Integrating Ground Penetrating Radar, Lidar, and Geologic Mapping to Image Fault Displacements at Mount Mazama (Crater Lake), Oregon

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Poster Session

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Geologic mapping indicates that normal faults on the western flank of Mount Mazama offset ~16 ka Last Glacial Maximum (LGM) till and underlying glaciated lava. Scarps are mantled by ignimbrite of the ~7.7 ka climactic, caldera-forming eruption. The timing of fault movement relative to the climactic eruption remains uncertain. If fault motion significantly predated the eruption, a stratified colluvial wedge should exist between the LGM till or lava and the ignimbrite. If most-recent fault motion closely predated the eruption, the colluvial wedge should be thin or non-existent and perhaps retain evidence of ground surface disruption.

In order to image colluvial wedge internal structure and identify optimal sites for trenching, lidar analysis was combined with geologic mapping to select sites for cross-fault ground penetrating radar (GPR) profiles. Optimal targets were characterized by scarps, typically 2‑3 m high, where geologic mapping suggested that mantling ignimbrite was likely to be less than a few meters thick above the hanging wall.

GPR profiles, 15-35 m long, were collected across 4 fault target sites with antenna frequencies of 50, 100, 200, and 500 MHz. The profiles suggest 2-3 meters of vertical offset on subhorizontal contacts at 2-20 meters depth, with offsets more abrupt than the current topographic slopes. Bright diffractions within the fault zone are recorded to 10 meters depth. At two sites, hanging wall contacts dip into the fault. Shallow (1-2 m depth) energy returns from the hanging wall are more locally disrupted and less continuous than returns from comparable depth on the footwall. These data and other subtle GPR returns may indicate colluvial fill over hanging-wall rocks and possibly reverse drag of the paleo ground surface. Trenching is clearly required for confirmation of lithologies between contacts associated with GPR energy returns. The data demonstrate, however, that the combination of lidar, geologic mapping, and selected GPR profiles can confirm the presence of a fault, reveal subsurface layering, and guide siting of trenches critical to understanding the relative timing of faulting and the climactic eruption.

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Citation / Publisher Attribution

Presented at the AGU Fall Meeting on December 15, 2016 in San Francisco, CA