Abstract

The pattern of faulting in southeastern Oregon is interpreted in terms of four major zones of right-lateral strike-slip faulting that separate blocks broken by normal faulting. The total amount of east-west extension is considered to decrease in the block north of each strike-slip fault zone. The right-lateral offset results from the decrease in extension. Extension essentially dies out across the northern two fault zones, which are thus considered the northern limit of the Basin and Range province. The greatest offset is apparently recorded on the next zone to the south by the displacement of the eastern edges of the Sierra Nevada and Idaho batholiths. The two southern zones offset the Pleistocene to Holocene trend of the High Cascades by 10 to 20 km in a right-lateral sense. The Brothers fault zone, one of the northern zones, is regarded as of special interest because both ends of the fault are interpreted to be exposed at the surface.

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