USING PETROLOGIC TECHNIQUES TO CONSTRAIN GEOPHYSICAL PROPERTIES: EXAMPLES FROM THE CENTRAL CASCADES, WASHINGTON

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 higher degree of understanding of seismic hazards. This modeling requires values of both the magnetic susceptibility and density relevant to rocks at depth, and a combination of field and laboratory measurements of basement units exposed at the surface can determine these properties. The basement rock of the central Cascades is made up of a complex series of metamorphic and igneous bodies, so we collected a representative suite of samples of the most volumetrically abundant members of each unit. We then performed laboratory analyses to determine the density value, and field measurements of the magnetic susceptibility, of each unit at surface conditions. Some units had highly variable magnetic susceptibility, such as the members of the Ingalls Ophiolite Complex, which vary from approximately 2.63K (Henrys/meter) for the freshest peridotite to 317 K (serpentinite). Flows of the Teanaway Basalt have a magnetic susceptibility of 38.4 K and density of 2.77 g/cm³, and dikes of the Teanaway have a magnetic susceptibility of 7.66 K and density that is 2.90 g/cm³. Other units, such as the members of the Easton Metamorphic Suite, had relatively low magnetic values of approximately 0.20 K (Darrington Phyllite) to 0.75 K (Shuksan Blusechist). The freshest Ingalls peridotite sample had a density of 3.12 g/cm³, while the most magnetic serpentinite samples had densities between 2.93 g/cm³ and 3.07 g/cm³. Densities from the Easton suite were 3.17 g/cm³ for the Shuksan Blueschist, and 2.60 g/cm³ for the Darrington Phyllite. The next step in this project is to use laboratory petrologic techniques to determine mineral abundances and to calculate corrections for the effect of near-surface alteration. The corrected density and magnetic values should best represent units in the sub-surface, improving the accuracy of geophysical models of buried structures relevant to regional seismic hazards.