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Geological Society of America Bulletin

doi: 10.1130/0016-7606(1998)110<0512:CTZWMU>2.3.CO;2 1998;110;512-522Geological Society of America Bulletin

Timothy Paulsen and Stephen Marshak salient's northern margin, Sevier fold-thrust beltCharleston transverse zone, Wasatch Mountains, Utah: Structure of the Provo

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ABSTRACT

The Charleston transverse zone of north-central Utah is an east-west–trending corridorof faults and folds that forms the boundary be-tween the Provo salient of the Sevier fold-thrust belt and the Uinta-Cottonwood arch.The zone trends nearly 90° to the regionalstructural grain of the fold-thrust belt. Struc-tural analysis and mapping in American ForkCanyon, near the western margin of theCharleston transverse zone, demonstrates thatthe zone contains an array of low-angle thrustfaults and high-angle reverse and normalfaults. Crosscutting relations indicate that thelow-angle thrust faults formed first, followedby the reverse faults and possibly selected nor-mal faults, and then by the majority of normalfaults. Locally, normal faults reactivate reversefaults. The overall pattern of faulting and fold-ing in the Charleston transverse zone suggeststhat the zone is a left-lateral strike-slip flowerstructure superimposed on an imbricatethrust fan. Trends of normal faults in thetransverse zone suggest that many formedduring a later phase of oblique extension thathad a component of right-lateral shear. Ourresults support a tectonic model in which theCharleston transverse zone initiated duringthe Sevier orogeny to accommodate along-strike changes in the geometry of the thrustbelt, caused in turn by along-strike changes inthickness of the stratigraphic sequence in-volved in thrusting. Thrust sheets in the Provosalient to the south of the Charleston trans-verse zone moved farther toward the forelandthan did thrust sheets to the north of the zone,so the Charleston transverse zone evolved intoa left-lateral strike-slip accommodation zone.

Preliminary paleomagnetic data support this proposal: they suggest that fault blockswithin the Charleston transverse zone locallyrotated counterclockwise about a vertical axis.Laramide uplift of the Uinta-Cottonwood archtilted structures of the Charleston transversezone to the south and may have locally reacti-vated faults within the zone. Subsequent ex-tensional tectonism reactivated the Charlestontransverse zone with an oblique component ofright-lateral shear.

INTRODUCTION

Fold-thrust belts consist of series of subparal-lel thrust faults and related folds, the traces of

which define a regional structural grain. Lo-cally, however, such arrays are interrupted bytransverse zones, in which structures strike at ahigh angle to the regional grain (e.g., Thomas,1990). One such zone, the Charleston transversezone, forms the northern boundary of the Provosalient in the Sevier fold-thrust belt, along thesouthern margin of the Uinta-Cottonwood archin north-central Utah (Figs. 1 and 2). Traces offaults and folds within the Charleston transversezone trend east-west, perpendicular to the re-gional grain of the Sevier fold-thrust belt. Previous authors have debated whether theCharleston transverse zone developed duringmovement of the Sevier fold-thrust belt, duringLaramide uplift of the Uinta-Cottonwood arch,

512

Charleston transverse zone, Wasatch Mountains, Utah: Structure of theProvo salient’s northern margin, Sevier fold-thrust belt

Timothy Paulsen*

Stephen Marshak

GSA Bulletin; April 1998; v. 110; no. 4; p. 512–522; 9 figures.

}

*Present Address: Byrd Polar Research Center,108 Scott Hall, 1090 Carmack Road, Columbus, Ohio43210; e-mail: [email protected]

Department of Geology, University of Illinois, 1301 West Green Street, Urbana, Illinois 61801

Wyoming

Utah

batholiths

ProvoSalient

U/C Arch

CSZ

NFT

SFT

Quatern

ary

Volcanics

Colorado

WyomingSalient

44°N

75 km

N

Laramide forelanduplift

Traces of folds andfaults in the Sevier

111°W

fold-thrust belt

CTZ

Figure 1. Map showing the trace of the Sevier fold-thrust belt and major Laramide forelanduplifts in the western U.S. Cordillera (modified from Beutner, 1977). Shading of Laramide up-lifts schematically represents different rock types. Precambrian crystalline basement rock isblack; Precambrian sedimentary rock is gray; CTZ—Charleston transverse zone; U/C—Uinta-Cottonwood arch; NFT—North Flank thrust; SFT—South Flank thrust.



or during Tertiary crustal extension (Beutner,1977; Riess, 1985; Bruhn et al., 1986). In thispaper we describe the structural geometry andkinematics of the Charleston transverse zone inorder to constrain a kinematic model for its de-velopment. We focus our effort on exposures ofthe Charleston transverse zone in the AmericanFork Canyon region, because incisement thereby the American Fork River and its tributariesprovides access to the deeper structure of thezone. Our results demonstrate that the architec-ture of the Charleston transverse zone initiallyformed during the Sevier orogeny. During thisevent, the Charleston transverse zone served asan accommodation zone linking segments of thefold-thrust belt that differed from one another interms of depth to detachment and amount of dis-placement. The structure of the Charlestontransverse zone, however, was modified later bytilting and extensional reactivation.

GEOLOGIC SETTING

The Sevier fold-thrust belt is an east-vergingzone of thin-skinned crustal shortening. Thisbelt formed along the foreland side of the con-vergent-margin orogen that fringed westernNorth America during Late Jurassic and earlyTertiary time (e.g., Armstrong, 1968, 1974;Hintze, 1973; Cowan and Bruhn, 1992; Milleret al., 1992). Thrusting in the belt carried latePrecambrian to Mesozoic age strata of theCordilleran passive margin eastward, up and

over the former hinge line. As the thrust beltgrew, a broad foreland basin developed along itseastern flank, and thrusting eventually incorpo-rated strata of this basin. In central Utah, thrust-ing also involved strata of the late PaleozoicOquirrh basin, which had formed during devel-opment of the Ancestral Rockies, as well as un-derlying basement rocks. Along its eastern mar-gin, the Sevier belt merges with the Laramidedeformation belt, a discontinuous series of LateCretaceous to Eocene uplifts bounded by base-ment-involved reverse faults that slipped whenthe locus of crustal shortening swept eastwardinto the cratonic platform (Beutner, 1977;Coney, 1978; Hamilton, 1988). The centralUtah and western Wyoming region exposes thetransition between the Sevier and Laramidebelts (Fig. 1).

The Uinta-Cottonwood arch, a Laramide up-lift, trends east-west across north-central Utah.The core of the arch exposes a 5–8-km-thickProterozoic sedimentary sequence of an east-west–trending fault-bounded basin that buriedmetamorphic basement (Crittenden, 1976; Searset al., 1982; Christie-Blick et al., 1989). Re-gional mapping indicates that Proterozoic strataextend significantly south of the arch in the sub-surface (Baker and Crittenden, 1961; Baker,1964, 1973). Regionally, the arch separates twomajor salients of the Sevier fold-thrust belt; theWyoming salient to the north and the Provosalient to the south (Fig. 1).

The Provo salient (Fig. 2) consists of a single

thrust sheet, the Charleston allochthon. Move-ment of the allochthon resulted in transport ofthicker units about 30 to 40 km eastward over anequivalent-age, but thinner, sequence of strata(Armstrong, 1968; Riess, 1985; Yonkee, 1998,written commun.). This movement occurredprimarily during Santonian to Campanian time,and thrusting possibly continued into Maas-trichtian time (Crittenden, 1974; Jefferson,1982; Lawton, 1982, 1985; Bruhn et al., 1983a).The Charleston thrust, the floor thrust of the al-lochthon, defines the present foreland limit ofexposed thrust faults within the salient (Riess,1985; Yonkee, 1998, written commun.). TheCharleston transverse zone, the subject of thispaper, defines the northern margin of the salient;the Wasatch front, which forms the eastern limitof the Basin and Range province, truncates thesalient to the west. Thrust-related folds in the in-terior of the salient verge to the east, indicatingthat the allochthon moved from west to east(Riess, 1985; Yonkee, 1998, written commun.).Fold traces within the salient roughly parallelthe overall convex-to-the-foreland shape of thesalient’s leading edge (Fig. 2).

The Charleston thrust probably correlates withthe Mount Raymond thrust, the basal detachmentof the Absaroka thrust system in the Wyomingsalient on the north side of the Uinta-Cottonwoodarch (Fig. 2; Bruhn et al., 1986). Both thrusts cutthrough the same stratigraphic units and were ac-tive contemporaneously (Crittenden, 1974;Royse et al., 1975; Dixon, 1982; Jefferson, 1982;Lawton, 1982, 1985; Bruhn et al., 1983a, 1986;Lamerson, 1983; Bradley, 1988; Bryant andNichols, 1988; Miller et al., 1992; DeCelles,1994). During or subsequent to Laramide upliftof the Uinta-Cottonwood arch, erosion removedthe link between the Mount Raymond andCharleston thrusts (Bruhn et al., 1986).

The Charleston transverse zone crops out in abelt that extends 22 km east of the Wasatch front(Fig. 2). In the region east of American ForkCanyon, the Charleston transverse zone consistsof an anastomosing array of faults bounded on thenorth by the Charleston thrust (Baker, 1964;Riess, 1985). Here, the thrust dips 0° to 40° to thesouth, and places Precambrian and Cambrianquartzite over younger footwall strata of theUinta-Cottonwood arch (Baker, 1964; Riess,1985). East-west–striking, south-dipping normalfaults are present to within 1 km south of theCharleston thrust (Riess, 1985). Displacementalong these normal faults has cut out 60 m to 1 kmof stratigraphic section by placing Mississippianlimestone against Precambrian and Cambrianstrata; locally, these faults displace Tertiary strata(Riess, 1985).

In American Fork Canyon, the Charlestontransverse zone consists of an east-west–trending

CHARLESTON TRANSVERSE ZONE, WASATCH MOUNTAINS, UTAH

Geological Society of America Bulletin, April 1998 513

Figure 2. Simplified structural map of the Provo salient, the southern portion of the Wyomingsalient, and the Uinta arch (modified from Constenius, 1996); CTZ—Charleston transversezone; boxed region, American Fork Canyon (location of study area); MRT—Mount Raymondthrust; NFT—North Flank thrust; SFT—South Flank thrust; OM—Oquirrh Mountains;WFZ—Wasatch fault zone; Ti—Tertiary intrusives; black shade—Precambrian crystallinerock; dark shade—Precambrian sedimentary rock; light shade—Phanerozoic sedimentaryrock; white region—Tertiary basins and sediments.



faulted anticlinorium (Fig. 3). Here, the basalCharleston thrust is not exposed, and instead theDeer Creek and Forest Gate faults define thenorthern and southern boundaries, respectively,of the transverse zone. The Deer Creek fault is asouth-dipping, right-oblique-slip normal faultthat places Mississippian, Pennsylvanian,and Tertiary strata against Paleozoic strata and Oligocene quartz monzonite (Baker and Crittenden, 1961; Parry and Bruhn, 1986; Constenius, 1995). This fault has a listric geome-try; it dips 50° to 60° south at high elevations and25° to 30° south at lower elevations (Baker andCrittenden, 1961; Constenius, 1995; Yonkee,1998, written commun.). The Forest Gate fault isa steep, south-dipping normal fault that separatesolder strata of the transverse zone from a thick sequence of younger formations to the south(Baker and Crittenden, 1961).

STRUCTURAL OBSERVATIONS

Methodology

In order to define the style and kinematics of de-formation in the Charleston transverse zone, weworked in the American Fork Canyon region,where two side canyons, the North Fork and TankCanyons, provide exposures of the Charlestontransverse zone. We mapped each canyon at ascale of 1:6000 (Fig. 4, A and B; see Paulsen,1997, for detailed versions), and examined thegeometry of fault and fold systems, kinematic indicators on faults, orientation of veins and cleav-age, and crosscutting relationships among struc-tures. For each canyon, we constructed a de-formed-state cross section parallel to the localvergence of folds and faults (Fig. 4, C and D).These sections utilize unit thicknesses based eitheron outcrop measurements or on published strati-graphic columns (Baker and Crittenden, 1961). Itis not possible to rigorously balance the cross sec-tions because they trend nearly perpendicular tothe regional eastward transport direction of theProvo salient, and thus rock may have been trans-ported into or out of the plane of the section bystrike-slip displacement. However, the sections are admissible cross sections (see Marshak andWoodward, 1988, for terminology), in that they il-lustrate realistic geometries.

Faults

Map-scale faults can be classified into threecategories on the basis of their geometry andsense of offset. They can be identified in the fieldby duplication of section, slickensided surfaces,detached mesoscopic folds, and/or breccia zones.Faulting occurred under brittle conditions.

1. Low-Angle Thrust Faults. Overall, low-an-

gle thrust faults in the study area strike west tonorthwest and verge north to northeast. For exam-ple, mesoscopic, layer-parallel detachment folds(Fig. 5,A and B) and duplicated strata indicate thepresence of a low-angle thrust (fault T1) in the shaly lower member of the Cambrian OphirFormation (Fig. 4A), a 1–2-m-thick breccia layer(fault T2) separates a duplex of northeast-vergenthorses above from gently south-dipping strata be-low (Fig. 4A), and a slickensided surface (faultT3) separates diamictite of the Precambrian Min-eral Fork Formation above from fractured Cam-brian Tintic Quartzite below (Fig. 4B).

2. High-Angle Reverse-Component Faults.High-angle reverse faults and high-angleoblique-slip faults having a reverse component ofdisplacement (Fig. 5C) strike west to northwestin American Fork Canyon. Dips of these faults

range between 60° to the north or northeast and60° to the south or southwest. Locally, pairs ofhigh-angle reverse-component faults dip towardeach other and bound an anticline, as illustratedby the association of faults R1 and R2 with foldFA in the southern portion of the North ForkCanyon map area (Fig. 4, A and C). In map view,high-angle reverse faults locally define an en ech-elon array, as exemplified by faults R3, R4, andR5 (Fig. 3).

Locally, high-angle reverse-component faultsoffset low-angle thrust faults. For example, faultR5 cuts across fault T1, and fault R1 cuts acrossfault T2 (Fig. 4A). However, such crosscutting re-lationships are not ubiquitous; some high-anglereverse faults terminate at the end points of low-angle thrust faults, thereby forming a corner. Forexample, in the northwest portion of the Tank

PAULSEN AND MARSHAK

514 Geological Society of America Bulletin, April 1998

Figure 3. Simplified geologic map of American Fork Canyon modified from Baker and Crit-tenden (1961). Older strata exposed in the area occur in the core of the east-west–trending anti-clinorium; closed dashed lines—location of study areas; R3, R4, R5—reverse faults referred toin text.

111°42'30"W

Deer Creek Fault

40°28'30"N

Forest Gate Fault

R4R3

R5

T

(1.6 km)1 mile

Fault (undifferentiated)Pennsylvanian-Mississippian

Precambrianstrata

Cambrian strata NNormal fault

Quaternarydeposits

Tertiarydeposits

Tertiaryintrusive

strata

Contact

Fold hinge

Fig. 4b

Fig. 4a

American Fork River



Canyon map area, fault R3 terminates at its inter-section with the southern end of fault T3; fault R3,therefore, does not cut across strata in the hangingwall of fault T3 (Fig. 4B). High-angle reverse-component faults also locally change dip direc-tion along strike. Consequently, stratigraphic rela-tionships across these faults change from olderover younger to younger over older along strike.For example, fault R7 changes from a southerlydip direction and reverse stratigraphic throw (see

Baker and Crittenden, 1961) to a northerly dip di-rection and a normal stratigraphic relationshipalong its trace from west to east (Fig. 4B).

3. High-Angle Normal-Component Faults.High-angle normal faults and oblique-slip faultshaving a normal component of movement occurwith three general strikes in American ForkCanyon; east-west to east-northeast, north-south,and southeast to east-southeast (Fig. 4, A and B).Dips of normal-component faults generally range

between 60° and 85°. High-angle normal-compo-nent faults cut across low-angle thrust faults andrelated folds. For example, fault N1 offsets faultT1, and fault N2 offsets fault T2 (Fig. 4A). Lo-cally, high-angle normal-component faults offsethigh-angle reverse-component faults and relatedfolds. For example, fault N3 offsets fault R6 (Fig.4B). In some cases, high-angle normal faultsmerge along strike with high-angle reverse faults,as shown by the intersection of faults N1 and R5



Figure 4. (A) Map of the North Fork Canyon area in American Fork Canyon. See Figure 3 for location of map area. (B) Map of the TankCanyon area in American Fork Canyon; S1 and S2 are paleomagnetic sampling sites. (C) Deformed-state cross section (A–A′) across the NorthFork Canyon map area. (D) Deformed-state cross section (B–B′) across the Tank Canyon map area; a.m.s.l.—above mean sea level.

A B

C D




Figure 5. (A) Photo of mesoscopic detachment folds in the lower member of the Cambrian Ophir Formation in the North Fork Canyon maparea. (B) Photo of northwest-striking slaty cleavage in lower shale member of the Cambrian Ophir Formation in the North Fork Canyon maparea. (C) Photo of northwest-striking surface of a high-angle reverse fault (fault R6); quartz fibers indicate oblique, left-lateral, top-to-the-north-east movement. (D) Photo of east-west–trending, north-verging anticline and thrust fault in the Tank Canyon area. (E) Photo of east-west–trend-ing, south-verging, tightly folded, slightly overturned syncline in Mississippian limestone that occupies the footwall of fault R7 in Tank Canyon.(F) Photo of east-west–trending, tightly folded syncline of Mississippian limestone in the Tank Canyon area of American Fork Canyon.

A B

C D

FE

SW

NW SE N

S N N S

S

SW

~1 ft (.3m)

~1 ft (.3m) ~10 ft (3m)

~30 ft (10 m) ~5 ft (1.6m)

~.5 ft (.15m)

NE NE

cleavage

bedding



and by the intersection of faults N4 and R3 (Fig. 4,A and B). Some high-angle normal-componentfaults appear to have mutual crosscutting relationswith high-angle reverse-component faults. For ex-ample, fault R3 dips south and offsets stratigraphywith a reverse sense of throw, whereas just south offault R3, fault N5 places Cambrian strata on Pre-cambrian strata (Fig. 4, B and D).

Folds

Folds in the Charleston transverse zone are inspatial association with both low-angle thrustfaults and high-angle faults, have hinges thattrend subparallel to fault traces, and have tracelengths that are comparable to those of the faults(Figs. 3, 4,A and B, and 6A). Thrust-related foldsverge north to northeast (Fig. 5, B and D) andthus have axial planes that dip steeply south tosouthwest. They typically have wavelengthsranging from about 50 to 300 m, amplitudesranging from about 10 to 150 m, and interlimbangles ranging from about 20° to 60°. Foldsalong high-angle reverse faults verge north tonortheast along southward-dipping faults, andsouth to southwest along northward-dippingfaults (Fig. 5E). We observed both kink-style andcurved fold-hinge zones (Fig. 5, A, B, D, E, andF). Folds are Ramsay class 1b (parallel) to class1c, and thus appear to have developed by flexuralslip (Ramsay, 1967).

Cleavage and Veins

Disjunctive tectonic cleavage (Borradaile et al., 1982; Engelder and Marshak, 1985) occursonly locally in the study area, specifically in clay-rich units. We found a northwest-striking axial-

planar slaty cleavage in diamictite and shale ofthe Precambrian Mineral Fork Formation in thehanging wall of fault R3 (Figs. 4B and 6B) and inthe lower shale member of the Cambrian OphirFormation along fault T1 (Figs. 4A and 5B); ananastomosing spaced cleavage that strikes east-west in the Mississippian Great Blue Formation;and an east-west–striking stylolitic cleavage lo-cally in the middle limestone member of theOphir Formation. Overall, cleavage strikes sub-parallel to the northwest-southeast– to east-west–trending folds and faults of the Charlestontransverse zone.

Calcite veining occurs in Mississippian lime-stones throughout American Fork Canyon. Wesystematically measured the orientation andcrosscutting relations of veins along a north-northwest–trending traverse in the southern halfof the Tank Canyon map area. Veins typicallyconsist of blocky calcite spar or, less commonly,fibrous calcite. Locally, veins compose roughly5% of the rock, as estimated by measuring thefrequency and thickness of veins cutting acrossthe traverse line. We recognized three vein setsalong the traverse. Set I veins strike northwestand are perpendicular to bedding. Set II veinscrosscut set I, strike north-northeast, have steepdips, and parallel normal faults or mesoscopicleft-lateral strike-slip faults. They crosscut slatycleavage in the Mineral Fork Formation, cutacross slip fibers on fault R6, and occur in sys-tematic arrays with 1–8 cm spacing and locally in en echelon arrays, indicating normal-sensemovement. Set II veins predominate in the south-ern portion of the Tank Canyon area (Fig. 6C).Set III veins crosscut set II, vary from northwestto northeast strikes, and are oriented at 60° to 90°relative to bedding.

INTERPRETATION OF STRUCTURAL OBSERVATIONS

Our structural analysis of the Charleston trans-verse zone in the American Fork Canyon areademonstrates that the zone contains a complex ar-ray of faults, including low-angle thrust faults,high-angle reverse faults, and high-angle normal-component faults, as well as folds, veins, andcleavage. The geometry and crosscutting relationsof the faults and folds indicate that the zone is notsimply an east-verging thrust system (i.e., a linkedarray of north-striking ramps, flats, and associatednorth-south–trending folds) that was tilted on itsside by uplift of the Uinta-Cottonwood arch andwas then eroded obliquely. Rather, the zone ap-pears to contain three distinct fault sets and tohave a polyphase deformation history.

Crosscutting relations indicate that low-anglethrust faults were the first faults to form in thearea, because they do not displace faults of othersets. Locally, low-angle thrust faults link to high-angle reverse faults in a geometry that closely re-sembles the linkage of frontal ramps and lateralramps in thrust systems (see the intersection offaults T3 and R3 at locality A in Fig. 4B). Thisgeometry implies that at least some high-anglereverse faulting occurred during low-anglethrusting and that high-angle reverse faults lo-cally served to link noncoplanar ramps of theearly low-angle thrust system. Most high-anglereverse faults in the study area, however, cutacross and displace low-angle thrust faults, sug-gesting that new high-angle reverse faults formedafter low-angle thrusting had initiated. In general,the youngest faults in the Charleston transversezone are high-angle normal-component faults,because they crosscut both low-angle thrustfaults and high-angle reverse faults. In sum, aninterpretation of the Charleston transverse zonemust explain the following sequence of events:(1) formation of low-angle northeast-vergingthrust faults, locally associated with east-west–trending lateral ramps; (2) formation ofhigh-angle reverse faults predominantly havingwest-northwest trends; and (3) formation of high-angle normal faults in a range of orientations.

The structural style of low-angle thrust faultsin the study area closely resembles that of low-angle thrust faults found in the interior of theProvo salient. Perkins (1955) correlated somenorthwest-southeast–trending low-angle thrustfaults in American Fork Canyon with northerly-trending structures in the Provo salient. Thus, weinterpret the low-angle thrust faults of theCharleston transverse zone to have formed dur-ing the same fold-thrust event responsible for for-mation of the Provo salient, i.e., the Sevierorogeny. The contrast between the vergence oflow-angle thrust faults in the Charleston trans-



n=62 n=42 n=95

A CB

Poles to VeinsTrends of Fold axes

Poles to CleavageMineral Fork Formation Mississippian limestones

Figure 6. Equal-area plots of structural data collected in the map areas in American ForkCanyon. (A) Trends of fold axes measured in the study areas. (B) Poles to slaty cleavage meas-ured in the Precambrian Mineral Fork Formation in the Tank Canyon map area. (C) Poles toextension veins (in situ orientations) measured in Mississippian limestones in the southern halfof the Tank Canyon map area.



verse zone (northeast) and the vergence of low-angle thrust faults in the interior of the Provosalient (due east) is a consequence of the salient’sinitial curvature (i.e., differential thrust propaga-tion between the apex of the salient and its limbs;e.g., Coward and Potts, 1983).

After the initial phase of low-angle thrustingand lateral ramping, the majority of high-angle re-verse-component faults and associated folds ofthe Charleston transverse zone developed. Thesefaults and folds predominantly trend east-west tonorthwest-southeast, and thus locally are at anoblique angle to the regional east-west trend of theCharleston transverse zone (Fig. 7A). Some ofthese faults have north to northeast dips, whereasothers have south to southwest dips. The en eche-lon pattern displayed by some high-angle reversefaults resembles the pattern of faulting found inleft-lateral strike-slip fault zones (Fig. 7B;Sylvester, 1988). The overall pattern of faultingand folding in the Charleston transverse zone re-sembles that of a flower structure in a strike-slipfault zone (cf. Sylvester, 1988). On the basis oftheir orientation, the northeast-trending normalfault set and the northeast-striking vein set of theCharleston transverse zone (Fig. 7A) also formedat this stage, but field data cannot prove this pro-posal. In sum, our observations suggest that theCharleston transverse zone evolved into a left-lat-eral strike-slip zone. This strike-slip zone was su-perimposed on a preexisting low-angle thrust sys-tem. Lateral ramps of the thrust systemreactivated during this stage. Northeast-southwestshortening across the zone yielded reverse fault-ing and associated folding.

Normal faults are the youngest structures of theCharleston transverse zone. They occur with bothnortheast and northwest strikes, and in places weinterpret them as reactivated reverse faults. Ac-cording to standard models of strain in strike-slipzones (Fig. 7B; Sylvester, 1988), northeast-strik-ing normal faults may have formed during left-lateral strike-slip movement, but northwest-trend-ing normal faults could not form in a left-lateralzone. Normal-sense movement along northwest-trending faults in the Charleston transverse zoneis, however, compatible with right-lateral shear inthe Charleston transverse zone. We suggest thatregional east-west extension reactivated theCharleston transverse zone as a right-lateralstrike-slip zone, and that this movement led tonormal-sense reactivation of preexisting north-west-trending high-angle reverse faults and to theformation of new northwest-trending normalfaults. The right-lateral component of movementthat Parry and Bruhn (1986) documented on theDeer Creek fault, along the border of theCharleston transverse zone north of the studyarea, and that Riess (1985) and Royse (1983) sug-gested for the Charleston transverse zone to the

east of the study area, represent this deformationphase. To the north and east of the study area, nor-mal faults of the Charleston transverse zone offsetTertiary strata and crosscut the Tertiary Cotton-wood stock (Baker and Crittenden, 1961; Riess,1985; Constenius, 1995), indicating that this ex-tensional phase of reactivation occurred after theSevier orogeny.

If the normal faults record a phase of lateEocene to early Miocene extension, this phase ofreactivation could represent postorogenic collapseof the Sevier fold-thrust belt, during which theCharleston thrust was reactivated as a west-south-west–verging extensional fault (Royse, 1983;Riess, 1985; Constenius, 1995, 1996). Our datado not constrain whether some normal faultingoccurred in association with Sevier orogeny fold-ing, Laramide orogeny tilting associated with up-lift of the arch, or Basin and Range extension, be-cause appropriate-age strata needed to date suchmovements do not crop out within the zone.

PALEOMAGNETIC TEST OF LEFT-LATERAL SHEAR

We conducted a preliminary paleomagneticanalysis to test whether thrust-sheet rotation oc-curred in the Charleston transverse zone during itsdevelopment (Paulsen et al., 1995), as would bepredicted if a significant component of movementin the zone involved left-lateral shear. For thisanalysis, we collected oriented samples of Missis-sippian Gardison Limestone on opposing limbs of an east-west–trending anticline within theCharleston transverse zone (see S1 and S2 in Fig. 4B), and then used a cryogenic magnetometerto evaluate the natural remanent magnetization ofeach sample. To isolate components of the rema-nent magnetization, the samples were cleaned us-ing both alternating field and thermal demagneti-zation techniques (for details of this analysis, seePaulsen, 1997). Sample-mean directions fromthermal treatment cluster after structural correction

PAULSEN AND MARSHAK


Strike of High-angleReverse Faults

Strike of High-angleNormal Faults

Strike of Veins

Trends of Fold axes

Strike of Cleavage

A

Figure 7. (A) Rose diagrams (each line represents one measurement) showing the map-viewtrend of faults, folds, cleavages, and veins found in the study areas. (B) Diagram showing faultand fold pattern, strain ellipse, and the sense of rotation that commonly develop by left-lateralstrike-slip movement (modified from Sylvester and Smith, 1976).

B



for the tilt of the strata, indicating that the magne-tization predates folding. The tilt-corrected meandirection calculated from the stable end points ofthe magnetization decay paths (110.7°/–32°) cor-responds to a calculated paleomagnetic pole of re-verse polarity positioned significantly off the ap-parent polar wander path (APWP) for NorthAmerica at 339.8°E/26.8°S (Fig. 8).

To evaluate the paleomagnetic results in termsof the tectonic evolution of the Charleston trans-verse zone, we first needed to determine the ageof the magnetic remanence. The inclination of themagnetization reflects the paleolatitude of thesampling site when the magnetization developed.Thus, to estimate the age of the remanence, wecompared the inclination of the mean paleomag-netic direction that we obtained after thermaltreatment to dated paleomagnetic inclinations thatwould be expected for poles on the APWP forNorth America (Van der Voo, 1990). This com-parison suggests that the paleomagnetic pole po-sition we obtained is Middle Jurassic (labeled*Jm in Fig. 8), but considering statistical error, themagnetization could be Lower Jurassic–UpperTriassic (labeled **Jl/Tru) or Upper Permian (la-

beled ***Pu) in age. In any of these cases, themagnetization is postdepositional. If the magneti-zation is Jurassic in age, it may reflect a remagne-tization caused by reaction of the limestone withhot brines driven to the foreland during thrusting,a process that has been recognized in other oro-genic belts and their forelands (cf. Bethke andMarshak, 1990).

We interpret the deviation of the pole positionfrom the North American APWP to be the resultof fault-block rotation about a vertical axis, aprocess that has been recognized in strike-slipbelts (Sylvester, 1988, and references therein).The smallest corrective rotation of the poleabout a vertical axis to the Jurassic pole on theNorth American APWP suggests that the sam-ple rotated in a counterclockwise sense by 59.3° ± 14.2° since its magnetization. Trial rota-tion back to the Early Jurassic–Late Triassicpole indicates a similar rotation, and a trial rota-tion back to the Late Permian pole indicates asmaller rotation (~39°). Such a counterclock-wise sense of rotation is compatible with ourproposal that left-lateral faulting occurred in theCharleston transverse zone. The data do not al-

low us to constrain when this rotation occurredrelative to formation of folds, cleavages, andveins. Furthermore, we do not know how wide-spread an area this rotation affected.

TECTONIC MODEL FOR THECHARLESTON TRANSVERSE ZONE

Linkage between east-west–striking high-an-gle faults in the Charleston transverse zone andthrust faults of the Sevier fold-thrust belt suggeststhat the Charleston transverse zone initiated as abelt of lateral ramps that connected noncoplanarlow-angle thrust faults during the Sevier orogeny.Fault and fold geometries within the transversezone indicate that the Charleston transverse zonesubsequently became a left-lateral strike-slipfault zone, an interpretation supported by prelim-inary paleomagnetic data. The structural archi-tecture of the Charleston transverse zone appearsto have developed during this stage. Mapping ofthe zone to the north and east of our study area in-dicates that most recently, the Charleston trans-verse zone reactivated as a zone of oblique exten-sion having a right-lateral component ofmovement. Considering this polyphase tectonichistory, we suggest that the Charleston transversezone has played several different roles during thetectonic evolution of north-central Utah.

Charleston Transverse Zone as a SevierOrogeny Lateral-Ramp Zone

Initially, the Charleston transverse zoneserved as a zone of lateral ramping (Fig. 9A).Lateral ramping linked noncoplanar low-anglethrust faults and transferred displacement be-tween ramps along the northern margin of theProvo salient. Development of such transferstructures typifies portions of fold-thrust belts inwhich regional thrust sheets curve sharply andthe magnitude of transport to the forelandchanges rapidly along the strike (Wilkerson,1992). We suggest that the Charleston transversezone formed where it did because at that locationthe pre-Sevier orogeny stratigraphic sectionthinned abruptly to the north across an east-west-trending basement-penetrating fault system(Beutner, 1977; Bruhn et al., 1983b, 1986; Smithand Bruhn, 1984; Bryant and Nichols, 1988).Considering that the Charleston transverse zonefollows an abrupt east-west–trending boundarybetween a thicker portion of the Oquirrh basin tothe south and a thinner portion of the Oquirrhbasin to the north (Bruhn et al., 1986), lateralramping in the Charleston transverse zone alsoprobably accommodated changes in the depth todetachment. In effect, the Charleston transversezone follows what was a major sidewall ramp inthe Sevier fold-thrust belt. Regional mapping



Figure 8. Position of paleomagnetic pole obtained from thermal treatment with respect to theposition of poles on the North American apparent polar wander path (poles in figure are plottedin normal polarity coordinates). Box contains paleomagnetic mean direction, latitude and lon-gitude of calculated pole, and Fisher statistical parameters (k, α95, dp, dm). Circles represent95% cones of confidence. K—Cretaceous; J—Jurassic; Tr—Triassic; P—Permian; C—Car-boniferous; u—upper; m—middle; l—lower; Ku—67–97 Ma; Kl—98–144 Ma; Ju—145–176Ma; Jm—177–195 Ma; Jl/Tru—196–215 Ma; Trm—216–232 Ma; Trl—233–245 Ma; Pu—246–266 Ma; Pl—267–281 Ma; Cu—282–308 Ma; Cm—309–365 Ma.



also indicates that the thrust belt progressivelyimbricated basement to the north of this bound-ary, whereas shortening involved only sedimen-tary rocks above basement to the south of thisboundary (Royse et al., 1975; Bruhn et al., 1986;Yonkee, 1992; Yonkee, 1998, written commun.).

Charleston Transverse Zone as a SevierStrike-Slip Accommodation Zone

As the Sevier fold-thrust belt evolved, theCharleston allochthon propagated significantlyfarther into the foreland than did either the south-ern portion of the Absaroka thrust system or theportion of the fold-thrust belt that formed over thesite of the present Uinta-Cottonwood arch(Dixon, 1982; Riess, 1985; Bradley, 1988; Yon-kee, 1998, written commun.). Consequently, theCharleston transverse zone evolved into a left-lat-eral strike-slip fault zone (Fig. 9A)—it became an

accommodation zone, in the sense that complexmovement within it accommodated abrupt along-strike contrasts in the magnitude of displacementtoward the foreland in the Sevier fold-thrust beltand in the position of the foreland edge of thethrust system. During this stage, numerous north-west-striking high-angle reverse faults developedin the Charleston transverse zone. Fault-boundedblocks underwent shortening in a northeast-south-west direction, resulting in folds and cleavage. Atthe same time, minor extension occurred alongthe strike of the Charleston transverse zone, as in-dicated by veining and northeast-striking normalfaults (cf. Coward and Potts, 1983; Marshak,1988). Fault-bounded blocks may have under-gone counterclockwise rotation around a verticalaxis during this time, as indicated by our prelimi-nary paleomagnetic data. Similar accommodationzones have been identified in many fold-thrustbelts—all are typified by arrays of faults, folds,

and cleavages that trend oblique or parallel to thethrust belt’s regional transport direction and by al-lochthonous strata that have rotated about a verti-cal axis (Wheeler, 1980; Coward and Potts, 1983;Eldredge and Van der Voo, 1988; Thomas, 1990).

Post-Sevier Orogeny Reactivation of theCharleston Transverse Zone

Although the overall structural architecture ofthe Charleston transverse zone was establishedduring the left-lateral strike-slip event at the timeof the Sevier orogeny, post-Sevier orogenydeformation modified the Charleston transversezone. Laramide uplift of the east-west–trendingUinta-Cottonwood arch tilted the zone at a shallowangle to the south (Fig. 9B) and may have reactiv-ated faults within the Charleston transverse zoneby north-side-up displacement. Tertiary exten-sional faulting reactivated the Charleston trans-

PAULSEN AND MARSHAK


Figure 9. Simplified schematic diagram showing the main stages in the structural evolution of the Charleston transverse zone. Shaded regionrepresents Precambrian crystalline basement.

A

C D

B



verse zone as a normal fault zone having anoblique right-lateral component of movement(Fig. 9C; Royse, 1983; Riess, 1985; Parry andBruhn, 1986; Constenius, 1995). Such movementimplies that during Tertiary extension, theCharleston transverse zone once again acted as anaccommodation zone, but during this phase, it ac-commodated differences in the magnitude of ex-tension between the region of the Provo salient andthe region of the Uinta-Cottonwood arch. Subse-quent erosion stripped thrust sheets overlying thearch (Fig. 9D).

Post-Sevier orogeny events undoubtedlymodified the geometry of the Charleston trans-verse zone, but we emphasize that they couldnot have been responsible for the establishmentof the Charleston transverse zone as a transversezone for the following reasons. (1) The trend ofthe Charleston transverse zone cannot be duesimply to tilting (followed by erosional expo-sure) of the Charleston allochthon around aneast-west axis during Laramide uplift of theUinta-Cottonwood arch (Beutner, 1977) and/oraround a north-south axis during possible east-ward tilting associated with Tertiary riftingalong the Wasatch front. Trial rotation ofnortherly-trending folds and cleavages using amaximum estimate of 20° about these axes doesnot change their trend to east-west or north-west-southeast. Likewise, corrective rotation of the east-west– to northwest-southeast–strikinghigh-angle reverse faults indicates that theirstrikes would not be significantly affected bysuch tilting events, although their dip anglesmay have decreased or increased, depending ontheir initial dip direction. (2) The trend of theCharleston transverse zone also cannot be dueto oroclinal bending of Sevier orogeny thrustsheets by interaction with a postthrusting strike-slip fault, as has occurred in the Makran belt ofPakistan (Marshak, 1988), because a strike-slipfault responsible for such bending would extendeast of the foreland edge of the Provo salient,and no evidence for such a fault exists.

CONCLUSION

Our work demonstrates that the kinematicsand structural geometry of transverse zones infold-thrust belts may be complex. Such zonesare localized by preexisting discontinuities inbasin geometry, which in turn may reflect evenolder basement structures. Transverse-zone de-formation results in arrays of high-angle faultshaving a range of orientations and vergences.Establishment of such a fault array creates alasting cross-strike weakness in a fold-thrustbelt, which can reactivate with an oppositeshear sense during subsequent orogenic col-lapse and result in fold and fault geometries

that resemble flower structures along strike-slip faults.

ACKNOWLEDGMENTS

Funding for this work was provided by the Col-orado Scientific Society, a Geological Society ofAmerica research grant, and the Morris Leightonfund of the University of Illinois. We thank John Stamatakos for assisting in all phases of the paleomagnetic analysis at the Univer-sity of Michigan paleomagnetics laboratory,Rick Allmendinger for stereonet programs that greatly simplified our data analysis,Scott Wilkerson, Adolph Yonkee, and Joe Meertfor useful discussions, Mark Fischer, Ed Beutner,and Gautam Mitra for helpful reviews of the man-uscript, and Christie Demosthenous for help indraft revisions.

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