Penetratively deformed rocks of the Yukon-Tanana terrane and Semenof block, and unfoliated Jurassic-Cretaceous intrusions are exposed in the southern Big Salmon Range of south-central Yukon. A newly mapped area, centred on the Boswell River, is divided into three structural panels by two regional-scale discontinuities, the Little Bear fault and the Sidney Creek fault. The panel on the northeastern side of the Little Bear fault includes units dominated by metasedimentary rocks (Slate Mountain succession), mafic volcanic and volcaniclastic rocks (Wiley succession), and a varied metasedimentary/metavolcaniclastic unit (Livingstone Creek succession). Each of these is crosscut by intermediate-mafic intrusions of the Simpson Range suite and is therefore Early Mississippian or older. Fossiliferous limestone is interpreted to unconformably overlie phyllite of the Slate Mountain succession. Other units that crop out in this region are Permian augen schist and Cretaceous plutons, including Late Cretaceous quartz monzonite that hosts the Red Mountain Mo porphyry deposit. The central part of the area, between the Little Bear and Sidney Creek faults, is dominated by mafic volcanic and plutonic rocks of the Sawtooth succession. These are along strike from, and provisionally correlated with the Moose formation of the Boswell assemblage. The Sawtooth succession is overlain by a carbonate and chert-bearing metasedimentary unit (Rosy succession), which hosts a small foliated metagranodiorite intrusion. A younger, as yet undated, mafic metavolcanic unit (Gunsight succession) is locally preserved above the Rosy succession. The metagranodiorite in the Rosy succession formed ca. 336 Ma and underwent metamorphic zircon growth in the Early Jurassic (~195 Ma). This zircon growth is interpreted to have accompanied regional, epidote-amphibolite to amphibolite-facies metamorphism and deformation. Mississippian rocks of the Simpson Range suite dominate much of the southwestern domain. These intrusions crosscut quartzose schist, graphitic phyllite and metabasaltic rocks of the Flat Creek succession, which also hosts Middle Triassic metagabbro. All pre-Jurassic rocks in the area are deformed, while post-tectonic intrusions include members of the Early Jurassic Lokken suite, the mid-Cretaceous Quiet Lake batholith and Iron Creek stock, and the aforementioned Red Mountain suite. Most Early Jurassic intrusions are undeformed, but deformation/hydration of some minor phases suggests they were intruded during the waning stages of deformation. The Sidney Creek fault is cut by the Early Jurassic Sawtooth stock while the Cretaceous Iron Creek stock intruded across the trace of the Little Bear fault. The Little Bear and Sidney Creek faults are interpreted to define tectonic boundaries between Yukon-Tanana terrane and rocks of the Semenof block and may form part of a regional detachment between these units.