The Trans-Hudson orogen is the preserved Early Proterozoic collisional zone between the Archean Superior and Hearne-Rae cratons and the focus of Lithoprobe's Trans-Hudson orogen transect where seismic reflection line S2b was recorded. The most intriguing structure in the data is a continuous band of strong reflections in the upper crust, herein named the Wollaston Lake reflector, which extends for 160 km across the northwestern hinterland of the Trans-Hudson orogen. The reflections range from 2.0 to 4.5 s two-way traveltime (6.0–13.5 km depth) and show a heterogeneous internal structure with multicyclic reflected arrivals and numerous diffractions. Our studies indicate that a series of high-velocity-high-density reflectors, having lengths of 800 m to a few kilometres and thicknesses from 50 to 150 m, most likely form the reflecting elements. Geologic mapping and dating document diabase intrusions associated with the Middle Proterozoic Mackenzie igneous event. Sills drilled in the region have thicknesses coinciding with those of our seismic model. We conclude that the Wollaston Lake reflector represents a series of diabase sheetlike intrusions emplaced during Mackenzie (i.e., post-Hudsonian) magmatic activity. Regionally, it provides constraints for the Middle Proterozoic paleo–stress field and for the late-stage tectonic history of the northwestern hinterland of the Trans-Hudson orogen. More generally, the detection of the reflector emphasizes the importance of large-scale horizontal injection of tabular bodies into the crust, over long distances and across tectonic boundaries.