Rapid, uniform fluid flow at the subduction interface: Lithium chronometry of amphibolite-facies melange blocks (Catalina Schist, CA)
Abstract
Mélange zones identified in many exhumed subduction complexes are thought to represent the subduction interface and preserve evidence of significant fluid transport and deformation, however the unknown duration of fluid transport along this regional-scale feature complicates connections between fluids and deformation. Previous studies have used Li chronometry to constrain fluid transport durations of <1000 years within cm- to m-scale eclogite-facies veins and shear zones. However, significantly longer durations of fluid transport might be expected within mélange matrix along the km-scale subduction interface. Two Li isotope transects were measured across reaction rinds around high-pressure amphibolite-facies (~700ºC and 1.4GPa) mélange blocks (Catalina Schist, CA). These profiles were inverted using a finite difference advection-diffusion model to obtain the duration of fluid transport driving Li exchange. Isotopic compositions and textures of both rinds are consistent with infiltration of metasedimentary-rock-derived fluids near peak metamorphic conditions. Durations of fluid transport are ~40 years (3 to 590 years, 1SD) for each block. Similarity in fluid source and transport duration between these two spatially distinct block-rind pairs suggests that they record the same regionally-pervasive fluid infiltration event. However, the similar fluid transport durations for the subduction interface and meter-scale veins suggests that the dynamic nature of the subduction interface may obscure the cumulative record of fluid transport. Instead, the modeled durations may reflect the last stage of fluid flow after deformation that disrupts and removes rind material has largely ceased. This deformation could have resulted in transient permeability that accommodated fluid transport, which in turn progressively sealed the mélange matrix over ~40 years as part of a cyclic process like fault sealing. Ongoing cycles of fluid transport and deformation within the subduction interface effectively remove Li diffusion profiles formed during prograde fluid-rock interaction, but reveal the temporal links between near-peak fluid flow and deformation and the potential for cyclic feedbacks between the two.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.T51A..08H