NEW SURFACE EXPOSURE AGES, SLIP RATES, AND PALEOSEISMIC EVIDENCE OF ACTIVE EXTENSIONAL FAULTING NEAR MOUNT HOOD, OREGON

The Oregon Cascade Range is subdivided into the Western Cascades, a dissected lower-relief Eocene to Miocene arc, and the Pliocene to modern High Cascade arc to the east. The Oregon Coast Range block, which includes the Coast Range, Willamette Valley, and the Western Cascades, has been rotating clockwise ~1°/My since ~40 Ma. Tectonic models show that rotation of this forearc block causes extension and concomitant magmatism on its trailing edge within the High Cascade arc. We analyze lidar topography to document numerous N-S trending extensional faults in the active High Cascades arc, cutting geologic units as young as latest Quaternary. Near Mt. Hood, we map a 102 km-long zone of normal faulting comprising multiple active fault segments. On Blue Ridge, north of Mt. Hood, youthful fault scarps up to 2.7 m-high cut Holocene alluvial and colluvial sediments as well as glacial moraines from the Last Glacial Maximum (LGM), of which we report a new ³He mean surface exposure age of 21.2 ± 3.5 ka. A 2.7 m scarp in this till yields a slip rate of 0.17 ± 0.04 mm/yr. In 2021, we excavated a paleoseismic trench on the Twin Lakes faultsegment, south of the summit, across an uphill-facing fault scarp with ~2.3 m of vertical separation that impounds a small drainage ~400 m east of the crest of the Cascade Range. The trench revealed that the scarp formed during two post-LGM surface-rupturing earthquakes. Faulted glacial till is overlain by a colluvial wedge interfingered with impounded marsh sediments. An ash bed we correlate to the ~1500 yrs BP Timberline eruption overlies this wedge and post-dates the penultimate earthquake. A younger colluvial wedge overlies post-Timberline sediments and is overlain by an ash bed we correlate to the ~250 years BP Old Maid eruption. These observations bracket the most recent earthquake to between ~1500 and ~250 years BP and suggest that surface ruptures are not contemporaneous with eruptions. Another trench site north of the summit confirms Holocene activity throughout the Mount Hood fault zone, likely independently in sequences of M 6.4 - 6.7-sized events. These active faults likely accommodate ongoing intra-arc extension related to continued regional clockwise rotation, consistent with tectonic models.