the metamorphic internal zone of wopmay orogen early proterozoic canada 30 km of structural relief in a composite section based on plunge projection in wopmay orogen an early proterozoic volcano sedimentary marginal prism and its underlying archean structural basement deformed together during the 1885 ma calderian orogeny are exposed in oblique view as a result of regional cross folding a composite cross section displaying over 30 km of structural relief provides a framework in which to examine the geologic relationships in the metamorphic internal zone of the orogen a sole thrust separates archean basement and a thin mantle of autochthonous early proterozoic strata from the eastward transported calderian allochthon the allochthon is characterized by a transition from external thrust fold belt geometries in the east to polyphase isoclinal recumbent folds and thrusts in the west the center of the zone contains the hepburn batholith which has been been transported along with the deforming hot allochthon over the exposed basement both basement and allochthon are deformed by late large scale upright folds the metamorphic internal zone is interpreted to record the closing of a rift basin during which the basin fill was shortened thickened to over 30 km metamorphosed and thrust eastward while still hot over the relatively cool archean craton the basement presently exposed beneath the sole thrust remained relatively nondeformed during thrusting but underwent large scale folding due to continued compression during uplift of the internal zone

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

erosion rates over millennial and decadal timescales at caspar creek and redwood creek northern california coast ranges comparing millennial scale denudation rates from cosmogenic nuclides with decadal scale sediment yields can shed light on erosional processes and on the effects of land use on sediment delivery to streams detailed measurements of sediment fluxes in the northern california coast ranges at caspar creek and redwood creek have provided estimates of physical erosion rates since 1963 and 1971 respectively we used cosmogenic 10 be to measure

evolution and thickness of the lithosphere beneath coastal california many of the tectonic features of the california coast ranges are directly related to the northward migration of the mendocino triple junction along the western edge of north america during the past 20 to 30 m y at the triple junction the western edge of the north american plate slides off the relative northward moving and subducting gorda plate leaving the thin western edge in direct contact with asthenosphere upwelling to fill the space vacated by the underlying gorda plate two dimensional time dependent thermal modeling of this process constrained by teleseismic delay studies is used to construct a map of lithospheric thickness of coastal california among the implications of this map are that 1 the high heat flow in the coast ranges can be almost entirely accounted for by the asthenospheric upwelling associated with the migrating mendocino triple junction 2 the general elevation and the late cenozoic volcanism in the california coast ranges are responses to a zone of unusually thin 20 to 45 km lithosphere that extends southward behind the mendocino triple junction and 3 the course of some segments of the san andreas fault in both central and southern california appear to deviate from a deeper more fundamental transform boundary separating the pacific and north american plates

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

reconciling tectonic shortening sedimentation and spatial patterns of erosion from 10 be paleo erosion rates in the argentine precordillera abstract the temporal evolution of erosion over million year timescales is key to understand the development of mountain ranges and adjacent fold and thrust belts while models of orogenic wedge dynamics predict an instantaneous response of erosion to pulses of rock uplift stream power based models predict that catchment wide erosion maxima significantly lag behind a pulse of rock uplift here we explore the relationships between rock uplift erosion and sediment deposition in the argentine precordillera fold and thrust belt at 30 s using a combination of 10be derived paleo erosion rates constraints on re exposure using 26al 10be ratios geomorphic observations and detrital zircon provenance we demonstrate that the attainment of maximum upland erosion rates lags the maximum rate of deformation over million year timescales the magnitudes and causes of the erosional delays shed new light on the catchment erosional response to tectonic deformation and rock uplift in orogenic wedges

u pb zircon geochronology of the yellowknife volcanic belt nwt canada new constraints on the timing and duration of greenstone belt magmatism volcanic and sedimentary rocks in the slave province collectively termed the yellowknife supergroup have long been considered to represent laterally continuous and correlative stratigraphically coherent homoclinal sequences correlation based on the lithologic analysis of certain distinctive units has been most intensely applied to the yellowknife volcanic belt in the southern slave province u pb zircon data for seven samples collected from within the belt are reported here the distinctiveness of the townsite formation a term applied to three lenticles of rhyodacitic to dacitic tuffs breccias and pillowed flows in comparison to the overlying and underlying tholeiitic basalts kam group has led to its use as a key marker unit for stratigraphic correlations and palinspastic restorations of offsets on proterozoic faults in the area a sample from each of the three segments was collected for u pb zircon geochronology in addition numerous thinner tuffaceous beds from the kam group were also dated

u pb geochronology of the hope bay elu inlet area bathurst block northeastern slave structural province northwest territories u pb ages for six felsic samples from the dominantly mafic hope bay and elu inlet metavolcanic belts of the bathurst block provide the first constraints on the timing of felsic volcanism 2685 2677 ma and plutonism 2672 2608 ma in the northeastern corner of the slave structural province gold veins that cut a pluton in the eastern hope bay belt are younger than 2672 ma early proterozoic magmatic activity is recorded in a pegmatite dyke that crosscuts metavolcanic rocks of the northern hope bay belt initial nd and sr isotopic ratios from the hope bay and elu inlet belts indicate a juvenile source for the magmas

tectonic and thermal history of the anialik river area northwestern slave province canada the anialik river area in the northwestern slave province comprises two geological domains of different age and origin that were tectonically juxtaposed at ca 2650 ma the older domain the kangguyak gneiss belt comprises ca 3300 2700 ma orthogneisses and paragneisses interpreted as the remnants of a mesoarchean continental margin the younger domain the anialik river greenstone belt consists of ca 2680 ma mafic to felsic volcanic rocks interpreted to have formed in an ensimatic island arc setting structural and geochronological evidence suggest collision of the two domains began around 2650 ma in a transpressive regime that involved oblique sinistral subduction of the greenstone belt beneath the kangguyak domain along the tokhokatak shear zone displacement continued until at least ca 2600 ma when late two mica granites intruded along and were deformed in the shear zone following ca 2600 ma rocks in both domains and along the fault cooled rapidly to about 350 c strongly overprinted musco

geochemistry of the late archean banting group yellowknife greenstone belt slave province canada simultaneous melting of the upper mantle and juvenile mafic crust this study investigates the geochemistry and tectonic setting of the 2 66 ga banting group the younger sequence of volcanic rocks in the yellowknife greenstone belt and its relationship to older tholeiitic volcanic rocks of the 2 72 2 70 ga kam group the banting group includes a much higher proportion of felsic volcanic and volcaniclastic rocks than the kam group but mafic to intermediate volcanic rocks are common banting basalts are tholeiitic and are melts of archean depleted upper mantle as are basalts of the kam group in contrast banting dacites and rhyolites have much lower heavy rare earth element abundances and generally have higher initial 143nd 144nd than kam felsic rocks the chemistry of the felsic rocks provides a geochemical signature to distinguish rocks of kam versus banting age where complex structures have obscured the stratigraphy whereas kam felsic rocks evolved from mafic parents by assimilation fractional crystallization processes banting felsic rocks have compositions sim

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

the control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment the study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape these measurements are done with in situ produced cosmogenic nuclides e g 10be 26al mostly in quartz from alluvial sediment cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the earth s surface they are produced within a characteristic depth scale of about 1 m which means that the measured concentrations record an integrated denudation history while material passed through this depth interval depending on the denudation rate the resulting integration time scales are 103 to 105 years and one obtains a robust long term estimate of natural denudation that is relatively insensitive to short term changes the last 10 years have seen significant research activity using these methods and an array of fascinating tectono geomorphologic and geochemical insights are emerging amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change a compilation of world wide denudation rates in non glaciated areas that however does not yet include some of the world s most active mountain belts has resulted in the following findings some of which have been unexpected 1 no obvious relationship between precipitation or mean annual temperature and total denudation is apparent 2 topographic relief alone does not result in high rates of denudation 3 denudation rates are high in areas of landscape rejuvenation that is triggered and controlled by tectonic activity faulting escarpment formation and retreat rifting surface uplift 4 rates of weathering using a combination of cosmogenic nuclides and zirconium normalised cation loss balances co vary primarily with physical erosion rates and much less with temperature or precipitation 5 in some areas of high land use short term rates from river load gauging exceed those from cosmogenic nuclides by several orders of magnitude which serves to highlight the severity of geomorphic change caused by human action in the future the control mechanisms over denudation will be determined on all spatial scales ranging from the single soil section to entire river basins the same analysis can be done back through time on well dated terraces lake records and marine sediment cores which is possible with 10be for the past 1 2 my the rates obtained will be used to develop a quantitative understanding of tectonic geomorphologic and geochemical landscape processes which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic climate and anthropogenic forcing

influence of tectonism and climate on lithofacies distribution and sandstone and conglomerate composition in the archean beaulieu rapids formation northwest territories canada abstract the 0 2 1 km thick archean beaulieu rapids formation is a late orogenic tectonically controlled sedimentary sequence overlying the beaulieu river volcanic belt unconformably on the west and bordering the major n trending beniah lake fault on the east local porphyry stocks characteristic of late orogenic successions were emplaced along strike of the fault four lithofacies comprise the beaulieu rapids formation 1 the 20 320 m thick conglomerate i 2 the 10 305 m thick siltstone sandstone 3 the 20 120 m thick conglomerate ii and 4 the 40 320 m thick quartz rich sandstone lithofacies stacking displays two distinct large scale fining upward sequences represented by the 300 375 m thick conglomerate i and sandstone siltstone sequence a and the 300 400 m thick conglomerate ii and quartz rich sandstone sequence b fining upward sequences a bounding unconformity and the beniah lake fault are compelling indicators of tectonic influence on sedimentation conglomerate i was deposited by debris flows traction currents and sheetfloods on the proximal medial and distal portions of alluvial fans or fan deltas respectively the overlying siltstone sandstone contains two distinct lithological units a sandstone dominated unit formed from fluvial dune migration and overbank deposition in a sandy braidplain setting with scattered ponds whereas sedimentary structures in the siltstone dominated unit are consistent with lacustrine deposition a second phase of tectonic uplift produced sequence b conglomerate ii was deposited in a proximal braided stream setting characterized by abundant longitudinal gravel bars the quartz rich sandstone developed from dune migration on a sandy braidplain mineralogy and textures reflect high sediment influx limited transport distance local sources and climatic influence clast populations in conglomerate indicate a prevalent mafic volcanic source whereas sandstones contain prominent quartz av 76 with subordinate feldspar 13 and lithic fragments 11 the disparity between clast and sandstone composition is a function of chemical weathering typical of a humid climate source areas are restricted to the mafic dominated beaulieu river volcanic belt the 2 8 ga gneisso plutonic sleepy dragon complex and fault related porphyry stocks the humid weathering aggressive archean climate was responsible for destroying labile minerals and lithic fragments in proximal source rocks rich in plagioclase accounting for the high quartz component in the sandstones the beaulieu rapids formation is similar to modern strike slip basins that form along major fault zones in terms of basin geometry faulted and unconformable contacts lithological units and sedimentary facies stacking and organization fault controlled basins of this nature commonly develop during the last stage of terrane accretion on archean cratons

geomorphic disequilibrium in the eastern korean peninsula possible evidence for reactivation of a rift flank margin abstract tectonically inactive since the middle miocene the korean peninsula is generally considered a typical passive continental margin with asymmetric relief with only low levels of seismic activity recent reports of uplift rates as high as 150 350 mm ka 1 from geomorphic markers using numerical dating methods e g radiocarbon optically stimulated luminescence osl and cosmogenic nuclides however cast doubt on the tectonic quiescence of korea thus we evaluate the geomorphic status of eastern korea using denudation rates at geomorphic timescales of 103 106 years which are equivalent to the rates previously reported at local points to infer regional denudation rates we measured catchment wide denudation rates cwdr in 13 watersheds using in situ cosmogenic 10be and 14c analyses of riverine sediment samples in eastern korea these cwdr data suggest that the mean denudation rate during the past 5 14 ka centers around 74 7 25 4 mm ka 1 which is consistent with the long term geologic exhumation rate we derive of 74 10 1 mm ka 1 since the middle miocene however our cwdr data are 2 3 times lower than rates of coastal uplift and river incision of 150 350 mm ka 1 derived from data on marine and fluvial terraces respectively in addition to isotopic analyses we quantified topographic indices such as elevation relief and slope and then compared these indices with cwdrs to examine morphological changes that might be related to tectonic disturbances since the beginning of late pleistocene the relationship between cwdrs and topographic indices indicates that most of the study area rests in a near steady state condition however in contrast to the smooth concave longitudinal profile expected for a steady state landscape topographic analyses reveal the presence of major knickzones confirmed by field reconnaissance these knickzones may have been produced by changes in base level caused by tectonic events lithologic variations or sea level changes our 1 d modeling of knickpoint propagation suggests that tectonic disturbance is the factor most likely to produce knickzones in the study area raising the possibility that erosional signals caused by late pleistocene tectonic disturbance affected channel gradients but that the knickpoints have not yet propagated through the entire catchment area our results suggest that the eastern part of the korean peninsula is approaching but has not completely reached a geomorphic steady state and that transient conditions have been induced by reactivation of the rift flank margin no later than the late pleistocene causing tilting of the central korean peninsula to the west at a rate of 7 0 10 10 m m per year over the past 125 ka

erosion of coastal drainages in the mendocino triple junction region mtj northern california abstract the mendocino triple junction mtj region in northern california is an archetype for studying landscape response to varying rock uplift rates as they increase toward the triple junction underpinning these studies lies the assumption that this landscape has reached a dynamic equilibrium in which rock uplift and erosion rates are equal however no study has shown that such an equilibrium actually exists around the mtj region here we report 10be and 26al derived erosion rates calculated from isotope concentrations in detrital fluvial sediment from coastal drainage basins we then compare these rates with uplift rates inferred from marine terraces that were formed and preserved by uplift during the last 305 ka erosion rates in the more slowly uplifting southern part of the region range from 0 21 0 32 mm yr and are consistent with rock uplift rates since 305 ka however in the northern transition zone where uplift rates apparently started to increase about 100 ka due to northward migration of the mtj erosion rates are higher 0 43 to 0 69 mm yr these rates are similar to uplift rates from 96 305 ka but substantially less than recent uplift rates of 3 5 4 mm yr inferred for the past 72 ka in the central part of the king range finite erosion rates cannot be determined due to i the presence of excess 10be that does not appear to have originated from in situ cosmic ray production during erosion and transport of the sediment and ii 26al concentrations below detection limits the difference between catchment wide erosion and recent uplift rates in coastal catchments may be related to a lag in response of hillslopes and tributaries to changes in rates of tectonically driven relative base level fall or the uncertainties in rock uplift rates inferred from marine terraces this study suggests that measurements of both erosion and uplift rates are crucial in assessing the equilibrium state of landscapes and in understanding the topographic features made by surface and tectonic processes

steepened channels upstream of knickpoints controls on relict landscape response 1 the morphology of a relict landscape provides important insight into erosion rates and processes prior to base level fall fluvial knickpoints are commonly thought to form a leak proof moving boundary between a rejuvenated landscape below and a relict landscape above we argue that fluvial rejuvenation may leak farther upstream depending on the rate and style of knickpoint migration the outer margin of a relict landscape should therefore be used with caution in tectonic geomorphology studies as channel steepening upstream of knickpoints could reduce the relict area we explore the response of the roan plateau to knickpoint retreat triggered by late cenozoic upper colorado river incision multiple knickpoints 100 m waterfalls separate a low relief upper landscape from incised canyons below two digital elevation model data sets 10 m u s geological survey and 1 m airborne laser swath mapping indicate steeper channels above waterfalls relative to concave channels farther upstream the steepened reaches are several kilometers long correspond to doubling of slope and exhibit channel narrowing and an increase in hillslope angle we compare two mechanisms for generating steepened reaches the first uses a recent model for erosion amplification due to flow acceleration at the waterfall lip the second acknowledges that waterfall lips may be limited to the outcrop of a resistant formation subtle structural warping of the stratigraphy can lead to lowering of the waterfall lip as it retreats thus lowering base level for upstream channels results of numerical modeling experiments suggest the latter mechanism is more consistent with our observations of long mildly steepened reaches

limits to quantifying climate driven changes in denudation rates with cosmogenic radionuclides abstract the hypothesis that climate change significantly modifies the denudation history of an orogen can be tested by reconstructing catchment wide denudation histories cosmogenic radionuclides measured in sediment deposits of known age e g a sequence of fluvial terraces offer one approach for quantifying catchment wide paleo denudation rates we present a numerical model and two case studies that evaluate the utility of this technique for reconstructing different denudation histories the concentration of cosmogenic beryllium 10 10 be is calculated for a sequence of sediments produced from climate driven variations in the mean denudation rate of a drainage basin at sea level and high latitude the model accounts for the nucleonic and muonic production and decay of 10 be as sediment is eroded and stored in deposits of different ages two scenarios for different climate driven denudation histories are considered 1 a sinusoidal variation in the input denudation rate with free parameters of periodicity 23 41 100 kyr amplitude 0 1 0 5 1 0 and mean denudation rate 0 01 0 1 1 0 mm yr 1 2 a climate driven denudation history based on oscillations of a global benthic foraminiferal oxygen isotope 18 o record over the last 2 myr results from both scenarios suggest cosmogenic radionuclides measured in a system with high denudation rates 0 5 mm yr 1 longer periodicity 100 kyr and low amplitude have the highest potential for reconstructing climate driven paleo denudation rates unfortunately the high denudation rates preferred in the theoretical consideration are not suitable for paleo denudation rate determination due to overprinting of nuclides accumulated during denudation by post depositional decay and irradiation hence a compromise between the capability to determine changes in paleo denudation rates and the determination of true denudation rates has to be accepted application of the model to previous data sets from northwest europe and southwest north america produce a good agreement but highlight the need for future work to consider how geomorphic processes e g landsliding respond to climate change and influence catchment wide denudation rates in addition other climate proxies than the benthic foraminiferal 18 o record should be used for specific sample locations

late neogene and quaternary landscape evolution of the northern california coast ranges evidence for mendocino triple junction tectonics a landscape records the surface response to tectonics at time scales intermediate between short time scale information derived from seismic imaging and global positioning systems and the long term geologic record we link late neogene and quaternary deposits and landforms in the northern california coast ranges to the tectonics of the mendocino triple junction in the northern california coast ranges the mendocino crustal conveyor geodynamic model describes crustal thickening thinning and dynamic topography that produce a double humped pattern of uplift that migrates northward with the mendocino triple junction the tectonics are manifest in the drainage system and elevation pattern of the coast ranges at long wavelengths the elevation pattern closely matches the predicted double peaked shape of mendocino crustal conveyor topography and the high points of uplift control the location of drainage divides presently the divide between the russian and eel rivers and the divide between the eel and van duzen rivers approximately correspond to the peaks of uplift predicted by the mendocino crustal conveyor model as the triple junction migrates northward the double humped pattern of uplift and subsidence migrates and the coast ranges emerge smaller drainages develop and evolve by stream capture and flow reversal and the two main divides migrate in concert with the triple junction in contrast to the systematic development of the small streams the largest trunk streams can maintain grade through regions of high uplift and coastal river mouths remain stationary despite the uplift moving north before ca 2 ma the majority of the coast range drainage flowed to a southern coastal outlet near the present mouth of the russian river at 2 ma facilitated by headwater stream capture at key locations the drainage direction reversed and the majority of coast range rivers now drain into the north flowing eel river the major drainage reorganization at 2 ma highlights the potential for complexity in geomorphic response to tectonics

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

stratigraphic control of landscape response to base level fall young womans creek pennsylvania usa abstract landscapes are thought to respond to changes in relative base level through the upstream propagation of a boundary that delineates relict from adjusting topography however spatially variable rock strength can influence the topographic expression of such transient landscapes especially in layered rocks where strength variations can mask topographic signals expected due to changes in climate or tectonics here we analyze the landscape response to base level fall in young womans creek a 220 km2 catchment on the appalachian plateau usa underlain by gently folded paleozoic sedimentary rocks we measured in situ 10be concentrations in stream sands from 17 nested watersheds and used a spatially distributed model of sediment and 10be production to constrain a threefold increase in the rate of base level fall propagating upstream from the catchment outlet using lidar topography and a nearby detailed stratigraphic section we map the extent of continuous blocky resistant sandstone strata that act as a caprock overlying more easily erodible sandstones and siltstones the caprock influences landscape response in two ways first it serves as a boundary between slowly eroding 11 5 m myr 1 low sloping 3 5 areas of relict topography and lower steeper portions of the landscape adjusting to base level fall second hillslopes supported by the overlying caprock are armored with coarse sediment and are significantly steeper 20 30 than hillslopes where the caprock has been eroded 10 despite having similar erosion rates 36 m myr 1 and bedrock substrate our results illustrate how gently dipping layered rocks engender complicated relationships between lithology topography and erosion rate highlighting the importance of understanding how rock material properties influence surface processes and landscape evolution

the slave province a distinct archean craton the slave province is a relatively small late archean craton that exhibits distinctive rock associations and structures by comparison with the much larger superior province differences are evident in i the abundance of sedimentary versus volcanic rocks and of felsic versus mafic volcanic rocks ii the greater evidence for sialic basement iii the higher proportion of more evolved potassium rich granite iv the type setting and timing of gold and base metal mineralization and v a regional zonation of gold deposits that seems to be a unique feature of the slave province contrasts in structure are also significant the large scale linear belts and boundaries sutures that characterize the superior province have no obvious counterpart in the slave province despite some similarities with other archean cratons the distinct features of the slave province are important for they imply that no single archean craton should be used to develop a universal paradigm for the genesis and tectonic evol

knickpoint initiation and distribution within fluvial networks 236 waterfalls in the waipaoa river north island new zealand if knickpoints transmit signals of base level fall in river networks then improvements in our understanding of their retreat rate and basin wide distribution helps constrain the transient response following perturbation many studies of knickpoint retreat focus on the response of trunk streams to base level fall here we examine the response of an entire fluvial network as recorded by 236 active knickpoints distributed within the waipaoa river on the north island of new zealand base level fall within the waipaoa catchment initiated 18 000 years ago in response to a climatically triggered and tectonically exacerbated pulse of incision using observations from field work aerial photo analysis and a digital elevation model dem we study the knickpoint positions within the network we find that 70 of the knickpoints are located at drainage areas between 1 10 5 m 2 and 1 10 6 m 2 and more than half are b1 km upstream of a large change in drainage area for the knickpoints b1 km upstream of large tributary junctions we find that the retreat distances were well correlated with the tributaries drainage areas in order to determine how a pulse of incision distributes itself throughout a fluvial network we develop two simple end member models and compare their behavior to the observed knickpoint distribution in the waipaoa in the first model we propose that a knickpoint initiated at the basin outlet retreats upstream and distributes the signal throughout the network at a rate that is a power law function of drainage area in the second model we propose that knickpoints form near a threshold drainage area below which channels cannot incise with the same efficiency as possible in downstream reaches though neither model addresses the along stream variability in substrate or knickpoint form the misfit between the modeled and the observed knickpoints along stream positions are surprisingly low b1 km 1 25 of the total stream length for knickpoints with drainage areas b1 10 6 m 2 large misfits up to 3 5 km are observed for knickpoints with present day drainage areas greater than 1 10 6 m 2 the large single step in channel elevation that characterizes knickpoints presently observed in tributaries of the waipaoa river may not characterize the base level fall signal that propagated through the trunk streams evidence for progressive rather than instantaneous incision in the trunk streams the knickpoints vicinities to tributary junctions and the equivalent success of the two end member models lead us to conclude that the present positions of the 236 observed knickpoints are largely a consequence of thresholds in channel incision at low drainage areas

relict landscape resistance to dissection by upstream migrating knickpoints expanses of subdued topographies are common at high elevation in mountain ranges they are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography however relict fragments could also possess intrinsic characteristics that make them resilient to dissection we document here the propagation of a wave of dissection across an uplifted relict landscape in puerto rico using 10be 26al burial dating on cave sediments we show that uplift started 4 ma and that river knickpoints have since migrated very slowly across the landscape modern detrital 10be erosion rates are consistent with these long term rates of knickpoint retreat analysis of knickpoint distribution combined with visual observations along the streambeds indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat the studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks earlier studies have shown that vegetation over the relict topography efficiently limits erosion allowing for the formation of a thick saprolite underneath such slow erosion reduces streambed load fluxes delivered to the knickpoints as well as bed load grain size both processes limit abrasion compounding the effect of slow abrasion wide joint spacing in the bedrock makes plucking infrequent thus the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream reducing the celerity at which the relict subdued landscape is dissected we conclude that similar top down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies

a new look at the stratigraphy of the yellowknife supergroup at yellowknife n w t implications for the age of gold bearing shear zones and archean basin evolution based on recent detailed mapping a revised stratigraphic column is proposed for the rocks of the archean yellowknife supergroup in the yellowknife greenstone belt the mafic volcanic rocks of the kam formation previously thought to represent the oldest supracrustal rocks of the belt overlap remnants of an earlier volcanic sedimentary sequence here referred to as the octopus formation as its enormous thickness makes it too unwieldy to be described as a single formation the kam formation is raised to group status and subdivided into four formations it is proposed that the kam group should replace the beaulieu group in the yellowknife area the chan formation at the base of the kam group consists of multiple gabbroic intrusions that were emplaced into a carapace of pillowed flows the intrusions locally resemble sheeted mafic dyke complexes in phanerozoic ophiolites thought to represent evidence for sea floor spreading the crestaurum formation which overlies the chan formation is characterized b

geodetic constraint on the motion of a slab window implication for the mendocino crustal conveyor model the migration of the slab window in the northern california coast ranges provides a unique setting to study the viscous coupling between crust and asthenosphere flow the mechanisms of these dynamic processes are explained by the mendocino crustal conveyor model which predicts a 2 d double humped surface uplift rate pattern on a 400 km long profile to evaluate the mendocino crustal conveyor mcc model using accurate geodetic measurements we derive the vertical velocity field from 43 continuous gps cgps stations in the coast ranges region and project it onto the profile along three possible orientations of the slab window the cgps measured uplift rates are in good agreement with the mcc prediction when the slab window orientation is parallel to the symmetry axis of the region of thickened crust thus the cgps solutions not only provide a complementary means to diagnose the mcc model but also provide an effective way to constrain the orientation of the slab window

fluvial dynamics and 14c 10be disequilibrium on the bolivian altiplano determining sediment transfer times is key to understanding source to sink dynamics and the transmission of environmental signals through the fluvial system previous work on the bolivian altiplano applied the in situ cosmogenic 14c 10be chronometer to river sands and proposed sediment storage times of 10 20 kyr in four catchments southeast of lake titicaca however the fidelity of those results hinges upon isotopic steady state within sediment supplied from the source area with the aim of independently quantifying sediment storage times and testing the 14c 10be steady state assumption we dated sediment storage units within one of the previously investigated catchments using radiocarbon dating cosmogenic 10be 26al isochron burial dating and 10be 26al depth profile dating palaeosurfaces appear to preserve remnants of a former fluvial system which has undergone drainage reversal reduction in catchment area and local isostatic uplift since 2 8 ma from alluvium mantling the palaeosurfaces we gained a deposition age of 580 ka while lower down fluvial terraces yielded 34 ka and floodplains 3 1 ka owing to restricted channel connectivity with the terraces and palaeosurfaces the main source of channel sediment is via reworking of the late holocene floodplain yet modelling a set of feasible scenarios reveals that floodplain storage and burial depth are incompatible with the 14c 10be disequilibrium measured in the channel instead we propose that the 14c 10be offset results from i non uniform erosion whereby deep gullies supply hillslope derived debris and or ii holocene landscape transience associated with climate or human impact the reliability of the 14c 10be chronometer vitally depends upon careful evaluation of sources of isotopic disequilibrium in a wide range of depositional and erosional landforms in the landscape

products of 2 65 2 58 ga orogenesis in the slave province correlated with slave northern cordillera lithospheric evolution snorcle seismic reflection patterns reflection patterns along lithoprobe slave northern cordillera lithospheric evolution snorcle seismic reflection line 1 in the southwestern slave province are interpreted as products of tectonic wedging during late archean lithospheric convergence the interpretation is aided by application of new seismic imaging techniques by correlation of upper crustal reflection patterns to known geology and by a comparison of lithospheric reflection patterns to similar convergent zones elsewhere in the yellowknife area reflection patterns consist of 1 east dipping reflections at 12 14 s that project into the upper mantle 2 a wedge shaped body in the lower crust with an east dipping reflection fabric that is truncated on the west by 3 a series of west dipping reflections that outline thrust and fold structures in the upper crust the similarity of these reflection patterns to those of the proterozoic fort simpson hottah collision zone 300 km to the west provides support for the interpretation that reflection patterns beneath the slave province are also products of collisional tectonics rocks within the slave province preserve evidence of a 2 65 2 58 ga pan slave orogenic event in which the 2 9 ga central slave basement collided with the 2 7 ga juvenile eastern slave province their suture is interpreted to be a west dipping surface at 4 5 s 12 15 km beneath yellowknife and to project to the surface east of the profile in the lower crust and upper mantle east dipping reflections are interpreted to delineate a coeval subduction zone and accretionary wedge the upper crustal thrust and fold structures are likely linked to gold bearing shear zones at yellowknife these results provide tantalizing evidence that processes similar to those of modern convergent zones were operational at 2 65 2 58 ga

neodymium isotopic evidence for the tectonic assembly of late archean crust in the slave province northwest canada the ca 2 7 2 5 ga slave province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks the nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km east west transect at 65 n across the structural grain of the province initial nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust supracrustal sequences ca 2 71 2 65 ga from the central and eastern parts of the province have positive nd 1 values 0 3 to 3 6 consistent with juvenile sources and formation remote from significantly older crust syn to late deformation ca 2 63 2 60 ga mantle derived diorites and related tonalites type i from the central and eastern parts of the province have similar initial nd values 0 1 to 2 7 in contrast samples from the westernmost plutons which intrude exposed pre 3 1 ga crust have much lower nd 1 values 1 0 to4 6 suggesting contamination of these magmas by older crust the nd 1 values of post deformation granites s s type ii also vary systematically across the province values for granites west of longitude 110 30 w range from 0 2 to 5 3 those to the east range from 0 6 to 3 7 these data suggest mixed crustal sources dominated by mid to early archean material nd 2 6 to 17 at 2 6 ga for the western granitoid rocks and juvenile sources for the eastern granites the nd isotopic data are consistent with the geology of the province in that exposures of mid to early archean crustal rocks predating the principal 2 7 2 5 ga orogenic event are restricted to the western part of the province the asymmetric pattern defined by the nd isotopic data indicates the presence of distinct crustal rocks beneath the slave province similar isotopic variations observed across phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin this process may also have been an important mechanism in the cratonization of the slave province

evolution of an archean basement complex and its autochthonous cover southern slave province canada abstract the sleepy dragon complex sdc is one of several antiformal exposures of a 4 0 2 85 ga basement block underlying the west central region of the archean slave province northwestern canada this basement block the central slave basement complex csbc is overlain by an autochthonous to locally parautochthonous dominantly quartz rich clastic sequence the 2 85 2 80 ga central slave cover group together these units comprised a fundamental building block during neoarchean growth and assembly of the slave craton the well exposed patterson lake morose lake area represents a type locality for study of the csbc central slave cover group and overlying rocks of the yellowknife supergroup field mapping and u pb geochronology employing both thermal ionisation mass spectrometry tims and sensitive high resolution ion microprobe shrimp document a prolonged crustal history oldest intact basement units consist of foliated to gneissic tonalite dated in two places at 2955 12 ma and 2944 9 ma indirect evidence for a ca 3150 ma basement component is obtained both from xenocrystic zircon in tonalite and detrital zircons from the overlying central slave cover group which here consists of the patterson lake formation a granite boulder from this formation is dated at 2934 3 ma and may be locally derived a sheared mafic volcanic unit along the basement cover contact contains 2942 3 ma metamorphic titanite which provides a minimum deposition age this volcanic unit is therefore also a part of the basement complex younger events in the sdc mainly reflect tonalite trondhjemite granodiorite plutonism during construction of the overlying cameron river and beaulieu river greenstone belts of the yellowknife supergroup a unit of k feldspar megacrystic granodiorite is dated at 2726 3 ma and tonalite and granodiorite bodies located toward the centre of the sdc have primary crystallization ages of 2683 3 2 ma 2677 2 ma and 2672 7 6 ma the core of the southern sdc is occupied by a late tectonic to post tectonic granite pluton previously dated at 2586 2 ma an intravolcanic unconformity along the southern margin of the sdc developed in part on 2 7 ga granodiorite is constrained by existing u pb data to have formed sometime between ca 2685 2660 ma an abundance of synvolcanic plutons of similar age and the presence of a 3 km thick mafic volcanic sequence overlying a less dense magmatically active felsic crust suggest that gravitational instability and magmatic diapirism may have played important roles in doming and uplift of the sdc prior to regional d 1 deformation the younger toward centre zonation of plutonic rocks in the sdc is documented in other domal granite gneiss complexes of archean age e g pilbara craton western australia where it is attributed to upward and outward displacement of older crust during repeated magma channeling into a growing mid crustal dome in the west central slave craton such a process might account for the regional presence of local unconformities of apparently similar age the domal basement plutonic complexes would have been further amplified after 2660 ma during the development of regional f 1 and f 2 fold belts

a zoned low p high t complex at the level of anatexis structural and plutonic patterns in metasediments of the archean yellowknife supergroup near bathurst inlet n w t canada abstract within metagreywackes and schists of the archean yellowknife supergroup the development of an anatectic stratigraphy during the late stages of regional deformation has resulted in the formation of a complex with a metamorphicplutonic structural zoning similar to that occurring in phanerozoic low p high t terrains from bottom to top the stratigraphy consists of 1 small folded migmatite grading with decreasing partial melting to 2 sillimanite schist strongly strained by the emplacement of two mica granites 3 andalusite cordierite schist the site of large folds related to the emplacement of the complex each zone is characterized by a different structural style resulting from the interplay between metamorphic grade and deformation for instance in the sillimanite zone the structure was influenced by the extraction of syntectonic melt and its emplacement into fractures at low effective pressure this zone was also the ductile envelope into which sills and plutons of two mica granites derived from the lower migmatites migrated the time taken for evolution of the complex has been constrained by u pb zircon and monazite dating of a member of the early tonalite diorite suite and members of the two mica granite suite this shows the evolution of the complex was rapid ca 10 ma but possibly polyphase the formation of a distinctively styled complex accompanied by syn tectonic anatexis and plutonism may be typical of lower crustal levels of the yellowknife supergroup and other low p high t terrains

correlation between the geologic record and computed plate motions for central california we have attempted to correlate geologic events in part of central california with computed motions of oceanic plates relative to cratonic north america three composite terranes are considered 1 the sierra nevada 2 the coast ranges northeast of the san andreas fault and 3 the salinian block in the sierra nevada jurassic plutonism ending about 147 m y b p and cretaceous plutonism 120 80 m y b p correlate with deduced farallon north america fa na convergence as does the nevadan orogeny 158 153 m y b p however the gap in plutonism 146 121 m y b p outlasted by far an apparent minimum in convergence 145 135 m y b p magmatism ceased in the sierra and uplift slowed during the laramide orogeny 75 45 m y b p when tectonic activity had moved east to the site of the rockies presumably because of a low angle of subduction but volcanisin resumed in the sierra about 33 m y b p indicating that a steeper angle was reestablished apparently east west compression slackened when the pacific north america pa na transform regime developed basalt was erupted in the southern sierra 12 3 m y b p and during this same time the range started its most rapid rise as a normal fault bounded block the coast ranges north east of the san andreas fault are underlain by the franciscan complex the older parts of the complex were probably assembled in a subduction zone far south of their present location but we believe that the same fa na convergence that was cited for sierran plutonism was responsible despite the disparity in latitude as the farallon plate was probably in contact with most of the western margin of north and south america a perceived strong pulse in convergence 100 85 m y b p evidently produced the coast range thrust which on geologic grounds alone most likely originated between 96 and 88 m y b p some of the northwesterly transport of franciscan rocks relative to north america is ascribed to oblique kula north america ku na convergence 85 43 m y b p during which period the coast range thrust was revived about 60 m y b p the farallon plate again influenced the california margin after the kuia plate had moved northward and fa na convergence caused the accretion of the coastal belt and king range terranes of the neo franciscan the advent of pacific north america pa na interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the mendocino triple junction dickinson and snyder 1978 in the salinian block plutons 107 82 m y old can be ascribed to the same fa na convergence as the contemporary cretaceous plutons of the sierra nevada although the salinian intrusives originated much farther south paleomagnetic results champion et al 1980 from the salinian related pigeon point formation 75 71 m y in age suggest about 2500 km of poleward essentially coastwise movement this postulated large movement during part or all of the last 75 m y poses the greatest dilemma in our study the kuia is the only known plate whose computed obliquely convergent motion is appropriate for the late cretaceous paleogene part of the migration and even this motion is inadequate for the estimated transport within the most likely time window furthermore the farallon plate seems ideally suited for the laramide orogeny 70 45 m y b p both plates could not have been in contact with the same part of the margin at the same time oblique convergence of the kula plate could possibly have caused large coastwise transport and also the laramide orogeny the coast ranges northeast of the san andreas fault and the salinian block both experienced strike slip faulting and en echelon folding correlative with pa na interaction indicating that they were close together by neogene time faulting and folding have been particularly marked since the inception of spreading in the gulf of california and attachment of westernmost california to the pacific plate about 4 m y b p the rise of individual coastal ranges commencing 3 1 m y b p may possibly have been caused by slight convergence which appears in computed pa na relative motions from 5 m y b p to the present

constant denudation rates in a high alpine catchment for the last 6 kyrs terrestrial cosmogenic nuclides tcn have widely been used as proxies in determining denudation rates in catchments most studies were limited to samples from modern active streams thus little is known about the magnitude and causes of tcn variability on millennial time scales in this work we present a 6 kyrs long high resolution record of 10be concentrations n 18 which were measured in sediment cores from an alluvial fan delta at the outlet of the fedoz valley in the swiss alps this record is paired with a three year time series n 4 of 10be measured in sediment from the active stream currently feeding this fan delta the temporal trend in the 10be concentrations after correction for postdepositional production of 10be was found to be overall constant and in good agreement with the modern river 10be concentration the calculated mean catchment wide denudation rate amounts to 0 73 0 18 mm yr this fairly constant level of 10be concentrations can be caused by a constant denudation rate over time within the catchment or alternatively by a buffered signal in this contribution we suggest that the large alluvial floodplain in the fedoz valley may act as an efficient buffer on holocene time scales in which sediments with different 10be signatures are mixed therefore presumable variations in the 10be signals derived from changes in denudation under a fluctuating holocene climate are only poorly transferred to the catchment outlet and not recorded in the 10be record however despite the absence of high frequency signals we propose that the buffered and averaged 10be signal could be meaningfully and faithfully interpreted in terms of long term catchment averaged denudation rate our study suggests that alluvial buffers play an important role in regulating the 10be signal exported by some alpine settings that needs to be taken into account and further investigated this article is protected by copyright all rights reserved

neodymium and lead isotope evidence for enriched early archaean crust in north america there has been considerable debate as to whether isotopically enriched continental crust was an important geochemical reservoir in earliest archaean times fundamental questions such as the volume of the early crust its composition and the effect of its formation and or recycling on mantle geochemistry have still to be answered the most direct clues to the nature of the earliest crust and mantle are obtained from the isotope systematics of old rocks1 5 here we report neodymium and lead isotope measurements and uranium lead zircon geochronology from archaean gneisses of the slave province northwest territories canada the gneisses contain zircons with cores older than 3 842 gyr one sample of tonalitic gneiss has a nd chondritic model age of 4 1 gyr and an end 3 7 gyr of 4 8 this is the oldest reported chondritic model age for a terrestrial sample and provides evidence for strongly enriched pre 3 8 gyr crust a reservoir complementary to the depleted mantle already in existence by 3 8 gyr before present2 6

detection of transience in eroding landscapes past variations in climate and tectonics have led to spatially and temporally varying erosion rates across many landscapes in this contribution i examine methods for detecting and quantifying the nature and timing of transience in eroding landscapes at a single location cosmogenic nuclides can detect the instantaneous removal of material or acceleration of erosion rates over millennial timescales using paired nuclides detection is possible only if one of the nuclides has a significantly shorter half life than the other currently the only practical way of doing this is to use cosmogenic in situ carbon 14 14c alongside a longer lived nuclide such as beryllium 10 10be hillslope information can complement or be used in lieu of cosmogenic information in soil mantled landscapes increased erosion rates can be detected for millennia after the increase by comparing relief and ridgetop curvature this technique will work as long as the final erosion rate is greater than twice the initial rate on a landscape scale transience may be detected based upon disequilibria in channel profiles or ridgetops but transience can be sensitive to the nature of transient forcing where forcing is periodic landscapes display differing behavior if forcing is driven by changes in base level lowering rates versus changes in the efficiency of either channel or hillslope erosion e g driven by climate change oscillations in base level lowering lead to basin averaged erosion rates that reflect a long term average erosion rate despite strong spatial heterogeneity in local erosion rates this averaging is reflected in 10be concentrations in stream sediments changes in hillslope sediment transport coefficients can lead to large fluctuations in basin averaged erosion rates which again are reflected in 10be concentrations the variability of erosion rates in landscapes where both the sediment transport and channel erodibility coefficients vary is dominated by changes to the hillslope transport coefficient copyright 2016 john wiley sons ltd

sequence and chronology of the cuerpo de hombre paleoglacier iberian central system during the last glacial cycle abstract the cuerpo de hombre paleoglacier occupies the upper sector of the cuerpo de hombre river basin located on the northwest slope of the sierra de bejar mountains iberian central system at the stage of the maximum ice extent during the last glacial cycle this paleoglacier was one of the longest tongues emerging from the sierra de bejar plateau glacier the study of the morphostratigraphic succession and the geometric and genetic relations between the geomorphological indicators of this paleoglacier has revealed its evolutionary sequence during the last glacial cycle the comparison between this sequence and the one previously established by a regional evolutionary pattern shows that although they both coincide in general terms some stages substages of this pattern must be corrected or more clearly defined the absolute chronology of the different stages was obtained using terrestrial cosmogenic nuclides 10 be the maximum ice extent of cuerpo de hombre paleoglacier has been dated to 25 0 ka mis2 and concurrent with the lgm this chronology coincides with date obtained for other paleoglaciers in the iberian central system but is slightly more modern than the regional chronology estimated as most likely for the maximum ice extent in these areas subsequent to reaching the maximum extent the glacier had a retreat minimum age 20 6 ka followed by another stage of expansion or readvance after which it stabilised until the start of the deglaciation stage 17 8 ka in all previous work the deglaciation stages in the iberian central system have been described as one continuous recession process however in the cuerpo de hombre paleoglacier all the data point to stabilisations of considerable magnitude and particularly to another stage of readvance of the glacier based on its chronology minimum age 11 1 ka and its evolutionary significance this new readvance has been correlated with the older dryas stadial finally the evolutionary context marked by the indicators and its chronology confirm that the whole of the late glacial sequence identified in cuerpo de hombre can be correlated with the traditionally established in northern europe climatic periods or stadials and interstadials oldest dryas bolling older dryas allerod younger dryas the minimum ages obtained for the cold periods stadials in this sequence in the cuerpo de hombre paleoglacier are respectively 17 5 ka 13 9 ka and 11 1 ka this complete sequence of the late glacial period including the stadials older and younger dryas had not been previously identified in any of the paleoglaciers in the iberian central system

landslides threshold slopes and the survival of relict terrain in the wake of the mendocino triple junction establishing landscape response to uplift is critical for interpreting sediment fluxes hazard potential and topographic evolution we assess how landslides shape terrain in response to a wave of uplift traversing the northern california coast ranges united states in the wake of the mendocino triple junction we extracted knickpoints landslide erosion rates and topographic metrics across the region modified by mendocino triple junction migration landslide erosion rates mapped from aerial imagery are consistent with modeled uplift and exhumation while hillslope gradient is invariant across the region suggesting that landslides accommodate uplift as predicted by the threshold slope model landslides are concentrated along steepened channel reaches downstream of knickpoints generated by base level fall at channel outlets and limit slope angles and relief we find evidence that landslide derived coarse sediment delivery may suppress catchment wide channel incision and landscape denudation over the time required for the uplift wave to traverse the region we conclude that a landslide cover effect may provide a mechanism for the survival of relict terrain and orogenic relief in the northern californian coast ranges and elsewhere over millennial time scales

concentration of gold during retrograde metamorphism of archean banded iron formations slave province canada gold bearing iron formations are widely distributed within extensive metasedimentary terranes of the archean slave province situated in the northwestern canadian precambrian shield mineralized iron formations occur within thick turbidite sequences overprinted by a protracted history of deformation metamorphism and plutonism economically significant gold prospects are specifically sited at structural culminations characterized by polyphase folding based on garnet biotite geothermometry on the stable prograde metamorphic assemblage of enveloping metapelites peak metamorphic conditions are approximated to be 570 c and 4 kbar 1 kbar 100 mpa diagnostic prograde mineralogy reveals that two facies of silicate iron formation are represented at the five gold occurrences investigated 1 amphibolitic iron formation aif characterized by quartz grunerite hornblende pyrrhotite garnet f graphite ilmenite and 2 pelitic iron formation pif consisting of quartz biotite garnet ilmenite f grunerite hornblende textures reveal that grunerite crystallization preceded hornblende and garnet within aif banded pyrrhotite is in textural equilibrium with prograde metamorphic minerals retrograde hornblende garnet zoisite apatite carbonate ferroactinolite and gold bearing sulphide minerals replace the prograde mineral assemblages on the margins of quartz veins that intensify at aif fold hinges it is hypothesized that the iron formation hosted gold deposits of the slave province are a result of multistage processes gold concentrated at high background levels within pyrrhotite bearing aif was remobilized during fluid migration into brittle aif fold hinges in subsequent metamorphic and deformational events metamorphic fluid ponded in fractured aif hinge domains caused retrogressive replacement quartz veining and gold bearing sulphide precipitation during waning temperature although the mineralized hinge zones commonly display evidence of late chloritization this alteration did not further affect gold distribution the gold precipitated with destabilization of thio complexes due to sulphidation prior to lowtemperature hydrothermal activity

impact of coarse sediment supply from hillslopes to the channel in runoff dominated dryland fluvial systems sediment supply from hillslopes to channels is an important control on basin functioning and evolution however current theoretical frameworks do not adequately consider processes of runoff driven hillslope sediment supply which affect river channels spatially and temporally mountainous dryland basins exhibit an important manifestation of these processes because their debris mantled hillslopes produce coarse sediment and because rainfall is delivered as infrequent high intensity short duration rainstorms this paper combines field measurements and modeling to explore runoff driven coarse sediment supply from hillslopes to the channel and assesses a range of plausible storms on the longitudinal patterns of sediment load and its caliber over a dryland basin reach our results show that modeled sediment load and its grain size distribution are determined by the nonlinear interaction between rainfall characteristics and hillslope attributes resulting in longitudinal fluctuations in sediment supply the relative magnitude and location of which varies between storms results suggest that long hillslopes are most sensitive to rainfall and they exhibit large variations in supplied sediment load and grain size for different runoff characteristics short and steep hillslopes are less sensitive to rainfall variations as gradient effects dominate over the role of length in modulating runoff accumulation furthermore the signal of the median fraction d50 of modeled sediment supplied by the hillslope is preserved in the coarse fraction of the measured in channel grain sizes d90 finally we propose a simple index which provides new insights into the effectiveness of different rainstorms in terms of the impact of hillslope sediment supply on the channel

a two stage model for the formation of the granite greenstone terrains of the kalgoorlie norseman area western australia abstract accurate dating of granites and greenstones from the kalgoorlie norseman area shows that there is considerable overlap in time between the formation of granites and greenstones however within any given area the oldest granites are always younger than the oldest greenstones it is suggested that the long lived 100 ma thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting leading to the production of granites the time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma where this happens the basaltic magma ponds at the base of the crust supplying additional heat and accelerating the melting process eventually the granitic melt layer becomes unstable and rises within the crust areas intruded by granite become topographic highs with topographic lows forming above areas where light melt has been withdrawn from the lower crust when the topographic highs protrude above sea level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones thus the change from a basalt dominated lower greenstone succession to a sediment and felsic volcanic dominated upper whitestone sequence seen in many granite greenstone terrains results from the intrusion of granitic magma into the upper crust the crust of the kalgoorlie norseman area formed in two distinct stages separated by a time break of up to 700 ma the first stage involved the formation of primitive crust probably above subduction zones this primitive crust was characterized by low rb sr and high sm nd ratios the second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents this led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas melting reset the rb sr sm nd and u pb systems giving the impression that the second event which occurred at 2 7 ga was a major crust forming event it was in fact a crustal reworking process although some basalt was added to the crust at this time although the two stage history of the kalgoorlie norseman area is clearly seen in the lead isotopes and in zircons it is not readily discernible from the rb sr and sm nd systematics due to mantle like ratios of those elements in the primitive crust

plutonism oblique subduction and continental growth an example from the mesozoic of california major episodes of mesozoic plutonism in california correlate with periods of oblique subduction and trench parallel transport of western california along intrabatholithic faults major episodes of plutonism occurred in the late jurassic during left oblique convergence and in the mid cretaceous during right oblique convergence in contrast a conspicuous lull in plutonism but continuation of volcanism in the earliest cretaceous coincides with a time when the north america farallon convergence vector although large in magnitude was oriented perpendicular to the trench this correlation suggests that plutonism is facilitated by strike slip faulting within the batholithic belt one explanation which helps to solve the plutonic room problem is that plutons are passively emplaced at releasing bends in the strike slip faults and volume is conserved by thrusting at the trench if this correlation is generally applicable then it implies that mid crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench continental growth in such areas may occur dominantly by volcanism

temporal distribution of granitoid plutonic rocks in the archean slave province northwest canadian shield granitoid rocks in the slave province consist of 4 0 2 8 ga granitoid gneisses that predate the 2 71 2 65 ga volcanic and turbiditic rocks of the yellowknife supergroup and 2 70 2 58 ga granitoid plutons that intrude the yellowknife supergroup u pb zircon ages and nd and pb isotopic data indicate that the older granitoids are restricted to the western part of the slave province granitoid gneisses in the eastern slave province previously suspected to predate the yellowknife supergroup are similar in age to the volcanic rocks in this paper the results of a detailed geochronological study of plutonic rocks of the contwoyto lake nose lake area of the northcentral slave province are reviewed and integrated with the available age data base for plutonic rocks elsewhere in the slave province the data indicate that the timing of the later 2 7 ga plutonism is bimodal consisting of two distinct magmatic periods separated by approximately 20 ma of apparent magmatic quiescence the first period was synvolc

structural development of angular volcanic belts in the archean slave province narrow belts of metavolcanics lying between metasediments and granitoid rocks in the southwestern part of the archean slave province display a distinctive angular map pattern steep homoclinal segm

late archean structural and metamorphic history of the wind river range evidence for a long lived active margin on the archean wyoming craton the archean rocks of the wind river range in western wyoming record a late archean history of plutonism that extends for more than 250 m y the range is dominated by granitic plutons including the 2 8 ga native lake gneiss the 2 67 ga bridger batholith the 2 63 ga louis lake batholith and late 2 54 ga granites these plutons provide a means of distinguishing the complex metamorphism and deformation that affected the range in the late archean five deformation events are recorded d1 is a penetrative deformation that occurred during the earliest granulite facies metamorphism d2 is a folding event probably in amphibolite facies that deforms porphyritic dikes that cut the d1 fabrics both d1 and d2 predate the intrusion of the ca 2 8 ga native lake gneiss d3 is a folding event accompanied by upper amphibolite to granulite metamorphism that deformed the medina mountain sequence a sequence of rocks that was either deposited or thrust upon the native lake gneiss d4 is a fabric forming event associated with the mount helen structural belt mhsb it is represented by mylonites in the mhsb a penetrative fabric in the bridger batholith and folding of the d3 structures in the medina mountain sequence we consider d3 and d4 to be coeval with the emplacement of the bridger batholith and hence to date at ca 2 67 ga the latest structures d5 are fabrics associated with the folding and thrusting of the 2 65 ga south pass sequence we recognize at least four metamorphic events m1 is associated with the d1 fabrics and occurred at high t 750 c and high p 7 8 kilobars m2 650 750 c and 4 5 5 kilobars is associated with the intrusion of the bridger batholith and formation of the d3 and d4 structures the d5 structures of the south pass sequence record m3 which is low p 2 3 kilobars and low t 500 c the final metamorphism m4 is a contact metamorphism around the louis lake batholith in the south against the south pass sequence the metamorphism occurred at 3 kilobars and at temperatures 700 c in contrast in the north where the louis lake batholith is charnockitic the metamorphism occurred at 6 kilobars and 800 c this pressure gradient is probably a reflection of tilting of the wind river block during the laramide orogeny the composition of the plutons and the structural and metamorphic history of the wind river range indicate that during the late archean this area occupied the active margin of the wyoming province this tectonic environment is similar to the long lived phanerozoic margins of north america the wind river range represents the best documented active margin of archean age

neoarchaean banded iron formation within a 2620 ma turbidite dominated deep water basin slave craton nw canada abstract banded iron formations bifs are iron and silica rich chemical metasedimentary rocks that span much of the archaean and early palaeoproterozoic rock record given that bifs precipitated from ambient seawater they provide an important archive of earth s ancient marine environments within the slave craton occurs one of the largest and well preserved achaean turbidite basins in the world interstratified within the greywacke mudstone turbidites is bif that is commonly associated with ferruginous sediments and locally with a felsic tuff bed field investigations were conducted at four bif locations from west to east across the slave craton these bifs are composed of micro and mesobands of quartz and magnetite with various amounts of metamorphic grunerite ferro actinolite ferro hornblende as well as greenalite and stilpnomelane the latter two are interpreted as retrograde metamorphic phases as the bif was deposited in a deep water turbidite dominated basin s the chemical component was occasionally diluted by clastic detritus as illustrated by highly variable al 2 o 3 content and al 2 o 3 tio 2 and cr ni that are similar to the clastic sediments on a layer by layer basis the iron content in the bif ranges from as low as 4 wt and up to 75 wt fe 2 o 3 whereas the sio 2 content ranges from 20 wt to as high as 94 wt this together with a hyperbolic germanium silicon distribution suggests that continentally sourced silica comprised the background precipitation which was periodically interrupted by pulses of hydrothermally sourced iron high temperature hydrothermal fluids contributed with an average shale normalized positive eu anomaly of 1 92 which indicates input of reduced eu 2 to the ambient seawater the bif shows characteristic aqueous fluid derived rare earths yttrium fractionation pr yb 3 to ce 4 the age and trace element signature of the felsic tuff is similar to arc type volcanic rocks potentially linking the bif and turbidite basin s to the defeat suite plutons ca 2630 2620 ma the slow chemical rainout behind bif deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump generated debris flows sourced and triggered by the tectonically active arc terrain episodically delivered clastic sediment

differences in channel and hillslope geometry record a migrating uplift wave at the mendocino triple junction california usa tectonic plate motion and the resulting change in land surface elevation has been shown to have a fundamental impact on landscape morphology changes to uplift rates can drive a response in fluvial channels which then drives changes to hillslopes because hillslopes respond on different time scales than fluvial channels investigating the geometry of channels and hillslopes in concert provides novel opportunities to examine how uplift rates may have changed through time here we perform coupled topographic analysis of channel and hillslope geometry across a series of catchments at the mendocino triple junction mtj in northern california usa these catchments are characterized by an order of magnitude difference in uplift rate from north to south we find that dimensionless hillslope relief closely matches the uplift signal across the area and is positively correlated with channel steepness furthermore the range of uncertainty in hillslope relief is lower than that of channel steepness suggesting that it may be a more reliable recorder of uplift in the mtj region we find that hilltop curvature lags behind relief in its response to uplift which in turn lags behind channel response these combined metrics show the northward migration of the mtj and the corresponding uplift field from topographic data alone

ephemeral crustal thickening at a triple junction the mendocino crustal conveyor as the north american crust interacts with the migrating mendocino triple junction the crust is first significantly thickened and then equivalently thinned over a distance of a few hundred kilometers within a time frame of 5 m y or less this process of ephemeral crustal thickening is proposed to result from viscous coupling between the northward migrating gorda slab and the base of north america south of the triple junction a time dependent thermal mechanical finite element model is developed to test this hypothesis of plate boundary tectonics results of the numerical simulations show patterns of crustal deformation consistent with the mapped sequence of folding and faulting in the area the observed crustal structure and triple junction regional seismicity and localized regions of crustal extension coincident with the position of a hypothesized lower crustal melt zone

arrested development erosional equilibrium in the southern sierra nevada california maintained by feedbacks between channel incision and hillslope sediment production tributary creeks of the southern sierra nevada have pronounced knickpoints that separate the landscape into an alternating sequence of gently sloped treads and steeply sloped risers these knickpoints and the surrounding stepped topography suggest that the landscape is still responding to pleistocene changes in base level on main stem rivers we tested this hypothesis using cosmogenic nuclides and uranium isotopes measured in stream sediment from widely distributed locations catchment scale erosion rates from the cosmogenic nuclides suggest that the treads are relict surfaces that have adjusted to a previous base level nevertheless erosion rates of relict interfluves are similar to canyon incision rates implying that relief is unchanging in the lower kings and san joaquin rivers in addition our results suggest that much of the southern sierra nevada is in a state of arrested development the landscape is not fully adjusted to and moreover is not responding to changes in base level lowering in the canyons we propose that this can be explained by a paucity of coarse sediment supply which fails to provide sufficient tools for bedrock channel incision at knickpoints we hypothesize that the lack of coarse sediment in channels is driven by intense weathering of the local granitic bedrock which reduces the size of sediment supplied from hillslopes to the channels our analysis highlights a feedback in which sediment size reduction due to weathering on hillslopes and transport in channels is both a key response to and control of bedrock channel incision and landscape adjustment to base level change

two distinct shortening events during late archean orogeny in the west central slave province northwest territories canada late archean structures in the west central part of the slave province formed during two separate orogenic events evidence for early folding and thrusting in an accretionary prism is confined to a narrow belt along the east margin of an older microcontinent the anton terrane in the west part of the province structures related to this event are overprinted by regional low pressure metamorphism subsequent shortening occurred in a continental arc setting in which folding and faulting was accompanied by calc alkaline magmatism and regional low pressure metamorphism although the entire region was affected the bulk of shortening during the second orogenic event occurred east of the early fold and thrust belt the first orogenic event produced a suture zone between old continental crust to the west and juvenile rocks to the east and during the second orogenic event rocks on either sides of the suture were tectonically underplated and intruded

interpreting quaternary uplift rates at the mendocino triple junction northern california from uplifted marine terraces analysis of the altitudinal spacing of 14 flights of marine terraces indicates a spatial pattern of varying uplift rates that agrees with that determined from previously dated terraces for the past 3 81 ka and temporal changes in uplift rates from 1 m ka to 3 5 m ka that may reflect response to changes in tectonic regime during passage of the mendocino triple junction mtj a possible mechanism for regional uplift is growth of a slab window south of the mtj the region of most rapid uplift is 25 43 km south of the mtj immediately south of the northern boundary of the slab window the coastline is tilted upward to the south in the region directly above the southern edge of the subducted gorda plate at point delgada 55 km south of the present mtj where the northern edge of the slab window passes approximately 300 ka uplift rates have been 1 2 m ka for at least 330 ka more than 1 4 m y after passage of the southern edge of the subducted slab at the mendocino coast uplift rates have been less than 0 4 m ka for at least 330 ka

evidence for a wide and gently dipping main himalayan thrust in western bhutan the main himalayan thrust mht is the source of great earthquakes that have been documented along the range its geometry is a key parameter that influences accommodation of tectonic loading and earthquake magnitudes along the himalayan arc although seismic images are available for both the western and the central part of the range this geometry remains poorly constrained for the bhutanese himalayas here we address this issue using a 10be cosmogenic nuclides denudation transect across western bhutan we observe a wide low denudation rate domain between 50 km and 110 km from the front followed by a strong northward increase using a joint inversion of denudation rates gps data and holocene uplift rates we interpret this pattern as a consequence of a flat ramp transition along the mht compared to central nepal and sikkim this location of the ramp suggests a wider decollement with implications for greater seismogenic potential of the mht in western bhutan

global analysis of the stream power law parameters based on worldwide 10be denudation rates abstract the stream power law expressed as e kamsn where e is erosion rate lt 1 k is an erodibility coefficient t 1l 1 2m a is drainage area l 2 s is channel gradient l l and m and n are constants is the most widely used model for bedrock channel incision despite its simplicity and limitations the model has proved useful for topographic evolution knickpoint migration palaeotopography reconstruction and the determination of rock uplift patterns and rates however the unknown parameters k m and n are often fixed arbitrarily or are based on assumptions about the physics of the erosion processes that are not always valid which considerably limits the use and interpretation of the model in this study we compile a unique global data set of published basin averaged erosion rates that use detrital cosmogenic 10be these data n 1457 enable values for fundamental river properties to be empirically constrained often for the first time such as the concavity of the river profile m n ratio or concavity index the link between channel slope and erosion rate slope exponent n and substrate erodibility k these three parameters are calculated for 59 geographic areas using the integral method of channel profile analysis and allow for a global scale analysis in terms of climatic tectonic and environmental settings in order to compare multiple sites we also normalize n and k using a reference concavity index m n 0 5 a multiple regression analysis demonstrates that intuitive or previously demonstrated local scale trends such as the correlation between k and precipitation rates do not appear at a global scale our results suggest that the slope exponent is generally 1 meaning that the relationship between erosion rate and the channel gradient is nonlinear and thus support the hypothesis that incision is a threshold controlled process this result questions the validity of many regional interpretations of climate and or tectonics where the unity of n is routinely assumed

what does a mean mean the temporal evolution of detrital cosmogenic denudation rates in a transient landscape in equilibrium landscapes 10 be concentrations within detrital quartz grains are expected to quantitatively refl ect basin wide denudation rates in transient landscapes though detrital quartz is derived from both the incising adjusting lowland and the unadjusted relict upland the integrated 10 be concentrations still provide a denudation rate averaged across the two domains because fi eld samples can provide only a snapshot of the current upstream averaged erosion rate we employ a numerical landscape evolution model to explore how 10 be derived denudation rates vary over time and space during transient adjustment model results suggest that the longitudinal pattern of mean denudation rates is generated by the river s progressive dilution of low volume high concentration detritus from relict uplands by the integration of high volume low concentration detritus from adjusting lowlands the proportion of these materials in any detrital sample depends on what fraction of the upstream area remains unadjusted because the boundary of the adjusting part of the landscape changes over time the longitudinal trend in cosmogenic nuclide derived erosion rates changes over time these insights are then used to guide our interpretation of geomorphic and longitudinal cosmogenic nuclide data from the south fork eel river sfer in the california coast range united states the northward propagating crustal thickening and rock uplift associated with the passage of the mendocino triple junction generates a mobile wave of uplift that progressively sweeps longitudinally down the sfer the consequences of this forcing can be both replicated in the model environment and observed in the fi eld the sfer contains transient landforms including knickpoints and river terraces along mainstem and tributary channels that defi ne a clear boundary between an incised adjusting lowland and an unadjusted relict upland we report nine nested basin wide denudation rates in the mainstem of the sfer using terrestrial cosmogenic 10 be in river borne sediment we fi nd that denudation rates increase in the downstream direction from 0 2 mm yr in the upper catchment to 0 5 mm yr at the outlet using comparisons to the modeled landscape we show that this pattern of denudation rates paired with the distribution of relict topography throughout the watershed refl ect the immaturity of the landscape s transient adjustment later in this modeled transient the predicted erosion rates decrease downstream before they become uniform this interpretation of our data has potentially far reaching implications for quantifying the uplift history and response time of transient landscapes using cosmogenic nuclides

u pb zircon ages from athapuscow aulacogen east arm of great slave lake n w t canada athapuscow aulacogen is an early proterozoic intracratonic basin located in the east arm of great slave lake between the slave and northwest churchill provinces athapuscow aulacogen comprises three stratigraphic sequences the wilson island group the great slave supergroup and the et then group new u pb zircon ages provide constraints on the development of the aulacogen the blachford lake intrusive suite consists of an older alkaline phase hearne channel granite dated at 2175 7 ma and a younger peralkaline phase thor lake syenite dated at 2094 10 ma confirming the suggestion that the two phases may not be related a felsite from the wilson island group has an age of 1928 11 ma the wilson island group is intruded by epizonal granites butte island intrusive suite one of which has an age of 1895 8 ma the wilson island group and the butte island instrusive suite are entirely allochthonous with respect to the slave craton rocks of the great slave supergroup overlie mylonitized wilson isl

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

geochemistry and origin of the regan intrusive suite and other granitoids in the northeastern slave province northwest canadian shield the regan intrusive suite of about 100 plutons of tonalite granodiorite and quartz diorite intruded the yellowknife supergroup and migmatite terrain in the northwest slave structural province 2 59 ga ago rare earth element ree trace element and major element analyses from 39 representative whole rocks from the suite suggest it was derived by batch melting of the crust producing a parental magma of tonalitic or granodioritic composition by analysing ree from different parts of a zoned pluton it was concluded that ree distribution was controlled by early separation of quartz diorite from the parent magma by flow differentiation and that the bulk of the ree were contained in early cumulate accessory apatite and monazite the residual magma was further fractionated in pipelike magma chambers during ascent into more leucocratic rocks chondrite normalized ree patterns of single lithology plutons are similar to lithologies in zoned plutons and it is proposed they initially segregated during ascent

on extracting sediment transport information from measurements of luminescence in river sediment accurately quantifying sediment transport rates in rivers remains an important goal for geomorphologists hydraulic engineers and environmental scientists however current techniques for measuring long timescale 102 106 yr transport rates are laborious and formulae to predict transport are notoriously inaccurate here we attempt to estimate sediment transport rates using luminescence a property of common sedimentary minerals that is used by the geoscience community for geochronology this method is advantageous because of the ease of measurement on ubiquitous quartz and feldspar sand we develop a model from first principles using conservation of energy and sediment mass to explain the downstream pattern of luminescence in river channel sediment we show that the model can accurately reproduce the luminescence observed in previously published field measurements from two rivers with very different sediment transport styles the model demonstrates that the downstream pattern of river sand luminescence should show exponential like decay in the headwaters which asymptotes to a constant value with further downstream distance the parameters from the model can then be used to estimate the time averaged virtual velocity characteristic transport lengthscale storage timescale and floodplain exchange rate of fine sand sized sediment in a fluvial system the sediment transport values predicted from the luminescence method show a broader range than those reported in the literature but the results are nonetheless encouraging and suggest that luminescence demonstrates potential as a sediment transport indicator however caution is warranted when applying the model as the complex nature of sediment transport can sometimes invalidate underlying simplifications

spatially averaged long term erosion rates measured from in situ produced cosmogenic nuclides in alluvial sediment spatially averaged erosion rates of small catchments can be accurately inferred from the concentrations of cosmogenic nuclides in stream sediment here we test this technique at two catchments by comparing erosion rates inferred from cosmogenic nuclides with rates of alluvial fan deposition over the past 16 000 years these two independent estimates agree within one standard error demonstrating that cosmogenic nuclide signatures of stream sediment can be used to measure spatially averaged long term erosion rates using this technique we show that long term erosion rates are an exponential function of average hillslope gradient at these sites

co 2 13 c 12 c and h 2 o variability in natural basaltic glasses a study comparing stepped heating and ftir spectroscopic techniques abstract a comparison of two independent techniques was used to assess the homogeneity of co 2 and h 2 o concentrations in a number of natural basaltic glasses variations in carbon concentration and isotopic ratio were determined by comparison of stepped heating data obtained in two different laboratories dissolved volatile concentrations were also obtained by stepped heating and fourier transform infrared ftir spectroscopy replicate stepped heating analyses of a mid ocean ridge basaltic glass show that the concentration and 13 c 12 c of bulk magmatic and dissolved co 2 vary by less than 10 and 0 5 respectively a similar degree of correlation is observed for replicate stepped heating analyses of mariana trough glasses conducted in two different laboratories dissolved co 2 concentrations determined by stepped heating also correlate well with concentrations measured by ftir spectroscopy the correspondence of results obtained in these experiments provide an upper limit to the degree of natural variation in concentrations and isotopic ratios of these volatiles in basaltic glasses and suggest that intrinsic magmatic carbon has a relatively homogeneous distribution in these glasses water concentrations determined through extraction by heating and ftir also show excellent agreement

crust mantle coupling revisited the archean slave craton nwt canada abstract lithospheric domains in the slave province inferred from regional variations in chemistry of mantle derived garnets are parallel with northeasterly structural trends in neoarchean surface rocks and were used previously to infer late neoarchean crust mantle coupling and argue against the presence of diamondiferous mesoarchean lithosphere under the western slave craton prior to its late neoarchean orogenic climax a review of more recent evidence does not support this view but concludes that a mesoarchean lithospheric root was coupled to mesoarchean crust of the central slave superterrane csst prior to accretion of adjacent juvenile neoarchean terranes a significant part of this root survived the neoarchean break up of the csst neoarchean terrane accretion and a complex sequence of post accretion deformation granitoid plutonism and metamorphism it was least modified in the ultra depleted harzburgitic layer udl under the central part of the contwoyto terrane where it is trapped above the deepest part of an underplated paleoproterozoic slab which stabilized the remnants of the archean root and caused downward younging of the lithosphere upper mantle domains recognized on the basis of the distribution of peridotitic garnets are not primary neoarchean features but were either inherited from the original mesoarchean root or resulted from neoarchean and paleoproterozoic tectonic erosion and or metasomatism of this root the slave province resembles other diamondiferous cratons in that its diamond endowment consists of both mesoarchean and paleoproterozoic stones

quantification of the transient response to base level fall in a small mountain catchment sierra nevada southern spain 1 we integrated optically stimulated luminescence dating and 10be cosmogenic nuclide measurements to quantify short to medium term 102 104 years catchment dynamics and response to active tectonics in the 27 km2 rio torrente catchment sierra nevada southern spain rapid base level fall has triggered knickpoint migration up both trunk and tributary channels resulting in two distinct geomorphic zones 1 a steep lower catchment with concordant rates of hillslope erosion and channel incision over both short 100 years 5 8 mm yr 1 and medium 12 ka 5 1 mm yr 1 timescales and 2 a low relief soil mantled upland topography with uniformly low bedrock and hillslope erosion rates 0 05 0 02 mm yr 1 the uniformity of erosion in this upland surface indicates that catchment topography was previously in steady state rapid river incision below the channel knickpoints has resulted in the development of steep landslide dominated hillslopes that are essentially tracking the incising channel the magnitude of base level fall required to generate these steep hillslopes is 100 m at least 50 m of this base level fall occurred during the past 21 ka these steep hillslopes are eroding back into the low angled upland surface at a much slower rate than the channel knickpoint consequently the trunk channel knickpoint has already reached the catchment headwaters while hillslopes continue to adjust to the new base level indicating that the channel profile will regain equilibrium form long before hillslopes thus hillslopes are the limiting factor for the duration of landscape transience in this small mountain catchment

effects of a tectonically triggered wave of incision on riverine exports and soil mineralogy in the luquillo mountains of puerto rico we document the long term response of a tropical mountain stream to tectonics and show how this response exerts a first order spatial control on the delivery of weatherable minerals to forest soils in its catchment these minerals in turn affect nutrient availability and biogeochemical cycles the luquillo mountains is a rainforest covered isolated massif on puerto rico island instead of displaying typical concave up long profiles of most mountain streams the rivers draining the southern flank of these mountains display a systematic downstream increase in gradient using concentrations of in situ produced cosmogenic 10be in quartz from river sediments we find that the downstream steepening is due to the propagation of an erosion wave along the rivers from the coastal plain upstream toward the headwaters decreased 10be concentration along the steepened reaches knickpoint faces or knickzones results from faster denudation of the valley sides around the steepened reaches the upper portions of the steepened reaches knickpoint lips cluster in elevation around the altitude of a flat lying dissected surface traceable around the luquillo mountains inspection of similar platforms over other parts of puerto rico and in the surrounding seas suggests that the dissected surface is an uplifted shore platform formed in the early pliocene 4 my ago upstream of the knickpoints rivers possess alluvial reaches still graded to this uplifted platform they represent relict profiles of the rivers that once drained an island that was surrounded by the now uplifted platform we conclude that the knickpoints initiated when the platform started to rise above the caribbean sea the knickpoints then started propagating upstream defining the front of a slow moving wave of erosion 10be derived catchment scale denudation rates measured above and below the knickpoints indicate a 30 210 increase in denudation associated to the passage of the knickpoints mineralogical analysis of the soils show that soils upstream of the knickpoints overlie deeply depleted saprolite and only contain recalcitrant minerals whereas soils downstream of the knickpoints experience an input of fresh weatherable minerals this influx considerably increases nutrient availability porder et al 2015 in a forest where atmospheric inputs are otherwise the main source of nutrients mcdowell et al 1990 zarin and johnson 1995 pett ridge 2009

investigation of the archaean crust by single grain dating of detrital zircon a greywacke of the slave province canada u pb dating of single zircon grains from a graywacke of the point lake area of the archaean slave structural province in canada indicates that this sediment is derived from two sources 1 the ancient shield parts of which are at least 3100 ma old which is the basement of the slave province supracrustal rocks and 2 younger rocks about 2700 ma old this implies the creation of large amounts of acidic to intermediate rocks during the 2700 ma event it also indicates that magma generation and intracontinental sedimentary basin formation and deepening are strongly related

archaean sedimentary sequences of the slave craton abstract the slave craton is composed of five archaean terranes that accreted during the neoarchean the central slave superterrane is the only tectonic block containing evidence of four major sedimentary successions including 1 a quartz arenite dominated succession deposited on an extensive tide influenced coastline along a rifted continental margin 2 a turbidite dominated succession that developed as submarine fans in a fore arc or intra arc basin 3 a second turbidite succession representing deep water deposition in a back arc basin and 4 late conglomerate dominated successions representing fluvial alluvial fan and shallow water depositional settings in strike slip basins deposition of the sedimentary rocks took place between ca 3 1 ga and 2 6 ga indicating a complex tectonic history an unconformity between the gneissic basement and the overlying quartz arenite succession marks a marine flooding surface consistent with a first order sea level change a major hiatus lies between the quartz arenite successions and arc backarc related volcanic deposits with interstratified and conformably overlying turbidite strata late conglomerate successions are exposed along north trending lineaments and overlie all sedimentary and volcanic rock units unconformably this depositional hiatus represents a fundamental change from compression and uplift to transcurrent motion during the final stages of craton stabilization between ca 2630 and 2580 ma the slave province experienced considerable pluton emplacement which disaggregated much of the once continuous stratigraphy a wheeler diagram illustrates the major tectonic events that occurred during formation of the central slave superterrane however remaining uncertainties concerning the evolution of the entire slave craton reflect the need for additional detailed field investigations

structural patterns and tectonic evolution of supracrustal domains in the archean slave province canada a new plate tectonic model accounts for lithological relations and regional structural patterns in late archean supracrustal domains of the slave province multiple dykes and pillowed mafic flows most common in the western part of the province suggest sea floor spreading the mafic volcanics lying in narrow homoclinal belts stratigraphically below more extensive turbidites are viewed as megaxenolithic remnants of oceanic crust preserved on the periphery of granitoid plutons and blocks of sialic crust closure of an oceanic basin was marked by emplacement of the granitoid plutons and coeval felsic volcanics the latter predominating over mafic volcanics in northeastern domains the felsic calc alkaline magmas may have risen from a shallow dipping subduction zone westerly verging folds westerly convex fold arcs and inclinations of later foliations particularly in lower level rocks of higher metamorphic grade are in accord with underthrusting to the east the zone of underthrusting shifted progressi

uranium lead zircon and titanite ages from the northern portion of the western gneiss region south central norway abstract the geochronologic history of the northern portion of the western gneiss region south central norway has been investigated by u pb analyses of zircon and titanite from various basement units and one supracrustal gneiss a six point four zircon and two titanite discordia line from a migmatite melt pod and its host gneiss astfjord migmatite gneiss defines upper and lower intercept ages of 1659 1 1 8 ma and 393 6 3 6 ma respectively the upper intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region a seven point three zircon and four titanite discordia line from the ingdal granite gneiss has similar upper and lower intercept ages 1652 9 1 7 ma and 396 1 4 9 ma respectively and the upper intercept age is interpreted as the time of granite crystallization the lower intercept age from both theastfjord migmatite gneiss and the ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for caledonian influence in the region titanite from these two gneiss units as well as from other tonalite granite and supracrustal gneisses throughout the western gneiss region displays a remarkably uniform pattern of discordance titanite discordance is variable from 6 to 100 and in general the degree of discordance is proportional to the caledonian metamorphic grade of nearby supracrustal schists episodic diffusional lead loss from titanite during caledonian metamorphism probably caused the discordance pattern but a combined mechanism of mixing and diffusional lead loss cannot be ruled out in any case the seventeen point titanite and zircon discordia line clearly indicates that 1 the bulk of the granitoid terrane in this portion of the western gneiss region was emplaced migmatized and cooled in a short time interval about 1657 ma ago 2 a second short lived thermal event that exceeded the blocking temperature of titanite occurred about 395 ma ago and 3 titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 ma or at any time after 395 ma

characterization of the paleoproterozoic hottah terrane wopmay orogen using multi isotopic u pb hf and o detrital zircon analyses an evaluation of linkages to northwest laurentian paleoproterozoic domains abstract the paleoproterozoic wopmay orogen is modeled as a simple arc continent collision involving the archean slave craton and an enigmatic continental arc terrane termed the hottah terrane hottah is a composite terrane that includes a juvenile 2 6 2 1 ga basement and a younger 1 93 1 90 ga continental arc the extent and crustal age structure of the terrane is poorly known which limits interpretations as to its origin and eventual role in the development of the orogen multi isotopic u pb hf o analyses of detrital zircon from rocks of the hottah terrane and its cover sequence are used to indirectly evaluate its age and crustal characteristics the data confirm that ca 2 6 2 1 ga crust is an important component of the terrane but hf isotopic data indicate mostly non juvenile compositions consistent with recycled neoarchean crust sources ages of detrital zircon are distinct from those expected of the slave craton but are similar to adjacent 2 0 1 90 ga paleoproterozoic terranes of northwestern laurentia the similarity to terranes such as the ksituan buffalo head and taltson thelon magmatic zones south and east of the wopmay orogen indicate that hottah could represent a terrane that initially evolved adjacent to the earliest assembled parts of the nuna continent between 2 0 and 1 9 ga and was rifted prior to the collision with the slave margin that produced the calderian orogeny

tectonic uplift of a middle wisconsin marine platform near the mendocino triple junction california an uplifted wave cut marine platform eroded across bedrock of the franciscan complex at point delgada northern california is overlain by 0 5 to 5 m of wave worked pea gravel which is in turn directly overlain by fluvial gravel and silt deposited as alluvial fans woody plant debris at the base of the fluvial deposits includes cones of brewer spruce picea breweriana which today are found only at higher elevations and latitudes fossil wood debris from this horizon yields a 14c date of 44 800 1 300 yr the 44 800 yr radiocarbon age for the base of the fluvial deposits establishes an approximate age for the immediately underlying marine gravels and wave cut platform we tentatively correlate this terrace with the middle wisconsin high sea level stand at 37 m dated at about 45 000 yr b p if this age is correct the tectonic uplift since middle wisconsin time has been 44 m and the average rate of uplift has been at least 1 0 m 1 000 yr this relatively high rate is probably related to interaction among the pacific gorda and north american plates at the mendocino triple junction

late archean tectono magmatic evolution of the central slave province northwest territories an early tectono stratigraphic and late plutonic assemblage are distinguished in the central part of the archean slave province contwoyto lake area on the basis of their relationship to deformation and metamorphic episodes and by geochemical characteristics the older assemblage which predates the onset of intense deformation and the thermal peak of metamorphism comprises 2 68 2 65 ga calc alkaline volcanic and plutonic rocks as well as extensive turbidites the younger assemblage comprises ca 2 61 2 58 ga plutonic units that were emplaced during and after deformation d d3 and the thermal peak of metamorphism compositions of the late plutonic units vary systematically with time of intrusion from an early calc alkaline diorite tonalite suite to late peraluminous granites the early assemblage is interpreted to represent remnants of an island arc or arcs overlain by turbidites and accreted during d1 and d2 the later assemblage is interpreted as a product of late to post accretion suprasu

the null hypothesis globally steady rates of erosion weathering fluxes and shelf sediment accumulation during late cenozoic mountain uplift and glaciation at the largest time and space scales the pace of erosion and chemical weathering is determined by tectonic uplift rates deviations from equilibrium arise from the transient response of landscape denudation to climatic and tectonic perturbations we posit that the constraint of mass balance however makes it unlikely that such disequilibrium persists at the global scale over millions of years as has been proposed for late cenozoic erosion we synthesize weathering fluxes global sedimentation rates sediment yields and tectonic motions to show a remarkable constancy in the pace of earth surface evolution over the last 10 ma and support the null hypothesis that global rates of landscape change have remained constant over this time period despite global climate change and mountain building events this work undermines the hypothesis that increased weathering due to mountain building or climate change was the primary agent for a decrease in global temperatures

integrated nd isotopic and u pb detrital zircon systematics of clastic sedimentary rocks from the slave province canada evidence for extensive crustal reworking in the early to mid archean abstract a combined u pb geochronology and nd isotopic analysis of clastic metasedimentary rocks from the archean slave province northwest territories canada is presented a series of clastic sedimentary rocks with deposition age of 3 13 ga to 2 58 ga was collected from the west central slave province these samples include conglomerates as well as finer sedimentary rocks such as greywacke arkose and quartzite although it is generally agreed that the nd model age t dm of clastic sedimentary rocks represents the average sedimentary provenance age t dm of samples studied here is generally older than the average u pb age of detrital zircons extracted from the same sample rather there seems to be a better 1 1 relationship between the t dm and u pb age of the oldest zircon from each sample a simple model relationship between u pb geochronology and t dm suggests that such correlation is a result of archean crustal evolution of the slave province being dominated by crustal reworking i e intracrustal processes rather than addition of juvenile materials from the mantle one exception is the period between 3 1 and 3 2 ga during which period a significant portion of detrital zircons have u pb ages exceeding the t dm indicating a large input of juvenile material and or decreasing level of crustal reworking an alternative explanation for the u pb vs t dm trend may be a mixing of older mafic juvenile crust containing no zircon and younger felsic juvenile crust however the general lack of extensive pre 2 8 ga mafic crust in the slave province makes this scenario unlikely the results of this study cannot constrain the change in the volume of continental crust at any given period of time since the addition of juvenile crust may always be counterbalanced by recycling of crustal material back into the mantle however a dominance of crustal reworking during the early to mid archean strongly implies that a large part of ancient crust was lost during the younger tectonomagmatic events since the majority of zircon may be lost during crustal reworking processes unless they are preserved as an inherited core absence of a large quantity of ancient detrital zircon cannot be used as evidence for the lack of large sialic crust existing on the early earth

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the