Showing papers in "The Journal of Geology in 1983"
TL;DR: The authors showed that rivers with large sediment loads (annual discharges greater than about $15 \times 10^{6}$ tons) contribute about $7 −times 10 −9$ tons of suspended sediment to the ocean yearly.
Abstract: New data and new estimates from old data show that rivers with large sediment loads (annual discharges greater than about $15 \times 10^{6}$ tons) contribute about $7 \times 10^{9}$ tons of suspended sediment to the ocean yearly. Extrapolating available data for all drainage basins, the total suspended sediment delivered by all rivers to the oceans is about $13.5 \times 10^{9}$ tons annually; bedload and flood discharges may account for an additional $1-2 \times 10^{9}$ tons. About 70% of this total is derived from southern Asia and the larger islands in the Pacific and Indian Oceans, where sediment yields are much greater than for other drainage basins.
3,409 citations
TL;DR: Sandstones of the Paleozoic turbidite sequences of eastern Australia show a large variation in their major element geochemistry, reflecting the distinct sedimentary provenance and tectonic setting as discussed by the authors.
Abstract: Sandstones of the Paleozoic turbidite sequences of eastern Australia show a large variation in their major element geochemistry, reflecting the distinct sedimentary provenance and tectonic setting ...
1,518 citations
TL;DR: In this paper, the development of pull-apart basins is discussed based on results of field studies in active and ancient strike-slip zones, and field observations from mainly active basins are synthesized into a qualitative model of the continuous growth of pullaparts from nucleation to extreme development.
Abstract: The development of pull-apart basins is discussed based on results of field studies in active and ancient strike-slip zones. The plate tectonic setting of active pull-aparts is described and field observations from mainly active basins are synthesized into a qualitative model of the continuous growth of pull-aparts from nucleation to extreme development. This model is compared with predictions from previously proposed qualitative, theoretical, and experimental models.
472 citations
TL;DR: In this article, a thin section and chemical study of 95 sand samples from the Amazon River system in Brazil, Peru, Ecuador, and Colombia, a river system with great contrasts in climate and rock types, reveals that sand derived from the Andes are lithic arenites whereas sands derived from Precambrian terrains are much richer in quartz.
Abstract: Key Idea: Following the spirit of P. D. Krynine, the sands of the world's largest tropical river system are studied to reveal much about the general origin of sand and sandstone. Thin section and chemical study of 95 sand samples from the Amazon River system in Brazil, Peru, Ecuador, and Colombia, a river system with great contrasts in climate and rock types, reveals that sands derived from the Andes are lithic arenites whereas sands derived from Precambrian terrains are much richer in quartz, as are sands in rivers draining Tertiary molasse. Sand becomes more mature downstream along the main stem of the Amazon-Solimoes-Maranon so that a sample analyzed at the mouth does not accurately reflect headwater composition. Dilution by quartz-rich tributaries and quartz-rich Tertiary outcrops, possible differential elimination of rock fragments, and even possible weathering on the modern flood plain of the Amazon contribute to enhanced maturity downstream. We speculate that, although data on modern, large subarti...
206 citations
TL;DR: Gases occluded in fracture zones of active faults are characterized by a high concentration of CO 2 and/or CO 2 in the lithofacies which the fault cuts Carbon dioxide concentration in sediments fluctuates with temperature as discussed by the authors.
Abstract: Gases occluded in fracture zones of active faults are characterized by a high concentration of $$H_{2}$$ and/or $$CO_{2}$$ A predominant gas species-$$H_{2}$$ or $$CO_{2}$$-is related to the lithofacies which the fault cuts Carbon dioxide concentration in sediments fluctuates with temperature This evidence and $$\sigma ^{13}C$$ of$$CO_{2}$$ (about -20‰) suggests that $$CO_{2}$$ originates from organic materials Carbon dioxide with $$\sigma ^{13}C$$ of 5‰ to - 17‰ in brecciated gneiss containing marble may have been produced by interaction between organically derived $$CO_{2}$$ and the marble, or alternatively, may have been magmatically derived Hydrogen usually occurs in sheared silicate rocks, and its concentration fluctuates a great deal The concentration of $$H_{2}$$ from active faults associated with historical earthquakes usually amounts to as high as several percent in maximum, whereas the concentration of $$H_{2}$$ from Quaternary faults not associated with historical earthquakes is at most 1
160 citations
TL;DR: In this paper, the authors identified 69 previously-recognized Paleozoic and Mesozoic storm depositional systems on global paleogeographic maps with the assumption that this latitudinal zonation of storm systems existed in the geological past.
Abstract: Present-day hurricane belts are confined to 5°-45° latitude along the western edges of oceans, whereas winter storms occur ubiquitously above 25° latitude. Both hurricanes and winter storms occur between 25° and 45° latitude in some areas. Storm zones can be identified on global paleogeographic maps with the assumption that this latitudinal zonation of storm systems existed in the geological past. Evaluation of 69 previously-recognized Paleozoic and Mesozoic storm depositional systems on such maps showed that only 70% of them occur in ancient hurricane, mixed hurricane and winter storm, and winter storm belts. The remainder occur on the western sides of continents and/or at low paleolatitudes, and therefore well may be of different origin. Mesozoic storm depositional systems containing hummocky stratification tend to occur in counterpart paleo-winter storm belts, whereas hummocky stratification in suggested Paleozoic examples occur in both winter storm and hurricane belts and in paleolatitudes where we in...
149 citations
TL;DR: In this article, the authors hypothesize that depression of pore pressure resulting from rebound during erosional unloading also may occur in certain geologic settings, such as some shales.
Abstract: When rocks deform in response to changes in stress, the small variations in pore volume that occur affect the pore fluid pressure. The changes in fluid pressure can be significant if the rate of change of stress is large relative to the rate at which pressure perturbations are dissipated by flow. It has been proposed previously that the gradually increasing loads on sediments undergoing burial can cause excess fluid pressures. We hypothesize that the opposite effect, depression of pore pressure resulting from rebound during erosional unloading, also may occur in certain geologic settings. Simple theoretical treatment of the problem indicates that rocks with small hydraulic diffusivity, such as some shales, could experience significant pore pressure decreases when unloaded by moderate rates of erosion. The analysis further indicates that decreases sufficient to produce negative pressures can occur. No theoretical bar to this condition exists, and it has been achieved in the laboratory. However, in subsurfa...
123 citations
TL;DR: In the intervening Merrimack-Fredericton Trough, metamorphosed and poly deformed turbidites and black shales record deep water conditions in Silurian as mentioned in this paper.
Abstract: Paleogeographic analysis of post-Taconian rocks in New England and adjacent parts of Canada has revealed the existence of two volcanic arcs which shut off at the time of the Acadian Orogeny. One was built on arc basement previously accreted to North America during the Taconic Orogeny, the other on Precambrian continental basement of Avalonia. In the intervening Merrimack-Fredericton Trough, metamorphosed and poly deformed turbidites and black shales record deep water conditions in Silurian. Following McKerrow and Ziegler (1971), this tract is interpreted as the site of an ocean which closed in Siluro-Devonian by simultaneous subduction beneath both continental margins. In the Molucca Sea in Indonesia, a comparable arc-arc collision is in an early stage of development; Moore et al. (1982) suggested that an accretionary prism built against one arc is overthrusting its counterpart, which developed on the other side of the ocean. An identical geometry, with an Avalonian accretionary prism overriding the conve...
119 citations
TL;DR: A revised denudational chronology based on K-Ar dating of basalts for much of southeastern Australia shows that many major topographic features are far older than was previously realized as mentioned in this paper.
Abstract: A revised denudational chronology based on K-Ar dating of basalts for much of southeastern Australia shows that many major topographic features are far older than was previously realized. Some upland surfaces originated in the Mesozoic and others in the Early Tertiary; in some areas the last major uplift and onset of canyon cutting occurred prior to the Mid-Oligocene. Apart from the slow headward extension of canyons and localized high level stripping, much of the region had taken on approximately its present-day form by the Mid-Miocene. Paradoxically, however, none of the mechanisms that have been proposed to explain the survival of paleoforms hold here. These instances may well be typical of large parts of the interior of tectonic plates, and a revision of widely held estimates of modal rates of denudation seems required.
117 citations
TL;DR: In this article, an index of inflection asymmetry is presented and applied to 15 rivers: the data confirm the statistical preference for delayed inflection, and an explanation of the phenomenon is offered based on the hydraulics of flow through bends.
Abstract: Inspection of the planform of meandering rivers indicates that they are generally not symmetric but display at least two forms of asymmetry indicative of flow direction. One is downchannel delay in the inflection point of meanders, producing traverses (the part of the channel trace that crosses from one side of the valley to the other) which are dominantly convex facing down valley. The other is upvalley skew in the axis of gooseneck loops. An index of inflection asymmetry is presented and applied to 15 rivers: the data confirm the statistical preference for delayed inflection. An explanation of the phenomenon is offered based on the hydraulics of flow through bends. Of major importance appears to be the persistence downchannel of patterns of helical circulation and cross-sectional distribution of longitudinal velocity well past the bend that forms them. The upvalley skew of goosenecks is, in many cases, nothing more than an extreme form of convex-downvalley looping, maintained as meanders have increased ...
115 citations
TL;DR: The physical and thermal properties of carbonatite magmas are estimated by analogy with alkali carbonate melts, which are ionic liquids, composed of discrete, unpolymerized ions as discussed by the authors.
Abstract: The physical and thermal properties of carbonatite magmas are estimated by analogy with alkali carbonate melts, which are ionic liquids, composed of discrete, unpolymerized ions. Carbonatite magmas are estimated to have low viscosity (on the order of 5 × 10-2 poise), low heat of fusion (175 J/gm), and large thermal diffusivity (4 × 10-3 J/cm-sec-K) compared to silicate melts. Dissolved water will reduce viscosity; dissolved orthosilicate will have little effect on viscosity because silicate tetrahedra will remain as unpolymerized, discrete anions. Solution of polymerized silicate compounds will increase melt viscosity as long-chain or network silicate polymers form; exsolution of polymerized silicate melt will limit the concentration of long-chain or network silicate polymers in the carbonate melt and provide an upper limit on viscosity. Given estimates of the physical and themal properties of carbonatite magma, it is possible to evaluate the importance of possible physical processes in carbonatite magma ...
TL;DR: The Eagle Peak pluton is zoned with an equigranular granodiorite margin and porphyritic granitic core; in contrast, the nearby, and coeval Red Lake granodiciorite pluton was relatively unzoned.
Abstract: The Eagle Peak pluton is zoned with an equigranular granodiorite margin and porphyritic granitic core; in contrast, the nearby, and coeval Red Lake granodiorite pluton is relatively unzoned. Major element abundance variations in both plutons can be explained by segregation of the early phases, plagioclase and hornblende. However, the preferential decrease of heavy rare-earth elements (HREE) from margin to core in the Eagle Peak pluton cannot be explained by fractional crystallization of a melt composition similar to the marginal rocks. The HREE decrease is best explained by segregation of hornblende from a mixture of melt and minerals equivalent in composition to the marginal rocks. Both plutons contain aplites that crystallized from highly evolved melts formed by segregation of mafic minerals, feldspars, and an accessory phase rich in light REE. Mafic inclusions are related to their host granitoids, and they may be remnants of restite or early accumulates. The small negative Eu anomalies in both plutons ...
TL;DR: Xenoliths of garnet-plagioclase clinopyroxenite, olivine websterite, and garnet anorthosite are relics of an igneous suite of olivines-normative alkali gabbros metamorphosed into granulites under Lashaine, Tanzania, at ∼1200 K and 14 kb.
Abstract: Xenoliths of garnet-plagioclase clinopyroxenite, garnet websterite, olivine websterite, and garnet anorthosite are relics of an igneous suite of olivine-normative alkali gabbros metamorphosed into granulites under Lashaine, Tanzania, at ∼1200 K and 14 kb. Most clinopyroxene megacrysts recrystallized into polygonal clusters of small grains, and plagioclase laths exsolved from the cores. Clinopyroxene (CATs 11 mole %, Jd 15) and plagioclase (An28–41, Or1–2) reacted into atolls of garnet. Meionitic scapolite with widely variable sulfate/carbonate developed from plagioclase, oxidized sulfides and CO2. Needles of bent, multiply-twinned kyanite pervade the plagioclase. Prolonged metamorphism homogenized most minerals, including plagioclase next to scapolite. Brown glasses (SiO2 38–56 wt %, Al2O3 15–23, K2O 0.5–8) and dark alteration products mainly occur as rims to garnet, clinopyroxene, and scapolite. One glass pocket with quench plagioclase and hollow clinopyroxene contains two glass populations attributed to...
TL;DR: Experiments conducted at 30 kbar, 850-1050°C, in piston-cylinder apparatus simulate hybridization between hydrous siliceous magma rising from subducted oceanic crust into overlying peridotite.
Abstract: Experiments conducted at 30 kbar, 850-1050°C, in piston-cylinder apparatus simulate hybridization between hydrous siliceous magma rising from subducted oceanic crust into overlying peridotite. Gold capsules containing granite and peridotite powders separated by sharp boundaries, and H_2O, were run in vertical and horizontal positions. The aqueous vapor caused minor metasomatic changes in the peridotite until it was withdrawn into the melting granite, producing H_2O-undersaturated granite liquid adjacent sintered, anhydrous peridotite. Minerals developed in the hybrid reaction zones between liquid and peridotite are orthopyroxene and jadeitic clinopyroxene in all runs, and one or more of garnet, phlogopite, and quartz in some runs. (The granite liquid moves upwards relative to the peridotite even in runs of 27 hours.) In the longest runs of 120 hours the hybridization zone remains narrow, the body of granite liquid remains crystal-free and only slightly changed in composition (lower SiO_2, MgO increased from 0.1% to about 1.5%). The products are consistent with phase equilibria in synthetic model systems, and the system peridotite-granite-H_2O (determined using mixtures). The hybridization process in subduction zones would produce discrete rock bodies dominated by pyroxenites without olivine. Na is fixed in jadeitic clinopyroxene. K is fixed in phlogopite dispersed through the pyroxenite, but there is a prospect that it may become concentrated into phlogopite-rich rocks by crystal fractionation. Partial melting of these source rocks would generate magmas different from those generated in peridotite or subducted oceanic crust.
TL;DR: In this article, the petrology of the pelitic-matrix melange matrix was compared to that of the high pressure/low-temperature metashales from the Diablo Range and the low-pressure/low temperature metashale from the base of the Great Valley sequence.
Abstract: The central belt of the Franciscan subduction complex of California is largely a zone of pelitic-matrix melange bearing exotic blocks of blueschist and eclogite. In outcrop, the matrix appears to be a black shale with an anastomosing fracture cleavage. In this study, the petrology of the melange matrix was compared to that of the high-pressure/low-temperature metashales from the Diablo Range and the low-pressure/low-temperature metashales from the base of the Great Valley sequence. The shale matrix and high-pressure metashales (7 < P < 10 kb) contain the assemblage quartz + albite + phengitic white mica + chlorite + sphene + trace amounts of lawsonite, pumpellyite, or calcite. By contrast, low-pressure Knoxville shales (2 < P < 3 kb) contain quartz + albite + mixed-layer clay minerals + calcite. Most matrix samples from the exotic block-bearing melange and high-pressure metashale have illite crystallinities of 4 to 10 mm, up to 100% 2M white mica, densities exceeding $$2.6 gms/cm^{3}$$, organic matter app...
TL;DR: The volcano Kerimasi lies on the Eastern Rift Zone of Africa and is built up of tuffs, agglomerates, and flows which range in composition from nephelinites to carbonatites.
Abstract: The volcano Kerimasi lies on the Eastern Rift Zone of Africa and is built up of tuffs, agglomerates, and flows which range in composition from nephelinites to carbonatites. Many of the surface rocks are tuffs which contain lapilli with microphenocrysts of melilite and calcite. A coarse-grained apatite-sovite containing monticellite and periclase overgrowths on magnesioferrite outcrops within the crater. A flow near the crater rim contains calcite phenocrysts which are single crystals and show complex zoning with cathodoluminescence. These crystals are often aligned giving a trachyitic texture and are thought to represent primary magmatic crystallization of calcite.
TL;DR: The mantle xenoliths from Olmani, Tanzania have deformation textures and fluid inclusions like those in olivine-rich peridotites from Hawaii.
Abstract: Nineteen mantle xenoliths from Olmani, Tanzania, have deformation textures and fluid inclusions like those in olivine-rich peridotites from Hawaii. Group A comprises 7 barren harzburgites (spinel, TiO2 <. 1 wt %) and group B (spinel, TiO2.8–2.6 wt %) contains 6 clinopyroxene-dunites, 2 lherzolites, 1 wehrlite, 1 two-pyroxene-dunite, and 1 olivine-orthopyroxenite. Both groups lack garnet and porphyroclastic textures and display fingerprint intergrowths of spinel with ortho-or clinopyroxene. Many secondary inclusions of several generations within olivine are rich in Co2. Alkali-rich glasses (Na2O.4–7.3 wt %, K2O 3.3–5.1, P2O5 ∼1) in B xenoliths are unrelated chemically to the olivine-augite-phyric ankaramite host. Aphyric inter-granular glass occurs along with larger patches of residual glasses associated with euhedral olivine, clinopyroxene and spinel. Clinopyroxenes of B xenoliths show decreasing Na2O (2.1–.3 wt %) and Cr2O3 from olivine-brown cores to clear margins of larger grains, and to finer grains a...
TL;DR: In this article, the Menderes Massif was studied to shed some light on its geological evolution and to compare it with that of the southern part of the Massif, and three lithologic groups were recognized on the basis of their lithology and structural position.
Abstract: The Menderes Massif represents the western part of the metamorphic axis of the Alpide orogen of Anatolia (Asia Minor). A restricted area in its poorly known northern part, around the village of Derbent, was studied to shed some light on its geological evolution and to compare it with that of the southern part of the Massif. In the Derbent area three lithologic groups are recognized on the basis of their lithology and structural position, namely the gneiss and the schist complexes and the marbles. Petrographic, mineralogic, and structural data indicate that the schist complex and the overlying marbles have undergone at least three phases of deformation and metamorphism. The first phase probably reflects the closure of the Karakaya marginal basin, located to the north of the present Menderes Massif, during the late Triassic. The traces of the first phase have been largely obliterated by the second phase, which led to amphibolite facies metamorphism. A pronounced, flat-lying foliation and a NNE-SSW trending ...
TL;DR: The fossiliferous Black Peaks Formation consists of fluviatile mudstones, silts, and sands, and the faunal succession of this deposit indicates an age ranging from middle Paleocene (Torrejonian) to early Eocene (Clarkforkian) as mentioned in this paper.
Abstract: The fossiliferous Black Peaks Formation consists of fluviatile mudstones, silts, and sands. The faunal succession of this deposit indicates an age ranging from middle Paleocene (Torrejonian) to early Eocene (Clarkforkian). A minimum of three separately oriented paleomagnetic samples were collected from each of 53 sites in the type and one reference section. Depending upon pilot studies and lithology, in the laboratory the paleomagnetic specimens were treated with either AF or thermal demagnetization (at, respectively, a minimum regime of 25 mT or 200° and 400°C), which appeared to isolate the characteristic component of magnetization. The magnetic polarity zonation is predominantly reversed punctuated by several normal intervals. Based on the faunal constraints and magnetostratigraphic pattern, the Black Peaks is correlated to the reversed zone below chron 26, to the base of chron 24, or an absolute interval of time ranging from about 62 to 53 Ma. Previous hypotheses that the Javelina-Black Peaks and Blac...
TL;DR: The Snow Mountain Volcanic Complex in the Franciscan terrain of northern California as mentioned in this paper is a 1 1/2 to 2 km-thick sequence of altered submarine pillow lavas, pillow breccias, diabase, and interbedded sediments, forming a klippe that structurally overlies nearby rocks.
Abstract: The Snow Mountain Volcanic Complex in the Franciscan terrain of northern California is a 1 1/2 to 2 km-thick sequence of altered submarine pillow lavas, pillow breccias, diabase, and interbedded sediments, forming a klippe that structurally overlies nearby rocks of the Franciscan assemblage, the Coast Range ophiolite, and Great Valley sequence. All of the Snow Mountain rocks bear a weak overprint of blue-schist-facies metamorphism. The volcanic rocks were deposited on top of 200-300 m of interbedded turbiditic greywacke, shale, and radiolarian chert, which are interpreted as an interfingering abyssal fan and pelagic sequence. The volcanic pile is therefore supracrustal and is not an ophiolitic fragment formed at a mid-ocean spreading center; rather, it is probably the remnant of a former seamount. The lavas are mostly titanium-rich transitional basalts; minor associated rocks include rhyolite and peralkaline rhyolite. Relict clinopyroxenes in the basaltic rocks are all titanium-aluminum-rich augites. Thes...
TL;DR: The structural, drainage, and depositional systems of the Lake Hazar Basin in southeastern Turkey exhibit an "instantaneous" picture of faulting and sedimentation within an actively evolving pull-apart basin this paper.
Abstract: The modern structural, drainage, and depositional systems of the Lake Hazar Basin in southeastern Turkey exhibit an "instantaneous" picture of faulting and sedimentation within an actively evolving pull-apart basin A general sedimentation model is proposed which consists of two types of drainage and depositional systems Streams entering the basin laterally cross numerous oblique-slip faults and are short, steep, associated with small drainage basins, and characterized by profiles with many knickpoints They build alluvial fan deltas which are small, steep, and composed of coarse-grained debris flow and fluvial deposits Fluvial deposits consist of braided channel and bar, sheetflood, and overbank facies Streams entering the basin longitudinally flow along the surface trace of single faults and are long, gently-sloping, associated with large drainage basins, and characterized by smooth profiles They build large, gently-sloping, alluvial fan deltas composed of fluvial and lacustrine deposits Lacustrine
TL;DR: In this paper, the authors evaluated the steps of ascent of basaltic magma including intersection and disruption of a repository followed by surface eruption in southern Nevada. But they focused on storage of high-level, radioactive waste in the area.
Abstract: Volcanic hazard studies, combining standard techniques of hazard appraisal and risk assessment (probability X's consequences), are being undertaken with respect to storage of high-level, radioactive waste in southern Nevada. Consequence studies, the emphasis of this work, are evaluated by tracing the steps of ascent of basaltic magma including intersection and disruption of a repository followed by surface eruption. outhern Nevada is cut obliquely by a N-NE trending belt of basaltic volcanism that contains a number of major volcanic fields separated by areas with scattered basalt centers. The basalts of the belt range in age from 8 m.y. to recent. They are primarily of hawaiite composition, and a number of the fields exhibit the straddle-type associations of the alkalic subsuite. The basalts are characterized by high $$^{87}Sr/^{86}Sr$$ isotopic ratios (~0.707) and high Sr contents and are evolved from parental compositions by crystal fractionation. Theoretical considerations suggest basalt magma ascends ...
TL;DR: In this paper, six separate K-bentonite beds in Middle Ordovician rocks of southwestern Ohio and northern Kentucky have been analyzed for three major and 22 trace elements and the data used as a basis for stratigraphie correlation.
Abstract: Six separate K-bentonite beds in Middle Ordovician rocks of southwestern Ohio and northern Kentucky have been analyzed for three major and 22 trace elements and the data used as a basis for stratigraphie correlation. Samples from eight cores distributed over a distance of 140 km were analyzed for Dy, Eu, Mn, Na, K, Sm, La, As, Ga, Lu, Sb, Fe, Yb, Cr, Tb, Se, Rb, Co, Ce, Ta, Th, Hf, Cs, Zn, and Zr by instrumental neutron activation analysis and X-ray fluorescence, and the results subjected to stepwise discriminant analysis. Within-layer and between-layer variations in composition were assessed, and a ranking of elements established on the basis of their ability to characterize individual layers. Eighteen elements are sufficient to establish 100% classification in a group matrix of the six beds with the principle assignment contributions coming from Th, Ta, Rb, Cr, Fe, Ga, Yb, and K. To test the stratigraphie usefulness of chemical fingerprinting over more extended distances, seven samples of K-bentonites e...
TL;DR: The Red Lake and Eagle Peak plutons are similar in age (K-Ar: 87-89 m.y.) and range in composition from granodiorite to granite as mentioned in this paper.
Abstract: The Red Lake and Eagle Peak plutons are similar in age (K-Ar: 87-89 m.y.) and range in composition from granodiorite to granite. The Red Lake pluton is equigranular, has a locally greisened marginal zone, and shows only minor mineralogical and chemical zoning, whereas the Eagle Peak pluton is mineralogically, compositionally, and texturally zoned, with an equigranular margin and a porphyritic core. Modal and normative trends within each pluton are consistent with separation of plagioclase and mafic minerals leading to a residual liquid enriched in quartz and alkali feldspar. The formation of the prominent alkali feldspar megacrysts in the Eagle Peak core was promoted by plagioclase separation, $H_{2}O$ undersaturation, and a slower cooling rate relative to the Red Lake magma. Early loss of volatiles and a resulting increase in magma (melt + crystals) viscosity inhibited crystal segregation within the Red Lake pluton and prevented the development of zoning. Late stage aplites associated with both plutons f...
TL;DR: Very accurate bathymetric data for five guyots from the newly named Dutton Ridge along with that of two associated nearby guyots are presented in this article, where they suggest that the Dutton ridge formed as a volcanic plateau at the extinct triple junction of the Phoenix, Farallon, and Pacific Plates.
Abstract: Very accurate bathymetric data for five guyots from the newly named Dutton Ridge are presented along with that of two associated nearby guyots. These new data are based on U.S. Naval Oceanographic Office swath mapping by the SASS multi-beam sonar system (Glenn 1970). The Dutton Ridge is a major east-west trending feature that intersects the junction of the Bonin and Mariana Trenches. Its trend parallels that of the Michelson Ridge, which lies further to the North at the intersection of the Izu and Bonin trenches. As of this writing no samples have yet been dredged from the Dutton guyots, but the new bathymetric data presented allows some interpretation of the geologic history of the ridge. I suggest that the Dutton Ridge formed as a volcanic plateau at the extinct triple junction of the Phoenix, Farallon, and Pacific Plates. This once-continuous plateau was then fractured and faulted to produce the present group of guyots and ridges. Later volcanism may have altered its morphology. Subsequently, the ridge...
TL;DR: In this article, the authors derived information on nucleation rates and processes in the solid-state transformation of aragonite to calcite by fitting Avrami models of nucleation and growth to existing data on the overall rate of reaction, using recent measurements of calcite growth rates.
Abstract: Information on nucleation rates and processes in the solid-state transformation of aragonite to calcite is derived by fitting Avrami models of nucleation and growth to existing data on the overall rate of reaction, using recent measurements of calcite growth rates. Aragonite samples of diverse origins and morphologies exhibit extremely rapid nucleation at the onset of the transformation, with negligible subsequent nucleation. Preferential nucleation on surfaces of transforming domains produces systematic discrepancies between observed reaction rates and the kinetics calculated from standard Avrami theory; these discrepancies may be resolved in large part by reformulating the kinetic equations to encompass both surficial and internal nucleation in domains of idealized shape and finite size. When fitted to a model of this sort, the data require that surficial nucleation greatly dominates internal nucleation, in agreement with petrographic observations and theoretical expectations. These models demonstrate t...
TL;DR: In this article, the transformation from opal A to opal CT, opal-CT to quartz, and opalA to quartz in these sediments, cored on DSDP Leg 64, can be used to infer the temperature history and order of emplacement of intrusives.
Abstract: Recent siliceous sediments of the Guaymas Basin, a rapid spreading center in the Gulf of California, are intruded by basaltic sills, which induced significant diagenetic and metamorphic changes in the sediments. The transformation from opal-A to opal-CT, opal-CT to quartz, and opal-A directly to quartz in these sediments, cored on DSDP Leg 64, can be used to infer the temperature history and order of emplacement of intrusives. At or near contacts with sills of 30 m or greater average thickness, opal-A inverts directly to authigenic quartz, but there is less quartz than would be expected from the amount of opal-A dissolution. Opal-CT forms in sediments sandwiched between adjacent sills. Based on high rates of quartz nucleation and growth at high temperatures (>150°C), and on considerations of convective solution transfer, opal-CT is thought to form only where temperatures were lower or at positions between sills, an environment which prevents rapid convective dissipation of silica in solution. Where temper...
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TL;DR: In this paper, a literature review shows that such maximum surficial concentrations, well away from sources, have been considered anomalies, and the significance for surficial prospecting is that the measured maximum ore concentration may be thus some distance from the ore body and only at depth it extends back to the bedrock source.
Abstract: Glaciers produce boulder trains, ore trails, and element fans when they override ore bodies or distinctive lithologies. Their utilization as two-dimensional map features has promoted the erroneous perception that maximum concentration of ore in till should be immediately above the ore body. Available studies of lithic, mineral, radioactive, element, and magnetic distributions in till, away from known sources, indicate that these are frequently three-dimensional configurations, with shapes resembling smoke plumes drifting laterally. The significance for surficial prospecting is that the measured maximum ore concentration may be thus some distance from the ore body (frequently 1-10 km) and only at depth it extends back to the bedrock source. Literature review shows that such maximum surficial concentrations, well away from sources, have been considered anomalies. In areas with thin or truncated tills or a high relief bedrock surface, the plume may approximate a two-dimensional fan or become irregular. The e...
TL;DR: The Bitterroot dome contains plutons of the northeastern Idaho batholith and their regionally metamorphosed country rocks as mentioned in this paper, which have ages ranging from 73 − 6 m.y. to 46 − 1m.y.
Abstract: The Bitterroot dome contains plutons of the northeastern Idaho batholith and their regionally metamorphosed country rocks. Several mesozonal to katazonal intrusions in the core of the dome have zircon U-Pb ages (lower intercept on concordia) which range from $$73 \pm 6 m.y. to 46 \pm 1 m.y$$. Epizonal plutons in the northern Idaho batholith region have Eocene K-Ar ages. An extensive mylonite zone developed on the eastern margin of the dome during the latter stages of uplift. Zircon (lower intercept) ages from three sheared plutons in this zone are $$52 \pm 1m.y., 49\pm 1 m.y.$$, and $$48 \pm 1 m.y.$$ The fact that significant shallow to deep-seated intrusion and mylonitization occurred during Eocene time requires a careful evaluation of Early Tertiary tectonism in the northern Idaho batholith region. Cretaceous events appear to include multiple deformation, metamorphism, and intrusion of mesozonal to katazonal plutons into the evolving root zone of the northern Rocky Mountain overthrust belt. Early Tertia...
TL;DR: In this article, the authors show that Cenozoic potassium-rich volcanism follows a northeast to southwest younging trend in the western U.S.A. They also show that the outer limits of arc-related magmatism may be related to deep subduction rather than intraplate tectonics.
Abstract: Cenozoic potassium-rich volcanism, characterized by $$K_{2}O > Na_{2}O$$ and $$SiO_{2} < 60$$ wt %, follows a northeast to southwest younging trend in the western U.S.A. Correlation of this trend with the outer limits of arc-related magmatism, based on variable dip subduction models, suggests that potassium-rich volcanism may be related to deep subduction rather than intraplate tectonics.