The Rocky Mountain Front, southwestern USA Charles E. Chapin, Shari A. Kelley, and Steven M. Cather New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA ABSTRACT northeast-trending faults cross the Front thrust in southwest Wyoming and northern Range–Denver Basin boundary. However, Utah. A remarkable attribute of the RMF is The Rocky Mountain Front (RMF) trends several features changed from south to north that it maintained its position through multi- north-south near long 105°W for ~1500 km across the CMB. (1) The axis of the Denver ple orogenies and changes in orientation from near the U.S.-Mexico border to south- Basin was defl ected ~60 km to the north- and strength of tectonic stresses. During the ern Wyoming. This long, straight, persistent east. (2) The trend of the RMF changed from Laramide orogeny, the RMF marked a tec- structural boundary originated between 1.4 north–northwest to north. (3) Structural tonic boundary beyond which major contrac- and 1.1 Ga in the Mesoproterozoic. It cuts style of the Front Range–Denver Basin mar- tional partitioning of the Cordilleran fore- the 1.4 Ga Granite-Rhyolite Province and gin changed from northeast-vergent thrusts land was unable to penetrate. However, the was intruded by the shallow-level alkaline to northeast-dipping, high-angle reverse nature of the lithospheric fl aw that underlies granitic batholith of Pikes Peak (1.09 Ga) faults. (4) Early Laramide uplift north of the RMF is an unanswered question. in central Colorado. The RMF began as the CMB was accompanied by southeast- a boundary between thick cratonic litho- ward slumping and décollement faulting of INTRODUCTION sphere to the east (modern coordinates) and upper Cretaceous sedimentary units. (5) The an orogenic plateau to the west and remains Boulder-Weld coal fi eld developed within Why the Southern Rocky Mountains trend so today. It was reactivated during the 1.1 the zone of décollement faulting. (6) The north-south while the Pacifi c–North Ameri- to 0.6 Ga breakup of the supercontinent huge Wattenberg gas field formed over a can convergent margin and its related tectonic Rodinia and during deformation associated paleogeothermal anomaly. (7) Apatite fi ssion and magmatic features mostly trend northwest with formation of both the Ancestral and track (AFT) cooling ages in the Front Range (Fig. 1) has been one of the enduring geologic Laramide Rocky Mountains. Its persistence north of the CMB are almost all associated mysteries of the southwestern United States. as a cratonic boundary is also indicated by with Laramide deformation (ca. 80–40 Ma), The imposing topographic escarpment along the emplacement of alkalic igneous rocks, gold- whereas south of the CMB, AFT ages in the eastern fl ank of the Southern Rocky Mountains telluride deposits, and other features that Front Range and Wet Mountains vary widely between Las Vegas, New Mexico, and south- point to thick lithosphere, low heat fl ow, and (ca. 449–30 Ma). Proterozoic rocks still retain ern Wyoming (Fig. 1) is often referred to as the episodic mantle magmatism from 1.1 Ga pre-Laramide AFT ages in a zone as much Rocky Mountain front (RMF). However, as a to the Neogene. Both rollback of the Faral- 1200 m thick south of the CMB, revealing tectonic feature the RMF approximately coin- lon fl at slab ca. 37 Ma and initiation of the comparatively modest uplift and erosion. cides with long 105°W for ~1500 km from near Rio Grande Rift shortly thereafter began A fourth step is a ~250 km defl ection of the the U.S.-Mexico border to southern Wyoming, near the RMF. Geomorphic expression of RMF from the Laramie Range to the Black where it is defl ected northeastward along the the RMF was enhanced during the late Mio- Hills of South Dakota along the southeastern boundary of the Wyoming Archean province to cene to Holocene (ca. 6–0 Ma) by tectonic boundary of the Wyoming Archean province. the Black Hills of South Dakota (Karlstrom and uplift and increased monsoonal precipita- Laramide synorogenic sedimentation Humphreys, 1998; Marshak et al., 2000). Since tion that caused differential erosion along the occurred mainly in Paleocene and early its origin in the Mesoproterozoic, the RMF has mountain front, exhuming an imposing 0.5– Eocene time on both sides of the Front Range been reactivated several times and has been 1.2 km escarpment, bordered by hogbacks in Colorado, but the timing and style of a signifi cant infl uence in development of the of Phanero zoic strata and incised by major basin-margin thrusting differed markedly. Ancestral and Laramide Rocky Mountains, as river canyons. Moderate- to high-angle thrusts and reverse well as Cenozoic magmatic and rifting events. Here we investigate four right-stepping faults characterized the east side beginning Here we utilize a variety of geological and geo- defl ections of the RMF that developed dur- in the Maastrichtian (ca. 68 Ma). On the west physical data to establish the timing, extent, and ing the Laramide orogeny and may reveal side, low-angle thrusts overrode the Middle tectonic character of the RMF from the Meso- timing and structural style. The Sangre Park and South Park basins by 10–15 km proterozoic onward. de Cristo Range to Wet Mountains and beginning in the latest Paleocene–early This paper began out of curiosity as to how Wet Mountains to Front Range steps are Eocene. This later contraction correlates and why the RMF makes progressive steps to related to reactivation of the eroded stumps temporally with the third major episode of the right as one moves northward along it in of Ancestral Rocky Mountain uplifts. In shortening in the Sevier fold and thrust belt, Colorado and Wyoming, and what that could northern Colorado, the Colorado Mineral when the Hogsback thrust added ~21 km of tell us about Laramide tectonics. The expecta- Belt (CMB) ends at the RMF; no signifi cant shortening to become the easternmost major tions were modest. However, as the investigation Geosphere; October 2014; v. 10; no. 5; p. 1043–1060; doi:10.1130/GES01003.1; 11 fi gures. Received 27 November 2013 ♦ Revision received 3 June 2014 ♦ Accepted 11 August 2014 ♦ Published online 5 September 2014 For permission to copy, contact [email protected] 1043 © 2014 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/5/1043/3332632/1043.pdf by guest on 24 September 2021 Chapin et al. Figure 1. Topography and seis- micity (circles, size refl ects mag- nitude) of the western United States. The Rocky Mountain BH Front is the remarkably straight, east-facing topographic bound- ary of the southern Rocky Mountains that parallels long WB 105°W. Shading mimics illumi- FR nation from the west. Colors D refl ect elevation; blue is near sea level and white is the highest (+2300 m). Reproduced from WM Simpson and Anders (1992). SC BH—Black Hills, FR—Front Range, D—Denver, WM—Wet SF CP Mountains, SC—Sangre de Cristo Mountains, SF—Santa Fe, CP—Colorado Plateau, WB—Wyoming basin. progressed, we realized that two more impor- ment map of Colorado compiled by Sims et al. as the 1.4 Ga Granite-Rhyolite Province (Karl- tant subjects were involved, the Precambrian (2001) from interpretations of aeromagnetic strom et al., 2004). ancestry of the RMF and the nature of the litho- anomalies. Sanders et al. (2006) estimated from 40Ar/39Ar spheric structure that underlies it. The ancestry The Neoproterozoic extensional events that thermochronometry that ~12 km of exhumation is known, but the underlying structure is yet a accompanied breakup of the Rodinia supercon- of Mesoproterozoic rocks occurred after 1.0 Ga mystery. So, the paper evolves from the ques- tinent between 1.1 Ga and 0.6 Ga (Karlstrom west of the RMF near Las Vegas, New Mexico, tion of the steps to the underlying lithospheric and Humphreys, 1998; Marshak et al., 2000; compared to ~3–5 km of exhumation between structure and ends with a list of constraints and Timmons et al., 2001; Keller et al., 2005; Luther 700 and 600 Ma east of the RMF. Sedimentary descriptions of three geophysical studies that et al., 2012) are also part of the tectonic ances- and volcanic rocks of the Mesoproterozoic Las provide some insight into possible lithospheric try of the RMF. This continental breakup estab- Animas Group (ca. 1.1 Ga) in southeastern controls. lished the structural framework of Laurentia Colorado (Tweto, 1980, 1987) and the Debaca (now the Precambrian core of North America), Group (ca. 1.26 Ga) in southeastern New Mex- TECTONIC ANCESTRY including the Cordilleran passive margin and ico (Karlstrom et al., 2004; Amarante et al., a series of northwest- and north-trending fault 2005) have been preserved in a stable cratonic Karlstrom et al. (2004, p. 23) pointed out, zones. Karlstrom and Humphreys (1998, fi g. 3 setting east of the RMF, in contrast to the appar- “An important but incompletely investigated therein) interpreted a north-trending generalized ent uplift and denudation west of the RMF. The Proterozoic north-striking boundary exists fault zone extending from southern New Mexico alkaline Pikes Peak granitic batholith (Barker along the Rocky Mountain front…East of this to northern Colorado at 1.1 Ga as the newly cre- et al., 1975; Wobus, 1976) in central Colorado boundary, 1.4 Ga rocks include shallow level ated Rocky Mountain trend. Similarly, Marshak was emplaced at shallow depths on the RMF plutons and volcanic rocks, whereas west of the et al. (2000) interpreted a north-trending eastern ca. 1.09 Ga (Smith et al., 1999; Karlstrom boundary, rocks of the same age were emplaced edge of the Rocky Mountain–Colorado Plateau et al., 2004) and is close to the present western at ~10 km depths…Thus, a Proterozoic fault province extending from the Mexico-U.S.