oxygen isotope evidence for hydrothermal alteration within a quaternary stratovolcano lassen volcanic national park california brokeoff volcano a quaternary stratocone located in the lassen volcanic center in northern california has been deeply eroded exposing a 10 km2 meteoric hydrothermal alteration zone at the core of the volcano portions of the former volcanic edifice are sufficiently well preserved that an unusual opportunity exists wherein the alteration pattern can be correlated with the position of the volcanic cone the 18o analyses of more than 100 whole rock samples consisting primarily of andesitic lavas vary from 9 8 to 0 6 per mil the highest 18o values occur in bleached solfatarically altered rocks that have interacted with low ph fumarolic hot springs associated with the present day hydrothermal system low 18o values are found in propylitically altered rocks that underwent isotopic exchange with meteoric hydrothermal fluids at elevated temperatures mostly during the stratovolcanic stage 650 400ka of the hydrothermal system but probably continuing today at depth electron microprobe analyses of secondary layer silicate minerals in strongly propylitized samples 18o 5 0 revealed the presence of discrete chlorite suggesting that temperatures up to 200 to 250 c were attained in the shallow levels of the system two zones of pervasive meteoric hydrothermal alteration defined by concentric 18o contours that are probably interconnected at depth are located within the original topographic edifice of the volcano the most intensely altered rocks within these equant zones of alteration define nnw trends that coincide with stream valleys and with regional structural patterns a comparison of the characteristics of the 18o depleted zone at brokeoff with those of more deeply eroded volcanic centers such as the comstock lode mining district criss and champion 1991 permits the construction of composite 18o cross sections through a hypothetical intact stratovolcano at both brokeoff and comstock hydrothermal fluids were strongly focused into plumelike zones of intense 18o depletion at comstock these low 18o plumes are associated with faults although major fault displacements are not observed at brokeoff the topographic and alteration patterns are consistent with the presence of a linear array of faults that acted as conduits for fluid flow up into the shallow levels of the volcano

the makran earthquake of 1983 april 18 a possible analogue to the puget sound earthquake of 1965 summary on 1983 april 18 a large earthquake mb 6 5 m0 1 2x1019 nm occurred in the makran region near the pakistan iran border this earthquake is the only event of mb 6 2 in the entire makran region since the occurrence of the great makran earthquake m 8 0 8 3 in 1945 we determined the source parameters of the 1983 event by formally inverting its p and sh waves recorded by the world wide standard seismograph network wwssn and the canadian seismograph network csn at teleseismic distances the best fitting centroidal solution shows predominantly normal faulting at a depth of about 65 5 km these results provide an additional constraint on the configuration of the down going slab previously inferred from the occurrence of three small to moderate sized events located 100 300 km to the west of the event reported here we also discuss the possibility of drawing an analogy between the occurrence of the 1983 makran event and that of the puget sound earthquake of 1965 m0 1 4x1019nm mb 6 5 and between the seismotectonic settings of the two regions if the very large m 8 0 8 3 earthquake off the makran coast in 1945 is a low angle thrust due to slip along the plate interface its occurrence calls for the possibility of similar seismic risk in the puget sound region however in contrast to inferences made from geological observations in the field about a dozen p wave first motions read from historical seismograms and those reported by the international seismological summary suggest that the 1945 makran earthquake has a large component of normal faulting which can be attributed to the relatively old age of the subducted lithosphere

the earliest open conduit eruptive center of the etnean region evidence from aeromagnetic geophysical and geological data at mount etna the present day active volcano is an open conduit structure characterized by continuous eruptive activity such conditions have been thought unique in the evolution of the etnean volcano as well as in the mediterranean region however a review study of available geophysical data and models combined with geological records petrographic and geochemical considerations has led us to consider that a large area of about 28 km2 located in val calanna on the eastern side of valle del bove can be interpreted as the site of an old open conduit volcano a dyke swarm outcrops in the area whose deep alteration and fumarolization can be attributed to the sustained passage of volcanic gases over long periods radiometric dating yields an age of about 129 ka this finding sheds new light on the evolution of mount etna volcano indicating that the tectonic conditions leading to an open conduit volcano must also have been active in the past

observations of strain accumulation across the san andreas fault near palmdale california with a two color geodimeter two color laser ranging measurements during a 15 month period over a geodetic network spanning the san andreas fault near palmdale california indicate that the crust expands and contracts aseismically in episodes as short as 2 weeks shear strain parallel to the fault has accumulated monotonically since november 1980 but at a variable rate improvements in measurement precision and temporal resolution over those of previous geodetic studies near palmdale have resulted in the definition of a time history of crustal deformation that is much more complex than formerly realized

oxygen isotope aeromagnetic and gravity anomalies associated with hydrothermally altered zones in the yankee fork mining district custer county idaho epithermal ag au vein and disseminated deposits in the yankee fork district are hosted in altered volcanic rocks having low delta 18 o values low magnetic susceptibilities low remanent magnetizations and relatively high densities these isotopic and physical quantities provide sensitive indices of rock alteration that can be contoured over areas that are much larger than zones of strong visible alteration such contour maps reveal a marked spatial association between the ore deposits the steepest delta 18 o gradients and the perimeter of zones where the rocks have extremely low total magnetizations the oxygen isotope results indicate that the ore zones coincide with steep gradients in temperature and water rock ratio in two fossil hydrothermal convective systems that involved fluids derived from tertiary meteoric waters our magnetic measurements suggest that these same high gradient zones also coincide with oxidation reduction boundaries low aeromagnetic intensities and positive bouguer anomalies are also associated with the altered zones in the yankee fork district the intensities of these geophysical features are consistent with the size of the altered zones and with the contrast in physical properties between altered and unaltered rocks contour maps of the magnetic and isotopic properties of host rocks should find application in exploration for a variety of types of hydrothermal ore deposits as well as in the study of ore controls and conditions of mineralization

a shallow fault zone structure illuminated by trapped waves in the karadere duzce branch of the north anatolian fault western turkey summary we discuss the subsurface structure of the karadere duzce branch of the north anatolian fault based on analysis of a large seismic data set recorded by a local passcal network in the 6 months following the mw 7 4 1999 izmit earthquake seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long period oscillations in both p and s wave trains that do not exist in nearby off fault stations examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone the anomalous features in fault zone seismograms produced by events not necessarily in the fault may be referred to generally as fault zone related site effects the oscillatory shear wave trains after the direct s arrival in these seismograms are analysed as trapped waves propagating in a low velocity fault zone layer the time difference between the s arrival and trapped waves group does not grow systematically with increasing source receiver separation along the fault these observations imply that the trapping of seismic energy in the karadere duzce rupture zone is generated by a shallow fault zone layer traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3 4 km the synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m a velocity decrease relative to the surrounding rock of approximately 50 per cent and an s wave quality factor of 10 15 the results are supported by large 2 d and 3 d parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones the inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower type structure the motion amplification associated with fault zone related site effects increases the seismic shaking hazard near fault zone structures the effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large

large scale stable isotope alteration around the hydrothermal carbonate replacement cinco de mayo zn ag deposit mexico carbonate hosted hydrothermal deposits typically show narrow visible mineralogical and textural alteration halos which inhibit exploration targeting in contrast hydrothermal modification of the country rock s stable isotope composition usually extends far beyond the limited visible alteration hence stable isotope studies should be an effective tool to aid exploration for carbonate hosted deposits here we present new insight into the development of a large stable isotope alteration halo based on 910 o and c isotope analyses of carbonate veins and hydrothermally altered limestone hosting the cinco de mayo pb zn ag au cu carbonate replacement deposit crd in chihuahua mexico our results demonstrate that stable isotope alteration is consistent with reactive magmatic fluid flow into unaltered limestone and represents a powerful tool for the characterization of these hydrothermal ore systems synmineralization veins are texturally and isotopically distinct from those formed during pre and postmineralization diagenesis and fluid flow and show distinct gradients along the direction of mineralizing fluid flow this appears to be a promising exploration vectoring tool downhole variations in wall rock isotope values reveal aquifers and aquicludes and outline the principal hydrothermal flow paths furthermore wall rock 18ovsmow systematically decreases toward mineralization from 23 to 17 over a distance of 10 km providing another vectoring tool the extent of the stable isotope alteration halo likely reflects the overall fluid volume and areal extent of a fossil hydrothermal system which may be expected to scale with the mineral endowment this suggests that constraining the size shape and degree of isotopic alteration has direct application to mineral exploration by outlining the system and indicating the potential size of a deposit

geology and isotope geochemistry c o s of the diyadin gold deposit eastern turkey a newly discovered carlin like deposit abstract diyadin mineralization is the first reported gold deposit located in a collisional tectonic environment in eastern anatolia the mineralization is related to n s and n10 20 w trending fault systems and hosted within the paleozoic metamorphic basement rocks of the anatolide toride microcontinent calc schist dolomitic marble and miocene and quaternary volcanic rocks comprise the exposed units in the mineralized area geochemical signatures alteration types and host rock characteristics of the diyadin gold deposit resemble those of carlin type deposits mineralization is constrained by alteration of overlying volcanic rocks to younger than 14 ma k ar carbon and oxygen stable isotope measurements of carbonate rocks were made on six drill holes n 81 with an additional four samples of fresh carbonate rocks from surface outcrops background carbonate rocks have 13cv pdb 1 8 and 18ov smow 27 isotopically altered host rock samples have decreased 18o down to 11 4 and variable 13c from 3 6 to 4 8 postore carbonate veins and cave fill material have distinctly different isotopic signatures particularly carbon from 13c 8 4 to 9 8 whether this post ore carbonate is simply very late in mineralization associated with the gold system or is a completely different younger system utilizing the same pathways is unclear at present within the host rock sample set there is no correlation between gold and 13c and a weak correlation between gold and 18o indicative of water rock interaction and isotopic alteration both the isotopic data and structural mapping suggest that the main upflow zone for the deposit is near the northern portion of the drill fence additional data at multiple scales are required to clarify the relationship s between fluid flow and mineralization

strain on the san andreas fault near palmdale california rapid aseismic change frequently repeated strain measurements near palmdale california during the period from 1971 through 1980 indicate that in addition to a uniform accumulation of right lateral shear strain engineering shear 0 35 microradian per year across the san andreas fault a 1 microstrain contraction perpendicular to the fault that accumulated gradually during the interval 1974 through 1978 was aseismically released between february and november 1979 subsequently november 1979 to march 1980 about half of the contraction was recovered this sequence of strain changes can be explained in terms of south southwestward migration of a slip event consisting of the south southwestward movement of the upper crust on a horizontal detachment surface at a depth of 10 to 30 kilometers the large strain change in 1979 corresponds to the passage of the slip event beneath the san andreas fault

crosswell seismic waveguide phenomenology of reservoir sands shales at offsets 600 m liaohe oil field ne china summary crosswell seismic data recorded at 620 650 m offsets in an oil bearing sand shale reservoir formation at the liaohe oil field northeast china provide robust evidence for waveguide action by low velocity reservoir layers crosswell section velocity models derived from survey well sonic logs and further constrained by observed waveguide seismic wavegroup amplitudes and phases yield plausible evidence for interwell reservoir sand continuity and discontinuity a pair of back to back liaohe crosswell vector seismic surveys were conducted using a source well between two sensor wells at 650 and 620 m offsets along a 200 m thick reservoir formation dipping 7 down to east between depths of 2 5 and 3 km a downhole orbital vibrator generated seismic correlation wavelets with frequency range 50 350 hz and signal noise ratio up to 5 1 over local downhole ambient noise the sensor wells were instrumented with a mobile 12 to 16 level string of clamped vector motion sensor modules at 5 m intervals using 5 m source depth increments crosswell surveys 1 and 2 cover source sensor well intervals above and through the reservoir of respectively 600 m 600 m 13 000 vector traces in 9 common sensor fans and 300 m 560 m 7000 vector traces in 7 common sensor fans survey 1 common sensor gathers show clear consistent high amplitude 20 ms waveletgroup lags behind the first arrival traveltime envelope such arrivals are diagnostic of seismic low velocity waveguides connecting the source and sensor wells observed survey 1 retarded wavegroup depths tally with source and sensor depths in low velocity layers identified in sonic well logs finite difference acoustic model wavefields computed for waveguide acoustic layers constrained by well log sonic velocity data match the observed waveguide traveltime and amplitude systematics model waveforms duplicate the observed m scale and ms scale sensitivity of waveguide spatio temporal energy localization survey 2 crosswell data in contrast provide no comparable evidence for waveguide action despite a sensor well sonic well log similar to that of survey 1 instead acoustic wavefield modelling of survey 2 data clearly favours an interpreted waveguide model with 10 downdip interrupted by a 75 100 m throw down fault near the sensor well the absence of clear waveguide arrivals is adequately explained by dispersal of waveguide energy at the fault discontinuity auxiliary well sonic velocity and lithologic logs confirm the model implied 75 100 m of down throw faulting near the sensor well

geochemistry and stable isotope composition of altered rocks at the golden cross epithermal au ag deposit new zealand golden cross is a late miocene volcanic hosted epithermal au ag deposit associated with quartz calcite adularia illite assemblages located in the coromandel peninsula new zealand gold and ag bearing quartz veins occur in intensely altered flows and lesser pyroclastic volcanic rocks of andesitic and dacitic composition the primary control of whole rock geochemistry at golden cross as at other volcanic hosted epithermal deposits associated with quartz calcite adularia illite assemblages is the destruction of igneous plagioclase by hydrothermal alteration and k metasomatism from hydrothermal fluids quantitative mass change calculations indicate that irrespective of the rock type andesite versus dacite hydrothermally altered rocks are enriched in k rb as and sb around the empire vein system and stockwork in contrast na and sr are strongly depleted around the veins with fe locally depleted silicon ca mg and ba are variably enriched and depleted with si enriched immediately adjacent to the veins and ca only locally enriched the enrichment and depletion trends for these elements are directly related to the degree of alteration and the alteration mineralogy intensely altered rocks show addition of k and rb coupled with loss of na and sr which reflect the replacement of plagioclase by adularia and or illite calcium depletion also reflects the destruction of plagioclase although ca loss is locally offset by the formation of hydrothermal calcite progressively higher na and sr away from the veins reflect greater preservation of plagioclase and the transition into less altered rocks on the margins potassium rb na and sr all define anomalies that extend 200 to 350 m east of the veins and are more widespread than as and sb which are anomalous within 150 to 200 m of the veins the geochemical zonation patterns identified at golden cross are very similar to those at other epithermal deposits associated with quartz calcite adularia illite assemblages potassium na rb sr and ca enrichment and depletion patterns provide insight into the intensity and type of alteration thereby complementing traditional analyses of au ag and other pathfinder elements such as as and sb that are used to explore for epithermal au ag deposits potassium na rb sr and ca enrichment and depletion patterns at golden cross are larger than halos defined by precious metals and other trace element pathfinders thereby providing a larger target area for exploration potassium metasomatism is particularly important because the empire vein system occurs in altered rocks that are significantly enriched in k potassium metasomatism can be evaluated by raw k 2 o data or more effectively by k mass changes molar element ratios of 2ca na k al versus k al wt k 2 o 100 ppm sr rb sr ratios and k numbers k 2ca k na molar whole rock oxygen isotope patterns at golden cross are similar to those described at other epithermal deposits associated with quartz calcite adularia illite assemblages however the area defined by low 18 o values is smaller than the area that can be defined by field mapping of alteration minerals or whole rock major and trace element geochemistry

three dimensional oxygen isotope imaging of convective fluid flow around the big bonanza comstock lode mining district nevada oxygen isotope analyses of propylitized andesites from the con virginia and california mines allow construction of a detailed three dimensional image of the isotopic surfaces produced by the convective fluid flows that deposited the famous big bonanza orebody on a set of intersecting maps and sections the 18o isopleths clearly show the intricate and conformable relationship of the orebody to a deep 500 m gyre of meteoric hydrothermal fluid that circulated along and above the comstock fault near the contact of the davidson granodiorite the core of this gyre 18o 0 to 3 8 encompasses the bonanza and is almost totally surrounded by rocks having much lower 18o values 1 0 to 4 4 this deep gyre may represent a convective longitudinal roll superimposed on a large unicellular meteoric hydrothermal system producing a complex flow field with both radial and longitudinal components that is consistent with experimentally observed patterns of fluid convection in permeable media

applications of algebraic reconstruction techniques to crosshole seismic data tomographic imaging techniques were applied to two crosshole data sets to determine the velocity structures and the reliability and resolution of the algorithms on real data the experiments were carried out at the retsoff salt mine in new york and at the underground radioactive waste study site at the stripa mine facility in sweden the traveltimes at restoff were high quality and were obtained over raypaths of up to 500 m in length the structure was quite complicated with velocity contrasts up to 50 percent the stripa site was in granitic rock with velocity contrasts of only a few percent the dimensions of the experiment were small with maximum ray lengths of just over 10 m the data at this site were collected with very high accuracy source and receiver locations were measured to better than 1 0 mm and traveltimes were read to 0 001 ms a number of algorithms similar to the algebraic reconstruction techniques art used in medical imaging have been applied to the data some modifications of the algorithms such as the application of weighting schemes damping parameters and curved raypaths were performed the resulting velocity fields were compared to the known fields and with each other to determine an optimal method the algorithms were found to be a rapid reliable means of reconstructing the slowness field of real data low velocity zones were recovered with accuracy in location and value it was also found that great care was necessary in application of the techniques to ensure that proper damping parameters are used and the proper number of iterations taken otherwise poor reconstructions will result

primary geochemical characteristics of mineral deposits implications for exploration extract the old adage mineral exploration is like finding a needle in a haystack recaps the difficulty of finding economic mineral deposits near or beneath the surface in general two main approaches can be used to detect the presence of mineral deposits near or beneath the ground surface one employs a mineral deposit model and the other aims at the identification of a primary halo about a mineral deposit a mineral deposit model provides systematic empirical descriptive and or theoretical genetic descriptions of the essential geological geochemical and geophysical features of a type of mineral deposits cf cox and singer 1986 and roberts et al 1988 the primary halo of a mineral deposit as defined originally by safronov 1936 is an area including rock surrounding mineral deposit ore bodies and enriched elements that make up that deposit thus descriptions of primary haloes may form part of a mineral deposit model but both deposit model and primary halo approaches to mineral exploration are based on studies of primary geochemical characteristics of mineral deposits because chemical processes during mineralization are the ones that ultimately bring about metal precipitation or mineral formation however a deposit model approach to mineral exploration is apt for identifying which haystacks contain a needle whereas a primary halo approach to mineral exploration is proper for deducing where in a needle bearing haystack is the needle located

seismic potential associated with subduction in the northwestern united states despite good evidence of present day convergence of the juan de fuca and north american plates there has been remarkably little historical seismic activity along the shallow part of the juan de fuca subduction zone although we cannot completely rule out the possibility that the plate motion is being accommodated by aseismic creep we find that the juan de fuca subduction zone shares many features with other subduction zones that have experienced great earthquakes

oxygen isotope map of the giant metamorphic hydrothermal system around the northern part of the idaho batholith u s a abstract determinations of 18o values from 100 outcrops of belt supergroup wallace fm metasedimentary rocks in the idaho panhandle reveal a regular regional pattern that was produced by pervasive fluid infiltration and isotopic exchange low grade argillites at large distances 60 km from the idaho batholith have high 18o values 15 compatible with their probable primary values pelitic rocks with anomalously low 18o values of 8 7 to 12 7 occur in the following zones 1 in a 5000 km2 zone of schist and gneiss peripheral to the idaho batholith generally coincident with high grade sillimanite bearing assemblages 2 in high grade metasedimentary roof pendants within the idaho batholith 3 peripheral to small cretaceous stocks and 4 within and near the scapolite bearing zone south west of st regis on 18o 18o plots data from coexisting minerals define trends with unit slopes indicating that the reductions in 18o occurred under high grade metamorphic conditions this metamorphism culminated in the emplacement of the idaho batholith probably as a consequence of profound crustal thickening associated with the cretaceous accretion of the wallowa seven devils arc terranes with north america the huge low 18o region is bounded by a steep 18o gradient 0 1 0 5 km that occurs in low grade rocks along and near the lewis and clark line well below the biotite isograd this boundary zone may be analogous to but is not nearly as sharp as those of meteoric hydrothermal systems in many regions the important ore deposits of the coeur d alene district are located in this peripheral zone suggesting that the metamorphic hydrothermal system may have been intimately involved in their formation in addition the metamorphic hydrotermal system in idaho is larger deeper and higher in temperature than typical meteoric hydrothermal systems and it involved fluids with much higher 18o values that were probably dominantly derived from formation waters accordingly this system produced rocks with 18o values similar to those of the idaho batholith and mineral assemblages that approach isotopic equilibrium under high temperature conditions

low sulfidation type au ag mineralization at bergama izmir turkey abstract bergama the center of bergama county is located in western turkey and includes the villages of ovacik narlica and saganci the ovacik epithermal gold silver deposit is located in the western anatolian volcanic and extensional province adjacent to the ene trending bergama graben some 100 km north of the city of izmir gold of economic grades at the ovacik deposit reserves 4 19 mt at 7 6 g t occurs in epithermal quartz veins which display low temperature epithermal textures including crustiform banding quartz pseudomorphs after bladed calcite and multiphase hydrothermal breccias alteration minerals at both ovacik and narlica are dominated by smectite mixed layer illite smectite chalcedonic quartz and adularia whereas major kaolinite and minor mixed layer smectite illite 13 4 a occur at saganci the total sulfide content at ovacik is low geochemical variations in altered wall rocks at ovacik and narlica are generally characterized by two fold enrichments in k rb cs and 25 to 93 depletions in sr ca mg na and even higher depletions 96 to 99 decrease in the quartz adularia vein zone lanthanum ce pr hf zr sm eu gd tb and ho also exhibit up to 50 depletions in the wall rock and even more so up to 90 depletion for la and nd in the vein structure wall rock enrichment in au ag as hg and sc is by factors of 60 150 88 8 and 3 respectively the ranges of ree in both the altered volcanic rocks and quartz adularia veins are wide and may reflect either significant mobilization of ree during alteration and mineralization or their dilution by the metasomatic processes from fresh volcanic rocks through montmorillonite illite adularia altered wallrock to quartz adularia veins high rb sr ratios in adularia illite altered areas are closely related to the presence of k rich alteration minerals low rb sr ratios and corresponding low k values at saganci are due to acid leaching of volcanic rocks positive correlation coefficients between au and ag pb zn cd cu and sb in epithermal quartz veins all of which are 0 51 are significant silver and sb are remarkably enriched at higher levels of the deposit no correlation occurs between au and as or ag and as the ovacik quartz in au ag bearing veins has 18o values ranging from 9 5 to 15 7 whereas d values of fluid inclusions in quartz range from 89 to 125 however the narlica quartz in au ag bearing veins returned 18o values ranging from 5 9 to 8 3 whereas the d values of fluid inclusion in quartz range from 82 to 99 18o results indicate that ore forming hydrothermal fluids at ovacik and narlica had 18oh2o values ranging from 2 9 to 3 5 average 0 24 and from 2 96 to 0 6 average 1 6 respectively they are thus 18o enriched in comparison with present day meteoric and hydrothermal meteoric waters 5 4 18o 18oh2o and d values suggest that mineralizing solutions were a mixture of meteoric and magmatic waters the 34s sulfide data at ovacik and narlica range from 2 1 to 5 3 average 1 7 and from 4 6 to 2 7 average 0 36 respectively these 34s sulfide values are consistent with a magmatic source for s

regional tectonic and earthquake hazard implications of a crustal fault zone in southwestern washington the st helens seismic zone shz is a crustal earthquake zone in southwestern washington that is defined by small to moderate magnitude 2 5 to 5 5 earthquakes and focal mechanisms this zone seismically active along a distance of 90 km is interpreted as a fault zone capable of generating a moderate to large magnitude shallow earthquake at the northern end of this seismic zone about 15 km north of the cowlitz river a large area of seismic quiescence for earthquakes above magnitude 3 0 has predominated since 1977 this quiescence leaves open the question whether the shz continues northward into southern puget sound earthquake focal mechanisms along the shz indicating nearly vertical strike slip faulting with the preferred fault planes striking north south from these focal mechanisms we infer that the direction of maximum compression is northeast approximately parallel with the direction of plate convergence between the north american and juan de fuca plates we interpret the focal mechanism data as evidence for locked subduction this tectonic model raises the possibility of a large magnitude subduction earthquake neither the possibility of a moderate to large shallow crustal earthquake nor a major subduction event is part of the current earthquake hazard assessment for washington

comparison of seismicity parameters in different subduction zones and its implications for the cascadia subduction zone the cascadia subduction zone is a young subduction zone with associated volcanic activity but with no history of large thrust earthquakes associated with it as is characteristic of other subduction zones this has led to questions about the potential for great earthquakes associated with this subduction the seismic potential associated with this subduction has been investigated in this study by examining and comparing the number of shallow earthquakes with magnitude 6 which have occurred in different subduction zones since 1910 this examination shows that all subduction zones exhibit a higher level of activity than cascadia furthermore two of these zones with similar rates of plate motion and age at subduction the rivera plate and southwest japan have not only experienced great earthquakes but show a higher rate of activity than the cascadia zone the activity rate in subduction zones with predominantly aseismic slip is also higher than the activity rate in the cascadia zone earthquake activity in the rupture zones of great earthquakes both before and after the occurrence of great earthquakes is higher than the activity rate in the cascadia zone these results therefore suggest that the low rate of activity in the cascadia zone is unusual and supports the idea that the subduction here is different and that perhaps the potential for great earthquakes cannot be extrapolated simply on the basis of certain similarities between the cascadia zone and other subduction zones with a history of great earthquakes the low rate of earthquake activity in the cascadia zone can however be explained on the basis of the combined effect of low rate of plate motion significant aseismic subduction and young age at subduction the activity rate in the cascadia zone is consistent with the activity rates in other subduction zones if it is assumed that much of the plate motion is accommodated aseismically the seismic slip rate associated with this subduction zone is a small fraction of the rate of plate motion and therefore even if the potential for great earthquakes may exist the return period for great earthquakes will be much longer 2 or 3 times than in other zones with similar characteristics such as the rivera plate or southwest japan

effects of hydrothermal alteration on the magnetization of the oligocene carpenter ridge tuff bachelor caldera san juan mountains colorado intracaldera oligocene carpenter ridge tuff fills the bachelor caldera in the central san juan caldera complex and hosts mineral deposits of the creede mineral district the carpenter ridge tuff and unaltered portions of its intracaldera bachelor mountain member have strong high coercivity reverse magnetizations with average magnetic susceptibility ms and natural remanent magnetization nrm of 6 10 3 volume si and 8 a m respectively oxide phenocrysts in these rocks are titanomagnetite and ilmenite however magnetization appears to be controlled by microcrystic titanomaghemite based on thermal demagnetization unblocking temperatures and curie temperatures between 580 c and 620 c much of the intracaldera tuff was affected by potassic metasomatism a type of hydrothermal alteration characterized by addition of k and loss of ca and na between 27 3 ma and 25 1 ma potassic metasomatism resulted in the replacement of original feldspars by potassium feldspar and quartz oxidation of the original oxide phenocrysts to hematite and rutile and consequent suppression of ms and nrm by a factor of 5 also present however are metasomatized rocks that have high magnetizations even though their original oxide phenocrysts were destroyed values of ms and nrm are similar to those of outflow tuff such rocks are suspected of containing secondary magnetite on the basis of 1 curie and thermal unblocking temperatures at and below 580 c indicating that magnetization is carried by magnetite but that primary microcrysts of maghemite were destroyed and 2 observation in one sample of magnetite rimming cores of hematite rutile the presence of secondary magnetite in metasomatized rocks requires a local shift to more reducing fluid chemistry although we are unable to determine whether this shift occurred during potassic metasomatism or during later alteration or ore deposition these altered rocks may record a change from a pervasive alteration by alkaline oxidizing fluids to a vein controlled alteration dominated by more reduced fluids in which magnetite was stable

origin of ore forming fluids of the haigou gold deposit in the eastern central asian orogenic belt ne china constraints from h o he ar isotopes abstract the haigou lode deposit contains 40 t of gold at an average grade of 3 5 g t and is one of the largest deposits in the jiapigou gold belt located along the eastern segment of the northern margin of the north china craton the deposit comprises 15 gold bearing quartz veins hosted in a carboniferous monzonite monzogranite stock cretaceous dikes consisting of diorite diabase and granodiorite porphyries are well developed in the deposit the diorite porphyry dikes 130 4 6 3 ma occur together with gold bearing quartz veins in nne and ne striking faults gold bearing quartz veins crosscut the diorite porphyry dikes and the veins are in turn crosscut by e w striking 124 6 2 2 ma granodiorite porphyry dikes the mineralization mainly occurs as auriferous quartz veins with minor amounts of sulfide minerals including pyrite galena chalcopyrite and molybdenite gold occurs as either native gold or calaverite common gangue minerals in the deposit include quartz sericite and calcite the deposit is characterized by various types of hydrothermal alteration including silicification sericitization chloritization potassic alteration and carbonatization three stages of hydrothermal activity have been recognized in the deposit 1 a barren quartz stage 2 a polymetallic sulfide gold stage 3 a calcite stage fluid inclusions in hydrothermal pyrites have 3he 4he ratios of 0 3 to 3 3 ra and 40ar 36ar ratios of 351 to 1353 indicating mixing of fluids of mantle and crustal origin hydrothermal quartz yielded 18o values of 1 3 to 7 2 and d values of fluid inclusions in the quartz vary between 80 and 104 these stable isotope data also suggest mixing of magmatic and meteoric fluids noble gas and stable isotopic data suggest that the ore fluids have a predominant mantle source with a significant crustal component based on the spatial association of gold bearing quartz veins with early cretaceous intrusions and the h o he ar isotopic data from the deposit and intrusions we propose that gold mineralization in the haigou deposit was formed in an extensional setting and was related to the early cretaceous mantle derived intrusions

high temperature glacial meltwater rock reaction in the neoproterozoic evidence from zircon in situ oxygen isotopes in granitic gneiss from the sulu orogen abstract to seek the occurrence of negative 18o magmas in the neoproterozoic we conducted in situ zircon o isotope analysis and u pb dating for granitic gneiss from the northeastern sulu orogen east central china zircon u pb dating yields protolith ages of 753 15 ma to 780 13 ma and metamorphic ages of 209 3 to 244 7 ma the neoproterozoic cores with concordant u pb ages exhibit a wide 18o range from 11 0 to 5 8 which is nearly the same as those for cores with discordant u pb ages the triassic rims of some samples have homogeneous 18o values of around 10 whereas the rims of the other samples show a wider range from 9 8 to 5 0 the 18o values as negative as 11 0 for zircons with concordant neoproterozoic u pb ages are reported for the first time representing the primary record of negative 18o magma in the neoproterozoic the continental subduction zone metamorphism in the triassic did not erase the abnormal 18o record in the protolith zircon cores despite metamorphic dehydration and partial melting under high pressure to ultrahigh pressure conditions a conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have 18o values lower than 9 2 corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation the spatial variation in the o isotope compositions of neoproterozoic zircons is a manifestation of the o isotope heterogeneity in the extinct hydrothermal magmatic system the hydrothermal alteration during the neoproterozoic was incongruent which was lately recorded by the wide range of 18o values in the metamorphic zircons of triassic age the extensive o isotope exchange between the surface water and the deep rock requires high temperature and high water rock ratios in continental rifting zones this is ascribed to neoproterozoic splitting of the south china block from the rodinia supercontinent

extra basinal fluid infiltration mass transfer and volume strain during folding insights from the idaho montana thrust belt finite strain and geochemical variations along strain gradients were used to study cleavage development in carbonates in the lost river range idaho deformation accommodating layer parallel shear was partitioned into thin deforma tion zones during folding of willow creek anticline cleavage intensity is strong to very strong in deformation zones and weak in surrounding rocks strain magnitudes range from e s0 32 1 15 0 93 0 73 x y z principal axes of strain outside to e s0 64 1 29 0 80 0 52 inside deformation zones we found a positive linear correlation between strain cleavage development and negative dilation volume loss at a cubic centimeter scale ranges from 12 percent to 49 percent cleavage selvages are depleted in ca and 18 o and enriched in other elements relative to microlithons and less deformed protolith carbonates mass balance considerations indicate that cleavage was formed by incongruent pressure solution leading to a passive concentration of less soluble components during ca loss and metasomatic additions of si al and k to produce authigenic clay minerals in selvages data for the willow creek locality and elsewhere in the lost river range davidson and others 1998 pioneer mountains and the tendoy range bebout and others 2001 show that across the sevier orogen fluid infiltration was heterogeneous at centimeter kilometer scales and resulted from positive feedback between deforma tion and far traveled surficial fluids metasomatic strain softening enhanced deforma tion zone development which generated increased permeability as the thrust belt evolved from a porosity based closed system to a discontinuity based open system increased fluid infiltration and isotopic exchange was associated with volume loss increasingly prolate strains and crystallization of clays to produce the cleavage selvages mass transfer was accommodated by diffusion early and advection later in the deformation history mesoscopic structures deformation zones faults veins focused fluid flow and were kinematically related to larger scale structures faults fault related folds the inferred addition of surficial fluids to depths of 7 kilometers in the thrust belt implies a fluid regime involving significant topographically driven recharge deformed whole rocks and microsamples are lower in 18 o than unde formed samples which have o and c isotope compositions similar to those of marine carbonates veins are even lower in 18 ov smow with minimum values of 5 permil reflecting penetration of the crust by ocean derived precipitation with 18 o near to somewhat lower than 0 permil range of 7 5 to 2 5 calculated for h2 oi n equilibrium with these veins the inferred penetration into the thrust belt of nearshore meteoric waters is consistent with proximity to the reconstructed western interior seaway later fluid infiltration locally lowered the 18 o of carbonates even further based on the o isotope compositions of veins related to younger compres sional deformation and to the onset of crustal scale extension in the eocene this progression toward lowered 18 o of the surficial waters is compatible with the retreat of the seaway during emergence of the thrust wedge and paleogene extension uplift and subaerial volcanism

forecasting southern california earthquakes since 1978 and 1979 california has had a significantly higher frequency of moderate to large earthquakes than in the preceding 25 years in the past such periods have also been associated with major destructive earthquakes of magnitude 7 or greater and the annual probability of occurrence of such an event is now 13 percent in california the increase in seismicity is associated with a marked deviation in the pattern of strain accumulation a correlation that is physically plausible although great earthquakes magnitude greater than 7 5 are too infrequent to have clear associations with any pattern of seismicity that is now observed the san andreas fault in southern california has accumulated sufficient potential displacement since the last rupture in 1857 to generate a great earthquake along part or all of its length

an explanation for contradictory geodetic strain and fault plane solution data in western north america in a few areas of western north america the principal strain azimuths from geodetic strain measurements differ dramatically from the principal stress directions inferred from fault plane solutions of earthquakes the reason for this difference may be related to the amount of strain measured by each technique the strain relieved in an earthquake is about two orders of magnitude greater than that observed by geodetic measurements taken over approximately a seven year time span strain measurements may thus sense short term fluctuations in strain while earthquakes may average these short term fluctuations responding to the total strain accumulated to produce a more reliable measure of the longer term tectonic strain

tertiary meteoric hydrothermal systems and their relation to ore deposition northwestern united states and southern british columbia tertiary meteoric hydrothermal systems have altered the rocks exposed over more than 5 of the land surface of the northwestern united states and southern british columbia including at least 25 000 km 2 in idaho the systems typically involved convective circulation of fluid derived from ordinary meteoric groundwaters around crystallizing calc alkaline epizonal plutons emplaced into coeval volcanic cover rocks these individual systems had widely ranging lifetimes of 10 3 to 10 6 years and operated locally throughout the cenozoic although the most profound development of such activity occurred during eocene time individual systems varied in size from a few tens of square kilometers yankee fork idaho or less to several thousand square kilometers sawtooth and castro ring zones idaho typically regional propylitization aacompanied the fluid circulation although the higher temperature alteration assemblages were developed locally as were intense alteration effects e g silicification sericitization etc near some veins and in mining districts a significant amount probably 25 50 of the mineral production and potential in the region is closely related to tertiary meteoric hydrothermal systems oxygen and hydrogen isotopic data clearly demonstrate the close geologic association of meteoric hydrothermal systems and mineralization in 1 the paleocene cu zn pb mn main stage mineralization at butte montana 2 numerous eocene epithermal deposits principally valued for au and ag but also including significant deposits of cu pb zn f sb etc as at republic washington and in several mining districts in the idaho batholith and the challis volcanic field 3 several eocene skarn deposits valued for w ima idaho and cu mackay idaho 4 important lead silver vein and replacement deposits of tertiary bluebell british columbia and of probable cretaceous and early tertiary age wood river idaho 5 several potentially economic mo be and u bearing eocene porphyry plutons and 6 miocene epithermal deposits most prominently the au and ag bearing veins at silver city and delamar idaho the hg deposits at the mcdermitt caldera nevada and oregon and at weiser idaho and au deposits in the western cascade range and lake county oregon a close spatial association has been demonstrated between ore deposits and rocks having anomalous 18 o values and low d values the most important deposits are associated with relatively small generally 5 300 km 2 zones of low 18 o values and they are particularly closely linked with zones of very steep 18 o 16 o gradients in the altered rocks these associations hold much promise for the use of 18 o and d contour maps in future exploration efforts

geodetic strain measurements in washington two new geodetic measurements of strain accumulation in the state of washington for the interval 1972 1979 are reported near seattle the average principal strain rates are 0 07 0 03 strain yr n 19 w and 0 13 0 02 strain yr n71 e and near richland south central washington the average principal strain rates are 0 02 0 01 strain yr n36 w and 0 04 0 01 strain yr n54 e extension is taken as positive and the uncertainties quoted are standard deviations a measurement of shear strain accumulation dilatation not determined in the epoch 1914 1966 along the north coast of vancouver island by the geodetic survey of canada indicates a marginally significant accumulation of right lateral shear 0 06 0 03 rad yr across the plate boundary n40 w strike although there are significant differences in detail these strain measurements are roughly consistent with a crude dislocation model that represents subduction of the juan de fuca plate the observed accumulation of strain implies that large shallow thrust earthquakes should be expected off the coast of washington and british columbia however this conclusion is not easily reconciled with either observations of elevation change along the washington coast or the focal mechanism solutions for shallow earthquakes in washington

geophysical characteristics of adularia sericite epithermal gold silver deposits in the waihi waitekauri region new zealand aeromagnetic airborne radiometric and surface gravity data from the adularia sericite epithermal province of the hauraki goldfield have delineated distinct anomalies associated with pervasive hydrothermal alteration and gold silver mineralization aeromagnetic derivative images particularly the analytic signal define the boundaries of several magnetic quiet zones that result from magnetite destruction in the volcanic host rocks six discrete zones each of 10 km2 areal extent are evident and are comparable in size to modern day geothermal systems in the taupo volcanic zone an extensive 20 km2 zone in the waitekauri valley is likely to be the result of multiple overlapping systems for which there are also analogues in the taupo volcanic zone that form single large scale alteration zones up to 100 km2 low pass filtering of the analytic signal data reveals still wider zones of relatively low magnetic intensity interpreted as areas of hydrothermally altered rock extending below younger unaltered cover many of these magnetic quiet zones are aligned along a north northeast south southwest structural corridor indicating that regional scale structures may have controlled the location of these geophysical features radiometric data delineate local high potassium anomalies that reflect up to 12 100 g potassium enrichment in the core of alteration zones in the form of adularia and illite a broad k th anomaly correlates with the extent of the magnetic quiet zones and indicates widespread potassium enrichment in the waitekauri maratoto area gravity data in the waihi district define a unique double peaked 50 gravity unit positive residual anomaly that correlates closely with the extent of the magnetic quiet zone and the locations of the waihi and favona deposits preliminary modeling indicates that the anomaly source body beneath the near surface low density altered andesite has a volume of up to 11 km3 and a minimum density of 2 900 kg m 3 the nature of this source body is enigmatic possible causes include a dense intrusion uplifted anomalously dense basement dense sulfide mineralization or some combination of these in addition the correlation of each of the two gravity peaks with a north northeast trending fault may indicate some structural focusing process active at the deposit scale this integration of geophysical data provides an outstanding case study of the district to regional scale geophysical characteristics of a classic epithermal province and demonstrates the strengths of geophysical surveys in the exploration for epithermal mineral deposits

geodetic evidence for active landward tilting of the oregon and washington coastal ranges nine leveling routes that extend inland from near the coast of oregon and washington indicate consistent landward tilting of the 600 km long coastal ranges at about 3 10 8 rad yr 1 analysis of tide gauge observations indicates tilting with roughly the same magnitude and sense as the leveling measurements these short term tilt rates 10 80 yr are comparable with longer term rates 100 000 yr deduced from tilted marine terraces contemporary landward tilting is interpreted to result from continuing subduction of the juan de fuca plate beneath north america the similarity between long and short term deformation suggests that present day subduction is occurring via aseismic creep as hypothesized by ando and balazs 1979 however recent measurements of horizontal deformation in northwest washington which have been interpreted to indicate elastic strain accumulation appear to be incompatible with aseismic subduction additional releveling observations along the oregon routes last observed in 1941 and further studies of late pleistocene deformation along the coastal region of oregon and washington could help determine present day tectonics and assess the risk of large thrust earthquakes in this area

magnetic effects of hydrothermal alteration in porphyry copper and iron oxide copper gold systems a review abstract magnetic anomaly patterns can be used as a tool for mapping lithology metamorphic zones and hydrothermal alteration systems as well as identifying structures that may control passage of magmas or hydrothermal fluids associated with mineralisation reliable geological interpretation of mineralised systems requires an understanding of the magmatic metamorphic and hydrothermal processes that create alter and destroy magnetic minerals in rocks predictive magnetic exploration models for porphyry copper and iron oxide copper gold iocg deposits can be derived from standard geological models by integrating magnetic petrological principles with petrophysical data deposit descriptions and modelling of observed magnetic signatures of these deposits even within a particular geological province the magnetic signatures of similar deposits may differ substantially due to differences in the local geological setting searching for look alike signatures of a known deposit is likely to be unrewarding unless pertinent geological factors are taken into account these factors include the tectonic setting and magma type composition and disposition of host rocks depth of emplacement and post emplacement erosion level depth of burial beneath younger cover post emplacement faulting and tilting remanence effects contingent on ages of intrusion and alteration and metamorphism because the effects of these factors on magnetic signatures are reasonably well understood theoretical magnetic signatures appropriate for the local geological environment can qualitatively guide exploration and make semiquantitative predictions of anomaly amplitudes and patterns the predictive models also allow detectability of deposit signatures to be assessed for example when deposits are buried beneath a considerable thickness of nonmagnetic overburden are covered by highly magnetic heterogeneous volcanic rocks or there is a strong regional magnetic gradient this paper reviews the effects of hydrothermal alteration on magnetic properties and magnetic signatures of porphyry copper and iron oxide copper gold systems and presents examples of predictive magnetic exploration models and their predicted signatures in various geological circumstances