SEPARATING MAGNETIC-ANOMALY CONTRIBUTIONS FROM SILETZIA AND UNDERLYING MANTLE, OREGON AND WASHINGTON, WITH REFLECTIONS ON CONTRIBUTIONS FROM ISIDORE ZIETZ

The advent of regional aeromagnetic compilations in the 1980s, pioneered by Isidore Zietz, illuminated a 1500-km-long alignment of high-amplitude magnetic anomalies inboard of the U.S. West Coast. Johnson, Zietz, and Bond (1990) speculated that this alignment, which they called the Puget Lowland--San Joaquin lineament (PLSJL), marks a single structure that controlled Mesozoic and Cenozoic evolution of the Western U.S. Finn (1991) disagreed, arguing that the PLSJL is a coincidental alignment of diverse accretionary terranes affected by post-accretionary rotation. We agree with Finn but also suggest that the PLSJL in Oregon and Washington partly originates from modern-day, deep-seated subduction processes.

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.