FORMATION OF OROGEN-PARALLEL MINERAL LINEATIONS BY MULTIPLE PROCESSES, NORTH CASCADES, WASHINGTON

The origin of subhorizontal orogen-parallel (NW-SE) mineral lineations that formed over a protracted time interval (pre-96 Ma to 45 Ma) in the crystalline core of the North Cascades is controversial. Of particular concern is the relationship of these lineations to finite strain axes and regional displacement directions. Our field and modeling (Paterson et al., this vol.) studies in the amphibolite-facies rocks and plutons of the core indicate that in many places folds are the dominant structures from the thin section to map scale. Mineral lineations are statistically subparallel to the fold hinge lines, but commonly lie at a small angle to the axis in the hinge and at a higher angle in the limbs. These patterns probably record finite strain due to nearly coaxial superposed folding where mechanically active layering played a key role. This interpretation is compatible with the better development of kinematic indicators in lineation (and fold axis)-normal- than in lineation-parallel surfaces. Stretch parallel to the lineation is indicated in metamorphic rocks by boudinage, fibrous overgrowths, and stretched clasts; and in syntectonic plutons by magmatic and high-T subsolidus lineations oriented normal to late dikes and parallel to the regional mineral lineation. During Paleogene exhumation, orogen-parallel non-coaxial shear on subhorizontal surfaces was superposed on earlier fold-related fabrics in the deepest exposed levels of the Cascades core. The resultant composite lineation is subparallel to the maximum stretch and slip direction in these surfaces, as indicated by boudinaged pegmatite dikes and sills, and by widespread development of kinematic indicators in lineation-parallel surfaces. We conclude that mineral lineations in the Cascades core are in many cases long-lived and composite, are typically parallel to the direction of maximum stretch but not regional displacement, and formed by multiple mechanisms that were partitioned at a variety of scales.