  Area-preserving nontwist maps are used to describe a broad range of physical
systems. In those systems, the violation of the twist condition leads to
nontwist characteristic phenomena, such as reconnection-collision sequences and
shearless invariant curves that act as transport barriers in the phase space.
Although reported in numerical investigations, the shearless bifurcation, i.e.,
the emergence scenario of multiple shearless curves, is not well understood. In
this work, we derive an area-preserving map as a local approximation of a
particle transport model for confined plasmas. Multiple shearless curves are
found in this area-preserving map, with the same shearless bifurcation scenario
numerically observed in the original model. Due to its symmetry properties and
simple functional form, this map is proposed as a model to study shearless
bifurcations.
