A recent Lithoprobe seismic-reflection transect in the Peace River arch region of the Western Canada sedimentary basin traverses continental crust that was assembled in a series of magmatic and collisional events during the interval 2.0–1.9 Ga. The seismic data reveal panels of dipping reflectivity at tectonic domain boundaries, but they are otherwise dominated by prominent, subhorizontal reflections that occur between 3.5 and 18.5 km depth over an area of at least 120 000 km2. These reflections, here named the Winagami reflection sequence, overlap several tectonic domains and are comparable in geometry, depth, and areal distribution to sills and discordant intrusive sheets imaged by previous seismic profiling in the Baltic shield and the Basin and Range province of the western United States. As many as five distinct reflections are present, some of which can be traced for more than 100 km. Contrasting reflection character in adjacent tectonic domains suggests an intrusive geometry controlled partly by the mechanical properties of the host rocks. Seismic modeling indicates that observed waveforms are consistent with reflections from tabular mafic bodies ∼70 m thick. The Winagami reflection sequence crosscuts the collisional fabric (ca. 1890 Ma), but the sequence is offset by a brittle fault with an estimated age of 1760 Ma. These timing constraints suggest that the Winagami reflection sequence is a previously unknown component of a regional magmatic event emplaced during a period of brittle indentation of the western Canadian shield by the Slave province.