Field-based constraints on finite strain and rheology of the lithospheric mantle, Twin Sisters, Washington

We present direct finite strain and rheological estimates of naturally deformed mantle materials using field observations in the Twin Sisters ultramafic body of Washington State. Folded and elongated (boudinaged or ductilely thinned) orthopyroxenite dikes within the host dunite provide strain markers that allow us to characterize the finite strain over an ∼15,000 m 2 area. Using dynamic instability analysis on the folded orthopyroxenite dikes, orthopyroxene is calculated to have ∼25 times the effective viscosity of olivine-rich host rocks (based on a power-law exponent of 3.0 for this dislocation creep process). Detailed mapping also indicates that inclusion of up to 15% orthopyroxene in the olivine-rich host rocks does not affect their viscosity, constraining the rheological behavior of two-phase, mantle material.