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| Titel |
Imaging Water in Deformed Quartzites: Examples from Caledonian and Himalayan Shear Zones |
| VerfasserIn |
Andreas Kronenberg, Kyle Ashley, Hasnor Hasnan, Caleb Holyoke, Lynna Jezek, Richard Law, Jay Thomas |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250122259
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| Publikation (Nr.) |
 EGU/EGU2016-1247.pdf |
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| Zusammenfassung |
| Infrared IR measurements of OH absorption bands due to water in deformed quartz grains
have been collected from major shear zones of the Caledonian and Himalayan orogens. Mean
intragranular water contents were determined from the magnitude of the broad OH absorption
at 3400 cm−1 as a function of structural position, averaging over multiple grains, using an IR
microscope coupled to a conventional FTIR spectrometer with apertures of 50-100 μm.
Images of water content were generated by scanning areas of up to 4 mm2 of individual
specimens with a 10 μm synchrotron-generated IR beam and contouring OH absorptions.
Water contents vary with structural level relative to the central cores of shear zones
and they vary at the grain scale corresponding to deformation and recrystallization
microstructures.
Gradients in quartz water content expressed over structural distances of 10 to 400 m from
the centers of the Moine Thrust (Stack of Glencoul, NW Scotland), the Main Central Thrust
(Sutlej valley of NW India), and the South Tibetan Detachment System (Rongbuk valley
north of Mount Everest) indicate that these shear zones functioned as fluid conduits.
However, the gradients differ substantially: in some cases, enhanced fluid fluxes appear to
have increased quartz water contents, while in others, they served to decrease water
contents. Water contents of Moine thrust quartzites appear to have been reduced during
shear at greenschist facies by processes of regime II BLG/SGR dislocation creep.
Intragranular water contents of the protolith 70 m below the central fault core are large
(4078 ± 247 ppm, H/106 Si) while mylonites within 5 mm of the Moine hanging
wall rocks have water contents of only 1570 (± 229) ppm. Water contents between
these extremes vary systematically with structural level and correlate inversely with
the extent of dynamic recrystallization (20 to 100%). Quartz intragranular water
contents of Himalayan thrust and low-angle detachment zones sheared at upper
amphibolite conditions by regime III GBM creep show varying trends with structural
level. Water contents increase toward the Lhotse detachment of the Rongbuk valley,
reaching 11,350 (± 1095) ppm, whereas they decrease toward the Main Central
Thrust exposed in the western part of the Sutlej valley to values as low as 170 (± 25)
ppm.
Maps of intragranular water content correspond to populations of fluid inclusions, which
depend on the history of deformation and dynamic recrystallization. Increases in water
content require the introduction of secondary fluid inclusions, generally by brittle
microcracking followed by crack healing and processes of inclusion redistribution
documented in milky quartz experiments. Decreases in water content result from dynamic
recrystallization, as mobile grain boundaries sweep through wet porphyroclasts, leaving
behind dry recrystallized grains.
Intragranular water contents throughout greenschist mylonites of the Moine thrust are
comparable to those of quartz weakened by water in laboratory experiments. However, water
contents of upper amphibolite mylonites of the Main Central Thrust are far below those
required for water weakening at experimental strain rates and offer challenges to our
understanding of quartz rheology. |
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