the messina layered intrusion limpopo belt south africa an example of in situ contamination of an archean anorthosite complex by continental crust the messina layered intrusion was emplaced prior to 3 0 ga into supracrustal rocks presently comprising the central zone of the limpopo belt analyses of relict cumulate plagioclase megacrysts in the intrusion and its tectonic setting show that it was emplaced as a thick body of batholithic dimensions composed of varying proportions of bytownite an77 and hypersthene the magma giving rise to the intrustion was heterogeneous characterized by positive sr values neutral to negative nd values and high 207pb 204pb ratios relative to 206pb 204pb it also was light ree enriched these characteristics probably originated by the interaction of a magma derived from a depleted mantle source with crustal material similar in composition to the rocks of the enclosing beit bridge group and sand river gneisses as a result most of the characteristics of the intrusion are closer to those of proterozoic massif type anorthosite complexes than to those of archean complexes the messina layered intrustion was metamorphosed and deformed prior to 3 0 ga when its composition was changed to quartz hornblende plagioclase gneisses by the introduction of water pb sr and rb were also added to most of these rocks probably derived from the host rocks but the primary isotopic characteristics of relict plagioclase megacrysts and of partially assimilated xenoliths of metapelite were preserved more recently the intrusion was metamorphosed under granulite facies conditions at 2 0 ga prior to the joining of the rocks of the central zone with the zimbabwe and kaapvaal cratons the emplacement metamorphism and deformation of the intrusion had nothing to do with early evolutionary events on those cratons

late archaean to early proterozoic granitoid magmatism and high grade metamorphism in the central limpopo belt south africa the central zone of the limpopo belt in southern africa has previously been interpreted as a segment of archaean crust which experienced its main deformation and metamorphism around 2 7 ga ago we report new single zircon u pb and pb pb ages for granitoid gneisses supracrustal rocks and anatectically derived granitic melt patches in the area around messina south africa the sand river gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2 6 and 3 2 ga the singelele gneiss a heterogeneous granodioritic to quartz monzonitic rock has protolith ages between 2 55 and 2 58 ga since both the sand river and singelele gneisses experienced polyphase high strain ductile deformation this must have occurred later than 2 55 ga ago granulite facies pelitic gneisses of the beit bridge complex contain abundant spherical multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism these zircons provide ages with a mean at 2026 5 6 3 ma which is interpreted as reflecting a high p t event 10 kbar 825 25 c granitic melt patches in the metapelites as well as in the sand river gneiss and anatectic granites are probably related to rapid near isothermal decompression to below 3 5 kbar and 600 750 c these rocks contain new magmatic zircons which yielded a mean age of 2005 6 4 4 ma and probably reflect a crustal melting event resulting from rehydration of the granulitic assemblage our zircon data support previous suggestions for only one single granulite facies event in the central zone and we suggest that this event occurred c 2027 ma ago since most of the deformation seen in the gneisses of the messina area must have occurred later than 2 55 ga ago it is likely that the limpopo orogeny at least in the central zone is not an archaean event but took place in the early proterozoic

the structural setting and deformation associated with pseudotachylite occurrences in the palala shear belt and sand river gneiss northern transvaal abstract two occurrences of shear related pseudotachylite from the limpopo metamorphic province are described in particular the structural setting as well as deformation textures in pseudotachylite inclusions and host rocks are presented for these occurrences in gabbroic anorthosite from the palala shear belt and dioritic to granodioritic rocks from the sand river gneiss locality the palala shear belt is a 13 km wide zone of intensely mylonitized rocks of the beit bridge and bushveld complexes situated at the southern boundary of the central zone of the limpopo metamorphic province the sand river gneiss comprises a bimodal suite of high grade dioritic and granodioritic rocks which were subjected to at least three periods of deformation in the context of comparisons between pseudotachylites from different geological settings i e cryptoexplosion structures meteorite craters and tectonic settings the major results are as follows 1 1 the deformation phenomena associated with pseudotachylite formation as recorded in quartz and feldspar occurring as inclusions or at the host rock contact are undulatory extinction in quartz brecciation mechanical twinning and kinking of plagioclase and characteristic formation of irregular and planar vredefort type fractures the latter occurring in quartz 2 2 frequency distributions of crystallographic orientations of planar fractures and their annealed manifestation planar fluid inclusion trails are very similar for the vredefort structure the sudbury structure the johannesburg dome the witwatersrand basin and the limpopo pseudotachylites it is therefore concluded that planar fractures in quartz are the typical microdeformation associated with pseudotachylite irrespective of the geological setting of the latter planar fractures alone do not provide unambiguous information on the genesis of the structure in which they are found

structural and p t evolution of a major cross fold in the central zone of the limpopo high grade terrain south africa the central zone cz of the limpopo complex of southern africa is characterized by a complex deformational pattern dominated by two types of fold geometries large sheath folds and cross folds the sheath folds are steeply ssw plunging closed structures whereas the cross folds are north south oriented with near horizontal fold axes in the area south of messina this complexly folded terrain grades continuously towards the south into a crustal scale ene wsw trending ductile shear zone with moderate dip towards the wsw all sheath folds document consistent top to the ne thrust movement of high grade material the timing of this shear deformational event d and thus of the gneissic fabric s is constrained at 2 6 ga by the syntectonic emplacement throughout the cz of precursors to quartzo feldspathic gneisses singelele type gneisses cross folds deform the s fabric and are characterized by a near vertical axial planar cleavage s recent single phase pb pb dating of garnet from a metapelitic gneiss with an s fabric from one of the largest cross folds in the cz constrains the timing of the deformational d and metamorphic m event at 2 0 ga mineral chemistry for metapelites from this cross fold shows a single peak on an n mg histogram for garnet reflecting a single phase of mineral growth metapelites from this cross fold also preserve evidence for only one well developed reaction texture grt sil qtz crd this reaction is accompanied by the simultaneously operating reaction grt fsp h o bt sil qtz both these divariant reactions belong to the univariant kfmash equilibrium crd grt fsp h o bt sil qtz the progress of the two divariant retrograde reactions leads to the consumption of grt and fsp k feldspar or never occurs with both cordierite and garnet microprobe profiling coupled with calculated isopleths for bt grt and crd in divariant equilibria define a decompression cooling p t path that reflects a single m high grade metamorphic event during the evolution of the cross fold this decompression cooling p t path traverses from 780 c 5 7 kbar to 600 c 3 3 kbar

a half graben setting for the proterozoic soutpansberg group south africa evidence from the blouberg area abstract the ca 1 85 ga soutpanberg group of the kaapvaal craton has in the past been interpreted as a volcano sedimentary sequence deposited either within an aulacogen rift basin or as a more extensive cover sequence preserved only within a post sedimentary graben evidence from the blouberg area of northern province south africa indicates that the soutpansberg strata were laid down within a half graben bound to the south by a northwards dipping normal fault palaeocurrent directions within the wyllie s poort formation of the soutpansberg group suggest that planar bedded ripple marked sandstones were deposited by transverse flowing rivers whilst small to large scale trough and planar cross bedded sandstones indicate transport in large low sinuosity longitudinal trunk rivers flowing approximately parallel to the axis of the half graben the n s oriented syn southpansberg extensional regime may reflect orogenic collapse after the cessation of n s orientated compression developed at ca 2 0 ga by collision or reactivation tectonics in the limpopo belt

the structural framework of the southern margin of the limpopo belt south africa abstract the southern marginal zone is characterized by two prominent regional structural features i massive crustal wedges 20 km 50 km containing large oval shaped closed fold structures which are related to an early d 1 phase of deformation and which are bounded by ii major ductile shear zones that strike in a general e w direction the hout river shear zone forms the southern boundary of the southern marginal zone where granulites are juxtaposed against lower grade granite greenstones of the kaapvaal craton this fault system also acted as the sole thrust for the other high grade shear zones in the southern marginal zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the southern marginal zone the hout river shear zone and related faults within the southern marginal zone clearly post date d 1 and a period of peak metamorphism m 1 they are composite structures showing dominantly reverse sense displacements and are responsible for the emplacement of the granulite terrane over and onto the kaapvaal craton in contrast to these older structures are the reactivated shear zones these structures clearly cross cut and displace the high grade shear zones in the southern marginal zone as well as the retrograde orthoamphibole isograd the fact that the hout river and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism i e 2670 ma barton and van reenen 1992

the timing of sub solidus hydrothermal alteration in the central zone limpopo belt south africa constraints from titanite u pb geochronology and ree partitioning abstract in the central zone of the limpopo belt south africa palaeoproterozoic granulite facies metamorphism was superimposed on an earlier archaean orogenic history previously determined ages of 2030 2020 ma obtained from high temperature chronometers zircon garnet monazite are generally thought to provide the best estimate of the peak of palaeoproterozoic granulite facies metamorphism in the central zone whereas ages as young as 2006 ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period we present a new mc icp ms 207 pb 206 pb age of 2030 9 1 5 ma for titanite found in amphibolite to greenschist facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed central zone metabasite this age could potentially date cooling of rocks at this locality to temperatures below the wet solidus alternatively the titanite could be inherited from the metabasite host and the age determined from it date the peak of metamorphism integration of the geochronology with la icp ms trace element data for minerals from the metabasite the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite facies hydrothermal alteration not at the metamorphic peak or during the greenschist facies phase of veining this suggests that high grade rocks in the central zone have cooled differentially through the wet solidus and provides timing constraints on when palaeoproterozoic reworking in the central zone began this study illustrates the potential of combined geochronological and high resolution geochemical studies to accurately match mineral ages to distinct crustal processes

pro and retrograde p t evolution of granulites of the beit bridge complex limpopo belt south africa constraints from quantitative phase diagrams and geotectonic implications interpretations based on quantitative phase diagrams in the system cao na2o k2o tio2 mno feo mgo al2o3 sio2 h2o indicate that mineral assemblages zonations and microstructures observed in migmatitic rocks from the beit bridge complex messina area limpopo belt formed along a clockwise p t path that path displays a prograde p t increase from 600 c 7 0 kbar to 780 c 9 10 kbar pressure peak and 820 c 8 kbar thermal peak followed by a p t decrease to 600 c 4 kbar the data used to construct the p t path were derived from three samples of migmatitic gneiss from a restricted area each of which has a distinct bulk composition 1 a k al rich garnet biotite cordierite sillimanite k feldspar plagioclase quartz graphite gneiss 2 a k poor al rich garnet biotite stauro lite cordierite kyanite sillimanite plagioclase quartz rutile gneiss and 3 a k al poor fe rich garnet orthopyroxene biotite chlorite plagioclase quartz rutile ilmenite gneiss preservation of con tinuous prograde garnet growth zonation demonstrates that the pro and retrograde p t evolution of the gneisses must have been rapid occurring during a single orogenic cycle these petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the beit bridge metasedimentary rocks resulted from an orogenic event during the palaeoproterozoic c 2 0 ga caused by oblique collision between the kaapvaal and zimbabwe cratons abbreviations follow kretz 1983

major influences on the evolution of the 2 67 2 1 ga transvaal basin kaapvaal craton abstract the transvaal is one of three structural basins of the transvaal supergroup preserved on the kaapvaal craton the evolution of the neoarchaean palaeoproterozoic transvaal basin is ascribed predominantly to magmatism palaeoclimate and eustasy with plate tectonics playing a sporadic role the supergroup comprises basal protobasinal rocks followed by the black reef formation chuniespoort group and uppermost pretoria group immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c 2 7 ga ventersdorp supergroup mantle plume individual protobasinal successions were laid down in separate fault bounded basins controlled at least partially by greenstone belt orientations in the kaapvaal basement post magmatic post rifting thermal subsidence accommodated black reef fluvial sheet sandstones and the subsequent thick carbonate bif epeiric platform succession of the chuniespoort group subordinate mechanical subsidence accompanied this long lived thermal relaxation intense weathering due to neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation globally enhanced sea levels due to enhanced mid ocean ridge growth consequent upon either global magmatic events or supercontinent break up also played a pivotal role in chuniespoort epeiric basin evolution pretoria group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence the first cycle appears to reflect plate tectonically induced rifting with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation the second pretoria cycle is most likely related to a major continental flood basalt event with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern kaapvaal craton evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic transvaal basin depends to a large degree on the age assigned to the limpopo orogeny during which the zimbabwe and kaapvaal cratons collided recent evidence for a c 2 0 ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during transvaal basin evolution

a numerical study of the fortnightly modulation of basin ocean water exchange across a tidal mixing zone abstract the authors examine the effect of fortnightly modulated tidal mixing over a sill on a bidirectional estuarine shear flow through it the study proceeds numerically by using two sets of vertically two dimensional nonhydrostatic numerical models in the first numerical model spatial and temporal resolutions are high enough to reproduce internal wave generation by strong tide sill interactions after higher frequency fluctuations such as semidiurnal tides are filtered out the density field demonstrates that basic stratification is significantly disturbed over the sill during spring tides producing horizontal density gradients this causes the secondary flows consisting of the convergent horizontal currents near the surface and bottom and divergent horizontal currents near the thermocline accompanied by upwelling and downwelling in the lower and upper layers within the mixing zone respectively except in the proximity of the sill these secondary flows are successfully reproduced in the second nu

internal tides and turbulence along the 3000 m isobath of the hawaiian ridge full depth velocity and density profiles taken along the 3000 m isobath characterize the semidiurnal internal tide and bottom intensified turbulence along the hawaiian ridge observations reveal baroclinic energy fluxes of 21 5k w m 1 radiating from french frigate shoals 17 2 5 kw m 1 from kauai channel west of oahu and 13 3 5 kw m 1 from west of nihoa island weaker fluxes of 1 4 2k w m 1 radiate from the region near necker island and east of nihoa island observed off ridge energy fluxes generally agree to within a factor of 2 with those produced by a tidally forced numerical model average turbulent diapycnal diffusivity k is 0 5 1 10 4 m 2 s 1 above 2000 m increasing exponentially to 20 10 4 m 2 s 1 near the bottom microstructure values agree well with those inferred from a finescale internal wave based parameterization a linear relationship between the vertically integrated energy flux and vertically integrated turbulent dissipation rate implies that dissipative length scales for the radiating internal tide exceed 1000 km

vegetation response to southern california drought during the medieval climate anomaly and early little ice age ad 800 1600 abstract high resolution studies of pollen in laminated sediments deposited in santa barbara basin sbb core spr0901 02kc reflect decadal scale fluctuations in precipitation spanning the interval from ad 800 1600 from ad 800 1090 during the medieval climate anomaly mca sbb sediments were dominated by xeric vegetation types drought resistant coastal sagebrush and chaparral implying reduced precipitation in the southern california region drought adapted vegetation abruptly decreased at ad 1090 and was rapidly replaced by mesic oak quercus woodlands associated with an increased pollen flux into the basin after a mesic interval lasting 100 years pollen flux and the relative abundance of quercus pollen dropped abruptly at ad 1200 when the rapid rise of chaparral suggests a significant drought similar to that of the mca ad 800 1090 this brief resurgence of drought adapted vegetation between ad 1200 1270 marked the end of the mca droughts a gradual increase in mesic vegetation followed characterizing cool hydroclimates of the little ice age lia in coastal southern california the presence of xeric vegetation in sbb coincides with major drought events recorded in tree rings and low lake levels elsewhere in california except for the brief drought between ad 1130 1160 correlative diatom and terrigenous sediment input proxy records from sbb are largely supportive of the pollen record predominantly linking the mca with drought and la nina like conditions and the lia with wetter more el nino like conditions differences between paleoclimate proxies pollen diatoms and terrigenous sediment in sbb exist however possibly reflecting the temporal and spatial differences in the generation of each proxy record as well as their individual sensitivity to climate change

single zircon ages for granitoid gneisses in the central zone of the limpopo belt southern africa and geodynamic significance abstract ion microprobe evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the central zone cz of the limpopo mobile belt south africa define three broad age groups namely at 3188 3314 2637 2734 and ca 2500 ma respectively all these granitoids including the youngest group display polydeformational features suggesting that much of the tectonic history of the cz occurred in the late archaean and early proterozoic the oldest group of predominantly tonalitic trondhjemitic granodioritic gneisses was found in relatively small domains from the extreme w to the e of the cz and probably represents remnants of a once more widespread early archaean terrain the most abundant granitoid gneisses belong to the 2 6 2 7 ga age group and were emplaced into already ductilely deformed gneisses of the oldest group several of these rocks were derived from intracrustal melting rare metamorphic zircons with ages of 2560 2575 ma reflect a high grade metamorphic event in the cz that has also been postulated by other workers a strongly deformed orthogneiss with a protolith emplacement age of 2510 ma proves beyond doubt that at least part of the structural history in the cz occurred in the early proterozoic an anatectic granite in the extreme w of the cz intruded at 2022 ma and is probably related to the peak of granulite facies metamorphism previously dated at 2027 6 ma in the messina area the second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite facies metamorphism in the northern and southern marginal zones of the limpopo belt broad contemporaneity of extensive granitoid magmatism in all three zones raises doubts about the exotic nature of the cz and suggests a revised evolutionary model for the limpopo belt

a holocene pollen record of persistent droughts from pyramid lake nevada usa pollen and algae microfossils preserved in sediments from pyramid lake nevada provide evidence for periods of persistent drought during the holocene age we analyzed one hundred nineteen 1 cm thick samples for pollen and algae from a set of cores that span the past 7630 years the early middle holocene 7600 to 6300 cal yr b p was found to be the driest period although it included one short but intense wet phase we suggest that lake tahoe was below its rim for most of this period greatly reducing the volume and depth of pyramid lake middle holocene aridity eased between 5000 and 3500 cal yr b p and climate became variable with distinct wet and dry phases lake tahoe probably spilled intermittently during this time no core was recovered that represented the period between 3500 and 2600 cal yr b p the past 2500 years appear to have had recurrent persistent droughts the timing and magnitude of droughts identified in the pollen record compares favorably with previously published y 18 o data from pyramid lake the timing of these droughts also agrees with the ages of submerged rooted stumps in the eastern sierra nevada and woodrat midden data from central nevada prolonged drought episodes appear to correspond with the timing of ice drift minima solar maxima identified from north atlantic marine sediments suggesting that changes in solar irradiance may be a possible mechanism influencing century scale drought in the western great basin d 2004 university of washington all rights reserved

geochronology of the hout river shear zone and the metamorphism in the southern marginal zone of the limpopo belt southern africa abstract in this paper monazite u pb and zircon evaporation dates stepleaching pb pb results on garnet staurolite and kyanite and hornblende ar ar data are presented which constrain the timing of granulite facies metamorphism in the southern marginal zone of the limpopo belt and its thrusting onto the kaapvaal craton the southern marginal zone of the limpopo belt is considered to be a lower crustal equivalent of the northern kaapvaal craton granulite exhumation is associated with southward thrusting along the hout river shear zone which is a set of thrust and strike slip shear zones zircon ages for the matok intrusive complex which was emplaced within the zone during this thrusting charno enderbites 2671 4 ma granodiorites between 2667 and 2664 ma have previously been interpreted as evidence for rapid exhumation of the southern marginal zone within only 7 ma we have obtained a u pb date of 2691 7 ma for monazite from the bandelierkop quarry in the southern marginal zone interpreted as the age of high grade metamorphism a single zircon evaporation pb pb date of 2643 1 ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation anatexis of metapelitic xenoliths within the matok intrusive complex was dated at 2663 4 ma by u pb on monazite indistinguishable from existing zircon ages for this complex pb pb step leaching dates obtained on synkinematically grown garnet 2691 20 ma staurolite 2712 37 ma and kyanite 2672 51 ma from the khavagari hills in the giyani greenstone belt in the immediate footwall of the hout river shear zone indicate that early thrusting was contemporaneous with peak metamorphism in the southern marginal zone ar ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations probably reflecting repeated or continuous tectonic activity of the hout river shear zone up to about 2600 ma

petrology and microthermometry of aluminous rocks in the botswanan limpopo central zone evidence for isothermal decompression and isobaric cooling phase analysis in the model k2o poor aluminous rock system fmash illustrates the following sequence of reactions during retrograde metamorphism in the botswanan limpopo central zone opx sil qtz crd opx sil spr crd grt qtz opx crd opx crd w ged qtz grt opx crd w ged and grt qtz w ged crd a quantitative petrogenetic grid with phase relations shows that sapphirine results from nearly isothermal decompression in the quartz undersaturated portions of the grid and that gedrite formation by reactions 4 6 records isobaric cooling from high temperature c 800 c after the decompression conditions for hydration in the western part of the area were 700 800 c and c 6 kbar based on microthermometric data and the available garnet cordierite geothermometer on the basis of these conditions and predicted thermodynamic properties of gedrite phase relations in t xmg space were constructed to investigate the isobaric cooling event the results are in good agreement with the hydration p t path further the t xmg topologies show that hydration of orthopyroxene in the central part of the area reaction 4 occurred at about 800 c and c 5 kbar therefore we conclude that the botswanan limpopo central zone has suffered isothermal decompression similar to the central zone in south africa and zimbabwe followed by isobaric cooling the isobaric cooling event in the western at c 6 kbar and central at c 5 kbar parts of the area commenced at nearly the same temperature c 800 c and appear to be consistent with a tectonic model that involved westward movement thrusting of the central zone

barotropic and baroclinic tides in the central north pacific ocean determined from long range reciprocal acoustic transmissions abstract travel times of reciprocal 1000 km range acoustic transmissions determined from the 1987 reciprocal tomography experiment are used to study barotropic tidal currents and a large scale coherent baroclinic tide in the central north pacific ocean the difference in reciprocal travel times determines the tidal currents while the sum of reciprocal travel times determines the baroclinic tide displacement of isotachs or equivalently isotherms the barotropic tidal current accounts for 90 of the observed differential travel time variance the measured harmonic constants of the eight major tidal constituents of the barotropic tide and the constants determined from current meter measurements agree well with the empirical numerical tidal models of schwiderski and cartwright et al the amplitudes and phases of the first mode baroclinic tide determined from sum travel times agree with those determined from moored thermistors and current meters the baroclinic tidal signals are consistent with a large

spatial decay of energy density of tidal internal waves 1 the spatial decay of energy density of tidal internal waves tiw was studied using field data taken in the indian ocean near the mascarene ridge and in the canary basin of the eastern atlantic near the heyres irving cruiser chain of seamounts several moorings were deployed at distances between 90 and 1745 km east of these topographic features with instruments located in the depth range 500 2500 m the energy densities of tiw averaged over the spring neap cycle were calculated using semidiurnal tidal components of current and temperature time series as well as local vertical gradients of temperature and density it was found that the horizontal component of tiw eh is less depth dependent compared to the vertical component e although both components showed a general decrease of magnitude with the distance from topography the decrease of total energy density etw eh e with distance from the topography is more rapid than that assumed in the work of morozov 1995 and followed an inverse power law at a distance of about x 10 from the topography where is the wavelength of the first mode etw in the main thermocline becomes equal to the energy density of the forcing barotropic tide whereas for x 2 etw exceeds the energy of the entire range of internal waves of the garrett munk spectrum a nonhydrostatic nonlinear two dimensional numerical model shows a reasonable agreement with the observations for x 2 3 but in the far field it predicts a faster spatial decay of etw than observed possibly because of topographic generation of tiw along the measurement swath the turbulent diffusivity estimates based on the mccomas and muller 1981 model exceeded 10 4 m2 s within the main pycnocline at x 100 km and suggest mixing enhancements due to tiw up to distances of 1000 km from the topography

tidally induced turbulent mixing at irving seamount modeling and measurements 1 the relationship of tidal currents and turbulence over irving seamount is examined by model and measurement surface tides are partially scattered into internal tides and particularly over the seamount summit current strength and shear greatly increase current shear causes the model s turbulent mixing at irving modeled turbulent mixing as represented by vertical diffusivity is largest o 10 3 m2 s 1 in the bottom boundary layer and in a turbulent column rising to the sea surface directly above the seamount pockets of less intense mixing occur within the turbulent column but their locations are highly variable field measurements confirm a turbulent mixing column atop irving of comparable intensity increased model diffusivity average of 7 10 5 m2 s 1 also occurs to a distance of several hundred kilometers from the seamount flanks in a band vertically centered at 800 m depth

internal tides in the strait of gibraltar abstract analysis of moored current measurements in the strait of gibraltar is provided here the vertical internal displacements double amplitudes with a semidiurnal frequency are extremely large and sometimes exceed 200 m it is found that these displacements are associated with forced tidal internal waves over camarinal sill which crosses the strait the separation of tidal components showed that the amplitude of internal tide with m2 frequency is 69 m while the amplitudes of other components of the internal tide s2 k1 o1 are 3 5 times smaller the amplitudes of the waves decrease with the distance from the sill and at a distance of 50 km from the sill the amplitudes are 3 times smaller than over the sill the energy of internal tides is transferred to internal bore and turbulent mixing the wavelength of the internal tide was estimated on the basis of moored measurements the wavelength of the east propagated wave is 90 km while the west propagated wave is 60 km numerical modeling demonstr

regional geological setting of the limpopo belt abstract the 2 7 ga limpopo belt granulite terrane is situated between two lower grade granite greenstone cratons the kaapvaal craton in the south and the zimbabwe craton in the north this granulite facies terrane is subdivided into three zones each with a distinctive geological signature and separated from each other and from the adjacent cratons by well defined shear zones 1 the southern marginal zone composed of granite greenstone material very similar to that of the kaapvaal craton has retrogressed granulites on its southern margin in the hanging wall of the northward dipping terrane boundary hout river shear zone separating the granulite terrane from the adjacent kaapvaal craton 2 the northern marginal zone a granite greenstone terrane at granulite grade which is separated from the zimbabwe craton by a southward dipping terrane boundary and 3 the central zone a unique crustal element with distinctive carbonate platform lithologies and a metagabbroic layered complex which intruded before or at 3250 ma this zone cannot be related to the kaapvaal craton or to the zimbabwe craton it is separated from the two marginal zones by mylonitic shear zones with horizontal stretching lineations the entire limpopo belt is characterized by a distinctive deformational and metamorphic style which contrasts markedly from that of the adjacent granite greenstone terranes of the kaapvaal and zimbabwe cratons this distinctive pattern developed during the limpopo orogeny probably as a consequence of continental collision during the late archaean

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

the role of internal tides in mixing the deep ocean abstract internal wave theory is used to examine the generation radiation and energy dissipation of internal tides in the deep ocean estimates of vertical energy flux based on a previously developed model are adjusted to account for the influence of finite depth varying stratification and two dimensional topography specific estimates of energy flux are made for midocean ridge topography weakly nonlinear theory is applied to the wave generation at idealized topography to examine finite amplitude corrections to the linear theory most internal tide energy is generated at low modes associated with spatial scales from roughly 20 to 100 km the richardson number of the radiated internal tide typically exceeds unity for these motions and so direct shear instability of the generated waves is not the dominant energy transfer mechanism it also seems that wave wave interactions are ineffective at transferring energy from the large wavelengths that dominate the energy flux instead it appears that most of

the significance of 3000 ma granulite facies mafic dikes in the central zone of the limpopo belt southern africa abstract structural petrographic and chemical relationships indicate that a suite of mafic dikes possibly 3000 ma old were emplaced at high crustal levels deformed and metamorphosed epicontinental supracrustal rocks of the central zone of the limpopo belt these dikes were subsequently metamorphosed at granulite grade conditions probably during the limpopo orogeny about 2700 ma ago the earlier metamorphism was under conditions of at least amphibolite grade and isotopic data suggest that it may have occurred between about 3200 ma and 3100 ma ago these relationships indicate that the rocks comprising the central zone did not follow a single p t time loop but instead were involved in two distinct orogenies the orogenies were separated by a period of crustal stability during which the supracrustal rocks presently and originally exposed at the surface were again moved near to the surface this tectonic history also implies that the supracrustal rocks of the central zone and the sand river gneisses are of great antiquity if the sand river gneisses with their unique suite of older mafic dikes form a previously deformed basement to the supracrustal rocks then the sand river gneisses have been involved in three orogenies pb isotopic data indicate that the mafic dikes were derived from a mantle source with a high average ratio at some stage in its evolution prior to 3200 ma all archean lithologic units within the central zone of the limpopo belt are characterized by having been derived from rocks that had a large mu ratio at some time in their history and this relationship suggests that a link existed between the crustal rocks of the central zone and this mantle source from at least 3200 ma ago until it was destroyed by or after the limpopo orogeny the tectonic history of the central zone is different from those of the lithologies forming the adjacent terranes and suggests that it evolved separately from these terranes until they were juxtaposed during the limpopo orogeny

global baroclinic tides an approximate estimate of the energy in the first mode m2 baroclinic tide has been made from satellite observations results based on topex poseidon t p precision altimetry indicate that the internal tide patterns are similar to those expected from mid ocean topographic features in the global oceans both the orthotide and harmonic analyses indicate that the total energy in global m2 baroclinic tide is approximately 50 pj for a variety of reasons m2 is the only component that can be obtained reliably from altimetric measurements even then the energy value may be an underestimate and the energy flux the dissipation rate cannot be deduced from altimetry since it is the tidal currents flowing over mid ocean topographic features that are responsible for generating internal tides a model calibrated by m2 observations is a plausible alternative currents from a high resolution 15 barotropic tidal model have therefore been used to obtain an estimate of both the energy and the dissipation rate in m2 s2 and k1 baroclinic tides a simple model of baroclinic tide generation has been used and the unknown constant in this model has been selected to yield a total energy of 50 pj in the first mode m2 baroclinic tide based on this calibration the total energy is 8 pj in s2 first mode baroclinic tide and 15 pj in k1 the total in all first mode baroclinic tides is 90 pj about 16 of the total energy 580 pj in barotropic tides the model results also suggest that about 360 gw of tidal energy are dissipated in m2 baroclinic tides alone and 520 gw are dissipated in all first mode baroclinic tides the latter value is approximately 15 of the power input into barotropic ocean tides 3490 gw by the lunisolar tidal forces we have preferred to be conservative and hence these are likely to be underestimates especially since the altimetric tracks do not often intersect mid ocean topographic features at optimum angles while these values are very much within the range of earlier estimates in literature they should be regarded as still uncertain to perhaps a factor of two the dissipation rate could be anywhere from 400 gw to 800 gw the most likely value being about 600 gw the small signal to noise ratio involved in altimetric measurements of the surface manifestation of internal tides and potential contamination by mesoscale signals are serious problems in situ measurements at least a few locations underneath altimetric tracks are essential for confirmation and or refinement of these preliminary estimates hopefully these very first estimates of the energy and dissipation rate in global baroclinic tides though rather crude will serve as a catalyst for a better estimation in the future since internal tides are likely to be a prominent source of mixing in the deep oceans and important to thermocline maintenance

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

reaction history of garnet sapphirine granulites and conditions of archaean high pressure granulite facies metamorphism in the central limpopo mobile belt zimbabwe sequential reaction textures in archaean garnet corundum sapphirine granulites from the central zone of the limpopo belt document a progression from early coarse grained high pressure p 9 5 kbar granulite facies assemblages m1 to late low pressure p 6 kbar granulite facies sub assemblages m2 the stable m1 assemblage was garnet 57 pyrope mg mg fe 62 sapphirine corundum gedrite phlogopite rutile late m1 boron free kornerupine grew at the expense of garnet and corundum and coexisted with garnet sapphirine and gedrite partial or complete breakdown of coarse garnet and kornerupine during m2 resulted in the development of pseudomorphs and coronas consisting of fine grained symplectic intergrowths of cordierite gedrite and sapphirine later spinel the majority of reaction textures can be explained in terms of a stable reaction sequence and a model time sequence of mineral facies can be constructed when compared with a qualitative petrogenetic grid of fe mg discontinuous reactions in the fmash multisystem sapphirine garnet corundum spinel cordierite gedrite kornerupine the facies sequence indicates decompression at essentially constant t assuming constant a h2o exhumation of m1 corundum inclusions during m2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite a second disequilibrium reaction of the spinel with cordierite produced sapphirine the operation of such reaction while pressure was decreasing the opposite dp from that implied by the texture if assumed to be the product of an equilibrium reaction has serious implications for the use of reaction textures in the construction of p t vectors garnet biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca 850 c for the m1 stage a minimum late m1 pressure of ca 7 kbar is indicated by the former association of kornerupine and corundum relict m1 kyanites reported by other workers indicate a minumum early m1 pressure of 9 5 kbar implying metamorphism at depths of at least 33 km probably 38km the high pressure granulite facies metamorphism was followed by an almost isothermal pressure decrease of 5 kbar indicative of rapid uplift the p t path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite facies terranes

structure of the deep spillway in the western part of the romanche fracture zone structure of the deep spillway was studied on the basis of conductivity temperature depth lowered acoustic doppler current profiler and mooring measurements in 2011 2014 in the romanche fracture zone 22 27 22 32 w the spillway exists quasi permanently in the regime of hydraulic control overflow and consists of two flows in the layer 1 20 c deeper than 4 150 m the first flow enters the fracture from the south through a gap in the southern wall the second flow is directed to the east along the narrow valley of the fracture the sill depths of these passages are 4 570 and 4 430 m respectively the depths over the surrounding ridges are shallower than 4 100 m both flows descend to the local depression of the fracture with 5 000 m depth bottom potential temperatures over both sills were 0 51 c and the measured velocities were 20 40 cm s when the flows descend down the slope they accelerate in supercritical regime maximal measured velocities were up to 65 cm s analysis of the 189 day time series of velocities on the mooring deployed in the flow descending from the gap showed a very high level of fluctuation energy 10 times greater than the energy of the barotropic tide at this point the only energy source of these fluctuations is the available potential energy of the deep thermocline in the west atlantic transforming to the kinetic energy of the deep spillway the total transport of the bottom water below 1 2 c based on the lowered acoustic doppler current profiler measurements is estimated at 0 29 0 35 sv which agrees with the predicted values

the triangle shearzone zimbabwe revisited new data document an important event at 2 0 ga in the limpopo belt abstract the limpopo belt in southern africa has been used to demonstrate that modern style continent continent collision operated during the late archaean 2 6 2 7 ga we have studied the age and pt conditions of strike slip tectonism along the important right lateral triangle shearzone our results substantiate existing proterozoic metamorphic mineral age data of prior uncertain significance using the pb pb and sm nd garnet chronometers and the ar ar step heating technique for amphibole we have dated pre and syn tectonic metamorphic minerals at 2 2 and 2 0 ga thus the triangle shearzone can now be regarded as an important proterozoic suture examination of corresponding high grade pt conditions reaching 800 c at 9 kbar indicates a clockwise metamorphic evolution with pronounced isothermal uplift although the evidence that thrusting of the marginal zones of the limpopo belt over the adjoining cratons occurred during the late archaean clearly remains it is now very uncertain to which event the various pt paths obtained in the limpopo belt may be assigned therefore the question of whether the 2 6 2 7 ga tectonism fits on its own a modern style continental collision model remains open and has to be reassessed

depositional system and palaeoclimatic interpretations of middle to late pleistocene travertines kocabas denizli south west turkey travertine deposits in western turkey are very well exposed in the area of kocabas in the eastern part of the denizli basin the palaeoclimatic significance of these travertines is discussed using u th dates stable isotope data and palynological evidence the kocabas travertine occurrences are characterized by successions of depositional terraces associated with palaeosols and karstic features the travertines have been classified into eight lithotypes and one erosional horizon namely laminated coated bubble reed paper thin raft intraclasts micritic travertine with gastropods extra formational pebbles and a palaeosol layer the analysed travertines mostly formed between 181 ka and 80 ka middle to late pleistocene during a series of climatic changes including glacial and interglacial intervals their 13c and 18o values indicate that the depositional waters were mainly of basinal thermal origin occasionally mixed with surficial meteoric water palynological results obtained from the palaeosols showed an abundance of non arboreal percentage and xerophytic plants oleaceae and quercus evergreen type indicating that a drought occurred marine isotope stage 6 is represented by grassland species but marine isotope stage 5 is represented by pinaceae pinus and abies quercus and oleaceae uranium thorium analyses of the kocabas travertines show that deposition began in marine isotope stage 6 glacial and continued to marine isotope stage 4 glacial but mostly occurred in marine isotope stage 5 interglacial the travertine deposition continued to ca 80 ka in the south west of the study area in one particular depression depositional system palaeoenvironmental indicators suggest that the travertine depositional evolution was probably controlled by fault related movements that influenced groundwater flow good correlation of the stable isotope values and dates of deposition of the travertines and palynological data of palaeosols in the kocabas travertines serve as a starting point for further palaeoclimate studies in south west turkey additionally the study can be compared with other regional palaeoclimate archives

himalayan tibetan analogies for the evolution of the zimbabwe craton and limpopo belt abstract the limpopo orogeny which lasted from 2700 to 2600 ma was characterised by nnw sse directed shortening this resulted in both crustal thickening accommodated by folding and nnw directed thrusting and associated crustal extrusion whereby a number of crustal blocks including the central zone of the limpopo belt was displaced to the west southwest along wsw ene trending shear zones this was part of a regionally distributed phase of orogenic deformation which affected the adjoining kaapvaal and zimbabwe cratons from the witwatersrand basin in the south to the harare greenstone belt in the north this deformation which migrated northwards with time postdated an earlier phase of crustal accretion during which the archaean crustal unit now marked by the kaapvaal and zimbabwe cratons and the limpopo belt grew towards the north northwest shortening of this unit during the limpopo orogeny may be regarded as the last stage of the crustal accretion process this tectonic process of continental growth by accretion followed by crustal shortening with associated crustal extrusion has many features in common with tibetan tectonics from the mesozoic to the present

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

archaean to proterozoic crustal evolution in the central zone of the limpopo belt south africa botswana constraints from combined u pb and lu hf isotope analyses of zircon a combined set of u pb and lu hf in situ laser ablation icp mc ms zircon analyses were obtained from orthogneisses and granitoids in the central zone of the limpopo belt which comprises the beit bridge and mahalapye complexes the results indicate that by combining the two isotope systems primary magmatic zircon domains can be distinguished from those formed during later metamorphic events even if the distinct zircon domains underwent multiple pb loss and the texture age relationships as obtained by cathodoluminescence images and u pb analyses are ambiguous furthermore the applied technique allows distinction of zircon grains formed in juvenile magmas from those generated by melting of older continental crust or affected by substantial crustal contamination the combined u pb and lu hf data reveal that the sand river gneiss suite of the beit bridge complex was emplaced at 3283 8ma and formed from melting of an older archaean crust which was derived from a depleted mantle source at around 3 65 ga the hafnium model age tdm is significantly older than those obtained from zircons from numerous neoarchaean granitoids of the beit bridge complex comprising the singelele gneiss 2647 12ma the bulai granite 2612 7ma the regina gneiss 2649 9ma and two samples of the zanzibar gneiss 2613 6ma these granitoids show initial ehf t values between 0 5 and 7 1 which correspond to initialtdm between 3 46 and 3 01ga these variable tdminitial and ehf t initial values are interpreted to be the result of different mixtures of reworked 3 65 ga palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the neoarchaean at 2 65 ga this conclusion is supported by results obtained from the mahalapye complex which was affected by migmatization and granite intrusions during the palaeoproterozoic at 2 02 2 06 ga the mokgware granite 2019 9ma contains zircon xenocrysts with pb pb ages of 2 52 2 65 ga and 2 93 ga and hafnium model ages of 3 0 3 4 ga indicating that this granite is derived from remelting of archaean crust in contrast uniformtdm hf initial ages of 2 61 2 67 ga obtained from a diorite gneiss 2061 6ma of the mahalapye complex indicate that its protolith may have been formed from remelting of a neoarchaean juvenile crust variable ehf t initial values from 3 7 to 6 3 of zircon cores 2711 11ma in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the neoarchaean

a 180 000 year pollen record from owens lake ca terrestrial vegetation change on orbital scales abstract pollen from the upper 90 m of core ol 92 from owens lake is a climatically sensitive record of vegetation change that indicates shifts in the plant associations representing warm and cold desertscrub pinyon juniper woodland and pine fir forest during the past 180 000 years these changes are synchronized with glacial interglacial cycles during glacial and stadial climates juniper woodland expanded downslope and replaced warm desert shrubs while upper montane and subalpine forests in the arid inyo mountains also expanded and those in the sierra nevada were displaced by the ice cap and periglacial conditions conversely during interglacial and interstadial climates warm desert plants expanded their range in the lowlands juniper and sagebrush retreated upslope and montane and subalpine forests expanded in the sierra nevada the reconstructed vegetation history demonstrates a regional climatic response and the congruence of the pollen sequence with marine and ice cap oxygen isotope stratigraphies suggests a link between regional vegetation and global climate change at orbital scales

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

observations of tidal internal wave beams at kauai channel hawaii abstract to observe the across ridge structure of internal tides density and velocity were measured using seasoar and a doppler sonar over the upper 400 600 m of the ocean extending 152 km on each side of the hawaiian ridge at kauai channel eighteen sections were completed in about 18 days with sampling intentionally detuned from the lunar semidiurnal m2 tide so that averaging over all sections was equivalent to phase averaging the m2 tide velocity and displacement variance and several covariances involving velocity and displacement showed one m2 internal wave beam on each side of the ridge and reflection of the beams off of the surface theoretical ray slopes aligned with the observed beams and originated from the sides of the ridge energy flux was in agreement with internal wave generation at the ridge inferred turbulent dissipation was elevated relative to open ocean values near tidal beams energy flux was larger than total dissipation almost everywhere across the ridge internal wave energy fl

a corundum and sapphirine paragenesis from the limpopo mobile belt southern africa an assemblage consisting of corundum sapphirine spinel cordierite garnet biotite and bronzite is described from the messina area of the limpopo mobile belt and consideration given to its petrogenesis various geothermometers and geobarometers have been applied in an attempt to determine the temperatures and pressures of metamorphism a former coexistence of garnet and corundum is suggested to have developed during the earliest high pressure phase of the metamorphism where temperatures exceeded 800 c and pressures as high as 10kbar may have been experienced subsequently continuous retrograding reactions from medium pressure granulite facies at about 800 c and 8kbar towards amphibolite facies generated spinel cordierite sapphirine and possibly also bronzite the most notable reaction was probably of the form garnet corundum cordierite sapphirine spinel

the significance of rb sr ages of biotite and phlogopite for the thermal history of the central and southern marginal zones of the limpopo belt of southern africa and the adjacent portions of the kaapvaal craton abstract generally rb sr biotite and phlogopite ages range from 2050 ma to 1950 ma between the murchison thabazimbi and triangle tuli sabi shear zones of southern africa in the rocks comprising the central and southern marginal zones of the limpopo belt and adjacent portions of the kaapvaal craton in some shear zones however similar ages ocurr as young as 800 ma in contrast within the kaapvaal craton south of the murchison thabazimbi shear zone rb sr biotite ages range between 3150 ma and 2100 ma and show no regular distribution in all cases the ages are interpreted to indicate the time elapsed since the biotite last cooled below its closure temperature approximately 320 c the regular pattern of 2050 ma to 1950 ma rb sr biotite ages north of the murchison thabazimbi shear zone suggests that the present erosional surface approximates a paleo isothermal surface this surface was exposed by uplift and erosion which was completed by 1900 ma to 1800 ma ago when the sedimentary and volcanic rocks of the soutpansberg group were deposited the regional uplift was accommodated by motion on the murchison thabazimbi and triangle tuli sabi shear zones and probably was related to tectonism associated with the emplacement of the anorogenic bushveld and phalaborwa complexes other anorogenic granitic magmatism accompanied uplift the depth represented by the paleo isothermal surface and the magnitude of the uplift that exhumed it cannot be constrained precisely it is suggested however that the majority of the 10 to 15 km by which crust in this area is thinner than that of the adjacent cratons is the manifestation of this uplift

westward continuation of the craton limpopo belt tectonic break in zimbabwe and new age constraints on the timing of the thrusting the umlali thrust zone has been mapped between the granite greenstone rocks of the zimbabwe craton and the granulite facies rocks of the limpopo belt a tectonic break had not been previously recognized in the southwest because of the intrusion of syn to post kinematic porphyritic granites these intrusions locally obliterate evidence for the tectonic break although in some localities the associated mylonites are preserved as xenoliths within the intrusive porphyritic granite bodies the porphyritic granite bodies were emplaced during and after the main regional fabric forming event a u pb zircon age of 2627 7 ma obtained from a late kinematic microgranite is interpreted to be the minimum time of thrusting a whole rock rb sr age of 2583 52 ma obtained from plutonic rocks and retrogressed zones within the north marginal zone falls within error of the zircon age suggesting widespread magmatic and hydrothermal activity coeval with the thrusting

pollen representation source area and basin size toward a unified theory of pollen analysis abstract the concepts of pollen source area and of production and dispersal biases in pollen representation are quantified by means of a simple theoretical model source areas and relative pollen representation are shown to depend on basin size according to functions that describe the amount of pollen remaining airborne at increasing distances from single pollen sources the form of these functions is determined by physical processes standard formulas for elevated sources do not apply but the integrated form of sutton s equation for particle dispersal from a ground level source gives useful approximations applicable to pollen transport over a forest canopy simulations using this equation yielded source areas that increased realistically with basin size showed substantial differences between source areas for pollen grains with different deposition velocities and predicted that lighter pollen grains should become better represented with increasing basin size all of these predictions are qualitatively consistent with present knowledge of the characteristics of pollen assemblages in different depositional environments the model further allows parameters that can be estimated by statistical calibration methods to be predicted from underlying physical quantities this extension suggests procedures for testing the theory with quantitative data on surface pollen and forest composition preliminary results showed reasonable agreement between estimated and predicted values of dispersal indices for the most abundant taxa in pollen spectra from the northern midwestern united states

sapphirine kornenipine bearing rocks and crustal uplift history of the limpopo belt southern africa sapphirine kornerupine bearing rocks occur within the anorthosites of the messina layered intrusion in the limpopo mobile belt of zimbabwe the xmg range of the major minerals is as follows cordierite 0 98 0 93 enstatite 0 97 0 86 chlorite 0 98 0 92 phlogopite 0 98 0 90 sapphirine 0 98 0 86 kornerupine 0 94 0 88 gedrite 0 96 0 85 spinel 0 92 0 78 there are four rock types the constituent minerals of which have different x m g values which decrease in the above mineral order other minerals are corundum sillimanite and relict kyanite we recognise twenty reactions without phlogopite and nine reactions involving phlogopite the textural relations and the plots of the microprobe data of coexisting minerals in the mgo al2o3 sio2 h2o system are consistent with the following sequence of main reactions 1 enstatite corundum cordierite sapphirine 4 sapphirine sillimanite cordierite corundum 8 kornerupine corundum cordierite sapphirine 13 kornerupine cordierite sapphirine enstatite 15 enstatite spinel chlorite sapphirine 18 cordierite sapphirine chlorite corundum 20 sapphirine chlorite corundum spinel the early reactions are shown by coarse grained reaction intergrowths kornerupine and gedrite breakdown is shown by finer grained symplectites and the latest reactions by very fine grained products in micro fractures these selected reactions illustrate a remarkably steep trajectory from thept peak close to 10 kbar and 800 c to the minimum observable at 3 5 4 5 kbar and 700 c as indicated by the pure mash system very rapid uplift took place under nearly isothermal conditions the protolith of this material was possibly sedimentary derived from altered volcanic rocks the bulk composition is close to the composition of kornerupine or to a mixture of alunite chlorite and pyrophyllite these texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history

proterozoic transpressive deformation in the northern marginal zone limpopo belt zimbabwe a major tectono metamorphic event affected the southern half of the northern marginal zone of the limpopo belt at 2 0 ga structural analysis along a 43 km long profile perpendicular to strike shows extension inclined toward the northeast extension parallel to axes of folds that uplift the southeast of the section i e the central zone of the limpopo belt and dextral shear these deformation patterns developed coevally the lineations form distinct tightly clustered groups that occur together in the profile there is no evidence for superimposed deformation or deformed lineations nor are lineations observed cross cutting each other this single episode of deformation can therefore be called transpression petrographic and thermobarometric evidence support this kinematic picture in that decreases of both pressure 2 5 kb and temperature 150 c are documented from southeast to northwest across strike 39 ar 40 ar step heating of metamorphic hornblende from amphibolites has yielded ages of

postglacial vegetation fire and climate history of the siskiyou mountains oregon usa the forests of the siskiyou mountains are among the most diverse in north america yet the long term relationship among climate diversity and natural disturbance is not well known pollen plant macrofossils and high resolution charcoal data from bolan lake oregon were analyzed to reconstruct a 17 000 yr long environmental history of high elevation forests in the region in the late glacial period the presence of a subalpine parkland of artemisia poaceae pinus and tsuga with infrequent fires suggests cool dry conditions after 14 500 cal yr b p a closed forest of abies pseudotsuga tsuga and alnus rubra with more frequent fires developed which indicates more mesic conditions than before an open woodland of pinus quercus and cupressaceae with higher fire activity than before characterized the early holocene and implies warmer and drier conditions than at present in the late holocene abies and picea were more prevalent in the forest suggesting a return to cool wet conditions although fire episode frequency remained relatively high the modern forest of abies and pseudotsuga and the present day fire regime developed ca 2100 cal yr b p and indicates that conditions had become slightly drier than before sub millennial scale fluctuations in vegetation and fire activity suggest climatic variations during the younger dryas interval and within the early holocene period the timing of vegetation changes in the bolan lake record is similar to that of other sites in the pacific northwest and klamath region and indicates that local vegetation communities were responding to regional scale climate changes the record implies that climate driven millennial to centennial scale vegetation and fire change should be considered when explaining the high floristic diversity observed at present in the siskiyou mountains d 2005 university of washington all rights reserved

the significance of cordierite hypersthene assemblages from the beitbridge region of the central limpopo belt evidence for rapid decompression in the archean

late glacial and early holocene vegetation and climate change near owens lake eastern california abstract pollen and algae from owens lake in eastern california provide evidence for a series of climatic oscillations late in the last glaciation juniper woodland which dominated the owens valley from 16 200 to 15 500 cal yr b p suggests much wetter conditions than today although still wetter and cooler than today the area then became fairly warm and dry with woodland being replaced by shrubs mainly sagebrush from 15 500 to 13 100 cal yr b p next chenopodiaceae shadscale increased woody species declined and lake levels fell all evidence for a brief ca 100 200 yr drought about 13 000 cal yr b p the climate continued to oscillate between wet and dry from 13 000 to 11 000 cal yr b p after 11 000 cal yr b p low lake levels and the increased dominance of desert shrubs indicate the beginning of warm dry holocene conditions the region s climate was unstable during the younger dryas but uncertainities in dating prevent identification of the younger dryas interval in the owens lake record comparison of the owens lake record with studies in the sierra nevada and great basin suggest that the climate was generally wetter between 13 000 and 11 000 cal yr b p with warmer summers although no consistent pattern of climate change emerges

shear zones of the limpopo belt and adjacent granitoid greenstone terranes implications for late archaean collision tectonics in southern africa abstract granulite metamorphism in the 2 65 ga limpopo belt of southern africa is related to crustal thickening due to late archaean collision tectonics the central zone of the belt is a major thrust sheet of distinctive lithological structural and isotopic character separated from adjacent terranes by crustal scale shear zones and the northern and southern marginal zones are up thrust deep level granulite facies terranes of the adjacent cratons geometrically the marginal zones are mirror images and the limpopo belt is essentially symmetrical in n s section conventionally regarded as a n s collisional orogeny the gross structural and lithological differences between the central zone and the marginal zones kinematic data from shear zones and geophysical data showing the three dimensional geometry of the central zone are here used to suggest that the direction of emplacement of the central zone was also the collision direction this was from northeast to southwest with the upthrust outwardly directed marginal zones developing in mechanical response to this collision

impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break the vertical and temporal structure of the dissipation of turbulent kinetic energy within the internal tide at a location 5 km shoreward of the shelf break on the malin shelf has been determined using a combination of the free falling light yo yo profiler and acoustic doppler current profilers two distinct internal wave regimes were encountered period i in which large amplitude high frequency nonlinear internal waves niws occurred around neap tides and period ii in which the internal wave spectral continuum was not dominated by any particular frequency band around spring tides empirical orthogonal function analysis shows that for the low frequency waves 76 of the variance was described by mode 1 rising to 95 for the high frequency waves during period i the dissipation and vertical mixing were characterized by the niws and 70 of the dissipation occurred in the bottom boundary layer during period ii the depth integrated dissipation was more evenly distributed throughout the tidal cycle whereas vertical mixing was greatly enhanced during a single hour long episode of elevated thermocline dissipation coincident with weakened stratification during both periods i and ii 30 of the total measured dissipation occurred within the thermocline when averaged over 12 4 hours the remainder occurred within the bottom boundary layer bbl tidal average values for depth integrated dissipation and vertical eddy diffusivity for period i ii were 1 1 10 2 w m 2 4 0 10 2 w m 2 and 5 cm2 s 1 12 cm2 s 1 respectively decay rates and internal damping are discussed and vertical heat fluxes are estimated observed dissipation rates are compared with a simple model for bbl dissipation

mono sample pb pb dating of pyrrhotite and tourmaline proterozoic vs archean intracratonic gold mineralization in zimbabwe gold sulfide tourmaline associations are characteristic of large gold resources worldwide such parageneses can rarely be dated hence genetic models are based on weak age constraints stepwise pb leaching enabled direct precise dating of individual samples of hydrothermal pyrrhotite 1 93 0 05 ga and paragenetically older tourmaline 2 02 0 02 ga from a shear zone hosted gold mineralization within the archean zimbabwe craton these ages reveal a multistage proterozoic hydrothermal event within the craton the presence of a scheelite tourmaline ore dated at 2 65 0 01 ga within the same shear zone may point to an archean precursor because of its capability to demonstrate pb isotope dis equilibrium between associated phases single mineral pb leaching also traces genetic processes

dating a variscan pressure temperature loop with staurolite staurolite with a complex internal metamorphic and structural relationship occurs in metapelite from the silvretta thrust sheet eastern alps stepwise leaching of a core and rim fraction of a single staurolite crystal resulted in a total pb pb isochron of 308 28 ma the variscan metamorphism has been confirmed by a further single bulk staurolite pb pb age of 298 42 ma which is in accordance with u pb ages of 306 8 ma and 304 6 ma respectively obtained from combined u pb leaching of the same crystals direct dating of the staurolite in reaction isograd was therefore successful our results date the prograde part of a variscan counterclockwise pressure temperature path in the silvretta thrust sheet proximity with white mica rb sr and k ar cooling ages suggests that the whole variscan episode was fast and took place within a few million years

interpretation of floodplain pollen in alluvial sediments from an arid region pollen was collected from modern alluvium and from the atmosphere to document the nature and amount of paleoenvironmental information reflected by alluvial pollen chronologies results indicate that pollen in alluvium is a homogeneous mixture derived almost entirely from the floodplain itself the few pollen grains derived from nonfloodplain plant communities and preserved in alluvial sediments are so well mixed that their frequencies no longer reflect the geographic distribution of the specific plant communities in which they originated in contrast the abundance of alluvial pollen grains derived from the major floodplain taxa chenopodiineae ambrosia type varies with summer and winter climate this annual variation is preserved in alluvial pollen assemblages through a combination of processes within sedimentation basins involving discontinuous deposition events and mechanical pollen degradation the high frequency wide amplitude pollen variance in alluvial pollen assemblages contrasts with the low frequency narrow amplitude pollen variance in sediments of lakes and ponds the slight geographic variance in alluvial pollen assemblages in contrast to the large variance in soil pollen allows use of alluvial pollen to infer climate throughout the watershed in which pollen is sampled

tidal data inversion interpolation and inference initial efforts in applying inverse methods to studies of ocean tides have focused on making the best use of a small number of observations to map tidal fields in a large area as such inversion can be viewed as an objective analysis scheme which uses a dynamically appropriate spatial covariance derived from the shallow water equations to interpolate and smooth a sparse data set data from recent altimetry missions are not sparsely distributed relative to tidal wavelengths in the open ocean apparently reducing the need for complicated dynamically based interpolation schemes altimetric data sets are also quite large making application of rigorous inversion methods to global tidal modeling a challenging computational problem we describe here a new iterative solution scheme which allows us for the first time to fit the full set of topex poseidon cross over differences the resulting solution tpxo 3 fits validation tide gauges significantly better than previous inverse solutions tpxo 3 also reduces residual cross over variances relative to other recent inverse and empirical solutions particularly in shallow water where improvements are dramatic with the new solution approach very significant improvements in global tidal models should be possible in shallow areas and in the vicinity of complex bathymetry where high accuracy tidal modeling remains a challenging problem with the recent improvements in the definition of tidal elevations in the open ocean it should now also be possible to resolve some long unanswered questions about tidal energetics and dynamics inverse methods provide a natural framework for addressing these issues and making inferences about tidal dynamics in particular by bringing data and dynamics together in a single solution we can rigorously test the consistency of the two we present results of global and local inversions which suggest that over elongated bathymetric features oriented perpendicular to tidal flows energy dissipation in the open ocean is significantly enhanced presumably due to conversion of barotropic tidal motions into baroclinic modes for the m2 tide our preliminary results suggest that perhaps as much as 0 5 tw of energy is dissipated in this manner however due to the simplified linear dynamics and limited spatial resolution used for our inversion there are significant uncertainties associated with these results a more careful application of inverse methods to make more rigorous inferences about tidal energetics including use of more reasonable prior dynamics and the highest possible spatial resolution should allow for closure of the tidal energy budget within the next few years

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

the generation of internal tides at the hawaiian ridge previous analyses of altimeter and acoustic data indicate that the hawaiian ridge is a significant generation site for semidiurnal m2 period internal waves owing to tidal flow over topography such waves likely play an important role connecting the dissipation of tidal energy to ocean mixing here a primitive equation numerical model with realistic stratification bathymetry and tidal forcing is used to simulate the generation of these waves in a manner consistent with altimeter sea surface observations the highest amplitude waves originate at three generation sites located at both the island and seamount portions of the chain these sites have in common both enhanced flows across ridge shaped topography and maxima in model calculated body force which couples barotropic and baroclinic motions the model vertical structure suggests the presence of multiple dynamical modes within 100 km of the ridge and not just the lowest modes that have been detected at the sea surface

geochronological problems related to polymetamorphism in the limpopo complex south africa abstract the integration of new and published geochronologic data with structural magmatic anatectic and pressure temperature p t process information allow the recognition of high grade polymetamorphic granulites and associated high grade shear zones in the central zone cz of the limpopo high grade terrain in south africa together these two important features reflect a major high grade d 3 m 3 event at 2 02 ga that overprinted the 2 63 ga high grade neoarchaean d 2 m 2 event characterized by sw plunging sheath folds these major d 2 m 2 folds developed before 2 63 ga based on u pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high grade gneissic fabric the d 3 m 3 shear event is accurately dated by u pb monazite 2017 1 2 8 ma and pbsl garnet 2023 11 ma age data obtained from syntectonic anatectic material and from sheared metapelitic gneisses that were completely reworked during the high grade shear event the shear event was preceded by isobaric heating p 6 kbar and t 670 780 c which resulted in the widespread formation of polymetamorphic granulites many efforts to date high grade gneisses from the cz using pbsl garnet dating resulted in a large spread of ages 2 0 2 6 ga that reflect the polymetamorphic nature of these complexly deformed high grade rocks

interference pattern and propagation of the m2 internal tide south of the hawaiian ridge abstract most of the m2 internal tide energy generated at the hawaiian ridge radiates away in modes 1 and 2 but direct observation of these propagating waves is complicated by the complexity of the bathymetry at the generation region and by the presence of interference patterns observations from satellite altimetry a tomographic array and the r p flip taken during the farfield program of the hawaiian ocean mixing experiment home are found to be in good agreement with the output of a high resolution primitive equation model simulating the generation and propagation of internal tides the model shows that different modes are generated with different amplitudes along complex topography multiple sources produce internal tides that sum constructively and destructively as they propagate the major generation sites can be identified using a simplified 2d idealized knife edge ridge model four line sources located on the hawaiian ridge reproduce the interference pattern of sea surface height and energy fl

lee wave resonances over double bell shaped obstacles lee wave resonance over double bell shaped obstacles is investigated through a series of idealized highresolution numerical simulations with the nonhydrostatic coupled ocean atmosphere mesoscale prediction system coamps model using a free slip lower boundary condition the profiles of wind speed and stability as well as terrain derive from observations of lee wave events over the sierra nevada and inyo mountains from the recently completed terrain induced rotor experiment t rex numerical experiments show that double bell shaped obstacles promote trapped lee waves that are in general shorter than those excited by an isolated ridge while the permissible trapped lee wave modes are determined by the upstream atmospheric structure primarily vertical wind shear the selected lee wave wavelengths for two obstacles that are close or equal in height are dictated by the discrete terrain spectrum and correspond to higher harmonics of the primary orographic wavelength which is equal to the ridge separation distance the exception is the smallest ridge separation distance examined one that corresponds to the owens valley width and is closest to the wavelength determined by the given upstream atmospheric structure for which the primary lee wave and orographic wavelengths were found to nearly coincide the influence two mountains exert on the overall lee wave field is found to persist at very large ridge separation distances for the nonlinear nonhydrostatic waves examined the ridge separation distance is found to exert a much stronger control over the lee wave wavelengths than the mountain half width positive and negative interferences of lee waves which can be detected through their imprint on wave drag and wave amplitudes were found to produce appreciable differences in the flow structure mainly over the downstream peak with negative interference characterized by a highly symmetric flow pattern leading to a low drag state

structural and compositional constraints on the emplacement of the bushveld complex south africa abstract despite a plethora of petrological studies the emplacement mechanics of the world s largest layered intrusion the 2 05 ga bushveld complex are still poorly understood early models considered the intrusion to comprise separate lopolithic intrusions or even concentric cone sheets but recently overwhelming support for a sill like intrusional form has emerged examination of the contact aureole reveals three groups of emplacement related structures interfinger deformation zones and bridges formed between intruding and dilating magma fingers preserved in both the western and eastern parts of the complex sub bushveld complex intrusions that reflect finger like emplacement into the underlying contact aureole and diapiric domes that characterize the eastern contact aureole formed by diapiric amplification of initial interfinger deformation zones associated with the earliest mafic ultramafic pulse of the bushveld complex most of these structures exhibit a strong nw se preferred orientation while longitudinal terminations and divergence of the sub bushveld intrusions away from their source horizons indicate magma emplacement towards the se this emplacement direction is supported by petrological data lateral lithological facies variations and mineral chemical variations within specific horizons the thickest and most chemically primitive accumulations of the lower zone of the complex are found adjacent to and thin towards the ne and se away from the thabazimbi murchison lineament tml a crustal scale lineament that has undergone polyphase reactivation since at least 2 7 ga the bushveld complex was most likely fed by a feeder dyke that utilised the tml spreading laterally from the dyke axis to form its current sill like geometry the stress field at 2 05 ga was suitably oriented to allow for dilation of the ene trending tml suggesting the kaapvaal craton was subject to a component of nw se extension during this time nw se directed collision of the kaapvaal and zimbabwe cratons is recorded in the limpopo belt which would suggest emplacement under conditions of far field extensional stress within a back arc setting this is corroborated by petrological data that supports a significant component of subducted crust in the bushveld magmas

geologic history of the central zone of the limpopo complex the west alldays area abstract new field structural petrologic and age data show that tectonic styles and metamorphic histories in the west alldays area of the central zone of the limpopo complex southern africa can be linked to neo archean and paleoproterozoic high grade granulite facies tectonometamorphic events the styles comprise 1 a regionally developed high grade s2 gneissic fabric that evolved into a regional d2 system of sheath folds mapped as circular to oval shaped structures with lineations plunging steeply to the wsw and 2 a system of mainly n s trending and north verging d3 shear zones characterized by high grade tectonites straight gneisses with well developed s3 gneissic fabrics in the west alldays area the superimposition of d3 onto the regional d2 fold pattern produced the kilometer scale n s trending baklykraal shear zone which before this study was mapped as the baklykraal fold and interpreted as part of the regional d2 fold pattern in the central zone the paleoproterozoic high

reply to rigby m j eriksson p g and mavimbela p k comments on structural and compositional constraints on the emplacement of the bushveld complex by b clarke r uken and j reinhardt

rapid oscillations in western north america vegetation and climate during oxygen isotope stage 5 inferred from pollen data from santa barbara basin hole 893a abstract chronologically controlled high resolution 200 yr average sampling interval pollen data from a core in the santa barbara basin sbb provide evidence of millennial scale climate variability on the southern california coast from 70 to 140 ka b p the first climate event inferred from a brief reversal in the glacial interglacial shift from montane conifer to lowland oak assemblages occurred during the oxygen isotope stage ois 6 5 transition in ois 5e oak dominated assemblages imply conditions similar to or slightly warmer than present cooler conditions that gradually replaced the relatively stable warm mediterranean climate that continued across the ois 5e 5d transition ended abruptly just before the end of ois 5d warming inferred from oak expansion continued in ois 5c this lengthy cyclic sequence of high amplitude warming events which peaked below interglacial values extended into the first part of ois 5b similar oscillations occurred in ois 5a these pollen data that are directly correlated with isotopic data from the same sediment samples from hole 893a support previous observations that the last interglacial sensu stricto on the west coast of north america was correlative with oxygen isotope substage 5e in addition pollen data show that significant warming events reflected in terrestrial vegetation also occurred in different background states of the climate system during processes of ice growth as well as ice decay

propagation of low mode internal waves through the ocean abstract the baroclinic tides play a significant role in the energy budget of the abyssal ocean although the basic principles of generation and propagation are known a clear understanding of these phenomena in the complex ocean environment is only now emerging to advance this effort a ray model is developed that quantifies the effects of spatially variable topography stratification and planetary vorticity on the horizontal propagation of internal gravity modes the objective is to identify baroclinic shoals where wave energy is spatially concentrated and enhanced dissipation might be expected the model is then extended to investigate the propagation of internal waves through a barotropic mesoscale current field the refraction of tidally generated internal waves at the hawaiian ridge is examined using an ensemble of mesoscale background realizations derived from weekly ocean topography experiment topex poseidon altimetric measurements the path of mode 1 is only slightly affected by typical cur

isotopic and ree characteristics of the intrusive charnoenderbite and enderbite geographically associated with the matok pluton limpopo belt southern africa abstract intrusive charnoenderbite and enderbite in the southern marginal zone of the limpopo belt are genetically linked with peak metamorphism m1 during the 2700 ma to 2650 ma limpopo orogeny they occur geographically associated with and are intruded by the granodiorite and granite of the matok pluton which were emplaced between the response phase m2 and the rehydration phase m3 of that orogeny rb sr pb ree and sm nd elemental and isotopic data for whole rock samples and u pb data for populations of zircon suggest that the charnoenderbite and enderbite were intruded 2671 ma ago and were derived from a crustal precursor the granodiorite and granite of the matok pluton were intruded at approximately the same time between 2667 ma and 2664 ma ago they also were derived from a crustal precursor nd model ages and ree patterns for the charnoenderbite and enderbite and the granodiorite are distinct this relationship may imply that they are not cogenetic alternatively these rock types could be cogenetic if the magma from which they were derived was assimilating older crustal material as it evolved nd isotopic data indicate that the magma giving rise to the charnoenderbite and enderbite formed by partial melting of rocks compositionally similar to some portions of the baviaanskloof gneiss data for the granodiorite are consistent with the possibility that this unit formed in a similar manner from rocks compositionally similar to other portions of the baviaanskloof gneiss the close temporal relationship between the emplacement of the charnoenderbite and enderbite and the granitic rocks of the matok pluton indicates that the retrograde portion of the limpopo orogeny beginning 2670 ma ago occurred rapidly perhaps over 7 ma

a magmatic age and four successive metamorphic events recorded in zircons from a single meta anorthosite sample in the central zone of the limpopo belt south africa abstract ion microprobe u th pb analyses combined with cathodoluminescence imaging of zircons from a meta anorthosite from the central zone of the limpopo belt record five distinct events the most concordant ages of cores of zircon yielded a weighted mean 207 pb 206 pb age of 3344 3 6 ma representing the protolith age the unzoned overgrowths yielded four discrete groups with weighted mean 207 pb 206 pb ages of 3150 15 ma 2614 4 ma 2488 22 ma and 2009 28 ma representing four possible metamorphic events in the central zone this is the first time that a succession of several metamorphic events has been demonstrated in a single rock type from the limpopo belt supplementary material analytical methods and data are available at http www geolsoc org uk sup18368

regional and local controls on postglacial vegetation and fire in the siskiyou mountains northern california usa article i nfo the siskiyou mountains of northwestern california and southwestern oregon are a floristic hotspot and the high diversity of conifers there likely results from a combination of geological ecological climatological and historical factors to evaluate how past climate variability has influenced the composition structure and fire regime of the siskiyou forests pollen charcoal and lithological evidence was examined from two lakes along a moisture gradient to reconstruct the vegetation fire and climate history the late glacial period was characterized by subalpine parkland and infrequent fire at both sites during the late glacial early holocene transition period subalpine parkland was replaced by a closed forest of pinus cupressaceae abies and pseudotsuga and more frequent fires a 1000 years earlier at the wetter site and it is likely that reduced pacific ocean upwelling created warmer drier conditions at the coast in the early holocene pinus cupressaceae were less abundant and fire less frequent at the coastal site during a period of increased coastal upwelling and fog production in the late holocene abies pseudotsuga pinus and quercus vaccinifolia increased in the forest at both sites suggesting a widespread response to cooling fewer fires at the wetter site may account for the abundance of picea breweriana within the last 1000 years the comparison of the two records implies that large scale controls in climate during the last 14 000 cal yr bp have resulted in major changes in vegetation and fire regime asynchrony in the ecosystem response of wetter and drier sites arises from small scale spatial variations in effective moisture and temperature resulting from topographically influenced microclimates and coastal to inland climate gradients

economic potential of the rooiberg group volcanic rocks in the floor and roof of the bushveld complex volcanic rocks of the rooiberg group are preserved in the floor and roof of the mafic rustenburg layered suite of the bushveld complex field and geochemical characteristics of these volcanic rocks imply that they are genetically related to the rustenburg layered suite four major ore forming events are identified in the rooiberg group the first phase was accompanied by volcanic hosted fault controlled hydrothermal copper mineralisation which is found in the lowermost portion of the rooiberg group underlying the rustenburg layered suite this type of mineralisation is tentatively linked to initial rustenburg layered suite intrusions stratabound arsenic mineralisation that possibly formed in response to contact metamorphism characterises the second phase and occurred after extrusion of the damwal formation possibly due to shallow granophyric intrusion the third mineralising event occurred in response to contact metamorphism during the final stages of the rustenburg layered suite where especially pb and zn were introduced into the felsite roof rocks this type of mineralisation affected the majority of the rooiberg group but is most pronounced towards the contact with the rustenburg layered suite the fourth phase is restricted to the rooiberg group in the nylstroom area and is linked to the granite intrusions of the lebowa granite suite from which sn and f were introduced into the uppermost felsite succession mineralisation in the rooiberg group appears to be controlled by the character and intrusion level of the associated bushveld magmas different styles of mineralisation in rooiberg group volcanic rocks are encountered at various stratigraphic levels major primary volcanogenic ore deposits appear to be absent

spatial patterns of atmospheric pollen transport in a montane region abstract annual pollen deposition pollen cm 2 yr 1 was sampled for 6 yr in the southern sierra nevada the owens valley and the inyo mountains of central california spatial pollen distributions were examined for anemophilous plants which are limited to growth sites 1 at high elevations on one or both mountain ranges 2 on lower mountain slopes and 3 on the owens valley floor pollen deposition values for high and low elevation plants changed by a factor of 5 to 10 between adjacent sample sites which straddle boundaries of the plant communities in which the source plants grow pollen deposition values were high and variable within communities in which the species grew pollen deposition beyond source populations was low and uniform despite great differences in distance of sites from pollen sources the step function that seems to characterize pollen distribution patterns implies that pollen from distant populations may be of little value in interpreting vegetation stability and change reflected by fossil pollen stratigraphy the physical isolation of pollen embedded in the local mountain valley wind system from that in the prevailing westerlies may explain part of the spatial distribution of the pollen

discrete metamorphic events in the limpopo belt southern africa implications for the application of p t paths in complex metamorphic terrains essentially all models for the tectonic evolution of the high grade limpopo belt of southern africa invoke a single orogenic event at 2 7 ga involving the collision of the zimbabwe and kaapvaal cratons with a distinct central zone by implication rocks within the belt must have followed parallel pressure temperature time p t t paths the main reasons for this situation are the belief that syn to postdeformational granitic magmatism in the belt was coeval and the fact that p t paths are similar new geochronological data indicate however that this magmatism occurred at distinct times in different parts of the belt over a span of at least 70 m y in addition the data indicate that high grade metamorphism and associated tectonism in the central and southern marginal zones occurred at about 2 0 and 3 15 ga respectively therefore p t t paths of these zones were independent until their juxtaposition more recently than 2 0 ga

internal tide generation by seamounts ridges and islands a numerical investigation is made into the generation of semidiurnal internal tides by tidal flow over steep topographic features in the deep ocean a fully three dimensional free surface nonlinear hydrostatic model the princeton ocean model is used with real stratification representative tidal forcing and gaussian shaped seamounts ridges and islands for the topography the efficiencies of the different topographies in extracting energy from the barotropic tide and in generating an internal tide are considered each topography produces an internal tide characterized by signals propagating away from the feature as beams that follow internal wave characteristic paths the strength of the signals however varies markedly for different topographies this is largely attributed to the way in which the barotropic tide interacts with the three dimensional topography internal wave generation requires a significant vertical displacement of stratified water by the barotropic tide as it flows over topography it is found that for symmetric seamounts and islands the barotropic flow tends to go around the feature producing only a weak internal tide when elongated into a ridge barotropic flow is forced across isobaths generating an energetic internal tide changing the horizontal aspect ratio of the topography from 1 1 a seamount to 3 1 a ridge increases the resulting baroclinic energy flux by nearly an order of magnitude the slope of the topography relative to that of the internal wave characteristics is also shown to be important with strongest generation occurring at regions of critical or supercritical topographic slope

constraints on the form of the p t loop in the southern marginal zone of the limpopo belt south africa abstract a generalised clockwise p t path consisting of a prograde portion a high grade isothermal decompression portion and an essentially isobaric cooling portion has been suggested for the metamorphic evolution of the southern marginal zone previous studies have concluded that the high grade decompression path started in the kyanite stability field peak metamorphic mineral compositions have been reset during the equilibrium decompression event and hydration during the cooling dominated isobaric portion occurred in the kyanite stability field the present petrographic study from the bandelierkop quarry a locality in the granulite zone of the southern marginal zone identified a series of sequential anatectic reactions which record a period of prograde heating from approximately 700 c to 850 c during this period pressure is constrained to within the sillimanite stability field by the production and preservation of prograde generations of sillimanite the maximum temperature melting event is closely followed by two divariant reactions ga qz sil cord and ga qz cord hy which occur in response to a period of general isothermal decompression a subsequent cooling dominated rehydration period is indicated by a crystallisation hydration reaction cord ksp melt bi ky qz which occurred at a minimum temperature of approximately 640 c and pressure greater than 6 5 kbar the present data confirms the clockwise form of the loop and also supports the previously suggested flat slope of the retrograde cooling portion additional mineral chemistry data relating to the second decompression reaction indicate that the zonation profiles of cordierite and garnet as well as the growth of small quantities of garnet after the reaction are in strong support of the suggested p t path the present mineral compositions in the southern marginal zone reflect a near total re equilibration during the high grade cooling dominated portion of the p t path and as a result reflect p t conditions considerably lower than those of the peak metamorphism

the sequence stratigraphic concept and the precambrian rock record an example from the 2 7 2 1 ga transvaal supergroup kaapvaal craton abstract sequence stratigraphic concepts are applied to the 2 7 2 1 ga transvaal supergroup which constitutes the sedimentary floor to the bushveld igneous complex the transvaal succeeds the foreland fill of the witwatersrand supergroup and in its lowermost portions is partly synchronous with the ventersdorp supergroup lavas that followed upon the witwatersrand sediments in the stratigraphic record the unconformable contacts at the base and the top of the transvaal supergroup mark significant changes in the overall tectonic setting which qualifies them as first order sequence boundaries sedimentation within the transvaal basin was controlled by cycles of extensional and or thermal subsidence resulting in the accumulation of sequences separated by stages of base level fall generating sequence bounding subaerial unconformities the transvaal first order sequence is subdivided into five second order unconformity bounded depositional sequences i e the protobasinal black reef chuniespoort rooihoogte timeball hill and boshoek houtenbek sequences the span of time of these second order sequences averaging 130 ma is much longer than the duration of the second order phanerozoic cycles which is explained by the less evolved and thus slower plate tectonic processes that dominated the late archaean early proterozoic times each transvaal second order sequence preserves a variable number of systems tracts mainly as a function of the strength of the erosional processes associated with the ravinement surfaces and subaerial unconformities in a complete succession a sequence would include a basal lowstand systems tract lst followed by transgressive tst highstand hst and falling stage fsst systems tracts the protobasinal and rooihoogte timeball hill sequences only preserve the lst and tst due to the strong post depositional erosion the black reef sequence is composed of lst tst and hst the chuniespoort sequence preserves the tst hst and fsst but lacks the lst due to ravinement erosion the boshoek houtenbek sequence develops the complete succession of systems tracts common features may be emphasized between the same types of systems tract developed within different sequences the lst includes high energy coarse prograding facies such as alluvial fans and fan delta systems that fill the irregularities of the pre existing topography and lead to the peneplanation of the depositional areas the tst may be recognized from retrogradational stacking patterns that are associated with the transgression of marine environments the hst marks the normal regression of the previous marine environments which assumes coeval aggradation of fluvial and basinal facies within an overall progradational framework the fsst would normally include only basinal facies age equivalent with the correlative subaerial unconformities in our case study the stages of base level fall resulted invariably in fully nonmarine environments within the transvaal area generally leading to the development of subaerial unconformities in particular circumstances the nonmarine environment may still accumulate and preserve fluvial alluvial fan or lacustrine sediments as a function of the relative position between topography and local equilibrium profiles and base levels the interpretations of base level rise and fall resulting from the application of the sequence stratigraphic concept to the 2 7 2 1 ga transvaal supergroup enable estimation of sea level changes during deposition of this succession with an understanding of local tectonic conditions on the kaapvaal craton and of coeval sedimentation elsewhere in africa eustatic and relative sea level changes and variation in continental freeboard conditions may be suggested for the transvaal supergroup

when was the limpopo orogeny abstract the term limpopo orogeny refers to the common high grade metamorphic and tectonic event that affected the archaean rocks of the central and southern marginal zones of the limpopo belt and its influence on the adjacent portions of the kaapvaal craton available geochronological data from muscovite kyanite prograde metamorphic assemblages zircon in igneous charnoenderbite and enderbite muscovite in syntectonic pegmatite bodies formed in shear zones partial melts and syn and post tectonic plutons of granodiorite and granite indicate that this orogeny occurred during the time interval from 2700 ma ago to 2650 ma ago the retrograde phase of this orogeny from the onset of the response phase m 2 until the rocks were subjected to rehydration m 3 was very rapid probably lasting 7 ma from 2671 ma to 2664 ma ago similar but limited data from the northern marginal zone and adjacent portions of the zimbabwe craton suggest that rocks there were subjected to a more recent interval of high grade metamorphism and tectonism extending from 2600 ma until 2550 ma

plio pleistocene climatic transition and the lifting of the teton range wyoming abstract fine grained lacustrine riverine and ash fall sediments of the shooting iron formation whose late pliocene age is established by blancan gastropods and vertebrates yield a pollen flora that is essentially similar in composition to the modern pollen rain in the jackson hole area the pliocene assemblage suggests a climate like that of the jackson valley and foothills today these spectra also resemble a pliocene pollen flora from yellowstone park dated at 2 02 ma however the underlying miocene teewinot sediments differ by containing pollen of four exotic deciduous hardwoods tertiary relicts that suggest a summer moist climate unlike that of today the shooting iron sediments lie with an angular unconformity on and above the miocene lake sediments of the teewinot formation both of these deposits probably preceded the main uplift of the teton range based on the absence of precambrian clasts in the tertiary valley deposits because the pliocene floras were modern in aspect a plio pleistocene transition would be floristically imperceptible here the sequence denotes a protracted period of relative stability of climate during teewinot time and a shift in vegetational state summer wet trees drop out sometime between the latest miocene and latest pliocene the pliocene spectra suggest a dry cooler climate toward the end of shooting iron time