X-ray scattering in a grazing-incidence geometry (GISAXS) was used to study relief microstructures in thin films of asymmetric polystyrene-block-poly(methyl methacrylate) P(S-b-MMA) that dewetted on SiOx/Si and PMMA-SiOx/Si after isothermal annealing at 210 degrees C for 12 h. The micro- and nanostructures of a P(S-b-MMA) film deposited on PS-SiOx/Si were studied for comparison. The diffuse scattering streaks observed at angles with respect to the film normal direction in the GISAXS patterns correlate with the formation of relief terraces comprising parallel PMMA cylinders packed within a PS matrix. The relief terraces reveal a facet-like wedge at the edge, resulting from partial wetting of the nonanchored P(S-b-MMA) top layer at contact angles in the range 4-6.5 degrees on a monolayer of autophobic P(S-b-MMA) brushes anchored onto SiOx/Si and PM MA-SiOx/Si.