LITHOTECTONIC MAPS OF THE NORTHERN ROCKY MOUNTAINS RECORD ARCHEAN TO CENOZOIC CONTINENTAL MARGINS

Lithotectonic maps of the northern Rocky Mountains were derived from 44 geologic source maps at scales of 1:100,000 to 1:250,000. Each lithotectonic unit combines geologic-map units representing a shared age-range and tectonic setting. These maps record Archean to Cenozoic continental margins, as summarized below.

Mafic crust (~3.5 Ga) initiated the Archean Wyoming proto-continent. Metasedimentary and subduction-related granitoid terranes later collided and sutured at ~3.0 to 2.8 Ga (during assembly of Kenorland). Post-orogenic rifting preceded intrusion of the mantle-plume-related Stillwater complex at ~2.7 Ga.

The foreland fold-thrust belt of the Paleoproterozoic Selway orogen parallels the northwestern margin of the Wyoming Province. The Selway seaway closed between convergent Wyoming and Hearn provinces at ~1.75 Ga (during assembly of Nuna).

The Mesoproterozoic Belt-Purcell basin rifted from ~1.5 to 1.45 Ga (during breakup of Nuna), and then sagged. Except for local folding and bimodal magmatism at ~1.37 to 1.35 Ga, sagging continued until ~1.25 Ga. Discordant Pb-isotopic ratios and garnets (~1.2 to 1.0 Ga) indicate regional metamorphism during the Grenville orogeny (and assembly of Rodinia).

The Windermere rift (<780 Ma) initiated breakup of Rodinia. In Early Paleozoic time, miogeoclinal strata overlapped Windermere strata along the rifted continental margin. The miogeoclinal wedge thickens westward and thins toward the continental interior. The Kootenai-Covada basin represents the outer miogeocline. The Sublett-Milligen basin was a forearc basin to the Antler island arc. The Pennsylvanian Sublett-Wood River basin contains a thick section of deltaic turbidites.

The Permian to Jurassic Blue Mountains island-arc complex was accreted to North America in Mesozoic time. Cordilleran compressional tectonism and subduction-related magmatism culminated with thrust faulting and emplacement of the Cretaceous Kaniksu, Idaho and Boulder batholiths.

Paleogene extension gave rise to metamorphic core complexes and the Challis magmatic belt. Neogene basalts filled the Columbia basin, and bimodal magmatism migrated eastward along the Snake River Plain as the continent overrode the Yellowstone hot spot, and basin-and-range faulting occurred inland from the San Andreas transform plate-margin.