Bell, T.H., and Newman, R. (2006) Appalachian orogenesis: the role of repeated gravitational collapse. Geological Society of America Special Papers, 414. pp. 95-118.

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Abstract

Orogenesis within the New England Appalachians has classically been regarded as occurring discontinuously even though the collision of plates driving it was essentially continuous for 200 million years from the Taconic through the Alleghanian orogenies. Structural, metamorphic, and age data obtained from the cores of porphyroblasts reveal a near continuous history of tectonism that is partitioned within and between outcrops as well as regionally. Very prolonged deformation and metamorphic histories predate the foliation parallel to bedding, and the oblique matrix foliations only reflect brief increments of the uplift path of these rocks back to the earth's surface. The matrix shows none of the structural effects of the path down into the crust. This deepening path is revealed by the sequences of foliations that developed about regionally consistent successions of foliation intersection axis trends preserved within porphyroblasts (FIAs). Indirect coupling between plates throughout the period of collision resulted in horizontal shortening accompanied by subvertical foliation development, followed by crustal instability, collapse, and the formation of subhorizontal foliation, repeated over and over until orogenesis ceased. These cycles repeat on time scales as short as 100,000 to 500,000 years, but because of partitioning of the deformation, only the weakest rocks preserve much of this history. Shifting directions of relative plate motion every 5 to 30 million years also results in easily deformed rocks being protected by more competent ones, with none of them seeing the total history. Furthermore, if the bulk composition is not suitable for porphyroblast growth, none of this history will be recorded. The recurring role of gravitational collapse and the variable scale of partitioning of this type of deformation is obscured by repeated reactivation of the bedding parallel foliation in multiply deformed rocks containing porphyroblasts in the New England Appalachians. It is also obscured by the lack of topographic relief relative to total crustal thickness.

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