Secular Evolution of the Seismic Structure of Continental Crust, Archean to Present

Abstract We summarize the seismic and density structure of continental crust as a function of crustal age and evaluate the secular changes in these physical properties. We subdivide the continental crust into six age provinces, from Archean to Mesozoic‐Cenozoic, and calculate the surface area, mean crustal thickness, volume, mass, and seismic velocity/density structure for each of these age provinces. Preserved Paleoproterozoic crust is anomalously abundant (18%) in comparison with Archean (11%) and Mesoproterozoic (7%) crust. The large volume of preserved Paleoproterozoic crust may be due to either a high rate of juvenile crustal formation during that Era, or the efficient reworking of Archean crust. The mean seismic P ‐wave velocity ( V p )‐depth structure of the crust is similar for Archean through Paleozoic crust, indicating that the process of crustal formation may also be broadly similar since the late Archean. However, the seismic velocity‐depth structure of Mesozoic‐Cenozoic middle and lower crust has seismic velocities that are 0.1–0.2 km/s lower than Paleozoic and older crust. We infer that the anomalous properties of the Mesozoic‐Cenozoic middle and lower crust is due to the local presence of partial melt. The mean S ‐wave velocity ( V s ) and V p / V s ratio of the crust has an age dependence, with higher values of V s and a lower V p / V s ratio for Archean and Paleoproterozoic crust. These observations are consistent with a more intermediate bulk crustal composition for Archean and Paleoproterozoic crust. Thick (150–250 km) lithosphere beneath Archean, Paleoproterozoic and Mesoproterozoic crust define the cratonic nuclei, the long‐lived, stable building blocks of continental crust.