DETERMINATION OF EARLY FOLD AXIS ORIENTATIONS IN MULTIPLY DEFORMED ROCKS USING PORPHYROBLAST INCLUSION TRAILS

Data on the original orientations of fold axes within long lived orogens are commonly preserved within porphyroblasts. This occurs in spite of matrix rotation and reactivation during non-coaxial deformation, because rigid objects that are large relative to the matrix do not normally rotate with respect to geographic coordinates. The orientation of these early fold axes can be determined from the intersection line between overprinting crenulation cleavages preserved as inclusion trails. The fold axis is located within the acute angle between two vertical thin sections of different orientation that show a change in crenulation asymmetry. Microfold axes adjacent to porphyroblasts generally form with nearly horizontal orientations during both crustal shortening and gravitational collapse, but may deviate from the macroscopic axial trend by up to 20°, because external foliations anastomose around rigid porphyroblasts prior to entrapment as inclusions. Concentric porphyroblast growth can repeat these deviation trends throughout multiple, colinear deformation stages. This consistant variation accounts for the preservation of well formed spirals, commonly with opposite crenulation asymmetries, in sections cut nearly parallel to the mean fold-axis trend. Previous methods for locating "rotation" axes are misleading because they assume that a section cut nearly parallel to the rotation axis will show no spiral-shaped inclusion trail geometries. Analysis of early orogenic fold-axis orientations from porphyroblast inclusion trails allows easy determination of the bulk compressive movement direction. This has considerable significance for determination of the amount and timing of matrix rotation during development of oroclines and pseudo-oroclines. Application of this technique allows correlation of different early orogenic episodes within a multiply deformed fold belt. © 1990.