tertiary extensional faulting and evolving ductile brittle transition zones in the northern snake range and vicinity new insights from seismic data new geologic mapping and high resolution seismic data from east central nevada shed light on the evolution of the northern snake range decollement nsrd and its relation to the present basins and ranges we suggest that the nsrd originated in oligocene time as a flat 20 km wide 7 km deep zone of decoupling between localized ductile stretching below and more widely distributed high angle normal faulting above younger east dipping normal faults cut the western part of the nsrd forming the tilted fault blocks and highly asymmetric grabens that constitute the duck creek range duck creek valley and schell creek range spring valley and folded the nsrd to its present antiformal geometry these youngest faults flatten into pervasive subhorizontal reflectors at a depth of 10 to 15 km that we interpret as the youngest ductile brittle transition

mesozoic tectonics and metamorphism in the pequop mountains and wood hills region northeast nevada implications for the architecture and evolution of the sevier orogen discussion and reply the pequop mountains wood hills east humboldt range region northeast nevada exposes a nearly continuous cross section of precambrian to mesozoic strata representing middle to upper crustal levels of the mesozoic hinterland of the sevier orogen these rocks preserve the transition from unmetamorphosed mesozoic upper crust to partially melted middle crust integration of new structural metamorphic and u pb thermochronologic data from the wood hills and pequop mountains coupled with a regional tectonic reconstruction reveals substantial cretaceous metamorphism contraction and extension in the sevier hinterland in northeast nevada we report two phases of contraction not previously recognized that are accommodated by top to the southeast thrust faults the windermere and independence thrusts contraction was succeeded by two phases of extension along west rooted normal faults the late cretaceous pequop fault and tertiary mary9s river fault system the earliest phase of thrust faulting resulted in as much as 30 km of crustal thickening and an estimated minimum of 69 km of shortening along an inferred fault called the windermere thrust the timing of this thrusting event is bracketed between late jurassic ca 153 ma and late cretaceous 84 ma relaxation of crustal isotherms following and perhaps during thrusting resulted in barrovian style metamorphism of footwall rocks and partial melting of metapelite at deep levels peak metamorphism was attained ca 84 ma and by this time hinterland crustal thickening had reached a maximum during 84 75 ma another minor pulse of shortening and thickening along the independence thrust was followed by partial exhumation of the metamorphic rocks and as much as 10 km of crustal thinning along the pequop fault thus the interval from 84 to 75 ma in northeast nevada marks a fundamental and apparently permanent change from horizontal contraction to extension in the upper to middle crust in the hinterland final exhumation of the metamorphic rocks was accomplished by the tertiary mary9s river fault system our data indicate that much of the metamorphism and some of the contraction in the sevier hinterland in northeast nevada which was previously thought to be largely late jurassic is actually cretaceous in age furthermore the data indicate that widespread metamorphism of the middle crust is a byproduct of tectonic burial and that hinterland and foreland thrust faulting were coeval suggesting that thrust faults in the sevier orogen do not form a simple foreland younging sequence

the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

large magnitude extensional deformation in the south mountains metamorphic core complex arizona paleomagnetic data are used to test controversial aspects of cordilleran metamorphic core complexes including the original dip of extensional structures origin of the mylonitic front and applicability of rolling hinge models we obtained paleomagnetic data 115 sites 82 accepted for analysis from the weakly deformed interior of a synkinematic footwall intrusive suite and proterozoic footwall rocks of the south mountains metamorphic core complex central arizona these rocks yield dual polarity high unblocking temperature and high to moderate coercivity magnetizations positive baked contact tests indicate that footwall rocks possess primary thermoremanent magnetizations trms or high temperature thermochemical remanent magnetizations tcrms acquired early in their cooling history and during ductile and brittle extensional deformation of structurally higher rocks this is consistent with thermochronologic data indicating rapid synkinematic cooling from crystallization through the range of laboratory unblocking temperatures for the magnetic mineralogy of these rocks between about 22 and 17 5 ma paleomagnetic data are considered as two populations based on the structural asymmetry of the south mountains metamorphic core complex 1 a front side characterized by northeast dipping 10 mylonitic fabrics and brittle extensional structures and 2 a back dipping side characterized by rollover of the mylonitic zone to form a southwest or back dipping 15 mylonitic front comparison of paleomagnetic data from these two sides suggests that the back dipping mylonitic front was synkinematically tilted about 10 down to the southwest the data support a folded shear zone hypothesis for origin of the mylonitic front and the interpretation that footwall rocks possess primary miocene age trms or tcrms a second regional fold test involved data from sites on both flanks of the topographically prominent northeast trending mountain range scale antiform the negative result from this fold test demonstrates that this structure formed early in the extensional history and prior to magnetization acquisition by the plutons we obtained a well grouped footwall grand mean from 62 front side and 20 back dipping site means n 82 d 1 0 i 51 7 k 41 8 95 2 5 we calculated this grand mean with the assumption that front side sites have remained structurally untilted whereas back dipping side sites require removal of 10 of southwest dip this grand mean is statistically indistinguishable 95 confidence level from time averaged miocene expected directions we thus conclude that the current gentle dip of front side mylonites and detachment faults is original therefore both ductile and brittle extensional deformations of the south mountains metamorphic core complex were accommodated along low angle structures dip of 15 our interpretation refutes the widespread applicability of models that predict metamorphic core complexes to represent tilted crustal blocks originally bounded by moderate angle normal faults and does not support rolling hinge models of metamorphic core complex evolution that require a moderate angle ramp

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

cretaceous to early tertiary paleogeography in the hinterland of the sevier thrust belt east central nevada the cretaceous to early tertiary hinterland of the sevier thrust belt in east central nevada underwent two periods of sedimentary basin development the older early cretaceous episode is characterized by basins that formed coeval with deformation within a belt of contractional structures basins were characterized by periods of through flowing fluvial drainage the younger late cretaceous to eocene episode of basin development was characterized by internally drained alluvial basins that subsequently expanded into more regional lacustrine basins basal alluvial detritus indicates that basin inception occurred as a result of rapid development of topographic relief syn sedimentary tectonism was characterized by horizontal extension due to attainment of equilibrium topography in response to crustal thickening

rapid eocene extension in the robinson district white pine county nevada constraints from 40ar 39ar dating precise 40ar 39ar ages on pretectonic syntectonic and posttectonic rhyolites from the robinson mining district which is within a highly extended domain in east central nevada indicate that extreme extension 400 occurred in fewer than 900 k y during the eocene the duration of extension is tightly delimited by an eocene lacustrine unit that was horizontal prior to extension and by younger rhyolitic units the rhyolites represent at least five eruptive episodes including the preextensional and synextensional rhyolite of white hill emplaced between 37 6 and 37 4 ma and the postextensional rhyolite of garnet hill emplaced at 36 7 ma although some of it is perhaps as old as 37 1 ma thus the duration of extreme extension at robinson was between 0 9 and 0 3 m y comparable to or shorter than other highly extended areas in western north america the extensional domain in east central nevada is a composite feature produced by localized episodic rapid extension that began in early eocene time and continued to at least middle miocene time

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

reactivation tendency analysis a theory for predicting the temporal evolution of preexisting weakness under uniform stress state abstract understanding the mechanical controls on reactivation of preexisting weakness is a fundamental problem in tectonic studies in this study we develop a theoretical framework for evaluating the likelihood of seismicity on active faults and the sequence of reactivation of multiple sets of preexisting weakness our analysis overcomes the restrictions of the early work that assumes uniform coefficient of friction across all preexisting planes with negligible cohesive strength and vertical or horizontal orientations of principal stresses using a coordinate system transformation we developed a new graphical technique that can express the reactivation problem in the mohr space with the principal stresses oriented neither vertically nor horizontally we verified the predicted sequence of reactivation on preexisting weakness and initiation of new fractures according to the byerlee law and the coulomb fracture criterion by conducting a simple sandbox experiment our proposed reactivation tendency analysis can be expanded to evaluate the likelihood of seismicity on a single fault with curvi planar geometry e g listric normal faults ramp flat thrusts and strike slip faults with restraining and releasing bend geometry our analysis also provides a new insight into the mechanical cause of temporal evolution of pre existing weakness in geologic records finally our work implies that the sequence of faulting reactivated from preexisting weakness provides critical information on the mechanical properties of the deforming lithosphere

geometry and kinematics of the grant range brittle detachment system eastern nevada u s a an end member style of upper crustal extension documenting the range of styles of normal faulting is fundamental to understanding crustal extension here geologic mapping field relationships and deformed and restored cross sections illustrate the geometry and kinematic development of a system of west vergent detachment faults in the grant range in eastern nevada faults exhibit brecciation and stratigraphic cutoff angles of 5 15 at all structural levels and deform a 10 km thick section of paleozoic and paleogene rocks the fault system is folded across an anticlinal culmination which grew during extension as indicated by progressively increasing interlimb angles and incision in the axial zone the eastern limb consists of an imbricate stack of faults that were emplaced from bottom to top in the western limb several faults exhibit apparent thrust relationships the oldest faults are cut by a 29 ma dike and the highest preserved fault cuts 32 ma volcanic rocks that restore to paleodepths of 1 km retrodeformation of folding and minimal structural relief and angularity across a paleogene unconformity indicate the faults were active at 5 15 angles retrodeformation of offset indicates 49 km 98 extension we propose a model of stationary sustained isostatic uplift and incision at the culmination axis a fixed hinge with updip excision producing bottom to top growth of the imbricate stack and downdip excision producing apparent thrust relationships the fault system exhibits similarities to core complex detachment systems though it is confined to upper crustal levels and there are no preserved high angle or listric normal faults indicating a unique extension style dominated by low angle excision

timing of tertiary extension in the railroad valley pioche transect nevada constraints from 40ar 39ar ages of volcanic rocks time space relations of extension and volcanism place critical constraints on models of basin and range extensional processes this paper addresses such relations in a 130 km wide transect in the eastern great basin bounded on the east by the ely springs range and on the west by the grant and quinn canyon ranges stratigraphic and structural data combined with 40ar 39ar isotopic ages of volcanic rocks document a protracted but distinctly episodic extensional history field relations indicate four periods of faulting only one of these periods was synchronous with nearby volcanic activity which implies that volcanism and faulting need not be associated closely in space and time based on published dates and the analyses reported here the periods of extension were 1 prevolcanic pre 32 ma 2 early synvolcanic 30 to 27 ma 3 immediately postvolcanic about 16 to 14 ma and 4 pliocene to quaternary the break between the second and third periods is distinct the minimum gap between the first two periods is 2 ma but the separation may be much larger temporal separation of the last two periods is only suggested by the stratigraphic record and cannot be rigorously demonstrated with present data the three younger periods of faulting apparently occurred across the entire transect the oldest period is recognized only at the eastern end of the transect but appears to correlate about 150 km northward along strike with extension in the northern snake range kern mountains area therefore the oldest period also is regional in extent but affected a different area than that affected by younger periods this relation suggests that distinct extensional structures and master detachment faults were active at different times the correlation of deformation periods of a few million years duration across the railroad valley pioche transect suggests that the scale of active extensional domains in the great basin may be greater than 100 km across strike

seismic attenuation due to wave induced flow analytical expressions for three p wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework two of the models concern wave induced flow due to heterogeneity in the elastic moduli at mesoscopic scales scales greater than grain sizes but smaller than wavelengths in the first model the heterogeneity is due to lithological variations e g mixtures of sands and clays with a single fluid saturating all the pores in the second model a single uniform lithology is saturated in mesoscopic patches by two immiscible fluids e g air and water in the third model the heterogeneity is at microscopic grain scales broken grain contacts and or micro cracks in the grains and the associated fluid response corresponds to squirt flow the model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus crack porosity and or frequency is reduced to zero it is shown that squirt flow is incapable of explaining the measured level of loss 10 sup 2 q sup 1 10 sup 1 within the seismic band of frequencies 1 to 10 sup 4 hz however either of the two mesoscopic scale models easily produce enough attenuation to explain the field data

seismic and electrical properties of sandstones at low saturations laboratory measurements of elastic wave velocities attenuation and electrical resistivity of sandstones all show distinct changes in the measured properties at very low levels of water saturation velocity decreases attenuation reaches a maximum and electrical resistivity decreases over a limited range in saturation we assume that all the water at low saturations is present in the rock as a layer of uniform thickness covering the entire internal surface area of the sample with this model we calculate the thickness of surface water that corresponds to the distinct low saturation region in all cases for both the seismic and the electrical data the thickness is equal to one to four monolayers of water this is interpreted as marking the transition in the behavior of water in a sandstone from that of a surface adsorbed phase to a bulk water phase the changes in seismic properties can be modeled by considering the effect of adding surface and bulk water to grain contacts in a similar treatment the role of contacts is shown to be a dominant factor in the electrical resistivity of sandstones at low saturations such that the addition of water to the contact area causes a dramatic decrease in the electrical resistivity

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

inversion of the seismic avf ava signatures of highly attenuative targets frequency dependent seismic field data anomalies appearing in association with low q targets have on occasion been attributed to the presence of a strong absorptive reflection coefficient this absorptive reflectivity represents a potent and largely untapped source of information for determining subsurface target properties it would most likely be encountered where a predominantly elastic nonattenuating overburden suddenly is interrupted by a highly attenuative target series expansions of absorptive reflection coefficients about small parameter contrasts and incidence angles can expose these anomalies to analysis either frequency by frequency amplitude variation with frequency avf or angle by angle amplitude variation with angle of incidence ava within this framework variations in p wave velocity and q can be estimated separately through a range of direct formulas both linear and with nonlinear corrections the latter come to the fore when a contrast from an incidence medium q i

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

seismic low frequency effects in monitoring fluid saturated reservoirs there is a complex relationship between seismic attributes including the frequency dependence of reflections and fluid saturation in a reservoir observations in both laboratory and field data indicate that reflections from a fluid saturated layer have an increased amplitude and delayed traveltime at low frequencies when compared with reflections from a gas saturated layer comparison of laboratory modeling results with a diffusive viscous theory model show that low 5 values of the quality factor q can explain the observations of frequency dependence at the field scale conventional processing of time lapse vsp data found minimal changes in seismic response of a gas storage reservoir when the reservoir fluid changed from gas to water lowfrequency analysis found significant seismic reflectionattribute variation in the range of 15 50 hz the field observations agree with effects seen in laboratory data and predicted by the diffusive viscous theory one explanation is that very low values of q are the result of internal diffusive losses caused by fluid flow this explanation needs further theoretical investigation the frequencydependent amplitude and phase reflection properties presented in this paper can be used for detecting and monitoring fluid saturated layers

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

field tests of rolling hinges existence mechanical types and implications for extensional tectonics in rolling hinge models a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large displacement tens of kilometers normal fault a fault bend fold that migrates through the footwall results causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip rolling hinge migration should leave a discernible geologic record but compelling tests are difficult to design because predictions of rolling hinge models are nonunique and alternative models commonly can explain individual data sets field tests based on thermochronology paleomagnetism and structural analysis are typically applied to footwall rocks whereas sedimentology geochronology of volcanic rocks and structural analysis are typically applied to upper plate rocks we review existing theoretical models and case studies in 14 detachment terranes some negative results are reasonably compelling but positive tests as yet tend to permit rather than demand a rolling hinge locally both positive and negative conclusions have been reached about the passage of a hinge within the same terrane such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area subvertical simple shear and flexural failure mechanisms with or without significant induced flow of lower crustal material are thought to dominate hinge related footwall deformation in individual sites although rolling hinge models offer an alternative to low initial fault dips hinge related deformation does not demand a steep initial fault dip few areas in which rolling hinge tectonics are permissible had initially steep faults 35 rather where the evidence most strongly favors a rolling hinge detachment faults had initial dips 35 in the future conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal at best the rolling hinge mechanism does not universally explain the observation that earthquakes are rare on low angle normal faults nor solve the mechanical problems of frictional slip on such faults

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

impact of fluid saturation on the reflection coefficient of a poroelastic layer the impact of changes in saturation on the frequencydependent reflection coefficient of a partially saturated layer was studied seismic attenuation and velocity dispersion in partially saturated i e patchy saturated poroelastic media were accounted for by using the analytical solution of the 1d white s model for wave induced fluid flow white s solution was applied in combination with an analytical solution for the normal incidence reflection coefficient of an attenuating layer embedded in an elastic or attenuating background medium to investigate the effects of attenuation velocity dispersion and tuning on the reflection coefficient approximations for the frequency dependent quality factor its minimum value and the frequency at which the minimum value of the quality factor occurs were derived the approximations are valid for any two alternating sets of petrophysical parameters an approximation for the normal incidence reflection coefficient of an attenuating thin compared to the wavelength layer was also derived this approximation gives insight into the influence of contrasts in acoustic impedance and or attenuation on the reflectivity of a thin layer laboratory data for reflections from a water saturated sand layer and from a dry sand layer were further fit with petrophysical parameters for unconsolidated sand partially saturated with water and air the results showed that wave induced fluid flow can explain low frequency reflection anomalies which are related to fluid saturation and can be observed in seismic field data the results further indicate that reflection coefficients of partially saturated layers e g hydrocarbon reservoirs can vary significantly with frequency especially at low seismic frequencies where partial saturation may often cause high attenuation

effect of anelastic patchy saturated sand layers on the reflection and transmission responses of a periodically layered medium based on analytic relations we compute the reflection and transmission responses of a periodically layered medium with a stack of elastic shales and partially saturated sands the sand layers are considered anelastic using patchy saturation theory or elastic with effective velocity using the patchy saturation theory we introduce a velocity dispersion due to mesoscale attenuation in the sand layer this intrinsic anelasticity is creating frequency dependence which is added to the one coming from the layering macroscale we choose several configurations of the periodically layered medium to enhance more or less the effect of anelasticity the worst case to see the effect of intrinsic anelasticity is obtained with low dispersion in the sand layer strong contrast between shales and sands and a low value of the net togross ratio sand proportion divided by the sand shale proportion whereas the best case is constituted by high dispersion weak contrast and high net to gross ratio we then compare the results to show which dispersion effect is dominating in reflection and transmission responses in frequency domain the influence of the intrinsic anelasticity is not negligible compared with the layering effect even if the main resonance patterns are the same the resonance peaks for anelastic cases are shifted towards high frequencies and have a slightly lower amplitude than for elastic cases these observations are more emphasized when we combine all effects and when the net to gross ratio increases whereas the differences between anelastic and elastic results are less affected by the level of intrinsic dispersion and by the contrast between the layers in the time domain the amplitude of the responses is significantly lower when we consider intrinsic anelastic layers even if the phase response has the same features for elastic and anelastic cases the anelastic model responses are clearly more attenuated than the elastic ones we conclude that the frequency dependence due to the layering is not always dominating the responses the frequency dependence coming from intrinsic visco elastic phenomena affects the amplitude of the responses in the frequency and time domains considering intrinsic attenuation and velocity dispersion of some layers should be analyzed while looking at seismic and log data in thin layered reservoirs

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

seismic signatures of permeability in heterogeneous porous media in homogeneous poroelastic systems the permeability tensor practically does not influence propagating seismic waves in the low frequency range 0 1000 hz see e g schmitt 1989 gelinsky and shapiro 1996 in this paper we show that this situation changes in heterogeneous systems such as layered or fractured sediments due to the heterogeneities of poroelastic structures the attenuation of p waves is influenced by the permeability in an enhanced way we show however that such a seismic permeability can differ very strongly from the hydraulic permeability

seismoacoustic signatures of fracture connectivity wave induced fluid flow wiff between fractures and the embedding matrix as well as within connected fractures tends to produce significant seismic attenuation and velocity dispersion while wiff between fractures and matrix is well understood the corresponding effects related to fracture connectivity and the characteristics of the energy dissipation due to flow within fractures are largely unexplored in this work we use oscillatory relaxation simulations based on the quasi static poroelastic equations to study these phenomena we first consider synthetic rock samples containing connected and unconnected fractures and compute the corresponding attenuation and phase velocity we also determine the relative fluid displacement and pressure fields in order to gain insight into the physical processes involved in the two manifestations of wiff in fractured media to quantify the contributions of the two wiff mechanisms to the total seismic attenuation we compute the spatial distribution of the local energy dissipation finally we perform an exhaustive sensitivity analysis to study the role played by different characteristics of fracture networks on the seismic signatures we show that in the presence of connected fractures both p wave attenuation and phase velocity are sensitive to some key characteristics of the probed medium notably to the lengths permeabilities and intersection angles of the fractures as well as to the overall degree of connectivity of the fracture network this in turn indicates that a deeper understanding of these two manifestations of wiff in fractured media may eventually allow for the extraction of some of these properties from seismic data

seismic attenuation in finely layered porous rocks effects of fluid flow and scattering scattering caused by fine layering is known to play an important role in seismic attenuation if the rock is also porous and permeable additional attenuation can be caused by a flow of the pore fluid induced by the passing wave see e g white 1983 at frequencies higher than 100 khz standard biot s attenuation biot 1962 bourbie et al 1987 may also contribute to the overall attenuation in a layered poroelastic rock while standard biot s attenuation has been well known for decades the effects of scattering by fine layering and interlayer flow have been studied extensively both theoretically and numerically in recent years however in all theoretical approaches proposed thus far these effects have been treated separately scattering was studied for purely elastic rocks burridge and chang 1989 shapiro et al 1994 while attenuation caused by interlayer flow was considered either for periodically stratified media that causes no scattering norris 1993 or for frequencies at which scatterin

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

seismic dispersion and attenuation in layered porous rocks with fractures of varying orientations abstract wave induced fluid flow plays an important role in affecting the seismic dispersion and attenuation of fractured porous rocks while numerous theoretical models have been proposed for the seismic dispersion and attenuation in fractured porous rocks most of them neglect the wave induced fluid flow resulting from the background anisotropy e g the interlayer fluid flow between different layers that can be normal in real reservoirs here according to the theories of poroelasticity we present an approach to study the frequency dependent seismic properties of more realistic and complicated rocks i e horizontally and periodically layered porous rock with horizontal and randomly orienting fractures respectively distributed in one of the two periodical layers the approach accounts for the dual effects of the wave induced fluid flow between the fractures and the background pores and between different layers the interlayer fluid flow because c33 i e the modulus of the normally incident p wave is directly related to the p wave velocity widely measured in the seismic exploration and its comprehensive dispersion and attenuation are found to be most significant we study mainly the effects of fracture properties and the stiffness contrast between the different layers on the seismic dispersion and attenuation of c33 the results show that the increasing stiffness contrast enhances the interlayer fluid flow of the layered porous rocks with both horizontal and randomly orienting fractures and weakens the wave induced fluid flow between the fractures and the background pores especially for the layered porous rock with horizontal fractures the modelling results also demonstrate that for the considered rock construction the increasing fracture density reduces the interlayer fluid flow while improves the dispersion and attenuation in the fracture relevant frequency band increasing fracture aspect ratio is found to reduce the dispersion and attenuation in the fracture relevant frequency band only especially for the layered porous rock with horizontal fractures

excision and the original low dip of the miocene pliocene bannock detachment system se idaho northern cousin of the sevier desert detachment geologic mapping and structural analysis in the bannock range se idaho indicate that the bulk of cenozoic extension in southeast idaho was accommodated by slip on the low angle bannock detachment system the bannock detachment system consists of low angle normal faults with a likely n s extent of 130 km it was active from ca 10 3 ma to ca 3 4 ma and accommodated 15 km of top to the wsw extension the master low angle detachment fault cuts steeper older normal faults and extensional folds in its hanging wall crosscutting relationships and prior work show that the hanging wall to the detachment system began as a cohesive block that later broke up along normal faults that either sole into or are cut by the master detachment fault after breakup began the master detachment fault was folded isostatically by a nnw trending extensional anticline the oxford ridge anticline the folding produced a new listric low angle normal fault east of the oxford ridge anticline to replace the back tilted portion of the detachment fault this new low angle normal fault excised part of the hanging wall cut hanging wall structures and served as a secondary breakaway for the detachment west of the oxford ridge anticline the master detachment fault initiated and slipped at a low angle is the youngest low angle normal fault of the system and has not been rotated to a low dip angle through time the exhumed bannock detachment system although somewhat smaller shorter lived and more disrupted by younger basin and range normal faults is similar to the buried sevier desert detachment system beneath central utah both top to the west systems formed near the transition between the sevier thrust belt and its hinterland collapsed broad thrust related culminations eroded to cambrian rocks are paired with partly coeval top to east metamorphic core complexes 100 160 km to the west lie east of major thrust ramps formed and slipped at low angles in the most extended part of the system and exhibit a translation and breakup phase of deformation both are late cenozoic in age and have voluminous lacustrine synrift deposits that record saline and alkaline conditions during early extension similar processes likely produced both detachment systems

acoustic signatures of partial saturation the relationship between elastic wave velocities and water saturation in a water gas reservoir depends strongly on whether saturation is heterogeneous patchy or homogeneous heterogeneity in saturation may result from lithologic heterogeneity because under conditions of capillary equilibrium different lithologies within a reservoir can have different saturations depending on their porosities and permeabilities we investigate this phenomenon by generating models of a reservoir in which we control the distribution of lithologic units and theoretically determine the corresponding velocity saturation relationship we assume a state of capillary equilibrium in the reservoir and determine the saturation level of each region within the reservoir from the corresponding capillary pressure curve for the lithologic unit at that location the velocities we calculate for these models show that saturation heterogeneity caused by lithologic variation can lead to a distinct dependence of velocity on saturation in a water gas saturated reservoir a patchy distribution of the different lithologic units is found to cause p wave velocity to exhibit a noticeable and almost continuous velocity variation across the entire saturation range this is in distinct contrast to the response of a homogeneous reservoir where there is only a large change in velocity at water saturations close to 100

preserving low frequency climate signals in growth records of geoduck clams panopea abrupta abstract the capacity of geoduck clams panopea abrupta to preserve detailed records of climatic change in their shells is limited primarily by longevity unlike the multi centennial growth records commonly available from trees shells rarely yield more than 160 years of data consequently low frequency signals may be lost when band width series are processed here we investigate use of regional curve standardization rcs to preserve decadal to centennial scale growth trends in the relatively short segment lengths available from individual clams advantages of this method over the more common exponential treatment are first explored through simulation experiments using sine waves embedded in geoduck growth curves both standardization methods are then used to remove biological trends from growth records of geoduck shells sampled near protection island in the strait of juan de fuca the resulting chronologies are compared in terms of their spectral properties their correlations with historical air temperature records and their agreement with historical observations of the marine environment simulation experiments illustrated the potential benefits of using the rcs growth model to standardize shell growth series but empirical results were inconclusive the rcs chronology exhibited enhanced spectral power at periods exceeding the length of growth segments and provided stronger correlation with coastal air temperature records especially during the earliest portion of the chronology over the full period of comparison however the correlation coefficients were essentially identical observational data from the mid to late 1800s appears to support the rcs results northward range expansions of several marine fish species coincide with a period of conspicuously high values in the rcs chronology and suggest that the coastal ocean was unusually warm along western north america from the 1850s through the 1870s

kinematic evolution of a large offset continental normal fault system south virgin mountains nevada the south virgin mountains and grand wash trough comprise a mid miocene normal fault system that defines the boundary between the unextended colorado plateau to the east and highly extended crust of the central basin and range province to the west in the upper 3 km of the crust the system developed in subhorizontal cratonic strata in the foreland of the cordilleran fold and thrust system the rugged topography and lack of vegetation of the area afford exceptional three dimensional exposures compact stratigraphy and well defined prefaulting configuration of the rocks permitted a detailed reconstruction of the system reconstruction of cross sections based on more than 300 km2 of detailed mapping at a scale of 1 12 000 shows that the fault system accommodated more than 15 km of roughly east west directed miocene extension extension was initially accommodated on moderately to steeply dipping listric normal faults as the early faults and fault blocks tilted steeply to moderately dipping faults initiated within the fault blocks soling into the early faults some of the early faults were active at dips of 20 isostatically driven tilting is superimposed on tilting due to active slip and domino style rotation of the fault blocks collectively these processes rotated originally steeply dipping faults to horizontal orientations the kinematics are inconsistent with the widely accepted view that many near horizontal normal faults were rotated to their present orientations by later crosscutting normal faults however reexamination of other areas suggests that the evolutionary sequence seen in the south virgin mountains may in fact be widely applicable

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

space time patterns and tectonic controls of tertiary extension and magmatism in the great basin of the western united states structural and stratigraphic relations in the great basin indicate widespread pre middle miocene crustal extension that appears to define two north trending belts most extension in these belts was oligocene age but locally it began earlier or lasted into early miocene time the eastern belt straddles the nevada utah border and includes the snake range nevada area with its southern end near 37 5 n and its western edge at the seaman butte mountains breakaway the southern boundary of the eastern belt is occupied by the 26 15 ma caliente caldera complex and approximately coincides with the present east west trending margin of the great basin north of saint george utah crust north of this boundary extended approximately east west before volcanism began at 30 32 ma but to the south extension began after about 15 ma this boundary may have been a rooted zone of left slip faults that allowed the footwall of the stampede detachment to move west relative to unextended terrain to the south the eastern margin of the eastern belt is probably located near the present eastern edge of the great basin but its northern end is poorly defined the western belt runs from the funeral and grapevine mountains california to the ruby mountains nevada and north northeast to the albion range idaho tens of kilometers of crustal extension occurred at least locally in both belts but magnitude of extension is poorly known for large areas of each tertiary volcanism in the great basin began in the north in eocene time with predominantly effusive volcanism and swept southward ending with voluminous oligocene miocene ignimbrite eruptions from calderas in an irregular discontinuous belt between marysvale utah and reno nevada a result of the south ward migration of volcanism is that the onset of extension in both belts was syn or post volcanic in the north but was pre volcanic in the south late paleogene extension and crustal magmatism coincided in both time and space only locally where south migrating magmatism overlapped active north south trending extensional belts most calderas in the southern great basin formed in previously extended belts or on their margins southward migration of ignimbrite sources was apparently blocked by unextended crust to the south in contrast volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to or during crustal extension this is consistent with observations in the southern basin and range where volcanism and crustal extension were generally synchronous and volcanism was dominantly effusive thus caldera formation may be controlled by the distribution of upper crustal extension although the physical mechanism for this control remains speculative the space time patterns of late paleogene extension in the great basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin but being driven by gravitational collapse of thick crust space time patterns of tertiary volcanism in the basin and range also appear to conform to patterns of thermal weakening or destruction of the subducted slab both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts because both predict close spatial and temporal association of extension and magmatism which is not generally observed

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

measurements of seismic attenuation and transient fluid pressure in partially saturated berea sandstone evidence of fluid flow on the mesoscopic scale s u m m a r y a novel laboratory technique is proposed to investigate wave induced fluid flow on the mesoscopic scale as a mechanism for seismic attenuation in partially saturated rocks this technique combines measurements of seismic attenuation in the frequency range from 1 to 100 hz with measurements of transient fluid pressure as a response of a step stress applied on top of the sample we used a berea sandstone sample partially saturated with water the laboratory results suggest that wave induced fluid flow on the mesoscopic scale is dominant in partially saturated samples a 3 d numerical model representing the sample was used to verify the experimental results biot s equations of consolidation were solved with the finite element method wave induced fluid flow on the mesoscopic scale was the only attenuation mechanism accounted for in the numerical solution the numerically calculated transient fluid pressure reproduced the laboratory data moreover the numerically calculated attenuation superposed to the frequency independent matrix anelasticity reproduced the attenuation measured in the laboratory in the partially saturated sample this experimental numerical fit demonstrates that wave induced fluid flow on the mesoscopic scale and matrix anelasticity are the dominant mechanisms for seismic attenuation in partially saturated berea sandstone

post laramide removal of the farallon slab western united states i propose that post laramide removal of the subhorizontally subducting farallon slab occurred by buckling downward along an approximately east northeast trending axis this process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the united states and propagation of the separated edges toward the central axis of downwelling accompanied by aesthenosphic upwelling behind the trailing edges initial buckling probably began near 50 ma and slab removal was complete by 20 ma this model is based primarily on the space time evolution of the ignimbrite flare up a major mid tertiary igneous event of mantle origin which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central nevada from the north and southeast post laramide uplift extension establishment of the cascadia subduction zone and active magmatism may be consequences of lithosphere scale modifications caused by the laramide removal of the slab and the resulting asthenospheric upwelling

influence of frequency and saturation on avo attributes for patchy saturated rocks abstractpartially saturated rocks are considered to be major sources of seismic wave velocity dispersion and attenuation in recorded real data from the physical description of partially saturated gas water and oil water reservoirs we use upscaling theories to compute an equivalent frequency dependent porous medium these homogenization methods are associated with mesoscale description of attenuation and dispersion coming from wave induced flow phenomena to compute wave propagation we use numerical codes in the frequency domain that allow us to take into account all the frequency dependent parameters without approximation or local time steps in this way the biot slow compressional wave is well modeled and its partially diffusive partially propagative behavior is completely considered the attenuation and dispersion of the waves in such media are coming partly from the wave mode conversion into diffusive slow waves not visible on seismograms but the amplitude of propagative p and s waves are mainl

fault mechanics and the kinematics of block rotations in many strike slip tectonic settings large rotations up to 100 of crustal blocks have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which accommodate the relative motion between the blocks as regional deformation progresses simple geometrical considerations require that the faults must also rotate in this paper we show that on the basis of mechanical considerations the amount of fault rotation permissible under a stationary stress field is limited to 20 to 45 consequently block rotations that are larger than 40 or 45 require more than one set of accompanying faults to accommodate the block rotation examples of such multiple sets with 40 to 45 between them as predicted by the model were recognized in sistan iran in yerington the lake mead area nevada and in southern california

development of active low angle normal fault systems during orogenic collapse insight from tibet active north trending rifts in tibet vary significantly in their character as a function of extension magnitude most rifts are characterized by internally drained basins bounded by high angle normal faults with paleogene or older rocks in the footwall however the central part of the yadong gulu rift near lhasa and the newly documented lunggar rift in west central tibet are bounded by low angle normal faults detachments with mylonitic rocks and miocene granites in the footwall and exhibit active basin incision and intrabasin topographic highs in areas of inferred maximum extension we suggest that tibetan rifts initiate as high angle normal fault and half graben or graben basin systems and evolve in response to increasing extension and footwall isostatic rebound into detachment systems that are active at uppermost crustal levels and above which rift basin fill is being uplifted and eroded

cretaceous crustal structure and metamorphism in the hinterland of the sevier thrust belt western u s cordillera combined structural and geochronologic studies to the west of the sevier belt at lat 39 degree 40 degree n indicate that peak metamorphism of the upper 15 km of crust occurred during the late cretaceous 70 90 ma overlapping in time with the intrusion of muscovite bearing granites and pegmatites metamorphism was accompanied by penetrative deformation top to the east layer parallel shear both were localized at the currently exposed structural levels resulting in impressive vertical and lateral strain gradients and t sub max gradients of 50 c km inception of thrust faulting in the sevier belt may have predated peak metamorphism and intrusion of granites in the hinterland by 10 15 m y but it continued during and ended simultaneously with the end of metamorphism at about 70 ma the rise of muscovite bearing granites and hot fluids generated by metamorphism and crustal anatexis at greater depth provides and explanation for the observed localized metamorphism and deformation at relatively high structural levels

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

exact expression for the effective acoustics of patchy saturated rocks seismic effects of a partially gas saturated subsurface have been known for many years for example patches of nonuniform saturation occur at the gas oil and gas water contacts in hydrocarbon reservoirs open pore boundary conditions are applied to the quasi static biot equations of poroelasticity to derive an exact analytic expression of the effective bulk modulus for partially saturated media with spherical gas patches larger than the typical pore size the pore fluid and the rock properties can have different values in the central sphere and in the surrounding region an analytic solution prevents loss of accuracy from ill conditioned equations as encountered in the numerical solution for certain input for a sandstone saturated with gas and water we found that the p wave velocity and attenuation in conventional models differ as much as 15 from the exact solution at seismic frequencies this makes the use of present exact theory necessary to describe patchy saturation although more realistic complex patch shapes and distributions were not considered we found that despite earlier corrections the white conventional model does not yield the correct low frequency asymptote for the attenuation

synconvergent surface breaking normal faults of late cretaceous age within the sevier hinterland east central nevada the hinterland of the sevier orogenic belt of western north america is widely interpreted as a cretaceous to paleogene orogenic plateau although evidence for mid crustal extension of late cretaceous age within the sevier hinterland is widespread coeval surface breaking normal fault systems have not been documented new 1 12 000 scale mapping within the type section of the latest cretaceous to eocene sheep pass formation of east central nevada suggests that deposition occurred in response to normal fault movement recording up to 4 km of late cretaceous and paleogene stratigraphic throw intrabasinal normal faulting caused lateral thickness variations within the basal sheep pass formation although upper members are largely unaffected an extensional basin setting best explains the fanning of bedding dips the deposition of megabreccia and the presence of syndepositional normal faults within the sheep pass formation deposition of the basal member of the sheep pass formation is bracketed between ca 81 3 3 7 ma and 66 1 5 4 ma based on the u th he cooling ages of detrital zircons and on a u pb carbonate age derived from the overlying lacustrine limestone member these new data provide the strongest evidence to date for the existence of late cretaceous surface breaking normal faults in the sevier hinterland normal faulting was coeval with mid crustal hinterland extension and with continued contraction within the sevier foreland to the east