Potential Fields

Magnetic Field over North-Central British Columbia

Figure 1.  Magnetic Map of north-central BC

        A simple visual inspection of this image shows significant variations in the character of the magnetic field across the region. These variations correspond closely with the mapped geology and with measured variations in the magnetic susceptibility of the surface exposures (see Tables 1 and 2 below).

        Over much of the Bowser Basin the amplitude of the magnetic field rarely exceed 350 nT and magnetic gradients are very gentle (typically less than 3 nT/km). This subdued magnetic character reflects the low magnetic susceptibility of the Bowser Lake Group and the considerable depth to basement (Stikine Terrane) beneath. The smooth character of the magnetic field over the basin is punctuated by two distinct types of anomalies, labelled "M" and "G" on the magnetic image. "M" type anomalies typically have intensities of 30-80 nT, wavelengths < 10 km, and gradients > 9 nT/km. Many correspond with mapped outcrops of Maitland volcanics basalt (outlined in grey on image) which have measured magnetic susceptibility values that are considerably higher than the Bowser Lake Group sedimentary rocks which they overlie (Table 1). In some cases the magnetic anomalies are more extensive than the mapped volcanic outcrops, suggesting that the volcanic rocks may be more extensive in the shallow subsurface. In most instances "M" type anomalies consist of a single peak or trough. Peaks predominate, and indicate intrusions during periods of normal magnetization, whereas troughs of decreased magnetic intensity indicate a reversed field at the time of emplacement. In a few cases, more complex magnetic responses are associated with individual outcrops of Maitland volcanics, suggesting multiple flows, or shallow intrusions magnetized at different times. These more complex magnetic responses and the existence of both normal and reversed magnetizations indicate extrusions of the Maitland volcanics spanned one or more field reversals.

Table 1: Magnetic susceptibility of geologic units in the overlap assemblages.

        "G" type anomalies are invariably normally magnetized, with wavelengths in excess of 10 km, peak magnetic intensities of 100-350 nT, and gradients of 8-30 nT/km. Many of the smaller such anomalies are associated with mapped outcrops of Mesozoic granitoids (outlined in white on magnetic image). Several of the larger anomalies, e.g. those labelled "A" and "B" on the magnetic image, show no correspondence with the mapped geology. Although these latter anomalies may simply reflect a shallowing of magnetic basement (Stikine Terrane, Table 1) beneath the basin we note that a) relatively high vitrinite reflectance indices have been measured in the sedimentary rocks that outcrop in this area (Evenchick et al., 2002) and b) that the northern part of anomaly A overlies the Groundhog Coalfield where high maturity anthracite occurs. Collectively, these observations suggest that anomalies A and B may result from intrusions in the deeper parts of the Bowser Basin or in the basement beneath.

Table 2: Magnetic susceptibility of geologic units in Stikinia.

        Outside of the Bowser Basin the magnetic field is considerably more complex reflecting the juxtaposition of rocks with contrasting magnetic properties. Magnetic amplitudes are typically 100-200 nT higher over the Sustut Basin compared with the Bowser Basin reflecting, in part, the higher magnetic susceptibilities of the Sustut Group, and the comparatively shallower depth to basement beneath the Sustut Basin. West of Bowser Basin exposed volcanic and volcaniclastic rocks in Stikine Terrane are typically several times more magnetic than the Bowser Lake Group, as are the granites, granodiorites, and quartz monzonties that intrude the western part of the Stikine Terrane and comprise most of the Coast Belt farther to the west (Lowe et al., 2003). Several of the short-wavelength, high intensity, sub-oval magnetic anomalies observed in these regions correspond with mapped intrusions. To the east and northeast of the Bowser Basin the dominant northwesterly structural fabric of the region is well reflected in magnetic data as numerous northwest-trending magnetic lineaments parallel mapped faults including the Pelly, Swannell and Kutcho faults identified in the magnetic image.

References:

Dods, S.D., Teskey, D.J., and Hood, P.J., 1985. The new 1:1,000,000 - scale magnetic anomaly maps of the Geological Survey of Canada: compilation techniques and interpretation; in The utility of regional gravity and magnetic maps, (ed.) W.J. Hinze; Society of Exploration Geophysicists, Tulsa, Oklahoma, p. 69-87.

Lowe, C., Dehler, S.A. and Zelt, B.C., 2003. Crustal architecture and density structure beneath the Strait of Georgia, British Columbia, Canadian Journal of Earth Sciences, 40, P. 965-981.