magnetotelluric multiscale 3 d inversion reveals crustal and upper mantle structure beneath the hangai and gobi altai region in mongolia

magnetotelluric imaging of upper crustal convection plumes beneath the taupo volcanic zone new zealand 1 broadband mt magnetotelluric data were recorded that form an array of measurements at the south eastern margin of the tvz taupo volcanic zone in the central north island of new zealand these array data are used to investigate mechanisms by which the tvz s extraordinarily high heat flux is transported to the surface taken together with seismological data these mt data show compelling evidence that support a model of hydrothermal convection within the brittle upper 6 7 km part of the crust both 2 d and 3 d inversion models of these mt data show vertical low resistivity zones that connect surface geothermal fields to an inferred magmatic heat source that lies below the brittle ductile transition

three dimensional electrical resistivity image of magma beneath an active continental rift taupo volcanic zone new zealand 1 magmatic activity in regions of continental extension may result in huge 400 km 3 explosive eruptions of viscous gas rich silicic magma geochemical and geological data suggest that the large volumes of magma erupted are produced by extracting interstitial liquid from a long lived mush zone a mixture of solid crystals and liquid melt that accumulates in liquid dominated lenses at the top of a much thicker region of lower melt fraction mush such lenses will be highly electrically conductive compared with normal mid crustal rocks here we use results of 220 magnetotelluric mt soundings to construct a 3 d electrical resistivity image of the northern silicic part of new zealand s taupo volcanic zone a young continental rift associated with very high heat flow and intense silicic volcanism the electrical resistivity image shows a plume like structure of high conductivity interpreted to be a zone of interconnected melt rising from depths 35 km beneath the axis of extension

compartmentalization of the coso east flank geothermal field imaged by 3 d full tensor mt inversion author s lindsey nj kaven jo davatzes n newman ga abstract the authors 2016 published by oxford university press on behalf of the royal astronomical society previous magnetotelluric mt studies of the high temperature coso geothermal system in california identified a subvertical feature of lowresistivity 2 5ohmm and appreciable lateral extent g1 km in the producing zone of the east flank field however these models could not reproduce gross 3 d effects in the recorded data we perform 3 d full tensor inversion and retrieve a resistivity model that out performs previous 2 d and 3 d off diagonal models in terms of its fit to the complete 3 d mt data set as well as the degree of modelling bias inclusion of secondary zxx and zyy data components leads to a robust east dip 60 to the previously identified conductive east flank reservoir feature which correlates strongly with recently mapped surface faults downhole well temperatures 3 d seismic reflection data and local microseismicity we perform synthetic forward modelling to test the best fit dip of this conductor using the response at a nearby mt station we interpret the dipping conductor as a fractured and fluidized compartment which is structurally controlled by an unmapped blind east flank fault zone

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

unraveling the electrical conductivity structure to decipher the hydrothermal system beneath the mt hasan composite volcano and its vicinity sw cappadocia turkey abstract to comprehend the hydrothermal potential beneath the mt hasan composite volcano and its vicinity three dimensional analyses of magnetotelluric data collected at 27 sounding locations were utilized to develop electrical conductivity or its reciprocal resistivity images in southwest cappadocia central anatolia turkey a low resistivity anomaly associated with a clay cap was observed in the resulting numerical models that followed the characteristics of the earlier theoretical models developed for the electromagnetic interpretation of hydrothermal systems the observed anomaly appeared in the form of a deep smectite smectite illite and illite chlorite sequence buried under the cover layers made up of welded ignimbrites contemporaneous sediments and monogenetic quaternary volcanism products further evidence was observed that indicated the presence of a relatively deep 4 6 km single magmatic chamber beneath mt hasan and the surrounding system the tuz golu fault and some other auxiliary faults within this system present a high correlation with the low to high resistivity interfaces in the vicinity of mt hasan

relationship between continental area and elevation continental hypsographic curves derived from data gathered by kossinna1 show a similarity of shape that suggests a correlation between continental area including the continental shelf and average height2 3 i have considered the varied geological histories of the continental blocks and suggest here that this relationship is not fortuitous and that changes in continental groupings will have profound effects not only on ocean volumes and hence sea level but also on the three dimensional shapes of the continental blocks

wide band magnetotelluric measurements across the taupo volcanic zone new zealand preliminary results the taupo volcanic zone tvz of new zealand is characterised by intensive geothermal activity and frequent rhyolitic volcanism sixteen wide band 0 01 1 800 s magnetotelluric soundings were measured along a 110 km long profile approximately perpendicular to the strike of the tvz a model obtained from 2d inversion of the soundings shows two near surface regions of high conductance which correspond to low density volcaniclastic sediments up to 3 km thick which infill a sequence of collapse calderas at deeper levels approximately 5 10 km a resistive layer underlies the entire tvz modelling shows other conductive zones occur beneath the tvz with the shallowest lying below the central part at a depth of 10 15 km given the high heat flux and volcanic history of the tvz the high conductivity at depth may indicate the presence of connected melt at greater depth 20 30 km the upper mantle beneath the tvz appears to be anomalously conductive consistent with observed high seismic attenuation

magnetotelluric imaging of the ohaaki geothermal system new zealand implications for locating basement permeability abstract closely spaced broadband mt magnetotelluric data measured on a profile through the ohaaki geothermal system show that the basement greywacke directly beneath the geothermal field is resistive there is no evidence in these mt data for the existence of an intrusive heat source directly beneath ohaaki however these data confirm that a dipping conductive zone links the near surface low resistivity anomaly associated with the geothermal field to a deep region of low resistivity that is offset to the northwest the resistivity of the dipping conductor is consistent with a zone of fracturing within the basement meta sediments and is interpreted to show the location of upwelling high temperature fluids that supply the ohaaki geothermal field from a deep magmatic source

evolution of a dynamic paleo hydrothermal system at mangatete taupo volcanic zone new zealand abstract recent quarrying and active faulting at mangatete taupo volcanic zone tvz new zealand illuminate a rare spatial and temporal window on a dynamic late quaternary geothermal system detailed geological mapping stratigraphic logging ams 14 c dating and textural and mineralogical analyses were used to construct a complex history of hydrothermal volcanological and tectonic activity from 36 to 2 ka extinct surface hydrothermal manifestations occur over a 2 km 2 area and include in situ siliceous sinters distributed on normal fault terraces an inferred hydrothermal eruption breccia heb containing acid etched sinter blocks another probable heb that was bathed in silicifying thermal fluids and sinter clasts that were entrained in a debris flow associated with a volcanic ash event preserved sinter textures typical of near neutral ph alkali chloride spring discharge channels aprons terraces and affiliated marshes comprise plant rich palisade tufted bubble mat and domal stromatolitic fabrics in addition a packed fragmental sinter facies is shown herein to constitute silicified microbial mats that were broken transported and deposited as point bar deposits in thermal spring fed streams moreover four unusual siliceous sinter fabrics vuggy globular spongy scalloped and arcuate wavy layered are interpreted to have formed from local acid sulfate chloride thermal springs possibly associated with paleo fumaroles the reconstructed history of paleo hydrothermal activity indicates that the oldest sinters 36 ka at mangatete developed in alkali chloride hot springs but then underwent post depositional alterion overprinting by acid sulfate steam condensate and were dismembered possibly by a hydrothermal eruption low ph hot spring discharges forming the unusual inferred acid sinter fabrics were localized in the same area a shift in paleo hydrology is evidenced by unaltered alkali chloride sinters dated between 22 and 3 ka a cluster of sinter dates between 22 7 and 20 7 ka denote thermal fluid emissions around the time of the okareka eruption 22 1 ka an 11 kyr gap in sinter ages followed which overlaps with a time of relative quiescence in local fault movement in between the rotorua and rotoma eruptions as determined from previous paleo seismicity studies on the whirinaki fault in the mangatete area at 9 4 ka hot spring activity in thermally fed streams slightly post dated the rotoma eruption 9 5 ka the locations and ages of the youngest post 9 4 ka alkali chloride sinters and silicified paleosols in relation to whirinaki fault movements indicate an overall westward migration of siliceous spring discharge dated charcoal 1 8 kyr in ash at the base of a locally extensive volcaniclastic debris flow containing sinter blocks of a range of ages and mineralogical maturity suggests its affiliation with the taupo eruption mangatete sinters occur along sw ne striking faults parallel to the regional trend of the tvz which dissect the topography into stepped terraces inferred basement cross faults in the study area i e structural lineaments orthogonal to this trend may have served as fluid pathways at depth initiating and maintaining hydrothermal activity at mangatete for more than 30 kyr within the larger ngakuru graben 150 km 2 the ages and distributions of sinters and hydrothermal eruption breccias including those at mangatete show that significant hydrothermal activity occurred for more than 60 kyr in an area lacking such activity today thus mangatete retains a rare stratigraphic record within the tvz sinter archive of changing geothermal fluids resulting from shifts in paleo phreatic water levels moreover although limited the presented age data imply a relationship among seismic activity volcanic ash fall events and the occurrence of permeable pathways for upward migration of thermal fluids finally whereas currently active tvz geothermal fields appear to be stable over tens of thousands of years or more the fossil hydrothermal systems in the wider mangatete area demonstrate the possibility of much greater temporal variability in the lateral expression of local surface manifestations of thermal fluid discharge through time

magnetotelluric evidence for thick skinned tectonics in central taiwan taiwan is the type example of an arc continent collision numerous tectonic models have been proposed for this orogen and include both thin skinned and thick skinned lithospheric deformation these models predict very different structures at middle and lower crustal depths but insuffi cient geophysical data exist to unequivocally distinguish between them long period magnetotelluric mt data were collected in central taiwan in 2006 2007 to constrain the crustal resistivity structure a two dimensional inversion of these mt data revealed a prominent electrical conductor that extends across the decollement predicted by the thin skinned model this feature is interpreted to be due to 1 2 saline fl uids and is inconsistent with the thin skinned model in contrast the thick skinned model predicts this feature since fl uids are generated in the crustal root through metamorphism quantitative correlation of the resistivity and seismic velocity models supports small volume high salinity fl uids in a thickened crust as the cause of this conductor

a new conceptual model of the icelandic crust in the hengill geothermal area based on the indirect electromagnetic geothermometry abstract application of the indirect electromagnetic geothermometer calibrated by available temperature well logs has enabled the construction of the three dimensional temperature model of the hengill geothermal area iceland up to a depth of 20 km its analysis showed that the background temperature of the icelandic crust above 20 km does not exceed 400 it is overlapped by a network of interconnected high temperature low resistive channels which braid through the crust mainly at a level of 10 15 km and root to a depth greater than 20 km accordingly the probable heat sources feeding the geothermal system are supposed to be the intrusions of the hot partially molten magma upwelling from the mantle through faults and fractures the comparison between the vertical temperature cross sections and the projections of the earthquake hypocenters showed that they all are located in the areas where temperature does not exceed 400 which is a gabbro solidus in a silica rich icelandic crust joint analysis of the temperature model together with the resistivity and residual bouguer gravity anomalies enabled us to explain the distribution of the earthquake hypocenters by different geothermal regimes in adjacent parts of the area and cooling of large massifs of the partially molten solidified magma beneath seismically active areas basing on the above inferences we suggest a new self consistent conceptual model of the icelandic crust which agrees with most of the previous geophysical results and provides an explanation for the facts that the previous models failed to explain

absolute reconstructions of the paleozoic oceans abstract the orientation of three intraplate magmatic belts the mongolian from 130 to 280 m y south siberian from 320 to 400 m y and baltic from 365 to 400 m y belts which can be considered as hot spot tracks are used together with apparent paleomagnetic pole wander paths to reconstruct true motions of continents during paleozoic times the reconstructions obtained show that the old late precambrian pangea continued to exist in the early paleozoic with an inner arrangement of constituent continents strongly different from that of the late paleozoic pangea the continents were constantly located in the eastern hemisphere only the western hemisphere was occupied by the paleo pacific oceans four oceans existed in paleozoic times between the continents in the eastern hemisphere 1 the iapetus which originated in the late precambrian and closed before devonian times 400 m y 2 the asiatic paleo ocean which originated in the late precambrian and closed 450 m y ago 3 the uralian paleo ocean which opened around 500 m y ago and closed in the latest permian 240 230 m y 4 the paleo tethys which opened in the ordovician 480 450 m y and became the mesozoic tethys life duration of the oceans was 200 400 m y the main trend in the earth s evolution during the paleozoic was a break up of the old precambrian supercontinent and construction of the new late paleozoic pangea

magnetotelluric study of the eastern margin of the bohemian massif relations between the cadomian variscan and alpine orogeny eastern margin of the bohemian massif is a geologically remarkable area where three different orogenic cycles are meeting the oldest cadomian building a basement for younger cycles a younger variscan and the youngest alpine covering the older units in the past this area was investigated by gravimetry and seismic methods recently we have supplemented broadband magnetotelluric measurements within the period range of 0 001 1000 s carried out on 29 stations distributed along a 140 km long west east regional profile the profile direction was based on local geology and then confirmed by a dimensionality and directionality analysis data showed moderate effects of cultural noise in the signals and could be successfully processed by robust methods we carried out a 2d inversion of the data using the rebocc approach the inversion results confirm the known near surface geology and reveal deeper structures on the west and in the central part of the profile units of the bohemian massif the moldanubian zone the brunovistulicum are interpreted in the east the western carpathians units are encountered short period data agree well with the known near surface geology of all inner smaller units and bring new knowledge in particular on their thickness in the shallow structure several conductive anomalies have been identified which are hypothesized to be related to graphitized layers in the moldanubian zone from long period data a new image of the moldanubian brunovistulicum contact and about the structure of the brunovistulicum especially the brunovistulian massifs was obtained

imaging the deep source of the rotorua and waimangu geothermal fields taupo volcanic zone new zealand abstract magnetotelluric data were recorded in a 45 10 km band crossing the rotorua and waimangu geothermal fields in the northern part of the taupo volcanic zone in the central north island of new zealand 3 d inverse modelling of these data show that beneath the low resistivity areas marking the near surface geothermal fields localised electrically conductive zones are present in the crust below about 2 5 and 3 5 km depth at rotorua and waimangu respectively at increasing depth these conductive zones broaden and appear to merge with a larger conductive zone at 8 km depth situated between the geothermal systems at rotorua the top of the conductive zone is situated directly beneath the area of greatest surface heat and gas discharge at waimangu the uppermost part of the deeper conductive zone is situated beneath the western part of lake rotomahana also an area of intense surface thermal activity and high heat flux the localised conductive zones are interpreted to be high temperature quasi magmatic fluids rising from a broader zone of partial melt at deeper levels

temperature and biotic crises in the marine realm climatic change has been a prominent cause of marine mass extinction but areal restriction of seafloor during global regression has not late eocene and pliocene pleistocene cooling for example caused major extinctions but profound global oligocene and pleistocene regressions had little or no direct effect on benthic diversity recurrent themes of pre cenozoic marine crises suggest that global temperature change also served as a major and perhaps dominant agent of extinction in these events 1 mass extinctions have frequently been concentrated in the tropics which seem to have become a refrigerated trap from which there has been no escape biotas previously occupying high latitudes have shifted equatorward to replace disappearing tropical biotas 2 some crises were not instantaneous but followed protracted and pulsatile temporal patterns as would be predicted for complex global climatic crises 3 several mass extinctions coincided with recognized intervals of climatic cooling

contemporary ground deformation in the taupo rift and okataina volcanic centre from 1998 to 2011 measured using gps the taupo volcanic zone tvz is one of the world s most productive regions of rhyolitic volcanism and contains the highly active okataina volcanic centre ovc within the tvz intra arc extension is expressed as normal faulting within a zone known as the taupo rift the ovc is located within a complex part of the rift where volcanism and deformation is considered influenced by rift structure and kinematics there has been significant research on the structural volcanic and geophysical properties of the rift and ovc but less focus on deformation using geodetic data the limited studies that have utilized geodetic data do not clearly resolve the distribution of deformation and strain rates within the rift and ovc this is essential to ensure that deformation signals from volcanic processes at the ovc are correctly identified and distinguished from those related to regional tectonic or local hydrothermal processes within the rift in this paper we present a picture of contemporary deformation at the ovc and within the surrounding rift in detail using existing and new gps campaign and continuous gps cgps data collected between 1998 and 2011 the results show a highly heterogeneous deformation and strain rate field both extension and shortening through the study area partitioned into different parts of the rift our results agree well with earlier geodetic studies as well as identify new features but some deformation patterns conflict with long term geological observations in the ovc we observe a locally rotated horizontal velocity field significant vertical deformation and variable strain rates across the caldera in the tarawera rift we identify elevated extension and shear rates which may have significant implications for volcanism there a shortening pattern is identified through the central rift which is unexpected in an intra arc rifting environment we attempt to explain the source s of shortening and extension and discuss their implications for geodetic monitoring efforts in the ovc

reactivation of fault systems by compartmentalized hydrothermal fluids in the southern andes revealed by magnetotelluric and seismic data in active volcanic arcs such as the andean volcanic mountain belt magmatically sourced fluids are channeled through the brittle crust by faults and fracture networks in the andes volcanoes geothermal springs and major mineral deposits have a spatial and genetic relationship with nne trending margin parallel faults and margin oblique nw trending andean transverse faults atf the tinguiririca and planchon peteroa volcanoes in the andean southern volcanic zone svz demonstrate this relationship as their spatially associated thermal springs show strike alignment to the nne oriented el fierro thrust fault system we constrain the fault system architecture and its interaction with volcanically sourced hydrothermal fluids using a combined magnetotelluric mt and seismic survey that was deployed for 20 months high conductivity zones are located along the axis of the active volcanic chain delineating fluids and or melt a distinct wnw trending cluster of seismicity correlates with resistivity contrasts considered to be a reactivated atf seismicity occurs below 4 km suggesting activity is limited to basement rocks and the cessation of seismicity at 9 km delineates the local brittle ductile transition as seismicity is not seen west of the el fierro fault we hypothesize that this structure plays a key role in compartmentalizing magmatically derived hydrothermal fluids to the east where the fault zone acts as a barrier to cross fault fluid migration and channels fault parallel fluid flow to the surface from depth increases in fluid pressure above hydrostatic may facilitate reactivation this site specific case study provides the first three dimensional seismic and mt observations of the mechanics behind the reactivation of an atf

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

phase difference between sea level and magnetic reversal rate in this letter i report a negative correlation and a phase difference of 10 myr between recent data on long term eustatic sea level change1 and geomagnetic reversal rate for the past 150 myr the phase difference is detectable both by visual inspection and through a correlation analysis i also show that the same phase difference but with a positive correlation could theoretically have existed between this eustatic sea level data and sea floor creation rate in light of the ridge volume hypothesis of long term eustasy the analysis suggests the view that sea floor creation rate and or subduction rate and geomagnetic reversal rate are closely associated and perhaps synchronized in their changes

taiwan orogeny thin skinned or lithospheric collision abstract the taiwan orogeny is young and presently very active it provides an excellent environment for studying ongoing orogenic processes especially since the region is monitored intensively with dense seismological and geodetic networks and new studies aiming at deciphering shallow and deep structures in and around taiwan have been recently conducted or are being planned the available data can be used continually to test critically hypotheses of the taiwan orogeny hypotheses dealing with the mechanics of mountain building are basic to the understanding of taiwan orogeny and are particularly amenable to testing the widely cited thin skinned tectonics hypothesis was formulated to explain mainly the geologic and relatively shallow

post triassic continental hypsometry and sea level application of modern hypsometric relationships to ancient continental groupings requires changes in ocean basin volumes through time and hence sea level a global sea level change curve based on current knowledge of continental evolution from the base of the jurassic is presented variation in sea level for most of the mesozoic requiring explanation by mechanisms other than the area elevation effect was of the order of 160 m a major lowering of sea level started in the late cretaceous and continued into the cenozoic this major change can be explained by changes in ocean ridge volumes

electrical resistivities of crustal fluids results of calculations on the electrical resistivities of crustal fluids as a function of temperature pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 m the dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures 300 c can not be readily explained as a product of variations in resistivities of crustal fluids these results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity

structural geodetic and seismological evidence for tectonic escape in sw taiwan abstract recent structural geodetic and seismological data in sw taiwan are analysed and discussed in terms of present day tectonic escape occurring in response to the active n100 collisional shortening the escaping area corresponds to the onland extension of the manila accretionary wedge this region comprises a rheologically weak thick muddy cover which is decoupled from the underlying basement by a decollement and which deforms mainly by aseismic creep it is separated from the northern actual collisional area by a major wnw to nw trending structural and kinematic transition zone oblique to the structural grain of the belt the chishan transfer fault zone geodetic data are further used to define several poorly deforming blocks undergoing nearly uniform displacement velocities and bounded by kinematic discontinuities that fit the major faults and to determine the present day across strike and along strike motions on these major faults although direct onland structural evidence of tectonic escape is poor reconstruction of quaternary paleostress patterns demonstrate that this escape probably began during the late pleistocene later than in northeastern taiwan as a result of the southward migration of the collision through time offshore structural data help to constrain the geometry and the southern extension of the escaping blocks finally a tentative model of lateral extrusion in sw taiwan is proposed

the pliensbachian and tithonian extinction events the claim by raup and sepkoski1 that an approximately 26 myr mass extinction periodicity can be recognized on a global scale at family level over the past 250 myr has excited much attention and provoked speculation about an ultimate extraterrestrial cause2 10 while the purported periodicity can be challenged on grounds of both the inaccuracy of the geological timescale11 and the method of extinction analysis12 there can be no doubting the reality of at least some of the 12 proposed extinction events such as those at or near the end of the permian triassic and cretaceous others are less widely acknowledged further analysis by raup and sepkoski13 confirms the importance of two of these events within the jurassic at the end of the pliensbachian and tithonian stages i demonstrate here that marine invertebrate mass extinctions at approximately these times occurred on a regional not a global scale they can be related to severe reductions in habitat area provoked either by regression of epicontinental sea or by widespread anoxia

phanerozoic shallow water diversity driven by changes in sea level shallow water 50 m marine diversity curves for the phanerozoic based on area changes due to sea level variation and continental movements are presented it is shown that a change in the degree of endemism can alter diversity more than the effect of sea level change or change in the number of faunal provinces the pattern produced for a constant degree of endemism shows some rapid but not periodic falls in diversity these occur in the middle cambrian at the end of the ordovician devonian permian cretaceous and eocene and in the messinian there is no recurring single cause for these falls

effect of degradation on seismic response of israeli continental slope constitutive models of soil behavior under cyclic loading that incorporate masing rules are often used to perform seismic site response analyses these analyses often ignore the effect of cyclic degradation of stiffness and strength on the seismic response of horizontal soft clay strata the authors recently proposed a normalized form of the shear stress strain relationship for undrained cyclic simple shear of soft clay cyclic degradation is considered via a single fatigue parameter the mean effective stress used in normalization of stresses and strains cyclic degradation of the small strain shear modulus though is neglected the normalized model is combined with an effective stress model for simple shear loading and boundary conditions developed within the framework of bounding surface kinematic and isotropic hardening plasticity to produce a model that is suitable for site response analysis this paper demonstrates the use of this model in one dimensional propagation analysis to compare the expected response in a soft clay profile to that response that would be estimated without accounting for cyclic degradation by neglecting cyclic degradation underestimation of velocities and accelerations developed in the profile are apparent

surface deformation and tectonic setting of taiwan inferred from a gps velocity field 1 we have determined the present day surface deformation of taiwan by computing the velocity gradient field and fault slip from 143 gps velocity vectors in southern taiwan the derived strain and rotation rates and fault slips are indicative of lateral extrusion toward the south in northern taiwan we infer the onset of gravitational collapse which is induced by the on land extension of the okinawa trough in the eastern central range the observed inverted nw se extension is consistent with geological observations and high heat flow measurements this could be the result of exhumation of crustal material the model further shows a significant decrease in slip rate northward along the longitudinal valley fault at 23 7 n the northern coastal range shows high strain rates and two oppositely rotating blocks by combining the surface deformation model with seismicity data and seismic tomography we are able to propose a coherent model for the present day tectonic activity both seismicity and tomography show further evidence for active southward propagating exhumation of a crustal slice in the eastern central range offshore east taiwan we deduce strong evidence of a southward propagating crustal tear fault accommodating most of the philippine sea plate eurasian plate convergence the tear is the crustal response to incipient northwestward subduction of the philippine sea plate thus the ryukyu trench is bending southward becoming almost perpendicular to the convergence direction while subduction of the philippine sea plate continues in this setting a sudden rapid southward propagation of the afore mentioned tear is conceivable

three dimensional conductivity model of the tendaho high enthalpy geothermal field ne ethiopia abstract tendaho is one of the high enthalpy geothermal fields at advanced stage of exploration which is located in the afar depression in north eastern ethiopia six deep and shallow geothermal wells were drilled in the field between 1993 and 1998 here we present the first 3d conductivity model of the tendaho high enthalpy geothermal field obtained from 3d inversion of magnetotelluric mt data mt data from 116 sites at 24 selected periods in the period range from 0 003 s to 1000 s were used for the 3d inversion the 3d conductivity model reveals three main resistivity structures to a depth of 20 km the surface conductive structure 10 m and 1 km thick is interpreted as sediments geothermal fluids or hydrothermally altered clay cap the underlying high resistivity structure in the afar stratiod basalts is associated with the deep geothermal reservoir at a depth 5 km a high conductivity is observed across the whole of the tendaho geothermal field this structure is inferred to be the partial melt heat source of the geothermal system the most striking feature in the 3d model is a fracture zone upflow zone in the afar stratoid basalts at the dubti area which acts as a pathway for geothermal fluids targeting the inferred fracture zone by directional drilling will likely increase the permeability and temperature of the deep reservoir in the basalts hence the inferred presence of a fracture zone and shallow magma reservoir suggest that there is a huge potential with temperature exceeding 270 c at 2 km depth at tendaho for conventional hydrothermal geothermal energy development

three dimensional resistivity image of the magmatic system beneath lastarria volcano and evidence for magmatic intrusion in the back arc northern chile lazufre volcanic center located in the central andes is recently undergoing an episode of uplift conforming one of the most extensive deforming volcanic systems worldwide but its magmatic system and its connection with the observed uplift are still poorly studied here we image the electrical resistivity structure using the magnetotelluric method in the surroundings of the lastarria volcano one of the most important features in the lazufre area to understand the nature of the magmatic plumbing the associated fumarolic activity and the large scale surface deformation results from 3 d modeling show a conductive zone at 6 km depth south of the lastarria volcano interpreted as the magmatic heat source which is connected to a shallower conductor beneath the volcano showing the pathways of volcanic gasses and heated fluid a large scale conductive area coinciding with the area of uplift points at a magma intrusion at midcrustal depth

crustal magma pathway beneath aso caldera inferred from three dimensional electrical resistivity structure at naka dake cone aso caldera japan volcanic activity is raised cyclically an example of which was a phreatomagmatic eruption in september 2015 using a three dimensional model of electrical resistivity we identify a magma pathway from a series of northward dipping conductive anomalies in the upper crust beneath the caldera our resistivity model was created from magnetotelluric measurements conducted in november december 2015 thus it provides the latest information about magma reservoir geometry beneath the caldera the center of the conductive anomalies shifts from the north of naka dake at depths 10 km toward naka dake along with a decrease in anomaly depths the melt fraction is estimated at 13 15 at 2 km depth moreover these anomalies are spatially correlated with the locations of earthquake clusters which are distributed within resistive blocks on the conductive anomalies in the northwest of naka dake but distributed at the resistive sides of resistivity boundaries in the northeast

visualizing preferential magmatic and geothermal fluid pathways via electric conductivity at villarrica volcano s chile abstract preferential fluid pathways along regional fault systems are crucial for the spatial localization of volcanic and geothermal manifestations differences in electric properties of fluids and hydrothermal alteration products against the unaltered matrix allow for visualization of such pathways unfavorable geometry for 2 d inversion resulting from the fluid pathways in regional faults often being aligned along the geo electric strike can be overcome by using 3d inversion of magnetotelluric data for a section in the 1400 km long liquine ofqui fault system lofs we demonstrate the potential of 3 d inversion of magnetotelluric data for visualization of fluid pathways six out of eight electric resistivity anomalies at intermediate depth are connected to volcanic or geothermal surface manifestations deep and highly conductive anomalies are detected in the vicinity of the volcanic chain and where the lofs is crosscut by a fault that belongs to the andean transversal fault system the anomaly related to the fault crossing reveals a vertical pathway extending to the surface phase tensor analyses indicate a structural origin of the latter anomaly that may be connected to a line of volcanic cones that occurs in the ne of the villarrica volcano

tiering in suspension feeding communities on soft substrata throughout the phanerozoic tiering of benthic marine suspension feeding communities on soft substrata has varied throughout the phanerozoic epifaunal tiering was most developed during the middle and late paleozoic and the triassic to jurassic with large scale reductions in tiering occurring during the permian triassic extinctions and after the jurassic infaunal tiering reached its highest level of organization after the paleozoic

melt distribution beneath a young continental rift the taupo volcanic zone new zealand 1 taupo volcanic zone tvz is a zone of intense volcanism and rifting associated with the subduction of the pacific plate beneath the continental crust of new zealand s north island an image of the conductivity structure beneath the central part of the tvz has been constructed using 2 d inverse modeling of magnetotelluric data a rapid increase in conductivity at a depth of 10 km beneath the tvz 3 km beneath the base of the seismogenic zone but well above the base of the quartzo feldspathic crust 16 km is interpreted to mark the presence of an interconnected melt fraction 4 within the lower crust beneath the quartzo feldspathic crust the model shows a zone of increased conductivity on the eastern side of the tvz consistent with an increased concentration of melt at deeper levels the pacific plate is resistive compared with the overlying mantle

application of modulus degradation model of clays a degradation model is applied in conjunction with different soil models and stress strain relations to site response analyses during earthquakes to evaluate the effects of degradation computations on two clay deposits subjected to both high and low level input excitations are conducted in addition modulus degradation with or without shear strength reduction is discussed in terms of surface response degradation has little effect in the case of low level excitation with high level excitatfons however degradation causes lower surface spectral response in the shorter period range and the difference is on the same order as that from different soil models where surface response differs the use of degradation with strength reduction is less conservative when compared to the use of degradation without strength reduction

kinematic analyses of the hsuehshan range taiwan a large scale pop up structure the hsuehshan range exposed in the northern and central taiwan slate belt is a fault bounded structural high cored by biotite grade slates and metasandstones syntectonic overgrowths in pyrite pressure shadows indicate that much of the eastern hsuehshan range experienced coaxial strain histories and that finite strain magnitudes generally increase toward the hinterland near the eastern boundary of the hsuehshan range however pressure shadows record noncoaxial strain histories consistent with a top to the east sense of shear along a steep nw dipping shear zone this noncoaxiality is attributed to se directed backthrusting on the lishan fault which separates the higher grade eo oligocene rocks of the hsuehshan range from the lower grade miocene rocks of the backbone range because it is bounded to the east by the se vergent lishan fault and to the west by a series of nw vergent thrusts e g the chuchih fault the hsuehshan range is envisaged as a pop up structure strain magnitudes measured from pressure shadows in the coaxial part of the range are consistently lower than those predicted by steady state wedge models that assume all deformation is accommodated by penetrative strain departure from the model predictions is attributed primarily to strain localization along discrete fault surfaces e g the lishan fault the hsuehshan range tapers in width to the south thus the pop up may be buried or die out to the south where the collision is younger

three dimensional resistivity structure of iwo yama volcano kirishima volcanic complex japan relationship to shallow seismicity surface uplift and a small phreatic eruption

active detachment of taiwan illuminated by small earthquakes and its control of first order topography we use 110 000 small earthquakes to locate and map active faults in three dimensions within the taiwan arc continent collision the structure is dominated by a nearly horizontal band of small earthquakes at 10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt with other illuminated fault zones abutting the detachment zone the zone steepens below eastern taiwan to 30 90 and reaches depths of 30 60 km the three dimensional shape of the detachment zone in relation to topography allows a new test of critical taper wedge mechanics and suggests that the reversal of topographic slope across taiwan is controlled by the shape of the detachment

magnetotelluric imaging beneath the taiwan orogen an arc continent collision 1 the taiwan orogen has formed since the late miocene by oblique collision between the luzon volcanic arc on the philippine sea plate and the eurasian continental margin this oblique collision has produced an orogen that decreases in age from north to south and permits study of the temporal evolution of an arc continent collision these factors make taiwan a favorable location to study the process of arc continent collision the first long period magnetotelluric mt measurements were recorded in taiwan as part of the taiwan integrated geodynamics research taiger project in 2006 7 measurements were made at 82 sites on three transects across south central and north taiwan that span the breadth of the orogen and cross all major tectonic boundaries robust remote reference processing of the mt time series data resulted in high quality soundings that were modeled in both 2 and 3 dimensions these mt models support predictions of lithospheric deformation i e thick skinned tectonics beneath the central ranges in south and central taiwan but are inconsistent with predictions of orogen scale thin skinned models the mt resistivity model for northern taiwan is consistent with dewatering of the subducting philippine slab and with deformation described by the subducting indenter tectonic model modeling the taiger mt data has definitively shown that conductive and seismically active crustal structures exist to 30 km beneath the orogen these conductive regions interpreted as interconnected fluid map pervasive zones of collisional deformation that are lithospheric in scale

magmatic hydrothermal system inferred from the resistivity structure of kusatsu shirane volcano abstract the kusatsu shirane volcano is a quaternary andesitic to dacitic active volcano located on the central honshu arc the volcano is known to have had repeated phreatic eruptions in last 200 years a number of geochemical and geophysical studies have been conducted around the yugama crater lake the focus of current volcanic activity however the 2018 unexpected phreatic eruption occurred at mt motoshirane a different pyroclastic cone from that which hosts yugama there were also frequent magmatic eruptions until 1500 years ago at this cone and the magma produced at that time has not yet cooled and solidified a better understanding of the magmatic hydrothermal system of this volcano requires structural information gathered over a larger area and to greater depths than has been achieved to date here we describe the three dimensional 3 d electrical resistivity structure around mt motoshirane down to a depth of 10 km using broadband magnetotelluric mt data 320 0 0005 hz collected in 2015 and 2016 the 3 d resistivity structure model shows an extensive low resistivity layer at depths of 1 3 5 km beneath the summit however structures characteristic of the presence of magma are not observed beneath this layer it could be too deep or too small to be detected combining the new data with results of previous geochemical and geophysical studies we interpret the conductor as a hydrothermal fluid reservoir that supplies fluids to the crater lake and to hot springs on the eastern and western flanks of the volcano the existence of a fluid reservoir that extends from the vicinity of the yugama crater lake to the subsurface beneath an inactive pyroclastic cone that has produced repeated magmatic eruptions in the past suggests that a large scale magmatic hydrothermal system is developing beneath the volcano

three dimensional electrical resistivity structure of the kuju volcanic group central kyushu japan revealed by magnetotelluric survey data abstract the kuju volcano group in the southwestern part of the oita prefecture japan consists of more than 20 volcanoes distributed over an area of approximately 400 sq km on october 11 1995 a phreatic eruption occurred on the eastern flank of mt hossho in the central part of the kuju volcano a second eruption occurred in mid december 1995 producing an ash volume of approximately 5000 m3 therefore clarifying the thermal and hydrologic processes occurring beneath the entire kuju volcano area is important because the geothermal areas beneath active volcanoes may be important to the development of geothermal energy the largest geothermal power plant in japan otake hatchobaru power plant 120 mw is located 5 km west of mt hossho and mt iwoyama to identify the structures associated with the hydrothermal systems beneath the kuju volcano area we performed a wide band magnetotelluric survey the survey data were collected at 63 observation points around the kuju volcano from september october 2015 the resulting primary three dimensional resistivity structure model clarified that a resistivity structure 50 m predominates the kuju volcano area a highly conductive structure 10 m exists around the iwoyama kuju area extending in depth towards the northeastern part of iwoyama based on the results of previous studies the former can be interpreted as a lava and pyroclastic deposit whereas the latter corresponds to a magmatic fluid pathway and hydrothermal altered layers

partial melt or aqueous fluid in the mid crust of southern tibet constraints from indepth magnetotelluric data summary the indepth project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the tibetan plateau in the lhasa block seismic reflection surveys in 1994 detected a number of bright spots at 15 20 km depths that indicate zones of crustal fluids aqueous fluids or partial melt coincident magnetotelluric mt data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots using constrained inversion the mt data require a minimum crustal conductance of 6000 s this abnormally high electrical conductance can be best explained by a layered model with fluids partial melt aqueous fluids or a combination of partial melt and aqueous fluids the non uniqueness of the mt method means that a wide range of melt fraction thickness combinations for the above models could all explain the 6000 s conductance to distinguish between these three models other geophysical and geological data are required reflection seismic data suggest that a high fluid content 15 per cent is present at the top of the layer the amplitude versus offset data suggest that the top of this layer may be aqueous fluids rather than partial melt passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt petrological studies suggest that concentrations of aqueous fluids above 0 1 per cent at mid crustal depth cannot be sustained taken together these data show that the high conductivity in southern tibet is most probably the result of a relatively thin layer of aqueous fluids 100 200 m overlying a thicker zone of partial melt 10 km

geodynamic processes of taiwan arc continent collision and comparison with analogs in timor papua new guinea urals and corsica abstract the taiwan arc continent collision involves four geodynamic processes intra oceanic subduction initial arc continent collision advanced arc continent collision and arc collapse subduction these processes now occur simultaneously in 19 24 30 n but have operated sequentially southward since the late miocene although the geological and geophysical features appear to change progressively from north to south across the island they are distinct within individual tectonic regimes using the present scenario as a basis for comparison it is suggested that timor is at the initial arc continent collision stage while papua new guinea and urals proceeded to the advanced arc continent collision stage however corsica represents an even more advanced stage than in taiwan where a previously accreted arc has collapsed and been totally removed

the ups and downs of the tvz geodetic observations of deformation around the taupo volcanic zone new zealand the 300 km long taupo volcanic zone tvz formed as a result of back arc rifting associated with subduction of the pacific plate beneath the australian plate with current extension rates of 8 15 mm yr 1 using gps and interferometric synthetic aperture radar insar data collected by the european esa and japanese jaxa space agencies we present ground deformation observations from 2003 to 2011 both insar and vertical gps data show widespread subsidence across the central tvz at rates of up to 20 mm yr 1 using simple elastic dislocation models to represent the contraction of a sill like body at 6 km depth we predict an annual volume change of 0 011 0 016 km3 beneath the central tvz we suggest that the majority of the observed subsidence is a result of the cooling and subsequent contraction of magma within the shallow crust

transition from oblique subduction to collision in the northern luzon arc taiwan region constraints from bathymetry and seismic observations we systematically study the detailed bathymetry seismicity and source parameters of large and moderate sized earthquakes that occurred in the northern luzon arc taiwan region between 1964 and 1996 the purpose is to characterize the transition from oblique subduction to regional collision in terms of the distribution of morphological features the characteristics of seismogenic structures and the corresponding state of strain to the south of 21 5 n the existence of a subduction zone is clearly shown by both bathymetry and seismicity the subducted eurasia slab reaches a maximum depth of 200 km and the depth decreases sharply as it approaches taiwan focal mechanisms indicate that the subducted slab is in downdip extension for depths 150 km but switches to downdip compression at greater depths to the north of 23 n collision is clearly the predominant process with broad deformation on both sides of the suture in between the transition is accommodated by a distributed thrust deformation zone in the frontal portion of the accretionary prism closure of the forearc basin and the right lateral ne sw striking taitung canyon fault zone tcfz these structures resolve the differential movements between the subduction and collision zones and will repeatedly change their positions as the collision propagates southwestward the present day plate boundary follows the axis of the manila trench in the south broadens significantly in the zone of transition and finally becomes a zone of several tens of kilometers in width across eastern taiwan the deformation front on the other hand is located between the western foothills and coastal plain in taiwan then connects to the manila trench to the south seismic strain tensors indicate that the region to the north of tcfz has a compression rate 15 times larger than that to the south the northern luzon arc shows characteristics of oblique subduction but the effects of collision are not as significant within the subducted eurasian lithosphere as comparing to the subducted philippine sea plate beneath the southernmost ryukyu arc

shallow water areas in space and time application of modern hypsometric relationships to former continental blocks allows calculation of the shallow sea area around each block for any given sea level using hallam9s 1984 sea level curve for the post triassic shallow sea area curves have been drawn for each continental block and a global curve of shallow sea area with time produced as the response of a block to sea level change depends upon the size of the block the cumulative curve does not vary in a straightforward manner with respect to sea level predictions of organic diversity change with sea level change must take account of this non linear relationship

the interaction between active crustal faults and volcanism a case study of the liquine ofqui fault zone and osorno volcano southern andes using magnetotellurics abstract the relationship between subduction in convergent margins crustal structures and magmatism is crucial to understand processes such as the type and frequency of volcanic activity in these areas although it is well known that the release of fluids by the subducting slab is the main cause of the arc volcanism in areas like the central and southern andes details such as the timing and pathways of magma ascent and storage are still not well understood a key factor that needs to be better studied in the southern volcanic zone of the andes of chile is the role of large tectonic features in fluid transport and magma ascent processes such as the liquine ofqui fault zone a n s strike slip crustal structure parallel to the main volcanic arc in this study we focus on osorno volcano a stratovolcano composed mainly by products of basaltic to basaltic andesite composition with minor dacites and with historical hawaiian strombolian eruptions through the measurement of magnetotellurics and the use of 3 d modeling tools and petrologic constraints two magmatic reservoirs have been inferred which suggest a complex magmatic system with reservoirs of different depths and compositions the shallowest magmatic reservoir 4 8 km has a dacitic composition while the deepest one 7 15 km has an andesitic composition instead the shallow reservoir is located 2 km to the e of the volcano and the deepest one is located 10 km to the e considering that the liquine ofqui fault zone is located 20 km to the e of the volcano we suggest that eruptions of osorno volcano are associated with the ascent of deep crustal basaltic magma enhanced by this master fault re activating the inferred reservoirs and the volcano

late ordovician extinctions and sea level change abstract if ordovician continental blocks had similar hypsometries to modern continental blocks glacially induced regressions would have increased shallow water areas thus species area effects cannot be invoked to explain extinctions during these regressions because late ordovician sea level was so high glacially induced regressions of similar magnitudes to quaternary regressions would have dramatically increased the input of dissolved load into the shallow sea this pollution effect may have been one cause of the extinctions

three dimensional resistivity structure and magma plumbing system of the kirishima volcanoes as inferred from broadband magnetotelluric data broadband magnetotelluric mt measurements were conducted in 2010 and 2011 in the vicinity of shinmoe dake volcano in the kirishima volcano group japan where sub plinian eruptions took place 3 times during 26 27 january 2011 by combining the new observations with previous mt data it is found that an anomalous phase in excess of 90 is commonly observed in the northern sector of the kirishima volcano group because the anomalous phase is not explained by 1 d or 2 d structure with isotropic resistivity media 3 d inversions were performed by applying small errors to the anomalous phase we successfully estimated a 3 d resistivity structure that explains not only the normal data but also the anomalous phase data the final model shows a vertical conductor that is located between a deep seated conductive body at a depth greater than 10 km and a shallow conductive layer by applying the findings of geophysical and petrological studies of the 2011 sub plinian eruptions we infer that the subvertical conductor represents a zone of hydrothermal aqueous fluids at temperatures over 400 c in which a magma pathway interconnected melt is partially and occasionally formed before magmatic eruptions to the north of the deep conductor earthquake swarms occurred from 1968 to 1969 suggesting that these earthquakes were caused by volcanic fluids

lithospheric dismemberment and magmatic processes of the great basin colorado plateau transition utah implied from magnetotellurics 1 to illuminate rifting processes across the transition zone between the extensional great basin and stable colorado plateau interior we collected an east west profile of 117 wideband and 30 long period magnetotelluric mt soundings along latitude 38 5 n from southeastern nevada across utah to the colorado border regularized two dimensional inversion shows a strong lower crustal conductor below the great basin and its transition zone in the 15 35 km depth range interpreted as reflecting modern basaltic underplating hybridization and hydrothermal fluid release this structure explains most of the geomagnetic variation anomaly in the region first measured in the late 1960s hence the transition zone while historically included with the colorado plateau physiographically possesses a deep thermal regime and tectonic activity like that of the great basin the deep crustal conductor is consistent with a rheological profile of a brittle upper crust over a weak lower crust in turn on a stronger upper mantle jelly sandwich model under the incipiently faulted transition zone the conductor implies a vertically nonuniform mode of extension resembling early stages of continental margin formation colorado plateau lithosphere begins sharply below the western boundary of capitol reef national park as a resistive keel in the deep crust and upper mantle with only a thin and weak moho level crustal conductor near 45 km depth several narrow steep conductors connect conductive lower crust with major surface faulting some including modern geothermal systems and in the context of other great basin mt surveying suggest connections between deep magma sourced fluids and the upper crustal meteoric regime the mt data also suggest anisotropically interconnected melt over a broad zone in the upper mantle of the eastern great basin which has supplied magma to the lower crust consistent with extensional mantle melting models and local shear wave splitting observations we support a hypothesis that the transition zone location and geometry ultimately reflect the middle proterozoic suturing between the stronger yavapai lithosphere to the east and the somewhat weaker mojave terrane to the west we conclude that strength heterogeneity is the primary control on locus of deformation across the transition zone with modulating force components

two dimensional crustal structures of taiwan from gravity data to delineate the tectonic character of taiwan an island wide gravity survey of taiwan was conducted between 1980 and 1987 the bouguer anomaly map shows that in general isogals trend nne in consonance with the overall structural trend of the island with seismic and other geophysical data as constraints the subsurface density structures were modeled along three profiles across the major structural trends the gravity data are consistent with average continental moho depths of 26 km in the coastal plain and the western foothills 28 km underneath the coastal range in eastern taiwan and 33 km under the central range a lack of significant correlation of the bouguer anomalies with the topography implies dynamic rather than isostatic support of topography in the taiwan region

probing the status of felsic magma reservoirs constraints from the p t h 2 o dependences of electrical conductivity of rhyolitic melt electrical conductivity of rhyolitic melts combined with magnetotelluric data can provide important constraints for the physicochemical conditions of active felsic magma reservoirs previous experimental investigations are limited to low h2o concentration 3 wt h2o and low pressure 0 2 gpa for hydrous melts we here report new results from electrical conductivity measurements of peralkaline rhyolitic melts with 4 7 wt na2o and 0 1 7 9 wt h2o at 868 1665 k and 0 5 1 0 gpa in piston cylinder apparatuses using sweeping frequency impedance analyses logarithmic electrical conductivity is found to correlate linearly with h2o concentration and reciprocal temperature and the influence of h2o has been considerably underestimated by previous work the negative pressure effect on electrical conductivity appears to attenuate with increasing h2o concentration based on the new data we develop the following electrical conductivity model for peralkaline rhyolitic melts under conditions up to 1665 k 1 0 gpa and 8 wt h2o log 2 983 0 0732w 3528 233 8w 763 7 5w2 pt where is electrical conductivity in s m t is temperature in k w is h2o concentration in wt and p is pressure in gpa according to the nernst einstein relation this conductivity model can also be applied to metaluminous and peraluminous rhyolitic melts by scaling down according to the diffusivity and concentration of na the primary charge carrier with the new model we suggest that the highest electrical anomaly beneath the tianchi volcano corresponds to a melt fraction of 9 if the melt contains 5 wt h2o or to a melt fraction of 32 for 2 wt h2o similarly the magma reservoir beneath the taupo volcanic zone new zealand is inferred to contain a fully molten rhyolite with 7 5 wt h2o or 36 water saturated melt with 10 wt h2o

episodic kinematics in continental rifts modulated by changes in mantle melt fraction gps measurements are used to investigate the continental rift system of north island new zealand and to develop a model for short term deformation involving flexing of an elastic upper crust caused by episodic changes in mantle viscosity and melt fraction direct observation of mantle flow dynamics beneath spreading oceanic crusts is difficult but similar processes occur below rifts in the continental crust and so can be inferred from satellite observations of the moving crust simon lamb et al use ten years of continuous gps measurements from a back arc continental rift system in north island new zealand to investigate the dynamics of mantle flow beneath seafloor spreading centres they observe deformation which they conclude to be the result of flexure of an elastic upper crust pulled downwards or pushed upwards along the rift axis by a driving force at depth they propose that this flexure is caused by melt induced episodic changes in mantle viscosity in their model a drop in melt fraction due to melt extraction raises the mantle viscosity and drives subsidence whereas melt accumulation reduces viscosity and allows uplift oceanic crust is created by the extraction of molten rock from underlying mantle at the seafloor spreading centres found between diverging tectonic plates modelling studies have suggested that mantle melting can occur through decompression as the mantle flows upwards beneath spreading centres1 but direct observation of this process is difficult beneath the oceans continental rifts however which are also associated with mantle melt production are amenable to detailed measurements of their short term kinematics using geodetic techniques here we show that such data can provide evidence for an upwelling mantle flow as well as information on the dimensions and timescale of mantle melting for north island new zealand around ten years of campaign and continuous gps measurements in the continental rift system known as the taupo volcanic zone reveal that it is extending at a rate of 6 15 millimetres per year however a roughly 70 kilometre long segment of the rift axis is associated with strong horizontal contraction and rapid subsidence and is flanked by regions of extension and uplift these features fit a simple model that involves flexure of an elastic upper crust which is pulled downwards or pushed upwards along the rift axis by a driving force located at a depth greater than 15 kilometres we propose that flexure is caused by melt induced episodic changes in the vertical flow forces that are generated by upwelling mantle beneath the rift axis triggering a transient lower crustal flow a drop in the melt fraction owing to melt extraction raises the mantle flow viscosity and drives subsidence whereas melt accumulation reduces viscosity and allows uplift processes that are also likely to occur in oceanic spreading centres

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

three dimensional imaging of a ag au rich epithermal system in british columbia canada using airborne z axis tipper electromagnetic and ground based magnetotelluric data abstractwe have evaluated results from a study combining airborne z axis tipper electromagnetic ztem and ground based magnetotelluric mt data to image an epithermal system in british columbia the spatially coincident use of these two methods allowed for a direct comparison of both data sets in the overlapping frequency band and showed that both measurements were consistent inversion of just the ztem data suffered from the lack of electric field amplitude information which could be provided by the mt data three dimensional inversion modeling of the two individual data sets was performed models of electrical resistivity derived from both data sets were consistent and could be correlated with the geological and structural setting of the mineralization gold is associated with disseminated pyrite and marcasite in quartz sericite altered felsic volcanic rocks and intrusions especially near the contact with mafic volcanic rocks and a late diorite intrusion the quartz sericite alteration yields a cond

earthquake swarm activity highlights crustal faulting associated with the waimangu rotomahana mt tarawera geothermal field taupo volcanic zone abstract the waimangu rotomahana mt tarawera geothermal field wrtgf in the taupo volcanic zone new zealand experiences frequent but sporadic earthquake swarms with durations of less than 1 day here we examine detailed locations of the seismic activity using precise double difference relative location techniques we utilize a combination of cross correlation derived arrival times and catalogue based arrival times from 582 earthquakes recorded in the area between 2004 and 2015 for the relocation analysis the new earthquake locations highlight a 6 km long ne sw lineation which we infer to represent a sub surface fault that extends along the northern side of waimangu geothermal system and the north western end of lake rotomahana we suggest that this structural feature acts as a permeable pathway for aqueous fluid and co2 release up to the surface geothermal field and lake rotomahana from a deeper magmatic source

geophysical characterization of the northwest geysers geothermal field california abstract the clear lake volcanic field in northern california is the youngest and northern most part of a long lived volcanic system that has produced recent 10 ka eruptions adjacent to the clear lake volcanic field is the world s largest energy producing geothermal field the geysers the hottest part of the geyser s geothermal field is in the northwest where temperatures reach 400 c at 3 km depth low permeability high thermal gradients and low steam saturation prescribed development of an enhanced geothermal system egs in the northwest geysers to increase energy producing capacity though the northwest geysers is known to be the hottest part of the field geophysical methods have failed to adequately image any inferred heat source this project aims to image the heat source of the northwest geysers using newly collected gravity and magnetotelluric mt measurements gravity data were jointly modeled with existing magnetic data along a two dimensional profile aligned with an existing geologic cross section the key feature of the potential field model is a low density low susceptibility body at 5 km depth bmsl under the egs mt data were modeled in three dimensions to characterize subsurface resistivity structure where the upper 3 km of the resistivity model agrees well with existing data lithologic and steam saturation are estimated from modeled resistivity values using existing geophysical data below 3 km depth bmsl the resistivity model images a possible young intrusion under the egs a possible zone of partial melt

strong motion seismology strong motion seismology is concerned with earthquake ground motion in the amplitude range that poses the threat of human injury or property damage because the occurrence of strong motion cannot be predicted in advance and because the time between significant events at any one place may range from tens to thousands of years strong motion data is recorded on triggered instruments that typically must be maintained in readiness for years between periods of significant recording as a consequence the strong motion data set is sparse because of the engineering need for estimates of future ground motion the scope of the field of strong motion seismology encompasses seismic source theory and any other aspect of seismology that can provide insights helpful in making the best possible estimates given the limited data this relationship with the rest of seismology is a reciprocal one in that strong motion data gives perhaps the best information available on earthquake source processes

paleomagnetic evidence for a diachronic clockwise rotation of the coastal range eastern taiwan abstract paleomagnetic results obtained from over 2100 cores sampled at 132 early pliocene to late pleistocene sites in the coastal range of eastern taiwan indicate that since the late pliocene the margin of the philippine sea plate has undergone a clockwise rotation of about 30 as a direct consequence of the plio pleistocene collision of this plate with the chinese continental plate the rotation is diachronic and started between 3 and 4 ma ago in the northern coastal range then propagated southward at an average speed of the order of 70 10 km ma this value is in very good agreement with estimates of the southward propagation of the collision between the philippine sea plate and the eurasian margin in taiwan it is suggested that the rotation reflects the deformation of the philippine sea plate at depth and does not results from the shallow deformations involved in the building of the coastal range orogen

fluid and deformation regime of an advancing subduction system at marlborough new zealand newly forming subduction zones on earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes the transpressional subduction regime of new zealand which is advancing laterally to the southwest below the marlborough strike slip fault system of the northern south island is an ideal setting in which to investigate these processes here we acquired a dense high quality transect of magnetotelluric soundings across the system yielding an electrical resistivity cross section to depths beyond 100 km our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops massive fluid release just inland of the trench induces fault fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates narrow strike slip faults in the shallow brittle regime of interior marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones deep subduction generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high angle thrusts that are poorly oriented for failure under dry conditions the fluid deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models

fragmentation of asia in the permian asia is a composite continent formed by the accretion of crustal blocks1 3 a likely sequence of events is that siberia collided with a kazakhstan block which together then collided with europe along the urals in permo triassic times2 this was followed by the sucessive accretion of various blocks of south and east asia to siberia culminating in the collision of india with asia during the cenozoic palaeomagnetic data have already demonstrated that the kolyma and sikhote alin blocks of eastern asia collided with the main continent along the late mesozoic fold belts of verkhoyansk and sikhote alin4 furthermore results from the malay peninsula5 and japan6 suggest that south east asia and japan were situated near the equator in permian times and were therefore separated from the asian continent we report here some palaeomagnetic results from the permian of the sino korean and yangtze blocks of china that show they were also situated near the equator were not in their present relationship to one another and were widely separated from siberia

electrical resistivity imaging of the inter plate coupling transition at the hikurangi subduction margin new zealand abstract inter plate coupling on the hikurangi subduction margin along the east coast of new zealand s north island changes from weakly coupled in the north to locked in the south 3 d inverse modeling of magnetotelluric mt data across the coupling transition shows that the electrical resistivity structure is correlated with the areal strain rate which provides a measure of plate coupling the correlation between strain rate and resistivity is seen parallel to the strike and dip directions of the subduction in the region where the strain is extensional the upper plate is more conductive than in the south where contraction is occurring and the plates are locked the increased mid crustal resistivity in the south can be interpreted to be a consequence of reduced fluid interconnectivity due to contraction or reduced fluid content due to decreased upward transport from sources beneath the subduction interface the increased mid crustal conductivity in the extensional area is interpreted to be a consequence of fluid released by subducted sediment that are also the cause of the extension seismicity in the upper 5 km of the subducting plate shows similar changes with a greater concentration of near interface seismicity beneath the conductive extensional region we interpret the concentration of seismicity below the subduction interface in the extensional region to show fluid sourced from dehydration reactions within the subducting plate or from the de watering of subducted sediments rising into the upper plate and decreasing the resistivity of the middle crust decreased near interface seismicity beneath the contractional region can be interpreted to be a consequence of decreased upper plate permeability preventing upward fluid escape or reduced lower plate fluid availability

pre quaternary sea level changes while the paramount importance of global sea level changes has been widely accepted by quaternary geologists throughout this century it is only within the last decade that such changes in the rest of the phanerozoic have received much attention there are two principal reasons for this arousal of interest first increased exploration of the stratigraphic record across the earth both on land and under the sea and significant improvement in biostratigraphic correlation have drawn attention to what appear to have been globally synchronous events second oceanographic research has demonstrated the importance of tectonically controlled changes in the cubic capacity of the ocean basins notably variations in ridge volume which provide a ready mechanism for sea level changes even at nonglacial times the adjective eustatic was first proposed for global changes of sea level by suess 1906 in his great treatise the face of the earth the corresponding noun is eustasy not eustacy as it is commonly misspelt the unconvinced reader need only think of ecstasy eustatic sea level changes can be inferred as suess recognized by a plotting the temporal spread of marine sediments over the continents and b estimating the depositional depth variations in marine stratal sequences which can be correlated across and between continents to these methods can be added seismic stratigraphy in which biostratigraphically age determined seismic sequences on the con tinental margins are analyzed in terms of cycles of coastal onlap and offiap when the results of these methods are broadly in accord confidence in a eustatic interpretation is increased but the range of confidence can vary considerably it is widely accepted that there was an exceptionally high sea

lithospheric structure of an active backarc basin the taupo volcanic zone new zealand seismic data from both explosive and earthquake sources have been used to model the crustal and upper mantle velocity structure beneath the taupo volcanic zone tvz an active backarc basin in central north island new zealand volcanic sediments with p wave velocities of 2 0 3 5 km s 1 reach a maximum thickness of 3 km beneath the central tvz underlying these sediments to 16 km depth is material with velocities of 5 0 6 5 km s 1 interpreted as quartzo feldspathic crust east and west of the tvz crust with similar velocities is found to depths of 30 and 25 km respectively beneath the tvz material with p wave velocities of 6 9 7 3 km s 1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust the base of the crust at 30 km depth under the tvz is marked by a strong seismic reflector interpreted as the moho modelling of arrivals from deep 40 km earthquakes near the top of the underlying subducting pacific plate reveals a region with low mantle velocities of 7 4 7 8 km s 1 beneath the crust of the tvz this region of low mantle velocities is best explained by the presence of partially hydrated upper mantle resulting from dehydration of hydrous minerals e g serpentinite carried down by the underlying subducting plate within the lower crust beneath the tvz a region of high 0 34 poisson s ratio is observed indicating the presence of at least 1 per cent partial melt this melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust where it provides a source for the voluminous rhyolitic magmas of the tvz

seismic studies of the crust under the hydrothermal areas of the taupo volcanic zone new zealand abstract observations of relative p wave travel time residuals for local mantle earthquakes and p s amplitude ratios for more distant events have been used to investigate the nature of the crust under the hydrothermal areas of the central taupo volcanic zone in contrast to some other regions of similar activity such as the yellowstone and geysers geothermal areas u s a there is no evidence in the data for an extensive region of semi molten or molten rock in the crust thus the large amounts of mafic intrusive rock and associated crustal melt inferred to be present on geological grounds must be now largely at temperatures below the appropriate solidus the data do provide further evidence for previously suspected lateral inhomogeneity in the underlying mantle

stratigraphy and structure of the ngatamariki geothermal system from new zircon u pb geochronology implications for taupo volcanic zone evolution abstract recent drilling at the ngatamariki geothermal field taupo volcanic zone new zealand has provided new constraints on the stratigraphy and volcanic evolution of the region over 2800 m thickness of volcanic products are present at ngatamariki mainly comprised of rhyolitic ignimbrites linked to large caldera forming events at sources outside the field area but locally sourced andesite and rhyolite lavas and domes are also encountered most of the rocks are allocated to the pre 0 35 ma tahorakuri formation crystallisation age spectra and consequent best estimates of eruption age have been obtained by u pb dating on zircons from otherwise severely hydrothermally altered magmatic rocks by secondary ion mass spectrometry techniques using a shrimp rg instrument the oldest rock dated is an ignimbrite which yields an eruption age estimate of 1 85 0 06 ma this ignimbrite plus comparable aged units dated at the adjacent rotokawa and ohaaki geothermal fields are interpreted to represent the oldest silicic volcanic deposits in the area and onlap the basal andesite lava pile that is best developed at rotokawa to the south other pyroclastic units and associated volcaniclastic sediments with another intercalated andesite lava unit return age estimates between 1 85 0 06 and 0 701 0 039 ma between 0 7 and 0 35 ma contemporaneous surface lithologies in the ngatamariki area are dominated by sediments with subordinate lava domes between 0 716 0 017 and 0 655 0 016 ma at least three shallow to 5 km 3 the intrusions generated a large alteration halo 25 km 3 minimum and intense silicification of the wall rocks at 0 35 and 0 34 ma the area was buried by two ignimbrite packages of the whakamaru group erupted from sources just west and well north respectively of the field ignimbrites of the waiora formation and several rhyolite lava domes were then emplaced over a period bracketed by domes dated at 0 282 0 011 and 0 257 0 011 ma coeval with more extensive volcanic activity in the maroa dome complex west of the field sediments of the huka falls formation and deposits of the 25 4 0 2 ka oruanui eruption then cap and seal the system the new u pb data coupled with detailed petrographical studies allow us to build the history of the area encompassed by the ngatamariki geothermal field the field despite 2 8 km of subsidence does not lie in a caldera and is the only one known to date to have a plutonic intrusive complex of quaternary age two chemically and temporally distinct hydrothermal events are located at the ngatamariki field with no evidence of continuity between the two