COMPARISON OF APPALACHIAN AND HIMALAYAN FORELAND CLASTIC WEDGES

Much of the history of an orogenic belt is written in its foreland stratigraphy—if we can decipher it. Clastic wedges (CWs) record tectonic events that produce deformation, metamorphism, plutonism, uplift in the hinterland, and diachroneity of CWs along orogenic strike. Diachroneity in CWs frequently parallels diachronous events in the interior. CW geometry also is subject to the serendipity of drainages that deliver sediment to the foreland or to internal depo-sites. Both the Appalachians (APs) and Himalayas (HMs) are products of continent-continent collision. Each of the three orogenies that built the AP orogen, the Taconic (TC, Middle Ordovician-Early Silurian), Acadian-Neoacadian (AN, Late Devonian-early Mississippian), and Alleghanian (AG, late Mississippian-Early Permian) produced a diachronous CW on the foreland that parallels the length of the orogen. A Late Cretaceous to Eocene arc collision is recorded in an early CW in the Lesser HM, but the clastic sediments preserved in the Siwalik Ranges comprise the principal CW of this orogen. It also is diachronous. Tracking detrital zircon ages in CWs has provided new insight into AP history. Southern-central AP CWs consistently contain detritus from the previous orogeny (or uplifted rifted margin), and not the event that produced the uplift, whereas the TC CW from New England (NG) northward contains heavy mineral-bearing detritus from TC thrust sheets obducted onto the early Paleozoic platform. The AN (Catskill-Bedford-Grainger) CW consistently youngs southwestward from NG paralleling the zippered dextral accretion of Avalon-Carolina exotic terrane to Laurentia, recorded in deformational and metamorphic events in the interior of the chain. The AG CW records the late Mississippian through Early Permian end of the orogeny, with parallel ages of metamorphism and plutonism in the hinterland related to dextral faulting, then forelandward thrusting. Of the AP CWs, the TC and AG wedges are the most analogous with those in the HM.