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, Sample assembly of the shear experiments using a Griggs-type apparatus. a) Schematic cross-1051 section of the assembly in a new generation Griggs rig at the University of Orléans (France). b), p.1052

, Sample powder is inserted between 45° pre-cut 1053 alumina forcing blocks. Modified after Tarantola et al., 2010. c) Sample geometry at the end of the 1054 experiment: th 0 = initial thickness

, Set of terms used to describe the 1057 mechanical data. b) Stress-strain curves showing the mechanical behaviour of two pure plagioclase 1058 samples deformed at 800 and 900 °C. The mechanical curve of a pure pyroxene sample deformed at 1059 900 °C with an initial coarser grain size (< 40 µm) is also plotted. c-d) Set of experiments deforming 1060 an Opx + Plag mixture at different temperatures (800, 850 and 900°C), and at constant confining 1061 pressure, strain rate and water content. These experiments are separated in two graphs according to the 1062 different duration of the "run-in" section, Differential stress (MPa) versus shear strain (?) plots. a)

, SEM-BSE images representative of microstructures documented in pure plagioclase (a) and manually (b), showing the overall strain gradient in the shear zone from top to bottom. This transect is 1072 characterized by a heterogeneity in the amount of reaction and deformation, p.1073

, cpx = clinopyroxene, pl = plagioclase, amph = amphibole, alumina FB = 1074 alumina forcing block

. Fig, SEM-BSE images representative of microstructures observed in the high-shear-strain 1077 experiments. a-b) The reaction products appear as coronas around the original Opx 1 and as aggregates 1078 in low-strain zones. Pyroxene is locally fractured and filled with reaction products, vol.5

, The transition zone shows the development of fine-grained tails and shear bands extending at the 1080 edges of Opx 1 . These tails form fine-grained mixed zones rich in Opx 2 , Pl 2 and Amph. e-f) High-strain 1081 zones textures are made of intercalated fine-grained polyphase shear bands. In these zones, an 1082 extensive phase mixing between Opx 2 , Pl 2 and Amph is observed. The original Pl 1 almost completely 1083 disappears, while some Opx 1 clasts, which are reduced in size, remains within these fine-grained 1084 mixed zones. opx = orthopyroxene, cpx = clinopyroxene

, Three 1088 samples deformed to high shear strain are documented: one deformed at 850°C (a-b) and two others 1089 deformed at 900°C (c-d-e), Fig. 6. SEM-BSE images representative of microstructures observed in the high-strain zones