Deep crustal structure and tectonic history of the Northern Kapuskasing Uplift of Ontario: An integrated petrological‐geophysical study

Abstract

The northeast trending Kapuskasing uplift transects the east‐west belts of the central Superior Province over a distance of some 500 km. Granulite to upper amphibolite facies rocks of the uplift form three distinct geological‐geophysical entities: from south to north, the Chapleau, Groundhog River, and Fraserdale‐Moosonee blocks. Uplift of the granulites along a moderately northwest dipping crustal‐scale thrust fault is attributed to an early Proterozoic compressional event. Major northeast‐striking faults that bound the Kapuskasing zone on the west were examined by modelling of geophysical anomalies to determine dip and by geobarometry of garnet‐orthopyroxene‐plagioclase‐quartz assemblages to determine vertical displacement. Granulites in the Kapuskasing zone have 7‐ to 9‐kbar signatures whereas those in the Quetico belt to the west indicate metamorphic pressure of 4–6 kbar. Individual calibrations of the barometer yield consistent pressure differences of 2–3 kbar, suggesting 7–10 km of west‐side‐down movement on the faults. Modelling of gravity and aeromagnetic gradients indicates westerly dips of 60°–65°, with west‐side‐down offset of up to 14 km. These major normal faults probably formed as collapse structures in response to crustal thickening which occurred during the preceding compressional uplift stage. Differences in the configuration of individual blocks of the Kapuskasing zone can be related to variable fault slip and intersection angles between normal and reverse faults. Thus the Groundhog River and southern Fraserdale‐Moosonee blocks are perched thrust tips analogous to the Sangre de Cristo Range of the Laramide uplift province, whereas the southern Chapleau block is a tilted slab with similar configuration to the Laramide Wind River Range. Pop‐up geometry deduced for the northern Fraserdale‐Moosonee block resembles the structure of the Laramide Uinta Mountains. A normal fault crosses the surface trace of the basal thrust fault between the Groundhog River and Fraserdale‐Moosonee blocks and causes a 65‐km “gap” without granulites. This article contains supplementary material.