Monocline development by oblique-slip fault-propagation folding ...

1304S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320extreme thinning of weak sedimentary rocks immediatelyoverlying the basement (Stearns, 1971). Rechesand Johnson (1978) determined that the PalisadesCreek branch of the East Kaibab monocline in theGrand Canyon region resulted from a combination ofbuckling and drape folding above a near-vertical fault.According to Reches (1978), the mechanism of deformationof the drape-folded cover is virtually independentof the type of basement deformation (e.g.faulting, igneous intrusion, or local steepening oflayers).1.2. Monoclines as fault-propagation foldsAccording to Suppe (1985), a fault-propagation foldrepresents deformation immediately in front of a propagatingfault tip. By this broad de®nition, drape foldsmight be considered as a subset of fault-propagationfolds. However, implicit in fault-propagation foldmodels is the idea that fault-accommodated o€set progressivelygives way to fold-accommodated o€set withhigher structural and stratigraphic levels, and that withcontinued deformation the fault will propagatethrough the fold (Suppe and Medwede€, 1984;Jamison, 1987). In drape folding, fault o€set simplydies out just above basement in the sedimentary cover.Although fault-propagation fold models were originallydeveloped to analyze `thin-skinned' fold±thrustbelt geometry (Suppe and Medwede€, 1984), the termfault-propagation fold has been extended to includefolding associated with basement-cored uplifts likethose of the Rocky Mountain foreland of the westernUnites States (e.g. Erslev, 1991; Erslev and Rogers,1993; Stone, 1993; Mitra and Mount, 1998). Stone(1984, 1993) proposed that use of the term fault-propagationfold be reserved for application to areas ofthin-skinned fold±thrust belt structures, and that theterm thrust-fold be adopted for basement-involved deformationlike that seen in the Rocky Mountain foreland.Admittedly the two structural styles di€er, butthe term `thrust-fold' implies dip-slip kinematics, aconnotation that might result in confusion if appliedto areas of wrench faulting or oblique deformation.The broad de®nition of fault-propagation fold capturesthe simultaneous development of fault and fold forboth thin-skinned and basement-cored structures withoutspecifying slip direction. Following Erslev (1991),Erslev and Rogers (1993) and Mitra and Mount(1998), the term fault-propagation fold is used here todescribe folding associated with basement-involved deformationin the study area.Colorado Plateau monoclines have not beenexcluded from basement-cored fault-propagation foldmodels, but they have not been cited as primeexamples of fault-propagation folding. One reasonmay be that o€set across Colorado Plateau structuresis small compared to structural relief in the RockyMountain foreland, so that fault-propagation foldcharacteristics (if present) are less developed. In addition,in Grand Canyon exposures of the monoclines,basement-rooted faulting gives way to unfaulted foldingvery low in the Paleozoic (above-basement) section(Huntoon, 1971, 1993; Reches, 1978; Reches andJohnson, 1978; Huntoon et al., 1996), a characteristicthat seems to support the drape fold model.It is important to note, however, that GrandCanyon exposures do not necessarily coincide with locationsof greatest structural relief, or greatest o€set,on the monoclines and their associated faults. Forexample, the East Kaibab monocline exhibits 1600 mof structural relief in southern Utah, but only 800 m ofvertical relief in the Grand Canyon (Babenroth andStrahler, 1945). In a fault-propagation fold, basementrootedfaulting should extend higher into the sedimentarycover in areas of greater structural relief than inareas of lesser o€set. Map relationships in southernUtah, where the East Kaibab monocline has its greateststructural relief, demonstrate that the structuredeveloped through fault-propagation folding, notdrape folding.1.3. Importance of oblique deformationDrape-fold and fault-propagation fold models areusually presented in vertical cross-section. This viewindirectly encourages the assumption that principalstress and strain directions are exactly parallel and perpendicularto the plane of the cross-section. For thesake of simplicity, oblique movement of material relativeto the cross-section plane is seldom considered.Such simpli®ed constructions may produce reasonableinterpretations when applied to individual structures,but can lead to confusion in interpretation of regionalkinematics.For example, basement-cored uplifts tend to occupya wide range of orientations, with no clear regionalsense of vergence (e.g. Colorado Plateau monoclines,Rocky Mountain foreland uplifts and AncestralRockies). No single compression direction seemscapable of producing reverse reactivation of structureswith such variable trends. Stearns (1978) promoted theidea that vertical uplift, perhaps caused by a verticallyoriented greatest principal stress (s 1 ), accounted forthe variable orientations and steeply dipping basementfaults associated with Colorado Plateau and RockyMountain uplifts. Since then, several authors haveshown that the Laramide stress which drove basementreactivation and monoclinal folding on the ColoradoPlateau was horizontal and compressive, not vertical(Reches, 1978; Huntoon, 1981; Anderson andBarnhard, 1986). Given a horizontal compressivestress, Chapin and Cather (1983) hypothesized two

S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320 1305Fig. 1. Location and geologic setting of the East Kaibab monocline.The southern Utah study area is outlined, and the location of Fig. 5(in the Grand Canyon) is shown.stages of Laramide deformation, marked by a changein the compression direction, to explain the disparatetrends of the uplifts.Despite the apparent diculty in reactivating a steeplydipping fault with a horizontal compressive stress,most studies of Colorado Plateau uplifts imply thatLaramide compression produced reverse, dip-slipmotion on the basement faults (e.g. Huntoon, 1971,1981, 1993; Davis, 1978; Reches, 1978; Stearns, 1978).Among the exceptions are studies by Stone (1969) andFig. 2. Structure contour map of the northern East Kaibab monocline.Structure contours, in feet, drawn on the base of theCretaceous Dakota Sandstone. Modi®ed from Gregory and Moore(1931).

1306S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320Fig. 4. Generalized stratigraphic column for the East Kaibab monoclinein southern Utah. Shaded units are used as markers to highlightstructural relationships on maps of Domains 1 through 4.Stratigraphy compiled from Hintze (1988).Fig. 3. Simpli®ed geologic map of the study area. Permian, Triassic,Jurassic, and Cretaceous rocks are shaded di€erently to emphasizethe slight northward plunge of the monocline. Note that faulting andfolding in the steep limb move from older stratigraphic units in thesouth into higher stratigraphic units northward. The relatively thinCretaceous Dakota Sandstone is shaded black to highlight left-lateralseparations on faults at the north end of the monocline.Geographical features and structural domains discussed in the textare labeled.Davis (1978) which pointed out that pre-existing basementfractures of many orientations could be reactivatedby a horizontal compressive stress. This wouldaccount for the wide range in structural trends, andwould also result in oblique deformation on somestructures. The possibility of basement-rooted obliquemotion across Colorado Plateau monoclines has beensuggested in studies by Barnes (1974, 1987), Ohlman(1982) and Karlstrom and Daniel (1993), but detailed®eld documentation is lacking. Fault relationships discussedhere not only suggest basement-rooted faultpropagationfolding, but also indicate that a signi®cantcomponent of right-lateral slip took place duringLaramide formation of the East Kaibab monocline insouthern Utah. This in turn opens up new possibilitiesfor interpreting the Colorado Plateau monoclines as asystem.

S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320 1307Fig. 5. The East Kaibab monocline and underlying Butte fault in the Grand Canyon. Lower Proterozoic and Cambrian rocks are shaded toemphasize the apparent normal o€set at the level of Precambrian sedimentary rocks, and reverse separation at the Proterozoic±Phanerozoicunconformity. Location of the cross-section is shown in Fig. 1. After Huntoon et al. (1996).2. Geological settingThe Kaibab Uplift of northern Arizona andsouthern Utah is a north±south trending, asymmetricalanticline near the western margin of the ColoradoPlateau. The moderately to steeply dipping east limbof the uplift, the East Kaibab monocline, meanders forapproximately 180 km from near Bryce, Utah to justnorth of Flagsta€, Arizona (Fig. 1). The 50-km longUtah segment of the monocline, which is the subject ofthis paper, shows structural relief of 1600 m betweenthe anticlinal crest of the uplift and the synclinaltrough of the monocline, based on structural contouringof the base of the Dakota Sandstone (Gregory andMoore, 1931; Fig. 2). The East Kaibab monoclinetrends N208E from the Arizona±Utah border to Bryce,where the monocline and the Kaibab Uplift die out;both structures plunge approximately 58 northward.The slight northward plunge of the Utah segment ofthe East Kaibab monocline creates an insightful perspectiveof the structure in map view (Fig. 3). Thesteep limb of the fold occupies progressively olderstrata when followed from north to south. Cretaceousunits form the steep limb in the north nearGrosvenor's Arch, Jurassic rocks are intenselydeformed at Paria Canyon, and Triassic and Permianrocks de®ne the steep limb where Highway 89 crossesthe fold. Although up to 2000 m of folded and faultedProterozoic and Paleozoic sedimentary rocks liebetween crystalline basement and the KaibabLimestone, these are not exposed in the study area(Fig. 4).The timing of monocline formation is poorly constrained.At the north end of the structure, CretaceousWahweap and Kaiparowits Formations have beeneroded from the crest of the uplift but are exposed onits ¯anks, where dips range from 408 near Grosvenor'sArch to 08 in the vicinity of Table Cli€ Plateau. TheseLate Cretaceous rocks were clearly deposited beforefolding. Paleocene rocks between Grosvenor's Archand Table Cli€ are synclinally folded, probably as aresult of Laramide deformation as well (Sargent andHansen, 1982). Eocene strata lie unconformably on theLate Cretaceous units at Table Cli€ (Gregory andMoore, 1931; Bowers, 1972) but have been strippedfrom the folded edges of the Kaibab Uplift (Sargentand Hansen, 1982). The Eocene rocks may or may nothave been a€ected by folding; their presence does notprovide an upper time limit for monocline formation.Deep exposures in the Grand Canyon reveal that asteeply west-dipping (708) basement fault zoneunderlies the East Kaibab monocline. Grand Canyonoutcrops provide clear evidence that the basementstructure originally formed as a normal fault inPrecambrian times (Walcott, 1890; Maxson, 1961;Huntoon, 1969, 1993; Huntoon and Sears, 1975) butthat the only Phanerozoic deformation on the faultresulted from Laramide compression (Fig. 5). This episodeproduced reverse separation across the fault atthe level of the Proterozoic/Phanerozoic unconformityin the Grand Canyon and formed the broad, asymmetricalKaibab Uplift in the Paleozoic and Mesozoiccover (Huntoon and Sears, 1975; Huntoon, 1993).Although the Grand Canyon provides the only exposureof the basement fault underlying the EastKaibab monocline, the fault (or a network of similarfaults) is assumed to underlie the fold for its entirelength (Davis, 1978; Stern, 1992).3. Structural data and observationsExamination of the northern 50 km of the EastKaibab monocline has revealed a continuous, N208Etrending,monocline-parallel zone of intense deformationexpressed at map scale by abundant, systematicfaulting within the steep limb. Map-scale and outcropscalestructures in the deformed zone indicate a signi®cantcomponent of reverse-right-lateral o€set. Whenfollowed south from Grosvenor's Arch to theArizona±Utah border, this narrow zone of faulting`steps' progressively southwestward and stratigraphicallydownward through Cretaceous, Jurassic, andTriassic strata. Structural style within the zone changes

1308S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320Fig. 6. Geology of Structural Domain 1. Short, northwest-striking, northeast-dipping faults o€set Cretaceous Dakota Sandstone (dark shading)in an apparent left-lateral fashion. At the northern and southern boundaries, northeast-striking, northwest-dipping faults accommodate reverse,right-lateral o€set. East of (and stratigraphically above) the northeast-striking faults at north and south ends, the right-lateral o€set results inbroad, z-shaped bends in the contact between Cretaceous Wahweap and Kaiparowits Formations. Equal-area plots summarize structural data:(a) Plot of poles to planes. Poles to faults are shown in black; poles to outcrop-scale slip surfaces are shown in grey. (b) Kamb contour plot ofpoles to faults and slip surfaces. Shades represent 2s contour intervals. White areas indicate fewer poles at contouring grid points than would befound in a uniform distribution minus 1s; light grey shading indicates grid points with number of poles within 21s of that found in a uniformdistribution; slightly darker grey shading indicates grid points with numbers of poles 1±3s more than in a uniform distribution, etc. (c)Slickenline orientations. (d) Kamb contour plot of slickenlines, emphasizing their low plunge and southeast trend.

S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320 1309Fig. 7. Southwest-directed photograph of the graded road surface inDomain 1 (outcrop location is circled on Fig. 6). Northwest-strikingfaults o€set northeast-striking, east-dipping shales and evaporites ofthe Carmel Formation. Geologist is Pilar Garcia.from north to south as well, allowing subdivision ofthe Utah portion of the East Kaibab monocline intofour domains based on style of deformation and stratigraphicinterval (Fig. 3). The fault pattern in eachdomain and in the transitions between domains providesevidence for oblique slip fault-propagation folding,as discussed in the following sections.3.1. Domain 1Structural Domain 1 begins near Grosvenor's Archin Grand Staircase±Escalante National Monument,and extends about 15 km toward S208W to PumpCanyon Spring (Fig. 6). A 10 km-long, monocline-parallelzone of short, closely spaced, northwest-strikingfaults occupies the stratigraphic interval of JurassicPage Sandstone through Cretaceous Tropic Shale.Strata within the zone are o€set by meters to tens ofmeters in apparent left-lateral fashion and are rotatedclockwise by the northwest-striking faults.More than 75 of these northwest-striking, northeastdippingfaults are visible in Domain 1 at 1:12 000scale. Trace lengths of the largest faults are on theorder of 0.5±1 km. The faulting is pervasive in outcropas well, with sub-map-scale faults evident on thegraded surface of the dirt road, where they o€setsteeply east-dipping, thin-bedded shales and evaporitesin the Carmel Formation (Fig. 7). Average strike anddip of map-scale and outcrop-scale faults are N508W,588NE with slickenlines (found on fault surfaces preservedin the Dakota Formation and the PageSandstone member of the Carmel Formation) thatrake 208SE. Fault and slickenline orientations suggestthat at least the latest slip along these short faults wasleft-lateral with a small reverse component.To the north, near Grosvenor's Arch, the monocline-parallelzone of northwest-striking faults endsabruptly at two northeast-striking faults, each with atrace length of about 3 km. These faults accommodateapparent right-lateral separation of the JurassicCarmel through Cretaceous Wahweap Formations.Strike-parallel o€set of the Cretaceous DakotaSandstone is on the order of 1 km across each fault,but appears to decrease to the northeast (stratigraphicallyupward) into the Straight Cli€s and WahweapFormations. Where preserved, the fault surfaces strikeN658E and dip 658NW. Slickenlines rake 15±208SW,disclosing at least a late-stage episode of reverse-rightlateraldisplacement. These northeast-striking faults occupya higher stratigraphic interval than do the northwest-strikingfaults between Grosvenor's Arch andPump Canyon Spring. Northeast of the faults themselves,in Cretaceous Wahweap and KaiparowitsFormations, lateral displacement is accommodated bya broad, z-shaped folding of the trend of the monocline,suggestive of right-handed shear (see contactbetween Kw and Kk, Fig. 6).Faults at the southern termination of Domain 1 aresimilar to the northeast-striking faults at the northernend, but occupy a lower stratigraphic interval. NearPump Canyon Spring, the zone of northwest-strikingfaults ends abruptly near a northeast-striking fault inPage Sandstone. Its polished surface strikes N558E,dips 608W, and displays grooves raking 20±308SW.The geometry again indicates reverse-right-lateral slip.This outcrop marks the north end of a lineation traceableon topographic maps and air photos for at least4 km toward S408W into gently dipping NavajoSandstone. The fault-controlled lineation and preservedfault surface occupy the upper Navajo andPage Sandstones, and the Jurassic Carmel throughCretaceous Wahweap Formations immediately to theeast form another broad, z-shaped bend in the trace ofthe monocline.As a whole, the map- and outcrop-scale faulting inDomain 1 de®nes a narrow, monocline-parallel zone ofintense deformation which constitutes a shear zone.From the north to the south end of Domain 1 theshear zone occupies progressively lower stratigraphicintervals within steeply east-dipping beds. Fault and

1310S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320Fig. 8. Geology of Domain 2. northeast-striking, northwest-dipping faults o€set the Page Sandstone (light shading) in reverse-right-lateralfashion. Dakota Sandstone, intensely faulted in Domain 1, is una€ected by faulting in Domain 2. Representative orientations of slip surfacesand deformation bands depict structural features too small to show at map-scale. (a) Equal-area plot of poles to faults (black) and slip surfaces(grey). (b) Kamb contour plot of poles illustrates tight clustering of northeast-striking, northwest-dipping fault and slip surface orientations. (c)Equal area plot and (d) Kamb contour plot of slickenline orientations. Slickenlines plunge gently toward the southwest, disclosing reverse-rightlateralslip on northeast-striking, west-dipping faults.

S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320 1311Fig. 9. Geology of Domain 3. Linear valleys, gouge, breccia, and exposures of polished fault surfaces reveal long, continuous faulting in Navajoand Kayenta Formations. Slip surfaces and deformation bands occupy both northeast-striking and northwest-striking orientations. (a) Equalareaplot of poles to faults (black) and slip surfaces (grey). (b) Kamb contour plot of poles to faults and slip surfaces, showing a primary set ofnortheast-striking, northwest-dipping surfaces and a secondary set of northwest-striking, northeast-dipping surfaces. (c) Equal-area plot of slickenlineorientations. (d) Kamb contour plot of slickenline orientations. Southwest-plunging slickenlines lie on northeast-striking faults, and southeast-plungingslickenlines lie on northwest-striking faults.

1312S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320slickenline orientations at the north and south ends ofDomain 1 indicate a large ratio (up to 5:1) of right-lateralstrike-slip to dip-slip o€set across the long, northeast-striking,west-dipping faults; the northweststrikingfaults between Grosvenor's Arch and PumpCanyon Spring also record a 5:1 ratio of left-slip toreverse-slip. Although slickenlines typically preserveonly the latest slip vector on a fault surface, theobserved orientations along this 15 km stretch of themonocline are consistent with interpretation as synthetic(northeast-striking) and antithetic (northweststriking)conjugates in a zone of oblique (i.e. reverseright-lateral) displacement.3.2. Domain 2Structural Domain 2 begins immediately south ofthe northeast-striking fault surface at the southern endof Domain 1 (Fig. 8). This 12 km-long interval ismarked by several map-scale northeast-striking faultsin the lower Carmel Formation, Page Sandstone, andupper Navajo Sandstone; the deformation is in aslightly lower stratigraphic interval. Map-scale faultsin Domain 2 have trace lengths on the order of 1 km.Map view reveals apparent right-lateral o€set on theorder of tens to a few hundred meters, and where canyonsincise the faults their reverse separation is evident.Average orientation of the northeast-strikingfaults in Domain 2 is N418E, 468NW, again with slickenlinesraking about 308SW. These faults are similar inorientation to those found at the north end of Domain1 and between Domains 1 and 2, but with shortertrace lengths and o€set on the order of only a fewmeters to tens of meters.At outcrop-scale, the Page and Navajo Sandstonesin Domain 2 are intensely fractured. Minor fault surfaces(slip surfaces) and deformation bands show twoprimary orientations: a prominent northeast-striking,northwest-dipping set and a secondary northwest-striking,northeast-dipping set (Fig. 8). Deformation inDomain 2 is still consistent with the interpretation as areverse-right-lateral shear zone, but long, en e chelonsynthetic faults rather than short, closely spaced antitheticfaults dominate Domain 2.3.3. Domain 3Domain 3 begins at Paria Canyon, and is distinguishedby evidence for continuous, through-goingfaulting in the Navajo Sandstone and KayentaFormation (Fig. 9). The mouth of Paria Canyonexposes a northeast-striking, northwest-dipping faultsurface similar to the one at the boundary betweenDomains 1 and 2, again with slickenlines and groovesthat rake 308SW. The cross-sectional view at themouth of the canyon reveals Navajo and PageFig. 10. North-directed photograph of the northeast-striking, northwest-dippingfault surface at the mouth of Paria Canyon. PageSandstone member of the Carmel Formation on the hanging walllies in fault contact above stratigraphically higher Carmel Formationredbeds. Stratigraphic relationship and drag folding in Carmel redbedsindicate a reverse component of faulting. Slickenlines on thefault surface (not visible) rake 20±308SW, disclosing a signi®cantright-lateral component of slip. Geologist is Bill Abbey.Sandstones in the hanging wall, above reverse dragfoldedCarmel Formation redbeds in the footwall (Fig.10). Jurassic Entrada Sandstone through CretaceousWahweap Formation east of the fault surface (up-section)again form a broad, z-shaped fold in map view.The fault surface exposed at Paria Canyon marks thenorth end of a series of linear valleys which trendS208W across N108E-striking, steeply east-dippingNavajo Sandstone. Evidence for through-going faultingis found in the valleys as fault gouge and breccia,intensely fractured Navajo Sandstone, and several exposuresof northeast-striking, steeply west-dippingpolished fault surfaces. Because the strike of the faultzone nearly parallels the strike of bedding in theNavajo, the zone of deformation crosses the NavajoSandstone at a very low angle; the steeply west-dippingfault requires 8 km of strike length to cross the (approximately)400 m thick, east-dipping sandstone. As aresult of this geometry, a large amount of right lateraldisplacement across the fault zone is theoretically possiblewithout causing a noticeable disruption of the surfacetrace of the Navajo Sandstone. Map-scale faultsurfaces measured in the Navajo in Domain 3 yield anaverage orientation of N368E, 598NW, with slickenlinesraking 308SW. At the south end of Domain 3 thefault zone o€sets Triassic/Jurassic Kayenta andTriassic Moenave Formations, in an apparent rightlateralfashion, on the east side of Fivemile Valleybefore it disappears beneath alluvium and colluviumon the valley ¯oor.Although at map scale northeast-striking (synthetic)

S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320 1313Fig. 11. Geology of Domain 4. Through-going faulting is inferred based on four key outcrops described in the text (locations circled). Intense deformationis obscured by alluvium in the valley, formed by Moenkopi and Chinle Formation shales. (a) Equal-area plot and (b) Kamb contourplot of poles to slip surfaces in Navajo, Kayenta, and Moenave Formations. (c) Equal-area and (d) Kamb contour plot of slickenline orientations.

1314S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320Fig. 12. Cross-sectional sketch based on outcrops visible in the canyonat Location 2 (above dashed line) and inferred subsurface structure(below dashed line). The inferred west-dipping fault with reverseseparation accounts for overturned bedding at Location 2 and theapparent absence of Chinle Formation in this part of Domain 4. Thesliver of Chinle Formation shown in the sketch just below the surfaceis a reverse- (and right-lateral?) fault-bounded block caught inthe shear zone, representing the relationship exposed at Location 3.faults are prevalent in Domain 3, a few short northwest-striking,northeast-dipping faults similar to thosefound in Domain 1 o€set the Navajo Sandstone eastof the through-going synthetic faults. These antitheticfaults accommodate meters to tens of meters ofreverse-left-lateral o€set within the Navajo Sandstone.Oblique slip is expressed by slickenlines that rake188E. Outcrop-scale deformation bands and slip surfacesalso show a bimodal distribution of syntheticand antithetic orientations (Fig. 9). Deformation inDomain 3 is concentrated in the Navajo Sandstone,with some fracturing and deformation bands a€ectingthe Page Sandstone; however, no map-scale faults o€setthe Page Sandstone south of Paria Canyon. Themost intense deformation has again moved stratigraphicallydown-section, from the interval of upperNavajo/Page/Carmel Formations in Domain 2 into theKayenta/Navajo Formations within Domain 3.3.4. Domain 4In Domain 4 the shear zone lies southward anddown-section in the Triassic Chinle and MoenkopiFormations in Fivemile Valley (Fig. 11). These shaleyTriassic units are sandwiched between resistantPermian Kaibab Limestone on the west side of the valley,dipping 25±358 east, and a ridge of 65±858 eastdippingMoenave, Kayenta, and Navajo Sandstoneson the east side. Most evidence for the continuation ofthe shear zone is hidden beneath alluvium and colluviumon the valley ¯oor, but a few key outcrops allowit to be traced southward almost to the Utah±Arizonaborder.Location 1 is a northeast-striking, steeply west-dipping,remarkably planar slope of Navajo, Kayenta,and Moenave Formations along strike with the linearvalleys described in Domain 3. Near the base of theslope, a sliver of Triassic Moenkopi Formation shaleseveral tens of meters long rests against TriassicMoenave sandstone; the Triassic Chinle Formation,which should separate the two, is missing. This olderon-youngerrelationship could be produced by faultingwith a reverse component of o€set.The exposure of interest at Location 2 follows adrainage that provides a transect into the ridge ofMoenave, Kayenta, and Navajo Formations on theeast side of Fivemile Valley (Fig. 12). Near the mouthof the wash, several outcrops of overturned MoenaveFormation beds are visible, striking N158E and dipping528NW. Towards the east, along the wash, dipsgradually steepen to vertical over the course of tens ofmeters in Moenave and Kayenta Formations. Within200 m of the overturned outcrops, at the mouth of thewash, bedding is upright, striking N108E and dipping658SE. The attitudes describe an overturned synclinethat may be the result of drag folding of beds immediatelyin the footwall of the shear zone assumed to liebeneath alluvium on the valley ¯oor.At Location 3, an isolated hill of northwest-striking,steeply east-dipping sandstone and conglomerate ofthe Chinle Formation (Shinarump Member) protrudesfrom the valley ¯oor. Triassic Moenave sandstone andshale on the east side of the valley, only a few tens ofmeters away, strike northeast. Triassic Moenkopi andPermian Kaibab Formations on the west side of thevalley also strike northeast. A northeast-striking, nearverticalfault surface with southwest-raking slickenlinesis preserved in the isolated Shinarump sandstoneblock. The outcrop is likely a sliver of ChinleFormation caught in the fault zone, which itself isobscured on the valley ¯oor.Evidence for faulting at Location 4 is similar to thatat Location 3. A wedge of distinctively stripedMoenkopi shale striking northwest is truncated at itssouthern edge by a ridge of Kayenta Formation strikingnortheast; Chinle Formation is absent between thetwo. Like the Chinle ridge at Location 2, the wedge ofstrangely oriented Moenkopi Formation here may be asliver of material caught in a reverse fault zone. Thefault contact between the two units is evident and themissing stratigraphic section discloses at least a reversecomponent of o€set; a right-lateral component alsomay be present. These outcrops make it possible totrack the presence of the shear zone almost to theUtah±Arizona border, south of which exposure iscompletely obscured by alluvium.4. Summary of ®eld observationsIn southern Utah the steep, east-dipping limb of

S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320 1315Fig. 13. Summary of structural and stratigraphic evidence for an oblique shear zone on the steep limb of the East Kaibab monocline.Progressively higher stratigraphic intervals are a€ected by intense deformation from south to north, and structural style changes from continuous,through-going faulting in the south to disjointed but pervasive fractures northward. Along the entire shear zone, northeast-striking, northwest-dippingsynthetic faults accommodate reverse-right-lateral slip, and northwest-striking, northeast-dipping antithetic faults accommodatereverse-left-lateral slip. The progression in structural style and stratigraphic level combined with consistent slip indicators suggests transpressivefault-propagation folding.

1316S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320the East Kaibab monocline hosts a narrow zone ofintense deformation marked by pervasive map-scaleand outcrop-scale faulting. This `shear zone' is seento move progressively down-section through steep,east-dipping Mesozoic strata from north to south,and the character of deformation changes with eachnew stratigraphic interval a€ected (Fig. 13). At thenorthern termination of Domain 1, northeast-striking,steeply west-dipping faults o€set Jurassic Carmelthrough Cretaceous Straight Cli€s Formations inreverse-right-lateral fashion. Slickenlines on fault surfacesrake 15±208SW. Cretaceous Wahweap andKaiparowits Formations east of (and stratigraphicallyabove) these faults are bent in map view into abroad, z-shaped fold.Domain 1 deformation occupies a slightly lowerstratigraphic interval: Jurassic Page Sandstone throughCretaceous Tropic Shale. Northwest-striking, northeast-dippingfaults with 208SE-raking slickenlines accommodatereverse-left-lateral o€set and clockwiserotation of intervening strata. Faults lie in a right-steppingen e chelon pattern and de®ne a narrow deformationzone that trends N208E, parallel to the trendof the monocline.Between Domains 1 and 2 another long, northeaststrikingfault lies just west of (and stratigraphicallybelow) a broad, z-shaped bend in steeply east-dippingJurassic and Cretaceous strata. Southward, deformationin Domain 2 a€ects the upper NavajoSandstone, Page Sandstone, and Carmel Formationredbeds. Map- and outcrop-scale, northeast-striking,steeply west-dipping faults accommodate reverse-rightlateraldisplacement of intervening strata. Slickenlineorientations on large fault surfaces average 308SW,implying a 3:1 ratio of strike-slip to dip-slip on thenortheast-striking faults. Domain 2 faults are left-steppingand slightly oblique to the trend of the monocline,but again de®ne a N208E-trending, monoclineparallelzone of deformation.Beyond yet another prominent northeast-strikingfault surface and z-shaped bend at the southern endof Domain 2, Domains 3 and 4 display evidence forreverse-right-lateral displacement on a single northeast-striking,west-dipping fault or series of long, continuousrelay faults. In Domain 3 intensedeformation is concentrated in the Navajo Sandstoneand Kayenta Formation. Major fault surfaces havean average strike and dip of N368E, 598NW withsouthwest-raking slickenlines. Evidence for continuous,through-going faulting continues to the south inDomain 4, moving down-section into the TriassicMoenave, Chinle, and Moenkopi Formations.Outcrops in these valley-forming shales are scarce,but several key exposures reveal the presence of anortheast-striking fault with at least a reverse componentof separation.Fig. 14. Equal-area and Kamb contour plots of shortening (S 3 ) andextension (S 1 ) axes calculated for 168 faults and slip surfaces usingthe kinematic analysis described by Marrett and Allmendinger(1990). The average shortening axis is consistent with ENE±WNWhorizontal compression, and the orientation of the extension axis indicatesreverse-right-lateral slip given a N208E-trending shear zone(the trend of the East Kaibab monocline).5. DiscussionThe continuous, narrow zone of deformationdescribed above is interpreted as a brittle to semibrittleshear zone occupying the steep limb of the EastKaibab monocline. Northeast-striking faults are syntheticto an overall reverse-right-lateral sense of shear,and northwest-striking faults are antithetic to the shearzone. The orientation and sense of o€set on map-scaleand outcrop-scale structures are consistent with areverse-right-lateral sense of shear for at least thenorthernmost 50 km of the monocline. Although slickenlineorientations typically record the slip vector ofonly the latest episode of movement on a fault, theclose agreement of fault attitudes and slickenline orientationsobserved at map and outcrop-scale over a full50 km distance strengthens the argument that a rightlateralcomponent of slip operated throughout shearzone development.

S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320 1317Fig. 15. (a) Lower-hemisphere equal-area projection showing theorientations of maximum, minimum, and intermediate stretch axes(S 1 , S 3 , and S 2 , respectively) and principal planes for a ®nite strainellipsoid in the shear zone. Axes were calculated from average orientationsof synthetic and antithetic faults. (b) Map-view projection ofa strain ellipsoid with the calculated S 1 and S 3 orientations within aN208E-trending shear zone. Map traces of synthetic and antitheticfaults are shown. Relative lengths of S 1 and S 3 axes have not beencalculated, but the orientation of the horizontal strain ellipse showsright-handed shear. (c) Cross-section view of the strain ellipsoidalong A±A', parallel to the S 1 ±S 2 plane. Orientation of the S 1 axiswith respect to the shear zone displays reverse, right-lateral shear.Fault-slip data were used to calculate the orientationsof shortening and extension axes using themethod described by Marrett and Allmendinger(1990). East Kaibab monocline fault-slip data includedstrike and dip of fault and slip surfaces, rake of slickenlines(representing the slip vector), and sense of slip.Shortening and extension axes were calculated for eachof 168 faults for which all of the above informationwas known. Orientations and Kamb contour plots ofthe axes are shown in Fig. 14. The average shorteningaxis trends 271.28 and plunges 3.48, and the averageextension axis plunges 47.38 toward 177.08. The nearhorizontal,east±west orientation of the shortening axisis consistent with Laramide, ENE-directed, compressivestress determined by Reches (1978) and Andersonand Barnhard (1986).Attitudes of synthetic and antithetic faults withinthe shear zone were used to determine the orientationof the ®nite strain ellipsoid. Three assumptions weremade concerning fault geometry: ®rst, that the line ofintersection of synthetic and antithetic faults is the intermediatestretch axis (S 2 ) of the ellipsoid; second,that the S 2 ±S 3 plane bisects the acute angle betweenthe fault sets; and third, that faults did not rotate considerablyduring progressive deformation. Based onthese assumptions, orientations of the principal axes ofthe ®nite strain ellipsoid in the deformed zone are(trend, plunge): 171, 41 (S 1 ); 261, 1 (S 3 ); and 350, 48(S 2 ) (Fig. 15). These values are remarkably similar tothe shortening (S 3 ) and extension (S 1 ) directions foundusing the Marrett and Allmendinger method. The geometricsolution also yields a minimum stretch (maximumshortening) axis that is horizontal with an ENEtrend, generally parallel to the direction of Laramidecontraction. Although the relative magnitudes of thestretch axes have not been determined, the orientationof S 1 implies reverse-right-lateral o€set across thezone. The sense of o€set indicated by the strain ellipsoidis consistent with the sense of o€set demonstratedby slickenline orientations observed on fault surfaces,which themselves imply a ratio of up to 5:1 of rightlateralslip to reverse slip across the shear zone.Despite the similarity in fault and slickenline orientationsalong the northern 50 km of the East Kaibabmonocline, deformation mechanisms are partitionedfrom one domain to the next. At map scale, en e chelonantithetic faults are favored in Domain 1, en e chelonsynthetic faults dominate Domain 2, and throughgoingfaulting is preferred in Domains 3 and 4. Thereasons for the changes in style are unclear. Di€erentstructures may result from di€erent mechanical responsesof the stratigraphic intervals involved, sinceboth structural style and stratigraphic interval changefrom north to south. It is also possible that thechanges are related to structural position within thefold. The slight northward plunge of the monoclinecreates an extremely elongated down-plunge view ofdeformation, such that each step towards the southwestexposes a deeper structural level, closer to thebasement fault. Considered in this way, it is relevantthat evidence for through-going faulting is present inthe structurally lower southern part of the study areabut gives way to more distributed deformation towardsthe north, at higher structural levels. The progressionfrom continuous faulting at depth to distributed fracturingat shallower levels is consistent with faultpropagationfolding (Suppe and Medwede€, 1984;Suppe, 1985; Jamison, 1987; Erslev, 1991). In the caseof the East Kaibab monocline, basement-rooted faultinghas propagated upward through Paleozoic and

1318S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320Fig. 16. The apparent diculty of reactivating a steeply dippingbasement fault with Laramide horizontal compressive stress as seenin cross-section (a) largely disappears when the perspective changesto map-view (b). An ENE-directed horizontal stress is ideally directedto cause right-lateral reactivation of a N108±208E-striking,steeply dipping basement fault such as the one underlying the EastKaibab monocline.Mesozoic strata to the level of the Navajo Sandstone.En e chelon faults in Domains 1 and 2 may representfractures immediately ahead of the propagating faulttip which would have joined and extended the basement-rootedfault if deformation had continued. In thedown-plunge perspective provided by the map, thesefractures are exposed over a distance of 25±30 km,whereas in vertical cross-section they would occupy astratigraphic thickness of less than 1500 m, possiblymaking them dicult to recognize and measure.The down-plunge view of the monocline and shearzone exposed in southern Utah invites interpretationof the East Kaibab monocline as a basement-rootedfault-propagation fold. Grand Canyon exposures showthat the fold form of the monocline widens upwardfrom basement, consistent with trishear fault-propagationfold models (Erslev, 1991). However, the brittleshear zone exposed along the steep limb in southernUtah remains narrow as it propagates up-sectionthrough the steep limb of the fold. The shear zone representsa frozen moment in the progressive developmentof fault and fold: it preserves intensedeformation that formed directly ahead of the fault tipas the propagating fault overtook the developing fold.The right-lateral component of slip in the shear zone isprobably tied to Laramide right-lateral displacementon the underlying basement fault. Thus the origin ofthe East Kaibab monocline should be considered inthe context of transpressional fault-propagation foldingrather than reverse-slip drape folding.The regional tectonic implications of these ®ndingsare signi®cant. Literature on Colorado Plateau monoclineshas commonly emphasized the role of reverseslipreactivation of Precambrian fault zones (e.g.Huntoon and Sears, 1975; Davis, 1978; Huntoon,1993). However, as seen in cross-section, a horizontalcompressive stress acting perpendicular to a near-verticalfault results in a high magnitude of normal stresson the fault plane, making reverse reactivation dicultto achieve. This limitation largely disappears when theperspective of viewing changes from cross-sectional tomap view (Fig. 16). A northeasterly directed horizontalcompressive stress acting on a N208E-striking, steeplywest-dipping Precambrian fault is suited ideally toreactivating the fault in a right-handed strike-slipfashion, with a component of reverse motion resultingfrom the steep westward dip of the fault. This is whatwe believe has occurred along at least the northern50 km of the East Kaibab monocline, and possiblyacross other basement-cored uplifts with structuraltrends oblique to the regional shortening direction.6. ConclusionsA long, narrow zone of concentrated map-scale andoutcrop-scale faulting de®nes a brittle to semi-brittleshear zone on the steep limb of the East Kaibabmonocline. The character of deformation in the shearzone varies from south to north: through-going faultingo€sets older strata at the south end of the studyarea, and more distributed, discontinuous deformationa€ects progressively younger strata to the north. Adown-plunge view of the northern 50 km of the northplungingmonocline resembles a fault-propagation foldin which the discrete fault rupture has propagatedthrough Triassic strata into Jurassic NavajoSandstone. Intense deformation directly ahead of thefault tip is seen in stratigraphically higher Jurassic andCretaceous strata. The orientations of fault surfacesexposed in the southern part of the study area closelyparallel the orientation of the underlying basementfault exposed in the Grand Canyon, leading to theassumption that the shear zone roots into the basementfault.Orientations of faults and slickenlines within theshear zone record at least a late-stage episode ofreverse-right-lateral slip. Northeast-striking and northwest-strikingfaults are interpreted as synthetic andantithetic, respectively, to a N208E-striking, steeplywest-dipping shear zone, parallel to both the monoclineand the Grand Canyon exposure of the under-

S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320 1319lying basement fault. Inversion of fault and slickenlinedata yields a ®nite strain ellipsoid with an orientationconsistent with reverse-right-lateral slip. The maximumshortening axis of the ellipsoid coincides with thenortheast-directed horizontal compressive stress determinedfor Laramide deformation on the ColoradoPlateau.Oblique displacement in the shear zone involved aratio of up to 5:1, strike-slip to dip-slip. If this ratiocharacterizes the slip vector throughout formation ofthe monocline (during initial folding and late-stagefaulting in Mesozoic strata), the observed structuralrelief of 1600 m would correspond to a right-lateralo€set of up to 8000 m between the structural crest ofthe Kaibab Uplift and the adjacent Kaiparowits Basin.AcknowledgementsWe acknowledge the valuable input received fromCharles F. Kluth and William G. Higgs, both ofChevron, Inc., with whom we discussed map relationshipsearly in the project. Conversations with the structuralgeologists at Mobil Oil in Dallas, TX wereinstrumental in clarifying many of the concepts presentedhere. We gratefully acknowledge the ®eld assistanceand input provided by Seth Gering, ShariChristo€erson, Danielle Vanderhorst, Pilar Garcia,William Abbey, and Jessica Greybill. Thanks to KarlKarlstrom, Ken McClay, Richard Allmendinger, DonFisher, and Scott Wilkerson for valuable input onearly versions of the manuscript. Field work was supportedthrough funding by the National ScienceFoundation, namely through NSF#EAR-9406208.ReferencesAnderson, R.E., Barnhard, T.P., 1986. Genetic relationship betweenfaults and folds and determination of Laramide and neotectonicpaleostress, western Colorado Plateau-Transition Zone, centralUtah. Tectonics 5, 335±357.Babenroth, D.L., Strahler, A.N., 1945. Geomorphology and structureof the East Kaibab monocline, Arizona and Utah.Geological Society of America Bulletin 56, 107±150.Barnes, C.W., 1974. Interference and gravity tectonics in the GrayMountain area, Arizona. In: Karlstrom, T.N.V., Swann, G.A.,Eastwood, R.L. (Eds.), Geology of Northern Arizona with Noteson Archaeology and PaleoclimateÐPart 2, Area Studies andField Studies. Geological Society of America Rocky MountainSection, pp. 442±453.Barnes, C.W., 1987. Geology of the Gray Mountain area, Arizona.In: Beus, S.S. (Ed.), Geological Society of America CentennialField Guide, 6. Geological Society of America, pp. 379±384.Bowers, W.E., 1972. The Canaan Peak, Pine Hollow and WasatchFormations in the Table Cli€s region, Gar®eld County, Utah,Bulletin 1331-B. United States Geological Survey.Chapin, C.E., Cather, S.M., 1983. Eocene tectonics and sedimentationin the Colorado Plateau±Rocky Mountain area. In:Lowell, J.D., Gries, R. (Eds.), Rocky Mountain Foreland Basinsand Uplifts. Rocky Mountain Association of Geologists, pp. 33±56.Davis, G.H., 1978. Monocline fold pattern of the Colorado Plateau.In: Matthews, V. (Ed.), Laramide Folding Associated withBasement Block Faulting in the Western United States, Memoir151. Geological Society of America, pp. 15±233.Dutton, C.E., 1882. Tertiary History of the Grand Canyon District,Monograph 2. United States Geological Survey.Erslev, E.A., 1991. Trishear fault-propagation folding. Geology 19,617±620.Erslev, E.A., Rogers, J.L., 1993. Basement±cover geometry ofLaramide fault-propagation folds. In: Schmidt, C.J., Chase, R.B.,Erslev, E.A. (Eds.), Laramide Basement Deformation in theRocky Mountain Foreland of the Western United States, SpecialPaper 280. Geological Society of America, pp. 125±146.Gregory, H.E., Moore, R.C., 1931. The Kaiparowits region: a geographicand geological reconnaissance of parts of Utah andArizona, Professional Paper 164. United States GeologicalSurvey.Hintze, L.F., 1988. Geologic History of Utah. Brigham YoungUniversity Geology Studies Special Publication 7.Huntoon, P.W., 1969. Recurrent movements and contrary bendingalong the West Kaibab fault zone. Plateau 42, 66±74.Huntoon, P.W., 1971. The deep structure of the monoclines in easternGrand Canyon, Arizona. Plateau 43, 148±158.Huntoon, P.W., 1974. Synopsis of Laramide and post-Laramidestructural geology of the eastern Grand Canyon, Arizona. In:Eastwood, R.L., Karlstrom, T.N.V., Swann, G.A. (Eds.),Geology of Northern Arizona, Part 1ÐRegional Studies.Northern Arizona University, Flagsta€, Arizona, pp. 317±335.Huntoon, P.W., 1981. Grand Canyon monoclines; vertical uplift orhorizontal compression? In: Boyd, D.W., Lillegraven, J.A. (Eds.),Rocky Mountain Foreland Basement Tectonics, 19. University ofWyoming Contributions to Geology, pp. 127±134.Huntoon, P.W., 1993. In¯uence of inherited Precambrian basementstructure on the localization and form of Laramide monoclines,Grand Canyon, Arizona. In: Schmidt, C.J., Chase, R.B., Erslev,E.A. (Eds.), Laramide Basement Deformation in the RockyMountain Foreland of the Western United States, Special Paper280. Geological Society of America, pp. 243±256.Huntoon, P.W., Sears, J.W., 1975. Bright Angel and EminenceFaults, eastern Grand Canyon, Arizona. Geological Society ofAmerica Bulletin 86, 465±472.Huntoon, P.W., Billingsley, G.H., Sears, J.W., Ilg, B.R., Karlstrom,K.E., Wiliams, M.L., Hawkins, D., Breed, W.J., Ford, T.D.,Clark, M.D., Babcock, R.S., Brown, E.H., 1996. Geologic mapof the eastern part of the Grand Canyon National Park, Arizona.Grand Canyon Association.Jamison, W.R., 1987. Geometric analysis of fold development inoverthrust terranes. Journal of Structural Geology 9, 207±220.Karlstrom, K.E., Daniel, C.G., 1993. Restoration of Laramide rightlateralstrike slip in northern New Mexico by using Proterozoicpiercing points: tectonic implications from the Proterozoic to theCenozoic. Geology 21, 1139±1142.Marrett, R., Allmendinger, R.W., 1990. Kinematic analysis of faultslipdata. Journal of Structural Geology 12, 973±986.Maxson, J.H., 1961. Geologic map of the Bright Angel quadrangle,Grand Canyon National Park, Arizona. Grand Canyon NaturalHistory Association.Mitra, S., Mount, V.S., 1998. Foreland basement-involved structures.American Association of Petroleum Geologists Bulletin 82, 70±109.Ohlman, J.R., 1982. A structural analysis of the Butte±Eminencefault system, eastern Grand Canyon, Coconino County, Arizona.MSc thesis, Northern Arizona University.Powell, J.W., 1873. Exploration of the Colorado River of the West

1320S.E. Tindall, G.H. Davis / Journal of Structural Geology 21 (1999) 1303±1320and its Tributaries Explored in 1869±1872. SmithsonianInstitution, Washington, DC.Prucha, J.J., Graham, J.A., Nickelsen, R.P., 1965. Basement-controlleddeformation in Wyoming province of Rocky MountainForeland. American Association of Petroleum Geologists Bulletin49, 966±992.Reches, Z., 1978. Development of monoclines: Part I, structure ofthe Palisades Creek branch of the East Kaibab monocline, GrandCanyon, Arizona. In: Matthews, V. (Ed.), Laramide FoldingAssociated with Basement Block Faulting in the Western UnitedStates, Memoir 151. Geological Society of America, pp. 235±273.Reches, Z., Johnson, A.M., 1978. Development of monoclines: PartII, theoretical analysis of monoclines. In: Matthews, V. (Ed.),Laramide Folding Associated with Basement Block Faulting inthe Western United States, Memoir 151. Geological Society ofAmerica, pp. 273±311.Sanford, A.R., 1959. Analytical and experimental study of simplegeologic structures. Geological Society of America Bulletin 70,19±52.Sargent, K.A., Hansen, S.E., 1982. Bedrock geologic map of theKaiparowits coal-basin area, Utah. United States GeologicalSurvey Miscellaneous Investigations Map I-1033-I.Stearns, D.W., 1971. Mechanisms of drape folding in the WyomingProvince. In: Renfro, A.R. (Ed.), Symposium on WyomingTectonics and Their Economic Signi®cance. 23rd FieldConference Guidebook. Wyoming Geological Association, pp.125±143.Stearns, D.W., 1978. Faulting and forced folding in the RockyMountain foreland. In: Matthews, V. (Ed.), Laramide FoldingAssociated with Basement Block Faulting in the WesternUnited States, Memoir 151. Geological Society of America,pp. 1±38.Stern, S., 1992. Geometry of Basement Faults Underlying theNorthern Extent of the East Kaibab Monocline, Utah. MSc thesis,University of North Carolina at Chapel Hill.Stone, D.S., 1969. Wrench faulting and Rocky Mountain tectonics.The Mountain Geologist 6, 67±79.Stone, D.S., 1984. The Rattlesnake Mountain, Wyoming, debate: areview and critique of models. The Mountain Geologist 21, 37±46.Stone, D.S., 1993. Basement-involved thrust-generated folds as seismicallyimaged in the subsurface of the central Rocky Mountainforeland. In: Schmidt, C.J., Chase, R.B., Erslev, E.A. (Eds.),Laramide Basement Deformation in the Rocky MountainForeland of the Western United States, Special Paper 280.Geological Society of America, pp. 271±318.Suppe, J., 1985. Principles of Structural Geology. Prentice-Hall,Englewood Cli€s, New Jersey.Suppe, J., Medwede€, D.A., 1984. Fault-propagation folding.Geological Society of America Abstracts with Programs 16,670.Walcott, C.D., 1890. Study of line of displacement in the GrandCanyon of the Colorado in northern Arizona. Geological Societyof America Bulletin 1, 49±64.