  With a view to understanding extended-BMS symmetries in the framework of the
$AdS_4/CFT_3$ correspondence, asymptotically AdS geometries are constructed
with null impulsive shockwaves involving a discontinuity in superrotation
parameters. The holographic dual is proposed to be a two-dimensional Euclidean
defect conformal field localized on a particular timeslice in a
three-dimensional conformal field theory on de Sitter spacetime. The defect
conformal field theory generates a natural action of the Virasoro algebra. The
large radius of curvature limit $\ell\to\infty$ yields spacetimes with
nontrivial extended-BMS charges.
