searching for travertines calcretes and speleothems in deep time processes appearances predictions and the impact of plants common models for modern calcite precipitation in and around caves soils springs and streams involve co 2 supplied by thick high p co 2 biogenic soils which were probably thin or non existent before vascular plants indeed plant influenced chemical weathering might have caused accelerated terrestrial carbonate production from the devonian onwards however terrestrial carbonates have also been documented from the archaean proterozoic cambrian ordovician and silurian mechanisms which could have caused non marine carbonates to precipitate without organic rich soils are described and some geological events likely to have influenced non marine carbonate precipitation up to the origin of vascular plants are highlighted as organisms have evolved so have the petrographic characteristics of non marine carbonates some examples of this are also given here

the effects of source rocks and chemical weathering on the petrogenesis of siliciclastic sand from the neto river calabria italy implications for provenance studies plutonic and gneissic rocks of the sila massif in the uppermost portion of the neto drainage basin calabria southern italy weather and erode under a humid mediterranean climate during the development of weathering profiles a combination of chemical weathering and granular disintegration processes occurred chemical weathering involves a loss of both plagioclase mainly during grus generation and k feldspar mainly during soil formation this loss is attributed to transformation of plagioclase to clay minerals and to leaching and dissolution of k feldspar sand composition is quartzofeldspathic and nearly homogeneous along the main channel of the neto river even where the river cuts across a blanket of sedimentary cover thus fluvial transport does not alter sand composition within the neto drainage basin petrographic indices are effective in 1 discriminating between contributions from similar granite and gneiss source rocks qm f 2 relating the provenance of plutoniclastic and gneissiclastic sand found in the headwaters to grus horizons qm f q rg and 3 distinguishing between upstream first cycle and downstream multicycle sand q rg this last distinction is further emphasized by considering both aphanitic and phaneritic varieties of rock fragments rgrmrs diagram chemical weathering is the main sand producer within the regolithic environment in northern calabria in addition rapid erosion resulting from steep slopes removes weathered products and rapid and short transport leads to minimal sediment maturation in general the f q index is climate and relief dependent thus it should be used in conjunction with palaeoclimatic and palaeophysiographic evidence for provenance interpretations of ancient quartzofeldspathic sandstones

magma accumulation rates and thermal histories of plutons of the sierra nevada batholith ca zircon u pb geochronology results indicate that the john muir intrusive suite of the central sierra nevada batholith california was assembled over a period of at least 12 ma between 96 and 84 ma bulk mineral thermochronology u pb zircon and titanite 40ar 39ar hornblende and biotite of rocks from multiple plutons comprising the muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure assembly of intrusive suites in the sierra nevada and elsewhere over millions of years favors growth by incremental intrusion estimated long term pluton assembly rates for the john muir intrusive suite are on the order of 0 001 km3 a 1 which is inconsistent with the rapid magma fluxes that are necessary to form large volume magma chambers capable of producing caldera forming eruptions if large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites it is doubtful that the intrusive suites represent cumulates left behind following caldera forming eruptions

weathering and sediment generation in the archean an integrated study of the evolution of siliciclastic sedimentary rocks of the 3 2ga moodies group barberton greenstone belt south africa abstract alluvial and braided fluvial deposits of the 3 2 ga moodies group in the barberton greenstone belt south africa are analyzed petrographically and geochemically to identify their source rocks provenance and assess the degree and type s of weathering required to produce the observed moodies sediment compositions conglomerate sandstone and shale data give somewhat differing pictures of provenance and weathering due to derivation from different components within the source terrane and size and compositional fractionation during transport the results suggest that shale geochemistry provides the most accurate estimate provenance although both rare earth element and trace element th sc zr cr and ti data must be used in combination to give the best results the source area for moodies group sediments was dominated by tonalite felsic volcanic rock komatiite basalt and granite based on mineralogical and major element divergence from estimated source area composition the moodies group sediments are remnants of an aggressive weathering environment labile materials such as komatiite basalt and coarse plagioclase grains decomposed almost entirely to clays and solutes and the chemical index of alteration for moodies shale is well above the global average an aggressive weathering environment in the archean may have been achieved by increased rainfall higher temperatures and or higher atmospheric p c o 2 more likely a combination of these conditions worked to offset the inhibitory weathering effects of a plant free environment

record of climatic fluctuations and high ph weathering conditions in a thick ordovician palaeosol developed in rhyolite of the dunn point formation arisaig nova scotia canada a thick ordovician intra rhyolitic palaeosol was studied to investigate the peculiarities of early palaeozoic continental environments shortly before the development of vascular plants and to compare its pedogenic patterns with those of previously studied intra basaltic profiles from the same succession well defined k eluviation from the top of the palaeosol is proportionally met by k illuviation at the base of the profile a pedogenic behaviour that was masked in the underlying intra basaltic profiles due to subsequent eodiagenetic k enrichment associated with rhyolite emplacement greater stability of k and eventually mg than si in the intra rhyolitic profile suggests high ph soil water at the time of pedogenesis despite the low base contents of the acidic host rock and despite evidence for humid weathering conditions we also observe that at the base and at the top of the profile al2o3 zr ratios are substantially lower than those of the host rock suggesting al leaching and therefore extreme weathering conditions whereas the middle portion of the profile shows al2o3 zr ratios that are similar to those of the host rock suggesting al stability and therefore less extreme conditions we interpret these variations in al2o3 zr ratios as signatures of a cyclic change from sub humid periods during which al was stable throughout the profile to more humid periods with a well defined seasonality during which only the juvenile zone of weathering at the base of the profile couldhavedevelopedsufficientlyhighphduringdryseasonsforaltobeleached leavingtheal2o3 zr signatures from previous stages untouched higher in the profile

multistage emplacement of the mount givens pluton central sierra nevada batholith california the ca 90 ma mount givens pluton is one of the largest granodioritic to granitic intrusions in the sierra nevada batholith of california emplacement of the pluton occurred during a critical time in the tectonic evolution of the central sierra nevada magmatic arc marked by a transition from regional contraction to dextral transcurrent shear a model for the emplacement of the intrusion is developed based on detailed mapping of the pluton and its wall rocks and characterization of its internal structure by measurements of the anisotropy of magnetic susceptibility ams at 351 stations one of the key results of the study is documentation of a strong correlation between petrologic and structural fabrics in the pluton and determination that these fabrics reflect internal magma chamber dynamics more than regional tectonic strain the 80 km long 15 30 km wide pluton crystallized from a multiphase three segment magma chamber marked by a bulbous northern lobe and linear central and southern segments the pluton is interpreted to be tabular in shape with a thickness of 5 km most of the space for the pluton was created by piecemeal block downdrop of the magma chamber floor along three principal fracture sets the most important of which were steeply dipping northwest trending fractures formed parallel to the structural grain of the arc and vertical north trending extension fractures formed in response to a component of arc parallel dextral shear some of these fractures acted as magma conduits episodically filling the pluton as source rocks became depleted in melt an initial voluminous intrusive event stage 1 quickly filled the southern chamber with granodiorite magma but only partially filled the northern and central chambers stage 2 magmatism involved underplating of megacrystic granite in the northern chamber and lateral flow of a large batch of this magma from the northern to the central chamber the latter delineated by a 20 km long belt of megacrystic granite containing subhorizontal magnetic lineations that connects the pluton segments floor downdrop eventually ceased to be an effective space making process in the northern lobe and renewed magmatism stage 3 led to expansion and doming of the chamber as the northern lobe cooled a ring fault ruptured within the viscoelastic stage 1 2 carapace allowing ring dike intrusion stage 4 and sinking of a central flap of consolidated material the temporal and spatial variations in emplacement mechanisms demonstrated for the mount givens pluton i e fracture generation floor downdrop underplating inflation ring diking suggest that end member models e g fracture vs diapir are oversimplifications of the pluton assembly process

lithologic and climatologic controls of river chemistry abstract the chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff in all 101 different rivers each draining a primary lithology were examined across the united states puerto rico and iceland to minimize seasonal effects only rivers with at least two years of data were used basaltic catchments were examined in the greatest detail in a survey of hawaiian island watersheds the average river chemistries could be related to the distribution of soil associations within each catchment an analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium ion exchange control on cation ratios among different lithologies unique weathering rate relationships were developed with yields at typical present day runoff rates 1 100 cm y increasing in the order sandstones granites basalts shales and carbonates the bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering thus a warm wet climate or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal

evidence from middle ordovician paleosols for the predominance of alkaline groundwater at the dawn of land plant radiation atmospheric carbon is estimated to have been 500 times more abundant in hadean time than at present and its concentration has been gradually decreasing since then due to its storage in sedimentary rocks consequently rain ph has been gradually increasing through geologic time leading to the common assumption that groundwaters are less acidic today than they were in the distant past however this assumption overlooks the fact that root forming land plants increase the carbonic acid concentration in soils by one or two orders of magnitude in the absence of rooted land plants reactions between minerals and rainwater are known to promote alkalinity it is hypothesized that groundwater ph must have been on average highest shortly before the late ordovician to silurian proliferation of root forming land plants to verify this hypothesis we studied the mineralogy and geochemistry of the youngest known pre silurian paleosols to have developed on primary rocks and provide evidence that they have evolved in predominantly alkaline groundwaters despite warm and humid paleoenvironmental conditions today a lush vegetation cover would thrive in such a climate and the system would be necessarily acidic due to large inputs of organic acids paired with previous observations indicating that early paleozoic sedimentary rocks are especially rich in detrital illite and k feldspar there is now enough evidence to believe that there was a greater tendency for alkalinity during this time period than during previous and subsequent geologic periods

a global scale plate reorganization event at 105 100 ma abstract a major plate reorganization is postulated to have occurred at approximately 100 ma however this reorganization has received limited attention despite being associated with the most prominent suite of fracture zone bends on the planet and many other geological events we investigate tectonic events from the period 110 to 90 ma and show that the reorganization occurred between 105 and 100 ma was global in scale and affected all major plates seafloor evidence for plate motion changes is abundant during this period with either fracture zone bends or terminations preserved in all ocean basins long lived eastern gondwanaland subduction ended along a 7000 km long section of the margin while elsewhere around the proto pacific rim subduction continued and there is evidence that compressional stresses increased in the overriding plates thrusting in western north america transpression and basin inversion in eastern asia and development of the present day andean style margin along western south america occurred contemporaneous with the development of an extensional regime in eastern gondwanaland basin instability in africa and western europe further demonstrates that lithospheric stress regime changes were widespread at this time considering the timing of the reorganization and the nature of associated plate boundary changes we suggest that eastern gondwanaland subduction cessation is the most likely driving mechanism for the reorganization subduction is the dominant driver of plate motion and therefore this event had the potential to strongly modify the balance of driving forces acting on the plates in the southwestern proto pacific and neighboring plates whereby producing widespread changes in plate motion and continental lithospheric stress patterns we propose that major changes in ridge trench interaction triggered the cessation of subduction the progressive subduction of two closely spaced perpendicular mid ocean ridges at the eastern gondwanaland subduction zone to the east of australia and new zealand respectively resulted in very young crust entering the trench and we suggest that by 105 100 ma there was insufficient negative buoyancy to drive subduction finally we propose that the plume push force of the bouvet plume that erupted near the african antarctic south american triple junction contributed to plate motion changes in the southern atlantic region

variations in deformation fields during development of a large volume magmatic arc central sierra nevada california mid to late cretaceous plutons in the central sierra nevada magmatic arc show widely preserved magmatic foliation whereas regionally developed solid state foliation is absent relatively slow cooling of these plutons and expected strain rates 10 14 suggest that the plutons were emplaced in a neutral or weakly extensional deformation regime domains of solid state ductile shear of only slightly younger age than the plutons on the other hand indicate a contractional regime timing of pluton emplacement and movement on the shear zones have been constrained using pb u zircon and 40 ar 39 ar hornblende and biotite geochronology both plutons and ductile shear zones become younger toward the east the four more westerly shear zones which were active between ca 100 and 90 ma show steeply plunging stretching lineations whereas the most easterly and or youngest zones active between ca 88 and 78 ma show mostly oblique and or subhorizontal stretching lineations indicating a change in kinematics at ca 90 ma the above events define a complex deformation pattern in which strain regimes fluctuated in time and space between neutral or weak extensional and contractional we propose a tectonic model in which thenospheric mantle corner flow produced eddy pairs in the mantle corner that transmitted a neutral or weak extensional regime to the overlying crust and facilitated the movement of granitic magma to mid and upper levels probably as dikes via fractures slab flattening caused the neutral or weak extensional regime to move eastward away from the trench increased coupling between upper and lower plates induced by the slab flattening promoted contractional strain in the cooling plutons and domains of ductile shear formed in progressively younger plutons to the east the above events were accompanied by an oblique convergence vector between north america and farallon plates engebretson et al 1985 which imposed a relatively small component of right lateral shear onto the arc that increased with time we estimate that at ca 100 ma the convergence vector made an angle obl 20 to the arc normal and we suggest that around ca 90 ma obl passed through a critical value conceivably 20 oblcrit 30 at this juncture the component of right lateral shear became sufficiently large to induce significant arc parallel strike slip movement on the most easterly shear zones these kinematics continued as the dominant scheme possibly as late as ca 78 ma

the effect of land plants on weathering rates of silicate minerals abstract land plants and their associated microbiota directly affect silicate mineral weathering in several ways by generation of chelating ligands by modifying ph through production of co2 or organic acids and by altering the physical properties of a soil particularly the exposed surface areas of minerals and the residence time of water in laboratory experiments far from equilibrium 1 mm oxalate a strong chelator of al has a negligible effect on the dissolution rate of alkali feldspars but some effect on calcic feldspars and olivine by analogy to oxalate the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small except perhaps in microenvironments adjacent to roots and fungal hyphae the effect of ph on silicate mineral dissolution rate depends on ph below ph 4 5 the rate increases with decreasing ph in the circumneutral region the rate is ph independent and at ph values above around 8 the rate increases with increasing ph vegetation should thus cause an increase in weathering rate through the ph effect only where the ph is below 4 5 as an overall generalization the effect of plants on weathering rate through changes in soil solution chemistry is probably small for granitic rocks it may be greater for more mafic rocks it is the release of ca and mg from mafic rocks that has the greatest influence on the global co2 budget the effect of changes in soil physical properties on weathering rate can be major by binding fine particles plants can greatly increase weathering rates in areas of high physical erosion where erosion rates are lower the effect of plants is less clear on long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water a major unknown in estimating the effect of the advent of land plants on weathering rates is the nature thickness particle size distribution permeability of the regolith on the pre silurian continents the indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

time span of plutonism fabric development and cooling in a neoproterozoic magmatic arc segment u pb age constraints from syn tectonic plutons sark channel islands uk abstract new u pb zircon and titanite dates from syn tectonic plutons on the british channel island of sark constrain the time span of plutonism fabric development and cooling in this part of the neoproterozoic cadomian magmatic arc the tintageu leucogneiss is a mylonitic unit that was dated previously at 615 6 4 2 2 3 ma the port du moulin quartz diorite which intruded the tintageu unit contains a high strain solid state deformation fabric that is less intense than but parallel to fabrics in the leucogneiss and yields a u pb zircon date of 613 5 2 3 1 5 ma the little sark quartz diorite also displays solid state deformation fabrics in addition to relict magmatic textures and yields a u pb zircon date of 611 4 2 1 1 3 ma the north sark granodiorite is largely penetratively undeformed exhibits mainly magmatic fabrics and textures and has a u pb zircon date of 608 7 1 1 1 0 ma two fractions of titanite from each intrusion are essentially concordant and are identical within error with mean dates of 606 5 0 4 ma port du moulin quartz diorite 606 2 0 6 ma little sark quartz diorite 606 4 0 6 ma north sark granodiorite the new u pb data in combination with previous u pb and 40 ar 39 ar data and previous field studies confirm the syn tectonic nature of the sark plutons and quantify the time span ca 7 m y required for intrusion and sufficient crystallization of each body to record incremental strain during waning deformation titanite u pb and hornblende 40 ar 39 ar dates mark final cooling about 2 m y after intrusion of the last pluton

constraints on the tectonic setting of the andaman ophiolites bay of bengal india from shrimp u pb zircon geochronology of plagiogranite abstract the andaman ophiolites are well exposed in the andaman group of islands which is part of the sunda burmese double chain arc system in the bay of bengal india plagiogranites occurring on the eastern margin of the southern part of south andaman island appear as interstitial vermicular and micrographic intergrowths of quartz and plagioclase they are tonalitic to trondhjemitic in composition and their rb yb ta and y abundances are characteristic of a volcanic arc affinity sensitive high resolution ion microprobe u pb dating of zircons from a plagiogranite within the andaman ophiolite has yielded a weighted mean 206pb 238u age of ma interpreted as the age of its crystallization the subduction related plagiogranite has intruded a gabbro unit of the andaman ophiolites as well as extrusives of the east coast volcanics at this time since the andaman ophiolitic rocks predate the plagiogranite they cannot have been generated in the currently active late miocene andaman java subduction zone and

direct effects of co2 and temperature on silicate weathering possible implications for climate control a critical uncertainty in models of the global carbon cycle and climate is the combined effect of organic activity temperature and atmospheric co2 on silicate weathering here we present new dissolution rates of anorthite and augite which indicate that silicate weathering in organic rich solutions is not directly affected by soil co2 but is very sensitive to temperature apparently co2 accelerates silicate weathering indirectly by fertilizing organic activity and the production of corrosive organic acids the weathering dependencies highlight the ability of silicate weathering to act as a global thermostat and damp out climate change when used as input in steady state carbon cycle and climate models

sedimentology of a wet pre vegetation floodplain assemblage descriptions of fluvial systems operating prior to significant terrestrial macrophyte vegetation have concentrated on assessing the impact of plant evolution on channel style this is probably in part due to the scarcity of well developed floodplain successions in fluvial assemblages of these ages this study describes wet pre vegetation floodplain deposits and processes observed and inferred from a continuous succession of drill core through an extensive 1 4 ga delta top channel floodplain assemblage forming a portion of the sibley group ontario canada sub aerial deposits are dominated by flaser wavy and lenticular bedded fine grained sandstones siltstones and mudstones with abundant small mudstone rip up clasts soft sediment deformation of these units is ubiquitous with loading and injection features being the most prominent thicker medium grained sandstone beds representing crevasse splays commonly have poorly developed protosols in their upper portions well laminated sediments with wave ripples and only rarely containing rip up clasts and soft sediment deformation were deposited in floodplain ponds these deposits differ from post vegetation floodplain sediments in having i better preservation of layering without rootlet bioturbation ii dominance of rippled sand on the floodplain probably due to lack of vegetation induced baffling iii large scale generation of small intraformational clasts iv desiccation crack fills consisting of peds and locally derived intraclasts probably delivered during horton overland flow during rainfall events v ubiquitous soft sediment deformation features in sub aerial deposits and vi well laminated commonly oxidized sediment that accumulated in floodplain ponds these attributes are the direct result of the lack of macrophyte vegetation and produce floodplain assemblages that are distinctly different from those presently forming in similar climatic settings

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

structural geology meets micromechanics a self consistent model for the multiscale deformation and fabric development in earth s ductile lithosphere abstract earth s lithosphere is made of rheologically heterogeneous elements of a wide range of characteristic lengths a micromechanics based self consistent multiorder power law approach is presented to account for lithospheric deformations and the accompanying multiscale fabric development the approach is principally based on the extended eshelby theory for the motion of a power law viscous ellipsoid in a power law viscous matrix and the idea of embedding inhomogeneities within inhomogeneities the extended theory provides a general means for investigating deformation partitioning in heterogeneous rocks the inhomogeneities within inhomogeneities method allows multi hierarchical levels of flow field partitioning and hence multiscale deformations to be investigated partitioned flow fields are used to investigate fabric development being based fully on micromechanics the approach generates model predictions of both kinematic quantities strain strain rates and vorticity and stress histories the former can be directly compared with field and laboratory structural observations while the latter can help to understand the physics of natural deformations the self consistent and multiscale approach is applied to a natural example of the cascade lake shear zone in the east sierra nevada of california the modeling shows that the fabrics are most consistent with a steeply dipping transpression zone with a convergence angle of 20 and a strike slip displacement about 26 km further the strength evolution of the model zone confirms that a transpression zone is a weakening system with respect to the simple shearing component and a hardening one for the pure shearing component this is consistent with slip partitioning in obliquely convergent plate boundaries boundary normal convergence tends to spread over a broad area whereas boundary parallel shear tends to localize in major strike slip zones

initial generation of sand across climate zones of the mojave sierra nevada and klamath batholiths in california u s a the generation of modern plutoniclastic sediment across california varies with climate resulting in significant mineralogical and geochemical differences between parent rock weathering profile material and low order stream sand with average precipitation of 15 cm yr in the arid zones and 120 cm yr in the temperate zones feldspar is relatively decreased in stream sand compared to parent rock by 2 and 20 respectively potassium feldspar is more resistant to weathering than plagioclase but is less stable than quartz in temperate climates hornblende and biotite are more easily weathered in temperate than arid climates although their occurrence in medium grained stream sands is affected by hydraulic sorting and comminution during transport temperate climate sands have lower concentrations of cao and na2o compared to k2o and al2o3 reflective of the expected faster loss of plagioclase than potassium feldspar during weathering with the exception of biotite the mineralogy of medium grained stream sand reflects that of medium grained weathering profile material in both climates confirming the findings from constant climate case studies that the composition of proximal sand is largely achieved within weathering profiles and during initial erosion we encourage continued attention to climate as a control on sand composition in provenance and paleoclimate studies where a moderate precipitation shift is capable of producing a significant loss of feldspar the differential response of common minerals to chemical weathering should be considered for studies and modeling related to bedrock erodibility sediment supply and landscape evolution

use of microgranitoid enclaves as solid state strain markers in deformed granitic rock an evaluation microgranitoid enclaves are common in many plutons and are potentially valuable as solid state strain markers in tectonically deformed granitoids we consider the parameters necessary to assess enclaves as two dimensional strain markers these include i their primary shapes ii differences in rheology between enclave and host granitoid iii the magmatic shape fabric ellipse of enclave populations iv the maximum fluctuation of enclave long axes developed during magmatic processes v the angle between the magmatic shape fabric ellipse and tectonic strain ellipse at the onset of tectonic deformation and vi the type of strain path undergone by the markers enclave populations show characteristic magmatic distributions as shown by plots of the magmatic shape fabric ellipse vs enclave axial fluctuation from several plutons in the sierra nevada california and in the adamello massif italy the plots of tectonically undeformed and deformed enclaves show nearly complete overlap creating substantial uncertainties in separating magmatic strain from tectonic strain if only the final enclave axial ratio is considered enclave host strength differences have been modeled using published mineral deformation data and theoretical approaches to simulate deformation of aggregates results indicate very similar flow properties of enclaves and felsic host between 500 and 800 c and possibly at lower temperatures as well provided that biotite bearing enclave and host compositions are present two dimensional strain modeling of the enclave populations indicates that values of the total shape fabric ellipse as determined from field measurements can be close to the true tectonic strain under some circumstances but most commonly the former substantially over or underestimates the latter if the parameters i v are assessed at each strain site however and each measured surface subsequently modeled using those parameters over and underestimates of solid state strain can be greatly reduced or eliminated and reasonable values for two dimensional tectonic strain of the granitoid and its enclaves can be obtained

weathering before the advent of land plants evidence from unaltered detrital k feldspars in cambrian ordovician arenites potassium chelation by land plants and the consequent depletion of k ion activity in soil waters make k feldspars unstable in present day soils other factors remaining constant the absence of land plants in pre silurian time would make k feldspars stable dissolution of plagioclase under the action of soil water would be the same in both pre and post silurian time the presence of abundant fresh k feldspars and the absence of plagioclase in the cambrian ordovician arenites of ohio wisconsin minnesota missouri texas wyoming and montana are probably due to the absence of potassium chelating agents in the soils of the time

intra basaltic soil formation sedimentary reworking and eodiagenetic k enrichment in the middle to upper ordovician dunn point formation of eastern canada a rare window into early palaeozoic surface and near surface conditions mafic flows of the middle to upper ordovician dunn point formation of eastern canada were deeply weathered under warm and relatively humid conditions before being buried by subsequent flows in the absence of superior plants and in the context of relatively low atmospheric carbon levels the soils developed alkaline groundwater conditions through mineral water interactions which resulted in an enhanced mobility of al relative to si in most palaeosols of that formation although the vegetation cover was volumetrically insignificant compared with that of subsequent geological times it was apparently producing very efficient chelates which for most palaeosols of the succession generated a well defined cheluviation pattern for not only al and fe but also and mainly ti which is typically immobile in modern soils the resulting soils developed an al fe ti depleted upper horizon that was enriched in si probably through periodic ground saturation long term climatic variations related to orbital cycles are inferred to have accounted for a second type of soil in the succession which contrasts with the former by showing a si depleted and less al fe ti depleted upper horizon some soil material was substantially reworked by surface runoff but such occurrences can be easily differentiated from in situ soil material in terms of texture structure and composition a thick overlying rhyolite flow is thought to be responsible for providing abundant k in solution which was incorporated in the underlying basalt palaeosols as exchangeable cations within a probably montmorillonitic clay precursor to the fe mg rich phengite that later developed during deep burial and orogenic compression

palaeomagnetic dating of sub torridon group weathering profiles nw scotland verification of neoproterozoic palaeosols a palaeoplain that cuts across lewisian gneiss in the cape wrath area northwest scotland is weathered to depths of 1 6 m beneath fluvial red conglomerates and sandstones of the applecross formation of the early neoproterozoic torridon group weathered lewisian rocks are enriched in k 2 o and fe 2 o 3 and contain hematite as an alteration product of biotite and magnetite palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration thermal step demagnetization of 87 oriented core specimens of biotite gneiss and amphibolite spanning four profiles at poll a mhurain and sheigra reveal a stable high temperature component with a mean direction of remanence of d 149 1 i 57 2 95 5 0 corrected for dip of the overlying applecross strata that gives a pole at 9 8 s 199 2 e dp 5 3 dm 7 3 this direction differs significantly from that for little weathered lewisian at those two localities and also from directions for the late mesoproterozoic stoer group the remanence direction for the weathered rocks is however near the dip corrected high temperature remanence direction d 122 3 i 50 5 95 10 8 obtained by us from the thermal step demagnetization of 39 sandstone specimens of the applecross formation in the cape wrath area furthermore the direction for the weathered rocks agrees closely with other dip corrected results for the torridon group such as the detrital remanent magnetization drm mean direction d 148 3 i 59 9 95 13 8 for the applecross formation on rum and that for the main outcrop of the aultbea formation d 139 7 i 52 6 95 14 5 the concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the cape wrath area acquired their remanence penecontemporaneously with deposition of the applecross and aultbea formations the palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of applecross age c 980 ma that formed at a palaeolatitude of 38 and constitute the oldest known palaeosols in the british isles views that the weathering is substantially older or younger than applecross deposition cannot be sustained our findings carry implications for applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early neoproterozoic time

quartz arenites review and interpretation abstract quartz arenites orthoquartzites are exceptional sands pettijohn et al 1972 that in the past have formed extensive and commonly thick formations they are somewhat enigmatic in that they do not appear to be forming at the present time this paper examines the origin of quartz rich sands documents the range of environments in which they are found and suggests a model for their genesis quartz sand is most commonly formed by tropical weathering of coarse grained quartz bearing rocks such as granite it may be deposited as texturally immature fluvial sandstones generally under a humid climate with drying of the climate quartz rich bauxitic soils and sandy alluvium can be a substantial source of poorly rounded dune sands for major deserts quartz sand when transported into shallow marine environments becomes very well rounded through reworking by tidal currents such sand may spill into deep oceanic environments and by sedimentary or tectonic transport may come to rest in both stable and active tectonic settings quartz rich sandstones can also be formed by diagenetic destruction of labile framework grains of immature sandstones precambrian orthoquartzites are commonly thick and widespread when compared to younger examples the thinness of post silurian quartz arenites may be largely due to the trapping effect of land vegetation and solution of quartz by organic acids in weathering profiles there is an abundance of shallow marine and fluvial quartz arenites in the early paleozoic and the cretaceous this is due to widespread continental flooding and a humid world climate following the disintegration of supercontinents the shallow marine examples are commonly associated with bituminous sometimes phosphatic shale glauconite and possibly chamositic ironstone there is an abundance of eolian quartz arenites in the permo triassic this is related to a widespread dry continental climate during assembly of the pangean supercontinent

tectonomagmatism in continental arcs evidence from the sark arc complex abstract the island of sark channel islands uk exposes syntectonic plutons and country rock gneisses within a precambrian cadomian continental arc this sark arc complex records sequential pulses of magmatism over a period of 7 ma ca 616 609 ma the earliest intrusion ca 616 ma was a composite sill that shows an ultramafic base overlain by a magma mingled net vein complex subsequently deformed at near solidus temperatures into the amphibolitic and tonalitic tintageu banded gneisses the deformation was synchronous with d 2 deformation of the paragneissic envelope with both intrusion and country rock showing flat top to the south ls fabrics later plutonism injected three homogeneous quartz diorite granodiorite sheets the creux moulin pluton 150 250 m ca 614 ma the little sark pluton 700 m 611 ma and the northern pluton 500 m 609 ma similar but thinner sheets in the south derrible hogsback dixcart and west port es saies brecqhou are interpreted as offshoots from the creux moulin pluton and little sark pluton respectively all these plutons show the same ls fabric seen in the older gneisses with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling the cooling rate increased rapidly with decreasing crystallisation age 9 ma for the oldest intrusion to cool to lower amphibolite conditions 7 8 ma for the creux moulin pluton 5 6 ma for the little sark pluton and 2 the initiation of the d 2 event is suggested to have been a response to the intrusion of the tintageu magma which promoted a rapid increase in strain rate 10 14 s 1 that focussed extensional deformation into the sark area the increased rates of extension allowed ingress of the subsequent quartz diorite granodiorite sheets although strain rate slowly declined as the whole complex cooled during exhumation the regional architecture of syntectonic cadomian arc complexes includes flat lying sark type and steep guernsey type domains produced synchronously in shear zone networks induced by oblique subduction a pattern seen in other continental arcs such as that running from alaska to california