Structure and magnetic fabric of the Quetico metasedimentary rocks in the Calm Lake - Perch Lake area, near Atikokan, Northwestern Ontario
Abstract
The Quetico metasedimentary rocks are the metamorphosed
equivalents of a turbidite sequence, comprised of a repetative
interstratification of sandstones and mudstones. The rocks are
metamorphosed to lower greenschist facies, but metamorphic grade
increases progressively from north to south. Anchimetamorphosed
phyllites and slates in the north part of the present study area grade
progressively into biotite schists in the south part.
Interpretation of structural features in the study area, mainly
bedding(S0) -cleavage(S1) relationships and structural facing
directions, has led to the delineation of a number of major folds.
These are tight to isoclinal, asymmetric sheath folds, with axial
planes arranged en echelon and slightly oblique to the dominant eastwest,
vertically-dipping, structural trend. The orientation, geometry
and disposition of the F1 folds suggest the Quetico rocks of the
present study area have experienced a regional dextral transpressional
tectonic evolution, with components of north-south
regional shortening and east-west regional dextral shear, with a
possible component of vertical (south side up) displacement.
The Quetico metasedimentary rocks have a polyminerallic
magnetic mineralogy comprised of significant proportions of a
ferrimagnetic (magnetite and pyrrhotite) and a paramagnetic
(chlorite, biotite, muscovite) component. The rocks possess a
predominantly tectonic magnetic fabric, which consists of a
deformational, and a metamorphic, magnetic fabric. In some
rocks (especially coarse-grained sandstones with wide-spaced
cleavage planes) a depositional magnetic fabric is partially
preserved. The complexities involved in having component
deformational, metamorphic and depositional magnetic fabrics
Indicate that the principal magnetic susceptibility directions of the
rock's magnetic susceptibility anisotropy cannot be considered
reliable Indicators of principal finite strain directions in the Quetico
metasedimentary rocks. Great care must be taken in interpreting the
significance of principal magnetic susceptibility directions.
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