Antler orogeny

There are two principal facies of lower Paleozoic rocks in Nevada.[1] 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.[2] Crafford assigned these two facies respectively to the shelf domain and the basin domain.[3] 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.[2][3] The western assemblage also differs from the eastern assemblage in its components of bedded chert, basalt bodies, barite deposits, and sulfide deposits.[3] 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.[1] 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.[2] 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.[2][3]

From an early date,[7] 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[5] and the nature and driving force of the orogeny remain uncertain.

This much is known concerning the Antler orogeny:

1. Great volumes of clastic rocks were deposited in Nevada and surrounding areas on both the western and eastern facies assemblages;[4][8]
2. Almost all of the orogenic deposits range in age from Late Devonian to mid-Pennsylvanian; some may be of Middle Devonian age;[8]
3. The orogenic deposits are in a generally disconformable relation to the underlying strata;[8]
4. Some areas within the western facies domain were first elevated and eroded, then were depressed and received a blanket of conglomeratic sediments;[8]
5. Some clasts in these deposits were derived from areas outside the western facies domain but the bulk were derived from the western facies assemblage;[4][5][8]
6. Blocky exposures of eastern facies carbonate rocks, of Cambrian to Devonian age are scattered across the western facies domain;[2]
7. No metamorphic rocks, volcanic arcs, or granitic intrusives associated with the Antler orogeny have been reported;
8. The age of the earliest-known orogenic deposits coincides approximately with the age of the Alamo impact event of early Late Devonian age[5] —a possibly significant coincidence.

Based on stratigraphic relations near Antler Peak, of the Battle Mountains, Roberts introduced the term Antler orogeny in an abstract as follows: The earliest orogeny, here named the Antler orogeny ... took place during Mississippian (?) and early Pennsylvanian time.[9] That abstract was followed in 1951 by his geologic map of the Antler Peak quadrangle in the text of which he described the Antler orogeny in detail and somewhat refined its age span: During the Antler orogeny, formations in Battle Mountain ranging in age from Ordovician to Mississippian (?) were complexly folded and faulted. As these rocks are unconformably overlain by the Battle Formation of Early Pennsylvanian (Des Moines) age, the orogeny probably took place during the Late Mississippian. The orogeny may have continued into Early Pennsylvanian, however, for the coarse conglomerates of the Battle Formation indicate derivation from a rugged highland area.[10] In a subsequent influential paper, Roberts and others adjusted the age of the Antler orogeny as follows: This belt is now known to have been the locus of intense folding and faulting during the Antler orogeny in latest Devonian or Early Mississippian time ...[1] In the same paper the authors established a connection between the Antler orogeny and a major thrust fault as follows: A belt along the 116°-118° meridians—the Antler orogenic belt—was the locus of intense folding and faulting that culminated in the Roberts Mountains thrust fault... That age range and connection with the Roberts Mountains thrust were confirmed in a widely quoted paper by Silberling and Roberts: During the Late Devonian or Early Mississippian ... the Antler orogenic belt was intensely folded and faulted, and during Mississippian time the Roberts Mountains thrust sheet was emplaced.[11] The effect of this revision in the age of the orogeny was to exclude the evidence in the Antler Peak quadrangle cited above for a Late Mississippian to mid-Pennsylvanian age, on which the concept of the Antler orogeny originally had been based, and to establish the conventional age of that orogeny as Late Devonian to Early Mississippian.

The original date of the Roberts Mountains thrust fault was post-Paleozoic.[12] However, with publication of the 1958 and 1962 papers cited above, the authors revised the age of the Roberts Mountains thrust to coincide with the Late Devonian to Mississippian Antler orogeny and to extend the name far beyond the Roberts Mountains.

Over a period of 22 years numerous reports relating the Antler orogeny and Roberts Mountains thrust to plate convergence were published in various journals, and because their basic tenets have been widely accepted, they are here termed the conventional theories. The earliest effort to relate plate tectonics specifically to the Antler orogeny was briefly outlined by E.M. Moores: A collision of this continental margin with a subduction zone dipping away from it in late Devonian-early Mississippian time ... resulted in deformation of the pre-existing continental marginal rocks in the Antler Orogeny.[13]

Two principal contrasting tectonic theories were published in greater detail between 1972 and 1992 as related below. One theory involved closure of a back-arc basin between the western continental margin and a volcanic arc over an east-dipping subduction zone. A second theory involved collision of the continent with an island arc above a west-dipping subduction zone. Both were based on the basic understanding that the western facies assemblage is composed of oceanic deposits and that it is underlain by an extensive thrust fault.