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

 
Nerak
03/28/2005 06:10 PM
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SUBDUCTION ZONE MAGNETIC ANOMALIES: IMPLICATIONS FOR MAPPING HYDRATED FOREARC MANTLE BENEATH CASCADIA
2003 Seattle Annual Meeting (November 2–5, 2003)
Paper No. 113-4
Presentation Time: 2:15 PM-2:30 PM
SUBDUCTION ZONE MAGNETIC ANOMALIES: IMPLICATIONS FOR MAPPING HYDRATED FOREARC MANTLE BENEATH CASCADIA
BLAKELY, Richard J.1, BROCHER, Thomas M.2, and WELLS, Ray E.2, (1) U.S. Geol Survey, 345 Middlefield Road, MS 989, Menlo Park, CA 94025, [email protected], (2) U.S. Geol Survey, 345 Middlefield Road, Menlo Park, CA 94025
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.

2003 Seattle Annual Meeting (November 2–5, 2003)
Nathan´s Son
12/08/2005 10:15 AM
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Re: SUBDUCTION ZONE MAGNETIC ANOMALIES: IMPLICATIONS FOR MAPPING HYDRATED FOREARC MANTLE BENEATH CASCADIA
Thank you for that piece.

What immediately came to my mind was the "curing" process that occurs when a slab of concrete is laid down and the hydration transfer of any object or insect that happened to be resting on the surface. This piece gave me a better idea of what kind of processes are actually at work in those zones--especially in the cold zones, because concrete "cures" altogether differently under cold conditions. It retains its hydration longer and, therefore, is more susceptible to slippage. Conversely, concrete that is heated both by speedier motion and use of material such as calcium will cure in minutes--much to the chagrin of he who is laying the slab.

It sounds like there are some important new tools are being discovered that may help us actually map the areas at risk. Thanks for the heads up.
Capt. Kirk
12/08/2005 10:15 AM
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Re: SUBDUCTION ZONE MAGNETIC ANOMALIES: IMPLICATIONS FOR MAPPING HYDRATED FOREARC MANTLE BENEATH CASCADIA
...thanks nordac...scotty was waiting for that info...





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