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

orogen response to changes in climatic and tectonic forcing despite much progress many questions remain regarding the potential dynamic coupling between atmospheric and lithospheric processes in the long term evolution of mountain belts as a complement to recent efforts to discover the interrelationships among climate topography erosion and rock deformation under conditions of mass flux steady state we explore orogen response to changes in climate and tectonic influx we derive an analytical model that predicts a powerful climatic control on orogen evolution and distinct potentially diagnostic responses to climatic and tectonic perturbations due to isostatic compensation the near surface rock uplift rate during transients is tightly coupled to climate modulated erosional efficiency system response is approximately exponential with a characteristic response timescale that is inversely proportional to the climate and lithology modulated erosional efficiency and is largely insensitive to initial conditions tectonic influx and both the sign and magnitude of perturbations

the lower eastern layered series of rhum the lower eastern layered series lels comprises approximately units 1 5 of previous workers remapping has revealed considerable along strike lithological variation in the units of the lels it is suggested on the basis of field and geochemical evidence that two layers formerly regarded as conformable intrusive sheets of fine grained olivine gabbro may be evolved alii valite layers rather than later intrusions xenolith suites in these layers and elsewhere indicate a component derived from the roof or walls of the magma chamber cryptic variation is more extensive in the lels than in other parts of rhum olivine forsterite content varies from 85 6 to 70 and clinopyroxene mg 100 mg fe varies from 88 to 74 postcumulus effects and subsolidus re equilibration have altered the initial compositions of the mineral phases the data are consistent with a model of repeated replenishment by picritic magma although the replenishing liquids may have been slightly less magnesian than those subsequently available during the formation of the upper els

the crystallization of lava lakes thermal convection driven by the kinetic undercooling of solidification can produce compositional and textural variations in an initially uniform lava flow that is cooled predominantly from above we show that such kinetic undercooling is sufficient to drive convection at high rayleigh numbers even in shallow lava lakes we then employ a theoretical model that includes the effects of convection at high rayleigh number to make predictions of the complete evolution of the lava we show that predictions of the rate of growth of solid crust at the surface of the lake do not differ much from predictions made by ignoring effects due to convection however convective motions do shorten the time for complete solidification because when they are driven by kinetic undercooling there is internal crystallization in addition to that occurring near the cooled upper boundary of the lava this important effect which is absent from purely conductive models predicts stratification and zonation of the lava in agreement with field observations we also examine the effect of taking into account the heat transfer to the country rock below the lava lake and determine how the evolution of the lava lake varies with the viscosity of the lava

topography relief and trmm derived rainfall variations along the himalaya 1 along the southern himalayan topographic front the indian summer monsoon modulates erosive processes and rates to investigate the influence of topography and relief on rainfall generation and resultant erosion we processed satellite rainfall amounts for the last 8 years 1998 2005 from the tropical rainfall measurement mission trmm based upon a spatial resolution of 5 5 km for the himalaya we identify 1 the spatial distribution of rainfall and 2 the large scale relationships between topography relief and rainfall locations our results show two distinct rainfall maxima along strike in the himalaya the first outer rainfall peak occurs along the southern margin of the lesser himalaya within a narrow band of mean elevation 0 9 0 4 km and mean relief 1 2 0 2 km the second discontinuous inner band typically occurs along the southern flank of the greater himalaya elevation and relief both 2 1 0 3 km

the influence of climate on the tectonic evolution of mountain belts simple physical arguments analogue experiments and numerical experiments all suggest that the internal dynamics of actively deforming collisional mountain ranges are influenced by climate however obtaining definitive field evidence of a significant impact of climate on mountain building has proved challenging spatial correlations between intense precipitation or glaciation and zones of rapid rock uplift have indeed been documented in numerous mountain ranges and are consistent with model predictions more compelling evidence such as tectonic changes in response to rather than just coincident with climate change has however rarely been documented triggered by a climate driven increase in erosion rate friction dominated mountain ranges are expected to show a number of simultaneous responses a decrease in the width of the range a temporary increase in sediment yield a persistent increase in the rate of rock uplift and a reduction in the subsidence rate of surrounding basins the most convincing field evidence for such a coordinated response of a mountain range to climate change comes from the european alps and the st elias range of alaska field studies and experimental research during the past two decades have provided considerable evidence for a significant influence of climate on tectonics recent advances suggest that model predictions can guide future fieldwork aimed at substantiating this view

climatic control on rapid exhumation along the southern himalayan front abstract along the southern himalayan front shf areas with concentrated precipitation coincide with rapid exhumation as indicated by young mineral cooling ages twenty new young 80 of the annual precipitation aft cooling ages reveal a coincidence between rapid erosion and exhumation that is focused in a 50 70 km wide sector of the himalaya rather than encompassing the entire orogen assuming simplified constant exhumation rates the rocks of two age vs elevation transects were exhumed at 1 4 0 2 and 1 1 0 4 mm a with an average cooling rate of 40 50 c ma during pliocene quarternary time following other recently published hypotheses regarding the relation between tectonics and climate in the himalaya we suggest that this concentrated loss of material was accommodated by motion along a back stepping thrust to the south and a normal fault zone to the north as part of an extruding wedge climatically controlled erosional processes focus on this wedge and suggest that climatically controlled surface processes determine tectonic deformation in the internal part of the himalaya

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

fluvial landscape response time how plausible is steady state denudation whether or not steady state topography and denudation are probable states depends on the timescale of system response to tectonic and climatic perturba tions relative to the frequency of those perturbations this paper presents analytical derivations of algebraic relations for the response time of detachment limited fluvial bedrock channel systems both to tectonic and climatic perturbations detachment limited fluvial erosion is described by the stream power incision model and the derivations are limited to the applicability of that model all factors likely to influence system response time that are not adequately captured by the stream power incision model will tend to increase the response time the calculations presented thus provide minimum estimates of landscape response time and therefore over predict the prob ability of attaining and sustaining steady state topography and denudation the central range of taiwan is used as a case study to estimate response times in a landscape often argued to be in steady state model parameters are fit to modern stream profiles by assuming that the topography represents a quasi steady state form estimated response times generally range from 0 25 to 2 5 ma depending on the non linearity of the incision rule and the magnitude and type of perturbation thus it may be reasonably argued that steady state topography and denudation are likely to prevail during periods of climatic stability response time is sufficiently short compared with plate tectonic timescales however rapid climatic fluctuation in the quaternary appears to preclude the attainment of steady state conditions in modern orogens motivation

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

pulses of plagioclase laden magmas and stratigraphic evolution in the upper zone of the bushveld complex south africa cumulate rocks of the upper main zone and upper zone uumz of the bushveld complex south africa contain the world s major resources of fe ti v hosted in ti magnetite and apatite and are commonly considered as having crystallized from the last major injection of magma into the magma chamber in this study we present the petrography modal proportions whole rock major element chemistry 260 samples electron microprobe data 10 000 analyses for plagioclase olivine and pyroxene and compiled analyses of cr in magnetite 239 samples for the uumz sampled over 2 1 km of the bierkraal drill cores in the western limb of the complex the uumz section exhibits a broad normal fractionation trend upwards but a series of reversals to more primitive anorthite contents in plagioclase mg in pyroxenes and olivine cr in whole rocks and cr in magnetite separates are observed accompanied by the appearance or disappearance of various minerals anorthosite or leucogabbro layers are closely linked to these reversals the reversals in an of plagioclase are used as boundaries to divide the uumz into 18 cycles these cycles are interpreted as indications of magma chamber replenishment by plagioclase laden magmas up to 20 vol plagioclase and are also marked by spikes in cr content in addition abundant fe ti oxide bearing plagioclase rich rocks are identified in the lower half of the uumz these have crystallized from a hybrid melt produced by the mixing of a new plagioclase bearing magma batch and the resident magma further crystallization of this hybrid liquid may lead to the formation of magnetite layers in the lower part of the uumz the bushveld uumz therefore grew by multiple emplacements of crystal laden magmas coming from deep seated chambers slow cooling in a shallow chamber explains the systematic bottom up compositional evolution in the cumulate pile within individual cycles the residual melt reached silicate liquid immiscibility soon after the saturation of apatite thereafter segregation of conjugate fe rich and si rich melts and crystallization of the paired melts produces cumulates with a smooth upward decrease in fe ti oxides whereas plagioclase mode increases in each apatite bearing cycle a comparison of systematic geochemical analyses and a detailed lithological stratigraphy between the bushveld limbs demonstrates the possible connectivity between the western and eastern upper zone but indicates notable differences from the bellevue section of the northern limb

geomorphic constraints on landscape sensitivity to climate in tectonically active areas abstract the geomorphology of fluvial landscapes is known to record information about uplift rate spatial patterns of faulting and tectonic history data is far less available when addressing the sensitivity of common geomorphological metrics such as channel steepness to climatic boundary conditions we test the relationship between channel steepness and precipitation rate by measuring a large number of channels in different mountainous areas these regions exhibit a tenfold variation in precipitation rate between them 100 1000 mm y 1 but have similar uplift rates allowing the tectonic variable to be controlled by accounting for the orographic coupling of rainfall with uplifted topography we find that channel steepness is significantly suppressed by higher precipitation rates in a measurable way that conforms to simple stream power erosion laws and empirical constraints on their parameters we demonstrate this using modern and estimated glacial precipitation rates and climate emerges as an important quantifiable control on channel geometry these findings help to explain why highly variable measurements of channel steepness are reported from different locations and provide important empirical constraints on how climate shapes tectonically active landscapes

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

exhumational variability within the himalaya of northwest india in the himalaya of chamba nw india a major orographic barrier in front of the greater himalayan range extracts a high proportion of the monsoonal rainfall along its southern slopes and effectively shields the orogen interior from moisture bearing winds along a 100 km long orogen perpendicular transect 28 new apatite fission track aft and 30 new zircon u th he zhe cooling ages reveal marked variations in age distributions and long term exhumation rates between the humid frontal range and the semi arid orogen interior on the southern topographic front very young elevation invariant aft ages of 4 ma have been obtained that are concentrated in a 30 km wide zone 1 d thermal modeling suggests a plio pleistocene mean erosion rate of 0 8 1 9 mm yr 1 in contrast aft and zhe ages within the orogen interior are older 4 9 and 7 18 ma respectively are positively correlated with sample elevation and yield lower mean erosion rates 0 3 0 9 mm yr 1 protracted low exhumation rates within the orogen interior over the last 15 myr prevailed contemporaneously with overall humid conditions and an effective erosional regime within the southern himalaya this suggests that the frontal dhauladar range was sufficiently high during this time to form an orographic barrier focusing climatically enhanced erosional processes and tectonic deformation there thrusting along the two frontal range bounding thrust the main central thrust and the main boundary thrusts was initiated at least 15 ma ago and has remained localized since then the lack of evidence for localized uplift farther north indicates either a rather flat decollement with no ramp or the absence of active duplex systems beneath the interior of chamba exhumational variability within chamba is best explained as the result of continuous thrusting along a major basal decollement with a flat beneath the slowly exhuming internal compartments and a steep frontal ramp at the rapidly exhuming frontal range the pattern in chamba contrasts with what is observed elsewhere along the himalaya where exhumation is focused in a zone 150 km north of the orogenic front in the nw himalaya preserved high himalayan crystalline nappes and lesser himalayan windows alternate on a relatively small scale of 100 km these alternations are closely correlated with the pattern of exhumation although the spatial distribution of high exhumation zones varies considerably between individual himalayan sectors all of these zones are closely correlated with locally higher rock uplift rates sharp topographic discontinuities and focused orographic precipitation suggesting strong feedbacks between tectonically driven rock uplift orographically enhanced precipitation and erosional processes

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

tectonics climate and mountain topography 1 by regressing simple independent variables that describe climate and tectonic processes against measures of topography and relief of 69 mountain ranges worldwide we quantify the relative importance of these processes in shaping observed landscapes climate variables include latitude as a surrogate for mean annual temperature and insolation but most importantly for the likelihood of glaciation and mean annual precipitation to quantify tectonics we use shortening rates across each range as a measure of topography we use mean and maximum elevations and relief calculated over different length scales we show that the combination of climate negative correlation and tectonics positive correlation explain substantial fractions 25 but 50 of mean and maximum elevations of mountain ranges but that shortening rates account for smaller portions 25 of the variance in most measures of topography and relief i e with low correlations and large scatter relief is insensitive to mean annual precipitation but does depend on latitude especially for relief calculated over small 1 km length scales which we infer to reflect the importance of glacial erosion larger scale averaged over length scales of 10 km relief however correlates positively with tectonic shortening rate moreover the ratio between small scale and large scale relief as well as the relative relief the relief normalized by the mean elevation of the region varies most strongly with latitude strong positive correlation therefore the location of a mountain range on earth with its corresponding climatic conditions not just tectonic forcing appears to be a key factor in determining its shape and size in any case the combination of tectonics and climate as quantified here can account for approximately half of the variance in these measures of topography the failure of present day shortening rates to account for more than 25 of most measures of relief raises the question is active tectonics overrated in attempts to account for present day relief and exhumation rates of high terrain

geo genic arsenic contamination in the kerman cenozoic magmatic arc kerman iran implications for the source identification and regional analysis kerman cenozoic magmatic arc kcma is located in the southeastern part of the central iranian volcano plutonic copper belt arsenic contamination from geo genic source is one of the most important environmental concerns in this area the main objective of this study was to determine the role of geothermal related activities in the arsenic contamination for this purpose the old and active geological indicators of the geothermal activities were investigated through the quaternary travertine deposits and the present hydrothermal warm springs respectively results showed that arsenic is highly concentrated ranged mainly from 12 400 to 90 500 mg kg in the reddish brown deposits of the travertine rocks arsenic showed geochemical association with co cu mo sb tl se fe and mn in these samples yukonite ca7fe3 12 aso4 10 oh 2015h2o a rare ca ferric arsenate hydrous mineral was the only as bearing mineral identified in the reddish brown deposits of the travertine rocks arsenic concentration in the hydrothermal warm springs 38 c ranged from 15 900 to 30 500 g l the dominant form was as h3aso30 hydrothermal contaminated waters also were characterized by na cl type and high values of ec 11 400 s cm tds 8300 mg l b 42 700 g l li 3000 g l fe 900 g l sb 82 8 g l and si 47 000 g l and natural anomalies of cs mn mo rb se and tl the obtained hydro geochemical results are similar to those reported in the literature for worldwide hydrothermal waters although natural attenuation processes such as adsorption co precipitation or mixing dilution reduce most of the arsenic contamination from hydrothermal source but some urban and rural communities are still depending on the arsenic contaminated waters with arsenic concentrations higher than recommended values for drinking or irrigation a subject that increases the risk of arsenic related diseases in some areas of the kerman province

time scales of tectonic landscapes and their sediment routing systems abstract in regions undergoing active tectonics the coupling between the tectonic displacement field the overlying landscape and the redistribution of mass at the earth9s surface in the form of sediment routing systems is particularly marked and variable coupling between deformation and surface processes takes place at a range of scales from the whole orogen to individual extensional fault blocks or contractional anticlines at the large scale the attainment of a steady state between the overlying topography and the prevailing tectonic conditions in active contractional orogens requires an efficient erosional system with a time scale dependent on the vigour of the erosional system generally in the range 10 6 10 7 years the catchment fan systems associated with extensional fault blocks and basins of the western usa are valuable natural examples to study the coupling between tectonic deformation landscape and sediment routing systems even relatively simple coupled systems such as an extensional fault block and its associated basin margin fans have a range of time scales in response to a tectonic perturbation these response times originate from the development of uniform steady state relief during the accumulation of displacement on a normal fault c 10 6 years the upstream propagation of a bedrock knickpoint in transverse catchments following a change in tectonic uplift rate c 10 6 years or the relaxation times of the integrated catchment fan system in response to changes in climatic and tectonic boundary conditions 10 5 10 6 years the presence of extensive bedrock or alluvial piedmonts increases response times significantly the sediment efflux of a mountain catchment is a boundary condition for far field fluvial transport but the fluvial system is much more than a simple transmitter of the sediment supply signal to a neighbouring depocentre fluvial systems appear to act as buffers to incoming sediment supply signals with a diffusive time scale c 10 5 10 6 years dependent on the length of the system and the extent of its floodplains stream channels and proximal gravel fans the vocabulary for explaining landscapes would benefit from a greater recognition of the importance of the repeat time and magnitude of perturbations in relation to the response and relaxation times of the landscape and its sediment routing systems landscapes are best differentiated as buffered or reactive depending on the ratio of the response time to the repeat time of the perturbation furthermore landscapes may be regarded as steady or transient depending on the ratio of the response time to the time elapsed since the most recent change in boundary conditions the response of tectonically and climatically perturbed landscapes has profound implications for the interpretation of stratigraphic architecture

petrology of the grenville swarm of gabbro dikes canadian precambrian shield the grenville tholeiitic dikes of late proterozoic or early paleozoic age cut marbles and gneisses of the grenville province within and adjacent to the ottawa rift structure where traversed the swarm consists of about 40 large dikes up to 100 m wide representing a crustal extension of 1 km the principal minerals are olivine augite pigeonite plagioclase magnetite ilmenite k feldspar and quartz crystals of pigeonite around olivine and complex augite pigeonite composite grains suggest that the reactions olivine melt pigeonite and pigeonite melt augite have taken place conspicuous zoning and grain to grain variation in the composition of augite are consistent with fractional crystallization but crystal melt equilibrium during crystal growth is indicated by a restricted range in pyroxene paleotemperatures of 1180 1060 c rock textures subophitic combined ophitic subophitic and equigranular in dike centres are related to dike width and are determined principally by the influence of

differentiation processes in an unusual iron rich alumina poor archean ultramafic mafic igneous body ontario detailed field and petrographic observation and geochemical study of the boston creek ferropicrite bcf a layered fe rich al poor archean ultramafic igneous body has yielded important insight into the controls on differentiation processes in ferropicrite liquids presence of flow top breccia spinifex vesicles amygdules and altered glass together with chert on its top and the fact that it is in direct contact with pillowed and massive basalt flows strongly suggest an effusive origin anomalous thickness 30 m of the spinifex textured layer compared to thick and differentiated komatiitic basalt and typical tholeiitic basalt flows of similar magnesium contents results from the great abundance of ca fe and mg relative to al and high contents of volatiles in the parental liquids and supercooling induced rapid crystallization which together increased the interval of clinopyroxene crystallization relatively anomalous lithological clinopyroxenite to diorite textural pegmatitiic to fine grained and geochemical mg numbers between 45 and 20 complexity in the gabbroic layer results from the high volatile content of the parental liquids and the crystallization of fe ti oxide prior to plagioclase and consequent displacement mixing and entrapment of low density volatile enriched residual liquids variations in the development of pegmatite liquid segregation pockets and pge mineralization in the gabbroic layer along strike reflect variations in the extent of within flow displacement migration and mixing of residual liquids

shrinking and splitting of drainage basins in orogenic landscapes from the migration of the main drainage divide climate and in particular the spatial pattern of precipitation is thought to affect the topographic and tectonic evolution of mountain belts through erosion numerical model simulations of landscape erosion controlled by horizontal tectonic motion or orographic precipitation result in the asymmetric topography that characterizes most natural mountain belts and in a continuous migration of the main drainage divide the effects of such a migration have however been challenging to observe in natural settings here i document the effects of a lateral precipitation gradient on a landscape undergoing constant uplift in a laboratory modelling experiment in the experiment the drainage divide migrates towards the drier leeward side of the mountain range causing the drainage basins on the leeward side to shrink and split into smaller basins this mechanism results in a progressively increasing number of drainage basins on the leeward side of the mountain range as the divide migrates such that the expected relationship between the spacing of drainage basins and the location of the main drainage divide is maintained i propose that this mechanism could clarify the drainage divide migration and topographic asymmetry found in active orogenic mountain ranges as exemplified by the aconquija range of argentina

a felsic end to bushveld differentiation the similarities in age c 2 06 ga and isotopic composition of the bushveld igneous complex and the overlying rooiberg group felsic volcanic rocks suggest the possibility that the two are cogenetic to investigate this possibility we derive a robust new estimate of the major and trace element bulk composition of the bushveld upper zone and upper main zone uumz using melts thermodynamic modeling we show that this composition fails to reproduce fundamental features of the cumulate sequence such as the presence of primary orthopyroxene at the base of the magma column we investigate the possibility that some amount of evolved magma escaped from the magma chamber and derive a new estimate of the uumz parent magma composition that reproduces generally the observed cumulate sequence our results constrain the amount and composition of the escaped magma and provide evidence of a genetic link between the uumz of the bushveld layered series and the felsic volcanic rocks immediately overlying it we conclude that between 15 and 25 of the original magma volume was expelled and is manifest either as part of the upper rooiberg group lava sequence or the rashoop granophyre distinction is hindered by their almost identical major and trace element compositions a parent magma in which 15 25 rooiberg or rashoop material is added back to the extant cumulate sequence is initially saturated in orthopyroxene and is in major and trace element equilibrium with the phases at the base of the uumz

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

origin of anorthosite and magnetitite layers in the bushveld complex constrained by major element compositions of plagioclase the bushveld complex a layered mafic intrusion in south africa shows extreme vertical differentiation in terms of mineral compositions and modal proportions from dunite to ferrodiorite in a continuous borehole core drilled through the uppermost 2 8k m of the intrusion typical rocks range upwards from troctolite through gabbronorite and ferrogabbronorite to ferrodiorite with extreme examples of anorthosite magnetitite and feldspathic pyroxenite the an content of plagioclase has previously been determined for 420 samples and decreases upward from an78 to an36 with six minor slow reversals variations in modal proportions of plagioclase have been calculated based on 2200 density determinations on whole rocks forty five anorthosite layers have been identified ranging from 1 to 23 m thick none of these layers is associated with the abovementioned reversals in an content in plagioclase and nearly all have leucocratic rocks below and above with more than the likely cotectic proportions of plagioclase these observations argue against an origin for anorthosite related to magma addition or to supersaturation and oscillatory nucleation rhythmically pulsed crystallization possibly associated with pressure changes followed by crystal settling and sorting of minerals of different densities is a hypothesis consistent with all the observations twenty layers of magnetitite have been identified there is a significant reversal in an content in the overlying plagioclase compared with the underlying sample across only one such layer again this observation challenges hypotheses that such layers result from magma addition but is consistent with a pressure change hypothesis for triggering magnetite crystallization the upper contacts of magnetitite layers that grade into anorthosite over many centimetres possibly also reflect settling and sorting rocks forming the uppermost 100 m of the intrusion contain the most sodic plagioclase compositions demonstrating that there is no downward crystallizing roof facies furthermore this uppermost 100 m section is depleted in plagioclase relative to its cotectic proportions hence we find no evidence supporting flotation or prolonged suspension of plagioclase

the emergence of topographic steady state in a perpetually dynamic self organized critical landscape we conducted a series of four physical modeling experiments of mountain growth at differing rates of uplift and three distinct climates ranging from relatively wet to relatively dry the spatial and temporal pattern of landscape behavior is characterized by f 1 scaling in sediment discharge and power law scaling in the magnitude and frequency of ridge movement in all four experiments we find that internally generated self organized critical soc processes generate dynamically stable catchment geometries after 1 relief depths of erosion these regularly spaced catchments have an average outlet spacing ratio of 2 16 well within the range of values reported in field studies once formed large catchment bounding ridges oscillate about a critically balanced mean location with occasional large scale changes in catchment size ridge movement appears to be driven by the competition for discharge as landslides push ridges back and forth these dynamics lead to the emergence of a complex twofold scaling in catchment dynamics that is fully established by 1 8 relief depths of erosion at this stage a clear threshold has emerged separating two distinct scaling regimes where large ridge mobility is insensitive to relief and small ridge mobility is relief dependent overall we demonstrate that the development of dynamically stable large scale landforms is related to the emergence of a complex system hierarchy in topographic dynamics once formed these landscapes do not evolve statistical properties such as average topography and discharge become stationary while topography remains highly dynamic at smaller length scales

dominance of tectonics over climate in himalayan denudation landscape denudation in actively deforming mountain ranges is controlled by a combination of rock uplift and surface runoff induced by precipitation whereas the relative contribution of these factors is important to our understanding of the evolution of orogenic topography no consensus currently exists concerning their respective infl uences to address this question denudation rates at centennial to millennial time scales were deduced from 10be concentrations in detrital sediments derived from 30 small basins 10 600 km2 in an 200 km wide region in central nepal along a northward strike perpendicular transect average denudation rates sharply increase from 0 5 mm yr in the lesser himalayas to 1 mm yr when crossing the physiographic transition and then accelerate to 2 3 mm yr on the southern flank of the high peaks in the greater himalayas despite a more than five fold increase in denudation rate between the southern and northern parts of this transect the corresponding areas display similar precipitation rates the primary parameter that presents a signifi cant co variation with denudation is the long term rockuplift rate that is interpreted to result from the ramp fl at transition along the main himalayan thrust we propose that in this rapidly uplifting mountain range landscapes adjust quickly to changing climatic conditions such that denudation is mainly limited by the rate at which material is pushed upward by tectonic processes and made available for removal by surface processes in this particular context variations in precipitation appear to have only a second order role in modulating the denudation signal that is primarily set by the background rock uplift rate 2014 geological society of america

slope and climate variability control of erosion in the andes of central chile climate and topography control millennial scale mountain erosion but their relative impacts remain matters of debate confl icting results may be explained by the infl uence of the erosion threshold and daily variability of runoff on long term erosion however there is a lack of data documenting these erosion factors here we report suspended load measurements derived decennial erosion rates and 10 be derived millennial erosion rates along an exceptional climatic gradient in the andes of central chile both erosion rates decennial and millenial follow the same latitudinal trend and peak where the climate is temperate mean runoff 500 mm yr 1 both decennial and millennial erosion rates increase nonlinearly with slope toward a threshold of 0 55 m m the comparison of these erosion rates shows that the contribution of rare and strong erosive events to millennial erosion increases from 0 in the humid zone to more than 90 in the arid zone our data confi rm the primary role of slope as erosion control even under contrasting climates and support the view that the infl uence of runoff variability on millennial erosion rates increases with aridity

mercury in the bushveld complex south africa and the skaergaard intrusion greenland abstract we present mercury concentrations for layered mafic rocks of the bushveld complex the skaergaard intrusion morb and a karoo dolerite sill as determined by combustion atomic absorption spectroscopy combustion aas with gold amalgamation pre concentration mercury in the bushveld complex and the skaergaard intrusion is generally low with about 1 1 and 0 8 ppb hg on average respectively the karoo dolerite sill chill margins and morb have average hg concentrations of 2 5 and 6 4 ppb respectively the reason for the low whole rock hg concentrations in layered mafic intrusions is that the element is incompatible with respect to the cumulus mineral assemblages our results show far lower hg concentrations for the skaergaard cumulates than previous measurements and we find no evidence for preferential enrichment in plagioclase rich rocks as previously reported we attribute this difference to inaccurate analytical methodology of the early measurements there are hg enrichments in distinct levels in both intrusions mercury enrichments in the bushveld complex are observed near the top of the lower zone 2 4 ppb in the merensky reef at 8 6 ppb the platreef at 3 ppb average and in the upper zone near the main magnetitite layer 4 7 ppb these enrichments are due to the concentration of hg by sulfides as suggested by a positive correlation between cu and hg there is no upward increase in hg with stratigraphic height or positive correlation between hg and zr in the bushveld complex as would be expected if hg acted as an immobile incompatible element the lack of correlation is explained by periodic degassing of the bushveld magma chamber in which volatile elements including hg and s escaped in contrast the skaergaard intrusion probably did not degas and there is hg enrichment in the sandwich horizon 53 ppb because of the combination of the incompatibility and the presence of sulfides

a review of the source behaviour and distribution of arsenic in natural waters the range of as concentrations found in natural waters is large ranging from less than 0 5 g l 1 to more than 5000 g l 1 typical concentrations in freshwater are less than 10 g l 1 and frequently less than 1 g l 1 rarely much higher concentrations are found particularly in groundwater in such areas more than 10 of wells may be affected defined as those exceeding 50 g l 1 and in the worst cases this figure may exceed 90 well known high as groundwater areas have been found in argentina chile mexico china and hungary and more recently in west bengal india bangladesh and vietnam the scale of the problem in terms of population exposed to high as concentrations is greatest in the bengal basin with more than 40 million people drinking water containing excessive as these large scale natural as groundwater problem areas tend to be found in two types of environment firstly inland or closed basins in arid or semi arid areas and secondly strongly reducing aquifers often derived from alluvium both environments tend to contain geologically young sediments and to be in flat low lying areas where groundwater flow is sluggish historically these are poorly flushed aquifers and any as released from the sediments following burial has been able to accumulate in the groundwater arsenic rich groundwaters are also found in geothermal areas and on a more localised scale in areas of mining activity and where oxidation of sulphide minerals has occurred the as content of the aquifer materials in major problem aquifers does not appear to be exceptionally high being normally in the range 1 20 mg kg 1 there appear to be two distinct triggers that can lead to the release of as on a large scale the first is the development of high ph 8 5 conditions in semi arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates this ph change leads either to the desorption of adsorbed as especially as v species and a range of other anion forming elements v b f mo se and u from mineral oxides especially fe oxides or it prevents them from being adsorbed the second trigger is the development of strongly reducing conditions at near neutral ph values leading to the desorption of as from mineral oxides and to the reductive dissolution of fe and mn oxides also leading to as release iron ii and as iii are relatively abundant in these groundwaters and so4 concentrations are small typically 1 mg l 1 or less large concentrations of phosphate bicarbonate silicate and possibly organic matter can enhance the desorption of as because of competition for adsorption sites a characteristic feature of high groundwater as areas is the large degree of spatial variability in as concentrations in the groundwaters this means that it may be difficult or impossible to predict reliably the likely concentration of as in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well arsenic affected aquifers are restricted to certain environments and appear to be the exception rather than the rule in most aquifers the majority of wells are likely to be unaffected even when for example they contain high concentrations of dissolved fe

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

chilled margins in igneous rocks abstract when hot magma contacts cold solid rock a chilled margin forms because the heat flux from the magma cannot balance the very large initial conductive flux in the rock however the conductive flux decreases with time and can eventually be exceeded by the heat flux from the magma in these circumstances the chilled margin begins to remelt and can even disappear this process is modelled quantitatively for the case of a constant heat flux from the magma the results are applied to the turbulent steady flow of komatiite magma in dykes and as lavas the initial chilled margin with cold rock only survives for a few tens of seconds and so does not inhibit thermal erosion significantly the theory can also be applied to magmatic intrusions where the heat flux varies with time results are presented for the remelting of a chilled margin formed at the roof of a basaltic sill for sills with thicknesses from 10 to 1000 m the remelting of initial chilled margins from a few tens of centimetres to a few tens of metres is possible three kinds of contact are predicted at the margins of intrusions first contacts are developed where the chilled margin grows continually without any meltback second meltback can occur but does not completely redissolve the initial chilled margin this causes an internal contact to form within the intrusion third meltback is complete and the country rock can then be melted and assimilated the marginal rocks of such intrusions need not be fine grained

geomorphic limits to climate induced increases in topographic relief recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross disciplinary investigation and debate on the question of whether tectonics have driven long term climate change or vice versa it has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision because isostasy acts to compensate mean elevations the debate hinges on the question of whether climate change can significantly increase topographic relief or more precisely increase the volume of missing mass between summits and ridges here we show that in tectonically active mountain ranges geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change thus although climate change may cause significant increases in denudation rates potentially establishing an important feedback between surficial and crustal processes neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift

arsenic in geothermal sources at the north central andean region of ecuador concentrations and mechanisms of mobility the arsenic content of geothermal hot springs and their sediments in the north central andean region of ecuador has been investigated the area of study is located between parallels 1 11 n and 1 30 s and includes five provinces the area is rich in geothermal surface manifestations that are mainly used for medicinal baths in recreational complexes unfortunately water residuals without treatment are released from the recreational facilities to surrounding water bodies the results indicate that total arsenic in geothermal waters in this region has a range of 2 969 g as l and sediments contain arsenic ranging from 1 6 to 717 6 mg kg chemical analyses of sediment samples show the presence of sulfur iron aluminum and calcium a high concentration of natural organic matter was also found in some samples 20 29 5 thus sorption and coprecipitation can be the main mechanisms of as immobilization on mineral phases and natural organic matter

rare earth element abundances in apatite in the bushveld complex a consequence of the trapped liquid shift effect reply the concentrations of the rare earth elements rees in apatite from a 600 m thick sequence toward the top of the bushveld complex south africa have previously been published they show two ranges of compositions in a lower section and an upper section the data were interpreted in terms of liquid immiscibility the evidence for this model is reassessed and an alternative explanation offered that involves the trapped liquid shift effect in this process the true cumulus phase compositions reequilibrated with trapped liquid as it solidified and ree abundances in apatite were variably increased the extent of the change depends on the relative proportions of cumulus apatite and trapped liquid calculations show that if there was 5 apatite and 10 trapped liquid as found in the lower section the ree abundance in the final apatite would have only increased marginally relative to the original cumulus composition however in the upper section where there was only 2 apatite and 25 trapped liquid the ree abundances were increased by a factor of 3 most of the rees are essentially incompatible in the other cumulus phases except apatite and the other phases have little influence on the final concentrations of rees in apatite however for eu which partitions into plagioclase this mineral acts as a major buffering influence and relatively less eu than the other rees entered apatite during the solidification process as a result the calculated final apatite in the upper section developed a large negative eu anomaly as observed in the actual mineral analyses the concept of large scale liquid immiscibility in the upper part of the bushveld complex based on rees in apatite is challenged and an interpretation involving the trapped liquid shift effect is considered more plausible

hafnium isotopic composition of the bushveld complex requires mantle melt upper crust mixing new evidence from zirconology of mafic felsic and metasedimentary rocks

high elevation of low relief surfaces in mountain belts does it equate to post orogenic surface uplift we present experiments of upraising and relaxing topographies showing that peneplanation can occur above the ultimate base level sea level after active uplift the erosion of a topography bounded by a piedmont generates a final smooth and highly elevated topography smoothing at high elevation is even possible during active uplift if the evolution of topography is disrupted by the deposition of the products of erosion on its piedmont which is the case at the transition from underfilled to overfilled conditions in foreland basins

frequency dependent landscape response to climatic forcing 1 whereas the existence of pronounced orbitally controlled periodicities is a major feature of earth climate its impact on landscape dynamics remains poorly understood we use a landscape evolution model lem to systematically investigate the response of landscapes to a range of periodic oscillations in precipitation the resulting sediment flux evolution displays a pronounced sensitivity to the period of the input precipitation signal such that for a given erodibility a specific periodicity maximizes the amplitude of the response this optimal period of resonance scales as the inverse of the erodibility but is progressively filtered out of the response when the intensity of hillslope diffusion increases this frequency dependent landscape behavior displayed by our model provides a mechanistic perspective on molnar s 2004 proposition that ubiquitous changes in late cenozoic continental denudation could result directly from modifications in the spectral content of the climatic signal

influence of piedmont sedimentation on erosion dynamics of an uplifting landscape an experimental approach models of relief development generally assume that eroded products are evacuated far from the landscape whereas in nature they are often deposited at the foot of mountain belts within continental environments because piedmont aggradation can modify the base level for erosion we investigate the influence of piedmont sedimentation on the dynamics of an upstream relief we developed an experimental study of relief dynamics using laboratory scale models submitted to uplift under runoff driven erosion we compare the dynamics of topographies surrounded or not by a depositional belt made of eroded products coming from upstream piedmont aggradation acts on the dynamics of the upstream relief by modifying the relative uplift rate applied uplift rate minus aggradation rate that denudation tends to balance relief denudes at a lower rate than the applied uplift rate so the mean elevation of the uplifting topography rises when the time scale of aggradation is higher than the time scale of relief development the topography cannot reach a steady state between denudation and the applied uplift rate as long as aggradation occurs however in this case denudation balances a continuously varying relative uplift rate during a dynamic equilibrium phase of the topography

effect of stirring on crystallization kinetics of basalt texture and element partitioning isothermal crystallization experiments on basalt have been carried out using an infrared heating furnace to investigate the effect of stirring when stirring was not applied static experiment the results agreed well with previous experiments but when stirring was applied and a flow of reynolds number 10 3 4 was present dynamic experiment considerably different results were obtained especially in respect to the nucleation rate and the morphology of crystals at t 25 c essentially similar results were obtained on the nucleation rates and morphologies of crystals in both static and dynamic experiments however at supercoolings larger than 45 c nucleation density increased drastically in dynamic experiments reaching up to ten times as large as that in static experiments crystals of plagioclase and clinopyroxene were small and adapted acicular morphology regardless of t in dynamic experiments and hyalopilitic textures were formed a ttt diagram shows that the nucleation incubation time is shorter in dynamic experiments than in static experiments no compositional difference in major elements was found in plagioclase and clinopyroxene produced in both static and dynamic experiments however minor element concentrations e g mg in plagioclase and ti al in clinopyroxene were found to increase with both t and flow velocity all these results imply that although chemical diffusion in the melts did not play an important role in the dynamic experiments interface kinetics were important it is suggested that hyalopilitic texture commonly seen in natural basalt is mainly due to flow in magma

phase equilibria constraints on relations of ore bearing intrusions with flood basalts in the noril sk region russia phase equilibria analysis has been performed to elucidate whether ore bearing noril sk and poorly mineralized lower talnakh type intrusions are co magmatic with the extremely ni cu and pges depleted and not depleted flood basalts respectively the parental magma of the intrusions is classified as silica undersaturated olivine basalt whereas that of the flood basalts is assigned to silica saturated tholeiite the plutonic equivalent of the former parental magma is olivine gabbro or melagabbro whereas the latter is gabbronorite phase equilibria relations clearly show that these two types of magmas cannot be co magmatic as there is no way to derive one liquid from another by fractional crystallization this provides no support for the current model considering the noril sk type intrusions as exit conduits for a great volume of volcanic magma which lost most of its chalcophile elements in the shallow settling chambers on its way to surface the association of a large volume of chalcophile element depleted basalts with the very sulfide rich intrusions in the noril sk region is most likely coincidental and cannot therefore be used as a regional criterion when prospecting for the noril sk type sulfides deposits in other flood basalts provinces

decoupling of erosion and precipitation in the himalayas the hypothesis that abrupt spatial gradients in erosion can cause high strain rates in active orogens has been supported by numerical models that couple erosional processes with lithospheric deformation via gravitational feedbacks1 2 3 most such models invoke a stream power rule in which either increased discharge or steeper channel slopes cause higher erosion rates spatial variations in precipitation and slopes are therefore predicted to correlate with gradients in both erosion rates and crustal strain here we combine observations from a meteorological network across the greater himalaya nepal along with estimates of erosion rates at geologic timescales greater than 100 000 yr from low temperature thermochronometry across a zone of about 20 km length spanning the himalayan crest and encompassing a more than fivefold difference in monsoon precipitation significant spatial variations in geologic erosion rates are not detectable decreased rainfall is not balanced by steeper channels instead additional factors that influence river incision rates such as channel width and sediment concentrations must compensate for decreasing precipitation overall spatially constant erosion is a response to uniform upward tectonic transport of greater himalayan rock above a crustal ramp

solidification front fractionation in phenocryst free sheet like magma bodies basal cumulate zones are common in sills formed of magma carrying phenocrysts upon emplacement and the process leading to these cumulates and the corresponding roofward depleted zones are well understood many large sills which seem to have been emplaced with phenocryst free magma are nearly uniform in composition and show little tendency to differentiate by crystal settling we examine whether for an initially crystal free sill solidifying inwardly from top and bottom crystals nucleated and grown along the roof can escape incorporation survive resorption in settling through the hotter central region and reach the basal solidification front to form a cumulate we call this process solidification front fractionation and build a relatively simple model to examine the rheologic dynamic and crystal growth rate conditions leading to the downward escape of newly grown single crystals from the roof of a sheet like body of mafic magma cooling by conduction our results suggest that whereas a thin micro

cyclicity in the main and upper zones of the bushveld complex south africa crystallization from a zoned magma sheet the major element composition of plagioclase pyroxene olivine and magnetite and whole rock sr sr data are presented for the uppermost 2 1 km of the layered mafic rocks upper main zone and upper zone at bierkraal in the western bushveld complex initial sr sr ratios are near constant 0 7073 0 0001 for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation the 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite magnetite ilmenite apatite layers and changes up section from gabbronorite an72 plagioclase mg 74 clinopyroxene to magnetite ilmenite apatite fayalite ferrodiorite an43 mg 5 clinopyroxene fo1 olivine the overall fractionation trend is however interrupted by reversals characterized by higher an of plagioclase higher mg of pyroxene and olivine and higher v2o5 of magnetite in the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite these reversals in normal fractionation trends define the bases of at least nine major cycles we have calculated a plausible composition for the magma from which this entire succession formed forward fractional crystallization modeling of this composition predicts an initial increase in total iron near constant sio2 and an increasing density of the residual magma before magnetite crystallizes after magnetite begins to crystallize the residual magma shows a nearconstant total iron an increase in sio2 and decrease in density we explain the observed cyclicity by bottom crystallization initially magma stratification developed during crystallization of the basal gabbronorites once magnetite began to crystallize periodic density inversion led to mixing with the overlying magma layer producing mineralogical breaks between fractionation cycles the magnetitite and nelsonite layers mainly occur within fractionation cycles not at their bases in at least two cases crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically and triggered the proposed density inversion resulting in close but not perfect coincidence of mineralogical breaks and packages of magnetitite layers

dynamic interactions between the gulf of guinea passive margin and the congo river drainage basin 1 morphology and mass balance 1 a sediment budget between the congo river drainage basin and the western african margin in the gulf of guinea is proposed on the basis of published and unpublished offshore tertiary isopach maps and onshore digital elevation analysis the overall denudation of that area may be as high as 3 5 106 km3 with a maximum of 10 coming from mesozoic and cenozoic covers the southern part of the congo river basin related to uplift of the south african and kalahari shields appears as the most immature from the morphologic standpoint but provides one third of the present day sediment production it represents a maximum denudation of 150 000 km3 on the easternmost part east african rift drainage basins show a more mature relief and a maximum denudation of 270 000 km3 which can increase to 570 000 km3 if the congo drainage basin had extended up to the east branch of the rift these values are confidently established from the existence of remnant geological surfaces and can explain the volume of tertiary sediment in the gulf of guinea during upper cretaceous the most important accumulations of sediments correspond to the ogooe and kwanza fans which shows that the organization of continental drainage was different as was potential source of sediments in the northern part of the present day congo river drainage basin there was a compressional episode during santonian that may have caused significant relief and erosion along a benue chad axis

scaling regimes of local slope versus contributing area in digital elevation models four scaling regimes are observed in digital elevation models dems when the average local slope is calculated for pixels grouped according to the values of the contributing area threshold criteria proposed by various researchers to identify the extent of the channel network are examined relative to the slope area scaling diagram the scaling response observed in dems is reproduced with a landscape evolution and channel network growth model originally developed by willgoose et al 1989 the threshold criterion proposed in this model is a useful tool to separate two different slope area scaling regimes observed in dem data

minimal climatic control on erosion rates in the sierra nevada california climate is widely thought to regulate erosion rates but the relationships among precipitation temperature and erosion rate have remained speculative because long term erosion rates have been difficult to measure we used cosmogenic nuclides to measure long term erosion rates at climatically diverse sites in the sierra nevada california spanning 20 180 cm yr in annual precipitation and 4 15 8c in mean annual temperature average erosion rates vary by only 2 5 fold across these sites and are not correlated with climate indicating that climate only weakly regulates nonglacial erosion rates in mountainous granitic terrain

reconstructing chemical weathering physical erosion and monsoon intensity since 25 ma in the northern south china sea a review of competing proxies reconstructing the changing strength of the east asian summer monsoon has been controversial because different proxies many being indirect measures of rainfall tell contrasting stories about how this has varied over long periods of geologic time here we present new sr isotope grain size and clastic flux data and synthesize existing proxies to reconstruct changing chemical erosion in the northern south china sea since the oligocene using the links between weathering rates and monsoon strength established in younger sediments as a way to infer intensity chemical proxies such as k rb l al and the chemical index of alteration cia together with clay proxies like kaolinite illite chlorite show a steady decline in alteration after a sharp fall following a maximum at the mid miocene climatic optimum mmco 15 5 17 2 ma probably as a result of cooling global temperatures in contrast physical erosion proxies including bulk ti ca and clastic mass accumulation rates mar show peaks at 21 23 ma similar to 19 ma and 15 5 17 2 ma implying faster run off in the absence of drainage capture rates increase again likely driven by slightly increased run off after 13 ma but decrease after 8 ma which is identified as a period of summer monsoon weakening sr isotope composition correlates with hematite goethite and the spectral proxy crat to show stronger weathering linked to more monsoonal seasonality these proxies argue for a strengthening of the east asian monsoon after 22 23 ma followed by an extended period of monsoon maximum between 18 and 10 ma then weakening there is some suggestion that the summer monsoon may have strengthened since 3 4 ma after reaching a minimum in the pliocene c 2014 elsevier b v all rights reserved

large scale geomorphology classical concepts reconciled and integrated with contemporary ideas via a surface processes model linear systems analysis is used to investigate the response of a surface processes model spm to tectonic forcing the spm calculates subcontinental scale denudational landscape evolution on geological timescales 1 to hundreds of million years as the result of simultaneous hillslope transport modeled by diffusion and fluvial transport modeled by advection and reaction the tectonically forced spm accommodates the large scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification the following three model scales are considered micro meso and macroscale the concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift at the larger meso and macroscales which represent individual interfluves and landscapes including a number of drainage basins respectively the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel for these linear models the response time and the transfer function as a function of scale characterize the model behavior numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape when tectonic timescales are much longer than the landscape response time the resulting dynamic equilibrium landscapes correspond to those envisaged by hack 1960 when tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses much shorter than the response time evolving landscapes conform to the penck type 1972 and to the davis 1889 1899 and king 1953 1962 type frameworks respectively the behavior of the spm highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing finally nonlinear behavior resulting from more general uplift geometries is discussed a number of model experiments illustrate the importance of fundamental form which is an expression of the conformity of antecedent topography with the current tectonic regime lack of conformity leads to models that exhibit internal thresholds and a complex response

compositional convection in a reactive crystalline mush and melt differentiation fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them to elucidate some physical principles of thermosolutal convection in a reactive porous medium we describe experiments on the directional solidification of aqueous ammonium chloride solutions the addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions the phase diagram and permeability solutions were cooled from below and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid released from the boundary layer at the top of the mush the interstitial fluid within the mush also became unstable eventually the onset of convection occurring when the porous medium rayleigh number of the mush reached a critical value this threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature composition and porosity convective motions locally caused dissolution and precipitation and hence fluctuations of porosity developed dissolution occurred preferentially in the central parts of upwellings and the upflow gradually focused being ultimately channelized into narrow chimneys devoid of crystals all else being equal the areal density of chimneys was greater the lower the viscosity chimney diameter increased with increasing solution viscosity in the liquid above the growing mush the convective plumes were very similar to salt fingers depending on solution viscosity and temperature gradient they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities the base plate temperature chosen was above the eutectic and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram the spatial distribution of crystals and the final porosity of the mush were however determined by the strength of compositional convection as measured by the porous medium rayleigh number when no convection occurred mush thickness finally became equal to the initial layer thickness and the system was homogeneous when compositional convection occurred the overlying reservoir underwent chemical evolution and mush growth slowed dramatically in experiments with progressively lower solution viscosity and hence more vigorous convection final mush thickness was progressively less and final porosity lower final mush thickness was found to scale with solution viscosity to the power 0 33 during fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks we speculate that fossil chimney structures can be found in the lower zone of the bushveld ultramafic complex in the form of iron rich platinum bearing dunite pipes this could explain the adcumulate nature of the lower zone rocks and the chemical and mineralogical similarities between the pipes and the overlying merensky reef the merensky reef would be interpreted as a hiatus in mush growth the results may also have applications to the flow structure in regions of partial melting and in the earth s core

simulation of primary phase relations and mineral compositions in the partridge river intrusion duluth complex minnesota implications for the parent magma composition in order to describe the composition and crystallinity of the initial parental magma of the partridge river intrusion of the keweenawan duluth complex and thereby understand the mode of emplacement and solidification of the intrusion we have applied a numerical simulation technique called geochemical thermometry frenkel et al 1988 the parental magma was a low alumina high ti p olivine tholeiite similar to typical keweenawan low alumina high ti p basalts associated with the duluth complex and from the nearby portage lake area of the lake superior region the parental magma was emplaced as a crystal liquid suspension followed by chilling of an evolved leading edge ferrodioritic liquid in the basal zone of the intrusion the conditions of emplacement at the present crustal location were 1 150 c 2 kbar and fo2 slightly above the wustite magnetite wm buffer the main differentiation process after emplacement was the sorting and redistribution of plagioclase and olivine crystals on a local scale accompanied by less efficient convection and minor settling of olivine calculated crystallization sequence for the parental magma is olivine plagioclase 1 240 c olivine plagioclase magnetite 1 146 c wm 0 5 olivine plagioclase magnetite augite 1 140 c wm 0 5 the calculated compositions of the cumulus olivine and plagioclase in equilibrium with the parent magma at 1 150 c are fo66 7 1 1 and an64 5 2 5 respectively and are similar to the estimated average composition of primary olivine fo69 1 2 8 and the average composition of plagioclase core an66 3 2 8 measured in drill core samples through the intrusion chalokwu and grant 1987

expression of active tectonics in erosional landscapes understanding the manner and degree to which topography in active mountain ranges reflects deformation of the earth s surface remains a first order goal of tectonic geomorphology a substantial body of research in the past decade demonstrates that incising channel systems play a central role in setting relationships among topographic relief differential rock uplift rate and climatically modulated erosional efficiency this review provides an introduction to the analysis and interpretation of channel profiles in erosional mountain ranges we show that existing data support theoretical expectations of positive monotonic relationships between channel steepness index a measure of channel gradient normalized for downstream increases in drainage area and erosion rate at equilibrium and that the transient response to perturbations away from equilibrium engenders specific spatial patterns in channel profiles that can be used to infer aspects of the forcing these aspects of channel behavior lay the foundation for a series of case studies that we use to illustrate how focused quantitative analysis of channel morphology can provide insight into the spatial and temporal dynamics of active deformation although the complexities of river response to climate lithology and uplift patterns mean that multiple interpretations of topographic data alone will always possible we show that application of stream profile analysis can be a powerful reconnaissance tool with which to interrogate the rates and patterns of deformation in active mountain belts

incompatible trace element behavior in the bushveld complex borehole core from the uppermost 500 m of the rustenburg layered suite of the bushveld complex has been analyzed for rb sr zr y and nb plots of absolute trace element abundance and trace element ratios against stratigraphic height are presented these are used to test for multiple injection of magma and postcumulus migration of residual liquid these and previously published data are used to support the hypotheses that there was a new injection of magma above the merensky reef and that large scale infiltration metasomatism did not occur no evidence from incompatible element data exists for an addition of magma near the pyroxenite marker where a distinct reversal in cumulus mineral composition occurs however this may be the result of relatively small degrees of fractionation of the host magma prior to the addition of new magma the ability to recognize the entry of new magma may be restricted by the analytical precision the uppermost 500 m of the layered sequence appears to behave as a single broadly homogeneous magma in showing a systematic predictable upward increase in incompatible element abundances consistent with a model assuming 10 to 30 percent trapped liquid in the cumulate deviations from the predicted trends for zr y and nb are attributed to these elements being compatible in magnetite for zr and apatite for y and nb which are present in the uppermost rocks

sediment supply to the orange sedimentary system over the last 150 my an evaluation from sedimentation denudation balance the south african plateau is bordered by passive margin basins preserving the terrigeneous sediment produced during onshore erosion as such these basins potentially provide a record of the variation in onshore elevation and relief over time here we bring new constraints on the uplift and erosion of the southern african plateau over the last 150 ma from the perspective of the stratigraphic architecture of these basins we review published data to quantify the terrigeneous supply eroded in the drainage area and preserved in the basins the novel aspect of our approach is the integration of the evolution of the whole domain in sedimentation i e not only the platform as well as the onshore eroding region along the south african and namibian atlantic margins we determined the long term signal of sedimentary supply at the 10 ma scale from 3d mass balance calculations comparing sedimentary volumes deduced from offshore isopach maps with a particular attention to associated uncertainties on one hand and denudation volumes deduced from thermochronology data on the other we show that in this case the volumes of denudation and of sediments preserved in the basin are consistent both in total and incremental amount the latter is therefore a reliable estimation of the denudation in the associated continental domain subject to certain assumptions such as the drainage area over time the long term signal of sedimentary supply of the south african and namibian atlantic margins shows that the major part of the denudation occurred during the post rift lower cretaceous and the upper cretaceous and subsequently decreased this is not the expected behavior for the simple relaxation of topography acquired during rifting that would show a progressive decrease of the sediment supply over time this suggests a significant rejuvenation of relief during the upper cretaceous 50 ma after the rifting event if erosion is directly linked to elevation or relief variation this implies 2 generations the first before break up i e before 130 ma presumably during rifting and the second during the upper cretaceous

the residual or roof zone of the bushveld complex south africa the geological succession of an area in the southeastern bushveld complex south africa is described here in which up to 700 m of monzonitic rocks more evolved than in currently proposed stratigraphic sections of the bushveld have been identified beneath the rooiberg group felsites the roof rocks to the intrusion a previous mass balance calculation for incompatible elements from the upper part of the bushveld complex indicated that some evolved rocks are missing from currently accepted sections through the stratigraphy most previous attempts to constrain the location of such evolved rocks have been based on one dimensional vertical modelling and the possibility of lateral magma migration within the complex has mainly been ignored by most researchers characterization of the currently accepted most evolved rocks is based on two challengeable assumptions that the crystallization surface was exactly parallel to the roof and that the bushveld granite was emplaced exactly at the contact between the uppermost rocks of the complex upper zone and the felsic volcanic roof rocks rooiberg group both assumptions are shown here to be questionable it is inferred that this body of monzonite represents this more evolved magma that migrated laterally not vertically these observations show that vertical summation of the compositions in the intrusion cannot give the correct answer for the original composition of the magma these evolved rocks contain abundant plagioclase up to 20 in total of quartz alkali feldspar plus hornblende and very minor olivine clinopyroxene and an oxide phase they contain up to 65 sio2 and can be called hornblende quartz monzonites they do not display good cumulate textures such rocks have been reported in the older pre 1970 literature but with different names from the eastern western and northern limbs of the bushveld and so potentially are volumetrically significant the more recent publications have focused on vertical sections where fairly melanocratic and oxide rich rocks form the top contact that contact is interpreted here to have been truncated by emplacement of the bushveld granite to emphasize that the present studied rocks are mineralogically and texturally distinctly different from those reported in previous sections it is proposed that they be designated as the residual or roof zone of the bushveld complex

the distribution of arsenic iii and arsenic v in geothermal waters examples from the massif central of france the island of dominica in the leeward islands of the caribbean the valles caldera of new mexico u s a and southwest bulgaria abstract data for as iii as v and total as for various types of geothermal spring water are given the different species of as were separated at the time of sampling after filtration acidification and dilution where necessary using an anion exchange method total as as iii and as v were determined by flameless atomic absorption spectrometry the total as content ranges from 5 to 1800 g l 1 samples from dominica show evidence of different sulfur arsenic oxidation kinetics as iii being the major component 85 99 in fluids from deep levels of the geothermal reservoir and also 99 in oxidized waters of the acid sulfate type the bicarbonate spring water of the valles caldera new mexico contains more as iii than as v but there as v is the major component of the acid sulfate springs as iii proves to be the predominant form 60 100 in the warm alkaline waters from southwest bulgaria a selection of co 2 rich spring waters from the central massif france cezallier is analysed two groups are distinguished on the basis of major element chemistry and total as content oxidized forms are mainly encountered in the first group including a well sample containing 0 5 as iii the as iii content of the second group ranges from 10 to 70 these preliminary results indicate that the fluids directly related to hydrothermal systems contain mainly as iii whereas acid sulfate springs have a variable as iii content bicarbonate waters are generally enriched in oxidized as in the absence of springs or sinks within a geothermal region the total as concentration may display conservative properties but both as iii and as v may be reactive during mixing and cooling

azimuth dependent amplification of weak and strong ground motions within a fault zone nocera umbra central italy 1 during three moderate magnitude earthquakes occurred in september october 1997 in the central apennines italy accelerations larger than 0 5 g were recorded in the town of nocera umbra 10 to 15 km n nw of the epicenters the accelerograph is sited in a fault zone close to a n30 e tectonic contact six temporary seismological stations installed across the fault recorded 82 aftershocks occurred in two seismogenic zones the colfiorito sellano area s se of the array and the gualdo tadino area to the north the array data reveal large variations in terms of both peak ground motions and spectral amplitudes within the fault zone amplifications show a strong dependence on the source azimuth at the accelerograph site the effects are particularly large for events from s se peak ground motions are a factor of 14 larger than those of a reference site and conventional spectral ratios attain amplitudes as large as 50 at 7 hz along the n30 e direction of motion parallel to the strike of the fault nineteen strong motion accelerograms were then used to compare ground motion properties between weak and strong events up to m 0 1 2 x 10 25 dyn cm a particle motion analysis shows that the directional effect is also present in the strongest motions even though the amplification of peak ground motion decreases when m 0 increases results from stochastic simulations indicate that such a behavior is not due to nonlinearity applying the empirical weak motion transfer functions in a purely linear model the observed peak ground motions of the largest events are fit satisfactorily

spatiotemporal trends in erosion rates across a pronounced rainfall gradient examples from the southern central andes abstract the tectonic and climatic boundary conditions of the broken foreland and the orogen interior of the southern central andes of northwestern argentina cause strong contrasts in elevation rainfall and surface process regimes the climatic gradient in this region ranges from the wet windward eastern flanks 2 m yr rainfall to progressively drier western basins and ranges 0 1 m yr bordering the arid altiplano puna plateau in this study we analyze the impact of spatiotemporal climatic gradients on surface erosion first we present 41 new catchment mean erosion rates derived from cosmogenic nuclide inventories to document spatial erosion patterns second we re evaluate paleoclimatic records from the calchaquies basin 66 w 26 s a large intermontane basin bordered by high 4 5 km mountain ranges to demonstrate temporal variations in erosion rates associated with changing climatic boundary conditions during the late pleistocene and holocene three key observations in this region emphasize the importance of climatic parameters on the efficiency of surface processes in space and time 1 first order spatial patterns of erosion rates can be explained by a simple specific stream power ssp approach we explicitly account for discharge by routing high resolution satellite derived rainfall this is important as the steep climatic gradient results in a highly nonlinear relation between drainage area and discharge this relation indicates that erosion rates er scale with er ssp1 4 on cosmogenic nuclide time scales 2 we identify an intrinsic channel slope behavior in different climatic compartments channel slopes in dry areas 0 75 m yr with equal drainage areas thus compensating lower amounts of discharge with steeper slopes 3 erosion rates can vary by an order of magnitude between presently dry 0 05 mm yr and well defined late pleistocene humid 0 5 mm yr conditions within an intermontane basin overall we document a strong climatic impact on erosion rates and channel slopes we suggest that rainfall reaching areas with steeper channel slopes in the orogen interior during wetter climate periods results in intensified sediment mass transport which is primarily responsible for maintaining the balance between surface uplift erosion sediment routing and transient storage in the orogen

macroscale dynamics of experimental landscapes abstract we review results from laboratory scale modelling of erosion and relief dynamics under variable uplift and rainfall rates under constant values of these forcing parameters an experimental landscape typically evolves towards a steady state between uplift and erosion and we show how the geometry of the steady state landscape adjusts to the rates of uplift and rainfall the comparison between these laboratory scale landscapes and the natural ones is not straightforward because contrary to analogue modelling in tectonics natural conditions of relief evolution cannot be downscaled to the laboratory without any scale distortions laboratory scale modelling in geomorphology is therefore only experimental not analogue despite these limitations experimental models may be used to provide physical tests for numerical models and they give insights into first order behaviours and directions for future research

platinum group element distribution in the main zone and upper zone of the bushveld complex south africa abstract the platinum group element pge contents of the upper portions of the bushveld complex were investigated with three questions in mind a in natural systems does magnetite concentrate os ir ru and rh ipge as has been observed in experimental systems b is there a au pd enriched layer present such as observed in the upper parts of the skaergaard intrusion c can changes in metal ratios be used for prospecting for pge deposits in the sulfide poor main magnetite layer of the eastern bushveld ir and rh are enriched relative to pt pd and au and this could be because magnetite preferentially concentrated ir and rh over pt pd and au however in most other magnetite layers no enrichment was observed this could be because most magnetite layers contain approximately 1 sulfides and the pge budget is dominated by the sulfides these sulfides obscured the effects of magnetite collecting ipge because sulfides collect all the pge and the partition coefficients for the pge into a sulfide liquid are much greater than the partition coefficient for ipge into magnetite the weighted average of the platinum group elements pge and au over the 2000 m of sampled stratigraphy is au 2 1 ppb pd 1 7 ppb pt 1 7 ppb rh 0 18 ppb ru comparison of the composition of the marginal rocks of the bushveld with a weighted average for the complete 6 km of the bushveld cumulates shows that the cumulate pile is much richer in compatible elements ir rh cr and poorer in incompatible elements sm and hf than the marginal rocks two possible solutions to this are a the magma emplaced into the chamber was a crystal mush and thus more mafic than marginal rocks to the intrusion or b fractionated magma has been removed from the cumulate sequence and either irrupted or intruded the country rocks as granites and granophyres

two expressions of the transition from mafic cumulates to granitoids in the bushveld complex south africa examples from the western and eastern limbs 2020 elsevier b v in the bushveld complex of south africa numerous granitic and granophyric rocks termed the lebowa granite and rashoop granophyre suites respectively overlie the layered ultramafic mafic rocks of the rustenburg layered suite despite their close spatial and temporal association the granites and granophyres are often interpreted as being unrelated to the rustenburg layered suite this paper describes the transition from the uppermost rustenburg layered suite into overlying granite and granophyre at three locations in the western the bierkraal drill core and eastern diepkloof farm and stoffberg town limbs of the bushveld complex in the western limb a 60 m thick transition zone bridges the petrological gap between the overlying nebo granite lebowa granite suite and upper zone rustenburg layered suite across the transition zone the composition of olivine changes from fo6 to fo1 clinopyroxene from mg 25 to mg 2 and plagioclase from an45 to an16 at stoffberg in the eastern limb the upper zone dioritic cumulates grade into the overlying monzonitic roof zone across the roof zone plagioclase compositions change from an42 to an4 clinopyroxene from mg 30 to mg 11 and olivine from fo9 to fo5 based on geochemistry and petrography we correlate the lowermost stratigraphy at the farm diepkloof with the stoffberg roof zone at diepkloof the roof zone grades into the overlying stavoren granophyre rashoop granophyre suite all units are indistinguishable in terms of bulk rock nd end 6 4 to 5 4 and hf ehf 9 2 to 6 6 isotopes corrected to 2055 ma similarly the upper main zone upper zone and transition zone are indistinguishable in terms of bulk rock sr isotopes 87sr 86sr 2055 ma 0 7071 to 0 7076 except one transition zone outlier at 0 7058 we submit that the transition zone represents the fossil record of bulk and or diffusional mixing between coexisting upper zone and nebo granite magmas we test this hypothesis by combining field petrographic and geochemical observations with forward modelling using the rhyolite melts algorithm our work on the bierkraal core western limb shows that at least a portion of the granitic magma was emplaced before the residual liquid of the upper zone had solidified at stoffberg and diepkloof in the eastern limb where the granite is absent the roof zone underwent uninterrupted fractional crystallization

petrography and geochemistry of an unusual fe rich basaltic komatiite from boston township northeastern ontario the boston creek flow is a thick archean layered basaltic komatiite lava flow located 16 km south of kirkland lake ontario within the abitibi greenstone belt it is superficially similar to the layered flows of munro township in that it consists of a basal pyroxenite overlain sequentially by peridotite pyroxenite gabbro and pyroxene spinifex textured rock and exhibits the typical cumulate komatiite crystallization sequence olivine clinopyroxene plagioclase the spinifex textured unit is however unusually thick 33 m and in comparison with other archean lavas of similar magnesium content the flow is fe and ti enriched and al and y depleted cao al2o3 and al2o3 tio2 ratios of 2 3 and 5 respectively also distinguish it from other archean magnesium rich volcanic rocks the preservation of typical tholeiitic chemistry in lavas underlying the boston creek flow precludes metamorphism and metasomatism as explanations of the flow s enigmatic geochemistry this is supported further by relict igneous te

the origin of basic ultrabasic sills with s d and i shaped compositional profiles by in situ crystallization of a single input of phenocryst poor parental magma an attempt is made to develop an in situ crystallization model based on the concept of soret fractionation to explain the origin of commonly observed s d and i shaped compositional profiles in sills formed from a single pulse of phenocryst poor parental magma the model envisages that the various compositional profiles observed in sills can be interpreted in terms of different combinations of four principal units basal zone and layered series forming the floor sequence and top zone and upper border series constituting the roof sequence the basal and top zones represent mirror images of the layered and upper border series respectively and therefore are referred to as basal and top reversals it is proposed that the formation of basal and top reversals takes place through the non equilibrium soret differentiation of liquid boundary layers which form as a consequence of the temperature gradient imposed by the cold country rock in contrast the layered and upper border series originate predominantly through the crystal liquid boundary layers developing in equilibrium conditions the model permits the production of s d and i shaped compositional profiles from the same magma composition all that is necessary to produce a specific shape of compositional profile is an appropriate temperature gradient imposed by the cold country rock on the liquid boundary layers of a parental magma of a given composition

landscape reorganization under changing climatic forcing results from an experimental landscape understanding how landscapes respond to climate dynamics in terms of macroscale average topographic features and microscale landform reorganization is of interest both for deciphering past climates from today s landscapes and for predicting future landscapes in view of recent climatic trends although several studies have addressed macro scale response only a few have focused on quantifying smaller scale basin reorganization to that goal a series of controlled laboratory experiments were conducted where a self organized complete drainage network emerged under constant precipitation and uplift dynamics once steady state was achieved the landscape was subjected to a fivefold increase in precipitation transient state throughout the evolution high resolution spatiotemporal topographic data in the form of digital elevation models were collected the steady state landscape was shown to possess three distinct geomorphic regimes unchannelized hillslopes debris dominated channels and fluvially dominated channels during transient state landscape reorganization was observed to be driven by hillslopes via accelerated erosion ridge lowering channel widening and reduction of basin relief as opposed to channel base level reduction quantitative metrics on which these conclusions were based included slope area curve correlation analysis of spatial and temporal elevation increments and wavelet spectral analysis of the evolving landscapes our results highlight that landscape reorganization in response to increased precipitation seems to follow an arrow of scale major elevation change initiates at the hillslope scale driving erosional regime change at intermediate scales and further cascading to geomorphic changes at the channel scale as time evolves

climatic control of denudation in the deglaciated landscape of the washington cascades erosion in the washington cascades mountains was dominated by glacier activity until the end of the last glacial maximum when fluvial incision and mass wasting became the dominant drivers an analysis of millennial scale rates of denudation in the range suggest erosion is sensitive both to spatial variations in precipitation and inherited topography

fluvial landscape evolution controlled by the sediment deposition coefficient estimation from experimental and natural landscapes the evolution of a fluvial landscape is a balance between tectonic uplift fluvial erosion and sediment deposition the erosion term can be expressed according to the stream power model stating that fluvial incision is proportional to powers of river slope and discharge the deposition term can be expressed as proportional to the sediment flux divided by a transport length this length can be defined as the water flux times a scaling factor this factor exerts a major control on the river dynamics on the spacing between sedimentary bedforms or on the overall landscape erosional behavior yet this factor is difficult to measure either in the lab or in the field here we propose a new formulation for the deposition term based on a dimensionless coefficient g which can be estimated at the scale of a landscape from the slopes of rivers at the transition between a catchment and its fan we estimate this deposition coefficient from 29 experimental catchment alluvial fan systems and 68 natural examples based on our data set we support the idea of davy and lague 2009 that g is a relevant parameter to characterize the erosional and transport mode of a fluvial landscape which can be field calibrated with a continuum from detachment limited g 0 to transport limited behavior g 0 4 from the studied examples

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

palisades sill origin of the olivine zone by separate magmatic injection rather than gravity settling the still widely cited view of the palisades sill northeastern united states being differentiated largely by vertically directed olivine dominated fractionation is not supported by available structural petrographic and geochemical data rather the sill can be viewed as a sheetlike composite intrusion possibly made up of multiple magma types common to the mesozoic eastern north america magmatic province the famous olivine zone of the palisades sill may have resulted from a separate late intrusion of olivine normative magma and not from gravity controlled olivine accumulation processes pyroxene dominated fractionation accounts for much of the vertical and lateral compositional variations in the palisades sill and other related intrusions from the province

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

patterns of nd and sr isotopic ratios produced by magmatic and post magmatic processes in the shiant isles main sill scotland the shiant isles main sill of the british tertiary igneous province is a classic example of a differentiated alkaline basic sill four separate intrusions each emplaced internally in rapid succession form a 165 m thick sill hosted by lower jurassic sedimentary rocks extensive nd and sr isotopic studies were conducted on samples from a vertical section through the sill where the relationships of samples to one another are well defined the results illuminate patterns of modification of isotopic ratios and clarify the petrogenesis magma sources crustal contamination magmatic processes bulk mixing interstitial liquid mixing and post magmatic alteration hydrothermal effects on sr and nd overall the whole rock initial 87sr 86sr ratios range from 0 7037 to 0 7061 while initial 143nd 144nd ratios vary from 0 51243 to 0 51286 nd 0 7 to 5 7 values that contrast markedly with those of the country rock acid leaching hcl of the whole rock samples that removes analcime indicates that most of the scatter in the 87sr 86sr is caused by the ubiquitous sub solidus aqueous alteration during which more radiogenic sr was introduced into the sill especially along the margins and also reveals magmatic isotopic ratios in contrast nd was immobile during fluid interaction so that the sill 143nd 144nd ratios were not affected even 1 m from the country rock contact using leached rock values 87sr 86sr and 143nd 144nd ratios are inversely correlated from magmatic processes magmas with two distinct isotopic compositions were involved a more primitive one with 143nd 144nd 0 51285 and 87sr 86sr 0 7035 that produced the first two intrusions and a more evolved one with 0 51252 and 0 7048 that produced the third intrusion mixing of the two magmas was very limited restricted to near contacts between units and apparently occurred by interstitial melt migration the more evolved crinanitic magma was probably produced from a batch of the more primitive picritic melt by a small degree of crustal contamination and crystal fractionation during a short crustal residence prior to ascent and emplacement

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

noritic dykes of southern west greenland early proterozoic boninitic magmatism two major swarms of early proterozoic ca 2 1 ga basic dykes occur within the archaean craton of southern west greenland one swarm comprises ophitic and sub ophitic tholeiitic dolerites while the other the bn dyke swarm constitutes mainly norites in which pyroxenes and olivine are enclosed by plagioclase oikocrysts the close geochemical similarity between a quenched norite and the coarser grained varieties indicates that the composition of the latter type has not been significantly modified by crystal accumulation the bn dykes are geochemically distinctive most having high mgo ca 16 cr and ni contents in conjunction with relatively high sio2 light rare earth ree and large ion lithophile lil element concentrations the texture mineral chemistry and petrochemistry of the quenched noritic dyke all bear strong resemblances to those features in modern boninitic lavas the bn dykes also correspond to proposed parental liquids of the bushveld complex and other major layered basic igneous intrusions the two dyke swarms are petrogenetically distinct the tholeiitic dolerites were derived from a relatively undepleted primordial mantle while the noritic dykes originated from a metasomatized harzburgitic source the wide spread distribution of similar proterozoic intrusions suggests crustal underplating by harzburgitic mantle on a world wide scale at this time

landscape response to climate change insights from experimental modeling and implications for tectonic versus climatic uplift of topography we present the results of an experimental investigation of the concurrent action of tectonic uplift and climate variation on relief evolution we designed an experimental apparatus that allows the study of erosion of laboratory scale topographies that evolve under given uplift and rainfall rates for constant uplift and rainfall rates the experimental topography evolves toward a statistical steady state defined by a mean elevation constant with time starting from such a steady state and keeping the input uplift rate constant a subsequent change in the rainfall rate yields a change in the mean elevation of the landscape to a new equilibrium elevation an increase in precipitation yields a lower mean steady state elevation whereas for a decrease in precipitation the surface is uplifted we define this phenomenon as a climatically induced surface uplift as opposed to a tectonically induced surface uplift the climatically and tectonically induced surface uplifts correspond to different dynamics of denudation so that it is theoretically possible to differentiate between the climatic or tectonic causes of surface uplift from records of output sediment fluxes

modulation of the erosion rate of an uplifting landscape by long term climate change an experimental investigation abstract whether or not climatic variations play a major role in setting the erosion rate of continental landscapes is a key factor in demonstrating the influence of climate on the tectonic evolution of mountain belts and understanding how clastic deposits preserved in sedimentary basins may record climatic variations here we investigate how a change in precipitation influences the erosional dynamics of laboratory scale landscapes that evolved under a combination of uplift and rainfall forcings we consider here the impact of a decrease in the precipitation rate of finite duration on the erosive response of a landscape forced by a constant uplift and initially at a steady state ss1 we performed several experiments with the same amplitude but different durations of precipitation decrease tp we observe that the decrease in precipitation induces a phase of surface uplift of landscapes to a new steady state condition ss2 however the details of the uplift histories timing rate differ between the experiments according to tp we also observe a decrease in the erosion rate induced by the precipitation change however the timing and amplitude of this decrease vary according to tp defining a delayed and damped erosion signal our data show that the landscape response to precipitation change is dictated by a critical water to rock ratio ratio of precipitation over uplift that likely corresponds to a geomorphic threshold our study suggests that variations in precipitation that occur at a geological time scale 106 years may have a weak impact on the erosion of landscapes and on the delivery of siliciclastic material to large rivers and sedimentary basins