摘要:

Assisted by detailed bathymetric and bottom photographic coverage, a series of closely spaced samples of rock were taken by dredging along a traverse from the center of the Median Valley to the adjacent crest mountains on the Mid-Atlantic Ridge. The specimens show a gradation from tholeiitic to alkali basalts. Chemical variations, and the alkali content in particular, can be correlated with the depth of extrusion and with the topographic relationship of the volcanoes to the axis of the Median Valley.Although the basalts show considerable evidence of gravity-controlled crystal fractionation, the trends so established are evidently not responsible for alkali enrichment, but appear to be superimposed onto the more fundamental, continuous trend from tholeiitic to alkali basalts.

ISSN:0008-4077    

DOI:10.1139/e68-001

出版商:NRC Research Press    

年代:1968

数据来源: NRC

摘要:

An analogue model for studying electromagnetic variations for an overhead oscillating line current is described. Measurements of electric and magnetic fields for model earth–sea interfaces are discussed and compared with results reported earlier for an overhead sheet current. A truncated cone representing a circular sea, a graphite wedge representing an ocean coastline, and a graphite wedge underlain by a graphite block representing an upwelling conducting zone in the earth's mantle near a sea coast are considered. In some cases the results for the line current model differ appreciably from those for the sheet current model, indicating that the nature of the source field can be an important factor in determining the behavior of the field components. The model measurements indicate that land–sea interfaces produce no appreciable anomalous magnetic fields at frequencies below 0.01 cps and hence the presence of the sea alone cannot account for coastal magnetic field anomalies. The results obtained for the model coastline structures tend to support the idea put forth by various workers that experimentally observed coastal magnetic field anomalies can in some cases be explained by assuming an upwelling conducting zone within the earth's mantle.

ISSN:0008-4077    

DOI:10.1139/e68-002

出版商:NRC Research Press    

年代:1968

数据来源: NRC

摘要:

The unpolarized optical absorption spectra of the sheet silicate minerals muscovite, lepidolite, and fuchsite have been examined. The spectra, interpreted on the basis of ligand field theory, indicate that all three minerals contain Fe3+and possibly Ti3+in octahedral sites. In addition, muscovite is shown to contain octahedrally coordinated Fe2+, lepidolite octahedral Fe2+and Mn2+, and fuchsite octahedral Cr3+. No evidence was found for transition metals in tetrahedral sites.Some comments are made on the similarity of the spectra of certain of the transition metal ions in several classes of minerals. This leads to the speculation that the cations Fe3+, Mn2+, and Cr3+may tend to 'mold' their immediate surroundings.

ISSN:0008-4077    

DOI:10.1139/e68-003

出版商:NRC Research Press    

年代:1968

数据来源: NRC

摘要:

Twenty-one uplift curves from Arctic Canada indicate a similar proportional response through time. The time/altitude relationship can thus be expressed as a per cent of uplift within a specific time period. A graph of the results is a smoothly decelerating curve. Uplift curves can be approximated by the formula:U = A(1 −it)/(1 −i) whereUis uplift,tis time with 0.0 years being the moment of site deglaciation,iis a constant for Arctic Canada equal to 0.677, andAvaries with time and amount of recorded uplift. Tables and a figure are presented so that uplift curves can be predicted on the basis of: (1)elevation of the marine limit; and (2)date of site deglaciation. A comparison between Arctic Canada, east Greenland, and Fennoscandia indicates that relaxation times for a comparable period (10 × 103years) are 2.5, 1.7, and 3.0 × 103years respectively. Velocities derived from uplift curves allow the amount of residual uplift to be calculated. A figure of 100 m is suggested for a sector of Hudson Bay.

ISSN:0008-4077    

DOI:10.1139/e68-004

出版商:NRC Research Press    

年代:1968

数据来源: NRC

摘要:

Geological field relationships and previously reported isotopic age determinations throughout the Appalachians provided evidence for a pattern of Lower Paleozoic tectogenesis. The regional isotopic age pattern indicates that intrusion of ultramafic and mafic igneous materials and major episodes of metamorphic recrystallization throughout the Appalachians were completed before the end of the Silurian Period.New K–Ar ages of metamorphic rocks from Vermont and adjacent regions in the Northern Appalachians are reported and confirm this pattern. Whole-rock ages of slates from the Taconic Klippe (460–445 m.y.) indicate that metamorphism and initial formation of slaty cleavage occurred during Middle to Late Ordovician times. Mica ages of Paleozoic schists exposed along the Green Mountain Anticlinorium (425–375 m.y.) are indicative of prolonged post-metamorphic uplift and cooling of this structure during the Silurian Period and Early Devonian Epoch. Local recrystallization and consequent loss of argon from the Taconic slates also occurred during this time. Ages obtained from a variety of metamorphic rocks from eastern Vermont fall in a narrow range (345 ± 7 m.y), indicating that renewed uplift and rapid cooling of this area, east of a line of ultramafic intrusions, occurred during Late Devonian time.Comparison of isotopic age data from the Appalachian and British Caledonian fold belts indicates that a broadly synchronous sequence of Lower Paleozoic tectonic and magmatic activity, lasting approximately 100 m.y., occurred in both regions.

ISSN:0008-4077    

DOI:10.1139/e68-005

出版商:NRC Research Press    

年代:1968

数据来源: NRC

摘要:

Heat flow measurements, with appropriate corrections for the effects of Wisconsin glaciation, from three widely separated locations in eastern Canada are reported. One value in the Grenville rocks of Ontario agrees with earlier published values, but, when corrected for the effects of glaciation, becomes close to the world average value. The heat flow found in the New Quebec part of the Superior Province is significantly lower than is found in the Grenville Province. This can be explained by a hypothesis based on geochemical analysis of the surface rocks.

ISSN:0008-4077    

DOI:10.1139/e68-006

出版商:NRC Research Press    

年代:1968

数据来源: NRC

摘要:

The location and depth to ore bodies lying along the hinge of either refolded or superposed folds can be determined by using simple geometric constructions. Methods are presented here for tracing hinge-line ore bodies for several common multiple fold situations.

ISSN:0008-4077    

DOI:10.1139/e68-007

出版商:NRC Research Press    

年代:1968

数据来源: NRC

摘要:

Carbonate-fluorapatite, the mineral of marine phosphorite, is found to form contemporaneously within the sediments by the replacement of skeletal carbonate in a core sample of foraminiferal ooze from the eastern tropical Pacific. A systematic downward increase in apatite within the core suggests that replacement is continuing at a decreasing rate in the sediments. The widespread distribution of foraminiferal ooze on the California borderland and its common association with phosphorite suggest that this material may play a role in the phosphorite accumulation there. The reaction of CaCO3with soluble phosphate in conditions favoring the slow dissolution of the carbonate mineral may result within the sediments in an increase of the carbonate ion concentration, which appears to control the rate of replacement.

ISSN:0008-4077    

DOI:10.1139/e68-008

出版商:NRC Research Press    

年代:1968

数据来源: NRC

摘要:

The optical absorption spectra of green vesuvianite crystals from Lowell County, Vermont, are reported in the energy range 12 000 cm−1to 30 000 cm−1. The principal absorptions have been attributed to octahedrally bonded Fe3+and the individual bands have been assigned to spin-forbidden electronic transitions from the6A1ground state to excited states in Fe3+. In particular, the6A1 → 4A14E(G) transition is marked by a relatively sharp band at 21 600 cm−1.

ISSN:0008-4077    

DOI:10.1139/e68-009

出版商:NRC Research Press    

年代:1968

数据来源: NRC

摘要:

A review of published data on flood basalts and diabase dike swarms suggests that they are related in origin, typically being developed in zones of crustal tension induced by sub-crustal flow. Perhaps, therefore, these basaltic effusions may be used to map convective cells in the mantle for various periods during geologic time. The inferred pattern of sub-crustal movements, particularly for the southern continents, is seen to be complicated, both temporarily and spatially, and is strongly suggestive of unsteady motion. Dikes of the Canadian Shield permit inferences about the site of convective cells back to 2150 million years.

ISSN:0008-4077    

DOI:10.1139/e68-010

出版商:NRC Research Press    

年代:1968

数据来源: NRC