Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: refolding of precleavage nappes during D₂ transpression

Abstract

Recent mapping in the western Wawa subprovince (the Vermilion district and its westward extensions in Minnesota) has identified a major, northeast-trending stratotectonic break, informally called the Leech Lake structural disconformity (LLSD), that separates two contrasting terranes. North of the LLSD are elongate, east-northeast-trending, fault-bounded panels of volcanic rocks, which are mostly north topping and homoclinal. South of the LLSD, large-scale, northwest-trending folds involve basaltic sequences that are stratigraphically overlain by thick sections of dacitic volcaniclastic and turbiditic rocks. However, the most prominent outcrop-scale deformational features are northeast-trending vertical folds and associated axial-planar cleavage related to transpression in D 2 . D 1 minor folds and cleavage are rare.New field data indicate that the large folds in a predominantly sedimentary part of the southern terrane are early formed (D 0 –D 1 ), and nappe-like. The precise form of the early folds is largely obscured by (i) superimposed folds and metamorphism contemporaneous with D 2 , (ii) faulting that began in D 2 and outlasted folding, and (iii) emplacement of the Giants Range batholith and associated plutons. Nevertheless, the presence in the southern terrane of large areas of shallow-plunging, downward-facing rock sequences and the map pattern of rock units imply that a large south-verging, northwest-plunging thrust nappe (or nappes) antedated D 2 . Where the nappe lacked thick, rigid volcanic layers, accommodation to D 2 transpression took the form of abundant Z folds. Much of the observed Z asymmetry of F 2 folds may have resulted from compression and shear oblique to the trend of rock units. In contrast, early thrusts are inferred to have positioned volcanic units north of the LLSD such that their strike was nearly perpendicular to D 2 compression, and therefore F 2 folds did not develop extensively.