shear wave model of southern africa from regional rayleigh wave tomography with 2 d sensitivity kernels summary rayleigh wave phase velocities are computed at the period range of 20 167 s in southern africa from a two plane wave tpw tomography method using 2 d born approximation sensitivity kernels these phase velocities are compared with the results of the tpw method using gaussian sensitivity functions and the correlation between the two methods is good up to 100 s the correlation coefficient becomes less than 0 5 at periods greater than 111 s indicating significant finite frequency effects at long periods the correlation of phase velocity between adjacent frequencies increases with period for inversions using the 2 d sensitivity kernels while such a systematic trend is not observed for inversions using gaussian sensitivity functions in addition low phase velocity anomalies at long periods 111 167 s are better and more consistently imaged from inversions using 2 d sensitivity kernels to account for finite frequency effects and improve resolution the 2 d sensitivity kernels are therefore recommended in regional tpw tomography a 3 d shear wave model is developed from the phase velocities obtained in southern africa the new model confirms the existence of a fast mantle lid underlain by a low velocity zone revealed in a previous model constrained from the tpw method using gaussian sensitivity functions one new feature in this model is the vertical alignment of a shallow low velocity anomaly with a deep high velocity anomaly at the western bushveld province the alignment provides new evidence to support the interpretation of the slow anomaly as the result of high iron content from the bushveld intrusion and the fast as a more depleted residual mantle a low velocity channel at the depths of 220 310 km from the namaqua natal belt to the northwest of the kaapvaal craton is also imaged it suggests that the hot asthenosphere beneath the mobile belt can migrate into the craton area through a weak channel and produce thermal disturbance at the base of the craton further more low velocity anomalies from 100 to 140 km in the 3 d model agree well with the localities of kimberlites erupted at 65 104 ma in the kaapvaal craton this observation supports that kimberlites are generated in the lithosphere by thermal disturbance and provides additional evidence for the depth extent of mantle xenoliths

determining crustal structure beneath seismic stations overlying a low velocity sedimentary layer using receiver functions the receiver function rf technique has been widely applied to investigate crustal and mantle layered structures using p to s converted ps phases from velocity discontinuities however the presence of low velocity relative to that of the bedrock sediments can give rise to strong reverberations in the resulting rfs frequently masking the ps phases from crustal and mantle boundaries such reverberations are caused by p to s conversions and their multiples associated with the strong impedance contrast across the bottom of the low velocity sedimentary layer here we propose and test an approach to effectively remove the near surface reverberations and decipher the ps phases associated with the moho discontinuity autocorrelation is first applied on the observed rfs to determine the strength and two way traveltime of the reverberations which are then used to construct a resonance removal filter in the frequency domain to remove or significantly reduce the reverberations the filtered rfs are time corrected to eliminate the delay effects of the sedimentary layer and applied to estimate the subsediment crustal thickness and vp vsusing a h k stacking procedure the resulting subsediment crustal parameters thickness and vp vs are subsequently used to determine the thickness and vp vs of the sedimentary layer using a revised version of the h k stacking procedure testing using both synthetic and real data suggests that this computationally inexpensive technique is efficient in resolving subsediment crustal properties beneath stations sitting on a low velocity sedimentary layer and can also satisfactorily determine the thickness and vp vs of the sedimentary layer

cemented sands under static loading cemented sands are found in many areas of the world one of their distinguishing characteristics is their ability to stand in steep natural slopes large deposits are located along the california coast and in a number of areas intense urban development has occurred near the crest of high steep slopes because of the hazards posed by slope failures in the cemented sands a test program was undertaken to define the nature of the cementation and its effect on behavior of the soils a total of 137 laboratory compression and tension load tests were performed on undisturbed samples of naturally cemented sands and artificially prepared cemented sands the materials exhibited friction angles similar to uncemented sands but had a cementation strength intercept and a tensile strength the primary cementing agents of the sands tested were silicates and iron oxides the strength of the cemented sands were found to be a function of density amount of cementing agent grain shape and grain arrangement

the genesis of the nissi peatland northwestern greece as an example of peat and lignite deposit formation in greece abstract the nissi fen is located in a 12 km 2 intramontane basin in northwestern greece since the last glacial limnotelmatic and pure telmatic conditions controlled mainly by karstic springs and partly by surface waters favoured peat formation in the basin resulting in the accumulation of a peat deposit up to 15 m thick the present fen occupies a large area of almost 9 km 2 flora cover comprises mainly cyperaceae cladium mariscus and carex species while phragmites australis extend along the banks of a river flowing through the basin as well as around a lake in the southern part of the fen these species also contributed to the peat formation the nissi peatland shows many genetic similarities to the philippi peat deposit eastern macedonia and may be considered as a recent analogue to the lignite deposits in the basins of ptolemais western macedonia and megalopolis the peloponnese

crustal structure of the kaapvaal craton and its significance for early crustal evolution high quality seismic data obtained from a dense broadband array near kimberley south africa exhibit crustal reverberations of remarkable clarity that provide well resolved constraints on the structure of the lowermost crust and moho receiver function analysis of moho conversions and crustal multiples beneath the kimberley array shows that the crust is 35 km thick with an average poisson s ratio of 0 25 the density contrast across the moho is f15 indicating a crustal density about 2 86 gm cc just above the moho appropriate for felsic to intermediate rock compositions analysis of waveform broadening of the crustal reverberation phases suggests that the moho transition can be no more than 0 5 km thick and the total variation in crustal thickness over the 2400 km 2 footprint of the array no more than 1 km waveform and travel time analysis of a large earthquake triggered by deep gold mining operations the welkom mine event some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the kimberley array and the event p and s wave velocities for the lowermost crust are modeled to be 6 75 and 3 90 km s respectively with uppermost mantle velocities of 8 2 and 4 79 km s respectively seismograms from the welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the moho correlation between observed and synthetic waveforms and phase amplitudes of the moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the moho transition zone is everywhere less than about 2 km thick while the extremely flat moho sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton wide ventersdorp tectonomagmatic event near the end of archean time d 2003 elsevier b v all rights reserved

gravity derived crustal thickness model of botswana its implication for the mw 6 5 april 3 2017 botswana earthquake abstract botswana experienced a mw 6 5 earthquake on 3rd april 2017 the second largest earthquake event in botswana s recorded history this earthquake occurred within the limpopo shashe belt 350 km southeast of the seismically active okavango rift zone the region has no historical record of large magnitude earthquakes or active faults the occurrence of this earthquake was unexpected and underscores our limited understanding of the crustal configuration of botswana and highlight that neotectonic activity is not only confined to the okavango rift zone to address this knowledge gap we applied a regularized inversion algorithm to the bouguer gravity data to construct a high resolution crustal thickness map of botswana the produced crustal thickness map shows a thinner crust 35 40 km underlying the okavango rift zone and sedimentary basins whereas thicker crust 41 46 km underlies the cratonic regions and orogenic belts our results also show localized zone of relatively thinner crust 40 km one of which is located along the edge of the kaapvaal craton within the mw 6 5 botswana earthquake region based on our result we propose a mechanism of the botswana earthquake that integrates crustal thickness information with elevated heat flow as the result of the thermal fluid from east african rift system and extensional forces predicted by the local stress regime the epicentral region is therefore suggested to be a possible area of tectonic reactivation which is caused by multiple factors that could lead to future intraplate earthquakes in this region

crustal structure and dynamics of botswana we present the first nationwide crustal thickness and vp vs maps for botswana based on the analysis of new p wave receiver functions of nars botswana network integrated with previous receiver function results in botswana using h k analysis we found crustal thickness values ranging from 34 km for the okavango rift zone to 49 km at the border between the magondi belt and the zimbabwe craton for stations with significant sediment cover a sediment correction was applied based on sequential h k stacking we observed distinct differences for the kaapvaal craton the eastern part has a high vp vs ratio typical of a predominantly mafic composition suggesting lateral extension of the bushveld mafic complex on the other hand the western part with a vp vs ratio of 1 67 is felsic probably as a result of delamination caused by proterozoic rifting processes further to the west of the kaapvaal craton we found a crustal thickness of 42 km and a vp vs ratio of 1 76 for the nosop basin these values are similar to other cratonic regions in botswana suggesting the presence of a buried craton as proposed by previous studies we confirm a relatively thin crust 34 39 km compared to the rest of botswana and high vp vs ratio 1 84 underneath the okavango rift zone found by previous receiver function studies notably we also found a relatively thin crust 37 km and high vp vs ratio 1 84 in central botswana underneath the passarge basin

deep seismic profiling in the nosop basin botswana cratons mobile belts and sedimentary basins abstract deep seismic reflection data from the kalahari of western botswana has been interpreted in terms of the tectonics of southern africa the 600 km of 12 fold data exhibit good quality reflections in the range 5 15 s twt the data outline the nosop basin a depositional centre with greater than 10 km of palaeozoic and younger sediments five prominent reflecting horizons are traced in the basement underlying the sediments on the basis of the seismic data the nosop is now interpreted as a single basin the previous multi basin interpretation of the aeromagnetic data esults from features within the magnetic basement the uppermost two basement units are interpreted as proterozoic sediments that rest on archean crust the seismic data show that the crust appears to thicken to the southeast as the kaapvaal craton is approached at the same time the sedimentary cover progressively thins suggesting that the western edge of the kaapvaal is an extended margin interpreted deep crustal shear zones indicate a major terrane boundary separating the proterozoic from the archean depths to basement inferred from aeromagnetic data are consistent with the depth to the seismic boundary associated with the top of the archean some of the aeromagnetic expression may be due to karoo volcanic intrusives in the basement a thrust belt in the northwest of the area now buried in the mid crust is correlated with the nearby ghanzi chobe belt and allows the mid crustal layering to be dated as late proterozoic

a seismic refraction investigation of the archaean kaapvaal craton south africa using mine tremors as the energy source summary the structure of the central kaapvaal craton of southern africa has been investigated by deep seismic sounding using mine tremors as energy sources seismometers were deployed at approximately 10 km intervals on two profiles stretching between major mine tremor source regions mine tremors are rich in shear energy enabling joint interpretation of p and s waves and produce substantial energy at frequencies as low as 1 hz record sections are presented for both p and s waves and the traveltimes and amplitudes interpreted using 2 d ray tracing techniques synthetic seismograms computed for a 1 d velocity model by the reflectivity method compare well with the observed data a generalized seismic model of the kaapvaal craton has the following features supracrustal strata 0 10 km thick upper crystalline basement with p wave velocities of 6 0 6 2 km s the boundary between upper and lower crust at a depth of 14 18 km a lower crust with a relatively uniform seismic velocity in the range 6 4 6 7 kms and the crust mantle transition over 1 3 km with the moho at a depth of about 35 km the lower crust is found to be seismically attenuating and has a poisson s ratio of about 0 28 it is also known to be electrically conductive these observations are in accord with the presence of hydrated mantle rock at the base of the crust the velocity depth model of the kaapvaal craton is similar to models derived for other archaean cratons the proterozoic provinces adjacent to the kaapvaal craton are significantly thicker and have an intermediate to high velocity layer developed at the base of the crust this is interpreted to indicate a change in the process of crustal growth with basaltic underplating becoming more important since the archaean this change is attributed to a change in the composition of the upper mantle the higher temperatures in the archaean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted mantle unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation

shear wave structure in the grenville province beneath the lower great lakes region from rayleigh wave tomography 1 we have constructed a high resolution 3 d shear wave velocity model of the upper mantle beneath the grenville province in the lower great lakes region a two plane wave tomography method was applied to fundamental mode rayleigh wave data recorded at 35 broadband seismic stations rayleigh wave phase velocities were obtained at 15 periods from 20 s to 143 s and then inverted for shear wave velocity variation a high velocity lid to 150 km depth is imaged beneath most of the studied grenville province one low velocity anomaly of 4 appears at a failed rift in the northeast above 100 km depth coinciding with the great meteor hot spot track and the high seismicity band which can be interpreted as high temperature and high iron and or volatile content related to the hot spot activity low velocity anomaly of 2 at the depths of 100 200 km are found beneath the east of lake ontario and lake huron where the lithosphere is relatively thin these anomalies follow the nw se trend of the slow indentation imaged on the continental scale models we conclude that the location of low velocity anomalies is not only related to the hot spot track but also controlled by the pre existing lithosphere morphology and strength a high velocity column appears between southern lake ontario and the appalachian front centered at 100 km depth it is likely caused by high mg content at the formation of the lithosphere or the presence of ecologite or garnet phases generated during the opening and closure of the iapetus ocean

fine structure of the lowermost crust beneath the kaapvaal craton and its implications for crustal formation and evolution high quality data from a dense seismic array covering an area of approximately 60 40 km2 are used to obtain tight quantitative estimates of the fine scale velocity and density structure of the lowermost crust and the crust mantle boundary moho beneath the kaapvaal craton in the vicinity of kimberley south africa results based on a modified receiver function waveform analysis of moho conversions and crustal reverberation phases show that the crust beneath the array is thin 35 4 km with an average poisson s ratio of 0 254 the minimum s wave velocity contrast across the moho is 17 3 while the contrast in density is 15 4 the density contrast across the moho is particularly diagnostic for an assumed uppermost mantle density beneath kimberley of 3 3 gm cc as determined from mantle xenoliths the density of the lowermost crust is 2 86 gm cc indicating rocks of felsic to intermediate composition analysis of waveform broadening of the crustal reverberation phases relative to that of the direct p wave shows the thickness of the moho transition zone to be less than 0 5 km and the maximum variation in crustal thickness over the region of the array to be less than 1 km the flat and almost perfectly sharp moho together with the absence of a mafic lower crust suggests large scale crustal reworking in the period between crustal formation and the time of cratonic stabilization

prediction of engineering properties of a selected litharenite sandstone from its petrographic characteristics using correlation and multivariate statistical techniques abstract the determination of rock strength and elastic parameters in laboratory tests requires high quality core samples which are not always available from fractured and or weathered strata the purpose of this study is to apply a correlation analysis in conjunction with multivariate statistical techniques to petrographical and laboratory geomechanical data for the investigation of correlations that exist between these properties of a selected sandstone and to suggest inexpensive predictive models in a preliminary investigation sandstone samples from a borehole drilled for a preliminary investigation of a shaft location at a hard coal region black sea turkey were tested to determine uniaxial compressive strength elastic parameters unit weight porosity point load index and quality index secondly fifteen selected samples were subjected to petrographic examination which included fifteen parameters and modal analysis based on the statistical analysis results polynomial prediction equations were developed for estimating mechanical properties from petrographic characteristics and index properties the study revealed that the influence of textural characteristics appear to be more important than mineralogy for predicting engineering properties it is also noted that type of contacts grain shape and size rock fragments with packing density and proximity are the petrographic characteristics which have significant influence upon engineering properties of the sandstones results of this study were presented in the form of correlation coefficient models and predictive models

the petrology of greek brown coals abstract results are given in the petrography of greek coals collected from most of the major coal bearing basins in greece rank was determined by measuring reflectances on the maceral varieties eu ulminite a and eu ulminite b and on the maceral textinite reflectances obtained from these components indicate a coalification stage of brown coal for all samples within this group of samples there is however a fairly wide scatter of reflectance values indicating for some of them the transition zone from peat to brown coal and for others a coalification stage close to the transition into bituminous coals reflectances obtained from eu ulminite a and eu ulminite b were found to correlate well with chemical rank parameters such as volatile matter and calorific value composition was determined by maceral analysis the coals are in general characterized by low amounts of macerals of the inertinite group low to intermediate amounts of macerals of the liptinite group and high amounts of macerals of the huminite group within the latter group densinite attrinite eu ulminite and textinite make up the bulk of the samples typical macerals observed in the coals are illustrated by two black and white and three colour plates cluster analysis based on maceral distribution mineral matter and reflectance indicates that the samples studied can be divided into three major groups the first one is dominated by eu ulminite and densinite with relatively high reflectances the second is dominated by attrinite textinite and texto ulminite with somewhat lower reflectances the third is represented by a single sample in which textinite and resinite are the most abundant macerals this sample also has the lowest reflectance

terrestrial heat flow in the malawi rifted zone east africa implications for tectono thermal inheritance in continental rift basins abstract rift basins show the widest range of heat flow values compared to passive continental margins and intracratonic sag basins elevated heat flow is often used to infer asthenospheric upwelling or subsurface magmatic activity for magma assisted rifts despite the lack of surface volcanism for magma poor rifts it is possible that magmatic bodies will be present at lower and mid crustal levels but yet to breach the surface thus necessitating consideration of heat flow from tectono thermal inheritance in addition to more recent magmatism we use aeromagnetic data to investigate the terrestrial heat flow distribution in the malawi rifted zone mrz in east africa defined as the neogene malawi rift and surrounding permo triassic karoo rift basins we use the two dimensional power density spectrum technique to estimate the curie point depth cpd geothermal gradient and heat flow beneath the mrz our results reveal predominance of shallow cpds 18 20 km high geothermal gradients 29 32 c km and elevated heat flow 70 82 mw m 2 within the karoo rift basins along the malawi rift geothermal gradient 25 27 c km and heat flow 60 66 mw m 2 are generally lower except pronounced high heat flow 70 82 mw m 2 at the rungwe volcanic province rvp in the north and at the central rift segment where the rift overprints karoo rift basins the surrounding cratonic blocks show deeper cpds 24 27 km lower geothermal gradients 22 24 c km and lower heat flow 53 63 mw m 2 we interpret that the elevated heat flow in the karoo rift basins are related to residual heat flow from the permo triassic rifting phase replenished by later jurassic cretaceous diking events apart from the thermal anomaly beneath the rvp which is due to neogene magmatism other important targets for geothermal explorations in the mrz are the residual thermal anomalies in the karoo rift basins we infer that in areas of active magma poor rifting pronounced heat flow may not only occur in regions of asthenospheric upwelling but may concentrate in the ancient magmatic rift segments our findings have implications for crustal stretching tectono thermal inheritance and geothermal energy potentials in east africa and other segments of magma poor rifting

fast evolving conduits in clay bonded sandstone characterization erosion processes and significance for the origin of sandstone landforms abstract in strelec quarry the czech republic an underground conduit network 300 m long with a volume of 10 4 m 3 and a catchment of 7 km 2 developed over 5 years by groundwater flow in cretaceous marine quartz sandstone similar landforms at natural exposures conduits slot canyons undercuts are stabilized by case hardening and have stopped evolving the quarry offers a unique opportunity to study conduit evolution in sandstone at local to regional scales from the initial stage to maturity and to characterize the erosion processes which may form natural landforms prior to stabilization a new technique was developed to distinguish erodible and non erodible sandstone surfaces based on measurements of relative erodibility drilling resistance ambient and water saturated tensile strength ts at natural and quarry exposures three distinct kinds of surfaces were found 1 erodible sandstone exposed at 60 of surfaces in quarry this sandstone loses as much as 99 of ts when saturated 2 sub vertical fracture surfaces that are non erodible already prior to exposure at ground surface and which keep considerable ts if saturated 3 case hardened surfaces that start to form after exposure in favorable conditions they became non erodible and reach the full ts in just 6 years an increase in the hydraulic gradient from 0 005 to 0 02 triggered conduit evolution based on long term monitoring of water table in 18 wells and inflows to the quarry rapidly evolving major conduits are characterized by a channel gradient of 0 01 a flow velocity 40 cm s and sediment concentration 10 g l flow in openings with a discharge 1 ml s and hydraulic gradient 0 05 exceeds the erosion threshold and initiates piping in the first phase of conduit evolution fast concentrated flow mobilizes erodible sandstone between sets of parallel fractures in the shallow phreatic zone in the second phase the conduit opening mainly expands vertically upward into the vadose zone by mass wasting of undercut sandstone slabs mass wasting is responsible for 90 of mobilized sandstone sides of the mature conduits are protected by non erodible fracture surfaces natural landforms were probably formed very rapidly by overland flow piping and possibly fluidization during or at the end of the glacial periods when sandstone was not yet protected by case hardening

crustal and upper mantle shear wave velocity structure of botswana the 3 april 2017 central botswana earthquake linked to the east african rift system rayleigh wave group and phase velocity measurements obtained from ambient noise and earthquake data at 51 broadband stations were used to construct the first 3 d crustal and upper mantle shear wave velocity model of botswana the model shows low crustal velocities associated with the passarge and nosop sedimentary basins whereas the kaapvaal zimbabwe maltahohe and congo cratons are recognized by high mantle velocities the lowest upper mantle shear wave velocity beneath northeastern botswana is associated with the southwestern branch of the east african rift system this low velocity mantle anomaly appears to be linked to the crust of the okavango rift zone and the location of the 3 april 2017 mw 6 5 earthquake in central botswana we suggest that fluids or melt at the base of the crust from the southward continuation of the east african rift zone triggered the intraplate earthquake in an extensional tectonic setting

upper mantle p and s wave velocity structure of the kalahari craton and surrounding proterozoic terranes southern africa new broadband seismic data from botswana and south africa have been combined with existing data from the region to develop improved p and s wave velocity models for investigating the upper mantle structure of southern africa higher craton like velocities are imaged beneath the rehoboth province and parts of the northern okwa terrane and the magondi belt indicating that the northern edge of the greater kalahari craton lithosphere lies along the northern boundary of these terranes lower off craton velocities are imaged beneath the damara ghanzi chobe belt and may in part reflect thinning of the lithosphere beneath the incipient okavango rift lower velocities are also imaged to the north and northwest of the bushveld complex beneath parts of the okwa terrane magondi belt and limpopo belt indicating that cratonic upper mantle in some areas beneath these terranes may have been modified by the 2 05 ga bushveld and or 1 1 ga umkondo magmatic events

shear wave velocity structure of the southern african upper mantle with implications for the uplift of southern africa summary broad band seismic data from the southern african seismic experiment and the africaarray network are used to investigate the seismic velocity structure of the upper mantle beneath southern africa and in particular beneath the kaapvaal craton a two plane approximation method that includes a finite frequency sensitivity kernel is employed to measure rayleigh wave phase velocities which are inverted to obtain a quasi 3 d shear wave velocity model of the upper mantle we find phase velocities for the kaapvaal craton and surrounding mobile belts that are comparable to those reported by previous studies and we find little evidence for variation from east to west across the namaqua natal belt a region not well imaged in previous studies a high velocity upper mantle lid is found beneath the kaapvaal craton and most of southern africa for the kaapvaal craton the thickness of the lid 150 200 km is consistent with the lid thicknesses reported in many previous studies the cratonic lid is underlain by a 100 km thick low velocity zone with a 3 9 per cent maximum velocity reduction by comparing the velocity model to those published for other archean cratons we find few differences and therefore conclude that there is little evidence in the shear wave velocity structure of the mantle to indicate that the southern african plateau is supported by an upper mantle thermal anomaly

evidence for a compositional boundary within the lithospheric mantle beneath the kalahari craton from s receiver functions abstract s and p receiver functions from the southern african seismic experiment are analyzed for lithospheric discontinuities beneath the kalahari craton besides the moho the most prominent feature is a discontinuous reduction in seismic velocity of about 4 5 at approximately 150 km depth the discontinuity appears to have a width of about 10 km termed the k discontinuity this feature is restricted to the northern half of the array extending from the zimbabwe craton south to the tml thabazimbi murchison lineament and in the west from botswana to the edge of the kalahari craton in the east it spans several archean sutures and is thus unlikely related to archean tectonics it does however appear to be related to subsequent magmatic episodes the strongest anomaly is coincident with the most intense karoo volcanism and it extends to the northern edge of the bushveld intrusion from mantle xenoliths and xenocrysts the entire lithosphere in this region appears to have experienced a long term infiltration of basaltic melt and metasomatic fluids we propose that the k discontinuity reflects the influence of this melt metasomatic infiltration which has over time intruded and refertilized the lithosphere based on kimberlites pipes that show obvious signs of melt metasomatism and likely karoo influence the observed reduction in seismic velocity is plausibly consistent with the observed major element and volatile enrichment at 150 km depth in such kimberlites if this interpretation is correct then the high temperature kimberlite nodules that most clearly reflect this perturbation likely represent the general state of the lower lithosphere rather than only reflecting local mantle properties in the immediate vicinity of the kimberlite eruption

a damara orogen perspective on the assembly of southwestern gondwana the pan african damara orogenic system records gondwana amalgamation involving serial suturing of the congo sao francisco and ro o de la plata cratons north gondwana from 580 to 550 ma before amalgamation with the kalahari antarctic cratons south gondwana as part of the 530 ma kuunga damara orogeny closure of the adamastor ocean was diachronous from the aracuao belt southwards with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580 550 ma in the kaoko belt and at 545 530 ma in the gariep belt peak deformation metamorphism in the damara belt was at 530 500 ma with thrusting onto the kalahari craton from 495 ma through to 480 ma coupling of the congo and ro o de la plata cratons occurred before final closure of the mozambique and khomas damara belt oceans with the consequence that the kuunga suture extends into africa as the damara belt and the lufilian arc and zambezi belt of zambia palaeomagnetic data indicate that the gondwana cratonic components were in close proximity by c 550 ma so the last stages of the damara kuunga orogeny were intracratonic and led to eventual out stepping of deformation metamorphism to the ross delamerian orogen c 520 500 ma along the leading edge of the gondwana supercontinental margin

sequential h κ stacking to obtain accurate crustal thicknesses beneath sedimentary basins abstract low velocity sedimentary basins introduce error in many standard receiver function rf analysis techniques including common conversion point dueker and sheehan 1997 and crustal thickness v p v s ratio h stacking zhu and kanamori 2000 we describe a simple rf analysis method for obtaining accurate crustal thickness below seismic stations located in sedimentary basins the method extends the methods of zhu and kanamori 2000 it employs an iterative two layer depth v p v s stacking approach that first characterizes sediment properties thickness and v p v s allowing for the accurate interpretation of moho conversions without accounting for sedimentary layers standard rf analysis can mischaracterize crustal thickness based on ps phase delay by 10 km beneath deep basins we test the technique with synthetic seismograms and with data from us array transportable array ta stations from regions with sediment thicknesses that are well determined through other means we find sequential h stacking for sediment properties to be a simple technique that can benefit many rf analysis studies and can play an important role in crustal seismic studies in areas with thick or variable sediments

shear wave structure in the northeastern tibetan plateau from rayleigh wave tomography 1 the northeastern tibetan plateau has undergone active shortening due to the india asia collision and has been an ideal place for studying the growth of the plateau to reveal deep structure that is responsible for geological observations in ne tibet we have constructed a 3 d shear wave velocity model from rayleigh waves recorded by 36 stations in the northeast tibetan plateau seismic experiment first the two plane wave tomography method was applied to compute the average and 2 d variations of phase velocities in a period range of 20 to 100 s then 1 d and 3 d shear wave velocity models were developed from the obtained rayleigh wave dispersions the entire region is characterized by a weak upper mantle with a relatively thin lithosphere of 80 100 km thick this observation helps to explain the active deformation at the kunlun mountains and the qinling orogen in ne tibet relative low velocity anomalies appear along and to the south of the kunlun fault kf in contrast to high velocity anomalies in the north and northeast one pronounced slow anomaly centered at 120 km depth is imaged right beneath the restraining bend of the kunlun fault which might be a northern extension of the large scale slow anomaly to the south of the fault we interpret it as a local feature due to its large magnitude and coincidence with the restraining bend it is probably related to anomalously high temperature and associated partial melting resulted from localized asthenosphere upwelling after the delamination of a thick lithosphere root the formation of the thick lithosphere root would require coherent shortening in the crust and mantle lithosphere caused by local compression due to the bend of the strike slip fault indicating that the kunlun fault is probably on lithosphere scale and has played an important role in accommodating the eastward extrusion of the tibetan plateau

bentheimer sandstone revisited for experimental purposes abstract bentheimer sandstone outcrop samples are ideal for laboratory studies due to their lateral continuity and block scale homogeneous nature over the years they have been used to investigate reservoir topics ranging from passive and active properties of oil gas water rock interaction and processes to flow and transport this work shows an evaluation of bentheimer sandstone properties and their characteristics to advance understanding on the mineral accessory and the physical and electrical transport properties on the basis of the nature of depositional environments and diagenesis we measure and characterize spatial attributes of the matrix mainly by qualitative analysis laboratory and stereological measurements and statistical 2d 3d reconstructions the main contribution of this paper is the impact of mineral composition on the petrophysical quality and block scale homogeneity of the reservoir with 3d techniques reconstructions of the new rock grain framework pore structures have been created based on measurements a review of literature data and interpretation of variation between the outcrops and flow relevant parameters we conclude that bentheimer sandstone is a rock that shows constant mineralogy and is largely free of paramagnetic impurities in accordance with the results of 3d reconstructions techniques and in line with the depositional settings they show a well sorted grain framework and a pore network that can be used to calculate the permeability and resistivity without conducting any direct laboratory measurements of either parameter it presents a porosity with a range of 0 21 0 27 and permeabilities varying between 0 52 and 3 02 darcy based on our work and literature it is found that high permeability together with similar distribution of pore throats and bodies make the sandstone an attractive and easy obtainable candidate for comparative experimental studies moreover we state by comparing various techniques that the efficiency and resolution accuracy of the applied method must be taken into consideration when planning the measurements

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

shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains 1 the nature of the lower crust across the southern african shield has been investigated by jointly inverting receiver functions and rayleigh wave group velocities for 89 broadband seismic stations located in botswana south africa and zimbabwe for large parts of both archaean and proterozoic terrains the velocity models obtained from the inversions show shear wave velocities 4 0 km s below 20 30 km depth indicating a predominantly mafic lower crust however for much of the kimberley terrain and adjacent parts of the kheis province and witwatersrand terrain in south africa as well as for the western part of the tokwe terrain in zimbabwe shear wave velocities of 3 9 km s are found below 20 30 km depth indicating an intermediate to felsic lower crust the areas of intermediate to felsic lower crust in south africa coincide with regions where ventersdorp rocks have been preserved suggesting that the more evolved composition of the lower crust may have resulted from crustal reworking and extension during the ventersdorp tectonomagmatic event at c 2 7 ga citation kgaswane e m a a nyblade j julia p h g m dirks r j durrheim and m e pasyanos 2009 shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains j geophys res 114 b12304 doi 10 1029 2008jb006217

moho depth and crustal composition in southern africa abstract we present new results on structure thickness and composition of the crust in southern africa based on 6300 seismic receiver functions at 85 stations application of hk stacking to the entire sase dataset and use of multi frequency bands improve resolution substantially we observe a highly heterogeneous crustal structure with short wavelength variations in thickness h vp vs ratio composition and moho sharpness which defines 20 blocks that do not everywhere coincide with surface tectonic features in the zimbabwe craton the tokwe block has h 35 38 km and vp vs 1 74 1 79 whereas the thicker crust in the tati block h 47 51 km may be related to deformation of the archean crust along the cratonic margin two distinct crustal blocks with similar crustal thickness 42 46 km but significantly different vp vs ratios are recognized in the limpopo belt extreme values of 1 90 1 94 at the dyke swarms in eastern limpopo and 1 84 at the olifants river dyke swarm and easternmost bushveld intrusion complex bic indicate voluminous magmatic intrusions in the whole crust we find no evidence for magmatic intrusions in the central inferred part of bic where the crust is thick 45 50 km and vp vs is low 1 68 1 70 this thick crustal root may have deflected rising magmas to form the two bic lobes most of central kaapvaal has thin 35 40 km crust and vp vs 1 74 these characteristics are similar to the tokwe block in zimbabwe craton and may indicate delamination of pre existing lower crust which is further supported by a very sharp moho transition the exposed cross section in the vredefort impact crater is non representative of cratonic crust due to shallow moho 34 km and high vp vs 1 80 attributed to shock metamorphism high vp vs 1 76 is typical of the witwatersrand basin and anomalously low vp vs 1 66 1 67 marks the kaapvaal kheis namaqua transition highly heterogeneous crust both in thickness and vp vs ratio is typical of the namaqua natal and cape fold belts

on the geodynamic setting of kimberlite genesis the emplacement of kimberlites in the north american and african continents since the early palaeozoic appears to have occurred during periods of relatively slow motion of these continents the distribution of kimberlites in time may reflect the global pattern of convection which forces individual plates to move faster or slower at different times two dimensional numerical experiments on a convecting layer with a moving upper boundary show two different regimes in the first when the upper boundary velocity is high heat is transferred by the large scale circulation and in the second when the upper boundary velocity is lower heat is predominantly transferred by thermal plumes rising from the lower boundary layer for a reasonable mantle solidus this second regime can give rise to partial melting beneath the moving plate far from the plate boundaries the transition between these modes takes place over a small range of plate velocities for a rayleigh number of 106 it occurs around 20 mm yr 1 we suggest that the generation of kimberlite magmas may result from thermal plumes incident on the base of a slowly moving plate

new seismic model of the upper mantle beneath africa we present a seismic model of the upper 400 km of the mantle beneath africa and surrounding regions this model is constructed by inverse modeling of fundamental mode rayleigh wave phase velocities 40 200 s for about 8000 propagation paths among the most pronounced anomalies are high shear velocity structures as much as 6 higher than in the preliminary reference earth model beneath the west african congo and kalahari cratons that extend to about 250 km depth these structures have near vertical margins across which the shear velocity changes by as much as 6 over 500 km distance anomalous low shear velocities 3 4 lower than in the preliminary reference earth model structures are observed beneath the east african red sea and gulf of aden rifts and beneath the northwestern indian ocean these structures extend to a depth of at least 250 km our model cannot be reconciled with models that invoke a large number of plumes that have impinged on the base of the lithosphere nor does our seismic model indicate that high temperature low density material beneath the lithosphere is responsible for the uplift of southern africa

velocity structure beneath the southern puna plateau evidence for delamination 1 the high elevation of the southern puna plateau the widespread melting of its crust the gap in intermediate depth seismicity and the recent eruptions of ignimbrite complexes can be explained by delamination of the lithospheric mantle beneath it to test this hypothesis an array consisting of 73 broad band and short period seismic stations was deployed in the region for a period of 2 years starting in 2007 we inverted the data using the two plane wave approach and obtained 1 d and 3 d rayleigh wave phase velocities our dispersion curve shows that at short periods 70 s the phase velocities are slightly higher than those of the tibetan plateau and lower than those of the anatolian plateau at periods of 100 140 s we observe a low velocity zone that might be remnant hot asthenosphere below a flat slab 7 10 ma we estimate the average continental lithosphere thickness for the region to be between 100 and 130 km our three dimensional rayleigh wave phase velocities show a high velocity anomaly at low frequencies 0 007 0 008 and 0 009 hz slightly to the north of cerro galan this would be consistent with the hypothesis of delamination in which a piece of lithosphere has detached and caused upwelling of hot asthenosphere which in turn caused widespread alkaline collision related volcanism this interpretation is also corroborated by our shear wave velocity model where a high velocity anomaly beneath the northern edge of cerro galan at 130 km depth is interpreted as the delaminated block on top of the subducting nazca slab

the petrographic and geomechanical properties of some sandstones from the newspaper member of the natal group near durban south africa abstract sandstones are by far the most important rock type in the natal group which is mainly a terrestrial succession formed during ordovician silurian times the most notable member of the group is the newspaper member from which sample material was obtained most of the material sampled was fine to medium grained arkosic sandstone they are well cemented with generally low porosity accordingly sandstones of the newspaper member tend to be very strong and of low deformability few of the petrological properties of these sandstones have a notable influence on the mechanical properties however increasing clay content tends to lower the unconfined compressive strength whilst increasing quartz content tends to enhance it on the other hand many of the mechanical properties are interrelated significant or highly significant relationships occur amongst the strength parameters and between hardness and strength young s modulus also has significant relationships with strength and hardness another property which has a significant influence on strength and deformability is porosity

upper mantle s wave velocity structure of the east anatolian caucasus region geodynamic processes occurring in the upper mantle such as slab break off and lithosphere delamination often result in high rates of lithospheric deformation and rapid tectonic uplift of large areas the continent continent collision zone between arabia and eurasia has been widely studied in this context but several different viable geodynamic models exist to explain the uplift and deformation of the anatolian plateau and the caucasus mountains we have imaged the uppermost mantle shear wave velocity structure of the east anatolian caucasus region using surface wave tomography to better understand the regional tectonic activity since the onset of the collision between the arabian and eurasian plates furthermore we used our tomographic models to better understand the processes which are responsible for the formation of the 2 km high plateau and the widespread volcanism in eastern turkey as well as reactivation of deformation and deep seismicity in the eastern greater caucasus our model of regional upper mantle shear wave velocity structure supports subduction of the northern and southern branches of neo tethys lithosphere between eurasia and gondwana and suggests a possible underthrusting of the kura basin lithosphere beneath the greater caucasus

seismic evidence of on going sublithosphere upper mantle convection for intra plate volcanism in northeast china abstract a 3 d crustal and upper mantle s wave velocity model of ne china is constructed by inversion of phase velocity dispersion curves at 6 140 s periods from ambient noise tomography and two plane surface wave tomography the seismic data used in this study are collected from 120 china earthquake administration cea permanent stations and 127 portable stations of necessarray we observe strong low s wave velocity beneath the changbaishan volcano in the upper mantle to at least 200 km depth which is interpreted as a mantle upwelling beneath the changbaishan volcano that is consistent with the body wave tomographic image the songliao basin is dominated by a high velocity extending to at least 200 km depth built upon the observed velocity anomalies we propose a sub lithosphere mantle convection model for ne china in which the upwelling of upper mantle materials from the mantle transition zone to the changbaishan volcano could induce a local sub lithosphere convection in the upper mantle and the strong high velocity of the upper mantle beneath the songliao basin corresponds to the downwelling limb of this convection cell the downwelling beneath the songliao basin could also induce secondary local convection in the asthenosphere to the west leading to local asthenospheric upwelling beneath the abaga and halaha volcanoes in the xing an mongolia orogenic belt

textural characterisation of rocks textural characteristics are a major factor in determining the mechanical behaviour of rocks and in the prediction of performance of rock cutting and drilling equipment the principal textural characteristics of rocks are grain size grain shape grain orientation relative proportion of grains and matrix material which were herein quantitatively measured using a modern image analysis system these features resulted in a texture coefficient represented by a single number for each rock specimen in this study a range of both textural and mechanical data for a range of rock types are given with some of the textural determination methodology correlation between textural and physical properties are also highlighted the results of drilling tests using polycrystalline diamond compact pin and hybrid and impregnated diamond core bits in the rocks are presented which demonstrate the influence of rock texture on drillability the rock texture can be used as a predictive factor for assessing the drillability and cuttability mechanical and wear performance of rocks

shear velocity structure and azimuthal anisotropy beneath eastern north america from rayleigh wave inversion 1 we have obtained shear velocity structure beneath the northeastern united states and southeastern canada using rayleigh wave phases and amplitudes thin crust 36 42 km is observed along the atlantic coast and in the eastern appalachian orogen and thick crust 42 46 km is imaged in the western appalachians and in the western new york portion of the grenville province the variation of crustal thickness correlates well with the observed bouguer gravity anomalies in the upper mantle the high velocity continental keel of cratonic north america is present in the western part of the study area while a broad low velocity region is imaged in new england from the hudson river valley to the white mountains this low velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel in addition to lateral variations in velocity we estimate the azimuthal dependence of phase velocity strong and relatively uniform shear wave splitting is observed in the study region but at periods of 100 s or less the average azimuthal anisotropy of rayleigh waves is less than 1 and is not significantly different from zero at any individual period this small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere much of the source of the shear wave splitting must lie deeper than 200 km

crustal and uppermost mantle structure in southern africa revealed from ambient noise and teleseismic tomography summary rayleigh wave phase velocity maps in southern africa are obtained at periods from 6 to 40 s using seismic ambient noise tomography applied to data from the southern africa seismic experiment sase deployed between 1997 and 1999 these phase velocity maps are combined with those from 45 to 143 s period which were determined previously using a two planewave method by li burke in the period range of overlap 25 40 s the ambient noise and two plane wave methods yield similar phase velocity maps dispersion curves from 6 to 143 s period were used to estimate the 3 d shear wave structure of the crust and uppermost mantle on an 1 1 grid beneath southern africa to a depth of about 100 km average shear wave velocity in the crust is found to vary from 3 6 km s 1 at 0 10 km depths to 3 86 km s 1 from 20 to 40 km and velocity anomalies in these layers correlate with known tectonic features shear wave velocity in the lower crust is on average low in the kaapvaal and zimbabwe cratons and higher in the surrounding proterozoic terranes such as the limpopo and the namaquanatal belts which suggests that the lower crust underlying the archean cratons is probably less mafic than beneath the proterozoic terranes crustal thickness estimates agree well with a previous receiver function study of nair et al archean crust is relatively thin and light and underlain by a fast uppermost mantle whereas the proterozoic crust is thick and dense with a slower underlying mantle these observations are consistent with the southern african archean cratons having been formed by the accretion of island arcs with the convective removal of the dense lower crust if the foundering process became less vigorous in arc environments during the proterozoic

an updated global review of solutional weathering processes and forms in quartz sandstones and quartzites abstract quartz is considered one of the less soluble minerals of the earth s crust and thus hardly affected by chemical weathering despite this for more than forty years it is clear that the formation of caves and peculiar solutional weathering dominated landforms in quartz rich lithologies is common and shares several similarities to the well known karstic ones in carbonate rocks in the last thirty years great strides have been made in furthering our knowledge of the distribution of these forms around the world and the geochemical processes involved these studies have clearly shown that solutional weathering is a fundamental process acting through intergranular dissolution of quartz increasing the rock porosity and decreasing the rock strength to erosion this process has been described in the concepts of arenization and phantomization and the widespread evidences of the fundamental role of quartz solution in landform genesis has even developed to the extent of several geomorphologists reassessing the definition of the term karst and its application to these peculiar lithologies nonetheless the process is complicated by several factors related both to environmental conditions water chemistry and availability as well as to the compositional and textural characters of the lithology presence of clays iron hydroxides carbonate cement etc all these aspects have to be taken carefully in consideration in order to understand if solution is a dominant or accessory process in the landscape evolution in this review the state of knowledge on the relevant chemical processes weathering mechanisms and speleogenesisk involved in the surface and underground karstification and clear examples of quartz solution and solutional landforms from different world locations are outlined and discussed

a geological approach toward developing a mudrock durability classification system relationships between durability and lithologic characteristics of 61 mudrock samples from north america were investigated with the objective of developing a mudrock durability classification based

converted waves reveal a thick and layered tectosphere beneath the kalahari super craton abstract thick and high velocity roots are generally observed beneath the archean cratons inside these high velocity keels velocity contrasts are detected neither by surface nor by body waves tomographies we present here evidences based on the s to p and p to s converted waves for the existence of an irregularly stratified and thick keel beneath the kalahari super craton to satisfy surface wave data and s to p conversions the velocity model should have beneath the moho a 160 km thick anisotropic structure with vertical slow axis and decreasing anisotropic parameters with depth such a structure may stem from the preferred orientation of olivine 100 axis in the horizontal plane under shearing deformation this structure together with the 100 km thick layer underlying it forms a 300 km thick continental root beneath the cratons inside this root the p and s velocities should be higher on average respectively by an amount of 6 and 4 than the ak135 velocity model beneath 300 350 km depth a low velocity zone is clearly detected that may be either the remainder of large magma reservoirs related to cratonic flood basalts or a melted silicate layer created by the transformation just above the 410 km deep discontinuity of wadsleyite assembly to olivine assembly

physical and mechanical properties of the repaired sandstone ashlars in the facing masonry of the charles bridge in prague czech republic and an analytical study for the causes of its rapid decay the facing masonry of the gothic charles bridge in prague czech republic has been largely altered during nineteenth and twentieth century repairs due to extensive replacements of weathered sandstone ashlar blocks natural stone varieties used during these replacements show different responses to a variety of weathering processes which lead to their rapid decay the decay of the newer stone has been accelerated by use of portland cement paste as the binding and fill material instead of the original hydraulic lime based and cocciopesto type mortars the hardened portland cement paste makes an almost impermeable barrier with permeability three to four orders of magnitude lower compared to the original mortars and natural stone the low permeability of this new alien material results in the accumulation of water soluble salts specifically nitrates but also sulphates and chlorides in the facing masonry ashlars with their crystallisation coming in the form of both sub and efflorescence as well as the development of various decay forms blistering granular disintegration and or scaling and flaking the source of the water soluble salts can be traced back to previous restoration attempts these included chemical cleaning black crust removal by nitric acid ammonium hydroxide ammonium water or ammonium hydrogen carbonate all which can be linked to high nitrate content in the efflorescence

radial anisotropy beneath northeast tibet implications for lithosphere deformation at a restraining bend in the kunlun fault and its vicinity three dimensional shear wave velocity and radial anisotropy models of the crust and upper mantle beneath the ne tibetan plateau are constructed from new measurements of love wave dispersions 20 77s and previously obtained rayleigh wave dispersions 20 87s using a two plane wave method the mid lower crust is characterized with positive anisotropy vsh vsv with large strength beneath the qinling and qilian mountains and small values beneath the anyemaqen mountain the large positive anisotropy can be explained by horizontal alignment of anisotropic minerals in the mid lower crust due to crustal flow the mantle lithosphere above 90 km is largely isotropic while weak positive anisotropy appears beneath 90 km which probably marks the lithosphere asthenosphere boundary lab a low shear wave velocity anomaly and relatively negative radial anisotropy are imaged in the entire lithosphere beneath the restraining bend in the eastern kunlun fault consistent with a weak lithosphere experiencing vertical thickening under horizontal compression the asthenosphere at the restraining bend is characterized by significant low velocity and positive radial anisotropy reflecting that the asthenosphere here is probably hotter has more melts and deforms more easily than the surrounding region we propose that the lithosphere at the restraining bend was vertically thickened and subsequently delaminated locally and induced asthenosphere upwelling this model explains the observations of velocity and anisotropy anomalies in the lithosphere and asthenosphere as well as geological observations of rapid rock uplift at the restraining bend of the kunlun fault

geophysical evidence for crustal and mantle weak zones controlling intra plate seismicity the 2017 botswana earthquake sequence abstract large earthquakes away from plate boundaries pose a significant threat to human lives and infrastructure but such events typically occur on previously unknown faults most cases of intra plate seismicity result from compression related to far field plate boundary stresses the april 2017 mw 6 5 earthquake in central botswana and subsequent events occurred in a region with no previously known large earthquakes occurred away from major present day tectonic activity and accommodate extension rather than compression here we present results from an integrated geophysical study that suggests the recent earthquakes may be a sign of future activity controlled by the collocation of a weak upper mantle and weak crustal structure between otherwise strong precambrian blocks magnetotelluric data highlights proterozoic continent accretion structure within the region and shows that recent extension and seismicity occurred along ancient thrust faults within the crust our seismic velocity and resistivity models suggest a weak zone in the uppermost mantle that does not persist to greater depths and is therefore unlikely to represent mantle upwelling the botswana events may therefore be indicative of top down extension as a response to large scale extensional forces

quantitative study of a rapidly weathering overhang developed in an artificially wetted sandstone cliff those factors controlling the weathering and erosion of sandstone on the field scale are still not well understood in this study a specific sandstone overhang and its surroundings with artificially induced and extremely high erosion rates was subjected to a complex investigation contrast between the erosion rate of the wet and dry portions of the same cliff enabled isolation of the factors responsible for rapid sandstone retreat erosion rates moisture and salt content as well as suction were monitored in the field mineral phases and water chemistry were analyzed the measurement of tensile strength laboratory frost weathering tests and numerical modeling of stress were performed the acquired data show that an increase of moisture content in pores in the area of the studied overhang decreased tensile strength of the sandstone to 14 of its dry value and increases the sandstone weathering and erosion rate by nearly 4 orders of magnitude compared to the same sandstone under natural moisture condition outside on the cliff area consequently frost weathering in combination with wetting weakening was found to play a major role in weathering erosion of the sandstone cliff and overhang frost weathering rate in both the laboratory and field increases up to 15 times with decreasing gravity induced stress the results also indicate that sandstone landforms in temperate climates may potentially develop very rapidly if the pore space is nearly saturated with water and will later remain relatively stable when the moisture content decreases as a general implication it is suggested that overhangs in central europe and elsewhere might be the result of rapid frost weathering of nearly saturated sandstone during the last glacial this article is protected by copyright all rights reserved

studies of the low rank greek coals 1 classification abstract physical and chemical measurements of greek coals lead to a classification by rank into five categories ranging from subbituminous c to peat this classification is further documented by esr spectroscopy and powder x ray diffraction in order to accommodate all the deposits a new category is introduced between lignite b and peat designated as lignitized peat which is clearly and operationally distinguished from peat and higher rank lignites

crustal structure beneath southern africa and its implications for the formation and evolution of the kaapvaal and zimbabwe cratons the formation of archean crust appears to in volve processes unique to early earth history initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern africa re vealsignicantdierences inthenatureof thecrustandthe crust mantle boundary between archean and post archean geologic terranes with the notable exception of the colli sional limpopo belt where the crust is thick and the moho complex the crust beneath undisturbed archean craton is typically thin 35 40 km unlayered and characterized byastrongvelocitycontrastacrossarelativelysharpmoho thiscrustalstructurecontrastsmarkedlywiththatbeneath post archeanterranesandbeneatharcheanregionsaected bylarge scaleproterozoicevents thebushveldcomplexand the okwa magondi belts where the crust tends to be rel atively thick 45 50 km and the moho is complex

upper mantle structure beneath southern african cratons from seismic finite frequency p and s body wave tomography abstract we present a 3d high resolution seismic model of the southern african cratonic region from teleseismic tomographic inversion of the p and s body wave dataset recorded by the southern african seismic experiment sase utilizing 3d sensitivity kernels we invert traveltime residuals of teleseismic body waves to calculate velocity anomalies in the upper mantle down to a 700 km depth with respect to the ak135 reference model various resolution tests allow evaluation of the extent of smearing effects and help defining the optimum inversion parameters i e damping and smoothness for regularizing the inversion calculations the fast lithospheric keels of the kaapvaal and zimbabwe cratons reach depths of 300 350 km and 200 250 km respectively the paleo orogenic limpopo belt is represented by negative velocity perturbations down to a depth of 250 km implying the presence of chemically fertile material with anomalously low wave speeds the bushveld complex has low velocity down to 150 km which is attributed to chemical modification of the cratonic mantle in the present model the finite frequency sensitivity kernels allow to resolve relatively small scale anomalies such as the colesberg magnetic lineament in the suture zone between the eastern and western blocks of the kaapvaal craton and a small northern block of the kaapvaal craton located between the limpopo belt and the bushveld complex

chemical weathering mechanisms and their effects on engineering properties of soft sandstone and conglomerate cemented by zeolite in a mountainous area abstract sandstone and conglomerate cemented by zeolite crystals in a humid mountainous area in japan are weathered and also weakened or strengthened by chemical interaction with percolating water that was originally in equilibrium with the atmosphere near the ground surface carbonic acid generated by the dissolution of carbon dioxide from the atmosphere and biological activity in the soil dissolves zeolite and thereby weakens the rocks at the oxidation front the base of the oxidized zone oxygen from the atmosphere reacts with pyrite in the rock to form sulfuric acid and iron oxide and or hydroxide the iron oxide and or hydroxide precipitates on zeolite surfaces and strengthens the rocks by cementing the grains to each other beneath the oxidation front and extending to the dissolution front sulfuric acid migrates downward dissolves zeolite and thus weakens the rocks this weathering mechanism and the resulting changes in mechanical properties of rocks generally are to be expected in marine sandstone and conglomerate with small amounts of clay in the matrix because the most common cementing mineral is carbonate which is also easily dissolved by acid and further because pyrite is a common rock forming mineral in marine sedimentary rocks the effects of chemical weathering on the mechanical properties of sandstone and conglomerate are different from those of mudstone because the latter contains much larger amounts of clay minerals the cementation of constituent grains by iron oxide and or hydroxide is inhibited by clay minerals because clay minerals have high specific surface areas and also because large amounts of iron can be incorporated in the minerals

geotechnical properties of cemented sands in steep slopes an investigation into the geotechnical properties specific to assessing the stability of weakly and moderately cemented sand cliffs is presented a case study from eroding coastal cliffs located in central california provides both the data and impetus for this study herein weakly cemented sand is defined as having an unconfined compressive strength ucs of less than 100 kpa and moderately cemented sand is defined as having ucs between 100 and 400 kpa testing shows that both materials fail in a brittle fashion and can be modeled effectively using linear mohr coulomb strength parameters although for weakly cemented sands curvature of the failure envelope is more evident with decreasing friction and increasing cohesion at higher confinement triaxial tests performed to simulate the evolving stress state of an eroding cliff using a reduction in confinement type stress path result in an order of magnitude decrease in strain at failure and a more brittle response tests aimed at examining the influence of wetting on steep slopes show that a 60 decrease in ucs a 50 drop in cohesion and 80 decrease in the tensile strength occurs in moderately cemented sand upon introduction to water in weakly cemented sands all compressive cohesive and tensile strength is lost upon wetting and saturation the results indicate that particular attention must be given to the relative level of cementation the effects of groundwater or surficial seepage and the small scale strain response when performing geotechnical slope stability analyses on these materials

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

sandstone landforms shaped by negative feedback between stress and erosion the formation and preservation of sandstone landforms such as pillars and arches is enigmatic experiments and numerical modelling show that load bearing material weathers more slowly and thus the internal stress field can shape and stabilize sandstone landforms

azimuthal anisotropy beneath southern africa from very broad band surface wave dispersion measurements summary seismic anisotropy within the lithosphere of cratons preserves an important record of their ancient assembly in southern africa anisotropy across the archean kaapvaal craton and limpopo belt has been detected previously by observations of sks wave splitting because sks splitting measurements lack vertical resolution however the depth distribution of anisotropy has remained uncertain end member interpretations invoked the dominance of either anisotropy in the lithosphere due to the fabric formed by deformation in archean or palaeoproterozoic orogenies or that in the asthenosphere due to the fabric formed by the recent plate motion each with significant geodynamic implications to determine the distribution of anisotropy with depth we measured phase velocities of seismic surface waves between stations of the southern african seismic experiment we applied two complementary measurement approaches very broad band cross correlation and multimode waveform inversion robust rayleigh and love wave dispersion curves were derived for four different subregions of the archean southern africa in a period range from 5 s to 250 400 s rayleigh and 5 s to 100 250 s love depending on the region rayleigh wave anisotropy was determined in each region at periods from 5 s to 150 200 s sampling from the upper crust down to the asthenosphere the jackknife method was used to estimate uncertainties and the f test to verify the statistical significance of anisotropy we detected strong anisotropy with a n s fast propagation azimuth in the upper crust of the limpopo belt we attribute it to aligned cracks formed by the regional e w extensional stress associated with the southward propagation of the east african rift our results show that it is possible to estimate regional stress from short period surface wave anisotropy measured in this study using broad band array recordings of teleseismic surface waves rayleigh wave anisotropy at 70 120 s periods shows that the fabric within the deep mantle lithosphere of the limpopo belt and northern kaapvaal craton is aligned parallel to the archean palaeoproterozoic sutures at block boundaries this confirms that the fabric within the lithosphere created by pervasive ancient deformation is preserved to this day suture parallel fabric is absent however in the deep lithosphere of the western kaapvaal craton suggesting that it was not reworked in the collision with the craton s core either due to its mechanical strength or because the deformation mechanism was different from those that operated in the north anisotropy at periods greater than 120 130 s shows fast directions parallel to the plate motion and indicates shear wave anisotropy in the asthenosphere the depth distribution of anisotropy revealed by surface wave measurements comprises elements of both end member models proposed previously anisotropy in the asthenosphere shows fast propagation directions parallel to the plate motion anisotropy in the limpopo and northern kaapvaal lithosphere shows fast directions parallel to the archean palaeoproterozoic sutures the distribution of sks splitting orientations across southern africa reflects anisotropic fabric both within the lithosphere dominating the splitting beneath the limpopo belt and northern kaapvaal craton and within the asthenosphere dominating beneath the western kaapvaal craton

wetting weakening of tertiary sandstones microscopic mechanism the micromechanism accounting for wetting weakening of tertiary sandstones was studied it was found that intragranular fracture prevails for all dry sandstones however when the sandstone is wet intergranular fracture occurs for type b sandstones therefore one sandstone from type a sandstones ms1 and another from type b tk were selected to further investigate the nature of the matrix it was found that 1 for both sandstones the major mineral components of the matrix are illite and kaolinite except that the ms1 sandstone has more chlorite 2 leaching of matrix induced an increase of porosity and consequently results in leaching softening and 3 among the mineral composition chlorite is easiest to be dissolved and leached out and induces a more significant increase of porosity which in turn results in a more significant leaching softening

a joint receiver function and gravity study of crustal structure beneath the incipient okavango rift botswana rifting incorporates the fundamental processes concerning the breakup of continental lithosphere and plays a significant role in the formation and evolution of sedimentary basins in order to decipher the characteristics of rifting at its earliest stage we conduct the first teleseismic crustal study of one of the world s youngest continental rifts the okavango rift zone orz where the magma has not yet breached the surface results from receiver function stacking and gravity modeling indicate that the crust mantle boundary beneath the orz is uplifted by 4 5 km and the initiation of the orz is closely related to lithospheric stretching possible decompression melting of the subcrustal lithosphere occurs beneath the orz as evidenced by a relatively low upper mantle density based on the gravity modeling

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

lithospheric boundaries and upper mantle structure beneath southern africa imaged by p and s wave velocity models we report new p and s wave velocity models of the upper mantle beneath southern africa using data recorded on seismic stations spanning the entire subcontinent beneath most of the damara belt including the okavango rift our models show lower than average velocities 0 8 vp 1 2 vs with an abrupt increase in velocities along the terrane s southern margin we attribute the lower than average velocities to thinner lithosphere 130 km thick compared to thicker lithosphere 200 km thick immediately to the south under the kalahari craton beneath the etendeka flood basalt province higher than average velocities 0 25 vp 0 75 vs indicate thicker and or compositionally distinct lithosphere compared to other parts of the damara belt in the rehoboth province higher than average velocities 0 3 vp 0 5 vs suggest the presence of a microcraton as do higher than average velocities 1 0 vp 1 5 vs under the southern irumide belt lower than average velocities 0 4 vp 0 7 vs beneath the bushveld complex and parts of the mgondi and okwa terranes are consistent with previous studies which attributed them to compositionally modified lithosphere resulting from precambrian magmatic events there is little evidence for thermally modified upper mantle beneath any of these terranes which could provide a source of uplift for the southern african plateau in contrast beneath parts of the irumide belt in southern and central zambia and the mozambique belt in central mozambique deep seated low velocity anomalies 0 7 vp 0 8 vs can be attributed to upper mantle extensions of the african superplume structure

influence of petrographic parameters on geotechnical properties of tertiary sandstones from taiwan while tunneling through tertiary sandstones in taiwan tunnel squeezing occurred in some of the sandstones e g mucha tunnel and chungho tunnel in northern taiwan but not in all of them this phenomenon indicates that the geotechnical properties of tertiary sandstones differ from one sandstone to another and further characterization is of interest laboratory experiments were conducted to explore the geotechnical characteristics including strength deformational behavior and wetting softening phenomena of more than 13 tertiary sandstones from northern taiwan it was found that these sandstones differ from hard rocks in having significant shear dilation and wetting softening behavior which is defined as the reduction of both strength and stiffness of dry sandstone due to wetting such distinct behavior can occur even when the sandstones have medium to moderate strength based on the degree of wetting softening behavior as well as the deformational behavior two types of sandstones types a and b were identified relative to type a sandstone the type b sandstone is characterized by more deformation shear dilation and more significant wetting softening in addition how the microscopic parameters or petrographic parameters affect the macroscopic mechanical behavior including the deformational behavior uniaxial compressive strength ucs and strength reduction r due to wetting was studied by petrographic analysis among the petrographic and physical parameters evaluated the grain area ratio gar and porosity are found to be the key parameters as a result empirical relations of ucs and strength reduction r due to wetting expressed in terms of porosity and grain area ratio respectively are proposed comparisons were made to sandstones worldwide from published data and it is found that these two empirical relations are also applicable to other sandstones finally a classification method based on ucs and r on the one hand and the petrographic parameters grain area ratio and porosity on the other is proposed for tertiary sandstones to indicate the geotechnical characteristics of sandstones this proposed classification method can also be expressed in terms of grain matrix porosity content of sandstones it is also found that porosity has more influence on the ucs than grain and matrix content does d 2004 elsevier b v all rights reserved

irregular silica cementation in sandstones and its implication on the usability as building stone abstract palaeocene sandstones outcropping in western spain have long been used for building purposes their use varies considerably in some cities they have been the main stones used in the construction of buildings such as churches and cathedrals zamora whereas in others such as in the historic district of salamanca which has been a world heritage site since 1988 they have been used in the lower part of the town s most striking monuments two cathedrals 12th and 16th centuries the main square 1755 and the house of shells 15th century for protection against erosion and weathering because they are more resistant than the eocene sandstones used for the higher parts of the monuments although in some cases this solution has been successful in others the results have been less satisfactory palaeocene sandstones have an important although irregular silica cement content according to their uniaxial compression strength ucs they may be strong or very strong rocks but under wet conditions some select samples display a significant decrease in their ucs furthermore two types of opal cement can be found one isotropic and the other birefringent the former being the most efficient strength is even higher when isotropic opal cement is found together with alunite kal3 so4 2 oh 6

finite frequency effects in global surface wave tomography summary we compare traditional ray theoretical surface wave tomography with finite frequency tomography using 3 d born sensitivity kernels for long period fundamental mode dispersion measurements the 3 d kernels preserve sidelobes beyond the first fresnel zone and fully account for the directional dependence of surface wave scattering and the effects of timedomain tapering and seismic source radiation tomographic inversions of love and rayleigh phase delay measurements and synthetic checkerboard tests show that 1 small scale s wave velocity anomalies are better resolved using finite frequency sensitivity kernels especially in the lowermost upper mantle 2 the resulting upper mantle heterogeneities are generally stronger in amplitude than those recovered using ray theory and 3 finite frequency tomographic models fit long period dispersion data better than ray theoretical models of comparable roughness we also examine the reliability of 2 d phase velocity sensitivity kernels in global surface wave tomography and show that phase velocity kernels based upon a forwardscattering approximation or previously adopted geometrical simplifications do not reliably account for finite frequency wave propagation effects 3 d sensitivity kernels with full consideration of directional dependent seismic scattering are the preferred method of inverting long period dispersion data finally we derive 2 d boundary sensitivity kernels for lateral variations in crustal thickness and show that finite frequency crustal effects are not negligible in long period surface wave dispersion studies especially for paths along continent ocean boundaries unfortunately we also show that in global studies linear perturbation theory is not sufficiently accurate to make reliable crustal corrections due to the large difference in thickness between oceanic and continental crust

geochronology of basement rocks in the kalahari desert botswana and implications for regional proterozoic tectonics abstract u pb 40 ar 39 ar and sm nd geochronological results from isolated basement exposures and drill core samples in the kalahari desert botswana constrain the distribution and temporal evolution of proterozoic crustal provinces in this part of africa there is evidence that a buried northeast trending mesoproterozoic orogen lies outboard of older crustal provinces in eastern botswana the subsurface part of this orogen includes a granitic gneissic terrain in northern botswana that records plutonism and amphibolite grade deformation between 1 2 and 1 1 ga limited exposures of bimodal volcanic rocks in the kalahari desert are interpreted to represent extensional magmatism superimposed on the buried mesoproterozoic belt previously published geochronological data and new u pb zircon crystallization ages presented here indicate that these volcanic rocks and associated subvolcanic intrusions were emplaced at 1 1 ga as part of the widespread umkondo within plate igneous province geophysical data sparse outcrops drill core samples and reconnaissance 40 ar 39 ar geochronology show the buried mesoproterozoic orogen to be separated from the congo craton by a continuation of the neoproterozoic lower paleozoic damara belt exposed in namibia limited outcrops of 2 05 ga augen gneiss within a largely buried basement massif in northwestern botswana are inferred to be part of a paleoproterozoic crustal province developed along the southern margin of the congo craton the massif was intruded by granites at 1 02 1 00 ga but the extent and tectonic significance of this plutonism are uncertain deformed granite in the subsurface extension of the damara belt has a protolith crystallization age of 0 72 ga complex structural patterns in northwestern botswana are inferred to reflect heterogeneous damaran deformation around a promontory extending from the southern margin of the congo craton

geotechnical properties of weak sandstones weak sandstones are here defined as having a saturated uniaxial compression strength of 0 5 20 mpa the mineralogy texture density porosity and saturated moisture content of these materials have

velocity conductivity relations for cratonic lithosphere and their application example of southern africa 1 seismic velocity is a function of bulk vibrational properties of the media whereas electrical resistivity is most often a function of transport properties of an interconnected minor phase in the absence of a minor conducting phase then the two should be inter relatable primarily due to their sensitivity to temperature variation we develop expressions between shear wave velocity and resistivity for varying temperature composition and water content based on knowledge from two kimberlite fields jagersfontein kaapvaal craton and gibeon rehoboth terrane we test the expressions through comparison between a new high resolution regional seismic model derived from surface wave inversion of earthquake data from africa and the surrounding regions and a new electrical image from magnetotelluric mt data recorded in samtex southern african magnetotelluric experiment the data defined robust linear regression between the two is found to be statistically identical to the laboratory defined expression for 40 wt ppm water in olivine cluster analysis defines five clusters that are all geographically distinct and tectonically relate to i fast cold and variably wet kaapvaal craton ii fast and wet central botswana iii slow warm and wet rehoboth terrane iv moderately fast cold and very dry southernmost angola craton and v slow warm and somewhat dry damara belt from the linear regression expression and the mt image we obtain predicted seismic velocity at 100 km and compare it with that from seismic observations the differences between the two demonstrate that the linear relationship between vs and resistivity is appropriate for over 80 of southern africa finally using the regressions for varying water content we infer water content in olivine across southern africa

tectonic model of the limpopo belt constraints from magnetotelluric data a b s t r a c t despite many years of work a convincing evolutionary model for the limpopo belt and its geometri cal relation to the surrounding cratons is still elusive this is partly due to the complex nature of the crust and upper mantle structure the significance of anatectic events and multiple high grade meta morphic overprints we use deep probing magnetotelluric data acquired along three profiles crossing the kaapvaal craton and the limpopo belt to investigate the crust and upper mantle lithospheric struc ture between these two tectonic blocks the 20 30 km wide composite sunnyside palala tshipise shear zone is imaged in depth for the first time as a sub vertical conductive structure that marks a fundamental tectonic divide interpreted here to represent a collisional suture between the kaapvaal and zimbabwe cratons the upper crust in the kaapvaal craton and the south marginal zone comprises resistive granit oids and granite greenstone lithologies integrating the magnetotelluric seismic and metamorphic data we propose a new tectonic model that involves the collision of the kaapvaal and zimbabwe cratons ca 2 6 ga resulting in high grade granulite limpopo lithologies this evolutionary path does not require a separate terrane status for each of the limpopo zones as has been previously suggested

fragmented indian plate and vertically coherent deformation beneath eastern tibet 1 using fundamental mode rayleigh waves from the indepth iv and namche barwa seismic experiments for periods between 20 and 143 s we have investigated the lithospheric structure beneath eastern tibet we have found a 200 km wide high velocity body starting at 60 km depth and roughly centered beneath the bangong nijuang suture which is most likely a piece of the underthrusting indian continental lithosphere the sub horizontal underthrusting of the indian lithosphere beneath eastern tibet appears to be accompanied by its lateral tearing into at least two fragments and subsequent break off of the westernmost portion at 91 e 33 n the uppermost mantle low velocity zone we observe beneath the n qiangtang and songpan ganzi terranes is most probably due to warmer and thinner lithosphere relative to southern tibet we attribute the low velocity zones concentrated along the northern and southern branches of the eastern kunlun fault at lithospheric depths to strain heating caused by shearing the azimuthal fast directions at all periods up to 143 s 200 km peak sensitivity depth beneath the n qiangtang and songpan ganzi terranes are consistent suggesting vertically coherent deformation between crust and uppermost mantle furthermore the low velocity zone below the kunlun shan reaching down to 200 km argues against a present southward continental subduction along the southern margin of qaidam basin

crustal structure beneath broad band seismic stations in the mediterranean region summary we have analysed receiver functions to derive simple models for crustal structure below 12 broad band seismological stations from the midsea project and 5 permanent broad band stations in the mediterranean region including northern africa to determine an accurate moho depth we have reduced the trade off between crustal velocities and discontinuity depth using a new grid search method which is an extension of recently published methods to determine crustal thickness in this method the best fitting synthetic receiver function containing both the direct conversion and the reverberated phases is identified on a model grid of varying moho depth and varying poisson s ratio the values we found for moho depth range from around 20 km for intra oceanic islands and extended continental margins to near 45 km in regions where the eurasian and african continents have collided more detailed waveform modelling shows that all receiver functions can be well fit using a 2 or 3 layer model containing a sedimentary layer and or a mid crustal discontinuity on comparing our results with moho maps inferred from interpolated reflection and refraction data we find that for some regions the agreement between our receiver function analysis and existing moho maps is very good while for other regions our observations deviate from the interpolated map values and extend beyond the geographic bounds of these maps

southern african crustal evolution and composition constraints from receiver function studies 1 stacking of approximately 1500 radial receiver functions recorded at about 80 broadband seismic stations deployed in southern africa reveals systematic spatial variations in the ratio of crustal p and s wave velocities crustal thickness h and the amplitude of the converted moho phases r the eastern zimbabwe and the southern kaapvaal cratons are characterized by small h 38 km small 1 73 and large r 0 15 values suggesting that the relatively undisturbed archean crust beneath southern africa is separated from the mantle by a sharp moho and is felsic in composition the limpopo belt which was created by a collisional event at 2 7 ga displays large h 43 km but similar and r values relative to the cratonic areas the bushveld mafic intrusion complex and its surrounding areas show large 1 78 large h 43 km and small r 0 11 values reflecting the intrusion of mafic material into the original crust as a result of the bushveld event at 2 05 ga excluding the bushveld the spatially consistent and age independent low accentuate the difference between felsic crustal composition and more mafic island arcs that are thought to be the likely source of continental material within such an island arc model our data combined with xenolith data excluding mantle delamination in cratonic environments suggest that the modification to a felsic composition e g by the partial melting of basalt and removal of residue by delamination is restricted to have occurred during the collision between the arcs and the continent

interpretation of seismic surface waves in regional networks joint estimation of wavefield geometry and local phase velocity method and numerical tests summary teleseismic surface waves in general strongly deviate from plane waves as is evident from the analysis of surface wave data recorded with dense networks this causes conventional ray based tomographic techniques to break down if applied to network surface wave data we present a new inversion method based on the acoustic wave equation and applicable to vertical component surface wave data which successfully deals with non plane wavefield geometries the basic idea of the method is a joint estimation of the incoming wavefield and heterogeneous structure within the network region crucial to the success of the method is an adequate parametrization of the incoming wavefield which is realized using hermite gaussian basis functions additionally we apply a constraint on the wavefield parameters that expresses the idea that the samples of the wavefield taken at the station locations should be representative for the wavefield in the whole network region in this way wavefields that show stronger fluctuations in spectral amplitude than observed at the stations are rejected to represent heterogeneous structure within the network region we use an expansion into 2 d hermite gaussian functions provided that the density of stations is sufficient the proposed method retrieves heterogeneous structure in the network region very well it is not sensitive to noise or non uniform azimuthal coverage of earthquakes moreover it yields smoothed versions of the true model if the roughness of the latter has been underestimated in the inversion conversely if the true model is much smoother than anticipated inspection of the trade off between model smoothness and data misfit allows us to find the correct model the limiting factor for the inversion is the density of stations which must allow for a reliable interpolation of the observed wavefield within the network therefore in order to perform regional surface wave studies it is essential to deploy seismic stations in the region of interest itself

lithospheric structure evolution and diamond prospectivity of the rehoboth terrane and western kaapvaal craton southern africa constraints from broadband magnetotellurics abstract a 1400 km long 2 d magnetotelluric mt profile consisting of 69 sites at 20 km intervals across the western part of the archaean kaapvaal craton the proterozoic rehoboth terrane and the late proterozoic early phanerozoic ghanzi chobe damara belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern african lithosphere the lithospheric structures of the rehoboth terrane and ghanzi chobe damara belt have not been imaged previously by geophysical methods temperature is the primary control on the resistivity of mantle minerals and the mt derived lithospheric thicknesses therefore provide a very reasonable proxy for the thermal thickness of the lithosphere i e the thickness defined by the intersection of a conductive geotherm with the mantle adiabat allowing approximate present day geotherms to be calculated the work indicates the following present day average lithospheric thicknesses to a precision of about 20 km for each of the terranes traversed inferred geotherms in brackets eastern kimberley block of the kaapvaal craton 220 km 41 mw m 2 western kimberley block 190 km 44 mw m 2 rehoboth terrane 180 km 45 mw m 2 and ghanzi chobe damara belt 160 km 48 mw m 2 a clear relationship between the electrical resistivity structure of the lithosphere and the tectonic stabilisation age of the terrane is evident good agreement between the inferred present day lithospheric geotherms and surface heat flow measurements indicate the latter are strongly controlled by variations in lithospheric thickness a significant difference in lithospheric thickness is observed between the eastern and western kimberley blocks and is consistent with previous seismic tomography images of the kaapvaal craton the present day lithospheric thickness and reduced depth extent into the diamond stability field accounts for the absence of diamondiferous kimberlites in the gibeon and gordonia kimberlite fields in the rehoboth terrane previously published mantle xenolith p t arrays from the gibeon gordonia and kimberley fields however suggest that the rehoboth terrane had equilibrated to a cooler conductive palaeo geotherm 40 42 mw m 2 very similar to that of eastern kimberley block of the kaapvaal craton at some time prior to the mesozoic eruption of the kimberlites the timing and nature of both the thermal equilibration of the rehoboth terrane and the subsequent lithospheric heating thinning event required to account for its present day lithospheric structure are not well constrained a model consisting of the penetration of heat transporting magmas into the lithosphere with associated chemical refertilisation at an early stage of mesozoic thermalism appears to be the most plausible model at present to account for both the present day lithospheric structure of the rehoboth terrane and an earlier cooler palaeo geotherm some problems however remain unresolved in terms of the isostatic response of the model based on a compilation of xenocryst cr ca in pyrope barometry observations the extent of depleted mantle in the rehoboth terrane is found to be significantly reduced with respect to the eastern kimberley block 117 km versus 138 167 km it appears most likely that the depletion depth in both terranes at least in the vicinity of kimberlite eruption is explained by refertilisation of the lower lithospheric mantle

crustal structure of precambrian terranes in the southern african subcontinent with implications for secular variation in crustal genesis summary new estimates of crustal thickness poisson s ratio and crustal shear wave velocity have been obtained for 39 stations in angola botswana the democratic republic of congo malawi mozambique namibia rwanda tanzania and zambia by modellingp wave receiver functions using the h stacking method and jointly inverting the receiver functions with rayleigh wave phase and group velocities these estimates combined with similar results from previous studies have been examined for secular trends in precambrian crustal structure within the southern african subcontinent in both archean and proterozoic terranes we find similar moho depths 38 39 3k msd standard deviation crustal poisson s ratio 0 26 0 01 sd mean crustal shear wave velocity 3 7 0 1 km s 1 sd and amounts of heterogeneity in the thickness of the mafic lower crust as defined by shear wave velocities 4 0 km s 1 in addition the amount of variability in these crustal parameters is similar within each individual age grouping as between age groupings thus the results provide little evidence for secular variation in precambrian crustal structure including between meso and neoarchean crust this finding suggests that 1 continental crustal has been generated by similar processes since the mesoarchean or 2 plate tectonic processes have reworked and modified the crust through time erasing variations in structure resulting from crustal genesis