ON THE SIGNIFICANCE OF FOLIATION PATTERNS PRESERVED AROUND FOLDS BY MINERAL OVERGROWTH

Simple inclusion trail geometries in porphyroblasts that vary symmetri cally around folds suggest the porphyroblasts have rotated during fold ing. However, they do not provide proof that the porphyroblasts have r otated as such geometries may also form by overgrowth during folding. This uncertainty results from the difficulty in establishing the relat ive ages of the fold and the porphyroblasts when the latter contain si mple trails. Simple inclusion trail geometries in porphyroblasts that do not change around a fold, remaining orthogonal to the axial plane f rom limb to limb, suggest that the porphyroblasts have not rotated dur ing folding. However, they can be rationalized as having rotated by ar guing that synthetic rotation due to buckling has been exactly counter balanced by antithetic rotation due to flexural flow. If the folds var y in tightness from open to isoclinal, and the trails still remain per pendicular to the axial plane, this can still be rationalized by argui ng for rotation during buckling balancing that due to flexural flow fo llowed by coaxial deformation with no subsequent rotation. Tests of po rphyroblast behaviour are only useful if they conclusively demonstrate that inclusion trail geometries around a fold are incompatible with r otation, 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 f rom limb to limb but lie oblique to the axial plane independent of lim b angle. Particularly powerful indicators of folding mechanisms involv ing no porphyroblast rotation are folds containing two different porph yroblastic phases that preserve contrasting geometries. For example, r ocks containing porphyroblasts preserving parallel trails from limb to limb in the earlier phase and symmetrically varying trails for the la ter phase. Such fords demonstrate that simple trails that vary symmetr ically around a fold cannot be used as proof of porphyroblast rotation . Where the rotation axes of such simple inclusion trail geometries li e at a high angle to the axial plane and vary around the fold, proof o f porphyroblast rotation may be possible. However, such proof can be m ore readily provided using complex inclusion trails with an orthogonal and/or truncational character that vary in orientation from limb to l imb.