Water loss and the origin of thick ultramylonites

Finch, Melanie; Weinberg, Roberto F.; Hunter, Nicholas J.R.

doi:10.1130/g37972.1

Cited by 41 publications

(48 citation statements)

References 31 publications

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“…With the introduction of efficient FTIR spectrometers and IR microscopes, studies of intragranular water and hydrogen defects in naturally deformed rocks have been enabled using apertures of 50-100 µm Nakashima et al, 1995;Gleason and DeSisto, 2008;Finch et al, 2016;Kilian et al, 2016) as well as FTIR mapping of OH contents (Seaman et al, 2013). Molecular water contents of naturally deformed quartz commonly have large grain-to-grain variations at this scale of observation, and it has not been possible to measure OH contents of fine, dynamically recrystallized grains without including OH bands associated with grain boundaries.…”

Section: Methodsmentioning

confidence: 99%

“…Fluid inclusions may be lost and intragranular water contents reduced during high temperature deformation by a variety of processes, ranging from pipe diffusion Jansen, 1990, 1994;Hollister, 1990;Cordier et al, 1994;Mavrogenes and Bodnar, 1994) to recrystallization and grain boundary sweeping (Faleiros et al, 2010;Seaman et al, 2013;Finch et al, 2016;Kilian et al, 2016), and partial melting (Seaman et al, 2013).…”

Section: Water Weakening In Nature?mentioning

confidence: 99%

“…With the advent of FTIR and IR microscopes, water and hydrogen defects in naturally deformed quartz have been reported with OH contents of 300 to > 10 000 ppm (molar H/10 6 Si; Nakashima et al, 1995;Gleason and DeSisto, 2008;Seaman et al, 2013;Finch et al, 2016;Kilian et al, 2016), comparable to those required for water weakening in laboratory experiments. In small granitic shear zones deformed at greenschist conditions, water contents of quartz grains appear to correlate with finite strain Gleason and DeSisto, 2008), and in the much larger Median Tectonic Line of Japan, intragranular water contents increase towards its center (Nakashima et al, 1995).…”

Section: Introductionmentioning

confidence: 99%

“…6. grain boundary diffusion (Faleiros et al, 2010;Seaman et al, 2013;Finch et al, 2016;Kilian et al, 2016).…”

Section: Wide Variations In Oh Contentmentioning

confidence: 99%

“…shear zone (Finch et al, 2016). Maps of OH content, constructed from FTIR spectra of deformed granitic rocks (Seaman et al, 2013), show compelling relationships between water content and microstructures generated during deformation, suggesting that water contents are reduced during recrystallization and partial melting.…”

Section: Introductionmentioning

confidence: 99%

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Synchrotron FTIR imaging of OH in quartz mylonites

et al. 2017

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Abstract. Previous measurements of water in deformed quartzites using conventional Fourier transform infrared spectroscopy (FTIR) instruments have shown that water contents of larger grains vary from one grain to another. However, the non-equilibrium variations in water content between neighboring grains and within quartz grains cannot be interrogated further without greater measurement resolution, nor can water contents be measured in finely recrystallized grains without including absorption bands due to fluid inclusions, films, and secondary minerals at grain boundaries.Synchrotron infrared (IR) radiation coupled to a FTIR spectrometer has allowed us to distinguish and measure OH bands due to fluid inclusions, hydrogen point defects, and secondary hydrous mineral inclusions through an aperture of 10 µm for specimens > 40 µm thick. Doubly polished infrared (IR) plates can be prepared with thicknesses down to 4-8 µm, but measurement of small OH bands is currently limited by strong interference fringes for samples < 25 µm thick, precluding measurements of water within individual, finely recrystallized grains. By translating specimens under the 10 µm IR beam by steps of 10 to 50 µm, using a software-controlled x − y stage, spectra have been collected over specimen areas of nearly 4.5 mm 2 . This technique allowed us to separate and quantify broad OH bands due to fluid inclusions in quartz and OH bands due to micas and map their distributions in quartzites from the Moine Thrust (Scotland) and Main Central Thrust (Himalayas).Mylonitic quartzites deformed under greenschist facies conditions in the footwall to the Moine Thrust (MT) exhibit a large and variable 3400 cm −1 OH absorption band due to molecular water, and maps of water content corresponding to fluid inclusions show that inclusion densities correlate with deformation and recrystallization microstructures. Quartz grains of mylonitic orthogneisses and paragneisses deformed under amphibolite conditions in the hanging wall to the Main Central Thrust (MCT) exhibit smaller broad OH bands, and spectra are dominated by sharp bands at 3595 to 3379 cm −1 due to hydrogen point defects that appear to have uniform, equilibrium concentrations in the driest samples. The broad OH band at 3400 cm −1 in these rocks is much less common. The variable water concentrations of MT quartzites and lack of detectable water in highly sheared MCT mylonites challenge our understanding of quartz rheology. However, where water absorption bands can be detected and compared with deformation microstructures, OH concentration maps provide information on the histories of deformation and recovery, evidence for the introduction and loss of fluid inclusions, and water weakening processes.

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Section: Methodsmentioning

confidence: 99%

Section: Water Weakening In Nature?mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…6. grain boundary diffusion (Faleiros et al, 2010;Seaman et al, 2013;Finch et al, 2016;Kilian et al, 2016).…”

Section: Wide Variations In Oh Contentmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Synchrotron FTIR imaging of OH in quartz mylonites

et al. 2017

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A Comparison of Geodetic and Geologic Rates Prior to Large Strike‐Slip Earthquakes: A Diversity of Earthquake‐Cycle Behaviors?

Dolan

Meade

2017

Geochem Geophys Geosyst

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Comparison of preevent geodetic and geologic rates in three large‐magnitude (Mw = 7.6–7.9) strike‐slip earthquakes reveals a wide range of behaviors. Specifically, geodetic rates of 26–28 mm/yr for the North Anatolian fault along the 1999 MW = 7.6 Izmit rupture are ∼40% faster than Holocene geologic rates. In contrast, geodetic rates of ∼6–8 mm/yr along the Denali fault prior to the 2002 MW = 7.9 Denali earthquake are only approximately half as fast as the latest Pleistocene‐Holocene geologic rate of ∼12 mm/yr. In the third example where a sufficiently long pre‐earthquake geodetic time series exists, the geodetic and geologic rates along the 2001 MW = 7.8 Kokoxili rupture on the Kunlun fault are approximately equal at ∼11 mm/yr. These results are not readily explicable with extant earthquake‐cycle modeling, suggesting that they may instead be due to some combination of regional kinematic fault interactions, temporal variations in the strength of lithospheric‐scale shear zones, and/or variations in local relative plate motion rate. Whatever the exact causes of these variable behaviors, these observations indicate that either the ratio of geodetic to geologic rates before an earthquake may not be diagnostic of the time to the next earthquake, as predicted by many rheologically based geodynamic models of earthquake‐cycle behavior, or different behaviors characterize different fault systems in a manner that is not yet understood or predictable.

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