failure process and modes of rockfall induced by underground mining a case study of kaiyang phosphorite mine rockfalls abstract the analysis of the failure processes and mechanisms of rockfalls that are associated with underground mining activities is presented the study area is located in kaiyang phosphorite mine in southwest china where the geological condition is dominated by anti dip slopes with layers inclined backward into the natural slope with decreasing strength in the rock mass from the upper dolomite rich to the lower shale rich strata the analysis is based on field investigation and laboratory experimental study using a gravitational simulation device the results support the proposed failure mechanism by demonstrating the process of failure from the deformation of the roof and floor in the stopes to the development of surface cracks near the top of slopes and eventually to the formation of rockfalls as the surface cracks propagate along pre existing joints the rockfalls are classified into one of three failure modes crack toppling crack sliding and crack slumping in which the failures are governed by the corresponding characteristics of the rock mass structure the study of the failure process and their spatial and temporal correlation with the underground workings reveals warning signs or indicators of impending slope instability improved understanding of the failure process and indicators can aid in early identification and timely warning of geohazards in phosphorite mines

comparison of synthetic pseudoabsolute response spectral acceleration psa for four crustal regions within central and eastern north america cena abstract this study contributes to the development of a new ground motion model for central and eastern north america cena we evaluate the impact of the seismic velocity structure for different crustal regions on ground motion measures we present an overview of the regionalization of the crustal structure of cena the physical parameters used for the simulation codes and the statistical method applied for the comparison of pseudoabsolute response spectral accelerations psas of four crustal regions defined within cena the four crustal regions are 1 the atlantic coastal plain acp 2 the appalachians app 3 central north america cna and 4 the mississippi embayment gulf coast mem for each region and its statistically representative velocity structure psa matrices were computed covering response oscillator frequencies from 0 5 to 20 hz and hypocentral distances up to 500 km psa matrices for earthquakes at four different focal depths 5 10 20 and 30 km were calculated the developed method of the normed mean differences gives a meaningful measure for inter regional differences of ground motions in comparison with intra regional variability in this study we compare the ln psa of the app acp and mem region with the reference region cna respectively the percentage of ln psa mean difference values that are smaller than the ln psa standard deviation of the reference region cna lies between 73 and 91 for the app region dependent on the focal depth for the acp region it is about 70 for all focal depths except for the 20 km source depth which was excluded because a layer boundary at 20 5 km causes unrealistically strong reflections the percentage for the mem region is less than 50 for each of the focal depths this analysis demonstrates that there are two distinct ground motion groups for cena group 1 cna app and acp group 2 mem electronic supplement figures of crustal velocity structures table of q studies and q studies illustrated for one region

integrating palaeocaves into palaeolandscapes an analysis of cave levels and karstification history across the gauteng malmani dolomite south africa abstract the drimolen palaeocave system in the fossil hominid sites of south africa unesco world heritage site is well known for numerous remains of early hominins such as paranthropus robustus and early homo these hominin fossils along with bone tools and notably diverse accumulation of non hominin primates and fauna have all been excavated from the main quarry area of the site where extensive lime mining took place here we report the first radiometric age of 1 712 0 269 ma for hominin bearing deposits associated with the dnh7 paranthropus robustus cranium in the main quarry area of the site which is consistent with recent biochronological estimates this age is similar to recent estimates for swartkrans member 1 hanging remnant somewhere between 2 3 and 1 8 ma which also contains paranthropus and early homo simultaneously we integrate the newly radiometrically dated main quarry deposits with a new fossil deposit the drimolen makondo discovered in 2013 that is situated some 50 m up the hill to the west from the main quarry it has experienced only limited disturbance from mining but much more extensive erosion preliminary excavations and analysis have revealed that the makondo infill is older than the main quarry dating to 2 706 0 428 ma its greater age is confirmed by biochronology the makondo thus overlap with the suggested end of deposition of australopithecus bearing sterkfontein deposits although it is yet to yield any hominin remains these new dates for the two drimolen palaeocave system deposits indicates that contrary to prior age estimates the drimolen site as a whole records the critical hominin and faunal turnover in south african palaeocommunities that occurred around 2 3 1 7 ma finally as the drimolen makondo represents a rare example of a pre 2 ma fossil bearing deposit in the gauteng exposures of the malmani dolomite we also integrate our results into the greater south african record of palaeodeposit formation most of which occur between 2 0 and 1 0 ma an analysis of the age of palaeocave infillings across the malmani dolomite suggests that as is classically the case with karst the height within the dolomite is broadly correlated to their age although with some notable exceptions that are likely related to localised geological features our analysis also indicates that most caves have undergone some form of secondary karstification related to a younger phase of cave formation contrasting with models that suggest the cavities all formed at the same time and that infill is related to erosion and the opening up of cave passages as such the reason that few pre 2 ma deposits have been identified in the gauteng exposures of the malmani dolomite is probably because these older caves have been eroded away identifying such early caves is critical in understanding whether earlier hominins may have once existed in south africa or if erosion of older deposits or an absence of speleogenesis at this time has made such early periods absent from the geological record

dating the shock wave and thermal imprint of the giant vredefort impact south africa u pb geochronology of single grains of zircon and monazite has been used to date an episode of intense postimpact metamorphism in the core of the deeply eroded vredefort impact structure of south africa results from two basement units exposed in the uplifted central region indicate that the impact and a later pyroxene hornfels metamorphic event were penecontemporaneous at 2020 3 ma discovery of a synimpact to postimpact dike of norite that intruded at 2019 2 ma is the first recognition of mafic igneous activity related to impact the dike is either derived from a sudbury type impact melt layer since eroded or is the product of decompression melting of kaapvaal mantle in response to the ablation of 15 km of crust at the center of the crater the combined heating effects of the shock wave and impact triggered magmas are thought to have created the 300 km 2 thermal imprint of the asteroid collision with kaapvaal craton and account for the nearly coeval timing relationship between core metamorphism and shock revealed by this study

precise u pb titanite age constraints on the emplacement of the bushveld complex south africa the c 2 06 ga bushveld complex intrudes the transvaal supergroup south africa calcsilicate xenoliths within the mafic phase of the bushveld complex rustenburg layered suite or rls preserve calcsilicate xenoliths with high temperature c 1200 c mineralogies that were later metasomatized by hydrous retrograde c 600 700 c fluids whose timing has been unconstrained new u pb isotope data from newly grown titanite within a completely retrogressed xenolith indicates that retrogression occurred at 2058 9 0 8 ma this suggests that retrogression was due to hydrothermal circulation associated with the cooling rls and not slightly later bushveld related granites the new data place a tight constraint on the minimum age of emplacement of the rls

fuzzy models for analysis of rock mass displacements due to underground mining in mountainous areas the displacement and deformations of rock mass due to underground mining has often resulted in major disasters throughout the world frequently inflicting heavy losses of life and damage to property and these disasters have motivated the development of rock mass mechanics the prediction of displacement of rock mass and their surface effects is an important problem of the rock mass mechanics in the excavation activities especially the coal and metal mining in mountainous areas based on results of the statistical analysis of a large amount of measured data in mining engineering the fundamental fuzzy model of displacements and deformations of rock mass is established by using the theory of fuzzy probability measures the theories of both two and three dimensional problems are developed and applied to the analysis of engineering problems in excavation and underground mining in mountainous areas the agreement of the theoretical results with the field measurements shows that our model is satisfactory and the formulae obtained are valid and thus can be effectively used for predicting the displacements and deformations and the safety evaluation of the buildings on the ground

structural controls on fe mineralization at thabazimbi mine south africa abstract chemical sedimentary rocks of the transvaal supergroup along the northern margin of the transvaal basin were imbricated during formation of the mohlapitsi fold and thrust belt between ca 2 55 and 1 93 ga high grade 63 fe stratiform fe ore bodies occur at or close to the base of the banded ironstone dominated penge formation which forms a distinct e w trending mountain range in the area the penge formation is separated from underlying malmani dolostone by a thin laterally continuous rheologically weak carbonaceous shale layer that has been intensely sheared this horizon denotes the position of a regional top to n low angle detachment that has been variably deformed during progressive development of the mohlapitsi belt several of kumba s thabazimbi iron ore deposits including kwaggashoek east donkerpoort west kumba donkerpoort west and buffelshoek west are confined to a zone directly above this detachment recent studies of ore petrography fluid alteration fluid composition and stable isotope geochemistry link economic iron ore mineralization to hydrothermal or hypothermal processes along with localized high grade mineralization a structural control on fluid flow has been suggested although a lack of structural data has hampered more concise mineralization models thus far in this study we present and analyse data from detailed structural mapping over a protracted period of mining across four opencast pits this mapping is combined with drillhole data to produce accurate intergrated 3d models of these deposits which reveal the true geometry of these ore bodies data and resultant models indicate that enhanced hydrothermal fluid flow associated with significant iron mineralization is linked to thrusting shearing in response to 1 fold amplification and flexural flow along the e w trending detachment between the malmani dolostone and banded iron formation 2 subsidiary fold accommodation structures such as conjugate shear joints normal and reverse faults and thrusts associated with inner and outer arc shortening and extension respectively 3 localization of fe at the intersections of major reverse faults the detachment and stratigraphically higher low angle thrusts which are preferentially developed at the contacts between diabase and banded iron formation collectively these features both aided and compartmentalized regional pervasive fluid flow along the detachment and in stratigraphically higher sites subsequent ore preservation was enhanced by reactivation of nw to nnw trending mesozoic normal faults and concomitant downthrow of fault bounded blocks structural relationships and timing of high grade iron ore mineralization further suggest that associated top to n kinematic fold and thrust formation of the mohlapitsi belt and subsequently the main detachment between malmani dolostone and banded iron formation of the penge formation could have been active during and immediately after intrusion of the bushveld complex at ca 2 05 ga

major influences on the evolution of the 2 67 2 1 ga transvaal basin kaapvaal craton abstract the transvaal is one of three structural basins of the transvaal supergroup preserved on the kaapvaal craton the evolution of the neoarchaean palaeoproterozoic transvaal basin is ascribed predominantly to magmatism palaeoclimate and eustasy with plate tectonics playing a sporadic role the supergroup comprises basal protobasinal rocks followed by the black reef formation chuniespoort group and uppermost pretoria group immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c 2 7 ga ventersdorp supergroup mantle plume individual protobasinal successions were laid down in separate fault bounded basins controlled at least partially by greenstone belt orientations in the kaapvaal basement post magmatic post rifting thermal subsidence accommodated black reef fluvial sheet sandstones and the subsequent thick carbonate bif epeiric platform succession of the chuniespoort group subordinate mechanical subsidence accompanied this long lived thermal relaxation intense weathering due to neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation globally enhanced sea levels due to enhanced mid ocean ridge growth consequent upon either global magmatic events or supercontinent break up also played a pivotal role in chuniespoort epeiric basin evolution pretoria group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence the first cycle appears to reflect plate tectonically induced rifting with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation the second pretoria cycle is most likely related to a major continental flood basalt event with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern kaapvaal craton evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic transvaal basin depends to a large degree on the age assigned to the limpopo orogeny during which the zimbabwe and kaapvaal cratons collided recent evidence for a c 2 0 ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during transvaal basin evolution

is air pollution causing landslides in china abstract air pollution in china often exceeds unhealthy levels but chinese air is not only a threat from being breathed the pollutants may also be causing fatal landslides very acid rain from severe air pollution falls widely in southwest china where coal is a major energy source we discuss where acid rain may provide an unsuspected link between mining and the fatal 2009 jiweishan landslide in southwest china it may have reduced the strength of a thin calcareous black sapropelic shale in jiweishan mountain by removing cementing carbonate minerals and sapropel matrix mining beneath the potential slide mass may not have directly triggered the landslide but collapse of abandoned adits drained a perched aquifer above a regional black shale aquiclude inflow of acid oxygenated water and nutrients into the aquiclude may have accelerated the reduction of strength of the weakest rocks and consequently led to rapid sliding of a large rock mass on a layer of weathered shale left composed largely of soft and slippery talc

squid sims is a useful approach to uncover primary signals in the archean sulfur cycle many aspects of earth s early sulfur cycle from the origin of mass anomalous fractionations to the degree of biological participation remain poorly understood in part due to complications from postdepositional diagenetic and metamorphic processes using a combination of scanning high resolution magnetic superconducting quantum interference device squid microscopy and secondary ion mass spectrometry sims of sulfur isotopes 32s 33s and 34s we examined drill core samples from slope and basinal environments adjacent to a major late archean 2 6 2 5 ga marine carbonate platform from south africa coupled with petrography these techniques can untangle the complex history of mineralization in samples containing diverse sulfur bearing phases we focused on pyrite nodules precipitated in shallow sediments these textures record systematic spatial differences in both mass dependent and mass anomalous sulfur isotopic composition over length scales of even a few hundred microns petrography and magnetic imaging demonstrate that mass anomalous fractionations were acquired before burial and compaction but also show evidence of postdepositional alteration 500 million y after deposition using magnetic imaging to screen for primary phases we observed large spatial gradients in 33s 4 in nodules pointing to substantial environmental heterogeneity and dynamic mixing of sulfur pools on geologically rapid timescales in other nodules large systematic radial 34s gradients 20 were observed from low values near their centers increasing to high values near their rims these fractionations support hypotheses that microbial sulfate reduction was an important metabolism in organic rich archean environments even in an archean ocean basin dominated by iron chemistry

the 2009 jiweishan rock avalanche wulong china precursor conditions and factors leading to failure abstract this study investigates the predisposing geological and anthropogenic factors and indicative precursors preceding the fatal 5 june 2009 jiweishan rock avalanche it began as a rockslide and transformed into a rock avalanche that killed 74 people and caused major property loss due to misdiagnosis of the failure mode and underestimation of volume and travel distance besides topography structure and stratigraphy that favoured failure mining below the failed rock mass had changed the state of stress and altered groundwater flow the anthropogenic factors caused deformation and accelerated failure of the slope and therefore were important contributions cracks and a karstic zone had developed along regional joints and defined several boundaries of the failed rock mass furthermore a 2 m wide crack developed since 1958 revealed a potential basal failure surface at the contact between karstic limestone and an underlying pre sheared thin shale aquiclude increasing magnitude and frequency of rockfalls and tension cracks at the front end of the sliding block appeared three days before the event and implied the likely movement direction of the sliding rock mass these could have warned of a developing giant failure topography included a large elevation difference between the toe of the failure surface and the cliff base a narrow valley to channel debris and the steep gradient of the downstream tiekuang creek could have suggested that failure might lead to a rapid and long runout rock avalanche however access to the densely vegetated source area before the failure was difficult and local investigators did not have all of the evidence that might have led them to recognize that potential danger was more than continuing rockfalls

zircon and whole rock nd pb isotopic provenance of middle and upper ordovician siliciclastic rocks argentine precordillera graptolite bearing middle and upper ordovician siliciclastic facies of the argentine precordillera fold thrust belt record the disintegration of a longlived cambro mid ordovician carbonate platform into a series of tectonically partitioned basins a combination of stratigraphic petrographic u pb detrital zircon and nd pb whole rock isotopic data provide evidence for a variety of clastic sediment sources four upper ordovician quartzo lithic sandstones collected in the eastern and central precordillera yield complex u pb zircon age spectra dominated by 1ae05 1ae10 ga zircons secondary populations of 1ae22 1ae30 and 1ae46 ga rare 2ae2 and 1ae8 ga zircons and a minor population 2 of concordant zircons in the 600 700 ma range archaean age grains comprise 1 of all zircons analysed from these rocks in contrast a feldspathic arenite from the middle ordovician estancia san isidro formation of the central precordillera has two well defined peaks at 1ae41 and 1ae43 ga with no grains in the 600 1200 ma range and none older than 1ae70 ga the zircon age spectrum in this unit is similar to that of a middle cambrian quartz arenite from the la laja formation suggesting that local basement rocks were a regional source of ca 1ae4 ga detrital zircons in the precordillera terrane from the cambrian onwards the lack of grains younger than 600 ma in upper ordovician units reinforces petrographic data indicating that ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins nd isotopic data n 32 for middle and upper ordovician graptolitic shales from six localities define a poorly mixed signal end 450 ma 9ae 6t o 4ae5 that becomes more regionally homogenized in upper ordovician rocks 6ae 2 1 ae0 tdm 1ae51 0ae15 ga n 17 a trend reinforced by the u pb detrital zircon data it is concluded that proximal recycled orogenic sources dominated the siliciclastic sediment supply for these basins consistent with rapid unroofing of the precordillera terrane platform succession and basement starting in mid ordovician time common pb data for middle and upper ordovician shales from the western and eastern precordillera n 15 provide evidence for a minor 30 component that was likely derived from a high l u pb terrane

chemical alteration within the volcanic roof rocks of the bushveld complex field relations petrography and geochemistry identify mobile and immobile elements within the rooiberg volcanic rocks of the floor and roof of the mafic rustenburg layered suite of the 2 06 ga bushveld complex the floor rocks show thermal metamorphism accompanied by dehydration two major alteration processes affected the roof rocks the first followed extrusion of the felsic damwal formation and is marked by veins of pyrite and arsenopyrite the second is related to the intrusion of the rustenburg layered suite which produced an aureole 1 4 km thick in the overlying felsic rocks pb zn and mn attain maximum concentrations of 335 ppm 929 ppm and 0 45 wt percent in the metasomatized roof rocks the economic significance of elevated pb and zn concentrations at the top of the aureole deserves future attention primary concentrations of si mn ca na k fe mg st th ba u hf ni cu zn pb nb zr and y were modified to varying degrees depending on locality and distance from the bushveld intrusions ti al p ga so and heavy ree were immobile concentrations of these elements in the rooiberg volcanic rocks indicate that at least one geochemically distinct magma type is present in both the floor and the roof sequences this confirms that the volcanic floor and root successions originally formed as a continuous sequence the color of the roof rocks is controlled by the contact metamorphic processes and cannot be used for stratigraphic subdivision nor should the red color of these rocks be used for inferences about the evolution of the proterozoic atmosphere hydrothermal alteration was also responsible for the aberrant radiometric dates in the rooiberg group

evolutionary aspects of lithosphere discontinuity structure in the western u s 1 we have produced common conversion point ccp stacked ps and sp receiver function image volumes of the moho and lithosphere asthenosphere boundary lab beneath the western united states using transportable array data the large image volumes and the diversity of tectonic environments they encompass allow us to investigate evolution of these structural discontinuities the moho is a nearly continuous topographic surface whereas the lab is not and the seismic images show a more complex expression the first order change in lab depth in the western u s occurs along the cordilleran hingeline the former laurasian passive margin along the southwestern precambrian north american terranes the lab is about 50 deeper to the east of the hingeline than to the west with most of the increase in lab thickness being in the mantle lithosphere we infer that the moho and the lab are late mesozoic or cenozoic everywhere west of the hingeline modified during farallon subduction and its aftermath between the hingeline and the rocky mountain front the lab and to a lesser extent the moho have been partially reset during the cenozoic by processes that continue today seismicity and recent volcanism in the interior of the western u s are concentrated along gradients in crustal and or lithospheric thickness for example the hingeline and the eastern edge of the coastal volcanic magmatic terranes to us this suggests that lateral gradients in integrated lithospheric strength focus deformation similarly areas conjectured to be the sites of convective downwellings and associated volcanism are located along gradients in regional lithosphere thickness

geochronological evidence for tectonically driven brine migration during the early proterozoic kheis orogeny of southern africa the 2 1 1 9 ga kheis orogeny on tne western margin of the kaapvaal craton of southern africa induced regional heating tilting and extensive fluid flow which isotopically reset and altered rocks and minerals of older proterozoic formations and parts of the archean cratonic basement this tectonic event is recognised in the ar rb sr pb pb and u pb zircon geochronological and palaeomagnetic studies on the transvaal and underlying basement rocks

bushveld aged fluid flow peak metamorphism and gold mobilization in the witwatersrand basin south africa constraints from in situ shrimp u pb dating of monazite and xenotime in situ u pb dating of monazite and xenotime in gold reefs and unmineralized greenschist facies sedimentary rocks from the witwatersrand basin south africa reveals two episodes of tectonothermal activity a major event between 2 06 and 2 03 ga is recorded in the wit watersrand and transvaal supergroups in the northwestern and central basin and broadly coincides with the ca 2 06 ga bushveld event in the central and southern basin a previously unrecognized event has been dated between 2 14 and 2 12 ga the widespread geographic and stratigraphic occurrence of bushveld aged monazite and xenotime including both auriferous reefs and unmineralized strata indicates that metamorphism and fluid flow associated with magmatism was pervasive affecting most of the succession 10 km thick in the central and northern parts of the basin the metamorphic phosphate dates which are younger away from the complex indicate a lag of 20 30 m y between emplacement and phosphate growth in the central basin 100 km south suggesting that heat related to magmatism was transferred southward at an average rate of 3 5 mm yr 1 the absence of 2 06 2 03 ga phosphates in the welkom goldfield at the southern end of the basin implies that bushveld related heating and fluid flow did not affect this part of the basin the intergrowth of ca 2 045 ga monazite with gold in quartz pebble conglomerate from the west rand goldfield indicates that fluid flow related to the bushveld event caused mobilization of gold in the witwatersrand basin

seismic vp vs tomography of texas oklahoma with a focus on the gulf coast margin abstract the northwestern gulf of mexico passive margin contains an extensive record of continental collision and rifting as well as deformation associated with orogenic events and heavy sedimentation seismic traveltime tomography that incorporates new data from 328 broadband seismic stations deployed throughout the region reveals features that correlate well with expected mantle structures as well as features that have no obvious expression at the surface among the former are a large fast anomaly that corresponds to the southern extent of the laurentia craton and a large slow anomaly associated with the southern oklahoma aulacogen among the latter are a slow layer that we interpret to be a shear zone at the base of the cratonic and transitional continental lithosphere a zone that is bounded at its top and bottom by discontinuities and high levels of seismic anisotropy identified in companion receiver function and shear wave splitting studies respectively a high velocity body underlying the gulf coastal plain may mark delaminating lower crust if this is true it could provide indirect evidence for an elevated geotherm during the rifting process that created the gulf of mexico

receiver function analysis of the north american crust and upper mantle summary data from 27 permanent broad band stations spread over north america and sampling diverse tectonic regimes of varying ages from the archaean to the phanerozoic were analysed using the receiver function approach receiver functions a few hundred at most of the locations provide valuable constraints on the composition of the crust shaped by diverse processes characteristic of their time of formation the chemically inert and stable nature of the archaean domains of canada is well manifested in their low poisson ratio 0 26 felsic lower than global averages for precambrian age crust accompanied by a thicker than global average 35 km archaean crust data from the canadian cordillera and innuitian orogens of phanerozoic times indicate the possible operation of two distinctly different crustal processes resulting in thin crusts 35 km with dominantly felsic and more mafic compositions respectively these processes are restricted to the region above 410 km without disturbing the integrity of the mantle transition zone mtz which is evident from the remarkably homogeneous nature of the 410 and 660 km discontinuities beneath the canadian landmass contrastingly the thermally complex phanerozoic southern california region suggests the presence of a heterogeneous mantle transition zone possibly owing to localized 660 topography with a relatively uniform 410 overlain by a crust heterogeneous in composition and perhaps maturing to that of a continent

layered structure in the upper mantle across north america from joint inversion of long and short period seismic data abstract we estimate crustal and uppermost mantle shear velocity structure beneath 30 stations in north america by jointly inverting the high frequency scattered wavefield observed in the p wave coda together with long period surface wave phase and group dispersion data several features distinguish our approach from previous such joint inversions 1 we apply a cross convolution method rather than more standard deconvolution approaches used in receiver function studies and consider both love and rayleigh wave dispersion allowing us to infer profiles of radial anisotropy 2 we generate probabilistic 1d radially anisotropic depth profiles across the whole uppermost mantle down to 350 km depth 3 the inverse problem is cast in a trans dimensional bayesian formalism where the number of isotropic and anisotropic layers is treated as unknown allowing us to obtain models described with the least number of parameters results show that the tectonically active region west of the rocky mountain front is marked by a lithospheric asthenosphere boundary and a lehmann discontinuity occurring at relatively shallow depths 60 150 km and 100 200 km respectively whereas further east in the stable craton these discontinuities are deeper 170 200 km and 200 250 km respectively in addition in the stable part of the continent at least two mid lithospheric discontinuities are present at intermediate depths suggesting the existence of strong lithospheric layering and a mechanism for lithospheric thickening by underplating of additional layers as cratonic age increases the moho across the continent as well as mid crustal discontinuities in the craton are also imaged in agreement with independent studies

a sharp lithosphere asthenosphere boundary imaged beneath eastern north america although a rigid lithosphere about 100 km of crust and uppermost mantle moving over a weaker asthenosphere a few hundred kilometres of more deformable rock is a widely accepted aspect of plate tectonics the properties of the boundary between them are poorly understood when compressional waves p waves from an earthquake encounter an interface at an angle some of the energy is converted to shear waves or s waves these converted waves can be used to construct an image of the interface based on converted waves recorded in eastern north america the lithosphere asthenosphere boundary has been resolved as a sharp gradient that cannot be explained by thermal gradients alone indicating that the boundary is controlled by melt or high water content in the asthenosphere plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere yet the nature of the lithosphere asthenosphere boundary remains poorly understood the gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers for example if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures mantle flow models1 2 indicate that the velocity gradient at its base would occur over tens of kilometres in contrast if the asthenosphere is weak owing to volatile enrichment3 4 5 6 or the presence of partial melt7 the lithosphere asthenosphere boundary could occur over a much smaller depth range here we use converted seismic phases in eastern north america to image a very sharp seismic velocity gradient at the base of the lithosphere a 3 11 per cent drop in shear wave velocity over a depth range of 11 km or less at 90 110 km depth such a strong sharp boundary cannot be reconciled with a purely thermal gradient but could be explained by an asthenosphere that contains a few per cent partial melt7 or that is enriched in volatiles relative to the lithosphere3 4 5 6

an automatic phase picker for local and teleseismic events an automatic detection algorithm has been developed which is capable of time p phase arrivals of both local and teleseismic earthquakes but rejects noise bursts and transient events for each signal trace the envelope function is calculated and passed through a nonlinear amplifier the resulting signal is then subjected to a statistical analysis to yield arrival time first motion and a measure of reliability to be placed on the p arrival pick an incorporated dynamic threshold lets the algorithm become very sensitive thus even weak signals are timed precisely during an extended performance evaluation on a data set comprising 789 p phases of local events and 1857 p phases of teleseismic events picked by an analyst the automatic picker selected 66 per cent of the local phases and 90 per cent of the teleseismic phases the accuracy of the automatic picks was ideal i e could not be improved by the analyst for 60 per cent of the local events and 63 per cent of the teleseismic events

a sharp cratonic lithosphere asthenosphere boundary beneath the american midwest and its relation to mantle flow abstract beneath the american midwest s to p sp converted wave imaging and multi mode surface wave tomography identify a north trending transition in seismic structure at 150 250 km depth to the east of this american midwest transition amt the lithosphere asthenosphere boundary lab is imaged as a 1 2 sp sv amplitude arrival at 200 240 km depth consistent with the depth of negative shear velocity and azimuthal anisotropy gradients imaged by surface wave tomography to the west of the amt sp conversions are much shallower at 150 190 km depth and are much weaker 0 7 or absent azimuthal anisotropy constrained by surface wave tomography also changes across the amt with stronger anisotropy to the east of the transition beneath the thicker lithospheric root we suggest that the seismic changes across the amt can be explained by considering the effects of asthenospheric flow beneath the leading edge of the thick lithospheric root the mantle flow is dominantly driven by the drift of the north america plate locally higher flow velocities are expected where the asthenosphere is forced to flow beneath the thicker root this mantle underflow could create a sharper seismic lab east of the amt via two effects first the local increase in flow velocities could steepen the thermal gradient at the base of the lithosphere and hence the isotropic velocity contrast second the increased strain rate along edge of the lithosphere could enhance the magnitude of azimuthal anisotropy our results suggest that seismically detectable lab sharpness variations could be used to constrain geographic variations in coupling between plates and mantle convection a secondary result is the image of a mid lithospheric discontinuity arrival at 80 110 km depth that is found primarily to the east of the amt this arrival is interpreted as produced by a layer of low velocity metasomatic minerals that have accumulated since the 1 8 ga creation of the lithosphere

green s functions source signatures and the normalization of teleseismic wave fields 1 we examine the canonical source green s function separation problem in the context of teleseismic p wave scattering from receiver side crust and upper mantle structure conventional receiver function analysis affords a leading order approximation to the s component of the green s function but provides no information on p to p scattering we demonstrate that an improved estimate of the three dimensional earth s green s function including scattered p contributions can be achieved through consideration of its theoretical spectral properties under conditions typical of the real earth the p component of the green s function is shown to be minimum phase this behavior is responsible for the success of receiver functions in mantle studies the minimum phase property is used here to normalize the source signature on p wave seismograms thereby facilitating implementation of multichannel multicomponent deconvolution of both green s function and source signature within the log spectral domain examples using both synthetic simulations and seismograms recorded on the canadian national seismograph network illustrate the recovery of accurate and reproducible estimates of the p wave green s function our approach can be adapted to a range of source receiver configurations in particular it may prove useful in the recovery of compressional properties beneath portable field arrays where calibration is provided by nearby permanent installations

the assessment of rockfall hazard at the base of talus slopes fragmental rockfall is characterized by the independent movement of individual rock fragments after detachment from a rock face the continued operation of the process leads to the accumulation of talus slopes on talus slopes the rockfall shadow extends beyond the base of the talus and consists of scattered boulders that have run out beyond the base of the slope the landing probability of boulders in the shadow is examined return periods of the order of 1000 years relative to a house site are typical rockfall behaviour particularly with respect to run out into the shadow can be assessed using geological evidence empirical methods physical modelling and computer based analytical models an empirical minimum shadow angle of 27 5 i e the angle between the distal limit of the shadow and the top of the talus slope is suggested and would be useful in rockfall vulnerability studies at the base of talus slopes as a first approximation to shadow limits it is preferable to the use of the rockfall fahrb

2 05 ga isotopic ages for transvaal mississippi valley type deposits evidence for large scale hydrothermal circulation around the bushveld igneous complex south africa abstract the known dimensions of hydrothermal fluid circulation associated with emplacement of the 2 055 ga bushveld igneous complex bic are extended with new isotopic ages for paleoproterozoic mineral deposits in wallrocks transvaal supergroup surrounding the bic a fluorite rich mississippi valley type mvt deposit in the zeerust district dated at 2 05 ga sm nd isochron age has nd isotopic compositions that are identical to coeval f rich lebowa granite late in the bic intrusive sequence suggesting transport of magmatic fluids 100 km beyond the present margins of the bic hydrothermal illite from the zn pb mvt bushy park deposit dated at 2 05 ga by k ar geochronology links widespread mvt deposits of the kaapvaal craton to large scale basinal brine circulation associated with emplacement of the bic these data in combination with gold vein phosphate mineral and paleomagnetic resetting ages indicate a regional scale bushveld age hydrothermal system circulating 700 km outward from the prese

geochemistry of contrasting siliceous magmatic suites in the bushveld complex genetic aspects and implications for tectonic discrimination diagrams abstract the intracratonic bushveld igneous province that formed 2 1 1 9 ga ago contains three contrasting suites of siliceous rocks that are demonstrably of magmatic origin the oldest of these are the high mg hmf and low mg lmf felsites which form interstratified flows in the rooiberg group bushveld granites intrude the rooiberg group and constitute the youngest component of the province well defined interelement variation trends illustrate that the granites do not share the same petrogenetic history as the rooiberg magmas nd isotope measurements indicate that the two eruptive suites probably formed under similar differentiation conditions but from parental magmas that were derived from compositionally different sources on trace element discrimination diagrams the bushveld granites and lmf are usually correctly assigned to a within plate setting but conversely the hmf are generally misclassified as subduction related eruptives it is argued that the trace element signatures of the granites and felsites do not identify their tectonic setting per se but rather point to the melting and crystallization histories of the source regions from which their parent magmas were extracted as such tectonic discrimination diagrams may provide valuable pointers to processes that have affected igneous source materials in much earlier magmatic cycles

geotechnical precursors to large scale ground collapse in mines abstract analysis of instances of large scale ground collapse in underground and open pit mines shows that geotechnical precursors to rock mass failure appear over time recorded geotechnical precursors in cases of ground collapse larger than 100 000 tonnes were examined results indicated that a sequence leading to rock mass collapse existed ground movement was observed at the periphery of each impending collapse over extended periods in some case years months before collapse the main precursor is deterioration of ground conditions in hard rock mass within weeks before collapse precursors in the form of high stress signs were noticed in close proximity to the centre of the eventual collapse area hours before the collapse the most common precursors were rock noises and large falls of ground close to the centre of the imminent collapse

how long can a hydrothermal system be sustained by a single intrusive event published numerical calculations show that the convective systems which produce near surface geothermal systems or ore deposits are likely to cool their intrusion heat source in at most a few tens of thousands of years even if the intrusion is large generally hydrothermal circulation near surface geothermal activity and magmatic intrusion should be closely tied in time and space long lived approximately 1 m y hydrothermal activity normally suggests multiple pulses of intrusion with associated pulses of hydrothermal circulation however if the permeability of the intruded environment is just above that which allows convection and the volume of the intrusion is very large calculations presented here show that a single episode of intrusion can sustain hydrothermal circulation and near surface geothermal activity for approximately 800 000 yr for a deep sill heat source these unusually long lived hydrothermal systems can vent through a few very widely separated discharge sites and have the potential to produce isolated massive sulfide deposits of unusually large size

samarium neodymium direct dating of fluorite mineralization the direct dating of many styles of hydrothermal mineralization has proved difficult limiting understanding of the geological processes that lead to crustal fluid flow and the formation of major ore deposits the hydrothermal mineral fluorite caf2 displays large variations in rare earth element ree abundance and samarium neodymium ratios within a single vein samarium neodymium dating of fluorite from the classic granite hosted tin deposits of southwest england demonstrates its use as a precise chronometer of mineralization the concentrations of light rare earth elements lrees in the fluorites are highly variable and suggest the coeval precipitation of an lree rich phase as the most likely cause of the extreme variation in samarium neodymium ratios

precise age of the platiniferous merensky reef bushveld complex south africa by the u pb zircon chemical abrasion id tims technique the bushveld complex south africa is the world s largest layered intrusion and host to the majority of the known resources of platinum group elements chromium and vanadium despite intensive study determining the precise crystallization age of the bushveld complex has proven to be difficult in this study zircon and rutile were identified in thin section and separated from a sample of pegmatitic feldspathic orthopyroxenite from the pge rich merensky reef west mine townlands shaft rustenburg section low u 21 105 ppm zircon occurs with interstitial biotite and is locally in direct contact with sulfide rutile occurs mainly as inclusions within or overgrowths on chromite especially in the chromitite stringers that bound the reef and also as acicular grains with biotite a u pb crystallization age of 2054 4 1 3 ma was determined from single crystal chemical abrasion id tims u pb dating of zircon and multigrain fractions of rutile yielded a u pb cooling age of 2055 0 3 9 ma integration of the merensky reef crystallization age with existing u pb age determinations for intrusions that pre and postdate emplacement and crystallization of the layered rocks of the bushveld complex indicate that magmatism occurred at ca 2054 ma and is contemporaneous with related magmatic activity across the northern kaapvaal craton of south africa and botswana

sp receiver function imaging of a passive margin transect across texas s gulf coastal plain abstract the gulf coast of texas has been the subject of intensive geological and geophysical investigation in pursuit of hydrocarbons but studies that penetrate beyond the upper crust are limited to a few refraction profiles and regional surface wave investigations the passing of earthscope s transportable array has facilitated regional investigations of the lithosphere but its 70 km station spacing does not allow many important tectonic features to be imaged a broadband seismic transect across the texas gulf coastal plain was therefore performed in order to image deep structure beneath this passive margin and the transition to the neighboring craton a 2d sp receiver function common conversion point ccp stacked image produced for this transect reveals several discontinuities in the sub crustal lithosphere the region nearest the shoreline is underlain by an anomalous 18 km thick low velocity layer that produces a strong negative pulse in the sp receiver functions the drop in velocity is too large to be due to any reasonable change in fe or mg content but could be produced by partial melt or mantle hydration it is unlikely that partial melt would still be found in a 160 180 year old passive margin such as the gulf coast but hydration possibly introduced by a through going balcones fault system and resulting serpentinization could produce the observed anomaly an event with negative polarity appears at a depth of 110 km which we interpret to be the lithosphere asthenosphere boundary lab thermal variations alone would not produce a sufficiently sharp discontinuity to be imaged by sp converted phases recent shear wave splitting studies revealed unusually large delay times in this region along with fast polarization directions that differ from measurements on the laurentian craton large delay times may imply significant flow which could also produce frictional heating due to shearing and partial melt which would steepen the velocity gradients an additional layer is bounded by discontinuities at 145 165 km depth with negative polarity and 185 205 km depth with positive polarity the appearance of these discontinuities suggests that the mantle beneath the lab is more complicated here than elsewhere and that the transition from lithosphere to asthenosphere occurs over a broad 85 100 km thick depth interval

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

melt generation and fluid flow in the thermal aureole of the bushveld complex granite sheets emplaced into the migmatite zone of the eastern contact aureole of the bushveld complex resulted from fluid enhanced incongruent biotite melting of the underlying silverton formation shales during prograde metamorphism ba concentrations are extreme in both the sheets 1000 ppm and the hornfels 800 ppm into which they have been emplaced we conclude that a ba rich hydrothermal fluid induced melting in the aureole and that fluid transport of ba2 and to a lesser extent sr2 and eu2 persisted in the melt zones under subsolidus conditions sr isotope systematics from high ba localities define an errorchron of 2161 106 ma with an initial 87sr 86sr ratio of 0 705 0 001 metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids there is no evidence of regional homogenization oxygen isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact suggesting the infiltrating fluid equilibrated isotopically with the metasediments their elevated 18o values 11 3 12 1 are consistent with a fluid source from devolatilization of the sedimentary lithologies textural analysis at both outcrop and thin section scale of arkose and psammites in the aureole lakenvalei and magaliesberg formations shows that the extent of melting was highly heterogeneous even on the grain scale and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt which preserve textures inherited from the regionally metamorphosed protolith simple finite difference thermal modelling of the aureole suggests that the width of the melt zone 500 m is inconsistent with conductive heat transfer and hence that the thermal structure has been modified by fluid advection

a contrast in anisotropy across mid lithospheric discontinuities beneath the central united states a relic of craton formation the mid lithospheric discontinuity mld is a seemingly sharp decrease in seismic velocity at depths internal to the lithosphere and appears to be a pervasive feature beneath continental interiors its presence within cratons which have remained relatively stable since formation suggests that the mld may result from processes associated with continent formation we use p to s wave receiver functions to interrogate seismic anisotropy across the mld within the ca 1 35 1 55 ga granite rhyolite province of the central united states our analysis reveals strong evidence for sharp changes in the orientation of anisotropy across multiple mlds with an approximately north to northwest fast orientation of anisotropy in the upper lithosphere the consistency of this signature over a large region suggests that the observed anisotropy is a relic of north american craton formation in addition the presence of several distinct anisotropic layers within the cratonic lithosphere supports models for craton formation via stacked subducted slabs or a series of underthrusting events

relationship between footwall composition crustal contamination and fluid rock interaction in the platreef bushveld complex south africa in the northern limb of the 2 06 ga bushveld complex the platreef is a platinum group elements pge cu and ni mineralized zone of pyroxenite that developed at the intrusion margin from north to south the footwall rocks of the platreef change from archaean granite to dolomite hornfels and quartzite where the footwall is granite the sr isotope system is more strongly perturbed than where the footwall is sr poor dolomite in which samples show an approximate isochron relationship the nd isotope system for samples of pyroxenite and hanging wall norite shows an approximate isochron relationship with an implied age of 2 17 0 2 ga and initial nd isotope ratio of 0 5095 assuming an age of 2 06 ga the nd values range from 6 2 to 9 6 ave 7 8 n 17 and on average are slightly more negative than the main zone of the bushveld these data are consistent with local contamination of an already contaminated magma of main zone composition the similarity in isotope composition between the platreef pyroxenites and the hanging wall norites suggests a common origin where the country rock is dolomite the platreef has generally higher plagioclase and pyroxene 18o values and this indicates assimilation of the immediate footwall throughout the platreef there is considerable petrographic evidence for sub solidus interaction with fluids and the plagioclase pyroxene values range from 2 to 6 which indicates interaction at both high and low temperatures whole rock and mineral d values suggest that the platreef interacted with both magmatic and meteoric water and the lack of disturbance to the sr isotope system suggests that fluid rock interaction took place soon after emplacement where the footwall is granite less negative d values suggest a greater involvement of meteoric water consistently higher values of plagioclase pyroxene in the platreef pyroxenites and hanging wall norites in contact with dolomite suggest prolonged interaction with co2 rich fluid derived from decarbonation of the footwall rocks the overprint of post crystallization fluid rock interaction is the probable cause of the previously documented lack of correlation between pge and sulfide content on the small scale the platreef in contact with dolomite is the focus of the highest pge grades and this suggests that dolomite contamination played a role in pge concentration and deposition but the exact link remains obscure it is a possibility that the co2 produced by decarbonation of assimilated dolomite enhanced the process of pge scavenging by sulfide precipitation

progressive crustal contamination of the bushveld complex evidence from nd isotopic analyses of the cumulate rocks we report the first nd isotopic data on the cumulate rocks of the bushveld complex south africa we analysed 17 whole rock samples covering 4700 m of stratigraphy through the lower critical and main zones of the intrusion at union section north western bushveld complex the basal ultramafic portions of the complex have markedly higher nd t 5 3 to 6 0 than the gabbronoritic main zone nd t 6 4 to 7 9 the rocks of the upper critical zone have intermediate values these results are in agreement with new nd isotope data on marginal rocks and sills in the floor of the complex that are generally interpreted as representing chilled parental magmas and with published sr isotopic data all of which show a larger crustal component in the upper part of the intrusion in contrast the concentrations of many highly incompatible trace elements are decoupled from the isotopic signatures the basal portions of the complex have higher ratios of incompatible to compatible trace elements than the upper portions the variations of isotopic and trace element compositions are interpreted in terms of a change in the nature of the crustal material that contaminated bushveld magmas those magmas that fed into the lower part of the complex had assimilated a relatively small amount of incompatible trace element rich partial melt of upper crust whereas magmas parental to the upper part of the complex had assimilated a higher proportion of the incompatible trace element poor residue of partial melting

mesothermal type mineralization in the sabie pilgrim s rest gold field south africa we describe here a different class of mesothermal gold deposit at sabie pilgrim9s rest which is probably associated with the bushveld igneous event in south africa pressure and temperature estimates indicate that the ore fluids of the sabie pilgrim9s rest gold field which occurs within the early proterozoic transvaal supergroup were similar to those of mesothermal gold deposits however a number of features distinguish the sabie pilgrim9s rest ores from typical mesothermal deposits 1 the presence of ubiquitous cu and bi in addition to au ag as and sb 2 high salinities mean asymp 12 wt nacl equiv 3 low mean bulk co 2 content asymp 5 mole 4 veins largely hosted within a platform carbonate sequence and 5 mineralization along both vertical and horizontal stratiform veins showing a clear genetic link characteristic features of the sabie pilgrim9s rest gold field that correspond to those within the above spectrum of mesothermal gold deposits include 1 comparable light stable isotope delta d delta 18 o delta 34 s ranges together with a lack of isotopic evidence for meteoric water involvement 2 high au ag ratios 3 presence of ore shoot structures which are highly characteristic of mesothermal gold deposits 4 variably deformed structures within individual sheeted veins which point toward cyclic opening and quartz accretion events separated by episodes of deformation and 5 fluid inclusion properties which indicate high fluid pressures during entrapment bushveld magmatism is thought to have been the main heat source responsible for generating the hydrothermal cell that resulted in the formation of the sabie pilgrim9s rest gold field mineralization involved an originally pristine fluid transporting metals that were probably derived from the same source as well as a measure of connate waters from the granitic basement and the overlying sedimentary rocks the fluid was relatively acidic and anoxic and equally efficient in transporting gold either as a bisulfide complex or a chloride complex gold precipitation took place as a result of 1 a decrease in sulfur activity within the ore bearing fluid because of sulfide precipitation and 2 changes in the fluid chemistry as a result of fluid immiscibility

lithospheric discontinuities beneath the u s midcontinent signatures of proterozoic terrane accretion and failed rifting abstract seismic discontinuities between the moho and the inferred lithosphere asthenosphere boundary lab are known as mid lithospheric discontinuities mlds and have been ascribed to a variety of phenomena that are critical to understanding lithospheric growth and evolution in this study we used s to p converted waves recorded by the usarray transportable array and the oiink ozarks illinois indiana kentucky flexible array to investigate lithospheric structure beneath the central u s this region a portion of north america s cratonic platform provides an opportunity to explore how terrane accretion cratonization and subsequent rifting may have influenced lithospheric structure the 3d common conversion point ccp volume produced by stacking back projected sp receiver functions reveals a general absence of negative converted phases at the depths of the lab across much of the central u s this observation suggests a gradual velocity decrease between the lithosphere and asthenosphere within the lithosphere the ccp stacks display negative arrivals at depths between 65 km and 125 km we interpret these as mlds resulting from the top of a layer of crystallized melts sill like igneous intrusions or otherwise chemically modified lithosphere that is enriched in water and or hydrous minerals chemical modification in this manner would cause a weak layer in the lithosphere that marks the mlds the depth and amplitude of negative mld phases vary significantly both within and between the physiographic provinces of the midcontinent double or overlapping mlds can be seen along precambrian terrane boundaries and appear to result from stacked or imbricated lithospheric blocks a prominent negative sp phase can be clearly identified at 80 km depth within the reelfoot rift this arrival aligns with the top of a zone of low shear wave velocities which suggests that it marks an unusually shallow seismic lab for the midcontinent this boundary would correspond to the top of a region of mechanically and chemically rejuvenated mantle that was likely emplaced during late precambrian early cambrian rifting these observations suggest that the lithospheric structure beneath the reelfoot rift may be an example of a global phenomenon in which mlds act as weak zones that facilitate the removal of cratonic lithosphere that lies beneath

the lithosphere asthenosphere boundary beneath the western united states summary s receiver functions from 67 broad band seismic stations in the western united states clearly reveal the existence of a mantle discontinuity with velocity reduction downward which we interpret as the lithosphere asthenosphere boundary lab the average depth of the lab is 70 km the boundary is relatively sharp with an overall sharpness of less than 20 km the boundary is more prominent south of the mendocino triple junction where the farallon plate has completely subducted this may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window a double low velocity zone is observed at base of the lithosphere beneath southern sierra nevada implying a second melting zone at a depth of 100 km well correlated with previous studies of lithospheric delamination in the area

a preliminary analysis of failure mechanisms in karst and man made underground caves in southern italy abstract natural and anthropogenic caves may represent a potential hazard for the built environment due to the occurrence of instability within caves that may propagate upward and eventually reach the ground surface inducing the occurrence of sinkholes in particular when caves are at shallow depth the effects at the ground surface may be extremely severe apulia region southern italy hosts many sites where hazard associated with sinkholes is very serious due to presence of both natural karst caves and anthropogenic cavities the latter being mostly represented by underground quarries the pliocene pleistocene calcarenite a typical soft rock was extensively quarried underground by digging long and complex networks of tunnels with time these underground activities have progressively been abandoned and their memory lost so that many apulian towns are nowadays located just above the caves due to urban expansion in the last decades therefore a remarkable risk exists for society which should not be left uninvestigated the present contribution deals with the analysis of the most representative failure mechanisms observed in the field for such underground instability processes and the factors that seem to influence the processes as for example those causing weathering of the rock and the consequent degradation of its physical and mechanical properties aimed at exploring the progression of instability of the cavities numerical analyses have been developed by using both the finite element method for geological settings represented by continuous soft rock mass and the distinct element method for jointed rock mass conditions both the effects of local instability processes occurring underground and the effects of the progressive enlargement of the caves on the overall stability of the rock mass have been investigated along with the consequent failure mechanisms in particular degradation processes of the rock mass as a consequence of wetting and weathering phenomena in the areas surrounding the caves have been simulated the results obtained from the numerical simulations have then been compared with what has been observed during field surveys and a satisfactory agreement between the numerical simulations and the instability processes as detected in situ has been noticed

facies relations depositional environments and diagenesis in a major early proterozoic stromatolitic carbonate platform to basinal sequence campbellrand subgroup transvaal supergroup southern africa the campbellrand subgroup and its correlative the malmani subgroup of the transvaal supergroup represent a major 2300 2600 ma old carbonate buildup it is on average between 1500 and 1700 m thick and covers an area of approximately 190 000 km2 on the kaapvaal craton the original depository probably extended across the entire 600 000 km2 surface area of the craton and beyond the buildup consists essentially of two major lithofacies assemblages namely a basinal non stromatolitic laminated carbonate and shale sequence with minor chert iron formation and mafic tuff interbeds off the craton to the west and a shallow water stromatolitic carbonate sequence on the craton proper growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite oolite and carbonate arenite shoal deposits these are flanked basinward by reef like giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites platform lagoonal deposits consist of dark grey laminoid fenestrate stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites laminoid fenestrae tepee structures and flat pebble conglomerates the buildup consisted primarily of limestone which was replaced by early diagenetic dolomite the chemical composition of the dolomite is related to depofacies platform margin and lagoonal dolomites are manganese rich basinal dolomites are iron rich and intertidal to supratidal deposits are virtually free of iron and manganeses indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the kuruman iron formation

hydrothermal controls on iron and lead mineralization on the farms leeuwbosch and cornwall thabazimbi district south africa abstract the paleoproterozoic malmani subgroup and penge formation transvaal supergroup exposed on the farms leeuwbosch and cornwall north of thabazimbi limpopo province south africa host hydrothermal pb ag cu zn and high grade hematite iron ore deposits which include the historic leeuwbosch lead mine based on ore petrography fluid inclusion and stable isotope analyses and geochemical modeling the structurally controlled stratabound leeuwbosch pb ag cu zn fe deposits formed from high salinity nacl cacl2 rich basinal brines with total salinities from 18 to 24 wt nacl equivalent which are similar to those found in the mississippi valley type mvt pb zn deposits the minimum formation temperatures homogenization temperatures for the lead deposits range between 120 and 185 c and the estimated formation conditions are 200 c at 1 kbar pressure stable isotope analyses of gangue siderite and calcite indicate that the deposits formed from base metal enriched brines derived from recycled seawater with 13cvs pdb from 0 46 to 3 17 and 18ovs smow from 2 95 to 1 79 hydrothermal transport of lead and silver as pbcl3 and agcl2 respectively took place at mildly acidic ph and elevated fo2 in the so42 predominance field extensive host rock interaction neutralized and reduced the ore fluid to precipitate galena hosting ag with chalcopyrite and sphalerite fluid inclusions in gangue quartz and calcite in the extensive structurally controlled and pervasive stratabound hydrothermal iron ore deposits on the farms leeuwbosch and cornwall chiefly consisting of high grade hematite with rare relict magnetite show a wide compositional variability due to episodic fluid mixing and successive hydrothermal activity in the presence of several distinct fluid end members total salinities of fluid inclusions in quartz from the hematite deposits range from 9 2 to 39 9 wt nacl eq with highly variable proportions of nacl and cacl2 and minimum formation temperatures are between 100 and 190 c the estimated formation conditions for the iron ore deposits are 175 c and 1 kbar stable isotopes of gangue calcite indicate that diagenetic fluids with 13cvs pdb from 3 33 to 2 09 and 18ovs smow from 2 52 to 5 60 and a second fluid with 13cvs pdb of 7 45 and 13cvs pdb of 17 45 were controlling the formation of the hematite deposits in agreement with oxidized conditions during hematite formation and characteristic host rock alteration patterns traced by carbon and oxygen isotopes the second low salinity fluid was derived from the overlying waterberg group sandstones and played an important role in the formation of the iron ore deposits in addition to high salinity diagenetic brines iron was leached from the bifs of the penge formation transported as fecl under mildly acidic conditions and likely deposited in response to the oxidation of the iron bearing fluids the lead and iron ore deposits formed by combinations of structural controls which influenced the regional fluid flow patterns and fluid compositions and fluid rock interaction with reactive carbonate lithologies of the malmani subgroup which acted as a sink for metals transported in acidic hydrothermal solutions

anatomy of a metamorphic core complex seismic refraction wide angle reflection profiling in southeastern california and western arizona the metamorphic core complex belt in southeastern california and western arizona is a nw se trending zone of unusually large tertiary extension and uplift midcrustal rocks exposed in this belt raise questions about the crustal thickness crustal structure and the tectonic evolution of the region three seismic refraction wide angle reflection profiles acquired and analyzed as a part of the u s geological survey s pacific to arizona crustal experiment were collected to address these issues the results presented here which focus on the whipple and buckskin rawhide mountains yield a consistent three dimensional image of this part of the metamorphic core complex belt the seismic refraction wide angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles these phases include refractions from the near surface and crystalline basement reflections from boundaries in the middle and lower crust and reflections and refractions from the upper mantle the final model consists of a thin veneer 2 km of upper plate and fractured lower plate rocks 1 5 5 5 kms 1 overlying a fairly homogeneous basement 6 0 km s 1 and a localized high velocity 6 4 km s 1 body situated beneath the western whipple mountains a prominent midcrustal reflection is identified beneath the whipple and buckskin rawhide mountains between 10 and 20 km depth this reflector has an arch like shape and is centered beneath or just west of the metamorphic core complex belt this event is underlain by a weaker approximately subhorizontal reflection at 24 km depth together these two discontinuities define a lens shaped midcrustal layer with a velocity of 6 35 6 5 km s 1 the apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift 10 km defined by surface geologic mapping and petrologic barometry studies the layer thins to the northeast and is absent in the transition zone the 6 35 6 5 km s 1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks this midcrustal layer is underlain by a higher velocity lower crustal layer that is modeled as only 3 6 km thick beneath the metamorphic core complex belt and regions to the southwest to the northeast however this layer thickens to 8 10 km as the midcrustal layer pinches out above it the velocity of the lower crust is constrained by traveltime modeling and is 6 6 0 15 kms 1 beneath the western transition zone and the metamorphic core complex belt higher velocities may be present farther to the southwest where the layer is thin the velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust instead we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle derived intrusions the crust mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26 28 km or in the case of the whipple mountains actually deepens a 3 km crustal root is modeled this lack of upward doming of the moho together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension related structures in the middle and lower crust suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension as one would expect if extension occurred along crust penetrating shear zones wernicke 1981 wernicke et al 1985 instead domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust

behavior of rock in slopes the behavioral modes to which a rock slope is susceptible are many and complex this article discusses a long but not exhaustive list of common failure styles highlighting observable characteristics that distinguish one from another slopes in both strong and weak rock masses are considered principles are stated to explain how and why different failure modes develop in different rocks the discussion is illustrated with examples of diverse failure modes in foliated metamorphic rocks developed in the walls of an eroded spillway runout

rock mass behaviour prior to failure abstract changes in stress around mining excavations can result in changes in the behaviour of the rock mass which in turn may lead to damage failure and consequent collapse of the rock mass analysis of documented case studies of regional scale collapses indicates that each rock mass failure is preceded by a precursory manifestation of rock mass behaviour structural damage and progressive failure are manifested by the presence of geotechnical warning signs indicators and precursors and can be exacerbated by triggers indicators suggest that the rock mass may be prone to damage whereas geotechnical precursors demonstrate that the rock mass has been disturbed possibly preceding failure geotechnical analysis of large scale rock mass behaviour also indicates that the failures do not happen at random and are not unpredictable in terms of the type of failure and its location indicators and precursors have to be interpreted in conjunction with mine design mining activities and potential triggers geotechnical monitoring of the precursory behaviour of the rock mass provides timely warning and allows for implementation of remedial measures in this paper these issues are discussed and illustrated in the context of geotechnical risk management

o isotope study of the bushveld complex granites and granophyres constraints on source composition and assimilation the 2 05 ma bushveld magmatic event culminated in the production of490 000 km 3 of granite and granophyres in these granitic rocks high temperature equilibrium o isotope fractionations are generally preserved between quartz and zircon but not between quartz and feldspar or between biotite and amphibole quartz separated from four granite samples shows no significant difference in core and rim d 18 o values which indicates that quartz is not significantly zoned and provides further evidence that it is unaffected by alteration quartz can therefore be used as a proxy for the magma d 18 o value leading to estimates of 6 9o for both the granites assuming quartz magma 1 11o and granophyres assuming quartz magma 0 62o similar magma d 18 o values 6 6o were obtained using zircon d 18 o values assuming zircon magma 1 3o the initial nd isotope ratio of the granitic rocks ranges from 0 509676 to 0 509822 with an average value of 0 509655 n 12 this corresponds to average end values of 5 9 and 4 8 for the granites and granophyres respectively the similarity in isotope composition between the granites and granophyres and between the granitic rocks from each of the three major lobes of the bushveld complex is consistent with a common origin the d 18 o values of the granitic rocks suggest derivation from mantle derived magmas by either fractional crystallization or partial melting but this hypothesis is incompatible with their crustal end values average 5 5 the associated rustenburg layered suite rls rocks have average d 18 o values of 7 1o which is within error of the average estimate for the bushveld granitic rocks and similar end values however granitic magma derived from the same paretal magmas that produced the rls would have had an average magma d 18 o of about 7 9o 1 o higher than observed we therefore suggest that the granitic magmas were produced by fractional crystallization of rls magma or by partial melting of solidified rls magma at depth followed by assimilation at a shallower level of a significant quantity of hydrothermally altered low d 18 o material from the since eroded volcanic edifice

an overview of the geology of the transvaal supergroup dolomites south africa in the neoarchaean intracratonic basin of the kaapvaal craton between approximately 2640 ma and 2516 ma two successive stromatolitic carbonate platforms developed deposition started with the schmidtsdrif subgroup which is probably oldest in the southwestern part of the basin and which contains stromatolitic carbonates siliciclastic sediments and minor lava flows subsequently the nauga formation carbonates were deposited on peritidal flats located to the southwest and were drowned during a transgression of the transvaal supergroup epeiric sea around 2550 ma ago this transgression led to the development of a carbonate platform in the areas of the preserved transvaal and griqualand west basins which persisted for 30 50 ma during this time shales were deposited over the nauga formation carbonates in the southwestern portion of the epeiric sea a subsequent period of basin subsidence led to drowning of the stromatolitic platform and to sedimentation of chemical iron rich silica precipitates of the banded iron formations bif over the entire basin carbonate precipitation in the archaean was largely due to chemical and lesser biogenic processes with stromatolites and ocean water composition playing an important role the stromatolitic carbonates in the preserved griqualand west and transvaal basins are subdivided into several formations based on the depositional facies reflected by stromatolite morphology and on intraformational unconformities interbedded tuffs and available radiometric age data do not yet permit detailed correlation of units from the two basins thorough dolomitisation of most formations took place at different post depositional stages but mainly during early diagenesis partial silicification was the result of diagenetic and weathering processes karstification of the carbonate rocks was related to periods of exposure to subaerial conditions and to percolation of groundwater such periods occurred locally at the time of carbonate and bif deposition main karstification however probably took place during an erosional period between approximately 2430 ma and 2320 ma

the early proterozoic mississippi valley type pb zn f deposits of the campbellrand and malmani subgroups south africa pb zn f deposits occur in the very late archaean 2 55 ga shallow marine dolostone of the relatively undeformed campbellrand and malmani sub groups which are overlain unconformably by the lower proterozoic postmasburg and pretoria group siliciclastics they consist of stratiform deposits formed by replacement and porosity filling as well as pipes ring shaped and irregular bodies associated with collapse breccia in the transvaal basin the latter were generated during the karst denudation period between the deposition of the chuniespoort group ending at 2 4 ga and of the pretoria group starting at 2 35 ga a part of these mineralisations were overprinted by the metamorphism of the bushveld complex intrusion at 2 06 ga in the transvaal basin the age of the mineralisation is constrained between the start of the pretoria group deposition and the bushveld intrusion it is concluded that although most of the mineralisations are characteristic of the mississippi valley type some of the northernmost occurrences rich in siderite are less typical a classic genetic model is proposed in an environment characterised by tensional tectonics and basin development brines of basinal origin were heated by circulation into pre chuniespoort rocks leached metals from the rocks they permeated and rose as hydrothermal plumes at relatively shallow depth they deposited minerals after mixing with water of surficial origin

upper mantle structure across the trans european suture zone imaged by s receiver functions we present a high resolution study of the upper mantle structure of central europe including the western part of the east european platform based on s receiver functions of 345 stations a distinct contrast is found between phanerozoic europe and the east european craton across the trans european suture zone to the west a pronounced velocity reduction with depth interpreted as lithosphere asthenosphere boundary lab is found at an average depth of 90 km beneath the craton no strong and continuous lab conversion is observed instead we find a distinct velocity reduction within the lithosphere at 80 120 km depth this mid lithospheric discontinuity mld is attributed to a compositional boundary between depleted and more fertile lithosphere created by late proterozoic metasomatism a potential lab phase beneath the craton is very weak and varies in depth between 180 and 250 km consistent with a reduced velocity contrast between the lower lithosphere and the asthenosphere within the trans european suture zone lithospheric structure is characterized by strong heterogeneity a dipping or step wise increase to lab depth of 150 km is imaged from phanerozoic europe to 20 22 e whereas no direct connection to the cratonic lab or mld to the east is apparent at larger depths a positive conversion associated with the lower boundary of the asthenosphere is imaged at 210 250 km depth beneath phanerozoic europe continuing down to 300 km depth beneath the craton conversions from both 410 km and 660 km discontinuities are found at their nominal depth beneath phanerozoic europe and the discontinuity at 410 km depth can also be traced into the craton a potential negative conversion on top of the 410 km discontinuity found in migrated images is analyzed by modeling and attributed to interference with other converted phases

engineering geology ground surface movement and fissures induced by underground mining in the jinchuan nickel mine the jinchuan nickel mine is the largest underground mine with cut and fill mining in china in 1999 there appeared evident ground surface movement and many ground fissures in jinchuan no 2 mine following an introduction to the engineering geology and mechanical properties of the rock mass in the jinchuan mine areas this paper reveals the features of the measured in situ stresses and puts emphasis on an analysis of the mechanism of surface movement and ground fissures induced by the underground mining with backfill it is indicated that both the fault structures and the alteration contact zones are well developed as the dominant engineering geological characteristics in the jinchuan mine areas the stress redistribution in surrounding rocks resulting from the mining the low stiffness of the filling body the existence of the void space and the influence of on going mining activities is the main reason for the occurrence of surface movement and ground fissures

seismic receiver functions and the lithosphere asthenosphere boundary abstract the lower boundary of the lithospheric plates has remained as an enigmatic boundary for seismologists since it is relatively poorly observed by seismic means there is traditionally a broad consensus that the asthenosphere is observable as a low velocity zone by seismic surface waves seismic techniques which use shorter period p to s or s to p converted body waves are now far enough developed to be successful in observing such a low velocity zone with a higher resolution the principle of this technique the so called receiver function technique is that a strong teleseismic mother phase e g p s pp or sks incident from below on any seismic discontinuity beneath a station produces a converted phase ps or sp which indicates its depth and properties we discuss details of this technique a sufficient number of such observations exist already to indicate that the top of the low velocity zone is a globally observable discontinuity and it is sharper than previously thought an intriguing observation is that in some cratons the new seismic data indicate that the low velocity zone exists already at shallower depths than obtained from surface waves this confirms earlier results from controlled source observations thybo and perchuc 1997 we discuss possible interpretations of this shallow low velocity zone in cratonic regions

stable isotope fluid inclusion and mineral chemistry constraints on contamination and hydrothermal alteration in the uitkomst complex south africa abstract the uitkomst complex is a satellite intrusion of the bushveld complex and contains the only mine currently producing ni as the main product in south africa previously reported sulfur isotopic data suggest that externally derived sulfur was important for the generation of sulfide mineralization in harzburgite units that comprise the lower portion of the complex we have determined oxygen isotopic compositions of minerals and variations in pyroxenes and plagioclase compositions and analyzed fluid inclusions in quartz from highly altered zones in order to evaluate the importance of country rock assimilation and hydrothermal processes in the evolution of the complex oxygen isotopic compositions of pyroxene and olivine from the sulfide bearing harzburgite in the complex range from 5 2 to 5 9 and suggest that bulk assimilation of high 18 o country rocks by parental magmas did not occur or was minimal sulfur transfer to magma was accomplished via a fluid which did not perturb the oxygen isotope composition of the magma harzburgite pyroxenite and gabbronorite that overlie the ore bearing harzburgites contain minor amounts of sulfide minerals that are characterized by mantle like 34 s values of 0 2 low abundances of sulfides in these units are consistent with segregation of cotectic proportions hydrothermal alteration is pronounced in the upper and lower marginal zones of the complex secondary quartz and albite are common in the upper portion of the main gabbronorite unit measurements of fluid inclusion in quartz indicate that alteration took place at 500 c and involved a moderately saline fluid of 20 to 25 eq wt nacl oxygen and hydrogen isotopes of secondary silicate minerals are most consistent with the involvement of an evolved long path length meteoric water that infiltrated the upper and lower contacts zones of the complex

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

pb and sr isotope and origin of proterozoic base metal fluorite and gold deposits transvaal sequence south africa the early proterozoic carbonate chuniespoort and ghaap groups of the transvaal sequence in south africa host a number of mississippi valley and vein type pb zn f deposits of variable size and grade the chuniespoort group carbonates and overlying pretoria group shales in the eastern transvaal also host a number of hydrothermal au deposits which are peripheral to the bushveld complex field evidence indicates that there are a number of deposits intermediate between the mississippi valley and hydrothermal au types pb in sulfides of the pb zn deposits becomes more radiogenic to the north and east in the transvaal subbasin 206 pb 204 pb 14 53 16 93 and toward the south in the griqualand west subbasin 13 43 14 20 the spread in the 87 sr 86 sr ratios at 2 0 ga of the gangue minerals of the pb zn deposits ranges from 0 7153 to 0 7305 compared with 0 7092 for the host rocks these data collectively reflect systematic districtwide variations in the sr and pb isotope signatures the data also imply that the gangue minerals derived their sr isotope compositions from sources exterior to or distant from the local dolomite sedimentary package in the case of the pb zn deposits the isotopic data on the ore and gangue minerals are consistent with a basinwide tectonic and hydrothermal event at ca 2 0 ga which concentrated metals in structurally prepared sites in the host rocks migration of the mineralizing brines responsible for the pb zn f deposits was possibly a response to tectonic overpressure resulting from the middle proterozoic kheis orogeny at the western margin of the kaapvaal craton the oxidizing ground water required for deposition of the ores was possibly provided by the red bed sequences overlying the transvaal sequence rocks on the other hand the isotopic data of the au deposits indicate more local derivation of the mineralizing fluids these deposits are probably associated mainly with the hydrothermal circulation generated by the bushveld complex magmatism intruded during the kheis orogeny therefore these mississippi valley and vein type pb zn and au deposits may be linked in space and time

inversion of receiver functions without deconvolution application to the indian craton s u m m a r y receiver functions are a powerful tool to isolate and interpret receiver side structure effects in teleseismic seismic records they are easily constructed by deconvolving one component of a seismogram by another deconvolution is the inverse of convolution and hence can be mathematically viewed as an inverse problem it is a numerically unstable procedure that needs to be stabilized i e regularized this points to a recurring problem in geophysical imaging there is a trade off between variance and resolution where the user needs to arbitrarily define a level of compromise here we propose a novel misfit function for inversion of converted phases that avoids deconvolution in this way the choice of regularization parameters e g water level width of a low pass filter is avoided and statistics of data errors can be correctly accounted for we use this misfit measure to construct a likelihood probability function and carry out a transdimensional bayesian inversion for shear wave structure after illustrating the method with a synthetic test a real data application is shown where teleseismic signals recorded at hyb station hyderabad india and surface wave dispersion measurements are jointly inverted to provide a probabilistic 1 d seismic model beneath the station the results help address the debate on the thickness of the lithosphere in this region we show that the sharp negative velocity jump at 110 km that was previously interpreted as the lithosphere asthenosphere boundary lab is actually a mid lithospheric discontinuity the actual lab is seen deeper as a milder gradient between 150 and 200 km

contrasting lithospheric signatures across the western united states revealed by sp receiver functions abstract to explore how lithospheric structure varies between tectonic and magmatic terranes of the western united states we use observations of shear to compressional wave conversions across lithospheric velocity interfaces with a newly developed automatic method designed to take advantage of the earthscope transportable array and other broadband seismic arrays deployed across the region and an optimized deconvolution technique based on the extended time multi taper method we map variations in the depth to the seismic moho lithosphere asthenosphere boundary lab and mid lithospheric discontinuities mlds beneath areas that have undergone substantial extension in the neogene including the eastern great basin the southern basin and range and the southern rio grande rift we detect strong sp conversions across a relatively shallow 60 80 km seismically defined lab we detect sp conversions across an 80 90 km depth discontinuity beneath the high rockies which we interpret as the seismically defined lab beneath areas that have remained relatively undeformed including the wyoming craton and the great plains craton we detect sp conversions in the 90 140 km depth range that are substantially weaker and more distributed in depth than those beneath less stable regions beneath the colorado plateau sp ccp stacks show that lithospheric thinning has accompanied magmatic encroachment and the lab beneath the margins of the plateau is indistinguishable from the lab of the surrounding extensional provinces in contrast beneath central and northern colorado plateau weaker sp conversions are observed in the 90 140 km depth range the observed variations in lithospheric structure across structural blocks with different magmatic and deformational histories imply that 1 both mechanical thinning and magmatic and thermal erosion have likely contributed to the thin lithosphere observed in the basin and range and southern rio grande rift 2 at the margins of the colorado plateau and beneath the high rockies where extension has been relatively minor strong and shallow seismic labs indicate magmatic alteration of the lithosphere 3 mlds seen in comparatively thick stable lithosphere produce weaker sp conversions and are often more distributed in depth than seismically observed labs in regions of thin lithosphere 4 mlds and weak deep phases consistent with a seismic lab are sometimes observed together

paleomagnetism of a neoarchean paleoproterozoic carbonate ramp and carbonate platform succession transvaal supergroup from surface outcrop and drill core griqualand west region south africa detailed paleomagnetic study across the archean paleoproterozoic boundary interval preserved in the ghaap group transvaal supergroup of the kaapvaal craton reveals a complex series of viscous and thermo chemical magnetic overprints despite this complex history a potential primary single polarity remanence direction was revealed at high temperature demagnetization steps in about 10 of orientated drill core samples only this component declination 132