JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. B6, PAGES 8437-8445, JUNE 10, 1990

PetrologicConstraints on the UnroofingHistory of the FuneralMountain MetamorphicCore Complex, Califomia

K.V. HODGES

Departmentof Earth, Atmospheric,and PlanetarySciences, Massachusetts Institute of Technology,Cambridge

J.D. WALKER

Departmentof Geology,University of Kansas,Lawrence

The northemFuneral Mountains, southeastern , comprise a metamorphiccore complex that lies within a regionof extremeNeogene extension but did not experiencea significantlate Cenozoic thermalevent. This circumstance,unusual among core complexesof the North AmericanCordillera, permitsdirect examination of the preextensionalthermal evolution of a portionof the Cordilleran hinterland.For pelitic schistsfrom the highest-gradeportions of the Funeralmetamorphic core, thermobarometryand thermodynamicmodeling of garnetzonation define a P-T trajectoryshowing: (1) attainmentof "peak"metamorphic conditions at 800-850K and 800-1000MPa (30-37 km depths); followedby (2) 400 to 600 MPa of decompression(15-22 km of exhumation)with no substantialchange in temperature.Available geochronologic data indicatethat peak metamorphismoccurred in Early Cretaceoustime andthat the decompression path developed over the Earlyto LateCretaceous interval. The maximumpressures indicated by thepetrologic data require substantial Late Jurassic(?) - Early Cretaceous tectonicburial even though most recognized thrust faults at this latitudethat have large stratigraphic throwsare assumedor demonstratedto haveearly Mesozoic ages. We postulatethat post-Early Cretaceous extensionalfaults may haveexcised the necessarymiddle Mesozoic thrust structures. While the majority of extensionalstructures responsible for this excisementis likely to be of Neogeneage, associated with Basinand Range extension, the Cretaceousdecompression path described in thispaper is similarto the theoreticalP-T pathsderived from numerical modeling of extensionalunroofing. Given recent evidence for the developmentof extensionalstructures in compressionalregimes like the Himalayanand Alpine orogens,it seemsprudent to searchfor evidenceof Mesozoicextensional structures in futurestudies of the hinterland of the North American Cordillera.

INTRODUCTION sedimentaryand volcanicunits [Troxel, 1988] (Figure2). The contact between these structural packages is marked by a Metamorphic core complexes represent the deepest diachronoussystem of extensionalstructures: the Boundary exposedlevels of the compressionalorogen that characterized Canyon detachment on the north and northwest, and the western North America in Mesozoic time [Armstrong, 1982; younger,predominantly dextral Keane Wonder fault on the west Anderson et al., 1988]; consequently,they offer an excellent [Troxel and Wright, 1989]. Geochronologicand stratigraphic opportunity to examine the thermal evolution of the constraintsindicate that both structuresdeveloped in the last 9 Cordilleran hinterland. Unfortunately,the thermal effects of m.y. [Reynoldset al., 1986]. Tertiary extension were so severe in many of the core The metamorphic core of the range includes (1) complexesthat evidenceof the Mesozoicpressure-temperature amphibolitic, granitic, and pelitic gneisses of early history has been obscured or obliterated. The Funeral Proterozoic(?) age; (2) metasedimentary rocks of late Mountainsof southeasternCalifornia (Figure 1) are unusualin Proterozoic-Cambrianage; (3) muscovite-bearinggranitoid this regardbecause available geochronologic data indicateno rocks of Cretaceousage; and (4) pegmatitic granites of importantTertiary thermalevents in the core complexdespite probablemiddle Tertiary age [Troxel, 1988; Wasserburget al., significant Neogene extensionin the region [DeWitt et al., 1959; DeWitt et al., 1988]. Pre-Cretaceous units contain 1988; Wernicke et al., 1988a]. We have studieda collectionof mineral assemblagesindicative of upper greenschistto upper pelitic schist samplesfrom the high-gradeportions of the in an effort to constrain the Cretaceous amphibolitefacies metamorphism;in general, the degreeof metamorphismincreases from southeastto northwest and pressure-temperatureevolution of the range. In this paper,we reachesa maximum(kyanite + sillimanite+ garnet+ biotite) in reportthe resultsof our researchand discusstheir implications early Proterozoic(?) gneisses and upper Proterozoic for the tectonicevolution of the region. metasedimentaryunits near the mouth of Monarch Canyon GEOLOGICSETTLNG (Figure2) [Labotka,1980]. The age of regional metamorphismin the Funeral TheFuneral Mouintains consist of a metamorphiccore Mountainsisconstrained by40Ar-39Ar mineral ages obtained overlain structurally by upper Proterozoic to Tertiary by DeWitt et al. [1988]. Although most of the spectra indicatedan extraneousargon component,hornblende samples withK20 > 0.6 wt % consistentlyyielded plateau ages of 130- Copyfight1990 by the AmericanGeophysical Union. 115 Ma. Plateauages for muscovitesranged from 110 to 55 Papernumber 89JB03310. Ma, the youngestages being obtained for metamorphic 0148-0227/90/89JB-03310505.00 muscovitesfrom samples collected near Upper Cretaceous

8437 8438 HODGESAND WALKER: UNROOFING OFFUNERAL MOUNTAINS, CALIFORNIA

DeathValley ExtensionalCorridor California-

ß 50 km "' .kFault ....

...

Fig.1. Simplifiedtectonic mapof the Death Valley extensional corridor, Calffomia-Nevada. FMindicates theposition of theFuneral Mountains. Rectangle shows the location of Figure2.

graniticbodies. DeWitt et al. [1988]interpreted these data in metamorphicgrade provide information about different periods termsof EarlyCretaceous regional metamorphism and a Late in thethermal evolution of thearea. For example, lower-grade Cretaceousthermal pulse associated with graniteintrusion. assemblages record more of the progradehistory than do Theabsence of post-earlyEocene muscovite ages in theFuneral higher-grade assemblages, whereas higher-grade assemblages Mountainsstrongly suggests that the metamorphic core had are better recordersof retrogradeprocesses after the cooledwell belowthe closure temperature for Ar in muscovitemetamorphic peak [Hodges and Royden, 1984]. For this study, (-625K [Jiiger,1979]) prior to Neogeneextension in theDeath we weremost interested in theunroofing history of theFuneral Valley region. Mountains. Consequently,we focussedour researchon the highestgrade portions of themetamorphic core. SAMPLESELECTION AND CHARACTERISTICS After examininga large numberof pelitic samples In metamorphicterrains exhibiting a varietyof facies petrographically, we selected seven for detailed types, thermobarometric studies in rocks of different thermobarometricstudy: FM-2, FM-12, FM-14, FM-16, FM- 17, FM-18, and FM-20. SampleFM-2 is a schistfrom the Upper ProterozoicCrystal Spring Formation.The other samplesare peliticgneisses from the basementcomplex of ;:.:•.•,".• i:z•. :::•!•'_-•::-";•:•'2_•';i•,'.•i '. presumedearly Proterozoic age [Troxel and Wright, 1989]. All samples were collected from the lower reaches of Monarch ::•;'.:2'•_•!•i','•-¾:,'..,'.,::,,:_•._xs•.x•_•..•:.•..,;. ..•':']i -•:-•: '.::..:•_•i:-,:..-.-..:::::"':':':':':':':':" -, :::::•..• _...;;• .... ::::::::::::::::::::::::::::. Canyon(Figure 2), andthey represent a single structural level ....:...•.:.:.-';'. to withinroughly 100 m. Thereis continuousoutcrop between thesample localities, and no majorpostmetamorphic structural discontinuitieswere observed. Granitic sills and dikes of .:.:.. :::!:!:: Canyon variousages occur in lower MonarchCanyon; the most abundantare weakly deformed, muscovite granites of probable Late Cretaceousage (E. DeWitt,personal communication, "5:5:5::':"'' .======...... ::i:i:. Keene ß 1988). SamplesFM-14 and FM-18 were collected adjacent to ß..:.:-:.:.:.:.:-:.:.:-:.:-:.:.:ß ...... _...:.:.:.:q.:.. oildel' .::::::.-. one of these bodies. Other sampleswere collectedfrom ...... ii!"0 Fault'.!i!i:•.. outcropsgreater than 5 m awayfrom the granites. Themodal mineralogy of the Monarch Canyon samples is indicatedin Table1. The samplescontain three deformational fabrics:(1) an early schistositypresent as inclusiontrails in [2':'i'"'/..'"•iii!:•":i'•i'"":•}•iiiii•ii]UnmetamorphosedStructuralCover garnet,staurolite, and kyaniteporphyroblasts (Si); (2)the predominantschistosity defined by micas(S1); and (3) late [ [ MetamorphicCore kinkbands best developed in biotiteand kyanite. Timing Fig.2. Simplifiedtectonic map of thenorthern end of theFuneral relationshipsbetween fabric development and porphyroblast Mountains(after Troxel and Wright [1989]). Solid circle indicates the growthare shownin Figure3. Althoughall sampleswere samplingarea in lowerMonarch Canyon. collectedfrom the high grade side of thekyanite + garnet+ HODGESAND WALKER: UNROOFING OF FUNERAL MOUNT•S, CALIFO• 8439

TABLE1. ModalMineraloily FM-6 FM- 12 FM- 14 FM- 16 FM- 17 FM- 18 FM-20 Quartz Muscovite Biotite ...... Garnet Plagioclase ...... Kyanite ...... Staurolite x i .... x Chlorite x Rutlie * * x x * x x ]]menite x ß ß ß x x ß Solid circles,present in sample;crosses, absent in sample;i, presentonly as inclusionsin muscovite.

biotite isograd defined by Labotka [1980], the majority 3 FeTiO3 + A12SiO5 + 2 SiO2 = containedstaurolite as part of the basic progradeassemblage ilmenite kyanite quartz that was stableduring the developmentof Si and SI: garnet+ Fe3A12Si3012+ 3 TiO2 (RAIL) biotite + kyanite+ muscovite+ quartz+ plagioclase.Six of the garnet futile samplesshowed evidence of minor, relatively low-temperature retrogression,including the spotty developmentof chlorite GARB describes Fe and Mg partitioning between reaction rims on garnet and white mica reaction rims on coexistinggarnet and biotite and is very strongly temperature kyanite. Petrographicstudy of otherMonarch Canyon samples dependent.GASP and GRAIL describethe pressuredependency indicated that the degree of low-temperature retrogression of Ca partitioning between garnet and plagioclase and Fe varies greatly from outcropto outcrop. partitioningbetween ilmenite and garnet, respectively. GASP is substantiallymore temperaturedependent than is GRAIL: the nominal Clapyronslope for GASP is 2.2 MPa/deg comparedto Si S • Kink Bands 1.0 MPa/deg for GRAIL [Hodges and McKenna, 1987; McKenna and Hodges, 1988]. Consequently,palcopressure Garnet estimatesmade using GASP are generally less precise than thosemade using GRAIL. Unfortunately,only sampleFM-12 containedthe GRAIL assemblage. Kyanite Experimental calibrations for the reactions considered here include (1) Ferry and Spear [1978] for GARB; (2) Hays Staurolite [1966], Hariya and Kennedy [1968], Goldsmith [1980], Gasparik [1984], and Koziol and Newton [1988] for GASP; and Plagioclase (3) Bohlen et al. [1983] for GRAIL. In this paper,we will use the thermodynamicconstants derived from these experiments Chlorite by Hodges and McKenna [1987] for GARB and GRAIL and thosederived by McKenna and Hodges [1988] for GASP. The Biotite use of other values derived through different statistical treatments of the same experimental data sets would not Muscovite substantiallyalter our results. Rim compositions of garnet, biotite, muscovite, and Fig. 3. Timing relationshipsbetween petrofabricsand principal plagioclasein all samples(as well as ilmenite in FM-12) were porphyroblastgrowth. analyzedusing the JEOL 733 Superprobeat the Massachusetts Institute of Technology. Pertinent mole fractions and uncertainties(reported at the 95% confidencelevel) for these THERMOBAROMETRY phases(Table 2) were calculatedusing the approachdescribed by Hodges and McKenna [1987]. Measuredrim compositions The mineral assemblages observed in the Monarch were very consistentacross the analyzedprobe mounts for each Canyon samplesare appropriatefor the applicationof several section,suggesting a close approachto equilibrium. pelitic thermobarometers.For this study,we choseto use three Final equilibration conditions for all samples were experimentallycalibrated equilibria: estimatedthrough simultaneous solution of GARB and GASP (Table 3 and Figure 4) using the solutionmodels described by Mg3A12Si3012+ KFe3A1Si3010(OH)2 = Hodges and Royden[1984]. For sampleFM-12, simultaneous garnet biotite solutionof GARB and GRAIL yields a pressureand temperature Fe3A12Si3012+ KMg3A1Si3010(OH)2 well within analyticaluncertainty for the GARB-GASP solution garnet biotite (Table 3). As expected, all P-T estimates plot within the kyanite stability field of Holdaway [1971]. Although Ca3A12Si3012+ 2 A12SiO5 + SiO2 = equilibration temperatures show limited variation between garnet kyanite quartz samples,the range of recordedpressures is 425 MPa. This 3 CaA12SiO8 (GASP) range far exceedsthe 2-s precisionlimits for individual plagioclase samples and cannot be explained by analytical uncertainty. 8440 HODGESAND W•R: UNROOFINGOFFUNERAL MOUNTAINS, CALIFORNIA

o GARB-GASP GARB-GRAIL I•< • , TABLET;K 3.ThermobarometrieP,MPa Results T,K , P, MPa •'•' [•+,9q• FM-6FM-12 803(22)810(48) 710(118)859(85) 811 (31) 739(46) 0000 O0 oß• ow o } [• .•• FM-16FM-14 771(42)851(43)646(102) 434(95) 000 • • • FM- 17 789(21) 590(95) t,• ,.-• 0 0 w o • FM-18 788(26) 439(61) o FM-20 822(62) 582(130) • -..... CScS cS [•II •o Num}•ersin'pare•th-e•es •mdi•t•-2-•'an•lard deviation precisioncal'

Comparedto theearlier work of Labotka[1980], who estimated conditions of 875-975 K and 720-960 MPa for the Monarch Canyonarea using GARB-GASP, our findings indicate greater variabilitybetween samples at a given structurallevel and substantiallylower temperaturesand pressures. We attribute much of the discrepancy between the two data sets to advancementsin our understanding of garnet solution behavior andto improvementsin the experimentalcalibration of GASP.

P-T PATHSMODEI.ED FROM GARNET ZONING Garnetporphyroblasts in the MonarchCanyon samples show relatively little compositionalzoning, but slight variationsin zoning profiles correlatewith the pressure variationsin Table3. Figure5 illustratesthe range in observed profilesßGarnets in sampleFM-6, whichyields the highest

•00 equilibrationpressure, have the leastsignificant variations in 0000 core-rimcompositions: Xgr and Xsp are essentially constant while Fe/Mg variesfrom roughly4.8 at the coreto 7.0 at the •00 rim. Sampleswith progressively lower equilibration pressures cScS :• cS • cScS showslightly greater core-rim variations in Fe/Mg, with a maximum of 4.5 to 7.5 for FM-14. Many of the MonarchCanyon garnets contain biotite and plagioclaseinclusions that have higher Xph 1 andXab, respectively,than the rims of theirmatrix counterparts. These 0000

00• •00 to + 1000 + e•0 FuneralSynoptic /1 40

l+ o 800 •00 0000 ß •oo • .• 600 •00 20

lO •' 200 00000 ,el' ß

0000 .•. 0000

400 600 800 1000 1200 Temperature (K) Fig.4. Thermobarometricdatafor the Funeral Mountains samples (FM prefixommitted for clarity). Ellipsesindicate 95% confidencefields for simultaneoussolutions of theGARB and GASP (unpattemed) or GARBand GRAIL (shaded)equilibria. Aluminum silicate stability fieldsare shown [Holdaway, 1971]. Note that samples FM-14 and FM- 18were collected near Late Cretaceous(?) granitic plutons. HODGESAND WALKER:UNROOFING OF FLrNERALMOUNTAINS, CALIFO• 8441

phasesand inclusionsof either biotite or plagioclasecould be used to model AP and AT (Table 4). Although the modeled pathsdisplay some variation in the sign and magnitudeof core- rim AT, all garnetsindicate substantialdecompression during their compositionalevolution (Figure 6).

• Xpy INTERPRETATION OF PETROLOGIC DATA -'- Xllr The variation in final equilibration pressuresshown in Figure 4 could be interpretedin severalways. It is temptingto suggestthat postmetamorphicfaults in Monarch Canyon have juxtaposed samples which were metamorphosedat variable depths. We feel that this interpretationis implausiblebecause 0 0.0 0.5 1.0 1.5 2.0 (1) the existingmap of the area [Troxel and Wright, 1989] and Distance(ram) our observationsreveal no appropriatestructures; (2) there is no variation in the geometryof mesoscopicand microscopic synmetamorphicfabrics between sample locations; and (3) there is no straightforward relationship between sample locationand calculatedpressure. A second possibility is that some or all of the P-T FM-16 estimates are spurious as a consequenceof disequilibrium

• x•l betweenthe phasesused for thermobarometry.It is effectively • xw impossible to prove that measuredmineral compositionsin any sample reflect equilibrium conditions,but the observed consistencyof rim compositionsin each Monarch Canyon sample and the corroborationof GASP by GRAIL in sample FM-12 argue againstsignificant disequilibrium. It is also possible that non-systematicvariations in the 0 0.0 0.5 1.o 1.5 2.0 2.5 chemistry of phases involved in the thermobarometric calculationsaffected some of the P-T estimates. For example, Distance(ram) lndares and Martignole [ 1985] suggestedthat significantTi and A1VI substitutionin biotite can substantially effect the accuracyof GARB geothermometry. Such errors also have a important effect on pressurescalculated by simultaneously solvingGASP and GARB becauseof the relatively high dP/dT slope of GASP. For the Funeral Mountainssamples, the mole FM-14 fractions of Ti and A1 in the octahedral site of biotite range from 0.037 to 0.052 and 0.126 to 0.156, respectively. These ---- Xpy -'- X• limited variationssuggest that empiricalcorrections for Ti and A1VIsuch as those suggested bylndares and Martignole [1985] would not significantly lessen the observed variability in equilibrationP-T. Moreover, regressionsof XTi and XA1VI versus estimated temperature and pressure suggest little

0 correlation, given reasonableuncertainties in the parameters 0.0 0.5 1.0 involved. Distance (mm) We believe that the scatterin Monarch Canyon pressures Fig. 5. Major element zoning profiles for the largest garnetsin indicates that different samples equilibrated at different P-T samplesFM-6, FM-16, and FM-14. Mole fractionabbeviations are conditions (and therefore at different times) during the defined in Table 2. unroofinghistory of the core complex[cf. Hodgesand Royden, 1984]. This interpretation is corroboratedby the P-T paths suggestedby Gibbs' method modeling for individual samples inclusionspermit applicationof the Gibbs' Method approach (Figure 6); for example, FM-18 recordsthe secondlowest rim of Spear and Selverstone [1983] in an effort to model the pressurebut yields a P-T path that extendsto the approximate directionand magnitudeof P-T variationsindicated by garnet P-T conditionsof FM-6, the sample that records the highest zoningin samplesFM- 12, FM- 14, FM- 17, FM- 18, and FM-20. rim pressure. The observationthat samplescollected near late For thispurpose, we assumedthat the samplescould be modeled stage granites (FM-14 and FM-18) yield the lowest rim in the systemK20-CaO-Na20-FeO-MnO-MgO-A1203-SiO 2- equilibrationpressures while the sample collected the greatest H20 and that the model assemblagegarnet + biotite + distanceaway from theseintmsives (FM-6) recordsthe highest muscovite+ plagioclase+ quartz+ kyanite + water was stable pressureimplies that reequilibrationof samples near the low- throughout the interval over which the garnet zoning pressureend of the P-T trajectorymay have been triggeredin developed. These assumptionsresult in a thermodynamic part by the thermal pulse associatedwith intrusion of the varianceof four,such that changes in Xalm, Xgr, Xsp in granites. The preservationof Early Cretaceousmica and garnet,along with differencesin Xann or Xan betweenmatrix amphibole40Ar-39Ar ages in samplescollected only a few 8442 HODGESAND WALKER:UNROOFING OF FUNERALMOUNTAINS, CALIFORNIA

TABLE 4. Gibbs' Method P-T Path Models Sample ModelAssembla[•e Variance Monitors DT(K) DP(MPa)

FM- 12 gr+bi+mu+pg+q+k+H20 4 gr + bi inc. -1 +57 gr + bi inc. +51 +37 gr + bi inc. -55 + 135 net path -5 +229

FM- 14 gr+bi+mu+pg+q+k+H20 4 gr + bi inc. +37 +262

FM-17 gr+bi+mu+pg+q+k+H20 4 gr + pg inc. +87 +215 gr + bi inc. -76 +234 net path + 11 +449

FM- 18 gr+bi+mu+pg+q+k+H20 4 gr + bi inc. -9 + 184 gr + bi inc. -9 +54 gr + bi inc. -6 + 126 gr + bi inc. +4 +18 net path -20 +382

FM-20 gr+bi+mu+pg+q+k+H20 4 gr + pg inc. +60 +93 gr + pg inc. +25 +36 net path +85 + 129

For modelassemblages: gr, garnet;bi, biotite;mu, muscovite;pg, plagioclase;q, quartz;k, kyanite. For monitors,gr: DXalm, DXgr, and DXspderived from the differencebetween garnet rim cooapositionand that measured at aninclusion; bi inc.:DXann derived from the difference betweenbiotite rim compositionand that measured for a biotiteinclusion; pg inc.:DXan derivedfrom the differencebetween plagioclase rim compositionand that measured for a plagioclaseinclusion. Multiple monitored inclusions are fisted in orderof relativedistance from rim toward core. Net pathindicates the cumulativeDT-DP for all monitoredinclusions. metersaway from theseLate Cretaceousgranites argues that temperaturesin excessof 775 K over muchof Cretaceoustime this thermalpulse was very localized[DeWitt et al., 1988]; as a consequenceof episodicgranitic intrusion. Its high- consequently,we interpretthe rim equilibrationconditions of temperaturedecompression path approachesthe kyanite = FM-6 and the highest P-T values implied by Gibbs' method sillimanitetransition; although none of the samplesexamined modeling as representative of Early Cretaceous peak in this study contained sillimanite, some Monarch Canyon metamorphicconditions. samples(notably those collected in the immediatevicinity of Our perceptionof the thermal evolutionof the Monarch granitedikes and sills) containlate fibrolite [Labotka, 1980]. Canyon area is illustratedschematically in Figure 7. Path A A sample tracking on path A does not cool below the representsthe P-T history of a sample that was initially muscovite closure temperature until roughly 75% of its metamorphosedin E•rly Cretaceous time but maintained overburdenhas been removed. We interpret the Paleocene-

40

20

lO

400 600 800 1000 1200 400 600 800 1000 1200 Temperature(IO Temperature(IO Fig. 7. InterpretiveP-T pathsfor the MonarchCanyon area. Path A Fig. 6. Thermodynamicmodeling results for the FuneralMountains holdsfor samplesin the area of graniticintrusion like thoseanalyzed samples.Solid squaresindicate calculated equilibration conditions for in this study (shown in gray for reference). Path B, entirely mineralrims. Open squaresindicate points on the modeledP-T paths hypothetical,holds for samplesunaffected by the thermal pulse that are constrainedby inclusiondata. Arrowsindicate progression associatedwith Cretaceousgranite intrusion. Constanttime lines are from garnetinterion to rims. Larger samplelabels, with FM prefix parallel to the temperatureaxis on this diagram. ApproximateAr omitted,indicate rim P-T; smallerlabels mark P-T conditionssuggested closuretemperatures for hornblende(Th) and muscovite(Tm) are by inclusionsmost distant from the garnetrims. indicatedfor reference.See text for furtherexplanation. HODGES^ND WALKER: UNROOFING OF FUNERAL MOUNTAINS, CALIFO• 8443

EoceneAr muscovitecooling agesof DeWitt et al. [1988] as Corbett et at., 1988], and it seemslikely that at least some of indicativeof cooling along such a path. the overburdenemplaced during Triassic thrus•g would have Althoughall of the petrologicdata obtainedin this study been lost due to erosion by Early Cretaceoustime. It is imply unroofingalong a trajectorysimilar to path A, the 130- tempting to relate part of the "missing" overburden to 115 Ma hornblendeplateau ages and Albian muscoviteplateau movement on thruststhat are strucmrally higher than the Last agesreported by DeWitt et at. [1988] suggestthat Monarch Chancethrust, but the EasternSierran systemis characterized Canyonsamples located some distance away from the granites by structureswith relatively little stratigraphicthrow [Dunne, (and not subject to their thermal effects) may have had 1986], and there is no evidence for Cretaceousmetamorphism trajectories more similar to path B. There is no in the Last Chancethrust plate. thermobarometricevidence that peak temperaturesduring the The lack of sufficient overburden in the Funeral Mountains Early Cretaceousevent were substantially greater than 800-850 leadsus to postulatethe occurrenceof one or more thrustplates K, only 25'-75' higher than the nominal closuretemperature of probableLate Jurassicor Early Cretaceousage betweenthe for Ar diffusion in hornblende [Jiiger, 1979]. A sample Last Chance thrust plate and the high grade exposuresat evolving along path B cools below this closure temperature Monarch Canyon. Existing maps of the northern Funeral early in its unroofinghistory, and it is never subsequently Mountains [Troxet and Wright, 1989] show no suitablethrust reheated. structures,and we are forced to conclude that the postulated Taken in conjunction with available geochronologic thrustplate c.•'plates have been entirely cut out by subsequent constraints,the petrologicdata presentedin this paper indicate extensionalstructures such as the Neogene Boundary Canyon roughly400-600 MPa of decompression,corresponding to 15- detachment(Figure 2). 22 km of unroofing, during the Early to Late Cretaceous interval in the Monarch Canyon area. Where the thermal MECHANISM FOR CRETACEOUS DECOMPRESSION effects of Cretaceous magmatic injection are strongest, temperaturesdo not appearto havevaried by more than 100 K Numerical models of P-T-t pathssuggest that they can be over the unroofing period; significant cooling of these powerfulindicators of tectonicprocesses during metamorphism horizonsdid not occuruntil the majority of unroofinghad been [England and Richardson, 1977; England and Thompson, accomplished. 1984; Royden and Hodges, 1984]. It is possibleto define two basic classesof mechanismsby which metamorphicrocks are broughtto the surfaceof the Earth, andeach of theseresults in a TECrONIC MECHANISMS FOR EARLY CRETACEOUSHIGH-PRESSURE distinctiveP-T-t trajectory[Hodges, 1988]. The fixstof these METAMORPHISM ("C-type") includes erosion as a consequenceof crustal Although Mesozoic metamorphism in the Funeral thickening [Engtand and Richardson,1977] and the upward Mountainswas almost certainly broadly related to development transportof material as a consequenceof the developmentof of the Sierran arc [Labotka and Albee, 1988], the causeof the ramp geometriesin structurallylower thrustfaults. Assuming tectonicburial required by paleopressuresin excess of 800 MPa purely conductiveheat transfer,C-type unroofingpaths are is enigmatic. WidespreadTertiary extensionaldeformation has characterizedby moderatedP/dT and dT/dt slopesafter the cloudedour understandingof the Mesozoictectonic architecture metamorphicpeak. Most of the cooling of samplestraversing of the Death Valley region. Recent attemptsto reconstructthe such paths occursduring exhumation. The secondclass ("E- regionto its preextensionalstructural configuration have been type") involves tectonic unroofing as a consequenceof basedon correlationsof Mesozoic fold and thrustgeometries as movement on structurally higher extensional structures well as isopachand faciestrends in Precambrianto Paleozoic [England and Thompson,1986; Ruppetet aI., 1988]. Because strata [Stewart, 1983, Wernicke et at., 1988a, Wernicke et at., the rate of tectonic denudationis generally fast comparedto 1988b]. Despitethe generalsuccess of theseefforts, important that of erosion, E-type paths commonly have high dP/dT questionsremain concerning the age and displacementof major slopes over much of the unroofing interval. Most of the Mesozoic thrust faults in this part of the Cordillera. Many cooling of samplessubjected to E-type denudationoccurs after foreland thrusts at this latitude have been intruded by Lower extensionand is characterizedby a very high dT/dt slope. Jurassicplutons or can otherwisebe reasonablyinferred to Petrologic and geochronologic data from Monarch havean early Mesozoicage [Dunne, 1986;Burchfiel and Davis, Canyon indicate nearly isothermal decompression of 1988]. West of the foreland thrusts,Dunne [1986] presents Cretaceousage followed by relatively rapid latest Cretaceous- evidencefor a protractedJurassic-Cretaceous movement history early Tertiary cooling, and it is tempting to suggest that on thrustfaults and folds of the East $ierran thrustsystem. CretaceousE-type processesplayed an importantrole in the In the pre-Tertiary reconstructionof the foreland thrust unroofinghistory of the FuneralMountains. Recentstudies in systemproposed by Wernicke et at. [1988a,b], the Funeral compressionalorogens such as the Himalaya [Burchfiet and Mountains occur in the footwall of the westernmostmajor Royden, 1985; Herren, 1987] and the Alps [Mancktetow, foreland structure,the Last Chance thrust [Stewart et at., 1966]. 1985; Platt, 1986; $etverstone, 1988] have led to the This structure is significant in that it has a very large recognition of large extensional structures that may have stratigraphicthrow and a minimum displacementof greater developedto accomodate gravitational collapse of topographic than 50 km [Corbett et at., 1988; Wernicke et at., 1988b]. fronts generated by crustal thickening. Several workers have Unfortunately,the thicknessof the Last Chance thrust plate proposedthat collapseof an "orogenicwelt" developedduring combined with the known stratigraphicthickness of rocks Mesozoic compressionwas largely responsiblefor Cenozoic resting above the Monarch Canyon samplesin the Funeral extensionin the North American Cordillera [e.g., Coney and Mountains is insufficient to account for more than half of the Harms, 1984; Sonder et al., 1987]. Wernicke et at. [1987] 30-37 km of overburdennecessary for the Early Cretaceous have presentedevidence that the time lag betweenthe end of paleodepthsindicated by our study. Moreover,the Last Chance compressionaldeformation and the beginningof extensional thrustis widely assumedto be of Triassicage [Dunne, 1986; deformation increased from north to south in the Cordillera, 8444 HODGESAND WALKER: UNROOFINGOF FUNERALMOUNTAINS, CALIFORNIA and was generallysmallest in areasof increasedCretaceous- Our data require tectonic burial of the Monarch Canyon early Tertiary magmatism. If the thermal structureof the area to depthsof more than 30 km in Early Cretaceoustime. overthickenedlithosphere governs the timing of extensionas The structuresresponsible for this burial remaincryptic, but we Wernickeet al. [1987] and Sonderet al. [1987] suggest,then it postulatethat one or more Early Cretaceousthrust plates above seems reasonableto speculatethat Mid-to-Late Cretaceous the Funeral Mountains have been excised by post-Early extensionmay have occuredin terrainsimmediately adjacent to Cretaceousextensional structures. These include Neogene the Sierranarc (e.g., the FuneralMountains). This hypothesisfaults like the Boundary Canyon detachment, but the gainssupport from the existenceof relativelyhigh-temperature morphology of the P-T path derived in this study raises the extensional fabrics in Monarch Canyon (E. Herren et al., intriguingpossibility that some tectonicunroof'mg may have unpublished data, 1988). The presence of Cretaceous occurred in Middle to Late Cretaceous time. extensional structuresin the Death Valley area could help Acknowledgments.We would like to expressour appreciationto explainthe "missing"overburden discussed previously. Tim Coonan, Peter Rowlands, Pete Sanchez,and the entire NPS staff at On the other hand, no large extensionalstructures of Death Valley National Monument,who make field work in the Funeral Mesozoicage have been recognized in the DeathValley region, Mountains logistically feasable. Extensive discussionswith Clark and comparisonof the MonarchCanyon P-T path with purely Burchild, Eveline Herten, Larry McKenna, Ed DeWitt, John Sutter, BennieTroxel, Brian Wemicke, and LaurenWright haveheightened our conductive thermal models [e.g., Ruppel et al., 1988] is understandingof the Death Valley region. Ed DeWitt and John Sutter hamperedby the presenceof Cretaceousintrusives. Although kindlygave us permission toquote their 40Ar-39Ar results inadvance it is not obviousthat granitic intrusionssignificantly add to of publication, and Lauren Wright provided preprints of map the large-scaleheat budgetin metamorphicterrains [England comilationsfor the Funeral Mountains. DeWitt, Sutter, and Wright and Thompson,1984], it is certainlytrue that advectiveheat made constructivecomments on early drafts of this manuscript. LawfordAnderson and Ron Frost provided thoughtfuland exhaustive transfer alters the thermal structure on a local scale [Lux et al., reviewsof the penultimateversion. None of the reviewersshould be 1986]. This effect is clearly demonstratedin the Monarch held responsiblefor the heretiesespoused in the final paper. Our work Canyonarea, where two distinctivelydifferent P-T histories in Death Valley is supportedby the National Science Foundation can be inferredfor samplesnear Cretaceousintrusions and for throughresearch grants EAR-8721258 and EAR-8816950. samplessome distance away. Eventhough path A in Figure7 is suggestiveof an E-type unroofingmechanism, path B is equallyconsistent with C-type denudation.Moreover, it is Anderson,J.L., A.P. Barth and E.D. Young, Mid-crustal Cretaceous possibleto generatetrajectories similar to path A in a C-type roots of Cordilleranmetamorphic core complexes,Geology, 16, situationby injectingmelts into the system(thereby adding 366-369, 1988. Armstrong,R.L., Cordilleran metamorphiccore complexes-- From heat) during uplift. Until the local effects of Cretaceous Arizona to southem Canada, Annu. Rev. Earth Planet. Sci., 10, intrusions can be subtracted from the overall thermal structure 129-154, 1982. of the FuneralMountains, perhaps by detailedstudies of the P- Bohlen, S.R., V.J. Wall, and A.L. Boettcher, Experimental T-t history of lower-grade, intrusive-free portions of the investigationsand geological apphcationsof equilibria in the systemFeO-TiO2-A1203-SiO2-H20., Am. Mineral., 68, 1049- metamorphiccore, the mechanismof Cretaceousunroofing 1058, 1983. remains enigmatic. Burchfiel, B.C. and G.A. Davis, Mesozoic thrust faults and Cenozoic low-angle normal faults, eastemSpring Mountains,Nevada, and LIMITS ON NEOGENE E-TYPE DENUDATION Clark Mountains thrust complex, California, in This Extended Land: GeologicalJourneys in the SouthernBasin and Range, edited Regardlessof the causeof Cretaceousuplift, the P-T data by D.L. Weide and M.L. Faber, pp. 87-106, Department of presentedhere place firm constraintson the amountof E-type Geoscience,University of Nevadaat ,Las Vegas,1988. unroof'mgthat could have occurredduring Neogene Basin and Burchfiel, B.C., and L.H. Royden, North-south extensionwithin the convergentHimalayan region., Geology,13, 679-682, 1985. Range extension. Final equilibrationof samplesFM-14 and Cemen, I., L.A. Wright, and R.E. Drake, Cenozoicsedimentation and FM-18 at pressuresof 430 to 440 MPa suggestsa depth of sequenceof deformationalevents at the southeasternend of the roughly 16 km for the MonarchCanyon area in Late Cretaceous Furnace Creek strike-slip fault zone, Death Valley region, time. Thick, largely preextensional Oligocene-Miocene California, in Strike-Slip Deformation, Basin Formation, and Sedimentation,edited by K.T. Biddle and N. Christie-Blick,pp. fanglomeratesequences in the southernFuneral Mountains and 127-141, Soc. Econ. Paleontol. Mineral., Denver, 1985. eastern [Reynolds, 1974; Cemen et al., Coney, P.J., and T.A. Harms, Cordilleran metamorphic core 1985] suggest that at least some of the Late Cretaceous complexes: Cenozoic extensional relics of Mesozoic overburdenwas eroded away prior to late Cenozoicextension. compression.,Geology, 12, 550-554, 1984. Consequently,an absolutemaximum of 15 km of cover may Corbeu, K., C.T. Wmcke, and C.A. Nelson, Structure and tectonic historyof the Last Chancethrust system, Inyo Mountainsand Last have been removed from the Funeral Mountains by tectonic Chance Range, California, in This Extended Land, Geological denudationin Neogene time. Journeysin the SouthernBasin and Range,Field Trip Guidebook, editedby D.L. Weide andM.L. Faber,pp. 269-292,Department of CONCLUSIONS Geoscience,University of Nevada at Las Vegas,Las Vegas,1988. DeWitt, E., J.F. Sutter, L.A. Wright, and B.W. Troxel, Ar-Ar Rim thermobarometryand P-T path modeling of pelitic chronologyof Early Cretaceousregional metamorphism, Funeral samples from the Monarch Canyon area suggest peak Mountains, Califomia--A case study of excessargon, Geol. Soc. Am. Abstr. w. Programs,20, A16-A17, 1988. metamorphicconditions of 800-850 K and 800-1000 MPa, Dunne, G.C., Geologic evolution of the southernInyo Range, Darwin followed by as much as 600 MPa of broadly isothermal Plateau,and Argus and Slate Ranges,east-central California: An decompression.Available geochronologicdata [DeWitt et al., overview, in Mesozoic and Cenozoic Structural Evolution of 1988] suggeststhat the metamorphicpeak occurredin Early SelectedAreas, East-Central California, Field Trip Guidebook, Cretaceoustime and that the low-pressureterminus of the P-T editedby G.C. Dunne,pp. 3-21, CordilleranSection, Geological Societyof America, Boulder, 1986. path correspondsto conditionsin Late Cretaceoustime. The England, P.C. and S.W. Richardson,The influenceof erosionupon average unroofing rate over the Early to Late Cretaceous mineral facies of rocksfrom different metamorphicenvironments, interval was approximately0.5 mm/yr. J. Geol. Soc. London, 134, 201-213, 1977. HODGESAND WALKER: UNROOFING OF FUNERAL MOUNTAINS, CAI•ORNIA 8445

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