New geomorphic mapping and cosmogenic 3He geochronology on the west Klamath Lake fault zone in southern Oregon, USA, constrain the age of glacial and post-glacial landforms and the rate of normal-fault slip over the past ca. 150 k.y. We focus on geomorphic features that record progressive offset over multiple time intervals in order to detect potential secular changes in fault slip rate. Thirty-nine new cosmogenic 3He surface exposure dates and a depth profile establish a late-Pleistocene glacial chronology in the southern Cascade Range of Oregon and constrain the timing of the last two major glacial advances in the region at 17.6 ± 2.1 ka (1σ) and 97.6 ± 12.1 ka. Additionally, these data provide evidence that an older glacial advance likely occurred during marine isotope stage (MIS) 6. Fault scarp profiles from high-resolution airborne lidar, in combination with the exposure age data, allow the calculation of fault slip rates over intervals of ca. 104−105 years. Our calculations indicate dip-slip rates of ∼0.3 mm/yr since ca.100 ka, which may represent an increase from <0.1 mm/yr prior to ca. 100 ka, depending on the absolute age of the older glacial moraines. These rates provide insights into the structure of the Klamath Basin and suggest that despite a complex surface expression, individual fault strands likely merge to a single fault at depth.

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