The late Tertiary basalt flows covering south-central Oregon have been intricately faulted to form a distinct fracture pattern and conspicuous horsts, grabens, and tilted fault blocks. The faults form a rhombic pattern and are in two principal sets—one striking approximately N. 35° W. and the other N. 20° E. Evidence indicates that the sets were contemporaneous in origin and that movements occurred concurrently along both sets. The latest movement was dip-slip; strike-slip movement had occurred previously. The traces of intersecting faults across escarpment faces, the presence of pivotal faults, and the results of a seismic-refraction study suggest that near-vertical faults may be dominant.
The contemporaneous origin of the two fault sets, the intersection angle of approximately 55°, and the nearly vertical dips indicate that the faults originally developed as conjugate strike-slip shears in a stress system characterized by a north-south maximum principal stress and an east-west minimum principal stress. Subsequent to the development of the rhombic fracture system, block faulting occurred in which the dominant movement on the shears was dip-slip. This movement does not reflect the stress system that produced the faults, but represents a redistribution of surface forces acting on individual fault blocks.