wrinkle ridge assemblages on the terrestrial planets the wrinkle ridge assemblage is a group of associated features that have been observed on the moon mars and mercury members of the wrinkle ridge assemblage are classified as either arches or ridges based on morphology and ridges are subdivided into first second or third order ridges on the basis of dimensions the ridge assemblages described in this study are known to occur in mare basalts on the moon and smooth plains units that may be volcanic in origin on mars and mercury the structures that occur in the continental flood basalts of the columbia plateau in the northwestern united states are morphologically and dimensionally similar to features in the ridge assemblages on the terrestrial planets the anticlinal ridges of the columbia plateau and many first order ridges on the moon mars and mercury are interpreted to be folds with reverse or thrust faulting developed as a result of buckling flexure fracture or buckling as a consequence of reverse or thrust faulting fracture flexure estimates of the unit shortening across first order ridges using simple geometric forms to approximate the ridge morphology are lower than those determined for the anticlines of the columbia plateau where displacements are the results of folding and reverse to thrust faulting evidence of layer parallel extension that often accompanies folds developed by fracture flexure mechanisms i e drape or fault bend folds is not found in the anticlinal ridges of the columbia plateau and has only been observed in one ridge on the moon a kinematic model involving buckling followed by reverse to thrust faulting flexure fracture is favored for the origin of the anticlinal ridges of the columbia plateau and many of the first order ridges on the moon mars and mercury

evidence for thin oceanic crust on the extinct aegir ridge norwegian basin ne atlantic derived from satellite gravity inversion 1 satellite gravity inversion incorporating a lithosphere thermal gravity correction has been used to map crustal thickness and lithosphere thinning factor for the ne atlantic predicted oceanic crustal thicknesses in the norwegian basin are between 4 and 7 km on the extinct aegir ridge increasing to 9 14 km at the margins consistent with volcanic margin continental breakup at the end of the paleocene the observation from gravity inversion and seismic refraction studies of thin oceanic crust produced by the aegir ridge in the oligocene may have implications for the temporal evolution of asthenosphere temperature under the ne atlantic during the tertiary thin oligocene oceanic crust may imply cool normal asthenosphere temperatures during the oligocene in contrast to elevated asthenosphere temperatures in the paleocene and miocene recent as indicated by the formation of volcanic margins and iceland respectively

influence of scale and matedness on the peak shear strength of fresh unweathered rock joints it is widely recognized that the mechanical parameters for unfilled and rough rock joints such as the peak shear strength can vary with scale however due to contradictory results concerning the extent and nature of the scale effect reported in the literature it is still a debated subject a conceptual model developed by johansson and stille 2014 suggests how roughness and matedness at different scales influences the peak shear strength for fresh rough and unweathered joint however the model s ability to predict how the roughness and matedness affects the peak shear strength at different scales was not verified the aim of this paper is to investigate the ability of the conceptual model to estimate the peak shear strength at different degrees of matedness and scales a series of direct shear test were carried out at two different scales and two different degrees of matedness the peak shear strength from the tests was compared to the peak shear strength calculated with the conceptual model the results showed that the model can predict the peak shear strength for both the perfectly mated and the unmated joints no scale effect was observed in the shear tests which is in line with the predictions using the model the influence of matedness in combination with scale might explain some of the contradictory findings regarding the scale effect

seismic tomographic imaging of the eastern mediterranean mantle implications for terminal stage subduction the uplift of anatolia and the development of the north anatolian fault

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

mid miocene propagation of the yellowstone mantle plume head beneath the columbia river basalt source region columbia river flood basalt volcanism is the product of mid miocene distortion of the yellowstone mantle plume head against the precambrian margin of north america the overall migration pattern of columbia river basalt vents reflects northward propagation of the plume head beneath the west southwest migrating north american plate thermal uplift mimics the vent migration over the 1 5 m y of grande ronde basalt extrusion the projected crest of the plume head migrated 105 km 7 cm yr from the chief joseph dike swarm east oortheast into the clearwater embayment where uplift occurred over the same period at a rate of 0 67 mm yr chemical variations reflect progressive melting of the plume and mantle lithosphere followed by the mixing of these components with an enriched source from the overriding craton rapid thickening of the plume head against the cratonic margin led to decompressional melting and magma rise beneath a thin oceanic lithosphere of accreted terranes

geologic studies of the columbia plateau part ii upper miocene basalt distribution reflecting source locations tectonism and drainage history in the clearwater embayment idaho the clearwater embayment of western idaho represents the easternmost encroachment of columbia river basalt against the western side of the rocky mountains several new units of the columbia river basalt group have been identified in the embayment and the distribution of these and other previously recognized flows reflects 1 the location of vents feeder dikes and source areas from which the flows were erupted 2 tectonic events during the eruptive episode and 3 the development of erosional channels that were filled subsequently by successive flows the lewiston and stites basins were filled by wanapum and saddle mountains flows erupted in the north fork of the clearwater river area flows poured into the stites basin from surrounding plateau surfaces but they spread into the lewiston basin after advancing down ancestral clearwater river canyons as an ancestral clearwater river would lose its canyon to an invading flow water would spill onto the low lying areas of the lewiston basin and deposit sediment this event was repeated several times and resulted in a sequence of alternating flows and intercalated sediment in the lewiston basin

paleomagnetism of middle miocene volcanic rocks in the mojave sonora desert region of western arizona and southeastern california paleomagnetic directions have been obtained from 190 early to middle miocene 12 20 ma mafic volcanic flows in 16 mountain ranges in the mojave sonora desert region of western arizona and southeastern california these flows generally postdate early miocene tectonic deformation accommodated by low angle normal faults but predate high angle normal faulting in the region after detailed demagnetization experiments 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations trm because of the episodic nature of basaltic volcanism in this region the 179 flows yielded only 65 time distinct virtual geomagnetic poles vgps the angular dispersion of the 65 vgps is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation the paleomagnetic pole calculated from the 65 cooling unit vgps is located at 85 5 n 108 9 within a 4 4 circle of 95 confidence this pole is statistically indistinguishable at 95 confidence from reference poles calculated from rocks of similar age in stable north america and from a paleomagnetic pole calculated from rocks of similar age in baja california the coincidence of paleomagnetic poles from the mojave sonora desert region with reference poles from the stable continental interior indicates that 1 significant vertical axis net tectonic rotations have not accompanied post middle miocene high angle normal faulting in this region 2 there has been no detectable post middle miocene latitudinal transport of the region and 3 long term nondipole components of the middle miocene geomagnetic field probably were no larger than those of the recent 0 5 ma geomagnetic field in contrast paleomagnetic data indicate vertical axis rotations of similar age rocks in the transverse ranges the eastern transverse ranges and the mojave block we speculate that a major structural discontinuity in the vicinity of the southeastward projection of the death valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

a case of simple magma mixing in the columbia river basalt group the wilbur creek lapwai and asotin flows saddle mountains formation three small sequential saddle mountains flows occupying similar areas on the columbia plateau were erupted over a short interval of time in the lewiston basin area the middle flow of the trio lapwai has intermediate mineralogy and lies on a straight mixing line between the other two flows for virtually all twenty five elements analyzed systematic changes in the ratios of incompatible elements demonstrate that these relationships are a result of magma mixing rather than either crystal fractionation or variable degrees of partial melting the lapwai flow has a composition approximately midway between the two homogeneous end members and is itself relatively homogeneous this implies efficient mixing between equal amounts of asotin and wilbur creek magmas and suggests that mixing was completed in a magma reservoir prior to eruption the wilbur creek and asotin end members have isotopic features which are believed to result from different degrees of assimilation of crustal material by magma derived from an enriched mantle source carlson 1984 the mixing processes described here cannot be related to that earlier mantle crust mixing process

rotation about an inclined axis three dimensional matrices for reconstructing paleomagnetic and structural data abstract we derive matrices for calculating the three dimensional finite rotation that restores a rigid block to its pre displacement attitude to use this technique an initial horizontal bedding is assumed and the following orientations must be known the expected field orientation of magnetization the situ orientation of the paleomagnetic vector and the bedding normal the restoring rotation is decomposed into two convenient steps represented by rotation matrices such that they restore the in situ paleomagnetic vector together with the tilted bedding normal to their original orientation these auxiliary rotations combine algebraically to a single finite rotation from which an inclined axis and amount of rotation arc obtained we demonstrate this method by reconstructing a distinct section of the mount sedom salt diapir dead sea rift valley israel here the conventional tilt correction that brings the bedding to horizontal fails to position the paleomagnetic vector in its expected orientation only a rotation of 125 about an axis which plunges 50 southward can restore both bedding and magnetization to their expected orientations

magnetic stratigraphy and tectonic rotation of the eocene oligocene keasey formation northwest oregon the upper eocene lower oligocene keasey formation in northwestern oregon consists of 700 m of deep marine forearc basin sediments exposed in stream beds and roadcuts the abundant molluscs of this unit were the basis for the keasey stage of the weaver et al 1944 timescale but the precise age of this fauna and formation have long been controversial analysis of over 130 magnetic samples from the keasey in five sections yielded a stable remanence held mainly in magnetite which passed a reversal test on the basis of calibration by benthic foraminifera and molluscs the keasey formation is correlative with chrons c15r c12r 35 0 33 0 ma so the unit spans the latest eocene and early oligocene the magnetic directions suggest a tectonic rotation of 73 7 4 7 which is consistent with rotation of the nearest volcanic basement rock reported by wells and coe 1985

post 12 m y rotation of southwest washington paleomagnetic field directions from the basalt of pack sack lookout are compared to those from the pomona member of the saddle mountains basalt of the columbia river basalt group the pomona crops out over a wide region on the columbia plateau east of the cascade range and the basalt of pack sack lookout crops out well to the west of the cascades about 30 to 60 km east of the washington coast our paleomagnetic results support existing geologic and geochemical evidence that both these units are part of a single 12 m y old flow that erupted in western idaho and flowed to the washington coast the magnetic data further indicate that southwest washington has undergone clockwise rotation with respect to the columbia plateau and stable north america the data suggest that both a large scale regional rotation of 16 and locally complex small scale rotations exist the two being present to different degrees in the eastern and western regions of southwest washington the cascade range appears to coincide with the tectonic boundary separating rotated and unrotated regions of washington state rotation of southwest washington appears to have been associated with the rotation of large pieces of western oregon and southern washington possibly as a result of either a ridge push force from the basin and range province or shear along the pacific north america plate boundary

central washington seismicity evidence for a reactivated buried continental rift and northwest trending structural zones analysis of central washington seismicity of the past two decades reveals some interesting features shallow seismicity and deep seismicity occur as different geographic distributions concentration of seismicity along north to northwest oriented trends appears to be related to a buried continental rift and possible associated fault zones hypothesized extensions of the chiwaukum graben and straight creek fault systems are plausible structural controls on the seismicity

application of paleomagnetism to accretionary tectonics and structural geology the use of paleomagnetism to determine translations and rotations of geologic terranes has contributed to a better understanding of deformation within accretionary complexes shear zones and rifts in this report to the i u g g we review the progress resulting from paleomagnetic studies during the last four years in the cordillera of north america the appalachian orogen and the circum pacific region

tillamook volcanic series further evidence for tectonic rotation of the oregon coast range paleomagnetic directions from the eocene tillamook volcanic series of the oregon coast range point 46 o clockwise from the expected eocene field direction potassium argon dating of six dikes and flows from this formation yields a mean age of 44 3 0 6 m y these results establish that the oregon coastal block of simpson and cox 1977 extends north to the oregon washington border and that this block has rotated clockwise 46 o 13 o during the past 44 m y the block has undergone no detectable north south translation a two phase model is developed to explain the tectonic history and anomalous magnetic field directions of the pacific northwest region phase i of the rotation active between 50 and 42 m y b p results from fragmentation of the farallon plate with rotation of a fragment during its accretion to north america phase ii of the rotation active between 20 m y b p and the present occurs in association with extension in the basin and range province

post 12 million year clockwise rotation in the central columbia plateau washington the pomona flow 12 m y b p of the columbia river basalt group was sampled for paleomagnetic analysis at 32 sites in the yakima fold belt of the central columbia plateau to assess the pattern of deformation field directions from anticlinal ridges are found to be rotated clockwise relative to sites from synclinal areas two geographic patterns of rotation are present the primary pattern is one in which the greatest amount of rotation occurs in the crest hinge area of the anticlines and decreases into synclines a secondary pattern in which the amount of rotation is consistent within a geometric segment but varies among them is superimposed on the primary pattern the pattern of clockwise rotation is interpreted to be the result of local tectonic rotation occurring along a closely spaced north northwest trending primary shear system and northeast trending secondary shear system the shear system and concurrent rotation are interpreted to have developed during the growth of the yakima folds the relationship between the paleomagnetic data and the geologic structures can be used to constrain tectonic models for the columbia plateau the data reported in this study indicate that clockwise rotation can not be attributed to rigid block or microplate rotation but support models that predict localized dextral shear furthermore it is postulated that this mechanism may contribute to clockwise rotation observed in other parts of the western united states

potential field data in integrated frontier basin geophysics successes and challenges on australia s continental margin abstract offshore frontier sedimentary basins are characterised by a lack of constraining geological and geophysical data this lack of data is generally the result of deep water 500 m difficult geology volcanics and salt remoteness and harsh met ocean conditions these characteristics present significant challenges to marine surveying which means that frontier basins tend to be underexplored with continuing interest in exploration for energy resources in frontier regions many frontier basins around the world have been the focus of increasingly sophisticated geophysical studies that integrate a range of methodologies including those based on potential field gravity and magnetic data underexplored frontier basins around australia s continental margin have received increased attention during the last decade largely as a result of government funded programs of precompetitive data acquisition and analysis the components of this work that have relied heavily on potential field data include first pass depth to basement estimation using spectral techniques applied to magnetic data enhancement of gravity and magnetic images to aid the identification of basin depocentres and to facilitate onshore offshore geological interpretation of basement structure multi scale edge detection applied to gravity and magnetic data to aid the interpretation of basement structure 3d forward and stochastic inverse modelling of gravity data to guide seismic interpretation of sediment thickness and basement structure and using supercomputers for high resolution regional scale 3d inverse modelling of magnetic and gravity data to constrain the physical properties of the crust despite the additional insight offered by this work efforts to understand frontier basins are not without challenges one of the most fundamental of which is to ensure that non specialists are not misinterpreting data e g wrongly interpreting artefacts arising from specific processing the other main challenge in australian frontier basins arises from a lack of constraints on crustal structure this leads to significant ambiguity when using gravity data to infer sediment thickness or to understand the nature of basement this ambiguity could be vastly reduced through the acquisition of seismic refraction data that focuses on imaging crustal structure further opportunities exist in using alternative methods for automated depth to basement estimation incorporating process oriented rather than static potential field modelling and in applying 3d forward and inverse gravity and magnetic modelling to other australian frontier basins

ct scanning and flow measurements of shale fractures after multiple shearing events abstract a shearing apparatus was used in conjunction with a hassler style core holder to incrementally shear fractured shale cores while maintaining various confining pressures computed tomography scans were performed after each shearing event and were used to obtain information on evolving fracture geometry fracture transmissivity was measured after each shearing event to understand the hydrodynamic response to the evolving fracture structure the digital fracture volumes were used to perform laminar single phase flow simulations local cubic law with a tapered plate correction model to qualitatively examine small scale flow path variations within the altered fractures fractures were found to generally increase in aperture after several shear slip events with corresponding transmissivity increases lower confining pressure resulted in a fracture more prone to episodic mechanical failure and sudden changes in transmissivity conversely higher confining pressures resulted in a system where after an initial setting of the fracture surfaces changes to the fracture geometry and transmissivity occurred gradually flow paths within the fractures are largely controlled by the location and evolution of zero aperture locations a reduction in the number of primary flow pathways through the fracture and an increase in their width was observed during all shearing tests

the influence of crustal structure on compositions of subduction related magmas abstract a survey of sr isotopic ratios and other compositional features of subduction related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust i e thickness composition and age these observations lead to the conclusion that crust and or mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas typically basalt or basaltic andesite it is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks the extent to which such processes manifest themselves will depend upon details of the local crustal structure in particular the composition and age of the crust will strongly influence such parameters as sr nd and pb isotopic compositions such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust e g central andes depending upon the level of magma stagnation and evolution within the crust different trends in isotopic composition may result these isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas and locally they may reflect subduction of continental sediments interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust cascades e eleutians and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes consequently most island arc magmas and especially those more evolved than basalt are probably not primary in the sense that they do not represent direct melts of the upper mantle studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered it appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle derived magmatic components are more mafic in composition than average continental crust

shear behaviors of real natural un matching joints of granite with equivalent joint roughness coefficients abstract contact planes of a natural unfilled rock joint often portray dissimilar roughness characteristics particularly at shallow depth some researchers investigated shear strength behaviors of un matching joints i e joints with dissimilar surface morphologies through laboratory experiments using mimicked discontinuities this study aimed to explore shear behaviors of real natural un matching joints with similar averaged joint roughness coefficients jrcs in the singhbhum granite india based on laboratory investigations direct shear tests were performed on a total of 18 encapsulated jointed samples split into three groups under a three different low normal stresses 0 70 mpa with increasing values b three different high normal stresses within a range of 0 98 1 99 mpa with increasing values and c one particular intermediate normal stress 0 99 mpa for five times where direction of shearing was kept same as the dip direction of the joints in situ shear behaviors of the investigated joints portray diverse patterns moreover most of the obtained behavioral patterns look different from the typical shear stress shear displacement graph in the literature that are based on direct shear test results in case of matching rock discontinuities influence of surface morphology on shear behavior shear stress fluctuations with displacement variation of jrc values with the increase in normal stresses and observations on peak friction angle were critically analyzed effect of cycles of shearing on shear strength was also examined plausible reasons behind the uniqueness of the results were explained

paleomagnetism and crustal rotations along a shear zone las vegas range southern nevada in southern nevada paleozoic isopachs mesozoic structures and mountain ranges trend north south but as they approach the right lateral west northwest trending las vegas valley shear zone lvvsz they are bent clockwise up to 90 in oroflexes 30 km long in the las vegas range the declination of the characteristic magnetization of the cambrian bonanza king formation has likewise been rotated clockwise more than 50 for samples from eight sites 13 23 samples per site in the las vegas range and in its extension south of the lvvsz stepwise thermal demagnetization to 600 c removed recent and tertiary overprints revealing a nearly horizontal late paleozoic viscous partial thermoremanent magnetization vptrm which predates both the tertiary oroflexural bending and the sevier thrusting in the area the declination of the characteristic magnetization after correcting for the dip of bedding varies from 133 2 at a site 24 km north of the shear zone to 196 4 13 km north of it and 204 4 11 km south of the shear zone this parallels a similar change in the structural trend the paleomagnetic data indicate that the las vegas range is broken into rather small blocks 5 km which rotate in response to more continuous deformation at depth this deformation of the upper crust can be modelled using a thin non newtonian viscous plate model which when applied to the las vegas valley shear zone predicts rotations similar to those seen in the las vegas range

ridge subduction kinematics and implications for the nature of mantle upwelling ridge subduction follows the approach of an oceanic spreading centre towards a trench and subduction of the leading oceanic plate beneath the overriding plate there are four possible kinematic scenarios 1 welding of the trailing and overriding plates e g aluk antarctic ridge beneath antarctica 2 slower subduction of the trailing plate e g nazca antarctic ridge beneath chile and pacific izanagi ridge beneath japan 3 transform motion between the trailing and overriding plates e g san andreas transform or 4 divergence between the overriding and trailing plates e g pacific north america in case 4 the divergence may be accommodated in two ways the overriding plate may be stretched e g basin and range province extension which has brought the continental margin into collinearity and therefore transform motion with the pacific north america relative motion or divergence may occur at the continental margin and be manifest as a change in rate and direction of sea floor

unloading induced rock fracture activation and maximum seismic moment prediction abstract hundreds of anthropogenic earthquakes have recently occurred worldwide due to underground space creation and energy extraction the mechanism behind the human induced geohazards is most likely associated with the reduction of normal stress on pre existing fractures and faults this study reports a series of laboratory experiments to investigate the mechanism of unloading induced fracture activation and proposes a simple approach to predict the maximum seismic moment for a critically stressed fracture the unloading driven shear test results exhibit that the unloading process induces the stress states of the sawcut and natural fractures to approach the mohr coulomb failure envelope and the normal stress unloading rate influences the peak slip rate the fracture instability is dependent on the relationship between the stiffness of the system and the slip weakening rate of the fracture and the shear dilation mainly occurs after the fracture activation the test results also show that the critical shear stress of the sawcut fracture during the unloading driven shear test is approximately equal to the residual shear strength after the displacement driven fracture slip this relationship inspires us to develop a new approach to estimate the maximum seismic moment our data demonstrate that the maximum seismic moments for both the fractures obtained from the unloading driven shear tests are all below the upper limit lines indicating that the proposed approach is reasonable the uncertainty analysis shows that the accurate estimation of fault size can improve the maximum seismic moment prediction

evidence from lava flows for complex polarity transitions the new composite steens mountain reversal record summary geomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava flow records by splicing together paleomagnetic results from earlier studies at steens mountain with those from three newly studied sections of oregon plateau flood basalts at catlow peak and poker jim ridge 70 90 km to the southeast and west respectively we provide support for this interpretation with the most detailed account of a magnetic field reversal yet observed in volcanic rocks forty five new distinguishable transitional t directions together with 30 earlier ones reveal a much more complex and detailed record of the 16 7 ma reversed r to normal n polarity transition that marks the end of chron c5cr compared to the earlier r t n t n reversal record the new record can be described as r t n t n t r t n the composite record confirms earlier features adds new west and up directions and an entire large n t r t segment to the path and fills in directions on the path between earlier directional jumps persistent virtual geomagnetic pole vgp clusters and separate vgps have a preference for previously described longitudinal bands from transition study compilations which suggests the presence of features at the core mantle boundary that influence the flow of core fluid and distribution of magnetic flux overall the record is consistent with the generalization that vgp paths vary greatly from reversal to reversal and depend on the location of the observer rates of secular variation confirm that the flows comprising these sections were erupted rapidly with maximum rates estimated to be 85 120 m ka 1 at catlow and 130 195 m ka 1 at poker jim south paleomagnetic poles from other studies are combined with 32 non transitional poles found here to give a clockwise rotation of the oregon plateau of 11 4 5 6 with respect to the younger columbia river basalt group flows to the north and 14 5 4 6 with respect to cratonic north america 95 per cent confidence interval

emplacement of columbia river flood basalt evidence is examined for the emplacement of the umatilla wilbur creek and the asotin members of columbia river basalt group these flows erupted in the eastern part of the columbia plateau during the waning phases of volcanism the umatilla member consists of two flows in the lewiston basin area and southwestern columbia plateau these flows mixed to form one flow in the central columbia plateau the composition of the younger flow is preserved in the center and the composition of the older flow is at the top and bottom there is a complete gradation between the two flows of the wilbur creek and asotin members erupted individually in the eastern columbia plateau and also mixed together in the central columbia plateau comparison of the emplacement patterns to intraflow structures and textures of the flows suggests that very little time elapsed between eruptions in addition the amount of crust that formed on the earlier flows prior to mixing also suggests rapid emplacement calculations of volumetric flow rates through constrictions in channels suggest emplacement times of weeks to months under fast laminar flow for all three members a new model for the emplacement of columbia river basalt group flows is proposed that suggests rapid eruption and emplacement for the main part of the flow and slower emplacement along the margins as the of the flow margin expands

paleomagnetic constraints on the geometry and timing of deformation at yucca mountain nevada paleomagnetic data from miocene ash flow sheets indicate that southern yucca mountain has undergone about 30 of clockwise vertical axis rotation since emplacement of the tiva canyon member of the paintbrush tuff at about 13 ma declinations of remanent magnetization from 32 sites in the reversely magnetized tiva canyon member display a systematic southward increase over the 25 km north south extent of yucca mountain probable errors in estimating structural attitudes are a major source of uncertainty in determining the amount of rotation between any two sites analysis of tilt corrected directions of remanent magnetization from the tiva canyon sites indicates that structural attitudes determined from outcrop patterns on geologic maps are generally more accurate than field measurements of attitudes of compaction foliation or contacts attitudes determined from map patterns apparently minimize effects of 1 initial dips 2 poorly defined compaction foliation and 3 tilting that occurred after compaction but before blocking of remanence however such attitudes cannot accurately represent tilting related to unmapped local structures rotations implied by data from three older ash flow sheets the topopah spring member of the paintbrush tuff seven sites and the prow pass four sites and bullfrog three sites members of the crater flat tuff are generally consistent with the amount and sense of rotation documented by data from the tiva canyon member uncertainties in the declinations of tilt corrected site mean directions from these older units preclude evaluating whether older units have undergone more rotation than the tiva canyon member

paleomagnetism of an east west transect across the cascade arc in southern washington implications for regional tectonism paleomagnetic data from a transect across the cascade arc in southern washington were collected to test models of vertical axis rotations for this part of the pacific northwest and to provide information on the tectonic history of the st helens seismic zone the 75 site mean directions are divided into three groups based on isotopic fission track and stratigraphic ages group 1 consists of samples from rocks deposited between 39 and 30 ma and shows a mean clockwise vertical axis rotation of 34 13 group 2 consists of samples from rocks deposited between 30 and 24 ma and shows a mean clockwise rotation of 17 11 and group 3 consists of samples from rocks between 24 and 16 ma and shows a mean clockwise rotation of 20 12 although these three values of rotation are statistically indistinguishable at the 95 confidence level we interpret them in combination with other data to indicate differential rotation across the st helens seismic zone shz the available paleomagnetic data for eocene oligocene rocks west of the seismic zone show clockwise vertical axis rotations of 30 8 and 35 9 consistent with the value for group 1 the rotational values for groups 2 and 3 and the intrusive suite of kidd creek 13 ma despite their different ages have similar values east of the shz comparing these groups of data east and west of the seismic zone indicates a differential rotation of 10 3 across it in addition the rates of rotation in southern washington are similar to those for rocks in the western cascades of oregon and indicate that rotational deformation of miocene age extends northward into southern washington and eastward into the axis of the arc

late cenozoic structure and correlation to seismicity along the olympic wallowa lineament northwestern united states discussion and reply the columbia plateau portion of raisz s 1945 olympic wallowa lineament owl is a geologically fascinating feature whose origin structural significance and age have been the subject of numerous geologic studies over the years because of its proximity to nuclear and hydroelectric facilities despite all the work that has been done some aspects of this complex feature are still not well understood therefore we recognize the need for the continued study of this feature and welcome new efforts that further our understanding however mann and meyer have overlooked an extensive body of published and publicly available data on the owl and this has resulted in numerous mistakes and erroneous interpretations conclusions in their paper with little new insight into the subject in the following sections we discuss some of the more significant problems with mann and meyer s paper based upon the existing literature and the results of our ongoing research

paleomagnetism of the tertiary clarno formation of central oregon and its significance for the tectonic history of the pacific northwest the clarno formation a mostly eocene and partly early oligocene sequence of andesitic lavas and volcaniclastic rocks is the oldest tertiary formation exposed in north central oregon remanent magnetization directions at 46 sites in the lavas provide a paleomagnetic pole at 84 n 278 e with a 95 confidence cone of 7 comparison of this pole with the north american reference pole for eocene time indicates that the clarno formation has rotated 16 clockwise with an uncertainty of 10 but has undergone no significant latitudinal displacement further comparison with paleomagnetic data from the miocene columbia river basalt group and steens basalt shows that large scale block rotations in the eastern columbia embayment ceased sometime between 38 m y and 15 m y a paleogeographic reconstruction for 38 m y is offered which is consistent with the observed rotation of the clarno and with the other paleomagnetic data from the pacific northwest which are briefly reviewed this reconstruction shows that it is possible to account for virtually all of the paleomagnetically indicated rotations in pre miocene tertiary rocks of the pacific northwest by an extensional tectonic model as a consequence only part of the rotations in the pre tertiary rocks of the eastern columbia embayment need to be the result of accretionary tectonics the initiation of magmatism in the eastern columbia embayment that is represented by the clarno formation itself may have been contemporaneous with the beginning of crustal extension in the pacific northwest

scale effect on the shear behaviour of rock joints based on a numerical study abstract a large number of studies have been carried out to investigate the effect of scale on the shear behaviour of rock joints however the nature of scale dependency of rock joints is still unknown and remains an ongoing debate this paper investigates the effect of joint length on the shear behaviour of rough rock joints using pfc2d in this numerical approach the intact material is simulated by a dense packing of bonded particles and the joint is represented by micro scale slip surfaces at contacts between particles the validity of this numerical approach in reproducing the shear behaviour of rock joints has been studied in previous studies of authors against the physical experiments analytical and empirical models in this study numerical direct shear tests on a natural tensile joint and subdivided joints were carried out and the shear behaviour of these joints at different scale was investigated results of this study show that as the joint length increases the peak shear strength peak dilation rate and shear stiffness decrease while the peak shear displacement increases in order to find out the reason of observed scale dependency the mechanism of asperity degradation at different scales was investigated and it was found that the mobilisation of different sized asperities at different scales resulted in the observed scale effect

physical modeling of deformation patterns in monoclines above oblique slip faults abstract many basement involved fault related folds are thought to form by reactivation of pre existing faults although oblique slip is expected on such structures it is often difficult to identify because the underlying faults are blind paleomagnetic records are ambiguous and mesoscopic deformation patterns in the associated monoclines are often incompletely exposed or difficult to interpret this study seeks to establish a relationship between oblique slip on a basement reverse fault and the deformation patterns in the overlying folded rock layers using scaled physical models of wet clay overlying a rigid basement we create a suite of laramide style folds over basement faults with variable obliquity precise displacements and strains on the surface of the clay are recorded using close range photogrammetry we use these measurements to predict deformation patterns that we would expect to find in analogous natural structures the results of our models show that there are three general strain zones that form in monoclines that form above oblique slip faults the upper hinge region of the monocline is dominated by extension the lower hinge region by contraction and the middle of the fold limb is dominated by shear strains the boundaries of these three zones as well as the magnitude of strain in each of the zones vary with the amount of oblique slip and fault throw deformation should be dominated by joints and extensional faults in the extensional zone by contractional faults and cleavage in the contractional zone and may contain extensional contractional or strike slip faults along with joints and cleavage arranged in riedel style geometries in the shear strain dominated zone at fault obliquities above 45 there is significant overlap of the different strain zones and deformation patterns in these regions can consequently be very complex involving reactivation or overprinting of earlier structures or the formation of mixed mode structures displacements on the surface of our model monoclines suggest that previous paleomagnetic interpretations of vertical axis rotations on natural monoclines may need to be re evaluated although such rotations increase with increasing fault obliquity the magnitude of these rotations will vary significantly with position across a monocline

intracanyon flows of the columbia river basalt group in the lower columbia river gorge and their relationship to the troutdale formation the priest rapids member wanapum basalt and the pomona member saddle mountains basalt of the columbia river basalt group crossed the miocene cascade range into western oregon and washington as intracanyon flows about 14 m y and 12 m y ago respectively a thick allogenic bedded palagonite complex underlying the priest rapids intracanyon flow originated when the first priest rapids flow rosalia chemical type interacted with a shallow lake on the columbia plateau displacing water that flushed hyaloclastic debris into an incipient ancestral columbia river channel the priest rapids flow then overfilled the canyon forcing the river northward where it established the bridal veil channel the bridal veil channel part of a river system that extended across the columbia plateau was only partly filled by the pomona flow allowing the river to remain in this channel through the cascade range the troutdale formation is made up of 1 older alluvial gravels deposited below and above the pomona intracanyon flow while the ancestral columbia river occupied the bridal veil channel and 2 younger more varied alluvial deposits characterized by clastic and hyaloclastic debris from boring and high cascade volcanism spread over a much wider area when aggradation overfilled the bridal veil channel less than 6 m y ago the shift of the lower columbia river to its present course took place during this time of maximum alluviation aided by scattered eruptions of younger volcanoes of the boring and high cascade lavas the present columbia river gorge was formed in post troutdale time by entrenchment of the columbia river during rapid uplift of the cascade range of northern oregon during the past 1 to 2 m y

shear wave splitting beneath the snake river plain suggests a mantle upwelling beneath eastern nevada usa the snake river plain srp a 90 km wide topographic depression in southern idaho is a topographically anomalous feature in the western u s previous seismic studies focused on the northeastern srp to study its relationship with the yellowstone hotspot we present new teleseismic shear wave splitting data from six broadband seismic stations deployed along the axis of the srp from june 2000 to september 2001 we also analyze splitting at hlid a permanent station of the national seismic network located f100 km north of the plain splitting of individual teleseismic phases is consistent at all stations within 2r errors and we favor the interpretation of anisotropy with a single horizontal fast axis although a dipping axis interpretation is statistically permitted at two of the stations our station fast directions as well as shear wave splitting data from numerous other stations throughout the basin and range are best explained by a lattice preferred orientation of olivine due to horizontal shear along the base of the plate associated with the gravitational spreading of buoyant plume like upwelling material beneath eastern nevada into a southwestward flowing asthenosphere with respect to a fixed hotspot reference frame this parabolic asthenospheric flow paf model for the great basin is attractive because it explains the observed high elevations high mantle buoyancy low velocity anomaly beneath eastern nevada high heat flow and depleted geochemistry of some erupted basalts the lack of pliocene recent major volcanism in eastern nevada suggests that a significant amount of the buoyancy flux is due to compositional buoyancy our splitting station delay times vary in a way not predicted by the paf model and can be explained by a zone of aligned magma filled lenses and or partially molten dikes beneath the srp lithosphere a depleted olivine rich residuum underneath the sides of the eastern srp and or the effect of lateral lower crustal flow from beneath the esrp toward its adjacent flanks d 2004 elsevier b v all rights reserved

influence of asperity degradation on the mechanical behavior of rough rock joints under cyclic shear loading a cyclic shear testing system was established to investigate the mechanical behavior of rough rock joints under cyclic loading conditions laboratory cyclic shear tests were conducted for two joint types of hwangdeung granite and yeosan marble saw cut and split tensile joints prior to test the roughness of each specimen was characterized by measuring the surface topography using a laser profilometer several important aspects of cyclic joint behavior such as high peak shear strength and non linear dilation in the first loading cycle different frictional resistance for the reversed shear loading direction and anisotropic shear behavior and its dependence on the normal stress level were identified from the cyclic shear test results these features and their variations in the subsequent loading cycles are mainly due to the effect of second order asperities and strength of rock material it was also observed from experimental results that degradation of asperities under cyclic shear loading also followed the exponential degradation laws for asperity angle and the mechanism for asperity degradation would be different depending upon the shearing direction and the type of asperities based on the experimental results an elasto plastic constitutive model which can consider the degradation of second order asperities was proposed numerical simulations for the monotonic and cyclic shear loading indicated agreement with the laboratory test results

the steens mountain oregon geomagnetic polarity transition 3 its regional significance study of the variations of direction and intensity of the geomagnetic field as recorded by the miocene lava flows on steens mountain southeastern oregon has resulted in a detailed description of total field behavior during a reversal in polarity in addition to information about the polarity reversal itself the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15 5 ma in order to test the feasibility of using this record as a means of correlation in this part of the western united states comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the steens basalt or rocks of equivalent age despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes convincing similarities among some of the records are evident the steens basalt paleomagnetic record does indeed have potential as a correlation tool during this time of widespread basaltic volcanism additionally paleomagnetic data from flows that were sampled in detail yield a middle miocene paleomagnetic pole at 88 3 n 209 0 95 6 3 for the high lava plains of oregon this pole position is statistically indistinguishable from the earth s rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted data from selected sites within the coeval part of the columbia river basalt group yield a paleomagnetic pole at 88 7 n 171 6 e 95 4 0 the columbia river basalt group pole is statistically indistinguishable from either the rotational axis or from the high lava plains pole these findings indicate no post 20 ma differential rotation between south eastern washington and south central oregon in contrast to previous interpretations

crustal thickness beneath libya and the origin of partial melt beneath as sawda volcanic province from receiver function constraints this study investigates crustal thickness and properties within the libyan region results obtained from 15 seismic stations belonging to the libyan center for remote sensing and space science are reported in addition to 3 seismic stations publically available using receiver functions the results show crustal thicknesses ranging from 24 km to 36 km with uncertainties ranging between 0 10 km to 0 90 km more specifically crustal thickness ranges from 32 km to 36 km in the southern portion of the libyan territory then becomes thinner between 24 km and 30 km in the coastal areas of libya and thinnest between 24 km and 28 km in the sirt basin the observed high vp vs value of 1 91 at one station located at the as sawda volcanic province in central libya indicates the presence of either partial melt or an abnormally warm area this finding suggests that magma reservoirs beneath the libyan territory may still be partially molten and active thereby posing significant earthquake and volcanic risks the hypothesis of an active magma source is further demonstrated though the presence of asthenospheric upwelling and extension of the sirt basin this study provides a new calculation of unconsolidated sediment layers by using the arrival time of the p to s converted phases the results show sediments thicknesses of 0 4 km to 3 7 km with the vp vs values ranging from 2 2 to 4 8 the variations in crustal thickness throughout the region are correlated with surface elevation and bouguer gravity anomalies which suggest that they are isostatically compensated

paleomagnetic results for eocene volcanic rocks from northeastern washington and the tertiary tectonics of the pacific northwest the direction of remanent magnetization for 102 sites in eocene volcanic and volcaniclastic rocks of the o brien creek formation sanpoil volcanics and klondike mountain formation suggests approximately 25 of clockwise rotation of a 100 by 200 km area in northeastern washington the volcanic rocks consist chiefly of rhyodacite and quartz latite flows with intercalated ash flow tuff and volcaniclastic layers these rocks have been sampled at 102 sites distributed among five volcanotectonic depressions the toroda creek republic keller and first thought grabens and the spokane enterprise lineament the volcanic rocks probably range in age from 55 m y to about 48 m y and the 50 to 48 m y old volcanic rocks within this suite appear to be rotated as much as the older rocks previous investigators have shown that 40 m y old and younger plutonic rocks of northwestern washington are not rotated hence we infer that the north central washington rocks were rotated to their present declination between 48 and 40 m y b p during the middle and or late eocene during early eocene time this region was extended in a westward direction through crustal necking gneiss doming diking and graben formation internal deformation of the region related to this crustal extension was extreme but most bedrock units that were formed concurrent with the crustal extension were probably in place prior to the rotation hence we infer that the rotation was chiefly accommodated by movement on faults peripheral to the sampled area faults active during paleogene time appear to define boundaries of a triangular crustal block the sanpoil block encompassing much of northeastern washington northern idaho northwestern montana and adjacent parts of british columbia the faults include the laramide thrusts of the rocky mountain thrust belt the strike slip faults of the lewis and clark line and strike slip faults of the straight creek fraser zone we suggest that during early eocene time the sanpoil block was extended westward through crustal necking and dilation and then during the middle eocene was rotated clockwise and thrust over the craton in a final stage of laramide thrusting the motor driving these deformations presumably was interaction of north america with oceanic lithosphere off its western margin such interaction probably involved right oblique underthrusting and dextral shear

paleomagnetism and tectonic significance of eocene basalts from the black hills washington coast range the black hills located about 15 km west of olympia constitute one of several large basement uplifts in the washington coast range the oldest rocks exposed in this west dipping homocline consist of early to middle eocene basalt flows and breccias with minor sedimentary interbeds which are correlated with the crescent formation in the northern olympic peninsula k ar geochronology of five basalt samples yields an average age of 53 1 2 0 m y b p or early eocene a total of 263 paleomagnetic cores were collected from 37 sites all sites were cleaned using af demagnetization and 35 sites were corrected for measured and inferred tilt the mean direction of these sites was virtually unchanged by applying tectonic corrections although the precision was slightly improved no flattening or steepening of inclination is apparent although the mean declination of the 35 sites is easterly discordant with respect to the expected early eocene declination for stable north america by 28 7 15 4 this discordance indicates that the black hills have rotated clockwise about 29 since early eocene time the black hills basalts show significantly less rotation than eocene rocks in the oregon coast range but they show nearly the same amount as analogous basalts in the southwestern washington coast range and latest eocene to late oligocene volcanic rocks in the oregon washington cascade range thus the rotations of coastal oregon and washington while identical in direction differ significantly in amount suggesting that the entire coast range block from the olympic peninsula to north of the klamath mountains has not been a coherent terrane since early eocene time several tectonic models are discussed that involve accretion and independent clockwise rotation of two or more coast range blocks or microplates in response to episodic periods of underthrusting and extensive right lateral shear along the farallon north america plate boundary during early and middle tertiary time the easterly discordances of declination that are observed in almost all tertiary rocks sampled in the oregon washington coast and cascade ranges reflect a composite of differential rotation during accretion of the coast range seamount terrane followed soon afterward by large scale rotation of the western margin of the pacific northwest in response to oblique subduction and consequent distributed shear

apparent tectonic rotations declination anomaly equations and declination anomaly charts the conventional method for tilt correction of paleomagnetic data assumes that rock bodies rotate about horizontal axes during deformations apparent tectonic rotations which may produce anomalous declination readings of paleomagnetic directions and other linear elements can result from the use of such a procedure if the rock bodies deform about inclined axes the error declination anomaly resulting from two apparent tectonic rotations 1 single rotations about an oblique axis and 2 continuous rotations about a precessing fold axis can be determined algebraically for given orientations of rotation axis and angles of rotation the declination anomaly does not depend on the orientation of the paleomagnetic vector or the trend of the rotation axis rather it is a function of the plunge of the rotation axis and the angle of rotation a declination anomaly chart dac is constructed for each of the two processes to determine graphically whether anomalous paleomagnetic data obtained from a study area can result from the process in the dacs the plunge of the bedding pole vector and the declination anomaly resulting from the apparent tectonic rotations are plotted against the plunge of the rotation axis and the angle of rotation the declination anomaly for an individual site is taken as the deviation of the measured remanent magnetic direction at the site from the reference direction for coeval rocks each data point is plotted at the intersection of the declination anomaly curve and the beddingpole curve in the dacs if the dispersion in remanent magnetic directions over a study area results from one of the apparent tectonic rotations the data points will lie along a straight line in the corresponding dac potential applications of the dacs include testing the nature of anomalous paleomagnetic directions determining the deformation style of folds and restoring structural orientations of tilted blocks

crustal structure and post rift evolution of the levant basin abstract petroleum exploration activity in the deep basins of the eastern mediterranean has been rejuvenated over the last decade with the discoveries of the giant tamar and leviathan gas fields made by noble energy and its partners and more recently with the discovery of the zohr field by eni with exploration efforts pursuing deeper and older leads a first principles quantitative study of the region seems timely while it is generally agreed that the levant basin formed along the northern passive margin of gondwanaland during the early mesozoic its complex post rift evolution has not been documented in detail located in a unique junction of three active tectonic plates and at the distal end of the nile one of the largest rivers on earth the levant basin is characterized by a thick sedimentary section more than half of which has accumulated since the oligocene in the last 15 of its existence this study implements a quantitative basin modelling workflow that assesses the three dimensional crustal geometry of the levant basin and its post rift evolution by integrating 3d gravity inversion and 3d flexural backstripping techniques the focus of the gravity inversion which includes corrections for the lithosphere thermal gravity anomaly resulting from lithosphere thinning and for the magmatic addition from decompression melting has been to predict both the 3d crustal structure and the magnitude of total lithospheric stretching for the levant basin the 3d flexural backstripping workflow using the outputs of the gravity inversion invokes three major processes in its calculations accompanying the layer by layer removal of the stratigraphy i 3d flexural isostatic unloading ii decompaction in response to the removal of overburden and iii reverse thermal subsidence modelling controlled by the stretching factor derived from the gravity inversion backstripping has produced a series of maps and cross sections depicting the evolution of post rift palaeobathymetry in the levant basin the integration of these techniques allows us to assess the evolution of the basin and the various sedimentary and tectonic events that have occurred since basin initiation although the calculated stretching factors indicate that while the lithosphere underlying the levant basin has been very highly stretched and thinned the stretching did not progress to the formation of new oceanic crust predicted palaeobathymetry portrays a thermally subsiding basin reaching maximum water depths at the onset of the cenozoic this deep basin provided ample amount of accommodation space for the large volumes of terrigenous clastics sourced from the contemporaneous uplift and exhumation of the basin margins and delivered in large part at least since the oligocene by the continental drainage of the nile these sediments have been filling and shallowing the basin ever since the basin evolution described here is consistent with available geophysical and geological data and has important implications for petroleum systems modelling within the basin

a paleomagnetic test of the proposed mojave sonora megashear in northwestern mexico abstract three upper triassic lower jurassic formations in southwestern united states and northwestern mexico have been investigated paleomagnetically as a means of testing for displacement along the proposed mojave sonora megashear luning formation of west central nevada and sil nakya formation of southern arizona lie north of the megashear whereas in northwestern sonora antimonio formation lies south of the megashear previously published results from the nazas formation of presumed lower jurassic age from north central mexico were incorporated into the interpretations these four units compose two sets of correlative formations on opposite sides of the proposed megashear restoration of 800 km of left lateral displacement along the megashear about the pole of rotation determined by anderson and schmidt 1983 improves the grouping of the two sets of paleomagnetic poles noticeably with the improvement having statistical significance at the 95 confidence level we feel that the data support but do not prove the postulated displacement along the megashear all four poles are displaced from irving s 1979 smoothed apparent polar wander path for cratonic north america in the intervals 190 and 200 ma the displacement needed to bring all of these poles into coincidence is a pure rotation of about 8 counterclockwise about the sampling area two probable cretaceous poles were obtained from the sand wells formation of southern arizona and from unnamed volcanics in northwest sonora these poles are essentially identical and suggest that displacement along the megashear had ceased by the cretaceous data from the early jurassic mulberry wash and pitoikam formations in southern arizona appear to have been remagnetized in a later cretaceous event and were not used in this study

shelf and deep sea sedimentation in eocene forearc basin western oregon fan or non fan the lower eocene tyee and flournoy formations in the southern and central coast range of oregon contain unusual associations of shelf and deep sea facies deposited in an elongate portion of a forearc basin the sandstone rich nature and abrupt transition of sedimentary facies suggest that a variation of existing deep sea fan models is present the 6 500 ft 2 000 m thick sequence calls for shelf sandstone virtually cascading into deep water along a line shelf edge rather than from a point source submarine canyon to feed a sand rich system facies are described in terms of a sand rich fan system from south to north these facies include deltaic shelf inner fan mid fan outer fan and basin plain the deltaic facies contains coarse grained cross bedded distributary channel sandstone and interbedded coal some prograding deltaic sand extended to the edge of the narrow eocene shelf and cascaded into the basin many complex nested channels up to 1 200 ft 350 m wide and 130 ft 40 m deep were incised into fine grained shelf and slope sediments at slope depths line source these channel and shelf slope deposits comprise an inner fan facies a single large canyon or feeder channel point source typical of some fan systems was not recognized mid fan facies consist of broad coalescing channels filled with thick am lgamated sandstone beds sandstone shale ratios in the mid fan facies are from 85 to 15 up to 95 to 5 outer fan facies have sandstone shale ratios of 60 to 40 and contain well developed graded turbidite beds about 5 ft 1 5 m or less in thickness interbedded with shale basin plain facies consist of fine grained strata sandstone shale ratios of 30 to 70 deposited at the north end of the basin these thin bedded deposits also contain graded fine grained turbidite sandstones paleogeographic reconstructions based on paleocurrent data facies distribution and sandstone composition indicate sediment derivation from the pre cenozoic klamath mountains terrane and a volcanic arc system to the south and southeast the average sandstone composition is 19 quartz 27 feldspar 19 volcanic rock fragments 7 other rock fragments and 28 accessories and cement paleocurrent data from cross bedding and groove and flute marks indicate transport directions from south to north instead of a radially dispersed fan pattern the stratigraphic succession of the tyee flournoy system indicates a general shoaling trend and a progradation of deltaic facies across a narrow shelf and out over the basin fill during the early eocene the predominance of sandstone and confinemen within a narrow basin account for a turbidite facies differing from more familiar mixed sandstone and shale submarine fan facies models

crustal stretching style variations in the northern margin of the south china sea linking deep seismic profiles with regional scale gravity inversion is a powerful tool to deduce the architecture of rifted margins and their structural evolution here we map upper and lower crustal thicknesses of the northern south china sea scs margin in order to investigate the occurrence of depth dependent crustal extension from the proximal to the distal margin by comparing upper and lower crustal stretching factors we find that the northern margin of the scs is segmented in three parts 1 sedimentary basins where upper crust is stretched more than lower crust 2 distal margin where lower crust is stretched more than upper crust 3 mostly proximal margin regions where the two layers have similar stretching factors our results suggest that sedimentary basins and distal margin prominently feature depth dependent extension however accommodated by different processes while differential thinning within sedimentary basins appears to be governed by lateral pressure variations inducing lower crustal flow we suggest the distal margin to be affected by a combination of mantle flow induced lower crustal shearing and sequential fault activity during crustal hyper extension

influence of paleorelief on the mid miocene climate variation in southeastern washington northeastern oregon and western idaho usa abstract terrestrial climate proxy records repeatedly show spatial heterogeneity because regional climate condition is largely influenced by the relationship between large scale atmospheric circulation patterns and regional topographic features the paleosols developed on the top of columbia river basalt flows emplaced between 15 3 and 15 8 ma in the northwestern united states show significant spatial variations in the mid miocene soil forming processes the paleosol in the northwestern part of the study area has the least degree of chemical weathering fine scale and very shallow penetrating depth of root traces with the estimated paleoprecipitation and paleotemperature of about 550 mm and 16 c respectively the paleosols in the southeastern part of the study area indicate more intense chemical weathering with the estimated paleoprecipitation and paleotemperature of approximately 1400 to 1500 mm and 17 c respectively the spatial variations in paleo pedogenesis were predominantly affected by spatial heterogeneity in the mid miocene regional climate and regional paleotopographic features taking the other ancient soil forming factors including time parent rock materials and biological activities into account we suggest that intrusions of hot columbia river basalt magma into the crust were responsible for the regional surface uplift and the paleoclimate heterogeneity because the major vent systems of the columbia river basalt eruptions were located in the area where development of the mid miocene high topography existed within the blue mountains paleotopography of the earth s surface can be reconstructed by the paleoprecipitation and paleotemperature estimations from several spatially distributed chrono stratigraphic paleosols although it is difficult to estimate the magnitude of the paleorelief precisely

geology of the deer butte formation malheur county oregon faulting sedimentation and volcanism in a post caldera setting abstract the deer butte formation accumulated during the middle miocene in fault controlled basins in an extensional setting the basins developed as regional faults asserted influence after eruption of ash flow sheets and collapse of calderas of the lake owyhee volcanic field the sequences of hurley flat dry creek and oxbow basin contain a lower basalt tephradominated unit formed by basalt hydrovolcanism overlain by fine grained fluvial and lacustrine volcaniclastic sedimentary units the sequence of freezeout creek was deposited in an erosional valley that was incised into older units and cut across the concurrently active wall rock ridge fault zone the sequence of hurley flat and dry creek contain alkaline tholeiitic basalt flows and tephra deposits whereas the sequences of freezeout creek and oxbow basin contain subalkaline calcalkaline basaltic andesite the compositional change occurred after local uplift due to faulting along the wall rock ridge fault zone the youngest unit well sorted medium grained muscovite bearing arkose of the arkose of dry creek buttes was deposited in a large river that drained westward from source areas in western idaho the deer butte formation was deposited between approximately 15 and 12 6 ma while basin and range type faulting dominated regional structural patterns

paleomagnetism and geology of eocene volcanic rocks of southwest washington implications for mechanisms of tectonic rotation paleomagnetic and geologic investigations in eocene volcanic rocks of the southwest washington coast range demonstrate a close relationship between tectonic rotations and the local structural geology the allochthonous middle eocene submarine basalt basement of the crescent formation consists of several fault bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation 20 to perhaps as much as 65 when compared to the eocene reference pole for north america structural analysis shows the differential rotations postdate middle eocene folding of the crescent formation against the continental margin and predate the unconformably overlying upper eocene goble volcanics which are rotated about 23 and do not show the same domains of rotation as the underlying crescent formation post goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults major north northwest trending faults with several kilometers of dextral displacement form the boundaries of cross faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west northwest trending sinistral r riedel shears thirty five to 100 of the observed post late eocene rotations could have occurred by this shear rotation mechanism other paleomagnetic study areas in the coast range and western cascades have a similar fault geometry and may also have undergone significant shear rotations long term northward oblique subduction of the farallon plate beneath the coast range throughout most of the tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large rigid plates

lava sediment interaction and drainage system development in a large igneous province columbia river flood basalt province washington state u s a abstract large igneous provinces lips have been implicated in some of the largest environmental perturbations to have affected the earth through geological time yet the impact of lip development on drainage and ecosystem development in the immediate vicinity of these provinces are still poorly constrained to date based on a detailed integrative facies scheme we characterize the interaction between volcanism and fluvial lacustrine and wetland environments in the miocene columbia river flood basalt province crbp lip exposed in central washington state usa the facies scheme proposed here comprises a detailed description and interpretation of siliciclastic bioclastic diatomite volcaniclastic and paleosol facies and subfacies intercalated with lavas of the columbia river basalt group crbg facies and facies associations of individual interbeds are further correlated to reconstruct changes in sedimentary settings and topography of lava fields during deposition field observations and facies analysis help to explain the effects of flood basalt volcanism on drainage system development we propose a generic model of lava drainage interplay and distribution of sedimentary settings in flood basalt provinces which will contribute to our understanding of sedimentological environmental and volcanic processes in the crbp hydrocarbon exploration in volcanic terrains requires detailed information on the distribution and development of sedimentary settings this model will help to better predict the character and distribution of sedimentary bodies and potential hydrocarbon reservoirs in volcanic terrains

predicting the shear characteristics of rock joints with asperity degradation and debris backfilling under cyclic loading conditions abstract this study presents a constitutive model to predict the shear behavior of rock joints under cyclic loading the model focuses on asperity degradation and debris backfilling during a complete shear cycle the degradation of two order asperity is associated with the evolution of waviness and unevenness based on the classic wear theory the produced debris significantly influences the joint shear behavior in the subsequent shear processes and its thickness is dependent on shear displacement joint length as well as worn area and wavelength of asperities the model particularly considers the dilation angles of waviness unevenness and backfilled debris at different processes during the shear cycle the validation results show that the model can well predict the shear behavior of regularly shaped joints but the accurate prediction for irregularly shaped joints depends on the quantitative description of joint surface roughness the proposed model uses the quantitative description of joint profiles to evaluate the shear behavior of rock joints which is more practical than previous models relying on empirical judgement or data back analysis

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

paleomagnetism and tectonics of the crescent formation northern olympic mountains washington a stable prefolding magnetization has been discovered in pillow basalts of the eocene lower crescent formation of the northern olympic mountains the curved outcrop pattern of the crescent formation has been the target of several unsuccessful studies to test for oroclinal bending the success of this study is due in part to the development of a small diameter electric core drill for sampling the fractured rims of basalt pillows thermal demagnetization produced stable endpoints by 580 c in 12 of the 34 sites sampled large within site scatter was common in the remaining sites among the accepted sites within site scatter was small and correction for bedding tilt significantly reduced the scatter between sites the mean paleomagnetic pole for newly sampled pillow basalts 86 4 north latitude 170 0 east longitude a95 16 5 is indistinguishable from the early to middle eocene pole expected for north america including previous results from sites in subaerial basalt of the upper crescent formation in and near the easterm olympic mountains results in a more regional paleomagnetic pole 80 7 north latitude 192 0 east longitude a95 8 0 n 46 that shows no significant rotation 0 8 14 4 or poleward displacement 3 6 8 5 analysis of the magnetic mineralogy suggests that the remanence is early probably primary the pole therefore should be valid for tectonic interpretation of the region a circular distribution of virtual geomagnetic poles after correction for bedding tilt supports the hypothesis that the northern crescent formation experienced deformation due to the rise in a domelike fashion of the sediments of the olympic core terrane erosion of a partial dome open to the west could have produced the curvature seen in the outcrop pattern of the crescent formation the lack of significant rotation of the northernmost coast ranges contrasts with the net clockwise rotation seen to the south the difference could be that irrotational northward translation paleomagnetically insignificant yet geologically important driven by oblique convergence was accommodated by the north south trending strike slip faults to the east of the olympic mountains

paleomagnetic and structural evidence for oblique slip in a fault related fold grayback monocline colorado significant fold axis parallel slip is accommodated in the folded strata of the grayback monocline northeastern front range colorado without visible large strike slip displacement on the fold surface in many cases oblique slip deformation is partitioned fold axis normal slip is accommodated within folds and fold axis parallel slip is resolved onto adjacent strike slip faults unlike partitioning strike parallel slip onto adjacent strike slip faults fold axis parallel slip has deformed the forelimb of the grayback monocline mean compressive paleostress orientations in the forelimb are deflected 15 37 clockwise from the regional paleostress orientation of the northeastern front range paleomagnetic directions from the permian ingleside formation in the forelimb are rotated 16 42 clockwise about a bedding normal axis relative to the north american permian reference direction the paleostress and paleomagnetic rotations increase with the bedding dip angle and decrease along strike toward the fold tip these measurements allow for 50 120 m of fold axis parallel slip within the forelimb depending on the kinematics of strike slip shear this resolved horizontal slip is nearly equal in magnitude to the 180 m vertical throw across the fold for 200 m of oblique slip displacement 120 m of strike slip and 180 m of reverse slip the true shortening direction across the fold is n90 e indistinguishable from the regionally inferred direction of n90 e and quite different from the s53 e fold normal direction recognition of this deformational style means that significant amounts of strike slip can be accommodated within folds without axis parallel surficial faulting

geochronology of age progressive volcanism of the oregon high lava plains implications for the plume interpretation of yellowstone 1 the high lava plains province hlp is a late cenozoic bimodal volcanic field at the northern margin of the basin and range province in southeastern oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the yellowstone snake river plain ysrp trend we present 40ar 39ar ages for 19 rhyolite domes 5 rhyolite ash flow tuffs and 34 basaltic lavas from the hlp the previously identified trend of westward migration of hlp rhyolites is confirmed the rate of propagation is 33 km m y from 10 to 5 ma slowing to 13 km m y after 5 ma the duration of silicic volcanism at any locus is 2 m y three older hlp dacite domes yielded ages of 15 5 ma basalts are not age progressive we identify several episodes of increased basaltic activity at 7 5 7 8 5 3 5 9 and 2 3 ma with the younger episode likely continuing into the recent the hlp and ysrp trends emerged from the axis of middle miocene basaltic volcanism of the columbia river and steens basalts we propose a model in which 1 miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin 2 the hlp trend is the result of westward flow originating at the craton margin and 3 the ysrp trend is the trace of the motion of the north american plate over the tail of the plume

comment on deflection of paleomagnetic directions due to magnetization of the underlying terrain by c baag c helsley shi zhe xu and b liernert

post folding remagnetization that passes the fold test summary the palaeomagnetism of folded cretaceous redbeds in the methow pasayten belt of washington was studied in an attempt to resolve the tilt versus translation origin of shallow inclinations found in most cretaceous plutonic rocks of the pacific northwest after elimination of results from apparently strained overturned beds correction to palaeohorizontal of the high temperature dominant components produced two distinct directions one from each of two areas treated separately the magnetizations from these two areas appear to pass the fold test both directions are anomalously shallow which is typical of the region but the discrepancy between them casts doubt that both could have been acquired when the strata were horizontal instead we propose that they date from at least two episodes of remagnetization after the planar strata were tilted but before development of the tighter folds observed today our caution is that a positive fold test only establishes that a magnetization likely was acquired when strata in an area were planar other arguments are required to demonstrate that this planar surface was in fact horizontal during magnetization

quickly erupted volcanic sections of the steens basalt columbia river basalt group secular variation tectonic rotation and the steens mountain reversal 1 the steens basalt now considered part of the columbia river basalt group crbg contains the earliest eruptions of this magmatic episode lava flows of the steens basalt cover about 50 000 km2 of the oregon plateau in sections up to 1000 m thick the large number of continuously exposed quickly erupted lava flows some sections contain over 200 flows allows for small loops in the magnetic field direction paths to be detected for volcanic rocks this detail and fidelity are rarely found outside of the holocene and yield estimates of eruption durations at our four sections of 2 5 ka for 260 m at pueblo mountains 0 5 to 1 5 ka for 190 m at summit springs 1 3 ka for 170 m at north mickey and 3 ka for 160 m at guano rim that only one reversal of the geomagnetic field occurred during the eruption of the steens basalt the steens reversal at approximately 16 6 ma is supported by comparing 40ar 39ar ages and magnetic polarities to the geomagnetic polarity timescale at summit springs two 40ar 39ar ages from normal polarity flows 16 72 0 29 ma 16 61 and 16 92 0 52 ma 16 82 equals 2 error place their eruptions after the steens reversal while at pueblo mountains an 40ar 39ar age of 16 72 0 21 ma 16 61 from a reverse polarity flow places its eruption before the steens reversal paleomagnetic field directions yielded 50 nontransitional directional group poles which combined with 26 from steens mountain provide a paleomagnetic pole for the oregon plateau of 85 7 n 318 4 e k 15 1 a95 4 3 comparison of this new pole with a reference pole derived from crbg flows from eastern washington and a synthetic reference pole for north america derived from global data implies relative clockwise rotation of the oregon plateau of 7 4 5 0 or 14 5 5 4 respectively probably due to northward decreasing extension of the basin and range

gravity inversion mapping of crustal thickness and lithosphere thinning for the eastern mediterranean the distribution of oceanic and continental crust in the eastern mediterranean region is not well understood but has major implications for tectonic evolution of this region and its petroleum systems in particular the location of the continent ocean boundary cob the ocean continent transition oct structure and crustal thickness within the basin regions is a topic of much debate while seismology especially refraction seismology is an ideal method for locally determining crustal thickness it is limited to 2d as 3d mapping of crustal thickness is not practical or affordable over large areas however a recent development in 3d crustal thickness mapping uses gravity anomaly inversion we illustrate the application of this technique using the example of the eastern mediterranean figure 1 the new 3d gravity inversion technique incorporating a lithosphere thermal gravity anomaly correction is used to map moho depth crustal basement thickness and continental lithosphere thinning we then use thi

elastic dislocation modeling of wrinkle ridges on mars wrinkle ridges are one of the most common landforms on mars although it is generally agreed that they are compressional tectonic features formed by folding and thrust faulting there is no consensus on the number of faults involved the geometry of the faults or the maximum fault depth the topography of martian wrinkle ridges in solis planum and lunae planum has been studied using mola data as determined in previous studies the topography shows that most wrinkle ridges are a composite of two landforms a broad low relief arch and a superimposed ridge constrained by mola topographic profiles the geometry and parameters of the faults associated with wrinkle ridges have been modeled the best fits are obtained with a blind listric thrust fault that flattens into a decollement the listric fault geometry is approximated by a series of linear connecting segments with varying dips the major morphologic elements of wrinkle ridges can be matched by varying the displacement on the different fault segments modeling of large scale wrinkle ridges indicates that the maximum depth of faulting or depth to the decollement is about 4 5 km this may correspond to the depth of the contact between the ridged plains volcanic sequence and the underlying megabreccia the results suggest that wrinkle ridge thrust faults are shallow rooted and reflect thin skinned deformation

crustal genesis on the oregon plateau the areal and temporal distribution and the chemical and isotopic characteristics of cenozoic volcanic rocks from the oregon plateau reflect the changing tectonic conditions affecting this area over the last 20 m y following subduction zone related calc alkaline volcanism flood basalt eruptions occurred in the time interval 18 14 ma this primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the juan de fuca ridge that resulted in changes in the geometry of subduction against the pacific northwest the compositions of the mid miocene flood basalts show signs of extensive crystal fractionation in some cases accompanied by crustal contamination roughly 11 m y ago basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions this implies that as extension in the oregon plateau proceeded the crust became less of an impediment to the passage of relatively unfractionated mantle melts initial sr nd and pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area these observations suggest that an area of some 130 000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern north america over the last 20 m y the intensity of basalt volcanism as manifest in the columbia river group and in the oregon plateau does not appear to reflect the presence of a hot spot beneath the area both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by back arc spreading in a manner analogous to the formation of the marginal basins of the western pacific this spreading initiated along the western border of the archean wyoming craton with the location of mantle ascent melt production and extraction controlled by the changing geometry of subduction the existence of a zone of lithospheric weakness and the presence of a thick low density subcontinental mantle keel existing beneath the archean craton in this area

extension tectonics the neogene opening of the north south trending basins of central thailand paleomagnetic samples were collected from late neogene basalt flows from thailand all of these flows are horizontal and are relatively unaltered in thin section these rocks possess a stable magnetization which is believed to be primary samples from 48 lava flows were collected from sites located within the khorat plateau the chao phraya phitsanulok basin and the mountainous terrane west of the chao phraya phitsanulok basin these data were combined with previously reported late neogene data from five flows from western thailand although the average inclination from the 53 sites is indistiguishable from the expected dipole inclination the average declination has a net clockwise rotation of 13 5 5 8 from the geocentric dipole field furthermore the mean declination values from the 29 flows from the khorat plateau are indistinguishable from the present dipole field direction dm 4 3 7 5 and indistinguishable from the mean declination from 28 late neogene volcanic flows from vietnam in contrast the mean declinations from 24 flows collected from central and western thailand are deflected significantly clockwise dm 24 4 7 7 from the geocentric dipole field direction the differential rotation between western and central thailand versus the khorat plateau suggests that indochina is composed of at least two structural blocks which underwent a different rotational history these observations when combined with geologic and geophysical data from the chao phraya phitsanulok basin gulf of thailand and the intermontane basins of western thailand suggest that the rotations are recording a late neogene phase of e w extension of these basins we suggest that the formation of these basins and the related basaltic volcanism developed in reponse to subduction of the indian plate under western burma we envision the tectonics of this region is similar in style to the basin and range region of the western united states last we have observed field relationships from some of the rhyolites located in the central basin although these rhyolites are reported to be mesozoic or paleozoic in age our field observations and a k ar age date show that at least some of these rhyolites are younger than the basalts we suggest that the rhyolites form a bimodal suite with the basaltic rocks which were erupted in the later stages of the extension

native terranes of the central klamath mountains california the klamath mountains of northern california and southern oregon contain several good examples of terranes which have developed in situ the term native is proposed for five of these terranes and all terranes whose development can be tied to an adjacent cratonal area the klamath terranes discussed herein include disrupted stratigraphic and metamorphic types three disrupted terranes the rattlesnake creek eastern hayfork and north fork contain fossiliferous blocks derived from both north american and exotic sources the unique mixed faunal assemblage stratigraphic ties to north america in the source terranes of the blocks and paleomagnetic evidence indicate that the tectonic and sedimentary processes responsible for mixing these blocks occurred in proximity to north america not distant from the terranes present positions coeval blueschist metamorphism in a fourth inboard terrane the stuart fork suggests that all four terranes developed during a late triassic to early jurassic subduction event a fifth stratigraphic terrane the western hayfork was constructed upon the assembled disrupted and metamorphic terranes in the middle jurassic disrupted terranes with similar mixtures of north american and exotic faunas occur throughout the cordillera from central california possibly as far north as british columbia late triassic deformation has been documented in several of these terranes suggesting that 1 subduction operated along at least this portion of north america during the late triassic early jurassic and 2 many additional cordilleran terranes should also be considered native

kinematics of plate convergence deduced from mesozoic structures in the western cordillera mesozoic structures in the western cordillera of the united states yield important constraints for kinematic reconstruction of plate motions relative to north america structures in the mesozoic arc s of the northern sierra nevada and of northeastern oregon are remarkably uniform in timing style and with late cretaceous and or cenozoic rotations removed orientation rocks of both regions experienced three major episodes of contraction occurring in the triassic early jurassic the late jurassic and the early cretaceous cross folds were developed after the first two deformations but were not associated with significant shortening the three major deformations are nearly homoaxial and have shortening axes oriented subperpendicular ne sw to the regional trend of the arc system in the back arc region of the northwestern great basin deposition was continuous during the triassic and early jurassic and the region was shielded from tectonism in the arc to the west development of a regional fold thrust belt associated with major nw se shortening was initiated in the back arc region during the middle or late jurassic and continued through the early cretaceous corresponding deformation did not occur in the arc and the arc and back arc were decoupled by a major left lateral fault system that subparallelled the arc axis in the mid cretaceous approximately 100 m y b p regional compression in the back arc abruptly changed to ne sw and both the arc and back arc experienced major shortening development of homoaxial structures in the sierran arc is probably analogous to shortening in present day arc systems undergoing oblique subduction in which the shortening axis is parallel to the normal component of convergence thus the dominant structures in the sierran arc simply may be a reflection of the orientation of the trench and may not constrain the sense of obliquity of convergence strike slip faults and related compressional structures such as the back arc left lateral fault and associated fold thrust belt are probably the response to the strike slip component of convergence and indicate a period of left oblique subduction relative plate motions constrained by paleomagnetic data are in agreement with regional structures and indicate strong right lateral components of motion for the last 100 m y before 100 m y b p relative plate motions are inadequately constrained by paleomagnetic data and regional structures indicate a period of left oblique convergence from the middle or late jurassic to approximately 100 m y b p during the triassic and early jurassic the relative sense of convergence is equivocal and the possibility exists that continuous left oblique convergence occurred throughout the late paleozoic and early mesozoic until 100 m y b p based on the available data we cannot discount the possibility that a complex convergence history existed however with changes from right to left convergence in the middle or late jurassic and from left to right convergence in the mid cretaceous

constitutive model for rock fractures revisiting barton s empirical model abstract barton s original model is modified in order to address the following limitations the peak shear displacement is independent of normal stress and is zero for sawed fractures for post peak shear strength barton suggested a zero mobilized joint roughness coefficient jrc after 100 times the peak shear displacement barton assumed zero dilation displacement at up to one third of peak shear displacement in this study a database of results of direct shear tests available in the literature was collected and analyzed an empirical equation is introduced to predict peak shear displacement which considers the effect of normal stress and works for all types of rock discontinuities even sawed fractures the post peak mobilized jrc can be obtained using a power base equation instead of employing barton s table this new empirical equation for post peak mobilized jrc works for all ranges of displacements the modified model can predict negative compressive dilatancy at small shear displacements

magnitude of global contraction on mars from analysis of surface faults implications for martian thermal history faults provide a record of a planet s crustal stress state and interior dynamics including volumetric changes related to long term cooling previous work has suggested that mars experienced a pulse of large scale global contraction during hesperian time here we evaluate the evidence for martian global contraction using a recent compilation of thrust faults fault related strains were calculated for wrinkle ridges and lobate scarps to provide lower and upper bounds respectively on the magnitude of global contraction from contractional structures observed on the surface of mars during the hypothesized pulse of global contraction contractional strain of 0 007 to 0 13 is indicated by the structures corresponding to decreases in planetary radius of 112 m to 2 24 km respectively by contrast consideration of all recognized thrust faults regardless of age produces a globally averaged contractional strain of 0 011 to 0 22 corresponding to a radius decrease of 188 m to 3 77 km since the early noachian

genesis of flood basalts and basin and range volcanic rocks from steens mountain to the malheur river gorge oregon the middle and south forks of the malheur river provide a unique mapping corridor connecting two flood basalt successions steens basalt to the south and the basalt of malheur gorge to the north each contains chemically defined subtypes which merge stratigraphically across the north south length of the map area the lowermost flows of steens basalt are stratigraphically equivalent to the lowermost flows of the basalt of malheur gorge the uppermost flows of steens basalt pinch out in the map area but are partly interbedded with the middle and uppermost flows of the basalt of malheur gorge which continue to thicken northward the upper part of the tholeiitic succession is interbedded with a group of previously unrecognized lavas the venator ranch basalt flows tholeiitic volcanism ceased at ca 15 3 ma the last tholeiitic unit erupted is the hunter creek basalt which also thickens northward subsequent younger than 15 3 ma more localized eruptions were dominated by calc alkaline to mildly alkaline lavas associated with basin and range extension local uplift generated deep canyons which were filled by andesitic lavas of the keeney sequence ca 13 10 ma the final eruptive products include the devine canyon tuff ca 9 7 ma the drinkwater basalt ca 6 9 ma and the voltage flow older than 32 000 yr b p major and trace element analyses demonstrate that 1 crystal fractionation was a universal process in the derivation of the

two ship deep seismic soundings in the basins of the eastern mediterranean sea pasiphae cruise summary new results from a two ship refraction and oblique reflection deep seismic cruise are presented to discuss the nature of the crust in the ionian sirte and herodotus abyssal plains these expanding spread profiles were processed and analysed in both the x t and p domains arrival times of reflected and refracted branches are matched by ray tracing in both domains in spite of a shallow evaporitic sequence messinian evaporites deposited on top of a thick sedimentary pile responsible for velocity inversions on many profiles we obtain excellent velocity control down to moho depth pmp and pn 8 4 8 6 km s 1 arrivals are observed the three basins have a relatively thin crust 8 to 11 km overlain by a thick sedimentary cover up to 10km in the herodotus abyssal plain the moho boundary and main crustal units identified in the basins can be followed beneath the calabrian prism to the west and beneath the mediterranean ridge to the east the crustal structure is of oceanic type for both the ionian and sirte basins where typical oceanic layer 2 and 3 are recognized the thin crust of the herodotus basin may be interpreted either as oceanic or thinned continental crust about 10 km thick the top of the crust of the herodotus basin is much deeper therefore the herodotus basin is probably significantly older than the ionian basin triassic versus early cretaceous in age

crustal configuration in the northern levant basin based on seismic interpretation and numerical modeling abstract the interpretation of five 2d pstm seismic reflection sections 14 s twt covering the northern levant basin revealed a total of 10 horizons among which one is interpreted as an interface that may represent the moho the interpretation of seismic packages and their bounding surfaces as well as the seismic facies analysis were constrained by published 2d seismic interpretations of the northern lebanese offshore a total of nine seismic packages are identified in the basin with ages varying from the mid jurassic to the quaternary the filling of the basin is made up of thick cenozoic and mesozoic strata deposited above rifted triassic early jurassic interval the sediments are deposited in deep water mixed settings resulting from high stand systems various types of carbonate platforms and low stand systems siliciclastic and carbonate deep water turbidite complexes carbonate and siliciclastic systems are sealed by 1 1 5 km of evaporites and underlie plio quaternary hemipelagic and pelagic sediments intercalated by turbiditic sheets the time horizons were converted into depth using two methods the first one is based on stacking velocities and the second one on velocities resulting from refraction data 2d crustal modeling was achieved by integrating free air gravity anomaly geoid heights and topography data on the five interpreted pstm seismic lines the models representing five sections across the northern levant basin show a progressively attenuated crystalline crust in an ew direction away from the basin s eastern margin the crystalline crust is best interpreted as a strongly thinned continental crust under the levant basin represented by two distinct components an upper and a lower continental crust the moho appears to be situated between 20 and 23 km in the central and southern lebanese offshore estimated surface heatflow in the basin is around 40 mw m2 which is lower than reported values for the onshore and the margin these differences in heatflow values between the offshore the margin and the onshore have an important impact on hydrocarbon maturation and assessment of potential petroleum systems

steady rotation of the cascade arc displacement of the miocene cascade volcanic arc northwestern north america from the active arc is in the same sense and at nearly the same rate as the present clockwise block motions calculated from gps velocities in a north american reference frame migration of the ancestral arc over the past 16 m y can be explained by clockwise rotation of upper plate blocks at 1 0 m y over a linear melting source moving westward 1 4 5 km m y due to slab rollback block motion and slab rollback are in opposite directions in the northern arc but both are westerly in the southern extensional arc where rollback may be enhanced by proximity to the edge of the juan de fuca slab similarities between post 16 ma arc migration paleomagnetic rotation and modern gps block motions indicate that the secular block motions from decadal gps can be used to calculate long term strain rates and earthquake hazards northwest directed basin and range extension of 140 km is predicted behind the southern arc since 16 ma and 70 km of shortening is predicted in the northern arc the gps rotation poles overlie a high velocity slab of the siletzia terrane dangling into the mantle beneath idaho united states which may provide an anchor for the rotations

a joint shear model incorporating small scale and large scale irregularities abstract the strength and dilation of rock joints in the field cannot be evaluated solely on the basis of parameters scaled from laboratory data but also requires assessment of large scale irregularities not present in the laboratory sample a constitutive model for rock joints has been developed that considers the dilation and strength along both small scale joint roughness scaled from laboratory data and large scale waviness determined from geologic observations the model s performance is illustrated by providing its correlation with experimental results taken from literature the degradation in dilation and post peak strength along small scale irregularities is modeled using the plastic work done in shear and the degradation along large scale irregularities is modeled using a sinusoidal function a dimensionless product of plastic work rock strength and wavelength of irregularities has been developed which fits the direct shear test results an approach to scaling shear strength and shear displacement from laboratory to field scale is also suggested

stratigraphic and structural evolution of the middle miocene synvolcanic oregon idaho graben the oregon idaho graben is a newly identified north south trending synvolcanic graben in southeastern oregon and southwestern idaho within the middle miocene backarc rift system that extends 1100 km from southern nevada to southeastern washington the graben formed along the western margin of the north american craton shortly after the largest volumes of tholeiitic flood basalt erupted columbia river basalt group steens pueblo basalt basalt of malheur gorge basalt and latite unit of ekren et al 1981 1 rhyolite flows and ash flow tuffs 16 1 14 0 ma erupted from northeastern oregon dooley volcanics to northern nevada mcdermitt volcanic field shortly after the flood basalt was emplaced subsidence of the oregon idaho graben 15 5 15 3 ma coincides with eruption of rhyolite flows and caldera related ash flow tuffs from vents along the margins and within the graben mafic and silicic intragraben volcanism accompanied sedimentation from about 15 3 to 10 5 ma sedimentary and volcanic rocks from extrabasinal sources especially southwestern idaho were introduced periodically after initial subsidence the evolution of the oregon idaho graben is divided into three stages stage 1 15 3 14 3 ma followed intragraben caldera collapse and was marked by deposition of fluvial and lacustrine sediment across the graben stage 2 14 3 12 6 ma movement on intragraben fault zones divided the graben into distinct subbasins and marked the onset of calc alkalic volcanism fine grained tuffaceous sediment derived from glassy rhyolite and pyroclastic deposits and basalt tuff cones interbedded with rhyolite ash and lapilli fall deposits and locally erupted basalt hydrovolcanic deposits predominated during synvolcanic subsidence synsedimentary hot spring alteration and precious metals mineralization of graben fill were controlled by the same intragraben fault zones that served as magmatic conduits during stage 3 12 6 10 5 ma the subbasins were filled and graben wide fluviatile and lacustrine sedimentation resumed at about the same time renewed rhyolitic volcanism occurred on both flanks and tholeiitic volcanism resumed within the oregon idaho graben subsidence in the oregon idaho graben ceased as west northwest striking faults related to the formation of the western snake river plain became active 1 ref 28

post oligocene tectonic rotation of the oregon western cascade range and the klamath mountains paleomagnetic data indicate that the western cascade range from northern california to central washington has undergone about 27 of clockwise rotation since 25 m y b p all the available regional paleo magnetic data fit a two phase rotation model the first phase being an eocene rotation that accompanied the accretion of the oregon coast range to the continent and the second phase being a rotation associated with post early miocene extension in the basin and range province

celadonite in continental flood basalts of the columbia river basalt group celadonite is a common alteration product of basalts in marine environments it has been argued that marine fluids are necessary for celadonite formation possibly by providing a source of k and other dissolved cations laterally extensive deposits of celadonite occur in basalts of the grande ronde basalt of the columbia river basalt group the celadonite is found in scoriaceous flow tops of layered basalt flows where it fills vesicles and replaces the surrounding groundmass evolved interstitial glasses are present in the basalts and dissolution of these glasses may provide sufficient k for celadonite formation whereas dissolution of groundmass augite provides a source of mg and fe these observations show that alteration by seawater or any other external source of dissolved ions is not necessarily required for celadonite formation

three dimensional gravity inversion for moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction summary this paper describes a method for determining moho depth lithosphere thinning factor 1 1 and the location of the ocean continent transition at rifted continental margins using 3 d gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere the lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of 100 mgal and for which a correction must be made in order to determine moho depth accurately from gravity inversion we describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present day temperature field from which we calculate the lithosphere thermal density and gravity anomalies for continental margin lithosphere the lithosphere thermal perturbation is calculated from the lithosphere thinning factor 1 1 obtained from crustal thinning determined by gravity inversion and breakup age for thermal re equilibration time for oceanic lithosphere the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere a correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading we investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the north atlantic

magnetic stratigraphy and tectonic rotation of the middle eocene coaledo formation southwestern oregon summary the coaledo formation in coos bay on the southern coast of oregon consists of over 2000 m of deltaic sediments that filled a forearc basin during the middle eocene this unit yields molluscs of the cowlitz coaledo fauna one of the oldest and best studied faunas in the pacific northwest but its precise age has been controversial over 100 magnetic samples spanning the formation were analysed using both thermal and alternating field demagnetization they yielded a stable remanence held mainly in magnetite which passed a reversal test and showed a tectonic rotation of 105 5 this is greater than any other tectonic rotation so far reported from the region it is consistent with the previously observed trends that the tectonic rotations in the coast ranges of oregon and washington increase with increasing age of the rock unit and increase towards the southwest the magnetic polarity pattern in the coaledo formation best correlates with chrons c18r c20r 40 0 44 0 ma which is significantly older than the magnetic pattern in the cowlitz formation 36 5 38 3 ma this suggests that the cowlitz coaledo fauna is very long lived at least 36 5 44 0 ma spanning most of the middle eocene