crustal geometry of the abitibi subprovince in light of three dimensional seismic reflector orientations we provide precise estimates of reflector orientations beneath the archean abitibi subprovince using two distinct approaches based on lithoprobe seismic reflection data in the first we use the dip of reflections observed on intersecting profiles to establish the three dimensional orientation of reflective structures in the second the strikes and dips of reflectors are estimated in the crooked parts of seismic reflection profiles by calculating a measure of coherency along the traveltime trajectories defined by a particular azimuth dip depth and medium velocity mid crustal reflectors define two areas with distinctive geometry reflectors beneath the southern abitibi belt are oppositely dipping and convergent at depth providing a v shape aspect to the greenstone rocks other reflectors beneath the northern abitibi belt are in general subparallel dipping at an average of 30 toward the north these north dipping reflectors are partly disrupted by a low reflectivity zone which is attributed to

a detachment fold model for fault zones in the late archean abitibi greenstone belt abstract the abitibi belt is one of the largest and most extensively studied late archean greenstone belts the structural geology of the abitibi belt consists of one generation of upright to slightly overturned doubly plunging first order folds with half wavelengths of 20 60 km and e w striking steeply dipping fault zones that are parallel to the fold limbs two of the main fault zones are continuous for hundreds of kilometers previous tectonic models for the abitibi belt interpret the fault zones to have formed as extensional growth faults bounding a volcanic sedimentary basin which were reactivated as thrusts during subsequent crustal shortening other models propose that the fault zones represent tectonic sutures implying that the abitibi belt is a collage of exotic terranes however distinct geological terranes have not been geologically demonstrated we propose a new detachment fold model for the deformational history of the southern abitibi belt in ontario that explains the formation of the fault zones during the single well documented folding event that deformed the entire region the internal structure of the fault zones documented here with emphasis on the porcupine destor fault zone consists of isoclinally folded strongly schistose highly metamorphosed rock cross cut by numerous fault segments we interpret that the upper crust greenstones was folded above a proposed detachment in the lower part of the volcanic stratigraphy the fault zones would be in essence highly evolved detachment anticlines ultramafic metavolcanic rock that crops out within the fault zones would represent material from the detachment horizon that was emplaced in the cores of the detachment anticlines the numerous segments that make up the mapped fault zones would be linked faults that formed within the isoclinal detachment anticlines to accommodate folding of the rheologically complex greenstones the detachment fold model is compared to the results of analogue experiments designed to investigate crustal scale folding using viscous and frictional materials detachment folds are produced in the brittle upper crustal analogue on the limbs of folds formed in the ductile middle and lower crust analogues the experimentally produced structures scale to the structures in the study area and indicate the detachment fold model for the southern abitibi is mechanically viable

mantle discontinuities and temperature under the north american continental keel a ubiquitous feature of upper mantle seismic velocity models has been the presence of high velocity keels beneath stable continental interiors1 2 3 4 5 uncertainty remains however regarding the maximum depth to which continental keels extend the degree to which they have cooled the mantle that surrounds them and their role in mantle flow here we investigate thermal anomalies across the eastern margin of the north american continental keel by imaging the seismic discontinuities at depths of 410 and 660 km with compressional to shear converted waves recorded by a 1 500 km long seismometer deployment in the eastern united states the thickness of the transition zone the region nominally between depths of 410 and 660 km and the depth to the 410 km discontinuity indicate that cold keel material and sub keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than 150 k a 20 km depression of the 660 km discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone2 and may be associated with the remnants of the subducted farallon plate1 2 4

seismic structure of the lithosphere from teleseismic converted arrivals observed at small arrays in the southern sierra nevada and vicinity california 21 well distributed teleseisms 30 to 100 distance were recorded by mixed broad band bb and short period sp seismic arrays at mineral king mk and horseshoe meadow hm in the southern sierra nevada and at darwin plateau dp between the inyo and argus ranges these arrays permit identification and separation of direct p and s arrivals reflections from topography scattered energy and arrivals from different back azimuths multipath arrivals p to s conversions can be identified from beamed bb or sp seismograms least squares time domain processing recovered single event receiver functions from these beamed seismograms converted phases attributable to the moho are clear and uncomplicated at dp ps p times of 3 9 4 2 s correspond to crustal thicknesses of 31 33 km assuming a mean p crustal velocity of 6 2 km s and a poisson s ratio of 0 255 ps p times at hm eastern sierra are 3 9 4 6 s 32 37 km excepting some unusual seismograms from events at back azimuths of 225 239 mk western sierra times for ps p show a strong e w asymmetry 3 9 4 1 s 31 33 km from the east and 4 8 5 3 s 39 42 km from the west the mk arrivals from the east are multiple and substantially weaker by comparison with hm and dp these results confirm the absence of a thick crust under the southern high sierra inferred from both a refraction experiment coincident with this experiment and some earlier studies at dp the moho event follows an intracrustal negative polarity event defining the top of an s wave low velocity zone this feature dips west under the sierran crest at hm but is absent farther west at mk this feature appears to be a manifestation of extensional strain and thus indicates that the surficially undeformed sierra overlies a tectonized lower crust sub moho energy is absent under the basin and range dp but is conspicuous under the high sierra at positive arrivals 7 3 s mk and 9 s hm after the p these arrivals might be from the base of a low velocity low density upper mantle body supporting the sierra

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

rethinking eastern australian caves there are some 300 bodies of cavernous limestone in eastern australia extending from precipitous bluff in southeastern tasmania to the mitchell palmer region in north queensland these impounded karsts developed in palaeozoic limestones of the tasman fold belt system contain many caves the caves have a suite of features in common that allows them to be thought of as a major group the tasmanic caves the tasmanic caves include multiphase hypogene caves such as cathedral cave at wellington and multiphase multiprocess caves such as jenolan with carboniferous hypogene and younger paragenetic and fluvial elements active hypogene caves occur at wee jasper and possibly at five other localities the tasmanic caves are one of the most complex suites of caves in folded palaeozoic limestones in the world field techniques developed to study these caves are now being applied to complex caves in central europe in the czech republic hungary poland slovakia and slovenia andrews 1911 recognized that there was a geographical unity of eastern australia although this may appear to be a quaint early 20th century idea later geologists recognized the tasman fold belt system as a continental scale tectonic fea ture in eastern australia similarly late 20th century geomorphologists such as ollier 1978 1982 recognized the existence of continental scale landforms such as the great escarpment running the length of eastern australia the tasman fold belt system in eastern australia has two palaeozoic sections the western lachlan thompson fold belt and the eastern new england fold belt both containing cavernous palaeozoic limestones if we take a continental scale view of the karst in these palaeozoic lime stones we find that despite some obvious climatic influences on surface landforms the caves have many characteristics in common caves in palaeo zoic limestones from the southeasternmost tip of tasmania to northern queensland are sufficiently similartobeviewedasacohesivegroup thetasma nic caves fig 1 the focus here is on caves because caves are the prominent features in many eastern australian karsts this paper begins by reviewing ideas about eastern australian caves from 1830 to 1995 the latter part of the paper dealing with developments in understanding eastern australian caves from 1996 onwards concentrates on the author s own work

anisotropy of the mantle inferred from observations of p to s converted waves summary seismic anisotropy has been previously studied at depths usually not exceeding 100 or 150 km in this paper we present a method of analysis of seismic records which is very sensitive to azimuthal anisotropy and is applicable at almost any depth range the idea of the method is to detect and analyse the sh component of the waves converted from p to s in the mantle the procedure of record processing includes frequency filtering axis rotation transformation of the record to a standard form stacking the standardized sh component records of many seismic events and the harmonic analysis of amplitude as a function of the direction of wave propagation when applied to the long period records of norsar the procedure detected a converted wave with the properties implying the possibility of its propagation in a transversely isotropic medium with a horizontal axis of symmetry our preferred model postulates anisotropy of 1 per cent in a layer 50 km thick at the base of the upper mantle

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

a summary of electromagnetic studies on the abitibi grenville transect electromagnetic surveys on the abitibi grenville lithoprobe transect have elucidated a number of conductivity signatures that can be genetically linked to precambrian tectonic processes some major fault zones are moderately conductive possibly signalling graphite deposition from a mantle co2 flux along crust penetrating fault systems however conductive graphitic metasedimentary rocks characteristic of foreland basins are apparently absent from the transect area a weak inverse correlation between metamorphic grade and electrical conductivity was observed by following rock units across the grenville front into high grade equivalents within the parautochthonous belt a uniformly conductive mid crustal layer extends across the grenville front apparently without change in character the existence of this ubiquitous mid crustal conductor has been interpreted to mean that electrical conductivity is controlled by the present day pressure temperature and fluid saturation of the lower crust independent o

depth localized azimuthal anisotropy from sks and p receiver functions the tien shan summary shear wave splitting in the seismic sks phase provides a unique possibility to judge on deformations at depths inaccessible for direct observations fast s wave polarization direction in collisional belts is often parallel to the trend of the belt although deformations of the mantle lithosphere in low angle thrusts would lead to the fast polarization direction normal to the trend of the belt these considerations suggested that the upper mantle in collisional belts is decoupled from the crust however sks technique is notable by a poor depth resolution and usually it assumes that the fast polarization direction is the same at any depth which is hard to justify here to investigate depth dependent azimuthal anisotropy in the mantle we invert jointly p receiver functions and sks particle motions at a number of seismograph stations the technique involves azimuthal filtering of the receiver functions and provides a criterion to discriminate between the effects of azimuthal anisotropy and lateral heterogeneity of isotropic medium a search for the optimum models is conducted with a technique similar to simulated annealing testing with synthetics demonstrates that this approach is robust the results for 10 seismograph stations in the tien shan the world s most active intracontinental collisional belt in central asia reveal a pronounced change in the patterns of azimuthal anisotropy at a depth around 100 km in the mantle lithosphere at depths less than 100 km anisotropy is relatively weak and fast wave polarization direction varies laterally in a broad range this layer is not necessarily decoupled from the crust its anisotropy can be a combined effect of present day thrusting and of deformations of the geologic past in the lower layer asthenosphere the average azimuth of fast wave polarization is close to the trend of the belt whereas magnitude of s wave anisotropy is stable and large between 5 and 6 per cent this anisotropy is a likely result of recent uniaxial shortening at right angle to the trend of the belt at some stations the data require anisotropy in the crust there is no evidence for anisotropy at depths exceeding 150 250 km

shear wave splitting in the mantle ps phases distribution with depth of mantle anisotropy is uncertain to better constrain this distribution we analyse records of the mantle ps phases at the grf array in germany two components of these phases sv and t are detected by stacking teleseismic records of events well distributed in azimuth and distance the detected t component signals are generated not so much by splitting of sv as by conversion from p to sh at the mantle discontinuities to reconcile these observations with previously known observations of splitting in sks at least three anisotropic layers are required the data favour a model with nne oriented fast velocity at the top of the mantle this is consistent with the results of other seismic studies beneath this layer the fast direction of anisotropy changes by around 90 our analysis suggests as well that a layer at the base of the mantle transition zone mtz is anisotropic this anisotropy might arise in the convective boundary layer between the upper and the lower mantle

dynamic surface topography a new interpretation based upon mantle flow models derived from seismic tomography the very long wavelength i e in the degree range l 1 8 dynamic topography at the earth s surface estimated by isostatic reduction of the observed topography is shown to be in good agreement with the topography predicted by a model of viscous flow in the mantle this flow model was previously derived by independent consideration of the observed nonhydrostatic geoid and is based upon a recent model of large scale seismic shear velocity heterogeneity in the mantle our estimate and prediction of the long wavelength dynamic surface topography reveals significant depressions 2 3 km of the continental shields relative to their hydrostatic reference positions

directional variations in travel time residuals of teleseismic p waves in the crust and mantle beneath northern tien shan we study the directional variation in travel time residuals using 13 820 p wave arrivals from 1 998 teleseismic events 15 d 98 4 1 mb 7 3 recorded in 1991 1997 by the kyrgyz digital seismic network knet based on a modified version of the iasp91 model that accounts for the kyrgyz crustal thickness beneath knet we convert p wave travel times to travel time residuals dt the de pendence of dt on backazimuth is modeled as one two and four lobed variations in a horizontal plane backus 1965 a least squares fit of the azimuthal variation of dt indicates that the crust in the northern tien shan is about 11 15 km thicker than it is in the kazakh shield and the chu depression from nine knet stations the one lobe model estimates that the slowest p wave travel time direction is 5 0 4 8 almost directly north and the magnitude of variation is 1 71 0 13 sec this result is consistent with an upwelling lower mantle plume for the two lobe model the slowest p wave travel time directions anisotropy term are 89 7 and 269 7 4 7 i e trending east west we find p wave velocity anisotropy of 2 0 2 9 associated with a layer with a thickness of 440 km at the top of the lower mantle the fast direction of the p wave travel time north south azimuthal anisotropy at the top of the lower mantle is 1 parallel to the absolute motion of the india plate and 2 close to the direction of the upwelling hot mantle flow the last result suggests that the azimuthal aniso tropy of the travel time residuals is due to the shape preferred orientation of middle mantle material that results from plume intrusion shear wave splitting studies mak eyeva et al 1992 wolfe and vernon 1998 estimated the fast polarization direction to be parallel to the strike of the geological structures of the northern tien shan 71 29 thus the fast polarization direction determined from these shear wave splitting studies using knet data contradicts differs by 90 the fast travel time direction 0 3 and 179 7 4 7 we determine here using p wave travel time residuals using knet data this suggests that the azimuthal anisotropy determined from p wave travel time variations and from shear wave splitting in sks and skks have different sources

azimuthal anisotropy in the lithosphere from observations of long period s waves summary the paper gives a detailed description of the technique for measuring azimuthal anisotropy which was suggested earlier by vinnik kosarev makeyeva 1984 and used by kind et ai 1985 the technique is based on the observations of long period converted phases like sks additionally we describe a generalization of this approach which makes use of long period s waves of arbitrary polarization both modifications were applied to records of the grf array in southern germany the results of this analysis if combined with the data on p velocities bamford 1977 suggest that the direction of the fast velocity in the lithosphere of the region varies with depth

integrated geophysical interpretation of crustal structures in the northern abitibi belt constraints from seismic amplitude analysis we present a processing sequence that attempts to balance geometrical and amplitude analyses in order to recover the maximum information from deep seismic reflection data the approach which is guided by the interpretation of other deep geophysical data sets magnetotellurics refraction is applied to lithoprobe seismic reflection line 28 across the central and northern abitibi belt we show in particular how amplitude analyses help to quantify the depth of penetration of seismic energy as well as the crustal reflectivity apparent lateral variations of deep structures e g the moho can be directly related to the high levels of noise that limit the signal penetration depth we propose a geological model that satisfies all deep geophysical constraints in this model the mid crust south of casa berardi tectonic zone consists of imbricated volcanic plutonic and sedimentary lithologies which are probably comparable to the mid crustal section of the kapuskasing structural zone and in this paper are

deep into the chibougamau area abitibi greenstone belt structure of a neoarchean crust revealed by seismic reflection profiling copper gold magmatic hydrothermal systems dominate in the chibougamau area of the neoarchean abitibi subprovince greenstone belt of the superior province craton whereas orogenic gold mineralization is more common in the rest of the abitibi understanding differences in metal endowment within the abitibi greenstone belt requires insights into the geodynamic evolution of the chibougamau area this was addressed by imaging the crust using seismic reflection profiling acquired as part of the metal earth project seismic reflection sections display shallowly south dipping reflectors located within the upper crust e g a possible deep continuation of the barlow fault and a northward dipping midcrust imbricated with older crust opatica subprovince to the north multiple reflectors characterize the upper part of the midcrust interpreted as faults superimposed on a major lithological boundary these structures likely formed during terrane accretion prior to craton stabilization combining the new seismic data with known stratigraphic structural and magmatic records we propose that the study area was initially a normal i e thick archean oceanic crust that formed at or before 2 80 ga and that evolved through terrane imbrication at 2 73 2 70 ga shortening caused rapid burial devolatilization and partial melting of hydrated mafic rocks to produce tonalite magmas that may have mixed with mantle derived melts to produce the diorite tonalite suite associated with observed cu au magmatic hydrothermal mineralization

inversion of teleseismic s particle motion for azimuthal anisotropy in the upper mantle a feasibility study summary we have developed a technique for the inversion of teleseismic s waveforms in terms of azimuthal anisotropy in the upper mantle we test different models of the earth upper mantle by transforming the observed horizontal components into a synthetic vertical component and comparing this with the observed vertical component the optimum model is found by minimizing the difference between the synthetic vertical component and the observed one using this method we explore the possibility of constraining the distribution of azimuthal anisotropy with depth we present examples of seismic observations where the data are clearly in favour of an anisotropic model these observations can be interpreted in terms of two anisotropic layers with different directions of fast velocity axes

how to relate body wave and surface wave anisotropy seismic anisotropy is one of the most efficient geological and geodynamical tools for understanding the dynamics of the earth upper mantle anisotropy is evident in seismic data sets for the last 30 years primarily from surface wave dispersion curves and body wave sks data we demonstrate in this paper that surface wave and body wave derived anisotropy can be explained by the same anisotropic parameters l gc gs in the simplest case of a horizontal fast symmetry axis one application of this method is to display the sks delay time and the corresponding azimuth which can be derived from surface wave global with a lateral resolution of around 2000 km and regional lateral resolution of 350 km anisotropy tomography a global scale comparison is disappointing since there are only a few areas where both data types are correctly retrieved at the same spatial scale same lateral resolution the anisotropy is well resolved in oceanic basins from surface waves whereas most measurements of sks splitting are available below continents however there is a good agreement between surface wave and body wave anisotropy in regions where large scale processes primarily tectonic are taking place such as the western united states and in central asia in particular we show that at high frequency 1hz the observed sks delay time and azimuth depend on the order in which s waves propagate through the layers a comparison between synthetic and observed sks phases in central asia highlights the varying sensitivity of sks waves at depth we can obtain a good correlation between synthetic and observed sks by considering anisotropy only between 80 and 200 km depth in the tomographic model rather than in the entire upper mantle however it is more difficult to explain the discrepancy between synthetic and observed sks under the eastern united states where a large scale coherent sks anisotropy is present such results call for more sophisticated sks waveform modeling and for enhanced lateral resolution in anisotropic tomographic models

shear wave splitting in the records of the german regional seismic network estimates of the parameters of shear wave splitting in the records of sks and skks of the new german regional seismograph network grsn for one layer model show large azimuthal variations at some stations of the network it is found that the variations are compatible with the presence of two anisotropic mantle layers in the region of the south german triangle sgt the fast direction in the upper layer is between n s and ne sw close to the previously reported estimates of the fast direction in the sub crustal lithosphere of the sgt the fast direction in the lower presumably asthenospheric layer is close to e w the estimates of the effective fast direction for one layer model at the stations of the grsn confirm the previously noted tendency of the fast direction of mantle anisotropy in central europe to change from 50 70 in the western part of the region to 100 120 in the eastern part a correlation of this change with a bending of the present day direction of maximum horizontal stress in the crust suggests that anisotropy in the asthenosphere of central europe can be related to the alpine orogeny

the role of fluids during formation and evolution of the southern superior province lithosphere an overview models for fluid flow and hydrothermal alteration for the abitibi greenstone belt are reviewed in the light of lithoprobe results in the region in the abitibi greenstone belt eruption of volcanic sequences over 2750 2700 ma was accompanied by pervasive low temperature hydrothermal alteration at high water rock ratios giving systematic 18o enrichment archean ambient ocean water bottom temperatures were likely ca 30 c and 18o 0 1 chert iron formations precipitated from low temperature hydrothermal discharge base metal massive sulphide deposits formed at or near the seafloor from focussed discharge of high temperature 300 400 c fluids in convective cells sited above subvolcanic intrusions the ore fluids were evolved seawater that had undergone compositional and isotopic evolution by high temperature low water rock exchange with the volcanic pile to nacl 3 7 wt or cacl2 nacl up to 30 wt brines of 18o 0 8 these volcanic associated hydrothermal deposits are associated with greenst