Abstract Talc can strongly influence the rheological properties of the subduction interface. Despite geologic evidence for talc at the interface, and proposed links between its rheology and slow slip events, few experiments have been conducted on talc at relevant conditions. We conducted constant‐load shear deformation experiments on talc aggregates at 1 GPa confining pressure, 420–700°C, and <150 MPa shear stress, and derived a low temperature plasticity flow law at lithospheric conditions. We show that tectonic displacement rates can be accommodated on talc‐rich shear zones with effective viscosities as low as 1013 ${10}^{13}$ Pa·s, and that slow slip displacement rates can be accommodated by a meters‐thick talc‐dominated shear zone, with strain rates orders of magnitude greater than antigorite at the same conditions. These observations indicate that talc can significantly weaken the lithosphere, and influence the dynamics of subduction and other tectonic processes when it is present in abundance.

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