Abstract

Regional fold patterns in PreCambrian rocks, particularly Archean, are usually difficult to outline by direct geological mapping techniques. Resorting to an indirect approach, it is believed that a broad picture of the folds in a district can be obtained by making use of the secondary rock foliations and bedding remnants, combined with a study of the outline and distribution of plutonic rocks.To illustrate how the method works and to demonstrate some conclusions that may be drawn from its application, a 10,000 square mile section of more-or-less typical Archean from Northwestern Ontario is analyzed. A number of "possible folds" are outlined, which trend in two directions approximately at right angles to one another. This cross-fold pattern is widespread in Northwestern Ontario (as it is in other parts of the Canadian Shield) and cross-folding may be a fundamental structure of the region. There is evidence to suggest that both members of the cross-folds were formed contemporaneously, which is what might be expected at the roots of mountains formed by compression in the crust.There is a tendency for mineral deposits to be concentrated in cross-folded zones, so that the method of plotting folds may be useful in locating areas for detailed prospecting.

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