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

Tsai-Wei Chen, University of Washington
Andrew Smye, Pennsylvania State University
Donald Fisher, Pennsylvania State University
Yoshitaka Hashimoto, Kochi University
Hugues Raimbourg, Institut des Sciences de la Terre d'Orléans
Vincent Famin, Université de La Réunion

Abstract

Estimating deformation in subduction fault zones during interseismic periods is critical for understanding the occurrence and magnitude of megathrust earthquakes. A significant mechanism of strain during interseismic periods along the plate interface is diffusive mass transfer, which is evidenced by the widespread presence of scaly fabrics and mineral veins in tectonic mélanges of ancient accretionary prisms. This process involves the dissolution of fluid-mobile elements (e.g., Si and Large‐Ion Lithophile Elements) along scaly folia, followed by their reprecipitation as veins, leading 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. In this study, we assess the magnitude of volumetric strain in a suite of mélange samples that span peak metamorphic temperatures of 130–340°C. Micro-chemical analyses reveal that the depletion of fluid-mobile elements and the enrichment of fluid-immobile elements in scaly fabrics increases with temperature. By assuming the conservation of Ti, we apply mass balance calculations to estimate the volumetric strain in scaly fabrics. Our results show average volumetric strain of 28% and 95% in the individual scaly fabrics of the Lower Mugi and Makimine mélanges in Japan, respectively, which record temperatures near the updip and downdip isotherms that bound the seismogenic zone. To estimate the total volumetric strain within an area of interest, we integrate the volume loss along individual microstructures across the network using image analysis, finding a range of 3% to 14% for the mélanges. This approach demonstrates the potential to comprehensively describe deformation associated with mass transfer by linking multi-scale characterizations with geochemical analyses.

Session

Subduction zone geology