  Microscopic effects in fragment mass distributions in fusion-fission
reactions of light projectiles (C, O and F) on deformed thorium and spherical
bismuth targets in near and below Coulomb barrier energies are investigated.
Precisely measured mass distribution shows a sudden anomalous increase in
variances of mass distributions ($\sigma_{m}^2$) near Coulomb barrier energies
for all three projectiles with deformed thorium target, in contrast to a smooth
variation of $\sigma_{m}^2$ with energy for spherical bismuth target.
Macroscopic effects of change in mass flow or prolonged mass equilibration time
can not explain the observed variation in $\sigma_{m}^2$ with energy.
Microscopic effects due to change in entrance channel shape compactness for
projectiles hitting the polar region of prolate thorium target is postulated to
reach a almost symmetric saddle without complete fusion for events of anomalous
fragment widths. Quantitative estimates of mass widths mixed for the two
processes explains the observed variation of $\sigma_{m}^2$ with excitation
energy.
