A study of the middle Miocene Hoh mélanges exposed on the western Olympic Peninsula, Washington, provides criteria for differentiating between shear-zone and diapiric mélanges. The shear-zone mélange is characterized by a consistent foliation orientation throughout the outcrop, intense pervasive scaly foliation, uniform fold vergence directions, exotic clasts, predominantly phacoidal clasts, and consistent clast long-axis lineations. The diapiric mélange is characterized by radial foliation in map view, well-developed scaly foliation at the contacts and poorly developed scaly foliation in the center of the mélange, opposite fold vergence directions across the mélange, and rare exotic clasts. Additionally, clasts vary in shape from phacoidal at the contacts to angular in the center. In the diapiric mélange, large clasts are found only in the center of the mélange, and clast long axes vary from tightly clustered at the contacts to near-random orientations in the center.
The systematic variation in clast size in the diapir may be due to increased shear at the margins and/or clast migration to the center of the diapir caused by a velocity-viscosity contrast. Interpretation of field data indicates that some clasts rotate continuously in the matrix, yet clast long axes have a definite preferred orientation. This contradiction may be resolved by theoretical models indicating that clasts rotate fastest at high angles to the shear plane. Thus, although clasts above a critical aspect ratio asymptotically approach stability parallel to the shear plane, the long axes of rotating clasts spend a proportionally greater amount of time close to the shear plane. Parasitic folds in the matrix indicate that clast long axes align parallel to transport direction. Small-scale folds in the matrix, clast long-axis lineations, and the mean foliation indicate that the Hogsback section of the Hoh mélange was emplaced by a high-angle oblique thrust directed to N10°E.