Sedimentary underplating at the Cascadia mantle-wedge corner revealed by seismic imaging
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| Authors: | Calvert, AJ; Preston, LA; Farahbod, AM |
| Year: | 2011 |
| Journal: | Nat. Geosci. 4: 545-548 Article Link (DOI) |
| Title: | Sedimentary underplating at the Cascadia mantle-wedge corner revealed by seismic imaging |
| Abstract: | Earth's largest earthquakes occur in subduction zones, along the boundary between the subducting and overriding plates(1). Non-volcanic tremor generated by slow slip between the plates is thought to originate on, or near, this boundary(2,3). Earthquakes also occur in the down-going plate as fluids are released(4), and zones of anomalously low seismic velocities observed beneath several subduction zones are interpreted to be the subducting oceanic crust(5-10). Yet, the exact location of the plate boundary remains uncertain(5). Here we interpret a three-dimensional seismic tomography model from the northern Cascadia subduction zone in the northwest USA. We find that the low-velocity zone varies considerably along the Cascadia margin. In places, we observe the low-velocity zone to crop out at the surface and separate from the descending plate at depths of 35-40 km. We argue that the low-velocity zone here cannot represent oceanic crust as previously suggested, and instead the zone mostly represents sediments that have been subducted and underplated beneath the North American continent. We also find that tremor signals correlate with the position of the low-velocity zone, implying that slow slip and tremor may be facilitated by trapped fluids and high pore fluid pressures in subducted sedimentary rocks at, or close to the plate boundary. Our results also imply that the plate boundary beneath Cascadia is much deeper than previously thought. |
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