The free-air gravity anomaly grid of Canada shows variations in the gravity field caused by lateral variations in the density of the Earth's crust and upper mantle that reflect variations in composition and thickness. Systematic gravity mapping began in Canada in 1944 and is ongoing. All data are tied to the International Gravity Standardization Network 1971. Local gravity anomalies result from the juxtaposition of relatively high- and low-density rock types. Longer wavelength anomalies such as the gravity low over the Cordillera and the relative gravity high over oceanic crust largely reflect variations in the thickness of the crust.

This grid presents free-air gravity gravity anomalies. The data were compiled from the holdings of the Canadian Geodetic Information System maintained by the Geodetic Survey Division, Geomatics Canada. They were collected to map the variation in gravitational attraction over the Canadian landmass and offshore areas. Variations in the force of gravity are due to variations in the mass of underlying materials. These data are useful for geological interpretation and have applications in oil, gas, and mineral exploration. The gravity field is also used to define the geoid, which is the ideal shape of the Earth, or mean sea level if the Earth were completely covered with water.

The data used to compile this grid consist of approximately 678 000 gravity observations, including 165 000 on land, acquired between 1944 and 2005. The data spacing ranges from less than 1 km to over 20 km, with an average spacing between 5 and 10 km. All measurements were reduced to the IGSN71 datum. Theoretical gravity values were calculated from the Geodetic Reference System 1980 (GRS80) gravity formula. The observation's coordinates are referred to NAD83. The data were gridded to a 2 km interval, with a blanking radius of 20 km.

The free-air gravity anomalies correspond to variations in lateral density and mass in the upper mantle and the crust. Most high-frequency anomalies are caused by near-surface contacts of rocks that have significantly different densities. For example, in the Superior Province of the Canadian Shield, granitic plutons are less dense than the volcanic rocks they intrude and are local gravity lows. The boundary between the Trans-Hudson Orogen and the Superior Province is marked by an extensive, curvilinear gravity high that corresponds to the denser material of associated magmatic arcs. Longer wavelength anomalies are generally associated with variations in crustal thickness. The offshore areas of the anomaly map have significantly higher values than those on land because oceanic crust has a higher average density and is thinner than continental crust. The anomalous low over the Cordillera is a response to lower density material in the upper mantle (Goodacre, 1972) that supports the excess mass of the topographic high. The Kapuskasing Uplift in the Superior Province is represented by a gravity high that corresponds to denser material underplating the uplifted crust. The gravity low associated with the Grenville Front is a response to thickening of the continental crust resulting from the collision of the Grenville and Superior provinces.

Goodacre, A.K. 1972: Generalized structure of the deep crust and upper mantle in Canada; Journal of Geophysical Research, v. 77, p. 3146-3161.

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