Fiordland, New Zealand, contains exposures of high-pressure (P = 12--14 kbar) granulite facies rocks that form one of Earth's largest exposed lower crustal roots of an Early Cretaceous magmatic arc. These exposures allowed us to examine the mechanisms and processes that controlled crustal thickening and large vertical displacements at the deepest levels of a deforming arc system. We present structural and metamorphic data that show how the root of this arc was tectonically thickened by imbricate, granulite facies thrust zones during and after the emplacement of sheeted plutons into the middle and lower crust. The imbricate thrust zones form part of a well-exposed, 12 km wide, two-sided fold-thrust belt that preferentially developed in crust that was thermally softened by magmatism. Changes in metamorphic mineral assemblages and microstructural data from the contact aureole of a composite arc batholith called the Western Fiordland Orthogneiss record thrust and pluton emplacement conditions of P ≈ 7--9 kbar (paleodepths of 25--30 km). These data show that thrust imbrication and tectonic loading at 30 km depth is a viable mechanism of large vertical displacements and crustal thickening at the deepest levels of magmatic arcs. This mechanism produced a characteristic up-pressure metamorphic history that is similar to that observed in many other large magmatic belts worldwide.