Lidar Mapping Documents Post-glacial Faulting West of the High Cascades Axis at Crater Lake National Park, Oregon

The Cascades magmatic arc lies mainly within the High Cascades graben system in the state of Oregon. Normal faults of the Klamath graben trend north into Mount Mazama, the volcano whose catastrophic eruption ~7700 cal y BP resulted in collapse of 8x10 km Crater Lake caldera. Geologic mapping of Mount Mazama (Bacon, USGS SIM 2832, 2008) delineated faults of the West Klamath Lake fault zone (WKLFZ) and their northern extensions through Crater Lake National Park west of the caldera. Outcrop patterns implied presence of normal faults farther west but dense conifer forest made discovery of subtle scarps impractical. Closer to the Cascades axis, successively decreasing offsets of mapped Mazama lava flows with decreasing age yielded a long-term vertical slip rate of ~0.3 mm/y on the principal fault segments of the WKLFZ near Crater Lake, where the youngest offset lavas are 35 ka in age. Other workers have found offset lateral moraine crests where Last Glacial Maximum (LGM) valley glaciers crossed the WKLFZ south of Crater Lake.

A lidar survey of Crater Lake National Park in 2010 supported by the Oregon Lidar Consortium (Robinson, USGS Data Series 716, 2012) revealed meter-scale, dominantly N–S trending fault scarps with down-to-the-east displacement west of most previously mapped faults at the latitude of Crater Lake, increasing the known width of the fault zone there to as much as 11 km. Fault segments as long as 7–16 km form a semi-continuous system for virtually the entire 32 km N–S extent of lidar coverage. Along the western part of the fault zone, scarp height is as great as ~20 m. Scarp length and height imply that several M>6–7 earthquakes have occurred in late Pleistocene–Holocene time. Field observations show that the ignimbrite of the Mazama climactic eruption banks against or covers scarps. One fault vertically displaces a lateral moraine ~3 m. The moraine contains clasts of ~50 ka andesite and therefore likely dates from the LGM so that the most recent displacement occurred between ~16 ka ice retreat and the ~7700 cal y BP Mazama eruption. The question arises, exactly when was the last major earthquake on the newly documented fault? Did it occur hundreds or thousands of years earlier, or was that earthquake possibly the trigger for the Mazama climactic eruption?