The Role of Climate in the Deformation of a Fold and Thrust Belt
Steen, Sean Kristian
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Theory and experiment show that the rate and geographic distribution of erosion control the rate and pattern of deformation in collisional mountain belts. Enhanced erosion reduces the mass of material that must be moved up and over ramps and uplifted in large folds. In order to test this and related ideas in a natural example, we have compared modeled rainfall to measured thrust sheet displacement, geometry, and internal deformation in the Appalachian fold and thrust belt. We use mean annual precipitation from a global climate model (GCM) as a proxy for rate of erosion. Deformation measurements were made on a portfolio of regional cross sections from Alabama to New England. During the Carboniferous Allegheny orogeny the Southern Appalachians moved from -30 ? to 0? latitude whereas the Central and Northern Appalachians lay between -15? and 5? latitude. Mean annual precipitation determined from the GENESIS 2 GCM (Grossman, per. comm.) varied from tropical to arid conditions as the collision both moved north and grew in breadth and height. The Southern Appalachians, which experienced more net rainfall than other regions, generally show more displacement, deeper present day exhumation, and shallower ramps than regions to the north. The vicinity of the Pine Mountain thrust sheet in the Southern Appalachians experienced the most displacement (~1.5X the Central Appalachian average) and bulk shortening (~1.6X the Central Appalachians) and produced the most eroded material (~1.5X the Central Appalachians). The latitude of the Pine Mountain thrust sheet in the Southern Appalachians received ~20% more rainfall than the Central Appalachians. Although the number of regional detachments and lithologies change from Southern to Central and Northern Appalachians, the change in rainfall both regionally at any one time and as the collision progressed may explain part of the change in structural style from south to north.