Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing

Abstract

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.

Crust and upper-mantle seismic anisotropy variations from the coast to inland in central and Southern Mexico

“…In most cases, high Poisson's and V p /V s ratios have been interpreted as regions with a high fluid content or high pore pressure (Kodaira et al 2004;Audet et al 2009;Peacock et al 2011). An alternate and complementary hypothesis is that the high V p /V s ratio due to a low S-wave velocity can be related to a preferred mineral alignment in the medium, a process expected to be particularly marked in the case of serpentine (Mainprice & Ildefonse 2009;Bezacier et al 2010).…”

Distribution of hydrous minerals in the subduction system beneath Mexico

“…In particular, a depth-section of RFs typically shows the oceanic crust as a dipping low-velocity layer, of which the top and bottom (plate interface and oceanic Moho) are outlined by strong negative and positive amplitudes, respectively. The estimates of the Vp/Vs by Audet et al (2009) and Peacock et al (2011) range between 2.0 and 2.4 in the shallowly dipping subducted oceanic crust beneath Vancouver Island from RFs. The timing of the reverberated phases from the top and bottom interfaces of the oceanic crust is used to constrain such high Vp/Vs of the downgoing crust (Audet et al, 2009).…”

“…The estimates of the Vp/Vs by Audet et al (2009) and Peacock et al (2011) range between 2.0 and 2.4 in the shallowly dipping subducted oceanic crust beneath Vancouver Island from RFs. The timing of the reverberated phases from the top and bottom interfaces of the oceanic crust is used to constrain such high Vp/Vs of the downgoing crust (Audet et al, 2009). Abers et al (2009) modeled and directly inverted RF waveforms, using a parameterization of structure with a few number of free parameters, and obtain Vp/Vs of 1.9 for the shallowly dipping Juan de Fuca crust beneath Washington, Cascades.…”

“…Hydrous minerals initially form within oceanic lithosphere as a result of hydrothermal circulation and alteration at spreading ridges, along fracture zones, and along faults in the region of the trench and outer rise (Maruyama and Okamoto, 2007;Audet et al, 2009). Such processes determine geophysical and mineralogical properties of the oceanic lithosphere with significant consequences for the mechanical behavior of subduction zones (Bach and Früh-Green, 2010).…”

“…As oceanic plates subduct, mineral-bound H 2 O is transported down into the deep mantle (Abers, 2000;Hacker et al, 2003b;Jacobsen and van der Lee, 2006;Maruyama and Okamoto, 2007;Mainprice and IIdefonse, 2009). The evidence has been seismically detected via tomography and receiver function (RF) based on teleseismic P-to-S converted (hereafter Ps) phases in various subduction environments as a lowvelocity layer atop of the subducting plate (e.g, Alaska-Ferris et al, 2003, Cascadia-Nicholson et al, 2005Abers et al, 2009;Audet et al, 2009;Peacock et al, 2011, central Andes-Yuan et al, 2000, southwest Japan-Shelly et al, 2006, northeast Japan-Kawakatsu and Watada, 2007Tsuji et al, 2008, andcentral Mexico-Pérez-Campos et al, 2008;Kim et al, 2010). Two above-mentioned seismic methods yield high P-to-S velocity ratio (Vp/Vs) exceeding 2.0 at the top interface of the subducting plate.…”