a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

chemical and mechanical processes during burial diagenesis of chalk an interpretation based on specific surface data of deep sea sediments burial diagenesis of chalk is a combination of mechanical compaction and chemical recrystallization as well as cementation we have predicted the characteristic trends in specific surface resulting from these processes the specific surface is normally measured by nitrogen adsorption but is here measured by image analysis of scanning electron micrographs this method concentrates on the micritic matrix alone deep sea sediments are ideally suited to the study of burial diagenesis because they accumulate in a relatively conservative tectonic setting we used material from the ontong java plateau in the pacific where a 1 km thick package of chalk facies sediments accumulated from the cretaceous to the present in the upper 200 300 m the sediment is unconsolidated carbonate ooze throughout this depth interval compaction is the principal porosity reducing agent but recrystallization has an equal or larger influence on the textural development in the chalk interval below compaction is not the only porosity reducing agent but it has a larger influence on texture than concurrent recrystallization below 850 m grain bridging cementation becomes important resulting in a lithified limestone below 1100 m this interpretation is based on specific surface data alone and modifies current diagenetic models

the relation between seismic p and s wave velocity dispersion in saturated rocks seismic velocity dispersion in fluid saturated rocks appears to be dominated by two mechanisms the large scale mechanism modeled by biot and the local flow or squirt mechanism the two mechanisms can be distinguished by the ratio of p to s wave dispersions or more conveniently by the ratio of dynamic bulk to shear compliance dispersions derived from the wave velocities our formulation suggests that when local flow dominates the dispersion of the shear compliance will be approximately 4 15 the dispersion of the compressibility when the biot mechanism dominates the constant of proportionality is much smaller our examination of ultrasonic velocities from 40 sandstones and granites shows that most but not all of the samples were dominated by local flow dispersion particularly at effective pressures below 40 mpa

viscosity structure of the crust and upper mantle in western nevada from isostatic rebound patterns of the late pleistocene lake lahontan high shoreline received 13 july 2005 revised 26 october 2006 accepted 31 january 2007 published 8 june 2007 1 large lakes can both produce and record significant crustal deformation we present an analysis of the isostatic rebound pattern recorded in the shorelines of paleolake lahontan in western nevada using a layered maxwell viscoelastic model the inferred viscosity structure depends on loading history we use three variants of a well documented lake surface elevation model as input and recover corresponding estimates of viscosity and density structure a simple two layer model with an elastic plate over an inviscid half space fits the observed elevation pattern quite well with a residual variance of 32 of the data variance using multilayered finite viscosity models the residual variance is reduced to 20 of the data variance which is very near to the noise level in the higher resolution models the viscosity is below 10 18 pa s over the depth range from 80 to 160 km the minimum viscosity is very similar to the value that has been seen in the eastern great basin from similar analyses of lake bonneville shorelines but the lowviscosity zone is thinner beneath bonneville making small adjustments to a seismically derived density structure allows an improved fit to the shoreline observations additionally we find that small variations in proposed loading models can result in presumably spurious density inversions and suggest that this modeling approach provides a test for loading histories

static and dynamic effective stress coefficient of chalk deformation of a hydrocarbon reservoir can ideally be used to estimate the effective stress acting on it the effective stress in the subsurface is the difference between the stress due to the weight of the sediment and a fraction effective stress coefficient of the pore pressure the effective stress coefficient is thus relevant for studying reservoir deformation and for evaluating 4d seismic for the correct pore pressure prediction the static effective stress coefficient n is estimated from mechanical tests and is highly relevant for effective stress prediction because it is directly related to mechanical strain in the elastic stress regime the corresponding dynamic effective stress coefficient is easy to estimate from density and velocity of acoustic elastic waves we studied n and of chalk from the reservoir zone of the valhall field north sea and found that n and vary with differential stress overburden stress pore pressure for valhall reservoir chalk with 40 porosity ranges between 0 98 and 0 85 and decreases by 10 if the differential stress is increased by 25 mpa in contrast for chalk with 15 porosity from the same reservoir ranges between 0 85 and 0 70 and decreases by 5 due to a similar increase in differential stress our data indicate that measured from sonic velocity data falls in the same range as for n and that n is always below unity stress dependent behavior of n is similar decrease with increasing differential stress to that of during elastic deformation caused by pore pressure buildup for example during waterflooding by contrast during the increase in differential stress as in the case of pore pressure depletion due to production n increases with stress while decreases

rise and fall of late pleistocene pluvial lakes in response to reduced evaporation and precipitation evidence from lake surprise california widespread late pleistocene lake systems of the basin and range province indicate substantially greater moisture availability during glacial periods relative to modern times but the climatic factors that drive changes in lake levels are poorly constrained to better constrain these climatic forcing factors we present a new lacustrine paleoclimate record and precipitation estimates for lake surprise a closed basin lake in northeastern california we combine a detailed analysis of lake hydrography and constitutive relationships describing the water balance to determine the influence of precipitation evaporation temperature and seasonal insolation on past lake levels at its maximum extent during the last deglaciation lake surprise covered 1366 km 2 36 of the terminally draining surprise valley watershed using paired radiocarbon and 230 th u analyses we dated shoreline tufa deposits from wave cut lake terraces in surprise valley california to determine the hydrography of the most recent lake cycle this new lake hydrograph places the highest lake level 176 m above the present day playa at 15 19 0 18 calibrated ka 14 c age this significantly postdates the last glacial maximum lgm when lake surprise stood at only moderate levels 65 99 m above modern playa similar to nearby lake lahontan to evaluate the climatic factors associated with lake level changes we use an oxygen isotope mass balance model combined with an analysis of predictions from the paleoclimate model intercomparison project 3 pmip3 climate model ensemble our isotope mass balance model predicts minimal precipitation increases of only 2 18 during the lgm relative to modern compared to an 75 increase in precipitation during the 15 19 ka highstand lgm pmip3 climate model simulations corroborate these findings simulating an average precipitation increase of only 6 5 relative to modern accompanied by a 28 decrease in total evaporation driven by a 7 c decrease in mean annual temperature lgm pmip3 climate model simulations also suggest a seasonal decoupling of runoff and precipitation with peak runoff shifting to the late spring early summer from the late winter early spring our coupled analyses suggest that moderate lake levels during the lgm were a result of reduced evaporation driven by reduced summer insolation and temperatures not by increased precipitation reduced evaporation primed basin and range lake systems particularly smaller isolated basins such as surprise valley to respond rapidly to increased precipitation during late heinrich stadial 1 hs1 post lgm highstands were potentially driven by increased rainfall during hs1 brought by latitudinally extensive and strengthened midlatitude westerly storm tracks the effects of which are recorded in the region9s lacustrine and glacial records these results suggest that seasonal insolation and reduced temperatures have been underinvestigated as long term drivers of moisture availability in the western united states

crustal accretion at high temperature spreading centres rheological control of crustal thickness abstract new determinations of lateral crustal thickness variations at anomalous oceanic spreading centres such as iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides bjarnason and schmeling 2009 to understand this behaviour crustal accretion models have been carried out solving the conservation equations of mass momentum and energy with melting melt extraction and feedback of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume the dynamics of rifting are thermally and rheologically controlled by the feedback due to accreted new crust four accretional modes with characteristic crustal thickness variations are identified depending on the width of the volcanic emplacement zone the accretional heating rate which can be associated with the thickness of the surface layer in which magmatic emplacement takes place and the spreading rate mode 1 zero crustal thickness at the spreading axis develops for cool accretion and a wide emplacement zone mode 2 strongly or moderately crustal thickening away from the axis develops in case of warm deep reaching accretion and wide emplacement zones mode 3 nearly constant crustal thickness develops in case of warm deep reaching accretion but narrow emplacement zones dynamic topography of mode 3 shows only a weak or no regional minimum at all near the axis modes 2 or 3 may be identified with the situation in iceland mode 4 a stagnating central crustal block evolves for cool accretion and narrow emplacement this mode disappears for increasing spreading rates no accretional mode with maximum crustal thickness above the plume at the rift axis has been found the absence of mode 1 accretion with zero crust at ridge axis on earth may be an indication that in general crustal accretion is not cold and shallow the model is also applied to other hotspot ridge settings azores galapagos and suggests modes 2 3 accretion

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

how burial diagenesis of chalk sediments controls sonic velocity and porosity based on p wave velocity and density data a new elastic model for chalk sediments is established the model allows the construction of a series of isoframe if curves each representing a constant part of the mineral phase contributing to the solid frame the if curves can be related to the progress of burial diagenesis of chalk which is revised as follows newly deposited carbonate ooze and mixed sediments range in porosity from 60 to 80 depending on the prevalence of hollow microfossils despite the high porosity these sediments are not in suspension as reflected in ifs of 0 1 or higher upon burial the sediments lose porosity by mechanical compaction and concurrently the calcite particles recrystallize into progressively more equant shapes high compaction rates may keep the particles in relative motion whereas low compaction rates allow the formation of contact cement whereby if increases and chalk forms rock mechanical tests show that when compaction requires more than in situ stress porosity reduction is arrested during subsequent burial crystals and pores grow in size as a consequence of the continuing recrystallization the lack of porosity loss during this process testifies to the absence of chemical compaction by calcite calcite pressure dissolution as well as to the porosity preserving effect of contact cementation at sufficient burial stress the presence of stylolites indicates that pressure dissolution takes place between calcite and silicates and depending on pore water chemistry and temperature pore filling cementation may occur over a relatively short depth interval limestone and mixed sedimentary rock form and porosity may be reduced to less than 20 isoframe increases to more than 0 6 in hydrocarbon reservoirs in north sea chalk relatively high porosity and high ifs are found the reason may be that recrystallization and porosity preserving contact cementation progress whereas pore filling cementation is small probably because pressure dissolution along stylolites is arrested pressure dissolution may be arrested for two reasons 1 the introduction of hydrocarbons causes a fall in effective burial stress and 2 adsorption of polar hydrocarbons on the silicates may shield calcite from the silicates

northwest basin and range tectonic deformation observed with the global positioning system 1999 2003 1 we use geodetic velocities obtained with the global positioning system gps to quantify tectonic deformation of the northwest basin and range province of the western united states the results are based on gps data collected in 1999 and 2003 across five new quasi linear networks in northern nevada northeast california and southeast oregon the velocities show 3 mm yr westward movement of northern nevada with respect to stable north america west of longitude 119 w the velocities increase and turn northwest parallel to sierra nevada great valley microplate motion and similar to velocities previously obtained to the south the observations are explained by a kinematic model with three domains that rotate around euler poles in eastern oregon and western idaho northeast california experiences internal dextral shear deformation 11 2 3 6 nstrain yr subparallel to pacific north america motion relative motions of the domains imply 2 5 mm yr approximately east west extension in northwest nevada and 1 4 mm yr approximately north south contraction near the california oregon border the northward decreasing approximately east west extension in northwest nevada is consistent with the northern termination of basin and range deformation faulting and characteristic topography no significant extension is detected in the oregon basin and range the oregon cascade arc moves north at 3 5 mm yr and is possibly influenced by the approximately eastward motion of the juan de fuca plate these results disagree with secular northwest trenchward motion of the oregon forearc inferred from paleomagnetic rotations south of latitude 43 however trenchward motion exists and is consistent with block rotations approximately east west basin and range extension and northwest sierra nevada translation

gravity field of the southeast central pacific from geosat exact repeat mission data efficient techniques for mapping the marine gravity field with geosat exact repeat mission erm data are demonstrated geosat has completed more than fifty two 17 day global erm repeat cycles and has collected roughly 50 million observations of geoid undulations or more precisely sea surface topography because the global ground track repeats every 17 days the erm observations are highly redundant by averaging the repeat cycles very precise profiles of alongtrack geoid undulations are obtained for a region of the southeast central pacific without any crossing arc orbit adjustments these profiles are shown to be repeatable to within 10 cm over their entire 3000km length i e to three parts in 108 this southeast central pacific region contains a portion of the east pacific rise as well as some cross grain gravity field lineations and is therefore of considerable geophysical interest in addition to profiles of alongtrack deflections of the vertical regularly gridded gravity anomalies are derived for this region and portrayed in color the procedure used for recovering gravity has two stages in stage 1 alongtrack deflections of vertical are derived by fitting cubic splines to alongtrack heights computing alongtrack slopes at standardized latitudes and then averaging slopes for coincident passes in stage 2 gridded band limited gravity anomalies are recovered from deflections by using fast fourier transform techniques to accomplish the necessary inverse vening meinesz transformation high precision gem t1 orbits are used and crossover adjustments are not performed the operational navy astronautics group orbits are shown to have systematic errors which may be as large as 10 m

patterns and timing of late quaternary faulting in the great basin province and relation to some regional tectonic features large scale surface faulting events each tens of kilometers long and involving more than a meter of displacement of late quaternary age have not been uniformly distributed but have been concentrated in subprovincial and smaller belts and areas within the great basin province furthermore faulting apparently has not been uniformly distributed over time the average recurrence interval of large scale faulting events on individual faults in the province is measured in thousands or tens of thousands of years in contrast to such long recurrence intervals in the central nevada and eastern california seismic belts large scale faulting events have occurred during the last 111 years in an apparently coherent sequential belt filling pulse of activity in which events were separated by only a few years or a few decades the questions are where are other pulses of large scale faulting likely to occur and what controls the localization of such belts and areas characterized by higher rates of faulting two examples of tectonic subprovinces that may play a role in delimiting smaller zones of faulting are the black rock carson sink zone of extension and the central nevada downwarp the central nevada seismic belt lies along the dividing line between these two subprovinces and the northwest margin of the black rock carson sink zone of extension coincides with the margin of a fingerlike belt of dense faulting in northwestern nevada

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

shoreline processes and the age of the lake lahontan highstand in the jessup embayment nevada the well developed shoreline record of pluvial lake lahontan in the jessup embayment nevada is used to refine the history of late pleistocene lake level fluctuations and to assess controls on shoreline development and distribution controls on the strength and type of shorelines developed include local slope the amount and characteristics of sediment available for transport the availability of accommodation space and length of time the lake level resides at a particular shoreline elevation at the sehoo highstand and during the early part of the regression strong storm winds and waves from the south southeast set up a clockwise net shore drift pattern near the head of the embayment although significant differences in local slope geometry of the shoreline and wave energy existed in the embayment crestal heights of constructional shoreline features formed at the highstand vary 2 6 m in elevation and hence provide a relatively precise marker of the highstand elevation radiocarbon dating of a camel bone preserved in high shoreline deposits indicates that the lake reached its highest elevation of 1338 5 m in the embayment and receded from that elevation immediately prior to 13 070 60 yr b p similar and slightly older radiocarbon ages on gastropod shells preserved in barrier deposits at 1327 m 13 280 110 yr b p and 1331 m 13 110 110 yr b p suggest that the final rise to the highstand was very rapid and that the lake maintained its highest stand for a very brief period of time perhaps only for years or decades the brevity of the highstand is reasonable in light of the recent formation of similar barrier features in modern pyramid lake which formed in less than seven months due to a rapid increase in lake level

chalk porosity and sonic velocity versus burial depth influence of fluid pressure hydrocarbons and mineralogy seventy chalk samples from four formations in the overpressured danish central north sea have been analyzed to investigate how correlations of porosity and sonic velocity with burial depth are affected by varying mineralogy fluid pressure and early introduction of petroleum the results show that porosity and sonic velocity follow the most consistent depth trends when fluid pressure and pore volume compressibility are considered quartz content up to 10 has no marked effect but more than 5 clay causes lower porosity and velocity the mineralogical effect differs between p wave and shear velocity so that smectite bearing chalk has a high poisson s ratio in the water saturated case but a low value in the dry case oil bearing chalk has up to 25 units higher porosity than water saturated chalk at similar depth but similar velocity probably because hydrocarbons prevent pore filling cementation but not pore structure stiffening cementation in this presumably water wet chalk these results should improve the modeling of chalk background velocity for seismic inversion analysis when describing the porosity reducing process pore volume compressibility should probably be disregarded when correcting for fluid pressure because the cementing ions originate from stylolites which are mechanically similar to fractures we find that cementation occurs over a relatively short depth interval

three dimensional flow beneath a slow spreading ridge axis a dynamic contribution to the deepening of the median valley toward fracture zones the axial valley along a slow spreading mid ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis along a ridge segment the axial valley floor deepens by as much as 2 3 km over distances of several tens of kilometers approaching a ridge transform intersection this deepening may be explained in part by a pressure gradient associated with horizontal flow in the ridge axis conduit vanishing vertical velocity on a conduit end wall at a ridge transform intersection results in reduced vertical flow within about one conduit width of the intersection horizontal flow along the conduit toward the intersection must occur to form lithosphere at a uniform rate along the ridge axis a simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge transform intersections is considered the model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls this model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection for reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection

the deep structure of ocean ridges in a convecting mantle abstract plate tectonic models have frequently assumed that mid ocean ridges are the surface expression of the hot rising limb of a convection cell measured half spreading rates show that in any reference frame the horizontal transverse velocity of some ridges is comparable to the individual plate velocities if the above assumption is correct this observation implies that the thermal structure beneath the ridge can adjust rapidly to the changing surface configuration of the plates numerical experiments in a two dimensional convecting layer of constant viscosity fluid show that such a rapid adjustment is probably unrealistic a symmetrically spreading ridge moving horizontally is simulated by specifying a velocity boundary condition on the upper surface of the layer the other boundary conditions are uncertain so several different combinations are used in the experiments including periodic or reflecting lateral boundaries and stress free or no slip lower boundary in all the experiments the rising sheets of the convection cells are anchored firmly in the mean horizontal flow of the layer unless the mean flow coincides approximately with the ridge velocity the ridge quickly moves away from a rising sheet which is initially beneath it if the aspect ratio width depth of the convection cells is significantly greater than one this conclusion is even more evident in the earth it seems unlikely that there is a systematic deep of the order of the convecting layer thickness structure beneath the worldwide network of ocean ridges

seismic scale petrophysical interpretation and gas volume estimation from simultaneous impedance inversion abstract simultaneous impedance inversion was performed to obtain the seismic scale p and s wave impedances as well as the bulk density volumes from seismic angle stacks at a siliciclastic turbidite offshore gas reservoir to translate these variables into seismic scale total porosity clay content and water saturation sw we used a deterministic rock physics model established at the well a crucial input into this model is the bulk modulus kf of the brine gas system we need to link it to sw at the seismic scale this link can be one of simple mixing laws such as harmonic and arithmetic alternately it can be derived at the well by using upscaled variables and ensuring that they obey the well data driven rock physics model these different kf sw relations produce different interpretations for and sw thus allowing one to assess the variability in the estimates of the gas volume in place that is proportional to the product of and 1 sw

comparison of geodetic and geologic data from the wasatch region utah and implications for the spectral character of earth deformation at periods of 10 to 10 million years the wasatch fault and adjacent fault zones provide an opportunity to compare present day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows ranging from the last millennium up to 10 myr the three easternmost gps sites of the basin and range geodetic network bargen at this latitude define a 130 km wide region spanning three major normal faults extending east west at a total rate of 2 7 0 4 mm yr with an average regional strain rate estimated to be 21 4 nstrain yr about twice the basin and range average on the wasatch fault the vertical component of the geologic displacement rate is 1 7 0 5 mm yr since 6 ka 0 6 mm yr since 130 ka and 0 5 0 7 mm yr since 10 ma however it appears likely that at the longest timescale rates slowed over time from 1 0 to 1 4 mm yr between 10 and 6 ma to 0 2 to 0 3 mm yr since 6 ma the cumulative vertical displacement record across all three faults also shows time variable strain release ranging from 2 to 4 mm yr since 10 ka to 1 mm yr averaged over the past 130 kyr conventional earthquake recurrence models reid type behavior would require an accordingly large variation in strain accumulation or loading rate on a 10 kyr timescale for which there appears to be no obvious geophysical explanation alternatively seismic strain release given a wide range of plausible constitutive behaviors for frictional sliding may be clustered on the 10 kyr timescale resulting in the high holocene rates with comparatively low uniform strain accumulation rates on the 100 kyr timescale wallace type behavior the latter alternative combined with observations at the million year timescale and the likelihood of a significant contribution of postseismic transients implies maxima of spectral amplitude in the velocity field at periods of 10 myr variations in tectonic loading 10 kyr clustered strain release and of 100 years postseismic transients if so measurements of strain accumulation and strain release may be strongly timescale dependent for any given fault system

mount mazama eruption calendrical age verified and atmospheric impact assessed geochemical identification of mount mazama ash in the greenland ice sheet project 2 gisp2 ice core gives a calendrical age of 7627 150 cal yr b p 5677 150 b c for the eruption thus providing a more accurate early holocene stratigraphic time line than previously available the gisp2 record of volcanically derived sulfate suggests a total stratospheric aerosol loading between 88 and 224 mt spread over an 6 yr period following the eruption of mount mazama taking into account the likelihood of some tropospheric aerosol transport to greenland realistic estimates of the resulting atmospheric optical depth range from 0 6 to 1 5 these values may have produced a temperature depression of 0 6 to 0 7 c at mid to high northern latitudes for 1 3 yr after the eruption these results indicate that the 5677 b c eruption of mount mazama was one of the most climatically significant volcanic events of the holocene in the northern hemisphere we also calculate a maximum stratospheric cl release of 8 1 mt by the eruption which may have led to substantial stratospheric ozone depletion

ubiquity of anomalous fading in k feldspars and the measurement and correction for it in optical dating anomalous fading rates were measured in k feldspars separated from 49 sediment samples mainly from north america the intensity of the optically stimulated luminescence was found to decrease linea

continuous gps measurements of contemporary deformation across the northern basin and range province we have acquired and analyzed data from the northern basin and range nbar continuous gps network since july 1996 the rms residual with respect to the best fitting lines through the individual station position estimates is 2 3 mm in the horizontal and 6 10 mm in the vertical after the first 395 days of operation uncertainties in horizontal velocity estimates are 1 2 mm yr 1 relative motion among nbar sites located in eastern nevada and in utah is small but east west extension is significant assuming uniform strain accumulation across the whole network the relative motion observed across the wasatch fault zone is 2 2 mm yr east west relative motions among stations in western nevada and california in contrast are dominated by northwest right lateral shear we infer an integrated deformation across the northern basin and range of 11 2 mm yr northwest these rates are consistent with previous geodetic measurements our gps velocity estimates however reveal a possibly abrupt transition from east west extension in eastern nevada and utah to right lateral shear in western nevada this transition is roughly coincident with the central nevada seismic belt and is consistent with the right oblique focal mechanisms of the 1954 dixie valley and fairview peak earthquakes the transition also appears to correlate spatially with a transition in upper mantle structure

chronology of latest pleistocene lake level fluctuations in the pluvial lake chewaucan basin oregon usa new accelerator mass spectrometer radiocarbon ages from gastropods in shore deposits within the pluvial lake chewaucan basin combined with stratigraphical and geomorpho logical evidence identify an abrupt rise and fall of lake level at ca 12 14 c ka the lake level high is coeval with lake level lows in the well dated records of palaeolakes bonneville and lahontan and with a period of relatively wet conditions in the more southerly owens lake basin this spatial pattern of pluvial lake levels in the western usa at 12 14 c ka indicates a variable synoptic response to climate forcing at this time copyright 2001 john wiley sons ltd

late quaternary paleoseismology of the southern steens fault zone northern nevada the 192 km long steens fault zone is the most prominent normal fault system in the northern basin and range province of western north america we use trench mapping and radiometric dating to estimate displacements and timing of the last three surface rupturing earthquakes e1 e3 on the southern part of the fault south of denio nevada coseismic displacements range from 1 1 to 2 2 0 5 m and radiometric ages indicate earthquake times of 11 5 2 0 ka e3 6 1 0 5 ka e2 and 4 6 1 0 ka e1 these data yield recurrence intervals of 5 4 2 1 k y between e3 and e2 1 5 1 1 k y between e2 and e1 and an elapsed time of 4 6 1 0 k y since e1 the recurrence data yield variable interval slip rates between 0 2 0 22 and 1 5 2 3 mm yr but slip rates averaged over the past 18 k y 0 24 0 06 mm year are similar to long term 8 5 12 5 ma slip rates 0 2 0 1 mm yr measured a few kilometers to the north we infer from the lack of significant topographic relief across the fault in bog hot valley that the fault zone is propagating southward and may now be connected with a fault at the northwestern end of the pine forest range displacements documented in the trench and a rupture length of 37 km indicate a history of three latest quaternary earthquakes with magnitudes of m 6 6 7 1 on the southern part of the steens fault zone

simultaneous melting and compaction in deformable two phase media summary melt generation and extraction are typically modelled using the two phase equations developed by mckenzie or scott and stevenson various approximations are often made to simplify the problem which may lead to some unphysical results e g thermodynamically inconsistent conditions of melting and unrealistic porosity profiles we discuss a generalized version of the set of equations introduced by bercovici et al that allows for mass transfer between the two phases in a single component system and consider a self consistent set of equations in our description the two phases solid and melt are submitted to individual pressure fields whose difference is related to the surface tension at the interfaces changes in porosity and the melting rate a kinetic relation for the melting rate arises from the second law of thermodynamics the condition of chemical equilibrium corresponds to the usual univariant equality of the chemical potentials of each phase when the matrix and melt are motionless in the most general form phase equilibrium is influenced by both the gibbs thomson effect that arises naturally from thermodynamic considerations on surface tension and by the viscous deformation of the phases we apply these new equations to a steady state problem of pressure release melting in a univariant system we treat melting and compaction simultaneously and observe several new effects including multiple domains near the onset of melting that correspond to various force balances a consequence of matrix compaction and melt expulsion or matrix dilation and melt accumulation is a pressure difference between melt and solid that facilitates inhibits melting for parameters corresponding to mid oceanic ridge magmatism compaction permits melting to start as much as 2k m below the standard solidus numerical solutions are necessary to determine the magnitude of the melt zone shift numerical results support the boundary layer solutions obtained analytically and suggest that in most of the melting zone the movement of melt and matrix should be close to the darcy equilibrium where the buoyancy of melt is balanced by the viscous drag between the phases the darcy equilibrium follows an initial stage where the matrix viscous stresses balance darcy drag in all situations the steady state porosity profile remains a monotonic function of depth the existence of a compaction layer following a melting zone where the porosity is maximum as described in various earlier publications has never been found

porosity mineralogy and pore fluid from simultaneous impedance inversion we find that in a quartz clay system p and s wave impedances ip and is respectively each uniquely depend on a different linear combination of the total porosity and clay content this implies that if the pore fluid is known we can resolve these two seismically derived impedances for porosity and clay content key to such interpretation is rock physics diagnostics that provide a theoretical rock physics model to quantitatively explain well data by relating ip and is to porosity clay content and pore fluid the well data conditioned according to this model serve as input to simultaneous impedance inversion poisson s or the ip is ratio serves as the pore fluid identifier we give an example of such rock physics based interpretation of seismically derived impedances for rock properties based on well and seismic data from an offshore oil field

fault orientations in extensional and conjugate strike slip environments and their implications seismically active conjugate strike slip faults in california and japan typically have mutually orthogonal right and left lateral fault planes normal fault dips at earthquake nucleation depths are concentrated between 40 and 50 the observed orientations and their strong clustering are surprising because conventional faulting theory suggests fault initiation with conjugate 60 and 120 intersecting planes and 60 normal fault dip or fault reactivation with a broad range of permitted orientations the observations place new constraints on the mechanics of fault initiation rotation and evolutionary development we speculate that the data could be explained by fault rotation into the observed orientations and deactivation for greater rotation or by formation of localized shear zones beneath the brittle ductile transition in earth9s crust initiation as weak frictional faults seems unlikely

structural controls on geothermal circulation in surprise valley california a re evaluation of the lake city fault zone faults and fractures play an important role in the circulation of geothermal fluids in the crust and the nature of that role varies according to structural setting and state of stress as a result detailed geologic and geophysical mapping that relates thermal springs to known structural features is essential to modeling geothermal systems published maps of surprise valley in northeastern california suggest that the lake city fault or lake city fault zone is a significant structural feature cutting obliquely across the basin and connecting thermal springs across the valley newly acquired geophysical data audio magnetotelluric gravity and magnetic combined with existing geochemical and geological data suggest otherwise we examine potential field profiles and resistivity models that cross the mapped lake city fault zone while there are numerous geophysical anomalies that suggest subsurface structures they mostly do not coincide with the mapped traces of the lake city fault zone nor do they show a consistent signature in gravity magnetics or resistivities that would suggest a through going fault that would promote connectivity through lateral fluid flow instead of a single continuous fault we propose the presence of a deformation zone associated with the growth of the range front surprise valley fault the implication for geothermal circulation is that this is a zone of enhanced porosity but lacks length wise connectivity that could conduct fluids across the valley thermal fluid circulation is most likely controlled primarily by interactions between n s trending normal faults

paleoseismic transect across the northern great basin 1 the relationship of strain accumulation to strain release over different timescales provides insight to the dynamics structural development and spatial and temporal pattern of earthquake recurrence in regions of active tectonics the great basin physiographic province of the western united states is one of the earth s broadest regions of ongoing continental extension encompassing an area reaching 800 km in width between the sierra nevada to the west and wasatch mountains to the east we present observations arising from excavations scarp profiling optically stimulated luminescence and radiocarbon dating to place limits on the late pleistocene paleoseismic history of faults bounding eight ranges across the interior of the northern great basin specifically the shawave hot springs humboldt sonoma shoshone tuscarora dry hills and pequop ranges combining the observations with similar previously published studies within and at the margins of the great basin yields a transect that extends eastward across the basin between the 40th and 41st parallels the sum of observations provides a picture of the patterns and rates of earthquake recurrence over the region during the last 20 45 kyr that may be compared to patterns of contemporary seismicity and recently reported measures of strain accumulation across the area using gps the recurrence rate of large surface rupture paleoearthquakes along ranges at the margins of the great basin is systematically greater than observed along ranges in the interior the pattern is similar to seismological and geodetic measurements that show levels of background seismicity and strain accumulation are also concentrated along the margins of the great basin an east west extension rate across the interior of the great basin on the order of 1 2 mm yr 1 strain rate of 1 nstrain yr 1 over the last 20 45 kyr is estimated by summing the record of paleoseismic displacements across the 400 km breadth of the transect as compared to 2m m yr 1 of strain accumulation indicated by a recently reported analysis of a collinear gps survey the comparison is hindered by significant uncertainties coupled to the geologic rate estimate the transect also crosses the northern limit of the central nevada seismic belt the central nevada seismic belt is defined by a north northeast trending alignment of historical surface rupture earthquakes increased levels of background seismicity and strain accumulation rates greater than observed elsewhere in the interior of the great basin the reported recurrence rate of late pleistocene surface rupture earthquakes within the central nevada seismic belt is also generally greater than observed along our transect the observations when taken together suggest that the characteristics of strain release observed historically within the central nevada seismic belt have been operative over the latest pleistocene and that the apparently greater rates of strain accumulation and release in the central nevada seismic belt are diminished or less localized in regions to the north and east thus while the historical alignment of surface ruptures that defines the central nevada seismic belt remains a unique clustering of earthquakes in time and space the likelihood of the cluster at its observed location appears greater than would be expected to the north or eastward in the interior of the great basin

dynamical consequences of compositional and thermal density stratification beneath spreading centers extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers this compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow the relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity for a low enough mantle viscosity buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone for a mantle viscosity of 1022 pa s upwelling due to plate spreading is dominant and crustal thickness is predicted to be a strong function of spreading rate however for a mantle viscosity of 1019 pa s buoyancy dominates mantle upwelling and the calculated crustal thickness like the oceanic crust is nearly independent of spreading rate thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling for a mantle viscosity of 1020 pa s and a low 1 cm yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single well defined period for even slightly lower mantle viscosity the frequency increases and crustal thickness oscillations become more complex with two or more frequencies present while numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneath spreading centers we speculate that these crustal thickness oscillations may explain important aspects of seafloor structure

the northwestern margin of the basin and range province part 1 reflection profiling of the moderate angle 30 surprise valley fault abstract seismic reflection profiling demonstrates that the active significant offset surprise valley fault that marks the western boundary of the basin and range province in northernmost california dips at a moderate angle only 30 to 2 km depth a nearby seismic refraction wide angle reflection profile albeit of lower resolution shows the fault plane remains approximately planar to at least 7 km depth variably tilted volcanic strata of known ages within the surprise valley basin demonstrate that 7 km of normal slip occurred along the surprise valley fault within the past 8 4 myr at a time averaged slip rate of 1 2 mm yr and that the fault rotated from an initial dip of as much as 60 to its present dip of 30 the rotation of this isolated fault was not accomplished by domino style rotation as seen elsewhere in the basin and range province but rather by flexural rift shoulder uplift aided by mid crustal flow

slip rate variations on normal faults during glacial interglacial changes in surface loads geologic and palaeoseismological data document a marked increase in the slip rates of the wasatch fault and three adjacent normal faults in the basin and range province during the late pleistocene early holocene epochs the cause of this synchronous acceleration of fault slip and the subsequent clustering of earthquakes during the holocene has remained enigmatic although it has been suggested that the coincidence between the acceleration of slip and the shrinkage of lake bonneville after the last glacial maximum may indicate a causal relationship here we use finite element models of a discrete normal fault within a rheologically layered lithosphere to evaluate the relative importance of two competing processes that affect fault slip postglacial unloading the removal of mass which decreases the slip rate and lithospheric rebound which promotes faster slip we show that lithospheric rebound caused by regression of lake bonneville and deglaciation of adjacent mountain ranges provides a feasible mechanism for the high holocene rates of faulting in the wasatch region our analysis implies that climate controlled changes in loads applied to earth s surface may exert a fundamental control on the slip history of individual normal faults

two phase neogene extension in the northwestern basin and range recorded in a single thermochronology sample we use a combination of apatite 4 he 3 he u th he and fission track thermochronology to date slip on the surprise valley fault in northeastern california by analyzing a single sample from the warner range in the footwall of the fault this sample a granitic clast from a conglomerate yielded a fission track age of 11 6 2 8 ma and a u th he age of 3 02 0 52 ma geologic relationships indicate that this sample was buried to a depth of 3 3 km prior to exhumation during slip on the surprise valley fault fission track age and length data indicate that the sample was fully reset 120 c prior to exhumation which began sometime after 14 ma a single aliquot of nine apatite grains was step heated for 4 he 3 he analysis modeling of the resulting 4 he distribution indicates that cooling from 80 c to 20 c occurred between 3 and 1 ma interconsistent time temperature t t solutions to the combined 4 he 3 he u th he and fission track data require two distinct periods of cooling consistent with non continuous slip on the surprise valley fault early cooling and fault slip took place between 14 and 8 ma followed by more recent fault slip ca 3 ma this timing is consistent with both local geologic relationships and with the regional timing of faulting along the western margin of the basin and range province these data demonstrate the resolving power of combined fission track u th he and 4 he 3 he thermochronometric data to extract low temperature t t information from a single sample close to earth9s surface

level of lake lahontan during deposition of the trego hot springs tephra about 23 400 years ago abstract the trego hot springs tephra bed is a silicic tephra about 23 400 yr old found at several localities in pluvial lake sediments in northern nevada southern oregon and northeastern california it has been characterized petrographically by the major and minor element chemistry of its glass and by its stratigraphic position with respect to other tephra layers at a newly described locality on squaw creek northwest of gerlach nevada at the north end of the smoke creek desert trego hot springs tephra has been found in sediments of the sehoo and indian lakes formations the depositional environments of these sediments show that when the tephra fell pluvial lake lahontan stood between 1256 and 1260 m and that immediately thereafter the lake rose to at least 1275 m these data corroborate earlier findings by benson quaternary research 9 300 318 from radiometric dating of calcareous tufa however the lake lahontan area has been affected by isostatic subsidence and rebound in response to changing water loads so that caution is required in the use of lakeshore elevations in correlation

pattern and rates of faulting in the central nevada seismic belt and paleoseismic evidence for prior beltlike behavior the central nevada seismic belt cnsb is a concentration of historical 1915 1932 1954 surface faulting in the western basin and range province forming a linear nearly continuous 300 km long rupture zone previous results are integrated in this study with new paleoseismic and exploratory trenching data from the historical zones to look for evidence of older similar beltlike patterns or elevated slip rates that could indicate whether the cnsb is a zone of focused long term crustal strain the data show that the continuous rupture belt produced by the seven earthquakes occurring between 1915 and 1954 is unique in the available paleoseismic record at the 1954 fairview peak fault the lack of prehistorical faulting in deposits containing the wilson creek bed 19 tephra eliminates the possibility of an identical seismic belt in the past 35 4 ka our studies also show that the faults have net slip rates ranging from a low of 0 09 mm yr on the fairview peak fault to a high of 0 7 mm yr on the 1932 cedar mountain fault these are considered moderate to low rates that are similar to most late quaternary faults in the western basin and range province a space time comparison shows that the paleoseismic histories for these multiple rupture zones are diverse and the number and timing of events in each of the zones indicate that there is little evidence for older contemporaneous ruptures of these same faults based on these results we reach several conclusions regarding the longer term holocene behavior of the cnsb although paleoseismic data preclude an older identical rupture belt among the historical zones consideration of associated holocene faults within the greater cnsb region indicates that several similar but not identical beltlike rupture patterns are plausible during the past 13 ka although each requires seismic gaps in the along strike pattern although long term strain represented by density of young faults does appear to increase from east to west into the cnsb the slip rate data demonstrate that the cnsb is not a belt of concentrated or elevated crustal strain compared with areas that extend west to the sierra nevada the increase in the distribution of holocene fault activity from east to west into the cnsb is consistent with a marked increase in the 1992 2002 gps velocity field at the latitude of the 1954 rupture sequence however a comparison of the geologic rates across the belt at this same latitude indicates that the extension rates 0 59 1 37 mm yr are systematically lower than both the campaign and continuous gps rates 2 20 3 13 mm yr by factors of 2 5 these discrepancies may be due to postseismic strain or to some form of off fault deformation we conclude that the results of our study of fault behavior in the cnsb best support the belt migration model proposed by wallace 1987 for the western basin and range province in which temporal tectonic pulses are believed to migrate regionally activating different beltlike combinations of late quaternary faults in an as yet unknown pattern of migration

energy dissipation rates at ridge and transform fault segments a model of oblique spreading we present an energy dissipation model that can easily be applied to oblique transform faults as well as to oblique spreading ridges by using a simple model of a rising incompressible newtonian fluid in a vertical conduit of the ridge transform fault system the energy dissipation rates of the ridge and transform fault segments are calculated as a function of the angles between the strikes of segments and the spreading directions and the spreading rates the stability is estimated for about 400 present day segments from the energy dissipation rates of ridge and transform fault segments it is concluded that most of the present day configurations of the ridge transform fault system are nearly stable even though the strikes of ridge and transform fault segments are oblique to the spreading directions several exceptions seem to be associated with extraordinary phenomena such as the occurrence of an extremely large earthquake and overlapping spreading centers

a physical model for porosity reduction in sandstones the experimental elastic moduli porosity trends for clean sandstones can be described by the modified upper hashin shtrikman muhs bound one geometrical but not necessarily geological realization is as porosity decreases the number of the pores stays the same and each pore shrinks while maintaining its shape this concept of uniform porosity reduction implies that permeability is proportional to the effective porosity squared and that formation factor is proportional to the inverse of the effective porosity the effective porosity here refers to the part of the pore space that dominates fluid flow the proposed relations for permeability and formation factor agree well with the experimentally observed values these laws are different from the often used forms of the kozeny carman equation and archie s law where permeability is proportional to the total porosity cubed and formation factor is proportional to the inverse of the total porosity squared respectively we suggest that the uniform porosit

crustal deformation across the sierra nevada northern walker lane basin and range transition western united states measured with gps 2000 2004 1 global positioning system gps data collected in campaigns in 2000 and 2004 were processed and interpreted with other gps data in the western basin and range province to provide new constraints on the rate style and pattern of deformation of the central and northern walker lane wl which lies near the western boundary of the basin and range across the central wl near 38n latitude the velocities with respect to north america increase westward by 10 mm yr inducing dextral shear farther north between 40 and 41n latitude a western zone of 7 mm yr relative motion undergoes dextral shear and an eastern zone of 3 mm yr relative motion undergoes extension and shear these data show that the northern wl is essentially a dextral shear zone experiencing minor net dilatation ed 2 6 0 8 nstrain yr near most holocene normal faults dilatation inferred from the velocity field is not greater than the uncertainties however near the central nevada seismic belt we detect significant dilatation expressed as extension in a direction approximately normal to the range fronts ed 23 0 3 9 nstrain yr some of which is attributable to transient postseismic deformation following large historic earthquakes a block model constrained by velocities corrected for transient effects shows that the sum of dextral slip rates across the honey lake warm springs east pyramid fault system and mohawk valley faults is 7 mm yr the wl is a zone whose width and dilatation rate increase northwestward consistent with counterclockwise rotation of the sierra nevada microplate and transfer of deformation into the pacific northwest

the northwestern margin of the basin and range province part 2 structural setting of a developing basin from seismic and potential field data abstract surprise valley in northeastern california offers an ideal opportunity to examine the structural setting of a developing extensional basin due to its late miocene to recent activity in isolation from other major normal fault bound basins seismic velocity and potential field modeling help determine the nature of basin fill and identify intra basin faults based on a detailed gravity and magnetic profile we identify shallow subsurface basalt flows and several faults within the valley that may accommodate hundreds of meters of vertical offset possibly cutting and offsetting the 30 east dipping surprise valley fault that rotated during footwall tilting of the adjacent warner mountains some of these intra basin faults correspond with mapped quaternary fault scarps but others have no surface expression these faults may represent the currently active fault system within the basin if so they would indicate that basin development is transitioning away from the main range front normal fault to a new set of steep intra basin faults that are more favorable for accommodating regional transtensional strain

unusually low rates of slip on the santa rosa range fault zone northern nevada the santa rosa range fault zone srrfz is one of the most topographically prominent normal fault systems in the northern basin and range province of the western united states it has been assigned high rates of vertical slip by others and has been identified as a possible site of the future extension of the central nevada seismic belt cnsb we use detailed trench mapping and luminescence dating to estimate displacements and timing of the last several large magnitude paleoearthquakes on the southern part of the srrfz at a trench site near orovada nevada coseismic vertical displacements ranged from 1 to 2 8 m for each of the last four events luminescence ages provide time limits for the last three events of 125 155 ka 90 108 ka and 11 16 ka these data yield recurrence intervals of 17 65 k y and 74 97 k y and an elapsed time of 11 16 k y since the youngest event slip rate determinations at the orovada site are complicated by multiple fault strands but rates calculated from a variety of data are surprisingly low 0 01 0 16 mm yr given the topographic prominence of the santa rosa range a lack of compelling patterns in a comparison of paleoseismic parameters indicate that the srrfz is no more likely a location for a large magnitude earthquake than previously identified seismic gaps or along faults that lie directly north of the cnsb

factors controlling elastic properties in carbonate sediments and rocks carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks in particular cementation and dissolution processes continuously modify the pore structure to create or destroy porosity in extreme cases these modifications can completely change the mineralogy from aragonite calcite to dolomite or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock figure 1 all these modifications alter the elastic properties of the rock and therefore the sonic velocity the result is a dynamic relationship among diagenesis porosity pore type and sonic velocity the result is a wide range of sonic velocity in carbonates in which compressional wave velocity vp ranges from 1700 to 6600 m s and shear wave velocity vs from 600 to 3500 m s

constraints on shallow mantle viscosity from morphology and deformation of fast spreading ridges 1 we show that the morphology and the deformation of a fastspreadingridgeconstrainshallowuppermantleviscosity a fast spreading center is simulated in a numerical model that couples tectonic deformation due to plate spreading and periodic dike emplacements the amount of magma intruded into dikes or extruded is enough to make a 7 km thick crust and the axial lithosphere is fixed at 2 km thickness for low viscosities 10 19 pa s the model spreadingcenter is marked by an axial high with relief governed by the axial density structure for high viscosities 10 20 pa s the axial morphology is a valley and extensional brittle deformation is distributed away from the near axis region the observed morphology and deformation of fast spreading ridges are not consistent with the high mantle viscosity estimated for very dry olivine as some suggest may result from partial melting related dehydration citation choi e and w r buck 2010 constraints on shallow mantle viscosity from morphology and deformation of fast spreading ridges geophys res lett 37 l16302 doi 10 1029 2010gl043681

burial stress and elastic strain of carbonate rocks burial stress on a sediment or sedimentary rock is relevant for predicting compaction or failure caused by changes in e g pore pressure in the subsurface for this purpose the stress is conventionally expressed in terms of its effect the effective stress defined as the consequent elastic strain multiplied by the rock frame modulus we cannot measure the strain directly in the subsurface but from the data on bulk density and p wave velocity we can estimate the rock frame modulus and biot s coefficient and then calculate the effective vertical stress as the total vertical stress minus the product of pore pressure and biot s coefficient we can now calculate the elastic strain by dividing effective stress with the rock frame modulus by this procedure the degree of elastic deformation at a given time and depth can be directly expressed this facilitates the discussion of the deformation mechanisms the principle is illustrated by comparing carbonate sediments and sedimentary rocks from the north sea basin and three oceanic settings a relatively shallow water setting dominated by coarse carbonate packstones and grainstones and two deep water settings dominated by fine grained carbonate mudstones and wackestones

matrix microcrystalline structure and acoustic properties of oomoldic dolograinstone previous studies have shown that carbonate rock with rounded moldic macropores has a higher compressional velocity at a given porosity than rock with interparticle or intercrystalline porosity and high aspect ratio macropores our study of permian dolograinstone challenge this comonlny accepted results petrophysical and acoustic velocities were measured on 55 mini core plugs collected along an approximately 1 kilometer km long horizontal transect within a single grainstone unit having lateral changes from grainstone with dominantly interparticle porosity to grainstone with mostly moldic porosity the studied dolograinstone samples have a large scattering of acoustic velocity up to 1 500 meters per second m s at equal porosity one some oomoldic grainstone samples have either higher velocity or significantly lower velocity than that of the intercrystalline grainstone samples petrographic analysis under plain light and uv light combined with mercury injection capillary pressure micp data and scann

moderate rates of late quaternary slip along the northwestern margin of the basin and range province surprise valley fault northeastern california 1 the 86 km long surprise valley normal fault forms part of the active northwestern margin of the basin and range province in northeastern california we use trench mapping and radiocarbon luminescence and tephra dating to estimate displacements and timing of the past five surface rupturing earthquakes on the central part of the fault near cedarville a bayesian oxcal analysis of timing constraints indicates earthquake times of 18 2 2 6 10 9 3 2 8 5 0 5 5 8 1 5 and 1 2 0 1 ka these data yield recurrence intervals of 7 3 4 1 2 5 3 2 2 7 1 6 and 4 5 1 5 ka and an elapsed time of 1 2 0 1 ka since the latest surface rupturing earthquake our best estimate of latest quaternary vertical slip rate is 0 6 0 1 mm a this late quaternary rate is remarkably similar to long term 8 14 ma minimum vertical slip rates 0 4 0 5 0 3 mm a calculated from recently acquired seismic reflection and chronologic and structural data in surprise valley and the adjacent warner mountains however our slip rate yields estimates of extension that are lower than recent campaign gps determinations by factors of 1 5 4 unless the fault has an unusually shallow 30 35 dip as suggested by recently acquired seismic reflection data coseismic displacements of 2 4 5 1 m documented in the trench and probable rupture lengths of 53 65 km indicate a history of latest quaternary earthquakes of m 6 8 7 3 on the central part of the surprise valley fault

the mid ocean ridge axial valley as a steady state neck abstract in this paper the mid ocean ridge axial valley is modelled as a steady state lithospheric neck in which lithospheric stretching balances lithospheric accretion conversely the axial high is a steady state lithospheric bulge the lithosphere is modelled as a thin plate with a newtonian rheology it is shown that an axial valley will occur if the rate of viscosity increase away from the ridge axis is faster than the rate at which accretion decreases an axial high will occur if the opposite condition holds this is consistent with the observation that axial valleys occur at low spreading rates and axial highs at high spreading rates by fitting our model to profiles across the mid atlantic ridge and the east pacific rise and assuming the lithospheric thickness at the ridge axis to be 5 km we find accretion widths of 6 8 km we find the width over which there is a significant increase in lithospheric viscosity to be also 6 8 km

porosity permeability relationships in miocene carbonate platforms and slopes seaward of the great barrier reef australia odp leg 194 marion plateau this paper reports a series of 700 porosity permeability analyses and supporting petrographic and sedimentologic descriptions from early to late miocene carbonate strata cored on the marion plateau offshore from north eastern australia during ocean drilling program odp leg 194 the samples analysed are not only mainly coarse bioclastic limestones and dolomitized equivalents from platform top facies but also include 79 plugs from deeper water slope to hemipelagic drift facies outstanding characteristics of this data set are the wide ranges of porosity and permeability in both limestones and dolostones the large degree of short range heterogeneity typical of these strata and the better porosity permeability correlation of dolostones than limestones the platforms have experienced widely varying calcite cementation dolomitization and dissolution but show little clear evidence of meteoric diagenesis suggesting that subaerial exposure may have played little role in porosity permeability evolution permeability for given porosity is controlled by grain size and calcite cement content in grainstones and by occurrence of larger shelter pores and vugs in mud rich samples dolomitization tends to reduce the variation of permeability for given porosity by recrystallizing mud matrix to form intercrystalline macroporosity that connects vugs and moulds to become integrated with the effective pore system as a result there are no differences in permeability porosity trends for different dolostone textures whether dominated by intercrystalline vuggy or preserved intergranular pore types two platform top sites separated by only 5 km display a major lateral variation in dolostone porosity permeability characteristics within the youngest dolostone units this difference is interpreted as reflecting a relatively windward current facing setting of the site with the overall higher permeability for given porosity site 1199 that led to less muddy depositional facies greater cementation and lesser grain dissolution pore geometry parameters measured by petrographic image analysis confirm that the windward dolostones have pores that are both larger and less intricate than dolostones comprising the more current protected location

geochronology structure and basin range tectonism of the warner range northeastern california the warner range of northeastern california is a north trending horst at the northwest margin of the great basin the 3 000 m thick bedrock section exposed in the southern part of the range is tertiary in age and it dips homoclinally 20 to 25 west southwest much of the section consists of volcanic rocks whose k ar ages range from about 32 m y near the base of the section to about 14 m y at the top rocks older than about 26 m y are principally andesitic lahars those between 26 and 17 m y are andesitic lava flows and rhyolitic welded ash flow tuffs and those younger than about 17 m y are principally basaltic and andesitic lava flows the homoclinal character of the warner section indicates that block faulting responsible for uplift and tilting of the range began after emplacement of the youngest exposed rocks drilling in the deeply alluviated graben adjacent to the eastern face of the range indicates that aggregate vertical offset across the range front fault is at least 3 600 m the difference in elevation between alluvium in the bottom of the deepest bore hole and the crest of the nearby part of the range thus vertical displacement has occurred at a minimum average rate of 26 cm 1 000 yr during the past 14 m y offset of alluvium along the eastern base of the range records the continuation of block faulting into holocene time the onset of block faulting somewhat less than 14 m y ago and the change in character of volcanic rocks from principally andesitic to principally basaltic in mid miocene time are similar to volcano tectonic events of a much broader region proposed by previous workers the warner range is remarkable for the nearly complete record of regional late cenozoic volcanic and tectonic events found within a single horst

geochemistry of chromitites and host rocks from the pindos ophiolite complex northwestern greece abstract representative chromite samples of schlieren and massive ore types from various localities of the pindos ophiolite complex were analyzed for platinum group elements pge s ni co and cu and mineral chemistry host dunites and harzburgite were also analyzed for the chalcophile elements the chromite ores show a wide compositional variation the cr cr al ratio ranges from 0 47 to 0 83 while mg mg fe 2 ranges between 0 42 and 0 73 but two populations can be distinguished the high cr and high al types which show a more restricted variation ranges of pge contents in ppb are os 8 150 ir 4 320 ru 15 550 rh 2 82 pt 3 150 and pd 1 5 20 gold ranges between 1 and 13 ppb the pge and less chalcophile elements ni co and cu the incompatible compatible element ratios the pge s ratio and pge patterns provide valuable information for the discrimination of chromite ores with a similar major element composition a fractionation trend in both chromitites and associated dunites can be used for stratigraphic orientation in the mantle sequences of ophiolite complexes and evaluation of their chromite potential

effects of partial melting on the uranium decay series distinction between possible partial melting processes using stable incompatible trace elements depends largely on the mass fraction of liquid produced at low degrees of melting less than 5 both incompatible element concentrations and interelement fractionations are greater for batch fractional and accumulated fractional melting than for dynamic or continuous melting at higher degrees of melting uncertainty in initial source composition and small fractionations between incompatible elements make it unlikely that the most realistic melting models batch and dynamic could be discriminated however it is possible for these melting processes to be distinguished by the magnitude of radioactive disequilibria that is produced within the 238u decay series even for durations of partial melting much longer than the half lives of 230th and 226ra both230th 238u and 226ra 230th disequilibria can occur because of differences in the bulk solid liquid partition coefficients for these elements the magnitude of these disequilibria depends on melting process the 230th 232th of a dynamic melt can differ significantly from that of a batch melt and will not necessarily indicate the original 238u 232th of the source if morb is formed by a dynamic melting process then the th u of the upper mantle near ridge axes may be higher than that calculated from the 230th 232th of the basalts

episodic kinematics in continental rifts modulated by changes in mantle melt fraction gps measurements are used to investigate the continental rift system of north island new zealand and to develop a model for short term deformation involving flexing of an elastic upper crust caused by episodic changes in mantle viscosity and melt fraction direct observation of mantle flow dynamics beneath spreading oceanic crusts is difficult but similar processes occur below rifts in the continental crust and so can be inferred from satellite observations of the moving crust simon lamb et al use ten years of continuous gps measurements from a back arc continental rift system in north island new zealand to investigate the dynamics of mantle flow beneath seafloor spreading centres they observe deformation which they conclude to be the result of flexure of an elastic upper crust pulled downwards or pushed upwards along the rift axis by a driving force at depth they propose that this flexure is caused by melt induced episodic changes in mantle viscosity in their model a drop in melt fraction due to melt extraction raises the mantle viscosity and drives subsidence whereas melt accumulation reduces viscosity and allows uplift oceanic crust is created by the extraction of molten rock from underlying mantle at the seafloor spreading centres found between diverging tectonic plates modelling studies have suggested that mantle melting can occur through decompression as the mantle flows upwards beneath spreading centres1 but direct observation of this process is difficult beneath the oceans continental rifts however which are also associated with mantle melt production are amenable to detailed measurements of their short term kinematics using geodetic techniques here we show that such data can provide evidence for an upwelling mantle flow as well as information on the dimensions and timescale of mantle melting for north island new zealand around ten years of campaign and continuous gps measurements in the continental rift system known as the taupo volcanic zone reveal that it is extending at a rate of 6 15 millimetres per year however a roughly 70 kilometre long segment of the rift axis is associated with strong horizontal contraction and rapid subsidence and is flanked by regions of extension and uplift these features fit a simple model that involves flexure of an elastic upper crust which is pulled downwards or pushed upwards along the rift axis by a driving force located at a depth greater than 15 kilometres we propose that flexure is caused by melt induced episodic changes in the vertical flow forces that are generated by upwelling mantle beneath the rift axis triggering a transient lower crustal flow a drop in the melt fraction owing to melt extraction raises the mantle flow viscosity and drives subsidence whereas melt accumulation reduces viscosity and allows uplift processes that are also likely to occur in oceanic spreading centres

chemical and isotopic variations along the superfast spreading east pacific rise from 6 to 30 s chemical data of 39 fresh basaltic glasses from the east pacific rise epr between 6 and 30 s and pb sr and nd isotopic compositions of 12 basalt glasses are presented major and trace element data indicate a wide compositional range including primitive basalts mg 0 67 and highly evolved feti basalts mg 0 34 molmg mg fe2 the compositional range can be attributed to low pressure fractional crystallization fractionation corrected major element concentrations provide evidence for varying mantle melting conditions calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar at temperatures between 1430 and 1280 c and total degrees of partial melting between 17 and 20 by weight leached and hand picked basalt glasses display large variations in 87sr 86sr 0 70235 0 70270 143nd 144nd 0 51312 0 51323 and 206pb 204pb 18 064 18 665 but are similar to other n type morb from the epr the isotopic ratios of basalts from 13 to 23 s show strong correlations and delineate two systematic trends from 23 to 17 s 87sr 86sr and pb isotope ratios increase and 143nd 144nd decrease in agreement with previous results mahoney et al 1989 a reverse trend is indicated by basalts from 17 to 13 s however k ti and la sm n continuously increase from 23 to 13 s this opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17 s north of 13 5 s garrett fracture zone isotopic data show no systematic variation with ridge location and display an overall weaker covariation the results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions p t f f degrees of melting there is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30 s we interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components in contrast variations in trace elements are attributed to young mantle differentiation processes

on the dynamics of mid ocean ridges most extant flow models for ocean ridges suffer either from unrealistic geometry vertical conduit models or a stress singularity at the ridge axis corner flow models i present a model which avoids these difficulties by incorporating a weak plate boundary zone pbz between diverging surface plates an analytical solution for the flow of a constant viscosity asthenosphere and a coexisting melt phase is obtained in elliptic coordinates and used to predict the dynamically supported topography axial valley on the pbz the force required to pull the plates apart and the rate of melt extraction at the ridge stress in the asthenosphere increases linearly with spreading rate but decreases as the plates become thinner and the dynamics of ridge crests is controlled by the balance between these opposing effects the depth of the axial valley is controlled by the plate thinning effect and is therefore a decreasing function of spreading rate the depth of the axial valley on the mid atlantic ridge can be explained if the viscosity of the asthenosphere is 6 1019 1021 pa s and the lower surface of the plate is the 600 c isotherm the plate separation force is controlled by the spreading rate effect and therefore increases with spreading rate about five times as much force is required to maintain a fast spreading ridge 9 cm yr 1 half rate as a slow spreading ridge 1 cm yr 1 melt extraction can produce 6 km of basaltic crust if the volume fraction of melt present in the asthenosphere total degree of melting ranges from about 2 5 4 0 for a slow spreading ridge to 4 0 6 5 for a fast spreading ridge the dynamics of melt extraction does not depend significantly on the finite width of the pbz

a controlled source seismic and gravity study of the high lava plains hlp of eastern oregon 1 this study employs data collected during the high lava plains hlp controlled source seismic experiment conducted in september 2008 in this experiment 2612 short period seismic recorders and 120 3 component recorders were deployed across eastern oregon and adjacent parts of nevada and idaho to record 15 seismic sources seismic and gravity data were integrated to create 2 d crustal scale p wave velocity and density models for the 400 km long nw se and n s profiles to provide a better understanding of the crustal and upper mantle structure and ultimately the magmatic and tectonic evolution of the region these models are the first high resolution images beneath the path of migratory bi modal volcanism that dotted the high lava plains beginning at 16 ma our models show that the crustal structure across the hlp region is similar to that of the northern basin and range which has experienced extension since 35 ma but with moderate magmatic modification of the crust a thick layer 5 7 km of sediments and volcanics extends over most of the area and is thickest in the harney basin area we interpret denser faster material in the lower to middle crust under the southern harney basin area to be mafic intraplating we have also identified a region of denser faster material in the upper crust in the vicinity of oregon idaho border the crust thickens from 34 to 37 km and the lower crust increases in density 2 8 2 85 g cm3 from west to east across eastern oregon in close proximity to the interpreted position of the 0 706 sr isotope line suggesting a decrease in extension in the lowermost crust below the southeastern hlp there is relatively high velocity 7 2 7 4 km s and density 2 95 g cm3 that suggests the presence of underplating the hlp region has undergone moderate extension and the average crustal velocity is somewhat higher than in the adjacent basin and range suggesting some magmatic modification in the lower crust but not as much as might be expected given the voluminous surface volcanism

geologic context of geodetic data across a basin and range normal fault crescent valley nevada geodetic strain and late quaternary faulting in the basin and range province is distributed over a region much wider than historic seismicity which is localized near the margins of the province in the relatively aseismic interior both the magnitude and direction of geodetic strain may be inconsistent with the holocene faulting record we document the best example of such a disagreement across the ne striking 55 nw dipping crescent normal fault where a nw oriented 70 km geodetic baseline records contemporary shortening of 2 mm yr orthogonal to the fault trace in contrast our geomorphic paleoseismic and geochronologic analyses of the crescent fault suggest that a large extensional rupture occurred during the late holocene epoch an excavation across the fault at fourmile canyon reveals that the most recent event occurred at 2 8 0 1 ka with net vertical tectonic displacement of 4 6 0 4 m at this location corresponding to the release of 3 m of accumulated nw se extension measured alluvial scarp profiles suggest a minimum rupture length of 30 km along the range front for the event implying a moment magnitude m w of at least 6 6 no prior event occurred between 2 8 ka and 6 4 0 1 ka the 14 c calender age of strata near the base of the exposed section assuming typical slip rates for basin and range faults 0 3 mm yr these results imply that up to one third or 1 m of the extensional strain released in the previous earthquake could have reaccumulated across the fault since 2 8 ka however the contemporary shortening implies that the fault is unloading due to a transient process whose duration is limited to between 6 years geodetic recording time and 2 8 ka the age of the most recent event these results emphasize the importance of providing accurate geologic data on the timescale of the earthquake cycle in order to evaluate geodetic measurements

on the universality of diagenetic trends for decades the main use of seismic data has been to delineate sedimentary bodies and tectonic features in the subsurface the mission of seeing inside the geologic body has been added recently mapping porosity lithology and other reservoir bulk properties inside the geologic body has become possible due to the recent dramatic improvement in seismic acquisition imaging and inversion quality as well as the accompanying progress of rock physics

platinum group element distribution in chromite ores from ophiolite complexes implications for their exploration abstract compilation of some new data on ophiolites for greece and yugoslavia and published data from previous studies indicate that platinum group element pge and gold concentrations in chromite ores are generally low ranging from less than 100 ppb to a few hundred ppb however samples from several ophiolite complexes exhibit an enrichment of a few ppm a only in os ir and ru b only in pt and or pd or c in all pge this enrichment up to 10s ppm is mainly related with chromitites hosted in supra moho dunites and dunites of the uppermost stratigraphic levels of the mantle sequence and it seems to be local independent of the chromitite major element composition and the chromite potential of the ophiolite complexes the contents of pge combined with less chalcophile elements ni co cu the ratios of incompatible compatible elements and pge patterns provide evidence for discrimination between chromitites derived from primitive magmas and those derived from partially fractionated magmas although they have a similar major element composition thus they can be used for a stratigraphic orientation in the mantle sequence and therefore for exploration targets moreover pge data offer valuable information for the evaluation of the chromite potential in ophiolite complexes the most promising ophiolites seem to be those which apart from the petrological and geochemical characteristics indicating extensive degree of partial melting in the mantle source contain only one chromite ore type the other type being only in small proportion of limited compositional variation in both major elements and pge low ratios of pd ir while pge enriched chromitites in the mantle sequence are only occasionally present in contrast ophiolites which contain both high cr and al chromitites and where their chalcophile element data implies relatively extensive fractionation trend are not good exploration targets for chromite ores although they are related with a ssz environment

latest quaternary paleoseismology and evidence of distributed dextral shear along the mohawk valley fault zone northern walker lane california the dextral slip mohawk valley fault zone mvfz strikes northwestward along the eastern margin of the sierra nevada in the northern walker lane geodetic block modeling indicates that the mvfz may accommodate 3 mm yr of regional dextral strain implying that it is the highest slip rate strike slip fault in the region however only limited geologic data are available to constrain the system s slip rate and earthquake history we mapped the mvfz using airborne lidar data and field observations and identified a site near sulphur creek for paleoseismic investigation at this site oblique dextral normal faulting on the steep valley margin has created a closed depression that floods annually during spring snowmelt to form an ephemeral pond we excavated three fault perpendicular trenches at the site and exposed pond sediment that interfingers with multiple colluvial packages eroded from the scarp that bounds the eastern side of the pond we documented evidence for four surface rupturing earthquakes on this strand of the mvfz oxcal modeling of radiocarbon and luminescence ages indicates that these earthquakes occurred at 14 0 ka 12 8 ka 5 7 ka and 1 9 ka the mean 4 kyr recurrence interval is inconsistent with slip rates of 3 mm yr these rates imply surface ruptures of more than 10 m per event which is geologically implausible for the subdued geomorphic expression and 60 km length of the mvfz we propose that unidentified structures not yet incorporated into geodetic models may accommodate significant dextral shear across the northern walker lane highlighting the role of distributed deformation in this region

three dimensional temperature structure of a ridge transform ridge system abstract we have solved the problem of three dimensional heat transfer by conduction and material transport within a ridge transform ridge system the boundary conditions are those of a finite plate with temperatures fixed at the top and bottom we do not solve for the pattern of flow so must arbitrarily describe the material transport within triangular regions beneath the spreading centers flow is assumed to be vertical and at constant velocity with the volume of upwelling matching the volume of horizontal transport by the plates the steady state solution is approached by repeated finite steps in time each involving two stages first to represent material transport temperatures in the grid of points are shifted horizontally and or vertically to the point at which the associated particle would arrive given the time period and assumed velocity field second a three dimensional fourier transform technique is used to calculate the cooling which would occur in the static case during the same period we find that the geometry of flow in the upwelling region here parameterized by velocity of upwelling or width of the upwelling zone is more important than spreading rate in determining the temperature structure the results indicate that conductive cooling cannot alone account for deepening of the median valley towards the fracture zone for a 10 m y offset on the transform fault conductive cooling across the fracture zone to the older lithosphere affects the upwelling material only to a distance of about 10 km from the fault in contrast to the deepening of the median valley over several tens of kilometers observed on the mid atlantic ridge

radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired 230 th 234 u 238 u and 14 c dates on pristine corals abstract radiocarbon dating is the most widely used dating technique in the world recent advances in accelerator mass spectrometry ams and sample preparation techniques have reduced the sample size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes today it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates the production of 14 c in the atmosphere varies through time due to changes in the earth s geomagnetic field intensity and in its concentration which is regulated by the carbon cycle as a result of these two variables a radiocarbon age is not equivalent to a calendar age four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12 000 calendar years before present this paper presents high precision paired 230 th 234 u 238 u and 14 c age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12 000 to 50 000 years before present we developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years taking full account of combined errors in input ages and calibration uncertainties our radiocarbon calibration program is publicly accessible at http www radiocarbon ldeo columbia edu along with full documentation of the samples data and our statistical calibration model

large magnitude late holocene earthquakes on the genoa fault west central nevada and eastern california the genoa fault a principal normal fault of the transition zone between the basin and range province and the northern sierra nevada displays a large and conspicuous prehistoric scarp three trenches excavated across this scarp exposed two large displacement late holocene events two of the trenches contained multiple layers of stratified charcoal yielding radiocarbon ages suggesting the most recent and penultimate events on the main part of the fault occurred 500 600 cal b p and 2000 2200 cal b p respectively normal slip offsets of 3 5 5 m per event along much of the rupture length are comparable to the largest historical basin and range province earthquakes suggesting these paleoearthquakes were on the order of magnitude 7 2 7 5 the apparent late holocene slip rate 2 3 mm yr is one of the highest in the basin and range province based on structural and behavioral differences the genoa fault is here divided into four principal sections the sierra diamond valley carson valley and jacks valley sections and is distinguished from three northeast striking faults in the carson city area the kings canyon carson city and indian hill faults the conspicuous scarp extends for nearly 25 km the combined length of the carson valley and jacks valley sections the diamond valley section lacks the conspicuous scarp and older alluvial fans and bedrock outcrops on the downthrown side of the fault indicate a lower activity rate activity further decreases to the south along the sierra section which consists of numerous distributed faults all three northeast striking faults in the carson city area ruptured within the past few thousand years and one or more may have ruptured during recent events on the genoa fault

stylolites porosity depositional texture and silicates in chalk facies sediments ontong java plateau gorm and tyra fields north sea comparison of chalk on the ontong java plateau and chalk in the central north sea indicates that whereas pressure dissolution is controlled by effective burial stress pore filling cementation is controlled by temperature effective burial stress is caused by the weight of all overlying water and sediments as counteracted by the pressure in the pore fluid so the regional overpressure in the central north sea is one reason why the two localities have different relationships between temperature and effective burial stress in the chalk of the ontong java plateau the onset of calcite silicate pressure dissolution around 490 m below sea floor bsf corresponds to an interval of waning porosity decline and even the occurrence of proper stylolites from 830 m bsf is accompanied by only minor porosity reduction because opal is present the pore water is relatively rich in si which through the formation of ca silica complexes causes an apparent super saturation of ca and retards cementation the onset of massive pore filling cementation at 1100 m bsf may be controlled by the temperature dependent transition from opal ct to quartz in the stylolite bearing chalk of two wells in the gorm and tyra fields the nannofossil matrix shows recrystallization but only minor pore filling cement whereas microfossils are cemented cementation in gorm and tyra is thus partial and has apparently not been retarded by opal controlled pore water a possible explanation is that due to the relatively high temperature silica has equilibrated to quartz before the onset of pressure dissolution and thus in this case dissolution and precipitation of calcite have no lag this temperature versus effective burial stress induced difference in diagenetic history is of particular relevance when exploring for hydrocarbons in normally pressured chalk while most experience has been accumulated in the over pressured chalk of the central north sea

estimating prehistoric earthquake magnitude from point measurements of surface rupture abstract we have developed a method for estimating the magnitude of prehistoric earthquakes using displacement data that usually can be collected from paleoseismic investigations this method is necessary because essentially all current magnitude estimates for prehistoric events rely on determining the total length of coseismic surface rupture which is rarely measurable or rely on segmentation scenarios for which uncertainties cannot be quantified although surface rupture length is a better predictor of magnitude than displacement for historic earthquakes paleoseismic investigations are better at providing measurements of the amount of displacement at a site along a fault the key to our method is to incorporate the variability in displacement observed in 14 modern events which allows a formal uncertainty in magnitude to be assigned to prehistoric ruptures we show how multiple measurements along a preserved fraction of a rupture can be combined to reduce the uncertainty in the estimate of magnitude our analysis shows that uncertainty asymptotically approaches the natural variability of ruptures so 5 to 10 displacement measurements are sufficient to characterize paleomagnitude we conclude that sampling of scarps with lengths of even 10 of the original rupture can provide magnitude values that reasonably estimate the earthquake tests of the method using randomly sampled data from the 1992 m w 7 3 landers and 1954 m s 6 8 dixie valley earthquakes provide close approximations of the actual magnitudes

radiocarbon ages and environments of deposition of the wono and trego hot springs tephra layers in the pyramid lake subbasin nevada abstract uncalibrated radiocarbon data from core plc92b taken from wizards cove in the pyramid lake subbasin indicate that the trego hot springs and wono tephra layers were deposited 23 200 300 and 27 300 300 14 c yr b p uncorrected for reservoir effect sedimentological data from sites in the pyramid lake and smoke creek black rock desert subbasins indicate that the trego hot springs tephra layer was deposited during a relatively dry period when pyramid lake was at or below its spill point 1177 m to the winnemucca lake subbasin the wono tephra layer was deposited when lake depth was controlled by spill across emerson pass sill 1207 m to the smoke creek black rock desert subbasin 18 o data from core plc92b also support the concept that the trego hot springs tephra fell into a relatively shallow pyramid lake and that the wono tephra fell into a deeper spilling lake

comment on radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired th 230 u 234 u 238 and c 14 dates on pristine corals by r g fairbanks et al quaternary science reviews 24 2005 1781 1796 and extending the radiocarbon calibration beyond 26 000 years before present using fossil corals by t c chin et al quaternary science reviews 24 2005 1797 1808

the effect of chemical and physical processes on the acoustic properties of carbonate rocks carbonate rocks have major economic significance 60 of the world s oil reserves lie in carbonate reservoirs and the potential for additional gas reserves is huge however the relationship in carbonates between measured geophysical data and rock properties is complex due to the large variety of textures that arise during postdepositional diagenesis and to the chemical processes i e dissolution and replacement by newly formed phases that characterize carbonate forming minerals most experimental and theoretical rock physics research has focused on siliciclastic and shaly rocks however applying insights gained from clastics to carbonates is rarely straightforward and thus is questioned in the literature

timing of cenozoic volcanism and basin and range extension in northwestern nevada new constraints from the northern pine forest range eocene middle miocene volcanic rocks in the northern pine forest range nevada are ideally situated for reconstructing the timing and style of volcanism and extensional faulting in the northwesternmost part of the basin and range province a conformable sequence of cenozoic volcanic and sedimentary strata in the northern pine forest range dips 30 w and 11 new 40 ar 39 ar ages from this sequence defi ne 3 major episodes of volcanic activity pre tertiary basement and older ca 38 ma tertiary intrusive rocks are overlain unconformably by oligocene ca 30 23 ma basalt fl ows and dacitic to rhyolitic ash fl ow tuffs interbedded with fi ne grained tuffaceous sedimentary rocks oligocene rocks are overlain by 550 m of ca 17 16 ma basalt fl ows equivalent to the steens basalt in southern oregon and basalt fl ows are capped by a thin 16 3 ma ignimbrite that likely is correlative with either the idaho canyon tuff or the tuff of oregon canyon the northern pine forest range is bounded to the east by a major down to the east normal fault that dips 40 e with well developed fault striations indicative of dip slip motion this fault initiated at an angle of 70 and was rotated 30 during uplift of the range a suite of 17 apatite fi ssion track ages from the pine forest footwall block demonstrates that exhumation uplift and slip on the range bounding fault began ca 12 11 ma and continued until at least 7 ma with moderate slip since then the pine forest range did not undergo signifi cant extension before or during peak oligocene and miocene volcanism and similar geologic relationships in nearby ranges suggest that a larger region of northwestern nevada was also little extended during this interval basin and range faulting in northwestern nevada appears to have begun no earlier than 12 ma making it distinctly younger than deformation in much of central and southern nevada where peak extension occurred in the middle miocene or earlier

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research