Acadian Slaty Cleavage Noncoplanar with Fold Axial Surfaces in the Northern Appalachians

Abstract

The Acadian orogeny though polyphase produced a single generation of folds and a single generation of slaty cleavage over wide areas of the northern Appalachians. Acadian slaty cleavage noncoplanar with fold axial surfaces is more widespread than has hitherto been recognized-Published examples from several areas are quoted. Cleavage coplanar with the axial surface in one part of a fold may transect the axial surface and both limbs elsewhere in the fold, and the time relations to folding are implicitly the same. Geometric relationships of regional Acadian slaty cleavage to axial surfaces of associated folds in Silurian sediments in northeast New Brunswick are described. Slaty cleavage is superimposed upon the folds whether coplanar or noncoplanar with axial surfaces. Convergent fans and refraction of the noncoplanar cleavage indicate compression later than the cleavage. Total shortening of Silurian strata is a combination of folding and of superimposed compression accompanying and succeeding initiation of regional slaty cleavage. A possible origin of the noncoplanar Acadian structures resulting from décollement of Silurian sediments upon Ordovician basement is discussed.