First Advisor

Ashley Streig

Term of Graduation

January 2023

Date of Publication

1-1-2023

Document Type

Thesis

Language

English

Subjects

Earthquake, Geologic mapping, Glaciology, Oregon, Seismology, Tectonics

Physical Description

1 online resource ( pages)

Abstract

Lidar topographic data for the Strawberry Mountains, Oregon has revealed youthful fault scarps that likely result from Holocene-age earthquakes. We have mapped 27 km of fault scarps striking roughly east-west across the northern foothills of the Strawberry Mountains (Strawberry Fault, SBF) and another scarp lineament striking north-south along the uppermost John Day River 10 km to the south. SBF scarps are aligned with the bedrock John Day Fault, a reactivated Miocene structure that is accommodating distributed extension north of the Basin and Range region. The Strawberry Mountains sit near the interface of the extensional Basin and Range province to the south and the tectonically stable Blue Mountains province to the north. We constrained Quaternary vertical slip rates on the SBF with 3He cosmogenic radionuclide (CRN) dating of glacial features. During the Last Glacial Maximum (LGM), the Strawberry Mountains contained numerous alpine glaciers which deposited prominent lateral and recessional moraines. The SBF offsets these moraines between 1 m and 3 m. Analysis of CRN data shows these glacial features to be LGM-aged with a median age 23.2 ± 0.5 ka. One glacial moraine of a pre-LGM glaciation was dated at 111.7 ± 13.5 ka, and is offset by the SBF ~19 m. When combined with measured offsets, the slip rate of the SBF is 0.18 ± 0.02 mm/yr. over the past 111,000 years.An earthquake history site on the SBF is located in a basin impounded by an uphill-facing scarp above a glacial valley. It records evidence for two surface-rupturing events since the LGM, and vertical separation of at least 3.3 meters. 14C dates from onlapping stratigraphy and surface rupture-correlative colluvium have bracketed the most recent earthquake to CE 200–1754. Youthful single-event fault scarps total 26 km in length and average 1.25 m tall, which empirical scaling relationships show could reflect a Mw 6.7 - 6.8 earthquake. Data from this study fills large gaps in knowledge of seismic hazard in Eastern Oregon and gives insight to the Pleistocene and Holocene behavior of the recently discovered SBF in addition to defining a glacial chronology stretching back ~110,000 years.

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