POSTGLACIAL FAULTING NEAR CRATER LAKE, OREGON, AND ITS POSSIBLE ASSOCIATION WITH THE MAZAMA CALDERA-FORMING ERUPTION

Volcanoes in subduction-related magmatic arcs occur in a variety of crustal tectonic regimes, including where active faults indicate arc-normal extension. Mount Mazama, the Cascades arc volcano that hosts Crater Lake caldera, overlaps with an ~10‑km-wide zone of ~north–south trending normal faults along its western margin. A lidar survey of Crater Lake National Park reveals several previously unrecognized, recently active faults within this zone. Post-Last Glacial Maximum (LGM; <16 ka) vertical separations measured from profiles across the primarily east-facing scarps in moraine and glaciated surfaces range from ~2 m to as much as 12 m. Scarp heights and beveled profiles commonly suggest two or more postglacial surface-rupturing events. Ignimbrite of the ca. 7.6 ka climactic eruption of Mount Mazama, during which Crater Lake caldera formed, appears to bury fault strands where they project into thick, valley-filling ignimbrite. Exposures in two ~1-m-deep trenches suggest that ignimbrite banks against and mantles scraps where the deposit is thin. Lack of lateral offset of linear geomorphic features suggests principally normal displacement, although predominant left stepping of scarp strands implies a component of dextral slip.

Full rupture of the longest fault (18 km) could have produced earthquakes of M_w≤6.5 from empirical scaling relationships. If several faults slipped in one event, prehistoric ruptures could have been a combined 44 km long and suggest an earthquake of M_w7.0. Scarps as high as 12 m in LGM moraine imply maximum vertical slip rates of 1.5 mm/yr for the zone west of Crater Lake, considerably higher than the ~0.3 mm/yr long-term rate for the nearby West Klamath Lake fault zone south of the caldera. An unanswered question is the timing of surface-rupturing earthquakes relative to the Mazama climactic, caldera-forming eruption. Apparent overlap of scarps by ignimbrite is permissive of the eruption having been preceded by a large earthquake. Alternatively, large surface-rupturing earthquakes may have occurred during the eruption, a result of decrease in east–west compressive stress during ejection of ~50 km³ of magma and concurrent caldera collapse.