Estella A. Nkwate
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Ph. D. Thesis
Gravity and Magnetic Interpretation of the Kapuskasing Structural Zone in the Val Rita and Groundhog River Blocks Northwestern Ontario, Canada
(PDF - Mb)
The Kapuskasing Structural Zone (KSZ) in Ontario consists of four separate tectonic blocks with distinctive gravity and magnetic anomalies. From south to north, these include the Chapleau Block (CB), the Groundhog River Block (GRB), the Val Rita Block (VRB) found west of the GRB and the Fraserdale-Moosonee Block (FMB). Previous gravity studies based on sparse data coverage, failed to satisfactorily explain the cause of the potential field anomalies within the VRB and the GRB in the central portion of the KSZ. Hence, a detailed gravity survey was conducted over the VRB and GRB in order to constrain interpretations of deep crustal structure based on seismic, geologic and physical properties data.
Gravity and magnetic models across the VRB suggest that uplift of dense granulites from depths of 10-25 km to shallow levels of the crust is responsible for the gravity anomaly observed over this block. These granulites are generally not exposed but are buried underneath a thin veneer of amphibolite facies tonalite gneiss. Furthermore, the models suggest that the crust is 48-50 km thick beneath the VRB and its associated gravity high compared to 40-43 km beneath adjacent areas.
Model results from the GRB suggest that the granulites associated with the GRB must be thin (1-4 km, increasing in thickness from north to south). Although magnetic susceptibilities are variable throughout this block, magnetic modelling indicates that they are sufficient to account for the strong magnetic anomaly associated with the block. Model results from the CB are consistent with previous results (e.g., Percival and Card, 1983) and indicate that uplift of dense granulites from depths of 15-30 km is responsible for its high gravity anomaly. A thick crust of 52 km underlies the CB and is also coincident with the region of high gravity anomaly.
The VRB is interpreted as a broad, southerly plunging antiform and the GRB as a thin thrust sheet. Arching within the VRB was caused by ramping and was probably coeval with thrusting in the GRB. The interpreted geometries of antiformal folding and thrusting within the VRB and GRB are analogous to the geometry of the Limpopo Belt of Southern Africa. The CB is interpreted as a west dipping slab consistent with previous interpretations, and is analogous to the Boothia Uplift in arctic Canada. The interpreted structures of the KSZ are similar to those in fold and thrust belts.
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Supervisor: Matt Salisbury