Abstract

Most rocks exposed in the Trout Lake Area are migmatitic gneisses severely deformed during the Hudsonian orogeny (± 1700 m.y. ago). Hornblende-bearing layers or lenses, mostly well lineated, are found throughout the entire area. The attitude(s) of the hornblende lineation (aligned prismatic grains) can be measured in every outcrop.Within cylindrical buckling folds and boudinage structures, involving hornblende-bearing rocks, the orientation of hornblende lineations is related to local directions of megascopic finite strain parallel to layering, for the main period of tectonic flow. Because the local strain ellipsoid is commonly oblique to layering, the strain ellipse parallel to layering is generally a non-principal section. The direction of its longest diameter is called M, regardless of whether it is a direction of extension, shortening or no longitudinal strain.Throughout all medium-scale folds and boudinage structures investigated, hornblende lineations are parallel to M. This relationship is considered as general, and hornblende lineations are employed as indicators of M within large-scale folds. (Other structural elements are used in distinguishing between finite extension and shortening parallel to M.) Finally, the strain pattern of the Trout Lake Area (west half) is analyzed by means of a contour map for the lineation plunge (non-statistical "plunge isogonic map" after D. Elliot 1965).The areal pattern of M contains a diagonal belt across which the plunge values tend to change "discontinuously." Because this narrow belt coincides roughly with a major branch of the Birch Rapids – Wepusko Bay fault zone, the angular "discontinuity" in the lineation map can be attributed to "late" rotational faulting.

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