We analyse the flow curves of a two-dimensional assembly of granular
particles which are interacting via frictional contact forces. For packing
fractions slightly below jamming, the fluid undergoes a large scale
instability, implying a range of stress and strainrates where no stationary
flow can exist. Whereas small systems were shown previously to exhibit
hysteretic jumps between the low and high stress branches, large systems
exhibit continuous shear thickening arising from averaging unsteady, spatially
heterogeneous flows. The observed large scale patterns as well as their
dynamics are found to depend on strainrate: At the lower end of the unstable
region, force chains merge to form giant bands that span the system in
compressional direction and propagate in dilational direction. At the upper
end, we observe large scale clusters which extend along the dilational
direction and propagate along the compressional direction. Both patterns, bands
and clusters, come in with infinite correlation length similar to the sudden
onset of system-spanning plugs in impact experiments
