ground motion attenuation relationships for cascadia subduction zone megathrust earthquakes based on a stochastic finite fault model the number of strong ground motion recordings available for regression analysis in developing empirical attenuation relationships has rapidly grown in the last 10 years however the dearth of strong motion data from the cascadia subduc tion zone has limited this development of relationships for the cascadia subduction zone megathrust which can be used in the calculation of design spectra for engi neered structures a stochastic finite fault ground motion model has been used to simulate ground motions for moment magnitude m 8 0 8 5 and 9 0 megathrust earthquakes along the cascadia subduction zone for both rock and soil site condi tions the stochastic finite fault model was validated against the 1985 m 8 0 mi choacan mexico and the 1985 m 8 0 valpariso chile earthquakes these two subduction zone megathrust earthquakes were recorded at several rock sites located near the fault rupture for the cascadia megathrust earthquakes three different rup ture geometries were used to model the m 8 0 8 5 and 9 0 events the geometries only differ in their respective fault lengths a fault dip of 9 to the east with a rupture width of 90 km was selected to represent average properties of the cascadia sub duction zone geometry a regional crustal damping and velocity model was used with the stochastic finite fault model simulations ground motions were computed for 16 site locations the parametric uncertainties associated with the variation in source path and site effects were included in the development of the ground motions a functional form was fit to the ground motion model simulations to develop region specific attenuation relationships for the cascadia megathrust rupture zone for both rock and soil site conditions the total uncertainty was based on a combination of the modeling and parametric uncertainties sigmas these newly developed atten uation relationships for cascadia subduction zone megathrust earthquakes can be used in both the probabilistic and deterministic seismic hazard studies for engineer ing design for the pacific northwest

atmospheric pumping a mechanism causing vertical transport of contaminated gases through fractured permeable media as the barometer falls gases are drawn upward out of the permeable earth into the atmosphere conversely a rising barometer pushes air downward in a homogeneous permeable medium these cyclical gas motions are piston like and nearly reversible so they contribute only modestly to the net transport of contaminant gases in a fractured permeable medium however the fractures will generally serve as breathing passages for all of the gas filled porosity greatly increasing the amplitude and nonuniformity of vertical motions the resulting transport process may be orders of magnitude more significant than molecular diffusion according to the theoretical analysis presented here analytical solutions are first derived for the sinusoidal pressure response of a medium containing identical vertical fractures equally spaced by slabs of permeable matrix material these solutions are then used to constrain the relationship between fracture aperture and fracture spacing based on field comparisons between surface and subsurface pressure variations the final phase of the analysis addresses the diffusive and advective transport of an inert trace gas which is carried by an oscillatory flow along a fracture having permeable walls a maximum rate of transport is predicted to occur for an intermediate fracture spacing which is typically a few meters

moment magnitude local magnitude calibration for earthquakes off canada s west coast local magnitude m l values of earthquakes off canada9s west coast are known to be underestimated by at least 0 5 magnitude units compared with other magnitude scales moment magnitude m w derived from moment tensor analysis provides the most robust estimate of the magnitude of earthquakes moment tensor analysis of regional seismic data in western canada is now possible due to the installation of more than 40 three component broadband stations in western canada the u s pacific northwest and southeast alaska moment tensor solutions are now possible down to m 4 0 more than 230 regional moment tensor solutions have been calculated off canada9s west coast at the geological survey of canada for 1995 2002 these solutions along with 14 previous solutions by oregon state university in 1994 1995 and 13 harvard solutions for 1984 1993 allow a systematic m w m l calibration for earthquakes in this region the study area extends from the queen charlotte islands region in the north to the area off the west coast of southern vancouver island at the northern end of the study area where there is little oceanic crust in the source receiver travel path m w is systematically larger than m l by 0 28 0 08 magnitude units at the southern end of the study area where there is a significant amount of ocean crust in the source receiver travel path m w is systematically larger than m l by 0 62 0 08 magnitude units calibration of m l with m w will allow the western canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

subduction zone anisotropy beneath corvallis oregon a serpentinite skid mark of trench parallel terrane migration 1 we studied the back azimuth dependence of ps converted phases at gsn station cor corvallis oregon using broadband p receiver functions computed from 602 teleseismic earthquakes the amplitudes and polarities of the transverse ps phases are largely two lobed which indicates anisotropy with a tilted symmetry axis a double peaked ps conversion at 4 5 6 5 s delay has the moveout of a dipping slab but is not consistent with simple deflection of the ps converted phase by the dipping interface a polarity flip on the transverse receiver function near north south back azimuth indicates an anisotropic symmetry axis aligned north south far from the convergence direction n68 e the ps phase is modeled using reflectivity synthetics with a highly anisotropic layer of depressed wave speed vp 6 0 6 5 km s near 40 km depth at the slab interface with the overriding north american plate one dimensional modeling suggests 10 anisotropy in the supraslab layer with a slow symmetry axis oriented n5 w at a 60 tilt from the vertical adjustments for the effect of slab interface dip on ps amplitude suggest a somewhat lower 7 anisotropy we infer a thin 7 km anisotropic detachment zone for the northward slippage of the siletz forearc terrane along the top of the descending slab a motion consistent with gps measurements and models of regional lithospheric dynamics serpentinite is a likely constituent for the deep anisotropic layer owing to its reduced vp and high poisson ratio the ductile rheology and hydrated composition of serpentinite make it a plausible lithology for a mechanical detachment zone

local relief and the height of mount olympus a three dimensional assessment of the net volume of rock differentially eroded from below mountain tops to form valleys yields a range wide constraint on feedback between valley development and the height of mountain peaks the superelevation of mountain peaks potentially attributable to differential removal of material from below peaks in the olympic mountains washington was constrained by fitting a smoothed surface to the highest elevation points on a 30 m grid digital elevation model of the range high elevation areas separate into two primary areas one centred on mount olympus in the core of the range and the other at the eastern end of the range the largest valleys and hence areas with the greatest volume of differentially eroded material surround mount olympus in contrast the highest mean elevations concentrate in the eastern end of the range calculation of the isostatic rebound at mount olympus attributable to valley development ranges from 500 to 750 m 21 to 32 per cent of its height for a 5 to 10 km effective elastic thickness of the crust comparison of cross range trends in mean and maximum elevation reveals that this calculated rebound for mount olympus corresponds well with its superelevation above the general cross range trend in mean elevation it therefore appears that the location of the highest peak in the olympics is controlled by the deep valleys excavated in the centre of the range copyright 2000 john wiley sons ltd

lithospheric instability beneath the transverse ranges of california recent high resolution seismic experiments reveal that the crust beneath the san gabriel mountains portion of the transverse ranges thickens by 10 15 km contrary to earlier studies associated with the transverse ranges there is an anomalous ridge of seismically fast upper mantle material extending at least 200 km into the mantle this high velocity anomaly has previously been interpreted as a lithospheric downwelling both lithospheric downwelling and crustal thickening are associated with the oblique convergence of pacific and north america plates across the san andreas fault though it seems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle we show by means of numerical experiment that the balance between buoyancy forces that drive deformation and viscous stresses that resist deformation determines the geometry of crustal thickening and mantle downwelling we use a simple two layered lithospheric model in which dense lithospheric mantle overlies relatively inviscid and less dense asthenosphere and is overlain by buoyant crust external plate motion drives convergence which is constrained by boundary conditions to occur within a central convergent zone of specified width a fundamental transition in the geometry of downwelling is revealed by our experiments for slow convergence or low crustal viscosity downwelling occurs as multiple sheets on the margins of the convergent zone for fast convergence or crust that is stronger than mantle lithosphere a single downwelling occurs beneath the center of the convergent zone this complexity in the evolution of the system is attributed to the interaction of crustal buoyancy with the evolving gravitational instability in order for a narrow downwelling slab to have formed beneath the transverse ranges within the last 5 myr the effective lithospheric viscosity of the convergent region is at most about 10 20 pa s

lithoprobe crustal reflection structure of the southern canadian cordillera 2 coast mountains transect the lithoprobe seismic reflection transect across the southern coast mountains of the canadian cordillera images fundamental crustal structures presumably related to collision of the intermontane and insular composite terranes and deep levels in the upper plate of the offshore cascadia subduction belt the eastern part of the coast mountains are characterized by east dipping upper crustal reflectors that project to exposed faults and east dipping lower crustal reflectors they are truncated by subhorizontal to west dipping middle and upper crustal reflectors these geometric relationships are interpreted to have formed during an early phase of primarily west directed contraction that created the east dipping structures of the upper and lower crust and a later phase of east directed shortening caused by wedging of the intermontane belt into the lower and middle crust of the tectonic stack subsequently the coast belt may have been displaced eastward on contractional faults that ascend from the lower crust beneath the intermontane belt and surface in the omineca and foreland belts extensional faults bounding the east flank of the coast mountains and west flank of the central nicola horst in the intermontane belt flatten into the middle and lower crust of the intervening region and geometrically outline crustal boudinage within the western coast mountains east dipping reflections spanning the middle crust to upper mantle are traced updip to vancouver island and the underlying cascadia subduction zone the c reflector on vancouver island is believed to separate wrangellia from underlying accreted terranes and is correlated to the mainland where it forms the upper boundary of a reflective lower crustal wedge that flattens into the moho if the moho is not a young feature then some accreted material appears to have wedged into the continental framework above the crust mantle boundary possibly causing shortening in the overlying crust and creating midcrustal ramps observed on the reflection data the structurally lower e reflections interpreted as shear zones originate at the subduction contact offshore and project landward into sub moho reflections within the upper plate on the mainland the region between the e reflector and the descending oceanic plate is interpreted to be subducted lower continental crust and mantle

crustal structure beneath the central oregon convergent margin from potential field modeling evidence for a buried basement ridge in local contact with a seaward dipping backstop models of magnetic and gravity anomalies along two e w transects offshore central oregon one of which is coincident with a detailed velocity model provide quantitative limits on the structure of the subducting oceanic crust and the crystalline backstop the models indicate that the backstop forming western edge of the siletz terrane an oceanic plateau that was accreted to north america 50 million years ago has a seaward dip of less than 60o3 seismic magnetic and gravity data are compatible with no more than 2 km of subducted sediments between the siletz terrane and the underlying crystalline crust of the juan de fuca plate the data also suggest the presence of a n s trending 200 km long basaltic ridge buried beneath the accretionary com plex from about 43on to 45on although the height and width of this ridge probably vary along strike it may be up to 4 km high and several kilometers wide in places and appears to be locally in contact with the siletz terrane beneath heceta bank several models for the origin of this ridge are discussed these include a sliver of siletz terrane detached from the main siletz terrane dur ing a late eocene episode of strike slip faulting imbrication and thickening of subducted oceanic crust in place an aseismic ridge rafted in on the subducting oceanic crust during the past 1 2 mil lion years and a series of ridges and or seamounts rafted in over a longer period of time and trans ferred from the subducting plate to the overlying plate the last model is the most consistent with the complicated history of local uplift subsidence and slope instability recorded in the ridges basins and banks of this part of the margin we speculate that the massive seaward dip ping western edge of the siletz terrane in this region inhibits subduction of seamounts and sedi ments resulting in fomation of buried ridge as the accumulated flotsam and jetsom of subduction this process may also be responsible for thickening of lower accretionary complex material over steepening of slopes leading to massive slumping and north south extension through strike slip faulting in the accretionary complex to the west of the buried ridge regardless of its origin the ridge may currently be acting as an asperity inhibiting subduction

stable carbon and oxygen isotopes in quaternary soil carbonates as indicators of ecogeomorphic changes in the northern chihuahuan desert usa stable carbon and oxygen isotopes in soil carbonates provide an additional technique for investigating quaternary ecogeomorphic changes in arid and semiarid regions this study presents 13c and 18o values in surface and buried soils in a basin and range region of southern new mexico and west texas three landform categories were investigated for their isotopic content 1 middle piedmont slopes 2 a lower piedmont slope and adjacent playa depressions and 3 an intermontane basin floor soils of the middle piedmont slopes had greatest 13c variability ranging from 0 6 in buried soils to 11 1 pdb in surface soils soils of the basin floor had greatest variability in 18o values ranging from 0 6 to 7 6 pdb fossil pollen patterns roughly paralleled 13c shifts with cheno am pollen exhibiting greatest changes both isotope and pollen data across stratigraphic discontinuities suggest that a conversion from c4 grasses to c3 desert shrubs accompanied holocene erosion

genesis of calcrete in quaternary soil profiles hutt and leeman lagoons western australia abstract soil profiles exposed along the margins of hutt and leeman lagoons contain a succession of calcrete horizons that exhibit oolitic pisolitic grainstone lithoclast breccia and pelloidal wackestone mudstone stone lithologies the typical coastal profile consists of an upper laminar calcrete horizon which is usually overlain by a transitional pisolitic loose soil horizon underlying the laminar calcrete is breccia and or massive calcrete downward gradation into the host is through a mottled calcrete horizon structure textures and fabrics are utilized to classify the following different calcrete types pisolitic loose soil is composed of sandy soils calcrete ooids and pisolites eolianite clasts and shell fragments laminar calcrete is composed of parallel laminated sheets of micritic carbonate that develop by segregation processes massive calcrete is dense relatively structureless calcrete with a secondary mudstone fabric resulting from diagenetic alteration of primary constituents calcrete mottles form in response to localized diagenesis in the host sediment breccia calcrete develops as a result of prolonged weathering involving repeated episodes of calcretization dissolution and brecciation diminution of permeability of the host sediment by development of massive calcrete is a major process in genesis of the profile and occurs in the lowermost part of the zone of alternate wetting and drying where the effect of water input overcomes the cumulative effects of evapotranspiration profile development is thereafter topographically controlled and restricted to horizons overlying the massive calcrete redevelopment of zones of alternate wetting and drying in older deposits may produce multiple soil profiles

subduction zone magnetic anomalies and implications for hydrated forearc mantle continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate magnetite is a significant byproduct of mantle hydration and forearc mantle cooled by subduction should contribute to long wavelength magnetic anomalies above subduction zones we test this hypothesis with a quantitative model of the cascadia convergent margin based on gravity and aeromagnetic anomalies and constrained by seismic velocities and find that hydrated mantle explains an important disparity in potential field anomalies of cascadia a comparison with aeromagnetic data thermal models and earthquakes of cascadia japan and southern alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide

the record of continental dust from australia in tasman sea sediments abstract the flux of aeolian dust from australia to sediments of the tasman sea has increased during glacial stages of the last four glacial cycles low fluxes of dust from australia before 350 ka were followed by an increase in stage 10 and high fluxes in peak glacial periods thereafter the initiation of higher dust fluxes in stage 10 is consisten with previous estimates of the age of onset of aridity in the source area dust flux to the tasman sea is thought to record the intensity of deflation from the source area in interior southeastern australia which was marked by arid conditions and possibly greater windiness during the last glacial a transect of five cores from the central tasman sea revealed consistently low dust fluxes north of 33 s increasing southward to approximately 40 s glacial to interglacial movement of the northern boundary of the dust plume governed by the position of the subtropical ridge was less than 6 latitude probably closer to 3 350 km the position of the dust plume determined in this study and its response to climate change are significantly different from a previous reconstruction based on the measurement of quartz content of the non biogenic sediment fraction

terrestrial stromatolites and laminar calcretes a review abstract laminated carbonates are prominent features of subaerial exposure profiles and many types of calcareous soils including calcretes previously much attention was given to differentiating these laminites from marine or lacustrine microbial stromatolites and oncoids however recent research has shown that many subaerial laminated carbonates are biogenically formed mainly but not wholly by microbial activity this paper reviews the criteria for distinguishing terrestrial carbonate laminites from non terrestrial stromatolites and oncoids and for differentiating biogenic from abiogenic terrestrial laminites

holocene sea level change on the central west coast of vancouver island british columbia a new holocene relative sea level rsl curve is proposed for the central west coast of vancouver island based on topographic surveys of coastal landforms and the stratigraphy of coastal deposits six study sites are described on vargas island and esowista and ucluth peninsulas and 23 previously unpublished radiocarbon ages that constrain mean sea level msl position are presented a long term rise in rsl from an early holocene 7000 bp lowstand below 3 m asl is documented from dates on tree stumps in the intertidal zone and salt marsh peats and forest floor deposits buried by marine sands and gravels submergence culminated in a stillstand from 6000 to 4800 bp marked by the development of a strandline at 6 m asl msl 3 2 m asl a fall in rsl is documented by a strandline at 4 0 4 5 m asl which dates from 2700 to 2000 bp msl 2 0 m asl little is known of the dynamics of emergence between 4800 and 2700 bp or from 2000 bp to the present although the absence of strandlines below 4 m asl sugges

stable isotopic composition of soil calcite o c and gypsum s overlying cu deposits in the atacama desert chile implications for mineral exploration salt sources and paleoenvironmental reconstruction abstract soils overlying two porphyry cu deposits spence gaby sur and the pampa del tamarugal atacama desert northern chile were collected in order to investigate the extent to which saline groundwaters influence soil chemistry in regions with thick miocene and younger sediment cover soil carbonate calcite was analyzed for c and o isotopes and pedogenic gypsum for s isotopes soil calcite is present in all soils at the spence deposit but increases volumetrically above two fracture zones that cut the miocene gravels including gravels that overlie the deposit the c isotope composition of carbonate from the soils overlying fracture zones is indistinguishable from pedogenic carbonate elsewhere at the spence deposit all 13 c vpdb values fall within a narrow range 1 40 4 23 consistent with the carbonate having formed in equilibrium with atmospheric co 2 however 18 o vpdb for carbonate over both fracture zones is statistically different from carbonate elsewhere average 18 o vpdb 0 82 vs 2 23 respectively suggesting involvement of groundwater in their formation the composition of soils at the tamarugal anomaly has been most strongly affected by earthquake related surface flooding and evaporation of groundwater 13 c vpdb values 4 28 to 2 04 are interpreted to be a mixture of dissolved inorganic c dic from groundwater and atmospheric co 2 at the spence deposit soils only rarely contain sufficient so 4 for s isotope analysis the so 4 bearing soils occur only above the fracture zones in the gravel results are uniform 3 7 4 9 34 s cdt which is near the middle of the range for so 4 in groundwater 0 9 7 3 sulfur in soils at the gaby sur deposit 3 8 6 1 34 s cdt is dominated by gypsum which primarily occurs on the flanks and tops of hills suggesting deposition from so 4 rich fogs sulfate in gaby sur deposit gypsum is possibly derived by condensation of airborne so 4 from volcanic so 2 from the nearby andes at the gaby sur deposit and tamarugal anomaly pedogenic stable isotopes cannot distinguish between s from porphyry or redeposited so 4 from interior salars the three sites studied have had different histories of salt accumulation and display variable influence of groundwater which is interpreted to have been forced to the surface during earthquakes the clear accumulation of salts associated with fractures at the spence deposit and shifts in the isotopic composition of carbonate and sulfate in the fractures despite clear evidence of relatively recent removal of salts indicates that transfer from groundwater is an ongoing process the interpretation that groundwaters can influence the isotopic composition of pedogenic calcrete and gypsum has important implications for previous studies that have not considered this mechanism

supergene gold transformation secondary and nano particulate gold from southern new zealand biogeochemical processes drive the transformation of gold au in surface environments in this study we assess the link between surface morphologies of au grains and supergene transformation processes with a focus on the formation of nano particulate au in temperate settings gold grains were collected from six localities across the south island of new zealand deposit styles vary from eluvial alluvial and beach placer deposits in areas of moderate to very high levels of precipitation gold grains were assessed using optical microscopy om field emission scanning electron microscopy feg sem focused ion beam scanning electron microscopy fib sem coupled with x ray dispersive analysis edxa and electron microprobe analyses epma morphologies indicative of au and ag dissolution e g grain boundary dissolution as well as abundant au neoformation and aggregation morphologies were observed on all grains the latter include a variety of secondary au morphotypes in particular nano particulate and crystalline forms as well as bacteriomorphic au sheet au and porous branched au networks pervasive dissolution features on grains from an outcropping quartz vein system as well as extensive nano particle formation on weathered quartz vein and placer grains from the west coast of new zealand s south island which is subject to very heavy orographic precipitation suggest that these climatic conditions enhance the transformation of au grains at these sites au nano particles are most abundant in the polymorphic layer i e a coating of biofilms secondary au siliceous and carbonaceous materials on the surface of transforming au grains and in soil materials associated with the grains nano particulate au is also highly abundant in carbonaceous likely exopolymeric coatings on au grains from orepuki beach suggesting that au dissolution in seawater and microbial biomineralization are important contributors to au alteration in beach placer deposits in conclusion surface morphologies of au grains from new zealand are the result of supergene transformations occurring in current environments the formation of nano particulate au which was previously thought to be evaporation driven is in these high rainfall environments likely due to other mechanisms such as biomineralization

initial stages of laminar calcrete formation by roots examples from the neogene of central spain thin carbonate laminae formed by root activity are common within miocene detrital deposits of the duero and madrid basins the laminae are about 3 cm thick extending several metres laterally and displace the original detrital sediment which ranges in grain size from fine gravel to sandy clay the thickness morphology microstructure and stable isotope compositions of the laminae indicate that they formed by the activities of roots and associated microorganisms within the soil the laminae are similar to those commonly recognised in thick laminar calcrete profiles three types of laminae are recognised differences in the micromorphology of the laminae are explained as reflecting the different organisms involved and whether calcification occurred when the root was alive or decaying the first type occurs in a profile at paracuellos the jarama where the laminae consist of a mosaic of 20 mm calcite crystals whose arrangement indicates that calcification took place in the medulla of the root and probably occurred while the plant was alive in a second type in the villacadima profile laminae comprise calcified root mats whose formation indicates the interplay of roots and fungi calcification of the cell walls and intracellular spaces took place in the cortices of the roots and not in the medullas as revealed by the presence of central pores in every calcified root a third type of lamina is recognised in profiles at vin egra de moran a and consists of mucilaginous sheets coated by needle fibre calcite crystals the occurrence of associated small root casts indicates that formation of the laminae occurred while the root was decaying and was also influenced by fungal activity these laminae occur within poorly developed soils and their formation was controlled by the relationship between sedimentation erosion and soil formation processes thus the occurrence of these laminae interbedded with detrital sediments reflects environments where sedimentation was relatively low and episodic so after the detrital sediment input surfaces were stable and root mats were able to develop renewed sedimentation accounted for the death of the root mats and the development of new ones on the new surfaces where the sedimentation rate is lower the laminae tend to amalgamate and thicker laminar calcrete profiles form with little or no interbedded detrital sediment

integrated crustal structure across the south central california margin santa lucia escarpment to the san andreas fault the continental margin of south central california has undergone different styles of deformation over the past 30 m y as the north america pacific plate boundary has evolved from subduction to transform tectonics results presented here identify strike slip and compressional structures in the lower crust that probably developed during this period of tectonic transition an integrated model of crustal structure along deep seismic line pg e 3 crosses the entire continental margin west of the san andreas fault near the latitude of san luis obispo a compressional wave velocity model was developed using coincident marine multi channel seismic and onshore offshore wide angle reflection refraction seismic data and was tested independently with gravity modeling a high velocity layer 6 8 7 0 km s about 6 km thick in the lower crust that extends east possibly as far as the san andreas fault is interpreted as tectonically underplated oceanic crust the upper and middle crust of the patton and sur obispo franciscan terranes extends to depths of 14 to 20 km the velocity and density modeling indicate lower average velocities and densities for the patton terrane supporting earlier interpretations that it is a distinct terrane within the franciscan complex abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the santa lucia escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 ma following the subduction of the pacific arguello ridge an imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust convergence between the monterey and arguello microplates during the final stages of farallon subduction about 24 to 16 ma probably caused the imbrication

the continental margin off oregon from seismic investigations in april and may 1996 a geophysical study of the cascadia continental margin off oregon and washington was carried out aboard the german rv sonne as a cooperative experiment between geomar the usgs and coas offshore central oregon which is the subject of this study the experiment involved the collection of wide angle refraction and reflection data along three profiles across the continental margin using ocean bottom seismometers obs and hydrophones obh as well as land recorders two dimensional modelling of the travel times provides a detailed velocity structure beneath these profiles the subducting oceanic crust of the juan de fuca plate can be traced from the trench to its position some 10 km landward of the coastline at the coastline the moho has a depth of 30 km the dip of the plate changes from 1 5 westward of the trench to about 6 5 below the accretionary complex and to about 16 further eastward below the coast the backstop forming western edge of the siletz terrane an oceanic plateau that was accreted to north america about 50 ma ago is well defined by the observations it is located about 60 km to the east of the deformation front and has a seaward dip of 40 at its seaward edge the base of the siletz terrane seems to be in contact with the subducting oceanic crust implying that sediments are unlikely to be subducted to greater depths the upper oceanic crust is thinner to the east of this contact than to the west at depths greater than 18 km the top of the oceanic crust is the origin of pre critical reflections observable in several land recordings and in the data of one ocean bottom instrument these reflections are most likely caused by fluids that are released from the oceanic crust by metamorphic facies transition

crustal structure of the borderland continent transition zone of southern california adjacent to los angeles we use data from the onshore offshore component of los angeles region seismic experiment larse to model the broad scale features of the midcrust to upper mantle beneath a north south transect that spans the continental borderland in the los angeles california region we have developed an analysis method for wide angle seismic data that consists primarily of refractions lacks near offset recordings and contains wide gaps in coverage although the data restrict the analysis to the modeling of broad scale structure the technique allows one to explore the limits of the data and determine the resolving power of the data set the resulting composite velocity model constrains the crustal thickness and location and width of the continent borderland transition zone we find that the mid to lower crust layer velocities of the inner borderland are slightly lower than the corresponding layers in the average southern california crust model while the upper mantle velocity is significantly higher the data require the moho to deepen significantly to the north we constrain the transition zone to initiate between the offshore slope and the southwest los angeles basin if the inner borderland crust is 22 km thick then the transition zone is constrained to initiate within a 2 km wide region beneath the southwest los angeles basin and have a width of 20 25 km the strong coherent and continuous pn phase suggests the moho is coherent and laterally continuous beneath the inner borderland and transition zone the inner california borderland seems to be modified and thickened oceanic crust with the oceanic upper mantle intact beneath it

isotope geochemistry of caliche developed on basalt enormous variations in oxygen and carbon isotopes occur in caliche developed on 3 ma basalts in 3 volcanic fields in arizona significantly extending the range of 18o and 13c observed in terrestrial caliche within each volcanic field 18o is broadly co variant with 13c and increases as 13c increases the most 18o and 13c enriched samples are for subaerial calcite developed on pinnacles knobs and flow lobes that protrude above tephra and soil the most 18o and 13c depleted samples are for pedogenic carbonate developed in soil atmospheres the pedogenic caliche has 18o fixed by normal precipitation in local meteoric waters at ambient temperatures and has low 13c characteristic of microbial soil co2 subaerial caliche has formed from 18o rich evapoconcentrated meteoric waters that dried out on surfaces after local rains the associated 13c enrichment is due either to removal of 12c by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over layered during burial by calcite carrying the isotopic signature of the soil environment the large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if mars had a warm wet early period and photosynthesizing microbes were present in the early soils the approach can be similarly applied to terrestrial precambrian paleocaliche in the search for the earliest record of life on land large variations reported for 18o of carbonate in martian meteorite alh84001 do not necessarily require high temperatures playa lakes or flood runoff if the carbonate is an example of altered martian caliche

a seismic reflection profile across the cascadia subduction zone offshore central oregon new constraints on methane distribution and crustal structure in 1989 we conducted an onshore offshore seismic experiment to image the crustal structure of the cascadia forearc in this paper we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort this profile reveals several features of the forearc that were not apparent in an earlier coincident reflection profile one of the most important of these features is a very strong bottom simulating reflection bsr beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m where it appears to crop out on the seafloor the pressure and temperature conditions at the bsr derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate seawater system over a broad range of temperatures and pressures assuming hydrostatic pressure and the temperature gradiant measured near the base of the continental slope during ocean drilling program odp leg 146 interval velocities and reflection coefficients derived from the data indicate that the bsr represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas consistent with results obtained during leg 146 although the regional distribution of the anomalously strong bsr beneath the midslope is poorly known we speculate that it may be related to apparent slope instability the data also provide constraints on the thickness and geometry of the siletz terrane which is the basement beneath the shelf and acts as the subduction zone backstop a deep reflection which might mistakenly be interpreted to be moho if coincident large aperture data were not available is interpreted to be the base of the siletz terrane a recently active strike slip fault zone that overlies the seaward edge of the siletz terrane suggests that the siletz terrane controls the location of decoupling of the subduc tion complex from the rest of the forearc

distribution and speciation of gold in biogenic and abiogenic calcium carbonates implications for the formation of gold anomalous calcrete abstract calcrete pedogenic ca carbonate is an important sampling medium for geochemical gold au exploration in semi arid and arid regions of australia because it is widespread easy to sample and calcium ca shows a strong positive correlation with au but not with base metals in calcrete overlying buried au mineralization in this study we show that the formation of au anomalous calcrete can be biomediated through the activity of resident microorganisms and may not simply be the result of passive nucleation on inactive cells or evapotransporative processes calcified microfossils are highly abundant in calcrete from the barns au prospect in south australia these microfossils are morphological analogues of calcified cells and biofilms formed in laboratory experiments conducted with active bacterial cultures enriched from au anomalous calcareous sand from the barns prospect calcium carbonates precipitated by these cultures consisted mostly of calcite which is the main carbonate mineral in calcrete synchrotron micro x ray fluorescence s xrf mapping was used to assess the distribution of au zn ca and other metals in ca carbonates precipitated by active bacterial cultures on a m scale the distribution of au was heterogeneous in these ca carbonates and differed from base metal distribution thus mimicking the spatial separation of these metals observed in calcrete the speciation of au in ca carbonates precipitated by active bacteria was measured using micro x ray absorption near edge structure spectroscopy xanes and resembled that observed in au anomalous calcrete closely while metallic au was observed in au hotpots ionic au was detected in the halo surrounding the hotspot in contrast the precipitates produced in the presence of dead bacterial cells or by raising solution ph or pco 2 i e hydroxylapatite portlandite and vaterite respectively did not reflect the mineralogy of calcrete gold distribution and speciation in vaterite formed by raising pco 2 were homogenous and did not reproduce the variation observed in calcrete and ca carbonates precipitated by active cells increasing the supersaturation with respect to ca in solution by incremental drying of the medium produced only x ray amorphous precipitates or hydroxylapatite in the presence heat killed cells in conclusion this study shows that active microbial processes that combine biogenic ca carbonatogenesis with au precipitation are likely to drive the formation of au anomalous calcrete

the biogenic origin of needle fibre calcite needle fibre calcite is one of the most ubiquitous habits of calcite in vadose environments caves deposits soil pores etc its origin either through inorganic indirect or direct biological processes has long been debated in this study investigations at 11 sites in europe africa and central america support arguments for its biogenic origin the wide range of needle morphologies is the result of a gradual evolution of the simplest type a rod this rod is the elementary brick which by aggregation and welding builds more complex needles the absence of cross welded needles implies that they are welded in a mould or under a longitudinal and unidirectional constraint before being released inside the soil pores the difference between the lengthening of the needles and the c axis can be explained by the existence of needles observed under a scanning electron microscope in organic sleeves which can act as a mould during rod growth complex morphologies with epitaxial outgrowths on straight rods cannot have grown entirely inside organic microtubes they must result from soil diagenesis after the release of straight rods in a soil free medium whisker crystals are interpreted as the result of growth and coalescence of euhedral crystals on a rod rhomb chains are considered to be the consequence of successive epitaxial growth steps on a needle during variations in growth conditions isotopic signatures for needle fibre calcite vary from 16 63 to 1 10 and from 8 63 to 2 25 for 13c and 18o respectively the absence of high 18o values for needle fibre calcite precludes a purely physicochemical origin evaporative for this particular habit of calcite as epitaxial growth cannot precipitate in the same conditions as initial needles needle fibre calcite stable isotopic signatures should be used with caution as a proxy for palaeoenvironmental reconstructions in addition it is suggested that the term needle fibre calcite should be kept for the original biogenic form the other habit should be referred to as epitaxial forms of needle fibre calcite

the association of gold with calcrete abstract the discovery that au accumulates in calcrete pedogenic carbonate or caliche was made in 1987 by csiro calcrete is a general term describing accumulation of alkaline earth metals in soils of arid and semi arid terrains around the world the principal constituent of calcrete is calcite while au is a noble metal calcrete has been a significant tool in a number of au deposit discoveries so understanding the mechanisms by which these diametrically different components come together is valuable for enhancing future discovery numerous laboratory experiments case histories and exploration models have been published most from australia yet we do not fully understand the mechanisms involved it is timely therefore twenty five years on since the first publication of this phenomenon to review this highly unusual but economically important association critical to any review on au in calcrete is to first consider calcretes themselves the nature of a particular calcrete where it has formed and mode of formation is relevant to how where and why au accumulates within it this review commences with a background nomenclature history classification and some examples of calcrete types found near au deposits how calcretes form their origins and the role of biota is considered their locations in the regolith and landscape as well as exploration models for au in calcrete are discussed a section on the chemistry of au in calcretes details what we know about possible mechanisms of formation and considers what laboratory experiments on microorganisms and abiotic experiments tell us following on is a summary of practical aspects of identifying collecting and analysing samples for exploration purposes selected mineral exploration case histories are described and how they fit into models of exploration and different regolith settings concluding sections include a summary and implications of this accumulated knowledge to discovering au deposits