Timing of Detachment Faulting in the Bullfrog Hills and Bare Mountain
Total Page:16
File Type:pdf, Size:1020Kb
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B2, PAGES 2815-2833, FEBRUARY 10, 1997 Timing of detachmentfaulting in the Bullfrog Hills and Bare Mountainarea, southwest Nevada' Inferences from 4øAr/39Ar, K-Ar, U-Pb, and fissiontrack thermochronology Thomas D. Hoisch Departmentof Geology,Northern Arizona University, Flagstaff Matthew T. Heizler New MexicoBureau of Minesand Mineral Resources, New MexicoInstitute of Miningand Technology, Soccoro Robert E. Zartmanl U.S. GeologicalSurvey, Denver, Colorado Abstract. Crustalextension in the BullfrogHills andBare Mountain area of southwestNevada is associatedwith movement along a regionaldetachment fault. Normalfaulting in theupper plate andrapid cooling (denudation) of the lowerplate were coeval with Miocenesilicic volcanism and with west-northwesttransport along the detachment fault. A west-northwestprogression of tilting alongupper plate normal faults is indicatedby agesof thevolcanic rocks in relationto angular unconformities.Near the breakaway, tilting in theupper plate occurred between 12.7 and 11.6 Ma, continuedless strongly past 10.7 Ma, andwas over by 8.2 Ma. Ten to 20 km westof the breakaway,tilting occurred between 10.7 and 10.33 Ma, continuedless strongly after 10.33 Ma, andwas over by 8.1 Ma. Thecooling histories of thelower plate metamorphic rocks were deter- minedby thermochronologicdating methods: K-Ar and4øAr/39Ar on muscovite, biotite, and hornblende,4øAr/39Ar onK-feldspar, U-Pb on apatite, zircon, and sphene, and fission track on apatite,zircon, and sphene. Lower plate rocks 10 km westof thebreakaway cooled slowly from Early Cretaceouslower-amphibolite facies conditions through 350_+50 øto 300-+50øCbetween 57 and38 Ma, thencooled rapidly from 205_+50øto 120-+50øCbetween 12.6_+1.6 and 11.1_+1.9Ma. Lowerplate rocks 20 km westof thebreakaway cooled slowly from Early Cretaceous upper- amphibolitefacies conditions through 500-+50øC at 78-67Ma, passedthrough 350-+50 øto 300-+50øCbetween 16.3_+0.4 and 10.5_+0.3 Ma, thencooled rapidly from 285_+50 øto 120-+50øC between10.2 and 8.6 Ma. Upperplate tilting and rapid cooling (denudation) of thelower plate occurredsimultaneously in the respective areas. The early slow-cooling part of thelower plate thermalhistories was probably related to erosionat theEarth's surface, which stripped off about9 kmof materialin 50 to 100m.y. Theresults indicate an initial fault dip _>30 øand a 12mm yr -1 west-northwestmigration of thelocus of rapidtilting in theupper plate. Introduction along a dipping fault segmentthat progressivelyrotates to a subhorizontalorientation; the rotatedsegment becomes inactive Many studieshave documented regional detachment (low- while the dippingsegment migrates in the directionof upperplate anglenormal) faults in areasof large-magnitudecrustal extension transport. Tilting of the lower plate rocks associatedwith the [e.g.,Crittenden et al., 1980;Davis and Coney, 1979]. The lower rotation of detachment faults has been inferred in field studies platetypically consists of myloniticplutonic and metamorphic [Axen, 1993;Axen et al., 1995;Bartley et al., 1990; Colemanand rocks that were rapidly denudedwhile the faults were active. Walker; 1994; Hoisch and Simpson,1993; Holm and Wernicke, Theorieshave been proposed to explainhow detachment faults, 1992; Manning and Bartley, 1994; McGrew and $nee, 1994; whichare presently horizontal or dipgently (<15 ø) at theEarth's Miller, 1991; $elverstoneet al., 1995; Spencerand Reynolds, surface,accommodated the denudationof middleor lower crustal 1989] and modeledin theoreticalor analogstudies [Brunet al., rocks. A numberof studies[Buck, 1988; Hamilton, 1988a,b; 1994; Buck, 1993; King and Ellis, 1990; Melosh, 1990; Spencerand Reynolds, 1991; Wernicke and Axen, 1988] have Wdowinskiand Axen, 1992]. The progressiveunroofing of the suggesteda rolling hinge model, in which movementoccurs lower plate is confirmedby thermochronologicstudies in which cooling ages become younger in the direction of upper plate •Nowat Departmentof GeologicalSciences, University of Cape transport[Hoisch and Simpson,1993; Holm and Dokka, 1993; Town,Rondebosch, Republic of SouthAfrica Holm et al., 1992; Foster et al., 1993; Lee, 1995; John and Foster, 1993; McGrew and $nee, 1994]. Copyright1997 by the AmericanGeophysical Union. Different studies have reached different conclusions concern- Papernumber 96JB03220. ing the dip angle of the fault at the time of movement. Moderate 0148-0227/97/96JB-03220 $09.00 angles(30-60 ø) have been inferredin somestudies [Davis, 1983, 2815 2816 HOISCH ET AL.: TIMING OF DETACHMENT FAULTING 116'30' 117'30' 116'45' 37'00 ILLFROG C82BM-455 JN85B-2 • BM-TH-14ab 73082-8 3•, BF-17 TSV-296-80 BF-12ab BF-TH-10 CRATER FLAT AMARGOSA A USWVH-2 DESERT PASS ,•' Low-angledashed wherenormal concealedfault, •I I Quaternaryvolcanic rocksalluvium and Moderate-tohigh-angle "----"•.......' Tertiary sedimentary and concealednormalfault, dashed where I-"-:--':.--:-:-"-/--'::'--'::•! volcanicrocks :'"'""---::'•Upper Proterozoic and ' Thrustfault F':'>-..'-'-:'...'..-".":J'':.:•:tPaleozoic sedimentary and DEATH metasedimentaryrocks Mineraldeposit 0 3 6 MILES VALLEY o Figure 1. Generalgeology of the studyarea, locations of all featuresmentioned in the text, andlocations of thermochronologysamples. Major upper plate normal faults in theFluorspar Hills andBullfrog Hills, andmajor normalfaults in the lowerplate at BareMountain are shown. TransectA-A' andfaults labeled B, C, and D are referredto in Figure8. OBM, OriginalBullfrog Mine; BGBM, BarrickGold BullfrogMine; MSM, Montgomery- ShoshoneMine; SPZ, SecretPass zone of mineralization.Geology based on work by Cornwalland Kleinhampl [1961], Frizzell and Shulters[ 1990],Maldonado [1990a], Maldonado and Hausback[1990], Mortsen et al. [1992], Wrightand Troxel[1993], and M.D. Carr(U.S. GeologicalSurvey, unpublished map, 1994). 1987;Hoisch and Simpson, 1995; McGrew and Snee, 1994; Lee, westNevada at Bare Mountain,in the Bullfrog Hills, and in the 1995; Richard et al., 1990] and dips lessthan 30ø have been FuneralMountains (Figure 1). The faultstrikes east-west along inferred in other studies[Axen, 1993; John and Foster, 1993; FluorsparCanyon in thenorthwest part of BareMountain [Carr Livacarri et al., 1993, 1995;Davis and Lister, 1988;Miller and and Monsen,1988] and along the southernpart of the Bullfrog John, 1988; Scott and Lister, 1992; Wernickeet al., 1984; Yin Hills [Ransomeet al., 1910]. In theFuneral Mountains, it strikes and Dunn, 1992]. A moderateangle is consistentwith earth- southeastalong the northeast flank and partly follows Boundary quakefocal mechanismswhich indicate that most,if not all, Canyon[Reynolds et al., 1986]. The threefault segments were activeextension occurs along moderately dipping normal faults initiallycorrelated by Carrand Monsen [ 1988]. [Jackson,1987]. Severalmechanisms have beenproposed to The goalof thisstudy was to evaluatethe temporal relation- overcomeproblems with the initiationand propagation of low- shipbetween normal faulting in theupper plate and denudation of anglenormal faults [Axen, 1992; Axen and Selverstone, 1994; the lower plate. In the upperplate, angularunconformities Brunet al., 1994;Melosh, 1990; Spencer and Chase,1989; Yin, between well-dated Miocene volcanic strataand other dated rela- 1989] andwith the apparentpaucity of low-anglenormal earth- tionshipsdescribed in previousstudies limit the age of tiltingand quakefocal mechanisms [Wernicke, 1995]. faulting.In the lowerplate, cooling histories are constructed This studyinvestigated a detachmentfault exposedin south- fromthermochronologic data(4øAr/39Ar, K-Ar, U-Pb, fission HOISCH ET AL.: TIMING OF DETACHMENT FAULTING 2817 track) generatedin this and previousstudies and usedto infer the detachment[Fridrich, 1997]. Tram Ridge is boundedon the east age of denudation.A basalticdike whichis cut by the Gold Ace and southby splaysoff the Bare Mountainfault [Fridrich, 1997]; fault (Figure 1) was alsodated in this studyin orderto limit the the south-boundingfault follows TatesWash (Figure 1). Relative age of movement. to lower plate rocks, the upper plate was extendedto the west- northwest [Maldonado, 1990b; Monsen et al., 1992], consistent Geologic Setting with shear indicatorsin lower plate mylonites in the Bullfrog Hills (T.D. Hoisch, unpublished data, 1993) and Funeral The volcanicsequences in the upperplate were eruptedmainly Mountains [Hoisch and Simpson,1993]. Maldonado [1990b] from several Miocene silicic volcanic centers located in the area interpretedupper plate relationshipsto indicatethat normalfaults north of Yucca Mountain [Byers et al., 1976; Carr et al., 1986; sole into the detachment;drilling has confirmedthis for normal Sawyer et al., 1994]. In the Bullfrog Hills, this includesthe faults at the Original Bullfrog and Montgomery-ShoshoneMines regionally extensive 14.0-Ma Lithic Ridge Tuff, 13.3 Ma (Figure 1) (D.R. Boden, written communication,1996). Carr Bullfrog Tuff, 12.7 Ma Tiva CanyonTuff, 11.6 Ma Rainier Mesa [1990] concluded that upper plate normal faulting in the Tuff, and the 11.45 Ma Ammonia Tanks Tuff (Table 1), which Fluorspar Hills is somewhatolder than in the Bullfrog Hills, total -1790 m in thickness [Maldonado, 1990b]. In addition to basedon an angularunconformity between Timber Mountain and these units, the 13.45 Ma Tram Tuff and 12.8 Ma Topopah PaintbrushTuffs in the FluorsparHills, and faulted post-Timber SpringTuff are presentin the FluorsparHills at Bare Mountain, Mountain volcanicrocks in the Bullfrog Hills. where they contributean