SEPARATING MAGNETIC-ANOMALY CONTRIBUTIONS FROM SILETZIA AND UNDERLYING MANTLE, OREGON AND WASHINGTON, WITH REFLECTIONS ON CONTRIBUTIONS FROM ISIDORE ZIETZ
The PLSJL in Oregon lies over the eastern margin of Siletzia, an early Eocene forearc terrane of oceanic origin, but is displaced >100 km east of all Siletzia exposures and their associated gravity highs. The PLSJL also lies above the wedge of continental mantle in contact with the subducting Juan de Fuca plate. S-wave perturbations and thermal models show that the mantle wedge in Oregon is ~60 percent serpentinized and cooler than the Curie-temperature of magnetite, and thus should be strongly magnetic. These observations led us to suggest that the PLSJL in Oregon is caused in part by hydrated, serpentinized mantle. In the present study, we applied matched filtering to regional magnetic anomalies to distinguish serpentinized mantle from Siletzia. We approximated the crust-mantle section by a stack of dipole layers overlying a half space, and determined layer depths by modeling in the Fourier domain. The half-space, which we interpret as mantle, lies at a depth of 33-38 km beneath Siletzia in Oregon, in general agreement with Moho depth (33 km) determined from S-wave inversion. With mantle depths thus determined, we applied matched filters to regional magnetic anomalies to evaluate mantle contributions. Remaining non-mantle magnetic anomalies show geologic details of Siletzia and a subdued PLSJL. While modern subduction processes help explain the PLSJL in Oregon and Washington, alternate explanations are required in California. We speculate that the PLSJL south of the Mendocino triple junction includes contributions from the relict mantle wedge left in the wake of the northward advancing triple junction.