  [Background] The fission mechanism of sub-lead nuclides remains unclear,
especially the types of fission modes involved and their corresponding shell
effects. [Purpose] The aim is to identify the different modes in the fission of
$^{187}$Ir, and investigate the corresponding mechanism. [Method] The
three-dimensional Langevin approach considering nucleus elongation,
deformation, and mass asymmetry is applied to simulate fission dynamics. The
macro-microscopic models are used to calculate the transport coefficients.
[Results] The fragment mass, deformation, and total kinetic energy (TKE) of
$^{187}$Ir fission at different excitation energies are calculated. Based on
the mass-TKE correlations, four fission modes are identified, namely two
asymmetric standard modes, a symmetric super-long mode, and a symmetric
liquid-drop mode. Strong excitation-energy resistance of two asymmetric modes
is found. The mass distributions show the dominance of single-peak shape, which
is in good agreement with experimental data. The fission potential energy
surface and the fission dynamics are analyzed to investigate the origins of the
modes and the competition between neutron and proton shell effects.
[Conclusions] Multiple fission modes are included in the $^{187}$Ir fission
behind the single-peak-like distribution of observables. The proton and neutron
magic numbers with different asymmetry parameter might heighten the sensitivity
to the uncertainties of shell corrections.
