collapse of archean orogens and the generation of late to postkinematic granitoids intrusion of late stage granitoids into archean granite greenstone terranes may be spatially and temporally related to late orogenic upper crustal extensional collapse and formation of metamorphic core complexes the late to postkinematic granitoid magmas are derived through partial melting of upper mantle and lower crustal sources and are locally associated with late stage extensional structures thick roots of depleted lithospheric mantle of archean age tectosphere beneath archean cratons makes melting of the upper mantle difficult it is proposed that the archean tectosphere is made of imbricated slabs of buoyant archean oceanic crust and lithosphere together with trapped wedges of fertile mantle this would explain several seemingly contradictory phenomena decompression from upper crustal extension generates significant amounts of basaltic melts from the trapped wedges of fertile mantle these basaltic magmas intrude and partially melt the lower crust becoming more silicic in composition they migrate upward to solidify in the middle and upper crust forming the late to postkinematic granitoid suite the tectosphere in this model is less dense and colder than surrounding asthenosphere and is consistent with eclogitic xenoliths from subducted oceanic lithosphere found entrained in kimberlites the direction and magnitude of shear wave anisotropy in the upper mantle is also explained in this model through the orientation of olivine crystals in the underplated slabs of oceanic lithosphere

basal platinum group element mineralization in the federov pansky layered mafic intrusion kola peninsula russia the 2501 to 2446 ma federov pansky layered mafic intrusion in the central part of the kola peninsula russia is situated along the northern contact of the early proterozoic imandra varzuga rift and archean granite gneiss the federov pansky intrusion has eight major zones from bottom to top 1 marginal zone mafic schists 50 to 100 m 2 taxitic gabbronorite zone bpca 150 to 300 m 3 norite zone bcpa pbc 200 m 4 main gabbronorite zone pacb 1 000 m 5 lower layered horizon zone bc pc pbac 100 m 6 gabbro zone pacb pbac 1 500 m 7 upper layered horizon zone pocab 250 m 8 upper gabbro zone pacb 400 m the taxitic gabbronorite zone is restricted to the federov and east lastjavr blocks this zone is distinctive due to its pegmatitic taxitic textures and abundant norite xenoliths concentrated near its upper contact with the norite zone chemically the taxitic gabbronorite is more feldspathic and iron enriched than the overlying mgo and cr 2 o 3 rich norite zone and norite xenoliths the zone hosts cu ni pge mineralization with disseminated sulfides which is continuous for over 2 5 km along strike and 150 to 200 m downdip 98 percent of the sulfides consist of varying amounts of pyrrhotite chalcopyrite and pentlandite the mineralization is divided into an upper mineralized section with two higher grade lenses a and b and the restricted lower mineralized section with six narrower lenses the lower mineralized section is less continuous and less uniform in terms of cu ni pge distribution the norite xenoliths and blocks occurring within the taxitic gabbronorite zone are practically barren of sulfide and can dilute higher pge and base metal grades in the gabbronorite matrix average total pge grades in rocks do not exceed a few grams per tonne the mineralization is palladium rich with a pd pt ratio from 4 22 to 4 34 rh and au grades are 0 1 to 0 5 g t os and ir grades are the most common pge bearing minerals are merenskyite moncheite kotulskite sobolevskite and michenerite less common are sperrylite and vysotskite braggite the cu ni pge sulfides are primarily magmatic in origin this is supported by s isotope compositions with a narrow range of 34 s of 0 2 to 1 4 percent and s se ratios of 1 050 to 3 300 a three stage process is postulated for the genesis of the federov mineralization this involves the intrusion of a more fractionated and sulfur saturated gabbronorite magma and its brecciation and mixing with a residual primitive magma forming immiscible sulfides that scavenged cu ni pge the federov resembles other contact type mineralization such as in the portimo intrusion in finland and the lac des iles and east bull lake intrusions in ontario canada

horizontal structures in granulite terrains a record of mountain building or mountain collapse in many high temperature 800 c granulite terrains the development of characteristic horizontal structures occurred during the metamorphic culmination and was followed by isobaric cooling the absolute magnitude of isobaric cooling commonly 300 c implies cooling intervals of the order of the thermal time constant of the continental lithosphere 100 m y such prolonged isobaric cooling implies that no significant erosional denudation followed the development of the horizontal structures and thus precludes the prograde deformation being responsible for significant crustal thickening rather the prograde deformation more probably records bulk crustal thinning during extensional collapse of a previously thickened crust possibly triggered by detachment of a thickened thermal boundary layer at the base of the lithosphere

archaean tectonic processes a case for horizontal shortening in the north pilbara granite greenstone terrane western australia abstract the archaean pilbara granitoid greenstone terrane ggt has been the focus of numerous studies on archaean geology especially the classic dome and basin area around marble bar in the east pilbara this area has been used as evidence for different tectonic processes i e that vertical tectonics or diapirism was a cause for archaean deformation this paper provides evidence to support regional horizontal plate interaction stresses as being largely responsible for the compressive deformation of the pilbara ggt at least from ca 3 2 ga the relative chronology of meso to macro scale structural elements are presented for a number of selected areas across the pilbara ggt these locally identified events are correlated with a regional pilbara wide structural framework of deformation events that are constrained by geochronological and stratigraphic controls the dome and basin geometry characteristic of the east pilbara was established after 3 2 ga and was successively modified by repeated orthogonal extensional and compressive subhorizontal events the result has been a locally complex development of polyphase structural elements with consistent overprinting relationships that can be correlated across much of the pilbara from 3 2 ga diapirism did not cause these deformation elements although it may have modified them

late paleoproterozoic 1900 1800ma nappe stacking and polyphase deformation in the hengshan wutaishan area implications for the understanding of the trans north china belt north china craton abstract the hengshan wutaishan area forms part of the trans north china belt developed in response to the final amalgamation of the north china craton the hengshan wutaishan domain and the adjacent fuping massif constitute the largest and the most representative rock exposure of the belt a structural study allows us to redefine the lithological and tectonic units that compose the architecture of the hengshan wutaishan domain and to propose that the trans north china belt was built up through a polyphase tectonic evolution within the period 1900 1800 ma the first event d1 corresponds to the emplacement of lower and upper nappes herein called the orthogneiss and volcanites unit ovu and the low grade mafic unit lgmu respectively the ovu mainly consists of rocks formed in an arc system and metamorphosed under amphibolite facies conditions whereas the lgmu represents oceanic crust rocks with an arc component that underwent greenschist facies metamorphism the syn metamorphic d1 deformation is characterized by a nw se stretching and mineral lineation with a top to the se sense of shear that corresponds to the sense of nappe motion u th pb epm chemical dating on monazite from three metapelites of the ovu gives isochron ages of 1884 11 ma 1886 5 ma and 1887 4 ma interpreted as the age of the prograde amphibolite facies metamorphism coeval with nappe stacking during the d1 event the d2 event corresponds to a widespread crustal melting of the ovu and underlying basement rocks in hengshan related to the exhumation of deep structures about 20 30 ma after nappe emplacement the migmatite contains blocks of retrograded eclogite or hp granulitic restites and is characterized by the development of a pervasive foliation and lineation a late d2 event represents the folding stage of the migmatitic foliation during the completion of a dome the orthogneiss and volcanics unit and the low grade mafic unit are unconformably covered by weakly metamorphosed to unmetamorphosed sediments belonging to the hutuo supergroup these rocks were deformed by a ductile d3 event characterized by a nw se trending stretching lineation south verging folds associated with an axial planar slaty cleavage and an e w crenulation lineation lastly a left lateral wrenching along the zhujiafang shear zone separates the northern and southern parts of hengshan our structural study shows that there is no significant difference between the southern part of hengshan and the lower part of wutaishan both areas belong to ovu and show a structural continuity we suggest that this polyphase deformation developed in response to a north westward subduction of an old ocean named the luliang ocean beneath the western block followed by collision with a micro continent named the fuping block at 1890 1880 ma

some thermal and mechanical consequences of rapid uplift an example from the southern alps new zealand abstract the thermal evolution of continental crust during active collision is modeled through numerical solutions of the two dimensional heat conduction equation for a rapidly moving medium the boundary conditions used in the modeling are derived from geological and geophysical observations from the active collision zone in the south island of new zealand the problem domain over which the solutions are obtained consists of a 40 km horizontal by 25 km vertical spatial plane with a vertical discontinuity at 10 km from the western boundary to the east of this discontinuity vertical uplift rates of up to 10 mm a occur over a timespan of up to 4 ma temperature distributions are calculated at 10 ka intervals over the 4 ma duration a two dimensional high temperature region is established upon initiation of uplift of the eastern block due to the advective component carrying heat upwards more rapidly than it can be dissipated laterally from the problem domain temperatures within the upper 5 km are greater than 400 c after 2 25 ma with geothermal gradients of up to 200 c km attained within the upper 3 km at times greater than 2 5 ma the vertical temperature distribution changes little while the anomalously high temperatures spread laterally into the stationary crust using rheological equations to describe the brittle behaviour of a water saturated upper crust and the ductile behaviour of a quartz dominated lower crust together with the thermal distribution of the conduction models the mechanical evolution of a collision zone is investigated in addition to high crustal temperatures and associated high heat flow rapid uplift produces a weakening of the crust by raising of the depth of transition from brittle to ductile behaviour within the zone of most rapid uplift the brittle ductile transition rises from 13 km to less than 5 km after 1 5 ma of uplift further uplift reduces the brittle layer to 3 km thickness and causes lateral spreading of the low strength zone the reductions in crustal strength caused by the thermal weakening produce a high strain zone within the region of maximum uplift which is incapable of sustaining large differential stresses this causes horizontal and vertical stress transfer and results in shallow seismicity increases in the adjacent crust as well as in intermediate depth seismicity within the high strength upper mantle because the thermal and mechanical anomalies discussed are a function of rapid uplift all regions of active continental collision may be expected to exhibit similar behaviour some mechanical and thermal characteristics of the himalayan collision zone are briefly examined in light of the numerical modeling

assessment of undiscovered metal resources in finland this paper summarises the results of probabilistic estimates of the amounts of cu zn pb ni co pt pd au ag and mo in undiscovered orogenic au volcanogenic massive sulphide vms porphyry cu outokumpu type cu zn co synorogenic intrusion related ni cu komatiite related ni and layered intrusion hosted contact type and reef type pge deposits in finland the assessments were carried out down to the depth of one kilometre using the three part quantitative assessment method permissive areas tracts within which mineral deposits can exist based on their geological properties were delineated separately for each deposit type total number of tracts delineated was 188 and excluding overlap total area covered by the tracts is 190 700 km2 this means that 57 of the land area of finland holds potential for the discovery of new mineral deposits of the types included in the assessments orogenic au tracts cover the largest area 110 000 km2 and the contact type pge and talvivaara type tracts cover the smallest areas both 310 km2 the number of undiscovered deposits was estimated at several levels of confidence for each permissive tract the total expected number of undiscovered deposits across all permissive tracts is 309 deposits the largest expected numbers of deposits are associated with the orogenic au 90 synorogenic intrusion related ni cu 66 and vms 45 tracts statistical comparisons indicated differences in tonnage and grade values between fennoscandian and global data sets for several deposit types and between global precambrian and phanerozoic porphyry cu deposit data sets the reason for the differences is inconclusive but probably related to both global variations in exploration maturity and availability of grade and tonnage information due to the differences grade tonnage models were constructed using data from well known deposits within the fennoscandian shield for most of the assessed deposit types the orogenic au model was constructed using fennoscandian and north australian deposits and the porphyry cu model using global data on precambrian deposits the sum of median estimates of undiscovered resources across all the assessed deposit types is 9 7 mt cu 5 0 mt ni 1 8 mt zn 0 15 mt pb 0 10 mt mo 86 000 t co 12 000 t pd 5600 t pt 2100 t ag and 1400 t au layered intrusion hosted pge deposits and porphyry cu deposits are estimated to host the largest undiscovered resources containing the majority of the undiscovered cu ni pt pd and mo most of the undiscovered zn is in vms deposits and more than half of the undiscovered au is in orogenic au deposits comparison between discovered and estimated undiscovered resources indicates that practically all mo pt and pd resources and more than half of the au and cu resources in finland exist in undiscovered or poorly known deposits undiscovered resources of ni pb and ag are smaller than the discovered resources most of the total endowment of zn and co appears to be in discovered resources on a global scale the finnish resources are small the discovered resources and estimated total endowment for most of the metals assessed are 1 of the corresponding global identified resources platinum group elements ni co and possibly au might be exceptions to this at least on a european scale as a caveat to the results of this work we stress that a few potentially significant deposit types e g kevitsa and talvivaara types precambrian epithermal au were excluded from the assessments due to the lack of statistically reliable grade and tonnage data

quantifying rates of dome and keel formation in the barberton granitoid greenstone belt south africa abstract the barberton granitoid greenstone belt is a classic dome and keel province characterized by kilometer scale gneiss domes and elongate keels of largely folded supracrustal rocks combined u pb shrimp data and structural mapping demonstrate that the geometry of the barberton belt reflects events that occurred over 30 million year interval from ca 3230 and 3203 ma early deformation with nw se shortening in the upper crust was accompanied by emplacement of tonalite trondhjemite granodiorite ttg magmas at 3234 12 and 3226 9 ma much of the structural grain of the greenstone belt relates to a long episode of post orogenic extension with ne directed extension in the lower crust leading to exhumation of high grade gneisses in the southern barberton terrane advective heat transfer during emplacement of kilometer scale ttg plutons around the margins of the greenstone belt facilitated the infolding of the relatively denser and colder greenstone sequence the end of this tectonic cycle is punctuated by the emplacement of the undeformed 3203 7 ma dalmein pluton which sharply truncates not only anticlines and synclines in the greenstone belt but also the dominant fabric in the high grade gneisses

origin of compositional layering and mechanism of crustal thickening in the high grade gneiss terrain of sri lanka abstract the highland complex of sri lanka comprises a 7 km thick succession of banded para and orthogneisses at granulite grade or partly retrograded we interpret this complex as a tectonostratigraphic unit in which no primary stratigraphy is preserved this succession with a probable depositional age of 2 ga evolved from a rift basin into a foredeep and then into a fold and thrust belt prior to severe flattening and transport into the lower crust during the final stages of collision some 550 620 ma ago granitoid rocks were emplaced into the supra crustal sequence at various stages of the tectonic evolution between 1950 and 670 ma ago and the original intrusive contacts became obscured or severely modified during progressive non coaxial ductile deformation structures that predate the presently observed layering are preserved in rootless isoclinal folds boudins and microstructures and in deformed pre granulite metamorphic minerals preserved in garnet porphyroblasts the evolution of the highland complex is similar to that of other precambrian granulite terrains notably parts of the grenville province of canada and probably resulted from collisional tectonics during which the original supracrustal assemblage was detached from its archaean to palaeoproterozoic basement the paucity of structures reflecting the early deformational history is due to severe flattening and ductile deformation during predominantly non coaxial deformation when the highland rocks were transported into the lower crust

spinifex biogeochemical expressions of buried gold mineralisation the great mineral exploration penetrator of transported regolith a major challenge for mineral explorers is to efficiently detect mineralisation beneath the weathered cover that extends across the landscape determination of the elemental composition of plants biogeochemical exploration can aid in the detection of buried ore deposits due to their root penetration through the weathered cover at the coyote prospect in western australia a range of plant species were sampled traversing a buried au orebody here it is shown that the soft spinifex triodia pungens accumulated important pathfinder elements related to the mineralisation which produced a multi element surficial expression of the underlying ore deposit spinifex grasses are one of the most widespread grasses over the australian continent and have vertical root systems that can extend for many 10s of metres through sedimentary cover and interact with buried mineral deposits and alteration zones in the underlying substrate spinifex biogeochemistry therefore has potential to be a low cost low environmental impact sampling medium for mineral exploration programs across large parts of arid australia the examination of similar grasses from other semi arid and arid regions from around the world therefore also warrants further investigation

a modern structural regime in the paleoarchean 3 64 ga isua greenstone belt southern west greenland abstract the footwall gneisses beneath the western part of the paleoarchean 3 8 ga isua greenstone belt southern west greenland are interpreted here in terms of a 3 64 ga stack of mylonitic crystalline thrust nappes the oldest example known on earth in present coordinates the kinematic history of the thrust nappe stack is couched in terms of initial longitudinal strike parallel thrusting towards the southwest followed by transverse thrusting to the northwest and subsequent extensional collapse of the thickened crust toward the southeast diorite and tonalite that form the western margin of granitoids structurally overlying the western part of the isua greenstone belt and its footwall contain 3 5 ga mafic dykes some of which are deformed and or truncated at fault contacts within the granitoids accordingly a component of the deformation structurally above the isua greenstone belt occurred after 3 5 ga significantly later than the formation of the underlying mylonitic nappes probably during the neoarchean the structural regime of mylonitic thrust nappe stacking is very similar to that in modern mountain belts it would appear that the deformational behaviour rheological constitution and overall strength of paleoarchean and modern continental crust were similar

the mara rosa arch in the tocantins province further evidence for neoproterozoic crustal accretion in central brazil the mara rosa volcano sedimentary sequence consists of several nne trending belts of metavolcanic metabasalts to metarhyolites and metasedimentary rocks micaschists quartzites cherts exposed over large areas of the tocantins province a neoproterozoic brasiliano orogenic region in the central part of brazil the supracrustal belts are separated by terrains dominated by metatonalites and metadiorites and intruded by several post orogenic granitic and dioritic bodies these volcano sedimentary and associated metaplutonic rock associations have been previously interpreted as a typical granite greenstone terrain of the archaean basement in this paper we present geochronological and isotopic data for rocks of the metavolcanic metaplutonic rock association samples of a felsic metavolcanic rock from the posse gold mine and a metatonalite yielded u pb zircon ages of 862 8 ma and 856 7 13 ma respectively these are interpreted as crystallization ages of the igneous protoliths titanites from the same metavolcanic rock sample yielded a concordant recrystallization age of 632 4 ma sm nd isotopes for these rocks indicate primitive compositions with nd t of 4 6 and 3 7 and tdm model ages of 1 0 ga a syn tectonic dioritic intrusion has an u pb crystallization age of 630 6 ma and nd t of 1 9 and tdm of 1 0 ga additional sm nd analyses on metasedimentary rocks and post orogenic granites of the mara rosa region yield tdm ages between 1 2 and 1 0 ga the isotopic data combined with preliminary trace element results suggest that the protoliths of the metavolcanic and metaplutonic rocks investigated were formed in an island arc system off the coast of the sao francisco congo continent at around 860 ma recrystallization and deformational ages of 630 ma may represent the timing of final ocean closure and continental collision the rock associations structural pattern geochronological and isotopic characteristics of the rocks from the mara rosa region are very similar to those of neoproterozoic arc terrains exposed 300 km to the southwest in the arenspolis area proving the regional importance of the neoproterozoic crustal accretion event in the central part of brazil

models for the archean thermal regime a variety of different regionalized thermal evolution models are presented all models include an oceanic style of heat loss in which mantle convection penetrates to the surface in the form of spreading and subducting plates in continental regions the lithosphere is assumed to act as a conductive lid on mantle convection that can be either layered or mantle wide the continental lithosphere is defined in different ways it can be of fixed thickness insensitive to temperature or extend to some fixed cutoff temperature a thermally controlled rheological lithosphere when a fixed temperature is used widespread melting occurs in the archean and one has the further option of allowing these melts to erupt onto the surface thus providing a new heat transfer mechanism and also a general subsidence of the continental lithosphere due to the continuous volcanic loading the models are first used to determine the concentration of heat producing elements required to have a present day mantle potential temperature of 1350 c starting from a high temperature 2000 c 4 5 billion years ago the results depend primarily on whether the mantle is layered if layered 15 30 ppb uranium normalized to the silicate portion of the earth and withk u 104 th u 3 8 are needed while mantle wide cases need uranium concentrations in excess of 35 ppb the models are also used to calculate the thermal structure of continental lithosphere through time only lithospheres of fixed thickness 175 km can explain the recent discovery of diamonds with model ages of 3 2 3 4 billion years for a thick lithosphere to have existed in the archean it would seem most likely that its thickness be the result of some chemical distinction rather than simply its rheological properties such chemically distinct lithosphere would have greater likelihood of surviving to the present which raises the issue of biased preservation

groundwater flow regime within the gambier embayment of the otway basin australia evidence from hydraulics and hydrochemistry a conceptual groundwater flow model for the cainozoic sequence of the gambier embayment of the otway basin south australia is developed using hydraulic head distribution environmental isotopes and hydrochemical data local flow cells occur within the upper sequence of the gambier unconfined aquifer and are recognised by local relief of the water table and as rapid lateral variations in chloride 14c 2h and 18o concentrations indicating that regional flow is not dominant in the upper portion of the unconfined aquifer these localised systems are topographically controlled and have developed in response to undulating stranded coastal dune ridges of the bridgewater formation recharge areas correspond to dune highs while discharge is focused towards the lower lying interdunal regions total dissolved solids concentrations are controlled by evapotranspiration processes during recharge and ion ratios are modified by water rock interactions within the groundwater system groundwaters are more dilute in the southern section of the basin where the surplus of precipitation over evapotranspiration in the winter months is greater than that in the northern section of the basin a regional flow system with interaquifer mixing has developed within the confined dilwyn aquifer system recharge to the dilwyn aquifer has been identified downgradient from the basin margin via downward leakage from an intermediate flow system within the gambier unconfined aquifer to the deeper regional flow system of the confined dilwyn aquifer interaquifer mixing appears to be geologically controlled in the south by the elevated gambier axis and in the north by the kanawinka fault downgradient of the zero head difference line lateral flow appears to be the dominant transport mechanism within the dilwyn aquifer as indicated by constant chloride concentrations monotonic decrease in radiocarbon concentration and more negative 2 and 18 o composition

initiation of leaking earth an ultimate trigger of the cambrian explosion abstract for life to have dramatically evolved and diversified during the so called cambrian explosion there must have been significant changes in the environmental conditions of earth a rapid increase in atmospheric oxygen which has been discussed as the key factor in the evolution of life cannot by itself explain such an explosion since life requires more than oxygen to flourish let alone survive the supply of nutrients must have played a more critical role in the explosion including an increase in phosphorus p and potassium k which are key elements for metabolisms to function so what happened at the onset of the cambrian to bring about changes in environmental conditions and nutrient supply and ultimately evolution of life an ultimate trigger for the cambrian explosion is proposed here the geotherm along subduction zones of a cooling earth finally became cool enough around 600 ma to allow slabs to be hydrated the subduction of these hydrated slabs transferred voluminous water from the ocean to the mantle resulting in a lowering of the sea level and an associated exceptional exposure of nutrient enriched continental crust along with an increase in atmospheric oxygen this loss of water at the surface of the earth and an associated increase in exposed landmass is referred to here as leaking earth vast amounts of nutrients began to be carried through weathering erosion and transport of the landmass rock fragments of the landmass would break down into ions during transport to the ocean through river providing life forms prokaryote sufficient nutrients to live and evolve also plume driven dome up beneath the continental crusts broadened the surface area providing a supply of nutrients an order magnitude greater than that produced through uplift of mountains by continental collision simultaneously atmospheric oxygen began to increase rapidly due to the burial of dead organic matter by enhanced sedimentation from the emergence of a greater landmass which ultimately inhibited oxidation of organic matter hence oxygen began to accumulate in the atmosphere which when coupled with a continuous supply of nutrients resulted in an explosion of life including an increase in the size an enhanced oxygen supply in the atmosphere resulted in the formation of an ozone layer providing life a shield from the uv radiation of the sun this enabled life to invade the land in addition to a change in the supply of nutrients related to a leaking earth the evolution of life was accelerated through mass extinction events such as observed during snowball earth possibly related to a starburst in our galaxy as well as mutation in the genome due to radiogenic elements sourced from carbonatite magma atomic bomb magma in rift valley there are two requirements to find a habitable planet 1 the initial mass of an ocean and 2 the size of a planet these two conditions determine the history of a planet including planetary tectonics and the birth of life this newfound perspective which includes the importance of a leaking planet provides a dawn of new planetary science and astrobiology

stratigraphy petrology and platinum group element mineralization of the early proterozoic penikat layered intrusion northern finland the penikat layered intrusion is part of an early proterozoic belt of layered intrusions 2 440 m y in age which extends across northern finland the intrusion is located between late archean granitoids and early proterozoic supracrustal rocks the marginal series with slightly contaminated chilled margin separates the penikat intrusion from the underlying basement complex the layered series is divided into five megacyclic units i to v from bottom to top each unit comprising an ultramafic lower part and a gabbroic upper part three major mineralized zones enriched in platinum group elements pge the sompujaervi sj ala penikka ap and paasivaara pv mineralizations have been delineated along almost 23 km of strike and occur in megacyclic unit iv the sompujaervi mineralization about 1 m thick is mostly associated with pyroxenitic rocks at the lower contact of the unit whereas the ala penikka zone about 0 35 m thick is associated with gabbronorite and poikilitic textured anorthosite in the middle part and the paasivaara zone about 1 m thick with leucogabbro and anorthosite very close to the upper contact the ratios of platinum group elements are relatively constant for each mineralization the paasivaara mineralization represents the most platinum rich variety and the ala penikka the most palladium rich one the ratio of pd to pt being typically about 2 for the sulfide free type and 1 5 3 5 and 0 7 for the sulfide bearing type for the sompujaervi ala penikka and paasivaara mineralizations respectively the sulfur free platinum group minerals pgm sperrylite and isomertieite are the dominant species encountered in the sompujaervi mineralization whereas sulfur bearing platinum group minerals together with sperrylite dominate in the ala penikka and paasivaara mineralizations these are represented by braggite and vysotskite in the ala penikka one and by braggite and cooperite in the more platinum rich paasivaara mineralization according to the new classification of platinum group element deposits proposed by cabri and naldrett 1984 the typically sulfide free sompujaervi mineralization may be included in the oxide silicate association as a possible new type whereas the ala penikka and paasivaara mineralizations may be classified as merensky reef type of the sulfide associations

pressures and temperatures in precambrian metamorphic rocks modelp t estimates from nearly 100 precambrian rocks have been derived by comparing published descriptions of metamorphic mineral assemblages to a standard set of mineral stability data pressures temperatures and metamorphic geotherms show large variations nevertheless modelp t data reveal two distinct secular trends average metamorphic geotherms have declined through time and average metamorphic pressures have increased through time metamorphic geotherms and pressures from archean rocks average 54 c km and 4 1 kbar respectively corresponding figures for early proterozoic terranes are 47 c km and 4 3 kbar and for late proterozoic terranes 35 c km and 6 kbar these secular trends may reflect gradual cooling and thickening of the earth s crust at least in orogenic belts there are exceptions to both trends found in localities where mineral assemblages indicate high pressures 6 12 kbar and low metamorphic geotherms 20 30 c km these assemblages formed in archean crust that at least locally was thicker than 20 km during metamorphism

numerical experiments on the onset of convective instability in the earth s mantle summary a simplified model of convection in the mantle is used to investigate the transient effect of cooling a fluid layer from above the model representing the mantle overlain by the lithosphere consists of a two dimensional fluid layer overlain by a solid conducting lid the initial temperature of both layers is the same with the top surface of the lid kept at 0 c throughout we observe the onset of small scale flow in the model in the absence of internal heating the behaviour of the system is controlled by the rayleigh number r and the ratio of the thicknesses of the two layers a the onset time of convection as defined by reference to conduction temperature profiles is related simply to a boundary layer critical rayleigh number the mean temperature profiles for the convection model are also compared with the observed depth age relation for oceanic lithosphere and the results are used to estimate the viscosity of the mantle

what caused the rise of atmospheric o2 abstract oxygenic photosynthesis appears to have evolved well before o 2 levels increased in the atmosphere at around 2 4 ga this has led to numerous suggestions as to what may have kept o 2 suppressed and then eventually allowed it to rise these suggestions include changes in the recycling of carbon and sulfur relative to water or hydrogen a switch from dominantly submarine to dominantly subaerial volcanism gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases a decline in deposition of banded iron formations a decline in nickel availability and various proposals to increase the efficiency of photosynthesis several of these different mechanisms could have contributed to the rise of o 2 although not all of them are equally effective to be considered successful any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates which shows relatively little change with time apart from transient but occasionally spectacular excursions the reasons for this constancy are explored here but are not fully resolved in the process of making these comparisons a self consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on earth like exoplanets

archean plate tectonics constraints and inferences the earth has been cooling since archean time the higher temperatures beneath archean ridges resulted in more partial melting which extended down to greater depths than at present the archean oceanic crust was much thicker 20 km than modern crust 5 km this inference is compatible with previous ideas of suspected oceanic crust the thicker oceanic crust in the archean tended to resist subduction similar to modern aseismic ridges but could not prevent it short lived episodes of intra arc spreading followed by cordilleran type compression may have produced archean greenstone belts archean meta tonalites represent a significant part of the continental crust and were probably produced by melting of the subducted slab which was favored by the higher mantle temperatures

insights into orogenic processes from drab schists and minor intrusions southern sao francisco craton brazil abstract minor altered intrusions and drab retrogressed schists can easily be overlooked in geological studies but this contribution explores these rocks within the archaean and palaeoproterozoic southern sao francisco craton ssfc brazil using geological relationships and accessory mineral in situ analyses in the context of cratonic assembly three magmatic pulses are documented i archaean and ii palaeoproterozoic felsic intrusions both hosted by archaean protoliths and iii palaeoproterozoic felsic intrusions in palaeoproterozoic supracrustal sequences archaean felsic intrusions confirm the palaeoarchaean age of the mafic ultramafic sequence of the rio das velhas greenstone belt and rhyacian intrusions mark the collisional stage of the mineiro belt with the sfc at c 2130 ma greenstone belt schists show a wide distribution of rounded soccer ball archaean detrital and metamorphic zircon grains ranging in age from 3200 to 2750 ma with an interpreted overprinting high grade metamorphic event at c 2700 to 2680 ma most high grade metamorphic rims have th u 0 1 negative ehf t values and ree pattern consistent with eclogite granulite metamorphic facies reinforcing the hypothesis of a dehydrated refractory crust formed during the stabilization of the ssfc even though no such protoliths are preserved this event links crustal thickening and partial melting of archaean lower crust archaean rutile crystals from the greenstone belt schist were reset during the palaeoproterozoic event but still preserve the early archaean high grade metamorphic signature the presence of unstable ilmenite replaced by rutile in the schist associated to felsic intrusions with the same age at c 2130 ma suggest high pressure low temperature prograde metamorphism during the collisional stage of the palaeoproterozoic orogen elongate and prismatic zircon grains from the rhyacian intrusions have low ehf t signature and crystallised from partial melting of sedimentary protoliths accretionary events produced thicker and more differentiated crust by the end of rhyacian time easily overlooked rocks in this study when studied have revealed a rich multi event history of cratonic evolution

vertical carbon 14 profiles for resolving spatial variability in recharge in arid environments summary groundwater age tracers are often measured to help constrain estimates of groundwater recharge especially in arid environments where other methods are unsuitable however multiple processes can influence the shape of vertical tracer profiles in an aquifer including 1 variation in tracer input concentrations from the unsaturated zone 2 the role of diffusion in transporting tracer into the aquifer when fluxes are low and 3 spatial variability in recharge this study demonstrates the influence of spatially variable recharge and spatially variable carbon 14 14 c activities in the unsaturated zone on vertical 14 c profiles in groundwater through groundwater flow and solute transport modelling we demonstrate that recharge estimated from single point measurements of 14 c may be wrong more than an order of magnitude when unsaturated zone 14 c activities and recharge vary spatially we then present a case study from the ti tree basin in arid central australia where detailed profiles of 14 c activity in unsaturated zone gas and groundwater have been measured and spatial variability in unsaturated zone 14 c is observed ranging from 54 to 106 pmc above the watertable through modelling our data we show that when unsaturated zone 14 c activities are known measurement of the 14 c profile can help constrain estimates of recharge and its spatial variability this approach improves our understanding of groundwater flow in the ti tree basin by showing mountain front recharge to be an important mechanism

regionalized models for the thermal evolution of the earth abstract traditional models for the heat loss in oceanic and continental regions are combined into a regionalized model for the thermal evolution of the earth the need for regionalization is obvious when one considers that the mantle loses 3 to 4 times as much heat per unit area in oceanic regions than in continental areas the present day rate of heat loss together with a geochemical estimate of the concentration of heat producing elements in the earth fixes the response time of the thermally convecting mantle the response time in turn can be used to select the most reasonable representation for mantle convection in terms of the sensitivity of viscosity on temperature and layering versus mantle wide circulation present geochemical estimates of the bulk composition of the earth are most easily reconciled with the observed heat flow if the mantle is layered and its rheology is slightly less temperature dependent than generally assumed the layered system can produce sufficiently high temperatures to explain the high magnesian komatiites of the archean one difficulty with the models is that they predict widespread melting at shallow depth in the early stages of earth history but do not address how such melting affects and alters the heat transfer mechanisms

carbon 14 and stable isotope data for an area in the murray basin its use in estimating recharge measurements of the carbon 14 concentration of water in 33 boreholes in a saline unconfined aquifer in southern australia have been interpreted to show that local recharge under natural conditions has a mean annual value which falls between 0 1 and 0 2 mm yr 1 the chloride concentration of groundwater is log normally distributed suggesting that its source is rainfall the estimate of mean annual recharge to the aquifer made using chloride is 0 25 mm yr 1

underplating of oceanic lithosphere in the archean a possible mechanism for the formation of archean komatiites abstract the thermal evolution of a subducting archean oceanic lithosphere suggests that amphibole and phlogopite can transport water to depths of about 100 km water released from the oceanic crust hydrates the overlying depleted peridotitic lithosphere lowering its melting temperature and causing a high degree of partial melting temperatures at the top of the archean asthenosphere only about 150 c higher than the present value are sufficient to produce over 50 melting in the bottom of the overlying lithosphere under wet conditions and generate peridotitic komatiites

the great whirl observations of its seasonal development and interannual variability 1 in situ measurements are used together with sea surface height data to study the development and variability of the great whirl gw a large quasi stationary anticyclone that appears off the coast of somalia during the southwest monsoon season we find that anticyclonic circulation indicative of the gw appears on average in april almost two months before the onset of the southwest monsoon winds this early initiation is coincident with the arrival of annual rossby waves at the western boundary with the onset of the monsoon winds in early june the gw proper intensifies quickly remaining at its peak throughout july august and september and dissipating about one month after the winds have died the gw is present for an average 166 30 days per year and the position of its northern flank close to 9 n coincides with the latitude of zero wind stress curl the intraseasonal variability of the gw is intense as a result of mutual advection with one to three flanking cyclones which accompany the gw 70 of the time and tend to circulate clockwise around it there is no consistent seasonal pattern for the development or dissipation of the gw although movement to the southwest is common toward the end of the season the gw of 1995 deepened from 200 m in june to over 1000 m in september and strengthened from a swirl transport of 10 to 60 sv cool waters in its core resulted from advection via the somali current and some local vertical mixing

conductive incubation and the origin of dome and keel structure in archean granite greenstone terrains a model based on the eastern pilbara craton western australia 1 the archean east pilbara granite greenstone terrane epggt of the pilbara craton in western australia preserves a prolonged record of voluminous mafic dominated volcanism and felsic plutonism commencing at circa 3515 ma and ultimately resulting in the development of spectacular dome and keel structures early crustal growth was dominated by basaltic magmatism and the regional development of a 12 18 km thick greenstone sequence by circa 3335 ma overlying widespread 3470 3430 ma tonalite trondhjemite granodiorite ttg suites emplaced into the middle crust stratigraphic and structural relationships imply that voluminous granitoid plutonism and crustal scale dome and keel formation in the southeastern epggt was not initiated until circa 3325 ma but proceeded rapidly over the interval 3325 3308 ma this scenario is consistent with a conductive incubation period of 10 100 myr duration following the burial of radiogenic granitic crust beneath the accumulated greenstone pile for heat production parameters appropriate to the 3515 3308 ma epggt we show that the burial of 3470 3430 ma granitoid rich crust beneath the 3335 ma euro basalt and its precursors led to long term crustal temperature increases of 170 234 c at 35 km depth with a corresponding reduction in effective viscosity of 2 5 orders of magnitude in combination with the greenstone over granitoid density inversion the changes in effective viscosity of the mid to deep crust due to the burial of heat sources may have provided the crucial trigger for the initiation of dome and keel formation similarly cooling and strengthening of the crust during syndoming exhumation of the heat production potentially terminated large scale dome and keel amplification

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

paleo to mesoarchean polymetamorphism in the barberton granite greenstone belt south africa constraints from u pb monazite and lu hf garnet geochronology on the tectonic processes that shaped the belt discussion cutts et al 2014 1 combined thermodynamic modeling of compositional change in metamorphic garnet with metamorphic peak and retrograde pressure temperature p t data derived from quantitative p t pseudosections to determine clockwise p t paths for several samples from the tjakastad

a basin model for massive banded iron formations and its geophysical applications because banded iron formation bif density evidently decreased toward upper asthenosphere density with increasing water depth feedback between sediment density and water depth by way of sedimentation driven subsidence should have modulated the sediments bulk density to the upper asthenosphere s this mechanism helps explain why massive bif s in longer wavelength continental shelf basins have a more or less uniform bulk composition and why their bulk densities closely correspond to the modern upper asthenosphere s the difference between bif and upper mantle densities in a short wavelength basin yields a rough estimate of the lithosphere s flexural rigidity the elastic lithosphere of the witwatersrand basin south africa 2 7 b y ago was about half as thick as typical modern continental lithosphere dynamic effects associated with isostatic equilibration yield estimates of the asthenosphere s dynamic viscosity to few hundred km depths the fluid mantle beneath the hamersley basin australia 2 5 b y

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

regional shortening followed by channel flow induced collapse a new mechanism for dome and keel geometries in neoarchaean granite greenstone terrains the lateral flow and extrusive exhumation of ductile migmatitic gneisses due to a horizontal gradient in lithostatic pressure a process termed channel flow or lateral protrusion has previously been proposed as an important tectonic process in large hot orogens centrifuge simulation of i layer parallel shortening followed by ii collapse of a cover sequence during ductile flow of underlying layers in an analogous manner to channel flow suggests a new mechanism for the development of structures within neoarchaean granite greenstone terrains the centrifuge model incorporates an upper package of silicone modelling clay microlaminates that simulate a greenstone sequence that overlies slightly less dense ductile silicone putties whose rheological properties simulate migmatitic felsic gneiss a low viscosity low density layer along half of the infrastructure superstructure interface simulates the presence of granitoid melt during initial layer parallel shortening upright folds form in the upper greenstone package and the interface with underlying ductile layers is folded in basal silicone layers recumbent to overturned non cylindrical and upright folds form with increasing distance from the moving end wall removal of material parallel to fold hinges at the foreland end of the model i e furthest from the ram used to shorten models in several stages simulates erosion and the difference in gravitational loading thus created induces ductile flow and lateral extrusion of basal silicone layers models aim to reproduce features comparable to those developed during extrusive channel flow and focused exhumation of basement migmatitic gneiss in nature early formed recumbent to inclined folds are then accentuated during simulated channel flow while new recumbent isoclinal folds in basal layers develop broad antiforms and tight synforms similar to the dome and keel geometry that typifies many archaean granite greenstone belts are produced as a late feature in the model channel flow and collapse of a fold thickened crust is therefore proposed as a potential alternative mechanism for the formation of structures in some neoarchaean granite greenstone terrains by analogy with other physical and numerical models channel flow in the neoarchaean may be enhanced by impingement of an upper mantle wedge into the base of the crust our results imply that both lithospheric shortening and non diapiric gravitational instabilities may be responsible for the formation of some archaean dome and keel structures and may account for the juxtaposition of some granite greenstone and high grade gneiss terrains in the archaean model results also show similarities to structures produced during lateral flow and withdrawal of salt in fold belts

impacts of cave air ventilation and in cave prior calcite precipitation on golgotha cave dripwater chemistry southwest australia abstract speleothem trace element chemistry is an important component of multi proxy records of environmental change but a thorough understanding of hydrochemical processes is essential for its interpretation we present a dripwater chemistry dataset pco2 alkalinity ca sicc mg and sr from an eight year monitoring study from golgotha cave building on a previous study of hydrology and dripwater oxygen isotopes treble et al 2013 golgotha cave is developed in quaternary aeolianite and located in a forested catchment in the mediterranean type climate of southwest western australia all dripwaters from each of the five monitored sites become supersaturated with respect to calcite during most of the year when cave ventilation lowers pco2 in cave air in this winter ventilation mode prior calcite precipitation pcp signals of increased mg ca and sr ca in dripwater are attributed to stalactite deposition a fast dripping site displays less evolved carbonate chemistry implying minimal stalactite growth phenomena which are attributed to minimal degassing because of the short drip interval 30 s we employ hydrochemical mass balance modelling techniques to quantitatively investigate the impact of pcp and co2 degassing on our dripwater initially we reverse modelled dripwater solutions to demonstrate that pcp is dominating the dripwater chemistry at our low flow site and predict that pcp becomes enhanced in underlying stalagmites secondly we forward modelled the ranges of solution mg ca variation that potentially can be caused by degassing and calcite precipitation to serve as a guide to interpreting the resulting stalagmite chemistry we predict that stalagmite trace element data from our high flow sites will reflect trends in original dripwater solutes preserving information on biogeochemical fluxes within our system by contrast stalagmites from our low flow sites will be dominated by pcp effects driven by cave ventilation our poorly karstified system allows us to highlight and quantify these in cave pcp processes which are otherwise masked at sites where karstification is more developed and hydrogeology is more complex our modelling also shows enhanced co2 source production in the unsaturated zone that is attributed to deeply rooted vegetation and increasing bioproductivity which we link to forest recovery after fires impacted our site during 2006 ce

structural style of basin inversion at mid crustal levels two transects in the internal zone of the brasilizno aracuai belt minas gerais brazil the araguai belt is the orogenic belt that directly borders the eastern margin of the silo francisco craton m eastern brazil detailed structural investigations in the governador valadares region of minas gerais indicate that the amphibolite to granulitegrade internal zones of the ara9uai belt contain several major west vergent crystalline overthrust sheets these thrust sheets contain approximately homoclinal east dipping gneissic banding and are separated from one another by zones of isoclinally and sheath folded ductiley sheared metasedimentary units that behaved as mechanically weak glide horizons during deformation we interpret this regionally imbricated sequence of basement and cover to be the mid crustal level manifestation of closure of a mid neoproterozoic rift basin that existed to the east of the sao francisco craton the major thrusts which are all cratonvergent are of brasiliano pan african age 650 450 ma because they cut the neoproterozoic galil6ia batholith older fabrics are locally preserved in the basement slices and these fabrics may be relicts of the transamazonian orogeny 2 0 ga discrete zones of ductile brittle extension that were identified in several localities in the study area suggest the occurrence of postorogenic collapse following brasiliano overthrusting alternations of rigid crystalline thrust sheets and highly deformed metasedimentary sequences such as those of the governador valadares region may be a common structural geometry at a depth of 15 20 km in modern regions of collision and basin closure

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

a review of single sample based models and other approaches for radiocarbon dating of dissolved inorganic carbon in groundwater abstract numerous methods have been proposed to estimate the pre nuclear detonation 14c content of dissolved inorganic carbon dic recharged to groundwater that has been corrected adjusted for geochemical processes in the absence of radioactive decay 14c0 a quantity that is essential for estimation of radiocarbon age of dic in groundwater the models approaches most commonly used are grouped as follows 1 single sample based models 2 a statistical approach based on the observed curved relationship between 14c and 13c data for the aquifer and 3 the geochemical mass balance approach that constructs adjustment models accounting for all the geochemical reactions known to occur along a groundwater flow path this review discusses first the geochemical processes behind each of the single sample based models followed by discussions of the statistical approach and the geochemical mass balance approach finally the applications advantages and limitations of the three groups of models approaches are discussed the single sample based models constitute the prevailing use of 14c data in hydrogeology and hydrological studies this is in part because the models are applied to an individual water sample to estimate the 14c age therefore the measurement data are easily available these models have been shown to provide realistic radiocarbon ages in many studies however they usually are limited to simple carbonate aquifers and selection of model may have significant effects on 14c0 often resulting in a wide range of estimates of 14c ages of the single sample based models four are recommended for the estimation of 14c0 of dic in groundwater pearson s model ingerson and pearson 1964 pearson and white 1967 han plummer s model han and plummer 2013 the iaea model gonfiantini 1972 salem et al 1980 and oeschger s model geyh 2000 these four models include all processes considered in single sample based models and can be used in different ranges of 13c values in contrast to the single sample based models the extended gonfiantini zuppi model gonfiantini and zuppi 2003 han et al 2014 is a statistical approach this approach can be used to estimate 14c ages when a curved relationship between the 14c and 13c values of the dic data is observed in addition to estimation of groundwater ages the relationship between 14c and 13c data can be used to interpret hydrogeological characteristics of the aquifer e g estimating apparent rates of geochemical reactions and revealing the complexity of the geochemical environment and identify samples that are not affected by the same set of reactions processes as the rest of the dataset the investigated water samples may have a wide range of ages and for waters with very low values of 14c the model based on statistics may give more reliable age estimates than those obtained from single sample based models in the extended gonfiantini zuppi model a representative system wide value of the initial 14c content is derived from the 14c and 13c data of dic and can differ from that used in single sample based models therefore the extended gonfiantini zuppi model usually avoids the effect of modern water components which might retain bomb pulse signatures the geochemical mass balance approach constructs an adjustment model that accounts for all the geochemical reactions known to occur along an aquifer flow path plummer et al 1983 wigley et al 1978 plummer et al 1994 plummer and glynn 2013 and includes in addition to dic dissolved organic carbon doc and methane ch4 if sufficient chemical mineralogical and isotopic data are available the geochemical mass balance method can yield the most accurate estimates of the adjusted radiocarbon age the main limitation of this approach is that complete information is necessary on chemical mineralogical and isotopic data and these data are often limited failure to recognize the limitations and underlying assumptions on which the various models and approaches are based can result in a wide range of estimates of 14c0 and limit the usefulness of radiocarbon as a dating tool for groundwater in each of the three generalized approaches single sample based models statistical approach and geochemical mass balance approach successful application depends on scrutiny of the isotopic 14c and 13c and chemical data to conceptualize the reactions and processes that affect the 14c content of dic in aquifers the recently developed graphical analysis method is shown to aid in determining which approach is most appropriate for the isotopic and chemical data from a groundwater system

constraining spatial variability in recharge and discharge in an arid environment through modeling carbon 14 with improved boundary conditions carbon 14 14c has been widely used to estimate groundwater recharge rates in arid regions and is increasingly being used as a tool to assist numerical model calibration however lack of knowledge on 14c inputs to groundwater potentially limits its reliability for constraining spatial variability in recharge in this study we use direct measurements of 14c in the unsaturated zone to develop a 14c input map for a regional scale unconfined aquifer in the ti tree basin in central australia the map is used as a boundary condition for a 3 d groundwater flow and solute transport model for the basin the model is calibrated to both groundwater 14c activity and groundwater level and calibration is achieved by varying recharge rates in 18 hydrogeological zones we test the sensitivity of the calibration to both the 14c boundary condition and the number or recharge zones used the calibrated recharge rates help resolve the conceptual model for the basin and demonstrate that spatially distributed discharge through evapotranspiration is an important part of the water balance this approach demonstrates the importance of boundary conditions for 14c transport modeling 14c input activity for improving estimates of spatial variability in recharge and discharge

secular trends in the thermal evolution of metamorphic terrains the thermal evolution of convergent orogens is sensitive to the deformation history of the mantle part of the lithosphere if the dense mantle lithosphere thickens as a consequence of convergent deformation and remains attached during subsequent denudation of the thickened crust then high pressure low to intermediate temperature glaucophane schist assemblages will form during both thickening and denudation the actual form of the p t paths for a given deformation depending primarily on the initial thermal state of the lithosphere and particularly the ratio of the thickness of the crust and mantle lithosphere however if mantle lithosphere is thinned during convergence by processes such as thermal boundary layer detachment then any early formed high pressure low to intermediate temperature glaucophane schist assemblages will be obliterated during subsequent high temperature overprints since the stability of the thermal boundary layer during convergent deformations is controlled by the rate of convergence and the convective vigour of the subjacent mantle any change in the convective vigour of the mantle through geologic time in response to for example changing global heat production may therefore be reflected in the thermal evolution of metamorphic terrains formed during convergent deformations models of the thermal evolution of deforming lithosphere appropriate to an earth in which the effect of declining global heat production and convective vigour is mediated by the process of thermal boundary layer detachment suggest that the preservation of high pressure low to intermediate temperature glaucophane schist assemblages in convergent orogens has become increasingly probable through geological time a suggestion in accord with the observed secular trend in the character of metamorphic terrains preserved in the geological record

co2 dynamics in the amargosa desert fluxes and isotopic speciation in a deep unsaturated zone 1 natural unsaturated zone gas profiles at the u s geological survey s amargosa desert research site near beatty nevada reveal the presence of two physically and isotopically distinct co2 sources one shallow and one deep the shallow source derives from seasonally variable autotrophic and heterotrophic respiration in the root zone scanning electron micrograph results indicate that at least part of the deep co2 source is associated with calcite precipitation at the 110 m deep water table we use a geochemical gas diffusion model to explore processes of co2 production and behavior in the unsaturated zone the individual isotopic species 12co2 13co2 and 14co2 are treated as separate chemical components that diffuse and react independently steady state model solutions constrained by the measured 13c in co2 and 14c in co2 profiles indicate that the shallow co2 source from root and microbial respiration composes 97 of the annual average total co2 production at this arid site despite the small contribution from deep co2 production amounting to 0 1 mol m 2 yr 1 upward diffusion from depth strongly influences the distribution of co2 and carbon isotopes in the deep unsaturated zone in addition to diffusion from deep co2 production 14c exchange with a sorbed co2 phase is indicated by the modeled 14c profiles confirming previous work the new model of carbon isotopic profiles provides a quantitative approach for evaluating fluxes of carbon under natural conditions in deep unsaturated zones

deformation style way back when thoughts on the contrasts between archean paleoproterozoic and contemporary orogens abstract in many important ways archean and paleoproterozoic older orogens differ structurally from contemporary examples this essay examines the premise that contrasts between older orogens and contemporary orogens reflect long term changes in the temperature of the continental crust in the density of supracrustal sections and in exhumation rates for example if continental crust were warmer and exhumation rates faster earlier in earth history then higher grade rocks would occur closer to the surface of older orogens and the orogens would be lower and wider this situation might contribute to the formation of wide belts of high grade gneiss found in ancient crust if the high strength layer of the crust were thinner and supracrustal sequences denser earlier in earth history then regional extensional tectonism might lead to crustal scale boudinage and diapirism this situation might explain formation of the extensive dome and keel provinces found in ancient crust testing such speculations through the application of structural analysis coupled with petrologic studies dating and rheological modelling will constrain models of earth s long term physical evolution

geology of an archaean accretionary complex the structural record of burial and return flow in the tartoq group of south west greenland abstract this study presents structural and petrographic data from supracrustal slivers and associated ttg rocks of the mesoarchaen tartoq group in southwestern greenland that point to the burial and subsequent return flow of the mainly metamafic supracrustal and ttg rocks during subduction accretion along an obliquely convergent margin locally preserved high grade metamorphic up to lower granulite facies fabrics record the burial of the supracrustal rocks to depths 25 km during southeast directed subduction retrograde amphibolite and regionally widespread greenschist facies fabrics and structures indicate the subsequent exhumation of the rocks pro and retrograde fabrics are largely coaxial but retrograde fabrics record a reversal of shear and uniformly developed top to the east and southeast kinematics along extensional shear zones and overprinting brittle ductile fault zones extensional shearing and the concomitant retrogression are interpreted to be related to the northwest directed expulsion of the rocks in the subduction channel intrusive relationships between supracrustal slivers and ttg s leucogranites and pegmatites document the synkinematic emplacement of voluminous felsic magmas throughout this structural evolution while the structural evolution of rocks of the tartoq group shows the salient attributes of a modern convergent margin high t metamorphism pervasive ductile strains partial melting of the supracrustal rocks and syntectonic intrusion of voluminous granitoids are interpreted to reflect processes operating in a hotter archaean subduction zone slab melting and the formation of granitoids at relatively shallow depths in the subduction zone channel move the magmatic front to the fore arc region of the convergent margin the emplacement of felsic melts low strength and progressive disintegration of the downgoing slab choke subduction this has most likely resulted in only irregular and shortlived subduction disrupted by episodes of slab break off and subduction step back promoting tectonomagmatic accretion of ttg greenstones at relatively shallow levels but preventing subduction of rocks to larger depths this mode of shallow structural and magmatic accretion leads to lateral crustal growth documented by the at least 80 km wide accretionary complex of the tartoq group

a unified view of craton evolution motivated by recent deep seismic reflection and refraction results summary an attempt is made to integrate recent continental seismic reflection refraction and geologic observations into a unified scheme of craton evolution 1 nascent continental crust of basaltic bulk composition is produced in island arcs as with oceanic crust moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below the petrologic crust mantle boundary lying 25 per cent deeper in the lithosphere has no seismic expression and corresponds to a depositional magmatic contact between cumulate ultramafic rocks above and residual mantle ultramafic tectonite below both contacts are presumably extensively disrupted by magmatic injection 2 island arcs are amalgamated into continents at collision zones delamination of the lower mafic ultramafic portion of the crust along with the mantle lithosphere during hard collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition 3 subsequent extensional collapse of the overthickened crust together with thermal re equilibration of the lithosphere results in youthful stable continental crust which is 30 km thick with its top surface awash at sea level and has the bulk composition of andesite this crust characteristically exhibits extensional features half grabens in the upper portion and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination the moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high strain zone or intrusive contact separating mafic intermediate composition material above from fertile mantle lherzolite below 4 subsequent cratonization is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of myr as manifest by the ubiquitous overlapping dike swarms of the precambrian shields this results in net long term thickening of the crust by underplating shifts the bulk crust back toward mafic composition periodically produces a new deeper moho and petrologic crust mantle boundary that do not coincide and through repeated dike injection disrupts pre existing laminated reflectors the first magmatic injection event sweats out the light melting fraction in the pre existing andesitic crust producing voluminous granitic magmatism typified by the great proterozoic anorogenic granite rhyolite provinces subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences the cumulative result is shield type crust in which intermediate crustal levels are exposed over wide areas crustal thickness commonly exceeds 40 km average crustal velocity is somewhat higher than in younger stable crust and the crust is commonly complexly reflective diffractive throughout

on the emergence of plate tectonics at present young oceanic lithosphere is positively buoyant and it does not become negatively buoyant until it is older than about 20 m y if in the past the mantle was hotter the oceanic crust would have been thicker and the lithosphere9s age of neutral buoyancy greater on the other hand the hotter mantle would have had a lower viscosity and convected faster so the average age of oceanic plates at subduction would have been less than the present 100 m y at some time in the past average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone it is estimated here that this condition occurred when the mantle was only about 50 c hotter than at present and as recently as 0 9 1 4 b y ago transformation of basaltic crust to eclogite might have enhanced the vigor of the plates but possibly only modestly and intermittently in earlier times plate tectonics could have operated but more slowly so that it could not by itself have accomplished the necessary rate of heat removal from the earth a different tectonic mode must also have operated perhaps subcrustal delamination or dripping plate tectonics would have gradually taken over from the earlier mode plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer and so they could have operated in conjunction with both regimes of the upper boundary layer

evolution of the archean continental crust some 20 years of plate tectonic theory combined with new insights into the fine structure of the lithosphere the application of multielement geo chemical and isotopic studies paleomagnetism and geophysical modeling of mantle processes have profoundly influenced present thinking on the origin and evolution of the earth s early continental crust previously our knowledge of the continental crust was based almost exclusively on field geological observations although there is now general agreement on how the earth worked for the last 200 m y because of observable evidence in the oceans and continents e g bird 1980 condie 1982 it has proved difficult to extend this history into more ancient times in view of the lost oceanic record and the ambiguity and complexity of the pre mesozoic rock relationships in the continents dewey 1982 however preserved characteristic rock as semblages uniquely identifying modern type wilson cycle processes i e opening and closure of oceans underlain by oceanic crust have now been recognized in continental terranes as old as 900 m y and these assemblages provide strong evidence for the conclusion that the present global tectonic regime has governed the evolution of the lithosphere at least since the late precambrian kroner 1977 1981a b goodwin 1981 profound disagreement on the older crustal history however prevails to the present day since typical features characterizing phanerozoic accretionary terranes such as obducted ophiolites blueschists and franciscan type melanges have not been found in more ancient regions thus two types of evolutionary models have been developed one type postulates uniformitarian development back to the earliest archean burke

formation of earth s early archaean continental crust abstract subduction of oceanic crust at an unusually low angle has been proposed as a model for the growth of continental crust older than about 2 5 ga at modern zones of low angle or flat subduction magmatic additions to new crust come from partial melting of both the subducting oceanic crust slab and the thin wedge of mantle above the slab evidence for both a slab and wedge source is preserved in most late archaean 3 0 2 5 ga terrains but we find little evidence that a mantle wedge contributed to crustal growth prior to 3 1 ga this lack of evidence in part reflects a dearth of exposed crust aged between 3 0 and 3 3 ga but also suggests that subduction enriched mantle source regions did not develop before 3 3 ga and possibly not before 3 1 ga in contrast to most modern terrains and some late archaean terrains early archaean 3 3 ga continental crust evolved through direct melting of thick mafic crust we invoke a process of subduction that does not include the development of a mantle wedge and call this process archaean flat subduction to distinguish it from modern low angle subduction

ni cu pge deposits in finland geology and exploration potential abstract several types of nickel sulphide deposits occur in finland some of them are also important sources of cu co and pge the main deposit types are related to archaean and palaeoproterozoic mafic to ultramafic rocks palaeoproterozoic metamorphosed black shales of the talvivaara type or the ultramafic and metasomatic calc silicate rocks of the ophiolitic outokumpu association several komatiite hosted ni cu pge deposits have been discovered in the archaean and paleoproterozoic greenstone belts in northern and eastern finland but only one has been briefly mined in 1989 many 2 44 ga palaeoproterozoic mafic ultramafic layered intrusions in finland host contact type pge ni cu deposits and significant reef type pge deposits typically the ni resources are of low grade but large tonnage the mafic ultramafic magmatism at ca 2 06 ga produced significant ni cu pge resources in the central lapland greenstone belt including the kevitsa and sakatti deposits the svecofennian domain in central and southern finland contains hundreds of small ca 1 88 ga mafic ultramafic orogenic intrusions many of these host nickel mineralization nickel exploration potential was evaluated in general based on identification of critical factors within a mineral system framework and a quantitative assessment of the potential occurrence of undiscovered nickel resources the mgo content of the parental magma was estimated for 28 deposits 6 komatiite hosted occurrences and 22 svecofennian orogenic intrusions the estimated mgo content of the parental magmas for the selected deposits varies from around 7 to 15 in basaltic to komatiitic basalt systems and up to 30 in more magnesian komatiitic systems the ni cu value varies mostly between 0 7 and 12 correlating in general positively with the mgo content of the magma exceptionally high or low ni cu values in some deposits may result from for example copper loss during post magmatic processes serpentinization and or metamorphism or part of the sulphide ore zoned by the ni cu value still being unknown recent advances in understanding of komatiites in finland indicates high exploration potential for nickel within the paleoproterozoic komatiitic formations which typically are also significantly enriched in cu and pge however the quantitative assessment of undiscovered nickel resources in finland suggests that the contact type ni cu pge deposits hosted by 2 44 ga mafic ultramafic layered intrusions in northern finland have the greatest potential for hosting undiscovered resources among the assessed nickel deposit types these occur within restricted areas resulting in a high deposit density finally considerable nickel resources might exist in undiscovered talvivaara kevitsa or sakatti type deposits these deposit types should be assessed as soon as more geological information becomes available

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

carbon dioxide respiration in the deep vadose zone implications for groundwater age dating in the deep vadose zone at the dalmeny site subsurface gas samples were collected and analyzed for co2 and the 13c and 14c ratios of that co2 high concentrations of co2 depleted in 14c near the water table necessitate the use of an open system model of calcite dissolution to match observed dissolved inorganic carbon 14c ratios just below the water table groundwater age dating models assuming closed system calcite dissolution predict incorrect groundwater age dates at the dalmeny site these results and our field observations suggest that such errors may generally occur where deep vadose generation of nonmodern co2 is not accounted for in groundwater age dating exercises

core complex model proposed for gneiss dome development during collapse of the paleoproterozoic penokean orogen minnesota gneiss domes preserved throughout the paleoproterozoic 1870 1820 ma penokean orogenic belt are commonly viewed as collisional features however 1700 ma mica 40ar 39ar ages from archean rocks of the mcgrath gneiss dome minnesota are in marked contrast with 1750 1760 ma mica ages obtained everywhere outside the dome suggesting the dome may have formed well after compression had ceased we hypothesize that the bimodal age pattern is a result of crustal excision between archean basement and paleoproterozoic cover during extensional collapse of the overthickened orogen removal of structural section and progressive unroofing may have led to the development of a domed basement cover shear zone in this respect the mcgrath gneiss dome would represent one of the oldest analogues of a metamorphic core complex postorogenic collapse of the penokean orogen was apparently long lived with significant pulses occurring at 1755 ma and at 1700 ma the sudden and widespread crustal fusion at 1770 1760 ma that preceded the proposed collapse suggests that rapid subcrustal lithospheric thinning may have triggered collapse as has been proposed recently for several phanerozoic collapsed orogens

emplacement and cooling of komatiite lavas we propose that komatiite lavas were emplaced as turbulent flows accompanied by vigorous forced convection with cooling rates often in excess of hundreds of c h 1 they melted and assimilated up to 10 of the ground over which they flowed forming deep channels nickel sulphide mineralization may have resulted from incorporation of sulphur rich sediment after emplacement high cooling rates persisted resulting in spinifex textures due to compositional convection

evolution of chemical and isotopic composition of inorganic carbon in a complex semi arid zone environment consequences for groundwater dating using radiocarbon abstract estimating groundwater age is important for any groundwater resource assessment and radiocarbon 14 c dating of dissolved inorganic carbon dic can provide this information in semi arid zone i e water limited environments there are a multitude of reasons why 14 c dating of groundwater and traditional correction models may not be directly transferable some include 1 the complex hydrological responses of these systems that lead to a mixture of different ages in the aquifer s 2 the varied sources origins and ages of organic matter in the unsaturated zone and 3 high evaporation rates these all influence the evolution of dic and are not easily accounted for in traditional correction models in this study we determined carbon isotope data for dic in water carbonate minerals in the sediments sediment organic matter soil gas co 2 from the unsaturated zone and vegetation samples the samples were collected after an extended drought and again after a flood event to capture the evolution of dic after varying hydrological regimes a graphical method han et al 2012 was applied for interpretation of the carbon geochemical and isotopic data simple forward mass balance modelling was carried out on key geochemical processes involving carbon and agreed well with observed data high values of dic and 13 c dic and low 14 c dic could not be explained by a simple carbonate mineral co 2 gas dissolution process instead it is suggested that during extended drought water sediment interaction leads to ion exchange processes within the top 10 20 m of the aquifer which promotes greater calcite dissolution in saline groundwater this process was found to contribute more than half of the dic which is from a mostly dead carbon source dic is also influenced by carbon exchange between dic in water and carbonate minerals found in the top 2 m of the unsaturated zone this process occurs because of repeated dissolution precipitation of carbonate that is dependent on the water salinity driven by drought and periodic flooding conditions this study shows that although 14 c cannot be directly applied as a dating tool in some circumstances carbon geochemical isotopic data can be useful in hydrological investigations related to identifying groundwater sources mixing relations recharge processes geochemical evolution and interaction with surface water

new u pb and 40ar 39ar ages from the northern margin of the barberton greenstone belt south africa implications for the formation of mesoarchaean gold deposits gold mineralization in the mesoarchaean new consort gold mine in the barberton greenstone belt south africa is situated in the immediate hanging wall of an exhumed composite granitoid gneiss dome known as the stentor pluton the granitoid greenstone contact is characterized by a contact parallel high strain zone that separates the mid crustal gneisses from the greenschist facies greenstone belt in contrast to the lower greenschist facies deposits in the central parts of the greenstone belt the mineralized shear zones at new consort are hosted by distinctly higher grade metamorphic rocks ranging from upper greenschist to upper amphibolite facies conditions a distinct change of ore and alteration assemblages with increasing metamorphic grade points to a temporal and spatial relationship between gold mineralization and the juxtaposition of the stentor gneiss dome against the overlying greenstone belt examination of the structural and petrologic evolution of the mineralized high strain zone combined with new u pb and ar ar data and previously published ages indicates a multistage tectono metamorphic evolution early high t medium p metamorphism ca 600 700 c and 5 1 kbar in the structurally deeper parts of the mine is interpreted to be related to an early period of orogen parallel ne sw extension and associated detachment shearing during the main d2 accretionary event in the centre of the greenstone belt between ca 3260 and 3225 ma recent work has shown that these early high t fabrics were reactivated at ca 3105ma during the subsequent d3 tectonism and associated emplacement of large potassic granite batholiths the gold mineralization was coeval with ne sw extensional exhumation and solid state doming of the stentor gneiss dome during the subsequent d4 deformation this late stage deformation and the gold mineralization postdate the emplacement of the d3 related potassic granites by more than 50 million years as indicated by 1 a u pb age of 3046 27ma for a retrograde titanite generation in a d2 related amphibolite sample 113 and 2 a 3027 7 5ma age for hydrothermal titanite from the so called footwall lens mineralization which temperature has been estimated at 590 40 c this high t event is also recorded by a gcorrected h 3040 4ma hornblende ar ar age from the same sample suggesting that cooling of the stentor gneiss dome shortly followed the exhumation a considerably younger ar ar age 2966 4 ma has been obtained for an amphibolite from the contact parallel high strain zone pointing to non uniform cooling following the mineralization the gold mineralization was largely contemporaneous with the emplacement of nnw trending syn kinematic granitic pegmatites and strain localization along the pegmatite swarm at ca 3030 3040 ma the mineralization is also considerably younger than the gold mineralization in the greenschist facies gold deposits in the barberton greenstone belt which has previously been dated at ca 3080 ma in conjunction with previously published ages the data indicate the au mineralization in the barberton greenstone belt to have occurred during at least two mineralization episodes at ca 3080 and 3040 ma the main phase of au mineralization was introduced during the end of an either protracted or episodic structural and thermal evolution of the granitoid greenstone terrain that may have lasted some 200 ma

the himalayas a modern analogue for archaean crustal evolution abstract present crustal evolution models fail to account for the generation of the large volumes of continental crust in the required time intervals if the oceanic crust was appreciably thicker in the archaean as geothermal models would indicate then oceanic crustal collision on the scale of the present day himalayan continental collision zone may have been a frequent occurrence in the archaean resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts leaving a depleted stabilised basic residuum

contrasted tectonic styles for the paleoproterozoic evolution of the north china craton evidence for a 2 1 ga thermal and tectonic event in the fuping massif structural analysis along with 40ar 39ar and u pb datings in the fuping massif provide new insight into the evolution of the eastern part of the trans north china belt north china craton from 2 7 ga to 1 8 ga d1 is responsible for the development of a dome and basin structure coeval with crustal melting giving rise to migmatite and nanying gneissic granites at 2 1 ga this dome and basin architecture resulted from the interference between a n s compression of a weak ductile crust and gravity driven vertical flow in a high thermal regime the next events involved flat lying ductile thrusting d2 and normal faulting d3 dated at around 1880 ma and 1830 ma respectively the d2 and d3 events belong to the trans north china orogeny that results in the final amalgamation of the north china craton the d1 deformation is considered as evidence for an earlier orogen developed around 2 1 ga prior to the trans north china orogeny the change in the deformation style between the 2 1 ga and 1 8 ga could be viewed as a consequence of the cooling of the continental crust in the north china craton

proterozoic crustal extension and the generation of dome and keel structure in an archaean granite greenstone terrane archaean granite greenstone terranes1 3 in which narrow belts of greenstone ultramafic and mafic volcanics and overlying sedimentary rocks occur in association with broad provinces of granite granitic igneous rocks gneiss and migmatite have long puzzled geologists because of the lack of any clear modern analogues not only is there uncertainty about how the greenstone formed in the first place4 6 but there is continuing debate about how and when the terranes developed their distinctive structural and metamorphic architecture many granite greenstone terranes display a dome and keel geometry in which belts of supracrustal volcanic and sedimentary rocks occur as structural troughs wedged between dome shaped bodies of the granite gneiss migmatite complex a metamorphic aureole typically occurs in the supracrustals adjacent to their contact with the domes and the contact itself is a shear zone here we report the results of field studies of a granite greenstone terrane in brazil showing that this architecture originated during the proterozoic more than 500 million years after extrusion of the greenstone we suggest that the thermal regime and deformation kinematics necessary to create this architecture could be generated during an episode of crustal extension when hot basement rocks were transported upwards along a transcrustal normal fault system to the base of the supracrustals

crustal heat production and style of metamorphism a comparison between two archean high grade provinces in the limpopo belt southern africa abstract different characteristics of late archean metamorphism in the northern and southern marginal zones of the limpopo belt in southern africa are examined in the light of differences between these provinces in contents of heat generating elements in the northern marginal zone intrusion of charnoenderbitic magmas lasted from 2 75 to 2 58 ga accompanied by repeated low to medium pressure granulite facies metamorphism with some anticlockwise p t paths in contrast the medium pressure granulites of the southern marginal zone experienced peak metamorphism at 2 69 ga without much evidence of plutonism at that time followed by near isobaric cooling at approximately 6 5 kbar as a basis for our approach we used steady state geotherms calculated for the deeply eroded marginal zones for late archean times on the basis of their average k th and u concentrations and assuming low basal heat flow values of 20 mw m 2 the southern marginal zone and the adjacent northern kaapvaal craton are both characterised by low average th and u concentrations th 1 1 and 1 5 ppm u 0 3 and 0 65 ppm which are not a consequence of high grade metamorphism the average th and u concentrations in the zimbabwe craton 8 8 and 2 2 ppm are much higher than those in the kaapvaal craton and those found in the northern marginal zone are even higher 13 8 and 2 6 ppm in the southern marginal zone peak metamorphic temperatures are much above reasonable steady state geotherms and peak conditions could only have been attained through transient very high basal heat flux which in the context of a subduction zone might have resulted from slab breakoff or slab windows the northern marginal zone in contrast has such a large internal heat generation that high grade metamorphic conditions plot close to steady state geotherms even with basal heat flux values of 20 mw m 2 and minor crustal thickening or basal heat flux increase would have sufficed to exceed fluid absent tonalite solidus temperatures in the lower crust the repeated high grade metamorphism and charnoenderbite intrusions over a period of 1 7 108 years in the province can be explained by this observation both marginal zones of the limpopo belt can be understood as archean active continental margins of the kaapvaal and zimbabwe craton respectively while the problems of explaining the metamorphism in the southern marginal zone i e the source of the required heat are similar to those faced in many present day margins or orogens the northern marginal zone is non uniformitarian in the sense that its extreme heat generation rate could not be a property of any geological province today

continental thermal and tectonic regimes during the archaean metamorphic pressures estimated from mineral assemblages in archaean high grade terrains may be used to constrain geothermal regimes in the archaean continental crust in two ways first they may be combined with temperature estimates to yield bounds on the heat flux during the thickening events that produced the metamorphism the pressure temperature data do not require any greater supply of heat to the base of the continental crust in the archaean than is supplied at the present day second the pressures alone may be used to infer the height of archaean mountains these mountains appear to have had elevations comparable with those of present day mountain belts it seems probable that the elevation of present day belts is limited by the creep strength of the lithosphere and in view of the strong temperature dependence of creep in earth materials this second observation also implies that archaean continental thermal regimes were similar to present day ones these observations together with the common

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

archean magmatic granulites diapirism and proterozoic reworking in the northern marginal zone of the limpopo belt the northern marginal zone nmz of the limpopo belt southern africa is a high grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite which intruded minor mafic ultramafic and metasedimentary rocks between 2 74 and 2 57 ga the intrusive rocks have crustal and mantle components and occur as elliptical bodies interpreted as diapirs peak metamorphism p 800 mpa t 800 850 c occurred at ca 2 59 ga the highly radiogenic nature of the rocks in the nmz supplemented by heat from mantle melts led to heating and diapirism culminating in the intrusion of distinctive porphyritic charnockites and granites horizontal shortening and steep extrusion of the nmz during which crustal thickening was limited by high geothermal gradients contrast with overthickening and gravitational collapse observed particularly in more recent orogens the granulites were exhumed by the end of the archean the pervasive late archean shortening over the whole of the nmz contrasts with limited deformation on the zimbabwe craton possibly owing to the strengthening effect of early crust in the craton in the southeast of the nmz strike slip kinematic indicators occur within the transition zone and the triangle shear zone where dextral shearing reworked the archean crust at ca 1 97 ga

speculations on the thermal and tectonic history of the earth summary the connection between the earth s thermal history and convection in the mantle is exploited to elucidate the early evolution of the earth it appears probable that convection extending over almost all of the mantle has dominated vertical heat transport throughout the whole of the earth s history only in boundary layers at the surface and at a depth of 650 700 km is conduction likely to be important the resulting evolution appears to be consistent with geological observations on early precambrian rocks various arguments are put forward in favour of two horizontal scales of convective flow in the mantle at depths less than 650 km the large scale flow is related to the motion of major plates and must be ordered over distances of more than 5000 km its evolution and energetics are discussed and there are no obvious problems in maintaining the proposed convective motions small scale flow with an extent of the order of 500 km appears necessary both to explain the heat flow through older parts of the earth s surface and to reconcile the geophysical observations with the results of numerical experiments though the existence of the small scale flow is at present speculative various tests of its presence are proposed

dome and keel provinces formed during paleoproterozoic orogenic collapse core complexes diapirs or neither examples from the quadrilatero ferrifero and the penokean orogen paleoproterozoic dome and keel provinces in which troughs of deformed and metamorphosed paleoproterozoic supracrustal rocks surround domes of archean basement continue to puzzle geologists in current literature some authors refer to the domes as diapirs implying basement flowed vertically upward and others consider them to be cordilleran type metamorphic core complexes implying that the contact between basement and cover is an upwarped detachment geochronological studies suggest that dome emplacement occurred during extensional collapse of contractional orogens seemingly supporting the core complex analogy however structural analyses in the quadrilatero ferrifero brazil and the penokean orogen michigan demonstrate that the domes resemble diapirs in shape and in terms of surface kinematics the domes differ from diapirs however in that they did not flow penetratively within but were emplaced by movement along steeply dipping shear zones we suggest that contrasts between dome and keel and core complex provinces reflect contrasts between paleoproterozoic and phanerozoic crustal structure specifically during the paleoproterozoic when crust was warmer and supracrustal assemblages denser core complex like detachment faulting and associated plutonism resulted in juxtaposition of hot basement beneath a denser tectonically thickened supracrustal layer the resulting viscosity contrast and density inversion triggered vertical rise of diapir shaped basement domes by slip on steep shear zones

steep extrusion of late archaean granulites in the northern marginal zone zimbabwe evidence for secular change in orogenic style the northern marginal zone nmz of the limpopo belt in southern zimbabwe is an essentially plutonic terrain now consisting of granulite gneisses that was assembled by extensive magmatic under and intraplating between ca 2 7 and 2 6 ga during late archaean crustal shortening in this study we present the first evidence for regional scale normal shear in the nmz normal shear has been identified along an up to 500m wide steeply dipping belt of mylonites the mtilikwe shear zone geochronologic and mineralogical textural data indicate that normal shear occurred coeval with crustal shortening that was accommodated along gently dipping thrust zones the bulk strain pattern consisted of horizontal shortening vertical stretching and sub vertical extrusion of a crustal segment horizontal shortening persisted throughout the orogenic evolution in the nmz and no features typical of gravitational collapse in modern orogens are seen this could imply that there was never major overthickening of the crust by thrusting which may be a characteristic feature of archaean orogeny

a neoarchean paleomagnetic pole from the kisii series of western kenya implications for crustal mobility abstract the kisii series lavas of western kenya were the target of a paleomagnetic study the kisii series is a volcano sedimentary sequence dated to 2531 3 ma u pb that rests unconformably over the mesoarchean neoarchean nyanzian and kavirondian series the paleomagnetic study reported in this paper expands on an earlier study by brock et al 1972 using modern demagnetization and analysis techniques in spite of the advanced methods the results of both our new study and the previous study are statistically indistinguishable we therefore combine the results of both investigations to arrive at a grand mean pole at 7 s 166 e a95 8 due to the relatively minor tilt corrections a fold test is inconclusive however the dual polarity magnetization passes a reversal test we argue for a primary remanence on the basis of a comparison to other paleomagnetic data in kenya and tanzania and geological information we examine the relationships between age equivalent paleomagnetic poles from the tanzanian zimbabwe and kaapvaal cratons based on this limited dataset we cannot conclude whether or not the observed apparent polar wander was due to true polar wander or modern style plate tectonics such as would be implied by the variable apparent polar wander path segments we favor the latter explanation based on the disparate lengths of the zimbabwe versus tanzanian apparent polar wander paths during the 2700 2500 ma interval the protopangea model is not consistent with paleomagnetic data from these three cratons

images of a lower crustal oceanic slab direct evidence for tectonic accretion in the archean western superior province the archean western superior province in canada is the type area for proposed archean plate tectonics seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust this slab with inferred garnet amphibolite composition adds a critical piece of evidence to previous suggestions that archean subduction was at a shallow angle and that some neoarchean tonalite trondhjemite granodiorite suites distinct from most modern day suprasubduction magmas are melts primarily derived directly from subducted slabs

evolution of dissolved inorganic carbon in groundwater recharged by cyclones and groundwater age estimations using the 14c statistical approach abstract the canning basin is the largest sedimentary basin in western australia and is located in one of the most cyclone prone regions of australia despite its importance as a future resource limited groundwater data is available for the basin the main aims of this paper are to provide a detailed understanding of the source of groundwater recharge the chemical evolution of dissolved inorganic carbon dic and provide groundwater age estimations using radiocarbon 14 c dic to do this we combine hydrochemical and isotopic techniques to investigate the type of precipitation that recharge the aquifer and identify the carbon processes influencing 14 c dic 13 c dic and dic this enables us to select an appropriate model for calculating radiocarbon ages in groundwater the aquifer was found to be recharged by precipitation originating from tropical cyclones imparting lower average 2 h and 18 o values in groundwater 56 9 and 7 87 respectively water recharges the soil zone rapidly after these events and the groundwater undergoes silicate mineral weathering and clay mineral transformation processes it was also found that partial carbonate dissolution processes occur within the saturated zone under closed system conditions additionally the processes could be lumped into a pseudo first order process and the age could be estimated using the 14 c statistical approach in the single sample based 14 c models 14 c 0 is the initial 14 c dic value used in the decay equation that considers only 14 c decay rate a major advantage of using the statistical approach is that both 14 c decay and geochemical processes that cause the decrease in 14 c dic are accounted for in the calculation the 14 c dic values of groundwater were found to increase from 89 pmc in the south east to around 16 pmc along the groundwater flow path towards the coast indicating ages ranging from modern to 5 3 ka a test of the sensitivity of this method showed that a 15 error could be found for the oldest water this error was low when compared to single sample based models this study not only provides the first groundwater age estimations for the canning basin but is the first groundwater dating study to test the sensitivity of the statistical approach and provide meaningful error calculations for groundwater dating

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

quantitative mineral resource assessment of undiscovered pge resources in finland abstract most of the known platinum group element pge resources in finland are in contact type and reef type deposits in 2 45 ga mafic ultramafic layered intrusions these intrusions are also estimated to contain the majority of possibly existing yet undiscovered pge resources in finland the undiscovered pt pd au ni and cu resources in contact type and reef type deposits were estimated down to 1000 m depth using a three part quantitative assessment method this included the delineation of 19 contact type and 24 reef type permissive tracts the formulation of a grade tonnage model for the contact type deposits and a separate model for each reef type tract estimation of the number of undiscovered deposits for each permissive tract and monte carlo simulation to estimate the frequency distributions of metal tonnages in the undiscovered deposits the expected number of undiscovered contact type deposits is 29 and that of reef type deposits is 23 at the 50 probability level the total undiscovered pge resources of finland are estimated to be at least 5600 t pt 12 000 t pd 430 t au 4 2 mt ni and 5 7 mt cu at the 90 probability level the corresponding tonnages are 720 t pt 1700 t pd 59 t au 0 87 mt ni and 0 91 mt cu more than 80 of the pt pd in the undiscovered resources are estimated to be in reef type deposits the known pge resources in finland are 259 t pt pd in six contact type deposits plus 69 t pt pd in the atypical kevitsa deposit these figures indicate that the presently known deposits account for approximately 2 of the total assessed pge resources in finland they suggest that significant potential exists for the discovery of additional pge resources in the country

structural evolution of the warrawoona greenstone belt and adjoining granitoid complexes pilbara craton australia implications for archaean tectonic processes abstract the pilbara granite greenstone terrain 3 6 2 8 ga australia is quoted as the key area formed by passive gravity driven solid state diapirism but contrasting views have been reported recently which led us to re examine the key area the warrawoona greenstone belt and the adjoining mt edgar and corunna downs granitoid complexes we found i an asymmetric non synclinal distribution of metamorphic grade and deformation intensity across the warrawoona greenstone belt ii a major shear zone on the margin of the mt edgar granitoid complex with a unidirectional stretching lineation pattern iii a ne trending transcurrent shear zone within the central gneiss complex of the mt edgar granitoid complex iv coeval magmatic activity and doming of the granitoid complexes and v the loss of about 9 km of stratigraphic units in the warrawoona greenstone belt the area has had a prolonged history with at least four deformation phases between ca 3 47 and 2 8 ga the main domal geometry of the mt edgar granitoid complex was obtained during active ne sw extension at 3 31 ga d x 2 which appears to been in response to e w compression possibly the e w compression resulted in crustal thickening and partial melting probably of a 3 46 3 43 ga granitic basement causing crustal weakening collapse and extrusion of the 3 31 ga granitoids extension at 3 31 ga was accommodated by the mt edgar shear zone a mid crustal detachment and by the associated beaton well zone which acted as a transfer zone within the gneissic basement doming of the mt edgar granitoid complex is interpreted to have occurred in an active extensional core complex type mode an indication for deformation prior to 3 32 ga comes from syntectonically intruded gabbro diorite and dolerite sills within the mt edgar shear zone which intruded possibly as early as 3 46 3 45 ga d x 1 ultramafic schistose xenoliths within the gabbro diorite intrusions are the oldest observed deformed rocks formed during d x post doming deformation involved ne sw compression d x 3 followed by dextral transcurrent deformation d x 4 during nw se compression the latter may be related to the regional phase of deformation at 2 93 ga during which the eastern and western pilbara were amalgamated comparison of this deformation sequence to others within the marble bar domain suggests a common tectonic history from 3 46 ga onwards our structural and kinematic data especially the 3 31 ga unidirectional stretching lineation pattern lead us to reject solid state diapirism due to convectional overturn as the main cause for doming of the mt edgar granitoid complex we suggest that the cause lies in a plate tectonic process

factors affecting carbon 14 activity of unsaturated zone co2 and implications for groundwater dating summary unsaturated zone processes may influence the carbon 14 14 c activity of infiltrating groundwater and thus introduce error in derived groundwater residence times however unsaturated zone 14 c activities are rarely measured and there is little understanding of how they may vary spatially in a groundwater basin in this study we measured 14 c activity in unsaturated zone gas at five sites with different watertable depths 8 2 31 5 m in the arid ti tree basin central australia we observed a relatively uniform decrease in 14 c activity of unsaturated zone gas with depth at most sites with variation in unsaturated zone depths leading to variation in 14 c activities directly above the watertable at each site ranging from 54 to 106 percent modern carbon pmc through modelling we show that the profiles are influenced by co 2 production at different depths from sources with different isotopic ratios including production of modern co 2 in the root zone and production of old co 2 above the watertable scenario modelling showed that these processes are independent of recharge when recharge is low 0 10 mm y 1 but that higher recharge rates 100 mm y 1 result in more advective transport of atmospheric co 2 to the watertable the variation in 14 c above the watertable was more sensitive to watertable depth and shallow and deep co 2 production rates these findings offer insight into how unsaturated zone 14 c activities may vary spatially and provide guidance as to when 14 c depletion in unsaturated zone co 2 may become important for groundwater dating particularly in arid settings

early precambrian crustal development changing styles of mafic magmatism the work of sutton watson 1951 was one of the cornerstones in the interpretation of the evolution of early precambrian archaean and early proterozoic gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted archaean cratons since then the concept of geological terranes and terrane accretion has been extended back to the archaean and different events can be distinguished by increasingly precise isotopic dating techniques there is growing evidence that with certain clear provisos the principle of uniformitarianism can be applied as equally to the early precambrian as it can through the rest of the geological record in terms of mafic magmatism there are similarities between some ancient and modern suites both intrusive and extrusive but there are also some major differences such as the presence of komatiites in the archaean and the formation of large scale layered intrusions and extensive dyke swarms at the end of the late archaean and in the early proterozoic the mantle has evolved continuously with the progressive extraction of mafic magmas but allowing for this continental tholeiites appear to have changed little with time ancient mid ocean ridge basalts are elusive because of their poor preservation record the geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present day arc systems the relative abundance of early precambrian intrusive mafic suites parallels the crustal growth curve reflecting the global crustal accretion differentiation superevent towards the close of the archaean bimodal tholeiitic noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle mafic crust continental crust system this in turn gives a possible link between the respective evolutions of archaean to recent komatiitic noritic and boninitic suites

metamorphism and melting of the lithosphere due to rapid denudation nanga parbat massif himalaya the nanga parbat haramosh massif of the western himalaya has undergone a complex metamorphic and denudational history over the past 1 8 ga metamorphism in the tato region is characterized by a barrovian type metamorphism followed by partial melting and recrystallization along shear zones during nearly isothermal decompression while the massif was still at temperatures 550 c mineralogical and textural evidence for this path includes garnet zoning patterns late stage migmatization abundant cordierite both in schists and leucogranite dikes replacement of high pressure assemblages by low pressure assemblages and abundant low density inclusions thermobarometry on the nonmigmatized gneisses reveals conditions of 540 740 c at 7 1 13 1 kbar thermobarometry in the migmat ized rocks reveals final equilibration at 608 675 c at 3 9 6 8 kbar early fluid inclusions occur in quartz inclusions within garnet porphyroblasts that grew during decompression the last fluid inclusions to be trapped occur in microfrac

implications of melting for stabilisation of the lithosphere and heat loss in the archaean abstract the increased depth and volume of melting induced in a higher temperature archaean mantle controls the stability of the lithosphere heat loss rates and the thickness of the oceanic crust the relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions the density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200 c or more hotter than modern upper mantle if archaean continental crust formed initially above oceanic lithosphere the compositional density gradients may be sufficient to preserve a thick archaean continental lithosphere within which the archaean age diamonds are preserved the amount of heat advected by melts at mid ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases archaean tectonics could have been dominated by spreading rates 2 3 times greater than now and with mantle temperatures between ca 1600 c and 1800 c at the depth of the solidus mid ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form protected from copious volcanism and high mantle temperatures

chemistry of unsaturated zone gases sampled in open boreholes at the crest of yucca mountain nevada data and basic concepts of chemical and physical processes in the mountain boreholes open to the unsaturated zone at the crest of yucca mountain nevada were variously sampled for co2 including 13c and 14c ch4 n2 o2 ar cfc 11 cfc 12 and cfc 113 from 1986 to 1993 air enters the mountain in outcrops principally on the eastern slope is enriched in co2 by mixing with soil gas and is advected to the mountain crest where it returns to the atmosphere the cfc data indicate that travel times of the advecting gas in the shallow tiva canyon hydrogeologic unit are 5 years the 14c activities are postbomb to depths of 100 m indicating little retardation of 14co2 in the shallow flow systems the 14c activities from 168 to 404 m in the topopah spring hydrogeologic unit are 85 90 pmc at borehole usw uz6 the cfc data show that the drilling of usw uz6 in 1984 has altered the natural system by providing a conduit through the paintbrush nonwelded unit allowing flow from topopah spring outcrops in solitario canyon on the west to usw uz6 upward in the borehole through the paintbrush to the shallow tiva canyon flow systems and out of the mountain

implications of centrifuge simulations of channel flow for opening out or destruction of folds abstract centrifuge models of modelling clay and silicone microlaminates upon silicone putty layers simulate folding and subsequent opening out and unfolding of a layered sedimentary cover sequence the superstructure upon a ductile substrate e g migmatitic gneiss in the infrastructure of large hot orogens or archaean terrains after initial layer parallel shortening a process akin to channel flow in nature is induced in models by removal of material to simulate foreland erosion isostatic readjustments due to lateral difference in gravitational loading without further shortening result in extrusion and exhumation of lower ductile layers and vertical collapse opening out and unfolding of microlaminate layers of the cover sequence many active folds developed in the superstructure during initial layer parallel shortening open out and unfold during subsequent ductile flow of the infrastructure especially where they are rounded in profile and a basal decollement to the cover sequence is present angular to chevron folds tend to be translated or passively rotated and undergo less increase in interlimb angle isolated synforms or synform antiform pairs separated by broad areas of gentle undulations or plateaux in nature may be indicative that unfolding during channel flow has taken place fold axial surfaces especially for folds of finely laminated layers may also be rotated so that folds become hinterland verging experiments suggest that where channel flow has occurred in nature if unfolding were not recognised through detailed field observations the amount of regional shortening would be underestimated field criteria to recognise unfolding in nature are suggested our models predict that veins and saddle reefs developed during folding may be potentially preserved in an area of open or sub horizontal strata following unfolding which has implications in exploration for fold related mineralisation in large hot orogens or archaean terrains

role of a thin komatiite rich oceanic crust in the archean plate tectonic process in the archean tectonic regime the time available for oceanic lithosphere to cool on its journey from ridge to convergent plate boundary was far less than at present under these conditions only an inherently dense lithosphere could have acquired the negative buoyancy necessary for it to subduct at this time plate tectonics could have operated only if 1 the oceanic crust was rich in komatiite lava and 2 the thickness of this crust was not much greater than the thickness of modern oceanic crust

neoproterozoic oceanic crustal thinning emergence of continents and origin of the phanerozoic ecosystem a model ophiolites emplaced in collisional orogens prior to 1 ga generally have a thicker magmatic crust than younger complexes this fact suggests a model in which mesoproterozoic oceanic crust was two or three times as thick as at present possibly owing to partial melting of a more fertile mantle earth had less continental freeboard and 90 95 marine inundation producing a much more uniform worldwide temperature distribution neoproterozoic oceanic crust thinning increased ocean basin volume and continental freeboard and caused a drop of average sea level relative to the level of the continents the resultant increased erosion sediment deposition upwelling and consequent biological productivity and burial led to increased atmospheric oxygen which permitted the evolutionary development of animals