rise and fall of the southern santa cruz mountains california from fission tracks geomorphology and geodesy the santa cruz mountains are closely associated with a left bend along the right lateral san andreas fault the loma prieta area on the northeast side of the san andreas consists of fault bounded blocks that rise along active deeply rooted reverse and oblique slip faults six samples from a transect across this area yield concordant apatite fission track ages averaging 4 6 0 5 ma these ages date the time of cooling below 110 c and suggest that about 3 km of unroofing has occurred over the last 4 6 m y allowing for current elevations of about 1 km this suggests an average uplift rate of the order of 0 8 mm yr over the last 4 6 m y to further define the extent and distribution of this young uplift we used morphometric analyses of the youthful topography of the area steep drainage slopes and high local relief indicate that the area northeast of the san andreas forms a well defined zone of high uplift in contrast the region on the southwest side of the san andreas is characterized by broad upwarping and folding more subdued topography old fission track ages and mean quaternary uplift rates of 0 1 0 4 mm yr geodetic data 1906 san francisco earthquake and subsequent strain transients 20 years of interseismic deformation 1989 loma prieta earthquake and 2 years of post loma prieta earthquake strain show that the southern santa cruz mountains repeatedly rise and subside through a complex sequence of bay area deformation events an additional deformation element that involves reverse slip averaging 2 3 mm yr along the foothills thrust system must occur to explain the longer term uplift pattern in the loma prieta area since the late pliocene the asymmetry in deformation on opposite sides of the san andreas probably reflects the contrasting rock types on opposite sides of the fault the influence of preexisting structures and the interaction with neighboring faults of the san andreas system

assessing the suitability of holocene environments along the central belize coast central america for the reconstruction of hurricane records since the belize coast was repeatedly affected by hurricanes and the paleohurricane record for this region is poor sediment cores from coastal lagoon environments along the central belize coast have been examined in order to identify storm deposits the paleohurricane record presented in this study spans the past 8000 years and exhibits three periods with increased evidences of hurricane strikes occurring at 6000 4900 4200 3600 and 2200 1500 cal yr bp two earlier events around 7100 and 7900 cal yr bp and more recent events around 180 cal yr bp and during modern times have been detected sand layers redeposited corals and lagoon shell concentrations have been used as proxies for storm deposition additionally hiatuses and reversed ages may indicate storm influence while sand layers and corals represent overwash deposits the lagoon shell concentrations which mainly comprise the bivalve anomalocardia cuneimeris and cerithid gastropods have been deposited due to changes in lagoon salinity during and after storm landfalls comparison with other studies reveals similarities with one record from belize but hardly any matches with other published records the potential for paleotempestology reconstructions of the barrier lagoon complexes along the central belize coast differs depending on geomorphology and deposition of washovers in the lagoon basins is limited probably due to the interplay of biological geological and geomorphological processes

major storm periods and climate forcing in the western mediterranean during the late holocene abstract big storm events represent a major risk for populations and infrastructures settled on coastal lowlands in the western mediterranean where human societies colonized and occupied the coastal areas since the ancient times the variability of storm activity for the past three millennia was investigated with a multi proxy sedimentological and geochemical study from a lagoonal sequence mappings of the geochemistry and magnetic susceptibility of detrital sources in the watershed of the lagoon and from the coastal barriers were undertaken in order to track the terrestrial or coastal marine origin of sediments deposited into the lagoon the multi proxy analysis shows that coarser material low magnetic susceptibility and high strontium content characterize the sedimentological signature of the paleostorm levels identified in the lagoonal sequence a comparison with north atlantic and western mediterranean paleoclimate proxies shows that the phases of high storm activity occurred during cold periods suggesting a climatically controlled mechanism for the occurrence of these storm periods besides an in phase storm activity pattern is found between the western mediterranean and northern europe spectral analyses performed on the sr content revealed a new 270 year solar driven pattern of storm cyclicity for the last 3000 years this 270 year cycle defines a succession of ten major storm periods sp with a mean duration of 96 54 yr periods of higher storm activity are recorded from 680 to 560 cal yr bc sp10 end of the iron age cold period from 140 to 820 cal yr ad sp7 to sp5 with a climax of storminess between 400 and 800 cal yr ad dark ages cold period and from 1230 to 1800 cal yr ad sp3 to sp1 little ice age periods of low storm activity occurred from 560 cal yr bc to 140 cal yr ad sp9 and sp8 roman warm period and from 820 to 1230 cal yr ad sp4 medieval warm period

late quaternary uplift rates of marine terraces on the pacific coast of north america southern oregon to baja california sur abstract marine terraces are common landforms along the pacific coast of north america from southern oregon to southern baja california we report 60 new uranium series ages of marine terrace corals hydrocorals and other fossils terraces representing the ca 80 ka and ca 125 ka high stands of sea are the most common along the pacific coast and we have used these ages along with terrace shoreline angle elevations and paleo sea level estimates to calculate late quaternary uplift rates uplift rates in the forearc region of the cascadia subduction zone range from 0 45 1 08 m kyr farther south along most of the coast west of the san andreas fault zone uplift rates range from 0 15 0 35 m kyr but some areas have significantly higher rates of uplift and others have apparently experienced little or no uplift in the last 125 ka in baja california sur west of the east pacific rise little or no uplift has occurred during the late quaternary on average the magnitude of rates of uplift for the tectonic regimes is as follows cascadia subduction zone san andreas fault zone region east pacific rise region

rapid weakening of subducting plates from trench parallel estimates of flexural rigidity abstract the negative buoyancy force of sinking lithosphere slabs is the principle driving force for subducting plates but transmission of this force to the subducting plate depends on the strength of the slab e g conrad and hager 2001 slab strength has been studied in the context of plate bending within subduction zones for a wide range of rheologies i e perfectly elastic perfectly viscous perfectly plastic brittle ductile layered because the applicability of these rheologic models cannot be distinguished based on trench perpendicular plate bending models forsyth 1980 a method was developed to directly measure variations in plate strength with distance from the trench and has found significant plate weakening within 100 km of the kermadec trench billen and gurnis 2005 using the same method we show that rapid plate weakening trenchward of the forebulge also exists at the tonga and japan izu bonin subduction zones the observed plate weakening provides further evidence for a plate rheology that leads to significant yielding loss of elastic strength and reduction in effective viscosity within the bending region of the subducting plate this rapid weakening within the shallow low curvature region of the plate may significantly decrease estimates of energy dissipation related to plate bending compared to recent calculations assuming high plate strength and constant plate curvature

paleotempestology database for the western north atlantic basin paleotempestology the study of past tropical cyclone activity has grown considerably over the past two decades and there is now a relatively dense network of sites across the western north atlan

late cretaceous polar wander of the pacific plate evidence of a rapid true polar wander event we reexamined the late cretaceous early tertiary apparent polar wander path for the pacific plate using 27 paleomagnetic poles from seamounts dated by 40ar 39ar geochronology the path shows little motion from 120 to 90 million years ago ma northward motion from 79 to 39 ma and two groups of poles separated by 16 to 21 degrees with indistinguishable mean ages of 84 2 ma the latter phenomenon may represent a rapid polar wander episode 3 to 10 degrees per million years whose timing is not adequately resolved with existing data similar features in other polar wander paths imply that the event was a rapid shift of the spin axis relative to the mantle true polar wander which may have been related to global changes in plate motion large igneous province eruptions and a shift in magnetic field polarity state

assessing surficial foraminiferal distributions as an overwash indicator in sur lagoon sultanate of oman abstract the identification of tsunami and storm deposits in arid coastal environments can be problematic as overwash sediments may not show significant contrasting lithologic characters with lagoonal sediments in this study foraminifera were evaluated as an overwash indicator in a small 12 km2 intertidal lagoon located at sur in the sultanate of oman the lagoon is shallow

deep scientific dives in the japan and kuril trenches abstract in the summer of 1985 during the french japanese kaiko program ten dives to depths of 6000 m in the japan and kuril trenches were made in the newly launched submersible nautile the sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving seabeam mapping geomagnetic and gravimetric measurements and single channel seismic profiling the results of the dives provide new constraints on the geodynamics of these subduction zones in the japan and kuril trenches huge slump scars were observed on the landward slopes of the trenches slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material the slump scars allowed us to observe the internal structure of the margin the monoclinal structure on the northern japan trench margin deduced from the seismic profiles and dsdp drilling was confirmed several dives on kashima seamount confirmed that this volcano has recently been split into two parts by a normal fault system comparisons of lithology and paleontology on the two separated parts of the seamount were made deep sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches it can be concluded that the whole margin is venting fluids from depths of 2 3 km which is consistent with the indications of overpressure observed in drill sites on the japan trench margin the fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor

evidence for possible precursor events of megathrust earthquakes on the west coast of north america megathrust earthquakes in western north america may be preceded by a precursor phase several years prior to megathrust induced earthquakes for example on 27 march 1964 a 9 2 magnitude on the richter scale earthquake occurred on the coast of alaska changes in foraminifera and diatom assemblages at girdwood flats alaska provide evidence of a precursor to this earthquake thereby detailing a previously unknown sequence of events we describe further evidence of precursor phases from marshes in turnagain arm alaska united states and farther south in netarts bay oregon united states this is the first time that two widely spaced locations have been examined for earthquake related precursor stages the alaska earthquake offers the possibility to compare a modern sequence a d 1964 of events with the geologic record the netarts bay marsh has experienced no modern earthquake that could be used for comparison but the nature of megathrust zones implies that the modern and ancient events should be physically similar new cores examined from turnagain arm include both the 1964 earthquake and an event identified and dated at 1800 yr b p prior to the 1964 event the foraminifera and thecamoebian assemblages changed from a forest phase to a mildly brackish stage this sequence was dated as being 15 yr or less in length using pb210 and cs137 dating techniques the event at 1800 yr b p was also associated with a similar precursor stage indicating a small subsidence prior to the megathrust earthquake related subsidence event in netarts bay a new core was taken at a previously cored site to make use of carbon 14 dates and the assurance that at least four events were known to have occurred over the past 3000 yr the new core contains four transitions each identified by a mineralic deposit often sharply bounded below and gradationally overlain by marsh peat transition in this context refers to the section of core analyzed 15 cm above and below each of the four sand layers foraminiferal assemblage analyses indicate that these units represented a high marsh prequake phase followed by a lower marsh precursor stage the earthquake related deposition sand layer and then a rebound back into postquake marsh deposits sand deposits with either no or few foraminifera are inferred as tsunami earthquake related deposition stages these transitions in two widely separated geographic areas alaska and oregon indicate that similar mechanisms operate for large megathrust earthquakes at subduction zones from alaska to northern california in the cascadia subduction zone thus implying that precursor events also occur and can be detected by foraminiferal zonation all along this area in a recent article dragert et al 2001 1 scientists from the west coast suggested that slow or silent earthquakes they measured with continuous geographical positioning systems might be indicators of megathrust earthquakes the transitions we document may be the prehistoric representations of these silent quakes foraminiferal evidence may help by allowing more accurate positioning of seismometers along the west coast of north america and thereby lead to more precise and timely earthquake prediction methods 1 ref 14

genes diversity and geologic process on the pacific coast abstract we examine the genetics of marine diversification along the west coast of north america in relation to the late neogene geology and climate of the region trophically important components of the diverse west coast fauna including kelp alcid birds e g auks puffins salmon rockfish abalone and cancer crabs appear to have radiated during peaks of upwelling primarily in the late miocene and in some cases secondarily in the pleistocene phylogeographic barriers associated with mio pliocene estuaries of the mid california coast the pliocene opening of the gulf of california tectonic and eustatic evolution of the california bight as well as the influence of pleistocene and holocene climate change on genetic structure are assessed in a geologic context comparisons to east coast and western freshwater systems as well as upwelling systems around the globe provide perspective for the survey

relationship between seismicity and subsurface fluids central coast ranges california seismicity is irregularly distributed in the central coast ranges of california with several large areas having virtually no seismic activity since 1968 aseismic areas correlate with zones of abnormally high pore fluid pressures ahps 130 of hydrostatic level line surveys suggest that aseismic areas have experienced significant historical uplift moreover these aseismic areas show a remarkable correlation with perennial saline springs that represent technically expelled formation fluid as indicated by their perennial flow 18o and d enrichment to 6 3 and 13 respectively elevated b to 331 ppm and chemistry dominated by na and cl aseismic areas are dominantly located at the center of structural blocks that are under compression and are bounded by major faults and seismicity belts volumetric strain within these blocks creates pockets of overpressured formation fluid that greatly reduce seismic activity while promoting aseismic uplift seismic cross sections reveal three dimensional pockets of aseismicity that are floored and surrounded by scattered hypocenters we suggest that overpressured formation fluids originate at depths of 1 to 7 km within these pockets on the basis of na k ca geothermometry estimates that are consistent with stratigraphic information and available drill hole data these overpressured fluid pockets may also influence seismicity on major faults such as the upper 3 km of the active greenville fault which is relatively aseismic where it separates two overpressured pockets decreasing flow rates observed at some saline springs may represent a local transition to a period of increased seismic activity while many studies have linked generation of earthquakes to man made fluid overpressures our data show that the dominant long term effect of ahps is to reduce seismicity and promote uplift

phytolith assemblages as indicators of coastal environmental changes and hurricane overwash deposition we demonstrate that phytolith assemblages are a useful proxy for reconstructing coastal environmental changes and for validating the overwash origin of sand layers in palaeotempestology studies phytolith analysis was conducted on 50 topsoil or surface sediment samples collected from a variety of coastal plant communities or depositional environments in the southeastern usa the data suggest that different coastal subenvironments can be distinguished by their modern phytolith assemblages for example coastal marsh samples contain a diverse phytolith assemblage dominated by smooth elongate and square morpho types and maritime forest samples are dominated by phytoliths from palms and broadleaf dicotyledonous plants remarkably the phytolith assemblages from sand dunes are characterized by high percentages of two horned tower flat tower spool horned tower and short saddle types phytolith analysis of three prehistoric sand layers in a sediment core from western lake northwestern florida shows that they contain a phytolith assemblage similar to those characteristic of sand dunes and interdune meadows these observations are confirmed by the results of principal components analysis and discriminant analysis on the modern and fossil phytolith data sets our study results support the interpretation that the sand layers in western lake were indeed formed by the erosion of sand dunes during overwash processes caused by landfalling catastrophic hurricanes

microhabitats of salt marsh foraminifera st catherines island georgia usa abstract salt marsh foraminifera of st catherines island georgia occupy a range of epibenthic to deep infaunal to 30 cm micro habitats freshly collected specimens of arenoparrella mexicana from 27 to 30 cm below the marsh surface extruded pseudopodial nets and actively collected foreign material about the aperture salt marsh foraminifera may be primarily epibenthic 0 1 cm epibenthic to intermediate infaunal 0 10 cm or deep infaunal 0 30 cm most of the twelve commonly occurring species in st catherines island marshes are epibenthic to intermediate infaunal species distributional patterns differ with regard to maximum depth of infaunal occurrences infaunal microhabitat utilization in different marsh zones and environments and the presence or absence of subsurface population maxima selective preservation impacts both calcareous and agglutinated foraminifera intense bioturbation primarily by halophytes and decapod crustaceans may contribute to the deep infaunal occurrences of foraminifera in these environments

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

normal faulting of the daiichi kashima seamount in the japan trench revealed by the kaiko i cruise leg 3 abstract a detailed topographic and geophysical survey of the daiichi kashima seamount area in the southern japan trench northwestern pacific margin clearly defines a high angle normal fault which splits the seamount into two halves a fan shaped zone was investigated along 2 4 km spaced 100 km long subparallel tracks using narrow multi beam seabeam echo sounder with simultaneous measurements of gravity magnetic total field and single channel seismic reflection records vertical displacement of the inboard half was clearly mapped and its normal fault origin was supported the northern and southern extensions of the normal fault beyond the flank of the seamount were delineated materials on the landward trench slope are displaced upward and to sideways away from the colliding seamount canyons observed in the upper landward slope terminate at the mid slope terrace which has been uplifted since start of subduction of the seamount most of the landward slope except for the landward walls aside the seamount comprises only a landslide topography in a manner similar to the northern japan trench wall this survey was conducted on r v jean charcot as a part of the kaiko i cruise leg 3 in july august 1984 under the auspices of the french japanese scientific cooperative program

the japan trench and its juncture with the kuril trench cruise results of the kaiko project leg 3 this paper presents the results of a detailed survey combining seabeam mapping gravity and geomagnetic measurements as well as single channel seismic reflection observations in the japan trench and the juncture with the kuril trench during the french japanese kaiko project northern sector of the leg 3 on the r v jean charcot the main data acquired during the cruise such as the seabeam maps magnetic anomalies pattern and preliminary interpretations are discussed these new data cover an area of 18 000 km2 and provide for the first time a detailed three dimensional image of the japan trench combined with the previous results the data indicate new structural interpretations a comparative study of seabeam morphology single channel and reprocessed multichannel records lead to the conclusion that along the northern japan trench there is little evidence of accretion but instead a tectonic erosion of the overriding plate the tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the japan trench axis which is a direct consequence of the downward flexure of the pacific plate in addition to these new faults ancient normal faults trending parallel to the n65 oceanic magnetic anomalies and oblique to the japan trench axis are reactivated so that two directions of normal faulting are observed seaward of the japan trench only one direction of faulting is observed seaward of the kuril trench because of the parallelism between the trench axis and the magnetic anomalies the convergent front of the kuril trench is offset left laterally by 20 km relative to those of the japan trench this transform fault and the lower slope of the southernmost kuril trench are represented by very steep scarps more than 2 km high slightly south of the juncture the erimo seamount riding on the pacific plate is now entering the subduction zone it has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches is it due to the collision of an already subducted chain of seamounts or does it correspond to one of the failure lines of the america eurasia plate boundary

rapid cooling and exhumation in the western part of the mesoproterozoic albany fraser orogen western australia abstract the albany fraser orogen of southwestern australia is an understudied orogenic belt which is interpreted to record the mesoproterozoic suturing of the yilgarn craton of western australia to the mawson craton of east antarctica during rodinia assembly previous u pb geochronology has dated peak amphibolite to granulite facies metamorphism in the orogen at ca 1180 ma here we report the first 40ar 39ar thermochronology of hornblende biotite and muscovite grains from a 360 km transect across the western albany fraser orogen and uncover a record of strikingly fast syn orogenic cooling and exhumation to the north muscovites from the northern foreland record cooling at ca 1159 ma in the central and southern domains of the orogen the biranup and nornalup zones hornblende yields ca 1169 ma cooling ages and biotite yields ca 1172 1144 ma cooling ages the new cooling ages imply that the three domains were exhumed rapidly following peak metamorphism at ca 1180 ma attained a similar structural level by ca 1159 ma and have experienced a uniform exhumation history since that time to constrain mineral closure temperatures and post peak metamorphic cooling rates we conducted a monte carlo simulation which fully propagates uncertainty and minimises error correlations modelling of cooling from hornblende to biotite closure temperatures ca 585 c and 365 c respectively in the nornalup and biranup zones yields fast cooling rates of 33 9 17 c ma and 22 5 7 c ma respectively these fast cooling rates imply rapid exhumation in an active tectonic setting undergoing peak metamorphism although the structural evolution of the albany fraser orogen remains poorly constrained the transpressional tectonic activity associated with deformation in the western part of the albany fraser orogen may have been an active driver of this fast exhumation this is distinctly different from exhumation models for granulite facies domains in other mesoproterozoic orogens which typically experience post orogenic slow 1 5 c ma cooling driven by mechanisms such as orogenic collapse and erosion we consider that the observed differences reflect the interpreted syn tectonic transpressional exhumation history of the albany fraser orogen which is an underrepresented tectonic regime in the mesoproterozoic cooling record

abundances distribution and sizes of volcanoes in the pacific ocean and implications for the origin of non hotspot volcanoes abstract in this paper i present data on the abundances sizes and crustal age for all volcanoes volcanic islands and seamounts which appear on published bathymetric charts of the pacific ocean these new data shed light on the origin of non hotspot volcanoes and are important in combination with data on the chemical compositions of seamounts and volcanic islands for estimates of the bulk composition of ocean crust these data also provide firm constraints on off ridge oceanic volcanism models results of this study show that the size frequency distribution of pacific volcanoes is poisson like and that the smallest volcanoes are much more abundant than large ones this study shows clearly that the most abundant volcanoes on the earth are the submerged oceanic volcanoes which comprise 5 25 of the oceanic volcanic layer on pacific crust of eocene age and younger the abundance of volcanoes number of volcanoes per unit area increases monotonically with increasing age assuming steady state the production rate of new off ridge volcanoes number of volcanoes per unit area per unit time is inversely proportional to the square root of the lithosphere age 1 on crust older than eocene the number of volcanoes per unit area of crust decreases monotonically with increasing age however the total volume of lava represented by these edifices increases with increasing age size frequency distributions of volcanoes on swaths of successively older crust indicate that these abundance patterns are partly due to the effect of sediment burial of small edifices on old pacific crust as well as the effect of increased lithosphere thickness on seamount size these general patterns are not appreciably changed by omitting from consideration known hotspot volcanoes 2 and volcanoes built at fossil constructional plate margins 3

a 1 d mechanistic model for the evolution of earthflow prone hillslopes 1 in mountainous terrain deep seated landslides transport large volumes of material on hillslopes exerting a dominant control on erosion rates and landscape form here we develop a mathematical landscape evolution model to explore interactions between deep seated earthflows soil creep and gully processes at the drainage basin scale over geomorphically relevant 10 3 year timescales in the model sediment flux or incision laws for these three geomorphic processes combine to determine the morphology of actively uplifting and eroding steady state topographic profiles we apply the model to three sites one in the gabilan mesa california with no earthflow activity and two along the eel river california with different lithologies and varying levels of historic earthflow activity representative topographic profiles from these sites are consistent with model predictions in which the magnitude of a dimensionless earthflow number based on a non newtonian flow rheology reflects the magnitude of recent earthflow activity on the different hillslopes the model accurately predicts the behavior of earthflow collection and transport zones observed in the field and estimates long term average sediment fluxes that are due to earthflows in agreement with historical rates at our field sites finally our model predicts that steady state hillslope relief in earthflow prone terrain increases nonlinearly with the tectonic uplift rate suggesting that the mean hillslope angle may record uplift rate in earthflow prone landscapes even at high uplift rates where threshold slope processes normally limit further topographic development

a new type of intra plate volcanism young alkali basalts discovered from the subducting pacific plate northern japan trench alkali pillow basalts were collected from the toe of the oceanward slope of the northern japan trench these alkali basalts formed as a result of a low degree of partial melting of pacific ocean mantle and rapid rise of the magma no fractionation in shallow magma chambers reconstructing pacific plate motion based on 40ar 39ar age dates of 5 95 0 31 ma for these basalts indicates that they erupted outboard of outer swell or forebulge of the japan trench in the nw pacific we suggest that these alkali basalts represent a new form of intra plate volcanism whereby magmatic activity occurs off the forebulge of the downgoing pacific slab perhaps using conduits related to fracturing of the slab during bending prior to subduction

mixing sedimentation rates and age dating for sediment cores abstract analytic solutions to the advection diffusion equation useful as a guide for the interpretation of mixing sedimentation rate and age dating effects in sediment cores have been derived various simplified mixing models appropriate to 14 c and 210 pb measurements have been considered the emphasis has been on a discussion of the various interrelations and dependencies among the mixing coefficient sedimentation rate and radioactive decay constant parameters in determining the 14 c and 210 pb concentrations as a function of depth porosity variations within the sediment column are also considered and the transformed advection diffusion equation including porosity effects is derived

uranium thorium and lead nuclides in a delaware salt marsh sediment natural isotopes of uranium 234 238 thorium 232 230 and lead 210 have been assayed in sediments from a lower delaware salt marsh uranium is concentrated in the top sections of the core and decreases with depth 3 to 1 d min 1 g 1 a significant proportion of this uranium has been authigenically extracted from seawater as demonstrated by an excess ratio of 234u 238u 1 03 1 14 thorium 232 is rather constant 2 4 4 7 parts 10 6 for all but one sample while the concentration of 230th is deficient with respect to its uranium parent in contrast to the deep sea the fixation of authigenic reduced uranium dominates over the scavenging of 230th in these tidal sediments excess 210pb shows an exponential decay over the length of the core and when corrected for a correspondingly significant exponential decrease in organic carbon and salt yields a sedimentation rate of 0 47 cm year 1 although this rate approximates the local apparent rate of sea level rise a lower limit for the rate of salt marsh accumulation the actual rate is higher suggesting net filling and acccretion of salt marshes the total integrated flux of 210pb to this and other salt marsh surfaces in the eastern u s is calculated to more often exceed the reported local atmospheric flux it appears that salt marshes may scavenge 210pb as other trace metals from flooding waters

spatio lateral continuity of hurricane deposits in back barrier marshes sedimentological and micropaleontological analysis of fifteen 3 m gouge auger cores revealed a lack of spatio lateral continuity for paleohurricane deposits from the back barrier marshes of folly island south carolina the offshore indicative calcareous microfossil content of some storm deposits was taphonomically altered or destroyed and in many cases cores taken 10 m apart provided significantly different storm records in several low marsh cores where bioturbation is more intense storm deposits were completely unrecognizable using foraminifer or sedimentological analyses in nearly all cores the sedimentary record of storm deposits was more extensive than the record produced using foraminifers and as a result the sedimentary proxy produced shorter hurricane return periods while offshore indicative foraminifers in the salt marshes behind folly island are good indicators of their original source and mode of deposition storm current transport from the shelf by overwash storm inlet or via lighthouse inlet their susceptibility to destruction from bioturbation and dissolution suggests that they provide a less accurate or less complete record of storm frequency than previously thought

historical nankai suruga megathrust earthquakes recorded by tsunami and terrestrial mass movement deposits on the shirasuka coastal lowlands shizuoka prefecture japan geological investigations of coastal sediment sequences play a key role in verifying earthquake and tsunami characteristics inferred from historical records in this paper we present a multi proxy investigation of a coastal lowland site facing the nankai suruga megathrust and appraise evidence for tsunamis and earthquake triggered terrestrial mass movements occurring over the last 800 years combining a high resolution chronology with x ray computed tomography and analyses of particle size diatoms pollen non pollen palynomorphs and aerial photographs we present the most compelling geological evidence of the 1361 ce koan also known as shohei tsunami reported to date from any site along the megathrust this finding is consistent with either of two recent hypotheses a single larger rupture of both the nankai and tnankai regions or two smaller ruptures separated by a few days enhancing the site chronology using bayesian age modelling we verify evidence for inundation during the 1498 ce mei tsunami while previous investigations identified evidence for historically recorded tsunamis in 1605 1707 and 1854 ce and a storm surge in 1680 or 1699 ce we encountered a thick sand layer rather than discrete extreme wave deposits in this interval the overprinting of evidence highlights the potential for geological records to underestimate the frequency of these events a terrestrial mass movement also deposited a sand layer at the site radionuclide dating and aerial photographs provide independent confirmation that this may have been triggered by intense shaking in 1944 ce during the most recent great nankai suruga megathrust earthquake

three dimensional crustal structure of the mendocino triple junction region from local earthquake travel times the large scale three dimensional geometry of the mendocino triple junction at cape mendocino california was investigated by inverting nearly 19 000 p wave arrival times from over 1400 local earthquakes to estimate the three dimensional velocity structure and hypocentral parameters a velocity grid 175 km n s by 125 km e w centered near garberville california was constructed with 25 km horizontal and 5 fan vertical node spacing the model was well resolved near cape mendocino where the earthquakes and stations are concentrated at about 40 6 n latitude a high velocity gradient between 6 5 and 7 5 km s dips gently to the south and east from about 15 km depth near the coast relocated hypocenters concentrate below this high gradient which we interpret as the oceanic crust of the subducted gorda plate therefore the depth to the top of the gorda plate near cape mendocino is interpreted to be 15 km the gorda plate appears intact and dipping 8 eastward due to subduction and flexing downward 6 12 to the south both hypocenters and velocity structure suggest that the southern edge of the plate intersects the coastline at 40 3 n latitude and maintains a linear trend 15 south of east to at least 123 w longitude the top of a large low velocity region at 20 30 km depth extends about 50 km n s and 75 km e w roughly between garberville and covelo and is located above and south of the southern edge of the gorda plate we interpret this low velocity area to be locally thickened crust 8 10 km due to either local compressional forces associated with north south compression caused by the northward impingement of the rigid pacific plate or by underthrusting of the base of the accretionary subduction complex at the southern terminous of the cascadia subduction zone south of cape mendocino and southwest of the garberville fault high velocities indicative of oceanic crust are detected at 15 km depth we interpret this evidence as tectonic underplating of the pacific oceanic crust beneath the coastal franciscan complex similar to the situation seismically imaged in central coastal california page and brocher 1994 this geometry indicates that the westernmost portion of the slab window has been filled by tectonic underplating and crustal thickening while deep seated mantle upwelling is confined to an area centered about 75 km north of clear lake benz et al 1992 zandt et al 1992

long river profiles tectonism and eustasy a guide to interpreting fluvial terraces along three rivers at the mendocino triple junction northern california strath cut and fill terraces have formed in response to tectonic and eustatic processes detailed surveying and radiometric dating at multiple sites indicate that lower reaches of the rivers are dominated by the effects of oscillating sea level primarily aggradation and formation of fill terraces during sea level high stands alternating with deep incision during low stands a eustasy driven deposi tional wedge extends tens of kilometers upstream on all rivers tapering to zero thickness this distance is greater than expected from studies of the effects of check dams on much smaller streams elsewhere due in part to the large size of these rivers however the change in gradient is nearly identical to other base level rise studies the depositional gradient is about half that of the original channel middle to upper reaches of each fiver are dominated by the effects of long term uplift primarily lateral and vertical erosion and formation of steep unpaired strath terraces exposed only upstream of the depositional wedge vertical incision at a rate similar to that of uplift has occurred even during the present sea level high stand along rivers with highest uplift rates strath terraces have steeper gradients than the modem channel bed and do not merge with marine terraces at the river mouth consequently they cannot be used to determine altitudes of sea level high stands strath formation is a continuous process of response to long term uplift and its occurrence varies spatially along a river depending on stream power and hence position upstream strath terraces are found only along certain parts of a coastal stream upstream of the aggradational effects of oscillating sea level and far enough downstream that stream power is in excess of that needed to transport the prevailing sediment load for a given size fiver the greater the uplift rate the greater the rate of vertical incision and consequently the less the like lihood of strath terrace formation and preservation

detrital zircon fission track ages for the hoh formation implications for late cenozoic evolution of the cascadia subduction wedge we report new fission track ft ages for detrital zircons for 34 sandstone samples and 2 volcanic ash beds from the hoh formation exposed along the western side of the olympic mountains of western washington state the hoh formation is now formally known as the coastal unit of the olympic structural complex or coastal osc for short about 35 zircons were dated per sample the fission track grain age ftga distributions are all strongly discordant grain ages range from 10 to older than 100 ma low vitrinite reflectance values short etch times for the zircons and a broad range of grain ages indicate that the zircon ft ages are unreset and thus preserve information about cooling events in the source region for these sedimentary rocks five areas were sampled repeatedly and yield similar ftga distributions demonstrating that sampling errors are not a problem we show that almost all of the samples contain a well defined young component that was probably derived from a contemporaneous active volcanic source presumably the adjacent cascadia arc binomial peak fitting was used to estimate the ft minimum age which is the age of the youngest concordant fraction of zircon ft grain ages in a ftga distribution in most cases minimum ages are similar to fossil ages where available this result supports our contention that zircon ft minimum ages from volcaniclastic sandstones commonly can be used as a proxy for depositional age our zircon ft minimum ages indicate that the coastal osc is made up mainly of lower miocene ca 24 to 16 ma sedimentary rocks we use these age data together with other geologic constraints to reconstruct a tectonic history sedimentary rocks of the coastal osc were derived from a mixed source region that included an active volcanic arc and also older units including cretaceous metamorphic rocks probably located in the omineca crystalline belt in the canadian rockies the upper part of the clallam formation located on the northern side of the olympic peninsula appears to be a remnant of the sedimentary system that fed the coastal osc the sediments that formed the coastal osc were initially deposited seaward of the cascadia trench at water depths of 2000 m this debris was deposited seaward of the cascadia trench at water depths of 2000 m and subsequently accreted beneath the frontal 50 to 100 km of the wedge owing to continued accretion at the front of the wedge and erosion of the forearc high in back of the wedge these lower miocene sediments were moved rearward within the cascadia subduction wedge a simple relationship based on the cross sectional area of the wedge and a steady accretion flux indicates that it would have taken 22 m y for the coastal osc to reach its present position 140 km landward of the toe of the wedge this estimate is in good agreement with the unit9s early miocene age

paleomagnetism of the joban seamount chain its origin and tectonic implications for the pacific plate the paleomagnetism of 10 seamounts from the joban seamount chain northwestern pacific were studied using a method that calculates mean magnetization parameters by an inversion of magnetic anomaly and edifice bathymetry of the 10 seamounts eight gave results consistent with other paleomagnetic studies of pacific seamounts joban seamounts appear to have formed at two different mean paleolatitudes contrary to what would be expected for a single hotspot origin furthermore six of the consistent poles plot along the 129 to 82 ma portion of the pacific plate apparent polar wander path apwp implying the seamounts formed mainly during the mid to late cretaceous two other poles from iwaki and hitachi seamounts are located northwest of the older end of the established pacific apwp possibly indicating early cretaceous ages because iwaki and hitachi seamounts are located in the middle of the chain age does not progress along the chain arguing against a single hotspot origin perhaps the chain formed by recurrent volcanism along a line of weakness or by another mechanism iwaki and hitachi seamounts display smaller northward drift compared to the others consistent with the pacific plate drifting southward from late jurassic or early cretaceous to mid cretaceous time

a summary of cenozoic tectonic history along the ipod japan trench transect along the international program of ocean drilling ipod japan trench transect a mid cenozoic period of tectonism without arc volcanism separated an earlier cretaceous to paleogene and a later neogene period of plate convergence and subduction the cretaceous to early paleogene period of convergence is evidenced by andesitic volcanism associated with a large syncline probably a forearc basin the seaward flank of the basin is a tectonically thickened sediment sequence inferred to be an accretionary complex the arc the syncline and the thickened sediment sequence resemble an arc trench system of the size and structure of the great valley forearc basin and franciscan accretionary complex of california and they were probably continuations of the yezo geosyncline and associated sequences exposed to the north on hokkaido the arc trench system is contemporaneous with the adjacent shimanto arc trench system southwest of tokyo bay in the early paleogene volcanism ceased and part of the thickened sediment sequence of the cretaceous paleogene margin was emergent as a 160 km wide landmass which is recognized in seismic records as a later paleogene sediment source at the end of the paleogene this landmass began to subside and volcanism began again but it was located first briefly on the east and then on the west of the cretaceous arc along the present arc of northern honshu the land mass erosion surface is clearly visible as an angular unconformity in multichannel seismic reflection records the sequence of neogene sediment explosive volcanism and benthic foraminiferal assemblages recording subsidence from subareal conditions to the present bathyal depths have been studied in deep sea drilling project dsdp cores cretaceous subduction appears to have resulted in a very extensive accretionary complex whereas the neogene subduction period appears to have been marked by little net accretion in the forearc area but massive subsidence and some erosion of the front of the convergent margin

foraminifera as an indicator of overwash deposits barrier island sediment supply and barrier island evolution folly island south carolina abstract micropaleontological analysis of four vibracores from back barrier marshes of folly island south carolina detected intervals of storm deposited sediment individual washover intervals were not detectable from lithology alone but only in conjunction with associated foraminifera washovers contained more modern offshore and oligo miocene genera and total calcareous foraminifera than background marsh samples the presence of tertiary foraminifera in the back barrier cores from folly island supports the hypothesis that much of the sediment for barrier island migration is derived from offshore foraminiferal analyses of back barrier cores were also used to calculate relative bioturbation rates by the method of officer and lynch officer c b lynch d r 1983 mixing sedimentation rates and age dating for sediment cores mar geol 46 261 278 eqs 10 14 in all cores the current depositional environment correlated well with the expected mixing parameter g the highest mixing parameter greatest signal attenuation was recorded in a core from a low marsh creek side environment whereas the lowest mixing parameter least smearing of signal was recorded in a high marsh core

the mendocino triple junction active faults episodic coastal emergence and rapid uplift a complex zone of rapid holocene surface uplift and deformation occurs at the mendocino triple junction the juncture of three plate bounding faults the cascadia subduction zone san andreas fault and mendocino fault within this mountainous structural knot up to 1 4 m of coastal emergence occurred during the 1992 cape mendocino m s 7 1 earthquake surveying and radiometric dating of ancient marine strandlines 7 5 along the cascadia subduction zone megathrust however another plausible interpretation of the data is that multiple earthquakes resulting in smaller amounts of net surface uplift per event similar to the 1992 earthquake occurred closely spaced in time giving the appearance in the geologic record of less frequent and larger events regardless of the number and timing of paleoearthquakes the number of platforms is a minimum of the number of events that resulted in sudden rapid uplift because platform preservation is also a function of rising holocene sea level at present rates and patterns of net surface uplift are better constrained than the timing and magnitude of paleoearthquakes periods of rapid holocene emergence also are identified as far south of the area of 1992 uplift and the mendocino fault as 30 km along the unlocated san andreas fault these also might be associated with coseismic uplift based on new mapping of active faults in the region it is proposed here that this uplift is the result of multiple discontinuous thrust and strike slip fault segments which distribute plate boundary deformation along a restraining bend at the termination of the san andreas fault this interpretation is consistent with the fact that a single prominent strike slip fault has not been identified in the mendocino triple junction region while several small reverse faults crossing holocene deposits and geomorphic surfaces have been mapped and dated

palaeohurricane reconstructions from sedimentary archives along the gulf of mexico caribbean sea and western north atlantic ocean margins hurricanes annually threaten the atlantic ocean margins historical hurricane records are relatively short and palaeohurricane sedimentary archives provide a geological and climatic context that sheds light on future hurricane activity here we review palaeo trends in hurricane activity elucidated from sedimentary archives we discuss dating methods site selection and stat istics associated with previously published records these archives have been useful for under standing the long term evolution of coastal systems and the response of intense hurricane activity to climatic changes regional shifts in hurricane overwash on centennial to millennial timescales have been linked to various climatic modes of variability including el nino southern oscillation and the north atlantic oscillation but could also reflect regional scale controls on hurricane activity

transform tectonics and thermal rejuvenation on the cote d ivoire ghana margin west africa formation of a pronounced basement ridge along many transform continental margins has been attributed to a variety of processes during continental break up including transpressional crustal thickening and thermal rejuvenation and igneous underplating during passage of a spreading ridge odp drill holes on the cote d ivoire ghana margin now provide the first opportunity to quantify the vertical motions along this type of margin apatite fission track dating of detrital sands suggests that large amounts of erosion occurred on the flanks of an intra continental wrench zone that predated margin formation rapid cooling of 120 c at 120 115 ma corresponds to erosion of 3 5 5 km along the conjugate brazilian margin reflecting c 1 km of tectonically driven uplift subaerial erosion and isostatic uplift due to unloading following rift initiation at 120 ma aptian an oceanic spreading axis passed adjacent to this part of the margin at 90 ma cenomanian maximum uplift during the ridge transform intersection was 390 m considerably less than the 2000 m predicted by heat conduction models in local isostatic equilibrium the modern ridge is partially the product of thicker crust 22 km underlying the ridge than the adjacent deep ivorian basin 19 km and partially related to flexural unloading of the transform ridge between the end of intra continental wrenching and ridge transform intersection flexural coupling between the continental and oceanic plates since ridge transform intersection has caused a progressive depression of the offshore margin estimated at about 650 m in the study area

sediment yield spatial characteristics and the long term evolution of active earthflows determined from airborne lidar and historical aerial photographs eel river california in mountainous landscapes with weak fine grained rocks earthflows can dominate erosion and landscape evolution by supplying sediment to channels and controlling hillslope morphology to estimate the contribution of earthflows to regional sediment budgets and identify patterns of landslide activity earthflow movement needs to be quantified over significant spatial and temporal scales presently there is a paucity of data that can be used to predict earthflow behavior beyond the seasonal scale or over spatially extensive study areas across 226 km 2 of rapidly eroding franciscan complex rocks of the eel river catchment northern california we used a combination of lidar light detection and ranging and orthorectified historical aerial photographs to objectively map earthflow movement between 1944 and 2006 by tracking the displacement of trees growing on earthflow surfaces we find that 7 3 of the study area experienced movement over this 62 yr interval preferentially in sheared argillaceous lithology this movement is distributed across 122 earthflow features that have intricate elongate planform shapes a preferred south southwesterly aspect and a mean longitudinal slope of 31 the distribution of mapped earthflow areas is well approximated by a lognormal distribution with a median size of 36 500 m 2 approximately 6 of the study area is composed of earthflows that connect to major channels these flows generated an average sediment yield of 19 000 t km 2 yr 1 rock erosion rate of 7 6 mm yr over the 62 yr study period equating to a regional yield of 1100 t km 2 yr 1 0 45 mm yr if distributed across the study area as such a small fraction of the landscape can account for half of the regional denudation rate estimated from suspended sediment records 2200 t km 2 yr 1 or 0 9 mm yr we propose a conceptual model for long term earthflow evolution wherein earthflows experience intermittent activity and long periods of dormancy when limited by the availability of readily mobilized sediment on upper slopes ultimately high order river channels and ephemeral gully networks may serve to destabilize hillslopes controlling the evolution of earthflow prone terrain

1906 surface faulting on the san andreas fault near point delgada california the northernmost onshore surface faulting on the san andreas fault in 1906 was reported at shelter cove a small embayment in the coastline east of point delgada between 1979 and 1985 several authors questioned both this location for the san andreas fault and the validity of 1906 surface faulting alternatively they proposed an offshore fault and a landslide origin for the 1906 ground fractures f e matthes 1874 1948 described and mapped the surface deformation at shelter cove in may and june 1906 documenting evidence of surface faulting for about 3 km between dead mans gulch and kaluna cliff detailed sketch maps in his notebook indicate a component of right lateral strike slip and a consistent fault dip to the southwest geomorphic and geologic evidence confirm a right oblique sense of slip and a southwest dip of about 45 the projected trace of the fault meets the sea at a point about 1 5 km northwest of kaluna cliff aligned scarps saddles and deflected stream courses along this fault imply repeated nearly colinear fault displacements during late quaternary time published observations of shear strain and fission track ages show that a narrow 1 km zone containing the mapped fault trace is slipping at depth in a right lateral sense and is also a profound tectonic boundary separating rocks with different thermal histories collectively the evidence supports both the reported faulting in 1906 and the interpretation that this fault is a northern extension of the san andreas fault

40ar39ar ages of igneous rocks recovered from daiichi kashima and erimo seamounts during the kaiko project abstract ten igneous rocks recovered from the daiichi kashima seamount the erimo seamount and near the japan trench with the use of the submersible nautile were dated by the 40 ar 39 ar and k ar methods the daiichi kashima seamount is estimated to have been formed about 120 ma based on the 40 ar 39 ar plateau ages of recovered rocks this result is consistent with the fossil age estimated from the limestone covering the top of the seamount a plateau age of 103 9 ma was obtained for a sample of the erimo seamount comparing the 40 ar 39 ar ages of these seamounts and the ages of the surrounding ocean floors based on magnetic anomaly patterns these seamounts are supposed to have been formed by the off ridge volcanism ages of rocks near the japan trench were determined to be 56 59 and 80 ma considering the reported age data and the circumstances of the sampling site these rocks are inferred to have not extruded at the sampling site

mineral parageneses regional architecture and tectonic evolution of franciscan metagraywackes cape mendocino garberville covelo 30 60 quadrangles northwest california 1 the franciscan complex is a classic subduction zone assemblage in northwest california it comprises a stack of west vergent thrust sheets westernmost eastern belt outliers central belt melange coastal belt yager terrane coastal belt coastal terrane coastal belt king range false cape terranes we collected samples and determined p t conditions of recrystallization for 88 medium fine grained metasandstones to assess their subduction exhumation histories and assembly of the host allochthons feebly recrystallized yager coastal and king range strata retain clear detrital features scattered neoblastic prehnite occurs in several coastal terrane metasandstones traces of possible pumpellyite are present in three yager metaclastic rocks pumpellyite lawsonite aragonite bearing central belt metasandstones are moderately deformed and reconstituted intensely contorted thoroughly recrystallized eastern belt affinity quartzose metagraywackes contain lawsonite jadeitic pyroxene aragonite glaucophane we microprobed neoblastic phases in 23 rocks documenting mineral parageneses that constrain the tectonic accretion and metamorphic p t evolution of these sheets quasi stable mineral assemblages typify eastern belt metasandstones but mm sized domains in the central and coastal belt rocks failed to achieve chemical equilibrium eastern belt slabs rose from subduction depths approaching 25 30 km whereas structurally lower central belt melanges returned from 15 18 km coastal belt assemblages suggest burial depths less than 5 8 km eastern and central belt allochthons sequentially decoupled from the downgoing oceanic lithosphere and ascended into the accretionary margin k feldspar rich coastal belt rocks were stranded along the continental edge without undergoing appreciable subduction probably during paleogene unroofing of the older deeply subducted units of the franciscan complex in east vergent crustal wedges

pilot study of fossil evidence of onshore directed storm events in estuarine sediments chezzetcook inlet nova scotianatural resources canada climate change impacts and adaptation program sst 20090027 sediment cores collected from the landward side of a baymouth barrier in a lower estuarine environment of chezzetcook inlet contain proxy signals of onshore directed storms that are manifested by the sudden appearance of benthic and planktic foraminifera species these taxa have been eroded and transported from adjacent inner shelf open marine environments one relatively strong onshore directed storm hurricane event appears to have caused a reduction in seawater exchange in the southeastern part of the inlet that persisted for at least several decades in this particular barrier protected lower estuarine depositional environment foraminiferal storm indicator species distributions in older sediments seem to be spatially patchy consequently in this chezzetcook depositional setting the successful reconstruction of a complete proxy storm record will require replicate coring and high frequency core subsampling strategies

foraminifera and tidal notches dating neotectonic events at korphos greece abstract fossil assemblages of foraminifera and thecamoebians from three salt marsh cores recovered at korphos greece provided evidence for five transgression events since the mid holocene marsh accretion rates based on radiocarbon dated peat and geomorphic evidence from a series of discrete v shaped submerged tidal notches indicated that these transgression events were rapid and episodic correlation of the tidal notches with the transgression horizons in the salt marsh stratigraphy revealed a stepwise pattern of relative sea level change at korphos which is best explained by coseismic subsidence related to fault displacement earthquakes associated with the hellenic subduction zone a comparison between the korphos data and a model of holocene sea level change for the peloponnesus reinforces this interpretation as sea level rose in a series of jumps by amounts greater than accounted for by eustatic and glacio hydro isostatic factors up to 2 0 m this study illustrates that by combining microfossil sedimentary and geomorphic records of past sea level change problems frequently encountered with each record individually e g dating submerged notches and autocompaction of marsh sediments may be overcome

major quaternary uplift along the northernmost san andreas fault king range northwestern california the king range is a rugged coastal mountain range that parallels the san andreas transform fault system just south of the mendocino triple junction point delgada is a small coastal headland that projects into the pacific ocean just southwest of the king range apatite fission track ages from parts of the king range are remarkably young averaging 1 2 ma indicating that a minimum of 2 5 km of uplift and unroofing have occured in the past 1 2 m y in contrast ages from point delgada are about 12 ma and fission track length data indicate that rocks there have resided at low temperatures le 50 degree c and thus at shallow depths since soon after 12 ma therefore point delgada has experienced relative vertical stability the contrast in uplift histories indicates that the two areas are separated by a major fault with a minimum of approximately 1 km of quaternary vertical offset the fault is probably part of the san andreas system and so may also have undergone major quaternary strike slip offset the uplift in the king range seems too great and too localized to have resulted from isostatic effects accompanying passage of the mendocino triple junction and development of a slab free window rather it is probably amore local response to space problems among the various moving crustal blocks around the triple junction and san andreas fault less

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

effects of subduction parameters on geothermal gradients in forearcs with an application to franciscan subduction in california geothermal gradients in forearcs are often suppressed below normal values because of the cooling effect of the relatively cold downgoing plate in this paper finite difference thermal modeling is used to evaluate the influence on forearc gradients of variations in six potentially important subduction zone parameters variations in forearc radiogenic heat production over the general range likely in forearc rocks 0 to 1 5 10 6 w m 3 cause only minor changes in temperatures at the base of the forearc about 2 to 35 depending on other parameters and thus have relatively little effect on pressure temperature pt conditions of metamorphism however their effect on gradients at shallow levels and thus on surface heat flows may be greater up to about 100 likely variations in forearc thermal conductivity 1 5 to 3 5 w m 1 c 1 similarly change temperatures at the base of the forearc only modestly 10 25 but change surface heat flows more substantially 1 45 1 75 times decreasing the subduction angle from 27 to 3 6 increases gradients by about a factor of 2 at both deep and shallow levels decreasing the subduction rate by a factor of 8 160 to 20 km m y increases gradients by about a factor of 2 when frictional heating at the plate contact is excluded and less when modest amounts of frictional heating are included forearc gradients increase virtually linearly with the presubduction geothermal gradient in the subducting slab and thus are higher above a younger slab when frictional heating is excluded forearc gradients are 2 0 2 8 times higher above a 10 m y old slab than above a 150 m y old slab again the relative effect is lessened when frictional heating is incorporated the amount of frictional heating at the plate contact potentially has the greatest influence on forearc gradients the frictional heat production h is given by h vs where is the shear stress along the plate contact and vs is the subduction rate the modeling shows that forearc gradients increase linearly with unless plate contact shear stresses are only a small fraction of those expected with frictional sliding of dry rocks outer parts of forearcs would be the sites of pervasive low p t metamorphism greenschist facies rather than high p t metamorphism blueschist facies very high fluid pressures along the plate contact are probably the way shear stresses are reduced pressure temperature conditions of blueschist facies metamorphism in the franciscan subduction complex of california are easily explained with typical subduction rates and slab ages with plate contact shear stresses of the order of 10 mpa equivalent to around 3 of lithostatic pressure but stresses within the range zero to a few tens of megapascals are probably permitted by the thermal constraints speculative application of the modeling results assuming a shear stress of 4 of lithostatic pressure to plate motion reconstructions for the franciscan forearc suggests that forearc gradients were about 8 c km around 85 ma when the subducting slab was perhaps 145 m y old and the subduction rate was perhaps 95 km m y gradients increased moderately through the latest cretaceous to middle tertiary as subduction became slower and the subducting slab became younger reaching about 16 c km at 28 ma when the slab age was about 11 m y and the subduction rate was about 48 km m y the slab age subduction rate and forearc gradient then remained fairly constant until 5 ma when subduction slowed to about 32 km m y and the slab age decreased to about 8 m y causing gradients to rise to about 20 c km

southward younging apatite u th he ages in the northern california coast ranges due to a northward migrating crustal welt abstract u th he in apatite dating of franciscan formation samples from between 39 47 n and 39 08 n in the northern california coast ranges nccr reveals a southward and downward younging pattern of ages 21 8 0 6 ma to 7 9 0 9 ma these ages are interpreted to represent a northward or northeastward dipping fossil he partial retention zone that was tilted and exhumed by a northward migrating pulse of crustal thickening that begins 150 200 km ahead of the migrating mendocino triple junction mtj k p furlong r govers ephemeral crustal thickening at a triple junction the mendocino crustal conveyor geology 27 1999 127 130 the 8 ma ages from the southern end of the sample transect are interpreted to come from rocks that were immediately below the he partial retention zone when crustal thickening began 5 my before the mtj passed by these rocks have now reached the surface 3 my after passage of the mtj the older ages from the central and northern portions of the sample transect are interpreted to represent rocks that were within the he partial retention zone prior to crustal thickening ahead of the mtj and which therefore record ages transitional from older subduction related cooling and exhumation to more recent transform related cooling and exhumation in addition to providing support for the model of a migrating crustal welt associated with the mtj the existence of an essentially intact gently tilted fossil partial retention zone in the northern california coast range raises the possibility of using isochronous surfaces as neotectonic structural markers in this geologically complex and heavily vegetated region

geomorphology denudation rates and stream channel profiles reveal patterns of mountain building adjacent to the san andreas fault in northern california usa relative horizontal motion along strike slip faults can build mountains when motion is oblique to the trend of the strike slip boundary the resulting contraction and uplift pose off fault seismic hazards which are often difficult to detect because of the poor vertical resolution of satellite geodesy and difficulty of locating offset datable landforms in active mountain ranges sparse geomorphic markers topographic analyses and measurement of denudation allow us to map spatiotemporal patterns of uplift along the northern san andreas fault between jenner and mendocino california emergent marine terraces found southwest of the san andreas fault record late pleistocene uplift rates between 0 20 and 0 45 mm yr 1 along much of the coast however on the northeast side of the san andreas fault a zone of rapid uplift 0 6 1 0 mm yr 1 exists adjacent to the san andreas fault but rates decay northeastward as the coast becomes more distant from the san andreas fault a newly dated 4 5 ma shallow marine deposit located at 500 m above sea level masl adjacent to the san andreas fault is warped down to just 150 masl 15 km northeast of the san andreas fault and it is exposed at just 60 110 masl to the west of the fault landscape denudation rates calculated from abundance of cosmogenic radionuclides in fluvial sediment northeast of and adjacent to the san andreas fault are 0 16 0 29 mm yr 1 but they are only 0 03 0 07 mm yr 1 west of the fault basin average channel steepness and the denudation rates can be used to infer the erosive properties of the underlying bedrock calibrated erosion rates can then be estimated across the entire landscape using the spatial distribution of channel steepness with these erosive properties the lower elevation areas of this landscape that show high channel steepness and hence calibrated erosion rate are distinct from higher elevation areas with systematically lower channel steepness and denudation rates these two areas do not appear to be coincident with lithologic contacts assuming that changes in rock uplift rates are manifest in channel steepness values as an upstream propagating kinematic wave that separates high and low channel steepness values the distance that this transition has migrated vertically provides an estimate of the timing of rock uplift rate increase this analysis suggests that rock uplift rates along the coast changed from 0 3 to 0 75 mm yr 1 between 450 and 350 ka this zone of recent relatively rapid crustal deformation along the plate boundary may be a result of the impingement of relatively strong crust underlying the gualala block into the thinner weaker oceanic crust left at the western margin of the north american plate by the westward migration of the subduction zone prior to establishment of the current transform plate boundary the warped pliocene marine deposits and the presence of a topographic ridge support the patterns indicated by the channel steepness analyses and further indicate that the zone of rapid uplift may herald elevated off fault seismic hazard if this uplift is created by periodic stick slip motion on contractional structures

first 40ar 39ar ages from low t mafic blueschist blocks in a franciscan melange near san simeon implications for initiation of subduction abstractthe franciscan complex of california is a type example of an accretionary prism with widespread high p t subduction zone metamorphism low t fine grained lawsonite bearing mafic blueschists encased in a shale matrix melange near san simeon have been dated for the first time 40ar 39ar ages were obtained for phengite separated from four blocks and from an actinolitic rind on the margin of one of the blocks the three blocks with lawsonite epidote yield ages of between 154 and ma while the actinolitic rind yields an age of ma these ages are from the part of the franciscan complex west of the san andreas fault that moved northward at least 300 km with respect to the extensive franciscan exposures east of the fault the ages obtained in this study show that some fine grained low t lawsonite epidote franciscan blueschist blocks formed at the same time as coarse grained high t garnet epidote franciscan blueschist blocks these dated rocks indicate that both high and low t mafic blueschi

contrasting sediment records of marine submersion events related to wave exposure southwest france abstract sediment records of two contrasting backshore coastal marshes extremely vulnerable to recent and historical marine flooding events located on the sw coast of france have been investigated using a multiproxy approach the studied marshes are 30 km apart and have been flooded by similar storm events 7 marine floods in the last 250 years one is located in a wave exposed coast but isolated from the sea by a sediment barrier whereas the other is located in a sheltered estuarine environment and isolated from the sea by a dike one core was collected in each marsh and information on grain size foraminifera shell contents and stable carbon isotopes was obtained along with an age model using 210 pb 137 cs and 14 c core data combined with historical maps give evidence of a typical estuarine backfilling part of the holocene regressive parasequence including an intertidal mudflat at the base and a backshore environment at the top despite the absence of grain size anomalies marine flood related sedimentation in the backshore area of both marshes is identified by a mixture of marine and terrestrial features including marine fauna vegetation debris and variation in the 13 c signature of the organic fraction very low sedimentation rates between flood events and or bioturbation prevents the identification of individual episodic marine floods in the sediment succession comparison of the two sedimentary successions shows that the foraminifera deposited by marine submersions are of two different types foraminifera are monospecific and originate from the upper tidal mudflat in the sheltered marsh whereas in the backshore marsh located in a wave exposed environment they show higher diversity and originate from both shallow and deeper water marine environments this study shows that wave exposure can control the faunal content of marine flood sediment records in coastal marshes

microfossils from coastal environments as indicators of paleo earthquakes tsunamis and storms abstract coastal risk assessment and hazard mitigation require datasets on centennial and millennial temporal scales to capture natural variability and multiple occurrences of the largest but least frequent events coastal sediments from low energy depositional environments archive geologic evidence of paleo earthquakes tsunamis and storms many of the best reconstructions of these events are derived from changes in microfossil diatoms foraminifera and pollen assemblages in this review we explain how microfossils are used to reconstruct records of paleoearthquakes by quantifying the amount of coseismic and interseismic vertical land movements along tectonically active coastlines examples from the united states alaska and the pacific northwest japan and chile show that microfossil based transfer functions may provide continuous records of vertical land movement during earthquake deformation cycles we discuss how microfossil habitat preferences and taphonomic character are used to constrain sediment provenance e g beach nearshore or offshore sources and identify overwash deposits and how this information can be used to reconstruct the recurrence of tsunamis and storms analysis of overwash deposits from thailand and malaysia indicates the ability of microfossils to resolve individual waves within tsunami sediments and an example from the sendai coastal plain in japan uses foraminifera to ascribe a beach to nearshore provenance for the 2011 tohoku tsunami deposit finally we present recent examples from the gulf of mexico on the use of foraminifera to estimate the volume and distance of transport of storm overwash from hurricanes

cenozoic northward translation of the kitakami massif in northeast japan paleomagnetic evidence abstract paleogene and early cretaceous welded tuffs have been sampled from eleven sites at the omoe peninsula 39 6 n 142 0 e in the kitakami massif of northeast japan for paleomagnetic study characteristic paleomagnetic directions with a high unblocking temperature component above 560 c are isolated westerly declinations with shallow inclination are identified after tilt correction in the 62 71 ma heizaki volcanics d 283 5 i 9 4 95 8 0 and in the 114 119 ma harachiyama formation d 274 2 i 20 4 the shallow inclination which was previously reported from early cretaceous plutonic rocks without tilt correction in the kitakami massif has been discovered in the tilt corrected paleomagnetic directions the presence of both normal and reversed polarities suggests reliability of this shallow and westerly paleomagnetic direction the shallow inclination indicates that the kitakami massif was located at low latitude 5 4 n at 71 62 ma taking into account a reference paleomagnetic pole of the paleocene expected from sikhote alin the kitakami massif was translated northward over 25 in latitude since 65 ma the kitakami massif can be subjected to latitudinal northward motion more than 24 during the last 70 ma when it was transported by push of the moving pacific plate and reached the asian continental margin near sikhote alin later than 30 ma we conclude that the kitakami massif was incorporated into northeast japan at some time between 30 and 22 ma after the cenozoic northward translation

bathymetry of the joban seamount chain northwestern pacific abstract we conducted a detailed bathymetric survey of a chain of seamounts named the joban seamount chain jsc extending n53 e over a distance of about 394 km on the ocean floor located east of the southern japan trench we surveyed a total of nine seamounts in the jsc and obtained their precise shapes and locations two of the seamounts hitachi and futaba seamounts were newly found and named two seamounts ryofu and futaba seem to lie in another line parallel to the jsc which probably also includes katori seamount two other seamounts iwaki and hitachi seamounts were classified as guyots and iwaki has a terrace on its northeastern side another isolated guyot daigo kashima is located close to the southeast of daiyon kashima seamount the origin of the jsc remains uncertain owing to the lack of reliable ages but formation at a congregation of hotspots or some combination with fissure eruption appear to be possible candidates

age dependent variation in the magnetization of seamounts the magnetization of pacific seamounts shows a surprisingly large variation when viewed as a function of time similar to normal upper oceanic crust the basalts that form the external shell of volcanic seamounts have a high average magnetization immediately after formation this high magnetization decays rapidly with time resulting in a factor of 5 reduction in intensity in less than 500 000 years because submarine volcanoes appear to consist of the same basalts that form the upper part of the seafloor low temperature oxidation of the magnetic mineral titanomagnetite is assumed to be responsible for the reduction in magnetization for both types of crust some large oceanic volcanoes are of sufficient volume that the upper capping rocks erupt subaerially and submerge completely due to thermal subsidence only after several million years these capping rocks can be locally abundant i e hawaii are exposed to a different post formation environment and can have a magnetic mineralogy that is different from submarine volcanoes in spite of these differences the magnetization of subaerially erupted rocks of the hawaii emperor chain also may show a similar rapid reduction in intensity with age finally seamounts of cretaceous age show a factor of 2 higher average magnetization when compared to tertiary seamounts similar to the previously observed increased magnetization of cretaceous seafloor basalts recovered from deep sea drilling project ocean drilling project cores this increase in cretaceous magnetization appears to be reflected in the increased amplitude of magnetic anomalies in some but not all of the ocean basins prior and subsequent to the cretaceous normal superchron the processes potentially responsible for the elevated cretaceous magnetization values include a high geomagnetic field intensity large scale variations in whole rock major element geochemistry of the extrusive rocks or a systematic acquisition of a secondary magnetization during the cretaceous normal superchron

proxy records of holocene storm events in coastal barrier systems storm wave induced markers abstract extreme storm events in the coastal zone are one of the main forcing agents of short term coastal system behavior as such storms represent a major threat to human activities concentrated along the coasts worldwide in order to better understand the frequency of extreme events like storms climate science must rely on longer time records than the century scale records of instrumental weather data proxy records of storm wave or storm wind induced activity in coastal barrier systems deposits have been widely used worldwide in recent years to document past storm events during the last millennia this review provides a detailed state of the art compilation of the proxies available from coastal barrier systems to reconstruct holocene storm chronologies paleotempestology the present paper aims i to describe the erosional and depositional processes caused by storm wave action in barrier and back barrier systems i e beach ridges storm scarps and washover deposits ii to understand how storm records can be extracted from barrier and back barrier sedimentary bodies using stratigraphical sedimentological micro paleontological and geochemical proxies and iii to show how to obtain chronological control on past storm events recorded in the sedimentary successions the challenges that paleotempestology studies still face in the reconstruction of representative and reliable storm chronologies using these various proxies are discussed and future research prospects are outlined

cretaceous paleomagnetic apparent polar wander path for the pacific plate calculated from deep sea drilling project and ocean drilling program basalt cores abstract the apparent polar wander path apwp of the pacific plate still has many uncertainties owing to the fact that paleomagnetic data are difficult to obtain for oceanic plates after more than three decades of coring by the deep sea drilling project dsdp and ocean drilling program odp there are now a large number of reliably dated basalt cores recovered from the pacific plate and this provides an opportunity to determine paleomagnetic poles based on igneous rock samples considered by many scientists to be the most reliable data type cretaceous pacific plate basalt core data were compiled corrected using a standard technique divided into groups based on age and combined to calculate five mean paleomagnetic poles with ages of 80 92 112 121 and 123 ma the latter two being for two different coeval regions in all pole analyses the lack of azimuthal orientation for cored samples leads to large uncertainties in pole locations along a nearly east west direction this difficulty was mitigated by using declination data from magnetic anomaly inversions of dated pacific seamounts for azimuth constraint the two nearly same age poles were calculated because paleocolatitudes from ontong java plateau ojp are discordant compared to those from other pacific locations i interpret the discordant ojp results to indicate that the plateau is on crust that had an early history as an independent plate the other poles 80 92 112 and 123 ma fall on a northeast trending line that suggests slow apparent polar wander during the early and mid cretaceous followed by rapid polar wander between 92 and 80 ma comparison of the 123 ma pole with previously published paleomagnetic data of jurassic age implies southward apparent polar wander followed by a turnaround because the 123 ma pole is the farthest from the geographic pole it implies the turnaround happened near that time and that the pacific plate has moved 40 northward since then the 80 ma pole stands 17 from the geographic pole indicating that 60 of the northward drift occurred prior to that time and 40 afterwards