Document Type

Thesis

Date of Degree Completion

Spring 2023

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Christopher Mattinson

Second Committee Member

Hannah Shamloo

Third Committee Member

Peter Davis

Abstract

High precision geophysical modeling can constrain the geometry of structures in the upper to middle crust beyond what is possible with surface data alone, which allows for a better understanding of seismic hazards. This modeling requires values of both the magnetic susceptibility and density relevant to rocks at depth; this study presents a combination of field and laboratory measurements of basement units to determine these properties. The basement rock of the Kittitas Valley is made up of a complex series of metamorphic and igneous bodies, so it was important to gather a representative suite of samples of the most volumetrically abundant members of each unit where exposed at the surface. Samples were grouped by geologic unit and rock type and then compiled into an average value of both density (in g/cm3) and magnetic susceptibility (1*10-3 SI) for all the units of interest. The sample suite displayed very little near-surface alteration, and a correction based on petrographic observations for one unit only changed the magnetic susceptibility value by ~1%. The unit-level average density and magnetic susceptibility values for the units of interest for this study are: Ingalls Ophiolite Complex, 2.96 g/cm3, 15.84*10-3 SI; Teanaway Formation, 2.64 g/cm3,18.51*10-3 SI; Kachess Lake inlier, 2.97 g/cm3, 0.37*10-3 SI Hicks Butte inlier, 2.86 g/cm3, 13.29*10-3 SI; and Horse Lake Mountain Complex, 2.56 g/cm3, 33.28*10-3 SI. The magnetic susceptibility value used in current models for basement units (Ingalls Ophiolite Complex, Kachess Lake Inlier, and Hicks Butte Inlier) is 0, but the results of this study demonstrate that there are likely several highly magnetic units in the sub-surface (Ingalls Ophiolite Complex, Hicks Butte Inlier, Horse Lake Mountain Complex). The density values of this study are also slightly higher than the values used in current models, since the currently used basement value is 2.72 g/cm3 and the lowest basement density value in this study is 2.80 g/cm3. It is also necessary to reconsider the Teanaway Formation, since the magnetic susceptibility value used in published models is 50-80*10-3 SI, which is more than double the value in this study.The calculated density of the Teanaway Formation is slightly lower (2.64 g/cm3) than the value used in recent models (2.70 g/cm3). The implications of these new values for the application and interpretation of potential fields models are: (1) The higher basement density values of this study suggest less topography on the basement-sediment interface (and therefore less inferred fault offset) compared to published models, and (2) magnetic susceptibility results suggest that significant magnetic remanence and/or presence of magnetic basement units has a more important role than weathering of magnetic minerals and therefore should be accounted for in future studies. The compiled values here are important for more precise geophysical modeling of the sub-surface of the Kittitas Valley and will assist scientists with understanding potential fault properties and the associated seismic hazards.

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