The dominant mode of deformation for anorthosite deformed experimentally at temperatures from 400 °C to 1000 °C and confining pressures from 5 to 17 kbar (5 × 105–17 × 105 kPa) at a constant strain rate of 8 × 10−5 s−1 is cataclasis, although plastic flow occurs abundantly at temperatures of 800 °C or above. The amount of plastic flow increases rapidly as temperature is increased. An anorthosite sample (AW14) containing abundant alteration prior to testing shows a further increase in alteration during testing, and is weak at temperatures of 800 °C or greater compared to an unaltered anorthosite sample (TL3), although their strength is similar below 800 °C. Some plagioclase in sample TL3 converted from the original low or low transitional structural state to a high or high transitional state during testing at 800 °C or above. Albite and pericline twins and deformation lamellae produced during tests concentrate in the regions of cores showing structural conversion, and are most common at temperatures of 800 °C or greater. The distribution of both alteration and structurally converted plagioclase produced during testing appears to be related to temperature gradients within cores.