CLASTIC WEDGES AS RESPONSES TO CRUSTAL RELAXATION FOLLOWING OROGENIC LOADING: EXAMPLES FROM THE APPALACHIAN FORELAND BASIN

It is well-known that clastic wedges are typical end products of orogenies, but flexural modeling and foreland-basin stratigraphy suggest that the wedges do not develop uniformly throughout orogeny. Rather, they develop at specific times and places and reflect very specific phases in the history of each orogeny. The most active, mountain-building phase of an orogeny reflects orogenic or deformational loading and apparently produces little in the way of clastic wedges. This is because much of the loading is in the subsurface, is subaqueous in the basin, or has had little time to develop drainage nets. Not until active loading (deformation) has ceased does this phase end, allowing the crust to relax (loading-type relaxation) in response to the new surficial load and generate a deeper adjacent basin. By this time, the surficial deformational load has had sufficient time to develop drainage nets, and the adjacent basin fills with deeper-water, flysch-like clastics. This phase continues until the orogen (load) has been largely planed. Once the orogen has been leveled and the load removed, the crust once again relaxes — this time, in response to the lost load (unloading-type relaxation), and with parts of the basin, rebounds upward. Parts of the old basin may be cannibalized, but a newer, shallower basin develops cratonward of the rebounding orogen and fills with shallow-water, marginal-marine (molasse-like) clastics. Hence, contrary to some literature, clastic wedges largely develop in the later, relaxational phases of an orogeny and reflect two different phases of relaxation, which can be distinguished in foreland-basin sequences. Clearly, foreland-basin clastic wedges offer critical information about orogenic timing, but their shape and distribution may contribute even more about the nature and location of the convergence event, and possibly, about interaction with other orogenies.