Miocene Unroofing of the Canyon Range During Extension Along the Sevier Desert Detachment, West Central Utah

TECTONICS, VOL. 20, NO. 3, PAGES 289-307, JUNE 2001

Miocene unroofing of the Canyon Range during extension along the Sevier Desert Detachment, west central Utah

Daniel F. Stockli • Departmentof Geologicaland EnvironmentalSciences, Stanford University, Stanford, California

JonathanK. Linn:, J.Douglas Walker Departmentof Geology, Universityof Kansas,Lawrence, Kansas

Trevor A. Dumitru Departmentof Geologicaland Environmental Scmnces, Stanford University, Stanford, California

Abstract. Apatite fission track resultsfrom Neoproterozoic 1. Introduction and Lower Cambrian quartzites collected from the Canyon Rangein west centralUtah reveal a significantearly to middle The Canyon Range in west central Utah lies within the Miocene cooling event (-19-15 Ma). Preextensional Mesozoic Sevier orogenic belt of Armstrong [1968] at the temperaturesestimated from multicompositionalapatite easternmargin of the Basin and Range extensionalprovince fissiontrack data suggest-4.5 to >5.6 km of unroofingduring (Figure 1). The geology of the Canyon Range and the the early to middle Miocene, assuminga geothermalgradient adjacentSevier Desert region has become the focusof intense of-25øC/km. The spatialdistribution of thesepreextensional scientificdebate concerning the regional tectonicevolution of temperaturesindicates -15ø-20ø of eastward tilting of the the easternGreat Basin and especiallythe mechanicaland Canyon Range during rapid extensionalunroofing along a kinematic viability of low-angle detachment faulting in moderately west dipping detachmentfault (-35ø-40ø). We extensional tectonics. Seismic reflection studies in the Sevier interpretthis fault to be the breakawayof the Sevier Desert Desert basin to the west of the Canyon Range (Figure 1) Detachment fault (SDD), the existence of which has been imaged a prominent -11ø-12ø west dipping reflector, the contested.The new thermochronologicdata presentedin this SevierDesert reflector (SDR). The SDR hasbeen interpreted study provide compelling evidencefor the existenceof the to representa top to the west, low-angleextensional fault, the SDD and thus the generalviability of low-angle detachment Sevier Desert Detachment (SDD) fault [McDonald, 1976; faulting. The data directly date the onset of extensional Wernicke,1981; Wernickeand Burchfiel, 1982; Allmendinger faulting along the SDD startingat -19 Ma and constrainthe et al., 1983; Von Tish et al., 1985; Mitchell and McDonald, fault slip rate in the SDD breakawayzone at 2.4-2.1 mm/yr 1987; Planke and Smith, 1991]. The magnitude of between-19 and 15 Ma. An early Miocene apatitefission displacementalong the proposed SSD ranges from <10 track age obtainedfrom a Proterozoicclast from the Tertiary [Planke and Smith, 1991] to 38-45 km [e.g., Von Tish et al., Oak City Formationconfirms that these conglomerateswere 1985; Coogan and DeCelles, 1996; Mitra and Sussman, depositedin a synextensionalbasin in the hangingwall of the 1997]. SDD. The timing of tectonicunroofing of the CanyonRange The hypothesizedSDD has become one of the most in response to faulting along the SDD appears to be famous and classic examples for low-angle detachment synchronouswith large-magnitudeextension along the Snake faulting challenging models that suggest that low-angle Range d•collement and with early extensionalong the Cave normalfaults only slippedat initially higherfault angles[e.g., Canyon detachment exposed in the Mineral Mountains, Buck, 1988]. This debate over the mechanicalviability of pointing to widespread east-west extension in the eastern low-anglenormal faulting continues,despite recent evidence Great Basin in the early and middle Miocene. for seismogenicand active low-angle normal faulting, for examplein the D'EntrecasteauxIslands [e.g., Hill et al., 1992; Abets et al., 1997], the Gulf of Corinth [e.g., Rietbrocket al., 1996; Rigo et al., 1996; Sorel, 2000], and the central Italian Apennines[e.g., Boncio et al., 2000]. •Nowat Divisionof Earthand Planetary Sciences, California In the past decadethe low-angle detachmentmodel for the Instituteof Technology,Pasadena, California. 2Nowat EaganMcAllister Associates, Inc., Lexington Park, prominentwest dippingreflector below the SevierDesert and Maryland. the existenceof the SDD has come under seriousscrutiny [Anders and Christie-Blick, 1994; Hamilton, 1994; Wills and Anders, 1996; Wills and Anders, 1999]. Several workers have Copyright2001 by the AmericanGeophysical Union. contested the existence of the SDD on the basis of lack of deformationaI fabrics in strata from boreholes near the Paper number2000TC001237. hypothesizeddetachment fault [Anders and Christie-Blick, 0278-7407/01/2000TC001237512.00 1994] and on aspectsof the surfacegeology near the eastern

289 290 STOCKLI ET AL.: MIOCENE UNROOFING OF THE CANYON RANGE

112ø15 ' 112ø7'30"

95BR024 CC95-1 o 20.0ñ2.0 Ma 96.8_+10.6 Ma Mountruns CROnyøn ge ! CC95-2 CQ95-7 41.7__.4.7 Ma 18.1ñ2.4 Ma Confusion 39' 30' Sevier I CQ95-5 t• 23.9ñ4.4 Ma

CO95-6 •t 17.3_+2.6 Ma / TKs C095-4 14.3ñ1.8 Ma

Mineral Mountains

CC95-3 112ø22'30" 63.4ñ6.7 Ma CRT

TKc OakCity

OC95-1 18.0ñ3.4 Ma ' ..'.- 97BR001 ß Ma 20.3_+3.0 Ma I 95BR021 125 19.0ñ2.0 Ma I CO95-1 20.0ñ2.4 Ma A« i -q A' 8.8_+2.7 Ma '• -.. 18.0ñ2.2Ma 18.7_+1.7 TKs ce95-3 95BR020

IQToalPlio-Pleistocene alluvium ....•..• FoolCreek conglomerate ':•.'• OakCity Formation • Tertiaryslide-block deposits "•!7.:ii?71'...:KTconglomerate andsandstone ,•?J•:.} KT conglomerate

•...... Cambrian- Devonian carbonates 19ø15 , Cambrianlimestone TinticQuartizite :i•-•] Precambrianquartzite 0 km 5 I I

Figure 1. Generalizedgeologic map of the CanyonRange showingapatite fission track samplelocalities and results.Unit "Toc" representsrocks in the hangingwall of the proposedSevier Desert Detachmentfault, whereas Proterozoicand Lower Cambrian units are in the footwall. Precambrianquartzites in the hanging wall of the CanyonRange thrust fault (CRT; barbson upperplate) are indicatedby pC. Stratigraphicunits: pCs, Precambrian quartzitesand minor limestone;Ct, CambrianTintic quartzite;Cu, undifferentiatedMiddle and Upper Cambrian limestoneand shale;CDs, Cambrianthrough Devonian limestone, dolomite, quartzite, and shale;TKc, Cretaceous and Tertiary conglomerate;TKs, Cretaceousand Tertiary conglomerate,sandstone, and shale;Toc, Tertiary Oak City Formation;Tfc, Tertiary Fool Creek conglomerate;Qtoa, Pliocene and Pleistocenealluvium. Pre-Cenozoic geologyis modifiedafter Hintze [1991a, 1991b, 1991c, 1991d, 1991e, 1991f], and Cenozoicgeology is modified after Otton [1995]. STOCKLI ET AL.: MIOCE• UNROOFING OF THE CANYON RANGE 291 margin of the Sevier Desert [Hamilton, 1994; Wills and resolution of the dispute over whether the SDR is an Anders, 1999]. According to their models, the seismically unconformityor a low-angle detachmentfault has important imaged SDR representsan unconformitybetween Paleozoic consequencesfor the ongoing debate on the mechanicsand and Tertiary strata along much of its extent, rather than an kinematicsof low-angle normal faulting. The hypothesized extensionallow-angle detachment fault. existence of the SDD also has important regional tectonic Most tectonic reconstructionsof the Sevier Desert region ramifications, strongly affecting both estimates of total place the surface projection of the SDR and therefore the Tertiary extensionin the northernBasin and Range province breakawayof the SDD near or along the western side of the [e.g., Wernicke, 1992] as well as thrust geometries and Canyon Range [e.g., Von Tish et al., 1985; Planke and Smith, shorteningestimates in the Mesozoic Sevier fold and thrust 1991; Otton, 1995; Coogan and DeCelles, 1996]. Several belt [e.g., Mitra and Sussman,1997; Coogan et al., 1995]. studieshave investigatedthe surfacegeology of the Canyon Thus understandingthe geologicalsignificance of the SDR is Range in order to shed light on the controversysurrounding imperativefor the tectonicreconstruction of the easternGreat the existence of the SDD [Otton, 1995; Morris and Basin in Mesozoic and Tertiary times. Hebertson, 1996; Wills and Anders, 1999]. Otton [1995] proposedthat the western foothills of the Canyon Range 2. Geologic Setting representthe breakawayzone of the extensionaldetachment systemhypothesized to floor the Sevier Desert basin, a view The Canyon Range is one of severalnorth-south trending shared by Coogan and DeCelles [1996] and Morris and rangesin the easternGreat Basin that exposestacked thrust Hebertson [1996]. In their model the Tertiary clastic sheetsof Sevier age containingdeformed Neoproterozoicto sediments in the western foothills were deposited in a Mesozoic rocks [Hintze, 1980, 1988]. The Canyon Range synextensionalsupradetachment basin above the SDD. This itself is composedof Neoproterozoicquartzites in the hanging hypothesishas been contestedby Wills and Anders [1999], wall and lower Paleozoicsedimentary rocks in the footwall of who proposedthat the CanyonRange is a simple horstblock the Canyon Range thrust fault (CRT) [e.g., Lawton et al., bound by Tertiary high-angle normal faults and that the 1997]. The CRT is folded into an eastvergentsynform by Tertiary sediments along the western range flank rest subsequentdeformation due to antiformalstacking associated unconformablyon pre-Mesozoicrocks. with the underlyingPavant thrust fault (PVT) (Figures 1 and This study presents apatite fission track 2) [Christiansen, 1952; Swank, 1978; Holladay, 1984; thermochronologicalconstraints from pre-Mesozoicrocks in Coogan et al., 1995; DeCelles et al., 1995; Mitra and the CanyonRange, the first directradiometric age constraints Sussman, 1997; Lawton et al., 1997]. Contractile deformation for the timingof unroofingof the range. Thesedata allow us purportedlyspanned latest Jurassicthrough Paleocenetime to critically evaluatethese contrastinghypotheses since the and had ceased by Eocene time, as evidenced by the differenttectonic models predict distinctly different cooling undeformedEocene Flagstaff Formation [Armstrong, 1968; historiesfor rocksfrom the CanyonRange. If the rangeis Villien and Kligfield, 1986; Coogan et al., 1995; DeCelles et indeed the breakawayzone of the hypothesizedSDD, the al., 1995]. Initial thrustingalong the CRT is constrainedby derived cooling histories should record exhumation and the age of the Cedar Mountain Formation (-130-120 Ma) unroofingrelated to the onsetof rapid fault slip along the along the easternflank of the range, which containsclasts SDD [e.g., Wernicke, 1985; Lister and Davis, 1989]. The derivedfrom the emergingCanyon Range thrustsheet (Figure Canyon Range W Sevier Desert E

• KT Sevierforeland SDD • basindeposits •, •" '• • TKs

Teaia•O•City Fm t

0 1 2 3 4 5

[km] Figure 2. Schematiccross section of the centralCanyon Range. Locationof crosssection is shownon Figure 1. Crosssection is modifiedfrom Otton [1995]. Stratigraphicunits: pCs, Precambrianquartzites and minorlimestone; Ct, Cambrian Tintic quartzite; Cu, undifferentiatedMiddle and Upper Cambrian limestone and shale; CDs, Cambrianthrough Devonian limestone, dolomite, quartzite, and shale;TKc, Cretaceousand Tertiaryconglomerate; TKs, Cretaceousand Tertiary conglomerate,sandstone, and shale. Faults:CRT, CanyonRange thrust;SDD, Sevier Desert Detachment. 292 STOCKLI ET AL.: MIOCENE UNROOFING OF THE CANYON RANGE

1) [DeCelleset al., 1995]. Subsequentmotion along the PVT depositshave been discussedin detail by Otton [1995], in late Aptian to Albian time (-115-98 Ma) resultedin Morris and Hebertson [1996], and Wills and Anders [1999]. deformationand exhumation of theoverlying Canyon Range Otton [1995] mappedthe moderatelyto steeplyeast dipping thrustsheet [DeCelles et al., 1995]. TheCanyon Range thrust Tertiary conglomeratesof the Oak City Formation and sheetmust have been deeplyeroded and subsequently proposeda synextensionalorigin for the depositsrelated to reburiedby severalkilometers of wedge-topsediments of the detachmentfaulting along the SDD. He also presented CanyonRange Formation during Late Cretaceousto Early evidencefor a low-anglenormal fault contactthat dips 18ø- Tertiaryeastward propagation of thrusting[Royse, 1993; 24ø to the west, juxtaposing synextensional,east tilted DeCelleset al., 1995]. The temporaland spatial succession Tertiary conglomeratesand Neoproterozoicand Cambrian of contractional deformation documents the overall eastward stratain the breakawayzone (Figure2). In contrast,Wills and migration of thrust faulting toward the Sevier foreland with Anders [1999] claimed that the Oak City Formation rests only minor out-of-sequencethrusting [Lawton et al., 1997]. unconformablyon pre-Mesozoicrocks. However, owing to The Canyon Range togetherwith the Pavant Range forms the weatheringcharacteristics of the Tertiary conglomerates the easternmargin of the Sevier Desert basin. The basin and the basal contact of the Oak City Formation is largely its origin have attractedsubstantial interest, with many studies obscuredand coveredby colluvium. focusingon the natureand origin of the SDR imagedon the Furthercomplicating the understandingof the depositional Utah-1 line of the COCORP seismic reflection profile and nature of the Oak City Formation and its regional tectonic several industrial seismic lines [Allmendingeret al., 1983; implicationsis the general lack of reliable age constraints. Von Tish et al., 1985; Mitchell and McDonald, 1987; Planke Age assignmentsare primarily derivedfrom tentativeregional and Smith, 1991; Coogan and DeCelles, 1996]. The low- correlationsand intuition. A Miocene-Plioceneage was angle reflector dips --11ø-12ø to the west and extends from proposedfor the Oak City Formationby Campbell [1979] on near the western side of the Canyon Range to beneath the the basis of general views on the age of Basin and Range Sevier Desert and Cricket Mountains (Figure 1). extension at the time. Otton [1995] assigneda tentative Allmendinger et al. [1983] and Von Tish et al. [1985] Miocene age to the Oak City Formationprimarily on the basis suggestedthat the reflector is a down to the west detachment of a 12.4+_1.4Ma zircon fissiontrack age for a tuff within an fault, the SDD, with up to 38-45 km of displacementon the Oak City Formationequivalent in the northernPavant Range basis of offset contractile structuresand other geological (Figure 1). Wills and Anders [1999], however,pointed out relationships.Coogan et al. [1995] and Cooganand DeCelles that even Otisage is questionablesince the tuff is not actually [1996] argue that major offset along the detachment is interbeddedwith these conglomeratesbut only coveredby required to restore distinctive upper Proterozoic and alluvium derivedfrom them. This observationimplies that Cambrianmiogeoclinal units in the Cricket Mountainsacross most of the conglomeratesof the Oak City Formationare the SevierDesert basinto the CanyonRange. Althoughsome older than -12 Ma, if they are indeed correlative with the controversyexists about the timing of fault slip along the depositsto the westof the PavantRange. hypothesizedSDD, most studiessuggest that faulting began The east side of the modemCanyon Range appears to be after middle Oligocene time (28-26 Ma) and has continued controlledby an active high-anglenormal fault [Christiansen, through the Pliocene and might in fact still be active [e.g., 1952; Campbell, 1979; Millard, 1983; Holladay, 1984; Von Tish et al., 1985]. The timing of onset of faulting is Hintze, 1991f; Wills and Anders, 1999]. Millard [1983] constrained by Oligocene lacustrine and volcanoclastic identified a normal fault with up to 450 m offset near the sediments from boreholes in the central Sevier Desert basin range front, and Holladay [1984] suggesteda connection that predate extensional faulting [Lindsey et al., 1981; between the range-bounding fault and an east side down Mitchell and McDonald, 1987; Cooganand DeCelles, 1996]. normal fault identified within the Late Cretaceous-early The Canyon Range has turned out to be particularly Tertiary Canyon Range Formation with -1.2 km of throw. important in the scientific discussionsurrounding the nature Along the westernrange front, geomorphicfeatures indicate and tectonic origin of the Sevier Desert basin. The updip active uplift along a west dipping, high-anglenormal fault projection of the SDR intersecting the surface near the that juxtaposes Neoproterozoic Pocatello Formation and western flank of the range, proposedoffsets of Mesozoic Tintic quartzite [Sussman, 1995; Wills and Anders, 1999]. contractionalstructures, and the tilted Tertiary sedimentsin Offset along this westernrange-bounding normal fault was the westernfoothills have made the Canyon Range the most estimatedat-1.25 km [Wills and Anders, 1999] (Figures 1 likely location for the extensional breakaway of the and 2). However, Otton [1995] interpretedthe samefault as hypothesizedSDD. Otton [1995] proposedthe presenceof an an early Tertiarynormal fault, and Holladay [1984] originally extensionalfault zone along the westernflank of the Canyon mappedthe contactas a Mesozoic thrustfault. Nevertheless, Range, which may represent the breakaway of the SDD these range-boundingfaults led Wills and Anders [1999] to (Figure 1), a model contestedby Wills and Anders [1999]. At propose a tectonic model envisaging the uplift and the core of these contrastingviews are the Tertiary coarse- exhumationof the Canyon Range entirely as a simple horst clastic sedimentsof the Oak City Formation in the western block bound by high-anglenormal faults on the westernand foothills of the Canyon Range and in particularthe age and easternflanks of the range. nature of the basal contact with the pre-Mesozoic basement [Hintze, 1991b; Otton, 1995; Morris and Hebertson, 1996; 3. Apatite FissionTrack Thermochronology Lawtonet al., 1997; Willsand Anders,1999] (Figures I and 2). The sedimentarycharacter and stratigraphicarchitecture Apatitefission tracks are lineardamage trails in the crystal of the Oak City Formationand its large rock avalanche latticethat form as the resultof spontaneousnuclear fission of STOCKLI ET AL.: MIOCENE UNROOFING OF THE CANYON RANGE 293

trace238U nuclei [Fleischer etal., 1975; Wagner and Van den that form at the polishedand etchedsurface of apatitegrains. haute, 1992]. Use of apatite fission track data for The size of thesefigures is in part a functionof the bulk etch reconstructingcooling and unroofinghistories relies on the rate of the particularapatite grain. The bulk etch rate is a factthat new 238U fission tracks form at an essentiallyfunction of apatite composition, with higher-Cl grains constantrate and with an essentiallyconstant initial track generallyetching faster than end-member(F, CI, OH)-apatite. length,while older tracksare simultaneouslyannealed (and Etch figure data are collectedby measuringand averagingthe ultimately erased) at elevated subsurfacetemperatures maximum dimensionsof etch figures of three or four near- [Gleadowet al., 1986; Green et al., 1989b; Dumitru, 2000]. verticalfission tracks in each apatitegrain. AbsoluteDpar The annealingprocess causes easily measuredreductions in values depend upon etching conditions;etching with 5.5 N both track lengths and fission track ages [Naeser, 1981; HNO3 for 20 s at 21ø-22øCplaces the boundarybetween large Gleadowet al., 1986; Greenet al., 1989a,1989b; Corrigan, Dpar,Cl-apatite and smallDpar, (F, C], OH)-apatiteat a Dpar 1993]. At temperatureshotter than -110ø-150øC, all fission value of-2.0 #m [Donelick et al., 1999]. In this study, we tracksare totally annealed,resetting the fissiontrack clock to etchedCQ, CO, and CC samplesusing 5.5 N HNO3 for 20 s at zero. Tracksare partially annealedbetween -60 ø and -110øC, 22øC, identical to the conditionsof Donelick et al. [1999]. All a temperaturerange termedthe partial annealingzone (PAZ). BR sampleswere etchedusing 5.0 N HNO3 for 20 s at room Below -60øC, fissiontracks in apatiteare effectivelystable, temperature(20ø-23øC) (Table 1). andannealing occurs only at very slowrates [e.g., Glea'dow et Track length distributionsare also affected by kinetic al., 1986; Vrolijk et al., 1992]. variationssince track lengthsin Cl-apatite grainsare reduced Apatitefission track thermochronologyis an effectivetool at higher temperatures. If Cl-apatite grains were partially for directly dating the exhumationand cooling of footwall annealedprior to exhumation,they would be characterizedby rocks in responseto extensionalfaulting and many studies shortermean track lengths,reducing the overall mean length have successfullyutilized this approachto investigatethe of the total population. However, both etch figure size and low-temperaturecooling historiesof exhumedfootwall rocks track lengthsare a function of bulk etch rate, which implies in the Basin and Range province [Foster et al., 1990; that large etch figure lengths in Cl-apatite correspondto Fitzgerald et al., 1991; Gans et al., 1991; Howard and Foster, slightly longer initial track lengths[see Burtner et al., 1994, 1996; Miller et al., 1999; Stockliet al., 2000]. In this study, Appendix 2; Carlson et al., 1999]. Therefore rapidly cooled the techniqueis employedto constrainthe exhumationhistory Cl-apatite grains, such as from volcanic samples, are of the CanyonRange and to evaluatethe implicationsof the characterizedby slightly longer mean track lengthsthan (F, cooling historyof the presumedfootwall of the SDD for the C1, OH)-apatites from the same sample. This is also regionaltectonic evolution. suggestedby the fact that track lengthsand etch figure values in the commonlyused Fish CanyonTuff apatitestandard are 3.1. Apatite CompositionalVariations larger than those in Durango apatite, which has a lower C1 The annealingrate of fissiontracks and the temperatureat content [Carlson et al., 1999]. which all tracks are totally annealedcan be correlatedwith apatitechemistry, principally C1 content[Green et al., 1985, 3.2. Fission Track Analysis Results 1986, 1989b], or etchingcharacteristics using the meanetch A total of 16 samples (-20-30 kg) were collected and pit diameterparallel to the crystallographicc axis (parameter analyzedfrom the westernfront of the Canyon Range, from Dpar)[Burtner et al., 1994;Carlson et al., 1999]. Estimates an east-westtransect across the centralpart of the rangealong of the total annealingtemperature of relativelychlorine-rich, Oak Creek, and from Cretaceoussynorogenic conglomerates large Dpar apatitesrange from -110 ø to 150øC[Green et al., from the northeasternflank of the range (Figures 1 and 2). 1989b; Corrigan, 1993; Ketcham et al., 1999], whereas Twelve of the samplesare quartzitesfrom the Neoproterozoic relativelyfluorine-rich, small Dparapatites are completely PocatelloFormation and the Lower CambrianTintic quartzite. annealedabove temperaturesof-90ø-110øC [Laslett et al., Apatite yields from these units were commonlyvery small. 1987; Green et al., 1989b; Carlson et al., 1999; Ketcham et In particular, the compositionallymature Tintic quartzite al., 1999]. Accountingfor this rangeof kinetic behaviorsis containslittle to no apatite. An additional 10 sampleswere importantfor the analysisof samplescontaining apatite grains collected but could not be analyzed owing to insufficient spanninga rangeof C1 and/orDpar values. In particular,if apatiteyields. One samplewas collectedfrom conglomerates suchrocks undergo a protractedcooling history, large Dpar, in the lower part of the Miocene Oak City Formation. Apatite Cl-apatitesyield olderfission track apparent ages and exhibit fission track ages and mean confinedtrack length data from a differenttrack length distribution than small Dpar, (F, C1, all samplesanalyzed are presentedin Table 1. Apatite fission OH)-apatiteseven thoughthey experiencedidentical thermal track data presentedin this studywere initially collectedas histories. Thus calculatingan apparentapatite fission track part of two independent studies using slightly different age or track length distribution of a sample without analytical parameters (Table 1). Specifically, etching determiningthe compositionalvariations within the sample conditionsvary, slightly affecting the etch figure technique may lead to erroneousinterpretations. This is particularly which was employed to estimate variability of apatite critical in sedimentaryrocks that commonlycontain detrital annealingkinetics. apatitederived from severalprovenance regions. The averaging effect of lumping multikinetic or In this study,the etchfigure methodof Donelick[1993; see multicompositionalage and length populationsmay obscure also Burtner et al., 1994; Carlson et al., 1999] was used to the thermal history information recorded by a sample. assessthe annealingkinetic variationsof individual apatite Apparent apatite fission track ages are plotted againstetch grains. Etch figures are the cross sectionsof fission tracks figure lengthsfor six selectedsamples (Figure 3), which were 294 STOCKLI ET AL.' MIOCENE UNROOFING OF THE CANYON RANGE

+l +l +l +l

oo • o.o.•.•.•.•oo •.• • • • o o o o • •• STOCKLI ET AL.' MIOCENE UNROOFING OF THE CANYON RANGE 295

140 CQ95-1 CQ95-2 CQ95-3 20.0 .+_2.4 Ma 18.8 _+2.7 Ma 18.0 _+2.2 Ma 120

100 P(X2)=35% •, • 80 P(X2):10%•,• X P(X2)=11% • 60 < 40

0 [ ß i ß [ ß [ ß , . .,I.•',..'..•' '' ...... ß i ß i ß 140 CQ95-5 X CQ95-6 CQ95-7 18.1 _+ 2.4 Ma 120 17.3 _+2.6 Ma P(X2) = 81% P(X2) = 1% P(X2)=100% 100 23.9_+4.4Ma •

•o 60 < 40 x 20 ,, ø• X ß o.s 1'.0 f.5 ' •.0 i.s 3:0 3'.s o.s i.o i.s' 2.0 2.5 3:0 i.s o.s f.0 1'.5 2.0 •.5 3'.0 3'.5 Etch Figure Length (pm) Etch Figure Length (!am) Etch Figure Length (•m) Figure3. Plotsof single-grainfission track age versus mean etch figure length. The meanetch figure length is an averageof threeto four etchfigures of subverticaltracks measured from eachapatite grain. Etchfigure length is inferredto be partlycontrolled by thechlorine concentration of the grains analyzed; that is largeretch figure lengths correspondto greaterchlorine concentration [Donelick, 1993; Burtneret al., 1994]. The diagramsillustrate the systematicrelationship between age dataand compositionalvariations in multicompositionalindividual samples. All Neoproterozoicand Cambrianapatite fission track ages(Table 1) were calculatedfor (F, C1, OH)-apatite populationsonly, omittinglarge Dparhigh-C1 grains (crosses) which retainolder and highertemperature age information.Ages given in diagramsare central ages of (F, C1,OH)-apatite population only. Thehatched area (etch figurelengths -2-2.3/xm) corresponds to the compositional transition between small Dpa r (F, C1,OH)-apatites and largeDpar Cl-apatite grains [Buttnet et al., 1994]. The samecriteria were applied to confinedtrack length data (see Figure 5). etchedusing etchingconditions of Donelick et al. [1999], in plot of all single-grain ages from 12 Neoproterozoic and order to assessthe influenceof kinetic variability and to Cambriansamples. Approximately95% of the grainscluster graphicallyseparate large Dpar Cl-apatite and small Dpar (F, C1, tightly aroundan age of-18.6 Ma, while -5% plot as outliers OH)-apatitepopulations. Dpar values were measured for all 12 with a mucholder averageof- 100 Ma. Neoproterozoicand Cambrian samplesand most of these Because of the relatively small apatite yields and young samples(except 97BR001) containfew of Cretaceoussingle- ages, the sample mounts used for age determinationsyield grain ageswith large etchfigure lengths(Figure 3). However, insufficienthorizontal confined track lengthsto permit useful the structurallydeepest sample from the lower Oak Creek track length data to be collected. Additional apatite grain area(CQ95-3) contains several large Dpar apatite grains that mountswere made from six samples and exposed to a 2S2Cf yield Miocenesingle-grain ages that are statisticallyidentical fission fragment source under vacuum and subsequently to agesfrom apatites with smallDpar values (Figure 3). This etched in order to reveal more spontaneousconfined tracks observationimplies that apatitesfrom the deepeststructural [Donelickand Miller, 1991; Donelick et al., 1992] (Figure 5). levelsexposed along the westernrange flank werecompletely The observed mean track lengths from these additional resetindependent of their compositionand annealing kinetics, mountsare listedin Table 1. Dparmeasurements were also indicatingtemperatures of >_150øC. carried out to investigatethe influence of kinetic variability In order to avoid compositionaleffects, single-grain ages on track length distributions. Comparisonof the track length characterizedby largeDpar values were not includedin the data from the two differentDpar populations suggests that calculationsof apparentfission track ages for the twelve meantrack lengthsare often statisticallyindistinguishable, but Neoproterozoicand Cambrian samples(Table 1). The older that length distributionsexhibit small differences(Figure 6). agepopulation that is characterizedby largeDpar values can (F, C1, OH)-apatite lengths are characterizedby symmetric be graphicallydistinguished using a radialplot, whichis used and unimodal distributions with a mode that tends to be to isolatedifferent fission track age populationsin a suiteof slightly shorter than the mode of the Cl-apatite lengths, single-grainages [Galbraith, 1990]. Figure4 showsa radial whereasthe Cl-apatite lengths are characterizedby broader 296 STOCKLI ET AL.' MIOCENE UNROOFING OF TIlE CANYON RANGE

500 Three samples from Cretaceous synorogenic smalllargeDp• high(F, CI,C1 OH)apatite apatite • 300 conglomeratesfrom the northeasternCanyon Range yield substantiallyolder apatitefission track apparentages ranging _+2.uncertainty .• 150.. 100 Ma from 96.8 _+10.6 to 41.7 _+4.7 Ma (Figure 7). The ages of &200 thesethree sampleswere calculatedusing all single-grainage X,X.., 100 e. -' -.80 • data, includinglarge Dpargrains. Apparentages appear to increasesystematically and continuouslyfrom ~ 18 to ~97 Ma ,,)(....• •'"-")• -760•. from west to east with increasingdistance from the western •::":ii!i!ii!i!:::.•:;:!::iiiii!i ...... :. •...... 0 go rangefront, exposingan exhumed,east tilted Miocenepartial iiiiii•;=::•,•!:!iiiii •il;::=•:=-..':...... =•i• 0• annealingzone (Figure 7). ß20 • 3.3. Apatite Fission Track Modeling Observed track length distributionsand age data were ...... •::7718.6•0.6 Ma modeled with the program Monte Trax [Gallagher, 1995] using the experimentally derived (F, C1, OH)-apatite annealing data of Laslett et al. [1987]. Model-predicted , , , , , , •10 apatite fission track parameters obtained from thermal 1 2 3 4 5 6 8 histories generated by Monte Carlo-style simulations are precisionindex comparedto the observedfission track data. In this study,(F, C1, OH)-apatite age and track length data were modeledin Figure 4. Radial plot of combinedapatite fission track order to constrainthe thermal history and the exhumationof single-grainages from all 12 Neoproterozoicand Cambrian the Canyon Range in a more quantitativefashion. Only age samplesof the CanyonRange, Utah. In radial plots, grain and track lengthdata from (F, CI, OH)-apatitewith Dpar agesare read by projectinga line from the origin throughthe values of <2.0 ktm were used in the model simulations. No data point onto the radial age scale [Galbraith, 1990]. All largeDpar high-C1 grains were observedin sample97BR001, grainsin radial plots have error bars of equal length, and so all available length data could be includedin the fission grains with more precise ages plot farther to the right. If track modeling. Figure 8 showstwo examplesof fissiontrack single-grainages are statisticallyconcordant, grains cluster modeling results representativeof the western and central within a _+20 wide swath. Radial plots are useful for Canyon Range. The resultscorroborate the inferencebased graphically separating and displaying different age on the track length distributions that the Canyon Range populationsin a suiteof single-grainages. Large Dpar high-C1 underwent rapid cooling and exhumation in the early to single-grainages plot as outliers relative to the rest of the middle Miocene. All modelruns for individualsamples yield ages,which define a linear age trend with a weightedmean consistentestimates for the timing of exhumationbetween age of 18.6_+0.6Ma. Tight clusteringof all single-grainages -19 and 15 Ma. The modeling resultsare discussedin detail in the plot indicate that the entire Canyon Range footwall in section 5. blockcooled very rapidly starting at -19 Ma. The older,large Dparhigh-Cl populationdefines an older age trend of-100 Ma, which appears to correlate with earlier cooling in 4. Exhumation During Mesozoic Sevier responseto thrustingin the Sevierfold and thrustbelt. Thrusting In Cretaceous time the Canyon Range thrust sheet underwentsignificant deformation and exhumationas a result and negativelyskewed distributions owing to the occurrence of motion alongthe PVT in late AptJanto Albian time (-115- of a few track lengthsbetween 9 and 12 /•m. Theseshort 98 Ma) [DeCelleset al., 1995]. In particular,the formationof tracklengths (<12 #m) comefrom highCl-apatites with etch antiformalduplexes along the Pavant thrustprobably caused figure diameters>3.0 /•m, suggestingthat those high-C1 folding of the CRT and exhumationof the overlying thrust apatitegrains were partiallyannealed before rapid coolingin sheet [Sussman,1995]. This is supportedby microstructural the Miocene or were more likely to preservea partially observations[Mitra et al., 1994; Sussman, 1995] that indicate annealedCretaceous thermal signal (Figure 6). rocks in the Canyon Range underwent deformation and The Neoproterozoicand Cambriansamples from alongthe folding at relatively shallow depths (<2 km) during westernrange front and from the east-westtransect across the progressiveCretaceous unroofing. This suggeststhat the Canyon Range at Oak Creek yield statisticallyconcordant Canyon Range and Pavant thrust sheets must have been apparentages (Figure 7) with a weightedmean age of 18.6 _+ deeply eroded before subsequent reburial by several 0.6 Ma. The samplesfrom the westernand centralCanyon kilometers of wedge-top sedimentsof the Canyon Range Rangeare characterizedby long mean track lengths,ranging Formationduring Late Cretaceousto early Tertiary eastward from 13.3 to 14.2/•m, indicatingthat theserocks underwent propagationof thrusting[Royse, 1993; DeCelles et al., 1995; rapid cooling starting at-19 Ma. A clast of Mutual Lawton et al., 1997]. Formationcollected from a conglomeratein the Oak City Apatite fissiontrack single-grainages from pre-Mesozoic Formationto the west of Oak Creek yields an age of 20.3 _+ rocksthat are characterized by largeDpar values yield ages of 6.0 Ma, statisticallyconcordant with agesobtained from the -100 Ma (Figure 4). These grainswere partially or not at all Neoproterozoicand Cambrianbedrock samples. reset during subsequentreburial and appear to record the STOCKLI ET AL.: MIOCENE UNROOFING OF THE CANYON RANGE 297

CQ95-1 I CQ95-2 F CQ95-3 I Mean13.81 + 0.15 pm • Mean 13.94 ñ 0.24 IJm Mean13.66 ñ 0.13pm I StdDev 1.31 pm • Std Dev 1.10 pm StdDev 1.25 pm I n=81 I n=21 I_n=87 I

I I ! I [ I I I

CQ95-6 Mean 13.70 ñ 0.19 pm • Mean 13.28 + 0.31 pm Mean13.70 ñ 0.25 pm • StdCQ95-4Dev 1.27 pm I Std Der 1.70 pm • CQ95-7StdDev 1.63 pm I n= 44 n= 30 _n=44 I _

I ! I I ! i I I I I [ I I I I I I I ! I I ,,I

97BR-001 I Mean 14.21 ñ 0.31 t•m 1 -- Sample number StdDev 1.41 pm I Mean track length _n=20 I Standarddeviation -- Tracks counted

! I I i [ I I ! I ,,I 0 5 10 15 iI Track length (IJm)

252 Figure 5. Apatitefission track length distributions of Cf-exposedsamples (CQ) andone sample(97BR001) used for agedetermination from the westernand central Canyon Range illustrating the longmean track lengths indicative of rapid coolingand exhumation.All lengthsare shown,independent of apatitecomposition. See Figure6 for lengthdistributions with respectto differentC1 contentsand Figure 8 for fissiontrack modelingresults and more quantitativetreatment of the Cenozoiccooling history of the CanyonRange.

timing of erosional unroofing of the Canyon Range and Range conglomeratein the northeasternCanyon Range record Pavant thrust sheets in response to motion along the an even more complete unroofing history of the Canyon underlying PVT. This is consistent with independent Range thrust sheet. Multicompositionalapatite fission track estimates for the onset of subsequent wedge-top data from quartzite cobbles within the Canyon Range sedimentation and deposition of the Canyon Range conglomerate(samples CC95-1, CC95-2, and CC95-3) were conglomerate starting in Cenomanian times (-95 Ma) modeled by Ketcham et al. [1996]. Their modeling results [DeCelles et al., 1995]. Samplesfrom portionsof the Canyon indicate that the CanyonRange thrust sheetunderwent a two- 298 STOCKLI ET AL.: MIOCENE UNROOFING OF THE CANYON RANGE

CQ95-1 (D.• <2 pm) CQ95-1 (D.•>2 pm) • Mean 13.88 _+0.19 Pm Mean13.93 _ 0.18 Pm I

Std Der 0.97 Pm StdDev 1.37 Pm •I skewness 0.037 skewness-0.795 I n=28 n:53 I I ,,

high CI apatite

I I I I I I I I

Figure 6. Representativeapatite fission track length distributions of (F, C1,OH)-apatite (etch figure width (Dpar) < 2 ttm) andCl-apatite (Dpar >2 ttm) froma sample(CQ95-l) in thecentral Canyon Range. Small Dpar (F, C1,OH)- apatitelengths exhibit a unimodaland symmetric distribution, whereas large Dpar Cl-apatite lengths are characterized by a negativelyskewed distribution, as a resultof trackshortening and partial annealing of high-C1apatite grains.

stageCretaceous cooling historyin responseto coolingand quantifythe maximum post 100 Ma burialtemperature and to exhumation associatedwith first thrusting along the CRT estimate the amount of Cretaceous and Tertiary sediment (-146 Ma) and subsequentlywith thrusting along the PVT depositedon top of the deeplyeroded Canyon Range and (-100 Ma) [Linn, 1998; this study]. PavantRange thrust sheets. Assuming a geothermalgradient The presented fission track data also have interesting of -25øC and a meanannual surface temperature of 10ø _+ 5øC, implicationsfor the reconstructionof the thrust belt in the estimatedmaximum burial temperaturesof - 110ø-130øC CanyonRange region. Whereaslarge Dparapatites from would require -4-4.5 km of Cretaceousand Tertiary samplesbelow the depositionalcontact of the CanyonRange overburdenprior to Tertiary unroofing. These new conglomerateare not fully or only partiallyreset, small Dpar thermochronologicalconstraints and publishedstratigraphic apatitesare totally annealed. This observationallows us to information [e.g., Lawton et al., 1997] contradictthe

110

lOO

80 exhumed

• 70 60 • 50

< 40

30 18.6 _+0.6 Ma /

W E o 5 ;, •, i• i0 f2 Distancefrom westernrange front

Figure7. Diagramof apparentapatite fission track ages plotted against horizontal east-west distance measured fromthe western range front of theCanyon Range. Apatitefission track ages within 8 km of therange front are essentiallyinvariant and indicate major cooling and fault slipalong the Sevier Desert Detachment (SDD) startingat -19 Ma. The weightedmean of thesecompletely reset samples yields an ageof 18.6_+ 0.6 Ma. Threepartially annealedsamples from the eastern flank of theCanyon Range define an exhumed partial annealing zone (PAZ). STOCKLI ET AL.' MIOCENE UNROOFING OF THE CANYON RANGE 299

o (>_140øC) translates into >_5.6 km of extensionalunroofing 97BR001 between -19 and 15 Ma and a vertical exhumation rate of 20

., -1.4 mm/yr for the structurallydeepest samples. Apparent 40 fission track agesincrease systematically from 18 to -97 Ma west to east with increasingdistance from the westernrange T(oc)60 front (Figure 7). This systematictrend in apparentages is 80 consistent with the existence of an exhumed east tilted 100 Miocene PAZ along the easternflank of the Canyon Range

120 (Figure 9). Asymmetriceastward tilting of the CanyonRange is alsoevident from the fact that the structurallydeepest levels 25 ;o ; o (antiformalduplex in the Pavant thrust sheet)outcrop along Time (Ma) the western flank and that the structurallyshallowest levels

0 (synorogenicCanyon Range Formation) are exposed to the CQ95-1 east of the range. This spatial distributionof fission track agessuggests that extensionalunroofing of the CanyonRange was accommodatedby a top down to the west normal fault alongthe westernrange flank (Figure7). T(oc)60- These apatite fission track data allow us to critically 80- evaluate the competing tectonic models for the Tertiary - unroofing of the Canyon Range and its implicationsfor the 100 - evolutionof the adjacentSevier Desert basin. A simplehorst 120 - model for the Canyon Range first proposedby Campbell

25 20 15 lOI I 51 i o [1979] and later by Holladay [1984] and recentlyrevived by Time (Ma) Wills and Anders [1999] suggeststhat uplift of the range is controlled on both flanks by high-angle normal faults with Figure 8. (F, C1, OH)-apatite fission track modeling results -1.2 and -1.5 km of fault offsets. However, thesefaults by of two representativesamples from the centralCanyon Range. themselvescan only accountfor a fraction of the observed Modeling resultsfor both samplesillustrate rapid coolingand extension-inducedcooling, although the presentphysiography exhumationof the Canyon Range between-19 and 15 Ma. of the rangeis in part controlledby theseyounger high-angle The suggestedcooling rates (-10-20øC/m.y.) are inconsistent normal faults that give it the appearanceof a symmetrical with erosional denudation and strongly support cooling horst block. However, the initial large-magnitudeunroofing becauseof tectonicunroofing of the CanyonRange relatedto of the CanyonRange cannot be wholly ascribedto thosehigh- motion along the SevierDesert Detachmentfault. Note that angle normal faults but requiresa moderatelywest dipping this modeling does not constrain cooling history at normal fault along the western range flank in order to temperatures> 110øC(all tracksin (F, C1, OH)-apatiteerased). accommodate the >_5.6 km of exhumation documented above. Given this observed magnitude of footwall exhumation,a 60ø high-angle normal fault would need to assessmentof Wills and Anders[1999], who suggestedthat accommodate>_6.5 km of normalslip. Such a scenario the Canyon Range did not experience substantiallate would likely result in a large half graben along the western Cretaceousand earlyTertiary reburial. range front and in significantfootwall rotation, inconsistent with geological and geophysicaldata. In fact, geometric 5. Tertiary Exhumationof the CanyonRange constraintson fault bedding cutoff angles argue against a high-anglenormal fault and suggesta west dipping normal 5.1. Unroofing History of the Canyon Range fault with an initial dip of 350-40ø . The easiest way of Apatite fission track age data and fissiontrack modeling accommodatingthe requiredfault slip is to feed displacement resultsfrom the CanyonRange constrainthe timing and into a low-angledetachment fault to the west of the Canyon magnitudeof coolingin responseto extensionalunroofing Range as suggestedby Otton [1995] and Coogan and subsequentto Late Cretaceousand early Tertiary reburial. DeCelles [ 1996]. The dataindicate that the rangeunderwent rapid exhumation In light of this, we proposea tectonicmodel that takesinto between-19 and 15 Ma (Figure8). The rapidcooling rate is accountthe geometricrelationships in the CanyonRange, the evidencedby the observedlong and unimodaltrack length constraints on timing of faulting, and magnitude of distributionsand by the fissiontrack modeling results, which exhumationand that honorsthe entire cooling history of the suggestthat pre-Mesozoicrocks from nearthe westernrange Canyon Range (Figure 10). Our model for the tectonic frontcooled from _>150øC to -50øCduring that time interval. evolutionof the CanyonRange is characterizedby (1) rapid The preextensionalMiocene paleogeothermalgradient for the early to middle Miocene cooling and exhumation of the northernBasin and Range provinceis estimatedat-25øC/km CanyonRange as the breakawayregion of the SDD startingat and appearsto be regionally uniform acrossmuch of the -19 Ma and (2) subsequentsmall-magnitude, high-angle province [Stockli, 1999]. Given this pre-extensional normal faulting along the easternand westernflanks of the geothermalgradient and assuminga mean annual surface Canyon Range giving the range a horst appearanceand temperatureof 10ø _+5øC, the observedamount of cooling overprintingthe SDD breakaway. The small degree of post 300 STOCKLI ET AL.' MIOCE• UNROOF!NG OF THE CANYON RANGE

20 t Cenozoicnormal fault t with ~1.2 km offset 4O W meanannualSUrfacetemperature10øñ5øc IJ [Holladay,1984] 60

lOO

120

140

Cl-apatitesreset below ~150øC 160 correctedprefaulting temerpatureestimates AT Az dT vertical +,XT offset: 180 Ax Ax dz dT +AT= 1.2km * •' 200 0 2 4 6 8 10 12 Distancefrom westernrange front [km] Figure9. Diagramshows preextensional temperature estimates derived from multicompositional apatite fission trackdata plotted against distance from western flank of theCanyon Range. Temperature estimates for thethree easternmostsamples are corrected for -1.2 km of offsetalong the eastern range-bounding normal fault [Holladay, 1984].Assuming a geothermal gradient, preextensional temperature estimates serve as a proxyfor structural depth andcan be used to calculatethe amount of footwalltilting. Given a geothermalgradient (dT/dz) of-25øC/km, the systematicspatial trend in temperatures(AT/Ax) translates into -15ø-20ø of footwalltilt.

15 Ma uplift of therange is alsosuggested by thefission track simplicity, we ignore small differencesin elevation between modelingresults and is consistentwith the displacement individualsamples, a reasonableassumption given the limited estimatesfor the high-anglenormal faults along both range topographicrelief along our sampling transectacross the fronts(Figure 8). CanyonRange (Table 1). Assuming a geothermalgradient, these preextensional 5.2. FlexuralTilting of the CanyonRange temperatureestimates can be used as a proxy for structural Multicompositionalapatite fission track data allow us to depth exposedin the footwall block (Figure 9). Thus the estimatethe preextensional temperatures experienced by each slopeof the line AT/Axthat is definedby the preextensional sample. The geographicdistribution of thesetemperature temperature estimates as a function of distance from the estimates permits us to constrain the amount of footwall westernrange front (Figure9) is given by: tilting in the CanyonRange (Figure 9). The structurally A T Az dT lowestsample (97BR001) does not contain any large Dpar CI- = (1) apatite,but thetotal resetting of all (F, C1,OH)-apatite prior Ax Ax to Mioceneexhumation indicates that the sampleresided at temperatures>_110øC before extension. The deepest andthe amount of footwalltilting (I) is givenby, multicompositionalsample from nearthe mouthof Oak Creek containscompletely reset Miocene (F, C1, OH)- and C1- Ax dT apatite,indicating temperatures _> 150øC (Figure 9). In O-sin-'IAT dzI (2) contrast,samples from the central and western Canyon Range showa differentbehavior, with (F, C1,OH)-apatite recording where T is the preextensionalburial temperature,x is Miocenecooling and Cl-apatiteonly partiallyor not at all horizontaldistance measured perpendicular to the western reset,suggesting that thesesamples resided in a relatively rangefront, dT/dz is theestimated geothermal gradient, and z narrow preextensionaltemperature window between -110 ø is the structuraldepth. A geothermalgradient of-25øC and >_150øC (Figure 9). In addition,partially annealed suggests15ø-20 ø of eastwardtilting of the CanyonRange in samplesfrom the CanyonRange conglomerate in the eastern responseto rapidexhumation between -19 and 15 Ma, given foothillsallow the estimation of preextensionaltemperatures a AT/Axvalue of-8øC/km (Figure9). decreasingfrom-80 ø _+ 10øC to about60 ø _+ 10øC up section. This amountof tilting is consistentwith the simple The amount of footwall rotation can be estimated from the observationthat structurallydeeper levels outcrop along the systematicincrease in maximumburial temperaturesfrom westernrange flank and the structurallyshallowest levels are westto east,assuming a steadystate geothermal gradient and exposedto the east of the range, suggestingan east tilted horizontal preextensionalisotherms (Figure 9). For crustalsection. The tilting estimateis alsosupported by the STOCKLIET AL.: MIOCENE UNROOFINGOF THE CANYON RANGE 301 geometry of Mesozoic contractionalstructures and by the antiform-synformpair in the breakawayregion (Figures 10 attitude of the postthrustingCanyon Range conglomerate and 11). alongthe easternflank of the range [e.g., Lawton et al., 1997]. The geometryof the Cricket Range can be reasonably inferred from the surface geology and geophysicaldata, 5.3. Initial Geometry of the SDD Breakaway which indicate a low-angle hanging wall cutoff by the detachmentand a subparallelismbetween pre-Mesozoic and If one assumesthat the contact between the Tertiary Oak City Formation and the underlying pre-Mesozoic strata is earlyTertiary strata. The hypothesizedfootwall cutoff in the indeed the SDD as suggestedby Otton [1995], then knowing CanyonRange is thesteeply dipping CRT, creatinga possible the amount of flexural tootwall rotation (15ø-20ø) from incompatibility between hanging wall and footwall unloadingallows us to estimatethe initial fault angle of the geometries. However, an initial fault dip of-35o-40 ø combined with -20 ø of flexural footwall rotation of the SDD in the Canyon Range breakaway region. The basal contact of the Oak City Formation dips -18ø-24% implying CanyonRange and -25o-30 ø of eastwardtilting of theCricket Mountains due to hanging wall rollover allow the that the SDD had an initial dip of-33o-44 ø. One arrives at similar initial fault dip estimates by projecting the KT reconciliationof potentialincompatibilities when restoring the conglomerateson the east side of the range upward, which preextensionalgeometry of the breakawayand suggestthat Otton [1995] demonstrated(Figure 2). The initial fault the CanyonRange syncline was originally a relativelyupright trajectoryof the SDD, basedon the cutoff angle betweenthe fold priorto Tertiarytilting (Figurel 0). fault and the postthrustingearly Tertiary conglomerates,is 5.4. Tertiary Oak City Formation and Fool Creek -40o-45ø . Furthermore, if the initial trajectory of the Conglomerate detachmentis drawn on the restoredpreextensional section such that the base of the Tintic appearsin the footwall, an As discussedabove, Orton [1995] assigneda tentative initial dip of 35o-45ø is achieved, suggestingthat the SDD Mioceneage to the Oak City Formation,but this -12 Ma age initially cut obliquely (-40 ø) across an upright (vertical) mostlikely representsa minimumage for the conglomerates

Cricket Mountains SevierDesert secondary CanyonRange E breakawaY(?)• / -20kin ,// • '

SDD 0 1 2 3 4 5

[km] (VE = 1:1)

SevierDesert basinsediments (Miocene- Pliocene) i•."•]Oak City Formation (Miocene) '-'Y•-•--•Fluvio-lacustrine sediments(Oligocene) X!.y•Sevier foreland basin(Cretaceous - earlyTertiary) Cri•1ock • x,,. [ • "---fu_/' • tutrar e ce -•/2--'• CanyonRange ----]Precambrian- Paleozoic strata(undifferentiated) block

Figure10. Simplifiedtectonic cross sections of thecentral Sevier Desert basin. (a) Present-daygeometry of the CanyonRange and Cricket Mountains. Separation of CanyonRange thrust (CRT) cutoffs in theCricket Mountains and CanyonRange indicates -45 km of normalfaulting along the SDD. Depthto SDD eastof the Cricket Mountainsblock is takenfrom VonTish et al. [ 1995]. The geometryof the fossil,preextensional apatite fission trackpartial annealing zone (Fr PAZ) illustratesthe amount of flexuralfootwall rotation of theCanyon Range and hangingwall rolloverin theCricket Mountains block in responseto faultslip along the SDD. (b) Restoredpost- Mesozoiccontractional geometries of the CanyonRange and Cricket'Mountains blocks prior to inceptionof extensionalfaulting (late Oligocene). The reconstruction indicates that the synform in theCanyon Range and the antiformin theCricket Mountains formed an uprightfold pairthat was cut obliquely by theSDD at an initialfault angleof -35o-40ø. Thicknessof Eocene-Oligoceneoverburden is estimated from multicompositional fission track data (see text for details). 302 STOCKLI ET AL.' MIOCENE UNROOFING OF THE CANYON RANGE

W secondarybreakaway? CanyonRange E \ .- •/HouseRange •I"Cricket Mtns •o SDD .o•'•"•øø •s •: .• •

::'.)':!::.::-'-c.:'.'..:.-.:: :..'::'..':;::.:::'..i,'.:..:.'.::...'.: :}::::.::.!;3:'"" ...... ' D CRT PVT PAX .-,•.,_-..';' '.••-•S•DD presentday(after Cenozoic extension)

Neogenebasin fill SevierCulmination CanyonRange Culmination •'?,•:d•..•Oligocene lake sediment I / CRT PVT PAX .:• Mesozoicforeland basin ['--'] undiff.Paleozoic

•.•]ß Eo-CambrianPrecambrian andsediments ":-q'i-- . -..'- [-• crystallinebasement 0 20 er Meso sting) I km I .:i.:"....:'•' -- e•ee(•aft Zoi•t•u (VE = 1:1)

modified after DeCelles at al. [1995] and Mitra and Sussrnan[1997]

Figure11. Crosssections of theCanyon Range-Sevier Desert area in west-centralUtah, illustrating the Mesozoic andCenozoic kinematic history. Thrusts shown on cross sections are the Canyon Range (CRT), Pavant (PVT), and Paxton(PAX) thrusts. Rocks in theCanyon Range were uplifted and eroded during Mesozoic thrust faulting (-130- 90 Ma) asconstrained by synorogenic sedimentary deposits and apatite fission track results [Ketcham et al., 1996; thisstudy]. In theearly to middleMiocene (-19-15 Ma) theCanyon Range underwent rapid cooling and exhumation.The tectonic unroofing ofthe Canyon Range is attributed tofootwall uplift in response tomotion along the Sevier Desert Detachment.

in theOak City Formation. An apatitefission track age from represents erosional remnants of Pliocene-Pleistocene(?) a clastof theMutual Formation collected from the Oak City alluviumreworked from the CanyonRange conglomerate and Formationyields a maximumage for the conglomerateof mightbe con'elativewith alluviumunconformably overlying 20.3_+ 6.0 Ma. Thisbrackets the depositional age of theOak the Oak City Formation[Otton, 1995]. City conglomeratesbetween --20 and -12 Ma, implyingthat they are synextensionalsediments that weredeposited in a supradetachmentbasin in the SDD breakawayzone [Otton, 6. Implicationsfor DetachmentFaulting Along 1995;Morris and Hebertson, 1996] (Figure 10). the SDD The enigmaticFool Creek conglomerate(Figure 1) has historicallyinfluenced many working hypothesesfor the 6.1. Timing Constraints on the SDD tectonicevolution of the CanyonRange [Wills and Anders, Temporalconstraints on the onsetof faultingin the Sevier 1999].Campbell [1979] correlatedthe poorlyexposed and Desertbasin are basedon subsurfaceEocene to Oligocene crudelybedded conglomerates outcropping near the top of the depositsfrom --2 km depthin theGulf Gronningwell near the rangewith basal units of theOak City Formation and assigned western margin of the basin [Lindsey ctal., 1981]. a tentativeOligocene age. In contrast,Otton [1995] argued Palynologicaland zirconand apatitefission track age data thatthe conglomerates, infilling modern drainages, are much indicate an Oligoceneage of-28-26 Ma. Mitchell and youngerthan Oligoceneand, in fact, are youngerthan the McDonald [1987] suggestedthe presenceof an additional--2 MioceneOak City Formation.Although the unit so far has km of pre-28 Ma sedimentsbelow the dated lacustrineand not beendirectly dated, the exhumationhistory of the pre- volcanicsediments and speculatedthat the oldestsediments Mesozoicstrata in theCanyon Range sheds new light on the may be as old as Eocene. These28-26 Ma depositsrest potentialage of the deposit.An Oligoceneage for the Fool unconformablyon Paleozoicand Precambrian rocks and dip Creekconglomerate is unlikely since the thermochronological 18ø-25 ø to the east,with no fanninggeometries or thickness data indicatethat pre-Mesozoicrocks were buriedunder >4- changesevident in seismicdata [Von Tish et al., 1985] 4.5 km of overburdenprior to the onset of Miocene (Figure 10). This has led most studies to conclude that exhumation.Therefore we suggestthat the poorly exposed faultingin the SevierDesert and extensional faulting along and poorly consolidatedFool Creek conglomerate SDD beganafter late Oligocenetime [Lindseyet al., 1981; substantiallypostdates unroofing of therange. It mostlikely Mitchelland McDonald,1987; Coogan and DeCelles,1996]. STOCKLI ET AL.: MIOCENE UNROOFING OF THE CANYON RANGE 303

This conclusionis also supportedby the fact that Oligocene The subsurfacefission track data reportedby Allmendinger fluvio-lacustrinesediments in the SevierDesert region are not and Royse [1996] are difficult to evaluate quantitatively limitedto thebasin but outcropoutside of thebasin along the becauseof the lack of apatite track length data. Such data east tilted easternflank of the HouseRange [Hintze, 1981] would be particularlyimportant since the samplemight have (Figure 11). Exposuresof Eocene to Oligocene fluvio- undergoneminor in situ annealing at elevated down hole lacustrine sediments and intercalated volcanic tuffs occur temperaturesat a depth of--1900 m in the Arco Meadow throughoutthe eastern Great Basin and appear to represent Federal well. Anders et al. [1995] further arguedthat these remnantsof regionallyextensive, long-lived lake systemsthat fission track agesmight have been resetowing to magmatic covereda large proportionof the easternGreat Basin [Gans activity. However, such a scenarioappears rather unlikely, and Calvert, 2000]. Depositionof theseEocene to Oligocene particularly since apatite and zircon fission track ages from lake sediments is thought to be the result of regional samplesin the Gulf Gronningwell in the adjacenthanging postcollisional subsidence [Wills and Anders, 1999] or of wall samplespreserve their volcanic agesand do not exhibit crustal thinning during localized Eocene to Oligocene any evidencefor resettingdespite their spatialproximity to a extensionand crustalcollapse in the easternGreat Basin [e.g., small Quaternaryvolcanic center [Lindsey et al., 1981]. Gans, 1987; Miller et al., 1999; Gans and Calvert, 2000]. The fission track data presentedfrom the breakawayzone 6.2. Magnitude and Rate of Fault Slip Along the SDD of the SDD corroboratethe post-Oligoceneonset of faulting The Sevier Desert basin formed as the result of low-angle and directlydate the onsetof footwall exhumationand large- normal faulting along the SDD with an estimatedcumulative magnitudedisplacement along the SDD startingat -19 Ma. displacementof-38 km [Allmendingeret al., 1983; Von Tish The timing appearsto be consistentwith the onsetof major et al., 1985] or -40-47 km [e.g., DeCelles et al., 1995; Mitra extensionalfaulting in the northernmostSevier Desert region. and Sussman, 1997] based on cross-sectionalreconstructions For example, in the Drum Mountains (Figure 1), Lindsey of the Mesozoic contractionaledifice of the Sevier orogenic [1982] documented a prominent angular unconformity belt (Figs. 10 and 11). Knowledge of the timing and betweenthe 21.3 Ma Spor Mountain rhyolite and the 6.3 Ma magnitude of exhumation of the Canyon Range makes it Topaz Mountain rhyolite, suggestingthat the major extension possible to estimate the time-integrated average rate of occurred between 21.3 and 6.3 Ma. displacement along the SDD in the breakaway region, Thus it appearsthat 28-26 Ma lacustrine sedimentsfound assuming a paleogeothermal gradient and subhorizontal in the subsurfaceof the SevierDesert basin predate extension isotherms. Using a Miocene paleogeothermalgradient along the SDD and do not constrain the onset of extension. estimate for the Great Basin of-25øC/km [Stockli, 1999], the Thereforethe fission track data do not imply a time lag of-8 coolingof the CanyonRange suggests-5.6 km of unroofing m.y. betweenthe onsetof slip along the SDD and the initial in responseto slip alongthe SDD between-19 and 15 Ma at a exhumation of the breakaway in the Canyon Range, as vertical rate of- 1.4 mm/yr. Given that the SDD in the suggestedby Wills and Anders [1999]. Rather, the timing of breakaway zone appears to dip -400-35ø to the west, the exhumationof the breakawayof the SDD in the Canyon estimatedunroofing of the CanyonRange translatesinto -12- Range as constrainedby apatitefission track dating directly 10 km of fault slip along the SDD between-19 and 15 Ma at datesthe inceptionof major extensionalfaulting along the a time-integrated dip-slip displacement rate of-2.4-2.1 SDD as beginning-19 Ma. mm/yr. A lower geothermalgradient estimate would result in Reportedlate Miocene zircon fissiontrack agesof 10.8 _+ a significant increasein fault displacementand slip rates. 0.9 and 13.0 _+1.0 Ma and an apatitefission track age of 5.8 _+ Theseestimates for the early to middle Miocene slip rate and 2.2 Ma from subsurfacesamples in the westernportion of the the magnitudeof displacementalong the SDD likely represent Sevier Desert [Allmendinger and Royse, 1995] are minimum values since the fault dip decreases during considerablyyounger than apatitefission track ages from the progressivefootwall rotation (•-20ø). Subsequent,post 15 Ma Canyon Range, implying that lowerplaterocks beneaththe cooling rates for the breakawayzone cannotbe constrained western Sevier Desert basin cooled later than rocks in the quantitativelyby apatitefission track data. However, fission breakawayzone. This observationmight point to progressive track modeling suggests-1.5 km of unroofing after 15 Ma, unroofingof the SDD lower plate either in a rolling hinge which in part might be attributableto younger,active high- setting [e.g., Buck, 1988; Hamilton, 1988; Wernicke and angle faulting along the present-dayrange front [e.g., Wills Axen, 1988] or simplydue to progressiveunroofing along the and Anders, 1999]. westdipping low-angle SDD [Wernicke,1981]. It mightalso We envisagea slip history for the SDD characterizedby suggestthe existenceof secondarybreakaway zones in the (1) fault initiation at -19 Ma with a minimum slip rate of centralSevier Desert and the progressiveabandonment of the -2.4-2.1 mm/yr and (2) continuedfault slip at a comparable primary breakawayin the CanyonRange during continued rate of-2.3-1.7 mm/yr, assumingthat the remaining26-35 slipafter -15 Ma (Figure10). Sucha scenariowould explain km of total slip occurredafter 15 Ma. These rate estimates the position of the broad basementarch beneath the Sevier agreewith the maximum ratesthat can be ascribedto present Desert to the west of the Canyon Range, which has been extension across the SDD based on geodesy and attributedto isostaticfootwall rebound[e.g., DeCelleset al., paleoseismology(Niemi et al., in preparation, 2001). A 1995] (Figure11). Post-15Ma slip alongthe SDD wouldalso constant-ratemodel for the Neogenewith an averageslip rate be consistent with the presence of late Miocene of ~2 mm/yr would explainthe middle to late Miocene fission synextensionalsediments in the Sevier Desert basin and with track ages from the subsurfaceof the Sevier Desert basin paleoseismologicand GPS data indicating continued [Allmendingerand Royse, 1995], the significantpost 10 Ma extensionalong the SDD (Niemi et al., in preparation,2001). evolution of the basin based on rollover and faulting of late 304 STOCKLI ET AL.: MIOCE• UNROOFING OF THE CANYON RANGE

Miocene intrabasin basalts [Von Tish et al., 1985], and the Range and northern Snake Range indicate two discrete active fault scarpsin the middle of the basin [Hoover, 1974]. episodesof extensionalfault slip and tilting in the Oligocene However,these data do not precludea modelin which slip (37-34 Ma) and the Miocene (18-14 Ma) [Gans et al., 1991; rates vary throughtime. The slip estimatesderived from our Lee, 1995]. Fission track data from the northern and southern thermochronologicaldata do not accountfor displacement SnakeRanges and sedimentaryand volcanicsequences in the alongfaults to the westof the CanyonRange that appearto hangingwall of the SnakeRange d6collement also support a sole into the SDD, such as along the western side of the two-stagemovement history [e.g., Miller et al., 1999]. The Cricket Mountains, suggesting that the cumulative data provide compellingevidence for a major episodeof displacementalong the SDD probablyincreases t¾om east to Miocene slip along the Snake Range d6collementat-20-16 west [Wernicke,1981]. Early to middleMiocene slip along Ma, accommodatinga minimumof 12-15 km of fault slip. thesefaults would result in a significantincrease in sliprates These data indicate that the east dipping Snake Range alongthe SDD duringthat time. d6collementwas active at the sametime as rapid fault slip occurredon the SDD. This temporal overlap of rapid 7. ExtensionalFaulting in the Eastern Great extensionalong the Snake Range d6collementand the SDD Basin suggestthat the two fault systemsof oppositepolarities formed a conjugate set of faults, accommodatinglarge- The constraints on the timing and magnitude of magnitude east-west extension during early to middle exhumationof the SDD breakawayand displacementalong Miocene times. the SDD proveimportant for the understandingof extension in the easternpart of the northernBasin and Range province. 8. Conclusions Thusit is importantto understandextensional faulting along the SDD in the contextof the regionalspatial and temporal Apatite fission track results from Proterozoicand Lower distribution of extension in the eastern Great Basin. Cambrianquartzites collected from the CanyonRange reveal .Areasto the northand southof the CanyonRange appear a significantearly to middle Miocene (-19-15 Ma) cooling to have undergonesubstantial east-west extension in Miocene event in responseto rapid extensionalunroofing. Samples timethat might be kinematicallylinked to slipalong the SDD. from an east-westtransect contain older, high Cl-apatite In the Drum Mountainsat the northernedge of the north of grains,indicating that preextensional burial temperatureswere the Sevier Desert, Lindsey [1982] documentedextensional sufficientto fully resetsmall Dpar (F, C1,OH)-apatite but not faultingbetween -21 and-7 Ma, on thebasis of a prominent largeDpar Cl-apatite. This compositionallyinduced spread in angular unconformity. It is unclear, however, whether single-grainages indicates that the pre-Mesozoicrocks in the faultingin the Drum Mountainsis directlyrelated to slip Canyon Range resided in a temperaturew•ndow between alongthe SDD (Figure1). To the southof the Canyonand --110ø and _>150øCprior to Mioceneexhumation. These Pavant Ranges,the west dipping Cave Canyon detachment preextensionaltemperature estimates suggest-4.5 to >5.6 km fault, exposedin the Mineral Mountains,projects beneath the of unroofingsince the earlyMiocene, assuming a geothermal southernmostSevier Desert region [Coleman and Walker, gradientaround -25øC/km. 1994;Coleman et al., 1997] (Figure1). Thermochronological The spatial distribution of preextensionaltemperature data from the central Mineral Mountains show the late estimatesacross the rangeindicates 15ø-20 ø of eastwardtilting Miocenebatholith was exhumedduring slip alongthe Cave of the CanyonRange during rapid exhumationbetween -19 Canyondetachment from -11 Ma to the present[Evans and and 15 Ma, assuming near-horizontal preextensional Nielson, 1982; Colemanet al., 1997]. However, Price [1998] isotherms. These data suggest top down to the west claimed that significant slip along the Cave Canyon extensionalunroofing of the Canyon Range accommodated detachment occurred before 11 Ma, on the basis of by a moderatelylow-angle, west dipping extensional structure crosscuttingrelations between -11 Ma dikes and cataclasites along the westernrange flank with an initial dip of-350-40 ø. alongthe easternsegment of the Cave Canyondetachment. It The data do not supportthe hypothesisof Wills and Anders is thereforepossible that the Mineral Mountainsunderwent [1999] that the CanyonRange was exhumedas a horstbound two periodsof significantfootwall exhumationbetween 18-11 by major high-angle faults. Rather, these observationsare Ma and 7.6 Ma to present. Conservative structural consistentwith rapid early to middle Miocene cooling and reconstructionsof the Mineral Mountainsregion argue for at exhumationof the Canyon Range occurringas a result of least 20 km and up to 30 km of east-westextension [Walker footwall unroofing along the low-angle Sevier Desert and Bartley, 1991; Coleman et al., 1997]. The lack of Detachmentfault and supportthe hypothesisthat the Canyon evidence for an east-west accommodation zone between the Rangeis the breakawayzone of the SDD. An earlyMiocene Sevier Desert and Mineral Mountains domains and the similar fission track age from the lower part of the Oak City magnitudesof extensionaccommodated by the SDD and the Formation bracketsits depositionalage at 20 to 12 Ma, Cave Canyon detachmentssuggest that the two detachment implying that it representsMiocene synextensionalsediments faults are kinematically linked and part of the same that were depositedin the hanging wall of the SDD in the extensionaldetachment system. breakawayzone. These constraintscorroborate the model of The easternGreat Basin west of the SevierDesert region is Otton [1995] and Coogan and DeCelles [1996] in which the dominatedby an east dipping low-angle detachmentfault west dipping contact between the Tertiary Oak City system,the Snake Range d6collement[e.g., Miller et al., Formationand the pre-Mesozoicstrata of the westernCanyon 1983;Bartley and Wernicke,1984; Lee et al., 1987] (Figure Range is the low-angle SDD. Trenching of the poorly 1). The4ømr/39Ar andfission track data from the Deep Creek exposedcontact between the Miocene Oak City Formation STOCKLIET AL.: MIOCENEUNROOFING OF THE CANYONRANGE 305 and the underlyingpre-Mesozoic strata might eliminate any in earlyand middle Miocene times, suggesting that the two ambiguityconcerning the natureand significanceof the faultsystems may have formed a conjugateset of faults.The contact. timingof tectonicunroofing of the CanyonRange appears to The thermochronologicaldata from the breakawayzone beroughly synchronous with large-magnitude extension along directlydate the onset of large-magnitudedisplacement along the SnakeRange d6collement and with an earlierphase of the SDD between -19 and 15 Ma. Given that the SDD in the extensionalong the Cave Canyondetachment, pointing to breakawayzone appears to dip -400-35ø to thewest in the widespreadeast-west extension in theeastern. Great Basin in CanyonRange, the estimatedunroofing of the SDD the earlyand middleMiocene. The SDD and the Cave breakawaytranslates into -12-10 km of faultslip along the Canyondetachment fault form a continuousnorth-south corridorcharacterized by large-magnitudeTertiary extension SDD at an averagerate of-2.4-2.1 mm/yr. Thuswe envisage andfigure prominently into strain estimates of theBasin and a sliphistory for theSDD characterizedby faultinitiation at Rangeprovince as a whole[e.g., Wernicke, 1992]. -19 Ma with a minimum slip rate of-2.4-2.1 mm/yr and Thisstudy presents compelling evidence for theexistence of followedby continuedfault slip at a rate of-2.3-1.7 mm/yr the SevierDesert Detachment, which has been at the centerof after 15 Ma. These rates are consistent with geodetic the controversysurrounding the viability of low-angle measurementsof present-dayextension across the SDD detachmentfaulting. Thermochronologicand geologic (Niemi et al., in preparation,2001). To testwhether the slip constraintsfrom the SDD supportthat it is mechanically rates along the SDD have been constantor have varied feasibleto accommodatelarge-magnitude extension along a throughtime, additional thermochronological constraints such moderatelylow to low-angledetachment fault. asfission track, 4ømr/39Ar K-feldspar, and (U-Th)/He titanite data would be requiredto comprehensivelyelucidate the Acknowledgments.This projectwas supported by NSF grant EAR-9417939(to E. L. Miller and T. A. Dumitru) and by a coolinghistory of the SDD footwallin the subsurfaceeast of GeologicalSociety of Americastudent research grant to J. K. Linn. the Cricket Mountains. We thankR. Donelickfor permissionto usehis etch figure technique Miocene large-magnitudeextension in the easternGreat andthe TexasA&M and OregonState University reactor facilities Basin is not limited to the Sevier Desert region west of the forsample irradiations. J. K. Linnwould like to thank the University CanyonRange. The CaveCanyon detachment exposed in the of Texasat Austinfission track laboratory. We also wouldlike to thankJ. Bartley,B. Currie,P. DeCelles,R. Donelick,L. Gilley,M. Mineral Mountains projects beneath the southern Sevier Hulver,S. Klemperer,T. Lawton,E. Miller,C. Naeser,J. Orton,and Desert and is characterizedby up to 30 km of Miocene A. Sussmanfor stimulatingdiscussions and helpful insights and B. extension. To the west the east dipping Snake Range Wernicke,D. Cowan,and M. Andersfor improvingthe final version d6collementexhibits evidence for large-magnitudeextension of the manuscript.

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