SUBDUCTION ZONE MAGNETIC ANOMALIES: IMPLICATIONS FOR MAPPING HYDRATED FOREARC MANTLE BENEATH CASCADIA

The seaward extent of continental mantle beneath subduction zones is significantly hydrated by release of water from the underlying, descending plate. Mantle rocks generally are not considered ferromagnetic, but subduction of oceanic lithosphere lowers the temperature of the forearc mantle wedge below the Curie point of magnetite, a significant by-product of mantle hydration. The existence of a magnetic mantle wedge beneath the Cascadia subduction zone would explain an important disparity between regional gravity and magnetic anomalies over the Oregon forearc, where high-amplitude magnetic anomalies have no comparable gravity signature.

To test this idea, we modeled characteristic gravity and magnetic profiles across the Oregon forearc using published seismic velocity models and density values as constraints. The model includes a thick (25 km) section of Siletzia underlain by lower crust and subducting lithosphere. The model is compatible with a mantle wedge corresponding in shape, location, and depth to a low-velocity zone identified in seismic models and having low-density (2750 kg/m3) and high-magnetization (1.4 A/m) consistent with serpentinite. Thus determined, magnetic anomalies allow us to map the presence of serpentinized mantle along the length of the Cascadia subduction zone and elsewhere. This mapping has important implications for earthquake hazards, as hydrated mantle is directly related to dehydration of the downgoing slab and slab embrittlement, and hence indirectly to inslab earthquakes, and, in cool subduction zones, to the downdip limit of rupture during megathrust earthquakes. In Cascadia, hydrated mantle is best expressed by long-wavelength magnetic anomalies in Oregon along a narrow swath from the Klamath Mountains to the Columbia River. A comparison with aeromagnetic data, thermal models, and inslab earthquakes in Japan and southern Alaska suggests that magnetic mantle may be a common occurrence in forearc settings.