  We study $C^1$-robustly transitive and nonhyperbolic diffeomorphisms having a
partially hyperbolic splitting with one-dimensional central bundle whose strong
un-/stable foliations are both minimal. {In dimension $3$, an important class
of examples of such systems is given by those with a simple closed periodic
curve tangent to the central bundle.} We prove that there is a $C^1$-open and
dense subset of such diffeomorphisms such that every nonhyperbolic ergodic
measure (i.e. with zero central exponent) can be approximated in the weak$\ast$
topology and in entropy by measures supported in basic sets with positive
(negative) central Lyapunov exponent. Our method also allows to show how
entropy changes across measures with central Lyapunov exponent close to zero.
We also prove that any nonhyperbolic ergodic measure is in the intersection of
the convex hulls of the measures with positive central exponent and with
negative central exponent.
