TRACKING THE ACCRETION OF SILETZIA WITH MAGNETIC ANOMALIES: HOW ANCIENT SPREADING CENTERS AND THRUST STRUCTURES MAY IMPACT MODERN TECTONIC DEFORMATION

Work over the last decade consistently shows that Siletzia, the basaltic basement of western Washington, originated as an oceanic plateau erupted at a spreading ridge and consequently accreted to North America in the Eocene. Combining new, higher-resolution aeromagnetic data from western Washington with older aeromagnetic surveys provides an updated, geographically comprehensive tool for deciphering the history of Siletzia from the Eocene to the present. In particular, it provides high-fidelity constraint on the boundaries of large-scale, deep-seated crustal blocks that produce long-wavelength anomalies not fully captured by individual aeromagnetic surveys. Interpreting these long-wavelength magnetic anomalies is a by-product of our work interpreting shorter-wavelength anomalies to better understand the near-surface geology. We use geologic, gravity, and aeromagnetic maps in conjunction with modeling the geophysics in 2-D cross sections to make our interpretations. The longer wavelength aeromagnetic and gravity anomalies reveal the location of the boundary between Siletzia and greater North America. Modeling suggests Siletzia subducted under Vancouver and Whidbey Islands, whereas it obducted at the latitude of Tacoma. Elongate pairs of high and low magnetic anomalies over the Seattle uplift suggest the duplication of Siletzia basalts in an accretionary fold-thrust belt during the Eocene. A high-amplitude, broad magnetic high within and south of the southern Puget Lowland is due to a strongly magnetic and moderately high-density block within the upper to mid crust. This block’s properties are most consistent with relict mantle of peridotite or pyroxenite preserved in an area with a shallow Eocene-age Moho. This region may have been a large-scale volcanic center during the formation of Siletzia. Active faults or seismic zones tend to traverse the edges of these major components of Siletzia rather than cut through them, suggesting these blocks focus stress and deformation within the crust. Crustal microseismicity tends to cluster within the basalts of Siletzia under the Puget Lowland but is almost completely absent within the large magnetic block to the south. These observations suggest that the heterogeneity of Siletzia imparted by past events continues to influence its deformation today.