Quasivibrational Bands at High Spins inYb158

High-spin states in the transitional nucleus ${}_{}{}^{158}{}_{}{}^{}\mathrm{Yb}$ were populated via several (HI, $4n$) reactions. Two yrast bands were established up to $I^{\pi }=\left({40}^{+}\right) \mathrm{and} \left({31}^{-}\right)$. In contrast to the negative-parity band up to $I=31\mathrm{}$ and the positive-parity states up to $I=24\mathrm{}$ which show collective rotational patterns, states with $I^{\pi }={26}^{+}-{36}^{+}$ exhibit a nearly vibrational excitation mode. Such a quasivibrational pattern suggests a gradual transition toward oblate shapes and is the first evidence for a "band termination" in a heavy nucleus.