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At least 649 records · Page 36Linked to original sources
USGS1971

Metamorphism of Precambrian granitic xenoliths in a mica peridotite at Rose Dome, Woodson County, Kansas: Part 2, petrologic and mineralogic studies

Field, drill-core, petrographic, and mineralogic studies show that the Precambrian granitic rocks exposed on Rose Dome, Woodson County, Kansas, were emplaced as inclusions of basement rock in a mica peridotite magma that intruded the Pennsylvanian section of Rose Dome during Late Cretaceous time. High temperatures of the alkaline ultramafic magma (probably greater than 800° C) led to metamorphism of the granitic inclusions and the formation of high sanidine and high albite from original microcline and albite. Contact metamorphic effects on country rock include the development of buchite-like sanidine-magnesian biotite hornfels from Weston Shale (Pennsylvanian) that was intruded by the mica peridotite. The high temperatures of the peridotite magma also led to partial melting of the granitic rocks; a quartzofeldspathic matrix shows volcanic textures and binds mineral and rock fragments together to produce the varied range of textures and structures of the granitic xenoliths. Partial melting of the granitic rocks accounts for those features that led earlier workers to conclude that the “granite” on Rose Dome had intruded the Pennsylvanian section. Those features included xenoliths of hornfels enclosed by granitic material and apparently intrusive relationships between the granitic rocks and metamorphosed shale. The lack of concordance in the Rb-Sr ages of the granitic rocks (Part 1) may stem partly from melting of the “granite” and exchange of ions with the mica peridotite magma, from the “sampling” of different levels of granitic crust by the upwelling peridotite magma, or from weathering.

Kansas↗
USGS1971

Mesozoic granitic rocks in northwestern Nevada: A link between the Sierra Nevada and Idaho batholiths

Extensive areas in northwestern Nevada are underlain by granodiorite and quartz monzonite plutons, as well as less common smaller bodies of quartz diorite. Twenty-six K/Ar age determinations on rocks from this suite range from about 175 to 85 m.y., but most of the plutons are between 105 and 85 m.y. old. This Late Cretaceous intrusive epoch extending from 105 to 85 m.y. ago is here named the Lovelock intrusive epoch. Twenty-three whole-rock chemical analyses show that the granitic rocks of northwestern Nevada form a homogeneous differentiation series with a narrow range in major element distribution. The granitic plutons of northwestern Nevada are chemically and petrographically indistinguishable from granitic intrusives of equivalent age in the Sierra Nevada and Idaho batholiths, and form a link between these two major batholiths.

California, Idaho, Nevada↗
USGS1971

Tectonics of the Mendocino triple junction

Interpretation of reflection profiles and of the magnetic anomaly pattern over the Gorda Basin and Escarpment gives broad agreement with the triple junction model of McKenzie and Morgan (1969). However, the basin has undergone internal deformation, a local departure from rigid plate tectonics, and the escarpment has had a component of underthrusting by the Gorda block. Faults in the Gorda Basin which disturb young turbidites parallel the trends of magnetic anomalies, suggesting deformation of the oceanic crust along lines of primary weakness. The northeast trends of the faults give a constraint on first-motion solutions for earthquakes within the basin and suggest left-lateral slip on the faults. Analysis of the geometry and timing of the Gorda Basin deformation based on the magnetic pattern gives an average gross tectonic strain rate of 10 −14 /sec. These observations give a measure of the mechanics of deformation of oceanic lithosphere very close to a spreading rise crest.

California, Oregon↗
USGS1971

Age of the Mesozoic oceanic crust in the California Coast Ranges

K-Ar ages of approximately 155 m.y. were measured on minerals from ophiolites at two localities in the California Coast Ranges. Upper Jurassic rocks of the Great Valley sequence are in depositional contact with underlying ophiolites, which are interpreted as Mesozoic oceanic crust that formed during differentiation of mantle material at a spreading ridge some 15 m.y. before deposition of the Great Valley sequence began.

California↗
USGS1971

Interpretation of an aeromagnetic strip across the northwestern United States

This report discusses the results of aeromagnetic survey bounded by latitudes 45°30′ N. and 47°00′ N. and extending from the Rocky Mountains, to approximately 120 mi offshore in the Pacific Ocean. East of the Rocky Mountains, a larger area has been surveyed in the Great Plains bounded approximately by latitudes 44°50′ N. and 48°10′ N. and by longitudes 104°W. and 110°W. Throughout the area of the survey, the magnetic map is marked by conspicuous northeast and northwest anomaly trends, lineaments, and breaks in the anomaly pattern. Their regional distribution, over-all magnetic character, and geologic evidence suggest that they are major structural features in the basement rocks. The close correspondence of structural and geologic features in younger rocks with these basement magnetic and structural trends suggest that basement trends controlled or at least greatly influenced intrusion, deposition, and structural history of younger rocks. In some cases, evidence suggests that basement structures have been reactivated during later tectonic activity. Perhaps even more striking than the northeast- and northwest-trending features are large east-west magnetic discontinuities which, in some cases, extend completely across the strip to the edge of the shelf, and which, in some cases, can be correlated with large-scale discontinuities dating back to the Precambrian.

Idaho, Montana, North Dakota, Oregon, South Dakota↗
USGS1971

Small plate tectonics in the northeastern Pacific

Lithospheric plate motions in the northeastern Pacific were complicated at about 2.5 m.y. B.P. by the movement along a major northeast-trending fault cutting Cascadia Basin. An estimate of the slip rate along this fault gives critical information on the relative motions of four geometrically interdependent blocks. The fault is presently inactive. Seventy km of slip along this fault during 2 m.y. or less gives an average slip rate of about 3-5 cm/yr or greater, and resulting plate motions suggest a significantly greater rate of net subduction along the continental margin off Oregon than off Washington and Vancouver Island. Subduction rate off Oregon is less sensitive to slip rate along this fault than is subduction off Washington.

British Columbia, Oregon, Washington↗
USGS1971

Tertiary igneous chronology of the Great Basin of western United States — Implications for tectonic models

The chronology of igneous activity in the Great Basin of western United States is used as a time framework for a simple plate model. This chronology suggests that a plate (Farallon plate) became underthrust to sufficient depth by the middle Tertiary to trigger the eruption of volcanic rocks of andesitic to rhyolitic composition in the central part of the Great Basin, 40 m.y. ago. This plate continued to be underthrust until about 19 m.y. ago, at which time it was completely consumed and volcanic activity ceased. When the oceanic ridge reached a certain point under the Great Basin about 16 m.y. ago, this resulted in the widespread eruption of olivine basalt and the main initial phase of Basin and Range faulting.

Arizona, California, Idaho, Nevada, Oregon, Utah↗
USGS1971

Upper Cenozoic basalts with high Sr87/Sr86 and Sr/Rb ratios, southern Great Basin, western United States

Upper Cenozoic basalts from southwestern Nevada and east-central California are unusually rich in both strontium (~ 1,200 ppm) and Sr 87 (initial Sr 87 /Sr 86 ~ 0.707). The average Rb/Sr ratio of these basalts is too low to have generated the observed Sr 87 /Sr 86 ratio during the 4.6 b.y. of the Earth's existence, and the high strontium contents and low Rb/Sr ratios effectively rule out introduction to the basalts of the high Sr 87 /Sr 86 values through contamination by more radiogenic material during ascent through the crust. Instead, the basalts must have been derived from unusual mantle material in which an originally high Rb/Sr ratio was markedly lowered during an earlier phase of magmatic activity.

Arizona, California, Nevada, Utah↗
USGS1971

Deformation of lee-side laminae in eolian dunes

Processes responsible for structures in sand dunes consist of (l) primary deposition by saltation and creep and by settling from suspension, (2) redeposition accompanying avalanching, and (3) penecontemporaneous erosion. Characteristics of dune structures were examined in the field by introducing marker beds of magnetite at times of sand deposition, thus recording original surfaces and making possible the determination of subsequent changes. Similar structures were examined in the laboratory by testing processes and comparing the resulting structural forms with corresponding natural features. Avalanching in sand is of two types: sand flow and slumping. Deformational structures characteristic of each were recorded in the field and were reproduced in the laboratory. Nine varieties of deformational structures are recognized and described. Analysis of these structures suggests criteria for distinguishing compressional types (lower dune slope) from tensional types (upper dune slope). The analysis of deformational structures also serves to distinguish between forms developed in cohesive sand and those in non-cohesive sand. Since the degree of cohesion is largely a function of the amount of moisture in the sand at the time of avalanching, the deformational structures provide a means for recognizing original dry sand, wet sand, sand crusts, and saturated sand surfaces in ancient deposits. A testing of these criteria was made by comparing laboratory samples with those of dry sand at White Sands, New Mexico, and with those of coastal dunes (probably wet sand) in southern Brazil.

New Mexico↗
USGS1971

Thin skin distension in Tertiary rocks of southeastern Nevada

Volcanic rocks of late Tertiary age, aggregating about 17,000 ft, accumulated on a surface of low relief cut on Precambrian rocks in the Basin and Range province south of Lake Mead, in Nevada and Arizona. They consist mostly of lava and flow breccia of intermediate composition with minor ash-flow tuff, bedded tuff, and lava of rhyolitic composition. The last of three main phases of volcanism was accompanied by widespread epizonal plutonism and intense faulting. All or parts of six similarly but separately fault-deformed structural units are recognized in a 92-sq-mi mapped area. The structural units are highly distended by a system of closely spaced north-to northwest-striking shingling normal faults (many of which are low angle) that displace younger over older rocks in a west to west-southwest direction. Cumulative amounts of distension approximate the breadth of the structural units and are as much as 20,000 ft, whereas cumulative vertical displacements are much less and in some places are minimal. The structural units are floored at or near the present level of exposure by complex low-angle zones of detachment or décollement into which the numerous shingling normal faults merge. Where the units abut along their strike, they are separated by complex zones of transcurrent faults that appear to merge with the detachment structures and thus mark the ultimate limits of the structural units. Displacement on the detachment structures has the same sense as, but in some places is much greater than, that of the cumulative offset on the shingling faults, thus indicating low-angle movement of the structural units as platelike or lobate masses. These relationships indicate remarkably thin-skinned, large-scale, fault-related tectonism of a type which is present in a broad belt south of Lake Mead and in numerous other areas in the Basin and Range province. The best exposed structural units exhibit a serial eastward progression from broad areas of steeply dipping strata, low-angle faults, and deep denudation to gently dipping strata, high-angle faults, and little denudation. Reverse-drag flexing, a volume-compensating mechanism for movement on concave-upward faults, is inferred to have produced the gentle to moderate dips of the strata, whereas the nearly vertical dips in the western parts of the units probably resulted from a combination of reverse-drag flexing and rotation related to uplift. Evidence of compression-related folding is absent. The extreme distension is viewed as a surficial feature of a crustal belt that was subjected to a brief episode of tensional rifting. Rifting at subjacent levels along the belt was compensated for by emplacement of plutons. The surficial rocks were stretched and thinned over the plutons.

Nevada↗
USGS1971

Related strontium isotopic and chemical variations in oceanic basalts

Sr 87 /Sr 86 values in oceanic basalts range from 0.7012 to 0.7057 and correlate with basalt composition as measured by the ratio K 2 O/(Na 2 O + K 2 O). The distribution of data points on this plot can be approximated by the following ranges in Sr 87 /Sr 86 and K 2 O/(K 2 O + Na 2 O) respectively: (l) ocean ridge tholeiites—0.7020 to 0.7035 (one value 0.7012), <0.10; (2) many island tholeiites and alkali basalts—0.7030 to 0.7045, 0.10 to 0.30; and (3) potassic island basaltic rocks—0.7040 to 0.7057, >0.30. If the volcanism occurring throughout much of geologic time preferentially depleted rubidium and potassium relative to strontium in the mantle, preservation of the resultant heterogeneities is necessary to explain the isotopic and chemical differences among oceanic basalts. As a corollary to this long-term depletion of rubidium and potassium of the mantle, the primitive mantle or total crust-mantle system would have an Sr 87 /Sr 86 value higher than many oceanic basalts derived from zones that have undergone multistage histories. Therefore, we suggest that the potassic lavas with Sr 87 /Sr 86 higher than those of ocean ridge tholeiites and many island basalts represent the least depleted or most primitive mantle sampled by young oceanic volcanism.

Bulletin of the Geological Society of America↗
USGS1971

Nomenclature and correlation of some upper Precambrian and basal Cambrian sequences in western Utah and southeastern Idaho

Recent stratigraphic studies in three widely separated localities in southeastern Idaho and western Utah have revealed a startling continuity of both individual rock units and of rock sequences over a distance of some 300 mi parallel to the strike of a late Precambrian and Cambrian depositional trough. Between 15,000 and 25,000 ft of beds were deposited in the axis of the trough, whereas only 1000 to 3300 ft of correlative rocks were laid down on the shelf to the east. In several areas a diamictite is present near the base of the sequence; this is underlain locally and overlain generally by argillites containing lenticular limestones and dolomites; these in turn are succeeded by quartzitic rocks containing a thick grayish-red to maroon unit—the Mutual Formation. In each area the sequence includes, at the top, quartzites typical of the basal Cambrian. Deposition in the basin was essentially continuous from late Precambrian into Cambrian time but was interrupted by uplift and erosion on the shelf. The hinge line of the ancient seaway is inferred to have coincided roughly with the present “Wasatch line,” but erosion prior to deposition of the Tintic Quartzite has removed most of the data needed to establish this with certainty. Rocks in each of the three areas described here in detail are regarded as allochthonous and appear to have been thrust eastward during the Sevier orogeny. A precise reconstruction of the sedimentary basin must therefore await not only additional stratigraphic studies in such areas as the Promontory Range of Utah and the Bannock and Malad Ranges of southern Idaho, but also final resolution of the structural events.

Idaho, Utah↗
USGS1971

An evaluation of procedures used in computing chemical denudation rates

Computations of chemical denudation rates (that is, the average rate of lowering of the earth's crust by chemical processes) should be based only upon those components of dissolved loads of streams that are derived from chemical weathering of rocks and soils, even though identification of those components is difficult and imprecise. Past computations of chemical denudation rates exaggerated by 1.4 to 2.4 times the significance of this process because they were calculated from total dissolved loads. Identification of separate denudation and nondenudation components in a water analysis can be done most readily for streams in crystalline silicate rock terrane where commonly 25 to 55 percent of the dissolved solids are HCO 3 −1 , SO 4 −2 , Cl −1 , and NO 3 −1 that cannot be derived from the underlying rock; instead these ions are derived either directly from the atmosphere or from atmosphere-biosphere interactions. Most of the remaining solutes are derived from chemical weathering or rainfall, or both. Existing data are often insufficient to separate these two components, but atmospheric contributions to dissolved loads of dilute natural waters are substantial. The commonly observed increased concentration of dissolved solids in chemically polluted rivers is produced by concentration of natural dissolved loads by evaporation, and by addition of solutes not present in natural waters. Concentration of natural solutes represents a modification of natural denudation processes that does not change the rate of denudation. Addition of new solutes represents man-induced, accelerated chemical denudation. Extrapolation of modern chemical denudation rates to geologic situations is extremely imprecise because of uncertainties in assessing man's influence on present dissolved loads, in reconstructing the number and magnitude of past climatic changes, and in evaluating the effects of evolving flora and fauna.

Bulletin of the Geological Society of America↗
USGS1971

Thrust and strike-slip faulting in the Plomosa Mountains, southwestern Arizona

Thrust and strike-slip faulting are recognized in the Plomosa Mountains, southwestern Arizona. The distribution of rock types and the geometry of the thrust faults necessitate that the upper plate moved from east to west. The amount of displacement is not known, but is considered to be large. Apparent separations along the strike-slip faults are in a right-lateral sense and are greater than 19,000 ft. Rhyodacite flows, dated by K-Ar methods at 19 to 20 m.y., unconformably overlie the thrust faults and are cut by the strike-slip faults.

Arizona↗
USGS1971

A further revision of the stratigraphic nomenclature of the Wissahickon Formation in Maryland

The Wissahickon Formation, the thickest and most extensive unit of the Glenarm Series, was divided into lithofacies several years ago. We suggest revision of two of these lithofacies and addition of another. We also suggest that the term lithofacies be shortened to facies. The added facies, the quartzite facies, is distinguished by metamorphosed orthoquartzites and protoquartzites. It corresponds in part to the former Peters Creek quartzite.

Maryland↗