Water loss and the origin of thick ultramylonites

Finch, Melanie A., Weinberg, Roberto F., and Hunter, Nicholas J.R. (2016) Water loss and the origin of thick ultramylonites. Geology, 44 (8). pp. 599-602.

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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.

Item ID: 73539
Item Type: Article (Research - C1)
ISSN: 1943-2682
Keywords: shear zone; ultramylonite; fluid migration
Copyright Information: © 2016 Geological Society of America.
Date Deposited: 12 May 2022 02:25
FoR Codes: 37 EARTH SCIENCES > 3705 Geology > 370511 Structural geology and tectonics @ 100%
SEO Codes: 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 100%
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