Relationships of rock cleavage fabrics to incremental and accumulated strain in the conococheague formation, U.S.A.

Abstract

Pressure solution coupled with twin gliding, and recrystallization acted to form two geometrically distinct cleavages in the Conococheague Formation, northwest Virginia and West Virginia. Limestone layers contain a penetrative cleavage ( S p ) formed by pervasive pressure solution (Coble creep), with twin gliding in more highly deformed zones. Dolomite layers contain a spaced solution cleavage ( S s ) formed by pressure solution, also with twin gliding in highly deformed zones. Recrystallization textures occur in highly deformed zones of limestones and dolomites. Incremental strain shows that layering was initially inclined as much as 30° to shortening, and that the deformation was locally non-coaxial, as viewed by the deforming material on the fold limbs. Spaced cleavage zones are curved, and closely match the initial and final orientations of incremental elongation. Penetrative cleavage parallels the elongation axis of accumulated strain (measured using deformed ooids). Pressure solution surfaces in dolomites concentrate insoluble materials. Deformation, together with twin gliding, and recrystallization are greatest in the hinge zones of folds and least on shallowly dipping limbs of asymmetric folds. The deformation mechanism path of calcite and dolomite are similar; changing from pressure solution to twin gliding with recrystallization as strain rate (strain energy density) increases.