Deformation processes at the down-dip limit of the seismogenic zone: the example of Shimanto belt
Résumé
The metamorphic terranes of Shimanto belt (southwestern Japan) have been recognized as a fossil accretionary
prism and offer a good opportunity to study the deep part of the seismogenic zone in subduction contexts. The
Hyuga and Makimine units have been strongly deformed for temperature conditions of 250-280 and 300C
(Mukoyoshi et al., Island Arc 2009), respectively, providing a lower limit in temperature to the seismogenic portion
of the plate interface. In both units, the ductile fabrics consist principally in the formation of a phyllosilicate-rich
foliation and the deformation of pre-existing quartz veins. The deformation mechanisms in the quartz from the two
units are nevertheless very contrasted: in the lower temperature Hyuga unit, quartz deformation results principally
from micro-fracturing and dissolution precipitation, with very limited plastic deformation and dynamic recrystallization.
In the higher temperature Makimine unit, most of the quartz domains are dynamically recrystallized. Both
deformation processes are associated with a strong crystallographic preferred orientation: the lower-temperature
processes result in c-axes being parallel to the elongation direction, while the higher-temperature processes result
in c-axes being perpendicular to foliation plane. Dissolution-precipitation of quartz constitutes therefore a transitional
process, for a temperature between 250 and 300C, between the low-temperature, brittle portion and the
high-temperature, plastic portion of the plate interface. We finally derive estimates of the shear stresses associated
with this transitional domain of the plate interface.