Seismic reflection imaging of the Chapleau block, Kapuskasing structural zone, Ontario
Wayne T. Geis
The granulite and upper amphibolite grade rocks of the Kapuskasing structural zone (KSZ) in Ontario represent an exposure of Archean crust that has been uplifted along a southeast verging thrust fault system. Regional and high resolution seismic reflection data recorded across the structure by LITHOPROBE image at least three low angle thrust faults that merge into a flat detachment towards the northwest. Along an east-west transect across the southern end of the KSZ the seismic geometry resembles a 'ramp and flat' style of thrusting, resulting in a thin upper plate above the 15-17 km (about 5.0 s two-way time) detachment. Other seismic profiles provide regional three dimensional coverage, allowing construction of time structure maps of the principal fault surface. These maps show that, on a regional scale, the time contours generally parallel the surface expression of the fault(s), the Ivanhoe Lake fault zone (ILFZ). Large structures imaged beneath the adjacent Abitibi belt also roughly parallel the ILFZ, suggesting they may have controlled the emplacement of the KSZ or are possibly deeper level detachments associated with the formation of the KSZ. Coupled with the surface geology and available geobarometry, three conceptual models are proposed that are consistent with the seismic geometry, but differ in respect to which of the faults is the principal detachment and where potential ramps are placed. From these models estimates of the minimum amount of horizontal shortening range between 55 and 85 km in a NW-SE direction. This large amount of shortening implies that much of the upper-middle crust of Superior province was detached from the lower crust during the formation of the KSZ. The proposed geometric interpretation, which includes ramps, flats and imbricate thrusts, further implies that rocks deep within the crust may deform into structures that strongly resemble those in layered media such as sediments in Phanerozoic supra-crustal foldthrust belts.
In addition to characterizing the geometric information from the reflection data, acquisition of data with both high resolution and regional parameters along a 17 km segment of line allowed comparison of these two modes of recording. The initial stacked section of the data recorded with regional parameters indicated that the high resolution data provided a far superior image to that of the regional data. However, reprocessing of the regional data with parameters that are comparable to the high resolution profile, reveals that significant improvements can be made on the regional profile image quality. A short offset range and a correlation statics window that encompasses early arrivals are the two most important considerations when imaging dipping shallow structures.