摘要:

Previous knowledge of the structure of the Cascadia subduction zone north of the Canada–United States border has been derived from a variety of geophysical studies that accurately delineated the downgoing Juan de Fuca plate from the offshore deformation front to depths of ~50–60 km beneath south-central Vancouver Island and the Georgia Strait. Little is known, however, of the structure of the Cascadia subduction zone farther westward and to greater depths in the upper mantle. We have assembled a set of some 1100 teleseismic traveltimes from events recorded on the Western Canadian Telemetered Network to augment a previously existing data set recorded on the Washington Regional Seismograph Network. The composite data set is inverted for upper mantle structure below Washington, Oregon, and southwestern British Columbia. We analyze the new northern portion of the model between 48.5–50°N and 118–127°W, which provides the first images of the deep slab structure in this region. The model is parameterized using splines under tension over a dense grid of knots. The nonlinearity of the inverse problem is treated by iteratively performing three-dimensional ray tracing and linear inversion. Resolution tests performed with a synthetic slab model indicate that the deep structure is resolved by the data north to at least 50°N. The inversions are characterized by a quasi-planar, high-velocity body inferred to represent the thermal and compositional anomaly of the subducted Juan de Fuca plate. This body exhibits velocity deviations of up to 3% from the background reference model and extends to depths of at least 400–500 km. The depth contours of the slab in the upper mantle mimic those of the shallow slab by changing strike, in the latitude range 48.0–48.5°N, from north–south in Washington to northwest–southeast in southern British Columbia. This forces the development of two arch-type structures: a main arch observed in previous studies trending east–west over Puget Sound and a possible second arch extending northeasterly from the Georgia Strait into the British Columbia interior. A steepening of the deep slab dip from British Columbia south towards Puget Sound and complexity in the evolution of the arches in depth may be the result of a change in plate motions at 3.5 Ma associated with the detachment of the Explorer plate.

ISSN:0008-4077    

DOI:10.1139/e95-001

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Icings, present in many small streams in the discontinuous permafrost environment, alter the seasonal water balance by redistributing a component of flow from the winter to the spring. Icing ablation continues to contribute to streamflow after the disappearance of the snowpack and constitutes an additional source of runoff. The temporal variability and significance of icing ablation to streamflow were examined with a combined water balance and isotope study in a tributary of Manners Creek, Northwest Territories. Icing accounted for 20% of the surface water available for melt. Over the spring study period, icing ablation accounted for 6% of total streamflow, while the total surface-water flux provided 8%. Shallow subsurface flow, which consisted of infiltrated snow meltwater and premelt groundwater, contributed the most to streamflow. At the end of the melt period, icing ablation was the only source of surface runoff to the stream and was similar in magnitude to the estimated evapotranspiration loss. Comparison with other studies revealed that on a regional scale, the hydrologic role of icing ablation is controlled by the interaction between the timing of melt, the magnitude of groundwater recharge, the spatial extent of permafrost, and the depth of seasonal frost.

ISSN:0008-4077    

DOI:10.1139/e95-002

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

A new species,Harrymys canadensis, referable to the Harrymyinae (Heteromyidae), is described from the Barstovian Wood Mountain Formation of Saskatchewan. The genusStratimusKorth et al. is included in the heteromyid subfamily Harrymyinae.Harrymys canadensisis the youngest species of the subfamily and is more derived thanStratimusor the Hemingfordian species ofHarrymys. The Harrymyinae may be derived from the latest ArikareeanProheteromys matthewiWood.

ISSN:0008-4077    

DOI:10.1139/e95-003

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The Ludlovian–Pridolian Cape Storm, Douro, and Somerset Island formations on eastern Prince of Wales Island form a transgressive–regressive succession that developed on the western flank of the active Boothia Horst. Sedimentation took place in a series of fault-bounded subbasins that were, at different times, characterized by abundant microbolites, rhodolites, brachiopods, and organisms that produced a diverse array of ichnofossils. Integration of paleoecological information from this succession provides a multifaceted paleoecological model, which can be applied to similar, lithologically monotonous successions throughout the world. Three main factors controlled the distribution of biota through the transgressive–regressive cycle. The most obvious factor was ever-changing depositional settings caused by progressive rise and fall in sea level. Superimposed on this general pattern, however, was the influence of water circulation and the influx of detrital quartz from the Boothia Horst. The microbolites and rhodolites were mutually exclusive; the microbolites developed in intertidal to shallow subtidal settings, whereas the rhodolites grew in subtidal settings. The exception is in the lower Somerset Island Formation, where oncolites and rhodolites are numerous, commonly in the same beds. The influx of detrital quartz grains into the intertidal and supratidal settings, common during the development of the lower part of the Cape Storm Formation and the upper part of the Somerset Island Formation, curtailed microbolite development. During the subtidal phase, circulation controlled the distribution ofAtrypoidea-dominated brachiopod faunas, rhodolites, microbolites, and ichnofossils. Even subtle changes in the nature of the rhodolite associations were accompanied by changes in theAtrypoideaspecies and changes in the ichnofossil assemblages.

ISSN:0008-4077    

DOI:10.1139/e95-004

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The Mont Logan Nappe is part of the Taconian internal domain of the Quebec Appalachians, and is entirely made up of synrift to passive margin elastics and volcanics of the Shickshock Group. Rocks of the Mont Logan Nappe were affected by both Taconian and Acadian deformations but regional prograde metamorphism is Taconian and limited to the D1deformational event. Thermobarometry and mineral assemblages indicate that the metasedimentary and metavolcanic rocks of the Mont Logan Nappe have recorded peak temperatures in the range 610–700 °C under pressures of approximately 600–700 MPa, and that prograde metamorphism was accompanied by the development of an inverted metamorphic gradient of −40 to −75 °C/km. The preferred interpretation of the cause of the inverted gradient is dissipative heating accompanying deformation along an intracontinental synmetamorphic thrust fault located at the top of the inverted metamorphic sequence.

ISSN:0008-4077    

DOI:10.1139/e95-005

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Hvitland and Svartfjeld peninsulas have been glaciated at least two or three times since the Late Pliocene. The earliest recognized glaciation is constrained by amino acid ratios on in situ pelecypods from marine sediments stratigraphically overlying till in Otto Fiord. High amino acid ratios suggest that the shells may be Pliocene and related to recently discovered Late Pliocene Hvitland beds at White Point. The same samples yielded finite radiocarbon ages in the 30.2–34.8 ka BP range, which cautions against their acceptance. A second, mid Quaternary glaciation is suggested by intermediate amino acid ratios from in situ pelecypods and shell-bearing till covering coastal uplands along Nansen Sound. During the last glaciation the peninsula was covered by coalesced ice caps, with glacial tongues descending tributary valleys and extending into the fiords. Deglaciation of a tributary valley in outer Otto Fiord proceeded before 11.6 ka BP and before 9.1 ka BP in mid Otto Fiord, but major ice margins remained at the coast until 8–8.5 ka BP. Lateral meltwater channels record systematic ice retreat up tributary valleys into the interior of the peninsulas. The highest recognized marine limits are marked by prominent deltas at least 164 m asl on western Hvitland Peninsula. The marine limit declines eastward inside Otto, Jugeborg, and Hare fiords. The highest strandlines could not be dated; however, by ~8.3 ka BP relative sea level stood at 103 m asl.

ISSN:0008-4077    

DOI:10.1139/e95-006

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The postglacial sedimentary environment of Moose Lake was investigated using 3.5 kHz subbottom profiles and cores from the sediment surface. A late phase in the deglaciation of the upper Fraser River valley involved a valley glacier that flowed east to west and that terminated near the present-day outlet of Moose Lake. The overdeepened Fraser valley was infilled with ice-contact sediments, including terminal and recessional moraines, thereby damming westward drainage and forming Moose Lake. Eastward glacier retreat resulted in the deposition of ice blocks in the shallow west zone of the lake, where subsequent melting disturbed conformable glaciolacustrine deposits. The main Moose Lake basin records a transition from proximal to distal proglacial environments. However, late glacial and postglacial sediments are dominated by settling from suspension. Coarse, but localized, sediment inputs are derived from the Thunder Creek fan-delta and avalanche activity on the north-facing slope of the south shore. Depths to major acoustic reflectors and known modern accumulation rates of 2.7 mm a−1indicate that the upper 15.5 m of sediment fill represents deposition during the Holocene and accounts for less than one third of the total late glacial and postglacial sediment yield. Sedimentary structures visible in cores and14C dating confirm the varved nature of the upper 3 m of sediment. Extreme positive departures in varve thickness are related to above-average discharge to Moose Lake following persistent, glacier-melt-augmented summer flows. The Moose Lake varve chronology confirms the general pattern inferred from other regional evidence of climate deterioration after 1600 AD, which led to glacier advance and moraine emplacement in the Canadian Rocky Mountains.

ISSN:0008-4077    

DOI:10.1139/e95-007

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The postglacial (ca. 13 000 BP) vegetational and climatic history is reconstructed from the pollen analytical study of the sediments of two lakes located in coastal northern Gaspé Peninsula. J'Arrive Lake (altitude: 56 m) was submerged by the Goldthwait Sea until about 11 400 BP. Euloge Lake, 83 m in altitude and less than 1 km from the shore, escaped the marine invasion. Before 10 400 BP, the vegetation, very scarce at first, evolved to a herb tundra that lasted for over a millenium. A cold climatic oscillation correlative with the Younger Dryas is identified at both sites between 10 700 and 10 400 BP, during this herb tundra phase. After 10 400 BP, dwarf birch and willows colonized the sites, leading to a shrub tundra. The afforestation of the area began only around 9300 BP, with the late arrival of spruce and aspen. This phase was interrupted at 8650 BP by the upsurge of green alder, in response to another cold oscillation. Since 7250 BP, the present forested landscape finally developed in three steps, characterized by (i) a high diversity of forest species until 2500 BP, (ii) a loss of diversity until 1000 BP that favoured spruce and balsam fir, and (iii) an increase of heliophilous trees, shrubs, and herbs. These reconstructions contribute to the late-glacial palaeogeography of Eastern North America through the unravelling of conditions close to the retreating ice.

ISSN:0008-4077    

DOI:10.1139/e95-008

出版商:NRC Research Press    

年代:1995

数据来源: NRC