 |
| Titel |
The influence of strain localisation on spine extrusion dynamics during the 1991-1995 eruption at Unzen volcano (Japan) |
| VerfasserIn |
Paul A. Wallace, Jackie E. Kendrick, James D. Ashworth, Rebecca Coats, Oliver Lamb, Takahiro Miwa, Elisabetta Mariani, Yan Lavallée |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250152424
|
| Publikation (Nr.) |
 EGU/EGU2017-17258.pdf |
|
|
|
|
|
| Zusammenfassung |
| The eruption of highly viscous magma at lava domes is commonly followed by the extrusion
of a spine, a typical manifestation of waning activity marking the cessation of a prolonged
eruptive episode. Exogenous dome growth can lead to catastrophic consequences on overall
dome stability and thus its understanding is central to the mitigation of associated risks. Spine
extrusion during the final stages of the 1991-95 lava dome eruption at Unzen volcano (Japan)
has provided a unique opportunity to investigate the contribution of the different deformation
textures and varying petrological phenomena associated with magma ascent. The
protraction of this dense magmatic plug formed a 6 m wide shear zone consisting of
four structurally discrete units including gouge (1), a highly sheared zone (2) to a
moderately sheared zone (3) and an undeformed magmatic core (4). We present the first
systematic study of the microstructures, mineralogy, crystal stability, geochemistry
and crystal size distribution across this shear zone. The data is complimented by
laboratory permeability measurements and seismic observations over the extrusion
period. The crystal-rich (∼70 vol.%) dacitic spine preserves an abundant record of
both brittle and ductile deformation. Gouge material is dominated by cataclastic
microstructures with comminution of crystals and formation of conjugate microfractures, a
result of loss of cohesion due to friction along the fault. A decrease in crystal size
and circularity with increasing shear indicates brittle processes dominate near the
spine margins. The porous network varies significantly with pore size decreasing
with increasing shear, a result of shear-enhanced compaction, consistent with a
decrease in permeability of one order of magnitude. The interaction of crystals (via
strain-partitioning) further facilitates in localising stress and strain along the conduit margin
leading to crystal-plastic deformation. Electron backscatter diffraction (EBSD)
enables the quantification of the degree of plasticity across this shear zone. Results
show significant strain localisation in the gouge and high shear zones as evidenced
by an increased degree of lattice misorientation, with the majority of strain being
accommodated in the groundmass plagioclase microlites and weak phenocryst phases
(i.e. biotite). Crystal-plastic behaviour may thus be used to unravel a history of
deformation in the magma, acting as a strain marker for the viscous-brittle transition
during ascent. Mineral destabilisation is evident across the entire shear zone, with
amphibole breakdown rims suggesting disequilibrium conditions. Textural variations in
reaction rims exist with granular rims being present only in the gouge, whereas
symplectitic rims situate in the sheared and bulk magma. These observations suggest
different breakdown triggering processes, with granular rims being specific to gouge
processes perhaps due to the increased effects of gas and fluid flow. These deformation
microstructures and associated processes that occur in the shallow conduit have important
rheological implications that can lead to permeability anisotropy, significantly altering
the degassing efficiency and therefore the style of activity we see at the surface. |
|
|
|
|
|
|