Barbara Ellen Seemayer HBSc thesis abstract

Thesis Title: 
Contact of a Late Archean Clastic Sequence
Barbara Ellen

This thesis examines the structural and stratigraphic relationships between an Archean clastic sequence of "Timiskaming-like" sedimentary and volcanic rocks and the adjacent Keewatin volcanic terrace.  Field and laboratory studies have shown that mylonitic rocks mark the sheared northern and southern boundaries of the sequence.  Similar clastic sequences are exposed to the west.  The intervening volcanic terrane is characterized by strongly sheared rocks.  The strike of these shear zones closely parallels the boundary shears of the clastic sequence, suggesting that all shear zones are related and form a large, east-west trending major zone of dislocation.  A faulted intrusive contact marks the contact with a granite to the south.  Proterozoic sedimentary rocks are exposed to the southeast and unconformably overlie the Archean rocks.

The rocks of the clastic sequence are cut by a pervasive, east-west striking subvertical cleavage.  Locally the rocks possess a second, northerly striking crenulation cleavage, which as resulted in the formation of kink bands, chevron folds, and minor folds in fine-grained rocks throughout the sequence.  The hinge lines of these minor folds plunge northward at moderate to steep angles.  The crenulation cleavage has not been observed in rocks of the adjacent terrane.

A model is proposed to account for the features found in these rocks.  Strike-slip motion along an irregularly oriented crustal shear zone resulted in the opening of pull-apart basins at releasing bends and uplift at restraining bends which led to local areas of high relief.  Sediments and volcanic debris filled the basin.  The crenulation cleavage is the result of complex motion and local reversals in the shear sense along portions of the fault, which caused compression in part of the basin.