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Post-Mazama (7 KA) Faulting Beneath Upper Klamath Lake, Oregon

U.S. Geological Survey
384 Woods Hole Rd.
Woods Hole, MA 02543
scolman{at}usgs.gov
(S.M.C.)

U.S. Geological Survey
MS 980, Box 25046
Denver Federal Center
Denver, CO 80225
(J.G.R., R.L.R.)

U.S. Geological Survey
MS 975, 345 Middlefield Rd.
Menlo Park, CA 94025
(A.M.S.W.)

High-resolution seismic-reflection profiles (3.5 kHz) show that a distinctive, widespread reflection occurs in the sediments beneath Upper Klamath Lake, Oregon. Coring reveals that this reflection is formed by Mazama tephra (MT), about 7 ka in age. The MT horizon is faulted in many places and locally displaced by as much as 3.1 m. Differential displacement of multiple horizons indicates recurrent fault movement, perhaps three episodes since deposition of the Mazama. The pattern of faulting indicates northeast–southwest extension beneath the lake basin.