euler deconvolution of the analytic signal and its application to magnetic interpretation euler deconvolution and the analytic signal are both used for semi automatic interpretation of magnetic data they are used mostly to delineate contacts and obtain rapid source depth estimates for euler deconvolution the quality of the depth estimation depends mainly on the choice of the proper structural index which is a function of the geometry of the causative bodies euler deconvolution applies only to functions that are homogeneous this is the case for the magnetic field due to contacts thin dikes and poles fortunately many complex geological structures can be approximated by these simple geometries in practice the euler equation is also solved for a background regional field for the analytic signal the model used is generally a contact although other models such as a thin dike can be considered it can be shown that if a function is homogeneous its analytic signal is also homogeneous deconvolution of the analytic signal is then equivalent to euler deconvolution of the magnetic field with a background field however computation of the analytic signal effectively removes the background field from the data consequently it is possible to solve for both the source location and structural index once these parameters are determined the local dip and the susceptibility contrast can be determined from relationships between the analytic signal and the orthogonal gradients of the magnetic field the major advantage of this technique is that it allows the automatic identification of the type of source implementation of this approach is demonstrated for recent high resolution survey data from an archean granite greenstone terrane in northern ontario canada

gravity gradiometry resurfaces the red october was heading southwest on gorshkov s railroad his speed was thirteen knots installed in the keel was a highly sensitive device called a gradiometer essentially two large lead weights separated by a space of one hundred yards a laser computer system measured the space between the weights down to a fraction of an angstrom distortion of that distance or lateral movement of the weights indicated variations in the local gravitational field with careful use of gravitometers he could plot the vessel s location to within a hundred meters

early cambrian back arc volcanism in the western taurides turkey implications for rifting along the northern gondwanan margin the lower cambrian tommotian go ebakan formation in western central anatolia is made up of slightly metamorphosed continental to shallow marine clastic rocks with pillowed and massive spilitic lavas and dolerite dykes spilitic lavas commonly amygdaloidal are albite and pyroxene phyric with the metamorphic mineral paragenesis albite calcite sericite epidote tremolite chlorite dolerite dykes mainly include plagioclase and pyroxene as primary minerals and tremolite epidote chlorite as low grade secondary minerals geochemical data show that the spilitic lavas and dolerite dykes are sub alkaline of oceanic tholeiitic basalt character and display a tholeiitic fractional trend characterized by an increase in feo mgo and zr and tio2 in variation diagrams they are characterized by relatively high zr y 2 4 5 relatively high th yb 0 15 1 0 and la nb 0 5 2 5 both show marked negative nb and ti anomalies relative to th and la ce implying a subduction related chemistry chondrite normalized ree patterns display slight enrichment of light ree spilitic lavas la yb n 0 79 1 56 dolerite dykes la yb n 0 89 3 50 fairly comparable with morb the geochemical similarity of the spilitic lavas and dolerite dykes suggests a co genetic origin la nb ratios of both types are slightly higher than average morb values and were possibly formed in the early stages of back arc basin development petrogenetic modelling suggests the mafic rocks of the formation were formed by 9 batch melting of spinel lherzolite in shallower depths c 60 km taken together the data suggest that the early cambrian mafic rocks of the taurus units were developed in a back arc basin along the northern edge of gondwana above the southward subducting oceanic lithosphere and may represent initial rifting that resulted in separation of the peri gondwanan terranes

geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

the rise and fall of early oil field technology the torsion balance gradiometer today elementary physics students take for granted such quantities as big g the universal gravitational constant in fact in the late 1700s the value of this quantity was unknown and the quest to determine it led to some of the earliest geophysical instrumentation just after the revolutionary war in the united states cavendish developed the first system to measure the universal gravitational constant the familiar big g unfortunately for geologists at this time still mostly gentlemen scientists this apparatus produced data which were difficult to interpret geologically and it was far too large and cumbersome for field use the geologic limitation was that the system only measured the horizontal derivative of a horizontal component of the gravity field a quantity which by itself is difficult to interpret thus no applications of this elegant yet laboratory bound instrument emerged

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

relics of an early panafrican metabasite metarhyolite formation in the brno massif moravia czech republic the brno massif in moravia czech republic is an important exposure of precambrian basement in central europe it includes large volumes of cadomian granitoids and a narrow fault bounded zone with metagabbros metadiorites and metabasalts this so called central basic belt also contains some metarhyolites one of these was dated by means of the zircon evaporation method at 725 15 ma chemical and isotope data show that the dated rock represents a mantle derived magma which is cosanguinous with surrounding morb type metabasites the data suggest that the brno massif hosts the oldest metabasite complex currently known in central europe its formation apparently coincides with the main period of ocean floor spreading and island arc formation in the panafrican orogens this lends further support to the theory that the brno massif is a gondwana derived element

the geology and gravity field in the central core of the vredefort structure the centre of the vredefort structure in south africa comprises an uplifted granitic basement with an associated 300 g u gravity high ringed by upturned sedimentary strata detailed geological mapping of the basement core has delineated a three fold concentric zoning of outer granite gneisses ogg steynskraal formation sf and inlandsee leucogranofels ilg an increase in metamorphic grade occurs toward the centre of the structure from greenschist relics on the basement perimeter to upper amphibolite and granulite facies in the sf and granulite facies in the ilg a marked lithological change occurs within the sf in which perimeter greenschists and inner amphibolites give way to paragneisses in the ilg paragneiss and mafic granulite relics predominate these divisions are consistent with the standing on edge model for the basement 1 three regional metamorphic episodes and two thermal events have been recognised m1 and m3 both attained granulite facies at low medium load pressures in the ilg m1 granulite facies assemblages extended into the ogg but the m3 facies was restricted to the ilg m2 was a k metasomatism event limited to the ogg an extensive thermal episode m4a is provisionally correlated with the collar metamorphism pyroxene hornfels facies t 800 c was attained in the centre as opposed to conditions of p 4 5 kbar and t 650 c for alkali granite stocks in the sedimentary collar 2 this event was thus pre or syn uplift but not post uplift a high level high temperature p 3 kbar and t 800 900 c event m4b occurred at a late stage in the overall sequence a suite of amphibole orthopyroxene rocks of basaltic komatiite type cao al2o3 1 are chemically related to younger high mg tholeiites these tholeiites are correlated with similar rocks in the sedimentary collar and were intruded syn to post m4a in several episodes of decreasing basicity shock metamorphic features granitic breccias and pseudotachylite are linked to the late stage history of the structure and a case is presented for a dynamic metamorphic front advancing outward and upward from the centre in response to unroofing of the basement the central gravity high3 is delineated by 1239 new gravity stations at 1 km and 0 5 km intervals a low frequency oval shaped anomaly with a long axis trending nw se has eccentrically positioned on it a plateau extending southwards as a spur a dyke is responsible for the spur but the source for the plateau is from a recent borehole thought to be largely due to sheared altered harzburgite modelling of the low frequency gravity field component was based on the mapped divisions of the ogg sf and ilg and a density contrast of 200 kg m3 was determined between the ogg and the sf and ilg lithologies a purely crustal solution for the excess mass distribution in the basement is conclusively demonstrated

feasibility of using full tensor gradient ftg data for detection of local lateral density contrasts during reservoir monitoring summary three dimensional full tensor gradiometry 3 d ftg acquires ultrasensitive measurements of the earth s vector gravity gradient field departures from simple weakening of the field in the vertical direction are due to subsurface variations in density we have undertaken a numerical examination of the feasibility of using this system for detecting time variations in local lateral density contrasts in subsurface layers during reservoir production monitoring our gravity modelling focuses on the value added by taking account of the horizontal components of gravity gradient in imaging local targets we have studied the sensitivity of these components to model and acquisition parameters iterative regularized inversion algorithms that can explain the behaviour of a hydrocarbon reservoir in terms of time lapse density changes have been described these algorithms are based on the homotopy method that utilizes the solution space topology to stabilize the inversion we have introduced the maximum compactness of anomalous sources along one or more directions this allows the use of a priori information natural to time lapse monitoring about the arbitrary shape and spatial locations of the sources the time evolving density contrast within the local target zone is recovered by means of an iterative procedure the treatment of either correlated or random noise fits naturally into this procedure the final scheme has been illustrated by several examples using synthetic ftg data generated from various density distributions of practical interest we have considered the effect of model and data uncertainties on the inversion solution it appears that the iterative process is effective numerically stable and rapidly convergent in the presence of both random and structural noise results show that it is relatively insensitive to the choice of starting model appropriate applications include direct monitoring of gas oil contact and temperature front expansion during steam injection in heavy oil reservoirs at shallow or moderate depths

window constrained inversion of gravity gradient tensor data using dike and contact models we have developed a constrained inversion technique for interpretation of gravity gradient tensor data for dike and contact models striking in the y direction the measured g xz and g zz com

paleomagnetic and rock magnetic study of the vredefort impact structure and the johannesburg dome kaapvaal craton south africa implications for the apparent polar wander path of the kaapvaal craton during the mesoproterozoic abstract new paleomagnetic and rock magnetic results are presented for various pre syn and post impact lithologies from the region of the 2023 4 ma originally ca 250 km diameter vredefort impact structure on the kaapvaal craton of south africa after removal of a viscous remanent magnetization vrm component the characteristic remanent magnetization chrm component from 16 vredefort granophyre and pseudotachylitic pt breccia samples d 18 3 i 54 8 95 8 1 was isolated and a paleomagnetic pole at 25 1 n and 43 5 e a 95 10 6 obtained since such a chrm component was isolated also from the archean basement rocks of the vredefort dome we consider its nature as a primary component of the impactites proven the pole falls onto the paleoproterozoic part of the apparent polar wander path of the kaapvaal craton rock magnetic analysis and scanning electron microscopy sem revealed that the pseudotachylitic breccia and granophyre samples contain two distinct magnetite phases one ultra small micrometer size and one altered and larger 50 m phase the ultra small magnetite is interpreted as the carrier of the chrm and the larger grains as the carriers of the vrm component additionally rock magnetic and petrophysical data reveal unusually high koenigsberger ratios q values in all pre impact lithologies in some vredefort impactite samples and in the much younger 1 1 ga anna s rust sheet ars gabbro samples as the high q values which had also been reported by previous studies of vredefort lithologies are now also seen in samples from the johannesburg dome a direct link to the vredefort impact can be ruled out this is also supported by the hysteresis data of this study as the observed magnetization is rather hard and shows multiple components of remanent magnetization we exclude lightning as a cause for all observed high q values except in case of ars gabbros it is instead suggested that the cause of the high q values could be related to the high temperatures of the rocks that were uplifted by the impact event from a mid crustal original setting and to fluid circulation within the two domes that made the rocks vulnerable to acquire high thermochemical remanence moreover paleomagnetic analysis of the rocks around the ars gabbro intrusion in the northern part of the vredefort structure revealed the presence of either a shallow north or a shallow south direction which is tentatively related to emplacement of the umkondo large igneous province analysis of all rocks including the vredefort impactites yields occasionally distinct great circle paths towards these shallow directions a likely explanation for this overprint direction is the heating caused by now eroded ars type gabbro in the area or by regional ca 1 1 1 0 ga orogenic effects no evidence of karoo type 0 18 ga overprint is seen

neoproterozoic and cambro ordovician magmatism in the variscan klodzko metamorphic complex west sudetes poland new insights from u pb zircon dating the klodzko metamorphic complex kmc in the central sudetes consists of meta sedimentary and meta igneous rocks metamorphosed under greenschist to amphibolite facies conditions they are comprised in a number of separate tectonic units interpreted as thrust sheets in contrast to other lower palaeozoic volcano sedimentary successions in the sudetes the two uppermost units the orla gologlowy unit and the klodzko fortress unit of the kmc contain meta igneous rocks with supra subduction zone affinities the age of the kmc was previously assumed to be early palaeozoic devonian based on biostratigraphic findings in the lowermost tectonic unit our geochronological study focused on the magmatic rocks from the two uppermost tectonic units exposed in the sw part of the kmc two orthogneiss samples from the orla gologlowy unit yielded ages of 500 4 3 1 and 500 2 4 9 ma interpreted to indicate the crystallization age of the granitic precursors a plagioclase gneiss from the same tectonic unit intimately interlayered with metagabbro provided an upper intercept age of 590 1 7 2 ma which is interpreted as the time of igneous crystallization from the topmost klodzko fortress unit a metatuffite was studied which contains a mixture of genetically different zircon grains the youngest 207pb 206pb ages which cluster at ca 590 600 ma are interpreted to indicate the maximum depositional age for this metasediment the results of this study are in accord with a model that suggests a nappe structure for the kmc with a middle devonian succession at the base and upper proterozoic units at structurally higher levels it is suggested here that the kmc represents a composite tectonic suture that juxtaposes elements of pre variscan basement intruded by the lower ordovician granite against a middle palaeozoic passive margin succession the new ages combined with the overall geochemical variation in the kmc indicate the existence of rock assemblages representing a gondwana active margin the recognition of neoproterozoic subduction related magmatism provides additional arguments for the hypothesis that equivalents of the tepla barrandian domain are exposed in the central sudetes

the margins of laurussia in central and southeast europe and southwest asia the continuation of the laurussian margin into central and southeast europe and southwest asia is defined by the rheic suture which becomes harder to trace to the southeast because of complex post variscan strike slip displacements and tectonic stacking linked to the cimmerian and alpine cycles the uncertain extent of the neoproterozoic baikalian orogeny fringing baltica ambiguous early paleozoic paleobiogeographic data and strike slip activity complicate the distinction of the tornquist suture from the transeuropean suture zone there is growing evidence for the incorporation of the brunovistulian malopolska and west moesian terranes into the late neoproterozoic cambrian baltican margin the istanbul zonguldak terrane along with the bitesh and east moesia terranes may have been part of the avalonian terrane assemblage although an arabian nubian shield or baltica provenance cannot be excluded the terranes of the armorica terrane assemblage bordering the rheic suture include the outer carpathian tatra getic and balkan terranes in southeast europe and sakarya and the anatolide tauride terrane in turkey further east the rheic suture may be present in the caucasus in all probability between the greater caucasus and the scythian platform or alternatively between the greater caucasus and transcaucasia alborz in northern iran is interpreted as a distal part of transcaucasia the location of the rheic suture in the southeast part of the turan domain of central asia is poorly constrained

relations between hinterland and foreland shortening sevier orogeny central north american cordillera the tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens the classic central cordilleran foreland fold and thrust belt in the united states late jurassic to early tertiary sevier belt and the more internal zones to the west central nevada thrust belt provide data critical to understanding the developmere of internal and external parts of orogens the garden valley thrust system part of the central nevada thnkst belt crops out in south central nevada within a region generally considered to be the hinterland of the jurassic to eocene sevier thrust belt the thrust system consists of at least four principal thrust plates composed of strata as young as pennsylvanian in age that are unconformably overlain by rocks as old as oligocene suggesting that contraction occurred between those times new u pb dates on intrusions that postdate contraction combined with new paleomagnetic data showing significant tilting of one area prior to intrusion suggest that regionally these thrusts were active before 85 100 ma the thrust faults are characterized by long relatively steeply dipping ramps and associated folds that are broad and open to close upright and overturned although now fragmented by cenozoic crustal extension individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike we correlate the structurally lowest thrust of the garden valley thrust system the golden gate mount irish thrust southward with the gass peak thrust of southern nevada this correlation carries the following regional implications at least some of the slip across jurassic to mid cretaceous foreland thrusts in southern nevada continues northward along the central nevada thrust belt rather than noaheastward into utah this continuation is consistent with age relations which indicate that thrusts in the type sevier belt in central utah are synchronous with or younger than the youngest thrusts in southern nevada this in turn implies that geometrically similar sevier belt thrusts in utah must die out southward before they reach nevada that slip along the southem nevada thrusts is partitioned

long lasting cadomian magmatic activity along an active northern gondwana margin u pb zircon and sr nd isotopic evidence from the brunovistulian domain eastern bohemian massif manuscript published version available in international journal of earth sciences 2016 doi 10 1007 s00531 016 1416 y

zircon and apatite u th he evidence for paleogene and neogene extension in the southern snake range nevada usa despite decades of study the timing rates and magnitude of extension in the basin and range are poorly quantified in some areas this study integrates new zircon and apatite u th he analyses zrnhe and aphe with published thermochronologic data to quantify these extensional parameters in the southern snake range ssr of east central nevada the new zrnhe dates range from 40 7 4 9 ma in the western ssr to 21 0 3 3 ma near the present day trace of the southern snake range decollement ssrd and the aphe dates range from 15 1 2 4 ma in the central ssr to 13 6 0 7 ma closest to the ssrd trace these new and previously published low temperature thermochronologic cooling ages were inverted for the extensional history of the ssr using a bayesian monte carlo method incorporating pecube the posterior extensional histories indicate three significant pulses of extension occurred during the paleogene and neogene 1 50 45 to 38 ma eocene 2 33 30 to 23 ma oligocene and 3 23 20 to 10 8 ma miocene modeled rates of extension were low at 0 5 mm a 1 however more rapid rates possibly occurred during the eocene and the miocene based on posterior histories net cumulative extension from posterior histories is 19 8 to 34 9 km with a mean of 29 7 km about 10 18 km of extension occurred during the eocene and oligocene model results indicate no relationship between extension and magmatism in the ssr our new model results and interpretations also indicate extensional collapse of the nevadaplano initiated prior to 17 ma

heterogeneous extrusion and exhumation of deep crustal variscan assembly geochronology of the western tatra mountains northern slovakia abstract the nature and style of mid crustal assembly and exhumation during continental collision has been investigated in the tatra mountains of the western carpathians the pre alpine basement of the western carpathians represents the easternmost exposure of the variscan orogen in europe which marks the collision of laurasia with gondwanian affiliated terranes during the palaeozoic the tatric crystalline unit of the western tatra in northern slovakia displays an inverted metamorphic sequence where a high grade unit comprising migmatites with relicts of eclogite has been thrust over a lower grade mica schist unit new geochronological and thermochronological data together with published thermobarometry illuminate the metamorphic history of the western tatra the upper unit eclogites with occasionally preserved omphacite record near isothermal decompression from 1 6 gpa to 1 0 1 2 gpa at 750 800 c which lead to intensive re equilibration at high pressure granulite facies conditions comparable to the peak metamorphic conditions of the host migmatite both eclogite and migmatite shared a retrograde p t path following the insertion of the eclogite assemblage into the migmatites the metamorphic evolution of the lower unit mica schist is constrained to peak p t conditions of 0 6 0 8 gpa and 640 and 660 c followed by retrogression this suggests that different rock types of the western tatra metamorphic core shared only their exhumation path from mid crustal levels id tims sm nd dating of garnet from eclogite yields a whole rock garnet isochron age of 337 10 ma with an initial e nd isotopic composition of 8 3 in situ u pb dating of monazite from a migmatite surrounding the eclogite shows one age population of c 380 ma whereas monazite from a migmatite away from the eclogite preserves a robust 340 11 ma age which is indistinguishable from sm nd garnet age and u pb age of zircons in the anatectic leucosome of the migmatite 347 7 ma a younger monazite age population from the migmatite of 300 16 ma is consistent with 40 ar 39 ar mica ages of c 310 ma this argues for a contemporaneous and likely shared exhumation path of the assemblage pair in situ monazite total pb analyses from the lower unit mica schists yields xenocrystic and c 370 ma ages but no geochronologic evidence for peak variscan tectonism exhumation of the deep crustal root occurred most probably in a two stage process the timing of the high pressure eclogite facies metamorphism before the onset of exhumation into the mid crust was likely between c 380 ma and 360 ma subsequent exhumation into the middle crust was coeval with migmatite generation at c 340 ma and garnet diffusion modeling suggest 30 c ma cooling rates the exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid crustal levels at a time coeval with anatexis and subsequently extruded with mid crustal material to the upper crust

understanding curvatures of the equipotential surface in gravity gradiometry the concept of curvatures of equipotential surfaces is of both theoretical and practice importance in gravity gradiometry the reason is that curvatures describe the shape of equipotential surfaces which can yield the information about the shape of the source although the fundamentals of curvatures are well established their connection to modern gravity gradiometry and the associated applications in exploration geophysics remain areas of active research in particular there is a misunderstanding in the calculation of the said curvatures directly from measured gravity gradient data that are now widely utilized in exploration geophysics the error stems from the incorrect use of the formulas in a fixed user coordinate system that are only valid in a rotated coordinate system we demonstrate that the gravity gradient tensor must be rotated to a local coordinate system whose vertical axis is aligned with the local anomalous gravity field direction so that the curvatures of the anomalous equipotential surf

petrogenesis and tectonic setting of cadomian felsic igneous rocks sandikli area of the western taurides turkey in the sandikli afyon region western taurides the late proterozoic rocks of the sandikli basement complex are composed of low grade meta sedimentary rocks guvercinoluk formation intruded by felsic rocks kestel cayi porphyroid suite kcps the kcps is a deformed and highly sheared dome shaped rhyolitic body with a granitic core quartz porphyry dikes intrude both the slightly metamorphic igneous and the sedimentary rocks of the basement complex both the quartz porphyries and rhyolites were converted into mylonites with relict igneous textures geochemical data show that these felsic igneous rocks are subalkaline and mainly granitic in composition with sio2 72 wt and al2o3 11 5 wt the chondrite normalized incompatible trace element patterns are characterized by distinct negative rb nb sr p ti and eu with enrichment in th u la ce nd sm and zr the ree patterns of the felsic rocks indicate a strong enrichment in lree but display slightly flat hree patterns according to geochemical characteristics and petrogenetic modeling extrusive and intrusive rocks of the kcps were probably derived from an upper continental crustal source partial melting of granites felsic rocks by 18 20 fractional melting plus 18 20 rayleigh fractional crystallization which seems to be the most effective igneous process during the crystallization of the kcps single zircon age data from the granitoids and fossils from the disconformably overlying sedimentary successions indicate that the metamorphism and the igneous event in the taurides are related to the cadomian orogeny based on the geological geochemical and petrogenetic correlation of the post collisional granitoids it is further suggested that the tauride belt in western central turkey was in a similar tectonic setting to the gondwanan terranes in north africa younger granitoids and southern europe spain france bohemia brno massifs during the late cadomian period

estimating magnetic dike parameters using a non linear constrained inversion technique an example from the sarna area west central sweden in this paper we describe a non linear constrained inversion technique for 2d interpretation of high resolution magnetic field data along flight lines using a simple dike model we first estimate the strike direction of a quasi 2d structure based on the eigenvector corresponding to the minimum eigenvalue of the pseudogravity gradient tensor derived from gridded low pass filtered magnetic field anomalies assuming that the magnetization direction is known then the measured magnetic field can be transformed into the strike coordinate system and all magnetic dike parameters horizontal position depth to the top dip angle width and susceptibility contrast can be estimated by non linear least squares inversion of the high resolution magnetic field data along the flight lines we use the levenberg marquardt algorithm together with the trust region reflective method enabling users to define inequality constraints on model parameters such that the estimated parameters are always in a trust region assuming that the maximum of the calculated gzz vertical gradient of the pseudogravity field is approximately located above the causative body data points enclosed by a window along the profile centred at the maximum of gzz are used in the inversion scheme for estimating the dike parameters the size of the window is increased until it exceeds a predefined limit then the solution corresponding to the minimum data fit error is chosen as the most reliable one using synthetic data we study the effect of random noise and interfering sources on the estimated models and we apply our method to a new aeromagnetic data set from the sarna area west central sweden including constraints from laboratory measurements on rock samples from the area

eigenvector analysis of gravity gradient tensor to locate geologic bodies we have developed a new method to locate geologic bodies using the gravity gradient tensor the eigenvectors of the symmetric gravity gradient tensor can be used to estimate the position of the source body as well as its strike direction for a given measurement point the eigenvector corresponding to the maximum eigenvalue points approximately toward the center of mass of the causative body for a collection of measurement points a robust least squares procedure is used to estimate the source point as the point that has the smallest sum of square distances to the lines defined by the eigenvectors and the measurement positions it s assumed that the maximum of the first vertical derivative of the vertical component of gravity vector gzz is approximately located above the center of mass observation points enclosed in a square window centered at the maximum of gzz are used to estimate the source location by increasing the size of the window the number of eigenvectors used in the robust least squares and

automatic interpretation of magnetic dike parameters using the analytical signal technique the analytical signal of the magnetic field is used to automatically determine the source parameters of dikelike structures the method is particularly useful for interpreting large amounts of data collected during airborne surveys because it makes full use of the high density of data along the flight lines while simultaneously checking for two dimensionality and strike directions by searching for coherent signals in neighboring profiles the maximum horizontal curvature of the amplitude of the analytical signal is used to locate the dikes along a given profile tests with synthetic data show that the dike s horizontal position is resolved accurately magnetic data from the siljan impact structure in sweden show that the estimated strikes are reliable and that dip depth and width estimates are coherent especially for well isolated dikelike structures

euler deconvolution of gravity tensor gradient data tensor euler deconvolution has been developed to help interpret gravity tensor gradient data in terms of 3 d subsurface geological structure two forms of euler deconvolution have been used in this study conventional euler deconvolution using three gradients of the vertical component of the gravity vector and tensor euler deconvolution using all tensor gradients these methods have been tested on point prism and cylindrical mass models using line and gridded data forms the methods were then applied to measured gravity tensor gradient data for the eugene island area of the gulf of mexico using gridded and ungridded data forms the results from the model and measured data show significantly improved performance of the tensor euler deconvolution method which exploits all measured tensor gradients and hence provides additional constraints on the euler solutions

enhancement and sharpening the migration images of the gravity field and its gradients potential field migration represents a rapid technique for imaging the subsurface based on gravity data however migration transformation usually produces a smooth and unfocused image of the targets due to the diffusive nature of the potential fields in this paper we introduce a method of the migration image enhancement and sharpening based on the application of the hybrid focusing stabilizer which combines the edge preserving smoothing filter with the minimum support functional the method is based on the model resolution matrix of the migration operator we also improve the migration image with a novel target oriented migration method the developed method of migration image enhancement and sharpening is illustrated by synthetic model studies and case studies the case study involves imaging the full tensor gravity gradient data collected in the nordkapp basin of the barents sea

large scale 3d inversion of potential field data inversion of gravity and or magnetic data attempts to recover the density and or magnetic susceptibility distribution in a 3d earth model for subsequent geological interpretation this is a challenging problem for a number of reasons first airborne gravity and magnetic surveys are characterized by very large data volumes second the 3d modelling of data from large scale surveys is a computationally challenging problem third gravity and magnetic data are finite and noisy and their inversion is ill posed so regularization must be introduced for the recovery of the most geologically plausible solutions from an infinite number of mathematically equivalent solutions these difficulties and how they can be addressed in terms of large scale 3d potential field inversion are discussed in this paper since potential fields are linear they lend themselves to full parallelization with near linear scaling on modern parallel computers moreover we exploit the fact that an instrument s sensitivity or footprint is considerably smaller than the survey area as multiple footprints superimpose themselves over the same 3d earth model the sensitivity matrix for the entire earth model is constructed we use the re weighted regularized conjugate gradient method for minimizing the objective functional and incorporate a wide variety of regularization options we demonstrate our approach with the 3d inversion of 1743 line km of falcon gravity gradiometry and magnetic data acquired over the timmins district in ontario canada our results are shown to be in good agreement with independent interpretations of the same data

late neoproterozoic to early palaeozoic palaoegeography of the holy cross mountains central europe an integrated approach study of geochemistry examination of isotope ages of detrital minerals palaeomagnetic analysis and a study of the trilobites were performed to provide constraints on the palaeogeographical position of the holy cross mountains in late ediacaran early palaeozoic time the geochemical results indicate an active continental margin or continental island arc provenance of the ediacaran sediments sediments from a passive continental margin were deposited here during the cambrian and ordovician the palaeomagnetic pole isolated from cambrian rocks of the malopolska region of the holy cross mountains corresponds to the cambrian segment of the baltic apparent polar wander path isotope age estimations indicate that cambrian sediments of the malopolska region contain detritus not only from a latest neoproterozoic source but also from sources with ages of c 0 8 0 9 ga 1 5 ga and 1 8 ga the malopolska brunosilesia dobrugea and moesia terranes which originally developed near the present southern edge of baltica and were partly involved in the cadomian orogen were dextrally relocated along its trans european suture zone margin the first stage of this movement took place as early as latest ediacaran time while baltica rotated anticlockwise anticlockwise rotation of baltica at the cambrian ordovician boundary implies further dextral movement of the malopolska block

impact induced melting of archean granulites in the vredefort dome south africa i anatexis of metapelitic granulites mid crustal archean pelitic granulites in the vredefort dome experienced a static low p granulite facies overprint associated with the formation of the dome by meteorite impact at 2 02 ga heating and exhumation were virtually instantaneous with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 c and locally even exceeded 1350 c this led to comprehensive melting of the precursor archean granulite assemblages grt bt qtz pl ksp crd opx sil followed by peritectic crystallization of aluminous alkali feldspar crd spl crn sil parageneses and the segregation of small evolved biotite leucogranite bodies however at a distance of c 6 km from the centre pre impact rock features are largely preserved although partial replacement of garnet by symplectitic coronas of crd opx spl pl and biotite by orthopyroxene indicate that peak temperatures approached 775 50 c thin interstitial moats of k feldspar are closely associated with the orthopyroxene coronas they are interpreted as the remnants of low proportion partial melts generated by biotite breakdown both the textures and mineral compositional data support reduced equilibration volumes in these rocks which reflect rapid isobaric cooling following shock heating and exhumation the high temperatures and strong lateral thermal gradient are consistent with the modelled impact induced isotherm pattern for a 200 300 km diameter impact crater

chronological constraints on the pre orogenic history burial and exhumation of deep seated rocks along the eastern margin of the variscan orogen bohemian massif czech republic key lithological units of the high grade eastern margin of the bohemian massif were dated using the u pb and pb pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the variscan orogenic root the protolith ages for metagranitoids orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from cambro ordovician to early devonian times probably related to discontinuous intracontinental rifting of neoproterozoic crust our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted neoproterozoic crust preserved farther to the east as the brunia microcontinent zircon ages for felsic granulites high grade gneisses of the lower crust and of a syn convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 ma exhumation of the lower crust to mid crustal levels was a localized process that occurred at 340 ma and was associated with crustal scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin both types of exhumation mechanisms were driven by deep level wedging indentation of the easterly brunia continent followed by deposition of heavy minerals and pebbles derived from high pressure rocks in the adjacent foreland basin final orogenic development was characterized by ne sw dextral transpressive shearing parallel to the brunia margin as well as dextral trans tension associated with activity along the elbe lithospheric fault these processes affected the marginal parts of the orogenic root and were accompanied by 330 to 325 ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries rotation of the assembled orogenic belt accompanied by lithospheric faulting driven by westerly subduction roll back may be the most plausible model to explain late deformation of the orogenic root

interpretation of aeromagnetic data using eigenvector analysis of pseudogravity gradient tensor this study has shown that the same properties of the gravity gradient tensor are valid for the pseudogravity gradient tensor derived from magnetic field data assuming that the magnetization direct

noise reduction through joint processing of gravity and gravity gradient data in mineral and oil exploration gravity gradient data can help to delineate small scale features that cannot be retrieved from gravity measurements removing high frequency noise while preserving the high frequency real signal is one of the most challenging tasks associated with gravity gradiometry data processing we present a method to reduce gravity and gravity gradient data noise when both are measured in the same area based on a least squares simultaneous inversion of observations and physical constraints inferred from the gravity gradient tensor definition and its mathematical properties instead of handling profiles individually our noise reduction method uses simultaneously measured values of the tensor components and of gravity in the whole survey area benefiting from all available information synthetic examples show that more than half of the random noise can be removed from all tensor components and nearly all the noise from the gravity anomaly without altering the high frequency information we apply our method to a set of marine gravity gradiometry data acquired by bell geospace in the faroe shetland basin to demonstrate its power to resolve small scale features

three dimensional regularized focusing inversion of gravity gradient tensor component data we develop a new method for interpretation of tensor gravity field component data based on regularized focusing inversion the focusing inversion makes its possible to reconstruct a sharper image of the geological target than conventional maximum smoothness inversion this new technique can be efficiently applied for the interpretation of gravity gradiometer data which are sensitive to local density anomalies the numerical modeling and inversion results show that the resolution of the gravity method can be improved significantly if we use tensor gravity data for interpretation we also apply our method for inversion of the gradient gravity data collected by bhp billiton over the cannington ag pb zn orebody in queensland australia the comparison with the drilling results demonstrates a remarkable correlation between the density anomaly reconstructed by the gravity gradient data and the true structure of the orebody this result indicates that the emerging new geophysical technology of the airborne gravity gradient observations can improve significantly the practical effectiveness of the gravity method in mineral exploration

geology and geochemistry of the pre early cambrian rocks in the sandikli area implications for the pan african evolution of nw gondwanaland abstract the pre early cambrian sandikli basement complex in western central anatolia comprises a low grade meta sedimentary succession guvercinoluk formation and meta rhyolites intruded by meta quartz porphyry rocks kestel cayi porphyroid suite the guvercinoluk formation consists of alternation of meta siltstones and meta sandstones with olistostromal conglomerates rare black chert and cherty meta dolomite lenses the kestel cayi porphyroid suite is a deformed highly sheared dome shaped rhyolitic body with quartz porphyry rocks quartz porphyry dykes intrude both the volcanic carapace and the meta sedimentary rocks of the guvercinoluk formation both the meta quartz porphyry rocks and meta rhyolites are typically mylonitic with relict igneous textures geochemical data indicate that the felsic rocks of the kestel cayi porphyroid suite are subalkaline and display characteristic features of post collisional i type granitoids the basement complex is unconformably overlain by variegated conglomerates mudstones and arkosic sandstones with andesitic lavas followed by siliciclastic rocks and carbonates that yielded early middle cambrian fossils based on the geochemical characteristics of the felsic rocks of kestel cayi porphyroid suite and the depositional features of the associated sediments it is suggested that the sandikli basement complex is related to a post collisional extension event in nw gondwanaland similar occurrences elsewhere have been related to a transition from continental plate convergence to continental plate divergence along the pan african belt

early cambrian location and affinities of the brunovistulian terrane central europe in the light of palaeomagnetic data the brunovistulian was one of the first tectonic units of central europe to be defined as a terrane in spite of extensive studies the early palaeozoic palaeogeographical position and provenance of this unit remains unclear a palaeomagnetic study of the lower cambrian red beds and a study of the trilobite fauna were performed to provide constraints on the palaeogeographical position of the terrane in early cambrian time good quality palaeomagnetic data obtained from the lower cambrian red beds suggest the mixed nature of the early cambrian geomagnetic field and a nearly equatorial position a palaeolatitude of c 7 of the brunovistulian terrane in the early cambrian comparison of this palaeolatitude with existing palaeogeographical models leads to the conclusion that at this time the brunovistulian terrane was separated by a great distance from the avalonian margin of gondwana the terrane was located within the cadomian belt occupying a position north of the present day northern margin of africa it was coupled to the present day southern margin of baltica during the cambrian when baltica moved along the cadomian margin of gondwana another possibility is that the brunovistulian terrane could have remained near this margin of baltica since grenvillian time and was incorporated into the cadomian orogen

geochemistry across an exposed section of archaean crust at vredefort south africa with implications for mid crustal discontinuities abstract the central region of the vredefort structure consists of a semi circular multi layered sequence of crystalline rocks which are nearly vertical in attitude and which increase in metamorphic grade towards the core of the structure together with the overlying precambrian strata this sequence provides a cross section through almost the entire crustal section of the kaapvaal craton 36 km the upper part of the vredefort crystalline crust consists of a 3 0 ga sequence of differentiated felsic rocks in amphibolite facies the lower crust consists of a complex and heterogeneous both chemically and isotopically high grade metamorphic terrain of charnockites granulites mafic and felsic and supracrustal rocks the upper crust is separated from the lower crust by the vredefort discontinuity a brittle ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock petrographic chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution this is indicative of different geological environments prior to their present juxtaposition we speculate that the upper and the lower parts of the vredefort crystalline crust were juxtaposed during intracratonic thrusting 2 8 ga ago and that the entire 36 km section of crust including the vredefort discontinuity were rotated and uplifted into their present vertical position during the 2 0 ga vredefort catastrophe

approximating edges of source bodies from magnetic or gravity anomalies cordell and grauch 1982 1985 discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks the final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes we attempt to automate their final step our method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources the method applies to both local surveys and to continent wide compilations of magnetic and gravity data e g zietz 1982 simpson et al 1983a kane et al 1982

aeromagnetic expression of faults that offset basin fill albuquerque basin new mexico high resolution aeromagnetic data acquired over the albuquerque basin show widespread expression of faults that offset basin fill and demonstrate that the aeromagnetic method can be an important hydrogeologic and surficial mapping tool in sediment filled basins aeromagnetic expression of faults is recognized by the common correspondence of linear anomalies to surficial evidence of faulting across the area in map view linear anomalies show patterns typical of extensional faulting such as anastomosing and en echelon segments depths to the tops of faulted magnetic layers showing the most prominent aeromagnetic expression range from 0 to 100 m sources related to subtler fault expressions range in depths from 200 to 500 m we estimate that sources of the magnetic expressions of the near surface faults likely reside within the upper 500 600 m of the subsurface the linear anomalies in profile form show a range of shapes but all of them can be explained by the juxtaposition of layers having different magn

risk faults in stable crust of the eastern bohemian massif identified by integrating gnss levelling geological geomorphological and geophysical data abstract it is difficult to identify and assess the recently active faults representing a risk factor for key human activities in the areas of stable crust with slow tectonic deformation and reduced seismicity we face the problem in the eastern bohemian massif central european variscides at a contact with the eastern alpine western carpathian orogenic belt through building its integrated geodynamic model based on the long term gnss data cross checked with the results from existing levelling geological surveys geophysical surveys and a new geomorphological analysis the model shows differently moving crustal blocks determining the intervening boundary zones as the main risk regions where weak earthquakes are also usually concentrated primarily diendorf cebin tectonic zone nectava konice fault hana faults b la fault and bulhary fault shear zone the maximum horizontal gnss differential velocities in the exposed bohemian massif reach up to 1 5 2 0 mm yr 1 the up thrusted segments of the western carpathians move individually 2 mm yr 1 the integration of a number of full area datasets corresponding to various dimensions and depth levels of crustal processes allowed us to highlight the important fault zones as driving elements of regional geodynamics

archean crustal structure of the kaapvaal craton south africa evidence from the vredefort dome abstract crystalline archean basement rocks in the core of the vredefort dome present a profile through a substantial part of the middle and lower crust of the kaapvaal craton previously this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal scale late archean brittle ductile thrust zone lithological and structural mapping across the dome indicates however that the basement lithologies share a common polyphase tectonic history culminating in high grade metamorphism and melting at 3 1 ga no evidence was found of the postulated tectonic terrane boundary but the alleged boundary does coincide with a 1 2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses the implications of these results for archean regional tectonic models for the kaapvaal craton are discussed

brunovistulian terrane bohemian massif central europe from late proterozoic to late paleozoic a review the brunovistulian terrane represents a microcontinent of enigmatic proterozoic provenance that was located at the southern margin of baltica in the early paleozoic during the variscan orogeny it represented the lower plate at the southern margin of laurussia involved in the collision with the armorican terrane assemblage in this respect it resembles the avalonian terrane in the west and the istanbul zone in the east there is a growing evidence about the presence of a devonian back arc at the margin of the brunovistulian terrane the early variscan phase was characterized by the formation of devonian extensional basins with the within plate volcanic activity and formation of narrow segments of oceanic crust the oldest visean flysch of the rheic rhenohercynian remnant basin protivanov andelska hora and horni benesov formations forms the highest allochthonous units and contains together with slices of silurian bohemian facies clastic micas from early paleozoic crystalline rocks that are presumably derived from terranes of armorican affinity although provenance from an active brunovistulian margin cannot be fully excluded either the development of the moravo silesian late paleozoic basin was terminated by coal bearing paralic and limnic sediments the progressive carboniferous stacking of nappes and their impingement on the laurussian foreland led to crustal thickening and shortening and a number of distinct deformational and folding events the postorogenic extension led to the formation of the terminal carboniferous early permian boskovice graben located in the eastern part of the brunovistulian terrane in front of the crystalline nappes the highest allochthonous westernmost flysch units locally with the basal slices of the devonian and silurian rocks thrusted over the silesicum in the nw part of the brunovistulian terrane may share a similar tectonic position with the giessen harz nappes the silesicum represents the outermost margin of the brunovistulian terrane with many features in common with the northern phyllite zone at the avalonia armorica interface in germany

the gradient tensor of potential field anomalies some implications on data collection and data processing of maps the full gradient tensor is presently not measured routinely onboard airplanes or on land this paper describes some improvements that can be made in strategies of data collection and in processing of potential field maps if such tensor measurements were available we suggest that in addition to producing for example standard total field anomaly maps the invariants of the tensor be mapped strikes of magnetic or gravimetric structures may be determined from minimizing the power in the first row and column of the tensor invariants can be looked upon as nonlinear filters enhancing sources with big volumes their lateral resolution is superior to that of the field proper and for a given resolution the flight altitude and separation between flight lines can be increased compared with the standard mode of operation in airborne surveys the distance between flight lines is normally much larger than the height above the ground this may introduce severe aliasing effects in the direction perpendicular to the

inversion of airborne gravity gradient data southwestern oklahoma the diagonal elements of the gravity gradient tensor as recorded by the bell airborne gravity gradient survey system ggss are used to determine the basement topography in southwestern oklahoma this determination is accomplished through a nonlinear inverse procedure based on the conjugate gradient algorithm the resulting model contains a ridge of shallow basement material or 3 0 km deep trending east southeast this ridge is bounded on the north and the south by basement troughs the northern trough extends as deep as 10 0 km the gradient field which results from this model fits most of the ggss observations within their estimated errors of 12 0 e 1 eotvos 10 super 6 mgal cm the model is in general agreement with a set of available oil well depths to the basement and with inferred faults in these igneous rocks to assess the derived solution the problem was linearized about the final model and the linear parameter resolution and parameter covariances were computed generally the basement depths are well resolved and the resolution matrix is diagonally dominant furthermore the parameter standard errors are small 72 parameters out of 98 have errors less than 1 0 km

understanding the structural setting in the southern apennines italy insight from gravity gradient tensor abstract the lucania apennines form the frontal part of the apennines in southern italy this orogenic belt was formed in response to late miocene early pleistocene shortening and allochthon emplacement toward the northeast and was subsequently affected by extensional faulting which migrated to northeast ahead of the thrust system as a result contraction structures are found associated to pliocene early pleistocene thrust top basins in the ne part of the area whereas extensional faults shape middle pleistocene holocene basin on the sw sector the main structural grain of the belt is nw se but e w structures are widespread in the area as a result of phases of non coaxial thrusting and strike slip faulting the analysis one by one of the components of the gravity gradient tensor ggt has proved to yield a fine image of the structural setting of the investigated area ggt is a second rank tensor containing the second spatial derivatives of the gravity potential and in this paper is used instead than the more traditional gravity data the tzz component allows an accurate description of the location and of the shape of basins and other structures but different components of the ggt provide even more detailed insights for such structures in particular we found that the tzy and tyy locate well those structures trending closer to the e w direction for both thrust top and extensional basins and their termination against the main nw se structures the combined use of components of ggt provides a finer definition of the anomaly sources particularly if a good knowledge of their strike and depth is independently estimated with other geological and geophysical investigations

interpretation of magnetic and gravity gradient tensor data using normalized source strength a case study from mcfaulds lake northern ontario canada in this paper we present a case study on the use of the normalized source strength nss for interpretation of magnetic and gravity gradient tensors data this application arises in exploration of nickel copper and platinum group element ni cu pge deposits in the mcfaulds lake area northern ontario canada in this study we have used the normalized source strength function derived from recent high resolution aeromagnetic and gravity gradiometry data for locating geological bodies in our algorithm we use maxima of the normalized source strength for estimating the horizontal location of the causative body then we estimate depth to the source and structural index at that point using the ratio between the normalized source strength and its vertical derivative calculated at two levels the measurement level and a height h above the measurement level to discriminate more reliable solutions from spurious ones we reject solutions with unreasonable estimated structural indices this method uses an upward continuation filter which reduces the effect of high frequency noise in the magnetic case the advantage is that in general the normalized magnetic source strength is relatively insensitive to magnetization direction thus it provides more reliable information than standard techniques when geologic bodies carry remanent magnetization for dipping gravity sources the calculated normalized source strength yields a reliable estimate of the source location by peaking right above the top surface application of the method on aeromagnetic and gravity gradient tensor data sets from mcfaulds lake area indicates that most of the gravity and magnetic sources are located just beneath a 20 m thick on average overburden and delineated magnetic and gravity sources which can be probably approximated by geological contacts and thin dikes come up to the overburden