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John C. Behrendt

Publications and source records attributed to John C. Behrendt.

At least 37 records · Page 2Linked to original sources
USGS1996

Patterns of late Cenozoic volcanic and tectonic activity in the West Antarctic rift system revealed by aeromagnetic surveys

Aeromagnetic surveys, spaced ≤5 km, over widely separated areas of the largely ice- and sea-covered West Antarctic rift system, reveal similar patterns of 100- to 1700-nT, shallow-source magnetic anomalies interpreted as evidence of extensive late Cenozoic volcanism. We use the aeromagnetic data to extend the volcanic rift interpretation over West Antarctica starting with anomalies over (1) exposures of highly magnetic, late Cenozoic volcanic rocks several kilometers thick in the McMurdo-Ross Island area and elsewhere; continuing through (2) volcanoes and subvolcanic intrusions directly beneath the Ross Sea continental shelf defined by marine magnetic and seismic reflection data and aeromagnetic data and (3) volcanic structures interpreted beneath the Ross Ice Shelf partly controlled by seismic reflection determinations of seafloor depth to (4) an area of similar magnetic pattern over the West Antarctic Ice Sheet (400 km from the nearest exposed volcanic rock), where interpretations of late Cenozoic volcanic rocks at the base of the ice are controlled in part by radar ice sounding. North trending magnetic rift fabric in the Ross Sea-Ross Ice Shelf and Corridor Aerogeophysics of the Southeast Ross Transect Zone (CASERTZ) areas, revealed by the aeromagnetic surveys, is probably a reactivation of older rift trends (late Mesozoic?) and is superimposed on still older crosscutting structural trends revealed by magnetic terrace maps calculated from horizontal gradient of pseudogravity. Long-wavelength (∼ 100-km wide) magnetic terraces from sources within the subvolcanic basement cross the detailed survey areas. One of these extends across the Ross Sea survey from the front of the Transantarctic Mountains with an east-southeast trend crossing the north trending rift fabric. The Ross Sea-Ross Ice Shelf survey area is characterized by highly magnetic northern and southern zones which are separated by magnetically defined faults from a more moderately magnetic central zone. Aeromagnetic data in the south delineate the Ross fault of unknown age. The extension of the southern Central Basin south of the Ross fault is associated with an 825-nT magnetic anomaly over the Ross Ice Shelf requiring inferred late Cenozoic volcanic rock essentially at the seafloor at its south end, as shown by magnetic models. Models show that the thickness of magnetic volcanic rocks beneath Hut Point Peninsula at McMurdo Station is probably <2 km. The detailed surveys, combined with data from > 100,000 km of widely spaced aeromagnetic profiles, led to the interpretation of the mostly subglacial West Antarctic flood basalts(?) or their subglacially erupted and intruded equivalent. The volume of the exposed volcanos is small in contrast to the much greater volume (> 10 6 km³) of late Cenozoic magmatic rock remaining at volcanic centers beneath the continental shelf, Ross Ice Shelf and West Antarctic Ice Sheet. We suggest as an alternative or supplemental explanation to the previously proposed mantle plume hypothesis for the late Cenozoic volcanism significantly greater lower lithosphere (mantle) stretching resulting in greater decompression melting than the limited Cenozoic crustal extension allows. However, this implies a space problem that is not obviously resolved, because the Antarctic Plate is essentially surrounded by spreading centers.

Tectonics↗
USGS1995

Glacial removal of late Cenozoic subglacially emplaced volcanic edifices by the West Antarctic ice sheet

Local maxima of the horizontal gradient of pseudogravity from closely spaced aeromagnetic surveys over the Ross Sea, northwestern Ross Ice Shelf, and the West Antarctic ice sheet, reveal a linear magnetic rift fabric and numerous subcircular, high-amplitude anomalies. Geophysical data indicate two or three youthful volcanic edifices at widely separated areas beneath the sea and ice cover in the West Antarctic rift system. In contrast, we suggest glacial removal of edifices of volcanic sources of many more anomalies. Magnetic models, controlled by marine seismic reflection and radar ice-sounding data, allow us to infer that glacial removal of the associated late Cenozoic volcanic edifices (probably debris, comprising pillow breccias, and hyaloclastites) has occurred essentially concomitantly with their subglacial eruption. "Removal' of unconsolidated volcanic debris erupted beneath the ice is probably a more appropriate term than "erosion', given its fragmented, ice-contact origin. The exposed volcanoes may have been protected from erosion by the surrounding ice sheet because of more competent rock or high elevation above the ice sheet. -from Authors

Geology↗
USGS1994

CASERTZ aeromagnetic data reveal late Cenozoic flood basalts(?) in the West Antarctic rift system

The late Cenozoic volcanic and tectonic activity of the enigmatic West Antarctic rift system, the least understood of the great active continental rifts, has been suggested to be plume driven. In 1991-1992, as part of the CASERTZ (Corridor Aerogeophysics of the Southeast Ross Transect Zone) program, an ∼25000 km aeromagnetic survey over the ice-covered Byrd subglacial basin shows magnetic "texture" critical to interpretations of the underlying extended volcanic terrane. The aeromagnetic data reveal numerous semicircular anomalies ∼100-1100 nT in amplitude, interpreted as having volcanic sources at the base of the ice sheet; they are concentrated along north-trending magnetic lineations interpreted as rift fabric. Models constrained by coincident radar ice soundings indicate highly magnetic sources, with a probable high remanent magnetization in the present field direction, strongly suggesting a late Cenozoic age. Magnetic anomalies over exposed late Cenozoic volcanic rocks along part of the rift shoulder and in coastal Marie Byrd Land are similar in form and amplitude. The CASERTZ aeromagnetic results, combined with >100 000 km of widely spaced aeromagnetic profiles, indicate at least 10 6 km 3 of probable late Cenozoic volcanic rock (flood basalt?) in the West Antarctic rift beneath the ice sheet and Ross Ice Shelf. Comparison with other plumes in active rift areas (e.g., Yellowstone and East Africa) indicates that this volume estimate lies in the range of magma generation found in these other low-extension continental rifts.

Geology↗
USGS1993

Active volcanism beneath the West Antarctic ice sheet and implications for ice-sheet stability

IT is widely understood that the collapse of the West Antarctic ice sheet (WAIS) would cause a global sea level rise of 6 m, yet there continues to be considerable debate about the detailed response of this ice sheet to climate change1-3. Because its bed is grounded well below sea level, the stability of the WAIS may depend on geologically controlled conditions at the base which are independent of climate. In particular, heat supplied to the base of the ice sheet could increase basal melting and thereby trigger ice streaming, by providing the water for a lubricating basal layer of till on which ice streams are thought to slide4,5. Ice streams act to protect the reservoir of slowly moving inland ice from exposure to oceanic degradation, thus enhancing ice-sheet stability. Here we present aerogeophysical evidence for active volcanism and associated elevated heat flow beneath the WAIS near the critical region where ice streaming begins. If this heat flow is indeed controlling ice-stream formation, then penetration of ocean waters inland of the thin hot crust of the active portion of the West Antarctic rift system could lead to the disappearance of ice streams, and possibly trigger a collapse of the inland ice reservoir.

Nature↗
USGS1991

Evidence of rapid Cenozoic uplift of the shoulder escarpment of the Cenozoic West Antarctic rift system and a speculation on possible climate forcing

The Cenozoic West Antarctic rift system, characterized by Cenozoic bimodal alkalic volcanic rocks, extends over a largely ice-covered area, from the Ross Sea nearly to the Bellingshausen Sea. Various lines of evidence lead to the following interpretation: the transantarctic Mountains part of the rift shoulder (and probably the entire shoulder) has been rising since about 60 Ma, at episodic rates of ~1 km/m.y., most recently since mid-Pliocene Time, rather than continuously at the mean rate of 100 m/m.y. Uplift rates vary along the scarp, which is cut by transverse faults. It is speculated that this uplift may have climatically forced the advance of the Antarctic ice sheet since the most recent warm period. A possible synergistic relation is suggested between episodic tectonism, mountain uplift, and volcanism in the Cenozoic West Antarctic rift system and waxing and waning of the Antarctic ice sheet beginning about earliest Oligocene time.

Geology↗
USGS1991

The West Antarctic rift system, a propagating rift "captured" by a mantle plume

The West Antarctic rift system, marked by a 3-5-kilometer high shoulder from northern Victoria Land to the Ellsworth Mountains, extends through the Ross Embayment and the Byrd Subglacial Basin. Geophysical data suggest that the ice covered area beneath the rift zone is underlain by Cenozoic volcanic rocks (flood basalts?), and extended crust about 20 km thick. Exposed bimodal alkaline volcanic rocks (mostly basalts, indistinguishable from ocean island basalts that have been interpreted to be mantle plume derived) range in age from Oligocene to the present. We propose a plume (approximately ellipsoidal and coincident with the West Antarctic rift system) defined by the distribution o f K/Ba ratios of basalts, the elevated tectonomagmatic dome of coastal Marie Byrd Land, the high topography marking the rift shoulder and the inferred flood basalts(?) beneath the ice covered Byrd Subglacial Basin. Although most extension in West Antarctica apparently occurred in the late Mesozoic with Gondwana rifting, all exposed rift related volcanic rocks are post early Oligocene; the time lag is possibly explained by the proposed mantle plume. As spreading centers surrounding the stationary Antarctic plate in the Cenozoic migrated away from Antarctica, continued rifting in West Antarctica to the present was focused by the mantle plume. The plume possibly caused reorganization of the existing ridge system through a ridge jump to the hot, extended, weakened lithosphere present in the Ross Embayment-Byrd Subglacial basin area at the end of the Cretaceous. The propagating rift may have been captured by the thermal plume.

Conference Paper↗
USGS1990

GLIMPCE Seismic reflection evidence of deep-crustal and upper-mantle intrusions and magmatic underplating associated with the Midcontinent Rift system of North America

Deep-crustal and Moho reflections, recorded on vertical incidence and wide angle ocean bottom Seismometer (OBS) data in the 1986 GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) experiment, provide evidence for magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension in the Midcontinent Rift system at 1100 Ma. The rift fill consists of 20-30 km (7-10 s) of basalt flows, secondary syn-rift volcaniclastic and post-basalt sedimentary rock. Moho reflections recorded in Lake Superior over the Midcontinent Rift system have times from 14-18 s (about 46 km to as great as 58 km) in contrast to times of about 11-13 s (about 36-42 km crustal thickness) beneath the surrounding Great Lakes. The Seismically complex deep-crust to mantle transition zone (30-60 km) in north-central Lake Superior, which is 100 km wider than the rift half-graben, reflects the complicated products of tectonic and magmatic interaction of lower-crustal and mantle components during evolution or shutdown of the aborted Midcontinent Rift. In effect, mantle was changed into crust by lowering Seismic velocity (through intrusion of lower density magmatic rocks) and increasing Moho (about 8.1 km s-1 depth.

Great Lakes↗
USGS1990

Recent geophysical and geological research in Antarctica related to the assessment of petroleum resources and potential environmental hazards to their development

During the 6-year negotiation of and adoption of the Convention on the Regulation of Antarctic Mineral Resources Activities on June 2, 1988, various countries have increased their attention to the possibility of exploitation of Antarctica's petroleum resources, which are covered by this treaty. However, there are no known petroleum resources in Antarctica, and scientific information is lacking to adequately assess any undiscovered resources or the possible environmental hazards to their development. Scientific research carried out in Antarctica since the International Geophysical Year (1957-1958) has provided a great deal of information on the geological framework of this ice-covered continent and its margin and has found general indications of the types of hazards that must be considered. Antarctica covers a vast area, and likely supergiant oil fields of the type to be exploited would be tiny (e.g., a few tens of kilometers across) in comparison. Any petroleum resources located in Antarctica will be found by applying the knowledge, techniques, and experience gained developing oil fields in other parts of the world and would, therefore, be other examples of general cases; in contrast, the Antarctic environment and its associated hazards must be considered unique.

Book chapter↗
USGS1990

Multichannel seismic reflection surveys over the Antarctic continental margin relevant to petroleum resource studies

More than 100,000 km of marine multichannel seismic profiles have been acquired over the continental margin of Antarctica since 1976 by scientific research programs of Australia, Brazil, France, Italy, Japan, Norway, Poland, United Kingdom, United States, U.S.S.R. and West Germany. Although scientific results are reported for most of these data, they also are relevant to petroleum resource assessment. Because of the one or two orders of magnitude greater cost of standard land survey techniques in Antarctica compared with marine techniques in areas of open water, there will likely be no great amount of coverage on the interior of the Antarctic ice sheet. Despite this, several countries are experimenting in a research mode using land systems, and deep crustal reflection sur eys at carefully selected interior sites will probably be made soon.

Book chapter↗
USGS1986

STRIKE SLIP ON REACTIVATED TRIASSIC(? ) BASIN BOUNDARY FAULT ZONES AS SOURCES OF EARTHQUAKES NEAR CHARLESTON, S. C.

Interpretation of several thousand kilometers of multifold seismic reflection data supports the old theory that earthquakes in the Charleston, S. C. area are associated with reactivated Triassic(? ) basin boundary extensional fault zones. The Gants-Cooke fault zone associated with the Jedburg basin in the 1886 meizoseismal area, an unnamed fault along the margin of the Branchville basin in the Bowman earthquake area and the offshore Helena Banks fault zone (no observed seismicity) along the margin of the Kiawah basin show evidence of reactivation of Triassic(? ) normal faults zones in a compressional, probably strike slip sense. The previously reported reverse separation of these faults observed on the seismic profiles in the late Cretaceous-Cenozoic Coastal Plain sediments is possibly produced by oblique slip with the horizontal component possibly 10 to 100 times the vertical. Earthquake recurrence intervals of several thousand years reported in the Charleston area appear consistent with ranges of magnitude of strike slip displacement inferred from the seismic reflection data, and are constrained by aeromagnetic data.

Conference Paper↗
USGS1985

Interpretations from multichannel seismic-reflection profiles of the deep crust crossing South Carolina and Georgia from the Appalachian Mountains to the Atlantic Coast

Over the past decade, the U.S. Geological Survey (USGS) has been investigating the cause of the Charleston, S.C., earthquake of 1886 and the likelihood of future earthquakes of similar magnitude (m b 6.9&ndash;7.2, Bollinger, 1977). As part of that work, multichannel reflection surveys were started in 1979 in the Charleston area, on 1 and (Behrendt and others, 1981; Hamilton and others, 1983) and offshore (Behrendt and others, 1983). The data for lines across the continental margin were tied into the USGS offshore seismic grid in the area discussed by Dillon and others (1979). At about the same time (1978&ndash;79), Consortium for Continental Reflection Profiling (COCORP) lines in Georgia and in the Charleston, S. C., area were recorded (Cook and others, 1979; Cook and others, 1981; Schilt and others, 1983). The COCORP data for Georgia (Cook and others, 1979) and other reflection data to the northeast, as discussed by Harris and Bayer (1979), indicated the presence of the Appalachian d&eacute;collement, extending seaward from the Appalachian Mountains. The authors of these papers inferred that the Appalachian d&eacute;collement might extend across the Piedmont and Coastal Plain to the continental shelf. Subsequently, Iverson and Smithson (1982) suggested, on the basis of their reprocessing of the COCORP line in Georgia, that the d&eacute;collement was rooted in the area of the Kings Mountain and the Carolina slate belts. The multichannel seismic-reflection data for the Charleston, S. C., area (Behrendt and others, 1981, 1983; Schilt and others, 1983) provided evidence, particularly strong offshore, of the existence of a reflecting surface at a depth of 11.4&plusmn;1.5 km that was suggested as a d&eacute;collement. Behrendt and others (1981, 1983) suggested that the Charleston earthquake of 1886 might have been caused by movement on the d&eacute;collement or on associated listric faults. Seeber and Armbruster (1981) suggested that movement on the Appalachian d&eacute;collement, if it continued coastward to Charleston, might have caused the Charleston earthquake of 1886. The best determined focal depths for recent seismicity, from data recorded by a seismograph network operated by the USGS in the Charleston, S. C., area since 1973, are shallower than 13+2 km (Tarr and others, 1981; and Tarr and Rhea, 1983), or above the suggested d&eacute;collement. The seismic-reflection data have also shown the existence of several Triassic (?) basins beneath the Coastal Plain Late Cretaceous and Cenozoic sedimentary rock section (Behrendt and others, 1981; Behrendt, 1983; Costain and Glover, 1983; Hamilton and others, 1983; Petersen and others, 1984). The basins, in several cases, appear to be bounded by high-angle normal faults. Some of these faults may have been reactivated in Late Cretaceous and Cenozoic time as apparently reverse faults. Also they are suggested to be listric onto the d&eacute;collement, thereby bearing a causal relation to Charleston seismicity (Behrendt, 1983; Behrendt and others, 1983). The question of whether the Appalachian d&eacute;collement is continuous to the coast is, therefore, important not only for the general understanding of the tectonics of the southeastern United States but for an understanding of the earthquake-hazard question as well. For these reasons, three long, deep-crustal, multichannel seismic-reflection profiles (S4, S6, and S8) were obtained by the USGS to address the problem. This report presents illustrations of interpretations of the profiles discussed by Behrendt (in press). The Appalachian d&eacute;collement does not appear continuous from the Appalachian Mountains to the coast but rather appears to extend southeastward only to the Carolina slate belt. A series of reflections on lines S4, S6, and S8 and on the COCORP line is interpreted as evidence of southeastward-dipping imbricate faults, from the Brevard fault on the northwest to beyond the Augusta fault, which marks the southeastern extent of the Eastern Piedmont fault zone. The Carolina slate belt is characterized on the four seismic profiles by a complex series of diffractions and reflections extending from less than 1 s to 8 s. These arrivals are possibly the result of layering in the metasedimentary rocks complexly disrupted by the imbricate faults. A number of Triassic (?) basins are apparent in the reflection data for the rifted Charleston terrane identified from low-gradient magnetic anomalies.

Miscellaneous Field Studies Map↗