A Geomagnetic Induction Study of the California Coast Range Near the Mendocino Triple Junction

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The PI has discovered a major electric conductivity anomaly related to a buried half spreading center at the south edge of the subducted Gorda Plate. The geometry of this anomaly shows that the motion of the Gorda Plate has been controlled by the orientation of the Blanco Fracture Zone and not the Mendocino Fracture Zone for at least several My. More mapping of the anomaly will provide further constraints on the plate motions. Understanding the detailed structure of the anomaly will provide insight into how the buried spreading center has produced the enormously important geothermal resources of the area and how the mechanical system must work. The anomaly, however, appears to be fundamentally 3-D. It probably involves conduction of currents through the otherwise resistive Coast Range, which are induced in nearby highly conductive surface structures such as the ocean. A 3-dimensional modeling effort to understand various features of the anomaly such as its lack of frequency- dependence will be carried out, as will a limited field effort to a really extend our array and to provide MT constraints for the modeling effort. These are both a necessary prelude to a major field effort designed to image the sturcture in detail using an innovative technique such as EMAP (Electro Magnetic Array Profiling).