The Precambrian basement rocks of the Boothia Arch exhibit three phases of folding characteristic of deep-level deformation in mobile belts. Structural cross-sections demonstrate that the geometry of compositional layering and regionally parallel axial-plane foliation of first-phase folds is controlled primarily by structures of the second phase.The basement gneisses contain assemblages of the amphibolite–granulite transitional facies. The felsic gneisses exhibit a planar fabric clearly defined by nearly oblate quartz grains, usually considered to be typical of "granulite" facies rocks. In the Boothia Arch this foliation is an axial surface structure formed during the second fold phase. Fabric studies suggest that anhydrous assemblages stable in the granulite facies prior to the second phase of deformation were replaced during this phase by hydrous assemblages characteristic of the amphibolite-granulite transitional facies.The Boothia Arch was a source of clastic sediments during three periods of cover-rock deposition. It was a mild positive feature in Aston times (late Proterozoic or early Paleozoic), and was rapidly uplifted along north-south trends during late Silurian and early Devonian times to produce continental clastic facies of the Peel Sound Formation symmetrically related to the arch. The limited Cretaceous–Tertiary record suggests irregular uplift and localized deltaic sedimentation. At other times the Boothia Arch was insignificant as a source of detritus, and carbonates were deposited over the area.The north–south strike of compositional banding and axial surfaces of the first- and second-phase folds in the basement parallels the axis of the arch and axial surfaces of folds in the cover rocks. Also, fracture patterns in the basement and cover rocks are similar. Beyond this there is no obvious relationship between the internal structure of the basement and the later development of the arch.