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The Geological Society of America Memoir 204 2009
George H. Davis Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
Alex P. Bump BP Exploration and Production Technology, Houston, Texas 77079, USA
The Colorado Plateau is composed of Neoproterozoic, Paleozoic, and Mesozoic sedimentary rocks overlying mechanically heterogeneous latest Paleoproterozoic and Mesoproterozoic crystalline basement containing shear zones. The structure of the plateau is dominated by ten major basement-cored uplifts and associated mono- clines, which were constructed during the Late Cretaceous through early Tertiary Laramide orogeny. Structural relief on the uplifts ranges up to 2 km. Each uplift is a highly asymmetric, doubly plunging anticline residing in the hanging wall of a (generally) blind crustal shear zone with reverse or reverse/oblique-slip displacement. The master shear zones are rooted in basement, and many, if not most, originated along reactivated, dominantly Neoproterozoic shear zones. These can be observed in several of the uplifts and within basement exposures of central Arizona that pro- ject “down-structure” northward beneath the Colorado Plateau. The basement shear zones, which were reactivated by crustal shortening, formed largely as a result of intracratonic rifting, and thus the system of Colorado Plateau uplifts is largely a product of inversion tectonics. The overall deformational style is commonly referred to as “Laramide,” and this is how we use this term here. In order to estimate the dip of basement faults beneath uplifts, we applied trishear modeling to the Circle Cliffs uplift and the San Rafael swell. Detailed and repeated applications of trishear inverse- and forward-modeling for each of these uplifts sug- gest to us that the uplifts require initiation along a low-angle shear zone (between ~20° and ~40°), an initial shear-zone tip well below the basement-cover contact, a propagation (p) to slip (s) ratio that is higher for mechanically stiffer rocks and lower for mechanically softer rocks, a planar shear-zone geometry, and a trishear angle of ~100°. These are new results, and they demonstrate that the basement uplifts, arches, and monoclines have cohesive geometries that reﬂ ect fault-propagation folding in general and trishear fault-propagation in particular. Expressions of the shear zones in uppermost basement may in some cases be neoformed shear zones that broke loose as “footwall shortcuts” from the deeper reactivated zones. Structural analysis of outcrop-scale structures permitted determination of hori- zontal-shortening directions in the Paleozoic and Mesozoic sedimentary cover of the uplifts. These arrange themselves into two groupings of uplifts, one that reveals NE/ SW-directed shortening, and a second that reveals NW/SE shortening. Because the
Davis, G.H., and Bump, A.P., 2009, Structural geologic evolution of the Colorado Plateau, in Kay, S.M., Ramos, V.A., and Dickinson, W.R., eds., Back- bone of the Americas: Shallow Subduction, Plateau Uplift, and Ridge and Terrane Collision: Geological Society of America Memoir 204, p. 99–124, doi: 10.1130/2009.1204(05). For permission to copy, contact [email protected]. ©2009 The Geological Society of America. All rights reserved.
99 100 Davis and Bump
strain in cover strata is localized to the upward projections of the blind shear zones, and because the measured shortening directions are uniform across a given uplift (independent of variations in the strike of the bounding monocline), it seems clear that the regional stresses ultimately responsible for deformation were transmitted through the basement at a deeper level. Thus, the stresses responsible for deforma- tion of the cover may be interpreted as a reﬂ ection of basement strain. The basement strain (expressed as oblique shear displacements into cover driven by reactivations of dominantly Neoproterozoic normal-shear zones) was a response to regional tectonic stresses and, ultimately, plate-generated tectonic stresses. The driving mechanism for the Sevier fold-and-thrust belt was coupled to sub- duction of the Farallon plate, perhaps enhanced by slab ﬂ attening and generation of higher traction along the base of the lithosphere. However, the disparate shortening directions documented here suggest that two tectonic drivers may have operated on the Colorado Plateau: (1) gravitational spreading of the topographically high Sevier thrust belt on the northwest side of the plateau adjacent to an active Charleston-Nebo salient of the Sevier thrust belt, which imparted northwest-southeast shortening; and (2) northeast-southwest shortening driven by the ﬂ at slab. The effect of the two driv- ers tended to “crumple” part of the region in a bidirectional vise, creating added com- plications to structural patterns. Testing of this idea will require, among other things, very precise age determinations of progressive deformation of the Colorado Plateau within the latest Cretaceous to early Tertiary time window, and sophisticated ﬁ nite- element modeling to evaluate the nature of the deformation gradients that would be induced by the two drivers.
The Raplee anticline, Monument Uplift, Utah. Photograph courtesy of Peter Kresan. Structural geologic evolution of the Colorado Plateau 101
INTRODUCTION posed rifting, thrusting, magmatism, core complex development, detachment faulting, and Basin and Range faulting. Viewed in the context of the “Backbone of the Americas The apparent cohesiveness of the Colorado Plateau derives Conference” and the papers presented within this volume on the importantly from the sharp tectonic boundaries on the west and Cordillera (from “top” to “bottom”), the Colorado Plateau appears south. On the west, the boundary is not simply one of abutting as a small “outcrop.” Yet, it is a phenomenal outcrop from the “against” the Basin and Range, but dropping off the Cordilleran standpoint of scenic beauty and tectono-structural insight, and hinge line from craton sediments on the east to passive-margin it is an informative outcrop in disclosing how distinct geologic basin sediments on the west. To the south, there are the north- structures and tectonic provinces reﬂ ect the interplay of mechani- west-trending Mogollon Highlands, which are fundamentally cal stratigraphy, rheology, shear zone deformation, and tectonic controlled by a tectonic fabric that originated at least as far back loading. Even though the Colorado Plateau was affected by and as the Jurassic. Boundaries to the north and east are more transi- responded to the major plate-generated movements and stresses, tional and are related to abrupt increases in structural relief of the it was never overwhelmed. The fact that this region is marked Laramide-style basement cored uplifts (Wyoming uplifts, Rocky by superb exposures of rocks and structures, and it resides as an Mountain uplifts). island of structural coherence surrounded by a sea of strain, pro- The tectonic and structural characteristics of the Colorado vides special opportunities in interpreting structure and tectonics. Plateau reﬂ ect a combination of the initial character of pre– Variously motivated by oil, uranium, and academic curios- Upper Cretaceous lithotectonic assemblages, including the Pre- ity, distinguished, now classic, geologic work has been carried cambrian basement; the heterogeneous nature of the basement, out on the Colorado Plateau for over a century (Powell, 1873; including the presence of crustal shear zones; a distinctive com- Gilbert, 1876; Dutton, 1882; Baker, 1935; Strahler, 1948; Eard- bination of loading conditions; and the changed rheologic condi- ley, 1949; Gilluly, 1952; Kelley, 1955a, 1955b; Kelley and Clin- tions brought about by plate dynamics in the late Mesozoic and ton, 1960). This work was done in a pre–plate tectonic era, and early Tertiary. We review and address these characteristics in this the results have had a relative lasting impact on interpretation paper, with the goal of explaining, both from structural and tec- because of the relative hiatus of investigations on the Colorado tonic perspectives, the variability in orientation of the uplifts, and Plateau when work along or near plate margins held such attrac- the shifts in vergence from some groupings of uplifts to others. tion. The early work, among other things, elucidated the highly These fundamental descriptive facts have posed major difﬁ cul- variable trends of Laramide folds, faults, and uplifts within the ties in achieving coherent tectonic deformation plans to explain Colorado Plateau, an observation that has proven to be an endur- them. There is irony in this, for this “outcrop” seems tectonically ing thorn for scientists trying to come up with a unifying tectonic so simple! To achieve our goals, we not only review pertinent model, and it is especially frustrating because the geology looks literature, but also contribute some new observations and ﬁ ndings so simple. Models that form the early core of tectonic interpre- that have emerged from our recent work. tations variously emphasized compressional shortening (Baker, 1935), differential vertical uplift (Stearns, 1978), compressional BASEMENT-CORED UPLIFTS OF THE COLORADO shortening with discrete shifts in regional stress directions (Kel- PLATEAU ley, 1955a), compressional shortening with incremental rotation of regional stress ﬁ elds (Gries, 1990), progressive rotation of The uplifts and folds of the Colorado Plateau were ﬁ rst stud- regional strain within a province-broad zone of distributed pro- ied in detail by Kelley (1955a, 1955b), who carried out compre- gressive transpression (Sales, 1968), and one self-described “out- hensive analyses of structures in the Colorado Plateau in relation rageous” hypothesis (Wise, 1963). to minerals (notably uranium), energy, and Laramide-style tec- P.B. King’s (1969) Tectonic Map of North America presents tonics. His work not only features the Colorado Plateau system the Colorado Plateau in an artistic and scientiﬁ cally informa- of uplifts and monoclines (Fig. 2), but also folds and fracture tive way. The simpliﬁ ed black-and white version here (Fig. 1) patterns (Kelley and Clinton, 1960). Kelly’s descriptions of the captures the lightly deformed cratonic assemblage of the Colo- uplifts were informed importantly through assembling structure- rado Plateau, bordered by the more substantial deformation of contour maps of the Colorado Plateau tectonic province (Kelley, the Wyoming Province to the north (Brown, 1988, 1993), the 1955b). Mapping reveals that the Colorado Plateau, Laramide- Rocky Mountains to the east and east-southeast (Tweto, 1979), style, basement-cored uplifts are not simply rectilinear blocks the Rio Grande rift on the southeast, and the Sevier fold-and- with sharp monocline margins, but they instead are more nuanced thrust belt on the west. Basin and Range faulting marks adjacent doubly-plunging asymmetric anticlinoria that typically have con- tectonic provinces to the south and west of the Colorado Plateau, spicuous monoclines along their steep ﬂ anks (e.g., see detailed and examples of this faulting include the several major normal descriptions by Davis, 1999) (Figs. 3 and 4). Over the entire faults that deﬁ ne the Western High Plateaus of southwestern span of the Colorado Plateau, shortening accommodated by these Utah (Fig. 1). Basin and Range faulting and volcanism in the uplifts is less than several percent; in fact, Davis (1978) ran a Basin and Range tectonic province proper are superimposed on rough calculation showing that shortening achieved by mono- intense older tectonic products of deformation, including super- clines in the Arizona part of the Colorado Plateau is no more than 102 Davis and Bump
-500 0 0 -2000 0
-3000 1 000 44o
-8000 -5000 100 km 0 0
0 State boundary 42o
Fault (dashed where buried) 0 -4000 Frontal trace of 2000 0 Sevier thrust belt
pC basement surface
contours (m above SL) 0
1000 - 0 Cenozoic volcanic -4000 40o cover -3000 2000
Exposed Precambrian -2000
basement -1000 0 0 0
o -1000 38
-1000 0 -10 0
0 1000 0 0 UT CO 0
AZ NM 0 Grande rift Grande 1000
-2000 o 0 36
000 1 -
1000 0 1000 34oN 114o 112o 110o 108o 106o 104oW
Figure 1. The Colorado Plateau tectonic province (light gray) in relation to the Wyoming province to the north, the Rocky Mountains to the east, the Rio Grande rift system to the southeast, and the front of the Sevier fold-and-thrust belt to the west. Not shown is the Basin and Range province, which borders the Colorado Plateau on the south and west. (Several Basin and Range faults that encroach upon the Colorado Plateau are shown in the lower left, where they demarcate the Western High Plateaus of Utah.) Map is from P.B. King (1969). pC—pre-Cambrian, SL—sea level. Structural geologic evolution of the Colorado Plateau 103
UTAH COLORADO A U San Rafael Uncompahgre Swell Uplift B
Miners Mountain WR DG Circle g Cliffs dr rd Monument u Uplift Uplift SR E UN bc 37°N sr Kaibab 109°W MM u G Uplift UT CO t e Echo Cliffs AZ NM uv w cr 37°N sj Uplift CC 109°W
M Defiance ek or h Uplift Zuni cr r ec Uplift rl cs l ed EC K D wd N gv cp na Apache ed N Z Uplift n i N 0 200 0150 km ARIZONA NEW MEXICO km
Figure 2. (A) Map of Colorado Plateau uplifts (from Davis et al., 1981, Fig. 42, p. 94; originally adapted from Kelley, 1955b). Reproduced by permission of the Arizona Geological Survey. Kelley did not include “Miners Mountain” as an uplift per se, though he did show the Teasdale faulted monocline on the southwest margin of this uplift. Furthermore, Kelley (1955b) was not aware of the “Apache uplift” (see text). (B) Map of monoclines of the Colorado Plateau (from Davis, 1978, Fig. 5, p. 222; originally adapted from Kelley, 1955b). Monoclines: bc—Book Cliffs; cp—Coconino Point; cr—Comb Ridge; cs—Cow Springs; dr—Davis Ranch; e—Escalante; ec—Echo Cliffs; ed—East Deﬁ ance; ek—East Kaibab; g—Grand; gv—Grandview; h—Hogback; i—Ignacio; l—Lukachukai; n—Nutria; na—Nacimiento; or—Organ Rock; r—Rattlesnake; rd—Redlands; rl—Red Lake; sj—San Juan; sr—San Rafael; t—Teasdale; u—Uncompahgre; uv—Upper Valley; w—Waterpocket; wd—West Deﬁ ance. Uplifts: CC—Circle Cliffs; D—Deﬁ ance; DG—Douglas; E— Elk; EC—Echo Cliffs; G—Gunnison; K—Kaibab; M—Monument; MM—Miners Mountain; N—Nacimiento; SR—San Rafael; U—Uinta; UN—Uncompahgre; WR—White River; Z—Zuni.
1%. The long back-limbs of the uplifts tend to be homoclinal or ern group consists of the Uncompahgre, Zuni, and Naciemiento almost-undetectably curviplanar, with dips ranging from 0.5° to uplifts. The monoclines themselves are more variable in orienta- several degrees, whereas the short forelimbs of the uplifts tend to tion than the uplifts (see Fig. 2B). NNW-trending monoclines are be curviplanar, with maximum dips as low as 10° (or less) and as the most abundant, but some are NE- and N- to NNE-trending high as 85° overturned. Individual uplifts tend to trend NNW, but within the system as a whole. there are N-trending and NNE-trending uplifts as well. Based on a variety of observations, the Colorado Plateau The Colorado Plateau uplifts are generally greater than 100 uplifts are interpreted as underlain by faulted basement. In the km in strike length and broader than 30 km. Structural relief asso- cases of the Kaibab and Uncompahgre uplifts, faulted basement ciated with the uplifts is as great as 2000 m (e.g., the Circle Cliffs is directly exposed (Lohman, 1963; Cashion, 1973; Huntoon, uplift!) (see Figs. 2 and 3). Kelley (1955a) noted that the system 1971; Huntoon and Sears, 1975; Stern, 1992; Huntoon, 1993). of Colorado Plateau uplifts can be subdivided into two systems. In the case of the San Rafael Swell, Allmendinger et al. (1987) In the western part of the Colorado Plateau, the uplifts are asym- demonstrated through the Consortium for Continental Reﬂ ection metric toward the east, and in the eastern part of the Colorado Proﬁ ling (COCORP) seismic data the presence of a basement Plateau, the uplifts are asymmetric toward the west (see Fig. high beneath this uplift. Furthermore, Cook et al. (1991) showed 2A). The western group consists of the San Rafael, Circle Cliffs, that Colorado Plateau uplifts express themselves as gravity highs, Kaibab, Monument, Echo Cliffs, and Deﬁ ance uplifts. The east- which they interpreted as expressions of uplifted basement. 104 Davis and Bump
-2000 Utah -2000 Colorado Map Sevier FTB Area River -1000 -1000 0 Colorado Plateau New Green Mexico thrust beltArizona 2000 0
39N River 1000 Uncompahgre Uplift 3000 Colorado
Swell San Rafael 2000
2000 N Miners Mt.
50 km 3000 Circle Cliffs 38N
Colorado 2000 2000 1000
1000 UT CO 37N AZ NM
111W110W 109W Figure 3. Structure contour map of the northern Colorado Plateau showing the approximate areas of the uplifts (shaded gray) in this study and their local shortening directions (black arrows) (from Bump, 2004, Fig. 1). Contours are drawn at 200 m intervals on the base of the Creta- ceous Dakota sandstone. Source for contours, modiﬁ ed after Baker (1935), O’Sullivan (1963), Williams (1964), Williams and Hackman (1971), Haynes et al. (1972), Cashion (1973), Hackman and Wyant (1973), and Haynes and Hackman (1978). Structural elevations are given in meters above sea level. The Colorado River and its tributaries (dark gray) are shown for reference. FTB—fold-and-thrust belt. Reprinted from Bump, A.P., 2004, Three-dimensional Laramide deformation of the Colorado Plateau: Competing stresses from the Sevier thrust belt and the ﬂ at Faral- lon slab: Tectonics, v. 23, doi: 10.1029/2002TC001424, with permission from the American Geophysical Union. Structural geologic evolution of the Colorado Plateau 105
Finally, the Colorado Plateau uplifts are similar in style to uplifts in Wyoming and Colorado, which are known to be basement- cored. Where exposed, these basement faults are ancient features that show multiple episodes of slip, further complicating the kinematic picture (e.g., Huntoon and Sears, 1975; Stone, 1977). The Colorado Plateau was modiﬁ ed by faulting related to (Miocene to present) regional extension to the west and to the east, but the uplifts shown in Kelly’s regional structure map (see Fig. 2A) for the most part escaped this superposed deformation. One exception within the Colorado Plateau proper is the Kai- bab uplift, which is cut by the Paunsaugunt and Sevier faults of Basin and Range origin (Davis, 1999). One more Colorado Pla- teau uplift completely escaped Kelly’s detection because of the effects of post-Laramide superposed extension. This uplift, the Apache uplift (see Fig. 2A), lies in the Arizona transition zone between the Colorado Plateau and Basin and Range. Davis et Figure 4. North-directed photograph of the East Kaibab monocline (from al. (1981) named this uplift, basing its presence on the work of Davis, 1999, Fig. 35A, p. 39). Gently dipping Navajo Sandstone (Juras- sic) caps the top of the monocline, on the far left. More readily erod- Finnell (1962), Granger and Raup (1969), Peirce et al. (1979), ed Carmel Formation (Jurassic), Dakota Sandstone, and Tropic Shale Peirce (1981, personal commun.), and their own structural map- (Lower Cretaceous) crop out in the foreground. Straight Cliffs Formation ping and analysis along the Salt River Canyon in central Arizona. ( Upper Cretaceous) is on the upper far right. Breadth of view is ~1 km. Much of this uplift lies within the Fort Apache Indian Reserva- tion, which is thus the context for its naming.
SHEAR ZONES IN COLORADO PLATEAU clines in the eastern Grand Canyon are expressions of parts of BASEMENT the original Precambrian fault-trace geometry in the underlying basement. Huntoon (1974) emphasized that the basement faults It has been known for decades that many monoclines in the formed originally as normal faults during the latest Mesopro- Grand Canyon “root” into Laramide reverse shear zones that terozoic and Neoproterozoic, but later, during the Laramide (late are reactivated Upper Proterozoic normal faults. Noble (1914) Mesozoic through early Tertiary), they were reactivated with recognized this in the Shinumo quadrangle, Maxson (1961) rec- a reverse sense of displacement to form monoclines involving ognized this in the Bright Angel quadrangle, and Huntoon and Neoproterozoic, Paleozoic, and Mesozoic strata (Fig. 5). In the Sears (1975) recognized it in their analysis of structures in the footwall immediately beneath the Butte fault, there is outcrop- eastern part of the Grand Canyon. In particular, Huntoon (1974) scale evidence of shortening in the form of thrust faults and asso- concluded that the abrupt shifts in trend of individual mono- ciated folding (Fig. 6).
Figure 5. Structure section showing the Butte fault in relation to basement and cover. During the Neoproterozoic, move- ment on the Butte fault created normal displacement. During the latest Cretaceous and early Tertiary, reactivation of the Butte fault produced reverse displacement, tipping out upward into monoclinal folding. The magnitude of reverse throw was less than Neoproterozoic normal throw, and thus net offset of Mesoproterozoic basement remains of normal displace- ment. From Tindall (2000a, Fig. A5, p. 54), used with permission of Sarah Tindall. 106 Davis and Bump
depending upon depth-level of observation, rheology, and superpo- sition of fabrics, reﬂ ecting histories of activation and reactivation (Davis and Reynolds, 1996). Thus, throughout the remainder of this paper, we have chosen to replace the term “fault” with “shear zone” in our descriptions and interpretations, though in referenc- ing past work of other workers, we will continue to use the term “fault” in the same way used by them in the literature.
NATURE OF BASEMENT SHEAR ZONES ADJACENT TO THE PLATEAU
Given the limited exposures of Precambrian rocks and structures within the Colorado Plateau, observations regard- ing Precambrian shear zones in the central Arizona “transition zone” between the Colorado Plateau (to the north) and the Basin Figure 6. Photograph of outcrop-scale thrust fault in Neoproterozoic strata (Uncar Formation) in the immediate footwall of the Butte fault. and Range (to the south) become very important. We here intro- This is an expression of Laramide compressional deformation during duce this kind of analysis as a variation on Mackin’s (1950) reactivation of the Butte fault. Photograph by Sarah Tindall. “down-structure” method to examine a geologic map as if it were a cross section and, in this case, to visualize Precambrian shear zones beneath the southern edge of the Colorado Pla- teau. The transition zone lends itself to this evaluation because Indeed, the structural history of the Butte fault, which is this part of Arizona was uplifted during the late Mesozoic and exposed in the Grand Canyon directly below monoclinally folded early Tertiary in such a way that Paleozoic and Mesozoic strata Paleozoic and Mesozoic strata, permits the salient relationships were eroded, extensively exposing the Precambrian basement between monoclines and basement faults to be understood. There (Kamilli and Richard, 1998). Furthermore, the region was tilted are well documented west-side-down offsets along the west- slightly northeastward (by ~1° to 2°). According to Peirce et al. dipping Butte fault in the Grand Canyon Supergroup (Walcott, (1979), and as summarized in Faulds (1986, p. 126–127), the 1890; Maxson, 1961; Huntoon, 1969, 1993; Huntoon and Sears, tilting was accomplished in two episodes, the ﬁ rst between the 1975; Tindall, 2000b). Reactivation of this fault, beginning pre- Late Jurassic and Late Cretaceous, and the second during the sumably in the latest Cretaceous, caused west-side-up reactiva- latest Cretaceous and Eocene. Because of this history, a very tion of the Butte fault during the creation of the Kaibab uplift low-plunging northward viewing of the transition zone in rela- and East Kaibab monocline (Huntoon, 1993; Huntoon and Sears, tion to the Colorado Plateau becomes an approximation of a 1975). As reemphasized by Tindall (2000b, p. 632): “The magni- structure-section. tude of reverse offset must have been smaller than the magnitude The framework for being able to identify shear-zone bound- of ancient normal offset, because normal separation is still pre- aries within the Precambrian of southwestern North America was served at the Precambrian level.” built by Lee Silver and his students and colleagues, based upon Davis (1978, p. 215) expanded the fault-speciﬁ c conclusions extensive geologic and geochronologic surveying (e.g., Cooper reached by Noble (1914), Maxson (1961), Huntoon (1974), and and Silver, 1964; Silver, 1965, 1967, 1969, 1978; Anderson et al., Huntoon and Sears (1975) (Fig. 7) and concluded that the mono- 1971; Anderson and Silver, 1976; Anderson and Silver, 2005). cline fold pattern as a whole in the Colorado Plateau reﬂ ects the This work led to the understanding that the late Paleoproterozoic expression of many elements of a provincewide basement-fault and early Mesoproterozoic of southwestern North America grew system. Davis further suggested that the shapes of many of the through accretionary crust-forming events, expressed importantly Colorado Plateau uplifts are the muted expressions of underly- as NE-SW–trending provinces of cogenetic suites of volcanic ing basement-block edges (Davis, 1978, p. 225). These conclu- and plutonic rocks (Anderson and Silver, 2005). Recognition of sions were not entirely new. Case and Joesting (1972), based on these distinctive provinces was based upon crystallization ages analysis of gravity and magnetic gradients evident in Precam- derived from developing and using the U-Pb system on zircon brian basement, interpreted a “fundamental” fracture pattern of (e.g., Silver, 1963; Silver and Deutsch, 1961, 1963). Anderson Precambrian age in Precambrian basement beneath a part of the and Silver (2005, p. 12, their Figs. 5A–5C) nicely summarized Colorado Plateau. the progressive delineation of two dominant provinces in south- We suspect that those who have studied the faults into which ern Arizona and southern New Mexico: the Pinal Province (to the the uplifts and monoclines root would agree that major deep-seated southeast), marked by crystallization ages of ca. 1.7–1.6 Ga, and “faults” associated with Colorado Plateau uplifts and monoclines the Yavapai Province (bordering the Pinal Province on the north- are “fault zones.” Strictly speaking, these “fault zones” are most west), marked by crystallization ages of ca. 1.8–1.7 Ga (Conway accurately described as “brittle, semibrittle, or ductile shear zones,” et al., 1987). Structural geologic evolution of the Colorado Plateau 107
Karlstrom and Humphreys (1998) and Nourse et al. (2005) presented interpretations of Proterozoic crustal provinces com- plementary to the aforementioned Yavapai-Pinal accretion map, but they used a “Yavapai-Mazatzal” province taxonomy (Fig. 8) and split out relationships in more detail based on the work of Karlstrom and Bowring (1988), Karlstrom and Williams (1998), Wooden and DeWitt (1991), Bender et al. (1993), Karl- strom (1993), Karlstrom and Daniel (1993), Ilg et al. (1996), and Eisele and Isachsen (2001). The conclusion remains: the Paleoproterozoic of central Arizona was assembled by tectonic shortening between ca. 1.8 and ca. 1.6 Ga (Karlstrom, 1993; Conway et al., 1987; Karlstrom and Humphreys, 1998; Ander- son and Silver, 2005). Figure 7. Experimental simulation of the relationship of The principal tectonic grain resulting from this tectonic monoclines to faulted basement (from Davis, 1978, Fig. accretion is northeast-oriented (see Fig. 8), as expressed by the 1, p. 216). In this case, the strata are alternating layers of powdery kaolin clay and modeling clay, the basement is trends of the provinces themselves (e.g., the Mojave province, the a wooden board, and the preexisting basement fault is a Mojave-Yavapai transition, the Yavapai province, the Yavapai- saw cut. The monocline was produced by compressional Mazatzal transition, and the Mazatzal province), but also by the end-loading of the board. “thrust-sense shear zones” (Karlstrom and Humphreys, 1998, p. 162) that separate each province from one another. From northwest to southeast, these late Paleoproterozoic–early Meso- proterozoic shear zones are the Gneiss Canyon shear zone, the Crystal shear zone, the Moore Gulch shear zone, and the Slate Creek shear zone (see Fig. 8). As emphasized by Karlstrom and Humphreys (1998), these shear zone boundaries inﬂ uenced the distribution and character of Laramide magmatism and metal- logenesis but were not reactivated in any conspicuous way dur- ing the formation of Laramide basement-cored uplifts. However, Karlstrom and Humphreys (1998) pointed out that it is possible for northeast grain to have been reactivated from place to place in the form of transfer zones or accommodation zones oriented parallel to the direction of Laramide shortening, i.e., NE/SW. We point out here that this may have occurred in fashioning the NE-trending Cow Springs monocline near Kayenta, Arizona, just southwest of the Monument uplift (see Fig. 2B) (Davis and Kiven, 1975; Davis, 1978, his Figs. 4 and 5). It was the late Mesoproterozoic and Neoproterozoic shear zones of normal-sense shear displacement that were reactivated preferentially during the formation of the Laramide-style base- ment-cored uplifts (Karlstrom and Humphreys, 1998). The best examples of such normal-sense displacement shear zones are the Canyon Creek and Butte faults (Karlstrom and Humphreys, 1998) (Fig. 9). Continental-scale rifting created these faults between 1.1 Ga and 700 Ma, which coincided with syntectonic diabase intrusions and rift-basin sediment accumulations (Silver, 1960; Shride, 1967; Silver, 1978; Granger and Raup, 1969; Elston, 1979). Indeed, the Canyon Creek “fault,” as a Paleoproterozoic shear zone, controlled the emplacement and distribution of dikes Figure 8. Map showing major northeast-striking tectonic boundar- and sills of Neoproterozoic diabase (Finnell, 1962; Shride, 1967; ies of latest Paleoproterozoic and earliest Mesoproterozoic age (1.8– Granger and Raup, 1969), which were dated by Silver (1960, 1.65 Ga), as projected beneath the Colorado Plateau. Note that the tec- 1978) as 1.1 Ga. This overall timing of rifting makes sense in the tonic boundaries are major shear zones. The Moore Gulch shear zone coincides with the north edge of 1.65 Ga deformation. The Slate Creek context of plate reconstructions for the Neoproterozoic (Burke shear zone coincides with the south edge of pre–1.7 Ga basement. and Dewey, 1973; Stewart, 1976; Stewart and Suczek, 1977; Figure is based on Karlstrom and Humphreys (1998, Rocky Mountain Dickinson, 1977). Geology, v. 33, no. 2, Fig. 1, p. 163). 108 Davis and Bump
modated at least two major reactivations in the Phanerozoic. During the latest Cretaceous and early Tertiary, the Canyon Creek fault experienced eastward-directed reverse displace- ment of at least 1.5 km (Peirce et al., 1979). Subsequently, as the result of extensional faulting in middle to late Tertiary time, this structural relief was countermanded when the Canyon Creek fault accommodated westward-directed normal displace- ment on the order of 1.5 km. Because the vertical component of the (older) reverse faulting exceeded the offset achieved by the superposed normal faulting, Tertiary rocks to the west of the Canyon Creek fault are structurally higher than equivalents to the east. Thus, the requisite throw related to reverse faulting must have exceeded 1.5 km of structural relief (H.W. Peirce, 1981, personal commun.). Faulds (1986, p. 231) placed the up-on-the-west reverse throw at 1650–1750 m and calculated a down-to-the-west nor- mal displacement of 750+ m. The Cherry Creek shear zone, which marks the western margin of the Apache uplift (see Fig. 10), also coincides with a Paleoproterozoic shear zone, a ca. 1.1 Ga reactivation history during diabase sill and dike emplacement, a shortening-induced reactivation history of down-to-the-west monocline development, an extension-induced down-to-the- west Oligocene normal faulting, and post–14 Ma normal fault- ing of Basin and Range tectonic origin (Faulds, 1986). Overall, as emphasized by Davis et al. (1981), the Apache uplift is less obvious because it lacks the stripped structural form so charac- teristic of typical Colorado Plateau uplifts. However, its breadth (~16 km), length (~100 km), and structural relief (at least ~1650 m) are quite comparable to that of other Colorado Plateau uplifts, such as the Circle Cliffs uplift (Davis et al., 1981, p. 94). The development of the Apache basement-cored uplift was followed by extensive erosion, as revealed by the nonconfor- mity atop the Apache uplift between late Paleoproterozoic and Figure 9. Map showing major NNW-striking shear zones of latest Mesoproterozoic basement beneath Tertiary sedimentary and Mesoproterozoic and Neoproterozoic age (1.1–0.6 Ga), as projected volcanic strata above (see Fig. 10). Monoclines associated with beneath the Colorado Plateau. Figure is based on Karlstrom and Hum- phreys (1998, Rocky Mountain Geology, v. 33, no. 2, Fig. 3, p. 168). the Apache uplift are shown in Figure 11. The orientations and locations of the monoclines associated with the Apache uplift are nicely compatible with the East Kaibab system of monoclines (see Fig. 11), and they convincingly match the Butte–Canyon The map relationships of the Apache uplift and its bound- Creek fault system (Fig. 9) as presented in Karlstrom and Hum- ing shear zones (Canyon Creek shear zone on the east, Cherry phreys (1998). Creek shear zone on the west) are quite pertinent to understand- ing Laramide-style basement-cored uplifts (Fig. 10), and thus OBLIQUE SLIP ALONG REACTIVATED BASEMENT we will discuss them here in some detail, drawing together some SHEAR ZONES dispersed literature that has tended to be “off the radar.” Finnell (1962) and Granger and Raup (1969) concluded decades ago As noted earlier, reactivation of the Neoproterozoic Butte that the Canyon Creek fault zone coincides with a major Pre- shear zone within the Grand Canyon is well documented, and cambrian shear zone. The presence of this shear zone was felt it contributed importantly to the development of the East Kai- tectonically in Neoproterozoic time, for it exerted control on bab monocline along the eastern margin of the Kaibab uplift. the emplacement of ca. 1.1 Ga diabase sills and dikes. Finnell Prior to the work of Davis and Tindall (1996), interpretations (1962) recognized that the Canyon Creek fault was the site of developed for the Butte shear zone exclusively reported vertical a major east-facing monocline that deformed Neoproterozoic throw component(s). It is important to recognize that some shear- and Phanerozoic strata. Peirce et al. (1979), more speciﬁ cally, zone reactivations associated with Colorado Plateau uplifts have determined that the Canyon Creek (Precambrian) fault accom- oblique-shear expressions. A case in point is the NNE stretch Structural geologic evolution of the Colorado Plateau 109
Tertiary “Rim Gravels” Paleozoic Strata
Canyon Creek Neoproterozoic Strata Tertiary Fault Sediments Volcanics Tertiary Conglomerate Mesoproterozoic Redmond Tertiary Formation Sediments Volcanics Cherry Creek Tertiary Fault Conglomerate
Ruin Granite alt Riv (1.44 Ga) S er
+ Canyon + Monocline Mule + Hoof Ro Monocline ose + ve lt Lake Neoproterozoic Apache Group and Diabase
Volcanics Tertiary Sediments Tertiary Conglomerate N Mesoproterozoic Sedimentary Mesoproterozoic Rocks of Hess Canyon Group Redmond 015 Formation Ruin Granite (1.44 Ga) km Figure 10. Geologic map showing the Apache uplift, bounded on the east by the Canyon Creek fault and on the west by the Cherry Creek fault. Schematic geological columns underscore that all Paleozoic sedimentary rocks as well as Neoproterozoic sedimentary rocks and associated diabase intrusions are completely missing atop the Apache uplift, where Tertiary sedimen- tary and volcanic rocks rest nonconformably upon late Paleoproterozoic and Mesoproterozoic basement. Figure is modiﬁ ed from Davis et al. (1981, Fig. 9, p. 58, and Fig. 27, p. 78). Reproduced by permission of the Arizona Geological Survey.
of the East Kaibab monocline in southern Utah (Babenroth and full 60 km stretch of the East Kaibab shear zone within Utah Strahler, 1945; Kelley, 1955a, 1955b; Davis, 1978, 1999). (Fig. 12) (Tindall, 2000a). The faults and fault sets are synthetic Mesozoic strata within the East Kaibab monocline (see and antithetic with respect to a major, overall, reverse right- Fig. 4) display an elegant, penetrative system of NNE-striking handed strike-slip shearing along this N20°E-trending segment and WNW-striking faults ﬁ rst identiﬁ ed by Sargent and Hansen of the East Kaibab monocline (Davis, 1999; Tindall and Davis, (1982). Based upon detailed structural analysis and geologic 1999; Tindall, 2000a, 2000b). Slickenlines and grooves along the mapping, these fault sets are recognized as right-handed and left- major shear zone consistently rake 30°S to 40°S, and the shear handed oblique slip faults, respectively (Davis and Tindall, 1996; zone surfaces dip steeply (~75°) westward. As ﬁ rst emphasized Tindall and Davis, 1998; Davis, 1999; Tindall, 2000a, 2000b). by Davis and Tindall (1996), this oblique right-handed strike- Sarah Tindall comprehensively mapped the faulting along the slip shearing is the natural consequence of reactivation of a 110 Davis and Bump
et al., 1963; Billingsley et al., 1987; Davis, 1999). Anderson and Barnhard (1986) documented that this faulted monocline accom- modated left-handed strike-slip movement. Bump et al. (1997) later studied this relationship and determined independently that the Teasdale faulted monocline is a reverse, left-handed transpres- sive structure, with perhaps 1–2 km of left-lateral offset.
COLORADO PLATEAU UPLIFTS AND INVERSION TECTONICS
The evidence is abundant to support the hypothesis that indi- vidual Neoproterozoic shear zones were reactivated as reverse faults and/or transpressive reverse/oblique-slip shear zones. These observations have led to an important provincewide per- spective, namely, that Colorado Plateau uplifts are an expression of “inversion tectonics,” and in particular “intracratonic rift inver- sion” (Marshak et al., 2000, p. 736). Marshak et al. (2000) made a compelling argument for this, building on some of their previous work (Marshak and Paulsen, 1996; Karlstrom and Humphreys, 1998; Timmons et al., 2001). Marshak et al. (2000) inserted an important and necessary concept into the story of structural evo- lution of the Colorado Plateau uplifts in particular. The mechanical basis for understanding reactivation of nor- mal faults and shear zones as reverse/thrust faults is long estab- lished (e.g., Donath, 1961), and it is perhaps best summarized in Byerlee (1978), the contribution for which “Byerlee’s law” was coined. Byerlee’s law describes the conditions necessary to cause Figure 11. Map of monoclines in the Salt River Canyon region (af- slip on a preexisting fault, namely, by assessing the product of ter Granger and Raup, 1969; Davis et al., 1981) and their relation to the coefﬁ cient of sliding friction and normal stress acting on a monoclines of the Colorado Plateau of northern Arizona (after Davis given preexisting fault, and comparing that product to the sum and Kiven, 1975). Map is from Davis et al. (1981, Fig. 23, p. 74). Re- of the shear strength and cohesive strength of the body of rock, produced by permission of the Arizona Geological Survey. where unfaulted. Certain products of the ﬁ rst faulting can reduce the coefﬁ cient of sliding friction, including breccia, gouge, and other fault-rock products; weak hydrothermally altered mineral assemblages; and ﬁ ner-grained and/or highly foliated rocks (e.g., NNE-striking basement shear zone in response to NE-SW short- in shear zones) (Etheridge, 1986). The ﬁ nal necessary condition ening (Davis, 1999; Tindall and Davis, 1999). A N65°E-trending for reactivation is suitability of orientation of the preexisting fault horizontal-shortening direction acting across a N10°E-striking, surface (or zone) within the prevailing stress ﬁ eld (Donath and steeply W-dipping basement shear zone is ideally suited to reac- Cranwell, 1981; Etheridge, 1986). Etheridge (1986) emphasized tivate the shear zone in right-handed strike-slip fashion (Fig. 13). that reverse (thrust) reactivation of normal faults is the most Reverse right-handed oblique slip also characterizes the likely circumstance of reactivation, since normal fault systems NNE-trending stretch of the East Kaibab monocline north of and thrust systems may have very similar geometries (see, for Flagstaff, Arizona, within Wupatki National Monument (see Fig. example, Cooper and Williams, 1989; Boyer and Elliott, 1982). 11). There, strata of the Permian Kaibab and Triassic Moenkopi Etheridge (1986) applied the conceptual framework of fault Formation exhibit abundant low-raking slickenlines on slicken- reactivation to deformation systems, using primarily examples lined fault surfaces. Where individual fault zones tip out, there are from southeastern Australia. He emphasized that insightful mod- relays that conform to right-handed (oblique) strike-slip shearing. els for lithospheric stretching (McKenzie, 1978; Le Pichon and Another good example of oblique-slip shearing associated Sibuet, 1981; and Dewey, 1988) provide a basis for understand- with monocline folding was recognized along the southwest mar- ing the formation of primary extensional fault systems in base- gin of Miners Mountain in Utah. This Colorado Plateau uplift is ment and cover, thus setting up conditions for reactivation during situated just north of the Circle Cliffs uplift (see Fig. 3). A doubly subsequent compressional deformation (Etheridge, 1986, p. 185). plunging NW-trending Colorado Plateau uplift, Miners Mountain Marshak et al. (2000) emphasized that intracratonic rift is bordered on the southwest by the N65°W-trending Teasdale inversion is expressed in uplift structures in the Rocky Moun- faulted monocline, the structural relief of which is 150 m (Smith tains, Colorado Plateau, and Midcontinent regions of North Structural geologic evolution of the Colorado Plateau 111
111°47’ 111°47’ 111°53’ A 37°35’ B 37°35’ C Ksc Kd 6000
5000 4000 K 5 5
J e 30
7000 J Grosvenor’s 31 Arch 8 30 Jc Grosvenor’s 36 Arch 5
10 42 N 6 Qal
8000 14 Ksc
Monocline 1 km 56 25
Pump Anticline Kk Canyon Jn Spring 49 3000 36 Tr 90 20 11 9000
Coyote K 33 Kaibab Paria 17 Canyon 55 Qal 10 4000 54
Kw Creek 13 East Jcp 71
89 0 89 500 31 49
Syncline J Pump Canyon 6000 Spring 67
0510 0510 45 Kk km km 25 17 36°59’Contour Interval = 250 feet 36°59’ 37°17’ 112°00’ 112°00’ 111°50’ Figure 12. Northern part of East Kaibab monocline. (A) Structural contour map. Structure contours (ft) are drawn on the base of the Cretaceous Da- kota Sandstone. From Tindall and Davis (1999, Fig. 2, p. 1305). (B) Simpliﬁ ed geologic map. Faulting and folding in the steep limb move from older stratigraphic units in the south into higher stratigraphic units northward. NE-striking faults are right lateral; NW-striking faults are left lateral. From Tindall and Davis (1999, Fig. 3, p. 1306). (C) Structural details. Short, NW-striking, NE-dipping faults are left lateral. Near Grosvenor’s Arch and Pump Canyon Spring, NE-striking faults dip NW and accommodate reverse right-handed offset. Jurassic stratigraphy from oldest to youngest: Navajo Sandstone (Jn), Page Sandstone Member (Jcp), Carmel Formation (Jc), and Entrada Formation (Je). Cretaceous stratigraphy from oldest to youngest: Dakota Formation (Kd), Straight Cliffs Formation (Ksc), Wahweap Formation (Kw), Kaiparowits Formation (Kk), and Qal—Quaternary alluvium. Figures 12A, 12B, and 12C reprinted from Tindall, S., and Davis, G.H., 1999, Monocline development by oblique-slip fault-propagation folding: The East Kaibab monocline, Colorado Plateau, Utah: Journal of Structural Geology, v. 21, no. 10, Figure 6, p. 1308, with permission from Elsevier. 112 Davis and Bump
could develop on different faults at the same time, depending on WSW ENE A the orientation of the faults (Marshak et al., 2003). Reactivation of Proterozoic normal faults and shear zones into reverse-slip and reverse/oblique-slip structures created Laramide Horizontal monocline fold vergence reﬂ ecting “antecedent fault dips” of the σ shortening basement structures (Marshak et al., 2000, p. 735). Absence of N σ Proterozoic rift strata in the hanging walls of many of the uplift- S bounding shear zones was not viewed as a problem by Marshak Cross-sectional trace of et al. (2000, p. 738)—they pointed out that regional erosion, now basement fault expressed in the Great Unconformity, removed at least 10 km of crust from mid-Proterozoic basement. Davis et al. (1981, p. 94–95) subscribed to the conclusions reached by Silver (1978) as to why the Neoproterozoic shear N Horizontal zones were prone to reactivation. Silver (1978) argued that intru- shortening sion of the 1.41 Ga, regionally extensive, Mesoproterozoic gra- nitic suite was a “cratonization process” that imparted a greater rigidity to the basement. Thus, when basement was subjected σ to horizontal compressive shortening, neither the relatively S σ incompressible granitic batholiths could shorten, nor could the ramide Map-View trace of mechanically softer pendants of metamorphic rocks insulated by La N basement fault B the granite. As a result, crustal shortening was accommodated Figure 13. (A) Cross-sectional diagram showing Laramide horizontal selectively on major, wide-spaced shear-zone discontinuities. σ compressive stress resolved on a steeply dipping basement fault. S is shear component; σ is normal component. (B) Map-view diagram N SUBSTANTIALLY DIFFERING VIEWS OF ORIGIN OF showing resolved shear stress on a steeply dipping basement fault, given N65°E-trending horizontal compressive stress. This conﬁ gura- THE FOLDS tion favors right-handed transpressive strike slip. Figure is from Davis (1999, Fig. 39, p. 42). The structural geologic literature underscores how difﬁ - cult it has been, over the decades, to work out the deformation mechanism(s) responsible for the formation of the monoclines and the regional anticlines (the uplifts!) with which they are America, and differences between uplifts across these provinces associated. For the longest time, there was no agreement that are only a matter of scale (Fig. 14). They reiterated that the the fundamental origin was due to crustal shortening, although Proterozoic normal faults and shear zones, some of which later this conclusion had been reached early and very compellingly became reactivated to form the Ancestral Rockies and Laramide- by workers such as Baker (1935). End-member compressional style basement-cored uplifts, were created during rifting of con- deformation mechanisms for folding of regional tectonic signiﬁ - tinental crust (Fig. 15). The cratonwide Proterozoic fault orienta- cance are “free folding” and “forced folding.” Products of free tions, according to the analysis by Marshak et al. (2000), were folding have geometric proﬁ les with characteristics that reﬂ ect marked by two dominant sets: N to NE, and W to NW. Marshak the physical and mechanical properties of the rock layers them- et al. (2000) suggested that these rifting-related faults and shear selves, especially thickness, stiffness, ductility contrast between zones evolved during the Mesoproterozoic and Neoproterozoic, layers, and cohesion along layer boundaries (Biot, 1957, 1959; speciﬁ cally in the interval 1.5 to 0.7 Ga (Marshak and Paulsen, Ramberg, 1959, 1962; Biot et al., 1961; Currie et al., 1962; John- 1996; Timmons et al., 2000). Of course, different kinematics son, 1977; Davis and Reynolds, 1996, p. 414).
+ Basement + + + + + + + + + + + + + + + + + Cover Fault Rocky Mtn. region Colorado Plateau Midcontinent Figure 14. Cross-sectional sketches that emphasize the similarity of style of basement uplifts in the Rocky Mountain region, the Colorado Plateau, and the Midcontinent. Differences are fundamentally a matter of scale. Figure is from Marshak et al. (2000, Fig. 1B, p. 735). Used with permission of the Geological Society of America. Structural geologic evolution of the Colorado Plateau 113
A Late Proterozoic Rifting
Laramide Inversion B Wind River Mts. Owl Creek Mts. Big Horn Mts. Black Hills
Moho 60 km
Figure 15. Illustrations showing intracratonic rift inversion. (A) Schematic cross section of Rocky Mountains: GRB—Green River Basin, WRB— Wind River Basin, BHB—Bighorn Basin, PRB—Powder River Basin. (B) Schematic E-W cross section illustrating how the Rocky Mountain section (in A) may have looked prior to Phanerozoic inversion. Figures are from Marshak et al. (2000, Figs. 3A and 3B, p. 737). Published with permission from the Geological Society of America.
Reches and Johnson (1978) emphasized, for example, that cover interface, only 122 m of slip could be generated along the buckling or (large-scale) kink folding are both “viable options” basement-cover interface during bending and formation of the for the formation of monoclines on the Colorado Plateau. Based antiform. If, more realistically, this ﬂ exural slip were distributed upon both mechanical and experimental modeling, they con- throughout the cover at the boundaries of major, thick, stiff for- cluded that the asymmetry—so pronounced in the case of mono- mations, the impact would be negligible. In fact, ﬁ eld observa- clines!—can be produced by layer-parallel shortening and layer- tions show that away from the monoclines proper, there is no parallel shearing, with or without differential movement along physical evidence for ﬂ exural slip along bedding planes. high-angle faults. Tikoff and Maxson (2001) elevated the scale of application Yin (1994) emphasized free folding as well, modeling the of free folding even higher and proposed that the initiation of the upper crust of the entire Colorado Plateau as an elastic thin plate arches and uplifts in the Colorado Plateau, Rocky Mountains, and (10 km thick) and having it become deﬂ ected into a broad NNW- Wyoming Province reﬂ ected buckling at a lithospheric scale. In trending antiform by the combination of horizontal compression particular, they envisioned that the Cordilleran foreland was later- and vertical edge loading. The structural relief of the antiform was ally stressed in such a way that the entire lithosphere experienced modeled as 1.5 km, which corresponds to limb-dip inclinations of a buckling instability, and that the “stiff layer,” which controlled ~1° or less. The bending of the crust was, according to Yin (1994), the dominant wavelength (~190 km), was the lithospheric man- accompanied by layer-parallel shear on the antiform’s broad, tle. The wavelength data reported by Tikoff and Maxson (2001) regional, western and eastern limbs, each of which would have are marked by a very high standard deviation, perhaps unaccept- been ~500 km wide. Yin proposed that the layer-parallel shear ably high in relation to the conclusions reached. They reported caused the monoclines to form as giant asymmetrical drag folds. distances between arches (measured along east-west transects) as We doubt that sufﬁ cient layer-parallel shear could have 140 km, 140 km, 80 km, 60 km, 300 km, and 230 km. been generated to achieve the ﬁ nal condition. Ramsay (1967, In contrast to free folding, products of forced folding have p. 392–393) has demonstrated that the actual amount of (ﬂ ex- geometric proﬁ les with characteristics that reﬂ ect the form of ural) slippage along the top of any layer within a ﬂ exurally folded the faults, and the amount of displacement along the faults with sequence can be determined by multiplying the thickness of the which the folding is associated. Stearns (1971, 1978) was a strong folded layer and the dip of the layer in radians (where 1° = 0.175 proponent of forced folding in interpreting deformation associ- radian). In Yin’s model, the folded basement layer within the ated with basement-cored uplifts in the central Rocky Moun- “thin elastic plate” is ~7 km thick, and it resides beneath a 3-km- tains, emphasizing the expression “drape folding” in describ- thick cover of Paleozoic, Mesozoic, and Cenozoic strata. If, for ing this deformation mechanism. Strata were “draped” over the Yin’s model, all of the slip concentrated itself at the basement- edges of basement blocks, which had differentially vertically 114 Davis and Bump moved with respect to one another by faulting. Descriptions of tice of modeling fault-propagation folding as if the fault-propa- the structural geometries and rock properties by Stearns and his gation fold was a migrating kink band (Suppe and Medwedeff, students and colleagues, through both ﬁ eld and laboratory work, 1990), establishing its shape early (as a function of fault-ramp are abundantly detailed (e.g., Jamison, 1979; Jamison and Stea- angle) and then simply growing in size as displacement along rns, 1982; Couples et al., 1994). Stearns (1978) emphasized that the master fault progressively increased. Instead Erslev (1991, the speciﬁ c fold geometries reﬂ ected such factors as ductility p. 617) emphasized that broad zones of folding in cover tighten contrast between basement and cover work, absolute rheologies, and constrict downward toward narrow shear zones in basement, and presence or absence of detachment between basement and and that the fold geometry throughout affected cover tends to be cover. Stearns’ work sparked great debate, not so much in rela- triangular in cross section. tion to the fold geometries and rock properties, but in relation to Erslev (1991) went on to provide the “trishear” kinematic his emphasis that the fault movements associated with basement model for fault-propagation folding, and this has proven to be uplifts were not generated by layer-parallel compression and a powerful contribution. “Trishear” (Erslev, 1991, p. 617–618) shortening, but by “vertical tectonics.” His emphasis on vertical was coined for the triangular geometry of the zone of deforma- tectonics was importantly derived from his view that the master tion within which strain-compatible shear is distributed (see, for faults steepen at depth. example, Fig. 7). Allmendinger (1998) took the precepts of tri- Forced folding geometries and mechanisms have now been shear deformation, and the mathematical analysis of “trishear” examined in excruciating detail, following the delineation of developed by Hardy and Ford (1997), and created software for the two dominant classes of forced folding: fault-bend folding both inverse and forward numerical modeling of trishear fault- (Boyer and Elliott, 1982; Suppe, 1983; Mitra, 1990), and fault- propagation folding (see also Cristallini and Allmendinger, propagation folding (Suppe, 1983, 1985; Mitra and Mount, 1998; 2001). The proﬁ le geometries that emerge from forward model- Mosar and Suppe, 1992; Poblet and McClay, 1996). “Fault-bend ing bear a very close relationship to the proﬁ les of Colorado folding” is germane to understanding the Sevier fold-and-thrust Plateau monoclines. One feature in particular stands out to us, belt west of the Colorado Plateau, where, for example, DeCelles both in the models and in ﬁ eld observations, namely, Erslev’s and Coogan (2006) reported 220 km of total shortening based on (1991, p. 617) emphasis that the synclinal hinges of monoclines their studies in central Utah. As is evident in the classical litera- are especially angular. In our experience, the synclinal hinges ture on structure-tectonics of the Canadian Rockies, the Appala- are seldom well exposed in the Colorado Plateau monoclines, chian Mountains, and the Sevier fold-and-thrust belt (including but where we have seen them (e.g., the Rock Canyon mono- the Wyoming-Idaho thrust belt), thick miogeoclinal sequences cline on the eastern margin of the Apache uplift, and the Nutria lend themselves to deformation by fault-bend folding. As empha- monocline along the Zuni uplift), the synclinal hinges are strik- sized in the following section, fault-propagation folding is the ingly angular. mechanism directly applicable to understanding the uplifts of the We take the opportunity in this paper to insert our own mod- Colorado Plateau. eling of Colorado Plateau folding using the trishear programs of Allmendinger (1998), using very carefully rendered ﬁ eld proﬁ les TRISHEAR DEVELOPMENT OF COLORADO of bedding dip and contact locations across uplifts for control PLATEAU MONOCLINES (Cardozo, 2005). The trishear modeling itself tracks ﬁ ve vari- ables: initial x and y locations of the fault tip, fault dip (ramp As is apparent, divergent views on the origin of the mono- angle), total fault slip, trishear angle, and propagation-to-slip clines and anticlines have commonly been inseparable from (p/s) ratio (Allmendinger, 1998). A common homogeneous tri- diverging views of the orientations of the faults with which these shear movement plan (a potential sixth variable) was assumed folds are associated. Advances in structural geology in the past throughout (Erslev, 1991). two decades have made it crystal clear that, almost always, the An example of our ﬁ ndings is revealed in the results of form of a major fold reﬂ ects the form of the major fault with forward modeling of the Waterpocket monocline, which is the which it is associated (often “blind” and at depth), and that folds eastern margin of the Circle Cliffs uplift (Fig. 16). Bedding dip of the type we are addressing are the products of incremental data were collected by University of Arizona students Darren progressive development over the course of thousands of earth- Green and Hillary Brown, who mapped a transect across the quake cycles. This recognition, not understood at the time when fold, measured dips using a meter-long digital level (leveling the “classical” Colorado Plateau studies were being carried out, with respect to expansive normal-proﬁ le dip exposures), and creates leverage in determining the orientations and shapes of compiled well logs made available by the Utah Geological faults at depth, even in the absence of subsurface data. Survey. Local stratigraphic thickness data were compiled from It is fault-propagation folding that best applies to formation well logs as well as from the measured sections of Smith et al. of Laramide-style uplifts and associated monoclines within the (1963), Billingsley et al. (1987), and Hintze (1988). Colorado Plateau. Such folding takes place above the tip of the We ran nearly 200 forward models of the Waterpocket blind shear zone as it propagates upward and laterally through monocline, experimenting with different trishear angles, ramp basement and into cover. Erslev (1991) took issue with the prac- angles, and p/s ratios. The best ﬁ t for the Waterpocket fold is one Structural geologic evolution of the Colorado Plateau 115
pC- Sea Pal Level
pC- Pal Sea Level Figure 17. Model of fault tip propagating from within basement Figure 16. Best-ﬁ t inverse and forward models for the Waterpocket into shallower basement, resulting in folding of basement and monocline (east margin of Circle Cliffs uplift). For the inverse model, cover. Figure was modiﬁ ed from García and Davis (2004, Fig. note the mismatches in both locations and dips of contacts at topograph- 3b, p. 1260). Published with permission of the American As- ic surface. Mes—Mesozoic; Pal—Paleocene; p–C—pre-Cambrian. sociation of Petroleum Geologists.
with a fault ramp angle of 30°, a trishear angle of 100°, and an REGIONAL STRESS AND STRAIN DIRECTIONS initial fault tip 2.3 km below the basement-cover contact. In our best solutions, we used a p/s ratio of 6.0 within basement, reduc- With few exceptions, the observations and interpretations ing it to 2.1 in cover for the remainder of the total 3.5 km of fault reported up to this point in this paper have skirted the challenge displacement (Fig. 16). of inferring shortening directions, and then perhaps stress ori- Where basement is found to be folded, the fault tip of the shear entations, on the basis of the structures themselves. It is one zone to be reactivated must have started out below the basement- thing to call attention to the geometry and deformation plan of cover contact (Fig. 17), perhaps originating as a footwall shortcut the Colorado Plateau basement-cored uplifts, but quite another on an inverted listric shear zone. Insight into the folding of granite, to invert products of the strain history in order to evaluate short- through trishear fault-propagation folding, was presented by Gar- ening directions and causal stresses. Methods have been avail- cía and Davis (2004, p. 1274), based on their analysis of the Sierra able to invert fault-slip data to regional stress (Angelier, 1979, de Hualfín basement-cored uplift in the Sierras Pampeanas: 1990, 1994; Suppe, 1985). There are limitations in applying these The very propagation of the tip of an advancing basement approaches to the Colorado Plateau uplifts. Although fracturing, fault creates, beyond the tip, a physical ground preparation that largely jointing, is penetrative and pervasive within and across all not only may more readily accommodate further tip advance, of the uplifts, mesoscale faulting is essentially absent, except in but may also accommodate folding of the material, even granite, the deformed zones in close proximity to the monoclines. There through which the tip may advance. Thus, in the trishear exam- are a number of exceptions, e.g., the Chimney Rock area of the ple, the trishear angle, combined with slip magnitude, inﬂ uences San Rafael Swell (Krantz, 1986, 1988), the steep limbs of the the amplitude dimensions of the eventual basement-cover folded Miner’s Mountain and Circle Cliffs uplifts, and the West Kaibab interface. The damage in the trishear zone, in combination with fault zone on the margin of the Kaibab uplift (Strahler, 1948), but predeformation anisotropy, renders even granite a macroductil- the mesofaulting record far aﬁ eld from the deformed zones is too ity. In short, before imagining folds as fault-propagation folds, it spotty for comprehensive analysis. was impossible to imagine a set of mechanisms that would incre- Nevertheless, Anderson and Barnhard (1986) applied Ange- mentally, ﬁ rst, damage the rock appropriately, and second, impart lier’s (1979) fault-inversion technique to mesofaults in the sedi- systematic movements of all constituent parts to create a folded mentary cover of the Waterpocket and Teasdale monoclines. form, even in granite. Although these are not the major uplift-bounding basement 116 Davis and Bump faults, they gave a maximum compressive stress direction of both instances, local conditions rule, and regional stress signa- N65°E/S65°W. tures remain elusive. Kelley and Clinton (1960) studied the regional joint patterns In spite of these collective challenges, Bump (2001) and within the entire Colorado Plateau and yet were not able to extract Bump and Davis (2002) analyzed, for purposes of stress inver- from those data substantive conclusions about the Laramide sion, mesoscale penetrative structures within and near the steep stress ﬁ eld. Ziony (1966) described joints in the southeastern part limb of the San Rafael, Miners Mountain, Monument, Uncom- of the Monument uplift, near the Four Corners region. Based on pahgre, Circle Cliffs, and Kaibab uplifts. A goal was to resolve the the orientations of regional sets of shear joints, he interpreted challenge of whether penetrative structures observed are products a Laramide minimum compressive stress direction of ~N20°E/ of monoclinal folding and/or manifestations of regional stresses S20°W, approximately parallel to the strike of the Comb Ridge that formed the monoclines in the ﬁ rst place. The approach was monocline, which bounds the Monument uplift. to gather structural data at diverse locations along each deformed Bergerat et al. (1992) reported a detailed study of joints zone and then determine whether the preferred orientations of the in selected areas of the Colorado Plateau. They identiﬁ ed both small-scale structures shifted in ways that conformed to compa- shear and tensile joints, from which they interpreted a progres- rable shifts in the local strike of the monocline, or whether they sive rotation of maximum compressive stress from 65° to 115°. remained ﬁ xed in preferred orientation even where the trend of They cautioned, however, that although jointing could be a useful the monocline itself shifted signiﬁ cantly. indicator of tectonic stress directions, it was difﬁ cult to use alone. The conventional mesostructures that proved useful were Swanberg (1999) analyzed joints in the Wingate Sandstone tectonic stylolites, en echelon vein arrays, deformation bands, across well-exposed cliff outcrops of the doubly plunging Cir- and mesoscale slickenlined fault surfaces, including crystal- cle Cliffs uplift. Her work also suggested that it may be produc- ﬁ ber lineations (Bump and Davis, 2002). We also used a rela- tive to invert joint system data to interpret regional compres- tively unconventional mesostructure, namely, Eshelby joints sive stress (Swanberg and Davis, 1999). The joints appear to (Eidelman and Reches, 1992). These are millimeter- to centi- “emerge” from strain energy stored from the time of folding, meter-spaced parallel, planar fractures that occur in stiff “inclu- for there is a tight relationship between the geometry of the sions” (stress concentrators such as chert nodules) within soft Circle Cliffs anticline and the geometry of the jointing. To date, layers (such as limestone). there have been no such systematic analyses across all of the For tectonic stylolites, we assumed that the orientation of σ many uplifts of the Colorado Plateau. maximum compressive stress ( 1) was parallel to the preferred ori- Davis (1999) evaluated the systems of deformation band entation of teeth and cones. Within certain strata in the Monument shear zones in the Colorado Plateau region of southern Utah, with uplift, the presence of conjugate semibrittle shear zones, composed an initial objective (among others) of determining if deformation of nested en echelon crystal-ﬁ ber gash veins (calcite) and en eche- band shear zones related to the Laramide-style uplifts were evi- lon tectonic stylolites (Fig. 18), revealed the direction of maximum dent across the uplifts, and not just within the deformed zones compressive stress (i.e., perpendicular to the line of intersection along the boundaries of uplifts. Deformation bands and deforma- of the two conjugate sets, and bisecting the acute angle between tion band shear zones typically are distinguished by millimeter- the sets). Deformation band shear zones of tectonic origin and to centimeter- to meter-scale bands or zones of cataclasis within slickenlined mesoscale faults commonly occur in conjugate sets porous sandstones (of ~20% porosity). They are shear phenom- as well, with demonstrable expression of sense-of-shear for each ena and fault-like in their structural and kinematic signiﬁ cance. set, permitting the direction of maximum compressive stress to be They “work harden” during their development and, unlike frac- deduced. We assumed that the strike azimuth of Eshelby joints (in tures and faults, do not become reactivated, thus creating a con- the form of penetrative parallel joints in chert nodules within lime- tinuous record of evolving strain, which is a distinct advantage in stone) was the direction of maximum compressive stress. stress-inversion studies. He found that deformation band shear Inferred directions of maximum compressive stress were zones of such origin are fundamentally restricted to the deformed found to be consistently oriented within each uplift, irrespec- zones, and although they contain an important local record of tive of changes in trends of monoclines associated with the progressive structural deformation, the regional tectonic signals uplift. The directions of maximum compressive stress were thus are much less clear. inferred to show the principal shortening directions (Table 1). Varga (1993) emphasized the challenge in using any meso- Viewed as a system, the data reveal that there are two group- scale structural data within the region of Rocky Mountain fore- ings of the Colorado Plateau uplifts, one that reveals a NE-SW land uplifts to interpret regional stresses within deformed zones, principal shortening direction and a second that reveals a NW-SE such as along the monoclines and faulted monoclines. He noted, principal shortening direction. The ﬁ rst group is composed of on the one hand, that the regional stress signature may be masked the Miners Mountain, Circle Cliffs, Kaibab, and Uncompahgre by the local stresses inverted from the local strain conditions. On uplifts. The second group is composed of the San Rafael Swell the other hand, he cautioned that if the deformed zones are weak and the Monument uplift. The conclusions reached regarding the σ zones, the far-ﬁ eld regional stresses will be rotated into princi- N60°W/S60°E direction of maximum compressional stress ( 1) pal planes parallel and perpendicular to the deformed zone(s). In in the San Rafael Swell are in good agreement with the work Structural geologic evolution of the Colorado Plateau 117
a direct manifestation of basement strain. “[T]he direction of greatest shortening in the basement was parallel to the maximum compressive stress direction in the cover” (Bump and Davis, 2002, p. 436). Stated differently, “[T]he interpreted cover stress directions can be viewed as basement strain directions” (Bump and Davis, 2002, p. 436). More broadly, the interpretation of ~65°-directed maximum compressive stress is consistent with the ﬁ ndings of many stud- ies in Wyoming and Colorado (Erslev, 1993; Erslev and Rogers, 1993; Erslev and Wiechelman, 1997). Our interpretation—that some uplifts shortened in an ~110° direction—is not consistent with these studies. Explanations for the origins of two shortening directions for uplifts in the Colorado Plateau, and why only one of these is seen elsewhere, require examination of the timing of the uplifts and consideration of inferred tectonic conditions west of and beneath the Colorado Plateau during the Cretaceous and early Tertiary.
TIMING OF UPLIFTS
Figure 18. Photograph of semiductile conjugate shear Over the years, there have been several attempts to deter- zones composed of nested en echelon crystal-ﬁ ber gash mine the exact timing of the uplifts in the Colorado Plateau. veins (calcite) and en echelon tectonic stylolites, in the These have involved several approaches, from sedimentologic Honaker Trail Formation, Monument uplift. Note lens cap for scale. Photograph by G.H. Davis. analysis to apatite ﬁ ssion-track analysis to (U-Th)/He thermo- chronology. The former have been most successful, but none has demonstrated any clear difference in timing among the different uplifts (Lawton, 1983, 1986; Dumitru et al., 1994; Goldstrand, of Davis (1999) and Christensen and Fischer (2000). The N60°- 1994; Stockli et al., 2002). σ 70°E/S60°-70°W maximum compressive stress ( 1) determined Lawton (1983, 1986) examined ﬂ uvial deposits of the Upper for the Miners Mountain uplift is similar to observations by Cretaceous Mesa Verde Group and Paleocene North Horn Forma- Anderson and Barnhard (1986). The N70°W/S70°E determina- tion in the Uinta Basin on the northern ﬂ ank of the San Rafael σ tion of maximum compressional stress ( 1) for the Monument Swell. Starting in the latest Campanian, he documented local thin- uplift is consistent with conclusions reached by Ziony (1966). ning of stratigraphy over the northern end of the San Rafael Swell The conclusions reached regarding a N50°-55°E/S50°-55°W ori- and the diversion of paleodrainages off its ﬂ ank. By the Maas- σ entation of maximum compressional stress ( 1) for the Uncom- trichtian, sediments were also ponding against the western ﬂ ank pahgre uplift are in general agreement with the work of Jamison of the swell. Based on these observations, Lawton interpreted the and Stearns (1982). A N55°E/S55W interpretation of maximum onset of uplift in the latest Campanian. Uplift was initially rapid, σ compressional stress ( 1) for the Kaibab uplift is consistent with waning later, and it continued until the late Paleocene. the ﬁ ndings of Reches (1978), Davis (1999), and Tindall and Goldstrand (1994) examined deposits of the Upper Creta- Davis (1999), and Tindall (2000a, 2000b). Finally, the interpreta- ceous Canaan Peak Formation in the Kaiparowits adjacent to the σ tion of maximum compressional stress ( 1) of N55°-65°E/S55°- Kaibab and Circle Cliffs uplifts. Like Lawton, he documented 65°W for the Circle Cliffs uplift is consistent with the work of stratal thinning onto the uplifts and diversion of the main axial Anderson and Barnhard (1986) and Swanberg (1999). drainages, starting in the latest Campanian or earliest Maastrich- Bump and Davis (2002) concluded that the compressive tian. The cessation of deformation was recorded by the presence stresses identiﬁ ed in their study, and those inferred to have been of middle Eocene lacustrine deposits of the Claron Formation, at work during the formation of the Laramide-style uplifts, were which onlap and overtop Laramide paleotopographic highs (Goldstrand, 1994). Dumitru et al. (1994) collected apatite-bearing samples TABLE 1. SUMMARY OF STRESS INVERSIONS spanning ~4 km of stratigraphy, starting with the Precambrian in σ Uplift Inferred direction of 1 the bottom of the Grand Canyon, continuing up to the Permian at San Rafael N60°W/S60°E the rim of the canyon, then shifting laterally to the Circle Cliffs Miners Mountain N60°-70°E/S60°-70°W Monument N70°W/S70°E uplift and continuing up through the Cretaceous. Fission-track Uncompahgre N45°-55°E/S45°-55°W analysis showed that Permian samples reached a temperature of Kaibab N60°-70°E/S60°-70°W 90–100 °C and began cooling at 75 Ma. Dumitru et al. (1994) 118 Davis and Bump
50 60 interpreted this as reﬂ ective of erosion triggered by the rise of 40 30 50 the Kaibab and Circle Cliffs uplifts. Stockli et al. (2002) hoped 50 to apply the (then) new technique of (U-Th)/He thermochronol- 50 20
ogy to the problem of dating structural deformation of many of 20 the Colorado Plateau uplifts. To that end, they collected and ana- lyzed samples from stratigraphic proﬁ les on the Circle Cliffs, San 30 4 Rafael, Monument, and Kaibab uplifts. Unfortunately, all of the 5 40 50
subduction samples yielded ages from 33 to 11 Ma, consistent with uplift and 0
denudation of the Colorado Plateau, but too young to be referenc- 40 ing the rise of individual Laramide uplifts (Stockli et al., 2002). 40
Though the constraints on timing are poorly deﬁ ned, ther- 30 mochronologic and sedimentologic observations suggest that the 40 uplifts examined all began to rise concurrently in the latest Cam- panian. Notably, this group includes the uplifts that developed Plate Farallon in response to NE-SW shortening and those that developed in zone Colorado response to NW-SE shortening. More broadly, these results are 20 Plateau 50 consistent with results of similar studies in Wyoming and Colo- 60 rado, namely, a rapid onset of deformation in the Campanian fol- lowed by quick eastward propagation of shortening, with little or 30 no north-south diachroneity (Bird, 1988; Dickinson et al., 1988; N 40 Roberts and Kirschbaum, 1995; Crowley et al., 2002).
5 300 km ORIGIN AND DEVELOPMENT OF THE SYSTEM OF UPLIFTS Basement outcrop Sevier thrust belt Internal zone 40 From the Late Cretaceous to the early Tertiary, coincident Crustal thickness with construction of the uplifts described here, the Colorado Pla- teau was subjected to compressive stress from two distinctly dif- Figure 19. Tectonic map of the western United States in the Late Creta- ceous and early Tertiary. Crustal thickness contours and state boundaries ferent sources. To the west and northwest, the thin-skinned Sevier are after Coney and Harms (1984) based on their palinspastic restora- thrust belt, over the course of the preceding 80 m.y., evolved into tions. The “internal zone” was deﬁ ned by Hodges and Walker (1992) on an enormous topographic ediﬁ ce (Fig. 19) (DeCelles and Coogan, the basis of Cretaceous anatexis and the locations of metamorphic core 2006). Balanced cross sections (Coogan et al., 1995; DeCelles complexes. Basement outcrops are shown only for the region east of the et al., 1995), pressure-time-temperature histories of midcrustal Sevier belt and serve to highlight the locations of major Laramide up- lifts. Location of the subduction zone is approximate. Note the geometry rocks now exposed in the hinterland (Hodges and Walker, 1992; of the Sevier belt and the “internal zone.” Stresses generated by them, Camilleri and Chamberlain, 1997; Lewis et al., 1999), ﬂ exural perpendicular to strike, would be southeast-directed in the Colorado Pla- modeling of the foreland basin (Jordan, 1981; Currie, 1998), teau and east- or northeast-directed elsewhere in the foreland. kinematic restorations of postorogenic extension (Coney and Harms, 1984), and paleoﬂ oral data (Chase et al., 1998) all suggest an average regional elevation of 3–4 km in western Utah. Like all thrust belts, or, for that matter, like all topographic highs, the tism sweeping far into the foreland (Coney and Reynolds, 1977; Sevier thrust belt exerted a compressive stress operating generally Dickinson and Snyder, 1978; Humphreys, 1995; Sterne and perpendicular to the thrust front (Elliott, 1976; Davis et al., 1983; Constenius, 1997). Forward and inverse modeling (Bird, 1984, Molnar and Lyon-Caen, 1988; Bada et al., 2001). The Charleston- 1998), together with investigations of modern analogs (Jordan Nebo salient of the Sevier belt strikes NE-SW (see Fig. 19), and and Allmendinger, 1986; Gutscher et al., 2000; Gutscher, 2002), thus the local topography-derived stress was probably directed indicates that low-angle subduction exerts a viscous shear on the southeastward into the Colorado Plateau (Bump, 2004). base of the overriding plate in a direction parallel to the rela- The second source of compressive stress acting on the Col- tive plate motion, northeast in the case of western North America orado Plateau was related to subduction of the Farallon plate, and the Farallon plate (Coney, 1978; Engebretson et al., 1985; enhanced by slab ﬂ attening and attendant generation of higher Cole, 1990; Severinghaus and Atwater, 1990). Additionally, the traction along the base of the lithosphere. A subduction zone had subducting slab weakened the North American crust and any sat off of the western coast of North America for much of the lithospheric mantle lid, both by hydration and magmatic heating Phanerozoic, with the slab plunging steeply into the mantle for (Humphreys et al., 2003). much or all of that time. Starting in the Late Cretaceous, how- In the Late Cretaceous and early Tertiary, the Colorado Pla- ever, the subducting slab shallowed, sending a pulse of magma- teau was thus caught in a tectonic squeeze, compressed NW-SE Structural geologic evolution of the Colorado Plateau 119 by the encroaching thrust belt, and compressed NE-SW by the which undoubtedly will be debated and lead to other discussions shallowly subducting Farallon plate and cratonic North America. and understandings. We infer that monoclines bounding the ten Paleostress magnitudes are very difﬁ cult to assess, but they were major uplifts of the Colorado Plateau are rooted downward into probably similar in magnitude. As Molnar and Lyon-Caen (1988) ancient basement shear zones with a long history of reactivation. pointed out, topographic highs are crude pressure gauges, in that These shear zones have their origins in different tectonic events the weight of the topography must be supported by an equal lateral and consequently span a broad range of orientations. Like most force at its margins. Assuming a bulk rock density of 2550 kg/m3, other workers, we have focused on reactivation of basement shear a 3–4-km-high thrust belt would exert a lateral stress of 75–100 zones that were originally created in the Proterozoic, but Marshak MPa on the surrounding region (Bump, 2004). Plate-interaction et al. (2003) rightly pointed out that faults created during Ances- stress magnitudes are less well known, but estimates range from tral Rocky Mountain deformation may have been reactivated as tens to hundreds of MPa (Govers et al., 1992; Richardson, 1992; well. The most likely areas for this are in the eastern part of the Zoback and Healy, 1992; Zoback et al., 1993; Richardson and Colorado Plateau (e.g., Paradox basin) and perhaps in the Four Coblentz, 1994; Coblentz and Richardson, 1996), about the same Corners region (e.g., Deﬁ ance uplift). During the formation of the order of magnitude as those results for a thrust belt. Farther to Laramide-style uplifts, some shear zones were reactivated in pure the east, the thrust-belt derived stress would have waned due to reverse slip; others were reactivated obliquely. The San Rafael friction on the basal detachment (be that within the crust or at the and Monument uplifts shortened toward the southeast. The Circle base of the lithosphere). Stress from the ﬂ at slab, on the other Cliffs, Kaibab, and Uncompahgre uplifts shortened toward the hand, would have increased to the east as the viscous coupling northeast. These two simultaneous, nearly orthogonal shortening acted over greater distances (Bird, 1988). directions were set up by a constrictional strain ﬁ eld that was the In the context of the Cordillera, the Colorado Plateau thus product of the northeastward-subducting ﬂ at slab and the south- sat in a unique location, subjected to unique stresses. By virtue eastward-advancing Sevier thrust belt. This constrictional stress of the northeastward-subducting Farallon slab below and the ﬁ eld was unique to the Colorado Plateau. Elsewhere in the Rocky southeastward-propagating thrust belt to the west, the plateau Mountains (e.g., in Wyoming), the directions of thrust-belt propa- was squeezed in two directions simultaneously. Furthermore, by gation and plate convergence were more nearly parallel. virtue of its position adjacent to the thrust belt, the Colorado Pla- An improved understanding of the origin of Laramide-style teau was subjected to the greatest possible stress from the thrust basement-cored uplifts in the Colorado Plateau will simultane- belt and a relatively low stress from the ﬂ at slab, such that the ously improve understanding of the plate-tectonic evolution of two stresses were uniquely similar in magnitude (Bump, 2004). continental interiors, including active tectonic phenomena far Caught in this constrictional stress ﬁ eld, existing weaknesses in removed from plate margins. In terms of testing the synthesis pre- the plateau deformed in a dominantly reverse-slip sense (Bump, sented here, there is an urgent need for a nuanced, sophisticated, 2004). NE-striking faults, such as those bounding the San Rafael very precise understanding of the timing and movement histories and Monument uplifts, slipped to the southeast, while NW- of individual uplifts, and for ﬁ nite-element modeling that would striking faults, such as those bounding the Waterpocket, Miners elucidate likely gradients of deformation across the plateau. Mountain, and Uncompahgre uplifts, slipped to the northeast. It is natural to think of the Colorado Plateau region as strong, The Kaibab uplift, which is bounded by a north-northeast–strik- but we believe it is more illuminating to think of its basement as ing fault that slipped obliquely to the northeast (Tindall and riddled with weak shear zones and tectonic displacements that Davis, 1999), did not slip to the southeast like those bounding made it quite unnecessary for the shear-zone–bounded blocks the San Rafael and Monument uplifts because it lies well to the themselves, each of which occupies thousands of square kilo- south of the Charleston-Nebo salient, out of the line of southeast- meters, to exercise layer-parallel strain. It was just a matter of directed thrust-belt stress (Bump, 2004). time before the Colorado Plateau thus deformed because the “tip zone” of the Sevier fold-and-thrust belt had been marching CONCLUSIONS steadily eastward toward the Cordilleran hinge line, ultimately advancing eastward beyond the last remaining passive-margin Our present understanding of the Colorado Plateau, its basin sediments. The encounter with “new” crustal structure structure, and its tectonic evolution is built on the shoulders of capped by basement-supported craton sediments triggered a giants and more than 100 yr of geologic ﬁ eld work, analysis, and “new” deformation mechanism, one that exploited crustal hetero- reanalysis. Beginning with the very ﬁ rst explorations by Pow- geneities, including preexisting shear-zone weaknesses, in order ell (Powell, 1873), Gilbert (Gilbert, 1876), and Dutton (Dutton, to achieve requisite shortening. 1882), structural knowledge of the Colorado Plateau has pro- gressed from basic (and elegant) description of the monoclines ACKNOWLEDGMENTS and their underlying shear zones, to confusion and debate over the tectonic signiﬁ cance of the range in orientations (Baker, We thank the conveners of the “Backbone of the Americas” 1935; Kelley, 1955b; Wise, 1963; Sales, 1968; Stearns, 1978; symposium, Suzanne Mahlburg Kay (Cornell University) and Gries, 1990; Yin, 1994), and to yet another attempt at synthesis, Víctor A. Ramos (Universidad de Buenos Aires), for inviting us 120 Davis and Bump to submit this paper to the symposium volume. The observations Inc.), headed by Jim Holmlund. Collaborations with industry and ideas presented are an outgrowth of multiple research proj- were invaluable, and I particularly acknowledge Chuck Kluth in ects over time. Research support came from The University of this regard. Susie Gillatt prepared ﬁ nal ﬁ gures and illustrations Arizona through a number of sources, notably the Department excellently. of Geosciences, the Laboratory of Geotectonics (with a special Finally, we thank John Geissman and Stephen Marshak for grant from the SOHIO Petroleum Company); the GeoStructure reviewing this manuscript, for providing helpful editorial sug- Partnership (sponsors of which include Mobil, Conoco, British gestions, and for providing valuable appraisals of strengths and Petroleum, British Petroleum/Amoco Exploration, ExxonMobil limitations of our conclusions and our arguments. Our paper is Upstream Research Company, GeoMap, Inc., and Midland Val- better as a result, but we take full responsibility for the short- ley Exploration); and the Ofﬁ ce of Arid Lands Studies (through comings in this ﬁ nal product, and we frankly remain humbled which George Davis received a National Aeronautics and Space by how such “simple” geology can be so difﬁ cult to “wire.” Administration grant directed toward analysis of monoclines in the Arizona sector of the Colorado Plateau). REFERENCES CITED Multiple grants were awarded to George Davis by the Tec- tonics Program of the Earth Sciences Division of the National Allmendinger, R.W., 1998, Inverse and forward numerical modeling of tri- Science Foundation (NSF). These include NSF grant EAR- shear fault-propagation folds: Tectonics, v. 17, p. 640–656, doi: 10.1029/ 98TC01907. 9406208 (use of deformation bands as a guide to regional tec- Allmendinger, R., Hauge, T., Hauser, E., Potter, C., Klemperer, S., Nel- tonic stress patterns within the southwestern Colorado Plateau); son, K., Kneupfer, P., and Oliver, J., 1987, Overview of the COCORP grant EAR-9706028 (structural analysis of Laramide strike-slip 40°N transect, western United States: The fabric of an orogenic belt: Geological Society of America Bulletin, v. 98, p. 308–319, doi: faulting along the eastern margin of the Kaibab uplift, Colo- 10.1130/0016-7606(1987)98<308:OOTCNT>2.0.CO;2. rado Plateau); grant INT-9908622 (basement-cored systemat- Anderson, C.A., and Silver, L.T., 1976, Yavapai Series—A greenstone belt: Ari- ics, a structural study of Sierra de Haulﬁ n and adjacent uplifts, zona Geological Society Digest, v. 10, p. 13–26. Anderson, C.A., Blacet, P.M., Silver, L.T., and Stern, T.W., 1971, Revision of Sierra Pampeanas, NW Argentina, with Pilar García as co–prin- Precambrian stratigraphy in the Prescott-Jerome area, Yavapai County, cipal investigator [PI]); grant INT-9907204 (the making of a Arizona: U.S. Geological Survey Bulletin 1323-C, p. C1–C16. high plateau, a ﬁ eld-based determination of tectonic strain and Anderson, R.E., and Barnhard, T.P., 1986, Genetic relationship between faults and folds and determination of Laramide and neotectonic paleostress, displacement across the Altiplano of Bolivia and implications western Colorado Plateau transition zone: Tectonics, v. 5, no. 2, p. 335– for modeling Andean plateau uplift, with Nadine McQuarrie as 357, doi: 10.1029/TC005i002p00335. co-PI); and grant EAR-0001222 (inverse and forward modeling Anderson, T.H., and Silver, L.T., 2005, The Mojave-Sonora megashear—Field and analytical studies leading to the conception and evolution of the of Utah system of Colorado Plateau basement–cored uplifts, hypothesis, in Anderson, T.H., Nourse, J.A., McKee, J.W., and Steiner, with Alex Bump as co-PI). M.B., eds., The Mojave-Sonora Megashear Hypothesis: Geological Soci- Furthermore, grants were also awarded to George Davis ety of America Special Paper 393, p. 1–50. Angelier, J., 1979, Determination of the mean principal directions of stresses by the American Chemical Society’s Petroleum Research Fund for a given fault population: Tectonophysics, v. 56, p. T17–T26, doi: (ACS-PRF). These include ACS-PRF support for “Thin-skinned 10.1016/0040-1951(79)90081-7. deformation in the Bryce-Zion region of southern Utah,” and Angelier, J., 1990, Inversion of ﬁ eld data in fault tectonics to obtain the regional stress. III. A new rapid direct inversion method by analytical means: Inter- “Characterization of deformation-band controlled connectivity national Journal of Geophysics, v. 103, p. 363–376, doi: 10.1111/j.1365 and compartmentalization in sandstone.” -246X.1990.tb01777.x. George Davis thanks former University of Arizona gradu- Angelier, J., 1994, Fault slip analysis and palaeostress reconstruction, in Hancock, P.L., ed., Continental Deformation: Oxford, Pergamon Press, ate students who participated with him in structural geological p. 53–100. research on the Colorado Plateau, and these include Steve Ahl- Babenroth, D.L., and Strahler, A.N., 1945, Geomorphology of the East Kaibab gren, Greg Benson, Pilar García, Lucy Harding, Bob Krantz, monocline, Arizona and Utah: Geological Society of America Bulletin, v. 56, p. 107–150, doi: 10.1130/0016-7606(1945)56[107:GASOTE]2.0.CO;2. Steve Lingrey, Chuck Kiven, Eric Lundin, Lisa McCallmot, Steve Bada, G., Horvath, F., Cloetingh, S., Coblentz, D., and Toth, T., 2001, Role Naruk, Scott Showalter, Karen Swanberg, Sarah Tindall, and of topography-induced gravitational stresses in basin inversion: The case Alex Bump. Undergraduate students who contributed to advance- study of the Pannonian Basin: Tectonics, v. 20, no. 3, p. 343–363, doi: 10.1029/2001TC900001. ment of structural geologic research in the program include Greg Baker, A.A., 1935, Geologic structure of southeastern Utah: American Associa- Belinski, Shari Christofferson, Erin Collin, David Boardman, tion of Petroleum Geologists Bulletin, v. 19, p. 1472–1507. Scott Grasse, Jessica Graybeal, Angela Smith, and Danielle Van- Bender, E.E., Morrison, J., Anderson, J.L., and Wooden, J.L., 1993, Early Pro- terozoic ties between two suspect terranes and the Mojave crustal block derhorst. I also call attention to colleagues Nick Nicelsen, Olivier of the southwestern U.S.: The Journal of Geology, v. 101, p. 715–728. Merle, Fred Cropp, and Gayle Pollock, with whom I beneﬁ ted in Bergerat, F., Bouroz-Weil, C., and Angelier, J., 1992, Paleostresses inferred participating together in Colorado Plateau research. from macrofractures, Colorado Plateau, western USA: Tectonophysics, v. 206, p. 219–243, doi: 10.1016/0040-1951(92)90378-J. We have beneﬁ ted over the years from close interaction with Billingsley, G.H., Huntoon, P.W., and Breed, W.J., 1987, Geologic Map of Cap- special colleagues in the Department of Geosciences, in particu- itol Reef National Park and Vicinity, Emery, Garﬁ eld, Millard, and Wayne lar, the late Peter Coney, Bill Dickinson, and Pete DeCelles. Counties, Utah: Utah Geological and Mineral Survey, colored geologic map, scale 1:62,500, 5 sheets. 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