the tectonothermal evolution of the cadomian belt of northern brittany france a neoproterozoic volcanic arc abstract the neoproterozoic cadomian belt northern brittany france results from the accretion of crustal blocks that record distinctive p t deformation paths in addition abundant gabbro dioritic to granitic intrusions during and after deformation influence the thermal pattern and are used as time markers as a whole the deformation history of the cadomian belt is characterised by a migration in time of the active zones which become younger towards the southeast relics of eocadomian rocks reveal that arc development took place at about 660 640 ma abundant calc alkaline magmatism intruding strongly deformed palaeoproterozoic orthogneisses characterise the northernmost unit of the cadomian belt i e the tregor unit which cooled before 600 ma the lanvollon basin a short lived intra arc rift containing abundant mafic and felsic volcanics followed by greywackes opened at about 600 580 ma the subsequent tectonothermal history of the cadomian belt is divided into two major episodes the first episode is related to the shortening of the intra arc rift and its southwestward thrusting over the guingamp unit this led to crustal thickening inducing partial melting in the guingamp unit exhumation of the latter rocks is accommodated by late extensional shearing along the boundary between the saint brieuc and guingamp units cooling of the saint brieuc unit is dated at about 570 560 ma the second episode is dominated by sinistral wrenching mainly localised in the eastern part of the cadomian belt at the boundary between the domnonean i e the arc domain and mancellian possibly a fore arc basin domains several shear zones developed during this episode some of them being associated with minor granitic intrusions dated around 550 ma the sinistral shear located southeast of the arc domain is reworking or crosscutting earlier high temperature fabrics within the saint brieuc unit in addition the vertical component of this shearing episode contributes significantly to the exhumation of the saint malo migmatites near the proterozoic cambrian boundary cordierite biotite granitoids intruded at shallow depths the fougeres unit their emplacement outlasting deformation of the sedimentary sequences minor stocks of muscovite leucogranites of cambrian age are possible equivalents of the abundant volcanism in an adjacent nw se trending rift this marks a major change in the kinematic framework with predominance of extension finally minor reactivation of the cadomian structures occurred during the carboniferous associated with the variscan orogeny

deep conductivity structure in abitibi canada using long dipole magnetotelluric measurements magnetotelluric data recorded in the period range 10 2 104s using 100 km telluric dipoles in western quebec canada yield impedance estimates free from galvanic distortion and static shift analysis of the apparent resistivity phase and magnetic transfer function data favors a regional 1d anisotropic model for the subsurface resistivity structure in the abitibi subprovince lower resistivity direction of the anisotropy is ew parallel to the main fault system joint 1d inversion of both principal impedances indicates a conductivity anisotropy in the upper and middle crust and in the upper mantle it might originate for all depths from the same tectonic mechanism mainly a ns shortening followed by an ew dextral shearing in the archean

transverse tectonics in the sikkim himalaya a magnetotelluric study abstract the tectonics of seismically active sikkim himalaya as inferred by numerous seismological studies is distinct from the conventional thrust tectonics proposed for the himalayan collision belt here focal mechanisms of several moderate magnitude earthquakes and composite fault plane solutions of microearthquakes have revealed strike slip motion along faults transverse to the northward convergence direction of the indian plate in the present study we analyze broadband magnetotelluric data of 12 sites located along an approximately n s profile cutting across major geological sub domains of sikkim to test whether magnetotelluric strikes also support such transverse tectonic nature of the region we have performed strike analysis of the data by two decomposition approaches as well as by phase tensor method the study has revealed local variations in the strike directions within the region consistent with the geological and tectonic setup and the presence of transverse tectonic features in the region of main central thrust zone mctz where major axis of phase ellipses align in nnw sse to nw se direction this trend coincides with the one obtained by microseismic data recorded after the september 18 2011 earthquake mw 6 9 magnetotelluric strike analysis thus supports the presence of nnw to nw trending transverse tectonic zone in mctz

preliminary results from a geophysical study across a modern continent continent collisional plate boundary the southern alps new zealand abstract the southern alps of south island new zealand is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement a joint us nz geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three dimensional model of the deformation the measurements undertaken include active source and passive seismology magnetotelluric and electrical studies and petrophysics preliminary models for the active source seismic measurements across south island confirm in general terms a thickened crust under the southern alps a high velocity lower crustal layer and a major crustal discontinuity associated with the alpine fault the anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements shear wave splitting and resistivity the mid crust under the southern alps coincides with a major electrical conductivity high which possibly corresponds to fluid in the crust the top lies at about 15 km close to the base of shallow seismicity east of the alpine fault offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to south island these data are showing complex structure particularly off western and southeastern south island the complexity in structure high quality data and consistency in results from several techniques indicates that the south island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular and the continental lithosphere in general

a magnetotelluric study of the damara belt in namibia 2 mt phases over 90 reveal the internal structure of the waterberg fault omaruru lineament abstract magnetotelluric mt observations at some sites in the vicinity of the waterberg fault omaruru lineament wf ol a major tectono stratigraphic zone boundary in the central zone of the damara belt show evidence for strong three dimensional 3d effects we observe very high skew values phases over 90 and a strong correlation of parallel components of the electric and magnetic fields at long periods because of the dense site spacing and good spatial coverage we can positively attribute these effects to local geology and are able to resolve structural detail within the wf ol mapping latorraca s electric characteristic vectors in form of ellipses proved particularly useful in identifying key elements of the conductivity structure for subsequent modelling 3d and 2d anisotropic modelling can reproduce most of the observed 3d effects the conductivity anomalies revealed in the area are i a conductive ring structure in the shallow crust along the northern part of the profile ii an anisotropic region in the upper crust with high conductivity parallel to the wf ol iii anisotropy in the lower crust with a different but undetermined strike direction and iv a shallow elongated conductor sub normal to the wf ol modelling studies further suggest that the anisotropic fault zone is approximately 10 km wide and may reach down to a depth of 14 km or more

electrical resistivity image of the south atlantic continental margin derived from onshore and offshore magnetotelluric data we present a deep electrical resistivity image from the passive continental margin in namibia the approximately 700 km long magnetotelluric profile follows the walvis ridge offshore continues onshore across the kaoko mobile belt and reaches onto the congo craton two dimensional inversion reveals moderately resistive material offshore atypically low for oceanic lithosphere reaching depths of 15 20 km such moderate resistivities are consistent with seismic p wave velocity models which suggest up to 35 km thick crust the neoproterozoic rocks of the kaoko mobile belt are resistive but nnw striking major shear zones are imaged as subvertical conductive structures in the upper and middle crust since the geophysical imprint of the shear zones is intact opening of the south atlantic in the cretaceous did not alter the middle crust the transition into the cratonic region coincides with a deepening of the high resistive material to depths of more than 60 km

an audio magnetotelluric investigation of the otjiwarongo and katima mulilo regions namibia abstractas an additional opportunistic component to the southern african magnetotelluric experiment natural source audio magnetotelluric amt data were acquired during phase iv to investigate the local scale electric conductivity subsurface structure in the otjiwarongo and katima mulilo regions namibia as an aid in locating the installation points for high voltage direct current earth electrodes the study showed that the shallow subsurface of areas containing one measurement site in the otjiwarongo region and three sites in the katima mulilo region have appropriate high conductivities for the optimal placement of the earth electrodes both of the amt surveys are situated close to the edge of the orogenic neo proterozoic damara mobile belt dmb previous studies all suggest the existence of a highly conductive midcrustal zone which correlates well with the spatial location of the dmb two dimensional inverse modeling of the otjiwarongo amt data confirms the existence of the high conductive zone at m

three dimensional magnetotelluric responses for arbitrary electrically anisotropic media and a practical application electrical anisotropy in earth media increases the complexity of magnetotelluric responses magnetotelluric models based on anisotropic media must be developed to fully understand observed data this paper presents a three dimensional algorithm for calculating magnetotelluric responses of arbitrary anisotropic media in the frequency domain using a staggered grid finite difference method the model space is discretized into rectangular blocks with electric fields on the edges of each block and the magnetic fields normal to the faces of each block the electric field helmholtz vector equation that considers a full 3 3 conductivity tensor is calculated numerically under two orthogonal polarizations in calculating the boundary values on the four sides of the three dimensional anisotropic model we adopt different procedures for calculating the two dimensional responses on the sides in the x and y directions the responses for a layered anisotropic model and a three dimensional isotropic model calculated with this algorithm are compared with the corresponding analytical and numerical solutions respectively the comparisons show that the algorithm s approximations are highly precise for a wide frequency band a typical two dimensional anisotropic model and a general three dimensional anisotropic model were also constructed and their responses were calculated these anisotropic models have ordinary structures but can produce phase rolling out of quadrant magnetotelluric responses which indicates that considering electrical anisotropy may improve our interpretation of observed data using this algorithm we can model the observed data from the northern qaidam basin in northern tibet where ultrahighpressure metamorphic rocks are exposed along an old suture and seismic anisotropy was indicated in neighbouring areas the phase tensors of the magnetotelluric sites at this location show large skew angles and the corresponding phase splits are distinct in the off diagonal impedance elements although the isotropic three dimensional electrical structure can model the profile data well the structure shows a sequence of conductive and resistive bodies in the mid lower crust of the north qaidam basin which is very spatially inhomogeneous and a simple intrinsic anisotropic body can also produce similar surficial responses using the three dimensional anisotropic algorithm we found an equivalent anisotropic replacement for this area the results of the three dimensional anisotropy modelling of the magnetotelluric data from the northern tibetan plateau show the valuable applicability of the threedimensional anisotropic algorithm in testing the qualitative presumption of electrical anisotropy

a simple 2 d explanation for negative phases in te magnetotelluric data summary we present magnetotelluric mt data collected in central australia that display unusual negative transverse electric te phases previous explanations for anomalous te phases on land have relied on anisotropy complicated 3 d geometries or coherent noise in contrast the central australian data are free from coherent noise while phase tensor analysis shows that the survey region is 2 d and that the negative phases are an inductive effect the survey was carried out in a grid that covers resistive basement rocks and conductive sedimentary cover stations located on the resistive basement display normal phase behaviour while stations located on the conductive cover display negative te phases at periods of 0 01 0 1 s forward modelling of the region and inversion of the data shows that a shallow laterally extensive bounded conductor that overlies a resistor can produce the observed negative te phases an investigation of te poynting vectors for such a system shows that there is a collision zone near the bounding edge of the conductor where energy that is diffusing downwards collides with energy that has been inductively coupled to the conductor and is diffusing upwards at the base of the conductor this causes a cusp in te apparent resistivity and phases that wrap through 360 a phenomenon previously observed in the ocean side of the te coast effect negative phases extend to the surface of the conductor where measurements are made in a land mt setting the resistivity contrast between the conductive and resistive zones must be at least 1000 to produce negative phases for land mt the period range and distance from the boundary of the conductor at which the negative phases are observed can be estimated from a combination of the thickness of the conductor and the resistivities of the two zones the results presented here are the first example of an isotropic 2 d setting producing negative phases on land and represent an alternative explanation for observations of anomalous data

tectonothermal chronology of early cadomian arc development in guernsey and sark channel islands the cadomian orogen exposed in nw france and the channel islands comprises a collage of variably displaced terranes which record the late proterozoic evolution of a continental margin within a complex subduction zone on the islands of guernsey and sark calc alkaline quartz diorite stocks were emplaced during early stages of the cadomian orogeny zircons from two stocks of deformed quartz diorite display evidence of inheritance in larger and more magnetic fractions upper intercept ages for these discordant concentrates are c 1300 ma sm nd whole rock model ages t dm suggest older ages of c 2000 ma conversely the smallest and least magnetic zircons exhibit a distinct lead loss pattern suggesting that the 207 pb 206 pb dates of c 700 ma correspond to the time of crystallization of the stocks and provide a lower age constraint for initiation of subduction related magmatism in the cadomian orogen hornblende concentrates from deformed quartz diorite stocks exposed on guernsey and sark display discordant 40 ar 39 ar age spectra five samples record well defined isotope correlation ages ranging between 596 ma and 606 ma a hornblende concentrate from host orthogneiss sark yields a plateau age of 608 ma the hornblende ages are interpreted as dating post metamorphic cooling following late proterozoic deformation and metamorphism within the arc system both magmatic and cooling ages from this early cadomian arc contrast with those of younger initial tectonothermal activity c 570 ma and c 540 ma in other cadomian terrane elements and suggest palinspastic separation until very late stages of the orogeny the polyphase nature of this late proterozoic activity is similar to that of related orogens which probably occupied similar tectonic settings along the margin of gondwana e g sw iberia west africa

magnetotelluric array in the central finnish lapland i extreme data characteristics abstract we present magnetotelluric mt data from the northern fennoscandian shield which show extreme behavior due to the complex geoelectric setting left behind by the tectonic reworking of the archaean bedrock during the palaeoproterozoic the core of the mt data set from the central finnish lapland were acquired in 2014 in the context of the masca project extreme data are defined by the condition where the determinant of the real and or imaginary part of the impedance tensor becomes negative this is related to but is a more anomalous condition than out of quadrant off diagonal impedance phases physically these are associated to reversal of e g the electric field as compared to its direction for one dimensional resistivity structure so far only few examples of extreme data have been explicitly reported in geophysical literature data previously classified as erroneous might belong to the class of extreme data instead and data quality analysis should take this into account a resistivity contrast of at least ca 1 1000 is required to observe extreme data as indicated by synthetic modelling studies extreme data are challenging from a modelling point of view and thus also drive the need for new developments in forward and inverse codes for mt in this study behavior of the phase tensor parameterization in case of extreme data is clarified the quadrants of the principal value phases a piece of information inherently missing from the phase tensor can be recovered if the parameters are followed as a function of period the behavior of the determinant average of the impedance tensor is investigated since out of quadrant determinant phases are sometimes observed in conjunction with extreme data a special condition is demonstrated which links out of quadrant determinant phases and extreme data to negative determinant of the galvanic distortion tensor

the cadomian granites of mancellia northeast armorican massif of france relationship to the st malo migmatite belt petrogenesis and tectonic setting abstract the north armorican shear zone nasz divides the cadomian belt of the armorican massif of france into two major terranes the north armorican composite terrane nact and the central armorican terrane cat the nact is a pastiche of displaced blocks and terranes st brieuc terrane st malo terrane and mancellian terrane which results from the amalgamation of cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone in the nact the neoproterozoic brioverian succession was deformed and metamorphosed during the cadomian orogeny and intruded by syn to post tectonic plutonic complexes the st malo migmatite belt is a syn tectonic suite of anatectic migmatites derived through partial melting of the brioverian succession using the t test on whole rock geochemical data there is no evidence to suggest that a sample of st malo diatexites comes from a population with a different mean than some samples of the brioverian succession sandstones resulting homogeneous diatexites anatectic granites of the core were emplaced syn kinematically into cadomian strike slip shear zones thus the age of anatexis of 540 ma is also the date of strike slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the nact the mancellian granites to the southeast of the st malo migmatites were emplaced at 540 ma and are overlain unconformably by cambrian sediments overall geochemistry and a variety of specific geochemical parameters show that the mancellian granites are similar to the diatexites and anatectic granites of the st malo migmatite belt using the t test on whole rock geochemical data there is no evidence to suggest that the sample of mancellian granites comes from a population with a different mean than the sample of st malo diatexites additionally both suites of rocks are peraluminous they have similar distributions of rb sr and ba they have k rb ratios around 225 and they plot largely within the vag field using trace element discrimination diagrams for the interpretation of tectonic setting samples of the brioverian succession sandstones the st malo migmatites and the mancellian granites all exhibit similar primordial mantle normalized element patterns preliminary results of nd and sr isotope studies reveal a similar restricted range of values for nd between 4 0 and 7 3 and sr between 3 and 42 and depleted model ages in the range 1 7 to 1 5 ga tdmur for both mancellian granites and st malo migmatites isotopic features which set these rock types apart from other cadomian granites within the nact the st malo migmatite belt and the mancellian granites are coeval at 540 ma and both developed by anatexis at intermediate crustal depths of the brioverian succession within an inverted but only moderately overthickened behindarc basin in comparison with the st malo migmatites the mancellian granites represent farther travelled products of intracrustal melting but their geochemistry still preserves the characteristic signature of the source

evidence for multiple high t metamorphism and regional migmatization within the cadomian belt of northern brittany france the cadomian orogeny is characterized by two major tec tonometamorphic events one that pre dates 570 ma and another at c 540 ma until recently the majority of migmatites occurring within the cadomian belt of northern brittany were interpreted as having been formed during the second event the discovery of the jouguet migmatites which have been hornfelsed by the st brieuc diorite emplaced at 533 l2 ma suggests that a migmatitic event occurred before 540 ma these data demand a reinterpretation of the structural sketch of the cadomian orogeny

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

a magnetotelluric study of the damara belt in namibia 1 regional scale conductivity anomalies abstract the namibian margin is dominated by the late proterozoic to early cambrian fold belts of the damara orogen which wrap around and separate the congo and kalahari cratons this mosaic of relatively soft fold belts and hard cratons apparently controlled the path for the opening of the south atlantic in the early cretaceous the continents split along the coast parallel fold belts of the damara orogen while the inland fold belt damara belt was effected by extension and widespread igneous intrusion but never developed to the rift stage this paper is concerned with the interpretation of magnetotelluric mt data along a 200 km nw se profile across the damara belt in nw namibia the regional two dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones i a generally very resistive upper crust which is typical for the granites and metasediments of the damara belt ii two subvertical conductors in upper to mid crustal levels which correlate with major tectonic zone boundaries and iii a highly conductive middle to lower crust in the southern part of the profile the geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust we interpret the high electrical conductivity in terms of graphite or other forms of mineralization enrichment along the shear planes this zone of crustal weakness may have originated in pre damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the cretaceous rifting and magmatism associated with the opening of the south atlantic

geoelectric structure of the great slave lake shear zone in northwest alberta implications for structure and tectonic history the study of ancient plate boundaries can provide insights into the past and present day tectonic processes here we describe a magnetotelluric mt study of the precambrian basement of the hay river fault hrf in northwest alberta which is the southwest segment of the great slave lake shear zone new broadband mt data were collected to give a clearer image of the crustal structure the western canada sedimentary basin was imaged as a low resistivity layer above the resistive crystalline basement four basement conductors were defined and correlate with the terrane boundaries delineated with aeromagnetic data these are 1 a major conductor in the kiskatinaw domain 2 a conductor on the boundary of the ksituan and chinchaga domains 3 a conductor on the boundary of the chinchaga and buffalo head domains and 4 a conductor near the hay river fault both 1 and 2 correspond to areas of high seismic reflectivity the low resistivity can be explained by interconnected grain boundary graphite or s

3 d inversion of complex magnetotelluric data from an archean proterozoic terrain in northeastern sao francisco craton brazil

ophiolite emplacement in ne oman constraints from magnetotelluric sounding summary magnetotelluric mt data have been acquired across the samail ophiolite oman mountains along a 115 km transect in january 2005 twenty five mt stations were deployed approximately every 4 km along a profile perpendicular to the coastline near muscat the profile extends from the huw l meeh subwindow in the northwestern part of the profile and crosses the saih hatat window a tertiary domal culmination that folds a major ne facing recumbent fold nappe and ends along the southern boundary between the dasir and ibra ophiolite blocks the survey aimed to investigate the tectonic evolution of the emplacement of the samail ophiolite by delineating major faults and geological boundaries on a crustal scale the data shows complex behaviour with the influence of static shift effects and anomalously high phases exceeding 90 phase tensor analysis and 3 d forward modelling hint that the upper plate lower plate up lp shear zone has a 3 d effect on the data in the northern part of the profile rotated mt impedances were inverted using a 2 d code major resistivity interfaces coincide with the location of the up lp shear zone near the surface and dip towards the arabian margin suggesting a southwest orientated underthrusting prior to the ophiolite emplacement the para autochthonous and less deformed upper plate is a resistor whereas the strongly deformed lower plate is more conductive

simple models for the beattie magnetic anomaly in south africa abstract the origin of the approximately 1000 km long beattie magnetic anomaly bma in south africa remains unclear and contentious key issues include the width depth and magnetization of its source in this study we use uniformly magnetized spheres prisms and cylinders to provide the simplest possible models which predict the 1 km altitude aeromagnetic measurements along a profile across the bma the source parameters are adjusted by forward modeling in case of a sphere an inversion technique is applied to refine the parameters our results suggest that two similarly magnetized and adjacent sources with a vertical offset can explain the observed magnetic anomaly the best fitting model corresponds to two highly magnetized 5 a m 1 sheet like prisms extending from 9 to 12 km depth and from 13 to 18 km depth respectively and with a total width reaching 80 km other less preferred models show thicker and deeper magnetized volumes associated magnetizations seem to be mostly induced although a weak remanent component is required to improve the fit we also compare our results with the interpretation of independent magnetotelluric and seismic experiments along the same profile it suggests that the geological sources for the bma are mostly located in the middle crust and may be displaced by a shear zone or a fault contrary to previous models suggesting a serpentinized sliver of paleo oceanic crust within the natal namaqua mobile belt we propose that granulite facies mid crustal rocks within this belt may cause the bma

numerical tsunami simulation including elastic loading and seawater density stratification systemic discrepancies between observed and modeled tsunami wave speeds were previously identified for two recent major tsunamis the 2010 maule and 2011 tohoku events to account for these discrepancies we developed a numerical tsunami propagation code solving the shallow water equation and including the effects of elastic loading of the seafloor by the tsunami as well as a linear density profile in the seawater column we show here that both effects are important to explain the commonly observed difference between observations and simulations we conclude that the density variation in the seawater column affects the wave speed without changing the waveform whereas the loading effect has an effect on the wave speed and the waveform showing a negative phase before the main arrival due to the depression of the seafloor surrounding the tsunami wave the combination of both effects is needed to achieve a better match between observations and simulations

electrical resistivity structures of the kunlun qaidam qilian system at the northern tibet and their tectonic implications abstract the northern tibetan plateau is characterized by northwest southeast trending basin range systems such as the kunlun shan ranges qaidam basin qilian shan ranges the cenozoic evolution and deformation of the qaidam basin and its neighboring ranges are important indications for the northward growth of the tibetan plateau the latest magnetotelluric data were collected along a profile in n16 5 e from the songpan ganzi block across the kunlun shan ranges east section of the qaidam basin and ending in the qilian shan ranges both two dimensional and three dimensional inversions of the data along the profile were performed and the results provided new evidence for lithospheric structures in the northern tibetan plateau with three dimensional isotropic inversion codes we are able to model the phase roll out of quadrant proq magnetotelluric data which exist at low frequencies in a number of continuous sites at the northern qaidam basin sensitivity studies of the three dimensional models indicate that the qaidam basin with low resistivity upper crust is the major contributor for the proq effect while a south north striking low resistivity gap intersecting with the qaidam basin at its northern part affects the range of the proq effect in the magnetotelluric resistivity models the qaidam basin is not in symmetric structure at upper crust level with its depocenter near the northern qaidam fault at mid lower crust level the compositions of both the kunlun shan ranges and the southern qaidam basin are of generally high resistivity which does not support large scale lower crust materials of the songpan ganzi block flow below the kunlun shan ranges or the qaidam basin instead the unsymmetrical qaidam basin and the shovel shape high resistivity anomalies in crust below the kunlun shan ranges and the southern qiadam basin indicate the regional northward push from the songpan ganzi block the south deepening low resistivity anomalies in the crust of the qilian shan ranges support the existence of an intracrustal detachment which controls the thrusts in northern qaidam basin and the qilian shan ranges the crustal deformation in northern tibetan plateau is consistent with mantle convergence or collision between the kunlun qaidam lithospheric mantle and the asian lithospheric mantle

magnetotelluric measurements across the beattie magnetic anomaly and the southern cape conductive belt south africa 1 the beattie magnetic anomaly bma and the southern cape conductive belt sccb two of earth s largest continental geophysical anomalies extend across the southern african continent in an east west direction to resolve structural details of the sccb a high resolution magnetotelluric study was conducted in march 2004 along a 150 km long n s profile across the karoo basin in south africa a two dimensional conductivity model at a scale of the entire crust exhibits three distinct zones of high conductivity 1 beneath the surface trace of the axis of the bma we observe a high conductivity anomaly at 5 to 10 km depth however it remains enigmatic whether both geophysical anomalies have the same source 2 a shallow regionally continuous subhorizontal band of high conductivity can be linked to a 50 to 70 m thick pyritic carbonaceous marker horizon intersected in deep boreholes 3 several highly conductive synformal features in the mid crust are newly imaged near the northern extremity of the profile our data generally support the existence of a deeper conductivity belt but allow for a much better definition of crustal conductivity in this region such laterally confined zones of crustal conductivity can only be resolved with a dense site spacing and broadband recordings

electrical structure of the tectonically active kalabsha fault aswan egypt in this work we use the magnetotelluric mt method to detect geoelectrical conductivity anomalies in the earth s crust and link them to local seismic activity this application affords the unusual opportunity to study the percolation of water from a lake into a fault system and its effect on the induced seismicity mt measurements were carried out in the period range 0 0046 420 s at nine sites along a 15 km long north south profile crossing the kalabsha fault on the western bank of lake aswan data were analysed by 2d simultaneous inversion of both polarisations the resulting model is compared with the local seismicity map and reveals the conductive signature of the fault as well as geological and tectonic stresses prevailing in the aswan area our mt investigations show the following features the measured mt strike aligns with the seismic epicentre axis corresponding to the kalabsha fault while crossing the fault enhanced conductivity is found down to depths of 5 km on a 1 2 km profile segment at mid crustal depths 20 km a very high conductive body is found to coincide with the main seismic cluster in the aswan area these observations indicate that seismic activity and high electrical conductivity are related the link between them is the presence of crustal fluids which are presumably the cause of the high conductivity observed their presence is also required to trigger the observed seismicity in addition we explain the lower conductivity of the local upper crust in terms of stress modulated rock porosity we believe that these results are of general significance as they could explain the mid crustal seismicity of tectonically active zones d 2005 elsevier b v all rights reserved

eastern termination of the altyn tagh fault western china constraints from a magnetotelluric survey the left lateral altyn tagh fault forms the northern boundary of the tibetan plateau the strike slip rate of the active altyn tagh fault decreases northeastward and reduces close to zero as it passes north of the qilian shan this geometry raises controversies on whether and how the fault terminates or extends further east to address these controversies wide band magnetotelluric data were collected along four profiles across the altyn tagh fault ranging from 135 to 261 km in length all four profiles are located in the foreland of the qilian shan ranges and are oriented perpendicular to the inferred fault zone that could be the continuation of altyn tagh fault both the two dimensional and three dimensional electrical resistivity models derived from our magnetotelluric data show that the hexi corridor crust is generally of low resistivity whereas the crust of the huahai jinta basin is in general of high resistivity with a local and isolated low resistivity anomaly within the mid lower crust the generally high resistivity crust of the huahai jinta basin may be rheologically unfavorable for the altyn tagh fault passing through the basin toward the northeast the entirely different electrical structure between the hexi corridor and its northern neighbors indicates the existence of a tectonic boundary that coincides with the altyn tagh fault in the west and reverse faults in the east the two dimensional electrical conductivity models suggest that the altyn tagh fault transfers from a single fault in the west to a branching set of mainly dip slip faults in the east

electrical anisotropy in the main central thrust zone of the sikkim himalaya inference from anomalous mt phase abstract the sikkim himalaya in the eastern sector of the himalayan collision belt shows some distinct features such as the occurrence of the main central thrust as a several km wide zone mctz of intense ductile deformation and its deviation from the general east west strike of the himalayan thrusts suggesting that the tectonics of this region could be different from the regional himalayan tectonics we have carried out a mt survey across mctz to understand the tectonics of the region the broadband mt data of the site at the northern boundary of the zone reveals anomalous phase exceeding 90 for frequencies smaller than 1 hz whereas other sites further north of this zone show normal behavior such anomalous phase behavior has been explained earlier in terms of anisotropy in the medium and special configuration of local conductors we have analyzed the observed anomalous phase behavior in terms of anisotropy based on geological consideration that alternating bands of graphitic schists are present within high grade crystalline gneisses at the northern boundary of mctz among various models analyzed a block model of the crustal structure with strong anisotropy contrast between the block and the underlying anisotropic layer provides good fit to the observed data we thus infer that the presence of a localized strongly anisotropic block at the northern boundary of mctz could be a viable explanation for the observed anomalous phase behavior

neoproterozoic shear zone tectonics within the icartian basement of guernsey and sark channel islands the channel islands of guernsey and sark are amongst the few localities within the neoproterozoic cadomian orogenic belt where palaeoproterozoic basement is exposed basement units collectively referred to as icartian comprise orthogneisses e g the c 2000 ma icart granite gneiss metasediments and amphibolites on guernsey the protolith to the icart granite gneiss was intruded into metasediments already deformed during a d 1 deformation event both were variably deformed during a d 2 event within an approximately north south trending steeply dipping oblique dextral shear zone on sark metasediments and amphibolites carry a d 1 fabric not necessarily correlatable with that on guernsey which was deformed during d 2 to form recumbent gently inclined folds and tectonic fabrics which locally carry top to the south kinematic indicators early cadomian quartz diorites which intruded the icartian basement carry variably developed magmatic and moderate temperature 400 550 c ls solid state fabrics which are similarly oriented to d 2 basement structures this indicates that the quartz diorites were emplaced and deformed during a protracted d 2 event published 40 ar 39 ar amphibole cooling ages of c 600 ma from guernsey and sark quartz diorites and a host icartian amphibolite are consistent with reworking of basement units during the cadomian orogeny the contrasting kinematic pattern during cadomian deformation implies that a flat lying shear zone on sark may have accommodated oblique dextral strike slip movement on guernsey with an overall southerly direction of tectonic transport deformation most likely occurred in the hanging wall of a major south dipping neoproterozoic subduction zone

chronology of cadomian tectonothermal activity in the baie de saint brieuc north brittany france evidence from 40ar 39ar mineral ages the late precambrian cadomian orogen exposed in the north armorican massif northwest france is a collage of displaced terranes that in part developed during amalgamation of continental arc and marginal basin complexes 40ar 39ar mineral ages reported here place new constraints on the timing of cadomian tectonothermal activity in the southern part of the st brieuc terrane in the baie de saint brieuc area brioverian supracrustal units were deformed metamorphosed and intruded by calc alkaline plutonic complexes metamorphic hornblende from a metabasic amphibolite sheet within brioverian rocks records an isotope correlation age of 568 4 2 6 ma interpreted to date postmetamorphic cooling through appropriate argon closure temperatures similar isotope correlation cooling ages are recorded by metamorphic hornblende within both an amphibolite sheet intrusive into the penthievre complex 567 5 1 2 ma and the la croix gibat amphibolite 574 8 2 1 ma igneous hornblende from the late tectonic to pos

magnetotelluric image linked to surface geology across the cape fold belt south africa terra nova 24 207 212 2012 abstract the subsurface structure of the cape fold belt cfb that traverses southern africa remains largely unknown because it lacks deep penetrating high resolution geophysics this hampers studies from local scale exploration for important resources such as water to continental scale modelling such as gondwana reconstructions our magnetotelluric mt soundings across the cfb provide a high resolution crustal scale electrical conductivity image inversion models of these data correlate well with surface geology and resolve deep resistive roots of the cfb s regional palaeozoic quartzite ridges swartberg and outeniqua mountains and thin tectonic wedges of proterozoic rocks of higher resistivity kango and kaaimans inliers separated by 2 3 km of cretaceous sediments in asymmetric intermontaine basins one of these basins the oudtshoorn basin is flanked by a listric normal fault rooted in an upper crustal detachment zone and is underlain by a massive conductivity anomaly that is likely to be a major saline water reservoir

macro anisotropy of the electrical conductivity of the crust a magnetotelluric study of the german continental deep drilling site ktb summary the interpretation of magnetotelluric mt data collected in the vicinity of the german continental drilling project ktb has revealed two major structures in the earth s crust first a large scale mid crustal layer of enhanced conductivity is required to fit the regional observations of the anomalous vertical magnetic field second the mt impedances of the area close to the ktb namely the crustal unit of the zone erbendorf vohenstraus zev support a model with a highly anisotropic upper to middle crust the extremely high values of anisotropy derived from the mt measurements exceed the values of anisotropy observed on rock samples in the laboratory by an order of magnitude and therefore exclude intrinsic features as a source for the observed anisotropy the regional mid crustal conductor is in agreement with central europe wide observations of high conductivity layers in the crust while the proposed model of well conducting subvertical dykes within a resistive host rock explains the mt observations and agrees with the steeply dipping seismic reflectors the downhole and core observations from the drilling and the model of the tectonic development of this area however the superposition of both structures leaves some unanswered questions

magnetotelluric data from the southeastern capricorn orogen western australia an example of widespread out of quadrant phase responses associated with strong 3 d resistivity contrasts

artefacts of isotropic inversion applied to magnetotelluric data from an anisotropic earth summary 2 d isotropic approaches are standard for magnetotelluric mt data modelling and inversion unfortunately the real subsurface structure is not isotropic everywhere and one should be aware of the possible consequences of applying an isotropic inversion to data from an anisotropic earth the work presented herein was motivated by discovering an unusual mid to lower crust conductor that appeared to be bent downward into the lithospheric mantle compared to the neighbouring terranes when applying routine 2 d isotropic inversion one major difference between the terranes is the presence of the okavango giant mafic dyke swarm northeastern botswana beneath which the conductor is imaged to be in the lithospheric mantle rather than the lower crust the limited width of the dykes makes them an anisotropic feature rather than a normal 2 d structure at mt scale to examine the possible effects of the dykes synthetic data were generated from an 1 d model accounting for the dyke swarm by using an anisotropic block and then inverted isotropically the synthetic tests showed that the normal 2 d decomposition and strike analysis techniques are not removing these large scale anisotropic effects and that an isotropic inversion result obtained in the presence of an anisotropic structure has to be treated with caution the comparison of the synthetic data with the presented case history strongly suggests that the lithospheric depths of the conductor is an artefact and it is most likely located in the lower crust as everywhere else in the area focused anisotropic inversion of the dyke swarm area supports the assumption of a crustal anisotropic structure related to the okavango dyke swarm that affects the isotropic inversion results

reexamination of magnetotelluric responses and electrical anisotropy of the lithospheric mantle in the grenville province canada magnetotelluric mt responses at the proterozoic grenville front in canada have been interpreted as being caused by lithospheric electrical anisotropy and the area is often noted as a classic example of lithospheric anisotropy this study reevaluates evidence for the electrical anisotropy using 56 mt stations the spatially uniform mt responses noted at the grenville front extend to 200 km southeast and for at least 400 km along strike and are associated with rocks at less than 150 km depth examination of induction arrows at longer periods shows arrows at high angle to the mt conductive direction consistent with the presence of macroscopic resistivity structures new 2 d anisotropic inversions show that electrical anisotropy is not required to fit the mt data the results indicate that in the resistive mantle lithosphere beneath the grenville front and in conductive lithosphere in adjacent areas the maximum horizontal resistivity anisotropy is 10 much less than the factor of 15 determined in earlier 1 d studies the results suggest that the upper lithospheric mantle in the area is devoid of significant electrical anisotropy and that the observed mt response directionality is due to large scale resistivity structure we interpret the spatially consistent mt responses observed at the grenville front as being associated with the resistive archean lithosphere extending southeast beneath the grenville front the obliquity between seismic and mt responses arises because the superior fabric is oblique to the seismic fast direction if dextral shearing occurred it appears to have not caused any significant shape preferred electrical anisotropy

a magnetotelluric profile across the german deep drilling project ktb area two and three dimensional modeling results previous interpretations of magnetotelluric data from the vicinity of the german deep drilling project ktb revealed two major structures a midcrustal layer of increased conductivity and large regional extent and a highly anisotropic upper crust nevertheless a satisfactory combination of both structures explaining all measurements has not yet been achieved mostly due to incomplete and qualitatively poor data simplified superposition of both structures could not yield an explanation of the observations on the basis of a carefully processed new data set we apply different modeling approaches to verify the existence of both structures models calculated with a two dimensional modeling program which allows for general anisotropy as well as a full three dimensional code show that an anisotropic upper crust is overlaying a regional east west striking high conductivity structure nevertheless this continuous conductive midcrustal layer with a conductance decreasing from north to south must be replaced by a quasi anisotropic one at least in the region of the ktb the final model may still be oversimplified considering the complexity of the true but unknown geology in this particular area but it demonstrates which major electrically effective structures could be resolved

a newly distinguished marine magnetotelluric coast effect sensitive to the lithosphere asthenosphere boundary the marine magnetotelluric mt method is a useful tool for offshore studies aimed at for example hydrocarbon exploration and the understanding of earth s tectonics marine mt data are often distorted by coastlines because of the strong resistivity contrast between the conductive ocean and the resistive land at mid ocean ridges the resistivity of earth s structure can be assumed to be two dimensional which allows mt data to be decomposed into a transverse electrical te mode with electric current flowing approximately along the ridge and a transverse magnetic tm mode with electric current flowing perpendicular to the ridge we collected marine mt data at the middle atlantic ridge which exhibited highly negative tm mode phases as large as 180 at relatively high frequencies 0 1 0 01 hz similar negative phases have been observed in other marinemtdata sets but have not been the subject of study we show here that these negative phases are caused by a newly distinguished coast effect the tm mode coast effect is not only a galvanic effect as previously understood but also includes inductive distortions tm mode negative phases are caused by the turning of the poynting vector the phase change of electromagnetic fields and vertically flowing currents in the seafloor the findings provide a new understanding of the tm mode coast effect which can guide our ability to fit the field data with the inclusion of coastlines and reduce misinterpretation of the data in offshore studies the study also shows that the tmmode coast effect is sensitive to the depth and conductivity of the asthenosphere an important feature of the earth s interior that was the object of our atlantic ocean study

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

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

the cadomian active margin north armorican massif france a segment of the north atlantic panafrican belt abstract within the european variscan belt the armorican massif northwestern france contains an isolated fragment of the north atlantic panafrican belt the cadomian block mostly unaffected by variscan deformation this block is composed of neoproterozoic rocks ranging in age from 620 to 540 ma brioverian locally overlying a composite basement of icartian 2000 ma or pentevrian 750 ma age the cadomian tectonics the last magmatic manifestations of which are dated at around 540 ma were sealed by a cover of early cambrian rocks in normandy and early ordovician rocks in brittany five units are distinguished in this cadomian block tregor saint brieuc guingamp saint malo and fougeres although the units are bound by major faults the overall coherence of their different protolithic characteristics suggests that they can be fitted into a simple geodynamic model the tregor unit represented by the north tregor batholith of crustal affinity and its icartian host rock probably reflects an early cadomian evolution the saint brieuc unit essentially made up of magmatic rocks of juvenile affinity includes two assemblages juxtaposed by the cadomian tectonics 1 a volcanosedimentary succession formed in a typical island arc setting reflecting subduction around 610 ma and 2 a volcanic plutonic complex emplaced around 590 ma in an extensional active continental margin of pentevrian age the guingamp unit predominantly migmatitic includes a tectonic slice of deep origin the extrusion of which accompanied the exhumation of the migmatites it may represent the trace of a possible suture the saint malo unit is composed of brioverian detrital rocks the significance of which is uncertain passive continental margin or accretionary prism it is structured around a late migmatitic dome 540 ma the fougeres unit corresponds to the outer domain of the cadomian orogeny it was only weakly deformed by cadomian tectonics and is intruded by the large mancellian batholith of crustal origin dated at 540 ma together the structural units are interpreted in the framework of an evolving active margin juxtaposing continental margin marginal basin and volcanic arc before being shortened during the cadomian convergent tectonics

late proterozoic zircon ages from a basic ultrabasic complex a possible cadomian orogenic complex in the hercynian belt of western europe the basic ultrabasic complex of belle isie en terre northwestern part of the armorican massif france has been dated at 602 4 1 m y by the u pb zircon method the calc alkaline trend of the metabasic rocks and the geologic context suggest that this ophiolite like complex is related to a back arc orogenic setting similar to the geodynamic environment that has been proposed for pan african calc alkaline ophiolite like complexes of late proterozoic age around the west african craton our results are compatible with the closing in late proterozoic time of a channel ocean north of the basement and formed during the cadomian orogeny rather than with possible caledonian or hercynian subduction