present day pattern of cordilleran deformation in the western united states we present the first detailed geodetic image of the entire western united states south of lat 42 n merging both campaign and continuous global positioning system gps and very long baseline interferometry vlbi data sets in a combined solution for station velocities having a single uniform reference frame the results are consistent with a number of features previously observed through local geodetic studies and very sparse space geodetic studies including a dominant pattern of right lateral shear associated with the san andreas fault rates of the westernmost sites along the california coast of 46 48 mm yr relative to a north america reference frame and some 11 13 mm yr of deformation accommodated east of the sierra nevada in the basin and range province north of lat 36 n south of 36 n the solution also shows that the southernmost san andreas fault system accommodates effectively all interplate motion and that the southern basin and range is not deforming significantly at lat 37 n the eastern california shear zone appears to exhibit simple shear oriented between n20 w and n40 w relative to north america with a fairly well defined transition zone from localized shear to diffuse spreading in the basin and range enigmatically this transition involves a significant component of contraction normal to the overall shear zone trend sites in the great basin move southwestward at up to 5 mm yr toward sites within the eastern california shear zone to the north in contrast there appears to be a relatively smooth transition from east west spreading within the eastern great basin to northwest southeast shear across the westernmost basin and range

constraints on the 29 june 1992 little skull mountain nevada earthquake sequence provided by robust hypocenter estimates abstract the m s 5 4 little skull mountain earthquake of 29 june 1992 occurred within 21 km of yucca mountain nevada the site of a proposed high level nuclear waste repository data from a dense portable seismic network have enabled us to examine the faulting process in detail standard linearized least squares procedures were used to determine hypocenter estimates for all the events a grid search technique was applied to test the robustness of estimates of the hypocenters of the mainshock and six of the largest aftershocks projection of the mainshock fault plane to the surface delineated by its focal mechanism and the distribution of aftershocks defines a strike subparallel to a north northeast structural trend within jackass flats and collinear with the mine mountain fault system this result implies reactivation of mapped faults with no known quaternary movement and growth or reactivation of buried faults having no clear surface manifestation the aftershock distribution suggests that postseismic strain release occurred up dip from the mainshock on the mainshock fault plane and on several secondary faults the mainshock focal mechanism and the aftershock distribution indicate a normal slip rupture on a plane with a strike of n55 e a dip of 56 and a rake of 72 results of the grid search analysis show that the mainshock and three of the largest aftershocks probably occurred on the same fault plane and that the other three occurred off the mainshock fault plane two on a steeply dipping n200 e striking plane with dextral strike slip displacements and the other possibly on a plane conjugate to the mainshock rupture plane focal mechanism unknown

stratigraphic control of hot fluids on anthracitization lackawanna synclinorium pennsylvania abstract the lackawanna synclinorium of pennsylvania contains abundant coal that was altered to anthracite rank during the late paleozoic alleghanian orogeny why did the coal in the synclinorium undergo anthracitization two alternatives have been suggested 1 the region was buried deeply during and or after the alleghanian orogeny and thus became hot in response to earth s geothermal gradient and 2 hot fluids migrating from the orogen toward the foreland carried heat into the coal and caused anthracitization if the second hypothesis is correct the region should contain evidence that hot fluids passed through the coal field observations illite crystallinity studies and fluid inclusion analyses indicate that the coal bearing pottsville and llewellyn formations and an underlying detachment fault called the pottchunk fault at or slightly below the base of the pottsville formation acted as a regional aquifer for the migration of hot fluids during the alleghanian orogeny the presence of quartz veins and the hydrothermal minerals tosudite and pyrophyllite in strata above the pottchunk fault and the apparent absence of an illite crystallinity burial gradient in the underlying strata suggest that fluids migrated through large pores in coarse grained sandstones through abundant fractures that developed in response to alleghanian deformation and along the pottchunk fault the quartz veins tosudite and pyrophyllite mineralization do not occur below the pottchunk fault except at one locality suggesting that fluid flow was greater above the fault perhaps because fracturing accompanying deformation increased permeability of the fault s hanging wall fluid inclusions in the pottsville and llewellyn formations indicate fluids achieved a minimum temperature of 270 c at a depth of 3 1 8 5 km heat flow calculations constrained by fluid inclusion data show the heat carried by the migrating fluids through the coal bearing strata was sufficient to elevate the rock temperature to anthracite grade conditions 250 c assuming that the fluid event lasted about 1 year thus deep burial 6 9 km of the coal bearing strata in the lackawanna synclinorium was not required for anthracitization anthracitization was likely the result of stratigraphically controlled hot fluid migration through the coal bearing strata at shallow depths 5 km

patterns and timing of late quaternary faulting in the great basin province and relation to some regional tectonic features large scale surface faulting events each tens of kilometers long and involving more than a meter of displacement of late quaternary age have not been uniformly distributed but have been concentrated in subprovincial and smaller belts and areas within the great basin province furthermore faulting apparently has not been uniformly distributed over time the average recurrence interval of large scale faulting events on individual faults in the province is measured in thousands or tens of thousands of years in contrast to such long recurrence intervals in the central nevada and eastern california seismic belts large scale faulting events have occurred during the last 111 years in an apparently coherent sequential belt filling pulse of activity in which events were separated by only a few years or a few decades the questions are where are other pulses of large scale faulting likely to occur and what controls the localization of such belts and areas characterized by higher rates of faulting two examples of tectonic subprovinces that may play a role in delimiting smaller zones of faulting are the black rock carson sink zone of extension and the central nevada downwarp the central nevada seismic belt lies along the dividing line between these two subprovinces and the northwest margin of the black rock carson sink zone of extension coincides with the margin of a fingerlike belt of dense faulting in northwestern nevada

strain accumulation at yucca mountain nevada 1983 1998 a 14 station 50 km aperture geodetic array centered on the proposed radioactive waste disposal site at yucca mountain nevada was surveyed in 1983 1984 1993 and 1998 to determine the rate of strain accumulation there the coseismic effects of the 1992 ms 5 4 little skull mountain earthquake which occurred within the array were calculated from a dislocation model and removed from the data the measured principal strain accumulation rates determined over the 1983 1998 interval are e1 2 12 nanostrain yr n87 w 12 and e2 22 12 nanostrain yr n03 e 12 extension reckoned positive and quoted uncertainties are standard deviations the n65 w extension rate is 2 12 nanostrain yr significantly less than the 1991 1997 n65 w rate of 50 9 nanostrain yr reported by wernicke et al 1998 the implied maximum right lateral engineering shear strain accumulation rate is e1 e2 23 10 nanostrain yr a marginally significant rate almost half e1 6 nanostrain yr n90 w e2 6 nanostrain yr n00 e and 12 nanostrain yr of the measured strain rate can be attributed to strain accumulation on the death valley furnace creek 50 km distant and hunter mountain panamint valley 90 km distant faults the residual strain rate after the removal of those fault contributions is not significant at the 95 confidence level

detecting strain in the yucca mountain area nevada from repeated surveys of the relative motions between geodetic stations claim black mile 67tjs and wahomie fig 1 from 1991 through 1997 wernicke et al 1 deduce a n65 w strain accumulation rate of 50 9 nanostrains per year 2 across the proposed high level radioactive waste disposal repository at yucca mountain nevada that strain rate is sufficient to indicate a higher than expected earthquake hazard at the repository an earlier 1983 1993 u s geological survey usgs measurement table 1 of the strain accumulation in the network black triangles in fig 1 found a n65 w strain rate of 8 20 nanostrains per year a rate not significantly different from zero 3 although the two measurements are marginally consistent at the 95 confidence level that is the difference 42 22 nanostrains per year is less than 2 standard deviations sd the earthquake hazard implications are somewhat different the usgs strain determination is the better of the two measurements wernicke et al did not include the effects of monument instability in their error budget and as a consequence have significantly underestimated the uncertainties in their measurements moreover in their interpretation of the data they did not give proper weight to the coseismic and postseismic effects of the little skull mountain earthquake june 29 1992 m 5 4

continuous gps measurements of contemporary deformation across the northern basin and range province we have acquired and analyzed data from the northern basin and range nbar continuous gps network since july 1996 the rms residual with respect to the best fitting lines through the individual station position estimates is 2 3 mm in the horizontal and 6 10 mm in the vertical after the first 395 days of operation uncertainties in horizontal velocity estimates are 1 2 mm yr 1 relative motion among nbar sites located in eastern nevada and in utah is small but east west extension is significant assuming uniform strain accumulation across the whole network the relative motion observed across the wasatch fault zone is 2 2 mm yr east west relative motions among stations in western nevada and california in contrast are dominated by northwest right lateral shear we infer an integrated deformation across the northern basin and range of 11 2 mm yr northwest these rates are consistent with previous geodetic measurements our gps velocity estimates however reveal a possibly abrupt transition from east west extension in eastern nevada and utah to right lateral shear in western nevada this transition is roughly coincident with the central nevada seismic belt and is consistent with the right oblique focal mechanisms of the 1954 dixie valley and fairview peak earthquakes the transition also appears to correlate spatially with a transition in upper mantle structure

strain accumulation near yucca mountain nevada 1993 1998 a 50 km aperture geodetic network centered on the proposed high level radioactive waste disposal site at yucca mountain nevada was surveyed with gps in 1993 and 1998 the average deformation rate across the area is described by the principal strain rates 22 8 8 8 nstrain yr 1 n77 6 w 13 5 and 8 8 11 9 nstrain yr 1 n12 5 e 13 5 extension reckoned positive and a clockwise rotation rate about a vertical axis of 9 6 7 4 nrad yr 1 relative to fixed north america quoted uncertainties are standard deviations those strain rates are consistent with the geodetic strain rates 2 12 nstrain yr 1 n87 12 w and 22 12 nstrain yr 1 n03 12 e previously reported by savage et al 1999 for the 1983 1998 interval and with the low extension rate 5 20 nstrain yr 1 marrett et al 1998 inferred from the geologic record none of those strain rates is consistent with the 50 9 nstrain yr 1 n65 w extension rate for the area reported by wernicke et al 1998

physical processes of shallow mafic dike emplacement near the san rafael swell utah some 200 shonkinite dikes sills and breccia bodies on the western colorado plateau of south central utah were intruded from approximately 3 7 to 4 6 ma contemporaneous with mafic volcanism along the nearby plateau margin thicknesses of dikes range to about 6 m the log normal mean thickness is 85 cm despite the excellent exposures of essentially all dikes in strata of the jurassic san rafael group their number is indeterminate from their outcrop and spacing because they are everywhere greatly segmented by our grouping of almost 2000 dike segments most dikes are less than 2 km in outcrop length the longest is 9 km because the san rafael magmas were primitive and probably ascended directly from the mantle dike lengths in outcrop are much less than their heights the present exposures probably lie along the irregular upper peripheries of dikes that lengthen and merge with depth orientations of steps on dike contacts record local directions of dike fracture propagation about half of the measurements plunge less than 30 showing that lateral propagation at dike peripheries is as important as the vertical propagation ultimately responsible for ascent the san rafael dikes now exposed after erosion of about 0 5 1 5 km appear to thicken and shorten upward probably because near surface vesiculation enhanced magmatic driving pressures propagation likely ceased soon after the first dike segments began to feed nearby sills or vented to initiate small volume eruptions most of the dikes are exposed in clastic strata of the jurassic san rafael group they probably acquired their strikes however while ascending along well developed joints in massive sandstones of the underlying glen canyon group rotation of far field stresses during the emplacement interval cannot account for disparate strikes of the dikes which vary through 110 most lying between north and n25 w rather the two regional horizontal principal stresses were probably nearly equal and so the dominant n75 e direction of dike opening was not strongly favored across the center of the swarm about 10 to 15 dikes overlap and produce 15 20 m of dilation many are in sufficient proximity that later dikes should be thinner than earlier ones if neither the magma pressures nor regional stresses were changing during the emplacement interval however dike thicknesses vary systematically neither along the length of the swarm nor in proportion to the number of neighboring dikes it appears that crustal extension during the magmatic interval relieved compressive stresses localized by intrusion

the little skull mountain nevada earthquake of 29 june 1992 aftershock focal mechanisms and tectonic stress field implications the ml 5 6 29 june 1992 little skull mountain earthquake occurred in the southern nevada test site nye county nevada within 20 km of a potential national nuclear waste repository at yucca mountain nevada while microseismicity in the immediate epicentral vicinity occurred for years prior to the mainshock foreshock activity ml 2 were also sufficiently well recorded to provide local network p wave focal mechanism solutions the mainshock and the majority of the aftershocks through september 1992 display predominantly normal slip focal mechanisms but unambiguous strike slip and oblique slip focal mechanism solutions are also present in the aftershock series the mainshock and immediate foreshock depths were 10 1 km below sea level and depths of focus of aftershocks range from about 6 to 12 km a large subset of the aftershock hypocenters define a southeast dipping structure probably the mainshock fault plane no variation of mechanism style with depth is apparent two mainshock and 84 aftershock focal mechanism solutions were input into gephart s 1990 stressfield inversion program and principal compressional stress directions j j 1 2 3 were determined as well as the stress difference ratio r 1 2 1 3 the results indicate a uniform stress field in which 3 minimum stress is oriented 116 5 azi plunge and 1 maximum stress is oriented 219 69 with r 0 35 the inversion s selection of preferred nodal planes invokes bott s 1959 criterion which states that slip occurs in the direction of resolved shear traction on preexisting fault surfaces for the predominantly normal slip little skull mountain earthquake mechanisms having northeast striking planes the southeast moderately to steeply dipping dip 55 nodal planes are selected however for the dip slip earthquake mechanisms exhibiting more northerly trending strikes shallowly to moderately west dipping 25 dip 65 nodal planes are selected when a coulomb mohr analysis was applied to the focal mechanisms using the above principal stress directions and an assumed ratio 1 3 the ratio of resolved shear stress to normal stress n on the southeast dipping nodal plane of the mainshock exhibited a near maximum level suggesting a high fault strength in contrast a wide range of fault strengths or apparent coefficients of friction was suggested by the n ratios for little skull mountain aftershock preferred nodal planes seismicity on misoriented faults i e those on which shear stress was relatively low may have been the result of a temporarily elevated fluid pore pressure following the mainshock

grouping and migration of surface faulting and variations in slip rates on faults in the great basin province abstract nonuniformity of the occurrence of large slip events producing surface ruptures on seismogenic faults and variations in slip rate probably characterize seismogenic faulting in the great basin province western united states examples include the grouping of faulting events along the lost river fault idaho changes in tilt rates of the east range and cortez mountains nevada extension of slip along a fault on the northwest flank of the humboldt range nevada and migration or shifting of slip back and forth from one fault to another along subparallel range front faults in dixie valley nevada

testing claims about volcanic disruption of a potential geologic repository at yucca mountain nevada 1 yucca mountain is a potential repository site for high level radioactive waste we examine claims that the frequency of future basaltic dike intersection could be 1 10 6 yr or higher i e on average one penetration every million yrs these claims fail simple tests at four time scales spatial temporal models that predict intrusion frequencies 2 10 7 yr are overly pessimistic because no dikes have been found in the 13 myr old potential repository footprint more realistic models are developed by considering this non detection of dikes and by applying probabilistic hazard assessments based on known patterns of pleistocene volcanism using a hazard assessment code we estimate the frequency of dike intersection at 5 4 10 8 yr with an upper 95 confidence bound of 9 8 10 8 yr

development of texture and seismic anisotropy during the onset of subduction 1 how reliable are shear wave splitting measurements as a means of determining mantle flow direction this remains a topic of debate especially in the context of subduction the answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation here we present an integrated model to simulate strain history dependent texture development and estimate resulting shear wave splitting in subduction environments we do this for a mantle flow model that in its geometry approximates the double sided molucca sea subduction system in eastern indonesia we test a single sided and a double sided subduction case results are compared to recent splitting measurements of this region by di leo et al 2012a the setting lends itself as a case study because it is fairly young and therefore early textures from the slab s descent from the near surface to the bottom of the mantle transition zone which we simulate in our models have not yet been overprinted by subsequent continuous steady state flow second it allows us to test the significance of the double sided geometry i e the need for a rear barrier to achieve trench parallel subslab mantle flow we demonstrate that although a barrier amplifies trench parallel subslab anisotropy due to mantle flow it is not necessary to produce trench parallel fast directions per se in a simple model of a type olivine lattice preferred orientation and one sided subduction trench parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the subslab mantle

interpretation of gravity data in a complex volcano tectonic setting southwestern nevada this regional gravity study based on an irregular 2 km data grid was conducted during the past few years at yucca mountain southern nye county nevada as part of a program to locate a suitable repository for high level nuclear waste about 100 surface rock samples three borehole gamma gamma logs and one borehole gravity study provide excellent density control a nearly linear increase in density of 0 26 g cm3 per kilometer of depth is indicated in the thick tuff sequences that underlie the mountain isostatic and 2 0 g cm3 bouguer corrections were applied to the observed gravity values to remove regional gradients and topographic effects respectively the bare mountain gravity high with an isostatic anomaly maximum of 48 mgal is connected with a greater gravity high over the funeral mountains to the southwest together these highs result from a continuous block of dense metamorphosed precambrian and paleozoic rocks that stretches across much of the walker lane from the east edge of death valley to bare mountain the calico hills gravity high appears more likely to originate from a northeast trending buried ridge of paleozoic rocks that extends southwestward beneath busted butte 5 km southeast of the proposed repository where two and three dimensional modeling indicates that the pre cenozoic rocks lie less than 1000 m beneath the surface tuff at least 4000 m thick fills a large steep sided depression in the pretuff rocks beneath yucca mountain and crater flat the gravity low and the thick tuff section lie within a large collapse area that includes the crater flat timber mountain silent canyon caldera complexes gravity lows in crater flat itself are interpreted to coincide with the source areas of the prow pass member the bullfrog member and the tram member of the crater flat tuff these source areas add nearly 350 km2 to the previously recognized extent of the local caldera complexes southward extension of the broad gravity low associated with crater flat into the amargosa desert is evidence for sector graben type collapse segments related to the formation of the timber mountain caldera and superimposed on the other volcanic and extensional structures within crater flat

the role of magma overpressure in suppressing earthquakes and topography worldwide examples in an extending terrane basaltic magma supplied at a pressure greater than the least principal stress overpressure may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting as vertical dikes intrude they press against their walls in the direction opposite the least principal stress and increase its magnitude the emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses this process tends to equalize the stresses and thus diminishes the deviatoric stress difference between maximum and minimum stresses that creates faults and causes earthquakes observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks

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

anomalous strain accumulation in the yucca mountain area nevada global positioning system gps surveys from 1991 to 1997 near yucca mountain nevada indicate west northwest crustal elongation at a rate of 1 7 0 3 millimeters per year 1 over 34 kilometers or 50 9 nanostrain per year global positioning system and trilateration surveys from 1983 to 1997 on a 14 kilometer baseline across the proposed repository site for high level radioactive waste indicate that the crust extended by 0 7 to 0 9 0 2 millimeter per year 50 to 64 14 nanostrain per year depending on the coseismic effect of the m s 5 4 1992 little skull mountain earthquake these strain rates are at least an order of magnitude higher than would be predicted from the quaternary volcanic and tectonic history of the area

strain accumulation near yucca mountain nevada 1983 1993 over the decade 1983 1993 the u s geological survey has measured the deformation of a 50 km aperture trilateration network centered on yucca mountain the proposed disposal site for high level nuclear waste in the united states the network was surveyed in 1983 1984 and 1993 the average annual principal strain rates are 0 010 0 020 strain yr n90 w 24 and 0 009 0 021 strain yr n00 e 24 indicating no significant strain accumulation the southeast corner of the network was disturbed on june 29 1992 by the little skull mountain earthquake ms 5 4 the epicenter of which is about 20 km southeast of the yucca mountain site using the seismically determined fault plane dip 54 s55 e we find that 0 580 0 075 m of normal slip on a 5 km square rupture surface at a depth of about 8 km provides a good fit to the observed deformation in the southeast corner of the network the inferred seismic moment is 4 4 0 6 1017 n m which compares well with the observed seismic moment of 4 1 1017 n m

global positioning system constraints on fault slip rates in the death valley region california and nevada we estimated horizontal velocities at 15 locations in the vicinity of yucca mountain nevada from global positioning system surveys conducted between 1991 and 1996 we used these velocity estimates to infer slip rates on two major quaternary faults within the eastern california shear zone ecsz the hunter mountain and death valley faults the sum of slip rates across the two faults is well determined at 5 1 mm yr 1 between 3 to 5 mm yr of this motion appears to be accommodated along the death valley fault implying 30 50 m of strain accumulation over the next 10 000 yr if so there is potential for 5 to 10 m w 6 5 7 5 earthquakes during this period a finding consistent with paleoseismological studies of the fault zone yucca mountain which lies 50 km east of the ecsz is the proposed location for the disposal of high level nuclear waste in the united states