On the significance of foliation patterns preserved around folds by mineral overgrowth

Abstract

Simple inclusion trail geometries in porphyroblasts that vary symmetrically around folds suggest the porphyroblasts have rotated during folding. However, they do not provide proof that the porphyroblasts have rotated as such geometries may also form by overgrowth during folding. This uncertainty results from the difficulty in establishing the relative ages of the fold and the porphyroblasts when the latter contain simple trails. Simple inclusion trail geometries in porphyroblasts that do not change around a fold, remaining orthogonal to the axial plane from limb to limb, suggest that the porphyroblasts have not rotated during folding. However, they can be rationalized as having rotated by arguing that synthetic rotation due to buckling has been exactly counterbalanced by antithetic rotation due to flexural flow. If the folds vary in tightness from open to isoclinal, and the trails still remain perpendicular to the axial plane, this can still be rationalized by arguing for rotation during buckling balancing that due to flexural flow followed by coaxial deformation with no subsequent rotation. Tests of porphyroblast behaviour are only useful if they conclusively demonstrate that inclusion trail geometries around a fold are incompatible with rotation, or alternatively non-rotation, of the porphyroblasts. Simple inclusion trail geometries in porphyroblasts that uniquely indicate a fold mechanism involving no rotation are those which remain parallel from limb to limb but lie oblique to the axial plane independent of limb angle. Particularly powerful indicators of folding mechanisms involving no porphyroblast rotation are folds containing two different porphyroblastic phases that preserve contrasting geometries. For example, rocks containing porphyroblasts preserving parallel trails from limb to limb in the earlier phase and symmetrically varying trails for the later phase. Such folds demonstrate that simple trails that vary symmetrically around a fold cannot be used as proof of porphyroblast rotation. Where the rotation axes of such simple inclusion trail geometries lie at a high angle to the axial plane and vary around the fold, proof of porphyroblast rotation may be possible. However, such proof can be more readily provided using complex inclusion trails with an orthogonal and/or truncational character that vary in orientation from limb to limb.