  We analyze the gravitational dynamics of a classical scalar field coupled to
gravity in asymptotically AdS spacetime, which leads to black hole formation on
the shortest nonlinear time scale for some initial conditions. We show that the
observed collapse cannot be described by the well-known process of a
random-phase cascade in the theory of weak turbulence. This implies that the
dynamics on this time scale is highly sensitive to the phases of modes. We
explore the alternative possibility of a coherent phase cascade and
analytically find stationary solutions with completely coherent phases and
power-law energy spectra. We show that these power-law spectra lead to
diverging geometric backreaction, which is the likely precursor to black hole
formation. In 4+1 dimensions, our stationary solution has the same power law
energy spectrum as the final state right before collapse observed in numerical
simulations. We conjecture that our stationary solutions describe the system
shortly before collapse in other dimensions, and predict the energy spectrum.
