 |
| Titel |
Brittle–viscous deformation of vein quartz under fluid-rich lower greenschist facies conditions |
| VerfasserIn |
H. J. Kjøll, G. Viola, L. Menegon, B. E. Sørensen |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1869-9510
|
| Digitales Dokument |
URL |
| Erschienen |
In: Solid Earth ; 6, no. 2 ; Nr. 6, no. 2 (2015-06-11), S.681-699 |
| Datensatznummer |
250115459
|
| Publikation (Nr.) |
 copernicus.org/se-6-681-2015.pdf |
|
|
|
|
|
| Zusammenfassung |
| We studied by Electron BackScatter Diffraction (EBSD) and optical microscopy a coarse-grained (ca. 0.5–6 mm)
quartz vein embedded in a phyllonitic matrix to gain insights into the
recrystallization mechanisms and the processes of strain localization in
quartz deformed under lower greenschist facies conditions, broadly
coincident with the brittle–viscous transition. The vein deformed during
faulting along a phyllonitic thrust of Caledonian age within the Porsa
Imbricate Stack in the Paleoproterozoic Repparfjord Tectonic Window in
northern Norway. The phyllonite hosting the vein formed at the expense of a
metabasaltic protolith through feldspar breakdown to form interconnected
layers of fine, synkinematic phyllosilicates. In the mechanically weak
framework of the phyllonite, the quartz vein acted as a relatively rigid
body. Viscous deformation in the vein was initially accommodated by quartz
basal <a> slip. Under the prevailing deformation
conditions, however, dislocation glide- and possibly creep-accommodated
deformation of quartz was inefficient, and this resulted in localized strain
hardening. In response to the (1) hardening, (2) progressive and cyclic
increase of the fluid pressure, and (3) increasing competence contrast
between the vein and the weakly foliated host phyllonite, vein quartz
crystals began to deform by brittle processes along specific, suitably
oriented lattice planes, creating microgouges along microfractures.
Nucleated new grains rapidly sealed these fractures as fluids penetrated the
actively deforming system. The grains grew initially by
solution precipitation and later by grain boundary migration. We suggest
that the different initial orientation of the vein crystals led to strain
accommodation by different mechanisms in the individual crystals, generating
remarkably different microstructures. Crystals suitably oriented for basal
slip, for example, accommodated strain mainly viscously and experienced only
minor fracturing. Instead, crystals misoriented for basal slip hardened and
deformed predominantly by domainal fracturing. This study indicates the
importance of considering shear zones as dynamic systems wherein the
activated deformation mechanisms may vary through time in response to the
complex temporal and spatial evolution of the shear zone, often in a cyclic
fashion. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|