摘要:

The uppermost Proterozoic–Lower Cambrian Hamill Group of southeastern British Columbia contains geologic evidence for a phase of extensional tectonism that led directly to the onset of thermally controlled subsidence in the Cordilleran miogeocline. Moreover, the Hamill Group contains the sedimentological record of the passage of the ancient passive margin from unstable tectonic conditions associated with rifting and (or) the earliest phases of thermal subsidence to post-rift conditions characterized by stabilization of the margin and dissipation of the thermal anomaly generated during the rift phase (the rift to post-rift transition). Widespread uplift that occurred prior to and during the deposition of the lower Hamill Group is indicated by an unconformable relation with the underlying Windermere Supergroup and by stratigraphic relations between Middle and Upper Proterozoic strata and unconformably overlying upper Lower Cambrian quartz arenites (upper Hamill Group) in the southern borderlands of the Hamill basin. In addition, the coarse grain size, the feldspar content, the depositional setting, and the inferred provenance of the lower Hamill Group are all indicative of the activation of basement sources along the margins of the Hamill basin. Geologic relations within the Hamill Group that provide direct evidence for extensional tectonism include the occurrence of thick sequences of mafic metavolcanics and rapid vertical facies changes that are suggestive of syndepositional tectonism.Evidence of extensional tectonism in the Hamill Group directly supports inferences derived from tectonic subsidence analyses that indicate the rift phase that immediately preceded early Paleozoic post-rift cooling could not have occurred more than 10–20 Ma prior to 575 ± 25 Ma. These data, together with recently reported isotopic data that suggest deposition of the Windermere Supergroup began ~730–770 Ma, indicate that the rift-like deposits of the Windermere Supergroup are too old to represent the rifting that leddirectlyto the deposition of the Cambro-Ordovician post-rift strata. Instead, Windermere sedimentation was apparently initiated by an earlier rift event, probably of regional extent, that was part of a protracted, episodic rift history that culminated with continental breakup in the latest Proterozoic – Early Cambrian.

ISSN:0008-4077    

DOI:10.1139/e88-001

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Kinematic simple-shear models have recently been used to provide qualitative explanations for tectonic features in the Basin and Range Province of the southwestern United States and on passive margins. In this paper, a general kinematic simple-shear model is presented. Explicit expressions for the subsidence and stretching factors across a simple-shear zone are derived for two important cases. The first case is one in which simple-shear rifting occurs along a major fault that cuts through the whole lithosphere. In the second case, simple-shear thinning takes place in a brittle zone overlying a regional ductile zone that is undergoing pure-shear thinning. In these cases the subsidence and stretching factors both have characteristic distributions across the stretched region, which can indicate the dominant mode of rifting. It is also shown that simple-shear rifting under the assumption of local isostatic compensation cannot lead to the production of uplifted metamorphic core complexes unless some additional mechanism such as crustal underplating is operating.

ISSN:0008-4077    

DOI:10.1139/e88-002

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Retrograde eclogite is found along the northern perimeter of ultramafic rocks in the Mont Albert complex, Gaspé, Quebec. Garnet–clinopyroxene geothermometry gives 840 °C as the peak temperature of metamorphism. Kyanite in associated metapelites requires that, at this temperature, pressure be a minimum of 12 kbar. Based on symplectic textures in the eclogitic and associated rocks and clinozoisite – white mica – quartz– plagioclase equilibria, a counterwise pressure–temperature trajectory that terminates near 500 °C and 8 kbar can be traced for the Mont Albert complex.

ISSN:0008-4077    

DOI:10.1139/e88-003

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

A comparative study of vacuum crushing methods for analyzing reactive gases in basaltic glasses shows that ball milling is an efficient means of releasing occluded gases but that blank production represents a potentially serious problem that must be accounted for when determining reactive-gas compositions. Production of H2and CH4in a stainless-steel ball mill in the absence of rock material increases with length of crushing time. However, test results presented here indicate that blank levels are reduced during the actual crushing process by the presence of rock powder, which may act as a cushion to reduce metal–metal contact. Crushing in copper tubes under a hydraulic press produces no blanks for these gases, but crushing efficiency and gas release are very low, and gas adsorption on rock powder becomes a significant problem. Experiments with methane adsorption on crushed basalt suggest that the loss of methane is a chemisorption process that is, for the most part, irreversible.Applying corrections for these effects, we find that H2/CH4ratios (~3 – 30) in mid-ocean-ridge basalt glasses are similar to those seen in high-temperature mid-ocean-ridge hydrothermal fluids. These data, arguments based on the similarity of water/rock mass ratios calculated from basalt gas data, and the uniformity of methane/helium ratios in divers high-temperature mid-ocean-ridge hydrothermal fluids support the contention that dissolved CH4and H2in these fluids are predominantly derived from leaching of mid-ocean-ridge basalt.

ISSN:0008-4077    

DOI:10.1139/e88-004

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Recent mapping of the Nord–Forez region, Massif Central, France, reveals a variety of complex field relationships along contacts between a large enclave of diorite and its enclosing monzogranite. Field relations, hybrid rocks, and a wide variety of mafic magmatic enclaves can be explained by a succession of processes between coexisting mafic and felsic magmas that either prevent or promote exchanges. The large volume of mafic magma, and consequently the large amount of heat it brought with it, favored exchange processes. Because the distribution of heat varied with both time and place, the nature and intensity of the compositional exchange processes also varied.Most of the features attributed to exchange are related to processes that occurred at the level of emplacement and after the beginning of crystallization of the two magmas. These processes affected both the granitic and dioritic magmas already thoroughly hybridized by nearly similar processes that took place earlier at depth.

ISSN:0008-4077    

DOI:10.1139/e88-005

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

A magnetotelluric (MT) survey using naturally occurring ultra low frequency (ULF) and extremely low frequency (ELF) sources was conducted in the frequency range of 0.5 mHz to 20 Hz in order to locate the western edge and the depth of the North American Central Plains (NACP) conductivity anomaly in the Bengough area of southeastern Saskatchewan. The data base was also used to evaluate the complex singular-value decomposition (CSVD) method of MT processing and to corroborate certain geologic interpretations in this part of the Williston Basin.Modelling of the resulting impedance tensors revealed a deep (10–15 km) zone with resistivity (35–85 Ω∙m) significantly lower than typical values (1000–1500 Ω∙m) obtained from a borehole resistivity log of the top 30 m of the Precambrian at a depth of 2.3 km. An increase in depth (to 20 km) and resistivity (150–275 Ω∙m) of this deep zone measured at the survey's west end was interpreted as indicating the anomaly's western edge. The CSVD processing of the data did not show any particular advantages over the conventional cross-spectral m

ISSN:0008-4077    

DOI:10.1139/e88-006

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Mean seasonal species composition of living molluscan communities was compared with the composition of current dead assemblages in the sediments of three sites located in the Delta Marsh of southern Lake Manitoba. Dead shells were more numerous in vegetated than in bare areas, resulting primarily from the affinities of living molluscs for vegetated areas. Redistribution patterns of empty shells were not significantly different for vegetated and bare areas, as judged from distributions of passively transported land shells in the sediments. Significant differences were observed at all sites between species frequencies in living and corresponding dead assemblages averaged for the season. Proportions of living to dead individuals per unit bottom area indicated higher attrition rates with increasing energy conditions as well as with increasing shell size. Differential attrition may result in overrepresentation of small species in fossil assemblages.

ISSN:0008-4077    

DOI:10.1139/e88-007

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

The early Mesozoic, quartz normative, North Mountain basalts in southwestern Nova Scotia (Digby area) form three units: a coarse massive lower flow (~190 m) bearing minor lenses of mafic pegmatite, a middle unit of thin amygdaloidal basaltic flows (~50 m), and an upper flow unit of massive phenocryst-rich basalt (~160 m). The two thick units show phenocrysts of orthopyroxene (bronzite) and (or) pigeonite, augite, and zoned plagioclase in a granular matrix of augite, pigeonite, and plagioclase. Variation diagrams and chondrite-normalized rare-earth-element patterns relate all chemical diversity between and within flows to removal and (or) accumulation of plagioclase and pyroxene phenocrysts (~1:1). High K, Rb, and Ba, appear related to assimilation of continental crust. Constancy of fractionation-independent element ratios and variations in phenocryst content vertically and along the 200 km strike of the basalts suggest (1) crystal settling and accumulation together with assimilation and mixing in a lower crustal magma chamber, (2) rise to upper crustal levels in a central conduit followed by northeast-ward emplacement along a tension-induced dyke system, and (3) extrusion along a fissure in two major and numerous minor pulses that formed the lower, upper, and middle units. Assimilation did not occur as magma moved through the dyke system, for assimilation-related variations in composition do not occur along strike.

ISSN:0008-4077    

DOI:10.1139/e88-008

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Two pollen diagrams describe the previously unknown vegetational history of the Black River and Little Black River drain ages of northeastern Alaska. Tundra grew throughout much of the region between ca. 19 800 and 15 000 years BP. Prior to 18 000 years BP, the vegetation was dominated perhaps by xeric Cyperaceae communities, but apparently between ca. 18 000 and 15 000 years BP a more diverse herb tundra characterized the vegetation. Herbaceous species continued to be important in the vegetation until ca. 9700 years BP, althoughBetula nana–glandulosaandSalixspecies became more common after 15 000 years BP. At ca. 9700 years BP a major change in the vegetation occurred with the spread ofPopulus(probablyP.balsamifera),B.nana–glandulosa, andSalix, possibly resulting in a mosaic of shrub tundra andPopulusgallery forest.Picea glaucamigrated into the Black River region ca. 7500 years BP, followed byAlnusca. 7200 years BP andPicea marianaca. 6000 years BP.Herb zone records from the Black River region support the hypothesis that eastern Beringian vegetation was characterized by a tundra mosaic prior to 14 000 years BP. Chronological and geographical patterns in thePopulussubzone from eastern Beringia suggest that the expansion ofPopuluspopulations may not have been exclusively in response to climate change. Dates ofPiceaarrival on the western Porcupine Plateau indicate that this area probably was not an early Holocene migration route forPicea.

ISSN:0008-4077    

DOI:10.1139/e88-009

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

More than 2000 line-kilometres of ice were sounded in 1981 using an 840 MHz pulsed radar system. The surveyed regions include Mt. Oxford ice cap, Disraeli Glacier, Milne Glacier, Ward Hunt Ice Shelf, and Milne Ice Shelf in northern Ellesmere Island, Arctic Canada. The maximum ice thickness that was reliably recorded was 750 m for Milne Glacier. In addition to ice thickness interpretation, the power reflection coefficient (PRC) and propagation loss rate were estimated from the data. Measured basal PRC's differ for ice shelves, floating glaciers, and grounded glaciers. Spatial variations of PRC have been used to infer a composite structure for the ice shelves. Ward Hunt Ice Shelf proved difficult to sound, probably because of its known saline and brackish chemistry. Excellent results were obtained for Milne Ice Shelf and its depth was found to average roughly 70 m, in places attaining 100 m. Bottom-side crevasses were observed within Milne Ice Shelf. The characteristic ridge and trough topography of the ice shelf surface has little or no bottom-side expression.

ISSN:0008-4077    

DOI:10.1139/e88-010

出版商:NRC Research Press    

年代:1988

数据来源: NRC