  Momentum decoupling develops when forward scattering dominates the pairing
interaction and implies tendency for decorrelation between the physical
behavior in the various regions of the Fermi surface. In this regime it is
possible to obtain anisotropic s- or d-wave superconductivity even with
isotropic pairing scattering. We show that in the momentum decoupling regime
the distortion of the $CuO_2$ planes is enough to explain the experimental
reports for s- mixing in the dominantly d-wave gap of $YBa_2Cu_3O_7$. In the
case of spin fluctuations mediated pairing instead, a large part of the
condensate must be located in the chains in order to understand the
experiments.
