The Antler orogeny was a tectonic event that began in the early Late Devonian with widespread effects continuing into the Mississippian and early Pennsylvanian. Most of the evidence for this event is in Nevada but the limits of its reach are unknown. A great volume of conglomeratic deposits of mainly Mississippian age in Nevada and adjacent areas testifies to the existence of an important tectonic event, and implies nearby areas of uplift and erosion, but the nature and cause of that event are uncertain and in dispute. Although it is known as an orogeny (mountain building event), some of the classic features of orogeny as commonly defined such as metamorphism, and granitic intrusives have not been linked to it. In spite of this, the event is universally designated as an orogeny and that practice is continued here. This article outlines what is known and unknown about the Antler orogeny and describes three current theories regarding its nature and origin.
Two facies of lower Paleozoic rocks
There are two principal facies of lower Paleozoic rocks in Nevada. In the eastern part of the state, a north-trending fossil-rich carbonate shelf of Ordovician to Devonian age, termed the carbonate or eastern assemblage, gives way westward to a contemporaneous expanse of siliceous sedimentary deposits and minor mafic volcanic rocks termed the siliceous or western assemblage. Crafford assigned these two facies respectively to the shelf domain and the basin domain. The dark color of the western assemblage, the scarcity of carbonate rocks, and a near absence of shelly fossils, are generally interpreted to indicate a relatively deep-water depositional environment. The western assemblage also differs from the eastern assemblage in its components of bedded chert, basalt bodies, barite deposits, and sulfide deposits. The nature of the two assemblages and their relation to one another are critical for an understanding of the Antler orogeny. The western facies assemblage is generally thought to be displaced from the west and to constitute the upper plate of an extensive thrust fault—the Roberts Mountains thrust. The eastern facies assemblage is thought to extend westward under the thrust plate. The basis for this belief is that the western facies domain is dotted with anomalous blocky exposures of contemporaneous eastern facies shelf strata, some of mountain size, surrounded by exposures of western facies rocks. These have been interpreted almost universally as exposures of the carbonate shelf facies in windows of the Roberts Mountains thrust sheet, and to prove the existence of that thrust sheet.
Plate tectonics
From an early date, geologists have struggled to explain the presence in Nevada and adjacent areas of the Antler orogenic deposits without achieving a consensus. The advent of plate tectonic theory provided a variety of possible mechanisms by which the Roberts Mountains thrust and the orogenic deposits could be explained, but none of them has been universally accepted. As outlined in the following paragraphs, plate motion along the western margin of the North American continent in the Late Devonian has been offered as the cause of the orogeny and three varieties of it have been tried—east dipping subduction, west-dipping subduction, and strike-slip motion. None of them is without serious problems and the nature and driving force of the orogeny remain uncertain.