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At least 181 records · Page 10Linked to original sources
USGS1971

On the nature of the Boulder Batholith of Montana

In a recent review of the nature of batholiths, Hamilton and Myers (1967) interpreted the Boulder batholith of western Montana to be "in effect a gigantic mantled lava flow .... only a few kilometers thick," that flowed, under a crust of its own ejecta, across a broad structural basin. Such an interpretation is inconsistent with abundant geologic and geophysical data. The main mass of the batholith, the Butte Quartz Monzonite, does not have the characteristics of a lava flow or a laterally emplaced sheet. Its volcanic cover was not a floating cap but a laterally stable roof that was part of a volcanic plateau which occupied at least twice the area of the batholith. It does not thin toward its edges but is generally steep sided. Its flow structures are predominantly steep rather than near horizontal. It is separated from two smaller flanking plutons by thin vertical septa kilometers long. Its emplacement required more than 4 m.y., a rate orders of magnitude too slow for a single sheet only a few kilometers thick, however extensive. The batholith is more than a few kilometers thick. Recent gravity studies (Burfeind, 1967; Bonini, 1969) suggest a maximum thickness of 9 to 15 km to their authors, but the calculations are based on (1) assumed lateral and vertical homogeneity of the batholith, whereas in reality the Butte Quartz Monzonite core is discontinuously rimmed by more mafic, denser plutons; and (2) inappropriate densities, leading to excessive apparent density contrasts. The gravity data suggest to us that the batholith is more than 15 km thick. Heat flow, cooling rate, and seismic data also are compatible with a thickness of at least 15 km, but are difficult to reconcile with a thickness of only a few kilometers. Convincing examples of extrusive or quasi-extrusive thin batholiths must be sought elsewhere. © 1971, The Geological Society of America, Inc.

Montana↗
USGS1971

Aeromagnetic study of the midcontinent gravity high of central United States

A composite map of detailed aeromagnetic surveys over the midcontinent gravity high provides coverage of the 600-mi-long buried belt of mafic rocks of the Keweenawan Series from their outcrop localities in Minnesota and Wisconsin through Iowa and Nebraska. A map of the subsurface extent of the mafic rocks, based on the intricate magnetic patterns, shows that the rocks form a long, semicontinuous block, averaging 40 mi wide and consisting mainly of a sequence of layered flows. This sequence is probably fault-bounded and has been tilted up along the margins, where the linearity of the anomalies indicates steeper dips. The associated clastic rocks, indicated by a smoother magnetic pattern, occur in basins along both sides of the mafic belt and in grabens and a series of axial basins on the upper surface of the block. The well-defined outliers of flows marginal to the main block and the truncation of some of the outermost flow units along a diagonal boundary striking at an angle to them suggest that the present boundaries of the block are postdepositional structural features. The basins and the edges of the block appear to have controlled later, largely vertical movement in the overlying Paleozoic and younger sedimentary cover. Calculated models based on coincident magnetic and detailed gravity profiles along typical cross sections of the midcontinent gravity high show that the block of mafic rocks is steep-sided and as much as several miles thick. The free-air gravity anomaly, which consists of a large positive maximum flanked by minima, averages very close to zero, indicating that this major crustal feature is regionally compensated, although locally each of its components shows a large departure from equilibrium. Remanent magnetization is a primary factor in the interpretation of the magnetic data. Magnetic property studies of Keweenawan mafic rocks in the Lake Superior region show that remanent magnetization may be five times the magnetization induced by the present Earth's field and differs from it radically in direction. This magnetization was acquired before the flows were tilted into their present positions. A computed magnetic profile shows that a trough of flows with such a magnetization and inward-dipping limbs can account for the observed persistent lows along the western edge of the block, the relatively low magnetic values along the axis of the block, and the large positive anomaly along the eastern side of the block. Flows as much as 1 mi thick near the base of the sequence have a remanent magnetization with a nearly opposite polarity. This reverse polarity has been measured on both sides of Lake Superior and is probably also present farther south, particularly in Iowa where the outer units of the block in an area north of Des Moines give rise to a prominent magnetic low. The axis of this long belt of Keweenawan mafic rocks cuts discordantly through the prevailing east-west-trending fabric of the older Precambrian terrane from southern Kansas to Lake Superior. This belt has several major left-lateral offsets, one of which produces a complete hiatus in the vicinity of the 40th parallel where an east-west transcontinental rift or fracture zone has been proposed. The axial basins of clastic rocks are outlined by linear magnetic anomalies and show a concordant relation to the structure of the mafic flows. These basins are oriented at an angle to the main axis, suggesting that the entire feature originated as a major rift composed of a series of short, linear, en echelon segments with offsets similar to the transform faults characterizing the present mid-ocean rift system. This midcontinent rift may well have been part of a Keweenawan global rift system with initial offsets consisting of transform faults along pre-existing fractures, but apparently it never fully developed laterally into an ocean basin, and the upwelling mafic material was localized along a relatively narrow belt.

Geological Society of America Bulletin↗
USGS1971

Age and chemistry of tertiary volcanic rocks in north-central Arizona and relation of the rocks to the Colorado Plateaus

During late Miocene (14.8 m.y.) to early Pliocene (10.1 m.y.) time, local latite and widespread basaltic flows accumulated with associated continental sedimentary deposits in north-central Arizona. Some of these rocks were displaced and tilted by normal faults, and new drainage, now occupied in part by basalt flows of late Pliocene age (average 5 m.y., maximum of 6 m.y.), was established at the southern margin of the Colorado Plateaus. The time of faulting and uplift of the Colorado Plateaus in this region is thus bracketed between about 10.1 and 6 m.y. ago. Five analyzed basalts fall within the tholeiitic basalt field of the alkali-silica diagram and twenty are in the alkalic basalt field; in general, they are similar to other alkali-olivine basalts from the western United States.

Arizona↗
USGS1972

Pliocene uplift of the grand canyon region - time of drainage adjustment

Tertiary gravel deposits in ancient stream channels along the southern margin of the Colorado Plateaus of northern Arizona show by composition and structure that these deposits came from sources to the south and southwest at a time when central Arizona stood higher than the present Grand Canyon region. Three cobbles of basalt included in the gravel deposits have K-Ar ages of about 10.0 m.y., 12.2, and 12.4 m.y. showing that the major uplift of the plateau in northern Arizona had not taken place at that time. The present south-flowing drainage of the Verde River and neighboring streams resulted from final elevation of the northern Arizona region relative to central Arizona, and must have developed well before about 5 m.y. - the age of some basalts that flowed into the Verde Valley. Thus, the major relative uplift of the southern part of the Colorado Plateaus must have occurred within the 5 to 10 m.y. interval, or in early to middle Pliocene time. This time of uplift also was the time of major canyon erosion, including the cutting of Grand Canyon, within the Plateaus province. © 1972, The Geological Society of America, Inc.

Arizona↗
USGS1972

Hawaiian-emperor chain and its relation to cenozoic circumpacific tectonics

The Hawaiian Ridge and Emperor Seamounts appear to form a single chain of tholeiitic shield volcanoes that erupted sequentially on the sea floor of the central Pacific Ocean during Tertiary and Quaternary time. The chain cuts obliquely across the older Cretaceous structural patterns of that sea floor. While the pattern of the chain as a whole is linear, the individual volcanoes lie on short, sigmoidal, en echelon loci that are subparallel with respect to each other and that may represent extensional features in the crust and upper mantle. In general, the order of eurption progressed from northwest to southeaśt along the chain, but the rate of progression of volcanism along individual loci is nonlinear where best studied in the southeastern part of the chain. Furthermore, simultaneous eruptions appear to have occurred within a distance along the chain of about 200 to 400 km. The available data are consistent with a genesis related to the motion of the Pacific crust over a melting spot in the mantle. This melting spot, which may be due to either excess heat or pressure release, appears to have a diameter of about 300 km and is presently centered slightly north of the island of Hawaii. We concur with the idea that the bend in the Hawaiian-Emperor chain probably reflects a significant change in the motion of the Pacific plate. Our best estimate of the age of the Hawaiian-Emperor bend, based on the existing radiometric data, is 24.6 ± 2.5 m.y., which correlates with a time of increased tectonic activity in the western Pacific island arcs and along the northern and eastern boundaries of the Pacific plate. The vector change in the motion of the Pacific plate (with respect to the melting spot) that is required to produce the bend is about 12 cm/yr in a west-southwest-ward direction. © 1972, The Geological Society of America, Inc.

Hawaii↗
USGS1973

Earthquakes near Mount St. Helens, Washington

Seismic monitoring around Mount St. Helens, Washington, for 28 days during the summer of 1970 showed that the frequency of local earthquakes was from 3 to 13 per day and very similar to the activity previously observed near Mount Rainier, Washington. The epicenters of the well-recorded earthquakes form a roughly linear pattern trending northeast from the summit area. This is the only obvious trend in the epicenter distribution. Recent geological investigations have not revealed faults or other major geological discontinuities along this trend. This pattern of epicenters seems to be approximately aligned with the remnants of five ancestral St. Helens plug domes that trend southwest from the summit region.

Washington↗
USGS1973

Potassium-argon ages and paleomagnetism of the Waianae and Koolau Volcanic Series, Oahu, Hawaii

Paleomagnetic and potassium-argon measurements on 786 oriented cores from 99 volcanic units at 18 sites in the Waianae and Koolau Ranges, Oahu, when combined with data from previous studies, show that the sub-aerial Waianae Volcano was active only from about 3.6 to 2.4 m.y. ago and the subaerial Koolau Volcano from about 2.6 to 1.8 m.y. ago. There is some evidence that Waianae Volcano was still active when Koolau Volcano emerged from the sea. The predominantly tholeiitic lower and middle members of the Waianae Volcanic Series are approximately contemporaneous and were extruded during the late Gilbert and early Gauss geomagnetic polarity epochs. They were followed within less than 0.2 m.y. by the alkalic lavas of the upper member, which were probably extruded largely during the later part of the Gauss normal polarity epoch. The Koolau Volcanic Series was extruded entirely during the early Matuyama reversed polarity epoch. Data from three thick stratigraphic sections in the Waianae and Koolau Volcanic Series indicate that stacks of lava flows as much as 470 m thick can be formed in less than 0.25 m.y. and that the maximum average period between superimposed lava flows is on the order of 103 yrs. Additional data on the hawaiite flow that led to the discovery of the Kaena reversed event indicate that this reversed flow is 2.85 ± 0.05 m.y. old. Angular dispersion of virtual geomagnetic poles (VGP) in the Hawaiian Islands appears to have decreased during the past 5 m.y. This may be caused by a decrease in dipole wobble, a decrease in the nondipole component of the Earth' magnetic field, or the accumulated effects of weathering, tectonism, and geomorphic processes in older rocks. The mean Waianae and Koolau VGPs are slightly on the side of the Earth's rotation axis away from Oahu. This supports, but does not prove, the hypothesis that the axial dipole is displaced slightly northward from the Earth's center. Three VGP “excursions” were recorded in sections of lava in the Waianae and Koolau ranges. During these excursions, the VGPs appear to have traveled away from or toward the geographic axis along great circle paths, suggesting they may be related to the dipole rather than the nondipole field and may record aborted reversals in polarity or rapid and infrequent dipole tilts.

Hawai'i↗
USGS1973

Reconnaissance study of the strontium isotopic composition of Cenozoic volcanic rocks in the northwestern Great Basin

Sixteen mafic and intermediate lava flows of Eocene to Pleistocene age from the northwestern Great Basin have initial Sr 87 /Sr 86 ratios of from 0.7029 to 0.7047. Seven upper Miocene mafic and intermediate lava flows have initial ratios of from 0.7037 to 0.7041, suggesting a common source for the Steens Basalt and contemporaneous rocks of the northwestern Great Basin. Values of 0.7047 and 0.7033 obtained on olivine basalts of early Miocene and Quaternary ages, respectively, suggest that these lavas were derived from different source materials than were the late Miocene rocks. Unusually low values of 0.7029 obtained on two specimens of andesite from the Eocene Cedarville Series of Russell (1928) suggest that these lava flows were derived from still another source material. Ten silicic volcanic rocks, most from widespread and voluminous ash-flow sheets and lava complexes, have initial ratios of approximately 0.7023 to 0.7057. Some of the more radiogenic values, obtained on very highly differentiated and strontium-poor rock units, provide only an upper limit for the initial Sr 87 /Sr 86 ratios of the parent magmas. The close similiarity of the strontium isotopic composition of the silicic rocks to those of spatially and temporally associated intermediate lavas supports the concept that the widespread Miocene silicic volcanic rocks of the northwestern Great Basin were produced by the fractional crystallization of enormous volumes of mantle-derived mafic magma. The data also provide indirect support for the interpretation that the relatively radiogenic character of many salic volcanic rocks from other parts of the Great Basin largely reflects the presence of an unusually radiogenic mantle rather than the involvement of crustal material.

California, Idaho, Nevada, Oregon↗
USGS1973

Submarine chert-argillite slide-breccia of Paleozoic age in the southern Klamath Mountains, California

A unique chert-argillite breccia—a breccia with an argillite matrix, in which nearly all the fragments are chert—underlies an area of at least 60 sq mi in the southern Klamath Mountains of California. Rocks of this composition have not been reported previously, in the Klamath Mountains or elsewhere, but in northwestern Trinity County they make up a large part of the western Paleozoic and Triassic belt. The remainder of the belt consists of northerly trending sinuous zones of two other principal lithologies: radiolarian chert, rhythmically interbedded with slate or phyllite, crops out in three zones in the eastern, central, and western parts of the area; and massive metabasalt and metadiabase intruded by gabbro and serpentinite occur in a zone about 3 mi wide in the eastern part of the area. Although the predominance of chert fragments makes the chert-argillite breccia apparently unique, rocks that are generally similar, consisting of angular to subangular clasts in a pelitic matrix, have been studied in many localities. Field relations and petrographic features of the chert-argillite breccia—specifically the angularity of clasts, absence of a sandy matrix, preservation of Radiolaria in both chert clasts and pelitic matrix, presence of contorted flow laminae in the matrix, large size of some of the exotic blocks, close association with chert, pillow lava, and limestone, absence of stretching or slickensiding of clasts, and overall dimensions of the unit measurable in miles—variously preclude origin of the rock as a normal sedimentary conglomerate, a tectonic breccia, a tillite, or a subaerial mudflow, but include features common to numerous submarine-slide deposits. We interpret the chert-argillite breccia as the result of uplift and subsequent sliding of a thick deposit of inter-layered chert and pelitic ooze. Brittle fracturing of the chert and mobilization of water-saturated ooze produced the observed features. Fossiliferous limestone blocks within the slide-breccia indicate that it is no older than Silurian or Devonian, and it predates Jurassic metamorphism and plutonism. The relation of the breccia to a major northwest-trending fault zone, and to serpentinite within this zone, suggests that the formation of the breccia was generally synchronous with the faulting and ultramafic intrusion. Rocks of the central metamorphic belt, east of the mapped area, were metamorphosed during a Devonian orogeny, and we suggest that the submarine slide-breccia was formed as a frontal effect of the same orogeny.

California↗
USGS1973

Mantle convection and volcanic periodicity in the pacific; Evidence from Hawaii

The thermal-feedback theory of mantle melting proposed by Shaw in 1969 is found to be quantitatively consistent with data pertaining to the evolution of the Hawaiian Ridge. Applicable rate factors are estimated from relations between lava volumes and position along the ridge given in this paper and the radio-metric age distributions given by Jackson and others in 1972. Rate curves derived from these data provide a new method of age extrapolation or interpolation; results indicate that previous methods used to estimate the age of the Hawaiian-Emperor Bend are in error. No definite age is established, but calculations suggest an age greater than 50 m.y. Much more extensive radiometric data are required to define kinematic relations between the Hawaiian Ridge and Emperor Seamount chain. It appears to be firmly established from the work of Jackson and others and from the present study that the evolution of the Hawaiian Ridge has been episodic, with episodes of several different time scales. Average growth rates of the entire ridge system are divided into two regimes with a discontinuity at a position roughly 1,000 km northwest of Kilauea; the estimated age of this discontinuity is about 10 m.y. Other episodes relate to the durations of eruptive sequences along individual or contiguous lines of volcanoes within the en échelon set of locus lines defined by Jackson and others. The latest of these episodes, beginning about 6 m.y. ago, is marked by accelerating volume rates of eruption and accelerating rates of ridge propagation; this episode appears to be approaching a culminating stage represented by the present activity of Kilauea Volcano. The calculated rate of eruption of Kilauea (0.11 km 3 per yr) is virtually identical with a rate independently estimated by Swanson in 1972 using different data. Calculated durations for older locus lines are generally greater than 6 m.y., but major time overlaps occur that are not adequately understood. Episodic behavior of shorter durations also exists relative to growth of individual shields or to synchronous activity on neighboring shields (for example, Mauna Loa and Kilauea). Some of these shorter term effects are partly explained in terms of isostatic factors acting on the lithosphere and asthenosphere. The longer episodes are explained in terms of variations of melting rates in the asthenosphere, governed by viscous heating produced by the interaction of lithosphere translation and both vertical and horizontal shear flows in the subjacent mantle. Accelerations of eruption and propagation rates are explained by melting instabilities in the upper zones of the asthenosphere as a result of thermal feedback. During the latest melting episode, shear stresses in the asthenosphere derived from the rate data as interpreted by the thermal feedback model are in the range 100 to 200 bars; apparent viscosities range from 2 × 10 21 to 4 × 10 20 poise, decreasing with increasing melting rate. In general, a thermomechanical model is shown to be consistent with the idea that oceanic melting spots can be fixed relative to the deep mantle, although this invariance is not completely established. The thermal plume model of Morgan is not definitely ruled out but does not seem to be required for internally consistent interpretations of oceanic chains of volcanism. It is concluded that motion vectors of the Pacific plate cannot be inferred directly from rates of propagation of volcanic chains, because these rates reflect local, not average, relative velocities of lithosphere versus mantle flow. During growth of the Hawaiian Ridge, propagation speeds calculated on the basis of rate data for the southeastern Hawaiian Islands ranged from less than 1 cm per yr near the Hawaiian-Emperor Bend to nearly 30 cm per yr at the present ridge front.

Hawaii↗
USGS1973

Petrology of the Vulcan Peak alpine-type peridotite, southwestern Oregon

The alpine-type peridotite in the area of Vulcan Peak, Oregon, is part of the larger Josephine ultramafic complex in the Klamath Mountains geologic province. Partially serpentinized, foliated harzburgite with 15 to 30 percent orthopyroxene makes up approximately 90 percent of the body. The remaining 10 percent is dunite that occurs in the harzburgite as concordant and discordant layers and as irregular bodies. In general, the peridotite at Vulcan Peak is similar in structure, texture, mineralogy, and chemistry to the peridotite at Burro Mountain, California. Structures, textures, and compositions of coexisting phases are consistent with high-temperature (1,000° to 1,200°C) deformation and recrystallization in the upper mantle, and tectonic emplacement into its present crustal position. Evidence to indicate whether the peridotite originated as a refractory residue during partial fusion processes that produced mafic melt or by crystallization from an ultramafic or picritic magma remains inconclusive; poikilitic clinopyroxene enclosing olivine in some dunites, and certain chromitite textures, may represent relict igneous features suggesting a magmatic stage in the history of the peridotite.

Oregon↗
USGS1973

Isotopic and paleontologic evidence for correlating three volcanic sequences in the Maine coastal volcanic belt

The correlation of the Castine Volcanics, the Thorofare Andesite–Vinalhaven Rhyolite sequence, and the Cranberry Island Series of Shaler (1889), suggested by their similar lithologies, appearance, and structural histories, is supported by the results of Rb-Sr whole-rock isotopic analyses and by the faunal assemblages from old and new fossil localities in the Castine Volcanics, Ames Knob Formation, and Thorofare Andesite. The volcanic rocks are partly Late Silurian, but mostly Early Devonian in age and yield an average radiogenic age of 390 ± 5 m.y. The use of calcite-bearing volcanic samples for whole-rock Rb-Sr dating degrades the method by greatly increasing the uncertainty of the isochron and initial Sr 87 /Sr 86 . Lower to Middle Devonian granitic plutons have initial ratios of Sr 87 and Sr 86 similar to those in the volcanic formations. The Castine Volcanics and the Lower Devonian granite of Sedgwick may be comagmatic, but the time interval between the extrusion of the Vinalhaven Rhyolite and its intrusion by the Middle Devonian granite of Vinalhaven Island is too long to support the comagmatic hypothesis.

Maine↗
USGS1973

River channel change with time : An example

Monumented channel cross sections were resurveyed over a period of 20 yrs (1953 to 1972) to determine the amount and kind of change of channel area and position on a 3.7-sq-mi basin, Watts Branch near Rockville, Maryland. For the first 12 yrs, the channel progressively but slowly became smaller as urbanization of the basin gradually proceeded. After 1966, a threshold of change apparently was passed and, probably as a result of an increased rate of land alteration upstream, large amounts of sediment were deposited within the channel and overbank. The number of floods exceeding channel capacity increased dramatically from an average of two to more than ten per year. Simultaneously, the channel area began to increase. Despite the trend toward increasing cross-sectional area, the net result after 20 yrs was a channel smaller by 20 percent than it had been originally. Urbanization did not alter the rate of channel migration.

Maryland↗
USGS1973

Radiometric ages of intrusive rocks in the Little Belt Mountains, Montana

Radiometric ages indicate that most, if not all, of the major intrusions in the Little Belt Mountains, central Montana, were emplaced during the Eocene epoch, between 48 and 54 m.y. ago. In the Hughesville area, igneous activity continued, or was episodic until 42 m.y. ago. As a result of the continued igneous activity, radiometric ages in the Hughesville area can be interpreted either as primary ages or as reset ages.

Montana↗
USGS1973

Petrogenesis of the Superstition-Superior volcanic area as inferred from strontium- and oxygen-isotope studies

Apparent initial Sr 87 /Sr 86 ratios of five ash-flow tuffs (0.7063 to 0.7139) and several mafic to silicic lavas (0.7055 to 0.7131) indicate that the magmas were derived below the base of the Precambrian granitic crust (0.7231 to 1.0906). Liquidus compositions in the system Q-Or-Ab-H 2 O and oxygen-isotope geother-mometry suggest that the silicic magmas started to crystallize quartz, magnetite, and two feldspars in a water-undersaturated environment of high pressure (∼10 kb) and moderate temperature of at least 830°C. During or after ascent into the crust, the magmas underwent varying degrees of crystal-melt re-equilibration. Measured plagioclase-biotite O 18 fractionations (0.5 to 0.7) imply a temperature that is too high for the observed mineral assemblage, and the inference is that the two minerals did not crystallize in equilibrium. Prior to eruption, the upper part of the magma column assimilated crustal Sr such that the base of each ash flow is now enriched in Sr 87 . In some cases, this assimilation was too rapid to allow crystal-melt equilibration of Sr isotopes. δO 18 values for the magmas are within the range typical of similar magma types, indicating that no significant interaction took place between the melts and meteoric water; however, some rocks have exchanged oxygen isotopes with meteoric water at low temperatures after eruption. Some of the magma appears to have had a long residence in the crust at lower pressure (1 kb) and temperature (750°C), because two of the ash-flow tuffs and one lava are greatly enriched in Sr 87 and have largely re-equilibrated under the P-T conditions of a shallow magma chamber. Even these, however, have retained evidence for a multistage genesis.

Geological Society of America Bulletin↗
USGS1973

Interpretation of a high-grade Precambrian terrane in northern Idaho

A terrane of high-grade metamorphic rocks in northern Idaho and northeastern Washington is almost completely surrounded by low-grade rocks of the Precambrian Belt Supergroup. The high-grade terrane includes both Belt and pre-Belt rocks. Four events of folding and metamorphism occurred in the high-grade terrane. The first three events may have been associated with the Late Cretaceous emplacement of quartz monzonite of the Kaniksu batholith; the fourth may have been associated with a slightly later emplacement of granodiorite or with a Tertiary plutonic and volcanic episode. A much older event of plutonism in the high-grade terrane is recorded by zircon, which was dated by the Pb-U method at 1,500 m.y. from pre-Belt meta-igneous augen gneiss. Evidence of regional events intermediate in age between 1,500 and 100 m.y. has been found in the surrounding low-grade rocks but not in the high-grade terrane.

Idaho↗