Geology of the Canadian Shield in Ontario: an update

The report reviews advances in understanding the geology of the Canadian Shield in Ontario since the 1992 compilation provided in the Ontario Geological Survey Geology of Ontario special volume. New information includes data and interpretations produced during several recent major initiatives: Operation Treasure Hunt, Discover Abitibi, the Abitibi-Grenville and Western Superior Lithoprobe transects, Western Superior NATMAP transect, and many updated maps and data sets. In addition to a descriptive overview, the report contains a set of digital geology and geophysical maps, along with distribution of point data from published databases of geochronology, lithogeochemistry, drill hole locations, and earthquake location-magnitude. Two main geological provinces of the Canadian Shield are represented in Ontario. The Superior Province and autochthonous cover sequences underlie approximately 80% of the province of Ontario, distributed mainly across the north and northwest. The Grenville Province, exposed in the southeast, and its autochthonous cover, make up the remainder. The Superior Province records about one billion years of geological history, from 3.6 to 2.6 billion years ago. Five microcontinental fragments evolved independently between 3.6 and 2.75 Ga, prior to a series of five discrete accretionary events between 2.72 and 2.68 Ga that assembled the continental and intervening oceanic crustal domains into a coherent Superior craton. The Northern Superior superterrane recorded 3.6 to 2.75 Ga events prior to 2.72 Ga collision with the 3.0 Ga North Caribou superterrane. Following rifting at 2.98 Ga, the Uchi margin of the North Caribou superterrane evolved in an upper plate setting before collision 2.72 to 2.70 billion years ago of the Winnipeg River terrane (<3.4 Ga), which trapped synorogenic English River turbidites in the collision zone. The Winnipeg River terrane was reworked in magmatic and tectonic events 2.75 to 2.68 billion years ago, including the central Superior orogeny (2.71-2.70 Ga) that marks accretion of the juvenile western Wabigoon terrane. In the south, the Wawa-Abitibi terrane evolved in a mainly oceanic setting until Shebandowanian collision with the composite Superior superterrane at 2.695 Ga. Synorogenic Quetico turbidites were trapped in the collision zone. The final accretionary event involved addition of the Minnesota River Valley Terrane (MRVT) from the south, and deposition and metamorphism of synorogenic turbidites of the Pontiac terrane during the Minnesotan orogeny (ca. 2.68 Ga). Seismic reflection and refraction images indicate north-dipping structures, interpreted as a stack of discrete, 10 to 15 km thick terranes. A slab of high-velocity material, possibly representing subcreted oceanic lithosphere, as well as Moho offsets, support a model of progressive accretion through plate-tectonic-like processes. Following stabilization in the Neoarchean, the craton was affected by emplacement of at least eleven dyke swarms (2.45-1.1 Ga), uplift of the Kapuskasing zone (ca. 1.9 Ga), ductile reworking of the northwestern margin (ca. 1.8 Ga), and attempted rifting (ca. 1.1 Ga). Minor seismicity occurs in the Lake Timiskaming seismic zone. The Grenville Province contains rocks ranging in age from 2.69 to 0.99 Ga, metamorphosed between 1.08 and 0.99 Ga. It is bounded to the northwest by the subvertical Grenville Front. Within the Grenville Province, there is an overall, generally shallow (20-40°), southeasterly dip to both surface geological structures and seismic reflectivity within the crust, suggesting northward thrusting of deep-level crustal rocks. Abitibi-Grenville LITHOPROBE seismic data suggests that the Superior Province continues at least 200 km southeast of the Grenville Front as a southward-tapering wedge in the Grenville lower crust. Tectonic stability has prevailed since circa 1.0 Ga in most of the Grenville Province, when it is thought that a mountain range and plateau existed, similar to the modern-day Himalayas and the Tibetan plateau. The mountains were peneplained by the time of deposition of Paleozoic limestones at circa 0.42 Ga. Neoproterozoic and younger tectonic activity is limited to localized rifting at circa 0.59 Ga along the Ottawa graben and along an east-trending corridor along the Mattawa, French and Pickerel rivers, and Lake Nipissing. In addition, subsequent rifting during the Jurassic, at circa 0.17 Ga, resulted in the formation of the Ottawa-Bonnechere graben system, as well as the localized injection of lamprophyric and kimberlitic dikes in the eastern Grenville Province in Ontario. In addition, emplacement of several mafic dyke swarms occurred prior to, during, and after the Grenville Orogeny. It has been suggested that significant neotectonic activity persists in the "Golden Horseshoe" area and along the north shore of Lake Ontario and the St. Lawrence River. At present, apart from localized, low-level seismicity in the "Golden Horseshoe" area near the subsurface trace of the Central Metasedimentary Belt boundary zone, there is no compelling evidence that significant neotectonic activity, has, or is, occurring in the Grenville Province.

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