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

new time constraints on dyke swarms related to the parana etendeka magmatic province and subsequent south atlantic opening southeastern brazil abstract 40 ar 39 ar incremental heating analyses were carried out on tholeiitic dyke swarms of santa catalina florianopolis and rio de janeiro and alkaline dykes from the proximity of rio de janeiro city southeastern brazil most of the plagioclases from the tholeiitic dykes display disturbed age spectra with younger apparent ages most often clearly correlated with alteration phases however a calculated weighted mean age from the less disturbed released fractions of the ne oriented santa catarina dyke swarm gives an age of 129 4 0 3 ma which is contemporaneous with the ponta grossa dyke swarm pgds and at least the northern parana continental flood basalts cfb the rio de janeiro tholeiitic dykes display age spectra affected by alteration and probably excess argon but with low temperature mini plateau ages between 130 6 and 129 8 ma which are concordant with previous data obtained on the pgds and rio de janeiro dyke swarms by renne et al 1996a and turner et al 1994 these new geochronological data when added to the previous ones show that the initiation of gondwana s breakup in the region was characterized by a volcano tectonic system with a geometry more complicated than the rift rift rift system suggested by many authors the different rift arms of this structural feature were operating during a brief time period mainly from 131 5 to 129 ma and were contemporaneous at least with the northern part of the parana etendeka province pep single grain and bulk sample biotite from the alkaline mainly coast parallel dyke swarm near rio de janeiro city display plateau ages around 80 ma 81 8 1 8 and 78 8 2 8 ma and 70 ma 72 5 0 5 and 69 7 0 2 ma these magmatic intrusions refer to late cretaceous flexuring and vertical movements and rift basin formations almeida 1976 due the northward progressing rifting motions separating south america and africa during south atlantic ocean formation

u pb zircon geochronology and tempo of the end permian mass extinction the mass extinction at the end of the permian was the most profound in the history of life fundamental to understanding its cause is determining the tempo and duration of the extinction uranium lead zircon data from late permian and early triassic rocks from south china place the permian triassic boundary at 251 4 0 3 million years ago biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the changhsingian pulse of the end permian extinction corresponding to the disappearance of about 85 percent of marine species lasted less than 1 million years at meishan a negative excursion in 13 c at the boundary had a duration of 165 000 years or less suggesting a catastrophic addition of light carbon

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

subsidence in late paleozoic basins in the northern appalachians during the interval between continental collision in the devonian and continental breakup in the triassic the northern appalachians became the site of a wide plate boundary zone of dominantly right lateral strike slip as is typical of intracontinental transforms tectonism was both diachronous and rapidly variable along strike through regimes of pure strike slip transpressional deformation and rapid subsidence of extensional basins up to 9 km of mainly nonmarine clastic sediments accumulated in these local depocenters which subsided episodically in two stages 1 an initial phase of stretching and thinning of the lithosphere when subsidence was rapid fault controlled and often accompanied by volcanism and 2 a subsequent phase of gradual thermal subsidence during which the depositional basins expanded to bury the earlier border faults and progressively younger sedimentary units onlapped basement the largest depocenter the magdalen basin opened as a pull apart between strike slip faults in newfoundland and new brunswick from late devonian to early carboniferous subsequent thermal subsidence affected a large area during medial and late carboniferous a phenomenon that is well recorded to the north and west where no later tectonism occurred in areas to the south and east of the basin strike slip on other faults continued into the time of thermal subsidence introducing complications such as localized transpressional deformation and rapid subsidence in smaller pull aparts

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

rapid eruption of silicic magmas from the parana magmatic province brazil did not trigger the valanginian event the valanginian stage is marked by a period of global positive 13c carbon cycle perturba tion and biotic crises which are collectively referred to as the valanginian event ve many attempts have been made to link the parana etendeka large igneous province volcanism with the ve however currently there is no conclusive proof to support this hypothesis since the timing and duration of the volcanic activity are not known with suf cient precision in this study we signi cantly revise the time scales of magmatism and environmental impact of the parana magmatic province pmp in brazil with new high precision zircon u pb ages from the low ti palmas and high ti chapeco sequences our data demonstrate that signi cant volumes of low ti silicic rocks from the pmp erupted rapidly at ca 133 6 ma within 0 12 0 11 k y the age of the high ti chapeco sequence from central pmp is constrained at ca 132 9 ma and thus extends the duration of magmatic activity by 700 k y our new ages are systematically younger than previous ages and postdate the major positive carbon isotope excursion indicating that pmp silicic magmatism did not trigger the ve but could have contributed to extending its duration within the framework of the stratigraphic column of the pmp the earliest low ti basalts could have been responsible for the ve if they are at least 0 5 m y older than the low ti silicic rocks dated herein

chemical stratigraphy of the parana lavas south america classification of magma types and their spatial distribution a new classification scheme has been developed to assign the lava flows of the parana continental flood basalt province south america into geochemically distinct magma types with six basaltic major and trace element abundances and or ratios by mapping out the spatial distribution of these magma types within the lava sequences it has been possible to determine the internal stratigraphy of the lava pile on a regional scale previous studies on road profiles traversing the well exposed coastal serra geral escarpment of southern brazil are summarised together with results from some new sampled sections more widespread stratigraphical investigations of the parana lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks however access to drill core chippings from nine boreholes in the central parana region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions the borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north which suggests that the main locus of magmatism moved northwards with time within the parana basin this migration could be related to the northward propagation of rifting during the initiation of the south atlantic ocean maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes although suggesting that the shift in magmatism may have been towards the northwest on the basis of geochemical similarities between magma types and their inferred stratigraphical relationships it is proposed that the parana can be divided into two principal magmatic centres 1 an older one in the south comprising the gramado esmeralda and urubici magma types and 2 a younger one developed about 750 km to the north formed by the pitanga paranapanema and ribeira magma types

mantle plumes and flood basalt stratigraphy in the parana south america geochemical studies of the parana continental flood basalts in brazil have led to the recognition of distinct magma types which have been used to infer the internal stratigraphy of the lava pile the overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying parana basin sedimentary rocks imply that the site of magnatism migrated 750 km toward the north during this volcanic event this has important implications for recent plume related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood basalt magmatism it is difficult to ascribe the magnitude and direction of the shift in the locus of the parana magmatism to movement of the brazilian lithosphere relative to the underlying tristan mantle plume instead it is suggested that the observed lava distribution and internal structure of the parana etendeka flood basalt province were imposed by the intracacies of the rifting process during the initial opening of the south atlantic ocean

the etendeka igneous province magma types and their stratigraphic distribution with implications for the evolution of the parana etendeka flood basalt province detailed geochemical and field data for the volcanic sequence and intrusions of the etendeka igneous province are used to construct a stratigraphic framework for petrogenetic interpretation of the evolution of the etendeka parana continental flood volcanic event geochemical and petrographic characterization of over 1 000 analyzed samples allows 8 mafic and 17 silicic magma types to be recognized both silicic and mafic types can be grouped into high ti and low ti suites on the basis of elevated ti relative to other elements the mafic magmas are khumib high ti tafelberg kuidas horingbaai huab tafelkop albin and esmeralda all low ti amongst the silicic types the goboboseb springbok wereldsend grootberg and beacon low ti quartz latites and the nil desperandum high ti latite have been described previously in addition the hoas low ti nadas sechomib and hoarusib all high ti latites and the fria low ti sarusas ventura khoraseb naude and elliott all high ti quartz latites are described for the first time here there is a marked provinciality in the distribution of the high and low ti suites with the former concentrated in the northern etendeka region and the latter dominant in the southern etendeka stratigraphic distribution of magma types allows two new formations to be defined in the northern etendeka the khumib formation of basaltic flows and the skeleton coast formation dominated by silicic sheets the geochemical provinciality hampers precise correlations between northern and southern etendeka available evidence suggests that the lower part of the awahab formation in the southern etendeka is coeval with the lower part of the khumib formation and that the silicic units in the upper part of the tafelberg formation probably correlate with the skeleton coast formation the paucity of khumib dykes in relation to tafelberg dykes and their field relationships with regard to the volcanic sequence in the northern etendeka suggests that the main khumib eruptive centers lay further north consistent with southward thinning of the khumib basalts in the southern etendeka the doros complex is the eruptive center of the plume derived tafelkop basalts which probably built a shield volcano within the regional flood lava field this work indicates that the etendeka has a greater variety of both silicic and low ti mafic magma types than are known from the se parana detailed comparisons reveal that all the important silicic types in the parana have geochemical equivalents in the etendeka new correlations are santa maria fria ourinhos khoraseb guarapuava sarusas thus extending the previously recognized correlations between southern etendeka quartz latites and the caixas do sul and anita garibaldi rhyolites these correlations emphasize that very large volume silicic systems develop with pronounced lithospheric rifting in continental flood basalt provinces

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

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

generation of block and ash flows at the onset of silicic volcanism in the parana magmatic province brazil evidence from photoanalysis of caxias do sul breccias in this work we perform textural analysis on volcanic breccias related to the silicic magmatism in the region of sao marcos southern brazil parana magmatic province the breccias are matrix supported coarse grained poorly sorted with angular to rounded fragments in a fine grained matrix juvenile fragments are dense to pumiceous massive to banded dacites lithic fragments were not recognised the analyses were performed on field photographs grain size distribution gsd analysis showed maximum fragment sizes between 8 and 9 5 and smallest recognised sizes between 3 and 1 5 due to the spatial resolution limit of the photographs and the weathered condition of the matrix the fine fragments from the matrix were not analysed the gsd fractality could be quantified by a single power law with low moderate d values of 1 9 2 3 vesicle size distribution vsd and shape parameters of both fragments and vesicles were also measured the fragments are typically low vesicularity with median vesicle area fractions around 5 for all samples but pumiceous fragments with up to 43 area are found vsds exhibit single exponential concave upwards curves with vesicles becoming more deformed complex with increasing size suggesting a single continuous process of nucleation and growth under low degrees of shear regarding the fragments shape parameters all samples have high values of solidity which together with the low vesicularity of the fragments suggest brittle fragmentation of poorly vesicular material and the operation of secondary fragmentation processes during transport that round and smooth the clasts however field evidence for local conduit systems together with the gsd fractality suggest that these may be proximal deposits without substantial transport history based on a comparison of the deposits characteristics with several types of volcanic breccias we propose a block and ash flow source for these breccias that indicate the presence of nearby lava domes providing evidence for high viscosity low flux dacitic volcanism in the parana magmatic province

stratigraphy and volcanic facies architecture of the torres syncline southern brazil and its role in understanding the parana etendeka continental flood basalt province abstract the torres syncline is a large structure that constitutes the eastmost outcrop of the parana etendeka cfb in south american side and this work focuses the stratigraphy and facies architecture of the volcanic pile in the syncline the volcanic sequence along the study area permits the division of three regions main valley intermediate zone and south hinge each of them with distinct stratigraphy which probably reflects the structural evolution of the syncline the stratigraphy of the torres syncline is composed by 1 botucatu palaeoerg 2 basic volcanic episode i 3 basic volcanic episode ii 4 acidic volcanic i 5 basic volcanic episode iii and 6 acidic volcanic episode ii the five volcanic episodes recognized in study area can be related to five volcanic facies architecture compound braided tabular classic tabular lobate escoriaceous dome field acidic lavas and tabular flows acidic lavas the basic episode i is composed by pahoehoe flows with a compound braided facies architecture that covered the botucatu palaeoerg the basic episode ii is a tabular classic facies architecture predominantly composed by simple flows 10 20 m thick reaching the total thickness of 500 m in main valley the acidic episode i is exposed in main valley and south hinge and is composed by acidic lavas forming lava dome field facies architecture with a thickness of 150 m the basic episode iii is predominantly constituted by a flows with tabular lobate escoriaceous facies architecture the acidic episode ii is constituted by tabular flow volcanic facies acidic flows and outcrops all along the study area the torres syncline constitute the eastmost on shore exposures of the parana etendeka cfb in south american side and detailed stratigraphic volcanological and structural studies in these area coupled with correlation with huab basin nw namibia africa will aim the understanding of the gondwana breakup process and the early stages of the south atlantic margin opening

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

protracted felsic magmatic activity associated with the opening of the south atlantic felsic magmatism is often observed in the final stages of the eruption of continental large igneous provinces the occurrence of these felsic extrusive igneous rocks potentially provides key information on the complex interplay of magmatism and tectonics in this paper we investigate the timing of eruption of early cretaceous felsic magmas associated with the parana etendeka flood basalt province using the 40 ar 39 ar technique and interpret the results in terms of the recently proposed petrogenetic histories and dominant stress factors there appears to be a link between lithospheric thinning and a change in the origin of the felsic magmas from open system fractional crystallization to mid lower crustal melting particularly in southern uruguay

correlation of widespread tephra deposits based on paleomagnetic directions link between a volcanic field and sedimentary sequences in japan abstract magnetic measurements were made of the yabakei and the imaichi pyroclastic flow deposits of the late matuyama chron in central kyushu and their correlative co ignimbrite ashes the pink and the azuki tephra deposits in the kinki district and the ku6c and the o7 tephra deposits in boso peninsula site mean magnetic directions of the imaichi deposit are all of reversed polarity with declinations deflected slightly to the east these directions are consistent with those of the azuki and the ku6c ashes magnetizations of the yabakei the pink and the o7 ashes are also consistent with one another all are characterized by shallow inclinations about 30 of normal polarity with slightly westerly declinations the identical paleomagnetic directions of these widespread tephra confirm correlations that have been proposed mainly on the basis of glass and mineral chemistry and provide a tephrochronological linkage between the source volcanic area and the lacustrine or marine sequences over distances of at least 1000 km this linkage allows us to correlate the oxygen isotope record of the kazusa group in boso peninsula with the alternating marine non marine lithology of the osaka group in the kinki district radiometric dates from the yabakei and the imaichi pyroclastic flow deposits are concordant with the astronomically calibrated geomagnetic polarity time scale

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

late palaeozoic basins of the southern u s continental interior the timing and geometry of late palaeozoic inhomogenous deformation in the southern u s continental interior from the appalachian foreland of kentucky tennessee through oklahoma and texas to the ancestral rockies of colorado wyoming utah can be definitively linked with a discrete sequence of collisional events in the appalachian ouachita marathon orogenic belt to the south permian coordinates a progressive staged collisional sequence beginning in late mississippian times in the southern appalachians and culminating in early permian times in the marathons led to a progressive deformation of the adjacent craton in a wide swath dominated by right lateral shear e g rough creek fault zone by which a mexican promontory of north america was displaced towards a pacific free face while the deformation in the appalachian marathon belt was dominated by vertical plane strain leading to crustal thickening the associated continental interior deformation can be averaged as a horizontal plane strain at or near sea level with localized deep extensions delaware and flexural arkoma basins and compressions uplifts amarillo wichita giving local source areas

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

early post collisional granitic and coeval mafic magmatism of medium to high k tholeiitic affinity within the neoproterozoic southern brazilian shear belt abstract the post collisional stage of the brasiliano pan african orogenic cycle in the garopaba region southern brazil began at 630 620 ma by the emplacement of large granitic massifs with associated mafic rocks the paulo lopes suite pls is composed of metaluminous to slightly peraluminous biotite hornblende paulo lopes and garopaba granitoids and by the silveira gabbro containing augite pigeonite and orthopyroxene mafic microgranular enclaves and mutual intrusive relations attest to the coeval character of mafic and felsic magmas the pls intruded during regional transcurrent shearing and microstructures indicative of high temperature solid state deformation are described in the mafic and granitic rocks mafic rocks are medium to high k tholeiitic and show mineralogical and geochemical features including isotopic ones that are compatible with magmas from sources modified by previous lithospheric subduction such as oib em1 the pls granitic magmas are interpreted as evolved from a mafic magma that was compositionally equivalent to the silveira gabbro contaminated by crustal melts or by more complex mechanisms hence they may be considered as granitic magmas of tholeiitic affinity transcurrent shearing seems to be essential for promoting the generation of mantle derived magmas and their interaction with crustal melts with consequent generation of abundant granitoids basic magmas have resulted from melting of heterogeneous mantle reservoirs and their highly evolved character is indicated by their low contents of mgo ni and cr the compositional similarity of pls granitoids particularly the garopaba granitoids with a type granitoids discriminates them from the medium to high k granitoids of mature magmatic arcs and this difference probably reflects the tholeiitic character of their primary magmas typical of the early magmatic stages of post collisional settings in southern brazil trace element and isotopic geochemical data indicate subduction modified oib mantle sources for the mafic magmas which is coherent with the post collisional character of this magmatism

relationships between deformation and basin evolution in the intracratonic amadeus basin central australia abstract tectonic subsidence curves based on the sedimentary record are used here to study basin evolution and to indicate deformational periods in the history of the intracratonic amadeus basin in central australia the basin developed during an initial period of extension at about 900 ma in the late proterozoic stage 1 was subjected to another period of extension at about proterozoic cambrian boundary time when there was also a shortening event on the southwest margin stage 2 and was shortened during the late devonian early carboniferous stage 3 stage 1 extension in the southwest and northwest corners of the basin was associated with rift basin sediments and volcanics previously regarded as basement to the amadeus succession in the metamorphic arunta complex on the northern margin of the basin a dolerite dyke swarm is considered to have been emplaced during this event extension in a n s direction associated with stage 2 occurred only in the northern part of the basin at the same time a major compressional event the petermann ranges orogeny was occurring along the southern margin in the north the sediment fill at this time firstly thickens rapidly towards the northern margin and then thins dramatically at the margin implying a half graben structure and the presence of a major bounding fault in the southwest the petermann ranges orogeny caused detachment of the sediments above the salt horizon in the bitter springs formation northward transport folding of the sediment pile and formation of major basement cored nappes during stage 3 a foreland basin associated with southward directed overthrust sheets of the alice springs orogeny formed in the northern part of the basin horizontal shortening was of the order of 50 100 km and hence the present basin margins are structural rather than depositional the structures that developed during this stage dominate the structural pattern of the basin