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At least 613 records · Page 34Linked to original sources
USGS1970

The Uralides and the motion of the Russian and Siberian Platforms

The Uralides—the late Precambrian and Paleozoic orogenic terrane between the Russian and Siberian Platforms—in part are exposed in the Ural Mountains, in the central Soviet Arctic, along the west edge of the Siberian Platform, and in southern Siberia and Kazakhstan, and in part are buried beneath the fill of the West Siberian Lowlands and other basins. Paleomagnetic orientations suggest that the Russian and Siberian Platforms were far apart during the early Paleozoic, converged during the middle Paleozoic, and collided in the Permian or Triassic. The geology of the Uralides accords with the concept that the two subcontinents approached and collided as the intervening oceanic plate slid beneath them along subduction (Benioff) zones. The medial eugeosyncline of the Uralides consists largely of what may be oceanic material scraped off against the edges of the opposed subcontinents. Basalt-and-spilite belts may represent ocean-floor abyssal tholeiite, and the manganiferous cherts and other sediments upon them may be pelagic oozes. Andesite belts may have formed as island arcs within the ocean, swept subsequently against the continents. Fossil subduction zones are recorded by great faults soled by, or containing tectonic injections of, mafic and ultramafic rocks from the lower oceanic crust and upper mantle, and containing high-pressure metamorphic rocks. Granitic and silicic-volcanic rocks may have formed above the subduction zones in the accreted parts of the continental plates. Both these continental-margin magmatic rocks and the island-arc complexes display ratios of potassium to silicon that vary across strike and so indicate the directions of dip of the subduction zones. From the distribution of such indicators of various ages, a history of the continental margins can be deduced. An active subduction zone dipped beneath the Siberian Platform during at least parts of late Precambrian and early, middle, and late Paleozoic time. The late Precambrian and Cambrian history of the Russian side is unclear, but in the Ordovician and Silurian the Russian continental margin was stable, while somewhere offshore an island arc was present whose trench was on the Russian side; the last of the intervening oceanic plate vanished down the subduction zone in about the Early Devonian, and the island arc became part of the continental margin. During the remainder of the Devonian and during the Carboniferous and Early Permian, a subduction zone was present along the margin of the enlarged Russian continent and dipped beneath it. Each subcontinent grew oceanward as oceanic material was accreted against it, and the subduction zones stepped oceanward correspondingly. The continental magmatic zones migrated oceanward behind the accreting edges of the continental plates, so the tectonic and magmatic progression with time at any one place is analogous to the variations present across the entire orogenic belt at any one time. Severe right-lateral deformation of the Uralides, the Russian side having moved northward relative to the Siberian side during Mesozoic and early Cenozoic time, is inferred from structural and magnetic-anomaly patterns. The deformation was accomplished by oroclinal folding, strike-slip faulting, and tensional thinning of the crust. The Uralides may have been continuous in early Mesozoic time with the Ellesmerides of North Greenland and the Canadian Arctic islands. The Cenozoic (and late Mesozoic?) opening of the Arctic Ocean was accomplished by spreading of the Eurasia Basin, and by opening of the Canada Basin behind a counterclockwise-rotating Alaska.

Siberia, Ural Mountains↗
USGS1970

Seismic refraction study of crustal structure in the western United States

A network of 64 seismic-refraction profiles recorded by the U.S. Geological Survey in California and Nevada and adjacent areas of Idaho, Wyoming, Utah, and Arizona from 1961 to 1963 was re-interpreted. From record sections compiled for all profiles, a basic travel-time diagram can be derived. In addition to the first arrivals on profiles in the Snake River Plain, the northern Basin and Range province, and the middle Rocky Mountains, two dominant phases can be correlated in secondary arrivals, whereas the profiles in other areas show only one dominant phase in later arrivals. Based on velocity-depth functions calculated for each profile after the method of Giese, the crustal structure of the western United States is presented on contour maps and on a fence diagram that is composed of 15 crustal cross sections. Crustal thickness reaches maxima under the Sierra Nevada (42 km), the Transverse Ranges of southern California (37 km), and in southwestern Nevada (36 km), whereas the crust is relatively thin under the Coast Ranges of California (24–26 km), under the Mojave Desert (28 km), and under parts of the central Basin and Range province in Nevada and Utah (29–30 km). The base of the crust dips generally from the Basin and Range province toward greater depths in the Colorado Plateau (43 km), the middle Rocky Mountains (45 km), and the Snake River Plain (44 km). The upper-mantle velocity is less than 8.0 kmps under the Great Basin of the Basin and Range province, the Sierra Nevada, and the Colorado Plateau, but it is equal to or greater than 8.0 kmps under the Coast Ranges of California, the Mojave Desert, and the middle Rocky Mountains. Velocity inversions within the upper crust are indicated under the southern Cascade Mountains and the middle Rocky Mountains, but not under the Sierra Nevada. The average velocity of the upper crust beneath the Basin and Range province is 6.1 to 6.2 kmps to a depth of 15 to 20 km. Only beneath the middle Rocky Mountains, the Snake River Plain, and the northern part of the Basin and Range province can a boundary zone between upper and lower crust be determined confidently.

Arizona, California, Idaho, Nevada, Utah, Wyoming↗
USGS1970

Radiometric ages and stratigraphic sequence of volcanic and plutonic rocks, southern Nye and western Lincoln Counties, Nevada

The geochronology of Tertiary igneous events at the Nevada Test Site and adjacent area is outlined by 36 recently determined K-Ar ages, together with other published K-Ar ages. The first evidence of Tertiary igneous activity is the ash-fall bedded tuffs in the Horse Spring Formation. One such tuff has been dated as 29 m.y. old (late Oligocene). Other ash-flow tuffs and lavas formed during the Miocene and Pliocene, according to radiometric age determinations. The youngest ash-flow tuff in this area is about 6 m.y. old. Great volumes of ash and lava were spewed forth 13 to 11 m.y. ago to form the Paintbrush and Timber Mountain Tuffs. Sixteen replicate age determinations on minerals from four densely welded ash-flow tuffs from these formations gave a pooled standard deviation of about ± 2 percent error, provided anomalous ages were rejected on the basis of rock alteration or analytical difficulties. In the Air Force Gunnery Range, just north of the test site, K-Ar ages suggest that the oldest ash flows, the Monotony Tuff, were emplaced 27.6 m.y. ago (late Oligocene) and were followed by outpourings of lava and ash throughout most of the Miocene. Youngest dated lava is about 13 m.y. old. In the southern Egan and northern Seaman Ranges of central Nevada, the Needles Range (?) Formation has an averaged K-Ar age of about 30 m.y., which compares closely with 29.2 m.y., the average of four earlier K-Ar ages determined by other investigators on known Needles Range Formation in eastern Nevada and western Utah. K-Ar ages given by micas from two exposed plutons in the Nevada Test Site suggest emplacement of these plutons at about 93 m.y. ago (early Late Cretaceous), although earlier emplacement in the Mesozoic would be more consistent with Pb-α ages

Nevada↗
USGS1970

Sea-floor spreading at the junction between Gorda Rise and Mendocino Ridge

A Quaternary deformation pattern revealed by new sub-bottom acoustic profiles supplements previous knowledge derived chiefly from magnetic and seismic evidence on the contemporary tectonics off northern California. An inferred age for the sedimentary cover along the axis of the southern part of Gorda Rise suggests that no volcanism has occurred along that part of the rise for more than 100,000 years. Sea-floor spreading at the rise crest has been accommodated by sinking of a keystone block that forms the floor of Escanaba Trough, the median valley of the rise. Differential movement between Pacific and American tectonic plates could have caused the deformation pattern, and during this movement, the displacement offshore beyond the northwest end of the San Andreas fault is inferred to have divided at Cape Mendocino between a fault along Mendocino Ridge and a fault segment that connects with the Blanco fault farther northwest.

California↗
USGS1970

A re-evaluation of basalt-obsidian relations at East Lake Fissure, Newberry Caldera, Oregon

Andesite scoria, agglutinate, and small flows formed by thin lava gushes that erupted from East Lake Fissure on the north wall of Newberry Caldera carry numerous inclusions of platy rhyolite, partly melted platy rhyolite, and frothy obsidian. This association of obsidian and “basalt” has been interpreted as the result of intermingling of mafic and siliceous magmas. The locality has been repeatedly cited as an example of a mixed intrusion of the “basalt-rhyolite association.” Field, petrographic, chemical, and experimental evidence suggest, however, that the inclusions of frothy and massive obsidian are melted fragments of platy rhyolite which were ripped from a rhyolite unit forming part of the caldera wall by uncontaminated andesite magma which rose and fountained from the fissure.

Oregon↗
USGS1970

Earthquake recurrence intervals on the San Andreas fault

Possible recurrence intervals between earth- quakes of different magnitude that may be generated along the San Andreas fault are derived by relating long-term offset rates since mid-Tertiary time, displacements, and lengths of breaks recorded for historic earthquakes, and tectonic creep rates. The recurrence interval for earthquakes of different magnitude at a given point on the fault is believed to follow the relation: (1) R x = D / (S - C) where: R x = recurrence interval at a point on the fault, D = displacement accompanying an earthquake of given magnitude (related empirically to Richter magnitude), S = long-term strain rate (from offset of geo- logic units), C = tectonic creep rate. The recurrence interval for earthquakes of different magnitudes for the total length of the fault is then derived by weighting equation (1) according to the number of break lengths in the total length as follows: (2) R t = DL / (S - C)L t where: R t = recurrence interval for entire fault, L = length of break (related empirically to Richter magnitude), L t = total length of fault. Tectonic creep is believed to be related to Richter magnitude, for example, small for segments of the fault characterized by earthquakes of large magnitude, and large for segments characterized by small earthquakes; and equations (1) and (2) can be weighted according to this relationship

California↗
USGS1970

Nature and origin of early and late cherts in the Leadville Limestone, Colorado

Two generations of chert have been observed in the Mississippian Leadville Limestone of west-central Colorado: (1) an early chert inferred to have precipitated from hypersaline marine waters of high pH after those waters seeped into carbonate muds prior to final burial and lithification of the carbonate; and (2) a late chert that appears to have precipitated from ground waters as amorphous silica after initial lithification but prior to or during karst erosion of the formation in Late Mississippian (?) and Early Pennsylvanian time. A third type of microcrystalline quartz, hydrothermal jasperoid, is associated with ore deposits of Laramide age in the Leadville, but it is not discussed here. Conditions in west-central Colorado at the time that the early cherts formed in the Leadville were probably similar to those associated with the formation of early diagenetic chert in modern sediments, and the early chert in the Leadville is believed to have formed in a manner similar to the modern early cherts. The silica of the late chert appears to have come from Paleozoic sandstones below the Leadville. It was carried by slightly acid artesian waters into the formation, probably in concentrations of less than 60 ppm SiO 2 Super-saturation of amorphous silica occurred within the Leadville and probably was obtained by evaporative concentration at the water-air interface during dry seasons. Dissolution of the limestone by the slightly acid waters may account for the removal of the calcite that is replaced by chert.

Colorado↗
USGS1970

Sr87/Sr86 ratios of Quaternary lavas of the Cascade Range, northern California

Quaternary lavas from Mount Lassen, Mount Shasta, and Medicine Lake, which range in composition from olivine basalt to rhyolite, have Sr 87 /Sr 86 values that fall between 0.7030 and 0.7043, corrected to 0.7080 for the Eimcr and Amend (MIT) SrCO 3 standard. These values are very close to average (corrected) strontium isotopic values of some other circum-Pacific volcanic suites and of lavas of oceanic islands. The low ratios suggest a mantle origin. The notably high strontium content (1400 ppm) in the andesites from Mount Shasta also indicates a mantle origin at depths where plagioclase is absent as a stable phase in the residual material.

California↗
USGS1970

Binary coefficients and clustering in biostratigraphy

The large data arrays common in biostratigraphy make subjective groupings difficult. Because of this, biostratigraphers have commonly based conclusions on the occurrences of relatively few species. The use of binary similarity coefficients, cluster analysis techniques, and digital computers allows a polythetic approach to biostratigraphy. The approach is free of circular and a priori reasoning in that it is established that assemblage zones (major clusters in a dendrogram) are present before the biostratigraphic utility of various taxa for the recognition of the zones is determined. A measure for determining the biostratigraphic fidelity of a species for established zones is proposed. Published data on the distribution of ostracodes and foraminifers from the Eocene and Oligocene of Mississippi and Alabama are used to demonstrate a method of using coefficients and clustering in establishing a zonation.

Alabama, Mississippi↗
USGS1970

A Permian disturbance of K-Ar radiometric ages in New England: Its occurrence and cause

Approximately 200 K-Ar mineral and whole rock ages from New England, half of which are previously unpublished, are used to delineate an area of Permian thermal disturbance. The disturbed area, as outlined by K-Ar mica ages, forms a north-northeast-trending belt 60–80 mi wide that extends from the coast of Long Island Sound in southern Connecticut to southwestern Maine, where it terminates against rocks displaying older radiometric ages. Several possible mechanisms that may have affected the radio-metric systems of pre-existing rocks are examined: (1) contact metamorphism related to contemporaneous igneous activity, (2) alteration associated with major faulting, (3) regional metamorphism in late Paleozoic time, and (4) burial followed by uplift and erosion. Evidence is given that each of these mechanisms was operative locally, especially in the southern portion of the belt. The general lack of late Paleozoic tectonism in New Hampshire and Maine suggests that only burial is a likely cause of the disturbance there.

Connecticut, Maine, Massachusetts, New Hampshire, ↗
USGS1970

Fission-track and K-Ar ages of Tertiary ash-flow tuffs, north-central Nevada

Ages obtained from three Tertiary ash-flow tuffs in central Nevada by fission-track and K-Ar dating are concordant. Samples dated by these methods from the same localities give the same age within the limits of analytical uncertainty. Samples of three units from widely separated localities were dated further to confirm the concordance of the dating methods and to establish that ages can be used, in conjunction with normal geologic techniques, as a criteria for correlation. The minerals dated by K-Ar were biotite and sanidine; and sphene zircon and apatite were used to determine the fission track ages. The Bates Mountain Tuff was dated at 24.0 m.y. (F.T.) and 24.0 m.y. (K-Ar). The Fish Creek Mountains Tuff has a fission-track age of 24.4 m.y., and a K-Ar age of 23.9 m.y. The average fission-track age for the Caetano Tuff is 34.9 m.y., and it has an average K-Ar age of 32.3 m.y. The amount of analytical uncertainty is slightly greater in the fission track method. Sampling, preparation, and determination of the age by the fission-track method is competitive with the K-Ar method.

Nevada↗
USGS1970

Use of Ar36 to Evaluate the Incorporation of Air by Ash Flows

The Ar 36 content of densely welded glasses from ash-flow units provides a means by which the amount of air entrapped and subsequently resorbed by the glasses during compaction and welding may be calculated. The amount of air measured in glasses from nine upper Tertiary ash-flow sheets in the western United States ranges from 0.033 to 13 ppm; median is about 1 ppm. These values are very small compared with the total amounts of volatiles which probably were incorporated during welding. The data strongly suggest that large volumes of air are not incorporated by ash flows during their eruption and lateral movement.

California, Idaho, Nevada↗
USGS1970

Sedimentary volumes and their significance

Sedimentary volumes are of prime interest in many fields of geology: as measures of erosional rates, of geochemical balance, and recently, with the virtual demonstration of continental drift, as measures of movement of the continental and oceanic plates. The Basement Map of the United States, published by the U.S. Geological Survey in 1968, provides a partial basis for an improved estimate of the volume of Phanerozoic rock in the center, minous United States. The map requires correction for this purpose, because all metamorphic rocks of whatever age have been classed as basement. We have, therefore, attempted to allow for the metamorphic rocks of Phanerozoic age. We have made estimates of volumes for areas not controlled by contours on this map and have used such offshore data as we have been able to assemble from the literature in order to extend our estimates to include offshore sediments reasonably attributable to erosion from the area of the contiguous United States. Our results are as follows: We consider this estimate to be within 10 percent of the true volume. Of it, we estimate about 3.2 ×10 6 km 3 to be volcanic rock, not representing erosion of pre-existing rock. The remaining 56.8 × 10 6 km 3 , rounded to 57 × 10 6 km 3 , we consider products of continental denudation. This volume is so large, representing, as it does, only 5.3 percent of the continental surface of the earth and only a sixth of recognizable geologic time, that it appears to invalidate schemes of geochemical balance such as those of Clarke, Goldschmidt, and others. These students assume that the salt in the sea is a measure of the amount of some “average igneous rock” that has been eroded during the whole of geologic time to produce some “average sedimentary rock.” Instead, our result points strongly toward the hypothesis of Livingstone, Gregor, Earth, and others that the oceanic salt is merely the cyclic salt not yet returned to the continents in a continuing cycle. Assuming that this volume was derived from erosion of the contiguous United States—an assumption that we recognize as invalid in detail, though not seriously in error—we obtain an ostensible average rate of Phanerozoic erosion of about 10 m/ m.y., about a sixth of the present rate. But inasmuch as present erosion is attacking a surface that exposes about 76 percent sedimentary rocks and only 24 percent igneous, most of its product is recycled rather than first-cycle sediment. An analysis of the broad features of the paleo-geography of the country indicates that a similar disproportion between first-cycle and recycled sediment has been characteristic of nearly all the Phanerozoic. The ostensible erosion rate is therefore spurious, and it is likely that the average erosion rate durin g the Phanerozoic was more than half that of the present, and perhaps was nearly or quite equal to it. The great disparity in volumes of sediment offshore in the Atlantic and Pacific—in a ratio of more than 5 to 1—is consonant with expectations if the continent has been moving westward and over-riding the Pacific Basin on a Benioff fault system activated at the beginning of the Mesozoic, though now dormant.

Bulletin of the Geological Society of America↗
USGS1970

Thermal infrared investigations, Arbuckle Mountains, Oklahoma

Thermal-infrared images obtained on flights over the Tishomingo anticline and South Flank areas near Mill Creek in the Arbuckle Mountains, Oklahoma, were used to study the possibility of identifying some common rock types from their diagnostic reflection and emission characteristics, and to evaluate the usefulness of infrared images in structural geologic investigations. The areas flown are underlain by folded and faulted Paleozoic dolomite, limestone, sandstone, shale, and Precambrian granite. Images were obtained at 6:00 a.m., 11:00 a.m., and 2:00 p.m. The predawn (6:00 a.m.) image is the most useful in distinguishing rock types. Of particular interest is a thermal contrast of dolomite (warm) and limestone (cool), sufficient to distinguish those rock types and to reveal facies changes between them. Theoretical considerations indicate that this thermal contrast arises from a combination of albedo and thermal-inertia characteristics distinctive of dolomites and limestones in many areas. The daytime images display much stratigraphic and structural detail. Small-scale bedding detail is enhanced in the morning images of low-relief areas, and contrasts of alternating formations that form hogbacks and valleys are enhanced in the afternoon images of higher relief areas. The difference in features displayed in morning and afternoon images appears to be a function of the insolation on sunward and shadowed slopes of differing scale. Fault or fracture zones are best displayed in the predawn image; they appear cooler than surrounding ground, because of greater water content and concomitant evaporation. The abundance and throughgoing nature of lineaments (which coincide for the most part with joint systems) are more obvious in the infrared images than in aerial photographs. Lineaments striking northwest are preferentially enhanced in the morning images, and lineaments striking northeast are preferentially shown in the afternoon images. This enhancement cannot be ascribed to the effects of topography, insolation, or wind; it may relate to a combination of ground-water and vegetation effects.

Oklahoma↗
USGS1970

Eocene age of the Adak ‘Paleozoic (?)’ rocks, Aleutian Islands, Alaska

In 1948, several specimens identified as the plant genus Annularia, a primitive horsetail of Pennsylvanian or Permian age, were found in tuffaceous sandstone exposed near the northern end of Adak Island, Alaska. These beds form the basal part of the Andrew Lake Formation, a newly named sequence of marine sedimentary rocks that is more than 850 m thick, and, in the main, consists of northwest-dipping tuffaceous sandstone, siltstone, shale, and siliceous siltstone and shale interbedded with basaltic flows or penecontemporaneous(?) sills (or both) a few tens of meters thick. This formation rests depositionally(?) on the Finger Bay Volcanics, the massive and intensely altered andesitic and basaltic flows and pyroclastic rocks that form the bulk of Adak Island. Mollusks, foraminifers, sponge spicules, and fish scales and skeletal remains occur in the lower 350 m of the section immediately overlying the basal “Annularia”-bearing beds. Included in this fauna is the pecten Pro-peamussium (cf. P. stanfordensis Arnold), of probable Eocene age; the associated foraminiferal fauna is provincially considered to be of late Eocene (Narizian) age, and the fish scales are similar to those found in the Narizian and Refugian (Eocene and Oligocene) of California. Examination of the matrix surrounding specimens of “Annularia” revealed a substantial dinoflagellate flora—establishing that the “Annularia”- bearing beds are themselves marine units of middle or late Eocene age. The Andrew Lake Formation probably accumulated in a perched basin along the crestal region of an early Tertiary Aleutian ridge. Accordingly, there is no evidence for a Paleozoic Aleutian ridge. There is only scant evidence that the ridge existed in Mesozoic time.

Alaska↗
USGS1970

Quartz gabbro and anorthositic gabbro: Markers of offset along the San Andreas fault in the California Coast Ranges

Large-scale lateral movement on the San Andreas fault zone is suggested by the distribution of gabbroic rocks that may be slivered remnants of oceanic crust. Distinctive and unusual hornblende quartz gabbro and anorthositic gabbro that are virtually identical both petrographically and chemically are exposed at Logan and Gold Hill in the Coast Ranges along the San Andreas fault. The hornblende quartz gabbro is made up of labradorite to bytownite plagioclase, hornblende, and quartz, with very minor biotite and pyroxene, and accessory metallic opaques and apatite. The coarse-grained anorthositic gabbro with anorthositic layers and associated gneiss is made up mainly of labradorite to bytownite plagioclase, hornblende, lesser clinopyroxene, and, locally, orthopyroxene. The present 100 mi of separation between these two gabbro bodies is probably due to displacement along the San Andreas fault of what was originally one gabbro mass. Somewhat similar quartz gabbro and anorthositic gabbro associated with ultramafic rocks near Eagle Rest Peak in the San Emigdio Mountains may represent a source for the Logan and Gold Hill slivers. This suggests a minimum right-lateral movement of about 200 mi on the San Andreas fault zone. Anorthositic gabbro clasts from Cretaceous conglomerate in the Gualala area have strong resemblance to gabbroic rocks at Logan, Gold Hill, and Eagle Rest Peak and speculatively suggest 350 mi of right-lateral movement on the fault zone. It is tentatively suggested that the gabbro clasts of Gualala may have been shed from a large area of exposed gabbroic oceanic crust, pieces of which occur as fault slivers at Gold Hill and Logan. It is further suggested that the Eagle Rest Peak locality may be a relatively in-place exposure of this gabbroic oceanic crust that lies near a continental-oceanic interface. This interface, thought to represent a fossil subduction zone between Franciscan and Sierran basements, appears to be overlain depositionally by Eocene sedimentary rocks. If these Eocene deposits do “seal over” the possible subduction zone, it suggests that movement on such a zone ceased before the Eocene deposition, and that the new and different pattern of right-lateral movement on the San Andreas fault zone probably began sometime later.

California↗