Homogenous deformation in precambrian supracrustal rocks of Kumlinge area, southwest Finland

Abstract

Supracrustal rocks form a trough-like prolonged syncline surrounded by granitic rocks. They consist of quartz-feldspar schists (leptites) overlain by amphibolites. The stratigraphic succession in the syncline is as follows: undermost lie leptites followed by amphibolitic pillow lavas, which are gradually superseded by pillow breccias and agglomerates. A small area of polygonally jointed amphibolites lies above the pillow breccias. The succession gives the impression of volcanic activity in a gradually declining depth of water. The pillow lavas and agglomerates are commonly flattened or elongated and only locally undeformed. Elongation-type deformation is most pronounced in the agglomerates of the central parts of the syncline, where it now reaches its maximum breadth. The axial ratios of the elongated agglomerate fragments are approx. 2/3/75. Around the margins of the syncline the pillow lavas are strongly flattened. The most flattened pillows have axial ratios of approx 1/60/60. The flattening deformation in the adjacent leptitic rocks can be estimated from boudinage and folding in numerous pegmatite dikes. The estimated deformation in the leptites agrees reasonably well with the observed deformation in the adjacent pillow lavas. A gravitative sinking of the dense amphibolites into the underlying granitic material could possibly account for variations in the type of deformation within the syncline.