EARLY DEFORMATIONAL HISTORY OF ARCHEAN ROCKS IN THE VERMILION DISTRICT, NORTHEASTERN MINNESOTA.

Two main episodes of folding and deformation, F//1 and F//2, have previously been recognized in the Tower area. The F//2 folds are open to tight, and for the most part plunge steeply to the east in near-vertical axial surfaces. The major F//1 folds are isoclinal, with near-horizontal hinge lines. A metamorphic fabric consists of a slaty cleavage parallel to the axial surfaces of the F//2 fold and a mineral lineation sub-parallel to the fold hinges. Finite strain was determined using clasts in the coarser sedimentary/volcanoclastic units. The maximum extension ( lambda //1) direction is parallel to the mineral lineation and the minimum extension ( lambda //3) direction is normal to the cleavage. The strain is of constrictional type where the mineral lineation is dominant and of flattening type where the cleavage is dominant. The strain data can best be accounted for in terms of a single deformation, that producing the F//2 folds. This is paradoxical because fold style indicates that the F//1 deformation was at least as intense as the F//2. The problem can be resolved if the F//1 folds were of soft sediment origin, resulting from downslope slump movements, such that the clasts remained internally unstrained during this deformation. A single tectonic deformation, involving lateral compression and vertical extension between the two intruding batholiths, satisfactorily accounts for the strain data, the development of the F//2 folds, and the present overall structural configuration.