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Date Permissions Signed
4-10-2020
Date of Award
Spring 2020
Document Type
Masters Thesis
Department or Program Affiliation
WWU Department of Geology
Degree Name
Master of Science (MS)
Department
Geology
First Advisor
Amos, Colin B.
Second Advisor
Clark, Douglas H., 1961-
Third Advisor
Meigs, Andrew J.
Abstract
New cosmogenic 3He chronologies and surficial geologic mapping constrain the age of glacial deposits and slip rates of predominantly normal faults in the White Branch and Dilman Meadows fault zones in central Oregon, USA, over the last ca. ~80 kyr. Our mapping of glacial landforms and deposits distinguishes three primary episodes of glacial deposition in the White Branch fault zone. Twenty-two new cosmogenic 3He surface exposure dates indicate that the youngest glacial unit represents a last glacial maximum deposit (ca. 19.4 +10.1/-6.2 kyr). Mapping of outwash terrace surfaces and deposits in the Dilman Meadows fault zone, constrained by two new cosmogenic 3He depth profiles, identifies an older outwash deposit (75.0 +11.3/-9.2 kyr) associated with MIS 5b (~75 ka), as well as a younger last glacial maximum outwash surface (20.8 +0.04/-0.03 kyr). For the first time, glacial chronologies are linked across the Cascades, where west of the crest, where erosion and precipitation rates are high and vegetation cover is dense, and east of the crest, where precipitation and erosion rates are significantly lower and tree cover is sparse. We use offset measurements from fault scarps that show primarily dip-slip motion in combination with the depth profile ages to determine a summed slip rate across the Dilman Meadows fault zone of 0.1 – 0.4 mm/yr since ~75 ka. Surface faulting evident in the White Branch fault zone represents deformation strictly since the last glacial maximum (< 20.3 +6.2/-5.3 ka). Combining offset values from scarp profiles that show purely dip-slip displacement with the cosmogenic 3He surface ages yields a summed dip slip rate across the White Branch fault zone of 0.6 ± 0.5 mm/yr since the last glacial maximum. The White Branch and the Dilman Meadows fault zone both accommodate mostly extension in the Cascade arc and could be mechanisms of Siletzia rotation or volcanic upwelling.
Type
Text
Keywords
Tectonics, Cosmogenic exposure dating, Cascades, Oregon
Publisher
Western Washington University
OCLC Number
1155521762
Subject – LCSH
Surface fault ruptures--Oregon, Central; Faults (Geology)--Oregon, Central; Geomorphological mapping--Oregon, Central; Geological time; Geology, Structural--Oregon, Central
Geographic Coverage
Oregon
Format
application/pdf
Genre/Form
masters theses
Language
English
Rights
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.
Rights Statement
http://rightsstatements.org/vocab/InC-EDU/1.0/
Recommended Citation
Alexander, Katherine, "Slip rates and kinematics of active crustal faults in the central Oregon Cascades" (2020). WWU Graduate School Collection. 947.
https://cedar.wwu.edu/wwuet/947