A Review of Geophysical Studies in the Canadian Cordillera

1971 ◽  
Vol 8 (7) ◽  
pp. 788-801 ◽  
Author(s):  
M. J. Berry ◽  
W. R. Jacoby ◽  
E. R. Niblett ◽  
R. A. Stacey
Keyword(s):  
British Columbia ◽  
Heat Flow ◽  
Rocky Mountain ◽  
The United States ◽  
Vancouver Island ◽  
Geophysical Data ◽  
Fault System ◽  
Canadian Cordillera ◽  
Crust And Upper Mantle ◽  
Juan De Fuca

Geophysical studies of the crust and upper mantle have been conducted in the Canadian Cordillera for over two decades, but only recently have sufficient data been collected to permit a synthesis and a correlation with the major geological units. The studies have included gravity, heat flow, and magnetotelluric observations, geomagnetic depth sounding, and high level aeromagnetics as well as both small and large scale refraction and reflection seismic surveys.It now appears that major crustal units may be recognized geophysically:(i) Seismic and gravity data suggest that the Plains and Rocky Mountains are underlain by two units of the North American craton with a crustal section 45–50 km thick. The northern unit appears to terminate at the Rocky Mountain Trench while the southern unit may extend to the Omineca Geanticline.(ii) The combined geological and geophysical data suggest that the Rocky Mountain Trench and possibly the Kootenay Arc near the 49th parallel mark the edge of the Precambrian continental margin and that the western Cordillera was formed by a complex succession of plate interactions with repeated reactivation of block boundaries.(iii) A combination of magnetic and heat flow data suggest that the region between the Rocky Mountain Trench and the Fraser Lineament is part of the Cordilleran Thermal Anomaly Zone recognized by Blackwell in the United States.(iv) Seismic data in Central British Columbia suggest that the Pinchi Fault system is a boundary between two crustal blocks.(v) The crustal thickness of the Coast Geanticline appears to increase gradually to the west to approximately 40 km and, at least in southern British Columbia, does not have a root zone below the mountains.(vi) The crustal section beneath Vancouver Island is abnormally thick and there is some paleomagnetic data which suggest that the Island may not have been formed in its present position, contiguous to the Cordillera. The crustal section for the northern part of the Insular Trough is significantly thinner.(vii) The active spreading of the Juan de Fuca Rise – Explorer Trench is now well documented. The geophysical data suggest active subduction of the Juan de Fuca plate beneath Oregon, Washing-ton, and southern Vancouver Island. However, further north there is no evidence for subduction.

Crustal temperatures near the Lithoprobe Southern Canadian Cordillera Transect

1992 ◽  
Vol 29 (6) ◽  
pp. 1197-1214 ◽  
Author(s):  
T. J. Lewis ◽  
W. H. Bentkowski ◽  
R. D. Hyndman
Keyword(s):  
British Columbia ◽  
Heat Flow ◽  
Heat Generation ◽  
Lower Crust ◽  
Volcanic Belt ◽  
The United States ◽  
Canadian Cordillera ◽  
Upper Crust ◽  
Thermal Data ◽  
Seismic Reflectors

Heat flow and radioactive heat generation have been measured and the data compiled across southern British Columbia in the region of the Lithoprobe Southern Canadian Cordillera Transect. Heat flow in the trench-arc zone between the continental margin and the Garibaldi volcanic belt is very low, but in the volcanic belt it is high and very irregular. Farther inland, to the east, the heat flow is moderately high, with the highest values in southeastern British Columbia, associated with high surface radioactive heat production. The thermal data from the central and eastern interior of southern British Columbia define a single heat-flow province with a reduced heat flow of 63 mW/m2 flowing into the upper crust. This indicates a warm, thin lithosphere similar to that of the Basin and Range of the United States to the south. Occurrences of seismic reflective bands in the lower crust of the Cordillera were compared with temperatures calculated from surface heat flow and heat generation using a simple one-dimensional conductive model. The 450 °C isotherm corresponds approximately to the brittle– ductile transition, and deeper crust may be rheologically detached from the upper crust. Where the thermal data approximately coincide with the transect seismic reflection lines, the 450 °C isotherm often corresponds to the top of characteristic sub-horizontal reflector bands, as found in Phanerozoic areas elsewhere around the world. The lower limit of the reflective band in a number of Cordilleran reflection sections is near the 730 °C isotherm, which corresponds to the transition from present "wet" amphibolite- to "dry" granulite-facies conditions. This control of the depth to the deep crustal reflective bands by present temperature provides support for the model of the reflectors being produced by fluids trapped at lithostatic pressure (layered porosity), a model that can also explain the high electrical conductivity in the deep crust of the area. The probable rheological detachment of the lower crust and a possible nonstructural origin of the deep reflectors require that interpreted lower crustal structural boundaries such as the top of the basement of the North American craton under the Lithoprobe Southern Canadian Cordillera Transect be treated with caution. However, there is no doubt that many seismic reflectors are related to crustal structures, and the model is presented as an explanation for bands of seismic reflectors in the lower Phanerozoic crust, not as a model for all seismic reflectors.

Review: Receiver function studies in the southern Canadian Cordillera

1995 ◽  
Vol 32 (10) ◽  
pp. 1514-1519 ◽  
Author(s):  
John F. Cassidy
Keyword(s):  
British Columbia ◽  
Upper Mantle ◽  
Receiver Function ◽  
Vancouver Island ◽  
Moho Depth ◽  
Canadian Cordillera ◽  
Low Velocity ◽  
Strait Of Georgia ◽  
Low Velocity Zone ◽  
Velocity Zone

Receiver function analysis has proven to be a powerful, yet inexpensive tool for estimating the S-wave velocity structure of the crust and upper mantle beneath three-component seismograph stations in the southern Canadian Cordillera. Receiver function studies using a portable broadband seismograph array across southwestern British Columbia provided site-specific estimates for the location of the subducting Juan de Fuca plate. The oceanic crust was imaged at 47−53 km beneath central Vancouver Island, and 60–65 km beneath the Strait of Georgia. Further, these studies revealed a prominent low-velocity zone (VS = −1.0 km/s) that coincides with the E reflectors imaged ~5–10 km above the subducting plate on Lithoprobe reflection lines. The E low-velocity zone was shown to extend into the upper mantle beneath the Strait of Georgia and the British Columbia mainland, to depths of 50–60 km. Combining the receiver function and refraction models revealed a high Poisson's ratio (0.27–0.38) for this feature. The continental Moho was estimated at 36 km beneath the Strait of Georgia, and a crustal low-velocity zone associated with the Lithoprobe C reflectors beneath Vancouver Island was interpreted to extend eastward, near the base of the continental crust, to the British Columbia mainland. Analysis of data from the recently deployed Canadian National Seismograph Network demonstrates the variations in crustal thickness and complexity across the southern Canadian Cordillera, with the Moho depth varying from 35 km in the Coast Mountains, to 33 km near Penticton, to 50 km near the Rocky Mountain deformation front.

Late Pleistocene history and geomorphology, southwestern Vancouver Island, British Columbia

1979 ◽  
Vol 16 (9) ◽  
pp. 1645-1657 ◽  
Author(s):  
Neville F. Alley ◽  
Steven C. Chatwin
Keyword(s):  
British Columbia ◽  
Continental Shelf ◽  
Late Pleistocene ◽  
Vancouver Island ◽  
Glacial Lakes ◽  
Small Lakes ◽  
Strait Of Georgia ◽  
Coast Mountains ◽  
Juan De Fuca ◽  
Juan De Fuca Strait

The major Pleistocene deposits and landforms on southwestern Vancouver Island are the result of the Late Wisconsin (Fraser) Glaciation. Cordilleran glaciers formed in the Vancouver Island Mountains and in the Coast Mountains had advanced down Strait of Georgia to southeastern Vancouver Island after 19 000 years BP. The ice split into the Puget and Juan de Fuca lobes, the latter damming small lakes along the southwestern coastal slope of the island. During the maximum of the glaciation (Vashon Stade), southern Vancouver Island lay completely under the cover of an ice-sheet which flowed in a south-southwesterly direction across Juan de Fuca Strait, eventually terminating on the edge of the continental shelf. Deglaciation was by downwasting during which ice thinned into major valleys and the strait. Most upland areas were free of ice down to an elevation of 400 m by before 13 000 years BP. A possible glacier standstill and (or) resurgence occurred along Juan de Fuca Strait and in some interior upland valleys before deglaciation was complete. Glacial lakes occupied major valleys during later stages of deglaciation.

Paleomagnetism of the Upper Cretaceous Nanaimo Group, southwestern Canadian Cordillera

2001 ◽  
Vol 38 (10) ◽  
pp. 1403-1422 ◽  
Author(s):  
Randolph J Enkin ◽  
Judith Baker ◽  
Peter S Mustard
Keyword(s):  
British Columbia ◽  
North America ◽  
Upper Cretaceous ◽  
Vertical Axis ◽  
Vancouver Island ◽  
Canadian Cordillera ◽  
Paleontological Data ◽  
The Mean ◽  
Prime Target

The Baja B.C. model has the Insular Superterrane and related entities of the Canadian Cordillera subject to >3000 km of northward displacement with respect to cratonic North America from ~90 to ~50 Ma. The Upper Cretaceous Nanaimo Group (on and about Vancouver Island, British Columbia) is a prime target to test the model paleomagnetically because of its locality and age. We have widely sampled the basin (67 sites from seven islands spread over 150 km, Santonian to Maastrichtian age). Most samples have low unblocking temperatures (<450°C) and coercivities (~10 mT) and strong present-field contamination, forcing us to reject three quarters of the collection. Beds are insufficiently tilted to provide a conclusive fold test, and we see evidence of relative vertical axis rotations. However, inclination-only analysis indicates pretilting remanence is preserved for many samples. Both polarities are observed, and reversals correlate well to paleontological data, proving that primary remanence is observed. The mean inclination, 55 ± 3°, is 13 ± 4° steeper than previously published results. Our new paleolatitude, 35.7 ± 2.6° is identical to that determined from the slightly older Silverquick and Powell Creek formations at Mount Tatlow, yet the inferred displacement is smaller (2300 ± 400 km versus 3000 ± 500 km) because North America was drifting southward starting around 90 Ma. The interpreted paleolatitude conflicts with sedimentologic and paleontologic evidence that the Nanaimo Basin was deposited near its present northern position.

Archaeocyathids and the Lower Cambrian Continental Shelf of the Canadian Cordillera

1975 ◽  
Vol 12 (12) ◽  
pp. 2014-2020 ◽  
Author(s):  
C. R. Stelck ◽  
A. S. Hedinger
Keyword(s):  
British Columbia ◽  
Continental Shelf ◽  
Lower Cambrian ◽  
Rocky Mountain ◽  
Canadian Cordillera ◽  
Western Canada ◽  
Early Cambrian ◽  
Structural Trend ◽  
East And West

The geographic occurrences of archaeocyathids are plotted for the Cordilleran region of western Canada. The archaeocyathids are found both east and west of, and within the Rocky Mountain Trench in British Columbia and are found east and west of the Tintina Trench in the southern Yukon. The overall pattern of the occurrences indicates that the shallow neritic portion of the continental shelf in Early Cambrian time traces a pattern widely diverse from that of the later, superimposed, Laramide structural trend. Portions of the continental shelf were already in existence west of the Rocky Mountain Trench by Early Cambrian time.

Notes on the Juniper Berry Mite, Trisetacus quadrisetus (Thomas) (Acarina: Eriophyidae), in British Columbia

1960 ◽  
Vol 92 (8) ◽  
pp. 608-610 ◽  
Author(s):  
C. V. G. Morgan ◽  
A. F. Hedlin
Keyword(s):  
British Columbia ◽  
Seed Production ◽  
Rocky Mountain ◽  
Vancouver Island ◽  
Indian Reserve ◽  
Juniperus Scopulorum ◽  
Forest Biology ◽  
Biology Laboratory ◽  
Heavy Infestation

The juniper berry mite, Trisetacus (Eriophyes) quadrisetus (Thomas), was unknown in Canada until 1956, when its occurrence was noted on Rocky Mountain juniper, Juniperus scopulorum Sarg., in the Tzouhalem Indian Reserve near Duncan on Vancouver Island, B.C.; it has not been found elsewhere in the Province (Fig. 1). Material was first submitted to the authors by Mr. W. G. Ziller, Forest Biology Laboratory, Victoria, B.C. Each year almost the entire crop of berries on the only two trees in the area is destroyed by the mite; these trees are 30 and 50 feet high. In 1956 and 1957, no normal berries were observed. In 1959, only four healthy berries were found amongst 715 examined from the two trees. Such a heavy infestation indicates that seed production by these trees was practically eliminated since feeding by the mite destroys the seed.

Monitoring temporal change in conductivity in the central Vancouver Island region, an area with past large earthquakes

1992 ◽  
Vol 29 (4) ◽  
pp. 601-608 ◽  
Author(s):  
D. R. Auld ◽  
S. E. Dosso ◽  
D. W. Oldenburg ◽  
L. K. Law
Keyword(s):  
Temporal Change ◽  
Vancouver Island ◽  
Crust And Upper Mantle ◽  
Juan De Fuca Plate ◽  
Juan De Fuca ◽  
Elevation Changes ◽  
Robust Processing ◽  
Processing Techniques ◽  
Large Earthquakes

Two major earthquakes, magnitude 7.0 in 1918 and magnitude 7.3 in 1946, have occurred this century in the central region of Vancouver Island, British Columbia, Canada. Levelling data in the region indicate relative uplift of 4 mm/year from 1977 to 1984, followed by subsidence at approximately the same rate over the next 2 years. In response to the observed elevation changes, a program was initiated to investigate if temporal changes in the geoelectrical conductivity might be associated with earthquake occurrence. Beginning in 1986, magnetotelluric (MT) data have been measured annually at a number of sites on central Vancouver Island to monitor the long-term variability of the conductivity of the crust and upper mantle in the region. Robust processing techniques now used in the analysis of MT data enhance the possibility of detecting changes in the conductivity.Past studies involving the monitoring of MT stations have considered temporal change only in terms of the measured responses. However, formulating the inverse problem of constructing conductivity–depth models that vary minimally from year to year allows quantitative investigation of the changes required in the models to accommodate the yearly variations in the data. This provides a method of evaluating the processes and depths involved in observed changes in the data. Our modelling study indicates a small but systematic yearly decrease in conductivity from 1987 to 1990 localized in a conductive zone overlying the subducting Juan de Fuca Plate.

Relocation of earthquakes west of Vancouver Island, British Columbia, 1965–1983

1992 ◽  
Vol 29 (5) ◽  
pp. 953-961 ◽  
Author(s):  
Rutger Wahlström ◽  
Garry C. Rogers
Keyword(s):  
Shear Zones ◽  
Vancouver Island ◽  
Fault System ◽  
Transform Fault ◽  
Arrival Times ◽  
Magnitude Range ◽  
Juan De Fuca ◽  
Internal Deformation ◽  
Seismograph Network ◽  
Hypocentre Locations

In the tectonically complex region of young plate interaction west of Vancouver Island, 360 earthquakes have been relocated. The earthquakes occurred in the years 1965 – 1983, when the Canadian seismograph network in the region did not significantly change configuration, and are in the magnitude range 3–5. A traveltime model was derived and applied to arrival times for a selected, limited set of station–phase combinations. Time corrections for these combinations were derived from joint-hypocentre locations of earthquakes in specific regions using independently located reference events. An algorithm for routine location of offshore earthquakes in this region is suggested.The correlation between seismicity and mapped bathymetrical features is strong along the Revere–Dellwood transform fault and the northern segments of the Explorer ridge – transform fault system. Considerable seismicity occurs inside the Explorer Plate, indicating internal deformation. The Sovanco and Nootka shear zones, the southern borders of the Explorer Plate, are characterized by broad belts of seismicity and evidently are not simple transform margins. The Explorer and northern Juan de Fuca ridges are aseismic in the investigated magnitude range.

The 1946 Mount Colonel Foster rock avalanche and associated displacement wave, Vancouver Island, British Columbia

1989 ◽  
Vol 26 (3) ◽  
pp. 447-452 ◽  
Author(s):  
Stephen G. Evans
Keyword(s):  
British Columbia ◽  
Rock Avalanche ◽  
Vancouver Island ◽  
Wave Crest ◽  
Canadian Cordillera ◽  
Mountainous Terrain ◽  
The North ◽  
Displacement Wave ◽  
Volcaniclastic Rocks ◽  
Landslide Lake

The 1946 Vancouver Island earthquake (M = 7.2) triggered a rock avalanche from the north face of Mount Colonel Foster, central Vancouver Island, British Columbia. Approximately 1.5 × 106 m3 of Triassic volcaniclastic rocks detached from between el. 1965 m and el. 1600 m. Although just over half of this volume was deposited in the upper part of the track above el. 1080 m, approximately 0.7 × 106 m3 descended the lower part of the track and entered the waters of Landslide Lake at el. 890 m. The resultant displacement wave ran up a maximum vertical distance of 51 m on the opposite shore and the wave crest was about 29 m high when it spilled over the lip of the lake. Water displaced during the event destroyed forest in the upper reaches of the Elk River valley up to 3 km from Landslide Lake. The wave at Landslide Lake is comparable to other waves generated by similar magnitude rock avalanches in Peru and Norway and it is the largest recorded in the Canadian Cordillera. The case history illustrates the conditions where substantial damage may be caused by a rock avalanche well beyond the limits of its debris when it produces a landslide-generated wave in the mountainous terrain of the Cordillera. Key words: rock avalanche, earthquake-induced landslides, landslide-generated waves, mountains.

Jurassic location of the Yukon-Tanana Terrane from paleomagnetism of the folded Mississippian Tatlmain Batholith and Ragged Stock

2019 ◽  
Author(s):  
D T A Symons ◽  
K Kawasaki
Keyword(s):  
British Columbia ◽  
North America ◽  
Fault Zone ◽  
Early Cretaceous ◽  
Rocky Mountain ◽  
Early Jurassic ◽  
Canadian Cordillera ◽  
Clockwise Rotation ◽  
Thrust Faulting ◽  
Host Rocks

Summary The extensive Yukon-Tanana terrane of the northern Canadian Cordillera has been considered controversially to be part of the allochthonous ‘Baja B.C.’ microcontinent or of the para-autochthonous North American cratonic margin during the Mesozoic. Paleomagnetic methods have isolated a very-stable Early Jurassic thermochemical remanent remagnetization in the terrane's felsic Tatlmain batholith and mafic Ragged stock after correction for: 1) rotation from northeast-plunging anticlinal deformation; 2) northerly dipping tectonic tilt of the host rocks; and, 3) northwestward regional translation on the adjacent Tintina transcurrent fault zone. The resulting 196 ± 6 Ma Tatlmain and Ragged paleopoles are 64.9° N, 44.8° E (A95 = 5.9°) and 64.2° N, 58.5° E (A95 = 7.7°), respectively. The YTT paleopoles support para-autochthonous tectonic models that have the YTT: 1) accreting to North America by the Early Jurassic; 2) undergoing non-significant orogen-perpendicular shortening by mid-Early Cretaceous from thrust-faulting; and, then 3) undergoing significant orogen-parallel northward translation of ∼500 km from mid-Early Cretaceous to the Eocene. In contrast, the paleopoles for Stikinia and Quesnellia of the Intermontane Belt show progressive northwestward translation relative to North America by ∼1000 km and a rotation of ∼55° cw since mid-Early Cretaceous. We speculate that ∼500 km of the northward translation is related to dextral motion on the Tintina and Northern Rocky Mountain Trench fault in British Columbia, and that the clockwise rotation is related to upper crustal tectonics in both Yukon and southern British Columbia.

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