preface archaean geology of the eastern goldfields of western australia regional overview abstract the eastern goldfields region forms part of the yilgarn craton which after the superior province of canada is one of the largest fragments of archaean granite greenstone terrain exposed at the surface of the earth this craton has much in common with the more intensely studied superior province but there are also significant differences between these two cratons both cratons were assembled from numerous smaller crustal fragments in a relatively brief period during the late archaean the superior province was assembled between 2725 and 2695 ma card 1990 and the yilgarn craton was assembled between c 2670 and 2650 ma myers 1993 both cratons contain fragments of older continental crust and volcanic arcs that formed just prior to collision and cratonisation the superior province contains extensive belts of metasedimentary rocks interpreted as accretionary prisms card 1990 whereas metasedimentary rocks form only a minor component of the yilgarn craton and most appear to postdate accretion both cratons contain significant concentrations of gold and support thriving gold mining industries the most prominent difference between the cratons lies in the nature of the granitoid rocks the superior province is dominated by tonalite and granodiorite whereas the yilgarn craton largely consists of granite this applies to granitoid rocks of all ages in each craton granite studies form a major part of this special volume of precambrian research

zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

laboratory experiments simulating the geomorphic response to tectonic uplift 1 we present the results of an experimental study of topography dynamics under conditions of constant precipitation and uplift rate the experiment is designed to develop a complete drainage network by the growth and propagation of erosion instabilities in response to tectonic perturbations the quantitative analysis of topographic evolution is made possible by using telemetric lasers that perform elevation measurements at an excellent level of precision we focus our study on the effect of initial surface organization and of uplift rate on both the transient dynamics and the steady state forms of topography we show that the transient phase is strongly dependent on the initial internally drained area which is found to decrease exponentially with time the topography always reaches a steady state whose mean elevation depends linearly on uplift rate with a strictly positive value when uplift is zero steady state surfaces are characterized by a well defined slope area power law with a constant exponent of 0 12 and an amplitude that depends linearly on uplift rate with a strictly positive value when uplift is zero these results are consistent with a stream power law erosion model that includes a nonnegligible threshold for particle detachment uncertainty regarding the sediment transport length is resolved by calibrating the transient dynamics with a surface process model reappraising published results on the linear dependency between mean elevation or relief and denudation rate we suggest that an erosion threshold is worth considering for large scale systems

granite greenstone crust formation on the archaean earth a consequence of two superimposed processes abstract new u pb zircon age data reveal that volcanism in the greenstones of the eastern part of the yilgarn craton of western australia was coeval with that in the abitibi subprovince of the superior craton of canada and that komatiites in the eastern goldfields and abitibi terranes were erupted synchronously at 2705 ma however major granitoid intrusion and compression episodes in the eastern yilgarn craton post date those in the southern superior craton by at least 15 ma intrusion of late tectonic syenites movement on major shear zones metamorphism and deposition of post tectonic clastic sequences also occurred 25 40 ma later in the eastern yilgarn craton than in the southern superior craton indicating that these cratons occupied different tectonic settings between 2720 and 2620 ma these comparisons and the synchronous eruption of ultramafic magmas in the southern superior pilbara and kaapvaal cratons at 2715 ma provide evidence of global scale magmatic eruption events during the late archaean these considerations indicate that two superimposed processes global scale catastrophic convective overturn of the earth s mantle and plate tectonic processes were in operation during formation of granite greenstone crust in the late archaean

a time scale for granitoid emplacement in the archean murchison province western australia by single zircon geochronology abstract in the late archean murchison province of western australia four major granitoid suites intruded into two stratigraphically distinct supracrustal sequences the luke creek and mount farmer groups single zircon u pb ages from five granitoid samples representing three of the four suites were measured by ion microprobe the earliest of the suites the pegmatite banded gneiss intruded luke creek group greenstones at 2919 12 ma 1 the luke creek group and the pegmatite banded gneiss protolith were subsequently deformed and then overlain by mount farmer group greenstones large volumes of monzogranite then intruded greenstones throughout the province two samples from this suite gave crystallization ages of 2704 51 and 2681 6 ma this package of greenstones gneiss and monzogranite was folded and metamorphosed and then intruded by two suites of post folding granitoids two samples from the more southern of these suites gave ages of 2641 5 and 2602 14 ma two additional samples were collected from a tonalite pluton previously assigned to the northern post folding granitoid suite they gave crystallization ages of 2760 8 and 2784 22 ma 1 this tonalite is now interpreted as a subvolcanic pluton associated with the andesite and dacite volcanism which produced the nearby mount farmer group greenstones no xenocrystic zircons with ages significantly in excess of 3 0 ga were found in any of the seven samples implying that little or no mid archean or older rocks are present in the region lower concordia intercept ages and 207 pb 206 pb ages from individual analyses indicate the presence of mid to late proterozoic activity throughout the murchison province concordia intercept ages were calculated using a least squares fit of a plane in three dimensions this approach obviates both the need to assign a common lead isotopic composition and the assumption that no pre recent events have disturbed the population s u pb system a further refinement of the data assessment process involved the use of the internal scan by scan scatter for defining the precision of the individual pb pb isotopic ratios compared with the observed scatter ion counting error estimates were found to frequently underestimate the uncertainties in the 207 pb 206 pb and 204 pb 206 pb ratios

the late archean history of the wyoming province as recorded by granitic magmatism in the wind river range wyoming abstract the wyoming province a small ca 500 000 km 2 archean craton is the most southwestern of the archean provinces in north america it is composed primarily of late archean potassium rich granitic rocks in contrast to many other archean provinces rocks of tonalite trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2 8 ga field petrologic geochemical and isotopic study of the late archean granites exposed in the wind river range have allowed us to identify at least four periods of potassic calc alkalic magmatism at 2 8 2 67 2 63 and 2 55 ga granitic rocks of these ages appear to be widespread across the wyoming province the oldest calc alkalic granites of the wind river range emplaced at ca 2 8 ga appear to be derived predominantly from pre existing crust however nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses during at least two other periods of plutonism at 2 67 and 2 63 ga generation of the wind river range batholiths involved the incorporation of substantial amounts of isotopically juvenile material either from depleted mantle or young continental crust the information presented below as well as data available from elsewhere in the wyoming province is interpreted to suggest that the wyoming province unlike other archean cratons is not composed of a tectonic amalgamation of smaller exotic terranes although the wyoming province did experience crustal addition in archean time it was not by lateral accretion but by incorporation of mantle derived melts into large granitic batholiths

archean diapirism recorded by vertical sheath folds in the core of the yalgoo dome yilgarn craton abstract in archean granite greenstone terrains both vertical and horizontal tectonic processes can generate the typical dome and keel structural geometries one of the classic archean domes interpreted to result from a horizontal tectonic regime is the yalgoo dome in the yilgarn craton of western australia in this contribution we reassess the structural evolution of the yalgoo dome by detailing the structures exposed in the migmatitic core the core contains multi scale domes and basins these structures were previously interpreted as the result of superposed folding our structural mapping shows that the dome and basin patterns result from vertical sheath folds developed in a single progressive deformation event d1 this first event most likely resulted from diapiric ascent of a buoyant partially molten tonalitic crust and was overprinted by two contractional episodes d2 d3 both reflecting bulk ew shortening the structural evolution proposed here points towards a shift from an early history of vertical tectonics dominated by diapirism to a regional regime dominated by horizontal tectonics

physical processes in a coupled bay estuary coastal system whitsand bay and plymouth sound abstract whitsand bay and plymouth sound are located in the southwest of england the bay and sound are separated by the 2 3 km wide rame peninsula and connected by 10 20 m deep english channel waters results are presented from measurements of waves and currents drogue tracking surveys of salinity temperature and turbidity during stratified and unstratified conditions and bed sediment surveys 2d and 3d hydrodynamic models are used to explore the generation of tidally and wind driven residual currents flow separation and the formation of the rame eddy and the coupling between the bay and the sound tidal currents flow around the rame peninsula from the sound to the bay between approximately 3 h before to 2 h after low water and form a transport path between them that conveys lower salinity higher turbidity waters from the sound to the bay these waters are then transported into the bay as part of the bay mouth limb of the rame eddy and subsequently conveyed to the near shore east going limb and re circulated back towards rame head the simpson hunter stratification parameter indicates that much of the sound and bay are likely to stratify thermally during summer months temperature stratification in both is pronounced during summer and is largely determined by coastal deeper water stratification offshore small tidal stresses in the bay are unable to move bed sediment of the observed sizes however the bay and sound are subjected to large waves that are capable of driving a substantial bed load sediment transport measurements show relatively low levels of turbidity but these respond rapidly to and have a strong correlation with wave height

geometric considerations of the three dimensional u pb data presentation abstract the three dimensional discordia plane is geometrically derived from basic assumptions it is shown that the projections of data which are located on a plane in xyz space onto the three coordinate planes yield different informations the projection from the common lead point yields the discordia line the yz projection the lead lead isochron plot and the xz projection the 206 pb 238 u isochron plot also discussed is the possibility to extend the wetherill discordia plot into three dimensions including the initial lead

crustal evolution of archaean granitoids in the murchison province western australia abstract four suites of granitoids intruded the supracrustal greenstone sequence in the murchison province of the archaean yilgarn craton during a 300 million year period the earliest granitoid suite intruded the base of the developing greenstone sequence as a series of thin subhorizontal tabular plutons of monzogranite and granodiorite at 2 9ga this suite has been deformed and metamorphosed and is now a pegmatite banded gneiss at about 2 7 ga thick subhorizontal tabular plutons of monzogranite intruded the base of the greenstone sequence this suite which now forms much of the regions between greenstone belts was folded and recrystallized during regional deformation and metamorphism two distinct but contemporaneous suites of post folding granitoids intruded the greenstone belts at 2 6 ga largely post dating regional metamorphism one suite of post folding granitoids comprises tonalite trondhjemite granodiorite and monzogranite plutons confined mainly to the north of the province the other suite comprises quartz rich monzogranite and syenogranite plutons confined mainly to the south of the province pegmatite banded gneiss recrystallized monzogranite and the northern suite of post folding granitoids were all derived by partial melting of mafic crustal rocks most post folding granitoids from the southern suite were derived by partial melting of siliceous crustal material at least as old as basal greenstones the modes and sites of intrusion of all granitoid plutons were controlled by active tectonic processes or by structural features of the crust widespread 2 6 ga rb sr ages of pegmatite banded gneiss and recrystallized monzogranite reflect post metamorphic cooling which was contemporaneous with intrusion of post folding granitoids

using hilltop curvature to derive the spatial distribution of erosion rates 1 erosion rates dictate the morphology of landscapes and therefore quantifying them is a critical part of many geomorphic studies methods to directly measure erosion rates are expensive and time consuming whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents if hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates in this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high resolution 1 m digital elevation data published and new cosmogenic radionuclide analyses in the feather river basin california suggest that erosion rates vary by over an order of magnitude 10 to 250 mm kyr 1 hilltop curvature increases with erosion rates allowing calibration of the hillslope sediment transport coefficient which controls the relationship between gradient and sediment flux having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature additionally we show that hilltop curvature continues to increase with rising erosion rates after gradient limited hillslopes have emerged hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient providing soil production on hilltops can keep pace with erosion finally hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels

high precision 207pb206pb zircon geochronology using a small ion microprobe abstract a small geometry ion microprobe cameca ims 4f has been successfully used for the routine measurement of 207 pb 206 pb ratios in individual zircons with a spatial resolution of 20 m in spite of the lower sensitivity of the small ion microprobe resulting from operating at high mass resolution m m 4600 in order to resolve isobaric molecular interferences we can discern archean components whose ages differ by 50 ma a single measurement requires 90 min of data acquisition thus a comprehensive study of a zircon population can be accomplished in a few days three sets of precambrian zircons analyzed earlier by other techniques yielded ages which are within 2 of their true values as our first unknown we analyzed zircons from a sample of orthogneiss collected from the aravalli craton of northwestern india for this sample we obtained a minimum age of 3281 3 ma 1 which we believe closely approximates the time of crystallization of the gneiss s igneous protolith

archean gold mineralization synchronous with the final stages of cratonization yilgarn craton western australia sm nd ages of pegmatite dikes that crosscut gold bearing structures in the southern yilgarn craton western australia provide minimum age constraints of 2640 6 11 ma 2628 6 10 ma and 2620 6 36 ma for gold mineralization at the westonia and nevoria yellowdine terrane and scotia norseman terrane gold deposits respectively similarly a post gold mineralization dike at westonia has a u pb zircon age of 2637 68 ma theseconstraintsareconsistentwith andprovidefurthersupportfor suggestionsthatthe majority of gold deposits in the yilgarn craton formed during a regionally extensive gold mineralization event at ca 2630 ma i e 2630 6 10 ma in combination with recent geochronological results these data also provide further evidence that although mineralization appears to significantly postdate the majority of magmatic and metamorphic activity at higher crustal levels widespread thermal reworking of the lower middle crust involvingpartialmelting amphibolitetogranulitefaciesmetamorphism andlowercrustal granitoid intrusion occurred concurrently with gold mineralization at ca 2630 ma it is probable that the large scale hydrothermalfluidflow that produced widespread gold deposition was also part of this tectonothermal event models developed for other archean terranes whereby gold mineralization postdates formation and cratonization of host granite greenstone terranes and is therefore unrelated to these processes are not required to explain the timing of the majority of gold deposits in the yilgarn craton

an overview of the relationship between granitoid intrusions and gold mineralisation in the archaean murchison province western australia in the archaean murchison province of western australia granitoid batholiths and plutons that intruded into the ca 2 7 2 8 ga and ca 3 0 ga greenstone belts can be divided into three major suites suite i is a ca 2 69 ga monzogranite granodiorite suite which was derived from anatexis of old continental crust and occurs as syn tectonic composite batholiths over the entire province suite ii is a trondhjemite tonalite suite termed i type derived from partial melting of subducted basaltic crust which intruded as syn to late tectonic plutons into the greenstone belts in the northeastern part of the province where most of the major gold deposits are situated one of the suite ii trondhjemite plutons has a pb pb isochron age of 2641 36 ma and one of the structurally youngest tonalite plutons has a minimum pb pb isochron age of 2630 1 4 3 ma suite iii is a ca 2 65 2 62 ga a type monzogranite syenogranite suite which is most abundant in the largely unmineralised southwestern part of the province gold deposits in the province are mostly hosted in brittle ductile shear zones and were formed at a late stage in the history of metamorphism deformation and granitoid emplacement at one locality mineralisation has been dated at 2636 8 4 2 ma through a pyritetitanite pb pb isochron lead and sr isotope studies of granitoids and gold deposits indicate that although most gold deposits have initial pb isotope compositions most closely similar to those of suite ii intrusions both suite i and suite ii intrusions or their source regions could have contributed solutes to the ore fluids these preliminary data suggest that gold mineralisation in the murchison province was temporally and spatially associated with suite ii i type granitoids in the northeastern part of the province this association is consistent with the concept that archaean gold mineralisation was related to convergent style tectonic settings as generation of both suite ii i type granitoids and hydrothermal ore fluids could have been linked to the dehydration and partial fusion of subducted oceanic crust and old sialic crust or its anatectic products may also contribute solutes to the ore fluids integration of data from this study with other geological and radiogenic isotope constraints in the yilgarn block argue against direct derivation of gold ore fluids from specific i type granitoid plutons but favour a broad association with convergent tectonics and granitoid magmatism in the late archaean

climate driven processes of hillslope weathering climate controls erosion and weathering on soil mantled landscapes through diverse processes that have remained diffi cult to disentangle due to their complex interactions we quantify denudation soil and saprolite weathering and soil transport near the base and crest of the western slope of the sierra nevada to examine how large differences in climate affect these processes depth profi les of fallout radionuclides and fi eld observations show relative differences in erosion and weathering processes at these two climatically diverse sites and our data suggest fundamentally different patterns of soil production and transport mechanisms biotically driven soil transport at low elevation and surface erosion driven by overland fl ow at high elevation soil production rates from cosmogenic 10 be decrease from 31 3 to 13 6 m ma with increasing soil depth at low elevation but show uncertain depth dependence at the high elevation site our data also show a positive correlation between physical erosion and saprolite weathering at both sites highly weathered saprolites are overlain by weakly weathered and rapidly eroding soils while chemically less depleted saprolites are overlain by slowly eroding more weathered soils our data are among the fi rst to quantify the critical role of saprolite weathering in the evolution of actively eroding upland landscapes and our results provide quantitative constraints on how different climates can shape hillslopes by driving processes of erosion and weathering

incremental pluton emplacement during inclined transpression abstract multidisciplinary investigations suggest that many igneous plutons were assembled incrementally through emplacement of discrete magma batches internal pluton contacts between different units can be remarkably cryptic and or complex in nature this presents a major challenge in understanding both how repeated intrusions were emplaced relative to each other and in their relationships with regional deformation here we present the results of a multidisciplinary study from a neoarchean pluton exposed in the yilgarn craton of western australia our new structural geophysical geochemical and geochronology datasets indicate that the pluton was emplaced incrementally along an active inclined transpressional shear zone structural relationships indicate that magmatic to high temperature solid state fabrics locally postdate a mid to upper greenschist tectonic fabric in an adjacent older unit we consider these relationships as diagnostic of incremental emplacement indicating that the shear zone recorded several cycles of strain softening and hardening in relation to the syntectonic emplacement of discrete magma pulses followed by syndeformational cooling down to upper greenschist facies conditions our data also suggest that pluton growth occurred through over accretion of progressively more evolved magma pulses

origin of granite greenstone patterns yilgarn block western australia the yilgarn block of western australia consists mainly of archean granite greenstone terrain with oval areas of granitoid rocks fringed by arcuate greenstone belts in a typical part of this terrain the outcrop pattern is shown to result from large scale dome and basin fold interference structures rather than diapiric emplacement of steep sided batholiths the greenstones and underlying granitoid rocks were initially deformed together in a subhorizontal tectonic regime as a result the granitoid rocks recrystallized and a gneissosity developed subparallel to the stratigraphy of the greenstones thick subhorizontal monzogranite sheets were later emplaced into the lower part of the greenstone sequence subsequent deformation generated two successive sets of folds with vertical axial surfaces at a high angle to each other these folds formed dome and basin interference structures ranging from a few centimetres to more than 50 km in diameter

stabilization of the aravalli craton of northwestern india at 2 5 ga an ion microprobe zircon study abstract in order to establish the stabilization age of the aravalli craton of northwestern india we have used an ion microprobe to measure 207 pb 206 pb ages of zircons from six samples collected from the southern aravalli mountains our analyses gave precise results for four granitoids whose minimum crystallization ages ranged between 2562 6 and 2440 8 ma 1 a single sample of gneiss gave an age of 2502 4 ma which is unexpectedly younger than an age of 3281 ma which we previously obtained from another gneissic sample we interpret the 2502 ma age as the time of emplacement of the sample s igneous protolith suggesting that the mewar gneiss contains multiple metamorphic components of differing ages based on our data we conclude that the southern segment of the aravalli craton had broadly stabilized by 2 5 ga this stabilized landmass formed the basement on which the younger aravalli supergroup was unconformably deposited this study represents the first application of a small geometry ion microprobe for routine geochronology

fluid chemical evolution as a factor in controlling the distribution of gold at the archean golden crown lode gold deposit murchison province western australia the golden crown lode gold deposit is one of a number of northwest southeast to north south trending quartz veins hosted by a steeply dipping 500 m thick differentiated dolerite sill the great fingall sill and adjacent graphitic shale in the day dawn district of the murchison province in the archean yilgarn craton western australia quartz veins of the golden crown deposit are shear zone hosted crosscut the dominant host rock stratigraphy at a high angle and are laminated partly boudinaged and generally bifurcate at their extremities veins are dominated by dark gray or milky white quartz with subordinate calcite and ankerite and in order of abundance arsenopyrite pyrrhotite pyrite galena chalcopyrite and minor sphalerite the vein sulfides coexist with native gold and are most abundant at vein margins textural evidence indicates that precipitation of the majority of these minerals overlapped and that most phases can be considered to have been in equilibrium during vein genesis mineral chemistry fluid inclusions alteration and ore assemblages show that there are significant systematic gradients in intensive variables within the golden crown deposit these are recorded over the approximately 450 m of vertical exposure of the vein system and over up to 30 m into the wall rocks based on proximal alteration assemblages and zoning patterns zoned wall rock alteration haloes are divisible into four sequences three of which are closely related to gold mineralization specific alteration sequences occur within different parts of the vein system biotite alteration predominates in the deeper portions whereas muscovite alteration characterizes the upper and southern portions of the system higher grade gold zones within the golden crown deposit are limited to the northern end of the system in the deeper portion of the veins and expand along strike to the south in the upper portions fluid inclusions interpreted to be trapped during gold mineralization are low salinity 10 wt nacl equiv h2o co2 ch4 inclusions with variable carbonic to aqueous contents that are interpreted to be due to fluid immiscibility inferred methane concentrations in the carbonic phase vary significantly from 19 7 mole percent in the vicinity of the shale contact to 5 2 mole percent in the shallower southern end of the deposit carbon and oxygen isotope compositions of carbonates 13ccalcite 10 5 to 1 9 13cankerite 5 3 to 4 1 18ocalcite 7 0 18 3 and 18oankerite 7 7 9 2 and oxygen isotope compositions of vein quartz 18oquartz 10 2 12 5 are well within the range of known archean orogenic lode gold deposits combining the results of all geothermometers and geobarometers a temperature of 300 to 400 c and pressure of 2 5 0 5 kbars are determined with temperature variations over the exposed quartz vein of less than 40 c the as content at of arsenopyrite and au ag ratios of native gold show significant compositional variation which appears to be controlled by parameters such as a h2s aq and f o2 not temperature gradients on the basis of the variations in fluid composition determined in the reefs the two major processes responsible for high grade gold mineralization are interpreted to have been phase separation in the deeper portions of the ore forming system approximately below 400 m level and above these levels mixing between two modified ore fluids one reduced fluid that had reacted with graphitic shale and a relatively oxidized fluid that had partly equilibrated with dolerite

observations of current flow through the mouth of the chesapeake bay abstract seven transects of the narrowly constricted mouth of the chesapeake bay u s a were made using an acoustic doppler current profiler adcp and conductivity temperature depth ctd sensor during the period 16 21 september 1996 data were acquired while participating in the chesapeake bay plume outfall experiment cope i seven ctd stations per transect were conducted across the 17 km wide mouth along with continuous high resolution adcp coverage although discontinuous the transect data covered nearly all phases of a generic tidal current cycle the ratio of water level anomaly to average depth was o 10 3 during the transects with atmospheric pressure and wind accounting for most of the anomaly freshwater discharge from the bay was above normal during this period the resultant flow structure obtained from the adcp is much more complete than was previously possible with point measurements analysis in all tidal phases revealed near isothermal conditions with horizontal and vertical temperature differences across the mouth rarely exceeding 1 0 c in any tidal phase higher surface salinity to the north coupled with higher subsurface salinity in the chesapeake channel and unidirectional flow from surface to depth horizontal and vertical current gradients were found to be quite strong o 10 3 s 1 and o 10 2 s 1 respectively a consistent feature found on nearly every adcp transect was a subsurface flood current core and a surface ebb current core both on the right side of the main channel in the direction of flow

neoarchaean metamorphic evolution of the yilgarn craton a record of subduction accretion extension and lithospheric delamination abstract the greater part of the metamorphic record of the yilgarn craton formed during a protracted middle neoarchaean orogenic cycle spanning from about 2750 to 2620 ma at least six distinct metamorphic events are defined by parageneses in different tectono stratigraphic settings this review characterizes the middle neoarchaean metamorphic response of the entire craton using large databases rock descriptions from 60 000 sites in government databases and 1040 pt determinations from all sources are used to construct metamorphic map patterns for each of these metamorphic events p t paths and timing of each metamorphic event are characterized using petrology and the well constrained pt determinations from this study n 287 combined with metamorphic age determinations from all sources n 114 the spatial patterns and metamorphic conditions of each event provides critical constraints on tectonic settings and changes in tectonics over 130 million years the record of metamorphism in conjunction with stratigraphic kinematic and magmatic constraints shows that the neoarchaean craton evolved from an accretionary crustal growth phase ma and m1 to thermal reworking of the crust during massive influx of granitic intrusions m2 and lithospheric extension after the termination of subduction m3 these events were followed by lithospheric delamination resulting in a craton wide diffusion delayed thermal pulse that coincided with renewed contraction m4 m1 metamorphism at 2748 19 2727 8 and 2706 10 ma experienced high p moderate t hairpin clockwise p t paths during burial of magmatic arc margins during terrane accretion events m2 regional contact type metamorphism at 2671 6 ma tracked low p moderate t clockwise p t paths in back arc settings during voluminous felsic magmatism related to shallow subduction m3 metamorphism at 2656 5 ma experienced low p moderate t anticlockwise p t paths in post volcanic rift basins formed during lithospheric extension m4 metamorphic peaks at 2644 4 and 2629 7 ma experienced low p high t clockwise p t paths during regional scale thermal pulses resulting from lower crust and mantle lithosphere delamination at 2665 2668 ma

surface processes recorded by rocks and soils on meridiani planum mars microscopic imager observations during opportunity s first three extended missions the microscopic imager mi on the mars exploration rover opportunity has returned images of mars with higher resolution than any previous camera system allowing detailed petrographic and sedimentological studies of the rocks and soils at the meridiani planum landing site designed to simulate a geologist s hand lens the mi is mounted on opportunity s instrument arm and can resolve objects 0 1 mm across or larger this paper provides an overview of mi operations data calibration and analysis of mi data returned during the first 900 sols mars days of the opportunity landed mission analyses of opportunity mi data have helped to resolve major questions about the origin of observed textures and features these studies support eolian sediment transport rather than impact surge processes as the dominant depositional mechanism for burns formation strata mi stereo observations of a rock outcrop near the rim of erebus crater support the previous interpretation of similar sedimentary structures in eagle crater as being formed by surficial flow of liquid water well sorted spherules dominate ripple surfaces on the meridiani plains and the size of spherules between ripples decreases by about 1 mm from north to south along opportunity s traverse between endurance and erebus craters

channel initiation and the problem of landscape scale since the 1940s it has been proposed that landscape dissection into distinct valleys is limited by a threshold of channelization that sets a finite scale to the landscape this threshold is equal to the hillslope length that is just shorter than that necessary to support a channel head a field study supports this hypothesis by showing that an empirically defined topographic threshold associated with channel head locations also defines the border between essentially smooth undissected hillslopes and the valley bottoms to which they drain this finding contradicts assertions that landscapes are scale independent and suggests that landscape response to changes in climate or land use depends on the corresponding changes in the threshold of channelization

an example of reverse discordance during ion microprobe zircon dating an artifact of enhanced ion yields from a radiogenic labile pb abstract sample 74459 contains 204pb rich zircons which when analyzed by ion microprobe yield u pb ages that are exceptionally older than their 207pb 206pb ages the 64 ion microprobe measurements define an age of 2681 6 ma 1 for the zircons with individual analyses giving dilution analyses were conducted on 12 single grains previously analyzed by shrimp the idtims measurements yielded radiogenic 238u 206pb ratios between 2 0 and 8 5 whereas the 34 ion microprobe analyses on the same 12 grains gave systematically lower values between 0 6 and 2 5 the discrepancy between the shrimp and idtims data has been traced to the labile pb by plotting the 204pb vs 206pb concentrations observed by the ion microprobe an unusually low 204pb 206pb ratio of 0 02 has been defined for the labile component apparently the secondary ion emission yield of the labile pb was significantly greater than that of in situ radiogenic pb thereby invalidating shrimp s interelement calibration and leading to inaccurate u pb ages these data provide important insights into the geochemistry of labile pb in 74459 the labile pb was locally derived and was well mixed at the kilogram scale prior to becoming housed within the zircon grains high resolution tem data suggest that the labile pb is contained within amorphous microdomains which are intimately associated with crystalline zircon another feature of the labile pb is its ubiquitous presence in the zircons whereas it was not taken by the coexisting feldspar this labile component has an age of 0 ma

magmatic and anatectic history of a large archean diapir insights from the migmatitic core of the yalgoo dome yilgarn craton abstract the genesis of domes in the archean was fundamentally different than those on modern earth as crustal reworking was far more voluminous and triggered by extensive reheating of the crust dictated by major pulses of mantle magmatic activity in this study we investigate how such mantle magmatism reflected by extrusion of mantle derived mafic ultramafic greenstones sequences 2825 2760 ma in the youanmi terrane of the yilgarn craton australia caused a cascade of crustal processes between 2760 and 2740 ma to form the 40 60 km diameter yalgoo dome voluminous deposition of the greenstone sequences initiated at 2800 2760 ma and followed by voluminous felsic magmatism lasting between 2760 and 2740 ma this process melted older basalts at depth and formed the goonetarra granodiorite in turn magmas caused water fluxed melting of older tonalite and amphibolite 2970 2900 ma that are preserved in the migmatitic core of the dome thermodynamic and geobarometry results on these rocks show that anatexis occurred between 3 and 9 kbar and 700 800 c with 0 5 2 wt h2o provided by the goonetarra granodiorite boosted melt production up to 15 30 vol the growth of this voluminous magmatic body accompanied by migmatization of country rocks caused dramatic crustal weakening allowing for sagduction of the thick overlying greenstones and the consequent diapiric rise of the buoyant hot and voluminous granitoid bodies partial convective overturn collins et al 1998 thus the internal evolution and the doming process of the yalgoo dome resulted from the arrival of a major thermal i e magma induced mantle anomaly into the crust this event first gave rise to a gravitationally unstable crustal stratigraphy greenstone over older felsic crust followed by a wave of conductive heat traveling upward through the crust this heat first produced voluminous and hydrous magmas as the goonetarra granodiorite then this melt advected heat and h2o to mid crustal levels causing water fluxed anatexis of older rocks and triggering partial convective overturn including diapiric doming of the felsic middle crust in a manner quite unlike the formation of phanerozoic domes that develop due to orogenic driven processes

granitoid associated orogenic intrusion related and porphyry style metal deposits in the archean yilgarn craton western australia abstract the yilgarn craton hosts three types of archean granitoid associated metallogenic systems orogenic gold intrusion related and porphyry systems these systems may occur in the same terrane and share many broad characteristics including spatial coincidence with granitoids gangue silicate mineralogy metal associations and local structural controls features of archean orogenic gold systems spatially associated with granitoids include i a broad range of granitoid host compositions ii gold mineralization is mostly late with respect to yilgarn wide granitoid emplacement peak metamorphism and deformation iii orebodies are structurally controlled and iv gold deposition occurs over a large range of temperatures pressures and crustal depths 2 to 15 km from a co 2 rich low to moderate salinity reduced ore fluid that was derived from a metamorphic and or distal magmatic fluid source these systems do not display deposit scale vertical or lateral zonation of metals away from granitoid contacts host granitoid age composition or oxidation state are not important in controlling gold mineralization processes whereas structural setting and fluid flux are paramount archean intrusion related au mo w and mo au systems in the yilgarn craton are commonly small 2 bearing moderate to high salinity fluid during ore deposition archean porphyry cu mo au cu mo and cu au systems are spatially and temporally associated with volumetrically small pervasively altered felsic plutons and dikes surrounded by altered and mineralized supracrustal countryrock lateral zonation of metals away from porphyry centers commonly includes a decrease in cu au and ag au ratios with distance from the source pluton microthermometry studies on fluid inclusions in gold bearing veins suggest that early ore fluids were high temperature and high salinity and that mineralization occurred at 2 o 3 feo are demonstrated in phanerozoic terranes but are presently ill defined in the yilgarn craton crustal depth estimation is important in defining areas likely to have preserve shallow forming archean porphyry systems

drainage basin responses to climate change recent investigations have shown that the extent of the channel network in some drainage basins is controlled by a threshold for overland flow erosion the sensitivity of such basins to climate change is analyzed using a physically based model of drainage basin evolution the golem model simulates basin evolution under the action of weathering processes hillslope transport and fluvial bedrock erosion and sediment transport results from perturbation analyses reveal that the nature and timescale of basin response depends on the direction of change an increase in runoff intensity or a decrease in vegetation cover will lead to a rapid expansion of the channel network with the resulting increase in sediment supply initially generating aggradation along the main network followed by downcutting as the sediment supply tapers off by contrast a decrease in runoff intensity or an increase in the erosion threshold will lead to a retraction of the active channel network and a much more gradual geomorphic response cyclic changes in runoff intensity are shown to produce aggradational degradational cycles that resemble those observed in the field cyclic variations in runoff also lead to highly punctuated denudation rates with denudation concentrated during periods of increasing runoff intensity and or decreasing vegetation cover the sediment yield from threshold dominated basins may therefore exhibit significant variability in response to relatively subtle environmental changes a finding which underscores the need for caution in interpreting modern sediment yield data

late archean granitoid hosted lode gold deposits yilgarn craton western australia deposit characteristics crustal architecture and implications for ore genesis granitoid hosted lode gold deposits constitute a sub group of late archean lode gold deposits in the yilgarn craton of western australia they share a number of common characteristics including 1 a strong structural control on a variety of scales 2 in most instances the presence of mineralisation in adjacent supracrustal sequence 3 wallrock alteration and vein assemblages consistent with the addition of sio2 k2o co2 s na2o but dependent on the p t conditions of the host environment and ore fluid 4 a metal association comprising au ag as bi te w mo and low concentrations of cu pb and zn and 5 deposition from a low moderate salinity near neutral h2o co2 ch4 fluid over a temperature and pressure range from 250 to 600 c and 1 to 4 kbar which corresponds to a range of crustal environments from lower greenschist to upper amphibolite facies these characteristics suggest that the granitoid hosted lode gold deposits represent a coherent genetic group that is equivalent to the more abundant greenstone hosted lode gold deposits of the craton in combination lode gold deposits irrespective of their host rocks and host terrane formed over a range of crustal levels and developed late in the tectonic sequence of the craton granitoids which host lode gold mineralisation are older than the inferred age of mineralisation by at least 10 million years and in combination with thermodynamic and isotope studies indicate that the host granitoids are not the dominant source of the ore fluid however the host granitoids do provide a structural and to a lesser extent chemical trap for gold bearing fluids in combination trace element and isotope studies of greenstone and granitoid hosted deposits suggest that the ore fluid originated from a deep mid to lower crustal source and at least interacted with granitic rocks at some stage the ore fluid composition is quite constant over the range of crustal levels from deep to shallow implying that the extent of modification by interaction with fluid conduit rocks is variable but not substantial

the age and pb loss behaviour of zircons from the isua supracrustal belt as determined by ion microprobe ion microprobe uthpb analyses of zircons from a conglomeratic metavolcanic unit in the isua supracrustal belt of west greenland show that their magmatic age is3807 1ma and failed to detect any older zircon cores or xenocrysts low u areas within zircons at a 30 m scale lost only small amounts of radiogenic pb and only in the recent past whereas high u areas within grains lost much greater proportions of their pb both recently and as early as the archaean some areas in the zircons have concordant upb and thpb ages the ion probe results agree with earlier determinations for the isua supracrustals and clarify minor discrepancies between previous multigrain and single crystal analyses

3820 ma zircons from a tonalitic armisoq gneiss in the godthab district of southern west greenland abstract zircons from a sample of the tonalitic amitsoq gneisses near godthhb west greenland exhibit several stages of crystal growth measurements of the u th pb isotopic composition of 30 m areas within individual grains using the ion microprobe shrimp reveal that components of magmatic zircon from the original igneous protolith are at least 3822 5 ma l old slightly but significantly older than the age of zircons from a fclsic volcanic unit in the isua supracrustal belt as measured at high precision by the same technique and older than all ages previously determined by mineral and whole rock isotopic techniques for amitsoq gneisses at godthab and at isukasia overgrowths of younger zircon formed at ca 3630 ma coinciding with an episode of major pb loss from the older grains these results indicate that previous whole rock isochron ages refer to later isotopic redistribution and or that other phases of the amitsoq gneisses intruded during this later period and dominate the isotopic systematics comparison of the ion probe results with previous conventional u th pb data for the same zircon sample demonstrates the strong influence of discordant high uranium parts of zircon on the mean isotopic composition of multi grain samples

chronology of the mount magnet granite greenstone terrain yilgarn craton western australia implications for field based predictions of the relative timing of granitoid emplacement abstract in this paper we report ion probe u pb ages for zircons from thirteen granitoids and five supracrustal rocks from the mount magnet area of the archaean murchison province of western australia the oldest granitoids in the area are two porphyry dykes and three small apparrently underformed bodies that intrude the greenstones the porphyries have ages of 2715 and 2752 ma and the undeformed granitoids range in age between 2696 and 2716 ma the external monzogranites that surround the greenstone belt are larger than the internal granitoids and show variable evidence of deformation from a foliated texture to recrystallization at grain boundaries with the exception of a gneissic sample which has an age of 2710 ma they are younger than the undeformed granites with ages between 2640 and 2694 ma a late leucocratic vein in one sample is 20 ma younger than the mesocratic foliated rock it cuts the young age for the external monzogranite is unexpected and it implies that the supracrustal rocks away from the margins of the mount magnet greenstone belt and the small granitoids that intrude them were cold and rigid at the time of emplacement of the monzogranites so that they behaved as a coherent block we suggest that the stress associated with the deformation even that produced foliation gneissic textures and recrystallization in the external granitoids was dissipated at the margins of the greenstone belts so that the cores of these belts were little affected by this event the age difference of 20 ma between the melanocratic and leucocratic phases in the same rock suggest slow cooling and crystallization in the lower or middle crust followed by tectonic emplacement into the upper crust the mount magnet granitoids contain abundant xenocrystic zircons that show three pronounced peaks in their age distribution a broad peak between 2910 and 2950 ma and sharp peaks at 2800 ma and 2730 ma these ages correlate with eruption of volcanic rocks in the murchison we suggest that the thermal events that produced these periods of volcanism were also recorded in the lower crust which later melted to produce the mount magnet granitoids two felsic volcanics from near the top of the stratigraphy yield ages of 2704 and 2727 ma and a chert the warramboo galtee moore chert of the windaning formation an age of 2798 ma not 3 0 ga as had previously been thought the 2727 ma felsic volcanic unit unconformably overlies the warramboo galtee moore chert and is in turn overlain by the wattle creek basalt of the mount farmer group this suggests that the mount farmer group has an age of 2 7 ga and that 2 7 ga mafic volcanism is present in the murchison the younger external monzogranites which make up the bulk of the granitoids in the mount magnet region may have derived their heat from the mantle plume head that is believed to be responsible for basaltic and komatiitic volcanism of the norseman wiluna greenstone belt however the smaller internal granitoids are older than the plume related volcanic activity of the eastern yilgarn and require an additional heat source to have affected the northern part of the yilgarn craton

u pb ages constraining batholith emplacement contact metamorphism and the formation of gold and w mo skarns in the southern cross area yilgarn craton western australia gold mines in the archean southern cross greenstone belt central yilgarn craton have produced more than 220 metric tons t of gold the deposits are characterized by a pyroxene rich gangue and have been interpreted as synmetamorphic amphibolite facies replacement deposits and as postmetamorphic intrusion related skarns we present ion microprobe u pb zircon data constraining the age of amphibolite facies contact metamorphism in the aureole of the ghooli dome the largest granitoid batholith intruding the southern cross belt drill cores from the copperhead gold mine provided zircons from an altered quartz porphyry sill located 300 m southwest of the ghooli dome and zircons from the monzogranite gneiss at the border of the batholith the porphyry sill contains embayed igneous zircons dated by a concordant u pb age of 2912 5 ma and euhedral zircons of metamorphic crystal habitus dated by a second concordant age of 2772 5 ma the igneous zircons are slightly younger than those recovered from another sill of quartz porphyry sampled at the southern star mine which define a single concordant u pb age of 2934 7 ma the metamorphic zircons in the porphyry at copperhead record the same age as the igneous zircons 2775 10 ma in the monzogranite gneiss of the ghooli dome this pluton extends more than 40 km to the southeast of copperhead and also forms the batholith border at the corinthian and fraser s mines these gold deposits too are hosted by amphibolite facies greenstones contact metamorphosed at 2772 5 ma the corinthian skarn 2620 6 ma formed 150 m y after high grade metamorphism and consequently cannot be classified as synmetamorphic or metamorphogenic a review of field relationships and of published geochronometric data suggests that the gold skarns in the southern cross area are related in space and time to a suite of magnetite series granite pegmatite complexes these granites dated regionally at 2 66 to 2 60 ga are also associated with gold bearing w mo skarn greisen or skarn systems of up to 6 km strike length the numerous skarns exposed in the deeply eroded 14 km paleodepth central yilgarn craton are members of a recently recognized group of intrusion related gold deposits which are part of continental margin tungsten tin provinces

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

lead isotope constraints on the age and source of gold mineralization in the archean yilgarn block western australia model mineralization ages based on whole earth lead isotope growth curves are used to date sulfides mainly galena with minor fe sulfides and pb tellurides related on one hand to stratiform base metal and ni cu mineralization and on the other to predominantly lode type gold mineralization in greenstone belts of the archean yilgarn block western australia galenas related to undoubted syngenetic volcanogenic base metal mineralization and fe sulfides to komatiite associated ni cu deposits yield pb model ages that are broadly consistent with sm nd pb pb and u pb zircon greenstone belt geochronology such deposits from the murchison province give model ages of ca 3 0 b y or greater whereas those of the eastern goldfields province give model ages of ca 2 85 to 2 80 b y for the northeastern part of the province and ca 2 75 to 2 70 b y for the highly mineralized norseman wiluna belt the majority of galenas related to gold mineralization record model ages of ca 2 85 to 2 75 b y for the murchison province and ca 2 75 to 2 70 b y for the southern cross province with most deposits from the norseman wiluna belt having model ages around or below ca 2 7 b y a few examples of gold mineralization all but one economically insignificant have galenas with more complex multistage lead isotope histories with mineralization ages lying in the proterozoic lead model ages of galenas from gold deposits in the murchison province are consistently younger then those related to syngenetic mineralization those from gold deposits in the southern cross province broadly coincide with the best estimates of the age of peak metamorphism and those from the norseman wiluna belt are indistinguishable or slightly younger than the ages of syngenetic base metal and ni cu mineralization in the same belt the lead isotope compositions of most gold related galenas yield 238 u 204 pb mu values that are consistent with a previously proposed epigenetic model for economically important gold mineralization in this model lead and by inference gold was derived from basalts and komatiites by major hydrothermal systems which developed due to devolatilization of the greenstone pile during regional metamorphism from between 250 m y to less than 50 m y after eruption the small deposits that give younger mineralization ages may be related to or affected by minor thermal events which occurred later in the history of the yilgarn block and which were also responsible for resetting rock and mineral rb sr systems the contrast in isotopic histories between galenas and possibly fe sulfides from major and minor gold deposits may have application as an exploration tool significantly higher 238 u 204 pb values for sulfides from the norseman gold deposits provide support for the suggestion that crust older than 3 0 b y existed prior to formation of the greenstone belts of the yilgarn block

smnd geochronology of greenstone belts in the yilgarn block western australia abstract sm nd geochronology has been used to date 3 widely spaced metavolcanic greenstone belts in the yilgarn block the measured isochron ages are 2 78 0 03 ga initial nd 2 5 0 3 for greenstones at kanowna in the eastern goldfields province 3 05 0 10 ga initial nd 0 9 0 7 for diemals marda in the southern cross province and 2 98 0 12 ga initial nd 0 7 1 2 for the warriedar fold belt in the murchison province these data show that substantial crustal components of age 3 0 ga exist in the southern cross and murchison provinces and support published evidence that the eastern goldfields province contains no crustal material 2 8 ga these ages are all significantly younger than ages commonly observed in the western gneiss terrain and so support the suggestion of a progressive crustal age trend across the yilgarn block the measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at 2 8 ga the age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas 0 2 ga so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences leaching experiments on altered warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system probably prior to the 2 6 ga regional metamorphic event the initial nd values do not conform to previously suggested mantle depletion curves when considered with published data for greenstone belts they suggest complex mantle heterogeneity with end diverging over time to both positive and negative values

discrimination and origin of the sanukitoid series geochemical constraints from the neoarchean western karelian province finland abstract neoarchean late to post tectonic sanukitoid intrusions are found in the western part of the karelian province in finland variable sized even grained to k feldspar porphyritic intrusions form a series of diorites tonalites and granodiorites that are calc alkaline to alkali calcic magnesian and mostly peraluminous the major and trace element geochemistry of the intrusions show typical sanukitoid affinities a mantle signature high content of mg ni cr and high mg and enrichment in lile especially k ba and sr the intrusions form a distinctive series that can be distinguished from the ttg series of the karelian province by their high ba sr signature low na 2 o k 2 o ratio and uniform hree pattern the sanukitoid series can be compositionally constrained as follows sio 2 55 70 wt na 2 o k 2 o 0 5 3 mgo 1 5 9 wt mg 45 65 k 2 o 1 5 5 0 wt ba sr 1400 ppm and gd er n 2 6 the probable source was enriched subcontinental lithosphere mantle with high lile concentrations in phlogopite and or sanidine the accumulation of k ba and sr in the mantle source may have occurred as a consequence of two separate metasomatic events 1 by subduction related fluids melts and 2 by metasomatism related to asthenospheric mantle upwelling caused by slab breakoff the slab breakoff at the end stage of subduction is also a viable trigger for the sanukitoid magmatism

controls on the spacing of first order valleys 1 many landscapes are composed of ridges and valleys that are uniformly spaced even where valley locations are not controlled by bedrock structure models of long term landscape evolution have reproduced this phenomenon yet the process by which uniformly spaced valleys develop is not well understood and there is no quantitative framework for predicting valley spacing here we use a numerical landscape evolution model to investigate the development of uniform valley spacing we find that evenly spaced valleys arise from a competition between adjacent drainage basins for drainage area a proxy for water flux and that the spacing becomes more uniform as the landscape approaches a topographic equilibrium valley spacing is most sensitive to the relative rates of advective erosion processes such as stream incision and diffusion like mass transport such as soil creep and less sensitive to the magnitude of a threshold that limits the spatial extent of stream incision analysis of a large number of numerical solutions reveals that valley spacing scales with a ratio of characteristic diffusion and advection timescales that is analogous to a peclet number we use this result to derive expressions for equilibrium valley spacing and drainage basin relief as a function of the rates of advective and diffusive processes and the spatial extent of the landscape the observed scaling relationships also provide insight into the cause of transitions from rill like drainage networks to branching networks the spatial scale of first order drainage basins the contributing area at which hillslopes transition into valleys and the narrow range of width to length ratios of first order basins

early archaean zircon ages from orthogneisses and anorthosites at mount narryer western australia abstract u th pb isotopic analyses have been made with an ion microprobe of 20 30 m areas within zircons from gneisses adjacent to the mount narryer quartzite western australia in which detrital zircons up to 4200 ma old have been found the cores of the zircons yield protolith crystallization ages of 3678 6 ma 95 confidence limits for the meeberrie banded monzogranite gneiss and 3381 22 ma for the dugel syenogranite leucogneiss in agreement with observed intrusive relationships zircons from inclusions of deformed leucogabbro and meta anorthosite within the dugel gneiss formerly part of the manfred layered igneous complex are 3730 6 ma old the remnants of the manfred complex are therefore the oldest rocks currently known on the australian continent analyses of discrete post magmatic grains and of overgrowths on older grains indicate an episode of subsolidus zircon growth in the region at 3296 4 ma which is interpreted as a time of local metamorphism of the gneisses subsequently the zircons have been rounded and embayed and have lost radiogenic pb in both ancient and recent times all the zircon ages determined in this study are within the age range for detrital zircons in the mount narryer quartzite it is therefore possible that these felsic gneisses and anorthositic rocks were a local source for some of the sedimentary material however the lateral extent of this early archaean terrain and the source rocks for the rare 4100 4200 ma old zircons in the metasediments are yet to be determined

shaping post orogenic landscapes by climate and chemical weathering the spacing of hills and valleys reflects the competition between disturbance driven or diffusive transport on hillslopes and concentrative or advective transport in valleys although the underlying lithologic tectonic and climatic controls have not been untangled here we measure geochemical and geomorphic properties of catchments in kruger national park south africa where granitic lithology and erosion rates are invariant enabling us to evaluate how varying mean annual precipitation map 470 mm 550 mm and 730 mm impacts hill valley spacing or landscape dissection catchment averaged erosion rates based on 10be concentrations in river sands are low 3 6 m m y and vary minimally across the three sites our lidar derived slope area analyses reveal that hillslopes in the dry site are gentle 3 and short such that the terrain is low relief and appears highly dissected with increasing rainfall hillslopes lengthen and increase in gradient 6 8 resulting in less dissected higher relief catchments the chemical depletion fraction of hilltop regoliths increases with rainfall from 0 3 to 0 7 reflecting a climate driven increase in chemical relative to physical erosion soil catenas also vary systematically with climate as we observe relatively uniform soil properties in the dry site that contrast with leached sandy crests and upper slopes coupled with downslope clay accumulation zones in the intermediate and wet sites the geomorphic texture of this slow eroding granitic landscape appears to be set by climate driven feedbacks among chemical weathering regolith fabric differentiation hydrological routing and sediment transport that enhance the vigor of hillslope sediment transport relative to valley forming processes for wetter climates 2013 geological society of america

erosion thresholds and land surface morphology we propose a graphical technique to analyze the entirety of landforms in a catchment to define quantitatively the spatial variation in the dominance of different erosion processes high resolution digital elevation data of a 1 2 km2 hilly area where the channel network had been mapped in the field were used in the digital terrain model topog to test threshold theories for erosion the land surface was divided into 20 m2 elements whose shapes were then classified as convergent planar or divergent the entire landscape plotted on a graph of area per unit contour length against surface gradient shows each planform plotting as a separate field a simple steady state hydrologic model was used to predict zones of saturation and areas of high pore pressure to mimic the extreme hydrologic events responsible for erosive instability of the land surface the field observation that saturation overland flow is rare outside convergent zones provided a significant constraint on the hydrologic parameter in the model this model was used in threshold theories to predict areas of slope instability and areas subject to erosion by saturation overland flow both of which can contribute to channel initiation the proportion of convergent elements predicted to exceed the threshold varies greatly with relatively small changes in surface resistance demonstrating a high sensitivity to land use such as cattle grazing overall the landscape can be divided using erosion threshold lines into areas prone to channel instability due to runoff and stable areas where diffusive transport predominates

regional scale polydiapirism predating the neoarchean yilgarn orogeny abstract gneiss domes occur in a wide variety of orogenic and anorogenic domains of the continental crust and play a major role in lithosphere dynamics by allowing upwards transfer of heat and mass they commonly contain a core of granites and or migmatites overlain by a mantle of lower grade rocks evolutionary models of many gneiss domes are controversial here we use new structural and zircon isotopic data to unravel the tectono magmatic evolution of the yalgoo dome and surroundings at the margins of the neoarchean yilgarn orogen western australia the study area includes at least seven granite migmatite domes 5 70 km in average diameter and several subdomes within the larger domes all showing comparable structural features and age in each dome a concentric domal foliation is concordant with granite greenstone contacts bearing a radial outward plunging lineation the bulk of the structures in each dome reflect magmatic flow with pervasive subsolidus fabrics occurring only along the outer dome margins and reflecting dome up kinematics narrow greenstone keels pinched between domes and subdomes display vertical constriction in an area regionally dominated by flattening along the steep sides of the deeply eroded domes and along the flat laying dome roofs the regional structural pattern and the kinematics of the high strain zones are best explained by sequential emplacement of nested diapirs which delivered large volumes of granitic magma in a c 20 myr time span the dome and keel regional architecture and the internal structures of each dome resulted therefore from multiscale polydiapirism these diapirs were later weakly overprinted by a tectonic fabric that developed during the neoarchean yilgarn orogeny the contrast between the yalgoo region and the rest of the craton which was strongly reworked by this orogeny highlights the dichotomy of tectonic styles in archean terranes demonstrating that diapirism dominated in times of tectonic quiescence

diversity within a unified model for archaean gold mineralization in the yilgarn craton of western australia an overview of the late orogenic structurally controlled gold deposits abstract the archaean yilgarn craton has produced 3000 tonnes of gold mainly from structurally controlled deposits that formed during the latest stages of an orogenic event that affected the entire craton and culminated in the period 2 66 2 63 ga as a group these late orogenic deposits encompass a wide range of host rocks structural settings and structural styles and alteration types however several consistent features justify grouping the deposits into a single class of mineralization these features include timing relative to orogenesis high gold to base metal ratios an association with potassium metasomatism and carbonation of host rocks low sulfide contents and a low salinity h 2 o co 2 ch 4 ore fluid most deposits formed in domains of low mean stress during east west regional compression and preserve evidence in the form of quartz vein arrays and hydraulic breccias for high fluid pressures and rock dilation the range of ore depositional conditions implied by alteration assemblages at various deposits makes the term mesothermal an inappropriate description for this class of mineralization the term late orogenic structurally controlled gold deposits is proposed as one which more completely encompasses the class of mineralization described in this overview late orogenic structurally controlled gold deposits formed in thermal equilibrium with high grade metamorphic rocks at the margins of the greenstone belts but were retrograde with respect to lower grade host rocks in the cores of greenstone belts these relationships probably reflect diachronous metamorphism and a single gold mineralization event which was contemporaneous with the later high grade metamorphism geochronological data indicate gold deposition at ca 2 63 ga in the western part of the yilgarn craton isotopic data suggest similar depositional ages in the eastern part of the craton eastern goldfields superterrane but this timing is difficult to reconcile with field relationships that suggest broad contemporaneity between regional metamorphism and gold mineralization at ca 2 66 ga the main competing genetic models for late orogenic structurally controlled gold mineralization in the yilgarn craton propose an orthomagmatic deeply sourced or metamorphogenic ore fluid none of these models can account satisfactorily for all the features of the late orogenic structurally controlled gold deposits

experimental evidence for hillslope control of landscape scale landscape evolution theory suggests that climate sets the scale of landscape dissection by modulating the competition between diffusive processes that sculpt convex hillslopes and advective processes that carve concave valleys however the link between the relative dominance of hillslope and valley transport processes and landscape scale is difficult to demonstrate in natural landscapes due to the episodic nature of erosion here we report results from laboratory experiments combining diffusive and advective processes in an eroding landscape we demonstrate that rainsplash driven disturbances in our experiments are a robust proxy for hillslope transport such that increasing hillslope transport efficiency decreases drainage density our experimental results demonstrate how the coupling of climate driven hillslope and valley forming processes such as bioturbation and runoff dictates the scale of eroding landscapes

gopher bioturbation field evidence for non linear hillslope diffusion it has generally been assumed that diffusive sediment transport on soil mantled hillslopes is linearly dependent on hillslope gradient fieldwork was done near santa barbara california to develop a sediment transport equation for bioturbation by the pocket gopher thomomys bottae and to determine whether it supports linear diffusion the route taken by the sediment is divided into two parts a subsurface path followed by a surface path the first is the transport of soil through the burrow to the burrow opening the second is the discharge of sediment from the burrow opening onto the hillslope surface the total volumetric sediment flux as a function of hillslope gradient is found to be qs cm3 cm 1 a 1 176 dz dx 3 189 dz dx 2 68 dz dx 34 dz dx 0 4 this result does not support the use of linear diffusion for hillslopes where gopher bioturbation is the dominant mode of sediment transport a one dimensional hillslope evolution program was used to evolve hillslope profiles according to non linear and linear diffusion and to compare them to a typical hillslope the non linear case more closely resembles the actual profile with a convex cap at the divide leading into a straight midslope section copyright 2000 john wiley sons ltd

landscapes in the lab rivers dissect much of earth s surface into conspicuous networks of valleys and hillslopes viewed from an airplane these erosional networks might appear fractal a close look at a part presents a view indistinguishable from the whole 1 but meter scale topographic measurements reveal that the extent of landscape dissection by down carving streams is finite and limited by the size of smooth undissected hillslopes that separate adjacent valleys 1 2 within a drainage basin hillslopes commonly have a characteristic size see the figure yet across different landscapes this scale can vary from meters to a kilometer 1 3 what imposes this scale on a landscape and why does it change from one landscape to another on page 51 1 of this issue sweeney et al 4 demonstrate using controlled laboratory experiments that landscape scale is set by a competition between river incision that cuts valley networks and diffusive hill slope processes that fill them in their experiments highlight that the extent of valley incision is an emergent dynamic landscape characteristic that depends on a delicate balance of forces shaping the landscape 1 lookup doi 10 1126 science aab0017

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

a three dimensional upb discordia plane to evaluate samples with common lead of unknown isotopic composition abstract the concordia method introduced by g w wetherill or in a modified fashion by f tera and g j wasserburg which can only be applied to the radiogenic leead fraction of u containing minerals can be extended to the measured total pb isotope ratios by a three dimensional discordia plane the three dimensional model does not require knowledge of the isotope ratios of the common pb used for correction if the assumption of only one type of common pb with an a priori unknown isotopic composition for the investigated samples is justified the theory is derived and formulae for calculation of best fit plane parameters as well as for their errors are derived with one synthetic set of data the applicability of the method is demonstrated

transverse structure of semi diurnal currents over a cross section of the merbok estuary malaysia abstract measurements are presented of the transverse distribution of longitudinal semi diurnal currents over a 900 m wide cross section in the seaward reaches of the merbok estuary malaysia data from two experiments one at four stations extending over 31 tidal cycles and the other at nine stations extending over one tidal cycle showed significant and systematic variations in the speeds and phases of the depth averaged semi diurnal currents a depth averaged cross estuary numerical tidal model was used to interpret these data larger frictional drag coefficients led to somewhat greater damping of modelled currents in the shallower water smaller drag coefficients led to larger modelled phase variations across the estuary both modelled and measured longitudinal semi diurnal current speeds were strongly dependent on local water depth current speeds in the deeper parts of the cross section were significantly faster than those in the shallower parts the modelled and measured longitudinal semi diurnal current phases were such that the times of maximum flood and ebb and changes of flow direction occurred earlier in the shallow waters near the shore as predicted from theoretical considerations

multiple sources for archean granitoids in the yalgoo area yilgarn craton western australia geochemical and isotopic evidence abstract in the yalgoo area of the north central yilgarn craton three small granitoid bodies named the lady lydia llg yalgoo east yeg and yalgoo north granitoids yng intruded 2 8 ga mafic supracrustal rocks they form two suites i granodiorites from both the llg and yeg with strongly fractionated ree patterns high lree contents low hree contents weakly positive eu anomalies and of archean ttg affinity and ii biotite monzogranites from the yng with very high lree contents and prominent negative eu anomalies all the granitoids are calc alkaline with the granodiorites recording ages of 2763 2752 ma and the biotite monzogranites recording ages of 2636 2634 ma the ree and hfse characteristics of the granodiorites favor an archean subduction environment abundant xenocrystic zircons 3 00 2 94 ga in the granodiorites and their lu hf isotopic characteristics imply a long history of crustal recycling in the north central yilgarn the lu hf isotopes indicate that the 2 76 ga granodiorites and xenocrysts were formed from a composite source ranging in age from 3 5 to 3 0 ga increased crustal thickening due to convergence after cessation of subduction caused melting in the upper mid crustal levels with generation of parental melt at 2 64 ga to source the biotite monzogranites the latter are characterized by strongly ve ehf t 10 to 12 and largely eoarchean tdm2 hf model ages 3 85 3 6 ga significantly they provide evidence of eoarchean crustal material in this part of the yilgarn craton

experimental channel response to tectonic uplift 1 we have performed a series of experiments to investigate the relationship between channel geometry and tectonic forcing in steady state landscapes at various uplift rates the experimental setup consists of uniformly uplifted silica paste eroded by artificial rainfall in this setup erosional landscapes evolve by growth and amalgamation of incisions which organize into a drainage network whose dynamics are a function of the interaction between vertical channel cutting hillslope erosion and sediment transport high precision 0 5 mm pixel size digital elevation models were constructed using a stereogrammetric camera system from this data channel bed slope was found to be independent of discharge and position on the experimental surface and to increase linearly with uplift rate geometric parameters of the flow such as channel width or depth could not be observed directly during the experiments using laminar flow equations for the mean flow velocity these parameters were back calculated and relationships to the imposed substrate uplift rate were derived channel width cross sectional area and wetted perimeter decrease with increasing uplift rate to a limit value hydraulic radius and flow depth increase slightly and flow velocity increases approximately linearly with increasing uplift rate these results are qualitatively consistent with recent field surveys and highlight the importance of channel width and slope variations in accommodating channel response to variable rock uplift rates