12-1 Why is Appalachian Crust Thinner than Grenville Crust?

Session: New advances in geological and geophysical research on the Appalachian orogen. (II)

Presenting Author:

Paul Karabinos

Authors:

Karabinos, Paul¹, Bourke, James R.², Masis Arce, Roberto Jose³, Luo, Yantao⁴, Espinal, Kimberly⁵, Long, Maureen⁶, Webb, Laura E.⁷

(1) Geosciences, Williams College, Williamstown, , (2) Yale University, Hamden, , (3) Rutgers State UniversityDept Geological Sci, Piscataway, , (4) Yale University, Earth & Planetary Sciences, New Haven, , (5) Yale University, New Haven, , (6) Yale University, New Haven, , (7) University of Vermont, Burlington, ,

Abstract:

The Mesoproterozoic Grenville orogen in eastern North America (ENA) was constructed during multiple events from ca. 1.4 to 1.0 Ga and contributed to the development of the supercontinent Rodinia. Neoproterozoic rifting formed the Laurentian passive margin of ENA. Multiple collisions along the Laurentian margin created the Paleozoic Appalachian orogen from ca. 480 to 270 Ma and culminated in the supercontinent Pangea, which later rifted apart in the Mesozoic 200 to 160 Ma to form the North American passive margin. Despite the similar tectonic histories of these two Wilson cycles, seismic evidence from ENA clearly indicates that Appalachian crust is commonly 10 to 20 km thinner than Grenville crust despite similar elevations.

Isostasy requires that the mantle lithosphere under Appalachian crust is thinner than under Grenville crust to account for the similar elevations. Espinal et al. (2024) reported shallow asthenosphere depths of 60-110 km while investigating the Northern Appalachian Anomaly, although other studies in the Appalachian suggest similar shallow depths to the LAB in regions without thermal anomalies. Appalachian crust is particularly thin in southern New England where an orogenic plateau existed from 380 to 330 Ma (Hillenbrand et al., 2021). Lithospheric delamination, thermal erosion by asthenospheric flow, and slab rollback have been proposed to explain thinning of the mantle lithosphere beneath orogenic plateaus. However, the Appalachian lithosphere appears to be consistently thinner than Grenville lithosphere, and there is evidence that orogenic plateaus formed within the Grenville orogen.

The greater age of the Grenville orogen is not a promising explanation for these observations because although the mantle lithosphere might have thickened over time, it would not explain the thicker crust. It is possible that differences in upper vs lower plate rifted margin behavior might have thinned margins differently, yet it is likely that both the Laurentian and North American passive margins included both. It is possible that Mesozoic rifting thinned Appalachian lithosphere more than Neoproterozoic rifting thinned Grenville lithosphere for unknown reasons. Finally, accreted Appalachian terranes may not rest on their original mantle lithosphere, but instead were detached and thrust onto rifted Grenville lithosphere during Paleozoic orogenies, which was further thinned during Mesozoic rifting.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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