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.

水合海洋岩石圈的俯冲可以将水带入地球深处，对一系列构造和岩浆过程产生影响。大部分流体在弧前释放，在控制俯冲大逆冲断层的力学性质和地震行为方面发挥着关键作用。在这里，我们展示了近 400 个长周期大地电磁场（包括 64 个近海）数据的三维反演结果，以提供对卡斯卡迪亚俯冲带前弧流体分布的洞察。我们限制了电阻性 Siletz 地体的几何形状，这是在始新世增生到北美的海洋地壳的增厚部分，以及沿边缘的导电增生复合体下冲。我们发现流体在超过 1 My 的时间尺度上以变质沉积单元在板块上方积累，而 Siletzia 的镁铁质岩石保持干燥。流体浓度往往在 17.5 和 30 公里的板片深度达到峰值，表明受变质过程的控制，但也集中在 Siletzia 的边缘周围，这表明该镁铁质块体是不可渗透的，脱水流体沿着巨型逆冲断层向上倾斜逸出。我们的研究结果表明，上覆地壳的岩性可以在控制俯冲带中的流体输送方面发挥关键作用。脱水液沿着大逆冲层向上倾泻而出。我们的研究结果表明，上覆地壳的岩性可以在控制俯冲带中的流体输送方面发挥关键作用。脱水液沿着大逆冲层向上倾泻而出。我们的研究结果表明，上覆地壳的岩性可以在控制俯冲带中的流体输送方面发挥关键作用。