NEW LIDAR AND PALEOSEISMOLOGIC DISCOVERIES OF HOLOCENE-ACTIVE FAULTS IN THE HIGH CASCADES GRABEN, OREGON

Abstract

The High Cascades Graben accommodates extension along the Cascade volcanic arc axis at the crest of the Cascade Range. Examination of high resolution lidar data in the Mt. Hood, Oregon region led to the recent discovery of Holocene activity on the ~55 km-long Mt. Hood fault zone (MHFZ) which forms the western margin of the northernmost portion of the High Cascades Graben. Trench studies on the Gate Creek and Blue Ridge fault segments north of Mt. Hood have identified one paleoearthquake between ~13,540 and 9,835 years BP and another between AD 1350 and 1500. Mt. Hood sits in the High Cascades Graben, locally defined by the Multorpor Mtn. and Twin Lakes fault segments. Paleoseismic evidence from buried soils offset by the Twin Lakes fault provide evidence for the most recent earthquake ~3,400 years BP. A new paleoseismic trench (late July 2021) on the Twin Lakes fault will evaluate the rupture history that formed 1-2 m of vertical separation on faults at the crest of the Cascade Range. South of the MHFZ, newly acquired lidar (summer 2021) covering the headwaters of the Clackamas River has revealed another Holocene-active fault zone in the High Cascades Graben, here named the High Rock fault zone (HRFZ). The HRFZ is at least 15 km-long, between the Salmon and Clackamas Rivers, and consists of about six NNE-striking strands. The two westernmost are east-side-down, while the three easternmost are west-side-down, defining a ~10 km-wide full graben. Individual scarps are typically 1-2 m high and are well preserved in steep terrain consisting of Quaternary glacial and volcanic deposits. The largest measured offsets are ~5 m of a fluvial terrace and ~7 m of a terminal moraine crest. Limited field reconnaissance has confirmed the presence of scarps at three locations. The northern end of the HRFZ almost reaches the southern end of the MHFZ. However, the grabens formed by these fault zones appear to be offset by a 6 km-wide right step. No E-W fault is evident in lidar topography or existing geologic mapping. South of the Clackamas River, the NNE-striking HRFZ appears to merge with the NW-striking Clackamas River fault zone (CRFZ), a Quaternary-active dextral normal-oblique fault zone that is oriented oblique to the High Cascades Graben. However, lidar data south of the Clackamas River is lacking and the relation between the HRFZ and CRFZ is unclear.