Diapiric structures and regional compression in an Archean greenstone belt, Island Lake, Manitoba

Abstract

The sequence of structures affecting rocks of the Island Lake greenstone belt suggests a progression from structures attributable to emplacement of granitic plutons to those that are of regional extent. Early homoclines (D0 deformation) and isoclinal folds (D1) are overturned or face away from marginal plutons which partly truncate the folds. Granitic offshoots display an S1 foliation. Syn-D1 emplacement of the granite is implied, and the pattern of homoclines and folds is consistent with deformation due to diapiric uplift.Later upright folds (D2) and a widespread subvertical schistosity S2 trend eastward throughout most of the region. S2 passes with little deflection into the granites. D1 folds are rotated towards S2 and clasts and volcanic vesicles are flattened, except within a zone of low strain extending 15 km east–west between two plutons. Evidently during D2 deformation the plutons acted as relatively rigid massifs between which a large 'pressure shadow' developed in response to regional north–south compression.