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At least 145 records · Page 8Linked to original sources
USGS1968

The age of the Puerto Rico Trench

The Puerto Rico Trench is parallel to and north of Puerto Rico and the Virgin Islands and reaches depths of more than 8000 meters. Puerto Rico, the closest land area, has a central longitudinal core of Cretaceous and early Tertiary volcanic rocks, some serpentinite of undetermined age, and numerous small intrusions. These rocks were folded into an anticlinorium and intensely faulted into hundreds of fault blocks in an orogeny that lasted from Late Cretaceous until middle Eocene time. Most fold axes and the major fault zones trend approximately west-northwest at an angle of about 15° to the general trend of the island and to the Puerto Rico Trench. Near the north coast, late Oligocene rocks rest on a truncated surface of Cretaceous and early Tertiary volcanic rocks. The Oligocene and overlying Miocene sedimentary rocks strike approximately east, parallel to the Puerto Rico Trench. Apparently the trench is not related to the tectonic movements that folded and faulted the Cretaceous and early Tertiary rocks, but it is closely related tectonically to the Oligocene and Miocene rocks. The trench probably first appeared in Oligocene or possibly late Eocene time and may have reached its maximum depth in late Miocene or Pliocene time.

Puerto Rico↗
USGS1968

Gneissic amphibolite at Las Palmas, Puerto Rico, and its significance in the early history of the greater antilles island arc

The basal complex of Puerto Rico consists principally of serpentinite, minor amounts of chert and spilite, and locally small blocks of amphibolite . A detailed structural and metamorphic study of a relatively large block of gneissic amphibolite at Las Palmas reveals that the rock has undergone repeated deformation and regional metamorphism prior to contact metamorphism by intrusive serpentinite. The first event recorded by the amphibolite is amphibolite facies regional metamorphism. Foliation and some hornblende lineation were formed at this time. An increase in the intensity of regional metamorphism then produced quartzo-feldspathic segregations from the host rock, and was accompanied by small-scale folding and development of several types of lineation oblique (o the early hornblende lineation. The metamorphic intensity then decreased slightly and tectonic movement radically decreased, as nonfoliated but metamorphosed mafic dikes cut the gneissic amphibolite . As the metamorphic grade decreased further, late-stage kink-bands were developed at high angles to the earlier small-scale folding. Later, the amphibolite was intruded by serpen-tinite, causing incipient low-grade contact metamorphism and some calcium metasomatism. H. H. Hess has maintained that Puerto Rico and other islands in the Greater Antilles rest directly on oceanic crust, and that the intrusive serpentinite is derived from the bottom layer of the crust. In the Las Palmas area, the amphibolite formed prior to intrusion of serpentinite, indicating a complex preserpentinite history . Three interpretations of the gneissic amphibolite are considered. It may represent: (1) metamorphosed gabbroic rocks genetically related to the serpentinite, (2) regionally metamorphosed mafic rocks that are part of the oceanic crust, or (3) older mafic rocks regionally metamorphosed as part of an extensive pre-Late Cretaceous orogeny. The evidence from Las Palmas is not conclusive, but when considered in the regional framework, either of the last two interpretations seems reasonable.

Puerto Rico↗
USGS1968

A further contribution to the petrology of Haleakala volcano, Hawaii

Sixteen new chemical analyses of the later rocks of Haleakala Volcano , on the island of Maui, Hawaii , add to the differentiation picture for that volcano . The early rocks of the volcano are tholeiitic. These are followed by dominant hawaiites with less abundant alkalic olivine basalts, picrite-basalts of ankaramite type, and a few mugearites. Still later rocks, separated from earlier ones by a profound erosional unconformity, include some hawaiites and ankaramites, but are dominantly alkalic olivine basalts (basanitoids) containing as much as 16.5 percent normative nepheline, some of them transitional to ankaramite. The progression toward ultramafic, strongly undersaturated rocks (nephelinites), characteristic of the post-erosional lavas of other Hawaiian volcanoes, appears to have just begun at Haleakala .

Hawaii↗
USGS1969

Seismic-refraction measurements in Jackson Hole, Wyoming

Three reversed seismic-refraction profiles were recorded in the Jackson Hole, Wyoming, area during July 1964. The seismic model which was developed consists of three layers with velocities of 2.4 km/sec for Tertiary and Cretaceous rocks above the Cleverly Formation (Lower Cretaceous), 3.8 km/sec for rocks from Lower Cretaceous down to lower Paleozoic, and 6.1 km/sec for lower Paleozoic (limestones and dolomites) and Precambrian rocks. The maximum thickness of sediments in Jackson Hole is 5 km, and the minimum throw of the Teton fault in the area covered by this survey is about 7 km.

Wyoming↗
USGS1969

Principal stress directions from plastic flow in crystals

Methods for determining orientations of principal stress axes in deformed rocks involve dynamic analysis of twin-gliding and of extinction bands produced by inhomogeneous translation gliding in crystals. The methods, beginning with Turner's (1953) technique for dynamic analysis of calcite twins, have been developed using as guides the results from experiments under controlled laboratory conditions. Structures induced by intragranular flow in calcite, dolomite, quartz, micas, orthopyroxenes, clinopyroxenes, olivine, and other common rock-forming materials, may now be used to derive orientations of principal stresses causing the deformation. The various methods, some new, are discussed in detail and examples of their application to tectonites are given. The usefulness of such studies is illustrated by evaluating the observed orientations of principal stresses around folds in light of new data from a theoretical analysis of large amplitude folding of viscous layers in a less viscous matrix. Other areas of research in structural geology in which these methods should prove useful have also been outlined.

Geological Society of America Bulletin↗
USGS1969

Errors in using modern stream-load data to estimate natural rates of denudation

The practice of calculating natural rates of denudation from routinely collected data on the loads of suspended and dissolved matter in modern rivers is subject to several significant errors. The sources of these errors are demonstrated by examples from the Atlantic drainage of the United States, where their total effect has apparently doubled the natural rate of erosion. The largest error is caused by assuming that modern sediment loads in populated areas represent natural erosion, whereas in fact they mainly reflect the influence of man. Conversion of forests to croplands in the middle Atlantic states causes about a tenfold increase in sediment yield. Coal mining, urbanization, and highway construction have added extra loads of sediment to the streams. Modern sediment loads in the Atlantic-draining rivers are probably 4 to 5 times greater than they would be if the area had remained undisturbed by man. Errors in calculating the chemical denudation are caused by atmospheric contributions to the dissolved loads of streams and by pollutants that are added directly to stream waters. About one-quarter of the salts in Atlantic-draining streams were contributed from the atmosphere, either as recycled sea salts or as pollutants and soil dust that originally became airborne as a result of the activities of man. Perhaps another one-tenth of the dissolved load consists of industrial and agricultural wastes or acid mine waters that have been added directly to the streams.

Connecticut, Delaware, Georgia, Maryland, Massachu↗
USGS1969

Alkalic and tholeiitic basaltic volcanism related to the Rio Grande depression, southern Colorado and northern New Mexico

Upper Cenozoic basaltic rocks in and near the northern Rio Grande depression, a major intracontinental tension-rift structure, vary systematically in petrology and chemistry with distance from the depression. Basalts and basaltic andesites of alkalic affinities, commonly showing evidence of crustal contamination, were erupted east and west of the depression concurrently with its formation, whereas little-contaminated tholeiitic basalts filled parts of the depression late in its history. Eruption of the contrasting basalt types was in part concurrent. The lateral change from alkalic to tholeiitic basaltic volcanism may reflect different conditions of magma generation in the mantle that are related to changes in crustal thickness and thermal gradient across the rift. Recent experimental studies suggest that the variations in magma composition may be due to differing depths of magma fractionation, the tholeiitic basalts originating at shallower depths than the alkalic basalts.

Colorado, New Mexico↗
USGS1969

Ultramafic and basaltic rocks dredged from the nearshore flank of the Tonga Trench

Deep dredging in the Tonga Trench (Southwest Pacific Ocean) at a depth of 9150 to 9400 m yielded fresh to granulated and serpentinized peridotite and dunite. Other rocks recovered there and at three stations deeper than 7000 m include basalts, tuffs, and tuffaceous agglomerates. Chemical analyses of the fresh peridotite, with combined H 2 O < 0.10 weight percent, indicate that the rock consists of Si, Mg, Fe (6 percent), and Cr + Ni about 0.7 percent. Mineralogically, the peridotite contains forsteritic olivine and enstatite with minor spinels. The ultramafic mass exposed at 9400 m probably is an accumulate exposed by faulting.

Pacific Ocean, Tonga-Kermedec Trench, Tonga Trench↗
USGS1969

Carbon isotopes in pelites of the Precambrian Uncompahgre Formation, Needle Mountains, Colorado

Carbon isotopic ratios and weight percentages of carbon were measured in 15 samples of slate, phyllite, and schist of the approximately 1500- to 1600-m.y.-old Uncompahgre Formation of the Needle Mountains, southwestern Colorado. Rocks with less than 1 percent total carbon, all of which is reduced, have δC 13 values of −23 to −28 per mil, whereas those with 1 to 6.4 percent carbon have δC 13 from −29 to −31 per mil. In general, the slates and phyllites contain more carbon and isotopically lighter carbon than do the schists of higher metamorphic rank. Increasing loss of C 12 -enriched methane with increasing intensity of metamorphism is suggested to account for these differences.

Colroado↗
USGS1969

Cretaceous, Tertiary, and early Pleistocene rocks from the continental margin in the Bering Sea

Rocks dredged from the continental margin in eastern Bering Sea in and near the Pribilof Canyon indicate that the acoustic basement represents the upper surface of thoroughly lithified turbidite beds of graywacke and siltstone of Late Cretaceous age. The stratified sequence covering the acoustic basement is gently deformed and includes marine clastic and diatomaceous sediments ranging in age from middle or late Miocene through early Pleistocene. Dense argillite, siltstone, and calcareous sandstone of early Tertiary age in and near the Zhemchug Canyon probably represent an older part of this sequence. The main layered sequence accumulated above the acoustic basement in shallow water, and, because the older beds now lie as much as 1000 m below sea level, the continental margin must have undergone considerable subsidence during late Tertiary and Quaternary time. A rich pollen flora indicates that the shoreline lay only a few tens of kilometers away from the site of the Pribilof Canyon during late Miocene time.

Alaska↗
USGS1969

Peridotite-gabbro complexes as keys to petrology of mid-oceanic ridges

Two suites of olivine-rich ultramafic and feldspathic rocks appear to be present in the Mid-Atlantic Ridge: one which seems to have alkalic affinities, and one similar to the chromitite- bearing alpine peridotite-gabbro complexes. The similarities of rocks in the two environments—continental and oceanic—imply that much about the petrology of mid-oceanic ridges may be learned from studies of continental complexes, and that silicic rocks have been formed in the mantle. Although gabbros in St. Paul Rocks and similar rocks at Tinaquillo, Venezuela, and Lizard, England, have been interpreted as not comagmatic with intimately associated peridotite by some petrologists, evidence to the contrary at Lizard is discussed. Association of fresh gneissic gabbro, some containing quartz, with talcose serpentinite, amphibole schist, quartz diorite and epidotic but unsheared basalts along the Mid-Atlantic Ridge is believed to indicate presence of alpine-type rocks that occur normally in eugeosynclinal belts. Gabbro, described as partly interlayered with peridotite by gravitational differentiation, forms major parts of three widely separated ultramafic complexes which have been interpreted as slices of oceanic crust and upper mantle: the Troodos massif in Cyprus, the Bowutu Mountains in Papua, and the Camagiiey complex in central Cuba. If, as Dietz has suggested, peridotite and related rocks in eugeosynclines represent fragments of ocean rind formed along mid-oceanic ridges and moved laterally by ocean-floor spreading, gabbro must be an essential constituent of the upper mantle. This could account for many geophysical anomalies, but would complicate some postulated mechanisms involved in ocean-floor spreading.

England↗
USGS1969

A geophysical study of North Park and the surrounding ranges, Colorado

A geophysical study in the North Park basin and surrounding mountains, Colorado illustrates the structural relationship of various sedimentary, metamorphic, and igneous rock units. Bouguer anomalies from 1330 gravity stations range from −210 mgal over Precambrian metamorphic rocks in the mountains to −260 mgal in the Walden syncline and —280 mgal in the North Park syncline. Steep gradients delineate a fault which strikes west-northwest along the north flank of the North Park syncline. Two models fitted to the gravity data show 1 to 2 km relief on this steeply dipping fault. Density contrasts between Precambrian metamorphic and igneous rocks produce anomalies of as much as 25-mgal amplitudes in the Park and Medicine Bow Ranges. A 30-km-long seismic refraction profile, parallel to the most negative Bouguer anomaly values in the North Park basin, shows velocities increasing from 2.5 to 3.4 km/sec within Tertiary rocks at depths ranging from 1.2 to 2.0 km. Mesozoic sedimentary rocks have a velocity of 4.0 to 4.5 km/sec, a very high velocity in view of the predominance of Upper Cretaceous rocks. Precambrian basement with a velocity of 6.25 km/sec underlies the profile at depths ranging from 3.5 to 4.5 km. Strong second arrivals across the profile, observed at distances of more than 14 km from the shotpoints and interpreted as SP reflections, verified the refraction model. An aeromagnetic survey shows numerous anomalies ranging from 100 to 200γ in the Park and Rabbit Ears Ranges and in the Never Summer Mountains, to 400γ in the Front Range, and to 1200γ over the Medicine Bow Range. Positive anomalies in the Park, Medicine Bow, and Front ranges overlie metamorphic rocks. Magnetic and gravity data suggest that the Never Summer Mountains are separated from the Front Range by a north-trending, steeply east-dipping reverse fault, extending beneath the Front Range along the Colorado River valley. The magnetic data indicate that this fault may connect with a possible fault that is parallel to the Laramie River valley. In the Rabbit Ears Range, a series of magnetic anomalies show that igneous rocks are present in the eastern part of the range. A northeast-trending positive magnetic anomaly, which is parallel to foliation trends reported in Precambrian rocks, extends from the Park Range across the North Park basin to the Medicine Bow Range. On the basis of this anomaly, the high seismic velocity of the Precambrian basement, and computed profiles fitted to the gravity and magnetic data, we infer that much of the basin is underlain by high-density metamorphic rock. As shown by gravity data, the deepest part of the basin is 2.7 km below sea level, resulting in a maximum relief of 6.7 km on the basement, relative to the Medicine Bow Range. A 25-mgal negative gravity anomaly and a zone of negative magnetic anomalies outline a large granitic intrusion in the Park Range, which probably extends northeast beneath the North Park basin and connects with granitic rocks in the Medicine Bow Range.

Colorado↗
USGS1969

Active metamorphism of upper Cenozoic sediments in the Salton Sea geothermal field and the Salton Trough, southeastern California

The Salton Sea geothermal system is entirely within Pliocene and Quaternary sediments of the Colorado River delta at the north end of the Gulf of California. At the time of deposition, these sediments consisted of sands, silts, and clays of uniform original mineralogic composition, but under the elevated temperatures and pressures of the geothermal system they are being transformed to low-grade metamorphic rocks of the greenschist facies. We have studied these transformations by X-ray, petrographic, and chemical analyses of cuttings and core from deep wells that penetrate the sedimentary section. Temperatures within the explored geothermal system range up to 360° C at 7100 feet. The wells produce a brine containing over 250,000 ppm dissolved solids, primarily Cl, Na, Ca, K, and Fe, plus a host of minor constituents.

California↗
USGS1969

Primary protodolomite in echinoid skeletons

Protodolomite has been identified in the teeth of Recent echinoids by means of electron microprobe and X-ray diffraction analyses. The analyses demonstrate a high degree of heterogeneity with respect to magnesium carbonate content in these skeletal parts; the entire spectrum of compositions between magnesium calcite and protodolomite was encountered.

Geological Society of America Bulletin↗
USGS1969

Aeromagnetic investigation of crustal structure for a strip across the western United States

This report represents part of a larger study undertaken to interpret the gross features of the earth's crust by aeromagnetic methods. The larger survey covers a 100-mile-wide strip along a great circle arc from Washington, D.C., to San Francisco, California. The area considered extends from about 200 miles east of the Rocky Mountains to 150 miles west of the coast of the Pacific Ocean. Inferences on gross structure and lithology are made from the magnetic patterns and trends shown on the map. At the continental margin, the magnetic data strongly suggest that the oceanic crust becomes much more deeply buried as the continent is approached. A large magnetic anomaly extending northwestward along the center of the Great Valley is probably caused by mafic intrusive rocks.

California, Colorado, Kansas, Nebraska, Nevada, Ut↗
USGS1969

Potassium-argon ages bearing on the igneous and tectonic history of the Elk Mountains and vicinity, Colorado: A preliminary report

K-Ar ages for epizonal plutonic rocks together with field studies indicate that uplift of the Sawatch Range began at least 72 m.y. ago. Vertical uplift of the Sawatch Range was followed or accompanied by gravity sliding of sedimentary rocks along the Elk Range thrust fault. The greatest volume of exposed intrusive rocks in the Elk Mountains, Ruby Range, and West Elk Mountains consists of granodioritic rocks of Oligocene age which are younger than the Elk Range thrust. Miocene and Pliocene(?) mafic dikes were emplaced after most of the structural features in the area had developed. The Treasure Mountain dome near Marble was formed by emplacement of a unique soda granite pluton about 12.5 m.y. ago. Discordant K-Ar and Rb-Sr ages for biotite from the Twin Lakes stock in the Sawatch Range suggest that this large pluton is of Late Cretaceous or early Tertiary age and lost argon during Oligocene plutonism, or is of Eocene age and represents a separate event not yet supported by other radiometric dates from the area.

Colorado↗