Unsteady rock uplift and erosion in a decaying orogen in response to surface and dynamic mantle processes (Invited)

The Appalachian Mountains in the eastern United States are constructed on a lithosphere that was assembled and modified during a long period of Paleozoic collisional tectonics, and then thinned by erosion and stretching with the opening of the Atlantic Ocean. Geologic, geomorphic, and geophysical data describe the long term erosion, persistence of mountainous topography, and coupling between surface and tectonic, isostatic, or eperiogenic processes of this ancient, decaying orogen. There are several key observables that must be reconciled to explain Appalachian post-orogenic (post-Triassic) evolution. These include subsidence of the former metamorphic core of the range and topographic inversion of the foreland, a long-term record of unsteady erosion preserved in the sediments of the Atlantic margin shelf-slope basins, the steepening of the longitudinal profiles of Atlantic slope rivers forming a zone of bedrock rapids as they approach the Atlantic, river knickpoints in the foreland with a common elevation that have no apparent relation to rock-type or structure, and evidence that the divide between Atlantic slope and Ohio drainages is not a static feature. The long term rate of Appalachian rock uplift (base level fall) and erosion is ~20-30 m/my., a result consistent with AHe thermochronology and the average sediment flux to the BCT, but misleading in terms of the unsteadiness demonstrated by river incision, growth of the Atlantic slope drainage, and short-term sediment flux to the BCT. This unsteadiness is likely influenced by several factors including climate change, sediment storage in the landscape and the Coastal Plain, and eustasy; however, we propose that the most important factor has been unsteady rock uplift as the result of a lithospheric flexural response to surface loads and sub-lithospheric mantle flow driven by the foundering Farallon slab.