structural evolution of the sarandi del yi shear zone uruguay kinematics deformation conditions and tectonic significance the sarandi del yi shear zone is a crustal scale shear zone that separates the piedra alta terrane from the nico perez terrane and the dom feliciano belt in southern uruguay it represents the eastern margin of the rio de la plata craton and consequently one of the main structural features of the precambrian basement of western gondwana this shear zone first underwent dextral shearing under upper to middle amphibolite facies conditions giving rise to the reactivation of pre existing crustal fabrics in the easternmost piedra alta terrane afterwards pure shear dominated sinistral shearing with contemporaneous magmatism took place under lower amphibolite to upper greenschist facies conditions the mylonites resulting from this event were then locally reactivated by a cataclastic deformation this evolution points to strain localization under progressively retrograde conditions with time indicating that the sarandi del yi shear zone represents an example of a thinning shear zone related to the collisional to post collisional evolution of the dom feliciano belt that occurred between the meso to neoproterozoic 600 ma and late ediacaran lower cambrian times

palynological analyses of eocene to oligocene sediments from dsdp site 338 outer voring plateau article i nfo against the background of the profound global climatic shift from greenhouse to icehouse conditions during the eocene oligocene transition major geographic and oceanographic changes were taking place in the norwegian greenland sea region the voring plateau was a prominent structural feature which influenced the evolution of water mass circulation in the nordic seas and we present detailed palaeoenvironmental reconstructions of this structure new palynological results suggest that shallow water inner neritic environments were developed across parts of the voring plateau during early eocene times with terrestrial and brackish water palynomorphs indicating that both basement highs to the north and the crestal part of the voring escarpment may have been emergent a transition from marginal marine to open marine conditions occurred around 44 ma ago with the complete subsidence of the voring plateau below sea level facilitating inter basinal surface water circulation and promoted a significant increase in photic zone fertility carbon sequestration associated with such enhanced productivity in the late eocene nordic seas may have contributed to declining cenozoic atmospheric carbon dioxide levels thence to declining global temperatures and the development of limited northern hemisphere continental ice on greenland in the latest eocene

crustal lineaments distribution of lower crustal intrusives and structural evolution of the voring margin ne atlantic new insight from wide angle seismic models abstract five lineaments on the volcanic voring margin ne atlantic have been identified in crustal scale models derived from ocean bottom seismograph obs data it is suggested that the voring basin can be divided in four compartments bounded by the jan mayen fracture zone lineament a new lineament defined from this study the gleipne lineament the surt lineament and the bivrost lineament the nw se trending jan mayen gleipne and bivrost lineaments probably represent old zones of weakness controlling the onset of the early eocene seafloor spreading whereas the surt and new lineaments rotated ca 30 symmetrically from the azimuth of the gleipne lineament may represent adjustment features related to the early cretaceous early tertiary rifting the longest landward extent of a lower crustal high velocity body assumed to represent intrusions related to the last phase of rifting is found between the new lineament and the gleipne lineament where the body extends across the helland hansen arch northeastwards in the voring basin the landward limit of the body steps gradually seawards closely related to the interpreted lineaments northeast of the gleipne lineament the body terminates close to the fles fault complex north of the surt lineament it extends across the nyk high and northeast of the bivrost lineament the intrusions terminate around the voring escarpment evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin the active component is evident through the decrease in magmatism with increased distance from the icelandic plume and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement the increased thickness of the continental crust on the seaward side of the voring escarpment the upwarping of moho and thinning of the lower crustal high velocity layer in the western part of the voring basin as well as a strong shallowing of the moho observed in parts of the area between the jan mayen fracture zone lineament and the new lineament can be explained by lithospheric delamination models

extension crustal structure and magmatism at the outer voring basin norwegian margin regional analysis of new 2d and 3d multichannel seismic data has improved interpretation of the crustal configuration and structural style along the norwegian margin five domains with different structural styles and evolutions are defined along the outer voring basin 1 the nyk high naglfar dome 2 the north gjallar ridge 3 the gleipne saddle 4 the south gjallar ridge 5 the ran ridge early campanian early paleocene and early to mid cretaceous extensional events are evidenced timing of deformation and structural styles observed along each segment reflect a lateral variation of the rifted system probably affected by magma tectonic processes correlation with the deep structures of the outer voring basin shows that the shallow structure in that basin is directly controlled by a deep seated strong high amplitude reflection the t reflection marking the top of a high velocity body v p 7 km s 1 the relation between the lower crustal architecture and the subsurface basin structures has implications for the margin evolution and for the nature of the high velocity body

brittle tectonic reconstruction of palaeo extension inherited from mesozoic rifting in west zagros kermanshah iran abstract the present day zagros mountain belt of sw iran corresponds to the former arabian passive continental margin of the southern neo tethyan basin which existed since a permian triassic rifting episode and underwent later collisional deformation in mid to late cenozoic time in west zagros the kermanshah radiolarian trough separated from the neo tethyan basin by the narrow continental platform of the bisotun limestone resulted from the triassic jurassic rifting of the passive margin brittle tectonic analyses of syndepositional normal fault slip data have been undertaken to study the extensional history of the passive margin in terms of palaeostress reconstruction dominant east west trends of the inherited rift related normal faults in west zagros indicate an approximately north south trend of extension that prevailed during the entire mesozoic syn to post rift evolution of the radiolarian trough this extension characterizes a stretched continental margin similar to the present day passive margin of the british isles considering the structural pattern of the inherited basement faults as revealed by the present day earthquake focal mechanisms an oblique crustal stretching model is proposed for the rifting process

a moderate melting model for the voring margin norway based on structural observations and a thermo kinematical modelling implication for the meaning of the lower crustal bodies high p wave velocities 7 1 7 8 km s lower crustal bodies lcbs imaged along volcanic margins are commonly interpreted as plume and breakup related thick mafic underplating this interpretation is partly challenged in this paper based on new seismic observations and modelling of the outer voring basin norway an exceptional strong amplitude reflection the t reflection is particularly well defined below the north gjallar ridge ngr between 7and 8 s twt the t reflection is located near the volcanic lava flows emplaced during the ne atlantic breakup 55 54 ma ago and coincides with the top of the lcb forming a mid crustal dome based on structural and temporal relationships we show that the dome clearly influences the structural development of the ngr and predates the continental breakup at least by 10 15 ma using a thermo kinematical model we tried also to investigate and quantify the relationships between the extension lcb and the magmatic production modelling suggests that significant paleocene early eocene magmatism can be produced without any temperature anomaly in the mantle if differential stretching occurs during the breakup initiation the conclusion of 2d thermo kinematical parametric analysis is that the magmatic model predicts either little extension 5 with significant melting along the outer voring basin we suggest that the continental part of the lcb could not necessarily be breakup related and so magmatic as has often been stated previously it is concluded here that the continental part of the lcb observed beneath the outer voring basin may be partly or fully attributed to inherited high pressure granulite eclogite lower crustal rocks the real amount of mafic material emplaced along the outer voring basin could be 20 40 less than thought

deep crustal structure and magmatic processes the inception of seafloor spreading in the norwegian greenland sea summary an extensive investigation into the deep crustal structure of the conjugate norwegian and e greenland margins utilizing two ship expanded spread and wide aperture cdp profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading on both margins a region of low velocity crust was found proximal to marginal escarpments immediately seaward of the low velocity crustal block seaward dipping reflectors occur within thick crust that is interpreted to be wholly igneous a product of voluminous melt production enhanced by convective partial melting processes this earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust normal oceanic crustal thicknesses are not produced until about 5 my after the initiation of spreading when convective partial melting has abated and the magma budget has reduced the extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter

upb geochronology of the central ribeira belt brazil and implications for the evolution of the brazilian orogeny abstract the ribeira belt in southeastern brazil was generated during the brazilian orogeny the south american equivalent of the pan african orogeny 700 450 ma the central sector of the belt is characterised by a series of northwest vergent crustal scale thrust stacks emplaced under amphibolite grade conditions major shear zones separate four lithotectonic domains andrel ndia to the northwest juiz de fora paraiba and coastal domains analyses of zircon monazite and titanite from a variety of rock types from the four domains lead to the following conclusions all domains record an important tectono thermal event at 590 565 ma represented by partial melting granitoid intrusion and remobilisation of older basement gneisses and corresponding to thrusting and development of dextral shear zones also recorded are older metamorphic ages 611 604 ma of unknown significance only the paraiba and coastal domains in the south record a younger metamorphic event m2 at 535 520 ma most likely associated with another episode of thrusting and shear zone development the absence of m2 metamorphism in the andrel ndia and juiz de fora domains can be explained by post 535 520 ma emplacement of the paraiba domain late to post tectonic activity is represented by 503 492 ma metamorphism precursors of the basement gneisses are 2185 2134 ma old rocks generated during the transamazonian orogeny some of which contain a significant archean component with minimum ages of 2981 2846 ma the ages presented above define specific metamorphic events for the brazilian orogeny for the first time in southeastern brazil in addition together with previously published data they indicate that the basement rocks found in the allochthonous domains of the central ribeira belt represent an extension of the units present in the southern sao francisco craton and indicate that the foreland was more extensive than the presently defined craton

zircon u pb shrimp dating of a neoproterozoic overprint in paleoproterozoic granitic gneissic terranes southern brazil the neoproterozoic florianopolis batholith provides the major clue for unraveling the pre brasiliano cycle evolution of santa catarina state southern brazil because it contains septa of basement units such as the camboriu and aguas mornas complexes reactivated during the younger brasiliano orogeny at about 600 ma we establish precise u pb zircon shrimp ages and nd isotopic evolution of four key samples from the septa the zircon population displays complex internal structures in three of the four dated samples the complex crystals have well preserved magmatic cores and altered rim domains the core populations were dated at about 2180 ma aguas mornas complex about 2160 ma camboriu complex amphibolite xenolith and 2000 ma camboriu complex orthogneiss whereas the ages of the measured rims and altered domains are about 590 ma in all three samples on the other hand the presidente nereu tonalite has a simpler zircon population without core rim separation and yields magmatic ages about 2200 ma the nd isotopic analyses of the three samples emphasize the polycyclic evolution of the batholith which is much more complex than previously realized the evolution is marked by two successive paleoproterozoic accretionary orogenic events at about 2200 2175 ma and by a crustal melting event at about 2000 ma which had an archaean source sm nd depleted mantle model age tdm 2860 ma these orogenic events were followed 1 4 billion years later by a neoproterozoic hydrothermal metamorphic overprint at about 590 ma this time gap permits us to identify a long intracratonic period the atlantica supercontinent the unraveling of the proterozoic evolution and the precise characterization of the basement remnants show that the paleoproterozoic was a crust building orogenic interval and was the major source of the much later neoproterozoic granites the integrated use of back scattered electron and cathodoluminescence imaging with shrimp spot dating and nd isotopes provide a clear understanding of the timing of crustal generation and deformation in southern brazil including strong evidence for a long lasting supercontinent cycle

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

geodynamic evolution of the offshore indus basin pakistan the western indian plate passive continental margin a complex meso cenozoic history characterizes the offshore indus basin pakistan including rifting drifting collision transpression and recent inverted extension that is poorly understood we use reflection seismic data to document the geometry of the structures that accommodated the deformation and offshore well data to correlate the sedimentary sequences which were affected by the different tectonic episodes synthetic seismograms have been prepared for calibrating seismic data with well information for identification of key reflections representative of major lithological horizons seismic stratigraphy helped in dividing the sedimentary successions into rifted margin and transpressive margin sequences additionally the basement represented by chaotic and semitransparent reflections and moho discontinuity by moderate amplitude bright reflections are also identified on seismic records syn depositional normal faulting in extensional regime cretaceous oligocene are identified on seismic profiles that converted into strike slip faults in miocene and listric faults in recent sediments forming inverted extensional regime we infer that the basin inversion from extension to inverted extension and transpression occurred in recent times number of depositional features including shelves deltaic deposits canyons and channel levee complexes have been identified that developed in different tectonic episodes five tectonic subsidence curves using back stripping procedure have been calculated in order to constrain the geodynamic evolution of the meso cenozoic sedimentary succession of the offshore indus pakistan the analysis of tectonic subsidence curves covering a time span of 140 ma allowed us to quantify the total subsidence and to document the activities of syn rift and post rift faulting systems different stages in terms of duration and magnitude of subsidence uplift trends have been identified in the evolution of the basin two uplift events are prominent in eocene similar to 40 ma and miocene time similar to 12 ma a rapid resumption of subsidence with non conformity in early palaeocene is seen along the margin

paleomagnetism of the sierra de las animas complex southern uruguay its implications in the assembly of western gondwana global paleogeographies in the neoproterozoic are poorly known in particular the assembly of gondwana at the end of the proterozoic is controversial in order to constrain the kinematic evolution of the rio de la plata craton a paleomagnetic study was carried out in the vendian to cambrian sierra de las animas magmatic complex exposed in southeastern uruguay 34 7 s 55 3 w one hundred and forty seven samples from 23 sites were studied these included two sites on the vendian glaciogenic sediments of the playa hermosa formation after stepwise af and thermal cleaning a characteristic remanence was isolated at several sites according to the available isotopic data for the complex and the correlation between remanence directions and lithologic type and age two paleomagnetic poles were computed for the sierra de las animas complex sa1 338 1 e 5 9 n dp 19 6 dm 26 7 n 7 sites mean declination 44 5 mean inclination 58 0 95 18 1 with a likely age around 520 ma shows a distribution of vgps that suggests a remanence acquisition that spanned a long time during a period of fast apparent polar wander for gondwana sa2 250 9 e 16 9 s dp 15 9 dm 21 5 n 6 sites mean declination 95 3 mean inclination 58 5 95 14 5 corresponds to a likely age around 550 ma and falls close to a pole of similar age from the congo craton this suggests that the rio de la plata craton was at that time part of gondwana and confirms previous postulates that a single apwp can be defined for the supercontinent since 550 ma a preliminary mean geomagnetic pole obtained from the playa hermosa formation ph 198 4 e 43 0 s dp 8 6 dm 16 0 n 6 samples mean declination 226 0 mean inclination 24 2 95 15 0 is consistent with a previous pole of 595 ma from the campo alegre ca lavas and may suggest if confirmed that the glaciogenic deposits were produced at low to intermediate latitudes despite high uncertainties a new speculative model on the process of assembly of gondwana is proposed

post collisional transition from calc alkaline to alkaline magmatism during transcurrent deformation in the southernmost dom feliciano belt braziliano pan african uruguay abstract in the southernmost dom feliciano belt of uruguay highly fractionated calc alkaline granites mildly alkaline granites shoshonitic volcanics and peralkaline intrusions and volcanics are spatially and temporal associated with the evolution of shear zones four representative magmatic unites of this diverse association were petrographic and geochemically investigated the solis de mataojo complex a medium to high k2o calc alkaline granite with signature typical of mature continental arcs and post collisional settings the maldonado granite highly fractionated calc alkaline to alkaline with characteristics that are transitional between both types of series the pan de azucar pluton with characteristics typical of post collisional alkaline granites and the las flores shoshonitic basalts geochemistry and geotectonic setting point out that slab breakoff was most likely the mechanism associated with the generation of high k calc alkaline magmas solis de mataojo and maldonado shortly after collision extension associated to the formation of molassic basins and emplacement of dolerites and basalt flows with shoshonitic affinity las flores 15and finally a shift to magmas with alkaline signatures pan de azucar simultaneous with a second transpressional phase were probably linked with lithospheric thinning through delamination this evolution took place between 615 and 575 ma according to available data contrary to previous proposals which considered this magmatism to represent the root of a continental magmatic arc a post collisional environment transitional from orogenic to anorogenic during transcurrent deformation is proposed

tectonic setting of the late proterozoic lavalleja group dom feliciano belt uruguay abstract the lavalleja group exposed along the dom feliciano orogenic belt is located in the southeast of uruguay this group consists of volcano sedimentary rocks developed during the neoproterozoic brasiliano cycle the geochemical signature of the igneous rocks of the lavalleja group mainly metagabbros and basic and acidic metavolcanic rocks indicates a back arc basin tectonic setting the metamorphic grade increases to the southeast from very low grade lower green schist facies in the minas formation to a medium grade amphibolite facies in fuente del puma and zanja del tigre formations the metamorphic mineral assemblages correspond to a low pressure regional metamorphism associated with a high thermal gradient a compressive deformational event that probably corresponds to the closure of the lavalleja basin during a continental collision was recognized the petrology geochemistry metamorphic grade and tectonic setting are consistent with a back arc basin setting for the lavalleja group

deepest exposed crust of brazil shrimp establishes three events the deepest exposed crust of brazil is in the western portion of the exposed precambrian shield of southernmost rio grande do sul state and comprises a bimodal sequence of mafic garnet granulites and metatrondhjemites intercalated with smaller volumes of metamorphosed pyroxenites anorthosites sillimanite gneisses and banded iron formation as determined by zircon u pb sensitive high resolution ion microprobe shrimp this granulite terrain accreted from the mantle at the end of the archean ca 2 55 ga and was deformed in high pressure granulite facies metamorphic conditions in the paleoproterozoic ca 2 02 ga a younger event possibly ca 900 ma caused local amphibolite facies retrogression in the complex zircons from a metabasalt and a metatrondhjemite show external morphologies typical of high grade metamorphic rocks whereas backscattered electron images reveal internal oscillatory zoning of their igneous precursors spot ages from zircon agree well with previous model nd and sm nd mineral isochron ages

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

structural evolution and tectonic setting of the porongos belt southern brazil the sw ne striking porongos belt located between juvenile neoproterozoic rocks in the west and the dom feliciano belt characterized by intense reworking of older crust in the east comprises a greenschist to amphibolite facies metavolcano metasedimentary succession porongos sequence of unknown age with some exposures of palaeoproterozoic gneisses encantadas gneisses high temperature ductile deformation of the basement gneisses comprises at least two magmatic events followed by three deformational phases including folding and shearing dt1 dt3 and can be attributed to the palaeoproterozoic trans amazonian orogeny the deformation of the porongos sequence occurred during the neoproterozoic brasiliano orogeny and comprises four ductile deformation phases db1 db4 including two phases of isoclinal folding associated with shearing recorded in mylonitic layers followed by closed nw vergent folding and thrusting leading to formation of a thrust stack uplift of the basement and formation of late tectonic sedimentary basins occurred as a result of semi ductile to brittle block faulting in a sinistral strike slip regime the porongos sequence can be subdivided into a southeastern and a northwestern part trace element analyses as well as sm nd and rb sr geochemical data indicate partial melting and significant contamination by old continental crust for the metavolcanic rocks the metavolcanic rocks show end t 780 ma values of 20 64 and 21 72 northwestern units and 6 87 southeastern sequence the metasedimentary rocks were derived from late palaeoproterozoic to archaean sources and the data indicate different sources for the northwestern and southeastern rock units of the porongos sequence end t 780 ma are 6 25 and 6 85 in the southeastern units with tdm model ages between 1734 and 1954 ma and vary between 14 72 and 17 96 in the northwestern parts which have tdm model ages between 2346 and 2710 ma high 87 sr 86 sr t values between 0 7064 and 0 7286 confirm reworking of older crust isotopic signatures of the porongos sequence do not show indications for a significant contribution from a neoproterozoic juvenile source a passive margin or continental rift environment is suggested for the tectonic setting of the porongos belt which is compatible with both deposition of shallow marine to deep marine sediments and stretching of continental crust leading to volcanism which is characterized by significant contamination by old continental crust

scandinavia s north atlantic passive margin 1 the tectonics of the eastern passive margin of the north atlantic are reexamined the scandinavian north atlantic passive margin includes not only the offshore exploration and basin domain but also large portions of the onshore domains of the scandinavian caledonides combined information from structural geology potential field data regional geology basin development and geomorphology made it possible to propose a new definition of the passive margin the rift shoulder is formed by linked faults defining the present rift flank these faults are the extensional structures located furthest to the east onshore norway sweden that can be linked to the rifting that led to the formation of the north atlantic this innermost boundary fault system ibf is formed by a set of normal west dipping crustal faults it extends over a distance exceeding 2000 km from the north sea across the caledonian mountain belt to the barents sea the passive margin width between the continent ocean boundary and the ibf ranges from 550 km in the south to over 700 km in mid norway to 165 km north of lofoten rifting and faulting on the ibf started in permo carboniferous and a succession of rift phases eventually culminated in continental breakup and the formation of the north atlantic basin development between permian and cretaceous was toward the future breakaway fault but faulting was also active on land from mesozoic to present onshore offshore crustal scale cross sections show the geometry of the passive margin the dip orientation of the major faults and the changes in crustal thickness

the effect of lithospheric phase transitions on subsidence of extending continental lithosphere abstract a new simple model for sedimentary basin formation which combines both stretching and phase transitions occurring together in the lithosphere is proposed and two important aspects in sedimentary basin formation are addressed 1 the explanation of the domal uplift preceding rifting and 2 the ability of the model to explain the contradiction between a small amount of crustal stretching and the large amount of thermal subsidence without the need to invoke differential stretching between the crust and mantle relationships for synrift initial and thermal post rift subsidence are derived the model predicts that extension of continental lithosphere could produce synrifting uplift up to a threshold amount of extension and then subsidence for greater extension three dimensionless parameters quantify the contribution of the phase transition to the subsidence history 1 the clapeyron slope normalized to the initial geothermal gradient 2 the ratio of depth of phase transition to the thickness of the lithosphere and 3 the ratio of density effect due to phase transition to the change in density due to thermal expansion an evaluation of these parameters has been carried out by using thermodynamical data for the garnet spinel transition for upper mantle rocks our simple model shows that by including the effects of lithospheric phase transition in existing stretching models of sedimentary basin formation we can obtain an extra significant parameter controlling the subsidence and the entire extensional tectonics of such basins

distal magmatic hydrothermal origin for the camaqua cu au ag and santa maria pb zn cu ag deposits southern brazil abstract the camaqua cu au ag and santa maria pb zn cu ag deposits are the largest base metal deposits discovered so far in the sedimentary clastic sequences of the neoproterozoic age camaqua basin the origin of the camaqua santa maria deposits has been the centre of dispute with three alternative genetic hypotheses proposed a syngenetic model a diagenetic model and a magmatic hydrothermal model in detail this mineralization has been suggested to be related to sedimentary clastic diagenetic processes volcanic related processes or deep granitic magmatism reevaluation of previous data and new studies in the area yield the following conclusions 1 mineralization is fracture controlled and magmatic hydrothermal in origin rather than stratiform syngenetic or diagenetic 2 the temperature of deposition of the main ore minerals was 210 to 300 c 3 the 34 s of sulphides of around 0 indicates an external magmatic hydrothermal source of sulphur 4 pb isotope ratios of sulphides indicate that metals were derived at the end of the brasiliano cycle from a large crustal source with very primitive pb and that 5 the age of mineralization is 594 ma as constrained by u pb shrimp sensitive high resolution ion microprobe determinations on zircons of the lavras granite thus the camaqua and santa maria deposits are interpreted to be of magmatic hydrothermal origin with the metals derived from an old crustal basement source during the end of dom feliciano collisional orogeny at 594 ma late in the brasiliano cycle the interpretation above is critical for base metal exploration in the sul riograndense shield previous exploration methodologies mainly followed models based on a sedimentary hypothesis for the origin of the deposits however the occurrence of mineralization along fractures within specific wall rocks requires consideration of alternative exploration parameters these include 1 ancient ew and nw trending regional fractures and their intersections which are potential structural sites for base metal mineralization and 2 preferential wall rock sites either rocks with high initial porosity or secondary fracture induced porosity or soluble rocks which are susceptible to replacement processes post collisional plutonism of the dom feliciano orogeny was the most likely heat source and also the source of sulphur and base metals gravity surveys and alteration studies would be useful to determine the presence of intrusive bodies at depth which may have the potential to host porphyry style cu au deposits

evolution of the major gercino shear zone in the dom feliciano belt south brazil and implications for the assembly of southwestern gondwana the dom feliciano belt developed during the brasiliano pan african orogenic cycle due to the tectonic interaction between the rio de la plata congo and kalahari cratons along with the amalgamation of smaller continental fragments together with its prolongations to the south the major gercino shear zone constitutes one of the main lineaments of the orogenic system establishing a more than 1000 km long ne trending tectonic boundary between a granitic batholith and a metavolcano sedimentary association based on combined field structural microstructural and textural data together with new and published geochronological data a refined model for the geological evolution of the major gercino shear zone is presented regional nw verging low dipping structures were generated between 650 and 615 ma preceding the formation of the shear zone this was followed by the main deformation phase corresponding to pure shear dominated dextral strike slip interpreted do be controlled by regional transpression during oblique convergence of the continental terranes this stage lasted until ca 585 ma and was coeval with the continuous emplacement of granitic magmatism along the structure strain partitioning and localization led to the development of mylonitic belts along the intrusion borders mostly under greenschist facies metamorphic conditions late stage ductile deformation along the structure was active during the cooling of the intrusions until ca 550 ma while the deformation front of the orogenic system migrated to counterpart orogenic belts in the african continent after tectonic stabilization the major gercino shear zone recorded episodic brittle reactivation possibly related to the phanerozoic evolution of the active margins and intracratonic basins in gondwana 2018 springer verlag gmbh germany part of springer nature

southernmost exposures of the arroyo del soldado group vendian to cambrian uruguay palaeogeographic implications for the amalgamation of w gondwana abstract the occurrence of the vendian to lowermost cambrian arroyo del soldado group asg is reported from an area located to the sw of minas which was formerly mapped as part of the lavalleja group the yerbal polanco and cerro espuelitas formations of the asg occur there as a rather continuous microfossil rich sedimentary cover intruded by granites of probable cambrian age minas granite organic walled microfossils recovered from the yerbal and polanco formations include bavlinella faveolata leiosphaeridia tenuissima soldadophycus bossii soldadophycus major siphonophycus solidum glenobotrydion aenigmatis and filament mats the acritarch dyctiotidium sp is described for the first time from the yerbal formation essentially the same microfossil assemblage occurs in five samples of this unit collected immediately to the north of minas no identifiable microfossils were found in fifteen samples of organic rich lithologies of the lavalleja group based on detailed geological mapping litho and biostratigraphic data we show that the contact between the arroyo del soldado and lavalleja groups is of tectonic nature arroyo la plata lineament the latter being pre vendian in age the villa serrana block is defined and consists of the carape and lavalleja groups las ventanas and playa hermosa formations an unnamed siliciclastic ultrabasic succession showing low grade metamorphism and various granitic intrusions mostly of neoproterozoic to cambrian age its boundaries are the sarandi del yi piriapolis shear zone to the w the sierra ballena shear zone to the e the arroyo la plata lineament to the n and the rio de la plata to the s the villa serrana block could be an allochthonous terrane or alternatively a part of the nico perez terrane that was thrusted onto the asg no continuity exists between the brusque and porongos groups of southern brazil and the lavalleja group according to available data these units are probably not coeval and do not represent the suture between the cratonic areas to the w rio de la plata craton and the remnants of a neoproterozoic magmatic arc to the e cuchilla dionisio pelotas terrane accretion of these blocks was controlled by sinistral continental scale megashears the geology of the area is far too complex to be explained in terms of a single orogeny following one wilson cycle

polyphase rift evolution of the voring margin mid norway constraints from forward tectonostratigraphic modeling the mid norwegian voring margin bounding the northern atlantic underwent several rift episodes after the caledonian orogeny permo triassic extension concentrated at the eastern inner margin followed by jurassic cretaceous extension at the center of the margin finally more westward the last rift episode led to continental breakup in the earliest eocene although the general architecture of the margin may be clear the timing and distribution of particularly the oldest rift structures are poorly constrained we use two dimensional forward tectonostratigraphic modeling for better quantitative understanding of the tectonic evolution of the margin two regional profiles crossing the margin have been analyzed modeling of the two profiles integrates the basin shape and fill as well as the underlying crustal structure of the margin the modeling allows us 1 to quantify the distribution and timing of extension of subsequent rift episodes 2 to predict the basin and crustal architecture at deep levels and 3 to test different interpretations of the seismic data the level of lithospheric necking during basin formation is constrained by basement topography and moho depth and lies within the middle crust 20 km furthermore we investigate the migration of the rift axes and the elastic lithospheric strength during the different rift episodes locations of previous extension are not reactivated during renewed rifting instead renewed rifting occurs preferentially in regions where the lithosphere has experienced less extension

tracking the sources and the evolution of the late neoproterozoic granitic intrusions in the brusque group dom feliciano belt south brazil la icp ms and shrimp geochronology coupled to hf isotopic analysis abstract the central domain of the neoproterozoic dom feliciano belt in the brazilian state of santa catarina is mainly composed of a metavolcano sedimentary sequence the brusque group and its crystalline basement such as the camboriu complex this association was intruded in its late deformational stage by voluminous granitic magmatism related to the metamorphic climax of the area the intrusions share a distinct crustal affinity and are divided into three suites from oldest to youngest as indicated by field criteria sao joao batista valsungana and nova trento we present seven new la icp ms and two shrimp u pb zircon ages coupled with in situ analysis of hf isotopes and use them to discuss the magmatic sources and evolution of the intrusions and propose an evolutionary model for this plutonic association intrusion mostly took place at 615 585 ma but the field stratigraphy is not necessarily reflected by u pb ages which establish considerable overlaps between intrusions from different suites inherited zircon crystals are common and reflect a long geological history that matches the geochronological record of the brusque group and the camboriu complex hf ratios are remarkably widespread but strengthen the characteristic crustal signature of the magmatism and partially overlap with whole rock isotopic data for the same rocks we propose that the intrusions were caused by a regional heating event within the central domain of the dom feliciano belt in santa catarina with limited direct mantellic contribution and more significant indirect thermal contribution through the voluminous magmatism of the recently juxtaposed florianopolis batholith this triggered large scale partial melting of the local continental crustal and formed the studied granites the long lasting magmatic event promoted the protracted generation and emplacement of numerous distinct intrusions synchronically leading to contrasts between the field and u pb stratigraphy the suites varied geochemical and isotopic characteristics probably reflect a combination of the heterogeneities of their source rocks with the impact of different magmatic processes during granitogenesis and emplacement while the valsungana and nova trento suites were probably generated within a deeper crustal level and underwent some degree of magmatic interaction the peraluminous sao joao batista suite has an upper crust signature that indicates the assimilation of metasedimentary rocks

geochemistry and crustal evolution of volcano sedimentary successions and orthogneisses in the sao gabriel block southernmost brazil relics of neoproterozoic magmatic arcs abstract precambrian metaplutonic rocks of the sao gabriel block in southernmost brazil comprise juvenile neoproterozoic calc alkaline gneisses cambai complex the connection with associated ultra mafic metavolcanic and metasedimentary rocks palma group is not well established the whole complex was deformed during the brasiliano orogenic cycle both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the cambai complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the sao gabriel block and its role during the assembly of west gondwana the major element compositions of the igneous rocks palma group and cambai complex indicate a subalkaline character most orthogneisses have a calc alkaline chemistry many metavolcanic rocks of the palma group show signatures of low k tholeiitic volcanic arc basalts trace element data especially ti zr y nb of most igneous samples from both the lower palma group and the cambai complex indicate origin at plate margins i e in a subduction zone environment this is corroborated by relative enrichment in lree low contents of nb and other high field strength elements and enrichment in lile like rb ba and th the data indicate the possible existence of two suites an oceanic island arc and a continental arc or active continental margin however some ultramafic samples of the lower palma group in the western sao gabriel block indicate the existence of another volcanic suite with intra plate character which possibly represents relics of oceanic island basalts oib trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks the patterns of chondrite and n morb normalized spider diagrams resemble the patterns of the igneous rocks i e lile and lree enrichment and hfs depletion the geochemical signatures of most igneous and metasedimentary samples and their low 87sr 86sr t ratios suggest only minor contribution of old continental crust a geotectonic model for the sao gabriel block comprises east ward subduction and following accretion of an intra oceanic island arc to the eastern border of the rio de la plata craton at ca 880 ma and westward subduction beneath the newly formed active continental margin between ca 750 and 700 ma the sao gabriel block represents relics of an early brasiliano oceanic basin between the rio de la plata and kalahari cratons this ocean to the east of the rio de la plata craton might be traced to the north and could possibly be linked with neoproterozoic juvenile oceanic crust in the western brasilia belt goias magmatic arc

chemostratigraphy of the lower arroyo del soldado group vendian uruguay and palaeoclimatic implications abstract new c and o isotopic determinations from the vendian lower arroyo del soldado group are reported and combined with sedimentologic and biostratigraphic data on the basis of different geochemical and petrographic criteria the primary nature of the c and sr isotopic signature is shown positive d 13 c values characterize the mainly siliciclastic upper yerbal formation which contains oxide facies bif and a diverse assemblage of skeletal fossils including cloudina riemkeae a series of positive and negative d 13 c excursions occur up section in the overlying polanco formation mainly composed of limestones and limestone dolostone rhythmites the transition to the overlying barriga negra and cerro espuelitas formation which consists of conglomerates and shales cherts bif respectively is marked by a further negative excursion on the basis of sedimentary structures a correlation of d 13 c with palaeobathymetry is established positive d 13 c peaks are associated with high sea level stand high organic carbon burial and relatively higher microfossil diversity while negative d 13 c excursions occur in carbonates with less organic matter less microfossil diversity and are always associated to regressions these observations can be readily explained by palaeoclimatic models which postulate that enhanced bioproductivity due to higher availability of nutrients p n fe was the key factor controlling neoproterozoic glaciations the mentioned models are discussed in view of the new data the occurrence of at least four cold periods in the upper vendian is envisaged these cold periods led to sea level fall and possibly glaciation at higher latitudes the absence of glacigenic rocks associated to negative d 13 c excursions in the arroyo del soldado group is probably due to the tropical setting of the basin finally the upper vendian age of the lower arroyo del soldado group is confirmed on the basis of c and sr isotopes the onset of carbonate deposition at the base of the polanco formation is estimated at 580 ma by comparison with existing global isotopic curves

subsidence of the voring basin and the influence of the atlantic continental margin the post cretaceous subsidence history of the voring basin part of the atlantic passive margin offshore mid norway has been investigated extension and factors related to rifting and continental break up during the palaeocene have been quantified using both forward and reverse basin modelling techniques in the preferred geological model it is assumed that rifting occurred in the voring basin during the palaeocene prior to break up following an earlier rift event during the late jurassic during palaeocene rifting the basin may have been dynamically uplifted by the iceland mantle plume in the east of the basin there was no palaeocene extension subsidence analysis shows that in the centre of the basin forward and reverse models converge to predict a modest palaeocene stretching factor of c 1 15 in the west of the basin closest to the atlantic margin forward models of upper crustal faulting also predict a of c 1 15 but reverse backstripped models of subsidence predict a of up to 1 75 we suggest that lower crustal and mantle lithosphere thinning close to the margin were greater than the extension accommodated by upper crustal faulting and that some lower crustal mantle lithosphere stretching associated with continental separation was partitioned below the voring basin up to 150 km landwards of the margin

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

0 3 byr of drainage stability along the palaeozoic palaeo pacific gondwana margin a detrital zircon study the palaeo pacific margin of gondwana in the present day south central andes is marked by tectonic activity related to subduction and terrane accretion we present detrital zircon u pb data encompassing the palaeozoic era in northern chile and northwestern argentina cathodoluminescence images reveal dominantly magmatic zircon barely affected by abrasion and displaying only one growth phase the main age clusters for these zircon grains are ediacaran to palaeozoic with an additional peak at 1 3 0 9 ga and they can be correlated with grenvillian age and the brasiliano pampean and famatinian orogenies the zircon data reveal main transport from the nearby ordovician famatinian arc and related rocks the silurian sandstone units are more comparable with cambrian units with brasiliano and transamazonian ages 2 2 1 9 ga being more common because the silurian deposits were situated within or east of the extinct famatinian arc hence the arc acted as a transport barrier throughout palaeozoic time the complete suite of zircon ages does not record the accretions of exotic terranes or the palaeozoic glacial periods we conclude that the transport system along the palaeo pacific margin of gondwana remained stable for c 0 3 byr and that provenance data do not necessarily reflect the interior of a continent hence inherited geomorphological features must be taken into account when detrital mineral ages are interpreted supplementary material u pb data cl images and detailed geological maps are available at www geolsoc org uk sup18796

comment on flank uplift and topography at the central baikal rift se siberia a test of kinematic models for continental extension by peter van der beek

ne atlantic continental rifting and volcanic margin formation the ne atlantic rift system which has been under extensional deformation since the caledonian orogeny experienced a series of rift episodes in late paleozoic to early cenozoic times fig 1 late paleozoic rifting is poorly constrained particularly with respect to timing however rifted basin geometries inferred to be of this age are observed at depth in seismic data from the flanks of the younger rift structures

geochronological constraints on the evolution of the southern dom feliciano belt uruguay abstract new u pb sensitive high resolution ion microprobe k ar and ar ar data from the southernmost dom feliciano belt allow the identification of four major events orthogneisses from the punta del este terrane indicate a magmatic episode at c 770 ma and high grade metamorphism at 641 17 ma granitoid emplacement at 627 23 ma was roughly coeval with peak metamorphism volcaniclastic rocks of the las ventanas formation dated at 573 11 ma can be correlated with the peripheral foreland basin 571 8 ma transpression and coeval high k calc alkaline magmatism is recorded in the maldonado granite dated at 564 7 ma the following events are postulated 1 magmatism at 850 750 ma related to rifting 2 metamorphism and granite emplacement at 650 600 ma 3 molasse sequences and foreland basins at c 573 ma 4 late magmatism at 580 560 ma associated with transpression the data indicate that 1 the punta del este terrane could be a portion of the coastal terrane of the kaoko belt 2 granitoid emplacement at 650 600 ma in the punta del este and nico perez terranes favours westward subduction and 3 widespread post collisional synkinematic magmatism occurred in the dom feliciano and kaoko belts between 580 and 550 ma supplementary material analytical methods and data are available at http www geolsoc org uk sup18369

the sierra ballena shear zone in the southernmost dom feliciano belt uruguay evolution kinematics and deformation conditions the sierra ballena shear zone sbsz is part of a high strain transcurrent system that divides the neoproterozoic dom feliciano belt of south america into two different domains the basement on both sides of the sbsz shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization grain boundary migration and fluid assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation age estimate of this episode is 658 ma the second stage corresponds to the start of transpressional deformation and the nucleation and development of the sbsz during this stage pure shear dominates the deformation and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites this event dates to 658 600 ma based on the age of these intrusions the third stage was a second transpressional event at about 586 to 560 ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening deformation in the sbsz took place during the late stages under regional low grade conditions as indicated by the metamorphic paragenesis in the supracrustals of the country rocks granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar a transition from magmatic to solid state microstructures is also frequently observed in syntectonic granites mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites which resulted in shear zone weakening quartz microstructures characteristic of the transition between regime 2 and regime 3 grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions 550 400 c on the other hand the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high t solid state deformation during cooling of the dikes

lower crustal composition and crustal lineaments on the voring margin ne atlantic a review abstract a prominent lower crustal high velocity layer observed in the voring basin ne atlantic has been investigated with regard to composition and spatial link to crustal scale lineaments in the southernmost part of the basin the lower crustal layer is interpreted as eclogites based on anomalously high p wave velocities 8 4 km s in the remainder of the basin the layer is preferably interpreted as mafic intrusions emplaced during the last phase of rifting but it cannot be excluded that the body consists of older caledonian mafic rocks direct link with high grade caledonian rocks to the east and westwards to east greenland cannot be made since these rocks are mainly of felsic composition the crustal scale lineaments inferred from wide angle seismic data appear to represent the oceanward extension of post caledonian detachments the spatial variation in the thickness of the layer is within the late rifting magmatic hypothesis partly interpreted in terms of pre magmatic variations in thickness of the lithosphere in addition lateral variations in extension mantle upwelling and decompressional melting during the last phase of rifting may also have contributed

dom feliciano belt orogenic cycle tracked by its pre collisional magmatism the tonian ca 800 ma porto belo complex and its correlations in southern brazil and uruguay abstract the porto belo complex orthogneisses are dominantly intermediate high k calc alkaline rocks of tonian age ca 800 ma that represent the hinterland basement of northern dom feliciano belt their significance and correlation to other pre collisional tonian orthometamorphic rocks of the same belt are discussed and integrated in an occam s razor model for the evolution of the kaoko dom feliciano gariep orogenic system the model proposes a single eastward subduction with temporal variations of subduction angle leading to migration of arc magmatism before the main collision in the following stages a 925 880 ma early subduction and forearc magmatism b 820 770 ma flat slab subduction and arc magmatism migration towards the back arc region with generation of arc related tonian magmatic protoliths c 765 700 ma steepening subduction and arc magmatism migration towards forearc as the back arc area is under extension and rifting is triggered d 700 660 ma accretionary phase after final consumption of the oceanic plate e 660 640 ma oblique collision leading to basin inversion and double verging transpression with hinterland high t low p metamorphism and exhumation f 640 580 ma post collisional magmatism and shear belt development

evolution of neoproterozoic ophiolites from the southern brasiliano orogen revealed by zircon u pb hf isotopes and geochemistry abstract u pb and lu hf isotopic analyses of zircon from albitites southern brasiliano orogen indicate neoproterozoic ages for two ophiolites the difficulty of dating ophiolites is overcome by finding zircon in albitites contained in serpentinite two key areas were selected for the study the cerro mantiqueiras and the ibare ophiolites both in the juvenile sao gabriel terrane rio grande do sul brazil two distinct u pb shrimp ages are registered in the cerro mantiqueiras ophiolite zircons respectively 923 3 ma in the cores and 786 13 ma in the rims in the ibare ophiolite the dating of zircon grains resulted in a single age of 892 3 ma distinctive depleted mantle hf isotope signatures characterize the zircons in the cerro mantiqueiras sample ehf 8 to 13 and hf tdm 0 91 1 23 ga whereas the ibare sample displays ehf 13 to 15 and hf tdm 0 75 0 93 ga ophiolite accretion in the sao gabriel terrane of the southern brasiliano orogen occurred in the tonian which establishes the age of a significant event in the tectonic evolution of the continent the hf data indicate that the ophiolites were derived from juvenile depleted mantle this first direct dating of ophiolites sets new limits to the evolution of the brasiliano orogen

shear zone evolution and timing of deformation in the neoproterozoic transpressional dom feliciano belt uruguay abstract new structural microstructural and geochronological u pb la icp ms ar ar k ar rb sr data were obtained for the dom feliciano belt in uruguay the main phase of crustal shortening metamorphism and associated exhumation is recorded between 630 and 600 ma this stage is related to the collision of the rio de la plata and congo cratons at ca 630 ma which also involved crustal reworking of minor crustal blocks such as the nico perez terrane and voluminous post collisional magmatism subsequent orogen parallel sinistral shearing gave rise to further deformation up to ca 584 ma and resulted from the onset of the convergence of the kalahari craton and the rio de la plata congo cratons sinistral shear zones underwent progressive strain localization and retrograde conditions of deformation during crustal exhumation dextral ene striking shear zones were subsequently active at ca 550 ma coeval with further sinistral shearing along n to nne striking shear zones the tectonothermal evolution of the dom feliciano belt thus recorded the collision of the rio de la plata and congo cratons which comprised one of the first amalgamated nuclei of gondwana and the subsequent incorporation of the kalahari craton into western gondwana

provenance of the arroyo del soldado group ediacaran to cambrian uruguay implications for the paleogeographic evolution of southwestern gondwana abstract the ediacaran to lower cambrian arroyo del soldado group represents a thick marine sequence cropping out in the eastern border of the rio de la plata craton in central uruguay this work addresses the provenance of different siliciclastic units in the arroyo del soldado group in terms of petrography geochemistry nd isotope and detrital zircon geochronologic data the lower part of the arroyo del soldado group the yerbal formation and the overlying carbonates of the polanco formation are related to an open shelf which formed after rifting along eastern border of the rio de la plata craton u pb zircon dating of reworked detrital material of the yerbal formation shows a provenance with mainly palaeoproterozoic ages 2 0 2 2 ga the yerbal formation shows t dm model ages of 2 1 ga and nd t of 15 the middle part of the arroyo del soldado group is characterized by deposition of coarse sediments barriga negra formation that could be related to a sea level fall during non global glaciation the zircon ages reflect a major contribution from the palaeoproterozoic and secondly from neoproterozoic basement rocks and constrain the maximum age of deposition to 566 ma t dm model ages reflect archaean 2 7 ga and palaeo mesoproterozoic 1 6 1 9 ga mantle extraction ages for the sources the archaean component shows geochemical similarities with a mafic source characterized by high concentration of cr 350 ppm and low th sc 0 7 up section during a sea level rise the deposition of bif shale and chert of the cerro espuelitas formation occurred t dm ages of 2 1 ga suggest a provenance mainly derived from the rio de la plata craton u pb zircon dating of the cerros san francisco formation representing the upper part of the arroyo del soldado group shows a provenance mainly from palaeoproterozoic 2 0 2 1 ga and archaean 2 5 2 9 3 5 ga rocks the scarcity of neoproterozoic zircons along with t dm ages of 2 2 2 9 ga and the extreme recycling that affected the cerros san francisco formation as indicated by petrography and geochemistry do not fit with the hypothesis that the arroyo del soldado group was deposited close to a syntectonic magmatic arc a tectonic scenario in which the arroyo del soldado group was deposited on a passive margin before strike slip collision with the cuchilla dionisio pelotas terrane at ca 530 ma is supported

oceanic crust and mantle evidence for the evolution of tonian cryogenian ophiolites southern brasiliano orogen abstract unravelling the complexity of tonian cryogenian 950 680 ma evolution of ophiolites requires the search for rare mineral systems and their quantification with varied techniques ophiolites in the brasiliano orogen are widely distributed over 2 000 km along the eastern half of south america we selected two ophiolites from different geotectonic settings of the sul riograndense shield southern brasiliano orogen to delimit the evolution of the oceanic phase of the orogen the southern portion of the bossoroca ophiolite is inserted in the sao gabriel juvenile terrane and contains rare metasomatic tourmaline in chloritite close to serpentinite and metamorphosed cr spinel the southern bossoroca ophiolite was intruded by cerro da cria and ramada granites and the u pb hf isotopic study of zircon from these rocks constrains the crustal evolution of the sao gabriel juvenile terrane capane ophiolite has similar age 793 715 ma as the bossoroca ophiolite and was inserted in the porongos fold thrust belt with preserved cr spinel of mantellic composition integrated use of cr spinel mineral chemistry b isotopes in tourmaline in the bossoroca ophiolite and zircon u pb hf isotopes of granites associated with the southern bossoroca ophiolite revealed several steps in the evolution of the ophiolites in the dom feliciano belt capane cr spinel cores have mantle derived compositions mg 0 66 0 69 cr 0 51 0 53 tourmaline from the bossoroca ophiolite is dravite and has 11b 0 to 3 and granites crystallization ages are 578 3 2 and 612 12 ma ehfzrn 10 to 25 zircon from other dravite occurrences of the bossoroca ophiolite were previously dated at 920 ma we unraveled the main steps in the evolution of ophiolites from the southern brasiliano orogen with emphasis on the bossoroca and capane ophiolites during their trajectory from mid ocean ridge 920 ma formation of dravite in oceanic crust preservation of mantellic cores in cr spinel and intrusion of craton generated granites at 612 578 ma

evidence of post gondwana breakup in southern brazilian shield insights from apatite and zircon fission track thermochronology abstract apatite and zircon fission track thermochronology studies are applied to basement and sedimentary rocks from the sul rio grandense shield to unravel the tectonic history of the onshore southernmost brazilian margin the sul rio grandense shield is a major geotectonic feature of southernmost brazil that includes paleoproterozoic basement areas and neoproterozoic fold belts linked to the brasiliano pan african orogeny crustal reworking and juvenile accretion events related to this cycle were dated in the region between 900 and 500 ma and were responsible for the assembly of southwestern gondwana in southeastern south america apatite fission track aft ages range from 340 33 to 77 6 ma and zircon fission track zft ages range from ca 386 to 210 ma based on thermal history modeling the most part of the samples record an early cooling event during the carboniferous which reflect the main tectonic activity of the final stages of the gondwanides at the pacific margin of west gondwana subsequently the permo triassic cooling event is related to the last stages of the gondwanides with convergence along the southern border of western gondwana and consequent reactivation of n s and ne sw trending basement structures the onset of initial breakup of southwestern gondwana with opening of the south atlantic ocean is mostly recorded in the eastern terrain and zft ages show that the temperature during this period was high enough for total or at least partial resetting of fission tracks in zircon the last cooling event of the sul rio grandense shield records the final breakup between south america and africa which began during the late cretaceous however the cenozoic rapid cooling episode probably is a result of plate adjustment after breakup and neotectonic reactivation of faults associated with south atlantic rift evolution

regional structure and polyphased cretaceous paleocene rift and basin development of the mid norwegian volcanic passive margin abstract the more and voring basins of the mid norwegian volcanic passive margin are characterized by thick accumulations of cretaceous to paleocene sedimentary strata they were formed during a series of late mesozoic early cenozoic extensional events and represent vast underexplored areas with a limited number of wells recently a new generation of long offset 2d seismic reflection lines and 3d seismic data together with new well data has permitted a significant improvement in the regional understanding of the more and voring basins this has enabled much better imaging of the deep cretaceous subbasins and sub basalt structures in light of this significant data improvement we performed a regional tectonostratigraphic synthesis of the pre breakup development of the more and voring basins we have interpreted eight regional cretaceous and paleocene horizons and constructed a series of structural and thickness maps the new interpretations allow us to examine the sequential evolution of the cretaceous to paleocene sedimentary infill and to discuss its relationship to the deep crustal structures and regional tectonic events we conclude that the long and polyphased development of the more and voring basins is partly controlled by deep seated structural highs we show that active deposition in the early cretaceous was mainly focused in the more basin while the main cenomanian and subsequent late cretaceous paleocene depocentres developed principally in the voring basin and migrated sequentially west towards the present continent ocean boundary we argue that the outer more and voring basins are likely underlain by a relatively thick continental crust compared to the inner part of the regional sag basin in this setting our observations do not support evidence for a large zone of exhumed upper mantle which has previously been proposed to have formed before magmatism and breakup

adamastor an ocean that never existed abstract existing models of tectonic evolution of the neoproterozoic orogenic system rimming the shores of the south atlantic ocean the aracuai ribeira congo and dom feliciano kaoko gariep belts interpret the belts as subduction related orogens and emphasize the role of the adamastor ocean in their pre collisional evolution a critical problem in such an interpretation is the confined nature of the northern termination of the orogenic system as well as a very short time span between the end of rifting and onset of convergence recognized in its southern part in this contribution we review the data for the pre and synorogenic evolution of this system of orogens here collectively called the south atlantic neoproterozoic orogenic system and show that the data speak against the presence of a large oceanic domain before the onset of its orogenic evolution we propose a new and simple intracontinental model suggesting that neoproterozoic oceanic crust played only a minor role in the development of the south atlantic neoproterozoic orogenic system and that its overall architecture and thermal evolution is the result of inversion of large scale rift structures with a protracted and probably episodic extensional history true oceanic crust probably developed only in the southern part of the rift system but it must have been narrow akin to the red sea gulf of aden stage of the adamastor rift evolution just before the onset of convergent thickening

deep structures and breakup along volcanic rifted margins insights from integrated studies along the outer voring basin norway three dimensional multichannel seismic survey and well data allows a deeper insight into rifting mechanisms and crustal structure of the norwegian volcanic rifted margin a surprising high velocity lower crustal dome marked by a strong amplitude reflection t reflection is described below the north gjallar ridge ngr near the volcanic plateau formed during the late paleocene early eocene breakup faulting along the ngr occurs during early campanian paleocene and is controlled by the crustal dome the t reflection was previously related to either the top of mafic underplated crust or soft lower crust updomed during the continental breakup the influence and timing of the crustal dome on the basin structure shows clearly that the dome predates the continental breakup based on the structural relationship and the results of the modelling it is therefore concluded that the t reflection does not necessarily originate from an anomalous tertiary magmatic event related to the breakup underplated material but could be also partly attributed to pre breakup mafic material or and inherited high pressure granulite eclogite rocks in the continental domain q 2004 elsevier ltd all rights reserved

symmetric conjugation of continent ocean boundary structures along the norwegian and east greenland margins abstract a characteristic structural feature of the norwegian continental margin that has been described from multichannel seismic data as well as dsdp and odp drilling is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin models for their origin describe them as either products of a late stage intra continental rift phenomenon with the volcanic rocks overlying attenuated continental crust or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the norwegian greenland sea from studies of the norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the east greenland margin was undertaken which used two ship multichannel seismic acquisition as its focus we found that the wedges were not only present on the east greenland margin but that details of their along strike variability are mirrored on both margins together with their spatial relationship to the oldest magnetic lineation pattern the wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation 24 moving south we see that the wedges broaden encroach into the pattern of lineation 24 then develop into two separate wedges the seaward of which is associated with lineation 23 lineations 24 and 23 on both margins are distorted when associated with wedges we believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary and we believe that the most likely mechanism to generate the excess volcanism is the convective partial melting scheme recently proposed by mutter and buck 1986 and mutter et al in press

the araa uaa west congo orogen in brazil an overview of a confined orogen formed during gondwanaland assembly abstract the neoproterozoic adamastor brazilide ocean was generated during the breakup of the rodinia supercontinent and remnants of its oceanic lithosphere have been found in the brasiliano pan african orogenic system that includes the aracuai west congo brasilia ribeira kaoko dom feliciano damara and gariep belts the aracuai and the west congo belts are counterparts of the same neoproterozoic orogen the first belt comprises two thirds of the aracuai west congo orogen this orogen is rather unique owing to its confined nature within the embayment outlined by the sao francisco and congo cratons in spite of this the presence of ophiolitic remnants and a calc alkaline magmatic arc indicate that the basin orogen evolution comprise both oceanic spreading and consumption it is assumed that coeval paramirim and sangha aulacogens played a key role by making room for the aracuai west congo basin sedimentary successions record all major stages of a basin that evolved from continental rift when glaciation related sedimentation was very significant to passive margin rifting started around 1 0 0 9 ga the oceanic stage is constrained by an ophiolitic remnant dated at 0 8 ga if the cratonic bridge that once linked the sao francisco and congo palaeocontinental regions did not hinder the opening of an ocean basin it certainly limited its width as a consequence only a narrow oceanic lithosphere was generated and it was subducted afterwards this is also suggested by orogenic calc alkaline granitoids occuping a small area of the orogen geochronological data for pre syn and late collisional granitoids indicate that the orogenic stage lasted from 625 ma to 570 ma a period of magmatic quiescence was followed by intrusion of postcollisional plutons at 535 500 ma the features of the aracuai west congo orogen suggest the development of a complete wilson cycle in a branch of the adamastor ocean which can be interpreted as a gulf with limited generation of oceanic lithosphere

mesozoic detachment faulting between greenland and norway inferences from jan mayen fracture zone system and associated alkalic volcanic rocks the spatial relation between the jan mayen fracture zone system and two occurrences of early tertiary mantle derived volcanic rock complexes indicates that an older continental fault system may have had hidden extensions in the deep lithosphere this is taken as indirect evidence that the inferred fracture zone precursor may have acted as a transfer fault system linking rifts formed by opposing simple shear detachment faulting through the lithosphere we infer that this contributed to continental rifting between greenland and norway in mesozoic to paleocene time before sea floor spreading began in the norwegian greenland sea region

re evaluation of extension across the pearl river mouth basin south china sea implications for continental lithosphere deformation mechanisms abstract the tertiary pearl river mouth basin in the south china sea is one of relatively few localities where independent estimates of extension from upper crustal normal faulting crustal thinning and subsidence have been compared previous work indicates that these methods all support extension factor 1 8 for this basin however the estimate from faulting used the controversial rigid domino method whereas summing normal fault heaves gives only 1 3 instead this study pursues the possibility that the true value for this basin is 1 3 and the published estimates of extension from subsidence and crustal thinning are exaggerated because they do not take account of outflow of lower crustal material from beneath this basin such outward channel flow will occur when the lithostatic pressure at the base of the brittle upper crust is greater beneath a basin than beneath its surroundings in response to such a pressure gradient material lost from beneath this basin has probably flowed to beneath the adjacent chinese landmass where it isostatically compensates the erosion that has supplied much of the basin s sediment

magma faults and gravitational loading at mount etna the 2002 2003 eruptive period 1 mount etna is characterized by a complex structural setting that influences its evolution in order to understand how the interaction between dike forming intrusions and faulting influences the kinematics of the volcanic edifice we developed a numerical model it takes account of the topography the medium heterogeneities the gravitational loading and the most active crustal discontinuities a parameterization of the apparent coefficient of friction as a function of the depth has been considered in the range from 0 01 to 0 5 the density values used young s modulus and poisson s ratio range from about 1700 to 3200 kg m3 8 5 to 140 gpa and 0 15 to 0 35 respectively the model was applied to the 2002 2003 etna eruption the resulting deformation pattern was in agreement with the data provided by the continuous gps stations and the seismological knowledge obtaining displacements up to 1 m some discrepancies between the recorded ground deformation field and the modeled displacements allowed us to make some hypotheses on the volcano tectonic e g for the pernicana fault or regional tectonic origin acireale s alfio fault system of the crustal discontinuities moreover our results suggested that the pernicana fault is characterized by a very low apparent coefficient of friction less than 0 1 furthermore we evaluated the contribution of the gravitational loading and found that it implies a variation of about 10 of the overall deformation pattern our results clearly showed that the weight of the volcanic edifice acts in opposition to the magmatic intrusions conversely we found that the presence of medium heterogeneities may favor the eastern flank movements toward the se and the spreading of the summit area playing a fundamental role in ascending magma finally we investigated the shallow sliding model and found that dike forming intrusions produce negligible displacements less than 1 mm along the subhorizontal detachment surface therefore the gravitational loading during dike forming intrusions is not able to trigger sliding processes along this plane

the cuchilla dionisio terrane uruguay an allochthonous block accreted in the cambrian to sw gondwana abstract the cuchilla dionisio terrane cdt of uruguay is a tectonostratigraphic unit defined as the block to the east of the sierra ballena shear zone sbsz it is composed of a palaeo to mesoproterozoic metamorphic basement cerro olivo and chafalote complexes intensely reworked and intruded by syncollisional to late orogenic granites between 680 and 550 ma dacitic and rhyolitic volcanism is recorded around 570 575 ma cerros de aguirre and sierra de rios formations the cdt is correlated with the pelotas terrane of southern brazil the western boundary of the block in brazil is defined by the cangucu and major gercino shear zones which are the northward extension of the sbsz the present position of the cdt is not a consequence of orthogonal collision after completion of a wilson cycle but of lateral sinistral accretion along the above mentioned megashear zones the allochthonous nature of the cdt is postulated on the grounds that a magmatism in the cdt is ca 120 ma younger than metamorphism of neighbouring volcanosedimentary successions e g porongos group in brazil and b while intense volcanism occurred in the cdt in the late vendian represented by the cerros aguirre and sierra de rios formations a passive continental margin deepening to the e existed on the eastern edge of the rio de la plata craton represented by the arroyo del soldado group therefore the cdt and rio del la plata craton were separated hundreds or thousands of kilometers by late vendian times accretion of the cdt took place at 530 ma by tangential collision and was one of the last events in the complicated amalgamation of w gondwana in uruguay the cdt collided with the nico perez terrane generating the sbsz and sinistrally reactivating the sarandi del yi piriapolis shear zone in brazil the cdt pelotas terrane collided with the parana block and a series of parautochthonous terranes curitiba apiai characterized by a transamazonian basement strongly reworked in the neoproterozoic the provenance of the cdt is still uncertain considering that both transamazonian and namaqua natal kibaran ages have been reported from its basement previously reported nd model ages tdm strongly suggest an african affinity the dom feliciano belt is thus a collage of diachronous units formed in different geotectonic settings and not the product of a single orogenic cycle terrane docking at different stages mainly by lateral accretion was the dominant process therefore usage of the name dom feliciano belt should be restricted only to descriptive purposes

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

deepest exposed crust of brazil geochemistry of paleoproterozoic depleted santa maria chico granulites abstract the deepest crustal processes of brazil are investigated in the santa maria chico granulitic complex of rio grande do sul state depth of ca 30 km is inferred from garnet bearing mafic granulite geobarometry for the deformation of the bimodal basic acid sequence of late archean age this low k tholeiitic magmatism occurred in an island arc environment which accreted to the atlantica supercontinent in a collisional episode at ca 2 1 ga during the transamazonian cycle although this magmatism is strongly depleted in lithophile elements its geochemistry is still magmatic and shows that the metamorphism did not disturb the original distribution of the elements in the host rocks significantly the associated trondhjemites could have been the differentiated portions of the basic magmas but more likely were the result of the partial melting of the garnet bearing mafic residue this suite of high grade rocks is one of several protoliths for the generation of the abundant k granites of the 600 ma brasiliano cycle from southern brazil this high grade rock association is rather comparable to the lewisian complex of scotland

west gondwana amalgamation based on detrital zircon ages from neoproterozoic ribeira and dom feliciano belts of south america and comparison with coeval sequences from sw africa

reassessment of copper mineral occurrence through electrical tomography and pseudo 3d modeling in camaqua sedimentary basin southern brazil the reassessment of mineral occurrences is a common and economically rational technical procedure because it involves studies directed to targets defined in previous studies in mineral exploration the targets definition can be based on indications of mineralization on the surface or in the contrast of measurable physical properties using geophysics and intrinsic to certain types of deposits this paper discusses the results by the combination of the use of geophysical methods dc resistivity and induced polarization on detailed study of a copper mineral occurrence from the adaptation of techniques of modeling of mineral deposits in mining the indications of mineralization in the area consist of malachite quartz and barite in veins recognized in volcanic tufts structurally embedded in andesites and metaconglomerates nine parallel lines of electrical tomography in schlumberger arrangement were performed which resulted in 2 d inversion models interpolated in models of blocks maps and pseudo 3 d isosurface models in a mineral deposits modeling program the results allowed the recognition of areas with high chargeability 2 5 mv v and high resistivity 800 m relating to areas with disseminated sulphide ores and silicification according to recognized evidence around the area distribution in regional recognition this is where the pseudo 3 d products enabled an estimate morphology of the deposit and clearly defined targets for sampling by drilling and quantification of results the results allow the recommendation of this procedure in mineral exploration activities supported by the quickness and possible global reduction of project costs with real increase in the chances of incorporation of new deposits this reduces the costs for research development and minimizes the environmental impacts associated with mining as well as the application of these methods for environmental diagnosis

time constraints on magmatism along the major gercino shear zone southern brazil implications for west gondwana reconstruction the dom feliciano belt situated in southernmost brazil and uruguay contains a large mass of granite gneissic rocks also known as florianopolis pelotas batholith formed during the pre syn and post orogenic phases of the brasiliano pan african cycle in the ne extreme of this granitic mass pre syn and post tectonic granites associated with the major gercino shear zone mgsz are exposed the granitic manifestation along the mgsz can be divided into pre kinematic tonalitic gneisses peraluminous high k calc alkaline early kinematic shoshonitic and metaluminous post kinematic granites u pb zircon data suggest an age of 649 10 ma for the pre tectonic gneisses and a time span from 623 6 ma to 588 3 ma for the early to post tectonic magmatism negative ehf t values ranging from 4 6 to 14 6 and hf model ages ranging from 1 64 to 2 39 ga for magmatic zircons coupled with whole rock nd model ages ranging from 1 24 to 2 05 ga and end t values ranging from 3 84 to 7 50 point to a crustal derivation for the granitic magmatism the geochemical and isotope data support a continental magmatic arc generated from melting of dominant paleoproterozoic crust and a similar evolution for the granitic batholiths of the eastern dom feliciano belt and western kaoko belt

multiple accretion at the eastern margin of the rio de la plata craton the prolonged brasiliano orogeny in southernmost brazil the neoproterozoic eoplalaeozoic brasiliano orogeny at the eastern margin of the rio de la plata craton in southernmost brazil and uruguay comprises a complex tectonic history over 300 million years the southern brazilian shield consists of a number of tectono stratigraphic units and terranes the sao gabriel block in the west is characterized by c 760 690 ma supracrustal rocks and calc alkaline orthogneisses including relics of older c 880 ma old igneous rocks both igneous and metasedimentary rocks have positive end t values and neoproterozoic tdm model ages they formed in magmatic arc settings with only minor input of older crustal sources a trondhjemite from the sao gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation d3 yield an u pb zircon age la icp ms of 701 10 ma giving the approximate minimum age of the sao gabriel accretionary event the encantadas block further east containing the supracrustal porongos belt and the pelotas batholith is in contrast characterized by reworking of neoarchean to palaeoproterozoic crust the 789 7 ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the porongos belt provides a time marker for the basin formation zircons of a sample from tonalitic gneisses constituting the palaeoproterozoic basement of the porongos belt form a cluster at 2 234 28 ma interpreted as the tonalite crystallization age zircon rims show ages of 2 100 2 000 ma interpreted as related to a palaeoproterozoic metamorphic event the porongos basin formed on thinned continental crust in an extensional or transtensional regime between c 800 700 ma the absence of input from neoproterozoic juvenile sources into the porongos basin strongly indicates that the encantadas and sao gabriel blocks were separated terranes that became juxtaposed next to each other during the brasiliano accretional events the tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the rio de la plata craton i accretion of an intra oceanic arc at c 880 ma passinho event and ii accretion of the 760 700 ma cambai vila nova magmatic arc sao gabriel event the latter event also includes the collision of the encantadas block with the rio de la plata craton to the west collision and crustal thickening was followed by sinistral shear along sw ne trending orogen parallel crustal scale shear zones that can be traced from southern brazil to uruguay and have been active between 660 and 590 ma voluminous granitic magmatism in the pelotas batholith spatially related to shear zones is interpreted as late to post orogenic magmatism possibly assisted by lithospheric delamination it marks the transition to the post orogenic molasse stage localized deformation by reactivation of preexisting shear zones continued until c 530 ma and can be assigned to final stages of the amalgamation of west gondwana

juvenile accretion at 750 700 ma in southern brazil a revista economic geology nao autoriza a publicacao de seus artigos em repositorios institucionais

tearing up rodinia the neoproterozoic palaeogeography of south american cratonic fragments rodinia was initially defined as a long lived supercontinent that assembled all the continental fragments around laurentia and remained stable from 1000 up to 750 ma nonetheless recent work has cast doubt on the rodinia palaeogeography and even on the timing of its assembly and break up the geochronological and palaeomagnetic databases accumulated for south america and africa in the last decade show that most of these continental fragments were not part of rodinia a wide brasiliano ocean separated most of the south american and african cratons from the laurentia amazonia rio apa west africa margin this ocean was closed between 940 and 630 ma along the pampean paraguay araguaia pharusian mobile belts moreover accretion along the south american and african platforms was a diachronous and long lived process that involved several intra oceanic and continental magmatic arcs and microcontinents this evolution started at around 1000 ma and ended at around 520 ma with the final assembly of gondwana

crustal thickening in an extensional regime application to the mid norwegian voring margin abstract the structure of the mid norwegian volcanic voring margin at the onset of the maastrichtian paleocene extension phase reflects the cumulative effect of earlier consecutive rifting events lateral structural differences present on the margin at that time are a consequence of migration of the location of maximum extension in time between norway and greenland the most important imprints moho depth thermal structure of these events on the lithosphere are incorporated in a numerical simulation of the final extension phase we focus on a possible mechanism of formation of the voring marginal high and address the relationship between spatial and temporal evolution of crustal thinning and thickening uplift of the surface and strength of the lithosphere it is found that the voring basin formed the strongest part of the margin which explains why the maastrichtian paleocene rift axis was not located here but instead jumped westward with respect to the earlier rift axes locations the modeling study predicts that local crustal thickening during extension can be expected when large lateral thermal variations are present in the lithosphere at the onset of extension negative buoyancy induced by lateral temperature differences increases downwelling adjacent to the rifting zone convergence of material at the particular part of the margin is mainly taken up by the lower crust the model shows that during the final phase of extension the crust in the voring marginal high area was thickened and the surface uplifted it is likely that this dynamic process and the effects of magmatic intrusions both acted in concert to form the marginal high

sm nd isotope geochemistry of metamorphic volcano sedimentary successions in the sao gabriel block southernmost brazil evidence for the existence of juvenile neoproterozoic oceanic crust to the east of the rio de la plata craton abstract juvenile neoproterozoic dioritic tonalitic trondhjemitic and granodioritic gneisses in the sao gabriel block southern brazil have been identified by geochronologic studies age proposals for associated ultra mafic metavolcanic and metasedimentary rocks however range from archean to neoproterozoic whole rock sm nd analyses presented here support a neoproterozoic age for these rocks tdm model ages of the ultra mafic metavolcanic rocks range between 0 65 and 1 35 ga with nd t positive values between 3 16 and 6 87 tdm model ages of metasedimentary and metavolcanoclastic rocks vary between 0 77 and 1 19 ga with nd t values between 1 2 and 6 23 tonalitic calc alkaline gneisses show nd t values of 4 34 and 6 3 and tdm model ages of 0 89 and 0 72 ga respectively a late kinematic granite santa zelia granite display slightly negative nd t values 1 6 and a higher tdm model age of about 1 4 ga these data support the existence of meso neoproterozoic juvenile oceanic crust and island arc rocks during the brasiliano orogenic events the main source rocks of the metasedimentary units are previously formed juvenile rocks the data also indicate minor assimilation of older crustal material and or contamination of the melts by radiogenic nd released from older rocks on the subducting slab existence of widespread old sialic crust in the subduction zone environment however can be ruled out indicating important orogenic accretion between 0 9 and 0 7 ga a geotectonic model for the sao gabriel block and the eastern margin of the rio de la plata craton comprises eastward subduction and following accretion of an intra oceanic island arc between 0 9 and 0 8 ga and a subsequent westward subduction with formation of an active continental margin at the eastern margin of the rio de la plata craton between 0 8 and 0 7 ga we postulate that the juvenile rocks of sao gabriel block represent relics of a neoproterozoic ocean between the rio de la plata craton and a continental block encantadas block possibly derived from the kalahari craton subduction and arc accretion began roughly coeval with the initial stages of the break up of rodinia 0 9 ga and indicate a peripheric rio de la plata craton in relation to the rodinia supercontinent with evolution from a passive margin to an active margin in the beginning of the neoproterozoic brasiliano orogenic events

origin of paleovalleys on the rio grande do sul shield brazil implications for the extent of late paleozoic glaciation in west central gondwana abstract the location longevity and geographic extent of late paleozoic ice centers in west central gondwana remain ambiguous paleovalleys on the rio grande do sul shield of southernmost brazil have previously been interpreted as fjords carved by outlet glaciers that originated in africa and emptied into the parana basin brazil in this study the sedimentology stratigraphy and provenance of sediments infilling two such paleovalleys the mariana pimentel and leao paleovalleys were examined in order to test the hypothesis that an ice center over present day namibia drained across southernmost brazil during the carboniferous and permian contrary to previous findings the facies assemblage from within the paleovalleys is inconsistent with a fjord setting and no clear evidence for glaciation was observed the facies show a transition from a non glacial lacustrine estuarine environment to a fluvial dominated setting and finally to a restricted marine estuarine environment detrital zircon results present a single population of neoproterozoic ages c 800 550 ma from the paleovalley fill that matches the ages of underlying igneous and metamorphic basement dom feliciano belt and is incongruent with african sources that contain abundant older mesoproterozoic paleoproterozoic and archean zircons furthermore results suggest that the formation of the paleovalleys and the deposition of their fill were controlled by the reactivation of neoproterozoic basement structures during the carboniferous and permian the lack of evidence for glaciation in these paleovalleys highlights the need for detailed studies of supposed late paleozoic glacial deposits these results are supportive of the hypothesis that well established glacial sediments on the rio grande do sul shield southern margin of the parana basin may be the product of a separate lobe extending north across uruguay rather than a single massive ice sheet draining west from africa

u pb and lu hf isotopes applied to the evolution of the late to post orogenic transtensional basins of the dom feliciano belt brazil abstract the santa barbara and guaritas groups located in southern brazil represent the last episode of deposition in the camaqua basin which included deposition of sandstone siltstone and conglomerates during the late neoproterozoic and eopaleozoic as part of the collisional period of the brasiliano pan african orogeny combined u pb and lu hf analyses of detrital zircon grains from outcrops and drill holes in both groups together with an analysis of paleocurrent data provide information regarding the sedimentary provenance and tectonic evolution the u pb geochronological data from both groups include concordant ages that are predominantly paleoproterozoic and neoproterozoic with an increasing contribution by early neoproterozoic and paleoproterozoic zircon grains in the upper unit the guaritas group the maximum depositional age of the basal siltstone units of the santa barbara group is 567 9 5 9 ma and the maximum depositional age of the guaritas group is 540 535 ma the lu hf model ages tdm indicate that the zircon grains in both groups display a predominantly archean and paleoproterozoic inheritance with predominantly negative hf values which indicates that the source rocks originated from the subcontinental lithosphere

structural modelling of regional depth profiles in the voring basin implications for the structural and stratigraphic development of the norwegian passive margin interpretation of key regional seismic lines across the voring basin has identified a number of seismic megasequences that have provided a framework for further basin analysis these megasequences have been classified as pre syn and post rift with respect to the late cretaceous early tertiary phase of extension forward and reverse structural and stratigraphic modelling of two of these regional lines has been performed in an attempt to test and validate the structural and stratigraphic interpretations of the syn and post rift megasequences reverse thermal subsidence modelling flexural backstripping of these lines to the base tertiary was performed using a laterally varying beta factor across the sections a number of features on each of the depth profiles are considered to be regional subaerial datums for the backstripped sections the backstripped templates were then forward modelled using the flexural cantilever model the forward models calculate a laterally varying beta profile along their length which can be compared to the reverse models thereby ensuring an internally consistent structural model between both the reverse and forward modelling techniques the modelling work has highlighted the existence of a significant discrepancy between the extension measured from the forward models fault heaves and the extension needed to produce geologically acceptable backstripped profiles this observation is of particular importance as it may place a significant constraint on the mode of lithospheric extension that can be used to explain the evolution of the norwegian continental margin

neoproterozoic igneous complex emplaced along major tectonic boundary in the kaoko belt nw namibia ion probe and la icp ms dating of magmatic and metamorphic zircons granitoid intrusions of the boundary igneous complex separate segments with different ages of high grade metamorphism in the kaoko belt nw namibia two granitoids of this complex were dated at 575 10 ma secondary ionization mass spectrometry sims or 571 9 ma laser ablation inductively coupled plasma mass spectrometry la icp ms and 562 11 ma sims or 572 4 ma la icp ms respectively the age of granulite facies metamorphism in the eastern part of the western kaoko zone was established at 549 5 ma sims by analysing metamorphic overgrowths of older c 1850 1000 ma zircons from melt segregations in amphibolites the coastal part of the western kaoko zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks zircons from felsic members of two bimodal suites have sims ages of 805 4 ma and 810 840 ma respectively that are interpreted as dating their respective igneous protoliths melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 5 ma sims or 645 5 ma la icp ms and 629 6 ma sims or 630 5 ma la icp ms respectively the later age is indistinguishable from the age of 630 4 ma sims or 625 10 ma la icp ms obtained from melt patches present in overlying metagreywackes the available age data suggest that the boundary igneous complex masks the suture between the coastal terrane and the rest of the kaoko belt ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 ma

age and magnetic fabric of the tres corregos granite batholith evidence for ediacaran transtension in the ribeira belt se brazil anisotropy of magnetic susceptibility ams and u pb shrimp zircon ages in the eastern part of the tres corregos batholith ribeira belt se brazil indicate a well defined fabric pattern acquired between 600 and 595 ma the batholith consists mostly of porphyritic granites distributed in the ribeirao branco barra do chapeu and itaoca plutons late fluorite bearing alkaline granites some containing sn polymetallic greisen type deposits intruded the ribeirao branco pluton and the low grade metasedimentary host rocks the magnetic fabric of the ribeirao branco granite is dominantly oblate and oblique to the pluton elongation while that in barra do chapeu is mostly concentric on both plutons ams records the preferred orientation of coarse homogeneous ti poor magnetite grains such fabric patterns indicate a partitioned strain field dominated by strike slip left lateral shear deformation in the ribeirao branco and extension in the barra do chapeu pluton the zircon ages of these plutons are 600 6 ma and 595 4 ma likewise the ages of alkaline plutons were in the range of 597 and 595 ma registering a fast transition between the typical syntectonic batholithic magmatism to the late highly evolved and specialized magmas these results indicate that the ribeira belt was deformed by transtension in the middle ediacaran the geological setting is consistent with a continental arc with the tres corregos batholith emplacing at the middle upper crust

north atlantic sea floor spreading rates implications for the tertiary development of inversion structures of the norwegian greenland sea the tertiary development of the norwegian continental margin was dominated by the opening of the arctic north atlantic ocean the correct identification of magnetic anomalies and their ages and the analysis of spreading rates during the formation of this ocean are important in understanding the development of the region and specifically the history of its passive margins three ocean domains the aegir reykjanes and mohns regions were investigated in an effort to understand the lateral changes in structural development of the passive margin after continental break up spreading rates generally slowed down from 2 cm a 1 after early eocene initiation of sea floor spreading to values around 0 5 cm a 1 in oligocene time an increase in spreading rates to around 1 cm a 1 coincided with the positioning of the iceland hotspot under the north atlantic mid ocean ridge at the same time the european plate changed its absolute plate motion from a north directed drift to a motion more towards the east the location of inversion structures in the voring and faeroes basin rather than in the more basin is related to differences in spreading rates the mohns and the reykjanes ridges produced more ocean floor than the aegir kolbeinsey ridges asymmetric ocean floor formation in the aegir ridge led to differential stress at the base of the lithosphere which probably explains the absence of inversion features in the more basin less mantle drag furthermore upper plate margins such as the voring basin and possibly the faeroe basin have a lower compressional strength than lower plate margins such as the more basin and therefore preferentially developed inversion structures along the transform boundaries separating the domains additional stress probably built up along extension of the transform zones into the extended continental crust this additional stress probably also assisted initiation of the inversion structures in the voring basin and the faeroes area the amplification of the inversion structures in the voring basin and the faeroes basin was subsequently caused by a variety of processes related to sedimentation and uplift erosion

palaeocene uplift and eocene subsidence in the northern north sea basin from 2d forward and reverse stratigraphic modelling structural and stratigraphic basin modelling has been used to examine quantitatively the cretaceous and tertiary post rift subsidence history of the northern north sea basin following late jurassic extension 2d forward and reverse syn rift and post rift modelling has been used to determine the magnitude and timing of departures from the mckenzie post rift thermal subsidence trend for regional stratigraphic profiles in the outer moray firth the south viking graben and the north viking graben observed stratigraphy has been reverse post rift modelled using flexural backstripping combined with decompaction and reverse thermal subsidence calculations and forward modelled through the syn rift and post rift period using the flexural cantilever model of continental rift basin formation long term global eustasy has been included in the analysis only reliable palaeobathymetric markers such as erosion surfaces and coals have been used to constrain subsidence history both forward and reverse 2d modelling show that a palaeocene regional uplift followed by a rapid eocene subsidence must be superimposed on mckenzie post rift cretaceous and tertiary basement subsidence in order to generate the stratigraphy observed the magnitude of palaeocene uplift deduced from modelling was of the order of 375 m in the outer moray firth central north sea increasing to 525 m in the north viking graben northern north sea rapid early eocene subsidence was of the order of 160 310 m the most likely mechanism for regional palaeocene uplift and rapid eocene subsidence in the northern north sea was the palaeocene development of the iceland plume situated at a distance of 700 900 km from the study area in the early tertiary forward and reverse modelling predict axial bathymetries in the graben of up to 1300 m for the early cretaceous and 900 m for the upper cretaceous the large palaeobathymetries at the end of the cretaceous provided much of the accommodation space for tertiary sedimentation

structural evolution of the voring basin norway during the late cretaceous and palaeogene the voring basin forms an integral part of the passive margin off central norway three phases of extension and three phases of compression record the transition from rifting to sea floor spreading between norway and greenland the first regional extensional event is of early cenomanian age causing relative uplift of the gjallar ridge and development of a regional depocentre in the ras basin a second extensional event is represented by late campanian faulting on the nyk high and along the fles fault complex a compressional event in the late maastrichtian led to inversion of the r s basin this was followed by a regional extensional event in late palaeocene leading to the continental break up between norway and greenland in earliest eocene after the onset of sea floor spreading the voring basin has remained in overall compression with two major compressional events identified in early eocene and mid oligocene

dating recurrent shear zone activity and the transition from ductile to brittle deformation white mica geochronology applied to the neoproterozoic dom feliciano belt in south brazil abstract argon geochronology of white mica has a good potential of recording recurrent dynamic recrystallization in shear zones in the transition from ductile to brittle deformation as temperature ranges for mineral crystallization and isotopic closure of different grain size fractions overlap between ca 275 425 c this study presents over 40 k ar ages of coarse grained muscovite and multiple clay sized white mica fractions extracted from mylonites and fault gouges in southernmost brazil complemented by detailed structural and xrd characterization the widespread obtained ages are best explained in the context of recurrent deformation under retrograde conditions during progressive strain localization and transition from low grade ductile deformation into brittle faulting the onset of the ibare shear zone is constrained in the tonian followed by reactivation during the ediacaran evolution of the adjacent dom feliciano belt which records a remarkable long term history of recurrent deformation for over 100 myr new data extends the geochronological constraints of the dorsal do cangucu shear zone into low temperature conditions estimating the onset of brittle activity at 525 470 ma by dating early fault gouges formed shortly after final syn kinematic white mica crystallization in the hosting mylonites dating of recurrent faulting events during the paleo mesozoic complement the regional sedimentary and thermochronological record

granites and the geodynamic history of the neoproterozoic brasilia belt central brazil a review abstract recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the neoproterozoic brasilia belt in central brazil this is an orogenic belt developed in response to the convergence between the amazon sao francisco congo and parana continental blocks the presence of neoproterozoic juvenile arc rocks and syn collisional peraluminous granites challenged previous intracontinental evolution models for the belt the granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt the main field and isotopic characteristics of four granite suites associated with the brasilia belt are reviewed i 1 77 1 58 ga old rift related a type granite intrusions ii ca 0 8 0 7 syn collisional granitoids iii arc metatonalites and metagranodiorites ca 0 9 to 0 63 ga and iv bimodal post orogenic suite ranging in age from ca 0 59 to 0 48 ga these rocks suggest that during most of the neoproterozoic the western margin of the sao francisco continent faced a large oceanic basin where subduction and oceanic lithosphere consumption started at ca 0 9 ga roughly coeval with the initial stages of the break up of rodinia final ocean closure happened at ca 0 63 0 60 ga with crustal thickening uplift and erosion post orogenic extension related magmatism took place between ca 0 6 and 0 5 ga and was partially contemporaneous with the deposition of the paraguay and tucavaca sedimentary successions resulting from the rifting event related to the break up of laurentia from southwestern gondwana

a connection between the neoproterozoic dom feliciano brazil uruguay and gariep namibia south africa orogenic belts evidence from a reconnaissance provenance study abstract a provenance study of neoproterozoic siliciclastic successions in the stratigraphically and tectonically lowermost and uppermost parts of the pan african gariep belt stinkfontein subgroup and oranjemund group respectively in southwestern africa as well as in the rocha group of the punta del este terrane dom feliciano belt in uruguay revealed that the oranjemund and rocha groups can be correlated and most likely formed in the same basin thus the rocha group is considered to represent the fill of the westernmost part of a re activated ediacaran gariep basin the lower parts of the oranjemund and rocha groups reflect erosion of mafic rocks whereas the upper parts are derived from a predominantly felsic source area first major and trace element geochemical data are consistent with oceanic islands of within plate geochemistry in the immediate vicinity as the most likely source of the mafic input into the lower part of the oranjemund group whereas a combination of continental margin and island arc is indicated as source for the bulk of the younger sediments in the upper oranjemund group age spectra obtained by shrimp u pb analyses of detrital zircon grains from the stinkfontein subgroup port nolloth group the oranjemund group and the rocha group are very similar except for a lack of the youngest age group around 600 ma in the stinkfontein subgroup in all three units zircon grains of 1000 1200 ma dominate with a further peak in the age distribution between 1700 and 2000 ma these ages compare well with the pre gariep basement geology in southwestern africa where the former age range corresponds to magmatic and high grade metamorphic activity in the mesoproterozoic namaqua natal belt and the latter to an extensive palaeoproterozoic andean type volcanic arc richtersveld terrane comparable ages are conspicuously absent in the basement of the rio de la plata craton in south america derivation of the rocha group sediments from a similar source as the contemporaneous oranjemund group sediments is therefore suggested the most likely source of the youngest detrital zircon grains in these two stratigraphic units is the 640 590 ma magmatic arc of the dom feliciano belt this interpretation explains the continental island arc component in the sediment provenance and takes into account the absence of comparable magmatism in the gariep belt two stages of basin evolution are distinguished for the gariep basin an early failed continental rift that subsequently became re activated as back arc basin related to the cryogenian dom feliciano arc consequently the gariep basin does not represent the main suture between the rio de la plata and the kalahari cratons this suture is now suspected to be west of the dom feliciano volcanic arc and might be connected with the sierra ballena major gercino lineament