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A geophysical analysis of the Kapuskasing Structural Zone

University of British Columbia

A crustal scale seismic refraction experiment was conducted over the Kapuskasing Structural Zone, Northern Ontario, in 1984. The zone cuts obliquely across the east-west structural grain of the Superior Province in the Canadian shield and has been proposed as a cross-section of Archean crust exposed by thrust faulting along the Ivanhoe Lake Cataclastic Zone during early Proterozoic time. Five seismic refraction lines of 360-450 km in length were shot over the area to obtain a velocity structure for the uplift region. There were 18 profile shots and two fan shots with a recorder spacing of 2 to 5 km. Single trace processing has been developed for enhancement of secondary arrivals. Homomorphic filtering was used to obtain an estimate of the wavelet and then spectral division and cross-correlation with that wavelet were implemented to enhance the later arrivals on the trace. The improved secondary arrivals were used to locate lower crustal refracted phases and wide-angle reflections which were combined with the first arrivals to construct the velocity models. The wide-angle reflections were also correlated with events from coincident Vibroseis reflection data. The travel-times and amplitudes of the data from the profile lines have been modeled using asymptotic ray theory methods. We have imaged a low velocity zone, ranging from 4-5 km to 9-12 km depth, under the Abitibi greenstone belt; it is underlain by a highly reflective zone. There is a considerable deepening of the Moho from 40-43 km to 50-53 km under and to the west of the southern end of the Kapukasing Structural Zone. Analysis of wide-angle reflections on the fan shots has corroborated this thickening of the crust under the structure. A high velocity anomaly of 6.5-6.6 km/s has been imaged in the upper crust down to 20 km depth beneath the Kapuskasing structure with a suggested dip of 15° ± 2° to the west. High Poisson's ratio values (0.26), determined from shearwave arrivals, were imaged in the structure, suggesting a high mafic content for that material. Gravity profile modeling has been performed along the five seismic lines. The observed Bouguer anomaly variations have been matched using the structure as delineated by the seismic modeling and velocity-density values that fall within the scatter of the Nafe-Drake set of data points. 2d frequency domain filtering techniques have been applied to the regional gravity and magnetic data. Directional, bandpass, continuation and derivative filtering have been used to locate faults and terrane boundaries. The main thrust fault, the Ivanhoe Lake Cataclastic Zone (ILCZ), has been more clearly delineated by this filtering and agrees well with the surface geological mapping. An extension of the Lepage Fault has been outlined and indicates that this normal fault may have been important in the post-thrust stabilising period. Seismic estimates of the soling depth for the ILCZ were further constrained by considering the rheological properties of the constituent lithologies. From the results of previous heat flow and heat production work in this region and the structures inferred from the seismic interpretation, geotherms and strength curves were constructed for the present, a period after the stabilisation of the Superior craton, and the time of uplift. Present day heat flows were successfully matched using a mantle heat flow of 20 mW m⁻². The formation pressures and temperatures of the Kapuskasing granulites were achieved using only a 25 % increase in the mantle heat flow. From analysis of the strength curves for the time of uplift, estimates of the depth to the brittle-ductile transition for dry quartz ranged from 17 to 21 km. This depth is corroborated by the results of the seismic refraction and reflection analysis. The results of this thesis support the Percival and Card thrust model but with a soling depth of 17-21 km. This corresponds to a granulite zone thrust up from the mid to lower crust and is indicative of large scale horizontal tectonic processes in late Archean to early Proterozoic times.

Text

2010-10-07

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http://hdl.handle.net/2429/29012

Geophysics

University of British Columbia

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