http://www.geo.arizona.edu/~reiners/geos474-574/Austrheim1987EPSL.pdf

Eclogitization of lower crustal granulites by fluid migration through shear zones

Exposed cross-sections through the continental crust: implications for crustal structure, petrology, and evolution

Seismic reflection profiles provide the highest resolution information on the insitu structure and physical properties of the lower continental crust and Moho. The seismic properties most readily obtained from reflection profiles are reflectivity patterns, which correlate with distinct geologic settings. When reflectivity patterns are interpreted with complementary seismic velocity and non-seismic crustal parameters, inferences regarding the composition and evolution of the lower crust can be made. The study of the lower continental crust with seismic reflection data is a rapidly evolving field; improved data sets continually provide new insights and alter initial concepts based on more limited data. In this chapter we summarize the most important observations and conclusions derived from reflection seismology and relate these to models of the physical properties and evolution of the lower crust. We focus particularly on the structure of the lower crust in diverse tectonic settings, the nature of the Moho, and the multi-genetic origin of crustal reflectivity. The reader should consult the companion papers by Holbrook et al. (this volume) and Jones (this volume) for the related topics of seismic velocities and conductivity in the lower crust.

Multi-genetic origin of crustal reflectivity: a review of seismic reflection profiling of the continental lower crust and Moho.

The question of the structure of the Wind River uplift, a Laramide foreland structure in Wyoming, has been answered by Consortium for Continental Reflection Profiling (Cocorp) deep crustal reflection profiling and by gravity interpretation. Wyoming uplifts are asymmetrical anticlines whose steep limb is commonly cut by a reverse fault or thrust. If the thrust continues into the crust with low dip, the uplift is caused by horizontal movements; and if the fault steepens into a high-angle reverse fault in depth, the uplifts are caused by vertical movement. The Wind River uplift is the largest of the Laramide structures in Wyoming, and the thrust fault has 14 km of vertical separation and 26 km of horizontal separation. About 168 km of crustal reflection profiling has been completed by Cocorp across the uplift. The Wind River thrust fault generates a continuous reflection from near the surface to a depth of about 24 km, and the thrust appears as a complex zone of faults. Apparent dip of the thrust is 30°–38°, and true dip may be up to 48°. Interpretation of deep crustal reflections is complicated by the presence of multiple reflections. A structure whose complexity approaches that of basement outcrops is found in the deep crust. Gravity modeling also suggests that the fault dips moderately and that dense rocks are uplifted in the deep crust. The Moho does not seem to be presently displaced by faulting. The crust in the Wind River uplift began to deform by large-scale folding and then broke and faulted as a rigid slab. Faulting is caused by crustal shortening from compression that is related to plate movements.

Structure of the Laramide Wind River Uplift, Wyoming, from Cocorp deep reflection data and from gravity data

Seismic reflection surveys carried out by the Consortium for Continental Reflection Profiling (COCORP) in the Rio Grande Rift near Socorro, New Mexico, have successfully mapped large-scale structural variations down to the base of the crust. A total of 155 km of 24-fold common reflection point coverage was obtained parallel and transverse to major rift structure, including a continuous east-west profile which spans the central rift from the Sierra Lucero on the west (at the edge of the Colorado Plateau) to the Manzano Mountains on the east. Stacked seismic reflection sections derived from these surveys indicate coherent reflected energy from depths as great as 35 km, with substantial evidence of structural complexity extending throughout the crust. Among the major features apparent on these sections are (1) large-scale relief on the floor of the Albuquerque Basin, including a major intragraben horst standing over 3 km above the surrounding deep basin floors, and an extensive, shallow structural bench beneath the southeast portion of the basin, (2) well-developed, antithetic Tertiary normal faulting on the east side of the rift, (3) marked lateral and vertical variation in intrabasement reflection character, including relatively long correlatable events, distinct bands of short, discontinuous reflections, and seismically transparent zones characterized by few, if any, reflections, (4) a complex band of reflections from depths appropriate for the Moho, and (5) unusually strong, coherent reflections from a depth of approximately 20 km, corresponding closely with the top of an active magma body previously postulated to exist beneath the rift in this area on the basis of microearthquake studies. The magma body reflections are among the most prominent deep events yet recorded by COCORP; their geometry and relationship to surrounding structural elements provide important constraints on the depth of brittle faulting and new insight on magma migration and accumulation. Extensional faulting appears to be primarily near vertical near the surface with no clear evidence of large scale listric faulting at depth. The eastern rift boundary at Abo Pass is marked by a high-angle planar zone devoid of coherent reflections, possibly representing intense structural disruption extending through the crust. In contrast, the seismic section across the western rift boundary is defined by a gently dipping, relatively simple reflection of uncertain origin, with little evidence of a throughgoing fault in the lower crust. The seismic characteristics of many of the intrabasement features suggest particular igneous and/or metamorphic origins and support models which emphasize lateral and vertical heterogeneity at depth.

Deep structure of the Rio Grande Rift from seismic reflection profiling

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.

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

As a part of the fundamental scientific investigation carried out in connection with the atomic bomb tests of 1946, a seismic refraction survey was made by Joint Task Force One to obtain information on the subsurface structure of Bikini Atoll. Naval depth charges were fired on the lagoon bottom along four lines extending across the atoll and the resulting seismic waves were picked up by water-coupled microphones near shore. A total of 126 charges were exploded in the survey. The time-distance curves indicate the existence of three zones with different sound velocity beneath the lagoon floor. For the uppermost layer, averaging about 2500 feet thick, the speed is 7000 ft/sec. Below this is a layer ranging from approximately 5000 to 10,000 feet in thickness with a seismic velocity of 11,000 ft/sec. Underlying the latter is a zone of undetermined thickness having a speed of 17,000 ft/sec. The recent boring, 2556 feet deep, on Bikini Island appears to confirm the conclusion reached from the seismic data alone that the uppermost zone consists of diverse calcareous materials such as are encountered on the sea floor within and around atolls at shallow depths,— i.e., a few hundred feet at the most. Results of a vertical velocity survey to 1800 feet in the same hole show a continuous transition from a velocity of 7000–11,000 ft/sec. It appears probable, therefore, that no essentially different rock materials enter into the composition of the atoll down to the top of the 17,000 ft/sec zone, and that the change in velocity is due mainly to progressive compaction and cementation of the calcareous sediments. In this case the 11,000 ft/sec layer indicated by the refraction data might consist of such sediments in a state of maximum cementation or of finer calcareous material, deposited in deeper water, that might have undergone greater compaction than the overlying coarser sediments. Parts of it, or even most of it, may be dolomitized; it might possibly consist mainly of pyroclastic sediments, although this seems less likely. Only a boring can provide a positive identification. The zone with a velocity of 17,000 ft/sec appears to represent the igneous basement. Its surface has considerable relief, the highest portion being near the center of the atoll about 7000 feet below sea level. From this summit a sharp nose plunges southeastward to the atoll9s edge, the greatest observed depth being about 13,000 feet. The seismic data suggest several possible histories of Bikini Atoll, but all require relative subsidence in the thousands of feet. If the material down to the 17,000 ft/sec zone is calcareous sand, the seismic evidence calls for a minimum of 7000 feet of subsidence. If a terrace on the surface of this zone at 13,000 feet is attributed to subaerial erosion, a minimum of 13,000 feet of subsidence is then required. If, on the other hand, the intermediate zone is identified as volcanic or pelagic material, it is still necessary to assume that there was at least 2000 feet of subsidence. Thus, for Bikini at least, the seismic results tend to favor the conception, first advanced in its simplest form by Darwin, that atolls are formed by long-continued subsidence of initial noncalcareous islands or shallow banks.

Subsurface constitution of bikini atoll as indicated by a seismic-refraction survey

The normal-mode theory of Pekeris has been applied to the water-wave dispersion recorded from the under-water explosions set off in the 1946 seismic-refraction shooting in Bikini lagoon. The aim was to ascertain from the dispersion data the velocity of sound in the sediments down to several hundred feet below the lagoon bottom and thereby gain information about the recent sedimentary history of the lagoon. Because of the lack of short shots this information could not be obtained from the refraction results. The principal merit of the dispersion method here is that the long refraction shots supplied some information about the superficial layers. Information on the first 100 feet or so of sediments below the lagoon bottom was sought from the “high-frequency” dispersion of water waves 50 cps and higher. The data from shots along the edge of the lagoon within 1-2 miles of the reefs and islands appear to be compatible both with normal-mode predictions for a liquid bottom and with refraction indications. From these data it is concluded that the speed within 20 feet of the water bottom averages 1.05 times that in water, with a rapid increase (probably from compaction) to a speed 1.3 times that in water at a depth of about 40 feet. Shots fired somewhat farther from the edge of the lagoon give dispersion curves indicating bottom speeds (on the basis of the Pekeris theory for liquid bottoms) much higher than the refraction data allow. Shots fired in the more central portion of the lagoon give dispersion patterns to which the Pekeris theory does not appear to apply. The author attributes the discrepancy to a lateral variation in sedimentary composition between the edge and center of the lagoon, possibly a downward continuation of the surface change from sand to Halimeda debris observed by Emery on going toward the lagoon9s center. The Airy waves (high-amplitude, low-frequency waves at minimum group velocity) give information on the bottom structure to about 400 feet. In virtually all parts of the lagoon their group velocity and frequency are as predicted by the Pekeris theory for a bottom velocity of 6500 ft/sec. This agrees with the first branch of the refraction travel-time curves, which were drawn from very limited data. That the Airy waves fit the theory for shots fired near the center of the lagoon while the higher-frequency waves do not suggests that the lateral change in sediment type within the uppermost layer disappears within a depth of about 100 feet. The change comes at a depth which can only be estimated roughly as 300 feet below sea level and was detected only in the central portion of the atoll.

Submarine geology of bikini lagoon as indicated by dispersion of water-borne explosion waves

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Bikini and nearby atolls, Marshall Islands; Part 3, Geophysics : Seismic studies of Bikini Atoll

During the past decade the digital computer has brought about revolutionary improvements in our capability to study geology by the seismic reflection method. New recording and processing technology has brought us much closer than anyone would have predicted ten years ago to our ultimate goal of extracting the same geological information that would be retrievable if there were a borehole to the maximum depth of interest at every shot point.

Milton B. Dobrin

Papers

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

Subsurface constitution of bikini atoll as indicated by a seismic-refraction survey

Submarine geology of bikini lagoon as indicated by dispersion of water-borne explosion waves

Bikini and nearby atolls, Marshall Islands; Part 3, Geophysics : Seismic studies of Bikini Atoll

Use of Computers in Seismic Reflection Prospecting