Interpretations of gravimetric and magnetic anomalies on traverses in the Canadian Shield in northern Ontario
Authors: Gagalova K K , Whitehill J G A , Culibrk L , Lin D , Levesque-Tremblay V , Keeling C I , Coombe L , Yuen M M S , Birol I , Bohlmann J , Jones S J M 
Published: March 2017

The Bouguer anomalies obtained on gravity traverses over the Canadian Shield in northern Ontario are separated into local and regional effects. The latter are evidently too extensive to be directly attributed to surface formations, and apparently result from
conditions deep within the crust. Two characteristic strike directions are exhibited by these regional trends, east-west and northeast-southwest, allowing them to be separated into two age groups. The first (cast-west striking) group of structures apparently dates from early Precambrian times, as it
closely parallels folding and thrust faulting in the Timiskaming sediments, while the second (northeast-southwest striking) group is believed to result from tectonic activity of Huronian age.
Interpretation of these anomalies has been based on the assumption of a layered crust, consisting of an upper, or granitic layer, underlain by a basaltic layer. It is shown that the majority of the regional anomalies cannot be explained by structures at a depth as great as the base of the basaltic
layer (36 kilometers), but could be caused by irregularities in the thickness of the granitic layer. For example, a marked gravity low, indicating a thickened granitic layer, trends east-west just south of the Kirkland Lake thrust zone. It would appear that this represents the remnant of the root of
an early Precambrian mountain range, as the surface rocks along the low are dominantly granite, while Precambrian sediments and. volcanics are preserved north and south of the low. A second root structure is suggested along the northwest limit of the Grenville province, where a line of thrust
mountains is believed to have formed during Huronian time.
In contrast to these areas of low gravity, regions of relatively high anomaly are found to be largely covered by Precambrian sediments. The implication is that these regions were low basins in Precambrian times, so that the surface rocks were preserved from the great erosion which bared such large
masses of granite elsewhere.
The conclusion is, therefore, that irregularities in the granitic layer exist as a result of Precambrian mountain-building processes, and that these irregularities produce the broad, regional gravity anomalies. A departure from isostatic equilibrium is thus indicated, since the root structures have
long outlived the topographic features they once supported.
The more local gravity anomalies of the Shield are found to have a direct correlation with the known surface geology. That is, small, well defined highs are observed over belts of relatively dense Keewatin lavas, with the geological contacts quite accurately demarcated by the inflection points of
the gravity profiles. By comparison of the observed curves with those calculated for type bodies, an estimate may be made of the depth to the base of the Keewatin rocks. Such estimates are of interest because of the ore deposits found within the lavas. For some of the broader lava belts, depths up
to ten thousand feet are indicated.