stable organic carbon isotope stratigraphy across oceanic anoxic event 2 of demerara rise western tropical atlantic ocean drilling program odp leg 207 recovered expanded sections of organic carbon rich laminated shales on demerara rise western tropical atlantic high resolution organic carbon isotope and total organic carbon toc records are presented which span the cenomanian turonian boundary interval ctbi including the oceanic anoxic event oae 2 from four sites oriented along a nw striking depth transect these records represent the first high resolution carbon isotope records across oae 2 from the south american margin of the tropical atlantic due to the scarcity of age significant fossils the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the ctbi across the depth transect and to tie this to biostratigraphically well defined sections in the western interior basin pueblo usa boreal shelf seas eastbourne england and western tethys oued mellegue tunisia all four sections studied document a 6 increase of 13corg values at the base of the ctbi which is followed by an interval of elevated 13corg values and a subsequent decrease our results supply an important stratigraphic base for subsequent paleoceanographic studies on late cenomanian to early turonian sediments from demerara rise and elsewhere

crustal deformation associated with glacial fluctuations in the eastern chugach mountains alaska the changes of the solid earth in south central alaska in response to two major glacial fluctuations on different temporal and spatial scales have been estimated and we evaluated their influence on the stress state and ongoing tectonic deformation of the region during the recent 1993 1995 bering glacier surge a large transfer of ice from the bagley ice field to the bering glacier terminus region occurred we estimated the elastic displacement of the solid earth due to ice mass redistribution from global positioning system gps measurements at sites near the surging glacier we can account for these displacements by transfer of an ice volume of about 14 km3 from the surge reservoir area to the terminus region we examined the background seismicity ml 2 5 before during and after the surge we found that the occurrence of small earthquakes ml 4 0 in the surge reservoir region increased during the surge time interval possibly in response to a decrease in ice mass this suggests that a small decrease in the vertical stress 3 could be enough to modulate the occurrence of small shallow earthquakes in this dominantly thrust fault setting during this century the southern alaska coastal glaciers have been undergoing an overall decrease in volume based on our compilation of changes in the extent and thickness of the coastal glaciers between the malaspina and bering we calculated surface displacements due to the earth s viscoelastic response to annual thinning and to the cumulative retreat over the last 100 years the uplift of the region due to an average annual thinning rate of 1 6 m yr in the ablation region is 1 12 mm yr for our reference model with a viscosity of 5 1019 pa s for depths between 40 and 200 km the total viscoelastic response due to the retreat over the last century may be as much as a couple of meters within the coastal ablation zone near icy bay the maximum decrease in v between 0 and 10 km was 1 0 mpa which is significant in relation to the stress drops in recent earthquakes 2 to 10 mpa but small in relation to the estimated tectonic stress magnitude therefore the occurrence of an earthquake such as the st elias 1979 ms 7 2 may have been advanced in time however most of the ongoing stress accumulation would be primarily due to tectonic forces

static stress transfer during the 2002 nenana mountain denali fault alaska earthquake sequence 1 on 23 october 2002 the mw 6 7 nenana mountain earthquake occurred in central alaska it was followed on 3 november 2002 by the mw 7 9 denali fault mainshock the largest strike slip earthquake to occur in north america during the past 150 years we have modeled static coulomb stress transfer effects during this sequence we find that the nenana mountain foreshock transferred 30 50 kpa of coulomb stress to the hypocentral region of the denali fault mainshock encouraging its occurrence we also find that the two main earthquakes together transferred more than 400 kpa of coulomb stress to the cross creek segment of the totschunda fault system and to the denali fault southeast of the mainshock rupture and up to 80 kpa to the denali fault west of the nenana mountain rupture other major faults in the region experienced much smaller static coulomb stress changes

unusual earthquakes in the gulf of alaska and fragmentation of the pacific plate two recent ms 7 6 strike slip earthquakes in the northern gulf of alaska ruptured a composite 250 km long north striking zone in the pacific plate these shocks are attributed to a combination of enhanced tensional stress in the pacific plate seaward of and following the great mw 9 2 alaska earthquake of 1964 and compressional stress resulting from collision of the yakutat terrane with north america the occurrence of these shocks reflects fragmentation of the northeast corner of the pacific plate possibly the initial step in establishing a new plate boundary seaward of the current boundary

model for the origin of the yakutat block an accreting terrane in the northern gulf of alaska a composite oceanic and continental terrane the yakutat block is currently colliding with and accreting to north america in the northern gulf of alaska marine geophysical data indicate that the block has moved with the pacific plate during pliocene and quaternary time but are indeterminant about its previous movement history a model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the kula farallon spreading center beneath north america in this model the spreading center that formed the basaltic basement of the yakutat block was offset from a spreading center to the south by a left lateral transform fault 400 500 km long subduction of the northern spreading center about 45 m y ago sliced off part of the north america continental margin attached it to the kula plate and initiated northward movement of the composite terrane as the yakutat block during eocene and oligocene time migration of the southern spreading center toward north america truncated the yakutat block on the south and left a fossil fracture zone that is now seen as the transition fault the southern spreading center subducted about 25 m y ago and the yakutat block has since traveled with the pacific plate to southern alaska

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

did the rifting of the atlantic ocean cause the cretaceous thermal maximum the cretaceous thermal maximum was a major turning point in the history of earth s climate this interval of peak warmth in the turonian has been attributed to very high atmospheric pco2 resulting from rapid outgassing rates although crustal cycling rates peaked in the aptian albian interval on the basis of coupled ocean atmosphere model simulations of the middle cretaceous we hypothesize that the formation of an atlantic gateway could have contributed to the cretaceous thermal maximum differences between prerifting and postrifting climate experiments demonstrate substantial regional oceanographic changes in the north and south atlantic basins that are consistent with oxygen isotopic evidence used to infer a cretaceous thermal maximum the model results help reconcile the paleoclimate record inferred from foraminiferal d 18 o with our understanding of climate dynamics and cretaceous tectonism

oceanographic controls on organic matter in miocene monterey formation offshore california abstract analyses of the type and amount of organic matter in tertiary through quaternary sediments drilled during dsdp leg 63 off the coast of california can be used to provide insights into the controls of deposition of the monterey shale the regional oceanography rather than basin silling controls the accumulation of organic matter in these sediments the laminated siliceous and organic rich monterey shales were deposited under anoxic conditions within a well developed oxygen minimum zone like that in today s gulf of california the oxygen minimum zone became strongly developed in response to increased upwelling and productivity caused by global cooling following development of an antarctic ice sheet 13 to 14 m y ago a drop in sea level 10 to 11 m y ago lowered the ba e of the anoxic oxygen minimum zone to water 2 500 to 3 000 m deep permitting substantial accumulation of organic matter in the late miocene and early pliocene of the california borderland at dsdp site 467 the base of the minimum stayed near this level until the quaternary then rose to 1 500 to 2 000 m where it remains today phosphorite indicative of a high rate of supply of nutrients formed at the same time as the monterey shale during the period of intensive upwelling from 13 to 14 m y into the pliocene a lessening of upwelling and supply of nutrients and development of the oxygen minimum zone from the early pliocene into the quaternary is implied by these data the changes are associated with a warming trend and a rise in sea level end of article last page 999

active tectonics of the st elias orogen alaska observed with gps measurements 1 we use data from campaign and continuous gps sites in southeast and south central alaska to constrain a regional tectonic block model for the st elias orogen active tectonic deformation in the orogen is dominated by the effects of the collision of the yakutat block with southern alaska our results indicate that 37 mm yr of convergence is accommodated along a relatively narrow belt of n nw dipping thrust faults in the eastern half of the orogen with the present day deformation front running through icy bay and beneath the malaspina glacier near the bering glacier the collisional thrust fault regime transitions into a broad northwest dipping decollement as the yakutat block basement begins to subduct beneath the counterclockwise rotating elias block the location of this transition aligns with the gulf of alaska shear zone implying that the pacific plate is fragmenting in response to the yakutat collision our model indicates that the bering glacier region is undergoing internal deformation and could correspond to the final stage of offscraping and accretion of sediments from the yakutat block prior to subduction predicted block motions at the western edge of the orogen suggest that the crust is laterally escaping along the aleutian fore arc

enhanced productivity led to increased organic carbon burial in the euxinic north atlantic basin during the late cenomanian oceanic anoxic event 1 three cenomanian turonian c t 93 5 ma black shale sections along a northeast southwest transect in the southern part of the proto north atlantic ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in 13c values of both bulk organic matter om and molecular fossils of algal chlorophyll and steroids all three sites show an increase in marine organic carbon oc accumulation rates during the c t oceanic anoxic event oae the occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria lack of bioturbation and high abundance of redox sensitive trace metals indicate sulfidic conditions periodically reaching up into the photic zone before as well as during the c t oae during the c t oae there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and mo al ratios the presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the c t oae however bottom water conditions did not dramatically change as indicated by more or less constant v al and zn al ratios at site 367 this suggests that the increase in oc burial rates resulted from enhanced primary productivity rather than increased anoxia which is supported by stable carbon isotopic evidence and a large increase in ba al ratios during the c t oae the occurrence of the productivity event during a period of globally enhanced organic carbon burial rates i e the c t oae points to a common cause possibly related to the formation of a deep water connection between north and south atlantic basins

deformation in the yakataga seismic gap southern alaska 1980 1986 a 60 by 40 km trilateration network in the yakataga seismic gap was surveyed in 1980 1982 1984 and 1986 with precise electro optical distance measuring equipment to measure strain accumulation the overall deformation is roughly approximated by a 0 24 0 03 strain yr n32 w 2 4 uniaxial contraction that is uniform in time however the spatial distribution of deformation shows some concentration of convergence in the neighborhood of the chugach st elias fault and of right lateral shear across the contact fault a simple dislocation model of the plate interaction in the yakataga gap fits the observed deformation reasonably well but seems to require that the motion of the pacific plate relative to the north american plate be directed more nearly n36 w than n15 w the generally accepted direction of relative motion for this location however the direction of plate motion inferred from the dislocation model depends upon details of the interaction at the plate boundary that may not have been modeled accurately a nearby but smaller trilateration network at icy bay was surveyed in 1982 1984 and 1986 this network spans the southwest corner of the rupture zone of the 1979 st elias earthquake the deformation at icy bay consists of left lateral shear across a northeast trending zone the relation of this deformation to strain accumulation in the yakataga gap postseismic relaxation associated with the 1979 earthquake or rebound from the unloading associated with the rapid recession of the guyot glacier is not understood

strain partitioning during oblique plate convergence in northern sumatra geodetic and seismologic constraints and numerical modeling global positioning system gps measurements along the subduction zone of northern sumatra 2 s to 3 n reveal that the strain associated with the oblique convergence of the australian plate with eurasia is almost fully partitioned between trench normal contraction within the forearc and trench parallel shear strain within a few tens of kilometers of the sumatran fault kinematic analyses of interplate earthquake slip vectors provide slip rates on the sumatran fault within a few millimeters per year of gps and geologic rates giving us more confidence in the use of slip vectors for inferring slip partitioning elsewhere the inferred slip rate on the sumatran fault is 1 3 less than the full margin parallel component of plate motion an across forearc rotation in the slip vectors suggests that the missing arc parallel shear occurs seaward of the geodetic network between the forearc islands and the trench simple finite element models are used to explore the conditions under which the change in the principal strain rate directions between the forearc and the arc region can occur modeling suggests that neither a preexisting strike slip fault nor a zone of thermally induced lithospheric weakness in the overriding plate is needed for strain partitioning to occur in general forearc slivers form over the region of interplate coupling and are driven along strike by the basal shear a volcanic arc can help the partitioning process by localizing the margin parallel shear strain in the upper plate if its crust and mantle are weaker than its surroundings interplate slip vectors and geodetic results from sumatra together suggest that the highest coupling on the plate boundary occurs beneath and seaward of the forearc islands consistent with inferences about the rupture zones of great nineteenth century earthquakes there the sumatra example suggests that geodetic measurements of interseismic margin parallel shear strain at oblique convergent margins can be used to map the landward extent of the relatively high basal stress beneath the overriding plate if one can correct for strain localization caused by weak upper plate strike slip faults

the earthquake sequence of november 1987 and march 1988 in the gulf of alaska a new insight new information has been obtained by reanalyzing the rupture process of three large strike slip earthquakes that occurred in the gulf of alaska on november 17 and 30 1987 and march 6 1988 we used spectral analysis of p waves complemented with a simple technique for inverting body waves using a fixed source mechanism our results show that the ms 7 0 event of november 17 which can be considered to be a foreshock to the november 30 event consisted of two pulses of large seismic moment release the first one with a duration of 15 sec coincides with those obtained by other studies the second which occurs 20 sec after the first arrival is clearly visible on the records but was not previously reported the ms 7 4 earthquake of november 30 is a complex event that ruptured on a fault plane trending north south the ms 7 7 shock of march 6 1988 was a simple event along the same north south trend south of the november events p wave far field spectra exhibit an 1 intermediate trend between the usual 0 and 2 slopes this trend is due to rupture along a long narrow fault so the corner frequency associated with this intermediate trend is related to the reciprocal of rupture propagation time we suggest that these events were due to a tear in the pacific plate

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

paleogene metamorphism of an accretionary flysch terrane eastern gulf of alaska reconnaissance field studies have identified a regional metamorphic complex at least 200 km long and 25 to 50 km wide in the eastern chugach mountains southern alaska the metamorphic complex was developed within the cretaceous valdez group a flysch sequence that was accreted to the continental margin during the late cretaceous or early tertiary the metamorphic complex shows the progressive development from subgreenschist and green schist facies slate phyllite and semischist that is regionally characteristic of the valdez group to biotite plagioclase quartz schist muscovite cordierite andalusite staurolite garnet and sillimanite in an intermediate zone and amphibolite facies muscovite biotite plagioclase quartz schist and gneiss sillimanite and garnet in a higher grade central zone k feldspar is characteristically absent in the amphibolite facies schists but it is present in migmatitic rocks that are common to the central zone the mineral assemblages and petrographic relations indicate that metamorphism took place under low pressure intermediate facies series conditions partial melting took place at the highest grade of metamorphism melting produced granodioritic magmas in migma titic rocks of the central zone that were emplaced as discrete plutons in both the intermediate zone and peripheral rocks of the complex the timing of metamorphism as indicated by field relations and k ar data is about 50 to 65 m y ago and overlaps or is coeval with the emplacement throughout the gulf of alaska region of early tertiary plutons that are thought to be anatectic in origin the distribution of these plutons suggests that a high temperature low pressure metamorphic belt about 1 800 km long developed along the leading edge of the continental margin in late paleocene to early eocene time

strain accumulation across the fairweather and totschunda faults alaska deformation across the fairweather and totschunda faults is estimated from repeated surveys of geodetic networks that span those faults surveys in 1967 1983 and 1986 of networks crossing the fairweather fault indicate that right lateral engineering shear strain is accumulating across the 1958 earthquake rupture at a rate of 1 6 0 2 rad yr this rate of deformation is one of the highest observed across a strike slip fault system and it is several times greater than the secular rate of deformation across locked parts of the san andreas fault the rate of strain accumulation across the fairweather fault was the same in the two intervals 1967 1983 and 1983 1986 and is thus believed to be a secular rate rather than a transient associated with the 1958 earthquake the 1983 1986 line length changes are best fit by a model fault that is locked from the surface to a depth of 7 to 9 km and is slipping in a right lateral sense at a rate of 41 to 51 mm yr below that depth models with the fault locked to any depth of 4 km fit the data almost equally well the best fitting models indicate a slip rate comparable to the 48 to 58 mm yr average late holocene slip rate indicated by offset glacial moraines and to the 54 mm yr component of relative motion between the pacific and north american plates that is parallel to the fairweather fault no significant contraction was measured across the fairweather fault only a small part of the relative plate motion is transferred across the saint elias mountains to the colinear totschunda fault surveys in 1909 1912 and 1985 of a geodetic network that extends for 72 km across the totschunda fault indicate right lateral shear strain is accumulating at a rate of 0 11 0 06 rad yr this rate of strain accumulation suggests a slip rate of about 10 mm yr on the totschunda fault a value at the lower end of the average holocene slip rate of 10 to 20 mm yr

controls on the formation of contact type platinum group element mineralization in the east bull lake intrusion the east bull lake intrusion is the type example of a series of 2 48 to 2 49 ga layered mafic intrusions that developed within the rifted southern margin of the superior craton central ontario the east bull lake intrusion contains contact type pge mineralization e g strata bound pge rich disseminated sulfide mineralization occurring within 200 to 300 m of the contact with footwall and sidewall units although the mineralization locally occurs in massive plagioclase cumulates anorthosite zone most of the sulfides are concentrated within an underlying heterolithic inclusion rich unit inclusion bearing zone that grades into an intrusive breccia disseminated sulfides also occur in fine grained gabbroic veins within brecciated footwall rocks border zone geochemical characteristics of the contact type sulfide mineralization and its host rocks include 1 high average pd abundances throughout the east bull lake intrusion including an average of ca 200 ppb in the lower series 2 low average s contents typically contact type pge mineralization in the east bull lake intrusion reflects flow enhanced gravitational settling of a cu rich sulfide liquid followed by local fractionation of ni rich monosulfide solid solution to produce minor amounts of cu and pt rich residual sulfide liquid most of the observed compositional variation in the sulfides can be modeled using variable silicate liquid sulfide liquid mass ratios field and geochemical observations suggest that the parental magmas to the mineralized parts of the east bull lake intrusion were s saturated and pge rich at the time of their emplacement these fertile parent magmas most likely originated as buoyant feldspathic residual magmas that escaped from a deeper more primitive subchamber although first described from the east bull lake intrusion contact type pge mineralization is now recognized in all of the major east bull lake suite intrusions similar styles of magmatic sulfide mineralization are developed in the contact environments of the bushveld igneous complex platreef deposits and the duluth complex minnesota e g dunka road deposit these contact type deposits are part of the family of basal accumulation magmatic sulfide deposits the economics of low sulfide tenor contact type pge cu ni mineralization such as that developed in the east bull lake intrusion will be most favorable in systems in which 1 the parent magmas possessed high initial pge tenors and reached s saturation prior to emplacement 2 the parent magmas experienced vigorous convection in the early stages of chamber development and 3 segregated sulfide was not diluted by coprecipitating silicate minerals a scenario that is most likely to occur in feldspathic magmas in which plagioclase is the principal high temperature liquidus mineral

wrangell benioff zone southern alaska the first unequivocal evidence for the existence of a benioff zone that may be related to the quaternary wrangell volcanoes comes from a set of 86 well located hypocenters for earthquakes smaller than about magnitude 4 in the region between lat 61 and 62 5 n and between long 142 and 145 5 w about half of the earthquakes occur at depths of 25 km or less below 40 km a clearly defined north northeast dipping zone of seismicity here termed the wrangell benioff zone extends to a depth of about 85 km and continues for about 115 km along strike subparallel to the volcanic trend the western end of the zone may be offset from the northern end of the much more active aleutian benioff zone where the benioff zone shoals to 30 to 40 km it becomes nearly horizontal and cannot be clearly distinguished from upper plate seismicity it is uncertain whether the subducted plate segment that contains the wrangell benioff zone is structurally part of the pacific plate or the yakutat block

high temperatures in the late cretaceous arctic ocean to understand the climate dynamics of the warm equable greenhouse world of the late cretaceous period it is important to determine polar palaeotemperatures the early palaeoceanographic history of the arctic ocean has however remained largely unknown because the sea floor and underlying deposits are usually inaccessible beneath a cover of floating ice a shallow piston core taken from a drifting ice island in 1970 fortuitously retrieved unconsolidated upper cretaceous organic rich sediment from alpha ridge a submarine elevated feature of probable oceanic origin a lack of carbonate in the sediments from this core has prevented the use of traditional oxygen isotope palaeothermometry here we determine arctic palaeotemperatures from these upper cretaceous deposits using tex86 a new palaeothermometer that is based on the composition of membrane lipids derived from a ubiquitous component of marine plankton crenarchaeota from these analyses we infer an average sea surface temperature of approximately 15 degrees c for the arctic ocean about 70 million years ago this calibration point implies an equator to pole gradient in sea surface temperatures of approximately 15 degrees c during this interval and by extrapolation we suggest that polar waters were generally warmer than 20 degrees c during the middle cretaceous approximately 90 million years ago

tectonic model of the pacific north american plate boundary in the gulf of alaska from broadband analysis of the 1979 st elias alaska earthquake and its aftershocks the st elias alaska earthquake of 28 february 1979 ms 7 2 is reanalyzed using broadband teleseismic body waves and long period surface waves because of unresolved questions about its depth focal mechanism seismic moment and location in a seismic gap teleseismic waveforms are simultaneously inverted to determine the source mechanism seismic moment rupture history and centroid depth these data are well modeled with a point source propagating in the ese direction with an average kinematic rupture velocity of 2 5 km s the best fitting source mechanism indicates underthrusting on a ne dipping plane the mainshock depth of 24 km and the depth of aftershocks determined from inversions are consistent with locations on the gently dipping main thrust of the pacific north american plate boundary these depths are substantially different from those of earlier body wave studies and regional seismic network aftershock depth determinations but are in accord with the harvard centroid moment tensor and international seismological centre determinations the seismic moment determined from body waves is 9 4 1019 n m mw 7 3 the spatial and temporal distribution of moment release indicates that the st elias earthquake was a complex rupture consisting of two distinct subevents within 38 s of the initial onset followed by low moment release during the next 34 s earlier studies indicated an unusual amount of surface wave energy at very long periods 200 s that led some workers to suggest that st elias was a slow earthquake our broadband modeling does not require more than 34 s of additional moment release after the first two subevents moreover we are able to match the phase and amplitude of 200 s love and rayleigh waves with a thrust fault point source of moment 1 3 1020 n m mw 7 4 located at the body wave centroid the moment difference is not discernible with body waves for moment evenly distributed over 72 s thus the st elias earthquake is not slow with respect to 200 s surface waves but is complex with regard to the broadband body waves upper plate structure apparently controlled the gross characteristics of rupture the rupture direction parallels mapped upper plate faults rupture propagated unilaterally to the ese with little initial moment release as a shallow north dipping thrust that later changed to more steeply ne dipping with a large right lateral strike slip component the locations and source mechanisms of these subevents and locations of aftershocks define a shallow dipping surface at the eastern edge of the pacific plate moreover the component of strike slip motion increases with time in the mainshock implying that the transition to strike slip faulting occurs along the plate interface the estimated nucleation point of the second subevent coincides with a large concentration of aftershocks interpreted as representing a barrier to continuous rupture associated with the northern most boundary of the yakutat terrane joint relocation of aftershocks suggests that the main plate boundary may be offset vertically by 5 10 km as a result of this structure the southern part of the aftershock zone while containing many aftershocks appears not to have ruptured coseismically but may have failed later by aseismic creep as seen in geodetic measurements faults associated with the malaspina fault system the onshore extension of the aleutian trench appear to be the surface expression of the underthrusting plate boundary however upper plate deformation is widespread because of the collision of the yakutat terrane the convergence direction may explain the lack of a highly active wadati benioff zone downdip of the st elias zone the neotectonic deformation of the chugach st elias mountains is probably related to collision and subduction of the yakutat terrane a terrane in the process of accreting and subducting will cause considerable upper plate deformation over a wide zone once subduction of a terrane has begun deformation may then become localized

discussion on displacement analysis detection of crustal deformation in using geodetic networks to detect crustal motion there is always a question about what to use as a reference if the stations in a subset of the network can be shown to remain fixed relative to one another then this subset can be used as a reference for determining the motion of the remaining stations in the network we derive such a solution for the displacement of a geodetic network referred to as a stationary station solution the advantage of this technique over other solutions to the same problem is that it neither depends on a single station nor makes assumptions about the movement of all the stations of the network application of the technique to a geodetic network near hollister california indicates that stations between the faults are stationary although for the block between the calaveras and san andreas faults the stations are only marginally stationary relative to one another the technique is then extended to give the total block motion between the faults

the paradox of drowned carbonate platforms and the origin of cretaceous pacific guyots geochemical stratigraphic and palaeolatitudinal data from deep boreholes drilled through pacific guyots flat topped seamounts help to explain the drowning of these cretaceous shallow water carbonate platforms that once thrived through the accumulation of biogenic and inorganic calcium carbonate sediment in mid oceanic regions the platforms drowned sequentially over a 60 million year interval while they were being transported northward by pacific plate motion through a narrow equatorial zone 0 10 s such platforms were apparently resistant to the effects of cretaceous oceanic anoxic events although the mechanism responsible for drowning remains unknown the tropics have not always been the refuge for atolls that they are today

tropical warming and intermittent cooling during the cenomanian turonian oceanic anoxic event 2 sea surface temperature records from the equatorial atlantic oceanic anoxic event 2 oae 2 occurring during the cenomanian turonian c t transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 myr however the impact of this major perturbation on and interaction with global climate remains unclear here we report new high resolution records of sea surface temperature sst based on tex86 and 18o of excellently preserved planktic foraminifera and stable organic carbon isotopes across the c t transition from black shales located offshore suriname french guiana demerara rise ocean drilling program leg 207 site 1260 and offshore senegal cape verde basin deep sea drilling project leg 41 site 367 at site 1260 where both sst proxy records can be determined a good match between conservative sst estimates from tex86 and 18o is observed we find that late cenomanian ssts in the equatorial atlantic ocean 33 c were substantially warmer than today 27 29 c and that the onset of oae 2 coincided with a rapid shift to an even warmer 35 36 c regime within the early stages of the oae a marked 4 c cooling to temperatures lower than pre oae conditions is observed however well before the termination of oae 2 the warm regime was reestablished and persisted into the turonian our findings corroborate the view that the c t transition represents the onset of the interval of peak cretaceous warmth more importantly they are consistent with the hypotheses that mid cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide co2 and that major oaes were capable of triggering global cooling through the negative feedback effect of organic carbon burial led co2 sequestration evidently however the factors that gave rise to the observed shift to a warmer climate regime at the onset of oae 2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 myr

estimates of long term probabilities for future great earthquakes in the aleutians we compute probabilities for future great earthquakes in the aleutian arc probability distributions are fitted to recurrence periods of great aleutian evens m sub w or 7 8 since 1788 given a probability distribution and data of the last grat earthquake in each arc segment time dependent conditional probabilities are determined for future periods of interest we obtain for the next two decades high probabilities 99 to 30 for great earthquakes in the shumagin yakataga unalaska and kommandorski seismic gaps these probabilities are higher than for any other assessed region of the u s believed to be capable of great earthquakes low probabilities 17 to 9 are found for segments that ruptured most recently in 1965 1964 and 1957 recurrence periods for great earthquakes measure on average about 80 years but vary substantially whether recurrence periods for great aleutian earthquakes follow a normal log normal or any other probability distribution is not resolved because in most arc segments the known seismic record embraces at best one or two recurrences the resulting uncertainty affects the magnitude of estimated probabilities especially in seismic gaps that have not ruptured for a long time in some instances a normal distribution yields a three times higher probability than the correspondingmore log normal distribution less

seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern alaska laboratory velocity measurements and structural field relations explain the observation of a significant seismic anisotropy within highly foliated rocks in southern alaska the orientation of the principal compressive stress indicates that this anisotropy is not related to extensive dilatancy but can be satisfactorily explained by the foliation of the metasedimentary rocks laboratory measurements indicate a significant anisotropy within these rocks even at pressures well above those thought to close microcracks these observations document that anisotropy related to preferred mineral orientation may be widespread in continental rocks thus pervasive foliation can play a more important role in causing anisotropy than dilatancy and inferences of principal stress orientations solely from anisotropy may be erroneous

possible atmospheric co2 extremes of the middle cretaceous late albian turonian 1 atmospheric carbon dioxide co2 estimates for the middle cretaceous mk have a range of 4000 ppm which presents considerable uncertainty in understanding the possible causes of warmth for this interval this paper examines the problem of mk greenhouse forcing from an inverse perspective we estimate upper ocean water temperatures from oxygen isotope measurements of well preserved late albian turonian planktonic foraminifera and compare these against temperatures predicted by general circulation model gcm experiments with co2 concentrations of 500 7500 ppm at least 4500 ppm co2 is required to match maximum temperatures inferred from well preserved planktonic foraminifera approximately 900 ppm co2 produces a good match between the model and the minimum temperature estimates for the mk an ocean model forced by these two extremes in surface conditions brackets nearly all available bottom water temperature estimates for this interval the climate model results support nearly the entire range of mk co2 estimates from proxy data the ocean model suggests possible mk oceanographic changes from deep water formation in the high latitude region of one hemisphere to the other hemisphere in response to changes in atmospheric temperatures and hydrologic cycle strength we suggest that rather than contradicting one another the various proxy co2 techniques especially those with high temporal resolution may capture true variability in co2 concentrations and that mk co2 could have varied by several thousand ppm through this interval

measurement of horizontal motions in alaska using very long baseline interferometry we have analyzed 6 years of very long baseline interferometry vlbi data beginning in 1984 from a network of sites in alaska the yukon and the conterminous united states the analysis of these 53 data sets includes constrained continuous piecewise linear estimation of the site atmospheres and clocks application of values and correlations from a self consistent earth orientation series derived from other vlbi data and simultaneous estimation of site positions and velocities from a solution that includes 21 007 observations with a fit of 40 ps in delay and 64 fs s in delay rate we have determined the horizontal velocities of seven sites with precisions of 0 5 2 mm yr 1 standard deviation the three vlbi sites within the alaska aleutian subduction boundary zone yakataga kodiak and sand point have higher instantaneous velocities relative to eastern north america than the interior sites of alaska fairbanks and sourdough the estimated velocity of yakataga relative to fairbanks is 37 8 1 3 mm yr at n27 w 2 modeling of the yakataga data suggests that the observed motion is the result of elastic straining of the overriding plate due to a locked main thrust zone with a component of oblique slip the estimated velocity of kodiak relative to fairbanks is 13 6 0 8 mm yr at n41 w 3 this result is lower than the rate predicted on the basis of simple elastic dislocation models the estimated velocity of sand point relative to fairbanks is 6 4 0 9 mm yr at n65 w 8 the component of this velocity in the plate direction 3 5 1 1 mm yr is not consistent with either an aseismic slip model i e no strain accumulation in the overriding plate or a model in which the site is undergoing compressional strain 10 9 mm yr at n28 w due to simple subduction at the predicted orientation of pacific plate convergence in a north america fixed reference frame the estimated velocities of fairbanks 1 5 0 5 mm yr at s12 e 20 and sourdough 3 9 1 0 mm yr at s13 w 13 are both southward these results may imply motion of this region relative to stable north america but more observations are required to reduce the uncertainties in the azimuths the time evolution of the estimated site positions from vlbi in alaska are in general consistent with constant velocity however in 1988 the yakataga site showed a significant departure from the position predicted from a constant velocity model obtained from the previously estimated position this departure may be static displacement associated with two major strike slip earthquakes that occurred in the gulf of alaska between the 1987 and 1988 measurement sessions

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

organic facies of middle cretaceous black shales in deep north atlantic the middle cretaceous black shales of the deep north atlantic consist of alternating layers of sediment rich in total organic carbon toc that contains abundant amorphous types of organic matter and sediment poor in toc that contains mostly woody and coaly types of organic matter the amorphous materials are derived mostly from marine organisms in the eastern north atlantic off africa in the caribbean and on the demerara rise off surinam and from higher land plants in the western north atlantic and off europe marine amorphous material is important off africa because this was an area of upwelling and highly productive surface water amorphous marine organic matter was deposited across the entire north atlantic basin in the cenomanian in response to an ocean wide upwell ng event connected with the opening of the equatorial atlantic the difference between the toc rich and background toc layers reflects deposition of the former under reducing conditions which allowed the preservation of labile amorphous materials of marine and terrestrial orgin and of the latter under oxidizing conditions when nearly all that was preserved was refractory organic material

midlatitude shelf seas in the cenomanian turonian greenhouse world temperature evolution and north atlantic circulation 1 an 8 million year record of subtropical and midlatitude shelf sea temperatures derived from oxygen isotopes of well preserved brachiopods from a variety of european sections demonstrates a long term cenomanian temperature rise 16 20 c midlatitudes that reached its maximum early in the late turonian 23 c midlatitudes superimposed on the long term trend shelf sea temperatures vary at shorter timescales in relation to global carbon cycle perturbations in the mid cenomanian and the late turonian two minor shelf sea cooling events 2 3 c coincide with carbon cycle perturbations and times of high amplitude sea level falls although this evidence supports the hypothesis of potential glacioeustatic effects on cretaceous sea level the occurrence of minimum shelf sea temperatures within transgressive beds argues for regional changes in shelf sea circulation as the most plausible mechanism the major carbon cycle event in the latest cenomanian oceanic anoxic event 2 is accompanied by a substantial increase in shelf sea temperatures 4 5 c that occurred 150 kyr after the commencement of the 13c excursion and is related to the spread of oceanic conditions in western european shelf sea basins our oxygen isotope record and published 18o data of pristinely preserved foraminifera allow the consideration of north atlantic surface water properties in the cenomanian along a transect from the tropics to the midlatitudes on the basis of fossil derived 18o estimated w ranges and modeled salinities temperature salinity density ranges were estimated for tropical subtropical and midlatitude surface waters accordingly the cenomanian temperate shelf seas waters have potentially the highest surface water density and could have contributed to north atlantic intermediate to deep waters in the preopening stage of the equatorial atlantic gateway

water mass characteristics in the cenomanian us western interior seaway as indicated by stable isotopes of calcareous organisms abstract stable oxygen isotopes taken from taxon specific planktic foraminifera are potential sources of water mass density proxy data and paleoclimate information material suitable for stable isotope analysis is uncommon in the middle to late cenomanian strata of the western interior of the usa however well preserved material from 17 correlative horizons at five localities across se montana and ne wyoming has provided an opportunity to obtain reliable 18 o data from foraminiferal samples fine fraction carbonate e g calcareous nannofossils and inoceramid prisms with the exception of inoceramids which for a benthic organism have stable isotopic compositions resembling surface dwelling calcareous organisms these data indicate that the water column was vertically stratified and water masses were anomalously depleted in 18 o within samples planktic and benthic foraminifera display strong vertical gradients in 18 o and 13 c of several per mil the vertical gradient of 18 o ranges from more negative nearer the sea surface to less negative at the sea floor with a gradient of about 4 this implies less saline warmer waters near the surface with increasingly more saline colder waters with depth this isotopic distribution indicates a stably stratified water column at least seasonally in addition a persistent vertical carbon isotope gradient of 2 3 suggests a strong nutrient gradient that if mixed would result in high surface water fertility preservation of high amounts of organic carbon in the sediments corroborates this inference although the water column was stratified overall the nutrients necessary to promote high productivity were clearly renewed at least seasonally nutrient renewal could have been achieved by 1 winter cooling of the upper layer allowing deeper seasonal mixing with nutrient rich deep waters 2 seasonal shallowing of the pycnocline into the photic zone 3 entrainment and decomposition of organic matter at the previously proposed frontal convergence 4 upwelling of nutrient rich deep waters along the previously proposed oceanic front or 5 mixing of shelf waters with more oceanic waters across an oceanic front

temperature dependent variation in the distribution of tetraether membrane lipids of marine crenarchaeota implications for tex86 paleothermometry 1 recently a new geochemical temperature proxy the tex86 was introduced this proxy is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraethers gdgts of the membrane lipids of marine crenarchaeota which changes as a response to temperature however until now only sediment data have been used to establish this proxy and experimental work is missing we performed mesocosm studies with marine crenarchaeota incubated at temperatures ranging from 5 to 35 c and salinities of 27 and 35 to test the validity of the tex86 proxy growth of marine crenarchaeota in these mesocosms was evident from the substantial increase in the concentration of marine crenarchaeotal membrane lipids with amounts up to 3400 ng l with increasing temperature an increase in the number of cyclopentane moieties in the crenarchaeotal membrane lipids was observed different salinities did not show any effect on the gdgt distribution the tex86 showed a significant linear correlation to incubation temperature tex86 0 015 t 0 10 r2 0 79 this equation has a similar slope to the correlation obtained from core tops but differs in the intersection tex86 0 015 t 0 28 r2 0 92 this difference is mainly determined by the smaller amount of the regioisomer of crenarchaeol in the incubation series compared to core top samples these incubation experiments indicates that water temperature is indeed the major controlling factor for the membrane distribution of marine crenarchaeota and confirms that the tex86 proxy depends on a physiological response to regulate membrane fluidity

relation of ongoing deformation rates to the subduction zone process in southern alaska the rate and orientation of ongoing strain associated with subduction of the pacific plate and the accretion of the yakutat terrane to southern alaska has been estimated at 13 sites from global positioning system measurements made in june 1993 and 1995 along the gulf of alaska coast near cape yakataga the average rate of deformation relative to fairbanks was 38 mm yr at n32 w further inland above the region where the dip of the downgoing pacific plate changes from about 10 to 30 the deformation rate was 12mm yr at n26 w in the sourdough paxson area the deformation rate drops to 2 5 mm yr and suggests a low short term deformation rate across the denali fault elastic straining of the overriding plate due to back slip on a main thrust zone with an average dip of about 10 can account for the overall rate and distribution of short term compressional strain across south central alaska above the transitional region between unstable and stable sliding we suggest that strain associated with 15 mm yr of right lateral strike slip occurs also if the strain accumulated since the two 1899 earthquakes both mw 8 1 from the offshore pamplona fault zone to south of the border ranges fault down dip width 100 km was seismically released on a single fault it would correspond to a m 8 1 earthquake

strain accumulation in the yakataga seismic gap southern alaska strain accumulation in the yakataga seismic gap has been estimated from the deformation of a 60 km 40 km trilateration network surveyed in 1979 1980 1982 and 1984 the contraction in the approximate direction n19 w of plate convergence is 0 19 0 04 strain yr and there is a minor 0 07 0 04 strain yr orthogonal extension a significant right lateral shear 0 09 0 02 strain yr occurs across a vertical plane striking n71 e a simple dislocation representation of the plate interaction model proposed by lahr and plafker for the yakataga seismic gap quantitatively accounts for the observed n19 w contraction and qualitatively accounts for the right lateral shear the two strains being associated with the normal and transverse components respectively of the oblique convergence between the yakutat block and the north american plate the measurements are consistent with strain accumulation building up to a great thrust earthquake in the yakataga gap in addition there is a suggestion of right lateral shear strain accumulation across the contact fault but the interval over which data are available is probably too short to prove such accumulation

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

black shale deposition on the northwest african shelf during the cenomanian turonian oceanic anoxic event climate coupling and global organic carbon burial high resolution geochemical records from a depth transect through the cenomanian turonian c t tarfaya basin northwest african shelf reveal high amplitude fluctuations in accumulation rates of organic carbon oc redox sensitive and sulphide forming trace metals and biomarkers indicative of photic zone euxinia these fluctuations are in general coeval and thus imply a strong relationship of oc burial and water column redox conditions the pacing and regularity of the records and the absence of a prominent continental signature suggest a dynamic depositional setting linked to orbital and higher frequency forcing determining the dominant frequency depends on the definition of the most pronounced oceanic anoxic event oae2 and its duration we propose that eccentricity is the main forcing factor at tarfaya and controlled fluctuations in wind driven upwelling of nutrient rich oxygen depleted intermediate waters from the adjacent atlantic and the periodic development of photic zone and bottom water euxinia on the mid cretaceous northwest african shelf accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six fold increase in oc burial from similar to2 g m 2 yr prior to the oae2 to similar to12 g m 2 yr during the oae2 photic zone and bottom water euxinia alternated with periods of greater oxygenation of the water column in response to climate forcing mass balance calculations imply that similar to2 of the overall global excess oc burial associated with the oae2 was deposited in the tarfaya basin an area that represented only similar to0 05 of the total global c t ocean floor in fact the lateral extent of similar black shales along the african continental margin indicates that this part of the ocean contributed significantly to the global increase in organic carbon burial during the oae2

trace element abundances in the rock canyon anticline pueblo colorado marine sedimentary section and their relationship to caribbean plateau construction and oxygen anoxic event 2 1 rapid eruption of submarine lava flows during formation of the caribbean plateau correlates closely with ocean anoxic event 2 oae2 which bracketed the cenomanian turonian boundary 93 5 ma these events also correspond with a positive excursion in carbon isotopic composition of seawater hydrothermal activity associated with large scale submarine volcanism may have been responsible for this abrupt change in ocean chemistry we determined the distribution of major minor and trace element abundances in the rock canyon marine sedimentary section pueblo colorado after normalizing element concentrations to zr to remove the variable contribution of terrigenous material to these sediments we detected an interval of concentrated metal abundance anomalies that coincides with the abrupt beginning of the positive 13c isotope excursion the metal abundance anomalies indicate that intermittent hydrothermal activity in the form of both water rock exchange and magmatic degassing introduced large concentrations of trace metals into the cretaceous ocean at the same time that extinctions of benthic species turnover in plankton communities and increases in isotopically light organic carbon burial occurred the stratigraphic position of this interval of trace metal anomalies matches events associated with oae2 and indicates that intermittent hydrothermal activity on a massive scale triggered abrupt changes in carbon burial and deep ocean oxygen contents

crustal deformation and the seismic cycle across the kodiak islands alaska 1 the kodiak islands are located 120 to 250 km from the alaska aleutian trench and are within the southern extent of the 1964 prince william sound mw 9 2 earthquake rupture and aftershock zone here we report new campaign gps results 1993 2001 from northeastern kodiak and reprocessed gps results 1993 1997 from southwestern kodiak the rate and orientation of the horizontal velocities relative to a fixed north america range from 29 7 1 7 mm yr at n30 3 w 3 3 located 120 km from the deepest point of the trench to 8 0 1 3 mm yr at n62 4 w 9 3 located 230 km from the trench we evaluated alternate models of coseismic and interseismic slip to test the importance of the mechanisms that account for surface deformation rates near the gulf of alaska coastal region of kodiak the horizontal velocity can be accounted for primarily by the viscoelastic response to plate motion and a locked main thrust zone mtz downdip creep and to a lesser extent slip in the 1964 earthquake farther inland the dominant mechanisms that account for post 1964 uplift rates are time dependent downdip creep and a locked mtz for the horizontal velocity component southwest translation of western kodiak may be important as well on the basis of the pre 1964 and post 1964 earthquake pattern of interseismic earthquakes we suggest that between the occurrences of great earthquakes like the 1964 event more moderate to large earthquakes occur in the southwestern kodiak region than near northeastern kodiak