Browsing by Author "Delph, Jonathan R."

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    Fluid Controls on the Heterogeneous Seismic Characteristics of the Cascadia Margin
    (Wiley, 2018) Delph, Jonathan R.; Levander, Alan; Niu, Fenglin
    The dehydration of oceanic slabs during subduction is mainly thermally controlled and is often expressed as intermediate‐depth seismicity. In warm subduction zones, shallow dehydration can also lead to the buildup of pore‐fluid pressure near the plate interface, resulting in nonvolcanic tremor. Along the Cascadia margin, tremor density and intermediate‐depth seismicity correlate but vary significantly from south to north despite little variation in the thermal structure of the Juan de Fuca Plate. Along the northern and southern Cascadia margin, intermediate‐depth seismicity likely corresponds to increased fluid flux, while increased tremor density may result from fluid infiltration into thick underthrust metasediments characterized by very slow shear wave velocities (<3.2 km/s). In central Cascadia, low intermediate‐depth seismicity and tremor density may indicate a lower fluid flux, and shear wave velocities indicate that the Siletzia terrane extends to the plate interface. These results indicate that the presence of thick underthrust sediments is associated with increased tremor occurrence.
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    Slab dehydration in warm subduction zones at depths of episodic slip and tremor
    (Elsevier, 2020) Condit, Cailey B.; Guevara, Victor E.; Delph, Jonathan R.; French, Melodie E.
    Non-volcanic tremor (NVT) and episodic slow slip events (SSEs) have been observed below the seismogenic zone of relatively warm subduction zones for the past 20 years. Geophysical and geologic observations show that this portion of the subduction interface is fluid-rich, and many models for these slip behaviors necessitate high pore fluid pressures. However, whether these fluids are sourced from local dehydration reactions in particular lithologies, or require up-dip transport from greater depths is not known. We present thermodynamic models of the petrologic evolution of four lithologies typical of the plate interface (average MORB, seafloor altered MORB, hydrated depleted MORB mantle, and metapelite) along predicted plate boundary pressure–temperature (P-T) paths at several warm subduction segments where NVT and SSEs are observed at depths between 25-65 km. The models suggest that 1-2 wt% H2O is released at the depths of NVT/SSEs in Jalisco-Colima, Guerrero, Cascadia, and Shikoku due to punctuated dehydration reactions within MORB, primarily through chlorite and/or lawsonite breakdown. These reactions produce sufficient in-situ fluid across a narrow P-T range to cause high pore fluid pressures at NVT/SSE depths. Dehydration of hydrated peridotite is minimal at these depths for most margins, and metapelite releases H2O (<1.5 wt%) gradually over a wide depth range compared to MORB. We posit that punctuated dehydration of oceanic crust provides the dominant source of fluids at the base of the seismogenic zone in these warm subduction zones, and up-dip migration of fluids from deeper in the subduction zone is not required.