 |
| Titel |
Strain localization in pseudotachylyte veins at lower crustal conditions |
| VerfasserIn |
Luca Menegon, Giorgio Pennacchioni, Nadia Malaspina |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143513
|
| Publikation (Nr.) |
 EGU/EGU2017-7240.pdf |
|
|
|
|
|
| Zusammenfassung |
| Viscous shearing in the dry and strong lower crust often localizes in pseudotachylyte veins
(i.e. quenched molten rocks formed by the frictional heat released during seismic slip),
and it has been suggested that brittle (coseismic) grain-size reduction and fluid
infiltration in the fractured domains are necessary to weaken the anhydrous granulitic
lower crust. However, the deformation mechanisms responsible for the associated
strain weakening and viscous shear localization in pseudotachylytes are yet to be
explored.
This study investigates the deformation microstructures of mylonitized pseudotachylytes
in anorthosites from Nusfjord, northern Norway, where ductile shear zones invariably
nucleate in pseudotachylyte veins. Thus, pseudotachylytes are weaker than the host rock
during superposed ductile deformation.
Pristine pseudotachylytes contain microlites of plagioclase, clinopyroxene, amphibole
and orthopyroxene, flow structures, and chilled margins. Some pseudotachylytes have lost the
pristine microstructure and have recrystallized into a fine-grained (< 10 μm) mixture of
plagioclase, amphibole, clinopyroxene, biotite, quartz ± K-feldspar ± orthopyroxene. Thus,
the fine grain size in the mylonites (< 20 μm) is not the product of progressive
grain-size reduction with increasing strain, but is an initial characteristic of the shear
zone (pseudotachylyte) precursor. The stable mineral assemblage in the mylonitic
foliation consists of plagioclase, hornblende, clinopyroxene ± quartz ± biotite ±
orthoclase. Geothermobarometry and thermodynamic modelling indicate that pristine
pseudotachylytes and their mylonitized equivalents formed at ca. 700˚ C and 0.6-0.9 GPa.
Diffusion creep and grain boundary sliding were identified as the main deformation
mechanisms in the mylonite on the basis of the lack of crystallographic preferred
orientations, the high degree of phase mixing, and the nucleation of hornblende in dilatant
sites.
In contrast with common observations that fluid infiltration is required to trigger viscous
deformation, thermodynamic modelling indicates that a limited amount of fluid (0.4 wt%,
similar to the bulk fluid content measured in the host rock) is sufficient to stabilize
the mineral assemblage in the mylonite. This suggests that cosesimic grain size
reduction resulted in fluid redistribution into the fractured domains and not necessarily
in fluid infiltration. Recent experiments suggest that very small amount of water
(tens of ppm) are effective in facilitating mineral reactions if sufficient porosity in
present. Coseismic fracturing and creep cavitation in the mylonitized pseudotachylytes
enhance the porosity of the shear zone and result in nucleation of new phases in
dilatant sites. This process keeps the grain size of the polymineralic aggregate in the
grain-size sensitive creep field, thereby stabilizing strain localization in the mylonitized
pseudotachylytes.
This study highlights that pseudotachylytes caused by brittle faulting can be precursors of
viscous, weak shear zones in the dry lower crust, indicating lower crustal earthquakes as
agents of rheological change from strong, brittle lower crust, to strong lower crust with
embedded fine grained, weak viscous shear zones. |
|
|
|
|
|
|