Elastic wave constraints on fluid migration along a shear zone above the inter-plate boundary in the northern Cascadia subduction zone
Abstract
P and S wave velocity models for southern Vancouver Island have been developed using 3-D double-difference tomographic inversion of travel times from local earthquakes, low frequency earthquakes (LFEs), the 1998 SHIPS wide-angle survey, and all post-2002 stations. These models extend from the landward end of the locked zone at 25-30 km depth to the forearc mantle. LFEs, which exhibit particle motions consistent with low-angle thrusting, occur immediately below a 6-12 km thick package of landward dipping high amplitude, normal incidence P wave reflections imaged in both onshore and offshore surveys, the so-called E reflections that are interpreted to be a shear zone. The inter-plate boundary may be the interface corresponding to the LFEs or alternatively LFEs may occur at the base of a vertically distributed active shear zone in the lowermost part of the E reflectors. The E reflectors occur in the lower part of a 9-12 km thick zone of relatively low S wave velocity, the top of which can be followed from 20 km to 35 km depth near the likely location of the continental Moho. At the base of the S wave low velocity zone, Poisson's ratio is elevated to 0.26-0.28 in a 3-6 km thick landward dipping zone, which correlates with an increase in P wave velocity, and may be due to the presence of mafic rocks within the E reflectors. This band of elevated Poisson's ratio extends downdip to a widespread, ~12 km thick anomaly mostly above the subducting oceanic crust at 40-50 km depth, which overlies a region of reduced, 0.23, Poisson's ratio and elevated seismicity in the oceanic plate at 55 km depth. This reduction in Poisson's ratio may be due to the creation of porosity by the basalt to eclogite phase transition in the subducting oceanic crust, with fluid released during this process rising into the over-riding plate where it has serpentinized the forearc mantle, consistent with the observed elastic properties. Fluids have likely risen through the mantle, but also migrated along the shear zone above the subducting plate, from which they have escaped into the shallower crust leading to porosity, and perhaps pore-pressure, related lowering of S wave velocity; however, there is no tomographic evidence of anomalously high pore fluid pressure in the shear zone, as there are no coincident downward reductions in P and S wave velocity where Poisson's ratio increases.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T42C..03C
- Keywords:
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- 8045 Role of fluids;
- STRUCTURAL GEOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS;
- 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS