HOLOCENE ACTIVITY ON THE KING MOUNTAIN FAULT, NORTHEASTERN OREGON

Characteristics of the King Mountain fault including recency of activity, type of displacement, potential rupture length, and activity rate are largely unknown or poorly constrained. The fault is recognized as one of several northwest-striking faults in the Unity basin. Past studies have largely been reconnaissance-level in nature and have variously described the fault as a normal fault dipping steeply (60⁰-70⁰) to the southwest to a right-lateral strike-slip fault that may accommodate regional dextral shear. The fault is exposed primarily in Tertiary volcanic and sedimentary rocks and evidence for late Quaternary activity has been inferred based on the presence and character of scarps, grabens, and prominent lineaments. Field mapping utilizing aerial photography and fault trench excavations were undertaken at a site located near Unity Dam about 65 km east of John Day, Oregon to better quantify these fault characteristics.

Mapping of scarps and lineaments associated with the fault indicates that the fault extends from about 1 km north of the North Fork of the Burnt River about 34 km to the southeast. This is about three times longer than previously reported. The map expression of the fault also suggests that the fault is steeper than previously portrayed being near-vertical and that displacement on the fault may be primarily lateral. Local bends and steps along the fault with associated extension and compression helps to more easily explain the overall expression of the fault and associated topographic features.

Two trenches were excavated across a low topographic scarp at the southeastern end of King Mountain. The stratigraphy exposed in both trenches exhibited a complex pattern of faulting with bedrock faulted against colluvium at the uphill end of each trench and a number of other faults within the colluvium along the length of the trench. The trenches were located about 150 meters apart with the southernmost trench crossing a low uphill facing scarp forming a shutter ridge across a small drainage that ponded sediment along the fault. Faults within the sequence of ponded sediment in this trench juxtaposed the Mazama ash (7.6 ka) against colluvium. Current interpretation of the faulting history suggests two events within this time frame. Additional numerical ages are pending to better constrain the timing of events.