Deformation processes at the down-dip limit of the seismogenic zone: theexample of Shimanto belt

International audienceThe metamorphic terranes of Shimanto belt (southwestern Japan) have been recognized as a fossil accretionaryprism and offer a good opportunity to study the deep part of the seismogenic zone in subduction contexts. TheHyuga 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 portionof the plate interface. In both units, the ductile fabrics consist principally in the formation of a phyllosilicate-richfoliation and the deformation of pre-existing quartz veins. The deformation mechanisms in the quartz from the twounits are nevertheless very contrasted: in the lower temperature Hyuga unit, quartz deformation results principallyfrom 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. Bothdeformation processes are associated with a strong crystallographic preferred orientation: the lower-temperatureprocesses result in c-axes being parallel to the elongation direction, while the higher-temperature processes resultin c-axes being perpendicular to foliation plane. Dissolution-precipitation of quartz constitutes therefore a transitionalprocess, for a temperature between 250 and 300C, between the low-temperature, brittle portion and thehigh-temperature, plastic portion of the plate interface. We finally derive estimates of the shear stresses associatedwith this transitional domain of the plate interface