deformation of the long valley caldera eastern california from mid 1983 to mid 1988 measurements using a two color geodimeter deformation of long valley caldera in eastern california has been closely monitored since june 1983 with a two color geodimeter which has a precision between 0 12 and 0 20 ppm of the baseline length initial trilateration measurements were concentrated in the south moat of the caldera the source of the january 1983 earthquake swarm since then the network has been expanded from 13 to 42 baselines the additional baselines span the resurgent dome and the western half of the caldera with a combination of measurements made several times per week on 7 11 baselines and monthly or yearly measurements on the remaining baselines we have measured both the temporal and spatial deformation of the caldera although the m 6 4 chalfant valley earthquake was located 35 km from the nearest baseline in the network we observed coseismic strain changes of up to 1 5 ppm on several baselines using fault planes selected to coincide with the hypo centers of the main shock and the m 5 9 foreshock we have inferred a combined moment of 3 0 1025 dyn cm from the two color data this cumulative moment of both earthquakes is equivalent to a magnitude 6 3 shock over the period from mid 1983 to mid 1988 the long term trends determined from the length changes of the baselines within the caldera show a general decrease in the rate of extension during 1983 measurements show a maximum extension rate of 5 0 ppm yr after mid to late 1984 the extension rates decreased by a factor of 2 4 relative to those in the first year with the exception of the coseismic step the extension rates have been fairly constant during the interval between late 1984 and mid 1988 the long term changes in rate can be modeled by using two mogi point sources representing inflation beneath the resurgent dome of the caldera and a dislocation surface representing slip in the south moat modeling indicates that both the rates of slip and inflation have decreased since mid 1983 but these two processes are still occurring at a measureable and constant rate

gravity changes and deformation due to a magmatic intrusion in a two layered crustal model we develop and extend theoretical and computational methods for the calculation of the deformation gravity and potential change due to a point source of magma injection into a multilayered elastic gravitational earth model in our calculations which are based upon the method outlined by rundle two distinct layers overlying a half space may be incorporated the source can be located in either of the layers or the half space the method is quite general and can be readily adapted to calculations in which stresses in either the layers or the half space relax by viscoelastic flow the results obtained indicate that the use of homogeneous half space to represent the earth may in some cases be too simple a model and that variations in elastic moduli have a more significant effect than variation in reference density on both the surface displacements and gravity changes as an example we calculate the displacement and gravity changes due to a subsurface mass injection in a crust mantle model appropriate to the volcanic island of lanzarote which is presently the subject of numerous geophysical experiments both historical and recent data indicate that lanzarote may be subject to some risk of volcanic eruption in the future thus our calculations may be useful in interpreting observations of preemption phenomena the results are discussed in terms of prediction versus measurement capabilities

study of volcanic sources at long valley caldera california using gravity data and a genetic algorithm inversion technique we model the source inflation of the long valley caldera california using a genetic algorithm technique and micro gravity data while there have been numerous attempts to model the magma injection at long valley caldera from deformation data this has proven difficult given the complicated spatial and temporal nature of the volcanic source recent work illustrates the effectiveness of considering micro gravity measurements in volcanic areas a genetic algorithm is a problem solving technique which combines genetic and prescribed random information exchange we perform two inversions one for a single spherical point source and another for two sources that might represent a more spatially distributed source the forward model we use to interpret the results is the elastic gravitational earth model which takes into account the source mass and its interaction with the gravity field the results demonstrate the need to incorporate more variations in the model including another source geometry and the faulting mechanism in order to provide better constraints on intrusion volumes future work should include the joint inversion of gravity and deformation data during the same epoch

a model for deformation in long valley california 1980 1983 geodetic data collected in long valley california from 1975 through 1983 define a pattern of uplift and strain which is evidently associated with a sequence of earthquakes occurring in may 1980 and subsequent swarm activity continuing until the present we have constructed a model to explain the deformation observed since may 1980 in terms of inflation of two subsurface magma chambers faulting in the south moat region of the caldera and slip on the hilton creek fault the most significant new feature of the model is the shallow magma chamber at 5 km depth located a few hundred meters to the east of the casa diablo hot spring area inflation of this chamber causes stresses which show consistency with various aspects of the seismicity in the south moat of the caldera calculations of stress across vertical planes over the magma chamber can be used together with failure criteria to estimate the inflation volume at which the rock layers intervening between the chamber and the surface will fail by extensional fracture

a late neoproterozoic paleomagnetic pole for the congo craton tectonic setting paleomagnetism and geochronology of the nola dike swarm central african republic abstract new structural geochronological and paleomagnetic data were obtained on dolerite dikes of the nola region central african republic at the northern border of the congo craton in this region metavolcanic successions were thrust southward onto the craton during the panafrican orogenic events our structural data reveal at least two structural klippes south of the present day limits of the panafrican nappe suggesting that it has once covered the whole nola region promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes paleomagnetic measurements revealed a stable dual polarity low inclination magnetization component in nine dikes 47 samples carried by pyrrhotite and magnetite this component corresponds to a paleopole at 304 8 e and 61 8 s d p 5 4 d m 10 7 graded at q 6 both metamorphism and magnetic resetting were dated by the 40 ar 39 ar method on amphibole grains separated from the dikes at 571 6 ma the nola pole is the first well dated paleomagnetic pole for the congo craton between 580 and 550 ma it marks a sudden change in direction of the congo craton apparent polar wander path at the waning stages of the panafrican orogenic events

geodetic methods for detecting volcanic unrest a theoretical approach in this paper we study the application of different geodetic techniques to volcanic activity monitoring using theoretical analysis this methodology is very useful for obtaining an idea of the most appropriate and efficient monitoring method mainly when there are no records of geodetic changes previous to volcanic activity the analysis takes into account the crustal structure of the area its geology and its known volcanic activity to estimate the deformation and gravity changes that might precede eruptions the deformation model used includes the existing gravity field and vertical changes in the crustal properties both factors can have a considerable effect on computed deformation and gravity changes topography should be considered when there is a steep slope greater than 10 the case study of teide stratovolcano tenerife canary islands for which no deformation or gravity changes are available is used as a test to avoid considering topography we worked at the lowest level of las canadas and examined a smaller area than the whole caldera or island therefore the results are only a first approach to the most adequate geodetic monitoring system the methodology can also be applied to active areas where volcanic risk is not associated with a stratovolcano but instead with monogenetic scattered centers especially when sites must be chosen in terms of detection efficiency or existing facilities the canary islands provide a good example of this type of active volcanic areas and we apply our model to the island of lanzarote to evaluate the efficiency of the monitoring system installed at the existing geodynamic station on this island topography is not important the results of our study show clearly that the most appropriate geodetic volcano monitoring system is not the same for all different volcanic zones and types and the particular properties of each volcano zone must be taken into account when designing each system

intrusive mechanisms at mt etna forerunning the july august 2001 eruption from seismic and ground deformation data in this work we present seismological and ground deformation evidence for the phase preparing the july 18 to august 9 2001 flank eruption at etna the analysis performed through data from the permanent seismic and ground deformation networks highlighted a strong relationship between seismic strain release at depth and surface deformation this joint analysis provided strong constraints on the magma rising mechanisms we show that in the last ten years after the 1991 1993 eruption an overall accumulation of tension has affected the volcano then we investigate the months preceding the 2001 eruption in particular we analyse the strong seismic swarm on april 20 24 2001 comprising more than 200 events mmax 3 6 with prevalent dextral shear fault mechanisms in the western flank the swarm showed a ca ne sw earthquake alignment which in agreement with previous cases can be interpreted as the response of the medium to an intrusive process along the approximately nnw sse volcano genetic trend these mechanisms leading to the july 18 to august 9 2001 flank eruption are analogous to ones observed some months before the 1991 1993 flank eruption and more recently in january 1998 before the february november 1999 summit eruption

new results at mayon philippines from a joint inversion of gravity and deformation measurements in this paper we detail the combination of the genetic algorithm ga inversion technique with the elastic gravitational model originally developed by rundle and subsequently refined by fernandez and others a sensitivity analysis is performed for the joint inversion of deformation and gravity to each of the model parameters illustrating the importance of proper identification of both the strengths and limitations of any source model inversion and this technique in particular there is a practical comparison of the theoretical results with the inversion of geodetic data observed at the mayon volcano in the philippines where there are gravity changes without significant deformation after the 1993 eruption

reviewing the puzzling intracontinental termination of the aracuai west congo orogenic belt and its implications for orogenic development abstract palinspastic reconstructions suggest that the late proterozoic cambrian brasiliano pan african orogenic belt in southeast brazil and west congo terminated northwards into an embayment within the sao francisco congo cratonic unit the orogenic shortening that created the aracuai west congo orogen in this embayment has been explained by tightening of the horseshoe shaped sao francisco congo craton in a fashion referred to as nutcracker tectonics we show that this model is incompatible with the general orogenic evolution proposed in recent literature which involves 1 50 m y of subduction of oceanic crust and associated arc formation followed by 2 collisional orogeny and crustal thickening quantitative considerations show that the original nutcracker model is too rigid to explain even the second crustal thickening part let alone any long pre collisional history to soften the model we suggest that the so called sao francisco congo bridge was broken by a 150 km wide orogenic corridor along the current african atlantic margin this corridor adds sufficient mobility to the system to explain the orogenic thickening of the crust to 60 65 km however even with this additional softening the confined nature of this orogen is incompatible with prolonged arc development we therefore suggest that oceanic crust was nonexistent or very limited in the macaubas basin and reject the widely published model involving 50 m y of subduction of oceanic crust and related arc development instead we find strong support for a hot intracontinental orogen model in the currently available p t geochronologic petrographic and structural data in this model extensive melting and flow of the middle crust is likely to have caused spreading of the upper crust in an orogenic setting that was created by collisions along the n w and s margins of the sao francisco craton from 630 ma

thermo mechanically coupled ice sheet response cold polythermal temperate classical mixture concepts are the appropriate vehicle for describing the dynamics of ice masses containing some water we review and derive respectively the theoretical formulations of cold polythermal and temperate ice masses emphasize the peculiarities of the model equations and point to difficulties that were encountered with the proposed models the focus is both on the adequate physical motivation of the models and the consistency of their mathematical representation

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

the pan african west congo belt in the republic of congo congo brazzaville stratigraphy of the mayombe and west congo supergroups studied by detrital zircon geochronology abstract the african part of the neoproterozoic aracuai west congo orogen forms a more than 1400 km long belt running from gabon in the north to angola in the south parallel to the south atlantic coast it consists of the west congo fold belt in the west and a largely undeformed sedimentary foreland basin in the east in the republic of congo congo brazzaville these are known as respectively the mayombe fold belt and the niari basin the mayombe fold belt is largely made up of strongly deformed metasedimentary rocks of the mayombe supergroup and its reworked granitoid basement complex the niari basin comprises the sedimentary strata of the west congo supergroup which contain a unit of diamictite and associated cap carbonate related to the marinoan ca 635 ma glaciation in this paper detrital zircon u pb data are employed to investigate the relationships between the mayombe and west congo supergroups in congo brazzaville as well as the relationships with similar stratigraphic units in the democratic republic of congo dr congo unfortunately major differences in stratigraphical nomenclature exist between congo brazzaville and dr congo which together with geological differences complicate comparisons the terminology used in this paper follows the congo brazzaville tradition u pb data have been obtained by la icp ms on 1674 zircon grains from samples of the mayombe and west congo supergroups seven and eight samples respectively two of the samples have palaeoproterozoic ages the other samples are of neoproterozoic age zircons from the latter fall into four main age groups group 1 500 800 ma forming 8 of the total population group 2 900 1200 ma 29 group 3 1800 2300 ma 19 and group 4 2500 3100 ma 31 only ca 13 of the zircons fall outside of these groups most zircons of group 1 600 800 ma are interpreted to have been derived from late neoproterozoic magmatic arc rocks and syn to post collisional granites of the aracuai orogen in brazil since rocks of these ages are rare in the congo craton to the east most of group 2 zircons have probably been derived from early neoproterozoic 1000 900 ma rhyolitic and granitic rocks exposed in dr congo the palaeoproterozoic and archaean zircons of groups 3 and 4 may have been derived both from eastern and western source regions none of the stratigraphical units beneath the marinoan diamictite contain the 500 800 ma zircons of group 1 this is interpreted to indicate deposition of these units during the earlier neoproterozoic rift and drift phases in the history of the aracuai west congo orogen while the strata that do contain group 1 zircons were deposited during the collisional phases and post collisional uplift a significant hiatus is likely to be present beneath the marinoan diamictite based on these results a revision of the stratigraphy of the mayombe and west congo supergroups is recommended three granitoid rocks and two metasedimentary samples from the reworked basement complex have also been investigated yielding palaeoproterozoic ages 2050 2100 ma for the granites and palaeoproterozoic and archaean detrital zircon ages for the metasedimentary rocks sm nd isotope data have been obtained on a number of meta argilitic and siltstone samples all samples yielded palaeoproterozoic model ages indicating mixing of sediments of different ages as also observed in the zircon age distributions

monitoring unrest in a large silicic caldera the long valley inyo craters volcanic complex in east central california recent patterns of geologic unrest in long valley caldera in east central california emphasize that this large silicic volcanic system and the adjacent geologically youthful inyo mono craters volcanic chain are still active and capable of producing locally hazardous volcanic eruptions a series of four magnitude 6 earthquakes in may 1980 called attention to this current episode of unrest and subsequent activity has included numerous earthquake swarms in the south moat of the caldera accompanied by inflation of the resurgent dome by more than 50 cm over the last five years the seismicity associated with this unrest is currently monitored by a network of 31 telemetered seismic stations with an automatic processing system that yelds hypocentral locations and earthquake magnitudes in near real time deformation of the ground is monitored by a a series of overlapping trilateration networks that provide coverage ranging from annual measurements of regional deformation to daily measurements of deformation local to the active southern section of the caldera b a regional network of level lines surveyed annually c a regional network of precise gravity stations occupied annually d local l shaped level figures surveyed every few months and e a network of fourteen borehole tiltmeter clusters two instruments in each cluster and a borehole dilatometer the telemetered signals from which provide continuous data on deformation rates additional telemetered data provide continuous information on fluctuations in the local magnetic field hydrogen gas emission rates at three sites and water level and temperatures in three wells continuous data on disharge rates and temperatures from hot springs and fumaroles are collected by several on site recorders within the caldera and samples for liquid and gas chemistry are collected several times per year from selected hot springs and fumaroles

repartition de la sedimentation dans le golfe proterozoique superieur du schisto calcaire au congo et gabon implications en afrique centrale abstract on the eastern border of the central africa craton the nw se directed nyanga niari basin formed a gulf cross cutting the precambrian chaillu massif this basin was linked to the ne sw directed comba basin the western extension of the sangha aulacogen in these two basins the late proterozoic schisto calcaire cycle comprises a transgressive zone with marls and conglomerates a condensed dolomite section pink dolomite and a high sea level stand zone with a carbonate ramp and barrier prograding sw ne towards the aulacogen between the comba and nyanga niari basins submarine dunes and infratidal giant stromatolites overlain by oncolitic dolomites were formed an internal shelf was isolated by an oolitic barrier an evaporitic dolomite lagoon with algal mats appeared in the nyanga niari basin and in the western part of the comba basin the lagoon was bordered by a shoal preceding a deepening of the internal shelf towards the aulacogen the foreshore sedimentation started with a thin conglomeratic layer and continued with the oolithe de kisantu characterized by a high frequency of talc oncolites in the nearshore zone black pseudo oolitic dolomite and algal mats were formed with thinly laminated algal deposits near the tidal flat in the northwestern part of the nyanga niari basin the continental serie de la biloua contains a continuous tidal breccia bar prograding towards the ne in the southern part of the comba basin fluvial channel deposits occur consisting of alluvial levees crevasse splays and tidal breccia located in the estuary in the neighbourhood of the channel mouth the sedimentation is disturbed most of the facies described here are present in many other proterozoic basins north and south of the central african craton everywhere shelf carbonate ramps seem prograding towards the sangha aulacogen the continental slope deposits of the same age cannot be interpreted as the lower part of the pan african molasse in all these sedimentary environments algal mats appear to be the main cause of organic matter productivity and of the primary carbonate precipitation finally the lagoonal environment was probably very similar to present day carbonate lagoons

sensitivity of crustal deformation instruments to changes in secular rate a variety of instruments including borehole strainmeters water wells creepmeters laser ranging and differential magnetometers are used to monitor crustal deformation in areas that are prone to geologic hazards such as earthquakes and volcanic eruptions in monitoring the deformation one typically examines the data for either a change in rate or a simple offset in the record however one needs to place a statistical confidence level that the detected signal differs from the background noise calculation of the statistical confidence level may be done using the formalism of the matched filter whose output is the signal to noise ratio two ingredients are needed to form a matched filter 1 the power density spectrum of the instrument and 2 the functional form of the signal that we desire to detect using the available crustal deformation data from the parkfield california network the background noise for individual instruments as a function of frequency f is estimated using the traditional method of the power density spectra except for two color laser distance ranging data the power spectra for most of the instruments have a frequency dependence of f n where 2 n 3 the confidence level with which a hypothesized signal is present is determined directly from the signal to noise ratio with the numerator being a function of the signal and the denominator being a function of the power spectrum using a creepmeter as an example a 0 04 mm change occurring over 1 hour a 0 06 mm occurring over 10 hours or 0 20 mm over 100 hours are all signals for which 2 and therefore have only a 5 confidence that these signals could be background noise

iron cycling and stable fe isotope fractionation in antarctic shelf sediments king george island iron fe fluxes from reducing sediments and subglacial environments are potential sources of bioavailable fe into the southern ocean stable fe isotopes 56fe are considered a proxy for fe sources and reaction pathways but respective data are scarce and fe cycling in complex natural environments is not understood sufficiently to constrain respective 56fe endmembers for different types of sediments environmental conditions and biogeochemical processes we present 56fe data from pore waters and sequentially extracted sedimentary fe phases of two contrasting sites in potter cove king george island antarctic peninsula a bay that is affected by fast glacier retreat sediments close to the glacier front contain more easily reducible fe oxides and pyrite and show a broader ferruginous zone compared to sediments close to the icefree coast where surficial oxic meltwater streams discharge into the bay pyrite in sediments close to the glacier front predominantly derives from eroded bedrock for the high amount of easily reducible fe oxides proximal to the glacier we suggest mainly subglacial sources where fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering processes likely pyrite oxidation or dissimilatory iron reduction dir our strongest argument for a subglacial source of the highly reactive fe pool in sediments close to the glacier front is its predominantly negative 56fe signature that remains constant over the whole ferruginous zone this implies in situ dir does not significantly alter the stable fe isotope composition of the accumulated fe oxides the nonetheless overall light 56fe signature of easily reducible fe oxides suggests pre depositional microbial cycling as it occurs in potentially anoxic subglacial environments the strongest 56fe depletion in pore water and most reactive fe oxides was observed in sediments influenced by oxic meltwater discharge the respective site showed a condensed redox zonation and a pore water 56fe profile typical for in situ fe cycling we demonstrate that the potential of pore water 56fe as a proxy for benthic fe fluxes is not straight forward due to its large variability in marine shelf sediments at small spatial scales 2 4 at the site proximal to oxic meltwater discharge vs 0 9 at the site proximal to the marine glacier terminus both at 2 cm sediment depth the controlling factors are multifold and include the amount and reactivity of reducible fe oxides and organic matter the isotopic composition of the primary and secondary ferric substrates sedimentation rates and physical reworking bioturbation ice scraping the application of 56fe geochemistry may prove valuable in investigating biogeochemical weathering and fe cycling in subglacial environments this requires however similarly to the use of 56fe for the quantification of benthic fluxes that the spatial and temporal variability of the isotopic endmember is known and accounted for since geochemical data from subglacial environments are very limited further studies are needed in order to sufficiently assess fe cycling and fractionation at glacier beds and the composition of discharges from those areas

inversion for p and s wave attenuation structure long valley caldera california p and s wave amplitude data from digital three component seismograms of local earthquakes recorded on a temporary seismic array have been used to tomographically invert for the three dimensional attenuation structure in the upper crust of long valley caldera california differential attenuation q 1 values are obtained through the use of a spectral ratio technique that minimizes the site effect on the spectral ratios the spectral ratios are inverted for q 1 using the lsqr algorithm zones of high positive p wave q 1 greater p wave attenuation are observed at a depth of 4 5 km beneath the east flank of mammoth mountain and at 6 8 km beneath the southwestern edge of the resurgent dome a region of positive s wave q 1 is seen at 7 8 km beneath the southern edge of the resurgent dome whereas at 6 7 km the s wave results are near average the region of positive s wave q 1 at 7 8 km beneath the resurgent dome may indicate the roof of a magmatic system the positive p wave q 1 at 4 5 km beneath mammoth mountain and 6 8 km beneath the resurgent dome may be due to the presence of a compressible fluid such as supercritical water these zones may represent some of the source regions for the long valley geothermal system

axi symmetric deformation of a thermo poro elastic half space inflation of a magma chamber summary the deformation observed at phlegraean fields italy during the recent 1982 1984 crisis is interpreted in terms of overpressure in a shallow magma chamber and migration of hot pressurized fluids elastic effects are studied first and previous results obtained from maruyama s strain nuclei are corrected poro elastic and thermo elastic effects are found to be as important as the elastic ones in a semi pervious medium the three terms are typically of the same magnitude results are applied to interpreting the bradyseismic crisis of 1982 84 at phlegraean fields pf near naples italy the concave shape of the first phase of the deformation history at pf can be reproduced by linearly increasing fluid pressure and or temperature at the boundary of a spherical cavity the convex shape of the second phase is interpreted in terms of a gradual return to normal conditions the late phase of slow subsidence is interpreted as the effect of diffusional dissipation of temperature and pressure excess the deconvolution of the real source time history at pf is largely non unique but in any case the importance of fluids in the deformation history is assessed furthermore the inclusion of fluid migration in the deformation model allows simple explanations of several geochemical and geophysical observations made during the bradyseismic crisis

reply to comment by ngako and njonfang on the adamawa yade domain a piece of archaean crust in the neoproterozoic central african orogenic belt bafia area cameroon by jacqueline tchakounte et al precambrian research 299 2017 210 229 abstract v ngako and e njonfang addressed in their comment an interesting and debated issue about the collision geodynamics of the pan african central african orogenic belt caob in cameroon however the reason for this comment is still not clear to us since it mainly deals with issues far away from the scope of our study namely the pre collisional set up of the belt the comment raises two main points the reworking process during plate collision and the pan african strain pattern not discussed in our paper for the simple reason that it was not our goal and because our data set does not even allow such discussion having said that the comment gives us the opportunity to clarify our view on the nature of the adamawa yade domain central cameroon and the early evolution of the caob we will successively discuss two other points raised in the comment 1 our fig 12 depicting a possible pre collisional configuration at the northern and southern margin of the congo craton in the early neoproterozoic and 2 the nature and extent of the adamawa yade domain

viscoelastic displacement and gravity changes due to point magmatic intrusions in a gravitational layered solid earth summary we present a method for the computation of time dependent geodetic and geophysical signatures deformation potential and gravity changes due to magmatic intrusions in a layered viscoelastic gravitational medium this work is an extension of a deformation model previously developed to compute effects due to volcanic loading in an elastic gravitational layered media the model assumes a planar earth geometry useful for near field problems and consists of welded elastic and viscoelastic layers overlying a viscoelastic half space every layer can either be considered elastic or viscoelastic the intrusion treated as a point source can be located at any depth in any of the layers or in the half space several examples of theoretical computations for different media are also presented we have found that in line with previous results obtained by other authors introducing viscoelastic properties in all or part of the medium can extend the effects displacements gravity changes etc considerably and therefore lower pressure increases are required to model given observed effects the viscoelastic effects seem to depend mainly on the rheological properties of the layer zone where the intrusion is located rather than on the rheology of the whole medium we apply our model to the 1982 1984 uplift episode at campi flegrei modelling simultaneously the observed vertical displacement and gravity changes the results clearly show that for a correct interpretation of observed effects it is necessary to include the gravitational field in the anelastic theoretical models this factor can change the value and pattern of time dependent deformation as well as the gravity changes explaining cases of displacement without noticeable gravity changes or vice versa cases with uplift and incremental gravity values and other cases the combination of displacement and gravity changes is found to be especially effective in constraining the possible characteristics of the magmatic intrusion as well as the rheology of the medium surrounding it

mathematical modeling and numerical simulation of polythermal glaciers 1 a mathematical model for polythermal glaciers and ice sheets is presented the enthalpy balance equation is solved in cold and temperate ice together using an enthalpy gradient method to obtain a relationship between enthalpy temperature and water content we apply a brine pocket parameterization scheme known from sea ice modeling the proposed enthalpy formulation offers two advantages 1 the discontinuity at the cold temperate transition surface is avoided and 2 no treatment of the transition as an internal free boundary is required fourier s law and fick type diffusion are assumed for sensible heat flux in cold ice and latent heat flux in temperate ice respectively the method is tested on storglaciaren northern sweden numerical simulations are carried out with a commercial finite element code a sensitivity study reveals a wide range of applicability and defines the limits of the method realistic temperature and moisture fields are obtained over a large range of parameters

analysis of two color geodimeter measurements of deformation within the long valley caldera june 1983 to october 1985 line length changes from several baselines in a trilateration network within the long valley caldera clearly define a decrease in strain rate from june 1983 through october 1985 the data consist of more than 1600 length measurements on 23 baselines using a two color geodimeter which has a precision of 0 2 ppm of the line length initial measurements made during the summer of 1983 show extension rates were as high as 6 ppm yr on some baselines the rates subsequently decreased steadily to less than 1 ppm yr during the summer of 1985 ih contrast the strain rates on a few other baselines were less than 0 2 ppm yr averaged over the 2 3 year interval a model is constructed using these observations as well as yearly observations of a trilateration network within and near the caldera the model contains two points of inflation located at 5 and 10 km beneath the resurgent dome of the caldera plus dextral slip on a fault plane within the south moat within an elastic half space to mimic the observed time dependence in the two color geodimeter data we conclude that the rates of both slip and inflation have steadily decreased over the survey interval because the two color geodimeter network is closer to all three sources than the single color geodimeter network these data are important for defining the factor of 2 3 rate decrease of slip and inflation comparing our estimates of slip and inflation with similar estimates for the year prior to july 1983 we conclude that the rate decrease is probably a result of postseismic and postinflation relaxation following the earthquake swarm in the south moat in january 1983

volcanic eruption prediction magma chamber physics from gravity and deformation measurements one of the greatest remaining problems in modern volcanology is the process by which volcanic eruptions are triggered it is generally accepted that eruptions are preceded by magma intrusion sigurdsson and sparks 1978 the degree of interaction between previously ponded magma in a chamber and newly intruded magma determines the nature and rate of eruption and also the chemistry of erupted lavas and shallow dykes here we investigate the physics of this interaction volcano monitoring at its most effective is a synergy between basic science and risk assessment while hazard mitigation depends on reliable interpretation of eruption precursors the simple and much used mogi model relates ground deformation h to changes in magma chamber volume gravity changes g combined with ground deformation provide information on magma chamber mass changes our new models predict how the g h gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity thus by simultaneous measurement of deformation and gravity at a few key stations magma chamber processes can be identified prior to the onset of conventional eruption precursors

a test on a neuro fuzzy algorithm used to reduce continuous gravity records for the effect of meteorological parameters abstract gravity measurements are utilized at active volcanoes to detect mass changes linked to magma transfer processes and thus to recognize forerunners to paroxysmal volcanic events continuous gravity measurements are now increasingly performed at sites very close to active craters where there is the greatest chance to detect meaningful gravity changes unfortunately especially when used against the adverse environmental conditions usually encountered at such places gravimeters have been proved to be affected by meteorological parameters mainly by changes in the atmospheric temperature the pseudo signal generated by these perturbations is often stronger than the signal generated by actual changes in the gravity field thus the implementation of well performing algorithms for reducing the gravity signal for the effect of meteorological parameters is vital to obtain sequences useful from the volcano surveillance standpoint in the present paper a neuro fuzzy algorithm which was already proved to accomplish the required task satisfactorily is tested over a data set from three gravimeters which worked continuously for about 50 days at a site far away from active zones where changes due to actual fluctuation of the gravity field are expected to be within a few microgal after accomplishing the reduction of the gravity series residuals are within about 15 gal peak to peak thus confirming the capabilities of the neuro fuzzy algorithm under test of performing the required task satisfactorily

spherical and ellipsoidal volcanic sources at long valley caldera california using a genetic algorithm inversion technique abstract we model the second inflation period at long valley caldera california using a genetic algorithm technique and high quality geodetic measurements of elevation changes and baseline extensions we compare two source inversions for both spherical mogi point sources and the finite prolate ellipsoid of yang and davis a sensitivity analysis for the genetic algorithm is performed based upon synthetic data set inversions on similar sources in order to better constrain the areal location orientation and volume of the potential sources the spherical sources are well constrained the larger located at 9 9 km beneath the resurgent dome with a volume of 0 036 km 3 while the second at only 0 008 km 3 is located at a depth of 7 3 km beneath the south moat the depths to the ellipsoidal sources are switched with the larger source at a depth of 9 6 km and the smaller at 11 8 km with volumes of 0 037 and 0 002 km 3 respectively

deformation gravity and potential changes due to volcanic loading of the crust a method for calculating deformation gravity and potential changes induced by volcanic loads imposed within and on the earth s crust is presented the method involves application of techniques developed for earthquake loading problems in elastic gravitational earth models by considering the elastic gravitational equilibrium of the layered system the gravitational and displacement response due to intruding masses and to pressurized magma chambers can be computed for applications involving seamount loading variations in the height of sea level can be calculated by requiring that seawater lie along an equipotential in continental loading problems the change in potential can be used to calculate the orthometric height cahnge as distinct from the geometric height change for some plausible loading geometries the difference between these two qunatities can be significant results for various loads and earth structures are shown and discussed in particular a magmatic mass intruding into an elastic gravitational lithosphere overlying a compressible gravitating asthenosphere produces distinct gravity anomalies which depend on the depth of intrusion applications of this result can be made to problems of isostatic compensation the intrusion of a magmatic mass together with the associated pressurization of the magma chamber produces distinctive surface gravity and deformation in the near field gravity gradients more defined as surface gravity divided by uplift vary with distance from the source moreover orthometric elevations can be significantly 25 less than geometric elevations these results have clear application to data from events such as the posteruption deflation of kilauea volcano less

dynamics of the ice cap on king george island antarctica field measurements and numerical simulations king george island is located at the northern tip of the antarctic peninsula which is influenced by maritime climate conditions the observed mean annual air temperature at sea level is 2 4 c thus the ice cap is regarded as sensitive to changing climatic conditions ground penetrating radar surveys indicate a partly temperate ice cap with an extended water layer at the firn ice transition of the up to 700 m high ice cap measured firn temperatures are close to 0 c at the higher elevations and they differ considerably from the measured mean annual air temperature the aim of this paper is to present ice flow dynamics by means of observations and simulations of the flow velocities during several field campaigns from 1997 98 to 2008 09 ice surface velocities were derived with repeated differential gps measurements ice velocities vary from 0 7 m a 1 at the dome to 112 1 m a 1 along steep slopes for the western part of the ice cap a three dimensional diagnostic full stokes model was applied to calculate ice flow parameters of the numerical model were identified with respect to measured ice surface velocities the simulations indicate cold ice at higher elevations while temperate ice at lower elevations is consistent with the observations

displacement of surface monuments horizontal motion measurements of tectonic deformation depend on both accurate instrumentation and adequate coupling of the apparatus to the earth s crust existing techniques capable of resolving the signals of interest strain rates of 10 n yr are mainly observatory based the limitation in the baselength of these instruments 1 km results in a requirement that the reference monuments be exceptionally stable 10 tzm yr however records from pition flat observatory california show that the actual horizontal displacements for massive near surface monuments eraplaced in competent weathered granite are of the order of 50 tzm yr the low noise level of the strain measurements at this site indicates that this magnitude of monument displacement is abnormally small until high accuracy geodetic techniques are developed sophisticated monuments or monument monitoring devices will be necessary to record faithfully continuous crustal deformation

numerical simulations of the ice flow dynamics of george vi ice shelf antarctica a diagnostic dynamic thermodynamic ice shelf model is applied to the george vi ice shelf situated in the bellinghausen sea antarctica the george vi ice shelf has a peculiar flow geometry which sets it apart from other ice shelves inflow occurs along the two longest and almost parallel sides whereas outflow occurs on the two ice fronts that are relatively short and situated at opposite ends of the ice shelf two data sources were used to derive the ice thickness distribution conventional radio echo sounding from the british antarctic survey was combined with thickness inferred from surface elevation obtained by the nasa glas satellite system assuming hydrostatic equilibrium we simulate the present ice flow over the ice shelf that results from the ice thickness distribution the inflow at the grounding line and the flow rate factor the high spatial resolution of the ice thickness distribution leads to very detailed simulations the flow field has some extraordinary elements e g the stagnation point characteristics resulting from the unusual ice shelf geometry

a comparative modeling study of the brunt ice shelf stancomb wills ice tongue system east antarctica two diagnostic dynamic thermodynamic ice shelf models are applied to the brunt ice shelf stancomb wills ice tongue system located off caird coast coats land antarctica the brunt ice shelf stancomb wilis ice tongue system is characterized as a thin unbounded ice shelf with an atypical and highly heterogeneous structure in contrast to other ice shelves a composite mass of icebergs that calved at the grounding line and were then locked within fast sea ice exists between the fast moving stancomb wills ice stream and the slow moving brunt ice shelf we simulate the present flow regime of the ice shelf that results from the ice thickness distribution and the inflow at the grounding line with two different models and compare the model results with feature tracking and insar flow velocities we then incorporate two observed features a rift and a shear margin into the models with two different approaches and demonstrate the effects of variations in numerical values for the shear strength and viscosity in these zones on the simulated velocity field a major result is that both kinds of implementation of the rifts lead to similar effects on the entire velocity field while there are discrepancies in the vicinity of the rifts

radar interferometry and its application to changes in the earth s surface geophysical applications of radar interferometry to measure changes in the earth s surface have exploded in the early 1990s this new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times the resulting interferogram is a contour map of the change in distance between the ground and the radar instrument these maps provide an unsurpassed spatial sampling density 100 pixels km 2 a competitive precision 1 cm and a useful observation cadence 1 pass month 1 they record movements in the crust perturbations in the atmosphere dielectric modifications in the soil and relief in the topography they are also sensitive to technical effects such as relative variations in the radar s trajectory or variations in its frequency standard we describe how all these phenomena contribute to an interferogram then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments including the four satellites currently in orbit ers 1 ers 2 jers 1 and radarsat the next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement respecting the limits of the technique assessing its uncertainty recognizing artifacts and discriminating different types of signal we then review the geophysical applications published to date most of which study deformation related to earthquakes volcanoes and glaciers using ers 1 data we also show examples of monitoring natural hazards and environmental alterations related to landslides subsidence and agriculture in addition we consider subtler geophysical signals such as postseismic relaxation tidal loading of coastal areas and interseismic strain accumulation we conclude with our perspectives on the future of radar interferometry the objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it

the marine record of deglaciation of the south shetland islands antarctica since the last glacial maximum abstract the marine geological record of the glacial retreat history of the south shetland islands is derived from the integration of seismic data core lithofacies radiocarbon ages and geomorphological data from two study areas on the northern shelf we present one of the first published multibeam bathymetric images of the seafloor this new image combined with descriptions of new and existing short sediment cores and high resolution seismic data from the continental shelf north of king george island are used to map a grounding zone wedge that marks the minimum seaward extent of the south shetland ice cap during the last glacial maximum seismic data multibeam bathymetry and sediment cores from maxwell bay on the opposite side of king george island are used to provide further spatial constrains on the history of grounding and retreat of the ice cap along the southern margin of the south shetland islands during the last glacial maximum the south shetland ice cap extended onto the outer continental shelf some 50 km north of its present location the ice grounded on the shelf in troughs in present water depths of almost 400 m it had a minimum thickness of 570 m and was also grounded in the deepest part of fjords and straits between the islands to the south the ice cap extended seaward of the mouths of fjords and straits to the steep northern boundary of the bransfield strait sediment eroded from the fjords and straits was deposited in prominent fans that extend into the bransfield strait and show evidence for an ice shelf that covered the bransfield strait during the last glacial maximum a long drill core shaldril in maxwell bay and several jumbo piston cores yielded carbonate material used to constrain the timing and spatial extent of glacial retreat within maxwell bay the oldest glacimarine sediments in the fjord date back to 14 1 to 14 8 ka and suggest an initial deglaciation earlier than most previous studies of the south shetland islands upper maxwell bay was ice free by 9 1 ka and most of maxwell bay proper was ice free by 5 9 ka except for the smaller tributary fjords of maxwell bay these smaller tributary fjords were ice free by 1 7 ka but may have been ice free earlier but reoccupied by a neoglacial advance that ended approximately 1 7 ka

analysis of borehole seismograms from long valley california implications for caldera structure a three component seismometer was emplaced in a borehole in long valley caldera for a period of 2 months to record local earthquakes to the south mostly in the sierra nevada the seismic records were plotted in depth versus time sections to produce vertical hypocentral profiles for both the vertical and horizontal components the data collected and displayed in this way provide good resolution of the crustal structure at depth and avoid the attenuation and complications introduced by the near surface caldera fill the record sections were then modeled with two dimensional ray tracing to match the observed travel times although nonreversal of the profiles and uncertainties in hypocentral locations introduce ambiguities the final interpretation reveals two separate low velocity bodies which we have tentatively identified as magma beneath the resurgent dome in long valley the first of these is a small 2 km by 6 km body with a depth to top of approximately 3 7 km beneath the surface at the southern end of the resurgent dome the other body appears larger and lies at a depth of 5 5 km beneath the northern end of the resurgent dome

mechanisms of magma eruption and emplacement at mt etna between 1989 and 1992 the present eruption sequence of mt etna began during september 1989 with extensive fracturing and culminated in 1991 93 with the largest lava eruption this century1 here we present measurements of gravity and ground deformation which in conjunction with seismic data provide a detailed record of the processes occurring before and during eruption these reflect a complex interplay between magma injection into the paths of lowest compressive stress through the volcano flanks forcing elastic deformation and fracture growth in response to gravitational stresses we find that after the end of the eruptive activity in 1989 fractures in the south southeast flank of the volcano remained unfilled until some months before the 1991 93 eruption these results indicate the utility of microgravity monitoring as the precursory processes that we identify were largely undetected by seismic and ground deformation monitoring

the 1989 earthquake swarm beneath mammoth mountain california an initial look at the 4 may through 30 september activity abstract mammoth mountain is a 50 000 to 200 000 year old cumulovolcano standing on the southwestern rim of long valley in eastern california on 4 may 1989 two m 1 earthquakes beneath the south flank of the mountain marked the onset of a swarm that has continued for more than 6 months in addition to its longevity noteworthy aspects of this persistent swarm include 1 an exponential like increase in the rate of activity through the first month 2 a vertically oriented planar distribution of hypocenters at depths between 6 and 9 km with a north northeast strike roughly perpendicular to the average t axis orientation for the swarm earthquakes 3 recurring spasmodic bursts rapid fire sequences of similar sized earthquakes with overlapping coda and occasional earthquakes with enhanced low frequency energy 4 a uniform temporal distribution of the four largest m 3 events over the first 4 months of the swarm with a cumulative seismic moment for the entire sequence through 30 september corresponding to a single m 4 earthquake 5 a b value of 1 2 and 6 submicrostrain perturbations on the nearby borehole dilatometer the first of which led the onset of swarm activity by more than 2 weeks these aspects of the swarm together with its location along the southern extension of the youthful mono inyo volcanic chain which last erupted 500 to 600 years ago point to a magmatic source for the modest but persistent influx of strain energy into the crust beneath mammoth mountain

the subsurface structure of long valley caldera mono county california a preliminary synthesis of gravity seismic and drilling information long valley caldera is a 0 7 m y old volcanic eruptive center associated with an elliptical collapse structure on the eastern margin of the sierra nevada two dimensional models of caldera fill are presented in this study resulting in structures compatible with existing seismic refraction and gravity data modifications to previous seismic models include shifting the north caldera rim to the south and decreasing the dip of the west and south boundaries densities in the model are not varied independently but are based on empirical velocity density relationships and measured densities from surface outcrops and drill cores these density values are adequate to account for observed gravity anomalies indicating that the assumed subsurface lithologies are approximately correct the large gravity anomaly in the shallow eastern caldera is modeled by assuming that the area is underlain by a layer of exceptionally low density

tectonic evolution of the cuvette centrale zaire the cuvette centrale central basin of zaire is an infra cambrian to recent basin with up to 9 km of sedimentary section basin subsidence can be largely explained by a postulated late proterozoic failed rift and subsequent thermal relaxation seismic reflection profiles and well data reveal a palaeozoic development of the basin punctuated by two regional deformational episodes one in the early palaeozoic late cambrian and a second in the late palaeozoic late permian triassic both episodes result from ne sw contraction with the late palaeozoic episode being largely a reactivation of the early palaeozoic structures the timing of the two deformations correlates with collisional events at continental margins some distance from the congo basin in the early palaeozoic the deformation appears to be related to the late stages of the west congolian fold and thrust belt and coincides regionally with the pan african event sensu stricto the late palaeozoic deformation coincides with collisional tectonics along the southern margin of gondwana and the generation of the cape and sierra ventana foldbelts it is postulated that in both the early and late palaeozoic africa and gondwana experienced widespread intracontinental deformation generated by distant collisional processes much as central asia is experiencing today

the influence of sedimentation on metal accumulation and cellular oxidative stress markers in the antarctic bivalve laternula elliptica recent rapid climate warming at the western antarctic peninsula wap results in elevated glacial melting enhanced sedimentary run off increased turbidity and impact of ice scouring in shallow coastal areas discharge of mineral suspension from volcanic bedrock ablation and chronic physical disturbance are anticipated to influence sessile filter feeders such as the antarctic soft shell clam laternula elliptica king and broderip 1831 we investigated effects of sedimentary run off on the accumulation of trace metals and together with physical disturbance the cumulative effect on oxidative stress parameters in younger and older l elliptica from two stations in potter cove king george island antarctica which are distinctly impacted by turbidity and ice scouring fe mn sr v and zn concentrations were slightly higher in sediments of the station receiving more sediment run off but not enriched in bivalves of this station the only element that increased in bivalves experimentally exposed to sediment suspension for 28 days was mn concentration of the waste accumulation biomarker lipofuscin in nervous tissue was higher in l elliptica from the exposed compared to the less exposed site whereas protein carbonyl levels in bivalve mantle tissue were higher at the less sediment impacted site tissue metal content and lipofuscin in nervous tissue was generally higher in older compared to younger individuals from both field stations we conclude that elevated sediment ablation does not per se result in higher metal accumulation in l elliptica instead of direct absorbance from sediment particles metal accumulation in gills seems to indicate uptake of compounds dissolved in the water column whereas metals in digestive gland appear to originate from enriched planktonic or detritic food accumulation of cellular waste products and potentially reactive metals over lifetime presumably alters laternula elliptica physiological performance with age and may contribute to higher stress susceptibility in older animals

modeling gravity and trilateration data in long valley california 1983 1984 during the period july 1982 through july 1984 a series of repeated high precision gravity measurements have been undertaken in and around long valley california data were collected at about 40 sites using three lacoste romberg model g gravity meters at the same time the u s geological survey was conducting a program of high precision line length measurements throughout the same area significant changes were observed with both types of data during the intervals 1982 1983 and 1983 1984 previous work by us has shown that deformation data during the interval 1975 1983 are consistent with a model for the inflation of long valley caldera which includes magma injection beneath the south central caldera at a depth of only 5 km simultaneous inversion of the trilateration and gravity data for the period 1983 1984 shows that most of the previous model is still reasonably consistent with the data and implies injected volumes of between 0 01 and 0 02 km3 additionally uplift during this period occurred along the inyo domes chain of vents and flows indicating perhaps a new source of inflation in the northwest part of the caldera this uplift amounts to a few centimeters and implies a volume increase of perhaps 0 004 km3 at reasonably shallow depth 3 km the total amount of uplift in the northwest is small in comparison to the total uplift centered on the central resurgent dome total vertical displacement well in excess of half a meter for purposes of comparison we examine several models one of which involves injection at the base of the crust this model implies much higher injected volumes as much as 0 15 0 2 km3

the interpretation of gravity changes and crustal deformation in active volcanic areas simple models like the well known point source of dilation mogi s source in an elastic homogeneous and isotropic half space are widely used to interpret geodetic and gravity data in active volcanic areas this approach appears at odds with the real geology of volcanic regions since the crust is not a homogeneous medium and magma chambers are not spheres in this paper we evaluate several more realistic source models that take into account the influence of self gravitation effects vertical discontinuities in the earth s density and elastic parameters and non spherical source geometries our results indicate that self gravitation effects are second order over the distance and time scales normally associated with volcano monitoring for an elastic model appropriate to long valley caldera we find only minor differences between modeling the 1982 1999 caldera unrest using a point source in elastic homogeneous half spaces or in elasto gravitational layered half spaces a simple experiment of matching deformation and gravity data from an ellipsoidal source using a spherical source shows that the standard approach of fitting a center of dilation to gravity and uplift data only excluding the horizontal displacements may yield estimates of the source parameters that are not reliable the spherical source successfully fits the uplift and gravity changes overestimating the depth and density of the intrusion but is not able to fit the radial displacements

a peritidal evaporite environment in the neoproterozoic of south gabon schisto calcaire subgroup nyanga basin the neoproterozoic 630 ma nsc3 or sciii formation schisto calcaire subgroup of the nyanga basin is a muddy peritidal carbonate dolomite shelf succession that is exposed in the old mouila quarry keri quarry south gabon its 20 m thick lower member is dominated by microbially laminated dolomites and domal stromatolites representing shallow subtidal to supratidal environments abundant pseudomorphs after gypsum and anhydrite relicts indicate carbonate precipitation in sulphate rich hypersaline environments dolomitization of the muddy microbial sediments probably occurred by the evaporite reflux of groundwaters silica precipitates constitute the ultimate diagenetic phase dolomites of the nsc3 formation retain a pattern of oxygenic isotopic variation that reflects the depositional environment shallow subtidal microbialites and stromatolites record the lightest values 18o 5 1 consistent with the isotopic values of neoproterozoic marine waters whereas strongly dolomitized facies display heavier oxygen isotope values 18o 1 3 with the heaviest values in subaerially exposed caps enrichment of 3 8 fungal hyphae and spores have been discovered in these upper parts the 18o pattern through the plurimetric recognized sequences thicknesses from 2 to 6 m suggests a constant exposure in a sabkha like environment although the timing of the series is poorly constrained the c isotopic values of the nsc3 carbonates with values of 2 and 3 approximate depositional conditions minor variations in c isotopes have been observed the evolution of the succession from the stromatolitic lower member to the oolitic upper member of the nsc3 formation could record the progradation of a low energy evaporitic tidal flat that migrated seaward from oolitic shoal crests

geochronological correlation of the late precambrian sequences on and around the stable zones of equatorial africa abstract very few radiometric age determinations have a bearing on the late precambrian sequences of the stable zones of equatorial africa a geochronological correlation of these sequences must therefore rest upon a compilation of ages from these stable zones and the corresponding mobile zones about 15 ages of varying precision more or less well tied up with the stratigraphical sequences together with results of the study of stromatolite assemblages allow a provisional correlation to be made one conclusion of this correlation is that most mixtites surrounding the congo basin are in a 950 50 ma age bracket and are presumably to be correlated

geometry and thermal regime of the king george island ice cap antarctica from gpr and gps king george island is the largest of the south shetland islands close to the tip of the antarctic peninsula the annual mean temperature on the island has increased by 1 c during the past three decades and the ice cap that covers the majority of the island is sensitive to climatic change we present data from two field campaigns 1997 and 2007 700 km of global positioning system gps and ground penetrating radar gpr profiles were collected on arctowski icefield and on the adjacent central part the data were analysed to determine the surface and bed topography and the thermal regime of the ice average ice thickness is 250 m and maximum thickness is 420 m the gpr profiles show isochrones throughout the ice cap which depict the uparching of raymond bumps beneath or close to the ice divides a water table from percolation of meltwater in the snowpack shows the firn ice boundary at 35 m depth the firn layer may be temperate due to the release of latent heat in the area below 400 m a s l backscatter by water inclusions is abundant for ice depths below the water table we interpret this as evidence for temperate ice scatter decreases significantly above 400 m ice temperatures below the water table in this part of the ice cap are subject to further field and modelling investigations

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

proterozoic first order sedimentary sequences of the sao francisco craton eastern brazil abstract the sao francisco craton in eastern brazil hosts sedimentary sequences deposited between the paleoarchean 3300 ma and late neoproterozoic 580 ma proterozoic successions occurring in this region comprise five 1st order sedimentary sequences which besides episodes of global significance record major basin forming events the ca 8000 m thick minas itacolomi 1st order sequence exposed in the brazilian mining district of the quadrilatero ferrifero and containing as marker bed the lake superior type caue banded iron formation tracks the operation of a wilson cycle in the paleoproterozoic era the quartz arenite dominated espinhaco i and ii sequences record at least two major rift sag basin forming events which affected the host continent of the sao francisco craton at around 1 75 ga and 1 57 ga the macaubas sequence and its correlatives in the extracratonic domains witness the individualization of a sao francisco congo plate in synchronicity with the break up of rodinia in the cryogenian period the sao francisco congo plate together with various fragments derived from rodinia reassembled to form gondwana in the ediacaran period in the course of the gondwana amalgamation orogenic belts developed along the margins of the craton its interior converted into foreland basins received the shallow water carbonates and pelites of the bambui 1st order sequence and its correlatives

principal component analysis of geodetically measured deformation in long valley caldera eastern california 1983 1987 typical geodetic measurements of deformation consist of repeated surveys of a particular geodetic network such deformation data can be interpreted as a consequence of one or more self coherent sources by means of principal component analysis a self coherent source is defined as any source that produces deformation that is time and space separable principal component analysis then gives the time and space factors that characterize the deformation attributed to each self coherent source geodetic measurements of deformation at long valley caldera provide two examples of the application of principal component analysis a 40 line trilateration network surrounding the caldera was surveyed in midsummer 1983 1984 1985 1986 and 1987 principal component analysis indicates that the observed deformation can be represented by a single coherent source the time dependence for that source displays a rapid rate of deformation in 1983 1984 followed by less rapid but uniform rate in the 1984 1987 interval the spatial factor seems consistent with expansion of a magma chamber beneath the caldera plus some shallow right lateral slip on a vertical fault in the south moat of the caldera an independent principal component analysis of the 1982 1983 1984 1985 1986 and 1987 leveling across the caldera requires two self coherent sources to explain the deformation the deformation pattern produced by the larger of these two sources appears to be roughly consistent with that found from the trilateration data the deformation due to the second source is a nearly uniform tilt in the uplift profile presumably that tilt is simply an artifact of systematic error in the leveling

mayon volcano philippines change of monitoring strategy after microgravity and gps measurements from 1992 to 1996 abstract mayon volcano is part of the bicol volcanic chain on the island luzon philippines during this century there were ten activity periods distributed almost regularly because of the density of population about one million people living in the vicinity of the volcano three seismological observatories are in operation measurements of gravity changes started in 1992 just before the eruption of february march 1993 two profiles at the slope were established connected to a regional network around the volcano in order to enable the determination of mass changes between the campaigns the height control was provided by parallel gps measurements in all the network consists of 26 points which were remeasured with three gravimetres at least three times within one campaign during five campaigns within 4 years the differential gps gives no significant changes of the elevation within 4 cm nevertheless the gravity increased significantly by up to 1500 nm s2 equivalent to 150 gal as no significant change of elevation is observed gps no extended shallow magma chamber system below the volcano can be proved this is in accordance with geochemical results indicating a rather undifferentiated magma the youngest lava which is of interest for the eruption dynamics belongs to the medium k basaltic andesite field of the k2o vs sio2 diagram a rather qualitative check of groundwater level changes reveals that these cannot be the sources for the observed gravity changes thus the increase of gravity after the eruption of february 1993 can be explained by a mass redistribution in the volcanic vent from above the level of the gravity points to below practical conclusions of these results lead to changes in the monitoring strategy deformation measurements did not reveal any volcanic activities at least for the eruption of 1993 no significant deformation was observed gravity could be an indicator for long term changes thus repeated gravity measurements gps at selected points could be used in parallel to seismic monitoring to detect slow mass movements prior to changes in seismicity

the effect of collapse structures on ground deformations in calderas ground deformations related to unrest episodes in calderas are generally interpreted like in other volcanic environments in terms of increased pressure within a magma chamber embedded in a continuous elastic medium in this framework the depth of the pressure source is inferred from the size of the deformed area this scheme works quite well for individual volcanoes for instance hawaiian shields where a good correspondence between inflation events and eruptive episodes has been observed ground deformation in calderas however show unusual features that are difficult to interpret within such a scheme because considerable positive deformation in the order of several meters and microfracturing i e intense seismicity may occur without eruptions altough the estimated depth for the magma source is sometime very shallow less than 1 2 km it has been recently suggested that the contact zones at the border of collapsed calderas could act as stress strain discontinuity zones and thus bias modeling results obtained for continuous elastic media we use both observations and theoretical modeling based on 3 d finite element techniques to show that ground deformations in collapsed calderas are strongly influenced by the caldera structure giving a new perspective in related geological and geophysical observations in such areas

shallow and peripheral volcanic sources of inflation revealed by modeling two color geodimeter and leveling data from long valley caldera california 1988 1992 we refined the model for inflation of the long valley caldera near mammoth lakes california by combining both geodetic measurements of baseline length and elevation changes baseline length changes measured using a two color geodimeter with submillimeter precision revealed that the resurgent dome started to reinflate in late 1989 measurements between late 1989 and mid 1992 revealed nearly 13 cm of extension across the resurgent dome geodetic leveling surveys with approximately 2 mm precision made in late 1988 and in mid 1992 revealed a maximum of about 8 cm of uplift of the resurgent dome two ellipsoidal sources satisfy both the leveling and two color measurements whereas spherical point sources could not the model s primary inflation source is located 5 5 km beneath the resurgent dome with the two horizontal axes being nearly equal in size and the vertical axis being 4 times the length of the horizontal axes a second ellipsoidal source was added to improve the fit to the two color measurements this secondary source is located at a depth between 10 and 20 km beneath the south moat of the caldera and has the geometry of an elongated ellipsoid or pipe that dips down to the northeast in addition the leveling data suggest dike intrusion beneath mammoth mountain during the 1988 1992 interval which is likely associated with an intense swarm of small earthquakes during the summer of 1989 at that location our analysis shows the dike intrusion to be the shallowest of the three sources with a depth range of 1 3 km below the surface to the top of the intrusion

joint interpretation of displacement and gravity data in volcanic areas a test example long valley caldera california volcanic activity produces deformation and gravity changes that many times can be used as precursors of future eruptions applying geodetic techniques to monitoring activity involves interpretation using deformation models usually gravity change data and displacement data are interpreted separately we show using modeling of deformation and gravity change data in long valley caldera california usa that this can lead to incorrect interpretations the results obtained show that displacements and gravity changes must be interpreted together whenever possible and that elastic gravitational models can be a far more appropriate approximation to problems of volcanic load in the crust than the more commonly used purely elastic models therefore it is necessary to change the philosophy normally used to interpret geodetic observations improving the possibility of predicting future eruptions

carbonate turbidite sequences deposited in rift basins of the jurassic tethys ocean eastern alps switzerland syn rift sediments in basins formed along the future southern continental margin of the jurassic tethys ocean comprise in the eastern alps of switzerland up to 500 m thick carbonate turbidite sequences interbedded with bioturbated marls and limestones in the fault bounded troughs no submarine fans developed in contrast the fault scarps acted as a line source and the asymmetric geometry as well as the evolution of the basin determined the distribution of redeposited carbonates the most abundant redeposits are bio and lithoclastic grainstones and packstones with sedimentary structures indicating a wide range of transport mechanisms from grain flow to high and low density turbidity currents huge chaotic megabreccias record catastrophic depositional events their main detrital components are upper triassic shallow water carbonates and skeletal debris from nearby submarine highs after an event of extensional tectonism sedimentary prisms accumulated in the basins along the faults each prism is wedge shaped with a horizontal upper boundary and consists of a thinning and fining upward megacycle within each megacycle six facies associations are distinguished at the base of the fault scarp an association of breccias was first deposited by submarine rockfall and rockfall avalanches a narrow approximately 4000 m wide depression along the fault was subsequently filled by the megabreccia association in which huge megabreccias interfinger with thin bedded turbidites and hemipelagic limestones the thick bedded turbidite association covered the megabreccias or formed farther basinward the base of the sedimentary column within the thick bedded turbidites thinning and fining upward cycles are common the overlying thin bedded turbidite association shows nearly no cyclicity and the monotonous sequence of fine grained calciturbidites covers most of the basin area with continuous filling and diminishing sediment supply a basin plain association developed comprising fine grained and thin bedded turbidites intercalated with bioturbated marls and limestones on the gentle slopes opposite the fault escarpment redeposited beds are scarce and marl limestone alternations as well as weakly nodular limestones prevail

kinematic evolution of the aracuai west congo orogen in brazil and africa nutcracker tectonics during the neoproterozoic assembly of gondwana abstract the neoproterozoic aracuai west congo a wc orogen is one of many brasiliano pan african orogens that developed during the assembly of west gondwana this orogen was split apart in mesozoic time due to opening of the south atlantic the aracuai orogen now underlies eastern brazil whereas the west congo belt fringes central africa s atlantic coast significantly at the time it formed the a wc orogen was bounded on the west north and east by the sao francisco congo craton a crustal block that had the shape of a lopsided upside down u thus the orogen was partially confined during tectonism in that it occupied an enclave surrounded on three sides by cratonic crust formation of the a wc orogen resulted in kinematically complex deformation substantial crustal shortening and production of a large volume of magma how such features could develop in this particular setting has long been a mystery our field studies in the aracuai orogen together with published data on the west congo belt characterize the kinematic picture of the a wc orogen and lead to a tectonic model that addresses its evolution in our model the a wc orogen formed in response to closure of the macaubas basin this basin was underlain by oceanic crust in the south but tapered northward into a continental rift which terminated against the cratonic bridge linking the eastern and western arms of the sao francisco congo craton closure occurred when the western arm now the sao francisco craton rotated counterclockwise towards the eastern arm now the congo craton this closure may have been driven by collision of the paranapanema amazonian and kalahari cratons against the external margins of the sao francisco congo craton rather than by slab pull associated with subduction of the macaubas basin s floor thus the process of forming the a wc orogen resembled the process of crushing of a nut between two arms of a nutcracker such nutcracker tectonics led to a series of kinematically distinct deformation stages initially internal portions of the orogen flowed northwards then substantial crustal thickening occurred in the orogen s interior and the deformation front migrated outwards producing thrust belts that overlapped the internal margins of the craton with continued closure space in the enclave became restricted and the orogen s interior underwent lateral escape to the south late stage extensional collapse triggered both production of late to post collisional granites and exhumation of high grade rocks from mid crustal levels

evolution sedimentaire et tectonique du bassin proterozoique superieur de comba congo stratigraphie sequentielle du supergroupe ouest congolien et modele d amortissement sur decrochements dans le contexte de la tectogenese panafricaine abstract the comba trough infilled by the upper proterozoic west congolian supergroup trends n60 e and cuts across the congo zaire archean craton the trough is perpendicular to the nyanga niari basin which itself represents the foreland of the pan african mayombe belt moreover the comba trough was connected to other troughs extending further east beneath the mesozoic cenozoic of the congolian basin the tectono sedimentary evolution of the comba basin was initiated between 950 and 700 ma the trough was formed by a nw se extension it was filled up by periglacial fluvio lacustrine sediments which correspond to the deltaic formations of the bouenza group and the upper diamictite tillite superieurerd respectively then a general climatic warm up produced a rise of the eustatic sea level which flooded the lake of comba as well as the lakes and shelves of the congo zaire craton this marine sedimentation is represented by the schisto calcaire group which in terms of sequence stratigraphy we interpret as a transgressive system tract red marls of the lower part of the scia facies a condensed section pink dolomite of the upper part of the scia facies a highstand system tract deposited along a carbonate ramp the latter corresponds to a lateral facies variation and not a vertical facies succession as it was classically assumed this highstand system tract was composed of an external shelf scib facies a barrier scic facies a lagoon scii facies a beach kisantu oolithe of the sciiia facies and sciiib facies a tidal flat sciiic facies with fluvial sciv facies and tidal niari breccia facies channels between 700 and 600 ma a first compressive phase was responsible further west for the eastward pan african mayombian thrusting this phase was accompanied by the deposition of a thick molassic formation mpioka formation on the foreland belt and extending into the comba basin a late shortening phase of the mayombian belt propagated into the comba basin which was affected by strike slip deformation along its southern edge this strike slip deformation induced important disruptions of the regional mayombian tensor and a particular structural pattern of the comba basin characterized by strike slip damping structures sections of these structures are interpreted as positive negative and dual flowers this particular structural pattern provides an explanation of the combian structures as they reflect local deviation of the pan african shortening axes as a conclusion the evolutionary stages of the comba basin are discussed in the wider context of the upper proterozoic of the congo zaire craton

unrest at campi flegrei a contribution to the magmatic versus hydrothermal debate from inverse and finite element modeling 1 we present results from the modeling of ground deformation and microgravimetric data recorded at campi flegrei in order to assess the causative phenomena of caldera unrest between 1981 and 2001 we find that residual gravity changes during ground uplift 1982 1984 are indicative of mass changes in a hybrid of magmatic and hydrothermal sources during deflation between 1985 and 2001 the inversion of gravity residuals for a single source does not provide convincing results we then performed the joint inversion of gravity and deformation data for multiple spherical sources and refined source parameters by finite element modeling in order to mitigate against limitations of the analytical solutions the data recorded during inflation and rapid deflation may be best explained by mass and pressure changes in a deep magmatic source at about 5 km depth and a shallow 2 km deep hydrothermal source both sources contribute equally to the gravity changes observed between 1982 and 1984 the contemporary uplift appears to be mainly caused by the shallow source the subsequent deflation is dominated by a pressure decrease in the hydrothermal source the magmatic source contributes chiefly to the observed gravity changes pressure and density variations within multiple shallow seated hydrothermal sources provide acceptable fits to the deflation and accompanying gravity changes recorded since 1988 these shallow level dynamics also appear to trigger spatially and temporarily random short term reversals of the overall mode of ground subsidence since 1985 our analysis does not support the idea of magmatic contributions to these short lived periods of inflation

three dimensional indirect boundary element method for deformation and gravity changes in volcanic areas application to teide volcano tenerife canary islands 1 most deformation models of volcanoes assume that the earth is a linear elastic isotropic and homogeneous half space although some volcanic areas are associated with significant relief we investigate the effects of topography on surface deformation and gravity changes caused by a magma intrusion in the earth s crust a three dimensional 3 d indirect boundary element method ibem that incorporates realistic topographic features is developed in order to perform this analysis our results show that the topography alters both the magnitude and pattern of the deformation and gravity signal as an example of realistic topography we consider a spherical source of dilatation located at 4 km depth below teide volcano summit tenerife canary islands in order to simulate the deformation and gravity changes that could be observed at tenerife if a hypothetical intrusion occurred in the volcanic system this approach gives a picture of the 3 d topographic effect at teide that can provide insight in order to improve the geodetic monitoring of the volcano

evidence for a neoproterozoic carbonate ramp on the northern edge of the central african craton relations with late proterozoic intracratonic troughs during late proterozoic times the archaean central african craton was affected by trough faulting which led to the formation of grabens the sangha aulacogen being the main structure of this type in the studied area this transverse basin connects with other basins on the northern and south western borders of the craton during the cryogenian this network of basins was filled with fluvio deltaic and lacustrine periglacial deposits the glacio eustatic transgression in neoproterozoic iii end proterozoic times flooded extensive areas of shelf on the northern edge of the craton leading to the development of carbonate sedimentation in a broad outer shelf environment associated with nearshore barriers and evaporitic lagoons these facies are similar to those developed in the west congolian schisto calcaire shale limestone ramp succession the north central african ramp succession sediment slope contains an example of tidal rhythmites in vertical accretion which occurs beneath the barrier deposits on the subtidal outer shelf mathematical analysis of the bedding pattern yields a period of 29 30 days for the lunar month a result which is in agreement with astrophysical evidence for this epoch i e 650 ma ago major subsidence and seismic activity on this gently sloping platform associated with the proximity of the sangha aulacogen caused the triggering of carbonate turbidites and mass flow deposits the proliferation of microbial mats under euphotic conditions on an extensive shelf led to the build up of a carbonate platform during early neoproterozoic iii times the west congolian and north central african ramps prograded northwards and southwards respectively into the sangha aulacogen the sea at that time was restricted to a long graben like basin while a remaining area of marine sedimentation persisted into the palaeozoic thus the pattern of end proterozoic carbonate sedimentation on the borders of the central african craton can be interpreted in terms of an overall gently sloping ramp model with progradation converging towards the sangha aulacogen

attenuation of p and s waves in a magma chamber in long valley caldera california shallow earthquakes around the southwest boundary of long valley caldera west of the hilton creek fault are characterized by lack of s waves at regional seismic network stations to the northwest north and northeast and p waves for these same station event combinations are deficient in frequencies higher than about 2 3 hz earthquakes east of the hilton creek fault and southeast of the caldera have normal p and s wave signatures at the same stations these effects are explained by propagation through a magma chamber in the south central part of long valley caldera at depth greater than 7 8 km

calving of fuerza aerea glacier greenwich island antarctica observed with terrestrial laser scanning and continuous video monitoring a short term series of quantitative observations of calving activity of fuerza aerea glacier greenwich island the south shetland islands antarctica was conducted in order to test new methods of monitoring calving the volume of single calving events was quantified by combining terrestrial laser scanning tls surveys with continuous video recording of the ice front an empirical formula for area volume scaling of the calved ice block was proposed based on the tls measured calved ice volume and the calved ice front area obtained by manual delineation on the images acquired with the video camera this combination of methods proves to be a valuable tool for glacier monitoring providing both high temporal resolution and precise quantitative measurements of the calving volume the size distribution of calving events is best approximated by a power law and within the short period of observations 14 d calving was found to be an intrinsic process not dependent on environmental forcings over the period of 21 january 04 february 2013 the ice flow velocity at the terminus of fuerza aerea glacier was 0 26 0 07 m d 1 and the calving rate was 0 41 0 07 m d 1

effects of geothermal development on deformation in the long valley caldera eastern california 1985 1994 long valley caldera in east central california has been the site of crustal unrest in the form of seismicity and ground deformation in response to magmatic inflation since 1980 uplift of the resurgent dome has totaled 0 6 m over the 1975 1992 period within this region of uplift and near the southwestern edge of the resurgent dome is the casa diablo area which experienced relative subsidence of 0 17 m between 1985 and 1992 geothermal fluid production began at casa diablo in 1985 to supply a binary electric power plant currently three such plants generate a total of about 40 mw of electricity the plants are supplied by 170 c water pumped from depths near 150m all the produced water is reinjected at cooler temperatures at depths near 600 m analyses of data from 1 regional leveling lines and a network of bench marks in the geothermal well field area 2 tilt observations from an l shaped array of bench marks and 3 calculations relating pressure and temperature changes to subsidence indicate that relative subsidence at casa diablo results mainly from reductions in pressure in the shallow production reservoir and overlying formations and reductions in temperature in the underlying injection reservoir secondary effects on deformation are caused by offsets along the taylor bryant fault bounding the west side of the well field

modeling of stress changes at mayon volcano philippines in this paper we analyze the stress change associated with the inverted volcanic source models at mayon volcano philippines where there are gravity changes without significant deformation after the 1993 eruption we detail the applicable data and the associated inversion techniques and models prior to calculating the appropriate stress changes it is determined that the stress change associated with the central magmatic source produces compressional stress changes at a secondary source to the northwest prompting a change in the local water storage in the underlying fractured rock

principles of drainage basin analysis from multisource data application to the structural analysis of the zaire basin abstract classifications which attempt to combine descriptive and genetic aspects of types of drainage basins must take into account the application of new techniques satellite imagery high precision topographic maps and digital terrain models in addition the use of new concepts such as neotectonics and drainage basin history will lead to a reassessment of classification schemes after a review of current classifications we propose a genetic scheme that is based on the development of drainage networks and which includes integration of local and regional scale perturbations state of the art methods of drainage basin analysis are applied to the zaire basin to provide new information on deep structures the influence of basin and block tectonics is revealed in the context of major late proterozoic shear faults which have been rejuvenated up to the present day this is demonstrated from an analysis of the drainage network in the light of geological and geophysical observations

genesis and evolution of unrest episodes at campi flegrei caldera the role of thermal fluid dynamical processes in the geothermal system we develop a model for describing water flow in a porous medium under the effect of thermal and pressure gradients the model simulates geothermal systems in calderas given the boundary conditions and the fluid dynamical properties of the medium the model allows computation in fluid dynamical stationary states of parameters characterizing the flow such as flow velocity and temperature and pressure distributions at depth the model is applied to investigate the effects of the local geothermal system on the unrest episodes at campi flegrei caldera using experimentally determined fluid dynamical parameters for the caldera rocks we show that changes of water flow in shallow aquifers under the effect of pressure and or temperature variations within the geothermal system can be very important in the genesis and evolution of unrest crises in particular they can strongly amplify the effect of pressure increase in the magma chamber on ground uplift they can also explain the timescales of evolution of ground movements in terms of transit times of the water front and of the connected temperature fronts due to advective transport on such grounds an integrated mechanic thermal fluid dynamical model was built allowing us to give a semiquantitative global explanation to the genesis and evolution of unrest phenomena results obtained here can be generalized to other similar calderas

evolution of early proterozoic passive margin carbonate platform rocknest formation wopmay orogen northwest territories canada abstract the superbly exposed rocknest formation wopmay orogen northwest territories canada is an early proterozoic 1 89 ga carbonate platform that formed along the eastern passive margin of slave craton archean the palinspastically restored shelf sequence is an eastward thinning prism 0 1 100 m thick extending for over 220 km parallel to strike and over 200 km perpendicular to strike it contains from west to east slope outer shelf shoal complex and inner shelf facies slope facies are carbonate rhythmites megabreccias and platy rhythmite breccias outer shelf facies include stromatolitic reefal boundstone and backreef ooid intraclast grainstore shoal complex facies are shallowing upward cycles of tufa cryptalgalaminite laminated dolosiltite lutite overlain by tepees b eccias and pisolite these formed a paleotopographic high restricting an eastern lagoon from western ocean inner shelf facies are dominantly asymmetric shallowing upward cycles of lagoonal mixed carbonates and siliciclastics that grade into intertidal stromatolitic dolomites cyclic facies pass eastward into noncyclic siliciclastics of central and eastern lagoon the rocknest lagoon spanned the width of slave craton and was bounded by a western carbonate shoreline shoal complex and by a tectonically active eastern siliciclastic shoreline the rocknest carbonate shelf developed on an underlying siliciclastic shelf an initial ramp rapidly evolved into a progradational accretionary rim and shoal complex followed by vertical aggradation of the rim concurrently the shoal complex underwent numerous west to east progradational events to produce many shallowing upward cycles in the shelf interior cyclic sedimentation was interrupted during three events of incipient shelf drowning associated with local backstepping of the rim and noncyclic sedimentation over most of the shelf probably caused by fourth order eustatic sea level oscillations the shelf was terminally drowned and overlain by deep water synorogenic siliciclastics during attempted subduction of the margin carbonates capping the shelf are stromatolitic patch re fs which overlie a possible unconformity suggesting emergence of the shelf prior to drowning

surface velocity and ice discharge of the ice cap on king george island antarctica glaciers on king george island antarctica have shown retreat and surface lowering in recent decades concurrent with increasing air temperatures a large portion of the glacier perimeter is ocean terminating suggesting possible large mass losses due to calving and submarine melting here we estimate the ice discharge into the ocean for the king george island ice cap l band synthetic aperture radar images covering the time span january 2008 to january 2011 over king george island are processed using an intensity tracking algorithm to obtain surface velocity measurements pixel offsets from 40 pairs of radar images are analysed and inverted to estimate a weighted average surface velocity field ice thicknesses are derived from simple principles of ice flow mechanics using the computed surface velocity fields and in situ thickness data the maximum ice surface speeds reach 225 m a 1 and the total ice discharge for the analysed flux gates of king george island is estimated to be 0 720 0 428 gt a 1 corresponding to a specific mass loss of 0 64 0 38 m w e a 1 over the area of the entire ice cap 1127 km 2

late precambrian tidal rhythmites in south australia and the history of the earth s rotation sedimentary rhythmites of siltstone and fine sandstone from late precambrian c 650 800 ma glaciogenic formations in south australia are interpreted as distal ebb tidal deposits that record variability in the velocity and range of palaeo ebb tides variations in lamina thickness encode a full spectrum of palaeotidal cycles including semidiurnal diurnal fortnightly and monthly tidal cycles as well as the lunar apsides perigee and nodal cycles a half yearly oscillation is attributable largely to a beat between the fortnightly tidal cycles of luni solar conjunction and lunar declination the lunar nodal cycle is discernible as an amplitude modulation of this beat oscillation the data allow determination of the earth9s palaeorotation and the past dynamics of the earth moon system with an accuracy previously unattainable for the precambrian the late precambrian c 650 ma year contained 13 1 0 5 lunar months and c 400 20 days and the late precambrian lunar month c 30 5 1 5 days these values suggest an average equivalent phase lag near 3 since late precambrian time rather than the present value of 6 the period of 19 5 0 5 years determined for the lunar nodal cycle c 650 ma ago indicates a lunar distance 96 9 1 7 of the present distance the low rate of lunar recession since late precambrian time revealed by the rhythmite data militates against a close approach of the moon during the proterozoic precambrian sedimentary rhythmites may hold a key to the early history of the earth9s rotation

location and configuration of magma bodies beneath long valley california determined from anomalous earthquake signals recordings of 281 small earthquakes which occurred recently near the southern edge of long valley caldera at depths from 1 to 15 km are used to map the subsurface geometry of shear wave attenuating bodies in the caldera signatures of these events recorded northwest north northeast and east of long valley with ray paths through the caldera are often anomalous in that s wave arrivals have very low amplitudes and high frequency p and s wave energy is missing for the same station event combinations the volcanic and geothermal history of the region suggests that ray paths for these anomalous signals have passed through magma which absorbed much of the s wave energy and also attenuated the higher frequencies with over 1200 normal and anomalous ray paths through the caldera we have located two apparently massive magma bodies in the central and northwest caldera as well as two more dispersed magma areas in the southern caldera and beneath crowley lake the central magma body extends from 4 5 to at least 13 km beneath the surface and the northwest body is as shallow as 5 5 km these two magma bodies are probably connected below about 8 km secondary arrivals seen on select seismograms are tentatively interpreted as reverberations inside the central magma chamber and are consistent with the interpreted lateral extent of the magma the northwest and central magma bodies lie beneath the northwest and southeast corners of the caldera s resurgent dome and probably connect beneath the dome s center also the shallower parts of these magma bodies and the southern anomalous area lie beneath the surface expression of the medial graben faults suggesting that structural features seen at the surface of the valley may be related to magma location at depth

origin and evolution of the late precambrian high grade yaounde gneisses cameroon abstract the yaounde series is composed of low to high grade garnet bearing schists and gneisses belonging to the pan african north equatorial fold belt the high grade rocks consist of kyanite garnet gneisses and garnet plagioclase gneisses containing layers of calcsilicate rocks marbles quartzites and magnetite rich orthopyroxenites the chemical patterns of these rocks are those of a sedimentary sequence of shales and greywackes and minor dolomite rocks dolomitic marls evaporitic beds quartzites and iron rich sediments some volcanogenic greywackes may have been of alkaline affinity this sequence was probably deposited in shallow water near shore semi barred conditions which can be related either to an intracontinental distensive environment or to a passive margin at the northern edge of the congo craton the yaounde gneisses were deformed during two main tectonic phases the first phase d1 is marked by a layering s0 s1 resulting from tectonic transposition the second one d2 corresponds to tangential tectonics with isoclinal folds and flat lying shear zones associated with a s2 schistosity and a l2 lineation a third phase late d2 is marked by the development of low temperature mylonitic shears scarce wrench faults indicate later e w compression the d1 d2 transition is characterized by the emplacement of mafic and ultramafic rocks and by the development of high pressure granulite facies conditions t 750 800 c p 10 12 kbar these peak metamorphic conditions triggered a widespread migmatization dated at 565 22 ma and coeval with the d2 event the gradual decrease of the metamorphic conditions toward the south and the existence of a syn d2 retrograde inverse metamorphic gradient suggest that the overall structure of the yaounde series is a large tectonic nappe thrust onto the congo craton a geotectonic scheme for the north equatorial fold belt is discussed