subsurface fault geometries and crustal extension in the eastern basin and range province western u s abstract we provide the first synthesis of seismic reflection data and active present day crustal deformation for the greater wasatch fault zone we analyzed a number of previously unpublished seismic reflection lines horizontal and vertical crustal velocities from continuous gps and surface geology to investigate the relationships between interseismic strain accumulation subsurface fault geometry and geologic slip rates on seismogenic faults across the eastern third of the northern basin and range province the seismic reflection data show recent activity along high angle normal faults that become listric with depth and appear to sole into preexisting decollements possibly reactivating them we interpret these listric normal faults as reactivated sevier age structures that are connected at depth with a regionally extensive detachment horizon these observations of subsurface structure are consistent with the mapped geology in areas that have experienced significant extension we modeled the crustal deformation data using a buried dislocation source in a homogeneous elastic half space the estimated model results include a low angle dislocation 8 20 at a locking depth of 7 10 km and slipping at 3 2 0 2 mm yr despite the model s relative simplicity we find that the predicted location of the dislocation is consistent with the interpreted seismic reflection data and suggests an active regionally extensive sub horizontal surface in the eastern basin and range this result may imply that this surface represents aseismic creep across a reactivated low angle fault plane or the onset of ductile flow in the lower crust at or beneath the brittle ductile transition zone under the present day basin and range extensional regime this result may also have implications for crustal rheology and suggests that geodesy might under some circumstances serve as an appropriate tool for inferring deeper crustal structure

viscoelastic coupling model for the earthquake cycle driven from below in a linear system the earthquake cycle can be represented as the sum of a solution which reproduces the earthquake cycle itself viscoelastic coupling model and a solution that provides the driving force we consider two cases one in which the earthquake cycle is driven by stresses transmitted along the schizosphere and a second in which the cycle is driven from below by stresses transmitted along the upper mantle i e the schizosphere and upper mantle respectively act as stress guides in the lithosphere in both cases the driving stress is attributed to steady motion of the stress guide and the upper crust is assumed to be elastic the surface deformation that accumulates during the interseismic interval depends solely upon the earthquake cycle solution viscoelastic coupling model not upon the driving source solution thus geodetic observations of interseismic deformation are insensitive to the source of the driving forces in a linear system in particular the suggestion of bourne et al 1998 that the deformation that accumulates across a transform fault system in the interseismic interval is a replica of the deformation that accumulates in the upper mantle during the same interval does not appear to be correct for linear systems

mechanical basis for low angle normal faulting in the basin and range province low angle normal faults seem to violate the criterion that relates fault dip to stress orientation to explain such faults one must postulate a 45 rotation of principal stress directions at mid crustal depths which is difficult to understand on the basis of previous elastic crust models such a stress rotation is shown to be a natural consequence of a low viscosity lower crust

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

role of crustal flexure in initiation of low angle normal faults and implications for structural evolution of the basin and range province the mechanics of low angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults gently dipping shear stress trajectories are necessary for nucleation and propagation of low angle faults in a structurally isotropic medium such trajectories are not produced by simple horizontal extension but do arise in the presence of flexural stresses we model stress conditions in a cross section through the strong upper crust using airy stress functions for a two dimensional elastic medium numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low angle normal fault initiation at intermediate depths in an elastic medium we propose that the gentle dip of cenozoic low angle normal faults in the basin and range province resulted from flexural stresses that were produced by isostatically uncompensated surface and especially moho relief resurgent mid cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust causing or promoting initiation of low angle normal faults temporal changes in structural style from dominantly low angle to dominantly high angle normal faulting may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the moho and cooling of the mantle lithosphere

localization of listric faults at thrust fault ramps beneath the great salt lake basin utah evidence from seismic imaging and finite element modeling reflection seismic data from the great salt lake basin utah show that the major basin bounding normal faults decrease in dip from 60 at the surface to 10 20 at depths as shallow as 4 6 km this rapid decrease in fault dip at depths shallower than the brittle ductile transition zone in the basin and range province suggests an explanation other than a gradual change of rheology and stress orientations with depth using a dense grid of seismic data gravity data borehole data and published geologic information from islands in the lake we constrain the position of the sevier age willard thrust and a footwall imbricate and show their reactivation as normal faults during tertiary extension in the absence of surface geologic information we use available subsurface information from the lake to draw an analogy with the ogden duplex in the wasatch front where cenozoic normal faulting was superimposed on an earlier sevier age thrust regime to give rise to listric normal faults our interpretations are consistent with finite element modeling results which demonstrate that extensional slip on preexisting thrust ramps leads to the formation of energetically favored synthetic normal faults some of which may merge with the thrust ramp and obtain listric geometries further slip on these listric faults gives rise to secondary synthetic and antithetic faults resulting in hanging wall grabens

depositional history and neotectonics in great salt lake utah from high resolution seismic stratigraphy high resolution seismic reflection data from great salt lake show that the basinal sediment sequence is cut by numerous faults with n s and ne sw orientations this faulting shows evidence of varied timing and relative offsets but includes at least three events totaling about 12 m following the bonneville phase of the lake since about 13 5 ka several faults displace the uppermost sediments and the lake floor bioherm structures are present above some faults which suggests that the faults served as conduits for sublacustrine discharge of fresh water a shallow fault controlled ridge between carrington island and promontory point underlain by a well cemented pavement separates the main lake into two basins the pavement appears to be early holocene in age and younger sediments lap onto it onlap offlap relationships reflection truncations and morphology of the lake floor indicate a low lake well below the present level during the early holocene during which most of the basin was probably a playa this low stand is represented by irregular reflections in seismic profiles from the deepest part of the basin other prominent reflectors in the profiles are correlated with lithologic changes in sediment cores related to the end of the bonneville stage of the lake a thick mirabilite layer in the northern basin and the mazama tephra reflections below those penetrated by sediment cores document earlier lacustrine cycles d 2002 elsevier science b v all rights reserved

sampling rate effects on the properties of dye breakthrough curves from glaciers dye tracer techniques are widely used to infer the character of subglacial drainage systems quantitative analysis of dye breakthrough curves focuses on the determination of the water throughflow velocity u the dispersion coefficient d and the dispersivity parameter d d u together these parameters describe the rate of passage of tracer through the drainage system and the extent to which the dye cloud becomes spread out during passage they have been used to infer the nature or flow conditions within a drainage system and temporal changes in system morphology estimates of all three parameters however are dependent upon the sampling interval at which measurements of dye concentration used to define breakthrough curves are made for a given breakthrough curve the dispersion coefficient increases with the sampling interval while the throughflow velocity shows no systematic variation as a result the dispersivity also tends to increase with the sampling interval investigations of the sensitivity of parameter estimates to the sampling interval reveal that reliable estimates can be obtained only if the sampling interval is less than 1 16 of the time from dye injection to peak dye concentration as a general guide we suggest that ideally quantitative analyses of dye breakthrough curves should therefore be conducted only when this criterion can be met

mechanics of extensional wedges the extensional wedges that overlie low angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold and thrust belt but with an opposite sense of shear on the basal fault such wedges are common in zones of crustal extension such as the basin and range province of the north american cordillera the north sea and the u s gulf coast this paper examines the mechanics of these upper crustal extensional wedges using a modification of the compressional coulomb critical taper model developed for fold and thrust belts and accretionary wedges if a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline a critically tapered extensional wedge is one that is on the verge of coulomb failure everywhere a wedge whose taper is narrower may be slid stably down the same incline without internal deformation a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical after testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges we apply the theory to a pair of active extensional wedges in the brazos area offshore texas both wedges are underlain by well imaged basal detachment faults dipping at 17 o down to depths of 10 km one wedge exhibits relatively little internal deformation whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical regional values for the coefficients of basal and internal friction in excess of 0 3 together with vertical cohesion gradients in excess of 1 kpa m are consistent with the observed pore pressures and the geometry of the wedges as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults such values are comparable to values inferred from mechanical and thermal models of compressional wedges and they are consistent with laboratory rock mechanics data

western frontal fault of the canyon range is it the breakaway zone of the sevier desert detachment geologic evidence developed from surface exposures demonstrates that the western frontal fault of the canyon range is a major structure representing the eastern breakaway zone of the sevier desert detachment the western frontal fault of the canyon range east of delta utah is a north trending west dipping low angle 18 24 normal fault that juxtaposes syntectonic conglomerates and large rock avalanches of the miocene oak city formation against footwall rocks that have been ductilely cataclastically and brittlely deformed the oak city strata dip variably to the east dips are shallow in eastern outcrops 5 15 steeper in central outcrops 20 60 and shallow in the western outcrops 10 20 in eastern exposures the shallow dips of the oak city formation 5 15 and the acute angle of 20 35 between the western frontal fault and oak city units limit past rotation of the fault to 15 the systematic variation in the dip of oak city units from east to west suggests that the western frontal fault probably flattens beneath present oak city outcrops linking the fault to the shallow reflector seen in the cocorp consortium for continental reflection profiling utah line 1 seismic line beneath the westernmost outcrops of the oak city formation upper plate movement is constrained by oak city clast compositions and rock avalanche lithologies fault geometry and lower plate reconstruction to a maximum of 6 7 km since oak city syntectonic deposition at 12 13 ma maximum upper plate displacement for the entire sevier desert detachment cannot be determined from these breakaway zone exposures

contemporary deformation of the wasatch fault utah from gps measurements with implications for interseismic fault behavior and earthquake hazard observations and kinematic analysis 1 contemporary strain accumulation rates of the 350 km long wasatch fault utah have been determined by gps measurements and provide key data for evaluating normal fault behavior and related earthquake hazard assessment this paper presents a complete description of the wasatch gps network data archiving and processing and the measured ground deformation the network spans the wasatch fault in an area 300 km long and 200 km wide in north and central utah and consists of 43 gps sites surveyed in 1992 1993 1994 1995 1999 2001 and 2003 plus 8 permanent gps stations operating continuously beginning 1997 observations across a 65 km wide area centered on the wasatch fault indicate the principal horizontal extension rate of 24 6 nstrain yr with the direction nearly perpendicular to the fault this strain rate corresponds to a horizontal displacement rate of 1 6 0 4 mm yr accommodating 50 of the crustal deformation across the 200 km wide eastern basin range analysis of the spatial variation of the strain rate field reveals that the strain accumulation is concentrated near the wasatch fault which suggests an abrupt transition in the horizontal deformation at the fault between the eastern basin range and the rocky mountains we employed a finite strain model that accounts for simple shear deformation of the hanging wall to compare horizontal extension rates measured by gps with vertical fault displacement rates determined by geologic data our results suggest that the average holocene strain release rate is higher than the contemporary strain loading rate for the wasatch fault if the fault dips less than 30 w but these two rates are consistent if the fault dips more steeply

electromagnetic images of regional structure in the southern canadian cordillera as part of lithoprobe s southern cordilleran transect investigations magnetotelluric mt soundings were made at 160 sites providing unprecedented coverage from the rockies to the west coast striking lateral variation which spatially correlates with the morphogeological belt boundaries is apparent at periods sensing the lower crust 10 s for the rockies mt phases are around 35 indicative of a moderately resistive 100 s 1000 s m north american basement foreland belt phases are transitional and increase from 60 in the east to 70 in the west omineca and coast belt phases are high 75 implying a conductive 10 30 m lower crust whereas intermontane belt phases are more than 10 lower equivalent to 150 m the regional variation in conductivity correlates to first order with surface heat flow changes along the profile and is also correlative with coincident seismic reflection sections in some aspects

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

sm nd systematics of hydrothermal scheelite from the mount charlotte mine kalgoorlie western australia an isotopic link between gold mineralization and komatiites

seismic reflection evidence for seismogenic low angle faulting in southeastern arizona focal mechanisms for large m 6 earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from approximately 30 degree to approximately 70 degree geologic relations suggest that low angle faults have accommodated large scale upper crustal extension these disparate observations are often reconciled by arguments that low angle faults move aseismically or rotate to low angles from initially high angles seismic reflection data from the tucson basin in southeast arizona image a low angle normal fault the santa rita fault that crops out along the trend of late quaternary fault scarps caused by large magnitude m approximately 6 7 7 6 earthquakes velocity independent dip analysis from shot records of the santa rita fault indicates that it has a true dip of approximately 20 degree to a depth of at least 6km this observation suggests that low angle extensional faults indeed may be seismogenic and that actual mechanisms for accommodation of upper crustal extension depend on local conditions of stress and preexisting geologic structure

on steady rate coupling between an elastic upper crust and a viscous interior although it is commonly accepted that the upper seismogenic layer of the earth s crust deforms elastically except in localized zones faults where deformation is discontinuous the sub seismogenic layer is not as well understand possibilities for the rheology include viscous or viscoelastic behavior and localization of shear to relatively thin zones that can sometimes be 1 km in width if the sub seismogenic layer is viscous or viscoelastic then during interseismic periods a steady rate of deformation will be achieved after transients associated with viscoelastic relaxation have decayed away for sufficiently long earthquake cycles i find that in this situation the motion in the elastic layer is decoupled from that deep within the viscous layer and they may be very different from each other this implies that one can not use contemporary geodetic data to infer steady motion in a viscous sub seismogenic layer although it may be used to study viscoelastic relaxation

comparison of geodetic and geologic data from the wasatch region utah and implications for the spectral character of earth deformation at periods of 10 to 10 million years the wasatch fault and adjacent fault zones provide an opportunity to compare present day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows ranging from the last millennium up to 10 myr the three easternmost gps sites of the basin and range geodetic network bargen at this latitude define a 130 km wide region spanning three major normal faults extending east west at a total rate of 2 7 0 4 mm yr with an average regional strain rate estimated to be 21 4 nstrain yr about twice the basin and range average on the wasatch fault the vertical component of the geologic displacement rate is 1 7 0 5 mm yr since 6 ka 0 6 mm yr since 130 ka and 0 5 0 7 mm yr since 10 ma however it appears likely that at the longest timescale rates slowed over time from 1 0 to 1 4 mm yr between 10 and 6 ma to 0 2 to 0 3 mm yr since 6 ma the cumulative vertical displacement record across all three faults also shows time variable strain release ranging from 2 to 4 mm yr since 10 ka to 1 mm yr averaged over the past 130 kyr conventional earthquake recurrence models reid type behavior would require an accordingly large variation in strain accumulation or loading rate on a 10 kyr timescale for which there appears to be no obvious geophysical explanation alternatively seismic strain release given a wide range of plausible constitutive behaviors for frictional sliding may be clustered on the 10 kyr timescale resulting in the high holocene rates with comparatively low uniform strain accumulation rates on the 100 kyr timescale wallace type behavior the latter alternative combined with observations at the million year timescale and the likelihood of a significant contribution of postseismic transients implies maxima of spectral amplitude in the velocity field at periods of 10 myr variations in tectonic loading 10 kyr clustered strain release and of 100 years postseismic transients if so measurements of strain accumulation and strain release may be strongly timescale dependent for any given fault system

possible seismogenic shallow dipping normal faults in the woodlark d entrecasteaux extensional province papua new guinea active normal faults dipping less than 30 have not been documented in the seismic record but they are commonly observed in geologic structures one interpretation is that seismogenic low angle faulting is physically impossible and that the geologic structures do not form at shallow dips to better examine this problem teleseismic body waves were inverted to determine source parameters for five large earthquakes in the woodlark d9entrecasteaux region a rare example of active continental extension associated with pliocene quaternary metamorphic core complexes the inversions show that at least two and possibly four events are consistent with normal dip slip on shallow dipping faults the largest earthquake m w 6 8 is located near the mapped core complexes the shallow dipping plane for this event determined to dip between 10 and 25 is nearly coplanar to the faults bounding the core complexes and may represent slip on these or other parallel low angle structures although slip on the conjugate high angle plane cannot be ruled out observations of quaternary uplift and the similarity to the adjacent core complex bounding structures favor motion on the low angle plane similar earthquake mechanisms have not yet been documented in most other extensional provinces perhaps because instances of active core complex exhumation are rare

gps constraints on vertical crustal motion in the northern basin and range 1 we estimated vertical velocities using an array of 12 continuous gps stations forming part of the pbo nucleus network we analyzed data spanning the period 1996 through 2006 the array has an aperture 400 km and crosses a major active intracontinental normal fault system in the northern basin and range province the root mean square rms of vertical rates relative to a local no net vertical frame is 0 3 mm yr indicating high relative rate precision the rms difference between rates based on t 7 year sub strands of the total data set and the full 10 year rate estimates varies as 2 4 t 1 3 mm yr the net vertical rate observed across the greater wasatch fault zone is consistent with the predictions of elastic half space dislocation models only when the deep slipping part of the fault has very shallow dip 30

active megadetachment beneath the western united states geodetic data interpreted in light of seismic imaging seismicity xenolith studies and the late quaternary geologic history of the northern great basin suggest that a subcontinental scale extensional detachment is localized near the moho to first order seismic yielding in the upper crust at any given latitude in this region occurs via an m7 earthquake every 100 years here we develop the hypothesis that since 1996 the region has undergone a cycle of strain accumulation and release similar to slow slip events observed on subduction megathrusts but yielding occurred on a subhorizontal surface 5 10 times larger in the slip direction and at temperatures 800 c net slip was variable ranging from 5 to 10 mm over most of the region strain energy with moment magnitude equivalent to an m7 earthquake was released along this megadetachment primarily between 2000 0 and 2005 5 slip initiated in late 1998 to mid 1999 in northeastern nevada and is best expressed in late 2003 during a magma injection event at moho depth beneath the sierra nevada accompanied by more rapid eastward relative displacement across the entire region the event ended in the east at 2004 0 and in the remainder of the network at about 2005 5 strain energy thus appears to have been transmitted from the cordilleran interior toward the plate boundary from high gravitational potential to low via yielding on the megadetachment the size and kinematic function of the proposed structure in light of various proxies for lithospheric thickness imply that the subcrustal lithosphere beneath nevada is a strong thin plate even though it resides in a high heat flow tectonic regime a strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern great basin deformation microstructures and low water content in dunite xenoliths in young lavas in central nevada and high temperature microstructures in analog surface exposures of deformed lower crust large scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere as inferred by such proxies as low p wave velocity and mafic magmatism

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

equivalent strike slip earthquake cycles in half space and lithosphere asthenosphere earth models by virtue of the images used in the dislocation solution the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike slip fault in an earth model consisting of an elastic plate lithosphere overlying a viscoelastic half space asthenosphere can be duplicated by prescribed slip on a vertical fault embedded in an elastic half space for the case in which each earthquake ruptures the entire lithosphere thickness h the half space equivalent slip rate is as follows depth interval 0 h slip identical to that in lithosphere asthenosphere model i e abrupt coseismic slip and no subsequent slip depth interval 2n 1 h to 2n 1 h n 1 2 slip rate uniform in space and dependent upon time as fn t exp t a where fn is a n 1 degree polynomial in t a is twice the asthenosphere relaxation time and t is measured from the instant after the preceding earthquake the slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion for reasonable values of a slip rates below 5h do not vary much from that mean value and can be treated as constant thus the surface deformation due to the earthquake cycle in the lithosphere asthenosphere model can be calculated very simply from the half space model with time dependent slip in the two depth intervals h 3h and 3h 5h and uniform slip at a rate equal to the secular relative plate velocity below depth 5h inversion of 1973 1988 geodetic measurements of deformation across the segment of the san andreas fault in the transverse ranges north of los angeles for the half space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm yr below 30 km one equivalent lithosphere asthenosphere model would have a 30 km thick lithosphere and an asthenosphere relaxation time greater than 33 years but other models are possible

tectonic control on coarse grained foreland basin sequences an example from the cordilleran foreland basin utah newly released reflection seismic and borehole data combined with sedimentological provenance and biostratigraphic data from upper cretaceous paleocene strata in the proximal part of the cordilleran foreland basin system in utah establish the nature of tectonic controls on stratigraphic sequences in the proximal to distal foreland basin during campanian time coarse grained sand and gravel were derived from the internally shortening charleston nebo salient of the sevier thrust belt a rapid regional campanian progradational event in the distal foreland basin 200 km from the thrust belt in 8 m y can be tied directly to active thrust generated growth structures and an influx of quartzose detritus derived from the charleston nebo salient eustatic sea level variation exerted a minimal role in sequence progradation

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

crustal velocity structure beneath the eastern flank of the ruby mountains metamorphic core complex results from normal incidence to wide angle seismic data abstract p wave velocity structure along the eastern side of the ruby mountains metamorphic core complex determined using 2 d ray inversion modeling shows that 1 the crust can be divided into three layers corresponding to the upper middle and lower crust 2 from east of ruby dome to the northern end of the seismic profile velocities of the upper crust range from 6 12 0 1 to 6 20 0 1 km s and near surface velocities range from 1 90 0 1 to 4 80 0 1 km s from this point to the southern end of the profile upper crustal velocities range from 5 80 0 1 to 6 25 0 1 km s and near surface velocities range from 3 01 0 1 to 4 80 0 1 km s 3 middle crustal velocities range from 6 35 0 15 to 6 45 0 15 km s 4 lower crustal velocities range from 6 60 0 15 to 6 80 0 15 km s and 5 depths to the moho vary irregularly between 30 5 and 33 5 km from interpretation of these results we conclude that 1 the transition in metamorphic grade from deep upper crustal rocks south of harrison pass to middle crustal rocks north of harrison pass does not correlate with an increase in seismic velocity near surface basement seismic velocities increase in the central part of the range near ruby dome well north of the major metamorphic transition 2 depths to the moho do not reflect local surface relief isostatic balance is achieved predominantly in the crust rather than by formation of a local crustal root and 3 the crust has been modified during extension by intracrustal processes rather than by large scale magmatic underplating

images of crustal variations in the intermountain west 1 we develop a map of crustal thickness variations across the great basin colorado plateau rocky mountain and great plains provinces of the western united states using common conversion point stacking of teleseismic receiver functions below the rocky mountains and high plains in colorado we find the thickest crust in the region at 45 50 km thick beneath the basin and range thinner between 30 and 40 km crust is found thin 30 km thick crust is present in the northern portion of nevada and utah despite elevations similar to those farther south crustal thickness across the colorado plateau can be characterized as a broad transitional region between the thin crust of basin and range to the thicker crust of the rocky mountains the impedance contrast across the mohorovicic discontinuity decreases below the colorado plateau as converted arrivals recorded in this region appear weak compared to surrounding areas variations in vp vs across the region indicate higher values along the western boundary of the basin and range in the rocky mountains and in the western great plains we are not able to characterize vp vs in the colorado plateau we find that crustal thickness does not closely correlate with surface topography within each region or across the region as a whole differences in crustal thickness in each tectonic province indicate the need for a mantle component to support the high elevations across the western united states

paleomagnetic evidence of large footwall rotations associated with low angle faults at the mid atlantic ridge exposures of gabbros and mantle derived peridotites at slow spreading oceanic ridges have been attributed to extension on long lived low angle detachment faults similar to those described in continental metamorphic core complexes in continental settings such detachments have been interpreted as having originated and remained active at shallow dips alternatively currently shallow dipping fault surfaces may have originated at moderate to steep dips and been fl attened by subsequent fl exure and isostatic uplift while the latter interpretation would be more con sistent with andersonian faulting theory it predicts large footwall tilts that have not been observed in continental detachment faults here we use the magnetization of oceanic gabbro and peridotite samples exposed near the fifteen twenty fracture zone on the mid atlantic ridge to demonstrate that substantial footwall rotations have occurred widespread rotations ranging from 50 to 80 indicate that original fault orientations dipped steeply toward the spreading axis

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

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

the mechanical feasibility of low angle normal faulting abstract low angle normal faulting is precluded by elementary rock mechanics arguments however the existence of many low angle normal faults in regions such as western north america has long suggested that this familiar theory is incomplete initiation of a low angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes which necessitates a substantial shear stress within the vertical plane nonetheless no one has previously identified any specific extensional stress field which is suitably oriented to allow shear failure of previously unfractured rock at a low angle dip of 30 in preference to the steeper dip of 50 60 predicted by standard theory despite previous claims to the contrary this study shows for the first time that appropriately oriented stress fields can exist however they require the shear stress to locally reach 100 mpa near the base of a 10 km thick brittle layer this is only possible under extreme conditions as it requires dramatic lateral variations in the state of stress across the extending region it is suggested that a shear stress of this order can develop due to the combined effects of lower crustal flow which imparts a horizontal shear traction at the base of the brittle layer and loading which imparts shear traction in the vertical plane associated with the isostatic response to changes in heat flow caused by changes to the geometry of subducting slabs beneath an extending region regional patterns of low angle normal faulting in western north america are thus interpreted in terms of changes to the geometry of subduction of the farallon plate

holocene slip rate of the wasatch fault zone utah from geodetic data earthquake cycle effects 1 gps data define a broad zone of present day deformation in the eastern basin and range province western us using finite element models with elastic upper crust over viscoelastic lower crust upper mantle and incorporating earthquake cycle effects we show that these data are consistent with a model whereby most contemporary fault slip is focused on the wasatch fault zone modeled rates of horizontal extension are 3 0 4 5 mm yr in agreement with holocene geologic data the models are non unique in part because much of the wasatch fault is in the late stages of the earthquake cycle when surface velocity gradients across the fault are low

strain accumulation across the wasatch fault near ogden utah deformation of a 70 by 40 km trilateration network spanning the north trending wasatch fault near ogden utah has been monitored from 1972 through 1990 all but nine of the 200 measurements are consistent with deformation that is linear in time we presume that those nine observations are contaminated by some blunder in making the measurements and that deformation is linear in time the strain rate over the 1972 1990 interval across a 40 km wide zone west of the wasatch fault hanging wall block is adequately approximated by a 0 04 0 01 strain yr n85 e 5 uniaxial extension adequate coverage of the footwall block is available only over the 1981 1990 interval and the strain rate across a 30 km wide zone there is approximated by a 0 07 0 03 strain yr n20 e 12 uniaxial extension the observed extension of the hanging wall block in the direction normal to fault strike suggests that the wasatch fault may be listric a planar fault would produce a zone of contraction normal to the fault at the surface of the hanging wall block the component of extension parallel to fault strike observed on the footwall block is unexplained

effect of annual signals on geodetic velocity 1 our analysis of global positioning system gps site coordinates in a global reference frame shows annual variation with typical amplitudes of 2 mm for horizontal and 4 mm for vertical with some sites at twice these amplitudes power spectrum analysis confirms that gps time series also contain significant power at annual harmonic frequencies with spectral indices 1 2 which indicates the presence of repeating signals van dam et al 2001 showed that a major annual component is induced by hydrological and atmospheric loading unless accounted for we show that annual signals can significantly bias estimation of site velocities intended for high accuracy purposes such as plate tectonics and reference frames for such applications annual and semiannual sinusoidal signals should be estimated simultaneously with site velocity and initial position we have developed a model to calculate the level of bias in published velocities that do not account for annual signals simultaneous estimation might not be necessary beyond 4 5 years as the velocity bias rapidly becomes negligible minimum velocity bias is theoretically predicted at integer plus half years as confirmed by tests with real data below 2 5 years the velocity bias can become unacceptably large and simultaneous estimation does not necessarily improve velocity estimates which rapidly become unstable due to correlated parameters we recommend that 2 5 years be adopted as a standard minimum data span for velocity solutions intended for tectonic interpretation or reference frame production and that we be skeptical of geophysical interpretations of velocities derived using shorter data spans

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

origin of regional rooted low angle normal faults a mechanical model and its tectonic implications rooted listric low angle normal faults 20 of regional extent have been recognized widely in the past few years in the north american cordillera and elsewhere the low angle geometry of these crustal scale normal faults conflicts with anderson s 1942 classic theory of faulting in that theory the orientations of principal stresses are assumed to be vertical and horizontal the predicted dip angle of normal faults is about 60 rather than 20 or less recent geological and geophysical studies in the mid tertiary extensional terrane of southeastern california and western arizona suggest that thick mylonitic gneisses in the lower plates of low angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust directed ductile flow possibly related to the gravitational spreading of thickened lower crust may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface a simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust this model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered in particular appropriate stress conditions for the formation of regional low angle normal faults 20 can be produced by the superposition of two stress fields a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant this superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust these results may explain both puzzling parallel belts of extension and compression and the occurrence of major low angle normal faults in some orogenic systems

subcontinental scale crustal velocity changes along the pacific north america plate boundary transient tectonic deformation has long been noted within approx100 km of plate boundary fault zones and within active volcanic regions but it is unknown whether transient motions also occur at larger scales within plates relatively localized transients are known to occur as both seismic and episodic aseismic events and are generally ascribed to motions of magma bodies aseismic creep on faults or elastic or viscoelastic effects associated with earthquakes however triggering phenomena and systematic patterns of seismic strain release at subcontinental 1 000 km scale along diffuse plate boundaries have long suggested that energy transfer occurs at larger scale such transfer appears to occur by the interaction of stresses induced by surface wave propagation and magma or groundwater in the crust or from large scale stress diffusion within the oceanic mantle in the decades following clusters of great earthquakes here we report geodetic evidence for a coherent subcontinental scale change in tectonic velocity along a diffuse 1 000 km wide deformation zone our observations are derived from continuous gps global positioning system data collected over the past decade across the basin and range province which absorbs approximately 25 per cent of pacific north america relative plate motion the observed changes in site velocity define a sharp boundary near the centre of the province oriented roughly parallel to the north northwest relative plate motion vector we show that sites to the west of this boundary slowed relative to sites east of it by 1 mm yr 1 starting in late 1999

movement along a low angle normal fault the s reflector west of spain 1 the existence of normal faults that moved at low angles less than 20 has long been debated one possible low angle fault is the s detachment at the west galicia spain margin and thought to occur at the top of serpentinized mantle it is unlikely that s was a large submarine slide as it was probably active over several million years without the development of any compressional features such as toe thrusts it appears to have rooted beneath the conjugate flemish cap margin and it is similar to structures elsewhere that also appear to be rooted detachments here we analyze depth images to identify synrift sediment packages above s and use the geometry of these synrift packages to constrain the angle at which s both formed and remained active we find that s must have remained active at angles below 15 too low to be explained simply by the low friction coefficient of partially serpentinized peridotites instead we suggest that transient high fluid pressures must have developed within the serpentinites and propose a model in which anastomosing fault strands are alternately active and sealed enabling moderately high fluid pressures to develop

origins of deep crustal reflections implications of coincident seismic refraction and reflection data in nevada we compare seismic refraction and reflection results along the passcal cocorp 40 n transect in the northern basin and range of nevada in order to determine the origin of the prominent reflections from the deep crystalline crust reflection data along the transect show a thick zone of discontinuous subhorizontal reflections beginning at 4 6 s two way traveltime 10 20 km depth and ending at 9 11 s 27 35 km two independently derived velocity models based on refraction data are largely similar and agree on many important aspects of the reflectivity velocity relation both models show that the top of the reflective zone lies 3 8 km above a prominent mid crustal velocity discontinuity which is interpreted to separate bulk silicic from bulk dioritic gabbroic crust in most places the silicic mid crust is more strongly reflective than the mafic lower crust this pattern is expected in areas where ductile shearing is the mechanism responsible for the reflectivity one of the velocity models however suggests that in places the strongest reflectivity spans both the middle 6 1 6 3 km s and lower 6 6 km s crust this pattern suggests that the combined influence of ductile strain fabrics and mafic intrusions gives rise to crustal reflections both models show that the lowermost crust and crust mantle transition are highly reflective also suggesting the presence of mafic and or ultramafic intrusions thus the observed reflection patterns suggest that ductile shearing and the intrusion of mantle derived magma both of which are likely to have accompanied the extreme cenozoic extension are important factors in generating deep crustal reflections

reevaluation of holocene faulting at the kaysville site weber segment of the wasatch fault zone utah the 1978 kaysville utah trench excavated by swan and others 1980 across a large graben of the weber segment of the wasatch fault zone was reexcavated in 1988 to reevaluate the timing and nature of holocene faulting relogging of the trench reveals evidence for five or six faulting events younger than the provo phase of lake bonneville circa 13 000 14c years b p geometric reconstruction of net vertical offset in the last three events suggests a variation in coseismic vertical displacement at this site ranging from a net of 1 4 3 4 m per event the three latest faulting events occurred at shortly before 0 6 0 8 ka 2 8 0 7 ka and circa 3 8 7 9 ka earlier events cannot be directly dated because older graben fill sediments yielded thermoluminescence ages older than the time of deposition and some scarp derived colluvial wedges beneath the trench floor were not exposed the two younger faulting events we recognize at kaysville correlate reasonably well with faulting events on the same segment 25 km north near east ogden utah at circa 0 8 1 2 ka and 2 5 3 0 ka forman and others 1991 whereas the earlier kaysville event is significantly older than the earliest 3 5 4 0 ka event dated at east ogden the 3 5 4 0 ka ground rupture recognized at east ogden may have died out at a subsegment boundary between the two trench sites within the 61 km long weber segment