THE MITCHELL FAULT
The Mitchell Fault is one of the most important geologic structures in central Oregon. It was first reported in an Oregon State College MS thesis by C. B. Howard (1955) and was represented as a strike-slip fault, 17 miles in length, on published geologic maps by W. D. Wilkinson, K. F. Oles, and H. E. Enlows (1968, 1971). The strike-slip nature of this fault has not been indicated on more recent geologic maps of Oregon (Walker and McLeod, 1991), probably because, where the fault intersects the Mitchell Anticline, the limbs of that fold (as represented by the well-mapped contact between Cretaceous and Tertiary rocks) appear to be symmetrically distributed on both sides of the fault. This presents a map pattern that is usually associated with dip-slip rather than strike-slip faulting.
However, near-vertical features such as steeply inclined strata, surface traces of fold axes, sub-volcanic intrusions, and older cross-faults that have been cut by the Mitchell Fault reveal the extent of dextral strike-slip displacement. This horizontal movement was 2.84 miles (4.57 km). Many distinctive units such as welded tuffs, porphyritic lavas, and a clustered group of Clarno intrusions can be matched across the fault only when this displacement is taken into account.
The fault is not an obvious feature of landscape except where it crosses Gable Creek canyon and has brought erosion-resistant conglomerates of the Gable Creek Formation against soft mudstones of the Hudspeth Formation.
Elsewhere, the fault is revealed only by careful geologic fieldwork tracing boundaries of distinctive formational units and lithologies that have been terminated against the fault. It should be noted that a generalized Bouguer gravity map of central Oregon (Fritts and Fisk, 1985) displays isolated regions of high and low gravity that are dextrally displaced along a line that coincides with the Mitchell Fault.
A vertical component of motion also occurred on the Mitchell fault and is best revealed by near-horizontal features such as bedding planes. Unfortunately, the Cretaceous and Tertiary rocks cut by the fault do not contain marker horizons that are sufficiently widespread to be represented on both sides of the fault. However, the K-T erosional unconformity has been mapped over a broad area and represents a depositional surface of very low relief, covered by a red regolith.
Reconstruction of this surface to a common level on both sides of the fault reveals that sedimentary beds east of Mitchell were dragged to a nearly vertical orientation on the south side of the fault as they were uplifted 2100 feet relative to the north side. Near Highway 26, 6.5 miles west of Mitchell, the south side of the fault was uplifted only 100 feet with no recognizable drag of rock adjacent to the fault. Consequently, while the south side of the Mitchell Fault moved nearly 4 miles westward, it was lifted trap-door fashion to a higher level on the east than on the west. This is why it now presents a symmetrical map pattern. Because the limbs of the Mitchell Anticline were subjected to differential uplift only on the south side of the fault and because minor folds of rocks south of the fault do not have corresponding folds north of the fault, it appears that the north side remained as a fixed and stable crustal block relative to a more mobile south side.
How old is the Mitchell Fault? The youngest rocks of known age that have been cut by the fault are 41 Ma Clarno lavas. The oldest rocks that cut the fault are part of the 28 Ma Lawson Mountain dome. Both ages are based upon Ar-Ar dating of fresh, bulk samples (Appel, 2001). My best guess is that the fault was produced between these events at 34.1 Ma (Ar-Ar) when the Mafic Dike System of that age was emplaced (Taylor, 1981).
This system includes 17.8 aggregate-miles of dike segments that have not been displaced by the Mitchell Fault but display an en-echelon pattern on both sides of the fault. This dike pattern is similar to that of gash fractures associated with right-lateral strike-slip faults. The Mafic Dike System is characterized by a high Fe-Ti composition and must represent an unusually dense mafic magma that might never have reached shallow levels without imposition of appropriate stresses. If the Mitchell Fault interacted with this magma, compressive stresses might have forced the mafic dikes toward the surface, not along the fault plane, but in the gash fractures where tensional stresses prevailed.
East of Lawson Mountain for about 5 miles, there are two parallel Mitchell Faults, separated by 2000 feet.
Between the faults there is a displaced “sliver” of earth’s crust that displays a pattern of east and west dips showing the axis of the Mitchell Anticline to have been displaced 0.76 miles to the west. Movement on the Mitchell Fault could have been initiated on the south fault, then transferred to the north during the last 0.76 miles of displacement. Or, more likely, 0.76 miles of movement could have been initiated on the north fault before the last 2.50 miles of displacement was transferred to the south. This probably would have reduced compressive stresses along the fault east of the “sliver” and might have been responsible for the Tertiary dip-slip faults that dropped several blocks of crust on the north side of the fault near Mitchell (see the illustrated blue-line faults).
The Mitchell Fault has not yet been traced to its extremities. On the highlands east of Mitchell, John Day beds, Picture Gorge basalt, and Quaternary deposits obscure the surface expression of the fault at least to the John Day River. To the west, I have traced the Mitchell Fault to Bear Creek near the boundary of the Lawson Mountain-Stephenson Mountain quadrangles. West of Lawson Mountain there is an east-west alignment of many dikes, plugs, and volcanic domes that are adjacent to (or in) the fault zone. None of these units has been dated to determine if they could have been intruded into a pre-existing fault zone. Farther west, there are many rhyolite dikes (one is 4 miles long) that trend northeast from Stephenson Mountain across the westward projection of the Mitchell Fault. These dikes have not been displaced by the fault but the older rocks between them might have been displaced. Additional field studies and age dating of the intrusive rocks will be necessary to clarify these relationships and to improve the known western extent of the Mitchell Fault.