Anisotropy of magnetic susceptibility on the nine hill tuff of northern Nevada and the Sierra Nevada of California and Nevada

Hager, Jaclyn Marie (2023) Anisotropy of magnetic susceptibility on the nine hill tuff of northern Nevada and the Sierra Nevada of California and Nevada. Masters thesis, Northern Arizona University.

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Abstract

Three depositional facies categories for pyroclastic density currents (PDCs) were developed using quantitative characteristics from anisotropy of magnetic susceptibility (AMS) analysis of the Nine Hill Tuff of Nevada and California. The PDC that emplaced the 25.38 Ma Nine Hill Tuff followed east-west-trending paleovalleys across central Nevada and into the Sierra Nevada, a distance of nearly 300 km from the source. AMS was studied in channeled deposits to determine facies variation at different locations within the paleovalleys. These data can be utilized to understand PDC flow relative to topography, which can lead to more accurate hazard maps and hazard preparedness for present-day explosive eruptions. AMS was determined for 30 sites (8 to 23 specimens per site) across central Nevada and the Sierra Nevada, and characteristic remanent magnetization was completed on 16 of these sites. Using the AMS data and the Ort et al. (2015a) model as a basis, three facies categories were created quantitatively to better understand how PDC flow interacted with valleys. The Non-Lineated / Non-Foliated facies (n = 8) has K3 clustering values > 8°; the ratio of the declination confidence error ellipse mean to the inclination confidence error ellipse mean (dec/inc) < 4; low to medium anisotropy (average P’ between 1.007 to 1.015 with one site average 1.030); oblate, prolate, or neutral ellipsoid shape; and predominantly scattered anisotropy factors (L, F, P’, T). K1 and K2 error ellipses on the lower-hemisphere equal-area projections could be overlapped. Due to the high standard deviation of anisotropy factor values and the scattering of data points on both the lower-hemisphere equal-area projections and anisotropy factor diagrams, the Non-Lineated / Non-Foliated facies shows disorganization. The Moderately Lineated / Moderately Foliated facies (n = 8) has K3 clustering values 3°-8°; dec/inc < 4; low, medium, or strong anisotropy (average P’ values between 1.005 and 1.028); clumping or scattering of anisotropy factors; oblate, prolate, or neutral ellipsoid shape; and no overlap in error ellipses. The Moderately Lineated / Moderately Foliated facies has less scattering of data points on lower-hemisphere equal-area projections and anisotropy factor diagrams compared with the Non-Lineated / Non-Foliated facies and therefore the data show greater organization. The Strongly Foliated facies (n = 8) has K3 clustering value < 3; dec/inc > 4; low, medium, or strong anisotropy (average P’ value between 1.011 and 1.044); clumping of anisotropy factors; a strongly oblate ellipsoid shape; and no overlap in the error ellipses. Clustering of data points and small anisotropy factor standard deviations show organization. The remaining sites (n = 6) have characteristics of two or more of the above facies and are classified as transitional. Using characteristic remanent magnetization data, vertical axis rotation was calculated on 11 sites; post-emplacement rotation ranged between 1.6° and 165.1°, with greatest rotation in the Basin and Range region of Nevada. Rotation correction was applied to sites within the Basin and Range, but the Sierra Nevada was determined to be in situ and no rotation correction was applied. The manner in which the PDC interacted with the paleovalley varies based upon distance from vent, depth above the base of the deposit, and topographic obstacles, which are reflected in the facies categories. The Non-Lineated / Non-Foliated facies is found in one deposit approximately 80 km from the caldera and in seven distal deposits, commonly close to paleovalley walls or floor. The disorganization in the Non-Lineated / Non-Foliated facies may show turbulent flow regime caused by high energy in deposits closer to the caldera and topographic obstacles in distal deposits. The Moderately Lineated / Moderately Foliated facies is commonly found in medial-to-distal deposits (over approximately 90 km from the vent prior to extension) that lack direct contact with the underlying stratigraphy. This facies has better organization compared with the Non-Lineated / Non-Foliated facies and likely indicates laminar flow due to lower sections of the deposit smoothing over topographic obstacles. The Strongly Foliated facies is found primarily in the Sierra Nevada, and may reflect channeled distal deposits where the flow was still laminar but energy waned. The transitional sites show changes in flow processes between facies categories. The data presented in this study can be used to develop a more thorough awareness of how PDC interaction with a valley can influence the runout distance of modern-day PDCs.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	Anisotropy of magnetic susceptibility; Ignimbrite; Pyroclastic density currents; Geological hazards; Nine Hill Tuff Nevada/California
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	15 Jul 2026 20:39
Last Modified:	15 Jul 2026 20:39
URI:	https://openknowledge.nau.edu/id/eprint/6295

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