1. Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
2. Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, Wisconsin 53706, USA
3. Present address: Seismological Laboratory, University of California Berkeley, 215 McCone Hall, Berkeley, California 94720, USA.

Correspondence to: Pascal Audet^1,3 Correspondence and requests for materials should be addressed to P.A. (Email: paudet@berkeley.edu).

Abstract

Water and hydrous minerals play a key part in geodynamic processes at subduction zones^{1, 2, 3} by weakening the plate boundary, aiding slip and permitting subduction—and indeed plate tectonics—to occur⁴. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization^{5, 6, 7}. 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.

MORE ARTICLES LIKE THIS

These links to content published by NPG are automatically generated.