The longitudinal structure of the quark-gluon plasma (QGP) remains poorly constrained. Using a transport-model calculation of two-particle elliptic-flow correlations $V_{2\mathrm{\Delta }}(\mathrm{\Delta }\eta )$ in relative pseudorapidity $\mathrm{\Delta }\eta$, we find two distinct initial-state components of flow decorrelation: a long-range component driven by the global geometry of participating nucleons and a short-range component reflecting local energy-deposition fluctuations. This short-range component extends to $|\mathrm{\Delta }\eta |\sim 4$ and can be misidentified as final-state nonflow. For isobarlike nuclei with different shapes, the calculation predicts an experimentally testable fingerprint of this component: it dilutes the deformation contribution at small $|\mathrm{\Delta }\eta |$, so that the $V_{2\mathrm{\Delta }}(\mathrm{\Delta }\eta )$ ratio between deformed and near-spherical systems, such as $\mathrm{U}+\mathrm{U}$-to-$\mathrm{Au}+\mathrm{Au}$, rises with $|\mathrm{\Delta }\eta |$. Separating long- and short-range flow decorrelations from nonflow gives access to the longitudinal initial structure and dynamics of the QGP.

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• Collective flow
• Nuclear structure & decays
• Particle correlations & fluctuations
• Quark-gluon plasma
• Relativistic heavy-ion collisions