CALCULATION AND IMPLICATIONS OF POISSON'S RATIO FOR UPPER CRUSTAL STRUCTURE AND COMPOSITION IN THE SOUTHWEST CASCADES, WASHINGTON
Here we present results from modeling P- and S-wave arrival times to obtain velocity structure for the upper 10 km of the crust. These models were then combined to calculate Poissons ratio, a more reliable indicator of rock type than either P- or S-wave velocity alone. The results show a strong correlation between Poissons ratio and known structure and stratigraphy in the region. Structures such as Morton and Skate Mountain Anticlines are clearly seen in the Poissons ratio model. Surficial deposits have Poissons ratios of 0.23 to 0.24 and are interpreted to represent sedimentary and volcanic units of the Puget Group and younger deposits. Beneath this lies a zone exhibiting Poissons ratio of 0.29 to 0.3 that we interpret to be an extension of the Summit Creek basalt. This zone appears to lie directly on top of the Southern Washington Cascades Conductor (SWCC), a major low-resistivity feature in the region that has previously been interpreted to be largely composed of lower Eocene marine sedimentary rocks. Our model shows the SWCC to have a Poissons ratio of 0.26, a value too high for young marine sedimentary rocks. We suggest that alternative interpretations for the SWCC including the presence of graphitic sedimentary rocks or the occurrence of hydrothermal fluids at depth are a better choice for the origin of this feature.