hillslope storage boussinesq model for subsurface flow and variable source areas along complex hillslopes 2 intercomparison with a three dimensional richards equation model 1 the boussinesq equation for subsurface flow in an idealized sloping aquifer of unit width has recently been extended to hillslopes of arbitrary geometry by incorporating the hillslope width function w x into the governing equation where x is the flow distance along the length of the hillslope troch et al 2003 introduction of a source sink term n allows simulation of storm interstorm sequences in addition to drainage processes while a function sc x representing the maximum subsurface water storage can be used to account for surface saturation response in variable source areas activated by the saturation excess mechanism of runoff generation the model can thus simulate subsurface flow and storage dynamics for nonidealized more realistic hillslope configurations in this paper we assess the behavior of this relatively simple one dimensional model in a series of intercomparison tests with a fully three dimensional richards equation model special attention is given to the discretization and setup of the boundary and initial conditions for seven representative hillslopes of uniform convergent and divergent plan shape drainage and recharge experiments are conducted on these hillslopes for both gentle 5 and steep 30 bedrock slope angles the treatment and influence of the drainable porosity parameter are also considered and for the uniform idealized hillslope case the impact of the unsaturated zone is examined by running simulations for different capillary fringe heights in general terms the intercomparison results show that the hillslope storage boussinesq model is able to capture the broad shapes of the storage and outflow profiles for all of the hillslope configurations in specific terms agreement with the richards equation results varies according to the scenario being simulated the best matches in outflow hydrographs were obtained for the drainage experiments suggesting a greater influence of the unsaturated zone under recharge conditions due to transmission of water throughout the hillslope in the spatiotemporal water table response a better match was observed for convergent than divergent hillslopes and the bedrock slope angle was not found to greatly influence the quality of the agreement between the two models on the basis of the intercomparison experiments we make some suggestions for further development and testing of the hillslope storage model

zircon u pb age hf isotopes and geochemistry of shuichang algoma type banded iron formation north china craton constraints on the ore forming age and tectonic setting abstract the shuichang iron deposit is an algoma type bif in the north china craton the shuichang bif is interlayered with plagioclase hornblende gneiss indicating that these two lithologies are temporally closely associated we use the content of major elements and their ratios in order to define the protolith of the plagioclase hornblende gneiss sims u pb dating of zircons from the plagioclase hornblende gneiss suggests that the shuichang bif was formed at ca 2547 7 ma and was subjected to metamorphism at 2513 4 ma the major elements and ree data of the ore as well as individual magnetite and chert bands indicate that the shuichang bif precipitated from hydrothermal fluids discharging on the sea floor and that iron and silica may be associated with submarine volcanic activity in situ lu hf analysis of zircons from the plagioclase hornblende gneiss yielded initial 176 hf 177 hf ratios from 0 280903 to 0 281197 and e hf t values from 10 5 to 0 4 with a weighted mean value of 8 72 0 38 the single stage hf model age ranges from 2836 ma to 3249 ma we infer that the protolith of the plagioclase hornblende gneiss originated from the enriched mantle mixed with a small amount of subducted crust materials the shuichang bif formed at 2 5 ga and is coeval with other bifs in north china and in other cratons in the world we propose that the shuichang bif was formed in a subduction related back arc basin or intra arc basin based on the geochemical signatures of the wall rocks we further point out that the 2 5 ga global bif formation peak not only predates the amalgamation of the north china craton but probably also record an ancient supercontinent event

comparing a three dimensional and a dupuit forchheimer solution for a circular recharge area in a confined aquifer abstract an exact closed form analytic solution is presented for three dimensional steady state flow due to a source disc the solution is used to model a circular recharge area at the upper boundary of a confined aquifer streamlines and contour plots of equipotentials are compared with those obtained from a dupuit forchheimer solution the latter solution is approximate in that resistance to vertical flow is ignored as a further illustration a groundwater contamination problem is solved that of a leaching circular pond in a uniform flow field some streamlines and the shape of the downstream plume are plotted for both the three dimensional and the dupuit forchheimer solution it appears that when the recharge area is sufficiently large the dupuit forchheimer solution is quite adequate particularly when tracing streamlines the results therefore confirm the validity of the dupuit forchheimer assumption when modeling regional flow even under circumstances of areal recharge however when the size of the recharge area is in the order of the aquifer thickness a truly three dimensional solution is needed

is the dupuit assumption suitable for predicting the groundwater seepage area in hillslopes many physically based hydrological hydrogeological models used for predicting groundwater seepage areas including topography based index models such as topmodel rely on the dupuit assumption to ensure the sound use of these simplified models knowledge of the conditions under which they provide a reasonable approximation is critical in this study a dupuit solution for the seepage length in hillslope cross sections is tested against a full depth solution of saturated groundwater flow in homogeneous hillslopes with horizontal impervious base and constant slope topography the comparison reveals that the validity of the dupuit solution depends not only on the ratio of depth to hillslope length d l as might be expected but also on the ratio of hydraulic conductivity to recharge k r and on the topographic slope s the validity of the dupuit solution is shown to be in fact a unique function of another ratio the ratio of depth to seepage length d ls for d ls 0 2 it increases dramatically in practice this criterion can be used to test the validity of dupuit solutions when d ls increases beyond that cutoff the ratio of seepage length to hillslope length ls l given by the full depth solution tends toward a nonzero asymptotic value this asymptotic value is shown to be controlled by and in many cases equal to the parameter r sk generalization of the findings to cases featuring heterogeneity nonhorizontal impervious base and variable slope topography is discussed

geochronological and geochemical constraints on the origin of clastic meta sedimentary rocks associated with the yuanjiacun bif from the luliang complex north china abstract the luliang complex is situated in the central part of the western margin of the trans north china orogen tnco in the north china craton ncc and consists of metamorphic volcanic and sedimentary rocks and granitoid intrusions the yuanjiacun formation metasediments occupy roughly the lowest part of the luliang group and are mainly represented by well bedded meta pelites chlorite schists and sericite chlorite phyllites banded iron formations bifs and meta arenites sericite schists which have undergone greenschist facies metamorphism the youngest group of detrital zircons from the meta arenite samples constrains their maximum depositional age at 2350 ma in combination with previous geochronological studies on meta volcanic rocks in the overlying jinzhouyu formation the depositional age of the yuanjiacun formation can be constrained between 2350 and 2215 ma the metasediments have suffered varying degrees of source weathering measured using widely employed weathering indices e g cia ciw pia and th u ratios source rocks of the low al meta pelites have undergone severe chemical weathering whereas those of the meta arenites and high al meta pelites have suffered relatively moderate chemical weathering significant secondary k addition is recognized in the a cn k diagram for most of the studied samples diagnostic geochemical features like the al 2 o 3 tio 2 values trace element ratios e g th sc and ree patterns suggest that the meta arenites and high al meta pelites are predominantly derived from felsic igneous sources whereas the low al meta pelites are sourced mainly from mafic rocks coupled with nd isotopic data it is proposed that the meta arenites and high al meta pelites were sedimentary erosion products of the less differential felsic terrain likely the old upper continental crust the low al meta pelites however have geochemical affinities with both pelite and bif like components suggesting that they were mixtures of these two sediments moreover the u pb ages of detrital zircons from the yuanjiacun metasediments yield three age populations of 2 8 2 65 ga 2 6 2 5 ga and 2 5 2 35 ga the dominant archean detrital zircons were most likely sourced from both the archean igneous rocks in the hengshan wutai fuping belt and old hidden archean basement rocks in the western block whereas the subordinate paleoproterozoic detrital zircons were probably derived from the granitoid plutons within this age range in the luliang complex taking the geochemical and geochronological data together the yuanjiacun formation is interpreted as having been deposited in a passive margin setting likely the eastern margin of the western block most probably on a stable continental shelf

mineral prospectivity analysis for bif iron deposits a case study in the anshan benxi area liaoning province north east china abstract banded iron formation bif deposits are the most important source of iron in china and they are dominantly distributed in the north china craton ncc the anshan benxi area is situated in the northeastern part of the ncc and is considered to be the most fertile and productive iron metallogenic province in china the recent successive discoveries of large bif deposits in this area especially the world class dataigou bif deposit indicates that there is great potential for iron resources in this area the bif deposits are well defined by magnetic and gravity anomalies and thereby extensive petrophysical and geophysical studies are critical for bif prospective mapping a data driven weights of evidence wofe analysis was used for bif prospectivity modeling the results indicated that prospective areas are generally confined to the precambrian greenstone belt basin and high prospectivity zones class a are mainly distributed in the middle and northeastern parts of the study area moderate prospectivity zones class b are mainly distributed in southwestern and northeastern parts of the study area finally we employed selective human intervention to redefine priority targets to be more reliable the results suggest the method is successful as 82 target areas were narrowed to class a 20 and 11 class b targets the total of class a and class b targets was just over one third of the total number of targets offering a much smaller but effective selection of targets for consideration by mineral explorers

reducing subjectivity in multi commodity mineral prospectivity analyses modelling the west kimberley australia predicting realistic targets in underexplored regions proves a challenge for mineral explorers knowledge driven prospectivity techniques assist in target prediction and can significantly reduce the geographic search space to a few locations the mineral prospectivity of the underexplored west kimberley region was investigated following interpretation of regional gravity and magnetic data emphasis was placed on identifying geological structures that may have importance for the mineral prospectivity of the region subsurface structure was constrained through combined gravity and magnetic modelling along three transects crustal scale structures were interpreted and investigated to determine their depth extent these interpretations and models were linked to tectonic events and mineralization episodes in order to map the distribution of minerally prospective regions using a knowledge driven mineral systems approach a suite of evidence layers was created to represent geological components that led to mineralization and then applied to each mineral system where appropriate this approach was taken to provide a more objective basis for prospectivity modelling the mineral systems considered were 1 magmatic ni sulphide 2 carbonate hosted base metals 3 orogenic au 4 stratiform hosted base metals and 5 intrusion related base metals including sn w fe oxide cu au and cu au porphyry deposits these analyses suggest that a geologically complex belt in the kimberley basin at the boundary to the king leopold orogen is prospective for magmatic related hydrothermal mineral systems including ni au and cu the lennard shelf is prospective for carbonate hosted base metals around a feature known as the 67 mile high and parts of the king leopold orogen are prospective for stratiform hosted base metals these results show that knowledge driven mineral system modelling is effective in identifying prospectivity in regional scale studies of underexplored areas as well as drastically reducing the search space for explorers working in the west kimberley

hillslope runoff processes and models abstract hillslope hydrology is concerned with the partition of precipitation as it passes through the vegetation and soil between overland flow and subsurface flow flow follows routes which attenuate and delay the flow to different extents so that a knowledge of the relevant mechanisms is important in the 1960s and 1970s hillslope hydrology developed as a distinct topic through the application of new field observations to develop a generation of physically based forecasting models in its short history theory has continually been overturned by field observation thus the current tendency particularly among temperate zone hydrologists to dismiss all hortonian overland flow as a myth is now being corrected by a number of significant field studies which reveal the great range in both climatic and hillslope conditions some recent models have generally attempted to simplify the processes acting for example including only vertical unsaturated flow and lateral saturated flows others explicitly forecast partial or contributing areas with hindsight the most complete and distributed models have generally shown little forecasting advantage over simpler approaches perhaps trending towards reliable models which can run on desk top microcomputers the variety now being recognised in hillslope hydrological responses should also lead to models which take account of more complex interactions even if initially with a less secure physical and mathematical basis than the richards equation in particular there is a need to respond to the variety of climatic responses and to spatial variability on and beneath the surface including the role of seepage macropores and pipes which call into question whether the hillside can be treated as a darcian flow system

formation mechanism of gongchangling high grade magnetite deposit hosted in archean bif anshan benxi area northeastern china abstract banded iron formations are main resources of global iron ore in which high grade ore is mainly composed of martite goethite and hematite they are also the major resource of iron ore in china mainly distributing in liaoning and hebei province in china the iron ore with fe greater than 50 is classified as high grade iron ore the high grade iron ore mainly consists of magnetite and displays its unique characteristics gongchangling iron deposit is one typical bif iron deposit which contains 150 mt of high grade iron ore in china the high grade magnetite ore bodies mainly occur around magnetite quartzite faults and the cores of folds and show positive relation to the development of the altered rocks in this deposit this research shows that high grade magnetite comes from magnetite quartzite and they are both formed with little or no addition of aluminum containing detrital material by marine chemical deposition in reduced environment and they are closely related to seafloor hydrothermal activity muddy silty rocks are original rocks of altered rocks of which the primitive mantle normalized ree pattern except eu is consistent with that of iron ore reflecting that their formation is related to the formation of high grade magnetite ore therefore the formation mechanism of high grade iron ore is proposed as following the regional metamorphism provides storage space for the formation of high grade magnetite ore and required temperature and pressure conditions for the mineral transformation the regional metamorphic hydrothermal fluid leaches feo out of magnetite quartzite when it passes by and the feo that leached out moves near faults or cores of folds together with the metamorphic hydrothermal fluid and aluminum containing rocks of which the original rocks are muddy silty in the formation of high grade iron ore aluminum containing rock appears in the intervals of sedimentation of iron containing rock series and consumes the silicon leached out of magnetite quartzite and forms garnet chlorite and biotite

types and general characteristics of the bif related iron deposits in china abstract bifs banded iron formations in china which account for approximately 64 of the total identified resources in the country belong to two categories a predominant neoarchean algoma type and a few paleoproterozoic superior type a minor group of bifs developed in the neoproterozoic is also representing having possible correlation with the snowball earth scenario spatially the bifs in china are mainly distributed in the north china craton they experienced intense metamorphism and deformation which led to the majority of iron oxides changing into coarse grained magnetites though their ores generally contain only 30 tfe the bifs are potential targets of industrial exploitation by magnetic beneficiation on the basis of ore forming age host formations mineral assemblages and grade of the ores the bif related iron deposits in china are classified into seven types 1 anshan type neoarchean volcanic sedimentary formations with magnetite as the main ore mineral 20 tfe

groundwater supported evapotranspiration within glaciated watersheds under conditions of climate change this paper analyzes the effects of geology and geomorphology on surface water groundwater interactions evapotranspiration and recharge under conditions of long term climatic change our analysis uses hydrologic data from the glaciated crow wing watershed in central minnesota usa combined with a hydrologic model of transient coupled unsaturated saturated flow hydrat2d analysis of historical water table 1970 1993 and lake level 1924 2002 records indicates that larger amplitude and longer period fluctuations occur within the upland portions of watersheds due to the response of the aquifer system to relatively short term climatic fluctuations under drought conditions lake and water table levels fell by as much as 2 4 m in the uplands but by 1 m in the lowlands the same pattern can be seen on millennial time scales analysis of holocene lake core records indicates that moody lake located near the outlet of the crow wing watershed fell by as much as 4 m between about 4400 and 7000 yr bp during the same time water levels in lake mina located near the upland watershed divide fell by about 15 m reconstructed holocene climate as represented by hydrat2d gives somewhat larger drops 6 and 24 m for moody lake and lake mina respectively the discrepancy is probably due to the effect of three dimensional flow a sensitivity analysis was also carried out to study how aquifer hydraulic conductivity and land surface topography can influence water table fluctuations wetlands formation and evapotranspiration the models were run by recycling a wet year 1985 87 cm annual precipitation over a 10 year period followed by 20 years of drier and warmer climate 1976 38 cm precipitation model results indicated that groundwater supported evapotranspiration accounted for as much as 12 10 cm of evapotranspiration the aquifers of highest hydraulic conductivity had the least amount of groundwater supported evapotranspiration owing to a deep water table recharge was even more sensitive to aquifer hydraulic conductivity especially in the lowland regions these findings have important implications for paleoclimatic studies because the hydrologic response of a surface water body will vary across the watershed to a given climate signal

formation age and tectonic setting of the shirengou neoarchean banded iron deposit in eastern hebei province constraints from geochemistry and sims zircon u pb dating abstract the shirengou iron deposit a typical bif deposit located in eastern hebei province in the north china craton is hosted in neoarchean metamorphic rocks the metamorphic rocks include hornblende plagiogneiss plagioclase amphibolite and magnetite quartzite we use contents and ratios of major elements for protolith reconstruction showing that protoliths of the hornblende plagiogneiss and plagioclase amphibolite should be dacite and basaltic andesite respectively based on geology and geochemistry of the hosting rocks we infer that the shirengou bif is an algoma type deposit formed in an arc related basin in the neoarchean constrains from sims u pb dating and oxygen isotopic analysis of zircon from wall rocks and interlayer of the shirengou iron bodies can be summarized as follow 1 igneous zircons from two samples of amphibolite and gneiss yield u pb ages of 2541 21 ma and 2553 31 ma respectively providing the age of the neoarchean protolith because the shirengou bif is an algoma type deposit determined u pb age 2541 2553 ma for zircons from hosting volcanic rocks can constrain the bif forming time 2 metamorphic overgrowth zircons from amphibolite and gneiss give two u pb ages of 2512 13 ma and 2510 21 ma respectively corresponding to the neoarchean matemorphism 3 the zircons with 18o values of 5 9 7 6 also indicate magmatic origin the zircons have 18o values of 6 8 9 9 suggesting that exotic high 18o fluids were involved during metamorphism based on the geology and geochemistry of bif and combing with zircon age and oxygen isotopic systematics we suggest that the shirengou iron deposit was formed in a submarine volcanic setting related to subduction of an oceanic slab

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

the archaean and early proterozoic banded iron formations of north china their characteristics geotectonic relations chemistry and implications for crustal growth abstract bif occurs extensively throughout the archaean and early proterozoic terrains of northern china there are four main iron metallogenic provinces anshan in n e china e hebei and wutai in n china and qingling dengfeng taihua on the south margin of the north china platform all bifs have undergone varying degrees of regional metamorphism from subgreenschist to granulite facies and deformation the chinese bifs have three marked characteristics 1 most bif are archaean in age their formation reaching a peak during the late archaean 2 bif in china occur in both high grade regions and greenstone belts the former are more significant their reserves accounting for more than three quarters of those of the precambrian bif in china 3 oxide iron formation is predominant in nearly all archaean bif silicate and carbonate iron formations are rare the few carbonate iron formations occur in lower proterozoic sediments there is no difference in chemical composition between bif in high grade regions and in greenstone belts however there are geochemical and petrological differences between their associated supracrustal sequences the presence of such large amounts of bif within high grade regions is unusual from their regional geology rock association and from the geochemistry of associated metavolcanics and orthogneisses we suggest that the high grade type of bif formed in arcs or back arc basins which were intruded by voluminous tonalites and granodiorites later converted by deformation and high grade metamorphism into orthogneisses such processes of accretion and crustal growth are different in kind but not in principle from those operating today

a continent wide study of australia s uranium potential part i gis assisted manual prospectivity analysis abstract this paper describes the approach to and outcomes of a manual analysis i e a cognitive assessment of spatial and non spatial data of the uranium potential of 90 geological regions in australia for this analysis the 14 principal uranium deposit types recognized by the international atomic energy agency were grouped in six uranium systems models i e surficial sedimentary igneous related metamorphic metasomatic unconformity related and vein related uranium systems on the basis of similar genetic processes environments of ore formation and ingredients mappable at the regional to continent scale the newly proposed uranium systems models are structured according to the mineral systems approach and focus on the critical mineralization processes that must occur for a uranium deposit to form in a particular region our manual prospectivity analysis employed this approach to assess the probability of the critical genetic processes having occurred in each geological region in this semi quantitative probabilistic evaluation technical quality and opportunity ranking schemes were used to rank each geological region based on the probability of occurrence of and potential for high quality uranium deposits and opportunity for securing prospective ground based on this assessment the geological regions with the greatest potential for discovery of potentially recoverable uranium resources are the ashburton broken hill litchfield mcarthur money shoal murphy paterson pine creek and northeast tasmania regions i e quality ranking of 10 0 the gawler and polda regions i e 9 0 and the amadeus georgetown stuart tanami regions i e 8 1 most of these regions contain known unconformity related or sandstone hosted uranium deposits although some of them are pure conceptual plays that have received relatively little attention in terms of uranium exploration maps based on the numerical output of the prospectivity analysis helped to inform area selection decisions and detailed follow up studies and focus time and resources the template developed in this study can easily be modified to suit prospectivity analyses for other metals or a similar investigation in another country as illustrated in part ii the best possible approach to a complex continent wide prospectivity analysis is to harness the strengths of both manual and automated i e sophisticated computational techniques applied to spatial data approaches as these methodologies essentially address each other s limitations

simulation of the seepage face limitations of a one dimensional approach abstract an approximate method for determining the length of seepage faces is presented based on the dupuit forchheimer approximations analytical solutions based on the method are developed for three idealized flow systems it is demonstrated that the d f based solutions represent the entire flow system inclusive of the seepage face and that incorporation of the d f assumptions implies a uniform discharge along the seepage face proportional to the slope of the land surface and the saturated hydraulic conductivity seepage face lengths predicted by the d f based solutions are compared to results from a two dimensional finite element model to establish error bounds under various boundary conditions it was found that the error in prediction of seepage face length increases as either the land surface slope or the thickness to length ratio of the flow system increases additionally discrepancies between the models were found to increase with increasing infiltration rates the effects of radial flow were not found to significantly affect the d f based solution considering the inadequate data base available to characterize most real world watersheds numerical studies may not be warranted and the d f based solution of seepage face length is a reasonable approximation

drainage of recharge to symmetrically located downstream boundaries with special reference to seepage faces summary the seepage face is an important feature of the drainage process when recharge occurs to a permeable region with lateral outlets examples of the formation of a seepage face above the downstream water level include agricultural land drained by open ditches and the ballast beneath railway tracks the first part of the paper considers vertical section two dimensional analytical and numerical solutions for recharge to permeable regions above horizontal impermeable bases having two symmetrically positioned downstream boundaries with zero or specified external depths of water for vertical downstream boundaries with seepage faces above the downstream water level empirical relationships are developed relating the seepage face height on the downstream boundary to the downstream discharge the same problems are also analysed using the one dimensional dupuit forchheimer approximation improved estimates of the entire water table profile are achieved when the downstream boundary head is specified as a function of the downstream discharge using relationships derived from the two dimensional solutions instead of the customary assumption that the water table is drawn down to the ditch water level finally time variant simulations based on the dupuit forchheimer approximation are introduced for a single layer and for a system of two horizontal layers having different hydraulic conductivities

gold predictivity mapping in french guiana using an expert guided data driven approach based on a regional scale gis the realistic estimation of gold mining in french guiana requires including the numerous illegal gold washing activities in predictivity mapping the combination of a classical approach based on the algebraic method of knox robinson and groves with innovative processing grid type geochemical and radiometric data as well as cluster analysis technique provides a better understanding of the structure of studied mineralized areas

ree geochemistry of carbonates from the guanmenshan formation liaohe group ne sino korean craton implications for seawater compositional change during the great oxidation event abstract the worldwide 2 33 2 06 ga positive 13 c carb excursion has been correlated with the great oxidation event goe and termed as the lomagundi event the 2 3 1 85 ga guanmenshan formation in the liaohe group of the northeastern sino korean craton is characterized by positive 13 c carb excursion and is a potential candidate to evaluate the lomagundi event using rey rare earth element and yttrium ree y chemical fingerprints here we present major and trace element analysis of 42 samples from the guanmenshan formation which are pure marine chemical sediments and use the data to trace the seawater composition during 2 3 1 85 ga 15 least altered dolomicrite samples 600 m strata have ree values of 0 739 4 175 ppm 2 414 1 184 ppm and the y ho ratios of 34 5 56 6 44 1 5 7 they show uniform positive la sn la sn 1 04 0 27 and gd sn gd sn 1 64 0 40 anomalies and notable lree depletions indicated by nd sn yb sn values of 0 24 0 92 average 0 56 0 19 these features are consistent with the geochemistry of well oxygenated shallow ambient seawater and suggest that these samples provide a robust record of the primary rey signature of seawater during the lomagundi event the rey patterns of 15 silicified dolomites marbles locally with veinlets from the pb zn mining camps in the region with average eu cn eu cn 1 56 0 95 are identical to those of high temperature hydrothermal fluids 250 c characterized by a flat pattern and marked positive eu anomalies indicating that these rocks were subjected to metasomatism by hydrothermal fluids the guanmenshan formation shows average ce sn ce sn of 0 93 0 09 and sm cn yb cn of 1 which are higher than those of the archean 2 33 ga chemical sediments generally f o 2 and p co 2 the rey in the dolomicrite were mainly sourced from fluxes of solutes from terrestrial weathering and also from seafloor hydrothermal processes on a subordinate scale the guanmenshan dolomicrites have eu sn eu sn values of 1 34 2 55 i e around 1 53 indicating that they were deposited during 2 33 2 06 ga as the eu sn eu sn 1 53 can be used as a proxy for the 2 33 2 06 ga marine chemical sediments our study shows that the guanmenshan formation was formed at a critical turning point in earth history when the global atmosphere hydrosphere system witnessed a dramatic change from reducing to oxidizing conditions

fault growth in brittle ductile experiments and the mechanics of continental collisions using laboratory models of continental collisions we study the mechanisms responsible for large scale deformation and the nature of the penetrative deformation localized or homogeneous and its characteristic scales and structure in order to focus the study one of the most spectacular cases of continental collisions namely the india asia collision is considered different models with varying rheologies are analyzed which attempt to respect the brittle ductile stratification corresponding to the crust and mantle structure in the earth each experiment is quantified by studying the strain field and the fault pattern as a function of the position p of the indenter within the system the strain field is characterized by 1 the second invariant of the two dimensional strain tensor and its variation with position for a fixed p the evolution of its average as a function of p 2 the evolution with p of the participation ratio s2 which quantifies the fraction of the system surface area which participates in the deformation 3 the evolution of the escape ratio calculated as the surface increase gained by the system during its eastward lateral escape divided by the surface covered by the indenter penetration qualitatively we find that the deformation first spreads out but later on undergoes localization once the fault pattern is mature the cumulative fault length does not increase anymore we observe that the strain field is essentially controlled by the kinematics of the larger faults each fault pattern is quantified by studying 1 the histograms of fault orientations 2 the capacity and barycenter fractal dimensions 3 the multifractal generalized dimensions and 4 the distribution of fault lengths the fractal dimension d is found to almost constant within experimental uncertainty d 1 7 0 1 and thus appears rather insensitive to the particular chosen rheology we find a correlation between the generalized multifractal dimensions and two exponents the barycenter fractal dimension b and the exponent a of the fault length distribution this shows that the scaling properties of fault patterns can be characterized by the knowledge of only two exponents b and a of the spatial and length distributions of faults our main results are 1 observation of the growth of self similar fault patterns 2 complete characterization of the fault patterns with two scaling exponents a and b 3 wide distribution of undeteriorated domains and large heterogeneity of the deformation field 4 maturation of the fault structure corresponding to a localization of the deformation on a few large faults in the late stage of the deformation

basement controlled faulting of paleozoic strata in southern ontario canada new evidence from geophysical lineament mapping abstract basement fault reactivation is now recognized as an important control on sedimentation and fault propagation in intracratonic basins in southern ontario the basement consists of complexly structured mid proterozoic ca 1 2 ga crystalline rocks and metasedimentary rocks that are overlain by up to 1500 m of paleozoic sedimentary strata reactivation of basement structures is suspected to control the location of paleozoic fault and fracture systems but evaluation has been hindered by a limited understanding of the regional structural characteristics of the buried basement new aeromagnetic and gravimetric lineament mapping presented in this paper better resolves the location of basement discontinuities and provides further evidence for basement controls on the distribution of paleozoic fault and fracture systems lineament mapping was facilitated by reprocessing and digital image enhancement micro levelling regional residual separation derivative filtering of existing regional gravity and aeromagnetic datasets reprocessed images identify new details of the structural fabric of the basement below southern ontario and delineate several previously unrecognized aeromagnetic and gravity lineaments and linear zones linear zones parallel the projected trends of mid proterozoic terrane boundaries identified by field mapping on the exposed shield to the north of the study area and are interpreted as zones of shearing and basement faulting mapped aeromagnetic and gravity lineaments show similar trends to paleozoic faults and fracture networks and broad zones of seismicity in southern ontario these new data support an inheritance model for paleozoic faulting involving repeated reactivation and upward propagation of basement faults and fractures into overlying cover strata

saturated area formation on nonconvergent hillslope topography with shallow soils a numerical investigation prediction of saturated area formation is important for hydrologic modeling of watersheds with shallow highly pervious soils this simulation study examines the relative importance of hillslope properties and rainfall rate on the evolution of saturated source areas during wetting the study focuses on homogeneous nonconvergent hillslope topography with constant slope the two dimensional variably saturated groundwater model vs2d healy 1990 lappala et al 1993 is used to simulate saturated area formation under the action of steady uniform rainfall the study methodology systematically varies four hillslope properties depth to impervious layer slope length slope angle and average saturated hydraulic conductivity and the rainfall rate results indicate that the fraction of the hillslope length that is saturated at equilibrium is a function of a parameter which is defined as the rainfall rate multiplied by the slope length divided by the slope angle soil thickness and saturated hydraulic conductivity the temporal evolution of saturated area is analyzed in terms of the equilibrium time nonlinearity in the unsaturated zone and differences in hillslope properties result in a nonunique time to equilibrium relation the time to equilibrium is maximum when factors that tend to cause surface saturation are in approximate balance with those that tend to dissipate surface saturation the temporal evolution of surface saturation during wetting follows a wide variety of trajectories equivalent hillslope soil water storage ratios on slopes with different properties can result in a wide range of surface saturation conditions hillslopes with shallower soils and smaller slope angles are most susceptible to saturated area formation however sudden changes in surface saturation are possible on steep hillslopes when 1 6

saturation regions in catchments and their relations to soil and topographic properties abstract criteria are derived for the existence of saturated areas on draining hillslopes in natural catchments these are given in terms of soil transmissivity the hillslope gradient and its wetness state characterised by the baseflow discharge from the catchment analyses are made for idealised hillslopes with uniform soils to obtain expression for the width of the saturated zone the size of saturated zones on undulating hillslopes with gradiational soils is shown to depend strongly on topographic convergence or divergence the effects of soil transmissivity hillslope geometry and drainage flux density are lumped into a dimensionless parameter whose value determines the location of seepage boundaries on slopes of specified geometry scaling relations for seepage area in catchments of different size are shown to be generally nonlinear and depend primarily on local hillslope gradients relative predictions of seepage zone size are confirmed by observed hydrographs from three catchments with different physical characteristics

rare earth element variations in mid archean banded iron formations implications for the chemistry of ocean and continent and plate tectonics abstract abundances of major and rare earth elements rees are reported for mid archean 3 3 3 2 gyr sedimentary rocks including banded iron formations bifs and ferruginous siliceous mudstone from the cleaverville area in the pilbara craton western australia geological lithological and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments ranging from that typical of mid oceanic spreading centers to convergent plate boundary settings a range of environments most likely caused by plate movements except for the mudstone the ree content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons europium anomalies decrease up section eu eu values normalized to nasc change from 3 5 to 1 1 as the ree contents and lree hree ratios increase the striking similarity in these ree signatures of bifs and modern hydrothermal sediments leads us to propose that the bifs were in situ hydrothermal precipitates near a mid ocean ridge significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon the observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics dominated by horizontal movement was already operating in the mid archean distal hydrothermal sediments without a eu anomaly when normalized to nasc suggest that mid archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no eu anomaly i e it had a negative eu anomaly when normalized to chondrite in addition to an absence of a eu anomaly mid archean seawater did not have a ce anomaly suggesting less oxic conditions in the mid archean than in the modern ocean

zircon ages and geochemistry of late neoarchean syenogranites in the north china craton a review abstract at the end of the neoarchean continental evolution voluminous syenogranites were emplaced in the north china craton together with other magmatic rocks trondhjemite tonalite granodiorite ttg monzogranite diorite gabbro syenogranites are widely distributed in anshan benxi qinhuangdao and western shandong and also occur in southern jilin northern liaoning northwestern hebei and central henan based on geological relationships degree of metamorphism deformation and magmatic zircon ages two phases of syenogranite magmatism are recognized rocks produced during the first phase show a gneissic texture and were formed between 2 53 and 2 52 ga and locally comprise abundant ttg rocks of the second phase cut late neoarchean ttg and supracrustal rocks display a massive structure and mainly formed between 2 52 and 2 50 ga all syenogranites share the same features in major element compositions being high in sio2 and low in cao total feo mgo tio2 and p2o5 however they are different in trace and ree compositions and can be subdivided into three types 1 type 1 shows a large variation in total ree contents low la yb n ratios strong negative eu eu anomalies and ba depletion 2 type 2 is similar to type 1 but has higher la yb n ratios 3 type 3 shows a large variation in total ree and la yb n ratios and significantly do not show strongly negative eu eu anomalies and ba depletion whole rock sm nd isotopic compositions show large variations in nd t values and tdm nd modal ages ranging from 9 49 to 4 72 and 3 70 to 3 25 ga type 1 0 55 1 03 and 2 77 2 71 ga type 2 and 2 35 to 1 23 and 2 93 2 66 ga type 3 respectively hf isotopic compositions of zircons from three samples have hf t values and tdm1 hf ages of 0 7 7 2 and 2 84 2 56 ga type 1 2 6 7 4 and 2 74 2 56 ga type 2 and 2 1 6 3 and 2 76 2 60 ga type 3 it is concluded that syenogranites were generated by melting of continental crust with different mean crustal residence ages and most of them were emplaced during the second phase 2 52 2 50 ga in an extensional tectonic regime the formation of these voluminous syenogranites marks a tectono magmatic event resulting in stabilization of the north china craton at the end of the neoarchean

mapping of prospectivity and estimation of number of undiscovered prospects for lode gold southwestern ashanti belt ghana in the southwestern part of the ashanti belt the results of fractal and fry analyses of the spatial pattern of 51 known mines prospects of mostly lode gold deposits and the results of analysis of their spatial associations with faults and fault intersections suggest different predominant structural controls on lode gold mineralisation at local and district scales intersections of nne and nw trending faults were likely predominantly involved in local scale structural controls on lode gold mineralisation whilst nne trending faults were likely predominantly involved in district scale structural controls on lode gold mineralisation the results of the spatial analyses facilitate the conceptualisation and selection of spatial evidence layers for lode gold prospectivity mapping in the study area the applications of the derived map of lode gold prospectivity and a map of radial density of spatially coherent lode gold mines prospects results in a one level prediction of 37 undiscovered lode gold prospects the applications of quantified radial density fractal dimensions of the spatial pattern of spatially coherent lode gold mines prospects result in an estimate of 40 undiscovered lode gold prospects the study concludes finally that analysis of the spatial pattern of discovered mineral deposits is the key to a strong link between mineral prospectivity mapping and assessment of undiscovered mineral deposits

sm nd isotope study of early archean rocks qianan hebei province china a sm nd study of mafic amphibolite which occurs as lensoid masses in migmatitic quartz dioriticto granitic gneiss was undertaken in order to determine if it has an early archean age six amphibolite samples define an isochron yielding an age of 3 50 0 08 b y with an initial nd value of 3 3 0 3 three samples of quartz dioritic gneiss lie close to this isochron and have sm nd model ages of ca 3 5 b y referenced to a epleted mantle source a sample of k feldspar rich granitic gneiss gives a younger model age of about 2 9 b y the non zero initial nd value of the mafic amphibolites can be compared with chondritic meteorite samples measured in this laboratory and is therefore precisely determined if the initial nd value can be regarded as representative of the original igneous rocks then it indicates that the mantle magma source was severely depleted with respect to magmaphile elements this suggests that the geochemistry of the early archean mantle may have been similar to that of the modern upper mantle and may require that a large pre archean continental crust have been present on the earth

hydrothermal plumes and the delivery of iron to banded iron formation most of the iron in banded iron formation bif was deposited during the period 2 7 1 8 ga within this period fe deposition rates were at least as high as 10 12 gm y according to new age models for the largest bifs peak deposition occurred between 2 4 2 3 ga it is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment estimates of global heat loss for the late archean early proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present so hydrothermal water rock exchanges were a more important source of fe to the oceans than today hydro thermal plumes depleted in o 2 and enriched in fe were generated at mid ocean ridge crests with substantially shallower depths than present assuming hydrothermal effluent dilution factors of 10 4 plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to

metallogeny of the north china craton link with secular changes in the evolving earth abstract the north china craton ncc has experienced a complex geological evolution since the early precambrian and carries important records of secular changes in tectonics and metallogeny here we synthesize the salient geological and tectonic features of the evolution and destruction of the ncc vis a vis major metallogenic events and the formation of potential ore deposits we identify a close relationship between the major geological events in the ncc and those reported elsewhere on the globe we trace the records of a regular change in the pattern of metallogeny mineral deposit character spatial distribution and genetic mechanisms which closely match the timing and styles of the major geological and tectonic events in this craton the ncc went through five major tectonic cycles 1 neoarchean crustal growth and stabilization 2 paleoproterozoic rifting subduction accretion collision with imprints of the great oxidation event goe 3 late paleoproterozoic neoproterozoic multi stage rifting 4 paleozoic orogenesis at the margins of the craton and 5 mesozoic extensional tectonics associated with lithospheric thinning and decratonization coinciding with these major geological events are five major metallogenic systems identified as follows 1 an archean bif system 2 paleoproterozoic cu pb zn and mg b systems 3 a mesoproterozoic ree fe pb zn system 4 a paleozoic orogenic cu mo system and 5 mesozoic intracontinental au and ag pb zn and mo systems the ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities from early precambrian through late precambrian to paleozoic and mesozoic the ncc records a transition from primitive to modern style plate tectonics evidence for imbricated oceanic plate stratigraphy in a subduction accretion setting and collisional orogenesis along at least three major zones of ocean closure are documented major transitions in tectonic style and surface environmental changes recorded in other parts of the world are also reflected in the geological history and metallogenic events in the ncc large scale gold deposits formed through intraplate tectonics during the mesozoic provide important insights into mantle dynamics and crust mantle interaction associated with lithospheric thinning and craton destruction the ncc provides one of the best examples for documenting secular changes in the geological history and metallogenic epochs of an evolving earth

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

hillslope storage boussinesq model for subsurface flow and variable source areas along complex hillslopes 1 formulation and characteristic response 1 hillslope response to rainfall remains one of the central problems of catchment hydrology flow processes in a one dimensional sloping aquifer can be described by boussinesq s hydraulic groundwater theory most hillslopes however have complex three dimensional shapes that are characterized by their plan shape profile curvature of surface and bedrock and the soil depth field studies and numerical simulation have shown that these attributes are the most significant topographic controls on subsurface flow and saturation along hillslopes in this paper the boussinesq equation is reformulated in terms of soil water storage rather than water table height the continuity and darcy equations formulated in terms of storage along the hillslope lead to the hillslope storage boussinesq hsb equation for subsurface flow solutions of the hsb equation account explicitly for plan shape of the hillslope by introducing the hillslope width function and for profile curvature through the bedrock slope angle and the hillslope soil depth function we investigate the behavior of the hsb model for different hillslope types uniform convergent and divergent and different slope angles under free drainage conditions after partial initial saturation drainage scenario and under constant rainfall recharge conditions recharge scenario the hsb equation is solved by means of numerical integration of the partial differential equation we find that convergent hillslopes drain much more slowly compared to divergent hillslopes the accumulation of moisture storage near the outlet of convergent hillslopes results in bell shaped hydrographs in contrast the fast draining divergent hillslopes produce highly peaked hydrographs in order to investigate the relative importance of the different terms in the hsb equation several simplified nonlinear and linearized versions are derived for instance by recognizing that the width function of a hillslope generally shows smooth transition along the flow direction or by introducing a fitting parameter to account for average storage along the hillslope the dynamic response of these reduced versions of the hsb equation under free drainage conditions depend strongly on hillslope shape and bedrock slope angle for flat slopes of the order of 5 only the simplified nonlinear hsb equation is able to capture the dynamics of subsurface flow along complex hillslopes in contrast for steep slopes of the order of 30 we see that all the reduced versions show very similar results compared to the full version it can be concluded that the complex derivative terms of width with respect to flow distance play a less dominant role with increasing slope angle comparison with the hillslope storage kinematic wave model of troch et al 2002 shows that the diffusive drainage terms of the hsb model become less important for the fast draining divergent hillslopes these results have important implications for the use of simplified versions of the hsb equation in landscapes and for the development of appropriate analytical solutions for subsurface flow along complex hillslopes

improvement of the hillslope storage boussinesq model by considering lateral flow in the unsaturated zone unsaturated flow is an important factor that affects groundwater motion among various drainage models the nonlinear hillslope storage boussinesq hsb model has been commonly used to predict water flux along a slope in this study we improved this model by considering lateral flow in the unsaturated zone using modified van genuchten functions we analytically expressed the concept of equivalent propagation thickness in the vadose zone this analytical expression was then incorporated into the hsb model to reflect two different stages of the drainage process and to simulate the hillslope drainage process more accurately the model results indicated that lateral flow has significant effects in the unsaturated zone during the hillslope drainage process even in sandy aquifers the amount of water contributed by the unsaturated zone is a key factor that enables a decrease in the water table during the middle and late stages of the process a comparison between the measured and simulated results based on both convergent type and divergent type hillslope drainage processes revealed that the thickness of the saturated zone decreases as the unsaturated flow increases this study emphasizes the necessity of considering unsaturated flow in the hsb model to improve the accuracy of predicting groundwater outflow rates and develop more accurate hydrographs the concept of equivalent propagation thickness also provides a criterion for assessing the importance of unsaturated lateral flow for future drainage research

evolution and effect of the stress concentration and rock failure in the deep multi seam coal mining supports crushing accident occasionally occurs in the protected seam exploitation of deep multi seam coal mining structure and results in adverse effect to the production to prevent its recurrence in a newly developed working field a 3d numerical extraction model was built based on the geologic and mining conditions of jining coal mine to evolve the changes state and characteristics of the reconstructed vertical and lateral stress in rock interlayer after protective seam exploitation stress release and increase zones of this mining structure were separated mining induced localized stress concentration and the interlayer rock failure behavior were explored the action of concentrated stress on the hydraulic supports in protected seam was discussed upon the major stress redistribution using the infinitesimal strain method a mechanical model was created to further explore from the vertical and lateral directions the cause and mechanism of localized stress concentration and rock failure behavior in rock interlayer the field investigation was finally performed to verify the numerical and mechanical results and the essential control measures were proposed to prevent this accident key findings of this study bring some new insights into the deep multi seam coal extraction and help to promote a more reliable underground mining

base flow recession from unsaturated saturated porous media considering lateral unsaturated discharge and aquifer compressibility unsaturated flow is an important process in baseflow recessions and its effect is rarely investigated a mathematical model for a coupled unsaturated saturated flow in a horizontally unconfined aquifer with time dependent infiltrations is presented the effects of the lateral discharge of the unsaturated zone and aquifer compressibility are specifically taken into consideration semi analytical solutions for hydraulic heads and discharges are derived using laplace transform and cosine transform the solutions are compared with solutions of the linearized boussinesq equation lb solution and the linearized laplace equation ll solution respectively a larger dimensionless constitutive exponent d a smaller retention capacity of the unsaturated zone leads to a smaller discharge during the infiltration period and a larger discharge after the infiltration the lateral discharge of the unsaturated zone is significant when d 1 and becomes negligible when d d 100 the compressibility of the aquifer has a non negligible impact on the discharge at early times for late times the power index b of the recession curve dq dt aqb is 1 and independent of d where q is the baseflow and a is a constant lumped aquifer parameter for early times b is approximately equal to 3 but it approaches infinity when t 0 the present solution is applied to synthetic and field cases the present solution matched the synthetic data better than both the ll and lb solutions with a minimum relative error of 16 for estimate of hydraulic conductivity the present solution was applied to the observed streamflow discharge in iowa and the estimated values of the aquifer parameters were reasonable

3800 to 2500 ma crustal evolution in the anshan area of liaoning province northeastern china shrimp u pb zircon geochronology has revealed several generations of granitoids and gneisses from the anshan area of northeastern china two samples of the baijiafen granite sensu lato 3804 5 and 3812 4 ma chentaigou granite 3306 13 ma lishan granite 3142 7 ma east and west anshan granites 3001 4 and 2994 8 ma tiejiashan granite 2962 4 and 2964 6 ma and qidashan granite 2475 ma the 3800 ma baijiafen granite consists of banded migmatitic trondhjemitic gneisses and forms only a small part of the area nonetheless they are the oldest known rocks in asia and are one of very few locations in the world where rocks of this antiquity have been found the younger suites are predominantly granite sensu stricto in composition and although foliated they have not been transformed into banded gneisses nd isotopic data suggest that the 3800 ma granites were produced at least in part by remelting of very ancient lree enriched sources perhaps like the exposed remnants of the 3800 ma rocks the chentaigou supracrustals contain layered mafic and ultramafic rocks banded iron formation bif and siliceous layered rocks of probable volcanogenic origin a sample of the latter has yielded a concordant u pb zircon age of 3362 5 ma interpreted as the age of deposition a trondhjemitic sheet cutting these rocks has yielded a u pb zircon age of 3342 6 ma the chentaigou supracrustals are the oldest recognised in the area younger supracrustal sequences collectively known as the anshan group contains the main economic bif units in the area bif is intruded by the 2475 ma qidashan granite indicating that the anshan group is archaean or earliest proterozoic in age

crustal structure beneath the paleozoic parnaiba basin revealed by airborne gravity and magnetic data brazil abstract the parnaiba basin is a large paleozoic syneclise in northeastern brazil underlain by precambrian crystalline basement which comprises a complex lithostructural and tectonic framework formed during the neoproterozoic eopaleozoic brasiliano pan african orogenic collage a sag basin up to 3 5 km thick and 1000 km long formed after the collage the lithologic composition structure and role in the basin evolution of the underlying basement are the focus of this study airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the parnaiba basin results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding neoproterozoic fold belts in the basement triangular shaped basement inliers are geophysically defined in the central region of this continental scale neoproterozoic convergence zone a 3 d gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36 40 5 km deep crustal root with borders defined by a high density and thinner crust forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by brasiliano shear zones representing lithospheric sutures of the amazonian and sao francisco cratons tocantins province and parnaiba block in addition coincident residual gravity residual magnetic and pseudo gravity lows indicate two complex systems of eopaleozoic rifts related to the initial phase of the sag deposition which follow basement trends in several directions