RECORDING OROGENESIS: MULTIPLE MINERALS, MULTIPLE METHODS

Over the past decade there has been an exponential rise in the number of studies utilizing detrital zircon U-Pb ages as a method of recording past orogenic events. It is a commonly held view in many of these studies that the abundance of zircon grains of a particular age interval is directly related to the magnitude of the orogenic event. This is often not the case, however, as some tectonic events are excessively recorded, whereas other orogenic events are only minimally recorded by detrital zircon. This is particularly true in the Appalachians where much modern alluvium and late Paleozoic sandstone have extremely abundant 1.3 – 1.0 Ga age peaks (i.e. a ‘Grenvillian’ signature), but virtually no 320 – 300 Ma age peaks (i.e. no ‘Alleghanian’ record).

A more advanced approach to accurately reading the past record of orogenesis is to apply multiple chronological, geochemical and isotopic methods to a variety of different minerals. An example of the very different tectonic record this approach provides compared to only detrital zircon ages is given by detrital minerals in alluvium from the French Broad River (TN + NC). The watershed of this river encompasses crust that records the Grenville Orogeny as well as the Taconic, Acadian, and Alleghanain orogenies. Th-Pb ages of detrital monazite from French Broad alluvium provide a full record of each of these tectonic events, without a massive bias toward the Grenville event. Combining crystal chemistry with ages may allow for even more refined tectonic discrimination. Detrital garnet has chemical compositions that allow for a direct link between the crystal and its source. Finally, detrital muscovite have 40Ar/39Ar ages that emphasize the importance of the younger Appalachian tectonic events. The combination of these minerals and methods, in conjunction with detrital zircon results, provides an orogenic record that plays in much higher tectonic fidelity.