organic haze in earth s early atmosphere source of low 13c late archean kerogens high concentrations of greenhouse gases would have been required to offset low solar luminosity early in earth9s history enhanced co 2 levels are probably at least part of the solution but ch 4 may have played a significant role as well particularly during the late archean era 2 5 3 0 ga when methanogenic bacteria were almost certainly present indeed biological ch 4 production should have led to co 2 drawdown by way of a negative feedback loop involving the carbonate silicate geochemical cycle we suggest here that the atmospheric ch 4 co 2 ratio approached the value of 1 needed to trigger formation of titan like organic haze this haze was strongly depleted in 13 c relative to 12 c and was produced at a rate comparable to the modern rate of organic carbon burial in marine sediments therefore it could provide a novel explanation for the presence of extremely low 13 c kerogens in late archean sediments

compositional gradients surrounding spherulites in obsidian and their relationship to spherulite growth and lava cooling spherical masses of crystal fibers spherulites crystalize from rhyolitic melt glass mainly in response to significant undercooling while lava cools spherulite growth should induce compositional gradients in the surrounding glass from expulsion of incompatible constituents and diffusion of those constituents away from the spherulite finite difference numerical modeling of one dimensional diffusion in which diffusivities are allowed to vary with temperature is used to investigate how compositional gradients reflect spherulite growth and lava cooling overall three forms of gradients are identified elements that diffuse quickly are expelled from the spherulite but then migrate away too quickly to become enriched at the boundary of the spherulite elements that diffuse slowly are trapped within the growing spherulite between those endmembers are elements that are not trapped yet diffuse slow enough that they become enriched at the contact their slow diffusion away then elevates their concentrations in the surrounding glass how enriched those elements are at the spherulite matrix interface and how far their enrichments extend outwards into the glass reflect how spherulites grow and thermal conditions during growth concentrations of h2o rb f li cl na k sr cs ba and be were measured in and around spherulites in obsidian from a 4 7 1 km3 rhyolite lava dome erupted from tequila volcano mexico measurable concentration gradients are found for h2o rb and f attributes of those gradients and the behaviors of the other elements are in accord with their experimentally constrained diffusivities spherulites appear to have grown following radial rather than volumetric growth the observed gradients and lack of others are more consistent with growth mainly below the glass transition which would necessitate the dome cooling at ca 10 5 to 10 7 c s 1 such slow cooling is consistent with the relatively large volume of the dome

nature of and controls on volcanism in the ca 2 32 2 06ga pretoria group transvaal supergroup kaapvaal craton south africa abstract the time of deposition of the pretoria group between 2 32 and 2 06 ga on south africa s kaapvaal craton was characterized by the first major increase in atmospheric oxygen it was accompanied by the extrusion of significant thicknesses of volcanic deposits namely the bushy bend lavas of the timeball hill formation the hekpoort formation and the machadodorp volcanic member of the silverton formation marking the three major volcanic events within the transvaal supergroup which are thought to be precursors to the succeeding bushveld complex magmatism the bushy bend lava member of the timeball hill formation is characterized by subaqueous basaltic andesitic fissure eruptions with fumarolic activity and probably minor subaerial explosive eruptions in the hinterland of the transvaal supergroup basin the subaerial volcanism of the hekpoort formation appears to have been dominated by fissure eruptions with a preponderance of lava flows and locally important pyroclastic material intermittent hiatuses in volcanism were marked by local lacustrine shale deposition finally the machadodorp lava member can be interpreted as the deposits of several seamounts aligned along a fissure probably within an extensional environment the geodynamic control inferred for the evolution of the pretoria group basin encompasses two cycles of prerift uplift subsequent mechanical rifting and long lived thermal subsidence the limited extent of the bushy bend lavas in the south of the pretoria group depository attests to the likelihood that volcanism accompanying the first rifting event was short lived in contrast the second rifting cycle postulated for the pretoria group basin has a strong association with widespread and large scale volcanism of the hekpoort ongeluk flood basalts which may have been plume related the scale of the second cycle volcanism and its importance in influencing the upper part of the pretoria group basin fill is underlined by the machadodorp volcanism probably related to hot spot volcanism as the ongeluk hekpoort plume waned

variolites of the paleoproterozoic hekpoort formation transvaal sub basin kaapvaal craton multistage undercooling textures abstract globular structures in volcanic and hypabyssal rocks called varioles or spherulites have been described from a wide variety of places and rock types including komatiites basalts dacites rhyolites as well as alkaline dykes but the processes leading to their formation are still debated here we present a field petrological and geochemical study of variole bearing lavas variolites in the paleoproterozoic volcanic hekpoort formation in the southern part of the transvaal sub basin of south africa variolites were only observed in a restricted area corresponding to a 3 5 km long basin or depression the variolites are locally up to 50 m thick and correspond to a succession of one meter thick lava flows most of the flows consist of two distinct layers with an upper layer rich in spherical varioles 1 32 cm in diameter overlying a layer generally devoid in varioles but often showing vertical jointing both layers show randomly oriented acicular textures of centimetre sized skeletal clinopyroxene crystals al rich augite that precede the formation of varioles the varioles themselves are mainly composed of radiating acicular augite 1 m in diameter the crystallization of which caused diffusion of incompatible elements which became enriched in the groundmass around the varioles the whole rock geochemistry of the hekpoort variolites is indistinguishable from that of non variolitic hekpoort lavas with the same mgo content this excludes liquid immiscibility as the origin of the varioles the acicular texture of al rich augite in combination with the incompatible element enrichment around the varioles suggests that both skeletal and acicular augite are related to undercooling rather than devitrification sub aqueous deposition is thought to be the cause of undercooling

globular ferropicritic rocks at pechenga kola peninsula russia liquid immiscibility versus alteration abstract the uppermost volcanic unit of the early proterozoic pechenga group comprises ferropicritic volcanic and hypabyssal rocks which possess well developed light weathering globular up to 10 cm in size or layer like structures mesoscopicly similar to those found in some archean ocellar komatiites these structures are superficially reminiscent of features attributed to liquid immiscibility including spheroidal aggregates in all stages of coalescence however several lines of field microscopical mineralogical and chemical evidence argue against the immiscibility hypothesis these include identical modal proportions grain sizes morphologies and orientations of crystalline phases in the globules and matrix as strikingly demonstrated for example by the spinifex textured varieties of the globular rocks a solid state hydrothermal metamorphic diffusional process that affected the originally glassy mesostasis is concluded to be the only feasable mechanism for the formation of the globular structures in the pechenga ferropicrites

nappes tectonics of oblique plate convergence and metamorphic evolution related to 140 million years of continuous subduction franciscan complex california this paper presents a new synthesis of franciscan complex tectonics with the emphasis on the pre san andreas fault history of these rocks field relations suggest that the franciscan is characterized by nappe structures that formed during sequential accretion at the trench the presence of these structures along with other field relations including the lack of evidence for large offset of conglomerate suites indicates that strike slip fault systems of large displacement 500 km did not cut the franciscan complex during subduction regional geology and comparisons to modern arc trench systems suggest that strike slip faulting associated with oblique subduction took place inboard east of the franciscan in the vicinity of the magmatic arc the franciscan varies along strike because individual accreted elements packets of trench sediment seamounts etc did not extend the full length of the trench different depths of underplating distribution of post metamorphic faulting and level of erosion pro

major and trace element geochemistry of the archaean ntem plutonic complex south cameroon petrogenesis and crustal evolution the ntem complex crops out in south cameroon and belongs to the congo craton it corresponds to a 2 8 ga basement which in the northern part is made up of plutonic rocks 1 charnockites and 2 soo tonalites both have similar compositions and define a typical tonalite trondhjemite and granodiorite ttg suite the study of isotopes isr major elements and ree allows us to propose a petrogenetic model that may summarized as follows 1 1 partial melting of the mantle gave rise to tholeiites 2 2 tholeiites transformed into garnet bearing amphibolites underwent partial fusion thus generating the parental magma of the ttg suite 3 3 fractional crystallization mainly of hornblende and plagioclase produced the differentiated ttg suite this scheme is typical of archaean ttg suites and is supposed to have occurred in a subduction like geodynamic environment where because of the high archaean geothermal gradients the subducted oceanic crust underwent fusion before dehydration operated it is also possible to show that the ttg suite was not emplaced during the granulite facies metamorphism but that this occurred later affecting only some parts of the basement consequently the ntem complex appears to be an archaean high grade terrane where the granulitic assemblages may be early proterozoic in age characteristic that seems to be more widespread among archaean high grade terranes

plate and intraplate processes of hercynian europe during the late paleozoic abstract it is speculated that until late carboniferous time the region of hercynian europe was occupied by an elongated island arc system underlain by a segment of continental crust in the upper carboniferous two subduction zones are assumed to have extended from the north and south beneath hercynian europe an extensive zone of hot partially molten upper mantle lay above and between these and diapiric uprise of portions of this material led to separation of mafic magmas widespread partial melting in the lower and middle crust high temperature low pressure metamorphism in crustal rocks and regional uplift and extension of the crust as indicated by intermontane troughs and their associated volcanic rocks in visean to westphalian time hercynian europe collided with both the large neighbouring plates north america europe and africa during these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of hercynian europe after the collision processes had been terminated processes of upper mantle activity continued causing further crustal uplift and even enhanced crustal extension for several tens of million years into the lower permian decline of the upper mantle activity beneath hercynian europe is indicated by crustal subsidence and formation of a peneplain in permian time followed by the upper permian transgression of both the zechstein sea and the tethys sea which mark the end of the hercynian geodynamic cycle

joint development normal to regional compression during flexural flow folding the lilstock buttress anticline somerset england alpine inversion in the bristol channel basin includes reverse reactivated normal faults with hanging wall buttress anticlines at lilstock beach joint sets in lower jurassic limestone beds cluster about the trend of the hinge of the lilstock buttress anticline in horizontal and gently north dipping beds j3 joints 295 2858 strike are rare while other joint sets indicate an anticlockwise sequence of development in the steeper south dipping beds j3 joints are the most frequent in the vicinity of the reverse reactivated normal fault responsible for the anticline the j3 joints strike parallel to the fold hinge and their poles tilt to the south when bedding is restored to horizontal this southward tilt aims at the direction of s1 for alpine inversion finite element analysis is used to explain the southward tilt of j3 joints that propagate under a local s3 in the direction of s1 for alpine inversion tilted principal stresses are characteristic of limestone shale sequences that are sheared during parallel flexural flow folding shear tractions on the dipping beds generate a tensile stress in the stiffer limestone beds even when remote principal stresses are compressive this situation favors the paradoxical opening of joints in the direction of the regional maximum horizontal stress we conclude that j3 joints propagated during the alpine compression caused the growth of the lilstock buttress anticline q 2001 published by elsevier science ltd

diffusion controlled spherulite growth in obsidian inferred from h2o concentration profiles spherulites are spherical clusters of radiating crystals that occur naturally in rhyolitic obsidian the growth of spherulites requires diffusion and uptake of crystal forming components from the host rhyolite melt or glass and rejection of non crystal forming components from the crystallizing region water concentration profiles measured by synchrotron source fourier transform spectroscopy reveal that water is expelled into the surrounding matrix during spherulite growth and that it diffuses outward ahead of the advancing crystalline front we compare these profiles to models of water diffusion in rhyolite to estimate timescales for spherulite growth using a diffusion controlled growth law we find that spherulites can grow on the order of days to months at temperatures above the glass transition the diffusion controlled growth law also accounts for spherulite size distribution spherulite growth below the glass transition and why spherulitic glasses are not completely devitrified

emplacement and deformation of granitoids during volcanic arc construction in the foothills terrane central sierra nevada california abstract emplacement of a series of late jurassic to early cretaceous tonalites and diorites at depths of 4 to 9 km accompanied volcanic arc construction in the foothills terrane central sierra nevada california emplacement of these plutons occurred after initiation of faulting and rigid rotation of bedding and immediately before widespread cleavage development pluton emplacement occurred by permissive emplacement in the hinges of large anticlines or domes or as structurally controlled sill like bodies in a ductile shear zone and was accompanied by widespread mingling of magmas subsequent deformation rotated but did not internally deform one pluton strongly deformed several others and shortened the overlying volcanic and sedimentary sequences by 30 to 50 we suggest that deformation was active throughout volcanic arc construction but evolved from early more brittle deformation to ductile flow immediately after pluton emplacement and back to brittle deformation as the arc was uplifted and cooled at this location arc magmatism lasted at least 30 ma whereas deformation lasted at least 35 ma and possibly much longer the different emplacement mechanisms changing style of deformation and differences in viscosity contrast between plutons and wall rocks resulted in an impressive variety of structural characteristics and apparent timing relationships for the subvolcanic plutons

early mesozoic southward migration of cordilleran transpressional terranes oblique plate convergence can cause forearc arc and back arc regions of active margin orogenic belts to behave as semi independent terranes relative to bordering continental and oceanic lithospheric plates these terranes called here transpressional terranes are bound by the subduction zone on one side by arc parallel strike slip faults in the forearc in the volcanic arc or in the back arc region on the other and are underlain by throughgoing basal decollements these terranes undergo shortening in a direction perpendicular to the subduction zone and migrate as coherent structural bodies parallel to the subduction zone along bounding strike slip faults many terranes of the north american cordillera may have behaved in this manner and perhaps did not travel across the pacific basin but rather were always peripheral to north america we propose that during the triassic to mid cretaceous portions of the cordillera borderland generally were displaced southward with respect to the north american craton due to left oblique convergence since the mid cretaceous they moved northward due to right oblique subduction the proposed displacements are consistent with structural paleomagnetic and paleontologic data and with the relative and absolute motions of north america and the pacific basin plates based on hotspot reference frame models

fluid enhanced deformation transformation of granitoids to banded mylonites western sierra nevada california and southeastern australia abstract deformation of tonalite and adamellite from the western sierra nevada california and southeastern australia has produced mylonite zones that show a broad range of microstructural development varying from strongly banded and or foliated to locally non foliated accompanying these fabrics are new mineral assemblages commonly rich in quartz and epidote whereas the banded foliated varieties appear to develop from progressive deformation of the granitic host they also occur in close proximity cm m to domains of non banded and or weakly to non foliated fabrics of similar mineralogy suggesting a complex timing and partitioning of deformation fluid flow and metamorphism in the mylonite zone whole rock major and trace element analyses of the tonalites show large enrichments of sio 2 cao fe 2 o 3 total fe and sr and strong depletions in na 2 o k 2 o feo mgo ba and rb with increasing stages of deformation although substantial chemical change appears to occur under static conditions locally chemical changes in the adamellite are much less dramatic all suites show systematic decrease in 18 o and convergence of oxygen isotopic compositions of whole rock and quartz with increasing deformation minimum fluid rock is estimated to lie between 0 1 and 10 increasing with intensity of deformation and depending on host composition isotopically exchanged seawater or connate water are the most plausible fluids chemical data from a number of mylonite zones in various orogenic belts show widely varying behavior gain loss of major and minor elements eleven factors control which and how much elements are gained lost during mylonitization of the granitoids the most influential of which are primary composition mineralogy and primary texture of the host as well as several aspects of fluid interaction active during deformation these influences may be a function of local rather than regional geological conditions

spherulites quench fractures and relict perlite in a late devonian rhyolite dyke queensland australia abstract a late devonian rhyolite dyke displays perlitic and other fracture sets as well as textures generated by crystallisation of the glass the dyke is less than a metre wide and has sharp contacts with ignimbrite although originally glassy no glass is preserved aligned magnetite after pyroxene microlites and trains of small 0 5 1 mm spherical spherulites crystallised early at temperatures above the glass transition temperature and before formation of the fracture sets long subplanar fractures oriented perpendicular to the dyke walls extend almost the full dyke width and end by merging with adjacent long fractures short subplanar cross fractures are perpendicular to and terminate at the long fractures well defined perlitic fractures are present within the volumes of rock generally

mesozoic reactivation of variscan thrusting in the bristol channel area uk new seismic reflection data from the bristol channel area provide direct evidence of a major variscan thrust in the palaeozoic basement beneath the mesozoic bristol channel syncline the thrust has an average southerly dip of about 25 and it rises into the major fault the central bristol channel fault zone cbcfz at the northern margin of the bristol channel syncline the cbcfz is attributed to reactivation in extension of the variscan thrust probably during the early cretaceous there is no evidence for major synsedimentary movement on the cbcfz and the bristol channel syncline does not represent a rollover structure section restoration suggests that the cbcfz has cut up through a previously tilted sequence of triassic and jurassic strata pervasive nnw sse cross faults are interpreted as original variscan transfer faults that have been reactivated in the mesozoic and tertiary

tectonic setting of the jurassic smartville and slate creek complexes northern sierra nevada california jurassic ophiolitic complexes in california and oregon are widely distributed outboard of a jurassic and older continental margin whether these outboard complexes were exotic and collided with the jurassic continental margin or whether they were simply part of a wide forearc to a jurassic continental magmatic arc has long been at issue we have studied two such complexes and crosscutting intrusions in the fault bounded central and western belts of the northern sierra nevada we report new and revised u pb zircon dates for the early jurassic and younger slate creek complex ca 203 171 ma in the central belt the late jurassic smartville complex 159 3 ma in the western belt and crosscutting intrusions ranging from middle jurassic to early cretaceous age the new ages imply that the central and western belts have been part of the same terrane since middle jurassic time and that the fault separating them has accommodated only minor displacements since ca 160 ma isotopic compositions of common pb indicate derivation from mantle sources but evidence for inheritance of pre cambrian zircon is common in plutons within the early and late jurassic complexes and in middle jurassic late jurassic and early cretaceous crosscutting intrusions although there are several potential sources for such contamination the ages of the inherited zircon are consistent with a north american origin this contamination would have been accomplished most plausibly near the north american margin

subsidence in late paleozoic basins in the northern appalachians during the interval between continental collision in the devonian and continental breakup in the triassic the northern appalachians became the site of a wide plate boundary zone of dominantly right lateral strike slip as is typical of intracontinental transforms tectonism was both diachronous and rapidly variable along strike through regimes of pure strike slip transpressional deformation and rapid subsidence of extensional basins up to 9 km of mainly nonmarine clastic sediments accumulated in these local depocenters which subsided episodically in two stages 1 an initial phase of stretching and thinning of the lithosphere when subsidence was rapid fault controlled and often accompanied by volcanism and 2 a subsequent phase of gradual thermal subsidence during which the depositional basins expanded to bury the earlier border faults and progressively younger sedimentary units onlapped basement the largest depocenter the magdalen basin opened as a pull apart between strike slip faults in newfoundland and new brunswick from late devonian to early carboniferous subsequent thermal subsidence affected a large area during medial and late carboniferous a phenomenon that is well recorded to the north and west where no later tectonism occurred in areas to the south and east of the basin strike slip on other faults continued into the time of thermal subsidence introducing complications such as localized transpressional deformation and rapid subsidence in smaller pull aparts

primitive os and 2316 ma age for marine shale implications for paleoproterozoic glacial events and the rise of atmospheric oxygen abstract re os dating of synsedimentary to early diagenetic pyrite from carbonaceous shale that straddles the boundary between the rooihoogte and timeball hill formations transvaal supergroup south africa provides a precise isochron age of 2316 7 ma 4 statistical uncertainty if error on decay constant is excluded and a chondritic initial 187os 188os ratio of 0 1121 0 0012 these units were deposited between what are interpreted as the second and third of three paleoproterozoic global glacial events and thus provide minimum and maximum ages respectively for these events the rooihoogte formation is correlative with the duitschland formation which records previously undated carbon isotope excursions because the pyrite samples show no evidence of mass independent fractionation of sulfur and have highly negative 34s values the rise of atmospheric oxygen most likely began prior to 2 32 ga the chondritic initial 187os 188os ratio requires that primitive hydrothermal sources of os dominated the os budget of seawater at 2 32 ga os is mobile only under oxidizing conditions therefore the chondritic initial 187os 188os ratio may reflect atmospheric oxygen levels too low to introduce sufficient riverine flux of dissolved radiogenic os to offset the primitive hydrothermal magmatic flux even if atmospheric oxygen levels had increased significantly by 2 32 ga anoxic conditions throughout the archean most likely limited os enrichment in archean marine shales so that their subsequent exposure and weathering was unable to provide a significant amount of radiogenic os to paleoproterozoic seawater

growth and deformation of porphyroblasts in the foothills terrane central sierra nevada california negotiating a microstructural minefield the main porphyroblastic minerals in schists and phyllites of the foothills terrane western metamorphic belt central sierra nevada california are cordierite and andalusite mostly chiastolite less commonly biotite muscovite chlorite garnet or staurolite are also present as porphyroblasts the variety of porphyroblast and matrix microstructures in these rocks makes them suitable for testing three modern hypotheses on growth and deformation of porphyroblasts 1 porphyroblast growth is always syndeformational 2 porphyroblasts nucleate only in low strain largely coaxially deformed quartz rich q domains of a crenulation foliation and are dissolved in active high strain non coaxially deformed mica rich m domains the spacing between which limits the size of the porphyroblasts and 3 porphyroblasts generally do not rotate with respect to geographical coordinates during deformation provided they do not deform internally so that they may be used as reliable indicators of the orientation of former regional structural surfaces even on the scale of orogenic belts some porphyroblast matrix relationships in the foothills terrane are inconsistent with hypotheses 1 and 2 and others are equivocal for example in many rocks it cannot be determined whether the porphyroblasts grew where the strain had already been partitioned into m and q domains whether the porphyroblasts caused this partitioning or both although most porphyroblasts appear to be syndeformational as predicted by hypothesis 1 observations that do not support the general application of hypotheses 1 and 2 to rocks of the foothills terrane include a lack of residual crenulations in many strain shadows and alternative explanations where they are present b absence of porphyroblasts smaller than the distance between nearest mica rich domains c nucleation of crenulations on existing porphyroblasts rather than nucleation of porphyroblasts between existing crenulations d presence of micaceous arcs in an undifferentiated matrix against some porphyroblasts suggesting static growth e absence of crenulations in porphyroblastic rocks showing sedimentary bedding and f porphyroblasts with very small random inclusions which are probably pre deformational similarly porphyroblasts that have overgrown sets of crenulations and porphyroblasts with micaceous arcs are probably post deformational at least on the scale of a large thin section and probably over much larger areas judging from mesoscopic structural evidence some porphyroblasts in rocks of the foothills terrane do not appear to have rotated with respect to geographical coordinates during matrix deformation in accordance with hypothesis 3 at least on the scale of a large thin section however other porphyroblasts evidently have rotated in some instances this appears to be due to mutual interference but many apparently rotational porphyroblasts are too far apart to have interfered with each other which indicates that the rotation was associated with deformation of the matrix the occurrence of planar bedding surfaces adjacent to porphyroblasts about which bedding and or foliation surfaces are folded suggests rotation of the porphyroblasts during non coaxial flow parallel to bedding rather than crenulation of the matrix foliation around static porphyroblasts it appears that porphyroblasts may rotate during deformation if the matrix is relatively homogeneous so that the strain is effectively non coaxial this may occur after homogenization of a matrix in response to the strongest degree of crenulation folding whereas the same porphyroblasts may have been inhibited from rotating previously when strain accumulation was partitioned in the matrix

palaeomagnetism of the early palaeoproterozoic volcanic hekpoort formation transvaal supergroup of the kaapvaal craton south africa

sequence stratigraphy and plate tectonic significance of the transvaal succession of southern africa and its equivalent in western australia the transvaal succession in the northeastern part of the kaapvaal province of southern africa is the erosional remnant of four 2 7 to 2 1 ga unconformity bounded stratigraphic units or sequences each of which is 102 to 103 m thick and spans 107 to 108 years three of these phanerozoic like cratonic sequences occur in the western part of the kaapvaal province and in the pilbara province of western australia thus the two provinces once were part of the same continent vaalbara the stratigraphically lowest sequence historically assigned to the transvaal succession is part of the ventersdorp supergroup in kaapvaal and the upper part of the fortesque group in pilbara it consists predominantly of arkosic sandstone and basaltic andesite it probably is 2687 2 ma the second sequence contains the quartz arenites dolomites and banded iron formations of the transvaal and griqualand west successions in the kaapvaal and the hamersley group in the pilbara the span of ages 2684 6 to 2432 31 ma lithologies and geographic extent of this sequence imply that the limpopo orogeny which welded the zimbabwe province to vaalbara is 2 47 ga not 2 67 0 05 ga the third sequence the pretoria and postmasburg groups in the kaapvaal and the lower part of the wyloo group in the pilbara probably is the clastic wedge generated by the limpopo orogeny the fourth sequence dominated by the felsites of the rooiberg group is restricted to northeastern kaapvaal sequence stratigraphy lithostratigraphy and lithofacies indicate that the pilbara rifted from the southern edge of western kaapvaal the penge iron formation in northeastern kaapvaal the asbesheuwels iron formation in southwestern kaapvaal and the brockman iron formation in the pilbara are the same formation in which cyclothemic units 0 6 to 15 0 m thick extended 1200 km

the ongeluk basaltic andesite formation in griqualand west south africa submarine alteration in a 2222 ma proterozoic sea abstract the ongeluk lavas form part of the palaeoproterozoic transvaal griqualand west supracrustal sequence of the archaean kaapvaal craton of south africa they form a thick shallow marine volcanic sequence of pillow lava massive flows and hyaloclastite which together with their comagmatic subaerial hekpoort correlate once covered most of the craton in this study the magmatic composition and alteration features of ongeluk samples were distinguished using geochemistry and radiogenic isotopes the ongeluk hekpoort magma was a basaltic andesite derived from anomalous kaapvaal craton lithosphere with a long history of high u pb high rb sr and chur like sm nd ratios little evidence for magmatic fractionation was found although contamination of the lower lavas with cr rich material is evident in both hekpoort and ongeluk sequences the alteration of ongeluk lavas is characterised using diagrams which compare pillow core rim pairs hyaloclastite and massive lava samples with the estimated original magma composition alteration is ascribed to a shallow 87 sr 86 sr ratios decreased from magmatic 0 7071 towards seawater 0 7025 during submarine alteration the rb sr system also reveals a much later 1200 ma phase of ca and rb mobility in a few samples in contrast the submarine alteration affected pb isotope ratios so little that a 2222 13 ma isochron age is regarded as a refinement of earlier dates for the ongeluk hekpoort extrusion the ongeluk hyaloclastites acted as a sink for k and rb in the overlying sea while the mn and ca entering the sea could have contributed to the overlying kalahari manganese deposits

prism c slip in the quartzites of the oakhurst mylonite belt california abstract quartz c axis fabrics from 21 amphibolite facies quartzites of the oakhurst mylonite belt california suggest that prism c prism a and basal a slip systems were activated during deformation qualitative two dimensional kinematic models consistent with meso and microscopic shear criteria are constructed based on a regime of combined pure and simple shear in this kinematic framework most of the observed c axis fabric patterns of the oakhurst mylonite belt can be explained if prism c slip was dominant during the early increments of deformation until a transition to dominant basal a slip occurred an apparent relationship between grain size and type of crystallographic fabric is interpreted as resulting from some quartz syntectonic vein quartz having recorded only the latest increments of deformation comparison with the metamorphic evolution of fault zones of similar tectonic origin in california suggests that prism c slip was enhanced by the circulation of hot fluids which hydrolytically weakened quartz and allowed prism c slip to occur at relatively low temperature

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

the origin of large varioles in flow banded pillow lava from the hooggenoeg complex barberton greenstone belt south africa exceptionally well preserved pillowed and massive phenocryst free metabasaltic lava flows in the uppermost part of the palaeoarchaean hooggenoeg complex of the barberton greenstone belt exhibit both flow banding and large leucocratic varioles the flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt robins et al in bull volcanol 72 579 592 2010a varioles occur exclusively in the dark chlorite mgo and feo rich metabasalt varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows in the central parts of some pillows they impinge to form homogeneous pale patches bands or almost homogenous cores individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite many varioles exhibit concentric zones and finer grained rims some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer grained rim the matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain like skeletal ca rich pyroxenes that are partly overgrown by the margins of varioles varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile tio2 cr zr and ree that are similar to the host metabasalts the shape distribution texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass nucleation of plagioclase was strongly inhibited and took place on vesicles on the bases of drainage cavities and along early fractures eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows

geochemistry and origin of early proterozoic volcanic rocks from the transvaal and soutpansberg successions south africa abstract the early proterozoic transvaal tr and soutpansberg sb successions in south africa are composed of sediments and variable amounts of volcanic rocks volcanic rocks are chiefly tholeiitic in character and in some formations range from mafic to felsic in composition except for machadodorp basalts all volcanics exhibit nb ta depletion relative to th and ree machadodorp basalts are relatively depleted in incompatible elements and some exhibit flat to depleted lree patterns like morb andesites and more felsic volcanics in the tr succession can be related to associated basalts by shallow fractional crystallization in terms of incompatible trace element ratios two and perhaps three mantle sources are required for tr sb basalts these sources may be part of an ancient subcontinental lithosphere beneath the kaapvaal craton which was variably enriched with a subduction zone component during one or more episodes of arc collision prior to 3 0 ga

volcanic influences on hydrothermal and diagenetic alteration evidence from highway reward mount windsor subprovince australia synvolcanic sills and cryptodomes hosting the highway reward volcanic hosted massive sulfide deposit invaded wet poorly consolidated silt sand and pumice breccia margins of the porphyriric units display perlitic and quench fractures suggesting that they were formerly glassy interiors of the rhyolites rhyodacites and dacites were originally glassy partly glassy or comprised crystalline domains exhibiting spherulitic lithophysae and micropoikilitic textures no glass is preserved and the sills and cryptodomes are now completely crystalline primary devitrification structures crystallized early above the glass transition temperature and before the formarion of perlite or quench fractures crystallization of residual glass proceeded by diagenetic alteration volcanic associated hydrothermal alteration and ore associated hydrothermal alteration massive sulfide ore is enclosed in a discordant alteration envelope that extends at least 60 m above ore and is known to a depth of 150 m in the footwall in general there is a progression from central quartz sericite pyrite zones giving way to zones of chlorite n anhydrite n gypsum chlorite sericite quartz and finally chloritesericite distal to ore the outermost alteration grades into ithofacies that have altered to various background assemblages of feldspar hematite sericite chlorite quartz epidote and carbonate compared to least altered lithofacies the quartz sericite pyrite zones are enriched in sion k20 fe o3 s and ba and depleted in n o cao mgo and sr elevated concentrations of mgo fe203 and s coupled with low na o cao and k o values characterize chlorirized zones the pattern of low n o cao and mgo values and high k o values passing outward through the chlorite sericite n quartz zones is more erratic the ishikawa alteration index ai value and the chlorite carbonate pyrite index ccpi value become progressively larger passing from background altered facies ai 60 ccpi 20 55 into the footwall and hanging wall alteration zones ai 89 97 ccpi 41 99 at ai values greater than 90 cu bi and mo are elevated the s n o ratio increases toward ore whereas sr ba ratios are lowest at ai values above 90 the geochemical halo patterns reflect the composite response of lithofacies to mineralogical and textural chan es accompanying diagenetic alteration volcanic associated hydrothermal alteration and ore associated hydromermal alteration most alteration zones around ore display two or more overprinting alteration mineral assemblages within single alteration zones the distribution of alteration minerals and assemblages varies spatially in response to contrasring lithofacies character formerly glassy coherent and volcaniclastic facies display mineralogical and textural changes that reflect the influence of fracture perlitic quench and or matrix permeability and porosity on fluid migration crystalline domains of volcanic facies generally preserve their original texture although recrystallization has partly destroyed primary devitrification microstructures alteration minerals are localized along cooling joints hydraulic fractures and veins or form mottled patchy or wispy domains in contrast siltstone that separates intrusions and forms the matrix in peperite is relatively unaltered or was baked during magma sediment mixing in zones of intense quartz sericite n pyrite chlorite n anhydrite gypsum and sericite alteration high water rock ratios an extended history of fluid flow and elevated temperatures promoted large scale mobilization of elements in all facies relics of the early alteration mineral assemblages are locally preserved in these zones and imply a progression from initial volcanic determined

primary clinopyroxene spherulites in basaltic lavas from the pacific antarctic ridge fresh glassy basaltic andesite samples recovered from the northern part of the pacific antarctic ridge contain abundant spherulites consisting of arrays of closely packed clinopyroxene fibers the spherulites frequently enclose elongated vesicles that are tear drop shaped or tailed the long axes of the elongated vesicles were found to be always parallel to the orientation of the surrounding crystal fibers in several cases elongated vesicles having different orientations are hosted by a single spherulite the existence of elongated vesicles provides unequivocal evidence that a significant proportion of the clinopyroxene fibers must have crystallized directly from a supercooled liquid at temperatures above the glass transition moreover the absence of fractures within the spherulites and the surrounding glass as well as the observed nucleation of clinopyroxene fibers at vesicle walls are interpreted to be consistent with a primary origin of the clinopyroxene spherulites based on these textural observations it has to be concluded that spherulites do not represent a diagnostic texture for the devitrification of volcanic glass that occurs below the glass transition temperature

middle triassic strike slip tectonics and igneous activity of the dolomites southern alps abstract strike slip faults and south vergent thrusts deformed a rhomb shaped area measuring about 20 20 km in the dolomites during the late ladinian deformation was caused by a right stepover of a nne ssw trending sinistral strike slip fault strike slip displacement amounted to more than 10 km thrusting caused a relief shortening of more than 3 km and can be demonstrated by the matching of particular ladinian carbonate facies boundaries these tectonic movements may account for the termination of a transgressive depositional megacycle which had begun in permian times rupture of the crust of the deformed terrain caused the emplacement of a magma chamber beneath its evacuation in the latest ladinian and early carnian was associated with the collapse of its roof and the extrusion of shoshonitic lavas they filled a volcano tectonic depression which overlaps with the deformed and eroded area the orientation of volcanic dikes suggests the absence of tectonic strain during igneous activity provided that they formed contemporaneously

the sticklepath lustleigh fault zone tertiary sinistral reactivation of a variscan dextral strike slip fault dextral strike slip movement on the sticklepath lustleigh fault zone slfz is indicated by displacements of permian and older rocks previous authors have inferred that the main dextral movement which caused these displacements was post permian and noting the presence of small tertiary pull apart basins along the fault zone probably of tertiary age however the geometry of these early tertiary pull apart basins indicates sinistral rather than dextral strike slip movement we present an alternative model for the history of the sticklepath lustleigh fault zone summarized below 1 late variscan strike slip movement with a total displacement of up to 10 km produced the slfz and offset dextrally an earlier variscan thrust 2 extensional reactivation of this thrust led to rapid permo triassic subsidence in the crediton trough and a dextrally offset neighbour the hatherleigh outlier 3 approximately 6 km of early tertiary sinistral movement on the slfz produced small pull apart sedimentary basins and reduced the net dextral offset across the fault 4 in mid tertiary times minor dextral movements on the slfz may have produced reverse faulting on the margins of the tertiary basins

modeling of continental weathering under high co2 atmospheres during precambrian times batch experiments were conducted to simulate abiotic weathering of the continental crust under high co2 atmospheric conditions during precambrian times i e corresponding to the general archaean conditions as well as to the immediate aftermath of the neoproterozoic snowball earth ice ages three types of rock basalt granodiorite and tonalite representative of the archaean to proterozoic continental crust were reacted in the form of powders for 1 year at 40 c with pure water under various water rock ratios in oxic and anoxic atmospheres containing 10 co2 that is under conditions assumed to characterize the greenhouse effect which prevailed at that time chemical and mineralogical data collected during the course of the experiments reflect alteration phenomena occurring in two steps 1 rapid leaching of the fresh surfaces probably related to the proton donor capacity of the co2 and 2 a steady state reaction under near neutral conditions these observed dissolution rates can be satisfactorily modeled by kinetic data available in the literature using a 1 d weathering model reproducing the two steps we evaluate the atmospheric co2 consumption rate at the snowball earth aftermath focusing on the relative contribution of surface leaching versus steady state reaction the results show that the process is dominated by surface exchange

chemical weathering in granitic environments abstract factors controlling chemical weathering in granitic environments are deduced using an extensive database from the literature based on a transition state theory model we evaluated the effects on chemical weathering of runoff temperature and ph the dependence between temperature and runoff and chemical weathering is complicated by other factors such as the presence depth of soil cover soil cover favors chemical weathering when it is thin and rich in weatherable minerals in contrast it reduces chemical weathering when is thick and poor in primary minerals processes that increase the contact time and the surface of the water rock interactions such as physical denudation increase chemical weathering rates in granitic environments the effect of temperature can be quantified assuming an apparent activation energy value of 48 7 kj mol to describe chemical weathering of silica in granitic crystalline environment under high runoff conditions

relationships between kinematics of arc continent collision and kinematics of thrust faults folds shear zones and foliations in the nevadan orogen northern sierra nevada california abstract the mesozoic nevadan orogeny in the northern sierra nevada metamorphic belt california may be attributed to arc continent collision stratigraphic data and macroscopic cross cutting relations suggest successive accretion of two arcs along an active continental margin the younger accretion event involved the early jurassic slate creek terrane which is a 3 5 km thick pseudostratigraphic arc fragment the slate creek thrust an isoclinally folded fault with a subhorizontal median surface carries the slate creek terrane at least 40 km eastward continentward over the pre existing continental margin terrane amalgam no rock units can be correlated across the slate creek thrust which is thus interpreted as an arc continent suture in addition to the slate creek thrust the nevadan orogen includes a major east vergent imbricate thrust set east of and beneath the slate creek thrust and steep west vergent reverse shear zones macroscopic upright folds and steep foliations that overprint and cut the east vergent structures these data suggest a model for the relationships between the kinematics of arc collision and nevadan orogenic structures the slate creek arc terrane accreted by westward partial subduction of the continental margin along the east vergent slate creek thrust the continental margin was imbricated along an east vergent thrust set the structurally higher inactive slate creek thrust suture was deformed by steep west vergent shear zones folds and foliations which may have accommodated shortening of the east vergent thrust sheet this deformation occurred within an active continental margin arc that probably initiated by subduction flip after collision of the slate creek arc this kinematic model is consistent with the structural geometry and chronology of the nevadan orogen while qualitatively maintaining lithosphere scale strain compatibility this model has implications for problems related to emplacement of large crystalline thrust sheets displacements beneath and at the margins of shortened crustal segments and interaction of oppositely verging structures the nevadan orogen is a slate belt and the structural plate tectonic model presented for the nevadan orogeny may be testable in slate belts of other orogens

basaltic volcanism mantle plumes and the mechanics of rifting the parana flood basalt province of south america dynamic modeling of continental extension between south america and africa shows that the mechanics of rifting played an important role in determining the pattern of volcanism within the parana and etendeka flood basalt provinces on the brazilian and namibian margins the key feature of the model is the development of a horizontal pressure gradient in the lower crust during the early stages of extension which provided a mechanism for transporting magma generated beneath the incipient sea floor spreading axis into the parana province 100 200 km distant the horizontal pressure gradient developed as a consequence of the dynamic interaction of preexisting weaknesses in the middle crust and upper mantle during rifting the model accounts for the large quantity of basalt the asymmetric distribution of basalt on the conjugate margins and the northward migration of the eruptive center with time the rapidity of magma genesis is in agreement with models of decompression melting during rifting the model indicates that although elevated asthenosphere temperatures associated with the tristan plume account for the volume of melt generated the mechanics of rifting control the location and style of emplacement the model suggests that extension in the region began ca 150 155 ma and lasted about 25 m y

magnetic susceptibility of the mont louis andorra ilmenite type granite pyrenees a new tool for the petrographic characterization and regional mapping of zoned granite plutons we have observed remarkably consistent patterns of concentric zonation in the values of low field magnetic susceptibility measurements over the variscan age mont louis andorra granite pluton of the eastern pyrenees of andorra spain and france this zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature as defined by chemical analysis and the magnetic susceptibility magnitude of the rocks it reflects the dominantly paramagnetic nature of the granite carried by fe bearing silicates and this is demonstrated by the fact that low field susceptibilities of representative specimens are almost equal to 1 their high field susceptibilities and 2 their calculated susceptibilities using fe contents assuming a null ferromagnetic contribution we conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons

evidence for the transition to an oxygen rich atmosphere during the evolution of red beds in the lower proterozoic sequences of southern africa abstract strata 2 5 to 1 8 ga in southern africa appear to span the transition from a non oxidizing atmosphere to one characterized by free oxygen red beds in 2 3 to 2 1 ga unconformity bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks ironstones or banded iron formations which evidently were sources of iron for the matrices however red beds in 1 9 ga unconformity bounded sequences not only matrix staining but also hematitic coatings on grains these beds also have far fewer interstratified ferruginous lithologies because red beds with stained clay matrices banded iron formations pyritic witwatersrand type conglomerates and iron depleted paleosols all occur in the 2 2 ga pretoria group formation of these lithologies may not have been time dependent until atmospheric oxygen approached 10 2 atm by 1 9 ga atmospheric oxygen was 10 2 atm causing lateritic weathering and red beds with hematitic coatings on grains

joints in the mesozoic sediments around the bristol channel basin abstract analysis has been carried out at four locations on the edges of the bristol channel basin to illustrate the later phases of deformation of a sedimentary basin and to illustrate the control on joint patterns of subtle changes in the stress system the characteristics of the joints are described and influences on joints are determined including the roles of faults folds and beds there is a low coefficient of correlation between joint spacing and bed thickness except in very thin limestone beds which have a high density of joints the lengths and spacings of earlier joint phases are usually greater than those of later phases later joints normally abut against earlier joints the joints abut the latest faults but are not displaced by them so the joints post date the main alpine contraction the joints formed in five main phases during reduction of the alpine stresses phase 1 joints are sub parallel to the regional compression direction 160 180 phase 2 joints are perturbed by faults often curving towards points of stress concentrations along the faults phase 3 joints are sub parallel to the earlier e w striking fold axes phase 4 joints are cross joints and phase 5 joints form polygonal patterns within joint bound blocks phases 2 and 3 do not occur in the absence of faults and folds and correspond with a reduction in horizontal compression and an increase in the importance of local factors phases 4 and 5 occur at all locations

the geology and geomorphology of the beacon cottage farm outlier st agnes cornwall the mid oligocene sediments forming the beacon cottage farm outlier at st agnes cornwall have a stratigraphical and geomorphological importance which is out of all proportion to their modest residual bulk some 300000 m3 during 1986 8 61 holes were sunk into the area surrounding beacon cottage farm many penetrated the palaeozoic floor beneath the outlier collectively these show that the outlier mostly covers rotten granite and killas the sub oligocene unconformity is uneven and varies in altitude by at least 20 m the local bovey formation succession comprises two members basal sand which is often pebbly and sandy silts known commercially as candle clay the whole totalling a maximum of 8 9 m there is indirect evidence either that slopes on the rotted palaeozoic floor locally exceed 45 or that several post mid oligocene faults affect the outlier we conclude that the beacon cottage farm sediments probably represent aeolian colluvially and fluvially redistributed rotten rock residues formed locally during a break in the long period of late palaeogene subtropical weathering and pediplanation the outlier is considered in relation to the geomorphological evolution of the cornubian massif

liquid immiscibility in the archean greenstone belt of piumhi minais gerais brazil abstract liquid immiscibility has been used for explaining several modern situations either in effusive or in intrusive rocks liquid immiscibility is also proposed for explaining the variolitic structures in the archean rocks of the abitibi greenstone belt ontario the volcanic sequence of the lower portion of the greenstone belt of piumhi minas gerais brazil shows the occurrence of variolitic structure that may have been formed by liquid immiscibility this possibility is supported by the following points a the contact between varioles and matrix is very sharp b spinifex texture occurs both in the varioles and in the matrix and the single crystal may cross the varioles c the varioles and the matrix show contrasting compositions the latter has the composition of a basaltic komatiite and the former varies form that of an alkali basalt to andesite and d a test by a thermodynamic model from literature shows that although with some doubt the free energy of the possible parent liquid is higher than that of the two unmixed liquids

dacitic ocelli in mafic lavas 3 8 3 7 ga isua greenstone belt west greenland geochemical evidence for partial melting of oceanic crust and magma mixing abstract mafic volcanic rocks in the 3 8 3 7 ga isua greenstone belt southern west greenland contain randomly distributed 1 to 10 centimeters long white spheroidal structures in this study these structures are called ocelli in the western part of the belt ocelli bearing lavas are enclosed in basaltic to picritic flows mgo 9 21 wt with a subduction zone geochemical signature the ocelli are composed predominantly of polycrystalline na plagioclase and quartz with minor hornblende and biotite whereas the surrounding amphibolite matrix basaltic host is composed mainly of hornblende ca plagioclase and quartz the ocelli are devoid of radial or concentric internal structure and display all stages of coalescence contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational compositionally the ocelli are calc alkaline dacites sio2 62 9 72 0 wt mgo 0 60 3 50 wt ni 58 143 ppm cr 250 510 ppm whereas the surrounding matrix is tholeiitic basalt sio2 46 6 50 6 wt mgo 8 70 12 30 wt ni 119 175 ppm cr 330 600 ppm in terms of major element composition the isua ocelli closely resemble plagiogranites in phanerozoic supra subduction zone ophiolites field and petrographic observations and geochemical data sio2 54 2 60 7 wt mgo 3 95 7 72 wt ni 127 158 ppm cr 500 570 ppm on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts on a chondrite normalized diagram the basaltic host is characterized by variably depleted lree patterns la smcn 0 30 0 94 gd ybcn 1 03 1 45 whereas the dacitic ocelli display lree enriched patterns la smcn 1 30 2 60 gd ybcn 1 32 2 58 the strongly depleted ree patterns in the basaltic host are attributed to lree loss during carbonate alteration partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host the geochemical characteristic of the ocelli cannot be explained by post magmatic alteration slab melting fractional crystallization of tholeiitic melts or liquid immiscibility we suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust high mg volcanic rocks that fell into the magma chamber suggesting magma crust interaction in the early earth formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early archean

an interpretation of the variscan structures in sw england the variscan structures in sw england apart from the high level structures in the culm synclinorium of n devon are recumbent with an overall gentle southward dip of cleavages thrusts and recumbent folds translation was towards the nnw the structures are interpreted on a thin skinned model with a basal decollement dipping gently s related to a s dipping subduction zone s of the lizard the granites are thought to have been generated to the s of their present position and injected northwards above the decollement zone

integrated geophysical modelling of a giant complex impact structure anatomy of the vredefort structure south africa abstract only three very large confirmed impact structures are known on earth the chicxulub crater mexico 65 ma ca 180 km wide the sudbury structure canada 1 85 ga 200 km in diameter and the vredefort structure in south africa 2 02 ga while extensive data on large impact structures have been obtained by remote sensing studies of such features on other planetary bodies only this small number of large terrestrial impact structures can provide data crucial to understanding these catastrophic impact processes on earth integrated modelling of gravity and magnetic data constrained by geological as well as refraction and reflection seismic data accomplished the reconstruction of the vredefort impact structure in south africa approximately 250 km wide the original vredefort impact structure covered the whole extent of the archaean witwatersrand basin distinguished by enormous gold resources as it is structurally preserved today in fact it is clear that the preservation of vast volumes of economically important witwatersrand strata is the direct result of the formation of the ring basin around the central uplift the vredefort dome of the impact structure this study is the first attempt to create an integrated and geophysically well constrained model of this very large complex impact structure

weathering of basaltic pebbles in a red soil from sardinia a microsite approach for the identification of secondary mineral phases abstract mineral weathering microsites in a basalt rock derived soil were investigated in the mediterranean environment of south western sardinia an in situ approach frequently used for saprolites and rock investigations was employed to examine the weathering microsites of a soil system this study compared clay phases that formed from the weathering of mineral bearing rock pebbles and soil crystal chemical composition with the goal of evaluating the evolutionary trends of the secondary minerals micromorphological mineralogical xrd and ft ir analyses and microscopic sem eds analyses were carried out on i plagioclase enstatite and glassy features weathered volcanic glass microsystems in rock pebbles embedded in the soil ii clay coatings microsystem connecting the soil rock environment and iii the soil itself eds data were plotted on phase diagrams to evaluate developmental and evolutionary trends of the weathered products the weathering of enstatite and volcanic glass resulted in primarily fe bearing montmorillonites while a mixture of fe bearing smectite and kaolinite developed from plagioclases our results indicated formation of secondary phases in open microsystems where transfers of elements among primary minerals and glass occur by slow diffusion along the network of inter mineral micropores cracks that developed inside the soil rock pebbles acted as direct connections between microsystems and the soil environment clay and fe mn coatings deposited in the cracks are an indication of clay migration the mineralogy of these clay coatings was similar to that of the soil matrix and the fine clays since it is likely that the clay migration processes remain active in guspini soils the further mineralogical similarities of clay coatings and soil fine clays suggest that the transformation of fe smectites into k s mlms occurs in the clay phases during transport prior to deposition into the soil environment indeed the similar mineralogy of the in situ analysed soil matrix and fine clays the youngest soil fraction is interpreted as due to same soil processes suggesting stable environmental conditions in the guspini area

post nevadan deformation along the bear mountains fault zone implications for the foothills terrane central sierra nevada california southern parts of the foothills terrane sierra nevada california consist of western and eastern sequences of volcanic rocks and overlying slate gray wacke units separated by a multiply deformed and lithologically chaotic central belt structural and strain studies suggest that the intensity of regional ductile deformation decreased from greater than 50 shortening in the western belt to less than 30 shortening in the eastern belt and that much of this ductile deformation is younger than the timing usually assumed for the nevadan orogeny these structures are in turn deformed by a large ductile shear zone representing the southern continuation of the bear mountains fault zone this shear zone separates the western and central belts has an oblique east over west sense of movement and deforms the western margin of the guadalupe igneous complex these observations suggest that the eastern and western volcanic sequences are pieces of arcs amalgamated along the bear mountains fault zone during and after nevadan deformation

geology and tectonic evolution of the bear mountains fault zone foothills terrane central sierra nevada california the late jurassic early cretaceous bear mountains fault zone bmfz is the westernmost strand of the foothills fault system in the western metamorphic belt of the sierra nevada the tectonic significance of this 300 km long fault zone has been downplayed in the past but we contend that it is a major discontinuity within the foothills terrane the bmfz is an 5 km wide shear zone consisting of slate metagraywacke matrix melange that experienced intense polyphase deformation and metamorphism small scale structures indicate reverse slip along most of the extent of this steeply east dipping shear zone a host of lithologically diverse tectonic blocks are enclosed in the bmfz blocks of sandstone metavolcanic rocks and plutonic rocks of intermediate composition have counterparts in the adjacent eastern and western zones of the foothills terrane whereas other block types are exotic exotic blocks include ultramafites that locally contain pods of gabbro garnet amphibolite greenschist sedimentary breccia and volcaniclastic rocks the breccia is composed primarily of clasts of amphibolite and minor garnet bearing impure quartzite metachert it probably accumulated at the base of fault scarps the exotic blocks comprise a broadly ophiolitic assemblage similar to that inferred to form basement to the foothills terrane and they record a deformation prior to incorporation in the slate metagraywacke matrix amphibolite blocks and quartzite preserved as clasts within the breccia probably were deformed during overthrusting of oceanic lithosphere in a setting similar to that beneath the tuolumne ophiolite east of the bmfz the presence of exotic blocks implies large scale mixing and large displacements in the bmfz as do differences in stratigraphy across the fault zone metamorphic grade does not vary significantly across the bmfz however suggesting that dip slip did not exceed several tens of kilometers if the fault zone originally dipped gently eastward as we propose there is no direct evidence for major strike slip along the bmfz as has been proposed by others any such slip must have occurred before the reverse slip related structures of the zone we thus interpret the bmfz as an intra arc reverse fault of moderate to large displacement with possible earlier strike slip that incorporates ophiolitic basement showing an older complicated history

the post variscan development of the british isles within a regional transfer zone influenced by orogenesis abstract the break up of pangaea after the variscan orogeny included rifting extending southwards from the barents sea via the norwegian greenland rift and into the north sea and northwards from the central atlantic these two major rift systems interacted to form an approximately 1200 km wide transfer zone across the british isles where a complex network of basins developed during the mesozoic fault patterns were commonly controlled by reactivation of precambrian caledonian and variscan structures the two main rift systems were unable to breach this regional transfer zone where the crust had been thickened by the caledonian and variscan orogenies until the eocene breaching did not occur down the north sea and through the english channel because of alpine contraction in nw europe instead breaching occurred around the west of ireland and nw scotland so the british isles remained connected to europe rather than to the north american plate

correlation of the rhenohercynian variscides recent structural and stratigraphical data from the rhenohercynian variscides of belgium germany and south cornwall indicate tectonic continuity within this zone prior to disruption along a late carboniferous dextral transcurrent fault this extends from the bristol channel to the vosges parallel to similar lineaments of variscan age in europe and north america consequently a tectonic model for south cornwall is extrapolated to europe and a plate tectonic setting for the evolution of the north european variscides by the closure of an oceanic basin is discussed

regional structural and strain analyses of terranes in the western metamorphic belt central sierra nevada california abstract a comparison of total strains in 225 samples from the western metamorphic belt sierra nevada shows that strain types and intensities strongly depend on rock type greywackes and clast supported conglomerates are least strained whereas more intense strains occur in lapilli tuffs pebbly mudstones and slates large belts of volcanic rock record plane strain whereas nearby slate and greywacke sequences record flattening type strain suggesting that volume losses of 45 65 occurred in slates and 0 26 in greywackes lapilli tuffs record less than 11 volume loss the differences in volume loss and strain from one rock type to another require that discontinuities in total strain occur at most lithologic contacts independently of the lithologic control regional structural patterns and strain fields support the delineation of three terranes foothills merced river and northern sierra and two distinct domains western and eastern foothills terranes at these latitudes within the western metamorphic belt these terranes each bordered by large ductile shear zones differ in average strain intensity and orientation but not in strain type in the foothills terrane strain intensities decrease from moderate values in the western foothills terrane to low values in the eastern foothills terrane strain intensities in the study area are highest in the melones and bear mountains fault zones and in terranes east of the melones fault structures associated with regional deformation in each terrane do not extend far across terrane boundaries multiple generations of regionally developed structures are therefore rare in this part of the western metamorphic belt outside of fault zones we see one regionally developed cleavage in the foothills terrane a single but older cleavage in the merced river terrane and two still older cleavages in the northern sierra terrane however younger deformations may have reactivated and rotated older structures in adjacent terranes the timing and kinematics of the deformation in the foothills terrane suggest that faulting and rigid body rotations preceded pervasive shortening continued fault movement and rotation of faults to steep dips if orientations of ductile structures reflect plate motions structures in the merced river terrane suggest jurassic triassic orthogonal nne convergence whereas structures in the foothills terrane support late jurassic orthogonal ne convergence changing to oblique ene convergence about 150 ma ago

subaerial volcanism in the palaeoproterozoic hekpoort formation transvaal supergroup kaapvaal craton abstract within a 30 km strike length of the palaeoproterozoic hekpoort formation pretoria group transvaal supergroup approximately equal proportions of basaltic andesitic flows and lenses of volcaniclastic rocks are preserved an overall intracratonic and subaerial setting is inferred for these rocks a complex volcanic environment is thought to have existed with relatively quiescent amygdaloidal massive lava flows and violent pyroclastic eruptions either succeeding each other or at times coexisting within the same setting identified physical processes of effusive volcanism include coarser and finer pyroclastic flows massive pyroclastic breccia and lapilli tuff breccia facies sedimentary reworking of inferred ash cloud fine material lapilli tuff facies distal sheetflood stratified lapilli tuff breccia facies and debris flow massive reworked lapilli tuff breccia facies reworking of pyroclastic debris two prominent lenses of volcaniclastic rocks preserved within the study area may lie either side of an eruptive centre and a palaeovalley may have confined lahar deposits the predominance of pyroclastic flow and lahar deposits in the hekpoort formation resembles the succession formed from the 1991 mount pinatubo eruption

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

possible variscan thrusting beneath the somerton anticline somerset vibroseis seismic data were acquired by goal petroleum plc during 1982 in the vicinity of the somerton anticline somerset these data show the simple form of the anticline but in addition show a southward dipping discontinuous seismic event beneath gravity modelling demonstrates that this event may be correlated with an e w trending gravity gradient which can be followed for approximately 150 km the dipping seismic event may represent a variscan thrust plane which dips to the ssw in contrast to similar planes seen to the east which dip to the sse the variation in dip seen between these postulated thrust planes may explain the curvature of structural trends seen within and to the east of the mendip hills the polden hills fault runs sub parallel to the proposed thrust plane beneath the somerton anticline and may be linked to a mesozoic reactivation of the postulated thrust

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

age and petrogenesis of the lundy granite paleocene intraplate peraluminous magmatism in the bristol channel uk the lundy granite forms part of the lundy igneous complex which is the southernmost substantive expression of magmatism within both the british cenozoic igneous province and the wider north atlantic igneous province its qz pl kfs bt grt tpz mineralogy and peraluminous character contrast with other british cenozoic igneous province granites farther north but are similar to the granites of the adjacent early permian cornubian batholith we present the results of mapping petrographical and mineral chemical analysis and the first u pb zircon ages for the granite 59 8 0 4 58 4 0 4 ma and cross cutting basic dykes 57 2 0 5 ma which confirm a paleocene age for magmatism zircon inheritance is limited but two cores imply the presence of early palaeozoic igneous rocks in the unexposed basement of sw england the anomalous southerly location of the lundy igneous complex is a consequence of mantle melting arising from the superimposition of localized lithospheric extension related to intraplate strike slip tectonics with the distal ancestral icelandic plume granite generation primarily reflects crustal partial melting during the emplacement of mantle derived melts the change in geochemical character between the lundy granite peraluminous and other british cenozoic igneous province granites metaluminous or subalkaline indicates a fundamental crustal source control between contrasting peri gondwanan and laurentian basement provinces supplementary material sample locations petrographic summaries mineral chemistry data and secondary ionization mass spectrometry u pb analytical data are available at https doi org 10 6084 m9 figshare c 3886783

late palaeozoic basins of the southern u s continental interior the timing and geometry of late palaeozoic inhomogenous deformation in the southern u s continental interior from the appalachian foreland of kentucky tennessee through oklahoma and texas to the ancestral rockies of colorado wyoming utah can be definitively linked with a discrete sequence of collisional events in the appalachian ouachita marathon orogenic belt to the south permian coordinates a progressive staged collisional sequence beginning in late mississippian times in the southern appalachians and culminating in early permian times in the marathons led to a progressive deformation of the adjacent craton in a wide swath dominated by right lateral shear e g rough creek fault zone by which a mexican promontory of north america was displaced towards a pacific free face while the deformation in the appalachian marathon belt was dominated by vertical plane strain leading to crustal thickening the associated continental interior deformation can be averaged as a horizontal plane strain at or near sea level with localized deep extensions delaware and flexural arkoma basins and compressions uplifts amarillo wichita giving local source areas

north american apparent polar wander plate motion and left oblique convergence late jurassic early cretaceous orogenic consequences the north american apparent polar wander apw path indicates an episode of unusually rapid absolute northward motion of western north america between 150 and 135 ma during this time the northward component of absolute motion of points along the washington oregon california coast was in excess of 150 km m y and perhaps as high as 230 km m y we believe that such high absolute northward velocity for north america probably ensures that relative motions of oceanic plates and terranes influenced by them were to the south at this time the inception of rapid northward motion and left oblique convergence was abrupt and should be recorded in the geology of the western cordillera it is tempting to correlate this period of unusual pacific basin north american interaction with the nevadan orogeny in the klamath mountains as well as with left lateral strike slip structures such as the pine nut fault and bear mountains fault zone significant differences exist between north american plate motion recorded by the late jurassic cretaceous apw path and that predicted by a fixed hotspot model we believe that this discrepancy reflects uncertainty associated with pre late cretaceous hotspot tracks and poorly constrained relative plate motions during the cretaceous normal polarity superchron

magmatic evolution of sulawesi indonesia constraints on the cenozoic geodynamic history of the sundaland active margin abstract tertiary and quaternary magmatic rocks from west sulawesi record the complex history of part of the sundaland margin where subduction and collision have been and are still active the present study based on petrographic data major and trace element chemistry and 40 k 40 ar dating aims to document the age and chemical characteristics of the magmatic formations from west sulawesi and to determine the corresponding constraints on the geodynamic evolution of the sundaland border the west sulawesi magmatic province includes the south arm of sulawesi ujung pandang area the western part of central sulawesi with the toraja and palu areas and finally the north arm extending from palu to manado which includes the tolitoli and manado areas paleocene magmatic activity seems to be restricted to an episode of calc alkaline magmatism in the ujung pandang area 61 59 ma the major eocene 50 40 ma magmatic event is tholeiitic and is documented in all areas except in ujung pandang it led to the emplacement of tholeiitic pillow lavas and basaltic dykes of back arc basin bab affinity these rocks are potential equivalents to the celebes sea basaltic basement from oligocene to miocene magmatic eruptions produced successively island arc tholeiitic iat and calc alkaline ca rock series the youngest iat activity occurred around 18 ma in the central part palu area and around 14 ma in the north arm tolitoli area while ca magmas were emplaced in the north arm at ca 18 ma tolitoli and manado areas typical calc alkaline activity resumed only in the north arm tolitoli and manado areas during the late miocene 9 ma and is still active in the manado region in other areas palu toraja and ujung pandang areas an important and widespread magmatic event occurred between 13 and 10 ma and emplaced k rich magmas either silica undersaturated alkali potassic basalts ak ultrapotassic basanites uk or shoshonites sh k rich activity continued in the south until the pleistocene 0 77 ma with alkali potassic ultrapotassic and shoshonitic magmas in central sulawesi toraja and palu areas the most recent magmatic event occurred between 6 5 and 0 6 ma the corresponding products are granitic rocks and widely distributed rhyolitic pyroclastic flow deposits all these rocks are acidic in character sio 2 60 with trace element and isotopic signatures sr nd pb typical of a strong continental imprint the most striking tectonic implication of this magmatic evolution is that west sulawesi can no longer be considered as a typical magmatic arc as previously assumed with the exception of the manado area beneath which subduction is still active calc alkaline and island arc tholeiitic lavas and plutonics are volumetrically minor with respect to k rich magmas their occurrence through time is also fairly restricted mostly to the period between 30 and 15 ma another important feature is the occurrence of island arc tholeiitic and calc alkaline magmas crosscutting an older terrane of bab affinity the tinombo formation manado tolitoli and palu areas as this formation is being regarded as an equivalent to the celebes sea floor the most likely explanation for this feature is the hypothesis of tectonic erosion linked to the nw dipping subduction beneath the north arm the late miocene high k magmatic activity in central and south sulawesi reflects the prevalence of a post collisional tectonic regime following the docking of microcontinents of australian origin to central sulawesi during neogene times the incompatible element enriched character of these high k rocks might reflect their derivation from a mantle source enriched through metasomatism related to a previous subduction event such a model cannot account for the plio pleistocene cak magmatism of central sulawesi the acidic composition of which does not support a derivation from an ultrabasic source the trace element patterns of the cak rocks are very similar to those of the high grade metamorphics of central sulawesi suggesting that the latter might represent their possible source such an anatectic model implies a collisional to post collisional tectonic regime limited to central sulawesi while a post subduction regime prevailed in the south

the british tertiary igneous province palaeomagnetism and ages of dykes lundy island bristol channel summary of 66 dykes collected 52 gave stable directions 44 reversed 4 normal and 4 intermediate the mean direction of magnetization was n3 6 e 62 7 down equivalent to a pole position of 82 6 on 155 0 e this is significantly different from the present pole and displays right handed farsidedness the bulk at least of the dykes fall within the apparent span 45 54 my yet they display approximately the same small proportion of normal dykes as do the older dyke swarms of the province which were formed during a different polarity period or periods

analysis and interpretation of composite foliations in areas of progressive deformation abstract in areas of progressive deformation where successive structures develop during a relatively continuous deformation within a geologically short time period traditional chronological notations of structural elements e g s 1 s 2 etc can give erroneous impressions of how large rock masses evolve in time and space we demonstrate from field examples that successive structures can develop which a are comparable in morphology and orientation but of different ages b are different in morphology but of comparable age and c show rapid morphological changes over short distances under such conditions correct identification of the relative age of structures is often difficult to impossible we consider the concepts of a composite foliation and of transposition cycles as vehicles to objectively evaluate the significance of different sets of structures in the evolution of larger rock masses we suggest that 1 structural elements be labeled using morphological notation adding numerical subscripts only when independent evidence is available 2 geologists more fully acknowledge and integrate the concept of deformation partitioning into their models and 3 when analyzing areas of multiple deformation more emphasis is placed on the relationship between domains of differing complexity integration of these three perspectives in the analysis will lead to a more realistic basis upon which to model the structural evolution of large rock masses

timescales of spherulite crystallization in obsidian inferred from water concentration profiles we determined the kinetics of spherulite growth in obsidians from krafla volcano iceland we measured water concentration profiles around spherulites in obsidian by synchrotron fourier transform infrared spectroscopy the distribution of oh groups surrounding spherulites decreases exponentially away from the spherulite glass border reflecting expulsion of water during crystallization of an anhydrous paragenesis plagioclase sio2 clinopyroxene magnetite this pattern is controlled by a balance between the growth rate of the spherulites and the diffusivity of hydrous solute in the rhyolitic melt we modeled advective and diffusive transport of the water away from the growing spherulites by numerically solving the diffusion equation with a moving boundary numerical models fit the natural data best when a small amount of post growth diffusion is incorporated in the model comparisons between models and data constrain the average spherulite growth rates for different temperatures and highlight size dependent growth among a small population of spherulites

cooling rates of mid ocean ridge lava deduced from clinopyroxene spherulites abstract we present a compositional analysis of spherulites and their surrounding groundmass in basaltic andesite lava samples collected from the pacific antarctic ridge the spherulites consist of radiating dendritic branches of clinopyroxene and contain abundant vesicles that are stretched parallel to the branches stretched vesicles require that the spherulites grew when the groundmass was liquid and thus while it was hotter than its glass transition although chemical components incompatible in clinopyroxene should have been enriched in the melt by spherulite growth no compositional variations occur at a scale larger than the 10 m wide slit aperture used for laser ablation icp ms the absence of large scale gradients implies that the spherulites grew faster than the elements could diffuse 10 m because diffusion is temperature controlled cooling must have been rapid enough to prevent movement if the spherulites nucleated when the melt was at between 1380 and 1223 k i e undercooled by 0 to 157 k then the constant concentrations of li na and k the fastest diffusing elements analyzed require temperature to have cooled faster than 10 6 7 k min 1 if instead the spherulites nucleated at undercoolings of 357 to 465 k or as cold as 915 k equal to the glass transition temperature then the cooling rate could have been as slow as those inferred for glassy rinds on submarine lavas by relaxation geospeedometry at all possible cooling rates however the spherulites must have grown as fast as 10 3 5 m s 1 to reach their final sizes of 1 4 to 3 2 mm we speculate that the rapid growth resulted from low kinetic barriers to clinopyroxene growth in the basaltic andesite melt