Seismic Evidence for Metamorphic Densification of the Lower Continental Crust in Eastern North America.

The composition of the lower continental crust, as well as its formation, growth, and evolution, remains a fundamental subject to be understood. In this study, we carry out a comparative and integrative analysis of seismic tomographic models, teleseismic receiver function results, and Airy isostasy in order to investigate the properties of the lower continental crust in eastern North America. We extract the depths for Vs of 4.0 km/s, 4.2 km/s, and 4.5 km/s from three selected tomographic models and calculate the differences between the Vs depth contours and the Moho depth defined by receiver functions. We then calculate the Airy isostatic Moho depth and its misfit with the receiver‐function‐defined Moho. Our analysis reveals three key features: (a) the deepening of the Vs depth contours and the strong negative Airy misfit within the U.S. Grenville Province; (b) a seismically faster‐than‐average and compositionally denser‐than‐average lowermost crust in the eastern North American Craton and the Grenville Province; and (c) the thickest, seismically fastest, and densest lowermost crust beneath the southern Grenville Front, the southern Grenville‐Appalachian boundary, and the U.S.‐Canada national border. We suggest that the lower crust of the craton and the Grenville Province has densified through garnet‐forming metamorphic reactions during and after orogenesis, contributing to the widely distributed fast‐velocity layer. The lower crust beneath the tectonic boundaries could have experienced more extensive garnet growth during orogenesis and emplacement of mafic magma. This study provides new constraints on the seismic and compositional properties of the lower crust in eastern North America. Plain Language Summary: The continental crust provides important clues about the formation and evolution of the Earth. However, the dominant composition of the lower part of the continental crust remains enigmatic, especially in the regions where the seismic velocities are significantly faster than the global average. In this paper, we investigate the properties of the lower continental crust in eastern North America by comparatively analyzing two types of data sets: (a) the variations of seismic velocity within the crust and (b) the variations of the crustal thickness. Our comparative and quantitative analysis reveals that the lower crust of the eastern North American Craton and the Grenville Province is seismically faster and compositionally denser than the global average. The thickest, seismically fastest, and compositionally densest lower crust is roughly along the orogenic fronts and the U.S.‐Canada national border. We suggest that garnet growth within the lower crust is a viable mechanism for forming the widely distributed fast‐velocity layer in eastern North America. The lower crust may have been further densified through extensive garnet growth and emplacement of mafic magma at a regional scale. Key Points: A high‐velocity lower continental crust distributes widely in eastern North AmericaThe seismically fastest and densest lower crust is roughly along the orogenic frontsThe lower crust densifies through the metamorphic growth of dense minerals [ABSTRACT FROM AUTHOR]