geomorphological significance of at many stations hydraulic geometry the critically important at a station hydraulic geometry ahg relationships relate hydraulic variables depth width or velocity to discharge in power law form the recently discovered at many stations hydraulic geometry amhg states that ahg exponents and coefficients are strongly correlated removing one parameter and lending amhgs to remote sensing discharge estimation despite the excitement there is an urgent need to clarify amhg s geomorphological significance for different hydraulic variables using data from 57 rivers in the u s and perturbation experiments we show that the width amhg is weak arising mainly from the mathematical construct involving the exponent in both the regressor and the regressand in contrast the depth amhgs result from geomorphological coevolution the similar time mean condition as argued for in the literature is sufficient but not necessary for rating curve convergence and in turn amhg for depth and velocity the predictive accuracy of amhgs whose coefficient of determination is unit dependent is similar to the flow percentile based downstream hydraulic geometry

verifying the prevalence properties and congruent hydraulics of at many stations hydraulic geometry amhg for rivers in the continental united states abstract hydraulic geometry hg has long enabled daily discharge estimates flood risk monitoring and water resource and habitat assessments among other applications at many stations hg amhg is a newly discovered form of hg with an evolving understanding amhg holds that there are temporally and spatially invariant congruent depth width velocity and discharge values that are shared by all stations of a river furthermore these river wide congruent hydraulics have been shown to link at a station hg ahg in space contrary to previous expectation of ahg as spatially unpredictable to date amhg has only been thoroughly examined on six rivers and its congruent hydraulics are not well understood to address the limited understanding of amhg we calculated amhg for 191 rivers in the united states using usgs field measured data from over 1900 gauging stations these rivers represent nearly all geologic and climatic settings found in the continental u s and allow for a robust assessment of amhg across scales over 60 of rivers were found to have amhg with strong explanatory power to predict ahg across space defined as r 2 0 6 118 191 rivers we also found that derived congruent hydraulics bear little relation to their observed time varying counterparts and the strength of amhg did not correlate with any available observed or congruent hydraulic parameters we also found that amhg is expressed at all fluvial scales in this study some statistically significant spatial clusters of rivers with strong and weak amhg were identified but further research is needed to identify why these clusters exist thus this first widespread empirical investigation of amhg leads us to conclude that amhg is indeed a widely prevalent natural fluvial phenomenon and we have identified linkages between known fluvial parameters and amhg our work should give confidence to future researchers seeking to perform the necessary detailed hydraulic analysis of amhg

partial thermal resetting of 40 ar 39 ar mineral ages in western spitsbergen svalbard possible evidence for tertiary metamorphism hornblende and muscovite within high grade caledonian metamorphic rocks exposed in albert i land northwest spitsbergen display internally concordant 40 ar 39 ar age spectra providing no record of any post silurian thermal history both minerals record 420 425 ma plateau ages indicating that relatively rapid cooling followed silurian metamorphism muscovite within some metamorphic sequences exposed within the tertiary tectonic zone of oscar ii land west central spitsbergen displays a low temperature 40 ar 39 ar age spectra discordance suggestive of a slight post caledonian thermal disturbance this is consistent with a post palaeozoic thermal overprint documented elsewhere in oscar ii land and indicates that at least locally metamorphism may have accompanied the tertiary orogeny

tracking ancient magmatism and cenozoic topographic growth within the northern andes forearc constraints from detrital u pb zircon ages a comparison of detrital zircon ages from the cenozoic forearc basins in sw ecuador nw peru and those derived from the amotapes massif offers a singular opportunity to shed some light on the evolution of the nw andean margin of south america an extensive set of pre andean u pb zircon ages older than late cretaceous was obtained from 18 detrital samples coming from the talara zorritos progreso and gulf of guayaquil tumbes forearc basins as well as from two metasedimentary rocks of the amotapes massif u pb age analysis on detrital zircon grains allows a general source to sink scenario to be determined for this section of the andes these analyses give consistent insights into the early topographic growth during the modern andean orogeny the occurrence of proto andean magmatic periods in the western margin of gondwana and the evolution of the amotapes massif within the geodynamical setting of northwestern south america zircon ages at ca 320 ma and neoproterozoic clusters for the amotapes metasediments show significant similarities along the strike of the metamorphic belt thus defining a common metasedimentary origin and can be compared with previously dated rocks from the eastern cordillera of peru these strong similarities demonstrate the presence of a wide polyphased metamorphic belt that includes the western parts of the eastern cordillera the northern section of the occidental cordillera of peru and the amotapes massif detrital sediments of the forearc are marked by a major gap in early cretaceous jurassic ages and the minor presence of ordovician famatinian orogeny and carboniferous permian 340 285 ma ages the age spectrum is not compatible with sediment input from the eastern andes and suggests that the external areas of the andean chain e g the western cordillera were already uplifted prior to or immediately after forearc initiation during the paleocene thus representing a pervasive drainage divide throughout all of the cenozoic the amotapes massif or other terrains sharing the same age affinities e g the olmos terrane occidental cordillera of peru are likely to represent the best potential sources for pre andean zircon grains in the forearc age clusters in both source and sink at ca 600 ma 570 560 ma and 520 ma most likely define magmatic periods and may represent first order constraints for the difficult correlation between proto andean magmatic and orogenic periods to the north and south of the huancabamba deflection it is unlikely that the neoproterozoic zircon grains came from the pan african braziliano belts thus suggesting the onset of significant magmatic activity in this section of the andes at around 570 560 ma and probably ca 600 ma the 520 ma cluster is compatible with the most prominent magmatic period in the southern andes related to the end of the pampean orogeny thus suggesting the existence of a regional proto andean magmatic arc during early cambrian times

low temperature thermochronology of the northern cordillera real ecuador tectonic insights from zircon and apatite fission track analysis the post early cretaceous tectonic development of the northern andean segment north of 5 s is loosely constrained by a series of partially and fully reset k ar ages new apatite and zircon fission track data from three traverses across the cordillera real of ecuador reveal distinct periods of accelerated crustal cooling during the cenozoic fission track ages range from 65 to 14 ma zircon and 44 to 8 ma apatite with apatite mean track lengths ranging from 14 6 to 12 1 m these results record rapid cooling rates during 43 30 ma 23 15 ma and 10 0 ma occurring at temperatures below 275 c the events reflect up to 9 km of crustal exhumation in various regions of the cordillera in response to relative changes in the kinematics of the farallon nazca and south american plates the exhumation history of the cordillera real was not continuous but rather evolved during separate phases these phases persisted for 10 myr and are partly coincident with tectonic phases of the central andes although extend for longer time periods possibly as a result of isostatically driven crustal exhumation large spatial variations in the zircon and apatite fission track data within the cordillera real indicate that local fault bounded massifs have experienced different thermal histories since 43 ma major variations are not restricted to the regional scale and exist within individual terranes faulted slivers within terranes demonstrate highly variable exhumation histories suggesting that deformation was not dominated by terrane bounding faults additionally n s variations in exhumation depths suggest that ne sw trending faults have been active since 10 9ma

oceanic plateau formation a cause of mass extinction and black shale deposition around the cenomanian turonian boundary the cenomanian turonian boundary 90 4 ma represents a major period of worldwide environmental disturbance the physical manifestations of this are elevated atmospheric and oceanic temperatures a significant sea level transgression and a period of widespread anoxia leading to the formation of oceanic black shales and the extinction of 26 of all genera elevated 13 c values and enrichment of trace elements in cenomanian turonian boundary sediments combined with a reduction in 87 sr 86 sr also imply a severe environmental perturbation at this time oceanic crustal production rates reached their highest level of the last 100 million years this was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere and the development of vast areas up to 1 10 6 km 2 of thickened oceanic crust in the pacific and indian oceans the anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c 90 ma these eruptions would also have resulted in the emission of large quantities of co 2 into the atmosphere leading to global warming additionally the emission of so 2 h 2 s co 2 and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate and further release of co 2 this run away greenhouse effect was probably put into reverse by the decline of the anomalous volcanic activity and by increased co 2 driven productivity in oceanic surface waters leading to increased organic carbon burial black shale deposition anoxia and mass extinction in the ocean basins

stratigraphic responses to a major tectonic event in a foreland basin the ecuadorian oriente basin from eocene to oligocene times the eocene to oligocene sediments of the ecuadorian oriente basin record two kinds of second order stratigraphic response to the tectonic evolution lower eocene shows evidences of local scale syntectonic deposits this tectonic activity can be related to right lateral convergent movements inverting pre cretaceous extensional structures upper eocene and oligocene sediments are integrated as the expression of an isostatic rebound characterizing a basin scale syntectonic deposition this response is evidenced by a reciprocal architecture of the depositional sequences identified in the sedimentary formations these data have allowed us to propose a new geodynamic model for the paleogene evolution of the oriente basin

a note on the iugs reclassification of the high mg and picritic volcanic rocks the purpose of this short note is to offer some comments and observations on the new iugs classification of high mg rocks le bas 2000 particularly its use of the terms komatiite and picrite with no reference to texture or the original usage of these terms at the outset of this discussion we would like to place on record that we are in agreement with the need to redefine the nomenclature of high mg rocks there are however important issues relating to both the use of the term komatiite for all extrusive volcanic rocks with 18 wt mgo and the definition and use of the term picrite neither of which are discussed by le bas

age and sources of gold mineralization in the marmato mining district nw colombia a miocene pliocene epizonal gold deposit abstract epigenetic gold mineralization occurs in the marmato mining district within the calima terrain of the setentrional andes colombia regional rocks associated with this mineralization include graphite and chlorite schists of the arquia complex metamorphosed during the cretaceous miocene sandstones shales and conglomerates of the amaga formation as well as pyroclastic rocks clasts of basalt andesites and mafic lavas and subvolcanic andesitic dacitic bodies of the combia formation 9 to 6 ma the subvolcanic marmato stock hosts mesothermal and epithermal low sulfidation au ag ores in the form of distensional veins stockwork and quartz veinlets within brecciated zones ore minerals are pyrite sphalerite and galena with subordinate chalcopyrite arsenopyrite pyrrhotite argentite and native gold electrum sericitized plagioclase from a porphyry dacite yielded a k ar age of 5 6 0 6 ma interpreted as the age of ore deposition this is in close agreement with the age of reactivation of the cauca romeral fault system 5 6 0 4 ma which bounds the calima terrain a porphyry andesite dacite 6 7 0 1 ma hosting the au ag veins shows a measured 87sr 86sr between 0 70440 and 0 70460 end between 2 2 and 3 2 and 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 18 964 to 19 028 15 561 to 15 570 and 38 640 to 38 745 respectively the 87sr 86sr and end values of rocks from the arquia group range from 0 70431 to 0 73511 and 12 91 to 10 0 respectively whereas the corresponding pb isotopic ratios 206pb 204pb 207pb 204pb and 208pb 204pb range from 18 948 to 19 652 15 564 to 15 702 and 38 640 to 38 885 respectively 87sr 86sr and end values obtained on sulfides from the gold quartz veins which occur at shallow and intermediate levels range from 0 70500 to 0 71210 and from 1 11 to 2 40 in the deepest veins end values lie between 1 25 and 3 28 and the 87sr 86sr of calcite and pyrite fall between 0 70444 and 0 70930 the 206pb 204pb 207pb 204pb and 208pb 204pb ratios of all mineralization are in the ranges 18 970 to 19 258 15 605 to 15 726 and 38 813 to 39 208 respectively carbonates have an average 87sr 86sr ratio of 0 70445 which is within the range of values measured in the host dacite the sr isotopic data indicate that carbonic fluids have a restricted hydrothermal circulation within the host igneous body while the sr pb and nd isotopic compositions of the sulfides suggest that the fluids not only circulated within the marmato stock but also throughout the arquia complex inferring that these rocks offer a potential target for mineral exploration based on geological and geochronological evidence the epizonal marmato gold ores formed during the miocene to pliocene as a result of cooling of the marmato stock and reactivation along a crustal scale fault zone related to thermal processes in an accretionary oceanic continental plate orogen

a model for the simulation of combined major and trace element liquid lines of descent abstract variations of major and trace element partition coefficients as functions of temperature and composition have severely limited the range of composition for quantitative modeling of the differentiation of natural magmatic systems application of a two lattice melt model greatly reduces compositional dependence for trace elements this allows the temperature dependence of isobaric trace element partitioning to be evaluated independently thus making it possible to model an expanded range of conditions and compositions the first goal of this investigation was to construct a model to simulate major and trace element systematics of natural differentiation processes by combining trace element partitioning with a major element phase equilibrium model nielsen 1985b the second goal was the application of this model to the simulation of magma chamber recharge assimilation eruption and fractional crystallization these simulations indicate that the proportions of minerals fractionating from an open system are not necessarily the same as those in a closed system recharge or assimilation will drive a magma composition off the cotectic along which it is evolving however fractionation will drive the liquid back towards the cotectic in a recharged magma chamber as the recharge flux increases the fractionating mineral proportions will be increasingly biased towards those phases which appear first from the primitive magma the bulk partition coefficient for an open system can be considerably different 5 10 or more for some elements compared to a closed system paired assimilation and fractional crystallization are often cited as the mechanisms responsible for the origin of high al basalts and andesites however the addition of an al rich assimilant into a fractionating basaltic magma will increase the proportion of plagioclase to the extent that al in the derived liquids will be effectively buffered or even decrease the assimilation of forsteritic olivine by reaction keleman and ghiorso 1986 increases the proportions of pyroxene and olivine thus reducing the fe enrichment and al depletion characteristic of low pressure basalt fractionation

mode and timing of terrane accretion in the forearc of the andes in ecuador the volcanic basement of the ecuadorian western cordillera pallatanga formation and san juan unit is made up of mafi c and ultramafi c rocks that once formed an oceanic plateau radiometric ages from these rocks overlap with a hornblende 40 ar 39 ar plateau age of 88 1 6 ma age obtained for oceanic plateau basement rocks of the pinon formation in coastal ecuador and with ca 92 88 ma ages reported for oceanic plateau sequences in the caribbean and western colombia these results suggest that the oceanic plateau rocks of the western cordillera and fl at forearc in ecuador are derived from the late cretaceous caribbean colombia oceanic plateau intraoceanic island arc sequences rio cala group overlie the plateau in the western cordillera and yield crystallization ages that range between ca 85 and 72 ma the geochemistry and radiometric ages of island arc lavas from the rio cala group combined with the age range and geochemistry of their turbiditic volcaniclastic products indicate that the arc was initiated by westward subduction beneath the caribbean plateau they are coeval with island arc rocks of coastal ecuador las orquideas san lorenzo and cayo formations and colombia ricaurte arc these island arc

alluvial channel geometry theory and applications the downstream hydraulic geometry of alluvial channels in terms of bank full width average flow depth mean flow velocity and friction slope is examined from a three dimensional stability analysis of noncohesive particles under two dimensional flows four governing equations flow rate resistance to flow secondary flow and particle mobility are solved to analytically define the downstream hydraulic geometry of noncohesive alluvial channels as a function of water discharge sediment size shields number and streamline deviation angle the exponents of hydraulic geometry relationships change with relative submergence four exponent diagrams illustrate the good agreement with several empirical regime equations found in the literature the analytical formulations were tested with a comprehensive data set consisting of 835 field channels and 45 laboratory channels the data set covers a wide range of flow conditions from meandering to braided sand bed and gravel bed rivers with flow depths and channe

origin and cretaceous tectonic history of the coastal ecuadorian forearc between 1 n and 3 s paleomagnetic radiometric and fossil evidence the presence of at least one block pallatanga block derived from the caribbean colombian oceanic plateau has been proposed in the western cordillera of ecuador new 40ar 39ar dating of the basement of the coastal blocks 90 87 ma and fossil ages in the overlying calentura fm 89 84 ma argue for late turonian coniacian extrusion of the basement of the pinon block paleomagnetic inclinations indicate that the mafic basement of the pinon and san lorenzo blocks extruded at equatorial low southern latitudes no subsequent significant latitudinal drift can be observed during the cretaceous rapid changes in paleomagnetic declination between 73 and 70 ma in both the pinon and the san lorenzo blocks indicate that the large igneous province which originally hosted the coastal blocks experienced a major tectonic event during late campanian times similar late turonian coniacian basement ages and late campanian tectonic events are reported in the pallatanga block of the western cordillera we propose that the pinon san lorenzo and pallatanga blocks all derive from a common large igneous province the caribbean colombian oceanic plateau and were incorporated into the south american margin by collision during the late campanian 73 70 ma

simple bouguer gravity anomaly field and the inferred crustal structure of continental ecuador the simple bouguer gravity anomaly field of continental ecuador corresponds closely to the physiographic provinces of the country the sierra which includes the andes and their foothills is characterized by a pronounced low with values to 292 mgal which reflects the deep andean root bouguer anomalies over the oriente become less negative away from the sierra chiefly in response to progressive thinning of continental crust eastward the costa between the sierra and the pacific shore in the north has the most positive on land bouguer anomalies 162 mgal so far known in the western hemisphere this part of the costa is underlain by an ancient oceanic plate now welded to the northwestern corner of the otherwise continental south american plate

labradorian and grenvillian crustal evolution of the goose bay region labrador new u pb geochronological constraints major initial crust formation in the northeastern grenville province of labrador occurred during the labradorian orogeny 1710 1620 ma prior to re deformation in the grenvillian orogeny between 10

an autochthonous geological model for the eastern andes of ecuador abstract we describe a traverse across the cordillera real and sub andean zone of ecuador poorly known areas with very little detailed mapping and very little age control the spine of the cordillera comprises deeply eroded triassic and jurassic plutons the roots of a major arc emplaced into probable palaeozoic pelites and metamorphosed volcanic rocks the w flank comprises a jurassic submarine basaltic andesitic volcanic sequence which grades up into mixed jurassic cretaceous volcanic and sedimentary rocks of the inter andean valley the sub andean zone on the e flank of the cordillera comprises a newly recognized cretaceous basin of cleaved mudrocks quartz arenites and limestones east of the syndepositional cosanga fault the cretaceous basin thins into a condensed sequence that is indistinguishable from the rocks of the adjacent hydrocarbon bearing oriente basin the principal penetrative deformation of the cordillera real was probably latest cretaceous palaeocene it telescoped the magmatic belts but shortening was largely partitioned into the pelites between plutons the plutons suffered inhomogenous deformation some portions completely escaped tectonism the pelites conserve two foliations the earliest comprises slaty cleavage formed under low or sub greenschist conditions the later is a strong schistosity defined by new mica growth it largely transposed and obliterated the first both foliations may have developed during a single progressive deformation we find inappropriate recent terrane models for the cordillera real and sub andean zone of ecuador instead we find remarkable similarities from one side of the cordillera to the other including a common structural history in place of sutures we find mostly intrusive contacts between major plutons and pelites triassic to cretaceous events occurred on the autochthonous western edge of the archaean guyana shield the latest cretaceous paleocene deformation is interpreted as the progressive collision of an oceanic terrane s with the south american continent young fault movements have subsequently juxtaposed different structural levels through the cordillera real orogen

the mera lahar deposit in the upper amazon basin transformation of a late pleistocene collapse at huisla volcano central ecuador abstract the sub andean zone east of the cordillera real ecuador and out to the amazon basin s western margin has been the depository of voluminous lahars related to volcanic activity in the andean highlands these lahars arrived to the sub andean zone via gravitational transport through narrow river canyons and emplaced volumes surpassing several cubic kilometers over robust inundation zones this paper discusses the origin flow route depositional zone terrace formation and geomorphic significance of the most important lahar deposit yet mapped in this region that of the mera lahar which likely formed from a late pleistocene collapse of huisla volcano followed by impounding by temporary dams behind drainage blocking debris avalanche deposits dad then subsequent rupture of the blockage the actual deposit of the mera lahar has a thickness between 30 and 70 m is mainly comprised of dad breccia is matrix supported c 70 of reddish gray color and is well consolidated based on geochemical and petrographic similarities huisla volcano s dad is the best candidate for the lahar s source huisla volcano is located some 90 km up valley of the bulk of mera lahar s mapped deposit clasts of mera lahar rocks and huisla s dad breccias have 57 61 wt sio2 corresponding to andesites of the calc alkaline series with mean values of 1 1 5 wt for k2o the mapped mera lahar deposit has an actual volume estimated in 1 2 km3 compared to its original estimated volume of 5 4 km3 cross sectional widths of 1 5 to 4 5 km span and extend laterally beyond the actual pastaza river channel where the lahar s deposition produced high standing isolated surfaces that are notable local geomorphic features of the upper sub andean zone the flow modeling program laharflow employing the modern landscape as the topographic base adequately simulates the flow route and inundation zones of the mera lahar

new geological and geochronological data from the colombian andes continental growth by multiple accretion the regional geology of colombia consists of a composite palaeozoic to cenozoic sedimentary basin overlying precambrian shield rocks which are exposed in the eastern cordillera magdalena valley and at least part of the extensive llanos plains a palaeozoic metamorphic schist belt with abundant post metamorphic igneous rocks comprises the central cordillera a mesozoic volcanic suite extends from the western flank of the central cordillera and accounts for the bulk of the western cordillera a cenozoic sedimentary sequence probably overlying a late cretaceous oceanic crust and fore arc complex forms the pacific coastal plain the continental edge of central and southern colombia is interpreted as a composite margin made up of successively accreted oceanic island arc related sequences of palaeozoic early mesozoic and late cretaceous ages the present day cycle which began in the miocene is not associated with island arc volcanism

40ar 39ar mineral age record of variably superimposed proterozoic tectonothermal events in the grenville orogen central labrador penetratively cleaved slate phyllite within supracrustal sequences seal lake and letitia lake groups in northernmost portions of the grenville parauthochthon in central labrador record 1018 15 and 985 14 ma whole rock 40ar 39ar plateau ages which are interpreted as closely dating deformation and a concomitant low grade metamorphism the 40ar 39ar signature of grenvillian tectonothermal effects is variably recorded in more southerly portions of the parautochthon locally within the extensive north pole brook igneous suite there is no significant metamorphic retrogression in these areas igneous hornblendes record ca 1650 ma plateau ages similar to those suggested for emplacement of most of the suite by previously reported rb sr whole rock isochron and u pb zircon analyses low temperature retrogression of igneous assemblages is observed throughout most of the north pole brook igneous terrane hornblende concentrates from five relatively undeformed samples are characterized by internally discordan

geology mineralization and geochronological constraints of the mirador cu au porphyry district southeast ecuador the mirador porphyry cu au district is located in the southernmost sector of the northern andean cordillera in zamora chinchipe province southeastern ecuador the district contains two significant porphyry cu au ag mo deposits mirador and mirador norte and an interconnected series of narrow mineralized structures known collectively as chancho the principal mineralization in the porphyries is disseminated to blebby chalcopyrite developed primarily in potassic alteration with overlying chalcocite supergene enrichment zones prior to radiometric dating presented in this study these deposits were considered late jurassic based on close similarity and therefore assumed age equivalence with the well dated panantza san carlos and sutzu porphyry copper deposits located 40 km to the north new u pb zircon ages confirm a middle jurassic age for granodiorite of the zamora batholith at this location 163 8 1 9 ma and late jurassic ages for subvolcanic intrusions 156 2 1 0 and 153 1 1 3 ma re os molybdenite ages average 156 1 0 ma and indicate coeval mineralization between mirador and mirador norte mineralization and related sub volcanic igneous activity are closely associated at mirador and represent the same temporal event recorded at panantza san carlos and sutzu as well as coeval porphyry skarn and epithermal au mineralization extending at least 80 km south defining a north south cu au metallogenic belt spanning over 120 km

distinguishing between in situ and accretionary growth of continents along active margins abstract active continental margins represent both sites of crustal loss by tectonic erosion and gain by juvenile magmatism and oceanic arc plateau accretion estimating the gain component depends on distinguishing between growth of margins by either accretion of oceanic crust or juvenile magmatic addition during extension distinguishing between these allochthonous and autochthonous models is frequently ambiguous due to difficulties in interpreting the tectonic origin of mafic rocks we present a detailed geochronological and isotopic study constraining the evolution of the continental margin of northwest south america during 190 113 ma semi continuous slab rollback resulted in crustal thinning juvenile magmatism and ultimately the removal of continental slivers negating previous hypotheses that the margin evolved by the accretion of an island arc however basic crustal growth estimates along with comparisons between rocks exposed in northern peru and the northern andes imply that uneven losses of continental crust occurred along northwest south america for mesozoic exposures and that the accretion of the caribbean large igneous province clip onto nw south america probably played a crucial role in maintaining and preserving mesozoic continental crust north of the huancabamba deflection

bam bayesian amhg manning inference of discharge using remotely sensed stream width slope and height the forthcoming surface water and ocean topography swot nasa satellite mission will measure water surface width height and slope of major rivers worldwide the resulting data could provide an unprecedented account of river discharge at continental scales but reliable methods need to be identified prior to launch here we present a novel algorithm for discharge estimation from only remotely sensed stream width slope and height at multiple locations along a mass conserved river segment the algorithm termed the bayesian amhg manning bam algorithm implements a bayesian formulation of streamflow uncertainty using a combination of manning s equation and at many stations hydraulic geometry amhg bayesian methods provide a statistically defensible approach to generating discharge estimates in a physically underconstrained system but rely on prior distributions that quantify the a priori uncertainty of unknown quantities including discharge and hydraulic equation parameters these were obtained from literature reported values and from a usgs dataset of acoustic doppler current profiler adcp measurements at usgs stream gauges a dataset of simulated widths slopes and heights from 19 rivers was used to evaluate the algorithms using a set of performance metrics results across the 19 rivers indicate an improvement in performance of bam over previously tested methods and highlight a path forward in solving discharge estimation using solely satellite remote sensing

40ar 39ar and u pb evidence for late proterozoic grenville age continental crust in north central cuba and regional tectonic implications renne p r mattinson j m hatten c w somin m onstott t c millfin g and linares e 1989 4 ar 39ar and u pb evidence for late proterozoic grenville age continental crust in north central cuba and regional tectonic implications precambrian res 42 325 341 central cuba is composed of fault bounded tectonostratigraphic terranes juxtaposed and deformed during plate collision and subsequent transform motion between the caribbean and north american plates in the late mesozoiccenozoic one of these the las villas terrane contains crystalline basement rocks thought to be pre upper jurassic on stratigraphic grounds the socorro complex occurs in the northwestern las villas terrane and consists of marbles and siliciclastic metasedimentary rocks and the rio carla granite an 4 ar 39ar plateau date of 903 5 7 1 ma for phlogopite from a marble corroborates previous k ar dates from this unit and establishes unambiguously a late proterozoic age for high grade metamorphism discordance of the 4 ar agar age spectrum can be reliably attributed to diffusive ar loss which if modeled as an episodic thermal event implies a reheating age that closely coincides with the late cretaceous paleogene collision between the caribbean and north american plates u pb zircon data indicate an intrusive age of 172 4 ma for the rio carla granite and reveal an inherited zircon component with an age of 900 ma radio isotopic data from the socorro complex display no evidence of pan african age thermal overprinting these observations combined with constraints provided by published results from nearly pangean landmasses suggest that the complex lay substantially to the southwest present day co ordinates during the early paleozoic following its genesis in the mid mesozoic the caribbean plate evidently transported fragments of an extensive grenville age belt that spanned the americas during the late proterozoic

metallogenic features of miocene porphyry cu and porphyry related mineral deposits in ecuador revealed by re os 40ar 39ar and u pb geochronology mineralization and alteration events at ten miocene porphyry cu and porphyry related epithermal mineral deposits in southern central and northern ecuador were dated by means of molybdenite re os biotite and alunite 40ar 39ar and titanite u pb geochronology most of these hydrothermal events show a spatio temporal correlation with porphyry intrusion emplacement as constrained by zircon u pb ages the total age range for these events spans the 23 5 6 1 ma period without displaying systematic along or across arc age distribution trends while epithermal deposits tend to be spatially associated with volcanic rocks of a similar age porphyry cu deposits in ecuador are frequently spatially associated with deeper seated basement units and batholith scale precursor intrusive systems assembled over 5 m y time periods in most cases formation of the porphyry cu deposits is related to the youngest magmatic hydrothermal event in a given area postdating batholith construction at a regional scale the majority of miocene deposits occurs in southern ecuador where areally extensive post mineralization late miocene to recent volcanic sequences with the potential to conceal mineralization at depth are lacking only few miocene deposits occur in northern central ecuador where they mainly crop out in the western cordillera west of the productive present day volcanic arc the surface distribution of post mineralization arc volcanism reflects along arc variations in subducting slab geometry porphyry cu and epithermal deposits in ecuador define a miocene metallogenic belt broadly continuous with its coeval counterpart in northern central peru although both belt segments were formed in an overall similar tectonomagmatic and metallogenic setting their respective metal endowments differ significantly

allochthonous terranes in northwestern ecuador abstract formations and lithostratigraphic units of the ecuadorian northwestern cordillera can be divided into three major groups the first group comprises lower cretaceous e morb basalts conformably overlain by tholeiitic to boninitic components of an ensimatic island arc the second group consists mainly of tholeiitic to calc alkaline volcanic and volcano sedimentary island arc deposits unconformably on top of the first group and of eocene to early oligocene age the third group is represented by calc alkaline continental volcanics ranging from late oligocene to recent two main tectonic events occur during early eocene and middle oligocene times the first event can be interpreted as an accretion of a slab of oceanic basalt with its overlying arc material the suture zone is marked by slices of morb type material delineating the southward extension of the colombian cauca patia fault zone the second event is related to a major rearrangement of plate tectonic regimes in the eastern pacific during oligocene times its nature is most probably transpressional accompanied by thrusting

transected folds from the western part of the bala lineament wales abstract fold cleavage relationships are described in the cadair idris area of the lower palaeozoic welsh basin which includes the western part of the bala lineament many fold hinges are transected by the broadly contemporaneous caledonian lower devonian cleavage transection angles are amongst the highest yet described from the welsh basin there is a close relationship between fold strike and sense and amount of transection n s folds are clockwise transected ne folds have an axial planar cleavage whilst ene folds are anticlockwise transected similar relationships occur throughout the central welsh basin a model of progressive deformation is described whereby weak regional transpression is modified by local factors viscosity contrasts are recognized as one of the most important controls on the timing of fold development and the amount of transection

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

40ar 39ar mineral ages within metamorphic clasts from the kuma group eocene central shikoku japan implications for tectonic development of the sambagawa accretionary prism abstract the sambagawa metamorphic belt exposed in central shikoku records a high p t metamorphism it is comprised by the oboke nappe and the structurally overlying internally imbricated besshi nappe complex the besshi nappe complex locally is unconformably overlain by the kuma group eocene which consists of conglomerates and subordinate sandstones and mudstones clasts within kuma conglomerates exposed in the kamegamori district include low grade schists derived from the unconformably underlying sambagawa complex and high grade metamorphic rocks of uncertain provenance including garnet amphibolite and oligoclase bearing pelitic schist 36 ar 40 ar vs 39 ar 40 ar isotope correlation ages recorded by hornblende from two amphibolite clasts are 131 1 4 9 ma and 156 8 4 3 ma these are interpreted to date post metamorphic cooling through temperatures required for intracrystalline retention of argon and are older than cooling ages previously reported from any presently exposed segments of the sambagawa terrane plateau ages of 108 8 0 7 ma and 115 7 0 6 ma are recorded by muscovite from two clasts of schist muscovite within a proximal basement exposure of unconformably underlying sambagawa pelitic schist records a plateau age of 78 7 0 5 ma combined with isotopic ages previously reported for the sambagawa terrane 40 ar 39 ar data from clasts in the kuma group suggest that metamorphic culmination within a high grade source terrane occured at c 185 145 ma sectors of this complex cooled through c 500 c at c 150 ma other portions cooled through c 500 c at c 130 ma contrasts in the cooling ages likely reflect internal imbrication within the source terrane the besshi nappe complex reached peak metamorphic conditions at c 100 90 ma and experienced relatively rapid uplift and cooling at c 85 75 ma the besshi nappe complex was structurally emplaced onto the oboke nappe which attained peak metamorphic conditions at c 75 ma subsequently the entire sambagawa section was exhumed and most high grade metamorphic rocks were removed the eocene kuma group was unconformably deposited over the sambagawa terrane at c 50 ma

geochemistry and tectonic setting of pre collision cretaceous and paleogene volcanic rocks of ecuador pre collision cretaceous lower tertiary volcanic rocks of western ecuador from the coastal area pinon and san lorenzo formations and western cordillera macuchi and celica formations consist of three rock suites the first suite includes tholeiites of ocean floor affinities that have distinct light ree depleted patterns and low la nb la hf and th hf ratios the island arc tholeiites of the second suite have lower contents of ti zr hf and nb the third suite comprises calc alkalic rocks mainly continental margin andesites and dacites the continental volcanic arc rocks crop out in an area east of the tholeiitic rocks the boundary between the two areas is the major suture separating continental south america from accreted terrenes to the west a model is suggested for the geodynamic evolution of what is now western ecuador during the late cretaceous it includes two east dipping subduction zones of turonian to santonian ages and the accretion of the oceanic island arc to the continent

oceanic plateau model for continental crustal growth in the archaean a case study from the kostomuksha greenstone belt nw baltic shield abstract field studies combined with chemical and isotope data indicate that the kostomuksha greenstone belt in the nw baltic shield consists of two lithotectonic terranes one mafic igneous and the other sedimentary separated by a major shear zone the former contains submarine komatiite basalt lavas and volcaniclastic lithologies and the latter is composed of shelf type rocks and bif komatiitic and basaltic samples yield sm nd and pb pb isochron ages of 2843 39 and 2813 78 ma respectively their trace element compositions resemble those of recent pacific oceanic flood basalts with primitive mantle normalized nb th of 1 5 2 1 and nb la of 1 0 1 5 this is in sharp contrast with island arc and most continental magmas which are characterized by nb th la n 1 calculated initial nd isotope compositions nd t 2 8 to 3 4 plot close to an evolution line previously inferred for major orogens momo which is also consistent with the compositions of recent oceanic plateaux the high liquidus temperatures of the komatiite magmas 1550 c and their al depleted nature require an unusually hot 1770 c mantle source for the lavas 200 c hotter than the ambient mantle at 2 8 ga and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet this plume had the thermal potential to produce oceanic crust with an average thickness of 30 km underlain by a permanently buoyant refractory lithospheric mantle keel nb u ratios in the komatiites and basalts calculated on the basis of th u pb relationships range from 35 to 47 and are thus similar to those observed in modern morb and oib this implies that some magma source regions of the kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle the mafic terrane is interpreted as a remnant of the upper crustal part of an archaean oceanic plateau when the newly formed plateau reached the active continental margin its upper part collided with the sedimentary terrane but was too buoyant to subduct as a result the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust the deeper zones were delaminated and tectonically underplated or subducted

downstream hydraulic geometry of alluvial channels this study extends the earlier contribution of julien and wargadalam in 1995 a larger database for the downstream hydraulic geometry of alluvial channels is examined through a nonlinear regression analysis the database consists of a total of 1 485 measurements 1 125 of which describe field data used for model calibration the remaining 360 field and laboratory measurements are used for validation the data used for validation include sand bed gravel bed and cobble bed streams with meandering to braided planform geometry the five parameters describing downstream hydraulic geometry are channel width w average flow depth h mean flow velocity v shields parameter and channel slope s the three independent variables are discharge q median bed particle diameter ds and either channel slope s or shields parameter for dominant discharge conditions the regression equations were tested for channel width ranging from 0 2 to 1 100 m flow depth from 0 01 to 16 m flow velocity from 0 02 to 7 m s

allochthonous terranes in the andes of ecuador and northwestern peru in ecuador and northwestern peru the andes and adjacent country particularly on the pacific side are composed of at least five distinctive geologic terranes the terranes are distinguished from one another and from cratonic south america to the east by dissimilar basements cover rocks intrusive rocks and bouguer gravity anomaly fields the pinon terrane occupying most of coastal ecuador has a basaltic basement characterized by the largest known on land positive bouguer anomalies in the western hemisphere the tahuin terrane occupies most of northwestern peru and the southwestern corner of ecuador the terrane has an especially complex basement and is the site of generally positive bouguer anomalies the small biron terrane has an unusual basement composed in part of cordierite gneiss and amphibolite that give consistent late cretaceous k ar mineral ages the wedge shaped chaucha terrane lies in part on the western andean slope between the oceanic pinon terrane on the north and the continental bir

cenozoic stages of forearc evolution following the accretion of a sliver from the late cretaceous caribbean large igneous province sw ecuador nw peru

magmatic dominated fluid evolution in the jurassic nambija gold skarn deposits southeastern ecuador the jurassic approximately 145 ma nambija oxidized gold skarns are hosted by the triassic volcanosedimentary piuntza unit in the sub andean zone of southeastern ecuador the skarns consist dominantly of granditic garnet ad20 98 with subordinate pyroxene di46 92hd17 42jo0 19 and epidote and are spatially associated with porphyritic quartz diorite to granodiorite intrusions endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone exoskarn has an outer k and na enriched zone in the volcanosedimentary unit gold mineralization is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulfide poor vugs and milky quartz veins and veinlets in association with hematite fluid inclusion data for the main part of the prograde stage indicate the coexistence of high temperature 500 c to 600 c high salinity up to 65 wt eq nacl and moderate to low salinity aqueous carbonic fluids interpreted to have been trapped at pressures around 100 120 mpa corresponding to about 4 km depth lower temperature 510 300 c and moderate to low salinity 23 2 wt eq nacl aqueous fluids are recorded in garnet and epidote of the end of the prograde stage the microthermometric data th from 513 c to 318 c and salinity from 1 0 to 23 wt eq nacl and 18o values between 6 2 and 11 5 for gold bearing milky quartz from the retrograde stage suggest that the ore forming fluid was dominantly magmatic pressures during the early retrograde stage were in the range of 50 100 mpa in line with the evidence for co2 effervescence and probable local boiling the dominance of magmatic low saline to moderately saline oxidizing fluids during the retrograde stage is consistent with the depth of the skarn system which could have delayed the ingression of external fluids until relatively low temperatures were reached the resulting low water to rock ratios explain the weak retrograde alteration and the compositional variability of chlorite essentially controlled by host rock compositions gold was precipitated at this stage as a result of cooling and ph increase related to co2 effervescence which both result in destabilization of gold bearing chloride complexes significant ingression of external fluids took place after gold deposition only as recorded by 18o values of 0 4 to 6 2 for fluids depositing quartz below 350 c in sulfide rich barren veins low temperature 300 c meteoric fluids 18owater between 10 0 and 2 0 are responsible for the precipitation of late comb quartz and calcite in cavities and veins and indicate mixing with cooler fluids of higher salinities about 100 c and 25 wt eq nacl the latter are similar to low temperature fluids 202 74 5 c with 18o values of 0 5 to 3 1 and salinities in the range of 21 1 to 17 3 wt eq cacl2 trapped in calcite of late veins and interpreted as basinal brines nambija represents a deep equivalent of the oxidized gold skarn class the presence of co2 in the fluids being partly a consequence of the relatively deep setting at about 4 km depth as in other au bearing skarn deposits not only the prograde stage but also the gold precipitating retrograde stage is dominated by fluids of magmatic origin

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

downstream variation in bankfull width of wadeable streams across the conterminous united states abstract bankfull channel width is a fundamental measure of stream size and a key parameter of interest for many applications in hydrology fluvial geomorphology and stream ecology we developed downstream hydraulic geometry relationships for bankfull channel width w as a function of drainage area a w a dhg wa for nine aggregate ecoregions comprising the conterminous united states using 1588 sites from the u s environmental protection agency s national wadeable streams assessment wsa including 1152 sites from a randomized probability survey sample sampled stream reaches ranged from 1 to 75 m in bankfull width and 1 to 10 000 km 2 in drainage area the dhg wa exponent which expresses the rate at which bankfull stream width scales with drainage area fell into three distinct clusters ranging from 0 22 to 0 38 width increases more rapidly with basin area in the humid eastern highlands encompassing the northern and southern appalachians and the ozark mountains and the upper midwest great lakes region than for the west both mountainous and xeric areas the southeastern coastal plain and the northern plains the dakotas and montana stream width increases least rapidly with basin area in the temperate plains cornbelt and southern plains great prairies in the heartland the coefficient of determination r 2 was least in the noncoastal plains 0 36 0 41 and greatest in the appalachians and upper midwest 0 68 0 77 dhg wa equations differed between streams with dominantly fine bed material silt sand and those with dominantly coarse bed material gravel cobble boulder in six of the nine analysis regions where dhg wa equations varied by sediment size fine bedded streams were consistently narrower than coarse bedded streams within the western mountains ecoregion where there were sufficient sites to develop dhg wa relationships at a finer spatial scale and ranged from 1 23 to 3 79 and 0 23 to 0 40 respectively with r 2 0 50 for 10 of 13 subregions range 0 36 to 0 92 enhanced dhg equations incorporating additional data for three landscape variables that can be derived from gis mean annual precipitation elevation and mean reach slope significantly improved equation fit and predictive value in several regions most notably the western mountains and the temperate plains channel width was also related to human disturbance we examined the influence of human disturbance on channel width using several indices of local and basinwide disturbance contrary to our expectations the data suggest that the dominant response of channel width to human disturbance in the united states is a reduction in bankfull width in streams with greater disturbance particularly in the western mountains where population density road density agricultural land use and local riparian disturbance were all negatively related to channel width and in the appalachians and new england where urban and agricultural land cover and riparian disturbance were all negatively associated with channel width

theoretical basis for at many stations hydraulic geometry citation gleason c j wang j 2015 theoretical basis for at many stations hydraulic geometry geophysical research letters 42 17 7107 7114 doi 10 1002 2015gl064935

variability of nb u and th la in 3 0 to 2 7 ga superior province ocean plateau basalts implications for the timing of continental growth and lithosphere recycling extensive volcanic sequences of tholeiitic basalts having near flat ree patterns and spatially associated with komatiites occur in many archean superior province greenstone belts they are considered to be fragments of intraoceanic volcanic plateaus derived from a plume basalts from the 2 9 3 0 ga lumby lake greenstone belt carefully screened for minimum alteration have variable nb u ratios of 36 to 58 least altered basalts from the abitibi and wawa greenstone belts also have variable nb u ratios of 25 to 50 and 28 to 42 respectively compared to an average value of 47 for modern ocean basalts in the abitibi suite nb u correlates positively with nb lapm but negatively with th lapm alteration can be ruled out as the cause of nb u variation as there are no correlations of nb u with loi or eu eu and nb u correlates with nb th in all three suites of basalts numerous lines of evidence indicate that crustal contamination can be eliminated as the cause of nb u variability especially for samples with nb u 36 high nb u ratios can be explained by recycling ocean crust processed through a subduction zone high nb u and nb lapm low th lapm into the mantle source of the basalts whereas low nb u ratios can be accounted for by recycling complementary subarc mantle lithosphere or continental crust low nb u and nb lapm high th lapm into the mantle source for the entire population of basalts th lapm ratios generally 1 may result from recycling the residue of slab derived archean type tonalites having high th la ratios nb u ratios as high as the 47 10 range of modern ocean basalts have been found in a 2 7 ga volcanic belt of the yilgarn craton taken together the results signify very early growth of the continental crust rather than episodic growth over several ga

u pb re os and 40ar 39ar geochronology of the nambija au skarn and pangui porphyry cu deposits ecuador implications for the jurassic metallogenic belt of the northern andes new u pb re os and 40ar 39ar dates are presented for magmatic and hydrothermal mineral phases in skarn and porphyry related ores from the nambija and pangui districts of the subandean zone southeastern ecuador nambija has been one of the main gold producing centers of ecuador since the 1980s due to exceptionally high grade ores average 15 g t but frequently up to 300 g t au pangui is a recently discovered porphyry cu mo district the geology of the subandean zone in southeastern ecuador is dominated by the i type subduction related jurassic zamora batholith which intrudes triassic volcanosedimentary rocks the zamora batholith is in turn cut by porphyritic stocks which are commonly associated with skarn formation and or porphyry style mineralization high precision u pb and re os ages for porphyritic stocks u pb zircon associated prograde skarn u pb hydrothermal titanite and retrograde stage skarn re os molybdenite from veins postdating gold deposition of the nambija district are all indistinguishable from each other within error 145 ma and indicate a late jurassic age for the gold mineralization previously gold mineralization at nambija was considered to be early tertiary based on k ar ages obtained on various hydrothermal minerals the new jurassic age for the nambija district is slightly younger than the 40ar 39ar and re os ages for magmatic hydrothermal minerals from the pangui district which range between 157 and 152 ma mineralization at nambija and pangui is associated with porphyritic stocks that represent the last known episodes of a long lived jurassic arc magmatism 190 to 145 ma a jurassic age for mineralization at nambija and pangui suggests that the northern andean jurassic metallogenic belt which starts in colombia at 3 n extends down to 5 s in ecuador it also adds a new mineralization style au skarn to the metal endowment of this belt

seismological evidence on the geometry of the orogenic system in central northern ecuador south america analysis of the spatial distribution of seismicity beneath central ecuador from a temporary network gives new insights on two main points of the ecuadorian tectonics major structures in the ecuadorian andes are east dipping late jurassic to paleogene sutures reactivated by present day compression the oceanic plate is plunging continuously down to a depth of about 200 km with a dip of 25 35 we also show that the coastal plain acts as a buttress transmitting stresses to the andes beneath which deformation is concentrated

full flow regime storage streamflow correlation patterns provide insights into hydrologic functioning over the continental us interannual changes in low median and high regimes of streamflow have important implications for flood control irrigation and ecologic and human health the gravity recovery and climate experiment grace satellites record global terrestrial water storage anomalies twsa providing an opportunity to observe interpret and potentially utilize the complex relationships between storage and full flow regime streamflow here we show that utilizable storage streamflow correlations exist throughout vastly different climates in the continental us conus across low to high flow regimes a panoramic framework the storage streamflow correlation spectrum sscs is proposed to examine macroscopic gradients in these relationships sscs helps form corroborate or reject hypotheses about basin hydrologic behaviors sscs patterns vary greatly over conus with climate land surface and geologic conditions data mining analysis suggests that for catchments with hydrologic settings that favor storage over runoff e g a large fraction of precipitation as snow thick and highly permeable permeable soil sscs values tend to be high based on our results we form the hypotheses that groundwater flow dominates streamflows in southeastern conus and great plains while thin soils in a belt along the appalachian plateau impose alimit on water storage sscs also suggests shallow water table caused by high bulk density soil and flat terrain induces rapid runoff in several regions our results highlight the importance of subsurface properties and groundwater flow in capturing flood and drought we propose that sscs can be used as a fundamental hydrologic signature to constrain models and to provide insights thatlead usto better understand hydrologic functioning

geological history of the cretaceous ophiolitic complexes of northwestern south america colombian andes abstract the western cordillera of colombia was formed by intense alpine type nappe forming folding and thrusting the cretaceous 80 120 ma b p tholeiitic material of the western cordilleran nappes has been obducted onto the paleozoic and precambrian polymetamorphic micaschists and gneiss of the central cordillera near yarumal the antioquia batholith 60 80 ma b p intrudes both obducted cretaceous oceanic material and the polymetamorphic basement rock of the central cordillera therefore nappe emplacement and obduction onto the central cordillera occurred during late senonian to early paleocene the nappes travelled from northwest to southeast so that the highest unit the rio calima nappe therefore has the most northwestern source whereas the lowest units originated from a more southeastward direction sedimentological analysis of the volcanoclastic and sandy turbidite material from each unit suggests a marginal marine environment during cretaceous times the opening of this marginal sea from now on called the colombia marginal basin probably originated by detachment of a block from the south american continent related to the farallon south america plate convergence in the popayan area southern colombia the central cordilleran basement exhibits glaucophane schist facies metamorphism this high pressure low temperature metamorphism is of early cretaceous 125 ma b p age and is related to an undated metaophiolitic complex the ophiolitic material originating from the western cordilleran is thrust over both the blueschist belt and the metaophiolitic complex these data suggest that the occidente colombiano suffered at least two phases of ophiolitic obduction during mesozoic time

geometrical control of subduction related magmatism the mesozoic and cenozoic plutonic history of western colombia radiometric age data from western colombia combined with geological evidence allow five main plutonic episodes ranging in age from triassic to tertiary to be recognized it is suggested that variation in the convergence angle of the oceanic plate relative to the continental edge was an important factor affecting the timing and spatial distribution of plutonic activity on a regional scale major breaks in activity are probably best attributed to either low angle parallel convergence or periods of accretion along the convergent margin

pre orogenic volcanic assemblages and structure in the western cordillera of ecuador between l 40 s and 2 20 s a major reverse fault with a separation of at least a few thousand meters is exposed in the western cordillera of ecuador near 2 s ree and other incompatible elements patterns show i that the rocks exposed in the upthrust block to the east of the fault aremorb that had not yet been identified and ii that a volcanic island arc assemblage extensively outcrops in the downthrust block west of the fault

geodynamic evolution of the northern and central andes during early to middle mesozoic times a tethyan model the early to middle mesozoic sedimentary and magmatic history of the northern and central andes i e the nne trending colombian ecuadorian segment and the nw trending peruvian segment exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings the late triassic to late liassic period began with a widespread marine transgression from the latest triassic the colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in peru during this time no significant magmatic activity is recorded except in the emerging colombian area this period is interpreted as the result of rifting of a tethyan oceanic arm which separated the colombian and palaeo mexican margins from latest liassic an important calc alkaline magmatic arc developed along the emergent colombian segment further south the north peruvian shelf probably emerged but marine sedimentation continued in southern peru during middle and early late jurassic times the colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation along the peruvian segment a turbiditic trough emergent areas and continental basins were created and the scarcity of calc alkaline magmatism suggests that only very local subduction took place this period is regarded as one of the southeastward subduction beneath the colombian segment and of tethyan oceanic crust originating in the newly formed colombian oceanic arm this pattern would have induced a chiefly left lateral transform motion along the peruvian segment by kimmeridgian tithonian times the palaeogeographic framework had drastically changed along the colombian segment magmatic activity ceased and continental accretions occurred along dextral strike slip sutures in peru tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation in the coastal area arc related volcanism indicated that subduction took place beneath this segment this geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the tethyan ridges and replacement by pacific spreading centres inducing a roughly northeastward convergence direction

cretaceous and tertiary terrane accretion in the cordillera occidental of the andes of ecuador new field geochronological geochemical and biostratigraphical data indicate that the central and northern parts of the cordillera occidental of the andes of ecuador comprise two terranes the older pallatanga terrane consists of an early to late cretaceous oceanic plateau suite late cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source and a tectonic melange of probable late cretaceous age the younger macuchi terrane consists of a volcanosedimentary island arc sequence derived from a basaltic to andesitic source a previously unidentified regionally important dextral shear zone named the chimbo toachi shear zone separates the two terranes regional evidence suggests that the pallatanga terrane was accreted to the continental margin the already accreted cordillera real in campanian times producing a tectonic melange in the suture zone the macuchi terrane was accreted to the pallatanga terrane along the chimbo toachi shear zone during the late eocene probably in a dextral shear regime the correlation of cretaceous rocks and accretionary events in the cordillera occidental of ecuador and colombia remains problematical but the late eocene event is recognised along the northern andean margin

quaternary sanukitoid like andesites generated by intracrustal processes chacana caldera complex ecuador implications for archean sanukitoids high mg diorites enriched in incompatible elements and their extrusive equivalents are rare subduction related rock types that have been found in modern arc settings and in late archean sequences where they are associated with trondhjemite tonalite granodiorite ttg suites archean rocks with these geochemical features are known as sanukitoids and despite their limited abundance are considered to be the indicators of the onset of modern plate tectonics because of their similarities to modern subduction related high mg andesites and diorites understanding the genesis of sanukitoid rocks is thus an essential step towards understanding crustal growth processes the accepted petrogenetic models for modern enriched high mg andesites and their archean equivalents the sanukitoids consist of metasomatic enrichment of the mantle wedge by slab components and its subsequent partial melting or the modification of siliceous slab components through continuous reaction with mantle peridotite during their ascent through the mantle wedge we present new data on the petrography mineral chemistry and whole rock geochemistry major and trace elements and sr nd pb isotopes of andesitic rocks from an 30 ka lava flow yuyos flow from the chacana caldera complex eastern cordillera of ecuador these rocks show a remarkable geochemical affinity with archean sanukitoids including high magnesium numbers 0 58 0 63 accompanied by high contents of incompatible elements e g th 17 23 ppm u 6 7 5 ppm ba 1600 1800 ppm sr 1430 1565 ppm la 74 94 ppm additionally the sanukitoid like andesites of yuyos are associated with predominant silica rich adakite like andesites which are widespread throughout the quaternary arc of ecuador this makes the quaternary ecuadorian magmatic province a close equivalent of the archean ttg sanukitoid association the bulk rock geochemistry petrography and mineral chemistry data indicate that the sanukitoid like features of the andesites of the yuyos flow derive from intracrustal recycling of the felsic intermediate to mafic ultramafic roots of the quaternary volcanic arc of ecuador by normal mantle derived basaltic magmas with the geochemical characteristics of continental arc basalts or high alumina basalts in view of the similarities between the yuyos andesites and archean sanukitoids in terms of geochemistry and lithological association we suggest that genetic models should consider the possibility of intracrustal recycling as a process responsible for the peculiar signatures of both archean sanukitoids and modern enriched high mg andesites

special paper major gold deposits and belts of the north and south american cordillera distribution tectonomagmatic settings and metallogenic considerations a compilation of economically viable gold concentrations containing 10 moz in the north and south american cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits most formed over the last 150 m y the gold concentrations are attributed to eight widely recognized deposit types of which porphyry sediment hosted and high sulfidation epithermal are economically the most important individual gold belts are typically several tens to hundreds of kilometers long dominated by single deposit types and metallogenically active for relatively brief periods 5 20 m y many of the gold belts and major isolated deposits were generated under extensional or transtensional tectonic conditions in either arc or back arc settings nevertheless the two main high sulfidation epithermal gold belts were generated in thickening or already thickened crust during low angle subduction eight other gold belts or districts also accompanied compression or transpression with two of them the main orogenic gold belts occupying fore arc sites there is a strong suggestion that the preeminent cordilleran gold concentrations formed either during or immediately following prolonged contraction the major gold deposits and belts occur along the craton edge as well as in adjoining accreted terranes but almost all are of postaccretionary timing many of the gold belts and isolated deposits were localized by crustal scale faults or lineaments which may be either parallel or transverse to the cordilleran margin the gold concentrations accumulated during active subduction commonly in close spatial and temporal association with intermediate to felsic medium to high k calc alkaline igneous rocks by contrast the low sulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc alkaline tholeiitic or alkaline affinity however a gold alkaline rock association is uncommon a genetic link between gold mineralization and coeval magmatism is widely accepted for most of the deposit types the exceptions being the sediment hosted and orogenic gold deposits notwithstanding the small cumulative extent of the gold concentrations relative to the entire cordilleran margin there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types to occur in close proximity within relatively restricted arc including fore and back arc segments in the case of the western united states for example six belts and four isolated major deposits make up a particularly prominent cluster if fortuity is discounted this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization an analogous situation is evident for other metals particularly copper and tin the reason for the recurrent generation of major deposits and belts dominated by one or more metals remains uncertain although heterogeneously distributed metal preconcentrations favorable redox conditions or other parameters somewhere above the subducted slab between the mantle wedge and upper crust are widely contemplated possibilities elucidation of the reason s for this metallogenic inheritance at the scale of limited arc segments poses an important and challenging series of research questions as well as being critical to the planning of potentially successful greenfield exploration programs

patterns of river width and surface area revealed by the satellite derived north american river width data set as hydraulic hydrologic and biogeochemical models evolve toward greater spatial resolution and larger extent robust morphometric data sets are essential to constrain their results here we present the landsat derived north american river width narwidth data set the first fine resolution continental scale river centerline and width database narwidth contains measurements of 2 4 105 km of rivers wider than 30 m at mean annual discharge we find that conventional digital elevation model derived width data sets underestimate the abundance of wide rivers to calculate the total surface area of north american rivers we extrapolate the strong observed relationship between river width and total surface area at different river widths r2 0 99 for 100 2000 m widths to narrower rivers and streams we conservatively estimate the total surface area of north american rivers as 1 24 0 15 0 39 105 km2 1 confidence intervals values 20 15 38 greater than previous estimates used to evaluate greenhouse gas efflux from rivers to the atmosphere

the geology and mesozoic collisional history of the cordillera real ecuador abstract the geology of the metamorphic rocks of the cordillera real of ecuador is described in terms of five informal lithotectonic divisions we deduce that during the mesozoic repeated accretionary events occurred and that dextral transpression has been of fundamental importance in determining the tectonic evolution of this part of the northern andes the oldest event recognised of probable late triassic age may be related to the break up of western gondwana and generated a regional belt of s type plutons during the jurassic major calc alkaline batholiths were intruded following this in latest jurassic to early cretaceous time a volcano sedimentary terrane of possible oceanic or marginal basin origin the alao division and the most westerly gneissic chaucha arenillas terrane were accreted to continental south america the accretion of the oceanic western cordillera took place in latest cretaceous to earliest tertiary time this latter event coincided with widespread thermal disturbance as evidenced by the large number of young k ar mineral ages recorded from the cordillera real

geomorphic evidence of active deformation and uplift in a modern continental wedge top foredeep transition example of the eastern ecuadorian andes abstract the eastern ecuadorian andes appear as a fold and thrust belt adjacent to a continental foredeep represented by one of the world s largest tropical alluvial megafans the pastaza megafan debouching into the amazonian lowland the apex of the pliocene pleistocene megafan situated in the present day wedge top subandean zone has been cut by an erosion surface the western part of which has been uplifted of 500 m along the frontal thrust forming a poorly dissected plateau the mera plateau this erosion surface erased most of the previous fluvial landscape but preserved a large thrust related anticlinal hinge deforming less erodible underlying strata the mirador fold and smaller sized anticlines this surface has been then incised by two antecedent major rivers the pastaza and the napo and few tributaries the plateau edge is marked by a series of large scale gently sloping landslides clustered along a 70 km long concave eastward line associated with the frontal thrust fault the newly formed immature rivers issued from the landslides or sourced within east dipping remnants of the erosion surface downstream of the landslide line constitute the greatest part of the streams feeding the ecuadorian amazonian basin at 70 to 100 km from the landslide line the drainage abruptly changes from highly immature to mature with a well defined hinge line representing the outer limit of landslide and tectonic control the diversions of the pastaza river indicate ongoing fold growth since at least the late pleistocene in the eastern cordillera and the early holocene in the mera plateau the preserved terraces of the pastaza valley are all degradational and are ascribed to periods of tectonic seismic activity alternating with periods of tectonic quiescence or decreased seismic activity rather than to climatic events 14c dating of the plateau erosion surface and of the upper pastaza terraces indicates that the minimum average incision rate since 18 000 years bp varies locally in the upper pastaza valley from 0 5 to 0 67 cm year 1 increasing from 18 000 years bp to now a comparison of these incision rates with fold and thrust fault uplift rates indicates that incision in the upper pastaza valley was a result of rapid uplift up to 1 cm year 1 along the mirador fold and thrust which caused a restoration of the local equilibrium profile of the upper reach combined with smaller local fault uplift along the westernmost thrust faults the uplift of the whole mera plateau with respect to the upper amazonian basin gives a minimum average uplift rate of 2 8 cm year 1 since 18 000 years bp the overall uplift of the mera plateau and the eastern cordillera is likely to have been caused by a regional scale low angle thrust ramp emerging as the frontal thrust fault

cooling and inferred uplift erosion history of the grenville orogen ontario constraints from 40ar 39ar thermochronology stepwise 40ar 39ar degassing experiments of 57 mineral separates of hornblende muscovite biotite and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the grenville orogen in ontario the 40ar 39ar data support an interpretation of slow nearly uniform cooling 1 4 c m y from temperatures of 500 c to below 150 c in the central gneiss belt cgb hornblendes cooled through ar closure between 930 and 1025 ma whereas in the central metasedimentary belt cmb hornblendes record the following range in 40ar 39ar cooling ages 1104 ma in the frontenac terrane 1007 1067 ma in the sharbot lake terrane 919 1026 ma in the elzevir terrane and 972 ma in the central metasedimentary belt boundary zone regional uplift erosion rates of 0 03 0 14 km m y have been estimated for the grenville orogen in ontario based on the 40ar 39ar data a model retrograde p t path for rocks of the cgb and an upper time constraint provided by flat overlying cambrian and ordovician sediments these erosion rates are consistent with rates estimated for other proterozoic or archean granulite terranes but are an order of magnitude slower than active orogens such as the alps and himalayas a regular variation in hornblende 40ar 39ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the cmb the pattern of 40ar 39ar ages is interpreted to reflect late tectonic extension consistent with field observations in the central metasedimentary belt boundary zone and elsewhere in the cmb up to 13 km of vertical displacement is inferred for some rocks in the cmb between the time they cooled below closure to argon diffusion in hornblende 500 c and their exposure at the surface 25 c

the nature and provenance of accreted oceanic terranes in western ecuador geochemical and tectonic constraints western ecuador consists of a complex tectonic melange of oceanic terranes accreted to the continental margin from late cretaceous to eocene time new geochemical data from these accreted terranes arising from a 5 year british geological survey mapping programme indicate that they comprise rocks from a variety of oceanic tectonic settings from thickened and relatively unsubductable oceanic plateau basalts through island arc tholeiites with occasional more calc alkaline lavas to back arc basin basalt sequences this study has enabled us to construct a new geodynamic model for the cretaceous tertiary evolution of the northern andes and has placed important new constraints on the extent of oceanic plateau sequences in colombia and around the caribbean the age and nature of sediments combined with evidence for the age of peak metamorphism suggests that a prolonged 15 20 ma accretionary event occurred in late cretaceous time and involved the collision of an oceanic plateau represented by the pallatanga unit with the continental margin this accreted unit can be correlated with similar oceanic plateau sequences from the western cordillera of colombia and those within and around the caribbean region the naranjal and macuchi island arcs and the associated la portada back arc basin developed along the accreted margin from late campanian to eocene time and these arcs accreted to the continental margin along with oceanic plateau material represented by the pinon unit and pedernales esmeraldas sequences during eocene time the development of island arcs which separate the two accretionary events implies that the most westerly coastal oceanic plateau sequences both in ecuador pinon and pedernales esmeraldas and in colombia gorgona and serrania de baudo cannot belong to the caribbean colombian oceanic plateau ccop it therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the northern andes it is possible that the ccop formed over the galapagos hotspot as previously proposed but the more westerly coastal plateau was derived from a more southerly hotspot source region such as sala y gomez in the se pacific

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

provenance of the upper cretaceous to upper eocene clastic sediments of the western cordillera of ecuador geodynamic implications abstract the late cretaceous eocene clastic deposits of the western cordillera of ecuador record significant changes in the source areas grain size and location of the depocenters related to the accretion of oceanic terranes that constitute the present day western cordillera and coast major changes in the source areas occurred in the late maastrichtian and late middle eocene they are interpreted as corresponding to the accretion of the guaranda and macuchi oceanic terranes respectively major increases in the grain sizes occurred in the late maastrichtian late paleocene and late middle eocene and seem to coincide with the accretion of the guaranda pinon and macuchi terranes respectively the increasing occurrence of plutonic or metamorphic fragments and the westward shift of the depositional areas through the paleocene upper eocene interval indicate an increasing uplift and erosion of the cordillera real continuous although jerky uplift of the latter during the maastrichtian eocene period supports the idea that the accreted oceanic material contributed to the crustal thickening and relief creation of the ecuadorian andes

the metamorphic basement of ecuador the metamorphic rocks of ecuador once thought to constitute a uniform and ancient basement belong to several distinct units of widely differing ages compositions and metamorphic histories the largest area of metamorphic rocks constitutes an unbroken belt on the eastern andean slope from the colombian to the peruvian border the bulk of the rocks in this belt are of cretaceous age and were metamorphosed in the greenschist facies under a barrovian facies series rocks in the extreme north and those south of lat 3 15 s are different and may be of paleozoic age a smaller terrain on the western andean slope in southwestern ecuador is yet more varied it includes four metamorphic terrains the polymetamorphic piedras group of precambrian age the low pressure tahuin group of paleozoic age the high pressure raspas formation of cretaceous age and a low pressure terrain of uncertain age north of the la palma fault several isolated outcrops of metamorphic rocks in ecuador are of uncertain significance

mafic pegmatites intruding oceanic plateau gabbros and ultramafic cumulates from bolivar colombia evidence for a wet mantle plume

modification of an oceanic plateau aruba dutch caribbean implications for the generation of continental crust the generation of the continental crust may be connected to mantle plume activity however the nature of this link and the processes involved are not well constrained an obstacle to understanding relationships between plume related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships particularly in ancient greenstone belts studies of younger analogous regions may help to clarify these relationships on the island of aruba in the southern caribbean a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed the mafic lavas show geochemical and isotopic affinities with other basaltic picritic and komatiitic rocks that crop out elsewhere in the caribbean these are well documented as belonging to an 88 91 ma plume related oceanic plateau which is allochthonous with respect to the americas and is thought to have been formed in the pacific region the 85 to 82 ma tonalitic rocks share some geochemical characteristics high sr and ba low nb and y with archaean tonalite trondhjemite granodiorite ttg suites field relationships suggest that deformation of the plateau sequence possibly related to collision with a subduction zone was synchronous with intrusion of the aruba batholith new incremental heating 40ar 39ar dates combined with existing palaeontological evidence show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence sr nd isotopic data for both rock suites are uniform 87sr 86sri 0 7035 endi 7 whereas pb isotopes are more variable plateau sequence 206pb 204pb 18 6 19 1 207pb 204pb 15 54 15 60 208pb 204pb 38 3 38 75 aruba batholith 206pb 204pb 18 4 18 9 207pb 204pb 15 51 15 56 208pb 204pb 38 0 38 5 this suggests that there has been a minor sedimentary input into the source region of the batholith however the limited time interval between basaltic and tonalitic magmatism makes a normal subduction related origin for the tonalites improbable instead models involving derivation of tonalite from partial melting of the plateau sequence or alternatively genesis in an unusual subduction zone environment are investigated

cretaceous basaltic terranes in western colombia elemental chronological and sr nd isotopic constraints on petrogenesis accreted terranes comprising mid to late cretaceous picrites basalts and dolerites occur in three north south trending belts in western colombia in the central cordillera western cordillera and along the pacific coast the geochemistry of these rocks is consistent with an oceanic plateau plume related origin and they most probably formed in the pacific as part of the caribbean oceanic plateau these igneous rocks display small but significant inter cordillera variations being younger and more depleted in incompatible trace element ratios and with more positive end values to the west the igneous rocks of the pacific coast serrania de baudo are dated at 73 78 ma 40ar 39ar and those of the western cordillera at 90 ma whereas the volcanics of the central cordillera are believed to be older than 100 ma most of the igneous rocks are basaltic and it is suggested that they have fractionated from picritic primary magmas generated by partial melting within a hot mantle plume variable and positive end values reveal that the plume must have been heterogeneous originating from a mantle source with a long term history of depletion partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of 20 mixing of melts from different depths either in the mantle melting column or during fractionation in lithospheric magma chambers can explain the relative homogeneity of basaltic lavas erupted to form this and other oceanic plateaux the caribbean colombian oceanic plateau may have formed at an oceanic spreading centre and valuable comparisons can be made between iceland and the caribbean colombian plateau

estimating reach averaged discharge for the river severn from measurements of river water surface elevation and slope summary an algorithm is presented that calculates a best estimate of river bathymetry roughness coefficient and discharge based on input measurements of river water surface elevation h and slope s using the metropolis algorithm in a bayesian markov chain monte carlo scheme providing an inverse solution to the diffusive approximation to the shallow water equations this algorithm has potential application to river h and s measurements from the forthcoming surface water and ocean topography swot satellite mission the algorithm was tested using in situ data as a proxy for satellite measurements along a 22 4 km reach of the river severn uk first the algorithm was run with gage measurements of h and s during a small in bank event in june 2007 second the algorithm was run with measurements of h and s estimated from four remote sensing images during a major out of bank flood event in july 2007 river width was assumed to be known for both events algorithm derived estimates of river bathymetry were validated using in situ measurements and estimates of roughness coefficient were compared to those used in an operational hydraulic model algorithm derived estimates of river discharge were evaluated using gaged discharge for the in bank event when lateral inflows from smaller tributaries were assumed to be known the method provided an accurate discharge estimate 10 rmse when lateral inflows were assumed unknown discharge rmse increased to 36 finally if just one of the three river reaches was assumed to be have known bathymetry solutions for bathymetry roughness and discharge for all three reaches were accurately retrieved with a corresponding discharge rmse of 15 6 for the out of bank flood event the lateral inflows were unknown and the final discharge rmse was 19 these results suggest that it should be possible to estimate river discharge via swot observations of river water surface elevation slope and width

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

multiple plume events in the genesis of the peri caribbean cretaceous oceanic plateau province the oceanic crust fragments exposed in central america in north western south america and in the caribbean islands have been considered to represent accreted remnants of the caribbean colombian oceanic plateau ccop on the basis of trace element and nd sr and pb isotopic compositions we infer that cumulate rocks basalts and diabases from coastal ecuador have a different source than the basalts from the dominican republic the latter suite includes the 86 ma basalts of the duarte complex which are light rare earth element ree enriched and display relative to normal mid ocean ridge basalts nmorb moderate enrichments in large ion lithophile elements together with high nb ta pb and low th contents moreover they exhibit a rather restricted range of nd and pb isotopic ratios consistent with their derivation from an ocean island type mantle source the composition of which includes the himu high 238u 204pb component characteristic of the galapagos hotspot in contrast the 123 ma ecuadorian oceanic rocks have flat ree patterns and relative to nmorb are depleted in zr hf th and u moreover they show a wide range of nd and pb isotopic ratios intermediate between those of ocean island basalts and nmorb it is unlikely on geochemical grounds that the plume source of the ecuadorian fragments was similar to that of the galapagos in addition because of the nne motion of the farallon plate during the early cretaceous the ecuadorian oceanic plateau fragments could not have been derived from the galapagos hotspot but were likely formed at a ridge centered or near ridge hotspot somewhere in the se pacific

a40ar 39ar study of post tectonic cooling in the britt domain of the grenville province ontario abstract we report the results of a 40 ar 39 ar study of rocks from the britt domain northwestern grenville province most of the age spectra obtained for micas and amphiboles have well defined age plateaus typically k feldspars yielded spectra with age gradients and arrhenius plots with kinks or offsets hence some of the data were interpreted in terms of a multi diffusion domain size model this 40 ar 39 ar data base 5 amphiboles 9 micas 4 k feldspar along with some recently acquired uzpb data provide reasonable constraints on the post peak metamorphic cooling history which began at depth in the upper amphibolite facies comparison of the data with exhumation paths generated by thermal models suggest that cooling taking place over 400 ma was for the most part due to erosionally controlled exhumation although the decompression may have initiated with a short period of rapid extensional uplift the entire cooling history may have been erosionally controlled there is an apparent difference in cooling ages between north and south in the 40 ar 39 ar mica and k feldspar data for the britt domain which may be due to somewhat higher exhumation rates in the north and or an elevated geotherm in the south

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

geochronological control for the main tectonic magmatic events of ecuador abstract a chronological outline of the principal tectonic magmatic events of ecuador is presented based upon approximately 100 k ar and rb sr age determinations eight magmatic episodes are recognized in southwestern ecuador evidence of plutonic and metamorphic activity in the late precambrian and in the middle to late triassic is found large granitic batholiths of middle to late jurassic age associated with widespread acidic volcanism are traceable along the eastern flank of the andes into colombia early cretaceous volcanism and plutonism is recognized only in southern ecuador although apparently equivalent units continue southwards into the coastal cordillera of peru a great orogenic belt characterized by metamorphism plutonism and volcanism was established in the late cretaceous and continued through the paleogene it is traceable into peru and colombia the macuchi fm of the western cordillera along with the pin on fm and associated sediments of the coastal zone represent the remnants of an island arc and the adjacent oceanic floor respectively that collided with the south american plate during the eocene feininger 1981 in the neogene quartz dioritic intrusions were emplaced in the western cordillera as well as in the cordillera de muelleturo although apparently at different times in the late oligocene and miocene acidic volcanism is noted in the southern ecuadorian andes but not in the north in the quaternary andesitic volcanism and large stratovolcanoes characterize the northern andes while widespread rhyolitic ashflow activity is noted in the southern andes in ecuador the post triassic magmatic episodes show a westerly migration similar to that found in colombia but opposite to that noted in peru

geochemical evolution of the quaternary chachimbiro volcanic complex frontal volcanic arc of ecuador abstract the chachimbiro volcanic complex cvc is a composite volcano located in the frontal part of the ecuadorian quaternary arc four periods of activity can be differentiated in chachimbiro 400 ka long volcanic history named ch1 405 7 20 0 298 6 32 9 ka ch2 121 75 23 2 36 08 2 8 ka ch3 36080 280 22730 120 ybp and ch4 5 760 30 ybp respectively the magmatic suite ranges in composition from andesites to rhyodacites and displays systematic increases of sio2 sr y la yb incompatible elements e g lree mree lile and depletion of hree with time these geochemical features coupled with radiogenic isotope data and textural observations e g several resorption and overgrowth cycles in plagioclase phenocrysts suggest an open system evolution for cvc including typical crustal magmatic processes such as recharge assimilation and fractional crystallisation to better constrain the magmatic crustal processes of the cvc we use a multi model approach including three open system models rfc recharge fractional crystallisation afc assimilation fractional crystallisation and ec rafc energy constrained recharge assimilation fractional crystallisation different simulations have been developed to best fit cvc s geochemistry including trace elements sr y ratios and sr isotopes which suggest predominant fractional crystallisation and variable assimilation as the main processes explaining the bulk of the geochemical data of cvc this is consistent with an overall thermal decline of the magmatic system through time the genetic model here presented involves the input of a magmatic recharge component matured in the low mid crust through dominant fractionation of amphibole garnet into intermediate crustal reservoir s where magma undergoes extensive fc

along strike variations in the thermal and tectonic response of the continental ecuadorian andes to the collision with heterogeneous oceanic crust oblique to strike geological segmentation in the andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates as well as the upperplate expressions of differing along strike subducted slab age strength and composition we present new white mica and biotite 40 ar 39 ar and zircon and apatite fission track data from several traverses across the cordillera real of ecuador in the northern andes that reveal distinct along strike differences in the timing of accelerated crustal cooling during the cenozoic the data record elevated cooling rates from temperatures of v380 c during v65 55 and v43 30 ma from all sampled regions of the cordillera real and at v15 ma and since v9 ma in the northern cordillera real each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust elevated cooling rates of up to v30 20 c myr were initiated during the palaeocene and eocene early oligocene along the entire contemporaneous margin of ecuador and were driven by the accretion of the oceanic pallatanga terrane and pin on macuchi block respectively onto northwestern south america both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the caribbean plateau and accreted onto the margin during the approximately northeastward migration of the plateau into its current position within ecuador the development of higher topography and elevated cooling rates of up to 50 c myr at v15 ma and since v9 ma are restricted to the region north of 1 30ps and is situated above the postulated subducted flatslab section of the aseismic carnegie ridge plate convergence rate calculations suggest the carnegie ridge collided with the ecuador trench at v15 ma which caused the pre existing coastal provinces to displace to the northeast subsequently driving extension and marine ingressions in southern ecuador and compression and uplift in northern ecuador fl 2001 elsevier science b v all rights reserved

partitioning of oblique convergence in the northern andes subduction zone migration history and the present day boundary of the north andean sliver in ecuador along the ecuadorian margin oblique subduction induces deformation of the overriding continental plate for the last 15 ma both exhumation and tectonic history of ecuador suggest that the northeastward motion of the north andean sliver nas was accompanied by an eastward migration of its eastern boundary and successive progressively narrowing restraining bends here we present geologic data earthquake epicenters focal mechanisms gps results and a revised active fault map consistent with this new kinematic model all data sets concur to demonstrate that active continental deformation is presently localized along a single major fault system connecting fault segments from the gulf of guayaquil to the eastern andean cordillera although secondary faults are recognized within the cordillera they accommodate a negligible fraction of relative motion compared to the main fault system the eastern limit is then concentrated rather than distributed as first proposed marking a sharp boundary between the nas the inca sliver and the subandean domain overthrusting the south american craton the nas limit follows a northeast striking right lateral transpressional strike slip system from the gulf of guayaquil isla puna to the andean cordillera and with the north south striking transpressive faults along the eastern andes eastward migration of the restraining belt since the pliocene abandonment of the sutures and reactivation of north south striking ancient fault zones lead to the final development of a major tectonic boundary south and east of the nas favoring its extrusion as a continental sliver accommodating the oblique convergence of the nazca oceanic plate toward south america

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

permo triassic anatexis continental rifting and the disassembly of western pangaea crustal anatectites are frequently observed along ocean continent active margins although their origins are disputed with interpretations varying between rift related and collisional we report geochemical isotopic and geochronological data that define an 1500 km long belt of s type meta granites along the andes of colombia and ecuador which formed during 275 223 ma these are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon u pb dates ranging between 240 and 223 ma a model is presented which places these rocks within a compressive permian arc setting that existed during the amalgamation of westernmost pangaea anatexis and mafic intrusion during 240 223 ma are interpreted to have occurred during continental rifting which culminated in the formation of oceanic crust and initiated the break up of western pangaea compression during 275 240 ma generated small volumes of crustal melting rifting during 240 225 ma was characterized by basaltic underplating the intrusion of tholeiitic basalts and a peak in crustal melting tholeiitic intrusions during 225 216 ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust and we assign this period to the onset of continental drift dissected ophiolitic sequences in northern colombia yield zircon u pb dates of 216 ma the permo triassic margin of ecuador and colombia exhibits close temporal faunal and geochemical similarities with various crustal blocks that form the basement to parts of mexico and thus these may represent the relict conjugate margin to nw gondwana the magmatic record of the early disassembly of pangaea spans 20 ma 240 216 ma and the duration of rifting and rift drift transition is similar to that documented in cretaceous tertiary rift settings such as the west iberia newfoundland conjugate margins and the taupo lau havre system where rifting and continental disassembly also occurred over periods lasting 20 ma

late glacial and holocene avulsions of the rio pastaza megafan ecuador peru frequency and controlling factors the geomorphological study by means of remote sensing imagery of the rio pastaza megafan ecuador and northern peru reveals the traces of numerous avulsions one hundred and eight avulsion sites have been defined the location of these sites the available radiocarbon ages as well as historical maps of the seventeenth century enable us to propose an evolution history of the migration and avulsions of the rio pastaza since the last glacial maximum the first avulsions of the rio pastaza occurred after the lgm in a zone close to and roughly parallel to the sudandean front where the developed avulsion gave a distributive pattern to the ancient stream of the rio pastaza in an area located between the modern rio morona and pastaza where they caused the rio pastaza to develop a fan like distributary pattern this is interpreted as a response to thrust related forelimb tilt progressively shifting eastward the rio pastaza and the apex of the megafan this sequence of events ended with the great diversion of the rio pastaza towards the modern rios corrientes and tigre avulsions occurred in the tigre corrientes area between 9200 and 8 500 years cal bp afterwards the rio pastaza was diverted to its present day north south course this last significant avulsion occurred before ad 1691 in the area located between the modern rio morona and pastaza avulsion frequency probably overestimated ranges between 100 and 200 years in the rios tigre and corrientes area avulsion frequency probably underestimated ranges from 300 to 400 years regional tectonics is likely to have triggered most of the avulsions in the morona pastaza area but its influence is restricted to this area the factors controlling the avulsions in the tigre corrientes area are less clear because the frequently described hydrologic driven avulsion as observed in areas characterized by contrasted hydrologic cycles are inconsistent with the characteristics of the hydrologic cycles of the rio pastaza

sediment budget of the napo river amazon basin ecuador and peru the upstream downstream sediment budget along the napo river 100 520 km2 6300 m3 s 1 was studied in the andean foothills of ecuador at the west of the amazon basin a comparative study was made during four hydrological cycles 2001 2005 for three hydrological stations located upstream and during one hydrological cycle 2004 2005 for the fourth one located near the mouth of the napo river region of iquitos in peru this analysis showed an unusual increase in the concentration of suspended sediment recorded for the western part of the amazon plain like the runoff 81 l s 1 km2 which is a world s maximum the erosion rate 1160 t km 2 year 1 i e 47 of total suspended solid tss export at the exit of ecuador one of the highest for a floodplain basin is the result of a stepper slope than in the rest of the andean foothills where typically sedimentation phenomena are predominant and can be explained in part by a greater tectonic activity similar phenomenes were evidenced in small mountainous rivers in new guinea milliman and syvitski 1992 milliman 1995 on the headwaters of the napo river drainage basin the tectonic uplift causes the pastaza megafan s existence this progressively diverts the course of napo river towards north and also provokes the remobilization of fine fluvial deposits moreover this geodynamic trend is completed by the impact of volcanic eruption earthquakes and landslides the combination of these phenomena so common in the region has provided a large sediment transfer not only at present but also in the past as can be confirmed by the presence of incised terraces mainly formed by volcanic materials then these results were compared with a similar study carried out further south in the madeira basin at the bolivian foothills these studies show the spatio temporal variability of the relation between sediment transfer and geodynamic processes at the andean piedmont copyright 2009 john wiley sons ltd

the petrogenesis of gorgona komatiites picrites and basalts new field petrographic and geochemical constraints gorgona island colombia is remarkable not only because it contains the only phanerozoic komatiites but also because it has mafic to ultramafic lavas with a wide range of compositions from moderately enriched to extremely depleted relative to bulk earth the komatiite flows are in many respects similar to archaean komatiites they formed from mgo rich 18 liquids and have upper spinifex zones and lower cumulate zones the cumulate zones of archaean komatiites contain many solid grains in contrast more than 90 of the olivine in the gorgona cumulates is highly skeletal this combined with the fact that the gorgona cumulate zones are thinner than those in archaean komatiites suggests that the komatiite magma became strongly superheated en route to the surface the komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs some basalts have isotope compositions indicative of long term enrichment in incompatible elements whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion it is apparent that the plume source region of the gorgona magmas was markedly heterogeneous with at least two source components contributing to the observed variation in composition this heterogeneity may have resulted from the incorporation of different components into the plume source or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basal new 4oar 39ar dating of basalts and gabbros are more consistent with all being generated at 87 ma during formation of the caribbean colombian plateau possibly at the galapagos hotspot