upper mantle anisotropy in se and central brazil from sks splitting evidence of asthenospheric flow around a cratonic keel abstract we present results of upper mantle anisotropy derived from measurements of core refracted shear wave splitting mainly sks phases recorded at 48 stations covering the major tectonic provinces in central and se brazil such as the tocantins province paraguay araguaia and brasilia belts between the amazon and sao francisco cratons the parana intracratonic basin the southern part of the sao francisco craton and the mantiqueira province with the coastal ribeira belt although the fast polarization directions vary across the region consistent orientations are observed over hundreds of kilometers the fast polarization directions tend to be close to the absolute plate motion given by the hot spot reference model hs3 nuvel1a however correlations with geological structures are also observed in the southern brasilia belt and in the ribeira belt respectively located sw and s of the sao francisco craton on the other hand in the northern tocantins province the fast shear wave direction n60 s is oblique to the sw ne trend of the geological units and faults and no anisotropy contribution from lithospheric sources can be clearly identified overall the fast polarization directions show a fan shaped pattern strongly suggesting asthenosperic flow around a thick and stiff keel in the southern part of the sao francisco craton consistent with the high velocity anomaly revealed by recent surface wave tomography the observed nw se directions in the southern part of the brasilia belt may also be interpreted as resulting from asthenospheric flow channeled between the sao francisco craton and a cratonic block beneath the parana basin the largest splitting delays observed in the southern brasilia belt and in the ribeira belt up to 2 4 s suggest contributions from both lithospheric and asthenospheric sources in those two areas our preferred model for the anisotropy causing the observed pattern of sks splitting consists of flow induced deformation in the asthenosphere caused by the absolute plate motion roughly towards w or wsw with flow channeled around a thick cratonic keel in the southern brasilia and in the ribeira fold belts the large delays and the parallelism with geological structures indicate additional anisotropy contribution from lithospheric sources

hydrothermal dolomitization of jurassic cretaceous limestones in the southern alps italy relation to tectonics and volcanism dolomitization has affected up to 750m of the jurassic and cretaceous pelagic carbonate sequence of the southern continental margin of the alpine tethys the sequence crops out in the southern alps of italy monti lessini late paleocene to miocene extrusion of basaltic tuffs breccias and lavas was contemporaneous with the dolomitization was was associated with extensive tectonism in an ancient back arc basin more than 200 samples were analyzed by x ray diffraction cathodoluminescence scanning electron microscopy stable isotope ratios carbon oxygen strontium and clay mineralogy the dolomite contains 40 to 50 mgco sub 3 in thin sections the crystal size distribution is unimodal about 100 mu m possibly indicating a single nucleation for the main crystallization phase the delta sup 13 c of the dolomite is not appreciably different from the undolomitized pelagic limestone 1 0 per thousand to 2 0 per thousand peedee belemnite pdb the delta sup 18 o variation 5 0 per thousand to 13 0 per thousand pdb is due to temperature variation in the system the sup 87 sr sup 86 sr ratio in the dolomite 0 70839 0 70867 is consistent with the ratio in late oligocene miocene marine water clay minerals in limestone and dolomite differ in the presence of neoformed mg chlorite indicating a maximum temperature of about 150c for dolomitization the dolomite is suggested more to have a hydrothermal origin the heat flow associated with the volcanism allowed marine water to penetrate the system and circulate in convective cells through the tectonic breccias locally dolomitizing the limestone less

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

signatures of continental collisions and magmatic activity in central brazil as indicated by a magnetotelluric profile across distinct tectonic provinces broadband and long period magnetotelluric soundings were collected along a 560 km e w profile in a region of central brazil subjected to neoproterozoic collision tectonics and archean to cretaceous magmatic events the profile crosses the northeastern portion of phanerozoic sediments and volcanics of the parana basin the southern extension of the neoproterozoic metasedimentary rocks of the brasilia belt locally pervaded by cretaceous alkaline magmas of the alto paranaiba igneous province apip and neoproterozoic sedimentary cover and exposed archean basement of the southern sao francisco craton 2d conductivity structures derived by joint inversions of the te and tm polarization modes and a separate inversion of the tipper components show signatures of the past tectonomagmatic events that affected the area a gravity defined suture zone beneath the parana basin is detected in the models as a subvertical conductor extending from crustal to upper mantle depths which strengthen the interpretation of a neoproterozoic collision of the sao francisco craton and a continental block beneath the sedimentary basin deep underthrusting of organic graphite bearing metasedimentary rocks composing a fossil suture zone is proposed to explain such increase in electrical conductivity a similar conductivity signature beneath the sedimentary covered region of the sao francisco craton can be tentatively interpreted as another similar cryptic suture zone other isolated high conductivity anomalies at midcrustal depths below the parana and apip provinces are interpreted as fossil residues representing precipitated graphite and sulfide derived from percolating volatiles during the emplacement of cretaceous mafic ultramafic volcanics a very high conductivity wedge into the lower lithosphere is highlighted in the deep mantle beneath the apip volcanic complex coincident with a zone of low velocity defined by seismic tomography geochemical evidence indicates that the alkaline magmatism stemmed from a metasomatized upper mantle at only slightly raised temperatures thus interconnected carbonatite melts of low melting point and graphite in the lithospheric mantle are the most likely candidates to explain this high conductivity the seismic and conductivity anomalies are probably triggered by the same source mechanisms incipient melting because they are plausibly related to the same magmatic process conductivity anomalies at crustal and mantle depths in the southern segment of the sao francisco craton suggest that its lithosphere was significantly affected by the several tectonomagmatic episodes it has experienced throughout its geological history consequently the enhanced conductivity in the lower crust can be genetically related to upwelling volatile rich intrusions whereas upper mantle high conductivity can be related to refertilization by infiltrations of low degree carbonatitic melts from deeper sourced metasomatic processes

joint inversion of tem and distorted mt soundings some effective practical considerations accurate layered earth interpretation of magnetotelluric mt apparent resistivity sounding curves distorted by the presence of near surface 3 d bodies in an otherwise horizontally stratified ground requires that the curves be shifted to the level they would have assumed in the absence of these anomalous small size bodies the most popular preinterpretation processing involves shifting the biased mt apparent resistivity curves to the levels determined by some additional data this paper examines the nature of tem field data their relationship with mt data and their usefulness in constraining the interpretation of mt data at locations affected by near surface 3 d effects the mt apparent resistivity curves are neglected in a joint inversion scheme involving the undistorted equivalent phase information and tem data thus eliminating the need for preinterpretation data processing or correction of statically shifted curves this approach is efficient and effective and since an overlapping tem observatio

resistance to mantle flow inferred from the electromagnetic strike of the australian upper mantle seismic anisotropy is thought to result from the strain induced lattice preferred orientation of mantle minerals especially olivine1 2 owing to shear waves propagating faster along the a axis of olivine crystals than along the other axes this anisotropy results in birefringence or shear wave splitting 3 which has been investigated in numerous studies1 4 although olivine is also anisotropic with respect to electrical conductivity5 with the a axis being most conductive few studies of the electrical anisotropy of the upper mantle have been undertaken and these have been limited to relatively shallow depths in the lithospheric upper mantle6 7 theoretical models of mantle flow have been used to infer that for progressive simple shear imparted by the motion of an overriding tectonic plate the a axes of olivine crystals should align themselves parallel to the direction of plate motion8 9 here however we show that a significant discrepancy exists between the electromagnetic strike of the mantle below australia and the direction of present day absolute plate motion10 we infer from this discrepancy that the a axes of olivine crystals are not aligned with the direction of the present day plate motion of australia indicating resistance to deformation of the mantle by plate motion

shear wave splitting in se brazil an effect of active or fossil upper mantle flow or both we investigated the structure of the upper mantle beneath southeastern brazil using teleseismic shear wave splitting measurements measurements were performed on seismic data recorded in the ribeira and brasilia neoproterozoic belts which wrap around the southern termination of the sa o francisco craton and disappear westward under the paranabasin in the northern ribeira belt dominated by thrust tectonics the fast shear wave polarization planes trend on average n080 e whereas in the central domain dominated by strike slip tectonics fast shear waves are polarized parallel to the structural trend n065 e stations located above the main transcurrent fault display large delay times s 2 5 s such values among the largest in the world require either an unusually large intrinsic anisotropy frozen within the lithosphere or a contribution from both the lithospheric and asthenospheric mantle within the southern brasilia belt fast split shear waves are polarized parallel to the structural trend of the belt at a high angle from the apm although part of our data set strongly favors an origin of anisotropy related to a fabric frozen in the lithospheric mantle since the neoproterozoic a contribution of the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters 2003 elsevier science b v all rights reserved

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

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

focal mechanism inversion in the giudicarie lessini seismotectonic region southern alps italy insights on tectonic stress and strain abstract a set of 41 focal mechanisms 1989 2006 from p wave first polarities is computed from relocated seismic events in the giudicarie lessini region southern alps estimated hypocentral depths vary from 3 1 to 20 8 km for duration magnitudes md in the range 2 7 5 1 stress and strain inversions are performed for two seismotectonic zones namely g giudicarie and l lessini this subdivision is supported by geological evidence seismicity distribution and focal mechanism types the available number of data 16 in g 22 in l does not make possible any further subdivisions seismotectonic zones g and l are undergoing different kinematic regimes thrust with strike slip component in g and strike slip in l principal stress and strain axes in each sub region show similar orientations the direction of maximum horizontal compressive stress is roughly perpendicular to the thrust fronts along the giudicarie belt in zone g and compatible with right lateral strike slip reactivation of the faults belonging to the schio vicenza system in zone l on the whole kinematic regimes and horizontal stress orientations show a good fit with other stress data from focal mechanisms and breakouts and with geodetic strain rate axes

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

relay ramp forms and normal fault linkages canyonlands national park utah fieldwork within a series of mesoscale grabens in southeast utah has revealed a particularly well exposed system of interlinked extensional faults a series of down faulted grabens are developed within a 460 m thick brittle layer of upper paleozoic sandstone and shale which overlies a ductile layer with a high gypsum content all the major grabens consist of two or more overlapping elements which are composed of fault segments these segments may be hard linked fault surfaces are joined or soft linked fault surfaces are isolated but linked by ductile strain of the rock volume between them in map view relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments in the canyonlands grabens the transfer of displacement between soft linked fault segments is characterized by well de fined dipping relay ramps commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments relay ramps develop as ephemeral structures eventually becoming breached by hard linkage of the fault segments breakdown of ramps by breaching is part of the process of fault growth by segment linkage within the canyonlands graben system four orders of segmentation are present this may be a consequence of different scales of mechanical heterogeneity but evidence from the canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage

hvsr technique in near surface thermal basin characterization the example of the caldiero district north east italy in the present work non invasive microtremor survey is applied for the preliminary characterization of the thermal district of caldiero in the eastern po plain italy a thermal resource known since the roman times for the presence of warm springs the work s aim was to test the suitability of the microtremor passive technique for the preliminary characterization of a thermal basin in terms of resonance properties between the alluvial deposit covers and the rocky bedrock in particular the horizontal to vertical spectral ratio hvsr single station technique is adopted to estimate the resonance properties of the geothermal basin more than 100 hvsr single station measurements are collected and analyzed in the area studied hvsr information is integrated with the findings of the analysis of the data relative to more than 20 deep wells such an integration permits the calibration for validation of the geophysical data the correlation between the results from microtremor survey lidar data analysis and wells information highlighted the main geological structures of the site which are potentially responsible for the thermal waters arising hvsr results are then interpreted in terms of regional geological setting textural and hydrogeological properties of the superficial deposits and the underlying rocky bedrock the results of this method confirm the existence of significant correlations between the resonance behavior of the basin as determined by the hvsr technique and the geological assessment this suggests that the single station technique could be a very promising tool for a rapid and preliminary non invasive method for the near surface general characterization of geothermal basins

electrical anisotropy of south african lithosphere compared with seismic anisotropy from shear wave splitting analyses abstract electrical anisotropy in southern africa inferred from the analysis of magnetotelluric mt data recorded as part of the southern african mt experiment samtex is compared with seismic anisotropy inferred from an sks shear wave splitting study in the same region given the vastly varying penetration depths in the survey area electrical anisotropy is derived in terms of approximate depth rather than frequency electrical anisotropy directions for crustal depths 45 km are not consistent with the fast axis directions inferred from the sks analyses upper mantle electrical results appear to be mostly a consequence of the geometry of large scale geological structures and provide evidence that some crustal structures are distinct at depth while others seem to be confined to the crust our results indicate that the causative region for the seismic anisotropy in the lithospheric mantle has either a correspondingly weak electrical anisotropic signature or is more prominent at greater lithospheric depths than those we investigate here

strike slip relay ramps areas of reorientated bedding at contractional oversteps between strike slip faults are here called strike slip relay ramps metre scale examples are described from the jurassic sediments at east quantoxhead somerset u k larger strike slip relay ramps occur in the rio de peixe basin ne brazil along the newport inglewood trend california and in the bovey basin sw england although the geometry and development of strike slip relay ramps are similar to those of relay ramps in normal fault systems there are differences in the structures which accommodate the transfer of displacement between the overstepping faults whereas strike slip relay ramps are typically transpressional with pressure solution often occurring relay ramps in normal fault systems are dominated by extension or transtension care needs to be taken when interpreting areas of reorientated bedding between overstepping faults particularly when displacement directions are unknown for example when using seismic data this is because relay ramps can occur in both strike slip and normal fault zones

comments on the interpretation of deformation textures in rocks abstract in rocks that undergo ductile deformation preferred orientation develops as a result of intracrystalline slip and mechanical twinning the orientation distribution is a consequence of the microscopic mechanisms and of the strain path it can be used to get some insight into the deformation history however it is never unique the interpretation relies largely on polycrystal plasticity theory the concepts of stress equilibrium and strain compatibility which are two extreme assumptions made to model deformation are discussed new approaches such as the viscoplastic self consistent theory are a compromise and may be applicable to mineral systems which display a high degree of plastic anisotropy important extensions allow for heterogeneous deformation in the polycrystal from grain to grain and even within grains in correspondence with microstructural observations all these theories defy the popular notion which is becoming entrenched in the geological literature that the microscopic slip plane normal aligns with the axis of maximum principal compressive stress and that in simple shear the crystallographic slip plane rotates into the macroscopic shear plane and the slip direction into the macroscopic shear direction an orientation referred to by geologists as easy glide it is emphasized that future work on texture development of rocks should be based on rigorous physics rather than ingenious intuition in accordance with an old recommendation of walter schmidt

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

on the combination of the magnetotelluric and the geomagnetic depthsounding method for resolving an electrical conductivity increase at 400 km depth the simultaneous use of two different passive electromagnetic em sounding techniques for resolving the upper mantle electrical conductivity structure is emphasized the geomagnetic gm method uses magnetic field variations at periods between 3 hours and 600 hours yielding penetration depths between 300 km and 1000 km the magnetotelluric mt method is applied to field variations at periods between a few seconds and 16 hours and yields penetration depths between a few km and 600 km this is an extension of both methods using additional d sub st data for mt and additional s sub q data for gm in application to field data from western europe the resulting overlap of the target depths includes an electrical conductivity increase from 10 sup minus 2 s m at 300 km to 1 s m at 700 km depth while a single conductivity jump at either 400 500 or 700 km cannot explain the broadband data although the olivine to spinel transition is a possible explanation of the conductivity enhancement at 400 km depth another mineralogy could also occur this conductivity increase has not been resolved in other sub continental profiles and lateral heterogeneity is probable in this depth range 27 refs 3 figs 2 tabs

shallow and deep rotations in the miocene alps mapping and the construction of balanced cross sections reveal dextral rotation in addition to n s contraction during the late middle to late miocene jura phase both at the northern and the southern front of the central alps jura and lombardic southern alps respectively these rotations may be ascribed to one and the same shallow domain or sublid rotated clockwise around a pivot somewhere at the southwestern end of the aar massif in a crude first approximation model the sublid may be considered rigid although the data indicate differential rotations and longitudinal stretching within the lid the eastern boundary of the sublid crosses the alps at the giudicarie brenner line which according to very recent data in the jura phase formed a kinematically linked system whereas from innsbruck to the northwest it is postulated to join as a diffuse or accommodation zone the eastern tip of the jura in the southwest the lombardic thrusts enter into the interior of the western alps from there to the southwestern end of the jura the lid boundary is tentatively chosen arbitrarily to pass somewhere near the northern end of the belledonne massif a similar sublid though less well substantiated may comprise the tauern and the venetian southern alps the rotational components are superimposed on elements of internal strain particularly transverse contraction and longitudinal stretching due to simple dextral shear and lateral escape the model of dextrally rotating sublids pulls together a number of hitherto unexplained structures and opens new perspectives on such recalcitrant problems as the eastward disappearance of the jura and the adige embayment in the southern alps dextral rotation of such shallow sublids may be considered to constitute mode 1 of the dextral component at the europa adria plate boundary there is also a mode 2 that affects its deep parts at the surface its manifestation is a dextral belt of en echelon folds and pull apart domains that crosses the sublids diagonally it consists of the massifs line in the west and the neo pustertal line in the east they approximately join in the brenner area where they cross and interfere with the sublid boundaries the combined massifs pustertal line describes an arcuate partial northern boundary of the adriatic plate with an apparent center of sinistral rotation in central italy this sinistral rotation may be responsible for the contractional features in the southern part of the western alps which cannot be accounted for by se nw adria europa convergence it could have caused the development of the fold and thrust belts of sisteron castellane and nice these arcs in turn suggest the action of more or less independent shallow sublids also in this part of the alps

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

tectonic implications of s wave anisotropy beneath se brazil summary sks shear wave splitting has been analysed for fast polarization direction and delay time at 14 broad band portable seismic stations arrayed in se brazil across the silo francisco craton the brasilia mobile belt the intracratonic parana basin the rio apa block to its west and the coastal transpressional ribeira belt splitting parameters are consistent with fossil mantle anisotropy resulting from major geological events and exhibit only coincidental correlation with absolute plate motion for the coastal ribeira belt the anisotropy while not unusually large 1 05 s parallels the local and regional fault trends at the surface consistent with anisotropy formed by regional transpressional tectonics in the region comprised of the siio francisco craton the brasilia mobile belt and the eastern margin of the parana basin fast polarization directions trend monotonously wnw ese parallel to the dominant regional faulting and consistent with their having formed during a ne sw brasiliano ca 600 ma collisional event between the silo francisco craton and the parana basin the delay time is smallest 0 6 s for station cdcb in the craton interior and largest 1 6 s for station rifb located near the north eastern edge of the parana basin and on the prominent bouguer gravity anomaly gradient inferred by lesquer et al 198 1 to mark the cryptic i e basement suture between craton and basin within the parana basin shear wave splitting is uniformly small 1 0 s indicating relatively weak mantle foliation in the horizontal plane fast polarization directions within the cratonic nucleus of the basin near the basin axis average approximately e w within the marginal regions of the basin fast polarization directions tend to be subparallel to the trend of the bordering brasiliano basement structures indicating the pervasive influence of the brasiliano accretionary events in the eastern and northern parana basin fast polarization directions are dominantly wnw ese whereas in the western basin the fast polarization direction is ne sw parallel to observed anisotropy in the rio apa block to the west minimal delay times 0 5 s are observed in the eastern parana basin above the axis of the seismic low velocity anomaly interpreted to be the fossil conduit of the parana plume head vandecar james assumpcilo 1995 while the splitting results are not definitive the regional anisotropy of the eastern parana basin may also reflect plume head spreading beneath the continental lithosphere and partial destruction of pre existing mantle fabric large shear wave splitting 1 55 s is observed for the station in the rio apa block adjoining the western margin of the parana basin the fast polarization direction ne sw is oriented parallel to that of the station in the western parana basin and almost normal to the fast polarization direction of the stations on the eastern margin of the parana basin the orientation of anisotropy in the rio apa block and western parana basin appears to be consistent with the inferred regional trends of brasiliano structures suggesting that the anisotropy may have been produced by nw se convergence and collision between the cratonic basement of the parana basin and the rio apa block

application of the 3d magnetotelluric inversion code in a geologically complex area the wsinv3dmt code makes the implementation of 3d inversion of magnetotelluric data feasible using a single pc audio magnetotelluric data were collected along two profiles in a cu ni mining area in xinjiang china where the apparent resistivity and phase curves the phase tensors and the magnetic induction vectors indicate a complex 3d conductivity structure 3d inversions were carried out to reveal the electrical structure of the area the final 3d model is selected from the inversion results using different initial lagrange values and steps the relatively low root mean square rms misfit and model norm indicate a reliable electrical model the final model includes four types of low resistivity areas the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone the second ones are not controlled by magnetotelluric sites and embody little information of the observed data they are considered as tedious structures the third one is near to the regional kangguer fault and should be treated carefully considering the effect of the fault the last ones are isolated and existing at a limited level as the first ones they should be paid more attention to

lehmann discontinuity as the base of an anisotropic layer beneath continents long period surface wave r 1 g 1 body wave s ss sss and scs reverberation data have been inverted to obtain anisotropic structures along seismic corridors that sample australia and the western pacific these models support the proposal that the lehmann discontinuity beneath stable continents represents a transition from an anisotropic lithosphere to a more isotropic material in the lower part of the continental tectosphere

heterogeneous crust and upper mantle across the se brazilian highlands and the relationship to surface deformation as inferred from magnetotelluric imaging 1 we provide evidence for lateral structural zoning of the crust and uppermost mantle across the southeast brazilian highlands from two dimensional magnetotelluric mt imaging of a 250 km long nw se profile extending from the archaean sao francisco craton to near the atlantic coast our model shows the presence of an electrically resistive cover layer of variable thickness 2 20 km and an underlying 5 15 km thick conductive marker horizon atop a resistive segmented layer extending to 60 70 km depth two structurally different lithospheric provinces are suggested based on conductor geometry and lateral segmentation in the model the northwestern half of the profile is characterized by flat to shallowly dipping crustal conductors interpreted as a fold and thrust belt while the southeastern half consists of major steeply dipping lithospheric conductors with spatially coincident high amplitude magnetic anomalies interpreted as a strike slip fault belt the boundary between both provinces may be associated with a suture but lies 35 km nw of a previously suggested boundary between the occidental and oriental terranes the marker horizon is coincident with a low p wave velocity layer and appears to be raised by about 5 km across a 85 km wide belt hosting a neoproterozoic magmatic arc in the southeastern province we propose a link between basement heterogeneity and surface deformation in the region

constraints on mantle flow at the caribbean south american plate boundary inferred from shear wave splitting 1 we measured shear wave splitting from sks and skks data recorded by temporary stations deployed as part of the broadband onshore offshore lithospheric investigation of venezuela and the antilles arc region project and the national seismic network of venezuela approximately 3000 station event pairs yielded 300 with visible sks and or skks phases we obtained 63 measurements at 39 of the 82 stations in the network using the method of silver and chan 1991 and conventional quality criteria we combined our results with previous measurements made by russo et al 1996 the most prominent feature in the data is an area of large 2 0 s lag times with roughly east west fast axes in northeastern venezuela mineral physics models show split times this large are difficult to explain with horizontal foliation but are more feasible with anisotropy characterized by a coherent vertical foliation and an east west fast axis extending over most of the upper 250 km of the mantle we interpret the large split times in northeastern venezuela as a consequence of eastward translation of the atlantic slab which has left a strong vertical foliation in its wake parallel to the plate boundary the peak split times correspond closely with the point the slab intersects the base of the anisotropic asthenosphere at 250 km away from this area of large split times the measured times fall to more standard values but an east west fast axis still predominates we suggest this is linked to the rapidly varying strain field at the southern edge of the atlantic which quickly disrupts the coherent strain field that causes the very large split times in northeastern venezuela

cordillera formation mantle dynamics and the wilson cycle we propose that the andes cordillera formed by a collision between the trenchward moving south american plate and the nazca slab and subslab mantle resistance to trenchward motion in the form of horizontal compressive normal stress deforms south america9s leading edge the advancing continent also perturbs the subslab mantle flow field as mantle is displaced this flow field detected via shear wave splitting includes trench parallel flow away from a central stagnation point and corner flow around the northern and southern ends of the continent the corresponding spatial variation in applied stress accounts for the large scale characteristics of andean deformation buckling at the stagnation point maximum normal stress forming the central andes and bolivian orocline and corner flow associated with the eastward motion of the caribbean and scotia plates the high stresses required to form and maintain the andes imply that south america9s trenchward motion is primarily driven by deep mantle flow coupled to its base no other driving force of sufficient magnitude is available similarities between north and south american plate motion history and large scale cordillera structure indicate that the western north american laramide rockies formed in a similar manner and that the north american plate is also driven westward by deep mantle flow we infer that atlantic spreading itself is similarly driven by contrast slab rollback in a closing ocean basin opens marginal basins at convergent margins as in the western pacific we suggest that there is a causal link between the opening and closing of ocean basins and the formation of cordilleras and marginal basins cordillera formation occurs at the leading continental margin during the spreading phase of the wilson cycle whereas marginal basins form during the closing phase

three dimensional geometry and growth of conjugate normal faults abstract good quality 2 d and 3 d seismic reflection data from the timor sea are used to determine the threedimensional geometry displacement patterns and development of intersecting conjugate normal faults these data are supplemented by data from previous physical modelling studies conjugate structures which comprise two intersecting opposed dipping normal faults or fault sets form synchronously on a geological time scale and develop due to the incidental intersection of the faults factors which affect both the formation and imaging of these structures include the fault density the spatial distribution of opposed dipping faults the seismic resolution and the vertical extent of the imaged fault data large conjugate structures grow from smaller ones larger conjugates are associated with more numerous and larger faults than small structures on the scale of the seismic data fault throws range from ca 10 400 m synchronous fault movements are accommodated by a reduction of displacements on discrete fault surfaces towards the fault intersection zone and a corresponding increase in ductile strain of this region high strains in the volume proximal to the fault intersection zone are expressed as thinning of stratigraphic units between the conjugate faults and are believed to be accommodated by numerous small sub seismic faults intersection of two opposed dipping faults does not prevent their continued synchronous movement and does not result in mechanical locking of the system

transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero abstract the paleoproterozoic transamazonian orogeny yielded a series of discrete orogens in brazil the present field study in the quadrilatero ferrifero qf indicates that the southern sao francisco craton region of the brazilian highlands preserves a portion of one of these orogens two sets of transamazonian structures occur in this region the first consists of northwest verging folds and thrusts affecting supracrustal sequences it is suggested that this set formed in a fold thrust belt setting shortly after 2 125 ga during the closure of a passive margin basin that had initiated along the margins of a preexisting continental mass at ca 2 5 ga the second set consists of structures defining the prominent dome and keel architecture of the qf this set a consequence of the emplacement of basement domes against supracrustal rocks at 2 095 ga may reflect the consequences of orogenic collapse narrow conglomerate filled intermontane basins may have formed coevally with dome emplacement formation of an ocean basin east of the present day sao francisco craton eventually occurred in late mesoproterozoic in effect the transamazonian orogen of the qf represents the collision and collapse stages of a paleoproterozoic wilson cycle the contractional phase of the transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present day sao francisco craton region the arc and an associated suture may be traced in the brasiliano pan african orogen east of the sao francisco craton northwards into the northeastern lobe of the sao francisco craton clearly initial assembly of crustal blocks to form a larger continent involving south america occurred during the paleoproterozoic 2 1 ga post transamazonian rifting of this continent created the basins which were later inverted during the brasiliano assembly of gondwana

the fluvial architecture of the upper buntsandstein in the iberian basin central spain the cercadillo sandstone and siltstone css formation forms the upper part of the buntsandstein fluvial facies in the western margin of the iberian basin sedimentation was controlled by two major pre triassic tectonic systems the iberian ranges system nw se and the central system ne sw fluvial sedimentation during this period took place in coexisting braided and meandering channels together with associated ephemeral episodes this unusual association was due to a combination of structural control by the main normal faults coupled with large discharge fluctuations the css formation consists of two major fluvial sequences with very similar vertical patterns comprising low sinuosity stream deposits that pass upwards into high sinuosity deposits associated with extensive floodplain sediments a widespread calcrete horizon is developed at the top of the lower sequence and a surface with pedogenic features is present at the top of the upper sequence these horizons are interpreted to be the result of subaerial exposure during a period of no subsidence and equilibrium in the erosion sedimentation processes in the area probably due to quiescent periods on the master faults that delineate the basin minor changes within the sequences are probably due to discharge fluctuations of a climatic origin whereas the general vertical trend of the sequences is suggested to be tectonically controlled palaeocurrent data show a general flux of sediments to the south east parallel to the axis of the iberian basin but the final part of the upper sequence palaeocurrents to the north east are interpreted to indicate a local reactivation of secondary transverse faults in the south east part of the study area

fractal analysis of a quaternary fault array in the central apennines italy abstract the monte vettore fault array in the central apennines fold and thrust belt is part of a major quaternary fault system displaying the main features of a regional negative flower structure associated with a roughly n s trending left lateral strike slip fault zone fractal analysis of the fault trace pattern of the area has shown that the box counting curve derived for the monte vettore array is defined by two straight line segments each showing a characteristic slope value the first segment which refers to a spatial distribution extending over about one order of magnitude is characterized by a fractal dimension of about 1 6 whereas the second one fits a euclidean i e a non fractal distribution the box counting analysis emphasizes that faults with variable sizes exhibit different scaling relations hence suggesting that fault linkage played a major role in the development and evolution of the monte vettore fault array the split point between the two segments falls at a value of log l s corresponding to a box size of 635 m hence suggesting that self similarity among faults within the array breaks down below this value fractal analysis of the monte vettore array also provides statistically based segmentation criteria for analysing a young immature fault structure capable of generating medium size earthquakes in a continental geotectonic setting

electrical resistivity of the northern australian lithosphere crustal anisotropy or mantle heterogeneity abstract interpretations of continental magnetotelluric mt data are increasingly invoking electrical resistivity anisotropy in the crustal and or mantle lithosphere or deeper in the asthenosphere to explain non uniform responses in this paper we show that seven shallow marine 10 000 m in the proterozoic western end of the survey grading to 1000 m beneath the eastern australian phanerozoic shear wave velocity tomographic models show 10 higher velocities beneath the proterozoic craton compared with phanerozoic continental lithosphere suggesting that the variations in resistivity may be due to higher temperatures beneath the eastern side and or an increased volatile content

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

geoelectric crustal structures off the sw border of the sao francisco craton central brazil as inferred from a magnetotelluric survey summary a magnetotelluric mt survey of the crust beneath sedimentary basins and thrust sheets along the southwestern edge of the sao francisco craton central brazil reveals intricate electrical characteristics that are interpreted in the context of proterozoic collision tectonics and horizontal transport of allochthonous rock units emplacement of cretaceous hypabyssals lavas and diatremes of ultrapotassic mafic composition and occurrence of induced seismicity the data exhibit strong distortions represented by 3 d induction effects and galvanic disturbances resulting from shallow structures frequency and site dependence of electrical strike and inhomogeneous anisotropic layers with smoothly varying phase split conductance and azimuth of the highly conductive direction geoelectrical and orthogonal phase difference directions 2 d inversion and forward modelling characterize three distinct subhorizontal sections showing two anisotropic conductors within a highly resistive crust laterally segmented into unique blocks the model for the uppermost crust section has e w geoelectric directions and a 15 30 s anisotropic zone at a depth of approximately 1 2 km along the entire profile this conducting layer is interpreted to represent a brine filled fracture layer possibly controlled by the present day state of crustal stresses as disclosed from reservoir triggered quakes the mid crust section presents a deeper conducting zone located at depths below 10 km it is defined by stronger mt responses having phase split directions oscillating from wnw ese to e w beneath sites in the central western area parana basin and allochthonous cover units and nne ssw in the northeastern region autochthonous platform units and sanfranciscana basin anisotropy is greater than an order of magnitude in the highly conductive direction with conductance in the range of 250 400 s conjecturally the source of this anomalous feature would come from interconnected grain boundary phases and hypersaline fluids exsolved and precipitated from upwelling cretaceous magma in the central western area favourable trapping of conductors was constrained along a nearly e w direction feasibly associated with relic structures inherited from brasiliano pan african continental collisions along the northeast however the coincidence with superficial nne ssw structural directions suggests a localized direct causal relationship with the trend of extension related to magma emplacement the lower crust has a highly resistive quasi 1 d section along the entire profile that prevails also at uppermost mantle depths thus whereas the brittle crust would have reconciled subhorizontal strain with fluid percolation related to uplift and magma emplacement a mechanically coupled and stronger lower crust upper mantle would have controlled the deep magma generation during cretaceous distention pulses

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

signification structurale des anomalies gravimetriques de la partie sud du craton de sao francisco bresil abstract the relationships between the cratonic area and orogenic belts of the southern part of the state of minas gerais brazil are analyzed using data from a recent bouguer anomaly map the interpretation was carried out using inverse anomaly approach and statistical analysis the stable area is characterized by elongate anomalies trending nne ssw which correspond to a higher level of the basement under the upper precambrian sedimentary cover bambui and macaubas groups they can be related to rejuvenated nne ssw trending fractures the gravity trends are either deformed or interrupted over the brasilian fold belt where an important negative linear anomaly is present this anomaly is produced by a thick sequence of folded sedimentary or metasedimentary terrains which can be interrupted as miogeoclinal units the alfenas mobile belt is characterized by a strong positive anomaly probably associated with heavy infracrustal rock complexes granulites charnockites a crustal thickening probably associated with a pre brazilian continental collision can be inferred from a long wavelength linear negative anomaly

lithospheric structures and precambrian terrane boundaries in northeastern botswana revealed through magnetotelluric profiling as part of the southern african magnetotelluric experiment 1 within the framework of the southern african magnetotelluric experiment a focused study was undertaken to gain improved knowledge of the lithospheric geometries and structures of the westerly extension of the zimbabwe craton zim into botswana with the overarching aim of increasing our understanding of southern african tectonics the area of interest is located in northeastern botswana where kalahari sands cover most of the geological terranes and very little is known about lithospheric structures and thicknesses some of the regional scale terrane boundary locations defined based on potential field data are not sufficiently accurate for local scale studies investigation of the nnw sse orientated 600 km long zim line profile crossing the zimbabwe craton magondi mobile belt and ghanzi chobe belt showed that the zimbabwe craton is characterized by thick 220 km resistive lithosphere consistent with geochemical and geothermal estimates from kimberlite samples of the nearby orapa and letlhakane pipes 175 km west of the profile the lithospheric mantle of the ghanzi chobe belt is resistive but its lithosphere is only about 180 km thick at crustal depths a northward dipping boundary between the ghanzi chobe and the magondi belts is identified and two middle to lower crustal conductors are discovered in the magondi belt the crustal terrane boundary between the magondi and ghanzi chobe belts is found to be located further to the north and the southwestern boundary of the zimbabwe craton might be further to the west than previously inferred from the regional potential field data

evidence of recent surface faulting and surface rupture in the fore alps of veneto and trentino ne italy abstract the fore alps of western veneto and trentino regions belong to the central southern alps ne italy in which there is little evidence of very fresh surface ruptures or surface faulting this does not seem to match historical data about earthquakes some of which have been very intense the strong influence of the inherited structures makes it difficult to detect a direct link between morphotectonic features and present day stress fields in the present study four areas orsara scandole naole and soran with surface faulting and surface rupture features were examined and models of morphotectonic evolution are discussed in the lessini mountains the orsara graben and scandole ridge show examples of surface faulting and surface rupture respectively reactivating paleogene normal faults and fractures within the orsara graben rocky bluffs displace the previous morphological features the bluffs are some decimetres to some metres high and are practically devoid of evidence of either physical or chemical weathering on the slopes above them are steep areas which may be interpreted as the remnants of previous strongly weathered bluffs the scandole ridge has many trenches some with rocky walls which may be the result of several episodes of morphotectonic rupture in the giudicarie belt the naole and mt soran surface faulting landforms are details within large frontal culmination walls of neogene thrusts the naole ridge corresponds to the southeastern sector of monte baldo here inside a fault angle valley a sinuous scarp originating from surface faulting marks the base of the fault scarp slope ridge splitting is the expression of the backward migration of separation niches due to slope tectonics also evidenced on the slope by several terrace like features and by a lower belt of very thick slope breccias on mt soran in the gruppo di brenta massif the surface faulting scarp faces uphill giving rise to a trench like feature downvalley of the scarp there is the niche of a large landslide dated to 3 kyears b p all these landforms are consistent with slope tectonic movements caused by intense earthquakes whereas the morphostructures in the lessini mountains are the result of responses by sensitive structures the naole and mt soran features express the evolution of frontal culmination walls of thrusts with clear evidence of present day tectonic activity on the basis of the weathering of the scarps and associated features the relative seismotectonic episodes probably occurred between the bronze and middle ages

apparent polar wander path for adria extended by new jurassic paleomagnetic results from its stable core tectonic implications abstract as a continuation of a systematic paleomagnetic research in the northern part of stable adria which provided a well defined apparent polar wander apw path for the cretaceous eocene we present new paleomagnetic results for the jurassic these new data were obtained from 15 geographically distributed localities from the trento platform eastern southern alps using standard paleomagnetic approach the lower jurassic shallow water carbonates are not considered for tectonic interpretation due to inconsistent inclinations the middle and upper jurassic rosso ammonitico provided excellent kinematic constraints with the new jurassic results an apw is now defined for adria for the 167 40 ma interval it is well constrained for timing of important changes like the speed and the sense of rotations this apw suggests that the cw rotation and southward shift changed to the opposite at 155 1 5 3 ma signifying a dramatic change in the life of the neo tethys from opening to closing the latter is manifested in fast ccw rotation and northward movement of adria up to 102 9 2 4 ma and moderate displacements in the same manner in post 103 ma times combined data sets from stable istria and the foreland of the southern alps point to an approximately 15 cw rotation with respect to africa at the end of the cretaceous and an about 25 in the ccw sense after the eocene

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

seismic and electrical anisotropies in the lithosphere across the grenville front canada the grenville front is a prominent tectonic feature of the canadian shield which has been the focus of a lithoprobe transect a strong electrical anisotropy has been detected in a frequency range of 0 1 0 01 hz 50 to 150 km the most conductive direction being oriented n80e in order to compare electrical and seismic anisotropies ten seismometers have been deployed across the grenville front to record teleseismic events s wave splitting measurements show an average delay of 1 54 0 21 s between the fast and the slow components the fast axis direction is approximately e w at all stations showing a strong coherence between the stations along the complete transect this azimuth correlates well with that of the electrical anisotropy the electrical anisotropy originates in the subcrustal lithosphere between 50 and 150 km the similarity of the magnetotelluric and seismic anisotropy directions across the grenville front is an argument in favor of an origin of the seismic anisotropy in the subcrustal lithosphere as advocated by silver and kaneshima 1993

obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones in the upper mantle teleseismic shear wave splitting and magnetotelluric experiments across the grenville front between the archean craton and the proterozoic grenville province in the regions of the pontiac subprovince and northwestern grenville province canada show a consistent obliquity between the polarization direction of the fast split shear wave and the most electrically conductive direction mt in the upper mantle transcurrent shear zones at all well recorded stations is nearly n103 e and mt is approximately n80 e the obliquity may be considered a potential kinematic indicator because the seismic and electrical anisotropies are thought to be controlled by lattice preferred and shape preferred orientations of mantle minerals mainly olivine respectively the dextral movement sense of the transcurrent shear zones in the mantle inferred from the observed obliquity is consistent with that inferred from surface geology of the crustal shear zones this consistency implies that deformation of the crust and the subcrustal upper mantle in the lithosphere was largely coherent in the study region

migrating cretaceous eocene magmatismin the serra do mar alkaline province se brazil melts from the deflected trindade mantle plume department of geological sciences university of durham south road durham dh1 3le uk department of earth sciences university of cambridge downing street cambridge cb2 3eq uk department of physics the university newcastle upon tyne ne1 7ru uk department of geology mcmaster university 1280 main street west hamilton ont canada l8s 4m1 departmento de geoquimica e recursos minerais instituto de geociencias universidade de brasilia

calibration of shear wave splitting in the subcontinental upper mantle beneath active orogenic belts using ultramafic xenoliths from the canadian cordillera and alaska abstract despite the abundance of measurements of shear wave splitting around the world some fundamental questions about its geological interpretation have not yet been answered in order to constrain 1 i the orientation and magnitude of s wave anisotropy 2 ii the thickness of the anisotropic layer and 3 iii the possible variation of the anisotropy with depth in the subcontinental upper mantle beneath active orogenic belts we have carried out a systematic investigation on shear wave properties of mantle xenoliths from recent alkaline basalts of the canadian cordillera and alaska it is found that the polarization direction of the fast s wave as well as the time delay t between the two arrivals are strongly dependent on the propagation direction with respect to the structural frame at a single station the variations of measured and t values for sks phases from different events can result from this dependence because the average grain size of olivine in upper mantle samples is commonly larger than 0 5 mm dislocation creep prevails over a thickness of at least 250 km and results in olivine lpo and hence anisotropy and shear wave splitting a single thick anisotropic layer in mountain parallel subvertical shear zones may cause high values of tgd while multiple subhorizontal layers will produce multiple splitting making the measurement and interpretation of s wave splitting difficult our results also suggest that the s wave splitting observed in cold old stable cratons such as the canadian shield is likely dominated by fossil anisotropy whereas that observed in hot active orogenic regions such as the northwest american cordillera and the tibetan plateau may be dominated by modern tectonic deformation in the lithosphere and flow in the asthenosphere

tertiary extension in the southern trento platform southern alps italy the southern trento platform southern alps ne italy was the locus of abundant mafic volcanism associated with paleogene extensional tectonics during the pal eocene and throughout the eocene a nnw sse trending graben the alpone agno graben aag developed in the eastern lessini mountains the graben was filled with basaltic volcaniclastics calcarenites and lava flows the aag is a half graben system probably bounded by rotational listric normal faults most faults in the graben dip towards the wsw paleostress analysis within the aag shows that during the eocene the direction of horizontal extension was ene wsw the volcanic activity continued into the oligocene however the relation between oligocene tectonics and volcanism remains unclear gravimetric and magnetic anomalies in conjunction with the crustal thickness variations suggest a locus of lithospheric thinning slightly off axis from the aag this can be explained by an overall asymmetric extensional geometry dominated by a wsw dipping detachment fault the paleogene geodynamic scenario of the trento platform suggests that the aag developed as a foreland extensional response to the active collisional convergence between the european and adriatic plates neogene compressional tectonics reactivated some of the paleogene normal faults as strike sup faults

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

architectural elements and bounding surfaces in fluvial deposits anatomy of the kayenta formation lower jurassic southwest colorado abstract three well exposed outcrops in the kayenta formation lower jurassic near dove creek in southwestern colorado were studied using lateral profiles in order to test recent regarding architectural element analysis and the classification and interpretation of internal bounding surfaces examination of bounding surfaces within and between elements in the kayenta outcrops raises problems in applying the three fold classification of allen 1983 enlarging this classification to a six fold hierarchy permits the discrimination of surfaces intermediate between allen s second and third order types corresponding to the upper bounding surfaces of macroforms and internal erosional reactivation surfaces within the macroforms examples of the first five types of surface occur in the kayenta outcrops at dove creek the new classifications is offered as a general solution to the problem of description of complex three dimensional fluvial sandstone bodies the kayenta formation at dove creek consists of a multistorey sandstone body including the deposits of lateral and downstream accreted macroforms the storeys show no internal cyclicity neither within individual elements nor through the overall vertical thickness of the formation low paleocurrent variance indicates low sinuosity flow whereas macroform geometry and orientation suggest low to moderate sinuosity the many internal minor erosion surfaces draped with mud and followed by intraclast breccias imply frequent rapid stage fluctuation consistent with variable seasonal monsonal ephemmeral flow the results suggest a fluvial architecture similar to that of the south saskatchewan river through with a three dimensional geometry unlike that interpreted from surface studies of that river

segmentation and linkage of the lessini mountains normal faults southern alps italy abstract the lessini mountains on the southern margin of the italian southern alps exhibit two tertiary graben systems at an angle of about 40 the nnw trending system is orthogonal to the ene trending tertiary extension direction the nne trending system parallels the neighbouring jurassic western margin of the trento platform and experienced sinistral transtension during the paleogene the interaction between the two normal fault trends defines a ca 8 km long and 6 km wide rhomb shaped structure containing igneous intrusions and dolomitized rock bodies suggesting that the structure controlled syn extensional fluid migration at a smaller scale fault segments of the nne trending set show two basic types of transfer zones i e conjugate and synthetic due to the sinistral and normal oblique slip on these faults left steps of en echelon segments had a kinematics similar to that of pull apart structures the location of intrusion centres within such transfer zones led to the collapse of the roof of the magma chambers and consequent deep basins development in contrast right stepping synthetic en echelon normal oblique fault segments developed relay ramps

lithospheric anisotropy structure inferred from collocated teleseismic and magnetotelluric observations great slave lake shear zone northern canada 1 accurate interpretation of sks shear wave splitting observations requires inherently indeterminate depth information magnetotelluric electrical anisotropies are depth constrained and thereby offer possible resolution of the sks conundrum mt and teleseismic instruments deployed across the great slave lake shear zone northern canada investigated lithospheric anisotropy and tested a hypothesis that seismic and electrical anisotropy obliquity can infer mantle strain shear sense lithospheric mantle mt strike n60 e differs significantly from crustal mt strike n30 e sks splitting vectors outside the shear zone exhibit single layer anisotropy with fast axis parallel to upper mantle mt strike and oblique to present day plate motion n135 w back azimuth sensitivity at sites within the 30 km wide shear zone imply more complex layering with two layer inversion yielding an upper layer of n20 e and a lower layer of n66 e the mt data help to constrain the depth location of sks anisotropy and taken together support a model of fossil lithospheric anisotropy

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

initiation and growth of strike slip faults within intact metagranitoid neves area eastern alps italy exhumed strike slip faults exposed in the neves area of the tauern window eastern alps italy formed in the lower brittle crust under hydrous conditions within intact metagranitoids where any precursor fractures were already healed due to earlier amphibolite facies metamorphism faults initiated as newly formed en echelon fractures delineating shear bands with segmentation occurring over scales of 10 3 to 10 2 m due to the initial en echelon pattern stepovers between fault segments were almost invariably contractional during subsequent slip accumulation in initial low slip centimeter to decimeter stages synthetic slip on the segments was associated with the development of a set of antithetic faults in the contractional stepovers oriented at an angle of 30 45 to the bounding faults slip on this antithetic set did not fully compensate for the decrease in slip on the main faults implying an additional deformation mechanism within the stepovers e g block rotation and or out of plane movement in larger faults with slip on the order of a few meters the antithetic faults within the stepover were crosscut by synthetic sigmoidal faults that connected the overstepped fault segments and accommodated most of the subsequent displacement transfer this second stage of evolution involving fault linkage is well documented in the mesule fault which has a current maximum offset of 10 m in the mesule fault the total horizontal slip summed along all secondary faults within the stepover accounts almost entirely for the net slip decrease toward the tips of the overstepping faults however the boundary faults remain nearly straight in spite of the sigmoidal ramp like geometry of the connecting faults since the adjacent blocks are little deformed and there is no evidence for appreciable volume loss or block rotation in the stepover significant out of plane movement is implied although it is difficult to quantify the neves area provides unusually detailed field constraints on fault initiation linkage and displacement accumulation within nonbedded and relatively isotropic granitoid rocks at the base of the brittle crust where neither a free upper surface nor substantial volume change e g by veining and pressure solution was a controlling factor in accommodating fault linkage and displacement transfer

miocene to present major fault linkages through the adriatic indenter and the austroalpine penninic collisional wedge alps of ne italy

continental rifting parallel to ancient collisional belts an effect of the mechanical anisotropy of the lithospheric mantle analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process rift propagation is not random but tends to follow the trend of the orogenic fabric of the plates systematically reactivating ancient lithospheric structures continental rifts often display a clear component of strike slip deformation in particular in the early rifting stage moreover although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts these observations suggest that the deformation of the lithosphere especially during rifting is controlled by its preexisting structure on the other hand 1 the plasticity anisotropy of olivine single crystal and aggregates 2 the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs and 3 seismic anisotropy data which require a tectonic fabric in the upper mantle coherent over large areas suggest that preservation within the lithospheric mantle of a lattice preferred orientation lpo of olivine crystals may induce a large scale mechanical anisotropy of the lithospheric mantle we use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process we assess the deformation of an anisotropic continental lithosphere in response to an axi symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head model results show that a lpo induced mechanical anisotropy of the lithospheric mantle may result in directional softening leading to heterogeneous deformation during continental rifting this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique 30 55 to the preexisting mantle fabric this directional softening associated with olivine lpo frozen in the lithospheric mantle may also guide the propagation of the initial instability that will follow the preexisting structural trend the preexisting mantle fabric also controls the deformation regime imposing a strong strike slip shear component a lpoinduced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting structural inheritance the plume rift paradox and the onset of transtension within continental rifts fl 2001 elsevier science b v all rights reserved

some remarks on the origin of foredeeps abstract geological observations show that there are two main types of foredeep basins on the base of their subsidence rate and their relationship with the associated orogen or accretionary wedge high rates of subsidence up to 1600 m myr and a ratio lower than 1 between the area of the elevated belt and the area of the basin characterize the foredeeps associated with w dipping subduction low rates of subsidence up to 300 m myr and a ratio higher than 1 between the area of the orogen and the area of the basin characterize the foredeeps associated with e or ne dipping subductions this observation enables us to interpret the slow filling of foredeeps of the first type and the much faster filling of foredeeps of the second type moreover w dipping subductions have a steeper and faster monocline bounding the base of the deep foredeep in contrast to e or ne dipping subductions which have a shallower monocline this differentiation supports the notion of the eastward mantle flow with respect to the lithosphere detected in the hot spot reference frame in this view the foredeep depth in w dipping subductions is mainly controlled by the roll back of the subduction hinge pushed by the relative eastward mantle flow while foredeep depth in e or ne dipping subductions is instead mainly generated by the load of the thrust sheets and by the roll back of the subduction hinge due to the advancing upper plate contrasting the upward push of the mantle the shape of the foredeeps is regularly arcuate in case of w dipping subduction while is linear or following the shape of the inherited continental margin in case of e or ne dipping subduction fold development in the two opposite foredeeps is significantly different in w dipping subduction the folds are transported down in subduction while they are forming and consequently they are poorly eroded syntectonic sediments drape a preserved fold in the e ne dipping subduction folds and thrust sheets are instead uplifted and deeply eroded this is also predicted by the envelope of the crests in the two distinct accretionary wedges in the w dipping case the envelope may dip toward the subduction while it is rising toward the hinterland in the other cases this approach may explain the differences in terms of geometries amount and rates of subsidence between the foredeeps around the adriatic plate for instance the foredeep of the southern alps back thrust belt foredeep is deformed in the western part by the later apenninic foredeep development associated with the sinistral oblique ramp of the w dipping adriatic plate subduction the relationship in the frontal parts of a thrust belt between accretion and eustacy has to be analyzed in terms of when where and how fast thrusts propagation took place with respect to the randomly distributed sea level third order fluctuations and by the type of subduction which is connected to the foredeep and thrust belt or accretionary wedge therefore the stratigraphic patterns in foredeep basins are peculiar and casual successions that record the combination of regional tectonic evolution with independent wider scale sea level changes

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

lithospheric and sublithospheric anisotropy beneath central southeastern brazil constrained by long period magnetotelluric data abstract electric anisotropy calculated from geoelectric strikes of magnetotelluric mt data and seismic anisotropy derived from shear wave splitting parameters are jointly analyzed to estimate the degree and orientation of strain in the subcontinental mantle of central southeastern brazil high quality long period mt soundings are available at 90 sites concentrated along four profiles at the southwestern and southern borders of the paleoproterozoic archean sao francisco craton this data set is complemented by a previous study of sks and skks splitting measurements available at more than 40 sites covering a slightly wider region for this study the mt data were processed with modern techniques including recovery of the undistorted em field polarizations through correction of static shift and phase mixing due to galvanic distortions three dimensional forward modelling of mt and gds geomagnetic depth soundings transfer functions supports interpretation of deep electrical anisotropy in the region magnetotelluric phase responses of orthogonal propagation modes present slight splitting at long periods for most of the sites indicative of overall electrically low mantle anisotropy to depths greater than 250 km the electrical strike azimuths are parallel to the fast nw directions of shear wave splitting along the southern borders of the sao francisco craton suggesting that the seismic anisotropy also resides within the same depth range since this direction is very different from that of present day south american westward absolute plate motion it is inferred that no significant lateral mantle flow or deformation related to the plate motion is observed under the study area either because it is absent or the rigid lithosphere is much thicker than expected to explain the prevailing electric and seismic common direction it is suggested that a mechanical coupling between lithospheric and sublithospheric mantle exists a relic strain possibly resulting from ancient continental collision processes is interpreted to have induced a general alignment of olivine down to mantle depths beneath the continental rigid plate the observed slightly enhanced conductive texture could be associated with hydrogen diffusion along the aligned olivine a axis especially within mantle patches subjected to metasomatic processes the coincidence of mantle strikes with the trend of surface deformation pattern also suggests that crust lithospheric and sublithospheric upper mantle have deformed and translated coherently preserving the regional nw direction since the tectono thermal event responsible for the deformation distinct electric azimuths from the general nw trend are observed in regions probably perturbed by cretaceous rift related intraplate magmatism and in zones of complex deep structures produced by superposed nearly simultaneous oblique neoproterozoic collisions in the southeast of the sao francisco craton

seismic velocity anomalies beneath se brazil from p and s wave travel time inversions 1 we present the result from teleseismic travel time inversions for p and s wave data mainly recorded at portable broadband stations in se brazil the stations were deployed at 45 sites within an area 1000 1700 km during the years 1992 1999 more than 10 000 relative p and s wave arrival times including core phases were obtained from the waveforms using a new coherence functional these p and s relative phase times are independently inverted for slowness perturbations earthquake relocations and station corrections the final models represent the least amount of structure required to explain the residuals within a defined standard error the robust and consistent features in the velocity anomaly models are interpreted and their resolution is tested with synthetic case inversions we confirm the existence of a cylindrical low velocity anomaly beneath the parana basin which has been interpreted by vandecar et al 1995 as the fossil conduit through which the initial tristan da cunha plume head traveled to generate the parana etendeka flood basalts about 130 ma we now show that this low velocity cylindrical structure seems to be confined to the upper mantle beneath the upper mantle the velocity anomalies show a n s oriented pattern which we interpret as due to the nazca plate subducted slab at lithospheric depths the archean southern part of the sao francisco craton shows high velocities down to 200 300 km all areas with late cretaceous postrift alkaline intrusions are characterized by low velocities at lithospheric depths