Crustal rheology and surface processes strongly influence strain distribution and shape of orogenic wedges at their front but how they influence the wedge rear is still unclear. Here, we analyse the coupled control of viscosity and sedimentation on forearc high growth during advanced stages of subduction accretion. We use 2D thermo-mechanical finite element models constrained with data of the south Anatolian margin. Our simulations show that forearc highs grow as a thermally-activated viscosity drop in the lower crust induces ductile deformation and viscous flow. Initial viscosity and the amount of sediments in the forearc basin control non-linearly the occurrence and timing of the thermally-activated viscosity drop, and thus of the growth of the forearc high. High sedimentation rates result in thicker forearc basins that stabilise the subduction wedge and delay the onset of uplift in the forearc high. Low viscosities promote earlier onset of forearc high uplift and lead to larger morphological variability along the subduction margin. Increasing either the sedimentation rate or viscosity may prevent forearc high formation entirely. The thermo-viscous forearc highs grow at an age set by wedge thermal state as a function of accretionary flux, wedge viscosity, and synorogenic sedimentation. Our models explain vertical motions in south Anatolia and potentially in other accretionary margins, like the Lesser Antilles or Cascadia, during the formation of their broad forearc highs.@en