a geophysically constrained litho structural analysis of the eburnean greenstone belts and associated granitoid domains burkina faso west africa abstract interpretation of airborne geophysical data integrated with field structural and lithological observation was successfully employed in the creation of the litho structural framework in a poorly exposed paleoproterozoic granite greenstone terrain of the west african craton the geophysical data portray with sufficient detail all of the lithological units and structural features present the results suggest that the granitoid domains are formed by numerous small to medium sized plutons the existence of several generations of magmatic episodes has a significant impact on the development of a regional tectonic model the magnetic data provided a better definition of the actual pluton shapes and several highly magnetic late orogenic plutons were reliably identified relic f1 fold hinges were recognized in the hounde and boromo greenstone belts the d1 penetrative structures are generally affected by the d2 transcurrent shear zones which often form as well the granite greenstone belt boundaries previously unreported s2 shear zones including the bossie shear zone transecting the sideradougou granitoid domain and abundant small scale shear zones affecting all of the lithologies represent prospective areas for gold exploration airborne gamma ray spectrometry data aided in the mapping process in areas with less regolith cover including erosional ridges and parts of the banfora greenstone belt the magnetic data revealed at least three generations of doleritic dykes crosscutting the western burkina faso

effects of bottom water warming and sea level rise on holocene hydrate dissociation and mass wasting along the norwegian barents continental margin 1 gas hydrate gh stability modeling results explain why some major holocene submarine landslides along the norwegian barents margin could have been triggered by gh dissociation during the early to middle holocene not during the lowest sea levels of the last glacial maximum lgm our model results show that subbottom depths of 170 260 m below the pre slide continental slope ca 350 475 m present water depth must have passed out of gas hydrate stability zone ghsz by 8 15 ka as the effect of warm bottom water inflow at 11 ka penetrated into the subbottom overcoming the effects of pressure increase due to sea level rise slr the component of local slr due to the isostatic response to fennoscandian deglaciation is shown to be relatively insignificant particularly for the part of the upper continental slope where the slide probably began the stability relations show that gh could have formed under the ice sheet before deglaciation and below deeper shelf areas after sea levels began to rise but before significant warming near the ghsz base to the extent water deeper than 800 m has remained cold 1 to 0 c since lgm times the ghsz continued to thicken in deep water and gh dissociation could not have triggered holocene failure in that regime the present distribution of gh stability is limited to water depths greater than about 400 m in the storegga slide area and the thickness of the ghsz increases with water depth

aeromagnetic mapping of precambrian geological structures that controlled the 1968 meckering earthquake ms 6 8 implications for intraplate seismicity in western australia abstract occurring in the southwest of western australia the 1968 meckering earthquake ms 6 8 resulted in the formation of an extensive surface rupture complex comprising faults with a range of orientations and reverse and dextral lateral offsets modeling of the seismological characteristics of the source showed that reverse failure occurred on a north south striking east dipping surface but how this is related to the local precambrian bedrock geology is not clear interpretation of new aeromagnetic data has allowed concealed bedrock lithology and structure to be mapped in previously unachievable detail these data show that the surface faulting correlates closely with linear magnetic anomalies interpreted as dykes faults and lithological contacts the complicated pattern of surface faulting contrasts with the more simple seismological fault model but can be explained in terms of the reactivation of northeasterly dykes and faults and northwesterly stratigraphic trending features in a stress regime with an east west oriented maximum principal stress space problems created where these two trends converge led to the creation reactivation of a linking north south trending thrust fault which accommodated the greatest displacements recorded for the 1968 event the district scale distribution of epicentres in the 3 years encompassing the meckering event shows the same northeasterly and northwesterly trends as seen in the aeromagnetic data the implied basement features controlling the seismicity will be prone to strike slip failure in the regional east west oriented stress field it is speculated that smaller events in the meckering area will tend to be strike slip and these account for most of the strain larger events all known examples of which involve predominantly thrusting are caused by stress build up where strike slip faults with the two orthogonal trends intersect this hypothesis provides an explanation for the lack of topography in the region which is incompatible with the high level of seismic activity and a predominance of thrust faulting

one dimensional thermal modelling of acadian metamorphism in southern vermont usa one dimensional thermal 1dt modelling of an acadian devonian tectonothermal regime in southern vermont usa used measured metamorphic pressures and temperatures and estimated metamorphic cooling ages based on published thermobarometric and geochronological studies to constrain thermal and tectonic input parameters the area modelled lies within the vermont sequence of the acadian orogen and includes i a western domain containing garnet grade pre silurian metasedimentary and metavolcanic rocks from the eastern flank of an acadian composite dome structure rayponda sadawga dome and ii an eastern domain containing similar but staurolite or kyanite grade rocks from the western flank of a second dome structure athens dome approximately 10 km farther east using reasonable input parameters based on regional geological petrological and geochronological constraints the thermal modelling produced plausible p t paths and temperature time t t and pressure time p t curves information extracted from p t t modelling includes values of maximum temperature and pressure on the p t paths pressure at maximum temperature predicted ar closure ages for hornblende muscovite and k feldspar and integrated exhumation and cooling rates for segments of the cooling history the results from thermal modelling are consistent with independently obtained pressure temperature and ar cooling age data on regional metamorphism in southern vermont modelling results provide some important bounding limits on the physical conditions during regional metamorphism and indicate that the pressure contemporaneous with the attainment of peak temperature was probably as much as 2 5 kbar lower than the actual maximum pressure experienced by rocks along various particle paths in addition differences in peak metamorphic grade garnet grade versus staurolite grade or kyanite grade and peak temperature for rocks initially loaded to similar crustal depths differences in calculated exhumation rates and differences in 40ar 39ar closure ages are likely to have been consequences of variations in the duration of isobaric heating or crustal residence periods and tectonic unroofing rates modelling results are consistent with a regional structural model that suggests west to east younging of specific acadian deformational events and therefore diachroneity of attainment of peak metamorphic conditions and subsequent 40ar 39ar closure during cooling modelling is consistent with the proposition that regional variations in timing and peak conditions of metamorphism are the result of the variable depths to which rocks were loaded by an eastward thickening thrust nappe pile rooted to the east new hampshire sequence as well as by diachronous structural processes within the lower plate rocks of the vermont sequence

kinematics of large syn orogenic intrusions example of the lower proterozoic saraya batholith eastern senegal the large saraya batholith intruded the lower proterozoic sequences of eastern senegal structural analysis indicates that this large granitic batholith corresponds to a composite body consisting of several coalescent plutons and interfering diapirs interpretation of the structural fabric patterns in these plutons and in the host rocks constitutes the basis of a hypothetical model of initiation ascent and emplacement as follows 1 development of a deep n s granitic ridge controled by a major crustal fracture generated by an earlier extensional phase re activated by a strike slip fault which probably would facilitate segregation collection and ascent of magmas 2 intrusion and splitting up of the ridge into several diapirs which ascended simultaneously and separately in a setting of a n s sinistral transcurrent deformation inducing a transverse nw se shortening and forceful emplacement of plutons and 3 final emplacement characterized by interference between regional deformation and pluton ballooning the latter leading to the coalescence of intrusions and to the formation of a larger apparently homogeneous body

hyperspectral analysis of the ultramafic complex and adjacent lithologies at mordor nt australia abstract the mordor complex consists of a series of potassic ultramafic rocks which were intruded into proterozoic felsic gneisses and amphibolite and are overlain by quartzite and unconsolidated deposits in situ and laboratory 0 4 to 2 5 m reflectance spectra show al oh absorption features caused by absorption in muscovite kaolinite and illite smectite in syenite granitic gneiss quartzite and unconsolidated sedimentary deposits and fe mg oh features due to phlogopite biotite epidote and hornblende in the mafic and ultramafic rocks ferrous iron absorption positioned near 1 05 m is most intense in peridotite reflectance spectra ferric iron absorption is intense in most of the felsic lithologies hymap data were recorded in 126 narrow bands from 0 43 to 2 5 m along a 7 km wide swath with approximately 6 m spatial resolution correction of the data to spectral reflectance was accomplished by reference to in situ measurements of an extensive alluvial plain spectral classes for matched filter processing were selected by using the pixel purity index procedure and analysis of in situ and laboratory spectra considering the spatial distribution of the resulting 14 classes some classes were combined which produced eight classes characterized by al oh absorption features and three fe mg oh absorption feature classes comparison of the distribution of these 11 spectral classes to a generalized lithologic map of the study area shows that the spectral distinction among the eight al oh classes is related to variations in primary lithology weathering products and vegetation density quartzite is represented in three classes syenite corresponds to a single scattered class quartz muscovite biotite schist defines a single very coherent class and unconsolidated sediments are portrayed in four classes the three mafic ultramafic classes are distinguished on the basis of generally intense fe mg oh and ferrous iron absorption features a single class represents the main mordor ultramafic mass epidote bearing rocks define another class which corresponds to biotite gneiss and in the southern part of the area to fracture zones the third class which exhibits al oh as well as fe mg oh features represents hornblende gneiss and other mafic gneisses these results indicate the importance of analyzing the vnir and swir spectral shape and albedo as well as analyzing specific spectral features for mapping lithologic units in this weathered terrain

origin and age of paleoproterozoic conglomerates and sandstones of the tarkwaian group in burkina faso west africa abstract the tarkwaian rocks of burkina faso overlie the birimian supergroup which is considered to be part of a juvenile crust formed during the eburnean orogen they comprise fluviatile pebbles quartz rhyolite chert and schist embedded in an acid volcaniclastic matrix during a sinistral transpressive shearing they underwent a low grade metamorphism characterized by a paragonitic muscovite assemblage formed during isoclinal folding fold axes are nearly horizontal parallel to a stretching lineation and oriented 035 215 a typologic study of zircons in the volcaniclastics coupled with radiometric datings enables the volcanism to be characterized and its age to be determined the typologic study has shown that 1 zircons from the rhyolitic pebbles the matrix of the conglomerates and from the sandstone are basically the same 2 the zircons morphology characterizes a calc alkaline to alkaline evolutionary trend implying three main episodes of volcanism 3 the morphology of this zircon population is very similar to those usually observed in post orogenic metasediments or sediments the tectonic setting is one of intracontinental extension associated with rifting and crustal thinning in the course of a major period of crustal growth around 2 1 ga the continental volcaniclastics accumulated in the rift at the same time as the sediments the radiometric study has shown that there are no reworked archean rocks the ages cover the period 2170 to 2124 ma three main volcanic episodes may be defined 2170 2150 and 2124 ma the last one was probably contemporaneous with the sedimentation deformation of tarkwaian rocks took place between 2124 9 ma the age of the youngest inherited zircon and 1991 12 ma the age of a post tarkwaian granite this period 2170 to 2124 ma fits well with the main birimian crustal growth and the ages reported for the tarkwaian rocks of ghana the same period of accretion is also recognized in south america the tarkwaian conglomeratic rocks of burkina faso do not have gold concentrations similar to those in ghana and guyana with respect to the tarkwaian rocks of ghana where gold is concentrated in the quartz pebbles they also differ because 1 there is a volcaniclastic contribution 2 the zircons from the tarkwaian rocks in burkina faso are euhedral indicating that transport distances were short and 3 they have been deformed by shearing with respect to guyana the rocks are very similar and probably have the same origin the short transport distance may not allow a secondary concentration of gold these differences at least may help explain the lack of economic gold concentration in the tarkwaian metasediments

the early proterozoic birimian supergroup of ghana and some aspects of its associated gold mineralization abstract rocks of the birimian supergroup of early proterozoic age sm nd isochron age 2166 66 ma form part of the west african craton ghana contains the eastern portion of the birimian terrane five parallel evenly spaced several hundred kilometre long volcanic belts consisting mainly of low grade metamorphic tholeiitic lavas morb chemistry characterize the birimian supergroup of ghana the belts are separated by basins containing isoclinally folded dacitic volcaniclastics wackes and argillitic sediments as well as granitoids lavas and sedimentary rocks were deposited contemporaneously as lateral facies equivalents within the sedimentary basins various lithofacies are distinguished in the transition zone between belt volcanics and basin sediments a chemical facies occurs this is defined by cherts manganiferous and carbon rich sediments fe ca mg carbonates and sulphide mineral disseminations after their emplacement birimian lavas and sediments were folded in the course of the eburnean tectono thermal event intruded by various types of granitoids uplifted and eroded the erosion products were deposited as sediments of the tarkwaian group in long narrow intramontane grabens which formed due to rifting preferentially in the central portions of all five birimian volcanic belts tarkwaian sediments were deformed by gravity tectonics two major types of eburnean granitoids in ghana are characterized as follows basin granitoids cape coast type occur as synorogenic foliated batholiths chiefly in the central portions of birimian sedimentary basins they are peraluminous and generally granodioritic in composition belt granitoids dixcove type mostly form late orogenic unfoliated intrusions in volcanic belts they possess a metaluminous character and commonly are of tonalitic composition bongo granitoids are of post tarkwaian age and k rich in contrast to these granitoid types which have strong mantle affinities the winneba type had an archean sialic precursor a model involving small scale equidimensional parallel and contemporaneously operating convection cells in the upper mantle which caused rifting of a highly attenuated protocrust as well as linear eruptions of tholeiitic magmas is proposed recent sm nd pb pb and rb sr data of volcanic and plutonic rocks are incorporated most ghanaian birimian gold occurrences and mines are concentrated in narrow corridors of 10 15 km width in the transition zone between volcanic belts and sedimentary basins as are the chemical facies and regionally extensive shear zones at the volcanics sediment interface birimian gold in ghana is present as two major types 1 the disseminated sulphide type which is generally lithofacies controlled i e controlled by chemical sediments and to a lesser extent by selvages of gold quartz veins and 2 the quartz vein type which is exclusively structure controlled data on geochemical underground traverses across both types of gold mineralization are briefly summarized

digital lithology mapping from airborne geophysical and remote sensing data in the melville peninsula northern canada using a neural network approach abstract feedforward neural networks were used to identify bedrock types from satellite images and airborne geophysical data of the melville peninsula northern canada the first experiment on rock outcrop pixels demonstrated that more than 90 classification accuracy was achieved with as few as 150 training samples 0 5 of rock outcrop pixels the second experiment consisted of re building the bedrock geology map based on all pixels except water pixels in the area the results demonstrated that greater than 85 classification accuracy can be obtained with only 150 training samples 0 17 of all pixels in training area tests with more training samples were also performed in both experiments and only a moderately higher accuracy was obtained in both experiments the geophysical data were the controlling factors in the classification the landsat tm data contributed to classification results only for the experiment on rock outcrop pixels

petrogenesis of juvenile type birimian paleoproterozoic granitoids in central cote d ivoire west africa geochemistry and geochronology abstract the birimian paleoproterozoic granitoids of central cote d ivoire are associated either with greenstone belts or with basin sedimentary rocks field data indicate two stages of granitoid emplacement which according to singlezircon and monazite 207 pb 206 pb age determinations occurred within a short period of time the first generation of granitoids 2123 2108 ma intruded and metamorphosed the greenstone belts whereas the second generation 2108 2097 ma intruded and metamorphosed the sedimentary pile and subordinate andesitic volcanic rocks of the bandama basin unconformably overlying the greenstone belts geochemical and isotopic data show that the various granitoids belong to four magmatic groups 1 sodic calc alkaline nacg 2 potassic calc alkaline kcg 3 peraluminous alg and 4 alkaline alkg all the granitoids show juvenile features with mantle like low initial sr isotopic compositions 0 700 0 702 and positive epsilon nd values 1 5 2 7 these data indicate that a the nacg may have formed by partial melting of a garnet amphibolite crust b the kcg and alkg derived from partial melting of crustal mafic to intermediate metaigneous rocks and c the alg epsilon nd 1 5 represent infracrustal melting of young sialic igneous materials during a regional crustal shortening in a transcurrent tectonic context sm nd model ages 2 25 2 4 ga preclude the existence of an archean crust reworked in the birimian magmas an underplating model is discussed

geochemical balance of lateritization processes and climatic signatures in weathering profiles overlain by ferricretes in central africa a simple geochemical balance of lateritization processes governing the development of several tens of meters of weathering profiles overlain by ferricretes is estimated on the basis of detailed mineralogical and geochemical data the lateritic weathering mantle of the haut mbomou area in central africa is composed of different weathering layers described from the base to the top of vertical profiles as a saprolite a mottled clay layer a soft nodular layer a soft ferricrete and a ferricrete in which kaolinite gibbsite goethite and hematite occur in various quantities incongruent dissolution of kaolinite leads to the formation of gibbsite in the upper saprolite whereas the hematite does not clearly replace the kaolinite according to an epigene process in the upper ferruginous layers of the profiles instead that kaolinite is also transformed into gibbsite according to an incongruent dissolution under hydrated and reducing conditions induced by a relatively humid climatic pattern the respective relations of the silica iron and aluminum balances and the al substitution rate of the hematite on the one hand and of rhg rhg 100 hematite hematite goethite and the kaolinite on the other hand to the consumption or the release of protons h permit differentiation of aggrading ferruginization and degradation processes operating in the different lateritic weathering profiles the al substitution rate of the fe oxyhydroxides varies according to the nature of lateritization processes e g saprolitic weathering and aggrading ferruginization vs degradation the observations and results indicate that the ferruginization process of the weathering materials of parent rocks is not a simple ongoing process as often thought this suggests that the actual lateritic weathering mantle of the haut mbomou area may result from different stages of weathering and erosion during climatic changes

lithologic mapping in the mountain pass california area using advanced spaceborne thermal emission and reflection radiometer aster data abstract evaluation of an advanced spaceborne thermal emission and reflection radiometer aster image of the mountain pass california area indicates that several important lithologic groups can be mapped in areas with good exposure by using spectral matching techniques the three visible and six near infrared bands which have 15 m and 30 m resolution respectively were calibrated by using in situ measurements of spectral reflectance calcitic rocks were distinguished from dolomitic rocks by using matched filter processing in which image spectra were used as references for selected spectral categories skarn deposits and associated bright coarse marble were mapped in contact metamorphic zones related to intrusion of mesozoic and tertiary granodioritic rocks fe muscovite which is common in these intrusive rocks was distinguished from al muscovite present in granitic gneisses and mesozoic granite quartzose rocks were readily discriminated and carbonate rocks were mapped as a single broad unit through analysis of the 90 m resolution five band surface emissivity data which is produced as a standard product at the eros data center three additional classes resulting from spectral angle mapper processing ranged from 1 a broad granitic rock class 2 to predominately granodioritic rocks and 3 a more mafic class consisting mainly of mafic gneiss amphibolite and variable mixtures of carbonate rocks and silicate rocks

short lived eburnian orogeny in southern mali geology tectonics u pb and rb sr geochronology abstract this work is based on a comprehensive field survey including mapping of the massigui degree sheet in southern mali the investigated area belongs to the northern part of the man shield which is mainly composed of lower proterozoic volcano sedimentary sequences birimian and of large areas of granitoids the whole being affected by the eburnian orogeny structural and geochronological u pb and rb sr results from this study point to a short period of time 30 ma for the eburnian orogenesis between 2100 and 2070 ma the events recorded later 1984 30 ma being due to tectonic reactivations on the other hand no trace of events older than 2100 ma have been found coupled with geochemical and rb sr isotopic systematics this leads to invoke a fundamental link between birimian volcanics and eburnian granitoids both being high k calc alkaline in composition and coming from a single evolving mantle source in a palaeo environment which has some of the characteristics of modern subduction including back arc or docking zones the main regional structure of the area strikes roughly nne ssw with an important strike slip component escorted by a greenschist metamorphism the deformation is the most intense in the n40 to n10 5 km large banifin shear zone where metamorphism reached the lower amphibolite facies here one observes subvertical strong foliation non cylindrical folding locally sheath folds and subhorizontal pencil lineations the deformation being regionally recorded by light subisoclinal folds with subvertical axes compatible with a sinistral movement this regional shear tectonics d2 2074 9 8 ma is preceded by an isoclinal deformation d1 bracketed between 2098 5 ma and d2 only known as relics from which it likely derived a later phase d3 occurred along the banifin shear belt at a more superficial level even if still ductile generated by a dextral movement and having induced retrograde metamorphism this last tectonic phase which is not accompanied by plutonism nor volcanism has been dated at 1982 30 ma being responsible for the complete resetting of the rb sr isotopic system in the plutons present in the banifin corridor

gold bearing fault zones related to late archean orogenic folding of upper and middle crust in the abitibi granite greenstone belt ontario abstract this work investigates the structures of the abitibi granite greenstone belt ontario with a focus on regional scale folds and on regional scale auriferous fault zones the folds and the fault zones resulted from late archean orogenesis the analysis is based on studies of surface geology and on modelling of the gravity field amphibolite grade orthogneiss was involved in the regional folding that also deformed the overlying greenstones folding resulted in uplift of the orthogneiss that preserves the metamorphic mineral assemblage ferropargasite garnet biotite andesine quartz geothermobarometric calculations indicate recrystallization of a metatonalite sample under peak paleopressures of 7 0 1 2 to 7 9 1 1 kb providing evidence that folding involved crust to depths of at least 18 25 km the regional bouger anomaly map reveals previously unrecognized first order folds interpretation of the bouger anomaly map also confirms that granitic batholiths are hosted by anticlines a wavelet based inversion method was used to determine the locations orientations and continuations at depth of the maxima of density contrasts the maximum density contrasts correspond to steeply dipping geological contacts on the limbs of the open to tight regional scale folds regional scale fault zones only have prominent gravity signatures where they correspond to sheared rocks on the limbs of folds modelled gravity profiles were used to produce profiles that visualize the shapes of batholiths and greenstone bodies within the present day upper crust the granitic batholiths have tabular shapes which are locally folded with preserved main body thicknesses of 1 3 km the greenstones form keels corresponding to synclines some of which reach 5 6 km depth the results of this study confirm that the principal deformational response of the abitibi granite greenstone belt to late archean collisional tectonics was large half wavelength 10 40 km folding of the upper and middle crust our results and interpretations also strongly suggest the regional scale fault zones which are loci for orogenic gold deposits are fold related structures they represent quasi continuous lineaments of shearing and faulting on the limbs of regional folds and locally in the cores of detachment anticlines as shown in a previous paper

multi scale analysis of proterozoic shear zones an integrated structural and geophysical study abstract structural mapping of poorly exposed shear zone outcrops is integrated with the analysis of aeromagnetic and bouguer gravity data to develop a multi scale kinematic and relative overprinting chronology for the palaeoproterozoic tallacootra shear zone australia d 2 mylonitic fabrics at outcrop record kimban aged ca 1730 1690 ma n s shortening and correlate with sz 1 movements overprinting d 3 sinistral shear zones record the partitioning of near ideal simple shear and initiated riedel to regional scale sz 2 strike slip on the tallacootra shear zone sz 2 previously undocumented ne sw extension led to the emplacement of aplite dykes into the shear zone and can be correlated to the ca 1595 1575 ma hiltaba magmatic event d 4 dextral transpression during the ca 1470 1450 ma coorabie orogeny reactivated the tallacootra shear zone sz 2 r4 exhuming lower crust of the northwestern fowler domain within a positive flower structure this latest shear zone movement is related to a system of west dipping shear zones that penetrate the crust and sole into a lithospheric detachment indicating wholesale crustal shortening these methods demonstrate the value of integrating multi scale structural analyses for the study of shear zones with limited exposure

graphitization of organic matter and fluid deposited graphite in palaeoproterozoic birimian black shales of the kaya goren greenstone belt burkina faso west africa palaeoproterozoic black shales form an essential part of the birimian volcanosedimentary belt in burkina faso west africa the mean rmax values and the atomic h c values of the bulk carbonaceous matter bcm together with rock structures and mineral assemblages indicate that these carbon rich rocks were metamorphosed to sub greenschist and low grade greenschist facies x ray diffraction reveals that the 002 graphite peak width in half maximum fwhm ranges from 0 43 to 0 71 2 in sub greenschist facies and from 0 27 to 0 41 2 in greenschist facies rocks but the d 002 values in both groups of rocks are approximately the same 3 35 a the bcm of individual samples is composed of particles with very variable shape reflectance and raman spectra type i particles that predominate in sub greenschist facies are fine grained irregular or elongate bodies 1 to 3 m in size their maximum reflectance rmax ranges between 2 5 and 8 2 and raman parameters r1 and r2 range from 0 5 to 1 4 and 0 5 to 0 8 respectively type ii particles are lath shaped up to 40 m large bodies commonly arranged parallel to white mica flakes the number of these particles increases from sub greenschist to greenschist facies maximum reflectance varies between 6 and 11 2 and r1 and r2 raman parameters range from 0 05 to 0 7 and from 0 1 to 0 5 respectively type iii particles occur in hydrothermally altered and sheared rocks these are nodular aggregates composed of grains up to 10 m in size this type of particles has very high reflectance rmax 11 15 and its raman spectra indicate a very high degree of structural ordering corresponding to well ordered graphite type i particles represent original organic matter in the metasediments type ii particles are believed to have been formed either in situ by solid state transformation of type i particles or by crystallization from metamorphic fluids gradual graphitization of the type i organic particles and the growth of lath shaped type ii particles from a fluid phase is assumed to have taken place under the peak metamorphic conditions associated with the burial of birimian sediments during thrust tectonism progressive tectonic accretion and crustal thickening during the d1 event of the eburnean orogeny the growth of equant high reflectance postkinematic nodular aggregates of type iii particles is ascribed to the reduction of co2 rich fluids during a hydrothermal event associated with late eburnean d2 exhumation and strike slip movements type i carbonaceous particles were only slightly affected by high temperature low pressure contact metamorphism during intrusion of late eburnean magmatic bodies whereas formation of type ii or iii particles was not recorded in contact metamorphosed rocks at all

architecture of proterozoic shear zones in the christie domain western gawler craton australia geophysical appraisal of a poorly exposed orogenic terrane abstract large scale mesoproterozoic crustal shear zones are partially exposed from beneath cover sequences in the christie domain of the northwestern gawler craton southern australia these structures are associated with gravity and magnetic anomalies that allow them to be mapped under cover and their three dimensional geometry and kinematics to be evaluated gravity and magnetic forward modelling indicate that these shear zones form an imbricate oblique thrust stack with combined top to the southeast left lateral transport the longest shear zones in the stack penetrate the crust to at least 15 km depth and dip to the northwest at 70 their inferred sense of motion are consistent with kinematic indicators from sparse outcrops from northwest to southeast the stack includes crustal slices bound by the karari fault zone tallacootra and blowout shear zones colona and coorabie fault zones and the muckanippie shear zone these structures separate discrete tectonometamorphic packages within the previously undifferentiated christie gneiss of the archean mulgathing complex and imply several generations of transport of material from the lower and middle crust three dimensional inversion of gravity data indicates that the muckanippie shear zone a shallowly rooted second order splay of the coorabie fault zone has an antithetic dip to the southeast and is part of a positive flower structure cross cutting relationships coupled with recent reconnaissance geochronology suggest that the western gawler craton part of the mawson continent was under a long lasting oblique slip deformational regime after ca 1590 ma this tectonic reworking is correlated with episodic pervasive metamorphism and deformation recorded elsewhere in the gawler craton between ca 1550 and 1450 coorabie orogeny pointing to a complex history of stabilization in proterozoic rodinia the tallacootra colona and coorabie structures are all cut at a high angle to their strikes by mesozoic rift faults of the southern rift system and may form piercing points in reconstructions of gondwana and earlier rodinia configurations

structure gravity models and stratigraphy of an early proterozoic volcanic sedimentary belt in northeastern ghana abstract the structure of an early proterozoic volcanic sedimentary belt in northeastern ghana is inferred from the distribution of lithologic units and interpretation of bouguer gravity anomaly associated with the belt it is shown from gravity modelling that the vertical thickness depth of the volcanic sedimentary succession is ca 3 km and that the structure of the western part of the belt is an overturned anticline an interpretation consistent with facing data this structure provides the basis from which the stratigraphic order of the mapped lithic units is deduced 1 fine grained epiclastic sediments interbedded with minor felsic tuffs followed by 2 tholeiitic basaltic lavas which are overlain by 3 calc alkaline andesitic and dacitic lavas and tuffs the youngest volcanic unit belonging to the sequence is a calc alkaline mafic tuff 4 a manganese rich chemical sediment is preserved at the boundary between the tholeiitic mafic lava and calc alkaline intermediate volcanic rock units the early proterozoic sequence which is unconformably overlain by coarse fluviatile sediments is estimated to be ca 8500 m thick the stratigraphic sequence in the study area contrasts strongly with the conceptual stratigraphic schemes which are currently held to be valid for similar lithologic associations of early proterozoic age birimian in the west african shield

detection and identification of north south trending magnetic structures near the magnetic equator long structurally undeformed north south trending structures show no magnetic anomaly at the magnetic equator except at the north and south truncations of the structure however folding faulting differential erosion or other structural deformation can produce detectable magnetic anomalies in a generally north south trending equatorial structure spatial variation in magnetic susceptibility or remanent magnetization can also produce anomalies in equatorial north south structures these anomaly patterns are often more complicated than patterns produccd by similar structures at high latitudes but interpretational insight can be gained through numerical modelling of common structures reduction to pole and analytic signal filters can aid in interpretation of equatorial anomalies but these must be applied carefully because of instabilities deriving from filter design and noise amplification

structure and emplacement of granite plutons in the paleoproterozoic crust of eastern burkina faso rheological implications the fada n gourma area in burkina faso is underlain by paleoproterozoic rocks that make the northeastern west african craton this region is composed of ne trending volcano sedimentary belts and foliated tonalites affected by several shear zones a generation of younger 2100 ma old non foliated biotite bearing granites intrudes the former rock units we have investigated the younger granite pluton of kouare that was previously considered as forming a single body with the pluton of satenga to the west a pluton which likely belongs to the 20 ma more recent tenkodogo yamba batholith magnetic fabric measurements have been combined with microstructural observations and the analysis of field and aeromagnetic data the granite encloses angular enclaves of the host tonalites magmatic microstructures are preserved inside the pluton and solid state high temperature deformation features are ubiquitous at its periphery the presence of steeply plunging lineations in the pluton of kouare and its adjacent host rocks suggests that large volumes of granitic magmas became crystallized while they were ascending through the crust that was softened and steepened close to the contact around kouare the foliation in the host tonalites conforms with a map scale z shaped fold in between nne trending shear zones implying a bulk clockwise rotation of the material contained in between the shear zones including the emplacing pluton regionally the fada n gourma area is concluded to result from nw shortening associated with transcurrent shearing and vertical transfer of granitic magmas this study concludes that the 2200 myears old juvenile crust of burkina faso was brittle before the intrusion of the biotite granites became softened close to them and that gravity driven and regional scale wrench tectonics were active together

mechanisms of pluton emplacement and structural evolution of a 2 1 ga juvenile continental crust the birimian of southwestern niger abstract the 2 1 ga birimian terranes of southwestern niger consist of a granite greenstone association the structures of the greenstone belts result from the interference between pluton emplacement and regional transcurrent deformation sinistral n s strike slip faults ne sw trending schistosity the regional cleavage trend is observed to change in the vicinity of elliptical plutons cleavage trajectories bend around their contacts resulting in triple junctions at the pluton extremities and narrow aureoles of intense penetrative schistosity and high grade amphibolitefacies metamorphism are developed around them structural features of plutons suggest syntectonic emplacement and age data indicate that all plutons were emplaced between 2 16 and 2 10 ga the plutons are assembled in two distinct types of batholith elongated either parallel or perpendicular to the inferred regional shortening direction a batholith parallel to the inferred regional shortening direction dolbel batholith consists of km sized subcircular alkali granite plutons except for the southernmost one which is of a larger size and exhibits an elliptical shape with a nw se short axis parallel to the shortening direction crosscutting relationships indicate a progressively northward emplacement trend for those plutons internal fabrics generally correspond to layering and magmatic flow fabrics and accessorily to schistosity and shear zones which represent late stage plastic deformation restricted to the pluton margin batholiths which are perpendicular to the inferred regional shortening direction are composed of large elliptical plutons 500 to 5000 km 2 in area consisting mainly of granodiorite and covering 70 of the region the plutons never show crosscutting relationships but are moulded against each other their internal fabric corresponds to a solid state foliation it either dips towards the centre of the pluton funnel or teardrop shaped intrusions or is subhorizontal in the centre and vertical at the margin dome shaped intrusions the foliation and the geometry of the largest pluton tera pluton were acquired during pluton ascent and inflation whereas the geometry of later shear zones indicates that deformation in the pluton was then passively acquired at the final emplacement level due to regional deformation the contrast in both the orientation and structural features of the batholiths reflects distinct emplacement mechanisms the small alkali granite plutons resulted from magma injection within nw se fractures parallel to the inferred regional shortening direction whereas the granodiorite plutons are assumed to be emplaced as diapirs forceful intrusions with long axes perpendicular to the shortening direction these two intrusion mechanisms for plutons emplaced during the same deformation event and at the same crustal level are considered to reflect a local change in the rheological behaviour of the crustal segment studied in connection with the thermal effect induced by the large granodioritic plutons this vertical tectonic regime appears to be typical of palaeoproterozoic juvenile continental crust areas and related to an interplay between emplacement of tremendous volumes of granodioritic magma and a regional transcurrent deformation

early proterozoic ore deposits and tectonics of the birimian orogenic belt west africa abstract the early proterozoic of the west african craton comprises a series of volcanic troughs and sedimentary basins the birimian with granitic terranes accreted on archean nuclei in the man and reguibat shields studies of the birimian of the man shield indicate a model of polycyclic evolution with a major collision event d 1 2 1 ga thrusting part of the proterozoic terrane over the archean before individualization of numerous volcanic troughs and clastic infill basins the proposed evolution for the birimian orogenic belt comprises 1 deposition of the sedimentary lower birimian b1 with minor tholeiitic volcano sedimentary intercalations containing chert and or mn formations and with most of the detritus being derived from early proterozoic sources apart from contamination near the proterozoic archean contact 2 pre b2 crustal thickening related to d 1 thrusting 3 formation over about 40 ma of the upper birimian b2 with numerous volcanic troughs of different composition tholeiitic and rare komatiitic bimodal tholeiitic to calc alkaline volcano plutonic and tarkwaian clastic infill basins and 4 major transcurrent tectonics with sinistral d 2 and dextral d 3 strike slip faults such a tectonic evolution from a collision d 1 phase to a transcurrent d 2 d 3 phase is typical of collision belts and in the present case the evolution of the birimian orogenic belt can be extended into guyana the metallogenic history of the birimian shows a three phase evolution coinciding with the orogenic evolution and extends over almost 150 ma from the perkoa massive zn ag sulfides 2 12 ga with a clear mantle affinity to the latemesothermal au quartz veins 2 ga with according to lead isotopes a high crustal participation the economic mineralization of belt thus consists of 1 1 pre orogenic pre d 1 deposits related to early extension zones this was diverse with stratiform au tourmalinite type 1 au loulo in mali dorlin in guyana stratiform fe cu faleme in senegal and mn nsuta in ghana tambao in burkina faso and a single massive zn ag sulfide deposit perkoa in burkina faso associated with regional volcanosedimentary variably tholeiitic stratigraphic marker beds 2 2 syn orogenic post d 1 to syn d 2 d 3 deposits with disseminated au sulfides type 2 au yaoure in the ivory coast in extensional zones of the b2 followed by auriferous paleoplacers type 3 au in b2 extensional zones tarkwaian banket conglomerate or syn d 2 transtensional zones debris flow of orapu in guyana 3 3 late orogenic post peak d 2 d 3 deposits with mesothermal au mineralization evolving from a disseminated gold bearing arsenopyrite and au quartz lode type type 4 au ashanti in ghana to a quartz vein type with free gold and cu pb zn ag bi paragenesis most of the gold in west africa comes from this phase finally the metallogeny of the birimian appears as rich in au and mn formations and poor in volcanogenic mineralization and bif it differs from the archean metallogeny of west africa and other regions through the presence of certain deposit types such as au stratiform tourmalinite gold bearing conglomerate with au and fe ti oxides but no uranium or sulfides and au arsenopyrite rich shear zone deposits that are very common in but not exclusive to the proterozoic and paleozoic

late paleo mesoproterozoic plate margin deformation in the southern gawler craton insights from structural and aeromagnetic analysis abstract the integration of structural analysis of outcrop and aeromagnetic interpretation has revealed a complex tectonic evolution for the southern gawler craton that reflects the regional tectonic signal during the late paleo to early mesoproterozoic results suggest that exposures at yarlbrinda hill comprise the margin of a highly deformed st peter suite pluton emplaced within the southwestern gawler craton at ca 1611 ma d 1 at outcrop records east to northeast directed ductile thrusting regionally this deformation overprints north south trending dextral yarlbrinda shear zone fabrics within ca 1622 ma st peter suite granite and ca 1690 1670 ma tunkillia suite granite progressive east to northeast directed thrust movements led to exhumation of the deformed ca 1611 ma granite d 2 deformation is recorded by the brittle reactivation of first generation mylonitic fabrics accompanied by cataclasis alteration and pseudotachylite development d 3 records a switch to northwest southeast shortening which led to the folding and tear faulting of the d 1 2 shear zone a switch to extension and subsequent rotations of the principle stress axes led to hiltaba suite granite emplacement and contemporaneous faulting under east west extension during d 4 and west northwest east southeast extension during d 5 deformation of ca 1622 ma st peter suite granite is consistent with movement on the yarlbrinda shear zone and requires the st peter suite to have been emplaced into or adjacent to the ca 1690 1670 ma tunkillia suite prior to shear zone activity and on the overriding plate during subduction d 1 d 3 deformation is constrained by the emplacement of ca 1580 ma hiltaba suite granite and correlates with the wartakan orogeny in the southeastern gawler craton and the olarian orogeny in the curnamona province a 52 disparity between the shortening directions in these respective areas suggests different degrees of mesoproterozoic rotation which likely aided the formation of an intervening basin province the remnants of which are presently preserved in the mount painter inlier the record of convergent deformation is geochronologically constrained to a 30 myr period and places this region of the central gawler craton near a late paleoproterozoic convergent plate margin prior to a period of transient extension beginning at ca 1580 ma the continental scale signal of tectonic events can be recognized at the outcrop scale highlighting the importance of detailed outcrop studies in sparsely exposed areas

basement controlled faulting of paleozoic strata in southern ontario canada new evidence from geophysical lineament mapping abstract basement fault reactivation is now recognized as an important control on sedimentation and fault propagation in intracratonic basins in southern ontario the basement consists of complexly structured mid proterozoic ca 1 2 ga crystalline rocks and metasedimentary rocks that are overlain by up to 1500 m of paleozoic sedimentary strata reactivation of basement structures is suspected to control the location of paleozoic fault and fracture systems but evaluation has been hindered by a limited understanding of the regional structural characteristics of the buried basement new aeromagnetic and gravimetric lineament mapping presented in this paper better resolves the location of basement discontinuities and provides further evidence for basement controls on the distribution of paleozoic fault and fracture systems lineament mapping was facilitated by reprocessing and digital image enhancement micro levelling regional residual separation derivative filtering of existing regional gravity and aeromagnetic datasets reprocessed images identify new details of the structural fabric of the basement below southern ontario and delineate several previously unrecognized aeromagnetic and gravity lineaments and linear zones linear zones parallel the projected trends of mid proterozoic terrane boundaries identified by field mapping on the exposed shield to the north of the study area and are interpreted as zones of shearing and basement faulting mapped aeromagnetic and gravity lineaments show similar trends to paleozoic faults and fracture networks and broad zones of seismicity in southern ontario these new data support an inheritance model for paleozoic faulting involving repeated reactivation and upward propagation of basement faults and fractures into overlying cover strata

evolution of the mount woods inlier northern gawler craton southern australia an integrated structural and aeromagnetic analysis structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships this approach is used to constrain the geometry and evolution of the sparsely exposed mount woods inlier in the northern gawler craton the mount woods inlier records a history of poly phase deformation high temperature metamorphism and syn and post orogenic magmatism between ca 1736 and 1584 ma the earliest deformation involved isoclinal folding and the development of bedding parallel and axial planar gneissic foliation s1 this was accompanied by high temperature upper amphibolite to granulite facies metamorphism at ca 1736 ma during subsequent north south shortening d2 open to isoclinal south southeast oriented f2 folds developed as the palaeoproterozoic successions of the inlier were thrust over the archaean nuclei of the gawler craton the syn d2 engenina adamellite was emplaced at ca 1692 ma the post d2 history involved shear zone development and localised folding exhumation of metamorphic rocks and deposition of clastic sediments prior to the emplacement of the ca 1584 ma granite balta suite the mount woods inlier is interpreted as the northern continuation of the kimban orogen

multi scale integrated structural and aeromagnetic analysis to guide tectonic models an example from the eastern musgrave province central australia abstract in many polydeformed precambrian provinces the understanding of their tectonic history is limited by insufficient exposure which leads to difficulty in developing links between local and regional architecture and results in poorly constrained models of their regional scale architecture high resolution aeromagnetic data allows interpretation and modelling at a scale comparable to structural mapping and by integrating these methods these links can be developed with high confidence as an example we integrate structural mapping aeromagnetic data interpretation and 3d magnetic inversions in a multi scale structural analysis of the polydeformed eastern musgrave province in central australia deformation during the ca 1200 ma musgravian orogeny is characterised by an originally northeast trending structural grain defined by shear zones and tight to isoclinal folds at all scales 3d magnetic inversion models of syn to post musgravian granites indicate that this structural grain is predominantly west dipping this architecture suggests that a northeast oriented fold and thrust belt developed as a result of northwest southeast compression in central australia during the musgravian orogeny the next deformation event is characterised by an array of originally east southeast trending shear zones that truncate and offset the musgravian structural grain with apparent dextral displacements of up to 1 km these shear zones are commonly co located with mafic and granitic dykes suggesting a dilational origin and they are interpreted to have initiated under northeast southwest extension during the ca 1080 ma giles event the architecture of these two mesoproterozoic events was later extensively overprinted by two deformation events during the pan african aged ca 550 ma petermann orogeny the first is characterised by the widespread reactivation of mesoproterozoic structures resulting in a network of shear zones of variable orientation and the second is characterised by the development of crustal scale transpressional shear zones motion on which has caused the rotation of lithospheric blocks and the reorientation of prior structural trends by up to 90 collectively the architecture of these two events suggests broadly north south crustal shortening in central australia as an intraplate response to gondwana assembly this example demonstrates that the integration of structural geology with aeromagnetic interpretation and modelling can be used to develop high confidence links between local and regional structures these links permit the derivation of better constrained models of major tectonic events particularly in regions where outcrop is poor and the structural architecture is complex

integration of landsat tm gamma ray magnetic and field data to discriminate lithological units in vegetatedgranite gneiss terrain abstract image classification of geological units in vegetated granite gneiss terrain from multispectral data is hampered by vegetation cover and limited spectral contrast of its lithological variations in this paper an alternative methodology is presented employing airborne gamma ray spectrometry and magnetic data the methodology includes the selection of combinations of data channels and their transformations to enhance the discriminative power of the lithology classification an analysis of the training set compiled from 2 795 field stations showed that the potassium thorium and uranium gamma ray spectrometry and total and residual magnetic field channels provided an overall classification success rate of 65 for a total of 10 lithological units parametric classifications based on this training set yielded 67 coincidence with the geological map whereas a neural network classifier provided only 23 coincidence exploiting the spatial autocorrelation of the geophysical signatures by adding averaged filtered channels slightly improved the coincidence percentage to 70 and enhanced the continuity of linear enclaves within larger lithological units the contribution of magnetic data to the classification depends on the extent to which the anomaly spectrum and its processed derivatives reflect surface geology we found that both the short and long wavelengths in the spectrum contributed to the classification performance this is explained by the geological structure of the area where both broad and narrow units extend downward subvertically a comparison of our results to regional geological map patterns identified targets for map refinement and exploration

new insights into magnetic derivatives for structural mapping magnetic and gravity derivatives can be likened to seismic attributes in that they can help define estimate the physical properties of the source structure causing the anomaly this contribution looks at the tilt derivative first reported in 1994 and more recently used to derive the local wavenumber 1997 we will show that the combination of the tilt derivative and its total horizontal derivative are highly suitable for mapping shallow basement structure and mineral exploration targets and that they have distinct advantages over many conventional derivatives we provide the simple theory behind the derivatives use a range of simple 2d models to illustrate their response and apply them to mapping a mineral target in namibia

lithologic mapping of the mordor nt australia ultramafic complex by using the advanced spaceborne thermal emission and reflection radiometer aster abstract spectral measurements made in the mordor pound nt australia study area using the advanced spaceborne thermal emission and reflection radiometer aster in the laboratory and in situ show dominantly al oh and ferric iron vnir swir absorption features in felsic rock spectra and ferrous iron and fe mg oh features in the mafic ultramafic rock spectra aster ratio images matched filter and spectral angle mapper processing sam were evaluated for mapping the lithologies matched filter processing in which vnir swir image spectra were used for reference resulted in 4 felsic classes and 4 mafic ultramafic classes based on al oh or fe mg oh absorption features and in some subtle reflectance differences related to differential weathering and vegetation these results were similar to those obtained by match filter analysis of hymap data from a previous study but the units were more clearly demarcated in the hymap image aster tir spectral emittance data and laboratory emissivity measurements document a wide wavelength range of si o spectral features which reflect the lithological diversity of the mordor ultramafic complex and adjacent rocks sam processing of the spectral emittance data distinguished 2 classes representing the mafic ultramafic rocks and 4 classes comprising the quartzose to intermediate composition rocks utilization of the complementary attributes of the spectral reflectance and spectral emittance data resulted in discrimination of 4 mafic ultramafic categories 3 categories of alluvial colluvial deposits and a significantly more completely mapped quartzite unit than could be accomplished by using either data set alone

a paleoproterozoic ultramafic mafic assemblage and associated volcanic rocks of the boromo greenstone belt fractionates originating from island arc volcanic activity in the west african craton abstract the loraboue birimian ultramafic mafic assemblage located in the boromo greenstone belt burkina faso is interpreted as the remains of a magma chamber that crystallized at the base of an island arc the ultramafic rocks exhibit an heteradcumulate texture being generally made up of wehrlites and more rarely dunites the crystallization sequence inferred from the cumulates is olivine chromite followed by clinopyroxene amphibole orthopyroxene biotite the gabbroic rocks are mainly layered and grade into more differentiated facies with sub pegmatitic texture containing up to 70 modal plagioclase including zircon and more commonly apatite crystals textural relationships and mineral phase chemistry are indicative of crystallization at elevated pressures 8 kbar the parental magma being generated by a moderate to high degree of partial melting of a mantle source affected by previous metasomatic events above a subducted oceanic slab the loraboue volcanic formations exhibit a range of geochemical features they consist dominantly of calc alkaline basalts pyroclastites and rhyolite and more rarely of basalt dolerite and gabbro of tholeiitic affinity these different types of basalt as well as the dolerite and the isolated massive gabbro show the classic features of arc magmatic suites namely lile and pb enrichment depleted hfse patterns and high ce nb and th nb ratios thus the calc alkaline plutonic and volcanic assemblages of loraboue could represent the roots of an island arc and the associated coeval volcanic rocks the paleoproterozoic crust of the west african craton was heterogeneous and was not the consequence of a single process of genesis as some modern igneous province the birimian crust was generated by both volcanic arc accretion and oceanic plateau accretion

age constraints on gold mineralization and paleoproterozoic crustal evolution in the ashanti belt of southern ghana abstract u pb isotopic dating in the region of the ashanti belt in ghana has been carried out to improve understanding of crustal evolution and gold mineralization in this paleoproterozoic terrane a u pb zircon age of ca 2174 2 ma for a belt type granitoid from sekondi accords with previous ages on belt type plutons in ghana but metamorphic titanite within this granitoid gives an age of 2092 3 ma coeval with intrusion of the late basin type granitoids the within belt granitoid at banso village has an age close to 2097 3 ma which falls into the age range typical for basin type granitoids 2116 2088 ma these results indicate that widespread plutonism in the birimian sedimentary basins also produced magmatic and metamorphic effects within the volcanic belts most dated detrital zircons from birimian metasediment samples at the ashanti gold mine agree in age at 2155 2 ma with others that are slightly older this gives a maximum estimate for the depositional age of the sediments and for gold mineralization at the ashanti gold mine hydrothermally altered and auriferous granitoid plutons along the western flank of the ashanti belt at anyankyerim nhyiaso yaomensakrom and esuajah ayanfuri all have intrusion ages within error of 2105 2 ma this is also the age of metamorphic titanite in an unmineralized basin type granitoid at kwapia rutiles from the hydrothermally altered yaomensakrom and the mineralized esuajah ayanfuri granitoid were dated at 2098 7 and 2086 4 ma respectively and probably provide minimum ages on gold mineralization these data indicate a close temporal association among auriferous granitoid plutons near the western margin of the ashanti belt at 2105 ma gold mineralization may be only slightly younger than magmatic emplacement of the plutons granitic plutonism in the sedimentary basins metamorphism and hydrothermal gold mineralization in southern ghana are all part of a major crust formation event the eburnean orogeny that can be constrained to the time range 2120 2080 ma

approximating edges of source bodies from magnetic or gravity anomalies cordell and grauch 1982 1985 discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks the final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes we attempt to automate their final step our method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources the method applies to both local surveys and to continent wide compilations of magnetic and gravity data e g zietz 1982 simpson et al 1983a kane et al 1982

a new metamorphic constraint for the eburnean orogeny from paleoproterozoic formations of the man shield aribinda and tampelga countries burkina faso abstract progressive metamorphic zones developed in pelites around the aribinda granite burkina faso indicate contact metamorphism at 5 6 kbar biotite chloritoid staurolite kyanite zones are distributed concentrically around the aribinda pluton on its northern margin but are displaced by later thrust faults in the south of the aribinda area and in the nearly tampelga area these metamorphic and structural characteristics provide new constraints for the geodynamical evolution of the eburnean orogeny in the north part of the baoule mossi domain they show the leading part of the granitoid emplacement in the thermic structuration of the greenstone belts and the post metamorphic character of the tangential tectonic in burkina faso the eburnean orogeny underwent 1 initial greenschist facies metamorphism 2 emplacement of granitoids at mid deep crustal levels leading to unusual medium pressure metamorphism 3 later thrusting

timing and structural controls on gold mineralization at the bogoso gold mine ghana west africa the bogoso gold deposit comprises 20 open pits scattered along 18 km of a major fault zone which separates birimian supergroup turbiditic sedimentary rocks to the northwest from tarkwaian group rocks to the southeast individual pits are centered on 200 to 400 m long ore shoots which are located in three broad structural settings steeply pitching relatively high grade orebodies are commonly localized in left hand dilational jogs in the strike of the controlling faults these jogs often coincide with intersecting minor faults gold is largely confined to the graphitic mylonite which defines the fault and imbricated slices of carbonate altered rock gently dipping mineralized quartz vein stockworks and disseminated auriferous pyrite and arsenopyrite form broader zones between more steeply dipping bounding faults in the vicinity of right hand bends and splays in these faults smaller zones of disseminated sulfide mineralization coincide with areas where bedding and or foliation in the adjacent birimian sedimentary rocks is strongly discordant with the adjacent mineralized fault generally where the faults cut the short limbs of tight upright east trending folds approximately 75 percent of the gold extracted from the bogoso district has been mined from a cluster of pits that coincides with the densest and most highly connected concentration of larger faults in the bogoso concession the majority of ore shoots are located along the largest of these faults the narrower faults generally host smaller and less frequent ore shoots the fault network originated during an early episode of regional scale shortening involving thrusting and isoclinal folding the structural setting and geometry of the ore shoots implies that mineralization occurred during a later minor episode of sinistral strike slip movement on the controlling faults after development of the fault network the undeformed nature of the auriferous sulfides implies that this was the last phase of ductile brittle deformation on the host faults this conclusion is consistent with earlier work indicating that the giant ashanti deposit 150 km along strike from bogoso also developed during a period of small displacement sinistral offset late in the history of the host fault network

early proterozoic crustal evolution in the birimian of ghana constraints from geochronology and isotope geochemistry abstract representative suites of ghanaian granitoids metavolcanics and metasediments were selected from birimian rock units for rb sr pb pb and sm nd analyses isotopic data on the igneous rocks provide evidence for a major early proterozoic crust forming episode which took place over a maximum time interval from 2 3 to 2 0 ga by a differention from a depleted mantle source there is little evidence for the involvement of significantly older e g archaean crust in the genesis of the igneous rock units the only exception so far found is the winneba granitoid which appears to contain a magmatic contribution from presumably underlying archaean basement sm nd model ages indicates that birimian sediments were derived from adjacent penecontemporaneous volcanic belts the birimian of ghana forms part of a major proterozoic eburnean episode of juvenile crustal accretion which has been recognised in the surrounding areas of mauretania senegal ivory coast burkina faso mali and niger where it has been dated at 2 1 2 2 ga abouchami et al the birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents

late eburnean granitization and tectonics along the western and northwestern margin of the archean kenema man domain guinea west african craton abstract recent mapping of the eastern part of guinea has revealed a vast plutonic belt that rims the margin of the archean kenema man craton and that is made up of a variety of granitic rocks granodiorite biotite granite monzogranite two mica granite granodiorite with common clinopyroxene being the most abundant single zircon dating by stepwise pb evaporation carried out on characteristic rock types granodiorite monzogranite two mica granite reveals their age to be late eburnean c 2090 2070 ma and indicates that their emplacement occurred within a relatively short time interval 20 ma the granodiorite biotite granite and monzogranite of the plutonic belt all present a calc alkaline and highly potassic nature and show numerous analogies with the neogene calc alkaline rocks of the central andes there is evidence to suggest strong contamination from the archean crust these analogies make it possible to envisage the emplacement of the plutonic belt directly above a subduction zone the occurrence of peraluminous two mica granite at the northern extension of the plutonic belt in the siguiri basin may suggest that the convergence here was associated with the local melting of metasedimentary rocks in the deep part of the basin contemporaneously with its emplacement the late eburnean plutonic belt accommodated regional convergence by major wnw ese sinistral movements along the northwestern margin of the archean block horizontal shortening towards the wsw is envisaged at the scale of eastern guinea in relation with these kinematics the late eburnean plutonic belt of eastern guinea is a key element of the archean proterozoic transition zone in the southern part of west africa evidence along the western and northwestern border of the archean kenema man domain leads us to interpret this transition zone as an active margin along which strike slip tectonism accommodated regional convergence towards the end of the eburnean cycle