apatite feldspar u pb thermochronometer a reliable mid range 450 c diffusion controlled system the u pb systematics of apatite are commonly dominated by simple volume diffusion without secondary growth and recrystallization that can affect u pb systematics of sphene monazite and zircon although apatite is a relatively low u mineral both u and pb are lattice bound and thus the u pb systematics are not controlled by micro inclusions as appears to be the case for other low u minerals such as garnet and staurolite closure temperature estimates from both experimental and empirical studies range from 425 c to as high as 500 c for typical diffusion radii 25 m and cooling rates 2 100 c ma respectively this range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks in addition the closure temperature for pb diffusion in apatite fills a gap between that for pb diffusion in sphene 650 c and diffusion of ar in hornblende 500 c muscovite 350 c and biotite 300 325 c apatite is a common accessory phase in many rock types and u pb apatite dating can be a reliable and powerful thermochronometer as it combines mid range cooling ages with the strength of the u pb system in testing for isotopic closure precisions of u pb apatite dates can range from 1 2 to as low as 0 3 for proterozoic or archean dates the main limitation to accuracy of u pb apatite dates is a moderate to low ratio of u to pb and corresponding dependence of concordia coordinates on the choice of initial pb isotopic composition for proterozoic aged samples with measured 206pb 204pb of 100 the range of geologically reasonable initial pb isotopic compositions can shift the calculated u pb age by 60 ma fortunately even though the diffusion rate of pb in alkali feldspar is less than in apatite it appears that the isotopic composition of pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings examples are given from an archean terrane in se wyoming that was unroofed and cooled during proterozoic uplift and from proterozoic terranes in sw us that have contrasting thermal histories

lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

crustal thicknesses in se brazilian shield by receiver function analysis implications for isostatic compensation hypothesis are proposed to explain the data 1 a lower density by 30 40 kg m 3 i n the lithospheric mantle under the archean block of the sao francisco craton relative to the proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area relatively low density and high p wave velocity are compatible with a depleted low feo composition for the archean lithosphere 2 alternatively if the density contrasts between archean and proterozoic lithospheres are smaller than the values above then the crust beneath the paranabasin must be more dense than that of the craton higher crustal density and high poisson s ratio would be consistent with magmatic underplating in the lower crust beneath the paranabasin as inferred from other studies index terms 7205 seismology continental crust 1242 9360 information related to geographic region south america 7218 seismology lithosphere and upper mantle keywords crust upper mantle brazil parana basin san francisco craton

u pb geochronology of zircon and polygenetic titanite from the glastonbury complex connecticut usa an integrated sem empa tims and shrimp study abstract u pb ages for zircon and titanite from a granodioritic gneiss in the glastonbury complex connecticut have been determined using both isotope dilution thermal ionization mass spectrometry tims and the sensitive high resolution ion microprobe shrimp zircons occur in three morphologic populations 1 equant to stubby multifaceted colorless 2 prismatic dark brown with numerous cracks and 3 elongate prismatic light tan to colorless cathodoluminescence cl imaging of the three populations shows simple concentric oscillatory zoning the zircon tims age weighted average of 207pb 206pb ages from group 3 grains 450 5 1 6 ma mswd 1 11 and shrimp age composite of 206pb 238 u age data from all three groups 448 2 2 7 ma mswd 1 3 are interpreted to suggest a relatively simple crystallization history titanite from the granodioritic gneiss occurs as both brown and colorless varieties scanning electron microscope backscatter bse images of brown grains show multiple cross cutting oscillatory zones of variable brightness and dark overgrowths colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning electron microprobe analysis empa clearly distinguishes the two populations brown grains contain relatively high concentrations of fe2o3 ce2o3 up to 1 5 wt nb2o5 and zr cerium concentration is positively correlated with total ree y concentration which together can exceed 3 5 wt oscillatory zoning in brown titanite is correlated with variations in ree concentrations in contrast colorless titanite both as discrete grains and overgrowths on brown titanite contains lower concentrations of y ree fe2o3 and zr but somewhat higher al2o3 and nb2o5 uranium concentrations and th u discriminate between brown grains typically 200 400 ppm u all analyses but one have th u between about 0 8 and 2 and colorless grains 10 60 ppm u th u of 0 0 17 in contrast to the zircon u pb age results shrimp u pb data from titanite indicate multiple growth episodes in brown grains oscillatory zoned cores formed at 443 6 ma whereas white in bse cross cutting zones are 425 9 ma colorless grains and overgrowths on brown grains yield an age of 265 8 ma using the total pb method or 265 5 ma using the weighted average of the 206pb 238u ages however empa chemical data identify zoning that suggests that this colorless titanite may preserve three growth events oscillatory zoned portions of brown titanite grains are igneous in origin white cross cutting zones probably formed during a previously unrecognized event that caused partial dissolution of earlier titanite and reprecipitation of a slightly younger generation of brown titanite colorless titanite replaced and grew over the magmatic titanite during the permian alleghanian orogeny these isotopic data indicate that titanite like zircon can contain multiple age components coupling shrimp microanalysis with empa and sem results on dated zones as presented in this study is an efficient and effective technique to extract additional chronologic data to reveal the complexities of igneous crystallization and metamorphic growth

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

is the ventersdorp rift system of southern africa related to a continental collision between the kaapvaal and zimbabwe cratons at 2 64 ga ago abstract rocks of the ventersdorp supergroup were deposited in a system of northeast trending grabens on the kaapvaal craton approximately 2 64 ga ago contemporary with a continental collision between the kaapvaal and zimbabwe cratons we suggest that it was this collision that initiated the ventersdorp rifting individual grabens strike at high angles toward the continental collision zone now exposed in the limpopo province where late orogenic left lateral strike slip faulting and anatectic granites are recognized we relate the ventersdorp rift province to extension in the kaapvaal craton associated with the collision and see some analogy with such rifts as the shansi and baikal systems associated with the current india asia continental collision

pre 3000 ma thermal history of the archean kaap valley puton south africa the kaap valley pluton is one of several early archean 3200 3500 ma tonalitetrondhjemite plutons that surround the barberton greenstone belt southern africa precise dating using single grain 207 pb 206 pb evaporation of zircon and 40 ar 39 ar laser step heating of hornblende and biotite indicates that in its interior the kaap valley pluton preserves a memory of its initial intrusion and cooling which spanned a time from 3225 to 3142 ma the pluton also records the effect of a low temperature thermal event at its margin as seen by a 40 ar 39 ar biotite age of 3035 ma which is perhaps related to hydrothermal activity and gold mineralization in the adjacent barberton greenstone belt these pre 3000 ma ages are not in agreement with results of dating studies from sedimentary rocks in the barberton greenstone belt and plutons south of the belt which show evidence of having been overprinted by late archean events 2650 2700 ma and the bushveld complex intrusion 2050 ma these events have been interpreted as affecting most of the kaapvaal craton that the kaap valley pluton has escaped these and all other events since 3035 ma with temperatures never reaching 250 c implies that these large scale events did not affect the entire craton and the overprinting seen elsewhere is of a more local nature thus it is possible to determine the intrusive and cooling history of the pluton these data can be used in developing models of heat flow paleomagnetic remanence acquisition and deformation events

accretionary history of the archean barberton greenstone belt 3 55 3 22 ga southern africa the 3 55 3 22 ga barberton greenstone belt south africa and swaziland and surrounding coeval plutons can be divided into four tectono stratigraphic blocks that become younger toward the northwest each block formed through early mafic to ultramafic volcanism onverwacht group probably in oceanic extensional island or plateau settings volcanism was followed by magmatic quiescence and deposition of fine grained sediments possibly in an intraplate setting late evolution involved underplating of the mafic crust by tonalitic intrusions along a subduction related magmatic arc yielding a thickened buoyant protocontinental block the growth of larger continental domains occurred both through magmatic accretion as new protocontinental blocks developed along the margins of older blocks and when previously separate blocks were amalgamated through tectonic accretion evolution of the barberton belt may reflect an early archean plate tectonic cycle that characterized a world with few or no large stabilized blocks of sialic crust

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

early archean 3 2 ga fe oxide rich hydrothermal discharge vents in the barberton greenstone belt south africa ironstone pods fe 2 o 3 72 5 97 2 wt interpreted to have formed by early archean sea floor related hydrothermal activity are described from the ca 3 5 3 1 ga barberton greenstone belt south africa most of the pods are elliptical in shape and have their longest dimensions subparallel to the local stratigraphy they rest on silicified ultramafic rocks and are overlain by ferruginous shales silicified sand stones other coarse clastics and barite horizons the ironstone pods grade along strike into laminated iron oxide facies banded iron formation that is inferred to represent periodic discharge of iron oxide and silica rich flocculates from the pods the massive texture of the iron stone pods and their lack of internal sedimentary features suggest that they formed directly on the sea floor relict hydrothermal discharge chimney structures have been recognized honeycomb like cavities and possible fluid flow channel textures attest to primary porosity the iron stone pods are dominated by massive locally coarse grained specular hematite and goethite with lesser amounts of quartz and an x ray amorphous fe ai bearing silicate the pods are distinct from typical massive sulfide gossans in total trace element concentrations including precious and base metals indicating a different origin in contrast the ironstone pods have similarities in major oxide and trace metal concentrations to iron oxide deposits presently forming in part from low temperature hydrothermal fluids on the sea floor for example red seamount iron oxide deposit fluid inclusion studies on quartz show dominant type i primary fluid inclusions with salinities of 4 7 to 15 8 wt nacl equiv and homogenization temperatures t h of 90 to 150 c no evidence for boiling is seen type i fluid inclusions are most likely dominated by nacl cacl 2 h 2 o although their measured eutectic temperatures are consistent with the presence of fecl 2 these inclusions represent the end member hydrothermal fluid more saline 24 29 6 wt cacl 2 equiv lower temperature t h 33 109 c type ii and ila fluid inclusions represent pulses of a hydrothermal fluid of nacl cacl 2 fecl 2 or mgcl 2 h 2 o or possibly fecl 3 bearing composition mixing with ambient sea water in the case of type ii inclusions the various fluid types are interpreted to be indicative of intermediate compositions of an evaporitic brine the lowest salinity measurement for a type i inclusion of 3 1 wt nacl equiv provides a constraint on the salinity of unenriched by evaporation sea water the t h data give a minimum trapping pressure of 7 bars which equates to a minimum sea water depth of 60 m the lack of precise pressure estimates has not enabled calculation of maximum water depths above the pods quartz 18 o analyses average 17 3 0 6 1 n 5 calculated 18 o h 2 o for two samples are 1 4 and 1 2 respectively these estimates are within error of the postulated barberton early archean sea water value of 0 quartz hematite goethite mineral pairs yield temperatures inconsistent with fluid inclusion t h data suggesting isotopic disequilibrium

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

dependence of the flexural rigidity of the continental lithosphere on rheology and temperature while the first order mechanical properties of oceanic and continental lithosphere are well defined and explain the observed increase in lithospheric flexural strength with thermal age the temporal variation of continental flexural rigidity following loading and its dependence on lithospheric composition and temperature structure is only just beginning to be appreciated we describe here the results of a quantitative theological model in which laboratory determined rock deformation data and transienttemperature distributions resulting from intra plate basin formation within continental interiors have been integrated continental flexural rigidity is shown to be controlled by crustal thickness lithospheric thickness through the temperature structure and the interaction of these factors during basin formation for thermally young lithosphere the flexural rigidity is dominated by the quartzo feldspathic theology of the crust while for the thermally older lithosphere it is dominated by the olivine rheology of the mantle flexural data through thermo mechanical models of lithospheric deformation provide a powerful constraint on both the transient and steady state temperature structure of the lithosphere

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

field geochemical and u pb isotopic constraints from hypabyssal felsic intrusions within the barberton greenstone belt south africa implications for tectonics and the timing of gold mineralization abstract field mapping geochemical and u pb zircon isotopic data are reported for two undeformed feldspar quartz porphyries from within the central region of the barberton greenstone belt 3450 ma and from a porphyry at the deepest level of one of the largest gold mines in the barberton area fairview mine one porphyry the mudpools porphyry has an age of circa 3230 ma and provides a minimum age on unusual bif ferruginous shale hosted mudpool structures field mapping constraints and similar geochemical signatures between the mudpools porphyry and a second porphyry the ironstone pod porphyry suggest that 3230 ma can also be considered the minimum age of the equally unusual ironstone pods the two porphyries cross cut fig tree sediments which are regionally deformed d 2 and intrude up a thrust fault s suggesting pre 3230 ma ages for d 2 deformation and thrusting geochemical data trace elements ree indicate that these two porphyries are high level co magmatic equivalents of the nearby 3227 ma kaap valley tonalite which in part intrudes the lower sequences of the greenstone bell since the edge of the kaap valley pluton is also deformed by the d 2 deformational event either d 2 is diachronous or d 2 in the center of the belt is not equal to d 2 at the edge of the greenstone belt zircon from the fairview mine porphyry indicates a maximum age of 3126 21 ma 2 a gold mineralized shear which cross cuts the fairview porphyryl is responsible for hydrothermal alteration of the porphyry u pb analysis of hydrothermal rutile associated with gold from the porphyry gives an age of 3084 18 ma 2 and may represent the age of gold mineralization associated with this late shearing it appears that 3230 ma is a important age in early barberton greenstone belt history signifying intrusion of the kaap valley tonalite adjacent to the greenstone belt together with the emplacement of high level co magmatic equivalents porphyries of the kaap valley tonalite along regional thrust faults shear zones within the greenstone belt gold bearing hydrothermal fluids have utilized similar shear zones 150 ma after emplacement of both the porphyries in the central region of the greenstone belt and the kaap valley tonalite

transcurrent shearing granite sheeting and the incremental construction of the tabular 3 1 ga mpuluzi batholith barberton granite greenstone terrane south africa structural petrographic and geochronological studies show that the tabular 3 1 ga mpuluzi batholith in the barberton granite gneiss terrane in south africa was emplaced via a combination of external and internal processes external structural controls are indicated by systematic variations in intrusive relationships and strain along the margins of the mpuluzi batholith and are consistent with an emplacement of the granite in a dilational jog within a ne ene trending system of dextral transcurrent synmagmatic shear zones internally the mpuluzi batholith is essentially made up of granite sheets the structurally higher parts of the granite are made up of shallowly dipping sheets that are underlain by an anastomosing network of steeply dipping variably deformed dykes and sheets these granite sheets at lower structural levels intruded either into the actively deforming shear zones or into extensional sectors between and along the bounding shear zones multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3 13 ma the spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting the transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets

crustal radiogenic heat production and the selective survival of ancient continental crust if the oldest terrestrial rocks are to be used as a representative sample of the early terrestrial crust it is important to consider the possibility that they may represent a biased sample chemical data indicate that archean crust is on average depleted in potassium uranium and thorium relative to younger crust radiogenic isotopes of these elements produce a significant portion of surface heat flow from continents and low mean heat flow in archean terrains relative to younger terrains also indicates that these elements are relatively depleted in archean crust in stable continental lithosphere the primary variable controlling variations in the geotherm is crustal radiogenic heat production qualitative evidence suggests that terrains with low radiogenic heat production and thus a relatively cool geotherm are less susceptible to magmatic and tectonic reactivation than terrains with higher heat production and a hotter geotherm since crustal radiogenic isotope abundances must have been greater in the past if the isotopes were immobile in stable crust the relative effect of crustal heat production must have been greater and thus the range in geotherms must have been greater for the same mantle heat flow it is suggested that crust with low heat production is less susceptible to orogenic reworking than high heat production crust and thus there may have been selective stabilization of low heat production low potassium uranium and thorium crust early in earth history either archean crustal generation was by a mechanism that perhaps fortuitously produced only low heat production crust or all high heat production crust was reworked by orogenesis during the archean and did not stabilize in significant volumes until the earliest proterozoic perhaps in association with a reduction in mantle temperatures and mean heat flow if selective crustal reworking did operate during the archean surviving archean terrains may represent a biased sample of the early terrestrial continental crust

using along strike variation in strain and kinematics to define the movement direction of curved transpressional shear zones an example from northwestern superior province manitoba a review of current understanding of structures in shear zones reveals that correlating the orientation of stretching lineations with movement direction once a common practice based on the simple shear model is not generally applicable to transpressional zones it is difficult to determine the movement direction of a transpressional zone based on internal structures we show that the movement direction can be better defined by considering the variation in strain geometry and kinematics of differently oriented and kinematically related shear zones or different segments of a curved shear zone application of this method to a shear zone in the northwestern superior province of manitoba shows that the movement direction there is approximately north south in contrast to a commonly held view for similarly oriented shear zones in the superior province that the movement direction is approximately east west

lateral variations in sh velocity structure of the transition zone beneath korea and adjacent regions 1 using seismic profiles of triplicated waveforms we show significant lateral variations in the sh velocity vs structure of the transition zone tz beneath korea and adjacent regions beneath sakhalin we detected a high vsanomaly 2 limited to middle regions of the tz mid tz and a largevsjump across the 660 km discontinuity a similar jump invs also occurs beneath the northern portion of the north china craton ncc beneath korea a high vs anomaly 2 in the lower tz is inferred accompanied by a relatively small vsjump across the 660 km discontinuity which is depressed by about 15 20 km the deep structure under the eastern part of northeast china nec also includes a slightvsanomaly 1 in the lower tz but does not exhibit significant depression of the 660 km discontinuity compared with previous study our observations reveal strong regional variations of the tz structure on a relatively short scale these variations most likely reflect the geometrical distribution of the subducting northwest pacific plate our results suggest that the subducting slab dips across the mid tz under sakhalin and becomes flattened atop of the 660 km discontinuity beneath korea while only the tip of the slab reaches the lower tz beneath the nec the tz beneath the ncc does not show evidence of the slab stagnation

sheeted intrusion of the synkinematic mcdoogle pluton sierra nevada california field microstructural and geochronologic evidence indicates that the late cretaceous mcdoogle pluton located near the eastern margin of the sierra nevada batholith was emplaced as a subvertically sheeted complex into a steep reverse sense shear zone evidence of internal subvertical sheeting includes abundant concordant wall rock inclusions and screens that separate the pluton from adjacent jurassic plutons preservation of a ghost tectonostratigraphy in the distribution of the inclusions and rare interphase contacts the solid state tectonic fabric in the wall rocks and the magmatic submagmatic and weak solid state fabrics in the mcdoogle pluton all are concordant and record northeast southwest horizontal shortening subvertical extension and a significant component of northeast side up simple shear intrusive contacts generally are concordant with the fabrics but where discordances occur the wall rock fabric invariably is truncated by the contact however late synkinematic emplacement of the mcdoogle pluton is indicated by synintrusive boudinage of apophyses of the pluton and by the overall concordance of pluton fabrics including magmatic lineation zircon u pb isotope dates were obtained from the quartz monzodiorite central phase of the mcdoogle pluton 94 4 ma the mafic granodiorite border phase 94 8 0 6 ma and an older hornblende granodiorite included in the central phase 97 6 0 4 ma granodiorite orthogneiss in the wall rock yielded a u pb zircon date of 164 2 1 6 8 ma which we interpret to be the crystallization age and which is indistinguishable from a new age of 164 5 2 3 ma for the twin lakes pluton a u pb titanite date of 94 4 0 9 ma from the 164 ma orthogneiss sample may reflect thermal effects of the intruding mcdoogle pluton synkinematic growth of titanite in the shear zone or perhaps both the age of the sawmill lake shear zone that hosts the mcdoogle pluton is bracketed between 148 ma the age of the independence dike swarm and 92 ma the age of the lamarck pluton that crosscuts both the mcdoogle pluton and the shear zone fabrics regional evidence suggests a shear zone age younger than 110 ma but older deformation is possible the predominance of horizontal shortening over transcurrent motion and an age older than 90 ma exclude a direct relationship to the dextral sierra crest shear system which has been proposed to pass near the study area the location age constraints and kinematics of the shear zone are consistent with both the later stages of movement in the east sierran thrust system and isostatic sinking of the arc magmatic complex into its substrate

gliding and overthrust nappe tectonics in the barberton greenstone belt abstract regional recumbent folds inverted stratigraphy nappes and olistostromes are described from the southern part of the 3 3 3 5 ga barberton greenstone belt overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km2 and up to 500 m in thickness occurred over minimum distances of 86 km more ductile and coherent units were overfolded up to at least 2 km during their emplacement the glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns driven by heat sources from igneous intrusions the upwelling areas of the geothermal convection cells were sites of mud pools and hydrothermal vents which may mark the trailing edges pull aparts of the overthrust units progressive silica and carbonate precipitation due to decreasing temperatures within the zones of fluid migration distant from the areas of high heat flow probably acted as built in braking systems below the travelling slabs active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt the recognition of nappe and overthrust tectonics in the barberton belt processes which may have been commonplace in archaean terrains necessitates a re evaluation of the stratigraphy of this belt

emplacement features of archaean ttg plutons along the southern margin of the barberton greenstone belt south africa abstract the barberton greenstone belt in south africa and its surrounding granitoid terrane represents one of the type localities for archaean granite greenstone tectonics contact relationships between early archaean granitoids and greenstone sequences along the southern margin of the greenstone belt illustrate multiple simultaneously operating emplacement mechanisms for the plutons these emplacement mechanism which can be identified in individual plutons comprise ductile wall rock deformation stoping assimilation and the intrusion of ring dykes the interaction between these various mechanisms is interpreted to be responsible for the complex and highly variable discordant and concordant contact relationships observed in the granite greenstone terrane as well as the deformation observed within wall rocks and country rock xenoliths the absence of regionally consistent fabrics within the plutons suggests considerable competence contrasts between the tonalitic trondhjemitic plutons and in many cases the hydrated greenstone cover sequence during later subhorizontal crustal shortening which occurred parallel to the greenstone granitoid interface this competence contrast is interpreted to have resulted in the accentuation of cuspate lobate folds along the granitoid greenstone interface being partly responsible for the typical arcuate dome and keel geometry of the barberton greenstone belt

late syndepositional deformation and detachment tectonics in the barberton greenstone belt south africa the thick bedded competent quartzose sandstones conglomerates and siltstones of the moodies group 3 225 ma influence structural style in the 3 1 3 5 ga barberton greenstone belt moodies group rocks are folded into a train of tight subparallel doubly plunging overturned synclines separated by narrow strike parallel fault zones structural analysis along a transect across the central part of the greenstone belt suggests that much of the volcanic sedimentary greenstone sequence was buckle folded above a detachment within altered ultramafic rocks located at a depth of 3 6 km early development of brittle structures concentrated shortening and displacement near the fold hinges and in the anticlinal fault zones penetrative strain during folding was low the sense of displacement along faults the original fold vergence and the provenance of moodies group sandstone and conglomerate suggest that tectonic transport was directed toward the interior of the belt from its northwestern and southeastern margins orogenic contraction due to folding alone was at least 56 and subsequent ductile shortening strain accounted for an additional contraction of approximately 10 buoyant rise of the kaap valley tonalite to the north at approximately 3 22 ga and lateral intrusions of sheetlike granitic plutons along both sides of the preserved greenstone belt at 3 1 and 3 0 ga respectively imposed significant ductile strain on the greenstone belt margins and obscured or displaced the orogenic hinterlands of the facing fold and thrust belts the syn and post moodies tectonic history of the central barberton greenstone belt involves the formation and deformation of several shortlived alluvial to shallow marine sedimentary basins on unstable protocontinental crust the probable extensional origin of at least some of these basins and their subsequent shortening buckle folding and displacement above a subhorizontal detachment emphasizes the dominance of horizontal tectonics in the late stage deformation of this archean greenstone belt

radioelement concentrations in the deep profile through precambrian basement of the vredefort structure the vredefort basement granite in south africa forms the core of an undomed and overturned sequence of precambrian strata slawson 1976 proposed that the granite core has also been overturned exposing to view a thick section through the archean crust incrasing in depth toward the center of the dome profiles across the vredefort basement provide an opportunity for studying the distribution of u th and k with depth in the upper and middle crust detailed radioelement concentration profiles across the granitic basement showed that 1 the central part of the basement core is depleted in th and u relative to the perimeter and the concentration of th and u falls off regularly from the basement margin inward 2 the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function 3 the felsic rocks of the central basement are characterized by high k u and th u ratios and the ratios k u k th and th u increase from the margin inward and 4 mafic granulites which occur in the central regions of the dome have normal k u k th and th u ratios high grade metamorphic environment resulted in the depletion of bothmore u and th relative to k in the greater proportion of the felsic rocks from the central regions of the vredefort basement depletion of u was far greater than that of th the normal th u k u and k th ratios in mafic rocks in close proximity to the felsic rocks suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the vredefort basement less

finite element modelling of the structure and evolution of the south kenya rift east africa abstract a finite element fe model was used in this study to investigate the intraplate rifting process in south kenya east africa the rifting model shows that the important factors influencing the amount of shoulder uplift and rift subsidence include the horizontal deviatoric extensional stresses the viscosity of the lower crust and the dimension and density contrast of the low velocity upper mantle anomaly thus for instance a reduced lower crust viscosity as well as an increased extensional far field stress favours the subsidence of the rift basin in turn for a reduced lower crust viscosity the magnitude of the applied extensional stress should be reduced in order to end up with the same topography of the rifted area as can be obtained with a more viscous lower crust combined with a relatively high extensional far field stress in addition it also became obvious that the uplift of the crust mantle boundary beneath the rift depends predominantly on the size and density contrast of the upper mantle anomaly an increased density contrast between the upper mantle anomaly and normal mantle results in increased buoyancy forces thus producing an increased uplift of the crust mantle boundary in turn in order to avoid too large values for the surface topography a reduced lower crust viscosity in the order of 1021 pa s is required models without an upper mantle low density anomaly have resulted in crustal thickening beneath the rift and not in crustal thinning in accordance with the geological field evidence we have modelled the evolution of the south kenya rift in two stages in the first stage from 16 to 10 ma ago a broad regional uplift of a few hundred metres produced by a relatively small but broad low density upper mantle anomaly was accompanied by an insignificant amount of faulting in the second stage from 10 ma ago to the present day significant faulting accompanied by the buoyancy effects of a larger low density low velocity upper mantle anomaly concentrated more beneath the rift itself have quite successfully reproduced the rift subsidence shoulder topography and crust mantle boundary uplift observed at the present day

injection of indian crust into tibetan lower crust a two dimensional finite element model study we propose a model in which the stronger indian crust injects into the weaker lower crust of tibet if the viscosity of the tibetan lower crust is below a critical value the continuous advancing of the indian crust beneath tibet would produce negligible variation in stresses on the bottom of the stronger brittle tibetan upper crust and would result in a spatially uniform uplift and a virtually undisturbed tibetan plateau surface this process is analogous to that of a piston rising in a hydraulic jack as additional fluid is forced into the chamber we find that if the viscosity of the tibetan lower crust is assumed to be constant throughout the computed critical viscosity is 6 1018 pa s for the present day lower crustal thickness of 58 km in addition to uniform viscosity two hypothetical depth dependent viscosities are also tested which give rise to essentially the same distribution of stress along the bottom of brittle upper crust our model also interprets the 4 km elevation contrast between the tibetan plateau and the tarim block a 1 160 viscosity contrast between the tibetan and tarim lower crust is required in order to maintain the current thickening rate of the tibetan crust 1 2 mm yr while keeping the thickness of the tarim crust unchanged

formation of new continental crust in western british columbia during transpression and transtension crustal growth in the coast mountains along the leading edge of the canadian cordillera was the result of processes associated with horizontal flow of material during transpression and subsequent transtension and the vertical accretion of mantle derived melts from 85 to 58 ma as exotic terranes were translated northward during transpression the crust was thickened to about 55 km and melt that originated from a mix of mantle derived basalt with partial melt of the thickened crust intruded into crustal scale transpressive shear zones when the orogen extended between 58 and 50 ma there was large scale decompression melting in the mantle and dehydration melting in the lower crust voluminous emplacement of sub horizontal sills facilitated by ductile flow of the gneissic country rocks partially filled space created as the crust was pulled apart and as 15 to 20 km of tectonic exhumation occurred across low angle normal ductile shear zones by 50 ma the final crustal thickness of the new continental crust was about 34 km comparison of seismic data with other crustal sections suggests that the crust forming processes identified in western british columbia have general applicability to models for the formation of continental crust

absolute ages aren t exactly the time scales of important events and processes in geology and archaeology are ascertained through measurements of radioisotopes the accuracy of the dating depends critically on how well known the radioactive decay constants are in their perspective renne karner and ludwig discuss the role that discrepancies in decay constants play in the accuracy of radioisotope dating as dating measurements become increasingly precise geochronologists will have to confront these kinds of systematic errors

a 40ar39ar geochronological study of komatiites and komatiitic basalts from the lower onverwacht volcanics barberton mountain land south africa abstract 40 ar 39 ar step heating analyses have been carried out on domatiites and komatiitic basalts from the komati formation the uppermost formation of the tjakastad subgroup in the barberton greenstone belt the results for the komatiites fall into two groups which have integrated ages either greater than 3 3 ga or less than 3 0 ga the most precise plateau ages in the first group correspond to an age of 3 49 ga which we take to be the age of metamorphism since the whole rocks analysed are composed of metamorphic minerals other samples in the first group appear to preserve a record of a later event at 3 43 ga evidence of this younger age is also seen in the only komatiitic basalt displaying a stable age spectrum the second group of komatiites integrated ages less than 3 0 ga on a 40 ar 39 ar correlation diagram suggest that a 3 3 ga event may have introduced paleoatmospheric argon with a 40 ar 39 ar ratio of 190 into some of the komatiites comparison of the results of detailed electron microprobe analyses with the ca k spectra obtained in the step runs suggests that tremolite is the mineral tenaciously recording the great ages of the komatiites while hornblende is the dominant argon retainer in the komatiitic basalts the argon ages are completely consistent with recently published u pb ages of zircons from the barberton greenstone belt and suggest that the original metamorphism of the komatiites occurred shortly after their eruption

role of melt during deformation in the deep crust deformation in the deep crust is strongly influenced by the presence of melt injected melt or magma weakens the crust because strain will tend to localize where melt is present the amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate for plutons that intrude into rocks which are near the solidus temperature of the melt crystallization times can be quite long 1myr partial melting of deep crustal rocks can lead to melt enhanced embrittlement this occurs because the volume change for most melting reactions is positive therefore when the rate of melt production outpaces the rate at which melt can leave the system the melt pressure increases eventually the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the central gneiss belt canada this suggests that melt enhanced embrittlement occurred in these rocks an expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived assuming that migmatite has a small tensile strength differential stresses are 20 mpa in migmatitic rocks at the time melt enhanced embrittlement occurs the occurrence of melt enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting furthermore repeated episodes of melt enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

iib trioctahedral chlorite from the barberton greenstone belt crystal structure and rock composition constraints with implications to geothermometry iib trioctahedral chlorite in the barberton greenstone belt bgb metavolcanic rocks was formed during pervasive greenschist metamorphism the chem ical composition of the chlorite is highly variable with the fe fe mg ratio ranging from 0 12 to 0 8 among 53 samples the chemical variation of the chlorite results from the chemical diversity of the host rock especially the mgo content of the rock but major details of the variation pattern of the chlorite are due to the crystal structure of the chlorite all major cation abundances in the chlorite are strongly correlated with each other sil icon increases with mg and decreases with fe while aliv and alvi decrease with mg and increase with fe2 a complex exchange vector explains over 90 of the chlorite compositional variation mg4sife2 3alvi 1 aliv 1 which has 3 parts fe mg substitution coupled with one part tschermakite substitution this ratio is required to maintain the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets the subtle change in al substitution in chlorite implies that alvi is preferentially ordered in the m 4 site and about 84 of the alvi present is in the m 4 sites when they are nearly filled with alvi based on 47 analyzed chlorite bearing rock samples chlorite chl composition is strongly correlated with the mgo content of the host rock calculated correlation coefficients are 0 91 for sio2chl mgorock 0 87 for al2o3chl mgorock 0 89 for mgochl mgorock and 0 85 for feochl mgorock only weak correlations have been found between chlorite oxides and other oxides of rock between same oxides in chlorite and rock sio2 0 67 al2o3 0 59 feo 0 41 however mgochl is saturated at about 36 wt in rocks that have mgo above 22 wt the mgochl is about 5 wt when the host rock approaches 0 wt of mgo this implies that mg substituting into the chlorite is approximately limited to 1 5 9 2 mg atoms per formula unit and 1 0 3 2 aliv chlorite geothermometers can not be applied to all bgb samples however the empirical chlorite geothermometer based on aliv of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition of the chlorite from basaltic and dacitic samples in this study an estimated temperature of about 320 c for the greenschist metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers

early archean foredeep sedimentation related to crustal shortening a reinterpretation of the barberton sequence southern africa abstract the upper dominantly terrigenous sedimentary successions of the early archean barberton sequence kaapvaal province record an evolution similar to that of phanerozoic foredeeps and may represent the oldest known foredeep sequence rather than a passive continental margin sequence as previously proposed pre foredeep ocean crust the 3 5 ga old onverwacht group consists of bimodal volcanics with intercalated shallow water sediments gravity modeling and boundary shear zones indicate that deformed onverwacht crust was obducted northward onto sialic continental crust immediately before during or after burial by the synorogenic terrigenous clastic fill foredeep basin deepening due to crustal loading from the south is recorded by basinal iron formation chert and mudstone of the basal fig tree group which accumulated paraconformably on the onverwacht overlying sediments of the fig tree group consist of 2 km of submarine fan greywackes and mudstones shoaling of the basin is indicated by the transition to the overlying braided alluvial and shallow marine sediments of the ca 3 3 ga old moodies group 3 km thick changing ree patterns and relative abundances of ni cr sr th and ur in the sediments of the fig tree and moodies groups indicate a significant ultramafic mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution a similar trend is reflected in sandstone petrography in which the ratio of extrusive to intrusive rock fragments decreases upwards in the fig tree group whereas the moodies group sediments are enriched in quartz and potassium feldspar a persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of phanerozoic recycled orogens the distribution of facies in the fig tree and moodies groups and paleocurrent petrographic and geochemical data indicate progressive unroofing of a southerly source terrane consisting of the onverwacht volcanics and intercalated sediments and the 3 5 3 2 ga old ancient gneiss complex agc in addition older components of the fig tree group were recycled into overlying stratigraphie units the agc consists of mantle derived 3 5 ga old bimodal metavolcanics and metasediments a tonalite gneiss batholith metabasite dikes and meta anorthosites its structures record progressive northwestward uplift beginning around 3 4 ga ago intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large scale northwestward simple shear followed by imbricate faulting and pseudotachylite formation by 3 3 ga ago the fig tree and moodies groups had also shortened with similar kinematics to those of the agc by partly syndepositional folding and volume loss during the formation of transecting cleavage further folding and slide formation followed during subsolidus diapiric rise of the initially tabular kaap valley tonalite by 3 2 ga and emplacement of trondhjemite plutons from 3 3 to 3 0 ga ago

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

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

fabric development in syn tectonic intrusive sheets as a consequence of melt dominated flow and thermal softening of the crust the temperature dependence of ductile deformational processes suggests that the thermal anomaly associated with a cooling syn tectonic intrusion will produce a crustal low viscosity horizon that will concentrate deformation in the vicinity of the pluton during cooling this effect should be most prominent when magma is present but should also occur after solidification of the magma when flow is by solid state processes this paper evaluates this hypothesis for sheet like intrusions using one dimensional time dependent thermal models and accompanying predictions of viscosity vs time history based on experimental flow laws these models predict that for systems with an initial normal geothermal gradient e g extensional systems strike slip systems or thrust systems with low displacement rates the base of a large plutonic sheet cools more slowly than the top and fabric development should be most pronounced on the floor of the pluton in contrast in megathrust systems where displacements are sufficiently rapid to produce temperature inversions cooling is also upside down and fabric development should preferentially occur along the top of plutonic sheets moreover when a pluton is emplaced within a zone of inverted isotherms the heat may be trapped within the inversion a natural system characterized by this history should show a sharp thermal front coincident with the top of a paleo temperature inversion if a pluton is weaker than its country rock it will form a weak horizon in the crust throughout its cooling history and the plutonic sheet should take up the bulk of the deformation throughout its cooling history if a pluton is stronger than its country rock under conditions of solid state flow however models predict a two phase deformational history prior to solidification the deformation should be concentrated in the pluton with the magma representing a weak horizon in the crust but upon crystallization the pluton should become relatively rigid with respect to its country rock and deformation should be concentrated in the wall rocks these two cases predict markedly different structural histories and the history could easily be misinterpreted in the first case static mineral growth might be prominent in the wall rocks and these textures could be misinterpreted as evidence for post tectonic emplacement in the second case the extensive high t deformation of the wall rocks could be readily ascribed to non tectonic emplacement related deformation rather than a consequence of syn tectonic emplacement the model predictions are tested with three examples of well exposed plutonic sheets a neogene extensional system in death valley california and two megathrust systems in southern alaska border ranges fault system and mclaren metamorphic belt structural observations from these systems are broadly consistent with the model predictions and lend support to first order characteristics of the models nonetheless more sophisticated 2 d models of the coupled thermal mechanical system are needed to thoroughly test the hypothesis

heterogeneity of the middle crust implications for strength of continental lithosphere paleoproterozoic rocks in the southwestern united states record tectonism at depths of 10 20 km during 1 75 1 65 ga amalgamation of arcs to north america these exposures of middle crust are characterized by great heterogeneity of composition structure and metamorphic grade the crust is dominated by subvertical fabric and is laterally segmented into 10 km scale blocks by subvertical shear zones metamorphic temperature gradients of 200 c occur within 10 km length scales at various 10 20 km depths hottest areas are near granitoid plutons 10 km depths or dike swarms 20 km depths and are dominated by subhorizontal foliations lowest temperature blocks depths 10 km show brittle ductile thrust belt geometries and all levels show a range of brittle semi brittle and ductile deformation features our synthesis suggests that the synorogenic middle crust exhibits a distinctive heterogeneity of tectonism influenced strongly by advective heating and resulting strength variations segmentation of the crust into tectonic blocks is interpreted to be related to the 10 km 10 m y space and time scales of plutonism and melt transfer in the middle crust by analogy to experimental data we hypothesize that the middle crust was a spatially and temporally complex strength beam during deformation with vertical fabric recording strain hardening during whole lithosphere failure heterogeneity and a dominance of vertical fabric are consistent with a general lack of middle crustal reflectivity and may have influenced differential uplift during subsequent tectonism

ages of altered granites adjoining the witwatersrand basin with implications for the origin of gold and uranium one of the main problems in understanding the origin of the witwatersrand basin mineralization concerns the source of the prodigious concentrations of gold and uranium in the basin another problem is the age and nature of the rocks in the source area recent data1 2 indicate that the source area was largely 3 200 2 900 myr old becoming progressively younger as the basin evolved granites adjoining the basin are commonly overprinted by pervasive and vein related hydrothermal alteration which resulted in an enrichment of both gold and uranium3 4 here we report uranium lead data demonstrating that these hydrothermally altered granites were intruded sporadically both before and during the 360 myr period5 constraining witwatersrand sediment deposition and that hydrothermal alteration was associated with granitoid intrusion and subsequent cooling thus fertilization of the crust occurred during basin evolution a feature that supports the idea19 that gold and uranium mineralization occurred simultaneously with sediment deposition

influence of lithological heterogeneity mechanical anisotropy and magmatism on the rheology of an arc north cascades washington abstract many aspects of crustal dynamics are dependent on changes in rheology and strength with depth in the lithosphere several of the controlling factors for rheology are difficult to study experimentally particularly lithological heterogeneity mechanical anisotropy and magmatism and we focus on these in a study of the deformation patterns in a thick crustal section 5 to 40 km paleodepth through the cretaceous cascades core of the nw cordillera this magmatic arc consists of metamorphosed oceanic and arc terranes intruded by magmatic bodies ranging from heterogeneous brittle deformation marked by serpentinite melange characterizes the shallowest part of the crustal section and the remainder of the section is characterized by heterogeneous fold dominated ductile deformation early tight to isoclinal recumbent folds and associated axial planar fabrics are refolded by one or more cycles of nearly coaxial open to isoclinal upright to overturned folds layering played a mechanically active role during folding at all levels as indicated by cleavage refraction boudinaged layers and kinematic indicators that record fold related shear ductile deformation intensifies in the narrow structural aureoles of plutons and sw directed reverse shear was partitioned into some of the aureoles the poor strain memory of these magmatic bodies makes it difficult to determine if deformation was focused in the pluton magma chambers before they reached the solidus as commonly predicted all of the plutons have magmatic foliations that at least in part reflect regional strains and these foliations are strong in the deeper plutons the thinner sheets acted as competent bodies during folding and boudinage after they reached the solidus but generally did not cause marked strain gradients in their hosts a relative strength profile constructed for the cascades crustal section shows an overall decrease in strength with depth for the ductile part of the arc that fits idealized strength profiles however in more detail relative strengths are markedly variable units were able to accumulate large ductile strains but even small variations in the physical properties of interlayered rock types exerted a strong influence on deformation patterns throughout the mid to deep crustal part of the profile this profile thus emphasizes the complex vertical rheological stratification of arcs at the crustal to thin section scale and should be applicable to many other magmatic arcs

the nature of the crust in southern india implications for precambrian crustal evolution 1 we present crustal thickness and poisson s ratio determinations from receiver function analyzes at 32 sites on the archaean and proterozoic terrains of south india the crustal thickness in the late archaean 2 5 ga eastern dharwar craton varies from 34 39 km similar crustal thickness is observed beneath the deccan volcanic province and the cuddapah basin the most unexpected result is the anomalous present day crustal thickness of 42 51 km beneath the mid archaean 3 4 3 0 ga segment of the western dharwar craton since the amphibolite grade metamorphic mineral assemblages 5 7 kbar paleopressures in this part of western dharwar craton equilibrated at the depths of 15 20 km our observations suggest the existence of an exceptionally thick 57 70 km crust 3 0 ga ago beneath the exhumed granulite terrain in southernmost india the crustal thickness varies between 42 60 km the poisson s ratio ranges between 0 24 0 28 beneath the precambrian terrains indicating the presence of intermediate rock type in the lower crust these observations of thickened crust suggest significant crustal shortening in south india during the archaean

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

deformation path partitioning within the transpressional white mountain shear zone california and nevada most numerical simulations of transpression zones predict a change in finite stretching direction from subhorizontal to subvertical for simple shear dominated zones we provide a detailed description of a dextral transpression zone the white mountain shear zone wmsz with a range of lineation orientations and compare these natural data to numerical models the wmsz is characterized by steeply dipping foliations with dominant shallowly plunging lineations and coeval subordinate domains of steeply plunging lineations within shallowly lineated domains foliation geometry shear sense indicators and quartz c axis fabrics indicate a large component of simple shear while microstructural and quartz c axis fabric data from steeply lineated domains indicate a large component of pure shear geometric relationships between foliations and lineations and quartz c axis fabrics demonstrate that lineation orientation has remained constant during much of the deformation history comparison of numerical models with the data collected from wmsz shows that the shear zone geometry and the observed strain path partitioning do not match any of these models we propose a conceptual kinematic model for the wmsz involving stable segregated coeval kinematic domains of simple shear dominated fabrics and pure shear dominated fabrics that accommodate the transcurrent and contractional components of deformation respectively

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

geochemical investigation of archaean bimodal and dwalile metamorphic suites ancient gneiss complex swaziland the bimodal suite bms comprises leucotonalitic and trondhjemitic gneisses interlayered with amphibolites based on geochemical parameters three main groups of siliceous gneiss are recognized i sio2 14 and fractionated light rare earth element ree and flat heavy ree patterns ii sio2 and al2o3 contents similar to i but with strongly fractionated ree patterns with steep heavy ree slopes iii sio2 73 al2o3 14 zr 500 ppm and high contents of total ree having fractionated light ree and flat heavy ree patterns with large negative eu anomalies the interlayered amphibolites have major element abundances similar to those of basaltic komatiites mg tholeiites and fe rich tholeiites the former have gently sloping ree patterns whereas the mg tholeiites have non uniform ree patterns ranging from flat 10 times chondrite to strongly light ree enriched the fe rich amphibolites have flat ree patterns at 20 30 times chondrite the dwalile metamorphic suite which is preserved in the keels of synforms within the bms includes peridotitic komatiites that have depleted light ree patterns similar to those of compositionally similar volcanics in the onverwacht group barberton basaltic komatiites and tholeiites the basaltic komatiites have ree patterns parallel to those of the bms basaltic komatiites but with lower total ree contents the dwalile tholeiites have flat ree patterns the basic and ultrabasic liquids were derived by partial melting of a mantle source which may have been heterogeneous or the heterogeneity may have resulted from sequential melting of the mantle source the fe rich amphibolites were derived either from liquids generated at shallow levels or from liquids generated at depth which subsequently underwent extensive fractionation

archean emplacement of eclogitic components into the lithospheric mantle during formation of the kaapvaal craton re os data for selected whole rock eclogites and eclogitic sulfide inclusions in diamonds from seven kimberlites on the kaapvaal zimbabwe cratons yield ages around 3 ga the presence of such old eclogitic components in the subcontinental lithospheric mantle is more common than previously thought overlap of these ages with those of many cratonic peridotites demonstrates that incorporation of eclogitic materials accompanied or closely followed cratonic lithosphere stabilization many of these eclogitic components have fractionated c n o and s isotopic compositions which could have been created at low temperatures implying that some style of archean subduction involving emplacement of oceanic lithosphere was part of craton keel development

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation