Early Proterozoic activity on Archean faults in the western Superior province - evidence from pseudotachylite

Late Archean granitic plutons (∼2700 to 2665 Ma; U-Pb zircon) of the Superior structural province are cut by a variety of brittle discontinuities, including joints, fractures, and faults, the latter of which show evidence of cataclasis and meter to tens of meters displacements. Fluid circulation, alteration of wall rocks, and neomineralization of epidote, actinolite, and chlorite occurred under greenschist facies conditions in these structures at the time of their formation. The Rb-Sr ages of host rocks are ∼2650 Ma, and the fracture-zone materials are 2300 Ma, indicating that most of this activity occurred long after Archean plutonism and was concentrated in Early Proterozoic time. The brittle structures formed in response to horizontal compression on a regional scale, possibly caused by tectonic processes at margins of the craton. The structures are seen as important clues to the tectonic evolution of the Superior province.