geology and tectonics of the betic zone se spain abstract the southern margin of the iberian continent comprises a complex geological history which can be traced back into the proterozoic when a gneissic basement was formed a cover of nevado filabride rocks was deposited onto this proterozoic basement during the palaeozoic the permian intrusion of the bedar granite suggests that the palaeozoic sedimentary rocks now exposed in the betic cordilleras have been affected by the hercynian orogeny similar to that of palaeozoic rocks in the adjoining ossa morena zone the hercynian basement was probably covered by alpujarride carbonates in the mesozoic when rifting opened tethys and intruded 140 ma old basic dyke swarms the palaeozoic basement of the betic cordilleras comprises a suture of late cretaceous subduction 80 85 ma b p this can be inferred from hp lt metamorphic facies obducted in coeval thrust slices of nevado filabride rocks the nevado filabride nappes include mesozoic carbonates transformed into marbles macael the suture was probably annnealed after the closure of tethys the mesozoic boundary between africa and europe and the rifting in tethys ceased during the palaeogene when the atlas mountains were formed 36 ma b p subsequently relative movements between africa and europe in the neogene were accommodated by strike slip faults these define the iberian microplate moving eastward by atlantic spreading and possibly subducting under italy neogene deformation of iberia is most intensive in southeastern spain due to emplacement of a mantle diapir under the alboran area this occurred early in the neogene aquitanian and caused nappe spreading involving alpujarride and malaguide rocks the subsequent neogene history has now been studied in great detail and is reviewed here it involves clear examples of calc alkaline volcanism most probably not related to subduction seismic motion along strike slip faults within the iberian microplate and syntectonic deposition in intermontane neogene basins

determination of present day stress tensor and neotectonic interval in the spanish central system and madrid basin central spain a brittle deformation tectonic analysis was performed in central spain spanish central system and madrid basin in order to decipher and understand the deformation processes that take place in a typical intracontinental zone 1174 fault slickensides obtained in materials with ages between late cretaceous and quaternary have been analyzed by means of fault population analysis methods to reconstruct paleostress tensors nine earthquake focal mechanisms have been determined with magnitudes ranging between 3 and 4 1 with regard to regional structural features and sedimentary record data the characteristics of present day and neotectonic stress fields have been figured out which determine the neotectonic period for this region thus we have established that the intraplate zone represented by central spain has been subjected to a stress field from the middle miocene until the present day with a largest horizontal shortening direction shmax located between n130e and n160e finally three paleostress maps with the main active structures are presented for a middle miocene to late miocene the period when the spanish central system was mainly formed b late miocene to quaternary and c the present day stress field deduced from earthquake focal mechanisms

strength of moist sand controlled by surface tension for tectonic analogue modelling abstract a technique has been developed to control the strength of moistened sand in a quantifiable accurate and reproducible way while other mechanical properties were maintained strength of dry sand was increased through adding a small amount of liquid in order to control the additional cohesion of moist sand the influence of the surface tension of the liquid was investigated direct shear experiments were performed on four granular materials at confining stress levels below 1 kpa it has been found that the surface tension of the added liquid controlled the additional apparent cohesion of sand with high accuracy the mechanical properties of moist sand show dynamic similarity towards natural brittle rock which enables analogue modelling of fault formation fault reactivation and tension fracture formation in the brittle regime with a controlled strength profile furthermore experimental results fit well to shear strength models from this followed the direct proportionality of the unsaturated shear strength parameter b relative to the matrix suction measured at low stress levels moreover shear strength turned out to be also a function of grain size and the grain shape

imaging and observing the electrical moho abstract defining the depth to the base of the crust is of profound importance for understanding and developing theories of the secular variation of our planet including crustal formation and evolution since its discovery 100 year ago the seismic moho is almost ubiquitously used as the crust mantle boundary reference laboratory studies on crustal and mantle rocks suggest that there should be a concomitant step lie change in electrical conductivity an electric moho emoho at the crust mantle boundary we examine resolution properties of electrical and electromagnetic methods for imaging and observing an emoho for three generic models one to represent the archean one for the proterozoic and the third for the phanerozoic we show that in all three cases given the existence in most localities of a conducting lower crust compared to the upper crust and upper lithospheric mantle the problem is difficult and at the edge and often beyond confident resolution for some highly unusual localities however the lower crust is more resistive than the underlying upper mantle and in those cases it is possible to resolve an emoho but very high quality data are required examples of emoho observations around the globe are discussed focussing on the results from a site on the southwestern part of the slave craton northwestern canada

determination of fe mg partitioning between perovskite and magnesiowustite the partition coefficient of fe and mg between perovskite and magnesiowostite k mg fe pv mw has been measured by adopting a b 2 o 3 catalyst technique the present experimental results demonstrate that k mg fe pv mw is a factor of two closer to unity than that obtained in previous studies k mg fe pv mw increases from 0 16 to 0 35 with increasing mg of magnesiowustite from 50 to 86 on the basis of these results the compositions of enstatite and magnesiowustite included in diamond imply that the temperature at the top of the lower mantle is 1870 2040 k the mg 100mg mg fe of perovskite and magnesiowustite at the top of the lower mantle are 93 and 82 83 respectively for a pyrolitic mantle model the upper bound of the lower mantle temperature estimated in this study do not allow a larger fraction of perovskite than the pyrolitic mantle

boat towed radio magnetotelluric and controlled source audio magnetotelluric study to resolve fracture zones at aspo hard rock laboratory site sweden boat towed radio magnetotelluric rmt measurements using signals between 14 and 250 khz have attracted increasing attention in the near surface applications for shallow water and archipelago areas

electric and magnetic galvanic distortion decomposition of tensor csamt data application to data from the buchans mine newfoundland canada electric and magnetic galvanic distortion decomposition of tensor csamt data application to data from the buchans mine newfoundland canada

comparison of induced polarization and controlled source audio magnetotellurics methods for massive chalcopyrite exploration in a volcanic area in this paper we compare and contrast the results of field experiments with the dipole dipole induced polarization ip and controlled source audio magnetotellurics csamt methods along the same survey profiles at a test area that was subject to extensive drilling and detailed geological investigation the ore bodies are interbedded between two series of dacitic tuff the depth and thickness of the massive chalcopyrite pyritesphalerite body vary between 25 and 100 m and 0 5 and 16 m respectively resistivity and ip phase measurements on the core samples collected from the test area provide some idea of the relative differences between the background rock units and the target the resistivity of the chalcopyrite samples varies between 0 6 to 2 ohm m and provides sufficient resistivity contrast with surrounding volcanic rock units for target detection the results of dipole dipole ip profiling with a 50 m dipole length conducted along two profiles are presented in the form of apparent resistivity and phase pseudosections csamt measurements were made at 13 frequencies from 2 hz to 8192 hz along three profiles the receiver dipole length was 25 m the csamt data are presented in the form of pseudosections using conventional and new definitions of apparent resistivity the elliptical contours of low apparent resistivity generated by the transition field notch can be misleading with respect to the real anomaly of the ore body these artificial anomaly patterns are suppressed by making use of an alternative apparent resistivity definition derived from the frequency normalized impedance the qualitative interpretation based on the ip and csamt pseudosections shows that the location and the extension of the ore body are indicated better in the csamt apparent resistivity data computed from the alternative definition the qualitative interpretation of the ip data is difficult because of the 3 d effect arising from the neighboring thicker parts of the ore body and pyrite particles within the basement at the final stage the far field range of the csamt apparent resistivity and phase data is identified by the visual inspection of the individual sounding diagrams the models derived from the 2 d occam inversion carried out on the far field data are compared with the drill hole information and are found to describe the actual geological situation

analysis of a gradual change in stress regime example from the eastern iberian chain spain abstract the compression extension passage in the eastern iberian chain during the miocene is analyzed structural data suggest that it is a gradual change in stress regime which does not involve rotation but interchange of 1 and 2 axes one of them remaining always vertical the gradual stress evolution is interpreted by a numerical graphic method y r diagram which optimizes combinations of y azimuth of the maximum horizontal axis and r stress ratio explaining groups of fault movements the y r diagrams of some fault populations measured in the castellon region provide optimum tensors appearing within a continuous cloud of solutions from the compressional field to the extensional one movement sequences obtained from superimposed striations indicate a systematic stress change between these two regimes

progressive deformation in anisotropic rocks abstract field observations show that the majority of crustal rocks possess a penetrative foliation defined by either compositional layering preferred orientations of crystals or both ductile deformation involving planar anisotropy of viscosity can be characterized by an anisotropy factor q n r s the ratio of the bulk viscosities in pure and simple shear respectively this ratio of the normal and shear viscosities may be determined analytically if the anisotropy is resolvable in a multilayer sequence with individual laminae of isotropic viscosity in that case the resistance to normal compression will be largely controlled by the competent layers whereas the resistance to shear is controlled by the soft layers more specifically the viscosities of the individual laminae add up like resistances in series and in parallel in the expressions for the normal and shear viscosities respectively the reorientation of the bulk stress within an anisotropic multilayer is systematically investigated for a range of anisotropy factors the mode of progressive deformation is controlled only by the anisotropy factor and the orientation of the principal deviatoric stress the rate of deformation can be scaled if the bulk normal viscosity and the magnitude of the principal deviatoric stress are also known the results are illustrated in a series of nomograms showing the spectrum of strain and rotation histories possible in rock volumes deforming with planar anisotropy it appears that with increasing anisotropy factor the deformation spectrum will narrow on simple shear irrespective of the orientation of the background or bulk deviatoric stress axes plane strain and isochoric conditions are assumed throughout the analysis

sandbox experiments of inverted listric and planar fault systems abstract the kinematic and geometric evolution of basement controlled inversion structures has been analyzed using a plane strain sandbox apparatus the results of two representative scaled analogue models comprising rigid listric and planar detachment faults that both have cut off angles of 60 to the horizontal are described in detail the hangingwall above these detachment geometries was constructed from alternate layers of sand and mica in order to simulate an anisotropic sedimentary sequence in both experiments initial shortening at the onset of inversion was accommodated by bulk strain this was followed by reactivation of the basal detachment together with the main basin controlling planar and listric faults which propagate upwards through the post rift sequence at approximately the same angle as the upper part of the detachment fault i e 60 reactivation of relatively steep intra basinal extensional faults within the cover was restricted to nucleation of reserve faults and thrusts in the region of the tips during approximately the first 10 to 15 contraction this resulted in gently dipping thrusts that propagated upwards through the post rift sequence to produce harpoon shaped structures later in the inversion phase after approximately 15 contraction backthrusts cut through and displaced the original antithetic extensional graben faults producing downward tapering wedge shaped structures at the margins of the basin although deformation in these experiments was limited to purely dip slip displacement the nature of the original extensional faults and their reactivated tip regions resulted in cross sectional geometries that have a marked resemblance to strike slip induced flower structures uplift was asymmetric and centred above the upper part of the main detachment above the listric fault but was more symmetrical and centred above the soling out point of the detachment in the case of the planar fault geometry the results of these experiments show distinct similarities to structures seen on seismic sections form the southern north sea and other inverted sedimentary basins

on results of a controlled source audiofrequency magnetotelluric survey at the puhimau thermal area kilauea volcano hawaii by l c bartel and r d jacobson geophysics 52 665 677 may 1987 a growing number of papers being published on the csamt mt curve transformation which as the authors state allows a simpler magnetotelluric interpretation of the corrected csamt curves the concept of near field corrections is based on electromagnetic relations over a homogeneous earth and the effects of subsurface layers or lateral inhomogeneities are usually neglected bartel and jacobson 1987 especially suppress the bounds of the near field correction after presenting several near field correction curves over a homogeneous earth in their figure 2 which includes an idealistic demarcation line instead of a gradual change between near field and far field regions they simply add that for a layered earth a similar demarcation occurs between the far and near field regimes further the problem of lateral inhomogeneities is not mentioned in the paper such a description might lead to an oversimplification i should like here to underline both limitations

mineral exploration in the thompson nickel belt manitoba canada using seismic and controlled source em methods seismic reflection and electromagnetic em data were acquired near thompson manitoba canada to map the subsurface extent of the paleoproterozoic nickel ore bearing ospwagan group these data are supplemented by surface and borehole geology and by laboratory measurements of density seismic velocity and electrical conductivity which indicate that ospwagan group rocks are generally more seismically reflective and electrically conductive than the archean basement rocks that envelop them the combined seismic em interpretation suggests that the thompson nappe cored by ospwagan group rocks lies blind beneath the archean basement gneisses to the east of the subvertical burntwood lineament in a series of late recumbent folds and or southeast dipping reverse faults the em data require that the shallowest of these fold fault structures occur within the basement gneisses or perhaps less conductive ospwagan group rocks the results of this study demonstrate how seismic and deep sounding em methods might be utilized as regional exploration tools in the thompson nickel belt

tensor csamt studies at the buchans mine in central newfoundland a novel application of the tensor controlled source audio magnetotelluric csamt method was part of a multidisciplinary geophysical study of an existing mine site at buchans newfoundland the orthogonal components of the horizontal electromagnetic fields used for magnetotelluric and csamt interpretation of the earth s conductivity structure were found to be inappropriate at buchans because of strong scattering in the electric fields instead the length of the major axes of the electric and magnetic field polarization ellipses and the vertical magnetic field were used as data the data from two bipole sources demonstrate that the bulk response of the earth in the vicinity of buchans is predominantly one dimensional 1 d these data were inverted to layered earth models with a first order correction for electric field distortions the parameter space considered during the inversion was contracted substantially by incorporating the vertical magnetic field data and by using depths to interfaces as determi

shear band foliation as an indicator of sense of shear field observations in central spain the macizo de nevera a paleozoic basement inlier surrounded by essentially flat lying mesozoic cover rocks of central spain exhibits an extremely consistent system of steep dextral strike slip faults different faults have a variable amount of displacement but their en echelon arrangement and similar sense of displacement imply a cogenetic origin a maximum displacement of 1 0 km corresponding to an averaged angular shear strain of 1 5 and peak values of 6 6 occurs along the central part of the 6 km long truchas ductile shear zone an early axial plane cleavage is locally deflected into the truchas shear zone and superposed by coarse shear band foliation the consistent angular relationship between the new foliation and the shear zone boundary confirms the megascopic sense of shear

x ray microanalysis of high pressure high temperature phases synthesized from natural olivine in a diamond anvil cell the transformation products of natural mg0 83fe0 17 2sio4 olivine have been prepared at high pressures and high temperature in a laser heated diamond anvil cell dac the study of the quenched high pressure phases has been performed using analytical transmission electron microscopy the iron magnesium partition coefficient between perovskite pv and magnesiowustite mw defined as kmw pv fe mg mw fe mgpv was measured in samples synthesized at pressures from 25 to 75 gpa under various heating conditions in all cases iron goes preferentially into magnesiowustite it was observed however that the partition coefficient decreases strongly between 25 and 40 gpa at pressures higher than 40 gpa it is constant 3 5 within experimental error although there is a systematic increase in k as t decreases the effect of temperature is weak at high pressures from the values of the iron partition coefficient and previously existing thermodynamic values we obtain a consistent thermodynamic data set for the major constituents of the lower mantle the equilibrium compositions of mg fe 2sio4 spinel mg fe sio3 perovskite and mg fe o magnesiowustite phases have been measured and a new high pressure phase of composition mg fe si2o5 is shown to occur in very high temperature regions probably due to high temperature breakdown or non stoichiometry of mg fe sio3 perovskite

pressure and temperature dependence of elastic wave velocity of mgsio3 perovskite and the composition of the lower mantle abstract simultaneous measurements of compressional p and shear s wave velocities and unit cell volume density have been conducted on mgsio 3 perovskite to 9 2 gpa and 873 k using ultrasonic interferometry in conjunction with x ray diffraction finite strain analysis of current data allowed us to determine the elastic moduli and their pressure derivatives independent of the pressure measurement the results are k s 253 2 gpa g 173 1 gpa k s p 4 4 1 and g p 2 0 1 the temperature derivatives of the bulk and shear moduli were tightly constrained from acoustic velocity measurements yielding k s t 0 021 2 gpa k and g t 0 028 2 gpa k p and s wave velocities and density profiles for a pyrolite compositional model consisting of magnesium silicate perovskite and magnesiowustite are calculated at lower mantle depths along a 1600 k adiabatic geotherm the results agree with those of seismic model prem within 0 5 0 5 and 0 3 respectively from 800 to 2600 km the effect of fe and mg partitioning between magnesium silicate perovskite and magnesiowustite on the calculated velocities and density is negligibly small compared to the effects of composition and temperature simultaneous constraints from p and s wave velocities and density support for a homogeneous lower mantle with pyrolitic composition

element partitioning between magnesium silicate perovskite and ferropericlase new insights into bulk lower mantle geochemistry n this study we investigated iron magnesium exchange and transition metal trace element partitioning between magnesium silicate perovskite mg fe sio3 and ferropericlase mg fe o synthetised under lower mantle conditions up to 115 gpa and 2200 k in a laser heated diamond anvil cell recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy atem and nanometer scale secondary ion mass spectroscopy nanosims iron concentrations in both phases were obtained from x ray energy dispersive spectroscopy measurements and nanosims our results are the first to show that recently reported spin state and phase transitions in the lower mantle directly affect the evolution of fe mg exchange between both phases mg perovskite becomes increasingly iron depleted above 70 80 gpa possibly due to the high spin low spin transition of iron in ferropericlase conversely the perovskite to post perovskite transition is accompanied by a strong iron enrichment of the silicate phase ferropericlase remaining in the fe rich phase though nanoparticles of metallic iron were observed in the perovskite bearing runs suggesting the disproportionation of ferrous iron oxide but were not observed when the post perovskite phase was present implications on the oxidation state of the earth and core segregation will be discussed transition trace element ni mn concentrations determined with the nanosims show similar trends and could thus be used to trace the origin of diamonds generated at depth this study provides new results likely to improve the geochemical and geophysical models of the earth s deep interiors

extensional tectonics in the central iberian peninsula during the variscan to alpine transition abstract the passage from the variscan cycle to the early alpine framework in the central part of the iberian peninsula can be explained in terms of a transitional process involving four clearly differentiated tectonic episodes 1 1 a first variscan compressional stage vi middle devonian to early carboniferous dominated by compressional conditions leading to the building up of the orogenic edifice the stress regime was relevant to what might be called variscan type compression e w oriented this stage was characterized by major himalayan type tectonics with frontal nappes thrusts overturned folds lateral transcurrent ramps and localized anatectic magmatism minor synorogenic extension and plutonism was also recorded during this stage in the tormes granitic dome 2 2 a second variscan stage v2 early to middle carboniferous was characterized by increasing extensional conditions leading to widespread plutonism adamellites granodiorites wanning compressional conditions were restricted to the eastern and southern realms of central iberia the eastern part of the spanish central system and the toledo mountains 3 3 a third stage here defined as late variscan lv developed from middle carboniferous to early permian as a result of n s late orogenic extension this episode is relevant to detachment tectonics and the gravitational collapse of the variscan orogenic edifice under combined simple pure shear conditions plutonism granites and leucogranites was still of major importance early permian andesitic to dacitic volcanism and sedimentary basins developed within the eastern part of the spanish central system 4 4 a fourth stage here defined as early alpine ea early permian to triassic marks the onset of the alpine framework this stage was characterized by what might be called an early alpine type regional stress regime i e e w extension and n s compression within a simple shear model and resulted in the configuration of the iberian peninsula into two contrasted realms a western inherited variscan block and an eastern alpine block subjected to post orogenic extension elements developed during this event include n s high angle normal faults nw se and ne sw conjugate strike slip faulting and asymmetric rifting involving listric low angle detachments

postspinel transformations and the width of the 670 km discontinuity a comment on postspinel transformations in the system mg2sio4 fe2sio4 and some geophysical implications by e ito and e takahashi in their recent paper dealing with postspinel transformations in the system mg2sio4 fe2sio4 ito and takahashi 1989 showed that the reaction mg fe 2sio4 mg fe sio3 mg fe o spinel perovskite magnesiowustite occurs over a very narrow pressure interval at 1600 c for values of fe fe mg of between 0 0 and 0 3 it is the purpose of this comment to point out that the argument for a narrow transition interval can be independently confirmed and strengthened using ito and takahashi s data on fe mg partitioning between the phases spinel perovskite and magnesiowustite ito and takahashi s contention that the postspinel transition in mg fe 2sio4 is sharp enough to produce the major or sole contribution to the 670 km discontinuity is reinforced from author

controlled source audiomagnetotellurics in geothermal exploration theoretical and field tests indicate that the controlled source audiomagnetotelluric csamt method provides an efficient means of delineating the shallow resistivity pattern above a hydrothermal system utilizing a transmitter overcomes the main limitation of conventional audiomagnetotellurics variable and unreliable natural source fields reliable csamt measurements can be made with a simple scalar receiver our calculations for a half space show that the plane wave assumption is valid when the transmitter is more than 3 skin depths away in the broadside configuration and more than 5 skin depths away in the collinear configuration three dimensional 3 d numerical modeling results for a bipole source 5 skin depths away compare well with those for a plane wave source showing that the method is valid a csamt survey at the roosevelt hot springs geothermal area in utah produced apparent resistivity contour maps at four frequencies 32 98 977 and 5208 hz these maps show the same features as those of a

joint inversions of three types of electromagnetic data explicitly constrained by seismic observations results from the central okavango delta botswana the okavango delta of northern botswana is one of the world s largest inland deltas or megafans to obtain information on the character of sediments and basement depths audio magnetotelluric amt

the effect of fe spin crossovers on its partitioning behavior and oxidation state in a pyrolitic earth s lower mantle system abstract geophysical interpretations of the earth s interior and its dynamics are significantly influenced by phase transitions of the constituting minerals and their chemical compositions pressure induced fe spin crossovers in the main mineral phases of the earth s lower mantle mg fe silicate perovskite and ferropericlase have been suggested to influence fe partitioning resulting in separate layers with distinct physical properties however previous results remain ambiguous regarding the exact effect of fe spin crossovers and the actual transition pressures we observe here a continuous decrease of the fe 2 mg partition coefficient k d between silicate perovskite and ferropericlase from 25 gpa to 79 gpa in a pyrolitic earth s lower mantle system at about 97 gpa the k d significantly increases with an accompanied decrease of the fe 3 fe ratio in perovskite which therefore leads to an amplified change in the fe 2 k d we conclude that the fe 2 high spin to low spin crossover in ferropericlase and the fe 2 high spin to intermediate spin crossover in perovskite at mid lower mantle pressures 30 80 gpa exert no control on k d but the fe 2 intermediate spin to low spin crossover in silicate perovskite at about 100 gpa preferentially partitions fe into silicate perovskite and reduces its fe 3 content the change in oxidation state and partitioning behavior of fe will increase thermal conductivity and probably could induce a thermal boundary layer at this depth

diffusion of transition metals in periclase by experiment and first principles with implications for core mantle equilibration during metal percolation diffusion of the divalent transition metals in the deep mantle is important for understanding length scales of mantle heterogeneity transport across the core mantle boundary and the degree of equilibration during core formation here we report the results of experiments and theoretical calculations on ni co fe and mn diffusion in periclase the primary repository of these elements in earth s lower mantle the periclase used in the experiments was doped with 550 ppm al 3 to fix the cation vacancy concentration and the experiments were performed at 2 gpa and temperatures between 1673 and 2073 k the variation in diffusion coefficients among the transition metals was

geological interpretation of a high resolution reflection seismic survey at the buchans mine newfoundland sixteen kilometres of high resolution vibroseis reflection seismic data have been acquired in the vicinity of the former buchans mine direct identification of the cause of several reflectors is possible because the geology is tightly constrained by underground workings and drill holes both of which locally exceed 1 km depth many of the mine scale thrust faults are imaged as reflectors but conformable and intrusive contacts generally responded poorly a significant shallow dipping thrust the powerline fault is recognized below the orebodies and traced throughout the buchans area primarily as a result of the seismic survey it truncates ore stratigraphy and forms the floor thrust of a large duplex stack which hosts all the orebodies its presence has negative implications for exploration in the immediate mine area several lines of evidence suggest that this fault has a significant component of out of sequence movement a strong reflector 4 5 km below buchans is correlated with the surface expression

origin of the permian triassic iberian basin central eastern spain abstract the iberian basin was an intracratonic rift basin in central eastern spain developed since early permian times the basin boundary faults were normal listric faults controlling an asymmetric extension propagating northeast with time hercynian or older lineaments controlled the orientation of the iberian basin and extension was accommodated basically in the hanging wall block by the formation of secondary grabens and a central high the basin was related with the coeval ebro catalan and cuenca mancha basins and their connections are discussed subsidence curves show that the early permian early jurassic period of extension can be subdivided into three rifting episodes and a flexural one extension factor increases from 1 17 in the northwest to 1 29 near the mediterranean coast the increasing extension rate was accommodated by transfer faults trending nne ssw more important in the levante area the rift evolution is intermittent and seems to reflect distinct stress fields the collapse of the late hercynian orogen and related increased heat flux extension and rifting is the most probable origin of the iberian basin and related basins the origin of the catalan and the valencia prebetic basins is related to the southwards migration of the hesse burgundy rift along the eastern margin of the iberian microplate

finite element modelling of tertiary paleostress fields in the eastern part of the tajo basin central spain three subsequent tertiary paleostress fields that are deduced from fault slip data for the eastern part of the tajo basin are analyzed by finite element studies the modelling results show that maximum horizontal stresses shmax are mainly controlled by the geometry of the model limits and the boundary conditions applied the models are used to test two hypotheses on the origin of the altomira range a local stress field responsible for its formation altomira can be modelled successfully by superposition in time and place of two major paleostress fields iberian and guadarrama stress trajectories have been modelled with respect to a homogeneous cover and heterogeneous basement to investigate the role of rheological contrasts between different basement blocks on the orientation of the stress field results of this kind of modelling suggest a mechanical decoupling between the cover and the basement especially for the altomira paleostress field

palaeostress evolution of the iberian peninsula late carboniferous to present day abstract three main stages can be differentiated in iberia from the upper carboniferous to the present the end of the variscan orogeny late carboniferous the permian early cretaceous rifting and the alpine orogeny late cretaceous to present in this contribution the main features of the stress fields determined in each of these stages are compiled and revised from the analysis of the brittle deformations mainly microfault population stresses are related to the development of the principal structures of each stage in the late variscan fracturing stage sinistral strike slip faults prevailed in a stress field with ne sw subhorizontal compression in the permian to early cretaceous rifting stage the extensional trends varied in each sector during the alpine orogeny primarily the margins of the iberian massif the betic and pyrenee mountain chains were deformed however numerous deformations are also identified in its interior where large mountain chains such as the central system and the iberian chain developed these deformations occurred in a complex stress field where zones of nw se to nne ssw horizontal compression prevailed albeit predominantly extensional zones existed as well at present the stress field in most of iberia has a nw se compressive trend associated with active deformations in the betic cordillera where a continental subduction developed however simultaneously there are zones in which extensional settings predominate such as the internal zones of the betic cordillera and the gulf of valencia region making for quite a complex current stress field

appalachian carboniferous dextral strike slip faults an example from brookneal virginia the northern appalachian dextral fault system of late paleozoic age is also present in the central and southern appalachians an example of a dextral strike slip fault is the 4 km wide brookneal shear zone in the southwest virginia piedmont the shear zone is in part superimposed on the melrose granite where an s c mylonite was produced by dynamic recrystallization of all constituent minerals the arvonia metasedimentary and charlotte belt metavolcanic rocks contain spaced dextral shear bands at a consistent 24 3 to the shear zone boundary a minimum displacement estimate of 17 km was obtained from rotated foliation measurements in the melrose granite the age of movement on the brookneal shear zone has been constrained by isotopic dating to between 324 and 300 ma other faults in the southern appalachians including the nutbush creek and modoc zones show similar ages and relative offsets possible plate tectonic models that could account for the late paleozoic dextral fault system throughout the appalachians include 1 tectonic escape resulting from the collision of a plate with north america to the north of the canadian appalachians 2 postcollision interplate readjustments involving counterclockwise rotation of africa relative to north america and 3 oblique convergence of eastern north america with an oceanic plate moving west

mesozoic extensional tectonics stratigraphy and crustal evolution during the alpine cycle of the eastern iberian basin abstract sequence stratigraphy subsidence analysis and the integration of the basin fill data allow to the identification of four successive evolutionary stages in the basins of the eastern iberian margin during mesozoic extension 1 triassic rift late permian hettangian 2 early and middle jurassic postrift sinemurian oxfordian 3 late jurassic and early cretaceous rift kimmeridgian middle albian and 4 late cretaceous postrift late albian maastrichtian the present day crustal structure of the eastern iberian range and evidence of its evolution are deduced from the analysis of a new gravity map and other geophysical data a regional gravity low along the iberian range is interpreted in terms of crustal thickening beneath the orogen as a result of the collision between the ebro block and the iberian plate during the paleogene the boundary between these blocks coincides with a magnetic lineament that is interpreted as an inherited signature of the thinning that occurred during mesozoic extension a geodynamic scenario for the crustal evolution of the eastern part of the iberian peninsula based on the evaluation of mesozoic extensional tectonics and an analysis of the available geophysical data includes three main successive evolutionary stages 1 1 mesozoic crustal thinning 2 2 paleogene crustal thickening 3 3 neogene crustal thinning based on new geological and geophysical arguments this study offers an alternative and a more complete geodynamic history of the eastern iberian basins related to the evolution of the iberian plate and the central north atlantic

a review of least squares inversion and its application to geophysical problems geophysical inversion involves the estimation of the parameters of a postulated earth model from a set of observations since the associated model responses can be nonlinear functions of the model parameters nonlinear least squares techniques prove to be useful for performing the inversion a common type of inversion applies iterative damped linear least squares through use of the marquardt levenberg method traditionally this method has been implemented by solving the associated normal equations in conventional ways however singular value decomposition svd produces significant improvements in computational precision when applied to the same system of normal equations iterative least squares modeling finds application in a wide variety of geophysical problems two examples illustrate the approach 1 seismic wavelet deconvolution and 2 the location of a buried wedge from surface gravity data more generally nonlinear least squares inversion can be used to estimate earth models for any set of geophysical observations for which an appropriate mathematical description is available

experiments on faulting in a two layer cover sequence overlying a reactivated basement fault with oblique slip abstract two series of experiments were undertaken to investigate the development of faults in a cover sequence overlying a reactivated basement fault with oblique slip the experiments represent physical models properly scaled to account for gravitational forces brittle behaviour of the upper cover and ductile behaviour of evaporites in the lower cover models were made with a coulomb layer sand overlying a newtonian ductile layer silicone putty in each series the thickness of the silicone layer was varied from each experiment to the next in all normal wrench experiments a domain of faults appeared in the brittle layer vertical offsets were mostly of normal sense but some reverse faults occurred in experiments with thick silicone layers the thicker the silicone the wider the fault domain the deformation zone was in all cases limited to the faulted domain in the reverse wrench experiments discrete reverse riedel faults appeared without a silicone layer as the silicone layer became thicker the importance of the reverse faults diminished with the thickest layer of silicone 3 cm no faults appeared in the overburden but a monoclinal flexure accommodated the dip slip component in the presence of silicone the deformation zone was not limited to a faulted domain and the strike slip component was accommodated by a bulk shearing of the overburden in the uplifted compartment these experiments demonstrate that in basins deformed in oblique slip mode any deformation that is localized along discrete faults at the basement level becomes distributed over a much wider zone of overburden when this overlies an interval of evaporites

a vibroseis reflection seismic survey at the buchans mine in central newfoundland we present results from the first major vibroseis seismic reflection survey at a mine site in north america it is demonstrated that conventional high stack fold reflection seismic techniques can image fault structures associated with volcanogenic massive sulfide bodies despite the fact that these structures are locally steeply dipping and produce records with low signal to noise ratios the new lines were recorded at the locations of two earlier experimental explosive surveys that failed to image many strong reflectors the principal reasons for the success of the vibroseis experiment were the proper choice of sweep frequencies for maximum signal to noise ratio the use of high stack fold and the careful analysis of velocities and statics during processing a comparison of the new seismic sections with borehole and other geophysical data indicates that the origin of reflections at buchans can be attributed to various mechanisms including contrasts in lithologies and rock competence and intrusion of diabase sills into pre existing fault zones the best reflections emanate from shallow dipping brittle ductile thrust fault zones characterized by fault gouge within broader fractured zones the two seismic lines presented have proven to be a useful and cost effective supplement to existing borehole and geophysical data and have provided enough new information on the nature of thrust faulting at the mine to significantly influence current exploration plans

mineral exploration using csamt data application to longmen region metallogenic belt guangdong province china abstracta controlled source audio frequency magnetotelluric csamt survey has been carried out to investigate potential iron fe and polymetallic pb zn cu deposits in longmen region which is one of the main metallogenic belts in southern china conducting geophysical surveys in this area is quite difficult due to mountainous terrain dense forest and thick vegetation cover a total of 560 csamt soundings were recorded along twelve surveying lines two dimensional occam s inversion scheme was used to interpret these csamt data the resulting electric resistivity models showed that three large scale highly conductive bodies exist within the surveying area by integrated interpretation combined with available geologic geophysical and geochemical data in this area three prospective mineral deposits were demarcated based on the csamt results a borehole penetrating approximately 250 m depth was drilled at the location of 470 m to the northwest end of line 06 defined with a massive pyrite from the de

natural rotated inclusions in non ideal shear abstract kinematic analysis of rotated winged inclusions is often couched in terms of ideal simple shear with reference to a specific model based upon porphyroclasts of circular section however many natural winged inclusions are not porphyroclasts commonly their geometries as well as the orientation distribution of the inclusions themselves reflect the non ideal nature of the shearing deformation during which they formed stair stepped sigma shaped geometries may reflect the immaturity of a structure developed in non ideal shear or a mature structure developed in ideal simple shear stair stepped delta shaped geometries indicate that deformation approximated to ideal simple shear in plane delta shaped geometries and skewed inclusion orientation distributions are indicators of non ideal shear

results of a controlled source audiofrequency magnetotelluric survey at the puhimau thermal area kilauea volcano hawaii the puhimau thermal area in the volcanoes national park hawaii has attracted interest because it represents an area with anomalously high heat flow and the heat source may be a shallow magma body at puhimau a variety of geophysical data including heat flow measurements have been acquired the self potential data suggest that the heat source is a result of a magma intrusion which plunges steeply northward the heat flow data can be explained in terms of a shallow relatively thick magma intrusion with a solidification rate sufficient to supply the surficial heat flow for the period of time since the appearance of the thermal area in 1936 in february of 1984 we performed a controlled source audiofrequency magnetotelluric csamt electromagnetic geophysical survey at the puhimau thermal area to investigate the electrical nature of the thermal anomaly and estimate the depth to any hot water and or remaining molten magma the csamt survey consisted of several lines of stations where two orthogonal prima

the electrical resistivity of canada s lithosphere and correlation with other parameters contributions from lithoprobe and other programmes over the last 30 years through lithoprobe and other programmes modern high quality magnetotelluric mt measurements probing deep into the lithosphere and underlying asthenosphere have been made at over 6000 sites across canada in all provinces and territories except nova scotia some regions are well covered particularly alberta southern british columbia and western ontario whereas others remain poorly covered such as quebec and large swaths of nunavut prior publications from individual studies have added significantly to the wealth of canada s geoscience knowledge and have demonstrated that mt can contribute significantly to understanding of the tectonic processes that have shaped canada however to date no continent scale maps of lithospheric electrical parameters have been constructed from the extensive mt database herein we review the contributions made by the mt components of lithoprobe and present new continental scale maps of various electrical parameters at crustal and upper mantle

experiments on fault reactivation in strike slip mode abstract experiments were undertaken to investigate how faults reverse or normal in a cover sequence become reactivated when an underlying basement fault is itself reactivated in strike slip mode physical models properly scaled for gravity were deformed in a large 2 1 5 m apparatus divided into two halves one half can be moved laterally or vertically past the other models were constructed with a coulomb layer sand overlying a newtonian ductile layer silicone three different configurations of the basement fault reverse normal or vertical defined three series of experiments each consisted of 1 brittle model 6 cm of sand and 2 brittle ductile models 5 cm of sand overlying 1 cm of silicone or 4 cm of sand overlying 2 cm of silicone each model was deformed in two steps 1 a pure dip slip stage and 2 a reactivation in pure strike slip mode in surface views little reactivation was observed but in all cases the deformation imposed during the strike slip episode was localized by pre existing dip slip structures in cross sections however faults were seen reactivated at depth on the basis of these experiments we suggest that 1 a previously faulted zone can control the deformation of a strike slip zone and 2 reactivation of faults in strike slip mode can occur at depth without being visible at the surface we attribute the higher probability of reactivation at depth to the mechanical properties of the sand especially cohesion

evolution of buntsandstein fluvial sedimentation in the northwest iberian ranges central spain abstract the buntsandstein sediments in central spain permian and triassic are mainly continental redbeds that form the base of a sequence that passes up into siliciclastic and carbonate tidal sediments the muschelkalk facies buntsandstein sedimentation began with conglomeratic and sandy fluvial facies whose detailed sedimentological analysis is the main subject of this work massive sheets are the most important conglomerate facies they were formed by deposition as longitudinal bars channel fill conglomerates are the second most important facies lateral accretion deposits also occur in the conglomerates they evolved from longitudinal bars because of decreasing flow or because the bars had reached a certain height sandstones are composed of eleven different facies large linguoid or transverse bars were the main bedforms in this environment these bedforms may be related downstream to large oblique troughs developed when the tabular bedform grew to a certain size or when a tabular bedform reached another one that acted as an obstacle compound and composite compound bars consisting of large sets of trough cross bedding with scoured wavy bases are al o recognized in the sandstone facies two major cycles occur in the complete conglomerate sequence which correspond to different physiographic models the lower cycle was mainly formed by channels and bars that were smaller than those in the upper cycle the upper cycle is more extensive than the lower one and has characteristics that indicate a higher stability paleocurrents for both cycles have a north northeasterly trend the vertical evolution of these sediments is clearly related to the different fault systems that controlled their distribution the paleogeography of the sandstone facies together with paleocurrents perpendicular to the conglomerate paleoccurents indicate deposition from a separate dispersal system that extensively overlies the previous one this unit which can be followed extensively over most of the iberian ranges and even outside the iberian ranges represents the growth of a fluvial braidplain over a large area of iberia during the lower triassic

an application of csamt for detecting weak geological structures near the deeply buried long tunnel of the shijiazhuang taiyuan passenger railway line in the taihang mountains abstract the shijiazhuang taiyuan shitai high speed railway line is designed to cross the taihang mountains the line will include the construction of a 27 85 km long tunnel with a maximum depth of 450 m the taihang mountain tunnel is the key to the successful construction of the railway the geological structure of the tunnel is highly complex and the lithology changes dramatically throughout fault zones karst and groundwater ingress are the main geological engineering problems for the tunnel construction however conventional geophysical methods are not effective for detecting these geological structures in a region with such a complex terrain therefore the controlled source audio frequency magnetotellurics csamt method was used to effectively characterize the resistivity distribution of the underground rock this technique is highly sensitive to low resistivity geological structures such as fault zones and it is effective for studying the geological engineering conditions of the tunnel various corrections designed denoising and inversion methods are used in csamt data processing to obtain the resistivity profile of the entire tunnel based on the resistivity results and existing core information the underground lithological distribution is inferred the location of the fault zone is determined and the distribution of dangerous geological bodies is delineated the interpretation of the csamt results is verified by boreholes and related geological information these results are important for understanding the subsurface geological engineering conditions and have provided stability safety and cost evaluation support for the construction of a long deeply buried tunnel in taihang mountains

anatomy of the mondonedo nappe basal shear zone nw spain this paper analyses a major shear zone from the iberian hercynian belt which forms the basal thrust of the mondonedo nappe the shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle ductile deformation in greenschists facies folds in the shear zone are asymmetric very tight 1c or similar class and frequently developing sheath geometries the sheath folds originated by non coaxial flow superimposed on earlier irregularities the fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non mylonitic tectonites c axis fabrics vary across the shear zone but show a dominant monoclinic symmetry the blastomylonitic rocks include the fabrics representing the highest temperatures the main foliation of the schists results from flattening of an earlier foliation recording occasional microfolds the use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen

elasticity composition and temperature of the earth s lower mantle a reappraisal the interpretation of seismological models for the earth s lower mantle in terms of chemical composition and temperature is sensitive both to the details of the methodology employed and also to uncertainties in key thermoelastic parameters especially for the dominant mg fe sio3 perovskite phase here the alternative approaches adiabatic decompression of the lower mantle for comparison at zero pressure with laboratory data and the projection of laboratory data to lower mantle p t conditions for direct comparison with seismological observations are assessed along with the equations of state on which they are based it is argued that adiabatic decompression of the lower mantle is best effected by a strategy in which consistent third order finite strain expressions are required simultaneously to fit the strain dependence of both the seismic parameter and the density this procedure accords due weight to the most robust seismological observations namely the wave speeds and reduces to an acceptable level the otherwise very strong covariance among the fitted coefficients it is demonstrated that this approach can readily be adapted to include the effects of relaxation from the hill average to the reuss lower bound of the aggregate bulk modulus which governs the radial variation of density for the projection of laboratory thermoelastic data to lower mantle p t conditions the preferred equation of state is of the mie gruneisen type involving the addition at constant volume of the pressure along a finite strain principal 300 k isotherm and the thermal pressure calculated from the debye approximation to the lattice vibrational energy a high degree of consistency is demonstrated between these alternative equations of state possible departures of the lower mantle from conditions of adiabaticity and large scale homogeneity are assessed these are negligible for the prem model bullen parameter b 0 99 0 01 but significant for ak135 b 0 94 0 02 reviving the possibility of a substantially superadiabatic temperature gradient experimentally determined thermoelastic properties of the major mg fe sio3 perovskite and mg fe o magnesiowustite phases are used to constrain the equation of state parameters employed in the analysis of seismological information although the scarcity of information concerning the pressure and temperature dependence of the elastic moduli for the perovskite phase remains a serious impediment nevertheless it is demonstrated that the combination of a pyrolite composition simplified to the three component system sio2 mgo feo with molar xpv 0 67 and xmg 0 89 and a lower mantle adiabat with a potential temperature of 1600 k consistent with the preferred geotherm for the upper mantle and transition zone is compatible with the prem seismological model within the residual uncertainties of the thermoelastic parameters for the perovskite phase in particular such consistency requires values for the perovskite phase of ks ks p t and q ln ln v t of approximately 3 8 and 2 and g t near 0 022 gpa k 1 for the lower mantle assemblage more silicic models which have sometimes been advocated can also be reconciled with the seismological data but require lower mantle temperatures which are much higher by about 700 k for pyroxene stoichiometry however the absence of seismological and rheological evidence for the pair of thermal boundary layers separating convection above and below which is implied by such models remains a formidable difficulty under these circumstances the simplest possible model that of grossly uniform chemical composition throughout the mantle is preferred

partitioning of iron between magnesian silicate perovskite and magnesiowustite at about 1 mbar abstract the disproportionation products of olivine fo 90 in diamond anvil cell experiments at about 1 mbar are magnesian silicate perovskite mg 94 and a series of magnesiowustites mg 85 90 yielding a recommended value of 0 4 0 1 for the distribution coefficient that defines the exchange of mg and fe 2 between the two phases k d the distinctive compositional trends which would signify that ferric iron had been stabilised during experiments are lacking pressure does not exert any significant effect on distribution behaviour despite thermodynamic predictions of major reallocation of fe 2 from magnesiowustite into and in favour of perovskite towards the base of the lower mantle nor does the presence of al 3 in magnesian silicate perovskite in amounts appropriate for the lower mantle substantially modify distribution behaviour and hence phase chemistry provided the oxygen content of the system remains essentially constant wood s earth planet sci lett 174 2000 341 multi anvil study has been used to link k d with temperature and to calculate that magnesian silicate perovskite mg 93 0 0 6 would coexist with magnesiowustite mg 82 6 0 9 just beyond the 660 km discontinuity 1800 k whilst at the core mantle boundary 2650 k the corresponding values would be 91 9 0 4 and 84 9 0 8

effects of the pyrenean collision on the paleocene stratigraphic evolution of the southern iberian margin southeast spain rapid sea level changes are generally interpreted in terms of glacio eustasy but they can also be explained by the lithospheric motions associated with changes in intra plate stress in eastern spain latest senonian paleocene time was characterized by a wide spread regression which culminated in the emergence of large parts of the carbonate platform new data indicate that the platform emergence was accompanied by slope faulting and a sudden appearance of olistostromes and sandy turbidites at the southern iberian margin of southeast spain these changes in basin geometry are the result of a tectonic event which is dated as middle paleocene ca 60 ma this tectonic event can be correlated with the onset of collision in the pyrenees regional geologic data and paleostress measurements provide evidence for far field transmission of the pyrenean compressive stresses through the iberian lithosphere it is concluded that the platform emergence and slope steepening at the southern iberian margin reflect the vertical lithospheric motions of a basin flank subjected to compression

temporal and spatial variations in tectonic subsidence in the iberian basin eastern spain inferences from automated forward modelling of high resolution stratigraphy permian mesozoic by subsidence analysis on eighteen surface sections and 6 wells which cover large part of the iberian basin e spain and which are marked by high resolution stratigraphy of the permian triassic jurassic and cretaceous we quantify the complex permian and mesozoic tectonic subsidence history of the basin backstripping analysis of the available high resolution and high surface density of the database allows to quantify spatial and temporal patterns of tectonically driven subsidence to a much higher degree than previous studies the sections and wells have also been forward modelled with a new automated modelling technique with unlimited number of stretching phases in order to quantify variations in timing and magnitude of rifting it is demonstrated that the tectonic subsidence history in the iberian basin is characterized by pulsating periods of stretching intermitted by periods of relative tectonic quiescence and thermal subsidence the number of stretching phases appears to be much larger than found by earlier studies showing a close match with stretching phases found in other parts of the iberian peninsula and allowing a clear correlation with discrete phases in the opening of the tethys and atlantic

squeeze box in a centrifuge abstract a squeeze box which operates in a centrifuge is presented in the experimental set up the centrifugal body force mimics the role of gravity in nature and at the same time drives a hydraulic press which simulates lateral forces the rate of lateral shortening is controlled by varying the loads placed on the hydraulic press utilization of a squeeze box in a centrifuge enhances the range of scaling and enables the use of coherent model materials without violating similitude requirements in the traditional squeeze box which operates under normal gravity similitude requirements severely limit the choice of model materials as a test of the apparatus simple experiments were carried out comparing models shortened in a centrifuge with similar models shortened under normal gravity the realistic form of decollement structures e g folds faults and diapirs shortened in a centrifuge in these tests as well as in other tests which will be reported later show the potential of this new technique for rigorous modelling of geodynamic systems which evolve under the influence of both gravity and a push from the rear the set up can also be used as a simple shear box in a centrifuge after a slight adjustment

on results of a controlled source audiofrequency magnetotelluric survey at the puhimau thermal area kilauea volcano hawaii by l c bartel and r d jacobson geophysics 52 665 677 may 1987 bartel and jacobson try to correct apparent resistivity values from csamt measurements where the plane wave approximation is not valid they want to convert csamt curves to passive mt curves cagniard curves which can be interpreted by 2 d modeling the measured csamt a value is projected onto the a axis in parallel with the half space curves of a horizontal electrical dipole in the given distance between receiver and transmitter the example looks convincing and it may be supposed to be a simple near field correction

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

sense of displacement on the cobequid chedabucto fault system nova scotia canada the cobequid chedabucto fault system of the canadian appalachians is a major anastomosing fault system over 300 km in length it separates the meguma terrane of southern nova scotia from the avalon terrane to the north these terranes are distinct tectonic and lithological entities in the appalachian orogen two areas at either end of this fault system have been examined in detail to determine the sense and history of offset along it both areas are situated on major component fault zones of the system and both exhibit structures due to early intense ductile shearing that are overprinted by semi brittle to brittle structures caused by later faulting along the eastern chedabucto fault zone area a ductile structures were examined this area is characterized by the progressive development of s c textures and shear bands rotated syntectonic porphyroblasts and asymmetric minor folds features indicative of and caused by ductile shearing along the western cobequid fault zone area b semi brittle and b

electromagnetic soundings in the sedimentary basin of southern ontario a case history the results of electrical and electromagnetic em soundings conducted in the sedimentary basin of southern ontario are presented the sounding sites are located strategically to take advantage of the dipping nature of the sediments a progressive study being undertaken from shallow to deep sections the vertical magnetic field transfer function estimated with a pseudonoise source em system and conventional schlumberger apparent resistivity are jointly inverted for each site the layered earth model containing the minimum number of layers is fitted to the data the joint inversion enables up to eight distinct layers to be identified at some sites the nonuniquenesses or ambiguities in each model for example s conductivity thickness product equivalence of a deep thin conductive layer are revealed through an eigensolution analysis the intrinsic ambiguities of the models of the deeper sections are resolved by a systematic progressive site to site correlation of electrical units across the basin for example the conductivity of a conductive unit is measurable when it is at a shallow depth in a deeper section if the unit appears as a thin layer it is assigned this value of conductivity removing the s equivalence ambiguity the individual inversions combined with the site to site correlation produce an overall electrical model of the basin consistent with the known geologic section

a precautionary note on shear bands as kinematic indicators the orientation of shear bands relative to foliations defined by elongated mineral aggregates is often used to determine the large scale sense of displacement in ductile faults data from contact strain zones at the bases of large overthrust complexes in the eastern alps and the betic cordilleras demonstrate that there is not always a simple geometrical relation between shear band orientation and sense of vorticity in bulk non coaxial flow in addition to single shear band sets that show displacements synthetic with the large scale displacement there are single sets with antithetic displacements and conjugate sets the last two observations are at variance with published data and interpretations and cast doubts on the general applicability of shear bands as indicators of large scale flow kinematics

composite fabrics in mid crustal gneisses observations from the oygarden complex west norway caledonides subhorizontal caledonian shear zones that transform migmatitic gneisses into layered gneissic tectonites in the western part of the oygarden complex show a number of composite fabrics the associated structures consist of two sets of related planar surfaces showing a consistent top to the west sense of shear the development of these structures depends mainly on the nature and variation of finite strain rheology strain heterogeneities and extent of metamorphic mineral growth and recrystallization in the shear zones the structures have one set of surfaces represented by slip surfaces or narrow high strain zones oblique to this is a complementary set of surfaces representing either a plane of mineral flattening or aligned metamorphic minerals in s c structures or rotated gneissic foliation in contractional and extensional composite structures rotation by folding and subsequent slip along limbs or axial planar cleavage results in contractional composite structures while rotation by downward cutting extensional type slip surfaces form extensional composite structures contractional and extensional composite structures involve slip transfer across the mylonitic layering commonly between terminating weak layers within strongly anisotropic layered tectonites combinations of the different types of composite structures are common but the method of shear sense identification is always the same

growth rate observation from the moss built checa travertine terrace central spain in situ observations of fossil and living specimens of the calcicolous mosses bryum pseudotriquetrum cratoneuron commutatum and catoscopium nigritum revealed very fast calcite depositional rates rhythmic layering in the fossil mosses corresponding with the seasonal climatic cycle suggests that the moss curtain occupied by these three mosses maintains the deposition of spongeous travertine layers at a mean rate of 4 cm a 1 a mean depositional rate of 4 2 cm a 1 may be calculated from measurements of the loss of bicarbonate from the springwater after it percolated through the moss curtain these rates suggest that the 8 m high travertine terrace of checa with a surface area of 800 m 2 did not exist two millennia ago mosses could be put to man s use for creating natural overgrowths on artificial surfaces an idea based on an allusion by wallner he observed that the thread forming blue green algae vaucheria builds travertine deposits at an annual rate of 0 7 1 4 cm we observed that the mosses cratoneuron commutatum and bryum pseudotriquetrum may form spongeous travertine layers at respective maximum rates of 11 and 14 cm a 1 this would reduce the time required to build natural overgrowths on artificial objects to a practical period of months

analogue modelling of a reactivated basement controlled strike slip zone sierra de albarracin spain application of sandbox modelling to polyphase deformation this paper presents the results of an analogue modelling study on the reactivation of riedel shears generated by basement induced sinistral strike slip faulting it is based on a natural example in the sierra de albarracin iberian range spain the area has a polyphase deformation history defined by the variscan and alpine orogenies late variscan deformation was concentrated in a wide nw se shear zone with accompanying kilometre scale e w riedel shears which divided the palaeozoic basement into large fault blocks alpine reactivation resulted in differential movements on the riedel shears as evidenced by a nw se chain of palaeozoic inliers surrounded by a mesozoic cover that generally shows minor deformations except near the e w riedel shears where strata locally appear in near vertical to overturned position sandbox analogue modelling was applied to improve insight into the structural history it focused on the kinematics of spontaneously developed en echelon riedel shears reactivated in a rotated stress field sand with a controlled added strength was used to form riedel shears in a first deformation phase to act as weak zones for a second phase the modelling showed that in the first deformation phase large pop up structures developed between the riedel shears in a basement induced sinistral strike slip zone later reactivation in the n060 e and n135 e shortening directions was taken up respectively by sinistral reverse and dextral reverse shear along the pre existing riedel shears but only if the sand on one side of the fault zone was allowed to move freely along the other scissor faulting along the riedel shears with their complex 3d geometry increased the height of the up squeezed blocks for experiments with fixed boundaries and no oil water emulsion layer between the base plate and sand pack thrusting at the backstop occurred rather than reactivation of the riedel shears this approach provided robust insights on the 4d development of the sierra de albarracin area the geological society of london 2006

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

evolving fluvial architecture during a marine transgression upper buntsandstein triassic central spain abstract the sedimentation in central spain during the late anisian middle triassic limos y areniscas de rillo formation recorded the evolution from an alluvial environment into a coastal one with a diversity of environments this evolution was the result of one of the episodes of the tethys sea transgression over iberia during the first and second stages of this evolution mixed load low sinuosity channels evolved upwards into a distal braidplain with ephemeral run off and poorly defined channels during a third stage the characteristics of the sediments indicate the existence of both continental and marine waters in a transitional zone with seasonal hydrological fluctuations the fourth stage records the sedimentation in an upper intertidal to supratidal environment in a semiarid coastline with the presence of thin cryptalgal sheets and chicken wire textures the final evolution consists of an environment with a high percentage of fluvial channel deposits probably related to a meandering pattern this evolution has been correlated with the depositional sequences established for the ne of spain where the anisian transgressive event resulted in the formation of an inter to supratidal carbonate ramp so lowstand transgressive and highstand system tracts from depositional sequence 1 have been recognized in the centre of spain

deflection of a compressional stress field by large scale basement faults a case study from the tertiary almazan basin spain abstract palaeostresses deduced from brittle mesostructures more than 50 sites and 1500 structures measured in the mesozoic and tertiary rocks of the almazan basin located in the northwestern iberian chain indicate a regional nne sse directed compression that lasted during all the tertiary shortening stage the stress regime corresponding to this compression ranges from thrust to wrench n s directed compression was the responsible for most macrostructures folds and thrusts forming a s vergent system with ne sw to e w and nw se orientations and mesostructures mainly faults stylolytes and impressed pebbles we propose that the stress trajectories are deflected following anderson s model in the western part of the almazan basin by the ne sw striking left lateral strike slip soria fault this basement fault was inherited from the late variscan fracturing stage and controlled the deposition in the mesozoic basins as well as the location of tertiary compressional macrostructures in the eastern border of the almazan basin palaeostress trajectories were deflected both by right lateral strike slip faults and nw se trending thrusts near which the regional n s directed compression becomes perpendicular to them three minor compression directions have also been recorded in several sites ne sw e w and nw se the ne sw directed compression is interpreted as a deviation of the main n s directed compression at a regional scale whereas the nw se directed stress field corresponds to the regional field responsible for the main tertiary shortening stage in the spanish central system the age of compression ranges from eocene to miocene

electromagnetic imaging of a complex ore body 3d forward modeling sensitivity tests and down mine measurements this study investigated the capability of audio magnetotelluricsamtnotonlytodetect butalsotodelineatecomplexconductive ore bodies at minable depths a detailed 3d numericalelectrical resistivity model of the bathurst no 12 deposit new brunswick canada was constructed using available geologic and geophysical information different geologic and data acquisitionconditionsweresimulated presenceofoverburden different geometries dimensions and positions of the ore body and different data sampling regimes the behavior of the surface 3d electromagneticfieldswascomparedwiththatfrombodiesofinfinite strike extent the 3d and 2d amt responses were similar at high frequencies so 2d modeling was shown to be both valid andsufficient however atlowfrequenciesonlythoseresponses for current flow perpendicular to the body the transverse magnetic mode in a 2d case were reasonably similar the 2d inversions showed that the position and the top of the 3d ore body were well resolved but the bottom of the ore body s resistivity were poorly resolved to increase resolution at depth below ore bodies and to possibly extend mine life we recommend that amt measurements are taken from within mines simple mod elsoforebodiesweresimulatedtoshowresponsesatdepth and to undertake body stripping of the overlying structures the tests showed that conductive structures above the measurement levelcanhaveastronginfluenceonimagingofconductingzones below and can produce distortion effects in apparent resistivity and phase although the body stripping approach reduces these effects and gives an indication of whether there are conductive structures below the resulting image is considerably different from that of a model without overlying conductive structures full 3d inversion holding known structures at constant is required

3d evolution of a pop up structure above a double basement strike slip fault some insights from analogue modelling in analogue modelling pop up structures have previously only been described in models of basement involved strike slip deformation with a stepover or restraining bend configuration we describe the results of an analogue experiment with an alternative model configuration to investigate the structural development of pop up structures this model consists of a cover sequence of sand overlying two parallel running basement faults which experience the same sense and the same amount of slip during deformation the ratio of basement fault separation 1 cm to overburden thickness 4 cm is small the experiment has been recorded in an x ray tomograph by which it was possible to investigate the 3d structural evolution of the model during deformation an elongated rhombic pop up structure developed centrally above the basement faults the faults bounding the pop up structure displayed convex upward as well as straight to concave upward shapes during deformation the topography above the basement faults increased to a maximum of c 4 8 mm compared to a maximum subsidence of c 1 5 mm on both sides of the basement faults during progressive deformation local maximum horizontal principal stresses displayed a considerable rotation from 45 in the initial stages to sub parallel in a more advanced stage and finally back to c 45 to the strike of the basement faults

an integrated magnetotelluric and gamma exploration of groundwater in fractured granite for small scale freshwater supply a case study from the boshan region shandong province china exploration and use of groundwater in hard rocks is a difficult but yet a significant issue especially when the local surface water source is polluted in this study integrated geophysical and nuclear methods are used to delineate groundwater bearing zones in a granite terrain in the boshan region shandong province china structural analysis showed that fractures extend in a nw se direction which is the same as the direction of river valley a controlled source audio frequency magnetotelluric survey conducted along the river valley indicated several different low resistivity anomalies or potential water bearing fracture zones they are seated at depth ranges down to 400 m from the surface along the river valley the locations of deep water bearing zones were further confirmed by higher gamma radiation measurements found near the river valley a fault with a nw se strike near the river was deduced as an anomalous water bearing zone two tube wells were drilled targeting at the water bearing zone that has achieved single well water yield greater than 360 m3 day the well water was sampled for water chemistry and stable isotope analyses shallow groundwater samples show similar water chemistry and isotopic feature to river water indicating connection between the two deep groundwater from the water bearing fracture zones is not hydraulically connected with shallow groundwater ensuring safety of the deep groundwater in the granite from being affected by the polluted shallow groundwater or river water integrated geophysical and gamma exploration methods helped to identify two disconnected groundwater systems offering an alternative freshwater source for the region if shallow groundwater or surface water is polluted