Print this pageAntler Tectonic System and Global Analogs in the Mediterranean and Asia
CARPENTER, JAMES A.
Antler orogenesis affected the western margin of North America from middle Frasnian to Meramecian time, and culminated with emplacement of the Roberts Mountains allochthon above a west dipping subduction zone. New paleostructural analyses suggest the allochthon advanced eastward 175-210 km, attaining a velocity greater than or equal to 5.25 cm/yr in the Kinderhookian. Biostratigraphic information from synorogenic deposits (pre- and post-emplacement Chainman-Diamond Peak clastics) constrains emplacement timing. The toe of the allochthon overrode clastics of Osagean age (lower anchoralis/latus zone/lower CM zone; approx. 355 Ma). The deformed toe was depositionally onlapped, subsequent to its eastward-most advance, by slightly younger Chainman-Diamond Peak clastics (upper anchoralis-latus zone/upper CM zone; approx. 353 Ma).
As the allochthon advanced, the autochthonous crust flexed as a result of loading, and experienced small-displacement reverse faulting and subtle folding as a result of compression. Mapping isopachs, unconformities, and the Roberts Mountains thrust subcrop shows eight newly recognized paleostructural elements in the autochthon. These include the early-formed north trending Antler forebulge, the east trending Monitor arch and Crescent depression, and the north trending ancestral Diamond, Blackburn, Willow Creek, Woodruff Creek and Dry Creek fault propagation anticlines. A broad uplift-subsidence couplet formed in the Frasnian. The stratigraphic record is used to track the eastward advance of the forebulge uplift, which migrated coincident with the allochthon. The syntectonic Pilot Shale back-bulge basin formed on the eastern side of the forebulge uplift. I wonder: Does the Yellow-weathering unit (lower Frasnian) of the Guilmette Limestone represent the earliest deposit of the incipient Pilot Shale back-bulge basin?
Downcutting on unconformities and isopach relations show that the fault propagation anticlinal features experienced renewed flexure in the Pennsylvanian-Permian Ancestral Rockies orogeny contemporaneous with deposition in the Dry Mountain basin. Some propagation faults were reactivated during the Cretaceous Eureka/Sevier orogeny and again, with normal sense movement, in the Cenozoic Basin-Range extensional orogeny. Crustal anisotropy is inferred to be inherited from late Proterozoic rift structures.
Convergent margin analogs such as the Banda/Irian Jaya system of Indonesia and the Apennine system of Italy can be profitably compared to the Antler. An enigma may be explained: A significant Antler volcanic arc would not be generated if shortening was small in relation to the dip of subducting oceanic crust.