Collisions between continents generate the world's most striking mountains; the deep structure of these collision zones, inferred from seismic reflection profiles1-3, can be equally dramatic, if less well understood. Improved knowledge of the structure of ancient collisional belts, now known to have developed through plate tectonic processes4,5, can provide important new insights into the mechanisms of crustal growth. Although many such belts have been partly destroyed by severe post-collisional deformation6,7 or magmatism8, some have been preserved in the stable interiors of continents9: one example is the Early Proterozoic Trans-Hudson orogen9,10, in western Canada. Here we present seismic reflection images across the entire orogen, which reveal a broadly symmetric structure, with reflections linked to island-arc rocks dipping beneath both bounding Archaean cratons. The observed reflection geometries imply a doubling of crustal thickness during the collision, along crust-penetrating faults. The presence of a significant amount of Archaean crust at depth, the extent of which could not have been determined from surface mapping, suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11,12.
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