| Comments |
| Geomatrix Consultants, Inc. (1996) cited 405 m of vertical offset of a 10.5-14.0 Ma basalt on the fault, and calculated a fault-parallel slip rate that would apply along the entire length of the fault. This vertical offset developed by slip along a ramp, on which the fault dips 60 degrees southward, and on which the fault cut through the thick, stiff Columbia River basalts. As the 405 m of vertical uplift accumulated, it grew an anticline atop the dipping fault ramp. However, Geomatrix Consultants, Inc. (1996) also reported an additional 640 m of horizontal slip measured along the flat part of the fault within the thick, soft sedimentary rocks and sediments that overlie the basalts. This 640 m of slip also took place on the ramp, where it would have been expressed as an additional (640 m)(sin 60 deg.) = 554 m of vertical uplift if the ramp had continued to cut upward through the sediments and sedimentary rocks. Instead, because the fault flattened at the top of the ramp, this 640 m of slip merely moved the front limb of the anticline farther north, widening the anticline instead of making it higher. Nonetheless, this 640 m represents additional slip on the ramp since 10.5-14.0 Ma. However, Geomatrix Consultants, Inc. (1996) was uncertain whether the 640 m of additional slip applied to the entire length of the fault. Accordingly, they assigned a probability of 0.6 to the possibility that the entire fault is best represented by only the 405 m of vertical uplift, and a probability of 0.4 to the alternative that the entire fault underwent both components of slip. We used these probabilities, offsets, and fault dip to replace the fault-parallel slip rate of Geomatrix Consultants, Inc. (1996) with a vertical slip rate of 0.052 mm/yr. |
