THE ANTLER OROGENY REINTERPRETED

We reinterpret the Antler orogeny as sinistral-oblique convergence at a dextral step in the southwest Laurentian continental margin. Mississippian rocks of the Antler foreland basin, plotted on palinspastically restored, time-slice paleogeographic maps, record the first arrival of synorogenic clastic rocks from the north or northwest in late Early Mississippian (Osagean) time. An orogenic highland formed in northern Nevada in Late Middle Mississippian (Meramecian) time, producing a regional angular unconformity there while submarine deposition continued in the southern part of the state. Structures truncated by the unconformity document NW-SE and W-E shortening. Late Mississippian (Chesterian) rocks of the Antler successor basin overlie the unconformity and bracket the age of Antler tectonism. There is no evidence of tectonic activity at the Devonian-Mississippian boundary, the time previously cited for the Antler orogeny.

Sr isotope data plotted on a palinspastically restored map document an abrupt dextral step in the Laurentian continental margin at the latitude of northern Nevada. The map trace of the Roberts Mountains thrust (RMT) lies southeast of and parallel to this Sr 0.706 line. We interpret the RMT as the basal contact of translated material from the north, the Roberts Mountains allochthon (RMA), emplaced onto the margin of Laurentia at this step. This contact was almost certainly not a single fault surface but multiple faults.

This model explains many of the anomalous features of the Antler orogeny and the RMT. There was no magma generation or regional metamorphism during the orogeny because there was no subduction. The RMT is unique along the Laurentian margin because it is limited to the location of a protruding step in the continental margin which resulted in local convergence. The model also eliminates the mechanically implausible 145 km slip previously cited for the RMT. This original estimate of offset assumed eastward emplacement, so was based on the mapped E-W extent of the RMT. Much less shortening is required if the rocks of the RMA were translated southward and emplaced against a protruding step in the margin. The resulting tectonic highland shed syntectonic sediments toward the continental margin.