| Summary: | The Precambrian core of North America was assembled in the Proterozoic by a series of collisions between Archean cratons. Among the orogenic belts, two stand out due to their significant spatial extent. The Paleoproterozoic Trans-Hudson Orogen (THO) and Mesoproterozoic Grenville Orogen extend for thousands of kilometers along strike and hundreds of kilometers across strike. Both have been compared to the present-day Himalayan-Karakoram-Tibetan Orogen (HKTO). Over the last 20–30 years, active and passive source seismic studies have contributed a wealth of information about the present-day crustal structure and composition of the two orogens in Canada. The Proterozoic orogenic crust is generally thicker than that of neighboring Archean terranes, with a more variable Moho character, ranging from relatively sharp to highly diffuse. Both orogens have a prominent high-velocity lower crustal layer, consistent with long-term preservation of a partially eclogitized root at the base of the crust and similar to that inferred beneath the western HKTO. Crustal structure in the northern THO strongly resembles the lower crustal structure of the HKTO, suggesting that Moho depths may have reached 60–70 km when the orogen was active. A prominent midcrustal discontinuity beneath the central Grenville Province and changes in the patterns of seismic anisotropy in the THO crust beneath Hudson Bay provide geophysical evidence that lower crustal flow likely played a role in the evolution of both orogens, similar to that inferred beneath the present-day HKTO. The seismic evidence from Canada supports the notion of tectonic uniformitarianism, at least as far back as the Paleoproterozoic.
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