KINEMATICS AND DEFORMATION HISTORY OF THE CENTRAL CROSS LAKE SHEAR ZONE

The tectonic environment in which Archaean greenstone belts have developed is of paramount importance to understand early Earth evolution. One effective way to unravel tectonics is to study the strain geometry and kinematics from geological structures preserved in the belts and then to examine likely tectonic environments that might produce the structural pattern. The Cross Lake greenstone belt in the northwestern Superior structural province, Canada, is characterized by two sets of high-strain zones, trending ~NE 060 and WNW – ESE respectively. On the basis of previous mapping and work on geochronology and metamorphism, we have conducted structural analysis of the Cross Lake greenstone belt, concentrating on the ESE-trending high-strain zone, the Central Cross Lake Shear Zone (CCLSZ). Five generations of deformation have been identified within the CCLSZ. D1/D2 deformation is defined by isoclinal folds, which are intrafolial to a dominant transposition foliation. D3 are generally open to tight asymmetrical folds overprinting the transposition foliation. Both D1/D2 and D3 have a dextral sense of movement, and are interpreted as a continuous progressive deformation. D4 is defined by en echélon veins observed in amphibolites or other competent layers. The veins indicate a sinistral sense of movement. D5 produces open to tight asymmetric folds indicating a dextral sense of movement. Three generations of folds are observed outside of the high-strain zones with variable styles and orientations. Combined with the structural analysis of the NE-trending zone, we suggest that the F1/F2 folds within the zones and outside the zones may have shared the same early deformation history. F1/F2 folds within the zones were inherited from pre D1/D2 deformation and were extensively transposed by D1/D2 deformation. The two sets of high-strain zones may have been conjugated to each other. The strain field of the high-strain zones is unique: lineation plunges almost vertically everywhere. Numerical modeling will be conducted to help understand this conjugation relationship and some special structural feature, e.g. L-tectonite.