Orientation history and rheology in slates, Kodiak and Afognak Islands, Alaska

Theoretical and experimental studies have shown that slates have a roughly parabolic yield curve in φ - σ and Φ - σ space, where φ is the orientation of cleavage relative to the maximum compressive stress, Φ is the orientation of cleavage relative to the incremental shortening direction and σ is the deviatoric stress. Deformation mechanisms that operate in low-grade rocks such as diffusive mass transfer in a grain boundary fluid, dislocation creep and frictional grain boundary sliding may have stress-strain rate relationships that depend on φ. During a progressive strain history, φ and Φ may vary as a function of the strain path. Thus, the sequence of dominant deformation mechanisms as well as the stress-strain history may depend on the orientation history or variations in Φ as strain accumulates. In the Kodiak Formation, a slate belt in southwest Alaska, the orientation history and deformation mechanisms were evaluated using incremental strain indicators and microstructures. There are four successive textural events: (1) development of a slaty cleavage (S₂) with antitaxial growth of fibrous quartz and mica in pressure shadows (Fi₂); (2) static growth of siderite and chlorite porphyroblasts; (3) 10-70° rotation of porphyroblasts relative to S₂ with growth of curved syntaxial siderite fibers; and (4) antitaxial growth of fibrous quartz and micas with local development of a crenulation cleavage (S₃). Cumulative incremental strain and progressive finite strain histories were determined in 23 samples from syntectonic fibers, and orientation histories were reconstructed from information provided by cumulative incremental strain histories. Textural and strain analyses indicate that: (1) the strain history of nearly all samples can be approximated by two coaxial strain accumulations separated by a rigid rotation; (2) development of crenulation cleavages was dependent on the orientation history and the magnitude of the early strain accumulation (i.e. the degree of mica alignment at the completion of D₂); (3) unstable behavior, or buckling, occurred when suggesting that the angle of internal friction (Ψ) was close to 0° (i.e. basal slip in micas may have been more important than frictional grain boundary sliding); and (4) diffusive mass transfer was accompanied by anisotropic plastic deformation, so the orientation history could be used to reconstruct the σ - ε{lunate} history.