We study the evolution of fission modes across the Th isotopic chain using a microscopic framework combining the time-dependent generator coordinate method and finite-temperature covariant density-functional theory. Theoretical fission-fragment charge distributions agree well with experiments, and reveal a rapid symmetric-to-asymmetric transition from $A=222$ to 234. By analyzing the collective potential-energy surfaces and time evolution of collective probability density distributions, we demonstrate that this fission mode transition is strongly correlated with the rapidly deepening asymmetric fission valley—a phenomenon driven by the reduction of deformation energies of both the heavy and light fragments formed in the asymmetric fission valley. Further analysis attributes the decrease of light-fragment deformation energies to the onset of a coexisting large-deformed minimum in neutron-rich Kr and Sr isotopes (the dominant isotopes for light asymmetric peak), which arises from a deformed proton $Z=38$ shell closure near ${\beta }_2\approx 0.46$. Notably, we identify, for the first time, the pivotal role of the light fragment and its shape coexistence structure on the fission mode transition in Th isotopes in a fully microscopic framework.

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• Fission
• Nuclear shapes and moments
• Shape coexistence
• A ≥ 220
• Collective models
• Density functional theory