TECTONIC EVOLUTION OF THE NORTHERN IDAHO SECTION OF THE NORTH AMERICAN CORDILLERAN INTERIOR: MAPPING CONSTRUCTION AND COLLAPSE

Geologic mapping by the Idaho Geological Survey (IGS) has yielded a treasure trove of geologic data that may help resolve outstanding problems regarding the tectonic evolution of the northern Idaho segment (~45°45’N) of the Jurassic to Paleogene North American Cordillera. The vast majority of pre-existing work in this part of the U.S. Cordillera has been focused on the peripheral tectono-structural boundaries: the retroarc fold-thrust belt to the east, and the exotic terrane boundary to the west; little research has been done on the geology between these two corridors. Recent 1:24,000-scale geologic mapping by IGS and new geochronology by IGS partners of the Elk City 30’x60’ quadrangle has identified multiple structural zones that vary in deformation styles and timing, providing insight into the tectonic evolution of the orogen.

The Elk City region of Idaho is characterized by amphibolite-facies Mesoproterozoic metasedimentary and metaigneous rocks, which are accompanied by migmatite and widespread penetrative fabric, and intruded by varying stages of Late Cretaceous Idaho batholith granitic rock and Eocene intermediate shallow intrusions. These rocks have undergone a protracted series of deformation, including 1) ductile contractional to dextral-transpressional shear fabrics that deform rocks of the Late Cretaceous Idaho batholith, 2) NE-striking brittle normal faulting that appears to be coeval and related to a similar trending Eocene dike suite, 3) N-striking brittle normal faulting and sedimentation during the Miocene, and 4) late-stage meter-scale E-striking brittle thrust faulting and folding that may be related to fault-controlled gold-bearing quartz veins.

The timing and geometry of these structural features allow for a preliminary explanation of the tectonic construction and demise of the heart of the U.S. Cordilleran orogen at this latitude. With IGS’s new detailed mapping and respective geochronology, we can storyboard the construction of the mountain belt to its eventual poly-episodic demise and collapse. This story is defined as a series of events beginning with Late Cretaceous crustal thickening and high-grade metamorphism to multiple stages of dismemberment and exhumation spanning the Eocene, Miocene, through to the present day, to the late-stage enigmatic small magnitude north-south shortening.