The plasticity of the dense hydrous magnesium silicate (DHMS) phase A, a key hydrous mineral within cold subduction zones, was investigated by two complementary approaches: high-pressure deformation experiments and computational methods. The deformation experiments were carried out at 11 GPa, 400 and 580 degrees C, with in situ measurements of stress, strain and lattice preferred orientations (LPO). Based on viscoplastic self-consistent modeling (VPSC) of the observed LPO, the deformation mechanisms at 580 degrees C are consistent with glide on the (00 01) basal and (01 1 0) prismatic planes. At 400 degrees C the deformation mechanisms involve glide on (211 0) prismatic, (0001) basal and {1 1 21} pyramidal planes. Both give flow stresses of 2.5-3 GPa at strain rates of 2-4 x 10(-5) s(-1\). We use the Peierls-Nabarro-Galerkin (PNG) approach, relying on first-principles calculations of generalized stacking fault (gamma-surface), and model the core structure of potential dislocations in basal and prismatic planes. The computations show multiple dissociations of the 1/3 [2 (1) over bar(1) over bar0] and [01 (1) over bar0] dislocations (< a >) and < b > dislocations) in the basal plane, which is compatible with the ubiquity of basal slip in the experiments. The gamma-surface calculations also suggest 1/3 [2 (1) over bar(1) over bar3] and [0 (1) over bar 11] dislocations (< a + c > or < c - b > directions) in prismatic and pyramidal planes, which is also consistent with the experimental data. Phase A has a higher flow strength than olivine. When forming at depths from the dehydration of weak and highly anisotropic hydrated ultramafic rocks, phase A may not maintain the mechanical softening antigorite can provide. The seismic properties calculated for moderately deformed aggregates suggest that S-wave seismic anisotropy of phase A-bearing rocks is lower than hydrous subduction zone lithologies such as serpentinites and blueschists. (C) 2015 The Authors. Published by Elsevier B.V.