the palos verdes anticlinorium along the los angeles california coast implications for underlying thrust faulting 1 the palos verdes anticlinorium pva is a 70 km wide nw trending pliocene quaternary transpressional high that separates the onshore los angeles basin from the offshore san pedro basin the pva southwest limb underlies a 70 km long sw facing seafloor escarpment this escarpment is separated into two segments by a 6 km right step it overlies two levels of ne dipping low angle thrust faults these faults project beneath los angeles toward known sw verging blind thrust faults and may be the same regional faults these conclusions are based on a detailed geometric representation constructed from a regional analysis of 5000 km of multichannel seismic reflection profiles logs and paleontology from 19 wells published seafloor geology and multibeam bathymetric data patterns of sedimentation and erosion indicate that folding of pva initiated during pliocene time and propagated southward with folding of part of the southern pva occurring during the quaternary structural relief continues to grow with regional long term rock uplift of the pva crest and ongoing subsidence in the adjacent basins the low angle ne dipping faults beneath the pva must be late quaternary active in order to maintain hanging wall rock uplift above subsiding footwalls structural geometry requires that the underlying faults have slipped at 1 1 1 6 mm yr over quaternary time

loading of the san andreas fault by flood induced rupture of faults beneath the salton sea the southern san andreas fault terminates in a stepover zone several small faults that separate major fault segments beneath the salton sea analysis of movements on the stepover zone faults indicates that periodic flooding of the palaeo salton sea during the late holocene could have triggered earthquakes on the san andreas fault

activity of the offshore newport inglewood rose canyon fault zone coastal southern california from relocated microseismicity an offshore zone of faulting approximately 10 km from the southern california coast connects the seismically active strike slip newport inglewood fault zone in the los angeles metropolitan region with the active rose canyon fault zone in the san diego area relatively little seismicity has been recorded along the offshore newport inglewood rose canyon fault zone although it has long been suspected of being seismogenic active low angle thrust faults and quaternary folds have been imaged by seismic reflection profiling along the offshore fault zone raising the question of whether a through going active strike slip fault zone exists we applied a waveform cross correlation algorithm to identify clusters of microseismicity consisting of similar events analysis of two clusters along the offshore fault zone shows that they are associated with nearly vertical north northwest striking faults consistent with an offshore extension of the newport inglewood and rose canyon strike slip fault zones p wave polarities from a 1981 event cluster are consistent with a right lateral strike slip focal mechanism solution manuscript received 23 july 2003

a combined in situ oxygen silicon isotopic and fluid inclusion study of a chert sample from onverwacht group 3 35ga south africa new constraints on fluid circulation abstract cherts have been extensively used as a proxy of precambrian environmental conditions of the earth cherts from the onverwacht group 3 5 3 3 ga south africa are considered as well preserved and have been used as a paleo oceanic thermometer especially by using the oxygen isotope composition of the chert nevertheless there are several lines of evidence size and number of the quartz veins that fluids circulated through these cherts after their deposition these cherts are composed mainly of microquartz with large quartz veins 100 m and iron carbonates as ankerites a combined study of oxygen silicon and fluid inclusions study have been performed on one chert sample from the onverwacht group in order to assess the effect of these fluid circulations on the chert oxygen and silicon isotopic compositions in situ oxygen and silicon isotopic compositions on microquartz and vein quartz were measured at high spatial resolution using a multicollector cameca 1270 ion probe crpg cnrs nancy moreover fluid inclusions present in the quartz veins are considered to determine the origin and conditions of the fluid circulation three types of primary fluid inclusions carbonic lc aqueous carbonic fluids lc w and aqueous fluid lw have been observed the temperatures of this fluid circulation were determined by microthermometry and using quartz carbonate pair thermometry at between 200 c and 300 c fluid pressures range from 100 mpa to 30 mpa which correspond to depths ranging from 4 to 1 km this evolution could be related to an exhumation associated with a dilution and cooling process this fluid circulation could be associated to the gold bearing fluid responsible of the sheba fairview ore system dated at 3074 6 ma the results of the isotopic study show that these fluid circulations homogenised the oxygen isotope composition but not the entire silicon isotope composition of the microquartz the 18 o value calculated for the fluid show that it equilibrated with carbonate rocks and is similar to the composition of the gold bearing fluid the silicon isotope composition of the microquartz allows determination of the initial 30 si of the microquartz precursor this precursor has been precipitated from a hydrothermal fluid hence a combined 18 o 30 si and fluid inclusion study allows the conditions of the fluid circulation to be constrained and the origins of chert to be determined

rethinking turbidite paleoseismology along the cascadia subduction zone a stratigraphic synthesis of dozens of deep sea cores most of them overlooked in recent decades provides new insights into deepsea turbidites as guides to earthquake and tsunami hazards along the cascadia subduction zone which extends 1100 km along the pacific coast of north america the synthesis shows greater variability in holocene stratigraphy and facies off the washington coast than was recognized a quarter century ago in a confluence test for seismic triggering of sediment gravity flows that test compared counts of holocene turbidites upstream and downstream of a deep sea channel junction similarity in the turbidite counts among seven core sites provided evidence that turbidity currents from different submarine canyons usually reached the junction around the same time as expected of widespread seismic triggering the fuller synthesis however shows distinct differences between tributaries and these differences suggest sediment routing for which the confluence test was not designed the synthesis also bears on recent estimates of cascadia earthquake magnitudes and recurrence intervals the magnitude estimates hinge on stratigraphic correlations that discount variability in turbidite facies the recurrence estimates require turbidites to represent megathrust earthquakes more dependably than they do along a flow path where turbidite frequency appears limited less by seismic shaking than by sediment supply these concerns underscore the complexity of extracting earthquake history from deep sea turbidites at cascadia

structure and paleoearthquake records of active submarine faults cook strait new zealand implications for fault interactions stress loading and seismic hazard 1 a new interpretation of active faulting in central cook strait new zealand reveals tectonic structures associated with the spatial transition from subduction to continental transform faulting marine seismic reflection profiles and multibeam bathymetric data indicate that there are no throughgoing crustal faults connecting the north island dextral fault belt and the marlborough fault system in south island the major faults terminate offshore associated with 5 20 km wide step overs and a change in regional fault strike this structure implies that propagation of strike slip earthquake ruptures across the strait is not probable faulted sedimentary sequences in the wairau basin marlborough shelf correlated to glacioeustatic sea level cycles provide a stratigraphic framework for fault analysis a high resolution study of the postglacial 20 ka vertical displacement history of the cloudy and vernon faults reveals up to six and five paleoearthquakes since 18 ka respectively these long timescale records indicate variable recurrence intervals and possibly variable stress drop thus conforming to the variable slip model of earthquake behavior integration of these data with other submarine and terrestrial paleoearthquake records indicates the presence of clustered earthquake sequences involving multiple faults different sequences do not always involve the same faults it appears that earthquake clustering is driven by fault interactions that lead to specific loading conditions favoring the triggering of earthquakes on major faults in relatively short time intervals present day regional coulomb stress distribution has been calculated in two scenarios considered to represent minimum and maximum loading conditions the models incorporating secular tectonic loading and stress changes associated with major crustal earthquakes indicate high stress loading in a large part of central cook strait these conditions may favor the triggering of future damaging earthquakes in this region

60 k y record of extension across the western boundary of the basin and range province estimate of slip rates from offset shoreline terraces and a catastrophic slide beneath lake tahoe deformation across three major fault strands within the lake tahoe basin has been mapped by using a novel combination of high resolution seismic chirp airborne laser and acoustic multibeam derived bathymetry and deep and shallow water sediment cores submerged erosional terraces of late pleistocene age 19 2 6 1 8 ka record vertical de formation across fault strands that ranges between 10 and 15 m offset of 10 m is observed across the southern part of the west tahoe fault avalanche deposits from the catastrophic mckinney bay slide ca 60 ka are offset across the stateline fault by at least 21 25 m the submerged shoreline terraces and debris avalanche provide marker beds with which to constrain the extensional history of the region for the past 60 k y this history is then used to assess the future seismic hazard of the region data on deformation across these two important marker beds combined with chronological control from 14 c and optically stimulated luminescence measurements yield an estimate of extension across the lake tahoe basin that is 0 4 0 5 mm yr on the basis of these measurements there exists the potential for a large seiche wave generating m7 earthquake every 3 k y late pleisto cene and holocene vertical deformation rates within the tahoe basin are characteristic of basin and range faulting and place the tahoe basin within the western limits of the extensional basin and range province

seismic reflection and geomorphic characterization of the onshore palos verdes fault zone los angeles california abstract two high resolution 25 to 110 hz bandwidth seismic reflection profiles acquired along roads crossing the palos verdes fault zone los angeles basin california strongly suggest that some geomorphic lineaments seen on aerial photographs coincide with steeply dipping faults at roughly the midpoint of the palos verdes peninsula the palos verdes fault zone consists of at least three major strands one of the northern strands which has the most apparent vertical displacement is interpreted to dip 55 sw down to 200 m steepening at greater depth other strands appear to dip from 50 to 90 just 3 km to the southeast however the fault zone is composed of at least five strands and is roughly 1 5 km wide apparently younger strands are located northeast of older less active strands across the fault zone recent right lateral offset on the northeastern fault strands is inferred from a deflection of an ancestral los angeles river channel approximately 1 8 km west of the los angeles harbor the channel incises deposits dated to be as young as 120 to 80 ka and is deflected approximately 300 m this deflection if attributed solely to slip on the palos verdes fault zone suggests an average horizontal slip rate between 2 5 and 3 8 mm yr because very little vertical displacement is detectable in the seismic data acquired near the channel offset that also shows little vertical displacement we infer that recent onshore movement in the palos verdes fault zone on the eastern palos verdes peninsula has been predominantly strike slip

high resolution mapping of two large scale transpressional fault zones in the california continental borderland santa cruz catalina ridge and ferrelo faults new mapping of two active transpressional fault zones in the california continental borderland the santa cruz catalina ridge fault and the ferrelo fault was carried out to characterize their geometries using over 4500 line km of new multibeam bathymetry data collected in 2010 combined with existing data faults identified from seafloor morphology were verified in the subsurface using existing seismic reflection data including single channel and multichannel seismic profiles compiled over the past three decades the two fault systems are parallel and are capable of large lateral offsets and reverse slip during earthquakes the geometry of the fault systems shows evidence of multiple segments that could experience throughgoing rupture over distances exceeding 100 km published earthquake hypocenters from regional seismicity studies further define the lateral and depth extent of the historic fault ruptures historical and recent focal mechanisms obtained from first motion and moment tensor studies confirm regional strain partitioning dominated by right slip on major throughgoing faults with reverse oblique mechanisms on adjacent structures transpression on west and northwest trending structures persists as far as 270 km south of the transverse ranges extension persists in the southern borderland a logjam model describes the tectonic evolution of crustal blocks bounded by strike slip and reverse faults which are restrained from northwest displacement by the transverse ranges and the southern san andreas fault big bend because of their potential for dip slip rupture the faults may also be capable of generating local tsunamis that would impact southern california coastlines including populated regions in the channel islands

a high resolution seismic chirp investigation of active normal faulting across lake tahoe basin california nevada we measured extension rates across lake tahoe basin for the last 60 k y based on measured displacement of offset marker surfaces across three active faults beneath lake tahoe seismic chirp imaging with submeter accuracy together with detailed multibeam and light detection and ranging lidar derived bathymetry was used to measure fault offset thickness of earthquake derived colluvial wedges depth of wave cut paleoterraces and other geomorphic features an analysis of these features provides refined estimates of extension rates and new information on holocene faulting and places tahoe basin deformation into the larger context of walker lane and basin and range tectonics measured offset marker surfaces include submerged wave cut paleoterraces of tioga age 19 2 1 8 ka mckinney bay slide deposits ca 60 ka and a winnowed boulder surface of tahoe age ca 62 ka estimated vertical offset rates across submerged geomorphic surfaces are 0 43 0 81 mm yr for the west tahoe fault 0 35 0 60 mm yr for the stateline north tahoe fault and 0 12 0 30 mm yr for the incline village fault these offset rates indicate a combined east west extension rate across lake tahoe basin assuming 60 fault dips of 0 52 0 99 mm yr this estimate when combined with the genoa fault slip rate yields an extension rate consistent with the magnitude of the extension deficit across carson valley and lake tahoe basin derived from global positioning system gps velocities the stateline north tahoe incline village and west tahoe faults all show evidence for individual holocene earthquake events as recorded by either colluvial wedge deposits or offset fan delta stratigraphy

late cenozoic tectonic evolution of the los angeles basin and inner california borderland a model for core complex like crustal extension we propose that the los angeles basin inner california borderland lab ib and the southern borderland off baja california formed by large magnitude crustal extension beginning in latest oligocene earliest miocene time and that these regions represent major late cenozoic rifts the southern borderland is analogous to the gulf of california rift but the onset of rifting in this region may have preceded the opening of the gulf by as much as 15 to 20 m y the lab ib rift is more complex than the southern borderland because it formed in the wake of the rotating western transverse ranges wtr a large crustal block that has undergone more than 90 of neogene clockwise rotation total extension in both rifted regions is estimated to be in excess of 200 km or comparable to the magnitude of late cenozoic crustal extension now estimated for the basin and range province also like the basin and range the crustal floors of the basins in these highly extended domains are characterized by metamorphic core complexes and detachment faults voluminous miocene volcanic rocks and hanging wall fragments of pre rift strata and basement rocks stranded atop detachments during extension in support of this model we synthesize a wide variety of existing geologic data and present interpretations of new geophysical data seismic reflection profiles these profiles image features that we infer are detachment faults and overlying remnants of fragmented and rifted hanging wall rocks we also suggest that a similar but slightly earlier onset of crustal extension affected the santa maria basin and adjacent regions in central california and that the southern margin of this santa maria basin rift was overridden during neogene clockwise rotation of the wtr

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

the palos verdes fault offshore southern california late pleistocene to present tectonic geomorphology seascape evolution and slip rate estimate based on auv and rov surveys the palos verdes fault pvf is one of few active faults in southern california that crosses the shoreline and can be studied using both terrestrial and subaqueous methodologies to characterize the near seafloor fault morphology tectonic influences on continental slope sedimentary processes and late pleistocene to present slip rate a grid of high resolution multibeam bathymetric data and chirp subbottom profiles were acquired with an autonomous underwater vehicle auv along the main trace of pvf in water depths between 250 and 600 m radiocarbon dates were obtained from vibracores collected using a remotely operated vehicle rov and ship based gravity cores the pvf is expressed as a well defined seafloor lineation marked by subtle along strike bends right stepping transtensional bends exert first order control on sediment flow dynamics and the spatial distribution of holocene depocenters deformed strata within a small pull apart basin record punctuated growth faulting associated with at least three holocene surface ruptures an upper shallower landslide scarp a buried sedimentary mound and a deeper scarp have been right laterally offset across the pvf by 55 5 52 4 and 39 8 m respectively the ages of the upper scarp and buried mound are approximately 31 ka the age of the deeper scarp is bracketed to 17 24 ka these three piercing points bracket the late pleistocene to present slip rate to 1 3 2 8 mm yr and provide a best estimate of 1 6 1 9 mm yr the deformation observed along the pvf is characteristic of strike slip faulting and accounts for 20 30 of the total right lateral slip budget accommodated offshore southern california

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

the silicon and oxygen isotope compositions of precambrian cherts a record of oceanic paleo temperatures oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperatures during the precambrian during the past decade the advance of in situ analysis of si isotopes has enhanced the interest on cherts as a paleo environmental proxy the coupled o and si isotope composition variations show secular and correlated trends that have been interpreted as a progressive cooling of the ocean however this reconstruction has been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation in this case the secular oxygen and silicon isotope variation are considered as reflecting a mixing between seawater and hydrothermal sources a key point in this discussion deals with the origin of cherts sedimentary hydrothermal or chemically silicified therefore several petrographical and geochemical criteria are proposed to recognize the pristine sedimentary origin of a chert namely they are 1 the occurrence of microquartz 2 a 18o rich bulk oxygen isotopic composition 3 the occurrence of large 18o ranges at a micrometer scale 4 variation of trace element compositions coupled with 30si 5 the occurrence of large ranges of 30si in pure microquartz these criteria should be regarded as guides to the identification of pristine diagenetic cherts in order to better constrain seawater paleo temperature reconstructions by taking into account the effect of diagenesis this article will review the different interpretations about o and si isotope variation and propose a model of formation based on ancient and modern chert studies

upper crustal structure and neogene tectonic development of the california continental borderland multichannel seismic reflection data sonobuoy seismic refraction data and regional geology are used to define the upper crustal structure of the southern california continental borderland and to delineate the characteristics of the main lithotectonic belts of the region the catalina schist belt is separated on its west side from the gently deformed nicolas forearc belt by faults that have steep west dips and pronounced normal separations on its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the peninsular ranges belt at the coastline near oceanside the catalina schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early miocene time in conjunction with about 10 of clockwise rotation of the western transverse ranges belt part of the uplift of the catalina schist could have occurred on the detachment fault but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt a large amount of uplift is required and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western transverse ranges and nicolas forearc belts extension accompanied by probable large amounts of right slip continued in the borderland region during and after middle miocene time the later stage of extension was accompanied by rapid clockwise rotation of the western transverse ranges of at least 90 most of the borderland including the belt of schist that was uplifted in early miocene time was further deformed into numerous basins and ridges during this stage of oblique extension the primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the pacific plate after it had become coupled to the farallon plate system which had previously been subducted beneath the borderland

natural variations of δ30si ratios during progressive basalt weathering hawaiian islands abstract silicon stable isotopes can be used to trace the biogeochemical pathways of si as it moves from its continental sources to its sink in ocean sediments along the way si is incorporated into clay minerals taken up by plants where it forms plant opal and leached into rivers the major land to ocean conduit compared to igneous rocks the waters that drain continents are enriched in heavy si isotopes but the mechanisms that control fractionation have not been elucidated we studied si isotope fractionation along a 4 million yr basaltic soil chronosequence on the hawaiian islands using the natural context of these samples in combination with laboratory experiments we demonstrate that the isotopic composition of dissolved si in weathering systems is determined by the combined effects of rock disintegration clay mineral neosynthesis and si biocycling weathering preferentially releases 28si into solution whereas secondary mineral formation preferentially removes 28si from solution in humid environments leached soils have lost large amounts of this soluble si thus creating a net loss of 30si from the entire soil system as soils develop and greater fractions of si reside in neoformed clay minerals 30sibulk soil values change progressively toward more negative values basalt 30si values are about 0 5 but older soils have 30si values up to 2 5 the difference between the solid and solution 30si values remains more or less constant with progressive weathering and therefore soil water from older soils has a more negative 30si composition in the upper horizons of the hawaiian soils this weathering driven 30si shift is modified by the addition of unweathered primary minerals via dust carrying 30si values of about 0 5 and by biocycling of si via plants producing negative 30si values in phytoliths and positive 30si values in soil solutions derived from upper horizons due to the high concentrations of dissolved si in these near surface layers rivers have more positive 30si values than predicted based on the weathering status of the lower horizons when combined with published 30si values from large rivers worldwide we find that the results from hawaii point to weathering control of si isotopes delivered to the oceans and thus to an important continent ocean linkage that warrants further investigation

structural geometry and slip of the palos verdes fault southern california implications for earthquake hazards abstract the palos verdes fault pvf is an active structure in southern california comprised of several segments that together form a complex fault system the fault has been active since the miocene and is a major regional seismic source using marine petroleum industry seismic data we define the geometry of the fault and offsets of tertiary stratigraphic units that constrain the along strike segmentation and slip rates on the fault system in san pedro bay patterns of deformed stratigraphic units suggest a deep fault geometry that dips moderately to the southwest and was initially formed as a miocene normal fault plio pleistocene transpression resulted in the reactivation of this normal fault and its subsequent inversion with oblique right lateral reverse displacement to the south the lasuen knoll segment dips steeply to the northeast and is not a reactivated miocene age structure but rather formed in middle pliocene time plio pleistocene transpression has linked these and other segments together to form the currently active pvf system the resulting complex fault geometry and slip patterns suggest that the pvf is a highly segmented fault which may rupture in moderate size earthquakes that involve individual segments or large events that are able to propagate across significant geometric discontinuities estimates of moment magnitudes based on empirical relationships with rupture fault area indicate the possibility of m w 6 6 6 9 earthquakes for single segment ruptures and m w 7 1 7 3 multisegment ruptures with recurrence intervals ranging from 181 to 534 yr given the high inferred slip rates on the fault and its proximity to the urban population these events pose significant earthquake hazards to metropolitan los angeles

stratigraphic changes of ge si ree y and silicon isotopes as insights into the deposition of a mesoarchaean banded iron formation in order to determine the origin of silicon si in banded iron formation bif we have undertaken a multi tracer study combining ree y data ge si ratios and si isotopes d 30 si on stratigraphically resolved layers from a 2 95 ga bif from the pongola supergroup south africa si in both si rich and fe rich layers has a common origin represented by a seawater reservoir strongly influenced by continent derived freshwaters 10 and very limited o 0 1 high t hydrothermal fluids as indicated by eu anomalies and y ho ratios the coevolution of d 30 si signatures of si and fe rich

another continental pool in the terrestrial silicon cycle silicon is the second most abundant element on earth and is involved in important terrestrial and marine biogeochemical processes however its terrestrial cycle is not well quantified and there is a missing reservoir in the budget of silicon isotopes water in seas and rivers is enriched in silicon 30 compared with igneous rocks suggesting that there is an unknown continental reservoir that is depleted in silicon 30 now with the help of a new generation mass spectrometer basile doelsch et al have obtained in situ measurements of silicon isotopes in quartz from sandstones in southern france the results showing greater silicon 30 depletion than in any previously reported terrestrial samples suggest that silicified rocks formed from quartz precipitates in soils and groundwaters can account for the missing reservoir silicon is the second most abundant element on earth it is an important nutrient for phytoplankton1 and is readily absorbed by terrestrial vegetation2 it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates3 but the continental cycle of silicon is not well known and only a few studies have attempted to use silicon stable isotopes 28si 29si and 30si 4 5 6 7 8 9 10 11 12 13 to quantify the continental silicon reservoirs dissolved silicon in sea and river waters forms a reservoir of mean isotopic value 1 1 refs 7 10 it is enriched in 30si with respect to the igneous rocks reservoir which has a mean isotopic value of 0 3 refs 4 9 this enrichment can only be produced by a major fractionation during weathering and should result in the formation of a continental 30si depleted reservoir such a reservoir however has not been identified to date here we analyse silicon isotopes of in situ quartz from a sandstone series in france using a new generation secondary ion mass spectrometry apparatus we show that quartz that precipitates as siliceous cements forms a strongly 30si depleted reservoir with isotopic values down to 5 7 a more negative value than any previously published for terrestrial samples our findings suggest that quartz re precipitation plays an important role in the biogeochemical cycle of silicon

the 18o 16o ratio of 2 billion year old seawater inferred from ancient oceanic crust an oxygen isotope profile of the 2 billion year old purtuniq ophiolite overlaps with similar profiles of younger ophiolites and the modern oceanic crust this overlap implies i that there was a similar style of seawater ocean crust interaction during the past 2 billion years ii that the oxygen isotope composition of early proterozoic seawater was similar to the modern value iii that early proterozoic sea floor spreading rates were similar to or greater than average modern rates and iv that early proterozoic carbonate rocks and cherts with low 18o 16o ratios do not reflect global scale 18o depletion of early proterozoic oceans

the palos verdes terraces california bathtub rings from a buried reverse fault uplift of the palos verdes peninsula has long been associated with a northwest trending southwest dipping reverse fault unfortunately the palos verdes hills fault has no obvious surface displacement and little background seismicity to substantiate its dimension orientation or earthquake potential in this paper we investigate the tectonic style and slip rate of the palos verdes hills fault and the uplift history of the palos verdes hills by analyzing the geometry of 13 marine terraces that encircle the peninsula in a bathtub ring configuration elevations of 211 terrace remnants constrain a fault model with 3 0 to 3 7 mm yr 1 of oblique dextral reverse slip on a fault dipping 67 at 6 to 12 km depth beneath the peninsula if the rate was constant through time fault inception would have occurred 2 4 3 0 ma we propose that the largest credible earthquakes on the fault have magnitude 6 and could revisit every 2000 years

si isotope systematics of meteorites and terrestrial peridotites implications for mg si fractionation in the solar nebula and for si in the earth s core abstract high precision si isotope ratios have been measured for a series of meteorites and terrestrial samples using high resolution multi collector icp ms our results differ from those reported in an earlier study georg et al 2007 si in the earth s core nature 447 1102 1106 in two important aspects first our data set reveals systematic differences in 30 si between different chondrite groups that are correlated with their mg si elemental ratio second in agreement with the previous study 30 si for the terrestrial samples are higher than values for chondrites but the difference between the bulk silicate earth bse and the carbonaceous chondrites 30 si bse c arbonaceous chondrites 0 08 0 04 1 standard deviation is about a factor of 2 smaller than previously reported the 30 si versus mg si trend defined by the chondrite groups can be explained by reaction of olivine with a sio rich vapor to form enstatite starting from a carbonaceous chondrite composition in contrast the difference between the bse and carbonaceous chondrites must reflect a different process and can be explained by incorporation of si into the earth s core during metal silicate equilibration in a deep magma ocean the observed si isotope fractionation is consistent with the temperatures and pressures of metal silicate equilibration derived from siderophile element abundances in the earth s mantle

stable isotope geochemistry of cherts and carbonates from the 2 0 ga gunflint iron formation implications for the depositional setting and the effects of diagenesis and metamorphism abstract 18 o values for all but three of 97 samples of chert along a 220 km transect through the 2 0 ga gunflint iron formation of the animikie basin north america range from 21 3 to 24 7 o there is no correlation between 18 o and the type of chert i e deeper water lutitic chert shallower water arenitic chert and peritidal algal chert the fact that the gunflint iron formation has been exposed to thermal conditions only slightly above burial diagenesis suggests that the values of the chert have not been significantly affected by metamorphism field and petrographic observations and the small 18 o range of all the chert types suggests that silicification lithification and any conversion to quartz chert from a hydrous silica precursor occurred close to the sediment water interface the uniform 18 o values of all the different chert types suggest that all cherts precipitated over a relatively common temperature interval 20 c from a common parent water having a relatively uniform 18 o the isotopic data suggest silica saturation and precipitation was not the result of the mixing of seawater and meteoric water or the result of evaporative processes siderite in the deep water banded facies of the gunflint displays a 18 o range of 6 0 to 2 6 o whereas siderite in the more shoreward organic rich shale facies is significantly enriched in 13 c 13 c 2 5 to 0 5 o 18 c of siderite does not appear to have been altered as a result of neomorphic recrystallization and authigenic microsparitic siderite along stylolites displays the same 13 c range as primary and neomorphosed siderite however neomorphic and authigenic siderite are depleted in 18 o by up to 7 o relative to primary unaltered microspheroidal siderite the lower 13 c of banded facies siderite relative to the shale facies siderite does not seem to be the result of thermal decarboxylation or anaerobic oxidation abiotic or bacterial of organic matter but rather is better explained by primary precipitation in an ocean system that was layered with respect to carbon isotopes the more basinward banded facies siderite precipitated from a 13 c depleted fe 2 rich hydrothermal water mass that was moved by advective upwelling from submarine regions of magmatic volcanic hydrothermal activity onto the continental shelf the siderite of the shale facies precipitated from a more near shore shallow seawater mass that was comparatively enriched in 13 c 13 c 0 2 o ankerite in the gunflint iron formation occurs as a replacement of primary siderite in the banded facies and as a replacement of greenalite and chert in the arenite facies many ankerite samples have 13 c values that are similar to those of the shale facies siderite which suggests that shallow seawater was a major component of the diagenetic fluid later neomorphism of the replacement ankerite at higher burial temperatures and probably in the presence of 18 o depleted waters resulted in a wide range of 18 o values 13 8 22 5 o the unique mineralogy of the upper most gunflint iron formation calcite serpentine and magnetite compared to the majority of the formation the very low values and overlying diabase sills suggest that the non ferroan nature of the carbonate in the upper gunflint is the result of contact metamorphism

strain partitioning in southeastern alaska is the chatham strait fault active abstract a 1200 km long transform plate boundary passes through southeastern alaska and northwestern british columbia and represents one of the most seismically active but poorly understood continental margins of north america although most of the plate motion is accommodated by the right lateral queen charlotte fairweather fault qcff system which has produced at least six m 7 earthquakes since 1920 seismic hazard assessments also include the chatham strait fault csf as a potentially active 400 km long strike slip fault that cuts northward through southeastern alaska connecting with the eastern denali fault nearly the entire length of the csf is submerged beneath chatham strait and lynn canal and has never been systematically imaged using high resolution marine geophysical approaches in this study we present an integrated analysis of new marine seismic reflection data acquired across lynn canal and tectonic block modeling constrained by data from continuous and campaign gps sites seismic profiles cross the csf at twelve locations spanning 50 km of fault length they reveal thick up to 300 m packages of glaciomarine sedimentary facies emplaced on an unconformity surface that formed during the last glacial maximum lgm localized warping of post lgm stratigraphy 13 9 kyr b p to present appears to correlate with sediment drape on basement topography and current controlled deposition there is no evidence for an active fault along the axis of lynn canal in the seismic reflection data crustal block models constrained by gps data allow but do not require a maximum slip rate of 2 3 mm yr along the csf higher slip rates on the csf result in significant misfit to gps data in the surrounding region based on the combined marine geophysical and gps observations it is plausible that the csf has not generated resolvable coseismic deformation in the last 13 ka and that the modern slip rate is

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

new constraints on deformation slip rate and timing of the most recent earthquake on the west tahoe dollar point fault lake tahoe basin california high resolution seismic compressed high intensity radar pulse chirp data and piston cores acquired in fallen leaf lake fll and lake tahoe provide new paleoseismic constraints on the west tahoe dollar point fault wtdpf the western most normal fault in the lake tahoe basin california paleoearthquake records along three sections of the wtdpf are investigated to determine the magnitude and recency of coseismic slip chirp profiles image vertically offset and folded strata along the southern and central sections that record deformation associated with the most recent event mre on the wtdpf three faults are imaged beneath fll and the maximum vertical offset observed across the primary trace of the wtdpf is 3 7 m coregis tered piston cores in fll recovered sediment and organic material above and below the mre horizon radiocarbon dating of organic material constrained the age of the mre to be between 3 6 and 4 9 k y b p with a preferred age of 4 1 4 5 k y b p in lake tahoe near rubicon point approximately 2 0 m of vertical offset is observed across the wtdpf based on nearby core data the timing of this offset occurred be tween 3 10 k y b p which is consistent with the mre age in fll offset of tioga aged glacial deposits provides a long term record of vertical deformation on the wtdpf since 13 14 k y b p yielding a slip rate of 0 4 0 8 mm yr in summary the slip rate and earthquake potential along the wtdpf is comparable to the nearby genoa fault making it the most active and potentially hazardous fault in the lake tahoe basin

earthquake locations in the inner continental borderland offshore southern california the inner continental borderland region offshore southern california is tectonically active and contains several faults that are potential seismic hazards to nearby cities however fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms to improve the accuracy of event locations in this area we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the borderland the regions are defined by either temporal or spatial clustering of seismic activity in the southern california seismic network scsn catalog obtaining accurate locations for these events is difficult due to the lack of nearby stations the limited azimuthal coverage and uncertainties in the velocity structure for this area our location procedure is based on the l 1 norm grid search waveform cross correlation method of shearer 1997 except that we use a nearest neighbor approach astiz et al 2000 to identify suitable event pairs for waveform cross correlation and we explore the effect of different velocity models on the locations and associated station terms in general our relocated events have small estimated relative location errors and the events are more clustered than the scsn catalog locations a quarry on the south tip of catalina island provides a test of our location accuracy and suggests that under ideal conditions offshore events can be located to within 1 to 2 km of their true locations our final locations for most clusters are well correlated with known local tectonic features we relate the 1981 santa barbara island m l 5 3 earthquake with the santa cruz fault the 13 july 1986 oceanside m l 5 3 sequence with the san diego trough fault zone and events near san clemente island with the known trace of the san clemente fault zone over 3000 of the offshore events during this time period are associated with the 1986 oceanside earthquake and its extended aftershock sequence our locations define a northeast dipping fault plane for the oceanside sequence but in cross section the events are scattered over a broad zone about 4 km thick this could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure events that occur near coronado bank in the scsn catalog are relocated closer to the san diego coast and suggest a shallow angle northeast dipping fault plane at 10 to 15 km depth manuscript received 12 february 1999

investigation of late pleistocene and holocene activity in the san gregorio fault zone on the continental slope north of monterey canyon offshore central california abstract we provide an extensive high resolution geophysical sediment core and radiocarbon dataset to address late pleistocene and holocene fault activity of the san gregorio fault zone sgfz offshore central california the sgfz occurs primarily offshore in the san andreas fault system and has been accommodating dextral strike slip motion between the pacific and north american plates since the mid miocene our study focuses on the sgfz where it has been mapped through the continental slope north of monterey canyon from 2009 to 2015 the monterey bay aquarium research institute collected high resolution multibeam bathymetry and chirp sub bottom profiles using an autonomous underwater vehicle auv targeted samples were collected using a remotely operated vehicle rov to provide radiocarbon age constraints we integrate the high resolution geophysical data with radiocarbon dates to reveal pleistocene seismic horizons vertically offset less than 5 m on nearly vertical faults these faults are buried by continuous reflections deposited after 17 5 ka and likely following erosion during the last sea level lowstand 21 ka bracketing the age of faulting to 32 21 ka clearly faulted horizons are only detected in a small area where mass wasting exhumed older strata to within 25 m of the seafloor the lack of clearly faulted holocene deposits and possible highly distributed faulting in the study area are consistent with previous interpretations that late pleistocene and holocene activity along the sgfz may decrease to the south this study illustrates the complexity of the sgfz offshore central california and demonstrates the utility of very high resolution data from combined auv geophysical rov seabed sampling surveys in offshore studies of fault activity

a petrographic and isotopic criterion of the state of preservation of precambrian cherts based on the characterization of the quartz veins the coupled o and si isotope variations of precambrian cherts show secular and correlated variations that have been interpreted as a progressive cooling of the oceans on earth however this reconstruction has been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation even pristine samples show evidence of fluid circulation by the presence of various quartz veins here we have determined the effect of fluid circulation on the isotopic composition of the microquartz chert matrix by studying the mineralogical relationships between quartz veins and microquartz in two distinctive chert samples these samples show different quartz habit and texture in their quartz veins which imply various fluid system conditions therefore the presence of quartz spherulites in the quartz veins indicates a more moderate fluid circulation than the presence of columnar quartz these observations were also confirmed by the oxygen isotope compositions of both microquartz matrix chert and quartz veins we have demonstrated that in some conditions fluid circulations do not modify the oxygen isotope composition of the chert matrix and thus the microquartz can still record paleo environmental conditions the petrography of quartz vein associated with in situ oxygen isotope composition provides one criterion to characterize the state of preservation of the microquartz

microplate capture rotation of the western transverse ranges and initiation of the san andreas transform as a low angle fault system tectonic rotation of the western transverse ranges block is explained by capture of the partially subducted monterey microplate by the pacific plate at about anomaly 6 time ca 20 ma as pacific monterey spreading slowed and eventually ceased the slip vector along the gently northeast dipping subduction interface beneath the california margin changed from slightly oblique subduction to transtensional dextral transform motion this change in slip vector and a shift of pacific plate motion eastward along the already subducted monterey plate interface imply that the san andreas transform began as a system of low angle faults that locally subjected the overriding continental margin to distributed basal shear and crustal extension this basal shear produced the rotated western transverse ranges this model helps explain the timing of initial rotation and basin formation the sudden appearance of widely distributed transform motion well inland of the margin in early miocene time why the western transverse ranges uniquely rotated as a large coherent crustal block and several fundamental structural characteristics of central and southern california the model also provides major constraints on the amount of pacific north america strike slip motion the position through time of offshore oceanic plates relative to onshore geology and a general explanation for what may happen as a spreading ridge approaches a trench and the subduction zone evolves into a transform system

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

geometry and late pleistocene displacement of the shoreline and oceano fault zones san luis obispo bay californiageometry and late pleistocene displacement of the shoreline and oceano fault zones

using high resolution multibeam bathymetry to identify seafloor surface rupture along the palos verdes fault complex in offshore southern california recently acquired high resolution multibeam bathymetric data reveal several linear traces that are the surficial expressions of seafloor rupture of holocene faults on the upper continental slope southeast of the palos verdes peninsula high resolution multichannel and boomer seismic reflection profiles show that these linear ruptures are the surficial expressions of holocene faults with vertical to steep dips the most prominent fault on the multibeam bathymetry is about 10 km to the west of the mapped trace of the palos verdes fault and extends for at least 14 km between the shelf edge and the base of the continental slope this fault is informally called the avalon knoll fault for the nearby geographic feature of that name seismic reflection profiles show that the avalon knoll fault is part of a northwest trending complex of faults and anticlinal uplifts that are evident as scarps and bathymetric highs on the multibeam bathymetry this fault complex may extend onshore and contribute to the missing balance of quaternary uplift determined for the palos verdes hills and not accounted for by vertical uplift along the onshore palos verdes fault we investigate the extent of the newly located offshore avalon knoll fault and use this mapped fault length to estimate likely minimum magnitudes for events along this fault

where is the real transform boundary in california 1 the zone of highest geodetically defined strain rate in california does not everywhere coincide with the surface trace of the san andreas fault saf to determine whether this reflects the pattern of long term permanent deformation we analyzed the velocity field on swaths across the transform located so as to avoid intersections among the major fault strands slip rates and flexural parameters for each fault were determined by finding the best fit to the velocity profile using a simple arctan model representing the interseismic strain accumulation our slip rates compare well with current geologic estimates which suggests that the present day velocity field is representative of long term motions we find that the transform is a zone of high strain rate up to 80 km wide that is straighter than the saf and has an overall trend closer to the relative plate motion vector than the saf most sections of the saf take up less than half of the total slip rate and slip is transferred from one part of the system to another in a way that suggests that the saf should not be considered as a unique locator of the plate boundary up to half of the total displacement takes place on faults outside the high strain rate zone distributed over several hundred kilometers on either side our findings substantiate previous suggestions that the transform has the characteristics of a macroscopic ductile shear zone cutting the continental lithosphere around which stress and strain rate decrease on a length scale controlled by the length of the transform

records of continental slope sediment flow morphodynamic responses to gradient and active faulting from integrated auv and rov data offshore palos verdes southern california borderland abstract variations in seabed gradient are widely acknowledged to influence deep water deposition but are often difficult to measure in sufficient detail from both modern and ancient examples on the continental slope offshore los angeles california autonomous underwater vehicle remotely operated vehicle and shipboard methods were used to collect a dense grid of high resolution multibeam bathymetry chirp sub bottom profiles and targeted sediment core samples that demonstrate the influence of seafloor gradient on sediment accumulation depositional environment grain size of deposits and seafloor morphology in this setting restraining and releasing bends along the active right lateral palos verdes fault create and maintain variations in seafloor gradient holocene down slope flows appear to have been generated by slope failure primarily on the uppermost slope 100 200 m water depth turbidity currents created a low relief

offset of latest pleistocene shoreface reveals slip rate on the hosgri strike slip fault offshore central california abstract the hosgri fault is the southern part of the regional hosgri san gregorio dextral strike slip fault system which extends primarily in the offshore for about 400 km in central california between morro bay and san simeon high resolution multibeam bathymetry reveals that the eastern strand of the hosgri fault is crossed by an 265 m wide slope interpreted as the shoreface of a latest pleistocene sand spit this sand spit crossed an embayment and connected a western fault bounded bedrock peninsula and an eastern bedrock highland a paleogeography resembling modern coastal geomorphology along the san andreas fault detailed analysis of the relict shoreface with slope profiles and slope maps indicates a lateral slip rate of 2 6 0 9 mm yr considered a minimum rate for the hosgri given the presence of an active western strand this slip rate indicates that the hosgri system takes up the largest share of the strike slip fault budget and is the most active strike slip fault west of the san andreas fault in central california this result further demonstrates the value and potential of high resolution bathymetry in characterization of active offshore faults online material 1 high resolution bathymetric data for the cross hosgri slope and 2 metadata that describes data collection processing and formatting

slip rate on the san diego trough fault zone inner california borderland and the 1986 oceanside earthquake swarm revisited the san diego trough fault zone sdtfz is part of a 90 km wide zone of faults within the inner california borderland that accommodates motion between the pacific and north american plates along with most faults offshore southern california the slip rate and paleoseismic history of the sdtfz are unknown we pre sent new seismic reflection data that show that the fault zone steps across a 5 km wide stepover to continue for an additional 60 km north of its previously mapped extent the 1986 oceanside earthquake swarm is located within the 20 km long restraining stepover farther north at the latitude of santa catalina island the sdtfz bends 20 to the west and may be linked via a complex zone of folds with the san pedro basin fault zone spbfz in a cooperative program between the u s geological survey usgs and the monterey bay aquarium research institute mbari we measure and date the coseismic offset of a submarine channel that intersects the fault zone near the sdtfz spbfz junction we estimate a horizontal slip rate of about 1 5 0 3 mm yr over the past 12 270 yr

kinematic analysis of secondary faults within a distributed shear zone reveals fault linkage and increased seismic hazard abstract complex multifault earthquake ruptures involving secondary faults emphasize the necessity to characterize their seismogenic potential better and study their relationship with major faults to improve the seismic hazard assessment of a region high resolution geophysical data were interpreted to make a detailed characterization of the averroes fault and the north averroes faults which are poorly known secondary right lateral strike slip faults located in the central part of the alboran sea western mediterranean these faults appear to have evolved since the pliocene as part of a distributed dextral strike slip shear zone in response to local strain engendered by the diverging movement of the carboneras fault to the north and the yusuf and alboran ridge faults to the south in addition the architecture of these faults suggests that the averroes fault may eventually link with the yusuf fault thus leading to a higher seismogenic potential therefore these secondary faults represent a hitherto unrecognized seismogenic hazard since they could produce earthquakes up to moment magnitude mw 7 6 our results highlight the importance of the role played by secondary faults in a specific kinematic framework their reciprocal linkage and their mechanical relationship with the main faults could lead to future complex fault ruptures this information could improve fault source and earthquake models used in seismic and tsunami hazard assessment in this and similar regions

style and rate of holocene slip palos verdes fault southern california recent studies of the palos verdes fault in the los angeles outer harbor characterize the fault s holocene activity the studies included areal and local geophysical surveys as well as drilling sampling and dating of the subsurface materials the studies identify the fault s location in the shallow subsurface document evidence of holocene strike slip quantify dip slip versus sediment age and quantify the net slip where an early holocene paleochannel crosses the fault the slip rate for the palos verdes fault is constrained to between 2 7 and 3 mm yr for the past 7 8 to 8 ka because the new evidence shows that the palos verdes fault has one of the highest slip rates in the los angeles basin the studies provide significant new insight into the regional tectonics and the seismic risk of the los angeles region using this rate of slip and segmentation models for the fault the coastal los angeles region can expect an mw 7 to 7 2 event about every 400 to 900 years from this potential seismic source

stratigraphy and sedimentary petrology of an archean volcanic sedimentary succession at mt goldsworthy in the pilbara block western australia implications of evaporite nahcolite and barite deposition abstract a approximately 3 3 ga sedimentary succession at mt goldsworthy in the northeastern pilbara block australia contains beds composed of silicified pseudomorphs of nahcolite nahco3 or barite baso4 this sedimentary succession correlates with the corboy formation and consists of lower middle and upper sedimentary units which conformably overlie mafic to ultramafic volcanic and volcaniclastic rocks the lower and middle units are predominantly siliciclastic whereas the upper unit is characterized by ferruginous cherts and banded iron formations with minor proportions of intercalated mature sandstone the succession defines an overall upward fining and deepening trend the detrital materials were derived from older greenstone successions volcaniclastics and reworked precipitative beds silicified pseudomorphs of nahcolite crystals up to 40 cm in length occur in a 20 m thick bed in the upper portion of the lower unit the formation of this unit was initiated by deposition of immature terrigenous clastic sediments and followed by the precipitation of nahcolite from na hco3 brines in a closed or semi closed evaporitic basin bladed barite deposits occur mainly in the middle unit which was deposited in a sub aerial to shallow marine environment barite may have precipitated as a result of mixing of so42 rich seawater and ba2 rich hydrothermal fluids the mt goldsworthy sedimentary succession records an archean shallow to sub aerial sedimentary environment that probably developed in a continental margin setting

kinematics and paleoseismology of the vernon fault marlborough fault system new zealand implications for contractional fault bend deformation earthquake triggering and the record of hikurangi subduction earthquakes the 40 km long vernon fault in the marlborough fault system of new zealand is characterized by dextral slip with subordinate reverse slip and exhibits abrupt variations in strike of up to 90 onshore fieldwork paleoseismic trenching and offshore high frequency seismic reflection data are integrated together to identify the kinematics and paleoseismic history of three sections of the fault 1 the nne striking vernon hills section which branches off from the awatere fault 2 the ne striking big lagoon section which borders big lagoon to the south and extends 9 km offshore to the east and 3 the e w striking wairau basin section which is entirely submarine the vernon fault can be shown to have a dextral slip rate of 0 8 4 9 mm yr with a preferred estimate of 0 9 mm yr on the big lagoon section we infer that a further unrecognized 3 4 mm yr of dextral slip has been accommodated off fault as a result of accumulated slip on small and or blind reverse faults adjacent to a 6 km wide restraining bend in the main fault the onshore and offshore paleoseismic records are in good agreement these indicate three to five events at eight sites and a mean recurrence interval of 3 9 1 2 ka over the past 16 kyr with the last event taking place at 3 3 ka earthquakes on the vernon fault are responsible for 25 of the holocene subsidence rate of big lagoon over the last 13 ka most of the subsidence of this lagoon has been the result of surface deformation related with southern hikurangi megathrust earthquakes

diagenetic trends in the siliceous facies of the monterey shale in the santa maria region california x ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the miocene monterey shale near santa maria california within many stratigraphic sections soft porous diatomaceous rocks change gradationally to underlying hard and brittle chert porcellanite and siliceous shale the accompanying silica mineral zones are in descending stratigraphic order 1 biogenic silica opal a 2 cristobalitic silica opal ct and 3 microcrystalline quartz boundaries between silica mineral zones and stratigraphic horizons are often discordant within the opal ct zone the d 101 spacing of opal ct decreases in a smooth non linear fashion from about 4 10 a to 4 04 a in the santa maria valley and bradley oil field areas the thicknesses of the opal ct zones are greater and the present thermal gradients less than in the adjacent orcutt oil field thin opal ct zones at shallow maximum burial depths apparently correlate with higher thermal gradients using present thermal gradients and reconstructed maximum burial depths from well data in the santa maria region the ranges in temperatures for the top and base of the opal ct zone are 38 54 c and 55 110 c respectively the temperature difference between these two boundaries ranges from 17 to 60 c in comparison temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal ct and quartz extrapolated experimental quartz water fractionations and assuming o18 0 o for the isotopic composition of the equilibrating fluid are 18 56 c and 31 80 c for the top and base of the opal ct zone respectively the temperature difference between these boundaries is 11 36 c using this method thermal gradients and sedimentation rates strongly influence rates of silica transformations reconstructed thermal and diagenetic histories of siliceous rocks of the monterey shale at four well sites in the santa maria region demonstrate that most silica conversions probably occurred during the last 3 4 myr in response to accelerated rates of sedimentation and therefore burial heating during the pliocene

holocene activity of the rose canyon fault zone in san diego california the rose canyon fault zone in san diego california has many well expressed geomorphic characteristics of an active strike slip fault including scarps offset and deflected drainages and channel walls pressure ridges a closed depression and vegetation lineaments geomorphic expression of the fault zone from mount soledad south to mission bay indicates that the mount soledad strand is the most active a network of trenches excavated across the mount soledad strand in rose creek demonstrate a minimum of 8 7 m of dextral slip in a distinctive early to middle holocene gravel filled channel that crosses the fault zone the gravel filled channel was preserved within and east of the fault but was removed west of the fault zone by erosion or possibly grading during development consequently the actual displacement of the channel could be greater than 8 7 m radiocarbon dates on detrital charcoal recovered from the sediments beneath the channel yield a maximum calibrated age of about 8 1 0 2 kyr the minimum amount of slip along with the maximum age yield a minimum slip rate of 1 07 0 03 mm yr on this strand of the rose canyon fault zone for much of holocene time other strands of the rose canyon fault zone which are east and west of our site may also have holocene activity based on an analysis of the geomorphology of fault traces within the rose canyon fault zone along with the results of our trenching study we estimate the maximum likely slip rate at about 2 mm yr and a best estimate of about 1 5 mm yr stratigraphie evidence of at least three events is present during the past 8 1 kyr the most recent surface rupture displaces the modern a horizon topsoil suggesting that this event probably occurred within the past 500 years stratigraphie and structural relationships also indicate the occurrence of a scarp forming event at about 8 1 kyr prior to deposition of the gravel filled channel that was used as a piercing line a third event is indicated by the presence of several fault strands that displace the channel but did not move during the most recent event other events may also have occurred but these data suggest that the return time for surface rupturing earthquakes is no more than about 4 kyr

distinguishing sediment waves from slope failure deposits field examples including the humboldt slide and modelling results abstract migrating sediment waves have been reported in a variety of marine settings including submarine levee fan systems floors of fjords and other basin or continental slope environments examination of such wave fields reveals nine diagnostic characteristics when these characteristics are applied to several features previously attributed to submarine landslide deformation they suggest that the features should most likely be reinterpreted as migrating sediment wave fields sites that have been reinterpreted include the humboldt slide on the eel river margin in northern california the continental slope in the gulf of cadiz the continental shelf off the malaspina glacier in the gulf of alaska and the adriatic shelf a reassessment of all four features strongly suggests that numerous turbidity currents separated by intervals of ambient hemipelagic sedimentation deposited the wave fields over thousands of years a numerical model of hyperpycnal discharge from the eel river for example shows that under certain alongshore current conditions such events can produce turbidity currents that flow across the humboldt slide serving as the mechanism for the development of migrating sediment waves numerical experiments also demonstrate that where a series of turbidity currents flows across a rough seafloor i e numerical steps sediment waves can form and migrate upslope hemipelagic sedimentation between turbidity current events further facilitates the upslope migration of the sediment waves physical modelling of turbidity currents also confirms the formation and migration of seafloor bedforms the morphologies of sediment waves generated both numerically and physically in the laboratory bear a strong resemblance to those observed in the field including those that were previously described as submarine landslides

the offshore palos verdes fault zone near san pedro southern california high resolution seismic reflection data are combined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the san pedro shelf near los angeles california prominent structures investigated include the wilmington graben the palos verdes fault zone various faults below the west part of the san pedro shelf and slope and the deep water san pedro basin the structure of the palos verdes fault zone changes markedly along strike southeastward across the san pedro shelf and slope under the north part of the shelf this fault zone includes several strands with the main strand dipping west under the slope the main fault strands exhibit normal separation and mostly dip east to the southeast near lasuen knoll the palos verdes fault zone locally is low angle but elsewhere near this knoll the fault dips steeply fresh seafloor scarps near lasuen knoll indicate recent fault movement we explain the observed structural variation along the palos verdes fault zone as the result of changes in strike and fault geometry along a master right lateral strike slip fault at depth complicated movement along this deep fault zone is suggested by the possible wave cut terraces on lasuen knoll which indicate subaerial exposure during the last sea level lowstand and subsequent subsidence of the knoll modeling of aeromagnetic data indicates a large magnetic body under the west part of the san pedro shelf and upper slope we interpret this body to be thick basalt of probable miocene age this basalt mass appears to have affected the pattern of rock deformation perhaps because the basalt was more competent during deformation than the sedimentary rocks that encased the basalt west of the palos verdes fault zone other northwest striking faults deform the outer shelf and slope evidence for recent movement along these faults is equivocal because we lack age dates on deformed or offset sediment manuscript received 26 february 2003

biological fractionation of silicon isotopes in southern ocean surface waters 1 the fractionation of silicon isotopes by diatoms during silicification i e opaline cell wall formation provides a new tool for paleoceanographic studies of the silicon cycle here we examine the natural variations of the 30si 28si ratio in silicic acid si oh 4 and biogenic silica bsio2 in surface waters of the antarctic circumpolar current acc along 170 w the results provide direct evidence of biologically mediated fractionation of silicon isotopes with an enrichment factor of between 1 1 and 1 9 depending on the model employed comparison of the mass flux of bsio2 captured in sediment traps deployed in the study area with the silicon isotopic composition of that material establishes a direct linkage between diatom dynamics in surface waters and the isotopic signature of exported particles we calculated si oh 4 no3 utilization ratios from variations in silicon and nitrogen isotopic ratios in surface waters and sediment traps that agree well with direct observations of 4 1 in this region this work supports the use of variations in silicon isotopes in sedimentary opal as a proxy for relative si oh 4 utilization in surface waters

paleoseismicity of the north branch of the newport inglewood fault zone in huntington beach california from cone penetrometer test data abstract application of cone penetrometer testing cpt is a promising method for studying subsurface fault zones in stratified unconsolidated sediment where trenching is not feasible analysis of data from 72 cpts spaced 7 5 to 30 0 m apart and 9 borings indicates that the north branch fault the active strand of the newport inglewood fault zone nifz in huntington beach has generated at least three and most likely five recognizable surface ruptures in the past 11 7 0 7 ka additional smaller earthquakes similar to the m w 6 4 1933 long beach earthquake may also have occurred but would not be recognizable with this method the minimum right lateral holocene slip rate of the nifz in the study area is estimated to be 0 34 to 0 55 mm yr the actual slip rate may be significantly higher

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

global positioning system constraints on fault slip rates in southern california and northern baja mexico we use global positioning system gps estimates of horizontal site velocity to constrain slip rates on faults comprising the pacific north america plate boundary in southern california and northern mexico we enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary we estimate a pacific north america relative plate motion rate of 49 3 mm yr one standard deviation consistent with nuvel 1a estimates we are able to resolve robust slip rate estimates for the southernmost san andreas san jacinto and elsinore faults 26 2 9 2 and 6 2 mm yr respectively and for the imperial and cerro prieto faults 35 2 and 42 1 mm yr respectively accounting for about 86 of the total plate motion the remaining 14 appears to be accommodated to the west of these fault systems probably via slip along the san clemente fault and or the san miguel vallecitos rose canyon and newport inglewood fault systems these results are highly consistent with paleoseismic estimates for slip rates implying that off fault strain accumulation within the deforming zone of the plate boundary is largely elastic we estimate that the seismically quiescent southernmost san andreas fault has incurred about 8 2 m of slip deficit over the last few hundred years presumably to be recovered during a future large earthquake

derivation of direct on fault submarine paleoearthquake records from high resolution seismic reflection profiles wairau fault new zealand direct on fault submarine paleoearthquake records can be derived from high resolution seismic reflection profiles of active fault growth sequences coseismic vertical increments of displacement are best preserved in the architecture of the growth sequence when the long term rate of sedimentation exceeds the rate of fault vertical displacement postseismic stratigraphic intervals can be recognized on vertical displacement history curves from which estimates of the earthquake timing and vertical displacements can be made the strike slip wairau fault is a major seismic hazard in central new zealand and is partially submarine analysis of 10 high resolution seismic profiles spanning a 20 km section of the offshore fault trace reveals two types of postseismic growth sequences and a composite paleoearthquake record with up to eight surface rupturing earthquakes since 18 ka the recurrence intervals range from 0 9 to 3 8 ka mean is 2 2 ka while the coseismic vertical displacements range from 0 5 to 5 3 m mean is 2 5 m these data conform to the variable slip model of earthquake behavior the vertical coseismic displacement is not always predictable from the recurrence interval preceding the event the data indicate that the seismic moment release has not been constant between earthquakes or that the ratio of horizontal to vertical coseismic displacement at a given site has varied over multiple earthquake cycles the offshore data are consistent with onshore paleoearthquake records long term dextral slip rate of 2 mm yr compared to 3 5 mm yr 40 km in land and paleoearthquakes of mw 7 5 the last earthquake occurred on the fault 2 0 0 3 kyr ago indicating that significant elastic strain has now accrued

can turbidites be used to reconstruct a paleoearthquake record for the central sumatran margin turbidite paleoseismology aims to use submarine gravity flow deposits turbidites as proxies for large earthquakes a critical assumption being that large earthquakes generate turbidity currents synchronously over a wide area we test whether all large earthquakes generate synchronous turbidites and if not investigate where large earthquakes fail to do this the sumatran margin has a well characterized earthquake record spanning the past 200 yr including the large magnitude earthquakes in 2004 mw 9 1 and 2005 mw 8 7 sediment cores collected from the central sumatran margin in 2009 reveal that surprisingly few turbidites were emplaced in the past 100 150 yr and those that were deposited are not widespread importantly slope basin deposits preserve no evidence of turbidites that correlate with the earthquakes in 2004 and 2005 although recent flow deposits are seen in the trench adjacent slope basins and adjacent pairs of slope basin and trench sites commonly have different sedimentary records and cannot be correlated these core sites from the central sumatran margin do not support the assumption that all large earthquakes generate the widespread synchronous turbidites necessary for reconstructing an accurate paleoearthquake record

geophysical evidence for the evolution of the california inner continental borderland as a metamorphic core complex we use new seismic and gravity data collected during the 1994 los angeles region seismic experiment larse to discuss the origin of the california inner continental borderland icb as an extended terrain possibly in a metamorphic core complex mode the data provide detailed crustal structure of the borderland and its transition to mainland southern california using tomographic inversion as well as traditional forward ray tracing to model the wide angle seismic data we find little or no sediments low 6 6 km s p wave velocity extending down to the crust mantle boundary and a thin crust 19 to 23 km thick coincident multichannel seismic reflection data show a reflective lower crust under catalina ridge contrary to other parts of coastal california we do not find evidence for an underplated fossil oceanic layer at the base of the crust coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge which represents the transition to the western transverse ranges on the shelf the palos verdes fault merges downward into a landward dipping surface which separates basement from low velocity sediments but interpretation of this surface as a detachment fault is inconclusive the seismic velocity structure is interpreted to represent catalina schist rocks extending from top to bottom of the crust this interpretation is compatible with a model for the origin of the icb as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks the basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until 13 m y ago subduction of the young and hot monterey and arguello microplates under the continental borderland followed by rotation and translation of the western transverse ranges may have provided the necessary thermomechanical conditions for this extension and crustal inflow

the unusual record of nereites wrinkle marks and undermat mining trace fossils from the late silurian earliest devonian of central western margin of gondwana argentina abstract silurian devonian deposits of central western argentina contain one of the most diverse palaeozoic records of nereites in gondwana nereites cf cambrensis nereites irregularis nereites jacksoni nereites macleayii nereites missouriensis and nereites pugnus are documented in a turbidite like succession of the san rafael block rio seco de los castanos formation the succession historically assumed as deep sea deposits or interpreted as a deltaic system also contains abundant microbially induced sedimentary structures miss such as wrinkle marks glossy surfaces indicate abundant biofilms where the fossil record comprises n macleayii n missouriensis scarce arthropod trackways diplichnites and undermat miner structures nereites the pioneer colonizer is in some cases cross cut by dictyodora while undermat miners are the late bioturbators the succession is dominated by gravity flow deposits including storm related turbidite like deposits and abundant wrinkle marks these suggest deposition on the shelf rather than the deep sea this study shows that microbial mats were not restricted to marginal marine environments during the mid palaeozoic and documents one of the most diverse palaeozoic records of nereites in western gondwana it also contributes to the record of nereites dictyodora and zoophycos found for first time in association with microbial mat structures in the late silurian earliest devonian the nereites ichnofacies together with abundant wrinkle marks

oceanside and thirtymile bank blind thrusts implications for earthquake hazards in coastal southern california we define an active blind thrust system in offshore southern california that extends from los angeles south to the united states mexico international border these blind thrusts formed by tectonic inversion of miocene extensional detachments we attribute the 1986 oceanside ml 5 3 earthquake local uplift of marine terraces seafloor fold scarps and observed geodetic convergence to motion on these faults single and multisegment fault rupture scenarios suggest the potential for large m 7 1 7 6 but infrequent earthquakes that would affect the los angeles and san diego metropolitan areas