Brittle to ductile transition due to large strains along the White Rock thrust, Wind River mountains, Wyoming

Abstract

The deformed foreland of the Rocky Mountain Cordilleras is characterized by major reverse faults that bring Precambrian basement rocks to the surface. The White Rock thrust in the Wind River mountains of Wyoming is one such fault that developed at fairly shallow levels in the crust. Detailed study of grain-size variations and textures within the fault zone provide a framework for discussion of the mechanics of movement along the fault. Textural criteria are suggested for distinguishing between dominantly brittle and dominantly ductile deformation. A transition from brittle fracturing to ductile deformation is seen with increasing strain and resultant decrease in grain-size along the fault zone. Diffusional creep in the final stages of deformation leads to strain softening and may allow large strains to take place along narrow zones at low deviatoric stresses. This may be one way of obtaining large displacements along major faults at shallow levels in the crust.