Failing under pressure: Exploring the frictional-viscous transition in cold subduction zones

De Caroli, Sara 2023. Failing under pressure: Exploring the frictional-viscous transition in cold subduction zones. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The rheology of epidote-blueschists deforming along the prograde path in the subduction zone downdip the seismogenic zone, and along the retrograde path in shear zones guiding exhumation, is poorly documented. In SW Japan, in the Ryukyu arc, the Tomuru block-in-matrix metabasites represent an exhumed subduction complex that registers prograde deformation at epidote-blueschist conditions (T~ 400 °C and P~ 8-9 kbar) and offers a natural laboratory to understand how microstructure and deformation mechanisms influence bulk rheology and fault behaviour along the plate interface during the prograde path. In the block-in-matrix complex, deformation occurred by progressively increasing strain from greenschist and epidote-blueschist blocks to epidote-blueschist matrix, reflected by increase in foliation development. Both blocks and matrix deform through distributed viscous creep by diffusive mass transfer under low shear stress, estimated at 11-28 MPa. Overall, microstructural studies indicate that the viscosity contrast between blocks and matrix must have been relatively low, but not negligible. The structural and mechanical heterogeneity during deformation results in epidote-blueschists likely accommodating plate creep at the P-T conditions of deformation. Retrograde blueschists exposed in the Lento unit (Alpine Corsica) register deformation during exhumation from blueschist to greenschist facies. Microstructural analyses indicate that bulk rheology during exhumation was governed by frictional-viscous flow, with the frictional component being accommodated by cataclasis of actinolite and epidote and frictional sliding along chlorite foliae, while the viscous component was accommodated by pressure solution of chlorite, albite, actinolite, and epidote, with minor dislocation creep of actinolite. Fluid-driven retrograde reactions accompany exhumation from blueschist to greenschist facies and lead to reaction and textural weakening, thus reducing rock strength. These mechanisms are important in limiting the stress magnitudes in retrograde shear zones, likely favouring fault creep in subduction zones if fluids are available to drive retrograde reactions.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Date of First Compliant Deposit:	16 August 2024
Last Modified:	16 Aug 2024 15:05
URI:	https://orca.cardiff.ac.uk/id/eprint/171437

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