tikoff_L97.indd

ABSTRACT We present direct fi nite strain and rheological estimates of naturally deformed mantle materials using fi eld observations in the Twin Sisters ultramafi c 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 fi nite 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 olivinerich host rocks does not affect their viscosity, constraining the rheological behavior of two-phase, mantle material