No abstract

Sequences of late Tertiary to Quaternary lava flows at six Aleutian Island sites were sampled for palaeosecular variation (PSV) measurement using palaeomagnetic methods. Due to limitations in probable time span represented by each sequence of flows and statistical constraints, data from only two sites are considered in themselves to be reliable indicators of PSV. When all the data are combined to give greater statistical significance, an angular standard deviation of palaeofield directions of 6 = 10.8" is obtained. This is compatible with other Pacific Ocean PSV data and points to the persistence of the present secular variation low in this region at least into Quaternary time.

array of 31 threecomponent magnetometers was operated over a known geothermal anomaly in the Rocky Mountains foothills of Alberta. The purpose of the array was to investigate the possibility of the existence of a geomagnetic induction anomaly coincident with the geothermal anomaly. Examples of magnetograms and Fourier transform anomaly maps are presented and show that although there is some indication of an anomaly this is to a great extent masked by the presence of large external source field gradients. A detailed transfer function analysis of the data is presented. Both single-station transfer functions and a solution of the induction matrix are calculated. The latter involves the representation of the normal fields by least-squares fitted planes and a justification of the validity of this method is given. Contour plots of the response of the anomalous vertical field to horizontal hypothetical events, and pseudosections of IZa/HnI for traverses across the array, are shown. These indicate the existence beneath the geothermal anomaly of a body of anomalously high electrical conductivity. By treating the observed values of the in-phase part of IZa/Ha I along a traverse across the anomaly as due to vertical dipoles, it is shown that the top of the conductive body probably lies at 10-20km depth. The pseudosections suggest that the body extends at least to lower crustal depths. The correlation of the conductivity and geothermal anomalies supports the inference that both are due to the presence of partial melt in the crust.

An array of 33 three-component recording magnetometers was operated in June and July 1980 in Alberta and British Columbia south of the Edmonton – Prince Rupert highway. This very large array, with its stations dispersed through 550 000 km2 and on average 150 km apart, had limited resolution and was designed to confirm known conductive structures, discover new ones, and locate them sufficiently for suitable placement of further arrays with closer spaced stations and, therefore, higher resolution. Magnetograms and three sets of Fourier transform anomaly maps are presented. They show the general attenuation of the vertical component of variation fields west of the Rocky Mountain Belt known from previous work and generally attributed to a conductive layer in the lower crust or upper mantle. Two prominent local anomalies are shown by variation fields of periods 15–30 min. The first indicates induced currents near Tête Jaune Cache, west of Jasper. The highly conductive structure carrying the induced currents may include wet sediments in the Rocky Mountain Trench and possibly partial melt at depth associated with recent volcanics. The second local anomaly appears to be associated with a crustal conductive structure that strikes northeast–southwest across southern Alberta and crosses the southeast corner of British Columbia into eastern Washington State. This may be associated with a Precambrian rift in the lower crust discovered by Kanasewich and his colleagues using deep crustal seismic reflections some 15 years ago. Both of these anomalies are under further investigation by means of arrays operated in 1981 in locations indicated by the results of the array reported here. The regional westward attenuation of the vertical fields has been quantified by means of single-station transfer functions and artificial event analysis, as developed by Bailey and others, to show the Z response to unit southwest–northeast horizontal field at three periods, along a profile from Squamish, near Vancouver, to Edmonton. These response curves will be used in model studies of the regional conductive structure.

Results are reported from 120 oriented samples collected from 40 stratigraphic horizons spanning the Akaitcho River Formation of the Great Slave Supergroup. Thermal demagnetization experiments reveal the presence of multiphase remanence. Difference vectors indicate that the lower blocking temperatures carry a magnetic component attributable to the present field. The remanence remaining above 500–600 °C defines two polarity groups, but the two groups are not exactly antiparallel. This is interpreted in terms of a previously recognised widespread magnetic overprint acquired during uplift and cooling in the Coronation Geosyncline. A compilation of appropriate overprint data from the Great Slave Supergroup yields a mean direction of D = 152°, I = +64 °(N = 75 sites, k = 30, α95 = 3°), with a corresponding pole at 91°W, 21°N (K = 14, A95 = 5°). A statistical procedure is employed to remove this overprint and obtain a best estimate of the original magnetization, which corresponds to a paleomagnetic pole on the western limb of the Coronation loop (92°W, 4°S, N = 35 sites, K = 13, A95 = 7°).