Significance of composite lineations in the mid- to deep crust: a case study from the North Cascades, Washington

Abstract

The origin of broadly orogen-parallel (NW–SE) mineral lineations in Cretaceous and Paleogene arc plutons and amphibolite–facies metamorphic rocks (paleodepths of <10–40 km) of the North Cascades (Cascades core) is controversial, particularly the kinematic significance of these lineations and their relationship to regional displacement fields. Outcrop- to map-scale structures are dominated by fold interference. Lineations are commonly parallel to the maximum finite strain axis, which is interpreted to result from nearly coaxial superposed folding and strain accumulation. This is compatible with the better development of kinematic indicators in lineation-normal sections rather than in lineation-parallel sections in much of the region. The fold-dominated strain pattern has been locally modified by steep SW-vergent, reverse shear zones with down-dip lineation that are localized next to plutons, and near the Windy Pass thrust where weak lineation is at a high angle to the inferred displacement direction. During Eocene exhumation, top-to-N to -NNE displacement on subhorizontal surfaces was superposed on fold-related fabrics in the deepest exposed levels of the orogen. The resultant lineation is subparallel to the inferred slip direction, compatible with widespread kinematic indicators in lineation-parallel surfaces. Mineral lineations in the Cascades core are thus composite, formed from pre-96 to ∼45 Ma, and are typically parallel to the direction of local maximum stretch and only locally to regional displacement. They formed by multiple mechanisms that were partitioned at a variety of scales, and under multiple boundary conditions, including: multi-scale folding, shear zone displacement, and flow in structural aureoles imposed by pluton emplacement. Local deformation, and not orogen-scale flow, dominated outcrop-scale structures, and pre-existing anisotropy and relative strengths of adjacent layers played an important role in development of final outcrop-scale structures.