Structural geology of the North Branch Big Sevogle River

Abstract

The deformation of Ordovician aged metasediments in the North Branch area has produced lower greenschist facies metamorphism of quartzose and pellitic sedimentary rocks. At least four generations of folds are present. The early folds with tight to isoclinal profiles are largely transposed into later upright F3 generation structures with kink or chevron profiles. The fold generations were identified principally on the basis of overprinting relationships with minor use of fold 'style' and orientation. The fold generations F 1 and F 2 have a generally shallow (less than 40°) southwestward dipping axial plane where they are preserved in outcrop. The third and fourth generation folds have steeply dipping axial planes. The first three fold generations have an associated axial planar foliation described by preferred orientation of layer silicates. The fourth phase has no distinct foliation associated. Axial planar foliations vary in style. The F1 folds have given rise to an axial planar schistosity (lrrinki, 1974) which may be a crenulation cleavage. This schistosity is seen to be folded by later folds and is a metamorphic differentiated layering described by alternating quartz and mica rich domains at this point. This foliation (S1) is crenulated in axial planes of F2 folds to produce a new axial planar parallel foliation S2. The foliations related to F3 vary in style from kink bands parallel to axial planes, through crenulation cleavage to a fracture cleavage. F 2 and F 3 folds have hinge parallel lineations created by intersection of axial plane parallel foliations with folded surfaces. The lineation associated with F2 strikes north northeasterly in this area. It is generally seen on a curved surface due to redistribution by later folding. The L2 lineation intersects a strong northwesterly striking L3 lineation at approximately 40°. The metamorphism, as stated, is low grade. The highest temperature mineral present was a spessartine garnet. This garnet is known to form at 410°C at 2 Kb pressure if the proper constituents are present (Deer et al., 1966). The presence of graphite in the rocks has probably affected the composition of the fluid phase and is known to lower equilibrium temperatures of metamorphic reactions (Winkler, 1979). The deformation of these rocks is typical of folded belts worldwide in both style of folding and sequence of fold styles created. The use of pelitic rocks for fold generation determination has proved a useful criteria for mapping in areas of polyphase deformation. Throughout the paper folds are referred to as F with a subscript number to represent fold generation. The foliation planes created by metamorphic differentiation are referred to by the letter 'S' with a subscript to identify the generation of folding with which the foliation is associated. Lineations are referred to by the letter 'L' with a subscript to identify generation. The designator S0 refers to bedding parallel foliation.