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Weakening Mechanisms in a Basalt‐Hosted Subduction Megathrust Fault Segment, Southern Alaska

Basaltic and gabbroic rocks that define the seafloor have been suggested to act as sources of rheological heterogeneity during subduction, with the capacity to enhance or dampen seismicity. Despite this, relatively little is known from the rock record regarding the progression and conditions of mafi...

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Autores principales: Braden, Zoe, Behr, Whitney M.
Formato: Online Artículo Texto
Lenguaje:English
Publicado: John Wiley and Sons Inc. 2021
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9285822/
https://www.ncbi.nlm.nih.gov/pubmed/35865263
http://dx.doi.org/10.1029/2021JB022039
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author Braden, Zoe
Behr, Whitney M.
author_facet Braden, Zoe
Behr, Whitney M.
author_sort Braden, Zoe
collection PubMed
description Basaltic and gabbroic rocks that define the seafloor have been suggested to act as sources of rheological heterogeneity during subduction, with the capacity to enhance or dampen seismicity. Despite this, relatively little is known from the rock record regarding the progression and conditions of mafic oceanic crust deformation during subduction, particularly in the shallow megathrust region of the seismogenic zone. We describe subduction‐related deformation structures and characterize deformation conditions from an exhumed, basalt‐hosted megathrust in the Chugach accretionary complex of south‐central Alaska. Rocks in the Chugach preserve a record of seafloor mineralogical changes from pre‐subduction, hydrothermal circulation that produced sheet silicates with a lower frictional strength than intact basalt. Pre‐subduction alteration also served to introduce hydrous phases that can expel water during deformation and raise the pore fluid pressure. Once strain localized within basalts onto a megathrust fault plane at lithostatic pore fluid pressures, the basalt weakened further through a combination of cataclasis, dilatational shear fracturing, and slip on chlorite‐rich shear bands. This process occurred in a narrower fault zone, and at higher maximum differential stress and greater pore fluid pressure fluctuations than recorded in some sediment‐hosted megathrusts at similar pressure and temperature conditions. Our data indicate that when the lower plate contains basalt bathymetric features, basalt dismembers during subduction into a chlorite‐rich fault gouge that surrounds lenses or slices of intact, less‐altered basalt.
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spelling pubmed-92858222022-07-19 Weakening Mechanisms in a Basalt‐Hosted Subduction Megathrust Fault Segment, Southern Alaska Braden, Zoe Behr, Whitney M. J Geophys Res Solid Earth Research Article Basaltic and gabbroic rocks that define the seafloor have been suggested to act as sources of rheological heterogeneity during subduction, with the capacity to enhance or dampen seismicity. Despite this, relatively little is known from the rock record regarding the progression and conditions of mafic oceanic crust deformation during subduction, particularly in the shallow megathrust region of the seismogenic zone. We describe subduction‐related deformation structures and characterize deformation conditions from an exhumed, basalt‐hosted megathrust in the Chugach accretionary complex of south‐central Alaska. Rocks in the Chugach preserve a record of seafloor mineralogical changes from pre‐subduction, hydrothermal circulation that produced sheet silicates with a lower frictional strength than intact basalt. Pre‐subduction alteration also served to introduce hydrous phases that can expel water during deformation and raise the pore fluid pressure. Once strain localized within basalts onto a megathrust fault plane at lithostatic pore fluid pressures, the basalt weakened further through a combination of cataclasis, dilatational shear fracturing, and slip on chlorite‐rich shear bands. This process occurred in a narrower fault zone, and at higher maximum differential stress and greater pore fluid pressure fluctuations than recorded in some sediment‐hosted megathrusts at similar pressure and temperature conditions. Our data indicate that when the lower plate contains basalt bathymetric features, basalt dismembers during subduction into a chlorite‐rich fault gouge that surrounds lenses or slices of intact, less‐altered basalt. John Wiley and Sons Inc. 2021-09-22 2021-09 /pmc/articles/PMC9285822/ /pubmed/35865263 http://dx.doi.org/10.1029/2021JB022039 Text en © 2021 The Authors. https://creativecommons.org/licenses/by-nc/4.0/This is an open access article under the terms of the http://creativecommons.org/licenses/by-nc/4.0/ (https://creativecommons.org/licenses/by-nc/4.0/) License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.
spellingShingle Research Article
Braden, Zoe
Behr, Whitney M.
Weakening Mechanisms in a Basalt‐Hosted Subduction Megathrust Fault Segment, Southern Alaska
title Weakening Mechanisms in a Basalt‐Hosted Subduction Megathrust Fault Segment, Southern Alaska
title_full Weakening Mechanisms in a Basalt‐Hosted Subduction Megathrust Fault Segment, Southern Alaska
title_fullStr Weakening Mechanisms in a Basalt‐Hosted Subduction Megathrust Fault Segment, Southern Alaska
title_full_unstemmed Weakening Mechanisms in a Basalt‐Hosted Subduction Megathrust Fault Segment, Southern Alaska
title_short Weakening Mechanisms in a Basalt‐Hosted Subduction Megathrust Fault Segment, Southern Alaska
title_sort weakening mechanisms in a basalt‐hosted subduction megathrust fault segment, southern alaska
topic Research Article
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9285822/
https://www.ncbi.nlm.nih.gov/pubmed/35865263
http://dx.doi.org/10.1029/2021JB022039
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