a vein and disseminated ba pb zn deposit in the appalachian thrust belt st fabien quebec the st fabien deposit occurs in the taconic thrust belt of the quebec appalachians it consists of intimately associated veins and disseminations of barite galena and sphalerite hosted in interdigitated sandstones and limestone conglomerates comprising a proximal deep submarine facies of the upper cambrian st damase formation at a late stage in the tectonic evolution of the host rocks hydraulic fracturing driven by the buildup of a geopressured zone took place in a dilatant stress field without mechanical relationships to folding episodic opening of the fractures enhanced corrosive fluid flow and led to the development of a secondary dissolution porosity followed by pervasive dolomitization of large volumes of host rocks and partial sealing of the fractures and pores by dolomite followed by ferroan dolomite in a reducing environment of low sulfur activity we propose that an interconnected network of incompletely cemented fracture and dissolution porosities channeled oxidizing sulfate bearing fluids into this reducing environment these oxidizing fluids mixed with deep orogenic metalliferous fluids set in motion by the taconic orogeny and were partially reduced to aqueous sulfide at this stage barite galena and sphalerite precipitated in the remaining open spaces of varying redox potential and partially replaced precursor cements and framework constituents remaining open spaces were cemented by calcite rare earth element patterns for the various generations of gangue dolomite and late calcite are characterized by positive europium anomalies ascribed to the partial dissolution of detrital potassium feldspars before during and after mineralization detrital feldspars are suggested as a major source of lead and barium in the st fabien deposit lead isotopes support this crustal source for the lead and their model age is consistent with the precipitation of galena during the late stages of or slightly after the taconic orogeny homogeneity of the lead isotope ratios of both vein and disseminated galenas throughout the st fabien deposit suggests that the lead isotopes were efficiently averaged throughout this part of the taconic belt by a large volume of fluid

holocene reef growth in torres strait the platform and fringing reefs of torres strait are morphologically similar to reefs of the northern great barrier reef to the south except that several are elongated in the direction of the strong tidal currents between the coral sea and the gulf of carpentaria surface and subsurface investigations and radiocarbon dating on yam warraber and hammond islands reveal that the initiation and mode of holocene reef growth reflect antecedent topography and sea level history on the granitic yam island fringing reefs have established in some places over a pleistocene limestone at about 6 m depth around 7000 years bp emergent holocene microatolls of porites sp indicate that the reefs have prograded seawards while sea level has fallen gradually from at least 0 8 m above present about 5800 years bp on the warraber island reef platform drilling near the centre indicated a pleistocene limestone foundation at a depth of about 6 m over which reefs established around 6700 years bp reef growth lagged behind that on yam island microatolls on the mature reef flat indicate that the reef reached sea level around 5300 years bp when the sea was around 0 8 1 0 m above present on the reef flat on the western side of hammond island bedrock was encountered at 7 8 m depth overlain by terrigenous mud a progradational reef sequence of only 1 2 m thickness has built seaward over these muds as sea level has fallen over the past 5800 years reef flat progradation on these reefs is interpreted to have occurred by a series of stepwise buildouts marked by lines of microatolls parallel to the reef crest marking individual coalescing coral heads detrital infill has occurred between these this pattern of reef progradation is consistent with the radiocarbon dating results from these reefs and with seismic investigations on the torres reefs

sedimentary facies and holocene depositional processes of laura island majuro atoll article i nfo article history the depositional processesthat formed lauraisland majuroatoll marshall islands were reconstructed basedon a facies analysis of island sediments and spine ratios and radiocarbon ages of foraminifera sedimentary facies were analyzed from trenches and drill cores excavated on the island and its adjacent reef flat depositional ages were obtained using benthic foraminifera calcarina whose spines had not been abraded the facies were classified into two types gravelly and sandy the initial sediments of these sites consisted of gravelly facies in the lower horizon and sandy facies in the upper horizon their ages were approximately 2000 cal bp and coincident with the onset of a 1 1 m decline in regional relative sea level which enabled deposition of the gravelly facies half of the sand fraction of the sediment was composed of larger benthic foraminifera the spine ratioshowedthat theirsupply source on the reef flatwaslocated oceansideof theisland the supplysource appears to have been caused by the relative sea level fall this indicates that the studied island was formed by a relative reduction in wave energy and enhanced foraminiferal supply both of which were triggered by the late holocene relative sea level fall

tectonic implications of early miocene extensional unroofing of the sierra mazatan metamorphic core complex sonora mexico sierra mazatan in northwest mexico is the southernmost metamorphic core complex in the north american cordillera new geologic structural and 4 0 ar 3 9 ar thermochronologic data demonstrate that the core complex detachment fault accommodated 15 35 km of slip at a rate of 3 3 7 7 mm yr between 20 5 and 16 ma these data also suggest that the lower plate was tilted eastward 20 50 since 20 ma indicating that the detachment fault initially dipped 30 60 rapid slip on the fault occurred concurrently with subduction in sonora and thus was not related to the ca 12 ma plate boundary change at this latitude or the late miocene opening of the gulf of california rapid extension at sierra mazatan was synchronous with core complexes along the length of the cordillera suggesting a distinct 20 15 ma core complex event the fundamental causes of which remain a mystery

escape hypothesis for the stikine block comparison of stratigraphic faunal and paleomagnetic characteristics of the stikine terrane of british columbia with other terranes in the cordilleran collage reveals broad similarities with a group of terranes that formed a volcanic belt marginal to north america in paleozoic and early mesozoic time unlike stikine these terranes lie inboard of another belt of terranes that represents an early mesozoic subduction complex or melange belt thus in british columbia the marginal volcanic belt is apparently doubled in the columbia embayment region the marginal volcanic and melange belts are missing and rocks of the outermost major component of the collage the wrangellia superterrane are juxtaposed directly against cratonic rocks we propose that stikine is the missing fragment from the columbia embayment its northward tectonic escape was driven by the early stages of collision of the wrangefia superterrane with north america in middle to late jurassic time the escaped fragment was then trapped between melange and more northerly later arriving parts of the wrangellia superterrane in early to mid cretaceous time

atoll reef island formation and response to sea level change west island cocos keeling islands abstract reef islands around the margin of coral atolls generally comprise unconsolidated holocene sands and gravels overlying a reef flat or cemented conglomerate platform such islands have accreted within the last 3000 4000 years since sea level has reached a level close to present and the reef flat and conglomerate platform have formed island morphology consists of an oceanward ridge a less distinct lagoonward ridge and low lying central depression several alternative models of how such reef islands might have developed are examined in relation to chronology and sediment provenance particularly in the context of the cocos keeling islands where this issue has been debated since darwin visited the atoll which of these models appears most appropriate for an elongate reef island on the atoll margin is assessed using conventional radiocarbon dating of coral shingle and accelerator mass spectrometry ams radiocarbon dating of individual sand grains from pits across west island the dating results suggest that both coral clasts and individual grains of various components are generally reliable and replicable indicators of the chronology of island accumulation implying rapid transport of skeletal material after death of the contributing organisms across the reef flat zone and relatively little reworking the central part of west island appears to have formed first with oceanward accretion up until about 2000 years bp gradual oceanward accretion with lesser lagoonward extension has continued beyond 2000 years bp at the northern and southern ends of the island and a sequence of lagoonward recurving spits has formed adjacent to the inter island passage at the southern end of the island radiocarbon dating of fossil microatolls indicates a gradual fall of sea level over this period which appears to have had little effect on the pattern of island accretion

growth limiting size of atoll islets morphodynamics in nature several linear models have been proposed for islet evolution these models do not address the non linear processes contributing to islet evolution or account for feedback between islet morphology and the fluid dynamics leading to sediment sequestration the non linear sediment allocation model sam has also been proposed for atoll islet evolution the sam is a quantitative model of atoll islet development implemented assuming that there is morphodynamic feedback between islet topography and the fluid dynamics leading to sediment sequestration in islet sinks the sam requires measurements of islet volume to recover parameters in the model using a data inversion algorithm the sam was developed and tested using synthetic data in order to determine the effect of errors in the islet volume measurements on model parameters in this study the model parameters are estimated using radiocarbon dated samples from published field studies of islet development on atolls these calibration procedures involved reanalysis of the data using methods consistent with the morphodynamic principles of islet growth underlying the sam the results support a pattern of islet growth characterized by rapid lateral expansion followed by diminishing vertical accretion and shoreface rotation due to morphodynamic feedback

new model of reef island evolution maldives indian ocean a new model of reef island evolution based on detailed morphostratigraphic analysis and radiometric dating of three islands in south maalhosmadulu atoll maldives is presented islands initially formed on a foundation of lagoonal sediments between 5500 and 4500 yr b p when the reef surface was as much as 2 5 m below modern sea level islands accumulated rapidly during the following 1500 yr effectively reaching their current dimensions by 4000 yr b p since then the high circum island peripheral ridge has been subject to seasonal and longer term shoreline changes while the outer reef has grown upward reducing the energy window and confining the islands this new model has far reaching implications for island stability during a period of global warming and raised sea level which will partially reactivate the energy window although it is not expected to inhibit upward reef growth or compromise island stability

thermal history of the ecstall pluton from 40ar 39ar geochronology and thermal modeling abstract new 40ar 39ar thermochronology results and thermal modeling support the hypothesis of hollister et al 2004 that reheating of the mid cretaceous ecstall pluton by intrusion of the coast mountains batholith cmb was responsible for spatially variable remagnetization of the ecstall pluton 40ar 39ar ages from hornblende and biotite from 12 locations along the skeena river across the northern part of the ecstall pluton decrease with proximity to the quottoon plutonic complex the nearest member of the cmb to the ecstall pluton the oldest 40ar 39ar ages are found farthest from the quottoon plutonic complex and are 90 3 ma for hornblende and 77 9 1 2 ma for biotite the youngest 40ar 39ar ages are found closest to the quottoon plutonic complex and are 51 6 1 2 ma for hornblende and 45 3 1 7 ma for biotite no obvious relationship between grain size and age is seen in the ecstall pluton biotites spatial trends in 40ar 39ar ages are consistent with model results for reheating by a thermal wall at the location of the quottoon plutonic complex although no unique solution is suggested our results indicate that the most appropriate thermal history for the ecstall pluton includes both reheating and northeast side up tilting of the ecstall pluton associated with intrusion of the quottoon plutonic complex estimates of northward translation from shallow paleomagnetic inclinations in the western part of the ecstall pluton are reduced to 3000 km consistent with the baja bc hypothesis when northeast side up tilting is accounted for

high frequency of super cyclones along the great barrier reef over the past 5 000 years understanding long term variability in the occurrence of tropical cyclones that are of extreme intensity is important for determining their role in ecological disturbances1 2 3 4 5 for predicting present and future community vulnerability and economic loss6 and for assessing whether changes in the variability of such cyclones are induced by climate change7 our ability to accurately make these assessments has been limited by the short less than 100 years instrumented record of cyclone intensity here we determine the intensity of prehistoric tropical cyclones over the past 5 000 years from ridges of detrital coral and shell deposited above highest tide and terraces that have been eroded into coarse grained alluvial fan deposits these features occur along 1 500 km of the great barrier reef and also the gulf of carpentaria australia we infer that the deposits were formed by storms with recurrence intervals of two to three centuries8 9 10 11 and we show that the cyclones responsible must have been of extreme intensity central pressures less than 920 hpa our estimate of the frequency of such super cyclones is an order of magnitude higher than that previously estimated which was once every several millennia12 13 14 and is sufficiently high to suggest that the character of rainforests and coral reef communities were probably shaped by these events

spatially correlated anomalous 40ar 39ar age variations in biotites about a lithologic contact near simplon pass switzerland a mechanistic explanation for excess ar forty four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40ar 39ar and demonstrate the importance of bulk ar diffusivity and system geometry factors not usually considered in the interpretation and collection of 40ar 39ar age data the resulting 40ar 39ar apparent ages range from 11 30 0 05 ma to 17 90 0 10 ma the ages and excess argon contents are spatially and lithologically correlated the pelite samples all yield ages clustering around 12 ma the age expected for cooling through biotite closure 360 c in this region of the alps ages in the amphibolite biotites are older showing a smooth trend between 15 ma at the contact with the pelite to 18 ma 34 cm from the contact this data shows that characterization of the ar closure age for biotite in a given system should not rest on a single sample as otherwise irresolvable differences in age between samples within the same outcrop can exist a generalized mechanistic model for excess argon is presented the presence or absence of excess ar depends on an intrinsic system parameter t the transmissive timescale which is the characteristic time for 40ar to escape through the local intergranular transporting medium itm to some sink for argon to prevent buildup of geochronologically significant excess 40ar t must be very short relative to the true closure age of the mineral a fortran code including radiogenic ar production diffusive loss of ar from biotite and bulk ar diffusion through the itm has been developed application of numerical modeling suggests that the time averaged effective bulk diffusivity deffar in the biotite amphibolite rock during early retrograde cooling is 2 2 1 0 10 8 m2 yr assuming steady state conditions the first such measurement available numerical modeling also provides information about the transmissivity and geologic history specific to the field site including a drop in deffar at 15 5 1 0 ma the timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack

argon diffusion in pyroxenes implications for thermochronometry and mantle degassing abstract the rate at which argon diffuses in pyroxenes is relevant to several problems in the earth and planetary sciences pyroxenes are among the most abundant minerals in the earth s upper mantle and are important hosts for potassium and radiogenic 40 ar 40 ar additionally pyroxenes commonly contain 25 of the total 40 ar within meteorites and therefore hold great potential for quantifying the timing duration and thermal conditions of events experienced by meteorites and their parent bodies we conducted detailed feedback controlled laser heating diffusion experiments using synthetically produced 37 ar in gem quality clinopyroxene cpx and orthopyroxene opx crystals stepwise heating between 850 and 1350 c yielded linear arrhenius arrays from which activation energies e a of 379 2 4 1 and 371 0 6 0 kj mol and pre exponential factors d o of 1 36 10 4 6 15 10 5 1 12 10 4 and 5 73 10 2 2 25 10 2 3 72 10 2 were determined for cpx and opx respectively these e a s are an order of magnitude higher than previously reported values obtained with a different experimental approach watson et al 2007 and indicate that ar diffusion within pyroxenes is strongly temperature dependent at high temperatures ar diffusion in pyroxenes is rapid which indicates that diffusive 40 ar distributions observed within meteoritic pyroxenes may provide information about brief high temperature shock heating events in contrast subsequent low t conditions should minimally influence these concentration gradients the experimental ar diffusion kinetics in opx and cpx correspond to closure temperatures of 600 700 and 700 800 c respectively for 10 c ma cooling these results suggest that diffusive equilibration through 0 1 0 5 mm pyroxene grains occurs in minutes to hours at basaltic melt generation temperatures 1300 c and is therefore unlikely to inhibit mantle degassing

basinal brines and groundwaters as possible metal carriers in the formation of sandstone hosted lead zinc deposits features of the two best documented worldclass sandstone hosted pb zn deposits laisvall sweden yava nova scotia canada and a few similar but smaller deposits se missouri quebec appalachians are compared with the purpose of contributing to the debate on the problem of the aqueous medium of metal transport basinal brines or groundwaters for the dolostone sandstone hosted deposits in se missouri it is concluded that basinal brines formed this medium in both host rocks the paleotopographic control of these missouri deposits is pronounced and leads to the conclusion that such control is insufficient evidence to establish the groundwater hypothesis for the yava deposit in the quebec appalachian deposits basinal brines expelled tectonically were the most likely metal carriers these deposits constitute an ore type that differs from other known sandstone hosted lead deposits

the importance of large benthic foraminifera to reef island sediment budget and dynamics at raine island northern great barrier reef low lying reef islands are among the most vulnerable environments on earth to anthropogenic induced climate change and sea level rise over the next century because they are low composed of unconsolidated sediment that is able to be mobilised by waves and currents and depend on sediments supplied by reef organisms that are particularly sensitive to environmental changes e g ocean temperatures and chemistry therefore the spatial and temporal links between active carbonate production and island formation and dynamics are fundamental to predicting future island resilience yet remain poorly quantified in this paper we present results of a detailed geomorphological and sedimentological study of a reef and sand cay on the northern great barrier reef we provide an empirical investigation of the temporal linkages between sediment production and reef island development using a large collection of single grain ams 14c dates large benthic foraminifera lbf are the single most important contributor to contemporary island sand mass 47 ranging from 36 to 63 at raine island reflecting rapid rates of sediment production and delivery standing stock data reveal extremely high production rates on the reef 1 8 kg m 2 yr 1 while ams 14c dates of single lbf tests indicate rapid rates of sediment transferral across the reef we also demonstrate that age is statistically related to preservation and taphonomic grade severely abraded tests moderately abraded tests pristine tests we construct a contemporary reef and island sediment budget model for raine island that shows that lbf baculogypsina marginopora and amphistegina contribute 55 of the sediment produced on the reef annually of which a large proportion 54 contribute to the net annual accretion of the island the tight temporal coupling between lbf growth and island sediment supply combined with the sensitivity of lbf to bleaching and ocean acidification suggests that islands dominated by lbf are likely to be very sensitive to short and long term climate change projections potential outcomes of this work relate to improving the understanding of the future change dynamics of reef islands in response to climate change

textural and fluid inclusion evidence for ore deposition in the pb zn deposit at laisvall sweden laisvall is a sandstone hosted lead zinc deposit on the eastern margin of the scandinavian caledonides it is part of a large ore province stretching over 1 000 km from the south of norway to north of sweden the ore occurs as interstitial filling between sand grains but textural and fluid inclusion data demonstrate that the ore was formed by open space filling of open micropores in the sandstone the textures of the cementing minerals also demonstrate that postore deformation and metamorphism are restricted to some twinning in calcite and fracturing no major recrystallization has taken place textures include growth zoning of sphalerite with systematic color changes traceable as a sphalerite stratigraphy over large areas of the deposit both sphalerite and calcite display several stages of mineral growth including dissolution breaks and etched surfaces fluid inclusion homogenization temperatures indicate that each stage was deposited in a distinct temperature interval during a generally overall temperature decrease the environment of ore deposition was characterized by a temperature of 120 degrees to 180 degrees c and a total salinity of 24 equiv wt percent nacl this resulted from mixing a hot highly saline incoming solution with a cool local ground water the principal minerals were deposited in the order calcite barite fluorite sphalerite and galena this process was repeated several times and sulfide deposition was accompanied by calcite dissolution deposition of the minerals was affected by different mechanisms during the mixing process 1 heating of ground water deposited calcite 2 cooling of brine deposited barite and 3 mixing which deposited sulfides and fluorite the mixing occurred as a result of a double diffusion process where the equalization of densities resulted from salt diffusion along an extended interface between the hot brine and the cool ground water in the porous sandstone

the source of lead in the osen sandstone lead deposit on the baltic shield norway the sandstone hosted lead deposits on the baltic shield occur in marine late precambrian to early cambrian sedimentary rocks a ground water model in which the basement is postulated as the metal source has been proposed for these deposits to test this model the osen deposit was selected because it lies directly on a homogeneous granite basement on the basis of u th pb whole rock isotopic analyses for three samples of unweathered basement granite trysil granite a basement age of 1 598 m y was established the lead isotopic compositions of four out of five specimens of galena from the osen deposit plot between isochrons calculated for the granite at 500 m y and 540 m y this is strong support for the hypothesis that the lead source was basement granite and suggests that mineralization took place soon after sedimentation a basement source for the lead is further substantiated by the regional pattern of lead isotopic compositions of sandstone lead deposits which reflects the character of adjacent basement terranes

the mode and timing of windward reef island accretion in relation with holocene sea level change a case study from takapoto atoll french polynesia abstract takapoto atoll northern tuamotu islands french polynesia central pacific was selected as a test area to clarify the conditions of atoll island accretion in relation to mid to late holocene sea level changes surveys were conducted along two distinct cross island profiles on the windward coast of the atoll in addition the stratigraphy of an ocean facing islet was described from an excavation in order to reconstruct the successive island accretionary stages at both sites the basement of the atoll rims consists of conglomerate pavements on which lie shingle ridges reaching 4 m in elevation stratigraphic analysis of the excavated ridge reveals alternation of gravelly sand supported to gravel dominated sediments the chronology of island accretion is based on dating of 41 u th surface and excavated coral specimens ridge initiation occurred from about 1000 yr bp when sea level was close to its present position shingle deposits progressively prograded from the lagoon margins oceanwards and were partially cemented at their bases cementation may have increased the resistance of the islets to erosion as a result some island lands accumulated and have persisted over the last millennium the modern gross island morphology was acquired during the last 500 years this model can be considered to be of regional value for the northern tuamotu islands adjusted for local thermal subsidence hydroisostasy and or lithospheric flexure compared to some other indo pacific reef islands island initiation at takapoto appears to be have been delayed by 2 to 4 millennia probably in response to retardation in the reef catching up with mid holocene sea level dating of individual coarse grained coral clasts allowed the major wave surge events that have hit takapoto to be identified for the last millennium the use of gravels results in the identification of a greater number of medium energy surge impacts when compared with megaclast based records the frequency of storm events identified is consistent with that derived from historical observations severe storms have a very low frequency of occurrence one to two events per century on average

new insights into thermal history from single grain 40ar 39ar analysis of biotite single grains of biotite extracted from two hand samples of two granitoid rocks from the red lake greenstone belt of northwestern ontario were examined using a high sensitivity 40ar 39ar laser dating system for variations in age for small grains a striking correlation is seen between the ages and the grain sizes as measured by the cylindrical radii with smaller samples yielding younger ages this effect saturates at a radius of about 225 m above which the integrated age is independent of grain size by combining this grain size effect with biotite diffusion parameters taken from the literature continuous cooling curves were derived for the region these curves suggest the region cooled at a rate of about 0 3 c myr between 2635 and 2570 ma with its ability to define a grain size effect the laser probe mass spectrometer system provides new insight into thermal history

age and sedimentary characteristics of beach sediments from yongle atoll south china sea implications for sediments supply in a coral reef system abstract low lying reef islands constructed from unconsolidated sediments are of growing interest due to concerns over the future stability of island landforms in response to climatic change and sea level rise sediment characteristics combined with the constituents and physical processes are fundamental to understand the links between sediment production and their supply to an island s beach reservoir the grain size constituents and 230th age of beach sediments from five islands at yongle atoll in the south china sea were analyzed data reveal that coral grains are the dominant constituents of the beach sediments followed by mollusks coralline algae foraminifera halimeda and bryozoans the mean grain sizes of the beach sediment were within a range of 226 4215 m and the coarsest sands were located near the passages in the atoll rim transportation by tropical cyclone generated waves and winnowing by tides are potentially the main factors that influenced the grain size characteristics calibrated 230th age dates for bulk sand samples date the beach sediment between 424 yr bp and 5953 yr bp dating of coral sand samples from the beach sediment provides ages of 686 5780 yr bp the coral sands from a vertical section 1 2 m depth located on the beach at jinqing island were dated at 1154 3824 yr bp the age reversal on the vertical section with similar ages of beach sediment on any single island and different age ranges on different islands in an atoll indicates that the coral sediment supply processes occurred within a relatively close time period and that different islands had different sediment supply rates the temporal link between the coral dominated sediment production and the beach reservoir provides an important insight into understanding the sediments supply mechanism of atoll islands in the northwestern pacific ocean

paleomagnetism and geochronology of the ecstall pluton in the coast mountains of british columbia evidence for local deformation rather than large scale transport 1 samples for geochronologic geobarometric and paleomagnetic analyses were collected across the northern portion of the ecstall pluton southeast of prince rupert british columbia al in hornblende geobarometry indicates pressures from 740 10 to 840 30 mpa corresponding to crystallization depths of 25 to 30 km u pb analyses of zircons from western central and eastern localities within the pluton yield crystallization ages of 91 5 1 0 ma 90 8 1 0 ma and 90 5 1 0 ma respectively rock magnetic experiments reflected light microscopy and thermal demagnetization behavior suggest that natural remanent magnetism is carried by low ti titanohematite unblocking temperatures of the characteristic remanent magnetization chrm are dominantly in the 560 c to 630 c range with age of magnetization approximated by the 40ar 39ar hornblende ages of 84 2 0 10 ma on the western margin and 76 4 0 6 ma in the center of the pluton site mean chrm directions were isolated for paleomagnetic samples from 23 sites and are distributed along a small circle with subhorizontal axis at 340 azimuth chrm directions from the central portion of the pluton are concordant with the expected cretaceous magnetic field direction while chrm directions from the western margin are discordant by 70 folding of the ecstall pluton either during late cretaceous west directed thrust transport above the convex upward prince rupert shear zone or during younger deformation of the pluton and underlying shear zone can account for the paleomagnetic data and is consistent with the geochronologic geobarometric and structural geologic observations

source and possible leaching process of ore metals in the uragen sandstone hosted zn pb deposit xinjiang china constraints from lead isotopes and rare earth elements geochemistry abstract the uragen world class zn pb deposit is located in the ulugqat basin in xinjiang china the deposit is characterized by low grade zn pb 3 wt and large tonnage of 4 84 mt metals in the southern ore zone and potentially 10 mt metals for the whole deposit the deposit is mainly hosted in sandstone and conglomerate of the fifth member of the lower cretaceous kezilesu group to provide better constraints on the source of ore metals systematic analyses of the pb isotopes and ree were conducted on sulfides and possible source terranes the sulfides have homogeneous pb isotopic ratios for 206pb 204pb of 18 528 18 690 for 207pb 204pb of 15 545 15 743 and for 208pb 204pb of 38 429 39 082 these ratios are different from those of the upper silurian upper devonian tangaitaer and paleocene aertashi formations the fourth member of the lower cretaceous kezilesu group and the permian granite despite partly overlapping with the values of sulfides the ratios of the proterozoic aksu metamorphic rocks are different in extent and variability however the clastic rocks of the fifth member of the lower cretaceous kezilesu group have almost the same pb isotopic compositions as the sulfides if the preferential leaching of radiogenic pb is taken into account the sulfides in the disseminated ores show lree enriched patterns with high lree hree ratios 3 50 12 06 variable changes of ce anomalies 0 67 1 22 and eu anomalies 0 52 1 33 that are also similar to those of the clastic rocks of the fifth member of the lower cretaceous kezilesu group but are significantly different from those of the basement rocks of the proterozoic aksu group and the evaporite sequences of the paleocene aertashi formation in the ce and eu anomalies petrographic observations indicate that the clastic rocks of the fifth member of the lower cretaceous kezilesu group have undergone sedimentation diagenetic reddening and bleaching reddening is related to the in situ oxidation of mafic minerals such as biotite to produce ferric iron oxide and hydroxide ioh that accompanied the liberation of ore metals from the silicate phase during subsequent crystallization of ioh to hematite preferential adsorption onto hematite could have been the main mechanism for separating zn pb from cu bleaching resulted from the dissolution of ioh during the oil gas infilling stage and based on mass balance calculations substantial amounts of leachable fe zn pb were extracted from the red clastic rocks by the ore forming fluid therefore we propose that the red clastic rocks of the fifth member of the lower cretaceous kezilesu group may be the predominant source of ore metals in the uragen deposit and that the leaching may be possibly related to the bleaching process

structural analysis of bic fault a thrust related strike slip fault of the external domain of the taconic orogen appalachians quebec and metallogenic implications the bic fault is exposed along the shoreline of the st lawrence river 21 km southwest of rimouski for 210 m at cap a l orignal and for 100 m at cap enrage the fault brings in contact two major thrust sheets the des seigneuries and the des iles cambrian lithologies of the former overlying ordovician rocks of the latter in the taconic orogen such contacts are normally thrust faults but the bic fault is a dextral strike slip fault striking east west and dipping southward a study of a narrow zone straddling the fault and an adjacent part of the des seigneuries thrust sheet has led to the recognition of four successive stages of deformation all compatible with a northeast southwest trending strain ellipsoid the two first stages are most important stage 1 brought about regional folding and faulting whereas stage 2 was characterized by the development of various structural elements c s fabrics stretching lineation riedel shears and kink bands exclusive to the fault zone we infer that i in

kinematics and tectonic significance of transpressive structures within the coast plutonic complex british columbia abstract structural data from the coast plutonic complex near prince rupert british columbia are consistent with a deformational history dominated by dextral transpression from campanian to paleocene time penetrative east side up southwest directed ductile shearing produced moderately northeast plunging overturned kilometer scale isoclinal folds these folds are dextrally sheared and refolded into kilometer scale upright northwest plunging folds and steeply dipping transposed foliations with moderate to shallow northwest plunging lineations along their western side an east side up component to the transcurrent shearing is kinematically compatible with east side up shearing found within the great tonalite sill kinematic and geometric gradients and the spatial distribution of the finite stretching direction are interpreted to result from partitioning of transpression the location of these structures and overprinting relationships suggest the great tonalite sill intruded late kinematically into a crustal scale dextral transpressive shear zone these results indicate this shear zone could form part of the baja b c fault system that would have accommodated large northward displacements of the terranes making up western british columbia and southeast alaska this conclusion is based on 1 it is favorably located to accommodate the proposed displacements 2 deformation occurred during the time period of proposed large displacement 83 59 ma 3 the 15 km thickness of the shear zone indicates it records large displacements

evidence for coral island formation during rising sea level in the central pacific ocean the timing and evolution of jabat island marshall islands was investigated using morphostratigraphic analysis and radiometric dating results show the first evidence of island building in the pacific during latter stages of holocene sea level rise a three phase model of development of jabat is presented initially rapid accumulation of coarse sediments on jabat occurred 4800 4000 years b p across a reef flat higher than present level as sea level continued to rise during the highstand island margins and particularly the western margin accreted vertically to 2 5 3 0 m above contemporary ridge elevations this accumulation phase was dominated by sand size sediments phase three involved deposition of gravel ridges on the northern reef as sea level fell to present position jabat has remained geomorphically stable for the past 2000 years findings suggest reef platforms may accommodate the oldest reef islands in atoll systems which may have profound implications for questions of prehistoric migration through pacific archipelagos

systematic variations of argon diffusion in feldspars and implications for thermochronometry abstract coupled information about the time dependent production and temperature dependent diffusion of radiogenic argon in feldspars can be used to constrain the thermal evolution attending a host of earth and planetary processes to better assess the accuracy of thermal models an understanding of the mechanisms and pathways by which argon diffuses in feldspars is desirable here we present step heating ar diffusion experiments conducted on feldspars with diverse compositions structural states and microstructural characteristics the experiments reveal systematic variations in diffusive behavior that appear closely related to these variables with apparent closure temperatures for 0 1 1 mm grains of 200 400 c assuming a 10 c ma cooling rate given such variability there is no broadly applicable set of diffusion parameters that can be utilized in feldspar thermal modeling sample specific data are required diffusion experiments conducted on oriented cleavage flakes do not reveal directionally dependent diffusive anisotropy to within the resolution limits of our approach approximately a factor of 2 additional experiments aimed at constraining the physical significance of the diffusion domain are presented and indicate that unaltered feldspar crystals with or without coherent exsolution lamellae diffuse at the grain scale whereas feldspars containing hydrothermal alteration and or incoherent sub grain intergrowths do not arrhenius plots for argon diffusion in plagioclase and alkali feldspars appear to reflect a confluence of intrinsic diffusion kinetics and structural transitions that occur during incremental heating experiments these structural transitions along with sub grain domain size variations cause deviations from linearity i e upward and downward curvature on arrhenius plots an atomistic model for arrhenius behavior is proposed that incorporates the variable lattice deformations of different feldspars in response to heating and compression the resulting implications for accurately extrapolating laboratory derived diffusion parameters to natural settings and over geologic time are discussed we find that considerable inaccuracies may exist in published thermal histories obtained using multiple diffusion domain mdd models fit to arrhenius plots for exsolved alkali feldspar where the inferred ar partial retention zones may be spuriously hot

holocene sea level change and coastal landscape evolution in the southern gulf of carpentaria australia a revised holocene sea level history for the southern gulf of carpentaria is presented based on new data from the south wellesley archipelago and age recalibration of previous research results confirm that rising sea levels during the most recent post glacial marine transgression breached the arafura sill ca 11 700 cal yr bp sea levels continued to rise to ca 30 m by 10 000 cal yr bp leading to full marine conditions by 7700 cal yr bp sea level reached present mean sea level pmsl and continued to rise to an elevation of between 1 5 m and 2 m above pmsl sea level remained ca 1 5 between 7000 and 4000 cal yr bp followed by rapid regression to within 0 5 m of pmsl by ca 3500 cal yr bp when placed into a wider regional context results from this study show that coastal landscape evolution in the tropical north of australia was not only dependent on sea level change but also show a direct correlation with holocene climate variability specifically the formation and preservation of beach rock deposits intertidal successions beach and chenier ridge systems hold valuable sea level and holocene climate proxies that can contribute to the growing research into lower latitude holocene sea level and climate histories

cooling rates and crystallization dynamics of shallow level pegmatite aplite dikes san diego county california pegmatites of the pala and mesa grande pegmatite districts san diego county california are typically thin sheet like composite pegmatite aplite dikes aplitic portions of many dikes display pronounced mineralogical layering referred to as line rock characterized by fine grained garnet rich bands alternating with albite and quartz rich bands thermal modeling was performed for four dikes in san diego county including the 1 m thick himalaya dike the 2 m thick mission dike the 8 m thick george ashley dike and the 25 m thick stewart dike calculations were based on conductive cooling equations accounting for latent heat of crystallization a melt emplacement temperature of 650 c into 150 c fractured gabbroic country rock at a depth of 5 km and an estimated 3 wt initial h 2o content in the melt cooling to 550 c at the center of each dike occurred in 9 years for the stewart dike 340 days for the george ashley dike 22 days for the mission dike and 5 days for the himalaya dike based on these calculations growth rates for large pegmatitic minerals such as the 10 cm long himalaya hanging wall tourmaline crystals may have been on the order of 10 5 cm s crystal size distribution csd studies of garnet from layered aplites suggest growth rates of about 10 6 cm s these results indicate that the dikes cooled and crystallized rapidly with variable nucleation rates but high overall crystal growth rates initial high nucleation rates coincident with emplacement and strong undercooling can account for the millimeter size aplite grains lower nucleation rates coupled with high growth rates can explain the decimeter size minerals in the hanging walls cores and miarolitic cavities of the pegmatites the presence of tourmaline and or lepidolite throughout these dikes suggests that although the melts were initially h 2o undersaturated high melt concentrations of incompatible or fluxing components such as b f and li h 2o aided in the development of large pegmatitic crystals that grew rapidly in the short times suggested by the conductive cooling models

incremental accretion of a sandy reef island over the past 3000 years indicated by component specific radiocarbon dating 1 low lying reef islands appear particularly threatened by anticipated sea level rise and determining how they formed and whether they are continuing to accumulate sediment is essential for their sustainable management depositional chronology of warraber island a small sand cay in torres strait australia is re examined based on ams radiocarbon dating of specific skeletal components whereas radiometric dating of bulk sand samples indicated one or more discrete phases of mid late holocene deposition component specific ams radiocarbon dating of sand grains indicates sustained incremental growth over the past 3000 years ages on gastropods that lived on the reef flat around the island indicate continuing sediment production and island progradation in contrast to bulk ages and other components produced at greater distances growth of sand cays depends on the rate and pattern of sediment supply which are functions of the local ecology of the surrounding reef and reef flat and hydrodynamic constraints

geochronology of the larderello geothermal field new data and the closure temperature issue the larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4 9 km depth hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones which implies that the field has always undergone a roughly monotonic cooling history fluctuations 40 k since intrusion of the granite at 4 ma the heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of 32000 km3 rooted in the mantle borehole minerals from larderello are thus a unique well calibrated natural example of thermally induced ar and sr loss under geological conditions and time spans the observations biotites retain ar above 450 c agree well with other albeit less precise geological determinations but contrast with laboratory determinations of diffusivity from the literature we therefore performed a hydrothermal experiment on two larderello biotites and derived a diffusivity dlab 370 c 5 3 10 18 cm2s 1 in agreement with published estimates of diffusivity in annite from dlab and the rejuvenation of the k ar ages we calculate maximum survival times at the present in hole temperatures they trend smoothly over almost two orders of magnitude from 352 ka to 5 3 ka anticorrelating with depth laboratory diffusivities are inconsistent not only with geological facts but also among themselves from the geologically constrained lifetime of the thermal anomaly we derive a diffusivity dg 370 c 3 81 1021 cm2s 1 3 1 orders of magnitude lower than dlab the cause of these discrepancies must be sought among various laboratory artefacts overstepping of a critical temperature t enhanced diffusivities in wet experiments presence of fast pathway dislocation and pipe diffusion and of dissolution reprecipitation reactions which we imaged by scanning electron microscopy these phenomena are minor in geological settings in the absence of mineral transformation reactions complete or near complete resetting is achieved only by volume diffusion therefore laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers

high precision u pb geochronology of the butedale pluton british columbia the butedale pluton a ca 100 km long compositionally zoned batholith is part of the coast plutonic complex that extends the length of the canadian pacific coast its age and thermal history are relevant to the butedale pluton s role as a test case of the baja bc hypothesis as paleomagnetic evidence suggests that it may have formed thousands of kilometres to the south and moved northward along what is now the coast shear zone high resolution u pb chemical abrasion thermal ionization mass spectrometry ca tims zircon analysis of rocks across the width of the butedale pluton indicates that it is actually made up of at least two distinct magmatic events that formed the west butedale pluton ca 95 ma and the east butedale pluton ca 85 ma the east butedale pluton was reheated by the emplacement of a younger adjacent pluton which may have caused partial pb loss and resulting excess scatter of 206pb 238u zircon ages within individual samples the west butedale pluton may be the same age as and

thermal modification of hematite ilmenite intergrowths in the ecstall pluton british columbia canada in this study we examine the effects of reheating on finely exsolved hematite ilmenite intergrowths from the 91 ma ecstall pluton using reflected light microscopy scanning electron microscopy sem and transmission electron microscopy tem as a result of the emplacement of the younger adjacent 52 ma quottoon pluton samples closer to a thermal boundary have experienced greater degrees of thermal alteration five main microstructural features characterize hematite ilmenite intergrowths from the ecstall i exsolution lamellae of hematite and ilmenite ii oxidation of ilmenite to form hematite rutile and other ti rich phases iii 20 50 nm magnetite precipitates in hematite iv rutile blitz texture and v exsolution of hematite in rutile based on spatial relationships textures ii through v appear to be related to reheating of the ecstall by the quottoon and samples up to 14 km from the thermal boundary intrusive contact have been affected we propose a mechanism similar to that of kontny and dietl 2001 in which reheating has driven t f o2 conditions across the hematite magnetite buffer to lower f o2 resulting in the reduction of hematite higher temperatures also enhanced oxidation in ilmenite the formation of magnetite altered the bulk magnetic properties of these samples increasing nrm intensity this study underscores the need to consider a pluton s post emplacement thermal history before making tectonic interpretations based on paleomagnetic data

spatial and temporal variations in wave characteristics across a reef platform warraber island torres strait australia a field experiment was conducted at warraber island torres strait australia to investigate spatial and temporal variations in wave characteristics and energy across a mesotidal coral reef platform measurements of water depth were obtained using five pressure sensors deployed across a 2 7 km section of reef flat from july 3 5 2001 the reef surface was uneven and consisted of an outer reef flat a central reef flat depression an inner reef ramp a palaeo reef surface and the shoreline water levels decreased landward across the platform with tide ranges at the shoreline being almost 50 lower than at the outer reef flat rising and falling tides were characterised by a bimodal energy distribution with both short period 0 3 s and wind 3 8 s waves present higher water levels were dominated by wind waves the highest waves occurred at high tides associated with nocturnal tidal cycles with hs decreasing from 0 5 to 0 2 m from the outer reef flat to the shoreline wave energy at swell 8 20 s and infragravity 20 s frequencies was negligible across the reef platform although there was evidence of wave groups at higher water levels reef geometry and changes in water level determine the magnitude of wave energy on the reef platform up to 85 95 of incident wave energy is attenuated by the central reef flat depression at high and low tide respectively and strong linear relationships exist between hs and h at all locations both wave height and wave type are strongly depth dependent critical reef rim depths required to produce hs of a given size vary spatially across the reef rim due to variations in reef topography a distinct depth related threshold exists at which short period and wind wave dominance reverses over a 14 day spring neap tidal cycle the time of occurrence of wave action diminishes across the reef platform to the shoreline larger waves hs 0 2 m occur for only 9 of time at the outer reef flat and for less than 0 5 over the remaining reef platform this implies that on mesotidal reef platforms sediment entrainment and transport are severely constrained under normal wave energy conditions and significant change is likely restricted to extreme events

the geomorphology development and temporal dynamics of tepuka island funafuti atoll tuvalu abstract the evolution sedimentary structure and morphodynamics of tepuka island a leeward vegetated sand cay on funafuti atoll tuvalu was examined based on morphostratigraphic analysis and radiometrically dated island sediments tepuka island formed rapidly on the leeward reef platform between 1100 and 500 cal years bp and presents the first evidence of island formation at current sea level postdating the holocene highstand the island is predominantly composed of foraminifera coralline algae and coral sands derived from the surrounding reef flat morphostratigraphic analysis suggests that the island formed as a central core and prograded rapidly through incremental accumulation results indicate that the vegetated core of tepuka has been stable since formation but the island margins have been morphologically dynamic over the past century the island has reduced in net area by approximately 23 substantial planform changes have been documented which have occurred through differential erosion of the shoreline in two phases lagoon shoreline erosion from 1897 to 1984 and ocean shoreline coupled with extension of the terminal ends of islands from 1984 to 2011 collectively these changes have caused a lengthening and clockwise rotation of the island on the reef platform the investigation highlights significant differences in the geomorphology sediment composition timeframe of formation and post formation island dynamics compared with the storm deposited and comparatively stable motu on the exposed eastern atoll rim

the genetic relationship between upper mississippi valley district lead zinc mineralization and minor base metal mineralization in iowa wisconsin and illinois minor base metal occurrences are present in paleozoic dolomites and to a lesser extent in slates and sandstones in a broad zone surrounding the main upper mississippi valley zinc lead district in iowa wisconsin and illinois these occurrences are of interest because they may be remnants of fluid pathways of upper mississippi valley zinc lead mineralization outlying base metal occurrences contain mainly pyrite marcasite sphalerite galena and calcite they occur in a variety of forms including vertical veins gash veins disseminated breccias vug linings and paleokarst replacement bodies two paragenetic types of mineralization are evident occurrences in which sulfides are generally early and calcite is late and occurrences in which carbonate either calcite or dolomite but predominantly calcite is early and sulfides are generally late similarities exist among mineralogy geologic setting and some geochemical aspects of the main upper mississippi valley district and outlying occurrences however paragenetic fluid inclusion and stable isotope data indicate major differences in the genetic history of the outlying occurrences fluid inclusion homogenization temperatures for outlying sphalerite and calcite range from 37 8 degrees to 115 8 degrees c these temperatures are generally lower than those reported previously for these minerals from the main upper mississippi valley deposits 46 degrees 220 degrees c although measured salinities for inclusions in sphalerite in outlying occurrences 15 6 23 8 equiv wt nacl are similar to those for sphalerite from the main upper mississippi valley district 19 6 23 equiv wt nacl salinities for inclusions in calcite in outlying occurrences 5 1 18 1 equiv wt nacl are considerably higher than those determined for calcite 4 8 5 1 equiv wt from the main upper mississippi valley district the delta 34 s values for outlying sulfates range from 21 to 31 per mil cdt and are similar to those of the main upper mississippi valley district 22 36ppm overall values of delta 34 s for sulfides from outlying occurrences range from 22 to 36 per mil and contrast to that obtained from pitch and flat deposits 6 30ppm and gash veins approximately 1ppm from the main upper mississippi valley district although sulfides from this district formed from reduced connate water sulfate the wide range in sulfur isotope data was caused by the ore forming fluid reacting to varying degrees with isotopically light diagenetic pyrite values of delta 13 c and delta 18 o for outlying calcites 9 to 5ppm pdb and 21 29ppm smow respectively tend to be heavier than calcites from pitch and flat deposits in the main upper mississippi valley district delta 13 o 13 to 2ppm delta 18 o 17 23ppm the delta 18 o values for fluid inclusions in outlying mineralization 8 to 6ppm are similar to those obtained from the main upper mississippi valley district mineralization 5 to 6ppm however delta d values 95 to 20ppm are lighter than values 47 to 2ppm for the main district these isotopic values support a connate seawater source for the mineralizing fluids from both areas sulfur carbon and oxygen isotope data suggest that some outlying mineralization was more strongly influenced by local diagenetic processes than by processes that formed the main upper mississippi valley deposits

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

tropical sand cays as natural paleocyclone archives sand cays are valuable paleo archives that can significantly increase our understanding of holocene tropical cyclone variability here we conducted detailed sedimentological and chronological analyses from a 195 cm depth pit excavated on guangjin island northern south china sea a cay influenced by frequent tropical cyclones radiometric dating of multiple deposits revealed that foraminifera soft coral spicules and gastropod shells yielded variable age distributions while u th ages of pristine acropora branches provided a clear record of deposition and cay formation based on this robust chronostratigraphy the proportions of 2 mm grain size fraction within the deposits corresponded with the frequency of paleotyphoons recorded by historical records in recent centuries u th ages ce 1687 12 ce 1735 6 and ce 1813 5 of acropora branches from the deposits matched with three known historical typhoon events our results highlight the potential of cyclone deposited sand cays as new archives for recording paleocyclones

coral reef island initiation and development under higher than present sea levels coral reef islands are considered to be among the most vulnerable environments to future sea level rise however emerging data suggest that different island types in contrasting locations have formed under different conditions in relation to past sea level uniform assumptions about reef island futures under sea level rise may thus be inappropriate using chronostratigraphic analysis from atoll rim islands sand and gravel based in the southern maldives we show that while island building initiated at different times around the atoll 2 800 and 4 200 calibrated years before present at windward and leeward rim sites respectively higher than present sea levels and associated high energy wave events were actually critical to island initiation findings thus suggest that projected sea level rise and increases in the magnitude of distal high energy wave events could reactivate this process regime which if there is an appropriate sediment supply may facilitate further vertical reef island building

permian diamictites in northeastern asia their significance concerning the bipolarity of the late paleozoic ice age despite a lack of detailed sedimentologic analyses diamictites in the middle permian atkan formation were previously interpreted as glaciomarine and glacially influenced marine deposits this interpretation allowed this unit to play a prominent role in paleoclimatic and biogeographical reconstructions associated with presumed bipolar glaciation during the late paleozoic ice age lpia in this sense the lpia is considered to be a close analog to bipolar glaciation and climate change during the cenozoic here results are presented that challenge the glacigenic interpretation for these strata and negate interpretations of the bipolar nature of the lpia the 400 to 1500 m thick atkan formation was deposited in back arc basins associated with activity of the okhotsk taigonos volcanic arc along the leading edge of pangea as it drifted across the north polar circle the occurrence of tuffs volcanic clasts and glass shards indicate derivation from a nearby arc cooling and solidification of some clasts during sedimentation is suggested by the occurrence of clasts with embayments and protrusions that extend into the surrounding matrix clasts with columnar like jointing and alteration of the matrix surrounding some clasts ca tims dating of tuff zircons indicate a late capitanian age which is consistent with fossils within the strata bedded diamictites deposited as debrites dominate these diamictites which occur as tens of m thick downlapping packages that thicken then thin upward were deposited as prograding and abandoning sediment gravity flow fans chaotic and folded strata formed as slumps graded sandstones and conglomerates were deposited as turbidites and mudstones were deposited as mudflows low density turbidites and hemipelagic deposits striated clasts and outsized clasts piercing bedding were not observed in the study area strata above and below the atkan formation contain abundant graded beds and deep water trace fossils indicating deposition as turbidites the combination of debrites turbidites slumps volcanic grains clasts glass and tuffs and an absence of glacigenic indicators suggest that atkan strata were deposited in deep water basins associated with the development of the volcanic arc rather than due to glacial activity these findings are significant as they require reconsideration of current views of lpia glaciation and suggest that ice sheets were limited to gondwana

the paleomagnetic effects of reheating the ecstall pluton british columbia abstract paleomagnetic data of plutons from the western canadian cordillera are currently interpreted to indicate thousands of kilometers of latitudinal transport in situ tilting or post intrusion folding for the ecstall pluton of british columbia the inclination of magnetic remanence directions steepens to the east from 16 to 81 butler et al j geophys res 107 2002 between 24 and 12 km west of a thermal boundary with the coast mountains batholith cmb the cmb was at 700 800 c between 60 and 52 ma the stable remanent magnetization is in ilmenohematite grains with exsolved lamellae of ferrian ilmenite it has the characteristic properties of lamellar magnetism robinson et al nature 418 2002 517 520 which is acquired with exsolution in ilmenohematite below 390 c ghiorso phys chem minerals 25 1997 28 38 thus it is thermal chemical remanent magnetization tcrm our heat flow calculations show that the thermal effects of eocene cmb on the adjacent 91 ma ecstall pluton were enough to reset the remanent magnetization directions reheating to the temperatures necessary for tcrm is supported by k ar and ar ar cooling dates on hornblende and biotite which young from west to east towards the cmb in concert with the change in inclinations thus the progressive steepening of magnetization inclinations is attributed to reheating during the eocene of the 91 ma ecstall pluton by the cmb this reheating followed post solidification northward latitudinal displacement of the pluton the amount of translational displacement of the coastal terranes of british columbia prior to 60 ma hinges on the interpretation of discordant magnetizations from plutons that may have cooled slowly after emplacement in the mid to lower crust as was the case for the ecstall pluton many of these plutons contain ilmenohematite as an accessory magnetic phase our interpretation for the low temperature acquisition of tcrm in the ecstall pluton may have far reaching implications for understanding enigmatic discordant paleomagnetic directions reported from plutons of the western canadian cordillera

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

driving mechanisms for 40 km of exhumation during contraction and extension in a continental arc cascades core washington 1 in the nw north american cordillera the cascades core region of the coast plutonic complex underwent late cretaceous 96 ma to locally 73 ma sw ne contraction and crustal thickening followed by dextral transpression 73 to 55 ma then transtension 3 mm yr by local thrusting in regions undergoing crustal thickening in the central part of the core chelan block 40 km of exhumation occurred between 91 and 45 ma about half of which occurred during early contraction driven by thrusting and half during top to north arc oblique shear during reactivation of a midcrustal cretaceous thrust the dinkelman decollement the footwall of this thrust consists of the swakane biotite gneiss a cretaceous metaclastic assemblage with recorded pressures of 10 12 kbar no arc related magmatism and structures dominated by pervasive top to north shearing the hanging wall consists of the napeequa complex an oceanic assemblage with recorded pressures of 6 12 kbar voluminous arc related magmatism and complex structures indicating early top to wsw shearing younger top to north shearing and widespread folding in the napeequa top to north shearing started by 73 ma during melt present conditions at pressures 6 kbar top to north shearing in both hanging wall and footwall continued during exhumation 1 6 mm yr and cooling to greenschist facies conditions during which slip became increasingly localized eventually resulting in formation of pseudotachylite on discrete slip surfaces we suggest that arc oblique extension was driven by along arc heterogeneity in displacements erosion initially during transpression and underplating of continental sediments and later during transtension

genesis and controls of hydrothermal dolomitization in sandstones of the appalachian thrust belt quebec canada implications for associated galena barite mineralization abstract the appalachian thrust belt of eastern quebec hosts six epigenetic barite galena sphalerite occurrences in interdigitated carbonate cemented sandstones carbonate clast bearing conglomerates and minor siltstones of the upper cambrian st damase formation two of these the st fabien and cap enrage deposits consist of veins and disseminations enveloped by dolomitized zones and contain ankerite as an essential gangue mineral in contrast the other occurrences consist exclusively of calcite cemented veins and are conspicuously devoid of dolomite the domains of the st fabien and cap enrage deposits are intensely fractured and contain abundant dolomite barite sulfide cemented interconnected fractures and pores whereas those of the other occurrences display relatively minor fractures and pores which are only rarely interconnected this difference in porosity leads to the conclusion that differences in paleo permeability were the most likely reasons for the presence of dolomite and disseminated mineralization in the two deposits and their absence from the vein only occurrences mass balance calculations relating altered and unaltered rock suggest that dolomitization was accompanied by large gains of cao mgo and co 2 and corresponding losses of sio 2 and k 2 o under conditions of constant volume the mass of silica removed from the altered sandstone at st fabien 14 5 g per 100 g of rock when considered in the context of quartz solubility implies a minimum water rock ratio of 2635 a similar value 2500 is indicated by the content of lead of a representative sample of the st fabien deposit and typical concentrations of lead in mississippi valley type fluids the ph of the mineralizing dolomitizing fluid was calculated from the solubility of dolomite and fluid inclusion estimates of m ca 2 m mg 2 and m co 2 to be between 3 9 and 4 6 fluid inclusion data suggest that dolomitization and mineralization at st fabien and cap enrage were the products of interactions between local sedimentary rocks and minor metabasaltic dikes with hydrothermal brines detailed x ray diffraction analysis of the phyllosilicates in the host rocks in and around the st fabien deposit mainly sandstones and minor metabasalt indicates that these brines could have acquired virtually all magnesium and all iron manganese needed for dolomite precipitation as a result of the replacement of chlorite by illite since p t conditions during deposition of the two types of mineralization were similar to each other it is concluded that these conditions did not govern the formation of dolomite

a doubly vergent fan structure in the peninsular ranges batholith transpression or local complex flow around a continental margin buttress 1 the jura cretaceous peninsular ranges batholith prb of southern and baja california contains a remarkable example of a crustal transition between juxtaposed oceanic and continental floored arcs jura cretaceous deformation in the prb is focused along this lithospheric discontinuity and defines a zone of contractional deformation that stretches at least 800 km along the batholith the western edge of this zone is associated with a dramatic eastward increase in maximum recorded metamorphic pressures from 2 to 6 kbar and temperatures from 650 c elsewhere in the north american cordillera similar lithospheric boundaries are thought to accommodate various amounts of orogen parallel displacements of outboard tectonic fragments to test this proposal in the prb we examined an unusually well preserved part of the transition zone in the southern sierra san pedro martir sspm here a spectacular 20 km wide doubly vergent fan structure which developed over a period 40 m y during both jurassic and cretaceous magmatism occurs along the transition zone this structure consists of moderately inward dipping outward vergent fold and thrust belts with widespread mylonitic fabrics that steepen toward the center where both mylonitic and magmatic fabrics occur in syntectonic vertically sheeted plutons mineral lineations change from steeply pitching on the sides of the fan to moderately pitching in the center a number of constraints on possible displacements rotation of units and duration of fabric formation limit the amount of orogen parallel displacement to a few kilometers across the fan structure in the sspm instead we propose that the fan structure is a local feature that results from a combination of tectonic wedging caused by a cusp of rigid pre mesozoic continental margin crust at this location and vise tectonics that arose from strong lateral rheological gradients resulting from extensive heating by the sheeted tonalite complex focused deformation along the lithospheric discontinuity and subsequent strain softening we suggest that these mechanical controls on deformation during orthogonal convergence may be common in other orogens where similar fan structures have been attributed to transpression emphasizing the need for caution in interpreting such features as zones of significant orogen parallel displacement

magnetic properties of ilmenite hematite single crystals from the ecstall pluton near prince rupert british columbia paleomagnetic studies of the 91 ma ecstall pluton and other cretaceous plutons of british columbia imply large northward tectonic movements 2000 km may have occurred during the tectonic evolution of western north america however more recent studies have shown that the eastern edge of the ecstall pluton experienced considerable mineralogical changes as younger eocene plutons such as the 58 ma quottoon pluton were emplaced along its margins we investigated changes in the rock magnetic properties associated with this reheating event by examining isolated grains of intergrown ilmenite and hematite the primary paleomagnetic recorder in the ecstall pluton measurements of hysteresis properties low temperature remanence and room temperature isothermal remanent magnetization acquisition and observations from magnetic force microscopy and off axis electron holography indicate that samples fall into three groups the groups are defined by the presence of mineral microstructures that are related to distance from the quotoon plutonic complex the two groups closest to the quottoon pluton contain magnetite within hematite and ilmenite lamellae reheating of the ecstall pluton led to an increase in coercivity and magnetization as well as to development of mixed phase hysteresis these results indicate that shallow paleomagnetic directions from the western ecstall pluton are not affected by reheating and are therefore likely to record original field conditions at the time of pluton emplacement in the absence of structural deformation these shallow inclinations are consistent with large scale northward translation suggested by the baja british columbia hypothesis

miocene unroofing of the canyon range during extension along the sevier desert detachment west central utah this is the published version copyright 2001 american geophysical union all rights reserved

argon retentivity of hornblendes a field experiment in a slowly cooled metamorphic terrane hornblende from samples of amphibolite and granitic gneiss collected within a single outcrop in the central adirondacks yield significantly different sup 40 ar sup 39 ark sub k dates of 948 plus minus 5 and 907 plus minus 5 ma assuming that this terrane cooled slowly following high grade metamorphism and that the samples have experienced the same thermal history the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples the fe fe mg mn of the hornblende samples are 0 8 and 0 6 the higher ratio corresponding to the younger sup 40 ar sup 39 ar sub k date transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution but contain zones of fibrous phyllosilicates approximately 0 1 to 2 mu m wide parallelling 100 and 110 these alteration zones probably formed during post metamorphic cooling as a result of the migration of fluids through the hornblendes and are obvious pathways for argon escape from hornblende as these features are more abundant in the hornblende sample with the younger sup 40 ar sup 39 ar date and higher fe fe mg mn they may influence the argon blocking temperature by effectively partitioning the hornblende grains intomore diffusion domains of varying size biotite from the granitic gneiss yields an sup 40 ar sup 39 ar sub k date of 853 plus minus 2 ma with a mildly discordant step heating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite plagioclase from the amphibolite yields a sup 40 ar sup 39 ar sub k integrated date of 734 plus minus 3 ma less

strain partitioning in an obliquely convergent orogen plutonism and synorogenic collapse coast mountains batholith british columbia canada 1 we describe the crustal structure of the coast mountains batholith between 54 and 55 n within the canadian cordillera with emphasis on emplacement of the 7 km thick kasiks sill complex ksc kinematic patterns that developed during emplacement of the ksc are the result of interactions between magma transport magma accumulation and regional deformation the sills were emplaced during nw directed normal shearing and flattening of country rocks that host the ksc a 2 km thick shallowly ne dipping mylonite zone cuts the eastern side of the ksc kinematic indicators within the mylonite zone record top to the east normal displacements structural analysis shows that mylonite formation occurred during subvertical shortening and east northeast subhorizontal extension u pb zircon age dates show that ene directed normal shearing along the eastern side of the ksc and wnw directed normal shearing within the ksc occurred contemporaneously between 54 and 51 ma indicating strong strain partitioning between the mylonite and the ksc this pattern of strain partitioning is interpreted to have been driven by return flow of melt laden crust in response to tectonic denudation of the upper crust seismic profiling shows that many of these structures extend to mid and lower crustal depths comparison of our results with other regions within the canadian cordillera indicates that orogen scale right lateral strike slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes thus the crustal structure of the coast mountains batholith was the result of early tertiary batholith construction during dextral oblique convergence and synorogenic collapse

u pb geochronology of late cretaceous and early tertiary plutons in the northern coast mountains batholith u pb geochronologic studies demonstrate that steeply dipping sheetlike tonalitic plutons along the western margin of the northern coast mountains batholith were emplaced between 83 and 57 perhaps 55 ma less elongate tonalitic granodioritic bodies in central portions of the batholith yield ages of 59 58 ma coeval with younger phases of the tonalitic sheets large granite granodiorite bodies in central and eastern portions of the batholith were emplaced at 51 48 ma trends in ages suggest that the tonalitic bodies generally become younger southeastward and that at the latitude of juneau plutonism migrated northeastward across the batholith at 0 9 km ma variations in the age shape location and degree of fabric development among the various plutons indicate that late cretaceous paleocene tonalitic bodies were emplaced into a steeply dipping dip slip shear zone that was active along the western margin of the batholith postkinematic eocene plutons were emplaced at shallow crustal levels inhe

building biogenic beachrock visualizing microbially mediated carbonate cement precipitation using xfm and a strontium tracer the fate of reef islands is a topic of ongoing debate in the face of climate change induced sea level rise and increased cyclone intensity increased erosion and changes to the supply of reef derived sediment may put sand reef cays at risk of dramatic morphological changes these changes may have negative implications for the existence of reef cay environments which host vital sea turtle and bird rookery habitats beachrock consolidated carbonate beach sediment in the intertidal zone forms naturally on many tropical beaches and reduces the erosion rates of these beaches when compared to unconsolidated sand in spite of the critical role beachrock plays in stabilizing some reef cay shores the method of beachrock formation is still incompletely understood in this investigation beachrock was synthesized using beach sand and beachrock samples from heron island great barrier reef australia in aquarium experiments in which natural beachrock formation conditions were simulated beachrock was produced in two aquaria wherein the water chemistry was influenced by microorganisms derived from the natural beachrock inoculum whereas no cementation occurred in an aquarium that lacked a microbial inoculum and was controlled only by physicochemical evapoconcentration the new cements in the synthesized beachrock were analyzed using synchrotron based x ray fluorescence microscopy and were identified using a sr tracer added to the experimental seawater the resulting precipitates cement sand grains together and contain abundant microfossils of the microorganisms on whose exopolymer they nucleated demonstrating the fundamental role microbes play in beachrock formation these results are of interest because beachrock could be utilized as a natural coastline stabilization strategy on sand reef cays and in turn protect the unique habitats reef islands support

fluid inclusion composition of baltic shield quartz associated with mineralization sweden abstract fluid inclusions in quartz from seven ore districts in sweden have been analyzed by the decrepitation icp method the geological setting of the deposits varies from a sedimentary sandstone environment to highly metamorphosed rocks in the amphibolite facies major components na k and ca give variable but interpretable information about fluids involved in ore disposition and subsequent metamorphism the k na and ca na ratios are very consistent even at low salt concentrations in the fluids and may be used to distinguish the origin of the involved mineralizing brine solutions feldspar contamination does not effect the ratios decrepitation icp data in the present study are consistent with the microthermometry and geochemistry of the deposits the base metals cu fe and zn give significant results agreeing with the mineralogy of the deposit

incremental heating of hornblende in vacuo implications for 40ar 39ar geochronology and the interpretation of thermal histories increasing evidence of the incompatibility between the geologic thermal histories deduced from the sup 40 ar sup 39 ar incremental heating technique and those derived form other methods have necessitated a careful evaluation of the assumptions inherent in the former approach sup 40 ar sup 39 ar age spectra are frequently used to infer the spatial distribution of ar in hydrous minerals and as such require that the mineral remains stable while heated in vacuo aliquots of mmhb 1 and inter laboratory hornblende standard used for k ar and sup 40 ar sup 39 ar dating were heated in vacuo at temperatures of 500 700 930 and 1 050c for 30 min up to 700c no physical changes in the hornblende were observed with the scanning electron microscope although x ray diffraction data indicate that it had dehydroxylated to oxyhornblende at this temperature the onset of the structural decomposition of the oxyhornblende as revealed by backscattered and secondary electron images occurs at approximately 930c and is directly correlated with the initial release of 6 of its radiogenic ar sup 40 ar the breakdown of the oxyhornblende by the internal nucleation of incongruent melt is almost 93 complete by 1 050c the reaction products being titano magnetite melt a pyribole phase and internal voids because ar is released simultaneously frommore these products and is subsequently homogenized in vacuo this breakdown can yield plateau age spectra regardless of any original ar spatial gradients in the mineral consequently independent evidence may be required to establish whether a plateau date from hornblende is truly a geologically significant age less

translation of terranes lessons from central baja california mexico baja california is one of the best modern examples of a continental block or terrane that is translating hundreds of kilometers along a highly obliquely divergent plate the central domain of baja california is particularly appropriate as a modern analogue for terrane translation because it has a relatively simple late neogene history and is a buoyant continental block that is likely to be preserved in any future accretion to the north american continent in contrast the deep dense oceanic crust of the gulf of california where the main transform faults of the plate boundary lie has little preservation potential it is surprising that there is little strike slip faulting in the central domain of baja california even though the plate boundary has a rift angle angle between rift trend and azimuth of plate motion alpha of only 20 recent field data confirm modeling studies of oblique rifting and show that there is a predictable change in fault type fault orientation and extension direction with changing rift angle that is from secondary structures alone one can predict the approximate obliquity of rifting and the orientation of the plate motion with rift angles of 0 20 strike slip faults at low angles to the rift trend will dominate the secondary structures with rift angles from 20 to 35 there are strike slip faults subparallel to the rift trend and normal faults 20 40 clockwise from the rift trend in dextral shear above 35 normal faults are dominant and there are few or no strike slip faults as secondary structures

evaluation of effective groundwater recharge of freshwater lens in small islands by the combined modeling of geoelectrical data and water heads 1 in small islands a freshwater lens can develop due to the recharge induced by rain magnitude and spatial distribution of this recharge control the elevation of freshwater and the depth of its interface with salt water therefore the study of lens morphology gives useful information on both the recharge and water uptake due to evapotranspiration by vegetation electrical resistivity tomography was applied on a small coral reef island giving relevant information on the lens structure variable density groundwater flow models were then applied to simulate freshwater behavior cross validation of the geoelectrical model and the groundwater model showed that recharge exceeds water uptake in dunes with little vegetation allowing the lens to develop conversely in the low lying and densely vegetated sectors where water uptake exceeds recharge the lens cannot develop and seawater intrusion occurs this combined modeling method constitutes an original approach to evaluate effective groundwater recharge in such environments

cyclone frequency during the last 5000 years at curacoa island north queensland australia abstract a series of storm deposits at curacoa island a steep bedrock island in the palm group north queensland record the passage of cyclonic events storm ridge accretion occurs when cyclonic waves and surge combine to transport coarse sediment mostly broken coral onto the shore and deposit it above normal tidal levels narrow fringing reefs at curacoa act as a rapidly replenished sediment source augmented by lithic gravel transported longshore beachface stormberm ridge platform and washover deposits can accrete during a single event a series of storm ridges were examined via stratigraphic pits and trenches on two traverses deposits from 22 events were identified and their ages were determined using 68 radiocarbon dates from coral clasts most dates from each storm deposit formed statistically accordant groups only 15 of the dates were older than the group ages and were interpreted as reworked specimens accordant ages within each group were combined and converted to calibrated radiocarbon ages ridge ages decrease uniformly with distance from the oldest ridge indicating uniform depositional processes over the last 5000 years a time series of storm deposit ages indicates that cyclone frequency was statistically constant over the last 5000 years this suggests that cyclone frequency may not have been affected by sea surface temperatures in the region which appear to have been about 1 c warmer about 5000 years ago gagan m k ayliffe l hopley d cali j mortimer g chappell j mcculloch m head m j 1998 temperature and surface water balance of the mid holocene tropical western pacific science 279 1014 1018

the durability of bioclastic sediments and implications for coral reef deposit formation the generation and composition of carbonate sediment within tropical carbonate settings is controlled by the skeletal production of caco3 in conjunction with physical biological and chemical processes which act to break down and disperse skeletal remains using the results of detailed tumbling barrel experiments this paper discusses the role the physical durability of common constituents of reef sediment plays in the composition of coral reef deposits the durability of the skeletal remains of six reef sediment constituents was determined experimentally using tumbling barrels results indicate that constituent durability varies considerably amongst common reef sediment constituents calcareous algae halimeda was the least durable constituent tested followed by larger benthic foraminifera baculogypsina sphaerulata and marginopora vertebralis and the pelecypod fragum fragum two species of branching coral acropora sarmentosa and acropora nasuta were found to be the most resistant to physical destruction these findings provide increased power to interpret reef and island deposits and the potential role skeletal durability plays in the retention of constituent skeletons within coral reef associated sedimentary systems

comparison between grain size and multi mineral 40ar 39ar thermochronology abstract in the case of volume diffusion the closure temperature of a mineral is function of among other factors the characteristic diffusion dimension which can be approximated by the grain size of the mineral analysed for grains smaller than or similar in size to the diffusion domains the theoretical possibility of single mineral grain size thermochronology had been demonstrated empirically in earlier studies mostly using biotite in order to examine the potential of this method it was tested alongside the widely used multi mineral 40 ar 39 ar thermochronology the sample comes from the granitic mclean pluton in the south section of the grenville orogeny seven grain size separates of biotite ranging between 90 and 1000 m eight size fractions of amphibole between 63 and 1000 m and three size fractions of k feldspar 250 600 m were extracted and dated by the laser step heating 40 ar 39 ar method the total gas ages obtained behave as theoretically predicted with increasing ages for increasing grain sizes including for k feldspar but with the exception of the smallest and the largest grains for biotite and amphibole the calculated cooling rates are ca 0 7 c ma for k feldspar ca 2 5 c ma for biotite and ca 11 c ma for amphibole corresponding very well to a monotonic cooling of the mclean pluton a quick initial thermal re equilibration with the cooler host rocks is followed by a much slower cooling on a thermal path parallel to that of the frontenac terrain situated immediately to the southeast the validity of the single mineral grain size thermochronology is demonstrated by comparison with the thermal evolution of the adjacent units and with the cooling history derived from a multi mineral thermochronology suggesting that it can be routinely used the application of this method can be hampered by insufficiently low analytical uncertainties

geomorphological changes in the coastal area of farasan al kabir island saudi arabia since mid holocene based on a multi proxy approach abstract the geomorphological evolution of the southeastern coastal area of farasan al kabir island saudi arabia is revealed by the mapping of modern landforms and a multi proxy and high spatial resolution study including grain size particulate organic carbon mineralogy element geochemistry benthic foraminifera analysis and radiocarbon dating of a 3 3 m long sediment core the modern geomorphological features comprise a variety of arid landforms such as plateau cliffs and pediments of pleistocene coral limestones playa depressions located on plateau surfaces alluvial fans butte and sandy beaches the mid holocene evolution of the borehole area is resulted from the detailed analysis of five sedimentary units detected along the core matar 1 and includes three distinct stages a from 5253 223 y cal bp to 3138 223 y cal bp carbonate coarse grained material consisting of coral fragments molluscs calcareous algae and benthic foraminifera are deposited on a shallow marine fringing reefal platform which becomes progressively a nearshore backreef around 3675 215 y cal bp and later around 3138 223 y cal bp a reef ramp b since 3040 220 y cal bp the borehole area obtains the characteristics of a high energy beach that receives increasing inputs of terrigenous material c subsequently a supratidal backshore setting is established influenced mostly by terrestrial processes and occasionally by marine processes as it is indicated by the decreasing and sometimes sporadic presence of benthic foraminifera and recently a sedimentary veneer consisting of terrigenous carbonate and evaporite material is formed by terrestrial mainly wadi and aeolian processes

late holocene sea level change and reef island evolution in new caledonia abstract in new caledonia numerous cays are distributed on platform reefs in the southwest lagoon behind the barrier reef at mba island a vegetated sand cay in this area we examined holocene sea level change and reef development and evolution the late holocene sea level curve for the area was updated using newly found fossil microatolls component specific dating of foraminifera tests in the island sediment provided reliable ages of island formation mba island initially formed around 4500 cal yr b p when sea level was 1 1 m higher than at present the island formed on the leeward side of the platform reef where the base had been constructed of lagoon and reef derived sediment of the platform reef sea level fall since 2800 cal yr b p has likely caused reef flat emergence at low tide and created a suitable habitat for foraminifera such a transition in ecology due to sea level fall has been observed in other pacific reefs suggesting that the interaction between reef flat formation sediment production and sea level fall is one of the critical factors determining island development

exploring carbonate reef flat hydrodynamics and potential formation and growth mechanisms for motu abstract atolls which develop as reef building coral platforms extend to near sea level typically consist of a shallow reef flat encircling a central lagoon often sub aerial islets known as motu or reef islands consisting of sand gravel and coral detritus can be found perched atop the reef flat here we use hydrodynamic numerical modeling xbeach to better understand the role of waves and wave driven currents on the reef flat and the processes driving motu formation and evolution by differing representative reef flat geometry e g width and water depth we investigate the effects of varying wave climate on hydrodynamics and resultant bed shear stresses across the reef flat model results suggest that as a reef flat shallows bed shear increases then after passing a critical value decreases again using these results we hypothesize that reef flats should attain a critical water depth just at the threshold for sediment mobilization resulting in a constant depth reef flat in both abrasional and depositional settings as reef flats widen prograding into the back reef lagoon shear stress decreases across the flat with a minimum in shear stress arising approximately midway on the reef flat motu formation would be expected to initiate at this mid flat nucleation site either from a storm when coarse sediment is mobilized and deposited or gradually as the reef flat widens a mid flat deposit need not be subaerial to form a motu sediment piled shallower than the critical depth would continue to accumulate once a motu is present reef flat shear stress directions reverse and the motu shoreline should prograde seawards until reaching a relatively narrow critical reef flat width 200 m together these results suggest that reef flats and motu spatially self organize through a series of morphodynamic feedbacks

effects of progressive mylonitization on ar retention in biotites from the santa rosa mylonite zone california and thermochronologic implications the santa rosa mylonite zone developed predominantly from a granodiorite protolith in the eastern margin of the peninsular ranges batholith a wide variation in k ar biotite dates within the zone is shown to reflect the times of cooling through closure temperatures whose variability is chiefly a result of deformation induced reduction in grain size we suggest that such variation generally may be exploited to place constraints on the timing of deformation episodes previous workers have shown that deformation in the santa rosa mylonite zone involved the formation of mylonites and an imbricate series of low angle faults which divide the area into structural units field work petrographic studies and tem analysis of deformation mechanisms in biotite show that the granodiorite mylonite the lowermost structural unit formed below the granodiorite solidus the granodiorite mylonite varies from protomylonite to ultramylonite with regions of high strain distributed heterogeneously within the zone previously reported biotite and hornblende k ar dates from the granodiorite protolith below 82 89 ma and the asbestos mountain granodiorite above 61 68 ma the mylonite zone indicate dramatically dissimilar thermal histories for the lowermost and uppermost structural units other workers fission track dates on sphene zircon and apatite from the granodiorite mylonite and the asbestos mountain granodiorite suggest thermal homogenization and rapid cooling to 100 c by ca 60 ma within and adjacent to the mylonite zone k ar dates on 5 samples of biotite from variably deformed granodiorite range from 62 76 ma dates are not correlated with structural depth but clearly decrease with degree of deformation and concomitant grain size reduction 40ar 39ar incremental heating analyses of biotite from the granodiorite protolith reveals no evidence of excess argon and produces a relatively flat age spectrum 40ar 39ar incremental heating analysis of biotite from the granodiorite mylonite discloses discordance consistent with 39ar recoil loss k analysis of samples allowing k ar dates to be calculated is therefore recommended as an adjunct to 40ar 39ar step heating analysis in rocks that have experienced similar deformation during mylonitization biotite grain size reduction through intracrystalline cataclasis results in estimated grain dimensions as small as 0 05 m locally within porphyroclasts as large as 1 mm because biotite compositions are relatively uniform fe fe mg mn ti alvi 0 47 0 52 and show no systematic variation with grain size compositional dependence of activation energy and diffusivity can be eliminated as sources of variation in ar retention ar closure temperatures calculated with appropriate diffusion parameters for the observed grain sizes are in the range 220 280 c and define a cooling curve consistent with a thermal history intermediate between those of the granodiorite protolith below and the asbestos mountain granodiorite above the mylonite zone changes in the slope of the cooling curve indicate that the main deformation episode initiated at or above ca 330 c 80 ma above the closure temperature for thermally activated diffusion of argon in biotite and continued to a minimum of ca 220 260 c 62 ma

p t t evidence for mechanisms of vertical tectonic motion in a contractional orogen north western us and canadian cordillera the 1800 km long coast mountains north cascades orogen of the canadian cordillera and north western us developed as a continental magmatic arc metamorphic rocks in the orogen contain widespread evidence for burial of supracrustal rocks to depths of c 40 km followed by nearly isothermal decompression to depths of 10 km near many shallowly emplaced mid cretaceous plutons low pressure contact metamorphic effects were overprinted by high pressure regional metamorphic minerals and textures as evidenced by kyanite staurolite pseudomorphs after andalusite in metapelitic rocks therefore near pluton rocks record the loading history of the orogen metapelitic rocks not associated with plutons only preserve evidence for high pressure conditions and or high temperature decompression as indicated for example by sillimanite and cordierite after kyanite and garnet respectively petrological evidence for burial and decompression is therefore recorded in different rocks various regions of the orogen differ in timing of metamorphism the overall shape of p t paths and the relative timing and regional extent of the high pressure event but most of these data and observations are consistent with thrusting and or pure shear thickening as primary loading mechanisms throughout the orogen as opposed to magma dominated loading this interpretation is further supported by comparison with thermal models which demonstrate that the p t paths are consistent with simultaneous thrusting and folding at a high initial geothermal gradient 35 40 c km 1 in much of the orogen a high geothermal gradient supports tectonic models invoking intra arc contraction and suggests that magmatism played an important role in regional temperature time paths this tectonic thermal history may be typical of other contractional orogens and illustrates the importance of large vertical displacement of crust in magmatic arcs

pb zn occurrences and their pb isotopic signatures bearing on metallogeny and mineral exploration paleozoic sedimentary rocks northern appalachians quebec galena pb isotope ratios of epithermal vein and disseminated sulfide occurrences in the taconian orogen and siluro devonian basin cluster around 17 90 18 05 for 206pb 204pb and 37 70 38 00 for 208pb 204pb the major source of pb in most if not all occurrences is a fairly common continental crust a characteristic found in published analyses of grenville feldspar pb a southwest to northeast increase in galena 206pb 204pb ratios is ascribed to the supply of several types of detritus from grenville basement during the cambro ordovician coarse grained k feldspar bearing in the southwest grading into fine grained phyllitic and relatively more highly radiogenic in the northeast emplacement i of pb zn barite veins and disseminations commonly of homogeneous crustal pb isotopic signature was late taconian ii of pb zn quartz veins of less homogeneous signature was post taconian and iii of pb zn carbonate veins relatively highly radiogenic and commonly homogeneous was late or post acadian signa

from sandstone to carbonate hosted stratabound deposits an isotope study of galena in the upper moulouya district morocco the upper moulouya district displays three main types of lead ores in the hercynian domain 1 aouli vein type deposits hosted in the hercynian basement 2 zeida triassic sandstone hosted and 3 mibladen carbonate hosted stratabound and karstic mineralisations in the mesozoic cover lead and sulfur isotope studies indicate that the aouli vein type and zeida sandstone hosted stratabound mineralisations are formed by a mixing of two fluids one formed by the leaching of the aouli granite massif and the other by leaching of the saharian basement these deposits may correspond to the same triassic metallogenic event focused along fault systems or within permeable sandstone deep fluids were mobilised during the early extensional movements associated with the opening of the atlas rifting basin the mississippi valley type mibladen mineralisation is related to a distinct metallogenic event superimposed on the earlier one and represents a remobilization of earlier concentrations or a more recent leaching of the same sources but with a more pronounced contribution of the local organic matter the upper moulouya area demonstrates the close relationship between vein type and sandstone hosted lead zinc mineralisations and their major differences with mississippi valley type deposits

radiocarbon evidence for the timing and rate of island development beach rock formation and phosphatization at lady elliot island queensland australia abstract lady elliot island great barrier reef is a coral cay constructed from near concentric shingle ridges composed of coral tridacna shells and bioclastic sand bound by guano derived phosphatic cement radiocarbon dating of individual tridacna samples is used to investigate the evolution of the island and place constraints on the variations in storminess of the last 3200 years a shallow platform reef existed at lady elliot since at least 6500 yrs b p shortly before 3200 yrs b p island growth by shingle progradation commenced and proceeded to the present at a rather uniform lateral rate 60 m ka 1 to leeward and 90 m ka 1 windward phosphatization may have occurred continuously or episodically at any time throughout the last 3200 years the youngest cemented ridge top is less than 770 years whereas at the base of uncemented ridges phosphatization is a modern 1950 a d process similarly low level beach rock has continued to form within the last 25 years a significant period of instability occurred less than 500 years ago during which marine erosion truncated the southwestern extremity of lady elliot island formed an eastern spit and stranded high level beach rock however the radiocarbon ages of tridacna samples from shingle ridges from lady elliot and curacoa islands are uniformly distributed kolmogorov smirnov test throughout the last 4000 years we interpret this to indicate no significant variation in ridge forming processes or fluctuation in average storminess during this interval

reef island accretion and soil development on makin kiribati central pacific the late holocene accretionary history of reef islands on makin the northernmost table reef of the gilbert atoll chain western kiribati has been reconstructed based on conventional radiocarbon dating of coral shingle and bulk sand samples and ams radiocarbon dating of individual grains makin is geomorphologically and sedimentologically simple comprising one main island with only a minor residual lagoon and composed of sand dominated by the reef flat foraminifera calcarina spengleri amphistegina lobifera and baculogypsina sphaerulata deposition commenced in mid platform around 2500 years ago in situ fossil coral heliopora dated at 2400 80 years bp indicates that sea level was 0 4 0 5 m above present at that time progradation occurred progressively to westward at a relatively constant rate of 200 300 m ka 1 however progradation was interrupted on the eastern side by lagoon encapsulation around 1400 years ago the soil and vegetation characteristics are uniform over most of the reef islands as a result of clearance and the planting and maintenance of coconuts and do not reflect reef island depositional history remnant stands of pisonia forest with an associated phosphatic soil are not related to any particular geomorphological feature or period of deposition the reef islands are continuing to accrete as a result of production of foraminifera on the reef flat

two stage exhumation of midcrustal arc rocks coast mountains british columbia 1 new geologic data from the central gneiss complex along douglas channel help delineate the burial and exhumation of the late cretaceous to eocene coast mountains magmatic arc arc plutonism was on going between 90 and 60 ma replaced by formation of dikes to 52 ma supracrustal rocks were buried to midcrustal levels by 90 ma and garnet and kyanite grew from 90 through 70 ma subsequent exhumation was nearly isothermal and took place in two stages the first occurred from 70 to 59 ma as the arc contracted obliquely and sillimanite replaced kyanite exhumation was slow 0 5 mm yr probably accomplished by erosion aided by coaxial crustal thinning exhumation rates about doubled in the second stage of exhumation after 59 ma and before 52 ma penetrative deformation ended prior to intrusion of the quottoon pluton at 59 ma and 6 5 kbar production of cordierite rims on garnet at 4 5 kbar and 700 c signaled the end of near isothermal decompression prior to 52 ma rapid cooling 100 c 106 years of the central gneiss complex followed cordierite growth temperatures dropped to 250 by 48 ma the increase in exhumation rate and the subsequent rapid cooling are attributed to excision of 6 km of crust on a detachment system on the northeastern side of the central gneiss complex comparison to other parts of the coast mountains arc the sierra nevada and fiordland new zealand shows that the amount and tempo of exhumation vary greatly within and between arcs suggesting that the processes accommodating exhumation vary significantly

rapid reef island formation and stability over an emerging reef flat bewick cay northern great barrier reef australia a model that resolves reef island formation in relation to both reef platform substrate development and mid holocene sea level change is presented for bewick island northern great barrier reef based on morphostratigraphic analysis and radiometrically dated island sediments and reef corals on bewick island microatolls record reef flat development at higher sea level 1 5 m by 6500 yr b p island building began on a partially emergent reef flat 5000 4000 yr b p when sea level was 0 5 m above present as sea level fell to its present level the reef platform process window closed and the island core stabilized results present the first unequivocal evidence of island building directly over a reef flat comprising microatolls and the first detailed model of island formation from the great barrier reef the model demonstrates that the interplay of sea level and reef surface elevation can vary between sites but their convergence is critical for island initiation future trajectories of island change will vary dependent on the unique relative sea level and substrate depth conditions that govern island formation

sand cay evolution on reef platforms mamanuca islands fiji abstract the sedimentary link between reef islands and their surrounding platforms is critical to understand if the morphological response of these systems to future boundary condition changes such as associated with human induced climate change is to be understood the sand cays of the mamanuca islands in fiji are an ideal system to investigate this link between reef and the islands that have developed on them the mamanuca s are a relatively simple sedimentary system being characterised by oval shaped platform reefs with a relatively uniform and horizontal reef crest and flat nine islands were investigated within the group through gps and auto level surveying with associated surface and subsurface sediment sampling the islands are composed of a typical chlorozoan sediment assemblage which is well mixed across each reef and island this mixing is related to the high transport capacity across the reef platforms with waves able to move sediment for at least 50 of the tidal cycle radiocarbon dating indicates the islands are young with modern ages returned on sand from the reef flat and ages of 500 600 years calbp from the island beaches dating from the centre of the islands at 2 m depth indicate island formation was occurring from at least 2260 2110 years calbp the young age of the sediments combined with the high transport potential suggests each cay is linked to the contemporary reefs that the islands are developed on it is inferred that the islands in the mamanuca group initially started as a subtidal gravel accumulation once wave energy is dissipated enough intertidal sands start to accumulate and eventually the islands reach a critical size where vegetation may colonise the contemporary development of the reef islands in the mamanuca s and their links to sediment production on the reef flat suggests that they may be able to adjust their morphology to future environmental conditions

new insights into the holocene development history of a pacific low lying coral reef island takapoto atoll french polynesia abstract low lying atoll islands are known to be made up of unconsolidated coral rich detritus together with high amounts of foraminifera derived from both outer reef and lagoonal environments while regarded as highly vulnerable to ongoing global changes these islands are poorly constrained in term of development history herein is presented a detailed chronostratigraphic study of tuamotu atoll islands french polynesia south central pacific based on analyses of sedimentary sequences through seven excavations along two transects across both windward and leeward rim areas around takapoto atoll the island accretionary chronology is supported by radiometric dating of 62 coral and molluscan clasts the sequences range between 3 80 m and 1 20 m in thickness from the oceanic shoreline lagoonwards four lithofacies were identified from sediment composition and texture a coral pocilloporid rich gravel supported preferentially located in the outermost rim areas a coral rich gravelly sand supported locally interbedded with gravel supported units a foraminifera amphisteginid rich sand dominated mainly located at the central and innermost rim settings and an organic rich sand facies atop of some sequences a model of atoll islet formation is drawn up in relation to mid to late holocene sea level changes the foundations of islets motus namely conglomerate platforms started to form with deposition of patchy rubble spreads over the upper reef rim surfaces from ca 4 500 yr bp as sea level was about 0 80 m above its present mean level on these platforms islets started to accrete not before ca 2 300 yr bp from isolated depocentres located midway between outer reef and lagoon margins at that time sea level at about 0 60 m above present mean sea level was starting to slowly decrease to its present position the major growth phases occurred in a context of continued sea level fall islets continued to accrete through concentric ridges mainly until the last 300 years accretion was dominantly driven by low frequency high energy wave surge events from dating of coral clasts a number of one to two events by century were identified as having apparently contributed to island formation at takapoto regionally predicted increase in the rates of sea level rise may have negative impacts on such islands since these have evolved under conditions of falling sea level

batholith emplacement at mid crustal levels and its exhumation within an obliquely convergent margin abstract emplacement of the central part of the coast mountains batholith of northern coastal british columbia occurred within a regime characterized by oblique convergence between the farallon kula and north american plates we use new structural kinematic and u pb isotopic data to show that the locations geometry and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones these data also show that the most active period of pluton emplacement from 67 to 51 ma coincided with a change in style of deformation within the batholith prior to 67 ma plutons were emplaced within an arc dominated by regional scale contractional shear zones in contrast emplacement of 67 51 ma plutons occurred in an arc increasingly dominated by normal faults with arc parallel to oblique displacement and by sinistral transtensional shear zones we have identified and mapped the structure of three plutonic complexes composed of 67 to 51 ma plutons the khyex sill complex arden lake plutonic complex and quottoon plutonic complex shear zone controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes during and after this latest cretaceous paleogene period of intense plutonic activity and accompanying deformation the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 ma

eco geomorphic processes that maintain a small coral reef island ballast island in the ryukyu islands japan abstract landform changes in ballast island a small coral reef island in the ryukyu islands were investigated by remote sensing analysis and a field survey the area of the island almost doubled after a mass coral bleaching event in 1998 coral branches generated by the mass mortality and broken by waves were delivered and stocked on a reef flat and accumulated to expand the area of the island in 2012 high waves generated by typhoons also changed the island s topography overall the island moved in the downdrift direction of the higher waves waves impacting both sides of the island piled up a large volume of coral gravels above the high tide level eco geomorphic processes including a supply of calcareous materials from the corals on the same reef especially during stormy wave conditions were key factors in maintaining the dynamic topographic features of this small coral reef island