the structural framework of the southern margin of the limpopo belt south africa abstract the southern marginal zone is characterized by two prominent regional structural features i massive crustal wedges 20 km 50 km containing large oval shaped closed fold structures which are related to an early d 1 phase of deformation and which are bounded by ii major ductile shear zones that strike in a general e w direction the hout river shear zone forms the southern boundary of the southern marginal zone where granulites are juxtaposed against lower grade granite greenstones of the kaapvaal craton this fault system also acted as the sole thrust for the other high grade shear zones in the southern marginal zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the southern marginal zone the hout river shear zone and related faults within the southern marginal zone clearly post date d 1 and a period of peak metamorphism m 1 they are composite structures showing dominantly reverse sense displacements and are responsible for the emplacement of the granulite terrane over and onto the kaapvaal craton in contrast to these older structures are the reactivated shear zones these structures clearly cross cut and displace the high grade shear zones in the southern marginal zone as well as the retrograde orthoamphibole isograd the fact that the hout river and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism i e 2670 ma barton and van reenen 1992

mt and reflection an essential combination summary at many localities in the world there have been coincident comprehensive electromagnetic em studies and seismic reflection profiles conducted unfortunately over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information this paper is an attempt to convince the reader that a collocated magnetotelluric mt study should in almost every case be made wherever a seismic reflection survey is undertaken examples are shown from six studies in which the em results aided the geological tectonic interpretations of the seismic sections also difficulties with the mt technique are discussed and the interpretations of conducting zones within the lower crust are examined finally a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust

seismic crustal structure of the limpopo mobile belt zimbabwe abstract a 145 km n s seismic traverse was deployed to determine the crustal structure of the limpopo mobile belt in southern zimbabwe and the nature of its northern boundary with the zimbabwean craton rockbursts from south african gold mines to the south and regional seismicity from the kariba south zambia belt to the north were used as seismic sources p wave relative teleseismic residuals were also measured to assess whether any velocity contrast between the craton and the mobile belt extended into the upper mantle interpretation of reduced travel times from the local buchwa iron ore mine blasts which were broadside to the traverse revealed an upper crustal interface in the limpopo mobile belt at a depth of 5 8 0 6 km dividing material with a velocity of about 5 8 km s from that of about 6 4 km s on the craton arrivals from the same source showed a 4 4 0 5 km thick 5 5 km s layer overlying crust of about velocity 6 5 km s p wave arrivals from the regional seismicity were used to construct a crustal cross section absolute crustal thickness was tentatively estimated from the identification of a moho reflection on the mine blast recordings to the south of rutenga the crust thins from around 34 km to 29 km in association with a positive gravity anomaly centred over the late karoo nuanetsi igneous province and karoo tuli syncline north of rutenga to the boundary with the zimbabwean craton the crust is about 34 km thick the craton boundary was found to be a steeply southerly dipping zone associated with high velocity material which could either be deep seated greenstones or mafic material associated with the margin in the region studied this zone divides cratonic crust which was found to be about 40 km thick from that typical of the mobile belt and implies a step in the moho of around 6 km analysis of relative teleseismic residuals showed that the velocity contrasts are not confined to the crust but extend into the uppermost upper mantle with the cratonic lithosphere being about 4 faster than that of the limpopo mobile belt the resolution of the technique is such that it is difficult to ascertain whether these differences are features of precambrian evolution or are due to reactivation of the upper mantle during karoo igneous and tectonic activity

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

the limpopo mobile belt a result of continental collision the 600 km long limpopo mobile belt is discussed within the frame of a proterozoic supercontinent model piper 1976 evidence is presented that the rhodesian and kaapvaal cratons may have been separated by distances of more than 1000 km of oceanic crust from about 3350 ma ago the kaapvaal craton appears to have been driven intermittently n nw and then wnw against the rhodesian craton forming the ne sw trending collision zone the limpopo mobile belt and all the major fold and fracture patterns found this movement would be similar to the oblique movement of the pacific plate into the aleutian trench when collision ceased around 2500 ma ago it is likely that the great dyke and other complexes intruded along release fractures formed at right angles to the compression

fluid inclusions in hydrated granulite facies rocks southern marginal zone of the limpopo belt south africa abstract the archean granulite terrane of the southern marginal zone of the limpopo belt south africa was rapidly uplifted and thrust over a low grade granite greenstone terrane van reenen et al 1987 during cooling the granulites were rehydrated in a zone parallel to the contact with the low grade terrane a sharply defined retrograde orthoamphibole isograd at conditions of 6 kbar 625 650 c x co 2 0 8 van reenen 1986 was established we made microthermometric analyses of over 1000 fluid inclusions in four samples representing a traverse across the isograd with the goal of characterizing the hydrating fluid we found patterns of composition and density of inclusions to be similar to those reported from other granulite facies terranes a few apparently pure co 2 inclusions with densities appropriate for the p t conditions many co 2 inclusions with lower densities and aqueous inclusions of variable salinity and containing no detectable co 2 the results suggest that the hydrating fluid may be represented by secondary co 2 rich fluid inclusions because of their small size most between 5 and 10 m and equant shapes h 2 o contents of the co 2 rich inclusions of up to 0 3 mole fraction may not be detectable a model for hydration of the granulites includes infiltration of a fluid into the partially cooled overthrust granulites with a temperature gradient decreasing towards the underthrust greenstone terrane the fluid was probably generated by devolatilization of the greenstones as they were heated by the overthrust granulites the high co 2 content of the calculated fluid and of the fluid inclusions suggests the hydrating fluid was co 2 rich at the p t conditions of the retrograde isograd

optimization of signal to noise ratio in dc soundings geoelectric dc sounding methods are usually limited to electrode spacings smaller than 1 km which restrict the depth of investigation to only several hundreds of meters greater depths of investigation require both a larger electrode spacing and motor generator driven transmitters in order to increase the depth of investigation into the middle crust the distance between transmitter and receiver dipole must be increased up to 100 km as well instead of a schlumberger electrode configuration we use a dipole dipole electrode arrangement which avoids cable connections of several tens of kilometers it takes less logistics in the field and requires less precautions to control the cable circuits however the electric field of a grounded dipole decays by the power of 3 even if a strong current source with a power of 30 kw is employed at distances beyond 10 km the amplitude of the transmitted dc signal becomes considerably smaller than the naturally inductive field and cultural noise here we present a technique which resolves the transmitted dc signal at sites up to 60 km apart using an electric dipole source the suggested technique removes the inductive part in the time series and therefore reduces the noise level it requires applying the magnetotelluric technique before and or after the geoelectrical sounding both the magnetic and electric variational fields are recorded during geoelectrical sounding if the magnetotelluric transfer function is obtained at a site then the natural inductive electric field can be predicted which in turn is used to remove the inductive electrical part from the receiver dipole record

origin of granulite terranes and the formation of the lowermost continental crust differences in composition and pressures of equilibration between exposed regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust as had been thought but rather middle and lower middle crustal levels application of well calibrated barometers indicate that exposed granulites record equilibration pressures of 0 6 to 0 8 gigapascal 20 to 30 kilometers depth of burial whereas granulite xenoliths which also tend to be more mafic record pressures of at least 1 0 to 1 5 gigapascals 35 to 50 kilometers depth of burial thickening of the crust by the crystallization of mafic magmas at the crust mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep seated mafic crust represented by many xenolith suites

diversity dynamics of bajocian middle jurassic ammonites and transgressions regressions in the caucasian sea northern neo tethys ocean a case high resolution test of probable dependence abstract ammonites commonly occur in the bajocian deposits of the greater caucasus basin from where 180 species and 52 genera are reported this back arc basin was located on the northern neo tethyan margin embraced by the caucasian sea an evaluation of diversity patterns for 7 bajocian ammonite zones documents high diversity in the beginning middle and end of the stage with a maximum in the humphriesianum zone and two diversity minima the mid early bajocian impoverishment of ammonite assemblages was caused by both an increase in the rate of disappearance and a decrease in the rate of appearance whereas the only latter was responsible for the mid late bajocian diversity drop an analysis of dominating facies types in the greater caucasus basin documents that the total diversity rose during regressions and fell during transgressions shoreline shifts corresponded better with changes in the number of appearances and worse with those of disappearances a comparison of results from this study with those obtained in the betic cordillera does not clarify the observed dependence changes in the exterior connections of the caucasian sea its depth and oxygen depletion of its waters are unlikely explanations of the regionally documented coincidence of ammonite diversity dynamics and transgressions regressions

hydration of cordierite and hypersthene and a description of the retrograde orthoamphibole isograd in the limpopo belt south africa

geodynamics seismicity and seismic hazards of the caucasus abstract being a part of ongoing continental collision between the arabian and eurasian plates the caucasus region is a remarkable site of moderate to strong seismicity where devastating earthquakes caused significant losses of lives and livelihood in this article we survey geology and geodynamics of the caucasus and its surroundings magmatism and heat flow active tectonics and tectonic stresses caused by the collision and shortening gravity and density models and overview recent geodetic studies related to regional movements the tectonic development of the caucasus region in the mesozoic cenozoic times as well as the underlying dynamics controlling its development are complicated processes it is clear that the collision is responsible for a topographic uplift inversion and for the formation of the fold and thrust belts of the greater and lesser caucasus tectonic deformations in the region is influenced by the wedge shaped rigid arabian block indenting into the relatively mobile region and producing near n s compressional stress and seismicity in the caucasus regional seismicity is analysed with an attention to sub crustal seismicity under the northern foothills of the greater caucasus which origin is unclear whether the seismicity associated with a descending oceanic crust or thinned continental crust recent seismic tomography studies are in favour of the detachment of a lithospheric root beneath the lesser and greater caucasus the knowledge of geodynamics seismicity and stress regime in the caucasus region assists in an assessment of seismic hazard and risk we look finally at existing gaps in the current knowledge and identify the problems which may improve our understanding of the regional evolution active tectonics geodynamics shallow and deeper seismicity and surface manifestations of the lithosphere dynamics among the gaps are those related to uncertainties in regional geodynamic and tectonic evolution e g continental collision and associated shortening and exhumation lithosphere structure deformation and strain stress partitioning and to the lack of comprehensive datasets e g regional seismic catalogues seismic gravity and geodetic surveys

role of fluids in the exhumation of the southern marginal zone of the limpopo complex south africa abstract the limpopo complex lc of southern africa displays structural metamorphic and geochronological evidence for a complex exhumation controlled evolutionary history that followed after burial and attainment of peak metamorphic conditions p 9 kbar t 900 c in the root zone central zone cz of the orogen before 2 72 ga the southern marginal zone smz of the lc in addition provides evidence for thrust controlled emplacement of a hot granulite facies nappe onto the adjacent low grade granite greenstone terrane of the northern kaapvaal craton nkvc in the interval 2 69 2 62 ga a tectono metamorphic scenario in which initial near vertical channeling exhumation at 2 69 ga was transferred into horizontal channeling thrusting at 2 67 2 64 ga is interpreted to have commenced with in situ partial melting of crustal rocks in the root zone of the orogen cz this resulted in formation of melt weakened crust migmatized gneisses that with continuous shortening led to vertical transfer of lower crustal material accompanied by fluid fluxed decompression melting that resulted in production of large volumes of leucocratic granitic material this material detached to form buoyant diapirs that accompanied the emplacement at 2 65 ga of mega sheath folds at the middle crustal level throughout the entire cz of the lc this early stage of near vertical exhumation is reflected i by the fact that linear elements of sheath folds and shear zones plunge consistently at steep angles to the sw throughout the entire cz and ii by decompression cooling dc segments of p t paths constructed for metapelitic rocks dc paths in the far northern part of the smz that are associated with the steep north dipping portion of the hout river shear zone hrsz traversed from p 8 5 kbar t 850 c to p 6 5 kbar t 720 c early near vertical exhumation transferred into horizontal channel flow that controlled emplacement at the middle crustal level of the granulite facies nappe smz horizontal channeling thrusting is recorded in metapelitic granulite by near isobaric cooling p 6 kbar t 700 550 c segments of earlier dc p t paths that are associated with the shallow north dipping portion of the hrsz devolitilization of underthrusted greenschist due to thrusting hot granulite over cool greenschist released large volumes of high temperature aqueous fluids from the underthrusted rocks that constantly infiltrated the overriding granulite during southwards migration of the nappe immiscible co 2 rich and brine fluids thus derived persistently infiltrated into and reacted with cooling granulite and triggered the main pulse of anatexis at p 7 5 kbar t 750 c similar fluids established a regional retrograde isograd and associated zone of 4500 km 2 retrograde hydrated granulite at p 6 kbar t 600 630 c located in the hanging wall of the shallow north dipping hrsz that bounds the smz in the south shear zone hosted metasomatic alteration including formation of lode gold deposits occurred at t 600 630 c p 6 kbar in the zone of rehydrated granulite and at higher p t conditions in the granulite zone the granulite facies smz of the lc thus preserves clear evidence for a direct link between persistent fluid flux and fluid rock interaction as an integral component of a conveyor like tectonic mechanism driven by vertical tectonics that operated from the core region of the orogen cz this mechanism explains the important link between the thrust controlled emplacement of the smz onto the nkvc and deep crustal fluid flow

reaction history of garnet sapphirine granulites and conditions of archaean high pressure granulite facies metamorphism in the central limpopo mobile belt zimbabwe sequential reaction textures in archaean garnet corundum sapphirine granulites from the central zone of the limpopo belt document a progression from early coarse grained high pressure p 9 5 kbar granulite facies assemblages m1 to late low pressure p 6 kbar granulite facies sub assemblages m2 the stable m1 assemblage was garnet 57 pyrope mg mg fe 62 sapphirine corundum gedrite phlogopite rutile late m1 boron free kornerupine grew at the expense of garnet and corundum and coexisted with garnet sapphirine and gedrite partial or complete breakdown of coarse garnet and kornerupine during m2 resulted in the development of pseudomorphs and coronas consisting of fine grained symplectic intergrowths of cordierite gedrite and sapphirine later spinel the majority of reaction textures can be explained in terms of a stable reaction sequence and a model time sequence of mineral facies can be constructed when compared with a qualitative petrogenetic grid of fe mg discontinuous reactions in the fmash multisystem sapphirine garnet corundum spinel cordierite gedrite kornerupine the facies sequence indicates decompression at essentially constant t assuming constant a h2o exhumation of m1 corundum inclusions during m2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite a second disequilibrium reaction of the spinel with cordierite produced sapphirine the operation of such reaction while pressure was decreasing the opposite dp from that implied by the texture if assumed to be the product of an equilibrium reaction has serious implications for the use of reaction textures in the construction of p t vectors garnet biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca 850 c for the m1 stage a minimum late m1 pressure of ca 7 kbar is indicated by the former association of kornerupine and corundum relict m1 kyanites reported by other workers indicate a minumum early m1 pressure of 9 5 kbar implying metamorphism at depths of at least 33 km probably 38km the high pressure granulite facies metamorphism was followed by an almost isothermal pressure decrease of 5 kbar indicative of rapid uplift the p t path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite facies terranes

late cenozoic burial history and dynamics of the northern caucasus molasse basin implications for foreland basin modelling abstract the collisional history of the caucasus segment of the alpine himalayan fold belt started at the end of the eocene the associated development of the northern caucasus foreland basin occurred during two syn collisional stages each displaying different subsidence patterns the 34 16 ma maikopian or pre foreland stage displays a long wavelength subsidence of a broad area whereas the 16 0 ma foreland stage displays asymmetrical foreland subsidence the first is correlated with the termination of subduction in the southern area and can be associated with mantle flow induced by the re equilibration of the subducted slab the along strike configuration of the second molasse basin stage contradicts the hypothesis in which topographical loading is considered to be the main control on foreland subsidence there is a clear anti correlation between basin depths and orogen heights the deepest parts of the basin are at the tips of the orogen where mountain heights are negligible while the area adjacent to the highest mountains central caucasus is uplifted the influence of other types of loading in foreland basin development has therefore been investigated and it has been possible to find a good fit with both gravity and basin architecture by including only two additional model parameters into the model these are crustal and lithospheric thickening thinning the results demonstrate that crustal thickening and removal of lithospheric roots are responsible for supporting the high central caucasus mountains and uplift of adjacent areas the subsidence of the basins at the orogen tips is explained by loading of lithospheric roots both effects are important in the geographically intermediate areas in general it is concluded that topography should not be considered as the main control on foreland subsidence but only as one of several counterbalancing mechanisms the existing flexural model therefore needs improvements such that it can be related directly to subsurface loading by real structures and processes during collision

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

evolution of the northern high grade margin of the kaapvaal craton south africa the northern kaapvaal craton is divided into three distinctive crustal zones 1 the high grade southern marginal zone smz of the limpopo belt in the north 2 a transitional zone of technically stacked metamorphic rocks which includes the area from the sutherland belt in the north to the pietersburg and murchison belts in the south and 3 a low grade granite greenstone terrane farther to the south the change across the smz boundary is expressed by an overall increase in the grade of metamorphism a strip of retrogression related to a zone of thrusting abrupt changes in intensity of deformation rock densities magnetic lineament patterns and isotopic ages in compositionally similar rocks regional metamorphism in the northern kaapvaal craton is demonstrably not a continuum in the sense that the highest grade rocks underwent all earlier stages of metamorphism rather that rapid local tectonic processes must have occurred a proposed evolutionary path for the smz was initiated with crustal thicke

evolution of the north caucasus foredeep constraints based on the analysis of subsidence curves abstract using a database of more than 130 wells the alpine evolution of the north caucasus foredeep can be described in three main periods 1 the early jurassic to middle late cretaceous including the cenomanian relates to the initial rifting phase and was characterised by succession of comparatively high rates of subsidence and uplift during this stage many events in the eastern and western parts were synchronous whereas some of them appeared to be smoothed in the stavropol high the southern border of the area had a more complex behaviour often moving in the opposite direction relative to the rest of the area the main peculiarities of evolution of the great caucasus region can be explained if we adopt the hypothesis of stamply and pillevuit 1993 and suppose that the early jurassic extension of the trough was accompanied by a strong left lateral transform movement as a result the central part of the great caucasus trough formed as a pull apart basin analysis of the style of movements of the southern border of the scythian plate as well as data on tectonics and volcanism in the great and lesser caucasus showed that other regional compressional and extensional mesozoic events could also have a transform component shear stresses within the lithosphere of the great caucasus can be due to oblique subduction or even transform movements at the plate boundary to the south of the caucasus region dercourt et al 1993 2 the period from the late cretaceous to the middle eocene from the turonian to the bartonian relates to the oceanic suture of the lesser caucasus and was characterised in the great caucasus area by alternating subsidence and uplift events of considerably lower amplitude at least up to the maastrichtian beginning in the late paleocene subsidence curves for almost all the area reflect the same events but in the western part and also in the north of the central part the rate of movements was considerably higher and since the late paleocene short term events in this area took place on the background of rather fast subsidence at nearly constant rate we believe that this subsidence and formation of the east black sea depression have the same origin we consider strong differences in evolution of the eastern western and central parts during the second stage to be due to closure of the lesser caucasus oceanic basin and arrival of the nakhichevan block this led to changes of configuration of the plate boundary and resulted in reorganisation of stress and displacements within the caucasus region this reorganisation was a reason for the opening of the east black sea depression and rapid subsidence of the western and central parts of the area at the end of the second stage 3 the period from the middle eocene to the present relates to the development of a foreland basin coeval with shortening and uplift in the adjacent great caucasus range it was characterised by alternation of relatively short uplift and longer subsidence events an important feature of this stage is that although the amplitude of movements varied from place to place these events were synchronous in all parts of the foredeep at the stavropol region sediments preserved only for the beginning of this stage formation and evolution of the foredeep during the third stage can not be explained exclusively by elastic flexure to do this we used the model of a small scale convection within the asthenosphere mikhailov et al 1996 mikhailov et al 1999 by the comparison of numerical results with the data on the evolution of the north caucasus foredeep we concluded that there were four main stages of compression in the processes of formation of the great caucasus mountain belt the first compression took place before the maykopian between 39 5 and 36 0 ma the other three were in the tarkhanian 16 6 15 8 ma konkian early sarmatian 14 3 12 3 ma and pontian 7 0 5 2 ma the different width of the great caucasus trough by the beginning of the compression as well as variations in thickness of the lithosphere and a different thermal state can cause interruption of the formation of the foredeep at the stavropol high

active stress field and fault kinematics of the greater caucasus abstract this work contributes to depict the current seismicity fault kinematics and state of stress in the greater caucasus territories of georgia azerbaijan and russia we merged and homogenized data from different earthquake catalogues relocated 1000 seismic events created a database of 366 selected focal mechanism solutions 239 of which are new and performed a formal stress inversion preferential alignments of crustal earthquake foci indicate that most seismic areas are located along the southern margin of the belt and in the north eastern sector this is consistent with the presence of dominant active wnw ese faults parallel to the mountain range in the entire greater caucasus a dominant nne ssw oriented greatest principal stress 1 controls the over all occurrence of earthquakes of minor and major magnitude main earthquakes are characterized by a vertical least principal stress 3 corresponding to reverse kinematics reverse slip is more common along the southwestern and north eastern foothills of the greater caucasus although in these areas there are also scattered strike slip events this suggests the presence of local stress fields with horizontal 1 and 3 in the central southern part of the mountain belt in correspondence of the local collision between the lesser and the greater caucasus 1 rotates to nnw sse the strike slip events instead dominate along the southern flank of the central eastern mountain range this is interpreted as the effect of the collision that promotes eastward escape of the tectonic blocks located to the east

reconnaissance electromagnetic induction study of the kapuskasing structural zone implications for lower crustal conductivity abstract a large scale electromagnetic induction study has been carried out over the kapuskasing structural zone ksz an upthrust section of lower crustal rocks in north central ontario the absence of an overall anomalous response from the ksz even though the lower crust in this region of the canadian shield is known to be anomalously conductive implies that enhanced lower crustal conductivity is related to intrinsic conditions at lower crustal depths and not to mineralogical composition

the holocene climatic optimum and pollen records of sapropel 1 in the eastern mediterranean 9000 6000 bp abstract the most recent sapropel in the deep eastern mediterranean sea has been deposited between 9 and 6 ka bp climate conditions as revealed by the pollen records of this sapropel in marine cores were most favorable for temperate deciduous trees which is in agreement with the inferences from records of peripheral land pollen sites the abundance of deciduous oak pollen is much higher than that of artemisia sage brush indicating that annual precipitation in the mid elevation borderlands was at least 550 mm without summer drought but more probably in the range 800 1300 mm the pollen of pistacia which formed a savanna at low elevations is also at its highest abundance and signals the absence of frost in winter while being capable of withstanding summer drought the early holocene therefore appears as the post glacial climatic optimum with the highest moisture and mildest winters in southwest asia this is also the time of the neolithic population explosion with incipient domestication of cereals possibly following natural selection of the tough rachis mutation in wheat and barley by the extreme aridity of the preceding younger dryas

structures in granitoid gneisses and associated migmatites close to the granulite boundary of the limpopo belt south africa abstract river platform exposures of quartzofeldspathic gneiss and associated migmatites occur close to the boundary of the southern marginal zone of the limpopo belt with the adjacent low grade granite greenstone terrane of the kaapvaal craton the complex gneisses and migmatites are described and are considered to represent the tectono thermally reworked equivalents of the pietersburg sutherland greenstone belts and early tonalitic and trondhjemitic intrusive granitoids structural features associated with the hout river shear zone and the limpopo orogeny 2700 2650 ma old and which include subhorizontal overthrust fault slices produced as a result of lateral extrusion of rock material following transpressional displacement along a strike slip fault or thrust zone support structural metamorphic and geophysical evidence suggesting southwestwards overthrusting of the high grade southern marginal zone rocks onto lower grade rocks of the kaapvaal craton

the deep structure of the limpopo belt from geophysical studies abstract geophysical studies in the limpopo belt and adjoining cratons contributed to the fundamental understanding of the deep structure in these tectonic provinces the seismic and geoelectrical data sets prove that the terrane boundaries of the limpopo belt dip in towards the belt to give it the appearance of a pop up structure the gravity data show that high grade metamorphic rock with densities that are high compared to the normal low grade upper crustal granitoids form the upper crust in the northern and southern marginal zones of the limpopo belt these rocks also show a low magnetic susceptibility the geophysical data clearly defined the position of the southern boundary of the limpopo belt the northern boundary of the central zone can be related to a southward dipping zone of seismic reflectors while the southern boundary of this terrane as formed by the palala shear zone seems to be near vertical the greenstone belts of the craton have a limited depth extent and are allochtonous the present position of these structures could be related to thin skinned tectonics that affected the upper crust on the kaapvaal craton and which were revealed by seismic reflection data

tectonic paleostress fields and structural evolution of the nw caucasus fold and thrust belt from late cretaceous to quaternary abstract the nw caucasus fold and thrust belt essentially corresponds to the inverted western flysch zone of the great caucasus mountains a deep basin that developed from late jurassic to eocene times between the scythian plate to the north and the transcaucasian terranes to the south the shatsky ridge sw of the nw caucasus zone the flysch basin was strongly affected by compression in late eocene times when the characteristic wnw trending folds and thrusts of the nw caucasus belt developed some authors regard the main compressive deformation as miocene in age by means of remote sensing analysis we elucidate the geometry of major structures in the belt wnw trending south vergent thrusts and folds and major vertical and transverse nne ssw to ne sw deep fault zones the later structures are interpreted as ancient faults that were active during the development of the flysch basin paleostress investigations reveal seven main tectonic episodes in the evolution of the nw caucasus since late cretaceous combining structural interpretation remote sensing analysis and paleostress field reconstruction we propose a model for the structural evolution of the belt during the late cretaceous paleocene the western caucasus zone was under transtensional regime with an e w to ne sw trending 3 that generated oblique normal movements along nne ssw transverse faults and wnw ese margins of the flysch basin this tectonism could correspond to rifting related to the formation of the eastern black sea basin at the paleocene eocene boundary a transpressional event with an e w to nw se trending 1 developed and the nne ssw to ne sw trending faults could have been inverted this event could correspond to an attenuation in the eastern black sea basin formation or to the incipient accretion of the transcaucasian terranes during the eocene another e w to nw se oblique extension transtensional event affected the flysch basin that could be related to a known rifting phase in the eastern black sea basin strong nne ssw to ne sw compression characterises late eocene tectonism the fold and thrust belt developed at this time as a result of the direct collision of the shatsky ridge with the scythian plate a ne sw extension followed the late eocene event related to basin development around the newly formed fold belt a wnw ese oblique contraction affected the belt during the early miocene as the result of arabian plate convergence with the caucasian system the latest inferred event is a compressional regime with nnw sse trending 1 that is affecting the nw caucasus belt from sarmatian times until the present under this oblique compression the belt has deformed as in a dextral shear zone and the thrust surfaces have acquired lenticular shapes this study highlights the occurrence of oblique movements in the nw caucasus area prior to and after the dominant late eocene compression from the late cretaceous until the eocene the structural development of the nw caucasus was closely related to the evolution of the eastern black sea basin from the late eocene until quaternary times it was rather related to the arabia eurasia plate convergence

tectonic nappes and horizontal layering of the earth s crust in the mediterranean belt carpathians balkanides and caucasus abstract the morphological and kinematic features and mechanism of nappe formation of the pre alpine and alpine tectonic nappes of the carpathians balkanides and caucasus are discussed detached basement nappes cover nappes and ophiolite obduction nappes are distinguished all detached nappes are shown to be genetically compressional squeezed out as a result of tangential compression at depth accompanied by active underthrusting of the autochthone under the allochthone amstutz type subduction ophiolite nappes are formed by squeezing out of oceanic crust and the occurrence of bilateral or unilateral obduction simultaneously with subduction this formation of ophiolite nappes is a very transient process occurring at lightning speed in the geological sense compared to subduction data on large horizontal displacements at various levels in the crust entirely confirm the idea of a proposition that horizontal crustal and lithospheric layering has recently been substantiated by geophysical data too

geodynamic evolution of the caucasus and adjacent areas in alpine time abstract the alpine geodynamics of the caucasus and adjacent areas is discussed palinspastic reconstructions are given on the basis of new geologic paleomagnetic and paleobiogeographic data the dynamic unity of extension and compression processes is shown to take place within the central segment of the mediterranean belt the closure of the paleotethys oceanic basin coincides approximately with the opening of the mesotethys owing to this the opening between afro arabia and eurasia persists as is confirmed by paleomagnetic and other data the closure of the mesozoic tethys is on the whole synchronous with the opening of the central and south atlantic the formation of the present structure of the caucasus and adjacent areas is connected with the unidirectional compression of this area and the rotation of the compressional axis from a northeastern to a sub meridional direction in the course of the cimmerian alpine cycle

imaging of deep fluids in archaean crust despite the importance of aqueous fluids in tectonic metamorphic and magmatic activity in the earth s crust few techniques are available for directly observing their distribution at depth even in stable continental crust such observations as have been made electromagnetic deep sounding which by inference1 2 images saline fluids and super deep drilling3 suggest that fluids are present in varying amounts throughout the crust in direct conflict4 with the normal petrological inference of a very dry deep crust here we report the results of an unusually high resolution electromagnetic sounding method to image the deep electrical conductivity structure below a 30 km profile in ontario which crosses archaean lower crustal material of high metamorphic grade exposed by a proterozoic thrust fault the ivanhoe lake cataclastic zone the electrical resistivity image obtained shows embedded in very resistive crystalline crust two nearly continuous sub horizontal zones of enhanced electrical conductivity at depths of 2 3 and 5 6 km below this the commonly observed2 5 7 mid crustal rise in conductivity is seen at depths of 15 20 km the image does not correlate well with the structure inferred from surface geology suggesting that the transport and distribution of fluids was not strongly controlled by lithological boundaries or the thrust fault itself

seismic studies in the limpopo belt abstract the structure of the archaean limpopo belt of southern africa has been investigated by means of a deep seismic reflection profile and the recording of the traveltimes of mining induced tremors at regional distances interpretation of the traveltimes suggests that a seismic low velocity zone exists in the upper mantle of the northern kaapvaal craton and limpopo belt beginning at a depth of about 42 km the seismic reflection profile provides evidence for a northward dip of the contact between the kaapvaal craton and the southern marginal zone of the limpopo belt no mid crustal decollement zone is detected the tectonic model of van reenen et al 1990 is in accord with these observations the lower crust is essentially seismically transparent and no reflection moho is observed

gravity signature of the limpopo kaapvaal fossil plate boundary in southern africa discussion discussion sur l article nld de emenike e a paru dans tectonophysics 1986 128 127 137

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

direct current resistivity and lotem model for the deep structure of the northern edge of the kaapvaal craton south africa abstract the transition from the high grade metamorphic southern marginal zone smz of the limpopo belt to the low grade kaapvaal craton was studied by means of geoelectrical techniques the d c data showed conclusively that the high grade rocks were thrust over the low grade rocks this contradicts earlier geological ideas but agrees with recent seismic reflection data the area where high grade rocks occur above low grade strata was mapped out and the southern boundary of the smz was established the conductor that occurs at lower crustal depths in the kaapvaal craton was traced into the smz using the lotem method the conductor shallows from 30 to 22 km over 12 km on the surface but does not reflect the difference in metamorphic depth of 30 km seen at the surface this indicates that the conductor is a transient feature of the crust the conductor is not associated with a seismically reflective crust the cause of the conductor remains enigmatic