摘要
Theoriginof3‐DseismicheterogeneityinPrecambrianlithospherehasbeenenigmatic,becausetemperature variations in old stable shields are expected to be small and seismic sensitivity to major‐elementcompositional variations is limited. Previous studies indicate that metasomatic alteration may significantlyaffect average 1‐D structure below shields. Here, we perform a grid search for 3‐D thermochemical structure,includingvariationsinalteration,tomodelpublishedRayleighwavephasevelocitiesbetween20and160sforthe 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 flow12–17mW/m 2 )andthethermalboundarylayerthickest(>250km)inthenortheasternSuperiorandwarmestin the southeastern Grenville (Moho heat flow 20–25 mW/m 2 , thermal boundary thickness 160–200 km).Compositionally,the phasevelocitiesformostoftheSuperiorwithin ourstudyregion require little alteration,but in a few regions, fast velocities need to overlie slower velocities. These can be modeled with an eclogitelayer in the midlithosphere, consistent with active seismic and xenolith evidence for remnants of subductedArcheancrust.Thephasevelocities fromtheGrenvilleProvincerequiresignificantmetasomaticmodificationto explain the relatively low velocities of the shallow lithosphere, and the required intensity of alteration ishighest in parts of the Grenville associated with arc accretion. Thus, the composition of the northeasternCanadian Shield appears to reflect different stages and styles of craton assembly.