NEW BUREAU OF GEOLOGY & MINERAL RESOURCES, BULLETIN 160, 2004 181 Laramide minor faulting in the

1 2 Eric A. Erslev, Steven M. Holdaway , Stephanie A. O’Meara , 3 4 Department of Earth Resources, Colorado State University, Fort Collins, CO 80523 Branislav Jurista , and Bjorn Selvig 1Now at Chevron USA Production Company, P.O. Box 36366, Houston, TX 77236 2Now at Colorado State University/National Park Service, 1201 Oak Ridge Drive, Suite 200, Fort Collins, CO 80525 3Now at Westshore Consulting, 2534 Black Creek Road, Muskegon, MI 49444 4Now at RETEC, 23 Old Town Square, Suite 250, Fort Collins, CO 80524


The nature of Laramide basement­involved deformation in the remains highly con­ troversial, sparking a continuing debate between advocates of vertical, thrust and strike­slip tectonic models. The Front Range of Colorado provides an important geographic link between the northwest­ trending arches of , which are commonly attributed to thrust deformation, and the north­ trending arches of the southern Rockies of New Mexico, which are commonly attributed to partitioned right­lateral transpression. separations and kinematics provide effective tests of Laramide structural models for the Front Range arch. In the center of the arch, exposures of crystalline rocks show no evidence of consistent strike­slip separations on north­striking faults, indicating that major right­lateral displace­ ments, if present, must be located on the margins of the arch. Fault kinematics were studied by meas­ uring nearly 5,000 slickensided faults in strata flanking the Front Range arch. The over­ whelming dominance of thrust and strike­slip minor faults with low­angle slickenline plunges falsi­ fies vertical tectonic hypotheses invoking dip­slip on high­angle faults. Average slickenline and ideal σ 1 axis trends for the minor faults range from N53°E to N80°E, with some variability due to localized folding and faulting. The high intersection angles of average slickenline and compression orientations with major fault strikes suggest a predominance of thrust motion on the bounding faults of the Front Range arch. The lack of north­striking, strike­slip minor faults of clear Laramide age indicates that slip partitioning was not a major contributor to regional Laramide deformation in the Colorado Front Range. The slight obliquity between the arch axis and average slickenline and compression orienta­ tions is consistent, however, with a minor component of right­lateral slip on the thrust faults that bound the Front Range of Colorado.

Introduction Geologic setting and structural models

The Colorado Front Range (Fig. 1) is an important yet con­ The Rocky Mountain province of the is a clas­ troversial part of the Rocky Mountain foreland. Its location sic basement­involved foreland orogen. Deformation dur­ adjacent to the geological community, with its ing the Late to Laramide creat­ strong academic, industry, and U.S. Geological Survey com­ ed an anastomosing system of basement­cored arches that ponents, has made the Front Range an important influence bound the northern and eastern margins of the Colorado in defining our understanding of basement­involved fore­ Plateau and define the elliptical sedimentary basins of the land arches. Field observations from the eastern Front Rocky Mountains. The tectonic mechanism for Laramide Range played a key role in the development of the vertical deformation remains controversial with proposed mecha­ uplift school of Laramide deformation (Prucha et al. 1965; nisms ranging from subcrustal shear during low­angle sub­ Matthews and Work 1978) and subsequent strike­slip duction (Bird 1988, 1998; Hamilton 1988) to detachment of hypotheses (Chapin and Cather 1981; Chapin 1983), which the upper crust during plate coupling to the west (Lowell used Front Range geologic relationships to support north­ 1983; Oldow et al. 1989; Erslev 1993). The Southern Rocky ward translation of the during the Mountains, here defined as the Rocky Mountains south of . Wyoming, have been further complicated by multiple stages This paper will synthesize 10 years of minor fault studies of Tertiary faulting, igneous activity and sedimentation cur­ at Colorado State University that were aimed at testing rently manifested by the . hypotheses for the Laramide formation of the Front Range The multitude of Laramide structural geometries in the of Colorado. The premise of this paper is that many struc­ Rocky Mountains has resulted in a mirroring multitude of tural models for the Laramide development of the Front kinematic hypotheses. In the 1970s and 1980s hypotheses Range predict significantly different stresses and strains, were polarized into the antagonistic horizontal compression which can be deduced from the characteristics of minor and vertical tectonics schools. The vertical tectonics school faults. Details of this research are partially contained in five was dominant in the 1970s, represented by upthrust (Prucha M.S. theses (Selvig 1994; Jurista 1996; Fryer 1997; Holdaway et al. 1965) and block uplift models (Stearns 1971, 1978; 1998) as well as several society field trip articles (Erslev and Matthews and Work 1978). In the 1980s incontrovertible Gregson 1996; Erslev and Selvig 1997; Erslev and Holdaway documentation of Precambrian basement thrust over 1999; Erslev et al. 1999). This work is part of the larger Phanerozoic sediments (e.g., Smithson et al. 1979; Gries Continental Dynamics—Rocky Mountain Project (CD­ 1983; Lowell 1983; Stone 1985) swung opinion back toward ROM) studying the Rocky Mountain lithosphere from its models invoking horizontal shortening and compression. Precambrian assembly to the present. The CD­ROM project Seismic profiles (Smithson et al. 1979; Gries and Dyer 1985), is combining the fault data presented here with diverse geo­ subthrust petroleum drilling at Laramide basin margins logical and geophysical studies to help delineate the geolog­ (Gries 1983), and balancing constraints (Stone 1984; Brown ic evolution of the . 1984; Erslev 1986; Spang and Evans 1988) have demonstrat­ 182

FIGURE 1—Geologic map of the Colorado Front Range (simplified from Tweto 1979) with boxes indicating areas of more detailed maps contained within subsequent figures. ed that the Laramide was dominated by lateral shortening and Snoke 1992), reactivation of pre­existing weaknesses in due to horizontal compression. the basement (Hansen 1986; Stone 1986; Blackstone 1990; The diversity of Laramide structural trends, with faults, Chase et al. 1993), transpressive motions (Cather 1999a,b), folds, and arches trending in nearly every direction, has indentation by the Colorado Plateau (Hamilton 1988), and been attributed to multiple stages of differently oriented detachment of the crust (Lowell 1983; Brown 1988; Kulik compression (Chapin and Cather 1981; Gries 1983; Bergh and Schmidt 1988; Oldow et al. 1990; Erslev 1993). Many of

183 these hypotheses are valid for indi­ vidual areas within the foreland but their relative regional importance is not always clear. Laramide structures of the Front Range basement arch (Fig. 1) are very diverse. Along the eastern flank of the range, the north­northwest­ striking, west­dipping Golden system (Berg 1962) west and south of Denver transitions to an array of northwest­striking, east­dip­ ping reverse faults to the north (Erslev 1993). Farther south, the Golden fault system merges into the Perry Park–Jarre Creek and thrust systems. These overlap southward with the –Chey­ enne Mountain thrust system, which dies southward into the Canon City embayment. On the western side of the range, Laramide geometries are overprint­ ed by more extensive post­Laramide fault and igneous systems. Still, exposures of the large displacement, low­angle Elkhorn, Williams Range, and Never Summer thrust faults indicate they defined the western limit of the basement arch during Laramide deformation. Horizontal fault displacements of at least 10 km (6 mi) are documented where post­ Laramide plutons bow the faults upward, forming tectonic windows exposing Cretaceous shales under­ neath Precambrian rocks of the Front Range hanging wall (Ulrich 1963; O’Neill 1981). Vertical tectonic interpretations invoking dip slip on high­angle, range­bounding faults provided an integrated hypothesis for the Front Range (Fig. 2a; Boos and Boos 1957; Matthews and Work 1978) by explaining the low­angle faults as gravity slides. This interpretation has been questioned on the basis of the magnitude of the overhangs, the faults’ consistency along strike, and their associated shortening struc­ a tures. Symmetric, concave­down­ FIGURE 2—Simplified cross­sections of tectonic models for the Front Range based on (b ) ward upthrusts on the arch margins vertical uplift, with gravity sliding on western flank, ( ) symmetric up­thrusts (Jacobs 1983)c (Fig. 2b) were proposed by Prucha et and positive strike­slip flower structures (Kelley and Chapin 1997), (d ) low­angle, symmet­ al. (1965) and Jacob (1983) as a means ric thrust faulting in the central Front Range (Raynolds 1997), and ( ) backthrust basement to explain the observed horizontal wedge in the northern Front Range (Erslev and Holdaway 1999). shortening within a vertical tectonic framework. logic evidence for thrust loading on the flanks of the central Hypotheses of strike­slip deformation have also been sug­ Front Range to propose a symmetric thrust model (Fig. 2c) gested, often using a cross­sectional geometry similar to that for the southern Front Range arch. Kluth and Nelson (1988) of the upthrust model but suggesting large amounts of used the timing of the sediments to propose an asymmetri­ strike slip on the bounding faults (Chapin 1983; Daniel et al. cal development of these arch­bounding thrust faults, with 1995; Kelley and Chapin 1997). Cather (1999b) provided a major movement on western thrust faults tilting the arch to potential explanation for the similar strikes of proposed the east first, followed by the eastern thrust faults breaking strike­slip and thrust systems by noting analogous patterns through to the surface. Erslev (1993) and Holdaway (1998) along major transform boundaries where the slip is parti­ showed that the northern Front Range arch can be modeled tioned into distinct thrust and strike­slip components. by an asymmetrical chip (Fig. 2d), where the northeastern Thrust models have also been proposed for the Front flank of the arch is underthrust by a wedge of lower crust Range arch (Warner 1956). Raynolds (1997) used sedimento­ whose roof thrust defines the western side of the arch. 184

185 Minor fault analysis methods In addition, multi­stage models for Laramide deforma­ tion (e.g., Chapin and Cather 1981; Gries 1983) have sug­ Slickensided minor fault surfaces were measured within gested that the Laramide foreland may have developed five areas (Fig. 1) along the flanks of the Colorado Front sequentially about differently oriented stress and strain Range (see the appendix for individual locality data). axes. Gries (1983) used structural geometries and sedimen­ Because this research focused on Laramide faulting, faults at tary basin characteristics to propose a counterclockwise localities of and older rocks were avoided rotation of shortening and compression axes from east­west because of the possible presence of faults related to the to north­south orientations during the Laramide. Chapin Ancestral Rocky Mountain and Precambrian . and Cather (1981) combined changes in dike orientations Similarly, localities exposing rocks younger than 40 Ma were and sedimentary basin geometries in the Southern Rocky not analyzed in detail by this paper because they are pre­ Mountains to predict an early east­northeast shortening sumed to record post­Laramide faulting, which was not the event followed by a northeast shortening event dominated focus of this study. by right­lateral strike­slip faulting. Chapin (1983) further In general, faults with the best­preserved slickenside lin­ developed the hypothesis of Laramide right­lateral faulting eations were found in silica­cemented quartz arenites, along the eastern margin of the Colorado Plateau and sug­ which are common in the Cretaceous Dakota, gested that a number of major north­striking faults in the Morrison, and Ingleside Formations. In general, Front Range may be dominated by strike­slip movement. slickenlines within major fault zones were rarely observed Bird (1998) combined fault geometries, kinematic informa­ because of poor exposures and pervasive cataclasis. Because tion and plate geometries to propose that the Southern more highly folded strata have complex slickenline orienta­ Rocky Mountains may have undergone multi­phase, multi­ tions caused by the added complexities of fold­related slip directional compression related to the different convergence (e.g., flexural slip) and rotation of early­formed slickenlines, directions of the Farallon and Kula plates as they were sub­ wherever possible this study concentrated on localities with ducted under . Unlike the counter­clockwise strata dipping less than 35°. interpretations of Gries (1983) and Chapin and Cather At each outcrop, slickenside strike and dip were meas­ (1981), however, Bird predicted clockwise rotation of com­ ured as well as slickenline trend, plunge, and shear sense. pression and shortening directions through time. Shear sense was determined using the criteria of Petit (1987) Recent fault analyses in (Erslev 1997; with most fault planes displaying excellent Riedel fractures Ruf 2000) and northern New Mexico (Erslev 2001) have doc­ and some containing calcite growth fibers. In this area, umented sequential, multi­directional shortening and com­ minor faults commonly form conjugate sets, with acute pression using minor fault analyses. In southwest Colorado bisecting angles between conjugate faults ranging from 60° near Durango, multiple conjugate fault sets indicate that to 30°. Acute bisectors of conjugate strike­slip and thrust northeast shortening was followed by northwest shorten­ faults commonly parallel bedding with the resulting discor­ ing, and both were overprinted by post­Laramide north­ dance of most faults to bedding ensuring that they did not northeast shortening and normal faulting (Ruf 2000). In reactivate pre­existing bedding weaknesses. northern New Mexico, clear cross­cutting relationships Where possible, approximately 20 fault surfaces were show a similar sequence, with faults indicating east­north­ measured for each fault set indicating horizontal shortening. east and east­west shortening cut by faults indicating north­ The possibility of multiple fault sets was considered where northeast shortening (Erslev 2001). At least some of the faults did not conform to simple conjugate patterns. A key faults associated with late stage north­northeast shortening to deducing the relative ages of conjugate fault sets is the are post Laramide because they cut rocks as young as 24 Ma. cross­cutting relationships between fractures which belong Similar mid­Tertiary strike­slip faults have been reported to only one conjugate set. In addition overprinting of slick­ from the southern Front Range at Cripple Creek enlines on individual fault surfaces can give timing relation­ (Wawrzyniec et al. 1999) and in eastern North Park (Erslev ships. Where cross­cutting and overprinting relationships et al. 1999). are not evident, the chronology of fault slip can be estimat­ Each of these hypotheses predict different stress histories, ed by the more complete preservation of the latest slicken­ which can be tested by minor fault analyses. Vertical tecton­ lines. ic hypotheses (Fig. 2a) predict a combination of high­angle Fault data for each area is presented in stereonets which σ dip­slip faults and lower­angle normal faults resulting from plot fault plane pole, slickenline and ideal 1 axis (Compton gravitational collapse of the range. Upthrust hypotheses 1966) orientations (Fig. 3). In addition, smoothed rose dia­ σ (Fig. 2b) predict a range of high­ to low­angle dip­slip fault­ grams of slickenline and ideal 1 axis trends are plotted in ing. Strike­slip hypotheses (Fig. 2b) suggest that at least Figure 4 to provide a graphical portrayal of their angular some of the minor faults paralleling the major faults should distribution and to identify multiple modes of slip and ideal be dominated by strike slip. If slip is partitioned between compression directions. Rose diagrams presented in this thrust and strike­slip systems, one might also expect to see paper are smoothed to clarify their interpretation, with the stress and strain refraction patterns adjacent to major strike­ petal length for each 1° increment proportional to the num­ slip faults. Thrust hypotheses (Figs. 2c,d) predict shortening ber of slickenlines within 5° of each increment. Rose dia­ and compression axes oriented perpendicular to the strike grams from north­central New Mexico (Fig. 4f) are provid­ of major faults and a lack slip parallel to the strike of these ed as an example of multi­stage, multi­directional faulting faults. Multi­stage, multi­directional hypotheses suggest with which to compare the Front Range data. The multi­ that Laramide fault patterns should reveal regionally­con­ stage nature of faulting in this area of New Mexico is con­ sistent multi­modal distributions of shortening and com­ firmed by cross­cutting relationships. For instance, the pression directions. north­south strike­slip faults cut post­Laramide strata as young as 24 Ma (Erslev 2001). In areas where conjugate shear bands without slickenlines are the only available faults, shear band strikes and dips σ FIGURE 3—Contoured stereonet plots of poles to faults, slicken­ 1 σ were used to estimate the orientation of the axis for each 1 lines and ideal axes (Compton 1966) for each area. The plots are locality. The average orientation of each conjugate was cal­ contoured using the Schmidt method (Vollmer 1992), with a 2% culated by eigenvector analysis and then used to find the contour interval calculated over 40 nodes.


FIGURE 4—Rose diagrams of minor fault slickenline trends, both plotting the number of trends within 5° of each 1° increment. In gen­ σ composite and separated by fault type, and ideal 1 axis trends for eral, lineations with plunges greater than 45° are not included in the Front Range localities and north­central New Mexico (Erslev 2001, rose diagrams with the exception of those from the eastern Front included here for comparison). Rose diagrams are smoothed by Range near Rocky Flats.

acute bisector of the conjugate pair, which is assumed to compression within the plane containing the pole to the σ σ parallel the 1 axis. Comparisons of conjugate acute bisec­ fault and the slickenline. For this paper, an angle of 25°, σ tors with ideal 1 axes for localities with slickensided faults consistent with Byerlee (1978), was used by our SELECT σ show that they are highly correlative (Fryer 1997). program to calculate the ideal 1 axis for each fault. Because Although the most direct interpretation of slickensided most of the fault sets are symmetrically weighted, with faults is as strain, the summation of a total strain from minor equal numbers of right­ and left­lateral faults, and equalσ faults requires knowledge of the amount of slip on each numbers of thrust conjugates, small changes in angles σ fault. Unfortunately, net slip could be estimated for only a cause less than 5° of change in average 1 axis orientations. very small fraction of the faults. This is particularly true for These ideal stress calculations assume fracture on ideal con­ strike­slip faults, whose bedding­parallel slip is difficult to jugate planes, an assumption which is violated by reactivat­ measure. In addition, the lack of slip data for larger, gouge­ ed fault planes and some cases of three­dimensional strain. filled faults casts doubt on the regional validity of any strain Additional methods for determining stress and strain value derived from the smaller slickensided faults. axes are presented in the theses of Selvig (1994), Jurista Consequently, this paper attempts to quantify minor fault (1996), Fryer (1997), and Holdaway (1998) and these will be σ σ data in terms of average slickenline and ideal 1 axis compared in a future paper. The ideal 1 axis technique has (Compton 1966) orientations, which parallel the acute bisec­ advantages over the Angelier (1990) least­squares algo­ tors of conjugate faults. Average slip directions are given by rithm, which uses fault slip directions to calculate a reduced the orientation of the first eigenvector of the slickenline stress tensor, in that outlying measurements do not have a trends and plunges (Table 1). The degree of clustering is large influence on stress directions. For the case of fault data given by the associated eigenvalue (E1), where E1 = 1.0 indi­ from the eastern and southern Front Range of Colorado cates perfect clustering about one orientation and E 1 = 0.33 (Jurista 1996; Fryer 1997; Erslev and Selvig 1997; Holdaway indicates the absence of a single preferred orientation. The trend of slickenlines varies with fault type. In the rel­ σ TABLE 1—Average slickenline and ideal 1 axes for the Front atively simple, unidirectional faulting in the northeast Front E1 n Range, showing average trend, plunge and eigenvalue ( ). = Range of Colorado (Fig. 4a; Erslev and Selvig 1997; Hold­ number of measurements. away 1998) right­lateral, thrust, and left­lateral faults form Slickenline Ideal σ1 axes Location n trend­plunge E trend­plunge E unimodal distributions, which can be combined into a tri­ 1 1 modal rose diagram representing all fault slip trends. This Northeast Front 2233 080­13 0.7232 079­11 0.8397 trimodal spread of slickenline trends complicates their use Range for discrimination of multiple shortening and compression Eastern Front 834 087­66 0.5513 075­25 0.5735 directions. Range For simple conjugate faulting, the spread in trend orienta­ σ Canon City 642 076­06 0.6697 075­04 0.7393 tions is largely eliminated by plotting ideal 1 axes, which embayment are assumed to parallel the acute bisector of the conjugate Webster Park area 543 231­06 0.6416 233­01 0.7635 sets. This technique was used by Compton (1966), who σ Western Front 612 250­06 0.6485 253­02 0.7189 determined ideal σ 1 axes by moving half the bisector angle (2 ) from the slickenline towards the direction of maximum Range


FIGURE 5—Stereonets of slickensided fault planes (plotted as arcs) with slickenside lineations (plotted as points),σ σ 1 stereonets of ideal A,B axes, and smoothed rose diagrams of ideal C–F 1 axis trends for individual localities from the northeastern ( ) and west­central ( ) Front Range σ 1998), the ideal 1 method gives higher precision (repro­ These basement­involved faults are overlain by fault­ ducibility) than the Angelier (1990) method. In addition, theσ propagation folds, which progressively transform the fault­ ideal 1 method aids in separation of faults into subsets withσ ing into folding of the sedimentary cover (Erslev and Rogers consistent 1 trends because faults are analyzed individual­σ 1993). Dip and slip­sense interpretations of the major faults ly. By plotting ideal 1 axes and analyzing their variability have varied dramatically through the years. Ziegler (1917) with rose diagrams and eigenvector analyses, data are por­ proposed motion on planar reverse and thrust faults, pre­ trayed in a format allowing relatively unbiased evaluation dicting that a 50° dipping reverse fault underlies Milner of the possibility of multiple directions of shortening and Mountain anticline (Fig. 8), the only fault large enough to compression. actually cut through the Cretaceous Dakota Group. Boos Minor fault analyses and Boos (1957) also showed a planar reverse fault in an σ analogous section but steepened the fault dip to around 80°. 1 The fault orientations and their ideal axis trends are pre­ Prucha et al. (1965) introduced the concept of concave sented as contoured stereonets (Fig. 3), rose diagrams (Fig. downward upthrusts to the area. By increasing fault dips 4), average orientations (Table 1), locality data (Appendix), downward, from 30° dipping thrusts in the sedimentary and data from selected individual localities (Fig. 5). These strata to vertical faults cutting basement, lower angle thrust orientations are sufficient to eliminate vertical tectonic mod­ faults in the cover were explained within a vertical tectonic els from consideration. Whereas fault plane orientations are framework. This upthrust geometry was adopted by quite variable from area to area, the slickenlines and theirσ Braddock et al. (1970) and Le Masurier (1970) for Milner 1 ideal axes strongly indicate subhorizontal slip and com­ Mountain anticline, which they interpreted as being under­ pression. The one exception is the high­angle slickenlines lain by a 60° dipping reverse fault that becomes vertical at from the eastern Front Range northwest of Denver. This depth. A further evolution of fault dip interpretations was data set is dominated by data from highly rotated strata in contributed by Matthews and Work (1978) who suggested the famous south of Boulder. that a planar normal fault with a dip of 80° underlies Milner Further interpretation of the faults requires the examina­ Mountain anticline. tion of the detailed setting of the data. This section will start The vertical tectonic interpretations of these structures are with a description of the northeast Front Range near Fort in clear conflict with the evidence for horizontal shortening Collins, for which we have the largest and simplest data and compression in the Rocky Mountain foreland. Recent sets, and move to the south and west before ending with a work at Colorado State University (Erslev and Rogers 1993; description of the west­central Front Range, which has the Erslev 1993; Selvig 1994; Holdaway 1998) has shown that o o Northeastern Front Rangemost complicated data sets. major fault dips in the Fort Collins area range from 20 to 70 to the northeast, consistent with the early interpretations of The eastern flank of the Front Range exposes a diverse set Ziegler (1917). These angles of faulting are compatible with of structures, with the major, north­northwest­striking fault the lateral contraction indicated by seismic information to systems west and south of Denver bringing the crystalline the north (Laramie Range: Brewer et al. 1982; Johnson and rocks of the core of the Front Range up relative to the Smithson 1985) and south (Golden Fault: Jacob 1983; DOE . However, to the north, a series of north­ 1993; Erslev and Selvig 1997; Weimer and Ray 1997), which northwest­striking, en echelon faults transect the north­ indicate large, west­dipping thrust faults overlapping the trending boundary of the range and bring the eastern basin western margin of the Denver Basin. side of the faults up relative to the western, arch side of the Curiously enough, the thrust and reverse faults in the faults (Figs. 6–9). northeastern Front Range cannot be directly responsible for


FIGURE 6—Simplified geologic map of the northeastern Front Range (after Tweto 1979) outlining the location of more detailed maps in subsequent figures.


FIGURE 7—Simplified geologic map (after Braddock et al. 1988a, 1989) of the eastern margin of the Front Range σ northwest of Fort Collins, Colorado. Smoothed rose diagrams show the trends of ideal 1 axis trends from individ­ ual outcrops (Holdaway 1998). the uplift of the range because their northeasterly dip brings the northeastern flank of the Front Range probably formed the basin up relative to the range. These faults were due to synclinal tightening during wedging at depth analo­ explained by Erslev and Selvig (1997) as second­order back­ gous to structures on the northeast side of the thrusts off a blind, east­northeast­directed thrust in the base­ Mountains (Erslev 1986). This back­limb tightening hypoth­ ment. More recent work by Holdaway (1998) has shown esis (Stanton and Erslev 2001) is consistent with the asym­ that an additional component of deep crustal wedging is metrical uplift of the arch previously proposed by Kluth and necessary to uplift the range (Fig. 2d). In this case, thrusts on Nelson (1988).


FIGURE 8—Simplified geologic map (after Braddock et al. 1970, 1989) of the eastern margin of the Front Range west of Fort Collins, Colorado. Smoothed rose diagrams show the trends of maximum compression directions from individual outcrops from Holdaway (1998).

Minor faults confirm the importance of horizontal com­ (Fig. 4a). Individual localities most commonly show conju­ σ pression and shortening in the northeastern Front Range. gate faulting about a single ideal 1 direction (e.g., Fig. 5a). Stereonets of fault plane and slickenline orientations (Fig. 3a) Two localities show bimodal distributions that are not due to form 4 modes consistent with two conjugate sets, one thrust reactivation of pre­existing weaknesses and are hard to attrib­ σ set and one strike­slip set. These share a common ideal 1 ute to local structural complications (e.g., Fig. 5b). At both of direction with an average orientation of N79°E­11 (trend­ these localities, thrust slickenlines indicating northwest­ plunge). This shortening and compression direction also is southeast shortening and compression overprint slickenlines σ evident in rose diagrams of slickenline and ideal 1 trends indicating more east­west shortening and compression.


FIGURE 9—Simplified geologic map (after Braddock et al. 1988b) of the eastern margin of the Front Range south­ west of Fort Collins, Colorado. Smoothed rose diagrams show the trends of maximum compression directions from individual outcrops from Holdaway (1998).

σ Rose diagrams of ideal 1 axis trends for individual local­ Flats plant, a deactivated nuclear weapons plant, the west­ ities plotted on geologic maps (Fig. 6–9) show little discern­ dipping Golden fault system brings the Precambrian base­ able influence of the local structures, with rose diagrams ment up and over the sedimentary strata of the Denver showing an east­northeast orientation regardless of bedding Basin. This thrust system is complicated, with recent struc­ σ strike. In a few places, rose diagrams indicate that ideal 1 tural balancing by Sterne (1999) indicating that it is a com­ axis trends immediately adjacent to major faults deviate plex triangle zone. from the average trends and instead are arrayed nearly per­ A seismic line through the Rocky Flats plant crossed the pendicular to the faults. On a regional scale, however, there mountain front and revealed both exposed and blind splays is a correlation between the average orientation of the range of the Golden fault system (DOE 1993; Erslev and Selvig front and the average trend direction. West of Fort Collins 1997; Weimer and Ray 1997). Balancing arguments indicate (Figs. 7, 8), where the average basement unconformity that displacement on these west­dipping thrust faults must σ strikes N20°W, the average ideal 1 direction trends N72°E. be transferred to the east­dipping backthrusts, which seg­ In contrast, in the area near Carter Lake (Fig. 9) where the ment the flat irons south of Boulder (Selvig 1994). Erslev and average basement unconformity strikes due north­south, Selvig (1997) suggested that the west­dipping thrusts of the σ the average ideal 1 direction trends S86°E. Golden fault system diverge in this area forming a broad The nature of the major faults is clear from the orienta­ zone of shear that tilted the flat irons toward the Denver tions of the rose diagrams of slickenline trends from individ­ basin by domino­style backthrusting. ual fault types. Strike­slip slickenline trends (Fig. 4a) do not The area near the Rocky Flats plant is further complicated parallel the major faults, suggesting minimal strike slip on by its intersection with the Wattenberg high. This feature these major faults. The dip directions of the largest faults in has many low­angle thrust faults which strike northeast and the area (e.g., the Milner Mountain and Redstone Creek repeat Cretaceous and strata (Kittleson 1992). The faults) are parallel to the thrust fault dip directions and the Wattenberg high is an area of uplift centered on a massive, σ average ideal 1 directions, again indicating a dominance of northeast­trending thermal anomaly paralleling a band of thrust movement over strike­slip movement. The dip direc­ Laramide intrusions to the southwest along the Colorado tions of several of the other major faults are more northeast­ Mineral Belt. The anomalous northeast­striking thrust faults σ erly than the average east­northeast 1 orientations, suggest­ may be because of gravity collapse of the sedimentary cover ing a slight left­lateral slip component on these faults. Left­ off the Wattenburg high into the center of the Denver Basin lateral slip cannot be a major component of the slip, howev­ to the southeast, perhaps aided by thermal maturation of er, because all of these faults generated right­lateral separa­ hydrocarbons during heating of the Wattenberg high (Erslev tions on the basement unconformity. and Selvig 1997). Eastern Front Range northwest of Denver Minor faults in the area of the range front near Rocky Flats plant (Fig. 3b) have an important but complicated his­ An intriguing structural transition is present in the eastern tory. Because of the scarcity of low­dipping beds with slick­ margin of the Front Range west of Denver. South of Boulder, ensided faults in the area (see appendix for bed and average the east­tilted hogbacks characteristic of the northeastern fault attitudes at each locality), trends greater than 45° have range margin steepen, as do the east­dipping en echelon not been culled from the data set plotted in Fig. 4b. In addi­ faults which transect them (Figs. 1, 10). South of the Rocky tion, the individual rose diagrams in Figure 10 have been


FIGURE 10— Simplified geologic map of the eastern margin of the Front Range south of Boulder. Smoothed σ rose diagrams show the trends of ideal 1 axis trends for three major domains from Selvig (1994). Southern Front Range plotted for domains, not individual localities, because of the complexity of the data set and the need for broader averag­ The southern Front Range arch plunges southward into the ing of the data. Canon City embayment where it disappears. Slip is presum­ Fault poles (Fig. 3b) show the dominance of high­angle ably taken up on the southeast­plunging Wet Mountain reverse faults, often approximately paralleling the rotated arch, whose relief also dissipates to the south. Tweto (1980) bedding orientation. Slickenlines show a diffuse, near verti­ used unconformities and distributions to cal maximum elongated in an east­northeast–west­south­ σ show that the Front Range, Wet Mountain, and southern west direction. Ideal 1 axes are more horizontal, but again Sangre de Cristo arches were uplifted in the Pennsylvanian are distributed about an east­northeast–west­southwest by the Ancestral Rocky Mountain orogeny (DeVoto 1980; direction. Rose diagrams showing distribution by fault type Tweto 1980; Kluth and Coney 1981). As a result, these ranges show the overwhelming predominance of dip­slip thrust were extensively eroded and then covered by Jurassic and and normal faults, which probably represent rotated thrust Cretaceous sediments before Laramide deformation. faults. Rose diagrams for the Hoosier and Livingston σ Laramide uplift of the and Front Range domains along the range front (Fig. 10) show ideal 1 axis was initiated in the , as indicated by synoro­ trends similar to those in the northeastern Front Range. genic sediments deposited on their flanks (Tweto 1975; Their oblique orientation to both the Hoosier and Kluth and Nelson 1988). Laramide uplift ended in early Livingston backthrusts as well as the north­south range Tertiary time, and the area formed an extensive late Eocene front suggests slip oblique to both structural elements. erosion surface, which truncates Laramide structures (Epis A minor, distributed mode of east­southeast­trending σ and Chapin 1975; Leonard and Langford 1994). slickenlines and ideal 1 axes (Figs. 3b, 4b) is contributed by The Wet Mountain arch is bounded by northwest­striking faults on the flank of the Wattenburg structural high (see thrust faults on its eastern margin (Burbank and Goddard small rose diagram in northeast corner of Fig. 10). Selvig 1937; Logan 1966). A similarly­striking, anastomosing net­ (1994) attributed these to gravity sliding because there is no work of connected faults, the Ilse fault system, is found in evidence that they are rooted below the Upper Cretaceous the central region of the Wet Mountain arch. The Ilse fault strata. and other northwest­striking faults that comprise the fault

193 system extend from the southern end of the Wet Mountains faults. Rose diagrams of the thrust and right­lateral faults northward into . are multi­modal, suggesting multiple directions of shorten­ σ The Ilse fault system is bound by Precambrian rocks along ing and compression. The left lateral and ideal 1 axis distri­ most of the system’s trace. In the northern Wet Mountains butions are more unimodal, although their rose diagrams west of Canon City and the Royal Gorge area, however, the show a marked asymmetry with a strong mode (N60°E for Ilse fault is bordered to the east by two sedimentary inliers ideal v axis trends) and more trends clockwise of the main at Webster Park and Twelvemile Park. The two inliers are mode than in the counterclockwise direction. separated from the Canon City embayment to the east by Whereas the multi­modal rose diagrams suggest multidi­ Precambrian crystalline rock of the south­plunging Royal rectional faulting, individual locality information (Fig. 11) Gorge anticline. The inliers themselves are separated by does not show consistent multi­modal pattern around the Precambrian crystalline rock exposures and are connected Canon City embayment. The asymmetrical distribution of σ by the Ilse fault system, the northeast­striking Mikesell ideal 1 axis trends is more consistent with variable influ­ Gulch fault, and several north­ to northeast­striking faults. ences of localized stresses. In several locations where beds Lovering and Goddard (1938) compared the northwest­ are oblique to the average N30°W trend (e.g., locations plot­ striking faults of the Wet Mountain arch to similarly­striking ted in Figure 11 south of Fort Carson Military Reservation σ faults bounding the Front Range arch to the east and con­ and immediately south of Wetmore), oblique ideal 1 axis cluded that these faults were formed by east­ or northeast­ trends suggest local control of fault slip patterns. trending horizontal compression. In northern Webster Park, To the west of the Canon City embayment at Webster and near the town of Parkdale, Johnston (1953) used fault and Twelve Mile parks (Fig. 12), faults are dominantly thrusts slickenline attitudes to infer an east to northeast Laramide (Figs. 3d, 4d). For the large number (approximately 75% of compression direction. the faults measured, see Fryer 1997 for details) of unslicken­ In contrast, Chapin (1983) noted that wide zones of sided faults, average conjugate bisectors are shown as crushed and sheared rocks adjacent to the Ilse fault are sim­ arrows in Figure 12. Again, thrust fault slickenline and ideal σ ilar to those adjacent to strike­slip faults. He proposed that 1 axis trends (Fig. 4d) do not yield a simple, symmetric the Ilse fault is a right­lateral strike­slip fault. But lateral sep­ mode of orientations. The dominant N60°E mode is nearly arations of Precambrian rocks adjacent to the Ilse fault sys­ identical to that from the Canon City embayment. Unlike tem do not support large­scale right­lateral slip. In the the faults around the embayment, however, the secondary σ southern Wet Mountains, the Ilse Fault system cuts through distribution of ideal 1 axis trends are rotated clockwise rel­ the Precambrian San Isabel pluton with no obvious strike­ ative to primary mode. slip separation. North of Webster Park, Taylor et al. (1975) Individual locality data at Webster and Twelve Mile parks mapped right­lateral separations of Precambrian rocks but also show a lack of consistent multi­modal patterns, σ these separations are only approximately 1.5 km. Approx­ although distinctly different ideal 1 axis trends can be dis­ σ imately 3–8 km north of Twelvemile Park, the geologic map cerned. Ideal 1 axis trends that deviate from the main of Wobus et al. (1979) shows either no lateral separations or N60°E mode are most common at localities within 1 km of minor left­lateral separations. the northeast­striking Mikesell Gulch and east­striking Chapin and Cather (1981) documented the existence of Gorge faults. In addition, three localities with multiple con­ narrow, elongate axial basins filled with Eocene clastic rocks jugate fault sets are within 1 km of these map­scale faults, along the eastern margin of the Colorado Plateau. Several of suggesting localized deviations in the strain and stress field. these basins, which they called Echo Park type­basins, have Separations on these major faults, and their strikes relative no Upper Cretaceous or strata and thus appear to to those of strike­slip minor faults, suggest that the Mikesell have been initiated in the Eocene, late in the Laramide Gulch fault is right lateral and the Gorge fault is left lateral. orogeny. Echo Park, Chapin and Cather’s type axial basin, The faults may have provided local stress releases, causing σ lies approximately 13 km west of Webster Park and localized stress refraction resulting ideal 1 axis trends that Twelvemile Park and the Ilse fault system. On the east side vary from the regional trend. of the Echo Park Basin, Chapin and Cather (1983) noted a West­central Front Range major shear zone containing highly sheared Precambrian rocks and, at several localities, slivers of Jurassic Morrison The western side of the Front Range is bounded by two and Cretaceous Dakota . Chapin and Cather basins, South Park Basin and Middle–North Park Basin, (1981) noted that Echo Park­type basins have numerous which probably represent the remnants of a once continuous similarities to wrench fault basins and proposed that these thrust­bounded axial basin. The bounding faults are the basins formed by a combination of horizontal shortening largest in the region. The Elkhorn fault extends northward and right­lateral shear. along the eastern margin of South Park until it appears to Unfortunately, slickensided minor faults were not found merge with the Williams Range thrust at the northern­most in the Eocene strata at Echo Park, probably because of a lack tip of South Park (Bryant et al. 1981). The Williams Range of sufficient cementation at the time of faulting. A small thrust extends northward along the eastern boundary of number of striated minor faults in the adjoining basement Middle Park towards Kremmling. The en echelon Never rocks yielded an average slip direction of S64°W­38 and an Summer thrust steps east from the Williams Range thrust σ average ideal 1 axis oriented S61°W­24 (Fryer 1997). The north of Kremmling where it defines the east side of North σ interpreted slip direction and ideal 1 trend estimates are Park. consistent with right­lateral oblique­slip. Unfortunately, The onset of faulting on the west side of the Front Range because these minor faults were measured in Precambrian is regarded to be marked by the age of the 70 Ma Pando rocks, they may not represent Laramide deformation. Porphyry near Minturn, about 30 km to the west (Tweto and Extensive minor fault measurements were made around Lovering 1977). Synorogenic Paleocene units are overridden the margin of the Canon City embayment (Fig. 11; Jurista by the Elkhorn thrust in South Park and by the Never 1996) and east of the Ilse fault at Webster and Twelvemile Summer thrust in North Park (O’Neill 1981), indicating that parks (Fig. 12; Fryer 1997). For the Canon City embayment, thrusting continued into early Eocene. Unlike the higher stereonets and rose diagrams of fault poles and slickenline angle reverse faults of the eastern margin of the Front lineations (Figs. 3c, 4c) show that the area is dominated by Range, thrusts on the west side of the range commonly have right­ and left­lateral faults, with a lesser number of thrust low angle to nearly horizontal dips. Displacements on these


σ FIGURE 11—Simplified geologic map of the southern margin of the Smoothed rose diagrams show ideal 1 axis trends for individual Front Range and the Canon City embayment after Tweto (1979). localities with slickensided faults from Jurista (1996).

faults are considerable judging by the existence of thrust The thrust nature of the low angle faults has not been windows bowed up by post­Laramide plutons. These win­ debated in recent years, although Chapin (1983) suggested dows expose 9 km from the Williams Range that they might conceal underlying and adjacent strike­slip thrust front east­southeast of Dillon, Colorado (Fig. 13; faults. Chapin (1983) suggested two zones of potential Ulrich 1963), and 10 km from the Never Summer thrust strike­slip deformation in the western Front Range that front north of Granby (O’Neill 1981). might transfer right­lateral slip from the Echo Park and Ilse


FIGURE 12—Simplified geologic map of the Webster Park–Twelvemile Park area south of the Front Range in the σ Wet Mountains after Tweto (1979). Smoothed rose diagrams show ideal 1 axis trends for individual localities with slickensided faults and arrows show average compression axes derived from shear bands (Fryer 1997).

fault systems to Wyoming. They are the Mt. Bross fault sys­ which is clearly defined by cliffs of the volcaniclastic tem, a north­northwest­striking zone of faults with right­lat­ Paleocene Windy Gap member of the Middle Park eral separations transecting the Middle–North Park Basin, Formation. Izett (1968) interpreted the Mt. Bross fault as a and a north­striking zone of deformation north­northwest reverse fault, a hypothesis that was challenged by Chapin of Granby, Colorado, which continues north in the Laramie (1983) who noted the presence of en echelon folds associat­ Basin of Wyoming (Figs. 1, 13). ed with the northern extension of the Mt. Bross fault. The Mt. Bross fault west of Granby dips east, bringing The entire western slope of the Front Range is complicat­ Pierre Shale over (Izett 1968). It ed by an active post­Laramide history. Mid­Tertiary volcanic forms the western margin of the Breccia Spoon syncline, rocks and plutonic bodies are abundant and appear to fol­


FIGURE 13—Simplified geologic map of the west­central Front Range after Tweto (1979). Smoothed rose diagrams σ show ideal 1 axis trends for lumped localities (see appendix for individual locality information). low the margin of the Front Range northward. The aptly­ Barclay 1973). Later tilting of basalt flows is consis­ named is well dated by mammal tent with normal faulting along the half and volcanic ash with fission track ages between 20 graben, the northern extension of the Rio Grande rift (West and 13 Ma (Izett and Barclay 1973). Near Kremmling (Fig. 1978; Kellogg 1999). 13), it was deposited in localized extensional basins whose Minor faults can be difficult to find in the west slope of faults follow Laramide thrust traces. This suggests that the the Front Range due to more extensive weathering and veg­ Troublesome Formation may have been deposited in local­ etative cover resulting from the increased precipitation on ized basins caused by back­sliding on thrust faults (Izett and


FIGURE 14—Geologic map of the Colorado Front Range (simplified from Tweto 1979) with rose diagrams of ideal σ 1 axis trends for each study area. The rose diagrams are scaled according to the number of measurements (see Table 1) except for diagram for the northeast Front Range, which is plotted at 50% scale.

the western slope. Locally, the area has extensive hydrother­ faulting and folding. Rather, minor faults just do not hold mal alteration associated with Laramide and post­Laramide up well in natural outcrops under these weathering condi­ magamtism and rifting. Areas without major river systems, tions. In addition, the west slope of the Front Range has less such as South Park, can be particularly frustrating because area of exposed Cretaceous , the most productive of the lack of fresh exposures with slickensided surfaces. strata in terms of slickenside preservation to the east. Many field days in South Park yielded only a few east­dip­ Another complication is provided by Neogene extension ping thrust planes with an average slickenline trend of along the northern Rio Grande rift. High­angle normal N75°E. Likewise, no fault planes were found in the northern faults can be easily excluded from the minor fault data but part of the North Park Basin. These areas do not lack defor­ associated strike­slip faults are included in our data sets. mation, as seismic profiles from both basins show intense Recent work on Neogene extension (Finnan and Erslev

198 2001) has identified at least two orientations of extension: 1) individual thrust planes locally show two slickenside lin­ northeast­southwest extension with associated north­strik­ eations with north­northeast­trending slickenlines over­ ing left­lateral and northwest­striking right­lateral faults, printing northeast and east­northeast slickenlines. and 2) eash­west extension, possibly with associated north­ A key locality north­northwest of Granby (stop B2 in northwest­striking right­lateral and north­northeast­strik­ Erslev et al. 1999) exposes mixed volcaniclastic and arkosic ing left­lateral faults. A mysterious, north­northeast­trend­ components (from Front Range crystalline rocks) in the ing shortening event, with north­striking right­lateral and Paleocene Middle Park Formation (Izett 1968). Three fault northeast­striking left lateral faults (Wawrzyniec et al. 1999; sets are exposed—early thrust faults with northeast­ and σ Erslev et al. 1999; Erslev 2001), may be related to Neogene west­northwest­trending ideal 1 axes and late strike­slip σ east­west extension. But cross­cutting relationships on faults with north­northwest­trending ideal 1 axes. The later north­striking faults commonly show strike­slip slickenlines set of faults is probably post Laramide since they roughly overprinted by normal fault slickenline, suggesting a sepa­ parallel strike­slip, north­striking faults (Fig. 4f) which cut rate phase of post­Laramide strike­slip faulting. This possi­ the 29 Ma (K­Ar; Corbett 1964) Mt. Richtofen batholith bility is supported by Neogene west­northwest­trending (O’Neal 1981) north of Granby at Cameron Pass. The post­ folds at State Bridge and North Park (Fig. 13; Erslev 2001). Laramide age of these faults is confirmed by the fact that the There are two areas with good exposures of Cretaceous batholith cuts Laramide thrusts forming the western margin sandstones in the west slope: near Dillon, Colorado, west of of the Front Range arch. Denver (Kellogg 1997) and along the near Discussion and conclusions Granby and Kremmling (Fig. 13). Maxima of fault poles, σ 1 slickenlines and ideal axes (Fig. 3e) are similar to those Minor faults within Cretaceous and Paleocene units flank­ from the northeastern Front Range (Figs. 3a), indicating con­ ing the Front Range are strongly dominated by thrust and jugate thrust and strike­slip faulting about common east­ strike­slip faults indicating east­northeast­trending shorten­ σ northeast­trending shortening and compression axes. Rose ing and compression. The most tightly defined ideal 1 axes, diagrams of west slope slickenline trends (Fig. 4e), however, as documented by the eigenvectors in Table 1, come from show more variability for thrust and strike­slip fault types, σ the northeastern Front Range, where en echelon map­scale resulting in a broad array of ideal 1 axis trends. σ faults transect the north­trending range front. Some local Rose diagrams of ideal 1 axes for three different sub­ control of fault strains and stresses is evident in a weak cor­ σ areas exposing Dakota sandstone near Dillon (Fig. 13) show relation of ideal 1 axes with changes in the dip directions of σ complex, multi­mode orientations with an average east­ the Phanerozoic strata. The least well­defined ideal 1 axes northeast orientation. In several spots, east­northeast­trend­ come from the complex transition area of the east­central ing thrust fault slickenlines are overprinted by north­north­ Front Range northwest of Denver. In this area, the disper­ east­trending thrust fault slickenlines. Additional complexi­ sion of fault data appears to be related to the steeper inclina­ ty is added by younger normal faulting of the Blue River tion of the strata, which suggests that early­formed faults half graben, which may contribute some of the strike­slip were rotated by subsequent deformation. In addition, the slickenlines. northwest­southeast thrust faulting east of the range front To the north­northwest, one third of the way from Dillon was probably caused by gravity sliding associated with the to Kremmling, exposures of sandstones with the Pierre Wattenberg high. σ Shale below the well­delineated Williams Range Thrust at Ideal 1 axes from the southern and western flanks of the Ute Pass (Figs. 13, 5c) show a complex array of strike­slip Front Range are more diffuse than those from the northeast­ and thrust faults. The two localities sampled gave average σ ern Front Range, with their first eigenvalues ranging 1 ideal axis trends of N62°E and S67°E. In combination between 0.64 and 0.67 as compared to 0.72 for the northeast with the faults from the Dillon area, this data suggests short­ Front Range (Table 1). The two study areas in the southern ening and compression roughly normal to the Williams plunge of the range have asymmetrical distributions of ideal σ Range thrust. Further north, near Kremmling at outcrops on 1 axes with opposite senses of asymmetry. For the Canon the road to and at exposures in , City embayment, a clockwise asymmetry relative to the pri­ strike­slip slickenlines (including two localities from Ehrlich mary east­northeast mode can be ascribed to local influ­ 1999) indicate east­trending to east­southeast trending ences. Strata at stations with more east­west slickenlines and σ shortening and compression oblique to the general east­ ideal 1 axes typically dip to the southeast, suggesting that northeast dips of the major faults and the sedimentary stra­ southeast­directed gravitational sliding and/or folding ta. combined with east­northeast tectonic stresses to control Along the Colorado River between Kremmling and fault orientations at these localities (Jurista 1996). In the Granby, complex faulting shows multiple orientations of Webster Park area, the counterclockwise asymmetry of σ shortening and compression. The Windy Pass member of slickenlines and calculated 1 axes relative to the primary the Middle Park Formation spans the Cretaceous–Paleocene east­northeast mode can be ascribed to stress refraction boundary and is locally composed of highly­indurated, very adjacent to map­scale strike­slip faults. Most stations with σ coarse grained volcaniclastic strata that probably originated more northeasterly ideal 1 axes are within 1 km of these from Laramide stratovolcanoes of the faults, and their deviations relative to the primary N60°E (Izett 1968). Conjugate thrust and strike­slip faults at a local­ mode increase as the faults are approached (Fryer 1997). ity on the west side of the Breccia Spoon syncline next to the σ The distribution of fault types is quite variable. For the Mt. Bross fault (Figs. 13, 5d) show two modes of ideal 1 northeast Front Range (Fig. 4a), approximately equal pro­ axes, a major east­northeast mode and a minor east­south­ portions of thrust, left­lateral and right­lateral faults sug­ east mode. The east­northeast mode is nearly perpendicular gests pure shear shortening. The abundance of strike­slip to the Mt. Bross fault, suggesting thrust motion. The east­ σ faults is surprising, considering the scarcity of parallel map­ 1 southeast mode is oblique to the fault and parallels ideal scale faults, and indicate a component of horizontal elonga­ axis modes west of Kremmling. tion perpendicular to average shortening orientations. An The Windy Pass member on the east side of the Breccia analogous situation may be the minor faulting associated Spoon syncline also contains excellent slickensides (see rose with the Northridge earthquake of 1994, in which thrust diagram closest to Granby in Fig. 13), although the 80° dip faulting at depth caused strike­slip faulting and fold axis­ of the strata makes their interpretation more difficult. Here, parallel extension in the hanging wall (Hudnut et al. 1996;


Unruh and Twiss 1998). Asimilar fault type distribution also strike­slip faults which cut Neogene strata. A vague σ is present in the Canon City embayment (Fig. 4c), an area bimodality in ideal 1 axis trends from the west­central which is also underlain by a system of southwest­directed Front Range (Fig. 4e) is permissive of multi­directional backthrusts (Jurista 1996). Laramide deformation, but the extensive evidence for mul­ In contrast, thrust and reverse faults dominate the minor tiple important stages of Laramide deformation seen in faults in the east­central Front Range and Webster Park northern New Mexico (Fig. 5f; Chapin and Cather 1981; areas. Both of these areas are more immediately adjacent to Erslev 2001) and southwestern Colorado (Erslev 1997; Ruf major thrust systems, the Golden and Ilse fault systems, 2000) is lacking in the Front Range. Suggestions of a late which may explain the dominance of minor thrust faults. northwest­southeast shortening and compression in the On the western part of the range, strike­slip faults become northern and western Front Range are supported by similar more dominant farther from the major thrust faults. observations from northwest (Gregson and Erslev 1997) and σ Slickenline and ideal 1 axis orientations give valuable southwest Colorado (Erslev 1997; Ruf 2000). This change in information on the type of strain during the Laramide compression directions was predicted by Bird (1998) on the orogeny. In the central and southern Front range, the gener­ basis of plate trajectories but may also be due to late­stage al symmetry of major thrust faults with respect to the range impingement by the Cordilleran thrust belt (Gregson and suggests a pure shear pop­up structure (Fig. 2c; Raynolds Erslev 1997). The age and significance of this northwest­ 1997). In the northern Front Range, surface faults are direct­ southeast shortening has not been well defined, although it ed mostly to the west­southwest, although wedge models has not been observed in post­Laramide rocks sampled to (e.g., Fig. 2d) suggest that overall transport was to the east­ date (Finnan and Erslev 2001). northeast. The question of whether the Front Range arch is consis­ If simple shear occurred in the horizontal plane due to tent with large, north­directed, right­lateral displacements strike­slip faulting, as proposed by Chapin (1983) and has major implications to our understanding of Laramide Cather (1999a), a consistent and substantial obliquity tectonics. The major debate over whether the geology of σ between the average slickenline and ideal 1 axis trends northern New Mexico indicates large (± 100 km: Chapin might be expected. This is the case for post­Laramide strike­ 1983; Cather 1999a) or minimal (< 20 km: Woodward et al. slip faulting in northern New Mexico (Fig. 4f; Erslev 2001) 1997) north­trending right­lateral separations of Laramide where right­lateral faults dominate late stage deformation. age is at least partially tested by fault slip in the Front Range In the Front Range, however, the closeness of average slick­ arch. If large Laramide right­lateral displacement did occur σ enline and ideal 1 axis trends (Table 1) suggests that pure in the Southern Rocky Mountains, there should be compati­ shear shortening dominated Laramide Front Range defor­ ble slip on faults in central Colorado. The Front Range is a mation. The only exception to this generalization is in the leading candidate to pass right­lateral slip from New eastern Front Range northwest of Denver, where the differ­ Mexico to Wyoming because the lower amount of throw on ence between the N87°E­trending average slickenline and the Gore Fault and the curvature on the Grand σ N75°E­trending average ideal 1 axes suggests a component limits the amount of strike­slip on these structures. of left­lateral motion. One degree differences between aver­ Geometrically, the possibility of a component of right­lat­ σ age slickenline and ideal 1 axis trends in the northeastern eral strike slip on concave­downward faults, forming a Front Range and Canon City embayment could be used to transpressive flower or palm­tree structure, is difficult to argue for a component of left­lateral shear. But the 2 and 3 dismiss because of the possible movement of material in degree differences for the Webster Park and west­central and out of the plane of section. But minor faults adjacent to Front Range areas suggest right­lateral shear, indicating that faults proposed as candidates for strike­slip motion (e.g., differences between average slickenline and stress axes may Ilse fault, Mt. Bross fault, and faults in the northeastern be more a function of non­representative data collection Front Range) indicate shortening and compression at high than a consistent asymmetry of deformation. angles to the fault traces. Laramide strike­slip faults are These results provide a clear test of Laramide models for common in the margins of the Front Range, but they gener­ the development of the Colorado Front Range. The over­ ally indicate shortening and compression in east­northeast­ whelming predominance of minor faults indicating horizon­ trending directions, not north­trending directions. In gener­ tal shortening and compression provides additional falsifi­ al, right­lateral minor faults (Fig. 4) do not parallel the cation of vertical tectonic models (e.g., Fig. 2a) invoking dip­ north­ to northwest­striking major faults that have been pro­ slip on high­angle reverse and normal faults. Upthrust mod­ posed to carry right­lateral slip northward away from New els using concave downward fault geometries to link sur­ Mexico. The exceptions to this generalization occur on the face thrusting with vertical tectonics in the lower crust (e.g., western slope of the range, but these faults are more logical­ Fig. 2b) could be thought to be consistent with the fault data, ly associated with transverse west­northwest­trending folds but they are impossible to balance at depth. In general, hor­ that deform the post­Laramide Browns Park and North Park izontal shortening in the upper crust is geometrically incom­ Formations of age (Erslev et al. 1999; Wawrzyniec patible with purely vertical slip causing a lack of horizontal et al. 1999; Erslev 2001). shortening in the lower crust without calling for an unreal­ In addition, major northward motion of the Colorado istic amount of basement expansion during uplift. One Plateau during the Laramide would suggest much more could conceive of major shallow extension to balance the northerly slip directions than the east­northeast–west­ shallow horizontal shortening, perhaps in the center of the southwest average slip directions given in Table 1. This arch, but there is no evidence of extension of the required could be explained by strain partitioning (Cather 1999b), magnitude (e.g., Kelley and Chapin 1997). where separate thrust and strike­slip systems develop about The possibility of regionally significant multi­stage, separate strain axes. Cather (1999b) argued that many multi­directional Laramide slip is supported only by a few Laramide structures may have been influenced by pre­exist­ locations in the northeastern Front Range and by locations ing weak faults which caused strain partitioning, and this is in the western Front Range. In the northeastern Front demonstratively true along the San Andreas fault (e.g., Range, west­northwest­trending slickenlines overprint east­ Zoback et al. 1987). Recent work on oblique Laramide arch­ northeast­trending slickenlines in three localities. In the es where there is strong geometric evidence for oblique slip western Front Range, these slip orientations are also seen, (Owl Creek Arch and : Molzer and Erslev and are overprinted by both north­striking normal and 1995; Uinta arch: Gregson and Erslev 1997; :


Erslev 1997; Ruf 2000; Erslev 2001), however, indicates that Range of Colorado. their slickenline trends are, on average, oblique to the arch­ Acknowledgments es and do not indicate strain partitioning. In each case, major sets of strike­slip or oblique­slip slickensides parallel This research was supported by grants from the National frontal faults of the arch, confirming an oblique slip origin. Science Foundation (EAR­9614787 and EAR­9814698), This is not the case, however, for our Front Range measure­ E.G.&G. Rocky Flats, and from the donors of the Petroleum ments, which show that the minor faults paralleling the Research Fund, administered by the American Chemical major range­bounding faults typically are dip­slip thrust Society. Student grants were provided by Geological Society faults. of America, American Association of Geologists, Rocky It can be argued that thrust slip on the margins of the Mountain Association of Geologists, and Chevron U.S.A. Front Range and Wet Mountain arches does not eliminate Insights from Donald Blackstone, Bruce Bryant, Steve the possibility of major strike­slip in the interior of the arch Cather, Chuck Chapin, Shari Kelley, Karl Kellogg, Vince where exposures are dominated by Precambrian crystalline Matthews, Ned Sterne, and Donald Stone provided impor­ rocks. A study of Precambrian contacts in the geological tant additions to this work. Reviews by Steve Cather, Chuck map of Colorado (Tweto 1979) shows only minor opportuni­ Chapin, and Basil Tikoff are gratefully acknowledged. ties for significant right­lateral strike­slip separations in the Finally, we would like to acknowledge the importance of the internal part of these arches. For instance, the Late multiple roles that Chuck Chapin has played in Front Range Precambrian Iron Dike (Chapin 1983) cuts diagonally across research, from the catalytic effect of his questioning of clas­ the Front Range arch from Denver to North Park, with the sical Front Range structural dogma to his open­minded sup­ only possible separations at the western arch flank. The con­ port of the sometimes contrary research of his colleagues. tacts of the and Silver Plume batholiths show References very little leeway for major dextral fault separations on the central and southern part of the Front Range arch. In addi­ Angelier, J., 1990, Inversion of field data in fault tectonics to obtain tion, for those fault zones that do show possible separations, the regional stress, part 3, a new rapid direct inversion method by many of them (e.g., the Ilse Fault of the Wet Mountains) analytical means: Geophysical Journal International, v. 103, no. 2, intersect the sedimentary strata at the southern plunge of pp. 363–379. the arches where hogbacks of Phanerozoic strata show no Berg, R. R., 1962, Mountain flank thrusting in the Rocky Mountain foreland, Wyoming and Colorado: American Association of major separations. Thus it appears that there is no room for major north­directed, right­lateral strike­slip in the Front Petroleum Geologists, Bulletin, v. 46, no. 11, pp. 2010–2032. Range of Colorado. Bergh, S., and Snoke, A., 1992, Polyphase Laramide deformation in the Shirley Mountains, south central Wyoming foreland: There is, however, evidence for a minor component of Mountain Geologist, v. 29, no. 3, pp. 85–100. oblique slip on many major faults in the Front Range area. Bird, P., 1988, Formation of the Rocky Mountains, a continuum Slight obliquities between the minor fault slip directions computer model: Science, v. 239, no. 4847, pp. 1501–1507. and the strike of the Ilse, Mt. Bross, and Williams Range Bird, P., 1998, Kinematic history of the Laramide orogeny in lati­ σ thrusts, where ideal 1 axes are counter­clockwise rotated tudes 35°–49°, : Tectonics, v. 17, no. 5, pp. relative to the fault dip directions, suggest a small compo­ 780–801. nent of right­lateral strike slip. These suggestions of right­ Blackstone, D. L., Jr, 1977, Independence Mountain thrust fault, lateral slip are at least partially counterbalanced by opposite North Park Basin, Colorado: University of Wyoming, obliquities suggesting left­lateral motion between faults in Contributions to Geology, v. 16, pp. 1–15. Blackstone, D. L., 1990, Rocky Mountain foreland structure exem­ the northeastern Front Range and the southern Canon City plified by the , Bridger Range, and Casper embayment. In the latter example, some of the discordance Arch, central Wyoming: Wyoming Geological Association, between the Wet Mountains fault and minor faults may be Guidebook 41, pp. 151–166. due to the likelihood that the Wet Mountains fault is a reac­ Boos, C. M., and Boos, M. F., 1957, Tectonics of eastern flank and tivated Ancestral Rocky Mountain structure of Pennsyl­ foothills of Front Range, Colorado: American Association of vanian age. Petroleum Geologists, Bulletin, v. 41, no. 12, pp. 2603–2676. In addition, the overall geometry of the Front Range arch Braddock, W. A., Connor, J. J., Swann, G. A., and Wolhford, D. D., σ relative to the average slip and ideal 1 axis trends does sug­ 1988a, Geologic map of the Laporte quadrangle, Larimer , gest a minor component of right­lateral motion. In the Colorado: U.S. Geological Survey, Geologic Quadrangle Map GQ­ 1621, scale 1:24,000. southern Front Range, the arch has a trend of N20°W, a Braddock, W. A., Calvert, R. H., Gawarecki, S. J., and Nutalaya, P., trend whose normal is 10° oblique to the N60°E principal σ 1970, Geologic map of the Masonville quadrangle Larimer 1 mode of fault slip and ideal axis trends in the southern County: U.S. Geological Survey, Geologic Quadrangle Map GQ­ Front Range. To the north, however, the arch trends due 832, scale 1:24,000. north toward its bifurcation into the Medicine Bow and Braddock, W. A., Nutalaya, P., and Colton, R. B., 1988b, Geologic σ 1 map of the Carter Lake Reservoir quadrangle, Boulder and Laramie arches. In this area, slickenline and ideal axis trends are more easterly, although still oblique to the arch Larimer Counties: U.S. Geological Survey, Geologic Quadrangle axis. This change in the arch trend may generate a small Map GQ­1628, scale 1:24,000. component of right­lateral slip on the bounding faults in the Braddock, W. A., Calvert, R. H., Wohlford, D. D., and O’Connor, J. T., 1989, Geologic map of the Horsetooth reservoir quadrangle, northern Front Range. Residual northerly slip may be Larimer County, Colorado: U.S. Geological Survey, Geologic absorbed in the more east­west­striking faults bounding the Quadrangle Map GQ­1625, scale 1:24,000. Hanna Basin in southern Wyoming, as proposed by Cather Brewer, J. A., Allmendinger, R. W., Brown, L. D., Oliver, J. 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