FTIR Maps and Spatial Distributions of OH in Caledonide and Himalayan Shear Zones: Implications for Dislocation Creep and Water Weakening

Abstract:

FTIR measurements of quartz mylonites from the Moine Thrust of the NW Scotland Caledonides and the Main Central Thrust of the Himalaya reveal spatial variations in OH absorptions that correspond to deformation temperature, tectonic level within the thrust, intragranular shear, and recrystallization. Infrared spectra measured with apertures of 10-100 µm exhibit OH absorptions characteristic of hydrogen point defects, fine fluid inclusions and micas within quartz grain interiors, and hydrous phases at grain boundaries.

Moine Thrust quartzites, deformed at greenschist conditions (T ~ 350°C) by Regime II, BLG/SGR dislocation creep, have pervasive broad OH absorptions due to fine fluid inclusions and sharp OH bands that appear locally due to finely dispersed micas. Mean OH contents vary from 2250 (+/-1500) ppm (10⁶ OH/Si) in mylonitic quartzites 2.5 m below the thrust surface to 4080 (+/-1400) ppm in weakly deformed Cambrian quartzites 70 m below the thrust. Spatial variations in OH due to fine fluid inclusions are complex, with water contents that differ significantly between neighboring grains and correlate with grain strain and recrystallization. OH absorption bands mapped at a resolution of 10 µm vary from 280 to 9000 ppm, well above and below concentrations known to weaken quartz.

Himalayan mylonites in the hanging wall of the Main Central Thrust of the Sutlej region were deformed at upper amphibolite conditions (T ~ 600°C) by Regime III, GBM dislocation creep. Given their extensive deformation and dislocation creep microstructures, quartz grains of these rocks are remarkably dry. Quartz grain spectra are dominated by sharp OH bands due to hydrogen interstitials with OH contents from 350 (+/-250) ppm (T = 600°C) to 119 (+/-62) ppm (T = 640 - 740°C).  These results offer challenges to the application of experimental flow laws for water-weakened quartz to the high-grade dislocation creep of Himalayan mylonites. Much lower water contents may be sufficient for water weakening at upper amphibolite conditions and natural strain rates than are needed at experimental strain rates. Alternatively, hydrogen defects that enhance dislocation motion at these conditions may be sourced from grain boundaries or micas, diffusing over longer distances than are possible at greenschist conditions or laboratory rates.