late cenozoic structure and tectonics of the northern mojave desert in the fort irwin region of the northern mojave desert late cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral thrust slip on northwest striking moderately dipping segments at the east ends of the blocks crustal blocks 7 10 km wide by 50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block faulting initiated after 11 ma and quaternary deposits are faulted and folded we document a minimum of 13 km cumulative sinistral offset in a north south transect from south of the bicycle lake fault to north of the drinkwater lake fault paleomagnetic results from 50 sites reveal two direction groups in early and middle miocene rocks the north to northwest declinations of the first group are close to the middle miocene reference pole however rock magnetic studies suggest that both primary and remagnetized directions are present in this group the northeast declinations of the second group are interpreted as primary and 63 5 7 6 clockwise from the reference pole and suggest net post middle miocene clockwise rotation of several of the east trending blocks in the northeast mojave domain the jurassic independence dike swarm in fort irwin may be rotated 25 80 clockwise relative to the swarm north of the garlock fault thus supporting the inference of clockwise rotation using a simple shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks we predict 23 clockwise rotation using the observed fault slip or one third that inferred from the paleomagnetic results the discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks where most of our paleomagnetic sites are located however at least 25 40 of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible rigid body rotation of the region evidenced by clockwise bending of northwest striking domain bounding faults our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the eastern california shear zone over the last 10 m y occurred within the northeast mojave domain the remainder must be accommodated in adjacent structural domains e g east of the avawatz mountains and west of the goldstone lake fault supporting appendices 1 and 2 are available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

geology of panamint valley saline valley pull apart system california palinspastic evidence for low angle geometry of a neogene range bounding fault geological studies support the interpretation that northern panamint valley and saline valley southeast california form paired pull apart basins on opposite sides of the right slip hunter mountain fault zone eight to ten km of late cenozoic net slip can be established on the hunter mountain fault zone palinspastic reconstruction of northern panamint valley indicates that the valley was formed by movement on a shallow crustal low angle normal fault of 0 15 degree west dip during the last 3 0 ma this interpretation appears to contradict the notions that little extension is accomodated in the uppermost crust by low angle faulting and that the most recent extension in the basin and range province is accomodated exclusively by high angle faulting saline valley however is interpreted to have formed by extension on closely spaced rotated planar normal faults thus within one geometric system of paired pull apart basins extension appears to have been accommodated in the shallow crust in two different ways

a newly recognized type of slickenside striation abstract this note summarizes current views on the origin of slickenside striations and points to the existence of a type of striation not readily explained by established genetic ideas such striations have been seen on fault surfaces in naturally deformed shale and in experimentally deformed paraffin wax they are defined by arrays of shallow slip parallel ridges and grooves of millimetric width and below on any given slip surface the ridges are as common as the grooves and display similar dimensions and profile shapes across any pair of slip surfaces in contact at the end of faulting ridges protruding from one surface fit into or nest with grooves on the other surface the origin of these features is not yet understood

elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the nankai trough subduction megathrust 1 recent seismic reflection and ocean bottom seismometer obs studies reveal broad regions of low seismic velocity along the nankai subduction plate boundary megathrust offshore sw japan these low velocity zones lvz s extend 55 km landward from the trench corresponding to depths of 10 km below sea floor here we estimate the in situ pore pressure and stress state within these lvz s by combining p wave velocities obtained from the geophysical surveys with new well constrained empirical relations between p wave velocity porosity and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment we document excess pore pressures of 17 87 mpa that extend 55 km into the subduction zone indicating that trapped pore fluids support 45 91 of the overburden stress along the base of the upper plate and surrounding major fault zones the resulting effective stresses in the lvz are limited to 1 3 of the values expected for non overpressured conditions these low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary the region of lowest effective stress coincides with precisely located very low frequency earthquakes providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure

stress induced smectite dehydration ramifications for patterns of freshening and fluid expulsion in the n barbados accretionary wedge abstract presented is new experimental evidence that smectites can partially dehydrate from 18 5 a to 15 4 a hydrates when they are subjected to effective mineral framework stresses above 1 3 0 3 mpa we show here that this process can substantially freshen saline pore fluids and impact our understanding of the hydrogeologic processes in convergent margins where smectites are abundant the general anomalous freshening of pore fluids in the n barbados accretionary wedge was previously proposed to relate to the widespread lateral expulsion of fluids derived from mineral dehydration reactions at great depth in contrast our analysis suggests that a significant part of the widespread anomalous freshening relates to stress induced smectite dehydration that occurred during the ocean drilling program odp pore water extraction process clear evidence for long distance lateral flow appears to be limited to localized chloride and isotopic anomalies developed in a 20 m thick locally mineralized region at the top of decollement horizon at site 948 our reinterpretation is consistent with the fluid flow in the decollement zone being 1 highly transient on time scales of several hundreds of years and 2 channeled laterally into heterogeneous finger like flow paths that are apparently not intersected and or sampled by drilling in most regions at the toe of the wedge the incipient stress induced dehydration of smectite may also be buffering in situ fluid pressures in the wedge maintaining the effective stresses at 1 3 mpa

the stratigraphic evolution of the el paso basin southern california implications for the miocene development of the garlock fault and uplift of the sierra nevada the ricardo group is a 1 700 m thick sequence of miocene volcanic rocks and continental sedimentary rocks deposited between 19 and 7 ma in the el paso basin near the junction of the garlock fault and the sierra nevada frontal fault in southern california the combination of stratigraphic and structural data from the ricardo group with chronologic and rotational histories derived from magnetostratigraphic and radiometric studies provides some new constraints for the morphotectonic emergence of the sierra nevada the initiation of strike slip movement on the garlock fault and related east west extension in the southern basin and range region the miocene sequence documents 1 volcanism and north south tension without net extension at 18 15 ma 2 relative uplift about 15 13 5 ma 3 the beginning of sinistral slip on the garlock fault and east west extension in the southwestern basin and range region about 10 9 ma and 4 emergence of the sierra nevada as a source area and topographic high by 8 ma stratigraphic evidence from the ricardo group does not indicate significant miocene dip slip movement along the large scale down to the north fault that has been previously hypothesized to have preceded the present garlock fault in the same location during the early tertiary the evolution of this sequence however can be explained by tectonic models which propose a time space linkage between basin evolution east west extension in southern california and the mendocino triple junction system

a self consistent mechanism for slow dynamic deformation and tsunami generation for earthquakes in the shallow subduction zone 1 dynamic pore pressure changes in the overriding wedge above a shallow dipping plate interface significantly affect the rupture dynamics of shallow subduction zone earthquakes and their tsunamigenesis for a wedge on the verge of coulomb failure everywhere including the basal fault the dynamic pore pressure increase due to up dip rupture propagation leads to widespread yielding within the wedge which is greatly enhanced by the shallow dip of the fault the widespread yielding reduces the stress drop slip velocity slip and rupture velocity giving rise to prolonged rupture duration thus explaining many anomalous features of shallow subduction zone earthquakes significant inelastic seafloor uplift occurs in the case of a shallow fault dip with the largest uplift located landward from the trench integrating this physical mechanism with existing seismic geodetic and tsunami observations can provide new insights into earthquake dynamics and deformation processes in shallow subduction zones

modern and ancient seismogenic out of sequence thrusts in the nankai accretionary prism comparison of laboratory derived physical properties and seismic reflection data 1 to investigate characteristics of a seismogenic out ofsequence thrust oost imaged as a strong reflection on seismic profiles in the nankai accretionary prism we determined acoustic properties of discrete samples from an fossil nobeoka oost outcrop under confining pressures and compared the acoustic properties with those of an active oost in the nankai accretionary prism we observed anisotropy of velocity and attenuation in the hanging wall of nobeoka oost attributed to foliation of pelitic phyllite in contrast the footwall is composed of brittlely deformed chaotic shales and fine sandstones and velocities in the footwall are lower than those in the hanging wall amplitude variation with offset avo modeling utilizing contrasts in p and s wave velocities and densities between the hanging wall and footwall of the nobeoka oost indicates that fractures filled with overpressured fluid likely account for angle dependent reflection amplitudes of the active oost in the nankai trough citation tsuji t g kimura s okamoto f kono h mochinaga t saeki and h tokuyama 2006 modern and ancient seismogenic out ofsequence thrusts in the nankai accretionary prism comparison of laboratory derived physical properties and seismic reflection data geophys res lett 33 l18309 doi 10 1029 2006gl027025

evidence for a long lived accommodation transfer zone beneath the snake river plain a possible influence on neogene magmatism geochronologic data compiled from 12 metamorphic core complexes and their flanking regions outline important differences in tectonic and magmatic histories north and south of the snake river plain yellowstone province srp y magmatism crustal flow metamorphism and extensional exhumation of core complexes north of the srp occurred mostly between 55 and 42 ma as compared to 42 25 ma south of the srp with final exhumation of the southern complexes occurring only during younger miocene 20 0 ma basin and range faulting these significant differences in the timing of events suggest that the now lava covered srp which is at a high angle to cordilleran trends may have at times operated as a steep shear or transfer zone accommodating difference in strain to the north and south following previous suggestions we infer that this proposed accommodation or transfer zone developed above an important lithospheric boundary localized above a tear in the subducting slab shallower slab angle to the south used to explain both the locus of late cretaceous paleocene magmatism and the different ages and mechanisms of slab reconfiguration and removal north and south of the srp during the cenozoic the details of these different histories help outline the complex evolution of this zone and also suggest that this zone of lithospheric weakness may have subsequently focused miocene srp y hot spot magmatism

seismic velocities from the barbados ridge complex indicators of high pore fluid pressures in an accretionary complex three wide aperture 0 5 km offset seismic profiles were shot perpendicular to the strike of the barbados ridge accretionary complex at 16 12 n line 465 14 23 n line 480 and 13 20 n line 484 to image the deep structure of the accretionary complex and obtain seismic velocity data velocity analyses on full fold common depth point cdp gathers were used to provide velocity depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex the sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity the velocity at the base of the wedge along line 480 is 2 4 km s at the toe the velocity at the wedge base at 60 km from the toe where the wedge thickness has increased from 1 8 km to 5 0 km has increased to greater than 3 0 km s however the velocities within the leading margins and within undeithrust sequences in each of the three sections are low as much as 1 0 km s less than velocities found in undeformed sedimentary sequences of equivalent thickness the low velocity regions are interpreted as sediments with high porosity where porosity is inferred to be high pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex the correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments velocities in the underthrust sequences where the differences between loading histories of wedge sediments and the underthrust sequences are largest are sufficiently low to produce velocity inversions low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge west of the wedge in the forearc basin along line 480 and near a seismically imaged mud diapir velocities at a depth of 3 5 km below the seafloor are 0 5 km s less than what is expected in a more normal sedimentary section immediately seaward of the complex the deepest undeformed sequences have a velocity anomaly of 0 5 km s velocities remain low for at least 12 km seaward of the complex but they become closer to a more normal sediment section farther from the wedge

compositional data analysis in the geosciences from theory to practice since karl pearson wrote his paper on spurious correlation in 1897 a lot has been said about the statistical analysis of compositional data mainly by geologists such as felix chayes the solution appeared in the 1980s when john aitchison proposed to use iogratios since then the approach has seen a great expansion mainly building on the idea of the natural geometry of the sample space statistics is expected to give sense to our perception of the natural scale of the data and this is made possible for compositional data using iogratios this publication will be a milestone in this process this book will be of interest to geologists using statistical methods it includes the intuitive justification of the methodology convincing through case studies and presenting user friendly software which includes a section for those who need to see the proof of the mathematical consistency of the methods used

the frictional hydrologic metamorphic and thermal habitat of shallow slow earthquakes slow earthquakes have recently been discovered on the shallow parts of some subduction zones a review of the conditions under which these quakes form reveals that they could be common in most subduction zones globally

geology and magmatic hydrothermal evolution of the giant pebble porphyry copper gold molybdenum deposit southwest alaska the pebble deposit is located 320 km southwest of anchorage alaska it is one of the largest porphyry deposits known with a total resource of 10 78 billion tons bt it comprises the east and west zones which are approximately equal in size with slightly lower grade mineralization in the center of the deposit where the peripheries of the two zones merge the west zone was discovered by cominco america in 1989 and the east zone was discovered by northern dynasty minerals in 2005 the oldest rock in the pebble district is the jurassic cretaceous kahiltna flysch unit which contains basinal turbidites interbedded basalt flows and associated gabbro intrusions these rocks were intruded between 99 and 96 ma by coeval granodiorite and diorite sills followed shortly thereafter by alkalic monzonite intrusions and related breccias subalkalic hornblende granodiorite porphyry plutons of the kaskanak batholith were emplaced at 90 ma similar smaller granodiorite plutons were emplaced around the margins of the batholith and are related to cu au mo mineralization re os dates on molybdenite are between 89 7 and 90 4 ma a late cretaceous volcanic and sedimentary cover sequence completely conceals the east zone whereas the west zone is overlain only by glacial sediments and is exposed in one small outcrop eocene volcanic rocks and subvolcanic intrusions occur east and southeast of the pebble deposit and unconsolidated glacial sediments are widespread the east and west zones represent two coeval hydrothermal centers within a single system the west zone extends from surface to 500 m depth and is centered on four small granodiorite plutons emplaced into flysch diorite and granodiorite sills and alkalic intrusions and breccias the much higher grade east zone extends to at least 1 700 m depth and is hosted by a larger granodiorite pluton and adjacent granodiorite sills and flysch the granodiorite plutons merge with depth on the east side of the deposit high grade mineralization has been dropped 600 to 900 m into the ne trending east graben where the deposit remains undelineated to the east and to depth variations in hypogene grade and metal ratios reflect multiple stages of metal introduction and redistribution hornfels related to the kaskanak batholith formed prior to hydrothermal activity at pebble and is most intensely developed in flysch disseminated and vein hosted cu au mo mineralization dominated by chalcopyrite and locally accompanied by bornite formed with potassic alteration in the shallow part of the east zone and with approximately coeval sodic potassic alteration in the west zone and at depth in the east zone illite kaolinite alteration overprinted potassic and sodic potassic alteration throughout the deposit and variably redistributed copper and gold high grade copper gold mineralization is related to advanced argillic alteration controlled by a synhydrothermal brittle ductile fault zone which overprinted potassic sodic potassic and illite kaolinite alteration in the east zone advanced argillic alteration comprises a core of pyrophyllite alteration associated with chalcopyrite bounded to the west by an upward flaring zone of sericite alteration which contains hypogene bornite digenite covellite and trace enargite and tennantite late quartz veins introduced additional molybdenum in several parts of the deposit grade destructive quartz sericite pyrite alteration forms a halo to the entire deposit and yields outward to propylitic alteration a quartz illite pyrite cap is preserved in the weakly mineralized center of the deposit mineralization at pebble is predominately hypogene a thin incompletely developed zone of supergene mineralization occurs in the west zone and is overlain by a thin leached capping there is no evidence for paleosupergene mineralization or leaching below the cover sequence in the east zone molybdenite contains high concentrations of rhenium throughout the deposit elevated palladium concentrations are associated with pyrophyllite alteration in the east zone the pebble deposit occurs in one of a number of large deep seated magnetic anomalies which are located at the intersection of crustal scale structures both parallel and at high angles to a mid cretaceous magmatic arc this setting is similar to fertile porphyry environments in northern chile and suggests that southwestern alaska is highly prospective for porphyry exploration the large size and high hypogene grades of the pebble deposit may reflect a combination of multiple stages of metal introduction with vertically restricted lateral fluid flow induced by hornfels aquitards in flysch

modern and paleofluid pathways revealed by cu isotope compositions in surface waters and ores of the pebble porphyry cu au mo deposit alaska copper isotope ratios measured in minerals and shallow groundwater and surface waters provide insight into high temperature mineralization and active weathering processes at the pebble porphyry cu au mo deposit alaska the west zone of the deposit contains hypogene mineralization with a supergene overprint and a thin oxide leached capping whereas the contiguous east zone contains only hypogene mineralization sulfide rich rock powders and mineral separates have 65cu values that range from 0 78 to 2 28 hypogene west 0 02 to 1 55 hypogene east 3 49 to 1 88 oxide west and 5 04 to 1 27 supergene west the results from hypogene samples show that there is a systematic increase in 65cu values from deeper to shallower portions of the deposit furthermore the 65cu values correlate with silicate alteration assemblages mostly positive values correspond to quartz illite pyrite sericite and quartz pyrophyllite alteration zones which formed at relatively lower temperatures whereas negative values characterize the higher temperature potassic and sodic potassic domains this empirical evidence could indicate that fractionation of cu isotopes during hypogene alteration is controlled by ph and or temperature variations shallow surface waters proximal to the deposit and which likely interacted with underlying concealed mineralization have heavy 65cu values which contrast with lighter values in waters distal from the deposit patterns measured in the copper isotope ratios of both solids and surface waters demonstrate the potential use of copper isotope distribution as a vectoring tool in mineral exploration and aid in understanding the sources of copper in the surface and near surface environments

nature of the southwestern boundary of the central mojave tertiary province rodman mountains california tertiary volcanic and sedimentary rocks in the central mojave desert are largely confined to the barstow bristol trough an elongate tectonic depression filled with lower miocene strata the southwestern boundary of the trough is sharply defined and separates thick 2 km tertiary sequences from an area to the southwest that is virtually devoid of tertiary rocks segments of this boundary have been interpreted as a buttress unconformity as a right slip sidewall fault that formed the lateral boundary of an early miocene extensional terrane and as a dip slip fault caused by flexure of the north american plate above the subducted mendocino fracture zone the silver bell fault in the rodman mountains central mojave desert is the best exposed segment of the boundary and relations exposed along it can be used to test these hypotheses the tertiary sequence north of the silver bell fault is at least 2 3 km thick and comprises at least 1 9 km of mafic lava flows overlain by and intercalated with coarse clastic rocks derived from the south side of the fault stratigraphic relations require faulting during volcanism and indicate that the minimum throw across the fault is at least equal to the thickness of the exposed tertiary section tuff layers intercalated in the lower part of the clastic deposits are 23 24 m y old indicating that faulting and volcanism were contemporaneous with extension and plutonism in the nearby central mojave metamorphic core complex structural and stratigraphic data indicate that the silver bell fault is primarily a dip slip fault the tertiary section is folded into a west trending upright to overturned syncline located near the southern edge of exposed tertiary rocks the syncline records late cenozoic north south shortening along the northwest striking calico fault which in this area accommodated about 10 km of right slip and perhaps 1 km of reverse slip southwestward dipping strata in the areally extensive north limb of the syncline resemble tilted strata in nearby highly extended areas and have been interpreted to reflect crustal extension however bedding dip does not decrease upward in the tertiary section paleocurrent directions trend obliquely updip and northeast dipping normal faults are rare stratal tilting postdated the accumulation of the exposed section which therefore was not deposited during rotational normal faulting the new data contradict interpretation of the silver bell fault as a major right slip fault bounding the central mojave extensional terrane although they permit small magnitude extension across the fault the amount of early miocene crustal extension in this area therefore is apparently small in spite of stratal tilting and the lateral limit of large magnitude crustal extension in the central mojave block is farther northwest the data are consistent with the fault having formed by failure during plate flexure above the subducted mendocino fracture zone

a low velocity zone with weak reflectivity along the nankai subduction zone three dimensional seismic reflection data reveal the presence of a low seismic velocity zone lvz with weak reflectivity character along the nankai accretionary prism this lvz is intercalated between an upper offscraped layer and a lower underthrusting layer in the outer accretionary wedge wide angle ocean bottom seismograph data also support the presence of the lvz which is estimated to be a maximum of 2 km thick 15 km wide and 120 km long the lvz could be an underthrust package underplated in response to the lateral growth of the nankai accretionary prism underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward the lvz could have elevated fluid pressure leading to rigidity reduction of the entire outer accretionary wedge the rigidity lowered outer wedge containing the lvz may be more easily uplifted and thus eventually foster tsunami generation during a nankai megathrust earthquake if the fluid rich lvz supplies a significant amount of the fluid to the megasplay fault zone at depth it may affect stick slip behavior of the fault

smectite diagenesis pore water freshening and fluid flow at the toe of the nankai wedge the presence of low chloride fluids in the lowermost sediments drilled at ocean drilling program site 808 at the nankai accretionary wedge has been considered as prime evidence for long distance lateral fluid flow from depth here we re evaluate the potential role of in situ reaction of smectite s to illite i in the genesis of this low chloride anomaly this reaction is known to be occurring at site 808 with both the s content and s to i ratio in the mixed layer clays decreasing substantially with depth we show that the bulk of the chloride anomaly can generate by in situ clay dehydration particularly if pre reaction smectite abundances ai approach 10 15 of the bulk sediment the ai values however are not well constrained at ai values 10 15 an additional source of low cl fluid centered close to the decollement could be required thus there remains the important possibility that the observed low cl anomaly is a compound effect of both lateral flow and in situ smectite dehydration

the age of lathrop wells volcanic center an 40ar 39ar dating investigation the sup 40 ar sup 39 ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the lathrop wells volcanic center nevada yield eruption ages of 77 3 plus minus 6 0 and 76 6 plus minus 4 9 ka 2 sigma for the ql sub 1 and ql sub 2 flows respectively basaltic samples were analyzed in four to six replicate runs and yield normally distributed plateau ages miocene ash flow tuff xenoliths were collected from the basaltic units with the majority of samples coming from the ql sub 2 the sanidines from the ql sub 2 xenoliths yielded apparent plateau ages n 10 ranging from 69 plus minus 17 to 309 plus minus 11 ka with a strong cluster n 6 at approximately 77 ka because the diffusion dimension of sanidine is approximated by the physical crystal size cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and hf acid etching lead to anomalously old apparent ages these new ages for the ql sub 1 and ql sub 2 flows are analytically indistinguishable and agree well with published eruption ages determined by sup 3 he and sup 36 cl cosmogenic surface exposure dating methods we conclude that the ql submore 1 and ql sub 2 flows are temporally equivalent additionally the agreement between our eruption ages for these units and the published 76 plus minus 20 ka ital zreda ital et al 1993 sup 36 cl exposure age for the lathrop wells scoria cone qs sub 3 suggests that the cone may be temporally equivalent to the ql sub 1 and ql sub 2 flows we believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic copyright 1999 american geophysical union less

elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust nankai trough subduction zone pore fluid pressure is a key parameter governing both the shear strength of faults and the conditions for seismic and tsunamigenic slip on subduction zone megathrusts however quantitative constraints on this parameter based on in situ data in these or any faults have proven elusive using seismic interval velocities derived from a three dimensional seismic reflection survey we compute porosity fluid pressure and effective stress at the plate interface in the nankai trough subduction zone offshore southwestern japan to 20 km down the fault dip using well constrained locally calibrated empirical relationships between velocity porosity and effective stress we show that the fault and immediately subjacent footwall are nearly undrained implying that the subduction megathrust slips under a shear stress of

thermal effects of hydrothermal circulation and seamount subduction temperatures in the nankai trough seismogenic zone experiment transect japan 1 we examine the thermal effects of seamount subduction seamount subduction may cause transient changes in oceanic crust hydrogeology and plate boundary fault position prior to subduction seamounts provide high permeability pathways between the basaltic crustal aquifer and overlying ocean that can focus fluid flow and efficiently cool the oceanic crust as the seamount is subducted the high permeability pathway is closed shutting down the advective transfer of heat if significant fluid flow occurs it would be restricted after seamount subduction and would result in a redistribution of heat warming the trench and cooling landward parts of the system additionally subducting seamounts can influence the position of the plate boundary fault that has thermal consequences by locally controlling the proportions of incoming sediment that subduct and accrete shifting the decollement to the seafloor at the trench in the wake of seamount subduction causes limited cooling focused at the toe of the margin wedge we apply these features of seamount subduction to a thermal model for the nankai trough seismogenic zone experiment transect on the margin of japan models with hydrothermal circulation provide an explanation for anomalously high surface heat flux observations near the trench they yield temperatures of 100 c 295 c for the rupture area of the 1944 tonankai earthquake temperatures in the region of episodic tremor and slip are estimated at 290 c 325 c 70 c cooler than a model with no fluid circulation

the application of heavy indicator mineralogy in mineral exploration with emphasis on base metal indicators in glaciated metamorphic and plutonic terrains abstract indicator mineralogy is used to explore for a wide variety of mineral commodities the method utilizes minerals which are sufficiently heavy to be readily concentrated in the laboratory often colourful and possess other useful physical and chemical properties the minerals also must be source specific some indicator minerals are true resistate minerals the others although less resistant are stable in oxidized glacial drift and many non glacial sediments a few such as gold grains are silt sized but most are coarse grained grain size has a major impact on indicator mineral dispersal patterns in glacial drift the coarse grained indicator minerals are of two main types 1 kimberlite indicator minerals kims and 2 metamorphosed or magmatic massive sulphide indicator minerals mmsims kims are enriched in mg and cr and most mmsims are enriched in mg mn al or cr these indicator elements cannot be diagnosed geochemically in anomalous heavy mineral concentrates because the concentrates contain other more plentiful non indicator minerals containing the same elements in the same chemical form chalcopyrite is also a very useful mmsim but the number of surviving grains in a dispersal train is too low for detection by selective geochemical analysis mmsims are derived from three main types of base metal deposits and their associated alteration or reaction zones 1 volcanosedimentary massive sulphides encompassing volcanogenic sedex and mississippi valley subtypes in medium to high grade regional metamorphic terrains 2 skarn and greisen deposits and 3 magmatic ni cu sulphides the variety of mmsims associated with ni cu deposits is astonishing apparently reflecting mineral hybridization related to assimilation of sulphurous sedimentary rocks by ultramafic magmas cr diopside is one of the best indicators of fertile ni cu environments although not necessarily of the actual ni cu deposits heavy indicator mineralogy is much more sensitive than heavy mineral geochemical analysis and offers many exploration benefits in regional exploration programs including 1 sampling efficiencies 2 enlargement of both the bedrock target and dispersal train 3 coverage of a wider range of mineral commodities 4 undiminished sensitivity in areas of overabundant non indicator heavy minerals 5 visual evidence of points of origin of dispersal trains and 6 indications of the economic potential of the source mineralization it is most effective as a reconnaissance exploration tool and is particularly well suited for testing gneissic volcanosedimentary and plutonic terranes where base and precious metal deposits are highly modified and difficult to recognize by other prospecting methods

geometallurgy of the pebble porphyry copper gold molybdenum deposit alaska implications for gold distribution and paragenesis the pebble cu au mo deposit in southwest alaska is one of the world s largest porphyry deposits the deposit contains over 80 billion pounds blbs cu 107 million ounces moz au and 5 6 blbs mo in all resource categories copper and gold grades vary across diverse hydrothermal alteration assemblages with higher grades associated with sericite and pyrophyllite rich assemblages in an advanced argillic alteration zone moderate grades are associated with the potassic and sodic potassic alteration assemblages that dominate the deposit and low grades are found in quartz illite pyrite assemblages these variations reflect fluctuations in both the temperature and composition of the magmatic hydrothermal fluids in time and space qemscan scanning electron microscope based mineral mapping and in situ laser ablation inductively coupled plasmamass spectrometry methods designed to document the spatial and temporal variations in metal deportment across alteration zones show that gold occurs as inclusions of electrum and high fineness gold in chalcopyrite and pyrite and as free grains hosted by silicate minerals with the proportion of each related to the principal host alteration assemblage gold associated with the earliest high temperature sodic potassic and potassic assemblages occurs as electrum inclusions in chalcopyrite and to a lesser extent pyrite and pyrite trace element signatures have high gold silver and copper concentrations pyrite 1 during cooling of the hydrothermal system lower temperature illite and illite kaolinite alteration overprinted the earlier assemblages with varying degrees of recrystallization and precipitation of more pyrite rich assemblages with pyrite having a gold poor trace element composition pyrite 2 gold released during recrystallization formed high fineness gold inclusions in newly formed pyrite and narrow gold rich pyrite rims pyrite 3 a second magmatic hydrothermal event resulted in a sericite and pyrophyllite rich advanced argillic overprint which introduced more copper and gold to the system and recrystallized preexisting sulfides into both high and low sulfidation assemblages with high fineness gold inclusions in pyrite and chalcopyrite and solid solution gold in bornite a second generation of pyrite 1 with elevated palladium concentrations is associated with this hydrothermal fluid pulse and also possibly high arsenic pyrite pyrite 4 understanding the controls on gold deportment provides genetic constraints on ore deposit genesis alteration and sulfide assemblages and gold compositions provide information on the hydrothermal fluid compositions ph and temperature evolution of the magmatic hydrothermal system the presence of electrum inclusions in chalcopyrite and pyrite in potassic and sodic potassic alteration is consistent with these zones forming from early high temperature magmatic fluids because both gold and silver are transported together in such fluids high fineness gold inclusions in younger pyrite related to low temperature clay alteration are evidence that the gold in early electrum was partially remobilized and reprecipitated the low temperature clay alteration most likely formed from a mixture of h2s rich vapor with meteoric waters conditions where gold is transported as a bisulfide complex and silver is not resulting in the separation of the two metals a second structurally controlled pulse of magmatic fluids formed a well mineralized advanced argillic alteration assemblage containing high fineness gold inclusions in chalcopyrite and pyrite the only part of the deposit where high fineness gold is hosted by chalcopyrite in the pyrophyllite stability field acidic fluid compositions transport gold and silver as different complexes and gold solubility is significantly lower than silver leading to the precipitation of highfineness gold inclusions palladium is transported by the same complex as gold under these conditions consistent with the elevated palladium content of pyrite in the advanced argillic alteration assemblage the temporal and spatial studies of variations in gold deportment across the pebble deposit provide critical inputs to optimization of mineral processing design the greatest influence on metallurgical gold recovery at pebble is the proportion of gold that is hosted by pyrite pyrite hosted gold may require different mineral processing methodologies compared with gold hosted by chalcopyrite therefore defining domains of consistent hydrothermal alteration and sulfide mineralogy and gold deportment is the key to the geometallurgical characterization of the deposit

coupled fluid flow and deformation modeling of the frontal thrust region of the kumano basin transect japan implications for fluid pressures and decollement downstepping 1 results of coupled fluid flow and deformation modeling illustrate the influence of frontal thrust slip on excess pore pressures and decollement downstepping in the kumano basin transect offshore japan although permeable trench sediments and a buried sandstone layer provide potential fluid escape pathways the numerical modeling suggests that excess pore pressure could build beneath the frontal thrust within hemipelagic clay of the shikoku basin facies simulated excess pore pressures range between 45 and 77 of the overburden weight 3 km landward of the deformation front and are very sensitive to the permeability of the shikoku basin clay and the coefficient of friction of the fault zone excess pore pressures increase as the fault coefficient of friction is increased suggesting that volume change due to shear within the footwall sediments is an important contributor to excess pore pressures raising the coefficient of friction also increases shear within the sediments beneath the fault zone suggesting downward migration of the decollement zone this shearing focuses in the weaker shikoku basin clays rather than an underlying turbidite unit suggesting that lithologic differences play an important role in controlling deformation location

tilt and rotation of the footwall of a major normal fault system paleomagnetism of the black mountains death valley extended terrane california paleomagnetic data have been obtained from miocene intrusions including crosscutting mafic and felsic dikes and proterozoic crystalline rocks to evaluate deformation during miocene and younger extension and unroofing of the black mountains death valley california synrift intrusions contain a well defined and at the site level well grouped magnetization interpreted to be of dual polarity the in situ direction of which is discordant in declination and inclination with an expected late cenozoic reference direction the oldest of these intrusions 11 6 0 2 ma gives a magnetite dominated remanent magnetization of high coercivity and high laboratory unblocking temperatures about 550 to 580 c that is interpreted to predate unroofing from midcrustal depths in situ site mean directions of this magnetization are directed toward the west and west northwest with moderate to shallow positive down and negative up inclinations the variation in direction of magnetization particularly inclination with site locality around the turtleback structures along the western flank of the black mountains is interpreted to result from folding of the intrusion after remanence acquisition younger intrusions 8 7 ma generally give magnetizations with inclinations similar to expected miocene values two populations of in situ site means are identified one with southwest declination and negative inclination the other with northward declination and positive inclination a preferred interpretation for footwall deformation involves from oldest to youngest 1 southwest side down tilting of the entire range block of some 20 to 40 to possibly 70 and at least locally folding of the crystalline rocks on a trend parallel to the death valley turtlebacks between 11 6 and 8 7 ma 2 progressive east to west footwall unroofing between 8 7 and ca 6 5 ma and 3 clockwise rotation 50 to 80 of much of the black mountains after the core detached from stable terrane to the west we interpret late rotation of the black mountains as oroflexure related to right lateral shear along the death valley fault zone

fold fault relationships in low angle detachment systems a close kinematic and timing relationship between low angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well exposed neogene detachment systems in the simplon region of the alps and in the death valley region of california fold amplitude is largest in the oldest mylonitic foliation of the footwall these folds are transected by younger mylonitic foliation s and cataclastic detachment faults which are themselves folded about similar axes in general the amplitude decreases and the wavelength increases for progressively younger fold structures consistent with a history of progressive folding during faulting the interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes this strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning

effective stress and pore pressure in the nankai accretionary prism off the muroto peninsula southwestern japan 1 we developed a theoretical method for predicting effective stress and pore pressure based on rock physics model we applied the method to reveal the pore pressure distribution within the nankai accretionary prism off southwestern japan and to investigate variations in pore pressure associated with evolution of the plate boundary decollement from the crack aspect ratio spectrum estimated from laboratory and well log data we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory by iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3d tomographic inversion we estimated in situ mean effective stress within the accretionary prism pore pressure is then the difference between the effective stress and the confining stress when we calculated pore pressure we considered compressive state of stress in the accretionary prism our results confirm that pore fluid pressure is high within the subducting sedimentary sequence below the decollement we determined a normalized pore pressure ratio of 0 4 0 7 abnormal pore pressures develop in the under thrust sequence as a result of the increase in overburden load because of the thickened overlying prism and a low permeability barrier across the decollement overpressuring within the accreted sequence is initiated at the deformation front and proceeds landward the increase in horizontal compaction within the accreted sequence may raise pore pressures within the accreted sequence and the pore pressure mean effective stress contrast at the decollement becomes smaller landward of the deformation front

transient strain accumulation and fault interaction in the eastern california shear zone satellite synthetic aperture radar interferometry reveals transient strain accumulation along the blackwater little lake fault system within the eastern california shear zone the surface strain map obtained by averaging eight years 1992 2000 of earth resource satellite ers radar data shows a 120 km long 20 kmwide zone of concentrated shear between the southern end of the 1872 owens valley earthquake surface break and the northern end of the 1992 landers earthquake surface break the observed shear zone is continuous through the garlock fault which does not show any evidence of left lateral slip during the same time period a dislocation model of the observed shear indicates right lateral slip at 7 6 3 mm yr on a vertical fault below 5 km depth a rate that is two to three times greater than the geologic rates estimated on northwest trending faults in the eastern mojave area this transient slip rate and the absence of resolvable slip on the garlock fault may be the manifestation of an oscillatory strain pattern between interacting conjugate fault systems

paleogeographic reconstruction of the death valley extended region evidence from miocene large rock avalanche deposits in the amargosa chaos basin california large rock avalanche deposits are extremely useful tools in paleogeographic and tectonic reconstructions these deposits are large 0 1 km3 semicoherent snapshots of source area that were emplaced instantaneously in a sedimentary basin a rock avalanche deposit with a known age in a sedimentary basin requires the restoration of that basin to a position adjacent to the rock avalanche source area at the time of emplacement the miocene sedimentary section of the amargosa chaos and sperry hills basins in the southern death valley region contains deposits of large rock avalanches from both spatially and temporally separated source terranes the rock avalanche deposits in the amargosa chaos basin require the restoration of the basin to within 10 km of the kingston range prior to 7 8 ma west derived alluvial fan sediments in this basin can be confidently tied to the southern panamint mountains with early east derived alluvial fan sediments pre 7 8 ma tied to the kingston range moreover the pre 7 8 ma stratigraphy of the amargosa chaos basin is equivalent to the pre 7 8 ma stratigraphy of the miocene sedimentary section in the sperry hills prior to 7 8 ma the amargosa chaos and sperry hills basins were both parts of a larger sedimentary basin sedimentologic constraints on the geometry of the amargosa chaos basin indicate that throughout its history it was a half graben opening along a west dipping normal fault with substantial vertical relief in the uplifted footwall evidence for this topography exists in the form of multiple large rock avalanche deposits derived from footwall bedrock the axis of this basin trends northwest with the active basin bounding normal fault on the northeast margin at 7 8 ma the extension direction in the southern death valley region changed from southwest to northwest younger sediments including three younger rock avalanche deposits record the motion of the amargosa chaos basin away from the sperry hills basin top to the northwest displacement of 30 km along the amargosa detachment system is required between 7 8 ma and 4 9 ma between the sperry hills basin and the location of amargosa chaos basin in the southern black mountains continued post 4 9 ma northwest directed extension is required between the amargosa chaos basin and the southern panamint mountains in addition 15 km of right lateral slip on the southern portion of the grand view fault is required from 7 8 to 3 ma to move the sperry hills basin away from its pre 7 8 ma position adjacent to the kingston range

seismic velocity anisotropy and fluid pressure in the barbados accretionary wedge from an offset vertical seismic profile with seabed sources 1 the state of compaction and fluid pressure in the barbados accretionary wedge near its toe at ocean drilling program site 949 were investigated by modeling travel times of seismic waves from ocean bottom shots to a borehole geophone array the model constrained by a three dimensional seismic survey and well logs shows 1 a velocity gradient of about 1 1 25 s 1 in the uppermost 180 230 m of the wedge 2 a zone of variable but no net change in velocity between 230 and 350 m depth 3 a low velocity zone 40 50 m thick just above the decollement at 391 m and 4 a displacement of the low velocity zone by thrust faults pore fluid pressure sections derived from p wave velocity show that the upper half of the wedge is normally pressured while the lower half is overpressured the 160 m thick underconsolidated basal zone shows anisotropy which increases downward the lowest 40 50 m has velocity varying 1 azimuthally 3 being fastest in the direction of plate convergence and 2 in the vertical plane 2 5 horizontal faster than vertical after correction for the effect of anisotropy in the derivation of effective stress from seismic velocity the calculated pore fluid pressure ratio does not exceed 0 9 and in the lowest 40 50 m of the basal zone is between 0 71 and 0 82 with fluid pressure hydrostatic lithostatic pressure hydrostatic between 0 5 and 0 65 in accordance with in situ measurements of fluid pressure in the decollement zone beneath these indicate that the accretionary wedge is stronger and less overpressured than was previously supposed

distribution and geochronology of oregon plateau u s a flood basalt volcanism the steens basalt revisited the timing and petrogenesis of mid miocene flood basalt volcanism in the northwest united states has been extensively addressed yet the chemical characteristics and temporal details of the steens basalt exposed on the oregon plateau are poorly defined steens basalt volcanism has generally been accepted to have occurred at 16 6 ma coeval and or just prior to the onset of columbia river basalt group volcanism to the north new major and trace element analyses and nine 40 ar 39 ar ages ranging from 15 51 0 28 to 16 58 0 18 ma were obtained on oregon plateau flood basalt lava flows from stratigraphic sections in close proximity to steens mountain additionally new 40 ar 39 ar ages were obtained on the uppermost and thirty first lava flow down from the top of the 1 km section of steens basalt exposed at steens mountain and yield eruption ages of 16 59 0 10 and 16 55 0 10 ma respectively field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of steens mountain the chemical and chronologic data presented in this study illustrate that flood basalts with the steens basalt chemical signature erupted across the southern oregon plateau over a much greater timespan than what is typically quoted for the steens mountain type section these data suggest that the main volume of steens basalt volcanism erupted over at least an 1 m y duration from eruptive loci in the vicinity of steens mountain while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for n2 ma across the oregon plateau these new geochemical and geochronological constraints verify a common temporal link between steens malheur gorge region and columbia river basalt group crbg volcanism this direct temporal link requires that petrogenetic and tectonic models of mid miocene northwestern u s flood basalt volcanism recognize that the northern columbia plateau and southern oregon plateau portions of this mid miocene basalt province were erupting simultaneously during portions of the regional event 2007 elsevier b v all rights reserved

the southern kahiltna terrane implications for the tectonic evolution of southwestern alaska regionally extensive and highly deformed jurassic to cretaceous basinal turbidite deposits occur along most of the inboard boundary of the talkeetna superterrane in southern alaska and western canada the southern kahiltna terrane of southwestern alaska consists largely of such basinal deposits regional and detailed studies in and adjacent to this terrane suggest several new conclusions regarding the origin of the basin and its role in the tectonic evolution of southwestern alaska stratigraphic sedimentologic compositional and structural evidence suggests that the upper jurassic to lower cretaceous strata of the southern kahiltna terrane were derived from and deposited on rocks of the adjacent talkeetna superterrane deposition of these clastic rocks apparently postdated arc magmatism in the talkeetna superterrane suggesting deposition in a basin developed on the suture between the talkeetna superterrane and north america if true this observation implies that collision of the talkeetna superterrane with north america began prior to latest jurassic time probably at a more southerly location the superterrane has since been translated northward probably by coast parallel strike slip displacement to its present location where it was structurally juxtaposed with middle to upper creteceous rocks of the kuskokwim group a magmatic overprint indicates that the talkeetna superterrane was in approximately its present location by latest cretaceous to paleocene time

porosity corrections for smectite rich sediments impact on studies of compaction fluid generation and tectonic history porosity is a fundamental parameter that must be correctly determined in order to relate physical property hydrologic and chemical flux studies to natural systems traditional porosity determinations generated from physical property or seismic data can greatly overestimate the true porosity of sediments in which hydrated minerals such as smectite are abundant to produce a true porosity distribution such data must be corrected to account for the h 2 o residing in smectite interlayers which can make up to 25 of the total hydrated mineral mass such h 2 o is easily removed from the mineral by oven drying and or exposure to low humidities standard physical property measurements can be corrected provided the weight percent smectite in the sediment is known and the interlayer h 2 o content of the mineral can be estimated we illustrate the significant consequences of this correction by comparing profiles of reported and corrected porosities for smectite rich barbados abyssal plain and accretionary wedge sediments from ocean drilling program leg 110 sites 671 and 672

slip rate of the western garlock fault at clark wash near lone tree canyon mojave desert california the precise tectonic role of the left lateral garlock fault in southern california has been controversial three proposed tectonic models yield significantly different predictions for the slip rate history orientation and total bedrock offset as a function of distance along strike in an effort to test these models we present the first slip rate estimate for the western garlock fault that is constrained by radiocarbon dating a channel referred to here as clark wash incised into a latest pleistocene alluvial fan has been left laterally offset at least 66 6 m and no more than 100 m across the western garlock fault indicating a left lateral slip rate of 7 6 mm yr 95 confidence interval of 5 3 10 7 mm yr using dendrochronologically calibrated radiocarbon dates the timing of aggradational events on the clark wash fan corresponds closely to what has been documented elsewhere in the mojave desert suggesting that much of this activity has been climatically controlled the range front fault located a few hundred meters northwest of the gar lock fault has probably acted primarily as a normal fault with a holocene rate of dip slip of 0 4 0 7 mm yr the record of prehistoric earthquakes on the garlock fault at this site though quite possibly incomplete suggests a longer interseismic interval 1200 2700 yr for the western garlock fault than for the central garlock fault the relatively high slip rate determined here indicates that the western and central segments of the garlock fault show similar rates of movement that are somewhat faster than rates inferred from geodetic data the high rate of motion on the western garlock fault is most consistent with a model in which the western garlock fault acts as a conjugate shear to the san andreas fault other mechanisms involving extension north of the garlock fault and block rotation at the eastern end of the fault may be relevant to the central and eastern sections of the fault but they cannot explain a high rate of slip on the western garlock fault

large magnitude transient strain accumulation on the blackwater fault eastern california shear zone we investigate the quaternary slip rate for the blackwater fault eastern california shear zone through mapping and geochronology of offset volcanic rocks basalt flows of the black mountains support the presence of faulting at 3 77 0 11 ma 1 8 0 1 km of subsequent slip and a well constrained long term slip rate of 0 49 0 04 mm yr total slip diminishes northward evidenced by a 0 3 1 8 km offset of a 7 23 1 07 ma dacite flow in the black hills and fault termination in the lava mountains 5 km short of the garlock fault slow long term slip rate together with sparse evidence for holocene rupture contradict predictions of rapid slip rate from tectonic geodesy these results support the conclusion that as much as 95 of geodetic strain accumulation across the blackwater fault and thus from 1 to 6 mm yr of geodetic strain measured across the eastern california shear zone is a transitory phenomenon discrepant geologic and geodetic results may indicate an increased near term seismic hazard but merit caution for interpretation of fault slip rates from geodesy alone

right lateral displacements and the holocene slip rate associated with prehistoric earthquakes along the southern panamint valley fault zone implications for southern basin and range tectonics and coastal california deformation the n20 w trending panamint valley fault zone is linked to the n60 w trending hunter mountain strike slip fault and the saline valley fault system which represents one of the three major fault systems accommodating active crustal extension in the southern great basin a 25 km long zone of fault scarps along the southern panamint valley fault zone is recognized as the surface rupture zone associated with the most recent prehistoric earthquake the displacement associated with the most recent event determined through six detailed topographic maps of offset features is 3 2 0 5 m and a number of larger offsets in range of 6 7 m and 12 m are also observed if the larger displacements represent respectively two and three events each of 3 m then the fault zone appears to be associated with a characteristic earthquake which we estimate from the length of the rupture zone and the displacement to be between ms 6 5 and 7 2 the holocene slip rate is 2 36 0 79 mm yr is determined from the displacement of two alluvial features whose maximum age is estimated from pluvial shorelines assuming a characteristic earthquake model the recurrence interval is between 860 and 2360 years the holocene slip rate appears to be similar to the 4 million year slip rate of 2 2 7 mm yr determined from the hunter mountain fault which we suggest reflects the relatively constant tectonics in this region over the last 4 million years we further speculate that this supports the san andreas discrepancy in that the holocene slip rate of the san andreas fault probably represents its very long term several ma slip rate the total slip vector of the southern panamint valley fault system is oriented toward n35 w making this a predominantly strike slip fault in conjunction with the n60 w orientation of the hunter mountain strike slip fault we suggest that the displacement vector for the southern great basin is toward the nw consistent with results from vlbi data rather than wnw as determined by combining vlbi and geological data this in turn suggests that the coastal california deformation component involves respectively less shortening and more strike slip displacement perpendicular and parallel to the san andreas fault than is currently proposed

elemental and isotopic chloride geochemistry and fluid flow in the nankai trough 1 the first transect of pore fluid cl profiles across the deformation front of an accretionary prism and underthrust section nankai trough and the first cl isotopic profile in this environment are presented these demonstrate that there is channelized horizontal episodic flow through most of the section arcward of the deformation front and in the lowermost section seaward of the deformation front in the underthrust sediments channels appear to be continuous fluid velocities within the channels based on cl gradients are 13 5cm year the near balance of depth integrated flow and the subduction of pore fluid indicates that flow episodes are frequent 37cl values are lower than seawater throughout the section averaging 6 1 per mil this demonstrates fluid mineral exchange of cl and suggests that subducted solids carry enriched 37cl that is efficiently recycled from deeper levels in the subduction zone back into seawater and may be responsible for the seawater mantle 37cl fractionation

initiation of deformation of the eastern california shear zone constraints from garlock fault geometry and gps observations 1 we suggest a 2 stage deformation model for the eastern california shear zone ecsz to explain the geometry of the garlock fault trace we assume the garlock fault was originally straight and then was gradually curved by right lateral shear deformation across the ecsz in our 2 stage deformation model the first stage involves uniform shear deformation across the eastern part of the shear zone and the second stage involves uniform shear deformation across the entire shear zone in addition to the current shape of the garlock fault our model incorporates constraints on contemporary deformation rates provided by gps observations we find that the best fitting age for initiation of shear in eastern part of the ecsz is about 5 0 0 4 ma and that deformation of the western part started about 1 6 myr later

pb sr nd isotopes in surficial materials at the pebble porphyry cu au mo deposit southwestern alaska can the mineralizing fingerprint be detected through cover the cretaceous pebble porphyry cu au mo deposit is covered by tundra and glacigenic sediments pb sr nd measurements were done on sediments and soils to establish baseline conditions prior to the onset of mining operations and contribute to the development of exploration methods for concealed base metal deposits of this type pebble rocks have a moderate range for 206pb 204pb 18 574 to 18 874 207pb 204pb 15 484 to 15 526 and 208pb 204pb 38 053 to 38 266 mineralized granodiorite shows a modest spread in 87sr 86sr 0 704354 0 707621 and 143nd 144nd 0 512639 0 512750 age corrected 89 ma values for the granodiorite yield relatively unradiogenic pb e g 207pb 204pb 15 52 low values of 87sr 86sr and positive values of nd 1 00 4 52 that attest to a major contribution of mantle derived source rocks pond sediments and soils have similar pb isotope signatures and 87sr 86sr and 143nd 144nd values that resemble the mineralized granodiorites glacial events have obscured the recognition of isotope signatures of mineralized rocks in the sediments and soils baseline radiogenic isotope compositions prior to the onset of mining operations reflect natural erosion transport and deposition of heterogeneous till sheets that included debris from barren rocks mineralized granodiorite and sulfides from the pebble deposit and other country rocks that pre and postdate the mineralization events isotopic variations suggest that natural weathering of the deposit is generally reflected in these surficial materials the isotope data provide geochemical constraints to glimpse through the extensive cover and together with other geochemical observations provide a vector to concealed mineralized rocks genetically linked with the pebble deposit

anomalous folds associated with the east central part of the garlock fault southeast california the east central part of the left lateral garlock fault in southeast california is associated with three broad folds that trend and plunge northeast the folds which lie north south and astride the fault postdate deposition of alluvial gravels that at one time formed a continuous northwest sloping fan the gravels rest conformably on lake beds that contain the lava creek b ash bed 0 62 ma the anticline named after christmas canyon has vertical relief of as much as 100 m topographic profiles suggest that the gravels exposed along the synclinal troughs to its southeast pilot knob valley syncline and northwest teagle wash syncline lie at about the same elevations as they did when part of the originally active fan implying that the synclines are passive results of the anticline development if so the original slope of the middle pleistocene alluvial sediment sheet was about 0 6 northwest the age and dimensional controls allow calculation of the rates of tilting and horizontal shortening corrected tilting rates vary from about 2 5 to 4 7 m y the total shortening of about 7 5 m developed at a minimum rate of 12 5 m m y the orientations of these folds imply northwest southeast compression which is anomalous when viewed in the context of regional tectonics the left lateral garlock fault and the north northwest trending searles valley syncline and slate range anticline imply northeast southwest compression which geophysical criteria confirm geologic evidence of northwest southeast compressional deformation dating to pre miocene paleocene time however is known from several other localities along the garlock fault these anomalous features are interpreted as evidence that episodes of north west southeast compression along the garlock fault do occur and although oriented about 90 from the average of conventional and presently observed compression directions they have occurred during much most of the cenozoic era including the past 0 6 m y

dynamic deformation of the accretionary prism excites very low frequency earthquakes 1 we have detected anomalous very low frequency earthquakes within the accretionary prism along the nankai trough southwestern japan centroid moment tensor inversion analysis reveals that the earthquake hypocenters are distributed at 10 km depth above the upper surface of the subducting philippine sea plate and within 50 70 km landward of the trough axis the focal mechanisms indicate reverse faulting their hypocenters are distributed beneath a deformation zone of an accretionary prism in sea floor topography these observations suggest that the occurrence of very low frequency earthquakes is related to numerous reverse fault systems within the accretionary prism and that the earthquakes reflect the dynamics of deformation within this accretionary prism

a distinctive pd hg signature in detrital gold derived from alkalic cu au porphyry systems abstract this study comprises the first systematic classification of native gold geochemistry within alkalic porphyry cu au systems and the placer expression of such systems the geochemistry and mineral associations of gold from four alkalic porphyry deposits in british columbia canada afton mount milligan mount polley and copper mountain have been compared to comment on the likely detrital gold expression of similar systems globally populations of gold grains collected from in situ hypogene mineralization as well as fluvial deposits downstream of these deposits have been characterized in terms of their alloy composition au ag cu hg and pd and associated mineral inclusions these data are combined to generate a microchemical signature gold compositions vary according to the alteration zone within a porphyry system previous compositional studies of gold in porphyry systems have focused on the most economically important ore associated with potassic alteration wherein native gold hosted either by bornite or chalcopyrite comprises tiny blebs typically 5 20 m containing detectable cu in the gold alloy to a maximum of around 5 but quite variable 2 30 wt ag the presence of such grains have been confirmed in hypogene ore from the four systems studied but they have been shown to be compositionally distinct from detrital gold collected from nearby fluvial placers which exhibit a strong pd and hg signature both in the alloy and as mineral inclusions several workers have described late stage veins associated with alkalic porphyries which contain distinctive pd hg bearing minerals in association with other sulphides and sulphosalts this unusual mineralogy has been observed in the mineral inclusion suites of populations of detrital gold grains collected in the environs of the porphyry systems we conclude that whilst the micron scale cu rich gold grains formed in potassically altered rocks are in general too small to be recovered during routine sediment sampling those formed in later stage hydrothermal systems are larger and exhibit a distinctive microchemical signature which may be differentiated from those of gold formed in other mineralizing systems consequently compositional studies of detrital gold could underpin a mineral indicator methodology in the exploration for alkalic porphyry cu au deposits

thermobarometric and40ar 39ar geochronologic constraints on eohimalayan metamorphism in the dinggye area southern tibet mineral assemblages in the dinggye area of southern tibet 28 n 88 e provide new insights regarding the poorly understood eohimalayan metamorphic event in the eastern himalayan orogen major element partitioning thermobarometry of pelitic rocks indicates temperatures of 750 830 k at depths of 14 3 km consistent with the presence of kyanite sillimanite and andalusite schists in the area laser and resistance furnace40ar 39ar analyses of hornblendes from intercalated amphibolites yield closure ages of 25 ma overlap between the probable range of ar closure temperatures for these hornblendes and the metamorphic temperatures estimated through thermobarometry suggests that eohimalayan metamorphism in the dinggye area occurred in late oligocene time no more than about 10 million years before the main or neohimalayan phase of metamorphism in early to middle miocene time muscovite biotite and k feldspar40ar 39ar ages indicate an important episode of rapid cooling between 16 and 13 ma which is interpreted as a signature of tectonic denudation related to movement on n dipping extensional structures of the south tibetan detachment system

mineral potential mapping in an accreted island arc setting using aeromagnetic data an example from southwest alaska the distribution of volcanogenic massive sulfide vms porphyry epithermal alaska type ultramafic mafic complexes intrusion related au and granitoid sn w ore deposits in southwest alaska supports current metallogenic models linking the formation of these deposit types to the emplacement of different suites of igneous rocks during the evolution of this convergent plate margin regional scale aeromagnetic data provide a continuous set of observations over the deposits and show contrasting patterns over the igneous rock suites hosting the various deposit types combined with surface geologic data and regional metallogenic constraints aeromagnetic data filtered to enhance the anomalous magnetic field and map magnetic domains were used to produce a mineral potential map across this accreted island arc setting the reduced to pole upward continuation and total horizontal gradient transform maps show anomalies that could represent porphyry epithermal deposits within the intraoceanic and continental arc terranes the tilt derivative transform highlights lineaments within the back arc that may represent zones with potential for vms deposits the truncations of tilt derivative lineaments outline a major magnetic domain boundary between the back arc and craton margin which is prospective for granitoid sn w deposits annular tilt derivative highs outline granitoids that could be associated with intrusion related au deposits within the craton margin shallow magnetite rich alaska type ultramafic mafic complexes are mapped by their short wavelength high amplitude anomalies successful mineral potential mapping across southwestern alaska as performed in the present study suggests that filtered aeromagnetic data can be effectively used in mineral exploration in convergent continental margin settings

response of natural smectite to seismogenic heating and potential implications for the 2011 tohoku earthquake in the japan trench the sensitivity of smectite during brief and protracted heating intervals can provide crucial information about the temperature history of faults during seismogenic slip and creep pelagic sourced smectite is the most abundant clay mineral that is incorporated into the slip zone that was drilled during the japan trench fast drilling project jfast expedition 343 in the japan trench located 820 m below seafloor this study investigates the potential for abundant smectite to preserve a record of coseismic frictional heating associated with the tohoku earthquake in may 2011 by laboratory examination of mineral hydration and dehydration in jfast drill core samples during brief maximum of 5 min and protracted 5 h heating sequences using a real time heating stage that is connected to an x ray diffractometer we observe that 1 both brief and protracted heating causes reduction of water interlayers in smectite 2 smectite recovers faster to the original hydration state after quick heating than long heating and 3 nonrecoverable collapse of all smectite occurs at 200 c for brief and protracted heating rates because hydrated smectite clays are widely present in the fault rocks we conclude that frictional heating within the slip zone of the tohoku fault zone cannot have reached significantly elevated temperatures

holocene slip rate of the central garlock fault in southeastern searles valley california a late pleistocene shoreline at the overflow level of searles lake has been displaced 82 to 106 m preferred value is 90 m in a left lateral sense and 2 5 m net north side up along the garlock fault at the southeastern corner of searles valley previously published radiocarbon dates from both surface and subsurface strata indicate that the most recent highstand of searles lake ended sometime between 10 000 and 13 800 14c years ago the maximum slip rate of the garlock fault in southeastern searles valley is thus 11 mm 14c yr if part of the offset of the shoreline occurred during older lakestands then the slip rate may be somewhat less a channel incised after the most recent highstand however is offset about 68 m indicating that the minimum slip rate is 5 mm 14c yr subjective evaluation of the constraints on the offset and on the age of the shoreline yields a preferred rate of 6 8 mm 14c yr at this site assuming bard et al s 1990 recent calibration of the radiocarbon time scale the calibrated slip rate of the garlock fault is between 4 and 9 mm yr with a preferred value of 5 7 mm yr this estimate is similar to a previous estimate of the holocene slip rate and is slightly less than an estimate derived from modelling of geodetic data extension north of the garlock fault in indian wells and searles valleys contributes no more than 3 mm yr left slip to the garlock fault

summary of exploration geochemical and mineralogical studies at the giant pebble porphyry cu au mo deposit alaska implications for exploration under cover exploration geochemical and mineralogical studies by the u s geological survey at the pebble porphyry cu au mo deposit were designed to 1 determine whether the concealed deposit can be detected with surface samples 2 better understand the processes of metal migration from the deposit to the surface and 3 test existing methods for assessing concealed mineral resources and or develop new ones surface water ponds streams and springs pond and stream sediment soils subjected to various leaching techniques and glacial till samples were collected the tilted nature of the undisturbed orebody varying depth of cover and later glacial processes strongly influence the geochemical responses and processes active on the various sample media the multimedia approach aids in identifying possible processes that caused the significant geochemical variations within and among the various media these processes include the following 1 in the pebble west zone thin cover and local exposure of the orebody have facilitated the oxidation of pyrite and other sulfides and associated ferrous ferric iron reactions resulting in the local natural acidification of ponds observed in the west zone and in associated metal anomalies in waters sediments and soils 2 in contrast the east zone is concealed by both glacial deposits and underlying thick cover rocks which precludes the oxidation of sulfides in the underlying orebody low level geochemical anomalies in circumneutral spring and pond waters from the east zone are discernible only by using high resolution inductively coupled plasma mass spectrometry with lower limits of determination two and perhaps three orders of magnitude lower than traditional methods a variety of partial leaches of soils over the east zone reveal geochemical anomalies in a similar suite of elements that may be related to upwelling waters from depth along graben bounding faults 3 the indicator minerals gold jarosite and andradite in till reveal a displaced mineralogical anomaly to the west and south of the pebble orebody as ore related minerals were scraped from the orebody and deposited in till downice of the deposit geochemical anomalies in pond water and sediment over the displaced till are attributed to the ore related minerals in till this orientation study demonstrates the strong control of local geologic and geochemical settings on the effectiveness of different traditional and newer reconnaissance geochemical exploration techniques and thus has important implications for exploration

cenozoic evolution of the mojave block of southern california the recorded cenozoic history of the mojave desert region of southern california began in the latest oligocene when intense volcanism and tectonism interrupted a long early tertiary silence volcanism commenced across the region in an east west band ca 24 22 ma northwest of barstow volcanism was accompanied by intense crustal extension and development of a metamorphic core complex outside of this relatively restricted area extension was minor or absent after extension ceased ca 18 ma volcanism shifted to small volume eruptions of basalt post extensional deformation has largely been by strike slip faulting along northwest striking dextral faults and west striking sinistral faults and total dextral slip across the mojave desert region since the early miocene is 45 60 km strike slip deformation has been overprinted locally by intense north south contraction that is the dominant style of deformation in the western mojave block paleomagnetic data indicate that parts of the mojave block were rotated clockwise although the magnitude and timing of this rotation are poorly determined the best evidence for large 45 rotation comes from the area east of barstow where large clockwise declination anomalies and mesozoic and cenozoic dikes with anomalous strikes may reflect early miocene clockwise deflection along the mojave river fault volcanism and tectonism in the mojave block resulted from interactions among the north american pacific and various oceanic plates patterns of volcanism and tectonism do not correlate with growth of slab windows beneath the continent but do correlate with the position of the subducted mendocino fracture zone plate circuit reconstructions suggest that the driving force for extension was divergence between the pacific and north american plates along the transform margin that separated the two this hypothesis accounts for the direction magnitude and rate of extension in the mojave block

stable cl isotopes in subduction zone pore waters implications for fluid rock reactions and the cycling of chlorine stable cl isotope ratios measured in marine pore waters associated with the barbados and nankai subduction zones extend significantly to 8 the range of 37 cl values reported for natural waters these relatively large negative values together with geologic and chemical evidence from barbados and nankai and recent laboratory data showing that hydrous silicate minerals i e those with structural oh sites are enriched up to 7 5 in 37 cl relative to seawater strongly suggest that the isotopic composition of cl in pore waters from subduction zones reflects diagenetic and metamorphic dehydration and transformation reactions these reactions involve clays and or other hydrous silicate phases at depth in the fluid source regions chlorine therefore cannot be considered geochemically conservative in these systems the uptake of cl by hydrous phases provides a mechanism by which cl can be cycled into the mantle through subduction zones thus stable cl isotopes should help in determining the extent to which cl and companion excess volatiles like h 2 o and co 2 cycle between the crust and mantle

exploring for deeply covered mineral deposits formation of geochemical anomalies in northern chile by earthquake induced surface flooding of mineralized groundwaters northern chile is a major producer of copper from porphyry deposits most ores worked to date were exposed near the surface exploration is now focused on deposits along a basement unconformity buried under piedmont gravels deposited by ca 10 ma two recent discoveries the spence and gaby sur deposits are covered by gravel soils above these deposits have anomalies for elements present in saline mineralized groundwaters the anomalies occur above fracture zones in the gravels believed to represent the upward propagation of basement faults that guided emplacement of the porphyries we propose that during earthquakes in this seismically active region there was pumping of saline basement waters up the faults and fracture zones entraining mineralized groundwaters from the deposits after surface flooding and evaporation elements were redistributed by infrequent rains sampling shows that surface active cations such as cu 2 1 were adsorbed and retained in the top few centimeters of soil whereas nacl and porphyry indicator elements that dissolve as anions se re and as are not adsorbed and are removed to 40 cm depth given the 10 m y interval since the gravels were deposited and the hyperarid climate of the atacama desert anomalies may have formed by repeated episodes of seismically induced flooding company geochemical surveys show that such features are numerous in northern chile producing linear zones of saline soils plus other constituents of local groundwaters e g b and i where faults controlling these features intersect mineralization elements specific to the mineralization are also present saline zones may be outlined in the field by measuring the conductivity of soil plus water slurries sampling for full geochemical analyses can be limited to areas so defined anomalies may be generated along fault lines in other seismically active wetter climates but only for elements that are adsorbed on soil minerals although the context of the study is mineral exploration the process may be relevant to the formation of geochemical features of environmental interest above major faults

chasing the garlock a study of tectonic response to vertical axis rotation vertical axis clockwise block rotations in the northeast mojave block are well documented by numerous authors however the effects of these rotations on the crust to the north of the northeast mojave block have remained unexplored in this paper we present a model that results from mapping and geochronology conducted in the north and central owlshead mountains the model suggests that some or all of the transtension and rotation observed in the owlshead mountains results from tectonic response to a combination of clockwise block rotation in the northeast mojave block and basin and range extension the owlshead mountains are effectively an accommodation zone that buffers differential extension between the northeast mojave block and the basin and range in addition our model explores the complex interactions that occur between faults and fault blocks at the junction of the garlock brown mountain and owl lake faults we hypothesize that the bending of the garlock fault by rotation of the northeast mojave block resulted in a misorientation of the garlock that forced the owl lake fault to break in order to accommodate slip on the western garlock fault subsequent sinistral slip on the owl lake fault offset the garlock creating the now possibly inactive mule springs strand of the garlock fault dextral slip on the brown mountain fault then locked the owl lake fault forcing the active leach lake strand of the garlock fault to break

tectonic setting of the pebble and other copper gold molybdenum porphyry deposits within the evolving middle cretaceous continental margin of northwestern north america the pebble cu au mo deposit in southwestern alaska containing the largest gold resource of any known porphyry in the world developed in a tectonic setting significantly different from that of the present day it is one of a series of metalliferous middle cretaceous porphyritic granodiorite quartz monzonite and diorite bodies evolved from lower crust and metasomatized lithospheric mantle melts which formed along much of the length of the north american craton suture with the peninsular alexander wrangellia arc the porphyry deposits were emplaced within the northernmost two of a series of ca 130 to 80 ma flysch basins that define the suture as well as into arc rocks immediately seaward of the two basins deposits include the ca 100 to 90 ma pebble neacola and other porphyry prospects along the kahiltna basin peninsula terrane boundary and the ca 115 to 105 ma baultoff carl creek horsfeld orange hill bond creek and chisna porphyries along the nutzotin basin wrangellia terrane boundary the porphyry deposits probably formed along the craton margin more than 1 000 km to the south of their present latitude palinspastic reconstructions of plate kinematics from this period are particularly difficult because magmatism overlaps the 119 to 83 ma cretaceous normal superchron a period when sea floor magnetic data are lacking our favored scenario is that ore formation broadly overlaps the cessation of sedimentation and contraction and the transition to a transpressional continental margin regime such that the remnant ocean basins were converted to strike slip basins the basins and outboard peninsular alexander wrangellia composite superterrane which are all located seaward of the deep crustal denali farewell fault system were subjected to northerly dextral transpression for as long as perhaps 50 m y beginning at ca 95 10 ma the onset of this transpression was marked by development of the mineralized bodies along fault segments on the seaward side of the basins geochemical and radiogenic isotopic data for igneous rocks associated with the pebble porphyry deposit suggest continuous melt derivation from enriched lithosphere of a recently metasomatized mantle these geochemical characteristics coupled with the arc continent related collisional setting suggest that lithospheric thickening and postcollisional lithospheric melting are the most likely cause of the ore related magmatism subsequent to translation of the alaskan margin terranes and early tertiary oroclinal bending of alaska the northernmost kahiltna basin and the pebble deposit as well as the other porphyry systems reached their present day locations along southern alaska

geochemical and geothermal evidence for fluid migration in the barbados accretionary prism odp leg 110 measured geochemical and geothermal effects of fluid migration in the northern barbados accretionary prism indicate that 1 fluid flows laterally along low angle active faults 2 the entire length of the leg 110 transect is a fluid discharge zone 3 migration results in heating of sediments across the transect but most strongly at the deformation front and 4 migration results in dilution of pore water cl and sup 18 o enrichment more in the arcward than the seaward side of the prism suggesting that the cl poor sup 18 o rich fluids that intrude the prism are derived from smectite illite reactions occurring arcward these conclusions have strengthened those derived from the initial appraisal of shipboard data modified ideas about where fluids are heated most and where the geochemistry is strongly altered and provided new ideas about the sources of fluids and their relative contributions

tsunamigenic potential of the shallow subduction plate boundary inferred from slow seismic slip at subduction zones slip along the shallowest parts of the plate boundary is generally thought to be aseismic observations of very low frequency earthquakes occurring at shallow levels on the plate boundary of the nankai trough subduction zone imply that slow but seismic slip can occur there and could potentially generate tsunamigenic earthquakes

mechanical and hydraulic properties of nankai accretionary prism sediments effect of stress path 1 we have conducted triaxial deformation experiments along different loading paths on prism sediments from the nankai trough different load paths of isotropic loading uniaxial strain loading triaxial compression at constant confining pressure pc undrained pc reduction drained pc reduction and triaxial unloading at constant pc were used to understand the evolution of mechanical and hydraulic properties under complicated stress states and loading histories in accretionary subduction zones five deformation experiments were conducted on three sediment core samples for the nankai prism specifically from older accreted sediments at the forearc basin underthrust slope sediments beneath the megasplay fault and overthrust upper shikoku basin sediments along the frontal thrust yield envelopes for each sample were constructed based on the stress paths of pc reduction using the modified cam clay model and in situ stress states of the prism were constrained using the results from the other load paths and accounting for horizontal stress results suggest that the sediments in the vicinity of the megasplay fault and frontal thrust are highly overconsolidated and thus likely to deform brittle rather than ductile the porosity of sediments decreases as the yield envelope expands while the reduction in permeability mainly depends on the effective mean stress before yield and the differential stress after yield an improved understanding of sediment yield strength and hydromechanical properties along different load paths is necessary to treat accurately the coupling of deformation and fluid flow in accretionary subduction zones

late cenozoic tectonic evolution of the north central mojave desert inferred from fault history and physiographic evolution of the fort irwin area california

37cl 35cl variations in a diffusion controlled groundwater system abstract groundwaters in the low permeability quaternary glacial deposits of southwestern ontario commonly exhibit increasing cl concentrations with depth because these deposits were formed in a freshwater glacial environment and because cl is not derived from weathering of these deposits previous investigations hypothesized that upward diffusion of saline formation waters from the shallow bedrock caused this cl trend this hypothesis was tested by measuring 37 cl 35 cl ratios in the groundwaters from two typical sites and comparing the results with theoretically derived isotopic distributions produced by diffusion 37cl measurements expressed relative to standard mean ocean chloride smoc ranged from 0 8 to 2 2 and vertical profiles showed a gradual enrichment in 35cl relative to 37cl with increasing distance upward from the bedrock till interface 537cl distributions were simulated with a mathematical solute transport model incorporating the effects of kinetic isotopic partitioning a sensitivity analysis indicated that the model results were affected by the groundwater velocity the isotope enrichment factor and diffusion times agreement between the simulated and observed cl isotope profiles was obtained using a groundwater velocity of 0 an isotopic enrichment factor of 1 0012 and diffusion times ranging from 14 000 to 16 000 years based on these results we conclude that the isotopic enrichment in 35cl was caused by upward diffusion over a period of about 15 000 years this diffusion period is consistent with what is known about the quaternary history of the area

evolution of the central garlock fault zone california a major sinistral fault embedded in a dextral plate margin the garlock fault is an integral part of the plate boundary deformation system inboard of the san andreas fault california usa however the garlock is transversely oriented and has the opposite sense of shear the slip history of the garlock is critical for interpreting the deformation of the through going dextral shear of the walker lane belt eastern california shear zone the lava mountains summit range lmsr located along the central garlock fault is a miocene volcanic center that holds the key to unraveling the fault slip and development of the garlock the lmsr is also located at the intersection of the nnw striking dextral blackwater fault and contains several sinistral wsw striking structures that provide a framework for establishing the relationship between the sinistral garlock fault system and the dextral eastern california shear zone new field mapping and geochronology data 40 ar 39 ar and u pb show five distinct suites of volcanic sedimentary rock units in the lmsr overlain by pliocene exotic clast conglomerates this stratigraphy coupled with fifteen fault slip markers define a three stage history for the central garlock fault system of 11 7 ma 7 3 8 ma and 3 8 0 ma pliocene to recent slip occurs in a 12 km wide zone and accounts for 33 km or 51 of the total 64 km of left lateral offset on the garlock fault in the vicinity of the lmsr since 3 8 ma this history yields slip rates of 6 9 mm yr for the younger stage and slower rates for older stages the lmsr internally accommodates northwest directed dextral slip associated with the eastern california shear zone walker lane belt via multiple processes of lateral tectonic escape folding normal faulting and the creation of new faults the geologic slip rates for the garlock fault in the lmsr match with and explain along strike variations in neotectonic rates

a new indicator mineral methodology based on a generic bi pb te s mineral inclusion signature in detrital gold from porphyry and low intermediate sulfidation epithermal environments in yukon territory canada porphyry epithermal and orogenic gold are two of the most important styles of gold bearing mineralization within orogenic belts populations of detrital gold resulting from bulk erosion of such regions may exhibit a compositional continuum wherein ag cu and hg in the gold alloy may vary across the full range exhibited by natural gold this paper describes a new methodology whereby orogenic and porphyry epithermal gold may be distinguished according to the mineralogy of microscopic inclusions observed within detrital gold particles a total of 1459 gold grains from hypogene eluvial and placer environments around calc alkaline porphyry deposits in yukon nucleus revenue casino sonora gulch and cyprus klaza have been characterized in terms of their alloy compositions au ag cu and hg and their inclusion mineralogy despite differences in the evolution of the different magmatic hydrothermal systems the gold exhibits a clear bi pb te s mineralogy in the inclusion suite a signature which is either extremely weak or most commonly absent in both yukon orogenic gold and gold from orogenic settings worldwide generic systematic compositional changes in ore mineralogy previously identified across the porphyry epithermal transition have been identified in the corresponding inclusion suites observed in samples from yukon however the bi te association repeatedly observed in gold from the porphyry mineralization persists into the epithermal environment ranges of p t x conditions are replicated in the geological environments which define generic styles of mineralization these parameters influence both gold alloy composition and ore mineralogy of which inclusion suites are a manifestation consequently we propose that this methodology approach can underpin a widely applicable indicator methodology based on detrital gold

the giant pebble cu au mo deposit and surrounding region southwest alaska introduction the pebble deposit is located about 320 km southwest of and 27 km northwest of the village of iliamna in alaska fig 1a it is one of the largest porphyry deposits in terms of contained cu fig 2a and it has the largest au endowment of any porphyry deposit in the world fig 2b the deposit comprises the pebble west and pebble east zones that represent two coeval hydrothermal centers within a single system lang et al 2013 together the measured and indicated resources total 5 942 million metric tons mt at 0 42 cu 0 35 g t au and 250 ppm mo with an inferred resource of 4 835 mt at 0 24 cu 0 26 g t au and 215 ppm mo in addition the deposit contains significant concentrations of ag pd and re northern dynasty minerals 2011 fig 1 a map of southwestern alaska showing extensive cover by tertiary and quaternary deposits and locations of known mineral deposits with outline of the pebble district distribution of tertiary and quaternary deposits is after wilson et al 2006 b map of the pebble district showing locations of deposits all deposits are named as zones for example the 37 zone regional mineral deposit locations are from u s geological survey 2008 descriptions of district deposits are in hawley 2004 and lang et al 2013 boundaries for the pebble west and pebble east zones are based on 0 3 and 0 6 cu equiv respectively fig 2 the 25 largest porphyry copper deposits based on a the tonnage of contained copper metal and b the tonnage of contained gold modified from cooke et al 2005 the classifications of deposits into cu mo cu au mo and cu au types are described in cooke et al 2005 unlike most porphyry deposits in southwest alaska which are 75 ma u s geological survey 2008 re os

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

evidence for post early miocene initiation of movement on the garlock fault from offset of the cudahy camp formation east central california the cudahy camp formation located in the el paso basin east central california consists of a 350 m thick volcanic and sedimentary sequence of early miocene age previous investigators determined from paleocurrent indicators that the source for these rocks was to the south and southeast but failed to suggest a specific location for that source restoration of 64 km of left slip on the garlock fault places cudahy camp rocks north and northwest of the early miocene eagle crags volcanic field the unique geochemical nature of basalts found in both locations coincident ages of volcanic rocks identical clast content of volcanic tuffs and identical stratigraphic sequence indicate that the source for the cudahy camp formation was the eagle crags volcanic field this is the first documented occurrence of miocene rocks offset by the garlock fault because the cudahy camp rocks and their sources appear to be offset by the full displacement on the garlock fault we conclude that movement on the garlock started after earliest middle miocene time further constraining initiation of movement on the garlock fault adds significantly to fully understanding the evolution of the western north american plate boundary in the middle to late cenozoic

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

process recognition in multi element soil and stream sediment geochemical data abstract stream sediment and soil geochemical data from the upper and lower coastal plains of south carolina usa were studied to determine relationships between soils and stream sediments from multi element associations characteristic compositions were determined for both media primary associations of elements reflect mineralogy including heavy minerals carbonates and clays and the effects of groundwater the effects of groundwater on element concentrations are more evident in soils than stream sediments a winnowing index was created using ratios of th to al that revealed differing erosional and depositional environments both soils and stream sediments from the upper and lower coastal plains show derivation from similar materials and subsequent similar multi element relationships but have some distinct differences in the lower coastal plain soils have high values of elements concentrated in heavy minerals ce y th that grade into high values of elements concentrated into finer grain size lower density materials primarily comprised of carbonates and feldspar minerals mg ca na k al these gradational trends in mineralogy and geochemistry are inferred to reflect reworking of materials during marine transgressions and regressions upper coastal plain stream sediment geochemistry shows a higher winnowing index relative to soil geochemistry a comparison of the 4 media upper coastal plain soils and stream sediments and lower coastal plain soils and stream sediments shows that upper coastal plain stream sediments have a higher winnowing index and a higher concentration of elements contained within heavy minerals whereas lower coastal plain stream sediments show a strong correlation between elements typically contained within clays it is not possible to calculate a functional relationship between stream sediment soil compositions for all elements due to the complex history of weathering deposition reworking and re deposition however depending on the spatial separation of the stream sediment and soil samples some elements are more highly correlated than others

broad weak regions of the nankai megathrust and implications for shallow coseismic slip abstract deep within the nankai trough subduction zone the plate boundary thrust slips along a well imaged megasplay fault system during the megathrust earthquakes that regularly strike southwest japan the routing of the active plate boundary thrust along an upward branching splay fault causes deep underthrusting of an unusually thick section of material attached to the subducting ocean crust here we present three dimensional seismic reflection data that shows this unusually thick section is fluid rich sediment that results in broad weakly coupled regions of the megathrust down into the updip end of the seismogenic zone the weakly coupled regions lie above an underthrust low seismic impedance presumed to be low velocity and low density sediment section that is between one and two kilometers thick and covers 3300 km2 at least one eighth of the total rupture area of the 1944 mw 8 1 tonankai earthquake this underthrust sediment section is broader and much deeper than inferred at other margins sediment underthrusting into the normally well coupled seismogenic zone likely releases fluids and elevates fluid pressure which reduces inter plate coupling along portions of the megasplay fault and allows coseismic rupture to propagate to unusually shallow depths and generate large tsunami as inferred for the 1944 tonankai event splay faults may be a common yet transient mechanism for developing weak subduction zone thrusts

hydrologic detection and finite element modeling of a slow slip event in the costa rica prism toe 1 we investigate transient fluid flux through the seafloor recorded near the costa rica trench during the 2000 costa rica seismogenic zone experiment using a 2 d fully coupled poroelastic finite element model we demonstrate that the observed hydrologic anomalies are consistent with a model of propagating slow slip at the subduction interface between the frontal prism and downgoing plate there are two sources of volumetric strain that drive fluid flux at the seafloor in response to fault slip at depth 1 compression and dilation in the vicinity of the tips of a slipping patch and 2 extension and compression due to flexure of the seafloor the superposition of these two effects results in distinctive spatial and temporal patterns of fluid flow through the seafloor in a forward modeling approach time series from shear ruptures with a range of fault length to depth ratios in a heterogeneous crust are generated and compared with flow rate observations assuming a constant propagation rate and an elliptical profile for the distribution of slip along the decollement the set of model predictions enables us to infer the probable rupture location extent propagation velocity and duration from a single flow rate time series the best fit model suggests that the slow slip event initiated within the toe at a depth of less than 4 km and propagated bilaterally at an average rate of 0 5 km d 1 this interpretation implies that stress in the shallow subduction zone is relieved episodically furthermore the costa rica data suggest that episodic slow slip events may initiate in the prism toe without being triggered by a seismic event further downdip

stratigraphy and geochemistry of volcanic rocks in the lava mountains california implications for the miocene development of the garlock fault volcanism in the lava mountains occurred between 11 7 and 5 8 ma and was contemporaneous with sinistral motion on the garlock fault volcanic rocks equivalent in age and chemistry to those in the lava mountains crop out 40 km to the southwest in the el paso mountains across the garlock fault three chemical groups of volcanic rocks erupted in the lava mountains over a period of 5 m y these are 1 andesite of summit diggings almond mountain volcanic section and lava mountains andesite 2 basalt of teagle wash and 3 tuffs in the northeastern lava mountains and dacite in the summit range volcanic rocks of each group have distinctive chemical signatures useful for correlation of units across the garlock fault our work demonstrated that tuffs in the almond mountain volcanic section may be equivalent to a tuff in member 5 of the miocene dove spring formation el paso mountains the basalt of teagle wash probably correlates with basalt flows in member 4 and tuffs in the northeast lava mountains may be equivalent to tuff of member 2 correlation of these units across the garlock fault implies that the lava mountains were situated south of the el paso mountains between 10 3 and 11 6 ma and that 32 40 km of offset occurred on the garlock fault in 10 4 m y resulting in a displacement rate of 3 1 to 3 8 mm yr projecting this rate to the total offset of 64 km on the garlock suggests that left lateral slip began at ca 16 4 ma smith e i sanchez a keenan d l and monastero f c 2002 stratigraphy and geochemistry of volcanic rocks in the lava mountains california implications for the miocene development of the garlock fault in glazner a f walker j d and bartley j m eds geologic evolution of the mojave desert and southwestern basin and range boulder colorado geological society of america memoir 195 p 151 160 data repository item 2002 contains additional material related to this article introduction the lava mountains lie just south of the active garlock fault a major continental strike slip fault that separates the southwestern basin and range to the north from the mojave block to the south the fault extends 250 km from the san andreas fault to the avawatz mountains just south of death valley fig 1 total sinistral displacement on the garlock fault is 64 km smith 1962 smith and ketner 1970 davis and burchfiel 1973 monastero et al 1997 estimates of the initiation of faulting vary from 10 to 9 ma burbank and whistler 1987 loomis and burbank 1988 to after 17 ma monastero et al 1997 faulting continued to the present with a minimum of 18 km of displacement occurring across the central garlock e i smith et al 152 figure 1 geologic map of the lava mountains and neighboring summit range and black hills modified from smith 1964 dibblee 1967 and keenan 2000 wlmv western lava mountains volcano inset map of california shows the following locations gf garlock fault sa san andreas fault lm lava mountains ec eagle crags volcanic field ep el paso mountains and cr coso range box indicates area covered by figure 3 ec

path and amount of dextral fault slip in the eastern california shear zone across the central mojave desert new total fault slip estimates for the central mojave desert portion of the eastern california shear zone support through going dextral shear of 18 km that is transferred northward to the garlock fault the eastern california shear zone accommodates 25 of the plate boundary shear with the transversely oriented garlock fault forming its northern boundary with the walker lane belt total fault slip estimates for the major faults in the central mojave desert were determined using detailed geologic mapping supplemented with aeromagnetic data offset data for the blackwater fault show a consistent 1 8 0 1 km of dextral slip along 55 km of strike length and they indicate that slip initiated at or after 3 8 ma the faults adjacent to the blackwater fault have dextral slip of 4 8 0 3 km harper lake fault 2 9 0 5 km mount general fault 1 0 0 7 km lockhart fault and 3 2 0 3 and 0 53 0 05 km paradise fault these slip data augment published data for the central mojave desert and allow evaluation of the strain path in the central mojave desert showing consistent along strike through going dextral slip contractional and extensional step overs in the central mojave desert accommodate a change from focused slip in the southern central mojave desert to a wider and more dispersed fault system northward with a shift in the locus of dextral shear westward we interpret these changes in the fault system to be a response to the garlock fault which inhibits through going faulting but allows through going dextral shear via clockwise deflection of its trace

effect of smectite dehydration on pore water geochemistry in the shallow subduction zone an experimental approach 1 the diagenetic smectite to illite transition is widely accepted to cause ubiquitous pore water freshening at convergent margins however experimental consolidation tests show that smectite also dehydrates partially with increasing effective stress to shed light on this process three hydrothermal consolidation tests on a silty smectite rich claystone were conducted with stresses p up to 70 mpa and at constant temperatures t of 20 c 60 c and 100 c fluids expelled during the tests were analyzed for major and trace elements to evaluate dehydration and fluid rock interaction with increasing pt conditions the results document that fluid freshening starts when the effective stress exceeds 1 3 mpa the smectite interlayer water content decreases from 27 wt to 20 wt during the experiments which is equivalent to an interlayer collapse from 18 5 to 15 4 a the released interlayer h20 accounts for up to 17 of the total fluid volume released from the consolidating sediment major and volatile element geochemistry is influenced by temperature and smectite interlayer collapse the interlayer collapse is characterized by k and ca uptake while is b released from smectite application of experimental results to the barbados accretionary margin shows that stress dependent smectite dehydration is an important fluid source mechanism and accounts for substantial fluid freshening modeled smectite dehydration suggests a decrease of smectite interlayer water content from 27 wt at the surface down to 22 wt at 2 km depth this leads to chlorinity values as low as 460 mm

extreme multi millennial slip rate variations on the garlock fault california strain super cycles potentially time variable fault strength and implications for system level earthquake occurrence abstract pronounced variations in fault slip rate revealed by new measurements along the garlock fault have basic implications for understanding how faults store and release strain energy in large earthquakes specifically dating of a series of 26 0 3 5 2 5 m fault offsets with a newly developed infrared stimulated luminescence method shows that the fault was slipping at 14 0 2 2 1 8 mm yr approximately twice as fast as the long term average rate during a previously documented cluster of four earthquakes 0 5 2 0 ka this elevated late holocene rate must be balanced by periods of slow or no slip such as that during the ca 3300 yr long seismic lull preceding the cluster moreover whereas a comparison of paleoseismic data and stress modeling results suggests that individual garlock earthquakes may be triggered by periods of rapid san andreas fault slip or very large slip events the on off behavior of the garlock suggests a longer term mechanism that may involve changes in the rate of elastic strain accumulation on the fault over millennial time scales this inference is consistent with most models of the geodetic velocity field which yield slip deficit rates that are much slower than the average latest pleistocene early holocene post 8 13 ka garlock slip rate of 6 5 1 5 mm yr these observations indicate the occurrence of millennia long strain super cycles on the garlock fault that may be associated with temporal changes in elastic strain accumulation rate which may in turn be controlled by variations in relative strength of the various faults in the garlock san andreas eastern california shear zone fault system and or changes in relative plate motion rates

compositional and fluid pressure controls on the state of stress on the nankai subduction thrust a weak plate boundary abstract we show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the nankai subduction plate boundary off southwest japan ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients r at stress levels between 1 0 and 40 mpa the direct shear r values for purified smectite are 0 14 0 02 for illite 0 25 0 01 and for chlorite 0 26 0 02 for point load velocities of 0 0001 mm s these clay minerals dominate the nankai subduction decollement zone illite plus quartz is mechanically important in the altered incoming muroto section and the predicted decollement r should lie between 0 2 and 0 32 this low residual strength together with elevated fluid pressure limits shear stresses to below 4 mpa within the frontal 50 km of the subduction system consistent with the low wedge taper in this region a higher wedge taper off the ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in r are predicted to occur as a result of variations in smectite and total clay content these variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite however our data suggest the low r values of the clay rich decollement still limit shear stresses to between 17 and 29 mpa within the frontal 50 km of the wedge consistent with other estimates of plate boundary weakness indeed we propose that it should be expected that subduction plate boundaries like nankai will be weak because of the intrinsic presence of clay rich faults and moderate fluid overpressures our data do not support the hypothesis that the smectite to illite reaction directly controls the onset of seismogenic behavior deep in the nankai system because there is already a mechanical dominance of illite rather than smectite in the shallow decollement zone and we find all the clay phases tend to velocity strengthen however temperature activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up dip limit of the seismogenic zone

temporally constrained tectonic rotations derived from magnetostratigiraphic data implications for the initiation of the garlock fault california examination of declination anomalies within a well controlled chronologic framework can permit precise delineation of the progression of a tectonic rotation in the past declination anomalies from numerous spatially separated and individually dated localities have been synthesized in order to define regional progressive rotations rarely however have magneto stratigraphic data from a single locality been used successfully to chronicle a sequential rotation that resulted from movement along a nearby fault magnetostratigraphic studies from the el paso basin in southern california reveal progressively smaller amounts of counterclockwise rotation for strata dated between 11 and 7 ma two tests indicate that this is a temporally controlled rather than a spatially controlled rotation this rotation is interpreted to have resulted from sinistral shear along the garlock fault as such it appears to delimit the initiation of miocene movement along the western segment of this major strike slip fault

mineral exploration and basement mapping in areas of deep transported cover using indicator heavy minerals and paleoredox fronts yilgarn craton western australia abstract glacial sediments have been thoroughly integrated into mineral exploration protocols in the northern hemisphere e g canada and fennoscandia but have received less attention in australia in western australia wa permo carboniferous diamictites buried by younger cenozoic sediments cover scattered areas that have potential to host gold and nickel mineralization in the ne of the yilgarn craton a systematic stratigraphic mineralogical and geochemical study was undertaken to determine whether diamictites in the agnew lawlers gold province have properties that may help target concealed mineralization at the agnew lawlers district rocks intersected in 31 drill holes ne of the waroonga gold mine were interpreted in terms of lithological textural and paleolandscape mineralogical and geochemical variations to select the best sampling media for mineral exploration and provenance assessment the paleotopographic variation asymmetry of the depositional basin polymictic composition matrix supported mixed angular and rounded and poorly sorted texture of the diamictite clasts indicate mechanical dispersal from proximal and distal heterogeneous source rocks ferromagnesian minerals chromite cr magnetite magnetite and ni cu fe sulfides are abundant in the diamictite close to mafic ultramafic source rocks monazite apatite zircon and clasts derived from pyrite bearing quartz veins increase in abundance in the diamictite close to the waroonga shear zone that separates granitoid gneiss terrain from the scotty creek metasediments ilmenite is extensively distributed in diamictites throughout the study area and its mn content can be used to differentiate between felsic and mafic source rocks alteration of ilmenite to titanite monazite to apatite and thorite and replacement and fracture filling of pentlandite to monazite suggest felsic intrusive related hydrothermal alteration of the source rocks prior to mechanical weathering late paleozoic tropical weathering associated with oscillation of water table and icehouse to greenhouse climatic shift has created paleoredox fronts in the permo carboniferous sequence below the redox fronts diamictites are unweathered rich in detrital sulfide and opaque oxide minerals and can be used in provenance studies and tracing mineralization these characteristics of unweathered diamictite are the optimal sampling medium target for provenance studies and mineral exploration these are cemented mainly by calcareous cement where they overlie mafic ultramafic rocks and by pyrite cement where they overlie the scotty creek metasediments above the redox fronts diamictites are variably weathered to ferruginous and bleached kaolinitic saprolites which are stable under oxidizing circum neutral conditions ferruginous diamictites are rich in recycled redox sensitive elements derived from oxidation of ferromagnesian sulfide and opaque oxide heavy minerals these weathering features can be used to identify weathered diamictite for sampling for hydromorphic metal dispersion studies

vp vs ratio and shear wave splitting in the nankai trough seismogenic zone insights into effective stress pore pressure and sediment consolidation to estimate variation of stress state and sediment consolidation in the nankai plate subduction zone off southwest japan we measured the p wave to s wave velocity ratio vp vs and s wave splitting along the seismic line extending from the trench to the seismogenic zone for this purpose we used active source seismic data recorded by multicomponent ocean bottom seismometers obs because it is difficult to identify the ps converted reflection waveforms for each of the geological boundaries in this deep offshore region we focused on the more easily identified pps refracted waveforms that register the conversion of the up going p waves to s waves at the igneous crust surface we estimated the average vp vs ratio within the sedimentary section by using the time lag between the p refracted waves and pps converted waves this vp vs ratio changes abruptly at the trough axis i e the deformation front of the accretionary prism arguably because of compaction associated with the accretion process we observ

the action of water films at a scales in the earth implications for the nankai subduction system abstract water properties change with confinement within nanofilms trapped between natural charged clay particles we investigated nanofilm characteristics through high stress laboratory compression tests in combination with analyses of expelled pore fluids we utilized sediments obtained from deep drilling of the nankai subduction zone at site c0002 of the integrated ocean drilling program iodp we show that below 1 2 km there should be widespread ultrafiltration of migrating fluids experiments to 100 mpa normal compression collapse pores below a few ion monofilm thicknesses a reduction towards a single condensing dehydrating ion monofilm occurs as stresses rise 100 200 mpa and clay separations are reduced to

consolidation state and strength of underthrust sediments and evolution of the decollement at the nankai accretionary margin results of uniaxial reconsolidation experiments 1 uniaxial reconsolidation experiments conducted on ocean drilling program drill cores along the muroto transect of the nankai accretionary margin demonstrate complex yield and postyield behavior and provide evidence for enhanced strengths within sediments beneath the decollement zone tests were conducted on samples collected from similar stratigraphic levels below the decollement and its seaward projection consolidation state of the samples increased landward and with depth and was tracked closely by sediment yield stress for all but one of the samples the sediments however exhibited substantial postyield strength up to 2 8 times the predicted in situ effective vertical stress beneath the protothrust zone this enhanced strength results from diagenesis that leads to matrix cementation during stable effective stress conditions within the underthrust section the close correspondence between yield stress and predicted in situ effective stress suggests that despite the cemented state the sediment matrix remains sensitive to in situ stress conditions the low yield stress of one sample collected within 40 m of the decollement fault at site 808 may reflect diagenesis under reduced effective stress conditions due to postconsolidation increases in pore pressure along the decollement due to their cemented state the strong underthrust sediments resist decollement downcutting beneath the toe of the prism leading to preferential incorporation of weaker continuously deforming accreted sediments during shear seismogenic slip along the decollement at depth may create stresses in excess of sediment strength causing shear failure and rapid release of trapped pore fluids feeding high pore fluid pressures along the decollement zone