Simon M. Peacock

Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing

Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction—and indeed plate tectonics—to occur. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson’s ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson’s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

High pore pressures and porosity at 35 km depth in the Cascadia subduction zone

In the Cascadia subduction zone, beneath southern Vancouver Island at 25–45 km depth, converted teleseismic waves reveal an ∼5-km-thick landward-dipping layer with anomalously high Vp/Vs averaging 2.35 ± 0.10 (2σ), interpreted as subducted oceanic crust of the Juan de Fuca plate. This layer is observed downdip of the inferred locked seismogenic zone, in the region of episodic tremor and slip. Laboratory velocity measurements of crystalline rock samples made at 200 MPa confining pressure and elevated pore pressures demonstrate that Vp/Vs increases with increasing fluid-filled porosity. The observed high Vp/Vs values are best explained by pore fluids under near lithostatic pressure in a layer with a high porosity of 2.7%–4.0%. Such large volumes of fluid take ∼1 m.y. to accumulate based on reasonable rates of metamorphic fluid production of ∼10 –4 m 3 /(m 2 yr) in subducting Juan de Fuca crust and mantle. Accordingly, the permeability of the plate interface at these depths must be very low, ∼10 –24 to ∼10 –21 m 2 , or the porous layer must have a permeability –20 m 2 .

Thermal and metamorphic environment of subduction zone episodic tremor and slip

[1]xa0Episodic tremor and slip (ETS) have been detected in the Cascadia and southwest Japan subduction zones, where the subducting crust is relatively warm because of the young incoming lithosphere (<20 Ma) and modest plate convergence rates (∼40–60 mm/a). In the southwest Japan subduction zone, low-frequency earthquakes occur on the plate interface at depths of 30–35 km beneath Shikoku where finite element thermal models predict temperatures of ∼425°C in the subducting oceanic crust and at depths of 30–40 km beneath the Kii Peninsula where predicted temperatures are ∼325°C. Warmer temperatures of ∼575°C are predicted at ETS depths beneath southern Vancouver Island in the Cascadia subduction zone, but here tremor also occurs within the overlying fore-arc crust where temperatures are lower. In the southwest Japan and Cascadia subduction zones, subducting oceanic crust passes through the blueschist, greenschist, and amphibolite metamorphic facies where mineral dehydration reactions are complex. The different temperatures predicted for the two subduction zones suggest that ETS does not coincide with a specific temperature or metamorphic reaction. Several lines of evidence indicate that a free H2O-rich fluid is present, at least transiently, in subducting oceanic crust and fluids released by prograde metamorphic dehydration reactions may help trigger or enable ETS within the subducting plate. Less clear is the role H2O may play in tremor observed in the Cascadia fore-arc crust where free H2O may exist locally in faults and fractures, but retrograde hydration reactions are expected to consume H2O.