GEOLOGIC MAPPING OF A POLYPHASE TECTONIC-MAGMATIC COMPLEX IN THE ROCK ROLL CANYON 7.5’ QUADRANGLE, SOUTH-CENTRAL IDAHO

In south-central Idaho, the Rock Roll Canyon 7.5' Quadrangle, encompasses a structurally complex area of the western flank of the Pioneer Mountains and the eastern part of the Boulder Mountains. The area has been the subject of a decades-long mapping effort by numerous Idaho State University Geoscience personnel. The recently completed map uniquely displays the upper-crustal structural stack of the extended upper plate of the Pioneer Mountains metamorphic core complex. Here, deformed Paleozoic rocks that underwent northeast-vergent crustal contraction during the Antler and the Sevier Orogenies, were subject to Paleogene and Neogene crustal extension. Ordovician through Devonian sedimentary rocks show low-grade metamorphism and penetrative deformation attributed to the Late Devonian-Early Mississippian Antler Orogeny. Conglomeratic Mississippian chert-lithic sandstone flysch of the Copper Basin Formation is exposed in the northeastern part of the quadrangle. Devonian and older rocks containing an Antler fabric are unconformably overlain by an overlap assemblage of Pennsylvanian – Permian rocks of the Wood River Formation (Sun Valley Group) which are not metamorphosed and which were deformed in large-amplitude overturned folds and minor thrust faults during northeast-vergent Sevier orogenic shortening. The Late Paleozoic rocks, are cut by gently southwest-dipping, oblique-slip sinistral normal faults including the Lake Creek Fault, forming three major extensional allochthons , that have undergone Paleogene northwest translation. Paleogene extensional faults translated Eocene granitic intrusive rocks and both cut- and are cut by Eocene Challis Volcanic related hypabyssal dikes. In the southwest part of the quadrangle, rocks are cut by steeply dipping, northwest striking Neogene normal faults. The area is notable for the lack of syn-extensional Paleogene sedimentary basin strata.