In the synthesis of superheavy element (SHE) via heavy-ion fusion reactions, quasifission is one of the major factors hindering superheavy nuclei (SHN) formation and the mechanism behind this process is intricate. We investigate dynamics of quasifission in hot fusion reactions synthesizing SHN with Z=112–120 using microscopic time-dependent Hartree-Fock theory in a total of 18 reactions. Remarkably, the nucleon numbers of heavy fragments distribute closely around certain quantum shells in these reactions, highlighting the crucial role of shell effects in fragment formation. In the reactions with 48Ca, 45Sc, 50Ti and 51V projectiles, the formation of heavy fragment is dominantly driven by the double spherical shells of 208Pb. In contrast, the influence of the double octupole deformed shells at Z=88 and N=136 is more pronounced in 54Cr-induced reactions, resulting in a tendency of producing pear-shaped 224Ra heavy fragment. Moreover, in the reaction with a heavier projectile, the colliding system tends to undergo a more rapid quasifission. This may be responsible for significantly reduced fusion probability and synthesis cross section observed in the reactions with projectiles heavier than 48Ca. These results elucidate quasifission mechanisms behind the reactions for synthesizing new SHEs Z=119 and Z=120.
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