The origin of 3‐D seismic heterogeneity in Precambrian lithosphere has been enigmatic, because temperature variations in old stable shields are expected to be small and seismic sensitivity to major‐element compositional variations is limited. Previous studies indicate that metasomatic alteration may significantly affect average 1‐D structure below shields. Here, we perform a grid search for 3‐D thermochemical structure, including variations in alteration, to model published Rayleigh wave phase velocities between 20 and 160 s for the eastern part of the Archean Superior and Canadian Proterozoic Grenville Provinces. We find that, consistent with constraints from surface heat flow and xenoliths, the lithosphere is coolest (Moho heat flow 12–17 mW/m2) and the thermal boundary layer thickest (>250 km) in the northeastern Superior and warmest in the southeastern Grenville (Moho heat flow 20–25 mW/m2, thermal boundary thickness 160–200 km). Compositionally, the phase velocities for most of the Superior within our study region require little alteration, but in a few regions, fast velocities need to overlie slower velocities. These can be modeled with an eclogite layer in the midlithosphere, consistent with active seismic and xenolith evidence for remnants of subducted Archean crust. The phase velocities from the Grenville Province require significant metasomatic modification to explain the relatively low velocities of the shallow lithosphere, and the required intensity of alteration is highest in parts of the Grenville associated with arc accretion. Thus, the composition of the northeastern Canadian Shield appears to reflect different stages and styles of craton assembly.

The thermal and compositional structure of lithospheric keels underlying cratons, which are stable continental cores formed during the Precambrian, is still an enigma. Mapping lithospheric temperatures and compositional heterogeneities is essential to better understand geodynamic processes that control craton formation and evolution. Here we investigate the northeastern part of North America which comprises the Superior Craton, the largest Archean craton in the world, and surrounding Proterozoic belts. We model Rayleigh-wave dispersion curves from a previous study, which were regionalized based on cluster analysis. Next, we perform a grid search for sub-crustal thermal and compositional structures that are consistent with the average dispersion curve for each cluster. We apply constraints on crustal structure and use thermodynamic methods to map thermo-compositional structures into seismic velocity. In agreement with previous studies, most regions require concentrations of metasomatic minerals over certain depth intervals to fit the seismic profiles. Our results further require vertical as well as lateral variations in compositional and thermal structures, which appear to reflect different stages of formation and modification of the lithosphere below the region, with distinct structures found under Archean cores, Archean/Paleoproterozoic collision belts, mid-late Proterozoic collision belts, and zones affected by rifting.

Introduction MotivationCratons are the stable continental cores formed during the Precambrian. Their formation, evolution and long-term stability is still debated (e.g., C.-T. A. Lee et al., 2011;van Hunen & Moyen, 2012;Sleep, 2005). Mapping lithospheric temperatures and compositional heterogeneity may shed light on their formation, evolution, and long-term stability. Cratonic mantle lithosphere is often described as relatively homogeneous, characterized by thick and high-wavespeed roots (Schaeffer & Lebedev, 2015), low surface heat flow (Cooper et al., 2004), and being approximately neutrally buoyant due to iron depletion as a result of melt extraction (Griffin et al., 2009;Jordan, 1978). However, recent studies have found heterogeneities within and between cratonic keels. S-to-P receiver function studies in cratonic regions have revealed sharp negative and/or positive wavespeed-depth gradients in the lithospheric mantle. The low or high wavespeed layers needed to explain such gradients have been

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<p>The thermal and compositional structure of the lithospheric keels underlying the Precambrian cratonic cores of the continents may shed light on their evolution and long-term stability. A number of seismic studies have found significant 3D seismic heterogeneity in cratonic lithosphere, which is enigmatic because temperature variations in old shields are expected to be small and seismic sensitivity to major-element compositional variations is limited. Previous studies show that metasomatic alteration may lead to significant variations in shield velocities with depth. Here we perform a grid search for thermo-chemical structures including metasomatic compositions, to model Rayleigh-wave phase velocities between 20 and 160 s for the northeastern part of North America comprising the Superior craton, the largest Archean craton in the world, and surrounding Proterozoic belts. We find smooth variations in thermal structure that include variations in thermal thickness within the Superior and decreasing thickness towards the edges of the shield. Four types of distinct compositional structures are required to match the long-period phase velocities. The different types appear to correlate with: (i) the unaltered oldest cores of the Superior, (ii) Archean and Proterozoic lithosphere modified by rifting and plume activity, and two distinct types of subduction signatures: (iii) an Archean/Paleo-Proterozoic signature that includes a high-velocity eclogite layer in the mid-lithosphere and (iv) a post Paleo-Proterozoic signature characterised by strongly altered shallow mantle lithosphere. Thus, processes that have affected the formation and modification of cratonic lithosphere and were previously recognised in xenoliths appear to have also left large-scale imprints in seismic structure.</p>