oxygen isotopic composition and u pb discordance in zircon we have investigated u pb discordance and oxygen isotopic composition of zircon using high spatial resolution 18o measurement by ion microprobe 18o in both concordant and discordant zircon grains provides an indication of the relationship between fluid interaction and discordance our results suggest that three characteristics of zircon are interrelated 1 u pb systematics and concomitant age discordance 2 18o and the water rock interactions implied therein and 3 zircon texture as revealed by cathodoluminescence and bse imaging a key observation is that u pb disturbed zircons are often also variably depleted in 18o but the relationship between discordance and 18o is not systematic 18o values of discordant zircons are generally lighter but irregular in their distribution textural differences between zircon grains can be correlated with both u pb discordance and 18o discordant grains exhibit either a recrystallized fractured or strongly zoned cl texture and are characteristic of 18o depletion we interpret this to be a result of metamictization leading to destruction of the zircon lattice and an increased susceptibility to lead loss conversely grains that are concordant have less expressed zoning and a smoother cl texture and are enriched in 18o from this it is apparent that various stages of water rock interaction as evidenced by systematic variations in 18o leave their imprint on both the texture and u pb systematics of zircon

pulling plates apart the salton sea is located in a sedimentary basin at the southern termination of the san andreas fault high resolution seismic data indicate that the basin formed and grew by active subsidence at its southern end

prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

distribution of organic and inorganic phosphorus compounds in mrine and lacustrine sediments a31nmr study abstract solution 31 p nmr spectroscopy has been used for the characterization of different classes of organic and inorganic phosphorus compounds in a number of different marine and lacustrine sediment environments all samples contain orthophosphate mono and di esters of which the former is always the major constituent of possible orthophosphate mono ester candidates inositol phosphates may be of importance the amount of orthophosphate di esters is much higher in the reduced sediments from the baltic sea than in those areas with anoxic suboxic conditions indicating a more efficient decomposition of di esters in oxic environments the same situation was also observed in the lake sediment profiles pyro and polyphosphate are found in the oxic sediments from the lakes whereas no sign of such compounds could be seen in the baltic sea sediment samples redox variations lake samples and cation strength variations between the sea water and the lacustrine water will most likely be the main explanation for the observed distribution patterns of pyro and polyphosphate small but sharp peaks indicate the occurrence of phosphoric acids in a sediment from the southern baltic sea several other unidentified peaks were also observed some of which could be assigned to aromatic di and mono esters

behavior of the southernmost san andreas fault during the past 300 years surficial creep occurs at low rates along the coachella valley segment of the san andreas fault which has not produced a large earthquake during the period of historical record geodetic data indicate however that the crust adjacent to this segment of the san andreas fault is accumulating strain at a high rate furthermore neotectonic and paleoseismic data indicate that the fault does produce very large earthquakes every two to three centuries in view of its long term behavior the occurrence of creep along the surficial trace of the fault in the coachella valley is of particular interest along two short reaches of the san andreas fault in the coachella valley measurements of offset geological deposits and man made structures and from alignment arrays and creep meters show that slip rates of 2 4 mm yr near indio and near the salton sea have persisted for the past three centuries this slow aseismic surficial creep is not a transient precursor to seismic failure of this segment of the fault we suggest that the coachella valley segment of the san andreas fault creeps in its upper few kilometers this behavior may be due to tectonically induced high pore pressures in the coarse sediments that abut the fault

a brine interface in the salton sea geothermal system california fluid geochemical and isotopic characteristics data from 71 geothermal production intervals in 48 wells from the salton sea geothermal system ssgs indicate that fluids in that system cluster into two distinct populations in terms of their salinity and their stable isotopic compositions the distinctive hot hypersaline brine typically 20 wt total dissolved solids for which the ssgs is known is overlain by a cooler 260 c fluid with distinctly lower salinity typically 10 wt total dissolved solids hypersaline brines have high and rather consistent 18o shifts produced by water rock interaction and have a very narrow range in d values low tds fluids on the other hand show a wide range in both d and 18o production of both types of fluid from closely spaced geothermal wells in many regions of the ssgs indicates that a relatively sharp salinity interface exists over much of the field the fluid interface typically cross cuts sedimentary bedding but is consistently found where reservoir temperatures are approximately 260 c at these temperatures hypersaline brines have densities of approximately 1 0 gm cm3 while the low tds fluids have densities as low as 0 85 gm cm3 this stable density stratified interface acts as a barrier to convective heat and mass transfer in the ssgs isolating the hypersaline reservoir from overlying dilute fluids a lithologic cap implied by previous ssgs models is unnecessary in such a stratified system since heat and mass transfer across the interface must occur by slow conductive diffusional and interface mixing processes regardless of local permeability

the heating duration and provenance age of rocks in the salton sea geothermal field southern california abstract 40ar 39ar step heating of single k feldspar crystals 500 g in a furnace with a low argon blank and accurate temperature control provide constraints on the thermal history of the salton sea geothermal field southern california estimates of 40ar loss from detrital k feldspars coupled with kinetic information gleaned from laboratory degassing of 39ar allow possible thermal histories to be evaluated samples from core and cuttings of sandstones from the state 2 14 landers 1 dearborn farms 1 magmamax 2 elmore 3 and sinclair 3 wells have been analyzed for this purpose no recent 40ar loss is recognized in any of the 17 crystals measured even for samples currently at temperatures greater than 275 c this observation suggests that the present temperature distributions in these wells have not been maintained for greater than 1000 years however aggregates 100 mg of two k feldspar samples do reveal recent 40ar loss from the state 2 14 well suggesting a time scale of 1000 to 5000 years for near peak thermal conditions these estimates of heating duration are broadly consistent with earlier results from thermal modelling but do not support interpretations that paleotemperatures were once hotter than present provenance ages span the tertiary through middle proterozoic but cluster at ages of 25 70 and 1250 ma these age peaks are very common k feldspar k ar ages in detachment terranes of southeastern california and southwestern arizona both drained by the colorado river this coincidence supports earlier suggestions that the sands of the salton trough were derived mainly from the colorado river unrealistically low apparent activation energies

nature and significance of igneous rocks cored in the state 2 14 research borehole salton sea scientific drilling project california the state 2 14 research borehole of the salton sea scientific drilling project penetrated 3 22 km of pleistocene to recent sedimentary rocks in the salton sea geothermal system located in the salton trough of southern california and northern baja california mexico in addition three intervals of igneous rocks were recovered a silicic tuff and two sills of altered diabase the chemical composition of the silicic tuff at 1704 m depth suggests that it is correlative with the durmid hill tuff cropping out 25 km nw of the geothermal system in turn both of these tuffs may be deposits of the bishop tuff erupted from the long valley caldera of central california at 0 7 ma the diabases are similar to basaltic xenoliths found in the nearby salton buttes rhyolite domes these diabase are interpreted as hypabyssal intrusions resulting from magmatism due to rifting of the salton trough as part of the east pacific rise gulf of california transtensional system the sills apparently intruded an already developed geo thermal system and were in turn altered by it

a case study of the seismic source function salmon and sterling reevaluated as part of a larger joint effort by the defense advanced research project agency and the department of energy to study the seismic source problem a comprehensive reevaluation of the 1964 salmon and 1966 sterling nuclear explosions in dome salt was carried out the sterling source function originally estimated by springer et al 1968 conveys the impression that the cavity was badly overdriven on reexamination this does not appear to be the case the work of glenn et al 1987 on the sterling free field data is expanded upon confirming that the cavity response was close to the theoretical expectation sterling s source function is estimated and is found to be comparable to patterson s 1966 slightly weakened salt model a source model for salmon is derived from the sterling source model and the five seismic stations that recorded both events the new source model has a reduced displacement potential of about half that previously estimated a temporary nonlinear two wave system developed during the salmon explosion as the compressional wave evolved from a shock wave the separation of these two waves resulted in a high frequency roll off of the reduced velocity potential of 3 in addition it is shown that the comer frequency is much higher and is created much closer to the cavity than the eigenfrequency for both salmon and sterling the radial stresses are approximately a low passed damped sinusoid superimposed on a small step function the decoupling value of 72 obtained by springer et al 1968 is confirmed a revision of patterson s 1966 partial decoupling curve shows that the value for full decoupling in a shot generated cavity would be only slightly higher contrary to previous studies decoupling as a function of frequency for the surface waves is found to be the same as for the p waves a new definition of decoupling appropriate to threshold test ban treaty monitoring is also proposed

equilibrium oxygen hydrogen and carbon isotope fractionation factors applicable to geologic systems as demonstrated by the chapters in this short course stable isotope techniques are an important tool in almost every branch of the earth sciences central to many of these applications is a quantitative understanding of equilibrium isotope partitioning between substances indeed it was harold urey s 1947 thermodynamically based estimate of the temperature dependence of 18o 16o fractionation between calcium carbonate and water and a recognition of how this information might be used to determine the temperatures of ancient oceans that launched the science of stable isotope geochemistry the approach pioneered by urey has since been used to estimate temperatures for a wide range of geological processes e g emiliani 1955 anderson et al 1971 clayton 1986 valley this volume in addition to their geothermometric applications equilibrium fractionation data are also important in the study of fluid rock interactions including those associated with diagenetic hydrothermal and metamorphic processes baumgartner and valley this volume shanks this volume finally a knowledge of equilibrium fractionation is a necessary first step in evaluating isotopic disequilibrium a widespread phenomenon that is increasingly being used to study temporal relationships in geological systems cole and chakraborty this volume in the fifty four years since the publication of urey s paper equilibrium fractionation data have been reported for many minerals and fluids of geological interest these data were derived from 1 theoretical calculations following the methods developed by urey 1947 and bigeleisen and mayer 1947 2 direct laboratory experiments 3 semi empirical bond strength models and 4 measurement of fractionations in natural samples each of these methods has its advantages and disadvantages however the availability of a variety of methods for calibrating fractionation factors has led to a plethora of calibrations not all of which are in agreement in this chapter we evaluate the major methods for determining fractionation factors

crustal and upper mantle velocity structure of the salton trough southeast california this paper presents data and modelling results from a crustal and upper mantle wide angle seismic transect across the salton trough region in southeast california the salton trough is a unique part of the basin and range province where mid ocean ridge transform spreading in the gulf of california has evolved northward into the continent in 1992 the u s geological survey usgs conducted the final leg of the pacific to arizona crustal experiment pace two perpendicular models of the crust and upper mantle were fit to wide angle reflection and refraction travel times seismic amplitudes and bouguer gravity anomalies the first profile crossed the salton trough from the southwest to the northeast and the second was a strike line that paralleled the salton sea along its western edge we found thin crust 21 22 km thick beneath the axis of the salton trough imperial valley and locally thicker crust 27 km beneath the chocolate mountains to the northeast we modelled a slight thinning of the crust further to the northeast beneath the colorado river 24 km and subsequent thickening beneath the metamorphic core complex belt northeast of the colorado river there is a deep apparently young basin 5 6 km unmetamorphosed sediments beneath the imperial valley and a shallower 2 3 km basin beneath the colorado river a regional 6 9 km s layer between 15 km depth and the moho underlies the salton trough as well as the chocolate mountains where it pinches out at the moho this lower crustal layer is spatially associated with a low velocity 7 6 7 7 km s upper mantle we found that our crustal model is locally compatible with the previously suggested notion that the crust of the salton trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust however we observe an apparently magmatically emplaced lower crust to the northeast outside of the salton trough and propose that this layer in part predates salton trough rifting it may also in part result from migration of magmatic spreading centers associated with the southern san andreas fault system these spreading centers may have existed east of their current loca tions in the past and may have influenced the lower crust and upper mantle to the east of the current salton trough

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

influence of water column anoxia on the elemental fractionation of carbon and phosphorus during sediment diagenesis abstract organic carbon oxidation and benthic total phosphorus flux measurements from sites covering a range of bottom water oxygen concentrations have been compiled the data show that p regeneration relative to carbon oxidation is much more extensive in sediments overlain by oxygen depleted waters these results are consistent with studies showing that low oxygen bottom water concentrations correlate with reduced total phosphorus burial efficiencies and enhanced organic carbon burial efficiencies based on these observations a positive feedback is proposed between water column anoxia enhanced benthic phosphorus regeneration and marine productivity this feedback may help explain the widespread accumulation of organic rich marine sediments from anoxic waters observed in the geologic record

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

sedimentary iron phosphorus cycling under contrasting redox conditions in a eutrophic estuary abstract phosphorus p is often a limiting nutrient within freshwater and estuarine systems thus excess inputs of p from anthropogenic activities dominantly agriculture can induce eutrophication in receiving water bodies the sequestration of p within estuarine sediments is controlled by sorption and precipitation processes which are impacted by local redox conditions and burial environment despite the global spread of oxygen depletion in coastal marine systems p burial under reducing conditions remains poorly understood we investigated p cycling in relation to iron fe redox chemistry in sediments from the eutrophic peel harvey estuary in western australia using a combination of porewater analysis sequential chemical p extractions synchrotron based micro scale x ray fluorescence mapping and fe k edge x ray absorption spectroscopy and po43 sorption experiments the sediments represented redox regimes varying from strongly reducing organic rich sediments with little or no reactive fe iii oxyhydr oxides to oxygenated sediments that were enriched in reactive fe iii phases organic p and fe associated p were the main p burial phases and the latter was quantitatively important even in sediments with an overall strongly reducing character we attribute this to adsorption of p onto micro scale fe iii oxyhydroxide enrichments and or fe bearing clay minerals the organic rich sediments showed a strong decline in p contents with depth p was released from organic matter and fe phases but apatite precipitation was apparently inhibited in these sediments despite greater and stronger po43 sorption capacity the oxic sediments contained relatively little p due to a lack of the primary p source in marine sediments organic matter our results provide detailed insight into p burial in dynamic estuarine sediments and show that micro scale spectroscopic analyses greatly advance our understanding of p sequestration processes

seismic decoupling with chemical and nuclear explosions in salt an extensive series of simulations was performed of underground explosions in salt using both chemical and nuclear explosives in both cases the radius of the initial emplacement cavity was varied from the fully tamped configuration to as large as 80 m kt sup 1 3 when not fully tamped the cavity was assumed initially to contain air at ambient temperature and pressure in the nuclear source case the simulations are shown to be in good agreement with the salmon sterling events conducted by the united states and with recently released russian data on a similar pair of explosions in an azgir salt dome simulation of the us cowboy series of chemical explosions in a louisiana salt mine are also shown to be in very good agreement with the experimental data however the constitutive model for the salt that best explains these data is different from that derived for salmon both salt models are amply supported by laboratory and field data the main result of these simulations is that cavity decoupling with chemical explosives is much less efficient than with nuclear explosives although maximum decoupling factors f sub max near 200 may be attainable with either of the two sources the cavity size required to achievemore this value appears to be 40 m kt sup 1 3 for cavity radii half as large f sub max is roughly 4 times lower with nuclear explosives and lower by another factor of 4 with chemical sources moreover if the initial cavity radius is a more modest 10 m kt sup 1 3 f sub max 3 even with a nuclear source 24 refs 8 figs less

shallow drilling in the salton sea region the thermal anomaly during two shallow thermal drilling programs thermal measurements were obtained in 56 shallow 76 2 m and one intermediate 457 3 m depth holes located both onshore and offshore along the southern margin of the salton sea in the imperial valley california these data complete the surficial coverage of the thermal anomaly revealing the shape and lateral extent of the hydrothermal system the thermal data show the region of high thermal gradients to extend only a short distance offshore to the north of the quaternary volcanic domes which are exposed along the southern shore of the salton sea the central thermal anomaly has an arcuate shape about 4 km wide and 12 km long across the center of the anomaly the transition zone between locations exhibiting high thermal gradients and those exhibiting regional thermal gradients is quite narrow thermal gradients rise from near regional 0 09 c m to extreme 0 83 c m in only 2 4 km the heat flow in the central part of the anomaly is greater than 600 mw m2 and in the two local anomalies exceeds 1200 mw m2 the shape of the thermal anomaly is asymmetric with respect to the line of volcanoes previously thought to represent the center of the field with its center line offset south of the volcanic buttes there is no broad thermal anomaly associated with the magnetic high that extends offshore to the northeast from the volcanic domes the salton sea thermal anomaly can be described as three nested thermal features superimposed on the regional salton trough heat flux a broad region of elevated conductive heat flow with constant temperature gradients of about 0 1 c m a central anomaly of high 0 4 c m near surface gradients observed through a conductive thermal cap underlain by a nearly isothermal zone of hydrothermal circulation and two local anomalies with shallow temperature gradients as high as 0 8 c m superimposed on the central geothermal system these observations of the thermal anomaly provide important constraints for models of the circulation of the hydrothermal system thermal budgets based on a simple model for this hydrothermal system indicate that the heat influx rate for local hot spots in the region may be large enough to account for the rate of heat flux from the entire salton trough

sedimentary organic carbon to phosphorus ratios as a redox proxy in quaternary records from the mediterranean abstract in this study organic carbon to organic phosphorus c org p org and total phosphorus c org p tot ratios as bottom water redox proxies are explored for two eastern mediterranean sediment records extending back 600 700 kyrs cores kc19c 19 6 m long 2750 m water depth and kc01b 37 0 m long 3643 m water depth contain numerous organic rich layers sapropels throughout the two cores variations found in the c org p ratios correspond to those for other redox proxies such as total s contents and v al ratios accordingly these are interpreted as variations in bottom water oxygenation and inter sapropel redox variability in core kc19c the original c org p signals seem to have been preserved despite intense post depositional sulfidization high resolution p fractionation data of sapropel s5 from a third core ps25pc 3304 m water depth showing high and relatively stable c org p org ratios indicate that sapropel s5 at this deep location was deposited under permanently reducing conditions while p regeneration from organic matter was intense net p burial increased in the sapropel due to enhanced burial of inorganic calcium phosphate likely biogenic ca p sediment p contents and c org p tot ratios in s5 are variable reflecting short term changes in marine conditions productivity redox and sediment chemistry the c org p org and c org p tot ratios thus provide valuable insight into paleoceanographic conditions and pathways of p cycling for mediterranean sediment records extending back at least 600 700 kyrs

deep water hydrocarbon seeps in guaymas basin gulf of california abstract acoustically discovered hydrocarbon seeps along a transform fault zone on the sonoran margin of guaymas basin and in the southern trough of guaymas basin were examined and sampled during dives of dsv alvin seepage of methane and heavier hydrocarbons occurs through shallow pockmarks along the eroding crest of a steep anticline 1600 m below sea level extensive ledges of aragonite crop out around the rims of the pockmarks isotopic analysis indicates that carbonate precipitation is a result of methane oxidation seepage zones within the pockmarks support dense communities of calyptogena half buried in the mud tubeworms lamellibracheae sp have colonized many of the aragonite outcrops though the margin site is only a few kilometers from a high temperature sulfide precipitating and petroleum discharging vent system at a nearby center its structural setting is more akin to pockmarks described from continental shelves and its chemosynthetic fauna is more like that around low temperature seeps on other continental slopes and margins e g oregon u s a and japan the seep in the southern trough is a condensate type petroleum c 1 c 40 and rises as a plume from a young vent mound system about 2000 m below sea level

hawaiian basalt and icelandic rhyolite indicators of differentiation and partial melting in spite of the voluminous basaltic volcanism on the island of hawaii rhyolite is not produced iceland on the other hand exhibits common rhyolitic volcanism amounting to some 10 12 of its surface rocks this contrast is investigated using the fundamental igneous processes exhibited by sheet like hawaiian lava lakes and shonkin sag laccolith in montana highly differentiated residual melts normally reside within inwardly advancing solidification fronts and are generally inaccessible to eruptive processes only when a large initial phenocryst population is present from which a thick basal cumulate can rapidly form is it possible to supply highly differentiated melt into the active i e eruptable portion of the magma chamber although there is protracted control of differentiation at hawaii by settling of olivine further differentiation occurs within the solidification fronts only by repeated transport and holding is it possible to differentiate beyond the critical composition of the leading edge of the solidification front 7 mgo and 51 5 sio2 crystal size distributions csds for hawaii and shonkin sag are used to demonstrate the inferred physical and chemical processes of solidification including the kinetics of crystallization

paleocurrent evidence for lateral displacement of the pliocene colorado river delta by the san andreas fault system southeastern california more than 600 paleocurrent measurements from fluvial paleochannels of the early colorado river delta plain preserved in the palm spring formation in the fish creek vallecito fcv section of southern california indicate that the fcv section was on the gulf of california southeastern flank of the delta plain as recently as 2 8 ma these measurements imply 130 km of tectonic translation to the northwest since 2 8 ma which can be accounted for by the displacement histories of baja california and component faults of the san andreas system stratigraphic data from the fcv section and other neogene localities when plotted on a palinspastic base indicate 1 marine transgression of a narrow gulf of california rift basin and bouse embayment site of modern lower colorado river by 5 5 ma and 2 uplift of the bouse embayment initiation of the lower colorado river and southward progradation of the delta to essentially its present position by 4 ma

alteration and remelting of nascent oceanic crust during continental rupture evidence from zircon geochemistry of rhyolites and xenoliths from the salton trough california abstract rhyolite lavas and xenoliths from the salton sea geothermal field southern california provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust salton buttes rhyolite lavas contain xenoliths that include granophyres fine grained altered rhyolites felsite and amphibole bearing basalts zircon is present in lavas and xenoliths surprisingly even in the basaltic xenoliths where it occurs in plagioclase rich regions interpreted as pockets of crystallized partial melt zircons in the xenoliths are exclusively late pleistocene holocene in age and lack evidence for inheritance u th isochron ages are 20 5 1 2 1 2 ka granophyres 18 3 3 5 3 6 ka felsite 30 1 12 4 14 1 ka and 9 2 6 6 7 0 ka basalts all errors 1 the dominant zircon population in the rhyolite lavas yielded u th ages between 18 and 10 ka with few pre quaternary xenocrysts present 18ozircon values are lower than typical crustal basement values thus ruling out rhyolite genesis by melting of continental crust moreover 18ozircon values are 0 5 1 0 lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid ocean ridge basalt suggesting that basaltic crust and silicic plutons in the subsurface of the salton sea geothermal field isotopically exchanged with meteoric waters this is evidence for deep reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma

age constraints for the present fault configuration in the imperial valley california evidence for northwestward propagation of the gulf of california rift system releveling and other geophysical data for the imperial valley of southern california suggest the northern section of the imperial brawley fault system which includes the mesquite basin and brawley seismic zone is much younger than the age of the valley itself a minimum age of 3000 years is calculated for the northern segment of the imperial fault from correlations between surface topography and geodetically observed seismic interseismic vertical movements calculation of a maximum age of 100 000 years is based upon displacements in the crystalline basement along the imperial fault inferred from seismic refraction surveys this young age supports recent interpretations of heat flow measurements and the evolution of geothermal systems which also suggest that the current patterns of seismicity and faulting in the imperial valley are not long lived the current fault geometry and basement morphology suggest a northwestward growth of the imperial fault and a northwestward migration of the brawley seismic zone if this localized process is representative of more regional tectonic processes along the extent of the salton trough we suggest that this migration is a manifestation of the propagation of the gulf of california rift system into the north american continent

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

why the phosphorus retention of lakes does not necessarily depend on the oxygen supply to their sediment surface in order to improve the trophic state of lake sempach a eutrophied lake in central switzerland its external phosphorus p load has been decreased and its hypolimnion has been artificially oxygenated to lower the lake internal p recycling based on more than 15 yr of experience we conclude that the reduction of the external p load resulted in a corresponding decrease of the lake s p concentration however contrary to initial expectations increased hypolimnetic dissolved oxygen concentrations neither 1 reduced the p release from sediments during summer nor 2 resulted in an increased permanent p retention these observations warrant a reevaluation of the well accepted management strategy of decreasing the lake internal p cycling by maintaining an aerobic hypolimnion and sediment surface we hypothesize that oxygenation only results in an increased permanent benthic p burial if because of the depressed sulfide production more ferrous phosphate e g vivianite and less fes is deposited in the anoxic sediment hence it is not the oxic sediment surface that directly affects the permanent redox dependent sediment p retention but the molar ratio of the available reactive fe ii s 2 po 4 in the anoxic sediment this ratio is driven by the settling rate and the nature of organic matter and particulate iron as well as the supply of oxygen nitrate and sulfate to the sediment

biogeochemical mechanisms controlling phosphorus diagenesis and internal loading in a remediated hard water eutrophic embayment abstract phosphorus p levels in eutrophic lakes with restricted external p inputs often show hysteresis resulting from sediment p diagenetic recycling hard water oxygenated lakes are often thought to be less susceptible to p release from sediment due to precipitation of p with calcium carbonate and p sorption on iron fe oxyhydroxides in oxidized surface sediments however perpetuated eutrophic conditions in many hard water oxygenated lakes persist despite drastic reduction of external p inputs what limits p sediment binding capacity and drives its release from sediments in such lakes is often less understood here we characterize sediment p diagenetic processes driving persistent internal loading in the bay of quinte a hard water polymictic embayment of lake ontario canada we used a multifaceted approach combining a comprehensive three year study of sediment p binding forms recent sediment stratigraphy and accumulation high spatial resolution measurement of dissolved oxygen redox potential and ph across the sediment water interface and depth profiles of nutrient and metals concentrations in pore water despite oxygenated bottom water conditions p diffusive fluxes during summer are substantial 1 6 5 mg p m2 day representing as much as 50 of external p inputs estimated rates of benthic organic carbon degradation indicate that anaerobic organic p mineralization in anoxic sediments is the main driver of p release which is strongly influenced by fe reduction estimated rates of benthic organic carbon degradation indicate that anaerobic organic p mineralization in anoxic sediments is the main driver of p release in surface sediment the redox sensitive p is the prevailing diagenetically reactive p phase emphasizing strong coupling with ferric fe while this indicates substantial p sorption on fe oxyhydroxides their spare binding capacity is limited as reducible fe p ratios are close to sorption limit the calcium carbonate bound p increased with sediment depth at the expense of redox sensitive p suggesting diagenetic sequestration into apatite with sediment burial however only 40 of diagenetically reactive p forms which account for 2 3 of the total p in surface sediment are permanently retained during burial importantly our data show that in fact significant p flux occurs in shallow areas with an apparent discordance with concurrent measures of long term p release from the sediments these results also demonstrate that short term measures cannot be extrapolated to long term estimates and vice versa

diffusion in zircon despite its low abundance in most rocks zircon is extraordinarily useful in interpreting crustal histories the importance of u th pb isotopic dating of zircon is well and long established davis et al this volume parrish et al this volume zircon also tends to incorporate trace elements useful as geochemical tracers such as the ree y and hf a number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations often on extremely fine scale which provide valuable insight into thermal histories and past geochemical environments the relative insolubility of zircon in crustal melts and fluids as well as its general resistance to chemical and physical breakdown often result in the existence of several generations of geochemical information in a single zircon grain the fact that this information is so frequently retained as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine scale zoning down to the sub micron scale has long suggested that diffusion of most elements is quite sluggish in zircon in this chapter we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon because of its importance as a geochronometer attempts have been made to measure diffusion especially of pb for over 30 years but only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies resulting in a plethora of new diffusion data these findings have important implications for isotopic dating interpretation of stable isotope ratios closure temperatures and formation and preservation of primary chemical composition and zoning in zircon efforts have been made for some time to quantify and characterize diffusion in zircon most notably of pb in deference to its significance in interpreting pb isotopic signatures and refining understanding of thermal histories as is evident from

simultaneous inversion of local and teleseismic data for the crust and mantle structure of southern california we determined a detailed three dimensional seismic velocity structure of the crust and mantle to a depth of 800 km beneath southern california using local and teleseismic data simultaneously we used 131 372 p wave arrival times from 6437 local earthquakes and 12 134 travel time residuals from 193 teleseismic events recorded by 251 seismic stations of the caltech us geological survey southern california seismic network compared with previous local and teleseismic studies the station coverage is considerably improved and the number of data used is greatly increased in the present study this together with the local and teleseismic joint inversion approach results in a unified three dimensional 3 d p wave velocity model of the crust and mantle in southern california with a higher resolution than the previous models the result obtained shows very heterogeneous structures in the crust and upper mantle in southern california shallow crustal structures correlate well with surface geological features sedimentary basins such as the los angeles basin ventura basin and santa maria basin are imaged well as low velocities and batholiths such as the peninsular ranges and san gabriel mountains are imaged as high velocities the velocity in the crust is generally low in the mojave desert the crustal velocities change abruptly across major faults such as the san andreas san jacinto elsinore and garlock faults in the upper mantle three major anomalies are found beneath the transverse ranges salton trough and southern sierra nevada the transverse ranges feature appears as a curtain like east trending and north dipping high velocity anomaly with a thickness of approximately 50 km and extends most deeply at its eastern end to about 250 km the salton trough feature is composed of low velocities which trend southeast and extend to approximately 200 km depth the southern sierra nevada feature is a slab like high velocity anomaly which extends to about 250 km the mantle velocity is generally low in the upper 100 200 km beneath the volcanic areas in southern california the seismic velocity anomalies in the mantle beneath southern california are considered to result from small scale mantle convection

recent and long term behavior of the brawley fault zone imperial valley california an escalation in slip rate the brawley fault zone bfz and the brawley seismic zone constitute the principal transfer zone accommodating strain between the san andreas and im perial faults in southernmost california the bfz ruptured along with the imperial fault in the 1940 mw 6 9 and the 1979 mw 6 4 earthquakes although in each case only minor slip apparently occurred on the bfz several other episodes of slip and creep have been documented on the bfz historically until this study it has been unclear whether the past few decades reflect average behavior of the fault two trenches were opened and a series of auger holes were bored across three strands of the bfz at harris road to compare the amount of slip observed historically with the displacements observed in the paleoseismic record evidence is presented across the westernmost strand of the bfz and across the entire bfz at harris road to show that both the average vertical slip rate observed in modern times since 1970 and the vertical creep rate excluding coseismic slip observed during the 1970s are sig nificantly higher than the long term average across the westernmost strand the long term vertical rate is 1 2 1 5 0 5 mm yr and the average rate since about a d 1710 is determined to be no greater than 2 0 mm yr in contrast the average vertical rate between 1970 and 2004 across that strand was at least 4 3 mm yr and the 1970s vertical aseismic creep rate was 10 mm yr likewise across the entire bfz the long term vertical rate is 2 8 4 1 1 4 mm yr whereas the rate between 1970 and 2004 was at least 7 2 mm yr and the 1970s aseismic creep rate was 10 mm yr the long term strike slip rate cannot be determined across any strands of the bfz but may be significant in contrast to the commonly accepted higher sedimentation rates inferred for the entire imperial valley we find that the average sedimentation rate on the downthrown side of the bfz adjacent to mesquite basin in the millennium preceding the onset of agricultural influences was at most 3 5 mm yr finally a creep event occurred on the bfz during our study in 2002 and is documented herein online material appendices 1 and 2 trench logs and color versions of figures 7 8 and 10

a deterministic and stochastic velocity model for the salton trough basin and range transition zone and constraints on magmatism during rifting as a high resolution addition to the 1992 pacific to arizona crustal experiment pace a 45 km long deep crustal seismic reflection profile was acquired across the chocolate mountains in southeastern california to illuminate crustal structure in the transition between the salton trough and the basin and range province the complex seismic data are analyzed for both large scale deterministic and fine scale stochastic crustal features a low fold near offset common midpoint cmp stacked section shows the northeastward lateral extent of a high velocity lower crustal body which is centered beneath the salton trough off end shots record a high amplitude diffraction from the point where the high velocity lower crust pinches out at the moho above the high velocity lower crust moderate amplitude reflections occur at midcrustal levels these reflections display the coherency and frequency characteristics of reflections backscattered from a heterogeneous velocity field which we model as horizontal intrusions with a von karman fractal distribution the effects of upper crustal scattering are included by combining the mapped surface geology and laboratory measurements of exposed rocks within the chocolate mountains to reproduce the upper crustal velocity heterogeneity in our crustal velocity model viscoelastic finite difference simulations indicate that the volume of mafic material within the reflective zone necessary to produce the observed backscatter is about 5 the presence of wavelength scale heterogeneity within the near surface upper and middle crust also produces a 0 5 s thick zone of discontinuous reflections from a crust mantle interface which is actually a first order discontinuity

diversity and spatial zonation of volcanic rocks from the east pacific rise near 21 n a substantial range of petrologic rock types has erupted on the accreting plate boundary near 21 n on the east pacific rise epr young olivine basalts have fo89 86 phenocrysts low bulk tio2 1 1 1 3 ba 7 10 ppm and high ni contents 100 ppm older plagioclase olivine pyroxene pop basalts have fo86 81 phenocrysts high tio2 1 4 1 7 ba 9 40 ppm and low ni 100 ppm the youngest olivine basalts erupt immediately around a segmented axial fissure system progressively older more fractionated pop basalts have spread farther from the same fissure system producing a stratigraphically controlled zonal pattern of basalt type distribution around the eruptive fissures a topographic and morphologic en echelon displacement of the ridge axis fissure of 1 7 km to the nw near 20 54 n offsets this zonal distribution pattern low pressure crystal fractionation of olivine plagioclase and clinopyroxene 2 5 1 from an olivine basalt parent would yield pop basalts of the observed zr ti y p and major element chemistry however very incompatible elements ba and k are too variably enriched in pop basalts for this model to be viable small variable degrees of mantle partial melting is not a viable model either because of the substantial depletion of ni which correlates with incompatible element enrichment and because of the precise low pressure cotectic character of pop basalts the in situ fractionation model of langmuir 1987 can explain these features the relative abundance of fractionated lavas their small scale areal chemical zonation the petrochemical correlation between types and geophysical evidence point to the existence of shallow fractionating magma reservoirs beneath the epr at 21 n

tectonic evolution of the salton sea inferred from seismic reflection data the salton sea is an evolving pull apart basin located between the san andreas and imperial faults in southern california seismic and geological data reveal a rapidly subsiding southern sub basin that is bounded by a hinge zone to the north and northwest dipping normal faults to the south oblique extension across strike slip faults causes subsidence and leads to the formation of pull apart basins such as the salton sea in southern california the formation of these basins has generally been studied using laboratory experiments or numerical models1 2 3 4 here we combine seismic reflection data and geological observations from the salton sea to understand the evolution of this nascent pull apart basin our data reveal the presence of a northeast trending hinge zone that separates the sea into northern and southern sub basins differential subsidence 10 mm yr 1 in the southern sub basin suggests the existence of northwest dipping basin bounding faults near the southern shoreline which may control the spatial distribution of young volcanism rotated and truncated strata north of the hinge zone suggest that the onset of extension associated with this pull apart basin began after 0 5 million years ago we suggest that slip is partitioned spatially and temporally into vertical and horizontal domains in the salton sea in contrast to previous models based on historical seismicity patterns5 the rapid subsidence and fault architecture that we document in the southern part of the sea are consistent with experimental models for pull apart basins1

phosphorus burial in sediments of the sulfidic deep black sea key roles for adsorption by calcium carbonate and apatite authigenesis abstract sedimentary burial of the essential nutrient phosphorus p under anoxic and sulfidic conditions is incompletely understood here we use chemical and micro scale spectroscopic methods to characterize sedimentary p burial along a water column redox transect six stations 78 2107 m water depth in the black sea from the shelf with its oxygenated waters to the anoxic and sulfidic deep basin organic p is an important p pool under all redox regimes accounting for up to 60 of p burial we find a general down core increase in the relative importance of organic p especially on the shelf where p bound to iron fe and manganese mn oxyhydr oxides is abundant in the uppermost sediment but rapidly declines in concentration with sediment depth our chemical and spectroscopic data indicate that the carbonate rich sediments unit i 3000 years 0 30 cm depth of the sulfidic deep black sea contain three major p pools calcium phosphate apatite organic p and p that is strongly associated with caco3 and possibly clay surfaces apatite concentrations increase from 5 to 25 of total p in the uppermost centimeters of the deep basin sediments highlighting the importance of apatite formation for long term p burial iron ii associated p ludlamite was detected with x ray absorption spectroscopy but was shown to be a minor p pool 5 indicating that lateral fe p transport from the shelf shuttling likely occurs but does not impact the p burial budget of the deep black sea the caco3 p pool was relatively constant throughout the unit i sediment interval and accounted for up to 55 of total p our results highlight that carbonate bound p can be an important sink for p in caco3 rich sediments of anoxic sulfidic basins and should also be considered as a potential p sink and p source in case of caco3 dissolution when reconstructing past ocean p dynamics from geological records

origin and diagenesis of polyphosphate in lake sediments a 31pߚnmr study polyphosphate poly p was detected with the use of 31 p nuclear magnetic resonance nmr spectroscopy in sediments from a large variety of lakes with different trophic state and morphometry in the top 0 5 cm of sediment poly p was 1 5 to 11 4 of total p nonreactive phosphorus nrp in the naoh fraction often classified as organically bound phosphate was up to 46 inorganic poly p in some surface sediments the poly p content equalled the iron fixed phosphorus determined by chemical phosphorus fractionation sediments were probably supplied with poly p by sedimentation because there were substantial amounts of poly p in plankton and settling seston as demonstrated with sediments of lake petersdorf benthic organisms can also contribute to the formation of poly p up to 0 11 mg p g dry weight 21 under favorable aerobic conditions poly p is more rapidly transformed into single orthophosphate during diagenesis than other inorganic and organic p species the transformation of organic p compounds and poly p can contribute significantly to the release of p during diagenesis and should be considered along with the reductive dissolution of p sorbed to iron oxihydroxides

petrology of alarcon rise lavas gulf of california nascent intracontinental ocean crust 1 the alarcon basin in the southern gulf of california originated by intracontinental rifting of the southwestern margin of north america and has evolved into an intercontinental ocean basin by 3 7 m y of seafloor spreading lava samples collected on the axial ridge along alarcon rise two of the many near axis seamounts on the northwest flank of the rise and an abyssal hill on 0 6 ma crust on the southeast flank were examined petrographically and analyzed for major and trace element and he sr nd pb isotope composition most samples are typical mid ocean ridge basalt morb but rocks at the axial ridge site closest to continent are more differentiated rocks from the axial ridge and abyssal hill are genetically related through separate episodes of fractional crystallization seamount samples are relatively more primitive than seafloor samples and may have been formed by smaller degree of melting of the same mantle source as the seafloor lavas tamayo transform fault has no apparent effect on the geochemistry of axial ridge lavas erupted close to it this is contrary to the postulated transform fault effect alarcon basin differs from the salton trough and guaymas basin in that its youngest seafloor shows no evidence for contamination with continental crust or a subduction component we propose that pacific morb source mantle underlies the gulf of california because the western north american margin has overridden the pacific suboceanic mantle miocene rifting and pliocene spreading in alarcon basin have created a window allowing the most recent morb melts to rise uncontaminated to the surface

cenozoic volcanism and two stage extension in the salton trough southern california and northern baja california pronounced differences in age and geochemistry between cenozoic volcanic rocks within the salton trough southern california and northern baja california imply their formation within two distinct tectonic settings rather than the previously proposed single phase of continental rifting the emplacement of oligocene miocene alkaline lavas overlapped with crustal extension and basin formation in the region during this time post 4 ma subalkaline magmatism related to the gulf of california east pacific rise divergent plate margin exploited crust already ruptured during miocene extension new rare earth element and sr nd isotopic data and k ar 40ar 39ar and u pb ages for basalt andesite and granophyric granite xenoliths contained within the salton buttes rhyolite domes part of the post 4 ma oceanic spreading regime indicate that they are the components of a differentiated magmatic system these data also refute the presence of crystalline basement beneath this part of the salton trough we suggest that the rupture of crystalline basement beneath the axis of the salton trough was more effective during oligocene miocene time

geochemistry of igneous rocks from the cerro prieto geothermal field northern baja california mexico abstract fractional crystallization of basaltic magma derived from an oceanic affinity source region present beneath the salton trough and emplaced into a pull apart basin of this continental rift regime produced a tholeiitic suite of hypabyssal rocks consisting of basalt andesite and dacite within the cerro prieto geothermal field located in northern baja california mexico higher light rare earth element abundances for a basalt from the cerro prieto geothermal field in comparison to basalts from the gulf of california and the east pacific rise suggest partial assimilation of crustal materials into the parental magmas generated beneath the salton trough the crustal contaminant may be present near the surface today in the form of granitoids of the peninsular ranges batholith at deeper levels as hydrothermally altered materials near the base of the salton trough or may be a relict feature of tertiary subduction contained within the upper mantle beneath the salton trough the sr isotopic compositions of dacites from the nearby cerro prieto volcano range from 0 7029 to 0 7036 indicating an oceanic affinity source for these rocks the suite of hypabyssal rocks of tholeiitic affinity present within the cerro prieto geothermal field related by fractional crystallization link the dacite volcano of cerro prieto to gabbroic plutons inferred to exist beneath the cerro prieto geothermal field

seismic characteristics of cavity decoupled explosions in limestone an analysis of soviet high explosive test data during the summer of 1960 soviet scientists conducted a series of high explosive cavity decoupling tests in a mine in kirghizia these decoupled tests were carried out in a variety of mined cavities in limestone including spherical cavities with diameters ranging from 3 5 to 10 m as well as nonspherical cavities with volumes of about 25 m3 the experiments of this test series consisted of 10 tamped and 12 decoupled explosions having yields of 0 1 1 0 and 6 0 t and seismic data were recorded at locations in the mine over a distance range extending from about 10 to 250 m from the sources these data provide valuable new insight into the dependence of decoupling effectiveness on variables such as cavity volume cavity shape and charge emplacement geometry in particular analyses indicate that chemical explosions at a depth of 290 m in limestone are essentially fully decoupled in spherical cavities with scaled cavity radii larger than about 27 m kt and that the low frequency decoupling effectiveness under such conditions is approximately independent of cavity shape for roughly cylindrical cavities with length to width ratios of as much as 6 12 these results suggest that the possibility of cavity decoupling in nonspherical cavities in hard rock media will have to be carefully evaluated in establishing the seismic verification regime for the comprehensive test ban treaty

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

obsidian hydration dating of volcanic events abstract obsidian hydration dating of volcanic events had been compared with ages of the same events determined by the 14c and k ar methods at several localities the localities ranging in age from 1200 to over 1 million yr include newberry craters oregon coso hot springs california salton sea california yellowstone national park wyoming and mineral range utah in most cases the agreement is quite good a number of factors including volcanic glass composition and exposuretemperature history must be known in order to relate hydration thickness to age the effect of composition can be determined from chemical analysis or the refractive index of the glass exposure temperature history requires a number of considerations enumerated in this paper

southern california hypocenter relocation with waveform cross correlation part 2 results using source specific station terms and cluster analysis we obtain precise relative relocations for more than 340 000 southern california earthquakes between 1984 and 2002 by applying the source specific station term ssst method to existing p and s phase picks and a differential lo cation method to about 208 000 events within similar event clusters identified with waveform cross correlation the entire catalog is first relocated by using existing phase picks a reference 1d velocity model and the ssst method of richards dinger and shearer 2000 we also perform separate relocations of imperial valley events by using a velocity model more suited to this region next we apply cluster analysis to the waveform cross correlation output to identify similar event clusters we re locate earthquakes within each similar event cluster by using the differential times alone keeping the cluster centroid fixed to its initial ssst location we estimate standard errors for the relative locations from the internal consistency of differential locations between individual event pairs these errors are often as small as tens of meters in many cases the relocated events within each similar event cluster align in planar features suggestive of faults we observe a surprising number of such faults at small scales that strike nearly perpendicular to the main seismicity trends in general the fine scale details of the seismicity reveal a great deal of structural com plexity in southern california fault systems

uranium thorium series radionuclides in brines and reservoir rocks from two deep geothermal boreholes in the salton sea geothermal field southeastern california naturally occurring u and th series radionuclides have been analyzed in high temperature brines 300 c 25 wt dissolved solids and associated rocks from two deep geothermal wells located on the northeastern margin of the salton sea geothermal field ssgf these data are part of a study of the ssgf as a natural analog of possible radionuclide behavior near a nuclear waste repository constructed in salt beds and permit evaluation of some characteristics of water rock interaction in the ssgf rock brine concentration ratios rc dpm g rock dpm g brine were found to vary from near unity for isotopes of ra pb and rn to about 5 105 for 232th the high sorptivity of 232th is closely followed by that of 238u and 234u rc 5 104 suggesting that u is retained in the 4 oxidation state by the reducing conditions in the brines the relatively high solubility of 210pb and 212pb is attributed to formation of chloride complexes while the high ra solubility is attributed to chloride complexing a lack of suitable adsorption sites due to the high brine salinity and temperature and the reducing conditions that prevent mno2 and raso4 from forming the 228ra226ra ratios in the brines are approximately equal to those of their parents 232th230th in associated rocks indicating that ra equilibration in the brine rock system is achieved within the mean life of 228ra 8 3 years the 224ra228ra ratios in these brines are about 0 7 indicating that either 1 brine composition is not homogeneous and 224ra decays in fracture zones deficient in ra and th as the brine travels to the wellhead or 2 ra equilibration in the brine host rock system is not complete within the mean life of 224ra 5 2 days because the desorption of 224ra from the solid phase is impeded the 228ac228ra activity ratio in the ssgf brines studied is 0 1 and from this ratio the residence time of 228ac in the brine before sorption onto solid surfaces is estimated to be 70 minutes indicating the potential for rapid removal of reactive isotopes fom brines the brine is greatly enriched in 226ra 2 3 dpm g about 104 105 times that of its parent 230th while reservoir rocks are about 10 deficient in 226ra relative to 230th material balance calculations for 2226ra and 18o suggest that brines reside in the reservoir for 102 103 years that the ssgf formed 10 000 40 000 years b p and that porosity cannot be more than 20