Structures in the Lower Paleozoic Meguma Group, Nova Scotia

Abstract

At least three generations of folds and faults affect slates, schists, and quartzites of the Meguma Group and other Lower Paleozoic rocks over a large part of mainland Nova Scotia. The main folds ( F 1 ) are upright and low-plunging and trend northeast to east closely parallel to the strike of a steep axial-plane foliation ( S 1 ). Mid-Devonian granite batholiths intersect the F 1 folds with little disruption of the trends. Cross-folds ( F 2 and F 3 ), although widespread, are less pervasive than the F 1 folds; most, but not all, are small-scale structures restricted to the foliated argillites. The cross-folds deform the S 1 foliation and are therefore younger than the F 1 folds. The F 2 folds are typically interfolial or interstratal, angular, and Z -shaped in profile; nearly vertical F 2 axial planes strike northward, and the axes of the F 2 folds in S 1 are steep. In metamorphic aureoles of granites, quartz-feldspar veins and andalusite porphyroblasts cross F 2 folds and crinkles, indicating that the granites are younger than the F 2 folds. Nearly all F 3 folds are open S -shaped kinks with steep axial planes striking northwest. The kinks deform porphyroblasts in the granite aureoles and pass into small sinistral faults which cut the granites. However, large sinistral faults parallel to the kinks offset the folded Meguma rocks, but not the granites. Thus the sinistral faults, and possibly the kinks, were formed before and after the emplacement of the granites. The Z -shape of the pregranite F 2 folds and the occurrence of postgranite dextral offsets along and across the S 1 foliation suggest that northeast- to easterly directed dextral movements may also have been repeated. Possibly the maximum compression was originally oriented northwest to north perpendicular to the F 1 axes and changed to an east-west direction to form the northerly trending F 2 folds; a continuation of this compression, both before and after granites were intruded, led to the F 3 S -kinks and sinistral faults striking northwestward, and to dextral faults striking northeast to eastward.