a middle cambrian trilobite faunule from the meguma group of nova scotia the meguma group of southern nova scotia is a thick succession of mostly deep water siliciclastic sedimentray rocks of uncertain age previous fossil finds have been in the halifax formation of the upper part of the sequence and have provided early ordovician ages a newly discovered trilobite bearing bed from the underlying goldenville formation yields middle cambrian taxa of acado baltic affinity specimens belonging to paradoxides s l agraulos dorypyge and ellipsocephalidae indicate that the meguma group is an exotic succession with faunal similarity to the adjacent avalon terrane and to middle cambrian successions in europe the middle east and west africa

finite element study of uplift and strain across vancouver island geological evidence for sudden coastal subsidence along the west coast of southern vancouver island points to the occurrence of great prehistorical subduction earthquakes contemporary uplift and crustal shortening patterns in southern vancouver island appear to indicate that the subduction megathrust fault is currently locked to understand better the dynamics of the observed surface deformation we develop a finite element model of earthquake cycles for the northern cascadia subduction zone across southern vancouver island using a linear viscoelastic rheology the model consists of the continental and oceanic lithospheres the asthenospheric mantle with a viscosity of 5 1019 pa s and a low viscosity 1018 pa s mantle wedge between the subducted oceanic plate and the overlying continental plate the shallow geometry of the subducted juan de fuca plate is well defined by the results of various geophysical surveys and the deep geometry is constrained by the results of seismic tomography the model me

late quaternary sea levels and crustal movements coastal british columbia late quaternary sea level fluctuations on the british columbia coast have been established from studies of terrestrial and marine sediments and landforms these studies indicate that the sea level history of mainland british columbia and eastern vancouver island is very different from that of the queen charlotte islands and western vancouver island specifically in the former areas there was a rapid rise of submerged coastal lowlands between about 13 000 and 10 000 years ago emergence culminated about 6000 9000 years ago depending on the locality when the sea relative to the land was 12 m or more lower than at present in some areas during middle and late holocene time relative sea level rose on the mainland coast and at least locally on eastern vancouver island resulting in inundation of coastal archaeological sites and low lying terrestrial vegetation tidal records and precise levelling suggest ongoing submergence of at least part of this region in contrast shorelines on the queen charlotte i

petrology and geochemistry of neoproterozoic volcanic arc terranes beneath the atlantic coastal plain savannah river site south carolina the piedmont of south carolina and georgia is a complex mosaic of exotic terranes of uncertain provenance farther south and east these terranes form the basement beneath several kilometers of cretaceous and cenozoic sedimentary rocks commonly referred to as the atlantic coastal plain the distribution and geologic history of this hidden crystalline basement can be inferred only on the basis of limited exposures at the margins of the coastal plain onlap aeromagnetic lineaments that define basement trends in the subsurface and core data from wells that penetrate basement during the past 40 years basement cores aggregating more than 6 miles 10 000 m have been recovered from 57 deep wells at the department of energy s savannah river site these cores provide the only known samples of basement terranes that lie southeast of the fall line in central south carolina cores from the 57 deep wells along with structural trends defined by aeromagnetic lineaments allow us to define four distinct units within the basement beneath the coastal plain 1 the crackerneck metavolcanic complex greenstones and felsic

upper mantle structure of the northern cascadia subduction zone previous knowledge of the structure of the cascadia subduction zone north of the canada united states border has been derived from a variety of geophysical studies that accurately delineated the downgoing juan de fuca plate from the offshore deformation front to depths of 50 60 km beneath south central vancouver island and the georgia strait little is known however of the structure of the cascadia subduction zone farther westward and to greater depths in the upper mantle we have assembled a set of some 1100 teleseismic traveltimes from events recorded on the western canadian telemetered network to augment a previously existing data set recorded on the washington regional seismograph network the composite data set is inverted for upper mantle structure below washington oregon and southwestern british columbia we analyze the new northern portion of the model between 48 5 50 n and 118 127 w which provides the first images of the deep slab structure in this region the model is parameterized using s

trace metal contamination in sediments from u k estuaries an empirical evaluation of the role of hydrous iron and manganese oxides abstract concentrations of reducible trace metals available to hydroxylamine hydrochloride acetic acid ha have been determined in sediments from contrasting estuaries around the united kingdom specifically estuaries receiving contemporary anthropogenic inputs from catchments which are urbanized and industrialized to varying degrees clyde dee forth loughor mersey and poole harbour and estuaries receiving inputs from mineralized catchments where working of sulphidic ore deposits was important in the past fal hayle helford and tavy mean estuarine ratios of fe mn ranged from less than 10 dee mersey loughor to about 100 fal poole harbour inter estuarine variations in the mass contributions of fe and mn were attributed to differences in geological and hydrological catchment characteristics while intra estuarine variations were attributed to local differences in hydraulic sorting of sediment and redox conditions the relative contributions of hydrous fe and mn oxides to the accumulation of trace metals cr cu pb zn was evaluated by applying different regression analyses to each estuarine dataset overall the most significant correlations were with fe and in estuaries draining mineralized catchments where well defined contaminant metal sources exist poor correlations in other cases were related to the dissolution of additional metal bearing phases including carbonates by the digest dee loughor or the inability of low energy environments clyde poole harbour to buffer the impacts of local sources of contaminant metals mean estuarine concentrations of pb and zn ranged from about 25 g g 1 poole harbour to 200 g g 1 hayle and 80 g g 1 forth to 1500 g g 1 hayle respectively pooled pb and zn data were defined by a single regression equation which was not controlled by the availability or abundance of oxidic material suggesting common environmental and anthropogenic sources and similar sorptive and post depositional behaviours for these metals pooled cr and cu data however exhibited distinct groupings which were attributed to the differential sources and sorptive behaviours of these metals naturally occurring cr is highly resistant to the ha digest and excess concentrations above a baseline of about 2 5 g g 1 which characterises sediment from mineralized catchments are the result of contamination from contemporary or historical industrial processes e g mean concentrations in the clyde and forth are about 30 g g 1 copper is available to the ha digest in sediments of estuaries draining mineralized catchments where concentrations locally exceed 750 g g 1 but is highly resistant to the ha digest in sediments of industrialized estuaries where concentrations are typically between 5 and 40 g g 1 this is because cu derived from the dissolution of sulphidic minerals adsorbs readily to hydrous fe oxides while cu derived from contemporary industrial and municipal sources appears to interact with anthropogenic organic matter and the resulting complexes partition into sediment organic matter which is relatively resistant to the ha digest although the ha digest cannot readily resolve geochemical and anthropogenic metals in hydrous oxides and other sorbents dissolved by ha the availability of metals like cr and cu may give some indication of the history of the contaminant e g its source and subsequent particle water interaction mechanism more generally from a management and assessment standpoint this approach affords a first order estimate of contaminant metal loading which is geochemically and biologically available

palaeogeographical controls on the cambrian trilobite immigration and evolutionary patterns reported in the western gondwana margin abstract southward drifting of the western gondwanan margin during the cambrian has been demonstrated by means of both palaeomagnetic methods and lithological indicators of climate such as carbonates and evaporites recent improvements in biostratigraphical correlations permit an enhanced understanding of the climatic and palaeobiogeographical constraints that controlled the distribution of cambrian benthic communities palaeogeographical and biogeographical reconstructions based on trilobites are reported in this paper in order to test interaction between migration speciation and extinction rates the variability of the documented biogeographical patterns is directly related to species diversity in which wider distribution coincides with transgressive trends and subsequent connection of neighbouring platforms early cambrian trilobite faunas show a high degree of both substrate control and endemicity although transgressions led to parallel shifts in faunal compositions by contrast mid cambrian trilobite faunas are relatively uniform across western gondwana and latest mid and late cambrian associations document influence of an increased similarity with asian trilobite faunas

the carolina terrane in northwestern south carolina u s a late precambrian cambrian deformation and metamorphism in a peri gondwanan oceanic arc the carolina terrane comprises an exotic volcanic island arc in the hinterland of the southern appalachian orogen the western portions of the carolina terrane consist of zoned mafic ultramafic plutonic complexes intruded into a pile of basalts and basaltic andesites this sequence of rocks has been interpreted to represent an episode of intra arc rifting prior to regional metamorphism and deformation new u pb zircon ages from the mean crossroads complex in northwestern south carolina along the central piedmont suture confirm relative ages inferred from detailed mapping two foliated metadiorites yield u pb dates of 579 4 and 571 16 ma interpreted to be crystallization ages a foliated metaquartz diorite yields a u pb date of 538 5 ma interpreted to be a crystallization age an undeformed unmetamorpbosed diorite intruding these metamorphosed zoned complex intrusive rocks yields an age of 535 ma hence we believe that intra arc rifting and regional metamorphism and foliation development both occurred between 580 and 535 ma while petrographic and ar ar studies support subsequent middle to late paleozoic regional metamorphic overprint s or at least static recrystallization and or uplift through hornblende biotite muscovite blocking temperatures for ar the 535 ma undeformed unmetamorphosed intrusion suggests late precambrian early cambrian regional metamorphism and deformation was the event responsible for the regional metamorphic fabric in this area of the piedmont these observations contradict the idea that this metamorphism and fabric development are related to presumed early paleozoic accretion of the carolina arc to laurentia as well as correlations with middle ordovician fabric elements in the eastern piedmont instead this fabric is interpreted to record a changing plate kinematics e g migration of triple junction or change in relative motion vectors during semicontinuous diachronous development of an island arc on the gondwanide margin

geochemistry of shales from the archean 3 0 ga buhwa greenstone belt zimbabwe implications for provenance and source area weathering abstract phyllites from the archean 3 0 ga buhwa greenstone belt zimbabwe were deposited on a stable cratonic platform analyses of the phyllites generally define a single geochemical group based on major and trace element abundances the phyllites are strongly depleted in cao na2o and sr with respect to average archean upper crust by contrast k2o ba and rb are enriched several times relative to average archean upper crust reflecting basin scale k metasomatism transition metals are somewhat depleted whereas high field strength elements are typically enriched relative to average archean upper crust samples generally have fractionated lree patterns average cen smn 2 4 with small negative eu anomalies average eu eu 0 78 and generally flat hrees average gdn ybn 1 3 such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite mixing calculations which massbalance the rees and th sc suggest 70 tonalite 15 mafic volcanic rocks and 15 granite as possible proportions for the source of the phyllites a nearby early archean 3 5 ga continental nucleus preserves similar lithologies and is the probable source intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95 100 nearly complete depletion of cao and na2o and high al2o3 na2o ratios fine grained sediments of comparable age and presumed tectonic setting elsewhere in southern africa show similar geochemical characteristics implying that source area compositions and weathering intensities were similar these mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at 3 0 ga across southern africa

crustal structure of the washington continental margin from refraction data abstract refraction data were collected in an onshore offshore experiment in 1978 near grays harbor washington two dimensional raytracing and synthetic seismograms were used to model the data along the refraction lines the resulting velocity model uses a broad scale subduction zone geometry to explain the major features of the data the three main contributions of the onshore offshore model are 1 evidence for an approximately 9 dip of the subducting oceanic lithosphere 2 a clear indication of the point at which the subducting slab begins to bend beneath the margin and 3 confirmation of the continuity of the slab from the buried trench offshore to beneath puget sound the model also helps explain the distribution and stress orientation of earthquakes within the subduction zone

the stress state implied by dislocation models of subduction deformation dislocation models of the earthquake cycle are often employed to predict surface deformations above a subduction zone however these models being based entirely on a kinematic formulation of subduction behaviour place no constraints on the subsurface stress field here the stress state predicted by an elastic dislocation model is shown to exhibit characteristics inconsistent with those anticipated based on physical models of subduction behaviour in particular the model predicts an unreasonable distribution of stress along the interface between the overriding and subducting plates the net shear traction integrates to zero along the interface so significant shear must act on the overriding plate in the updip direction opposite to that of the relative direction of motion of the subducting slab quantitative calculations using an elastic half space representation of the earth are employed to illustrate the inconsistencies inherent in the dislocation model analytic expressions for this case are extended to allow for calculation of the stress components over the entire half space

influence of provenance weathering and sedimentary processes on the elemental ratios of the fine grained fraction of the bedload sediments from the vembanad lake and the adjoining continental shelf southwest coast of india abstract geochemical studies comprising twelve major elements and twenty four trace elements including the rare earth elements ree have been carried out on the concentrations normalized to the average upper continental crust ucc show that the sediments studied are depleted in rb k ba ta hf na and ca and enriched in cs u th rees sc fe co and cr ucc normalized patterns are strikingly similar for all the three sediment types probably because of efficient mixing of sedimentary material during weathering transport and deposition the depletion of na k ca and ba in sediments relative to ucc could be attributed to their mobility during weathering the depletion of ta and hf and enrichment of th ree and sc are probably related to provenance characteristics and enrichment of u is attributed to oxidation reduction processes shale normalized ree values reveal steep hree patterns tbsample tbshale ybsample ybshale lasample lashale smsample smshale and flat lree patterns the shale normalized patterns for all the three types of sediments are remarkably similar with variations in the magnitude of lree enrichment lan ybn varies between 1 72 to 2 57 times that of paas lree enrichment and steep hree patterns are attributed to felsic source rock characteristics other geochemical characteristics such as high la th and low la sc and th sc ratios compared to ucc also support a felsic provenance eu eu values for all the three types are little more than 1 which is also a characteristic feature of archean crust furthermore data plotted on discrimination diagrams of la th vs hf and la th sc cluster closer to granite and away from basalt and komatiite and are consistent with source rock geology geochemical data have also helped in ascertaining the weathering trends the chemical index of alteration cia has been used to quantify the degree of weathering cia values range between 81 and 94 on a scale of 40 100 indicating a high degree of alteration fluvial sediments seem to be more altered on an a cn k diagram the data fall closer to the compositional fields of highly weathered minerals kaolinite gibbsite and chlorite a high degree of weathering is also indicated by other plots such as k2o fe2o3 al2o3 and log k vs log rb the geochemical data are consistent with the studies on clay mineralogy higher kaolinite carried out by other authors on the sediments of the same area secondary processes seem to affect other geochemical discriminants such as co cr ni and u as revealed by plots of 1 la sc vs co th and 2 th hf co and co th cr th ratios and makes them unsuitable for determining provenance and tectonic setting

deep in the heart of dixie pre alleghanian eclogite and hp granulite metamorphism in the carolina terrane south carolina usa the central part of the carolina terrane in western south carolina comprises a 30 to 40 km wide zone of high grade gneisses that are distinct from greenschist facies metavolcanic rocks of the carolina slate belt to the se and amphibolite facies metavolcanic and metaplutonic rocks of the charlotte belt to the nw this region termed the silverstreet domain is characterized by penetratively deformed felsic gneisses granitic gneisses and amphibolites mineral assemblages and textures suggest that these rocks formed under high pressure metamorphic conditions ranging from eclogite facies through high p granulite to upper amphibolite facies mafic rocks occur as amphibolite dykes as metre scale blocks of coarse grained garnet clinopyroxene amphibolite in felsic gneiss and as residual boulders in deeply weathered felsic gneiss inferred omphacite has been replaced by a vermicular symplectite of sodic plagioclase in diopside consistent with decompression at moderate to high temperatures and a change from eclogite to granulite facies con ditions all samples have been partially or wholly retrograded to amphibolite assemblages we infer the following p t t history 1 eclogite facies p t conditions at 1 4 gpa 650 730 c 2 high p granulite facies p t conditions at 1 2 1 5 gpa 700 800 c 3 retrograde amphibolite facies p t conditions at 0 9 1 2 gpa and 720 660 c this metamorphic evolution must predate intrusion of the 415 ma newberry granite and must postdate formation of the charlotte belt and slate belt arcs 620 to 550 ma comparison with other medium temperature eclogites and high pressure granulites suggests that these assemblages are most likely to form during collisional orogenesis eclogite and high p granulite facies metamorphism in the silverstreet domain may coincide with a 570 535 ma event documented in the western charlotte belt or to a late ordovician early silurian event the occurrence of these high p assemblages within the carolina terrane implies that prior to this event the western carolina terrane charlotte belt and the eastern carolina terrane carolina slate belt formed separate terranes the collisional event represented by these high pressure assemblages implies amalgamation of these formerly separate terranes into a single composite terrane prior to its accretion to laurentia

paleoenvironmental and paleobiogeographic implications of a new ediacaran body fossil from the neoproterozoic carolina terrane stanly county north carolina new partial swartpuntiid from neoproterozoic age rocks of stanly county north carolina herein assigned to cf swartpuntia sp along with previously described pteridinium carolinaensis aspidella other metazoan trace fossils and previously reported radiometeric ages suggest nama assemblage type affinities for the carolina terrane the new specimen cf swartpuntia sp and associated radiometric ages add further information pertaining to proposed geographic linkages between the carolina terrane and the avalon terrane this new swartpuntiid may be evolutionarily convergent in terms of habitat preference to the infaunal or partially infaunal habitat preferred by other members of the nama type ediacaran biota such as pteridinium and rangea

on the mechanical role of smectite in subduction zones smectites a group of hydrated phyllosilicate minerals are often a main component of sediments entering a subduction zone they are common in hemipelagic sediments and consist of an authigenetic component that arises from the alteration of volcanic ash intercalated in the sediment and a detrital component smectite rich strata are structurally weak retain water and are highly porous a review of sediment lithology in subduction zones around the world suggests that decollement zones may preferentially form in smectite rich horizons one consequence of this observation is that the assignment of high frictional resistance to materials in a decollement may be unwarranted at deeper structural levels smectite is transformed to illite a mechanically stronger mineral estimates of the depth of this transition coincide with the depth at which earthquakes are first detected along plate interfaces i speculate that the smectite illite transition in subduction zones may be an important component of sediment consolidation processes that trigger interplate seismicity

nd isotopic evidence for juvenile crust in the carolina terrane southern appalachians nd isotopic analyses of whole rock samples from the older portion of the carolina terrane one of the largest terranes in the appalachian orogen demonstrate that part of this terrane is composed of juvenile mantle derived crust these data suggest that the terrane may not have originally been built upon old evolved basement material but rather may have been built upon oceanic crust a recent study by other workers demonstrates a more crustally evolved nd isotopic signature for younger components of the carolina terrane these data may indicate that the terrane interacted with evolved crust at a later time possibly by amalgamation with a more evolved crustal fragment before final accretion to laurentia rather than indicating a primary old basement a juvenile nature for the older portion of the terrane contrasts with models that suggest it is an evolved crustal fragment that formed in a continental margin setting a scenario proposed to explain the high proportion of felsic volcanic rocks within the terrane it is herein suggested that carolina is a chemically evolved but isotopically juvenile crustal fragment because it remained in an oceanic setting for an unusually long time in this regard the carolina terrane is similar to some of the large accreted terranes in the canadian cordillera such as wrangellia and alexander the presence of juvenile crust in the carolina terrane documents that at least part of the southern appalachian orogen is not composed solely of reactivated pre existing continental crust the importance of this part of the orogen in terms of the volume of juvenile phanerozoic crustal material in north america may be larger than previously thought however until additional major appalachian terranes have been isotopically characterized the volume of juvenile crust in the whole orogen remains unknown the isotopic make up of a terrane can be an important aspect of terrane analysis as different terranes may have significantly different isotopic compositions while even widespread pieces of a single terrane should have very similar isotopic characteristics the nd isotopic data for the carolina terrane form the beginning of an isotope database for terranes in the southern appalachians

the earthquake deformation cycle at the nankai trough southwest japan geodetic measurements made landward of the nankai trough site of great subduction zone thrust earthquakes in 1854 and 1946 provide a uniquely detailed picture of the strain buildup process supply constraints on the mechanism of strain accumulation and allow for improved estimates of earthquake recurrence provided the two most recent movement cycles are similar the observations dating from about 1890 may be used to reconstruct a single complete deformation cycle coseismic strain release postseismic transients interseismic strain accumulation very complete leveling and tidal gage data indicate that postseismic deformation extends more than 300 km inland from the plate boundary persists for at least 30 years and shows a clear tendency to become longer wavelength with increasing time the transient movements have two timescales the first of about a year or less corresponds to deformation largely uplift concentrated close to the coseismic fault and is most easily explained by aseismic slip or very localized deformation downdip of the earthquake rupture plane the second longer timescale is associated with a diffusion like spread of the deformation further landward an effect qualitatively similar to that first predicted by elsasser to be an expected consequence of faulting in an elastic plate overlying a viscoelastic asthenosphere cumulative uplift since 1890 correlates well with the distribution of uplifted marine terraces although average post 1890 tilt rates exceed late quaternary and holocene averages by at least a factor of three because of the nonlinearity of strain buildup and the significant permanent deformation simple recurrence calculations typically overestimate the true interval between great earthquakes by a factor of 2 to a factor of 3 strict application of the time predictable model assumed correct overcomes these difficulties provided the cumulative transient deformation and the proportion of permanent deformation per cycle can be estimated

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

thermal regime of the southwest japan subduction zone effects of age history of the subducting plate abstract during a mid miocene at about 15 ma tectonic reorganization an ocean spreading ridge perpendicular to the strike of the nankai trough subduction zone stopped spreading since that time there has been subduction of the cooling fossil spreading ridge in the present study we have developed a time dependent thermal subduction model for this region using the finite element method there are good seismological and geological constraints for the plate geometry and for the subduction history of the past 15 ma boundary conditions are specified so that the oceanic plate subducting at the nankai trough becomes cooler as it gets older the model results agree with the present heat flow trend that decreases landward and explain the paleothermal regime of high mid miocene temperatures inferred from land geological studies the tectonics of the region prior to 15 ma has some uncertainties assuming subduction of an active spreading ridge or an 80 ma old lithosphere prior to 15 ma give the same results for the present thermal regime of the seaward portion of the forearc but different results for the most landward region 250 km the thermal history of the forearc since 15 ma can be summarized as a rapid warming period as a consequence of ridge subduction followed by a cooling trend to the present as a result of the aging of the subducting plate the results illustrate the thermal consequences of one type of ridge subduction they also demonstrate that the thermal regime of a subduction zone depends critically on the age history of the subducting oceanic lithosphere especially if it is young as well as such parameters as the subducting plate dip angle and thickness of insulating sediments on the incoming oceanic crust this dependence is especially important when the thermal regime is used to constrain the seismogenic behaviour of the subduction thrust fault

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

paleoseismicity of the cascadia subduction zone evidence from turbidites off the oregon washington margin cores from cascadia deep sea channel contain sequences of turbidites that can be correlated and dated by the first occurrence of volcanic glass from the mount mazama eruption 6845 50 radiocarbon yrbp turbidity currents from the tributaries appear to have occurred synchronously to form single deposits in the main channel there being only 13 turbidite deposits in the lower main channel since the mazama eruption instead of the twice as many expected if the tributaries had behaved independently in addition to the cascadia channel 13 post mazama turbidites have been deposited in the astoria canyon and at two sites off cape blanco sample locations that span 580 km of the oregon washington margin pelagic intervals deposited between the turbidites suggest that in each place the turbidity currents occurred fairly regularly every 590 170 years on average the best explanation of the spatial and temporal extent of the data is that the turbidity currents were triggered by 13 great earthquakes on the cascadia subduction zone the variability of turbidite timing is similar to that for great earthquake cycles the thickness of the topmost pelagic layer suggests the last event was 300 60 years ago from three places along the margin but this number may be a biased underestimate it is however consistent with the youngest sudden subsidence event on the washington coast the turbidite data demonstrate that the near term hazard of a great earthquake on the cascadia subduction zone is of the order of 2 10 in the next 50 years

three dimensional geometry of the gorda plate beneath northern california the isostatic residual gravity field over northern california displays a gravity gradient interpreted to reflect the south edge of the gorda plate where it is subducted eastward beneath the north american plate the locus of points of maximum slope defines a line trending s60 e from a coastal point approximately 20 km south of cape mendocino a point where the buried plate boundary is inferred from magnetic and seismicity data southeast from the coast for a distance of 120 km the gravity anomaly parallels the strike of the blanco fracture zone and the present direction of relative motion between the pacific and northern gorda plates calculations from the form of the anomaly yield depth estimates that fit an east southeast plunge of approximately 9 for the top of the gorda south edge the sense of the anomaly higher gravity to the south supports the hypothesis that a window developed in the subducted slab east of the san andreas fault and south of the gorda plate south of the gorda boundary the base of the north american plate is thus in contact with hot material from the asthenosphere that invaded the window because the overlying north american plate has been moving relatively south across the gorda boundary the north american plate beneath the coast ranges east of the san andreas fault in central california may be decoupled from the underlying material at a depth slightly deeper than the depth to the top of the boundary at the time the north american plate passed over it

a comparison of the receiver structure beneath stations of the canadian national seismograph network three component broadband data from the recently deployed canadian national seismograph network provide a new opportunity to examine the structure of the crust and upper mantle beneath the canadian landmass receiver function analysis is an ideal method to use with this data set as it can provide constraints on the s velocity structure beneath each station of this seismograph network this analysis method is particularly useful in that it provides site specific information i e within 5 15 km of the station low velocity layers can be identified and it is possible to examine structure to upper mantle depths in this study receiver functions were computed for each of the 19 stations that made up the seismograph network in june 1994 five stations sampling a variety of tectonic environments including the appalachian orogen the canadian shield the western canadian sedimentary basin and the cascadia subduction zone were chosen for detailed modelling the results presented here are the first estimates of the s velocity structure beneath these five stations for those stations where comparisons can be made with seismic reflection and refraction results there is excellent agreement in eastern canada simple receiver functions and clear moho ps conversions at most stations indicate a relatively transparent crust and a moho depth of 40 45 km in northwestern canada moho ps phases indicate a crustal thickness of 33 38 km beneath inuvik northwest territories the moho is interpreted as two velocity steps separated in depth by 5 km and an upper mantle low velocity zone is near 47 km depth in western canada the data indicate a mid crustal low velocity zone beneath edmonton the moho of the subducting juan de fuca plate is interpreted at 52 krn depth beneath southern vancouver island several stations exhibiting complex receiver functions warrant further study they include stations at scheffewille quebec in the canadian shield deer lake newfoundland on the boundary of the grenville province and the appalachian orogen and yellowknife northwest territories at the intersection of the churchill and slave provinces and the western plains

inventory fluxes and residence times from the depth profiles of naturally occurring 210pb in marine sediments of mumbai harbor bay measurements of the vertical concentration profile of deposited 210pb and its parent 226ra in cores of recent marine sediment of mumbai harbor bay mhb were made to determine the various depth distribution parameters sedimentation rate age of sediment total inventory fluxes residence times scavenging rate linear attenuation coefficient and relaxation mass depth overall the profile distributions of 226ra and 210pb showed a decreasing trend with increasing the depth the mean of sedimentation rate in the studied area was obtained to be about 0 76 cm year 1 which corresponds to average age of sediment of about 56 years the average total deposited 210pb for the studied sediment profile was 9 68 k bq m 2 and correspondingly the average value of fluxes of total 210pb and excess 210pb atmospheric flux were 0 59 and 0 32 k bq m 2 year 1 respectively the residence time of 210pb was in the range of 26 226 years throughout the sediment profile and showed generally an increasing trend from top to bottom the mean relaxation mass depth h0 derived from the depth profile of 210pb at the studied area was about 190 kg m 2 the negative association between clay content of cores and the h0 of 210pb indicates that as the clay content increases the penetration depth of deposited 210pb in sediment profile decreases

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

continuous gps monitoring of elastic strain in the northern cascadia subduction zone previous monitoring and modeling of crustal deformation across the northern cascadia margin at vancouver is has provided strong evidence that the subduction thrust fault is locked and may generate future great earthquakes the recent establishment of the western canada deformation array wcda a network of continuous gps trackers in southwestern british columbia provides a new tool for monitoring crustal strain and thereby helps to assess the earthquake hazard in this region 17 months of continuous data from the three longest running wcda sites penticton victoria holberg indicate 1 a 7 mm yr easterly motion of victoria relative to penticton victoria is located in the forearc at the southeastern end of vancouver is 230 km from trench axis whereas penticton is located behind the arc 520 km from the trench axis and assumed fixed on the stable north american plate 2 a 3 mm yr northwesterly motion and 10 mm yr uplift with a large uncertainty of holberg relative to penticton holberg is located on the northern most part of vancouver is 50 km from the trench and near the plate triple junction and 3 the presence of probably seasonal variations in the apparent relative positions of the two coastal sites with respect to penticton the landward motion of victoria agrees with the deformation predicted by elastic dislocation models of the interseismic period for a great subduction thrust earthquake as constrained by previous deformation data although not as well resolved the motion of holberg roughly parallel to the margin is significantly different and is inconsistent with simple subduction models the non linear variations in measured position are dominated by an annual period probably resulting from seasonal biases in the precise orbit estimates or in the modeling of tropospheric delays or tidal effects

seismicity of the gorda plate structure of the continental margin and an eastward jump of the mendocino triple junction analysis of 9 years of data from the humboldt bay seismic network sheds new light on the structure and evolution of the gorda plate and mendocino triple junction significant findings include the buttressing effect of the pacific plate which demonstrates that there is no underthrusting of the gorda plate along the mendocino fault and the pattern of left lateral northeast trending faults which demonstrates how the gorda plate accommodates n s shortening without such underthrusting focal mechanisms are consistent with northward compression of the gorda plate by the pacific plate until the gorda plate passes the triple junction beyond which the n s compressive stress is effectively removed and the focal mechanisms show a change from strike slip to normal faulting with downslab tension another significant feature is the shallow benioff zone 10 40 km which shows a double seismogenic layer unlike most other double seismic layers in deep subduction zones this one appears to be due to underplating with a new subduction zone being developed east of the former one a corner of continental margin material has been partially subducted we attribute this double seismic zone to reactivation of both the old and new subduction boundaries under n s compression produced by pacific gorda plate interaction at the triple junction because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the san andreas fault has migrated eastward we have constructed a model that accounts for both the eastward migration of the san andreas fault and the doubling of the seismic zone in the gorda plate this can be done by assuming that at the time of the last eastward jump of the fault the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the san andreas fault other scenarios for the evolution of the mendocino triple junction do not require that the subduction zone migrate eastward however the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place this is consistent with other evidence for an eastward jump of the san andreas fault

heavy metal concentrations in surface sediments from the two coastal inlets golden horn estuary and izmit bay of the northeastern sea of marmara a total of 57 surficial sediment samples collected in the golden horn estuary and izmit bay northeastern marmara sea was analyzed for the heavy metals fe mn cr ni co zn pb and cu and the results were compared with various natural and anthropogenic sources it was observed that the concentrations of zn cu pb and to some extent cr in the golden horn sediments are comparable with those in most other seas in the vicinity of highly industrialized and densely populated regions based on calculations from the metal al and metal fe ratios the bottom sediments of golden horn were found to be enriched in zn cu pb and to a lesser extent cr by factors of up to 192 compared with the regional background levels of these metals fe mn co and to some extent ni are mostly at natural levels in contrast the metal levels of the bottom sediments in izmit bay are significantly lower and they appear to be controlled by lithogenic rather than the anthropogenic influences highly significant correlation coefficients between the metals zn cu and pb are widely attributed to the common sources of these metals the metal smelters and shipyards in the vicinity of the areas studied based on the numerical value of the geoaccumulation index of heavy metals it was found that the bottom sediments of izmit bay can be regarded as basically uncontaminated exceptionally high igeo values are found for the bottom sediments of the golden horn indicating moderate to extreme contamination in this region by the metals pb zn cu and cr the presence of a large number of anthropogenic metal point sources higher river run off via the two major creeks particularly coastal topography and hydrodynamic conditions associated with the very high sedimentation rates all strongly favour the accumulation of anomalously high metal concentrations in the golden horn estuary while the opposite is true for izmit bay

u pb zircon ages and pb isotope geochemistry of gold deposits in the carolina slate belt of south carolina volcanic rocks of the persimmon fork formation host the largest known gold mines of the carolina slate belt u pb shrimp zircon ages have been obtained from rocks closely associated with pyrite enargite gold deposits at brewer quartz topaz rhyolite breccia from the argillic alteration zone in the brewer pit and felsic ash flow tuff from the quartz sericite alteration zone from the disseminated and semimassive pyrite gold deposits at haile crystal lithic rhyolitic ash flow tuffs from the champion pit and from the ridgeway deposit felsic ash flow tuff from the stratigraphic host of the north pit gold deposit generally the zircons are fine grained fractured and contain crystal imperfections corrosion inclusions and pits 206 pb 238 u zircon spot ages for all deposits span a wide range mostly from 400 to 760 ma inclusions and cores indicative of inherited domains in the zircons were not found and only a few analyses range from 1 1 to 1 8 ga a distinct xenocrystic zircon population was not identified the 206 pb 238 u weighted age averages of zircon indicate the following crystallization dates for the volcanic and volcaniclastic rocks closely associated with the gold deposits 550 3 ma for brewer 553 2 ma for haile and 556 2 ma for the ridgeway deposit these zircon crystallization ages represent close estimates of the age of the original gold mineralizing events younger zircon spot ages can be attributed to the effects of paleozoic regional metamorphism pb isotope compositions of sulfide minerals galena pyrite enargite sphalerite chalcopyrite and molybdenite and silicate minerals k feldspar and sericite in the gold deposits help to constrain the sources of fluids and metals during the mineralizing events the deposits are pyrite rich containing multiple generations of pyrite including early crystallized pyrite that is closely associated with the original gold mineralizing event as well as recrystallized pyrite formed in response to paleozoic metamorphism pb isotope compositions of pyrite span a wide range including the most radiogenic values for the sulfides galena and k feldspar are not abundant but where present they are typically the least radiogenic minerals galena has a limited range of pb isotope compositions that are representative of the gold deposits as a group 206 pb 204 pb 18 020 18 326 207 pb 204 pb 15 550 15 639 208 pb 204 pb 37 605 38 286 values of 207 pb 204 pb straddle the average crustal pb growth curve consistent with contributions involving the mantle and continental crust whole rock pb isotope compositions of volcanic and volcaniclastic rocks of the persimmon fork formation nearly match the range for sulfides in the gold deposits subtle regional contrasts in pb isotope compositions exist among the deposits sulfide minerals from barite hill e g galena 206 pb 204 pb 206 pb 204 pb 18 169 haile 206 pb 204 pb 18 233 and brewer 206 pb 204 pb 18 311 in northern south carolina because pb isotope compositions of basement rocks from grenville massifs in the southern appalachians and sulfide minerals from the gold deposits do not match a direct genetic connection cannot be established diversity in values of 206 pb 204 pb and the relatively high values of 207 pb 204 pb suggest that the deposits evolved adjacent to or closely related to continental blocks perhaps linked to a back arc tectonic setting among potential younger analogues of the slate belt gold deposits are the sulfide deposits of the okinawa trough in the western pacific mantle derived isotopic contributions were more important at barite hill in southern south carolina the least radiogenic among the deposits where oceanic crust had developed than at brewer haile and ridgeway in northern south carolina where rifting thinned the continental crust

contemporary movements and tectonics on canada s west coast a discussion abstract evidence from published tidal records and geodetic relevelling data in british columbia indicates that there is a consistent pattern of contemporary uplift on the outer coast 2 mm yr and subsidence on the inner coast 1 2 mm yr the zero uplift contour or hinge line runs through hecate strait georgia strait and victoria this pattern continues southwards into washington state but is interrupted to the north by considerable uplift in southeastern alaska although glacio isostatic recovery has dominated vertical movements in the region over the last 10 000 years the distribution and trend of the observed contemporary movements are not compatible with the pattern to be expected from this source and are most probably tectonic in origin there is however no clear distinction between the movements seen opposite the queen charlotte transform margin and the vancouver island convergent margin comparison with movements observed at other active plate margins show that the pattern is essentially similar to that seen in association with subduction and convergence the paradox that the vertical movement rates are much too great to explain observed geology and topography may be soluble by assuming that discontinuous lateral shifts of the movement pattern occur on a scale of hundreds of thousands of years

p waveform analysis for local subduction geometry south of puget sound washington the local subduction geometry at a site south of puget sound in western washington is investigated using teleseismicp waveforms recorded on a three component event triggered seismograph the data are processed using source equalization deconvolution in order to isolate locally convertedp to s arrivals and stacked to improve the signal to noise ratio stable arrivals in the radial component indicate an oceanic moho within the subducted slab at a depth of about 53 km beneath the station observed amplitude variations with azimuth in the radial data as well as qualitative aspects of the tangential data are used to establish a slab dip of 16 to the southeast our results are compatible with previous results from a site 60 km to the west and further confirm a substantial warp in the regional geometry of the subducted juan de fuca plate

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

slab geometry of the cascadia subduction zone beneath washington from earthquake hypocenters and teleseismic converted waves using earthquake hypocenters obtained from the washington regional seismograph network and analysis of ps converted phases from teleseisms at special temporary stations located south of the olympic peninsula we have developed a preliminary model of the structure of the subducted slab beneath western washington and southern british columbia the slab is smoothly arched beneath puget sound subducting at a dip of about 10 12 at the top of the arch north and south of the arch crest the slab dips 15 20 subcrustal earthquakes are localized near the top of the arch this deformation from planar slab geometry is caused by the change in azimuth of the trench or hinge line of the slab from almost ns south of 47 north latitude to approximately n30 w north of 48 north the slab appears to be easily flexed probably due to small elastic thickness but its geometry reflects conservation of surface area and minimization of in plane strain crustal seismicity in the puget sound basin may result indirectly from the presence of the arch in the underlying juan de fuca plate

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

strain accumulation in western washington the juan de fuca plate is subducted beneath the north american plate off the coast of washington at a rate of about 40 mm yr n68 e the average principal strain rates extension reckoned positive measured in northwestern washington are as follows olympic peninsula 25 km south of port angeles from 1982 through 1990 e 1 0 011 0 027 strain yr n31 e 6 6 and e 2 0 092 0 028 strain yr n59 w 6 6 and near seattle from 1972 through 1985 e 1 0 027 0 019 strain yr n22 e 6 4 and e 2 0 036 0 013 strain yr n68 w 6 4 both strain measurements are consistent with uniaxial contraction in the direction of plate convergence uplift rates inferred from tide gage recordings are about 4 mm yr on the pacific coast and near 0 mm yr farther inland near seattle these deformation rates are consistent with a model of the cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward the limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth 20 km of the isotherm 450 c that defines the brittle ductile transition small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths the principal strain rates measured from 1972 through 1983 in the back arc region near richland washington are e 1 0 016 0 013 strain yr n03 w 34 and e 2 0 024 0 013 strain yr n87 e 34

evidence for a large earthquake and tsunami 100 400 years ago on western vancouver island british columbia abstract a peaty marsh soil is sharply overlain by a sand sheet and intertidal mud at tidal marshes near tofino and ucluelet vancouver island british columbia foraminifera and vascular plant fossils show that the buried soil was submerged suddenly and was covered quickly by sand radiocarbon ages place this event between 100 and 400 yr ago the coastal subsidence suggested by the submergence occurred in an area of net late holocene emergence perhaps during the most recent great earthquake on the northern part of the cascadia subduction zone the sand sheet overlying the peaty soil records the tsunami triggered by this earthquake similar stratigraphic sequences of about the same age have been reported from estuaries along the outer coasts of washington and northern oregon suggesting that hundreds of kilometers of the cascadia subduction zone may have ruptured during one or a series of plate boundary earthquakes less than 400 yr ago

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

three dimensional dislocation model for great earthquakes of the cascadia subduction zone there have been no historical cascadia great subduction thrust earthquakes but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event geodetic measurements in the coastal region from northern california to southern british columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault the segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two dimensional 2 d elastic dislocation modeling of interseismic deformation geodetic data in this study a general 3 d dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture the model is based on die surface deformation due to shear faulting in an elastic half space the 3 d model of the cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin the bend in the margin trend and subducting slab end effects are included there is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement an initial 3 d model based upon 2 d dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data the best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km the two zones lie mainly offshore beneath the continental shelf and slope the locked and transition zone widths vary smoothly along the margin being greater off northern washington where the thrust dip is shallow and narrower off central oregon assuming that the locked plus transition zones approximate the maximum coseismic rupture area these widths permit amw 9 earthquake

northward extension of carolina slate belt stratigraphy and structure south central virginia results from geologic mapping geologic mapping in south central virginia demonstrates that the stratigraphy and structure of the carolina slate belt extend northward across a steep thermal gradient into upper amphibolite facies correlative gneiss and schist the neoproterozoic greenschist facies hyco aaron and virgilina formations were traced northward from their type localities near virgilina virginia along a simple upright northeast trending isoclinal syncline this syncline is called the dryburg syncline and is a northern extension of the more complex virgilina synclinorium progressively higher grade equivalents of the hyco and aaron formations were mapped northward along the axial trace of the refolded and westwardly overturned dryburg syncline through the keysville and green bay 7 5 minute quadrangles and across the northern end of the carolina slate belt as interpreted on previous geologic maps hyco rocks including felsic metatuff metawacke and amphibolite become gneisses upgrade with areas of local anatexis and the segregation of granitic melt into leucosomes with biotite selvages phyllite of the aaron formation becomes garnet bearing mica schist aaron formation rocks disconformably overlie the primarily felsic volcanic and volcaniclastic rocks of the hyco formation as evidenced by repeated truncation of internal contacts within the hyco on both limbs of the dryburg syncline at the aaron hyco contact east northeast trending isograds defined successively by the first appearance of garnet then kyanite staurolite in sufficiently aluminous rocks are superposed on the stratigraphic units and synclinal structure at moderate to high angles to strike the textural distinction between gneisses and identifiable sedimentary structures occurs near the kyanite staurolite in isograd development of the steep thermal gradient and regional penetrative fabric is interpreted to result from emplacement of the goochland terrane adjacent to the northern end of the slate belt during alleghanian orogenesis this mapping study indicates that the carolina slate belt does not terminate on the north against through going faults or rest on higher grade basement as previously suggested

three dimensional model of vertical deformation at the southern cascadia subduction zone western united states recent studies indicate that there have been several episodes of coseismic subsidence along the coasts of central and southern oregon and northern california during the past several thousand years a three dimensional dislocation model is used to predict the amount of coseismic subsidence produced by different magnitude earthquakes the magnitude of coseismic subsidence is equal to the interseismic uplift in an elastic earth model the model is constrained mainly by interseismic uplift data from repeated leveling surveys in western oregon over the past 50 60 yr the results suggest that earthquakes with moment magnitude of 8 5 are required to produce the coseismic subsidence observed the model also suggests that larger earthquakes mw 9 0 would produce coseismic subsidence in areas where none is observed in the geologic record

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

spatial geochemical variation of major and trace elements in the marine sediments of mumbai harbor bay the resulting concentration data sets of major na k mg ca and fe and trace elements cu ni co and mn in bed and suspended sediments were used to evaluate the enrichment factor for anthropogenic influences and principal component analysis for identifying the origin of source contributions in the studied area normalization of metals to fe indicated that high enrichment factors in the bed sediment were in the order of co cu na ca ni except mg k and mn while for suspended sediments only co has a high enrichment factor high enrichment of co and cu reflected the contamination of sediments from anthropogenic sources the high influence of na and ca in sediments may be caused for seawater salinization factor a significant positive correlation among enrichment factors of various elements of interest suggests a common origin identical behavior during transport in the sediment system

ephemeral plate tectonics at the queen charlotte triple junction three plate boundaries the queen charlotte transform the cascadia subduction zone and the juan de fuca ridge meet in a complex triple junction offshore vancouver island some interpretations of the plate tectonics of the region have included the explorer ridge the dellwood knolls and the tuzo wilson volcanic field as part of the spreading system requiring up to three triple junctions in the region new seabeam bathymetry interpreted with existing regional data sets indicate that the dellwood knolls and tuzo wilson volcanic field are not independent plate boundaries but the result of leaky transform tectonics between two overlapping transform faults seismicity shows that the explorer plate is being deformed by pacific north american relative motion as a new transform plate boundary forms and cuts off the explorer ridge the system is evolving to a single triple junction at the northern terminus of the juan de fuca ridge where it meets the nootka deformation zone thus the explorer microplate which was spawned 5 ma is an ephemeral adjustment to mechanical difficulties at the triple junction this new model implies that the explorer subduction zone is no longer active ephemeral oceanic microplates have also existed at pacific north american triple junctions off southern california and baja california

structural analysis of the carolina inner piedmont terrane boundary implications for the age and kinematics of the central piedmont suture a terrane boundary that records paleozoic laurentia gondwana interactions new field mapping along the carolina inner piedmont terrane boundary in south carolina and eastern georgia reveals preaccretionary synaccretionary and postaccretionary faults the dextral strike slip lowndesville shear zone is adjacent to a 50 km long segment of the terrane boundary however the lowndesville shear zone is correlated eastward with the dextral strike slip beaver creek shear zone which is within the carolina terrane and predates 415 ma the lowndesville shear zone is overprinted by a dextral strike slip phyllonite zone named the deal creek shear zone the deal creek shear zone is correlated eastward with the gold hill silver hill shear zone which is also within the carolina terrane and records dextral strike slip motion between 400 and 325 ma the cross anchor and mulberry creek faults both truncate the lowndesville and the deal creek shear zones and form the terrane boundary the mulberry creek fault probably of triassic jurassic age juxtaposes the lowndesville shear zone adjacent to the carolina inner piedmont terrane boundary west of waterloo south carolina the cross anchor fault is the terrane boundary east of waterloo south carolina and forms the southeastern boundary of the whitmire reentrant crosscutting relationships indicate that the cross anchor fault is the oldest fault which juxtaposes the carolina and inner piedmont terranes in the study area these structural interpretations and available geochronological data indicate that the cross anchor fault is a 325 ma thrust fault and may be the central piedmont suture an early alleghanian suture resolves the problem of inserting the carolina terrane into the western iapetus ocean

an explanation for the double seismic layers north of the mendocino triple junction we propose that the gently eastward dipping double planed seismic zone observed at 15 25 km depths in the southern cascadia subduction zone just north of the mendocino triple junction is a direct consequence of the thermally controlled rheology as the oceanic lithosphere subducts to a depth of about 15 km the temperature regime causes a brittle plastic transition to occur within the oceanic crust thus a ductile layer forms in the lower oceanic crust sandwiched between the brittle upper crust and brittle upper mantle the very high strain rates near the triple junction caused by the northward push of the pacific plate on the gorda plate increase the seismicity and thus accentuate the double seismic zone in this region this model explains the focal mechanisms observed in the seismic zone and their spatial change the double seismic layers clearly define the position of the subducting gorda plate previously uncertain in the cape mendocino region

present day vertical deformation of the cascadia margin pacific northwest united states we estimate present day uplift rates along the cascadia subduction zone in california oregon and washington in the pacific northwest united states by utilizing repeated leveling surveys and tide gauge records these two independent data sets give similar profiles for latitudinal variation of contemporary uplift rates along the coast uplift rates are extended inland through east west leveling lines that connect the north south line along the coast to the north south line along the inland valleys just west of the cascades the results are summarized as a contour map of present day uplift rates for the western pacific northwest we find that rates of present day uplift vary latitudinally along the coast and inland valleys as well as longitudinally along transects connecting the coast to the inland valleys long term tidal records of neah bay astoria and crescent city indicate uplift of land relative to sea level of 1 6 0 2 0 0 0 2 and 0 9 0 2 mm yr respectively 1 standard error unlike previous estimates of relative sea level change at astoria we adjust for discharge effects of the columbia river including human management influences after approximating an absolute framework by using 1 8 0 1 mm yr to compensate for global sea level rise results indicate that much of the western pacific northwest is rising at rates between 0 and 5 mm yr the most rapid uplift rates are near the coast particularly near the olympic peninsula the mouth of the columbia river cape blanco and cape mendocino two axes of uplift are identified one trends northeast from the southwest oregon coast and the other trends south southeasterly from the olympic peninsula to the columbia river the puget sound vicinity and a small east west region from the north central oregon coast to the inland willamette valley are subsiding at rates up to 1 mm yr we interpret the overall pattern of rapid present day uplift to be generated by interseismic strain accumulation in the subduction zone this interseismic elastic strain accumulation implies significant seismic hazard

tsunami generation by horizontal displacement of ocean bottom tsunami generation by an earthquake is generally modeled by water surface displacement identical to the vertical deformation of ocean bottom due to faulting the effect of horizontal deformation is usually neglected however when the tsunami source is on a steep slope and the horizontal displacement is large relative to the vertical displacement the effect becomes significant we show this for two recent earthquakes which generated much larger tsunamis than expected from seismic waves in the case of the 1994 june 2 java indonesia earthquake the focal mechanism was a very shallow dipping thrust and the source was near a very steep trench slope in the case of the 1994 nov 14 mindoro philippines earthquake strike slip faulting extended from ocean to land perpendicular to the coast line in both cases we found that the horizontal motion of slope had an important contribution to the tsunami generation

coupling of tectonic loading and earthquake fault slips at subduction zones because of the viscoelastic behaviour of the earth accumulation of elastic strain energy by tectonic loading and release of such energy by earthquake fault slips at subduction zones may take place on different spatial scales if the lithospheric plate is acted upon by distant tectonic forces strain accumulation must occur in a broad region however an earthquake releases strain only in a region comparable to the size of the rupture area a two dimensional finite element model of a subduction zone with viscoelastic rheology has been used to investigate the coupling of tectonic loading and earthquake fault slips a fault lock and unlock technique is employed so that the amount of fault slip in an earthquake is not prescribed but determined by the accumulated stress the amount of earthquake fault slip as a fraction of the total relative plate motion depends on the relative sizes of the earthquake rupture area and the region of tectonic strain accumulation as well as the rheology of the rock material the larger the region of strain accumulation is compared to the earthquake rupture the smaller is the earthquake fault slip the reason for the limited earthquake fault slip is that the elastic shear stress in the asthenosphere induced by the earthquake resists the elastic rebound of the overlying plate since rapid permanent plate shortening is not observed at subduction zones there must be either strain release over a large region or strain accumulation over a small region over earthquake cycles the former can be achieved only by significant aseismic fault slip between large subduction earthquakes the most likely mechanism for the latter is the accumulation of elastic strain around isolated locked asperities of the fault which requires significant aseismic fault slip between asperities

review receiver function studies in the southern canadian cordillera receiver function analysis has proven to be a powerful yet inexpensive tool for estimating the s wave velocity structure of the crust and upper mantle beneath three component seismograph stations in the southern canadian cordillera receiver function studies using a portable broadband seismograph array across southwestern british columbia provided site specific estimates for the location of the subducting juan de fuca plate the oceanic crust was imaged at 47 53 km beneath central vancouver island and 60 65 km beneath the strait of georgia further these studies revealed a prominent low velocity zone vs 1 0 km s that coincides with the e reflectors imaged 5 10 km above the subducting plate on lithoprobe reflection lines the e low velocity zone was shown to extend into the upper mantle beneath the strait of georgia and the british columbia mainland to depths of 50 60 km combining the receiver function and refraction models revealed a high poisson s ratio 0 27 0 38 for this feature the contin

interseismic deformation at the nankai trough japan subduction zone uplift along the coast of southwestern japan following the 1944 tonankai ms 8 0 and 1946 nankaido ms 8 2 earthquakes has been inferred from the 1950 1985 tide gage records although uplift rates vary with geographic position the temporal dependence at each site can be described as the superposition of an exponentially decaying time constant 5 years transient significant only in the first decade postseismic and a steady interseismic rate the steady interseismic uplift rate is defined only over the 1956 1985 interval but the linearity in time ofthat uplift record is such that uplift over the remainder of the interseismic cycle 80 years might reasonably be expected to occur at the same rate moreover the spatial pattern and magnitude of the steady uplift rate are reasonably close to those predicted by a simple dislocation model of subduction however the currently observed steady uplift rate is not consistent with the uplift rate inferred from geodetic leveling for the final half of the previous earthquake cycle thus either the hypothesis of a steady interseismic uplift rate after the first decade postseismic is wrong or the strain accumulation pattern for one earthquake need not be the same as for the preceding earthquake on the same segment

biogeographical significance of cambrian trilobites from the carolina slate belt abundant middle middle cambrian trilobites of the ptychagnostus atavus interval zone are described from the upper asbill pond formation informal name near batesburg south carolina the fauna containing at least nine species has strong affinities with cool water faunas of both armorica and baltica but little affinity with warm water shelf faunas of laurentia this is compatible with previous conclusions that the carolina slate belt is an exotic terrane that was accreted to north america during the early or middle paleozoic further evaluation however introduces an alternative possibility that the fauna could have lived in deep cool water environments along the periphery of laurentia and subsequent displacement need not necessarily have been great combined faunal and stratigraphic components of the carolina slate belt do not precisely match those of any known region but may be closest to those of bohemia although a southpolar location in proximity to armorica baltica and gondwana seems likely a definitive placement in a reconstruction of the cambrian world is not yet possible included in the trilobite fauna are the agnostoids hypagnostus mammillatus h parvifrons peronopsis fallax ptychagnostus sp and tomagnostus fissus the polymeroids include agraulos sp paradoxides cf polonicus paradoxides sp and skreiaspis sp