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John C. Behrendt

Publications and source records attributed to John C. Behrendt.

At least 55 records · Page 3Linked to original sources
USGS1983

Studies related to the Charleston, South Carolina, earthquake of 1886 — Tectonics and seismicity

Since 1973, the U.S. Geological Survey (USGS), with support from the Nuclear Regulatory Commission, has conducted extensive investigations of the tectonic and seismic history of the Charleston, S.C., earthquake zone and surrounding areas. The goal of these investigations has been to discover the cause of the large intraplate Charleston earthquake of 1886, which dominates the record of seismicity in the Southeastern United States, through an understanding of the historic and modern seismicity at Charleston and of the tectonic setting of the seismicity. This goal is being pursued to evaluate the potential for additional large earthquakes in the Charleston area and surrounding regions and to determine whether the Charleston area differs tectonically in any significant fashion from other parts of the Southeastern United States. An understanding of the specific cause for the 1886 event and of the regional distribution of any structures that are generically related to or geometrically and mechanically similar to the source structure is essential for evaluation of seismic hazards throughout the Southeast. The results given herein represent significant progress toward understanding the tectonic setting of the Charleston-area seismicity. Several chapters in the volume address the distribution and origin of pre-Cretaceous rocks and structures beneath Coastal Plain sediments in the Charleston area and regionally beneath the southern Atlantic Coastal Plain and adjacent Continental Shelf. The modern seismicity at Charleston is occurring at depths equal to or greater than the known extent of these older structures, and rejuvenation of an older fault in the modern stress field is a possible cause of the seismicity. Accordingly, several chapters discuss the possible relationships of the various pre-Cretaceous structures to faults identified near Charleston that have a known Cretaceous and Cenozoic movement history and to the historic and instrumentally recorded seismicity. However, at the present time, none of the young structures can be related unequivocally to the seismicity because earthquake fault-plane solutions and hypocenter distributions do not agree with the locations and orientations of these structures. Therefore, a major emphasis of continuing USGS investigations near Charleston will be to identify additional faults, if any exist, to delineate fault movement histories, and to further refine earthquake locations, focal mechanisms, and related seismological interpretations.

Florida, Georgia, South Carolina↗
USGS1980

Aeromagnetic and radio echo ice-sounding measurements show much greater area of the Dufek intrusion, Antarctica

A combined aeromagnetic and radio echo ice-sounding survey made in 1978 in Antarctica over the Dufek layered mafic intrusion suggests a minimum area of the intrusion of about 50,000 square kilometers, making it comparable in size with the Bushveld Complex of Africa. Comparisons of the magnetic and subglacial topographic profiles illustrate the usefulness of this combination of methods in studying bedrock geology beneath ice-covered areas. Magnetic anomalies range in peak-to-trough amplitude from about 50 nanoteslas over the lowermost exposed portion of the section in the Dufek Massif to about 3600 nanoteslas over the uppermost part of the section in the Forrestal Range. Theoretical magnetic anomalies, computed from a model based on the subice topography fitted to the highest amplitude observed magnetic anomalies, required normal and reversed magnetizations ranging from 10 -3 to 10 -2 electromagnetic units per cubic centimeter. This result is interpreted as indicating that the Dufek intrusion cooled through the Curie isotherm during one or more reversals of the earth's magnetic field.

Science↗
USGS1980

High-sensitivity aeromagnetic survey of the U.S. Atlantic continental margin

The U.S. Geological Survey contracted a high-sensitivity, digital aeromagnetic survey that was flown over the U.S. Atlantic continental margin over a period of 15 months between 1974 and 1976. The 185,000 km of profile data have a relative accuracy approaching a few tenths of a nanotesla, which allowed compilation into maps at a scale of 1:250,000, with a contour interval of 2 nT. Automatic data processing using the Werner method allowed calculations of apparent depth to sources of the magnetic anomalies on all of the profiles, assuming a dike or interface as a source. Comparison of the computed depths to magnetic basement with multichannel seismic profiles across the survey area helped to reduce ambiguities in magnetic depth estimates and enabled interpolation of basement structures between seismic profiles. The resulting map showing depth to basement of the Atlantic continental margin is compatible with available multichannel seismic data, and we consider it a reasonable representation of the base of the sedimentary column.

Geophysics↗
USGS1979

Seismic refraction study of the continental edge off the eastern United States

Three long, strike-parallel, seismic-refraction profiles were made on the continental shelf edge, slope and upper rise off New Jersey during 1975. The shelf edge line lies along the axis of the East Coast Magnetic Anomaly (ECMA), while the continental rise line lies 80 km seaward of the shelf edge. Below the unconsolidated sediments (1.7–3.6 km/sec), high-velocity sedimentary rocks (4.2–6.2 km/sec) were found at depths of 2.6–8.2 km and are inferred to be cemented carbonates. Although multichannel seismic-reflection profiles and magnetic depth-to-source data predicted the top of oceanic basement at 6–8 km beneath the shelf edge and 10–11 km beneath the rise, no refracted events occurred as first arrivals from either oceanic basement (layer 2, approximately 5.5 km/ sec) or the upper oceanic crust (layer 3A, approximately 6.8 km/sec). Second arrivals from 10.5 km depth beneath the shelf edge are interpreted as events from a 5.9 km/sec refractor within igneous basement. Other refracted events from either layers 2 or 3A could not be resolved within the complex second arrivals. A well-defined crustal layer with a compressional velocity of 7.1–7.2 km/sec, which can be interpreted as oceanic layer 3B, occurred at 15.8 km depth beneath the shelf and 12.9 km beneath the upper rise. A well-reversed mantle velocity of 8.3 km/sec was measured at 18–22 km depth beneath the upper continental rise. Comparison with other deep-crustal profiles along the continental edge of the Atlantic margin off the United States, specifically in the inner magnetically quiet zone, indicates that the compressional wave velocities and layer depths determined on the U.S.G.S. profiles are very similar to those of nearby profiles. This suggests that the layers are continuous and that the interpretation of the oceanic layer 3B under the shelf edge east of New Jersey implies progradation of the shelf outward over the oceanic crust in that area. This agrees with magnetic anomaly evidence which shows the East Coast Magnetic Anomaly landward of the shelf edge off New Jersey and with previous seismic reflection data which reveal extensive outbuilding of the shelf edge during the Jurassic and Lower Cretaceous, probably by carbonate bank-margin accretion.

East Coast Magnetic Anomaly↗
USGS1976

Regional geologic framework off northeastern United States

Six multichannel seismic-reflection profiles taken across the Atlantic continental margin off the northeastern United States show an excess of 14 km of presumed Mesozoic and younger sedimentary rocks in the Baltimore Canyon trough and 8 km in the Georges Bank basin. Beneath the continental rise, the sedimentary prism thickness exceeds 7 km south of New Jersey and Maryland, and it is 4.5 km thick south of Georges Bank. Stratigraphically, the continental slope–outer edge of the continental shelf is a transition zone of high-velocity sedimentary rock, probably carbonate, that covers deeply subsided basement. Acoustically, the sedimentary sequence beneath the shelf is divided into three units which are correlated speculatively with the Cenozoic, the Cretaceous, and the Jurassic-Triassic sections. These units thicken offshore, and some have increased seismic velocities farther offshore. The uppermost unit thickens from a fraction of a kilometer to slightly more than a kilometer in a seaward direction, and velocity values range from 1.7 to 2.2 km/sec. The middle unit thickens from a fraction of a kilometer to as much as 5 km (northern Baltimore Canyon trough), and seismic velocity ranges from 2.2 to 5.4 km/sec. The lowest unit thickens to a maximum of 9 km (northern Baltimore Canyon), and velocities span the 3.9 to 5.9-km/sec interval. The spatial separation of magnetic and gravity anomalies on line 2 (New Jersey) suggests that in the Baltimore Canyon region the magnetic-slope anomaly is due to edge effects and that the previously reported free-air and isostatic gravity anomalies over the outer shelf may be due in part to a lateral increase in sediment density (velocity) near the shelf edge. The East Coast magnetic anomaly and the free-air gravity high both coincide over the outer shelf edge on line 1 (Georges Bank) but are offset by 20 km from the ridge on the reflection profile. Because the magnetic-slope-anomaly wavelength is nearly 50 km across, a deep source is likely. In part, the positive free-air gravity anomaly likewise may represent the significant lateral density increase within the sedimentary section toward the outer edge of the shelf.

American Association of Petroleum Geologists Bulle↗
USGS1974

Geophysical evidence for the intersection of the St Paul, Cape Palmas and Grand Cess fracture zones with the continental margin of Liberia, West Africa

PUBLISHED reconstructions of Gondwana continent 1 (Fig. l a ) show a gap in fit near the junction of the Americas and Africa. To study this critical area, the Unitedgeo I made geophysical measurements and collected rock samples across the continental margin of Liberia (USGS-IDOE cruise leg 5) in November 1971. Figure I b indicates the location of the 5,400 km of ship track on a generalised bathymetric map 2 . We shall discuss the data in detail elsewhere. Here we present the evidence for the existence of three fracture zones, two of which have not been reported previously, intersecting the continental margin at the north end of the South Atlantic, which remained closed probably until Cretaceous time. We suggest that Precambrian structures on the African continent controlled the location of these fracture zones. Figure Ic compares gravity and magnetic profiles and interpretations of the seismic profiles for three selected lines (27, 30 and 34) crossing the Grand Cess, Cape Palmas and St Paul fracture zones, respectively.

Nature↗
USGS1974

Structure of the continental margin of Liberia, West Africa

Geophysical surveys made by R/V Unitedgeo I (USGS–IDOE Cruise Leg 5), combined with earlier surveys and available geologic information, provide the basis for interpreting the structure of the continental margin of Liberia. This area lies at the junction of the Americas and Africa in published reconstructions of Gondwanaland prior to the opening of the North and South Atlantic in Jurassic and Cretaceous time, respectively. Three fracture zones (St. Paul, Cape Palmas, and Grand Cess) are inferred in the area southeast of 9°30′ W. on the basis of magnetic and gravity data, which is supported by bathymetric and seismic reflection data. The three fracture zones appear to exist as separate lineaments near the African coast. Farther seaward, they may be part of the same transform fault crossing the Atlantic (St. Paul fracture zone). The magnetic anomalies associated with these fracture zones, which may have originated in Cretaceous time at the opening of the South Atlantic, are continuous with magnetic anomalies over crust of Eburnean age (∼2,000 m.y.) in southeast Liberia and its continental shelf. This suggests that Eburnean age structures may have been zones of weakness that were reactivated in Cretaceous time. A positive gravity anomaly (∼50 mgal) along the coast and continental shelf of Liberia is attributed to deep crustal rocks that were uplifted and exposed in Pan-African time (∼550 m.y.). The land boundary of this anomaly coincides with a shear zone that marks the boundary between the Pan-African and the Liberian age province (∼2,700 m.y.); the shearing (in a thrust-fault sense) may be the result of compressive stress associated with the closing of a proto-Atlantic ocean. Liberian age magnetic anomalies in the area northwest of 9°30′ W. cross the Pan-African province (and the positive coastal gravity anomaly) and continue over the continental shelf and slope to about the 3,000-m bathymetric contour; the seaward limit of the anomalies is interpreted as representing the seaward limit of the old continental crust. This westward extension of the continental crust does not completely fill the gap in fit in various published reconstructions of Gondwanaland, and we suggest that the northern Florida block may have been located near the Liberian margin at one time. Magnetic data indicate a thick section of sedimentary rock, possibly as great as 8 km, on the continental slope. Comparison of gravity data over magnetically inferred basins in the shelf, slope, and rise suggests that low-density sedimentary rocks constitute a greater proportion of the section in basins beneath the shelf and beneath the slope and rise northwest of 9°30' W. than beneath the slope and rise in the area of the fracture zones. The gravitational attraction that corresponds to a crust-mantle boundary dipping 45° to 60° can be computed to fit observed data – as might be expected at a rifted continental margin. A shallow high-density block beneath the coast and continental shelf is required to fit the coastal positive anomaly; this block is represented by exposures on land of granulite-grade metamorphic rock of the Pan-African province.

Atlantic Ocean↗