Wright, James E.¹, and Groves, Marty J.², (1) Department of Geology, University of Georgia, Athens GA 30602, (2) Department of Geological and Environmental Sciences, Stanford University, Stanford CA 94305
The Late Devonian/Early Mississippian Antler Orogeny has been interpreted in terms of a collision between either an east or west facing arc with the western Laurentian continental margin resulting in the emplacement of deep water marine facies (Roberts Mountains Allochthon; RMA) over the shelf terrane. These models assume margin parallel arc terrane(s). The Alexander terrane was interpreted to represent a fragment of this arc terrane. Later studies, however, showed that the Alexander terrane is exotic to western Laurentia, and many reasoned that it could not be the Antler arc. Paleozoic arc and subduction complex terranes within the Klamath Mountains and Sierra Nevada were also interpreted as vestiges of this arc. Detrital zircon studies have revealed the presence of ca. 1.5 Ga non-Laurentian detrital zircon (DZ) as well as abundant Neoproterozoic DZ indicating that the arc terrane is also exotic to western Laurentia. This DZ signature is present in the Alexander terrane and is also found in the Sierra City melange of the Sierra Nevada foothills belt. DZ from the Yreka terrane of the eastern Klamath Mountains also contain the 1.5 Ga and Neoproterozoic zircon components as well as Late Devonian DZ indicating a Late Devonian or younger assembly of the Yreka terrane thrust system. Should we also abandon the Klamath and Sierran terranes as remnants of the Antler Arc? Perhaps not. Relatively recent paleontologic finds within the coarse immature clastics of the Harmony formation of the RMA indicate a Late Devonian/Early Mississippian age instead of a Cambrian age. DZ from the Harmony are very similar to that of the British Columbia miogeocline, as well as other strata contained within the bulk of the RMA. The Harmony also contains another sandstone facies with a bimodal Neoproterozoic/ Grenville age DZ population not likely derived from western Laurentia but potentially sourced by the exotic arc and its basement. We suggest that a north facing (in current coordinates) Early Devonian Caribbean style arc formed between Laurentia and Gondwana . The arc was constructed on a basement of older exotic convergent margin assemblages most likely derived from Baltica. Late Devonian collision of the arc terrane and its exotic basement with a fragment of continental crust left behind following Rodinia breakup resulted in uplift of the crustal fragment leading to development of submarine fan deposits of the Harmony as well as similar age deposits in the Sierra Nevada (Picayune Valley Formation). This crustal block sourced most of the other sandstone units within the RMA, as well as the bulk of the Shoo Fly Complex of the Northern Sierra terrane prior to collision. Lateral tectonic escape from the collision zone resulted in the emplacement of the RMA, and formation of the Antler Foredeep. The collision also resulted in the formation of the Yreka terrane thrust belt and its foredeep, the Late Devonian/Early Mississippian Bragdon Formation of the Klamath Mountains that also contains abundant 1.5 Ga and Neoproterozic DZ.