MINERALOGICAL CONTRIBUTIONS TO MAGNETIC SUSCEPTIBILITY AND THE EFFECTS OF NEAR SURFACE ALTERATIONS ON GEOPHYSICAL MODELING

Geophysical modeling is a tool to constrain the subsurface properties of faults, and thus to understand possible seismic hazards in the American west. In the Kittitas Valley, Washington, potential field geophysical modeling has revealed concealed or poorly characterized faults, but well-constrained values for magnetic susceptibility and density are needed to improve confidence in these models. Magnetic susceptibility values are especially inclined to vary based on differences in the rock unit present as well as potential near-surface alteration.

We use field measurements to understand unit-level differences in magnetic susceptibility, and thin section analysis to assess and correct for potential near surface alteration. We collected samples of rock units that make up the basement of the Kittitas Valley and created SEM X-ray maps of the most magnetic samples to identify the minerals responsible for magnetism and any alterations present. We use the alteration-corrected values for each rock type and the relative abundances of each rock type based on geologic maps to calculate weighted averages of each unit.

Units investigated in this study include the Ingalls Ophiolite Complex, the Teanaway formation, the Easton Metamorphic Suite, and the Horse Lake Mountain Complex. The Ingalls complex has a unit level average of 15.8 (in units of 1*10^-3 SI) and the primary magnetic phase is magnetite with minor contributions from chromite; there was no evidence of surface alterations. The Teanaway unit has a value of 20.6 and the magnetic phase (ulvöspinel) appears unaltered despite the altered appearance of outcrops. The Easton suite has values of 0.24 to 0.78; minor hematite is only present as an alteration product of sulfides so should have no significant effect on the magnetic signal. The Horse Lake Mountain Complex yielded 33.3; if minor hematite rims on magnetite are an alteration product rather than a magmatic feature, the corrected value is 33.6. Although textural evidence was insufficient to determine alteration timing, the potential effect of this alteration is minor compared to the variation between different units. In comparison, previous models used a magnetic susceptibility value of 0 for basement units and 50 for the Teanaway unit; our new values are recommended to improve the reliability of future modeling.