Nature of the crust beneath northwest Basin and Range province from teleseismic receiver function data

July 2007; revised 12 April 2008; accepted 2 July 2008;   We utilized teleseismic receiver function techniques complemented by active source refraction seismology to study the crustal structure and continental rift processes responsible for the development of the northwest corner of the Basin and Range province in northwest Nevada. Our passive seismic array of 28 short-period stations, spanning 70 km west to east, and 5 broadband USArray transportable array stations that extended our aperture to 230 km provided data on crustal properties independent of the results of our active source refraction experiment. Combining data from the two experiments provides better constraints on Moho depth, Vp/Vs ratio, and the structure and composition of the crust beneath the northwestern Basin and Range province than possible from either experiment alone. Our new data indicate Moho depths that vary from 29.5 on the east to 36.5 km in the west along the passive source array and Vp/Vs ratios that systematically span a wide range from 1.68 to 1.83. Velocity modeling of data collected from the 300 km, wide-angle refraction/reflection survey provides comparable Moho depths of 32–37 km under the same region. Decreasing Vp/Vs is correlated with decreasing crustal thickness, with Vp/Vs ratio > 1.80 in northeast California and Vp/Vs <1 .74 in the thinner crust of the Basin and Range province. We associate the high Vp/Vs of northeast California with mafic additions to the crust during formation of the Modoc Plateau in a region of accreted Paleozoic island arc terrans similar to those exposed in the eastern Klamaths and northwestern Sierra Nevada. Unusually low Vp/Vs in conjunction with low absolute Vp and Vs is observed beneath the central part of our array and may be due to the roots of the Sierra Nevada batholith, now dismembered by Basin and Range faults. Back azimuthal stacking of our receiver function data suggests a northwest oriented crustal anisotropy fast direction beneath this region, parallel to the northwest to southeast oriented Cenozoic extension experienced by the Basin and Range province and suggestive of a similar flow direction in the middle to lower crust.

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abstract: Data from 27 permanent broad-band stations spread over North America and sampling diverse tectonic regimes of varying ages from the Archaean to the Phanerozoic, were analysed using the receiver function approach. Receiver functions, a few hundred at most of the locations, provide valuable constraints on the composition of the crust shaped by diverse processes characteristic of their time of formation. The chemically inert and stable nature of the Archaean domains of Canada is well manifested in their low Poisson ratio (0.26, felsic, lower than global averages for Precambrian age crust) accompanied by a thicker than global average (>35 km) Archaean crust. Data from the Canadian Cordillera and Innuitian orogens of Phanerozoic times indicate the possible operation of two distinctly different crustal processes resulting in thin crusts (<35 km) with dominantly felsic and more mafic compositions, respectively. These processes are restricted to the region above 410 km without disturbing the integrity of the mantle transition zone (MTZ), which is evident from the remarkably homogeneous nature of the 410 and 660 km discontinuities beneath the Canadian landmass. Contrastingly, the thermally complex Phanerozoic southern California region suggests the presence of a heterogeneous mantle transition zone possibly owing to localized 660 topography with a relatively uniform 410 overlain by a crust heterogeneous in composition and perhaps maturing to that of a continent.

title: Structure of the Earth's crust from spectral behavior of long‐period body waves
abstract: Long-period P waves from distant earthquakes have been analyzed from seismograms recorded at Albuquerque and Bermuda in the light of Haskel's theory of the spectral response of a layered crust. By using the ratio of the vertical spectrum to the horizontal component spectrum, we obtain a function which depends on structure beneath the station. Because of the poorly understood nature of the signal which follows the first P wave motion, the methods of power spectrum analysis are applied and a lag window selected to discriminate against long time correlations within the signal. Corrections for the differing responses of the three components are made by using the power spectral matrix of calibration signals. A range of crustal models has been found which agrees with the data. We are restricted primarily in choosing models for the lower half of the crust; variations of structure in the upper half do not noticeably affect the theoretical curves in the period range considered (0.02 to 0.20 cps). At Albuquerque the crust is about 40 km thick and the lower crust has velocities in the range 6.6 to 7.0 km/sec. The Mohorovicic (M) discontinuity under Bermuda is 12 km below sea level, and the structure appears to be a normal oceanic crust depressed elastically by the weight of the volcanics which compose the island.

title: Detection of waves converted from P to SV in the mantle
abstract: A method is described for the detection of P to SV converted waves in the long-period P coda. The procedure involves axis rotation, transformation of records to a standard form and stacking of processed records from events of various epicentral distances. When applied to NORSAR records, the procedure detected converted phases corresponding to the boundaries in the 410–440- and 640–690-km depth ranges.

title: Moho depth variation in southern California from teleseismic receiver functions
abstract: The number of broadband three-component seismic stations in southern California has more than tripled recently. In this study we use the teleseismic receiver function technique to determine the crustal thicknesses and V_p/V_s ratios for these stations and map out the lateral variation of Moho depth under southern California. It is shown that a receiver function can provide a very good “point” measurement of crustal thickness under a broadband station and is not sensitive to crustal P velocity. However, the crustal thickness estimated only from the delay time of the Moho P-to-S converted phase trades off strongly with the crustal V_p/V_s ratio. The ambiguity can be reduced significantly by incorporating the later multiple converted phases, namely, the PpPs and PpSs+PsPs. We propose a stacking algorithm which sums the amplitudes of receiver function at the predicted arrival times of these phases by different crustal thicknesses H and Vp/Vs ratios. This transforms the time domain receiver functions directly into the H-V_p/V_s domain without need to identify these phases and to pick their arrival times. The best estimations of crustal thickness and V_p/V_s ratio are found when the three phases are stacked coherently. By stacking receiver functions from different distances and directions, effects of lateral structural variation are suppressed, and an average crustal model is obtained. Applying this technique to 84 digital broadband stations in southern California reveals that the Moho depth is 29 km on average and varies from 21 to 37 km. Deeper Mohos are found under the eastern Transverse Range, the Peninsular Range, and the Sierra Nevada Range. The central Transverse Range, however, does not have a crustal root. Thin crusts exist in the Inner California Borderland (21–22 km) and the Salton Trough (22 km). The Moho is relatively flat at the average depth in the western and central Mojave Desert and becomes shallower to the east under the Eastern California Shear Zone (ECSZ). Southern California crust has an average V_p/V_s ratio of 1.78, with higher ratios of 1.8 to 1.85 in the mountain ranges with Mesozoic basement and lower ratios in the Mojave Block except for the ECSZ, where the ratio increases.

title: Structure under Mount Rainier, Washington, inferred from teleseismic body waves
abstract: Teleseismic long-period P waves recorded at the World-Wide Standard Seismograph Network station LON (Longmire, Washington) are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction. In particular, a large and azimuthally smoothly varying tangential component is observed after vector rotation of horizontal P waves into the ray direction and after application of a deconvolution technique which equalizes effective source time functions and removes the instrument response. These tangential waves attain amplitudes comparable to the radial component and demonstrate wave form antisymmetry about a NNE azimuth. A model which contains a single high-contrast interface dipping toward the NNE at a depth of 15–20 km can explain most of the characteristics of the long-period P wave data, provided dips are greater than about 10° and only the interference of P and Ps generated at the interface is considered. The model breaks down for later arrivals which are presumably multiples or scattered waves. Examination of long-period S waves from several deep teleseisms shows a prominent Sp arrival 18 s before S. The timing of this phase conversion suggests an interface at about 145-km depth, and its sense of polarity suggests that the velocity contrast is from higher to lower velocities as depth decreases. This interface may correspond to the bottom of the upper mantle low-velocity zone in the area.

title: Shear-Wave Velocity Structure in the Northern Basin and Range Province from the Combined Analysis of Receiver Functions and Surface Waves
abstract: A new method based on the joint inversion of receiver functions and surface-wave phase velocities results in well-determined shear-velocity structures that are consistent with the compressional-wave structure, gravity, heat flow, and elevation data in the northern Basin and Range. This new inversion method takes advantage of average-velocity information present in the surface-wave method and differential velocity information contained in the receiver function method, thus min- imizing the nonuniqueness problem that results from the velocity-depth trade-off. An unusually thick (38 kin) and relatively faster crust and upper mantle are found in central and eastern Nevada compared to the thin (28 to 34 km) and slower crust and upper mantle of the western Basin and Range. We interpret the regions of thicker and faster crust and upper mantle as zones that have undergone less Cenozoic ex- tension relative to the surrounding regions to the west and north. The thick crust and consequently greater depth to the dense mantle material is consistent with the gravity low pattern in central and eastern Nevada. Simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface, indicating a near-classical Airy type of compensation in the province. We analyze in detail the shear-wave (S-wave) velocity model derived from the receiver functions at station BMN and compressional-wave (P-wave) velocity models derived from the 1986 PASSCAL experiment in northwestern Nevada. The most interesting feature of these models is the presence of negative-velocity gradients in the S-wave model with no corresponding velocity decrease in the P-wave models between depths of 10 and 24 km. This combined velocity model may be explained by high pore fluid pressures at these depths. This model favors a layered fluid porosity model proposed in the literature to explain extensive middle- to lower-crust conti- nental seismic reflections and high electrical conductivity. An upper-mantle, grada- tional low-velocity zone is present between 32 and 38 km in the S-wave model. This upper-mantle, shear-wave, low-velocity zone is consistent with partial melt, which may be the source material for magmatic underplating in this region.

title: Geophysical inversion with a neighbourhood algorithm—I. Searching a parameter space
abstract: This paper presents a new derivative-free search method for finding models of acceptable data fit in a multidimensional parameter space. It falls into the same class of method as simulated annealing and genetic algorithms, which are commonly used for global optimization problems. The objective here is to find an ensemble of models that preferentially sample the good data-fitting regions of parameter space, rather than seeking a single optimal model. (A related paper deals with the quantitative appraisal of the ensemble.) The new search algorithm makes use of the geometrical constructs known as Voronoi cells to derive the search in parameter space. These are nearest neighbour regions defined under a suitable distance norm. The algorithm is conceptually simple, requires just two ‘tuning parameters’, and makes use of only the rank of a data fit criterion rather than the numerical value. In this way all diYculties associated with the scaling of a data misfit function are avoided, and any combination of data fit criteria can be used. It is also shown how Voronoi cells can be used to enhance any existing direct search algorithm, by intermittently replacing the forward modelling calculations with nearest neighbour calculations. The new direct search algorithm is illustrated with an application to a synthetic problem involving the inversion of receiver functions for crustal seismic structure. This is known to be a non-linear problem, where linearized inversion techniques suVer from a strong dependence on the starting solution. It is shown that the new algorithm produces a sophisticated type of ‘self-adaptive’ search behaviour, which to our knowledge has not been demonstrated in any previous technique of this kind.

title: Genetic algorithm inversion for receiver functions with application to crust and uppermost mantle structure
abstract: Genetic algorithm (GA) inversion, a nonlin- ear global optimization technique, has been applied to determine crustal and uppermost mantle velocity struc- ture from teleseismic receiver functions. With a new vicinity of the receiver. The influence of the source can be largely eliminated by source equalization in which the radial component of motion is deconvolved with the vertical component to generate a receiver function (Langston,1979) which isolates conversions from P to $ generated at boundaries beneath the recording site. The receiver function waveform can be inverted in the

title: Structure of the crust beneath the southeastern Tibetan Plateau from teleseismic receiver functions
abstract: Southeastern Tibet marks the site of presumed clockwise rotation of the crust due to the India–Eurasian collision and abutment against the stable Sichuan basin and South China block. Knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region. Here, we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method. We find that the crustal thickness decreases gradually from the Tibetan Plateau proper to the Sichuan basin and Yangtze platform but that significant (intra-)crustal heterogeneity exists on shorter lateral scales ( 0.30) are detected beneath the southeastern margin of Tibet but in the Sichuan basin and southeastern Yangtze platform the values are close to the global average. Combined with high regional heat flow and independent evidence for mid-crustal layers of high (electric) conductivity, the large intra-crustal S-wave velocity reduction (12–19%) and the intermediate-to-high average crustal Poisson's ratios are consistent with partial melt in the crust beneath parts of southeastern Tibet. These results could be used in support of deformation models involving intra-crustal flow, with the caveat that significant lateral variation in location and strength of this flow may occur.

title: Seismic velocity structure and composition of the continental crust: A global view
abstract: Seismic techniques provide the highest-resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high-pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5-km depth intervals, both in the form of histograms and as an average velocity-depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5-km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low-grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

title: Seismic structure of the lithosphere from teleseismic converted arrivals observed at small arrays in the southern Sierra Nevada and vicinity, California
abstract: 21 well-distributed teleseisms (30° to 100° distance) were recorded by mixed broad-band (BB) and short-period (SP) seismic arrays at Mineral King (MK) and Horseshoe Meadow (HM) in the southern Sierra Nevada and at Darwin Plateau (DP) between the Inyo and Argus ranges. These arrays permit identification and separation of direct P and S arrivals, reflections from topography, scattered energy, and arrivals from different back azimuths (multipath arrivals). P-to-S conversions can be identified from beamed BB or SP seismograms. Least squares time domain processing recovered single-event receiver functions from these beamed seismograms. Converted phases attributable to the Moho are clear and uncomplicated at DP; Ps-P times of 3.9–4.2 s correspond to crustal thicknesses of 31–33 km, assuming a mean P crustal velocity of 6.2 km/s and a Poisson's ratio (ν) of 0.255. Ps-P times at HM (eastern Sierra) are 3.9–4.6 s (32–37 km) excepting some unusual seismograms from events at back azimuths of 225°–239°. MK (western Sierra) times for Ps-P show a strong E-W asymmetry: 3.9–4.1 s (31–33 km) from the east, and 4.8–5.3 s (39–42 km) from the west. The MK arrivals from the east are multiple and substantially weaker by comparison with HM and DP. These results confirm the absence of a thick crust under the southern High Sierra inferred from both a refraction experiment coincident with this experiment and some earlier studies. At DP the Moho event follows an intracrustal negative polarity event defining the top of an S wave low-velocity zone. This feature dips west under the Sierran crest at HM but is absent farther west at MK. This feature appears to be a manifestation of extensional strain and thus indicates that the surficially undeformed Sierra overlies a tectonized lower crust. Sub-Moho energy is absent under the Basin and Range (DP) but is conspicuous under the High Sierra at positive arrivals ∼7.3 s (MK) and ∼9 s (HM) after the P. These arrivals might be from the base of a low-velocity, low-density upper mantle body supporting the Sierra.

title: Nature and composition of the continental crust: A lower crustal perspective
abstract: Geophysical, petrological, and geochemical data provide important clues about the composition of the deep continental crust. On the basis of seismic refraction data, we divide the crust into type sections associated with different tectonic provinces. Each shows a three-layer crust consisting of upper, middle, and lower crust, in which P wave velocities increase progressively with depth. There is large variation in average P wave velocity of the lower crust between different type sections, but in general, lower crustal velocities are high (>6.9 km s−1) and average middle crustal velocities range between 6.3 and 6.7 km s−1. Heat-producing elements decrease with depth in the crust owing to their depletion in felsic rocks caused by granulite facies metamorphism and an increase in the proportion of mafic rocks with depth. Studies of crustal cross sections show that in Archean regions, 50–85% of the heat flowing from the surface of the Earth is generated within the crust. Granulite terrains that experienced isobaric cooling are representative of middle or lower crust and have higher proportions of mafic rocks than do granulite terrains that experienced isothermal decompression. The latter are probably not representative of the deep crust but are merely upper crustal rocks that have been through an orogenic cycle. Granulite xenoliths provide some of the deepest samples of the continental crust and are composed largely of mafic rock types. Ultrasonic velocity measurements for a wide variety of deep crustal rocks provide a link between crustal velocity and lithology. Meta-igneous felsic, intermediate and mafic granulite, and amphibolite facies rocks are distinguishable on the basis of P and S wave velocities, but metamorphosed shales (metapelites) have velocities that overlap the complete velocity range displayed by the meta-igneous lithologies. The high heat production of metapelites, coupled with their generally limited volumetric extent in granulite terrains and xenoliths, suggests they constitute only a small proportion of the lower crust. Using average P wave velocities derived from the crustal type sections, the estimated areal extent of each type of crust, and the average compositions of different types of granulites, we estimate the average lower and middle crust composition. The lower crust is composed of rocks in the granulite facies and is lithologically heterogeneous. Its average composition is mafic, approaching that of a primitive mantle-derived basalt, but it may range to intermediate bulk compositions in some regions. The middle crust is composed of rocks in the amphibolite facies and is intermediate in bulk composition, containing significant K, Th, and U contents. Average continental crust is intermediate in composition and contains a significant proportion of the bulk silicate Earth's incompatible trace element budget (35–55% of Rb, Ba, K, Pb, Th, and U).

title: Empirical relations between elastic wavespeeds and density in the Earth's crust
abstract: A compilation of compressional-wave ( V p) and shear-wave ( V s) velocities and densities for a wide variety of common lithologies is used to define new nonlinear, multivalued, and quantitative relations between these properties for the Earth's crust. Wireline borehole logs, vertical seismic profiles, laboratory measurements, and seismic tomography models provide a diverse dataset for deriving empirical relations between crustal V p and V s. The proposed V s as a function of V p relations fit V s and V p borehole logs in Quaternary alluvium and Salinian granites as well as laboratory measurements over a 7-km/sec-wide range in V p. The relations derived here are very close to those used to develop a regional 3D velocity model for southern California, based on  data, and thus provide support for that model. These data, and these relations, show a rapid increase in V s as V p increases to 3.5 km/sec leading to higher shear-wave velocities in young sedimentary deposits than commonly assumed. These relations, appropriate for active continental margins where earthquakes are prone to occur, suggests that amplification of strong ground motions by shallow geologic deposits may not be as large as predicted by some earlier models.

title: Characteristics and consequences of flow in the lower crust
abstract: In some places, there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening. In order to better understand the processes involved, we investigate the behavior of a fluid layer over a fluid half-space to see how such a system responds to the deformation of its upper and lower boundaries. This simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet. The changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically. The most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front, which may cause a topographic step above it at the Earth's surface. Deformation within the channel will be principally by simple shear. The clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur. The smoothing of crustal thickness contrasts leads to differential vertical motions, and is thus a method by which substantial tilting can occur without faulting. This differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the Basin and Range province of the western United States. It is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults.

title: Lower crustal anisotropy or dipping boundaries? Effects on receiver functions and a case study in New Zealand
abstract: We examine the effects on receiver functions of transverse anisotropy and of dipping isotropic boundaries. Splitting of the Moho Ps phase predicts the anisotropy from a postulated 10-km-thick layer of highly anisotropic crustal material only when other phases can be well isolated and when allowance is made for the rotation of the incident energy out of the plane of energy propagation expected for isotropic models. We examine azimuthal variations of the synthetic radial and transverse receiver functions. For both transversely anisotropic layers and dipping isotropic boundaries, radial receiver functions are symmetric and transverse receiver functions are antisymmetric about a given back azimuth (the horizontal projection of the axis of symmetry or the dip direction). Differences include the following: (1) Transverse energy from dipping boundaries arrives at the time of the initial P phase no matter at what depth the dipping boundaries occur, while transverse energy arrives at the time of the initial P phase for anisotropic material only when the anisotropy is at the surface. (2) Transversely anisotropic systems with horizontal symmetry axes have waveforms with 180° periodicity as a function of back azimuth, while dipping symmetry axes or dipping boundaries just have 360° periodicity. A consequence of the symmetry is that stacking the initial P and Ps phases from transverse receiver functions for back azimuths that are 180° apart tends to decrease the arrivals from shallowly dipping isotropic boundaries and doubles the arrivals from anisotropic layers with horizontal symmetry axes. We examine receiver functions for station SNZO in New Zealand, above a subducting plate dipping at 15° beneath the station. Opposite polarities of transverse receiver functions separated by 180° in back azimuth suggest that either dipping isotropic layers or dipping axes of symmetry of anisotropic layers are present. The 15° dip of the subduction zone is insufficient to explain the energy on the transverse receiver functions by the isotropic structures previously determined from refraction and earthquake travel time inversion. However, modifying the isotropic models for the region by anisotropy determined via shear wave splitting measurements and refraction studies, allows many of the features of both the radial and transverse receiver functions to be explained. A relatively highly anisotropic layer about 7 km thick at the base of the crust with an axis of symmetry dipping at 20° - 40° may be caused by metamorphosed oceanic crust.

title: The effect of planar dipping structure on source and receiver responses for constant ray parameter
abstract: A geometrical ray method is developed for wave calculations involving three-dimensional planar dipping interfaces. Justification for the method is based on analogy with first-motion approximations derived from generalized ray theory where frequency dependence in the reflection-transmission coefficients is related to changes in the complex ray parameter. The method is applied to finding the teleseismic response of an arbitrarily oriented dislocation source in dipping layered media and for receiver calculations which assume an impinging P or S wave beneath a stack of dipping layers. Source results indicate that wave forms from fast azimuthally varying sources, such as strike-slip faults, are significantly distorted from the plane layered case for simple structures. A simple dipping Moho for dips up to 10° does not significantly distort vertical and radial P waves for the receiver response. However, due to azimuth anomalies introduced by interface dip a significant tangential P component is produced. In addition, the S-wave response becomes a function of source mechanism due to the need for specifying the incident polarization angle. Polarization studies are suggested for finding dipping structure.