244-4 Investigating geological conditions conducive for slow slip and tremor: field and structural observations from Ios Island, Greece

Session: Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology

Presenting Author:

Tshering Sherpa

Authors:

Sherpa, Tshering¹, Condit, Cailey², Poulaki, Eirini³, Tewksbury-Christle, Carolyn⁴, Soukis, Konstantinos⁵, Amanatidou, Dimitra⁶, Alley, Claire⁷, Hammons, Julie⁸, Saylor, Justin⁹

(1) University of Washington, Seattle, WA, USA, (2) University of Washington, Seattle, WA, USA, (3) Louisiana State University, Baton Rouge, LA, USA, (4) Fort Lewis College, Durango, CO, USA, (5) National and Kapodistrian University of Athens, Athens, Greece, (6) National and Kapodistrian University of Athens, Athens, Greece, (7) Louisiana State University, Baton Rouge, LA, USA, (8) Louisiana State University, Baton Rouge, LA, USA, (9) Fort Lewis College, Durango, CO, USA,

Abstract:

Deep slow slip and tremor (SST) events along the subduction plate interface, down-dip of the seismogenic zone, play a key role in subduction slip budgets, yet they remain poorly understood. Recent geophysical and rheological work suggest that coeval brittle-ductile deformation of metamafic rocks in underplated, rheologically/lithologically heterogeneous packages (duplex structures) along the plate interface may host SST. To investigate these processes in the rock record, we examine exhumed subduction plate interface rocks of the Cycladic Blueschist Unit (CBU) on Ios Island, Greece which have experienced P-T conditions of modern SST. Here, back-arc extension in the Hellenic subduction zone exposed Permian to Late Cretaceous CBU metasedimentary and metavolcanic rocks that reached peak high-pressure, low-temperature (HP/LT) metamorphism of up to ~1.8-2.5 GPa and 500-540°C ca. 50-40 Ma. CBU rocks on Ios also record subsequent greenschist facies metamorphism ca. 30-20 Ma associated with syn-subduction exhumation. Prior studies have used detrital and igneous zircon U-Pb geochronology to identify cryptic thrusts, associated with duplexes, as potential fossil SST sources. We present three new detailed maps and cross-sections representing the macroscale distribution of metamafic rocks and other lithologies along three transects in Ios Island, aiming to investigate these cryptic thrusts. We also focus on preliminary field evidence of transient brittle-ductile deformation within metamafic rocks and contextualize these exposures with the locations of cryptic thrusts and the broader structural architecture of the fossil subduction plate interface. Field observations include but are not limited to vein injections and fractures in metabasitic rocks, which indicate changing stress states and high pore-fluid pressures, potentially related to SST events. Vein injections are both parallel to and crosscut foliation, suggesting that these are associated with deformation mode switching during subduction-related deformation. We propose that these brittle-ductile deformation features are likely related to syn-subduction underplating, highlighting the importance of regional structural and lithologic architecture in potentially facilitating SST events.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9315

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