Veins occupying tensile fractures are a common feature of metamorphic rocks of all grades. It can be shown in many cases that such veins are synmetamorphic and that they underwent repeated cycles of fracturing and healing throughout deformation and foliation development. Theoretical failure models have predicted that tensile failure is limited to differential stresses less than four times the tensile strength of the material (σ1−σ3 ⩽ 4T), and this condition can therefore be used to place an upper bound on differential stress intensities during deformation and foliation development where they are concurrent with vein formation. The tensile strengths of rocks are generally less than 10 MPa, and in the presence of a high temperature metamorphic fluid, a value of 5 MPa may be more reasonable, due to subcritical crack growth. It is thus concluded that differential stress intensities during crustal orogenesis will be less than 40 MPa, and they may be lower than 20 MPa.