The structure, stratigraphy and strain history of the Seine Group and related rocks near Mine Centre, Northwestern Ontario

Abstract

One or possibly two overlapping periods of deformation are responsible for the structures observed in the rocks in the Seine River area. This deformation has resulted in the formation of tight to isoclinal, non-plane approximately cylindrical major F1 folds with steeply dipping E-W striking axial surfaces. A late stage deformation has resulted in the formation of a crenulation clsavage, kink bands and minor faulting. There is also I imited evidence of a possible pre-F 1 folding event. Two major I ithological groups are present in the study area: shallow water metasedimentary rocks bf the Seine Group and metavolcanic rocks. The Seine Group metasedimentary rocks are younger than the metavolcanic rocks in the western part of the area but may be older than similar metavolcanic rocks in the eastern part of the area. Two ages of metavolcanic rocks therefore appear to be present: older metavolcanic rocks in the west which under! ie the Seine Group, and younger metavolcanic rocks in the east which overlie the Seine Group. Regional metamorphism to the chlorite to biotite zone greenschist facies was synkinematic with the deformation of the rocks but may have outlasted the folding in plates. Strain analysis from the metasedimentary rocks reveals that the conglomeratic units are more intensely strained than arenite units, although alI the strain elI lpsoids are of the flattened (K < 1) type. Average shortening in Z ranges from 52% for arenite units to 75% for conglomerate units. A new empirical approach suggested by the writer and Dr. Borradai le for assessing competence contrasts between strain markers and matrix is outlined. This method uses the effects of competent markers on cleavage traces in the matrix of conglomerates.