The Rocky Mountain Front, southwestern USA

Abstract

The Rocky Mountain Front (RMF) trends north-south near long 105°W for ∼1500 km from near the U.S.-Mexico border to southern Wyoming. This long, straight, persistent structural boundary originated between 1.4 and 1.1 Ga in the Mesoproterozoic. It cuts the 1.4 Ga Granite-Rhyolite Province and was intruded by the shallow-level alkaline granitic batholith of Pikes Peak (1.09 Ga) in central Colorado. The RMF began as a boundary between thick cratonic lithosphere to the east (modern coordinates) and an orogenic plateau to the west and remains so today. It was reactivated during the 1.1 to 0.6 Ga breakup of the supercontinent Rodinia and during deformation associated with formation of both the Ancestral and Laramide Rocky Mountains. Its persistence as a cratonic boundary is also indicated by emplacement of alkalic igneous rocks, gold-telluride deposits, and other features that point to thick lithosphere, low heat flow, and episodic mantle magmatism from 1.1 Ga to the Neogene. Both rollback of the Farallon flat slab ca. 37 Ma and initiation of the Rio Grande Rift shortly thereafter began near the RMF. Geomorphic expression of the RMF was enhanced during the late Miocene to Holocene (ca. 6–0 Ma) by tectonic uplift and increased monsoonal precipitation that caused differential erosion along the mountain front, exhuming an imposing 0.5–1.2 km escarpment, bordered by hogbacks of Phanerozoic strata and incised by major river canyons. Here we investigate four right-stepping deflections of the RMF that developed during the Laramide orogeny and may reveal timing and structural style. The Sangre de Cristo Range to Wet Mountains and Wet Mountains to Front Range steps are related to reactivation of the eroded stumps of Ancestral Rocky Mountain uplifts. In northern Colorado, the Colorado Mineral Belt (CMB) ends at the RMF; no significant northeast-trending faults cross the Front Range–Denver Basin boundary. However, several features changed from south to north across the CMB. (1) The axis of the Denver Basin was deflected ∼60 km to the northeast. (2) The trend of the RMF changed from north–northwest to north. (3) Structural style of the Front Range–Denver Basin margin changed from northeast-vergent thrusts to northeast-dipping, high-angle reverse faults. (4) Early Laramide uplift north of the CMB was accompanied by southeastward slumping and decollement faulting of upper Cretaceous sedimentary units. (5) The Boulder-Weld coal field developed within the zone of decollement faulting. (6) The huge Wattenberg gas field formed over a paleogeothermal anomaly. (7) Apatite fission track (AFT) cooling ages in the Front Range north of the CMB are almost all associated with Laramide deformation (ca. 80–40 Ma), whereas south of the CMB, AFT ages in the Front Range and Wet Mountains vary widely (ca. 449–30 Ma). Proterozoic rocks still retain pre-Laramide AFT ages in a zone as much 1200 m thick south of the CMB, revealing comparatively modest uplift and erosion. A fourth step is a ∼250 km deflection of the RMF from the Laramie Range to the Black Hills of South Dakota along the southeastern boundary of the Wyoming Archean province. Laramide synorogenic sedimentation occurred mainly in Paleocene and early Eocene time on both sides of the Front Range in Colorado, but the timing and style of basin-margin thrusting differed markedly. Moderate- to high-angle thrusts and reverse faults characterized the east side beginning in the Maastrichtian (ca. 68 Ma). On the west side, low-angle thrusts overrode the Middle Park and South Park basins by 10–15 km beginning in the latest Paleocene–early Eocene. This later contraction correlates temporally with the third major episode of shortening in the Sevier fold and thrust belt, when the Hogsback thrust added ∼21 km of shortening to become the easternmost major thrust in southwest Wyoming and northern Utah. A remarkable attribute of the RMF is that it maintained its position through multiple orogenies and changes in orientation and strength of tectonic stresses. During the Laramide orogeny, the RMF marked a tectonic boundary beyond which major contractional partitioning of the Cordilleran foreland was unable to penetrate. However, the nature of the lithospheric flaw that underlies the RMF is an unanswered question.