Abstract

The survey area lies close to the continental margin and includes parts of the Insular Tectonic Belt and the Coast Mountains igneous and metamorphic complex, which are part of the Cordilleran (geological) Region. In an endeavor to clarify the structure of the Insular Tectonic Belt and the Coast Mountains complex, gravity measurements have been made using Worden or LaCoste and Romberg meters at 12–15 km intervals throughout the Queen Charlotte Islands, Vancouver Island, and the coastal areas of the British Columbia mainland. Measurements have been made at the same interval using a LaCoste and Romberg underwater gravity meter wherever the depth of water is less than 300 fathoms (< 540 m) along the fiords of the mainland coast and over the continental shelf. The observed gravity values have been reduced to Bouguer anomalies and terrain corrections have been calculated using either Bible's tables or a computer system based on the attraction of the rectangular prismatic block.The major features of gravity field are: (1) a positive Bouguer anomaly along the western edge of the area, which is associated with the change from continental to oceanic crust, and (2) a negative anomaly along the Coast Mountains, which is attributed to the thickening of the continental crust below these mountains. On the eastern side of the Queen Charlotte Islands, Hecate Strait, Queen Charlotte Sound, and Vancouver Island, the average Bouguer anomaly is approximately zero, with local anomalies superimposed on a fairly flat gravity field. Several of these local anomalies are related to density variations in the surface rocks.

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