Vertical and fold-axis parallel extension within a slate belt in a transpressive setting, northern Appalachians

An incremental and finite strain study of a Mid-Paleozoic slate belt in a transpressive setting in Gaspé, Canadian Appalachians, has been undertaken using syntectonic fibers from pyrite-type pressure shadows. Regionally, strain estimates are quite consistent and reflect only slightly higher strain in the central part of the anticlinorial structure defining the slate belt. Extensions of up to 160% vertically and 110% parallel to the fold axis were recorded in the slate belt combined with an estimated overall regional shortening of 80%. The fibers first rotated toward the final position of the cleavage within a vertical section normal to the cleavage, and then rotated from a vertical to a horizontal orientation. This change in fiber orientation observed in cleavage-parallel sections indicates an abrupt change in the incremental stretching direction from sub-vertical to sub-horizontal (fold-axis parallel). The vertical extension is related to folding and cleavage development during coaxial flattening. The horizontal extension reflects the strain imposed on the rocks during regional simple shear which produced strike-slip faulting and further tightening of the folds. Folding, cleavage development, fold-axis parallel extension and faulting are all related to oblique convergence during the Middle Devonian Acadian orogeny in this part of the Canadian Appalachians and clearly indicate an overall transition from a pure shortening deformation to a simple shear dominant deformation in an overall transpressive setting.