A tale of two orogens: crustal processes in the Proterozoic Trans-Hudson and Grenville Orogens, eastern Canada

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 mid-crustal 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.