seismic structure of the crust and uppermost mantle of north america and adjacent oceanic basins a synthesis we present a new set of contour maps of the seismic structure of north america and the surrounding ocean basins these maps include the crustal thickness whole crustal average p wave and s wave velocity and seismic velocity of the up permost mantle that is pn and sn we found the following 1 the average thickness of the crust under north america is 36 7 km standard deviation s d 8 4 km which is 2 5 km thinner than the world average of 39 2 km s d 8 5 for continental crust 2 histograms of whole crustal p and s wave velocities for the north amer ican crust are bimodal with the lower peak occurring for crust without a high velocity 6 9 7 3 km sec lower crustal layer 3 regions with anomalously high average crustal p wave velocities correlate with precambrian and paleozoic orogens low average crustal velocities are correlated with modern extensional regimes 4 the average pn velocity beneath north america is 8 03 km sec s d 0 19 km sec 5 the well known thin crust beneath the western united states extends into north west canada 6 the average p wave velocity of layer 3 of oceanic crust is 6 61 km sec s d 0 47 km sec however the average crustal p wave velocity under the eastern pacific seafloor is higher than the western atlantic seafloor due to the thicker sediment layer on the older atlantic seafloor

new york alabama lineament a buried right slip fault bordering the appalachians and mid continent north america the new york alabama ny al lineament recognized in 1978 is a magnetic anomaly that delineates a fundamental though historically enigmatic crustal boundary in eastern north america that is deeply buried beneath the appalachian basin data not in the original aeromagnetic data set particularly the lack of any information available at the time to constrain the southern continuation of the anomaly southwest of tennessee left the source of the lineament open to conjecture we use modern digital aeromagnetic maps to fill in these data gaps and for the first time constrain the southern termination of the ny al lineament our analysis indicates that the lineament reflects a crustal scale right lateral strike slip fault that has displaced anomalies attributed to grenville orogenesis by 220 km palinspastic restoration of this displacement rearranges the trace of the grenville belt in southern rodinia and implies only passive influence on later formed appalachian structures the precise timing of dextral movement on the ny al structure is not resolvable from the existing data set but it must have occurred during one of or combinations of the following events 1 a late postcontractional post ottawan stage of the grenville orogeny 2 late neoproterozoic to cambrian rifting of laurentia or 3 right slip reactivation during the late neoproterozoic cambrian rifting of laurentia or during appalachian movements our palinspastic reconstruction also implies that the host rocks for modern earthquakes in the eastern tennessee seismic zone are metasedimentary gneisses and it provides an explanation for the spatial location and size of the seismic zone

sediment source tracing using environmental magnetism a new approach with examples from australia environmental magnetism can be used for sediment source tracing in situations such as stream junctions where two catchments deliver magnetically distinguishable well combined average source mixes to a trunk stream consistent mineral magnetic parameter relationships show that spatially and temporally constant magnetic mineral assemblages occur along reaches of natural streams for most of the confluence mixing situations examined it is possible to distinguish between tributaries using magnetic parameter relationships using these relationships relative tributary contributions to the resultant downstream mix can be quantified the method can be used for tracing suspended and bedload sediments so that the principal source catchments can be determined by a sequence of measurements at stream junctions along a drainage network there is considerable potential for using this new and easily applied method for studies of sediment delivery processes and sediment movement in fluvial environments

automatic interpretation of gravity gradiometric data in two dimensions vertical gradient1 the magnetic and gravity field produced by a given homogeneous source are related through poisson s equation starting from this consideration it is shown that some 2d interpretation tools widely applied in the analysis of aeromagnetic data can also be used for the interpretation of gravity gradiometric data vertical gradient this paper deals specifically with the werner deconvolution analytic signal and euler s equation methods after a short outline of the mathematical development synthesized examples have been used to discuss the efficiency and limits of these interpretation methods these tools could be applied directly to airborne gravity gradiometric data as well as ground gravity surveys after transformation of the bouguer anomalies into vertical gradient anomalies an example is given of the application of the werner deconvolution and euler s equation methods to a microgravity survey

grenville basement structure associated with the eastern tennessee seismic zone southeastern usa the eastern tennessee seismic zone extends more than 300 km from as far north as southeastern kentucky southward into alabama southeastern united states we propose that a large scale shear zone which originated as a continental transform fault during the grenville orogeny and assembly of supercontinent rodinia constitutes the framework for earthquake activity in the eastern tennessee seismic zone this new seismotectonic model is based on a diverse set of geophysical and geological observations including paleomagnetic and isotopic constraints on the growth of southeastern laurentia during the grenville orogeny

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

particle size related mineral magnetic source sediment linkages in the rhode river catchment maryland usa characterization of both the source materials and the bottom sediments of an estuary by magnetic characters on a particle size basis allows the source of the bottom sediments to be determined and indicates a shift in sediment sources corresponding to changing land use patterns the major source of the sediments in the lower part of cores from the middle of the estuary is eroding shorelines soil derived particulates from the uplands dominate the upper part of the cores this change in source occurred in the early part of the 19th century the magnetic character of the most recent sediments suggests a progressive depletion through time of the upland soils in the finest fractions

crustal velocity structure associated with the eastern tennessee seismic zone vp and vs images based upon local earthquake tomography we present three dimensional p and s wave velocity models for the active eastern tennessee seismic zone etsz using arrival time data from more than 1000 local earthquakes a nonlinear tomography method is used that involves sequential inversion for model and hypocenter parameters we image several velocity anomalies that persist through most of the inversion volume some anomalies support the presence of known features such as an ancient rift zone in northern tennessee other anomalies reveal the presence of basement features that can be correlated with regional gravity and magnetic anomalies we image a narrow ne sw trending steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the prominent new york alabama magnetic lineament the low velocity zone may have an apparent dip to the se at depths exceeding 15 km the low velocity zone is interpreted as a major basement fault juxtaposing granite rhyolite basement to the nw from grenville southern appalachian basement to the se relocated hypocenters align in near vertical segments suggesting reactivation of a distributed zone of deformation associated with a major strike slip fault we suggest that the etsz represents reactivation of an ancient shear zone established during formation of the super continent rodinia

seismotectonics of the southern appalachians abstract the southern appalachians of eastern tennessee and western north carolina is a region for which high quality instrumental seismic data have been very limited in this paper we report more than 100 new or relocated hypocenters obtained from the initial 2 1 2 yr of operation of the southern appalachian regional seismic network sarsn a 16 station four state telemetered network of short period vertical instruments these data provide a first order snapshot of southern appalachian seismicity that confirms some previous observations and adds new results for the region the most important new observations are well constrained focal depths and focal mechanisms for southern appalachian earthquakes the level of activity exceeds expectations and concentrates more heavily in the valley and ridge province of eastern tennessee as opposed to the blue ridge or inner piedmont than is evident in historical listings the large majority of these events lie between the new york alabama lineament and the clingman ocoee lineaments magnetic anomalies produced by deep seated basement structures an analysis of the uniqueness and stability of hypocentral depth yields reliable depths for 58 earthquakes and demonstrates that most southern appalachian seismic activity occurs beneath the decollement thus lending quantitative support to the argument that appalachian seismicity is unrelated to surficial geological or tectonic features several new focal mechanisms of a higher quality than previously available indicate that the basement of the valley and ridge is under ne sw compression and the principal mode of faulting is right lateral strike slip on nearly north south faults in both depth and type of faulting the seismogenic model we propose for the southern appalachians closely emulates the giles county zone to the north of our study area

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

paleorift structure constrained by gravity and stratigraphic data the statherian arai rift case abstract gravimetric and stratigraphic data were used to investigate the paleoproterozoic arai paleorift a failed statherian continental rift located in the western margin of the sao francisco craton where basement and cover were affected by the neoproterozoic brasiliano orogeny euler deconvolution tilt total horizontal gradient amplitude and upward continuation technics were applied to terrestrial gravimetric data in order to investigate the rift s main faults location direction and depth allowing to identify its main horsts grabens volcanic and plutonic centers we found that rift faults occur to a maximum depth of ca 38 km but major fault throw occurs from 4 to 8 km deep and attenuates from 8 to 12 km probably the brittle ductile transition zone at the time of rifting practically disappearing at 20 km stratigraphic data and basement mapping were used in order to constraint gravimetric results we classify the arai rift as a passive three armed failed rift narrow to divergent type that produced preferably anorogenic rapakivi related magmas most of it still lodged in the crust from surface down to ca 19 km deep and subsidiary mafic magmatism the results indicate the deep occurrence of low density magmas beneath the rift s main axis detected up to 20 km deep correlation to other global statherian rifts show that the sao francisco craton was strongly affected by taphrogenesis during the statherian together with siberia north america and north china cratons finally by comparing our results to recent rifts we found that the ethiopian rift s morphology is quite similar to the arai surrounding the tanzanian craton the cenozoic east africa rift system morphology is compared to the arai espinhaco rift system which surrounds the sao francisco craton the major contribution of this paper is the recognition of arai paleorift surface and subsurface morphology up to now unknown over an area of ca 45 000 km2

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

crustal and upper mantle velocity structure in the vicinity of the eastern tennessee seismic zone based upon radial p wave transfer functions teleseismic transfer function analysis is used to investigate crust and upper mantle velocity structure in the vicinity of the active eastern tennessee seismic zone etsz the etsz is associated with the new york alabama ny al magnetic lineament a prominent aeromagnetic anomaly indicative of grenville age basement structure radial component p wave transfer functions for 10 short period stations operated by the center for earthquake research and information are inverted for velocity structure velocity profiles are also determined for three broadband stations by converting the instrument response to that of an s 13 short period seismometer distinct differences in the velocity profiles are found for stations located on either side of the ny al magnetic lineament velocities west of the lineament are lower than velocities to the east of the lineament in the upper 10 km and in the depth range 30 to 50 km a gradational moho boundary is found beneath several stations located in the valley and ridge province a moho boundary is absent at four valley and ridge stations located east of the magnetic lineament and south of 35 5 n

group velocity tomography of the upper crust in the eastern tennessee seismic zone from ambient noise data abstract the eastern tennessee seismic zone etsz is the second most seismically active area in the central and eastern united states after the new madrid seismic zone but the relatively weak seismicity and the absence of correlation between the seismicity distribution and the surface geology make its seismogenic potential controversial in this work we investigate the structure of the upper crust in the etsz by means of group velocity tomography maps from seismic noise data results show that the seismic activity is associated with a relatively low velocity anomaly mainly located in one or more basement blocks these blocks bounded to the nw by the ny al lineament and to the se by the clingman lineaments are buried beneath low velocity strata consistent with the presence of a relatively thick sedimentary cover the imaged low velocity anomaly migrates towards the se at increasing periods suggesting a possible se dipping weak structure where most of the seismic activity takes place the correlation between the ny al magnetic signature and the position of the seismic velocity anomalies supports the interpretation of the low velocity zone as a major basement fault projected to the surface as the ny al magnetic lineaments the fault juxtaposes granite rhyolite basement to the nw with grenville southern appalachian basement to the se

cumulatic diorites related to post collisional brasiliano pan african mafic magmatism in the vila nova belt southern brazil abstract the capivaras diorite in the vila nova region nw of the sul rio grandense shield is composed of six ne to nne oriented rock bodies of late tectonic emplacement relative to the d3 deformation phase which forms subvertical high strain zones in basement gneiss sequences within these intrusive bodies a shape foliation is present generally parallel to contacts and displaying a local solid state deformational component the internal structure of the capivaras diorite main intrusion is marked by a zone of intense flow and mingling characterized by strong shape foliation and layers of variable texture and composition which result from cumulative processes heterogeneous flow and interaction of coeval compositionally contrasting magmas the central part of this intrusion is texturally homogeneous and slightly foliated even though cumulative processes have remained important during its formation along the contact with basement gneisses fine grained diorites are found which are considered to be compositionally close to the parental magma of the capivaras diorite this magma has a mildly alkaline affinity and shows moderate to high contents of zr ti and p highly fractionated ree patterns low nb contents as well as high contents of k sr ba and rb are suggestive of its provenance from mantle sources which have been previously affected by subduction processes such as those of mature magmatic arcs or post collisional settings magmatic evolution was controlled by cumulative processes and gave origin to pyroxene orthocumulates plagioclase pyroxene orthocumulates pyroxene adcumulates and more rarely plagioclase adcumulates the cumulative origin of these rocks is indicated by field textural and geochemical features which are distinct from those of crystallized liquids the compositional diversity of cumulates has led to the generation of compositionally different melts the early formed pyroxene cumulates have caused ca al na ba and sr enrichment in the magmatic liquid leading to plagioclase crystallization and accumulation coarse grained mafelsic cumulates were formed during the late stages of magmatic crystallization due to volatile enrichment of the intercumulus liquid considering geological relations as well as tectonic and compositional features of the capivaras diorite it is interpreted as part of neoproterozoic magmatism related to the post collisional stage of brasiliano pan african orogenic cycle in southern brazil

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

characterisation of soils by mineral magnetic measurements processes of iron mineral authigenesis diagenesis and dissolution in interaction with primary inputs of iron minerals act to produce vertical differentiation of soil magnetic properties thus magnetic iron oxides in common with other iron forms may both respond to and reflect soil forming processes investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance in several areas of environmental study for example the observed magnetic variation can be used to discriminate between individual soils and soil horizons for the purposes of soil surveying sediment tagging and tracing and empirical modelling of source sediment linkages this paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance the effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters including magnetic susceptibility frequency dependent susceptibility and anhysteretic and saturation remanences contrary to current pedological thought but in accord with other magnetically based studies e g mullins the presence of magnetite maghemite typically of superparamagnetic single domain grain size within soils has been found to be an extremely widespread phenomenon its contemporary neoformation within the soil environment is inferred however the input of artificially generated ferrimagnetic material is indicated for one of the magnetically enhanced soils examined here and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic processes of magnetic enhancement are inferred from the data presented discrimination of industrially derived magnetic particles appears possible on the basis of their very low levels of frequency dependent susceptibility fd low anhysteretic remanence arm and dissociation from the finer clay sized not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons within gleyed waterlogged soils processes of magnetite dissolution appear to be grain size specific preferential reduction of those grains of ultrafine superparamagnetic and fine single domain magnetic grain size is indicated where soil forms a major contributor to catchment sediment loads this has significance in the area of source sediment modelling soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments further only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record

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

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

bacterial magnetite and the magnetic properties of sediments in a swedish lake a comparison of the magnetic properties of lake sediments and catchment materials from pajep njakajaure north sweden demonstrates that high concentrations of single domain sd magnetite in the upper 70 cm of gyttja in the deepest part of the basin are not of detrital origin tem and light microscope examination revealed the existence of bacterial cells and magnetosomes in the upper sediments which are responsible for the presence of the sd magnetite most of the fossil magnetosomes appear to have been dissolved through reductive processes at lower depths although some are preserved in the lower sediments and carry a post depositional remanent magnetization pdrm the results highlight the complex effects of post depositional processes on mineral magnetic assemblages and natural remanent magnetization aquisition and demonstrate how mineral magnetic measurements may be used to identify magnetic grain size changes brought about by sediment diagenesis

tracer properties of eroded sediment and source material this paper examines the conservativeness of tracers through the sediment generation process this is done by comparing a selection of tracer properties of sediment eroded from large plots by simulated rainfall with the corresponding properties of the source materials within the plots sediment was generated using three simulated rainfall events for each of five selected erosion source types in the tarago catchment victoria australia as there were particle size and organic content differences between the source material and the generated sediment the measured tracer properties of the source material were corrected for these differences the possible role of analytical errors in this investigation was also addressed the geochemical property concentration of fe2o3 was not conservative for any of the process sources investigated concentration of al2o3 was not conservative for three of the four process sources investigated and the sum of molecular proportions of cao na2o k2o and al2o3 was not conservative for two of the four process sources investigated mineral magnetic properties irm850 and were also found to be not conservative although this may be the result of the complex relationship between particle size and mineral magnetic properties not being adequately accommodated in this analysis the radionuclide tracers 137cs and 210pbex were found to be conservative through the sediment generation process copyright 2002 john wiley sons ltd

whole rock pb and sm nd isotopic constraints on the growth of southeastern laurentia during grenvillian orogenesis the conventional view that the basement of the southern and central appalachians represents juvenile mesoproterozoic crust the final stage of growth of laurentia prior to grenville collision has recently been challenged new whole rock pb and sm nd isotopic data are presented from mesoproterozoic basement in the southern and central appalachians and the granite rhyolite province as well as one new u pb zircon age from the granite rhyolite province these data combined with existing data from mesoproterozoic terranes throughout southeastern laurentia further substantiate recent suggestions that the southern and central appalachian basement is exotic with respect to laurentia sm nd isotopic compositions of most rocks from the southern and central appalachian basement are consistent with progressive growth through reworking of the adjacent granite rhyolite province however pb isotopic data including new analyses from important regions not sampled in previous studies do not correspond with pb isotopic compositions of any adjacent crust the most distinct ages and isotopic compositions in the southern and central appalachian basement come from the roan mountain area eastern tennessee western north carolina the data set indicates u pb zircon ages up to 1 8 ga for igneous rocks inherited and detrital zircon ages 2 0 ga sm nd depleted mantle model t dm ages 2 0 ga and the most elevated 207 pb 204 pb observed in southeastern laurentia the combined u pb geochronologic and sm nd and pb isotopic data preclude derivation of southern and central appalachian basement from any nearby crustal material and demonstrate that grenville age crust in southeastern laurentia is exotic and probably was transferred during collision and assembly of rodinia these new data better define the boundary between the exotic southern and central appalachian basement and adjacent laurentian granite rhyolite province

the effects of particle breakage and abrasion on the magnetic properties of two soils as mineral magnetism is used as a tracer for sourcing river and dam sediments changes in the magnetic properties that may occur during transport between the source and sink must be considered abrasion and breakage of particles will occur during transport these processes were examined in simulation experiments with a granitic and a sedimentary soil the effects of these processes on the magnetic properties of a granitic and a sedimentary soil were examined using a simulated breakage abrasion experiment breakage and or abrasion had substantial effects on the magnetic properties of both soils all particle sizes were affected but the magnitude varied through the size range of derived particles the major effect was on the concentration of magnetic minerals with differences between the concentrations in particle sizes of the original material and those generated by the experiments being as much as 20 times the effect on the granite soil was to reduce the concentrations i e derived material was less than original material whereas for the sedimentary soil the derived particles had higher concentrations the effect on magnetic grain size as indicated by the magnetic ratios was less than the effect on the mass magnetic properties but still substantial for some ratios for some sizes 1998 john wiley sons ltd

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

the effects of particle breakage and abrasion from simulated fluvial transport on phosphorus sorption by two soils phosphorus sorption studies were carried out on particle size fractions of soils collected from the walls of gullies through a granitic and a sedimentary soil as well as on particle size classes derived from breakage and abrasion of the 500 to 1400 m components of these soils sorption of phosphorus by the particle size classes of the sedimentary soil was much greater than those of the granite soil and this also applied to the particles derived from breakage and abrasion for the original sedimentary soil sorption of phosphorus by the particle size components was strongly associated with the iron content and less strongly associated with the aluminium content and this also applied to the particles derived from breakage and abrasion for the granitic soil these relationships were much less precise a period of vigorous mixing after 165 hours of gentle mixing caused release of a proportion of the sorbed phosphorus from all particle sizes of the original granitic soil and for most of the breakage abrasion derived particles the released phosphorus was re sorbed during a subsequent 48 hour period of settling relationships were evident between labile p and sorbed p for the particle size classes of the soils and treatments tested bacause particle size lithology of the transported sediments and flow dynamics affect the distribution of phosphorus between water and sediments they can also be expected to effect bio availability of phosphorus particle breakage and abrasion during sediment transport is another factor likely to influence the bioavailability of p

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

semi automatic mapping of dyke and dyke related fractures using uav based photogrammetric data a case study from sijiao island coastal southeastern china abstract mapping dyke and dyke related fractures is important for magma propagation and emplacement mechanisms analysis here we propose a semi automatic workflow for dyke and fracture mapping using unmanned aerial vehicle uav based photogrammetric data our workflow integrates mean shift and grabcut in one cut gcoc algorithms to segment dykes and harnesses the phase symmetry algorithm and a series of follow up actions to detect fractures sijiao island of zhejiang province se china is examined as a case study to validate the approaches with our workflow the generated dyke map attains more than a 97 recall rate when compared with the results of expert interpretation and the ratio of the number of detected false fracture segments to the number of total segments is less than 18 the dyke strike and fracture strike calculation match well with field measurements moreover by combining visual interpretation and the resulting geometric information our results manifest that dyke magma invaded along pre existing fractures to form dykes on sijiao island and the difference between dyke fractures and host rock fractures may be because of the effect of major changes of regional stress this workflow has great potential for high resolution terrestrial mapping

sediment tracing in a reservoir catchment system using a magnetic mixing model magnetic mixing models are developed for specific susceptibility and a demagnetisation parameter which demonstrate how the magnetic properties of soils are altered by mixing these phenomena are used to interpret the composition of reservoir sediments in terms of the relative proportions of two types of source materials top soil and sub soil which are eroded from the catchment area and deposited in the reservoir the model provides a black box approach to the reservoir catchment system dependent on four simplifying assumptions which are the subject of current research application of the model to an afforested catchment suggests a marked change in the types of erosion processes associated with the maturation of the forestry plantation over a period of 50 yr

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

petrogenesis of syntectonic granites emplaced at the transition from thrusting to transcurrent tectonics in post collisional setting whole rock and sr nd pb isotope geochemistry in the neoproterozoic quatro ilhas and mariscal granites southern brazil abstract the neoproterozoic post collisional period in southern brazil 650 580 ma is characterized by substantial volumes of magma emplaced along the active shear zones that compose the southern brazilian shear belt the early phase syntectonic magmatism 630 610 ma is represented by the porphyritic high k metaluminous to peraluminous quatro ilhas granitoids and the younger heterogranular slightly peraluminous mariscal granite quatro ilhas granitoids include three main petrographic varieties muscovite biotite granodiorite mbg biotite monzogranite bmz and leucogranite lcg that although sharing some significant geochemical characteristics are not strictly comagmatic as shown by chemical and sr nd pb isotope data the most primitive muscovite biotite granodiorite was produced by contamination of more mafic melts possibly with some mantle component with peraluminous crustal melts the biotite monzogranite although more felsic has higher ca mgo tio 2 and ba and lower k 2 o feo t sr and rb contents possibly reflecting some mixing with coeval mafic magmas of tholeiitic affinity the leucogranite may be derived from pure crustal melts the mariscal granite is formed by two main granite types which occur intimately associated in the same pluton one with higher k 5 6 5 wt k 2 o high rb and lower cao na 2 o ba and zr as compared to the other 3 5 wt of k 2 o the two mariscal granite varieties have compositional correspondence with fine grained granites fgg that occur as tabular bodies which intruded the quatro ilhas granoitoids before they were fully crystallized and are inferred to correspond to the mariscal granite feeders an interpretation that is reinforced by similar u pb zircon crystallization ages the initial evolution of the post collisional magmatism marked by the emplacement of the quatro ilhas granitoids varieties activated sources that produced mantle and crustal magmas whose emplacement was controlled both by flat lying and transcurrent structures the transition from thrust to transcurrent related tectonics coincides with the increase in the proportion of crustal derived melts the transcurrent tectonics seems to have played an essential role in the generation of mantle derived magmas and may have facilitated their interaction with crustal melts which seem to be to a large extent the products of reworking of orthogneiss protoliths

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

volcanic cycles and setting in the neoproterozoic iii to ordovician camaqua basin succession in southern brazil characteristics of post collisional magmatism the camaqua basin comprises a volcano sedimentary succession located in southernmost brazil and represents a molasse basin formed at the post collisional stage of the brasiliano pan african orogenic cycle in the neoproterozoic iii to ordovician period this basin is one of the most well preserved ancient volcano sedimentary sequences undeformed and unmetamorphic in the world dominantly developed on a continental setting under subaerial conditions it is composed of five major stratigraphic units four of them with a distinct volcanic character from the bottom to the top as 1 marica 2 bom jardim 3 acampamento velho 4 santa barbara and 5 guaritas allogroups a concise sight of geochemical and isotopic rock data is presented as well as stratigraphic correlation and description of rock structures and textures that lead to the identification of their genetic processes the aim of this paper indicating a relation with a coeval plutonism and volcanism that evolved from high k calc alkaline to shoshonitic and ended with a silica saturated sodic alkaline magmatism with a crustal component represented by peraluminous granites volcanic deposits from bottom to top are made mostly of volcanogenic sedimentary deposits succeeded by basic to intermediate lava and pyroclastic flows of shoshonitic affinity followed by intermediate and acid lava flows and ignimbrites of sodic alkaline affinity the last volcanic event is represented by basalt pahoehoe flows probably of mildly alkaline sodic affinity

tracing movements of topsoil by magnetic measurements two case studies an array of magnetic measurements is used to trace the occurrence of magnetically enhanced topsoil in two different environmental settings the first is a cultivated slope section where the results confirm that movements of topsoil have occurred but that identification of specific erosional processes is problematic the second is a lake catchment system where measurements on soils and sediments allow a partial reconstruction of soil movements from different sources over the last 200 y the study highlights both the need to analyse the magnetic mineralogies of different particle size fractions and the difficulties that may be encountered in trying to discriminate between topsoil and subsoil material on the basis of demagnetization parameters alone

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

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

lithofacies analysis of basic lava flows of the parana igneous province in the south hinge of torres syncline southern brazil abstract the parana igneous province records the volcanism of the earlier cretaceous that preceded the fragmentation of the gondwana supercontinent historically investigations of these rocks prioritized the acquisition of geochemical and isotopic data considering the volcanic pile as a monotonous succession of tabular flows this work provides a detailed analysis of the emplacement conditions of these basic volcanic rocks applying the facies analysis method integrated to petrographic and geochemical data the torres syncline is a nw se tectonic structure located in southern brazil where a thick sequence of the parana etendeka volcanic rocks is well preserved this study was performed in the south hinge of the syncline where the basaltic lava flows are divided into three lithofacies associations early compound pahoehoe early simple pahoehoe and late simple rubbly the first lavas that erupted were more primitive compound pahoehoe flow fields composed of olivine basalts with higher mgo contents and covered the sandstones of the botucatu formation the emplacement of compound pahoehoe flow fields is possibly related to intermittent low effusion rates whereas the emplacement of simple pahoehoe is related to sustained low effusion rates with continuous supply the thick simple rubbly lavas are associated with high effusion rates and were formed during the main phase of volcanism in the area the absence of paleosoils between the lavas and lithofacies associations suggests that the successive emplacement of the lava flows occurred in a relatively short time gap geochemically the lithofacies associations are low tio2 and belong to gramado magma type the lavas of the south hinge of the torres syncline have a similar evolution when compared to other continental basaltic provinces with earlier compound flows at the base and thicker simple flows in the upper portions

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

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

geological geochemical and isotope diversity of 134 ma dykes from the florianopolis dyke swarm parana magmatic province geodynamic controls on petrogenesis abstract the florianopolis dyke swarm fds one of the major dyke swarms belonging to the early cretaceous 135 131 ma parana magmatic province is largely dominated by high sr ti p basalts that are confirmed here as feeders of the unique urubici khumib lavas of the parana and edendeka lava piles on the basis of their age and geochemistry our study integrates field petrographic whole rock geochemistry and sr nd pb isotope geochemistry of representative samples from three main areas of exposition santa catarina island garopaba and pinheira beaches thus encompassing the whole extension of the fds compared to the urubici lavas the dykes have usually higher contents of lile and lree more radiogenic sr and pb and more unradiogenic nd features attributed to a more pronounced interaction with melts derived from the country rocks registered in the basic magmas that remained in the conduits some of these dykes show strongly interactive contacts that must be part of a wider zone of crustal melting probably more developed at greater depths small volumes of intermediate to acidic rocks form the cores of some composite dykes and correspond to products of fractional crystallization from urubici basalts contaminated with high rb sr and u th crustal melts probably derived from neoproterozoic granites as indicated by geochemical and sr nd pb isotope data the chemical and isotope signatures of the less contaminated fds basalts and related urubici lavas do not show clear evidence of inputs from primitive mantle and seem heavily influenced by enriched mantle this suggests that the mantle wedge that was affected by subduction during the neoproterozoic may have been frozen and coupled to the base of the lithospheric plate where the early cretaceous magmatism occurred a control of previous tectonic limits on the sources of the urubici basalts seems evident since they seem to be related to the younger lithosphere from the south domain related to the florianopolis batholith and no influence from the older cratonic lithosphere of the central domain can be identified in their feeders

nacr14 a 3d model for the crustal structure of the north american continent abstract based on the large number of crustal seismic experiments carried out in the last decades we create nacr14 a 3d crustal model of the north american continent at a resolution of 1 1 we present maps of thickness and average velocities of the main layers that comprise the north american crystalline crust obtained from the most recent seismic crustal models within the usgs crustal structure database however the crustal data are unevenly distributed and in some cases discrepancies exist between published models in order to construct a consistent 3d crustal model with three layers in the crystalline crust we refrained from a direct interpolation of the crustal seismic parameters in the database instead we implemented the following sequence of steps 1 definition of the geometry of the main tectonic provinces of north america 2 selection and evaluation of the reliability of seismic crustal models in the database 3 estimation of the p wave seismic velocity and thickness of the upper middle and lower crust for each tectonic province 4 estimation of the interpolated pn velocity distribution the resulting average velocity of the crystalline crust is mostly consistent with that of the seismic points the main variations of the structure of the crystalline crust of north america displayed in the model can be related to its tectonic evolution the model available in a digital form can be used in various geophysical applications such as the correction for the crustal effects in gravity and seismic tomography and models of dynamic topography in order to detect heterogeneities characterizing the underlying upper mantle

the upper crust of the eastern tennessee seismic zone insights from potential fields inversion abstract the study investigates the crustal structure of the eastern tennessee seismic zone etsz by means of potential field inversion through the located euler deconvolution method inversion of magnetic field data shows that the top of the magnetic basement ranges between 6 and 12 km depth in the valley and ridge physiographic province while it is shallower

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

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

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

granulitic xenoliths from the french massif central petrology sr and nd isotope systematics and model age estimates the protoliths of granulitic xenoliths found in tertiary quaternary alkali volcanics from the french massif central are shown to have been principally immature sediments and calc alkaline intrusives the association may indicate formation of the crust in an island arc or continental margin environment pressure and temperature estimates indicate that they have undergone intermediate to high pressure granulite facies metamorphism 9 15 kbar followed by uplift to lower pressures 4 9 kbar rb sr and sm nd isotopic data and ree abundances are presented for both meta igneous and meta sedimentary suites esr present day values for the granulitic xenoliths range from 2 7 to 325 with end values between 0 2 and 12 0 there are no isochron relationships within the sr and nd data but possible mixing lines in the rb sr system may indicate sedimentation at 700 1000 myr t nd dm model ages indicate a proterozoic 800 2100 myr origin for the protoliths of the meta igneous samples and a similar provenance age for the meta sediments 1986 the geological society