LOCATION-SPECIFIC TIMING AND STYLE OF PLIOCENE AND QUATERNARY BASIN-RANGE FAULTING, FRENCHGLEN AREA (WEST OF STEENS MOUNTAIN), SOUTHEAST OREGON

Normal faults with dip separation as great as 500 m have disrupted middle and upper Miocene volcanic and volcaniclastic strata on the west side of the Blitzen Valley, north of Frenchglen, eastern Oregon. The resulting fault blocks are capped by the Rattlesnake ignimbrite, about 7.1 Ma in age. Nothing in the distribution of older units in the fault blocks, including the Steens Basalt at the base of the exposure, indicates faulting prior to Rattlesnake time (between about 16 and 7 Ma). Thus the offset is latest Miocene or younger, probably chiefly Pliocene and Pleistocene. Deformation associated with these fault offsets continued into late Pleistocene time or later, as indicated by gently folded tephra-fall beds once exposed in a rustic dumpsite excavation on an alluvial fan 3 km southwest of Frenchglen (the site has subsequently been destroyed). One fault with trifling offset was found in upper Pleistocene pluvial lake beds 15 km to the southwest.

Mapped outlines of the Rattlesnake and underlying Devine Canyon ignimbrites are similar and show that these pyroclastic flows were basin-covering except in their eastern reach, where they narrowed and filled paleovalleys up the ancestral slope of the Steens Mountain shield volcano. This transition is near the longitude of Frenchglen. The easterly, narrow traces of the valley-filling deposits serve as linear elements, creating piercing points where offset by predominantly north-striking faults, thus allowing the long-term slip, and not just separation, to be specified. That motion is dip slip, with virtually no lateral component.

Location-specific timing and style of faults, when gathered from numerous sites, will ultimately lead to maps that specify the age of faults and perhaps demonstrate a temporal progression of tectonism. This knowledge leads to more precise estimates of strain and provides field evidence for paleostress fields.