Timing and mode of emplacement of the Roberts Mountains allochthon, Antler orogeny
This Antler model involves incipient subduction of continental crust and miogeoclinal strata beneath an accretionary wedge composed primarily of oceanic sedimentary rocks and lesser outer shelf strata (under-thrusting and sediment obduction). Three phases of associated tectonism are: (1) first emplacement phase, in which only oceanic crust is subducted, evidenced primarily by foreland epeirogeny; (2) second emplacement phase, in which incipient subduction of continental crust occurs and a flysch trough is formed; (3) post-emplacement phase of isostatic uplift of continental margin and allochthon, accompanied by molasse sedimentation. The first two phases, associated with compressive movements of the Antler orogeny of Nevada, occurred between mid-Frasnian and mid-Osagean time. This interval equals about 23 m.y., based on C. A. Sandberg9s estimates of the duration of conodont zones. Overlap of the surface trace of the Roberts Mountains thrust by the Kinderhookian Webb Formation preceded final emplacement of the allochthon. Ordovician, Devonian, and Kinderhookian units in the allochthon, as mapped in the Carlin-Pinyon Range area by Smith and Ketner, overlie the oldest Antler flysch (Kinderhookian age) and are interpreted to have been encased within the Antler clastic wedge during Osagean time.