shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

temporal variation of the stress field during the construction of the central andes constrains from the volcanic arc region 22 26 s western cordillera chile during the last 20 ma in order to understand the response of the stress field state to intrinsic processes during the construction of the andes such as thickening of the continental crust lithospheric delamination and or thermal weakening we investigate the stress field evolution of the arc region since the last 20 my in the central andes 22 26 5 s the 43 reduced paleostress tensors derived from inversion of 682 fault slip data reveal a complex pattern of stress states during the last episode of orogenic construction and topographic uplift we identify two geodynamic stages the first stage corresponds to the construction of the altiplano puna plateau and the second one to its gravitational collapse four stress states that have prevailed in the altiplano puna plateau since middle miocene times characterize the transition from one stage to the other along the study latitudes a spatiotemporal change in stress state is clearly observed which led to an understanding that a change in the stress field may be related not only to the boundary conditions but also to intrinsic factors associated with the construction of the andean orogeny our results suggest that ca 13 10 ma and ca 8 5 ma in the southern altiplano and northern puna and in the southern puna respectively regional elevation and crustal thicknesses reached threshold values necessary to generate the orogenic collapse

late cenozoic tectonism collapse caldera and plateau formation in the central andes abstract the evolution of andean volcanism including the formation of late miocene to recent collapse calderas on the puna plateau is generally interpreted in terms of the kinematic framework of the nazca and south american plates we present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first order nw se striking zones of left lateral transtension on the local and regional scales consequently ages of collapse calderas indicate activity of these fault zones which initiated at about 10 ma on the puna plateau the onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen parallel stretching upon reaching maximum crustal thickness and critical surface elevation horizontal magma sheets that formed at mid crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub vertical faults this accounts well for the observed tectono magmatic phenomena at surface it follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the andean plateau rather than to changes in the geometry of the wadati benioff zone or plate boundary kinematics

ridge segmentation and the magnetic structure of the southwest indian ridge at 50 30 e 55 30 e and 66 20 e implications for magmatic processes at ultraslow spreading centers 1 the aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow spreading southwest indian ridge contrary to faster spreading ridges magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities there is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities we suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase result in thinner extrusive lavas and thus control the along axis magnetization variations by thinning the magnetic source layer a unique segment centered at 50 28 e shows a marked low magnetization anomaly at its center similarly to the segments of the slow spreading mid atlantic ridge we suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated a higher rate of melt production to the west of gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low magnetization lavas to the east magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas finally we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley off axis in thin crust areas upper mantle rocks may become progressively more altered as distance from the axis increases the strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization which is observed along the traces of the largest amagmatic discontinuities by contrast in thicker crust areas the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting as on the mid atlantic ridge in slightly more positive magnetization values along the traces of axial discontinuities regardless of polarity

the portland purbeck junction portlandian berriasian in the weald and correlation of latest jurassic early cretaceous rocks in southern england ammonites and palynology are described at the portland purbeck junction in the weald a major disconformity exists between glaucolithus zone portland beds and the overlying basal purbeck facies the basal purbeck beds of the weald are correlated with the middle purbeck strata of dorset the cinder bed of dorset and wiltshire is described as one of several marine incursions entering wessex from further south in the anglo paris basin not from the spilsby basin the use of cinder bed as a convenient jurassic cretaceous portlandian ryazanian boundary is critically reviewed as is the placing of the base of the berriasian stage in the type purbeck section the rival biostratigrahic schemes based on ostracods and palynology are discussed and the discrepancies between the two schemes in southern england are examined

decadal volcanic deformation in the central andes volcanic zone revealed by insar time series 1 decadal trends of volcanic deformation in the central andes volcanic zone cvz are identified with interferometric synthetic aperture radar insar stacks and time series velocity maps covering an area 19 s 27 s and 66 w 69 w we combine over 750 ers and envisat interferograms from two descending and three ascending tracks these tracks cover 100 000 km 2 and span 1992 2011 our analysis extends observations at cerro blanco uturuncu and lazufre volcanic centers and uncovers two previously undocumented deformation centers cerro overo in northern chile and putana volcano in southwest bolivia cerro overo exhibits a transition from steady 0 4 cm yr deflation to 0 5 cm yr inflation over several years putana volcano underwent a short lived episode of uplift between 13 september 2009 and 31 january 2010 with a maximum uplift of 4 0 cm cerro blanco continues 1 0 cm yr deflation since 1995 uplift at lazufre began between 1997 and 2000 and has gradually accelerated to 3 5 cm yr since 2005 uturuncu volcano continues 1 0 cm yr monotonic uplift since 1992 and shows evidence for a broad moat of subsidence surrounding the uplifting region four of the nine deformation events in the cvz are not obviously associated with a particular volcanic edifice furthermore there is significant spatial and temporal variability of these deformation events within a small geographic area

diapiric ascent of silicic magma beneath the bolivian altiplano 1 the vertical transport of large volumes of silicic magma which drives volcanic eruptions and the long term compositional evolution of the continental crust is a highly debated problem in recent years dyking has been favored as the main ascent mechanism but the structural connection between a distributed configuration of melt filled pores in the source region and shallow magma reservoirs remains unsolved in the central andes inversion of a new high resolution bouguer anomaly data over the altiplano puna magma body apmb reveals 15 km wide vertically elongated low density 3d structures rooted at the top of the apmb at 20 km depth we integrate our gravity inversion with the available geophysical geological and petrological observations and in agreement with petrological mechanical considerations propose that in this region of the andes partially molten granitic bodies ascend diapirically through the hot ductile mid upper crust

sombrero uplift above the altiplano puna magma body evidence of a ballooning mid crustal diapir the altiplano puna ultralow velocity zone in the central andes south america is the largest active magma body in earth s continental crust space geodetic observations reported an uplift in the altiplano puna proper at a rate of 10 mm year however the nature of the inferred inflation source has been uncertain we present data showing that the uplift has persisted at a nearly constant rate over the past two decades and is surrounded by a broad zone of subsidence we show that the ongoing uplift and peripheral subsidence may result from a large mid crustal diapir fed by partial melt from the altiplano puna magma body

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

variations in earthquake size distribution across different stress regimes the earthquake size distribution follows in most instances a power law1 2 with the slope of this power law the b value commonly used to describe the relative occurrence of large and small events a high b value indicates a larger proportion of small earthquakes and vice versa statistically significant variations of b values have been measured in laboratory experiments mines and various tectonic regimes such as subducting slabs near magma chambers along fault zones and in aftershock zones3 however it has remained uncertain whether these differences are due to differing stress regimes as it was questionable that samples in small volumes such as in laboratory specimens mines and the shallow earth s crust are representative of earthquakes in general given the lack of physical understanding of these differences the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b 1 is a universal constant for earthquakes in general5 here we show that the b value varies systematically for different styles of faulting we find that normal faulting events have the highest b values thrust events the lowest and strike slip events intermediate values given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress

simulation and modeling of ultrasonic pitch catch through tubing logging abstractcased petroleum wells must be logged to determine the bonding and hydraulic isolation properties of the sealing material and to determine the structural integrity status although ultrasonic pitch catch logging in single casing geometries has been widely studied and is commercially available this is not the case for logging in double casing geometries despite its increasing importance in plug and abandonment operations it is therefore important to investigate whether existing logging tools can be used in such geometries using a finite element model of a double casing geometry with a two receiver pitch catch setup we have simulated through tubing logging with fluid between the two casings we found that there appears a cascade of leaky lamb wave packets on both casings linked by leaked wavefronts by varying the geometry and materials in the model we have examined the effect on the pulse received from the second wave packet on the inner casing sometimes known as the third interface echo th

kimmeridge clay sedimentation a new cyclic model abstract the kimmeridgian upper jurassic fine clastic sequences of the boulonnais northern france and dorset southern england represent oxygen controlled environments five facies types with increasingly deteriorating living conditions ranging from normal shelf environments to submarine lifeless desert have been distinguished the widespread occurrence of this facies in the late jurassic north atlantic shelf sea and the palaeogeographic situation require a large scale model to explain the kimmeridge clay sedimentation anoxic conditions are generally explained by one of three models none of which applies directly to the kimmeridge clay a new model derived from the overall palaeogeographic and palaeoclimatic situation is therefore proposed in this model water mass stratification and anoxic bottom conditions refer to a high latitude monsoonal type wind circulation causing a water current and counter current system in the north atlantic shelf sea and the arctic ocean variations in the amount of oxygen in the bottom waters allow recognition of four different cycles with periods ranging from seasonal to 3 107 years they influenced the counter current system and the resulting anoxic conditions

palynostratigraphy of the classic portland and purbeck sequences of dorset southern england and the correlation of jurassic cretaceous boundary beds in the tethyan and boreal realms

plate motions slab dynamics and back arc deformation back arc extension or compression is often related to a particular behavior of the slab dip change and or forward backward migration with respect to the upper plate slabs act either passively when anchored in the deep mantle whereas back arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench or actively when slab pull increasing with slab age forces the trench to rollback in addition to these two mechanisms i e slab anchor and slab rollback numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab causing its forward or rearward migration with respect to the arc based on a compilation of upper plate absolute motion trench absolute motion back arc deformation rate upper plate strain regime and slab age for all oceanic subduction zones excluding any kind of collision with continents arcs or plateaus we have examined how the combined effects of these parameters can account for the observed back arc deformations our main results are 1 a global correlation exists between upper plate absolute motion and back arc deformation i e back arc extension when upper plate retreats and vice versa 2 there are as many advancing trenches as retreating ones with trench motion globally limited to 50 mm y 1 furthermore there is no positive correlation between trench retreat and slab age and 3 upper plate absolute motion often fails to explain the back arc deformation rates and the trench motions observed at several subduction zones as tonga new hebrides sandwich or ryukyu we propose that the trench migration of these subduction zones are on the influence of mantle flows we conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion back arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab trench motion is limited to 50 mm y 1 even if locally mantle flows force slab to move

uturuncu volcano bolivia volcanic unrest due to mid crustal magma intrusion uturuncu volcano sw bolivia is a dormant stratovolcano 85 km3 dominated by dacitic lava domes and flows 39ar 40ar ages show that the volcano was active between 890 ka and 271 ka with the lavas becoming younger and less extensive at higher elevations there are current signs of unrest between 1992 and 2006 geodetic satellite measurements record an ongoing 70 km deformation field with a central uplift rate of 1 to 2 cm yr deformation indicates volume changes of 400 x 108 m3 over 14 years an average of 1 m3 s 10 2 km3 yr the deformation is attributed to magma intrusion into the altiplano puna regional crustal magma body deformation models indicate a source at depths of 17 to 30 km beneath current local relief in a reconnaissance survey persistent seismic activity mean of 2 6 earthquakes per hour with a maximum of 14 per hour was recorded at about 4 km depth below the center of the uplift 4 km sw of the volcano s summit the seismic events have a normal b value 1 04 and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion the porphyritic dacite lavas 64 68 sio2 have a plagioclase orthopyroxene biotite magnetite ilmenite assemblage and commonly contain juvenile silicic andesite inclusions cognate norite nodules and crustal xenoliths temperature estimates are in the range 805 to 872 c for the dacites and about 980 c for the silicic andesites the dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite reversely zoned orthopyroxene phenocrysts in the andesitic end member are explained by changing oxidation states during crystallization fe3 fe2 ratios from orthopyroxene crystals and fe3 in plagioclase provide evidence for a relatively reduced melt that subsequently ascended degassed and became more oxidized as a consequence of degassing the geophysical and petrological observations suggest that dacite magma is being intruded into the altiplano puna regional crustal magma body at 17 km or more depth consistent with deformation models in the late pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together the current unrest together with geophysical anomalies and 270 ka of dormancy indicate that the magmatic system is in a prolonged period of intrusion such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation

an insar based survey of volcanic deformation in the central andes we extend an earlier interferometric synthetic aperture radar insar survey covering about 900 remote volcanos of the central andes 14 27 s between the years 1992 and 2002 our survey reveals broad 10s of km roughly axisymmetric deformation at 4 volcanic centers two stratovolcanoes are inflating uturuncu bolivia and hualca hualca peru another source of inflation on the border between chile and argentina is not obviously associated with a volcanic edifice here called lazufre and a caldera cerro blanco also called robledo in northwest argentina is subsiding we explore the range of source depths and volumes allowed by our observations using spherical ellipsoidal and crack like source geometries we further examine the effects of local topography upon the deformation field and invert for a spherical point source in both elastic half space and layered space crustal models we use a global search algorithm with gradient search methods used to further constrain best fitting models inferred source depths are model dependent with differences in the assumed source geometry generating a larger range of accepted depths than variations in elastic structure source depths relative to sea level are 8 18 km at hualca hualca 12 25 km for uturuncu 5 13 km for lazufre and 5 10 km at cerro blanco deformation at all four volcanoes seems to be time dependent and only uturuncu and cerro blanco were deforming during the entire time period of observation inflation at hualca hualca stopped in 1997 perhaps related to a large eruption of nearby sabancaya volcano in may 1997 although there is no obvious relation between the rate of deformation and the eruptions of sabancaya we do not observe any deformation associated with eruptions of lascar chile at 16 other volcanoes that had recent small eruptions or fumarolic activity or associated with a short lived thermal anomaly at chiliques volcano we posit a hydrothermal system at cerro blanco to explain the rate of subsidence there for the last decade we calculate the ratio of the volume of magma intruded to extruded is between 1 10 and that the combined rate of intrusion and extrusion is within an order of magnitude of the inferred geologic rate

are volcanic seismic b values high and if so when abstract the gutenberg richter exponent b is a measure of the relative proportion of large and small earthquakes it is commonly used to infer material properties such as heterogeneity or mechanical properties such as the state of stress from earthquake populations it is well known that the b value tends to be high or very high for volcanic earthquake populations relative to b 1 for those of tectonic earthquakes and that b varies significantly with time during periods of unrest we first review the supporting evidence from 34 case studies and identify weaknesses in this argument due predominantly to small sample size the narrow bandwidth of magnitude scales available variability in the methods used to assess the minimum or cutoff magnitude m c and to infer b informed by this we use synthetic realisations to quantify the effect of choice of the cutoff magnitude on maximum likelihood estimates of b and suggest a new work flow for this choice we present the first quantitative estimate of the error in b introduced by uncertainties in estimating m c as a function of the number of events and the b value itself this error can significantly exceed the commonly quoted statistical error in the estimated b value especially for the case that the underlying b value is high we apply the new methods to data sets from recent periods of unrest in el hierro and mount etna for el hierro we confirm significantly high b values of 1 5 2 5 prior to the 10 october 2011 eruption for mount etna the b values are indistinguishable from b 1 within error except during the flank eruptions at mount etna in 2001 2003 when 1 5 b b 1 our results confirm that these volcano tectonic earthquake populations can have systematically high b values especially when associated with eruptions at other times they can be indistinguishable from those of tectonic earthquakes within the total error the results have significant implications for operational forecasting informed by b value variability in particular in assessing the significance of b value variations identified by sample sizes with fewer than 200 events above the completeness threshold

modeling of highly anisotropic crust and application to the altiplano puna volcanic complex of the central andes 1 the altiplano puna volcanic complex apvc is located in the central andes and covers an area over 50 000 km2 the style of volcanism is predominately caldera forming ignimbrite eruptions active between 10 and 1 ma chmielowski et al 1999 used teleseismic events recorded from seven portable broadband seismic stations deployed in the apvc to identify a 1 km thick very low velocity zone at a depth of 19 km based on the correlation with surface volcanics the extremely low shear velocities of 1 km s or less and the depth of the layer the low velocity zone was interpreted to be a magma body using local events recorded during the same seismic deployment we have improved the crustal model for the apvc and detected seismic anisotropy in the crust above the low velocity zone using a hexagonal symmetry anisotropy code we computed synthetic receiver functions for many models of layered anisotropic media and found two different models consistent with the data in both models approximately 20 30 anisotropy is present in a 3 km thick surface low velocity layer and 15 20 anisotropy is present in the remaining crust above the buried low velocity zone the first model has the 1 km thick low velocity zone at 17 km while the alternate model has the low velocity zone at 14 km depth overlain by a 4 km thick transition zone of intermediate velocities this anisotropy may be due to the fracture system by which magma migrates to the surface from the midcrustal sill like magma body

shear wave anisotropy spatial and temporal variations in time delays at parkfield central california summary shear wave splitting is analysed on data recorded by the high resolution seismic network hrsn at parkfield on the san andreas fault central california during the three year period 1988 1990 shear wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress at some 70 to the fault strike whereas at station mm in the immediate fault zone shear wave polarizations are aligned approximately parallel to the fault normalized time delays at this station are found to be about twice as large as those in the rock mass either side this suggests that fluid filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks and that the alignment of fault parallel shear wave polarizations are associated with some fault specific phenomenon temporal variations in time delays between the two split shear waves before and after a ml 4 earthquake can be identified at two stations with sufficient data mm within the fault zone and vc outside the immediate fault zone time delays between faster and slower split shear waves increase before the ml 4 earthquake and decrease near the time of the event the temporal variations are statistically significant at 68 per cent confidence levels earthquake doublets and multiplets also show similar temporal variations consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass this study is broadly consistent with the behaviour observed before three other earthquakes suggesting that the build up of stress before earthquakes may be monitored and interpreted by the analysis of shear wave splitting

characterizing borehole plumbing using full waveform ultrasonic data application to data from a north sea well abstractwe have analyzed ultrasonic flexural data acquired in a north sea well using a commercial tool optimized for generating such data and found how one might separate refractions along successive layers of casings and from these separated refractions characterize successive pipes and annuli from the timing of refracted events we determined the shape of a pipe and by examining the amplitudes of these refracted events a measure of the conductance of a pipe wall to transverse movement we characterized the material in the annuli around a pipe data from two separate depth intervals were analyzed demonstrating that a well plan might not give a sufficiently accurate description of the well in the deeper interval in which the inner pipe was supposed to be free we found that the annulus was most likely filled with sedimentation and debris we could also see that the inner 7 in tubing was touching or nearly touching the outer 9 in casing over the entire interval for the shallower interval bel

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

ascent and emplacement of buoyant magma bodies in brittle ductile upper crust 1 the emplacement of silicic magma bodies in the upper crust may be controlled by density such that there is no buoyancy to drive further ascent or temperature such that surrounding rocks are too cold to deform significantly over geological timescales evidence for the latter control is provided by negative gravity anomalies over many granitic plutons conditions of diapir ascent and emplacement in this case are studied with a numerical model for deformation and heat transport allowing for ductile elastic and brittle behavior a large strain formulation is used to solve for temperature stress strain and strain rate fields as a function of time for a range of diapir sizes density contrasts and background geotherms the method allows for large viscosity contrasts of more than 6 orders of magnitude and determines the dominant deformation mechanism depending on the local values of temperature strain and strain rate emplacement depth and final deformation characteristics depend on diapir size and buoyancy small diapirs less than about 5 km in diameter cannot reach shallow crustal levels and do not involve brittle deformation in the ductile regime the diapir flattens significantly upon emplacement due to stiff roof rocks and to the free surface above late stage deformation proceeds by horizontal spreading with little upward displacement of roof rocks and is likely to be interpreted as ballooning large diapirs more than about 5 km in diameter rapidly rise to shallow depths 1 5 km and induce brittle faulting in the overlying rocks in this regime buoyancy forces may lead to faulting in roof rocks in this case late stage ascent proceeds by vertical intrusion of a plug of smaller horizontal dimensions than the main body buoyant diapirs keep on rising after solidification long after the relatively short lived high temperature magmatic stage this may account for some phases of late caldera resurgence in extinct volcanic systems

widespread uplift and trapdoor faulting on galapagos volcanoes observed with radar interferometry volcanic uplift caused by the accumulation of magma in subsurface reservoirs is a common precursor to eruptions1 2 but for some volcanoes uplift of metres or more has not yet led to an eruption3 here we present displacement maps of volcanoes in the galapagos islands constructed using satellite radar interferometry that might help explain this dichotomy we show that all but one of the seven volcanoes on the islands of isabela and fernandina deformed during 1992 99 cerro azul and fernandina erupted4 5 6 during the observation period and show evidence of inflation co eruptive deflation and shallow dyke intrusion in contrast the largest volcano sierra negra has not erupted yet exhibits spatially and temporally variable deformation with a maximum uplift of 2 7 m between 1992 and 1999 which can be modelled by a shallow inflating sill inflation during 1997 98 however was accompanied by trapdoor faulting on a steeply dipping fracture system within the caldera repeated trapdoor faulting over geological time has formed an arcuate intra caldera ridge within sierra negra and may have acted to relax stresses above the magma chamber inhibiting summit eruptions similar processes may help explain large uplift unaccompanied by eruptive activity at other volcanoes

the central andean altiplano puna magma body receiver function analysis of 14 teleseismic events recorded by 6 temporary passcal broadband stations within the altiplano puna volcanic complex apvc shows a consistent 2 s negative polarity p to s conversion for all stations for all available azimuths forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of 19 km with a vs 0 5 km s and a thickness of 750 810 m we interpret the characteristics of the low velocity zone low vs areal extent and flatness to be consistent with a sill like magma body on the basis of additional data from the german ancorp experiment the altiplano puna magma body appears to underlie much of the apvc and it may therefore be the largest known active continental crustal magma body

seismicity associated with magmatism faulting and hydrothermal circulation at aluto volcano main ethiopian rift the silicic volcanic centres of the main ethiopian rift mer play a central role in facilitating continental rifting many of these volcanoes host geothermal resources and are located in heavily populated regions insar studies have shown several are deforming but regional seismic networks have detected little seismicity a local network of 12 seismometers was deployed at aluto volcano from 2012 to 2014 and detected 2142 earthquakes within a 24 month period we locate the events using a 1d velocity model that exploits a regional model and information from geothermal boreholes and calculate local magnitudes b values and focal mechanisms event depths generally range from the near surface to 15 km with most of the seismicity clustering in the upper 2 km a significant amount of seismicity follows the artu jawa fault zone which trends in alignment with the wonji fault belt nne ssw and is consistent with previous studies of strain localisation in the mer focal mechanisms are mostly normal in style with the mean t axes congruent to the orientation of extension in the rift at this latitude some show relatively small left lateral strike slip components and are likely associated with the reactivation of ne ene structures at the southern tip of the aluto gedemsa segment events range from 0 40 to 2 98 in magnitude and we calculate an overall b value of 1 40 0 14 this relatively elevated value suggests fluid induced seismicity that is particularly evident in the shallow hydrothermal reservoir and above it subdividing our observations according to depth identifies distinct regions beneath the volcanic edifice a shallow zone 2 0 km of high seismicity and high b values that corresponds to the hydrothermal system and is influenced by a high fluid saturation and circulation a relatively aseismic zone 0 2 km with low b values that is impermeable to ascending volatiles a region of increased fluid induced seismicity 2 9 km that is driven by magmatic intrusion from below and a deeper zone below 9 km that is interpreted as a partially crystalline magmatic mush these observations indicate that both the magmatic and hydrothermal systems of aluto volcano are seismically active and highlight the need for dedicated seismic monitoring at volcanoes in the mer

the jurassic cretaceous transition of the fararp 1 core southern sweden sedimentological and phytological indications of climate change the 116m deep fararp 1 core drilled in the vomb trough in southernmost sweden is dated by integrated terrestrial and marine palynostratigraphy the lower part of the succession ca 84 m encompasses uppermost jurassic to lowermost cretaceous uppermost tithonian to valanginian strata an unconformity separates the valanginian strata from an overlying ca 1 m thick interval of upper albian to cenomanian arnager greensand formation the uppermost part of the core is a repetitive succession of lowermost cretaceous sediments during the jurassic cretaceous u k transition nw europe was located in mid latitudes and comprised an archipelago of large and small islands separated by deeper grabens and epicontinental seaways that connected the boreal sea to the north with the warmer tethys ocean to the south boundary strata in england france the netherlands and germany are characterised by relatively prominent climatic change from arid semi arid to subhumid humid conditions southernmost sweden was located on the margin of a large landmass comprising most of the fennoscandian shield bordering a large epicontinental sea to the west by combining sedimentology clay mineralogy and palynofacies the tithonian to valanginian cored succession of the fararp 1 core provides complementary information on how marginal deposits from the eastern part of the epicontinental sea reflect the climatological and environmental changes observed in other parts of nw europe the fararp 1 core shows that during the tithonian to earliest berriasian deposition took place in a terrestrial but near marine depositional setting in coastal lakes or lagoons with little marine influence a dry climatic regime favoured stagnant water conditions with common algal blooms of primarily bonyococcus and zygnemataceae palynofacies and sedimentology indicate limited transport of freshwater and material to the basin the stagnant depositional environment was terminated by a marine flooding in the early berriasian during the remaining berriasian and the early valanginian conditions shifted between near marine and marine settings in a dynamic coastal environment similar to contemporaneous assemblages reported from the danish island of bornholm a shift in clay mineralogy from a dominance of 10 a minerals to increasing amounts of mixed layer and kaolinite indicates a change to more humid conditions in the latest tithonian cheirolepidacean pollen classopollis are present but never common in the cored succession and a similar conspicuous decrease of these pollen as previously reported from england germany and france is not evident in the fararp 1 core instead a subsequent shift in both palynofacies and palynoflora marked by an increase in abundance of heavy terrigenous material i e wood and coal particles upland pollen grains and reworked palynomorphs is also observed in the uppermost tithonian lowermost berriasian interval at the same level spores and pollen classified as warmer drier elements decrease in abundance this is interpreted as representing a shift to more humid climatic conditions with increased runoff from the hinterland thus the combined sedimentological and palynological data from the fararp 1 core suggest that climatic conditions in the area changed from more seasonally dry semi arid to more humid semi humid across the j k boundary latest tithonian to earliest berriasian and hence earlier than the mid berriasian climatic shift recorded from e g england and the netherlands c 2011 elsevier b v all rights reserved less

the large scale surface uplift in the altiplano puna region of bolivia a parametric study of source characteristics and crustal rheology using finite element analysis 1 this paper focuses on the driving mechanism behind a 70km wide region of ground uplift centered on uturuncu volcano in the altiplano puna region of southern bolivia we present a series of forward models using finite element analysis to simultaneously test for first order parameters that help constrain a viable model for the observed maximum line of sight uplift rate of 1 2cm yr between 1992 and 2006 stresses from pressure sources with finite geometries are solved numerically accounting for both homogeneous and heterogeneous mechanical rock properties in elastic and viscoelastic rheologies crustal heterogeneity is constrained by seismic velocity data that indicate the presence of a large low velocity zone the altiplanopuna magma body at depths of 17km below the surface a viscoelastic rheology is employed to account for time dependent deformation and an inelastic crust comparing homogeneous and heterogeneous models demonstrates the significant impact of a mechanically weak source depth layer which alters surface displacement patterns by buffering subsurface deformation elastic model results guide the source parameters tested in the viscoelastic models and demonstrate a range of possible causative source geometries our preferred model suggests that pressurization of a magma source extending upward from the altiplano puna magma body is causing the observed surface uplift and alludes to a continued increase in this pressure to explain both the spatial and temporal patterns we also demonstrate how a pressure time function plays a first order role in explaining the observed temporal deformation pattern

crustal origin of trench parallel shear wave fast polarizations in the central andes abstract in this study sks and local s phases are analyzed to investigate variations of shear wave splitting parameters along two dense seismic profiles across the central andean altiplano and puna plateaus in contrast to previous observations the vast majority of the measurements reveal fast polarizations sub parallel to the subduction direction of the nazca plate with delay times between 0 3 and 1 2 s local phases show larger variations of fast polarizations and exhibit delay times ranging between 0 1 and 1 1 s two 70 km and 100 km wide sections along the altiplano profile exhibit larger delay times and are characterized by fast polarizations oriented sub parallel to major fault zones based on finite difference wavefield calculations for anisotropic subduction zone models we demonstrate that the observations are best explained by fossil slab anisotropy with fast symmetry axes oriented sub parallel to the slab movement in combination with a significant component of crustal anisotropy of nearly trench parallel fast axis orientation from the modeling we exclude a sub lithospheric origin of the observed strong anomalies due to the short scale variations of the fast polarizations instead our results indicate that anisotropy in the central andes generally reflects the direction of plate motion while the observed trench parallel fast polarizations likely originate in the continental crust above the subducting slab

crustal anisotropy and state of stress at uturuncu volcano bolivia from shear wave splitting measurements and magnitude frequency distributions in seismicity abstract the physical signatures of unrest in large silicic magma systems are commonly observed in geophysical surveys yet the interactions between magmatic processes and crustal stresses are often left unconstrained stresses in the mid and upper crust exert a strong control on the propagation and stalling of magma and magma ascent can in turn change the magnitude and orientation of these stresses including those associated with hydrothermal systems this study assesses the state of stress at the restless uturuncu volcano in the bolivian andes with space depth and time using observations of seismic anisotropy and the magnitude frequency distributions of local earthquakes shear wave splitting measurements are made for 677 events in the upper crust 1 25 km below sea level between june 1 2009 and march 10 2012 and b values are calculated using the aki maximum likelihood method for a range of catalog subsets in the entire crust 5 to 65 km below sea level the b value of the crustal events is unusually low b 0 66 0 09 indicating that the seismogenic region features strong material with high stresses that are released with limited influence from hydrothermal fluids the 410 good quality shear wave splitting results have an average delay time of 0 06 0 002 s and an average percent anisotropy ranging from 0 25 0 04 to 6 2 0 94 with a mean of 1 70 0 32 fast shear wave polarization directions are highly variable and appear to reflect a combination of tectonic and magmatic stresses that overprint the regional e w compressive stress associated with the convergence of the nazca and south american plates the shear wave splitting results and b values suggest that the upper crust beneath uturuncu 0 7 km below the summit is characterized by a weak and localized hydrothermal system in a poorly developed fracture network we conclude that stresses imposed by crustal flexure due to magmatic unrest above the altiplano puna magma body activate crack opening on a pre existing fault beneath the volcano generating seismicity and a spatially variable 1 10 anisotropy above the brittle ductile transition zone these results suggest that strong stresses in relatively unfractured upper crustal rocks may locally inhibit fluid migration in large silicic magma systems leading to pluton emplacement and effusive volcanism rather than explosive eruptions

comments on the paper the portland purbeck junction portlandian berriasian in the weald and the correlation of latest jurassic early cretaceous rocks in southern england by w a wimbledon c o hunt

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

shallow seismicity triggered seismicity and ambient noise tomography at the long dormant uturuncu volcano bolivia using a network of 15 seismometers around the inflating uturuncu volcano from april 2009 to 2010 we find an average rate of about three local volcano tectonic earthquakes per day and swarms of 5 60 events a few times per month with local magnitudes ranging from 1 2 to 3 7 the earthquake depths are near sea level more than 10 km above the geodetically inferred inflation source and the altiplano puna magma body the mw 8 8 maule earthquake on 27 february 2010 triggered hundreds of earthquakes at uturuncu with the onset of the love and rayleigh waves and again with the passage of the x2 x3 overtone phases of rayleigh waves this is one of the first incidences in which triggering has been observed from multiple surface wave trains the earthquakes are oriented nw se similar to the regional faults and lineaments the b value of the catalog is 0 49 consistent with a tectonic origin of the earthquakes we perform ambient noise tomography using love wave cross correlations to image a low velocity zone at 1 9 to 3 9 km depth below the surface centered slightly north of the summit the low velocities are perhaps related to the hydrothermal system and the low velocity zone is spatially correlated with earthquake locations the earthquake rate appears to vary with time a seismic deployment from 1996 to 1997 reveals 1 5 earthquakes per day whereas 60 events day were seen during 5 days using one seismometer in 2003 however differences in analysis methods and magnitudes of completeness do not allow direct comparison of these seismicity rates the rate of seismic activity at uturuncu is higher than at other well monitored inflating volcanoes during periods of repose the frequent swarms and triggered earthquakes suggest the hydrothermal system is metastable