Structural analysis of a similar-type fold : Booster Lake, Manitoba

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Lamb, Craig Forbes
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A similar-type fold is outlined by a unit of meta-conglomerate in the Booster Lake area of Manitoba. This anticline merges into a major syncline which extends 25 miles westward to Lac du Bonnet. The anticline has deformed an Archean sequence of metagreywacke, meta-conglomerate and a second unit of metagreywacke. A granite pluton was intruded into the core of the fold. The oreintation of the Booster Lake fold is defined by its plunge and the dip of its axial surface. On the basis of its orientation and closure, it is a tight inclined anticline. Assuming cylindrical folding, a right section is constructed to enable classification of the style of folding. Three parameters: dip isogons; orthogonal thickness, t; and axial planar thickness, T, show the fold style to be complex. It almost ideally belongs to Ramsay's (1967) Class 2 in the hinge area but is of Class 1C in the limbs. The style of folding, foliation, lineations, and textures are inconclusive in defining the type of strain which produced the fold, but four types of strain are considered possible: heterogeneous pure shear; homogeneous flattening superimposed on a flexural fold; flexural folding followed by heterogeneous or homogeneous pure shear and heterogeneous simple shear; and heterogeneous or homogeneous pure shear following by heterogeneous simple shear. The mechanism was that of passive flow contemporaneous with metamorphism. The granite pluton may have been involved in the folding as it was either syn-folding or post-folding.