improved imaging with phase weighted common conversion point stacks of receiver functions summary broad band array studies frequently stack receiver functions to improve their signal to noise ratio while mapping structures in the crust and upper mantle noise may produce spurious secondary arrivals that obscure or mimic arrivals produced by p to s conversions at large contrasts in seismic impedance such as the moho we use a hilbert transform to calculate phase weights which minimize the constructive stacking of erroneous signal in receiver function data sets we outline this approach and demonstrate its application through synthetic data combined with different types of noise a previously published example of signal generated noise and a large data set from the sierra nevada earthscope project these examples show that phase weighting reduces the presence of signal generated noise in receiver functions and improves stacked data sets

seismic reflections from the basal contacts of batholiths although seismic reflection data in batholithic terranes are scarce datasets available to us from three different regions show surprising similarities all three profiles one across precambrian rocks in texas and two across mesozoic batholiths in california and nevada show strong subhorizontal reflections at depths of 6 10 km these reflections are interpreted to originate from the base of the batholiths and indicate that these batholiths are tabular in shape the large amplitudes of the reflections require a marked contrast in acoustic impedance product of velocity and density between the batholith and the underlying rocks two of the profiles show reflective zones having appreciable thicknesses 0 5 2 5 km while the third profile shows a single continuous reflection the four most favored candidates for sub batholithic reflections are 1 underplated gabbro 2 cumulate layers 3 migmatites and 4 thrust faults cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance

crustal structure across northeastern tibet from wide angle seismic profiling constraints on the caledonian qilian orogeny and its reactivation abstract the northeastern tibetan plateau results from the superposition of the caledonian qilian orogenic belt and cenozoic reactivation following continental collision between the indian and asian plates in order to identify the constraints on the qilian orogenic mechanism and the expansion of the plateau we carried out a 430 km long wide angle seismic experiment between jingtai and hezuo we herein present the crustal p wave velocity structure model resulting from the interpretation of this wide angle seismic dataset the principal characteristics of the crustal velocity model include 1 thinning of the crust from south to north within a range of about 48 54 km with some undulation of the moho beneath the northern qilian block 2 the thickness of the sedimentary layer and its average p wave velocity exhibit the obvious tectonic division of the qaidam kunlun west qinling belt the central and northern qilian and the alax blocks 3 the lower crustal layer has a p wave velocity of 6 6 6 8 km s beneath the qaidam kunlun west qinling belt northern qilian orogenic belt and alax block but has a very low value of 6 4 6 5 km s beneath the central qilian block and 4 the central qilian block shares a similar crustal p wave velocity depth relationship with the sierra nevada arc or the accretionary crust the qaidam kunlun west qinling and the northern qilian orogenic belts have a crustal p wave velocity depth relationship characteristic of the crust of the global average continent the particular vp depth relationship beneath the central qilian orogenic belt may result from a caledonian accretionary orogeny or from the delamination of the ecologized lower crust during the arc continental collisional orogeny the lower velocity layer lvl exists at the bottom of the upper crust to the north of the middle of the central qilian block we infer that this lvl plays an important role as an intracrustal decollement to expand the plateau

moho orientation beneath central california from regional earthquake travel times table al is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 011 2 50 payment must accompany order analysis of relative pn travel times recorded by 238 stations of the u s geological survey central california seismic network calnet from 77 regional earthquakes and explosions shows that arrivals are progressively delayed to the northeast in both the coast ranges and sierra nevada foothills by dividing the calnet into 10 subarrays and assuming laterally uniform velocities for a crust over an upper mantle half space the orientation of a planar moho beneath each subarray is calculated by inversion of relative pn arrival times the results show that the moho dips to the northeast between 3 and 6 beneath the coast ranges and 8 17 beneath the great valley and is horizontal beneath the foothills pn residuals west of the san andreas fault are relatively early by 0 35 s but are interpreted as resulting from a lateral velocity contrast across the fault rather than variations in crustal thickness unexplained pn travel time variance improves by approximately 60 after correction for the travel time due to dip and mean pn residual at each station the pn data corrected for the travel time due to a dipping moho indicate an isotropic upper mantle velocity of 7 97 0 05 km s beneath the coast ranges great valley and foothills evidence of upper mantle velocity anisotropy in the azimuthal variation in pn apparent velocity corrected for dip is only marginally significant at the 5 level however large azimuthal variations in corrected apparent velocity do exist and may be attributed to velocity heterogeneities in the crust unmodeled higher order mantle structure and gradients in upper mantle velocity arrival times of cape mendocino earthquakes recorded in the coast ranges south of san francisco bay indicate lateral variations in upper mantle velocity the 7 4 km s apparent velocity estimate from travel times of the oroville earthquake recorded in the coast ranges out to distances greater than 400 km is anomalous in that pn is normally observed at distances greater than 200 km these arrivals may originate from updip propagation along a 6 9 km s layer in the lower crust of the coast ranges subparallel to the moho

the crustal structure of the axis of the great valley california from seismic refraction measurements abstract in 1982 the u s geological survey collected six seismic refraction profiles in the great valley of california three axial profiles with a maximum shot to receiver offset of 160 km and three shorter profiles perpendicular to the valley axis this paper presents the results of two dimensional raytracing and synthetic seismogram modeling of the central axial profile the crust of the central great valley is laterally heterogeneous along its axis but generally consists of a sedimentary section overlying distinct upper middle and lower crustal units the sedimentary rocks are 3 5 km thick along the profile with velocities increasing with depth from 1 6 to 4 0 km s the basement upper crust consists of four units 1 1 a 1 0 1 5 km thick layer of velocity 5 4 5 8 km s 2 2 a 3 4 km thick layer of velocity 6 0 6 3 km s 3 3 a 1 5 3 0 km thick layer of velocity 6 5 6 6 km s and 4 4 a laterally discontinuous 1 5 km thick layer of velocity 6 8 7 0 km s the mid crust lies at 11 14 km depth is 5 8 km thick and has a velocity of 6 6 6 7 km s on the northwest side of our profile the mid crust is a low velocity zone beneath the 6 8 7 0 km s lid the lower crust lies at 16 19 km depth is 7 13 km thick and has a velocity of 6 9 7 2 km s crustal thickness increases from 26 to 29 km from nw to se in the model although an unequivocal determination of crustal composition is not possible from p wave velocities alone our model has several geological and tectonic implications we interpret the upper 7 km of basement on the northwest side of the profile as an ophiolitic fragment since its thickness and velocity structure are consistent with that of oceanic crust this fragment which is not present 10 15 km to the west of the refraction profile is probably at least partially responsible for the great valley gravity and magnetic anomalies whose peaks lie about 10 km east of our profile the middle and lower crust are probably gabbroic and the product of magmatic or tectonic underplating or both the crustal structure of the great valley is dissimilar to that of the adjacent diablo range suggesting the existence of a fault or suture zone throughout the crust between these provinces

extraction of deep crustal reflections from shallow vibroseis data using extended correlation lower crustal seismic reflections can be extracted from shallow crustal seismic profiles through the application of extended correlation to uncorrelated vibroseis seismic data fixed bandwidth extended correlation shortens the correlation operator before crosscorrelation producing reflections over an increased correlation time range all with lowered bandwidth self truncating extended correlation preserves the full bandwidth in the original seismic reflection times but loses bandwidth in a predictable manner at the additional later arrival times correlation wavelet shape and extra correlation time are directly related and can be calculated for specific acquisition parameters pre correlation tapering is necessary to avoid undue wavelet distortion at extended correlation times seismic data collected in the basin and range province illustrate the application of the method the results are verified with conventional correlations of long sweep records and with impulsive source data

overthrust model for the sierra nevada the structure of the northern sierra nevada california is a series of east directed thrust faults modified by upright and west directed folds and faults the highest tectonic unit is a nappe composed of the smartville ophiolitic complex and related rocks these structures collectively represent the middle jurassic nevadan orogeny this overthrust backfold model resembles middle jurassic events in british columbia and the cretaceous tertiary tectonics of the western european alpine belt and it is consistent with the hypothesis that the nevadan orogeny reflects the collision of an oceanic volcanic arc with north america

frac sand crushing characteristics and morphology changes under high compressive stress and implications for sand pack permeability sand consisting of round quartz grains is widely used as a proppant during hydraulic fracturing to produce natural gas from tight shale formations this paper presents results from sand characteriz

crustal structure and composition of the southern foothills metamorphic belt sierra nevada california from seismic data the foothills metamorphic belt is an accreted terrane consisting of paleozoic and mesozoic metamorphic rocks that separates the great valley from the sierra nevada batholith in northern and central california until recently the only available geophysical data for this area were reconnaissance refraction surveys and gravity and magnetic data new insights into the structure of the deep crust are provided by the interpretation of a seismic reflection profile cc 2 acquired in 1984 by the u s geological survey at the southern end of the foothills metamorphic belt our interpretation is constrained by a new seismic velocity model derived from coincident microearthquake data earthquake hypocenters that occur at unusually great depths of 12 to 30 km make the data set particularly useful for obtaining deep crustal velocity information the velocity model shows velocities of 5 2 to 6 3 km s 1 for the upper 12 km of the crust and 6 7 to 6 8 km s 1 from 12 km to an estimated moho at 32 km the upper crustal velocities correspond to metamorphic rocks and serpentinites of the foothills metamorphic belt as well as to diorites and granodiorites of the sierra nevada batholith while the lower crustal velocities are interpreted to represent intermediate to mafic granulites the majority of the earthquake hypocenters as well as a 6 7 km s 1 layer in the velocity model corresponds in depth to thick zones of west dipping midcrustal reflections that may represent major shear zones formed during the late jurassic nevadan orogeny or synbatholithic ductile shear zones that accommodated crustal extension associated with batholith intrusion these reflections are truncated updip by an inferred subvertical contact that coincides with the western edge of the sierra nevada batholith and the southward trace of the bear mountains fault zone the updip truncation of midcrustal shear zones and high lower crustal velocities indicate that strike slip faulting and magmatic underplating can be important processes during the docking and welding of an accreted terrane

crustal structure in the northwestern foothills of the sierra nevada from seismic refraction experiments abstract analysis of travel times from a reversed active seismic refraction experiment in the western foothills of the sierra nevada reveals a localized layer of high velocity material within 10 km of the surface this layer with a velocity of 7 0 km sec has a five fold increase in dip approximately 25 km south of the epicenter of the 1 august 1975 magnitude 5 7 earthquake near oroville california we interpret this material to be a remnant of buried ophiolite these data also suggest a 5 per cent anisotropy in the near surface p wave velocity the compressional velocity is 6 3 km sec parallel to the strike of the sierra range and 6 0 km sec normal to it a series of aftershocks to the magnitude 5 7 oroville earthquake recorded on a linear array of portable seismographs extending 300 km southward along the west edge of the sierra nevada foothills show an apparent p n velocity of 7 8 km sec the mohorovicic discontinuity is located between 30 and 39 km depth this wide range of depth reflects the facts that the profile is unreversed the receivers are widely spaced and the origin times and depths of the sources are not exactly known

elastic wave velocities in rocks from a lower crustal section in southern calabria italy compressional and shear wave velocities vp and vs have been measured on 14 rock samples recovered from a high grade metamorphic terrane in the southern part of the calabrian massif which is considered to represent a continuous section of a former deep continental crust the experiments have been carried out in a cubic anvil apparatus at constant confining pressures up to 600 mpa and temperatures ranging from 20 to 700 c using the ultrasonic transmission technique at 600 mpa the compressional and shear wave velocities range from 6 33 km s 1 to 7 74 km s 1 and from 3 54 km s 1 to 4 30 km s 1 respectively variations in velocity are strongly related to variations in mineralogy both vp and vs increase with increasing amphibole pyroxene garnet and sillimanite content the range of velocities measured in the granulite facies rocks under conditions of lower crustal depth is similar to the range of seismic velocities for the crust beneath calabria importantly the metapelite rocks with intermediate silica and high alumina contents have in situ compressional and shear wave velocities which are equivalent to the mafic rocks or even significantly higher the detected high velocity layer at a depth between 15 and 20 km can be explained by a sequence of garnet cordierite sillimanite gneisses for which actual velocities between 6 6 and 7 6 km s 1 have been determined experimentally

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

origin of reflections from the brevard fault zone the brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from alabama into virginia seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth the origin of these reflections has been investigated by measuring compressional wave velocities to 800 mpa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m densities are reported as well as chemical analyses at selected depths anisotropy at elevated pressures ranges within the fault zone from less than 5 to over 26 similarly the fault zone rocks show significant chemical variability with sio2 ranging from 58 0 to 70 4 by weight synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain essential to this study has been the continuous coring of a major section of the brevard fault zone which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies

deep intraplate seismicity in the western sierra nevada central california abstract a microearthquake monitoring program was conducted from 1976 to 1979 in the eastern san joaquin valley and sierran foothills of central california this program revealed one of the deepest occurrences of intraplate earthquakes observed in the conterminous united states earthquakes in the m l range of approximately 0 to 3 2 were confined to the lower crust of the sierran block at depths from 12 to 38 km the distribution of epicenters was diffuse and widespread with some clustering in space and time two particulary active areas that were consistent with the historical seismicity record were recognized an area 15 km south of the town of madera and an area 15 km south of mariposa no association of the seismicity with faults or other geologic structures expressed at the surface was apparent based on regional seismic monitoring by the u s geological survey and the detailed monitoring performed in this study this occurrence of deep seismicity does not appear to extend farther north along the foothills than jamestown although two deep events have been observed near oroville the southern limit is not as well defined due to the lack of seismographic coverage focal mechanisms of these deep crustal earthquakes exhibited 1 strike slip faulting combined with some reverse and normal components and 2 a regional tectonic stress field characterized by a maximum compressive stress generally trending north south and a minimum compressive stress trending east west these observations support the view that 1 the sierra nevada represents a transition between a region of primarily shear deformation along the san andreas fault zone and extensional tectonics of the basin and range province and 2 the orientation of the tectonic stress field is generally uniform throughout the crust of the sierran block

two dimensional velocity structure along the synclinal axis of the great valley california abstract in 1981 and 1982 the u s geological survey conducted a seismic refraction study of the great valley central california in order to obtian a better understanding of the deep crustal structure and tectonic evolution of the region we present the interpretation of a 147 km long seismic refraction profile recorded along the synclinal axis of the valley the northern 98 km of the profile are reversed whereas the remaining southern portion is unreversed observed travel times and amplitudes of primary and secondary arrivals from three shot points have been modeled using two dimensional ray tracing techniques the resultant p wave velocity cross section consists of a 27 km thick crust whose isovelocity contours generally rise gently to the southeast the crust is described in six layers with average velocities average layer thickness in parentheses of 2 8 km sec 3 km 4 25 km sec 3 km 5 5 km sec 2 km 6 1 km sec 6 km 6 75 km sec 6 km and 7 2 km sec 7 km the second through fifth layers are separated by 1 km thick high gradient transition zone the lower crustal boundary 6 75 7 2 km sec and the moho 7 2 8 1 km sec are modeled with first order discontinuities the first layer cenozoic sedimentary deposits is characterized by a velocity gradient of 0 7 km sec km starting with a velocity of 1 7 km sec at the surface the second layer 4 25 km sec consists of the cretaceous great valley sequence and shows a very modest velocity gradient of 0 2 km sec km the third layer 5 5 km sec basement may consist of metasedimentary rocks of the franciscan assemblage fractured crystalline rocks or possibly serpentinized ophiolite the fourth layer 6 1 km sec apparently has no velocity equivalent 30 km to the west in the diablo range it and the layer beneath it 6 75 km sec may be correlated with the metamorphic and igneous rocks of the eastern foothills of the sierra nevada the basal crustal layer 7 2 km sec is evidently mafic igneous rock an independent interpretation of this profile using the reflectivity method produced a similar model an interpretation of a parallel profile 10 to 15 km east of our line shows a structure similar to our model below 15 km but a thinner sedimentary section and velocities as much as 0 9 km sec faster 6 to 15 km deep the crustal velocity structure of the central great valley is atypical of either continental or oceanic crust but is consistent with the geologic hypotheses that the valley is a thickened marginal basin formed in either a forearc or backarc setting located west of the mesozoic sierra nevadan arc

fragments of the mantle and crust from beneath the sierra nevada batholith xenoliths in a volcanic pipe near big creek california a miocene volcanic pipe that cuts cretaceous granodiorite is exposed in the central sierra nevada near the town of big creek california the volcanic magma began its ascent from a mantle source as an alkalic basalt but was extensively contaminated by incorporation and assimilation of silicic plutonic rocks during ascent resulting in a pseudo andesitic composition the conduit cut through the entire thickness of the sierra nevada batholith as well as the underlying lower crust and upper mantle and the magma entrained a diverse polygenetic assemblage of igneous and metamorphic xenoliths including peridotites eclogites and granulites schists and hornfels and gabbroids and granitoids some xenoliths of the high grade metamorphic xenolith group are of particular interest in elucidating the nature of the uppermost mantle and crust beneath the sierra nevada garnet granulites the most abundant rock type of the mafic high grade metamorphic assemblage and eclogites are restricted to the central area of the pipe these xenoliths are essentially bimineralic consisting of colorless to pale green clinopyroxene and garnet eclogites contain brilliant emerald green clinopyroxene the clinopyroxenes which have a broad range of compostions mg mg fe 0 45 0 90 have a 20 jadeite component in eclogites garnets also have a range of compositions mg mg fe 0 30 0 70 with those from eclogites being the most fe rich one of the xenoliths contains ca rich garnet grs adr 90 feldspathic granulites contain plagioclase of an 45 70 and some lack garnet hornblende occurs in some granulites and a few amphibolites are present likely protoliths are ocean floor basalts for the eclogites and precambrian limestone for the grossular clinopyroxene rock the big creek xenoliths attest to the former presence of subducted oceanic lithosphere beneath the western sierra nevada

a model of velocity structure beneath calabria southern italy based on laboratory data abstract laboratory velocity measurements of representative rock samples from across a nearly complete crustal section from the serre mountains of southern calabria have been carried out as an aid to interpreting in situ seismic velocity data the section is composed of interlayered acidic mafic and pelitic rocks metamorphosed to granulite and amphibolite facies conditions in addition a suite of post tectonic granitoids intrudes the section the experimental data include the measurement of compressional wave vp and shear wave vs velocities at confining pressure in conjunction with structural and stratigraphic field data the experimentally determined data are used to construct a seismic velocity model for the crust beneath calabria in the upper crustal levels vp and vs vary in different manners below the uppermost amphibolite facies gneisses a weak velocity inversion of vp is associated with the post tectonic granites at the granite granodiorite and the tonalite dioritic gneiss boundary vp increases in steps of about 200 and 300 m s respectively in contrast vs remains constant or even decreases slightly through this same sequence of rocks this contrasting behaviour is primarily related to the quartz feldspar ratio in the rocks the most pronounced velocity jump occurs at the top of the granulite facies metapelite unit here both vp and vs increase markedly due to mineralogic changes that accompany increasing metamorphic grade and the greater abundance of mafic and garnet bearing restites of partially melted pelite the mafic rocks of the lowermost crustal layer have p and s wave velocities that are significantly lower than those in the overlying metapelites the variation of poisson s ratio with depth suggests that this parameter which is a combination of vp and vs is more sensitive to lithology than the p and s wave velocities alone in rock units of upper and mid crustal levels with average p wave velocities

geology and tectonic evolution of the bear mountains fault zone foothills terrane central sierra nevada california the late jurassic early cretaceous bear mountains fault zone bmfz is the westernmost strand of the foothills fault system in the western metamorphic belt of the sierra nevada the tectonic significance of this 300 km long fault zone has been downplayed in the past but we contend that it is a major discontinuity within the foothills terrane the bmfz is an 5 km wide shear zone consisting of slate metagraywacke matrix melange that experienced intense polyphase deformation and metamorphism small scale structures indicate reverse slip along most of the extent of this steeply east dipping shear zone a host of lithologically diverse tectonic blocks are enclosed in the bmfz blocks of sandstone metavolcanic rocks and plutonic rocks of intermediate composition have counterparts in the adjacent eastern and western zones of the foothills terrane whereas other block types are exotic exotic blocks include ultramafites that locally contain pods of gabbro garnet amphibolite greenschist sedimentary breccia and volcaniclastic rocks the breccia is composed primarily of clasts of amphibolite and minor garnet bearing impure quartzite metachert it probably accumulated at the base of fault scarps the exotic blocks comprise a broadly ophiolitic assemblage similar to that inferred to form basement to the foothills terrane and they record a deformation prior to incorporation in the slate metagraywacke matrix amphibolite blocks and quartzite preserved as clasts within the breccia probably were deformed during overthrusting of oceanic lithosphere in a setting similar to that beneath the tuolumne ophiolite east of the bmfz the presence of exotic blocks implies large scale mixing and large displacements in the bmfz as do differences in stratigraphy across the fault zone metamorphic grade does not vary significantly across the bmfz however suggesting that dip slip did not exceed several tens of kilometers if the fault zone originally dipped gently eastward as we propose there is no direct evidence for major strike slip along the bmfz as has been proposed by others any such slip must have occurred before the reverse slip related structures of the zone we thus interpret the bmfz as an intra arc reverse fault of moderate to large displacement with possible earlier strike slip that incorporates ophiolitic basement showing an older complicated history

stress breakdown time and slip weakening distance inferred from slip velocity functions on earthquake faults we estimate the critical slip weakening distance on earthquake faults by using a new approach which is independent of the estimate of fracture energy or radiated seismic energy the approach is to find a physically based relation between the breakdown time of shear stress tb the time of peak slip velocity tpv and the slip weakening distance dc from the time histories of shear stress slip and slip velocity at each point on the fault which can be obtained from dynamic rupture calculations using a simple slip weakening friction law numerical calculations are carried out for a dynamic shear crack propagating either spontaneously or at a fixed rupture velocity on a vertical fault located in a 3d half space and a more realistic horizontally layered structure with finite difference schemes the results show that tpv is well correlated with tb for faults even with a heterogeneous stress drop dis tribution except at locations near strong barriers and the fault edges we also inves tigate this relation for different types of slip weakening behavior we have applied the method to two recent strike slip earthquakes in western japan the 2000 tottori and the 1995 kobe events we integrated the slip velocity functions on the vertical fault obtained from kinematic waveform inversion of strong motion and teleseismic records from the arrival time of rupture tr to the time of the peak slip velocity tpv and we then corrected the slip obtained at tpv for the errors expected from the dynamic calculations it was found that the slip weakening dis tance d c estimated in the frequency window between 0 05 and 0 5 hz ranges between 40 and 90 cm on the two earthquake faults however if we consider the limited frequency resolution of the observed waveforms probable time errors in the slip velocity functions obtained from kinematic inversion and the uncertainty of the slip weakening behavior the above estimates may be those located between the minimum resolvable limit and the upper bound of their real values the estimated dc values do not necessarily seem to indicate larger values in the shallower part and smaller values in the deeper part of the fault but rather a spatially heterogeneous distribution that appears to be dependent on the local maximum slip this possible dependence might be interpreted by the frictional properties of the fault such as the degree of roughness or the thickness of gouge layers in addition to stress heterogeneities

nature and timing of deformation in the foothills terrane central sierra nevada california its bearing on orogenesis detailed mapping and structural analysis combined with new age dates using u pb and 40 ar 39 ar techniques have allowed us to constrain the timing of pre ductile and ductile deformation in the foothills terrane of the central sierra nevada by using strain and other data it can be shown that rigid rotation of beds folding faulting predated the onset of ductile deformation and probably occurred between 160 and 151 ma ductile structures consisting of continuous and secondary cleavages and associated folds and lineations started forming ca 151 ma in the bear mountains fault zone and then ca 145 ma began moving away from the fault zone forming diachronously over an 30 m y period the last documented ductile structure formed ca 123 ma although some secondary structures may be younger metamorphism of these rocks is generally upper greenschist facies although higher grade belts one bearing staurolite andalusite and sillimanite are present strain was preferentially partitioned into one of these belts of higher metamorphic grade and sporadically elsewhere the structural history here is much more complex and at least one and locally two complete transpositions of the original cleavage have occurred in these zones of complex deformation it is in most cases possible only to identify a composite foliation consisting of new continuous cleavage and relicts of earlier phases all lying mutually parallel timing constraints indicate that the pre ductile structures may correspond to a very late stage of the nevadan orogeny that is 155 3 ma but the ductile structures postdate that orogeny as defined by as much as about 25 30 m y models which relate the ductile structures in the central foothills terrane to nevadan plate tectonic events are untenable in addition recent work indicates that late cretaceous ductile deformation in the central and southern sierra nevada may be relatively widespread indicating that tectonic models for the sierra nevada need to be reassessed

rayleigh and s wave tomography constraints on subduction termination and lithospheric foundering in central california the crust and upper mantle structure of central california have been modified by subduction termination growth of the san andreas plate boundary fault system and small scale upper mantle convection since the early miocene here we investigate the contributions of these processes to the creation of the isabella anomaly which is a high seismic velocity volume in the upper mantle there are two types of hypotheses for its origin one is that it is the foundered mafic lower crust and mantle lithosphere of the southern sierra nevada batholith the alternative suggests that it is a fossil slab connected to the monterey microplate a dense broadband seismic transect was deployed from the coast to the western sierra nevada to fill in the least sampled areas above the isabella anomaly and regional scale rayleigh and s wave tomography are used to evaluate the two hypotheses new shear velocity vs tomography images a high velocity anomaly beneath coastal california that is sub horizontal at depths of 40 80 km east of the san andreas fault a continuous extension of the high velocity anomaly dips east and is located beneath the sierra nevada at 150 200 km depth the western position of the isabella anomaly in the uppermost mantle is inconsistent with earlier interpretations that the isabella anomaly is connected to actively foundering foothills lower crust based on the new vs images we interpret that the isabella anomaly is not the dense destabilized root of the sierra nevada but rather a remnant of miocene subduction termination that is translating north beneath the central san andreas fault our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern sierra nevada where we find a lower crustal low vs layer consistent with a small amount of partial melt the high elevations relative to crust thickness and lower crustal low vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a 3 4 ma pulse of basaltic magmatism

three dimensional seismic model of the sierra nevada arc california and its implications for crustal and upper mantle composition a three dimensional p wave velocity model of south central california from the coast ranges to the sierra nevada shows that the crust under most of the southern sierra nevada batholith has seismic velocities 5 9 6 3 km s below the continental average the crust is not much thicker on average about 35 km than in the adjacent great valley and basin and range province apart from a small northward thickening crustal root under the western sierra nevada that reaches a depth of 42 km crustal velocities above the continental average are observed beneath much of the great valley due to a high velocity body underlying the sedimentary basin and the foothills metamorphic belt 6 4 7 0 km s upper mantle velocities are generally low 7 8 km s but span a wide range 7 4 8 2 km s we display the velocity model in several cross sections and maps of moho depth and average crustal velocity the measured velocities in the upper and mid crust of the sierra nevada batholith are in good agreement with laboratory measurements on sierra nevada tonalites after corrections for density and temperature peridotite xenoliths from the eastern sierra nevada suggest strong upper mantle anisotropy which could explain some of the velocity heterogeneity in the sierra nevada mantle by the time cretaceous subduction related magmatism ceased the sierra nevada arc must have had a thick mafic lower crust yet a principal result of our work is that today the batholith has a crust of mainly felsic composition throughout a subcrustal layer with velocities below normal pn velocities 7 6 km s may indicate the presence of lower crustal material in eclogite facies

polygenetic ophiolite belt of the california sierra nevada geochronological and tectonostratigraphic development the assumption that ophiolite sequences are generated at essentially one point in geologic time by the process of sea floor spreading is critical for modern concepts in the tectonics of ophiolites and for topics dealing with their structure and petrology however this assumption has only been verified in a few locations by an integrated geochronological and structural stratigraphic approach many ophiolite sections are reconstructed from structurally disrupted sequences with the idealized ocean floor model in mind such reconstructions are prone to error without adequate age control on each of the reconstructed fragments this is a significant problem in structurally complex regions where more than one generation of ophiolite may be present in this paper new pb u zircon ages are presented for key locations along a 375 km segment of the western sierra nevada ophiolite belt these age data are combined with structural stratigraphic observations and published ages and significant tectonic implications for the ophiolite belt emerge three different ophiolitic assemblages are recognized with igneous ages of about 300 200 and 160 m y b p rocks of the 300 m y assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite matrix melange fragments of upper paleozoic seamounts occur in association with the ophiolitic melange and together these assemblages constitute the basement framework for the western sierra pb u and k ar isotopic systematics are complex within this framework and indicate a polymetamorphic history systematics in the 200 and 160 m y assemblages are less complex and give tighter igneous age constraints rocks of the 200 m y assemblage are in a semi intact state with only local tectonite and melange zones rocks of the 160 m y assemblage are intact but nevertheless deformed both the 200 and 160 m y assemblages have equivalent age basinal volcanic sedimentary sequences that lie unconformably above the ophiolitic melange basement in each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age equivalent ophiolite sections these relations along with vestiges of intrusive contacts between the edges of both younger ophiolites and the melange basement indicate that the younger ophiolites underwent igneous formation in proximity to the melange basement the sierran ophiolite belt is considered to have formed by a multistage process initiated by the early mesozoic tectonic accretion of upper paleozoic sea floor in general proximity to the ancient continental margin regional metamorphism and ophiolitic melange resulted this accretionary nucleus became the basement of jurassic age primitive volcanic arc terranes which underwent rifting episodes during the production of the 200 and 160 m y ophiolites the rifting episodes resulted in the formation of sedimentary basins which were the depositional sites of volcanic sedimentary sequences non volcanic sources for the basinal sedimentary rocks include the melange basement and continental margin terranes contact zones between pre existing basement and the juvenile ophiolitic sequences created during the rifting episodes consist of dynamothermal metamorphic aureoles protoclastic deformation zones and cross cutting dikes such edge zone assemblages are in most localities obscurred or destroyed by superimposed deformations resulting from convergent and perhaps transform motions along basin edges both the 200 and 160 m y basins were destroyed by compressional orogenic episodes shortly after their formational episodes destruction of young ophiolite floored basins may be a common course of events when small oceanic type plates are generated along continental margin environments such tectonic settings are ideal for the emplacement of young ophiolite sheets

timing and structural expression of the nevadan orogeny sierra nevada california the nevadan orogeny was a very short lived event in the late jurassic that involved the deformation of a great variety of rock types and paleozoic and mesozoic terranes throughout the extent of the sierra nevada the nevadan structures show great variation in style but relatively constant orientations these relations can be explained by considering the prior histories of the various terranes slaty cleavages and tight folds are the characteristic main phase structures in the western belt of jurassic island arc volcanic rocks and flysch type sedimentary rocks a strip of phyllites and greenschists along the eastern edge of the belt apparently represents similar jurassic rocks that were deformed and metamorphosed at greater depths probably during underthrusting of the western belt beneath the central belt the central belt of paleozoic metasedimentary and metavolcanic rocks shows the most extreme variation in style of main phase structures from weak spaced to crenulation cleavages in the south where polyphase deformed rocks formed a structural basement to slaty and phyllitic cleavages and asymmetric to isoclinal folds in the north where most of the paleozoic basement rocks lack penetrative pre nevadan fabrics eastward directed thrust faulting apparently was important only in the northern part of the range where main phase deformation was most intense the eastern belt of jurassic and triassic magmatic arc volcanic and sedimentary rocks defines the core of a major synclinorium and the rocks contain penetrative slaty cleavages and asymmetric tight to isoclinal folds a late phase of nevadan structures consisting of northeast trending cleavages and minor folds also shows a marked variation in style from relatively intensely developed in the north to very weakly developed in the south the regional extent and geometry of the nevadan structures indicate that the nevadan orogeny involved underthrusting of island arc rocks on the west and significant crustal shortening in the central and eastern belts these features suggest that the nevadan orogeny resulted from the collision of the island arc western belt with an andean type arc eastern belt situated at the western edge of north america

lower crustal seismic velocities from lofoten vesteralen north norway abstract the archipelago of lofoten vesteralen comprises archaean and proterozoic rocks in granulite or amphibolite facies the area is characterised by a large gravity anomaly 120 mgal and thin crust c 25 km and it has been suggested that it represents a deep section into the continental crust compressional and shear wave velocities have been measured in the laboratory on over 70 samples from the area most of them to 0 4 gpa confining pressure velocity profiles are then deduced for sections of the crust assuming a restored horizontal moho and it is shown that upper and lower crustal divisions of approximately 5 9 6 2 km s 1 and 6 5 6 6 km s 1 can be recognised which are similar to those found on seismic refraction lines in scandinavia the velocities of the lofoten granulites are however higher than the velocities observed on the lofoten seismic refraction experiment possible reasons for this including the opening of cracks at low pressure retrograde metamorphism the interpretation of the refraction data or major tectonic dislocation are explored

a transect across the mesozoic accretionary margin of central california subductive accretion formed along the pacific margin of north america in the mesozoic we are exploring deep structure across this accreted margin using coordinated reflection and detailed refraction profiling gravity and magnetic interpretation together with surface and well geology although none of these techniques alone is sufficient to define the critical elements of structure in complement they place important constraints on geologic interpretation for both the foothill rocks and particularly the franciscan assemblage we find evidence of obductive emplacement overthrusting from the west in contrast to the eastward subduction of most previous views these surprises emphasize the importance of such geophysical studies to geologic analysis