Water loss and the origin of thick ultramylonites

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Abstract

Hydrolytic weakening has been suggested as a major process facilitating strain localization, in line with many studies that found a positive correlation between water content and intensity of deformation. We examine the role of water in the unusually thick ultramylonite of the El Pichao shear zone, northwestern Argentina. We used Fourier transform infrared spectroscopy to measure water content in quartz and feldspar, comparing ultramylonitic rocks to mylonites and weakly deformed rocks. Quartz and feldspar in ultramylonites contained half the water of weakly deformed rocks, contrary to findings in previous studies. We propose that the kilometer-thick ultramylonite formed in three stages: (1) localized deformation and recrystallization caused release of intracrystalline water to grain boundaries, which promoted grain-boundary sliding, forming the ultramylonite; (2) high pressure in the shear zone continuously expelled intercrystalline water to the surroundings, drying the boundaries and leading to strain hardening; and (3) water migrated to less deformed rocks causing hydrolytic weakening, repeating the cycle and widening the ultramylonite.

Original languageEnglish
Pages (from-to)599-602
Number of pages4
JournalGeology
Volume44
Issue number8
DOIs
Publication statusPublished - 2016

Cite this

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title = "Water loss and the origin of thick ultramylonites",
abstract = "Hydrolytic weakening has been suggested as a major process facilitating strain localization, in line with many studies that found a positive correlation between water content and intensity of deformation. We examine the role of water in the unusually thick ultramylonite of the El Pichao shear zone, northwestern Argentina. We used Fourier transform infrared spectroscopy to measure water content in quartz and feldspar, comparing ultramylonitic rocks to mylonites and weakly deformed rocks. Quartz and feldspar in ultramylonites contained half the water of weakly deformed rocks, contrary to findings in previous studies. We propose that the kilometer-thick ultramylonite formed in three stages: (1) localized deformation and recrystallization caused release of intracrystalline water to grain boundaries, which promoted grain-boundary sliding, forming the ultramylonite; (2) high pressure in the shear zone continuously expelled intercrystalline water to the surroundings, drying the boundaries and leading to strain hardening; and (3) water migrated to less deformed rocks causing hydrolytic weakening, repeating the cycle and widening the ultramylonite.",
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Water loss and the origin of thick ultramylonites. / Finch, Melanie; Weinberg, Roberto; Hunter, Nikk.

In: Geology, Vol. 44, No. 8, 2016, p. 599-602.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Water loss and the origin of thick ultramylonites

AU - Finch, Melanie

AU - Weinberg, Roberto

AU - Hunter, Nikk

PY - 2016

Y1 - 2016

N2 - Hydrolytic weakening has been suggested as a major process facilitating strain localization, in line with many studies that found a positive correlation between water content and intensity of deformation. We examine the role of water in the unusually thick ultramylonite of the El Pichao shear zone, northwestern Argentina. We used Fourier transform infrared spectroscopy to measure water content in quartz and feldspar, comparing ultramylonitic rocks to mylonites and weakly deformed rocks. Quartz and feldspar in ultramylonites contained half the water of weakly deformed rocks, contrary to findings in previous studies. We propose that the kilometer-thick ultramylonite formed in three stages: (1) localized deformation and recrystallization caused release of intracrystalline water to grain boundaries, which promoted grain-boundary sliding, forming the ultramylonite; (2) high pressure in the shear zone continuously expelled intercrystalline water to the surroundings, drying the boundaries and leading to strain hardening; and (3) water migrated to less deformed rocks causing hydrolytic weakening, repeating the cycle and widening the ultramylonite.

AB - Hydrolytic weakening has been suggested as a major process facilitating strain localization, in line with many studies that found a positive correlation between water content and intensity of deformation. We examine the role of water in the unusually thick ultramylonite of the El Pichao shear zone, northwestern Argentina. We used Fourier transform infrared spectroscopy to measure water content in quartz and feldspar, comparing ultramylonitic rocks to mylonites and weakly deformed rocks. Quartz and feldspar in ultramylonites contained half the water of weakly deformed rocks, contrary to findings in previous studies. We propose that the kilometer-thick ultramylonite formed in three stages: (1) localized deformation and recrystallization caused release of intracrystalline water to grain boundaries, which promoted grain-boundary sliding, forming the ultramylonite; (2) high pressure in the shear zone continuously expelled intercrystalline water to the surroundings, drying the boundaries and leading to strain hardening; and (3) water migrated to less deformed rocks causing hydrolytic weakening, repeating the cycle and widening the ultramylonite.

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