significance of late paleozoic fission track ages in volcanic rocks from the lavras do sul region southernmost brazil abstract this paper describes the application of apatite fission track thermochronology to four outcrop samples of the basaltic to andesitic volcanic rocks of the hilario formation located in the precambrian sul rio grandense shield in southernmost brazil the investigated unit was generated during the late stages of the neoproterozoic brasiliano pan african cycle 900 500 ma in a context of strike slip motion along ne trending structures and consequent basin inception the apparent fission track ft ages fall in the range 270 1 to 298 2 ma late carboniferous to early permian the mean confined fission track lengths range between 10 76 and 12 20 mm and the estimate for initial fission track recording below 110 c is positioned around 359 and 344 ma considering this the neoproterozoic volcanic rocks of the hilario formation were buried under at least 4 km of volcanic and sedimentary rocks resulting in thermal fading of all the original fission tracks and remained so until the late paleozoic then the investigated rocks and the whole region acted as the southern margin and source area for the paleozoic mesozoic intraplate parana basin a long lived cooling trend from the early carboniferous 359 344 ma to the permian triassic boundary ca 250 ma can be inferred from the analysis of the thermal history curves this tendency can be associated to the conjunction of uplift and denudation of this region during the carboniferous it probably reflects a response to the compressive events in the southwestern gondwanan margin in the context of the eo hercynian chanic orogeny this period in the studied region was also characterized by extensive ice caps related to the gondwanan glaciation and by lack of sedimentation with the collapse of the glaciers alluvial fan and glaciogenic sedimentation took place during the late carboniferous and early permian this cooling phase is probably recorded by the apparent ft ages a subsequent uplifting event is revealed by the analysis of the thermal history curves which show a strong inflection between 90 and 70 ma these data are here tentatively correlated with significant re arrangements of the brazilian southeastern coast such as the serra do mar and serra da mantiqueira uplifts

correlations between the classic parana and cape karoo sequences of south america and southern africa and their basin infills flanking the gondwanides du toit revisited

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

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

paleo and neoproterozoic magmatic and tectonometamorphic evolution of the isla cristalina de rivera nico perez terrane uruguay the isla cristalina de rivera crystalline complex in northeastern uruguay underwent a multistage magmatic and metamorphic evolution based on shrimp u pb zircon th u pb monazite chime epma method and k ar age data from key units several events can be recognized 1 multistage magmatism at 2 171 2 114 ma recorded on zircon of the granulitic orthogneisses and their 2 093 2 077 ma overgrowths 2 a distinct amphibolite facies metamorphism at 1 980 ma recorded by monazite 3 greenschist facies reworking and shearing at ca 606 ma monazite and k ar on muscovite along the rivera shear zone and finally 4 intrusion of the post tectonic sobresaliente and las flores granites at around 585 ma lithological similarities geographic proximity and coeval magmatic and metamorphic events indicate a similar tectonometamorphic evolution for the isla cristalina de rivera the valentines block in uruguay and the santa maria chico granulitic complex in southern brazil since at least 2 1 ga

a helium based model for the effects of radiation damage annealing on helium diffusion kinetics in apatite abstract widely used to study surface processes and the development of topography through geologic time u th he thermochronometry in apatite depends on a quantitative description of the kinetics of 4he diffusion across a range of temperatures timescales and geologic scenarios empirical observations demonstrate that he diffusivity in apatite is not solely a function of temperature but also depends on damage to the crystal structure from radioactive decay processes commonly used models accounting for the influence of thermal annealing of radiation damage on he diffusivity assume the net effects evolve in proportion to the rate of fission track annealing although the majority of radiation damage results from recoil while existing models adequately quantify the net effects of damage annealing in many geologic scenarios experimental work suggests different annealing rates for the two damage types here we introduce an alpha damage annealing model adam that is independent of fission track annealing kinetics and directly quantifies the influence of thermal annealing on he diffusivity in apatite we present an empirical fit to diffusion kinetics data and incorporate this fit into a model that tracks the competing effects of radiation damage accumulation and annealing on he diffusivity in apatite through geologic time using time temperature paths to illustrate differences between models we highlight the influence of damage annealing on data interpretation in certain but not all geologic scenarios the interpretation of low temperature thermochronometric data can be strongly influenced by which model of radiation damage annealing is assumed in particular geologic scenarios involving 1 2 km of sedimentary burial are especially sensitive to the assumed rate of annealing and its influence on he diffusivity in cases such as basement rocks in grand canyon and the canadian shield u th he ages predicted from the adam can differ by hundreds of ma from those predicted by other models for a given thermal path involving extended residence between 40 80 c

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

phanerozoic low temperature evolution of the uruguayan shield along the south american passive margin the crystalline basement of uruguay was assembled during the brasiliano orogeny in the neoproterozoic era and was later affected by discrete tectonic activity a new multi method low temperature dataset including u th he ages from both zircon and apatite t t modelling and k ar dating of fine sericite fractions and fault gouge reveal a detailed post orogenic geological history spanning the phanerozoic eon the juxtaposition of the terranes that compose the area was achieved in the ediacaran period and post collision was marked by intense exhumation in which the crystalline basement reached near surface conditions by the early to mid palaeozoic regional subsidence promoted sedimentation in the parana basin until the permian covering and reheating much of the basement that is at present exposed afterwards deposition and volcanism were mostly confined to its current limits regional exhumation of the shield during the permo triassic exposed much of the northern portion of the basement and the south was further affected by the opening of the south atlantic ocean during the mesozoic little exhumation affected the uruguayan shield during the cenozoic as reflected in its modest topography the reactivation of inherited neoproterozoic structures influenced the development of mesozoic basins and the present day landscape supplementary material supplementary data sample locations u th he data and k ar data are available at https doi org 10 6084 m9 figshare c 3702043

fundamentals of fission track thermochronology fission track ft analysis has developed into one of the most useful techniques used throughout the geologic community to reconstruct the low temperature thermal history of rocks over geological time the ft method is based on the accumulation of narrow damage trails i e fission tracks in uranium rich mineral grains e g apatite zircon titanite and natural glasses which form as a result of spontaneous nuclear fission decay of 238u in nature price and walker 1963 fleischer et al 1975 the time elapsed since fission tracks began to accumulate is estimated by determining the density of accumulated tracks in a particular material in relation to the uranium content of that material chemical etching can be used to enlarge fission tracks that have formed within a mineral in order to make them readily observable under an ordinary optical microscope price and walker 1962 if a host rock is subjected to elevated temperatures fission tracks that have formed up to that point in time are shortened progressively and eventually erased by the thermal recovery i e annealing of the damage fleischer et al 1975 because thermal diffusion basically governs the annealing process the reduction in ft length is a function of heating time and temperature importantly fission tracks are partially annealed over different temperature intervals within different minerals this characteristic allows for the construction of time temperature paths of many different rock types by a plotting ft and other isotopic ages from different minerals versus their closure temperatures which is applicable in the case of a monotonous cooling history e g wagner et al 1977 zeilter et al 1982 and or by b the inverse modeling of observed ft age and confined track length data e g corrigan 1991 lutz and omar 1991 gallagher 1995 ketcham et al 2000 see also ketcham 2005

fission track lengths in the apatite annealing zone and the interpretation of mixed ages abstract measurements of confined fission tracks in apatites from deep boreholes show that their mean length is reduced and the length distribution becomes progressively broader through the fission track annealing zone at subsurface temperatures of around 100 c and above the length distribution characteristically becomes very broad without a pronounced peak the lengths of the longest tracks remain essentially constant at all stages of natural annealing observed in the boreholes this pattern is similar to that found in laboratory annealing of spontaneous fission tracks in apatite from an outcrop sample of the same formation these observations show that confined track lengths can be used as important indicators of the type of thermal history that a sample has experienced in the temperature zone of increasing track stability the length distributions reported here provide a basis for interpretation of fission track ages that might otherwise be ambiguous an apatite age which results from a uniform slow cooling history will have a broad negatively skewed length distribution while a bimodal distribution provides clear evidence of a two stage history involving partial annealing by a discrete thermal event the apparent fission track age associated with a bimodal length distribution will be a mixed age intermediate between the original age and that of the later event

climate change and the episodicity of sediment flux of small california rivers abstract we studied the streamflow and sediment flux characteristics of the 20 largest streams entering the pacific ocean along the central and southern california coast extending for 750 km from monterey bay to just south of the u s mexico border drainage basins ranged in area from 120 to 10 800 km2 with headwater elevations ranging from 460 to 3770 m annual streamflow ranged from 0 to a maximum of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 1 times 10 9 end document m3 yr for the santa clara river in 1969 with a

the influence of burial heating on the u th he system in apatite grand canyon case study article history thermochronological data can constrain the cooling paths of rocks exhumed through the uppermost 1 2 km of earth s crust and have thus been pivotal in illuminating topographic development over timescales 0 1 ma however in some cases different methods have led to conflicting conclusions about timing of valley scale exhumation here we investigate the case of western grand canyon usa where different thermochronological datasets have been interpreted to record very different timings of canyon incision 70 ma versus 5 ma we present a method to assess key assumptions in these constraints and demonstrate that burial heating conditions of basement rocks in the mesozoic can result in incomplete annealing of radiation damage in apatite in turn this has a dramatic effect on the temperature sensitivity of the apatite u th he system and its ability to record post burial exhumation the possibility of incomplete annealing resolves the apparent conflict in time temperature paths inferred over the last 70 ma although it requires temperatures during burial that are lower than predicted by apatite fission track data a refinement of parameters that prescribe the kinetics of damage annealing and related control on 4 he diffusivity in apatite would account for this discrepancy specifically if alpha recoil damage anneals at a lower rate than fission tracks at a given temperature these effects will be important for any application of the apatite u th he system in geologic settings that experienced prolonged residence 10 ma between 50 150 c the approaches developed here provide means to assess these effects

apatite u th sm he thermochronology on the wrong side of the tracks abstract despite the increasing application of apatite u th sm he thermochronology in a range of settings the technique suffers from two major unresolved problems which preclude reliable thermal history interpretations on a routine basis one problem is the common but unexplained over dispersion of single grain ages compared to predictions based on accepted models of diffusion systematics including enhanced he retentivity due to the buildup of radiation damage in the apatite crystal lattice a second related problem is that the widely adopted rdaam model does not provide an accurate representation of the degree to which helium retentivity is enhanced as a result of the accumulation of radiation damage until these problems are resolved and eliminated thermal history interpretations derived from this method cannot be regarded as reliable routine application of the technique requires a more rigorous quantitative understanding of these factors further experimental studies are required to identify all significant sources of variation in measured ages and to develop more accurate models of the thermal response of the system combined with rigorous calibration and validation against independent methods in well constrained natural settings only once this is achieved will the technique be capable of producing robust and reliable interpretations

the continental margin of uruguay crustal architecture and segmentation abstract the uruguayan continental margin comprises three sedimentary basins the punta del este pelotas and oriental del plata basins the genesis of which is related to the break up of gondwana and the opening of the atlantic ocean herein the continental margin of uruguay is studied on the basis of 2d multichannel reflection seismic data as well as gravity and magnetic surveys as is typical of south atlantic margins the uruguayan continental margin is of the volcanic rifted type large wedges of seaward dipping reflectors sdrs are clearly recognizable in seismic sections sdrs flat lying basalt flows and a high velocity lower crust hvlc form part of the transitional crust the sdr sequence subdivided into two wedges has a maximum width of 85 km and is not continuous parallel to the margin but is interrupted at the central portion of the uruguayan margin the oceanic crust is highly dissected by faults which affect post rift sediments a depocenter over oceanic crust is reported deepwater pelotas basin and volcanic cones are observed in a few sections the structure of continental crust sdrs flat flows oceanic crust is reflected in the magnetic anomaly map the positive free air gravity anomaly is related to the shelf break while the most prominent positive magnetic anomaly is undoubtedly correlated to the landward edge of the sdr sequence given the attenuation interruption and or sinistral displacement of several features most notably sdr sequence magnetic anomalies and depocenters we recognize a system of nw se trending transfer faults here named rio de la plata transfer system rpts two tectono structural segments separated by the rpts can therefore be recognized in the uruguayan continental margin segment i to the south and segment ii to the north

apatite fission track analysis apatite fission track aft analysis has been widely used during the past 30 years to constrain the low temperature thermal histories of many igneous metamorphic and sedimentary rocks in a wide range of geological settings applicable geological settings include orogenic belts rifted margins faults sedimentary basins cratons and mineral deposits the types of geologic problems that can be addressed include the timing and rates of tectonic events sedimentary basin evolution the timing of hydrocarbon generation and ore mineralization the absolute age of volcanic deposits the effects of major climatic changes on the near surface geothermal gradient and long term landscape evolution early work by naeser 1967 and wagner 1968 1969 first established the basic procedures that enabled the fission track dating method to be applied routinely to these geologic problems fleischer et al 1975 summarized the early studies of the broader discipline of nuclear track detection in different solid state materials more recent comprehensive overviews of fission track applications have been provided by naeser and mcculloh 1989 wagner and van den haute 1992 gallagher et al 1998 van den haute and de corte 1998 dumitru 2000 and gleadow et al 2002 etched natural fission tracks in several apatite grains are shown in figure 1 successful aft analysis is limited by the following 1 the availability of apatite from which useful aft data can be obtained often due to a lack of apatite of sufficient grain size and quality within available rock types or alternatively the lack of available rock samples due to minimal outcrop exposure or other reasons 2 economic considerations in terms of the time and money required to obtain sufficient aft data 3 the inherent limitations of aft data to resolve geological thermal history information often related to limited numbers of accumulated spontaneous fission tracks due to

fluvial sediment transport and deposition following the 1991 eruption of mount pinatubo the 1991 eruption of mount pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows bedload sampling in the pasig potrero river one of the most heavily impacted rivers revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth fine grained bed dimensionless bedload transport rates in the pasig potrero river differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments the similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply in arid rivers this is attributed to a flashy hydrograph whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery this work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced d 2002 elsevier science b v all rights reserved

swift snowmelt and floods lahars caused by great pyroclastic surge at mount st helens volcano washington 18 may 1980 the initial explosions at mount st helens washington on the moring of 18 may 1980 developed into a huge pyroclastic surge that generated catastrophic floods off the east and west flanks of the volcano near source surge deposits on the east and west were lithic sorted lacking in accretionary lapilli and vesiculated ash not plastered against upright obstacles and hot enough to char wood all attributes of dry pyroclastic surge material deposited at the surge base on steep slopes near the volcano transformed into high concentration lithic pyroclastic flows whose deposits contain charred wood and other features indicating that these flows were hot and dry stratigraphy shows that even the tail of the surge had passed the east and west volcano flanks before the geomorphically distinct floods lahars arrived this field evidence undermines hypotheses that the turbulent surge was itself wet and that its heavy components segregated out to transform directly into lahars nor is there evidence that meters thick snow slab avalanches intimately mixed with the surge to form the floods the floods must have instead originated by swift snowmelt at the base of a hot and relatively dry turbulent surge impacting hot pyroclasts probably transferred downslope momentum to the snow surface and churned snow grains into the surge base melting snow and accumulating hot surge debris may have moved initially as thousands of small thin slushflows as these flows removed the surface snow and pyroclasts newly uncovered snow was partly melted by the turbulent surge base this and accumulating hot surge debris in turn began flowing a self sustaining process feeding the initial flows the flows thus grew swiftly over tens of seconds and united downslope into great slushy ejecta laden sheetfloods gravity accelerated the floods to more than 100 km h as they swept down and off the volcano flanks while the snow component melted to form great debris rich floods lahars channeled into valleys

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

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

the chronology and tectonic style of landscape evolution along the elevated atlantic continental margin of south africa resolved by joint apatite fission track and u th sm he thermochronology atlantic type continental margins have long been considered passive tectonic settings throughout the entire postrift phase recent studies question the long term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin s evolution the namaqualand sector of the western continental margin of south africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity here we present the first systematic study using joint apatite fission track and apatite u th sm he thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region apatite fission track ages range from 58 3 2 6 to 132 2 3 6ma with mean track lengths between 10 9 0 19 and 14 35 0 22 m and mean u th sm he sample ages range from 55 8 31 3 to 120 6 31 4ma joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150 130ma and 110 90ma with limited cooling during the cenozoic estimates of denudation based on these thermal histories predict approximately 1 3 km of denudation coinciding with two major tectonic events the first event during the early cretaceous was driven by continental rifting and the development and removal of synrift topography the second event during the late cretaceous includes localized reactivation of basement structures as well as regional mantle driven uplift relative tectonic stability prevailed during the cenozoic and regional denudation over this time is constrained to be less than 1 km

40ar 39ar and rb sr geochronology of the uruguayan dike swarm rio de la plata craton and implications for proterozoic intraplate activity in western gondwana abstract the uruguayan dike swarm intrudes country rocks of the rio de la plata craton that were deformed and metamorphosed to high grade during the 2200 1900 ma transamazonian orogeny slow cooling of the continental crust following this event is suggested by the 40 ar 39 ar plateau ages of hornblende 2016 11 ma and biotite 1817 10 ma from the pintos post tectonic granodiorite there are seven 40 ar 39 ar age determinations on mafic dikes and felsic veins from the swarm between 1727 and 1700 ma one dike yielded concordant plateau ages for igneous hornblende 1727 10 ma and biotite 1725 10 ma these ages are taken as the best estimate for the emplacement of the uruguayan swarm a rb sr whole rock best fit line calculated for 15 dikes yielded 1766 124 ma in agreement within error with the 40 ar 39 ar plateau ages the origin of the uruguayan swarm is tectonically related to the emplacement of the illescas and minas de corrales rapakivi granites that crop out in the nico perez domain the tectonic setting of these units is also compatible with bimodal magmatism and basin formation associated with extensional events in the uruguayan and brazilian shields on the whole the geologic framework suggests that the uruguayan dike swarm marks a transcontinental intraplate episode within the south america continent which occurred shortly after the transamazonian orogeny the results of initial release portions of 40 ar 39 ar spectra dikes and felsic veins coupled with a rb sr mineral isochron from one felsic vein are between 1370 and 1170 ma eight k ar ages range from 1600 to 1170 ma suggesting variable argon loss in the systems these isotopic disturbances were probably favored by low grade hydrothermal overprints exemplified by differences in the 40 ar 39 ar spectra from one felsic vein muscovite and biotite and suggested also by the secondary mineralogy that may be present in the dikes these post emplacement ages could result from mesoproterozoic crustal shortening recognized in the nico perez domain interpreted to be tectonically associated with the namaqua orogeny of southern africa this inference is important in understanding the late mesoproterozoic evolution of western gondwana and brings potential implications for a paleogeographic reconstruction between the paleocontinent plata and amazonia at the time of the grenville orogeny

chemostratigraphy of mesoproterozoic and neoproterozoic carbonates of the nico perez terrane rio de la plata craton uruguay abstract increasing evidence shows that mesoproterozoic rocks are widespread in the rio de la plata craton carbon and strontium isotope analyses were carried out for three different carbonate bearing successions in the southern nico perez terrane the parque ute group is erected comprising from base to top the mainly volcanogenic canada espinillo formation the dolomitic mina valencia formation and the mixed carbonate siliciclastic cerro del mastil formation a 13 c curve was obtained for carbonates of the parque ute group which is characterized by a plateau at 1 to 1 6 v pdb bracketed between two negative excursions 1 8 v pdb at the base and 3 3 v pdb at the top these values are consistent with a mesoproterozoic depositional age for the unit as indicated by u pb ages of synsedimentary volcanics and gabbros of 1429 21 and 1492 4 ma respectively the mataojo formation and the marcos de los reyes formation were previously thought to be of similar age the chemostratigraphic data however show that there is a hiatus of at least 700 myr between these units and that neither is correlative of the parque ute group dolomitic marbles of the mataojo formation yielded a 13 c curve that varies within a narrow range around 0 v pdb 0 6 to 0 4 this along with the youngest u pb detrital zircon ages of 1802 ma constrains deposition between 1 8 and 1 5 ga late palaeoproterozoic early mesoproterozoic in contrast with the latter unit the marco de los reyes formation yielded 13 c values defining moderate to high amplitude 13 c excursions between 4 4 v pdb and 3 2 v pdb corresponding 87 sr 86 sr values range between 0 7070 and 0 7080 both c and sr chemostratigraphy point to a later neoproterozoic possibly ediacaran age for the marco de los reyes formation it is here proposed that modest 13 c seawater enrichment and low amplitude 13 c excursions following a long period of isotopic invariance around 0 v pdb began already around 1 50 1 45 ga thereafter the amplitude of 13 c excursions increased gradually until the late neoproterozoic one important conclusion emerging from the data presented in this paper is that mesoproterozoic rocks are common in the rio de la plata craton and probably fringe it at both its western and eastern side this suggests a rather central position of the craton within rodinia as also suggested by other lines of evidence such as detrital zircon ages and a widespread thermal overprint at 1 25 ga

low temperature thermochronology of the south atlantic margin along uruguay and its relation to tectonic events in west gondwana abstract the geodynamic forces acting during jurassic cretaceous south atlantic rifting provoked intense transformations in west gondwana such as the reactivation of ancient basement structures voluminous magmatism and general uplift of the new continental margins low temperature thermochronology records cooling associated with uplift syn and post breakup along the brazilian margin while further south in uruguay mostly pre breakup uplift is identified thermochronometry data are scarce in uruguay but previous studies suggest that basement cooling and exhumation preceded west gondwana breakup by hundreds of millions of years to improve our knowledge of uruguay s thermotectonic evolution in this study we present 19 apatite fission track ages 42 apatite and 40 zircon u th he single crystal ages for the uruguayan shield uys from which we modeled 19 inverse thermal histories our results suggest that the uys temperatures were below 200 c since the early paleozoic and that cooling below 110 c started during the carboniferous with continuous exhumation of the basement until early cretaceous the onset of this long term uplift is correlated with orogenesis and terrane accretions in the sw margin of west gondwana during the paleozoic lithosphere thinning and uplift preceding breakup contributed to the continuous late paleozoic to middle mesozoic exhumation until the voluminous volcanism of the parana etendeka large igneous province c 133 ma this magmatic event combined with the thermal influence of the tristao da cunha mantle plume and rift spreading likely raised the basement geotherm during the late cretaceous models suggest a slight increase in temperatures of the uys from late cretaceous until the oligocene when a final cooling to surface temperatures took place our findings corroborate a long and complex thermal history for uruguay with crustal uplift occurring essentially before west gondwana breakup

constraining south atlantic growth with seafloor spreading data recent models of south atlantic opening history focus on early plate divergence by incorporating intracontinental deformation which is poorly constrained aiming to avoid the uncertainties in this approach we model the entire divergence history with a joint inversion for seafloor spreading data for this history the pre campanian motion parameters are the first to feature formal uncertainty estimates we date the onset of spreading at 138 ma with movement along intracontinental accommodation zones leading to the assembly of south america by 123 ma and africa by 106 ma part of the ridge in the agulhas basin jumped westward soon afterward toward the bouvet plume initiating the motion of a short lived malvinas plate the ne georgia and maud rises and agulhas plateau formed as a large igneous province over the plume farther north part of the ridge jumped eastward toward the tristan plume around 94 93 ma but seems not to have resulted in independent plate motion our results show that the south atlantic grew by diachronous breakup of continents on just two plates cretaceous intracontinental deformation in south america and africa can be interpreted in terms of the accommodation of stress associated with northward propagation of this process the pattern of accommodation is usually envisaged as focusing all of the strain in narrow belts with our rotations a commonly used set of such belts accounts instead for just 42 67 of the implied total strain we suggest that the remainder was accommodated at all scales within the continental interiors and the extended continental margins

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

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

sedimentary and geomorphic responses to ignimbrite emplacement readjustment of the waikato river after the a d 181 taupo eruption new zealand abstract the posteruption readjustment of landscapes to major explosive volcanic eruptions is one of the most dramatic processes in physical sedimentology and geomorphology large ignimbrite emplacing eruptions from rhyolitic caldera volcanoes can form new landscapes over 104 km2 create new depocenters in calderas and bury existing topography and drainage systems beneath hundreds of meters of pyroclastic material subsequent remobilization of this debris constitutes a widespread and enduring hazard in the central north island of new zealand the a d 181 taupo eruption destroyed hydrologic systems over 20 000 km2 when emplacement of the climactic ignimbrite filled valleys and depressions with up to 70 m of pyroclastic material and blocked the waikato river outlet to the intracaldera lake basin posteruption remobilization of this material in the waikato catchment was associated with axial stream reestablishment vertical lateral and proximal distal changes in volcaniclastic lithofacies preserved al

isotope geochemistry and geochronology of the nico perez terrane rio de la plata craton uruguay u pb shrimp zircon geochronology bulk rock geochemistry and sr nd isotopic data are herein presented in order to better constrain the tectonic evolution of the nico perez terrane npt uruguay the studies are focused on two key geological units located in the southern part of the npt namely carape complex cc and lavalleja metamorphic complex lmc the carape complex previously interpreted as part of a remnant magmatic arc formed at 1 75 ga mainly by partial melting of archean crust t dm model ages between 2 8 and 3 0 ga with strong negative e nd t values the lavalleja metamorphic complex previously interpreted as a back arc basin has volcanic units formed at 0 59 ga during the brasiliano cycle but may also contain older paleoproterozoic units the geochemistry of the lmc meta igneous rocks indicates a complex scenario two distinct signatures have been recognized one similar to morbs and the other similar oibs sm nd t dm model ages of basic rocks present a bimodal paleoproterozoic and archean distribution which is also recognized in several other units of the npt in the light of new results we conclude that the npt represents a cratonic margin setting characterized by the stacking of lithotectonic assemblages with archean paleoproterozoic and neoproterozoic age at the margin of the rio de la plata craton

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

evolution of the south atlantic passive continental margin in southern brazil derived from zircon and apatite u th sm he and fission track data abstract the area between sao paulo and porto alegre in southeastern brazil plays a key area to understand and quantify the evolution of the south atlantic passive continental margin sapcm in brazil in this contribution we present new thermochronological data attained by fission track and u th sm he analysis on apatites and zircons from metamorphic sedimentary and intrusive rocks the zircon fission track ages range between 108 4 15 0 and 539 9 68 4 ma the zircon u th sm he ages between 72 9 5 8 and 525 1 2 4 ma whereas the apatite fission track ages range between 40 0 5 3 and 134 7 8 0 ma and the apatite u th sm he ages between 32 1 1 5 and 93 0 2 5 ma the spatial distribution of these ages shows three distinct blocks with a different evolution cut by old fracture zones while the central block exhibits an old stable block the northern and especially the southern block underwent complex post rift exhumation the sample of the northern block shows two distinct cooling phases in the upper cretaceous and the paleogene to neogene after sedimentation of the permian sandstones the samples of the central block were never heated up over 100 c with a following moderate to fast cooling phase in cretaceous to eocene time and a fast cooling between oligocene to miocene the five thermal models obtained in the southern block indicate a complex evolution with three cooling phases the exhumation events of the three blocks correspond with the parana etendekka event the alkaline intrusions due to the trinidad hotspot and the evolution of the continental rift basins in se brazil and are therefore most likely to be the major force for the post rift evolution of the passive continental margin in se brazil which therefore corresponds to the three main phases of the andean orogeny

neoproterozoic magmatic and metamorphic events in the cuchilla dionisio terrane uruguay and possible correlations across the south atlantic abstract the cuchilla dionisio or punta del este terrane of the southern dom feliciano belt eastern uruguay experienced a prolonged neoproterozoic to cambrian tectonothermal evolution zircon and monazite age data from orthogneisses of the basement of the terrane cerro olivo complex provide evidence of widespread magmatism at c 780 ma and high grade up to granulite facies metamorphism between 655 and 632 ma isotope data imply that the orthogneiss protolithic melts formed in a within plate rift related setting by neoproterozoic re melting of lower crustal material that was extracted from the mantle during the palaeoproterozoic we show that the terrane extends further north than previously thought the presence of inherited meso and palaeoproterozoic zircon grains indicates the existence of older crustal components in the terrane the ages of the detrital zircon grains and their hf isotope signatures cannot be explained by a rio de la plata craton provenance but are consistent with a southern african kalahari craton origin ar ar ages of 628 and 625 ma were obtained on magnesiohornblende in amphibolite from the northern cuchilla dionisio terrane whose protolith formed in an ediacaran back arc basin these ages are interpreted as the time of amphibolite facies metamorphism thus crustal convergence between the kalahari or congo and the rio de la plata craton and subduction of the intervening oceanic basin must have been underway between 655 and 625 ma the early neoproterozoic basement of the cuchilla dionisio terrane is part of the southern african namaqua province that was separated from the kalahari craton during rodinia break up and opening of the southern adamastor ocean in the tonian closure of this ocean in the late cryogenian to early ediacaran was associated with arc magmatism and high temperature metamorphism of the cuchilla dionisio terrane basement a back arc basin formed and was closed between this arc and the western african margin relics of this basin were obducted onto the cuchilla dionisio terrane at c 625 ma late neoproterozoic intrusive igneous rocks from the cuchilla dionisio and the coastal terranes were emplaced in a supra subduction setting at least in uruguay and have the same hf isotope signature

influence of crystal size on apatite u th he thermochronology an example from the bighorn mountains wyoming near surface tectonic and geomorphic processes involve cooling of rocks through low temperatures 50 200 c because rates of helium diffusion in apatite titanite and zircon are sensitive to temperature variations in this range uranium thorium helium thermochronometry u th he dating is well suited to establishing the timing and rates of these processes in the geologic record however because fractional loss of he is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic he u th he ages must vary not only with thermal history but also with crystal size here we present crystal size correlated he ages from co existing apatites from the bighorn mountains wyoming that range from 100 to 350 ma these correlations are a sensitive indicator of the rock s thermal history in a temperature range below the system s nominal closure temperature t c 70 c for apatite and are consistent with a thermal history involving residence in the upper 2 3 km of crust since the precambrian with maximum temperatures of 65 80 c just prior to laramide orogenic exhumation the influence of crystal size on he ages will be most apparent in rocks where temperatures have been in the range of partial he retention for long periods of time 30 70 c for 10 7 years in such cases accurate interpretation of u th he ages must incorporate the effect of crystal size and this method may provide insights to thermal histories of rocks in previously inaccessible low temperature ranges

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

tectonics and sedimentation in southern south america during triassic abstract triassic continental sedimentation in southern south america is related to the gondwanides orogenesis directly there were two distinct phases of sedimentation 1 the wide alluvial basins during early triassic and 2 the restricted extensional basins during middle and late triassic the main early triassic deposits are the sanga do cabral supersequence southernmost brazil and uruguay and the talampaya and tarjados sequences western argentina these deposits represent poorly channelized braided rivers with source areas in the gondwanides units under compressional tectonic setting the main middle and late triassic strata are rifts in the back arc setting such as the ischigualasto and cuyo basins western argentina and in the intracontinental setting santa maria supersequence southernmost brazil these basins are mainly composed of alluvial lacustrine and deltaic deposits with a great pyroclastic contribution in the case of the basins of argentina two rift sequences were identified in the western argentina basins composed of syn rift alluvial deposition followed by lake transgression the sag phase is marked by the progradation of deltaic and high sinuosity rivers the southernmost brazil deposits were divided into three rift sequences composed of low sinuosity rivers followed by lake transgression the progradation of deltaic and fluvial systems marks the sag phase the accommodation space generation within the back arc rifts was high if compared to the intracontinental coeval basin due to higher base level and higher tectonic subsidence rates the development of deep organic matter rich lakes suggests a more humid palaeoclimate in the back arc region than in the intracontinental setting where playa lakes dominated the back arc rifts are related to sinistral transtensional stresses in the western branch of gondwanides region the intracontinental rifts the southernmost brazil deposits and coeval rift basins of southern africa were generated by transtensional stresses within a dextral shear zone the shear is related to stress transmission from the southern branch of the gonwanides where oblique compression occurred

archaean crustal evolution in west africa a new synthesis of the archaean geology in sierra leone liberia guinea and ivory coast abstract a new synthesis of the geology and geochronology of the little known archaean rocks in sierra leone liberia guinea and ivory coast is presented in order to better understand the processes of archaean crustal evolution in this region and to attempt to interpret these data in the light of our current understanding of archaean crustal evolution in addition this study seeks to identify those aspects of archaean crustal evolution which are currently not known in this area and which need to become the subject of future studies given the economic importance of this region in terms of the mineral deposits hosted in the archaean rocks these include greenstone belt hosted iron ore lode gold chromite and columbite tantalite and younger diamondiferous kimberlites intrusive into archaean felsic gneisses the new results show that this cratonic nucleus comprises of four main geological units 1 the oldest crust is made up of 3 5 3 6 ga ttg tonalite trondhjemite granodiorite gneisses these only outcrop in the east of the craton in guinea but their presence is indicated elsewhere in the central part of the craton though xenocrystic zircon cores in younger rocks 2 the major rock type found throughout the craton is 3 26 2 85 ga ttg gneiss in detail these magmas are thought to have formed in two episodes one between 3 05 3 26 ga and the other between 2 85 2 96 ga the presence of inherited zircons in the younger suite indicate that this event represents the partial reworking of the older gneisses 3 4 ga eclogite xenoliths in kimberlite derived from the sub continental lithospheric mantle are thought to be the restite after the partial melting of a basaltic protolith in the production of the ttg magmas 3 supracrustal rocks form linear belts infolded into the ttg gneisses and metamorphosed to amphibolite and granulite grade they are of different sizes contain a variety of lithological sequences and may be of several different ages the larger supracrustal belts in sierra leone contain a thick basalt komatiite sequence derived by the partial melting of two different mantle sources unconformably overlain by a sedimentary formation they are seen as an important resource for gold iron ore chromite and columbite tantalite 4 a suite of late archaean granitoids formed by the partial melting of the ttg gneisses in a craton wide deformation metamorphic partial melting event at 2800 20 ma this thermal event is thought to be responsible for the stabilisation of the craton this new synthesis highlights major geological and geochronological similarities between the archaean rocks of sierra leone liberia guinea and ivory coast and those in the reguibat shield in the northern part of the west african craton suggesting that the two regions were once more closely related

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

natural age dispersion arising from the analysis of broken crystals part i theoretical basis and implications for the apatite u th he thermochronometer abstract over the last decade major progress has been made in developing both the theoretical and practical aspects of apatite u th he thermochronometry and it is now standard practice and generally seen as best practice to analyse single grain aliquots these individual prismatic crystals are often broken and are fragments of larger crystals that have broken during mineral separation along the weak basal cleavage in apatite this is clearly indicated by the common occurrence of only 1 or no clear crystal terminations present on separated apatite grains and evidence of freshly broken ends when grains are viewed using a scanning electron microscope this matters because if the 4 he distribution within the whole grain is not homogeneous because of partial loss due to thermal diffusion for example then the fragments will all yield ages different from each other and from the whole grain age here we use a numerical model with a finite cylinder geometry to approximate 4 he ingrowth and thermal diffusion within hexagonal prismatic apatite crystals this is used to quantify the amount and patterns of inherent natural age dispersion that arises from analysing broken crystals a series of systematic numerical experiments were conducted to explore and quantify the pattern and behaviour of this source of dispersion using a set of 5 simple thermal histories that represent a range of plausible geological scenarios in addition some more complex numerical experiments were run to investigate the pattern and behaviour of grain dispersion seen in several real data sets the results indicate that natural dispersion of a set of single fragment ages defined as the range divided by the mean arising from fragmentation alone varies from c 7 even for rapid c 10 c ma monotonic cooling to over 50 for protracted complex histories that cause significant diffusional loss of 4 he the magnitude of dispersion arising from fragmentation scales with the grain cylindrical radius and is of a similar magnitude to dispersion expected from differences in absolute grain size alone spherical equivalent radii of 40 150 m this source of dispersion is significant compared with typical analytical uncertainties on individual grain analyses c 6 and standard deviations on multiple grain analyses from a single sample c 10 20 where there is a significant difference in the u and th concentration of individual grains eu the effect of radiation damage accumulation on 4 he diffusivity assessed using the rdaam model of flowers et al 2009 is the primary cause of dispersion for samples that have experienced a protracted thermal history and can cause dispersion in excess of 100 for realistic ranges of eu concentration i e 5 100 ppm expected natural dispersion arising from the combined effects of reasonable variations in grain size radii 40 125 m eu concentration 5 150 ppm and fragmentation would typically exceed 100 for complex thermal histories in addition to adding a significant component of natural dispersion to analyses the effect of fragmentation also acts to decouple and corrupt expected correlations between grain ages and absolute grain size and to a lesser extent between grain age and effective uranium concentration eu considering fragmentation explicitly as a source of dispersion and analysing how the different sources of natural dispersion all interact with each other provides a quantitative framework for understanding patterns of dispersion that otherwise appear chaotic an important outcome of these numerical experiments is that they demonstrate that the pattern of age dispersion arising from fragmentation mimics the pattern of 4 he distribution within the whole grains thus providing an important source of information about the thermal history of the sample we suggest that if the primary focus of a study is to extract the thermal history information from u th he analyses then sampling and analytical strategies should aim to maximise the natural dispersion of grain ages not minimise it and should aim to analyse circa 20 30 grains from each sample the key observations and conclusions drawn here are directly applicable to other thermochronometers such as the apatite rutile and titanite u pb systems where the diffusion domain is approximated by the physical grain size

post breakup tectonics in southeast brazil from thermochronological data and combined inverse forward thermal history modeling the continental margin of southeast brazil is elevated onshore tertiary basins and late cretaceous paleogene intrusions are good evidence for post breakup tectono magmatic activity to constrain the impact of post rift reactivation on the geological history of the area we carried out a new thermochronological study apatite fission track ages range from 60 7 1 9 ma to 129 3 4 3 ma mean track lengths from 11 41 0 23 mm to 14 31 0 24 mm and a subset of the u th he ages range from 45 1 1 5 to 122 4 2 5 ma results of inverse thermal history modeling generally support the conclusions from an earlier study for a late cretaceous phase of cooling around the onshore taubate basin for a limited number of samples the first detectable period of cooling occurred during the early tertiary the inferred thermal histories for many samples also imply subsequent reheating followed by neogene cooling given the uncertainty of the inversion results we did deterministic forward modeling to assess the range of possibilities of this tertiary part of the thermal history the evidence for reheating seems to be robust around the taubate basin but elsewhere the data cannot discriminate between this and a less complex thermal history however forward modeling results and geological information support the conclusion that the whole area underwent cooling during the neogene the synchronicity of the cooling phases with andean tectonics and those in ne brazil leads us to assume a plate wide compressional stress that reactivated inherited structures the present day topographic relief of the margin reflects a contribution from post breakup reactivation and uplift

effective sediment transporting discharge from magnitude frequency analysis the long term effectiveness of a geomorphic event of a particular magnitude is the product of the effect of a single event of that magnitude times the frequency with which it recurs magnitude frequency analysis if discharge frequency of a stream is assumed to be log normally distributed and if sediment transport rate is assumed to be a power function of discharge then transport effectiveness attains a maximum value at some discharge termed here the effective discharge that moves the most sediment over the long term the effective discharge and its recurrence interval may be predicted if both assumptions are correct analysis of 55 u s streams demonstrates that the observed and the predicted recurrence intervals of the effective discharge are in poor agreement this lack of agreement results from the failure of the power function of discharge to predict adequately the sediment transport rate above some high discharge threshold or thresholds the observed recurrence interval of the effective discha

variability of apatite fission track annealing kinetics ii crystallographic orientation effects a method is presented that permits the length of any horizontal confined fission track inclined at a specified angle to the crystallographic c axis in apatite to be converted to an equivalent track length parallel to the crystallographic c axis the model is based on the results of annealing experiments for six selected apatites five calcian fluorapatites and durango apatite representing a subset of the 15 total apatite specimens studied an iterative process of calculation is required to project fission track lengths onto the c axis and computer source code implementing the solution to this problem is presented this method of projecting apatite fission track lengths onto the crystallographic c axis is shown to remove effectively fission track length variation within single fission track populations due to anisotropic track length reduction for all 15 apatites studied in addition a model is developed that offers predictions that closely reproduce published experimental data concerning the relationship between fission track density etched fission tracks per unit area of apatite surface and the arithmetic mean fission track length finally it is shown that natural fission track populations exhibit fission track length anisotropy similar to that of fission track populations created and annealed in the laboratory this observation implies that the same process by which apatite fission tracks anneal in the laboratory is responsible for annealing of apatite fission tracks in the geological environment

lahars at merapi volcano central java an overview abstract merapi volcano in central java is one of the most active volcanoes in the world at least 23 of the 61 reported eruptions since the mid 1500s have produced source deposits for lahars the combined lahar deposits cover about 286 km 2 on the flanks and the surrounding piedmonts of the volcano at merapi lahars are commonly rain triggered by rainfalls having an average intensity of about 40 mm in 2 h most occur during the rainy season from november to april and have average velocities of 5 7 m s at 1000 m in elevation a wide range of facies may be generated from a single flow which may transform downvalley from debris flow to hyperconcentrated streamflow because of the high frequency and magnitude of the lahar events lahar related hazards are high below about 450 600 m elevation in each of the 13 rivers which drain the volcano hazard zone maps for lahar were produced by pardyanto et al volcanic hazard map merapi volcano central java 1 100 000 geol surv of indonesia bandung ii 4 1978 and the japanese indonesian cooperation agency master plan for land conservation and volcanic debris control in the area of mt merapi jakarta 1980 but these maps are of a very small scale to meet modern zoning requirements more recently a few large scale maps 1 10 000 and 1 2000 scale and risk assessments have been completed for a few critical river systems

timing of deformation in the sarandi del yi shear zone uruguay implications for the amalgamation of western gondwana during the neoproterozoic brasiliano pan african orogeny u pb and hf zircon sensitive high resolution ion microprobe shrimp and laser ablation inductively coupled plasma mass spectrometry la icp ms ar ar hornblende and muscovite and rb sr whole rock muscovite isochron data from the mylonites of the sarandi del yi shear zone uruguay were obtained in order to assess the tectonothermal evolution of this crustal scale structure integration of these results with available kinematic structural and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630 625 ma during the collision of the nico perez terrane and the rio de la plata craton the shear zone underwent dextral shearing up to 596 ma under upper to middle amphibolite facies conditions which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 ma after emplacement of the cerro caperuza granite at 570 ma the shear zone underwent only cataclastic deformation between the late ediacaran and the cambrian the sarandi del yi shear zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the rio de la plata craton and the nico perez terrane furthermore the obtained data reveal that strain partitioning and localization with time magmatism emplacement and fluid circulation are key processes affecting the isotopic systems in mylonitic belts revealing the complexity in assessing the age of deformation of long lived shear zones

abrupt plate accelerations shape rifted continental margins rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved it is well known that strain rate dependent processes control rift evolution yet quantified extension histories of earth s major passive margins have become available only recently here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions we find that rifted margins feature an initial slow rift phase less than ten millimetres per year full rate and that an abrupt increase of plate divergence introduces a fast rift phase plate acceleration takes place before continental rupture and considerable margin area is created during each phase we reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions the extension models suggest that the two phase velocity behaviour is caused by a rift intrinsic strength velocity feedback which can be robustly inferred for diverse lithosphere configurations and rheologies our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force this mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes offering new insights into the balance of plate driving forces through time

sediment yield from disturbed earth systems changes of sediment yield rates through time reflect evolutionary changes within a landscape when a drainage basin is perturbed significantly by base level climatic or tectonic change sediment yields increase dramatically but with no further disturbance they decline rapidly these sediment yield changes have been documented at all scales from small experimental studies to incised channels to the colorado river basin and to the himalaya mountains thus the shape of the sediment yield curve can be used to estimate future sediment yields and to interpret past tectonic events

the cretaceous opening of the south atlantic ocean abstract the separation of south america from africa during the cretaceous is poorly understood due to the long period of stable polarity of the geomagnetic field the cretaceous normal superchron cns lasted between 121 and 83 6 myr ago we present a new identification of magnetic anomalies located within the southern south atlantic magnetic quiet zones that have arisen due to past variations in the strength of the dipolar geomagnetic field using these anomalies together with fracture zone locations we calculate the first set of magnetic anomalies based finite rotation parameters for south america and africa during that period the kinematic solutions are generally consistent with fracture zone traces and magnetic anomalies outside the area used to construct them the rotations indicate that seafloor spreading rates increased steadily throughout most of the cretaceous and decreased sharply at around 80 myr ago a change in plate motion took place in the middle of the superchron roughly 100 myr ago around the time of the final breakup i e separation of continental oceanic boundary in the equatorial atlantic prominent misfit between the calculated synthetic flowlines older than anomaly q1 and the fracture zones straddling the african plate in the central south atlantic could only be explained by a combination of seafloor asymmetry and internal dextral motion

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

interpreting data dispersion and inverted dates in apatite u th he and fission track datasets an example from the us midcontinent abstract new apatite u th he ahe and apatite fission track aft data were acquired for cratonic basement samples from an 80 m span of drillcore in northeastern kansas the short depth interval over which the samples were collected indicates that they should have undergone thermal histories that would be indistinguishable using low temperature thermochronometry techniques individual ahe dates from four samples range from 99 to 464 ma three samples yield dates 300 ma higher eu 39 113 ppm and substantial data dispersion uncorrelated with eu and grain size these apatites have strong and variable eu zonation aft dates for five samples range from 242 to 291 ma the sample with the highest eu apatites and oldest ahe dates yields the youngest aft results these results are inverted with ahe dates distinctly older than the corresponding aft date we explore both the causes of data dispersion and the overall compatibility of this cratonic dataset we find that geologically reasonable thermal histories can 1 explain the distribution of the moderate eu ahe data when accounting for the influence of radiation damage grain size and eu zonation on apatite he diffusivity 2 reproduce the observed dispersion in the high eu ahe data when using a viable range of eu zonation and grain size and 3 explain the aft data for the same samples the ahe and aft data are mutually consistent and viable thermal histories successfully predict the observed pattern of older ahe than aft dates for the high eu apatites together these results suggest that appropriately accounting for the known controls on apatite he diffusivity can explain the observed dispersion and inverted ahe and aft results in some thermochronometry datasets a range of ahe dates should be especially common in cratonic data because small differences in apatite he diffusivity are amplified by the thermal histories that typify cratonic settings we use these results to develop some guidelines for interpreting dispersed ahe datasets first date eu and date grain size correlations should be evaluated and if these patterns occur they can be used to better resolve the thermal history second for samples that yield inexplicably large dispersion of ahe dates uncorrelated with eu and crystal size the appropriate strategy is either to reject these samples from the suite used for thermal history interpretation or to acquire additional data to help decipher the significance of the age distribution

eruption triggered avalanche flood and lahar at mount st helens effects of winter snowpack an explosive eruption of mount st helens on 19 march 1982 had substantial impact beyond the vent because hot eruption products interacted with a thick snowpack a blast of hot pumice dome rocks and gas dislodged crater wall snow that avalanched through the crater and down the north flank snow in the crater swiftly melted to form a transient lake from which a destructive flood and lahar swept down the north flank and the north fork toutle river

consolidation and break up of the south american platform in southeastern brazil tectonothermal and denudation histories the different tectonic stages that occurred at the end of the proterozoic and during the phanerozoic have an important bearing on the tectonothermal history of the south american platform and its consolidation geochronological data u pb monazite 40ar 39ar whole rock and apatite fission track analysis from precambrian rocks of the southeastern brazilian coastline permit the modeling of a long term thermal history of the crust and constrain variable denudation rates using these data a temperature time diagram reflects a period of accelerated exhumation during the end of the brasiliano orogeny followed by long stability and reactivation of the platform during the rifting phase of the south atlantic ocean u pb zircon and monazite blocking temperature of ca 650 c data from a series of igneous bodies suggest that a tangential and transpressional tectonic regime occurred between 625 and 610 ma during the following escape tectonics between 610 and 590 ma the exhumation process indicates cooling rates of ca 12 c ma 40ar 39ar biotite ages between 540 and 510 ma ca 300 c and a corrected fission track age on apatites 100 c of 480 ma indicate an exhumation event related to block tectonics with huge vertical displacement along shear zones a long stabilization phase with low exhumation and cooling rate around 0 25 c ma was recorded from the cambro ordovician to the mesozoic at 65 ma an acceleration of the exhumation through denudation and reworking of the south american surface with cooling rate of 1 5 c ma is observed the uplift of the mantiqueira and serra do mar mountain ranges along the southeast brazilian coastline works as a climatic barrier provoking lateral erosional processes causing long term scarp retreat combined with intense but progressive denudation towards the continent a denudation of 2 5 to 4 km was calculated for such processes this lateral retreat of escarpments and flexural response can provide important insights regarding marginal isostatic uplift and the evolution of offshore sedimentary basins of southeast brazil

thermochronological constraints on the cambrian to recent geological evolution of the argentina passive continental margin abstract passive continental margins are geo archives that store information from the interplay of endogenous and exogenous forces related to continental rifting post breakup history and climate changes the recent south atlantic passive continental margins sapcms in brazil namibia and south africa are partly high elevated margins 2000 m a s l and the recent n s trending sapcm in argentina and uruguay is of low elevation in argentina an exception in elevation is arising from the higher topography 1000 m a s l of the two nw se trending mountain ranges sierras septentrionales and sierras australes precambrian metamorphic and intrusive rocks and siliciclastic rocks of ordovician to permian age represent the geological evolution of both areas the sierras australes have been deformed and metamorphosed incipient greenschist during the gondwanides orogeny the low temperature thermochronological ltt data

conflicting u th he and fission track ages in apatite enhanced he retention not anomalous annealing behaviour while initial studies showed a high degree of consistency between afta and apatite u th he dating an increasing number of studies are reporting apatite u th he ages which are older than expected on the basis of apatite fission track data from the same region we present data from a range of geological settings to document a systematic discrepancy between the two systems which becomes more pronounced in samples with older fission track ages except for samples with very low uranium contents results from a granite pebble and enclosing volcanogenic sandstone provide a well controlled test case in which independent constraints are available on the underlying thermal history these demonstrate that the progressive discrepancy between the two techniques arises not from anomalous fission track annealing behaviour as has been suggested but as a result of a change in the he retention properties of apatite this change appears to be linked to the degree of accumulated radiation damage within the crystal lattice although underlying mechanisms remain unclear we suggest detailed experiments should be performed to delineate and quantify the processes responsible until this is achieved we suggest that apatite u th he studies should also incorporate apatite fission track data as well as other low temperature indicators in order to monitor the u th he system response and to guard against the anomalous behaviour described here

radiation damage control on apatite u th he dates from the grand canyon region colorado plateau individual detrital apatite grains from the esplanade coconino and moenkopi formations in the grand canyon region of the colorado plateau yield u th he dates from 104 to 5 ma the range of dates within each unit far exceeds analytical uncertainty but correlates with both he concentration he and effective u concentration eu these dates are all signifi cantly younger than the sandstone units indicating partial to complete he loss following deposition recently published laboratory diffusion data suggest that he retentivity in apatite increases with radiation damage forward models predict that the consequences of this effect will be manifested most clearly as a correlation between u th he dates and the he and eu in suites of apatites that 1 are characterized by a large span of eu and 2 had thermal histories in which suffi cient time elapsed for the apatite he diffusion kinetics to diverge prior to reheating and partial resetting apatites in the sedimentary units investigated fi t these cri teria using geologically reasonable deposition burial and unroofi ng histories simulations that include the effect of radiation damage on apatite he retentivity can reproduce the observed distributions of apatite dates and correlations with parent and daughter concentrations these results suggest that a span of u th he dates positively correlated with eu may provide important information regarding a sample s thermal history

post rift reactivation of the onshore margin of southeast brazil evidence from apatite u th he and fission track data abstract on the continental margin of southeast brazil there is good geological evidence for post rift tectonic reactivation in the form of onshore tertiary basins up to 800 m deep and cretaceous to paleocene alkaline intrusive bodies at the surface with the aim of constraining the mesozoic and cenozoic exhumation history of this region we present new apatite u th he ages on basement samples which one of us previously used for apatite fission track aft analysis the new he ages range from 115 5 2 2 ma to 47 0 6 4 ma whereas the aft ages range from 330 3 22 ma to 64 5 3 ma inverse thermal history modelling of all the data provides more detail on the post rift cooling history than do the aft data alone furthermore models incorporating radiation damage effects on he diffusion produce thermal histories that are more consistent with independent geological evidence than models without these effects the results imply that only a few samples record an early cooling phase around the time of opening of the south atlantic ocean the new u th he data also imply a late cretaceous phase of exhumation between 90 and 60 ma subsequently the region underwent slow cooling i e erosion rate two or three samples seem to indicate a period of rapid cooling during the neogene although this is at the limit of resolution of the methods the post rift cooling phases imply exhumation from depths between 2 and 5 km this activity we attribute to compression in the south american plate between the andean subduction zone and the mid atlantic ridge in southeast brazil compression acted on a crust that was thermally or mechanically susceptible to reactivation during the late cretaceous deformation in the study area was of regional extent whereas during the tertiary it concentrated along pre existing shear zones of the precambrian brasiliano orogeny

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

long term distributions of annual sediment yields from small watersheds annual sediment yields from small 20 km2 mountain watersheds in the northwestern united states are approximately log normally distributed over time stochastic sediment yield models based on brief infrequent transport events reproduce the log normal property observed cumulative distributions of annual yields from several pacific northwest drainages also appear to be log normal the property gives a statistical basis for interpreting annual yield data for example impacts of watershed disturbance should be assessed by median rather than mean annual yield

posteruption suspended sediment transport at mount st helens decadal scale relationships with landscape adjustments and river discharges received 11 december 2002 revised 30 september 2003 accepted 18 november 2003 published 17 january 2004 1 widespread landscape disturbance by the cataclysmic 1980 eruption of mount st helens abruptly increased sediment supply in surrounding watersheds the magnitude and duration of the redistribution of sediment deposited by the eruption as well as decades to centuries old sediment remobilized from storage have varied chiefly with the style of disturbance posteruption suspended sediment transport has been greater and more persistent from zones of channel disturbance than from zones of hillslope disturbance despite the severe landscape disturbances caused by the eruption relationships between discharge magnitudes and frequencies and suspended sediment transport have been remarkably consistent discharges smaller than mean annual flows generally have transported 5 but locally 15 of the annual suspended sediment loads and infrequent p 0 01 large floods have transported as much as 50 of the annual suspended sediment loads in a single day however moderate magnitude discharges those greater than mean annual flows but less than 2 year floods have transported the greatest amounts of sediment from all disturbance zones such discharges have transported on average 60 to 95 of the annual suspended sediment loads usually within cumulative periods of 1 3 weeks each year although small magnitude and largemagnitude discharges have locally and episodically transported considerable amounts of suspended sediment there has not been any notable change in the overall nature of the effective discharges moderate magnitude flows have been the predominant discharges responsible for transporting the majority of suspended sediment during 20 years of posteruption landscape adjustment index terms 1824 hydrology geomorphology 1625 1815 hydrology erosion and sedimentation 1860 hydrology runoff and streamflow keywords sediment yield suspended sediment transport mount st helens erosion volcano geomorphology

west gondwana orogenies and pangaea break up thermotectonic effects on the southernmost mantiqueira province brazil through the joint use of apatite fission tracks aft and u th he analysis in apatite ahe and zircon zhe we evaluate the thermotectonic evolution of the sul rio grandense shield srgs southernmost mantiqueira province brazil formed during the assembly of west gondwana neoproterozoic the shield comprises four tectonostratigraphic terranes separated by regional faults and shear zones the central and eastern terranes present mesozoic aft ages whereas the western terrane ages are mostly late paleozoic ahe ages show considerable dispersion although most are mesozoic zhe ages from the east are early permian whereas the west presents devonian ages inverse thermal modelling indicates a devonian to carboniferous cooling phase in the west time correlated with orogenies occurring at the sw margin of gondwana which affected the regional geodynamics and are possibly linked to limited uplift of the shield from the permian to jurassic a major cooling phase took place in the srgs probably related to lithosphere thinning and uplift preceding the south atlantic rifting samples closer to the atlantic coast suggest a subtle reheating after this event provisionally linked to a geothermal disturbance related to ocean opening and associated magmatism a final post paleocene cooling phase towards surface conditions affected the entire srgs supplementary material the final inverse thermal model for each sample is available at https doi org 10 6084 m9 figshare c 4579547

incomplete retention of radiation damage in zircon from sri lanka a suite of 18 zircon gemstones from placers in the highland southwestern complex sri lanka were subjected to a comprehensive study of their radiation damages and ages the investigation included x ray diffraction raman and pl spectroscopy electron microprobe pixe and hrtem analysis as well as u th he and shrimp u th pb age determinations zircon samples described in this study are virtually homogeneous they cover the range from slightly metamict to nearly amorphous generally concordant u th pb ages averaging 555 11 ma were obtained late ordovician zircon u th he ages scattering around 443 9 ma correspond reasonably well with previously determined biotite rb sr ages for rocks from the hswc slightly to moderately metamict zircon has retained the radiogenic he whereas only strongly radiation damaged zircon calculated total fluences exceeding 3 5 10 18 events g has experienced significant he loss when compared to unannealed zircon from other localities sri lanka zircon is about half as metamict as would correspond to complete damage accumulation over a 555 m y lasting self irradiation period suggesting significant annealing of the structural radiation damage insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon we suggest to estimate effective doses for sri lanka zircon by multiplying total fluences which were calculated using the zircon u th pb age by a correction factor of 0 55 this conversion may be applied to literature data as well because all gem zircon samples from sri lanka this work and previous studies seem to reveal the same general trends of property changes depending on the radiation damage the use of effective doses for sri lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation damaged zircon

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

effects of u th rich grain boundary phases on apatite helium ages abstract single grain apatite u th sm he ages he ages from non detrital samples sometimes show larger dispersions than can be explained by known sources of age dispersion such as grain size radiation damage parent zonation fracturing or intracrystalline inclusions we present observations and model age bias effects of an additional source of apatite he age dispersion observed in some cases u th rich grain boundary phases gbps precipitated on apatite crystal surfaces apatite grains from several samples with dispersed apatite he ages are coated or partially coated by reddish orange gbps rich in fe u and th these gbps are heterogeneous in thickness grain coverage and composition and have effective u concentrations eu up to 1000 ppm these phases may have large effects on the bulk eu and 4he compositions of apatite grains and can produce significant he age biases the direction and magnitude of this age bias is primarily a function of four factors 1 the host grain s size 2 the gbps thickness and eu relative to the host grain 3 the timing of gbp formation relative to the cooling age of the host grain and 4 whether the gbps are preserved and analyzed with the grain during he dating some of the most severe effects occur when gbps formed before or near the time of the apatite cooling age but were lost just prior to analysis for example during mineral separation in this case gbps of commonly observed thicknesses 1 10 m with eu 2 10 times that of the host apatite grain implant enough 4he into typical sized apatite grains to produce hundreds of percent positive age biases in contrast when u th rich gbps are preserved and analyzed with apatite grains the resulting he ages can be negatively biased though the heterogeneity variable preservation and ambiguous formation ages of gbps preclude a general quantitative and practical solution to this problem our model demonstrates that observed age dispersions in some samples are consistent with the effects of u th rich grain boundary phases and illustrates the conditions under which and by how much they will bias he ages