  The Milky Way's (MW) inner stellar halo contains an [Fe/H]-rich component
with highly eccentric orbits, often referred to as the "last major merger."
Hypotheses for the origin of this component include Gaia-Sausage/Enceladus
(GSE), where the progenitor collided with the MW proto-disk 8-11 Gyr ago, and
the Virgo Radial Merger (VRM), where the progenitor collided with the MW disk
within the last 3 Gyr. These two scenarios make different predictions about
observable structure in local phase space, because the morphology of debris
depends on how long it has had to phase mix. The recently-identified
phase-space folds in Gaia DR3 have positive caustic velocities, making them
fundamentally different than the phase-mixed chevrons found in simulations at
late times. Roughly 20\% of the stars in the prograde local stellar halo are
associated with the observed caustics. Based on a simple phase-mixing model,
the observed number of caustics are consistent with a merger that occurred 1--2
Gyr ago. We also compare the observed phase-space distribution to FIRE-2 Latte
simulations of GSE-like mergers, using a quantitative measurement of phase
mixing (2D causticality). The observed local phase-space distribution best
matches the simulated data 1--2 Gyr after collision, and certainly not later
than 3 Gyr. This is further evidence that the progenitor of the "last major
merger" did not collide with the MW proto-disk at early times, as is thought
for the GSE, but instead collided with the MW disk within the last few Gyr,
consistent with the body of work surrounding the VRM.
