Evidence for a complex archean deformational history; Southwestern Michipicoten Greenstone Belt, Ontario

Abstract

The Michipicoten Greenstone Belt extends for about 150 km ENE from the northeastern angle of Lake Superior. In common with many other Archean greenstone belts, it is characterized by generally steep bedding dips and a distribution of major lithologic types suggesting a crudely synclinal structure for the belt as a whole. Detailed mapping and determination of structural sequence demonstrate that the structure is much more complex. The Archean history of the belt includes formation of at least three regionally significant cleavages, kilometer‐scale overturning, extensive shearing, and diabase intrusion. Most well defined, mappable “packages” of sedimentary rocks appear to be bounded by faults. These faults were active relatively early in the structural history of the belt, when extensive overturning also occurred. Steepening of dips, NW‐SE shortening, development of steep NE cleavage, and pervasive shearing all postdate the early faulting and the regional overturning, obscuring much of the detail needed to define the geometry of the earlier structures. The results obtained so far suggest, however, that the Michipicoten Greenstone Belt underwent an early stage of thrusting and associated isoclinal folding, probably in a convergent tectonic environment.

Metamorphism of the Canadian Shield, Ontario, Canada. I. The Superior Province

“…All exhibit a metamorphic zonation similar to that observed in other granite-greenstone subprovinces, with greenschist-facies cores and amphibolite-facies zones developed marginal to large internal and external plutons (e.g., Ayres 1969, Studemeister 1983, McGill & Shrady 1986). In the Wawa area, Studemeister (1983Studemeister ( , 1985 suggested that pretectonic tonalite stocks produced greenschistfacies contact aureoles at 450-550°C and <2 kbar, with regional metamorphism affecting both the volcanic and plutonic rocks under conditions of 325-450°C and 2-3 kbar.…”

Structure and kinematics of a major tectonic contact, Michipicoten greenstone belt, Ontario

“…The structural differences are almost certainly in part a reflection of the lithologic contrasts, but it is by no means clear that lithology is the sole controlling factor. Can McGill and Shrady (1986), and Arias and Helmstaedt (1990). Township locations shown in Fig.…”

“…2) is characterized by lithologic assemblages separated by sharp contacts inferred to be faults (McGill and Shrady 1986). Evidence used to support a fault interpretation for assemblage contacts includes one or more of the following: truncated layering on one or both sides of the contact; abrupt tops reversals; enhanced stretching or flattening of clasts; local cleavages; disaggregation of silt layers at mesoscopic and microscopic scales; gossans; mafic sills; networks of quartz veins; vein quartz deformed into rootless folds; and narrow linear topographic depressions.…”

“…Three regionally significant cleavages were described by McGill and Shrady (1986): a C l (S1) cleavage defined by oriented phyllosilicates and the XY plane of deformed clasts; a C2 (S2) spaced cleavage, striking northwest, that crenulates S1; and a C3 (S3) spaced cleavage, striking northeast, that crenulates both older cleavages. S1 is commonly nearly parallel to bedding and may be associated with tight to isoclinal folds at centimetre to tens of metres scale.…”

“…Additional detalled mapping has, " however, turned up several localities where these two fabrics appear to be separated by a small angle and one or two outcrops In which they occur at high angles to each other. The designations SI, and S l h are intended to emphasize the close geometric 4% relationship between these two fabrics and also to facilitate correlation with fabric numbering schemes already published (McGill and Shrady 1986;Arias and Helmstaedt 1989).…”