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Fluid transport and storage in the Cascadia forearc influenced by overriding plate lithology

Abstract

Subduction of hydrated oceanic lithosphere can carry water deep into the Earth, with consequences for a range of tectonic and magmatic processes. Most of the fluid is released in the forearc where it plays a critical role in controlling the mechanical properties and seismic behaviour of the subduction megathrust. Here we present results from three-dimensional inversions of data from nearly 400 long-period magnetotelluric sites, including 64 offshore, to provide insights into the distribution of fluids in the forearc of the Cascadia subduction zone. We constrain the geometry of the electrically resistive Siletz terrane, a thickened section of oceanic crust accreted to North America in the Eocene, and the conductive accretionary complex underthrust along the margin. We find that fluids accumulate over timescales exceeding 1 My above the plate in metasedimentary units, while the mafic rocks of Siletzia remain dry. Fluid concentrations tend to peak at slab depths of 17.5 and 30 km, suggesting control by metamorphic processes, but also concentrate around the edges of Siletzia, suggesting that this mafic block is impermeable, with dehydration fluids escaping up-dip along the megathrust. Our results demonstrate that the lithology of the overriding crust can play a critical role in controlling fluid transport in a subduction zone.

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Fig. 1: Map of the study area.
Fig. 2: 3D view of the forearc portion of the model from NNW.
Fig. 3: 2D views derived from the 3D resistivity model.
Fig. 4: Profiles of conductance in a 10-km-thick layer above the upper plate interface.
Fig. 5: Conceptual model for fluid transport and storage.

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Data availability

The data that support the findings of this study are publicly available online at http://ds.iris.edu/ds/products/emtf/. The electrical resistivity model file can be accessed online at https://doi.org/10.5281/zenodo.6303537Source data are provided with this paper.

Code availability

The 3D inversion code used for this study (ModEM) is freely available online at https://sites.google.com/site/modularem.

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Acknowledgements

This work was supported by NSF grant numbers EAR1053632 and EAR1460437 (G.D.E. and A.S.), EAR1053207 and EAR1459067 (K.K.) and EAR1053202 and EAR140552 (D.W.L.), NSFC grant number 41774079 and National Key R&D Program of China 2018YFC0603604 (B.Y.). We thank R. Blakely, J. Delph and G. Schmalzle for providing data and models for comparison studies.

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D.W.L., K.K., A.S. and P.A.B. collected the MOCHA magnetotelluric data. A.K. processed the land data and K.K. the marine data. B.Y. and B.P. ran the 3D inversions. G.D.E., B.Y. and P.A.B. developed the interpretation and wrote the manuscript with input from D.W.L. and K.K.

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Correspondence to Bo Yang.

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Extended data

Extended Data Fig. 1 Graphical summary of prior model.

(a) E-W cross-section, showing deep layered structure. (b) 3D geometry of resistive (1000 Ohm-m) subducting plate. Data from refs. 61,73. (c) surface layer of prior model, showing top layer of ocean and actual grid resolution (11 × 9.5 km (N-S × E-W). (d) Thickness of ocean sediment layers. Data from ref. 71.

Extended Data Fig. 2 Observed (columns 1 and 3) and computed (columns 2 and 4) apparent resistivities for 4 representative periods.

Left two columns are for the nominal TE mode, with current flow parallel to the coastline. Right columns are for nominal TM mode, with current flow perpendicular to coast. Plotted values are interpolated from values at sites using natural neighbor scheme.

Extended Data Fig. 3 Observed (columns 1 and 3) and computed (columns 2 and 4) phases for 4 representative periods.

Left two columns are for the nominal TE mode, with current flow parallel to the coastline. Right columns are for nominal TM mode, with current flow perpendicular to coast. Plotted values are interpolated from values at sites using natural neighbor scheme.

Extended Data Fig. 4 Normalized root-mean-square (nRMS) misfit for each site.

(a) total; (b) impedance components only; (c) VTF (Vertical magnetic Transfer Function) only; (d-f) nRMS for individual impedance and VTF components for coast parallel electric mode (nominal TE); (g-i) nRMS for individual components for coast perpendicular electric mode (nominal TM). In all cases nRMS is summed over period.

Extended Data Fig. 5 Curved sections of the 3D resistivity model just above the upper interface of the subducting slab from various inversion runs.

(a) run#36. (b) run#44. (c) run#45. Panel (d) shows a 3D view of the result shown in panel (c). For run#36 & run#44, the conductive ocean was allowed to vary during inversion, while for run#45 it was frozen. Note that run#44 & run#45 have more sites in Southwestern Oregon.

Extended Data Fig. 6 Comparison between Vp from seismic tomography24 (a) and (b) and resistivity from the MT (c) and (d), along two profiles in Western Washington.

Top panels: an east-west profile across the Olympic Mountains; lower panels, the CAFE profile (MT sites along this profile34 are shown in Fig. 1). Note that for the Vp plots cool colors are low velocities (a-b), and are interpreted as subducted sediments and metasediments. These should have low resistivities (hot colors in (c-d). There is a very good agreement between geometries imaged by the two geophysical methods.

Extended Data Fig. 7 Comparison of resistive bodies to other geophysical data.

(a) Pseudo-gravity derived from magnetics79 with contours (20 and 30 km) for depth to bottom of resistive body from Fig. 3a overlain. There is a good correlation between the 3D geometry of the core of Siletzia inferred from MT, and magnetic anomalies converted to pseudo-gravity. There is no clear correlation with resistive block ‘e’ but this body is under the thick (8 km) sedimentary Seattle basin, and is also outside our area of good data coverage. Deeply extending resistive bodies ‘g’ and ‘f’, also do not exhibit strong magnetic anomalies but these are likely not part of Siletzia per se, and may have different composition. (b) Crustal seismicity (M > 2, 1990–2020) from ANSS catalogue with the same resistivity contours overlain. Resistive blocks in the core of Siletzia are mostly aseismic, while block ‘e’ has little seismicity below the level of the Seattle basin. As is well known, there is almost no crustal seismicity in central-southern Oregon. This seismic gap includes the main thick block of Siletzia, but extends further south to the California border.

Extended Data Fig. 8 Shear wave velocity averaged over 10 km thick layer above plate interface25.

Contours (20 and 30 km) of depth to bottom of resistor, and block labels from Fig. 3a are overlain. Updip of the FMC, where the 10 km thick layer is in the overriding crust, deep resistors inferred from the MT are generally seismically fast.

Extended Data Fig. 9 Conductance (\({{{\mathrm{C}}}} = {\upsigma}_{bulk}H\)) of a fluid layer, as a function of porosity (ϕ) and layer thickness (H).

We assume a fluid of conductivity \({\upsigma}_{fluid} = 30\;S/m\) (salinity of seawater at ambient temperatures76) and compute bulk resistivity using Archie’s law (\({\upsigma}_{bulk} = \sigma _{fluid}\phi ^m\)) for \({{{\mathrm{m}}}} = 1.5,1.75,2\) for the three panels (a, b and c). Colormap for conductance is identical to that used for Fig. 3b. Even with an Archie’s law exponent of \({{{\mathrm{m}}}} = 2\) (panel c) conductance exceeds \(\sim 300\;{{{\mathrm{S}}}}\) for layer thickness51 and porosities29 previously postulated. The MT conductance maps (Fig. 3b) show that such a layer is not present everywhere, but a thinner layer, or one with lower \({\upsigma}_{fluid}\) could be. Note also that conductance of observed anomalies integrated over a 10 km layer, exceed peak values shown here, requiring either higher \({\upsigma}_{fluid}\), lower values of m or some combination.

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Egbert, G.D., Yang, B., Bedrosian, P.A. et al. Fluid transport and storage in the Cascadia forearc influenced by overriding plate lithology. Nat. Geosci. 15, 677–682 (2022). https://doi.org/10.1038/s41561-022-00981-8

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