the incorporation of pb into zircon abstract the incorporation of pb into zircons grown from pb rich solutions was evaluated using three different approaches 1 high temperature growth of large crystals from pb silicate melts 2 hydrothermal overplating of thin epitaxial layers on substrates of natural zircon and 3 growth of small homogeneously nucleated crystals from aqueous fluids the melt grown zircons 50 400 m were crystallized from pbo sio2 zro2 p2o5 liquid at atmospheric pressure by cooling from 1430 to 1350 c in the p2o5 free system despite 66 wt pbo in the melt these zircons contain 3 atom pb with apparent zircon fluid partition coefficients of 4 2 and 2 6 respectively for pb4 and pb2 in contrast to the case of melt grown zircons available p is excluded from the aqueous epitaxial zircon suggesting that charge balance is accomplished by h instead small 2 5 m zircons grown by cooling aqueous solutions pbo sio2 zro2 p2o5 from 800 c or 900 c contain 0 25 0 5 atom pb 2 4 wt pbo yielding apparent dpb values of 0 2 0 3 available p5 is incorporated in a 2 1 ratio with pb2 suggesting a specific charge balance mechanism 2p5 pb2 2si4 zr4 however pb enters the zircon even when p is unavailable so h may again play a charge balancing role because of the rapid polythermal modes of zircon growth and the high pb content of the experimental systems the apparent partition coefficients should not be viewed as equilibrium values but as qualitative indicators of pb compatibility under various growth circumstances the overall results are consistent with the low but variable levels of non radiogenic common pb in natural zircons the increased compatibility of pb in fluid grown low temperature zircons suggests a possible fingerprint for zircons from hydrothermal and wet metamorphic rocks i e high concentrations of common pb

the juan de fuca ridge hot spot propagating rift system new tectonic geochemical and magnetic data underway geophysics deep tow camera work and preliminary geochemical and magnetic studies from a 1980 cruise to the juan de fuca ridge allow the following interpretations 1 the central third between 45 5 n and 46 5 n of the ridge is now actively spreading from a zone which is 10 to 20 km west of the axis bisecting the brunhes matuyama magnetic reversal boundary 2 the apparent terminus of surficial igneous activity at the northern tip of the active cobb propagator has been identified by dredging and camera work the tip appears curved or offset slightly to the west does not correspond to the northernmost tip of the central magnetic anomaly elvers et al 1974 and spreading has been initiated beneath sediments farther north and 3 iron and titanium in fresh glasses from the active volcanic zone exhibit a progressive enrichment northward then a pronounced reversal to more normal concentrations in the vicinity of the cobb offset

seismicity at the cascadia plate boundary beneath the oregon continental shelf since 2003 39 small earthquakes have been detected off the coast of central oregon in the nominally locked part of the cascadia subduction zone where very little seismic activity has been recorded in spite of a paleoseismic record of great subduction events although the regional earthquake bulletin reports depths of 29 and 28 km for the two largest events m w 4 9 and m w 4 7 which occurred in 2004 analysis by trehu et al 2008 indicates that they were low angle thrust events that occurred on the plate boundary at depths of 9 11 and 16 km respectively because of sparse onshore station coverage most of the smaller events have large location uncertainties double difference relative location of 30 of these earthquakes reveals two tight clusters approximately 30 km apart each cluster is associated with one of the two larger events within each cluster relocation reduces the hypocenter depth spread from 15 km to 3 km with uncertainties on the order of 0 1 km the relocations combined with independent absolute hypocenter locations for two 2008 events using a deployment of land and ocean bottom seismometers suggest that the seismicity occurred at plate boundary depths possibly on the cascadia megathrust this concentrated activity in the seismogenic zone may represent patches on the fault plane with anomalous frictional characteristics possibly caused by subducted topographic features which can affect the propagation of a large megathrust rupture

segmentation of the nazca and south american plates along the ecuador subduction zone from wide angle seismic profiles we describe the deep structure of the south colombian northern ecuador convergent margin using travel time inversion of wide angle seismic data recently collected offshore the margin appears segmented into three contrasting zones in the north zone affected by four great subduction earthquakes during the 20th century normal oceanic crust subducts beneath the oceanic cretaceous substratum of the margin underlined by seismic velocities as high as 6 0 6 5 km s in the central zone the subducting oceanic crust is over thickened beneath the carnegie ridge a steeper slope and a well developed high velocity cretaceous oceanic basement characterizes the margin wedge this area coincides with a gap in significant subduction earthquake activity in the south zone the subducting oceanic crust is normal the fore arc is characterized by large sedimentary basins suggesting significant subsidence velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing even if the distance between the three profiles exceeds 150 km the structural segmentation obtained along the ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation

seismic reflection imaging of the juan de fuca plate from ridge to trench new constraints on the distribution of faulting and evolution of the crust prior to subduction we present prestack time migrated multichannel seismic images along two cross plate transects from the juan de fuca jdf ridge to the cascadia deformation front df offshore oregon and washington from which we characterize crustal structure distribution and extent of faults across the plate interior as the crust ages and near the df in response to subduction bending within the plate interior we observe numerous small offset faults in the sediment section beginning 50 70 km from the ridge axis with sparse fault plane reflections confined to the upper crust plate bending due to sediment loading and subduction initiates at 120 150 km and 65 80 km seaward of the df respectively and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity most bend faulting deformation occurs within 40 km from the df on the oregon transect bright fault plane reflections that extend through the crust and 6 7 km into the mantle are observed if attributed to serpentinization 0 12 0 92 wt water within the uppermost 6 km of the mantle is estimated on the washington transect bending faults are confined to the sediment section and upper middle crust the regional difference in subduction bend faulting and potential hydration of the jdf plate is inconsistent with the spatial distribution of intermediate depth intraslab seismicity at cascadia a series of distinctive ridgeward dipping 20 40 lower crustal reflections are imaged in 6 8 ma crust along both transects and are interpreted as ductile shear zones formed within the ridge s accretionary zone in response to temporal variations in mantle upwelling possibly associated with previously recognized plate reorganizations at 8 5 ma and 5 9 ma

wave field decomposition of ocean bottom seismograms seismograms recorded on the seafloor are affected by reverberations in the overlying ocean layer we examine the feasibility of removing these reverberations through a 1d wave field decomposition based on a reflection transmission formulation of the problem two decomposition schemes are presented the first scheme involves a simple manipulation of the fundamental matrix relating stress and displacement of plane harmonic waves to upgoing and downgoing wavevector coefficients and requires measurement of both ocean bottom displacement and pressure the second approach involves only displacement recordings and knowledge of the water column depth this latter quantity will generally be known a priori from deployment logs or alternatively it may be estimated using vertical displacement and pressure seismograms in a preprocessing step both approaches require prior information on seabed properties in the case of p wave incidence the decomposition depends primarily on the seabed s velocity this quantity can be determined by examining trial decompositions over a range of s and selecting the value that minimizes the energy of the upgoing s wave component at the arrival time of the incident wave we apply this approach to synthetic seismograms for a simple earth structure and to recordings of two large events from the central oregon locked zone array colza on the continental margin of oregon the real data indicate that the wave field decompositions are largely successful at lower frequencies 0 1 hz but that 3d scattering likely originating near the sediment basement contact is manifested at higher frequencies

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

chronology of multiphase emplacement of the salmi rapakivi granite anorthosite complex baltic shield implications for magmatic evolution the u pb dating of 18 samples representing the principal rock types of the 4000 km2 salmi anorthosite rapakivi granite complex and its satellite uljalegi pluton southeastern baltic fennoscandian shield reveals that six temporally distinct episodes of igneous activity occurred in a timespan of 17 million years from oldest to youngest they are 1 gabbronorite and monzonite at 1546 7 ma 2 syenogranite at 1543 4 ma 3 early wiborgite and pyterlite at 1540 6 1537 9 ma 4 biotite granite and more evolved granite at 1538 4 1535 ma 5 late pyterlite at 1535 2 ma 6 olivine gabbro and biotite amphibole granite at 1530 ma the resolvable intervals between magmatic episodes are 3 5 5 0 million years early wiborgite and pyterlite 3 above and biotite granite 4 above probably crystallized from multiple magma intrusions age differences of 3 4 1 5 million years between zircon and baddeleyite in olivine gabbro 6 above are probably a result of xenocrystic origin of baddeleyite extracted from an earlier mafic phase of the salmi complex the ages and chemical features of early and late zircon populations together with our modeling of magma crystallization and zircon growth show that the duration of magma crystallization and pb diffusion in zircon was short lived and insignificant compared to the precision of dating of about 1 2 million years hence the range of u pb ages for each of the major rock types may approximate the emplacement intervals of their respective magmas average rate of magma emplacement was about 0 01 km3 year for the most voluminous phase of early biotite amphibole rapakivi granite and about 0 0024 km3 year for the salmi complex as a whole compositional changes of the salmi magmas over time are in agreement with the model of magmatism related to lithospheric extension

cleopatra crater on venus venera 15 16 data and impact volcanic origin controversy cleopatra structure is a 100 km diameter feature having a morphology similar to that of double ring basins of the moon and mercury and dissimilar to that of volcanic calderas on mars earth and venus the 2 4 km depth of cleopatra is anomalously large compared to venusian and terrestrial impact craters of equivalent diameters an impartial summary of the situation is as follows if cleopatra is a volcanic caldera it is a strange caldera if cleopatra is an impact crater it is a strange crater

late holocene tectonics and paleoseismicity southern cascadia subduction zone holocene deformation indicative of large subduction zone earthquakes has occurred on two large thrust fault systems in the humboldt bay region of northern california displaced stratigraphic markers record three offsets of 5 to 7 meters each on the little salmon fault during the past 1700 years smaller and less frequent holocene displacements have occurred in the mad river fault zone elsewhere as many as five episodes of sudden subsidence of marsh peats and fossil forests and uplift of marine terraces are recorded carbon 14 dates suggest that the faulting subsidence and uplift events were synchronous relations between magnitude and various fault offset parameters indicate that earthquakes accompanying displacements on the little salmon fault had magnitudes of at least 7 6 to 7 8 more likely this faulting accompanied rupture of the boundary between the gorda and north american plates and magnitudes were about 8 4 or greater

reports venus identification of banded terrain in the mountains of ishtar terra high resolution images obtained with the arecibo radar system at a wavelength of 12 6 centimeters reveal numerous parallel 10 to 20 kilometer wide bands of high radar backscatter situated on and oriented parallel to the major mountain ranges of the ishtar terra region of venus geometric and morphological characteristics suggest that the bands represent earth like tectonic deformational features such as folds and faults

seismic evidence for widespread serpentinized forearc upper mantle along the cascadia margin petrologic models suggest that dehydration and metamorphism of subducting slabs release water that serpentinizes the overlying forearc mantle to test these models we use the results of controlled source seismic surveys and earthquake tomography to map the upper mantle along the cascadia margin forearc we find anomalously low upper mantle velocities and or weak wide angle reflections from the top of the upper mantle in a narrow region along the margin compatible with recent teleseismic studies and indicative of a serpentinized upper mantle the existence of a hydrated forearc upper mantle wedge in cascadia has important geological and geophysical implications for example shearing within the upper mantle inferred from seismic reflectivity and consistent with its serpentinite rheology may occur during aseismic slow slip events on the megathrust in addition progressive dehydration of the hydrated mantle wedge south of the mendocino triple junction may enhance the effects of a slab gap during the evolution of the california margin

crustal structure of the colorado plateau arizona application of new long offset seismic data analysis techniques the colorado plateau is a large crustal block in the southwestern united states that has been raised intact nearly 2 km above sea level since cretaceous marine sediments were deposited on its surface controversy exists concerning the thickness of the plateau crust and the source of its buoyancy interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick a thick crust could support the observed topography of the colorado plateau isostatically while a thinner crust would indicate the presence of an underlying low density mantle this paper reports results on long offset seismic data collected during the 1989 segment of the u s geological survey pacific to arizona crustal experiment that extended from the transition zone into the colorado plateau in northwest arizona we apply two new methods to analyze long offset data that employ finite difference travel time calculations 1 a first arrival time inverter to find upper crustal velocity structure and 2 a forward modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals we find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern basin and range province to about 42 km beneath the northern transition zone and southern colorado plateau margin we observe some crustal thinning to 37 km thick and slightly higher lower crustal velocities farther inboard beneath the kaibab uplift on the north rim of the grand canyon the crust thickens to a maximum of 48 km we observe a nonuniform crustal thickness beneath the colorado plateau that varies by 15 and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations crustal compositions as inferred from seismic velocities appear to be the same beneath the colorado plateau as those in the basin and range province to the southwest implying that the plateau crust represents an unextended version of the basin and range some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the proterozoic era

mantle downwelling and crustal convergence a model for ishtar terra venus the ishtar terra region contains the highest topography known on venus over 10 km above mean planetary radius as well as abundant tectonic features many of apparently compressional origin these characteristics suggest that ishtar is a crustal convergence zone overlying downwelling mantle in order to explore quantitatively the implications of this hypothesis for ishtar s origin we present models of viscous crustal flow driven by gradients in topography assuming a free slip surface boundary condition we find that if the crustal convergence hypothesis is correct then the crustal thickness in the plains surrounding ishtar can be no more than about 25 km thick this upper bound assumes a cold 10 k km 1 geotherm and the stiffest available diabase flow law if the geothermal gradient is larger or the rheology is weaker the crust must be even thinner for net crustal convergence to be possible this upper bound is in good agreement with several independent estimates of crustal thickness of 15 30 km in the plains of venus based on modeling of the spacing of tectonic features and of impact crater relaxation if the surface layer of venus provides a no slip boundary then our models allow the crustal thickness in the plains to be up to 50 km but the likely existence of faults that cut through the crust makes a no slip surface layer unlikely our upper bound on crustal thickness is much less than that derived from an airy isostasy model of ishtar s gravity anomaly much of the observed gravity anomaly must be due to density anomalies in the mantle beneath ishtar although we treat ishtar as a crustal convergence zone our crustal flow model shows that under some circumstances near surface material may actually flow away from ishtar providing a possible explanation for grabenlike structures in fortuna tessera

nanoscale occurrence of pb in an archean zircon abstract we report for the first time a direct atomic scale characterization of pb in zircon 4 4 3 1 ga from the early archean yilgarn craton in australia using high resolution haadf stem two forms of pb have been identified pb concentrated at 3 atom as a nanoscale patch in zircon structure and pb concentrated within the amorphous domain created by fission fragment damage the first result suggests that the pb atoms directly substitute for zr4 in the zircon structure and the latter observation demonstrates that pb diffusion can occur through amorphous regions created by radiation damage although volume diffusion is typically considered to be the dominant mechanism for pb diffusion beyond the first percolation point i e when the amorphous domains overlap and form a fully interconnected network of amorphous domains there is a new pathway for the diffusion of pb that is faster than volume diffusion through crystalline zircon

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

tectonic control on sediment accretion and subduction off south central chile implications for coseismic rupture processes of the 1960 and 2010 megathrust earthquakes based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the south central chile margin 33 46 s we study the processes of sediment accretion and subduction and their implications on megathrust seismicity in terms of the frontal accretionary prism fap size the marine south central chile fore arc can be divided in two main segments 1 the maule segment south of the juan fernandez ridge and north of the mocha block characterized by a relative large fap 20 40 km wide and 2 the chiloe segment south of the mocha block and north of the nazca antarctic south america plates junction characterized by a small fap 10 km wide in addition the maule and chiloe segments correlate with a thin 1 km thick and thick 1 5 km thick subduction channel respectively the mocha block lies between 37 5 and 40 s and is configured by the chile trench mocha and valdivia fracture zones this region separates young 0 25 ma oceanic lithosphere in the south from old 30 35 ma oceanic lithosphere in the north and it represents a fundamental tectonic boundary separating two different styles of sediment accretion and subduction respectively a process responsible for this segmentation could be related to differences in initial angles of subduction which in turn depend on the amplitude of the down deflected oceanic lithosphere under trench sediment loading on the other hand a small fap along the chiloe segment is coincident with the rupture area of the trans pacific tsunamigenic 1960 earthquake mw 9 5 while a relatively large fap along the maule segment is coincident with the rupture area of the 2010 earthquake mw 8 8 differences in earthquake and tsunami magnitudes between these events can be explained in terms of the fap size along the chiloe and maule segments that control the location of the updip limit of the seismogenic zone the rupture area of the 1960 event also correlates with a thick subduction channel chiloe segment that may provide enough smoothness at the subduction interface allowing long lateral earthquake rupture propagation

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

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

a crystal chemical basis for pb retention and fission track annealing systematics in u bearing minerals with implications for geochronology this study develops an empirical crystal chemical framework for systematizing the kinetics of pb loss and fission track annealing in u bearing minerals ionic porosity z the fraction of a mineral s unit cell volume not occupied by ions potentially accounts for kinetic behavior by monitoring mean metal oxygen bond length strength various tests of a general kinetics porosity relationship are presented based upon diverse mineral data including 1 pb diffusion parameters 2 measured closure temperatures tc for fission track annealing and 3 retentivities of both pb and fission tracks from apparent age data every kinetic parameter including tc and mineral age for both the u pb and fission track systems is inversely correlated with z within the sub assemblage zircon z 29 titanite 34 and apatite 38 assuming a diffusional closure model pb isotopic transport phenomena are described by a tc z scale calibrated with field based tc data for titanite 680 20 c and apatite 500 c extrapolation of this scale yields tc estimates for the following minerals staurolite tc 1060 c z 25 garnet 1010 c 26 5 zircon 900 c monazite xenotime and epidote 750 c 32 and ca clinopyroxene 670 30 c 34 1 depending on composition these empirical results imply that a u pb pb date for staurolite or garnet records the time of mineral growth not post growth isotopic closure as also concluded in recent field studies because z systematizes fission track annealing this recrystallization process like volume diffusion must also be rate limited by the strength of chemical bonds the extent to which other recrystallization processes are likewise rate limited is important to u pb geochronology because they potentially compete with diffusion as mechanisms for pb isotopic resetting in nature

oblique strike slip faulting of the central cascadia submarine forearc at least nine wnw trending left lateral strike slip faults have been mapped on the oregon washington continental margin using sidescan sonar seismic reflection and bathymetric data augmented by submersible observations the faults range in length from 33 to 115 km and cross much of the continental slope five faults offset both the juan de fuca plate and north american plates and cross the plate boundary with little or no offset by the frontal thrust left lateral separation of channels folds and holocene sediments indicate active slip during the holocene and late pleistocene offset of surficial features ranges from 120 to 900 m and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2 2 to 5 5 km of total slip near their western tips fault ages range from 300 ka to 650 ka yielding late pleistocene holocene slip rates of 5 5 2 to 8 5 2 mm yr the geometry and slip direction of these faults implies clockwise rotation of fault bounded blocks about vertical axes within the cascadia forearc structural relationships indicate that some of the faults probably originate in the juan de fuca plate and propagate into the overlying forearc the basement involved faults may originate as shears antithetic to a dextral shear couple within the slab as plate coupling forces are probably insufficient to rupture the oceanic lithosphere the set of sinistral faults is consistent with a model of regional deformation of the submarine forearc defined to include the deforming slab by right simple shear driven by oblique subduction of the juan de fuca plate

stonewall anticline an active fold on the oregon continental shelf stonewall bank is the site of a growing west verging anticline striking north northwest on the continental shelf at 44 5 n southwest of newport oregon to the east are pliocene pleistocene strata of the newport syncline onlapping eastward against gently west dipping late miocene and older rocks of the oregon coast range folding of stonewall anticline results in fine grained middle miocene strata being exposed at the sea floor along the anticlinal crest rocks as old as eocene are encountered in the unocal p 0093 1 grebe well drilled at the anticlinal crest rates of folding are based on deformation of an unconformity between pliocene and miocene strata pm unconformity and of a stream channel that crossed stonewall bank during the last glacial maximum the bed length of the unconformity is shortened across the stonewall anticline and adjacent folds by about 400 m between structures west of stonewall bank and the newport syncline the pm unconformity has a vertical separation of about 1000 m between the anticline and the first syncline to the west the horizontal shortening and vertical separation imply that stonewall anticline is underlain by a blind reverse fault retrodeforming the pm unconformity shows that this fault dips 65 70 e a vertical separation of 1000 m on a fault with this dip yields a slip of 1070 1080 m along the fault if folding of the pm unconformity is assumed to have begun 2 3 ma this would give a long term slip rate of 0 4 0 6 mm yr if most folding began after deposition of the entire pliocene pleistocene sequence the slip rate would be 1 0 1 1

migration of lead in non metamict zircon abstract the problem of activation energy calculations of lead loss from zircon and the determination of the lead diffusion coefficient in zircon based on the kinetics of diffusion is dealt with the experimental technique for the evaluation of the diffusion parameters used in this study is thermal ionization mass spectrometry thims the results of experiments on the kinetics of lead loss from several zircons under linear heating step heating and isothermal heating regimes are considered the study of effects of grain size on the kinetics of lead loss is performed under all regimes a comparison of various mechanisms of lead diffusion in zircon is also made the experimental results may be much better explained by the principles of volume diffusion than by those of first order reactions the values of activation energies calculated from volume diffusion are in the range 7 8 to 9 5 ev for crystalline zircons the suitability of thims technique for lead migration studies is considered

impact craters of venus analysis of venera 15 and 16 data from description of impact craters on venus and crater size and depth distributions models are developed that analyze the size distribution curve and radar bright haloes around fresh craters the dense venusian atmosphere has a strong influence on cratering the estimated age of the surface of the area under study is from 0 5 to 1 0 109 years the estimated rate of resurfacing during this time interval is about 0 04 to 0 8 cm 106 years the lithosphere of venus seems to be sufficiently rigid to sustain craters of 100 140 km diameter and 0 8 2 5 km depth for about 108 years

impact craters of venus a continuation of the analysis of data from the venera 15 and 16 spacecraft additional studies of venusian impact craters have been made based on an analysis of venera 15 16 radar imagery and altimetry the crater population on venus has been subdivided into groups representing different morphological classes and types the craters display the size dependent variations in morphology which are well known from other planets assuming a crater production rate based on estimates by hartmann et al 1981 their areal density indicates an age for the total population of approximately 1 b y alternative estimates based on the assumption that the recent cratering rates on the earth and venus were similar would give a significantly lower age the areal and size frequency distribution of a number of circular features of unclear origin have also been analyzed and it is proposed that some of these features may be highly degraded and modified impact craters corresponding to a population approximately 3 b y in age a suggested model of the effect of the atmosphere on the crater diameter frequency curve indicates that cometary impacts played only a minor role in forming the contemporary venus environment a comparative study of craters in the area where the arecibo earth based radar and venera 15 16 images overlap indicates that the radar dark inner zones of the craters in the high incidence angle arecibo images correspond to the flat bottoms of the craters seen in the venera images

postimpact modification by volcanic or tectonic processes as the rule not the exception for venusian craters 1 we present three investigations that use the venusian impact crater population to constrain the planet s resurfacing history we evaluate stereo derived topography for 91 venusian craters that have a diameter d greater than 15 km craters with radar bright floors have greater rim floor depths and rim heights than craters with radar dark floors for the bright floored craters rim floor depths are dbf 0 483 d0 165 and rim heights are rhbf 0 056 d0 483 trends for dark floored craters are ddf 0 424 d0 108 and rhdf 0 181 d 0 025 for a 60 km crater this represents differences of 290 m in rim floor depth and 240 m in rim height we interpret these results to indicate that dark floored craters have experienced postimpact volcanic embayment and filling we examine the population of craters with d 20 km that have radar dark halos surrounding their continuous ejecta 114 craters we find that a portion of the halo has been removed for almost all dark floored craters consistent with our interpretation that dark floored craters have been affected by postimpact volcanism finally we assessed geologic histories of 12 large impact structures with stereo coverage all but one of these structures has experienced postimpact volcanism or tectonic deformation often in multiple episodes in summary widespread volcanic and tectonic activity occurred throughout the time period of emplacement of the crater population postulated resurfacing histories that consider the majority of craters to be at the top of the stratigraphic column are invalid and the mean surface age of venus is young 150 my

estimating the thickness of the free gas zone beneath hydrate ridge oregon continental margin from seismic velocities and attenuation recent ocean drilling program odp results in the oregon accretionary prism and on the blake ridge indicate that the zone containing free gas beneath the hydrate bearing near surface sediments is considerably thicker than previously thought in this paper we present results from travel time inversion of refracted seismic waves that show very low 100 in the normal accretionary complex sediments west of hydrate ridge these results suggest that refraction seismic techniques are a powerful way to constrain the depth to which free gas is present in sediments beneath the hydrate stability zone defining the extent of the free gas zone is an important factor for estimating the total volume of gas present and for evaluating its impact on slope stability and potential contribution to global climate change

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

identifying low frequency earthquakes in central cascadia using cross station correlation abstract we use stations in the willamette valley and oregon coast ranges to identify low frequency earthquakes that occurred during the august september 2009 episodic tremor and slip event while autocorrelation techniques are often successful at identifying lfe templates in central cascadia this technique systematically fails due to the lack of densely spaced high quality stations instead we use cross station methods that have been successful at identifying lfes in northern cascadia to register initial candidate templates network cross correlation to register additional lfe detections and stacking to refine the identified templates using this procedure we detect nearly 16 thousand events comprising a total of 18 lfe families located in central oregon between 30 and 40 km depth either at or near the plate boundary the time history of detections between families is consistent with the slip front in the 2009 sse migrating from north to south at a velocity of 5 km day the templates we identify have more complicated waveforms than those previously identified in northern cascadia these differences in waveform character likely a consequence of the small number of contributing stations

transport of uranium thorium and lead in metamict zircon under low temperature hydrothermal conditions abstract understanding the stability of radiation damaged zircon under low temperature hydrothermal conditions is crucial to the application of zircon for u pb geochronology and as a host phase for the disposal of plutonium waste we report the results of an investigation of the stability of partially metamict zircon by leaching experiments at 175 c with a 2 m alcl 3 and a 1 m hcl cacl 2 solution as hydrothermal fluids for 1340 h cathodoluminescence cl and backscattered electron bse images show that the zircon grains have developed a reaction rim several micrometers thick or deeply penetrating reticulated alteration zones with sharp boundaries to unaltered metamict zircon these zones have experienced severe loss of si u th and pb and gain of al or ca and a water species as revealed by electron microprobe sensitive high resolution ion microprobe shrimp analyses and infrared spectroscopy micro raman and infrared measurements on the altered areas show that disordered crystalline remnants of the partially metamict zircon structure were partially recovered whereas recrystallization of the embedding amorphous phase was not observed no detectable structural or chemical changes were detected inside the unaltered areas intensive fracturing which was most intense in the hcl cacl 2 experiment occurred inside the altered areas due to the volume reduction associated with the recovery of the disordered crystalline material and probably with the leaching reactions we explain the formation of deep penetrating alteration zones by a percolation type diffusion model which is based on the assumption that percolating interfaces or areas of low atomic density between crystalline and amorphous regions as well as between amorphous domains exist along which fast chemical transport is possible the idea of the existence of fast diffusion interfaces is supported by the sharp chemical gradients at the margin to unreacted zircon the model was used to estimate for the first time diffusion coefficients for u th and pb diffusion in amorphous zircon at 175 c by assuming that volume diffusion inside the amorphous domains is the loss rate limiting process these estimates show that volume diffusion in metamict zircon can cause significant loss of u th and especially loss of radiogenic pb over geological time scales even at temperatures as low as 175 c our results show that recent pb loss discordias can be generated 1 by predominate pb loss from metamict zircon through volume diffusion at low temperatures where thermal healing of the structure is insignificant and 2 by leaching of pb and u and th from metamict zircon through an external fluid

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

impact craters on venus initial analysis from magellan magellan radar images of 15 percent of the planet show 135 craters of probable impact origin craters more than 15 km across tend to contain central peaks multiple central peaks and peak rings many craters smaller than 15 km exhibit multiple floors or appear in clusters these phenomena are attributed to atmospheric breakup of incoming meteoroids additionally the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across ejecta is found at greater distances than that predicted by simple ballistic emplacement and the distal ends of some ejecta deposits are lobate these characteristics may represent surface flows of material initially entrained in the atmosphere many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid craters are absent from several large areas such as a 5 million square kilometer region around sappho patera where the most likely explanation for the dearth of craters is volcanic resurfacing there is apparently a spectrum of surface ages on venus ranging approximately from 0 to 800 million years and therefore venus must be a geologically active planet

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

interseismic uplift rates for western oregon and along strike variation in locking on the cascadia subduction zone 1 we quantify the spatial pattern of uplift rate in western oregon and northernmost california using tidal and leveling records to better understand the pattern of interseismic locking on the cascadia subduction zone we extend relative sea level time series of the six primary noaa tide gauges to include all observations from 1925 to 2006 previously unidentified bench mark instability biases portions of tidal records by 1 6 mm a 1 before correction we determine precise relative uplift rates at the six tidal sites with an adjustment that includes rates of differenced time series as additional constraints our analysis of national geodetic survey leveling data between tide gauges corrects errors in 1941 leveling and 184 secondary ties double the number of highest quality uplift rate estimates relative uplift rates from leveling are adjusted to the tidal rates accounting for uncertainties in both data types tidal and leveling uplift rates agree within error for all but one of the coastal segments where we infer systematic leveling error affects the 1988 line uplift rates are made absolute using an interval and location specific geocentric sea level rise rate of 2 3 0 2 mm a 1 total propagated one sigma errors for the absolute uplift rates of bench marks are 0 4 mm a 1 along strike changes in uplift rate near 45 n and 42 8 n require two distinct changes in locking depth as inferred from elastic dislocation modeling the along strike changes in locking on this portion of the cascadia subduction zone interface correlate to the western and southern extent of the mafic siletzia block in the fore arc

deep structure of an island arc backstop lesser antilles subduction zone 1 we present the results from a coincident seismic reflection refraction grid conducted at the lesser antilles subduction zone near 16 n this paper focuses on the seismic refraction data and constraints these data place on the three dimensional structure of the island arc backstop we find that the backstop in this region contains considerable topography in both the strike and dip directions two ridges each 25 35 km in length and 10 km in width rise 1 6 km above the adjacent basement the eastern edge of one of the ridges deepens by 4 6 km over a horizontal distance of 10 km and forms the eastern edge of the backstop in contrast to the complex nature of the backstop the adjacent accretionary wedge displays little lateral variability at large scales this may be a consequence of the spatial scales involved the backstop topography is 10 35 km in width while the accretionary wedge extends 125 km from the deformation front to the backstop the top of the subducting oceanic crust as identified by an increase in velocities to 6 6 5 km s intersects the backstop at a depth of 14 15 km the updip limit of plate boundary seismicity is located 75 100 km west and downdip of the backstop however two earthquake clusters are observed at the intersection of the subducted barracuda ridge and tiburon ridge with the backstop suggesting active deformation associated with the backstop edge at these locations

studying lower crustal flow beneath mead basin implications for the thermal history and rheology of venus abstract mead the largest crater on venus has low topographic relief at the surface and at the crust mantle boundary due to high surface temperatures viscous deformation could play an important role in crustal structure using the finite element method we simulate the long term viscoelastic deformation of mead crater and investigate the role of lower crustal flow in the evolution of the surface and subsurface topography we examine the thermal states that allow this evolution to occur and determine the background heat flux our study constrains the background heat flux in the vicinity of mead basin to 55 90 mw m 2 this surface heat flow is generally higher than the average venusian global values suggested by recent thermal models in addition by applying hydrous and anhydrous creep rheological parameters we demonstrate that the venus s interior is rheologically dry and that the crust near mead could be relatively high in plagioclase

venus detailed mapping of maxwell montes region from october 1983 to july 1984 the north hemisphere of venus from latitude 30 to latitude 90 was mapped by means of the radar imagers and altimeters of the spacecraft venera 15 and venera 16 this report presents the results of the radar mapping of the maxwell montes region one of the most interesting features of venus surface a radar mosaic map and contour map have been compiled

trench parallel fluid flow in subduction zones resulting from temperature differences differences in the thermal state of subducting crust along the trench of a subduction zone cause differences in subduction zone temperature that persist to tens of kilometers down dip of the trench the resulting differences in fluid viscosity permeability and hydraulic conductivity can lead to trench parallel variations in fluid pressure on the plate boundary fault temperature differences in locations with low decollement temperature 75 c at the trench result in large differences in fluid viscosity and fluid flow from the cold to the hot side of the system margins characterized by high decollement temperature at the trench are probably dominated by temperature controlled differences in intrinsic permeability resulting in fluid flow from the hot to the cold side of the system margins with large trench parallel temperature differences support considerable trench parallel fluid flow and the effect is accentuated for cases where compaction driven dewatering is concentrated near the trench type locations for along strike hydrologic differences include margins with subducting crust with differences in plate age along the trench or patchy hydrothermal circulation in such cases the three dimensional pattern of subduction zone fluid flow should be considered when inferring the location of the source of fluids sampled from boreholes or seeps additionally trench parallel differences in fluid pressure control effective stress on the plate boundary fault reduced effective stress on the plate boundary on the low hydraulic conductivity side of a subduction zone reduces the effective friction coefficient which both reduces frictional heating and may delay the onset of frictional instability

new seismic images of the cascadia subduction zone from cruise so108 orwell abstract in april and may 1996 a geophysical study of the cascadia continental margin off oregon and washington was conducted aboard the german r v sonne this cooperative experiment by geomar and the usgs acquired wide angle reflection and refraction seismic data using ocean bottom seismometers obs and hydrophones obh and multichannel seismic reflection mcs data the main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the cascadia subduction zone and to image the downgoing plate coincident mcs and wide angle profiles along two tracks are presented here the plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at ca 13 km depth thus the downgoing plate dips more shallowly than previously assumed the dip of the plate changes from 2 to 4 at the eastern boundary of the wedge on the northern profile where approximately 3 km of sediment is entering the subduction zone on the southern profile where the incoming sedimentary section is about 2 2 km thick the plate dips about 0 5 to 1 5 near the deformation front and increases to 3 5 further landwards on both profiles the deformation of the accretionary wedge has produced six ridges on the seafloor three of which represent active faulting as indicated by growth folding the ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement thus the entire incoming sediment package is being accreted at least two phases of accretion are evident and the rocks of the older accretionary phase s forms the backstop for the younger phase which started around 1 5 ma ago this documents that the 30 to 50 km wide frontal part of the accretionary wedge which is characterized by landward vergent thrusts is a pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods velocities increase quite rapidly within the wedge both landward and downward at the toe of the deformation front velocities are higher than 4 0 km s indicating extensive dewatering of deep oceanic sediment further landward considerable velocity variation is found which indicates major breaks throughout the accretionary history

geologic evidence for earthquakes during the past 2000 years along the copalis river southern coastal washington evidence for several prehistoric earthquakes is present in deposits less than 2000 years old that crop out at the copalis river estuary 47 07 2 n 124 09 7 w the deposits show that land subsided decimeters into the intertidal zone on as many as three occasions and that sand vented while the land underwent little or no subsidence on another occasion the evidence for earthquake induced subsidence consists of buried marsh and forest soils that evince sudden estuarine submergence attended on at least one occasion by a landward directed surge of sandy water this combination of submergence and surge implies subsidence and tsunami exposed evidence for subsidence and tsunami along the copalis river is strong for a time within a few decades of 300 years ago moderate for a time 1400 1900 years ago and weak subsidence or lacking tsunami for an intermediate time also about 1400 1900 years ago the history of sudden submergence and tsunami in the past 2000 years at the copalis river estuary resembles that shown by buried soils farther south in coastal washington at grays harbor and willapa bay the history can be explained most simply by great mw of 8 or 9 thrust earthquakes at the boundary between the juan de fuca and north america plates with one or two earthquakes occurring 1400 1900 years ago and another close to 300 years ago the evidence for earthquake induced venting along the copalis river consists of sand intrusions and vented sand volcanoes the sand rose through muddy estuarine deposits at least 3 m thick entraining these deposits in fragments as much as 0 3 m long intrusion occurred at one or more times in the past 2000 years and at least some of the venting occurred 900 1300 years ago the sand bodies do not imply artesian flow of flood pressured groundwater because the sand vented forcefully and infrequently through a lowland that probably lacked natural levees the earthquake implied by the venting of sand 900 1300 years ago could have come from a source below above or at the boundary between the juan de fuca and north america plates if from the subducted juan de fuca plate the implied earthquake was farther west or larger than the largest earthquakes within that plate during the past 100 years evidence permitting a source within the north america plate includes a fault scarp an uplifted tideflat and landslides near puget sound dating from the interval 700 1700 years ago a plate boundary source would explain buried soils whose estuarine submergence and burial imply coseismic subsidence for parts of coastal washington in the interval 600 1300 years ago however stratigraphy shows that little or no coseismic subsidence occurred along the copalis river during the venting 900 1300 years ago if a plate boundary earthquake struck coastal washington 900 1300 years ago the earthquake produced a different distribution of land level change than did the plate boundary earthquakes inferred for 1400 1900 and 300 years ago

subducted seamounts and recent earthquakes beneath the central cascadia forearc bathymetry and magnetic anomalies indicate that a seamount on the juan de fuca plate has been subducted beneath the central cascadia accretionary complex and is now located 45 km landward of the deformation front passage of this seamount through the accretionary complex has resulted in a pattern of uplift followed by subsidence that has had a profound influence on slope morphology gas hydrate stability and sedimentation based on potential field data and a new three dimensional seismic velocity model we infer that this is the most recent of several seamounts subducted over the past several million years beneath this segment of cascadia more deeply subducted seamounts may be responsible for recent earthquake activity on the plate boundary in this region and for along strike variations in the thickness of the subduction channel which may affect coupling across the plate boundary

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

probable low angle thrust earthquakes on the juan de fuca north america plate boundary in 2004 two clusters of earthquakes occurred in the central part of the cascadia forearc which displays several characteristics indicative of along strike and downdip variations in plate coupling moment tensor analysis for the main shock in each cluster indicates that both events which had magnitudes of 4 9 and 4 8 were compatible with low angle thrust motion on fault planes dipping 6 15 to the east consistent with the plate boundary dip of 12 by tracing rays through a high resolution two dimensional crustal velocity model to match arrival times of secondary arrivals pp and pmp we estimate that the source depth was 9 11 km for the m4 9 event and 15 17 km for the m4 8 event we conclude that these earthquakes probably represent seismogenic thrust motion on the nominally locked or transitional part of the cascadia megathrust updip of where episodic tremor and slip ets have been documented continued high resolution observations of seismicity and improved crustal models are needed to confirm whether apparent temporal correlations between ets and continued seismic activity in these clusters indicate stress transfer along the megathrust

kinetics of pb release during the zircon evaporation technique abstract the release of silica and pb from zircon by the thermal evaporation technique is shown to relate to the conversion of zircon to a porous baddeleyite structure the zircon baddeleyite reaction occurs at a sharp reaction front which progressively migrates towards the centre from the grain margin significant reaction in non metamict zircon commences in the mass spectrometer at temperatures of the order of 1500 c the progressive migration of zircon baddeleyite transformation towards grain interiors explains why the evaporation technique with its inherent advantage of not requiring chemical separation of pb and u progressively samples pb in zones towards the centre of the grain and gives 207 pb 206 pb ages as accurate as conventional u pb dating methods the rates of pb and silica loss are primarily determined by the rate of advance of this de silicification reaction front towards the interior of the grain most estimates of pb diffusion coefficients in zircon imply that diffusion distances in the unreacted zircon are negligible 10 2 m at the temperatures and duration of the pb evaporation experiments isothermal experiments demonstrate the rate of zircon baddeleyite transformation with time and temperature and provide an estimate of 130 10 1 kcal mol 1 for the activation energy of the process significantly higher than estimates of pb diffusion activation energy

slow cooling of deep crustal granulites and pb loss in zircon abstract we attempted to obtain the magmatic crystallization age of a metamorphosed granulite facies t 800 90 c p 8 1 kbar massif type anorthosite from the ankafotia body of southwest madagascar the sample studied is a coarse grained leuconorite with good preservation of igneous texture and mineralogy although plagioclase which contains abundant rutile and zircon inclusions has been slightly recrystallized thirty three isotope dilution u pb analyses of zircons representing single grain fragments 29 analyses and multi grain fractions 4 analyses yield a spectrum of concordant ages from 631 to 549 ma a time span of more than 80 myr back scattered electron and cathodoluminescence images show that most grains are either homogeneous structureless fragments 35 or are permeated to a variable degree by anastamosing cracks occupied by relatively u and or th enriched zircon 45 a smaller percentage of grains show relict magmatic zoning 20 thin u and or th rich overgrowths occur on about 25 of grains raman spot analyses demonstrate that all fragments are highly crystalline and non metamict there are marked correlations between zircon grain size and internal features such that the oldest grains are larger and show relict magmatic zoning the youngest grains are small fragments containing high u crack networks ion microprobe spot analyses show that each zircon grain preserves a distinct trace element signature rare earth element patterns show heavy ree enrichment with negative eu anomalies and positive ce anomalies we suggest that the ca 80 myr spread in concordant u pb ages in this sample is indicative of high temperature pb loss during one or more protracted periods of granulite facies metamorphism with only minor episodic or continuous metamorphic zircon growth volume diffusion and or fracture assisted diffusion seems to be the dominant mechanism of pb loss cooling curves calculated using recently measured pb diffusion parameters conform to the age size relationship and imply very slow cooling rates 1 2 c myr or less as might be expected for a terrane in which granulite conditions were maintained for an extended period of time our results therefore suggest a note of caution for interpretation of concordant zircon ages in meta igneous rocks affected by high grade metamorphism of long duration

three dimensional velocity structure of the outer fore arc of the colombia ecuador subduction zone and implications for the 1958 megathrust earthquake rupture zone in 2005 an onshore offshore 3 d refraction and wide angle reflection seismic experiment was conducted along the convergent margin at the border between colombia and ecuador over the rupture zone of the 1958 mw 7 6 subduction earthquake a well defined vp velocity model of the plate boundary and upper and lower plates was constructed down to 25 km depth using first arrival traveltimes inversion the model reveals a several kilometers thick low velocity zone in the upper plate located immediately above the interplate contact this low velocity zone might be related to alteration and fracturing of the mafic and ultramafic rocks which composed the upper plate in this area by fluids released by the lower plate with possible contributions from sediment underplating near the toe of the margin the model shows a low velocity gradient in the outer wedge which is interpreted as highly faulted and fractured rocks this low velocity low gradient region appears to limit the oceanward extension of the rupture zones of the 1958 and 1979 earthquakes possibly because coseismic deformation and uplift of the outer margin wedge dissipates most of the seismic energy

prediction of self diffusion and heterodiffusion coefficients in zircon abstract an empirical model interconnecting point defect parameters with bulk properties of solids has been employed to calculate the self diffusion coefficients of o and si in zircon meanwhile the model is extended to heterodiffusions for the first time in this study which are estimated for pb ti li trivalent cations rees sm dy yb and tetravalent cations th u hf diffusing in zircon for most cases our empirically estimated diffusivities under anhydrous conditions agree well with the experimental measurements over the temperature ranges investigated in the laboratory as compared to the estimates derived from the anion porosity although the estimated activation energies of the diffusion have a little bit large uncertainties in some cases

a new view into the cascadia subduction zone and volcanic arc implications for earthquake hazards along the washington margin in light of suggestions that the cascadia subduction margin may pose a significant seismic hazard for the highly populated pacific northwest region of the united states the u s geological survey usgs the research center for marine geosciences geomar and university collaborators collected and interpreted a 530 km long wide angle onshore offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation we observed 1 an increase in the dip of the juan de fuca slab from 2 7 to 12 where it encounters a 20 km thick block of the siletz terrane or other accreted oceanic crust 2 a distinct transition from siletz crust into cascade arc crust that coincides with the mount st helens seismic zone supporting the idea that the mafic siletz block focuses seismic deformation at its edges and 3 a crustal root 35 45 km deep beneath the cascade range with thinner crust 30 35 km east of the volcanic arc beneath the columbia plateau flood basalt province from the measured crustal structure and subduction geometry we identify two zones that may concentrate future seismic activity 1 a broad because of the shallow dip possibly locked part of the interplate contact that extends from 25 km depth beneath the coastline to perhaps as far west as the deformation front 120 km offshore and 2 a crustal zone at the eastern boundary between the siletz terrane and the cascade range

cascadia onshore offshore site response submarine sediment mobilization and earthquake recurrence local geologic structure and topography may modify arriving seismic waves this inherent variation in shaking or site response may affect the distribution of slope failures and redistribution of submarine sediments i used seafloor seismic data from the 2011 to 2015 cascadia initiative and permanent onshore seismic networks to derive estimates of site response denoted sn in low and high frequency 0 02 1 and 1 10 hz passbands for three shaking metrics peak velocity and acceleration and energy density sn varies similarly throughout cascadia and changes primarily in the direction of convergence roughly east west in the two passbands sn patterns offshore are nearly opposite and range over an order of magnitude or more across cascadia sn patterns broadly may be attributed to sediment resonance and attenuation this and an abrupt step in the east west trend of sn suggest that changes in topography and structure at the edge of the continental margin significantly impact shaking these patterns also correlate with gravity lows diagnostic of marginal basins and methane plumes channeled within shelf bounding faults offshore sn exceeds that onshore in both passbands and the steepest slopes and shelf coincide with the relatively greatest and smallest sn estimates at low and high frequencies respectively these results should be considered in submarine shaking triggered slope stability failure studies significant north south sn variations are not apparent but sparse sampling does not permit rejection of the hypothesis that the southerly decrease in intervals between shaking triggered turbidites and great earthquakes inferred by goldfinger et al 2012 2013 2016 and priest et al 2017 is due to inherently stronger shaking southward

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

a 2 d tomographic model of the juan de fuca plate from accretion at axial seamount to subduction at the cascadia margin from an active source ocean bottom seismometer survey we report results from a wide angle controlled source seismic experiment across the juan de fuca plate designed to investigate the evolution of the plate from accretion at the juan de fuca ridge to subduction at the cascadia margin a two dimensional velocity model of the crust and upper mantle is derived from a joint reflection refraction travel time inversion to interpret our tomography results we first generate a plausible baseline velocity model assuming a plate cooling model and realistic oceanic lithologies we then use an effective medium theory to infer from our tomography results the extent of porosity alteration and water content that would be required to explain the departure from the baseline model in crust of ages 1 ma and away from propagator wakes and regions of faulting due to plate bending we obtain estimates of upper crustal hydration of 0 5 2 1 wt and find mostly dry lower crust and upper mantle in sections of the crust affected by propagator wakes we find upper estimates of upper crustal lower crustal and upper mantle hydration of 3 1 0 8 and 1 8 wt respectively at the cascadia deformation front we find that the amount of water stored at uppermost mantle levels in the downgoing jdf plate is very limited 0 3 wt with most of the water carried into the subduction zone being stored in the oceanic crust

structural factors controlling the rupture process of a megathrust earthquake at the nankai trough seismogenic zone summary the nankai trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100 200 yr the 1946 nankaido earthquake is well known as an unusual event among these earthquakes because the rupture zone estimated from long period geodetic data is more than twice as large as that derived from seismic wave data in the summer of 1999 an onshore offshore deep seismic survey was performed along a 355 km long profile in the western nankai trough seismogenic zone seismic signals both from an airgun array 207 l and land explosions maximum of 500 kg were recorded simultaneously by 98 oceanbottom seismographs and 93 land seismic stations conventional 2 d seismic reflection data were also acquired along part of the offshore profile from the wide angle seismic data we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth the seismic velocity image also shows that the seamount is now colliding with the japanese island arc crust from this significant structure this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake i e the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount and then the rupture was deflected around the subducted seamount at the down dip end of the locked zone between cape muroto and cape ashizuri another significant structure a highly reflective layer is obtained beneath shikoku island a very slow p wave velocity 3 km s 1 is necessary in a thin layer at the base of the island arc crust in order to explain the observed high amplitude reflection phases an area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer this suggests a possible water layer at the base of the island arc crust the water may be generated by dehydration of the downgoing probably partially serpentinized mantle which is implied by a low p wave velocity 7 5 km s 1 beneath the subducted seamount a locally observed non slip region during the 1946 earthquake at the eastern part of shikoku island is interpreted as a result of weak coupling at the possible water layer

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

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

crustal structure beneath the strait of juan de fuca and southern vancouver island from seismic and gravity analyses 1 wide angle and vertical incidence seismic data from seismic hazards investigations in puget sound ships gravity modeling and seismicity are used to derive two dimensional crustal models beneath the strait of juan de fuca the eocene volcanic crescent siletz terrane is significantly thicker than previously recognized and extends from near the surface to depths of 22 km or greater for the northern strait a weak midcrustal reflector dipping east from 12 to 22 km depth is inferred from wide angle reflections a stronger deeper reflector dipping eastward from 23 to 36 km depth is associated with the top of reflector band e a zone of high reflectivity on coincident multichannel seismic mcs data interpreted as a shear zone a high velocity zone 7 60 0 2 km s 1 between these reflectors is interpreted as a localized slice of mantle accreted with the overlying crescent siletz terrane for the southern strait no deep high velocity layer is observed and the e band reflectivity is weaker than to the north a strong deep reflector interpreted as the oceanic moho dips eastward from 35 to 42 km seismicity within the subducting slab occurs mainly above the inferred oceanic moho gravity modeling constrained by the wide angle seismic models and seismicity is consistent with the inferred large thickness of crescent siletz and high density rocks 3030 kg m 3 in the lower crust

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

time and size of a giant earthquake in cascadia inferred from japanese tsunami records of january 1700 geological evidence shows that great earthquakes have occurred in the recent prehistoric past in the cascadia subduction zone off the pacific coast of north america the most recent event or series of events is dated at about 300 years ago1 4 but the precise date and magnitude have not been determined geological investigations have not been able to distinguish a single giant earthquake from a series of great earthquakes occurring over a timespan of a decade or two4 although this information is important for the assessment of future hazard5 we have found several tsunami records in japan from ad 1700 with no indication of a local cause historical earthquake records and palaeoseismic evidence indicate the absence of a large earthquake in 1700 in south america alaska or kamchatka leaving cascadia as the most likely source of this tsunami the estimated time of the earthquake is the evening about 21 00 local time of 26 january 1700 the magnitude is estimated as 9 from the tsunami heights in which case the earthquake ruptured the entire length of the cascadia subduction zone2 these estimates are consistent with native american legends that an earthquake occurred on a winter night6

coseismic strengthening of the shallow portion of the subduction fault and its effects on wedge taper 1 according to the dynamic coulomb wedge model coseismic strengthening of the shallowest part of the subduction interface can cause permanent deformation of the overlying outer wedge so that the cumulative effects of great interplate earthquakes control the geometry of the wedge in this work we use a numerical model which is a hybrid of the frictional contact model and the classical crack model to study how stress is coseismically transferred from the seismogenic zone to the strengthening updip zone to cause wedge deformation in this static model the critical strengthening required to prevent the rupture from breaking the trench depends on the force drop of the seismogenic zone defined as the product of the average shear stress drop and the area of the seismogenic zone in a simple model of uniform material properties with a few megapascals average stress drop over a seismogenic zone of 120 km downdip width the critical strengthening for a 30 km wide updip zone is an increase in the effective friction coefficient by about 0 05 using the coulomb wedge theory we demonstrate that this level of strengthening can readily push the overlying wedge into a critical state of failure with much greater strengthening the rupture is able to extend into the updip zone only slightly causing localized wedge compression in the area of slip termination we examined wedge geometry of 22 subduction zones in the light of the model results we found that the surface slope of these wedges is generally too high to be explained using the classical wedge theory but can be explained using the dynamic coulomb wedge model including coseismic strengthening of the shallow portion of the megathrust

a marine electromagnetic survey to detect gas hydrate at hydrate ridge oregon gas hydrates are a potential energy resource and hazard for drilling and infrastructure yet estimates of global volume vary by over three orders of magnitude hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations a marine electromagnetic survey was carried out at hydrate ridge oregon usa with the aim of testing the use of controlled source electromagnetic csem and magnetotelluric mt methods to map gas hydrate and free gas below the gas hydrate stability zone a 2 d csem inversion supports the scenario deduced from previous seismic and drilling results which indicate two mechanisms of hydrate emplacement a transport dominated and reaction dominated regime a prominent resistive region of 2 5 4 m at a depth of about 130 mbsf near the seismic bottom simulating reflector bsr suggests that 27 to 46 per cent of the bulk volume is filled with hydrate depending on whether archie s law or the hashin strikman bounds are used this is representative of a reaction dominated regime for hydrate emplacement and where a significant low velocity zone exists based on a seismic tomography inversion suggests large quantities of free gas below the bsr electrical resistivity logging while drilling lwd data show general agreement with the csem inversion model except for a csem derived resistive region at seismic horizon a known to transport free gas into the gas hydrate stability zone inversion of mt data collected simultaneously during the csem survey provides a complimentary low resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary