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

the proterozoic of eastern bolivia and its relationship to the andean mobile belt abstract the previously unmapped bolivian sector of the central brazil shield 220 000 km in area has been studied during an anglo bolivian technical co operation programme the metamorphic basement comprises the lomas maneches granulitic complex the chiquitania gneissic complex and the san ignacio schist supergroup which crops out as metasedimentary belts rb sr dates on some of the granulites suggest a protolith age of 2000 ma but the outcrop pattern of the metamorphic rocks as a whole is due to the 1300 ma san ignacio orogeny which was accompanied by widespread granite emplacement and metamorphism up to granulite facies the sunsas cycle began with the deposition of the supracrustal sunsas vibosi group and closed with the development of the sunsas and aguapei mobile belts at 1000 ma the upper proterozoic brasiliano cycle is represented by a bifurcating sedimentary basin in the se of the area with a closing tectonic event of 520 ma correlation is suggested with units in the central brazil shield the bolivian belts are extrapolated east into brazil and their limits indicated to the west the belts show trends parallel to the phanerozoic andean chain and they reflect the major deflection in the chain the arica elbow furthermore when the belts are traced towards the andes their ages broadly correspond to those of the sub andean precambrian basement this leads to the proposal of an enlarged andean orogenic belt system 1000 km wide comprising a 1300 ma old proto andean orogen which has controlled the trends of subsequent mobile belts

chemostratigraphy of the tamengo formation corumba group brazil a contribution to the calibration of the ediacaran carbon isotope curve abstract the corumba group cropping out in the southern paraguay belt in brazil is one of the most complete ediacaran sedimentary archives of palaeogeographic climatic biogeochemical and biotic evolution in southwestern gondwana the unit hosts a rich fossil record including acritarchs vendotaenids vendotaenia eoholynia soft bodied metazoans corumbella and skeletal fossils cloudina titanotheca the tamengo formation made up mainly of limestones and marls provides a rich bio and chemostratigraphic record several outcrops formerly assigned to the cuiaba group are here included in the tamengo formation on the basis of lithological and chemostratigraphical criteria high resolution carbon isotopic analyses are reported for the tamengo formation showing from base to top 1 a positive 13c excursion to 4 pdb above post glacial negative values 2 a negative excursion to 3 5 associated with a marked regression and subsequent transgression 3 a positive excursion to 5 5 and 4 a plateau characterized by 13c around 3 a u pb shrimp zircon age of an ash bed interbedded in the upper part of the 13c positive plateau yielded 543 3 ma which is considered as the depositional age babinski et al 2008a the positive plateau in the upper tamengo formation and the preceding positive excursion are ubiquitous features in several successions worldwide including the nama group namibia the dengying formation south china and the nafun and ara groups oman this plateau is constrained between 542 and 551 ma thus consistent with the age of the upper tamengo formation the negative excursion of the lower tamengo formation may be correlated to the shuram wonoka negative anomaly although 13c values do not fall beyond 3 5 in the brazilian sections sedimentary breccias occur just beneath this negative excursion in the lower tamengo formation one possible interpretation of the origin of these breccias is a glacioeustatic sea level fall but a tectonic interpretation cannot be completely ruled out

the paranapanema lithospheric block its importance for proterozoic rodinia gondwana supercontinent theories abstract this work considers the tectonics of the southeastern portion of the south american platform based on new geological and geophysical grounds for the last decade only three amazonic sao francisco and la plata of the many other cratonic blocks have been attributed remarked to the south america portion for most of the usual rodinia reconstitutions the possibility of the existence of other blocks has rarely been mentioned the postulation of the presence of a considerable paleoproterozoic pre brasiliano fragment as part of parana basin basement is highly probable in order to infer the basement structure of parana basin previous to the sedimentation process an isostatic modeling was applied to a large scale gravity survey looking to correlate topographic and gravity anomalies caused by sub surface loads the bouguer anomaly obtained from the gravity survey represents the crustal contribution of crystalline basement in addition to the sedimentary and volcanic layers of the basin following the isostatic modeling and the basin load stripping the residual anomaly allows observing similarities between the basement gravity signature and outcropping units besides the stress pattern of the two earlier events obtained through the back stripping analysis presents a geographically coincident maximum and a new e se high emerging for the second event suggesting continuous change of the stress field as a precursor for south american plate rotation the evident correlation between gravity highs and main attenuation suggests the presence of some pre existing suture zones the weakened lithosphere during ordovician and carboniferous provided the magma conduits to form in early cretaceous tectonic stress field pattern the resultant mosaic of gravity blocks and the main faults site give support to the presence of this cratonic proterozoic unit here on referred to as the paranapanema block which had been neglected in most of the models reported for the reconstruction of gondwana and rodinia

c o sr isotopic stratigraphy of cap carbonates overlying marinoan age glacial diamictites in the paraguay belt brazil abstract the neoproterozoic paraguay belt located along the southeastern border of the amazon craton and eastern border of the rio apa block comprises a thick passive margin succession of glaciomarine turbidites carbonates and siliciclastic sedimentary rocks that were deformed during the brasiliano pan african orogeny the carbonate sequence comprises a 1300 m thick succession of platformal carbonate rocks araras formation which directly overlie marinoan glacial diamictites puga formation the late neoproterozoic age of this carbonate sequence is indicated by the presence of cloudina lucianoi and corumbela werneri found in the upper part of the corumba group southern paraguay belt carbon and sr isotope data were obtained on marine carbonate samples across three different sections of the araras formation 13 c pdb values of the carbonate from the border of the basin range between 10 5 and 2 7 including a short stratigraphic interval of cap dolomite 22 m directly overlying diamictites the isotopic profile across the basal 200 m of deep shelf laminated microcrystalline limestone and clay limestone cap carbonate that overlie glacial diamictites also show negative 13 c pdb values 5 3 and 0 6 in contrast with the lower portion of the carbonate sequence a 800 m thick unit of shallow water dolostones forming the upper part of the carbonate sequence present uniform positive 13 c pdb values ranging from 1 9 to 2 4 dolostone and arenaceous dolostone of the upper parts of the araras formation have high 13 c pdb values 4 1 to 9 6 which are followed by an abrupt decrease in 13 c values down to 1 0 the carbon isotope profile of the paraguay belt is quite different from the bambui group an other important neoproterozoic sequences in central brazil which also overlie glacial sediments negative 13 c pdb values are found only within the lowermost meters of these sequence and are followed by an important and extensive positive carbon isotope excursion values ranging from 6 9 to 16 0 on the other hand the 13 c pdb profile observed across the araras formation exhibits an approximately 200 m thick section of carbonates with low 13 c pdb values carbonates of the araras formation have 87 sr 86 sr ratios ranging from 0 70753 to 0 70803 comparable to cap carbonates that overly marinoan glacial deposits from other neoproterozoic sequences

the neoproterozoic cambrian paraguay belt central brazil part i new structural data and a new approach on the regional implications abstract together with the araguaia and brasilia belts the paraguay belt forms in central brazil the tocantins province that is one of the largest orogens of western gondwana the corumba area occupies the site where the northern and southern parts of the paraguay belt form together with the chiquitos tucavaca aulacogen stretching e w in the adjacent bolivian territory an r r r basin system opened filled in the 700 650 540 ma interval within the amazon rio apa paleo continent the sedimentary volcanic rocks of the jacadigo and corumba groups found around the corumba city record part of the neoproterozoic cambrian passive margin precursor of the paraguay belt our pioneer structural analysis reveals that these rocks experienced progressive deformation phases d 1 d 2 d 3 and low grade metamorphism during the brasiliano cycle 540 513 ma the crystalline basement was also involved according to structural data and k ar ages in the literature the paleo passive margin was thickened during the d 1 d 2 deformation and was lately shortened d 3 in two orthogonal directions se nw d 3p and sw ne d 3t developed co axially and verging to nw d 1 d 2 d 3p structures record the closure of the basin precursor of the paraguay belt whereas d 3t structures seem related to the inversion of the aulacogen although the tectonic transport to nw as observed in the corumba area matches the reported transport of paraguay belt s supracrustal rocks towards the eastern margin of the rio apa block and araguaia belt s rocks towards the amazon craton the transport direction is opposite in other parts of the paraguay belt our comprehensive discussion of these facts brings to light profound regional implications

crustal framework of the northwest parana basin brazil insights from joint modeling of magnetic and gravity data abstract the transbrasiliano lineament tbl is a major proterozoic strike slip shear zone crossing the parana basin brazil yet the relationship between the tbl and basin structures is poorly understood magnetic and gravity data are available from the whole northwestern region of the parana basin and provide insights into crustal partitioning and basement geometry joint 2d modeling of these data was performed in three strategic sections of the study area two smaller sections were constrained by available borehole data and receiver function analysis modeling of these sections facilitated modeling of the third larger nw se section which crosses the basin in an area with significant lack of data these three sections together form the basis for a preliminary tectonic model of the northern parana basin the tbl includes strong n30 e trending gravity and magnetic gradients represented by the serra negra fault zone this structure separates two crustal tectonic domains the paranapanema block to the east and the amazon paleocontinent to the west the tectonic domain northeast of the fault is represented by the arenopolis magmatic arc the n60 70 e trending general carneiro fault also reflected in gravity anomalies forms another important magnetic contact our tectonic model shows a region of crustal thinning between the two fault zones where structures associated with possible reactivation form small cretaceous basins the baliza fault may accommodate significant vertical displacements of devonian and carboniferous sections of the parana basin the main structures revealed in the sections are associated with grabens and horsts interpreted as reactivated structures that controlled parana basin sedimentation the northwestern portion of the basin exhibits basement asymmetry caused by dip and strike slip faulting along the tbl

timing and role of the maranhao river thrust in the evolution of the neoproterozoic brasilia belt and tocantins province central brazil abstract a low angle thrust fault places high pt granulites hangingwall of the internal zone of the neoproterozoic brasilia belt tocantins province central brazil in contact with a lower grade footwall external zone comprised of nappes of distal passive margin and back arc basin related supracrustals the footwall units were emplaced at 750 ma onto proximal sedimentary rocks paranoa group of the sao francisco paleo continent passive margin the high pt belt is comprised of 645 630 ma granulite facies paragneiss and orthogneiss and mafic ultramafic complexes that include three major layered intrusions and metavolcanic rocks granulitized at 750 ma these complexes occur within lower grade metasedimentary rocks in the hangingwall of the maranhao river thrust which forms the internal zone external zone boundary fault to the north of the pirineus zone of high strain detailed lithostructural studies carried out in maranhao river thrust hangingwall and footwall metasedimentary rocks between the niquel ndia and barro alto complexes and also to the east of these indicate the same lithotypes and sm nd isotopic signatures and the same d1 d2 progressive deformation and greenschist facies metamorphism additionally footwall metasedimentary rocks exclusively display a post d2 deformation indicating that the maranhao river thrust propagated through upper crustal rocks of the paranoa group relatively late during the tectonic evolution of the belt fault propagation was a consequence of intraplate underthrusting during granulite exhumation the results allow for a better tectonic understanding of the brasilia belt and the tocantins province as well as explaining the presence of the pirineus zone of high strain

the kaapvaal craton seismic anisotropy petrophysical analyses of upper mantle kimberlite nodules a dense network of seismic stations has been deployed on the kaapvaal craton south africa to investigate the upper mantle seismic structures in order to bring independent petrophysical constraints we analyze a direct sampling of the cratonic upper mantle and determine the seismic properties of 48 mantle nodules brought up to the earth s surface by kimberlite eruptions seismic properties of these nodules are calculated from the olivine and pyroxene crystal preferred orientations and the single crystal elastic constants despite variations in the nodules compositions microstructures and crystallographic preferred orientations seismic anisotropy is rather homogeneous throughout the craton mean s wave anisotropy is weak 2 64 which is compatible with the small measured sks wave splitting mean delay time of 0 62 s

crustal structure beneath the paleozoic parnaiba basin revealed by airborne gravity and magnetic data brazil abstract the parnaiba basin is a large paleozoic syneclise in northeastern brazil underlain by precambrian crystalline basement which comprises a complex lithostructural and tectonic framework formed during the neoproterozoic eopaleozoic brasiliano pan african orogenic collage a sag basin up to 3 5 km thick and 1000 km long formed after the collage the lithologic composition structure and role in the basin evolution of the underlying basement are the focus of this study airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the parnaiba basin results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding neoproterozoic fold belts in the basement triangular shaped basement inliers are geophysically defined in the central region of this continental scale neoproterozoic convergence zone a 3 d gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36 40 5 km deep crustal root with borders defined by a high density and thinner crust forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by brasiliano shear zones representing lithospheric sutures of the amazonian and sao francisco cratons tocantins province and parnaiba block in addition coincident residual gravity residual magnetic and pseudo gravity lows indicate two complex systems of eopaleozoic rifts related to the initial phase of the sag deposition which follow basement trends in several directions

the paleoproterozoic campinorte arc tectonic evolution of a central brazil pre columbia orogeny abstract the campinorte arc is a poorly exposed 2 19 2 07 ga paleoproterozoic terrane in contact with the neoproterozoic goias magmatic arc by the rio dos bois fault in the northern brasilia belt central brazil the campinorte arc is divided into pau de mel suite metatonalites to metamonzogranites and the campinorte volcano sedimentary sequence pau de mel suite whole rock geochemistry indicates at least three separate coeval parental magmas compatible with arc signatures u pb geochronology data of paragranulite and mafic granulite exposed in the region as part of the campinorte arc provide additional information on this paleoproterozoic orogenic cycle the formation and preservation of these granulites was due to tectonic switching of the back arc basin and consequent lithospheric thinning from 2 14 to 2 09 ga with metamorphic peak from 2 11 to 2 08 ga a pau de mel suite granodiorite sample dated at ca 2 08 ga indicates post peak magmatism the arc was thereafter rapidly contracted preserving paleoproterozoic high metamorphic grade mineral assemblages additionally the campinorte arc and the neighboring crixas guarinos faina greenstone belts may have shared the same source of sediments as attested by geographic proximity coeval maximum sedimentation felsic volcanism and the occurrence of similar rock types gravimetric and seismic data also support a common basin hypothesis the formation of the campinorte arc is contemporaneous to other northern brasilia belt basement terranes and along with similar arcs within and at the sao francisco craton edges indicate a continental crust formation event that eventually led to the assemblage of columbia

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

journey beneath southern africa seismic analyses of the lithosphere undertaken as part of the kaapvaal project provide an unprecedented view of cratons the earliest parts of continental landmasses

surface crusts on soils sediments of the southern aral sea basin uzbekistan soils and sediments of the southern aral sea basin of central asia are commonly covered by salt bearing surface crusts these crusts represent a potential source of salt rich dust that can contribute to salinisation of soils in a large area crust types include takyric crusts surface crusts on solonchaks and salt crusts covering dry parts of the lake bed the takyric crusts are structural crusts showing features related to flooding and shrinkage gypsum formed within the sediment matrix is the only evaporite mineral they contain one studied solonchak crust is fine grained with a thin thenardite and bloedite coating along the irregular surface and halite coatings along the sides of pores all salts in this crust formed by evaporation of solutions contained in the groundmass a solonchak crust with a sandy texture contains prismatic gypsum crystals that formed in a water saturated surface layer as well as bloedite eugsterite and halite formed by evaporation of interstitial brines the lake bed crust is a pure accumulation of salt minerals bloedite hexahydrite halite kainite with synsedimentary and diagenetic features salt mineral assemblages in the three crust types are those that form during successive stages of evaporation of aral sea waters in agreement with published theoretical predictions

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

continental collision and accretion recorded in the deep lithosphere of central china abstract basaltic andesitic volcaniclastic diatremes erupted in the mesozoic 160 ma in the xinyang area henan province of central china the locality lies on the western margin of the eastern block of the north china craton and the southern margin of this craton the diatremes carry a suite of xenoliths including lower crustal mafic to felsic granulites eclogites meta gabbros pyroxenites and mantle peridotites the petrological and geochemical data show that the crust in the area is temporally and compositionally zoned exposed rocks up to ca 2 85 ga old underlain by felsic granulites and rare pyroxenites with zircon ages of 3 6 3 4 ga to ca 30 km depth deeper ca 30 45 km parts of the lower crust consist of high pressure mafic to felsic granulites and meta gabbro which give paleoproterozoic 2 0 1 8 ga zircon ages our data show the significance of underplating vertical crustal growth in the paleoproterozoic which was related to the amalgamation of the eastern and western block of this craton and a global columbia supercontinent assembly hf isotope data indicate that both juvenile material and remelting of older 3 8 3 0 ga crustal rocks were involved in this important event paleozoic 440 260 ma and early mesozoic 228 219 ma zircons are also found in xenoliths from the deeper part of the lower crust and the uppermost mantle i e eclogite and peridotite they are interpreted as reflecting geodynamic processes related to the continental collision between this craton and the yangtze craton respectively the xenoliths from xinyang diatremes thus record the growth and modification of the old paleo mesoarchean continental lithosphere by magma underplating during several tectonic events assembly of the southern and northern parts of the eastern block in neoarchean time collision of the western and eastern blocks along the trans china orogen in the paleoproterozoic and subduction and collision of the yangtze craton with the north china craton in early paleozoic and triassic times respectively

using nd isotopes to understand the provenance of sedimentary rocks from a continental margin to a foreland basin in the neoproterozoic paraguay belt central brazil abstract the neoproterozoic paraguay belt pb in central south america includes a thick sequence of sedimentary rocks that were deposited and deformed along the southeastern margin of the amazonian craton the stratigraphy is divided into four major successions 1 a lower succession of fine grained sedimentary rocks in the cuiaba group 2 a glacial diamictite turbidite succession in the cuiaba group and puga formation 3 a carbonate succession in the araras group and 4 an upper succession composed of sandstones siltstones and arkoses in the alto paraguay group this sequence of sedimentary rocks represents a passive margin that was deformed during late neoproterozoic cambrian epoch we investigated the nd isotope signature of the sedimentary rocks while considering the different lithostratigraphic sequences and their geographic extent within the pb nd isotopic ratios and tdm model ages suggest a dominant continental source where are related to paleoproterozoic to mesoproterozoic rocks of the amazonian craton the nd isotopic compositions do not display a major contribution from the younger neoproterozoic juvenile magmatic arc terrane of the brasilia belt tdm model ages plotted against the stratigraphy show old tdm model ages of ca 2 3 1 5 ga and strongly negative nd t values of 11 to 6 revealing sedimentary changes in the source the changing nd t values are interpreted to be function of the erosion deposition ratio that occurred due to uplift during orogenic processes causing variations in the source area compared to the lower glacial and carbonate successions the upper siliciclastic succession shows lower nd isotopic ratios that is consistent with a source from the paraguay belt or another neoproterozoic continental source this can be accommodated by the change from an extensional continental margin to a compressional foreland basin setting this provenance study of the paraguay belt sedimentary rocks has important implications for the west gondwana collision history prior to its final amalgamation at the end of the neoproterozoic and beginning of the cambrian epoch

40ar 39ar constraints on the age and thermal history of the urucum neoproterozoic banded iron formation brazil urucum is one of the youngest banded iron formations bifs yet its exact age remains uncertain 40 ar 39 ar geochronology on late diagenetic to early metamorphic cryptomelane from the urucum sequence reveals a minimum depositional age of 587 7 ma metamorphic braunite age spectra yield flat segments defining apparent ages of 547 3 ma to 513 4 ma interpreted as recrystallization or cooling ages metamorphic muscovite grains from a meta arkose interbedded with the bif yield reproducible plateau ages of 513 3 ma cryptomelane ages are interpreted to record the minimum timing of the extensional tectonic events that generated the graben systems at the southeastern border of the amazon craton including the chiquitos tucavaca aulacogen and the jacadigo basin which hosts the urucum banded iron formation structurally controlled hydrothermal alteration resulted in crystallization of braunite and muscovite during the interval between 547 and 513 ma coeval with post collisional decompression that resulted in granite emplacement e g sao vicente granite in the paraguay belt subsequent uplift and erosion of the precambrian sequence possibly succeeded metamorphism a chemically distinct cryptomelane generation identified in surface samples yield ca 60 ma results these ages provide evidence for supergene recrystallization after exhumation in the mesozoic cenozoic 40 ar 39 ar systematics implies that the oldest cryptomelane samples experienced ar loss probably via thermally induced diffusion this suggests that the urucum bif was buried and heated to significant temperatures in the late proterozoic consistent with present mineralogy and isotopic composition that do not reflect the original characteristics of the depositional environment

structural and compositional constraints on the emplacement of the bushveld complex south africa abstract despite a plethora of petrological studies the emplacement mechanics of the world s largest layered intrusion the 2 05 ga bushveld complex are still poorly understood early models considered the intrusion to comprise separate lopolithic intrusions or even concentric cone sheets but recently overwhelming support for a sill like intrusional form has emerged examination of the contact aureole reveals three groups of emplacement related structures interfinger deformation zones and bridges formed between intruding and dilating magma fingers preserved in both the western and eastern parts of the complex sub bushveld complex intrusions that reflect finger like emplacement into the underlying contact aureole and diapiric domes that characterize the eastern contact aureole formed by diapiric amplification of initial interfinger deformation zones associated with the earliest mafic ultramafic pulse of the bushveld complex most of these structures exhibit a strong nw se preferred orientation while longitudinal terminations and divergence of the sub bushveld intrusions away from their source horizons indicate magma emplacement towards the se this emplacement direction is supported by petrological data lateral lithological facies variations and mineral chemical variations within specific horizons the thickest and most chemically primitive accumulations of the lower zone of the complex are found adjacent to and thin towards the ne and se away from the thabazimbi murchison lineament tml a crustal scale lineament that has undergone polyphase reactivation since at least 2 7 ga the bushveld complex was most likely fed by a feeder dyke that utilised the tml spreading laterally from the dyke axis to form its current sill like geometry the stress field at 2 05 ga was suitably oriented to allow for dilation of the ene trending tml suggesting the kaapvaal craton was subject to a component of nw se extension during this time nw se directed collision of the kaapvaal and zimbabwe cratons is recorded in the limpopo belt which would suggest emplacement under conditions of far field extensional stress within a back arc setting this is corroborated by petrological data that supports a significant component of subducted crust in the bushveld magmas

the araguaia fold belt brazil a reactivated brasiliano pan african cycle 550 ma geosuture abstract the araguaia tocantins geosuture which separates the araguaia fold belt afb from the archean amazonian craton was active in the late middle proterozoic the baixo araguaia supergroup was deposited consisting of the estrondo group lower quartzites with intercalated schists xambioa formation schists and canto da vazante formation upper feldspathic schists and the tocantins group consisting of the couto magalhaes formation phyllites quartzites slates limestones and metacherts and pequizeiro formation upper chlorite schists and associated mafic ultramafic bodies the deformational history includes four regional phases of deformation within this supracrustal sequence recumbent folds with vergence to the west refolding with a n s trend an intense crenulation episode and late thrusting from east to west metamorphism is of intermediate or intermediate high pressure type with garnet biotite chlorite and sericite isograds succeeded by a slightly or non metamorphosed zone from east to west rocks surrounding sparse gneissic cored domes contain isograds of staurolite kyanite and fibrolite these isograds are believed to be associated with the 1100 ma uruacuano event the brasiliano orogeny strongly affected the afb with displacements due to transcurrent reactivation of great and old faults of the basement slight folding in the supracrustal sequence intrusion of small granite bodies and development of domes with associated normal faults the area underlain by the estrondo group was uplifted at this time causing the deposition of the rio das barreiras polymictic conglomerate of the central area k ar and rb sr analyses date this thermo tectonic event at 550 100 ma the archean basement is exposed in the cores of domes as a granite gneiss association the colmeia complex which shows thermo tectonic features that may be interpreted as polycyclic imprints jequie transamazonian uruacuano and brasiliano events

the araripe basin in ne brazil an intracontinental graben inverted to a high standing horst abstract the mesozoic sediments of the araripe basin peak at ca 1000 m more than 500 m above the host precambrian basement this means that an intracontinental graben has been inverted to a topographically high standing horst which raises four fundamental questions 1 where are the inversion structures 2 what is the age of the inversion 3 how can inversion occur in a thick lithosphere 4 what forces are responsible for the inversion here we show that displacement on inverted normal faults increases towards the core of the horst and new reverse faults thrust basement over basin field observations indicate that inversion has occurred on faults striking between nne ssw and e w following the structure of the host basement inversion is reverse dextral in the nne ssw system and reverse sinistral in the ene wsw system this is consistent with the mid atlantic ridge and andean pushes from the tectonic analysis of topography and gravity anomaly the nw se faults seem critical to the inversion although not clearly recognized in the field the inversion of high angle normal faults occurred by oblique slip and injection of clay rich breccias which apparently reduced friction sufficiently to allow for major inversion fault scarps with a prominent topographic expression and faults cutting through colluvium indicate that the latest stage of inversion is quaternary although older inversion episodes are likely the new tectonic data the timing and the stress field altogether indicate that inversion was due to plate wide compression which preferentially concentrated along inherited shear zones and reactivated them to form the main inversion faults of the araripe basin the high degree of inversion in the araripe basin is most likely due to its position at a bend in the major patos shear zone which worked as a restraining bend under the maximum compressive stress acting in ne brazil

archean emplacement of eclogitic components into the lithospheric mantle during formation of the kaapvaal craton re os data for selected whole rock eclogites and eclogitic sulfide inclusions in diamonds from seven kimberlites on the kaapvaal zimbabwe cratons yield ages around 3 ga the presence of such old eclogitic components in the subcontinental lithospheric mantle is more common than previously thought overlap of these ages with those of many cratonic peridotites demonstrates that incorporation of eclogitic materials accompanied or closely followed cratonic lithosphere stabilization many of these eclogitic components have fractionated c n o and s isotopic compositions which could have been created at low temperatures implying that some style of archean subduction involving emplacement of oceanic lithosphere was part of craton keel development

the southern border of the guapore shield in western brazil and bolivia an interpretation of its geologic evolution abstract the geology of the southeastern and southwestern margins of the guapore shield is described the shield rocks comprise high grade granulite and low grade greenschist sequences these rocks are poorly known oldest recorded ages include transamazonian datings 2 ma the margins of the shield show the development of a mobile belt formed by rocks of upper proterozoic to cambrian age within this mobile belt the cuiaba group of uncertain but possibly upper proterozoic age outcrops to the east and shows tectonic contacts with younger rocks that span the vendian upper proterozoic to cambrian age the rocks of vendian to cambrian age form three isolated elongated basins which show evidence of fault control the axes of these basins are equally separated by angles of 120 and meet at a triple junction near corumba the stratigraphy of the vendian to cambrian basins is correlated from bolivia to brazil based on tectonic continuity sedimentological similarity and fossil content the central location at corumba shows instability since the middle proterozoic when important igneous activity is recorded at rincon del tigre bolivia and in brazil this igneous activity is interpreted as being related to hot spot activity in an intercratonic setting from which the younger upper proterozoic basins originated