This chapter contains sections titled: The Mantle The Oceanic Layer (Layer 3) The Basement (Layer 2) The Sediments (Layer 1)

Low‐frequency sound propagation in shallow water environments is not restricted to the water column but also involves the subbottom. Thus, as well as being important for geophysical description of the seabed, subbottom velocity/attenuation structure is essential input for predictive propagation models. To estimate this structure, bottom‐mounted sources and receivers were used to make measurements of shear and compressional wave propagation in shallow water sediments of the continental shelf, usually where boreholes and high‐resolution reflection profiles give substantial supporting geologic information about the subsurface. This colocation provides an opportunity to compare seismically determined estimates of physical properties of the seabed with the “ground truth” properties. Measurements were made in 1986 with source/detector offsets up to 200 m producing shear wave velocity versus depth profiles of the upper 30–50 m of the seabed (and P wave profiles to lesser depths). Measurements in 1988 were made with smaller source devices designed to emphasize higher frequencies and recorded by an array of 30 sensors spaced at 1‐m intervals to improve spatial sampling and resolution of shallow structure. These investigations with shear waves have shown that significant lateral and vertical variations in the physical properties of the shallow seabed are common and are principally created by erosional and depositional processes associated with glacial cycles and sea level oscillations during the Quaternary. When the seabed structure is relatively uniform over the length of the profiles, the shear wave fields are well ordered, and the matching of the data with full waveform synthetics has been successful, producing velocity/attenuation models consistent with the subsurface lithology indicated by coring results. Both body waves and interface waves have been modeled for velocity/attenuation as a function of depth with the aid of synthetic seismograms and other analytical techniques. Some results give strong evidence of anisotropy and lateral heterogeneity in shear velocity of the upper 5–10 m of sediments and of extremely high velocity gradients in the topmost 1–2 m, possibly exceeding 30 s−1.

Bottom-mounted sources and receivers have been used to measure shear wave velocity and attenuation in sediments of the U.S. East Coast continent shelf. Experimental results are compared with borehole and other subseafloor geologic information. Shear sources were designed primarily for generating SH waves but also produced SV and P waves. Receiver nodes containing orthogonal geophone or accelerometer sensors, plus hydrophone, provided four-component data, each component producing unique information on wave type, velocity/attenuation structure, scattering, lateral heterogeneity, and anisotropy. Measurements were made with two systems; one with a large source and 4–8 m sampling interval to a maximum range of 200 m, the other with a smaller source and 1-m sampling to a range of 30 m. Velocity and attenuation are estimated by matching recorded data with full-waveform synthetic seismograms. [Work supported by ONR.]

Data from a newly developed 30-element accelerometer/hydrophone array are used to study seismoacoustic propagation in shallow-water locations off New Jersey and Martha's Vineyard. The elements of the array have 1-m spacing; each consisting of three orthogonal accelerometers (flat from 2 to 500 Hz), a hydrophone, and a vertical direction sensor. The 120 accelerometer/hydrophone signals and 30 vertical direction signals are digitized and transmitted via 1 km of fiber optic cable to a PC-type recording system. The A/D converters digitize all hydrophones at 2048 samples/s and all accelerometers at 512 samples/s. Good coupling to the bottom is obtained since sensor symmetry has been maximized; coupling to the water has been minimized; and density is matched to the sediments. There are 10-m extensions between the 30-m sensor section and the A/D converter housing at either end of the array. Units containing port-starboard paired, shotgun-shell sources were mounted within the array in these extensions and fired through the array circuitry, providing precise time and distance control and reverse profile capability. Differenced shot pairs emphasize transverse horizontal shear (SH) and Love wave signals; summed pairs emphasize P, SV, and Rayleigh/Stoneley/Scholte waves. Shear boundary wave energy is observed with frequencies greater than 60 Hz and velocities and wavelengths as low as 20 m/s and 1 m, respectively. Considerable energy appears to result from lateral heterogeneity and/or anisotropy. [Research supported by ONR.]

Abstract Several notable papers have been published recently that review our knowledge of the seismic structure of oceanic crust (e.g. Christensen and Salisbury, 1975; Tréhu and others, 1976; Kennett, 1977; Lewis, 1978; Ewing and Houtz, 1979; Spudich and Orcutt, 1980a; Houtz, 1980; White, 1984). To reduce duplication of those previous efforts, this paper will emphasize evaluation of the resolving power of existing seismic data in the Western Atlantic Ocean and will compare this evaluation with the scale of local and regional variations in crustal structure predicted by reasonable geological models. We shall not focus attention on the relationship between the seismic and petrological structure of the crust and will use the terms Layer 2, Layer 3, and Moho only in their original seismic sense. Layer 2 is a region of high velocity gradients (perhaps as high as 3s−1 at its upper surface), with velocities in the range 3 to 6.5 km/s that extends from the seafloor to depths of 2–3 km. (We neglect sediments in this chapter, restricting ourselves to the igneous crustal column.) Layer 3 is typically about 4 km in thickness, has velocities of 6.7 to 7.2 km/s, and is characterized by low velocity gradients, generally less than 0.1 s−1. The Mohorovicic discontinuity (Moho) is actually a transition zone varying in thickness from a few hundred meters (or less) to 1–2 km and is almost always the most prominent seismic characteristic of the ocean crust. Velocities increase by about 1 km/s

Mr. President, fellow members of the American Geophysical Union, and members of the U.S. Navy, it gives me great pleasure to present the citation for the 1984 AGU/USN Maurice Ewing Medal, to be awarded to Dr. Xavier Le Pichon.After receiving diplomas in several disciplines of geology, physics, and geophysics from the University of Strasbourg during the 1950s, Xavier came to the Lamont‐Doherty Geological Observatory as a visiting scientist where he put his knowledge to practice until 1968. In 1966 he received the Doctor of Sciences degree from the University of Strasbourg. Returning to France in 1968, Xavier spent the next five years at the Centre Océanologique de Bretagne in Brest where he founded the Research Group. From Brest he moved to the headquarters of CNEXO in Paris for 5 years and then to the University of Paris to found the new Laboratoire de Géodynamique. From his present position of professor at the university he will move next year to become director of the Geology Laboratory in the Ecole Normale Supérieure, one of the French Grandes Ecoles.

Velocity solutions from 165 airgun-sonobuoy reflection/refraction stations on the continental margin of southernmost South America are presented in 10 seismic structure sections. Also included are an isopach map of Cretaceous and Tertiary sediments (seismic velocities generally less than 4.2 km/sec) in the Magellan, Malvinas, Falkland Plateau, and Falkland Trough basins, and three north-south single-channel seismic reflection traverses of the Falkland Plateau, Falkland Trough, and North Scotia Ridge. The Falkland Trough is a silver of oceanic crust between the Falkland Plateau and the North Scotia Ridge. The acoustic basement and deep sedimentary layers of the plateau extend beneath the trough and then are subducted beneath the northern flank of the ridge. Movement of the ridge and the plateau toward each other has apparently resulted in the deformation and uplift of the overlying sediments to form the northern flank of the ridge.

Knowledge of continental margins advanced rapidly during the 1970s. Multichannel seismic reflection whose cost formerly restricted its use largely to the immediate vicinity of shallow-water prospects has become more common in deeper waters. The use of the technique by government and academic groups helped solve basc structural and evolutionary problems of rocks of the deeper offshrore. Better sources and more sophisticated processing yielded better and deeper resolution of the data. To better disseminate new knowledge of continental margins, AAPG held three meetings in 1977 to review the current status of knowledge. The papers presented at those meetings are contained in this volume. There are 32 chapters divided into the following sections: Rifted Margins; Convergent Margins; Small Basin Margins; and Resources, Comparative Structure, and Eustatic Changes in Sea Level.

Recently obtained high-energy multichannel seismic reflection profiles across the Blake Escarpment show that the oceanic basement, identified as a hyperbolic reflector, forms a deep sediment-filled trough at the base of the escarpment. The western boundary of the trough is formed by a steep rise (45°-60°) of acoustic basement producing an apparent structural relief of more than 2.5 sec for the trough. Seismic velocities were determined as 3.23 to 3.95 km/sec and indicate 4.03 to 4.94 km of sediments in the trough, which deepens to approximately 11 to 12 km. The steep west side suggests fault control and deposition in a fault-bounded half-graben trough. Undisturbed sediments lie over Horizon s which passes across the trough without deflection on one profile, nd which is the shallowest disturbed and upturned reflector on another profile. Correlation of reflectors involved in the structure of the trough with the nearby DSDP Site 391 indicates that the deeper seismic layers include sediments older than Late Jurassic. These old sediments filling the fault-bounded trough at the base of the Blake Escarpment should include facies deposited during the earliest opening of the Atlantic, perhaps in the Early Jurassic. The seismic reflection profiles also show reflectors at the edge of the Blake Plateau which can be correlated with DSDP Drill Site 390. Important reflectors of earliest Eocene, Campanian/Albian and Barremian age can be traced westward to depths of 3.4 km (2.8 sec) under the Blake Plateau. Hyperbolic reflectors, not associated with igneous basement, occurring over acoustically opaque zones are interpreted to be well-cemented, high-velocity carbonate bank-margin complexes as drilled at DSDP Site 392.

Knowledge of continental margins advanced rapidly during the 1970s. Multichannel seismic reflection whose cost formerly restricted its use largely to the immediate vicinity of shallow-water prospects has become more common in deeper waters. The use of the technique by government and academic groups helped solve basc structural and evolutionary problems of rocks of the deeper offshrore. Better sources and more sophisticated processing yielded better and deeper resolution of the data. To better disseminate new knowledge of continental margins, AAPG held three meetings in 1977 to review the current status of knowledge. The papers presented at those meetings are contained in this volume. There are 32 chapters divided into the following sections: Rifted Margins; Convergent Margins; Small Basin Margins; and Resources, Comparative Structure, and Eustatic Changes in Sea Level.

Each Lamont-Doherty sonobuoy located on well-dated crust has been carefully analyzed to determine crustal structure down to oceanic layer 3. Results from the Atlantic and the Pacific are compiled separately in order to study crustal structure as a function of plate age in both oceans, since they have very different spreading rates. Layer 2A (refraction velocity about 3.6 km/s) in the North Atlantic is 1.5 km thick at the ridge crest and thins consistently to about 100 m as the crust ages to about 60 m.y. Layer 2A in the east Pacific is 0.7 km thick at the ridge and thins to about 100 m at about 30 m.y. This difference in thickness is probably attributable to the much faster spreading rate in the Pacific. A poorly refractive acoustic basement layer about 200 m thick with similarities to layer 2A but not necessarily composed of the same materials is measured sporadically in the Pacific M-Series plates and even less consistently in the Atlantic. This layer is not recorded in the Cretaceous or the Jurassic quiet zones. Regressions of refraction velocities in layer 2A as a function of age show that its velocity increases from about 3.3 km/s at the ridge crests to that of layer 2B on crust about 40 m.y. old. There is no corresponding increase of velocity with age in any of the deeper layers. The high resolution of the air gun/sonobuoy records shows that the layer 2B refraction line breaks directly to layer 3 velocities (46 times in the present work) or to a line with a velocity of 6.1 km/s (114 times in the present work), which we call layer 2C. The variance of the velocities in 2C is one fourth that of 2A and 2B, which indicates relative lithological uniformity in 2C. It does not seem likely that layer 2A really thins; what appears to be a thinning of the layer may actually be the result of an increase of its refraction velocity with age. However, the 2A/2B interface is well-defined by large amplitude refractions from 2B on crust that is younger than about 30 m.y., which seems to rule out a transitional zone at the base of layer 2A where it ‘converts’ to 2B. The seismic observations seem to require a diagenetic process or repeated basaltic intrusions; both processes raise serious objections.

Research Article| October 01, 1975 Vema fracture zone transform fault Stephen Eittreim; Stephen Eittreim 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar John Ewing John Ewing 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 109642Present address: (Ewing) U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information Stephen Eittreim 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 John Ewing 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 109642Present address: (Ewing) U.S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1975) 3 (10): 555–558. https://doi.org/10.1130/0091-7613(1975)3<555:VFZTF>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Stephen Eittreim, John Ewing; Vema fracture zone transform fault. Geology 1975;; 3 (10): 555–558. doi: https://doi.org/10.1130/0091-7613(1975)3<555:VFZTF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Airgun reflection profiles indicate that the relict Aleutian abyssal plain consists of two tongues of well-stratified sediments deposited from turbidity currents which entered the region from a northerly direction. The distribution of the turbidites appears to have been largely governed by a topographic grain imparted to the basement surface by Late Cretaceous-Paleocene plate motions associated with the generation of the Great Magnetic Bight. High resolution 3.5 kHz sub-bottom profiles and sedimentation rate data suggest that the last channelized routes of turbidity current flows to the southern portion of the area were severed during the Late Miocene, 6.9 (±0.9) m.y. ago, and since then only pelagic sediments have accumulated. The change in the depositional regime can thus be associated in time with the change in the direction of sea-floor spreading in the northeastern Pacific and to the start of a period of severe deformation in southern Alaska. The pattern of sedimentation and the time at which the Aleutian plain became isolated from its source of terrigenous sediments are both consistent with models recently proposed for plate motions in the northeastern Pacific since the close of the Mesozoic. The present sedimentary data are not sufficient, however, to determine whether large relative motions between the Pacific and North American plates may have occurred during Tertiary time.