Quantifying Interseismic Volume Strain from Chemical Mass‐Balance Analysis in Tectonic Mélanges

Abstract

Estimating interseismic deformation in subduction fault zones can offer insights into the frequency and magnitude of megathrust earthquakes. Diffusive mass transfer is a significant mechanism of strain during interseismic periods along the plate interface, observed through the pervasive scaly fabrics and mineral veins in tectonic mélanges of ancient accretionary prisms. The dissolution of fluid‐mobile elements (e.g., Si and Large‐Ion Lithophile Elements) along scaly folia and subsequent reprecipitation as veins lead to the enrichment of fluid‐immobile elements (e.g., Ti and High Field Strength Elements) in scaly fabrics. The kinetics of dissolution‐precipitation is temperature‐dependent, suggesting depth‐dependent mass transfer along subduction interfaces. Here, we evaluate the magnitudes of volume strain in a suite of mélange samples that span peak metamorphic temperatures of 130–340°C. Micro‐chemical analysis shows that the depletion of fluid‐mobile elements and enrichment of fluid‐immobile elements in scaly fabrics increases with temperature. Assuming the conservation of Ti, we apply mass balance constraints to calculate the volumetric strain in scaly fabrics. Results indicate average volumetric strain of 28% and 95% in the individual scaly fabrics of the Lower Mugi and Makimine mélanges in Japan, which record temperatures near the updip and downdip isotherms bounding the seismogenic zone, respectively. To determine the total volume strain within an area of interest, we integrate the amount of volume loss along individual microstructures across the network using image analyses, which ranges from 3% to 14% for the mélanges. Our approach demonstrates the potential to fully describe the deformation related to mass transfer by connecting characterization in different scales with geochemical analyses.

Frictional Properties and Healing Behavior of Tectonic Mélanges: Implications for the Evolution of Subduction Fault Zones

“…The Lower Mugi and the Makimine mélanges record temperatures that correspond to the updip and downdip limits of the seismogenic zone based on previous studies using a variety of geothermometers (Ikesawa et al, 2005;Kiminami & Ohno, 1999;Matsumura et al, 2003;Raimbourg et al, 2019Raimbourg et al, , 2021. The Makimine mélange exhibits enhanced mobility of elements compared to the Lower Mugi mélange due to the temperature dependence of DMT, as evidenced by the amplified depletion of Si along scaly fabrics (Chen et al, 2024;Ramierz et al, 2021). The release of Si from mudstone involves the breakdown of quartz and feldspar, coupled with the dehydration of smectite, leading to the formation of illite, chlorite, and muscovite.…”

“…Field observations document anastomosing networks of scaly microfaults in the mudstones to accommodate slip, while the sandstones experience opening-mode cracking and precipitation of mineral veins (e.g., Fisher et al, 2021;Kimura et al, 2012;. A major source of elements distributed in the veins comes from local diffusive mass transfer (DMT; i.e., pressure solution), supported by the evidence that adjacent scaly fabrics are depleted with fluid-mobile elements (e.g., Si, Li, Al, K, and LILEs) and enriched in fluid-immobile elements (e.g., Ti, REEs, and HFSEs) while the veins show an opposite pattern (Chen et al, 2024;Kawabata et al, 2007;Ramirez et al, 2021;Figure 4). This dissolution-reprecipitation process occurring during interseismic periods is likely to restrengthen subduction fault zones microscopically by reducing pore space and welding grain-to-grain contacts (e.g., Olsen et al, 1998;Yasuhara et al, 2005) and macroscopically, by increasing the coupling areas across plate interfaces (Fisher et al, 2019(Fisher et al, , 2021.…”

“…The release of Si from mudstone involves the breakdown of quartz and feldspar, coupled with the dehydration of smectite, leading to the formation of illite, chlorite, and muscovite. Notably, in the quartz veins of the Makimine mélange, it has been observed that, in addition to locally redistributed Si, there is an extra source of silica supplied through long-distance transport via up-dip fluid flow (Chen et al, 2024;Ujiie et al, 2018).…”

“…This phenomenon results from the local diffusion of mobile elements from scaly fabric to cracks in sandstone blocks driven by its temperature-dependent pressure solution. Moreover, quartz veins may include silica that is derived from an external fluid related to coseismic fluid advection (Chen et al, 2024;Ujiie et al, 2018). The Makimine mélange sample provides a record of these transformative processes.…”