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

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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
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