pollen record of the past 60 ka bp in the middle okinawa trough terrestrial provenance and reconstruction of the paleoenvironment abstract core dgks9602 931 cm in length was retrieved from the middle okinawa trough the age model was correlated with the adjacent core dgks9603 li et al 2001 for the upper part and to the standard specmap chronology down to 60 ka bp using marine oxygen isotope control points pollen analysis was carried out and seven pollen zones were identified the results showed that the pollen spectra in mis 3 were very different to mis 2 but comparable with the holocene the saccate pollen principally pinus predominated in most parts of the core except for the interval of the last glacial maximum lgm and the late mis 3 when artemisia and other herbs increased prominently the ratios of p a pinus artemisia and of pb nap pollen with bladders versus non arboreal pollen were used as a proxy for sea level change and for estimating distances from the coastline to the study site the provenance study using a modern analog technique based on the east asian modern pollen database and fossil pollen assemblages revealed that the pollen were derived from diverse source areas during different stages since 60 ka bp the pollen zones equivalent to mis 1 and 3 were characterized by uncertain and disparate areas of provenance and the surrounding continents including eastern mainland china korea and southern japan possibly made equivalent contributions which may be the result of enhanced pollen transport by the northwesterly wind however the lgm represented in core dgks9602 was distinguished by restricted sources from the exposed east china shelf where the vegetation was quite similar with the herb coverage in the present day north china plain the pollen assemblages sensitively reflected the coastline changes marked by transport patterns between fluvial runoff and northwesterly winds plus ocean currents some pollen changes and limits of pollen zones coincided with the timing of heinrich events implying that the rapid climate fluctuations during mis 3 and mis 2 may have partially resulted in the forest changes in eastern asia the pattern of relative abundance of subtropical and total arboreal taxa is consistent with that of 18 o from hulu cave implying monsoonal climatic control over vegetation change the estimated temperature mat by the means of pollen ratios for the lgm was 5 8 c lower than that in the changjiang river mouth today

hazards singularities in geomorphic systems abstract the development of landscapes is commonly interpreted as the outcome of the operation of a few simple phenomenological principles amongst which the antagonism principle is the most fundamental one it states that there are two types of processes active in the formation of a landscape at the same time the endogenic tectonic and the exogenic climate driven processes these two types of processes may more or less balance each other in which case the actual aspect of a landscape corresponds to the instantaneous dynamic equilibrium between them if one of the external parameters is changed the equilibrium is supposed to be re established by a corresponding continuous change of the other parameters process response theory the dependences between various landscape parameters may become multivalued at junctions cusps etc in that case the system can jump from one branch of the process response curve to another a natural disaster occurs however modern views on the behavior of complex systems allow a somewhat different interpretation of the above phenomenological landscape interpretation but also in the context of the principle of antagonism however a stationary landscape state does now not correspond to a dynamic equilibrium but to self organized exogenic order at the edge of chaos under conditions of such self organized criticality there are no natural space and time scales so that fractal statistics are applicable spatial and temporal correlations follow a power law extremely small accidental perturbations can cause the system to become unstable leading to rapid changes in the landscape which are experienced as hazard events these follow a power law distribution for size versus frequency provided the endogenic input occurs at a constant rate they are therefore simply part and parcel of geomorphic systems

magnetotelluric analysis using mohr circles the mohr circle most commonly met in the analysis of mechanical stress is used to depict magnetotelluric impedance information taking the real and quadrature parts of magnetotelluric tensors separately the magnetotelluric concepts of two dimensionality three dimensionality skew and anisotropy are then all given quantitative expression on a diagram as are various magnetotelluric invariants in particular a new invariant the central impedance becomes evident in a discussion of effective impedances some insight is gained into impedance rotations and an anisotropy angle is defined analogous to skew angle mohr circles are also tested to depict the effects of the shear and twist operations on a regionally two dimensional structure generally the application of shear or twist results in an impedance tensor with a mohr circle of typical three dimensional form

electrical resistivity cross section across northern part of saurashtra region an insight to crystallized magma and fluids abstract saurashtra region forms the western part of the deccan volcanic province dvp in india and is bounded by the cambay rift basin crb in the east narmada rift basin nrb in the south and kachchh rift basin krb in the north this region is uplifted due to different stages of inter continental breakup madagascar india seychelles india different stages of rifting reactivation of fault zones and deccan volcanism the major portion of saurashtra region is covered by deccan basalts underlain by mesozoic sediments detailed magnetotelluric mt and long period magnetotelluric lmt soundings have been carried out in the northern part of saurashtra with an aim to bring out the geoelectrical configuration of different basins jamnagar jasdan and cambay along the profile 2d inversion carried out for different modes transverse electric te transverse magnetic mode tm and te tm independently the inversion of different modes shows distinct electrical signatures due to the 3d nature of the data we have considered the tm mode of interpretation because it is less prone to 3d distortions the final model represents large scale heterogeneities in the crust due to the presence of different resistive and conductive blocks high conductive lithosphere denotes the thermal influx of the mantle material to the base of the crust exchange of heat with the lower crust during basaltic magmatic underplating released carbonate fluids and or dehydration of minerals resulting in high conductivity anomalies a prominent conductive feature on the east side of the profile at upper crustal depths may be due to partial melts associated with mafic ultra mafic intrusions related to the reunion plume activity different resistive blocks in conjunction with seismic tomography studies represent recrystallized older precambrian crust that forms the uplifted blocks of jamnagar jasdan and western part of cambay basins

sea level changes in the tectonically stable malay thai peninsula abstract the malay thai peninsula is part of sundaland a geological region of relative crustal stability therefore the positions of old shorelines of the past 20 000 years or so can be considered to indicate positions of actual past sea levels over 200 shoreline indicators at positions below as well as above present sea level have been radiocarbon dated these dated positions indicate that prior to 6 ka the sea rose from at least 90 m below present sea level at rates that initially amounted to 15 mm year and later became 6 mm year at ca 6 ka the rising sea surpassed its current position and at ca 5 ka it reached 5 m during this maximum marine inundation the peninsula was narrower since the low coastal plains did not yet exist in the period following this maximum transgression the available shoreline data indicate that regional sea level subsided through a series of fluctuations that were characterized by amplitudes of 2 m and periods of about 2000 years the peaks and troughs of the fluctuations became progressively lower the latest peak was at about 1 7 m or 0 7 m above present high tide a few hundred years ago the cause of these fluctuations is not yet known but the trend suggests that in the near future regional sea level may subside at annual rates between 1 5 mm and 2 mm this rate of descent is 25 to 30 of the estimated rate of sea level rise that may result from global warming in other words for the malay thai peninsula the effect of sea level rise on account of manmade greenhouse gases will be partly compensated by actual subsidence of regional sea level

flow and electrical anisotropy in the upper mantle finite element models constraints on the effects of olivine crystal preferred orientation and microstructure long period magnetotelluric mt data shows that electrical conductivity in the upper mantle is highly anisotropic agreement between high electrical conductivity directions and seismic anisotropy fast directions suggests that anisotropic diffusion of hydrogen along oriented olivine crystals controls the anisotropy of electrical conductivity in the upper mantle since both seismic waves and hydrogen diffusion are faster along the 1 0 0 axis of olivine thus mt electrical anisotropy data like seismic anisotropy may be used to map flow patterns in the upper mantle however observed electrical anisotropies are significantly higher than seismic ones to quantify the influence of strain induced crystal preferred orientations of olivine on upper mantle bulk electrical conductivities we calculate the macroscopic electrical conductivity anisotropy of a series of naturally and experimentally deformed peridotites using an anisotropic finite element model these models which fully take into account the microstructure crystal and shape preferred orientations based on orientation maps obtained by indexation of electron back scattered diffraction ebsd patterns show macroscopic electrical anisotropy factors ranging from 3 to 16 the intensity of electrical anisotropy depends to first order on the intensity of the olivine crystal preferred orientations but the relation saturates for strong crystal preferred orientations in addition the spatial distribution of the various crystal orientations may significantly enhance influence the anisotropy in strongly textured mantle rocks the strongest anisotropy factors 10 occur in mantle rocks in which deformation by dislocation creep has produced not only crystal but also strong shape preferred orientations even if the latter is masked by recrystallization higher anisotropy factors 100 observed in a few mt experiments imply however an additional as yet unknown mechanism controlling electrical conduction at asthenospheric depths in these regions

magnetotelluric tensor decomposition part ii examples of a basic procedure the decomposition procedure suggested for magnetotelluric mt data in a companion paper is illustrated by examples adjacent pairs of stations are examined to test for common features in their mt response the method analyzes the real and quadrature parts of an impedance tensor separately and the results for the real and quadrature parts are compared and in some cases combined at the end values are obtained which are interpreted directly in terms of nearest 2 d principal impedances local geologic strike and regional geologic strike the requirement for valid data derived in the companion paper that zxyrzyxr zxxrzyyr together with zxyqzyxq zxxqzyyq is shown to be a useful criterion in the winnowing of erroneous data the examples presented typically show the direction interpreted as local geologic strike to be well determined period independent and consistent between real and quadrature mt matrices in these examples however the direction interpreted as regional geologic strike is period depen

anisotropy and phase splits in magnetotellurics abstract several recent papers suggesting that the electrical conductivity of the lower crust and upper mantle are anisotropic have been based upon magnetotelluric mt observations of phase splits the mt analogue of shear wave splitting the emphasis on the phase rather than the amplitude response results from the distorting effect of conductivity variations in the overlying crust these variations distort the amplitude response from deeper levels and make inferences based on the amplitude difficult at the long periods needed to probe the conductivity at upper mantle depths the mt phase response a tensor is negligibly affected by small scale conductivity variations in the overlying crust however the mt phase response of a half space with a uniform but anisotropic conductivity is independent of polarization direction and no phase split occurs using simple anisotropic layered models we demonstrate that a phase split is produced by the conductivity change at the interface between the isotropic and anisotropic layers the maximum phase difference reflecting the conductivity contrast gradient between the principal conductivities of the anisotropic layer and the isotropic layer s conductivity in the general case where none of the principal axes of the anisotropic conductivity layer lie in the plane of the surface the orientation at which the maximum phase difference occurs i e the principal axis of the phase tensor is determined by the horizontal projection of conductivity tensor s ellipsoid in such cases the orientation of the maximum phase split will not be aligned parallel to any of the conductivity tensor s principal axes mt phase splits are a consequence of spatial differences or gradients in conductivity not the intrinsic bulk properties of the conductivity tensor itself and in this respect are fundamentally different from shear wave splitting

lithospheric structure of an archean craton and adjacent mobile belt revealed from 2 d and 3 d inversion of magnetotelluric data example from southern congo craton in northern namibia 1 archean cratons and the stitching proterozoic orogenic belts on their flanks form an integral part of the southern africa tectonic landscape of these virtually nothing is known of the position and thickness of the southern boundary of the composite congo craton and the neoproterozoic pan african orogenic belt due to thick sedimentary cover we present the first lithospheric scale geophysical study of that cryptic boundary and define its geometry at depth our results are derived from two dimensional 2 d and three dimensional 3 d inversion of magnetotelluric data acquired along four semiparallel profiles crossing the kalahari craton across the damara ghanzi chobe belts dgc and extending into the congo craton two dimensional and three dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger dgc orogen to the older adjacent cratons we find damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent congo craton the congo craton is characterized by very thick to depths of 250 km and resistive i e cold lithosphere resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during pan african magmatism graphite bearing calcite marbles and sulfides are widespread in the damara belt and account for the high crustal conductivity in the central zone the resistivity models provide new constraints on the southern extent of the greater congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south

variability of the path of the kuroshio ocean current over the past 25 000 years the kuroshio current is the strong northwestern component of the subtropical north pacific ocean gyre and advects a large amount of heat from the tropics to northern mid latitudes the kuroshio has bimodal stationary flow patterns with small and large meander paths east of central japan1 2 which switch on annual and decadal timescales3 4 these switches seem to be caused by changes in current velocity and volume transport of the north equatorial current that are associated with variations in the trade wind intensity in the eastern equatorial north pacific ocean5 6 here we present alkenone derived sea surface temperature records at multicentennial resolution from sediment cores from the nishishichitou ridge off central japan these 25 000 year records show that the kuroshio path has also fluctuated on millennial timescales this variability resembles that of the subtropical high pressure of the north pacific reconstructed from terrestrial pollen distributions water levels in north american lakes and marine micropalaeontological records7 together these data indicate that climate variability off central japan over the past 25 000 years may be part of a circum pacific phenomenon reflecting the rate of subtropical surface circulation in the north pacific ocean

anomalous phases exceeding 90 in magnetotellurics anisotropic model studies and a field example summary a study of synthetic anisotropic models that explain phases exceeding 90 in magnetotellurics is presented the basic model comprises an anisotropic layer overlain by a shallow local anisotropic block with both structures inserted in a 2 d model the 2 d strike and the anisotropy strikes layer and block differ the influence of the following parameters was analysed anisotropy strike geometry of the block and the layer and anisotropy ratios of the block and the layer we show that according to this model the anomalous phase effect is limited to those sites above the shallow block and does not influence the regional structure which can therefore be recovered these results were applied to field data from a magnetotelluric profile in sw iberia where phases greater than 90 occurred in an area in which alternating bands of schist and graphite rich blackschists crop out giving rise to strong macroanisotropy

analytical investigations of the magnetotelluric directionality estimation in 1 d anisotropic layered media abstract inferring geoelectric dimensionality 1d 2d or 3d and directionality strike directions from the impedance tensor is a basic procedure in magnetotelluric data processing given that electrical anisotropy is increasingly recognized in observations it is valuable to understand the imprint of anisotropy in these analyses in this paper we analytically investigate the estimation of strike directions based on rotational invariants in 1d anisotropic layered media we first show that if anisotropy axes are identical in all anisotropic layers the estimated strike coincides with that direction we then derive an analytical formula of the strike angle at long periods for general anisotropic layers with an isotropic basement this formula shows a clear physical interpretation that the strike angle points where the conductance integrated along depth takes a maximum value

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

3 d interpretation of short period magnetotelluric data at furnas volcano azores islands s u m m a r y accurate geophysical imaging of shallow subsurface features provides crucial constraints on understanding the dynamics of volcanic systems at furnas volcano azores intense circulation of volcanic fluids at depth leading to high co 2 outgassing and flank destabilization poses considerable threat to the local population presented is a novel 3 d electrical resistivity model developed from 39 magnetotelluric soundings that images the hydrothermal system of the furnas volcano to a depth of 1 km the resistivity model images two conductive zones one at 100 m and another at 500 m depth separated by a resistive layer the shallow conductor has conductivity less than 1 s m 1 which can be explained by clay mineral surface conduction with a mass fraction of at least 20 per cent smectite the deeper conductor extends across the majority of the survey area this deeper conductor is located at depths where smectite is generally replaced by chlorite and we interpret it as aqueous fluids near the boiling point and infer temperatures of at least 240 c the less conductive layer found between these conductors is probably steam dominated and coincides within the mixed clay zone found in many volcanic hydrothermal systems

cryptic signatures of neoproterozoic accretionary events in northeast brazil imaged by magnetotellurics implications for the assembly of west gondwana abstract the borborema province in northeast brazil is a complex orogenic system severely affected by deformational metamorphic and magmatic processes mostly during the gondwana convergence in late neoproterozoic early phanerozoic brasiliano pan african orogeny new magnetotelluric mt data collected along the northwestern part of the province and eastern part of the contiguous parnaiba basin are combined with previous mt data to assess the regional deep electrical resistivity structure dimensionality analysis shows that a 3d electrical structure predominates in the subsurface and thus 3d inversion was carried out the final geoelectric model allows delineating the geometry and variation in physical properties of different lithospheric blocks bounded by major electrical discontinuities these lithospheric blocks constitute a coalesced mosaic made up of four main terrane compartments a resistive cratonic keel detected along the western part of the study area currently hidden beneath the parnaiba basin parnaiba block two complex domains in the center characterized by several resistive and conductive zones throughout the crust and upper mantle ceara central and rio grande do norte domains and a conductive block in the east with the geoelectric response being controlled by possible remains of late neoproterozoic subduction activity to the south central sub province the interfaces between these blocks are interpreted as suture zones correlated to their neoproterozoic collage one curved conductor concealed by the sediments of the parnaiba basin and bordering the eastern margin of the basin another huge conductor corresponding to the location of the oros jaguaribe subdomain on the surface and a third interface coinciding with the position of the patos shear zone the presence of these proposed sutures could be a conspicuous evidence of a neoproterozoic accretion system in northeast brazil and would support tectonic evolution models for the west gondwana assembly involving oceanic closure and collision of continental blocks during the brasiliano orogeny

the north american central plains conductivity anomaly and its correlation with gravity magnetic seismic and heat flow data in saskatchewan canada abstract the north american central plains conductivity anomaly nacp lies virtually in its entirety within the trans hudson orogen accordingly should these two features prove to be contemporaneous then the geometrical relationship between these two is of foremost importance to any evolutionary tectonic model proposed to explain the collision of the superior and churchill provinces in the hudsonian and a model that does not include a mechanism for the generation of this anomaly is obviously untenable in order to map better the trend of the nacp in the province of saskatchewan canada two magnetotelluric mt profiles were conducted over the nacp previously defined by magnetometer arrays and profiles data from these two profiles along with an earlier mt profile just north of the u s canadian border suggest that the nacp is not a continuous feature but rather that it exhibits a definite break at latitude 51 n other geophysical evidence examined herein is concordant with this characteristic if a second mt anomaly mapped to the northwest of this break is in fact a manifestation of the same geological structure then one possible interpretation is of a major nw se trending sinistral fault in the deep crust previously undetected with a movement of some 100 150 km along strike the mt data from the southernmost profile after correction for static shift are modelled in a 2d manner and it is shown that the nacp is consistent with an arcuate structure in vertical section of high conductivity 2 s m 1 beginning at a depth of 10 km centred on 103 w dipping down to the west reaching possibly the base of the crust it is also shown that such an arcuate shape in section can explain an observed gravity high of 40 mgal such high conductivities cannot be explained in terms of connected fluids as it would require implausibly high porosities 12 20 a comprehensive interpretation of all the geophysical data would have to account for the observed characteristics of the anomalous region which are high electrical conductivity positive density contrast no magnetization and high heat flow in the basement in terms of a single causative body one possible explanation is presented in terms of a zone of lithospheric weakness along which was emplaced low density differentia from a mantle derived body an alternative explanation could be the phase transformation of obducted material into eclogite or serpentinite however difficulties exist with both of these explanations and further data are required

magnetotelluric response of a laterally inhomogeneous anisotropic inclusion abstract the field of h polarisation is studied for an anisotropic two dimensional conducting inclusion embedded in a less conducting isotropic surrounding using a finite difference approximation technique contour lines for the re h x have been plotted laterally across the inclusion it is shown that 1 for larger anisotropic factors in the inclusion the contour lines converge in the embedded inclusion and 2 the effects of changing lateral conductivity variations are reflected significantly in the apparent resistivity versus lateral distance variation patterns our results may provide an insight into the interpretation of magnetotelluric investigations over anisotropic inhomogeneities frequently encountered in mapping natural resources in sedimentary and fissured zones in geothermal fields

deep sea pollen from the south china sea pleistocene indicators of east asian monsoon abstract a high resolution pollen record sampling interval averages 820 years has been obtained from odp site 1144 water depth 2037 m northern south china sea the 504 m sequence in composition length covers the last 1 03 million years according to micropaleontological and isotopic stratigraphy the pollen assemblages are characterized by high proportions of pinus and herb pollen and by their frequent alternations based on these alternations 29 pollen zones have been recognized that are closely correlated to the marine oxygen isotope stages mis 1 29 pinus dominant pollen zones correspond to interglacial periods with lighter 18o values while herb marked ones relate to the heavier 18o stages assigned to glacials judging from the pollen data the exposed northern continental shelf of the south china sea during the glacials was covered by grassland and the extensive northern shelf has formed only since mis 6 ca 150 ka probably as a result of tectonic subsidence tree pollen influx values are indicative of winter monsoon which began to intensify 600 ka ago the summer monsoon variations can be approximated by the fern percentage within the total pollen and spore abundance and the result shows high values in general occurring at interglacials with the maxima at mis 15 5e and 1 the relatively high fern percentage with smaller amplitude in variations before 600 ka might suggest more stable humid conditions before the intensification of winter monsoon

magnetotelluric images of paleoproterozoic accretion and mesoproterozoic to neoproterozoic reworking processes in the northern sao francisco craton central eastern brazil abstract broadband and long period magnetotelluric mt data were collected along an east west oriented 580 km long profile across the northern sao francisco craton where extensive proterozoic and phanerozoic sedimentary cover and lack of deep probing geophysical surveys have prevented to establish unequivocally the cratonic character of the archean paleoproterozoic lithosphere following dimensionality analyses the mt dataset was interpreted using both 2 d and 3 d inversion procedures the near surface structure is better resolved in the 2 d model due to its finer resolution a huge upper crustal conductor is found all along the shallow early neoproterozoic irece basin in the central domain of the craton extending laterally for approximately 150 km and restricting signal propagation below the basin its high conductance is explained by a combination of high porosity and high fluid salinity in the sedimentary package another upper crustal conductor is observed on the west side of the profile interpreted as fractured metasedimentary rocks of the rio preto belt thrusted on top of the craton basement during neoproterozoic marginal collision the 3 d model explains significantly better the measured data related to deep structure contrary to what is expected for a stable cratonic block the geoelectric model shows pronounced electrical complexity and heterogeneity an indication that the cratonic lithosphere was multiply reworked in the past by tectonothermal events different lithospheric resistive blocks bounded by major conductive zones are identified constrained by geochemical and isotopic data these vertical conductive interfaces are interpreted as cryptic suture zones due to large scale amalgamation of continents and microcontinents leading to the assembly of the sao francisco craton in the paleoproterozoic the conductivity enhancement is more likely explained by emplacement of sulfides along previous suture zones during mafic magmatism at upper mantle depths high conductivity observed below most of the profile indicates that metasomatism or refertilization processes with incompatible elements caused by the paleoproterozoic subducting slabs and mesoproterozoic to neoproterozoic upwelling of deep fluids and melts reworked this portion of the craton mantle

tectonic fabric of the subcontinental lithosphere evidence from seismic magnetotelluric and mechanical anisotropy abstract knowledge of anisotropy is key to our understanding of tectonic fabrics in the lithosphere and sublithospheric mantle anisotropy provides a unique constraint on the character of past and present deformation and thus is critical to our understanding of dynamics of the plate tectonic system in particular how continents with their thick lithospheric mantle roots form stabilize and interact with underlying mantle regions improved observational methods and rapid developments of geophysical inversion techniques are contributing to more realistic earth models that include various types of anisotropy and heterogeneity a more robust characterization of anisotropic parameters can be achieved by integrating complementary datasets incorporating constraints from seismology magnetotellurics geodynamics and mineral physics valid comparisons between different measures of anisotropy however require a solid understanding of the principles and limitations of the various techniques to this end we briefly summarize a new compilation of original articles with a common focus on anisotropy of continental lithosphere including theory laboratory experiments and observations that span a range of different geophysical techniques

magnetotelluric inversion for anisotropic conductivities in layered media abstract electrical anisotropy in the earth s crust and upper mantle has recently gained attention as a significant linking factor between electrical models and underlying structural and tectonic patterns this interest has also motivated new methodological studies into the modelling and inversion for electrically anisotropic structures we present an algorithm for the inversion of magnetotelluric data over layered anisotropic conductors which is a straightforward extension of the standard occam 1 d inversion to anisotropic models owing to the essential limitation of magnetotellurics to resolve the complete conductivity tensor we formulate the inversion for azimuthal anisotropy only we treat the non linear inverse problem as a multi criterion minimization of the structure complexity data misfit and anisotropy to constrain the structure complexity we employ the standard roughness penalty as well as non quadratic penalties of the total variation and gradient support type that produce more focused model sections and thus conform better to the idea about sharp non diffuse boundaries of anisotropic structures in the earth application of the anisotropy penalty is crucial for suppressing spurious anisotropy in the inverse models we use a 2 d extension of the heuristic l curve method to estimate the quasi optimal penalty weights with two non linear iteration solvers specifically the reweighted conjugate gradient method and the lagged diffusivity iteration we can arrive at the minimum of the target functional for one selected pair of regularization weights typically after a few tens of iteration steps to demonstrate the inverse solution we present two simple yet not completely trivial synthetic examples the first one based on data generated by a model with two anisotropic layers with discordant strikes and the other showing possible misinterpretations in case a 1 d inversion with anisotropy is formally applied to data produced by simple 2 d block structures field data from the variscan structures in southern portugal bear indications both geological and geophysical on possible crustal anisotropy due to shearing and graphitization we present results of a 1 d anisotropic imaging and a preliminary 2 d modelling for a short section of five sites over an old suture between the south portuguese zone and ossa morena zone aimed at explaining some essentially non 2 d effects in the data as e g systematic frequency variations of the regional strike or phases rolling out of their natural quadrants

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

dimensionality imprint of electrical anisotropy in magnetotelluric responses abstract dimensionality analysis of magnetotelluric data is a common procedure for inferring the main properties of the geoelectric structures of the subsurface such as the strike direction or the presence of superficial distorting bodies and enables the most appropriate modeling approach 1d 2d or 3d to be determined most of the methods currently used assume that the electrical conductivity of individual parts of a structure is isotropic although some traces of anisotropy in data responses can be recognized in this paper we investigate the imprints of anisotropic media responses in dimensionality analysis using rotational invariants of the magnetotelluric tensor we show results for responses generated from 2d synthetic anisotropic models and for field data that have been interpreted as showing the effects of electrical anisotropy in parts of the subsurface structure as a result of this study we extend the wal dimensionality criteria to include extra conditions that allow anisotropic media to be distinguished from 2d isotropic ones the new conditions require the analysis of the strike directions obtained and take into account the overall behavior of different sites in a survey

a finite element algorithm for electromagnetic induction in two dimensional anisotropic conductivity structures summary electromagnetic fields are computed for a 2 d electrically anisotropic earth using a finite element technique the models consist of a background layered structure containing anisotropic blocks each block and layer may be anisotropic by assigning to them 3 3 conductivity tensors the forward modelling problem leads to a coupled system of two partial differential equations for the strike parallel field components ex and hx they are solved numerically using the finite element fe method the resulting system of linear fe equations is solved using a preconditioned conjugate gradient method subsequently strike perpendicular field components ey and hy at the surface are found by numerical differentiation of ex and hx using spline interpolation the 2 d fe algorithm has been validated by comparison with a 2 d finite difference solution three model types are used to demonstrate the effect of anisotropy upon the magnetotelluric responses horizontal vertical and dipping anisotropy a fourth model simulates the effect of anisotropy in the context of shear and subduction zones the model responses simulate the splitting of apparent resistivity curves at long periods as well as the existence of tensor impedances with significant diagonal elements as has been observed previously

north american central plains conductivity anomaly within the trans hudson orogen in northern saskatchewan canada magnetotelluric data acquired across the paleoproterozoic trans hudson orogen northern saskatchewan image one of the world9s longest crustal features the north american central plains conductivity anomaly modeling shows the anomaly at this latitude to comprise two distinct westward dipping bodies of high conductivity lying structurally above a late collisional feature the guncoat thrust the shallower of these bodies correlates with the western part of the la ronge belt the deeper body at mid crustal depths underlies the wathaman batholith and its western boundary is close to the inferred subsurface extension of the sub vertical needle falls shear zone the anomaly is identified with interleaved biotitic metasedimentary rocks of the nemeiben zone and cree lake belt and is interpreted to have been thrust beneath the margin during collision of the la ronge arc with the rae hearne continent via westward directed subduction

deep magnetotelluric signatures of the early neoproterozoic cariris velhos tectonic event within the transversal sub province of the borborema province ne brazil abstract the borborema structural province in northeast brazil consists of a complex set of crustal blocks of different ages origin and evolution with final amalgamation during the late neoproterozoic brasiliano orogeny in the central part of the province its transversal sub province is characterized by elongate ne sw terranes largely affected by the brasiliano metamorphic overprinting and by a suite of early neoproterozoic metavolcanic and metaplutonic rocks reflecting a distinct tectonic event cariris velhos magnetotelluric mt data were collected at 64 stations along four profiles across this central region of the province to assess its deep electrical resistivity structure dimensionality analysis shows that the subsurface is electrically dominated by 3d structure and 3d inversions of the full impedance tensor and vertical magnetic transfer function tipper were carried out data fit is relatively good with an acceptable distribution of the misfit between the sites and the dominant features in the final model are north dipping crustal conductors which can be tentatively correlated between the different mt profiles and cross contrasting terranes at the surface of the province considering the geometry and distribution of the conductors it is proposed that they might reflect remnants of a north directed subduction zone with enhanced conductivity associated with graphitized biogenic material in metasedimentary rocks originally deposited in a restricted oceanic environment and subsequently metamorphosed and introduced into the deep crust by the subduction of the oceanic lithosphere constrained by isolated outcrops of early neoproterozoic rocks with oceanic affinity the proposed subduction event may have taken place during the cariris velhos orogeny with the crustal remobilization by the brasiliano orogeny being restricted to the brittle upper crust this interpretation gives support for an accretionary model to the neoproterozoic geodynamic evolution of the borborema province