  We describe a new approach for driving GeV-scale plasma accelerators with
long laser pulses. We show that the temporal phase of a long, high-energy
driving laser pulse can be modulated periodically by co-propagating it with
low-amplitude plasma wave driven by a short, low-energy seed pulse. Compression
of the modulated driver by a dispersive optic generates a train of short pulses
suitable for resonantly driving a plasma accelerator. Modulation of the driver
occures via well-controlled linear process, as confirmed by good agreement
between particle-in-cell (PIC) simulations and an analytic model. PIC
simulations demonstrate that a 1.7 J, 1 ps driver and a 140 mJ, 40 fs seed
pulse can accelerate electrons to energies of 0.65 GeV in a plasma channel with
an axial density of 2.5 x 10$^{17}$ cm$^{-3}$. This work opens a route to
high-repetition-rate, GeV-scale plasma accelerators driven by thin-disk lasers,
which can provide joule-scale, picosecond-duration laser pulses at
multi-kilohertz repetition rates and high wall-plug efficiencies.
