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Abstract The impact of past tectonic events on the formation and modification of continental lithosphere remains as an open question of fundamental importance. Eastern North America provides a complete record of supercontinent assembly and breakup over the past 1.3 Ga, serving as a natural laboratory for our understanding of continental crust and mantle lithosphere and for integrating geologic and geophysical observations. In this study, we used teleseismic Ps receiver functions to image the detailed distribution of crustal thickness beneath eastern North America. The radial‐component receiver functions were calculated from seismic waveforms recorded by a total of 659 broadband stations during 2010–2017, yielding a high‐resolution image of Moho depth distribution. The depths of the Moho and intracrustal layers vary within and across the major tectonic units. Specifically, there are distinct differences in crustal thickness between the northern and southern Grenville Province. A dipping intracrustal feature can be seen within the central Grenville Province, with the depth increasing eastward from 5 to 27 km. The Moho depth decreases southeastward across the Grenville‐Appalachian boundary, with a sharp Moho offset of up to 12–15 km in the central segment and a more gradual variation to the north and south. The thickness difference between the southern and northern Grenville‐aged crusts suggests different tectonic and/or exhumation histories during and after the Grenville Orogeny. The low‐angle eastward dipping crustal feature is interpreted to be a Grenville‐aged collisional structure. Differences in the steepness of the Moho offset along the strike of Appalachians probably reflect variation of the steepness of the subsurface boundary between Laurentia and accreted terranes with different intensities of postorogenic modification. The observed spatial relation between the geologically defined tectonic boundaries and crustal thickness variations provides new constraints on the depth extent of the tectonic units within the crust.
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Seismic Evidence for Metamorphic Densification of the Lower Continental Crust in Eastern North America
Abstract 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.
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- Award ID(s):
- 1930014
- PAR ID:
- 10426710
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 128
- Issue:
- 6
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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