EVALUATION OF THE GEOCHRONOLOGIC EVIDENCE FOR THE TIMING OF PALEOZOIC OROGENIC EVENTS ALONG THE WESTERN FLANK OF THE SOUTHERN APPALACHIANS

The tectonic assembly of the southern Appalachians has traditionally been ascribed to three tectonothermal episodes: the Ordovician Taconian, Middle Devonian Acadian and Carboniferous-Permian Alleghanian orogenic events. The potential significance of a Neoacadian event has been a matter of much recent discussion. Although orogeny is defined in terms of geography and the rock record, time also plays a role in the sense that the plate tectonic forces that caused crustal stresses and thermal perturbations have a beginning and an end. In the southern Appalachians, it is common to “bin” the effects of orogenesis into one of three orogenic time slots based on available geochronology. This study is an evaluation of data used to define those “bins” for the western flank of the southern Appalachians.

A comprehensive database was constructed from 528 journal articles, abstracts, and other sources (1959 to 2006) providing 454 Ar/Ar, EPMA, K-Ar, Rb-Sr, Sm-Nd, and U-Pb, ages. All ages taken together at face value show a distinct Taconian maximum at 460-480 Ma, an asymmetric Alleghanian peak at ~340 Ma tailing off to 250 Ma, and between the two lies a broad swath of Acadian or Neoacadian ages. This general pattern is retained through a stepwise quality control process that yields a surprisingly small subset of highly robust ages. The Ar/Ar dataset reveals virtually no Taconian cooling ages, little distinction between Acadian and Neoacadian cooling ages, and an Alleghanian 335 Ma spike. Only 18 U-Pb zircon ages have bearing on the distinction between Neoacadian vs. Acadian plutonism and metamorphism. Ion-microprobe ages from two Acadian granites (one of which, Rabun, is now known to be Alleghanian) are questionable. Two Spruce Pine intrusions have high-precision Acadian ages of ca. 377 Ma. Fourteen reported ages, evenly divided between Acadian and Neoacadian, await publication before their significance can be evaluated.

Although there are many potential biases in this type of evaluation, it does highlight historical trends in thinking about the timing of tectonothermal events and it potentially points the way to future progress in better deciphering the history of southern Appalachian orogenic effects. The database is available in spreadsheet format from the author's website.