Potential Fields

Gravity Field over North-Central British Columbia

Figure 1.  Gravity Anomaly Map of north-central BC

        The northwest-oriented structural grain of the this region is well reflected in observed gravity data. Gravity values decrease by more than 100 mGal from the southwestern to the northeastern part of the map area and distinct northwest-trending Bouguer anomaly lows are associated with the Hogem (HB) and Cassiar (CB) batholiths in the northeastern map area and with thick sedimentary accumulations in the Sustut Basin. Although the amplitude of the gravity field varies considerably over the Bowser Basin, here too gravity values are considerably lower than observed to the west, south, and north of the basin where volcanic and volcaniclastic rocks in the Stikine Terrane are exposed.

        Lowe et al. (2004) conducted several hundred density determinations on rock samples from the region. Analysis of these density data reveals: a) minimal spatial variation in the density of the Bowser Lake Group; and b) no difference in the average density of the Bowser Lake Group and the underlying Hazelton Group (see Tables 1 and 2 below). The observations imply that much of the northeasterly decrease in gravity observed across the Bowser Basin must be attributed to lateral density contrasts in pre-Hazelton rocks.

Table 1: Density of geologic units in the overlap assemblages.

Table 2: Density of geologic units in Stikinia.

        In general, observed gravity values are lower over the Sustut Basin compared to the Bowser Basin. This is consistent with the density information which indicates that the mean density of the Sustut Group is 50 kg/m³ lower than that of the Bowser Lake Group.

        Several short-wavelength, low amplitude anomalies are observed throughout the map area, many of which correspond with small granitic intrusions (outlined in white on gravity image). Despite their relatively high densities, the Maitland and younger volcanic rocks that overlie the Bowser Lake Group (outlined in grey on gravity image) do not generate positive anomalies because of their small spatial extents. Steep, linear gravity gradients parallel several mapped faults, including the Pelly, Swannell and Kutcho faults (east of the Sustut Basin), indicating lateral contrasts in density (lithology) and/or unit thicknesses across the faults.