brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

observational evidence for predictions of tropical cyclone propagation relative to environmental steering abstract theories of vortex motion due to the variations of the coriolis parameter and environmental vorticity are compared to observations of tropical cyclone motion relative to computed steering flows using previously published composite data the composite results are manipulated to obtain a vector quantity for the difference between tropical cyclone motion and steering and this vector difference is termed propagation the properties of these propagation vectors within various composite data stratifications provide tentative support for nonlinear numerical results such as i the general magnitude and direction of the induced propagation ii the dependence of such propagation on the outer wind strength of the tropical cyclone and iii the dependence of such propagation on the direction of the environmental vorticity gradient ambiguities in the composite data are discussed with respect to linear and nonlinear theories of tropical cyclone propagation and several new composite data stratific

rapid growth and strain rates inferred from synkinematic garnets penokean orogeny minnesota recent reinterpretations of the early proterozoic penokean orogeny in minnesota invoke a plate tectonic model with characteristics similar to phanerozoic collisional belts deformation during collision produced large flattening strains 89 and the development of a tectonic foliation and progressive metamorphism up to amphibolite facies as indicated by the presence of synkinematic s1 garnets biotite garnet thermometry and plagioclase garnet biotite muscovite barometry give southward increasing temperatures and pressures and indicate burial to depths of 21 25 km similar to medial and internal zones of collisional orogens worldwide gibbs method modeling on zoned synkinematic garnets indicates virtually no change in pressure p or temperature t during garnet growth growth and rotation of garnets at a point in p t space in an actively evolving deformational and thermal regime are surprising and suggest rapid garnet growth rates and high rates of strain during orogeny this is consistent with previous studies that indicate more rapid rates of tectonometamorphic processes in the precambrian relative to younger orogenic belts

new insights into the southern margin of the archean proterozoic boundary in the north central united states based on u pb sm nd and ar ar geochronology abstract new geophysical analysis of the precambrian basement in minnesota iowa wisconsin indicates that an archean proterozoic boundary spirit lake trend previously recognized in nw iowa can be continued eastward into central wisconsin and farther east as the spirit lake tectonic zone sltz to test the age of paleoproterozoic crust south of this structure several subsurface samples of precambrian basement from the north central united states have been analyzed or re examined using modern techniques of u pb sm nd and 40ar 39ar geochronology the results fill in a major data gap for the region and show that all u pb crystallization ages for samples south of the sltz are geon 17 1700 1800 ma bedrock core samples from eastern nebraska are ca 1760 1800 ma and two samples from se south dakota immediately south of the sltz yield ages of 1762 28 ma vermillion and 1733 2 ma elk point xenoliths from impact breccia in the manson structure in north central iowa yield a similar age of ca 1705 30 ma and metagabbro from se minnesota yields an age of 1760 9 ma farther to the northeast zircons from paleoproterozoic gneiss in the basement of manitoulin island only a few km south of the superior craton in ontario also yield a geon 17 age 1714 10 ma sm nd model ages tdm for samples immediately south of the sltz fall in the range 1 9 2 2 ga indicating limited involvement of archean crust in contrast sm nd tdm ages for samples north of the sltz typically range from 2 5 to 3 0 ga for both paleoproterozoic plutons and archean gneisses ion microprobe analyses of zircons from the elk point and manson samples also show the presence of geon 16 overgrowths indicating a strong regional thermal overprint during geon 16 accretion this is supported by mid geon 16 hornblende 40ar 39ar ages for samples from se south dakota and se minnesota although no u pb ages are available for juvenile basement beneath the ca 1760 ma granite rhyolite suite of southern wisconsin south of the sltz sm nd model ages are typically ca 1 9 2 0 ga consistent with basement to the rhyolites being geon 17 in age collectively the data require that most if not all of the paleoproterozoic crust immediately south of the sltz formed during geon 17 and probably represents eastward continuation from colorado through nebraska of the yavapai crustal province in the sw united states penokean geon 18 crustal rocks are limited mainly to northern and central wisconsin east central minnesota and northern michigan these results also show that medium grade 500 c tectonothermal effects of the subsequent geon 16 mazatzal orogeny to the south continue into the north central united states both terranes probably also continue eastward into ontario canada and farther east into protolith of the grenville province

the mara rosa arch in the tocantins province further evidence for neoproterozoic crustal accretion in central brazil the mara rosa volcano sedimentary sequence consists of several nne trending belts of metavolcanic metabasalts to metarhyolites and metasedimentary rocks micaschists quartzites cherts exposed over large areas of the tocantins province a neoproterozoic brasiliano orogenic region in the central part of brazil the supracrustal belts are separated by terrains dominated by metatonalites and metadiorites and intruded by several post orogenic granitic and dioritic bodies these volcano sedimentary and associated metaplutonic rock associations have been previously interpreted as a typical granite greenstone terrain of the archaean basement in this paper we present geochronological and isotopic data for rocks of the metavolcanic metaplutonic rock association samples of a felsic metavolcanic rock from the posse gold mine and a metatonalite yielded u pb zircon ages of 862 8 ma and 856 7 13 ma respectively these are interpreted as crystallization ages of the igneous protoliths titanites from the same metavolcanic rock sample yielded a concordant recrystallization age of 632 4 ma sm nd isotopes for these rocks indicate primitive compositions with nd t of 4 6 and 3 7 and tdm model ages of 1 0 ga a syn tectonic dioritic intrusion has an u pb crystallization age of 630 6 ma and nd t of 1 9 and tdm of 1 0 ga additional sm nd analyses on metasedimentary rocks and post orogenic granites of the mara rosa region yield tdm ages between 1 2 and 1 0 ga the isotopic data combined with preliminary trace element results suggest that the protoliths of the metavolcanic and metaplutonic rocks investigated were formed in an island arc system off the coast of the sao francisco congo continent at around 860 ma recrystallization and deformational ages of 630 ma may represent the timing of final ocean closure and continental collision the rock associations structural pattern geochronological and isotopic characteristics of the rocks from the mara rosa region are very similar to those of neoproterozoic arc terrains exposed 300 km to the southwest in the arenspolis area proving the regional importance of the neoproterozoic crustal accretion event in the central part of brazil

upb and smn edvidence for eburnian and pan african high grade metamorphism in cratonic rocks of southern cameroon abstract u pb analyses of zircons and sm nd mineral and whole rock analyses of samples from southern cameroon document three distinct rock forming or high grade metamorphic events 2900 ma 2050 ma and 600 ma zircons from the ebolowa charnockite in southern cameroon yield an age of 2896 7 ma consistent with published rb sr total rock data for the region sm nd crustal formation ages for these rocks are 3000 ma indicating that the granitoids and ntem series gneisses in the north end of the congo craton formed 2900 ma ago and do not contain major contributions from older archean crust a second metamorphic event of eburnian age is identified from re equilibration of sm and nd in gneisses and metadolerites of the ntem complex at 2050 ma zircons from the younger nyong series gneisses are a complex mixture of cores and overgrowths the overgrowths in most cases formed during a high grade metamorphic event between 2100 and 2000 ma and clearly postdate the ebolowa charnockite the ages of the cores are not precisely known but we believe most represent detrital zircons from 2900 ma protoliths sm nd crustal formation ages for samples of the nyong series gneisses are generally 2500 to 2900 ma indicating that their precursors were apparently derived mainly from the archean craton with variable amounts of younger component sm nd garnet total rock isochrons for metamorphosed mafic rocks in the nyong series yield ages of 2050 ma and zircons from one of them yield an upper intercept age of 2037 10 ma further documenting the existence of a major event with magmatism and charnockitization at 2000 to 2100 ma we interpret the nyong series metasedimentary gneisses as post 2900 ma cratonic cover that was deformed and metamorphosed during the 2000 to 2100 ma eburnian orogeny we cannot define precise limits on the ages of sedimentation preliminary data on metamorphosed intrusive rocks associated with the yaounde series yield u pb concordant ages around 620 ma consistent with sm nd garnet whole rock ages from the same series this dates the pan african tectono metamorphic activity in the southern end mobile belt the pan african lower intercepts of zircons and sm nd garnet whole rock ages from thnyong series are the consequences of this event which is further represented by a 590 ma zircon age from a late syenitic pluton results from cameroon are consistent with recent results from the western part of the congo craton farther south and from the northeastern part of the sao francisco craton in brazil one interpretation of these results is that the congo and sao francisco cratons were joined during the ebumian trans amazonian orogeny 2100 ma and are bordered to the north by a late mesoproterozoic rift basin that was closed during the pan african brasiliano orogeny 600 ma

paleoproterozoic basin development and sedimentation in the lake superior region north america abstract the peneplaned archean craton in the lake superior region was the platform upon which a continental margin assemblage was deposited extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform seas transgressed onto the continent several times and an ocean basin opened south of the present day lake superior island arcs that formed during subduction collided with the craton margin as the ocean basin closed oceanic crust is poorly preserved as a dismembered ophiolite sequence the arc volcanics are preserved as the wisconsin magmatic terranes the collision resulted in a fold and thrust belt known as the penokean orogen to the north of the fold and thrust belt a northward migrating foreland basin the animikie basin developed thick turbidite successions were deposited along the basin axis and terrigenous clastics and lake superior type iron formation were deposited on the shelf along the northern margin of the basin the primary paleoclimatic indicators are 1 glaciogenic rocks at the base of the paleoproterozoic succession in michigan indicating ice house conditions 2 remnants of a paleosol on the glaciogenic rocks indicative of deep weathering probably under subtropical conditions and therefore of greenhouse conditions and 3 carbonate minerals after gypsum halite and anhydrite in stromatolitic dolomite indicative of aridity three second order depositional sequences are bounded by major unconformities and can be correlated throughout the lake superior region

petrological and stable isotope studies of carbonate and sulfide minerals from the gunflint formation ontario evidence for the origin of early proterozoic iron formation abstract the 1 9 ga gunflint formation in the thunder bay district of ontario principally comprises iron formation overlain by an iron poor limestone dolostone member the upper limestone member rocks are virtually unmetamorphosed retaining primary textures except locally around diabase sills the gunflint presents an opportunity to study both unmetamorphosed and metamorphosed iron formation and to study the changes from iron rich to iron poor sedimentation of the carbonate minerals in iron rich units siderite formed at or near the sediment water interface with ankerite and calcite forming later during diagenesis and during metamorphism around sills heaviest 13c values of siderite and of dolomite in the upper limestone member are consistently near 0 throughout the stratigraphic section indicating that marine bicarbonate was the source of carbon a spread of values between 0 and 7 for unmetamorphosed rocks is the result of the incorporation of oxidized organic matter during diagenesis whereas 13c values more negative than 7 are the result of metamorphism the heaviest 18o values from unmetamorphosed rocks 5 3 pdb for siderite 6 1 for ankerite and 6 7 for calcite are considered to represent the original marine composition lighter values to 17 represent isotopic exchange reactions with pore fluids at higher temperatures and or isotopic exchange with 18o depleted meteoric water low s c ratios and a relatively narrow range of positive 34s values in pyrite throughout most of the formation are consistent with bacterial sulphate reduction of seawater containing sulphate at a significantly lower concentration than the modern ocean locally higher sulfide contents and a wider range of 34s values were the product of the introduction of sulfur after deposition it is proposed that the gunflint formation was deposited in a stratified water column with anoxic bottom water the depositional basin had restricted communication with the open ocean and was affected by distal volcanism hydrothermal activity associated with the volcanism provided a large source of dissolved iron and possibly silica which helped buffer o2 and sulphate in water to low levels low concentrations of sulphate limited the generation of h2s that would otherwise have restricted the solubility of iron during periods of increased hydrothermal activity the anoxic oxic water boundary moved upwards permitting transport of iron to the shallow shelf where it was precipitated as siderite iron hydroxides iron silicates or pyrite depending on physico chemical conditions the transition to the overlying limestone marks a decrease in hydrothermal activity with a contraction of the redox boundary

potassic and sodic metasomatism in the southern province of the canadian shield evidence from the paleoproterozoic serpent formation huronian supergroup canada abstract major element geochemistry petrographic observations and microprobe analyses of the paleoproterozoic serpent formation huronian supergroup canada suggest that these strata experienced both potassium and sodium metasomatism major element data plotted in al2o3 cao na2o k2o compositional space form an array nearly identical to data from sub huronian paleosols reported elsewhere suggesting that serpent fluvial systems captured materials from entire weathering profiles potassium metasomatism was concentrated in shales and represents the illitization of kaolinite which is a residual product of plagioclase weathering sodium metasomatism principally affected silt and sand sized particles microprobe analyses of plagioclase grains indicate that they are nearly pure albite ab96 corresponding whole rock major element data plot in bulk albite space on an al2o3 k2o cao na2o ternary diagram whereas basement rocks plot in bulk andesine and oligoclase space end member compositions and the absence of luminescence when bombarded with an electron beam indicate that the albites are of secondary origin extremely pure albite cements ab99 commonly fringe detrital grains there has been little replacement of detrital k feldspars despite the complete albitization of plagioclase grains the timing of metasomatism at least the last stage is constrained to the period between 1700 and 1750 ma based on geochronologic data reported elsewhere when the southern province of the canadian shield experienced considerable felsic plutonism possibly related to arc collision this plutonism may have resulted in the heating of deep basinal brines stresses associated with orogenic build up or gravity driven groundwater circulation working in concert with the plutonism expelled these brines through the serpent formation and the entire huronian supergroup thoroughly metasomatizing the region such processes associated with collisional tectonics are akin to the development of mississippi valley type pb zn mineralization in the mid continent of the united states

palaeogeographical reconstructions of the pan african brasiliano orogen closure of an oceanic domain or intracontinental convergence between major blocks abstract a reconstruction of the pan african brasiliano orogen has become possible thanks to new data from the southern pan african belt and by using published data geological mapping which included structural thermobarometric and geochemical studies in togo and neighbouring countries have enabled to define the existence of a preorogenic continental rift whose closure followed soon after its opening continental crust convergence induced an oblique collision that caused nappe stacking this involved external nappes thrust over the west african craton intermediate nappes including metabasic rocks with eclogitic assemblages and internal nappes composed of highgrade anatectic gneiss and intrusive granite and charnockite comparison with data from other regions hoggar adrar des iforas northeastern brazil nigeria central africa has led to the recognition of a pre orogenic continental rift that bordered the west africa sao francisco congo cratons in the southern part of the chain farther north in the hoggar and adrar des iforas this continental rift opened on a wider oceanic domain which involved a passive margin along the west african craton and an active margin to the east after these rifting and subduction stages continental collision followed that was dominated by strike slip movement oblique to the plate boundaries the general organization of the pan african brasiliano belt can be considered as the result of a confrontation between three major continental domains these were the west african and sao francisco congo cratons and a reworked shield composed in its inner part of polycyclic basement rocks and bounded by a fringe of monocyclic metasedimentary thrust belts that faced the cratons

tropical cyclones in the southwest pacific spatial patterns and relationships to southern oscillation and sea surface temperature abstract an analysis of a 20 yr dataset of tropical cyclones in the southwest pacific has been made to determine the spatial patterns of cyclone occurrence and how these depend on the southern oscillation so and sea surface temperature a local measure of cyclone incidence is defined as the number of cyclones that enter into a 2 lat long square during some period eg within one season and this quantity is mapped for four southern oscillation index soi categories the maps show that the geographical distribution of cyclone incidence shifts eastward and northward during negative soi phases and vice versa the mean annual incidence for the whole region is 28 higher than average for our strongly negative so category and 16 below average for our near zero category to explore the roles of the so and local sst correlation analysis was applied to mean values of cyclone incidence for four subregions of 20 longitude width between 10 and 22 s markedly different soi sst responses were found on either s

the chengwatana volcanics wisconsin and minnesota petrogenesis of the southernmost volcanic rocks exposed in the midcontinent rift the southernmost exposed rocks of the north american midcontinent rift system 1100 ma consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the st croix river in minnesota and wisconsin the flows are mostly high fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene abundant secondary chlorite epidote and actinolite indicate the group was metamorphosed to greenschist facies 350 c low sodium m4 site and tetrahedral aluminum aliv contents of actinolite indicate low pressure metamorphism 0 25 gpa and imply a geothermal gradient of 45 50 c km low magnesium mg 0 37 0 58 and ni contents 36 185 ppm indicate the basalts have undergone significant fractionation and are not primary mantle melts incompatible element abundances are inversely correlated with mg and most samples plot within either high or low trace element groups e g ti p zr the basalts are enriched in the light rare earth elements and th and are v

where are the eburnian transamazonian collisional belts the reconstruction of early proterozoic crustal evolution and geodynamic environments in africa and south america is incomplete if cratonic areas alone are studied if the presence of high grade

geochemical and nd pb isotopic evidence for the provenance of the early proterozoic virginia formation minnesota implications for the tectonic setting of the animikie basin the early proterozoic animikie basin is located at a major crustal boundary between archean crust of the superior craton and penecontemporaneous early proterozoic crust of the wisconsin magmatic terranes the provenance of sedimentary rocks within the animikie basin constrains the tectonic evolution of the early proterozoic margin of the superior craton published geological petrographic and nd pb isotopic data indicate that the pokegama quartzite lower passive margin unit of the animikie group has a late archean provenance in contrast the virginia formation upper deep water turbidite shale unit of the animikie group was derived from dominantly early proterozoic sources with isotopic compositions similar to igneous rocks of the wisconsin magmatic terranes shales of the virginia formation have nd depleted mantle model ages 2 14 to 2 35 ga and mu 1 of 7 81 to 8 09 ashes within the virginia are more evolved geochemically than shales and have younger model ages of 1 86 and 1 99 ga and simila

the penokean orogeny in the lake superior region abstract the penokean orogeny began at about 1880 ma when an oceanic arc now the pembine wausau terrane collided with the southern margin of the archean superior craton marking the end of a period of south directed subduction the docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back arc extension both in the initial arc and adjacent craton margin to the north a belt of volcanogenic massive sulfide deposits formed in the extending back arc rift within the arc synchronous extension and subsidence of the superior craton resulted in a broad shallow sea characterized by volcanic grabens menominee group in northern michigan the classic lake superior banded iron formations including those in the marquette gogebic mesabi and gunflint iron ranges formed in that sea the newly established subduction zone caused continued arc volcanism until about 1850 ma when a fragment of archean crust now the basement of the marshfield terrane arrived at the subduction zone the convergence of archean blocks of the superior and marshfield cratons resulted in the major contractional phase of the penokean orogeny rocks of the pembine wausau arc were thrust northward onto the superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 ma in the south baraga group rocks and 1835 ma in the north rove and virginia formations a thick succession of arc derived turbidites constitutes most of the foreland basin fill along with lesser volcanic rocks in the southern fold and thrust belt tectonic thickening resulted in high grade metamorphism of the sediments by 1830 ma at this same time a suite of post tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the penokean orogeny the penokean orogen was strongly overprinted by younger tectonic and thermal events some of which were previously ascribed to the penokean principal among these was a period of vertical faulting in the archean basement and overlying paleoproterozoic strata this deformation is now known to have post dated the terminal penokean plutons by at least several tens of millions of years evidence of the penokean orogen is now largely confined to the lake superior region comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive penokean orogen have been removed or overprinted by younger tectonic events

diagnosis of the mjo modulation of tropical cyclogenesis using an empirical index the modulation of tropical cyclone activity by the madden julian oscillation mjo is explored using an empirical genesis potential gp index composite anomalies of the genesis index associated with the different mjo phases are consistent with the composite anomalies in tc genesis frequency that occur in the same phases indicating that the index captures the changes in the environment that are at least in part responsible for the genesis frequency changes of the four environmental variables that enter the genesis potential index the midlevel relative humidity makes the largest contribution to the mjo composite gp anomalies the second largest contribution comes from the low level absolute vorticity and only very minor contributions come from the vertical wind shear and potential intensity when basin integrated mjo composite anomalies of the gp index are regressed against basin integrated composite anomalies of tc genesis frequency the results differ quantitatively from those obtained from the analogous calculation performed on the annual climatologies in the two quantities the gp index captures the mjo modulation of tc genesis to a lesser degree than the climatological annual cycle of genesis to which it was originally tuned this may be due to weaknesses of the reanalysis or indicative of the importance of precursor disturbances not well captured in the gp index computed from weekly data to the intraseasonal tc genesis frequency fluctuations

geochronology of the proterozoic basement of southwesternmost north america and the origin and evolution of the mojave crustal province the proterozoic baldwin gneiss in the central transverse ranges of southern california a part of the mojave crustal province is composed of quartzofeldspathic gneiss and schist augen and granitic gneiss trondhjemite gneiss and minor quartzite amphibolite metagabbro and metapyroxenite sensitive high resolution ion microprobe shrimp data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 12 and 1675 19 ma adjacent to or through thinned archean crust high u th rims on zircons in most samples suggest an early metamorphic event at 1741 ma but peak amphibolite facies metamorphism and penetrative west vergent deformation occurred after 1675 ma the baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a proterozoic shelf slope sequence joshua tree terrane we hypothesize that emplacement of this regional allochthon occurred during a late early or middle proterozoic arc continent collision along the western margin of laurentia

tropical cyclone climatology of the south pacific ocean and its relationship to el nino southern oscillation abstractclimatological features of tropical cyclones in the south pacific ocean have been analyzed based on a new archive for the southern hemisphere a vortex tracking and statistics package is used to examine features such as climatological maps of system intensity and the change in intensity with time average tropical cyclone system movement and system density an examination is presented of the spatial variability of these features as well as changes in relation to phase changes of the el nino southern oscillation phenomenon a critical line is defined in this study based on maps of cyclone intensity to describe the statistical geographic boundary for cyclone intensification during el nino events the critical line shifts equatorward while during la nina events the critical line is generally displaced poleward regional variability in tropical cyclone activity associated with el nino southern oscillation phases is examined in relation to the variability of large scale atmospheric or oceanic varia

systematic variation of summertime tropical cyclone activity in the western north pacific in relation to the madden julian oscillation abstract the variability of observed tropical cyclone tc activity i e genesis track and landfall in the western north pacific wnp is examined in relation to the various categories of the madden julian oscillation mjo during summer june september for the period 1979 2004 the mjo categories are defined based on the empirical orthogonal function analysis of outgoing longwave radiation data the number of tcs increases when the mjo related convection center is located in the wnp the axis of a preferable genesis region systematically shifts like a seesaw in response to changes in the large scale environments associated with both the eastward and northward propagation of the mjo and the intraseasonal variability of the wnp subtropical high furthermore the authors show that the density of tc tracks in each mjo category depends on the systematic shift in the main genesis regions at first order also the shift is affected by the prevailing large scale steering flows in each mjo category when th

electrical conductivity and paleo proterozoic foredeeps sedimentary facies associated with paleo proterozoic foredeeps are apparently highly conductive the conductive lithology appears to be a euxinic carbonaceous shale deposited on preforedeep continental margin facies the shale is overlaid by axial turbidites and often incorporated into the succeeding fold and thrust belt carbonaceous shales can be made highly conducting if graphitized by exposure to temperatures exceeding 400 c although foredeep facies develop diachronously the most conductive unit is usually found in the core of the fold and thrust belt suggesting that deformation may be essential for the complete graphitization of the carbon the foredeep hypothesis offers an attractive explanation for several highly conductive features detected in the upper and middle crust in laurentia and possibly throughout the world the euxinic facies represent highly specific tectonic marker horizons that are readily delineated by using electromagnetic methods

the age of the gunflint formation ontario canada single zircon u pb age determinations from reworked volcanic ash the gunflint formation a paleoproterozoic chemical clastic sedimentary assemblage outcropping to the immediate northwest of lake superior became famous in 1954 as containing the oldest fossil assemblage known at that time older microfossils have since been discovered but the gunflint procaryotes remain one of the most diverse precambrian fossil communities the finding of possible multicellular organisms in correlative lithic units in michigan has recently added to the need for an exact age of the formation zircons were extracted from rainout and storm reworked volcanoclastic beds in the upper portion of the gunflint formation a euhedral zircon population has yielded a 1878 3 1 3 million years bp u pb age believed to be nearly synchronous with the depositional age this not only provides a precise age for the community of organisms but also strongly supports a back arc extensional setting for the animikie basin rather than a foreland trough resume la formation de gunflint un assemblage sedimentaire chimique et clastique datant du paleoproterozoique affleurant immediatement au nord ouest du lac superieur est devenue celebre en 1954 car elle contenait les plus an ciens assemblages fossiliferes connus a cette epoque de plus anciens microfossiles ont ete decouverts depuis mais les procaryotes de gunflint demeurent l une des communautes fossiles precambriennes les plus diversifiees la decouverte

static recrystallization and preferred orientation of phyllosilicates michigamme formation northern michigan usa abstract the michigamme formation of the marquette district in michigan s upper peninsula comprises a sequence of cleaved rocks of increasing metamorphic grade because metamorphism in the area occurred after cleavage formation the rocks provide an opportunity to study preferred orientation development of phyllosilicates under conditions of static recrystallization x ray texture goniometry on samples from the greenschist facies zone that were collected at varying distances from the bounding biotite in and garnet in isograds shows that 1 the preferred orientation of phyllosilicates is always parallel to the mesoscopic cleavage and 2 the degree of preferred orientation of phyllosilicates improves as a function of increasing metamorphic grade from 9 m r d scanning electron microscopy on these samples shows that 1 the length width ratio increases with increasing grade and 2 grain shapes are better defined with increasing grade previous work on slates showed mechanical processes dominate at very low grade metamorphism whereas chemical processes are favored at higher grades the michigamme samples show that improvement of preferred orientation occurrred by grain dissolution and crystallization noncleavage parallel phyllosilicate grains were preferentially dissolved probably facilitated by internal strain energy from mineral defects aided by chemical energy whereas cleavage parallel phyllosilicates were hosts for new growth along their basal planes these results show that significant fabric strengthening can be achieved by grain dissolution and crystallization in the absence of tectonic stress

sudden tropical cyclone track changes over the western north pacific a composite study abstracttropical cyclones tcs over the western north pacific wnp are usually embedded in the multitime scale summer monsoon circulation and occasionally experience sudden track changes which are currently a challenge in tc forecasting a composite analysis of 15 sudden north turning cases and 14 west turning cases that occurred during the period 2000 10 was conducted with a focus on influences of low frequency monsoon circulations it is found that tcs in the two specific categories of track changes are embedded in a monsoon gyre of about 2500 km in diameter on the quasi biweekly oscillation qbw time scale which is also embedded in a larger scale cyclonic gyre or monsoon trough on the madden julian oscillation mjo time scale the two types of track changes are closely associated with interaction between low frequency and synoptic flows two different types of asymmetric flow patterns are identified on the synoptic time scale in the vicinity of these tcs in the north turning case enhanced winds

tectono stratigraphic evolution of the early proterozoic wisconsin magmatic terranes of the penokean orogen the early proterozoic penokean orogen developed along the southern margin of the archean superior craton the orogen consists of a northern deformed continental margin prism overlying an archean basement and a southern assemblage of oceanic arcs the wisconsin magmatic terranes the south dipping niagara fault suture zone separates the south facing continental margin from the accreted arc terranes the suture zone contains a dismembered ophiolite the wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure the northern pembine wausau terrane contains a major succession of tholeiitic and calc alkaline volcanic rocks deposited in the interval 1860 1889 ma and a more restricted succession of calc alkaline volcanic rocks deposited about 1835 1845 ma granitoid rocks ranging in age from about 1870 to 1760 ma intrude the volcanic rocks the older succession was generated as island arcs and or closed back arc basins above the south dipping subducti

quantifying the timing and rate of crustal evolution global compilation of radiometrically dated detrital zircon grains abstractmultiple models steady state episodic and early growth followed by crustal reworking have been postulated to explain the evolution of earth s continental crust an independent assessment of these models is now possible as a result of the massive numbers of detrital zircon grains that have been dated individually over recent years using u pb and hf isotopes a compilation of 200 000 published zircon dates reveals a global age distribution u pb with six prominent statistically significant peaks 3 2 3 0 2 7 2 5 2 0 1 7 1 2 1 0 0 7 0 5 and 0 3 0 1 ga in most cases these peaks are recorded on all seven continents most of the peaks are detectable also in modern sediments a comparison of grains with both u pb ages and hf isotope model ages available for 5100 detrital zircons suggests that the age frequency distribution of detrital grains reflects predominantly episodic crustal recycling rather than crustal growth the zircon age distributions and associated data provide consistent ev

incipient weathering rind development on introduced machine polished granite discs in an arctic alpine environment northern scandinavia weathering rinds zones of alteration on the exterior surfaces of rock outcrops and coarse unconsolidated surficial debris are widely used by geomorphologists and quaternary geologists as indicators of the relative age of landforms and landscapes additionally they provide unique insights into the earliest stages of rock and mineral weathering yet the origin of these alteration zones is relatively poorly understood this lack of understanding applies especially to the initial stages of rind formation the study reported in this paper has two principal objectives the first is to use lightly polished granite discs inserted in soil profiles under several different plant communities in an arctic alpine environment for a period of four or five years to investigate the nature of incipient weathering rind development the second is to investigate the factors responsible for spatial variability in the nature and rates of rind formation incipient weathering rind development on the outer edges of the granite discs is observable and measurable over a period of time as short as four years in the mild arctic alpine environment of swedish lapland the earliest stages of rind development involve the development of a porous structure consisting of a combination of pits and fractures which have been solutionally enlarged and modified solution appears to be preferentially concentrated on the surfaces of feldspars and to a lesser extent quartz in addition iron oxides are present along grain boundaries and in grain interiors and are interpreted to have been derived from the oxidation of ferromagnesian minerals spatial variability in weathering rind development appears to be particularly driven by differences in moisture but is not related to soil ph copyright 2005 john wiley sons ltd

tropical cyclone motion a comparison of theory and observation abstract the analytic predictions of tropical cyclone motion by holland are shown to be in very good agreement with observations in the australian southwest pacific region these results indicate that a combined linear asymmetric advection and divergence of earth and cyclone vorticity provides the main mechanism for tropical cyclone motion it is also shown that an accurate prediction requires a consideration of horizontal and vertical asymmetries in the wind field hence care needs to be taken in defining a steering current

age of volcanic rocks and syndepositional iron formations marquette range supergroup implications for the tectonic setting of paleoproterozoic iron formations of the lake superior region a rhyolite in the hemlock formation a mostly bimodal submarine volcanic deposit that is laterally correlative with the negaunee iron formation yields a sensitive high resolution ion microprobe shrimp u pb zircon age of 1874 9 ma but also contains inherited archean zircons as old as 3 8 ga this precise age determination for the classic paleoproterozoic stratigraphic sequence of northern michigan the marquette range supergroup mrs necessitates modification of previous depositional and tectonic models our new data indicate that the menominee group previously ascribed to continental rifting during early pre collision phases of the penokean orogenic cycle is coeval with arc related volcanic rocks now preserved as accreted terranes immediately to the south and is more aptly interpreted as a foredeep deposit we interpret these to be second order basins created by oblique subduction of the continental margin rather than basins formed on a rifting margin along with a recently reported age for the gunflint formation in ontario of 1878 2 ma our data suggest that an extensive foredeep in the western lake superior region was the locus of iron formation deposition during arc accretion from the south further we interpret the lower mrs chocolay group a glaciogenic and shallow marine succession that lies atop archean basement to be equivalent to the upper part of the huronian supergroup of ontario and to represent the original continental rifting and passive margin phase of the penokean cycle the upper mrs baraga group represents deeper marine basins dominated by turbidites and lesser volcanic rocks resulting from increased subsidence and continued collision a stitching pluton which cuts correlatives of the hemlock formation in a thrust sheet yielded a u pb zircon age of 1833 6 ma consistent with other post tectonic plutons in wisconsin and northern michigan indicating that penokean convergence lasted no longer than 40 million years

rock based measurement of temperature dependent plagioclase weathering long term 105 years weathering can be quantified by measuring microsopic dissolution of minerals in exposed rock surfaces digital backscattered bse electron microscope images of plagioclase porosity in field exposures of known age resolves weathering at finer scales and over longer time spans than conventional solute budget and laboratory studies rock based bse imaging is therefore a potentially useful tool for quantifying steady state weathering fluxes occurring over geologic time here we compare results of the rock based method against solute budget and experimental data using plagioclase weathering rates from 14c dated basalt flows on hualalai volcano in hawaii usa a rock based field activation energy of 26 2 kcal mol 1 is somewhat higher than solute budget and laboratory measurements of plagioclase weathering

the influence of the madden julian oscillation on tropical cyclone activity in the fiji region abstract this study examines the modulation of tropical cyclone tc activity by the madden julian oscillation mjo in the fiji samoa and tonga regions fst region using joint typhoon warning center best track cyclone data and the mjo index developed by wheeler and hendon results suggest strong mjo tc relationships in the fst region the tc genesis patterns are significantly altered over the fst region with approximately 5 times more cyclones forming in the active phase than in the inactive phase of the mjo this modulation is further strengthened during el nino periods the large scale environmental conditions i e low level relative vorticity upper level divergence and vertical wind shear associated with tc genesis show a distinct patterns of variability for the active and inactive mjo phases the mjo also has a significant effect on hurricane category and combined gale and storm category cyclones in the fst region the occurrences of both these cyclone categories are increased in the active

proterozoic anorogenic magmatism and its orogenic connections there is a major belt of proterozoic so called anorogenic igneous rocks that extends from the baltic shield through southern greenland and eastern canada to southern usa all rocks are situated within a zone of 1 9 1 6 ga arc accretionary orogens that consist of juvenile mantle derived material especially accretionary prism sediments that provided the fertile source for abundant granitic crustal melts there were two main periods of formation of the anorogenic rocks 1 76 1 55 ga rocks are mainly rapakivi granites and rhyolites that formed about 60 200 ma after the last compressional events in the svecofennian ketilidian and penokean orogens the time lag being caused by the slow thermal relaxation of thickened lower crust that had commonly been depleted by the extraction of earlier granitic crustal melts 1 45 1 4 ga anorogenic rocks are mainly rhyolites and peraluminous granites in central southern usa and anorthosites in eastern canada that formed in the continental margin of the grenvillian ocean the subduction of which led to the grenville orogen these anorogenic igneous rocks formed in extensional regimes during the formation and breakup of a 1 5 ga supercontinent

the serido group of ne brazil a late neoproterozoic pre to syn collisional basin in west gondwana insights from shrimp u pb detrital zircon ages and sm nd crustal residence tdm ages abstract the serido group is a deformed and metamorphosed metasedimentary sequence that overlies early paleoproterozoic to archean basement of the rio grande do norte domain in the borborema province of ne brazil the age of the serido group has been disputed over the past two decades with preferred sedimentation ages being either paleoproterozoic or neoproterozoic most samples of the serido formation the upper part of the serido group have sm nd tdm ages between 1200 and 1600 ma most samples of the jucurutu formation the lower part of the serido group have tdm ages ranging from 1500 to 1600 ma some basal units have tdm ages as old as 2600 ma reflecting proximal basement thus based on sm nd data most if not all of the serido group was deposited after 1600 ma and upper parts must be younger than 1200 ma cathodoluminescence photos of detrital zircons show very small to no overgrowths produced during ca 600 ma brasiliano deformation and metamorphism so that shrimp and isotope dilution u pb ages must represent crystallization ages of the detrital zircons zircons from meta arkose near the base of the jucurutu formation yield two groups of ages ca 2200 ma and ca 1800 ma in contrast zircons from a metasedimentary gneiss higher in the jucurutu formation yield much younger ages with clusters at ca 1000 ma and ca 650 ma zircons from metasedimentary and metatuffaceous units in the serido formation also yield ages primarily between 1000 and 650 ma with clusters at 950 1000 800 750 and 650 ma thus most if not all of the serido group must be younger than 650 ma because these units were deformed and metamorphosed in the ca 600 ma brasiliano fold belt during assembly of west gondwana deposition probably occurred ca 610 650 ma soon after crystallization of the youngest population of zircons and before or during the onset of brasiliano deformation the serido group was deposited upon paleoproterozoic basement in a basin receiving detritus from a variety of sources the jucurutu formation includes some basal volcanic rocks and initially received detritus from proximal 2 2 2 0 ga transamazonian to late paleoproterozoic 1 8 1 7 ga basement provenance for the upper jucurutu formation and all of the serido formation was dominated by more distal and younger sources ranging in age from 1000 to 650 ma we suggest that the serido basin may have developed as the result of late neoproterozoic extension of a pre existing continental basement with formation of small marine basins that were largely floored by cratonic basement subjacent oceanic crust has not yet been found immature sediment was initially derived from surrounding land as the basin evolved much of the detritus probably came from highlands to the south present coordinates alternatively if the patos shear zone is a major terrane boundary the basin may have formed as an early collisional foredeep associated with south dipping subduction in any case within 30 million years the region was compressed deformed and metamorphosed during final assembly of west gondwana and formation of the brasiliano pan african fold belts

styles of proterozoic crustal growth on the southeast margin of laurentia evidence from the central grenville province northwest of lac st jean quebec the southeast margin of laurentia was a very long lived active continental margin part of whose history is recorded in the grenville province of the canadian shield within this province nd isotope mapping can be used to define the boundaries between terranes with a variety of crustal formation ages and can also distinguish between crustal growth by oceanic and continental arc magmatism the former gives rise to large terranes with homogeneous nd isotope signatures and well defined boundaries whereas the latter leads to areas with heterogeneous nd isotope signatures one of the best examples of continental arc magmatism in the grenville province is provided by the region northwest of lac st jean quebec eighty new nd isotope analyses are used along with aeromagnetic data to divide this area into three blocks bounded by abrupt changes in nd model age the western block consists almost exclusively of tonalitic grey gneisses with archean model ages the eastern block is composed almost exclusively of

sedimentary phosphate and associated fossil bacteria in a paleoproterozoic tidal flat in the 1 85 ga michigamme formation michigan usa abstract phosphorus is a nutrient fundamental to life and when it precipitates in modern environments bacteria are intimately involved in its release concentration and mineralization preserved fossil bacteria in phosphate crusts and grains from the ca 1850 million year old bijiki iron formation member of the michigamme formation michigan provide insight into the longevity and nature of this relationship the michigamme formation accumulated near the end of the earth s initial phosphogenic episode ca 2 2 and 1 8 ga to produce one of the first granular phosphorites phosphatic lithofacies consist of fine to medium sand sized francolite peloids concentrated on bedding surfaces in peritidal facies granular beds are up to 2 cm thick and peloids are often partially to completely replaced by dolomite and chert the grains contain organic matter and pyrite framboids that suggest bacterial breakdown of organic matter and bacterial sulfate reduction the peritidal nature of phosphorite in the michigamme formation is in sharp contrast to phanerozoic phosphogenic environments in deeper coastal upwelling settings peritidal settings were well suited for phosphogenesis under the very low oxygen and low dissolved sulfate levels of the paleoproterozoic as cyanobacteria produced oxygen in shallow water and evaporation led to increased sulfate concentrations such concomitant processes helped establish focused redox interfaces in the sediment that chemosynthetic bacterial communities sulfur oxidizers reducers forms that concentrate p and possibly iron oxidizers could exploit phosphate released from organic matter by heterotrophic bacteria and fe redox pumping was further concentrated by these chemotrophs a process that forms late neoproterozoic to phanerozoic phosphorites but on a much larger scale this early example of a granular phosphorite demonstrates that like their phanerozoic counterparts paleoproterozoic phosphorites are the concentrated indirectly biomineralized products of bacterial communities but unlike younger analogs which accumulated across subtidal shelves and shelf margins these ancient deposits formed only in tidal flat settings where phosphogenic redox processes could be established in the sediment from this early beginning the zone of phosphogenesis likely migrated into deeper water settings as oxygen and sulfate levels rose expanding the zone of chemosynthetic bacterial and associated phosphogenesis across the shelf

lead isotope analyses of leached feldspars constraints on the early crustal history of the grenville orogen abstract lead isotopic data on leached k feldspars plagioclases and muscovite provide constraints on the premetamorphic evolution of the grenville orogen in ontario and new york a boundary between reworked archean crust to the northwest and new mantle derived crust generated during mid proterozoic times to the southeast is identified within the northern portion of the gneiss belt of ontario the identification of juvenile mid proterozoic crust at the southeastern margin of the orogen is consistent with previous investigations based on neodymium model ages however near the grenville front lead isotopes indicate the presence of reworked archean crust rather than primary early proterozoic crust postulated on the basis of neodymium model ages this discrepancy between the lead and neodymium isotopic terranes underscores the necessity of using a multidisciplinary approach to identify possible terrane boundaries only through a combination of the lead with neodymium isotopes is it possible to distinguish between crust that formed during a single major orogenic cycle and crust that represents a mixture of old with new mantle derived material the combined neodymium and lead data imply that the grenville orogen in ontario consists essentially of reworked archean crust and juvenile mantle derived material generated around 1 5 ga the grenville orogen involved archean crust that was depleted in thorium and uranium several hundred ma before high grade metamorphism at ca 1 1 ga such a depletion may have a significant impact on the geotherm achieved during orogeny and may explain the lower metamorphic thermal gradients observed for the gneiss belt

microtextural controls of weathering of perthitic alkali feldspars the relationship between the microtexture and dissolution behaviour of fresh hf acid etched and naturally weathered alkali feldspar phenocrysts from the lower devonian shap granite has been investigated by sem and tem a novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface further electron microscope work suggests that shap phenocrysts are representative of the alkali feldspar in the protolith of many soils fresh and unweathered shap feldspars have a complex microtexture comprising areas of pristine cryptoperthite and lamellar microperthite cross cut by volumes of microporous altered feldspar or patch perthites cryptoperthites are made up of 75 nm wide albite films the platelets are coherent but albite films have numerous edge dislocations along their interface with orthoclase 001 and 010 cleavage surfaces intersect 2 3 edge dislocations m2 in three dimensions these edge dislocations form an orthogonal net in the murchison plane of easy fracture close to 601 patch perthites are irregular semicoherent to incoherent intergrowths of albite and irregular microcline subgrains with 0 65 0 70 sub m to m sized pores m2 microporous patch perthites form by dissolution reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime dissolution of shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture especially by dislocations and exsolution lamellae the core and strain energy associated with dislocation outcrops on 001 and 010 cleavage surfaces promotes rapid dissolution at those sites and formation of nn sized etch pits after 30 s of laboratory etching with hf acid vapour with progressive hf etching crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite naturally weathered feldspars were collected from the glacial erratic boulders fine gravels surrounding exposed granite surfaces and from peat soil overlying the granite during natural weathering etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate 15 m below the cleavage surface forming an interconnecting ladder like grid of submicrometer wide channels in the murchison plane coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly but faster than orthoclase this is probably because the albite lamellae have significant elastic coherency strain but this is much less per unit volume of albite than the core and strain energy associated with edge dislocations patch perthites etch in hf vapour and weather rapidly in nature to produce a honeycomb like texture of interconnecting nanometer to micrometer sized pits which nucleate at preexisting micropores or incoherent subgrain boundaries the size and density of etch pits on microperthite surfaces which is determined by rates of growth and coalescence may be a useful progress variable for natural and experimental dissolution all alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale these microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains the controls on absolute dissolution rates and why they commonly change over time the nonlinear variation of dissolution rate with grain size the ratio of alkali ions released into solution and disparities between laboratory dissolution rates and those observed in the field

tectonometamorphic evolution of ghana togo and benin in the light of the pan african brasiliano orogeny abstract the south pan african belt of west africa ghana togo and benin province can be divided into three domains 1 1 the external nappes thrust over the west african craton derived from passive margin sedimentary deposits middle to late proterozoic and displaying middle to high pressure metamorphism such as the kante and atacora nappes 2 2 the intermediate nappes including the suture zone rocks with eclogitic assemblages protoliths ca 800 ma and mainly monocyclic metasedimentary rock and orthogneiss that are found in central togo and northwest benin 3 3 the internal nappes composed of high grade commonly anatectic gneiss anatexis ca 600 ma including orthogneiss eburnian plutons elongated granulite belts reworked archean basement and an intrusive granite and charnockite association ca 600 ma which are exposed in benin and nigeria the p t t d paths calculated from rocks in these three domains show a prograde retrograde evolution 1 1 an initial clockwise evolution related to an early burial stage hp ht in the intermediate nappes followed by oblique thrusting in a sw direction that caused nappe stacking and subsequent uplift 2 2 a second anti clockwise evolution with late temperature and pressure increases only recorded in the more internal units this temperature increase was a consequence of the previous crustal thickening with subsequent doming extension and anatexis the pressure increase was related to movement along the kandi fault working as a dextral strike slip fault with a transpressional component under late e w shortening the retrograde trend corresponds to the final uplift the evolution of the belt can thus be divided into three successive stages 1 1 a first stage was characterized by an oblique collision inducing sw oblique thrusting with associated syn foliation reverse metamorphism 2 2 during a second stage anatectic doming took place followed by 3 3 a third stage at still high temperature with dextral wrench faulting and associated n s post foliation folding under late e w shortening comparison between the west african north brasiliano and central african late proterozoic belts shows very strong similarities between their lithology kinematics and metamorphism and indicates that the pan african brasiliano chain resulted from collision between three major continental domains the west african and sao francisco congo cratons and a late proterozoic mobile domain

the southern limit of archean crust and significance of rocks with paleoproterozoic model ages nd model age mapping in the grenville province of western quebec abstract detailed nd model age mapping in the grenville province of western quebec has identified biotite gneisses with crustal formation age intervals of 2 5 2 9 ga and 1 8 2 0 ga although the presence of archean crust in the grenville parautochthonous belt has been known for some time the results of nd model age mapping combined with geochemical and aeromagnetic data indicate that the archean parautochthon extends more than 20 km to the south of the renzy shear belt this belt which was previously defined as the southern limit of archean crust corresponds to an isolated allochthon with nd model ages ranging from 2 0 to 1 4 ga and does not demarcate the allochthon boundary thrust rocks with model ages of 1 8 2 0 ga exposed to the south of the archean crust geochemically possess an arc affinity and are interpreted as a paleoproterozoic arc a model age boundary between archean crust and this arc has been defined by this study the age boundary probably represents a suture along which the southern margin of laurentia and the arc converged

paleoproterozoic crustal history of the southwestern grenville province evidence from nd isotopic mapping nd isotopic mapping in the north bay area of the central gneiss belt southwestern grenville province has revealed the precise trend of a tdm model age line developed between the uplifted southern margin of the archean superior craton tdm 2 7 ga and a paleoproterozoic allochthon tdm 1 9 ga separating these two crustal blocks is a narrow zone of gneisses with intermediate tdm ages these transitional gneisses are interpreted to reflect a remnant fault or ductile shear zone of uncertain age along which crustal material from both blocks mechanically mixed during their juxtaposition accordingly the nature of the tdm line in the north bay area is interpreted to be tectonic in the temiscaming area widespread exposures of mature metasedimentary gneisses are shown by their tdm ages to be dominantly of paleoproterozoic provenance these results are consistent with the existing detrital zircon geochronology inferring a maximum depositional age of 1 7 ga the anorogenic chemistry of the north bay o

two paleoproterozoic statherian siliciclastic metasedimentary sequences in central wisconsin the end of the penokean orogeny and cratonic stabilization of the southern lake superior region abstract statherian siliciclastic sedimentary rocks in the southern lake superior region are key to understanding the evolution and stabilization of paleoproterozoic crust in the midcontinent deposited during an important period of significant crustal growth controversy exists regarding the conditions of sandstone deposition and the relations between magmatism and sedimentation in central wisconsin the hamilton mounds inlier although restricted in areal extent is significant in that it reveals the existence of two separate statherian units an 1800 1760 ma immature meta arkose and a post 1760 ma baraboo interval supermature quartzite the meta arkose is crosscut by 1762 ma granite and contains abundant penokean age detrital monazite grains the younger quartzite contains detrital monazite as young as 1750 ma and was deposited unconformably upon these older units all units experienced 1630 ma low grade recrystallization and isotopic disturbance associated with the mazatzal orogeny in addition the supermature quartzite experienced k metasomatism and brecciation at 1470 ma ascribed to regional scale hydrothermal alteration related to the influence of the nearby 1470 ma wolf river batholith collectively these data suggest an early statherian episode of arkosic deposition and magmatism during yavapai orogenesis followed by a longer interval of deposition of supermature quartz arenite associated with crustal stabilization the existence of two separate statherian units reconciles apparent conflicts in depositional models and provides a greater understanding of the paleoproterozoic evolution of the midcontinent

variation of tropical cyclone activity in the south indian ocean el nino southern oscillation and madden julian oscillation effects 1 the present study examines variation of tropical cyclone tc activity in the south indian ocean sio during tc seasons december march for the period 1979 2004 the impact of the el nino southern oscillation and the madden julian oscillation mjo on the variation is revealed through a composite analysis during el nino periods tc genesis was shifted westward enhancing the formation west of 75 e and reducing it east of 75 e these changes in the genesis correspond to a westward shift of convection it may be explained by a remote effect on the sio that is the increase in sea surface temperature in the central eastern pacific alters the walker circulation and forms an anomalous anticyclonic circulation in the east sio during el nino the spatial difference in tc passages between el nino and la nina shows a significant decrease to the southeast of madagascar but a moderate increase in the central midlatitude sio indicating that tcs move farther east during el nino this change is possibly due to the anomalous southwesterlies east of madagascar variation of tc activity also depends on various mjo phases frequent tc passages for phases 2 4 strong convective activity straddles along the equatorial indian ocean versus infrequent tc passages for other phases tc tracks tend to be more south oriented in phase 3 compared with those in phases 2 and 4 this is possibly caused by the increased steering northerlies which are a part of the anticyclonic rossby wave of gill type in response to the suppressed mjo related convection in the maritime continent

sem analysis of weathered grains pretreatment effects fresh microcline albite and almandine along with soil grains were treated with various traditional pretreatments prior to observation with scanning electron microscopy sem the grains were observed with sem and given ratings for each of several surface properties to determine which pretreatments produced clean surfaces on soil grains without laboratory induced damage on the fresh mineral grains gentle overnight shaking in 2 sodium bicarbonate at ph 9 5 produced the most effective cleaning of soil grains with the least amount of induced damage to fresh mineral samples this pretreatment was equivalent to that given the control samples shaken overnight in distilled water for observations of etch pits on fresh mineral samples within a 25 equivalence or negligible difference interval ultrasonification hydrogen peroxide and boiling hydrochloric acid caused the most damage to mineral samples mainly in the form of 0 5 m etch pits boiling hydrochloric acid boiling nitric acid and stannous chloride resulted in increased coated surfaces on soil grains

neoproterozoic crustal accretion in central brazil recent geochronological studies have demonstrated the existence of several neoproterozoic orthogneiss and metavolcanic are suites ca 900 to 600 ma in the southern part of the tocantins province a major neoproterozoic orogenic area in central brazil exposed between the amazon and sao francisco cratons nd and sr isotopic characteristics of these metamorphic suites are primitive initial 87 sr 87 sr ratios vary from 0 7024 to 0 7042 and initial nd values are positive varying between 0 2 and 6 9 the most primitive initial sr and nd isotopic compositions are very similar to model depleted mantle compositions at the time of formation of the original magmas the mantle like characteristics of these are suites in western goias contradict previous ensialic evolution models for the tocantins province and reveal that crustal accretion processes were important in large areas of central brazil during neoproterozoic time

seismic images and three dimensional architecture of a proterozoic shear zone in the sudbury structure superior province canada a broad zone of intense ductile to brittle deformation transects the paleo proterozoic sudbury igneous complex and likely formed during the final stages of the penokean orogeny on the southern margin of the superior province herein we present a three dimensional image of this deformation zone derived from four seismic reflection profiles additional information from rock property measurements drill results and published structural mapping suggests that the seismic data define the three dimensional disposition of listric shear zones the seismic image of the deformation is best developed within the onaping formation however the images of the deformation zone envelope are imperfect being most clear where truncating known contacts within the sudbury igneous complex and its contact with huronian host rocks the southern edge of the seismically imaged shear zones does not extend significantly beyond the exposed sudbury igneous complex suggesting limits to the extent of deformation in the sudbury structure

geoscience impact a synthesis of studies of the sudbury structure geophysical probing results are synthesized into a three dimensional framework necessary for understanding the genesis of the sudbury structure based primarily on seismic reflection results centred on the sudbury igneous complex remnants of crustal melting from a catastrophic meteorite impact are superimposed on the juxtaposition of mid crustal rocks exhumed during the archean against deformed paleoproterozoic sedimentary rocks sedimentation metamorphism deformation and metasomatic overprints are all part of the post impact history of sudbury and tend to dominate the geophysical response of the structure pre impact deformation although certainly preserved in some aspects of sudbury geology is not clearly expressed in the geophysical data nor are any elements of impact induced deformation geophysical views of the sudbury igneous complex are thus somewhat biased in representing mostly the post impact but pre grenvillian history of the region with the exception of igneous events establishing th

tropical cyclone track direction climatology and its intraseasonal variability in the australian region aspects of tropical cyclone tc activity such as the influence of the el nino southern oscillation and the variability of tc genesis location have been examined in numerous previous studies however relatively few studies have examined aspects such as the influence of intraseasonal variability on tc track direction here we focus on a number of knowledge gaps relating to observed tc track directionality and intraseasonal variability in the australian region climatological examinations are presented for tc track directional variability throughout the australian region in contrast to previous studies that have focused on the mean direction of tc movement in this region tc tracks are examined here based on the full spectrum of track directions for a given location or region variability in initial tc track directions is investigated including an examination of the influence of the madden julian oscillation mjo it is demonstrated that there is a considerable degree of seasonal and intraseasonal variation in tc motion in this region these variations result from variations in genesis location throughout the tc season as well as zonal wind anomalies associated with the influence of the mjo on the steering flow winds anomalous westerly steering flow occurs during mjo phase 4 5 and anomalous easterly flow during phase 8 1 in the western basin with resulting changes in the proportion of tc tracks in each direction during these phases of the mjo the results presented here are intended to provide improved seasonal tc activity guidance and enhanced resilience to tc impacts 2016 american geophysical union all rights reserved

u pb and lu hf isotope systematics of zircons from the mississippi river sand implications for reworking and growth of continental crust we carried out in situ u pb and lu hf isotope analyses of detrital zircons from the mississippi river in order to understand crustal reworking and continental growth rates the u pb analyses for 416 zircons reveal three major peaks of crust formation at 2 8 2 6 ga 1 8 0 9 ga and after 0 2 ga initial hf isotope ratios were obtained for 402 of the dated zircons and only 8 of the zircons have batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document hf mathit t dm end document values less negative than 2 5 these data correspond to a crustal residence time of 120 m y this finding indicates that crustal reworking was a very important process in continental crust formation the batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document hf mathit t dm end document population demonstrates that reworking was predominant at 2 5 2 0 ga and after 0 9 ga whereas juvenile crust formation dominated between 2 0 and 1 6 ga we calculated the mantle extraction model ages to estimate the continental growth rate approximately half of the grains have model ages between 2 0 and 1 3 ga indicating rapid crustal growth during this time the continental growth rate suggests that 15 and 78 crust in the source region of the zircons formed by 2 5 and 1 3 ga respectively

nd isotopes and the origin of 1 9 1 7 ga penokean continental crust of the lake superior region nd isotopic data on 26 samples demonstrate the origin of the early proterozoic crust of the penokean orogen the three major components are 1 the marquette range supergroup a predominantly miogeoclinal continental margin sequence deposited before 1 85 ga on archean basement 2 ca 1 88 ga felsic metavolcanic rocks of the wisconsin magmatic terrane to the south of evolved island arc affinity and 3 1 87 1 76 ga granitoids that intrude the metavolcanic rocks initial nd values for the metavolcanic rocks of the magmatic terrane range between 0 0 and 2 4 and t dm ages between 1 9 and 2 2 b y the volcanic rocks primarily represent new crustal material that had only a limited archean input probably through mixing of subducted sediments into the magma source area nd t values for sedimentary rocks in the lower part of the marquette range supergroup indicate an archean source most likely the 2 7 ga superior province to the north on the other hand the sedimentary rocks in the upper part of the marquette range supergroup graywackes of the upper michigamme formation have initial nd values between 0 8 and 1 5 indicative of an early proterozoic source these graywackes probably are foredeep deposits derived from the volcanic rocks of the wisconsin magmatic terrane to the south thus the time of deposition of the upper michigamme formation dates the final convergence of the wisconsin magmatic terrane with the continental margin the granitoids of the wisconsin magmatic terrane have a wide range of nd t values from 4 5 to 4 0 they represent mixtures of variable amounts of new crustal material and recycled archean detritus the more negative nd t values occur in samples close to the northern boundary of the wisconsin magmatic terrane which is marked by a mylonitic shear zone the niagara fault zone we suggest that the miogeoclinal lower marquette range supergroup rocks having an archean nd signature became involved in magma genesis close to the collisional boundary whereas increasingly lesser amounts of this older material were available for mixing farther away from the suture the 1 9 1 7 ga penokean events involved major growth of new crust from the mantle

a neodymium isotope study of plutons near the grenville front in ontario canada abstract sm nd analysis of meta plutonic rocks near the grenville front in ontario allows their division into two groups granitoid plutons with model ages of 2 ga are intruded into a crustal section formed largely of archean or re worked archean basement plutons with model ages of plutons south of the proposed suture line can be explained by anatexis of penokean age crust alone while plutons north of this line contain mixtures of archean and proterozoic nd these mixed signatures suggest that the nw grenville province may be analogous to northern wisconsin and michigan in this region a very early proterozoic miogeocline supplied by the superior province is overlain by a formation of mixed superior penokean provenance which heralded the collision of a penokean arc with the archean craton despite very strong sm nd evidence for the presence of old crustal components in the plutons u pb analysis did not reveal the presence of inherited zircon hence the absence of such zircons should not be taken in isolation as evidence against crustal re working

pb isotope constraints on the provenance and diagenesis of detrital feldspars from the sudbury basin canada abstract residues after hf leaching of detrital feldspar grains from the early proterozoic chelmsford formation of the sudbury basin ontario record isotopic compositions very close to the initial pb isotopic composition of their late archean superior province source these grains have been recrystallized to albite and k feldspar residues of chelmsford feldspar grains lie within the range of reported 206 pb 204 pb compositions of feldspars from plutons of the adjacent abitibi subprovince of the superior province with a steep 207 pb 206 pb slope of 0 35 0 15 3 7 0 7 ga leaches are more radiogenic than residues but leach residue pairs do not record in situ u and th decay since either the late archean time of feldspar crystallization in the superior province or the early proterozoic time of recrystallization in the chelmsford formation instead leach residue pairs lie along an average 207 pb 206 pb slope of 0 250 0 036 representing mixing trends most whole rock samples from the chelmsford formation lie close to a 2 7 ga slope collinear with detrital feldspar compositions we interpret the pb data from the leaches as recording the passing of altering fluids these fluids likely carried pb derived from rocks or minerals with relatively high u pb th pb and th u within the sedimentary basin itself the residues appear to record very close to initial values because they lie along the very steep trend defined by feldspars of the superior province

observational relationship of climatologic beta drift with large scale environmental flows 1 while many numerical studies suggested that large scale environmental flows significantly affect beta drift this influence was not verified observationally and included in assessing changes of tropical cyclone tc tracks under the background of global warming this study investigates the influence of large scale environmental flow on climatologic beta drift in the northern west pacific using the best track dataset from joint typhoon warming center jtwc and the ncep monthly reanalysis over the period from 1965 to 2007 consistent with most previous numerical studies the meridional shear of zonal wind vertical shear of zonal wind and meridional gradient of relative vorticity which are mainly associated with the zonal environmental wind play an important role in affecting beta drift for the meridional shear of zonal flows the northward beta drift in an anticyclonic shear is faster than that in a cyclonic shear a poleward equatorward gradient of relative vorticity leads to faster slower westward beta drift than in a resting environment tcs tend to move to the left of the vector of vertical wind shear

the role of tropical waves in tropical cyclogenesis abstract this paper analyzes relationships between tropical wave activity and tropical cyclogenesis in all of the earth s major tropical cyclone basins twenty nine years of outgoing longwave radiation data and global reanalysis winds are filtered and analyzed to determine statistical relationships between wave activity in each basin and the corresponding cyclogenesis composite analyses relative to the storm genesis locations show the structures of the waves and their preferred phase relationships with genesis five wave types are examined in this study including mixed rossby gravity waves tropical depression type or easterly waves equatorial rossby waves kelvin waves and the madden julian oscillation the latter is not one of the classical tropical wave types but is a wavelike phenomenon known to have a strong impact on tropical cyclogenesis tropical cyclone formation is strongly related to enhanced activity in all of the wave filter bands except for the kelvin band in each basin the structure o

cluster analysis of tropical cyclone tracks in the southern hemisphere a probabilistic clustering method is used to describe various aspects of tropical cyclone tc tracks in the southern hemisphere for the period 1969 2008 a total of 7 clusters are examined three in the south indian ocean three in the australian region and one in the south pacific ocean large scale environmental variables related to tc genesis in each cluster are explored including sea surface temperature low level relative vorticity deep layer vertical wind shear outgoing longwave radiation el nino southern oscillation enso and the madden julian oscillation mjo composite maps constructed 2 days prior to genesis show some of these to be significant precursors to tc formation most prominently westerly wind anomalies equatorward of the main development regions clusters are also evaluated with respect to their genesis location seasonality mean peak intensity track duration landfall location and intensity at landfall enso is found to play a significant role in modulating annual frequency and mean genesis location in three of the seven clusters two in the south indian ocean and one in the pacific the enso modulating effect on genesis frequency is caused primarily by changes in low level zonal flow between the equator and 10 s and associated relative vorticity changes in the main development regions enso also has a significant effect on mean genesis location in three clusters with tcs forming further equatorward poleward during el nino la nina in addition to large shifts in mean longitude the mjo has a strong influence on tc genesis in all clusters though the amount modulation is found to be sensitive to the definition of the mjo

samarium neodymium elemental and isotopic systematics in sedimentary rocks abstract trace element sm nd th sc and nd isotopic data for sedimentary rocks are examined in order to evaluate 1 1 possible secular trends in the composition of the sedimentary mass and upper continental crust 2 2 the role of tectonic setting in controlling the chemical and nd isotopic composition of the sedimentary mass when plotted against either sedimentation age or average provenance age determined mainly from nd model ages sm nd in sedimentary rocks decreases abruptly from about 0 21 to 0 19 at the archean proterozoic transition consistent with a change towards a more felsic upper crustal composition the change is only somewhat less pronounced for turbidite sediments than for those of continental affinity a trend toward increasing sm nd approaching or exceeding archean values is observed for phanerozoic turbidites this latter change is interpreted as indicating undifferentiated young crust as a component in the sediment which has been lost from the older post archean record due to sedimentary and igneous intracrustal recycling processes when plotted against sedimentation age th sc ratios exhibit a trend directly comparable to that of sm nd in this case th sc increases abruptly at the archean proterozoic boundary from about 0 5 to 1 0 and then decreases to about 0 7 during the phanerozoic nd isotopic composition of modern sediments is affected by tectonic association with those deposited at active margin settings tending to have higher nd reflecting younger provenance and higher and more scattered sm nd reflecting less homogenized and differentiated provenance it is likely that these distinctions can be traced back for about 1 2 ga but with the current data base are not readily resolved in older sedimentary rocks the degree to which the preserved sedimentary mass is open to new additions from the mantle either through replacement during sedimentary recycling or sediment mass growth is highly variable but averages about 15 25 during the phanerozoic

u pb geochronology of baddeleyite from the belleview metadiabase age and geotectonic implications for the negaunee iron formation michigan abstract the belleview metadiabase sill crosscuts the siamo slate and the negaunee iron formation of the marquette range supergroup michigan it is one of a series of metamorphosed diabase sills that intrudes the menominee group and possibly the baraga group chloritic sills within the marquette range supergroup have yet to be officially grouped as one or more series or swarms and therefore stratigraphic correlations and temporal relationships between these sills remain speculative the recovery of the first sensitive high resolution ion mass spectrometry id tims u pb date of 6 baddeleyite grains from the belleview sill yielding an age of 1891 3 ma is a positive step to classifying these chloritic metadiabases this new date has three important implications 1 it gives a definitive minimum age of negaunee iron formation deposition 2 it confirms recent stratigraphic correlations that suggest that the negaunee iron formation is older than the mesabi gunflint and gogebic iron ranges and not contemporaneous and 3 it does not support recent correlations of the menominee group chloritic sills with the 1874 ma hemlock formation within the baraga group of the marquette range supergroup this new date does not support a more recently proposed synorogenic foredeep depositional environment for the deposition of the negaunee iron formation instead this age date supports an earlier interpretation that the negaunee iron formation is part of continental shelf deposits laid down prior to onset of 1875 1835 ma collision the age of the belleview sill suggests that it is the result of extension related magmatism that is prior to the currently defined penokean orogenic event

sedimentary and metamorphic lithofacies of the lower negaunee iron formation marquette district michigan usa the base of the proterozoic negaunee iron formation is exposed in the open pit at tilden mine marquette michigan juxtaposed against the archean aged palmer gneiss it is bounded by the regional scale southern shear zone and cut by two sets of dykes an older chloritic and schistose set and a younger 1 1 ga keweenawan set tilden mine is dominated by a 100 m scale plunging northwest anticline and is cut by a growth fault locally termed the tower hill fault that intersects the southern shear zone the base of the exposed iron formation is composed of three lithofacies including lower clastics that grade into the overlying banded iron formation that in turn grades upward into granular iron formation this succession is capped by chloritic metadiabases locally termed the summit hill sill and pillar intrusive petrographic and mineral chemical investigations document primary or early diagenetic hematite siderite and possibly ferri hydrite metamorphic and related hydrothermal magnetite chlorite late mar

early paleozoic island arc accretion to the north china craton and the shang dan fault zone a major paleoplate boundary in eastern asia the early paleozoic tectonic evolution of part of the qinling dabie shandong orogenic belt is reconstructed on the basis of the results of field mapping geochemical and geochronological studies the qinling orogenic belt separates the archean proterozoic north china craton ncc from the south china craton scc also yangtze craton the study area is located about 90 km southwest of the city of xi an three lithologically distinctive zones separated from each other by fault zones can be distinguished passive continental margin deposits of the ncc zone 1 have end t values between 6 16 and 12 54 and tdm ages between 1 29 and 2 20 ga they are separated from an early paleozoic island arc sequence zone 2 by the sinistral chenche fault mafic volcanic lithologies in the southern part of zone 1 are associated with the development of a passive continental margin in late proterozoic time similiar lithologies of zone 2 have geochemical characteristics of an oceanic island arc accretion was accompanied by a strong deformation event between early ordovician and late silurian time magmatic intrusions in zone 2 and the southern part of zone 1 are associated with a short lived magmatic arc that developed above a north dipping subduction zone on the modified continental margin of the ncc in late silurian to early devonian time the end t values of felsic intrusions from the northern part of zone 2 decrease in a northern direction from 1 78 to 1 05 mafic to intermediate intrusions in the southern part of zone 2 have positive end t values from 1 28 to 2 04 as well as low 87sr 86sr i values between 0 705 and 0 706 indicating an increasing mantle component toward the south low grade metamorphic pelites are separated from the island arc sequence of zone 2 by a 2 km wide shang dan shear zone isotopic studies show strong similarities between a granite sample from zone 3 and samples from the scc 87sr 86sr i values increase from 0 709 to 0 713 whereas end t values decrease from 1 90 to 5 34 in a southern direction the shear zone that separates zone 2 from 3 is of fundamental importance and marks the boundary between the ncc and the scc the results presented in this paper document a previously unknown early paleozoic tectonic evolution of the qinling orogenic belt and delineate one of the fundamental paleoplate boundaries in eastern asia

u pb geochronology of the wolf river batholith north central wisconsin evidence for successive magmatism between 1484ma and 1468ma abstract single grain zircon u pb age determinations were completed on individual phases of the mesoproterozoic anorogenic wolf river batholith wrb of wisconsin and the nearby wausau syenite complex in order to a define the age more precisely and accurately b determine whether there is a resolvable age difference within units of the wrb and c determine whether the wausau syenite complex is coeval with any or all parts of the wrb u pb results for many individual zircon grains from the wrb plot near concordia and yield ages ranging from 1468 4 ma to 1484 2 ma plutons of the northeastern lobe of the wrb peshtigo monzonite hager porphyry belongia granite and high falls granite have a weighted average crystallization age of 1470 3 1 2 ma which is younger than the main complex plutons wolf river granite wiborgite porphyry red river granite and waupaca granite which yields a weighted average crystallization age of 1475 6 1 7 ma excluding one sample of wolf river granite from hogarty which yields an age of 1484 2 ma two samples from the main complex also contain zircons with younger apparent ages of 1470 1471 ma these younger grains may reflect late stage alteration of some zircons coeval with emplacement of the northeastern lobe the wausau syenite and the nine mile granite of the nearby wausau syenite complex 1522 ma and 1506 ma respectively are significantly older than the wrb and definitely not coeval or comagmatic the syenite complex may be a precursor to the larger wrb magmatism sm nd t dm ages for most wrb samples indicate crust formation ages from 1 8 ga to 1 9 ga compatible with formation of the magmas from late paleoproterozoic penokean crust one unit which locally intrudes archean gneiss has a small component of that older rock involved in its evolution the two wausau complex units have younger tdm ages and appear to have incorporated slightly greater amounts of juvenile constituents

etch pit size and shape distribution on orthoclase and pyriboles in a loess catena abstract etch pits on 50 100 m orthoclase and pyribole grains from the e b and c horizons of a catena or drainage sequence of well drained somewhat poorly drained and poorly drained loessal soils were analyzed using sem and digital image analysis the objective was to determine the influence of soil drainage conditions and the associated geochemical environments on dissolution of primary silicates as determined from the distribution of etch pits on the grain surfaces etch pit size frequency distributions indicated that the poorly drained soil contained the smallest and most numerous etch pits for both minerals this indicated low growth rates for etch pits in the poorly drained soil analysis of etch pit shapes indicated that etch pits on pyriboles were more elongated and irregular in the poorly drained soil possibly resulting from a greater growth rate whereas etch pits on orthoclase grains were more elongated and irregular in the well drained soil the larger more elongated and irregularly shaped etch pits on orthoclase from the well drained soil indicative of a greater pit growth rate is consistent with a flushing model of mineral weathering in soils the etch pit size distribution on pyriboles is consistent with the flushing concept but the shape data indicates that other mechanisms of weathering may be involved

transpressive tectonism in the eastern penokean orogen canada consequences for proterozoic crustal kinematics and continental fragmentation abstract results of a structural analysis of archean basement huronian cover and various igneous rocks in the sudbury area ontario and regional tectonometamorphic correlations suggest that structures in the eastern penokean orogen formed dominantly by dextral transpression bulk horizontal shortening started with the ca 2 4 to 2 2 ga blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism during the 1 89 to 1 83 ga penokean orogeny horizontal dextral shear parallel to the curved axis of the eastern penokean orogen gained in importance and is most likely related to docking of proterozoic magmatic arc terranes to the superior province margin this tectonic activity led to distributed heterogeneous deformation of archean basement rocks and exhumation of deep crustal rocks in the sudbury area and the kapuskasing structural zone but may have failed to accrete paleoproterozoic terranes in the lake huron area the lack of evidence for horizontal extension following blezardian and penokean contraction suggests that crustal fragmentation of the archean supercontinent started at 2 45 ga finally the great lakes tectonic zone and the murray fault master dislocations in the penokean orogen may have acted as crustal weakness zones which influenced the orientation and development of 1 1 ga midcontinent rift segments

impact of intraseasonal oscillation on the tropical cyclone track in the south china sea this study investigates the impact of the intraseasonal oscillation iso on tropical cyclone tc tracks in the south china sea scs during 1970 2010 about one third of tcs in the scs move eastward while the other two thirds move westward in the tc genesis peak seasons of june october jjaso the westward moving tcs are controlled by the background tc steering flow of easterly and the eastward moving tcs by the tc steering flow induced by the iso the outgoing longwave radiation and wind fields show that the eastward moving tcs were mostly along the main axis of strong tc steering flow anomaly of westerly associated with the iso while the westward moving tcs were only weakly associated with the iso an experiment performed with a simple two level model further confirmed the result the interannual variation of tc tracks in the scs is also discussed it is found that the steering flow anomalies in the scs mostly favor eastward moving tcs in central pacific cp el nino and eastern pacific ep el nino years however the eastward flow anomalies are too weak to have strong influence on the majority of the tcs during la nina years tcs in the scs tend to move westward possibly related to the westward steering flow anomalies

temporal changes in nd isotopic composition of sedimentary rocks in the sevier and taconic foreland basins increasing influence of juvenile sources initial 143 nd 144 nd ratios of clastic sedimentary rocks in the ordovician taconic and sevier foreland basins increase with decreasing stratigraphic age this shift in isotopic composition reflects an increased influx of juvenile sediment to both basins for the taconic basin geologic and isotopic data are consistent with the juvenile sediment being derivedfromupliftedgrenvillecrust upthrustaspartofthetaconicallochthons sediment derivedfromthebronsonhillarc oracombinationofboth forthesevierforelandbasin there are few geologic constraints on source regions but the similar shift in isotopic ratios with time suggests a similar increase in the influence of juvenile sources such as exotic terranes sevier basin sedimentary rocks have higher nd values than those within the taconic basin thus juvenile source areas were of greater importance to sevier basin sediments

detrital zircon ages from early proterozoic quartzites wisconsin support rapid weathering and deposition of mature quartz arenites abstract the ages of more than 200 detrital zircons from two proterozoic quartzites the baraboo and hamilton mounds quartzites from central wisconsin have been measured utilizing a laser ablation icpms technique both quartzites contain penokean 1 83 1 89 ga and archean components as well as a younger post penokean component the 69 detrital zircons from hamilton mounds contain major pre penokean age populations at 3 2 2 9 and 2 55 ga penokean ages at 1895 1853 and 1811 ma and a post penokean population at 1772 ma most detrital zircon ages are similar to previously measured ages of locally underlying gneisses and intrusive rocks of the marshfield terrane the 147 detrital zircons from two samples of the baraboo quartzite show a grossly similar age distribution with populations at 1754 and 1850 ma and 2 55 ga however the zircon populations in the baraboo quartzite differ from those of hamilton mounds in having a large population at 1920 ma a broad distribution of ages from 2 0 to 2 4 ga a