Abstract
A regional, closely spaced and continuous array of strike-slip fault-en echelon fractures is observed in three dimensions in Archean rocks in the Wawa and Quetico belts north of the Niagara fault and Great Lakes tectonic sutures in Minnesota, Michigan and Ontario, Canada and this pervasive rock fabric parallels the radial pattern of the 2.2 Ga KenoraKabetogama (KK) mafic dike swarm. Within the region, the composite fault-fracture array is everywhere vertical, and offsets the following Archean rock types: Lake Vermilion Formation, Ely Greenstone, Giants Range Granite, Soudan Iron-Formation, Puritan Quartz Monzonite, Kitchi Schist, and unnamed gneisses and amphibolites in the Huron Mountains, MI. Adjacent Penokean-and Keweenawan-aged rocks are not cross-cut by these fault-fracture arrays.
The composite strike-slip fault-fracture arrays are closely spaced (~1 per meter) vertical planes a few millimeters in width that display both dextral and sinistral offsets (<1 m) within an outcrop, and are usually oriented normal to an outcrop’s foliation (~E-W) and exposed on flat, glacially polished surfaces. Dextral fault offsets predominate both in fault number and total right-lateral offset in an outcrop and the region. Within weaker rocks, such as metaturbidites like the Lake Vermilion Formation, en echelon fracture arrays are developed along strike of the fault-fracture array providing a unique field test of the relationship between absolute fault offset and the kinematic sense of shear of the contemporaneous en echelon fracture array. In most cases, the en echelon kinematic sense and the absolute foliation offset are in agreement, but not always; dextral and sinistral offsets are also observed along strike of an individual fault-fracture array. The sense of offset on the strike-slip fault (Mode 1 fractures) is also opposite the sense of offset along faults within en echelon arrays associated with the strike-slip faults. Our interpretation is that the fault-fracture array was initially a Mode 1 extension fracture which, along its undulating strike, developed contemporaneously with one or two sets of Mode 2 shear fractures. Analysis of the cross-cutting field relations and mechanical twins in fault-fracture calcite fillings indicate that σ1 was horizontal and parallel to the strike of the fault-fracture arrays during fault displacement. Twinned calcite and deformation lamellae in quartz in the fault-fracture fillings indicates a differential paleostress level of ~100 MPa during faulting. The orientation of al parallel to a strike-slip fault is a unique result compared to the principal stress orientations associated with strike-slip faults proposed by Anderson (1951; 45 °) and Mount and Suppe (1987; 90 °).
Where the composite fault-fracture array and the KK dikes are exposed together, they are parallel suggesting that the dikes utilized this anisotropy during magma injection. Petrographic observations did not reveal any obvious internal dike deformation; however, in one locale the edge of one dike preserves some horizontal, dike-parallel striations. We hypothesize that the fault-fracture array preceeded, and greatly influenced the the KK dike injection.
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Craddock, J.P., Moshoian, A. (1995). Continuous Proterozoic Strike-Slip Fault-En Echelon Fracture Arrays in Archean Rocks: Implications for Fault Propagation Mechanics and Dike Injection. In: Ojakangas, R.W., Dickas, A.B., Green, J.C. (eds) Basement Tectonics 10. Proceedings of the International Conferences on Basement Tectonics, vol 4. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-0831-9_36
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