Florence-Niagara terrane: An early Proterozoic accretionary complex, Lake Superior region, U.S.A.

Abstract

The Florence-Niagara terrane is an elongate fault-bounded block ∼ 10 km wide and 80 km long, composed of miogeoclinal assemblage lithologies, bounded on the north by a generally coherent miogeocline extending to the Superior craton, and on the south by a probably exotic, magmatic arc terrane. The Florence-Niagara terrane is composed of a collage of eight fault-bounded packets. All fault packets are characterized by faults that trend west to west-northwest, with a dominant west-northwest-striking, deeply dipping foliation. Elongation lineations plunge down the dip of foliations in all cases except in packet 1, where bulk extension locally parallels shallowly plunging fold axes, a probable product of superimposed strains. Fold axes are subhorizontal, or girdled in the mean foliation plane, or plunge steeply. Packets containing girdled fold axes probably were subjected to greater strain than were packets with subhorizontal fold axes. Steeply plunging fold axes are not easily explained, but they were, perhaps, formed by noncoaxial deformation. One of the packets contains a younger generation of folds formed during east-west shortening. Another packet contains folds associated with later strike-slip faulting. Convergence between the magmatic terrane and the miogeocline probably resulted in accretion of miogeoclinal sediments in the forearc area of the magmatic terrane. The Florence-Niagara terrane may represent such a zone of forearc accretion formed prior to, and during, collision, and thus it may be referred to as an accretionary complex.