TEMPERATURES AND THE ROLE OF WATER IN CASCADIA SUBDUCTION ZONE DEFORMATION AND MAGMATISM

In the Cascadia subduction zone, young (5-10 Ma) warm lithosphere of the Juan de Fuca plate subducts beneath North America at a modest rate of ~40 mm/yr. In contrast to cool subduction zones like NE Japan, numerical thermal models for Cascadia predict that subducting oceanic crust undergoes warm greenschist to epidote blueschist/amphibolite facies metamorphism. At 35 km depth (P = 1GPa), temperatures in the subducting crust are predicted to be ~400-650 ˚C depending on the extent of hydrothermal circulation in the subducting crust. At ~50 km depth, subducting oceanic crust transforms to eclogite, with relatively little bound H₂O predicted to persist to greater depths. Beneath the arc, predicted slab surface temperatures range from 890-960 ˚C (Syracuse et al., 2010); predicted temperatures in the subducting mantle are ~100 ˚C. In Cascadia, the H₂O in hydrous arc magmas may be derived from dehydration reactions in the subducting slab mantle or from dehydration melting of mantle-wedge chlorite that is dragged down from shallow levels (Till et al., 2012).

High fluid pressures appear to be a fundamental feature of subduction-zone plate boundaries. Beneath accretionary prisms, high fluid pressures along the plate boundary promote low friction and enable great thrust earthquakes. Magnitude 9 earthquakes on the Cascadia plate interface occur every few hundred years, with the last great earthquake occurring in 1700 AD. Downdip of the seismogenic zone, at ~35-45 km depth, episodic tremor and slip events occur every ~15 months on or near the Cascadia plate interface. S-wave diffraction tomographic profiles (receiver functions) of the Cascadia subduction zone forearc reveal a prominent landward-dipping low-velocity layer interpreted as subducting oceanic crust of the Juan de Fuca plate. At 25-45 km depth beneath southern Vancouver Island, the ~5-km-thick layer is estimated to contain 2.7-4.0 volume % fluid in crack porosity suggesting episodic tremor and slip occurs in a very water-rich environment. Beneath the forearc Moho, there is strong geophysical evidence for extensive serpentinization of the forearc mantle wedge in the Cascadia that require substantial quantities of H₂O to migrate upward through the plate boundary over the history of the subduction zone.