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The Temagami magnetic anomaly (Figure 1)
is located within the Cobalt Embayment area of the Southern
Province, Ontario, Canada. With an areal extent of 50×15
km, the Temagami anomaly reaches a magnitude of ~10,000 nT making
it one of the largest anomalies in North America. In the east,
short wavelength linear NE-SW trending highs coincide with outcropping
Archean iron formation. The central section has the greatest
amplitude and an E- W strike. The western portion appears smoother
in character than the eastern sections with values <2500 nT
and an ENE-WSW trend. Reflection seismic data acquired along
N-S lines over portions of the Temagami anomaly have imaged
a 20 km-wide early Huronian rift basin bounded by north- and
southerly dipping faults (Milkereit and Wu, 1996). Prominent
subhorizontal reflectors exist at depths of 3-6 km, coincident
with the maximum of the anomaly, and may represent lower Huronian
volcanics of the Elliot Lake Group. However, modeling suggests
that the limited depth extent of this unit precludes it as the
source of the observed anomalies (Milkereit and Wu, 1996). The
origin is more likely located within the basement.
The observed data (Figure 1) were modeled
using the preconditioned CGA to solve equation (5). The data
were sampled every 2 km resulting in a model subsurface of dimensions
32×32×16 blocks. Assuming only induced magnetization,
the appropriate values of inclination 74°, declination
-10° and magnitude 58250 nT were used. The results of the
inversion are shown along east-west (Figure
2) and north-south (Figure 3) cross-sections
as indicated by the white arrowheads in the magnetic field map
(Figure 1). The cross-sections show a
slight widening of the body with depth and an accompanying shift
in the position of highest susceptibility from the center to
the west. N-S cross-sections (Figure 3)
indicate dips of <10° for both the shallow and deep parts
of the body. These estimated dips are, however, valid only if
any remanent magnetization is in the inducing field direction.
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