laramide exhumation of the bighorn mountains wyoming an apatite u th he thermochronology study we report u th he apatite ages from the crystalline core of the bighorn mountains and compare the exhumation history derived from those ages with the exhumation history determined from sedimentary rocks in the adjacent powder river and bighorn basins our u th he apatite ages range from 62 to 369 ma and represent a pre laramide he partial retention zone that was deformed and uplifted at ca 65 5 ma the geometry of the he partial retention zone indicates that the basement in the bighorn mountains is deformed into a doubly plunging anticline the preservation of a pre laramide partial retention zone in the upper few hundred meters of precambrian basement indicates that in general the temperature at the cambrian unconformity did not exceed the apatite he closure temperature this is difficult to reconcile with evidence from adjacent basins for thick sequences of sedimentary rocks 3 4 km prior to 65 ma and normal modern geothermal gradients either the range was never deeply buried 2 3 km its geothermal gradient has been low 20 c since at least the mesozoic or our apatites have higher u th he closure temperatures 80 90 c than those measured for other apatites

heat flow along the nantroseize transect results from iodp expeditions 315 and 316 offshore the kii peninsula japan mal conductivity varies between about 1 0 w m 1 k 1 near the seafloor to 1 6 w m 1 k 1 at a depth of 1 km thermal conductivity generally increases with depth and correlates with variations in porosity and lithology thermal gradients along the transect are characterized by 48 sediment temperature measurements from 6 sites thermal corrections for the effects of bathymetric relief and sedimentation improve the confidence with which the heat flow values can be interpreted heat flow generally decreases with landward distance from the deformation front and varies from 70 mw m 2 just landward of the deformation front to 54 mw m 2 at sites characterizing the outer fore arc high and to 57 mw m 2 at the kumano basin site iodp heat flow measurements are significantly lower than nearby seafloor heat flow measurements this difference is most likely due to variations in bottom water temperature that have a large effect on values of seafloor heat flow thus the heat flow of the nankai accretionary prism is lower than previously thought we present thermal models of subduction along this transect and explore the impact of the initial geotherm conductive plate cooling based on the age of subducting seafloor 20 myr under predicts the observed heat flow we find a good fit to the data using a geotherm appropriate for 10 myr seafloor the extra heat is interpreted in terms of back arc thermal environments

estimates of heat flow derived from gas hydrates a gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data from the depth of the reflectors heat flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments this method is applied to the data in the nankai trough around central america and along the blake outer ridge the estimated values consistent with those measured by conventional means suggest that derivation of heat flow by this method is feasible on the landward slope of the nankai trough we find that heat flow increases downslope toward the floor of the trough

u th he thermochronology of the ottawa embayment eastern canada the temperature time history of an ancient intracratonic rift basin abstractthe ottawa embayment is an intracratonic rift basin in eastern north america that preserves a unique and eventful history through deep time we report single grain zircon and apatite u th he and biotite 40ar 39ar data from samples collected along a 250 km transect across the crystalline rift flank and through the sedimentary sequences of the embayment the data sets show significant intrasample age dispersions and we investigate the influence of zircon crystallinity and nonideal crystal shapes on he diffusion through the application of raman spectroscopy and x ray microcomputed tomography ultimately we utilize the relationship between radiation damage and helium diffusion to quantify observed age dispersion we integrate our age data from the basement and overlying sedimentary unit of the embayment with regional geology to utilize multisample inverse modeling to reveal viable thermal histories that we then further evaluate by using the forward modeling method to improve our understanding of t

paleothermal structure of the point san luis slab of central california effects of late cretaceous underplating out of sequence thrusting and late cenozoic dextral offset late cretaceous shale and sandstone turbidites of the point san luis slab are isoclinally folded locally sheared and faulted but their severity of stratal disruption is relatively mild when compared to adjacent polymictic melange of the franciscan complex we tested the interpretation of a trench slope basin origin for these strata by documenting their paleothermal structure including contacts between turbidites and melange values of mean random vitrinite reflectance rm from turbidites are 0 9 1 7 estimates of maximum paleotemperature are 135 200 c melange matrix samples yield rm values of 1 1 2 5 with an average of 1 5 and peak temperatures between 160 and 240 c the turbidite over melange contact is locally cooler over warmer and was folded after peak heating the relatively high paleotemperatures cast doubt on a shallow slope basin model i e 1 2 km burial depth we suggest instead that thermal maturation of the point san luis slab occurred much deeper in an accretionary prism 10 15 km where offscraped trench wedge deposits were faulted against underplated melange the paleothermal structure was offset and tilted after peak heating by two out of sequence faults late oligocene to pliocene strata rest unconformably above the franciscan and there is a significant gap in thermal maturity across this unconformity with no evidence to show that franciscan rocks were reset thermally following the main episode of uplift and erosion three dimensional orientations of isoreflectance surfaces on opposite sides of the san gregorio san simeon hosgri fault system also can be used to test interpretations of strike slip neotectonics the failure to match these geometries among suspected piercing points at point san luis cambria and point sur favors a suggestion that differential post thermal peak dextral offset of franciscan basement has not exceeded 10 15 km

advection of fluids at the front of the sicilian neogene subduction complex abstract the sicilian accretionary prism shows a large southward imbricated system of thrust sheets the frontal part of the belt has been tectonically emplaced in plio pleistocene times major decollement surfaces and sole thrusts at the base of and inside the prism consist of melanges with a blocks in matrix fabric in the frontal part two superposed decollement levels are recognized the upper one located at the base of the paleogene neogene sedimentary series and the basal one at the base of duplexes triassic miocene sedimentary series syntectonic dewatering of the sedimentary sections occurred along decollements and thrust faults involving hydrofracture and mineralized vein development inside the deep sole thrusts rooted in the basal decollement hydrofractured blocks show different generations of syntectonic quartz and calcite extensional veins while in the upper decollement only calcite veins have been observed both quartz and calcite veins show aqueous primary fluid inclusions of low salinity with low trapping temperature for fluids in the upper decollement if we assume that at the time of deformation the main heat source was the regional conductive geothermal gradient the trapping temperatures of the fluids inside veins of the deep sole thrusts are higher than those predicted by the pilo pleistocene gradient for this subduction zone and correspond to a thermal anomaly we suggest that the frontal sicilian prism supported localized transient fluid flow of deep freshwater sourced from two different origins a shallow one for fluids now trapped in the upper decollement and a deep internal origin 6 10 km minimum depth for the hot fluids channelized along the basal decollement and now trapped inside the sole thrust of the mt scalpello duplex area

phanerozoic low temperature evolution of the uruguayan shield along the south american passive margin the crystalline basement of uruguay was assembled during the brasiliano orogeny in the neoproterozoic era and was later affected by discrete tectonic activity a new multi method low temperature dataset including u th he ages from both zircon and apatite t t modelling and k ar dating of fine sericite fractions and fault gouge reveal a detailed post orogenic geological history spanning the phanerozoic eon the juxtaposition of the terranes that compose the area was achieved in the ediacaran period and post collision was marked by intense exhumation in which the crystalline basement reached near surface conditions by the early to mid palaeozoic regional subsidence promoted sedimentation in the parana basin until the permian covering and reheating much of the basement that is at present exposed afterwards deposition and volcanism were mostly confined to its current limits regional exhumation of the shield during the permo triassic exposed much of the northern portion of the basement and the south was further affected by the opening of the south atlantic ocean during the mesozoic little exhumation affected the uruguayan shield during the cenozoic as reflected in its modest topography the reactivation of inherited neoproterozoic structures influenced the development of mesozoic basins and the present day landscape supplementary material supplementary data sample locations u th he data and k ar data are available at https doi org 10 6084 m9 figshare c 3702043

zircon titanite and apatite u th he ages and age eu correlations from the fennoscandian shield southern sweden craton cores far from plate boundaries have traditionally been viewed as stable features that experience minimal vertical motion over 100 1000 ma time scales here we show that the fennoscandian shield in southeastern sweden experienced several episodes of burial and exhumation from 1800 ma to the present apatite titanite and zircon u th he ages from surface samples and drill cores constrain the long term low temperature history of the laxemar region single grain titanite and zircon u th he ages are negatively correlated 104 838 ma for zircon 160 945 ma for titanite with effective uranium eu u 0 235 th a measurement proportional to radiation damage apatite ages are 102 258 ma and are positively correlated with eu these correlations are interpreted with damage diffusivity models and the modeled zircon he age eu correlations constrain multiple episodes of heating and cooling from 1800 ma to the present which we interpret in the context of foreland basin systems related to the neoproterozoic sveconorwegian and paleozoic caledonian orogens inverse time temperature models constrain an average burial temperature of 217 c during the sveconorwegian achieved between 944 ma and 851 ma and 154 c during the caledonian achieved between 366 ma and 224 ma subsequent cooling to near surface temperatures in both cases could be related to long term exhumation caused by either post orogenic collapse or mantle dynamics related to the final assembly of rodinia and pangea our titanite he age eu correlations cannot currently be interpreted in the same fashion however this study represents one of the first examples of a damage diffusivity relationship in this system which deserves further research attention

the measurement of thermal conductivity of solids with a transient linear heat source on the plane surface of a poorly conducting body abstract during the last ten years the transient hot wire method for measuring thermal conductivity of ocean bottom muds and other soft materials has been adapted for measuring the conductivity of solid rocks because it is cheaper and quicker than the steady state methods such as the divided bar this is done by embedding the hot wire in the form of a hypodermic needle or a ribbon flush into the surface of a block of a poorly conducting material when a flat surface of a specimen is placed on the surface of such a sensor the line source can be said to diffuse heat into an infinite half space if one neglects the heat absorbed by the poorly conducting block into which the needle is embedded the rate of the temperature rise of the line source with respect to the logarithm of time elapsed since turning on the heater is inversely proportional to the conductivity in practice the rates of temperature rise are compared for the unknown and for a standard specimen a fused silica plate in our case the percentage of the total heat flowing into the sensor block is in general different for the standard and for the unknown thus possibly creating an error when the conductivity of the sensor block is neglected it was determined experimentally that the temperature rise for a silica plate is decreased only by 1 3 by the heat absorbed by the material of the sensor block an empirical equation is proposed to correct errors arising from this source this correction needs to be used only for materials of conductivity less than 2 mcal cm c s the equation gives somewhat larger but still acceptable conductivity values for substances in the range of 0 208 3 25 mcal cm c s as shown in table 1 and fig 2 for measurements on common rocks corrections for the heat diffused into the material of the sensor block can be neglected

long term tectonothermal history of laramide basement from zircon he age eu correlations abstract the long term 1 ga thermal histories of cratons are enigmatic with geologic data providing only limited snapshots of their evolution we use zircon u th he zircon he thermochronology and age composition correlations to understand the proterozoic phanerozoic thermal history of archean wyoming province rocks exposed in the northern laramide ranges of western north america zircon he ages from the wind river range 54 dates and bighorn mountains 32 dates show negative correlations with effective uranium eu a proxy for radiation damage zircon dates from the bighorns are between 960 ma low eu and 20 ma high eu whereas samples from the wind rivers are between 582 ma low eu and 33 ma high eu we applied forward modeling using the zircon radiation damage and annealing model zrdaam to understand this highly variable dataset a long term t t path that is consistent with the available geologic constraints successfully reproduced age eu correlations the best fit to the wind rivers data involves two phases of rapid cooling at 1800 1600 ma and 900 700 ma followed by slower cooling until 525 ma during the phanerozoic these samples were heated to maximum temperatures between 160 and 125 c prior to laramide cooling to 50 c between 60 and 40 ma data from the bighorn mountains were successfully reproduced with a similar thermal history involving cooler phanerozoic temperatures of 115 c and earlier laramide cooling between 85 and 60 ma our results indicate that age eu correlations in zircon he datasets can be applied to extract long term thermal histories that extend beyond the most recent cooling event in addition our results constrain the timing magnitude and rates of cooling experienced by archean wyoming province rocks between recognized deformation events including the 1 ga period represented by the regionally extensive great unconformity

contrasting responses of rb sr systematics to regional and contact metamorphism laramie mountains wyoming usa archean supracrustal sequences of pelitic quartzitic calcareous and mafic compositions in the central laramie mountains wyoming have been affected by two metamorphic events a 1 78 ga amphibolite grade regional metamorphism and a 1 43 ga contact metamorphism resulting from the intrusion of the laramie anorthosite complex lac rb sr whole rock isotopic data from both outside and within the lac contact aureole define a linear array that lies along a 1 78 ga isochron this date has been independently established as the time of amphibolite facies regional metamorphism associated with collision of the archean wyoming province and the proterozoic colorado province along the cheyenne belt the rb sr isotopic data require that sr was redistributed during regional metamorphism on a scale of at least tens of metres although within the 2 km wide aureole of lac the pelitic rocks were thermally metamorphosed at temperatures greater than 800 c none of the whole rock rb sr data from samples within the lac aureole show evidence of resetting at 1 43 ga it is interpreted that the regional metamorphism involved fluid transport which facilitated sr isotopic resetting whereas the contact metamorphism occurred in a relatively dry environment in which isotopic mobility was restricted to centimetre scale or less rb sr data for biotite feldspar and whole rock from a regional metamorphosed pelitic schist give an isochron age of 1450 40 ma which is interpreted as a cooling age resulting from crustal uplift rb sr data for biotite quartz feldspar and whole rock from a pelitic schist affected by contact metamorphism give an isochron age of 1420 43 ma the time of isotopic re equilibration in response either to crustal uplift or to both contact metamorphism and crustal uplift this study demonstrates that although the response of isotopic systems to metamorphism is complex isotopic data provide insight into metamorphic processes that is difficult to obtain by other means

statistical analysis for thermal data in the japanese islands abstract we calculated statistical average of thermal data to speculate regional thermal structure of the forearc area of the japanese islands the three thermal statistical averages show a difference of a high thermal regime in the western part of forearc inner zone and a low in the kanto forearc outer zone the kanto zone marks 18 k km 1 for mean geothermal gradient 44 mw m 2 for mean heat flow while the western inner zone shows 27 k km 1 for mean geothermal gradient 63 mw m 2 for mean heat flow the geothermal gradients of the nobi plain and the osaka plain in the western inner zone are 29 and 36 k km 1 respectively while the value of the kanto plain in the kanto zone is 21 k km 1 taking account of the effect of accumulation of sediments we see the difference in the thermal regime between the plains and conclude that the difference is significant heat flux in the crust depends on the volume of granite rich in radioactive elements there are few granitic rocks in the kanto zone while granitic rocks are dominant in the western inner zone the heat flow of 20 mw m 2 is attributed to the granitic rocks of about 8 km in thickness there are two oceanic plate subductions of the pacific plate and the philippine sea plate under the kanto zone while only the philippine sea plate has been subducting under the western inner zone the model simulation based on thermal and subduction model shows a heat flow ranging 50 60 mw m 2 in the southwest japan forarc area and a low value of about 20 mw m 2 in the northeast japan forearc area the heat flux from the cooling oceanic lithosphere depends on the age of plate the shikoku basin a part of the philippine sea plate off the western inner zone is 15 30 ma while the pacific plate off the kanto zone is 122 132 ma theoretically heat flux values of 15 and 50 ma oceanic plates range 60 120 mw m 2 and those of 122 132 ma could be about 10 mw m 2 if the heat flux contribution from the philippine sea plate under the kanto zone is smaller than the plate under the western inner zone there could be a thermal regime difference in order of several tens of mw m 2 conclusively the cause of the difference of heat flux could be the uneven granitic rocks distribution and or the difference of heat flux between the two subducting plate

dewatering and extensional deformation of the shikoku basin hemipelagic sediments in the nankai trough the shikoku basin hemipelagic sequence which underlies the nankai trough wedge s w japan thins by 50 between the outer edge of the trench wedge and dsdp site 582 14 km arcward a sedimentation model which incorporates changes in sedimentation rates with time and with distance from the trench wedge toe indicates that 57 of the total thinning occurs as a result of temporally varying sedimentation rates and a time transgressive facies boundary between the trench wedge turbidites and the underlying hemipelagites burial induced consolidation beneath the wedgeshaped turbiditic overburden accounts for the remaining 43 of arcward thinning within the hemipelagic unit rapid dewatering modeled as one dimensional consolidation suggests that the excess pore water pressures are quite low during this progressive dewatering thus high pore water pressures should not be assumed to occur universally in convergent margin settings normal faults and vein structures in the hemipelagites suggest near horizontal extension in addition to vertical consolidation the estimates of excess pore water pressures together with fault geometries and horizontal extensional strains determined from the geometry of the subducting oceanic plate can be used to constrain the stress conditions at failure the expulsion of hot water from the rapidly dewatering sediments in the nankai trough may help to explain anomalously high heat flow in the central part of the trough

sediment deformation and fluid activity in the nankai izu bonin and japan forearc slopes and trenches a variety of sediment and crustal deformation associated with fluid activity has been observed in the present forearc slopes trenches and ancient onland outcrops in the japanese island arcs the nankai forearc represents a typical elastic dominated accretionary prism where the expulsion of pore fluids from sediments seems to have occurred intermittently through both channelized and diffusive mechanisms some of which appears to be pulsed mud diapirs occur within the trench near the toe and upper slope of the accretionary prism the widespread development of a gas hydrate phase boundary bottom simulating reflector bsr may indicate pervasive fluid advection throughout the prism the japan trench forearc is characterized by tectono gravity slope instability and collapse resulting in zones of fluid escape along normal faults no bsr is observed in the japan forearc despite the presence of organic rich sediments the izu bonin forearc apparently unlike the japan and nankai trench forearcs contains recently discovered serpentinite diapirs in which there are blocks of basic ultrabasic rocks together with blueschist fossil calyptogena beds in the pliocene basins of the izu collision zone provide good examples of fluid activity associated with venting in an ancient accretionary prism now exposed onland contrasting tectonic processes in the three forearcs may be explained by differences in the plumbing systems and hydrogeologic characteristics of the forearc basements dewatering sediments in the nankai trough prism impermeable and consolidated sediments in the japan trench prism and the very permeable pillow lava volcaniclastic complex of the izu bonin trench forearc

proterozoic multistage ca 1 1 and 0 8 ga extension recorded in the grand canyon supergroup and establishment of northwest and north trending tectonic grains in the southwestern united states the grand canyon supergroup records at least two distinct periods of intracratonic extension and sedimentation in the late mesoproterozoic and neoproterozoic new 40 ar 39 ar age determinations indicate that the mesoproterozoic unkar group was deposited between ca 1 2 and 1 1 ga basins in which the unkar group was deposited and the related northwest striking faults were created by northeast southwest extension which was contemporaneous with regional northwest southeast grenville contraction new u pb data indicate that the neoproterozoic chuar group was deposited between 800 and 742 ma sedimentary and tectonic studies show that chuar deposition took place during east west extension and resulting normal slip across the butte fault this event is interpreted to be an intracratonic response to the breakup of rodinia and initiation of the cordilleran rift margin laramide monoclines of the grand canyon region have north and northwest trends reactivate faults that originated at the time of unkar and chuar deposition and can be traced for great distances hundreds of kilometers from the grand canyon we use the distribution of monoclines in the southwest to infer the extent of proterozoic extensional fault systems the 1 1 ga northwest trending structures and ca 800 700 ma north trending extensional structures created regional fault networks that were tectonically inverted during formation of the ancestral rocky mountains and laramide contraction and reactivated during tertiary extension

magnetic anomalies in the shikoku basin a new interpretation kaiko surveys over the nankai trough made available new magnetic and structural data for the northern shikoku basin a survey of the oceanic lithosphere subducting below southwest japan along the central nankai trough revealed the existence of several north south basement troughs they are probably transform faults related to a north south spreading system we exanune the possibility of a late phase of north south spreading limited to the axial northernmost shikoku basin active between 14 and 12 ma if this system was already active before that time i e during the n55 opening of the southeastern basin then a triple junction should be found between both areas based on these data and previous studies we present a new interpretation of magnetic anomalies over the whole basin from early east west rifting to late north south spreading opening of the shikoku basin proceeded through multiple episodes of spreading the analysis of magnetic anomalies constrains the kinematic evolution of the basin through time and space two successive counter clockwise rotations of the spreading direction are postulated at anomaly 6 19 ma and at anomaly 5b 14 ma involving segmentation and rotation of the spreading ridge

criteria for discretization of seafloor bathymetry when using a stairstep approximation application to computation of t phase seismograms acoustic solutions for numerical models in which an overly coarse discretization of a stairstep boundary is employed to simulate smoothly varying bathymetry are degraded in a way that simulates scattering geometrical optics approximations are used to derive discretization criteria for simulating a smoothly sloping interface for the case of a source embedded in either an acoustic or an elastic seafloor and applied to modeling t phases a finite difference time domain modeling approach is used to synthesize t phases for both smoothly sloping and rough seafloor boundaries it is shown that scattering at a rough seafloor boundary yields ocean borne acoustic phases with velocities near those of observed t phase while smooth seafloor models yield t phases with slower horizontal velocities the long duration of the computed t phases for both the rough acoustic and elastic models is consistent with energy being scattered into the sound channel both as it transits the ocean crust boundary as well as at several subsequent seafloor reflections however comparison between the elastic and acoustic modeling solutions indicates that the t phase wavetrain duration decreases with decreasing impedence contrast between the ocean and seafloor

fluid flow in accretionary prisms evidence for focused time variable discharge accretionary prisms are wedges of saturated sediment that are subject to intense deformation as a result of lithosphere convergence compressive stress and rapid burial of the accreted deposits result in sediment compaction and mineral dehydration these latter processes in conjunction with fermentation or thermal maturation of entrained organic matter yield hydrocarbon bearing pore fluids that are expelled from the prism regional fluxes of heat and a number of dissolved chemical species most notably carbon are controlled by the advective expulsion of the pore waters numerical modeling observation and monitoring of flow patterns and rates and recent in situ hydrogeological tests quantify the conditions that control rates of fluid flow dispersed intergranular flow 10 8 to 10 11 m s controlled by the vertical permeability of the prism 10 14 to 10 20 m is limited by low permeability lithologies and seems not to vary much from margin to margin focused flow 10 1 to 10 8 m s above the decollement is controlled by fault zones or sedimentary intrusions diapiric structures at low fluid pressures fault zone permeability may be similar to that of adjacent wall rock but as fluid pressure increases from hydrostatic 0 to near lithostatic levels 1 0 fault zones dilate and fracture permeability increases by 2 4 orders of magnitude 10 10 to 10 16 m similarly mud volcanoes and diapirs provide high permeability fluid conduits to the sediment water interface as a result faults and intrusions become primary flow paths and support surface vents at which syntectonic deposits carbonate and gas hydrates accumulate and chemosynthetic organisms cluster models of thermal and chemical anomalies and epigenetic deposits indicate that flow is temporally variable that conclusion has been quantified by extended 1 10 months seafloor and borehole experiments that measured temperature anomalies associated with flow events on the cascadia prism flow estimated velocity 3 10 5 m s 950 m yr confined to a thrust fault that cuts upward through the prism has brought thermogenic hydrocarbons from depths 1 5 km to the surface within the last 400 years

shear strength of sediments approaching subduction in the nankai trough japan as constraints on forearc mechanics 1 the mechanical behavior of the plate boundary fault zone is of paramount importance in subduction zones because it controls megathrust earthquake nucleation and propagation as well as the structural style of the forearc in the nankai area along the nantroseize kumano drilling transect offshore sw japan a heterogeneous sedimentary sequence overlying the oceanic crust enters the subduction zone in order to predict how variations in lithology and thus mechanical properties affect the formation and evolution of the plate boundary fault we conducted laboratory tests measuring the shear strengths of sediments approaching the trench covering each major lithological sedimentary unit we observe that shear strength increases nonlinearly with depth such that the apparent coefficient of friction decreases in combination with a critical taper analysis the results imply that the plate boundary position is located on the main frontal thrust further landward the plate boundary is expected to step down into progressively lower stratigraphic units assisted by moderately elevated pore pressures as seismogenic depths are approached the decollement may further step down to lower volcaniclastic or pelagic strata but this requires specific overpressure conditions high taper angle and elevated strengths in the toe region may be local features restricted to the kumano transect

diffusion of helium in radiation damaged zircon abstract diffusion of helium was characterized in natural zircon that had been implanted with kr at a range of energies to simulate the effects of alpha recoil in natural radiation damage polished slabs of zircon oriented either normal or parallel to c were implanted with kr ions at a series of energies to produce a damaged region 1 m in thickness then implanted with 100 kev 3he at a dose of 1 1015 3he cm2 which placed the 3he within the region damaged by the kr ion implantation the implanted zircons were annealed isothermally in pt capsules in 1 atm furnaces at temperatures ranging from 51 to 252 c and times ranging from 15 min to 3 weeks 3he distributions following experiments were measured with nuclear reaction analysis using the reaction 3he d p 4he for diffusion in zircon normal to c we obtain the following arrhenius relation d 1 7 10 8 exp 71 2 kj mol 1 rt m2 sec 1 this activation energy for diffusion is about half of the value for he diffusion in zircon from the same sample locality but without kr implantation measured with an otherwise similar experimental and analytical approach cherniak et al 2009 he diffusivities in the kr implanted material are also significantly faster further he diffusivities normal and parallel to c in the kr implanted zircon are similar in contrast to the marked diffusional anisotropy with differences of two orders of magnitude for diffusion normal and parallel to c observed for he diffusion in this zircon in samples not implanted with kr these data indicate that heavily damaged zircon will have very limited retentivity of helium only those zircons larger than a few hundred m will have mean closure temperatures exceeding typical earth surface temperatures mean closure temperatures for he in highly damaged zircon will be about 200 c lower than those for relatively undamaged zircon at 200 c equivalent amounts of he will be lost from severely damaged zircon at a rate about a million times faster than from relatively undamaged zircon for retentivity of he comparable to that for low damage zircon at 200 c the severely damaged zircon would require residence temperatures of 10 c

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

statistical modelling of annealing kinetics of fission tracks in zircon reassessment of laboratory experiments abstract we remodel the annealing kinetics of fission tracks fts in zircon to fit data from 43 laboratory experiments in three earlier publications using spontaneous ft lengths in zircon from the nisatai dacite heating temperature and time are in the ranges of 350 912 c and 10 3 104 h respectively the data sets illustrate a transition in annealing process at a certain temperature zone in order to express the annealing behavior involving the possible transitional process with improved goodness of fit two types of models are constructed the hybrid linear model that consists of the fanning linear model at high temperature and the parallel linear model at low temperature connected with a transitional temperature zone and the parallel curvilinear model that approximates smoothly the transition in the annealing process the validity of the extrapolation of the model to geological time scale is confirmed by comparison with some deep borehole core data the length estimates by the hybrid linear model agree well with the borehole data whilst the estimates by the parallel curvilinear model show a significant disagreement with highly annealed borehole samples although this model provides good agreement with the experimental data for the hybrid linear model estimates of the partial annealing zone corresponding to isothermal heating for 106 and 107 years are 281 352 c and 262 330 c where the top and bottom of the partial annealing zone are defined by the mean length reduction ratio of 0 4 and 0 8 respectively

mesoproterozoic burial of the kaapvaal craton southern africa during rodinia supercontinent assembly from u th he thermochronology abstract we present the and zhe data for basement samples across the kaapvaal craton to investigate the cratonic burial and erosion history during the assembly and breakup of the rodinia supercontinent the and zhe dates vary from 1187 to 135 ma and 997 to 32 ma respectively both are negatively correlated with effective u concentration consistent with lower he retentivity at higher radiation damage dose the oldest the dates combined with volcanic rocks that document the basement was at the surface at 1 4 ga require heating to 130 c at 1 0 1 2 ga to cause complete he loss from the titanites we attribute this mesoproterozoic heating event to widespread burial of the craton during rodinia supercontinent assembly by sediments shed from the namaqua natal orogen on the craton s margin later neoproterozoic paleozoic erosion re exhumed the basement to the surface spatial variability in zhe data patterns can be explained by geologically reasonable differences in phanerozoic karoo basin burial and erosion the mesoproterozoic and karoo burial phases are analogous in that their detritus was derived from similarly located orogens during two different supercontinent cycles mesoproterozoic deposition across the kaapvaal craton may be part of a more global signal of burial and erosion during rodinia assembly and breakup our dataset illustrates the power of the and zhe thermochronology for detecting missing proterozoic depositional events in the rock record

influence of radiation damage on titanite he diffusion kinetics titanite is a common u th bearing accessory mineral that previous study suggested has a u th he closure temperature tc of 210 175 c although it has been recognized that radiation damage influences the he retentivity of apatite and zircon this effect has not been documented in titanite we acquired 51 the analyses from 11 archean basement samples across the kaapvaal craton south africa the the dates range from 20 to 1200 ma and are negatively correlated with their 10 970 ppm span of effective uranium concentration eu raman spectra acquired for 4 of these samples display peak broadening with increasing eu 4he diffusion experiments on the same 4 samples suggest tc values that vary by 175 c estimated alpha dose versus tc for our results and published the diffusion data document tc values of 150 210 c at alpha doses 50 1016 g with a sharp decrease in he retentivity above this threshold this damage diffusivity pattern is similar to zircon but the damage level at which the reduction in retentivity occurs appears to be lower for titanite because titanite typically has substantially lower eu than zircon for the same protracted thermal history the dates are likely to record higher temperature portions of the history than zircon u th he dates the results demonstrate that the dating can access a much larger temperature range and potentially be used to decipher more detailed time temperature paths than previously thought

effect of alpha damage annealing on apatite u th he thermochronology recent data on he diffusion challenge the temperature sensitivity of apatite u th he thermochronology the damage induced by recoil of u and th decay series during emission of particles recoil damage has been proposed to modify he diffusion properties through time however we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly cooled or reheated basement rocks to test this hypothesis we developed a quantitative model including an explicit treatment of recoil damage annealing and their effect on he diffusion kinetics and calibrate it against literature data our model is based on two hypotheses 1 helium is in equilibrium between an apatite crystal and its defects and 2 alpha recoil damage annealing can be described analogously to fission track annealing this model has been embedded into a monte carlo simulation of helium production ejection diffusion and applied to data from the french massif central a complex slowly cooled terrain with burial reheating where the thermal history has been constrained by previous fission track ft data including ft length distributions u th he ages are close to the ft ages from the same samples and are generally reproducible among replicates but some samples present he age dispersion that is not correlated with crystal size our model reproduces the massif central data very well except for three samples where he ages are older than corresponding ft ages we show that annealing of irradiation damage has an important impact on retentivity of helium and that the he content he is only a rough approximation of the damage level in particular our results show that independence of he ages on crystal sizes in case of reheated samples is a clear indication of the higher he retentivity induced by recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of u th he ages in such a case more generally a correlation with the crystal size can bring information on the thermal path only if the age of defects well represented by the fission track age is available due to the dependence of the partial retention zone on damages conversely in case of rapid cooling or for samples having low u and th contents damage effects can be ignored without significant effects on he ages

living microbial ecosystems within the active zone of catagenesis implications for feeding the deep biosphere earth s largest reactive carbon pool marine sedimentary organic matter becomes increasingly recalcitrant during burial making it almost inaccessible as a substrate for microorganisms and thereby limiting metabolic activity in the deep biosphere because elevated temperature acting over geological time leads to the massive thermal breakdown of the organic matter into volatiles including petroleum the question arises whether microorganisms can directly utilize these maturation products as a substrate while migrated thermogenic fluids are known to sustain microbial consortia in shallow sediments an in situ coupling of abiotic generation and microbial utilization has not been demonstrated here we show using a combination of basin modelling kinetic modelling geomicrobiology and biogeochemistry that microorganisms inhabit the active generation zone in the nankai trough offshore japan three sites from odp leg 190 have been evaluated namely 1173 1174 and 1177 drilled in nearly undeformed quaternary and tertiary sedimentary sequences seaward of the nankai trough itself paleotemperatures were reconstructed based on subsidence profiles compaction modelling present day heat flow downhole temperature measurements and organic maturity parameters today s heat flow distribution can be considered mainly conductive and is extremely high in places reaching 180 mw m2 the kinetic parameters describing total hydrocarbon generation determined by laboratory pyrolysis experiments were utilized by the model in order to predict the timing of generation in time and space the model predicts that the onset of present day generation lies between 300 and 500mbelow sea floor 5100 5300mbelow mean sea level depending on well location in the case of site 1174 5 10 conversion has taken place by a present day temperature of ca 85 c predictions were largely validated by on site hydrocarbon gas measurements viable organisms in the same depth range have been proven using 14c radiolabelled substrates for methanogenesis bacterial cell counts and intact phospholipids altogether these results point to an overlap of abiotic thermal degradation reactions going on in the same part of the sedimentary column as where a deep biosphere exists the organic matter preserved in nankai trough sediments is of the type that generates putative feedstocks for microbial activity namely oxygenated compounds and hydrocarbons furthermore the rates of thermal degradation calculated from the kinetic model closely resemble ratesof respiration and electron donor consumption independently measured in other deep biosphere environments we deduce that abiotically driven degradation reactions have provided substrates for microbial activity in deep sediments at this convergent continental margin

heat flow and fluid flow regime in the western nankai accretionary prism the thermal structure of the western nankai subduction zone has been studied through surface heat flow measurements with conventional geothermal probes and estimation of heat flow from depths of gas hydrate bottom simulating reflectors temperature and thermal conductivity measurements at odp site 808 located at the toe of the nankai accretionary prism added important information on any variation in heat flow with depth and thus on thermal effects of pore fluid flow in the prism surface heat flow is generally higher than 130 mw m 2 on the floor of the nankai trough such high heat flow cannot be explained simply by conductive cooling of the subducting shikoku basin lithosphere recent closely spaced probe measurements revealed that the heat flow on the trough floor shows local variation with particularly high values near the deformation front the heat flow distribution suggests that warm fluid flows seaward possibly along the decollement and locally rises toward the seafloor along some channels resulting in both the regional and local high heat flow anomalies contrary to this inference no active localized fluid flow was detected at site 808 the temperatures measured at six depths down to 347 m below seafloor mbsf just below the frontal thrust can be well explained by conductive heat flow of 125 130 mw m2 no geochemical anomaly associated with present day pore fluid movement was observed in cores recovered from the frontal thrust or the decollement on the other hand the high heat flow at site 808 requires some heat source below 350 mbsf there is a possibility that the fluid flow in the nankai accretionary prism is localized and transient and thus was not detected at this single drill site

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

hydrothermal circulation in subducting crust reduces subduction zone temperatures most thermal models of subduction zones assume no advection of heat by fluid flow because slow flow through underthrusting sediment the decollement and wedge likely transports only a minor amount of heat we model coupled fluid and heat transport in a subduction zone and show that hydrothermal circulation in subducting basaltic basement rocks can have a great influence on subduction zone temperatures fractured basaltic basement has permeability several orders of magnitude higher than a typical decollement allowing fluid circulation to redistribute and extract heat from a subduction zone we simulate systems with upper basaltic basement permeability ranging from 10 13 to 10 10 m2 in addition we incorporate the effect of permeability reduction within the basaltic basement as it is subducted the models with fluid transport show suppressed temperatures along the subducting slab relative to models with no fluid transport with continuous sediment cover heat is extracted from under the margin wedge to the trench in models where faulted ocean crust exposes high permeability basement to the ocean floor cooling from ocean bottom water results in highly suppressed heat flow relative to conductive models hydrothermally cooled ocean crust also acts to slow thermally controlled diagenetic reaction progress within subducting sediment

erosion and sedimentation as processes controlling the bsr derived heat flow on the eastern nankai margin in 2000 a 3d mcs survey was carried out on the eastern nankai trough a strong and widespread bottom simulating reflector bsr was observed and mapped after a preserved amplitude 3d prestack migration of the seismic data we use this bsr to calculate a heat flow map over the 3d survey area this map presents some high and low heat flow areas that cannot be correlated to known active wedge faults and previously observed fluid venting sites since the heat flow anomalies are not correlated to known major faults or fluid venting sites large fluid circulation is not likely to explain the varying bsr derived heat flow distribution in order to explain this heat flow distribution quantitatively we calculate the rates of erosion and sedimentation that are necessary to create these anomalies these rates are in good agreement with observations on seismic profiles and are locally consistent with a sedimentation rate calculated from slope basin depth and reflect the distribution of erosion landsliding and deposition on the margin slope we therefore propose that the observed bsr derived heat flow anomalies are mainly due to active erosion and sedimentation and that heat flow distribution on the margin section of the 3d survey is controlled by surface processes fluid migration could occur where a bsr is absent but is probably episodic or limited where a bsr is present and does not affect the bsr derived heat flow in this part of the margin these surface processes are the consequence of the tectonic and seismic activity of the wedge and are influenced by the subduction of a wide oceanic ridge below the eastern nankai margin

geochemistry of a sealed deep sea borehole on the cascadia margin the deep sea borehole seal cork was deployed for the first time on a modern accretionary prism during odp leg 146 to the cascadia margin ten months after the deployment the fluid flow and geochemistry of the borehole fluids was investigated during several dives by dsrv alvin the chemical analysis of the borehole fluids revealed methane concentrations of more than 3 5 mm whereas oxygen and dissolved ions as cl no3 or po4 are still close to the ambient seawater composition the exceedingly high methane content measured at the top of the sealed borehole and the observed degassing during the ascent of the submersible indicates that the sampled fluid was initially saturated or close to saturation with respect to ch4 the hydrocarbons are characterized by ratios of 170 200 and 13c values of 59 5 to 62 4 o which indicates a considerable admixture of thermogenic hydrocarbon gases the occurrence of methane of partly thermogenic origin demonstrates that ch4 enters the sealed borehole in the lower perforated section 94 178 mbsf and accumulates at the top of the borehole this suggests the occurrence of free gas within the encapsulated borehole considering the stability field of ch4 hydrates the formation of these ice like structures may take place and potentially results in a clogging of the top of the borehole such precipitates could result in a decoupling of the top of the borehole from the hydraulic and geochemical regime of the accretionary complex an important aspect for future plans of cork deployments

influence of crystal size on apatite u th he thermochronology an example from the bighorn mountains wyoming near surface tectonic and geomorphic processes involve cooling of rocks through low temperatures 50 200 c because rates of helium diffusion in apatite titanite and zircon are sensitive to temperature variations in this range uranium thorium helium thermochronometry u th he dating is well suited to establishing the timing and rates of these processes in the geologic record however because fractional loss of he is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic he u th he ages must vary not only with thermal history but also with crystal size here we present crystal size correlated he ages from co existing apatites from the bighorn mountains wyoming that range from 100 to 350 ma these correlations are a sensitive indicator of the rock s thermal history in a temperature range below the system s nominal closure temperature t c 70 c for apatite and are consistent with a thermal history involving residence in the upper 2 3 km of crust since the precambrian with maximum temperatures of 65 80 c just prior to laramide orogenic exhumation the influence of crystal size on he ages will be most apparent in rocks where temperatures have been in the range of partial he retention for long periods of time 30 70 c for 10 7 years in such cases accurate interpretation of u th he ages must incorporate the effect of crystal size and this method may provide insights to thermal histories of rocks in previously inaccessible low temperature ranges

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

fluid venting along japanese trenches tectonic context and thermal modeling abstract large benthic chemosynthetic communities have been observed at four main locations during the kaiko submersible dives in the japanese trenches they appear to be associated with venting along fractures the first site for our observation was along the japan and kuril trenches where the continental margin is eroded by the subducting plate and collapses into the trench the benthic communities there seem to be related to tension gashes parallel to the subduction vector the other communities were found on the toe of the nankai accretionary prism along the frontal thrust and tension gashes the temperature anomaly associated with one of the communities is modeled to constrain the upward flow of interstitial water as the anomaly has a small spatial extent and as the peak thermal gradient is high the best fitting model is to be found in a vertical upward flow at a velocity of 100 m yr in a cylindrical conduit leading out of an underlying shallow thrust

late cretaceous early eocene laramide uplift exhumation and basin subsidence in wyoming crustal responses to flat slab subduction low angle subduction of the farallon oceanic plate during the late cretaceous early eocene is generally considered as the main driver forming the high rocky mountains in wyoming and nearby areas how the deformation was transferred from mantle to upper crust over the great duration of deformation 40 myr is still debated here we reconstruct basin subsidence and compile paleoelevation thermochronology and provenance data to assess the timing magnitude and rates of rock uplift during the laramide deformation we reconstruct rock uplift as the sum of surface uplift and erosion constrained by combining paleoelevation and exhumation with regional stratigraphic thickness and chronostratigraphic information the amount and rate of rock uplift of individual laramide ranges was less than 2 4 4 8 km 0 21 0 32 mm yr during the early maastrichtian paleocene stage 1 and increased to more than 3 km 0 38 0 60 mm yr during the late paleocene early eocene stage 2 our quantitative constraints reveal a two stage development of the laramide deformation in wyoming and an increase of rock uplift during stage 2 associated with enhanced intermontane basin subsidence exhumation and uplift during stage 1 is consistent with eastward migration of cordilleran deformation associated with low angle subduction whereas the change in exhumation during stage 2 seems to follow a southwestward trend which requires an alternative explanation we here suggest that the increase of rock uplift rate during the late paleocene early eocene and the southwestward younging trend of uplift may be a response to the rollback and associated retreating delamination of the farallon oceanic slab

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

fission track thermochronology of the wind river range wyoming evidence for timing and magnitude of laramide exhumation fission track analysis of wind river range basement rocks provides information about the timing magnitude and cooling that occurred in response to laramide uplift eighty samples were collected from precambrian rock along six traverses in the northern central and southern parts of the range over 5 km of elevation apatite fission track ages and track length data suggest that the range did not have a uniform thermal history during late mesozoic and cenozoic time zircon fission track ages are precambrian implying that these rocks have not been at temperatures higher than 200 c 9 10 km burial depth since precambrian time apatite ages from the northeastern and southwestern flanks of the range suggesst that cooling may have initiated in the wind river range by about 85 ma but was definitely occurring by 75 ma data from the green river lakes area and fremont peak in the northern part of the range indicate that cooling was occurring by 62 ma and was most rapid between 60 and 57 ma apparent rates of exhumation vary from 94 m m y to 246 m m y in this part of the range the central part of the range records this same period of rapid cooling but structural and topographic relief are not as great ages from the southern part of the range suggest that uplift and erosion may have been initiated earlier there but continued at much slower rates 34 m m y over a longer period of time than in the north samples from a drill hole in the central wind river range have apatite ages ranging from 37 ma to 86 ma and suggest a significant cooling event at approximately 42 ma involving nearly 2 km of the rock column timing magnitudes and apparent rates of exhumation have varied spatially in the wind river range

on the use of stair steps to approximate bathymetry changes in ocean acoustic models stair step discretization criteria have been established for accurately representing smoothly varying bathymetry in numerical models it is shown that the strictest criterion applies to backscatter calculations where the horizontal stair step size must be a small fraction of an acoustic wavelength x 4 the forward scatter problem assuming that backscatter is weak can be accurately solved with an order of magnitude larger step sizes a coupled mode approach is used to illustrate solution convergence by computing backscattering from a single bottom facet as well as backscattering in the benchmark wedge used for checking ocean acoustic models in the 1987 benchmark session of the acoustical society of america asa published field solutions for the asa wedge are used to illustrate forward scatter results

heat flow in the rifted continental margin of the south china sea near taiwan and its tectonic implications abstract temperature measurements carried out on 9 hydrocarbon exploration boreholes together with bottom simulating reflectors bsrs from reflection seismic images are used in this study to derive geothermal gradients and heat flows in the northern margin of the south china sea near taiwan the method of horner plot is applied to obtain true formation temperatures from measured borehole temperatures which are disturbed by drilling processes sub seafloor depths of bsrs are used to calculate sub bottom temperatures using theoretical pressure temperature phase boundary that marks the base of gas hydrate stability zone our results show that the geothermal gradients and heat flows in the study area range from 28 to 128 c km and 40 to 159 mw m 2 respectively there is a marked difference in geothermal gradients and heat flow beneath the shelf and slope regions it is cooler beneath the shelf with an average geothermal gradient of 34 5 c km and 62 7 mw m 2 heat flow the continental slope shows a higher average geothermal gradient of 56 4 c km and 70 9 mw m 2 heat flow lower heat flow on the shelf is most likely caused by thicker sediments that have accumulated there compared to the sediment thickness beneath the slope in addition the continental crust is highly extended beneath the continental slope yielding higher heat flow in this region a half graben exists beneath the continental slope with a north dipping graben bounding fault a high heat flow anomaly coincides at the location of this graben bounding fault at the jiulong ridge indicating vigorous vertical fluid convection which may take place along this fault

a synthesis of heat flow determinations and thermal modeling along the nankai trough japan 1 we review marine heat flow data along the nankai trough and show that observations 30 km seaward of the deformation front are 20 below conductive predictions 129 94 mw m 2 but consistent with the global heat flow average for oceanic crust of the same age 16 28 ma heat flow values 30 km seaward of the deformation front are generally 20 higher than conductive predictions this heat flow pattern is consistent with the advection of heat by fluid flow in the subducting oceanic crust and explains both the high heat flux in the vicinity of the trench 200 and 140 mw m 2 and steep landward declines to values of approximately 60 mw m 2 over distances of 65 and 50 km along the muroto and kumano transects respectively along the ashizuri transect the lack of heat flow data precludes a definitive interpretation we conclude that fluid flow in the subducting oceanic crust leads to temperatures that are generally 25 c higher near the toe of the margin wedge and 50 100 c lower near the downdip limit of the seismogenic zone than estimated by purely conductive models

combining 2d basin and structural modelling to constrain heat transport along the muroto transect nankai trough japan abstract the nankai trough located southeast of shikoku island japan exhibits a zone of exceptionally high heat flow in the central part of the nankai trough the fossil spreading centre of the shikoku basin is subducted beneath the southwest japan arc we have modelled the temperature and maturation history along the muroto transect reaching from the tip of the thrust zone out into nearly undeformed quaternary and tertiary sediments seawards of nankai trough we used two balanced cross sections defining the sections before and after overthrusting as input for 2d basin modelling we can show that rapid burial and overthrusting during the quaternary in combination with a heat flow history following the cooling curve of a 15 ma old oceanic plate is not sufficient to explain the measured maturity of organic material in the sediments several heat flow scenarios derived from theoretical concepts yamano m kinoshita m goto s matsubayashi o 2003 extremely high heat flow anomaly in the middle part of the nankai trough physics and chemistry of the earth parts a b c 28 487 497 and previous modelling approaches e g brown k m saffer d m bekins b a 2001 smectite diagenesis pore water freshening and fluid flow at the toe of the nankai wedge earth and planetary science letters 194 97 109 spinelli g a underwood m b 2005 modeling thermal history of subducting crust in nankai trough constraints from in situ sediment temperature and diagenetic reaction progress geophysical research letters 32 l09301 doi 10 1029 2005gl022793 steurer j underwood m b 2003 clay mineralogy of mudstones from the nankai trough reference sites 1173 and 1177 and frontal accretionary prism site 1174 in h mikada et al eds pp 1 37 available from were tested the best match between observed maturity levels temperature and heat flow measurements is reached for a heat flow history which initially assumes the cooling of a 15 ma old oceanic lithosphere but is reheated to 170 180 mw m 2 during the phase of rapid burial in the quaternary this can be achieved either by assuming the onset of hydrothermal circulation in the cooling crust or by reheating caused by off axis volcanism at about 6 ma yamano m kinoshita m goto s matsubayashi o 2003 extremely high heat flow anomaly in the middle part of the nankai trough physics and chemistry of the earth parts a b c 28 487 497

sediment compaction and fluid migration in the makran accretionary prism the makran continental margin in the gulf of oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland a recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge as seen in previous sonobuoy wide angle data accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment a prominent bottom simulating reflector appears 500 800 m beneath the sea bed several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile the hydrate reflector appears to shallow in the region of some faults suggesting a temperature anomaly due to the presence of warm pore fluids a heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly the inferred presence of overpressured pore fluids in the makran suggests that accreted sediments have a low permeability the seismic evidence suggests a two stage compaction process with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones

contrasting geothermal regimes of the barbados ridge accretionary complex three e w transects of closely spaced heat flow measurements across the barbados accretionary complex are reported one transect about 100 km in length at 14 20 n and a shorter 15 km line at 14 35 n show that heat flow decreases arcward roughly in proportion to the thickening of the wedge over the actively accreting zone this decrease is due to the combined thermal effects of the underthrusting oceanic lithosphere and thickening of sediments that have been ingested into the wedge except for anomalously high heat flow over a mud volcano on the eastern end of the 100 km transect fluid expulsion appears to have only a secondary effect on the seafloor heat flow in this region an increase of heat flow is observed at the western end of the 100 km transect where high angle arcward vergent thrusting is occurring we attribute this increase primarily to a decrease in the arcward thickening of wedge material and heat generated by friction on the decollement in strong contrast a short line of stations at 15 30 n the deep sea drilling project ocean drilling project dsdp odp drilling transect found anomalously high heat flow a 40 increase over the heat flux from the lithosphere with large variability over the most recently accreted material and also in the adjacent basin the thermal regime in the vicinity of the dsdp odp transect is dominated by a strong flow of pore fluids along the decollement and upward flow along fault planes in the toe of the wedge and adjacent basin sediments heat transport by pore fluids also appears to dominate the subseafloor thermal regime in a zone near 13 50 n the thermal effects of dewatering of the sediments by upward diffusive flow were examined using a simple one dimensional model which showed that the resulting effects are too small to produce detectable heat flow anomalies consequently the flow of fluids must be channelized in subseafloor conduits to produce the observed anomalies in heat flow

midcontinent u s fault and fold zones a legacy of proterozoic intracratonic extensional tectonism the u s continental interior midcontinent contains numerous fault and fold zones seismic and drilling data indicate that some of these zones first formed as proterozoic eocambrian rift faults but the origin of most remains enigmatic we propose that the enigmatic fault and fold zones also began as proterozoic eocambrian normal faults we base our hypothesis on the following 1 enigmatic zones parallel known rifts 2 the structural style of enigmatic zones mirrors the structural style of known rifts 3 the map pattern of some enigmatic zones e g the la salle deformation belt of illinois resembles the map pattern of contemporary rifts and 4 it is easier to rupture an intact craton by normal faulting than by reverse or strike slip faulting these zones along with known rifts represent the legacy of widespread extensional tectonism that brittlely broke up the craton into fault bounded blocks prior to deposition of phanerozoic platform cover once formed midcontinent fault and fold zones remained weak allowing cratonic blocks to jostle relative to one another during the phanerozoic thereby inverting faults and creating transpressional or transtensional structural assemblages localizing seismicity and channeling or releasing ore generating fluids

heat flow tectonics and fluid circulation at the toe of the barbados ridge accretionary prism detailed patterns of near surface tectonics and heat flow were investigated at two locations at the toe of the barbados ridge accretionary prism at the two locations in the thickly sedimented southern portion of the prism near 12 20 n and in the northern area drilled during ocean drilling program odp leg 110 near 15 32 n relative highs of heat flow were associated with zones of thrusting we attribute them to upward advection of water rising from the decollement along faults through the sediments in the odp area a significant heating plate effect from warm fluids flowing along the decollement and thrusts is required to account for the high regional heat flow this effect implies high rates of fluid flow at the decollement which can only be non steady state or would require massive lateral transport of fluids along strike

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

post laramide oligocene uplift in the wind river range wyoming in late oligocene time the crest of the wind river range was uplifted from an erosion surface that formed during laramide late cretaceous early eocene deformation this uplift created the highest peaks in the wyoming foreland and is indicated by rejuvenation of the range as a sediment source and by the physiography of the peaks differential movement of basement blocks in the core of the range is identified by apatite fission track data we distinguish this deformation from earlier laramide compression and neogene normal faulting and speculate that it occurred when precambrian shear zones were reactivated in response to local flexural stress in the laramide subsidence uplift couple

transient fluid flow through the toe of the barbados accretionary complex constraints from ocean drilling program leg 110 heat row studies and simple models thirty four sediment and mud line temperatures were collected from six drill holes on ocean drilling program odp leg 110 near the toe of the barbados accretionary complex when combined with thermal conductivity measurements from sediment cores and results from earlier surveys these data delineate the complicated thermal structure on the edge of this convergent margin heat flow values at the seafloor of 92 192 mw m sup 2 are 80 300 higher than those predicted by standard heat flow versus age models for oceanic crust but are compatible with earlier seafloor measurements made in this area at the same latitude heat flow tends to decrease downhole at four sites suggesting the presence of heat sources within the sediments these results are consistent with the flow of warm fluids through the complex along high permeability conduits including thrust faults the major decollement zone and sandy intervals simple calculations suggest that this fluid flow is transient occurring on time scales of tens of thousands of years fluid flow velocities along the decollement zone estimated with a simple thermal model are about 10 sup minus 7 m s 100 times faster than predicted by numerical steady state analyses the estimated maximum sediment permeability within the decollement zone which was basedmore on the above fluid flow velocity is about 10 sup minus 12 m sup 2 also 100 times higher that that calculated numerically high heat flow in the vicinity of 15 degree 30 prime n and not elsewhere along the deformation front suggests that the leg 110 drill sites may be situated over a prism water discharge zone with dewatering more active here than elsewhere along the accretionary complex less

melange forming processes in the development of an accretionary prism evidence from fission track thermochronology to give thermotectonic constraints on melange forming processes and to understand the significance of melange in the evolution of accretionary complexes fission track analysis was carried out on samples collected from sandstone blocks in melange matrix and coherent sandstone sequences adjacent to melange units in the cretaceous to miocene shimanto accretionary complex southwest japan which is one of the typical accretionary complexes with an accumulation of geological information nineteen zircon ages and 15 track length distributions from melanges indicate with some local variation that materials with higher maximum paleotemperatures are exposed at the rear part of the prism as is the case of the coherent units two apatite ages are contemporaneous with previously reported apatite ages of 10 ma obtained from coherent units in the region emphasizing the regional cooling through the apatite closure temperature of 100 c at that time there is no evidence for substantial relative motion of the melanges with respect to the surrounding coherent units melange formation is explained by processes that exclude large vertical transport i e 5 km of melange forming materials these thermotectonic characteristics observed in the shimanto accretionary complex are maintained during subsequent local internal deformation caused by the successive growth of the wedge

a coupled mode solution for acoustic propagation in a waveguide with stepwise depth variations of a penetrable bottom a coupled mode solution is formulated for the problem of acoustic propagation in a cylindrically symmetric ocean divided in range into a finite number of adjoining pekeris waveguides of differing water depths attenuation is included in the bottom and the problem is discretized by assigning a pressure release boundary condition at a depth which is sufficiently far removed to prevent significant energy from returning to the water this formulation includes backscatter from the depth variations of the water column and full coupling between a finite number of modes propagating in the water column and in the bottom numerical results based on an implementation of this solution are presented

deep sea bottom simulating reflectors calibration of the base of the hydrate stability field as used for heat flow estimates ocean drilling program and deep sea drilling project downhole data from three areas the southwestern japan nankai margin the continental slope off peru and the blake bahama outer ridge provide temperature calibrations for bottom simulating reflectors bsr that mark the base of a clathrate hydrate stability field the inferred temperatures at bsrs provide an important reference for the mapping of geothermal gradient and heat flow from subduction zone accretionary sedimentary wedges the borehole results provide information on which stability field is applicable for the bsrs and thus calibrate the heat flow estimates while an ideal calibration has not been possible the bsr temperatures at the three sites in the temperature range 25 27 c have been estimated with uncertainties of 0 7 to 2 0 c the temperatures correspond closely to the laboratory dissociation temperatures for pure water pure methane hydrate at equivalent pressures no laboratory data are available for seawater salinity and methane at equivalent pressures but extrapolation from lower pressures gives temperatures 1 2 c lower which is just significantly different the data also could be explained by the stability curve for seawater salinity and methane with about 7 co2 or with a small amount of higher hydrocarbons but most hydrate samples that have been recovered by deep sea drilling have contained almost pure methane the uncertainties in the temperature at the bsr should contribute no more than 5 error in heat flow estimates from bsr depths if the pure water methane stability field is used

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

methane hydrate accumulation model for the central nankai accretionary prism abstract a two dimensional transient model of methane hydrate accumulation is designed and applied in the nankai accretionary prism model results show that the formation and distribution of methane hydrate and free gas is related to value of the thermal flux the fluid flux the methane flux supplied from below and methane source within the sediment the duration of the methane flux is of significant importance the older the sediments implying a longer period of methane flux supply from below and within the sediments of the model the larger the volume percentage of free gas will be this can explain the occurrence of bsr in a area with lower methane flux observed but absence of the bsr in the other area with higher methane flux observed modeling also predicts that the maximum saturation of methane hydrate is about 1 of the pore space in the area of odp site 1178 and the methane hydrate occurrence zone is probably 90 200 m below the sea floor mbsf which is supported by both direct and indirect evidence for hydrate in the area near odp site 808 the fault zones having high fluid flow and methane flux may possibly result in methane hydrate formation without bsr occurrence

implications of basement rock alteration in the nankai trough japan for subduction megathrust slip behavior abstract the nankai trough is an exceptionally well studied convergent margin known to host damaging megathrust earthquakes as well as various forms of slow fault slip outcrop studies of exhumed analogues for the modern subduction thrust as well as seismic reflection images of the active subduction zone suggest that the basaltic basement participates in plate boundary shearing at depths of a few km and greater we obtained altered basaltic upper basement from the modern nankai trough seaward of the trench recovered by drilling on iodp expedition 333 we performed laboratory friction experiments on bare surfaces and gouge powders of this material in addition to and in combination with other materials for comparison the altered basalt exhibits predominantly velocity strengthening frictional behavior indicating a tendency for stable slip for bare surface experiments few instances of velocity weakening friction occur and are restricted to slip rates of

thick skinned deformation of the archean wyoming province during proterozoic arc continent collision two major mylonite zones are exposed in the southeastern archean wyoming province the cheyenne belt which marks the boundary between the wyoming and colorado provinces and the laramie peak shear zone across which archean rocks have been uplifted differentially at least 10 km between the two mylonite zones is a 60 100 km wide belt of archean migmatitic gneiss which has been intruded by diabase dikes and peridotites and contains enclaves of high grade metasedimentary rocks we interpret this belt of high grade rocks as a tectonically reactivated block of archean crust uplifted during the development of the cheyenne belt collisional zone at ca 1 8 ga this block does not exhibit a decreasing metamorphic gradient away from the mylonite zone but is at uniformly high grade thus rather than a thick skinned thrust ramp model for uplift we invoke vertical uplift along a high angle reverse fault without rotation this style of uplift may be a tectonic response to transpression underplating or tectonic interfingering at depth during collisional orogeny and it may be a more common feature of basement uplifts within foreland fold and thrust belts than has been previously recognized

fault slip and exhumation history of the willard thrust sheet sevier fold thrust belt utah relations to wedge propagation hinterland uplift and foreland basin sedimentation

deciphering a 2 gyr long thermal history from a multichronometer u th he study of the phalaborwa carbonatite kaapvaal craton south africa