Robert A. Phinney

Origin of High Mountains in the Continents: The Southern Sierra Nevada

Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.

Seismic structure of the lithosphere from teleseismic converted arrivals observed at small arrays in the southern Sierra Nevada and vicinity, California

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 Poissons 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.

Continuous seismic reflection profiling of the deep basement, Hardeman County, Texas

Our understanding of the crust and upper mantle would be enhanced if geophysical studies of the deep basement rocks provided information of resolution and character more nearly like that of geological observations of basement rocks at and near the surface. A test of the continuous seismic reflection profiling technique, the geophysical method with by far the highest resolution and the best potential in this regard, at a site in the midcontinent provided abundant information on intrabasement diffractors and reflectors to depths as great as about 45 km. Conventional equipment and techniques, including nonexplosive vibratory sources, were used with minor modification. In the upper part of the section below the sediments, there are reflectors continuous over the entire length of a profile that give evidence for warping, faulting, unconformities, and other structural features. An age of 1,265 ± 40 m.y. for a sample from a nearby hole indicates that these are Precambrian rocks and not part of the Cambrian basement rocks of the Wichita Province. Detailed correlation with the Precambrian section is inhibited by scarcity of geological information. In the lower part of the section, reflections are not, in general, continuous over more than a few kilometres, but zones and discontinuities within the basement may be distinguished on the basis of spatial density, length, and dip of reflectors. Zones of low reflector density may be plutons; curvature of reflections may indicate deep folded structures. The scale of such features is a few kilometres, and it contrasts with the markedly larger scale of the smallest features of the deep basement that can be resolved by other methods. The method appears to have outstanding potential.

Seismic imaging of deep crust

We introduce here an integral two‐dimensional (2-D) scheme for the processing of deep crustal reflection profiles. This approach, in which migration occurs before stacking, is tailored to the unique character of the data in which nonvertically propagating energy is as important as vertically propagating energy. Since reflector depths range beyond 30 km, the horizontal displacement of reflections which occurs in migration can be as large as reflector depths; under these circumstances, the common‐midpoint (CMP) stack is inadequate. In our scheme, each common‐source trace gather is transformed into a set of traces (beams) corresponding to set of different incidence angles. A correction for wavefront curvature similar to the normal moveout (NMO) correction yields traces (focused beams) which are focused at image points along the direction of arrival. While the method is equivalent to the Kirchhoff integral migration method, and therefore to any complete continuation method, it gives rise to an intermediate da...

A nonlinear signal detector for enhancement of noisy seismic record sections

In an area of complicated structure a stacked record section is likely to be characterized by a low signal‐to‐noise ratio, even after substantial velocity analysis and other processing. The interpreter identifies signals showing phase coherence across many traces at physically allowable velocities and compiles them into a line drawing. We have developed a nonlinear filter designed to mimic this process, which passes only signals showing spatial coherence and having slowness within an allowed range. In this algorithm, called the “SSD filter,” overlapping M-trace windows are converted into a p-τ representation, obtained by multiplying the stack along the relevant slant line by the smoothed semblance. The results from all windows are composited in the p-τ domain, then retransformed into x-t. The principal tunable parameter is the width M of the correlation window, adjusted to provide an output which agrees well with the event picks made by an experienced interpreter on a test panel of data. The method was de...

Full-waveform inversion of plane-wave seismograms in stratified acoustic media: Theory and feasibility

We have implemented an inversion procedure for obtaining velocity and density profiles from multioffset data in a layered acoustic medium. Using an iterative modeling technique and the p-τ representation, the procedure is derived from the nonlinear least‐squares formalism of Tarantola and Valette. The feasibility of this method depends upon obtaining Frechet derivatives during the modeling process and on vectorizing the Thompson‐Haskell reflectivity algorithm. Test data sets for this study are gathers of two to four plane‐wave synthetic seismograms which may contain both precritical and postcritical arrivals generated by a seismic wavelet. These examples demonstrate convergence to the correct velocity and density profiles with reasonable accuracy.

Surveyor v: discussion of chemical analysis.

Material of basaltic composition at the Surveyor V landing site implies that differentiation has occurred in the moon, probably due to internal sources of heat. The results are consistent with the hypothesis that extensive volcanic flows have been responsible for flooding and filling the mare basins. The processes and products of lunar magmatic activity are apparently similar to those of the earth.

The crustal structure in central Maine from coherency processed refraction data

In 1984, the U.S. Geological Surcvey and the Canadian Department of Energy, Mines and Resources conducted an extensive seismic refraction experiment, the Quebec-Maine Transect Project. As part of this project, a 180-km-long seismic refraction profile was recorded along the axis of the Central Maine synclinorium. High near-surface velocities of 6.0–6.2 km/s made a conventional refraction analysis (including first arrivals) uninformative below about 10 km. Peak signal frequencies around 30 Hz, recorded at all offsets, and a trace spacing of 800 m led to severe spatial aliasing effects for secondary arrivals on the record sections. To allow a reliable deep structural interpretation, a coherency processing technique was applied to the data to enhance phase coherent secondary arrivals. In addition, interpretation of a coincident wide-angle reflection study provided constraints on the deep crustal velocities and structure. The crust beneath the Central Maine synclinorium can be described by three major structural units. The upper crust is interpreted as the metasedimentary rocks of the Central Maine synclinorium with velocities that range from 6.2 to 6.3 km/s. Regions of relative lower velocity (6.0–6.1 km/s) within the upper crust are interpreted as granitic intrusions. Low velocity regions, modeled at 2.7 km depth with velocities around 5.4 km/s, are interpreted as seismic evidence for folding and doubling within the upper crustal metasedimentary sequence. At 12–15 km depth, the base of the synclinorium appears alternately as a strong reflector, indicating the presence of layering, and also as a weak reflector, especially NE of the Sebago pluton. A 4 km thick boundary layer at 24 km depth is interpreted as a major intracrustal shear zone in which the velocity increases from 6.4 to 6.8 km/s. The crust thickens from 36 to 40 km going from refraction experiment, the Quebec-Maine Transect Project. As part of this project, a 180-km-long seismic refraction profile was recorded along the axis of the Central Maine synclinorium. High near-surf ace velocities of 6.0–6.2 km/s made a conventional refraction analysis (including first arrivals) uninformative below about 10 km. Peak signal frequencies around 30 Hz, recorded at all offsets, and a trace spacing of 800 m led to severe spatial aliasing effects for secondary arrivals on the record sections. To allow a reliable deep structural interpretation, a coherency processing technique was applied to the data to enhance phase coherent secondary arrivals. In addition, interpretation of a coincident wide-angle reflection study provided constraints on the deep crustal velocities and structure. The crust beneath the Central Maine synclinorium can be described by three major structural units. The upper crust is interpreted as the metasedimentary rocks of the Central Maine synclinorium with velocities that range from 6.2 to 6.3 km/s. Regions of relative lower velocity (6.0–6.1 km/s) within the upper crust are interpreted as granitic intrusions. Low velocity regions, modeled at 2.7 km depth with velocities around 5.4 km/s, are interpreted as seismic evidence for folding and doubling within the upper crustal metasedimentary sequence. At 12–15 km depth, the base of the synclinorium appears alternately as a strong reflector, indicating the presence of layering, and also as a weak reflector, especially NE of the Sebago pluton. A 4 km thick boundary layer at 24 km depth is interpreted as a major intracrustal shear zone in which the velocity increases from 6.4 to 6.8 km/s. The crust thickens from 36 to 40 km going from the NE to the SW end of the profile. Surface rocks at the SW end of the profile have a metamorphic depth of burial of at least 15 km, indicating that the crust below the Central Maine synclinorium may have been thickened to at least 55 km during Paleozoic collisions.

Full-waveform inversion of plane-wave seismograms in stratified acoustic media: Applicability and limitations

A nonlinear waveform inversion procedure derived from Tarantola and Valette’s inversion formalism has been implemented in the horizontal slowness and intercept time (p-τ) domain. It infers simultaneously the velocity (bulk modulus) and density profiles of layered acoustic media. An arbitrary partition of the full wavefield information, i.e., an arbitrary selection of traces (and time ranges) of the whole p-τ seismic section is used as input to the inversion procedure. To explore its applicability and limitations, the algorithm has been tested against a series of “noise‐free” and noisy minimum‐phase band‐limited p-τ data sets synthesized from models of many microlayers which are meant to emulate the behavior of well logs. For a given data covariance, we confirmed that the specification of the starting model covariance determines the “generalized” damping factor which predicts the resolution of the final inverted model and the convergence rate of the inversion procedure. We found three major obstacles in th...

Structure of the crust and upper mantle beneath the Great Valley and Allegheny Plateau of eastern Pennsylvania 1. Comparison of linear inversion methods for sparse wide‐angle reflection data

We use τ(p) data extracted from a small number of wide-angle recordings of quarry blasts to construct averaged, one-dimensional velocity models of the crust beneath portions of the Great Valley, Newark Basin, Valley and Ridge, and Allegheny Plateau of the central Appalachians of Pennsylvania. Using the linear form of the equations for τ(p) in terms of slowness-depth structure, we compare inversion results for the extremal, Backus-Gilbert, and generalized least squares methods. Although the uncertainties are large, the models do show a well-constrained increase in midcrustal velocities and crustal thickness beneath the Allegheny Plateau. Inversion of precritical reflections from the Moho suggests bounds of 7.1–7.6 km/s on P wave velocity and 3.9–4.2 km/s on S wave velocity near the base of the crust. P wave velocity models for the Great Valley show a crustal thickness between 40 and 45 km, with an average velocity between 6.4 and 6.6 km/s. Models for the Allegheny Plateau show a larger crustal thickness (47–52 km) and a much higher average velocity (6.8–6.9 km/s). Estimates of average shear wave velocities for the Great Valley range from 3.6 to 3.8 km/s, with crustal thickness estimates between 37 and 44 km. For the relatively small number of singular values retained, standard errors in depth for models derived by generalized least squares range from ±1 to 2 km for the P wave slowness models and from ±2 to 4 km for the S wave models; extremal depth bounds in general are 2 to 3 times as wide. Corresponding uncertainties in interval velocity, estimated from resolving kernels in slowness, range from 0.15 to 0.45 km/s for P wave models and from 0.15 to 0.40 km/s for S wave models. T2–X2 inversion of PmP data gives similar estimates for total crustal thickness and average velocity after correction for refraction effects. T2–X2 inversion of PmP data for a fourth profile suggests the possibility of a slight thickening of the crust (47–48 km) directly beneath the axis of the Great Valley gravity low. Estimates of average VP/VS for the crust based on average velocities for models derived by generalized least squares inversion range from 1.73 to 1.77. Estimates based on travel time ratios for events interpreted as P wave and S wave reflections from the Moho lie between 1.75 and 1.79. For a crust in which the effects of velocity anisotropy can be neglected, these estimates correspond to crustal averages between 0.25 and 0.27 for Poissons ratio. The one-dimensional velocity models derived here provide estimates of the long-wavelength component of velocity structure. Besides demonstrating the maximum resolving power inherent in a limited τ(p) data set, these averaged models can be used for migrating reflection data and as starting models for determining two-dimensional velocity structure.