284 The Deformation-Metamorphism-Fluid Triplet Governing Plate Boundaries and Orogens

Investigating orogens requires establishing relationships between deformation, fluids, metamorphism, and timing from within and adjacent to faults and shear zones. We seek presentations that combine new and traditional structural, petrochronology, and other microanalytical techniques.

Wednesday, 22 October 2025
1:30 PM - 5:30 PM
HBGCC, 217B

Is it a mess or just a little messy? Subduction zone fabrics and fluid migration controlled by pre-subduction slab geometry on Andros Island Greece

Paper number: 284-1
Program: Technical Sessions
Time: 1:40 PM – 2:00 PM

Authors

Abstract

Slow slip and tremor (SST) events contribute to subduction zone slip budgets, but the geologic record and exact mechanisms of these events are still poorly understood. We present macro- and micro-structural and geochemical data from Andros Island, Greece, which represents a fossil subduction interface that reached the conditions of deep SST in the Hellenic subduction zone during the Eocene. This subduction complex represents a paleo-oceanic lithosphere with sediments, volcanic rocks, and carbonates that produced a rheologically heterogeneous interface. These rocks reached peak metamorphic temperatures of ~500 °C (M1) in the Eocene followed by extensive metasomatism and retrogression into greenschist facies during early exhumation at ~440 °C (M2) in the Oligo-Miocene. 

Within exhumed metamafic rocks from the subducted slab, field mapping and structural measurements show that the dominant ductile foliation wraps around inherited flattened meta-pillow structures that control the orientation of this main structural fabric. A NE-SW stretching lineation is aligned with NE-SW trending fold axes, and along the fold noses epidote boudins form quartz-filled crack seal veins, that opened perpendicular to the stretching lineation. Top to the NE shear sense indicators suggest that these processes occurred during early exhumation under greenschist facies conditions (M2). We propose that pre-subduction seafloor fabrics and lithologic heterogeneities are controlling both the structural and rheological behavior of the slab during peak to early exhumation metamorphism, leading to multiple stages of ductile and brittle deformation. Geochemical analyses reveal deformation modes switching is activated by infiltration of sedimentary-sourced fluids (H2O and CO2) and fluid reactions with mica growth and calcite precipitation around the pillow basalt rims. These fluids have contributed to both metasomatic reactions and promoted brittle failure. The observations of fluid mediated deformation agree with the geophysical evidence of slow slip environments which occur within fluid-rich environment along the plate interface. Collectively our field mapping, macro and micro-structural, and geochemical data showcase the importance of the pre-subduction slab geometry and structures in controlling subduction zone processes including the development of major structural fabrics (e.g., foliations) at the conditions of slow slip and tremor events.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413739

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