continental oceanic crustal transition off southwest africa more than 1 426 mi 2 300 km of 48 trace 12 fold seismic reflection profiles were used to examine the nature of the continent ocean boundary off southwest africa south of the orange river faulted blocks which we interpret as rifted continental crust can be traced seaward to the 3 km isobath the transition to oceanic basement occurs in a zone 19 to 25 mi 30 to 40 km wide beneath the continental rise although structural details are obscured by a thick seaward dipping wedge of pre aii pre late early cretaceous overburden oceanic crust at the presumed contact is not older than magnetic anomaly m9 126 to 121 m y b p north of the orange river a pronounced hinge in continental crust correlates with magnetic anomaly g of rabinowitz 1976 seaward of the hinge a complex fault block terrane is evident oceanic basement can not be traced with any certainty landward of anomaly m4 intracontinental stretching and associated volcanism appear to have been important in the early history of the cape basin rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea floor spreading related extrusives form at least part of the pre aii wedge just seaward of the hinge zone our interpretation of the seismic data suggests that the initiation of normal spreading in the cape basin postdates by 4 to 9 m y the valanginian age derived from prevailing plate tectonic reconstructions of the south atlantic

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

kinematic model for postorogenic basin and range extension the raft river extensional shear zone is exposed in the albion raft river grouse creek metamorphic core complex several studies of ductile deformation have shown that it accommodated crustal stretching in tertiary time during late orogenic collapse of the thickened cordilleran crust progressive deformation that results from mixed pure and simple shear produces a complex strain pattern along the shear zone the authors propose a numerical kinematic model that relates strain variations in the shear zone to the different amounts of extension between the brittlely extended upper plate rocks and the undeformed lower plate rocks a possible hypothesis for postorogenic crustal extension involves diffuse heterogeneous extension and flow in the ductile lower crust and localized extensional shear zones in the middle and upper crust

remanent hysteresis study of dolerite dykes saturation remanent hysteresis studies were carried out on numerous dolerite dyke samples from the peninsular india these studies result in four types of remanent hysteresis curves which indicate that the magnetic material is magnetite or titanomagnetite with variable grain size having remanent coercive forces hcr of 8 to 30 mt requiring saturating fields hs up to 250 mt two extreme types of samples with 1 low coercive forces requiring high saturating fields and 2 high coercive forces requiring low saturating fields are noticed along with the generally observed ones the granulometric and lowrie fuller tests on these samples indicated that the magnetic material i e magnetite or titanomagnetite in these rocks is in the form of multi domain md cation deficient cd and a mixture of these two forms md cd within

basalt magmatism along the passive continental margin of se brazil campos basin the se brazil passive continental margin is characterized by tholeiitic magmatism that is particularly widespread in the marginal campos basin facing the inland flood basalts of the adjacent parana basin campos magmatism is represented by early cretaceous ec 134 122 ma flood basalts and minor upper cretaceous early tertiary basalt flows and intrusives which were emplaced in a basin with attenuated crustal thickness 20 km petrography mineral chemistry wholerock geochemistry and sr nd isotope composition emphasize that the ec campos basalts have suffered extensive seawater interaction which caused enrichment in mgo feo total k2o rb and ba and depletion in sio2 and cao while zr nb y and ree remained virtually constant in samples with loss on ignition values less than 4 wt in general campos basalts have bulkrock chemistry similar to those of the inland parana tholeiites 140 130 ma with relatively low concentrations of incompatible elements and tio2 16 morb mid ocean ridge basalt mantle in simple binary mixing models on the whole the early cretaceous campos basalts appear as an easterly younger extension of the northern parana volcanism and probably erupted during early stages of the major riftingprocesses which caused continental thinning it is notable that in the campos marginal basin both the basalt magmatism contemporaneous with the continental break up as well as that which occurred after the s america africa separation appears substantially related to subcontinental lithosphere and a dupal like oib ocean island basalt e g tristan da cunha source components

lower cretaceous tholeiitic dyke swarms from the ponta grossa arch southeast brazil petrology sr nd isotopes and genetic relationships with the parana flood volcanics the lower cretaceous dykes of the ponta grossa arch the most important dyke swarms in brazil are associated with the flood basalts and rare acid flows of the northern parana basin the ponta grossa pg dykes are formed by two pyroxene tholeiites and rare acid rocks the basaltic dykes may be distinguished into two main groups a dominant high tio2 2 wt hti group and a subordinate low tio2 2 wt lti group characterized for similar mgo content by high and low incompatible element contents respectively most pg dykes do not show chemical and isotope evidence supporting important crustal contamination pg dykes with 87sr86sr 0 0 7060 plot in the mantle array sr 17 and nd 3 and can be considered virtually uncontaminated intra dyke chemical variations suggest that the normal or reverse differentiation trends may be related to pre emplacement low pressure differentiation processes the important chemical differences between incompatible element poor and incompatible element rich basaltic dykes cannot be accounted for in terms of fractional crystallization but are compatible with different melting degrees of a garnet peridotite mantle source e g 9 and 20 melting for the hti and lti tholeiites respectively field distribution of the magmatism chemistry and sr nd isotope compositions support that the pg dykes may be related to the flood volcanics of northern parana and not to those of the central and southern parana paleomagnetic data indicate that pg dykes are younger than the parana volcanics all the results suggest that pg dykes were probably feeders of the stratoid volcanics erupted in northern parana towards the continental margin and later eroded chemistry and isotopic data reveal that asthenospheric mantle components were not significant factors in the genesis of pg dykes dyke emplacement occurred during early phases of rifting and or flexuring cutting flood volcanic suites

late cretaceous alkaline complexes southeastern brazil paleomagnetism and geochronology late cretaceous alkaline rocks from pocos de caldas itatiaia passa quatro and sao sebastiao island were studied with the main purpose of improving the south american apparent polar wander path apwp rbsr age determinations for passa quatro and alkaline stocks from sao sebastiao island are around 70 and 81 ma respectively the magnetic carriers in the rocks studied were maghemite and highly oxidized titanomagnetites the paleomagnetic poles obtained in this work are 320 1 e 83 2 s n 47 a95 2 7 ksc 2609 8 for pocos de caldas 360 0 e 79 5 s n 18 a95 5 7 ksc 607 4 for passa quatro and itatiaia and 331 9 e 79 4 s n 18 a95 4 9 ksc 862 4 for sao sebastiao island the island set

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

tectonic significance of the florianopolis dyke swarm parana etendeka magmatic province a reappraisal based on precise u pb dating abstract the first precise id tims u pb baddeleyite zircon dating of dykes representative of the main activity in the florianopolis dyke swarm parana etendeka magmatic province south brazil reveals a restricted age range 134 7 0 3 to 133 9 0 7 ma this is at least 5 ma older than indicated by previous 40 ar 39 ar stepwise heating dating and precludes a direct relationship with syn rifting magmatism connected with opening the south atlantic ocean instead as indicated by their age and geochemical signature these dykes are feeders to the nearby urubici lavas and coeval khumib lavas in etendeka these new geochronological results also cast doubt on a large 10 ma time span for the emplacement of the florianopolis dyke swarm implied by 40 ar 39 ar dates but the determination of its upper age limit still requires accurate dating of younger low volume fine grained dykes with distinct low ti chemistry crystals suitable to u pb dating were not obtained thus far from these dykes

long duration 4 ma and steady state volcanic activity in the early cretaceous parana etendeka large igneous province new palaeomagnetic data from namibia there is long standing correlation between large igneous provinces lips and major mass extinction events in the geological record postulated to be due to the emission of large quantities of volcanic gases over a geologically short period of time causing major climatic perturbations within the earth system the 135 ma parana etendeka volcanic province of brazil and namibia represents something of an enigma amongst lips despite an erupted volume 1 mkm3 comparable to other lips associated with mass extinctions such as the siberian or deccan traps it is not linked to a known mass extinction event this suggests that the parana etendeka volcanic province was emplaced over longer timescales than other lips and or emitted a lower concentration of volatiles directly or indirectly during its emplacement we present a new detailed magnetostratigraphy for the etendeka portion of the province that suggests emplacement took place over longer timescales 4 ma than those associated with other lips palaeomagnetic analysis of 893 specimens from 99 sites in sections that encompass nearly the complete etendeka stratigraphy yielded high quality data from 70 sites 612 specimens these record 16 individual polarity intervals which can be correlated with chrons 15 to 11 of the geomagnetic polarity time scale gpts while also providing two new high quality palaeopoles for south africa at 130 135 ma our magnetostratigraphy reveals a minimum period of volcanic activity in excess of 4 myrs and importantly we find no evidence for major changes in the rates of volcanic activity through that time period in contrast to other lips where volcanism seems to be concentrated in major pulses this suggests that the anomalously feeble environmental impact of parana etendeka volcanism may be due to lower effusion rates reducing the atmospheric loading due to volcanogenic volatiles

eocene paleomagnetic pole for south america northward continental motion in the cenozoic opening of drake passage and caribbean convergence 1 a paleomagnetic study of eocene volcanic rocks in patagonia yields high unblocking temperature and high coercivity magnetizations combining these results with those of a previous study on patagonian eocene basalts yields a high precision high quality pole located at latitude 81s longitude 337 4e a95 5 7 critically this paleopole is indistinguishable from that of the late cretaceous circa 85 65 ma pole position of south america indicating that the plate was essentially motionless with respect to the spin axis for a period of 45 m y the pole position places south america at higher 5 than present day latitudes during the eocene indicating that northward continental motion toward present day latitudes must have been accomplished sometime since the late eocene paleomagnetic and tectonic correlation admits the hypothesis that cenozoic northward drift was associated with oligocene miocene extension in the southern continental edge leading to the opening of the drake passage and it agrees with the timing of foredeep formation and development of fold thrust belts in the northern continental edge this positive correlation between the paleomagnetically predicted drift of a major continent with extension at its trailing edge and convergence at its leading edge during times for which seafloor tectonic fabric and the geological record are particularly well preserved illustrates the utility of paleomagnetism in constraining paleogeographic and tectonic reconstructions for pre cretaceous times

paleomagnetism and dating of the early cretaceous florianopolis dike swarm santa catarina island southern brazil abstract the florianopolis fl basaltic dikes are well exposed on santa catarina island close to the present southeastern border of the parana magmatic province in southern brazil and their intrusion was related to the tectonic and magmatic evolution of rifting in the southern atlantic the dikes are vertical or subvertical and cut the crystalline basement rocks mainly late proterozoic granites most of the dikes exhibit ne trends corresponding to structural trends of the crystalline basement a few nw trending dikes can also be observed occasionally crosscutting the ne dikes samples from 73 dikes were collected for paleomagnetic work and subjected to both alternating field and thermal demagnetization characteristic magnetizations of normal and reversed polarities interpreted as original thermoremanences are carried by ti poor magnetites showing remanent coercive forces in the range 18 45 mt the corresponding paleomagnetic pole fl pole is located at 3 3 e 89 1 s n 65 95 2 6 k 47 samples for 40 ar 39 ar dating were collected at the same paleomagnetic sites plagioclase samples from nine dikes were analyzed by stepwise heating the apparent age spectra are strongly discordant due to excess 40 ar contamination but five samples yielded plateau ages ranging from 119 0 0 9 ma to 128 3 0 5 ma one additional sample yielded an isochron age of 121 5 0 5 ma the inferred ages apparently define two modes at 119 122 ma and 126 128 ma although the data set is probably insufficient to determine whether dike intrusion was continuous or episodic the ages of 128 119 ma are coeval with the final stages of the rifting at this latitude suggesting that the emplacement of the fl dikes is related to extension of the continental crust just prior to the formation of oceanic crust by comparing the fl pole with the existing paleomagnetic poles for the nw ponta grossa dike swarm mainly 130 5 ma to the north the parana volcanics mainly 133 132 ma and the central alkaline province 127 130 ma on the western side of the parana province it is concluded that the differences in pole positions reflect plate displacements and therefore the fl pole is probably dominated by the dikes with ages younger than 127 ma

emplacement of dike swarms from the island of ilhabela se brazil and its relationship with the south atlantic ocean opening revealed by magnetic fabrics abstract magnetic fabric and rock magnetism studies were performed on 153 dikes from diabase lamprophyre and alkaline dike swarms that outcrop on the island of ilhabela ne sao paulo state the dikes crosscut archean and proterozoic polymetamorphic rocks their thicknesses range from a few centimeters to 4 or 5 m they trend predominantly n30 60e with a very steep 60 to vertical dip magnetic fabrics were determined using anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm rock magnetism measurements reveal that the magnetic mineral of the three swarms is magnetite mainly with grains in the range of 2 5 m for a few dikes these grains are the carriers of bulk magnetic susceptibility but are not responsible for the ams which is due to fe bearing minerals as shown by aarm the main ams fabric recognized in the swarms is due to magma flow in which the kmax kint plane is parallel to the dike s plane and the magnetic foliation pole kmin is perpendicular to it the analysis of the kmax inclination showed that the dikes were fed by horizontal to vertical flows however for the minority of the dikes the ams and aarm tensors are not coaxial the aarmmax is oriented n30 60 w approximately perpendicular to the kmax the aarmmax orientation is similar to the direction of a fault system located mainly in the santos marginal basin which was formed in the cretaceous rifting during the south atlantic opening the aarm fabric is tectonic in origin and is parallel with the extensional paleostress responsible for the gondwana break up the comparison of ams and aarm fabrics suggests that diabase and lamprophyre dikes were emplaced in three distinct events in the earliest stages of the south atlantic opening later the alkaline dikes were emplaced in the final stages of the atlantic opening

paleomagnetism of the patagonian plateau basalts southern chile and argentina a total of 505 paleomagnetic samples were collected from 65 sites volcanic flows of the patagonian plateau basalts at four locations in southern chile and argentina k ar analyses indicate that 38 flows from two locations form a late cretaceous group 64 79 ma while the remaining 27 flows are eocene in age 42 56 ma progressive demagnetization indicates that a characteristic remanent magnetization chrm has been successfully isolated from 59 flows rock magnetic properties and analysis of chrm directions within and between sites allow the secure inference that the chrm is a thermoremanent magnetization acquired during original cooling the eocene flows yield 15 independent samplings of the paleomagnetic field however these data fail the reversals test and probably do not adequately average geomagnetic secular variation the late cretaceous flows yield 18 independent samplings of the paleomagnetic field six of normal polarity and 12 of reversed polarity these data pass the reversals test and have angular dispersion of virtual geomagnetic poles vgps consistent with adequate sampling of geomagnetic secular variation the mean of these 18 vgps yields a late cretaceous paleomagnetic pole for south america latitude 78 7 s longitude 358 4 e a95 6 3 k 31 6 s 14 5 this pole is consistent with similar age poles from north america and africa when the atlantic is reconstructed to 70 ma contrary to previous indications the south american apparent polar wander path does not reach the present rotation axis until after late cretaceous time

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

new paleomagnetic pole for the upper jurassic chon aike formation of southern argentina south america testing the tectonic stability of patagonia with respect to south america and implications to middle jurassic early cretaceous true polar wander abstract a paleomagnetic pole and a new ar ar date of 156 08 0 05 ma are reported for the chon aike formation an extensive plateau of ignimbrites outcropping in the deseado massif southern patagonia argentina the geographic coordinates and statistical parameters for the paleomagnetic pole validated by a reversal test are lat 84 3 s long 191 3 e a95 8 6 k 13 3 n 23 this pole shows a complete coherence with late jurassic and early to mid cretaceous poles of south america obtained exclusively from igneous rocks avoiding any potential inclination shallowing it is suggested that a southern patagonia was an integral part of south america in the late jurassic b any reported vertical axis tectonic rotations must pre date the late jurassic in this region c according to updated paleomagnetic data from igneous rocks south america had very limited latitudinal drift between about 160 and 140 ma and therefore d its apparent polar wander apw path when compared with global apparent polar wander paths does not support a large global shift or true polar wander tpw between 160 and 140 ma

south american reference pole for the mid cretaceous further constraints in the interpretation of andean paleomagnetic data a paleomagnetic study of the aptian cerro barcino formation in central patagonia showed the presence of a stable magnetization with a positive tilt test the obtained pole position lat 85 s long 181 e 95 5 6 n 11 differs from reliable early and late cretaceous south american cratonic poles the cerro barcino pole overlaps coeval poles from the chilean andean fore arc previously interpreted to record crustal block rotations this new result indicates that previously published andean tectonic models must be reviewed

3 d 40ar39ar geochronology in the parana continental flood basalt province new 40ar39ar analyses of borehole samples from the parana etendeka continental flood basalt province provide critical evidence of eruption rates in unexposed regions when combined with surface samples the new data clarify the duration of this major volcanic province new ages from both surface and borehole samples confirm a 10 12 million year duration of magmatism which post dates the tithonian mass extinction and provide a unique 3 dimensional picture of the spatial and temporal eruption of the lava pile chemically defined magma types are diachronous and the onset of magmatism occurred 500 1000 km inland migrating southeast towards the incipient south atlantic ocean calculated eruption rates increased from 0 03 km3 yr 1 between 138 and 135 ma through 0 13 km3 yr 1 between 135 and 133 ma to 0 21 km3 yr 1 between 133 and 131 ma supporting the notion that the rate of magmatism increased with time and proximity to the developing south atlantic after 131 ma the continental eruption rates dropped dramatically to 0 01 km3 yr 1 presumably because rifting had occurred and eruption centred in the oceanic rio grande rise with eruption rates of around 0 3 0 5 km3 yr 1 given the available constraints on the potential temperature of the plume thickness of the continental lithosphere and degrees of extension through time predicted melt production rates within the plume seriously underestimate the calculated eruption rates on the continent however the eruption rates can be reconciled if melting occurred by conductive heating of volatile enriched mantle the diachronous nature of the magma types supports the idea that melting occurred over a wide area and that the different magma types reflect different source regions rather than the temporal evolution of magmas from a single source given that 10 million years is appropriate to the timescales for conductive heating above the mantle plume it is suggested that these distinct source regions were located within the lithospheric mantle consistent with geochemical studies

structures and lithofacies of inferred silicic conduits in the parana etendeka lip southernmost brazil abstract extensive silicic units in the parana etendeka lip have been long interpreted as pyroclastic density currents rheomorphic ignimbrites derived from the messum complex in namibia in recent literature however they have been characterized as effusive lava flows and domes in this paper we describe structures and lithofacies related to postulated silicic lava feeder conduits at mato perso sao marcos and jaquirana cambara do sul areas in southern brazil inferred conduits are at least 15 25 m in width and the lithofacies include variably vesicular monomictic welded and non welded breccias in the margins to poorly vesicular banded spherulitic and microfractured vitrophyres in the central parts flat lying coherent vitrophyres and massive obsidian are considered to be the subaerial equivalents of the conduits large scale regional tectonic structures in southern brazil include the ne sw aligned porto alegre suture leao and acotea faults besides the antas lineament a curved tectonic feature accompanying the bed of antas river south of the antas lineament smaller scale nw se lineaments limit the exposure areas of the inferred conduits ne sw and subordinate nw se structures within this smaller scale lineaments are represented by the main postulated conduit outcrops and are parallel to the dominant sub vertical banding in the widespread banded vitrophyre lithofacies upper lava flows display flat lying foliation pipe like and spherical vesicles and have better developed microlites petrographic characteristics of the silicic vitrophyres indicate that crystal poor magmas underwent distinct cooling paths for each inferred conduit area the vitrophyre chemical composition is defined by the evolution of trachydacitic dacitic vitrophyres with 62 65 wt sio 2 to rhyodacite and rhyolite with 66 68 wt sio 2 the more evolved rocks are assigned to the latest intrusive grey vitrophyre outcropping in the center of the conduits degassing pathways formed during fragmentation and fracturing episodes within the conduits may have helped to inhibit the explosivity of the eruptions based on the documented lithofacies architecture we attribute the source of the silicic lava flows in the studied localities to tectonic controlled local conduits rather than pyroclastic density currents from distant vent areas

an early cretaceous paleomagnetic pole from ponta grossa dikes brazil implications for the south american mesozoic apparent polar wander path paleomagnetic results from the early cretaceous ponta grossa dike swarm are reported these dikes crosscut sediments and basement rocks associated with the ponta grossa arch a tectonic feature in the eastern border of the paleozoic mesozoic parana basin these dikes are vertical or subvertical and trend mainly nw and ne samples from 127 dikes were submitted to both alternating field and thermal demagnetizations 121 of them yielded characteristic directions interpreted as original thermal remanent magnetizations both normal and reversed polarities as well as intermediate directions 6 dikes are recorded the paleomagnetic pole is located at 30 3 e 82 4 s n 115 95 2 0 k 43 8 this pole along with the available paleomagnetic poles for the parana basin volcanics serra geral formation recalculated in this paper in the light of reported 40 ar 39 ar ages describe an apparent polar wander path segment corresponding to a clockwise rotation 6 9 of the south american plate this movement is compatible with the initial phase of the south atlantic opening with rifting progressing from south to north however the calculated plate velocity is higher than the predicted velocities reported so far based on oceanic floor magnetic anomalies the new data reported in this paper allowed the calculation of a mean early cretaceous pole 60 3 e 83 8 s n 7 95 2 9 which can be discriminated from the middle late jurassic 191 3 e 86 2 s n 3 95 7 8 and late cretaceous 346 5 e 84 1 s n 5 95 4 9 mean poles for south america indicating a more complex movement of the continent than the simple east west drift generally proposed

synchronous negative carbon isotope shifts in marine and terrestrial biomarkers at the onset of the early aptian oceanic anoxic event 1a evidence for the release of 13c depleted carbon into the atmosphere 1 a common feature of records of the early aptian oceanic anoxic event oae 1a is the sharp negative 13c excursion displayed in both carbonate and organic matter at the onset of this event a synchronous negative 13c excursion has also been noted for terrestrial organic matter this negative excursion has been attributed to either an injection of 13c depleted light carbon into the atmosphere or in case of marine sediments recycling of 13c depleted co2 however most studies were done on separate cores and no information on the relative timing of the negative spikes in terrestrial versus marine records has been obtained here we examine early aptian core sections from two geographically distal sites italy and the mid pacific to elucidate the causes and relative timing of this negative spike at both sites increased organic carbon corg and decreased bulk carbonate contents characterize the interval recording oae 1a variously referred to as the selli event the organic material within the selli level is immature and of autochthonous origin measured 13c values of marine and terrestrial biomarkers largely covary with those of bulk organic carbon with lowest values recorded at the base of the organic rich section by contrast sediments enveloping the selli level exhibit very low corg contents and their extractable corg is predominantly of allochthonous origin hydrous pyrolysis techniques used to obtain an autochthonous pre selli 13c value for algal derived pristane from corresponding sample material yielded a negative 13c shift of up to 4 a negative 13c shift of similar magnitude was also measured for the terrigenous n alkanes the results are collectively best explained by means of a massive syndepositional rapid input of 13c depleted carbon into the atmosphere and surface oceans likely delivered either via methane produced from the dissociation of sedimentary clathrates or perhaps by widespread thermal metamorphism of corg rich sedimentary rocks

palaeomagnetism of the mesozoic serra geral formation southern brazil abstract palaeomagnetic results from 20 volcanic sequences and 11 intrusive bodies sills and dykes of the serra geral formation parana basin are reported in this paper the sequences are widespread all over the basin while sills and dykes ponta grossa arch come from the northeastern portion three mean palaeomagnetic poles were computed for the serra geral formation which account for a time interval of 15 ma pole sg1 is located at 85 s 108 e 95 1 1 n 18 and represents the main phase of the magmatic activity in the basin with a mean age of 135 ma pole sg2 is located at 82 s 38 e 95 7 8 n 2 and represents a younger magmatic phase with an associated age of 130 ma pole sg3 72 s 37 e 95 6 8 n 10 is the youngest pole it is computed from the intrusive rocks and its age is assumed to be not younger than 118 ma the lower limit of the cretaceous normal magnetic interval these three poles describe a shifting path which suggest that the south american platform moved 5 southwards and rotated 10 clockwise during the lower cretaceous preceding the south atlantic opening

mid cretaceous polar standstill of south america motion of the atlantic hotspots and the birth of the andean cordillera article i nfo a paleomagnetic study on mid cretaceous rocks from the san bernardo foldbelt patagonia yields high unblocking temperature and high coercivity magnetizations the results indicate absence of relative vertical axis rotations during development of the foldbelt with the associated pole position being highly concordant with coeval poles from brazil and patagonia taken together mid cretaceous poles derived from studies in widely distributed localities provide supportive evidence that south america was essentially motionless with respect to the paleomagnetic axis from ca 125 to at least 100 ma the paleolatitudes of south america are not consistent with the occurrence of mid cretaceous true polar wander suggesting that the previously observed discrepancy between the paleomagnetic and the fixed indo atlantic hotspot reference frames be related to motion of the atlantic hotspots in agreement with this the discrepancy is diminished by half when the cretaceous poles of the americas are observed in a moving hotpots reference frame with the residual offset being comparable to that seen for younger time intervals the south american paleopoles and the moving hotspot framework provide a kinematic scenario that allows relating the extensional tectonics in the early stages of andean evolution with episodic divergence between the trench and the continental interior likewise the beginning of contractional events correlates with model predicted westward acceleration of south america in the late cretaceous suggesting that the continent episodically overrode the andean trench by those times we argue that this change in andean tectonic regime is associated to major plate reorganization at ca 95 ma

pre rift tectonic scenario of the eo cretaceous gondwana break up along se brazil sw africa insights from tholeiitic mafic dyke swarms