparent resistivity and phase at hollister are greatly influenced by the ocean with the mantle branch beginning at a period of about 12 h in typical continental areas the mantle branch usually begins at periods of 10s or less a maximum likelihood inversion algorithm was used to invert the observed data and to determine the sensitivity of the model parameters the data required a resistivity thickness product of approximately 1 x lo9 q m2 1 x lo6 q m km for the oceanic lower crust and upper mantle the oceanic mantle shows a rather small gradient of conductivity from approximately 200 to 640 km in depth and a rapid conductivity increase below 640km the sensitivity analysis shows the data are most sensitive to the oceanic parameters except for the electric field amplitudes which are also sensitive to the resistivities under the observation site

numerical recognition of alignments in monogenetic volcanic areas examples from the michoacan guanajuato volcanic field in mexico and calatrava in spain abstract identification of geological lineaments using numerical methods is a useful tool to reveal structures that may not be evident to the naked eye in this sense monogenetic volcanic fields represent an especially suitable case for the application of such techniques since eruptive vents can be considered as point like features application of a two point azimuth method to the michoacan guanajuato volcanic field mexico and the calatrava volcanic province spain demonstrates that the main lineaments controlling the distributions of volcanic vents 322 in calatrava and 30 in michoacan approach the respective main compressional axes that dominate in the area i e the cocos north america plates convergence and the main betics compressional direction respectively considering the stress fields that are present in each volcanic area and their respective geodynamic history it seems that although volcanism may be a consequence of contemporaneous extensional regimes the distribution of the volcanic vents in these kinds of monogenetic fields is actually controlled by reactivation of older fractures which then become more favourable for producing space for magma ascent at near surface levels

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

reconstructing the eruption magnitude and energy budgets for the pre historic eruption of the monogenetic 5 ka mt gambier volcanic complex south eastern australia understanding explosive volcanic eruptions especially phreatomagmatic eruptions their intensities and energy budgets is of major importance when it comes to risk and hazard studies with only a few historic occurrences of phreatomagmatic activity a large amount of our understanding comes from the study of pre historic volcanic centres which causes issues when it comes to preservation and vegetation in this research we show that using 3d geometrical modelling it is possible to obtain volume estimates for different deposits of a pre historic complex monogenetic centre the mt gambier volcanic complex south eastern australia using these volumes we further explore the energy budgets and the magnitude of this eruption vei 4 including dispersal patterns eruption columns varying between 5 and 10 km dispersed towards north east to south to further our understanding of intraplate monogenetic eruptions involving phreatomagmatic activity we also compare which thermodynamic model fits best in the creation of the maar crater of mt gambier the major explosion dominated model or the incremental growth model in this case the formation of most of the craters can best be explained by the latter model

40ar 39ar geochronology of terrestrial pyroxene abstract geochronological techniques such as u pb in zircon and baddeleyite and 40ar 39ar on a vast range of minerals including sanidine plagioclase and biotite provide means to date an array of different geologic processes many of these minerals however are not always present in a given rock or can be altered by secondary processes e g plagioclase in mafic rocks limiting our ability to derive an isotopic age pyroxene is a primary rock forming mineral for both mafic and ultramafic rocks and is resistant to alteration process but attempts to date this phase with 40ar 39ar has been met with little success so far in this study we analyzed pyroxene crystals from two different large igneous provinces using a multi collector noble gas mass spectrometer argus vi since those machines have been shown to significantly improve analytical precision compared to the previous single collector instruments we obtain geologically meaningful and relatively precise 40ar 39ar plateau ages ranging from 184 6 3 9 to 182 4 0 8 ma 2 uncertainties of 1 8 0 4 and 506 3 3 4 ma for tasmanian and kalkarindji dolerites respectively those data are indistinguishable from new and or published u pb and 40ar 39ar plagioclase ages showing that 40ar 39ar dating of pyroxene is a suitable geochronological tool scrutinizing the analytical results of the pyroxene analyses as well as comparing them to the analytical result from plagioclase of the same samples indicate pure pyroxene was dated numerical models of argon diffusion in plagioclase and pyroxene support these observations however we found that the viability of 40ar 39ar dating approach of pyroxene can be affected by irradiation induced recoil redistribution between thin pyroxene exsolution lamellae and the main pyroxene crystal hence requiring careful petrographic observations before analysis finally diffusion modeling show that 40ar 39ar of pyroxene can be used as a powerful tool to date the formation age of mafic rocks affected by greenschist metamorphism and will likely play an important role in high temperature thermochronology

an evidence based approach to accurate interpretation of 40ar 39ar ages from basaltic rocks abstract since its inception in the mid 1960s the 40 ar 39 ar dating technique has been the premier method for determining the eruption ages of basaltic rocks providing valuable insights into a plethora of terrestrial and planetary processes advances in multi collector mass spectrometry and improved sample preparation procedures are enabling ever improving analytical precision and clearer evaluation of the isotopic disturbances that affect many basaltic samples and cause discordant 40 ar 39 ar age spectra here we present 40 ar 39 ar step heating data for multiple samples from two quaternary basalt flows 0 8038 0 0017 and 2 309 0 009 ma of the intraplate newer volcanic province southeast australia a small proportion of these samples give concordant 40 ar 39 ar results but most are variably discordant the factors controlling these disturbances and implications for accurate age determination are examined and modelled in both step heating spectra and inverse isochron space we demonstrate that the proportion of radiogenic 40 ar 40 ar present in these samples strongly influences the nature of the discordance reflected in 40 ar 39 ar data mass dependent fractionation appears to have a major influence on low 40 ar samples whereas 39 ar recoil loss redistribution effects are evident in samples with higher 40 ar proportions the impact of mass fractionation is quantified via step heating analyses of unirradiated basalt whereby a 4 difference in 38 ar 36 ar ratios is observed between low and high temperature heating steps on an inverse isochron plot 39 ar 40 ar vs 36 ar 40 ar isotopic disturbance for groundmass samples primarily manifests as isochron rotation leading to a negative correlation between initial 40 ar 36 ar 40 ar 36 ar i values and associated 40 ar 39 ar ages we propose a new framework for the interpretation of 40 ar 39 ar step heating data for basaltic samples through judicious evaluation of inverse isochron data 40 ar 36 ar i ratios and inverse isochron ages results from this study suggest that only samples exhibiting both flat 40 ar 39 ar age spectra and atmospheric 40 ar 36 ar i ratios yield accurate eruption ages in the case of more discordant age spectra intermediate temperature steps with atmospheric 40 ar 36 ar i ratios may provide the closest approximation of the eruption age

intraplate deformation and 3d rheological structure of the rhine rift system and adjacent areas of the northern alpine foreland the lithosphere of the northern alpine foreland has undergone a polyphase evolution during which interacting stress induced intraplate deformation and upper mantle thermal perturbations controlled folding of the thermally weakened lithosphere in this paper we address relationships among deeper lithospheric processes neotectonics and surface processes in the northern alpine foreland with special emphasis on tectonically induced topography we focus on lithosphere memory and neotectonics paying special attention to the thermo mechanical structure of the rhine graben system and adjacent areas of the northern alpine foreland lithosphere we discuss implications for mechanisms of large scale intraplate deformation and links with surface processes and topography evolution

spatial distribution and alignments of volcanic centers clues to the formation of monogenetic volcanic fields monogenetic basaltic volcanic fields occur worldwide in tectonic environments ranging from extensional to convergent understanding similarities and differences between these fields may help to characterize key controls on their generation such volcanic fields consist of numerous volcanic centers each of which represents a pathway of magma from its source to the surface we analyzed the spatial distribution of volcanic centers in 37 monogenetic volcanic fields and assuming that the distribution of volcanic centers relative to each other is matched by a similar source pattern within the mantle applied the following methods for each 1 the poisson nearest neighbor pnn analysis representing the degree to which the distribution of the volcanic centers departs from a predicted poisson distribution and 2 a volcanic alignment analysis to ascertain the preferential pathways if any used by the magma to reach the surface this is the first comprehensive global comparison of such analyses magma pathways within the brittle upper crust are influenced to various degrees by two end member situations 1 formation of new extension fractures perpendicular to the least compressive stress 3 and 2 re activation of pre existing fractures that are near parallel to the maximum principal stress 1 the results of the pnn analysis show that independently of the tectonic environment most volcanic fields display a clustered distribution of their volcanic centers alignment analysis shows that either the ambient tectonic environment exerts a strong influence on the preferential orientations of the volcanic alignments or that it is in competition with other factors e g pre existing structures local stress changes due to older intrusions overall these results indicate that the propagation of the magma and therefore the spatial distribution of the volcanic centers within volcanic fields is the product of an interplay between deep level influences i e nature of the magma source and shallow level influences i e stress field pre existing crustal structures of the lithosphere

a three dimensional magnetotelluric investigation of the california basin and range band limited magnetotelluric measurements were collected at several sites across a wide area in the california basin and range and northern mojave desert from early 1988 until march 1989 due to the large spacing between sites and the limited frequency range of the data there are limits to what the data can resolve however a three dimensional 3 d magnetotelluric modeling study and sensitivity analysis of a subset of the data support the following conclusions 1 electrical currents from the pacific ocean are channelled across the western mojave desert and into the southern basin and range 2 the 3 d geometry of the basins significantly influences the fit to the data mode aligned along the strike of the basins transverse electric mode 3 the lower crust in the california basin and range is more conductive than the lower crust under the mojave desert and pacific ocean the resistivity values we modeled for the lower crust in the california basin and range though inconsistent with mineral semiconduction can be explained by electrical conduction through small amounts of saline fluids

basalt lava flows of the intraplate newer volcanic province in south east australia melbourne region 40ar 39ar geochronology reveals 8 ma of episodic activity abstract intraplate basaltic volcanism is one of the most common yet poorly understood types of volcanism intraplate continental basaltic volcanic provinces icbvps typically comprise tens to hundreds of usually monogenetic small volume eruption centres often cumulatively producing significant volumes of mainly primitive basaltic magmas a wide range of mechanisms have been invoked to account for icbvp melt generation ranging from deep mantle plumes to local tectonic perturbations of the lithospheric mantle the key to understanding the cause s of intraplate volcanism is understanding the geochemical evolution of magmatism within a regional tectonic framework unfortunately many icbvps contain complex arrays of overlying lava flows with uncertain eruption sources and a lack of precise age constraints the newer volcanic province nvp in south eastern australia is a young 4 6 ma 5 ka geochemically and volcanologically diverse icbvp comprising 416 volcanic centres covering an area of 23 000 km2 as with other icbvps elucidating the underlying controls on magmatism has proved contentious with recent studies favouring edge driven convection but divided as to the changing nature of volcanism at the eastern boundary of the nvp in the melbourne area previous work has indicated a complex network of lava flows representing almost the entire age and geochemical range of the nvp making this an ideal microcosm for studying the evolution of the nvp in this study a holistic approach to lava flow mapping is utilised to unravel the complex network incorporating diagnostic petrography geochemistry and precise 40ar 39ar geochronology this work reveals seven major lava flows here named the tullamarine redstone hill aitken hill mount kororoit fenton hill tulloch hill and mount fraser flows along with eight smaller volume flows here named the pretty sally green hill bald hill mount cooper mount ridley crowe hill springs hill and summerhill road flows 40ar 39ar ages for eleven lava flows and eruption centres span an age range of 7 9 0 8 ma these new data reveal that alkalic small volume eruptions in the melbourne area occurred exclusively between 8 and 3 8 ma post 3 8 ma large volume tholeiitic eruptions dominated a local progression from alkalic to tholeiitic volcanism is contrary to a province wide progression from tholeiitic to alkalic volcanism inferred by previous studies the earliest four eruptions ca 8 ma were aligned with the cosgrove hotspot track and are probably unrelated to the broader nvp the ages and geochemical variations of the younger flows are consistent with melt generation due to edge driven convection magma composition might have been controlled by local perturbations within the convective cell with tholeiitic magmas generated during high degree melting in regions of more intense upwelling and alkalic melts reflecting lower degree melting and less intense upwelling the episodic nature of volcanism is likely due to restricted magma transport to the surface which was only possible during brief periods of favourable tectonic conditions