The Rocky Mountain Front, Southwestern USA

Home , Colorado Mineral Belt, Cuchara Formation, Denver Basin, Geology of Colorado, Laramie Mountains, Poison Canyon Formation

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 Phanerozoic 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

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Figure 1. Topography and seis- micity (circles, size reﬂ ects magnitude) of the western United States. The Rocky Mountain BH Front is the remarkably straight, east-facing topographic boundary 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. bor- margin of the continental interior craton. Thus, (post 1.4 Ga) with up to 10 km of east-down der region to South Dakota ca. 0.9 to 0.7 Ga. there seems ample evidence to conclude that the dip slip seems to be required to explain the rock The incipient RMF transected the northeast- RMF occupies a linear lithospheric boundary distribution.” Yet, geological and geophysical trending Yavapai (1.8–1.7 Ga) and Mazatzal that originated between 1.4 and 1.1 Ga during cross sections typically cross the RMF with- (1.7–1.6 Ga) provinces that were progressively the Mesoproterozoic. out sensing anything unusual in the subsurface. added to the southern boundary of the Wyoming Further evidence comes from the North The same is true of the Precambrian base- Archean craton during the Proterozoic, as well American Cordilleran belt of alkaline igneous

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rocks that follows the RMF, and is known as the Kimberlite diatremes, State Line District, Rocky Mountain alkalic province (Lindgren, 377–620 Ma 1933; Wooley, 1987; Allen and Foord, 1991; Hausel, et al., 1985; Lester and Farmer, 1998 Mutschler et al., 1991; McLemore, 1996; Cappa, Estes Park diatreme, 377–395 Ma 1998; Kelley and Ludington, 2002; Jensen and Smith, et al., 1979 Barton, 2000, 2007). Figure 2 summarizes the Green Mtn. diatreme, 367–570 Ma alkaline igneous rocks along the RMF in Colo- Larson and Amini, 1981 rado (modifi ed from Cappa, 1998). The igne- Colorado Mineral Belt, gold deposits ous rocks and their associated mineral deposits 40 Davis and Streufert, 1990 range in age from 1090 Ma (Mesoproterozoic) Kelly and Goddard, 1969 for the Pikes Peak granitic batholith to 27–20 Ma (Oligocene–Miocene) at Spanish Peaks and the Boulder Co., Central City, gold, 63–59 Ma numerous synrift intrusions of the Sangre de Sims, et al., 1963; Rice, et al., 1985 Cristo Range (Miggins, 2002). Important min- CMB Schwartzwalder uranium mine, ca. 69 Ma eral deposits include diamond-bearing kimber- Ludwig, et al., 1985 lite diatremes in the State Line district (other Wallace and Whelan, 1986 kimberlite intrusives occur near Estes Park and Pikes Peak granite, ca. 1090 Ma Boulder), major gold deposits of the northeast- Barker, et al., 1975; Smith, et al., 1999 39 ern Colorado Mineral Belt (CMB) at Central Cripple Creek, 32–27 Ma, giant gold deposit City and in Boulder County, the largest vein-type in a diatreme complex uranium deposit in the U.S. near Ralston Buttes, Kelley and Ludington, 2002 a world-class gold deposit in an Oligocene dia- Jensen and Barton, 2007 treme complex at Cripple Creek, rare earth and thorium deposits associated with Cambrian Wet Mountain alkalic intrusions, alkaline intrusives in the Wet Mountains, and 517–534 Ma, niobium, rare earths, precious metal deposits in Eocene–Oligocene thorium, carbonatite dikes volcanic centers at Silver Cliff and Rosita (see Olson, et al., 1977; Bickford, et al., 1989 38 Cappa, 1998; Abbott and Cook, 2012, for well- Silver Cliff-Rosita volcanic centers, 35–27 Ma organized summaries and references). Similar breccia pipes, small cauldrons, Au and Ag alkaline rocks and gold deposits are present Siems, 1968; Sharp, 1978 along the RMF in New Mexico (McLemore, Spanish Peaks, 27–21 Ma 1996; Kelley and Ludington, 2002). stocks,dike swarms Along the convergent-margin volcanic arcs of Johnson, 1968; Penn and Lindsey, 2009 both North and South America, igneous rocks closest to the thicker, colder lithospheres of the Sangre de Cristo Mtns. intrusions, 33–20 Ma continental interiors tend to be strongly alkaline 40 km Johnson, 1968; Miggins, 2002 (Mutschler et al., 1987, 1991; Allen and Foord, 1991; Déruelle, 1991; Kay and Gordillo, 1994; Figure 2. Map showing alkalic igneous rocks (black) and associated mineral deposits along Kelley and Ludington, 2002). The RMF paral- the RMF in Colorado (modifi ed from Cappa, 1998). Isotopic ages are mostly ranges from lels the western margin of the thick, cold cra- multiple sources. See Cappa (1998) and Abbott and Cook (2012) for summaries and addi- tonic interior of North America (Lee and Grand, tional references. 1996; Lerner-Lam et al., 1998; West et al., 2004; Yuan and Romanowicz, 2010) with its ≥200-km-thick lithosphere, low heat fl ow, and fast P-wave velocities (Gao et al., 2004; Eaton, metal and silver deposits (Mutschler et al., 1987; formed the Ouachita-Marathon orogenic belt in 2008). The CMB provides a good example of Bookstrom, 1990; Chapin, 2012). The alkaline Oklahoma and Texas (Kluth, 1986; Dickinson the effects of thick lithosphere and low heat fl ow compositions are thought to be due to smaller and Lawton, 2003; Kues and Giles, 2004; Keller on the compositions of igneous rocks and asso- degrees of partial melting at greater depth, prob- and Stephenson, 2007; Nance and Linnemann, ciated mineral deposits. The northeastern end of ably within the lithospheric mantle and/or lower 2008; Soreghan et al., 2012). Far-fi eld stresses the CMB is on the RMF near Boulder (Fig. 2), crust, and possibly with enrichment of source imposed on the Laurentian foreland (now south- and the southwestern end is on the Colorado rocks by metasomatic activity (Mutschler et al., western North America) resulted in widely dis- Plateau near the Four Corners intersection of 1987; Stein and Crock, 1990; Kelley and Lud- tributed intracratonic basement-cored uplifts contiguous states. Both ends have relatively ington, 2002; Pilet et al., 2008; Chapin, 2012). and adjoining basins. The overall pattern of thick lithospheres, low heat fl ow, and alkaline Ancestral Rocky Mountains (ARM) uplifts and intrusions with associated gold deposits (e.g., REACTIVATION: ANCESTRAL ROCKY basins trends northwestward ~2000 km from the Boulder County and Central City at the north- MOUNTAINS DEFORMATION Llano uplift in southern Texas to southern Idaho eastern end; Allard stock in the La Plata Moun- (Fig. 3A) with a width of 750–1000 km (Kluth, tains near the southwestern end). Between, the Progressive westward collision and sutur- 1986; Dickinson and Lawton, 2003; Soreghan CMB is characterized by mainly granodiorite– ing of Gondwana with Laurentia (Fig. 3B) et al., 2012). The individual uplifts trend mainly quartz monzonite stocks with associated base from Late Mississippian to early Permian time west-northwest to north-northwest except along

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A 110° 105° B

Figure 3. (A) Isopach map showing preserved Pennsylvanian strata of the Ancestral Rocky Mountains (ARM). Note the large (>1000 m) thicknesses of Pennsylvanian strata adjacent to the Wichita (WU), Uncompahgre, Apishapa, and Front Range uplifts. Bold black lines are faults. From Soreghan et al. (2012) with 105°W meridian and outlines of state boundaries of Colorado (CO) and New Mexico (NM) (our addition). (B) Inset illustrating oblique rotational collision between Laurentia and Gondwana culminating in head-on Laurentia-Gondwana collision in the early Permian (from Nance and Linnemann, 2008).

the RMF (105°W), where northward trends faulting, and the lack of magmatism are all prob- The orientation of ARM uplifts and basins dominate (Kluth, 1986; Dickinson and Lawton, lematic. The tectonic setting of the ARM defor- with northwest and northward trends crossing 2003; Soreghan et al., 2012). The northward mation is controversial; interpretations range each other in southern Colorado and northern alignment along the RMF is also visible in from a continental-scale collisional system asso- New Mexico is puzzling. But when the effect of Figure 4, the residual gravity map of Soreghan ciated with suturing of Laurentia and Gondwana reactivation of older structures is considered, the et al. (2012), and on the aeromagnetic map (not (Moores, 1991; Nance and Linnemann, 2008) to causes of the pattern become clear. The failed shown) of Karlstrom et al. (2004). A series of a Laramide-style foreland contractional system rift of the Cambrian southern Oklahoma aulaco- little-known ARM structures mapped by Kel- related to subduction along the southwest margin gen, with related mafi c and alkaline intrusions ley (1972) extends for more than 300 km south of Laurentia (Ye et al., 1996). The contempora- that extend intermittently for ~1500 km north- of the Sangre de Cristo Range, along 105°W in neity in timing of the intracratonic deformation westward to the Uncompahgre uplift in south- southeastern New Mexico. with the progressive southwestward collision western Colorado (Figs. 3 and 4), apparently The origin of ARM deformation remains and suturing of Laurentia and Gondwana and controlled the major northwest-trending ARM enigmatic in several respects in spite of abundant the lack of magmatism makes this the generally structures of Pennsylvanian and early Perm- stratigraphic and structural data gathered during accepted model. For a detailed space-time analy- ian age (Kluth, 1986; Keller and Stephenson, intense petroleum exploration and many decades sis of the time-transgressive, westward-young- 2007; Soreghan et al., 2012). The north-trending of geologic and geophysical research. The great ing history of block uplifts and basin fi lling, see ARM structures refl ect basement control by the areal extent of the intracratonic deformation, the Kluth (1986), Dickinson and Lawton (2003), RMF. Figure 5 illustrates how these competing overall pattern of uplifts and basins, the type of and Kues and Giles (2004). northwest and northward trends have infl uenced

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constrained by the preservation of synorogenic strata in the adjacent basins (Appendix 1). The change is often manifest in the transition from marine shales to delta front sands overlain by coastal plain terrigenous deposits with interbed- ded coal seams and the fi rst signs of detritus shed from the Precambrian basement (Weimer, 1976; Raynolds, 2002, 2003). The tectonic interpretations of Paleocene to middle Eocene terrestrial deposits are not so straightforward; they are often separated by signifi cant lacunae from the upper Cretaceous–lower Paleocene transitional sequence, and are more diffi cult to date due to scarcity of fossils and datable volcanic ashes. We selected four key localities along the RMF for their potential to help elucidate the history and tectonic development of the RMF. The fi rst three are localities where the RMF steps 40–50 km to the right while maintaining the overall trend paralleling 105°W. At the fourth locality, the RMF was defl ected ~250 km to the northeast along the southeastern margin of the Figure 4. Residual gravity anomaly map calculated by subtracting a regional gravity fi eld Archean Wyoming province to the Black Hills from the complete Bouguer anomaly values (see Soreghan et al., 2012, for details). UU— of South Dakota. The locations of the steps are Uncompahgre uplift, WU—Wichita-Amarillo uplift, AU—Arbuckle uplift, WM—Wet shown in Figure 6. Mountains, CBP—Central basin platform, AB—Arkoma Basin, MCR—Midcontinent Rift, RGR—Rio Grande Rift, SU—Sierra Grande uplift, AGM—Abilene gravity minimum, Step 1. Sangre de Cristo Range to O1Z—Ouachita orogenic belt interior zone that marks Cambrian margin of Laurentia. Wet Mountains From Soreghan et al. (2012); 105°W meridian and state boundaries of Colorado (CO) and New Mexico (NM) are our additions. In southern Colorado, the RMF steps to the right ~40 km from the Sangre de Cristo Range to the Wet Mountains (Figs. 6 and 7); between are ARM, Laramide, and Rio Grande Rift structures and Pilger, 1978b; Hamilton, 1981; Gries, 1983; the northern end of the Laramide Raton Basin in southern Colorado and northern New Mexico Bird, 1984; Cross, 1986; Dickinson et al., 1988). and its northern extension, the Paleogene Huer- from Pennsylvanian to Neogene time (Chapin Deformation of the Rocky Mountain foreland fano Park basin (Fig. 7). Kleinkopf et al. (1970) and Seager, 1975). Transmission of compres- coincided temporally with an increase in the showed a 15 mgal, north-northwest–trending, sive stresses from collision and suturing along rate of Farallon–North American convergence closed Bouguer gravity low over Huerfano Park. the Ouachita-Marathon orogenic belt (Fig. 3B), from ~100 km/m.y. to as much as 150 km/m.y. The syntectonic sedimentary fi ll of Huerfano and/or from Andean-type subduction in north- during the interval ca. 75–45 Ma (Coney, 1978; Park consists of the Cuchara and Huerfano For- eastern Mexico (Ye et al., 1996), activated pre- Jurdy, 1984; Engebretson et al., 1985; Chapin, mations and the Farasita facies, with an aggre- existing structures with many uplifts overlap- 2012, fi g. 2 therein). Contraction in the Sevier gate thickness of more than 1500 m (Cather, ping in time and some in space. Deformation fold and thrust belt west of the foreland con- 2004). The Huerfano and Farasita were consid- tended to be along high-angle thrusts or reverse tinued intermittently until ca. 50 Ma (Jordan, ered by Briggs and Goddard (1956), Robinson faults, some accompanied by strike-slip move- 1981; DeCelles, 1994; DeCelles and Mitra, (1963, 1966), and Scott and Taylor (1975) to be ment, but normal faults are also present. The 1995). Thus, the Rocky Mountain–style base- laterally equivalent facies. The Huerfano For- lack of volcanism indicates far-fi eld stresses ment-cored arches (Erslev, 1993) and block mation contains red sandstone and mudstone regardless of origin. uplifts, including those along the RMF, over- derived mainly from sedimentary redbeds of lapped signifi cantly in time with contraction in Pennsylvanian and Permian age in the Sangre REACTIVATION: LARAMIDE ROCKY the Sevier fold and thrust belt (Dickinson et al., de Cristo Range. The Farasita conglom erate MOUNTAIN FRONT 1988; Kulik and Schmidt, 1988). This raises contains yellowish-gray coarse conglomer- the interesting possibility that Laramide defor- ate and sandstone derived mainly from granitic The Late Cretaceous–Paleogene (ca. 75–45 mation was driven concurrently at two levels: crystalline rocks in the Wet Mountains, and the Ma) partitioning of the Cordilleran foreland (1) end loading of the middle and upper crust by Cuchara Formation is a mixture of Huerfano and from the Sevier fold and thrust belt to the RMF far-fi eld forces from the Cordilleran convergent Farasita lithologies (Scott and Taylor, 1975). (Fig. 6) refl ects northeastward-directed com- margin, and (2) northeastward drag imparted to Early-middle Eocene vertebrate faunas have pressive stresses from the Cordilleran conver- the basal lithosphere through viscous coupling been found in the Huerfano Formation (Robin- gent margin and the northeastward subhori- with the subducting Farallon plate. son, 1966; Lindsey, 1998). zontal subduction of the Farallon plate (Coney, The beginning of Laramide deformation The Huerfano Formation unconformably over- 1972, 1978; Coney and Reynolds, 1977; Cross and initial reactivation of ARM uplifts are well lies the Paleocene Poison Canyon Formation of

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105°W Range were deposited in the central Colo- rado trough, a long, narrow Ancestral Rocky Mountain basin between the huge San Luis–

Front Range Uncompahgre uplift on the southwest and the Apishapa and Frontrangia uplifts (“Frontrangia” is a feature distinguished from the Laramide Front Range) on the northeast (Figs. 5 and 6). The Laramide Wet Mountains and Wet Moun- tain Valley were superimposed on the Apishapa Uncompahgre uplift uplift, which was denuded to the Precambrian basement during the Ancestral Rocky Mountain Wet Mnts. orogeny (Tweto, 1975; Lindsey et al., 1983).

Sangr Three alkalic intrusive complexes of Cambrian to Early Ordovician age occur in the northern e Wet Mountain Valley and were exposed during Pennsylvanian–Permian uplift and erosion of the Wet Mountain block (Naeser, 1967; Taylor et al., 1975; Olson et al., 1977). The complex thrust Aplshapa uplift faulting of the northern Sangre de Cristo Range is prominent on geologic maps of Colorado as

de the most highly deformed thrust-faulted terrain Mnts. Cristo in the state. During northeast-oriented Laramide compression, the Pennsylvanian–Permian sedimentary fi ll of the central Colorado trough was shortened between the San Luis–Uncompahgre uplift on the southwest and the Wet Mountain block on the northeast. Lindsey et al. (1983) estimated the magnitude of shortening as ~8–14 km (or 40%–70%) across an original width of 20 km. Burbank and Goddard (1937) estimated 15–17 km shortening for Paleozoic and Meso- zoic rocks in Huerfano Park. Northeast-directed Laramide compression was transmitted across the Wet Mountain block, resulting in the eastern margin of the Wet Moun- tains being thrust over upper Cretaceous rocks of the Canon City embayment (Scott et al., 1978; Weimer, 1980; Jacob and Albertus, 1985). Several anticlines and synclines involving Pre- cambrian to upper Cretaceous rocks closely parallel the thrusted eastern margin of the Wet Mountains. The eroded core (~22 × 6 km) of the Red anticline (Fig. 7) reveals patches of Ordo- vician rocks overlying Precambrian basement Figure 5. Map showing overprinting of the Rio Grande Rift on structures of Laramide and and overlain by arkosic redbeds of the Penn- late Paleozoic age. Modifi ed from Chapin and Seager (1975). N.M.—New Mexico; Ariz.— sylvanian–Permian Fountain Formation and Arizona; Colo—Colorado. the Jurassic Morrison Formation (Scott et al., 1978; Jacob and Albertus, 1985). Interpreta- the Raton Basin and is overlain by volcani clastic in McIntosh and Chapin, 2004) range from ca. tions of the Wet Mountain thrust range from a fl uvial and debris-fl ow deposits of the lower 35 to 32 Ma. (For a perspective of how the Huer- low-angle thrust near the surface, with several Oligocene Devils Hole Formation (Johnson and fano Park basin relates to other Laramide basins kilometers of overhang (Jacob and Albertus, Wood, 1956; Scott and Taylor, 1975). The vol- in northern New Mexico and southern Colorado, 1985), to a high-angle reverse fault (Weimer, canic constituents of the Devils Hole Formation see Cather, 2004, fi g. 2 therein.) 1980). Reactivation of the Wet Mountain block were derived from the late Eocene–early Oligo- Huerfano Park merges to the northwest occurred in the early-middle Eocene, recorded cene Silver Cliff and Rosita Hills volcanic cen- with the Wet Mountain Valley graben, which by the southwestward-prograding alluvial fans ters, ~20 km to the north-northwest (Fig. 7) in separates the northeast-vergent thrusts of the of Precambrian detritus that formed the Farasita the Wet Mountain Valley, and the Deer Peak vol- east fl ank of the Sangre de Cristo Range from facies of the Huerfano Formation (Scott and canic center ~10 km to the northeast in the Wet the normal-faulted western margin of the Wet Taylor, 1975). Reactivation occurred again in Mountains (Scott and Taylor, 1975; McIntosh Mountains. The thrust-faulted Pennsylvanian Miocene–Pliocene time during development of and Chapin, 2004). The Ar40/Ar39 ages (reported and Permian formations of the Sangre de Cristo the Wet Mountain Valley graben as a subsidiary

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with Paleozoic and Mesozoic AFT ages to be removed by erosion. At Greenhorn Mountain, the high peak at the south end of the Wet Moun- tains (Fig. 7), ~1200 m of Precambrian rocks are still present with pre-Laramide, partially reset AFT ages between 228 and 166 Ma (Kelley and Chapin, 2004). A recent study of the erosion and uplift history of the central and southern Rocky Moun- tains (Cather et al., 2012) indicates that the Wet Mountains and adjacent high country of the Colorado Rocky Mountains were not strongly uplifted and dissected by erosion until late IDAHO BATHOLITH Miocene to Holocene time (ca. 6–0 Ma), which helps explain preservation of the old AFT ages. Today, the middle Eocene erosion surface (Epis and Chapin, 1975) that MacGinitie (1953) esti- Hogsback Thrust mated was carved at elevations <900 m (see (4) also Cather et al., 2012) rises southward along the crest of the Wet Mountains until it reaches modern elevations as high as 3500 m at Green-

(3) horn Peak. The apparent step to the northeast of the RMF from the Sangre de Cristo Range to the Wet Mountains was simply reactivation of the Apishapa uplift, beveled during and after (2) the Pennsylvanian–Permian ARM orogeny.

(1) Step 2. Wet Mountains to Front Range CMB

The RMF steps northeastward ~50 km from the eastern edge of the Wet Mountains to the eastern edge of the southern Front Range (Figs. 6 and 7); between are the Cañon City embayment, its northern extension, the Garden Park syncline, and the Cañon City–Florence basin (Fig. 7; Appendix 1). A broad plateau composed mostly of Precambrian rocks is to the northwest; it is thinly covered by upper Eocene to lower Oligocene volcanic rocks of the Thirty- nine Mile volcanic fi eld, now part of the Central Colorado volcanic fi eld (McIntosh and Chapin, 2004). The area discussed here is mostly within 0 500 km the Pueblo (Scott et al., 1978) and the Denver 1° × 2° geologic quadrangles (Bryant et al., 1981b). The Central Colorado volcanic fi eld has Figure 6. Basement-cored uplifts and basins of Laramide age in been deeply dissected by erosion; widely scat- the U.S. Rocky Mountains. Precambrian basement rocks east of tered remnants are present in the northern Wet the foreland thrust belt and north of the Basin and Range Province Mountain Valley, South Park, the southern Front are stippled. Modifi ed from Hamilton (1981). Trend of Colorado Range, and the High Plains of eastern Colorado. Mineral Belt is from Chapin (2012). Locations of apparent Rocky The Central Colorado volcanic fi eld is mainly Mountain Front (RMF) right steps 1, 2, 3, and 4 are indicated. in an elongate belt that extends north-northwest from the Raton Basin to southern Wyoming basin of the Rio Grande Rift containing alluvial apatite fi ssion track (AFT) cooling ages in Kel- and includes the Laramide intermontane basins fi ll correlated with the Santa Fe Group (Scott ley and Chapin (2004) show that the stump of Huerfano Park, Echo Park, South Park, and and Taylor, 1975). of the Wet Mountain block (~2–3 km in the Middle Park–North Park (called the Colorado Tweto (1975, p. 1) stated that “… eroded subsurface) had cooled below ~120–100 °C Headwaters Basin by Cole et al., 2010). The stumps of late Paleozoic mountain ranges made before burial by upper Cretaceous sedimentary basins were classifi ed as axial basins by Dickin- up of Precambrian rocks” were present at the deposits, but was not buried deeply enough to son et al. (1988). The belt containing the basins beginning of the Laramide orogeny beneath a reset the AFT ages (Fig. 7). The Wet Mountain is essentially the lower elevation terrain between blanket of Cretaceous sedimentary rocks. The block was not uplifted suffi ciently for the rocks the high frontal ranges on the east and high inte-

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Figure 7. Simplifi ed geologic map of central Colorado showing apatite fi ssion track (AFT) cooling ages in the Wet Mountains, southern Front Range, and adjacent areas. Triangles—AFT ages older than 100 Ma, x—AFT ages 99–30 Ma, closed circles—AFT ages younger than 30 Ma. Fission track ages are from Kelley and Chapin (2004), Bryant and Naeser (1980), and Lindsey et al. (1986). Yellow—Paleogene park basins (Huerfano, Echo, South) and Canon City–Florence basin (CC-F). Bold black lines—Paleogene strike-slip faults, dashed where inferred. Dash-dot lines—Eocene paleovalley mar- gins. CCV—Central Colorado volcanic fi eld, F—Florissant, J—Jefferson, RA—red anticline. Base adapted from Tweto (1979) and the Colorado geologic highway map (Christiansen, 1991). See text for details and additional references.

rior ranges like the Sangre de Cristo, Sawatch, and Sierra Madre on the west (Figs. 6 and 7). The old AFT cooling ages that characterize the Precambrian rocks of the Wet Mountain block extend northward across the Arkansas River gorge (Fig. 7). A sample traverse along the Arkansas River revealed that AFT ages between 92 and 174 Ma extend ~30 km westward from the range front before giving way to Laramide cooling ages (75 Ma or younger) near the Texas Creek fault (Fig. 7). The old AFT ages are near the southern edge of a broad area in the Front Range, south of the CMB, with AFT ages older than 100 Ma. Near the center of the southern Front Range, near Bailey, Colorado, we (Kelley and Chapin, 2004) found ~1200 m of Protero- zoic rocks preserved above the base of the Late Cretaceous partial annealing zone (PAZ) of fi ssion tracks in apatite. North of the Arkansas River gorge, on the plateau-like surface beneath the Central Colorado volcanic fi eld, an inlier of Ordovician rocks and patches of Jurassic Morrison Formation and Cretaceous Dakota Sandstone directly overlie Proterozoic rocks. Thus, the southern Front Range and its west- faults (Fig. 7), some of which bound narrow Echo Park basin in 1977 (Chapin et al., 1982) ern fl ank compose the reactivated core of the strike-slip basins that contain as much as 600 m led to extensive drilling programs between the ARM Frontrangia uplift, as noted by Mallory of prevolcanic arkosic alluvium (Chapin and Arkansas River and South Park. The drilling (1972), Tweto (1975), and Kluth (1997). The Cather, 1981; Chapin et al., 1982). The best documented the existence of other narrow, fault- geographic extent and thickness (as much as exposed basin, named after Echo Park canyon bound basins containing arkosic alluvial fi ll, but 1200 m) of Proterozoic rocks that cooled during on the north side of the Arkansas River gorge, much of the data remain proprietary. A late-early uplift and denudation in Paleozoic and early to extends ~65 km northward to near Hartsel in to early-middle Eocene age of the Echo Park middle Mesozoic time, but are still preserved in the South Park basin. The Echo Park basin is Alluvium (Appendix 1) was estimated based the Laramide Rocky Mountains, are surprising. ~1.5–5 km wide and as deep as 600 m. The fault on pollen assemblages in core samples (R.H. The broad plateau of mostly Proterozoic zone bounding the east side contains lenses of Tschudy, U.S. Geological Survey, 1983, written rocks that underlies the Central Colorado vol- Morrison and Dakota sandstones and exhibits commun.). The Echo Park Alluvium is overlain canic fi eld north of the Arkansas River is cut dextral kinematic indicators (Chapin and Cather, by the Wall Mountain Tuff, 39Ar/40Ar dated as by a series of north-northwest–trending major 1981). Large uranium deposits discovered in the 36.7 ± 0.07 Ma (McIntosh and Chapin, 2004).

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Northeast-directed Laramide compressive stresses were transmitted across the remnants of Frontrangia uplift just as they were across the Wet Mountains block. The east ﬂ ank of the Fort southern Front Range was crowded against Pha- Collins

nerozoic sedimentary formations underlying the Steamboat southern Denver Basin along an echelon series Springs of moderately steep thrusts (Tweto, 1979; Trim- ble and Machette, 1979; Kluth, 1997; Weimer and Ray, 1997). The scarcity of seismic lines and drill holes has resulted in a wide variety of con-

fl icting structural interpretations of the range- Boulder bounding faults. In a detailed, model-based structural analysis, Sterne (2006) interpreted the southeastern fl ank of the Laramide Front Range Lakewood as a complex series of stacked triangle zones. Denver Nesse (2006) included a foldout sheet containing 21 cross sections of the Front Range from 38°30′N to 41°00′N. The cross sections reveal the structural asymmetry of the Front Range, characterized by low-angle, west-vergent thrusts along the west fl ank and relatively short, moderate-angle, east-vergent thrusts and high-angle reverse faults along the east fl ank (see Appendix 1). The Laramide Front Range does not coincide completely with the Ancestral Rocky Mountain Frontrangia uplift, as shown in Figure 8; the Pikes Colorado Frontrangia uplift trends more to the northwest, Peak Springs so that the two uplifts diverge in northern Colo- CMB rado (Kluth, 1997; Erslev, 2005). The east fl ank of the Laramide Front Range changes character north of its intersection with the Colorado Min- eral Belt (discussed in the following). km Pueblo 04020 Step 3. Front Range–Colorado Mineral Belt Intersection Figure 8. Map of Front Range area showing ages of rock units A very interesting and important change younger than Pennsylvanian–Permian that are in depositional con- in structural style of the eastern margin of the tact with the Proterozoic basement. The area of Jurassic Morrison Laramide Front Range occurs between Golden Formation (J) deposited directly on basement (diagonal ruling) was and Boulder, Colorado (Fig. 9). The transition not covered by sedimentary rocks for ~100 m.y. following exposure in AFT ages is surprisingly abrupt, occurring and probably represents the highest part of the late Paleozoic Front- within a 3.5 km interval between Golden Gate rangia uplift (TR—Triassic rocks). Modifi ed from Kluth (1997). and Van Bibber Canyons northwest of Golden CMB—trend of Colorado Mineral Belt. (Kelley and Chapin, 2004). No major faults cross the margin of the Denver Basin to account for the abruptness; however, the transition faults need cross the Proterozoic-Phanerozoic (Erslev 1993; Nesse, 2006; Cole and Braddock, occurs across the trend of the northern CMB. boundary of the range front. For alternative 2009) north of the CMB intersection. The axis As pointed out by Tweto and Sims (1963), the hypotheses see Tweto and Sims (1963), McCoy of the Denver Basin is close to the range front CMB cuts indiscriminately across the geologic et al. (2005), and Jones et al. (2011). south of the CMB due to loading of the basin by grain of Colorado with remarkable continuity, The Front Range–Denver Basin margin overhanging northeast-vergent thrusts. Starting seemingly independent of the tectonic elements changes from a north-northwest trend south at the CMB intersection, the basin axis veers to it crosses. In Chapin (2012) the origin of the of the CMB intersection to a northward trend the northeast to near Greeley, Colorado (Fig. 9); CMB was interpreted as due to an underlying (Figs. 8 and 9) within ~10 km of the intersection. between Greeley and the Wyoming border the segment boundary in the subhorizontally sub- The Golden thrust is no longer mappable at the axis is 35–40 km east of the range front. This ducting Farallon plate that became dilated as surface north of this zone. The structural style is the area in which the structural style of the the Farallon slab was overridden by the thicker of the range front also changes from northeast- Front Range changes from northeast-vergent lithospheres of the Wyoming and continental vergent thrusts with triangle zones characteristic thrusts to northeast-dipping, high-angle reverse interior cratons. Thus, the control of the CMB of the southern margin to northwest-trending, faults, also called southwest-vergent backthrusts was not in the North American plate and no northeast-dipping, high-angle reverse faults by Erslev (1993) and Erslev and Selvig (1997),

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Figure 9. Simplifi ed map of the northeastern margin of the Front Range and its intersection with the Colorado Mineral Belt (CMB). The trend of the range margin changes from north-northwest to north across the CMB and the structural style changes from northeast-vergent thrusts to northeast-dipping, high-angle reverse faults, also called backthrusts (Erslev, 1993). The histograms of apatite fi ssion track ages show contrasts in cooling history of the northern versus southern Front Range and Wet Mountains (Kelley and Chapin, 2004). Associated features in the Denver Basin are defl ection of the basin axis to the northeast away from the range margin (Weimer, 1996); the location of the Boulder-Weld coal fi eld (yellow) in a northeast-trending zone of décollement faulting shed off the Longmont fault (Kittleson, 2009); and the location of the huge Wattenberg gas fi eld (gray) above a geothermal anomaly outlined by vitrinite refl ectance values Ro = 1.0 and 1.4 (Higley et al., 2003). Intrusives at the northeast end of the CMB are shown in red; ISRS—Idaho Springs–Ralston shear zone; U—upthrown; D—downthrown. Heavy black line is Precambrian–Phanerozoic contact along the basin margin; thinner black lines are faults. Base is modifi ed from Weimer (1996).

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and possibly caused by basement wedging and basement beneath the PAZ. In contrast, the end- overlying delta plain coal-bearing Laramie For- backthrusting above a blind northeast-directed Cretaceous PAZ south of the CMB was within mation (213–262 m). The coal beds were gen- thrust in the basement (Erslev 1993; Erslev the resistant basement rocks and preserved from erally thicker on the graben blocks (Davis and and Holdaway, 1999). The Denver and Chey- erosion. However, the abruptness of the transi- Weimer, 1976), indicating that the detachment enne Basins contain asymmetric thrust-loaded tion (~3.5 km) across the CMB, the northeast- occurred in a terrestrial near-surface environ- deeps in the Denver and Cheyenne areas (Fig. ward defl ection of the axis of the Denver Basin, ment with peat bogs accumulating organic mat- 10) separated by a relatively shallow saddle in the shallowing of the basin, and the change in ter contemporaneously with faulting. Davis and the Greeley area known as the Greeley arch structural style suggest that uplift of the Front Weimer (1976) described the faulting as growth (Weimer, 1996). Range north of the CMB may also have been faulting and also reported unusually thick sec- Figure 9 displays histograms of AFT cool- a factor. tions of Fox Hills Sandstone. The Golden– ing ages for areas of the Front Range north There is additional evidence of early Laramide Boulder area has the greatest thickness of Pierre and south of the CMB intersection (Kelley and uplift across the CMB. The Boulder-Weld coal Shale (2295–2448 m) in the Denver Basin Chapin, 2004). South of the CMB, the AFT ages fi eld (Fig. 9) is a 40-km-long, 10–15-km-wide (Davis and Weimer, 1976). High sedimentation range from ca. 30 Ma to older than 300 Ma, northeast-trending zone of horst and graben rates may have been a contributing factor in the and many samples show relatively short track faulting from which more than 100 × 106 t of gravity sliding. The importance to this study is lengths indicative of long residence within the coal were produced between 1859 and 1979 the evidence supporting early Laramide uplift AFT partial annealing zone. In contrast, north from the Maastrichtian Laramie Formation (Car- (Maastrichtian) of the Front Range north of the of the CMB the AFT ages span a narrow range roll, 2009). The faults have been variously inter- CMB. The gravity sliding may also have been from ca. 80 to 40 Ma and characteristically have preted as normal, reverse, strike slip, growth, or aided by magmatic infl ation and seismic activ- relatively long track lengths indicative of fast décollements. Kittleson (2009) appears to have ity in the northern CMB. A cluster of fi ve intru- cooling. In Kelley and Chapin (2004), we inter- solved the problem via correlation of sedimen- sive centers (Fig. 9) at the northeast end of the preted the narrow range of AFT ages and their tary units and faults in ~1450 geophysical well CMB are within 20–30 km of the Boulder-Weld correlation with the Laramide orogeny north logs that resulted in recognition of down-to-the- coal fi eld and range in age from ca. 75 to 45 Ma of the CMB as being due primarily to the PAZ basin gravity sliding away from the Longmont (Gable, 1984; Mutschler et al., 1987). of AFT ages being up within the thicker and fault (Fig. 9) above a bedding-plane detachment Figure 9 also shows the outlines of the huge relatively low thermal conductivity Pierre Shale ~76 m below the top of the Pierre Shale. The Wattenberg gas fi eld and its associated geo- that was subsequently eroded away, revealing detached blocks included the regressive, delta thermal anomaly, shown by contours of vitrinite the Laramide cooling ages in the Proterozoic front Fox Hills Sandstone (30–67 m) and the refl ectance values of Ro 1.0 and 1.4 (Higley et al., 2003). The Wattenberg fi eld is a continuous-type gas accumulation located along the axis of the Denver Basin adjacent to its intersection with the CMB. Production is from the lower Cretaceous Muddy Sandstone at depths of 2070–2830 m and from overlying upper Cre- taceous units, Codell Sandstone, Niobrara For- mation, and the Hygiene and Terry Sandstone Members of the Pierre Shale. Stratigraphic traps, tight sands, and a paleostructural high provided the trapping mechanisms (Weimer, Figure 10. Structure contour 1996; Higley et al., 2003; Nelson and Santus, map of Denver and Cheyenne 2011). The Wattenberg fi eld is pertinent to this Basins (Matuszczak, 1976). study of the RMF because of (1) its location at Contours are on top of Pre- the intersection of the CMB with the Denver cambrian basement. Contour Basin; (2) its geothermal anomaly and residual interval is 304.8 m. Wheatland- magnetic intensity anomaly that may indicate a Whalen fault zone and the Hart- magmatic intrusion at depth (T. Grauch, in Hig- ville uplift are from Love and ley et al., 2003); (3) its location where the axis Christiansen (1985). COCORP of the Denver Basin is defl ected northeastward (Consortium for Continental to a position 35–40 km east of the basin mar- Refl ection Profi ling) seismic line gin where the axis parallels the north-trending, 3 is from Brewer et al. (1982). reverse-faulted segment of the Front Range (Fig. 9); and (4) the structural and stratigraphic control provided by thousands of drill-hole logs. The fi ve small-displacement, oblique-slip fault zones mapped across the Wattenberg fi eld by Weimer (1996) were not included in Figure 9 because they appear to have little tectonic signifi cance, other than causing compartmentaliza- tion effects within the gas fi eld. No northeast- trending faults cut the Front Range–Denver

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Basin margin; this is another example of the CMB cutting indiscriminately across the geologic grain of Colorado, independent of the tectonic elements it crosses, as pointed out by Tweto and Sims (1963).

Step 4. Laramie Range–Black Hills

The RMF continues northward along the east Figure 11. Generalized geologic edge of the Front Range, which merges almost map of the Laramie Mountains imperceptively with the Laramie Range of south- showing apatite fission track ern Wyoming. In like manner, the Denver Basin (AFT) cooling ages in relation merges northward with the Cheyenne Basin to the Cheyenne suture belt and across a broad saddle known as the Greeley other structures. PAZ—partial arch (Weimer, 1996). The Denver and Cheyenne annealing zone. Circles repre- Basins are two halves of a 500-km-long basin sent samples analyzed by Kelley (if measured at the –900 m contour level in Fig. (2005). Squares mark samples 10) with basin deeps near Denver and Cheyenne analyzed by Cerveny (1990). (Matuszczak, 1976). The axis of the combined Standard errors of AFT ages basins is close to the range front (the RMF) at are in parentheses (from Kelley, the Denver and Cheyenne deeps due to loading 2005; with permission of the of the basin by east-vergent thrusts (Fig. 10). American Geophysical Union). The relatively shallow middle of the Denver and Base is after Blackstone (1996). Cheyenne Basins is opposite the section of the Front Range dominated by northeast-dipping, high-angle reverse faults (or backthrusts) that bring the basin up relative to the range (Erslev, 1993; Erslev and Selvig, 1997). This section of the Front Range contains the Indian Peaks Wilder ness, Rocky Mountain National Park, and the Continental Divide, with many peaks in the 3600–4000 m range. The area is largely under- lain by the massive Mesoproterozoic granite of the Longs Peak–St. Vrain batholith (Cole and Cheyenne Basin margin (Fig. 11) revealed a ered by the upper Eocene White River Group Braddock, 2009), the southern margin of which series of westward-dipping refl ections traceable and Miocene Ogallala Formation (Love and approximately coincides with the CMB and the as deep as 10–12 km and arranged en echelon Christiansen, 1985) and there is little agreement changes in structural style of the Front Range with dips of 20°–50° (Brewer et al., 1982). on the nature of the faulting. The Precambrian and Denver Basin as outlined here. Refl ectors inferred to be sedimentary formations basement is at shallow depths along a subsur- The Laramide AFT cooling ages (ca. 80–40 could be traced as far as 3 km west of the moun- face arch connecting the Laramie and Black Ma), highlighted by the histogram in Figure tain front under the overhang of Precambrian Hills uplifts (Blackstone, 1993; Erslev, 1993), 9 for the northern Front Range, are similar to rocks (Brewer et al., 1982). Thus, the RMF at so petroleum exploration has been minimal. For those in the Laramie Range except for ~15 km the latitude of the Laramie Range is similar to these reasons, we end our coverage of the RMF on either side of the boundary with the Archean that of the Front Range south of the CMB. at the Laramie Range. Wyoming province (Fig. 11). Old AFT ages Northeast-directed compressional stresses ranging from 378 to 172 Ma (Kelley, 2005) are transmitted across the Wyoming Archean prov- SUMMARY present near the boundary, generally referred to ince during the Laramide orogeny generated the as the Cheyenne belt (Karlstrom and Houston, large (320 × 110 km) north-northwest–trending Orogenic activity along the RMF occurred in 1984; Karlstrom et al., 2005). One of us (Kelley, Black Hills uplift (Figs. 1 and 6). The Laramide several episodes of differing intensity and causes 2005) concluded that the distribution of AFT evolution of the Black Hills uplift is covered in during the Late Cretaceous and the Cenozoic. ages in the Laramie Range is indicative of long- the map by Lisenbee (1985) and in Shurr et al. Evidence for four episodes of Cenozoic erosion wavelength folding of the basement, similar to (1988) and Lisenbee and DeWitt (1993). How- and uplift in southwestern North America was other Laramide uplifts in southern Wyoming. ever, the Cheyenne belt between the Laramie presented in Cather et al. (2012). In episode Two range-parallel doubly plunging anticlines Range and the Black Hills has received scant one, the Laramide orogeny (ca. 75–45 Ma) with Laramide AFT ages are separated by a low attention in the geologic literature. For discus- reactivated the RMF through regional north- area along the Cheyenne belt (Fig. 11), where sions of the Precambrian tectonics and metal- east-directed compression. Intermontane basins old AFT ages and short track lengths character- logeny of the Hartville uplift, Wyoming, see remained at low elevations with the RMF mark- ize rocks that cooled within the Late Cretaceous Sims et al. (1996) and Sims and Stein (2003). ing the eastward limit to basement-cored uplifts, PAZ. Refl ection seismic lines recorded by the The Wheatland-Whalen fault zone (Fig. 11) inferred here to be due to its proximity to the Consortium for Continental Refl ection Profi l- trends northeast from the Laramie uplift toward thick (≥200 km) continental interior lithosphere ing (COCORP) across the Laramie Range– the Black Hills, but the surface is largely cov- (Lee and Grand, 1996; Lerner-Lam et al., 1998;

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West et al., 2004; Gao et al., 2004; Yuan and Peak (Naeser et al., 2002) to ca. 27 Ma (Oligo- the Front Range (Appendix 1), but the structural Romanowicz, 2010). In episode two, late-middle cene) near Cripple Creek (Kelley and Chapin, style was quite different. The eastern margin is Eocene erosion (ca. 43–37 Ma) was centered in 2004). The pre-Laramide AFT samples exhibit crowded against the Denver Basin by east-ver- Wyoming and northern Colorado and is inferred short track lengths indicative of slow cooling gent moderate- to high-angle thrusts (~35°–70°) to be in response to lithospheric rebound follow- and lengthy presence within the end-Cretaceous and triangle zones south of the CMB, and high- ing cessation of Laramide dynamic subsidence. PAZ. At Greenhorn Peak in the Wet Mountains angle, northeast-dipping reverse faults north of Carving of the Rocky Mountain erosion surface and near the town of Bailey in the central Front the CMB. However, along the western margin, and shallowing of the Denver Basin north of the Range, ~1200 m of Proterozoic rocks with pre- contraction culminated in low-angle, west-ver- CMB were two effects along the RMF. Laramide cooling ages still reside above the gent thrusts that advanced over the Middle Park The third episode of erosion and uplift (dis- base of the end-Cretaceous PAZ. The inescap- and South Park basins by as much as 10–15 km cussed in Cather et al., 2012) occurred in late able conclusion that the southern Front Range (Erslev et al., 1999; Kellogg et al., 2004; Nesse, Oligocene–early Miocene time (ca. 27–15 Ma) has undergone modest exhumation compared to 2006; Cole et al., 2010). In contrast, documented in response to increased mantle buoyancy the northern third is also suggested by the pres- overhangs of the Denver Basin by thrusts along related to major volcanism in the southern ervation of several inliers of rocks ranging in the east margin of the Front Range are only in Rocky Mountains and Sierra Madre Occiden- age from Cambrian to Cretaceous in the south- the range of 2–3 km (Weimer and Ray, 1997; tal of Mexico. Early development of the Rio ern part of the range. Nesse, 2006). The short overhangs were appar- Grande Rift overlapped with this episode and There are also conspicuous changes in the ently adequate to cause subsidence and create rifting increased markedly during the interval adjacent Denver Basin that relate to structural accommodation space that was rapidly infi lled ca. 16–6 Ma (Chapin and Cather, 1994). Uplift changes along the Front Range. South of the by proximal sedimentary facies, leading to fur- and outward tilting of the east fl ank of the Rio CMB, the Front Range trends north-northwest ther subsidence and an asymmetric basin. Grande Rift provided source areas for aggrada- and the axis of the Denver Basin is close to the The Middle Park and South Park basins along tion of the Ogallala Formation on the western range front due to gravitational loading of the west side of the Front Range may have ini- Great Plains (Chapin and Cather, 1994). the basin by northeast-vergent thrust faults. The tially developed in a similar manner, with mod- Accelerated uplift and erosion during the late Laramide synorogenic sedimentary deposits erately steep, west-vergent thrusts and asym- Miocene–Holocene (ca. 6–0 Ma) episode four of of the Laramie Formation and the D1 and D2 metric basin fi lls. Volumetrically, the bulk of Cather et al. (2012) resulted in major enhance- sequences of Raynolds (2002, 2003) occupy Laramide synorogenic sedimentation occurred ment of the RMF along the Front Range, Wet the resultant asymmetric syncline. However, as in the Paleocene (Appendix 1). However, in lat- Mountains, and Sangre de Cristo Range. The the structure of the Front Range margin changes est Paleocene–early Eocene time, a fi nal episode erosion was increased by opening of the Gulf to northeast-dipping, high-angle reverse faults of horizontal compression drove the west-ver- of California ca. 6 Ma (Oskin and Stock, 2003; north of the CMB, the axis of the Denver Basin gent basin-margin thrusts to override the Mid- Chapin, 2008) which, together with the effect of is defl ected ~60 km to the northeast (Fig. 9), the dle Park and South Park basins by 10–15 km higher elevation, intensifi ed the North American basin becomes shallower and more symmetrical, (Erslev et al., 1999; Kellogg et al., 2004; Cole monsoon. The east-facing, ~500–1200 m topo- and the synorogenic sedimentary sequences D1 et al., 2010). This intense interval of horizontal graphic escarpment along the east fl ank of the and D2 of Raynolds (2002, 2003) are no lon- compression coincided temporally with the third southern Rocky Mountains owes its existence to ger present. This leaves the Laramie Formation major episode of shortening in the Sevier fold a combination of late Miocene–Holocene uplift (Maastrichtian) as the only Laramide synoro- and thrust belt, as the Hogsback thrust (Fig. 6) and erosional exhumation (Leonard and Lang- genic sedimentary unit in the northern Denver added ~21 km of additional shortening at a ford, 1994; Chapin and Cather, 1994; Leonard Basin. When and where did the other synoro- rate of ~3 mm/yr during late Paleocene–early et al., 2002). The intensifi ed erosion removed genic sediments go? The Laramie Formation in Eocene time (ca. 56–50 Ma; DeCelles, 1994). the alluvial fans along the mountain fronts, the northern Denver Basin is much thicker than to The east fl ank of the Front Range was little eroded hogbacks of tilted Paleozoic and Meso- the south (~540 m vs. ~260 m) and is dominantly affected except for modest uplift and deposition zoic formations, and beheaded the Ogallala fl uvial with several relatively thick channel sand- of the D2 sequence (Raynolds, 2002, 2003) in Formation from its source terrains in the moun- stones (Kirkham et al., 1980). Unfortunately, the Denver Basin. The Middle Park and South tains. Strike valleys paralleling the mountains the middle Eocene (ca. 43–37 Ma) episode of Park basins, however, are internally complex, are home to the larger cities of the Front Range uplift and erosion (Cather et al., 2012) removed with intrabasin, west- to southwest-vergent urban corridor with the Rocky Mountain Front much of the synorogenic sedimentary record. thrusts and tight folding (Izett, 1968; Izett and as their scenic backdrop. The record was further obscured by the upper Barclay, 1973; Beggs, 1977; Wellborn, 1977). The Front Range dominates the RMF in Eocene–lower Oligocene (ca. 35.5–30 Ma) blan- A remarkable attribute of the RMF is its Colorado with its ~280 km length, ~60–100 km ket of eolian and fl uvial tuffaceous sediments ability to maintain its position through multi- width, and ~2 km of topographic relief; how- of the White River Group (Larson and Evanoff, ple orogenies and changes in orientation and ever, it is not a monolithic feature. The northern 1998). The Front Range and Denver Basin have strength of tectonic stresses. During the fi nal half differs signifi cantly from the southern half, also undergone changes in regional tilting, as paroxysm of Laramide contraction in lat- the CMB being the dividing line. As illustrated refl ected by changes in direction of stream fl ow, est Paleocene–early Eocene time, the RMF in Figure 9, north of the CMB the AFT cooling stream capture, and tilting of the Rocky Moun- remained in place as thrusts advanced from east ages are nearly all Laramide in the narrow range tain erosion surface from southeastward in the and west on opposite sides of a foreland parti- of 80–40 Ma; the Front Range trends north and late Eocene to northeastward and then eastward tioned by basement-cored uplifts, sedimentary is dominated structurally by northeast-dipping, in the Neogene to Holocene (Steven et al., 1997). basins, and the Colorado Plateau. That the RMF high-angle reverse faults. South of the CMB, the Laramide synorogenic sedimentation was is a deeply penetrating, fundamental fl aw in the AFT ages cover a wide range, from ca. 458 Ma similar in character and timing (dominantly North American lithosphere is also indicated by (partially reset) at the summit (4300 m) of Pikes Paleocene–early Eocene) on opposite sides of the wide range in age of alkaline igneous rocks

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and mineral deposits (summarized in Fig. 2). to the Laramie Range in southern Wyoming. profi le crosses the RMF at an oblique angle, The Colorado Front Range is a composite struc- Gough (1989, Fig. 2 therein) extended the but projections constructed approximately nor- ture with signifi cant differences from north to NACP conductor southward along the RMF mal to the RMF (Reiter and Chamberlin, 2011) south and east to west, and with geologic time. to near the U.S.-Mexico border with a similar show an abrupt transition over ~50 km from low It represents a microcosm of the overall RMF. conductor mapped along the Wasatch Range heat fl ow (~41–50 mW/m2) and relatively fast that borders the Colorado Plateau on the west. shear wave velocity of the southern High Plains DISCUSSION Gough (1989, p. 148) stated “…there is little to the high heat fl ow (~60 to >100 mW/m2) and doubt that the NACP marks a major fracture relatively slow shear wave velocity of the RMF An important question remains. What is the zone.” That the NACP extends over such a long and eastern shoulder of the Rio Grande Rift. The nature of the lithospheric fl aw that underlies distance, including areas that have not under- La Ristra experiment documented present-day the RMF? It may not be possible to answer this gone magmatism since the Proterozoic, sug- partial melting and mantle convection beneath question given the present lack of defi nitive sub- gests that saline waters in fractures may be the the southern Rocky Mountains and Rio Grande surface data, but we can outline some constraints dominant cause of the enhanced conductivity. Rift with a remarkably sharp seismic velocity and list some features that suggest a solution. That the geomagnetic conductivity anomalies and heat fl ow boundary at the RMF. For example, the RMF has three features that are tracked the eastern and western boundaries of In summary, the lithospheric fl aw underlying found in many suture zones: (1) a long, narrow the Rocky Mountain–Colorado Plateau prov- the RMF has provided a tectonic boundary and tectonic belt that follows a cratonic boundary, ince rather than the zone of partial melting a pathway to the surface for alkaline magmas, (2) a long-term existence measured in hundreds beneath the southern Rocky Mountains and Rio kimberlite intrusions, and hydrothermal fl uids of millions of years, and (3) the presence of a Grande Rift (Gao et al., 2004; West et al., 2004; under varying tectonic conditions over more variety of alkaline igneous intrusions. However, Karlstrom et al., 2012) also suggests the pres- than a billion years of geologic time. However, exposures of Proterozoic rocks along the RMF ence of saline waters in fracture zones. the nature of the lithospheric structure underly- lack the complex structural deformation, high- The second series of geophysical studies ing the RMF remains elusive. grade metamorphic rocks, and indications of began with the PASSCAL (Program for Array ACKNOWLEDGMENTS major strike-slip faulting that are found in many Seismic Studies of the Continental Lithosphere) suture zones. Several geologic features strongly Rocky Mountain Front experiment in 1991– We thank Virginia McLemore and Mike Tim- indicate that the RMF is a major lithospheric 1992, in which 36 seismographs were deployed mons for informal reviews of an early version of the boundary: (1) during the Laramide orogeny from eastern Utah to western Kansas and north- manuscript. Chapin benefi ted from discussions with (ca. 75–45 Ma) contractional basement-cored south within the state borders of Colorado. The Virginia McLemore concerning her work on the Great Plains margin gold deposits. James Cole and Jeremiah uplifts ended at the RMF; (2) igneous intrusions two-dimensional array recorded broadband Workman kindly provided copies of the Estes Park and volcanism also ended at the RMF; (3) the teleseismic data from 446 earthquakes; the data quadrangle and a preliminary copy of the Fort Col- fl atly subducted Farallon slab broke at, or near, were processed using receiver-function tech- lins 1:100,000 quadrangle. The patience and skills of the RMF by ca. 37 Ma and rolled back toward niques to estimate crustal thickness and seismic Lynne Hemenway in word processing and Leo Gabal- don in computer graphics were extremely helpful. We the southwest as it sank into the mantle, causing velocities in the crust and upper mantle (see also thank the reviewers E.A. Erslev, G.R. Keller, and widespread late Eocene–Oligocene ignimbrite Lerner-Lam et al., 1998, for details). Regional associate editor R.M. Flowers for helpful comments volcanism; (4) the Rio Grande Rift formed in body wave and shear wave tomography (Lee and suggestions that resulted in a better paper. close proximity and approximately parallel to and Grand,1996; Lerner-Lam et al., 1998) the RMF; (5) the RMF has been, and remains, a imaged the transition from seismically slow APPENDIX 1. LARAMIDE SYNOROGENIC major heat fl ow boundary; and (6) the RMF sep- upper mantle beneath the Colorado Rocky SEDIMENTATION AND THRUSTING ON OPPOSITE SIDES OF THE FRONT RANGE arates regions with different patterns of anoma- Mountains to the seismically fast cratonic struc- AND WET MOUNTAINS, COLORADO lies on the residual gravity map of Figure 4. ture beneath eastern Colorado and western Kan- The results of three types of geophysical stud- sas. The transition was seen as gradational east FORMAT ies are especially pertinent to the question of of the RMF with an abrupt increase in velocity Location what underlies the RMF. The fi rst concerns the near the Colorado-Kansas border, interpreted Basin Structural style use of magnetometer arrays to map anomalies as the western edge of the continental interior Laramide sedimentary units in magnetovariation fi elds in the western United craton (Lee and Grand, 1996; Lerner-Lam et al., Age, thickness, paleontologic, paleomagnetic, and/or States and Canada (see review by Gough, 1998). A temperature contrast of at least 350 °C, isotopic dating 1989). The natural magnetovariation fi elds are as well as partial melt beneath the Rocky Moun- Key references produced by primary currents in the ionosphere tains, was estimated by Lee and Grand (1996) to WEST SIDE Huerfano Park and magnetosphere and by secondary induced be required to match the contrast in shear wave East-vergent, low-angle thrusts from Sangre de Cristo currents in the solid Earth. The induced cur- velocity between the mantle beneath Kansas Range rents fl ow preferentially in the more conductive and the Colorado Rocky Mountains. Huerfano Formation (Fm.), lower-middle Eocene, rocks. High conductivity can be generated by The third series of geophysical studies was 0–1500 m, vertebrate faunas Poison Canyon Fm., Paleocene, 0–590 m, overlies either saline hot waters in fractures or by a few based on the La Ristra (Colorado Plateau/Rio upper Cretaceous formations percent of silicate melt in interconnected spaces Grande Rift Seismic Transect Experiment) Johnson (1959); Scott and Taylor (1975); Lindsey in the crust or lithospheric mantle. Alabi et al. deep-imaging seismic profi le, generated using (1998) (l975) and Camfi eld and Gough (1977) identi- naturally occurring earthquake sources, and Echo Park fi ed a narrow zone of high conductivity, which extending from the Four Corners area to south- Dextral strike-slip faults, north-northwest trending Echo Park Fm., early-middle Eocene, 0–600 m, pollen, was named the North American Central Plains eastern New Mexico and west Texas along a narrow elongate basins (NACP) conductor, and mapped it over a total S45°E line (Gao et al., 2004; West et al., 2004; capped by 37 Ma Wall Mountain Tuff length of ~1800 km from ~57°N in Canada Reiter and Chamberlin, 2011). The La Ristra Chapin and Cather (1981); Chapin et al. (1982)

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