Polyphase ultramylonite development during shear zone evolution

by Alexander Lusk and John P. Platt
We present a mechanism for ultramylonite development in polyphase rocks. Main points as follows:

(1) Quartz CPOs record consistent transitions with changing temperature. Transitions are likely caused by changes in the active slip system(s), which we attribute to (a) a decrease in grain boundary mobility (and quartz recrystalliza- tion mechanism) due to lower temperature, and (b) finite strain.
(2) Quartz CPOs in the hanging wall of the Moine Thrust show evidence for inheritance of grain orientations from deeper regions, indicating incomplete transitions to the easy slip system at shallower levels.
(3) We postulate that transitions in easy slip system may transiently strengthen quartz aggregates, causing strain to become increasingly parti- tioned into phyllosilicate and polyphase layers. This partitioning could trigger the development of fine-grained polyphase interconnected weak networks, which likely deform by grain size sensitive mechanisms including diffusion- accommodated grain boundary sliding.

Cite as:

Lusk, A., & Platt, J. P.. (2020, July 21). LUSK_CIG_2020.pdf (Version 1). Tectonics Community Science Workshop 2020.

author = "Alexander Lusk and John P. Platt",
title = "{LUSK_CIG_2020.pdf (Version 1)}",
year = "2020",
month = "7",
url = "",
doi =}



Posted by Greg Hirth on Jul 28th, 2020

In reference to the Ti thermometry. Our results with Will N. indicate that the sub-grain boundaries were migrating, which is likely an underappreciated process.




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