POSTGLACIAL FAULTING NEAR CRATER LAKE, OREGON

Quaternary Cascade arc volcanoes in Oregon are spatially associated with ~arc-parallel normal faults. Situated at the head of the Klamath graben, Mount Mazama volcano overlaps on its west an ~10‑km-wide zone of ~N–S to ~NNW–SSE-trending, en echelon, mainly down-to-east normal faults. An outstanding question is the timing of surface-rupturing earthquakes relative to the ~7700 yr BP Mazama climactic eruption during which Crater Lake caldera collapsed. From geologic mapping and Ar geochronology it was known that several faults were active well into the late Pleistocene with long-term vertical slip rate of ~0.3 mm/y for the northward continuation of the West Klamath Lake fault zone (WKLFZ). The 2010 lidar survey of Crater Lake National Park revealed additional faults, northwest of the WKLFZ, in the heavily forested fault zone west of the caldera. The latter faults displace Last Glacial Maximum features, such as moraine crests, till, glaciated lava surfaces, and valley walls, by as much as 5–10 m. Scarp profiles commonly have rounded crests that suggest two or more surface-rupturing events. Unconsolidated ignimbrite of the climactic eruption appears to bury ends of fault strands, and scarps have not been identified in thick, valley-filling ignimbrite. Although the en echelon scarp patterns might be ascribed to strike-slip components of motion, inconsistency in stepping sense and lack of lateral offset of linear features suggest that displacement is virtually entirely normal. These relations indicate that two or more earthquakes occurred between 16 ka ice retreat and 7.7 ka, and suggest that surface rupturing earthquakes may have occurred immediately before or during the climactic eruption. Postglacial surface rupture lengths (SRL) of individual faults suggest characteristic earthquakes of M_w5.9–6.5. If several faults slipped in one event, SRL of 44 km suggests an earthquake of M_w7.0. Postglacial scarps 5–10 m high, developed over an ~8 ky interval, imply vertical slip rates of 0.6–1.3 mm/y for the zone west of Crater Lake, considerably higher than for the WKLFZ. We consider it plausible that the most recent slip was syneruptive, a result of radical decrease in east–west horizontal compressive stress during rapid eruption of ~50 km³ of magma and concurrent caldera collapse.