(U-Th)/He thermochronologic constraints on the evolution of the northern Rio Grande Rift, Gore Range, Colorado, and implications for rift propagation models Rachel L. Landman* and Rebecca M. Flowers Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA ABSTRACT INTRODUCTION ard, 2002; Heller et al., 2003; Duller et al., 2012). In addition, reference to a northward- The Rio Grande Rift system tapers north- The Rio Grande Rift is a zone of intraconti- propagating rift is relatively common in studies ward into the center of the southern Rocky nental extension that extends >1000 km from located in the vicinity of the rift where the point Mountains. How rift initiation and evolu- Mexico into central Colorado (Fig. 1). The rift is only peripherally relevant to the investigation tion are related to development of the Rocky bisects the southern Rocky Mountains, and for (e.g., Penn and Lindsey, 1996; Porreca et al., Mountains is not well understood. The Gore most of its length separates the Colorado Pla- 2006; Reiter and Chamberlin, 2011), but where Range and adjacent Blue River Valley of cen- teau to the west from the Great Plains to the the mention demonstrates the extent to which tral Colorado are the northernmost signifi - east. There is no consensus on the geodynamic this concept is entrenched in the literature. cant fault-related manifestations of the rift. mechanisms responsible for the origin of the Although the Upper Arkansas Basin is the New apatite (U-Th)/He data from the range rift. Models include extensional collapse of the northernmost of the major Rio Grande Rift record only middle and late Cenozoic exhu- southern Rocky Mountains (e.g., Cordell, 1978; basins containing deep Tertiary fi ll (Fig. 1; mation phases, unlike neighboring Rocky Eaton, 1986), clockwise rotation of the Colorado Tweto, 1979), basins lacking that fi ll but shar- Mountain uplifts that record a single phase Plateau (e.g., Hamilton, 1981; Cordell, 1982; ing other structural similarities with the south- of Late Cretaceous–early Tertiary unroof- Steiner, 1986), and mantle upwelling associated ern rift basins extend north to the Wyoming ing. Middle Tertiary dates require that with sinking of the Farallon slab (Moucha et al., border (Chapin and Cather, 1994; Kellogg, Gore Range basement remained covered by 2008). How the rift is related to the evolution of 1999). The evolution of these less-studied rift sedimentary rocks until at least the Middle the Rocky Mountains is similarly controversial; basins to the north is not as well constrained as Eocene. Normal faulting initiated during there are inconsistencies in the literature as to the history of those to the south. Determining the Oligocene, inducing exhumation of base- whether the rift propagated spatially through whether the northern basins evolved over a time ment to the surface and deposition of synrift time, and how this bears on broader uplift in frame similar to that of their southern counter- fi ll in the Blue River Valley. Major cooling the southern Rocky Mountains region. Studies parts, or are somewhat younger and developed and unroofi ng in the Miocene were restricted suggest that onset of rifting was broadly coeval as the rift expanded northward, is key for evalu- to the southern Gore Range and continued ca. 29–26 Ma from at least central New Mexico ating hypotheses of a propagating rift and the until at least 7 Ma, with ~2.3 km of total through southern Colorado (e.g., Chapin and proposed link with late Tertiary Rocky Moun- displacement along the Blue River normal Cather, 1994). This synchroneity in rift develop- tain uplift. fault. This middle to late Tertiary unroofi ng ment has been used to support Colorado Plateau The Gore Range in central Colorado is the history is strikingly similar to that inferred rotation as the cause of rift initiation (Chapin most signifi cant of the northern Rio Grande for other rift fl ank uplifts to the south, and and Cather, 1994). However, the rift’s north- Rift fl ank uplifts (Fig. 2). The Blue River nor- suggests broad synchroneity in the middle ward-tapering character has led to the persis- mal fault separates the Blue River Valley from Tertiary onset and subsequent evolution of tent notion that rifting initiated in the south and the range and accommodates ≥1.4 km of verti- >700 km of the Rio Grande Rift. The results then propagated north into Colorado. This idea cal displacement at its southern end (Kellogg et highlight that the notion of a northward- is most notable in work inferring late Cenozoic al., 2011). The ~1.4 km of relief in the south- propagating rift, as suggested by its north- elevation gain of the southern Rocky Mountains ern Gore Range stands out against the neigh- ward-tapering geometry, is incorrect, and based on tilting of late Cenozoic units on the boring and lower relief western Front Range preclude models invoking rift propagation Great Plains. These studies specifi cally invoked and Park Range. The range is also distinctive to explain late Cenozoic elevation gain of the northward rift propagation as a plausible cause when evaluating patterns of apatite fi ssion track Rocky Mountains. of surface uplift (McMillan et al., 2002; Leon- (AFT) dates in the southern Rocky Mountains *Corresponding author: [email protected] Geosphere; February 2013; v. 9; no. 1; p. 170–187; doi:10.1130/GES00826.1; 10 fi gures; 1 table; 3 supplemental fi gures. Received 22 May 2012 ♦ Revision received 11 September 2012 ♦ Accepted 12 September 2012 ♦ Published online 13 December 2012 170 For permission to copy, contact [email protected] © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/1/170/3343905/170.pdf by guest on 03 October 2021 (U-Th)/He Gore Range 110°0'0"W 105°0'0"W Colorado Middle Park 40°0'0"N Blue River Valley 40°0'0"N HIGH PLAINS Upper Arkansas Basin ROCKY MOUNTAINS San Luis Basin COLORADO PLATEAU Texas Española Basin Albuquerque New Basin 35°0'0"N Mexico 35°0'0"N Oligocene - La Jencia and Pliocene synrift deposits Socorro Basins Tertiary volcanic rocks Proterozoic basement AFT Date < 20 Ma 20 - 30 Ma 30 - 45 Ma > 45 Ma N 0 50 100 200 Kilometers 110°0'0"W 105°0'0"W Figure 1. Topography, geology, and apatite fi ssion track (AFT) dates in the Rio Grande Rift and southern Rocky Mountain region. Black box marks location of the Gore Range and Blue River Valley study area, shown in Figure 2. Data for the uplifts adjacent to the Albuquerque Basin include the previously published AHe [apatite (U-Th)/He] dates from the Rio Grande Rift (House et al., 2003); no other detailed AHe data set from the southern Rocky Mountains or rift has yet been published. AFT dates are from Bryant and Naeser (1980); Bryant et al. (1990); Church and Bickford (1971); Kelley and Duncan (1986); Kelley et al. (1992); Kelley and Chapin (1995, 1997, 2004); Kelley (2005); Lindsey et al. (1986); Lipman et al. (1986); Marvin and Dobson (1979); Naeser (1971); Nae- ser et al. (1990, 2002); Olson et al. (1977); Wilks and Chapin (1997). Geologic maps are from Green (1992) and Green and Jones (1997). Geosphere, February 2013 171 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/1/170/3343905/170.pdf by guest on 03 October 2021 106°30'0" W Landman and Flowers 106°0' 0" W ColoradoColorado RiverRiver MiddleMiddle ParkPark Quaternary sediments PParkark RRangeange e Tertiary volcanic rocks or on GoreG y an CanyonC Tertiary intrusive rocks 40° 40° 0' Latest Cretaceous - 0 0' "N Tertiary sedimentary rocks 0 "N Jurassic - Late Cretaceous sedimentary rocks Pennsylvanian - Triassic sedimentary rocks B Cambrian - BlueB River Mississippian sedimentary rocks lu e Proterozoic metamorphic ′ C R and plutonic basement iv e r C GoreG Fault cross-fault o ′ r D BBluelue e F RiverRiver a u VValleyalleyalley l t FrontFront RangeRange GoreGore BlueB River Fault l D u ′ RangeRange e A R iv e r F a CentralCentral ColoradoColorado TTroughroughrough u lt A ′ B WWilliamsilliamsilliams 39 MosquitoMosquito RangeRange 39 ° SawatchSawatch RangeRange 3 ° 0 ult FaultFa ThrustTh 3 ' rust 0 0 "N ' 0 "N Thrust s e g SW A A′ n 5 km NE a R Gore Range Williams Range Gore Fault Front Range Central Colorado Trough Blue River Fault landslide deposit e l Blue River Valley i m n 0 km enmilee Range TTenmile Range N 5 km 05102.5 -5 km Kilometers 106°30'0" W 106°0 0" W Figure 2. Geologic map of the Gore Range, Blue River Valley, and adjacent regions after Tweto (1979), Green (1992), and Kellogg et al. (2011). Inset is cross section adapted from Kellogg et al. (2004) along section line A–A′ (no vertical exaggera- tion). Dashed gray box is the outline of Figure 3. Section line B–B′ marks the location of the geologic cross section in Figure 5. Section lines C–C′ and D–D′ mark the locations of the cross-section reconstructions in Figure 10. 172 Geosphere, February 2013 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/1/170/3343905/170.pdf by guest on 03 October 2021 (U-Th)/He Gore Range (Fig. 1). Middle to late Tertiary AFT dates for on rift fl anks suggests initial basin development the cross fault have a relief of no more than the Gore Range (Naeser et al., 2002) contrast and sedimentation in the Late Oligocene, with 700 m and typically <200 m, with no more than with Late Cretaceous and early Tertiary results the onset of rifting broadly synchronous from ~200 m of offset across the Blue River fault from adjacent areas (Bryant and Naeser, 1980; at least the Socorro Basin northward through based on the local juxtaposition of Jurassic units Naeser et al., 2002; Kelley and Chapin, 2004; the San Luis Basin (Chapin and Cather, 1994; (Fig.