ALPS-APPALACHIANS COMPARISON: WORTHWHILE EXERCISE OR “APPLES AND ORANGES?”

Orogenic belts are constructed on continental margins as products of opening and closing of oceans (Wilson cycle): the Tertiary Alpine chain and Paleozoic Appalachians (–Variscides) are no different. Both are commonly treated as collisional orogens that were constructed via orogenies extended through time, but the similarities end there. The Alps involved closing of a small Tethyan ocean with Africa-Europa collision, whereas the Appalachians formed as the product of closing of three Paleozoic oceans that involved island-arc accretion (Ordovician) and Peri-Gondwanan terrane collision (Late Devonian-early Mississippian), then final collision between Gondwana and Laurentia to form Pangea. Trümpy’s Alpine events, “Paleoalpine” (Cenomanian-Paleocene?), “Mesoalpine” (Eocene-Oligocene main event), and “Neoalpine” (Miocene?) were preceded by Pangea breakup, but were largely built by the Eocene-Oligocene event that involved subduction of a small ocean, yielding few plutons, minor volcanism, and continent-continent collision. The later events produced dextral strike slip along major faults within the chain, followed by collapse during final uplift. The forelandward shapes of both orogens were influenced by the old continental margins on which they were built, and have the classic wedge shape in cross section, but the forelands and internides of both orogens are markedly different kinematically and are diachronous along strike. Both contain a forelandward gravity low, which likely maps the distribution of thickened continental crust. Parts of each contain a tectonic lid above an infrastructure (Eocene-Oligocene Alps: Austroalpine–Pennines; mid-Paleozoic Appalachians: Carolina–Avalon superterrane–Inner Piedmont/Connecticut Valley). Comparisons of the two chains, however, is not a comparison of “apples and oranges,” but a worthwhile exercise in understanding the evolution of continental crust—for both contain similarities that form the attributes of mountain chains.