John F. Dehls MSc thesis abstract

Thesis Title: 
The Magnetic Fabrics and Strain History of the Archean Seine Group Metasedimentary Rocks Near Mine Centre, Northwestern Ontario
John F.

The Seine Group metasedimentary rocks are situated between two converging major dextral transcurrrent faults, the Quetico Fault to the north and the Seine River Fault to the south.  To the west lies the Bad Vermilion Intrusive Complex (B.V.I.C).  The Seine Group shows one major period of deformation resulting in major F1 folds.  Regional metamorphism of chlorite to biotite zone greenschist facies was synkinematic with the deformation.  Later minor deformation include localized crenulation cleavage and faults and shear zones.

Strain analysis of deformed conglomerates indicates 50-67% shortening in a north-south direction.  The B.V.I.C. has behaved rigidly during the deformation and produced a strain shadow in which only 18-42% shortening has occurred, deflected to a WNW-ESE direction.

The schistosity has an average trend of 252.3/85.7°NW, and the average trend of the stretching lineations is 063.4/43.4°.  However in the strain shadow of the B.V.I.C. the strikes are deflected to the NNE.

The AMS fabric in the Seine Group is controlled by paramagnetic chlorite and biotite.  The AMS foliation has an average orientation of 251.6/81.4°NW.  The AMS lineation has a peak trend of 059.7/41.7°.  The AMS ellipsoids are mostly oblate, with the least anisotropic fabric located in the strain shadow.  The AMS fabrics are deflected in the strain shadow of the B.V.I.C. in the same sense as the schistosity.

The anisotropy of isothermal remanent magnetization (ASIRM) of each sample was determined, using a saturating field of 500 mT, and then repeated at 800 mT.  The ASIRM fabric in the Seine Group is controlled by the ferromagnetic minerals magnetite and pyrrhotite.  The ASIRM lineation has a peak trend of 060.3/33.3°.  These fabrics are deflected in the same sense as the schistosity in the strain shadow of the B.V.I.C.

Although the three steep planar fabrics measured are similar in orientation, they are offset in plan view.  The angle between the schistosity and the AMS foliation is 4.4°.  The angle between the AMS and ASIRM foliation is 4.2°.  The schistosity, defined by deformed quartz and feldspar, is the oldest fabric.  The AMS fabric is younger, as the chlorite and biotite formed during metamorphism.  The magnetite and pyrrhotite which define the ASIRM formed later still, and thus represent the youngest fabric.

The successive offsets between the three fabrics is consistent with a dextral transpressional strain history operating throughout the fabric development.  The strain history had components of simple shear and non-coaxial shortening.  The initial shortening probably took place in a SE-NW direction.

A copy of the thesis can be downloaded here