mapping the 3d extent of the northern lobe of the bushveld layered mafic intrusion from geophysical data abstract geophysical models image the 3d geometry of the mafic portion of the bushveld complex north of the thabazimbi murchison lineament tml critical for understanding the origin of the world s largest layered mafic intrusion and platinum group element deposits the combination of the gravity and magnetic data with recent seismic mt borehole and rock property measurements powerfully constrains the models the intrusion north of the tml is generally shallowly buried generally 12 000 m averaging 4000 m a feeder suggested by a large modeled thickness 10 000 m and funnel shape for lower zone magmas could have originated near the intersection of ns and ne trending tml faults under mokopane the tml has been thought to be the feeder zone for the entire bushveld complex but the identification of local feeders and or dikes in the tml in the models is complicated by uncertainties on the syn and post bushveld deformation history however modeled moderately thick high density material near the intersection of faults within the central and western tml may represent feeders for parts of the bushveld complex if deformation was minimal the correspondence of flat high resistivity and density regions reflect the sill like geometry of the bushveld complex without evidence for feeders north of mokopane magnetotelluric models indicate that the transvaal sedimentary basin underlies much of the bushveld complex north of the tml further than previously thought and important because the degree of reaction and assimilation of the transvaal rocks with the mafic magmas resulted in a variety of mineralization zones

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

rheological differences between archaean and younger crust can determine rates of quaternary vertical motions revealed by fluvial geomorphology surface uplift is widely observed in subaerial regions of continental crust that are distant from plate boundaries often revealed by river terrace staircases with altitude ranges of several hundred metres which typically span the quaternary this characteristic geomorphology has been explained by forcing of flow in the weak lower crustal layer by surface processes non steady state erosion and cyclical loading by ice sheets and sea level fluctuations this study tests this interpretation by showing that analogous river terrace staircases are absent in archaean crust which lacks the weak lower crustal layer found elsewhere

thermal hydrous and mechanical states of the mantle transition zone beneath southern africa abstract observations of p to s conversions from the seismic discontinuities near 410 and 660 km depth reveal the shoaling of the 410 km discontinuity and the deepening of the 660 km discontinuity beneath the archaean cratons in southern africa consequently the mantle transition zone is 20 km thicker than beneath post archaean regions and the global average the discontinuity structure combined with tomographically imaged seismic velocities suggests a relatively cold t 100 k and water enriched oh 20 35 saturation transition zone beneath the cratons the thicker than normal transition zone correlates with surface geology and does not follow the track of african plate motion as numerical simulations of small scale convection imply the observations suggest that cold thermal downwelling is not the primary cause of the anomalously thick transition zone beneath the cratons although it may exist in a confined area near the boundary between the kheis thrust belt and the namaqua natal belt if thermal downwelling exists its temperature anomaly at the 660 km discontinuity must be below the levels detectable by the method used in this study t 60 k the spatial correlation between surface geology high velocities beneath the cratons and the transition zone structure indicates that the cratonic keels extend to the base of the upper mantle substantially deeper than most estimates our results demonstrate the need to closely consider the effect of water content on seismic velocities beneath deep continental roots

crustal structure of iceland and greenland from receiver function studies 1 we used data from both permanent and temporary seismic networks on iceland and greenland to investigate the crustal thickness by partly reinterpreting earlier data p receiver functions and adding s receiver functions to better constrain the results we obtained good conversions from the moho and also crustal multiples in both iceland and greenland the central ice covered part of greenland has an average crustal thickness of 40 km typical for a craton at the edges of greenland the crustal thickness decreases to 30 40 km on the east coast of greenland where the track of the iceland plume is thought to have affected the lithosphere the crustal thickness is only 24 26 km in contrast to previous studies we find that the crustal thickness in the east and the northwest coastal regions of iceland is more than 40 km similar to beneath the active volcanic region in the southwest region of iceland and along the mid ocean ridge the crustal thickness is only 25 km or less also in contrast to earlier receiver function interpretations which deduced a broad crust mantle transition zone for iceland we find indications for a normal sharper moho beneath a number of sites

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

geological modeling of the new champ magnetic anomaly maps using a geographical information system technique 1 reliable global crustal field anomaly maps produced from magnetic measurements of the champ satellite mission now allow for quantitative geological studies of crustal structure and composition we have developed a gis based forward modeling technique to model these anomaly maps on the basis of the geologic and tectonic maps of the world laboratory susceptibility values of the occurring rock types and the seismic thickness of the crust a vertically integrated susceptibility grid is generated in the gis system in addition a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates combining the global vis and remanent magnetization grids the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding champ magnetic anomaly map over the oceans induced and remanent magnetization explains well the prominent observed anomalies over the cretaceous quiet zones we also find a good agreement between predicted and observed anomalies over the continents remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried precambrian provinces until the recomputed anomalies fit the observed anomalies the feasibility of this approach is demonstrated on greenland the west african craton bangui in central africa and the kolyma omolon block in siberia we conclude that quantitative information on the lateral extent the composition and the thickness of the lower crust within a precambrian province can thus be inferred from the new satellite magnetic anomaly maps

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

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

pn arrivals and lateral variations of moho geometry beneath the kaapvaal craton abstract pn arrivals from mining induced earthquakes on the edge of the witwatersrand basin show that the p wavespeeds in the uppermost mantle are almost constant throughout most of the kaapvaal craton the presence of only small wavespeed variations allows the use of a simple method of estimating crustal thicknesses below the stations of the kaapvaal broad band network using pn times that has been compared with results from receiver functions one thousand three hundred thirty seven pn arrivals were used to derive crustal thicknesses at 46 stations on the kaapvaal craton the average crustal thicknesses for 19 centrally located stations on each of the northern and southern regions of the craton that yielded well constrained thicknesses were 50 52 0 88 km and 38 07 0 85 km respectively in contrast the corresponding average thicknesses determined from receiver functions were 43 58 0 57 km and 37 58 0 70 km respectively the systematically lower values for receiver functions in the northern part of the kaapvaal craton that was affected by the bushveld magmatism at 2 05 ga suggest that the receiver functions do not enable the petrological crust mantle boundary to be reliably resolved due to variations in composition and metamorphic grade in a mafic lower crust the pn times also suggest pervasive azimuthal anisotropy with maximum wavespeeds of about 8 40 km s at azimuths of about 15 and 217 in the northern and southern regions of the craton respectively and minimum wavespeeds of about 8 25 km s

aeromagnetic gravity and differential interferometric synthetic aperture radar analyses reveal the causative fault of the 3 april 2017 m w 6 5 moiyabana botswana earthquake on april 3 2017 a mw 6 5 earthquake struck moiyabana botswana nucleating at 20 km focal depth within the paleoproterozoic limpopo shashe orogenic belt separating the archean zimbabwe and kaapvaal cratons we investigate the lithospheric structures associated with this earthquake using high resolution aeromagnetic and gravity data integrated with differential interferometric synthetic aperture radar dinsar analysis here we present the first results that provide insights into the tectonic framework of the earthquake the ruptured fault trace delineated by dinsar aligns with a distinct nw striking and ne dipping magnetic lineament within the precambrian basement the fault plane solution and numerical modeling indicate that the cause of the earthquake was 1 8 m displacement along a nw striking and ne dipping normal fault rupturing at 21 24 km depth we suggest that this seismic event was due to extensional reactivation of a crustal scale precambrian thrust splay within the limpopo shashe orogenic belt

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

waning buoyancy in the crustal roots of old mountains when mountains form through the collision of lithospheric plates uplift of the earth s surface is accompanied by thickening of the crust and the buoyancy of these deep crustal roots relative to the surrounding mantle is thought to contribute to the support of mountain topography once active tectonism ceases continuing erosion will progressively wear away surface relief here i provide new constraints on how crustal roots respond to erosional unloading over very long timescales in old collisional mountain belts ratios of surface relief to the thickness of the underlying crustal root are observed to be smaller than in young mountains on the basis of gravity data this trend is best explained by a decrease in the buoyancy of the crustal root with greater age since the most recent mountain building episode which is consistent with metamorphic reactions1 2 produced by long term cooling an approximate balance between mountain and root mass anomalies suggests that the continental lithosphere remains weak enough to permit exhumation of crustal roots in response to surface erosion for hundreds of millions of years the amount of such uplift however appears to be significantly reduced by progressive loss of root buoyancy

small scale variations in seismic anisotropy near kimberley south africa summary we have exploited a new data set of broad band seismic waveforms recorded near the region of kimberley south africa to place new constraints on seismic anisotropy in an area of extensive mantle modification within an archaean cratonic setting the kimberley region is significant in that it represents archaean cratonic mantle that has been studied extensively via petrologic geochemical and seismic means and provides a unique opportunity to meld the results of a broad range of data sets to examine processes of crust and mantle deformation shear wave splitting observations using sks phases recorded by a dense broad band seismic array in the kimberley region exhibit consistent fast polarization directions of ne sw and splitting times that range from 0 15 s in the se to nearly 0 75 s in the nw regions of the array multichannel cross correlation of relative arrival times of teleseismic sks phases across the array reveals clear azimuthal variations and a relative arrival time range comparable to the shear wave splitting delay time range a like analysis of teleseismic pkpdf phases does not exhibit significant differences in relative arrival times across the array the combined relative arrival time and splitting analyses indicate that a significant change in the strength of anisotropic structure is required across the region our results are most consistent with a model in which variations in seismic anisotropy near the kimberley region are constrained to the lithosphere with an average anisotropic strength of 1 8 per cent extend to depths no greater than 150 km and are primarily controlled by a significant change in the strength of anisotropy within the lateral bounds of the seismic array this inferred structural boundary is near the location of the ca 2 9 gyr old colesberg magnetic lineament a prominent aeromagnetic feature interpreted as the primary suture zone joining the eastern meso archaean and western neo archaean blocks of the kaapvaal craton the proposed boundary is also located within the southern exposure of the ca 2 7 gyr old ventersdorp volcanic sedimentary sequence a key continental rifting feature in the region we hypothesize that the observed seismic anisotropy is the result of straininduced lattice preferred orientation of olivine caused by the amalgamation and deformation of the eastern and western blocks of the kaapvaal craton this collisional process mechanically weakened the region to generate the observed mantle fabric which was exploited by significant rifting events that may have subsequently enhanced the regional fabric this model is also consistent with sks splitting results for the whole of southern africa which suggest either weak anisotropic or isotropic mantle structure in the eastern kaapvaal and coherent mantle anisotropy across much of the western kaapvaal our results support the notion that seismic anisotropy beneath archaean continental regions is not created or alternatively not preserved during initial continental formation but is instead generated during subsequent significant mantle deforming events

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

crustal structure of the arabian plate new constraints from the analysis of teleseismic receiver functions an edited version of this paper was published by elsevier science copyright 2005 elsevier science see also http dx doi org 10 1016 j epsl 2004 12 020 http atlas geo cornell edu saudiarabia publications al damegh 202005 htm

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

crustal structure beneath ne china imaged by necessarray receiver function data abstract we analyzed tens of thousands of receiver function data recorded by 185 broadband seismic stations to study the crustal structure beneath the northeast china moho depth and average crustal v p v s ratio were measured at each station using the h grid searching technique for stations located above unconsolidated sediments we applied the h method to remove strong shallow reverberations and generate subsurface receiver functions that allow for effective h analysis the measurements show that the songliao basin has a relatively thin crust 31 km and the moho depth increases significantly from southeast 27 km to northwest 35 km the northwestward tilting of the songliao basin may suggest that it was initiated by lithospheric flexure due to the load of the great xing an range in the jurassic before the large scale extension in the cretaceous moho depth varies from 26 7 km to 42 3 km across the study area with the shallowest and deepest moho being located at the eastern flank of the songliao basin and the great xing an range respectively the moho depth correlates well with the surface topography in the western part but not the central and eastern parts of the study area the residual topography computed based on the airy s isostasy model indicates that the high topography at the eastern flank of the songliao basin the changbaishan volcanic center and the southern end of the great xing an range is likely dynamically supported by the upper mantle

reconstruction of the east africa and antarctica continental margins the early jurassic separation of antarctica from africa plays an important role in our understanding of the dispersal of gondwana and pangea previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent ocean boundary cob and determining the amount and distribution of extended continental crust this study focuses on the evolution of the african margin adjacent to the mozambique basin and the conjugate antarctic margin near the riiser larsen sea satellite derived gravity data have been used to trace the orientations and landward limits of fracture zones a 3 d gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust crustal thicknesses together with fracture zone terminations reveal cobs that are significantly closer to the african and antarctic coasts than previously recognized correlation of fracture zone azimuths and identified cobs suggests antarctica began drifting away from africa at approximately 171 ma in a roughly sse direction an areal balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins both margins reveal a trend of increasing extension from east to west our results suggest africa underwent extension of 60 120 km while antarctic crust was stretched by 105 180 km various models tested to determine the direction of extension during rifting suggest that antarctica moved away from africa in a wnw ese direction during the period between 184 and 171 ma prior to the onset of seafloor spreading

lithospheric structure of the malawi rift implications for magma poor rifting processes this project was supported by the national science foundation continental dynamics nsf cd grant ear 1255233 and office of international and integrative activities oiia grant iia 1358150 this project is also supported by the nsf earthcube integration grant 1740627 we thank the computational infrastructure for geodynamics geodynamics org which is funded by the national science foundation under award ear 0949446 and ear 1550901 for supporting the development of advanced solver for problems in earth s convection

rudists as gregarious sediment dwellers not reef builders on cretaceous carbonate platforms abstract it has sometimes been claimed that rudist bivalves competitively displaced corals from reef frameworks during the cretaceous this hypothesis combines two assertions 1 that the autecology of rudists was convergent with that of reef building corals and 2 that rudist formations commonly developed as reefs we dispute both assertions and thus reject the hypothesis of competitive displacement we argue instead that mobile sediments rather than frameworks dominated the margins and tops of most of the extensive carbonate platforms of the period and that it was on these non reefal biotopes that the rudists flourished definitions of reefs tend to combine two major elements 1 a robust biogenic framework with accompanying sedimentary and diagenetic components and 2 topographical relief such definitions are clearly rooted in recent coral reefs in which endosymbiotic zooxanthellae permit the extensive growth of colonial coral frameworks in shallow but relatively nutrient poor waters coralline algae and cementation may contribute significantly to the growth of rigid structures and topography is largely the legacy of pleistocene changes in sea level in rudist formations in contrast individual rudist congregations are volumetrically limited relative to sediment they are often loosely constructed and they evidently showed little if any original relief tabular and small lenticular units predominate these differences in structure and palaeoenvironmental situation between rudist and modern coralgal associations reflect the different autecologies of the constituent organisms the clonal growth of corals predisposes them to the development of frameworks projecting above the sediment surface herein termed superstratal growth by contrast the aclonal development of rudists was better suited to the opportunistic occupation of a variety of temporarily available substrata by large numbers of individuals in particular elevator rudists in which the entire commissure exhibited upward growth evidently grew with their shells largely embedded in and supported by the ambient sediment herein termed constratal growth moreover the tolerances and growth responses of rudists to such factors as water turbidity nutrients and current regime were quite different from those of the majority of reef building corals despite repeated assertions in the literature that rudists possessed zooxanthellae only a few species show any evidence for such a symbiosis and other evidence suggests that most lacked them in view of these differences in their preferred biotopes competition between rudists and corals is doubtful even though members of both groups co occur in many areas the relative decline of coral frameworks in the cretaceous was thus probably independently caused

crustal structure of a proterozoic craton boundary east albany fraser orogen western australia imaged with passive seismic and gravity anomaly data abstract we use passive seismic and gravity data to characterize the crustal structure and the crust mantle boundary of the east albany fraser orogen in western australia a proterozoic orogen that reworked the southern and southeastern margin of the archean yilgarn craton the crustal thickness pattern retrieved from receiver functions shows a belt of substantially thickened crust that follows the trend of the orogen but narrows to the southwest common conversion point profiles show a clear transition from a wide symmetric moho trough in the northeast to a one sided northwestern moho dip in the southwest where the moho appears to underthrust the craton towards its interior this change appears to coincide with the inferred trace of the ida fault a major terrane boundary within the yilgarn craton forward modelling of gravity anomaly data using the retrieved moho geometry as a geometric constraint shows that a conspicuous elongated gravity low on the northwestern side of the east albany fraser orogen is almost certainly caused by thickened archean crust to obtain a model that resembles the regional gravity pattern the following assumptions are necessary high density rocks occur in the upper crust of the fraser zone at depth inside the moho trough and in parts of the eastern nornalup zone east of the moho trough although our gravity models do not constrain at which crustal level these high density rocks occur active deep seismic surveys suggest that large extents of the east albany fraser orogen s lower crust include a mesoproterozoic magmatic underplate known as the gunnadorrah seismic province the simplest interpretation of the imaged crustal structure is that the gunnadorrah seismic province is underthrust beneath the yilgarn craton likely as a consequence of crustal shortening during accretion further east the imaged geometry overall appears to show a wedge of archean lower crust that was driven between the exhumed fraser zone and the gunnadorrah seismic province effectively splitting the paleo to mesoproterozoic crust of the east albany fraser orogen the vertical splitting of proterozoic crust by a cratonic crustal wedge comparable to what we image in this study may be a process that contributed to forming many craton margins around the world

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

comment on a crustal thickness map of africa derived from a global gravity field model using euler deconvolution by getachew e tedla m van der meijde a a nyblade and f d van der meer summary we discuss a recent publication by tedla et al which presents a crustal thickness map of africa determined by euler deconvolution of gravity in our comment we first outline the limitations of the data set used by the authors and the deleterious effects of working with free air gravity next we discuss the grievous parameter choices grid interval euler window size structural index and incorrect assumptions made by the authors and their serious effects on the results finally we demonstrate with two examples why their results are not helpful in understanding crustal thickness and consequently the tectonic history this is not specific to africa but also true for the application to other parts of the earth

long lasting epeirogenic uplift from mantle plumes and the origin of the southern african plateau 1 we investigate under what conditions the present day long wavelength anomalous elevation 500 m of the southern african plateau can be attributed to mesozoic plume events the anomalous long wavelength topography of the southern african plateau cannot be easily explained by recent heating of the upper mantle as opposed to other areas of the african superswell and geomorphological studies provide few hard constraints on the timing of the uplift a mesozoic origin for the plateau uplift is attractive in that southern africa experienced several large magmatic events in the mesozoic we formulate numerical and scaling models to investigate if plume material ponded beneath cratonic lithosphere in the mesozoic could produce 500 m of present day elevation we find that starting plume heads are ineffective at producing much long lasting uplift because the material spreads into a thin layer tens of km thick the ponded plume material also suppresses secondary convection by having a stable boundary at its base over time heat continues to flow out of the top of the lithosphere and a net imbalance of heat flow in the lithosphere develops resulting in subsidence even after the plume material has cooled to the temperature of the mantle adiabat secondary convection supplies less heat to the lithosphere than is lost upward by conduction the cratonic lithosphere returns to its original thermal structure on a timescale of less than 200 m y even two mantle starting plume heads one at the time of karoo volcanism ca 183 ma and the other at the time of kimberlite eruption ca 80 90 ma in our models cannot produce much 200 m of the present day anomalous elevation however significant uplift can be generated by plume tails provided they linger beneath the lithosphere for 25 30 m y and if the uplift effects of mesozoic plume heads and tails are considered together then it is possible to account for 500 m of present day elevation consequently a mesozoic plume model for plateau uplift in southern africa appears to be a viable model if plume tails delivered heat to the southern african lithosphere over a period of 25 m y or more

cretaceous reactivation and intensified erosion in the archean proterozoic limpopo belt demonstrated by apatite fission track thermochronology abstract cratons are generally assumed to be regions of long lasting tectonic stability in particular the study of the phanerozoic exhumation history of cratons has been largely hampered by the scarcity of suitable stratigraphic controls onshore this fact is even more pronounced in terranes lacking mesozoic or younger penetrative structural fabrics and metamorphic overprinting our study in the limpopo belt shows that modern apatite fission track thermochronology provides a hitherto unavailable perspective in the study of these rocks and has profound implications for the crustal evolution of the zimbabwe craton apatite fission track data from 35 samples taken along two transects in the southern edge of the zimbabwe craton and in the central zone of the limpopo belt suggest that extensive regions experienced kilometer scale exhumation in two discrete events as recently as the cretaceous the first occurred at around 130 ma and the second at around 90 ma basin subsidence and sedimentation loads on the mozambique margin support the timing of these events and provide strong indications of the source and pathways for the eroded material further the results indicate that young and old surfaces in a geomorphological sense may be structurally juxtaposed in regions of high elevation in zimbabwe this is contrary to early ideas of surface chronologies based on summit accordances or invoking pediplanation

regional geological setting of the limpopo belt abstract the 2 7 ga limpopo belt granulite terrane is situated between two lower grade granite greenstone cratons the kaapvaal craton in the south and the zimbabwe craton in the north this granulite facies terrane is subdivided into three zones each with a distinctive geological signature and separated from each other and from the adjacent cratons by well defined shear zones 1 the southern marginal zone composed of granite greenstone material very similar to that of the kaapvaal craton has retrogressed granulites on its southern margin in the hanging wall of the northward dipping terrane boundary hout river shear zone separating the granulite terrane from the adjacent kaapvaal craton 2 the northern marginal zone a granite greenstone terrane at granulite grade which is separated from the zimbabwe craton by a southward dipping terrane boundary and 3 the central zone a unique crustal element with distinctive carbonate platform lithologies and a metagabbroic layered complex which intruded before or at 3250 ma this zone cannot be related to the kaapvaal craton or to the zimbabwe craton it is separated from the two marginal zones by mylonitic shear zones with horizontal stretching lineations the entire limpopo belt is characterized by a distinctive deformational and metamorphic style which contrasts markedly from that of the adjacent granite greenstone terranes of the kaapvaal and zimbabwe cratons this distinctive pattern developed during the limpopo orogeny probably as a consequence of continental collision during the late archaean

was phanerozoic reef history controlled by the distribution of non enzymatically secreted reef carbonates microbial carbonate and biologically induced cement throughout most of the phanerozoic reef rigidity resulted as much or more from early lithification by microbial carbonates and biologically induced cements non enzymatic carbonates than from biological encrustation of or by large enzymatically secreted metazoan skeletons reef framework is divided into four categories 1 skeletal metazoan 2 non skeletal microbialite stromatolite and thrombolite 3 calcimicrobe and 4 biocementstone in which small or delicate organisms serve as scaffolds for rigid cement crusts the last three categories are dominated by non enzymatic carbonates skeletal framework and non skeletal microbialite framework were the most abundant framework types through the phanerozoic the composition and abundance of skeletal framework was controlled largely by mass extinction events but most reefs consisted of both microbialite and skeletal organisms in a mutually beneficial relationship microbialite framework was abundant throughout the palaeozoic and early mesozoic but declined after the jurassic calcimicrobe framework was important during the cambrian early ordovician and devonian and biocementstone framework was important from the late mississippian to the late triassic the phanerozoic history of reefs does not correlate well with the stratigraphic distribution of large skeletal reef builders or with a variety of physicochemical parameters including sea level history wilson cycle or global climate cycles because non enzymatic carbonates result from induction by non obligate calcifiers and not enzymatic precipitation by obligate calcifiers the distribution of these carbonates was controlled to a larger extent by temporal changes in physicochemical parameters affecting the saturation state of sea water with respect to carbonate minerals changes in pco ca mg ratios cation concentrations and temperature may have affected the abundance of non enzymatic carbonates and hence reefs independently from the effects of these same parameters and mass extinction events on skeletal reef biota the decline in abundance of reefal microbialite and absence of calcimicrobe and biocementstone reef framework after the jurassic may be a result of relatively low saturation states of sea water owing to increased removal and sequestration of finite marine carbonate resources by calcareous plankton since the jurassic reef history is difficult to correlate with temporal changes in specific global parameters because these parameters affect skeletal biota and biologically induced carbonate precipitation independently hence reef history was regulated not just by skeletal reef biota but by parameters governing non enzymatic carbonates

the descriptive nomenclature and classification of growth fabrics in fossil scleractinian reefs abstract framework sensu stricto is a concept originally relating to modern reefs and is defined by structural ecological and sedimentological criteria however in the geological literature the term framework has come to be applied to almost any organically influenced in situ accumulation including non calcareous non rigid facies associated with sea grasses this drastic departure from the original meaning is misleading and scientifically unhelpful the concept of wave resistance is integral to that of framework but in the geological record it is not easy to evaluate and the term may be of little comparative use between reef environments it is suggested that the term growth fabric be generally applied as a descriptive term for the presence of aggregated in situ corals or other organisms in growth position within a facies the term growth fabric should be used as an alternative to framework as a purely descriptive term the term framework should be restricted to its original definition and used only where the criteria for framework can be clearly demonstrated two end member types of scleractinian growth fabric genesis are recognised termed superstratal and constratal terms adapted from gili e masse j p skelton p w 1995 rudists as gregarious sediment dwellers not reef builders on cretaceous carbonate platforms palaeogeogr palaeoclimatol palaeoecol 118 245 267 in a superstratal growth fabric the constituent organisms collectively projected decimetres to metres above the substratum creating positive topographic relief in contrast constratal growth is where vertical organic accretion occurred at a similar rate to sediment accumulation hence the majority of the skeletal material remained continuously embedded in the sediments and only a small proportion of the skeleton projected centimetres above the sediment surface the architectural palaeoecological sedimentological and diagenetic implications of different types of growth fabric genesis is discussed a review of the classification of reef fabrics suggests that there is currently no adequate system to describe fossil scleractinian growth fabrics the most commonly used classification of reefal fabrics is that of embry and klovan 1971 embry a f klovan j e 1971 a late devonian reef tract on northeastern banks island northwest territories bull can pet geol 33 730 781 there are a number of shortcomings in this scheme which may be grouped into three categories 1 the interpretative nature of the classification 2 problems in interpreting biological effect from form and 3 insufficient categories to adequately describe mesozoic and cenozoic growth fabrics moreover there appears to be a lack of a standardised nomenclature for growth fabrics which has hindered meaningful comparisons of scleractinian growth fabrics through time and space a descriptive system based on a modification and expansion of the embry and klovan system 1971 is proposed and a revised nomenclature for growth fabrics presented the system is designed to be flexible in its application it can be used simply to describe a growth fabric or through the use of genetic and non genetic modifiers to imply types of reef building processes and growth fabric heterogeneity although the concepts and terminology discussed in this paper relate to scleractinian growth fabric they are equally applicable to fabrics comprising other organisms

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

zircon u pb strain chronometry reveals deep impact triggered flow abstract large 100 km meteorite impact cratering events play important roles in surface and biosphere evolution however their potential for widespread ductile modification of the lithosphere has been difficult to assess due partly to our inability to isotopically age correlate deep mineral fabrics with surface records we have integrated benchmark u pb zircon dating methods id tims shrimp with new microstructural techniques ebsd xrd to demonstrate that crystal plastic deformation can cause rapid out diffusion of radiogenic pb and accompanying trace element alteration in crystalline zircon we have used this phenomenon to directly date fabric in archean zircons and xenoliths of the lower crust of south africa at 2023 15 million years coeval with the 2020 3 million year old vredefort cratering event at surface with extent 20 000 km 2 our findings indicate that regional exogenic fabrics similar to high temperature tectonic fabrics exist in ancient crust moreover our results establish that crystal plastic deformation in the lithosphere can now be directly dated and linked to planetary evolution by zircon u pb strain chronometry

the p wavespeed structure in the lowermost 700 km of the mantle below the central part of the indian ocean abstract earthquakes from the indonesian region recorded at distances between 78 and 104 from broad band stations of the kaapvaal network were used to investigate the p wavespeed structure of the lowermost 700 km of the mantle below the central part of the indian ocean analysis of arrival times indicates a maximum in slowness gradients at a distance of 88 and wavespeeds in the lowermost 700 km of the mantle that are lower than in the reference model iasp91 these results suggest that the lowermost mantle is relatively hot measurements of p wave amplitudes also show a pronounced maximum at a distance close to 88 phase weighted stacking of waveforms from three deep focus earthquakes do not show any later arrivals that might identify a triplication in travel times associated with the presence of a discontinuity at the top of d however synthetic seismograms produced by both the wkbj and reflectivity methods for models having a small discontinuity at the top of d show the presence of an amplitude maximum due to constructive interference between the closely spaced signals near the cross over point of a triplication whose branches are not clearly resolvable visually the observed amplitude maximum however is too large to be explained by the smooth model midind01 fitted to the travel times stacking of synthetic seismograms constructed from models with small discontinuities at the top of d gives similar results to those for the real seismograms the observed amplitudes travel times results of phase weighted stacking and synthetic seismograms together suggest the presence of a small p wave discontinuity of 0 4 0 6 that defines the top of d 210 20 km above the core mantle boundary

preserved and modified mid archean crustal blocks in dharwar craton seismological evidence abstract we report significant lateral variability in shear wave velocity and moho depth in the archean crust beneath the dharwar craton using earthquake waveform data recorded over 50 broadband seismographs the craton is a continuously exposed archean continental fragment divided into the west dharwar craton wdc of age 2 7 3 36 ga and the east dharwar craton edc of age dominantly 2 5 ga the craton progressively transitions into the southern granulite terrain sgt with age of metamorphism around 2 6 ga the inversion and modeling of receiver function data reveal significant variation of moho depth viz 38 54 km in the wdc and 40 46 km in the sgt and 32 38 km in the edc the average shear wave velocity vs of crust beneath the wdc is 3 85 km s as compared to 3 6 km s in the edc we infer highly variable thickness 16 30 km of mafic cumulate vs 4 0 km s and vp 7 0 km s beneath the wdc in contrast with a thin one 50 km as compared to felsic to intermediate composition for the edc crust with almost flat moho down to 38 km these results suggest preserved mafic crustal root beneath the middle archean terrain in the wdc that has remained inert since then on the other hand felsic to intermediate composition of crust with a nearly flat moho beneath the late archean edc could be a consequence of regional delamination of lower crust the preserved distinct moho topography across the archean terrains suggests it as compositional boundary considering the surface exposure of 15 20 km crust based on p t condition in the granulite segment of the wdc we speculate a himalaya like crustal thickness 50 70 km beneath the middle archean crust pointing toward a plate tectonic like scenario at 3 0 ga

structure of the kaapvaal craton from surface waves phase delays of rayleigh and love waves from teleseismicearthquakesrecordedduringthesouthernafrica seismic experiment have been inverted for upper mantle seismic velocities along the propagation paths and beneath the station array successful models of subarray struc turedisplayedsignicantradialanisotropyintheuppermost mantle but no shear wave low velocity zone supporting the hypothesisthatthelithospherebeneaththekaapvaalcraton isthick theazimuthalvariationsofrayleigh waveslowness are not consistent with simple models based onskssplit tingobservationsandanisotropy measurementsofkaapvaal mantlexenoliths thisdiscrepancymayimplysomeamount of small scale heterogeneity in anisotropy

the initial gondwana break up a synthesis based on new potential field data of the africa antarctica corridor abstract the initial opening of the africa antarctica corridor in the heart of gondwana is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the jurassic therefore in 2014 new ship borne magnetic data were systematically acquired in the northern mozambique basin and across beira high which we merged with all accessible magnetic data in the mozambique basin herein distinct magnetic lineations are observed which allow a refined identification of a whole set of jurassic magnetic spreading anomalies constraining the timing of the onset of oceanization beginning at m38n 2n 164 1 ma in combination with high resolution potential field data from the conjugate antarctic margin well expressed fracture zones can be traced throughout the africa antarctica corridor and allow the precise rotation of antarctica back to africa the initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account within a tight gondwana fit the beira high can be restored along the major sinistral namama orvin shear zone of the east african antarctic orogen the beira high represents a continental block which was detached from antarctica by 157 ma at the latest simultaneously the antarctic plate cleared the area of the mcp however the crustal nature of the southern mcp remains ambiguous the northern natal valley and the mozambique ridge consist of thick oceanic crust being emplaced between m26r m18n 157 1 144 ma and m18n m6n 144 131 7 ma respectively about the half of this crust was won from the antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the explora wedge a refined kinematic break up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the africa antarctica corridor and the somali basin

himalayan tibetan analogies for the evolution of the zimbabwe craton and limpopo belt abstract the limpopo orogeny which lasted from 2700 to 2600 ma was characterised by nnw sse directed shortening this resulted in both crustal thickening accommodated by folding and nnw directed thrusting and associated crustal extrusion whereby a number of crustal blocks including the central zone of the limpopo belt was displaced to the west southwest along wsw ene trending shear zones this was part of a regionally distributed phase of orogenic deformation which affected the adjoining kaapvaal and zimbabwe cratons from the witwatersrand basin in the south to the harare greenstone belt in the north this deformation which migrated northwards with time postdated an earlier phase of crustal accretion during which the archaean crustal unit now marked by the kaapvaal and zimbabwe cratons and the limpopo belt grew towards the north northwest shortening of this unit during the limpopo orogeny may be regarded as the last stage of the crustal accretion process this tectonic process of continental growth by accretion followed by crustal shortening with associated crustal extrusion has many features in common with tibetan tectonics from the mesozoic to the present

connectivity between the western and eastern limbs of the bushveld complex abstract the mafic layered rocks of the bushveld complex are 6 8 km thick and crop out over an area of 65 000 km 2 previous interpretations of the bouguer gravity anomalies suggested that the intrusion consisted of two totally separate bodies however the mafic sequences in these arcuate western and eastern limbs are remarkably similar with at least six petrologically distinctive layers and sequences being recognisable in both limbs such similarity of sequences in two totally discrete bodies 200 300 km apart is petrologically implausible and it is suggested that they formed within a single lopolithic intrusion all previous bouguer gravity models failed to consider the isostatic response of the crust to emplacement of this huge mass of mafic magma isostatic adjustment as a result of this intrusion would have caused the base of the crust to be depressed by as much as 6 km with this revised whole crustal model it becomes possible to construct a gravity model consistent with observed data which includes a 6 km thick sequence of mafic rocks connecting the western and eastern limbs of the bushveld complex the exact depth at which the mafic rocks of the bushveld complex lie in the centre of the structure cannot be constrained by the gravity data such a first order model is an approximation because there have been subsequent deformation and structural readjustments in the crust some of them probably related to the emplacement of the bushveld complex specifically the observed geometry of the rocks around the crocodile river dennilton marble hall and malope domes suggests that major upwarping of the crust occurred on a variety of scales triggered by emplacement of the bushveld complex

a crustal thickness map of africa derived from a global gravity field model using euler deconvolution summary we develop a new continental scale crustal model for africa by modelling the free air gravity anomaly eigen gl04c which was developed from 30 months of grace level 1b data covering the period from 2003 february to 2005 july and surface gravity data from seven different sources from this gravity model crustal thickness is estimated using 3 d euler deconvolution a method that does not rely on ap rioridepth and density constraints the results are in good agreement i e within 5km of seismically determined moho depth estimates from across the continent except for narrow tectonic regions such as rift valleys and areas where seismic velocity models of the crust indicate a gradational moho the results show that crustal thickness is fairly homogeneous with an average crustal thickness for the whole continent of 39 2 sd km the average moho depth for most terrains is within 5km of the continental average and there is little variability between terrains of different age the average thickness for archean proterozoic and palaeozoic crust is 39 39 and 41km respectively crustal thickness in sedimentary basins across northern and central africa varies between 33 and 36km through comparison with global averages for similar aged terrains we find that african crustal thickness does not deviate significantly from the thickness of crust in other parts of the world

crustal structure of the southern margin of the african continent results from geophysical experiments 1 a number of geophysical onshore and offshore experiments were carried out along a profile across the southern margin of the african plate in the framework of the inkaba yeafrica project refraction seismic experiments show that moho depth decreases rapidly from over 40 km inland to around 30 km at the present coast before gently thinning out toward the agulhas falkland fracture zone which marks the transition zone between the continental and oceanic crust in the region of the abruptly decreasing moho depth in the vicinity of the boundary between the namaqua natal mobile belt and the cape fold belt lower crustal p wave velocities up to 7 4 km s are observed this is interpreted as metabasic lithologies of precambrian age in the namaqua natal mobile belt or mafic intrusions added to the base of the crust by younger magmatism the velocity model for the upper crust has excellent resolution and is consistent with the known geological record a joint interpretation of the velocity model with an electrical conductivity model obtained from magnetotelluric studies makes it possible to correlate a high velocity anomaly north of the center of the beattie magnetic anomaly with a highly resistive body

facies analysis of an upper cretaceous high energy rudist dominated carbonate ramp matese mountains central southern italy subtidal and peritidal cycles abstract upper cretaceous shallow water limestones rich in rudists have been studied in the matese mountains they show a preponderance of skeletal components molluscs and benthic foraminifers and a lack of non skeletal grains the most prominent sediments of the studied stratigraphic interval are rudist dominated fine to coarse grained lithologies most of the senonian successions are characterized by massive neritic limestones rich in molluscs mostly rudists and subordinate gastropods and a few corals the sediments were generated in situ on shelves where rudist bivalves were the primary sediment producers these sediments were actively moved by storms waves and swells the finer fractions were probably winnowed out and deposited in deeper water well bedded dolomitized mudstones wackestones microbial laminites and benthic foraminifer wackestones characterize some successions where they rhythmically alternate and testify to deposition in tidal flat environments the lithofacies faunal and taphonomic characters and the sedimentary structures are typical of a ramp like open shelf with local peritidal deposition the vertical lithofacies organization consists of numerous decimetre to metre scale shallowing upward subtidal and peritidal cycles a complete transition from peritidal to subtidal cycles has been recognized and documented the peritidal cycles are usually bounded by desiccation surfaces and or dolomitized crusts followed by shallow subtidal to peritidal facies of the next cycle the subtidal cycles are typified by rudist floatstones passing to large scale cross bedded bioclastic packstones grainstones current and storm structures are distinctive features in shallower water environments the effects of submarine erosion prevail hardgrounds commonly developed and the resulting cycles are usually truncated the formation of the peritidal cycles appears to have been controlled by accommodation space in response to fluctuations in relative sea level the formation of the subtidal cycles was determined by sedimentation rates in response to fluctuations in the zones of fair weather and storm wave reworking in turn controlled by relative sea level this indicates that sediment production and supply were important controlling factors in the cycle formation during the early senonian stratigraphic interval the recognition of similar depositional styles in other early senonian successions cropping out in the southern apennines suggests that this type of sedimentation is a recurring attribute of the upper cretaceous open shelves

the use of velocity spectrum for stacking receiver functions and imaging upper mantle discontinuities summary in order to improve the signal to noise ratio of receiver function data it is typical to stack receiver functions calculated from events at similar distances and back azimuths we have adapted the velocity spectrum stacking vss technique used extensively in reflection seismology to the receiver function method in order to stack data with different ray parameters thereby improving further the signal to noise ratio perhaps more importantly by producing the velocity spectrum stacks we take advantage of the differences in the shapes of the moveout curves of converted phases and reverberations to identify and separate the various phases and to infer velocity structure by comparison of velocity spectrum stacks produced from the observed data at the iris ida broad band station at obninsk russia obn with those produced from prem synthetics we have identified ps phases from the 400 and 670 km discontinuities the p to s ps converted phase from the 400 km discontinuity observed at obn is much larger compared with that of the 670 km discontinuity than is predicted by prem this suggests a higher velocity contrast at 400 km than in prem by adapting a bootstrap method to assess the depth and velocity estimates determined by vss we find that the ps phase from the 670 km discontinuity is best imaged with higher than prem upper mantle p and s velocities and at a depth of 663 3 km we find no evidence of a 220 km discontinuity beneath obn in these data

thickness composition and evolution of the indian precambrian crust inferred from broadband seismological measurements abstract we present seismological constraints to model the evolution of indian precambrian crust through receiver function analysis of teleseismic waveforms recorded on 22 broadband seismographs along a n s corridor from southern granulite terrain to delhi on the aravalli craton archean terrains eastern dharwar bastar bundelkhand and aravalli cratons are underlain by crust with felsic intermediate composition vp vs 1 65 1 75 in contrast to mafic composition vp vs 1 78 of crust beneath vindhyan basin godavari basin and the narmada son lineament nsl of proterozoic age the moho depth varies between 33 and 43 km with no definite relationship with age of the crust duplex moho with highly mafic underplated lower crust is characteristic feature of the nsl signifying its paleo rift character present day felsic character of the indian archean crust could be a consequence of post formation modification of the initial mafic lower crust through the process of lithospheric delamination this major tectonic event also created layered lithosphere beneath parts of india imprinted as positive velocity contrast at a depth of 80 km usually referred to as hales discontinuity the upper part of the lithosphere has shear velocity vs 4 52 km s and the lower one 4 77 km s we speculate that such a layered lithosphere could be due to a low density depleted spinel peridotite with archean affinity lying over higher density fertile garnet peridotite of proterozoic age

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

evidence for an early archaean granite from bastar craton india abstract granitoids in the early archaean are believed to be potassium poor tonalite trondhjemite granodiorite rocks only after continental crust attained sufficient thickness did true relatively potassium rich granites form no record of true granite prior to 3 4 ga is available we report a 3 6 ga true granite from the archaean bastar craton in india in contrast to the typical early archaean granitoids which are commonly deformed into gneisses this granite is relatively undeformed the age and composition of the granite implies that continental crust of the bastar craton attained sufficient thickness to permit intracrustal melting at 3 6 ga supplementary material representative major element trace element and ree composition of the dalli rajhara granite samples and a summary of shrimp u pb zircon data for the granite sample d 9 are available at http www geolsoc org uk sup18337

crustal velocity structure of the neoarchean convergence zone between the eastern and western blocks of dharwar craton india from seismic wide angle studies abstract the dharwar craton in the southern part of the indian shield comprises two distinct blocks viz the meso neoarchean western and the predominantly neoarchean eastern blocks which are popularly referred to as western dharwar craton wdc and eastern dharwar craton edc the boundary between the wdc and edc is marked by a prominent shear zone the chitradurga eastern margin shear zone csz several studies suggested neoarchean convergence between the edc and wdc although there is diversity of opinion on the tectonic process we present here the results of a new seismic refraction wide angle reflection experiment along a 200 km long profile across the wdc and edc encompassing the csz to elucidate the crustal velocity structure of the convergence zone travel time inversion and amplitude modeling of the seismic data delineate a five layered crustal velocity model for the dharwar craton with a 42 km minimum thick crust in the wdc grading to a distinctly thinner crust up to 38 km in the edc the transect is also characterized by a high velocity 7 0 7 1 km s 1 mafic lower crustal layer but with distinctly variable thickness 10 km in wdc and 8 km in edc the transition from thick to thinner crust coincides with the csz the thick crust with a high velocity basal layer is atypical of archean cratons but more akin to proterozoic terrains the crustal velocity structure across the edc wdc boundary zone is consistent with the neoarchean plate convergence model that invoke tectonic scenarios such as accretion of a hot orogen the edc and or westward subduction of the latter beneath the wdc spatial variation in the high velocity lower crustal layer across the craton is ascribed to post accretional mafic magmatism and underplating mainly during the paleoproterozoic and phanerozoic

crustal structure beneath china from receiver function analysis 1 we collected and processed a large amount of high quality broadband teleseismic waveform data recorded by the 48 chinese national digital seismic network stations to estimate large scale lateral variations of crustal thickness and vp vs ratio hence poisson s ratio beneath china a statistical method was used to select mutually coherent receiver functions at each station which yielded over 200 traces for most of the stations with the conventional h the crustal thickness and vp vs ratio approach there is a large trade off between h and consequently multiple maxima are frequently observed in the h domain we introduced a weight function that measures the coherence between the p to s conversion and the reverberation phases at each h grid to reduce the trade off a 4th root stacking method was further applied to reduce uncorrelated noise relative to the linear stack these modifications turned out to be very effective in reducing the h trade off and yielded reliable estimates of crustal thickness and vp vs ratio the crust beneath eastern china is as thin as 31 33 km and the underlying moho is relatively flat and sharp in the western part of china the crust is considerably thicker and shows large variations the moho is observed at about 51 km depth along the tian shan fold system and about 84 km deep beneath the central part of the tibetan plateau the transition occurs at the so called n s belt between about 100 and 110 e which is featured by unusually high seismicity and large gravity anomalies the average vp vs ratio over the mainland china crust is about 1 730 0 249 significantly lower than the global average 1 78 0 27 of the continental crust this lower vp vs ratio may suggest a general absence of mafic lowermost crustal layer beneath china

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

a revised position for the rotated falkland islands microplate the early stages of transform margin formation are associated with crustal fragmentation and block rotation the restricted size of the resultant microcontinental blocks precludes palaeogeographical reconstructions and reliable estimations of the amount of rotation they can undergo an example considered here is the falkland plateau this is located adjacent to the agulhas falkland fracture zone and its westernmost province is the falkland islands microcontinent the position of the plateau and the islands prior to gondwana break up remains contentious this study integrates seismic reflection and gravity data to propose a revised position of the falkland islands microcontinent constrained by 1 the presence of a mega decollement controlling the gondwanide orogen described north of the falkland islands and underneath south africa and the outeniqua basin and 2 the similar architecture of fault networks mapped north of the islands and in the northernmost outeniqua basin this revised position requires a re evaluation of the timing and rate of rotation of the falkland islands microcontinent and affects the expected crustal architecture adjacent to the islands our model yields rotation rates between 5 5 and 8 ma 1 and two potential times for rotation and predicts more unstretched crust beneath the basin east of the falkland islands than previous models

exploration geophysics of the bushveld complex in south africa the bushveld complex consists of three roughly coeval components namely the lebowa granite suite the rashoop granophyre and the rustenburrg layered suite rls the latter being the only economically important unit the rls underlies an area of 65 000 km2 in the northeast sector of south africa and is the oldest and largest mafic layered complex in the world figure 1 it is also the largest repository of magmatic ore deposits and hosts inter alia 80 of the world s platinum group metal pgm resources and most of the chromitite resources over an east west distance of roughly 350 km the rls outcrops as three apparently discrete crescent shaped lobes the western eastern and northern limbs and lies below cover rocks along the southeastern limb the far western limb is currently not of economic significance lithologies range across mafic and ultramafic cumulates plus anorthosites pseudo stratigraphic layering exhibits dips of 8 20 toward the center of the complex the ug2 and merensky reefs constitute the major pgm bearing horizons of mining interest and fall close to the top of the 1500 m thick critical zone an assemblage of alternating leuconorite anorthosite and pyroxene rocks outcropping midway through the rls succession

p s wave velocity model of the crust and upper most mantle of albania region abstract this paper describes the one dimensional 1d velocity model computed by velest in the seisan seismic analysis system inverting re picked p wave and s wave arrival times recorded during 2002 2006 by the albanian montenegro thessalonica and macedonia seismic networks the re picked data yield p wave and s wave velocities proved to be more suitable compared to bulletin data for this detailed inversion study seismic phases recorded by the albania seismic network and integrated with data from the montenegro thessalonica and macedonia networks are used to prepare the albanian seismic bulletin earthquake hypocenters from the albanian bulletins have also location errors that are negligible for civil protection purposes large scale seismotectonic analyses and more accurate hypocentral determinations which are necessary for detailed seismotectonic and geodynamic studies it was noted that the smoothness of the velocity variation increased with depth a velocity of 5 5 km s was calculated for the upper crust 6 1 km s was calculated for the middle crust and 6 9 km s was computed for the lower crust p wave velocity was 7 85 km s at depth of 50 km and for the upper mantle it is 8 28 km s using the improved velocity model the earthquakes which occurred in albania in the past 5 years were able to be relocated achieving constrained hypocentral determinations for events in albania the interpretation of the 1 d velocity models infers interesting features of the deep structure of albania these results represent an important step towards more detailed seismotectonic analyses

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

upper mantle low velocity zone structure beneath the kaapvaal craton from s wave receiver functions summary the southern african plateau is marked by anomalously high elevations reaching 1 2 km above sea level and there is much debate as to whether this topography is compensated by a lower mantle source or by elevated temperatures in the upper mantle in this study we use s wave receiver functions srfs to estimate the lithospheric thickness and sublithospheric mantle velocity structure beneath the kaapvaal craton which forms the core of the plateau to fit the srf data a low velocity zone lvz is required below a 160 km thick lithospheric lid but the lvz is no thicker than 90 km although the lid thickness obtained is thinner than that reported in previous srf studies neither the lid thickness nor the shear velocity decrease 4 5 associated with the lvz is anomalous compared to other cratonic environments therefore we conclude that elevated temperatures in the sublithospheric upper mantle contribute little support to the high elevations in this region of southern africa

wavelet transform mapping of effective elastic thickness and plate loading validation using synthetic data and application to the study of southern african tectonics 1 we present a new wavelet transform method to map spatial variations in effective elastic thickness te and plate loading ratio f the method assumes a model of thin plate flexural isostasy to describe the mechanical response of the lithosphere to vertical loading in this model the rheological properties of the lithosphere are aggregated into the effective elastic thickness te of an equivalent thin plate overlying an inviscid fluid a number of methods have been developed to map spatial variations in te in an attempt to assess regional patterns of flexural strength our new method first obtains local coherence and local admittance through wavelet cross spectral analysis of surface topography and bouguer gravity anomaly wavelet coherence is used to obtain the local characteristic wavelength which is a function of both te and the degree of relative loading f of the plate by subsurface loads and surface loads loading is the combined effect of erosion sedimentation intrusion faulting and metamorphism wavelet admittance in a normalized form is used to resolve this f te ambiguity and maps are made of the spatial variations in te and f we carry out extensive tests of the wavelet method on simulated topography and bouguer gravity anomaly data that we generate through finite difference simulations of flexural isostasy with spectrally realistic loads and simple spatial variations in te these tests demonstrate that the wavelet inversion method is reasonably robust to uncertainties in loading and in crustal thickness and is able to recover te to within 25 50 of its correct value we apply the wavelet method to southern africa and recover estimates of te principally in the range 25 50 km in good agreement with existing estimates from forward modeling and fourier coherence analyses we find that our te estimates often exceed estimates of crustal thickness pointing to a strong upper mantle in parts of southern africa

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

precambrian crustal structure in africa and arabia evidence lacking for secular variation we review the thickness and shear wave velocity structure of precambrian crust in africa and arabia where over 90 of the landmass is comprised of archean and proterozoic terranes and examine the data for evidence of secular variation the data come from many published 1d shear wave velocity profiles obtained by jointly inverting receiver functions and surface wave dispersion measurements 35 new 1d shear wave velocity profiles for locations in eastern africa and a new map of crustal thickness for africa and arabia derived from modeling satellite gravity data we find for both archean and proterozoic terranes a similar range of crustal thicknesses 33 45 km similar mean crustal shear wave velocities 3 6 3 7 km s and similar amounts of heterogeneity in lower crustal structure as reflected in the thickness of lowermost crust with shear wave velocities 4 0 km s there is little evidence for secular variation in crustal structure indicating that there may have been few changes over much of earth s history in the processes that form the continental crust post formation tectonic events also may have modified many of the terranes to such an extent that secular trends arising from crustal genesis may be difficult to recognize

shear wave velocity structure of the bushveld complex south africa the structure of the crust in the environs of the bushveld complex has been investigated by jointly inverting high frequency teleseismic receiver functions and 2 60 s period rayleigh wave group velocities for 16 broadband seismic stations located across the bushveld complex group velocities for 2 15 s periods were obtained from surface wave tomography using local and regional events while group velocities for 20 60 s periods were taken from a published model 1 d shear wave velocity models obtained for each station show the presence of thickened crust in the center of the bushveld complex and a region at the base of the crust where shear wave velocities exceed 4 0 km s the shear wave velocity models also suggest that velocities in some upper crustal layers may be as high as 3 7 3 8 km s consistent with the presence of mafic lithologies these results favor a continuous sheet model for the bushveld complex in which the outcropping mafic layers of the western and eastern limbs are continuous at depth beneath the center of the complex however detailed modeling of receiver functions at one station within the center of the complex indicates that the mafic layering may be locally disrupted due to thermal diapirism triggered by the emplacement of the bushveld complex or thermal and tectonic reactivation at a later time

vertical deformation monitoring at axial seamount since its 1998 eruption using deep sea pressure sensors pressure measurements made on the seafloor at depths between 1500 and 1700 m at axial seamount an active submarine volcano on the juan de fuca ridge in the northeast pacific ocean show evidence that it has been inflating since its 1998 eruption data from continuously recording bottom pressure sensors at the center of axial s caldera suggest that the rate of inflation was highest in the months right after the eruption 20 cm month and has since declined to a steady rate of 15 cm year independent campaign style pressure measurements made each year since 2000 at an array of seafloor benchmarks with a mobile pressure recorder mounted on a remotely operated vehicle also indicate uplift is occurring in the caldera at a rate up to 22f1 3 cm year relative to a point outside the caldera the repeatability of the campaign style pressure measurements progressively improved each year from f15 cm in 2000 to f0 9 cm in 2004 as errors were eliminated and the technique was refined assuming that the uplift has been continuous since the 1998 eruption these observations suggest that the center of the caldera has re inflated about 1 5f0 1 m thus recovering almost 50 of the 3 2 m of subsidence that was measured during the 1998 eruption this rate of inflation can be used to calculate a magma supply rate of 1410 6 m 3 year if this rate of inflation continues it also suggests a recurrence interval of 16 years between eruptions at axial assuming that it will be ready to erupt again when it has re inflated to 1998 levels d 2005 elsevier b v all rights reserved

archean magmatic granulites diapirism and proterozoic reworking in the northern marginal zone of the limpopo belt the northern marginal zone nmz of the limpopo belt southern africa is a high grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite which intruded minor mafic ultramafic and metasedimentary rocks between 2 74 and 2 57 ga the intrusive rocks have crustal and mantle components and occur as elliptical bodies interpreted as diapirs peak metamorphism p 800 mpa t 800 850 c occurred at ca 2 59 ga the highly radiogenic nature of the rocks in the nmz supplemented by heat from mantle melts led to heating and diapirism culminating in the intrusion of distinctive porphyritic charnockites and granites horizontal shortening and steep extrusion of the nmz during which crustal thickening was limited by high geothermal gradients contrast with overthickening and gravitational collapse observed particularly in more recent orogens the granulites were exhumed by the end of the archean the pervasive late archean shortening over the whole of the nmz contrasts with limited deformation on the zimbabwe craton possibly owing to the strengthening effect of early crust in the craton in the southeast of the nmz strike slip kinematic indicators occur within the transition zone and the triangle shear zone where dextral shearing reworked the archean crust at ca 1 97 ga

the jurassic history of the africa antarctica corridor new constraints from magnetic data on the conjugate continental margins abstract finding the best fit for east and west gondwana requires a detailed knowledge of the initial jurassic movements between africa and antarctica this study presents results of systematic and densely spaced aeromagnetic measurements which have been conducted in 2009 2010 across the astrid ridge antarctica and in the western riiser larsen sea to provide constraints for the early seafloor spreading history between both continents the data reveal different magnetic signatures of the northern and southern parts of the astrid ridge which are separated by the astrid fracture zone the southern part is weakly magnetised corresponding to the low amplitude anomaly field of the southwestern riiser larsen sea in contrast the northern astrid ridge bears strong anomalies of positive value furthermore several sets of trends are visible in the data in the mozambique channel we extended the existing magnetic spreading anomaly identifications combined with the spreading anomalies in the conjugate riiser larsen sea they were used to establish a new model of the relative movements of africa and antarctica after the breakup of gondwana in jurassic times a detailed model for the emplacement of the mozambique ridge is now incorporated the model postulates a tight fit between africa and antarctica and two stages of breakup the first of which lasting until 159 ma m33n during this stage antarctica rotated anticlockwise with respect to africa the grunehogna craton cleared the coastal plains of mozambique to a position east of the mozambique fracture zone the southern astrid ridge is interpreted to consist of oceanic crust also formed during this first stage prior to the riiser larsen sea during the second stage antarctica moved southward with respect to africa forming the mozambique basin and the conjugate riiser larsen sea the mozambique ridge and the northern natal valley were formed at different spreading centres being active subsequently

heterozoan carbonates when where and why a synthesis on parameters controlling carbonate production and occurrences abstract in modern and phanerozoic times heterozoan carbonates group a large array of depositional environments from the poles to the tropics this global assessment reviews the critical parameters and controlling factors of heterozoan carbonates i stratigraphic and global distributional trends ii oceanographic and trophic relationships and iii biological and sedimentary processes well documented case studies n 129 have been investigated when facies and stratigraphic attributes were available and when environmental settings such as oceanography climate and paleogeography were clearly identified these case studies occur during specific periods of time in the sedimentary record e g late ordovician early carboniferous early permian and neogene while they are absent during others e g triassic and jurassic periods during greenhouse to icehouse transitions and those with active thermohaline circulation are particularly conducive for heterozoan carbonates based on global oceanographic patterns and existing ecological and hydrodynamic regime classifications hallock 2001 pedley and carannante 2006 james and jones 2015 a process based classification scheme is proposed that distinguishes two main heterozoan carbonate systems i a highly productive system characterized by suspension feeding biota and ii an oligo mesotrophic warm temperate system characterized by red algal and seagrass derived biota eight depositional models are proposed that combine i the source of energy for metabolic activities that allow the carbonate producing biota to thrive and ii hydrodynamics and physiography of the depositional system that control the sediment accumulation the local and platform scale based profiles can be used to complement the facies approach of james and jones 2015

architecture and paleogeography of the early paleozoic carbonate systems in the east central tarim basin china constraints from seismic and well data abstract the evolution of the cambrian middle ordovician carbonate systems in the east central tarim basin northwestern china has been investigated using 2d seismic profiles and well data interpretation was performed on seismic profiles flattened along an originally horizontal surface base of the cambrian to remove the effects of post depositional deformations six seismic facies differently arranged in six seismic stratigraphic units record four major stages of architectural evolution from ramp to flat topped systems with differences from north to south 1 the onset of a homoclinal ramp over a clayey unit at the beginning of early cambrian 2 the development of a prograding carbonate platform characterized by restricted evaporitic facies in the inner platform domain during early cambrian to late cambrian 3 the development of a first prograding and a aggrading retrograding to the north platform during lower ordovician 4 the development of a prograding retrograding to aggrading final platform during middle ordovician the basinward eastward progradation of the carbonate platform edge has been reconstructed in time steps highlighting differences from the north to the south of the study area in terms of amount of progradation platform to basin relief and depositional surfaces likely controlled by differences in subsidence and environmental factors the identification of the four major stages in the evolution of the lower paleozoic succession of the tarim basin was likely controlled by changes in the rate of creation destruction of accommodation space due to the interplay between eustatic changes and tectonic events the demise of the cambrian to middle ordovician carbonate system in the study area is associated with a major tectonic event recorded by a tilting of the succession toward the east onlapped by sediments coming from the east and an uplift bulge of the western part of the study area

an intrusive origin of some ug 1 chromitite layers in the bushveld igneous complex south africa insights from field relationships abstract the ug 1 chromitite in the bushveld complex is a 1 m thick massive layer overlain by orthopyroxenite 4 9 m thick and underlain by anorthosite 1 5 2 0 m thick which is strongly interleaved with numerous thin layers of ug 1 chromitite mm to 10 s of cm thick the chromitite bearing anorthosite is further underlain by mottled anorthosite which is devoid of chromitites sometimes the massive ug 1 chromitite may be hosted entirely within orthopyroxenite in this study however the orthopyroxenite occurs intermediate to the massive ug 1 layer and the footwall interleaved anorthosite unit which has a 10 cm thick chromitite layer at the contact though the ug 1 chromitites display complex features in the field outcrops e g presence of numerous anorthosite autoliths bifurcations and dimpled contacts with anorthosite the ones discussed in this study are distinctive and diagnostic of the emplacement mechanism for this chromitite layer in this study of the ug 1 chromitite at the impala platinum mine the chromitite is seen to form a small pothole 1 5 m deep 3 6 m wide that cuts down into the underlying orthopyroxenite exposed only on the right side of the pothole and anorthosite layers a 15 cm thick ug 1 chromitite layer 85 modal chromites is observed to protrude or branch out of the pothole margin and laterally extend into the anorthosite with which it shows several crosscutting features examples include truncation of layering defined by interstitial pyroxene mottles within the footwall anorthosite against the protruding ug 1 layer and truncation of several thinner ug 1 layers in the hanging wall anorthosite against erosive outlines of this chromitite layer these observations strongly attest to the emplacement of the protruding ug 1 chromitite layer as a sill into the anorthosite the presence of thin anorthosite lenses with high aspect ratios within this ug 1 layer along with lateral variations in thickness of the layer its irregular contact with the host anorthosite and disturbances observed for the ug 1 layers in the footwall anorthosite adjacent to the erosive contact implies that the superheated melt parental to the chromitite chemically dissolved the anorthosite and made space for it to flow as a sill within the latter the probable mode of emplacement of the laterally protruding ug 1 layer would be through the opening of fractures related to the event of pothole formation where there is ponding of hot thick and dense melt above the anorthosite the melt subsequently filled the fractures and flowed laterally into the anorthosite thereby eroding it an open magmatic system ensured continuous addition of melts in the pothole and their supply into the sill this study thus emphasizes that some of the ug 1 chromitite layers within anorthosite can be emplaced as sills and not all need to form from discrete cyclical pulses of chromite saturated and plagioclase saturated melts being brought into the chamber

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

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

an integrated geophysical study of the beattie magnetic anomaly south africa abstract the source of the beattie magnetic anomaly bma still remains unclear with several competing hypotheses here we add a piece to the puzzle by investigating available potential field data over the anomaly filtered magnetic data show the bma as part of a group of linear magnetic anomalies as the linear anomaly north of the bma is associated with exposed supracrustals migmatites and shear zones within the natal thrust terranes we assume a similar source for the bma this source geometry constrained by seismic and mt data fits potential field data over the bma and other magnetic linear anomalies in the south central and south western karoo in these models the bodies deepen from 5 km towards the south with horizontal extents of 20 60 km and vertical extents of 10 15 km densities range from 2800 to 2940 kg m 3 and magnetic susceptibilities from 10 to 100 10 3 si these magnetic susceptibilities are higher than field values from supracrustal rocks 10 60 10 3 si but could be due to the fact that no remanent magnetisation was included in the model the lithologies associated with the different linear anomalies vary as is evident from varying anomaly amplitudes the strong signal of the bma is linked to high magnetic susceptibility granulite facies supracrustals 10 50 10 3 si as seen in the antarctic where the mobile belt continued during gondwana times