Geophysical observations have illuminated a spectrum of fault slip styles from continuous aseismic sliding to fast earthquake slip. We study exhumed intercalated lenses of oceanic crust and sedimentary rocks, deformed to high shear strains. Deformation was partitioned between fractured, rigid blocks, with lengths of tens to hundreds of meters, and surrounding metapelites characterized by interconnected phyllosilicate networks. Under inferred conditions of low effective stress at temperatures > 500°C, locally and transiently elevated shear strain rate in phyllosilicates deforming by dislocation creep can reach those needed for transient slow slip. Concurrently, increased matrix strain rate likely stimulates brittle failure in rigid lenses. The ubiquitous presence of quartz veins and microfractures within rigid material provides evidence for brittle deformation occurring coincident with viscous shearing flow. We suggest that geophysically observed tremor and slow slip may be a manifestation of strain partitioning, where deformation is accommodated viscously in a matrix enveloping rigid lenses.