dislocation models of interseismic deformation in the western united states 1 the gps derived crustal velocity field of the western united states is used to construct dislocation models in a viscoelastic medium of interseismic crustal deformation the interseismic velocity field is constrained by 1052 gps velocity vectors spanning the 2500 km long plate boundary zone adjacent to the san andreas fault and cascadia subduction zone and extending 1000 km into the plate interior the gps data set is compiled from u s geological survey campaign data plate boundary observatory data and the western u s cordillera velocity field of bennett et al 1999 in the context of viscoelastic cycle models of postearthquake deformation the interseismic velocity field is modeled with a combination of earthquake sources on 100 known faults plus broadly distributed sources models that best explain the observed interseismic velocity field include the contributions of viscoelastic relaxation from faulting near the major plate margins viscoelastic relaxation from distributed faulting in the plate interior as well as lateral variations in depth averaged rigidity in the elastic lithosphere resulting rigidity variations are consistent with reduced effective elastic plate thickness in a zone a few tens of kilometers wide surrounding the san andreas fault saf system primary deformation characteristics are captured along the entire saf system eastern california shear zone walker lane the mendocino triple junction the cascadia margin and the plate interior up to 1000 km from the major plate boundaries

contemporary tectonic motion of the eastern snake river plain a campaign global positioning system study a comparison of precision campaign gps data from 1995 and 2004 from ten benchmarks on the eastern snake river plain esrp has revealed that the province moved 2 8 0 3 mm yr to the sw 232 4 6 3 relative to a fixed north american reference frame the benchmarks had no measurable displacement relative to one another at the resolution of the gps during the nine year study evidence that the province moves as a rigid non extending block this scenario is supported by the aseismic nature of the province and the lack of measurable horizontal stress in boreholes however an additional small component of intra plain extension must also be invoked to account for the observed nw trending volcanic rift zones that transect the esrp we suggest that intra plain extension is too slow 1 mm yr to measure using our campaign gps methods but may be sufficient over millennial time scales to accommodate rift zone formation slower velocities measured on three benchmarks within the neighboring basin and range seismic parabola are consistent with this region serving as a zone of detachment between the north american craton and the faster moving esrp

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

block kinematics of the pacific north america plate boundary in the southwestern united states from inversion of gps seismological and geologic data 1 the active deformation of the southwestern united states 30 41 n is represented by a finite number of rotating elastic plastic spherical caps gps derived horizontal velocities geologic fault slip rates transform fault azimuths and earthquake derived fault slip vector azimuths are inverted for block angular velocities creep on block bounding faults permanent strain rates within the blocks and the rotations of 11 published gps velocity fields into to a common north american reference frame gps velocities are considered to be a combination of rigid block rotations recoverable elastic strain rates resulting from friction on block bounding faults and nonrecoverable strain rates resulting from slip on faults within the blocks the resulting pacific north america angular velocity is similar to some published estimates and satisfies transform azimuths and one spreading rate in the gulf of california earthquake slip vectors in the gulf of california and alaska and gps velocities along coastal california and within the pacific basin published fault slip rates are satisfied except in the southern mojave desert where the motion of the mohave block relative to north america is faster than can be explained by mapped faults the largest blocks the sierra nevada great valley and the eastern basin and range show permanent strain rates after removing elastic strain of only a few nanostrain per year demonstrating approximately rigid behavior observed horizontal strain rates correlate strongly with predicted strain rates from known faults suggesting that the short term strains evident in gps velocities are largely elastic in only about 20 of the region is distributed deformation needed to match the data indicating that a plate tectonic style description of the deformation of the western united states is plausible most blocks rotate about vertical axes at approximately the same rate as the pacific relative to north america suggesting that locally spin rates are communicated from block to block arguing against both floating block and ball bearing mechanisms of block rotation the similarities of the blocks spin rates to that of the pacific suggests that the pacific strongly influences their motions through edge tractions however it is shown that the blocks cannot rotate about the pacific north america pole without spinning counter to the sense of pacific north america shear unlike some other broad plate boundaries in the western united states vertical axis rotations take up very little of the slip rate budget across the region

northwest basin and range tectonic deformation observed with the global positioning system 1999 2003 1 we use geodetic velocities obtained with the global positioning system gps to quantify tectonic deformation of the northwest basin and range province of the western united states the results are based on gps data collected in 1999 and 2003 across five new quasi linear networks in northern nevada northeast california and southeast oregon the velocities show 3 mm yr westward movement of northern nevada with respect to stable north america west of longitude 119 w the velocities increase and turn northwest parallel to sierra nevada great valley microplate motion and similar to velocities previously obtained to the south the observations are explained by a kinematic model with three domains that rotate around euler poles in eastern oregon and western idaho northeast california experiences internal dextral shear deformation 11 2 3 6 nstrain yr subparallel to pacific north america motion relative motions of the domains imply 2 5 mm yr approximately east west extension in northwest nevada and 1 4 mm yr approximately north south contraction near the california oregon border the northward decreasing approximately east west extension in northwest nevada is consistent with the northern termination of basin and range deformation faulting and characteristic topography no significant extension is detected in the oregon basin and range the oregon cascade arc moves north at 3 5 mm yr and is possibly influenced by the approximately eastward motion of the juan de fuca plate these results disagree with secular northwest trenchward motion of the oregon forearc inferred from paleomagnetic rotations south of latitude 43 however trenchward motion exists and is consistent with block rotations approximately east west basin and range extension and northwest sierra nevada translation

the ghost of an earthquake seismic activity can cause earth9s crust and upper mantle to deform and then relax slowly and this relaxation can be detected decades after the original event in his perspective hammond discusses results reported in the same issue by gourmelen and amelung in which satellite based radar insar was used to detect the surface deformation in the basin and range province of the western united states caused by earthquakes in the early to mid 20th century the data indicate that disagreements between geodetic mapping such as obtained by global positioning system measurements and paleoseismic observations may be explained by postseismic relaxation moreover the results suggest that magnitude information about past earthquakes might be retrieved by such postseismic analysis

density and lithospheric strength models of the yellowstone snake river plain volcanic system from gravity and heat flow data abstract the structure and composition of the yellowstone snake river plain ysrp volcanic system were analyzed using gravity data taken at over 30 000 stations in the ysrp and surrounding region additional constraints were provided by tomographic seismic velocity models new heat flow and temperature information gps derived strain rates earthquake locations and chemical analyses of volcanic rocks p wave velocity models and velocity density data based on petrologic information were used to constrain three dimensional density models rheology and strength properties were calculated at selected locations and compared to earthquake focal depths results of this study suggest that the lower crust of the snake river plain srp has been thickened by the addition of an underplated layer composed primarily of clinopyroxene having a density of 3200 kg m3 a mid crustal high density sill is composed of a series of gabbroic lenses inter fingering with the granitic upper crust this geometry yields a bulk composition comparable to diorite and a density of 2900 kg m3 the mid crustal sill varies from 4 to 11 km in thickness resulting in a series of se nw trending gravity highs observed along the axis of the srp the mid crustal sill extends up to 20 km southeast of the srp volcanic field and causes asymmetry of the gravity field the yellowstone plateau volcanic field density model reveals low density partial melt 10 km beneath the caldera that shallows under the northeastern caldera and continues laterally 20 km north of the caldera boundary and notably increases the previously estimated size of the magma reservoir by 20 the caldera melt body has a density of 2520 kg m3 and a significantly lower value of 2470 kg m3 for the northeastern caldera melt body southwest of yellowstone the crustal section occupied by the mid crustal sill in the srp and the partial melt in yellowstone constitute a transition area between the active yellowstone magma system and the now volcanically quiescent srp with a density of 2820 kg m3 strength models show that the crust of the ysrp becomes progressively stronger and cooler with increasing distance from yellowstone and tectonic earthquakes within the yellowstone caldera are unlikely to nucleate below 4 6 km depth thus limiting the maximum magnitude of earthquakes to m 6 5

an assessment of source parameters of earthquakes in the cordillera of the western united states abstract source parameters for 50 earthquakes occurring between 1915 and 1988 have been used to examine the faulting characteristics of magnitude 5 5 to 7 8 earthquakes within the western u s cordillera an area extending from the eastern sierra nevada to the great plains and from northwestern montana to trans pecos texas source parameters used in this study include seismic information from first motion analyses body and surface waveform modeling geodetic studies and geological studies of surface faulting the principal results of this analysis include 1 all earthquakes occurred on faults dipping 38 or more and no evidence for listric of low angle planar faulting was found 2 all m 7 0 earthquakes occurred at depths 12 km and were composed of multiple subevents 3 most earthquakes 70 per cent had unilateral ruptures and 4 no individual subevent had a rupture length 21 km

geodetic constraints on areal changes in the pacific north america plate boundary zone what controls basin and range extension using 1500 geodetic velocities we model the present day spatial patterns of areal changes inside the pacific north america plate boundary zone from this model we show that between the central gulf of california and the queen charlotte islands there is no significant net change in surface area this zero net areal change result allows us to relate regions of areal growth to areas of equivalent contraction elsewhere within the plate boundary zone we find that areal growth of the basin and range province brp and its eastern margin 5 2 0 1 10 3 m 2 yr is balanced by areal reduction near northwestern california between 38 n and 42 n the san andreas fault system south of 38 n and the plate boundary zone north of 42 n including the juan de fuca and gorda ridge systems each have no significant net areal change our results suggest a kinematic relationship between extension in the brp and contraction near the northern california coast ranges and klamath mountains from these observations we propose that although brp extension may be caused by internal forces the southernmost cascadia subduction zone provides a window of escape that acts as a stress guide to brp extension as well as northwestward sierra nevada motion such a dynamic model is consistent with independent findings that 1 the least principal horizontal stress orientations in the brp are toward northern california 2 extension directions in the brp have changed orientation to track the northward migration of the mendocino triple junction and 3 the southernmost cascadia subduction zone is a relatively weak plate boundary

present day kinematics of the eastern california shear zone from a geodetically constrained block model we use global positioning system gps data from 1993 2000 to determine horizontal velocities of 65 stations in eastern california and western nevada between 35 and 37 n we relate the geodetic velocities to fault slip rates using a block model that enforces path integral constraints over geologic and geodetic time scales and that includes the effects of elastic strain accumulation on faults locked to a depth of 15 km the velocity of the sierra nevada block with respect to nevada is 11 1 0 3 mm yr with slip partitioned across the death valley 2 8 0 5 mm yr panamint valley 2 5 0 8 mm yr and airport lake owens valley 5 3 0 7 4 6 0 5 mm yr faults the western mojave block rotates at 2 1 0 8 my clockwise with 3 7 0 7 mm yr of left lateral motion across the western garlock fault we infer 11 2 mm yr of right lateral motion across the mojave region of the eastern california shear zone

extension of the yellowstone plateau eastern snake river plain and owyhee plateau formation of the late cenozoic volcanic province comprising the owyhee plateau eastern snake river plain and yellowstone plateau has been accompanied by east northeast directed crustal extension a new vector of 45 mm yr n56 degree e for the migration of silicic volcanism across the volcanic province is calculated if migration of volcanism reflects west southwest continental drift over a mantle plume a zone of crustal extension must separate the volcanic province from the more slowly moving north american craton space time relations of basin fill in the adjacent basin and range province provide evidence for a zone of extension about 125 km wide coincident with and east of coeval silicic volcanism since 16 ma the zone of extension has migrated along with silicic volcanism maintaining its position between the province and the unextended craton

crustal structure of the eastern snake river plain determined from ray trace modeling of seismic refraction data ray trace travel time modeling of the seismic refraction record sections for a profile from near soda springs idaho to near mckay idaho was used to derive a crustal model across the eastern snake river plain esrp the derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the esrp the interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the esrp when compared with the upper crust beneath the adjoining northern rocky mountain and basin and range provinces the most prominent features of the crustal structure inferred by the ray trace modeling are as follows 1 the northwest margin of the esrp was modeled as a fault structure downthrown on the srp side se with an offset of greater than 4 km the southeast margin conversely appears to be downwarped with possible minor faulting paleozoic sedimentary rocks are present beneath the cenozoic volcanics of the esrp possibly to and beyond 40 km from the southeastern margin of the esrp near blackfoot idaho 2 the modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the esrp 3 an intermediate 6 5 km s layer occurring beneath the esrp interpreted from a refraction profile located along the axis of the esrp was found to be localized within the esrp margins this layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region 4 a density model of the crust across the esrp was prepared with the densities selected using the interpreted seismic velocities as a constraint the gravity field calculated from this model resulted in a good match to observed gravity data over the eastern snake river plain

subsidence of a volcanic basin by flexure and lower crustal flow the eastern snake river plain idaho the eastern snake river plain esrp is a linear volcanic basin interpreted by many workers to reflect late cenozoic migration of north america over the yellowstone hotspot thermal subsidence of this volcanic province with respect to yellowstone has been documented by several workers but no one has characterized subsidence with respect to the adjacent basin and range province this paper documents crustal flexure along the northwest edge of the esrp uses flexure to model the dimensions of a dense load beneath the basin and presents evidence in support of density driven subsidence and lower crustal flow away from the basin crustal flexure adjacent to the esrp is reflected by the attitudes of mesozoic fold hinges and neogene volcanic rocks fold hinges formed with a subhorizontal plunge and a trend perpendicular to the esrp but now show a southward plunge near the esrp of as much as 20 25 we present a contour map of equal fold plunges proximal to the esrp that shows flexure is roughly parallel to and extends 10 20 km north of the average edge of the esrp flexural profiles indicate the minimum amount of esrp subsidence with respect to the basin and range subsidence ranges from 4 5 to 8 5 km the structural contour map and published seismic and gravity data were used to develop and constrain flexural subsidence models these models indicate the flexed crust is very weak flexural parameter of 4 10 km interpreted to be a result of the high heat flow of the esrp assuming subsidence was induced by emplacement of a dense crustal layer beneath the esrp a midcrustal sill identified in previous seismic surveys is too wide and probably too thin to produce the measured flexure new dimensions include a thickness of 17 25 km and a half width of 40 50 km which place the edge of the sill beneath the edge of the esrp the dimensions of the esrp sill are based on isostatic compensation in the lower crust because compensation in the asthenosphere requires an unreasonable sill thickness of 30 km and because esrp seismic gravity and heat flow data support lower crustal compensation density driven lower crustal flow away from the esrp is proposed to accommodate subsidence and maintain isostatic equilibrium timing of subsidence is constrained by esrp exploratory wells where 6 6 ma rhyolites at a depth of 1 5 km indicate most subsidence occurred prior to their emplacement and by strong spatial correlations between plunge contours and quaternary volcanic rift zones two processes interpreted to contribute to the load include an extensive midcrustal mafic load emplaced at 10 ma which provided the heat source for the initial rhyolitic volcanism on the esrp and continuing localized loads from dikes and sills associated with quaternary basalts widespread 10 ma magmatism and subsidence conflicts with simple time transgressive migration of the yellowstone hotspot indicating a need for revision of the hotspot paradigm

rheology of the lithosphere inferred from postseismic uplift following the 1959 hebgen lake earthquake 1 we have modeled the broad postseismic uplift measured by geodetic leveling in the epicentral area of the 1959 mw 7 3 hebgen lake montana earthquake a normal faulting event in the northern basin and range province to fit the observed uplift we calculate synthetic postseismic deformation using the relaxation response of a gravitational viscoelastic earth to the earthquake for a model with an elastic plate overlying a viscoelastic half space we find that the elastic thickness is 38 8 km which is close to the local crustal thickness the half space viscosity is estimated at 4 1018 0 5 pa s the leveling data do not require a viscous lower crust but permit a lower bound viscosity of 1020 pa s the observed broad uplift cannot be explained by physically plausible afterslip on and below the coseismic fault however local deformation across the coseismic surface rupture requires shallow afterslip reaching the surface the postseismic deformation induced by the estimated viscoelastic structure decays exponentially with a time constant of 15 years because of coupling between the elastic layer and the viscoelastic substrate this relaxation time is significantly longer than the 2 year maxwell relaxation time of the viscous half space itself our result suggests the importance of postseismic relaxation in interpreting high precision global positioning system velocities for example our model results suggest that postseismic transient velocities from both the 1959 hebgen lake and the 1983 mw 6 9 borah peak earthquakes are currently as large as 1 2 mm yr

the yellowstone hotspot in space and time nd and hf isotopes in silicic magmas abstract over the course of its 16 m y history the yellowstone hotspot has produced silicic magmas exhibiting systematic and often sympathetic variations in isotopic and chemical composition temperature and frequency of eruption nd and hf isotopic ratios vary systematically from initial eruptions at 16 ma contemporaneous with basaltic volcanism in eastern oregon and washington to the present day yellowstone volcanic plateau nd and hf isotopic ratios co vary and span the range of most terrestrial samples reflecting mixing of mantle and crustal sources earliest erupted silicic magmas were hot in excess of 1050 c relatively less evolved and have isotopic ratios within the range of contemporaneous columbia river flood basalts the transit of the hotspot across the lithospheric boundary between the western accreted oceanic terrain and the precambrian craton at 15 ma is marked by shifts in e nd from 4 to 11 and in e hf from 10 to 10 the duration of the transit yields a crustal magma source diameter of 70 km in the interval from 14 to 9 ma e nd systematically increases from 11 to 7 recording a minimum increase in the mantle component from 5 to 30 the mantle component could be twice as great depending upon the isotopic composition of crust and mantle reservoirs in this same interval peak temperatures of 1000 c occurred at 9 ma the last 8 m y are characterized by less frequent eruption of lower temperature 830 900 c and more compositionally evolved magmas

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

strain accumulation and rotation in western nevada 1993 2000 1 the positions of 44 gps monuments in an array extending from the sierra nevada at the latitude of reno to near austin nevada have been measured several times in the 1993 2000 interval the western half of the array spans the walker lane belt whereas the eastern half spans the central nevada seismic zone cnsz the principal strain rates in the walker lane belt are 29 6 5 3 nstrain yr 1 n88 4 e 5 4 and 12 8 6 0 nanostrain yr 1 n01 6 w 5 4 extension reckoned positive and the clockwise as seen from above the earth rotation rate about a vertical axis is 13 6 4 0 nrad yr 1 the quoted uncertainties are standard deviations the motion in the walker lane belt can then be represented by a zone striking n35 w subject to 16 8 4 9 nstrain yr 1 extension perpendicular to it and 19 5 4 0 nstrain yr 1 right lateral simple shear across it the n35 w strike of the zone is the same as the direction of the local tangent to the small circle drawn about the pacific north america pole of rotation the principal strain rates for the cnsz are 46 2 11 0 nstrain yr 1 n49 9 w 6 0 and 13 6 6 1 nstrain yr 1 n40 1 e 6 0 and the clockwise rotation rate about a vertical axis is 20 3 6 3 nrad yr 1 the motion across the cnsz can then be represented by a zone striking n12 e subject to 32 6 11 0 nstrain yr 1 extension perpendicular to it and 25 1 6 3 nstrain yr 1 right lateral simple shear across it the n12 e strike of the zone is similar to the strikes of the faults rainbow mountain fairview peak and dixie valley within it

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

a revised dislocation model of interseismic deformation of the cascadia subduction zone 1 cas3d 2 a new three dimensional 3 d dislocation model is developed to model interseismic deformation rates at the cascadia subduction zone the model is considered a snapshot description of the deformation field that changes with time the effect of northward secular motion of the central and southern cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and the forearc horizontal deformation data including strain rates and surface velocities from global positioning system gps measurements provide primary geodetic constraints but uplift rate data from tide gauges and leveling also provide important validations for the model a locked zone based on the results of previous thermal models constrained by heat flow observations is located entirely offshore beneath the continental slope similar to previous dislocation models an effective zone of downdip transition from locking to full slip is used but the slip deficit rate is assumed to decrease exponentially with downdip distance the exponential function resolves the problem of overpredicting coastal gps velocities and underpredicting inland velocities by previous models that used a linear downdip transition a wide effective transition zone etz partially accounts for stress relaxation in the mantle wedge that cannot be simulated by the elastic model the pattern of coseismic deformation is expected to be different from that of interseismic deformation at present 300 years after the last great subduction earthquake the downdip transition from full rupture to no slip should take place over a much narrower zone

viscoelastic shear zone model of a strike slip earthquake cycle i examine the behavior of a two dimensional 2 d strike slip fault system embedded in a 1 d elastic layer schizosphere overlying a uniform viscoelastic half space plastosphere and within the boundaries of a finite width shear zone the viscoelastic coupling model of savage and prescott 1978 considers the viscoelastic response of this system in the absence of the shear zone boundaries to an earthquake occurring within the upper elastic layer steady slip beneath a prescribed depth and the superposition of the responses of multiple earthquakes with characteristic slip occurring at regular intervals so formulated the viscoelastic coupling model predicts that sufficiently long after initiation of the system 1 average fault parallel velocity at any point is the average slip rate of that side of the fault and 2 far field velocities equal the same constant rate because of the sensitivity to the mechanical properties of the schizosphere plastosphere system i e elastic layer thickness plastosphere viscosity this model has been used to infer such properties from measurements of interseismic velocity such inferences exploit the predicted behavior at a known time within the earthquake cycle by modifying the viscoelastic coupling model to satisfy the additional constraint that the absolute velocity at prescribed shear zone boundaries is constant i find that even though the time averaged behavior remains the same the spatiotemporal pattern of surface deformation particularly its temporal variation within an earthquake cycle is markedly different from that predicted by the conventional viscoelastic coupling model these differences are magnified as plastosphere viscosity is reduced or as the recurrence interval of periodic earthquakes is lengthened application to the interseismic velocity field along the mojave section of the san andreas fault suggests that the region behaves mechanically like a 600 km wide shear zone accommodating 50 mrn yr fault parallel motion distributed between the san andreas fault system and eastern california shear zone in the viscoelastic coupling model savage and prescott 1978 time dependent deformation around a strike slip fault zone is realized by loading subsequent rupture and relaxation of a fault present in a schizosphere elastic upper crust overlying a ductile plastosphere assuming a two dimensional 2 d geometry periodic rupture of the fault and identical slip interseismic deformation is obtained as the superposition of viscoelastic relaxation of the plastosphere summed over the infinity of past periodic earthquakes this model yields far field interseismic motions equal to 1 2 of the average slip rate v0 imposed on the fault thus even when no external mechanism is invoked to load the fault i e when the locked portion of the fault penetrates the entire schizosphere the model satisfies the expectation of rigid plate like behavior in the far field in addition when periodic coseismic offsets are factored in the model predicts that the time averaged fault parallel velocity of each point is vol2 depending on location

strain accumulation across the eastern california shear zone at latitude 36 30 n the motion of a linear array of monuments extending across the eastern california shear zone ecsz has been measured from 1994 to 1999 with the global positioning system the linear array is oriented n54 e perpendicular to the tangent to the local small circle drawn about the pacific north america pole of rotation and the observed motion across the ecsz is approximated by differential rotation about that pole the observations suggest uniform deformation within the ecsz strike n23 w 26 nstrain yr 1 extension normal to the zone and 39 nstrain yr 1 simple right lateral shear across it with no significant deformation in the two blocks the sierra nevada mountains and southern nevada on either side the deformation may be imposed by right lateral slip at depth on the individual major fault systems within the zone if the slip rates are death valley furnace creek fault 3 2 0 9 mm yr 1 hunter mountain panamint valley fault 3 3 1 6 mm yr 1 and owens valley fault 6 9 1 6 mm yr 1 however this estimate of the slip rate on the owens valley fault is 3 times greater than the geologic estimate

postseismic mantle relaxation in the central nevada seismic belt holocene acceleration of deformation and postseismic relaxation are two hypotheses to explain the present day deformation in the central nevada seismic belt cnsb discriminating between these two mechanisms is critical for understanding the dynamics and seismic potential of the basin and range province interferometric synthetic aperture radar detected a broad area of uplift 2 to 3 millimeters per year that can be explained by postseismic mantle relaxation after a sequence of large crustal earthquakes from 1915 to 1954 the results lead to a broad agreement between geologic and geodetic strain indicators and support a model of a rigid basin and range between the cnsb and the wasatch fault

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

geodetic estimates of fault slip rates in the san francisco bay area bourne et al 1998 have suggested that the interseismic velocity profile at the surface across a transform plate boundary is a replica of the secular velocity profile at depth in the plastosphere on the other hand in the viscoelastic coupling model the shape of the interseismic surface velocity profile is a consequence of plastosphere relaxation following the previous rupture of the faults that make up the plate boundary and is not directly related to the secular flow in the plastosphere the two models appear to be incompatible if the plate boundary is composed of several subparallel faults and the interseismic surface velocity profile across the boundary known each model predicts the secular slip rates on the faults which make up the boundary as suggested by bourne et al the models can then be tested by comparing the predicted secular slip rates to those estimated from long term offsets inferred from geology here we apply that test to the secular slip rates predicted for the principal faults san andreas san gregorio hayward calaveras rodgers creek green valley and greenville faults in the san andreas fault system in the san francisco bay area the estimates from the two models generally agree with one another and to a lesser extent with the geologic estimate because the viscoelastic coupling model has been equally successful in estimating secular slip rates on the various fault strands at a diffuse plate boundary the success of the model of bourne et al 1998 in doing the same thing should not be taken as proof that the interseismic velocity profile across the plate boundary at the surface is a replica of the velocity profile at depth in the plastosphere

microplate versus continuum descriptions of active tectonic deformation whether deformation of continents is more accurately described by the motions of a few small rigid plates or by quasi continuous flow has important implications for lithospheric dynamics fault mechanics and earthquake hazard assessment actively deforming regions of the western united states central asia japan and new zealand show features that argue for both styles of movement but new observations are necessary to determine which is most appropriate and at what scale the description applies geologic geodetic seismic and paleomagnetic measurements tend to sample complementary aspects of the deformation field so an integrated observation program can utilize the strengths of each method and overcome their separate spatial or temporal biases provided the total relative motion across each region is known and the distribution of active faults is well mapped determination of fault slip rates can provide potentially decisive constraints reconnaissance geological studies supply useful slip rate estimates but precise values depend upon detailed intensive investigation of individual sites geodetic survey measurements can determine the spatial pattern of contemporary movements and extract slip rate information but the sometimes elusive effects of cyclic elastic strain buildup and relief must be accounted for in relating current movements to the long term deformation pattern earthquake catalogs can be applied to determine seismic strain rates and relative velocities but must be averaged over large regions and are usually limited by the inadequate duration of historical or instrumental seismicity catalogs paleomagnetic determinations of vertical axis rotations provide estimates of block rotation rates but are often locally variable and averaged over many millions of years which of the two descriptions of continental tectonics is more nearly correct depends on the local rheological stratification of the lithosphere especially the strength and thickness of the elastic crust relative to the ductile lithosphere and dynamical models can provide contrasting forecasts of observable features with testable consequences

intraplate deformation and microplate tectonics of the yellowstone hot spot and surrounding western u s interior 1 contemporary deformation of the yellowstone hot spot and surrounding western united states is analyzed using tectonic microplate modeling employing constraints from gps observations corrected for postseismic deformation of m7 earthquakes fault slip rates and earthquake focal mechanisms we focus primarily on the kinematics of the yellowstone hot spot and the eastern snake river plain volcanic field esrp and secondarily on basin range and columbia plateau provinces our results reveal southwest motion of the yellowstone plateau excluding localized volcanic deformation at 0 9 0 1 mm a that decreases to 0 8 0 1 mm a in the esrp block the southwest to west motion of the yellowstone esrp introduces shear in the northern rocky mountain block which is translating east at 0 78 0 08 mm a there is 0 5 mm a differential motion between the esrp and the block at its northern boundary and none at the southern boundary the eastern basin range block moves west at 3 0 0 1 to 4 6 0 1 mm a while velocities of the western basin range microplates rotate to a northwest direction accompanied by a transition from normal to oblique shear deformation columbia plateau block velocities are notably eastward an effect enhanced by postseismic relaxation following the m9 1700 cascadia subduction zone paleoearthquake this study reveals that the overall motion of the western united states is characterized by clockwise rotation with westward extension and northwest shear in the basin range province northeast to east contraction in the columbia plateau and southwest extension of the yellowstone snake river plain block that is driven by the high gravitational potential of the yellowstone swell

summary of fossil vertebrate taxa named by richard c fox with an annotated list of taxa named between 1962 and 2012 and new photographs for non mammalian therapsid and mammalian holotypes erected between 1968 and 1994 beginning in 1962 and extending to the present richard c fox and colleagues have named 87 species of fossil vertebrates 1 fish 4 amphibians 2 choristoderes 12 lizards 1 crocodile 1 dinosaur 2 pelycosaurs 2 non mammalian therapsids and 62 mammals and numerous new supraspecific taxa virtually all of these species continue to be accepted although the higher level assignments of several have been altered the vast majority of the named species were founded on specimens collected during the mid 1960s to early 2000s by field parties under fox s direction from the late cretaceous late santonian to late maastrichtian and paleocene of alberta and saskatchewan canada and that are housed at the university of alberta laboratory for vertebrate paleontology here we present i an annotated list of all fossil vertebrate species named by richard fox between 1962 and 2012 ii updated information on the stratigraphic nomenclature and age estimates for the eight localities in alberta that yielded ho

crustal deformation across the sierra nevada northern walker lane basin and range transition western united states measured with gps 2000 2004 1 global positioning system gps data collected in campaigns in 2000 and 2004 were processed and interpreted with other gps data in the western basin and range province to provide new constraints on the rate style and pattern of deformation of the central and northern walker lane wl which lies near the western boundary of the basin and range across the central wl near 38n latitude the velocities with respect to north america increase westward by 10 mm yr inducing dextral shear farther north between 40 and 41n latitude a western zone of 7 mm yr relative motion undergoes dextral shear and an eastern zone of 3 mm yr relative motion undergoes extension and shear these data show that the northern wl is essentially a dextral shear zone experiencing minor net dilatation ed 2 6 0 8 nstrain yr near most holocene normal faults dilatation inferred from the velocity field is not greater than the uncertainties however near the central nevada seismic belt we detect significant dilatation expressed as extension in a direction approximately normal to the range fronts ed 23 0 3 9 nstrain yr some of which is attributable to transient postseismic deformation following large historic earthquakes a block model constrained by velocities corrected for transient effects shows that the sum of dextral slip rates across the honey lake warm springs east pyramid fault system and mohawk valley faults is 7 mm yr the wl is a zone whose width and dilatation rate increase northwestward consistent with counterclockwise rotation of the sierra nevada microplate and transfer of deformation into the pacific northwest

solubility of manganotantalite zircon and hafnon in highly fluxed peralkaline to peraluminous pegmatitic melts the behavior of tantalum and zirconium in pegmatitic systems has been investigated through the determination of ta and zr solubilities at manganotantalite and zircon saturation from dissolution and crystallization experiments in hydrous li f p and b bearing pegmatitic melts the pegmatitic melts are synthetic and enriched in flux elements 0 7 1 3 wt li2o 2 5 5 wt f 2 8 4 wt p2o5 and 0 2 8 wt b2o3 and their aluminum saturation index ranges from peralkaline to peraluminous asili al na k li 0 8 to 1 3 with various k na ratios dissolution and crystallization experiments were conducted at temperatures varying between 700 and 1 150 c at 200 mpa and nearly water saturated conditions for dissolution experiments pure synthetic end member manganotantalite and zircon were used in order to avoid problems with slow solid state kinetics but additional experiments using natural manganotantalite and zircon of relatively pure composition i e close to end member composition displayed similar solubility results zircon and manganotantalite solubilities considerably increase from peraluminous to peralkaline compositions and are more sensitive to changes in temperature or asi of the melt than to flux content a model relating the enthalpy of dissolution of manganotantalite to the asili of the melt is proposed hdiss kj mol 304 asili 176 in the peralkaline field and hdiss kj mol 111 asili 245 in the peraluminous field the solubility data reveal a small but detectable competitivity between zr and ta in the melt i e lower amounts of zr are incorporated in a ta bearing melt compared to a ta free melt under the same conditions a similar behavior is observed for hf and ta the competitivity between zr or hf and ta increases from peraluminous to peralkaline compositions and suggests that ta is preferentially bonded to non bridging oxygens nbos with al as first neighbors whereas zr is preferentially bonded to nbos formed by excess alkalies as a consequence zr ta ratios when buffered by zircon and manganotantalite simultaneously are higher in peralkaline melts than in peraluminous melts